FADS3

FADS3 is a membrane-bound desaturase whose best-established physiological function is as a sphingolipid Δ14Z-desaturase that introduces a cis double bond into the long-chain base of ceramides to generate 4,14-sphingadiene (sphingadienine) sphingolipids (PMID:31862735, PMID:31916624). Its obligate substrate is sphingosine-containing ceramide rather than free sphingosine, with selectivity for C16–C20 sphingosine moieties but not the N-acyl chain; catalysis uses NADH or NADPH with cytochrome b5 as the electron carrier, and the resulting sphingadiene flux is directed preferentially into sphingomyelin and away from lipid microdomains (PMID:31916624, PMID:37209771). Through this activity FADS3 also detoxifies the cytotoxic atypical base 1-deoxysphinganine by desaturating it to 1-deoxysphingosine, conferring resistance to deoxysphingolipid toxicity (PMID:31862735). FADS3 was originally characterized as a fatty acid Δ13-desaturase that converts trans-vaccenic acid to a trans11,cis13-conjugated linoleic acid isomer in vitro and in mammary epithelial cells (PMID:24070791, PMID:27865506), although this activity was not detectable in liver or heart of knockout mice, indicating it does not operate in those tissues under physiological conditions (PMID:28838557). At the organismal level, Fads3 supports brain DHA accretion via liver n-3 PUFA synthesis (PMID:28838557), and its alternative transcripts are induced by dietary long-chain PUFA through a PPARγ-dependent mechanism distinct from FADS1/FADS2 (PMID:22398025). In clear-cell renal cell carcinoma, FADS3 drives proliferation and EMT by generating lipid-droplet-derived acetyl-CoA that fuels Smad2 acetylation and autocrine TGF-β/Smad2/3 signaling (PMID:41314810).

1. 2009 Medium

   Before any catalytic role was defined, it was unknown whether FADS3 was expressed as a single product; demonstrating multiple tissue-dependent protein isoforms established FADS3 as a structurally complex gene with potential for distinct functional forms.

   Evidence Isoform-specific polyclonal antibody Western blotting and qRT-PCR across rat, mouse, and human tissues

   PMID:19752397

   Open questions at the time

   • Functional differences between isoforms not defined
   • Catalytic identity of isoforms unknown at this stage
2. 2012 Medium

   The regulatory logic of FADS3 was unknown; showing dietary PUFA induce FADS3 alternative transcripts via PPARγ placed FADS3 in a regulatory regime opposite to that of FADS1/FADS2.

   Evidence Dietary LCPUFA intervention in baboon neonate liver and HepG2 cells with PPARγ antagonist GW9662

   PMID:22398025

   Open questions at the time

   • Direct PPARγ binding to FADS3 regulatory elements not demonstrated
   • Functional consequence of transcript induction not measured
3. 2013 High

   FADS3 had no assigned enzymatic activity; in vitro desaturation of trans-vaccenate to a trans11,cis13-CLA isomer provided the first functional characterization of a mammalian methyl-end fatty acid desaturase.

   Evidence In vitro desaturase activity assay with rat FADS3, product structural characterization, and siRNA knockdown in rat hepatocytes

   PMID:24070791

   Open questions at the time

   • Physiological relevance in vivo not established
   • Electron donor and cofactor requirements not defined here
4. 2014 Medium

   The subcellular behavior of FADS3 was unknown; finding that the 51 kDa isoform is cytosolic, secreted on fibronectin fibers via exosome-like vesicles, and present in serum revealed an unexpected extracellular distribution distinct from other desaturases.

   Evidence Subcellular fractionation, confocal imaging, exosome characterization, and serum immunodetection in rat hepatocytes

   PMID:23966218

   Open questions at the time

   • Functional role of secreted FADS3 not established
   • Mechanism of secretion not defined
5. 2017 Medium

   Whether FADS3 contributes to PUFA homeostasis in vivo was unclear; knockout mice revealed altered DHA/DPA balance supporting brain DHA accretion, while showing the in vitro Δ13-CLA activity does not operate in liver or heart.

   Evidence Fads3 knockout mouse fatty acid profiling by gas chromatography and qRT-PCR of related genes

   PMID:28838557

   Open questions at the time

   • Mechanism by which Fads3 loss alters DHA synthesis unresolved
   • Tissue context where Δ13-desaturation operates not identified
   • Negative in vivo result conflicts with in vitro activity
6. 2020 High

   The principal physiological substrate of FADS3 was redefined; identification as a Δ14Z long-chain base and ceramide desaturase producing sphingadiene sphingolipids established a sphingolipid rather than fatty-acid-centric role, and linked FADS3 to detoxification of 1-deoxysphinganine.

   Evidence Metabolic labeling, overexpression/knockdown, KO mouse LCB profiling, GWAS association, and biochemical ceramide desaturase identification with microdomain fractionation

   PMID:31862735 PMID:31916624

   Open questions at the time

   • Structural basis of substrate selectivity not resolved
   • Cofactor requirements not yet defined
7. 2023 High

   The enzymatic mechanism was incompletely defined; establishing that ceramide (not free sphingosine) is the obligate substrate, with chain-length specificity for the sphingosine moiety and NADH/NADPH-cytochrome b5 cofactor dependence, completed the biochemical model and traced flux toward sphingomyelin.

   Evidence Cell-based assay with ceramide synthase inhibitor, in vitro enzymatic assay with defined substrates, and cofactor requirement analysis

   PMID:37209771

   Open questions at the time

   • No structural model of the catalytic site
   • Regulation of substrate access not defined
8. 2025 Medium

   A role in disease was unknown; in ccRCC FADS3 was shown to drive proliferation and EMT through lipid-droplet-derived acetyl-CoA promoting Smad2 acetylation and autocrine TGF-β/Smad2/3 signaling.

   Evidence Co-immunoprecipitation, mass spectrometry, proliferation/EMT assays, and Smad2/3 phosphorylation/acetylation biochemistry

   PMID:41314810

   Open questions at the time

   • Link between desaturase activity and acetyl-CoA generation not mechanistically resolved
   • Single study without independent replication
   • Direct FADS3 interaction partners not validated reciprocally

• It remains unresolved how FADS3 sphingolipid desaturase activity, the tissue-restricted fatty acid Δ13-desaturase activity, and the secreted/oncogenic functions are mechanistically connected within a single protein, and whether distinct isoforms underlie these distinct activities.
• No structural model linking activities
• Isoform-specific function not mapped to catalytic roles
• Physiological substrate hierarchy across tissues unknown