Ligand-dependent corepressor acts as a novel androgen receptor corepressor, inhibits prostate cancer growth, and is functionally inactivated by the Src protein kinase

The activated androgen receptor (AR) promotes prostate cancer (PCa) growth. AR antagonists repress the AR by recruitment of corepressors. Not much is known about the inactivation of AR by corepressors in the presence of agonists (androgens). Here we show that the corepressor LCoR acts as an androgen-dependent corepressor that represses human PCa growth in vivo. In line with this, progressive decrease of ligand-dependent corepressor expression was observed in the PCa TRAMP mouse model with increasing age. LCoR interacts with AR and is recruited to chromatin in an androgen-induced manner. Unexpectedly, the LXXLL motif of LCoR is dispensable for interaction with the AR. Rather, the data indicate that LCoR interacts with the AR DNA binding domain on DNA. Interestingly, the interaction of LCoR with AR is inhibited by signaling pathways that are associated with androgen-independent PCa. Here we also show that the Src kinase inactivates the corepressive function of LCoR. Interfering with endogenous Src function by a dominant negative Src mutant, the growth inhibitory activity of LCoR is enhanced in vivo in a xenograft mouse model system. Thus, our studies indicate a role of LCoR as an AR corepressor and a tumor suppressor. Further, the decreased expression or inactivation of LCoR is as an important step toward PCa carcinogenesis in vivo.     

9-cis-retinoic acid inhibits androgen receptor activity through activation of retinoid X receptor

Although the retinoic X receptor (RXR) forms heterodimers with many members of the estrogen receptor subfamily, the interaction between RXR and the members of the glucocorticoid receptor subfamily remains unclear. Here we show that the RXR can form a heterodimer with the androgen receptor (AR) under in vitro and in vivo conditions. Functional analyses further demonstrated that the AR, in the presence or absence of androgen, can function as a repressor to suppress RXR target genes, thereby preventing the RXR binding to the RXR DNA response element. In contrast, RXR can function as a repressor to suppress AR target genes in the presence of 9-cis-retinoic acid, but unliganded RXR can function as a weak coactivator to moderately enhance AR transactivation. Together, these results not only reveal a unique interaction between members of the two nuclear receptor subfamilies, but also represent the first evidence showing a nuclear receptor (RXR) may function as either a repressor or a coactivator based on the ligand binding status.     

Anti-androgen receptor ASC-J9 versus anti-androgens MDV3100 (Enzalutamide) or Casodex (Bicalutamide) leads to opposite effects on prostate cancer metastasis via differential modulation of macrophage infiltration and STAT3-CCL2 signaling

Despite androgen deprivation therapy (ADT) suppression of prostate cancer (PCa) growth, its overall effects on PCa metastasis remain unclear. Using human (C4-2B/THP1) and mouse (TRAMP-C1/RAW264.7) PCa cells–macrophages co-culture systems, we found currently used anti-androgens, MDV3100 (enzalutamide) or Casodex (bicalutamide), promoted macrophage migration to PCa cells that consequently led to enhanced PCa cell invasion. In contrast, the AR degradation enhancer, ASC-J9, suppressed both macrophage migration and subsequent PCa cell invasion. Mechanism dissection showed that Casodex/MDV3100 reduced the AR-mediated PIAS3 expression and enhanced the pSTAT3-CCL2 pathway. Addition of CCR2 antagonist reversed the Casodex/MDV3100-induced macrophage migration and PCa cell invasion. In contrast, ASC-J9 could regulate pSTAT3-CCL2 signaling using two pathways: an AR-dependent pathway via inhibiting PIAS3 expression and an AR-independent pathway via direct inhibition of the STAT3 phosphorylation/activation. These findings were confirmed in the in vivo mouse model with orthotopically injected TRAMP-C1 cells. Together, these results may raise the potential concern about the currently used ADT with anti-androgens that promotes PCa metastasis and may provide some new and better therapeutic strategies using ASC-J9 alone or a combinational therapy that simultaneously targets androgens/AR signaling and PIAS3-pSTAT3-CCL2 signaling to better battle PCa growth and metastasis at castration-resistant stage.     

Hormone depletion-insensitivity of prostate cancer cells is supported by the AR without binding to classical response elements

A need for androgen response elements (AREs) for androgen receptor (AR)-dependent growth of hormone depletion-insensitive prostate cancer is generally presumed. In such cells, androgen-independent activation by AR of certain genes has been attributed to selective increases in basal associations of AR with putative enhancers. We examined the importance of AR binding to DNA in prostate cancer cells in which proliferation in the absence of hormone was profoundly (～ 90%) dependent on endogenous AR and where the receptor was not up-regulated or mutated but was predominantly nuclear. Here, ARE-mediated promoter activation and the binding of AR to a known ARE in the chromatin remained entirely androgen dependent, and the cells showed an androgen-responsive gene expression profile with an unaltered sensitivity to androgen dose. In the same cells, a different set of genes primarily enriched for cell division functions was activated by AR independently of hormone and significantly overlapped the signature gene overexpression profile of hormone ablation-insensitive clinical tumors. After knockdown of endogenous AR, hormone depletion-insensitive cell proliferation and AR apoprotein-dependent gene expression were rescued by an AR mutant that was unable to bind to ARE but that could transactivate through a well-established AR tethering protein. Hormone depletion-insensitive AR binding sites in the chromatin were functional, binding, and responding to both the wild-type and the mutant AR and lacked enrichment for canonical or noncanonical ARE half-sites. Therefore, a potentially diverse set of ARE-independent mechanisms of AR interactions with target genes must underlie truly hormone depletion-insensitive gene regulation and proliferation in prostate cancer.