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Cytokine regulation of glycogen synthase kinase-3 (GSK-3) Vilimek, Dino Alexander Henry

Abstract

Cytokines are soluble growth factors that are essential for the continued survival of numerous hematopoietic cell lines. For example, in cells of the erythromyeloid lineage, interleukin-3 (IL-3) and granulocyte-macrophage colony stimulating factor (GM-CSF) are powerful anti-apoptotic agents. Glycogen synthase kinase-3 (GSK-3) is an ubiquitous and yet enigmatic protein kinase implicated in various cellular functions, including glycogen metabolism, embryonic development, cell survival, proliferation, and protein translation. Numerous extracellular stimuli can promote the inactivation of GSK-3 through serine phosphorylation of the amino-terminal region. Growth factors are among the agents capable of inactivating GSK-3, presumably to promote cell survival and proliferation. Interestingly, an extensive literature search has found no studies examining GSK-3 in the context of cytokine-mediated signaling. This study is the first to demonstrate the elevation of GSK-3α and GSK-3β serine phosphorylation by IL-3, IL-4, and GM-CSF in several hematopoietic cell lines. IL-4 required PI3-K activity, and presumably PKB activity, for full GSK-3 modification, while IL-3 and GM-CSF did not. Although MAPK and p70S6K did not regulate the control of GSK-3 by IL- 3, IL-4 or GM-CSF, PKC did demonstrate a strong effect. Inhibition of PKC activity, through the addition of pharmacological compounds, led to a dramatic abrogation of GSK-3 serine phosphorylation induced by all three cytokines. However, the lack of specificity of these inhibitors made it difficult to identify the class of PKC or the individual isoforms responsible for GSK-3 regulation. Furthermore, the inhibition of diacylglycerol production also impacted on the cytokines' ability to phosphorylate GSK-3. Diacylglycerol is a necessary component for the activation of numerous PKC isoforms. However, like the PKC inhibitors, some evidence suggests that these PLC inhibitors may be acting on non-specific targets. Interestingly, increased serine phosphorylation of GSK-3 did not appear to correlate with a decrease in catalytic activity, although the quality of the assay system is questionable. Regardless, this study is the first to examine GSK-3 in cytokine-mediated signaling, and to implicate PI3-K and PKC as mediators of this event. Further studies will help develop a clear signaling model for the regulation of GSK-3 and determine GSK-3' s role in the cellular effects induced by cytokines.

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