Constitutively active androgen receptor splice variants expressed in castration-resistant prostate cancer require full-length androgen receptor

Abstract

Androgen receptor (AR) splice variants lacking the ligand binding domain (ARVs), originally isolated from prostate cancer cell lines derived from a single patient, are detected in normal and malignant human prostate tissue, with the highest levels observed in late stage, castration-resistant prostate cancer. The most studied variant (called AR-V7 or AR3) activates AR reporter genes in the absence of ligand and therefore, could play a role in castration resistance. To explore the range of potential ARVs, we screened additional human and murine prostate cancer models using conventional and next generation sequencing technologies and detected several structurally diverse AR isoforms. Some, like AR-V7/AR3, display gain of function, whereas others have dominant interfering activity. We also find that ARV expression increases acutely in response to androgen withdrawal, is suppressed by testosterone, and in some models, is coupled to full-length AR (AR-FL) mRNA production. As expected, constitutively active, ligand-independent ARVs such as AR-V7/AR3 are sufficient to confer anchorage-independent (in vitro) and castrationresistant (in vivo) growth. Surprisingly, this growth is blocked by ligand binding domain-targeted antiandrogens, such as MDV3100, or by selective siRNA silencing of AR-FL, indicating that the growthpromoting effects of ARVs are mediated through AR-FL. These data indicate that the increase in ARV expression in castrate-resistant prostate cancer is an acute response to castration rather than clonal expansion of castration or antiandrogen-resistant cells expressing gain of function ARVs, and furthermore, they provide a strategy to overcome ARV function in the clinic.

Identification of Androgen Receptor Splice Variant Transcripts in Breast Cancer Cell Lines and Human Tissues

“…Generally, one or two ARVs were expressed at levels of <5.0 % compared to that of the wild type AR transcript. This data is also consistent with observations of ARV expression in non-cancerous prostate tissues [10,12,30]. While such low expression of the ARVs challenges their physiological significance, a recent study of prostate cancer bone metastases shows that mRNA levels may not accurately reflect protein levels [18].…”

“…Inserts were bidirectionally sequenced using primers T7 and Sp6 (Invitrogen). A recent study with VCaP Xenografts in mice showed a significant increase in ARV expression just 2 days postcastration, and that this increase was completely abrogated by testosterone replacement [30]. We demonstrated that dihydrotestosterone (DHT) negatively regulated ARV expression in the MDA-MB-453 cell line in a similar manner.…”

“…The C-terminal short peptide of 1-53 amino acids encoded by the cryptic exons is the only structural difference that would contribute to their reported varying capacity for transcriptional activity [17,30] and differing cellular localization [9,10,12,30]. Thus, polymorphisms that change this peptide sequence would be expected to have potential impact on ARV biological function.…”

Beyond Castration—Defining Future Directions in the Hormonal Treatment of Prostate Cancer

“…A series of AR splice variants lacking the LBD are upregulated in hormone-resistant prostate cancer cell lines and tissues [25,26] and promote castration-resistant growth, however their impact in hormone resistance remains to be defined as they still may be dependent on full-length receptor (and hence sensitive to MDV3100) for function although this remains controversial [27,28].…”

Abiraterone acetate, a first-in-class CYP17 inhibitor, establishes a new treatment paradigm in castration-resistant prostate cancer

“…It has been reported that the transcriptional activity of these AR variants can be blocked by MDV3100, a highaffinity AR antagonist [20]. This compound was rationally designed using data from a cocrystal structure of bicalutamide bound with mutant AR to create a chemical library that was screened for AR antagonistic activity [10].…”