A Novel Androgen Receptor Splice Variant Is Up-regulated during Prostate Cancer Progression and Promotes Androgen Depletion–Resistant Growth

Abstract

The androgen receptor (AR) plays a key role in progression to incurable androgen ablation-resistant prostate cancer (PCA). We have identified three novel AR splice variants lacking the ligand-binding domain (designated as AR3, AR4, and AR5) in hormone-insensitive PCA cells. AR3, one of the major splice variants expressed in human prostate tissues, is constitutively active, and its transcriptional activity is not regulated by androgens or antiandrogens. Immunohistochemistry analysis on tissue microarrays containing 429 human prostate tissue samples shows that AR3 is significantly up-regulated during PCA progression and AR3 expression level is correlated with the risk of tumor recurrence after radical prostatectomy. Overexpression of AR3 confers ablation-independent growth of PCA cells, whereas specific knockdown of AR3 expression (without altering AR level) in hormone-resistant PCA cells attenuates their growth under androgen-depleted conditions in both cell culture and xenograft models, suggesting an indispensable role of AR3 in ablation-independent growth of PCA cells. Furthermore, AR3 may play a distinct, yet essential, role in ablation-independent growth through the regulation of a unique set of genes, including AKT1, which are not regulated by the prototype AR. Our data suggest that aberrant expression of AR splice variants may be a novel mechanism underlying ablation independence during PCA progression, and AR3 may serve as a prognostic marker to predict patient outcome in response to hormonal therapy. Given that these novel AR splice variants are not inhibited by currently available antiandrogen drugs, development of new drugs targeting these AR isoforms may potentially be effective for treatment of ablation-resistant PCA. [Cancer Res 2009;69(6):2305-13]

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].…”

“…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.…”

“…In addition, at least 7 other so-called "cryptic" exons (CE1-5, CE1b, and CE9) have been identified in the AR gene [9][10][11][12][13]. These cryptic exons can be incorporated into AR transcripts by altered splicing events [14] or as a result of genomic rearrangements [15,16], yielding at least 14 variant AR transcripts (termed AR-V1 to AR-V14) [10].…”

“…Most ARVs have been shown to be constitutively (AR-V3, -V4, -V7, and -V12) [9,10,12,17] or conditionally (AR-V1 and -V9) [10,12] active androgen-independent transcription factors in a prostate cancer context. Pre-clinical and clinical studies have supported the concept that ARVs are biologically relevant in prostate cancer, particularly in the genesis of a CRPC [11,12,[17][18][19][20].…”

SOCS3 Modulates the Response to Enzalutamide and Is Regulated by Androgen Receptor Signaling and CpG Methylation in Prostate Cancer Cells

“…These facts clearly demonstrate that SOCS3 is an androgen-regulated gene. To further investigate the effect of the androgen receptor on SOCS3 expression in CRPC, we performed experiments in presence of the constitutively active androgen receptor splice variant V7 which frequently contributes to the development of castration resistance (43,44). It is still a matter of debate, whether the AR-V7 variant has a different target gene specificity (39,45).…”