A Glucocorticoid-Responsive Mutant Androgen Receptor Exhibits Unique Ligand Specificity: Therapeutic Implications for Androgen-Independent Prostate Cancer

Abstract

The cortisol/cortisone-responsive AR (ARccr) has two mutations (L701H and T877A) that were found in the MDA PCa human prostate cancer cell lines established from a castrated patient whose metastatic tumor exhibited androgen-independent growth. Cortisol and cortisone bind to the ARccr with high affinity. In the present study, we characterized the structural determinants for ligand binding to the ARccr. Our data revealed that many of the C17, C19, and C21 circulating steroids, at concentrations that are found in vivo, functioned as effective activators of the ARccr but had little or no activity via the wild-type AR or GRα. Among the synthetic glucocorticoids tested, dexamethasone activated both GRα and ARccr, whereas triamcinolone was selective for GRα. In MDA PCa 2b cells, growth and prostate-specific antigen production were stimulated by potent ARccr agonists such as cortisol or 9α-fluorocortisol but not by triamcinolone (which did not bind to or activate the ARccr). Of the potential antagonists tested, bicalutamide (casodex) and GR antagonist RU38486 showed inhibitory activity. We postulate that corticosteroids provide a growth advantage to prostate cancer cells harboring the promiscuous ARccr in androgen-ablated patients and contribute to their transition to androgen-independence. We predict that triamcinolone, a commonly prescribed glucocorticoid, would be a successful therapeutic agent for men with this form of cancer, perhaps in conjunction with the antagonist casodex. We hypothesize that triamcinolone administration would inhibit the hypothalamic-pituitary-adrenal axis, thus suppressing endogenous corticosteroids, which stimulate tumor growth. Triamcinolone, by itself, would not activate the ARccr or promote tumor growth but would provide glucocorticoid activity essential for survival.