The T-Cell Receptor Regulates Akt (Protein Kinase B) via a Pathway Involving Rac1 and Phosphatidylinositide 3-Kinase

Abstract

The serine/threonine kinase Akt (also known as protein kinase B) (Akt/PKB) is activated upon T-cell antigen receptor (TCR) engagement or upon expression of an active form of phosphatidylinositide (PI) 3-kinase in T lymphocytes. Here we report that the small GTPase Rac1 is implicated in this pathway, connecting the receptor with the lipid kinase. We show that in Jurkat cells, activated forms of Rac1 or Cdc42, but not Rho, stimulate an increase in Akt/PKB activity. TCR-induced Akt/PKB activation is inhibited either by PI 3-kinase inhibitors (LY294002 and wortmannin) or by overexpression of a dominant negative mutant of Rac1 but not Cdc42. Accordingly, triggering of the TCR rapidly stimulates a transient increase in GTP-Rac content in these cells. Similar to TCR stimulation, L61Rac-induced Akt/PKB kinase activity is also LY294002 and wortmannin sensitive. However, induction of Akt/PKB activity by constitutive active PI 3-kinase is unaffected when dominant negative Rac1 is coexpressed, placing Rac1 upstream of PI 3-kinase in the signaling pathway. When analyzing the signaling hierarchy in the pathway leading to cytoskeleton rearrangements, we found that Rac1 acts downstream of PI 3-kinase, a finding that is in accordance with numerous studies in fibroblasts. Our results reveal a previously unrecognized role of the GTPase Rac1, acting upstream of PI 3-kinase in linking the TCR to Akt/PKB. This is the first report of a membrane receptor employing Rac1 as a downstream transducer for Akt/PKB activation.

ACKNOWLEDGMENTS

This work was supported by a Wellcome Foundation Grant DMIH 3468. E.M.G. is a member of the INSERM organization. C.A. was supported from a fellowship from the Fondation pour la Recherche Medicale.

We are grateful to Sarah Beach for initial technical assistance, Ludowijk Dekker for help with radioactive experiments, Alan Hall and Julian Downward for various plasmids, Alain Trautmann and Peter Parker for helpful discussions, Bart Vanhaesebroeck and Jacques Nunes for advice, and Doreen Cantrell and Robert Lechler for constant support.