Abstract PR04: HIF-2α dependent lipid storage promotes endoplasmic reticulum homeostasis in clear cell renal cell carcinoma
Authors: Qiu Bo , Ackerman Daniel , Sanchez Danielle J. , Li Bo , Ochocki Joshua D. , Grazioli Alison , Bobrovnikova-Marjon Ekaterina , Diehl J. Alan , Keith Brian , Simon M. Celeste 
Published: May 2016

Abstract
Clear cell renal cell carcinoma (ccRCC) is the most common form of renal cancer, defined pathologically by abundant intracellular lipid droplets (LDs) that impart the clear cell phenotype, and molecularly by constitutive activation of the hypoxia inducible factors (HIFs). The primary aim of this study was to identify the mechanisms driving enhanced neutral lipid storage and the function of this phenotype in ccRCC. Our work demonstrates that HIF-2α promotes neutral lipid storage in ccRCC through up-regulation of the LD coat protein PLIN2. Expression profiling in multiple cohorts of primary ccRCC and normal kidney samples revealed that PLIN2 was overexpressed in all stages of disease and correlated with constitutive activation of HIF-2α, but not HIF-1α. Using ccRCC cell lines, 3D tumor spheroids, and tumor xenografts, we showed that HIF-2α dependent PLIN2 expression was required for neutral lipid storage and tumor cell viability. We demonstrated, for the first time, that PLIN2 dependent lipid storage promoted ccRCC tumor growth by maintaining integrity of the endoplasmic reticulum (ER), an organelle that is functionally and physically associated with LDs. Recent work indicates that cellular transformation commits tumors to growth programs that strain ER homeostasis, including elevated protein and lipid synthesis. Such ER stress is exacerbated by conditions of oxygen and nutrient deprivation characteristic of solid tumor microenvironments, which further disrupt cellular protein and lipid homeostasis. Consistent with this theme, we found that the enhanced requirement for PLIN2 in ccRCC was due to heightened ER stress downstream of cellular transformation, including elevated protein synthesis and growth within nutrient and oxygen limited tumor microenvironments. In these settings, PLIN2 depletion elicited irremediable ER stress and triggered a cytotoxic unfolded protein response. Remarkably, suppression of protein synthesis via cycloheximide or pharmacologic inhibitors of mTORC1, which is activated in &gt;80% of ccRCC, prevented ER stress and cell death in PLIN2 depleted cells. On the other hand, inhibition of lipid synthesis was selectively toxic to PLIN2 depleted cells, consistent with an adaptive function of lipid synthesis downstream of ER stress. Our results suggest that PLIN2 is not merely a marker of lipid accumulation in ccRCC. Instead, it promotes lipid storage and maintains integrity of the ER, a vital hub for tumor cell protein and lipid metabolism. Consistent with this model, HIF-2α/PLIN2 dependent lipid storage also protected ccRCC cells against pharmacological ER stress inducing agents, including the proteasome inhibitor Bortezomib (Velcade). These findings reveal a novel function of the well documented clear cell phenotype in ccRCC and identify ER stress as a targetable vulnerability created by HIF-2α/PLIN2 suppression in this common renal malignancy.
Citation Format: Bo Qiu, Daniel Ackerman, Danielle J. Sanchez, Bo Li, Joshua D. Ochocki, Alison Grazioli, Ekaterina Bobrovnikova-Marjon, J. Alan Diehl, Brian Keith, M. Celeste Simon. HIF-2α dependent lipid storage promotes endoplasmic reticulum homeostasis in clear cell renal cell carcinoma. [abstract]. In: Proceedings of the AACR Special Conference: Metabolism and Cancer; Jun 7-10, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(1_Suppl):Abstract nr PR04.