PHOSPHOINOSITIDE 3-OH KINASE ACTIVATES THE BETA(2) INTEGRIN ADHESION PATHWAY AND INDUCES MEMBRANE RECRUITMENT OF CYTOHESIN-1

Signal transduction through phosphoinositide 3-OH kinase (PI 3-kinase) has been implicated in the regulation of lymphocyte adhesion mediated by integrin receptors, Cellular phosphorylation products of PI 3-kina ses interact with a subset of pleckstrin homology (PH) domains, a modu le that has been shown to recruit proteins to cellular membranes. We h ave recently identified cytohesin-1, a cytoplasmic regulator of beta(2 ) integrin adhesion to intercellular adhesion molecule 1. We describe here that expression of a constitutively active PI 3-kinase is suffici ent far the activation of Jurkat cell adhesion to intercellular adhesi on molecule 1, and for enhanced membrane association of cytohesin-1. U p-regulation of cell adhesion by PI 3-kinase and membrane association of endogenous cytohesin-1 is abrogated by overexpression of the isolat ed cytohesin-1 PH domain, but not by a mutant of the PH domain which f ails to associate with the plasma membrane. The PH domain of Bruton's tyrosine kinase (Btk), although strongly associated with the plasma me mbrane, had no effect on either membrane recruitment of cytohesin-1 or on induction of adhesion by PI 3-kinase. Having delineated the critic al steps of the beta(2) integrin activation pathway by biochemical and functional analyses, we conclude that PI 3-kinase activates inside-ou t signaling of beta(2) integrins at least partially through cytohesin- 1.