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Background: Overcoming taxane resistance remains a major clinical challenge in metastatic castrate-resistant prostate cancer (mCRPC). Loss of DNA repair proteins is associated with resistance to anti-microtubule agents. We propose that alterations in DNA damage response (DDR) pathway contribute to taxane resistance, and identification of these alterations may provide a potential therapeutic target to resensitize docetaxel-refractory mCRPC to taxane-based therapy., Methods: Alterations in DDR gene expression in our prostate cancer cell line model of docetaxel-resistance (DU145-DxR) derived from DU-145 cells were determined by DDR pathway-specific polymerase chain reaction array and immunoblotting. The PRKDC gene encoding DNA-PKc (DNA-dependent protein kinase catalytic unit), was noted to be overexpressed and evaluated for its role in docetaxel resistance. Cell viability and clonogenic survival of docetaxel-treated DU145-DxR cells were assessed after pharmacologic inhibition of DNA-PKc with three different inhibitors-NU7441, LTURM34, and M3814. Response to second-line cytotoxic agents, cabazitaxel and etoposide upon DNA-PKc inhibition was also tested. The impact of DNA-PKc upregulation on DNA damage repair was evaluated by comet assay and analysis of double-strand breaks marker, γH2AX and Rad51. Lastly, DNA-PKc inhibitor's effect on MDR1 activity was assessed by rhodamine 123 efflux assay., Results: DDR pathway-specific gene profiling revealed significant upregulation of PRKDC and CDK7, and downregulation of MSH3 in DU145-DxR cells. Compared to parental DU145, DU145-DxR cells sustained significantly less DNA damage when exposed to etoposide and docetaxel. Pharmacologic inhibition of DNA-PKc, a component of NHEJ repair machinery, with all three inhibitors, significantly resensitized DU145-DxR cells to docetaxel. Furthermore, DNA-PKc inhibition also resensitized DU145-DxR to cabazitaxel and etoposide, which demonstrated cross-resistance. Inhibition of DNA-PKc led to increased DNA damage in etoposide- and docetaxel-treated DU145-DxR cells. Finally, DNA-PKc inhibition did not affect MDR1 activity, indicating that DNA-PKc inhibitors resensitized taxane-resistant cells via an MDR1-independent mechanism., Conclusion: This study supports a role of DDR genes, particularly, DNA-PKc in promoting resistance to taxanes in mCRPC. Targeting prostatic DNA-PKc may provide a novel strategy to restore taxane sensitivity in taxane-refractory mCRPC., (© 2021 Wiley Periodicals LLC.)

Background: The central role of androgen receptor (AR) signaling is established in prostate cancer growth and progression. We propose CYP3A5 is part of a feedback loop that modulates the sensitivity of AR to androgen exposure. The purpose of this study is to elucidate the mechanism of regulation of AR expression by CYP3A5., Methods: To identify the role of CYP3A5 in regulating AR signaling, CYP3A5 protein expression was inhibited using CYP3A5 siRNA and azamulin. Both cell fractionation and immunocytochemical approaches in combination with dihydrotestosterone (DHT) and R1881 treatment were used to evaluate changes in AR nuclear translocation., Results: CYP3A5 siRNA blocked growth of LNCaP and C4-2 cells by 30-60% (P ≤ 0.005). Azamulin, a CYP3A pharmacologic inhibitor, reduced the growth of LNCaP, C4-2 and 22RV1 lines by ∼ 40% (P ≤ 0.005). CYP3A5 siRNA inhibited growth in response to DHT and R1881 treatment in LNCaP and C4-2 by decreasing nuclear AR localization and resulting in diminished PSA and TMPRSS2 expression. Decreased AR nuclear localization resulting from CYP3A5 inhibition resulted in growth inhibition comparable to IC60 and IC40 of bicalutamide in LNCaP and C4-2 cell lines. Conversely, the CYP3A inducer rifampicin enhanced AR nuclear localization., Conclusion: As CYP3A5 regulates the nuclear translocation of AR; co-targeting CYP3A5 may provide a novel strategy for enhancing the efficacy of androgen deprivation therapy. Consequentially, these data suggest that concomitant medications may impact androgen deprivation therapy's efficacy., (© 2015 Wiley Periodicals, Inc.)

Background: Neutral endopeptidase (NEP) is a transmembrane cell surface peptidase present on prostatic epithelial cells that catalytically inactivates small peptide substrates. Neutral endopeptidase loss is associated with prostate cancer growth, progression, and increased angiogenesis. We examined whether NEP expression is regulated by hypoxia, frequently encountered in the tumor microenvironment., Methods: NEP expression was compared in prostate cancer cell lines cultured in normoxic and hypoxic conditions. The NEP activity, protein levels, and mRNA levels were determined using enzyme assay, Western blotting and q-PCR analysis, respectively. Electrophoretic mobility shift assay and chromatin immunoprecipitation assay (ChIP) was used to confirm the negative regulation of NEP at the transcriptional level by hypoxia responsive elements (HREs)., Results: The results indicate that NEP expression was inhibited under hypoxic conditions in the NEP positive LNCaP, C4-2, and 22RV1 cells and human umbilical vascular endothelial (HUVEC) cells. NEP regulation appeared to be predominantly at the transcriptional level as NEP mRNA expression significantly reduced with hypoxia, concordant with the kinetics of protein levels, and NEP enzyme activity. A search of the NEP gene sequence revealed three putative HREs upstream of the NEP promoter. Two of the HREs demonstrated a specific reduction of shift in the presence of cobalt chloride; specificity of the binding sites was confirmed by ChIP., Conclusions: Our data indicate a novel mechanism where hypoxia negatively regulates the tumor suppressor function of NEP in prostate cancer. The negative regulation of NEP is mediated by binding of the HIF1-α protein binding to the HREs present upstream of the NEP promoter., (Copyright © 2012 Wiley Periodicals, Inc.)