Heat Shock Protein 70 (Hsp70) Acts as a Guardian Against Multiple Cell Death Pathways.

Abstract

The proper balance between protein synthesis, folding, and turnover is termed protein homeostasis, or proteostasis. Cancer cells are especially reliant on the activities of molecular chaperones, such as heat shock proteins 70 (Hsp70) and 90 (Hsp90), which are critical to protein quality control. Hsp70 and Hsp90 collaborate to buffer cancer cells against unstable oncogenic mutations, and these chaperones are thought to prevent apoptosis during rapid metabolism and cell division. While Hsp90’s specific roles in cancer are relatively well described and inhibitors are being studied in multiple clinical trials, less is known about Hsp70. In this thesis, we aimed to characterize Hsp70’s roles in cancer signaling. Towards this goal, we developed the first selective and potent inhibitors of Hsp70, based on the rhodocyanine MKT-077. We found that these molecules, including JG-98 and JG-13, have potent cytotoxic activity in multiple cancer cell lines, with minimal effect on normal fibroblasts. Moreover, these compounds were highly synergistic with inhibitors of either Hsp90 or the proteasome. Using JG-98 as a chemical probe, we found that Hsp70 suppresses apoptotic cell death through a novel process dependent on RIP1 kinase. Further, under conditions in which apoptosis was inhibited, JG-98 triggered a strong necroptotic phenotype, suggesting that Hsp70 helps regulate a key cellular “decision” to proceed with either apoptosis or necrosis. Thus, Hsp70 shares some cellular roles in common with Hsp90, but also has important differences that might be exploited in the pursuit of anti-cancer agents with new mechanisms-of-action. Hsp70 is actually a family with thirteen members in humans, and it is still unclear if these isoforms play distinct or overlapping roles in cancer. To better understand this question, we have made significant efforts towards designing small molecules that target cytosolic and mitochondrial Hsp70s specifically. These probes have the potential to further transform or understanding of Hsp70 as a cancer chaperone. Taken together, these studies reveal that Hsp70 operates in a much more global cytoprotective fashion in cancer cells than previously suggested. This work also makes significant strides towards the development of improved chemotherapeutics targeting the protein quality control machinery.