{"gene":"ACAT1","run_date":"2026-04-28T17:12:37","timeline":{"discoveries":[{"year":2020,"finding":"Cryo-EM structure of human ACAT1 reveals it forms a dimer-of-dimers tetramer, with each protomer containing nine transmembrane segments enclosing a cytosolic tunnel and a transmembrane tunnel that converge at the catalytic site; structure-guided mutagenesis indicates acyl-CoA enters through the cytosolic tunnel while cholesterol enters through the transmembrane tunnel, rationalizing preference for unsaturated acyl chains.","method":"Cryo-EM structure determination + structure-guided mutagenesis","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure with functional mutagenesis validation in a single rigorous study","pmids":["32433614"],"is_preprint":false},{"year":2020,"finding":"Cryo-EM structure of human ACAT1 in complex with the inhibitor nevanimibe shows the holoenzyme as a tetramer of two homodimers, with each monomer containing nine transmembrane helices (TM4-TM9 forming the inhibitor-binding cavity); the cavity contains a catalytically essential histidine residue and accommodates an endogenous acyl-CoA, providing a structural basis for cholesterol esterification and inhibitor interaction.","method":"Cryo-EM structure determination + biochemical analysis","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — independent cryo-EM structure with biochemical validation, orthogonally replicates findings of companion paper","pmids":["32433613"],"is_preprint":false},{"year":2008,"finding":"Mutagenesis identified the putative catalytic triad of ACAT1 as S456, H460, and D400; mutation of any of these residues abolished enzymatic activity, and ACAT1 is sensitive to serine-modifying reagents, supporting a serine-based catalytic mechanism. Additionally, Y518 and the conserved FYXDWWN motif tyrosine are required for ACAT1 activity.","method":"Site-directed mutagenesis + in vitro enzyme activity assays","journal":"Journal of lipid research","confidence":"High","confidence_rationale":"Tier 1 — in vitro mutagenesis with functional readout across multiple residues","pmids":["18480028"],"is_preprint":false},{"year":2014,"finding":"Mitochondrial ACAT1 functions as an acetyltransferase that acetylates PDHA1 at K321 (inhibiting it by recruiting PDK1) and PDP1 at K202 (inhibiting it by dissociating its substrate PDHA1), thereby suppressing the pyruvate dehydrogenase complex (PDC) and promoting the Warburg effect; SIRT3 acts as the corresponding deacetylase, and Y381 phosphorylation of PDP1 toggles recruitment between SIRT3 and ACAT1 at the PDC.","method":"Co-IP, mass spectrometry, in vitro acetyltransferase assay, site-directed mutagenesis, ACAT1 knockdown with tumor growth readout","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (Co-IP, in vitro assay, mutagenesis, KD phenotype) in a single study","pmids":["24486017"],"is_preprint":false},{"year":2005,"finding":"ACAT1 has two distinct sterol-binding sites: a substrate-binding site and an allosteric activator site. Stereochemistry of the 3-hydroxyl group is critical for a sterol to serve as substrate but less critical for activation; enantiomeric cholesterol fails to activate ACAT1, demonstrating stereospecific interaction at the allosteric site independent of membrane biophysical effects. Cholesterol loading of macrophages increases ACAT1 activity without increasing ACAT1 protein, consistent with allosteric activation via increased ER cholesterol.","method":"In vitro ACAT enzyme assay with sterol analogs + intact cell cholesterol loading experiments","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with stereospecific substrate analysis and intact-cell validation","pmids":["15992359"],"is_preprint":false},{"year":1998,"finding":"Immunodepletion with anti-ACAT1 antibodies demonstrated that ACAT1 protein is responsible for ~90% of cholesterol esterification activity in human liver, ~98% in adrenal gland, ~91% in macrophages, and ~80% in kidney, but only ~19% in intestine, establishing the tissue-specific catalytic role of ACAT1.","method":"Immunodepletion of ACAT1 protein from tissue homogenates followed by residual enzyme activity measurement","journal":"Journal of lipid research","confidence":"High","confidence_rationale":"Tier 2 — quantitative immunodepletion enzyme assay across multiple human tissues","pmids":["9717734"],"is_preprint":false},{"year":2000,"finding":"Immunoelectron microscopy and immunofluorescence of human macrophages showed ACAT1 localizes predominantly to tubular rough endoplasmic reticulum under normal conditions; upon cholesterol loading, ~30-40% of ACAT1 immunoreactivity redistributes into ER-derived small vesicles also enriched in GRP78, linking ER vesiculation to foam cell formation.","method":"Immunoelectron microscopy, immunofluorescence, subcellular fractionation with GRP78 co-localization","journal":"The American journal of pathology","confidence":"High","confidence_rationale":"Tier 2 — direct localization by immunoelectron microscopy with functional consequence (foam cell formation)","pmids":["10623671"],"is_preprint":false},{"year":2010,"finding":"In cholesterol-loaded macrophages, ACAT1 redistributes from high-density ER membranes into lower-density ER-derived vesicles bearing both ER and trans-Golgi network markers; when normalized per equal ACAT1 protein mass, vesicle-associated ACAT1 shows ~3-fold higher enzymatic activity than ER membrane-associated ACAT1, revealing a mechanism by which macrophages increase cholesterol esterification capacity without upregulating ACAT1 protein.","method":"Subcellular fractionation, in vitro reconstituted ACAT enzyme assay, immunoblotting, cholesterol loading experiments","journal":"Journal of lipid research","confidence":"High","confidence_rationale":"Tier 1-2 — biochemical fractionation combined with in vitro enzyme activity reconstitution","pmids":["20460577"],"is_preprint":false},{"year":2004,"finding":"Human ACAT1 can be produced as a novel 56-kDa isoenzyme (in addition to the normal 50-kDa form) via interchromosomal trans-splicing of RNAs from chromosomes 1 and 7; the 56-kDa ACAT1 localizes to the ER and retains enzymatic activity, and uses GGC (glycine) as its translation initiation codon.","method":"Expression in CHO cells, mutagenesis, mass spectrometry, anti-peptide antibodies, immunolocalization","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — multiple methods in one lab establishing novel isoenzyme production and function","pmids":["15319423"],"is_preprint":false},{"year":2014,"finding":"ACAT1 inhibition (gene KO or K604 inhibitor) in microglia stimulates autophagosome formation and transcription factor EB (TFEB)-mediated lysosomal proteolysis, increasing phagocytic uptake and lysosomal degradation of oligomeric Aβ1-42; this autophagy induction is mTOR-independent and can be modulated by agents disrupting cholesterol biosynthesis.","method":"Acat1 gene KO in mouse, pharmacological ACAT1 inhibitor K604, autophagy assays, lysosomal proteolysis assays, in vitro and in vivo Aβ uptake/degradation experiments","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — genetic KO and pharmacological inhibition with multiple cellular readouts","pmids":["25339759"],"is_preprint":false},{"year":2010,"finding":"ACAT1 gene ablation in triple-transgenic AD mice causes a 32% increase in 24-hydroxycholesterol content, a 65% decrease in HMG-CoA reductase protein, and a 28% decrease in sterol synthesis rate in the brain, and reduces full-length APP and its proteolytic fragments by >60%, ameliorating cognitive deficits; treating hippocampal neurons with 24-hydroxycholesterol recapitulates these reductions in APP and HMGR.","method":"Acat1 KO mouse cross with 3XTg-AD model, biochemical assays, sterol synthesis rate measurement, hippocampal neuron treatment experiments","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — genetic model with multiple biochemical endpoints and cell-based mechanistic follow-up","pmids":["20133765"],"is_preprint":false},{"year":1996,"finding":"A point mutation at codon 265 (Ser→Leu) of ACAT1 in SRD-4 hamster cells results in an inactive enzyme; complementation with wild-type ACAT1 cDNA restored cholesteryl ester synthesis but did not restore sterol-mediated SREBP cleavage inhibition, demonstrating that ACAT deficiency and the sterol-regulatory defect are caused by independent mutations and can be uncoupled.","method":"Mutagenesis characterization of SRD-4 cells, transfection with wild-type cDNA, enzyme activity and SREBP processing assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — molecular identification of loss-of-function mutation with complementation and functional dissection","pmids":["8662991"],"is_preprint":false},{"year":2003,"finding":"Acyl-CoA binding protein (ACBP) regulates microsomal ACAT activity: in the presence of exogenous cholesterol, ACBP stimulates ACAT more potently than SCP-2 or L-FABP in proportion to their fatty acyl-CoA binding affinities; in the absence of exogenous cholesterol, these proteins inhibit ACAT. ACBP co-localizes with ACAT2 and ER markers in cells.","method":"In vitro microsomal ACAT assay with recombinant lipid-binding proteins, immunolocalization","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro reconstitution with purified proteins, single lab","pmids":["12518025"],"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, leading to increased cholesteryl ester accumulation and suppression of HDL-mediated cholesterol efflux; ACAT1 inhibitor K604 reversed the leptin-induced suppression of cholesterol efflux.","method":"JAK2/PI3K inhibitor treatment, ACAT activity assay, protein expression (Western blot), cholesterol efflux assay","journal":"American journal of physiology. Endocrinology and metabolism","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological pathway dissection with multiple readouts in single lab","pmids":["19625677"],"is_preprint":false},{"year":2009,"finding":"TNF-α, through NF-κB pathway activation at a unique NF-κB element in the human ACAT1 proximal promoter, specifically enhances ACAT1 (but not ACAT2) gene expression in differentiating human monocytes, increasing cholesteryl ester accumulation and promoting lipid-laden cell formation.","method":"NF-κB inhibitor treatment, promoter reporter assay, gene expression analysis, CE accumulation measurement","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 — promoter mapping combined with pharmacological pathway inhibition and functional readout","pmids":["19189937"],"is_preprint":false},{"year":2023,"finding":"25-Hydroxycholesterol (25HC) activates ACAT in the ER to create an imbalance in accessible cholesterol distribution between ER and plasma membrane, triggering rapid internalization of accessible cholesterol from the PM; this is sustained by concurrent SREBP suppression. In ACAT-deficient cells, 25HC fails to suppress Zika virus, coronavirus, or Listeria infection, placing ACAT activation mechanistically upstream of oxysterol-mediated antimicrobial immunity.","method":"ACAT-deficient cell lines, viral/bacterial infection assays, cholesterol trafficking assays, epistasis with SREBP pathway","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — genetic loss-of-function with multiple orthogonal functional readouts (viral, bacterial, cholesterol distribution)","pmids":["36695568"],"is_preprint":false},{"year":2023,"finding":"Acute ACAT1/SOAT1 blockade increases cholesterol content at the mitochondria-associated ER membrane (MAM), leading to enrichment of ACAT1 itself at the MAM and strengthening ER-mitochondria connectivity by increasing contact site number and shortening inter-organelle distance, as shown by MAM proteomics, confocal microscopy, and electron microscopy.","method":"Biochemical fractionation, MAM proteomics, confocal microscopy, electron microscopy, pharmacological ACAT1 inhibition","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 — multiple imaging modalities with proteomics in single lab","pmids":["36982602"],"is_preprint":false},{"year":2022,"finding":"Acat1/Soat1 knockout in mutant Npc1 mice prolongs lifespan by 34% and improves motor function, hepatosplenic pathology, and Purkinje neuron survival; in mutant NPC1 fibroblasts, ACAT1 blockade increases cholesterol at TGN-rich membranes and mitochondria, decreases cholesterol at late endosomes, and restores proper localization of syntaxin 6, golgin 97, cathepsin D, and ABCA1, placing ACAT1 at the intersection of cholesterol distribution among multiple organelles.","method":"Genetic Acat1 KO in Npc1 mouse model, fibroblast cholesterol fractionation, immunolocalization of organelle markers","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — in vivo genetic model with lifespan readout and mechanistic cell biology follow-up","pmids":["35507892"],"is_preprint":false},{"year":2019,"finding":"Myeloid-specific Acat1 KO (Acat1-M/-M) mice are resistant to Western diet-induced obesity; mechanistically, Ly6Chi monocytes from Acat1-M/-M mice express reduced integrin-β1, impairing their interaction with inflamed endothelium and infiltration into white adipose tissue; ACAT1 inhibition in RAW264.7 macrophages also reduces LPS-induced inflammatory responses.","method":"Myeloid-specific Acat1 KO mouse, adoptive transfer experiment, flow cytometry, pharmacological ACAT1 inhibition, gene expression analysis","journal":"American journal of physiology. Endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with adoptive transfer experiment and pharmacological validation","pmids":["29533741"],"is_preprint":false},{"year":2013,"finding":"Global Acat1 knockout mice show a significantly higher proportion of Lin-Sca-1+c-Kit+ hematopoietic stem/progenitor cells in proliferation, resulting in elevated myeloid progenitor numbers and leukocytosis, demonstrating that ACAT1 plays a role in restraining hematopoietic progenitor proliferation in bone marrow.","method":"Acat1 KO mouse model, flow cytometry of bone marrow populations, cell proliferation assays","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with defined cellular phenotype by flow cytometry","pmids":["23846496"],"is_preprint":false},{"year":2007,"finding":"RNAi-mediated knockdown of ACAT1 (~50% reduction in protein) reduced cholesteryl ester levels by 22% with a slight increase in ER free cholesterol, correlating with ~40% reduction in Aβ secretion from APP-expressing cells, demonstrating that even partial reduction of ACAT1 activity is sufficient to suppress amyloidogenic APP processing.","method":"ACAT1 siRNA knockdown, cholesteryl ester measurement, Aβ ELISA","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 — RNAi KD with quantitative mechanistic readouts; single lab","pmids":["17412327"],"is_preprint":false},{"year":2025,"finding":"ACAT1 acetylates malate enzyme 2 (ME2) at lysine 156, potentiating ME2 enzyme activity and facilitating lactate production from glutamine-derived malate; this occurs when decreased intracellular glucose levels (under chemotherapy) reduce glucose uptake, triggering ME2-K156 acetylation by ACAT1, which drives lactylation of homologous recombination proteins and chemoresistance in ovarian cancer.","method":"Co-IP, mass spectrometry identification of acetylation site, mutagenesis of K156, in vitro ME2 enzyme activity assay, in vivo tumor models","journal":"Advanced science","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP, site mutagenesis, and enzyme activity assay from single lab","pmids":["39951294"],"is_preprint":false},{"year":2023,"finding":"ACAT1 acetylates METTL3 protein in triple-negative breast cancer cells; this acetylation stabilizes METTL3 by inhibiting ubiquitin-proteasome-mediated degradation, and the NR2F6/ACAT1/METTL3 axis suppresses TNBC cell migration and invasion.","method":"Co-IP, GST pulldown, IP-based acetylation detection, ubiquitination assays, functional migration/invasion assays","journal":"Genes and immunity","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP and pulldown with functional cellular readout; single lab","pmids":["36890220"],"is_preprint":false},{"year":2001,"finding":"ACAT1 deletion in macrophages reduces total cellular cholesterol efflux by 25% despite upregulation of ABCA1, while increasing efflux of lipoprotein-derived cholesterol by 32% and increasing accumulation of free cholesterol from acetylated LDL by 26%, accompanied by a 75% increase in intracellular vesicles, demonstrating ACAT1's role in routing cholesterol toward efflux pathways.","method":"Acat1-/- peritoneal macrophages, radiolabeled cholesterol efflux assay, ABCA1 expression analysis, electron microscopy","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with quantitative cholesterol trafficking readouts","pmids":["15499044"],"is_preprint":false},{"year":1998,"finding":"The adrenocortical lipid depletion (ald) phenotype in AKR inbred mice is caused by a deletion of the first coding exon and two missense mutations in the ACAT1 (Acact) gene; genetic non-complementation with Acact-/- mice and immunoblotting confirmed the ald allele encodes a truncated ACAT protein; despite structural differences, the mutant protein retained cholesterol esterification activity, suggesting the adrenal phenotype arises from altered susceptibility to post-translational modifying factors.","method":"Genetic mapping, complementation cross, immunoblotting, cDNA sequence analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — genetic complementation and molecular characterization of endogenous mouse mutation","pmids":["9422770"],"is_preprint":false},{"year":2023,"finding":"SCD1 inhibition reduces oleic acid and ACAT1-generated esterified cholesterol in CD8+ T cells, enhancing IFN-γ production and cytotoxic activity; restoration of cholesteryl oleate reverses the enhanced T cell function, establishing an SCD1-ACAT1 axis in which ACAT1-mediated cholesterol esterification suppresses CD8+ T cell effector functions.","method":"SCD1 inhibitor treatment, ACAT1 inhibitor treatment, cholesterol/esterified cholesterol measurement, T cell functional assays, in vivo tumor models","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological dissection with rescue experiments and in vivo validation","pmids":["37879607"],"is_preprint":false},{"year":2018,"finding":"NLRP3 inflammasome activation downstream of free cholesterol accumulation in ACAT1-deficient macrophages is the primary driver of cutaneous xanthomatosis in hyperlipidemic mice; loss of NLRP3 completely reversed the cutaneous xanthoma caused by bone marrow ACAT1 deficiency, while ACAT1-null macrophages showed enhanced CHOP and TNF-α expression upon cholesterol loading.","method":"Bone marrow transplantation with Acat1/Nlrp3 double KO, histology, cytokine analysis, LDLR-null mouse model","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis (double KO) with clear phenotypic reversal","pmids":["30354239"],"is_preprint":false}],"current_model":"ACAT1 (SOAT1) is an ER-resident integral membrane enzyme that forms a tetramer (dimer of dimers) with nine transmembrane helices per protomer, catalyzing transfer of an unsaturated acyl chain from acyl-CoA (entering via a cytosolic tunnel) to cholesterol (entering via a transmembrane tunnel) at a catalytic site requiring H460, S456, D400, and Y518; beyond cholesterol esterification it functions as a mitochondrial acetyltransferase that acetylates PDHA1 and PDP1 to suppress the pyruvate dehydrogenase complex and promote the Warburg effect, and also acetylates additional substrates (ME2, METTL3), with its activity and localization dynamically regulated by cholesterol loading (allosteric activation, redistribution into catalytically hyperactive ER-derived vesicles), inflammatory signals (TNF-α/NF-κB, leptin/JAK2-PI3K), and oxysterols, and its esterification activity plays a central role in macrophage cholesterol homeostasis, hematopoietic progenitor proliferation, CD8+ T cell effector function, and protection against neurodegenerative and metabolic disease."},"narrative":{"teleology":[{"year":1996,"claim":"Identifying that a single-residue substitution (S265L) abolishes ACAT1 enzymatic activity, while complementation restores cholesteryl ester synthesis but not SREBP regulation, established that ACAT1 cholesterol esterification is genetically separable from sterol-sensing signaling pathways.","evidence":"Molecular characterization and cDNA complementation of SRD-4 hamster cells","pmids":["8662991"],"confidence":"High","gaps":["Identity of the sterol-regulatory defect gene in SRD-4 cells remained unresolved","No structural explanation for why S265L inactivates the enzyme"]},{"year":1998,"claim":"Quantitative immunodepletion across human tissues established ACAT1 as the dominant cholesterol-esterifying enzyme in liver, adrenal gland, macrophages, and kidney, while revealing a minor role in intestine where ACAT2 predominates.","evidence":"Immunodepletion of ACAT1 from tissue homogenates with residual activity measurement","pmids":["9717734"],"confidence":"High","gaps":["Mechanism of tissue-specific ACAT1 vs. ACAT2 expression not addressed","Post-translational regulation not examined"]},{"year":1998,"claim":"The adrenocortical lipid depletion (ald) phenotype in AKR mice was mapped to mutations in the Acat1 gene, providing a natural mammalian loss-of-function model and showing that a truncated ACAT1 protein can retain partial activity.","evidence":"Genetic mapping, complementation cross with Acat1-KO, immunoblotting, cDNA sequencing in AKR mice","pmids":["9422770"],"confidence":"Medium","gaps":["The post-translational modifying factors proposed to explain the adrenal phenotype were not identified","Only one inbred strain examined"]},{"year":2000,"claim":"Immunoelectron microscopy resolved the subcellular site of ACAT1 action to rough ER tubules and showed that cholesterol loading induces redistribution into ER-derived vesicles, providing a cell-biological framework for foam cell cholesterol ester accumulation.","evidence":"Immunoelectron microscopy and immunofluorescence with GRP78 co-localization in human macrophages","pmids":["10623671"],"confidence":"High","gaps":["Mechanism of ER vesiculation was unknown","Whether vesicle-associated ACAT1 has altered enzymatic properties was not tested"]},{"year":2001,"claim":"ACAT1-deficient macrophages revealed that cholesterol esterification by ACAT1 routes cholesterol away from efflux pathways, as knockout cells accumulated free cholesterol and showed altered vesicular trafficking despite ABCA1 upregulation.","evidence":"Acat1−/− peritoneal macrophages with radiolabeled cholesterol efflux assay and electron microscopy","pmids":["15499044"],"confidence":"Medium","gaps":["Mechanism by which ACAT1 influences vesicular cholesterol routing was not defined","Compensatory ABCA1 upregulation mechanism not elucidated"]},{"year":2005,"claim":"Demonstration that ACAT1 possesses a stereospecific allosteric activator site distinct from its substrate site explained how macrophage cholesterol loading amplifies esterification activity without increasing ACAT1 protein levels.","evidence":"In vitro enzyme assay with enantiomeric cholesterol and sterol analogs combined with intact-cell cholesterol loading","pmids":["15992359"],"confidence":"High","gaps":["Structural location of allosteric site was unknown at this time","Identity of endogenous allosteric activator(s) in vivo not confirmed"]},{"year":2008,"claim":"Systematic mutagenesis defined the ACAT1 catalytic triad (S456, H460, D400) and the essential residue Y518, establishing a serine-based catalytic mechanism for acyl transfer.","evidence":"Site-directed mutagenesis with in vitro enzyme activity assays","pmids":["18480028"],"confidence":"High","gaps":["No crystal or cryo-EM structure was available to place these residues architecturally","Role of the FYXDWWN motif beyond Y518 was not mechanistically dissected"]},{"year":2009,"claim":"Inflammatory signals — TNF-α via NF-κB and leptin via JAK2/PI3K — were shown to transcriptionally upregulate ACAT1 in monocytes/macrophages, linking inflammatory milieu to foam cell formation and cholesterol ester accumulation.","evidence":"Promoter-reporter assays with NF-κB site mapping; JAK2/PI3K inhibitor experiments in human monocyte-derived macrophages","pmids":["19189937","19625677"],"confidence":"Medium","gaps":["In vivo relevance of inflammatory ACAT1 upregulation not demonstrated genetically","Epigenetic regulation of the ACAT1 promoter not explored"]},{"year":2010,"claim":"Two parallel advances showed that (1) vesicle-associated ACAT1 has ~3-fold higher specific activity than ER-resident ACAT1, explaining amplified esterification in cholesterol-loaded macrophages, and (2) ACAT1 deletion in AD model mice reduces APP and amyloid burden while altering brain cholesterol homeostasis via 24-hydroxycholesterol.","evidence":"Subcellular fractionation with reconstituted enzyme assay in macrophages; Acat1 KO crossed with 3XTg-AD mice with sterol synthesis and APP measurements","pmids":["20460577","20133765"],"confidence":"High","gaps":["Mechanism by which vesicle environment enhances ACAT1 activity was not established","Downstream link between 24-hydroxycholesterol and APP reduction was correlative"]},{"year":2013,"claim":"Global Acat1 knockout revealed a role in restraining hematopoietic stem/progenitor cell proliferation, expanding the known functions of ACAT1 beyond cholesterol esterification into immune cell homeostasis.","evidence":"Acat1 KO mice with flow cytometric analysis of bone marrow LSK populations","pmids":["23846496"],"confidence":"Medium","gaps":["Whether the proliferation phenotype is cell-autonomous or due to altered bone marrow niche cholesterol was not resolved","Downstream signaling mechanism not identified"]},{"year":2014,"claim":"Discovery that mitochondrial ACAT1 functions as a protein acetyltransferase — acetylating PDHA1 (K321) and PDP1 (K202) to suppress the pyruvate dehydrogenase complex — established a second enzymatic activity for ACAT1 that promotes the Warburg effect in cancer.","evidence":"Co-IP, mass spectrometry, in vitro acetyltransferase assay, mutagenesis, ACAT1 knockdown with tumor growth readout","pmids":["24486017"],"confidence":"High","gaps":["Mechanism by which ACAT1 is targeted to mitochondria was not defined","Relationship between ER cholesterol esterification and mitochondrial acetyltransferase activities was unexplored"]},{"year":2014,"claim":"ACAT1 inhibition in microglia was shown to stimulate TFEB-mediated autophagy and lysosomal degradation of oligomeric Aβ, providing a mechanistic basis for the neuroprotective effects of ACAT1 loss in Alzheimer's models.","evidence":"Acat1 gene KO and pharmacological inhibitor K604 with autophagy and Aβ uptake/degradation assays in microglia","pmids":["25339759"],"confidence":"High","gaps":["How cholesterol ester depletion triggers TFEB activation (mTOR-independent pathway) was not fully resolved","Long-term in vivo microglial-specific effects not tested"]},{"year":2018,"claim":"Genetic epistasis demonstrated that free cholesterol accumulation in ACAT1-deficient macrophages drives NLRP3 inflammasome activation, which is the primary cause of cutaneous xanthomatosis in hyperlipidemic settings, clarifying the deleterious consequences of macrophage ACAT1 loss under high-cholesterol conditions.","evidence":"Bone marrow transplantation with Acat1/Nlrp3 double KO in LDLR-null mice","pmids":["30354239"],"confidence":"High","gaps":["Whether this inflammasome activation contributes to atherosclerotic plaque instability was not tested","Threshold of free cholesterol accumulation needed for NLRP3 triggering was not quantified"]},{"year":2019,"claim":"Myeloid-specific Acat1 knockout protected against diet-induced obesity by impairing monocyte integrin-β1-mediated infiltration into adipose tissue, extending ACAT1's functional scope to metabolic inflammation and adipose tissue homeostasis.","evidence":"Myeloid-specific Acat1 KO mice with adoptive transfer, flow cytometry, and pharmacological ACAT1 inhibition","pmids":["29533741"],"confidence":"High","gaps":["How ACAT1 loss reduces integrin-β1 expression at the molecular level was not determined","Whether lipid composition of monocyte membranes mediates the integrin effect was not tested"]},{"year":2020,"claim":"Two independent cryo-EM structures of the ACAT1 tetramer revealed the architecture of the nine-transmembrane protomer with dual substrate tunnels converging at the catalytic histidine, providing a structural explanation for acyl-chain selectivity and enabling rational understanding of inhibitor binding.","evidence":"Two independent cryo-EM structures (with and without nevanimibe inhibitor) with structure-guided mutagenesis","pmids":["32433614","32433613"],"confidence":"High","gaps":["No structure with bound cholesterol substrate was obtained","Mechanism of allosteric activation by cholesterol was not structurally resolved","Conformational dynamics during catalysis remain unknown"]},{"year":2022,"claim":"ACAT1 knockout in Niemann-Pick type C (Npc1 mutant) mice extended lifespan by 34% and corrected cholesterol misdistribution across late endosomes, TGN, and mitochondria, establishing ACAT1 as a critical node governing inter-organelle cholesterol routing in lysosomal storage disease.","evidence":"Genetic Acat1 KO crossed with Npc1 mutant mice; fibroblast cholesterol fractionation and organelle marker immunolocalization","pmids":["35507892"],"confidence":"High","gaps":["Whether ACAT1 inhibitors can replicate the lifespan extension pharmacologically was not tested","Mechanism by which ACAT1 loss redirects cholesterol from late endosomes was not defined"]},{"year":2023,"claim":"Multiple 2023 studies expanded ACAT1's functional reach: oxysterol-activated ACAT1 depletes accessible plasma membrane cholesterol to mediate antimicrobial defense; ACAT1 blockade enriches cholesterol at the MAM and strengthens ER-mitochondria contacts; and ACAT1-mediated cholesterol esterification suppresses CD8+ T cell effector function via the SCD1-ACAT1 axis.","evidence":"ACAT-deficient cell lines with viral/bacterial infection assays; MAM proteomics with electron microscopy; SCD1/ACAT1 inhibitors with T cell functional assays and tumor models","pmids":["36695568","36982602","37879607"],"confidence":"High","gaps":["Structural basis for oxysterol-specific allosteric ACAT1 activation is unknown","Whether MAM enrichment of ACAT1 has functional consequences for mitochondrial metabolism was not shown","In vivo confirmation of SCD1-ACAT1 T cell axis in human patients is lacking"]},{"year":2025,"claim":"ACAT1 was shown to acetylate ME2 at K156, potentiating malate-to-lactate conversion and driving lactylation of DNA repair proteins to promote chemoresistance, extending the mitochondrial acetyltransferase function beyond the pyruvate dehydrogenase complex.","evidence":"Co-IP, mass spectrometry, K156 mutagenesis, in vitro ME2 activity assay, in vivo ovarian cancer models","pmids":["39951294"],"confidence":"Medium","gaps":["Independent replication in a second cancer type is needed","Whether SIRT3 opposes ME2-K156 acetylation as it does for PDHA1 was not tested","Structural basis for ACAT1 substrate recognition of diverse acetyltransferase targets is unknown"]},{"year":null,"claim":"Key unresolved questions include the structural basis for cholesterol allosteric activation, the determinants of ACAT1 mitochondrial targeting versus ER retention, whether the ER cholesterol esterification and mitochondrial acetyltransferase activities are coordinated or independent, and how ACAT1 activity is precisely tuned in immune cells to balance protective cholesterol esterification against pathological free cholesterol accumulation.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structure of ACAT1 with cholesterol bound at the allosteric site","Mitochondrial targeting signal or import mechanism not identified","Regulatory logic connecting the two enzymatic activities is unstudied"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1,2,4,5]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[3,21,22]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[4,0]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[6,7,8,16]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[6,7]},{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[3,16]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,2,4,5,7]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[15,18,25,26]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[9]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[10,17,21]}],"complexes":[],"partners":["PDHA1","PDP1","SIRT3","ME2","METTL3","NLRP3"],"other_free_text":[]},"mechanistic_narrative":"ACAT1 (also called SOAT1) is an ER-resident integral membrane acyltransferase that esterifies cholesterol using acyl-CoA substrates, serving as the predominant cholesterol-esterifying enzyme in liver, adrenal gland, macrophages, and kidney, while also functioning as a mitochondrial acetyltransferase that modifies metabolic enzymes including PDHA1, PDP1, ME2, and METTL3 [PMID:9717734, PMID:24486017, PMID:39951294, PMID:36890220]. Structurally, ACAT1 assembles as a dimer-of-dimers tetramer with nine transmembrane helices per protomer, enclosing a cytosolic tunnel for acyl-CoA entry and a transmembrane tunnel for cholesterol access that converge at a catalytic site defined by residues H460, S456, D400, and Y518 [PMID:32433614, PMID:32433613, PMID:18480028]. ACAT1 activity is allosterically activated by cholesterol at a stereospecific site distinct from the substrate site, and upon cholesterol loading in macrophages the enzyme redistributes into ER-derived vesicles where it exhibits approximately three-fold higher specific activity, amplifying esterification capacity without changes in protein level [PMID:15992359, PMID:20460577]. Beyond lipid metabolism, ACAT1 loss alters intracellular cholesterol distribution across organelles with broad physiological consequences including enhanced microglial Aβ clearance via TFEB-dependent autophagy, restraint of hematopoietic progenitor proliferation, suppression of CD8+ T cell effector function, NLRP3-dependent cutaneous xanthomatosis, and amelioration of neurodegeneration in Alzheimer's and Niemann-Pick type C disease models [PMID:25339759, PMID:20133765, PMID:35507892, PMID:23846496, PMID:37879607, PMID:30354239]."},"prefetch_data":{"uniprot":{"accession":"P24752","full_name":"Acetyl-CoA acetyltransferase, mitochondrial","aliases":["Acetoacetyl-CoA thiolase","T2"],"length_aa":427,"mass_kda":45.2,"function":"This is one of the enzymes that catalyzes the last step of the mitochondrial beta-oxidation pathway, an aerobic process breaking down fatty acids into acetyl-CoA (PubMed:1715688, PubMed:7728148, PubMed:9744475). 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obesity.","date":"2018","source":"American journal of physiology. Endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/29533741","citation_count":23,"is_preprint":false},{"pmid":"32639091","id":"PMC_32639091","title":"C. elegans ACAT regulates lipolysis and its related lifespan in fasting through modulation of the genes in lipolysis and insulin/IGF-1 signaling.","date":"2020","source":"BioFactors (Oxford, England)","url":"https://pubmed.ncbi.nlm.nih.gov/32639091","citation_count":23,"is_preprint":false},{"pmid":"18618086","id":"PMC_18618086","title":"Novel N-terminal cleavage of APP precludes Abeta generation in ACAT-defective AC29 cells.","date":"2008","source":"Journal of molecular neuroscience : MN","url":"https://pubmed.ncbi.nlm.nih.gov/18618086","citation_count":23,"is_preprint":false},{"pmid":"36473091","id":"PMC_36473091","title":"Targeting Sterol O-Acyltransferase/Acyl-CoA:Cholesterol Acyltransferase (ACAT): A Perspective on Small-Molecule Inhibitors and Their Therapeutic Potential.","date":"2022","source":"Journal of medicinal chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/36473091","citation_count":22,"is_preprint":false},{"pmid":"35507892","id":"PMC_35507892","title":"Acat1/Soat1 knockout extends the mutant Npc1 mouse lifespan and ameliorates functional deficiencies in multiple organelles of mutant cells.","date":"2022","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/35507892","citation_count":22,"is_preprint":false},{"pmid":"27485769","id":"PMC_27485769","title":"5-Hydroxymethylcytosine in E-box motifs ACAT|GTG and ACAC|GTG increases DNA-binding of the B-HLH transcription factor TCF4.","date":"2016","source":"Integrative biology : quantitative biosciences from nano to macro","url":"https://pubmed.ncbi.nlm.nih.gov/27485769","citation_count":22,"is_preprint":false},{"pmid":"23430882","id":"PMC_23430882","title":"Three Japanese Patients with Beta-Ketothiolase Deficiency Who Share a Mutation, c.431A>C (H144P) in ACAT1 : Subtle Abnormality in Urinary Organic Acid Analysis and Blood Acylcarnitine Analysis Using Tandem Mass Spectrometry.","date":"2011","source":"JIMD reports","url":"https://pubmed.ncbi.nlm.nih.gov/23430882","citation_count":22,"is_preprint":false},{"pmid":"29097366","id":"PMC_29097366","title":"Quantitative Trait Locus Mapping of Macrophage Cholesterol Metabolism and CRISPR/Cas9 Editing Implicate an ACAT1 Truncation as a Causal Modifier Variant.","date":"2017","source":"Arteriosclerosis, thrombosis, and vascular biology","url":"https://pubmed.ncbi.nlm.nih.gov/29097366","citation_count":22,"is_preprint":false},{"pmid":"36890220","id":"PMC_36890220","title":"ACAT1-mediated METTL3 acetylation inhibits cell migration and invasion in triple negative breast cancer.","date":"2023","source":"Genes and immunity","url":"https://pubmed.ncbi.nlm.nih.gov/36890220","citation_count":21,"is_preprint":false},{"pmid":"20460577","id":"PMC_20460577","title":"Cholesterol loading in macrophages stimulates formation of ER-derived vesicles with elevated ACAT1 activity.","date":"2010","source":"Journal of lipid research","url":"https://pubmed.ncbi.nlm.nih.gov/20460577","citation_count":20,"is_preprint":false},{"pmid":"7630037","id":"PMC_7630037","title":"Studies on acyl-CoA: cholesterol acyltransferase (ACAT) inhibitory effects and enzyme selectivity of F-1394, a pantotheic acid derivative.","date":"1995","source":"Japanese journal of pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/7630037","citation_count":20,"is_preprint":false},{"pmid":"32404369","id":"PMC_32404369","title":"The ThiL enzyme is a valid antibacterial target essential for both thiamine biosynthesis and salvage pathways in Pseudomonas aeruginosa.","date":"2020","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/32404369","citation_count":19,"is_preprint":false},{"pmid":"9422770","id":"PMC_9422770","title":"Adrenocortical lipid depletion gene (ald) in AKR mice is associated with an acyl-CoA:cholesterol acyltransferase (ACAT) mutation.","date":"1998","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/9422770","citation_count":19,"is_preprint":false},{"pmid":"9140228","id":"PMC_9140228","title":"Inhibitory effects of oren-gedoku-to and its components on cholesteryl ester synthesis in cultured human hepatocyte HepG2 cells: evidence from the cultured HepG2 cells and in vitro assay of ACAT.","date":"1997","source":"Planta medica","url":"https://pubmed.ncbi.nlm.nih.gov/9140228","citation_count":19,"is_preprint":false},{"pmid":"37157042","id":"PMC_37157042","title":"Inhibition of ACAT as a Therapeutic Target for Alzheimer's Disease Is Independent of ApoE4 Lipidation.","date":"2023","source":"Neurotherapeutics : the journal of the American Society for Experimental NeuroTherapeutics","url":"https://pubmed.ncbi.nlm.nih.gov/37157042","citation_count":18,"is_preprint":false},{"pmid":"15712998","id":"PMC_15712998","title":"Polyacetylenic compounds, ACAT inhibitors from the roots of Panax ginseng.","date":"2005","source":"Journal of agricultural and food chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/15712998","citation_count":18,"is_preprint":false},{"pmid":"27748876","id":"PMC_27748876","title":"Exon 10 skipping in ACAT1 caused by a novel c.949G>A mutation located at an exonic splice enhancer site.","date":"2016","source":"Molecular medicine reports","url":"https://pubmed.ncbi.nlm.nih.gov/27748876","citation_count":17,"is_preprint":false},{"pmid":"8555254","id":"PMC_8555254","title":"Lack of correlation between ACAT mRNA expression and cholesterol esterification in primary liver cells.","date":"1996","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/8555254","citation_count":16,"is_preprint":false},{"pmid":"22243772","id":"PMC_22243772","title":"Sex-specific association of ACAT-1 rs1044925 SNP and serum lipid levels in the hypercholesterolemic subjects.","date":"2012","source":"Lipids in health and disease","url":"https://pubmed.ncbi.nlm.nih.gov/22243772","citation_count":16,"is_preprint":false},{"pmid":"30354239","id":"PMC_30354239","title":"Inflammasome Activation Aggravates Cutaneous Xanthomatosis and Atherosclerosis in ACAT1 (Acyl-CoA Cholesterol Acyltransferase 1) Deficiency in Bone Marrow.","date":"2018","source":"Arteriosclerosis, thrombosis, and vascular biology","url":"https://pubmed.ncbi.nlm.nih.gov/30354239","citation_count":15,"is_preprint":false},{"pmid":"32942042","id":"PMC_32942042","title":"Acidic extracellular pH promotes accumulation of free cholesterol in human monocyte-derived macrophages via inhibition of ACAT1 activity.","date":"2020","source":"Atherosclerosis","url":"https://pubmed.ncbi.nlm.nih.gov/32942042","citation_count":15,"is_preprint":false},{"pmid":"15242859","id":"PMC_15242859","title":"Acyl-coenzyme A:cholesterol acyltransferase-2 (ACAT-2) is responsible for elevated intestinal ACAT activity in diabetic rats.","date":"2004","source":"Arteriosclerosis, thrombosis, and vascular biology","url":"https://pubmed.ncbi.nlm.nih.gov/15242859","citation_count":15,"is_preprint":false},{"pmid":"23846496","id":"PMC_23846496","title":"Acat1 gene ablation in mice increases hematopoietic progenitor cell proliferation in bone marrow and causes leukocytosis.","date":"2013","source":"Arteriosclerosis, thrombosis, and vascular biology","url":"https://pubmed.ncbi.nlm.nih.gov/23846496","citation_count":15,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":51676,"output_tokens":6725,"usd":0.127951},"stage2":{"model":"claude-opus-4-6","input_tokens":10383,"output_tokens":4659,"usd":0.252585},"total_usd":0.380536,"stage1_batch_id":"msgbatch_01TqNM6p6FsoMBT2jzQTwKBr","stage2_batch_id":"msgbatch_01Ya473eco8sTHbvRymQaU8o","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2020,\n      \"finding\": \"Cryo-EM structure of human ACAT1 reveals it forms a dimer-of-dimers tetramer, with each protomer containing nine transmembrane segments enclosing a cytosolic tunnel and a transmembrane tunnel that converge at the catalytic site; structure-guided mutagenesis indicates acyl-CoA enters through the cytosolic tunnel while cholesterol enters through the transmembrane tunnel, rationalizing preference for unsaturated acyl chains.\",\n      \"method\": \"Cryo-EM structure determination + structure-guided mutagenesis\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with functional mutagenesis validation in a single rigorous study\",\n      \"pmids\": [\"32433614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cryo-EM structure of human ACAT1 in complex with the inhibitor nevanimibe shows the holoenzyme as a tetramer of two homodimers, with each monomer containing nine transmembrane helices (TM4-TM9 forming the inhibitor-binding cavity); the cavity contains a catalytically essential histidine residue and accommodates an endogenous acyl-CoA, providing a structural basis for cholesterol esterification and inhibitor interaction.\",\n      \"method\": \"Cryo-EM structure determination + biochemical analysis\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — independent cryo-EM structure with biochemical validation, orthogonally replicates findings of companion paper\",\n      \"pmids\": [\"32433613\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Mutagenesis identified the putative catalytic triad of ACAT1 as S456, H460, and D400; mutation of any of these residues abolished enzymatic activity, and ACAT1 is sensitive to serine-modifying reagents, supporting a serine-based catalytic mechanism. Additionally, Y518 and the conserved FYXDWWN motif tyrosine are required for ACAT1 activity.\",\n      \"method\": \"Site-directed mutagenesis + in vitro enzyme activity assays\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro mutagenesis with functional readout across multiple residues\",\n      \"pmids\": [\"18480028\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Mitochondrial ACAT1 functions as an acetyltransferase that acetylates PDHA1 at K321 (inhibiting it by recruiting PDK1) and PDP1 at K202 (inhibiting it by dissociating its substrate PDHA1), thereby suppressing the pyruvate dehydrogenase complex (PDC) and promoting the Warburg effect; SIRT3 acts as the corresponding deacetylase, and Y381 phosphorylation of PDP1 toggles recruitment between SIRT3 and ACAT1 at the PDC.\",\n      \"method\": \"Co-IP, mass spectrometry, in vitro acetyltransferase assay, site-directed mutagenesis, ACAT1 knockdown with tumor growth readout\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (Co-IP, in vitro assay, mutagenesis, KD phenotype) in a single study\",\n      \"pmids\": [\"24486017\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"ACAT1 has two distinct sterol-binding sites: a substrate-binding site and an allosteric activator site. Stereochemistry of the 3-hydroxyl group is critical for a sterol to serve as substrate but less critical for activation; enantiomeric cholesterol fails to activate ACAT1, demonstrating stereospecific interaction at the allosteric site independent of membrane biophysical effects. Cholesterol loading of macrophages increases ACAT1 activity without increasing ACAT1 protein, consistent with allosteric activation via increased ER cholesterol.\",\n      \"method\": \"In vitro ACAT enzyme assay with sterol analogs + intact cell cholesterol loading experiments\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with stereospecific substrate analysis and intact-cell validation\",\n      \"pmids\": [\"15992359\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Immunodepletion with anti-ACAT1 antibodies demonstrated that ACAT1 protein is responsible for ~90% of cholesterol esterification activity in human liver, ~98% in adrenal gland, ~91% in macrophages, and ~80% in kidney, but only ~19% in intestine, establishing the tissue-specific catalytic role of ACAT1.\",\n      \"method\": \"Immunodepletion of ACAT1 protein from tissue homogenates followed by residual enzyme activity measurement\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — quantitative immunodepletion enzyme assay across multiple human tissues\",\n      \"pmids\": [\"9717734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Immunoelectron microscopy and immunofluorescence of human macrophages showed ACAT1 localizes predominantly to tubular rough endoplasmic reticulum under normal conditions; upon cholesterol loading, ~30-40% of ACAT1 immunoreactivity redistributes into ER-derived small vesicles also enriched in GRP78, linking ER vesiculation to foam cell formation.\",\n      \"method\": \"Immunoelectron microscopy, immunofluorescence, subcellular fractionation with GRP78 co-localization\",\n      \"journal\": \"The American journal of pathology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct localization by immunoelectron microscopy with functional consequence (foam cell formation)\",\n      \"pmids\": [\"10623671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"In cholesterol-loaded macrophages, ACAT1 redistributes from high-density ER membranes into lower-density ER-derived vesicles bearing both ER and trans-Golgi network markers; when normalized per equal ACAT1 protein mass, vesicle-associated ACAT1 shows ~3-fold higher enzymatic activity than ER membrane-associated ACAT1, revealing a mechanism by which macrophages increase cholesterol esterification capacity without upregulating ACAT1 protein.\",\n      \"method\": \"Subcellular fractionation, in vitro reconstituted ACAT enzyme assay, immunoblotting, cholesterol loading experiments\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — biochemical fractionation combined with in vitro enzyme activity reconstitution\",\n      \"pmids\": [\"20460577\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Human ACAT1 can be produced as a novel 56-kDa isoenzyme (in addition to the normal 50-kDa form) via interchromosomal trans-splicing of RNAs from chromosomes 1 and 7; the 56-kDa ACAT1 localizes to the ER and retains enzymatic activity, and uses GGC (glycine) as its translation initiation codon.\",\n      \"method\": \"Expression in CHO cells, mutagenesis, mass spectrometry, anti-peptide antibodies, immunolocalization\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods in one lab establishing novel isoenzyme production and function\",\n      \"pmids\": [\"15319423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"ACAT1 inhibition (gene KO or K604 inhibitor) in microglia stimulates autophagosome formation and transcription factor EB (TFEB)-mediated lysosomal proteolysis, increasing phagocytic uptake and lysosomal degradation of oligomeric Aβ1-42; this autophagy induction is mTOR-independent and can be modulated by agents disrupting cholesterol biosynthesis.\",\n      \"method\": \"Acat1 gene KO in mouse, pharmacological ACAT1 inhibitor K604, autophagy assays, lysosomal proteolysis assays, in vitro and in vivo Aβ uptake/degradation experiments\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO and pharmacological inhibition with multiple cellular readouts\",\n      \"pmids\": [\"25339759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"ACAT1 gene ablation in triple-transgenic AD mice causes a 32% increase in 24-hydroxycholesterol content, a 65% decrease in HMG-CoA reductase protein, and a 28% decrease in sterol synthesis rate in the brain, and reduces full-length APP and its proteolytic fragments by >60%, ameliorating cognitive deficits; treating hippocampal neurons with 24-hydroxycholesterol recapitulates these reductions in APP and HMGR.\",\n      \"method\": \"Acat1 KO mouse cross with 3XTg-AD model, biochemical assays, sterol synthesis rate measurement, hippocampal neuron treatment experiments\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic model with multiple biochemical endpoints and cell-based mechanistic follow-up\",\n      \"pmids\": [\"20133765\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"A point mutation at codon 265 (Ser→Leu) of ACAT1 in SRD-4 hamster cells results in an inactive enzyme; complementation with wild-type ACAT1 cDNA restored cholesteryl ester synthesis but did not restore sterol-mediated SREBP cleavage inhibition, demonstrating that ACAT deficiency and the sterol-regulatory defect are caused by independent mutations and can be uncoupled.\",\n      \"method\": \"Mutagenesis characterization of SRD-4 cells, transfection with wild-type cDNA, enzyme activity and SREBP processing assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — molecular identification of loss-of-function mutation with complementation and functional dissection\",\n      \"pmids\": [\"8662991\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Acyl-CoA binding protein (ACBP) regulates microsomal ACAT activity: in the presence of exogenous cholesterol, ACBP stimulates ACAT more potently than SCP-2 or L-FABP in proportion to their fatty acyl-CoA binding affinities; in the absence of exogenous cholesterol, these proteins inhibit ACAT. ACBP co-localizes with ACAT2 and ER markers in cells.\",\n      \"method\": \"In vitro microsomal ACAT assay with recombinant lipid-binding proteins, immunolocalization\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with purified proteins, single lab\",\n      \"pmids\": [\"12518025\"],\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, leading to increased cholesteryl ester accumulation and suppression of HDL-mediated cholesterol efflux; ACAT1 inhibitor K604 reversed the leptin-induced suppression of cholesterol efflux.\",\n      \"method\": \"JAK2/PI3K inhibitor treatment, ACAT activity assay, protein expression (Western blot), cholesterol efflux assay\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological pathway dissection with multiple readouts in single lab\",\n      \"pmids\": [\"19625677\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"TNF-α, through NF-κB pathway activation at a unique NF-κB element in the human ACAT1 proximal promoter, specifically enhances ACAT1 (but not ACAT2) gene expression in differentiating human monocytes, increasing cholesteryl ester accumulation and promoting lipid-laden cell formation.\",\n      \"method\": \"NF-κB inhibitor treatment, promoter reporter assay, gene expression analysis, CE accumulation measurement\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — promoter mapping combined with pharmacological pathway inhibition and functional readout\",\n      \"pmids\": [\"19189937\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"25-Hydroxycholesterol (25HC) activates ACAT in the ER to create an imbalance in accessible cholesterol distribution between ER and plasma membrane, triggering rapid internalization of accessible cholesterol from the PM; this is sustained by concurrent SREBP suppression. In ACAT-deficient cells, 25HC fails to suppress Zika virus, coronavirus, or Listeria infection, placing ACAT activation mechanistically upstream of oxysterol-mediated antimicrobial immunity.\",\n      \"method\": \"ACAT-deficient cell lines, viral/bacterial infection assays, cholesterol trafficking assays, epistasis with SREBP pathway\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic loss-of-function with multiple orthogonal functional readouts (viral, bacterial, cholesterol distribution)\",\n      \"pmids\": [\"36695568\"],\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), leading to enrichment of ACAT1 itself at the MAM and strengthening ER-mitochondria connectivity by increasing contact site number and shortening inter-organelle distance, as shown by MAM proteomics, confocal microscopy, and electron microscopy.\",\n      \"method\": \"Biochemical fractionation, MAM proteomics, confocal microscopy, electron microscopy, pharmacological ACAT1 inhibition\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple imaging modalities with proteomics in single lab\",\n      \"pmids\": [\"36982602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Acat1/Soat1 knockout in mutant Npc1 mice prolongs lifespan by 34% and improves motor function, hepatosplenic pathology, and Purkinje neuron survival; in mutant NPC1 fibroblasts, ACAT1 blockade increases cholesterol at TGN-rich membranes and mitochondria, decreases cholesterol at late endosomes, and restores proper localization of syntaxin 6, golgin 97, cathepsin D, and ABCA1, placing ACAT1 at the intersection of cholesterol distribution among multiple organelles.\",\n      \"method\": \"Genetic Acat1 KO in Npc1 mouse model, fibroblast cholesterol fractionation, immunolocalization of organelle markers\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic model with lifespan readout and mechanistic cell biology follow-up\",\n      \"pmids\": [\"35507892\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Myeloid-specific Acat1 KO (Acat1-M/-M) mice are resistant to Western diet-induced obesity; mechanistically, Ly6Chi monocytes from Acat1-M/-M mice express reduced integrin-β1, impairing their interaction with inflamed endothelium and infiltration into white adipose tissue; ACAT1 inhibition in RAW264.7 macrophages also reduces LPS-induced inflammatory responses.\",\n      \"method\": \"Myeloid-specific Acat1 KO mouse, adoptive transfer experiment, flow cytometry, pharmacological ACAT1 inhibition, gene expression analysis\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with adoptive transfer experiment and pharmacological validation\",\n      \"pmids\": [\"29533741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Global Acat1 knockout mice show a significantly higher proportion of Lin-Sca-1+c-Kit+ hematopoietic stem/progenitor cells in proliferation, resulting in elevated myeloid progenitor numbers and leukocytosis, demonstrating that ACAT1 plays a role in restraining hematopoietic progenitor proliferation in bone marrow.\",\n      \"method\": \"Acat1 KO mouse model, flow cytometry of bone marrow populations, cell proliferation assays\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined cellular phenotype by flow cytometry\",\n      \"pmids\": [\"23846496\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"RNAi-mediated knockdown of ACAT1 (~50% reduction in protein) reduced cholesteryl ester levels by 22% with a slight increase in ER free cholesterol, correlating with ~40% reduction in Aβ secretion from APP-expressing cells, demonstrating that even partial reduction of ACAT1 activity is sufficient to suppress amyloidogenic APP processing.\",\n      \"method\": \"ACAT1 siRNA knockdown, cholesteryl ester measurement, Aβ ELISA\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — RNAi KD with quantitative mechanistic readouts; single lab\",\n      \"pmids\": [\"17412327\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ACAT1 acetylates malate enzyme 2 (ME2) at lysine 156, potentiating ME2 enzyme activity and facilitating lactate production from glutamine-derived malate; this occurs when decreased intracellular glucose levels (under chemotherapy) reduce glucose uptake, triggering ME2-K156 acetylation by ACAT1, which drives lactylation of homologous recombination proteins and chemoresistance in ovarian cancer.\",\n      \"method\": \"Co-IP, mass spectrometry identification of acetylation site, mutagenesis of K156, in vitro ME2 enzyme activity assay, in vivo tumor models\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP, site mutagenesis, and enzyme activity assay from single lab\",\n      \"pmids\": [\"39951294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ACAT1 acetylates METTL3 protein in triple-negative breast cancer cells; this acetylation stabilizes METTL3 by inhibiting ubiquitin-proteasome-mediated degradation, and the NR2F6/ACAT1/METTL3 axis suppresses TNBC cell migration and invasion.\",\n      \"method\": \"Co-IP, GST pulldown, IP-based acetylation detection, ubiquitination assays, functional migration/invasion assays\",\n      \"journal\": \"Genes and immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP and pulldown with functional cellular readout; single lab\",\n      \"pmids\": [\"36890220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"ACAT1 deletion in macrophages reduces total cellular cholesterol efflux by 25% despite upregulation of ABCA1, while increasing efflux of lipoprotein-derived cholesterol by 32% and increasing accumulation of free cholesterol from acetylated LDL by 26%, accompanied by a 75% increase in intracellular vesicles, demonstrating ACAT1's role in routing cholesterol toward efflux pathways.\",\n      \"method\": \"Acat1-/- peritoneal macrophages, radiolabeled cholesterol efflux assay, ABCA1 expression analysis, electron microscopy\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with quantitative cholesterol trafficking readouts\",\n      \"pmids\": [\"15499044\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The adrenocortical lipid depletion (ald) phenotype in AKR inbred mice is caused by a deletion of the first coding exon and two missense mutations in the ACAT1 (Acact) gene; genetic non-complementation with Acact-/- mice and immunoblotting confirmed the ald allele encodes a truncated ACAT protein; despite structural differences, the mutant protein retained cholesterol esterification activity, suggesting the adrenal phenotype arises from altered susceptibility to post-translational modifying factors.\",\n      \"method\": \"Genetic mapping, complementation cross, immunoblotting, cDNA sequence analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic complementation and molecular characterization of endogenous mouse mutation\",\n      \"pmids\": [\"9422770\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SCD1 inhibition reduces oleic acid and ACAT1-generated esterified cholesterol in CD8+ T cells, enhancing IFN-γ production and cytotoxic activity; restoration of cholesteryl oleate reverses the enhanced T cell function, establishing an SCD1-ACAT1 axis in which ACAT1-mediated cholesterol esterification suppresses CD8+ T cell effector functions.\",\n      \"method\": \"SCD1 inhibitor treatment, ACAT1 inhibitor treatment, cholesterol/esterified cholesterol measurement, T cell functional assays, in vivo tumor models\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological dissection with rescue experiments and in vivo validation\",\n      \"pmids\": [\"37879607\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"NLRP3 inflammasome activation downstream of free cholesterol accumulation in ACAT1-deficient macrophages is the primary driver of cutaneous xanthomatosis in hyperlipidemic mice; loss of NLRP3 completely reversed the cutaneous xanthoma caused by bone marrow ACAT1 deficiency, while ACAT1-null macrophages showed enhanced CHOP and TNF-α expression upon cholesterol loading.\",\n      \"method\": \"Bone marrow transplantation with Acat1/Nlrp3 double KO, histology, cytokine analysis, LDLR-null mouse model\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis (double KO) with clear phenotypic reversal\",\n      \"pmids\": [\"30354239\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ACAT1 (SOAT1) is an ER-resident integral membrane enzyme that forms a tetramer (dimer of dimers) with nine transmembrane helices per protomer, catalyzing transfer of an unsaturated acyl chain from acyl-CoA (entering via a cytosolic tunnel) to cholesterol (entering via a transmembrane tunnel) at a catalytic site requiring H460, S456, D400, and Y518; beyond cholesterol esterification it functions as a mitochondrial acetyltransferase that acetylates PDHA1 and PDP1 to suppress the pyruvate dehydrogenase complex and promote the Warburg effect, and also acetylates additional substrates (ME2, METTL3), with its activity and localization dynamically regulated by cholesterol loading (allosteric activation, redistribution into catalytically hyperactive ER-derived vesicles), inflammatory signals (TNF-α/NF-κB, leptin/JAK2-PI3K), and oxysterols, and its esterification activity plays a central role in macrophage cholesterol homeostasis, hematopoietic progenitor proliferation, CD8+ T cell effector function, and protection against neurodegenerative and metabolic disease.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"ACAT1 (also called SOAT1) is an ER-resident integral membrane acyltransferase that esterifies cholesterol using acyl-CoA substrates, serving as the predominant cholesterol-esterifying enzyme in liver, adrenal gland, macrophages, and kidney, while also functioning as a mitochondrial acetyltransferase that modifies metabolic enzymes including PDHA1, PDP1, ME2, and METTL3 [PMID:9717734, PMID:24486017, PMID:39951294, PMID:36890220]. Structurally, ACAT1 assembles as a dimer-of-dimers tetramer with nine transmembrane helices per protomer, enclosing a cytosolic tunnel for acyl-CoA entry and a transmembrane tunnel for cholesterol access that converge at a catalytic site defined by residues H460, S456, D400, and Y518 [PMID:32433614, PMID:32433613, PMID:18480028]. ACAT1 activity is allosterically activated by cholesterol at a stereospecific site distinct from the substrate site, and upon cholesterol loading in macrophages the enzyme redistributes into ER-derived vesicles where it exhibits approximately three-fold higher specific activity, amplifying esterification capacity without changes in protein level [PMID:15992359, PMID:20460577]. Beyond lipid metabolism, ACAT1 loss alters intracellular cholesterol distribution across organelles with broad physiological consequences including enhanced microglial Aβ clearance via TFEB-dependent autophagy, restraint of hematopoietic progenitor proliferation, suppression of CD8+ T cell effector function, NLRP3-dependent cutaneous xanthomatosis, and amelioration of neurodegeneration in Alzheimer's and Niemann-Pick type C disease models [PMID:25339759, PMID:20133765, PMID:35507892, PMID:23846496, PMID:37879607, PMID:30354239].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Identifying that a single-residue substitution (S265L) abolishes ACAT1 enzymatic activity, while complementation restores cholesteryl ester synthesis but not SREBP regulation, established that ACAT1 cholesterol esterification is genetically separable from sterol-sensing signaling pathways.\",\n      \"evidence\": \"Molecular characterization and cDNA complementation of SRD-4 hamster cells\",\n      \"pmids\": [\"8662991\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the sterol-regulatory defect gene in SRD-4 cells remained unresolved\", \"No structural explanation for why S265L inactivates the enzyme\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Quantitative immunodepletion across human tissues established ACAT1 as the dominant cholesterol-esterifying enzyme in liver, adrenal gland, macrophages, and kidney, while revealing a minor role in intestine where ACAT2 predominates.\",\n      \"evidence\": \"Immunodepletion of ACAT1 from tissue homogenates with residual activity measurement\",\n      \"pmids\": [\"9717734\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of tissue-specific ACAT1 vs. ACAT2 expression not addressed\", \"Post-translational regulation not examined\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"The adrenocortical lipid depletion (ald) phenotype in AKR mice was mapped to mutations in the Acat1 gene, providing a natural mammalian loss-of-function model and showing that a truncated ACAT1 protein can retain partial activity.\",\n      \"evidence\": \"Genetic mapping, complementation cross with Acat1-KO, immunoblotting, cDNA sequencing in AKR mice\",\n      \"pmids\": [\"9422770\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The post-translational modifying factors proposed to explain the adrenal phenotype were not identified\", \"Only one inbred strain examined\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Immunoelectron microscopy resolved the subcellular site of ACAT1 action to rough ER tubules and showed that cholesterol loading induces redistribution into ER-derived vesicles, providing a cell-biological framework for foam cell cholesterol ester accumulation.\",\n      \"evidence\": \"Immunoelectron microscopy and immunofluorescence with GRP78 co-localization in human macrophages\",\n      \"pmids\": [\"10623671\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of ER vesiculation was unknown\", \"Whether vesicle-associated ACAT1 has altered enzymatic properties was not tested\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"ACAT1-deficient macrophages revealed that cholesterol esterification by ACAT1 routes cholesterol away from efflux pathways, as knockout cells accumulated free cholesterol and showed altered vesicular trafficking despite ABCA1 upregulation.\",\n      \"evidence\": \"Acat1−/− peritoneal macrophages with radiolabeled cholesterol efflux assay and electron microscopy\",\n      \"pmids\": [\"15499044\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which ACAT1 influences vesicular cholesterol routing was not defined\", \"Compensatory ABCA1 upregulation mechanism not elucidated\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Demonstration that ACAT1 possesses a stereospecific allosteric activator site distinct from its substrate site explained how macrophage cholesterol loading amplifies esterification activity without increasing ACAT1 protein levels.\",\n      \"evidence\": \"In vitro enzyme assay with enantiomeric cholesterol and sterol analogs combined with intact-cell cholesterol loading\",\n      \"pmids\": [\"15992359\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural location of allosteric site was unknown at this time\", \"Identity of endogenous allosteric activator(s) in vivo not confirmed\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Systematic mutagenesis defined the ACAT1 catalytic triad (S456, H460, D400) and the essential residue Y518, establishing a serine-based catalytic mechanism for acyl transfer.\",\n      \"evidence\": \"Site-directed mutagenesis with in vitro enzyme activity assays\",\n      \"pmids\": [\"18480028\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal or cryo-EM structure was available to place these residues architecturally\", \"Role of the FYXDWWN motif beyond Y518 was not mechanistically dissected\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Inflammatory signals — TNF-α via NF-κB and leptin via JAK2/PI3K — were shown to transcriptionally upregulate ACAT1 in monocytes/macrophages, linking inflammatory milieu to foam cell formation and cholesterol ester accumulation.\",\n      \"evidence\": \"Promoter-reporter assays with NF-κB site mapping; JAK2/PI3K inhibitor experiments in human monocyte-derived macrophages\",\n      \"pmids\": [\"19189937\", \"19625677\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo relevance of inflammatory ACAT1 upregulation not demonstrated genetically\", \"Epigenetic regulation of the ACAT1 promoter not explored\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Two parallel advances showed that (1) vesicle-associated ACAT1 has ~3-fold higher specific activity than ER-resident ACAT1, explaining amplified esterification in cholesterol-loaded macrophages, and (2) ACAT1 deletion in AD model mice reduces APP and amyloid burden while altering brain cholesterol homeostasis via 24-hydroxycholesterol.\",\n      \"evidence\": \"Subcellular fractionation with reconstituted enzyme assay in macrophages; Acat1 KO crossed with 3XTg-AD mice with sterol synthesis and APP measurements\",\n      \"pmids\": [\"20460577\", \"20133765\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which vesicle environment enhances ACAT1 activity was not established\", \"Downstream link between 24-hydroxycholesterol and APP reduction was correlative\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Global Acat1 knockout revealed a role in restraining hematopoietic stem/progenitor cell proliferation, expanding the known functions of ACAT1 beyond cholesterol esterification into immune cell homeostasis.\",\n      \"evidence\": \"Acat1 KO mice with flow cytometric analysis of bone marrow LSK populations\",\n      \"pmids\": [\"23846496\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether the proliferation phenotype is cell-autonomous or due to altered bone marrow niche cholesterol was not resolved\", \"Downstream signaling mechanism not identified\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Discovery that mitochondrial ACAT1 functions as a protein acetyltransferase — acetylating PDHA1 (K321) and PDP1 (K202) to suppress the pyruvate dehydrogenase complex — established a second enzymatic activity for ACAT1 that promotes the Warburg effect in cancer.\",\n      \"evidence\": \"Co-IP, mass spectrometry, in vitro acetyltransferase assay, mutagenesis, ACAT1 knockdown with tumor growth readout\",\n      \"pmids\": [\"24486017\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which ACAT1 is targeted to mitochondria was not defined\", \"Relationship between ER cholesterol esterification and mitochondrial acetyltransferase activities was unexplored\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"ACAT1 inhibition in microglia was shown to stimulate TFEB-mediated autophagy and lysosomal degradation of oligomeric Aβ, providing a mechanistic basis for the neuroprotective effects of ACAT1 loss in Alzheimer's models.\",\n      \"evidence\": \"Acat1 gene KO and pharmacological inhibitor K604 with autophagy and Aβ uptake/degradation assays in microglia\",\n      \"pmids\": [\"25339759\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How cholesterol ester depletion triggers TFEB activation (mTOR-independent pathway) was not fully resolved\", \"Long-term in vivo microglial-specific effects not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Genetic epistasis demonstrated that free cholesterol accumulation in ACAT1-deficient macrophages drives NLRP3 inflammasome activation, which is the primary cause of cutaneous xanthomatosis in hyperlipidemic settings, clarifying the deleterious consequences of macrophage ACAT1 loss under high-cholesterol conditions.\",\n      \"evidence\": \"Bone marrow transplantation with Acat1/Nlrp3 double KO in LDLR-null mice\",\n      \"pmids\": [\"30354239\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this inflammasome activation contributes to atherosclerotic plaque instability was not tested\", \"Threshold of free cholesterol accumulation needed for NLRP3 triggering was not quantified\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Myeloid-specific Acat1 knockout protected against diet-induced obesity by impairing monocyte integrin-β1-mediated infiltration into adipose tissue, extending ACAT1's functional scope to metabolic inflammation and adipose tissue homeostasis.\",\n      \"evidence\": \"Myeloid-specific Acat1 KO mice with adoptive transfer, flow cytometry, and pharmacological ACAT1 inhibition\",\n      \"pmids\": [\"29533741\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How ACAT1 loss reduces integrin-β1 expression at the molecular level was not determined\", \"Whether lipid composition of monocyte membranes mediates the integrin effect was not tested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Two independent cryo-EM structures of the ACAT1 tetramer revealed the architecture of the nine-transmembrane protomer with dual substrate tunnels converging at the catalytic histidine, providing a structural explanation for acyl-chain selectivity and enabling rational understanding of inhibitor binding.\",\n      \"evidence\": \"Two independent cryo-EM structures (with and without nevanimibe inhibitor) with structure-guided mutagenesis\",\n      \"pmids\": [\"32433614\", \"32433613\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure with bound cholesterol substrate was obtained\", \"Mechanism of allosteric activation by cholesterol was not structurally resolved\", \"Conformational dynamics during catalysis remain unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"ACAT1 knockout in Niemann-Pick type C (Npc1 mutant) mice extended lifespan by 34% and corrected cholesterol misdistribution across late endosomes, TGN, and mitochondria, establishing ACAT1 as a critical node governing inter-organelle cholesterol routing in lysosomal storage disease.\",\n      \"evidence\": \"Genetic Acat1 KO crossed with Npc1 mutant mice; fibroblast cholesterol fractionation and organelle marker immunolocalization\",\n      \"pmids\": [\"35507892\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ACAT1 inhibitors can replicate the lifespan extension pharmacologically was not tested\", \"Mechanism by which ACAT1 loss redirects cholesterol from late endosomes was not defined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Multiple 2023 studies expanded ACAT1's functional reach: oxysterol-activated ACAT1 depletes accessible plasma membrane cholesterol to mediate antimicrobial defense; ACAT1 blockade enriches cholesterol at the MAM and strengthens ER-mitochondria contacts; and ACAT1-mediated cholesterol esterification suppresses CD8+ T cell effector function via the SCD1-ACAT1 axis.\",\n      \"evidence\": \"ACAT-deficient cell lines with viral/bacterial infection assays; MAM proteomics with electron microscopy; SCD1/ACAT1 inhibitors with T cell functional assays and tumor models\",\n      \"pmids\": [\"36695568\", \"36982602\", \"37879607\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for oxysterol-specific allosteric ACAT1 activation is unknown\", \"Whether MAM enrichment of ACAT1 has functional consequences for mitochondrial metabolism was not shown\", \"In vivo confirmation of SCD1-ACAT1 T cell axis in human patients is lacking\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"ACAT1 was shown to acetylate ME2 at K156, potentiating malate-to-lactate conversion and driving lactylation of DNA repair proteins to promote chemoresistance, extending the mitochondrial acetyltransferase function beyond the pyruvate dehydrogenase complex.\",\n      \"evidence\": \"Co-IP, mass spectrometry, K156 mutagenesis, in vitro ME2 activity assay, in vivo ovarian cancer models\",\n      \"pmids\": [\"39951294\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Independent replication in a second cancer type is needed\", \"Whether SIRT3 opposes ME2-K156 acetylation as it does for PDHA1 was not tested\", \"Structural basis for ACAT1 substrate recognition of diverse acetyltransferase targets is unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis for cholesterol allosteric activation, the determinants of ACAT1 mitochondrial targeting versus ER retention, whether the ER cholesterol esterification and mitochondrial acetyltransferase activities are coordinated or independent, and how ACAT1 activity is precisely tuned in immune cells to balance protective cholesterol esterification against pathological free cholesterol accumulation.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of ACAT1 with cholesterol bound at the allosteric site\", \"Mitochondrial targeting signal or import mechanism not identified\", \"Regulatory logic connecting the two enzymatic activities is unstudied\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1, 2, 4, 5]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [3, 21, 22]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [4, 0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [6, 7, 8, 16]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [6, 7]},\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [3, 16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 2, 4, 5, 7]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [15, 18, 25, 26]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [9]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [10, 17, 21]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"PDHA1\",\n      \"PDP1\",\n      \"SIRT3\",\n      \"ME2\",\n      \"METTL3\",\n      \"NLRP3\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}