Inhibition of glycogen synthase kinase-3 in androgen-responsive prostate cancer cell lines: are GSK inhibitors therapeutically useful?

The glycogen synthase kinase 3 (GSK-3) is a serine/threonine kinase widely expressed in mammalian tissues. Initially identified by its ability to modulate glycogen synthesis, GSK-3 turned out to be a multifunctional enzyme, able to phosphorylate many proteins, including members of the steroid receptor superfamily. Although GSK-3 was shown to phosphorylate the androgen receptor (AR), its effects on AR transcriptional activity remain controversial. Analysis of short hairpin RNA (shRNA)-mediated downmodulation of GSK-3 proteins in prostate cancer cells showed a reduction in AR transcriptional activity and AR protein levels. Pharmacological GSK-3 inhibitors such as the maleimide SB216763 or the aminopyrazole GSK inhibitor XIII inhibited AR-dependent reporter gene activity and AR expression in vitro. Analysis of androgen-induced nuclear translocation of the AR was performed in PC3 cells transfected with pAR-t1EosFP coding for EosAR, a green fluorescent AR fusion protein. When grown in presence of androgens, EosAR was predominantly nuclear. Incubation with SB216763 before and after androgen treatment almost completely reduced nuclear EosAR. In contrast, the thiazole-containing urea compound AR-A014418 increased rather than decreased AR-expression/function. Although not all GSK inhibitors affected AR-stability/function, our observations suggest a potential new therapeutic application for some of these compounds in prostate cancer.