Pleiotropic Role of GSK3 Kinase in Myeloma Cell Biology.

GSK3 is a cellular serine-threonine kinase discovered because of its involvement in insulin, growth factor and Wnt signalling, downstream of which it is inhibited through the action of the PI3K/AKT cell survival pathway or through other Wnt signalling-dependent mechanisms. GSK3, therefore, has been included in the group of “tumor suppressors”, as it can antagonize cell proliferation triggered by these cascades. Recent findings, however, have challenged this notion in that GSK3β has been found central for cell survival and NF-κB signalling. Since growth factor, Wnt and cytokine-dependent signalling pathways have been implicated in MM pathogenesis, we decided to investigate the role of GSK3 in multiple myeloma cell biology. GSK3 kinase activity was found slightly higher in malignant plasma cells as compared to normal resting B-lymphocytes and normal in vitro generated plasmablasts. GSK3 enzymatic activity was hampered by stimulation of MM cells with IL-6 and IGF-I but, remarkably, not with TNFα. IL-6 and IGF-I driven MM cell proliferation was significantly increased by GSK3 blockade as it was MM cell survival upon serum starvation or contact with bone marrow stromal cells (BMSC). At molecular level, IL-6-dependent STAT3 phosphorylation was unaffected by GSK3 inhibition, however, ERK1, 2 phosphorylation was increased. Importantly, NF-κB activation and transcriptional activity downstream from TNFα were only slightly affected when GSK3 function was inhibited in MM cells. However, when GSK3 inhibitors were added to MM cell cultured with BMSC, IL-6 secretion in the medium was reduced and the expression of NF-κB-dependent antiapoptotic genes was altered. Lastly, the addition of GSK3 inhibitors in MM cells-BMSC cultures led to an increased expression of Wnt/β-catenin-dependent genes both in MM and in BMSC cells. Our data indicate a different involvement of GSK3 downstream from growth factors or TNFα-induced signalling pathways in MM cells; the observed effects of GSK3 inhibition on the Wnt-signalling pathway indicate that, whereas they would be desirable in the BMSC compartment (i.e. antagonisms of Wnt-inhibitors released in the MM bone marrow, such as Dikkopf-1 (DKK1) or secreted Frizzled-Related Protein (sFRP)-2 and favouring of the osteoblast maturation) on the other hand they could lead, together with the loss of a brake downstream from growth factor signals, to uncontrolled and enhanced MM cell proliferation; thus, the inhibition of this kinase for therapeutic purposes in MM is likely to be accompanied by dangerous and unwanted side effects that may promote the progression of this disease.