{"gene":"CERS5","run_date":"2026-06-09T22:57:18","timeline":{"discoveries":[{"year":2003,"finding":"TRH4 (CERS5), when overexpressed in HEK 293T cells, elevates dihydroceramide synthesis selectively enriched in palmitic acid (C16), and in cell homogenates preferentially utilizes sphinganine (rather than sphingosine) as the sphingoid base substrate; in vitro dihydroceramide synthase activity using various fatty acyl-CoA substrates confirmed palmitoyl-CoA selectivity in a fumonisin B1-sensitive manner.","method":"Overexpression in HEK 293T cells, [3H]-serine metabolic labeling, LC-ESI-MS/MS lipidomics, in vitro dihydroceramide synthase activity assay with acyl-CoA substrates, fumonisin B1 inhibition","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro enzymatic assay with substrate specificity profiling, confirmed by multiple orthogonal methods (metabolic labeling, mass spectrometry, cell-free homogenate assay), replicated in follow-up study (PMID:16100120)","pmids":["12912983"],"is_preprint":false},{"year":2005,"finding":"LASS5 (CERS5) is a bona fide dihydroceramide synthase that does not require additional protein subunits for activity. HA-tagged LASS5 was solubilized with digitonin, immunoprecipitated to near homogeneity (single band by SDS-PAGE, identity confirmed by MS), and the purified protein retained dihydroceramide synthase activity that was strictly dependent on exogenous phospholipids, highly selective for palmitoyl-CoA as acyl donor, and inhibited by fumonisin B1.","method":"Immunoprecipitation/purification of HA-tagged LASS5, SDS-PAGE, mass spectrometry identity confirmation, in vitro dihydroceramide synthase activity reconstitution with defined lipid substrates, fumonisin B1 inhibition","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution of enzymatic activity from purified protein with mutagenesis-equivalent substrate profiling, multiple orthogonal methods in single rigorous study","pmids":["16100120"],"is_preprint":false},{"year":2005,"finding":"LASS5 (CERS5) is the predominant ceramide synthase isoform in lung epithelia; it is membrane-associated when exogenously expressed, and its knockdown by siRNA reduced ceramide synthase activity by ~45%, while fumonisin B1 reduced it by ~78%. Overexpression of LASS5 reduced phosphatidylcholine (PtdCho) synthesis, linking LASS5-driven ceramide production to inhibition of surfactant PtdCho metabolism.","method":"siRNA knockdown, overexpression in lung epithelial cells, ceramide synthase activity assay, [14C]-PtdCho synthesis measurement, subcellular fractionation (membrane association)","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KD with defined enzymatic and metabolic phenotypes, two orthogonal perturbations (siRNA and pharmacological inhibitor), single lab","pmids":["15772421"],"is_preprint":false},{"year":2016,"finding":"LASS5 (CERS5) physically interacts with succinate dehydrogenase subunit B (SDHB); the interaction was identified by yeast two-hybrid screen, confirmed by mammalian two-hybrid, GST pull-down, and co-immunoprecipitation assays. The C-terminal fragment of SDHB is required for binding. LASS5 and SDHB co-localize in COS-7 cells and synergistically repress p53 and p21 transcriptional reporter activity when co-expressed.","method":"Yeast two-hybrid screen, mammalian two-hybrid, GST pull-down, co-immunoprecipitation, co-localization (fluorescence microscopy), p53/p21 luciferase reporter assay","journal":"Current molecular medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal binding assays (GST pulldown + Co-IP) plus functional reporter assay, multiple orthogonal methods in single lab study","pmids":["27280497"],"is_preprint":false},{"year":2017,"finding":"Fenretinide decreases long-chain ceramide (C16, synthesized by CerS5) levels and increases very long-chain ceramide levels in cystic fibrosis lung epithelial cell lines by transcriptionally downregulating Cers5 expression, not through direct enzymatic inhibition.","method":"RT-PCR/transcriptional analysis of Cers5 in CF lung epithelial cell lines treated with fenretinide, ceramide species quantification by mass spectrometry","journal":"Journal of molecular medicine (Berlin, Germany)","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — defined transcriptional mechanism for Cers5 downregulation with ceramide lipidomics readout, single lab, single method for mechanistic link","pmids":["28695226"],"is_preprint":false},{"year":2014,"finding":"17β-estradiol and GPER1 upregulate CerS5 promoter activity in MCF-7 breast cancer cells via activation of the AP-1 transcription factor (c-Jun/c-Fos dimerization), as demonstrated by promoter deletion and mutation constructs; this effect is blocked by the anti-estrogen fulvestrant.","method":"CerS5 promoter-luciferase reporter assays, promoter deletion and mutation constructs, co-transfection with GPER1, fulvestrant inhibition in MCF-7 and MDA-MB-231 cells","journal":"Biochemical pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — promoter deletion/mutation mapping plus pharmacological validation, two orthogonal perturbations, single lab","pmids":["25451689"],"is_preprint":false},{"year":2022,"finding":"CerS5 silencing in mouse skeletal muscle (reducing C16:0-ceramide) did NOT improve insulin pathway activation or glucose uptake under high-fat diet conditions, in contrast to CerS1 silencing (which reduces C18:0-ceramide and does improve insulin sensitivity). This establishes that C16:0-ceramide generated by CerS5 is not the critical ceramide species driving skeletal muscle insulin resistance.","method":"In vivo electroporation-mediated shRNA-based gene silencing in gastrocnemius muscle of HFD-fed mice, insulin pathway activation assays, ceramide species quantification, glucose uptake measurement","journal":"Cells","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — negative finding for CerS5 with clean KO and defined phenotypic readouts, single lab in vivo study","pmids":["35053322"],"is_preprint":false},{"year":2023,"finding":"Hepatocyte-specific CerS5 knockout in mice fed a CDAHFD diet worsened liver fibrosis progression (increased α-SMA, COL1α, TGF-β) without affecting hepatic steatosis or inflammation. Mechanistically, CerS5 knockout decreased expression of cyp27a1 (a key enzyme in the alternative bile acid synthesis pathway), shifting the hepatic bile acid pool toward hydrophobic 12α-OH bile acids that promote fibrosis.","method":"Hepatocyte conditional CerS5 knockout mice, CDAHFD feeding, RT-PCR, IHC, Western blot for fibrosis/inflammation markers, RNA-seq transcriptome analysis, metabolomics for bile acid profiling","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional KO with multiple downstream readouts (transcriptomics + metabolomics + protein markers), single lab with multiple orthogonal methods","pmids":["37216827"],"is_preprint":false},{"year":2017,"finding":"Downregulation of LASS5/CERS5 by siRNA in HUVECs attenuated ceramide production and increased expression of AMPK-alpha target genes, suggesting CERS5-derived ceramide negatively regulates AMPK signaling in endothelial cells.","method":"siRNA-mediated gene silencing in HUVECs, differential gene expression analysis of AMPK-alpha and its target genes, AMPK activator treatment","journal":"Turk Kardiyoloji Dernegi arsivi","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single siRNA knockdown experiment with expression-level readout, limited mechanistic resolution","pmids":["28424433"],"is_preprint":false}],"current_model":"CERS5 (LASS5/TRH4) is a bona fide, autonomous dihydroceramide synthase that resides in the endoplasmic reticulum membrane and selectively uses palmitoyl-CoA to produce C16-ceramide in a phospholipid-dependent, fumonisin B1-inhibitable reaction requiring no additional protein subunits; its expression is transcriptionally upregulated by estradiol/GPER1 via AP-1, and the C16-ceramide it generates regulates downstream processes including phosphatidylcholine synthesis in lung epithelia, bile acid metabolism and liver fibrosis via cyp27a1, and p53/p21 transcription through physical interaction with SDHB, while its activity is dispensable for skeletal muscle insulin resistance."},"narrative":{"mechanistic_narrative":"CERS5 (LASS5/TRH4) is an endoplasmic-reticulum-resident dihydroceramide synthase that selectively condenses palmitoyl-CoA with sphinganine to generate C16-ceramide [PMID:12912983]. It is a bona fide, autonomous enzyme: HA-tagged CERS5 purified to near homogeneity retains full activity that is strictly phospholipid-dependent, highly selective for palmitoyl-CoA, and inhibited by fumonisin B1, establishing that no additional protein subunit is required for catalysis [PMID:16100120]. CERS5 is the predominant ceramide synthase isoform in lung epithelia, where its C16-ceramide output suppresses surfactant phosphatidylcholine synthesis [PMID:15772421]. The C16-ceramide it produces governs distinct downstream programs in a tissue-specific manner: hepatocyte CERS5 sustains cyp27a1 expression and the alternative bile acid pathway, restraining accumulation of fibrogenic hydrophobic bile acids [PMID:37216827], whereas its product is dispensable for skeletal muscle insulin resistance, unlike CerS1-derived C18-ceramide [PMID:35053322]. Beyond catalysis, CERS5 physically interacts with succinate dehydrogenase subunit B (SDHB) through the SDHB C-terminus and the two proteins synergistically repress p53 and p21 reporter activity [PMID:27280497]. CERS5 expression is transcriptionally controlled, being upregulated by 17β-estradiol/GPER1 signaling through AP-1 (c-Jun/c-Fos) at its promoter [PMID:25451689] and downregulated by fenretinide [PMID:28695226].","teleology":[{"year":2003,"claim":"Whether CERS5/TRH4 was a ceramide synthase and which substrates it used was unknown; this work defined it as a dihydroceramide synthase with strict acyl-chain and sphingoid-base preference.","evidence":"Overexpression in HEK 293T, [3H]-serine metabolic labeling, LC-ESI-MS/MS lipidomics, and cell-free synthase assay with acyl-CoA panel plus fumonisin B1","pmids":["12912983"],"confidence":"High","gaps":["Did not establish whether activity required partner proteins","Subcellular site of catalysis not directly resolved"]},{"year":2005,"claim":"It was unclear whether CERS5 catalysis is intrinsic or depends on accessory subunits; purification to homogeneity showed the enzyme is autonomous and lipid-dependent.","evidence":"Digitonin solubilization, immunoprecipitation/purification of HA-tagged LASS5, MS identity confirmation, reconstituted synthase assay with defined phospholipids and acyl-CoA","pmids":["16100120"],"confidence":"High","gaps":["Catalytic residues and structural basis of palmitoyl-CoA selectivity not mapped","Regulation of activity in intact membranes not addressed"]},{"year":2005,"claim":"The physiological consequence of CERS5 activity in a specific tissue was undefined; it was identified as the dominant lung-epithelial isoform whose ceramide output suppresses surfactant phosphatidylcholine synthesis.","evidence":"siRNA knockdown and overexpression in lung epithelial cells, synthase activity assay, [14C]-PtdCho synthesis, membrane fractionation","pmids":["15772421"],"confidence":"Medium","gaps":["Mechanism linking ceramide to PtdCho suppression not resolved","Residual ~55% activity after knockdown implies isoform redundancy"]},{"year":2014,"claim":"How CERS5 expression is regulated was unknown; estrogen/GPER1 signaling was shown to drive its promoter via AP-1.","evidence":"CerS5 promoter-luciferase with deletion/mutation constructs, GPER1 co-transfection, fulvestrant inhibition in MCF-7 and MDA-MB-231 cells","pmids":["25451689"],"confidence":"Medium","gaps":["Downstream consequence of estrogen-induced CERS5 in breast cancer not defined","AP-1 binding shown by reporter, not endogenous chromatin occupancy"]},{"year":2016,"claim":"Whether CERS5 has functions beyond lipid synthesis was unaddressed; a physical interaction with SDHB linking it to p53/p21 repression was identified.","evidence":"Yeast and mammalian two-hybrid, GST pull-down, co-immunoprecipitation, co-localization in COS-7, p53/p21 luciferase reporters","pmids":["27280497"],"confidence":"Medium","gaps":["Whether catalytic activity is required for transcriptional repression unknown","Functional consequence on endogenous p53 targets and SDH activity not tested"]},{"year":2017,"claim":"It was unclear whether fenretinide alters C16-ceramide enzymatically or transcriptionally; the drug was shown to downregulate Cers5 transcription rather than inhibit the enzyme.","evidence":"RT-PCR of Cers5 and ceramide mass spectrometry in fenretinide-treated CF lung epithelial lines","pmids":["28695226"],"confidence":"Medium","gaps":["Transcription factors mediating fenretinide repression not identified","Single-method mechanistic link"]},{"year":2017,"claim":"A possible link between CERS5-derived ceramide and metabolic signaling in endothelium was probed, indicating it negatively regulates AMPK target gene expression.","evidence":"siRNA knockdown in HUVECs with AMPK-alpha target gene expression analysis","pmids":["28424433"],"confidence":"Low","gaps":["Single siRNA knockdown with expression-only readout, no functional AMPK activity measurement","No rescue or orthogonal perturbation"]},{"year":2022,"claim":"Whether C16-ceramide is the species driving muscle insulin resistance was tested; CERS5 silencing failed to improve insulin signaling, excluding its product as the critical mediator.","evidence":"In vivo electroporation shRNA silencing in HFD-fed mouse gastrocnemius, insulin pathway and glucose uptake assays, ceramide quantification, with CerS1 comparison","pmids":["35053322"],"confidence":"Medium","gaps":["Does not exclude roles in other insulin-target tissues","Negative result specific to skeletal muscle context"]},{"year":2023,"claim":"The hepatic role of CERS5 was undefined; conditional knockout revealed it protects against liver fibrosis by sustaining cyp27a1 and steering bile acid composition.","evidence":"Hepatocyte-specific CerS5 knockout mice on CDAHFD, RT-PCR/IHC/Western for fibrosis markers, RNA-seq, bile acid metabolomics","pmids":["37216827"],"confidence":"Medium","gaps":["How C16-ceramide regulates cyp27a1 expression is not mechanistically resolved","Whether the SDHB/p53 axis contributes to the hepatic phenotype untested"]},{"year":null,"claim":"How CERS5 catalytic output is mechanistically transduced into its varied downstream effects (PtdCho suppression, cyp27a1/bile acid control, p53/p21 repression) and whether the SDHB interaction depends on enzymatic activity remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of CERS5 or its catalytic mechanism","Causal chain from C16-ceramide to transcriptional and metabolic outputs not established","Catalysis-independent (scaffolding) functions not separated from enzymatic ones"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[2]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,1,2,7]}],"complexes":[],"partners":["SDHB"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8N5B7","full_name":"Ceramide synthase 5","aliases":["LAG1 longevity assurance homolog 5","Sphingoid base N-palmitoyltransferase CERS5","Sphingosine N-acyltransferase CERS5"],"length_aa":392,"mass_kda":45.8,"function":"Ceramide synthase that catalyzes the transfer of the acyl chain from acyl-CoA to a sphingoid base, with high selectivity toward palmitoyl-CoA (hexadecanoyl-CoA; C16:0-CoA) (PubMed:16951403, PubMed:18541923, PubMed:22144673, PubMed:22661289, PubMed:23530041, PubMed:26887952, PubMed:29632068, PubMed:31916624). Can use other acyl donors, but with less efficiency (By similarity). N-acylates sphinganine and sphingosine bases to form dihydroceramides and ceramides in de novo synthesis and salvage pathways, respectively (PubMed:31916624). Plays a role in de novo ceramide synthesis and surfactant homeostasis in pulmonary epithelia (By similarity)","subcellular_location":"Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/Q8N5B7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CERS5","classification":"Not Classified","n_dependent_lines":20,"n_total_lines":1208,"dependency_fraction":0.016556291390728478},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000139624","cell_line_id":"CID001883","localizations":[{"compartment":"er","grade":3}],"interactors":[{"gene":"HMGCS1","stoichiometry":0.2},{"gene":"TMEM57","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001883","total_profiled":1310},"omim":[{"mim_id":"615336","title":"CERAMIDE SYNTHASE 6; CERS6","url":"https://www.omim.org/entry/615336"},{"mim_id":"615335","title":"CERAMIDE SYNTHASE 5; CERS5","url":"https://www.omim.org/entry/615335"},{"mim_id":"615334","title":"CERAMIDE SYNTHASE 4; CERS4","url":"https://www.omim.org/entry/615334"},{"mim_id":"606920","title":"CERAMIDE SYNTHASE 2; CERS2","url":"https://www.omim.org/entry/606920"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nuclear membrane","reliability":"Approved"},{"location":"Endoplasmic reticulum","reliability":"Additional"},{"location":"Primary cilium","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/CERS5"},"hgnc":{"alias_symbol":["Trh4","MGC45411","FLJ25304"],"prev_symbol":["LASS5"]},"alphafold":{"accession":"Q8N5B7","domains":[{"cath_id":"-","chopping":"40-76_136-342","consensus_level":"high","plddt":95.0014,"start":40,"end":342},{"cath_id":"1.10.10.60","chopping":"91-134","consensus_level":"high","plddt":93.0202,"start":91,"end":134}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N5B7","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N5B7-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N5B7-F1-predicted_aligned_error_v6.png","plddt_mean":84.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CERS5","jax_strain_url":"https://www.jax.org/strain/search?query=CERS5"},"sequence":{"accession":"Q8N5B7","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8N5B7.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8N5B7/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N5B7"}},"corpus_meta":[{"pmid":"12912983","id":"PMC_12912983","title":"Two mammalian longevity assurance gene (LAG1) family members, trh1 and trh4, regulate dihydroceramide synthesis using different fatty acyl-CoA donors.","date":"2003","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12912983","citation_count":260,"is_preprint":false},{"pmid":"16100120","id":"PMC_16100120","title":"LASS5 is a bona fide dihydroceramide synthase that selectively utilizes palmitoyl-CoA as acyl donor.","date":"2005","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16100120","citation_count":110,"is_preprint":false},{"pmid":"25451689","id":"PMC_25451689","title":"Ceramide synthases CerS4 and CerS5 are upregulated by 17β-estradiol and GPER1 via AP-1 in human breast cancer cells.","date":"2014","source":"Biochemical pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/25451689","citation_count":41,"is_preprint":false},{"pmid":"15772421","id":"PMC_15772421","title":"LASS5 is the predominant ceramide synthase isoform involved in de novo sphingolipid synthesis in lung epithelia.","date":"2005","source":"Journal of lipid research","url":"https://pubmed.ncbi.nlm.nih.gov/15772421","citation_count":41,"is_preprint":false},{"pmid":"28695226","id":"PMC_28695226","title":"Fenretinide differentially modulates the levels of long- and very long-chain ceramides by downregulating Cers5 enzyme: evidence from bench to bedside.","date":"2017","source":"Journal of molecular medicine (Berlin, Germany)","url":"https://pubmed.ncbi.nlm.nih.gov/28695226","citation_count":32,"is_preprint":false},{"pmid":"35053322","id":"PMC_35053322","title":"CerS1 but Not CerS5 Gene Silencing, Improves Insulin Sensitivity and Glucose Uptake in Skeletal Muscle.","date":"2022","source":"Cells","url":"https://pubmed.ncbi.nlm.nih.gov/35053322","citation_count":12,"is_preprint":false},{"pmid":"26800871","id":"PMC_26800871","title":"The MHC Class I Cancer-Associated Neoepitope Trh4 Linked with Impaired Peptide Processing Induces a Unique Noncanonical TCR Conformer.","date":"2016","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/26800871","citation_count":11,"is_preprint":false},{"pmid":"29507106","id":"PMC_29507106","title":"The Immunogenicity of a Proline-Substituted Altered Peptide Ligand toward the Cancer-Associated TEIPP Neoepitope Trh4 Is Unrelated to Complex Stability.","date":"2018","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/29507106","citation_count":8,"is_preprint":false},{"pmid":"27280497","id":"PMC_27280497","title":"LASS5 Interacts with SDHB and Synergistically Represses p53 and p21 Activity.","date":"2016","source":"Current molecular medicine","url":"https://pubmed.ncbi.nlm.nih.gov/27280497","citation_count":7,"is_preprint":false},{"pmid":"38244710","id":"PMC_38244710","title":"Identification of CERS5 as a molecular biomarker in pan-cancer through multiple omics integrative analysis.","date":"2024","source":"Cellular signalling","url":"https://pubmed.ncbi.nlm.nih.gov/38244710","citation_count":5,"is_preprint":false},{"pmid":"28424433","id":"PMC_28424433","title":"[Investigating the role of ceramide metabolism-associated CERS5 (LASS5) gene in atherosclerosis pathogenesis in endothelial cells].","date":"2017","source":"Turk Kardiyoloji Dernegi arsivi : Turk Kardiyoloji Derneginin yayin organidir","url":"https://pubmed.ncbi.nlm.nih.gov/28424433","citation_count":2,"is_preprint":false},{"pmid":"37216827","id":"PMC_37216827","title":"CerS5 deficiency promotes liver fibrosis development in non-alcoholic fatty liver disease.","date":"2023","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/37216827","citation_count":1,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8986,"output_tokens":2664,"usd":0.033459,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9967,"output_tokens":3163,"usd":0.064455,"stage2_stop_reason":"end_turn"},"total_usd":0.097914,"stage1_batch_id":"msgbatch_01ACAxJQabDsht39JXeWG22d","stage2_batch_id":"msgbatch_01QEU22A9U5sbPTgsMzTmvSf","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2003,\n      \"finding\": \"TRH4 (CERS5), when overexpressed in HEK 293T cells, elevates dihydroceramide synthesis selectively enriched in palmitic acid (C16), and in cell homogenates preferentially utilizes sphinganine (rather than sphingosine) as the sphingoid base substrate; in vitro dihydroceramide synthase activity using various fatty acyl-CoA substrates confirmed palmitoyl-CoA selectivity in a fumonisin B1-sensitive manner.\",\n      \"method\": \"Overexpression in HEK 293T cells, [3H]-serine metabolic labeling, LC-ESI-MS/MS lipidomics, in vitro dihydroceramide synthase activity assay with acyl-CoA substrates, fumonisin B1 inhibition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro enzymatic assay with substrate specificity profiling, confirmed by multiple orthogonal methods (metabolic labeling, mass spectrometry, cell-free homogenate assay), replicated in follow-up study (PMID:16100120)\",\n      \"pmids\": [\"12912983\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"LASS5 (CERS5) is a bona fide dihydroceramide synthase that does not require additional protein subunits for activity. HA-tagged LASS5 was solubilized with digitonin, immunoprecipitated to near homogeneity (single band by SDS-PAGE, identity confirmed by MS), and the purified protein retained dihydroceramide synthase activity that was strictly dependent on exogenous phospholipids, highly selective for palmitoyl-CoA as acyl donor, and inhibited by fumonisin B1.\",\n      \"method\": \"Immunoprecipitation/purification of HA-tagged LASS5, SDS-PAGE, mass spectrometry identity confirmation, in vitro dihydroceramide synthase activity reconstitution with defined lipid substrates, fumonisin B1 inhibition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution of enzymatic activity from purified protein with mutagenesis-equivalent substrate profiling, multiple orthogonal methods in single rigorous study\",\n      \"pmids\": [\"16100120\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"LASS5 (CERS5) is the predominant ceramide synthase isoform in lung epithelia; it is membrane-associated when exogenously expressed, and its knockdown by siRNA reduced ceramide synthase activity by ~45%, while fumonisin B1 reduced it by ~78%. Overexpression of LASS5 reduced phosphatidylcholine (PtdCho) synthesis, linking LASS5-driven ceramide production to inhibition of surfactant PtdCho metabolism.\",\n      \"method\": \"siRNA knockdown, overexpression in lung epithelial cells, ceramide synthase activity assay, [14C]-PtdCho synthesis measurement, subcellular fractionation (membrane association)\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KD with defined enzymatic and metabolic phenotypes, two orthogonal perturbations (siRNA and pharmacological inhibitor), single lab\",\n      \"pmids\": [\"15772421\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"LASS5 (CERS5) physically interacts with succinate dehydrogenase subunit B (SDHB); the interaction was identified by yeast two-hybrid screen, confirmed by mammalian two-hybrid, GST pull-down, and co-immunoprecipitation assays. The C-terminal fragment of SDHB is required for binding. LASS5 and SDHB co-localize in COS-7 cells and synergistically repress p53 and p21 transcriptional reporter activity when co-expressed.\",\n      \"method\": \"Yeast two-hybrid screen, mammalian two-hybrid, GST pull-down, co-immunoprecipitation, co-localization (fluorescence microscopy), p53/p21 luciferase reporter assay\",\n      \"journal\": \"Current molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal binding assays (GST pulldown + Co-IP) plus functional reporter assay, multiple orthogonal methods in single lab study\",\n      \"pmids\": [\"27280497\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Fenretinide decreases long-chain ceramide (C16, synthesized by CerS5) levels and increases very long-chain ceramide levels in cystic fibrosis lung epithelial cell lines by transcriptionally downregulating Cers5 expression, not through direct enzymatic inhibition.\",\n      \"method\": \"RT-PCR/transcriptional analysis of Cers5 in CF lung epithelial cell lines treated with fenretinide, ceramide species quantification by mass spectrometry\",\n      \"journal\": \"Journal of molecular medicine (Berlin, Germany)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — defined transcriptional mechanism for Cers5 downregulation with ceramide lipidomics readout, single lab, single method for mechanistic link\",\n      \"pmids\": [\"28695226\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"17β-estradiol and GPER1 upregulate CerS5 promoter activity in MCF-7 breast cancer cells via activation of the AP-1 transcription factor (c-Jun/c-Fos dimerization), as demonstrated by promoter deletion and mutation constructs; this effect is blocked by the anti-estrogen fulvestrant.\",\n      \"method\": \"CerS5 promoter-luciferase reporter assays, promoter deletion and mutation constructs, co-transfection with GPER1, fulvestrant inhibition in MCF-7 and MDA-MB-231 cells\",\n      \"journal\": \"Biochemical pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — promoter deletion/mutation mapping plus pharmacological validation, two orthogonal perturbations, single lab\",\n      \"pmids\": [\"25451689\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CerS5 silencing in mouse skeletal muscle (reducing C16:0-ceramide) did NOT improve insulin pathway activation or glucose uptake under high-fat diet conditions, in contrast to CerS1 silencing (which reduces C18:0-ceramide and does improve insulin sensitivity). This establishes that C16:0-ceramide generated by CerS5 is not the critical ceramide species driving skeletal muscle insulin resistance.\",\n      \"method\": \"In vivo electroporation-mediated shRNA-based gene silencing in gastrocnemius muscle of HFD-fed mice, insulin pathway activation assays, ceramide species quantification, glucose uptake measurement\",\n      \"journal\": \"Cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — negative finding for CerS5 with clean KO and defined phenotypic readouts, single lab in vivo study\",\n      \"pmids\": [\"35053322\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Hepatocyte-specific CerS5 knockout in mice fed a CDAHFD diet worsened liver fibrosis progression (increased α-SMA, COL1α, TGF-β) without affecting hepatic steatosis or inflammation. Mechanistically, CerS5 knockout decreased expression of cyp27a1 (a key enzyme in the alternative bile acid synthesis pathway), shifting the hepatic bile acid pool toward hydrophobic 12α-OH bile acids that promote fibrosis.\",\n      \"method\": \"Hepatocyte conditional CerS5 knockout mice, CDAHFD feeding, RT-PCR, IHC, Western blot for fibrosis/inflammation markers, RNA-seq transcriptome analysis, metabolomics for bile acid profiling\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional KO with multiple downstream readouts (transcriptomics + metabolomics + protein markers), single lab with multiple orthogonal methods\",\n      \"pmids\": [\"37216827\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Downregulation of LASS5/CERS5 by siRNA in HUVECs attenuated ceramide production and increased expression of AMPK-alpha target genes, suggesting CERS5-derived ceramide negatively regulates AMPK signaling in endothelial cells.\",\n      \"method\": \"siRNA-mediated gene silencing in HUVECs, differential gene expression analysis of AMPK-alpha and its target genes, AMPK activator treatment\",\n      \"journal\": \"Turk Kardiyoloji Dernegi arsivi\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single siRNA knockdown experiment with expression-level readout, limited mechanistic resolution\",\n      \"pmids\": [\"28424433\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CERS5 (LASS5/TRH4) is a bona fide, autonomous dihydroceramide synthase that resides in the endoplasmic reticulum membrane and selectively uses palmitoyl-CoA to produce C16-ceramide in a phospholipid-dependent, fumonisin B1-inhibitable reaction requiring no additional protein subunits; its expression is transcriptionally upregulated by estradiol/GPER1 via AP-1, and the C16-ceramide it generates regulates downstream processes including phosphatidylcholine synthesis in lung epithelia, bile acid metabolism and liver fibrosis via cyp27a1, and p53/p21 transcription through physical interaction with SDHB, while its activity is dispensable for skeletal muscle insulin resistance.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CERS5 (LASS5/TRH4) is an endoplasmic-reticulum-resident dihydroceramide synthase that selectively condenses palmitoyl-CoA with sphinganine to generate C16-ceramide [#0]. It is a bona fide, autonomous enzyme: HA-tagged CERS5 purified to near homogeneity retains full activity that is strictly phospholipid-dependent, highly selective for palmitoyl-CoA, and inhibited by fumonisin B1, establishing that no additional protein subunit is required for catalysis [#1]. CERS5 is the predominant ceramide synthase isoform in lung epithelia, where its C16-ceramide output suppresses surfactant phosphatidylcholine synthesis [#2]. The C16-ceramide it produces governs distinct downstream programs in a tissue-specific manner: hepatocyte CERS5 sustains cyp27a1 expression and the alternative bile acid pathway, restraining accumulation of fibrogenic hydrophobic bile acids [#7], whereas its product is dispensable for skeletal muscle insulin resistance, unlike CerS1-derived C18-ceramide [#6]. Beyond catalysis, CERS5 physically interacts with succinate dehydrogenase subunit B (SDHB) through the SDHB C-terminus and the two proteins synergistically repress p53 and p21 reporter activity [#3]. CERS5 expression is transcriptionally controlled, being upregulated by 17\\u03b2-estradiol/GPER1 signaling through AP-1 (c-Jun/c-Fos) at its promoter [#5] and downregulated by fenretinide [#4].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Whether CERS5/TRH4 was a ceramide synthase and which substrates it used was unknown; this work defined it as a dihydroceramide synthase with strict acyl-chain and sphingoid-base preference.\",\n      \"evidence\": \"Overexpression in HEK 293T, [3H]-serine metabolic labeling, LC-ESI-MS/MS lipidomics, and cell-free synthase assay with acyl-CoA panel plus fumonisin B1\",\n      \"pmids\": [\"12912983\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish whether activity required partner proteins\", \"Subcellular site of catalysis not directly resolved\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"It was unclear whether CERS5 catalysis is intrinsic or depends on accessory subunits; purification to homogeneity showed the enzyme is autonomous and lipid-dependent.\",\n      \"evidence\": \"Digitonin solubilization, immunoprecipitation/purification of HA-tagged LASS5, MS identity confirmation, reconstituted synthase assay with defined phospholipids and acyl-CoA\",\n      \"pmids\": [\"16100120\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Catalytic residues and structural basis of palmitoyl-CoA selectivity not mapped\", \"Regulation of activity in intact membranes not addressed\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"The physiological consequence of CERS5 activity in a specific tissue was undefined; it was identified as the dominant lung-epithelial isoform whose ceramide output suppresses surfactant phosphatidylcholine synthesis.\",\n      \"evidence\": \"siRNA knockdown and overexpression in lung epithelial cells, synthase activity assay, [14C]-PtdCho synthesis, membrane fractionation\",\n      \"pmids\": [\"15772421\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking ceramide to PtdCho suppression not resolved\", \"Residual ~55% activity after knockdown implies isoform redundancy\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"How CERS5 expression is regulated was unknown; estrogen/GPER1 signaling was shown to drive its promoter via AP-1.\",\n      \"evidence\": \"CerS5 promoter-luciferase with deletion/mutation constructs, GPER1 co-transfection, fulvestrant inhibition in MCF-7 and MDA-MB-231 cells\",\n      \"pmids\": [\"25451689\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Downstream consequence of estrogen-induced CERS5 in breast cancer not defined\", \"AP-1 binding shown by reporter, not endogenous chromatin occupancy\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Whether CERS5 has functions beyond lipid synthesis was unaddressed; a physical interaction with SDHB linking it to p53/p21 repression was identified.\",\n      \"evidence\": \"Yeast and mammalian two-hybrid, GST pull-down, co-immunoprecipitation, co-localization in COS-7, p53/p21 luciferase reporters\",\n      \"pmids\": [\"27280497\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether catalytic activity is required for transcriptional repression unknown\", \"Functional consequence on endogenous p53 targets and SDH activity not tested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"It was unclear whether fenretinide alters C16-ceramide enzymatically or transcriptionally; the drug was shown to downregulate Cers5 transcription rather than inhibit the enzyme.\",\n      \"evidence\": \"RT-PCR of Cers5 and ceramide mass spectrometry in fenretinide-treated CF lung epithelial lines\",\n      \"pmids\": [\"28695226\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Transcription factors mediating fenretinide repression not identified\", \"Single-method mechanistic link\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"A possible link between CERS5-derived ceramide and metabolic signaling in endothelium was probed, indicating it negatively regulates AMPK target gene expression.\",\n      \"evidence\": \"siRNA knockdown in HUVECs with AMPK-alpha target gene expression analysis\",\n      \"pmids\": [\"28424433\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single siRNA knockdown with expression-only readout, no functional AMPK activity measurement\", \"No rescue or orthogonal perturbation\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Whether C16-ceramide is the species driving muscle insulin resistance was tested; CERS5 silencing failed to improve insulin signaling, excluding its product as the critical mediator.\",\n      \"evidence\": \"In vivo electroporation shRNA silencing in HFD-fed mouse gastrocnemius, insulin pathway and glucose uptake assays, ceramide quantification, with CerS1 comparison\",\n      \"pmids\": [\"35053322\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not exclude roles in other insulin-target tissues\", \"Negative result specific to skeletal muscle context\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"The hepatic role of CERS5 was undefined; conditional knockout revealed it protects against liver fibrosis by sustaining cyp27a1 and steering bile acid composition.\",\n      \"evidence\": \"Hepatocyte-specific CerS5 knockout mice on CDAHFD, RT-PCR/IHC/Western for fibrosis markers, RNA-seq, bile acid metabolomics\",\n      \"pmids\": [\"37216827\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How C16-ceramide regulates cyp27a1 expression is not mechanistically resolved\", \"Whether the SDHB/p53 axis contributes to the hepatic phenotype untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CERS5 catalytic output is mechanistically transduced into its varied downstream effects (PtdCho suppression, cyp27a1/bile acid control, p53/p21 repression) and whether the SDHB interaction depends on enzymatic activity remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model of CERS5 or its catalytic mechanism\", \"Causal chain from C16-ceramide to transcriptional and metabolic outputs not established\", \"Catalysis-independent (scaffolding) functions not separated from enzymatic ones\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 1, 2, 7]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"SDHB\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}