HILPDA Regulates Lipid Metabolism, Lipid Droplet Abundance, and Response to Microenvironmental Stress in Solid Tumors

Abstract

Accumulation of lipid droplets has been observed in an increasing range of tumors. However, the molecular determinants of this phenotype and the impact of the tumor microenvironment on lipid droplet dynamics are not well defined. The hypoxia-inducible and lipid droplet associated protein HILPDA is known to regulate lipid storage and physiologic responses to feeding conditions in mice, and was recently shown to promote hypoxic lipid droplet formation through inhibition of the rate-limiting lipase adipose triglyceride lipase (ATGL). Here, we identify fatty acid loading and nutrient deprivation-induced autophagy as stimuli of HILPDA-dependent lipid droplet growth. Using mouse embryonic fibroblasts and human tumor cells, we found that genetic ablation of HILPDA compromised hypoxia-fatty acid-and starvation-induced lipid droplet formation and triglyceride storage. Nutrient deprivation upregulated HILPDA protein posttranscriptionally by a mechanism requiring autophagic flux and lipid droplet turnover, independent of HIF1 transactivation. Mechanistically, loss of HILPDA led to elevated lipolysis, which could be corrected by inhibition of ATGL. Lipidomic analysis revealed not only quantitative but also qualitative differences in the glycerolipid and phospholipid profile of HILPDA wild-type and knockout cells, indicating additional HILPDA functions affecting lipid metabolism. Deletion studies of HILPDA mutants identified the N-terminal hydrophobic domain as sufficient for targeting to lipid droplets and restoration of triglyceride storage. In vivo, HILPDAablated cells showed decreased intratumoral triglyceride levels and impaired xenograft tumor growth associated with elevated levels of apoptosis.Implications: Tumor microenvironmental stresses induce changes in lipid droplet dynamics via HILPDA. Regulation of triglyceride hydrolysis is crucial for cell homeostasis and tumor growth.

The diversity and breadth of cancer cell fatty acid metabolism

“…Knockdown of HILPDA increased lipid droplet-derived fatty acid mobilization, due to less suppression of ATGL activity, resulting in increased fatty acid oxidation and ROS production, and impaired the growth of HCT116 colon carcinoma and HeLa tumor xenograft growth [ 130 ]. Similar observations were reported in HILPDA loss-of-function studies in HCT116 colon carcinoma models [ 132 ]. Overall, reduced ATGL activity, via knockdown of ATGL or ABHD5, has similar effects on cancer cell proliferation and viability as increased ATGL activity, via overexpression of ATGL or knockdown of HILPDA.…”

“…ATGL overexpression did suppress melanoma, lung, and liver cancer cell proliferation [ 131 ]. We and others have shown that ATGL protein levels, and TG levels, are increased in colon, breast, and prostate cancer cells in response to high levels of extracellular fatty acids [ 3 , 4 , 132 ] and co-culturing with adipocytes [ 133 ], which also increase cell proliferation. Importantly, we also showed that the increase in ATGL protein and intracellular TG levels increased the rate of mitochondrial oxidation of TG-derived fatty acids in breast and prostate cancer cells [ 3 , 4 ].…”

“…Several proteins act as co-suppressors of ATGL, including G0/G1 switch gene 2 (G0S2) [ 134 ], and hypoxia-inducible lipid droplet associated protein (HILPDA; also known as hypoxia-inducible gene 2, HIG2) [ 130 , 132 ]. Knockdown of HILPDA increased lipid droplet-derived fatty acid mobilization, due to less suppression of ATGL activity, resulting in increased fatty acid oxidation and ROS production, and impaired the growth of HCT116 colon carcinoma and HeLa tumor xenograft growth [ 130 ].…”

Altered lipid metabolism marks glioblastoma stem and non-stem cells in separate tumor niches

“…43 Additionally, studies show that HILPDA is involved in triglyceride fatty acid secretion 44 and regulates lipid metabolism and hypoxia-induced lipid droplet biogenesis. 45 When we investigated lipid droplet accumulation in our 3D GBM organoid model, we discovered that lipid droplets were exclusively enriched in the hypoxic core region of the 3D organoids. We further traced this finding back to patients, showing that all patients in our panel with clear pseudopalisading necrosis by pathology, and most patients overall, accumulate lipid droplets in the hypoxic regions of their tumors.…”

Metabolic Escape Routes of Cancer Stem Cells and Therapeutic Opportunities

“…This phenomenon results from FA oxidation process, activated by patatin like phospholipase domain containing 2 (PNPLA2) and mitochondrial electron leakage, which both lead to oxidative stress. To counteract this phenomenon, cancer cell downregulate PNPLA2 through the hypoxia-inducible protein 2, to promotes cancer cell survival [130,131]. The resistance to oxidative stress in hypoxic condition is also driven by a deregulated membrane lipid saturation, with increased saturated and monounsaturated FAs, which are less susceptible to peroxidation [132].…”