Increased levels of calprotectin in obesity are related to macrophage content: impact on inflammation and effect of weight loss

Calprotectin has been recently described as a novel marker of obesity. The aim of this study was to determine the circulating concentrations and expression levels of calprotectin subunits (S100A8 and S100A9) in visceral adipose tissue (VAT), exploring its impact on insulin resistance and inflammation and the effect of weight loss. We included 53 subjects in the study. Gene expression levels of the S100A8/A9 complex were analyzed in VAT as well as in both adipocytes and stromovascular fraction cells (SVFCs). In addition, circulating calprotectin and soluble receptor for the advanced glycation end product (sRAGE) concentrations were measured before and after weight loss achieved by Roux-en-Y gastric bypass (RYGB) (n = 26). Circulating concentrations and VAT expression of S100A8/A9 complex were increased in normoglycemic and type 2 diabetic obese patients (P < 0.01) and associated with markers of inflammation (P < 0.01). Oppositely, concentrations of sRAGE were significantly lower (P < 0.001) in both obese groups compared to lean volunteers. Elevated calprotectin levels in obese patients decreased (P < 0.00001) after RYGB, whereas sRAGE concentrations tended to increase. Calprotectin was mainly expressed by SVFCs, and its expression was significantly correlated (P < 0.01) with mRNA levels of the monocyte-macrophage-related molecules macrophage-specific antigen CD68 (CD68), monocyte chemotactic protein 1 (MCP1), integrin α-M (CD11B), and NADPH oxidase 2 (NOX2). Tumor necrosis factor-α treatment significantly enhanced (P < 0.05) the mRNA levels of S100 calcium-binding protein A8 (S100A8) of human visceral adipocytes. The increased levels of calprotectin in obesity and obesity-associated type 2 diabetes, its positive association with inflammation as well as the higher expression levels in the SVFCs in VAT suggests a potential role of this protein as a chemotactic factor in the recruitment of macrophages to VAT, increasing inflammation and the development of obesity-associated comorbidities.