Qiu, Bo
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Publication Tumor Promoting Functions of Lipid Storage in Clear Cell Renal Cell Carcinoma(2015-01-01) Qiu, BoTwo hallmarks of clear cell renal cell carcinoma (ccRCC) are constitutive hypoxia inducible factor (HIF) signaling and abundant intracellular lipid droplets (LDs). However, regulation of lipid storage and its role in ccRCC are incompletely understood. In this study, we explored the function of the LD coat protein perilipin 2 (PLIN2) within the context of ccRCC. Transcriptional profiling of primary ccRCC samples revealed that expression of PLIN2 was elevated in tumors and correlated with HIF-2α, but not HIF-1α, activation. HIF-2α dependent PLIN2 expression promoted lipid storage, proliferation, and viability in xenograft tumors. Mechanistically, lipid storage maintained integrity of the endoplasmic reticulum (ER), which is functionally and physically associated with LDs. Specifically, PLIN2 dependent lipid storage suppressed cytotoxic ER stress responses that otherwise result from elevated protein synthetic activity characteristic of ccRCC cells. Thus, in addition to promoting ccRCC proliferation and anabolic metabolism, HIF-2α modulates lipid storage to sustain ER homeostasis, particularly under conditions of nutrient and oxygen limitation, thereby promoting tumor cell survival. We also examined the mechanisms underlying the tumor promoting functions of PLIN2-dependent lipid storage in ccRCC. Most phospholipid and triacylglycerol (TAG) biosynthetic enzymes reside in the ER, and lipids are exchanged between the ER and LD. The phospholipid composition of the ER membrane is tightly regulated to support biosynthetic functions and protein homeostasis within the ER lumen. In addition, perturbation of the phospholipid membrane is sufficient to induce ER stress. We tested the hypothesis that (TAG) synthesis and storage prevents ER stress by guarding against perturbations in ER phospholipid composition. Phospholipid and TAG biosynthesis share a common pathway involving the synthesis of diacylglyerol. We found that ablation of the acyl-CoA:diacylglycerol (DGAT) enzymes necessary for TAG biosynthesis enhances sensitivity of ccRCC cells to conditions that perturb phospholipid homeostasis and trigger ER stress. These include exposure to the saturated fatty acid palmitate, hypoxia, and importantly, growth as a sub-cutaneous xenograft tumor. Collectively, our results reveal a novel function for the well-documented “clear cell” phenotype and identifying ER stress as a targetable vulnerability created by HIF-2α/PLIN2 suppression in this common renal malignancy.