Visfatin – an activator of the insulin signalling pathway?

Visfatin, also known as PBEF, was recently identified as adipocytokine. It was found to act as a ligand of the insulin receptor and to exert insulinomimetic cellular effects. In this study our group examined the signalling of visfatin through the insulin receptor and subsequent activation of the insulin signalling pathway. Different cell modells including R-IR (IGF-1R knock-out mouse fibroblasts overexpressing the human insulin receptor isoform A), HepG2 (expressing both insulin receptor isoform A and B) and SGBS (a human preadipocyte cell line mainly expressing the insulin receptor isoform B) were stimulated with commercially available visfatin expressed in E. coli or in Hek293 as well as with supernatant from COS-7-PBEF (transiently transfected with the plasmid pSRα-PBEF). Activation of components of the classical insulin signalling pathway was assessed by immunoprecipitation and/or immunoblotting. In contrast to stimulation with 10 nM insulin, we could not detect any tyrosine-phosphorylation of the insulin receptor after administering 10 or 100 nM visfatin in R-IR, HepG2 or SGBS cells. There was also no detectable activation of the downstream signalling molecule Akt/PKB. To exclude the possibility that the failure to stimulate signalling was due to inactivation of the purified visfatin, we exposed R-IR cells to COS-7-PBEF conditioned medium either freshly taken from cell culture or after one freeze-and-thaw cycle. This did also not result in activation of insulin signalling.

In conclusion, our group cannot confirm activation of the insulin receptor or Akt/PKB by visfatin stimulation.