Targeting the ubiquitin-proteasome pathway to overcome anti-cancer drug resistance.

Drug resistance is a major obstacle in the field of pre-clinical and clinical therapeutics. The development of novel technologies and targeted therapies have yielded new modalities to overcome drug resistance, but multidrug resistance (MDR) remains one of the major challenges in the treatment of cancer. The ubiquitin-proteasome system (UPS) has a central role in regulating the levels and activities of a multitude of proteins as well as regulation of cell cycle, gene expression, response to oxidative stress, cell survival, cell proliferation and apoptosis. Therefore, inhibition of the UPS could represent a novel strategy for the treatment and overcoming of drug resistance in chemoresistant malignancies. In 2003, bortezomib was approved by the FDA for the treatment of multiple myeloma (MM). However, due to its limitations, second generation proteasome inhibitors (PIs) like carfilzomib, ixazomib, oprozomib, delanzomib and marizomib were introduced which displayed clinical activity in bortezomib-resistant tumors. Past studies have demonstrated that proteasome inhibition potentiates the anti-cancer efficacy of other chemotherapeutic drugs by: i) decreasing the expression of anti-apoptotic proteins such as TNF-α and NF-kB, ii) increasing the levels of Noxa, a pro-apoptotic protein, iii) activating caspases and inducing apoptosis, iv) degrading the pro-survival protein, induced myeloid leukemia cell differentiation protein (MCL1), and v) inhibiting drug efflux transporters. In addition, the mechanism of action of the immunoproteasome inhibitors, ONX-0914 and LU-102, suggested their therapeutic role in the combination treatment with PIs. In the current review, we discuss various PIs and their underlying mechanisms in surmounting anti-tumor drug resistance when used in combination with conventional chemotherapeutic agents.