The T-cell receptor regulates Akt (protein kinase B) via a pathway involving Rac1 and phosphatidylinositide 3-kinase

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