Insulin inhibits leptin receptor signalling in HEK293 cells at the level of janus kinase-2:: a potential mechanism for hyperinsulinaemia-associated leptin resistance

Aims/hypothesis. Leptin resistance in obese humans seems to be predominantly caused by signalling abnormalities at the post receptor level. Leptin resistance in obese individuals is frequently associated with insulin resistance and pronounced hyperinsulinaemia indicating a negative crosstalk of the insulin and leptin signalling chain. Methods. This hypothesis was tested using a cell model of peripheral leptin signalling, i.e. insulin-secreting cell lines (RINr1046-38). Mechanisms for a crosstalk between the insulin and leptin signalling pathway were also studied in rat-1 and HEK293 cells overexpressing elements of the insulin and leptin signalling chain. Results. The effects of leptin on insulin secretion are completely cancelled by a 4-h preincubation with 1 nmol/l insulin, supporting the hypothesis of a negative crosstalk of insulin and leptin signalling. We investigated the potential molecular mechanisms in more detail in HEK293 cells and Rat-1 fibroblasts that overexpressed proteins of the insulin and leptin signalling chain. Leptin (60 ng/ml) stimulated autophosphorylation of JAK-2 in HEK 293 cells. This leptin effect could be inhibited by simultaneous treatment of cells with insulin. Furthermore, overexpression of the insulin receptor in HEK 293 cells clearly reduced JAK-2 phosphorylation and led further downstream to a diminished phosphatidylinositol 3-kinase activity. The inhibitory effect of the insulin signal could be partially prevented by transfection of the cells with an inactive mutant of the tyrosine phosphatase SHP-1. Conclusion/interpretation. In summary, our data suggest that the insulin receptor signalling pathway interferes with leptin signalling at the level of JAK-2. Inhibition of JAK-2 phosphorylation might occur through SHP-1-dependent pathways, indicating that hyperinsulinaemia contributes to the pathogenesis of leptin resistance.