Inhibition of Glycogen Synthase Kinase-3 in Androgen-Responsive Prostate Cancer Cell Lines: Are GSK Inhibitors Therapeutically Useful?

Abstract

The glycogen synthase kinase 3 (GSK-3) is a serine/threonine kinase widely expressed in mammalian tissues. Ini-tially 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 pres-ence of androgens, EosAR was predominantly nuclear. Incubation with SB216763 before and after androgen treat-ment 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 pros-tate cancer.