Ser312 Fetuin-A Phosphorylation and Its Association with Serum Lipids in Metabolic Syndrome

Abstract

Fetuin-A (also known as alpha 2-HS-glycoprotein), secreted by the liver into circulation, interacts with the insulin receptor, and is a physiological inhibitor of insulin receptor tyrosine kinase activity in vitro, in intact cells and in vivo in animals. Serum fetuin-A levels have been shown to be associated with insulin resistance, obesity, and metabolic syndrome in animals and humans. Fetuin-A null mice demonstrate improved insulin sensitivity and resistance to diet-induced obesity. Phosphorylation status of fetuin-A has been shown to be critical for its inhibitory activity on insulin action. However, there are no reports on fetuin-A phosphorylation status in insulin resistant conditions or on the molecular characterization of two phosphorylation sites (Ser120 and Ser312). This study examines serum total fetuin-A and Ser312-fetuin-A phosphorylation status in individuals with metabolic syndrome, both before and after treatment with the lipid-lowering drug, Niaspan, and correlates these with changes in serum lipids and markers of insulin sensitivity and inflammation. Additionally, using mutation analysis, we have examined the role of phosphorylation on Ser312-fetuin-A on insulin action. Fifteen sedentary, male participants, who met the NCEP ATP III criteria for metabolic syndrome, were treated with extended-release niacin (Niaspan) for 6 weeks. Serum concentrations of phosphorylated (Ser312) fetuin-A were positively correlated with triglycerides in MetS. Serum total fetuin-A and phosphofetuin-A concentrations decreased significantly following Niaspan treatment. Changes in fetuin-A concentrations were correlated with changes in triglyceride concentrations after Niaspan treatment. Additionally, the change in high density lipoprotein cholesterol following Niaspan intervention was negatively correlated with the change in serum fetuin-A and phosphorylated fetuin-A concentrations. Interestingly, serum cortisol levels were significantly elevated following Niaspan intervention. The change in cortisol was significantly correlated with the change in serum non-esterified fatty acids (NEFA), suggesting that increased cortisol and NEFA concentrations may contribute to the mild hyperglycemia and insulin resistance observed in niacin-treated individuals. Molecular characterization using Ser312Ala-fetuin-A mutant, which was devoid of phosphorylation, suggested that phosphorylation on Ser312-fetuin-A is critical for fetuin-A’s inhibition of insulin signaling through MAPK and Akt. This is the first report demonstrating that serum total- and phosphorylated Ser312-fetuin-A levels are amenable to intervention by Niaspan treatment and are correlated with changes in serum lipids. We demonstrate that while phosphorylation on Ser312 site of fetuin-A is critical, phosphorylation on Ser120 may be required for fetuin-A’s full inhibitory activity.