Retinol binding protein 4 affects the adipogenesis of porcine preadipocytes through insulin signaling pathways

Retinol binding protein 4 (RBP4), a novel cytokine, is mainly secreted by hepatocytes and adipocytes. RBP4 reportedly induces insulin resistance and RBP4 secretion is increased in the adipocytes of animals or humans with type 2 diabetes, obesity, and metabolic syndrome, but its role in preadipocyte differentiation remains unclear. In this study, we investigated the effect of RBP4 on the differentiation of porcine preadipocytes into adipocytes. The results suggest that RBP4 significantly suppresses the differentiation of porcine preadipocytes into adipocytes, including those treated with the hormone cocktail methylisobutylxanthine-dexamethasone-insulin. RBP4 also weakened the activity of normal threonine 308, the phosphorylation of serine/threonine kinase AKT, and downstream insulin signaling, including the mammalian target of rapamycin (mTOR) and β-catenin. Moreover, the activation of insulin signaling mediated by knockdown RBP4 in porcine preadipocytes was recovered in the suppression of LY294002. RBP4 also had a suppressive effect on the differentiation of porcine preadipocytes by decreasing the activation of insulin signaling pathways.