Differential roles of glycogen synthase kinase-3 isoforms in the regulation of transcriptional activation

Glycogen synthase kinase-3 represses cyclic AMP response element-binding protein (CREB)-targeted immediate early genes in quiescent cells

Despite its central role in cell survival and proliferation, the transcriptional program controlled by GSK-3 is poorly understood. We have employed a systems level approach to characterize gene regulation downstream of PI 3-kinase/Akt/GSK-3 signaling in response to growth factor stimulation of quiescent cells. Of 31 immediate-early genes whose induction was dependent on PI 3-kinase signaling, 12 were induced directly by inhibition of GSK-3. Most of the GSK-3-regulated genes encoded transcription factors, growth factors, and signaling molecules. Binding sites for CREB were highly over-represented in the upstream regions of these genes, with 9 genes containing CREB sites that were conserved in mouse orthologs. Binding sites predicted in 6 genes were confirmed by CREB chromatin immunoprecipitation and forskolin induction of CBP binding. Moreover, CREB siRNA substantially blocked induction of 5 genes by forskolin and of 3 genes following inhibition of GSK-3. These results indicate that GSK-3 actively represses gene expression in quiescent cells, with inhibition of CREB playing a key role in this transcriptional response.

Regulation and function of glycogen synthase kinase-3 isoforms in neuronal survival

Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase consisting of two isoforms, alpha and beta. The activities of GSK-3 are regulated negatively by serine phosphorylation but positively by tyrosine phosphorylation. GSK-3 inactivation has been proposed as a mechanism to promote neuronal survival. We used GSK-3 isoform-specific small interfering RNAs, dominant-negative mutants, or pharmacological inhibitors to search for functions of the two GSK-3 isoforms in regulating neuronal survival in cultured cortical neurons in response to glutamate insult or during neuronal maturation/aging. Surprisingly, RNA interference-induced depletion of either isoform was sufficient to block glutamate-induced excitotoxicity, and the resulting neuroprotection was associated with enhanced N-terminal serine phosphorylation in both GSK-3 isoforms. However, GSK-3beta depletion was more effective than GSK-3alpha depletion in suppressing spontaneous neuronal death in extended culture. This phenomenon is likely due to selective and robust inhibition of GSK-3beta activation resulting from GSK-3beta Ser9 dephosphorylation during the course of spontaneous neuronal death. GSK-3alpha silencing resulted in reduced tyrosine phosphorylation of GSK-3beta, suggesting that tyrosine phosphorylation is also a critical autoregulatory event. Interestingly, GSK-3 inhibitors caused a rapid and long-lasting increase in GSK-3alpha Ser21 phosphorylation levels, followed by a delayed increase in GSK-3beta Ser9 phosphorylation and a decrease in GSK-3alpha Tyr279 and GSK-3beta Tyr216 phosphorylation, thus implying additional levels of GSK-3 autoregulation. Taken together, our results underscore important similarities and dissimilarities of GSK-3alpha and GSK-3beta in the roles of cell survival as well as their distinct modes of regulation. The development of GSK-3 isoform-specific inhibitors seems to be warranted for treating GSK-3-mediated pathology.