Prostate cancer: molecular biology of early progression to androgen independence

To improve the therapy for prostate cancer, it will be necessary to address the problems of progression to androgen independence and the process of metastatic spread of tumour. The complexity of the latter condition is likely to mitigate against the immediate development of relevant therapeutic approaches. However, the basis of androgen independence appears to be a problem of simpler dimensions and more amenable to treatment with current therapeutic technology. Since early tumour progression can be detected by an incomplete prostate-specific antigen (PSA) response to androgen withdrawal therapy, a study of the molecular biology of PSA gene regulation may well provide insight into new methods for preventing or delaying this problem. Mounting evidence suggests that ligand-independent activation of the androgen receptor may be one underlying mechanism of androgen independence. In the absence of androgen, a compensatory increase in the activity of cAMP-dependent protein kinase (PKA) enhances the ability of the androgen receptor to bind to the response elements regulating PSA gene expression. The activation of the androgen receptor through up-regulation of the PKA signal transduction pathway involves the amino-terminus of the androgen receptor, the function of which may be altered either by modifications such as phosphorylation, or through interactions with co-regulators or other proteins. Of therapeutic interest is the fact that this effect can be counteracted experimentally by the anti-androgen, bicalutamide, and clinically by several other similar agents. We speculate that the inhibition of PKA-activated androgen receptor might also be accomplished by decoy molecules that can bind to the relevant activated site on the amino-terminus or competitively interact with proteins recruited by the PKA pathway that are responsible for activating the receptor in the absence of androgen. Such molecules might include small mimetic substances or agents that can gain access to the nucleus of the cell.