Glycogen Synthase Kinase-3 (GSK-3) alpha and beta are two serine-threonine kinases controlling insulin, Wnt/beta-catenin, NF-kappaB signaling and other cancer-associated transduction pathways. Recent evidence suggests that GSK-3 could function as growth-promoting kinases, especially in malignant cells. In this study, we have investigated GSK-3alpha and GSK-3beta function in multiple myeloma (MM).

GSK-3 alpha and beta expression and cellular localization were investigated by Western blot (WB) and immunofluorescence analysis in a panel of MM cell lines and in freshly isolated plasma cells from patients. MM cell growth, viability and sensitivity to bortezomib was assessed upon treatment with GSK-3 specific inhibitors or transfection with siRNAs against GSK-3 alpha and beta isoforms. Survival signaling pathways were studied with WB analysis.

GSK-3alpha and GSK-3beta were differently expressed and phosphorylated in MM cells. Inhibition of GSK-3 with the ATP-competitive, small chemical compounds SB216763 and SB415286 caused MM cell growth arrest and apoptosis through the activation of the intrinsic pathway. Importantly, the two inhibitors augmented the bortezomib-induced MM cell cytotoxicity. RNA interference experiments showed that the two GSK-3 isoforms have distinct roles: GSK-3beta knock down decreased MM cell viability, while GSK-3alpha knock down was associated with a higher rate of bortezomib-induced cytotoxicity. GSK-3 inhibition caused accumulation of beta-catenin and nuclear phospho-ERK1, 2. Moreover, GSK-3 inhibition and GSK-3alpha knockdown enhanced bortezomib-induced AKT and MCL-1 protein degradation. Interestingly, bortezomib caused a reduction of GSK-3 serine phosphorylation and its nuclear accumulation with a mechanism that resulted partly dependent on GSK-3 itself.

These data suggest that in MM cells GSK-3alpha and beta i) play distinct roles in cell survival and ii) modulate the sensitivity to proteasome inhibitors.

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