A Novel Chemotherapy Combination to Enhance Proteotoxic Cell Death in Hepatocellular Carcinoma Experimental Models Without Killing Non-Cancer Cells

Inhibitors of the ubiquitin-proteasome system increase proteotoxic stress and have achieved clinical success for multiple myeloma but not for solid cancers such as hepatocellular carcinoma. Our objective is to identify a combination with proteasome inhibitors that enhances proteotoxic stress and apoptotic cell death in hepatocellular carcinoma but with less toxicity to non-cancer cells. We found that rencofilstat, a pan-cyclophilin inhibitor, combined with ixazomib, a proteasome inhibitor, increased apoptotic cell death in hepatocellular carcinoma but not in umbilical vein or dermal fibroblast non-cancer cells. We then analyzed the effects of rencofilstat + ixazomib on XBP1s and PERK, critical factors in the unfolded protein response used by cells to survive proteotoxic stress. Rencofilstat + ixazomib maintained higher expression of XBP1s and genetic models suggested that XBP1s was a pro-survival protein early and pro-death protein at later times. Simultaneously, decreased PERK expression prevented the block in protein synthesis via phospho-eIF2α and likely further amplified proteotoxic stress. Rencofilstat + ixazomib did not have effects on XBP1s or PERK in non-cancer cells. Further genetic experiments revealed the pro-survival roles for cyclophilin A and B in mediating rencofilstat + ixazomib-induced cell death. In the Hep3B xenograft model, rencofilstat + ixazomib significantly inhibited tumor volumes/weights without general toxicity. We conclude that rencofilstat + ixazomib amplified proteotoxic stress in hepatocellular carcinoma past a threshold pro-survival pathways could not tolerate, whereas non-cancer cells were less affected.