{"gene":"ACAT1","run_date":"2026-06-09T22:02:38","timeline":{"discoveries":[{"year":2020,"finding":"Cryo-EM structure of human ACAT1 revealed it forms a dimer-of-dimers tetramer; each protomer has nine transmembrane segments enclosing a cytosolic tunnel and a transmembrane tunnel that converge at the predicted catalytic site. Structure-guided mutagenesis showed acyl-CoA enters through the cytosolic tunnel and cholesterol likely enters from the side through the transmembrane tunnel, rationalizing ACAT1's preference for unsaturated acyl chains.","method":"Cryo-EM structure determination plus structure-guided mutagenesis and biochemical analyses","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure with mutagenesis validation, two independent papers published simultaneously in the same journal confirming tetrameric architecture","pmids":["32433614"],"is_preprint":false},{"year":2020,"finding":"Cryo-EM structure of human ACAT1 in complex with the clinical-stage inhibitor nevanimibe confirmed a tetrameric (dimer-of-dimers) holoenzyme; each monomer has nine transmembrane helices with TM4–TM9 forming a cavity that accommodates nevanimibe and an endogenous acyl-CoA; this cavity contains the catalytically essential histidine residue. The structure provides a physical model for cholesterol esterification and inhibitor binding.","method":"Cryo-EM structure determination with bound inhibitor, biochemical analyses","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure with bound ligand, corroborated by the parallel Qian et al. structure in the same issue","pmids":["32433613"],"is_preprint":false},{"year":1998,"finding":"Recombinant human ACAT1 purified to homogeneity exhibits allosteric (sigmoidal) kinetics with respect to cholesterol substrate whether assayed in mixed micelles or reconstituted vesicles, while the oleoyl-CoA saturation curves are hyperbolic, supporting the hypothesis that ACAT1 is an allosteric enzyme regulated by cholesterol.","method":"Protein purification (~7000-fold), in vitro enzyme kinetics in mixed micelles and reconstituted vesicles","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — purified enzyme reconstitution with rigorous kinetic analysis in two different lipid systems","pmids":["9857049"],"is_preprint":false},{"year":2000,"finding":"ACAT1 topology study using glycosylation reporters and FLAG epitope tags showed ACAT1 spans the ER membrane five times with its N-terminus in the cytosol and C-terminus in the ER lumen. The putative active-site serine (Ser269 in ACAT1) is positioned on the cytosolic face; mutation of Ser269 inactivated ACAT1, demonstrating its catalytic essentiality.","method":"Glycosylation reporter constructs, FLAG epitope accessibility assays, in vitro microsomal assembly, immunofluorescence microscopy, site-directed mutagenesis","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal topology methods plus active-site mutagenesis confirming functional role of Ser269","pmids":["11071899"],"is_preprint":false},{"year":2006,"finding":"Histidine residues H386 and H460 in ACAT1 are essential for catalytic activity; H386A and H460A substitutions abolished enzymatic activity without altering substrate-binding affinity, indicating these residues are catalytic rather than structural. Partial restoration of H386A activity with 25-hydroxycholesterol (but not cholesterol) as substrate further defined the catalytic role.","method":"Site-directed mutagenesis of histidine residues, baculovirus expression, enzyme activity assays, substrate-binding affinity measurements","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 / Strong — active-site mutagenesis with multiple substitutions and substrate specificity analysis","pmids":["16647063"],"is_preprint":false},{"year":2010,"finding":"Purified human ACAT1 binds oleoyl-CoA with Kd ~1.9 μM, inducing significant intrinsic fluorescence changes indicating structural rearrangement; cholesterol binding produces larger fluorescence changes than its diastereomer epicholesterol, demonstrating stereospecificity (3β-OH on steroid ring A) in substrate recognition by ACAT1.","method":"Protein purification from H293 cells, difference intrinsic fluorescence spectroscopy","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — purified protein with direct binding measurements and stereospecificity controls, single lab","pmids":["20964445"],"is_preprint":false},{"year":2014,"finding":"Mitochondrial ACAT1 was identified as an acetyltransferase for PDHA1 (K321) and PDP1 (K202) in the pyruvate dehydrogenase complex; K321 acetylation of PDHA1 recruits PDK1 to inhibit PDC activity, and K202 acetylation of PDP1 dissociates its substrate PDHA1, both promoting the Warburg effect. SIRT3 acts as the opposing deacetylase. Y381 phosphorylation of PDP1 dissociates SIRT3 and recruits ACAT1 to the PDC.","method":"Mass spectrometry, Co-IP, in vitro acetyltransferase assays, site-directed mutagenesis, siRNA knockdown with tumor growth readout, EGF stimulation in cancer cells","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — multiple orthogonal methods (MS, Co-IP, in vitro assay, mutagenesis, KD with phenotype) in a single rigorous study","pmids":["24486017"],"is_preprint":false},{"year":2000,"finding":"In human macrophages, ACAT1 localizes to the tubular rough ER under normal conditions; upon cholesterol loading, ~30–40% of total ACAT1 immunoreactivity shifts into small ER-derived vesicles that are also enriched in the ER marker GRP78, suggesting cholesterol overload activates an ER vesiculation process.","method":"Immunoelectron microscopy, immunoblot, immunofluorescence microscopy, subcellular fractionation","journal":"The American journal of pathology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal localization methods with direct functional context in human macrophages","pmids":["10623671"],"is_preprint":false},{"year":2010,"finding":"Cholesterol loading of macrophages causes ACAT1 to redistribute from high-density ER membranes to lower-density ER-derived vesicles positive for both ER and trans-Golgi network markers; these vesicles exhibit ~3-fold higher ACAT1-specific enzymatic activity than ER membranes, and reconstitution assays showed this is not due to increased cholesterol content in the vesicles.","method":"Subcellular fractionation, in vitro ACAT enzyme activity assays on isolated membranes, reconstituted ACAT assay","journal":"Journal of lipid research","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — direct biochemical fractionation with reconstituted enzyme activity measurements, mechanistically rigorous","pmids":["20460577"],"is_preprint":false},{"year":1999,"finding":"The human ACAT1 4.3-kb mRNA is produced from sequences on two different chromosomes (exons 1–16 and P1 promoter on chromosome 1; exon Xa and P7 promoter on chromosome 7), requiring a novel interchromosomal trans-splicing mechanism of two discontinuous precursor RNAs.","method":"Genomic cloning, gene mapping, RNase protection assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — gene mapping plus RNase protection assays establishing interchromosomal origin of chimeric mRNA","pmids":["10196189"],"is_preprint":false},{"year":2004,"finding":"The chimeric 4.3-kb human ACAT1 mRNA (derived from chromosomes 1 and 7 via trans-splicing) produces a 56-kDa ACAT1 isoform using a GGC (glycine) start codon upstream of the normal AUG; this 56-kDa isoform localizes to the ER and is enzymatically active. Both chromosomal sequences are required for its production.","method":"Expression in CHO cells, site-directed mutagenesis, mass spectrometry, immunoblot with isoform-specific antibodies, subcellular localization, enzyme activity assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — mutagenesis, MS, enzyme activity, and localization data in a single comprehensive study","pmids":["15319423"],"is_preprint":false},{"year":2008,"finding":"Production of the 56-kDa ACAT1 isoform from the chimeric mRNA requires two specific RNA secondary structures (stem-loops at nt 1255–1268 from chromosome 7 and nt 1286–1342 from chromosome 1) flanking the GGC start codon; translation initiation from GGC is mediated by an IRES mechanism requiring an upstream AU-constituted structure and downstream GC-rich structure.","method":"Deletion and point mutation analyses of stem-loop sequences, monocistronic and bicistronic vector experiments with stable hairpin, western blot","journal":"Cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple mutational analyses in single lab establishing IRES mechanism","pmids":["18542101"],"is_preprint":false},{"year":1998,"finding":"Immunodepletion experiments demonstrated that ACAT1 protein accounts for ~90% of ACAT activity in adult human liver, ~98% in adrenal gland, ~91% in macrophages, and ~80% in kidney, but only ~19% in intestines, establishing tissue-specific catalytic roles.","method":"Immunodepletion with anti-ACAT1 antibodies followed by residual activity measurement in solubilized tissue homogenates","journal":"Journal of lipid research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct quantitative immunodepletion-activity assays across multiple human tissues, single lab","pmids":["9717734"],"is_preprint":false},{"year":2000,"finding":"In adult human liver, ACAT1 (not ACAT2) accounts for 85–90% of total ACAT activity as measured by immunodepletion. ACAT1 and ACAT2 do not form hetero-oligomeric complexes. In intestinal enterocytes, ACAT2 (not ACAT1) is the dominant isoenzyme, concentrated at villus apices.","method":"Specific polyclonal antibody immunodepletion, immunohistochemistry of human intestine and liver, activity assays in HepG2 and Caco-2 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — quantitative immunodepletion, cell-based activity assays, histological localization; replicated key findings from prior immunodepletion study","pmids":["10846185"],"is_preprint":false},{"year":2003,"finding":"Purified ACAT1 expressed in insect cells exists preferentially as oligomers (dimer to tetramer); ACAT1 esterifies cholesterol more rapidly than ACAT2, which more efficiently esterifies cholic acid derivatives, demonstrating distinct substrate specificities. Pyripyropene A selectively inhibits ACAT2 (IC50 = 0.64 μM) but not ACAT1.","method":"Baculovirus-insect cell expression, protein purification, enzyme kinetics, inhibitor screening","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — purified enzyme kinetics but single lab, limited mechanistic follow-up","pmids":["13679053"],"is_preprint":false},{"year":2003,"finding":"ACAT1 selectively esterifies oleic acid over polyunsaturated fatty acids in intact cells and microsomes (THP-1 macrophages), while ACAT2-expressing cells show broader unsaturated fatty acid utilization; K-604 competitively inhibits ACAT1 with respect to oleoyl-CoA (Ki = 0.378 μM), establishing the oleoyl-CoA binding site as part of the active site.","method":"NBD-cholesterol fluorescence assay, specific inhibitor kinetics in AC29 cells stably transfected with ACAT1 or ACAT2, microsomal activity assays","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 1–2 / Moderate — cell-based and microsomal assays with specific inhibitor kinetics, replicates fatty acid preference findings","pmids":["14617738","16820149"],"is_preprint":false},{"year":2014,"finding":"Pharmacological blockade (K604) or genetic knockout of ACAT1 in microglia stimulates autophagosome formation and TFEB-mediated lysosomal proteolysis in an mTOR-independent manner, increasing phagocytic uptake and lysosomal degradation of oligomeric Aβ1-42. The effect can be modulated by agents that disrupt cholesterol biosynthesis.","method":"Acat1 KO mouse primary microglia, specific ACAT1 inhibitor K604, autophagosome formation assays, TFEB reporter assays, lysosomal degradation assays, phagocytosis assays, mTOR signaling analysis","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic KO plus pharmacological inhibition with multiple cellular readouts in one study","pmids":["25339759"],"is_preprint":false},{"year":2023,"finding":"Acute ACAT1/SOAT1 blockade increases cholesterol content at the mitochondria-associated ER membrane (MAM), where ACAT1 is enriched; this increases the number of ER-mitochondria contact sites and shortens the distance between these organelles, strengthening ER-mitochondria connectivity.","method":"Biochemical fractionation (MAM isolation), MAM proteomics, confocal and electron microscopy of ER-mitochondria contact sites","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — orthogonal methods (fractionation, proteomics, EM) but single lab","pmids":["36982602"],"is_preprint":false},{"year":2023,"finding":"ACAT1/SOAT1 blockade in microglia alters the intracellular fate of TLR4 by increasing TLR4 endocytosis and enhancing its trafficking to lysosomes for degradation, thereby suppressing LPS-induced pro-inflammatory signaling. Myeloid-specific Acat1 KO mice showed markedly attenuated LPS-induced neuroinflammation in hippocampus and cortex.","method":"Myeloid-specific Acat1 KO mice, pharmacological ACAT1 inhibitor K604, TLR4 localization by microscopy, endocytosis assays, cytokine measurements","journal":"International journal of molecular sciences","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic KO in vivo plus pharmacological inhibition with defined molecular mechanism (TLR4 trafficking)","pmids":["36982689"],"is_preprint":false},{"year":2018,"finding":"Myeloid-specific Acat1 knockout reduces Ly6Chi monocyte integrin-β1 expression, impairing monocyte adhesion to inflamed endothelium and infiltration into adipose tissue; this attenuates Western diet-induced obesity and adipose tissue inflammation. ACAT1 inhibition in RAW264.7 macrophages also reduces LPS-induced inflammatory responses.","method":"Myeloid-specific Acat1 KO mouse model, adoptive transfer, flow cytometry, integrin-β1 expression analysis, small-molecule ACAT1 inhibitor in cell culture","journal":"American journal of physiology. Endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic KO with adoptive transfer and mechanistic cell biology, multiple readouts","pmids":["29533741"],"is_preprint":false},{"year":2010,"finding":"In cholesterol-loaded macrophages, approximately 20% of total ACAT1 co-localizes with the late endosome/lysosome marker LAMP2, and ACAT1-positive membranes isolated by immunoadsorption contain LAMP2; cholesterol-loaded macrophages can re-esterify LDL-derived cholesterol even when cholesterol egress from late endosomes is blocked by U18666A, suggesting ACAT1-positive LE/LS facilitate direct esterification of lipoprotein-derived free cholesterol.","method":"Confocal fluorescence microscopy, immunoelectron microscopy, subcellular fractionation, immunoadsorption, cholesterol esterification assay with pharmacological block","journal":"Journal of atherosclerosis and thrombosis","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods establishing localization and functional consequence, single lab","pmids":["20523008"],"is_preprint":false},{"year":2005,"finding":"ACAT1 deficiency in peritoneal macrophages increases de novo cholesterol synthesis by 134% and upregulates SREBP1a mRNA 6-fold, indicating that ACAT1 normally suppresses the SREBP-driven cholesterol/fatty acid synthesis program; total cellular cholesterol efflux increases proportionally but esterification of new cholesterol is reduced by 93%.","method":"Radiotracer cholesterol synthesis and efflux assays in ACAT1−/− peritoneal macrophages, qPCR for SREBP1a","journal":"Atherosclerosis","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct biochemical measurements in KO cells, single lab, limited pathway validation","pmids":["16144700"],"is_preprint":false},{"year":2004,"finding":"ACAT1 deficiency in macrophages reduces overall cellular cholesterol efflux by 25% despite upregulated ABCA1 expression, while efflux of lipoprotein-derived (acLDL) cholesterol is increased by 32%; ACAT1-deficient macrophages accumulate 26% more free cholesterol from acLDL and show a 75% increase in intracellular vesicles.","method":"ACAT1−/− peritoneal macrophages, radiolabeled cholesterol efflux assays, ABCA1 expression analysis, vesicle quantification","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct KO with quantitative efflux assays and ABCA1 expression; replicated in subsequent bone marrow transplant studies","pmids":["15499044"],"is_preprint":false},{"year":2023,"finding":"ACAT1 acetylates ME2 (malate enzyme 2) at lysine 156, potentiating ME2 enzyme activity and promoting conversion of glutamine-derived malate to pyruvate and subsequently lactate; this ME2-derived lactate facilitates lactylation of homologous recombination repair proteins, contributing to chemoresistance in ovarian cancer. Glucose deprivation triggers this ACAT1-ME2 axis.","method":"Co-IP, site-directed mutagenesis of ME2-K156, in vitro ME2 enzyme activity assays, metabolomics, in vitro and in vivo cancer models","journal":"Advanced science","confidence":"Medium","confidence_rationale":"Tier 1–2 / Weak — multiple methods but single lab, novel substrate identification needing independent replication","pmids":["39951294"],"is_preprint":false},{"year":2023,"finding":"ACAT1 acetylates METTL3, and this interaction (demonstrated by Co-IP and GST pulldown) stabilizes METTL3 protein by inhibiting ubiquitin-proteasome-mediated degradation; ACAT1-mediated METTL3 stabilization suppresses TNBC cell migration and invasion. NR2F6 transcriptionally activates ACAT1 to regulate this axis.","method":"Co-IP, GST pulldown, IP for ubiquitination, qPCR, western blot, migration/invasion assays","journal":"Genes and immunity","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP/pulldown, single lab, mechanistic acetylation not directly demonstrated","pmids":["36890220"],"is_preprint":false},{"year":2023,"finding":"ACAT1 loss in lung tumor cells leads to mitochondrial protein hypersuccinylation and enhanced mitochondrial oxidative metabolism; this increases ROS, which impedes tertiary lymphoid structure (TLS) formation. ACAT1 knockdown reduced ROS and promoted B-cell aggregation and TLS construction, sensitizing tumors to anti-PD1 therapy.","method":"In vivo CRISPR screening in orthotopic lung tumor model, succinylation proteomics, ROS measurements, B-cell aggregation assays, NAC treatment, NSCLC tissue microarray (305 patients)","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo CRISPR screen plus mechanistic follow-up with multiple methods, single lab","pmids":["40166933"],"is_preprint":false},{"year":2024,"finding":"SIRT5 acts as a desuccinylase of ACAT1, removing succinyl groups from ACAT1 and enhancing its enzymatic activity; SIRT5-mediated ACAT1 activation then activates the NRF2 pathway and inhibits secretion of CCL5 and CXCL10 chemokines that recruit CD8+ T cells, contributing to an immunosuppressive tumor microenvironment in EGFR-mutant LUAD.","method":"Desuccinylation assay, ACAT1 activity measurements, chemokine secretion assays, T-cell recruitment assays","journal":"Heliyon","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, limited mechanistic validation of direct succinylation site on ACAT1","pmids":["39524872"],"is_preprint":false},{"year":2023,"finding":"ACAT1 inhibition suppresses fatty acid oxidation and acetyl-CoA production while increasing free fatty acids in glioblastoma cells; this restores mitochondrial function and negatively regulates the choline metabolic pathway, which is required for GBM cell differentiation into astrocytes. Chlorogenic acid inhibits ACAT1 phosphorylation to achieve this effect.","method":"ACAT1 KD in GBM cells, Seahorse metabolic assays, metabolomics, in vitro and in vivo differentiation assays","journal":"International journal of biological sciences","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, indirect mechanism linking ACAT1 inhibition to choline metabolism","pmids":["39494339"],"is_preprint":false},{"year":2023,"finding":"UBE3A ubiquitin ligase ubiquitinates ACAT1 (identified by orthogonal ubiquitin transfer), promoting its degradation; on a high-fat diet, reduced hepatic UBE3A expression is associated with increased ACAT1 protein and ketone body accumulation; forced UBE3A expression in mouse liver decreases ACAT1 protein content.","method":"Orthogonal ubiquitin transfer platform, overexpression in HEK293 cells, mouse liver forced expression, western blot, metabolite measurements","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — orthogonal ubiquitin transfer (novel method) plus in vivo mouse liver validation, single lab","pmids":["36920305"],"is_preprint":false},{"year":2024,"finding":"ACAT1-deficient preadipocytes show inhibited SREBP2-mediated cholesterol uptake, reduced intracellular and plasma membrane cholesterol, and impaired PPARγ transcription and adipogenesis; rescue with catalytically functional but not catalytic-dead ACAT1 restores cholesterol levels and PPARγ transcription, demonstrating that ACAT1 enzymatic activity is required for normal adipogenesis.","method":"ACAT1 KO, overexpression of WT vs. catalytic-dead ACAT1, cholesterol measurements, PPARγ transcription analysis, cholesterol replenishment rescue experiments","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — catalytic-dead rescue experiment plus KO with multiple mechanistic readouts, single lab","pmids":["39481851"],"is_preprint":false},{"year":2019,"finding":"Myeloid-specific Acat1 KO in the ApoE-KO mouse model for advanced atherosclerosis significantly reduced lesion cholesterol crystal content, lesion size, and macrophage content without increasing apoptotic cell death; cell culture studies showed that ACAT1 inhibition in macrophages reduces pro-inflammatory responses to cholesterol loading by acetyl-LDL.","method":"Myeloid-specific Acat1 KO in ApoE-KO mice, lesion analysis (cholesterol crystal quantification, macrophage immunostaining), macrophage cell culture inflammatory response assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic KO in advanced disease model with mechanistic cell culture follow-up, builds on prior work","pmids":["31495784"],"is_preprint":false},{"year":2013,"finding":"Global Acat1 knockout causes a higher proportion of Lin−Sca-1+c-Kit+ hematopoietic stem/progenitor cells to proliferate, resulting in increased myeloid progenitor cell numbers and leukocytosis in normal mice and elevated monocytosis in Apoe−/− mice during atherosclerosis, demonstrating that ACAT1 plays a role in regulating hematopoiesis.","method":"Acat1−/− mouse bone marrow analysis, flow cytometry of Lin−Sca-1+c-Kit+ population, myeloid progenitor cell quantification","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct KO phenotype with bone marrow cell population analysis, single lab","pmids":["23846496"],"is_preprint":false},{"year":2010,"finding":"ACAT1 gene ablation in a 3xTg-AD mouse model increases 24(S)-hydroxycholesterol content by 32% in the brain at 4 months, decreases HMG-CoA reductase protein by 65%, reduces sterol synthesis rate by 28%, and decreases full-length hAPP and its proteolytic fragments by >60%; treating hippocampal neurons with 24(S)-hydroxycholesterol recapitulates reductions in hAPP and HMGR, placing cholesterol esterification upstream of APP processing.","method":"Acat1 KO in 3xTg-AD mice, 24(S)-hydroxycholesterol measurement, HMGR activity assay, neuronal cell treatment with oxysterol, amyloid quantification","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic KO in disease model plus mechanistic cell culture validation with oxysterol, single lab","pmids":["20133765"],"is_preprint":false},{"year":2007,"finding":"ACAT1 RNAi (~50% protein reduction) decreases cholesteryl ester levels by 22%, causes slight increase in ER free cholesterol, and reduces Aβ secretion by 40%, demonstrating that ACAT1 activity influences amyloidogenic APP processing.","method":"RNAi knockdown, cholesteryl ester quantification, Aβ ELISA","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single method KD with defined biochemical readouts, replicated in principle by subsequent KO studies","pmids":["17412327"],"is_preprint":false},{"year":2015,"finding":"ACAT1 inhibition in SK-N-SH neuronal cells increases free cholesterol at the plasma membrane (PM-FC), and this increased PM-FC reduces APP α-processing; the effect on PM-FC and α-processing is independent of total cellular cholesterol and persists even when the NPC-dependent cholesterol trafficking pathway is blocked, identifying an ACAT1-dependent pathway for shuttling PM-FC to the intracellular pool.","method":"ACAT1 siRNA and pharmacological inhibitor, free cholesterol measurement by filipin staining, APP α-processing assay, NPC pathway blockade","journal":"Acta biochimica et biophysica Sinica","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — two perturbation approaches with mechanistic controls, single lab","pmids":["26474739"],"is_preprint":false},{"year":2009,"finding":"Leptin increases ACAT1 protein expression (~1.9-fold) and ACAT activity (~1.8-fold) in human monocyte-derived macrophages via JAK2 and PI3K signaling pathways, promoting cholesteryl ester accumulation and suppressing HDL-mediated cholesterol efflux; the efflux suppression is reversed by the ACAT1 inhibitor K604.","method":"Leptin treatment of differentiating monocytes, JAK2 and PI3K inhibitors, ACAT activity assay, western blot, cholesterol efflux assay","journal":"American journal of physiology. Endocrinology and metabolism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological pathway dissection with multiple readouts, single lab","pmids":["19625677"],"is_preprint":false},{"year":2009,"finding":"TNF-α, through NF-κB, specifically enhances ACAT1 (but not ACAT2) gene expression in differentiating human monocytes; a functional NF-κB element in the human ACAT1 proximal promoter is required for this effect; increased ACAT1 promotes cholesteryl ester accumulation and lipid-laden cell formation.","method":"TNF-α treatment, NF-κB inhibitors, promoter reporter assays, mutational analysis of NF-κB binding site, cholesteryl ester accumulation assay","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — promoter mutagenesis plus inhibitor studies defining the NF-κB element, single lab","pmids":["19189937"],"is_preprint":false}],"current_model":"ACAT1 (mitochondrial acetoacetyl-CoA thiolase / acyl-CoA:cholesterol acyltransferase 1) is a multi-functional enzyme: in the ER membrane it forms an allosteric, cholesterol-regulated tetrameric enzyme (resolved by cryo-EM) that esterifies free cholesterol using long-chain fatty acyl-CoA—with an essential cytosolic-facing catalytic serine (Ser269) and active-site histidines (H386, H460)—thereby maintaining ER cholesterol homeostasis; in mitochondria it functions as an acetyltransferase that acetylates PDHA1 (K321) and PDP1 (K202) to inhibit the pyruvate dehydrogenase complex and promote the Warburg effect, with SIRT3 as the opposing deacetylase; ACAT1 is enriched at MAM/ER–mitochondria contact sites, and its inhibition strengthens ER–mitochondria connectivity; in macrophages, ACAT1 controls cholesterol efflux, TLR4 turnover, and inflammatory responses, with NF-κB-driven transcriptional regulation by TNF-α; and in neurons, ACAT1 activity influences APP processing and amyloid-β generation via control of plasma membrane and ER cholesterol pools."},"narrative":{"mechanistic_narrative":"ACAT1 is a cholesterol-esterifying enzyme that maintains intracellular cholesterol homeostasis and, through this lipid-handling role, governs membrane sterol pools in macrophages, neurons, and adipocytes [PMID:32433614, PMID:9857049, PMID:39481851]. In the ER membrane it assembles as a dimer-of-dimers tetramer in which each protomer presents converging cytosolic and transmembrane tunnels that admit acyl-CoA and cholesterol, respectively, to a shared catalytic site; long-chain unsaturated acyl-CoA (preferentially oleoyl-CoA) is conjugated to free cholesterol, and the enzyme behaves allosterically with sigmoidal kinetics toward its cholesterol substrate [PMID:32433614, PMID:32433613, PMID:9857049]. Catalysis depends on a cytosolic-facing active-site serine (Ser269) and on histidines H386 and H460, and substrate recognition is stereospecific for the 3β-hydroxyl steroid configuration [PMID:11071899, PMID:16647063, PMID:20964445]. ACAT1 accounts for the dominant fraction of cholesterol-esterifying activity in human liver, adrenal, macrophage, and kidney tissue, and is enriched at mitochondria-associated ER membranes, where its inhibition raises local cholesterol and strengthens ER–mitochondria contacts [PMID:9717734, PMID:36982602]. In macrophages, ACAT1 controls the balance between cholesterol esterification and efflux and is transcriptionally driven by TNF-α through an NF-κB element in its promoter; myeloid ACAT1 ablation attenuates atherosclerotic lesion formation, adipose inflammation, and TLR4-dependent inflammatory signaling [PMID:15499044, PMID:19189937, PMID:31495784, PMID:36982689]. In neurons, ACAT1 activity controls plasma-membrane and ER free-cholesterol pools and thereby modulates APP processing and amyloid-β generation, placing cholesterol esterification upstream of amyloidogenesis [PMID:20133765, PMID:26474739]. Beyond its esterase function, mitochondrial ACAT1 acts as a protein acetyltransferase that acetylates PDHA1 (K321) and PDP1 (K202) to inhibit the pyruvate dehydrogenase complex and promote the Warburg effect, opposed by the deacetylase SIRT3 [PMID:24486017]. The human enzyme is unusual in arising from a chimeric mRNA assembled by interchromosomal trans-splicing, which additionally yields a 56-kDa ER-localized isoform via IRES-dependent translation from a GGC start codon [PMID:10196189, PMID:15319423].","teleology":[{"year":1998,"claim":"Established that ACAT1 is the dominant cholesterol-esterifying enzyme in most human tissues and that it behaves as an allosteric enzyme regulated by its cholesterol substrate, defining its homeostatic role.","evidence":"Immunodepletion-activity assays across human tissues and purified-enzyme kinetics in micelles and reconstituted vesicles","pmids":["9717734","9857049"],"confidence":"High","gaps":["Structural basis of allostery not yet resolved","Tissue partitioning relative to ACAT2 not fully defined"]},{"year":2000,"claim":"Resolved the membrane topology and identified the catalytically essential cytosolic-facing Ser269, and distinguished ACAT1 from ACAT2 as non-interacting isoenzymes with tissue-specific dominance.","evidence":"Glycosylation reporter and FLAG accessibility assays with active-site mutagenesis; immunodepletion and immunohistochemistry of human liver and intestine","pmids":["11071899","10846185"],"confidence":"High","gaps":["Later cryo-EM revised the transmembrane segment count","Mechanism coupling Ser269 to the histidine residues not yet established"]},{"year":2006,"claim":"Defined H386 and H460 as catalytic rather than structural residues, refining the active-site chemistry of cholesterol esterification.","evidence":"Histidine site-directed mutagenesis with baculovirus expression, activity and substrate-binding assays, and oxysterol substrate rescue","pmids":["16647063"],"confidence":"High","gaps":["Precise catalytic mechanism (general base vs. nucleophile roles) not fully resolved"]},{"year":2010,"claim":"Provided direct binding evidence for stereospecific substrate recognition, showing acyl-CoA and cholesterol binding induce conformational change.","evidence":"Difference intrinsic fluorescence spectroscopy on purified human ACAT1 with cholesterol vs. epicholesterol","pmids":["20964445"],"confidence":"High","gaps":["Binding stoichiometry per tetramer not determined","Order of substrate addition not established"]},{"year":2020,"claim":"Cryo-EM structures resolved the tetrameric architecture and the converging substrate tunnels, giving a physical model for catalysis, acyl-chain preference, and inhibitor binding.","evidence":"Cryo-EM of human ACAT1 alone and with the inhibitor nevanimibe, plus structure-guided mutagenesis","pmids":["32433614","32433613"],"confidence":"High","gaps":["Conformational basis of the allosteric cholesterol response not captured","Dynamics of substrate entry not directly visualized"]},{"year":1999,"claim":"Demonstrated the human ACAT1 transcript arises by interchromosomal trans-splicing, a novel origin for a metabolic enzyme mRNA.","evidence":"Genomic cloning, gene mapping, and RNase protection assays","pmids":["10196189"],"confidence":"High","gaps":["Trans-splicing machinery not identified","Physiological significance of the chimeric origin unclear"]},{"year":2008,"claim":"Showed the chimeric mRNA produces an enzymatically active 56-kDa ER isoform via IRES-dependent initiation from a GGC codon flanked by required stem-loops.","evidence":"Expression, mutagenesis, MS and localization for the isoform; deletion/point mutation of stem-loops in mono- and bicistronic reporters","pmids":["15319423","18542101"],"confidence":"Medium","gaps":["IRES trans-acting factors not identified","Functional role distinct from the canonical isoform unresolved"]},{"year":2004,"claim":"Defined the macrophage cholesterol balance role, showing ACAT1 deficiency reroutes lipoprotein-derived cholesterol toward efflux while reducing net efflux despite ABCA1 upregulation.","evidence":"Radiolabeled cholesterol efflux and esterification assays in ACAT1−/− peritoneal macrophages with ABCA1 and SREBP1a readouts","pmids":["15499044","16144700"],"confidence":"Medium","gaps":["Mechanism linking esterification to SREBP suppression not fully dissected","In vivo efflux consequences not measured here"]},{"year":2010,"claim":"Mapped the dynamic subcellular redistribution of ACAT1 upon cholesterol loading into more active ER-derived and late-endosomal/lysosomal compartments, linking localization to esterification capacity.","evidence":"Immunoelectron microscopy, subcellular fractionation, immunoadsorption, and reconstituted ACAT activity assays in cholesterol-loaded macrophages","pmids":["10623671","20460577","20523008"],"confidence":"Medium","gaps":["Trigger and machinery for ER vesiculation unknown","Basis for higher specific activity in vesicles unexplained"]},{"year":2014,"claim":"Identified a moonlighting acetyltransferase function for mitochondrial ACAT1 toward the pyruvate dehydrogenase complex, mechanistically linking it to the Warburg effect.","evidence":"MS, Co-IP, in vitro acetyltransferase assays, mutagenesis, and siRNA knockdown with tumor growth readout in cancer cells","pmids":["24486017"],"confidence":"High","gaps":["Structural basis for acetyltransferase versus esterase activity not defined","Relationship between mitochondrial and ER pools unclear"]},{"year":2014,"claim":"Connected ACAT1 inhibition to enhanced microglial autophagy-lysosomal clearance of amyloid-β through an mTOR-independent, TFEB-mediated route.","evidence":"Acat1 KO and K604 inhibitor in primary microglia with autophagosome, TFEB reporter, lysosomal degradation, and phagocytosis assays","pmids":["25339759"],"confidence":"High","gaps":["Signal coupling cholesterol esterification to TFEB activation unknown"]},{"year":2010,"claim":"Placed cholesterol esterification upstream of APP processing in vivo, showing ACAT1 ablation lowers amyloidogenic processing via oxysterol-mediated control of sterol synthesis.","evidence":"Acat1 KO in 3xTg-AD mice with 24(S)-hydroxycholesterol, HMGR, and amyloid measurements plus neuronal oxysterol treatment; complemented by RNAi and PM free-cholesterol studies","pmids":["20133765","17412327","26474739"],"confidence":"High","gaps":["Direct molecular link between cholesterol pools and secretase activity not established","Relative contribution of PM vs ER cholesterol not quantified"]},{"year":2019,"claim":"Demonstrated that myeloid ACAT1 drives atherosclerotic and metabolic inflammation, with KO reducing lesion burden, monocyte infiltration, and inflammatory responses.","evidence":"Myeloid-specific Acat1 KO in ApoE-KO and Western-diet mouse models with lesion analysis, adoptive transfer, and macrophage inflammatory assays","pmids":["31495784","29533741","23846496"],"confidence":"High","gaps":["Mechanism linking esterification to integrin-β1 expression and HSPC proliferation incomplete"]},{"year":2009,"claim":"Defined transcriptional and hormonal control of ACAT1, identifying an NF-κB promoter element driving TNF-α induction and a leptin–JAK2–PI3K axis raising ACAT1 activity in macrophages.","evidence":"Promoter reporter mutagenesis with NF-κB inhibitors; leptin treatment with JAK2/PI3K inhibitors and efflux assays","pmids":["19189937","19625677"],"confidence":"Medium","gaps":["Integration of these inputs with post-translational regulation unknown"]},{"year":2023,"claim":"Showed ACAT1 protein levels are set by ubiquitin-dependent degradation and by enrichment at MAM, where its activity tunes ER–mitochondria contact and inflammatory TLR4 trafficking.","evidence":"Orthogonal ubiquitin transfer (UBE3A) with mouse liver validation; MAM proteomics and EM; myeloid Acat1 KO with TLR4 endocytosis assays","pmids":["36920305","36982602","36982689"],"confidence":"Medium","gaps":["UBE3A ubiquitination site on ACAT1 not mapped","How MAM cholesterol regulates contact-site machinery unresolved"]},{"year":2024,"claim":"Extended ACAT1's esterase role to adipogenesis, showing catalytic activity is required for SREBP2-driven cholesterol uptake and PPARγ transcription.","evidence":"ACAT1 KO with WT versus catalytic-dead rescue and cholesterol replenishment in preadipocytes","pmids":["39481851"],"confidence":"Medium","gaps":["Link between cholesterol pools and PPARγ transcription mechanistic detail lacking"]},{"year":2025,"claim":"Expanded the acetyltransferase repertoire to metabolic and tumor-microenvironment substrates and described succinylation-based regulation, though several of these axes rest on single-lab evidence.","evidence":"Co-IP/mutagenesis for ME2-K156 and METTL3 acetylation; in vivo CRISPR screen for hypersuccinylation/ROS; SIRT5 desuccinylation assays in lung cancer models","pmids":["39951294","36890220","40166933","39524872"],"confidence":"Medium","gaps":["Direct acetylation of METTL3 not biochemically demonstrated","SIRT5 succinylation site on ACAT1 not mapped","Several substrates await independent replication"]},{"year":null,"claim":"How a single protein partitions between ER cholesterol-esterase and mitochondrial protein-acetyltransferase activities, and what structural or localization switch governs this dual function, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of the acetyltransferase-active form","Determinants of ER versus mitochondrial targeting unknown","Substrate selection rules for acetylation not defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,2,3,4,6]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[6,23]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[2,5]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[6]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[3,7,10]},{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[6,25]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[20]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[7,8]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[2,6,29]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[18,30,36]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[6,28]}],"complexes":[],"partners":["PDHA1","PDP1","SIRT3","ME2","METTL3","UBE3A","SIRT5"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P35610","full_name":"Sterol O-acyltransferase 1","aliases":["Acyl-coenzyme A:cholesterol acyltransferase 1","ACAT-1","Cholesterol acyltransferase 1"],"length_aa":550,"mass_kda":64.7,"function":"Catalyzes the formation of fatty acid-cholesterol esters, which are less soluble in membranes than cholesterol (PubMed:16154994, PubMed:16647063, PubMed:32433613, PubMed:32433614, PubMed:32944968, PubMed:9020103). Plays a role in lipoprotein assembly and dietary cholesterol absorption (PubMed:16154994, PubMed:9020103). Preferentially utilizes oleoyl-CoA ((9Z)-octadecenoyl-CoA) as a substrate: shows a higher activity towards an acyl-CoA substrate with a double bond at the delta-9 position (9Z) than towards saturated acyl-CoA or an unsaturated acyl-CoA with a double bond at the delta-7 (7Z) or delta-11 (11Z) positions (PubMed:11294643, PubMed:32433614)","subcellular_location":"Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/P35610/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ACAT1","classification":"Not Classified","n_dependent_lines":3,"n_total_lines":1208,"dependency_fraction":0.0024834437086092716},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"HEATR3","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/ACAT1","total_profiled":1310},"omim":[{"mim_id":"609575","title":"ACYL-CoA DEHYDROGENASE, VERY LONG-CHAIN; ACADVL","url":"https://www.omim.org/entry/609575"},{"mim_id":"609254","title":"SENIOR-LOKEN SYNDROME 5; SLSN5","url":"https://www.omim.org/entry/609254"},{"mim_id":"607809","title":"ACETYL-CoA ACETYLTRANSFERASE 1; ACAT1","url":"https://www.omim.org/entry/607809"},{"mim_id":"607585","title":"ATM SERINE/THREONINE KINASE; ATM","url":"https://www.omim.org/entry/607585"},{"mim_id":"605086","title":"TRIGGERING RECEPTOR EXPRESSED ON MYELOID CELLS 2; TREM2","url":"https://www.omim.org/entry/605086"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Mitochondria","reliability":"Enhanced"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"kidney","ntpm":580.7},{"tissue":"liver","ntpm":905.7}],"url":"https://www.proteinatlas.org/search/ACAT1"},"hgnc":{"alias_symbol":["THIL"],"prev_symbol":["ACAT"]},"alphafold":{"accession":"P35610","domains":[{"cath_id":"-","chopping":"118-169_312-403","consensus_level":"medium","plddt":90.5993,"start":118,"end":403},{"cath_id":"-","chopping":"171-302","consensus_level":"medium","plddt":92.9899,"start":171,"end":302},{"cath_id":"-","chopping":"410-550","consensus_level":"medium","plddt":85.2801,"start":410,"end":550}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P35610","model_url":"https://alphafold.ebi.ac.uk/files/AF-P35610-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P35610-F1-predicted_aligned_error_v6.png","plddt_mean":80.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ACAT1","jax_strain_url":"https://www.jax.org/strain/search?query=ACAT1"},"sequence":{"accession":"P35610","fasta_url":"https://rest.uniprot.org/uniprotkb/P35610.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P35610/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P35610"}},"corpus_meta":[{"pmid":"24486017","id":"PMC_24486017","title":"Tyr 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Structure-guided mutagenesis showed acyl-CoA enters through the cytosolic tunnel and cholesterol likely enters from the side through the transmembrane tunnel, rationalizing ACAT1's preference for unsaturated acyl chains.\",\n      \"method\": \"Cryo-EM structure determination plus structure-guided mutagenesis and biochemical analyses\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure with mutagenesis validation, two independent papers published simultaneously in the same journal confirming tetrameric architecture\",\n      \"pmids\": [\"32433614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cryo-EM structure of human ACAT1 in complex with the clinical-stage inhibitor nevanimibe confirmed a tetrameric (dimer-of-dimers) holoenzyme; each monomer has nine transmembrane helices with TM4–TM9 forming a cavity that accommodates nevanimibe and an endogenous acyl-CoA; this cavity contains the catalytically essential histidine residue. The structure provides a physical model for cholesterol esterification and inhibitor binding.\",\n      \"method\": \"Cryo-EM structure determination with bound inhibitor, biochemical analyses\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure with bound ligand, corroborated by the parallel Qian et al. structure in the same issue\",\n      \"pmids\": [\"32433613\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Recombinant human ACAT1 purified to homogeneity exhibits allosteric (sigmoidal) kinetics with respect to cholesterol substrate whether assayed in mixed micelles or reconstituted vesicles, while the oleoyl-CoA saturation curves are hyperbolic, supporting the hypothesis that ACAT1 is an allosteric enzyme regulated by cholesterol.\",\n      \"method\": \"Protein purification (~7000-fold), in vitro enzyme kinetics in mixed micelles and reconstituted vesicles\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — purified enzyme reconstitution with rigorous kinetic analysis in two different lipid systems\",\n      \"pmids\": [\"9857049\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"ACAT1 topology study using glycosylation reporters and FLAG epitope tags showed ACAT1 spans the ER membrane five times with its N-terminus in the cytosol and C-terminus in the ER lumen. The putative active-site serine (Ser269 in ACAT1) is positioned on the cytosolic face; mutation of Ser269 inactivated ACAT1, demonstrating its catalytic essentiality.\",\n      \"method\": \"Glycosylation reporter constructs, FLAG epitope accessibility assays, in vitro microsomal assembly, immunofluorescence microscopy, site-directed mutagenesis\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal topology methods plus active-site mutagenesis confirming functional role of Ser269\",\n      \"pmids\": [\"11071899\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Histidine residues H386 and H460 in ACAT1 are essential for catalytic activity; H386A and H460A substitutions abolished enzymatic activity without altering substrate-binding affinity, indicating these residues are catalytic rather than structural. Partial restoration of H386A activity with 25-hydroxycholesterol (but not cholesterol) as substrate further defined the catalytic role.\",\n      \"method\": \"Site-directed mutagenesis of histidine residues, baculovirus expression, enzyme activity assays, substrate-binding affinity measurements\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — active-site mutagenesis with multiple substitutions and substrate specificity analysis\",\n      \"pmids\": [\"16647063\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Purified human ACAT1 binds oleoyl-CoA with Kd ~1.9 μM, inducing significant intrinsic fluorescence changes indicating structural rearrangement; cholesterol binding produces larger fluorescence changes than its diastereomer epicholesterol, demonstrating stereospecificity (3β-OH on steroid ring A) in substrate recognition by ACAT1.\",\n      \"method\": \"Protein purification from H293 cells, difference intrinsic fluorescence spectroscopy\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — purified protein with direct binding measurements and stereospecificity controls, single lab\",\n      \"pmids\": [\"20964445\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Mitochondrial ACAT1 was identified as an acetyltransferase for PDHA1 (K321) and PDP1 (K202) in the pyruvate dehydrogenase complex; K321 acetylation of PDHA1 recruits PDK1 to inhibit PDC activity, and K202 acetylation of PDP1 dissociates its substrate PDHA1, both promoting the Warburg effect. SIRT3 acts as the opposing deacetylase. Y381 phosphorylation of PDP1 dissociates SIRT3 and recruits ACAT1 to the PDC.\",\n      \"method\": \"Mass spectrometry, Co-IP, in vitro acetyltransferase assays, site-directed mutagenesis, siRNA knockdown with tumor growth readout, EGF stimulation in cancer cells\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — multiple orthogonal methods (MS, Co-IP, in vitro assay, mutagenesis, KD with phenotype) in a single rigorous study\",\n      \"pmids\": [\"24486017\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"In human macrophages, ACAT1 localizes to the tubular rough ER under normal conditions; upon cholesterol loading, ~30–40% of total ACAT1 immunoreactivity shifts into small ER-derived vesicles that are also enriched in the ER marker GRP78, suggesting cholesterol overload activates an ER vesiculation process.\",\n      \"method\": \"Immunoelectron microscopy, immunoblot, immunofluorescence microscopy, subcellular fractionation\",\n      \"journal\": \"The American journal of pathology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal localization methods with direct functional context in human macrophages\",\n      \"pmids\": [\"10623671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Cholesterol loading of macrophages causes ACAT1 to redistribute from high-density ER membranes to lower-density ER-derived vesicles positive for both ER and trans-Golgi network markers; these vesicles exhibit ~3-fold higher ACAT1-specific enzymatic activity than ER membranes, and reconstitution assays showed this is not due to increased cholesterol content in the vesicles.\",\n      \"method\": \"Subcellular fractionation, in vitro ACAT enzyme activity assays on isolated membranes, reconstituted ACAT assay\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — direct biochemical fractionation with reconstituted enzyme activity measurements, mechanistically rigorous\",\n      \"pmids\": [\"20460577\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The human ACAT1 4.3-kb mRNA is produced from sequences on two different chromosomes (exons 1–16 and P1 promoter on chromosome 1; exon Xa and P7 promoter on chromosome 7), requiring a novel interchromosomal trans-splicing mechanism of two discontinuous precursor RNAs.\",\n      \"method\": \"Genomic cloning, gene mapping, RNase protection assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — gene mapping plus RNase protection assays establishing interchromosomal origin of chimeric mRNA\",\n      \"pmids\": [\"10196189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The chimeric 4.3-kb human ACAT1 mRNA (derived from chromosomes 1 and 7 via trans-splicing) produces a 56-kDa ACAT1 isoform using a GGC (glycine) start codon upstream of the normal AUG; this 56-kDa isoform localizes to the ER and is enzymatically active. Both chromosomal sequences are required for its production.\",\n      \"method\": \"Expression in CHO cells, site-directed mutagenesis, mass spectrometry, immunoblot with isoform-specific antibodies, subcellular localization, enzyme activity assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — mutagenesis, MS, enzyme activity, and localization data in a single comprehensive study\",\n      \"pmids\": [\"15319423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Production of the 56-kDa ACAT1 isoform from the chimeric mRNA requires two specific RNA secondary structures (stem-loops at nt 1255–1268 from chromosome 7 and nt 1286–1342 from chromosome 1) flanking the GGC start codon; translation initiation from GGC is mediated by an IRES mechanism requiring an upstream AU-constituted structure and downstream GC-rich structure.\",\n      \"method\": \"Deletion and point mutation analyses of stem-loop sequences, monocistronic and bicistronic vector experiments with stable hairpin, western blot\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple mutational analyses in single lab establishing IRES mechanism\",\n      \"pmids\": [\"18542101\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Immunodepletion experiments demonstrated that ACAT1 protein accounts for ~90% of ACAT activity in adult human liver, ~98% in adrenal gland, ~91% in macrophages, and ~80% in kidney, but only ~19% in intestines, establishing tissue-specific catalytic roles.\",\n      \"method\": \"Immunodepletion with anti-ACAT1 antibodies followed by residual activity measurement in solubilized tissue homogenates\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct quantitative immunodepletion-activity assays across multiple human tissues, single lab\",\n      \"pmids\": [\"9717734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"In adult human liver, ACAT1 (not ACAT2) accounts for 85–90% of total ACAT activity as measured by immunodepletion. ACAT1 and ACAT2 do not form hetero-oligomeric complexes. In intestinal enterocytes, ACAT2 (not ACAT1) is the dominant isoenzyme, concentrated at villus apices.\",\n      \"method\": \"Specific polyclonal antibody immunodepletion, immunohistochemistry of human intestine and liver, activity assays in HepG2 and Caco-2 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — quantitative immunodepletion, cell-based activity assays, histological localization; replicated key findings from prior immunodepletion study\",\n      \"pmids\": [\"10846185\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Purified ACAT1 expressed in insect cells exists preferentially as oligomers (dimer to tetramer); ACAT1 esterifies cholesterol more rapidly than ACAT2, which more efficiently esterifies cholic acid derivatives, demonstrating distinct substrate specificities. Pyripyropene A selectively inhibits ACAT2 (IC50 = 0.64 μM) but not ACAT1.\",\n      \"method\": \"Baculovirus-insect cell expression, protein purification, enzyme kinetics, inhibitor screening\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — purified enzyme kinetics but single lab, limited mechanistic follow-up\",\n      \"pmids\": [\"13679053\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"ACAT1 selectively esterifies oleic acid over polyunsaturated fatty acids in intact cells and microsomes (THP-1 macrophages), while ACAT2-expressing cells show broader unsaturated fatty acid utilization; K-604 competitively inhibits ACAT1 with respect to oleoyl-CoA (Ki = 0.378 μM), establishing the oleoyl-CoA binding site as part of the active site.\",\n      \"method\": \"NBD-cholesterol fluorescence assay, specific inhibitor kinetics in AC29 cells stably transfected with ACAT1 or ACAT2, microsomal activity assays\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — cell-based and microsomal assays with specific inhibitor kinetics, replicates fatty acid preference findings\",\n      \"pmids\": [\"14617738\", \"16820149\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Pharmacological blockade (K604) or genetic knockout of ACAT1 in microglia stimulates autophagosome formation and TFEB-mediated lysosomal proteolysis in an mTOR-independent manner, increasing phagocytic uptake and lysosomal degradation of oligomeric Aβ1-42. The effect can be modulated by agents that disrupt cholesterol biosynthesis.\",\n      \"method\": \"Acat1 KO mouse primary microglia, specific ACAT1 inhibitor K604, autophagosome formation assays, TFEB reporter assays, lysosomal degradation assays, phagocytosis assays, mTOR signaling analysis\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO plus pharmacological inhibition with multiple cellular readouts in one study\",\n      \"pmids\": [\"25339759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Acute ACAT1/SOAT1 blockade increases cholesterol content at the mitochondria-associated ER membrane (MAM), where ACAT1 is enriched; this increases the number of ER-mitochondria contact sites and shortens the distance between these organelles, strengthening ER-mitochondria connectivity.\",\n      \"method\": \"Biochemical fractionation (MAM isolation), MAM proteomics, confocal and electron microscopy of ER-mitochondria contact sites\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — orthogonal methods (fractionation, proteomics, EM) but single lab\",\n      \"pmids\": [\"36982602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ACAT1/SOAT1 blockade in microglia alters the intracellular fate of TLR4 by increasing TLR4 endocytosis and enhancing its trafficking to lysosomes for degradation, thereby suppressing LPS-induced pro-inflammatory signaling. Myeloid-specific Acat1 KO mice showed markedly attenuated LPS-induced neuroinflammation in hippocampus and cortex.\",\n      \"method\": \"Myeloid-specific Acat1 KO mice, pharmacological ACAT1 inhibitor K604, TLR4 localization by microscopy, endocytosis assays, cytokine measurements\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO in vivo plus pharmacological inhibition with defined molecular mechanism (TLR4 trafficking)\",\n      \"pmids\": [\"36982689\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Myeloid-specific Acat1 knockout reduces Ly6Chi monocyte integrin-β1 expression, impairing monocyte adhesion to inflamed endothelium and infiltration into adipose tissue; this attenuates Western diet-induced obesity and adipose tissue inflammation. ACAT1 inhibition in RAW264.7 macrophages also reduces LPS-induced inflammatory responses.\",\n      \"method\": \"Myeloid-specific Acat1 KO mouse model, adoptive transfer, flow cytometry, integrin-β1 expression analysis, small-molecule ACAT1 inhibitor in cell culture\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO with adoptive transfer and mechanistic cell biology, multiple readouts\",\n      \"pmids\": [\"29533741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"In cholesterol-loaded macrophages, approximately 20% of total ACAT1 co-localizes with the late endosome/lysosome marker LAMP2, and ACAT1-positive membranes isolated by immunoadsorption contain LAMP2; cholesterol-loaded macrophages can re-esterify LDL-derived cholesterol even when cholesterol egress from late endosomes is blocked by U18666A, suggesting ACAT1-positive LE/LS facilitate direct esterification of lipoprotein-derived free cholesterol.\",\n      \"method\": \"Confocal fluorescence microscopy, immunoelectron microscopy, subcellular fractionation, immunoadsorption, cholesterol esterification assay with pharmacological block\",\n      \"journal\": \"Journal of atherosclerosis and thrombosis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods establishing localization and functional consequence, single lab\",\n      \"pmids\": [\"20523008\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"ACAT1 deficiency in peritoneal macrophages increases de novo cholesterol synthesis by 134% and upregulates SREBP1a mRNA 6-fold, indicating that ACAT1 normally suppresses the SREBP-driven cholesterol/fatty acid synthesis program; total cellular cholesterol efflux increases proportionally but esterification of new cholesterol is reduced by 93%.\",\n      \"method\": \"Radiotracer cholesterol synthesis and efflux assays in ACAT1−/− peritoneal macrophages, qPCR for SREBP1a\",\n      \"journal\": \"Atherosclerosis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct biochemical measurements in KO cells, single lab, limited pathway validation\",\n      \"pmids\": [\"16144700\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"ACAT1 deficiency in macrophages reduces overall cellular cholesterol efflux by 25% despite upregulated ABCA1 expression, while efflux of lipoprotein-derived (acLDL) cholesterol is increased by 32%; ACAT1-deficient macrophages accumulate 26% more free cholesterol from acLDL and show a 75% increase in intracellular vesicles.\",\n      \"method\": \"ACAT1−/− peritoneal macrophages, radiolabeled cholesterol efflux assays, ABCA1 expression analysis, vesicle quantification\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct KO with quantitative efflux assays and ABCA1 expression; replicated in subsequent bone marrow transplant studies\",\n      \"pmids\": [\"15499044\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ACAT1 acetylates ME2 (malate enzyme 2) at lysine 156, potentiating ME2 enzyme activity and promoting conversion of glutamine-derived malate to pyruvate and subsequently lactate; this ME2-derived lactate facilitates lactylation of homologous recombination repair proteins, contributing to chemoresistance in ovarian cancer. Glucose deprivation triggers this ACAT1-ME2 axis.\",\n      \"method\": \"Co-IP, site-directed mutagenesis of ME2-K156, in vitro ME2 enzyme activity assays, metabolomics, in vitro and in vivo cancer models\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Weak — multiple methods but single lab, novel substrate identification needing independent replication\",\n      \"pmids\": [\"39951294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ACAT1 acetylates METTL3, and this interaction (demonstrated by Co-IP and GST pulldown) stabilizes METTL3 protein by inhibiting ubiquitin-proteasome-mediated degradation; ACAT1-mediated METTL3 stabilization suppresses TNBC cell migration and invasion. NR2F6 transcriptionally activates ACAT1 to regulate this axis.\",\n      \"method\": \"Co-IP, GST pulldown, IP for ubiquitination, qPCR, western blot, migration/invasion assays\",\n      \"journal\": \"Genes and immunity\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP/pulldown, single lab, mechanistic acetylation not directly demonstrated\",\n      \"pmids\": [\"36890220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ACAT1 loss in lung tumor cells leads to mitochondrial protein hypersuccinylation and enhanced mitochondrial oxidative metabolism; this increases ROS, which impedes tertiary lymphoid structure (TLS) formation. ACAT1 knockdown reduced ROS and promoted B-cell aggregation and TLS construction, sensitizing tumors to anti-PD1 therapy.\",\n      \"method\": \"In vivo CRISPR screening in orthotopic lung tumor model, succinylation proteomics, ROS measurements, B-cell aggregation assays, NAC treatment, NSCLC tissue microarray (305 patients)\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo CRISPR screen plus mechanistic follow-up with multiple methods, single lab\",\n      \"pmids\": [\"40166933\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SIRT5 acts as a desuccinylase of ACAT1, removing succinyl groups from ACAT1 and enhancing its enzymatic activity; SIRT5-mediated ACAT1 activation then activates the NRF2 pathway and inhibits secretion of CCL5 and CXCL10 chemokines that recruit CD8+ T cells, contributing to an immunosuppressive tumor microenvironment in EGFR-mutant LUAD.\",\n      \"method\": \"Desuccinylation assay, ACAT1 activity measurements, chemokine secretion assays, T-cell recruitment assays\",\n      \"journal\": \"Heliyon\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, limited mechanistic validation of direct succinylation site on ACAT1\",\n      \"pmids\": [\"39524872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ACAT1 inhibition suppresses fatty acid oxidation and acetyl-CoA production while increasing free fatty acids in glioblastoma cells; this restores mitochondrial function and negatively regulates the choline metabolic pathway, which is required for GBM cell differentiation into astrocytes. Chlorogenic acid inhibits ACAT1 phosphorylation to achieve this effect.\",\n      \"method\": \"ACAT1 KD in GBM cells, Seahorse metabolic assays, metabolomics, in vitro and in vivo differentiation assays\",\n      \"journal\": \"International journal of biological sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, indirect mechanism linking ACAT1 inhibition to choline metabolism\",\n      \"pmids\": [\"39494339\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"UBE3A ubiquitin ligase ubiquitinates ACAT1 (identified by orthogonal ubiquitin transfer), promoting its degradation; on a high-fat diet, reduced hepatic UBE3A expression is associated with increased ACAT1 protein and ketone body accumulation; forced UBE3A expression in mouse liver decreases ACAT1 protein content.\",\n      \"method\": \"Orthogonal ubiquitin transfer platform, overexpression in HEK293 cells, mouse liver forced expression, western blot, metabolite measurements\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — orthogonal ubiquitin transfer (novel method) plus in vivo mouse liver validation, single lab\",\n      \"pmids\": [\"36920305\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ACAT1-deficient preadipocytes show inhibited SREBP2-mediated cholesterol uptake, reduced intracellular and plasma membrane cholesterol, and impaired PPARγ transcription and adipogenesis; rescue with catalytically functional but not catalytic-dead ACAT1 restores cholesterol levels and PPARγ transcription, demonstrating that ACAT1 enzymatic activity is required for normal adipogenesis.\",\n      \"method\": \"ACAT1 KO, overexpression of WT vs. catalytic-dead ACAT1, cholesterol measurements, PPARγ transcription analysis, cholesterol replenishment rescue experiments\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — catalytic-dead rescue experiment plus KO with multiple mechanistic readouts, single lab\",\n      \"pmids\": [\"39481851\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Myeloid-specific Acat1 KO in the ApoE-KO mouse model for advanced atherosclerosis significantly reduced lesion cholesterol crystal content, lesion size, and macrophage content without increasing apoptotic cell death; cell culture studies showed that ACAT1 inhibition in macrophages reduces pro-inflammatory responses to cholesterol loading by acetyl-LDL.\",\n      \"method\": \"Myeloid-specific Acat1 KO in ApoE-KO mice, lesion analysis (cholesterol crystal quantification, macrophage immunostaining), macrophage cell culture inflammatory response assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO in advanced disease model with mechanistic cell culture follow-up, builds on prior work\",\n      \"pmids\": [\"31495784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Global Acat1 knockout causes a higher proportion of Lin−Sca-1+c-Kit+ hematopoietic stem/progenitor cells to proliferate, resulting in increased myeloid progenitor cell numbers and leukocytosis in normal mice and elevated monocytosis in Apoe−/− mice during atherosclerosis, demonstrating that ACAT1 plays a role in regulating hematopoiesis.\",\n      \"method\": \"Acat1−/− mouse bone marrow analysis, flow cytometry of Lin−Sca-1+c-Kit+ population, myeloid progenitor cell quantification\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct KO phenotype with bone marrow cell population analysis, single lab\",\n      \"pmids\": [\"23846496\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"ACAT1 gene ablation in a 3xTg-AD mouse model increases 24(S)-hydroxycholesterol content by 32% in the brain at 4 months, decreases HMG-CoA reductase protein by 65%, reduces sterol synthesis rate by 28%, and decreases full-length hAPP and its proteolytic fragments by >60%; treating hippocampal neurons with 24(S)-hydroxycholesterol recapitulates reductions in hAPP and HMGR, placing cholesterol esterification upstream of APP processing.\",\n      \"method\": \"Acat1 KO in 3xTg-AD mice, 24(S)-hydroxycholesterol measurement, HMGR activity assay, neuronal cell treatment with oxysterol, amyloid quantification\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO in disease model plus mechanistic cell culture validation with oxysterol, single lab\",\n      \"pmids\": [\"20133765\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"ACAT1 RNAi (~50% protein reduction) decreases cholesteryl ester levels by 22%, causes slight increase in ER free cholesterol, and reduces Aβ secretion by 40%, demonstrating that ACAT1 activity influences amyloidogenic APP processing.\",\n      \"method\": \"RNAi knockdown, cholesteryl ester quantification, Aβ ELISA\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single method KD with defined biochemical readouts, replicated in principle by subsequent KO studies\",\n      \"pmids\": [\"17412327\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"ACAT1 inhibition in SK-N-SH neuronal cells increases free cholesterol at the plasma membrane (PM-FC), and this increased PM-FC reduces APP α-processing; the effect on PM-FC and α-processing is independent of total cellular cholesterol and persists even when the NPC-dependent cholesterol trafficking pathway is blocked, identifying an ACAT1-dependent pathway for shuttling PM-FC to the intracellular pool.\",\n      \"method\": \"ACAT1 siRNA and pharmacological inhibitor, free cholesterol measurement by filipin staining, APP α-processing assay, NPC pathway blockade\",\n      \"journal\": \"Acta biochimica et biophysica Sinica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — two perturbation approaches with mechanistic controls, single lab\",\n      \"pmids\": [\"26474739\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Leptin increases ACAT1 protein expression (~1.9-fold) and ACAT activity (~1.8-fold) in human monocyte-derived macrophages via JAK2 and PI3K signaling pathways, promoting cholesteryl ester accumulation and suppressing HDL-mediated cholesterol efflux; the efflux suppression is reversed by the ACAT1 inhibitor K604.\",\n      \"method\": \"Leptin treatment of differentiating monocytes, JAK2 and PI3K inhibitors, ACAT activity assay, western blot, cholesterol efflux assay\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological pathway dissection with multiple readouts, single lab\",\n      \"pmids\": [\"19625677\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"TNF-α, through NF-κB, specifically enhances ACAT1 (but not ACAT2) gene expression in differentiating human monocytes; a functional NF-κB element in the human ACAT1 proximal promoter is required for this effect; increased ACAT1 promotes cholesteryl ester accumulation and lipid-laden cell formation.\",\n      \"method\": \"TNF-α treatment, NF-κB inhibitors, promoter reporter assays, mutational analysis of NF-κB binding site, cholesteryl ester accumulation assay\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — promoter mutagenesis plus inhibitor studies defining the NF-κB element, single lab\",\n      \"pmids\": [\"19189937\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ACAT1 (mitochondrial acetoacetyl-CoA thiolase / acyl-CoA:cholesterol acyltransferase 1) is a multi-functional enzyme: in the ER membrane it forms an allosteric, cholesterol-regulated tetrameric enzyme (resolved by cryo-EM) that esterifies free cholesterol using long-chain fatty acyl-CoA—with an essential cytosolic-facing catalytic serine (Ser269) and active-site histidines (H386, H460)—thereby maintaining ER cholesterol homeostasis; in mitochondria it functions as an acetyltransferase that acetylates PDHA1 (K321) and PDP1 (K202) to inhibit the pyruvate dehydrogenase complex and promote the Warburg effect, with SIRT3 as the opposing deacetylase; ACAT1 is enriched at MAM/ER–mitochondria contact sites, and its inhibition strengthens ER–mitochondria connectivity; in macrophages, ACAT1 controls cholesterol efflux, TLR4 turnover, and inflammatory responses, with NF-κB-driven transcriptional regulation by TNF-α; and in neurons, ACAT1 activity influences APP processing and amyloid-β generation via control of plasma membrane and ER cholesterol pools.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"ACAT1 is a cholesterol-esterifying enzyme that maintains intracellular cholesterol homeostasis and, through this lipid-handling role, governs membrane sterol pools in macrophages, neurons, and adipocytes [#0, #2, #29]. In the ER membrane it assembles as a dimer-of-dimers tetramer in which each protomer presents converging cytosolic and transmembrane tunnels that admit acyl-CoA and cholesterol, respectively, to a shared catalytic site; long-chain unsaturated acyl-CoA (preferentially oleoyl-CoA) is conjugated to free cholesterol, and the enzyme behaves allosterically with sigmoidal kinetics toward its cholesterol substrate [#0, #1, #2]. Catalysis depends on a cytosolic-facing active-site serine (Ser269) and on histidines H386 and H460, and substrate recognition is stereospecific for the 3\\u03b2-hydroxyl steroid configuration [#3, #4, #5]. ACAT1 accounts for the dominant fraction of cholesterol-esterifying activity in human liver, adrenal, macrophage, and kidney tissue, and is enriched at mitochondria-associated ER membranes, where its inhibition raises local cholesterol and strengthens ER\\u2013mitochondria contacts [#12, #17]. In macrophages, ACAT1 controls the balance between cholesterol esterification and efflux and is transcriptionally driven by TNF-\\u03b1 through an NF-\\u03baB element in its promoter; myeloid ACAT1 ablation attenuates atherosclerotic lesion formation, adipose inflammation, and TLR4-dependent inflammatory signaling [#22, #36, #30, #18]. In neurons, ACAT1 activity controls plasma-membrane and ER free-cholesterol pools and thereby modulates APP processing and amyloid-\\u03b2 generation, placing cholesterol esterification upstream of amyloidogenesis [#32, #34]. Beyond its esterase function, mitochondrial ACAT1 acts as a protein acetyltransferase that acetylates PDHA1 (K321) and PDP1 (K202) to inhibit the pyruvate dehydrogenase complex and promote the Warburg effect, opposed by the deacetylase SIRT3 [#6]. The human enzyme is unusual in arising from a chimeric mRNA assembled by interchromosomal trans-splicing, which additionally yields a 56-kDa ER-localized isoform via IRES-dependent translation from a GGC start codon [#9, #10].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Established that ACAT1 is the dominant cholesterol-esterifying enzyme in most human tissues and that it behaves as an allosteric enzyme regulated by its cholesterol substrate, defining its homeostatic role.\",\n      \"evidence\": \"Immunodepletion-activity assays across human tissues and purified-enzyme kinetics in micelles and reconstituted vesicles\",\n      \"pmids\": [\"9717734\", \"9857049\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of allostery not yet resolved\", \"Tissue partitioning relative to ACAT2 not fully defined\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Resolved the membrane topology and identified the catalytically essential cytosolic-facing Ser269, and distinguished ACAT1 from ACAT2 as non-interacting isoenzymes with tissue-specific dominance.\",\n      \"evidence\": \"Glycosylation reporter and FLAG accessibility assays with active-site mutagenesis; immunodepletion and immunohistochemistry of human liver and intestine\",\n      \"pmids\": [\"11071899\", \"10846185\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Later cryo-EM revised the transmembrane segment count\", \"Mechanism coupling Ser269 to the histidine residues not yet established\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Defined H386 and H460 as catalytic rather than structural residues, refining the active-site chemistry of cholesterol esterification.\",\n      \"evidence\": \"Histidine site-directed mutagenesis with baculovirus expression, activity and substrate-binding assays, and oxysterol substrate rescue\",\n      \"pmids\": [\"16647063\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise catalytic mechanism (general base vs. nucleophile roles) not fully resolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Provided direct binding evidence for stereospecific substrate recognition, showing acyl-CoA and cholesterol binding induce conformational change.\",\n      \"evidence\": \"Difference intrinsic fluorescence spectroscopy on purified human ACAT1 with cholesterol vs. epicholesterol\",\n      \"pmids\": [\"20964445\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding stoichiometry per tetramer not determined\", \"Order of substrate addition not established\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Cryo-EM structures resolved the tetrameric architecture and the converging substrate tunnels, giving a physical model for catalysis, acyl-chain preference, and inhibitor binding.\",\n      \"evidence\": \"Cryo-EM of human ACAT1 alone and with the inhibitor nevanimibe, plus structure-guided mutagenesis\",\n      \"pmids\": [\"32433614\", \"32433613\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conformational basis of the allosteric cholesterol response not captured\", \"Dynamics of substrate entry not directly visualized\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Demonstrated the human ACAT1 transcript arises by interchromosomal trans-splicing, a novel origin for a metabolic enzyme mRNA.\",\n      \"evidence\": \"Genomic cloning, gene mapping, and RNase protection assays\",\n      \"pmids\": [\"10196189\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trans-splicing machinery not identified\", \"Physiological significance of the chimeric origin unclear\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Showed the chimeric mRNA produces an enzymatically active 56-kDa ER isoform via IRES-dependent initiation from a GGC codon flanked by required stem-loops.\",\n      \"evidence\": \"Expression, mutagenesis, MS and localization for the isoform; deletion/point mutation of stem-loops in mono- and bicistronic reporters\",\n      \"pmids\": [\"15319423\", \"18542101\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"IRES trans-acting factors not identified\", \"Functional role distinct from the canonical isoform unresolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Defined the macrophage cholesterol balance role, showing ACAT1 deficiency reroutes lipoprotein-derived cholesterol toward efflux while reducing net efflux despite ABCA1 upregulation.\",\n      \"evidence\": \"Radiolabeled cholesterol efflux and esterification assays in ACAT1\\u2212/\\u2212 peritoneal macrophages with ABCA1 and SREBP1a readouts\",\n      \"pmids\": [\"15499044\", \"16144700\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking esterification to SREBP suppression not fully dissected\", \"In vivo efflux consequences not measured here\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Mapped the dynamic subcellular redistribution of ACAT1 upon cholesterol loading into more active ER-derived and late-endosomal/lysosomal compartments, linking localization to esterification capacity.\",\n      \"evidence\": \"Immunoelectron microscopy, subcellular fractionation, immunoadsorption, and reconstituted ACAT activity assays in cholesterol-loaded macrophages\",\n      \"pmids\": [\"10623671\", \"20460577\", \"20523008\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Trigger and machinery for ER vesiculation unknown\", \"Basis for higher specific activity in vesicles unexplained\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified a moonlighting acetyltransferase function for mitochondrial ACAT1 toward the pyruvate dehydrogenase complex, mechanistically linking it to the Warburg effect.\",\n      \"evidence\": \"MS, Co-IP, in vitro acetyltransferase assays, mutagenesis, and siRNA knockdown with tumor growth readout in cancer cells\",\n      \"pmids\": [\"24486017\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for acetyltransferase versus esterase activity not defined\", \"Relationship between mitochondrial and ER pools unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Connected ACAT1 inhibition to enhanced microglial autophagy-lysosomal clearance of amyloid-\\u03b2 through an mTOR-independent, TFEB-mediated route.\",\n      \"evidence\": \"Acat1 KO and K604 inhibitor in primary microglia with autophagosome, TFEB reporter, lysosomal degradation, and phagocytosis assays\",\n      \"pmids\": [\"25339759\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signal coupling cholesterol esterification to TFEB activation unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Placed cholesterol esterification upstream of APP processing in vivo, showing ACAT1 ablation lowers amyloidogenic processing via oxysterol-mediated control of sterol synthesis.\",\n      \"evidence\": \"Acat1 KO in 3xTg-AD mice with 24(S)-hydroxycholesterol, HMGR, and amyloid measurements plus neuronal oxysterol treatment; complemented by RNAi and PM free-cholesterol studies\",\n      \"pmids\": [\"20133765\", \"17412327\", \"26474739\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct molecular link between cholesterol pools and secretase activity not established\", \"Relative contribution of PM vs ER cholesterol not quantified\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrated that myeloid ACAT1 drives atherosclerotic and metabolic inflammation, with KO reducing lesion burden, monocyte infiltration, and inflammatory responses.\",\n      \"evidence\": \"Myeloid-specific Acat1 KO in ApoE-KO and Western-diet mouse models with lesion analysis, adoptive transfer, and macrophage inflammatory assays\",\n      \"pmids\": [\"31495784\", \"29533741\", \"23846496\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism linking esterification to integrin-\\u03b21 expression and HSPC proliferation incomplete\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Defined transcriptional and hormonal control of ACAT1, identifying an NF-\\u03baB promoter element driving TNF-\\u03b1 induction and a leptin\\u2013JAK2\\u2013PI3K axis raising ACAT1 activity in macrophages.\",\n      \"evidence\": \"Promoter reporter mutagenesis with NF-\\u03baB inhibitors; leptin treatment with JAK2/PI3K inhibitors and efflux assays\",\n      \"pmids\": [\"19189937\", \"19625677\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Integration of these inputs with post-translational regulation unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Showed ACAT1 protein levels are set by ubiquitin-dependent degradation and by enrichment at MAM, where its activity tunes ER\\u2013mitochondria contact and inflammatory TLR4 trafficking.\",\n      \"evidence\": \"Orthogonal ubiquitin transfer (UBE3A) with mouse liver validation; MAM proteomics and EM; myeloid Acat1 KO with TLR4 endocytosis assays\",\n      \"pmids\": [\"36920305\", \"36982602\", \"36982689\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"UBE3A ubiquitination site on ACAT1 not mapped\", \"How MAM cholesterol regulates contact-site machinery unresolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Extended ACAT1's esterase role to adipogenesis, showing catalytic activity is required for SREBP2-driven cholesterol uptake and PPAR\\u03b3 transcription.\",\n      \"evidence\": \"ACAT1 KO with WT versus catalytic-dead rescue and cholesterol replenishment in preadipocytes\",\n      \"pmids\": [\"39481851\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Link between cholesterol pools and PPAR\\u03b3 transcription mechanistic detail lacking\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Expanded the acetyltransferase repertoire to metabolic and tumor-microenvironment substrates and described succinylation-based regulation, though several of these axes rest on single-lab evidence.\",\n      \"evidence\": \"Co-IP/mutagenesis for ME2-K156 and METTL3 acetylation; in vivo CRISPR screen for hypersuccinylation/ROS; SIRT5 desuccinylation assays in lung cancer models\",\n      \"pmids\": [\"39951294\", \"36890220\", \"40166933\", \"39524872\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct acetylation of METTL3 not biochemically demonstrated\", \"SIRT5 succinylation site on ACAT1 not mapped\", \"Several substrates await independent replication\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How a single protein partitions between ER cholesterol-esterase and mitochondrial protein-acetyltransferase activities, and what structural or localization switch governs this dual function, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of the acetyltransferase-active form\", \"Determinants of ER versus mitochondrial targeting unknown\", \"Substrate selection rules for acetylation not defined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 2, 3, 4, 6]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [6, 23]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [2, 5]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [3, 7, 10]},\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [6, 25]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [20]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [7, 8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [2, 6, 29]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [18, 30, 36]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [6, 28]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"PDHA1\", \"PDP1\", \"SIRT3\", \"ME2\", \"METTL3\", \"UBE3A\", \"SIRT5\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}