GPAT4

GPAT4 is an NEM-sensitive, endoplasmic reticulum-localized glycerol-3-phosphate acyltransferase that catalyzes the first committed step of de novo glycerolipid synthesis, generating lysophosphatidic acid (LPA) from glycerol-3-phosphate and acyl-CoA, thereby channeling exogenous fatty acids into triacylglycerol and away from β-oxidation (PMID:18238778, PMID:18192653, PMID:25918168). Its enzymatic activity is stimulated by the N-myristoylated binding partner CHP1, enhanced by insulin-dependent phosphorylation via PI3K, and transcriptionally controlled by PPARγ and FXR (PMID:30846317, PMID:20181984, PMID:36398416, PMID:41530783). GPAT4 is required for lipid droplet biogenesis and milk fat synthesis in mammary epithelium, and in brown adipose tissue it accounts for ~65% of total GPAT activity, limiting exogenous fatty acid oxidation (PMID:16449762, PMID:25918168). Loss of GPAT4 in endocardial cells triggers ER stress, mitochondrial DNA escape, and cGAS-STING-dependent type-I interferon signaling that causes cardiac developmental defects—a phenotype rescued by ablation of the cGAS-STING pathway (PMID:40199910).

1. 2006 High

   Establishing that AGPAT6/GPAT4 localizes to the ER and is essential for lipid droplet formation and triacylglycerol production resolved its basic cellular function and tissue requirement, as Agpat6-/- mice showed dramatic loss of milk fat and lipid droplets in mammary epithelium.

   Evidence Gene-trap knockout mouse, histology, subcellular localization, milk lipid analysis

   PMID:16449762

   Open questions at the time

   • Enzymatic activity of the protein had not been directly measured
   • Whether the enzyme acted as a GPAT or AGPAT was unresolved
   • Mechanism of lipid droplet loss was unclear
2. 2008 High

   Demonstrating that purified AGPAT6 possesses GPAT but not AGPAT activity reclassified the enzyme as GPAT4 and established it as the major NEM-sensitive microsomal GPAT, resolving a long-standing misannotation.

   Evidence Purified protein GPAT activity assay, substrate specificity studies, 13C-oleic acid labeling/MS, KO tissue enzyme assays in two independent labs

   PMID:18192653 PMID:18238778

   Open questions at the time

   • Post-translational regulation of GPAT4 activity was unknown
   • Substrate selectivity among different acyl-CoA chains was not fully defined
   • How GPAT4-generated lipid intermediates differed from GPAT1-derived pools remained unclear
3. 2010 Medium

   Discovery that insulin stimulates GPAT4 phosphorylation in a PI3K-dependent manner, increasing its activity, connected glycerolipid biosynthesis to anabolic hormone signaling, while showing GPAT4 plays a modest role in adipocyte differentiation relative to GPAT3.

   Evidence Insulin stimulation of GPAT4-overexpressing cells, wortmannin inhibition, shRNA knockdown in 3T3-L1 adipocytes

   PMID:20181984

   Open questions at the time

   • Specific phosphorylation sites mediating activation were not mapped
   • Kinase(s) directly phosphorylating GPAT4 were not identified
   • Physiological consequence of insulin-driven GPAT4 activation in vivo was not tested
4. 2013 High

   Establishing that GPAT4 channels exogenous—but not de novo synthesized—fatty acids into TAG clarified the division of labor between GPAT1 and GPAT4, showing they access distinct fatty acid pools.

   Evidence Primary hepatocytes from Gpat1-/- and Gpat4-/- mice, de novo FA synthesis labeling, exogenous [14C]FA incorporation

   PMID:23908354

   Open questions at the time

   • Molecular basis for selective access to exogenous vs. de novo fatty acid pools was unknown
   • Whether this selectivity applies in non-hepatic tissues was not tested
5. 2015 High

   Quantifying GPAT4 as ~65% of total GPAT activity in brown adipose tissue and showing that its loss redirects exogenous fatty acids from TAG synthesis into β-oxidation defined GPAT4 as a metabolic gatekeeper in thermogenic fat.

   Evidence Gpat4-/- mice, GPAT activity in BAT, differentiated primary brown adipocytes, fatty acid oxidation/incorporation assays

   PMID:25918168

   Open questions at the time

   • Whether increased β-oxidation in GPAT4-null BAT affects whole-body thermogenesis was not conclusively shown
   • Compensatory changes in GPAT1 or GPAT3 expression were not fully addressed
6. 2019 High

   Identification of N-myristoylated CHP1 as a direct binding partner and activator of GPAT4 revealed the first protein-level regulatory mechanism for microsomal GPAT activity.

   Evidence CRISPR genetic screens, lipidomics, CHP1-GPAT4 binding assay, N-myristoylation mutagenesis in mammalian cells and invertebrates

   PMID:30846317

   Open questions at the time

   • Structural basis for CHP1-GPAT4 interaction was not resolved
   • Whether CHP1 regulation is modulated by calcium or other signals was not determined
   • Tissue-specific importance of CHP1-GPAT4 axis was unexplored
7. 2020 Medium

   Showing that GPAT4-generated saturated LPAs accumulate at ER-mitochondria contact sites and block autophagosome maturation linked GPAT4 lipid products to autophagy regulation beyond simple TAG storage.

   Evidence SCD-knockout VSMCs, lipid metabolite analysis, GPAT4 co-localization with omegasome markers, autophagic flux assays

   PMID:32408172

   Open questions at the time

   • Direct manipulation of GPAT4 (rather than SCD) to confirm the autophagy phenotype was not performed
   • Whether saturated LPA effects on autophagy are generalizable beyond VSMCs was untested
   • Physical mechanism by which saturated LPAs block omegasome resolution was not defined
8. 2022 Medium

   Identifying PPARγ/RXRα as a transcriptional activator of GPAT4 and linking GPAT4-produced PA to mTORC1 activation established a signaling axis from nuclear receptor to lipid intermediate to nutrient sensing in mammary epithelial cells.

   Evidence Promoter-luciferase reporter assay with deletions/mutations, PA rescue, mTORC1 phosphorylation readouts in dairy cow mammary epithelial cells

   PMID:36398416

   Open questions at the time

   • Whether PPARγ regulation of GPAT4 occurs in non-mammary tissues was not tested
   • ChIP confirmation of PPARγ occupancy at the endogenous promoter was lacking
   • Whether PA-mediated mTORC1 activation is GPAT4-specific or shared with other GPATs was unclear
9. 2025 High

   Demonstrating that GPAT4 loss in endocardial cells triggers ER stress, mitochondrial DNA escape via enhanced ER-mitochondria contacts, and a cGAS-STING-interferon inflammatory cascade causing cardiac defects—rescued by cGAS-STING ablation—established GPAT4 as an ER homeostasis factor whose absence activates innate immune pathways during development.

   Evidence Global and endocardial-specific Gpat4 KO mice, ER stress markers, MAM communication assays, mtDNA escape measurement, genetic epistasis rescue via cGAS-STING ablation

   PMID:40199910

   Open questions at the time

   • Whether ER stress from GPAT4 loss is due to lipid composition change or protein misfolding was not distinguished
   • Whether the cGAS-STING pathway is activated in other GPAT4-deficient tissues was not examined
   • Human genetic evidence linking GPAT4 mutations to congenital heart disease is lacking
10. 2025 Medium

    Identifying FXR as a transcriptional repressor of GPAT4 and showing that GPAT4 upregulation drives hepatic lipid droplet accumulation in MAFLD models established a second nuclear receptor axis controlling GPAT4 expression with disease relevance.

    Evidence siRNA knockdown, dual-luciferase reporter assay, HFD mouse model, in vitro hepatocyte model

    PMID:41530783

    Open questions at the time

    • Direct FXR binding site on the GPAT4 promoter was not mapped by ChIP
    • Whether FXR-GPAT4 axis operates independently of PPARγ regulation was not addressed

• Key unresolved questions include the structural basis of GPAT4 catalysis and CHP1 activation, the mechanism by which GPAT4 selectively accesses exogenous versus de novo fatty acid pools, whether GPAT4 mutations cause human disease, and the in vivo significance of GPAT4 nano-confinement on lipid droplet surfaces.
• No crystal or cryo-EM structure of GPAT4
• No human Mendelian disease linked to GPAT4 mutations
• Molecular basis for selective fatty acid pool access is unknown
• Functional consequence of LD surface nano-confinement is untested in vivo