REQUIREMENT FOR PHOSPHOINOSITIDE 3-OH KINASE IN GROWTH-HORMONE SIGNALING TO THE MITOGEN-ACTIVATED PROTEIN-KINASE AND P70(S6K) PATHWAYS

Pituitary growth hormone (GH) co-ordinately stimulates three distinct signalling pathways in 3T3-F442A preadipocytes, the STAT (signal trans ducer and activator of transcription) pathway, the mitogen-activated p rotein (MAP) kinase cascade and p70(s6k). The mechanisms linking the G H receptor to these signals have not been fully identified. In this st udy we have examined the role of phosphoinositide 3-OH kinase (PI 3-ki nase). Pretreatment of cells with wortmannin, a specific inhibitor of PI 3-kinase, prevented the activation of p70(s6k) and partially inhibi ted the activation of p42 and p44 MAP kinases by GH. In contrast, wort mannin failed to appreciably affect the GH-stimulated tyrosyl phosphor ylation of JAK-2 or STAT-1, GH transiently increased the activity of P I 3-kinase recovered in antiphosphotyrosine immunoprecipitates. In add ition, several tyrosyl-phosphorylated proteins were specifically adsor bed from lysates of cells exposed to GH by a glutathione S-transferase fusion protein containing the 85 kDa regulatory subunit of PI 3-kinas e. GH also induced an increase in the PI 3-kinase activity associated with both JAK-2 and insulin receptor substrate-1 (IRS-I) immunoprecipi tates. These results establish PI 3-kinase as an important mediator of GH signalling to the MAP kinase and p70(s6k) pathways and suggest tha t PI 3-kinase is activated by a mechanism involving JAK-2 and IRS-1.