HILPDA

HILPDA is a hypoxia- and lipid-inducible lipid droplet-associated protein that functions as a physiological inhibitor of ATGL-mediated intracellular lipolysis, thereby promoting triglyceride storage in hepatocytes, macrophages, adipocytes, and tumor cells (PMID:31308147, PMID:32049012, PMID:37422000). Transcriptionally regulated by HIF-1α, HIF-2α, and PPARα via direct promoter binding, HILPDA localizes to the ER–lipid droplet interface through an N-terminal hydrophobic domain that also mediates ATGL interaction, and its lipid droplet targeting is partly ATGL-dependent (PMID:24876382, PMID:28760743, PMID:36420951). By controlling lipid droplet abundance and fatty acid partitioning, HILPDA modulates ferroptosis sensitivity through redistribution of polyunsaturated fatty acids into membrane phospholipids, promotes mitophagy via stabilization of CLS1 and cardiolipin accumulation, and buffers eicosanoid precursors for prostaglandin synthesis (PMID:37061197, PMID:37552373, PMID:28760743, PMID:41422655). In certain cancer contexts, secreted HILPDA binds FZD10 to activate Wnt/β-catenin signaling, functioning as an autocrine growth and immune-evasion factor (PMID:15930302, PMID:39633113).

1. 2005 Medium

   The first functional assignment for HILPDA established it as a secreted ligand for FZD10 that activates Wnt signaling and promotes renal cell carcinoma cell survival, answering how a hypoxia-inducible gene product could act as an autocrine growth factor.

   Evidence Co-expression in COS7, siRNA knockdown, and anti-HIG2 antibody-induced apoptosis in RCC cells

   PMID:15930302

   Open questions at the time

   • Direct biophysical demonstration of HILPDA–FZD10 binding not provided
   • Secretion mechanism uncharacterized
   • Wnt activation not confirmed in non-RCC contexts at that time
2. 2014 High

   Identification of HILPDA as a direct PPARα target gene with a functional PPRE, and demonstration that hepatic overexpression drives triglyceride accumulation without altering lipogenesis genes, established HILPDA as a lipid storage effector rather than a lipogenic transcription factor target.

   Evidence ChIP for PPARα binding, transactivation assay, AAV-mediated hepatic overexpression in mice with triglyceride secretion assays

   PMID:24876382

   Open questions at the time

   • Molecular target of HILPDA-mediated triglyceride retention unknown at this point
   • Mechanism of impaired TG secretion not delineated
3. 2017 High

   Myeloid-specific knockout revealed HILPDA as essential for hypoxia-driven lipid droplet accumulation in macrophages and demonstrated that these lipid droplets serve as reservoirs for prostaglandin-E2 synthesis, linking lipid storage to inflammatory eicosanoid production.

   Evidence Tie2-Cre conditional KO mice, subcellular fractionation showing ER–lipid droplet localization, PGE2 ELISA after LPS stimulation

   PMID:28760743

   Open questions at the time

   • Direct lipolytic target not yet identified in macrophages
   • Whether HILPDA loss affects other eicosanoid species unknown
4. 2019 High

   Genetic ablation and inhibitor rescue experiments identified ATGL as the direct functional target of HILPDA, establishing HILPDA as a bona fide ATGL inhibitor; domain mapping showed the N-terminal hydrophobic region suffices for lipid droplet targeting and triglyceride restoration.

   Evidence HILPDA KO MEFs and tumor cells, Atglistatin rescue of lipolysis, deletion-mutant mapping, lipidomics

   PMID:31308147

   Open questions at the time

   • Direct binding affinity and stoichiometry of HILPDA–ATGL not measured
   • Relationship to G0S2 inhibition not resolved
5. 2020 High

   Confirmation that HILPDA inhibits ATGL in macrophages in vivo, and the unexpected finding that macrophage lipid droplet loss does not ameliorate obesity-driven inflammation, separating lipid storage from inflammatory signaling in adipose tissue macrophages.

   Evidence Myeloid-specific HILPDA KO in diet-induced obesity mouse model with ATGL inhibitor rescue and metabolic profiling

   PMID:32049012

   Open questions at the time

   • Whether HILPDA affects macrophage ATGL post-translationally or via protein–protein occlusion not resolved
   • Impact on tissue-resident macrophage subtypes beyond adipose not tested
6. 2022 Medium

   Structural comparison with G0S2 showed HILPDA lacks a G0S2-like hairpin and that its lipid droplet localization depends on ATGL co-expression, defining HILPDA as an ATGL-dependent LD resident rather than an autonomous LD-targeting protein.

   Evidence Structure prediction, deletion-mutant localization studies, ATGL co-expression rescue in cells

   PMID:36420951

   Open questions at the time

   • No crystal or cryo-EM structure of HILPDA or HILPDA–ATGL complex
   • Whether ATGL-dependence applies in all cell types untested
7. 2023 High

   Three studies collectively expanded HILPDA's role beyond lipolysis: (1) in hepatocellular carcinoma, HILPDA deficiency redirects PUFAs to membrane phospholipids and saturated FAs to ceramides, exacerbating lipid peroxidation and apoptosis under hypoxia; (2) in adipocytes, HILPDA participates in a fatty acid–FFAR4–ER stress negative feedback loop that downregulates ATGL; (3) in nasopharyngeal carcinoma, HILPDA stabilizes CLS1 against PINK1-mediated ubiquitination, increasing cardiolipin and promoting mitophagy-dependent radioresistance.

   Evidence Hepatocyte-specific KO mice on Western diet + CCl4, shotgun lipidomics with ceramide inhibitor rescue (PMID:37061197); WT/KO/OE adipocytes with FFAR4 pharmacology and in vivo lipolysis (PMID:37422000); Co-IP of HILPDA–CLS1, ubiquitination assays, mitophagy inhibitor + irradiation in NPC cells (PMID:37552373)

   PMID:37061197 PMID:37422000 PMID:37552373

   Open questions at the time

   • Direct mechanism by which HILPDA inhibits PINK1-mediated CLS1 ubiquitination unclear
   • Whether ceramide-mediated apoptosis is a general feature of HILPDA loss beyond HCC not established
   • FFAR4-to-HILPDA transcriptional intermediaries not fully mapped
8. 2024 Medium

   Multiple 2024 studies extended HILPDA biology to renal fibrosis, ferroptosis suppression via lipid droplets, Wnt signaling in glioblastoma, and phagosome regulation: HILPDA-driven triglyceride overload causes mitochondrial dysfunction and G2/M arrest in tubular cells; lipid droplet formation downstream of HIF-1α–HILPDA suppresses ferroptosis in NPC; HILPDA–FZD10 activates Wnt/β-catenin in glioma stem cell microparticles; and HILPDA-dependent LD growth restricts RAC1-mediated phagosome formation in macrophages.

   Evidence HILPDA OE/KD in HK-2 cells and UUO/UIRI mouse models (PMID:36990128); AGT–HIF-1α Co-IP, luciferase, ferroptosis markers in NPC xenografts (PMID:39709027); HIF-1α ChIP, FZD10 binding, microparticle Wnt reporter in glioma (PMID:39633113); HILPDA-KO macrophages with RAC1 GTPase assay and in vivo Candida model (preprint: bio_10.1101_2024.06.11.598578)

   PMID:36990128 PMID:39633113 PMID:39709027

   Open questions at the time

   • FOXS1–HILPDA interaction (PMID:38780613) lacks reciprocal validation
   • Phagosome/RAC1 mechanism from preprint awaits peer review
   • Whether ferroptosis modulation by HILPDA is cell-type-general or context-specific is unresolved
9. 2025 Medium

   HIF-2α (distinct from HIF-1α) was identified as the transcriptional inducer of HILPDA in trophoblasts, where HILPDA sensitizes cells to ferroptosis rather than suppressing it, revealing cell-type-dependent directionality of HILPDA's effect on ferroptosis.

   Evidence siRNA and CRISPR KO of HILPDA in trophoblasts with HIF-2α agonist, C11-BODIPY lipid peroxidation, MDA/GSH quantification

   PMID:41422655

   Open questions at the time

   • Mechanism by which HILPDA promotes ferroptosis in trophoblasts while suppressing it in NPC cells is unexplained
   • Direct HIF-2α ChIP on HILPDA promoter in trophoblasts not shown

• Key unresolved questions include: the atomic-resolution structure of HILPDA and the HILPDA–ATGL complex; the basis for opposing ferroptosis outcomes across cell types; whether the secreted/Wnt-activating and intracellular lipolysis-inhibiting functions are mediated by distinct pools or post-translational isoforms; and in vivo validation of the RAC1/phagosome axis.
• No structural model of HILPDA or HILPDA–ATGL complex exists
• Dual ferroptosis phenotype (pro- vs. anti-) mechanistically unexplained
• Secretory pathway for HILPDA and FZD10 binding interface uncharacterized