Mitochondrial Hsp90s suppress calcium-mediated stress signals propagating from mitochondria to the ER in cancer cells

Background: Resistance to cell death in the presence of stressful stimuli is one of the hallmarks of cancer cells acquired during multistep tumorigenesis, and knowledge of the molecular mechanism of stress adaptation can be exploited to develop cancer-selective therapeutics. Mitochondria and the endoplasmic reticulum (ER) are physically interconnected organelles that can sense and exchange various stress signals. Although there have been many studies on stress propagation from the ER to mitochondria, reverse stress signals originating from mitochondria have not been well reported. Methods: After inactivation of the proteins by pharmacologic and genetic methods, the signal pathways were analyzed by fluorescence microscopy, flow cytometry, MTT assay, and western blotting. A mouse xenograft model was used to examine synergistic anticancer activity and the action mechanism of drugs in vivo. Results: We show in this study that mitochondrial heat shock protein 90 (Hsp90) suppresses mitochondria-initiated calcium-mediated stress signals propagating into the ER in cancer cells. Mitochondrial Hsp90 inhibition triggers the calcium signal by opening the mitochondrial permeability transition pore and, in turn, the ER ryanodine receptor, via calcium-induced calcium release. Subsequent depletion of ER calcium activates unfolded protein responses in the ER lumen, thereby increasing the expression of a pro-apoptotic transcription factor, CEBP homologous protein (CHOP). Combined treatment with the ER stressor thapsigargin and the mitochondrial Hsp90 inhibitor gamitrinib augmented interorganelle stress signaling by elevating CHOP expression, and showed synergistic cytotoxic activity exclusively in cancer cells in vitro and in vivo. Conclusions: Collectively, mitochondrial Hsp90s confer cell death resistance to cancer cells by suppressing the mitochondria-initiated calcium-mediated interorganelle stress response.