Autophagy and oxidative stress modulation mediate Bortezomib resistance in prostate cancer

Abstract

Proteasome inhibitors such as Bortezomib represent an established type of targeted treatment for several types of hematological malignancies, including multiple myeloma, Waldenstrom’s macroglobulinemia, and mantle cell lymphoma, based on the cancer cell’s susceptibility to impairment of the proteasome-ubiquitin system. However, a major problem limiting their efficacy is the emergence of resistance. Their application to solid tumors is currently being studied, while simultaneously, a wide spectrum of hematological cancers, such as Myelodysplastic Syndromes show minimal or no response to Bortezomib treatment. In this study, we utilize the prostate cancer cell line DU-145 to establish a model of Bortezomib resistance, studying the underlying mechanisms. Evaluating the resulting resistant cell line, we observed restoration of proteasome chymotrypsin-like activity, regardless of drug presence, an induction of pro-survival pathways, and the substitution of the Ubiquitin-Proteasome System role in proteostasis by induction of autophagy. Finally, an estimation of the oxidative condition of the cells indicated that the resistant clones reduce the generation of reactive oxygen species induced by Bortezomib to levels even lower than those induced in non-resistant cells. Our findings highlight the role of autophagy and oxidative stress regulation in Bortezomib resistance and elucidate key proteins of signaling pathways as potential pharmaceutical targets, which could increase the efficiency of proteasome-targeting therapies, thus expanding the group of molecular targets for neoplastic disorders.

The ERK1/2-Elk1, JNK-cJun, and JAK-STAT Transcriptional Axes as Potential Bortezomib Resistance Mediators in Prostate Cancer

“…The RB40 clone also exhibited cross-resistance to Carfilzomib; however, the IC 50 value was significantly lower. These results were confirmatory of a previous study conducted in our laboratory, where DU-145 cells were used as a model for Bortezomib resistance, and the resulting phenotype was a cell line resistant only against proteasome inhibitors (Bortezomib and Carfilzomib) and not against anthracyclines (27).…”

“…The molecular chaperon Hsp70 is a protein that assists in UPS-mediated degradation and its expression in stress conditions. Bortezomib is a known Hsp family inducer, and this was documented in the naïve clone (27,69). Incubation with Bortezomib increased Hsp70 accumulation in a dose-dependent way (Figure 7a, 7b).…”

“…The idea that autophagy can substitute for dysregulated UPS has been documented in several studies of different cancer types, including prostate cancer (27,35,56,57). To monitor the autophagic activity, the lysosomal marker Lysotracker RED was used.…”

“…However, those mutations are not the sole ways for resistance to emerge, since most of the relapsed patients did not have alterations regarding β5 structure or expression. The current hypothesis is that, besides genetic mutations that may alter the proteasome subunits’ abundance and structure, thus rendering Bortezomib insufficient, a great role in the resistant phenotype is also played by changes in the signaling cascades that regulate apoptosis, autophagy, and oxidative stress (22–27).…”

“…The STAT family is another class of transcription factors that have been found active in resistant clones, as well as the ERK1/2 signaling pathway that mostly regulates cell survival and proliferation (29–34). Recent advances in Bortezomib resistance, focused on prostate cancer, report the activation of autophagy in the resistant cells and describe a regulation mechanism that substitutes the impaired UPS and abolishes the cells’ dependence on it to maintain their proteostasis (27,35–38). Though Beclin-1 regulates autophagic flux to successfully downregulate pro-apoptotic pathways and diminish oxidative stress levels, given the complexity of resistance emergence and the multiple layers that a resistant phenotype can exhibit, the need for a new approach is of paramount importance.…”