Changmeng Cai

Androgen receptor functions in castration-resistant prostate cancer and mechanisms of resistance to new agents targeting the androgen axis.

The metabolic functions of androgen receptor (AR) in normal prostate are circumvented in prostate cancer (PCa) to drive tumor growth, and the AR also can acquire new growth-promoting functions during PCa development and progression through genetic and epigenetic mechanisms. Androgen deprivation therapy (ADT, surgical or medical castration) is the standard treatment for metastatic PCa, but patients invariably relapse despite castrate androgen levels (castration-resistant PCa, CRPC). Early studies from many groups had shown that AR was highly expressed and transcriptionally active in CRPC, and indicated that steroids from the adrenal glands were contributing to this AR activity. More recent studies showed that CRPC cells had increased expression of enzymes mediating androgen synthesis from adrenal steroids, and could synthesize androgens de novo from cholesterol. Phase III clinical trials showing a survival advantage in CRPC for treatment with abiraterone (inhibitor of the enzyme CYP17A1 required for androgen synthesis that markedly reduces androgens and precursor steroids) and for enzalutamide (new AR antagonist) have now confirmed that AR activity driven by residual androgens makes a major contribution to CRPC, and led to the recent Food and Drug Administration approval of both agents. Unfortunately, patients treated with these agents for advanced CRPC generally relapse within a year and AR appears to be active in the relapsed tumors, but the molecular mechanisms mediating intrinsic or acquired resistance to these AR-targeted therapies remain to be defined. This review outlines AR functions that contribute to PCa development and progression, the roles of intratumoral androgen synthesis and AR structural alterations in driving AR activity in CRPC, mechanisms of action for abiraterone and enzalutamide, and possible mechanisms of resistance to these agents.

c-Jun enhancement of androgen receptor transactivation is associated with prostate cancer cell proliferation

Androgens and the androgen receptor (AR) are involved in the growth and progression of prostate cancer. Our previous studies suggest that the proto-oncoprotein c-Jun is an AR coactivator that stimulates AR transactivation by mediating receptor dimerization and subsequent DNA binding. To study the physiological relevance of this c-Jun activity on AR, we have generated stable LNCaP cell lines expressing different levels of c-Jun. These cell lines exhibit a direct correlation between endogenous c-Jun levels and AR transcriptional activity and expression of endogenous androgen-regulated genes. Disruption by antisense RNA of endogenous c-Jun expression in LNCaP cells strongly compromises the androgen-dependent proliferation of these cells. In contrast, expression of a c-Jun mutant, which is fully active in coactivation of AR but deficient in AP-1 transactivation, significantly enhances androgen-dependent proliferation. This finding indicates that the coactivation function of c-Jun is sufficient for regulating androgen-induced growth of LNCaP cells. c-Jun also enhances AR transactivtion in androgen-independent LNCaP cells, which closely mimic hormone-refractory prostate cancer cells in gene expression and growth behavior. Importantly, siRNA-mediated repression of endogenous c-Jun expression results in markedly reduced growth of these cells, strongly suggesting an important biological role for c-Jun in hormone-refractory prostate cancer.

Androgen induces expression of the multidrug resistance protein gene MRP4 in prostate cancer cells.

Multidrug resistance-associated proteins (MRPs) may mediate multidrug resistance in tumor cells. Using a gene array analysis, we have identified MRP4 as an androgen receptor (AR)-regulated gene. Dihydrotestosterone induced MRP4 expression in both androgen-dependent and -independent LNCaP cells, whereas there was little detectable expression in PC-3 or normal prostate epithelial cells. Disruption of MRP4 expression renders LNCaP cells more sensitive to the cytotoxic effects of methotrexate but not etoposide. Analysis of human tissues showed detectable MRP4 expression only in metastatic prostate cancer. These results suggest that AR induction of MRP4 mediates resistance of PC cells to nucleotide-based chemotherapeutic drugs.

Androgen regulation of soluble guanylyl cyclaseα1 mediates prostate cancer cell proliferation

The growth and progression of prostate cancer are dependent on androgens and androgen receptor (AR), which act by modulating gene expression. Utilizing a gene microarray approach, we have identified the α1-subunit gene of soluble guanylyl cyclase (sGC) as a novel androgen-regulated gene. A heterodimeric cytoplasmic protein composed of one α and one β subunit, sGC mediates the widespread cellular effects of nitric oxide (NO). We report here that, in prostate cancer cells, androgens stimulate the expression of sGCα1. A cloned human sGCα1 promoter is activated by androgen in an AR-dependent manner, suggesting that sGCα1 may be a direct AR target gene. Disruption of sGCα1 expression severely compromises the growth of both androgen-dependent and androgen-independent AR-positive prostate cancer cells. Overexpression of sGCα1 alone is sufficient for stimulating prostate cancer cell proliferation. Interestingly, the major growth effect of sGCα1 is independent of NO and cyclic guanosine monophosphate, a major mediator of the sGC enzyme. These data strongly suggest that sGCα1 acts in prostate cancer via a novel pathway that does not depend on sGCβ1. Tissue studies show that sGCα1 expression is significantly elevated in advanced prostate cancer. Thus, sGCα1 may be an important mediator of the procarcinogenic effects of androgens.

Soluble Guanylyl Cyclase α1 and p53 Cytoplasmic Sequestration and Down-Regulation in Prostate Cancer

Our laboratory has previously identified soluble guanylyl cyclase α1 (sGCα1) as a novel androgen-regulated gene essential for prostate cancer cell proliferation. sGCα1 expression is highly elevated in prostate tumors, contrasting with the low expression of sGCβ1, with which sGCα1 dimerizes to mediate nitric oxide (NO) signaling. In studying its mechanism of action, we have discovered that sGCα1 can inhibit the transcriptional activity of p53 in prostate cancer cells independent of either classical mediators of NO signaling or the guanylyl cyclase activity of sGCα1. Interestingly, sGCα1 inhibition of p53-regulated gene expression was gene specific, targeting genes involved in apoptosis/cell survival. Consistent with this, overexpression of sGCα1 makes prostate cancer cells more resistant to etoposide, a chemotherapeutic and apoptosis-inducing drug. Immunoprecipitation and immunocytochemistry assays show a physical and direct interaction between sGCα1 and p53 in prostate cancer cells. Interestingly, sGCα1 induces p53 cytoplasmic sequestration, representing a new mechanism of p53 inactivation in prostate cancer. Analysis of prostate tumors has shown a direct expression correlation between sGCα1 and p53. Collectively, these data suggest that sGCα1 regulation of p53 activity is important in prostate cancer biology and may represent an important mechanism of p53 down-regulation in those prostate cancers that express significant levels of p53.