Non-steroidal antiandrogens act as AF-1 agonists under conditions of high androgen-receptor expression.

BACKGROUND: The mechanism of resistance acquisition to antiandrogens in prostate cancer is not fully understood. Numerous clinical and basic research studies have shown expression of androgen receptors (ARs) increases in hormone-refractory prostate cancer and therefore we explored possible molecular mechanisms by which prostate cancer acquires resistance to antiandrogens under conditions of increased AR expression. METHODS: In order to study resistance to antiandrogens at the AR transactivation level we used a human AR (hAR) reporter assay system. In addition, we utilized an hAR deletion mutant to determine the functional domain responsible for the acquisition of resistance. RESULTS: Increased hAR protein expression enhanced the sensitivity of AR transactivation to low concentrations of DHT, and also reduced the inhibitory activity of the non-steroidal antiandrogens, hydroxyflutamide, and bicalutamide on DHT-induced AR transactivation. Moreover, these antiandrogens acquired agonistic activity under conditions of high hAR protein expression. Such agonistic activity of antiandrogens was not detected in an hAR deletion mutant (hAR-DeltaA/B) that lacked an A/B domain with AF-1 activity. CONCLUSIONS: We found that non-steroidal antiandrogens act as AF-1 agonists under conditions of high AR protein expression. This partial antagonistic property of antiandrogens may be a molecular mechanism by which prostate cancer develops resistance to these drugs.

Ligand-selective potentiation of rat mineralocorticoid receptor activation function 1 by a CBP-containing histone acetyltransferase complex.

The rat mineralocorticoid receptor (MR) has two activation functions in distinct regions of the A/B domain, designated activation function 1a (AF-1a; amino acids 1 to 169) and AF-1b (amino acids 451 to 600). Since the p160 family protein TIF2, a known component of the AF-2 coactivator complex, potentiates the transactivation function of AF-1b but not that of AF-1a, it is likely that some other, novel protein complex interacts with the AF-1a region. Therefore, we attempted to identify such coactivator complexes from HeLa nuclear extracts by biochemical purification using a glutathione S-transferase-MR AF-1a fusion protein. Purified AF-1a region-interacting proteins were found to contain RNA helicase A (RHA) and CBP. Further analysis showed that RHA interacted with the AF-1a region directly and then recruited a complex with histone acetyltransferase (HAT) activity that contained CBP. For full-length MR, aldosterone, but not hydrocortisone, was found to induce the binding of RHA/CBP complexes to the AF-1a region, as well as to allow the cooperative potentiation of MR transcriptional activity by RHA and CBP. In addition, a chromatin immunoprecipitation assay showed that aldosterone-bound MR, but not hydrocortisone-bound MR, recruited RHA/CBP complexes to native MR target gene promoters. Our results suggested that an altered conformation of the A/B region induced by aldosterone, but not hydrocortisone, might determine the accessibility of MR AF-1a to RHA/CBP complexes.