The inhibition of endoplasmic reticulum chaperone protein GRP78 enhances the anti-tumor effect of sunitinib in renal cell carcinoma model

Abstract

Tumor microenvironments are characterized by less oxygen and nutrition than normal tissues due to the rapid and progressive nature of the tumors. These intense cell stressors trigger a protective cell survival mechanism heralded by the unfolded protein response (UPR). The UPR is induced by an accumulation of unfolded proteins in the endoplasmic reticulum (ER) after cell starvation and leads to the global suppression of mRNA translation to protect cells from excessive amounts of unfolded proteins. Although the ER stress response is implicated in cytoprotection, its precise role remains unclear in renal cell carcinoma. One of the major proteins involved in ER stress is glucose-regulated protein 78 (GRP78), which binds to unfolded proteins and dissociates from membrane-bound ER stress sensors. To determine the role of ER stress responses during anti-angiogenic therapy and the potential role of GRP78 for use in single or combined therapy in renal cell carcinoma (RCC), we used transient and stable GRP78-knockdown RCC cells under hypoxic or hypoglycemic stress conditions in vitro and in animal models treated with sunitinib. Here, we report that ER protein GRP78 plays a crucial role in protecting RCC cells from hypoxic and hypoglycemic stress in tumor microenvironments. GRP78 overexpression led to increased tumor proliferation following hypoxic and/or hypoglycemic stress by activating the double-stranded RNA-activated protein kinase-like ER kinase/eukaryotic initiation factor-2α pathway and protected tumor cells from stress-induced apoptosis. Knockdown of GRP78 using small interfering RNA (siRNA) inhibited cancer cell survival and induced apoptosis in RCC cells in vitro. GRP78 knockdown showed more potent suppression of tumor progression and enhanced the anti-tumor effect on sunitinib in vitro and in vivo. Our findings suggest that knockdown of GRP78 enhances apoptotic cell death from anti-angiogenic stress and that GRP78 may be a novel therapeutic target in RCC management.