Abstract
BackgroundGlycogen Synthase Kinase-3 (GSK-3) α and β are two serine-threonine kinases controlling insulin, Wnt/β-catenin, NF-κB 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-3α and GSK-3β function in multiple myeloma (MM).MethodsGSK-3 α and β 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 α and β isoforms. Survival signaling pathways were studied with WB analysis.ResultsGSK-3α and GSK-3β 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-3β knock down decreased MM cell viability, while GSK-3α knock down was associated with a higher rate of bortezomib-induced cytotoxicity. GSK-3 inhibition caused accumulation of β-catenin and nuclear phospho-ERK1, 2. Moreover, GSK-3 inhibition and GSK-3α 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.ConclusionsThese data suggest that in MM cells GSK-3α and β i) play distinct roles in cell survival and ii) modulate the sensitivity to proteasome inhibitors.
Highlights
Glycogen Synthase Kinase-3 (GSK-3) a and b are two serine-threonine kinases controlling insulin, Wnt/b-catenin, NF-B signaling and other cancer-associated transduction pathways
We found that treatment of MM cells with GSK-3 inhibitors and GSK3b knock down caused growth arrest and apoptosis by perturbing pivotal signaling pathways
Since MM cells receive signals that activate these two kinases, we investigated the expression of total and serine-phosphorylated GSK-3 in these cells
Summary
Glycogen Synthase Kinase-3 (GSK-3) a and b are two serine-threonine kinases controlling insulin, Wnt/b-catenin, NF-B signaling and other cancer-associated transduction pathways. Recent evidence suggests that GSK-3 could function as growth-promoting kinases, especially in malignant cells. Two major isozymes (GSK-3a and GSK-3b) are known and conserved throughout the species [1] This kinase is involved in cell proliferation and survival by controlling the Wnt/b-catenin and growth factors (GFs)-dependent pathways [2]. Upon activation of Wnt signalling, GSK-3 activity is hampered and unphosphorylated b-catenin accumulates in the cytosol, translocates to the nucleus and promotes gene transcription and cell growth by acting as a co-activator of the transcription factors TCF/LEF [3,4]. Since Wnt/b-catenin and PI3K/AKT-dependent signaling pathways promote cell growth, GSK-3 has been considered a growth-suppressor. A caveat for many of these studies is that GSK-3a and b isoforms were not evaluated separately
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