Detection of androgen receptor (AR) and AR-V7 in small cell prostate carcinoma: Diagnostic and therapeutic implications

Objective:Small cell prostate carcinoma(SCPC)is a rare and highly malignant subtype of prostate cancer.SCPC frequently lacks androgen receptor(AR)and prostate-specific antigen(PSA)expression,and often responds poorly to androgen deprivation therapy(ADT).AR splice variant-7(AR-V7)is a truncated AR protein implicated in resistance to AR-targeting therapies.AR-V7 expression in castration-resistant prostate cancers has been evaluated extensively,and blood-based detection of AR-V7 has been associated with lack of response to abiraterone and enzalutamide.However,whether AR-V7 is expressed in SCPC is not known.Methods:Using validated antibodies,we performed immunohistochemistry(IHC)assay for the full-length AR(AR-FL)and(AR-V7)on post-ADT surgical SCPC specimens.Results:Seventy-five percent(9/12)of the specimens showed positive staining for the AR-FL with various intensities.Thirty-three percent(4/12)of the specimens showed positive staining for AR-V7.Among the specimens with positive AR-V7 staining,two samples displayed very weak staining,one sample showed weak-to-moderate staining,and one sample showed strong staining.All positive specimens displayed a heterogeneous pattern of AR-FL/AR-V7 staining.All specimens positive for AR-V7 were also positive for AR-FL.Conclusion:The study findings support the existence of measurable AR-FL and AR-V7 proteins in SCPC specimens.The results also have implications in detection of AR-V7 in specimens obtained through systemic sampling approaches such as circulating tumor cells.A positive AR-V7 finding by blood-based tests is not impossible in patients with SCPC who often demonstrate low PSA values.

Regulation of androgen receptor variants in prostate cancer

Aberrant activation of androgen receptor(AR)signaling occurs in patients treated with AR-targeted therapies,contributing to the development of castration-resistant prostate cancer(CRPC)and therapeutic resistance.Over the past decade,many AR variants(AR-Vs)have been identified in prostate cancer cell lines and clinical CRPC specimens.These AR-Vs lack the COOH-terminal ligand-binding domain(LBD),and may mediate constitutively active AR signaling acquired following AR-targeting therapies.AR splice variant-7(AR-V7),one of the most well characterized AR-Vs,can be reliably measured in tissue and liquid biopsy specimens,and blood-based detection of AR-V7 is a reliable indicator of poor outcome to relatively novel hormonal therapies(NHT)such as abiraterone and enzalutamide in men with metastatic CRPC(mCRPC).Given the important clinical implication of AR-Vs,this short review will focus on studies addressing how AR-Vs are regulated in prostate cancer.With regard to the molecular origin of AR-Vs,it is established that expression of AR-Vs is highly correlated with androgen deprivation and suppression of AR signaling.Therapeutic targeting of the AR axis may result in active transcription of the AR gene,elevated activities of certain components of the mRNA splicing machinery,as well as AR genomic alterations,all of which may explain the molecular origin of AR-Vs.Although a unified hypothesis is currently lacking,existing data suggest that elevated expression of AR-Vs,which in general occurs quite specifically in a cellular environment where the canonical AR signaling is suppressed,is driven by both genomic and epigenomic features acquired in the development of CRPC.