SIGNAL TRANSDUCER AND ACTIVATOR OF TRANSCRIPTION-3 SERINE PHOSPHORYLATION BY INSULIN IS MEDIATED BY A RAS RAF/MEK-DEPENDENT PATHWAY/

We recently reported that insulin stimulation results in the serine ph osphorylation of STAT3 (signal transducer and activator of transcripti on-3). In the present study, we identified serine 727 as the site of i nsulin-stimulated STAT3 serine phosphorylation. This phosphorylation e vent occurs independent of tyrosine phosphorylation. Furthermore, inte rleukin-6-induced tyrosine phosphorylation can occur independent of se rine phosphorylation, demonstrating that these two phosphorylation pat hways are mechanistically unrelated. Selective activation of the JNK a nd p38 family of mitogen-activated protein (MAP) kinases by anisomycin treatment did not result in the phosphorylation of STAT3. In contrast , activation of the ERK MAP kinase pathway with both insulin and osmot ic shock resulted in the serine phosphorylation of STAT3. In addition, expression of a dominant-interfering Ras mutant (N17Ras) or treatment with the specific MEK inhibitor (PD98059) prevented the insulin stimu lation of STAT3 serine phosphorylation. Blockade of ERK activation by expression of the MAP kinase phosphatase (MKP-1) had no effect on insu lin-stimulated STAT3 serine phosphorylation. Together, these data demo nstrate that the insulin-stimulated serine phosphorylation of STAT3 oc curs by a MEK-dependent pathway that is independent of ERK activation.