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 Ser 133 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. (8, 9). Previous studies have demonstrated that multiple signaling pathways in different cell lineages can mediate the phosphorylation and activation of CREB. These include a protein kinase A (PKA)-dependent pathway that is activated by increased intracellular cAMP (2, 3), a calcium/calmodulin-dependent pathway in which CREB can be phosphorylated by CaM kinases II and IV (10), and a Ras-dependent pathway in which the serine/threonine kinase RSK2 is thought to phosphorylate CREB on Ser 133 (11).Recent studies have demonstrated that transcriptionally active CREB is required for the activation of normal murine T cells following engagement of the T cell antigen receptor (12). Resting T cells contain exclusively unphosphorylated and inactive CREB. TCR cross-linking leads to the rapid and transient phosphorylation of CREB on Ser 133 . More important, transgenic mice expressing a dominant-negative unphosphorylatable form of CREB display a profound T cell proliferative defect characterized by G 1 cell cycle arrest, markedly decreased IL-2 production, and defective transcriptional induction of multiple Fos and Jun proteins (12). These results were consistent with other reports that demonstrated that T and B cell activation results in CREB phosphorylation and increased CREB DNA-binding activity ...