Interaction between leptin and insulin signaling pathways differentially affects JAK-STAT and PI 3-kinase-mediated signaling in rat liver

Chronic leptin treatment markedly enhances the effect of insulin on hepatic glucose production unproportionally with respect to body weight loss and increased insulin sensitivity. In the present study the crosstalk between insulin and leptin was evaluated in rat liver. Upon stimulation of JAK2 tyrosine phosphorylation, leptin induced JAK2 co-immunoprecipitation with STAT3, STAT5b, IRS-1 and IRS-2. This phenomenon parallels the leptin-induced tyrosine phosphorylation of STAT3, STAT5b, IRS-1 and IRS-2. Acutely injected insulin stimulated a mild increase in tyrosine phosphorylation of JAK2, STAT3 and STAT5b. Leptin was less effective than insulin in stimulating IRS phosphorylation and their association with PI 3-kinase. Simultaneous treatment with both hormones yielded no change in maximal phosphorylation of STAT3, IRS-1, IRS-2 and Akt, but led to a marked increase in tyrosine phosphorylation of JAK2 and STAT5b when compared with isolated administration of insulin or leptin. This indicates that there is a positive crosstalk between insulin and leptin signaling pathways at the level of JAK2 and STAT5b in rat liver.