There is emerging evidence that the balance between estrogen receptor-A (ERA) and androgen receptor (AR) signaling is a critical determinant of growth in the normal and malignant breast. In this study, we assessed AR status in a cohort of 215 invasive ductal breast carcinomas. AR and ERA were coexpressed in the majority (80-90%) of breast tumor cells. KaplanMeier product limit analysis and multivariate Cox regression showed that AR is an independent prognostic factor in ERApositive disease, with a low level of AR (less than median of 75% positive cells) conferring a 4.6-fold increased risk of cancer-related death (P = 0.002). Consistent with a role for AR in breast cancer outcome, AR potently inhibited ERA transactivation activity and 17B-estradiol-stimulated growth of breast cancer cells. Transfection of MDA-MB-231 breast cancer cells with either functionally impaired AR variants or the DNA-binding domain of the AR indicated that the latter is both necessary and sufficient for inhibition of ERA signaling. Consistent with molecular modeling, electrophoretic mobility shift assays showed binding of the AR to an estrogenresponsive element (ERE). Evidence for a functional interaction of the AR with an ERE in vivo was provided by chromatin immunoprecipitation data, revealing recruitment of the AR to the progesterone receptor promoter in T-47D breast cancer cells. We conclude that, by binding to a subset of EREs, the AR can prevent activation of target genes that mediate the stimulatory effects of 17B-estradiol on breast cancer cells. [Cancer Res 2009;69(15):6131-40]

The androgen receptor (AR) gene contains a polymorphic trinucleotide repeat region, (CAG)(n), in its N-terminal transactivation domain (NTD) that encodes a polyglutamine (polyQ) tract in the receptor protein. Whereas the length of the CAG repeat ranges from 6 to 39 in healthy individuals, the variations in repeat length both within and outside the normal range are associated with disease, including impaired spermatogenesis and Kennedy's disease, and with the risk of developing breast and prostate cancer. Whereas it has been proposed that the inverse relationship between polyQ tract length within the normal range and AR transactivation potential may be responsible for altered risk of disease, the molecular mechanisms underlying polyQ length modulation of AR function have not been elucidated. In this study, we provide detailed characterization of a somatic AR gene mutation detected in a human prostate tumor that results in interruption of the polyQ tract by two non-consecutive leucine residues (AR-polyQ2L). Compared with wtAR, AR-polyQ2L exhibits disrupted inter-domain communication (N/C interaction) and a lower protein level, but paradoxically has markedly increased transactivation activity. Molecular modeling and the response to cofactors indicate that the increased activity of AR-polyQ2L results from the presentation of a more stable platform for the recruitment of accessory proteins than wild-type AR. Analysis of the relationship between polyQ tract length and AR function revealed a critical size (Q16-Q29) for maintenance of N/C interaction. That between 91 and 99% of AR alleles in different racial-ethnic groups encode a polyQ tract in the range of Q16-Q29 suggests that N/C interaction has been preserved as an essential component of androgen-induced AR signaling.

Steroid receptors are hormone-activated transcription factors, the expression and activities of which are also highly dependent upon posttranslational modifications including phosphorylation. The remarkable number of phosphorylation sites in these receptors and the wide variety of kinases participating in their phosphorylation facilitate integration between cell-signaling pathways and steroid receptor action. Sites have been identified in all of the functional domains although the sites are predominantly in the amino-terminal portions of the receptors. Regulation of function is receptor specific, site specific, and often dependent upon activation of a specific cell-signaling pathway. This complexity explains, in part, the early difficulties in identifying roles for phosphorylation in receptor function. With increased availability of phosphorylation site-specific antibodies and better means to measure receptor activities, numerous roles for site-specific phosphorylation have been identified including sensitivity of response to hormone, DNA binding, expression, stability, subcellular localization, and protein-protein interactions that determine the level of regulation of specific target genes. This review summarizes current knowledge regarding receptor phosphorylation and regulation of function. As functional assays become more sophisticated, it is likely that additional roles for phosphorylation in receptor function will be identified.

While the clinical benefit of androgen-based therapeutics in breast cancer has been known since the 1940s, we have only recently begun to fully understand the mechanisms of androgen action in breast cancer. Androgen signalling pathways can have either beneficial or deleterious effects in breast cancer depending on the breast cancer subtype and intracellular context. This review discusses our current knowledge of androgen signalling in breast cancer, including the relationship between serum androgens and breast cancer risk, the prognostic significance of androgen receptor (AR) expression in different breast cancer subtypes and the downstream molecular pathways mediating androgen action in breast cancer cells. Intracrine androgen metabolism has also been discussed and proposed as a potential mechanism that may explain some of the reported differences regarding dichotomous androgen actions in breast cancers. A better understanding of AR signalling in this disease is critical given the current resurgence in interest in utilising contemporary AR-directed therapies for breast cancer and the need for biomarkers that will accurately predict clinical response.