The novel mechanism by hich proteaome inhibitors induce apoptosis in triple-negative breast cancer

Abstract

Triple-negative breast cancer (TNBC) is a challenging malignancy due to a lack of targeted therapeutic options. Genomic studies have found that the proteasome may be a viable therapeutic target in TNBC, suggesting that proteasome inhibitors (PIs) could be a potential therapeutic option. In this dissertation, I wanted to find the mechanism by which proteasome inhibitors induce apoptosis in TNBC. Proteasome inhibitors are currently approved for the treatment of multiple myeloma (MM) and mantle cell lymphoma and have been shown to induce proteotoxic stress due to a buildup of misfolded proteins in MM cells. However, the exact mechanism by which proteasome inhibitors induce apoptosis is debated. In this dissertation, I demonstrate that, in response to PIs, TNBC cells have a comparable sensitivity profile to MM. Although the mechanism of action in TNBC is distinct from the proposed mechanisms in MM, there is an increase in proteotoxic stress observed in TNBC cells. Proteotoxic stress can be regulated by changes in protein synthesis and protein breakdown. Given that protein breakdown is impaired due to proteasome inhibition, we investigated the role protein synthesis plays in mediating the effects of PIs. eIF2α (eukaryotic initiation factor 2α) promotes global protein synthesis, and its phosphorylation inhibits it. I show that eIF2α does not get phosphorylated in response to PIs and thereby does not decrease protein synthesis rates. In some cases, we observed a decrease in eIF2α phosphorylation. We investigated the role eIF2α phosphatases play in the response of TNBC cells to PIs and found that they contribute to apoptosis in TNBC cells in an eIF2α-independent manner. This suggests that the eIF2α phosphatases regulate apoptosis through a novel mechanism. The effector of PI-induced apoptosis in MM cells is NOXA. We found the same in TNBC cells. In MM, upstream regulators of NOXA in response to PIs are debated. In this study, we found that c-myc upregulates NOXA expression, regulating apoptosis in response to PIs in TNBC cells. This has not been previously reported in response to PIs in TNBC cells. The findings in this dissertation establish key mediators involved in PI-induced apoptosis in TNBC cells and lay a foundation for identifying combination therapeutic strategies to translate PIs to TNBC treatment.