Insulin stimulates the serine phosphorylation of the signal transducer activator of transcription (STAT3) isoform

Insulin stimulation of Chinese hamster ovary cells expressing the human insulin receptor differentiated 3T3L1 adipocytes resulted in a time-dependent reduction in the SDS-polyacrylamide gel electrophoretic mobility of STAT3. The decreased STAT3 mobility initially occurred by 2 min and was quantitative by 5 min. In addition, the change in STAT3 mobility was concentration-dependent and was detectable at 0.3 nM insulin with maximal effect between 1 and 3 nM. Although both these cell types also express the STAT1α, STAT1β, STAT5, and STAT6 isoforms, only STAT3 was observed to undergo an insulin-dependent reduction in mobility. Immunoprecipitation of STAT1 and STAT3 from ³²P-labeled cells demonstrated that only STAT3 was phosphorylated in response to insulin whereas phosphoamino acid analysis indicated that this phosphorylation event occurred exclusively on serine residues. Furthermore, treatment of cell extracts with alkaline phosphatase reversed the insulin-stimulated decrease in STAT3 mobility. Together, these data demonstrate that insulin is a specific activator of STAT3 serine phosphorylation without affecting the other STAT isoforms.