Continued androgen receptor (AR) signaling is an established mechanism underlying castration-resistant prostate cancer (CRPC), and suppression of androgen receptor signaling remains a therapeutic goal of CRPC therapy. Constitutively active androgen receptor splice variants (AR-Vs) lack the androgen receptor ligand-binding domain (AR-LBD), the intended target of androgen deprivation therapies including CRPC therapies such as abiraterone and MDV3100. While the canonical full-length androgen receptor (AR-FL) and AR-Vs are both increased in CRPCs, their expression regulation, associated transcriptional programs, and functional relationships have not been dissected. In this study, we show that suppression of ligand-mediated AR-FL signaling by targeting AR-LBD leads to increased AR-V expression in two cell line models of CRPCs. Importantly, treatment-induced AR-Vs activated a distinct expression signature enriched for cell-cycle genes without requiring the presence of AR-FL. Conversely, activation of AR-FL signaling suppressed the AR-Vs signature and activated expression programs mainly associated with macromolecular synthesis, metabolism, and differentiation. In prostate cancer cells and CRPC xenografts treated with MDV3100 or abiraterone, increased expression of two constitutively active AR-Vs, AR-V7 and ARV567ES, but not AR-FL, paralleled increased expression of the androgen receptor-driven cell-cycle gene UBE2C. Expression of AR-V7, but not AR-FL, was positively correlated with UBE2C in clinical CRPC specimens. Together, our findings support an adaptive shift toward AR-V-mediated signaling in a subset of CRPC tumors as the AR-LBD is rendered inactive, suggesting an important mechanism contributing to drug resistance to CRPC therapy.
"For example, the study of Wang et al. 23 identified UBE2C, an AR-driven cell cycle regulated gene that overrides an M-phase checkpoint, as a marker of androgen-independent growth. Interestingly, by comparing the AR cistrome in CaP cells expressing full-length (FL) AR vs. a splice variant (V) of AR typically upregulated in abiraterone and enzalutamide-resistant CR-CaP cells and tumors, Hu et al. 24 also identified UBE2C expression as a marker of specific for CR-CaP. A more recent study by Sharma et al. 25 analyzed the AR cistrome in human clinical CR-CaP samples, identifying a 16-gene signature with stronger correlation to CR-CaP than the aforementioned Chinnaiyan and Brown lab studies. "
[Show abstract][Hide abstract] ABSTRACT: There is growing appreciation that castration-recurrent prostate cancer (CR-CaP) is driven by the continued expression of androgen receptor (AR). AR activation in CR-CaP through various mechanisms, including AR overexpression, expression of AR splice variants or mutants, increased expression of co-regulator proteins, and by post-translational modification, allows for the induction of AR-regulated genes in response to very low levels of tissue-expressed, so-called intracrine androgens, resulting in pathways that mediate CaP proliferation, anti-apoptosis and oncogenic aggressiveness. The current review focuses on the role played by Src-family (SFK) and Ack1 non-receptor tyrosine kinases in activating AR through direct phosphorylation, respectively, on tyrosines 534 or 267, and how these modifications facilitate progression to CR-CaP. The fact that SFK and Ack1 are central mediators for multiple growth factor receptor signaling pathways that become activated in CR-CaP, especially in the context of metastatic growth in the bone, has contributed to recent therapeutic trials using SFK/Ack1 inhibitors in monotherapy or in combination with antagonists of the AR activation axis.
International journal of biological sciences 06/2014; 10(6):620-626. DOI:10.7150/ijbs.8264 · 4.51 Impact Factor
"Most ARΔLBDs are products of alternative splicing (AR-V) , , albeit other mechanisms like nonsense-mutations leading to premature chain termination and enzymatic cleavage were also shown to give rise to ARΔLBDs , , . Although many ARΔLBDs were shown to activate AR-target genes under androgen deprived conditions in vitro, they are unable to activate the full panel of AR-dependent genes , , , , , . While in vivo both, the AR and ARΔLBD are expressed in CRPC cells, it was suggested that ARΔLBD- receptors must act in concert with full length AR to activate AR-dependent genes in CRPC . "
[Show abstract][Hide abstract] ABSTRACT: Background
Advanced castration resistant prostate cancer (CRPC) is often characterized by an increase of C-terminally truncated, constitutively active androgen receptor (AR) variants. Due to the absence of a ligand binding domain located in the AR-C-terminus, these receptor variants (also termed ARΔLBD) are unable to respond to all classical forms of endocrine treatments like surgical/chemical castration and/or application of anti-androgens.
In this study we tested the effects of the naturally occurring stilbene resveratrol (RSV) and (E)-4-(2, 6-Difluorostyryl)-N, N-dimethylaniline, a fluorinated dialkylaminostilbene (FIDAS) on AR- and ARΔLBD in prostate cancer cells. The ability of the compounds to modulate transcriptional activity of AR and the ARΔLBD-variant Q640X was shown by reporter gene assays. Expression of endogenous AR and ARΔLBD mRNA and protein levels were determined by qRT-PCR and Western Blot. Nuclear translocation of AR-molecules was analyzed by fluorescence microscopy. AR and ARΔLBD/Q640X homo-/heterodimer formation was assessed by mammalian two hybrid assays. Biological activity of both compounds in vivo was demonstrated using a chick chorioallantoic membrane xenograft assay.
The stilbenes RSV and FIDAS were able to significantly diminish AR and Q640X-signalling. Successful inhibition of the Q640X suggests that RSV and FIDAS are not interfering with the AR-ligand binding domain like all currently available anti-hormonal drugs. Repression of AR and Q640X-signalling by RSV and FIDAS in prostate cancer cells was caused by an inhibition of the AR and/or Q640X-dimerization. Although systemic bioavailability of both stilbenes is very low, both compounds were also able to downregulate tumor growth and AR-signalling in vivo.
RSV and FIDAS are able to inhibit the dimerization of AR and ARΔLBD molecules suggesting that stilbenes might serve as lead compounds for a novel generation of AR-inhibitors.
PLoS ONE 06/2014; 9(6):e98566. DOI:10.1371/journal.pone.0098566 · 3.23 Impact Factor
"Although ARVs function as transcription factors independent of androgens, there is conflicting evidence with regard to their transcriptional output. Some reports suggest that ARVs mediate transcriptional programs distinct from the full length (FL) AR (AR-FL),5155 whereas others claim that ARVs re-activate the transcriptional program of the AR-FL.56 Importantly, the expression of ARVs is increased upon androgen withdrawal and treatment with new generation antiandrogen therapies555758 and can mediate resistance to enzalutamide in cell culture.56 "
[Show abstract][Hide abstract] ABSTRACT: Alternative splicing (AS) is a crucial step in gene expression. It is subject to intricate regulation, and its deregulation in cancer can lead to a wide array of neoplastic phenotypes. A large body of evidence implicates splice isoforms in most if not all hallmarks of cancer, including growth, apoptosis, invasion and metastasis, angiogenesis, and metabolism. AS has important clinical implications since it can be manipulated therapeutically to treat cancer and represents a mechanism of resistance to therapy. In prostate cancer (PCa) AS also plays a prominent role and this review will summarize the current knowledge of alternatively spliced genes with important functional consequences. We will highlight accumulating evidence on AS of the components of the two critical pathways in PCa: androgen receptor (AR) and phosphoinositide 3-kinase (PI3K). These observations together with data on dysregulation of splice factors in PCa suggest that AR and PI3K pathways may be interconnected with previously unappreciated splicing regulatory networks. In addition, we will discuss several lines of evidence implicating splicing regulation in the development of the castration resistance.
Asian Journal of Andrology 05/2014; 16(4). DOI:10.4103/1008-682X.127825 · 2.60 Impact Factor
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