PHOSPHATIDYLINOSITOL 3-KINASE IS AN EARLY INTERMEDIATE IN THE G-BETA-GAMMA-MEDIATED MITOGEN-ACTIVATED PROTEIN-KINASE SIGNALING PATHWAY

The beta gamma-subunit of G(i) mediates mitogen-activated protein (MAP ) kinase activation through a signaling pathway involving Shc tyrosine phosphorylation, subsequent formation of a multiprotein complex inclu ding Shc, Grb2, and Sos, and sequential activation of Ras, Raf, and ME K. The mechanism by which G beta gamma mediates tyrosine phosphorylati on of Shc, however, is unclear. This study assesses the role of phosph atidylinositol 3-kinase (PI-3K) in G beta gamma-mediated MAP kinase ac tivation, We show that G(i)-coupled receptor- and G beta gamma-stimula ted MAP kinase activation is attenuated by the PI-3K inhibitors wortma nnin and LY294002 or by overexpression of a dominant negative mutant o f the p85 subunit of PI-3K, Wortmannin and LY294002 also inhibit G(i)- coupled receptor-stimulated Ras activation. The PI-3K inhibitors do no t affect MAP kinase activation stimulated by overexpression of Sos, a constitutively active mutant of Ras, or a constitutively active mutant of MEK. These results demonstrate that PI-3K activity is required in the G beta gamma-mediated MAP kinase signaling pathway at a point upst ream of Sos and Ras activation.