Prohibitin (PHB) is a cell cycle regulatory protein, known to repress E2F1-mediated gene activation via recruitment of transcriptional regulatory factors such as retinoblastoma and histone deacetylase 1 (HDAC1). We previously identified PHB as a target protein of androgen signaling in prostate cancer cells and showed that downregulation of PHB is required for androgen-induced cell cycle entry in these cells. We now present evidence that PHB, which has 54% homology at the protein level to the oestrogen receptor corepressor REA (repressor of oestrogen receptor activity), can repress androgen receptor (AR)-mediated transcription and androgen-dependent cell growth. Depletion of endogenous PHB resulted in an increase in expression of the androgen-regulated prostatespecific antigen gene. The repression appears to be specific to androgen and closely related receptors, as it is also evident for the glucocorticoid and progesterone, but not oestrogen, receptors. In spite of interaction of PHB with HDAC1, HDAC activity is not required for this repression. Although AR and PHB could be co-immunoprecipitated, no direct interaction was detectable, suggesting that PHB forms part of a repressive complex with the AR. Competition with the co-activator SRC1 further suggests that formation of a complex with AR, PHB and other cofactors is the mechanism by which repression is achieved. It appears then that repression of AR activity is one mechanism by which PHB inhibits androgendependent growth of prostate cells. Further, this study implies that the AR itself could, by mediating downregulation of a corepressor, be involved in the progression of prostate tumours to the hormone refractory stage.

Prostate tumor growth initially depends on androgens, which act via the androgen receptor (AR). Despite androgen ablation therapy, tumors eventually progress to a castrate-resistant stage in which the AR remains active. The mechanisms are poorly understood but it may be that changes in levels or activity of AR coregulators affect trafficking and activation of the receptor. A key stage in AR signaling occurs in the cytoplasm, where unliganded receptor is associated with the heat shock protein (HSP)90 foldosome complex. p23, a key component of this complex, is best characterized as a cochaperone for HSP90 but also has HSP90-independent activity and has been reported as having differential effects on the activity of different steroid receptors. Here we report that p23 increases activity of the AR, and this appears to involve steps both in the cytoplasm (increasing ligand-binding capacity, possibly via direct interaction with AR) and the nucleus (enhancing AR occupancy at target promoters). We show, for the first time, that AR and p23 can interact, perhaps directly, when HSP90 is not present in the same complex. The effects of p23 on AR activity are at least partly HSP90 independent because a mutant form of p23, unable to bind HSP90, nevertheless increases AR activity. In human prostate tumors, nuclear p23 was higher in malignant prostate cells compared with benign/normal cells, supporting the utility of p23 as a therapeutic target in prostate cancer.

Anti-androgens used in prostate cancer therapy inhibit AR (androgen receptor) activity via largely unknown mechanisms. Although initially successful in most cases, they eventually fail and the disease progresses. We need to elucidate how anti-androgens work to understand why they fail, and prolong their effects or design further therapies. Using a cellular model, we found different anti-androgens have diverse effects on subcellular localization of AR, revealing that they work via different mechanisms and suggesting that an informed sequential treatment regime may benefit patients. In the presence of the anti-androgens bicalutamide and hydroxyflutamide, a significant proportion of the AR is translocated to the nucleus but remains inactive. Receptor inhibition under these conditions is likely to involve recruitment of co-repressor proteins, which interact with antagonist-occupied receptor but inhibit receptor-dependent transcription. Which co-repressors are required in vivo for AR repression by anti-androgens is not clear, but one candidate is the Notch effector Hey1. This inhibits ligand-dependent activity of the AR but not other steroid receptors. Further, it is excluded from the nucleus in most human prostate cancers, suggesting that abnormal subcellular distribution of co-repressors may contribute to the aberrant hormonal responses observed in prostate cancer. A decrease in co-repressor function is one possible explanation for the development of anti-androgen-resistant prostate cancer, and this suggests that it may not occur at the gross level of protein expression.