Enzalutamide is a second-generation nonsteroidal antiandrogen clinically approved for the treatment of castration-resistant prostate cancer (CRPC), yet resistance to endocrine therapy has limited its success in this setting. Although the androgen receptor (AR) has been associated with therapy failure, the mechanisms underlying this failure have not been elucidated. Bioinformatics analysis predicted that activation of the Wnt/β-catenin pathway and its interaction with AR play a major role in acquisition of enzalutamide resistance. To validate the finding, we show upregulation of β-catenin and AR in enzalutamide-resistant cells, partially due to reduction of β-TrCP-mediated ubiquitination. Although activation of the Wnt/β-catenin pathway in enzalutamide-sensitive cells led to drug resistance, combination of β-catenin inhibitor ICG001 with enzalutamide inhibited expression of stem-like markers, cell proliferation, and tumor growth synergistically in various models. Analysis of clinical datasets revealed a molecule pattern shift in different stages of prostate cancer, where we detected a significant correlation between AR and β-catenin expression. These data identify activation of the Wnt/β-catenin pathway as a major mechanism contributing to enzalutamide resistance and demonstrate the potential to stratify patients with high risk of said resistance. Wnt/β-catenin inhibition resensitizes prostate cancer cells to enzalutamide. .

Enzalutamide, a nonsteroidal second-generation antiandrogen, has been recently approved for the management of castration-resistant prostate cancer (CRPC). Although patients can benefit from enzalutamide at the beginning of this therapy, acquired enzalutamide resistance usually occurs within a short period. This motivated us to investigate the mechanism involved and possible approaches for overcoming enzalutamide resistance in CRPC. In the present study, we found that 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGCR), a crucial enzyme in the mevalonate pathway for sterol biosynthesis, is elevated in enzalutamide-resistant prostate cancer cell lines. HMGCR knockdown could resensitize these cells to the drug, and HMGCR overexpression conferred resistance to it, suggesting that aberrant HMGCR expression is an important enzalutamide-resistance mechanism in prostate cancer cells. Furthermore, enzalutamide-resistant prostate cancer cells were more sensitive to statins, which are HMGCR inhibitors. Of note, a combination of simvastatin and enzalutamide significantly inhibited the growth of enzalutamide-resistant prostate cancer cells and tumors Mechanistically, simvastatin decreased protein levels of the androgen receptor (AR), which was further reduced in combination with enzalutamide. We observed that the decrease in AR may occur through simvastatin-mediated inhibition of the mTOR pathway, whose activation was associated with increased HMGCR and AR expression. These results indicate that simvastatin enhances the efficacy of enzalutamide-based therapy, highlighting the therapeutic potential of statins to overcome enzalutamide resistance in CRPC.

Edited by Xiao-Fan Wang Prostate cancer is the second leading cause of cancer death among men in the United States. The androgen receptor (AR) antagonist enzalutamide is a Food and Drug Administrationapproved drug for treatment of patients with late-stage prostate cancer and is currently under clinical study for early-stage prostate cancer treatment. After a short positive response period, tumors will develop drug resistance. In this study using RNA-Seq and bioinformatics analyses, we observed that NOTCH signaling is a deregulated pathway in enzalutamide-resistant cells. NOTCH2 and c-MYC gene expression positively correlated with AR expression in samples from patient with hormone refractory disease in which AR expression levels correspond to those typically observed in enzalutamide resistance. Cleaved NOTCH1, HES1 (Hes family BHLH transcription factor 1), and c-MYC protein expression levels are elevated in two enzalutamide-resistant cell lines, MR49F and C4-2R, indicating NOTCH signaling activation. Moreover, inhibition of the overexpressed ADAM metallopeptidase domain 10 (ADAM10) in the resistant cells induces an exclusive reduction in cleaved NOTCH1 expression. Furthermore, exposure of enzalutamide-resistant cells to both PF-03084014 and enzalutamide increased cell death, decreased colony formation ability, and resensitized cells to enzalutamide. Knockdown of NOTCH1 in C4-2R increased enzalutamide sensitivity by decreasing cell proliferation and increasing cleaved PARP expression. In a 22RV1 xenograft model, PF-03084014 and enzalutamide decreased tumor growth through reducing cell proliferation and increasing apoptosis. These results indicate that NOTCH1 signaling may contribute to enzalutamide resistance in prostate cancer, and inhibition of NOTCH signaling can resensitize resistant cells to enzalutamide.

Olaparib is a FDA-approved PARP inhibitor (PARPi) that has shown promise as a synthetic lethal treatment approach for BRCA-mutant castration-resistant prostate cancer (CRPC) in clinical use. However, emerging data has also shown that even BRCA-mutant cells may be resistant to PARPi. The mechanistic basis for these drug resistances is poorly understood. Polo-like kinase 1 (Plk1), a critical regulator of many cell cycle events, is significantly elevated upon castration of mice carrying xenograft prostate tumors. Herein, by combination with Plk1 inhibitor BI2536, we show a robust sensitization of Olaparib in 22RV1, a BRCA1 deficient CRPC cell line, as well as in CRPC xenograft tumors. Mechanistically, monotherapy with Olaparib results in an override of the G1/S checkpoint, leading to high expression of Plk1, which attenuates Olaparib’s overall efficacy. In BRCA1 wild type C4-2 cells, Plk1 inhibition also significantly increases the efficacy of Olaparib in the presence of p53 inhibitor. Collectively, our findings not only implicate the critical role of Plk1 in PARPi resistance in BRCA-mutant CRPC cells, but also shed new light on the treatment of non-BRCA-mutant patient subgroups who might also respond favorably to PARPi.

Castration-resistant prostate cancer (CRPC) is the later stage of prostate cancer (PCa) when the disease has stopped responding to androgen deprivation therapy (ADT). It has been established that androgen receptor (AR) re-activation is responsible for the recurrence of PCa after ADT. Thus targeting different pathways that regulate AR stability and activity should be a promising strategy for treatment of CRPC. Heat shock proteins (HSPs) are chaperones that modify stability and activity of their client proteins. HSP90, a major player of the HSP family, regulates stabilities of many proteins, including AR and Polo-like kinase 1 (Plk1), a critical regulator of many cell cycle events. Further, HSP90 is overexpressed in different cancers, including PCa. Herein, we show that co-treatment of PCa with AR antagonist enzalutamide and HSP90 inhibitor leads to more severe cell death due to a synergistic reduction of AR protein. Interestingly, we show that overexpression of Plk1 rescued the synergistic effect and that co-targeting HSP90 and Plk1 also leads to more severe cell death. Mechanistically, we show that E3 ligase CHIP, in addition to targeting AR, is responsible for the degradation of Plk1 as well. These findings suggest that co-targeting HSP90 and some of its client proteins may be a useful strategy in treatment of CRPC.