The cAMP-responsive element-binding protein (CREB) transcription factor is required for normal T cell activation following stimulation through the T cell antigen receptor (TCR). CREB is present in resting T cells in an unphosphorylated and inactive state. TCR engagement results in the rapid phosphorylation of CREB on Ser133 and its concomitant activation. In the studies described in this report, we have investigated the signaling pathway(s) that are responsible for CREB activation in normal T cells. Using pharmacological agonists, we show that protein kinase C (PKC)-, calcium/calmodulin-, and protein kinase A-dependent pathways are each capable of independently eliciting CREB phosphorylation in T cells and thymocytes. Pharmacological inhibitor studies demonstrated that the PKC-mediated signaling pathway is required for TCR-mediated activation of CREB. In contrast, inhibitors of protein kinase A and calmodulin kinases had no effect on CREB phosphorylation following TCR cross-linking. T cells lacking the p56(lck) tyrosine kinase failed to phosphorylate CREB in response to TCR engagement. Overexpression of dominant-negative mutant Ras and Raf-1 proteins in Jurkat T cells abolished TCR-mediated CREB phosphorylation, whereas overexpression of the RSK2 serine/threonine kinase significantly potentiated TCR-mediated CREB phosphorylation. Taken together, these experiments are consistent with a model in which TCR engagement leads to the rapid phosphorylation and activation of CREB via a signaling pathway involving the activation of p56(lck), PKC, Ras, Raf-1, MEK, and RSK2. Given the importance of CREB phosphorylation in normal T cell activation, this pathway may be an attractive target for the development of novel immunosuppressive agents.
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