GSK-3β Inhibition Enhances Sorafenib-induced Apoptosis in Melanoma Cell Lines*

Abstract

Glycogen synthase kinase-3β (GSK-3β) can participate in the induction of apoptosis or, alternatively, provide a survival signal that minimizes cellular injury. We previously demonstrated that the multikinase inhibitor sorafenib induces apoptosis in melanoma cell lines. In this report, we show that sorafenib activates GSK-3β in multiple subcellular compartments and that this activation undermines the lethality of the drug. Pharmacologic inhibition and/or down-modulation of the kinase enhances sorafenib-induced apoptosis as determined by propidium iodide staining and by assessing the mitochondrial release of apoptosis-inducing factor and Smac/DIABLO. Conversely, the forced expression of a constitutively active form of the enzyme (GSK-3β^S9A) protects the cells from the apoptotic effects of the drug. This protective effect is associated with a marked increase in basal levels of Bcl-2, Bcl-x_L, and survivin and a diminution in the degree to which these anti-apoptotic proteins are down-modulated by sorafenib exposure. Sorafenib down-modulates the pro-apoptotic Bcl-2 family member Noxa in cells with high constitutive GSK-3β activity. Pharmacologic inhibition of GSK-3β prevents the disappearance of Noxa induced by sorafenib and enhances the down-modulation of Mcl-1. Down-modulation of Noxa largely eliminates the enhancing effect of GSK-3 inhibition on sorafenib-induced apoptosis. These data provide a strong rationale for the use of GSK-3β inhibitors as adjuncts to sorafenib treatment and suggest that preservation of Noxa may contribute to their efficacy.