Synergistic activation of a family of phosphoinositide 3-kinase via G-protein coupled and tyrosine kinase-related receptors

Abstract

Phosphoinositide 3-kinase (PI 3-kinase) is a key signaling enzyme implicated in a variety of receptor-stimulated cell responses. Stimulation of receptors possessing (or coupling to) protein-tyrosine kinase activates heterodimeric PI 3-kinases, which consist of an 85-kDa regulatory subunit (p85) containing Src-homology 2 (SH2) domains and a 110-kDa catalytic subunit (p110α or p110β). Thus, this form of PI 3-kinases could be activated in vitro by a phosphotyrosyl peptide containing a YMXM motif that binds to the SH2 domains of p85. Receptors coupling to αβγ-trimeric G proteins also stimulate the lipid kinase activity of a novel p110γ isoform, which is not associated with p85, and thereby is not activated by tyrosine kinase receptors. The activation of p110γ PI 3-kinase appears to be mediated through the βγ subunits of the G protein (Gβγ). In addition, rat liver heterodimeric PI 3-kinases containing the p110β catalytic subunit are synergistically activated by the phosphotyrosyl peptide plus Gβγ. Such enzymatic properties were also observed with a recombinant p110β/p85α expressed in COS-7 cells. In contrast, another heterodimeric PI 3-kinase consisting of p110α and p85 in the same rat liver, together with a recombinant p110α/p85α, was not activated by Gβγ, though their activities were stimulated by the phosphotyrosyl peptide. Synergistic activation of PI 3-kinase by the stimulation of the two major receptor types was indeed observed in intact cells, such as chemotactic peptide ( N-formyl–Met–Leu–Phe) plus insulin (or Fcγ II) receptors in differentiated THP-1 and CHO cells and adenosine (A1) plus insulin receptors in rat adipocytes. Thus, PI 3-kinase isoforms consisting of p110β catalytic and SH2-containing (p85 or its related) regulatory subunits appeared to function as a ‘cross-talk’ enzyme between the two signal transduction pathways mediated through tyrosine kinase and G protein-coupled receptors.