Glycogen Synthase Kinase 3 Regulates Cell Death and Survival Signaling in Tumor Cells under Redox Stress

Targeting tumor-specific metabolic adaptations is a promising anticancer strategy when tumor defense mechanisms are restrained. Here, we show that redox-modulating drugs including the retinoid N-(4-hydroxyphenyl) retinamide (4HPR), the synthetic triterpenoid bardoxolone (2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid methyl ester), arsenic trioxide (As2O3), and phenylethyl isothiocyanate (PEITC), while affecting tumor cell viability, induce sustained Ser9 phosphorylation of the multifunctional kinase glycogen synthase kinase 3 beta (GSK3 beta). The antioxidant N-acetylcysteine decreased GSK3 beta phosphorylation and poly(ADP-ribose) polymerase cleavage induced by 4HPR, As2O3, and PEITC, implicating oxidative stress in these effects. GSK3 beta phosphorylation was associated with upregulation of antioxidant enzymes, in particular heme oxygenase-1 (HO-1), and transient elevation of intracellular glutathione (GSH) in cells surviving acute stress, before occurrence of irreversible damage and death. Genetic inactivation of GSK3 beta or transfection with the non-phosphorylatable GSK3 beta-S9A mutant inhibited HO-1 induction under redox stress, while tumor cells resistant to 4HPR exhibited increased GSK3 beta phosphorylation, HO-1 expression, and GSH levels. The above-listed findings are consistent with a role for sustained GSK3 beta phosphorylation in a signaling network activating antioxidant effector mechanisms during oxidoreductive stress. These data underlie the importance of combination regimens of antitumor redox drugs with inhibitors of survival signaling to improve control of tumor development and progression and overcome chemoresistance.