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. 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.

The Fibroblast Growth Factor 14·Voltage-gated Sodium Channel Complex Is a New Target of Glycogen Synthase Kinase 3 (GSK3)

“…These compounds, including indirubin-3Ј-monoxime, kenpaullone, and the GSK3 inhibitors XIII, XII, X, IX, and VIII ( Fig. 2A), were then run through a counter-screening to account for potential interference of test compounds with the luciferase enzymatic activity (27,28), and three were declared hits (Fig. 2B, black dots).…”

“…2, C and D; pubChem number 649766), a potent cell-permeable inhibitor used in cell culture studies (27,28), because of its robust effect (reduction of the LCA signal to 30 Ϯ 0.1% S.D. compared with DMSO control, n ϭ 2, see Fig.…”

6BIO Enhances Oligonucleotide Activity in Cells: A Potential Combinatorial Anti-androgen Receptor Therapy in Prostate Cancer Cells

“…47 Congruent with our results, recent studies have also implicated GSK-3a/b inhibition in the downregulation of AR signaling. [60][61][62] Silencing of both GSK3a and GSK-3b has also been demonstrated to produce a significant reduction in AR transcript levels, in addition to influencing AR stability and AR cellular localization. 61 Conversely, the AR can be phosphorylated by ectopically expressed GSK-3b or by purified GSK-3b in vitro.…”

“…[60][61][62] Silencing of both GSK3a and GSK-3b has also been demonstrated to produce a significant reduction in AR transcript levels, in addition to influencing AR stability and AR cellular localization. 61 Conversely, the AR can be phosphorylated by ectopically expressed GSK-3b or by purified GSK-3b in vitro. 62 GSK-3a and GSK-3b are also aberrantly expressed in many human prostate cancer cells.…”

“…46,60 Several GSK-3 inhibitors have been shown to repress AR signaling. These small molecular compounds include SB216763, 61 lithium chloride (LiCl), 63 CHIR99021, 46 and TDZD-8. 63 We compared the potency of 6BIO with CHIR99021 and LiCl, and found that 6BIO is more potent on a molar basis than these other two inhibitors with respect to AR silencing ( Figures 7C and 7D).…”