INSULIN ACTIVATES STAT3 INDEPENDENTLY OF P21RAS-ERK AND PI-3K SIGNAL-TRANSDUCTION

The binding of insulin to its receptor initiates multiple signal trans duction pathways regulating such diverse processes as proliferation, d ifferentiation, glucose transport, and glycogen metabolism, The STAT-f amily of transcription factors has been demonstrated to play a critica l role in gene induction by a variety of hemopoietic cytokines and hor mones, Furthermore, constitutive activation of STATs is observed in tr ansformed cells, Here we describe activation of a transcriptional comp lex binding to a consensus STAT-transcriptional element in response to insulin challenge, This complex is induced rapidly after tyrosine aut ophosphorylation of the insulin receptor, and is sustained for several hours, Supershift analysis of the insulin-induced complex reveals tha t it specifically contains the transcription factor Stat3, DAN binding of this complex is inhibited by pre-incubation with tyrosine, but not serine/threonine protein kinase inhibitors, whereas transcriptional a ctivation is inhibited by both, Utilising a dominant negative mutant o f p21ras we demonstrate that both insulin-induced Stat3 DNA-binding an d also transactivation do not require p21ras, Furthermore, although pr evious studies have suggested a role for MAP kinases (ERKs) and PI-3K in STAT activation, utilising the specific MEK inhibitor PD098059 and the PI-3K inhibitor wortmannin, we demonstrate that activation of ERKs or PI-3K are not required for insulin induced Stat3 phosphorylation o r transactivation.