A PROTEIN-KINASE C-DEPENDENT, RAS-DEPENDENT, AND RSK2-DEPENDENT SIGNAL-TRANSDUCTION PATHWAY ACTIVATES THE CAMP-RESPONSIVE ELEMENT-BINDING PROTEIN TRANSCRIPTION FACTOR FOLLOWING T-CELL RECEPTOR ENGAGEMENT

The cAMP-responsive element-binding protein (CREB) transcription facto r is required for normal T cell activation following stimulation throu gh the T cell antigen receptor (TCR), CREB is present in resting T cel ls in an unphosphorylated and inactive state, TCR engagement results i n the rapid phosphorylation of CREB on Ser(133) and its concomitant ac tivation. In the studies described in this report, we have investigate d 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 p athways are each capable of independently eliciting CREB phosphorylati on in T cells and thymocytes, Pharmacological inhibitor studies demons trated that the PKC-mediated signaling pathway is required for TCR-med iated 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 faile d to phosphorylate CREB in response to TCR engagement. Overexpression of dominant-negative mutant Ras and Raf-1 proteins in Jurkat T cells a bolished TCR-mediated CREB phosphorylation, whereas overexpression of the RSK2 serine/threonine kinase significantly potentiated TCR-mediate d CREB phosphorylation. Taken together, these experiments are consiste nt with a model in which TCR engagement leads to the rapid phosphoryla tion and activation of CREB via a signaling pathway involving the acti vation of p56(lck), PKC, Ras, Raf-1, MEK, and RSK2, Given the importan ce of CREB phosphorylation in normal T cell activation, this pathway m ay be an attractive target for the development of novel immunosuppress ive agents.