Insulin regulates the novel adipokine adipolin/CTRP12: in vivo and ex vivo effects.

There has been intense interest in the adipokines of the C1q complement/TNF-related protein (CTRP) superfamily. Adipolin (CTRP12) has been described as a novel adipokine, abundantly expressed in adipose tissue with insulin-sensitising and anti-inflammatory effects. We wanted to investigate the effects of acute and chronic hyperinsulinaemia on circulating adipolin concentrations (ELISA) via a prolonged insulin-glucose infusion in humans. We also examined the effects of insulin and the insulin sensitiser, rosiglitazone, on adipolin concentrations (western blotting) in human adipose tissue explants. We found that hyperinsulinaemic induction in healthy lean human subjects significantly increased circulating levels of adipolin (P<0.05 and P<0.01). Furthermore, in subcutaneous adipose tissue explants, insulin significantly increased adipolin protein expression and secretion (P<0.05 and P<0.01). This effect was attenuated by the phosphatidylinositol 3-kinase inhibitor, LY294002 (P<0.05). Moreover, the insulin-sensitising peroxisome proliferator-activated receptor γ (PPARγ) agonist, rosiglitazone, significantly increased adipolin protein expression and secretion in subcutaneous adipose tissue explants (P<0.05 and P<0.01). This effect was inhibited by the PPARγ antagonist, GW9662 (P<0.05). Our data provide novel insights into adipolin physiology in human subjects.