Stat 5B, activated by insulin in a Jak-independent fashion, plays a role in Glucokinase gene transcription

Stat proteins are SH2 domain-containing transcription factors that are acti vated by various cytokines and growth factors. In a previous work, we have identified Stat 5B as a substrate of the insulin receptor based on yeast tw o-hybrid and mammalian cell transfection studies. In the present study, we have approached the biological relevance of the interaction between the ins ulin receptor and the transcription factor Stat 5B. Firstly, we show that b oth insulin and insulin-like growth factor I lead to tyrosine phosphorylati on of Stat 5B, and this promotes binding of the transcription factor to the beta-casein promoter containing a Stat 5 binding site. Further, we demonst rate that insulin stimulates the transcriptional activity of Stat 5B. Activ ation of Stat 5B by insulin appears to be Jak2-independent, whereas Jak2 is required for GH-induced Stat: 5B activation. Hence the pathway by which St at 5B is activated by insulin is different from that used by GH. In additio n, by using Jak1- and Tyk2-deficient cells we exclude the involvement of bo th Jak1 and Tyk2 in Stat 5B activation by insulin. Taken together, our resu lts strengthen the notion that insulin receptor can directly activate Stat 5B. More importantly, we have identified a Stat 5 binding site in the human hepatic glucokinase promoter, and we show that insulin leads to a Stat 5B- dependent increase in transcription of a reporter gene carrying this promot er. These observations favor the idea that Stat 5B plays a role in mediatin g the expression of the glucokinase gene induced by insulin. As a whole, ou r results provide evidence for the occurrence of a newly identified circuit in insulin signaling in which the cell surface receptor is directly linked to nuclear events through a transcription factor. Further, we have reveale d an insulin target gene whose expression is, at least in part, dependent o n Stat 5B activation and/or binding.