No abstract

Persistent androgen receptor (AR) signaling is the key driving force behind progression and development of castration-resistant prostate cancer (CRPC). In many patients, AR COOH-terminal truncated splice variants (ARvs) play a critical role in contributing to the resistance against androgen depletion therapy. Unfortunately, clinically used antiandrogens like bicalutamide (BIC) and enzalutamide (MDV), which target the ligand binding domain, have failed to suppress these AR variants. Here, we report for the first time that a natural prenylflavonoid, icaritin (ICT), can co-target both persistent AR and ARvs. ICT was found to inhibit transcription of key AR-regulated genes, such as KLK3 [prostate-specific antigen (PSA)] and ARvs-regulated genes, such as UBE2C and induce apoptosis in AR-positive prostate cancer (PC) cells. Mechanistically, ICT promoted the degradation of both AR and ARvs by binding to arylhydrocarbon-receptor (AhR) to mediate ubiquitin-proteasomal degradation. Therefore, ICT impaired AR transactivation in PC cells. Knockdown of AhR gene restored AR stability and partially prevented ICT-induced growth suppression. In clinically relevant murine models orthotopically implanted with androgen-sensitive and CRPC cells, ICT was able to target AR and ARvs, to inhibit AR signaling and tumor growth with no apparent toxicity. Our results provide a mechanistic framework for the development of ICT, as a novel lead compound for AR-positive PC therapeutics, especially for those bearing AR splice variants.

Neuroendocrine prostate cancer (NEPC) has a poor prognosis, with a median survival of less than 1 year after diagnosis. Following androgen deprivation therapy, prostate adenocarcinoma cells have been observed to develop an androgen receptor-negative, terminally differentiated and indolent neuroendocrine-like phenotype. However, several molecular events, including interleukin 6 (IL-6) stimulation, in the prostate microenvironment result in the appearance of aggressive, highly proliferative castrate-resistant NEPC. In this study, we examined the mechanistic effects of a natural prenylflavonoid, icaritin (ICT), on neuroendocrine differentiation in IL-6-induced LNCaP cells and NEPC development in the male transgenic adenocarcinoma of the mouse prostate (TRAMP) model. TRAMP mice received daily intraperitoneal injection of ICT or vehicle. ICT induced apoptosis in prostate tumor, suppressed NEPC development and, accordingly, improved overall survival in TRAMP mice. Expression of neuroendocrine markers (synaptophysin) and androgen receptor in TRAMP mice and neuroendocrine-like LNCaP cells were inhibited by ICT. Suppression of neuroendocrine and NEPC development by ICT was associated with dose-dependent inhibitory effects on abnormally elevated IL-6/STAT3 and Aurora kinase A in vitro and in vivo Since ICT demonstrated favorable pharmacokinetic and safety profiles with marked enrichment in prostate tissues, our study provides evidence for the development of prenylflavonoid as a multimodal therapeutic agent against NEPC.