TNF-alpha Antagonizes the Effect of Leptin on Insulin Secretion through FOXO1-Dependent Transcriptional Suppression of LepRb in INS-1 Cells

Proinflammatory cytokines play a causal role in the development of hyperinsulinemia and T2MD. FOXO1, a transcription factor which is known to enhance proinflammation, was recently shown to be involved in obesity-induced beta cell dysfunction. However, molecular mechanisms for the association remained elusive. In this study, we first found that both leptin (10 nM) and TNF-alpha (20 ng/ml) significantly inhibited glucose-stimulated insulin secretion (GSIS) of INS-1E cells. When in combination, the GSIS function of INS-1E cells was significantly increased compared with that of the leptin alone treatment, indicating that TNF-alpha attenuated the inhibiting effect of leptin on GSIS of INS-1E cells. Similarly, we found that TNF-alpha has the same inhibitory effect on leptin in regulating insulin synthesis and secretion, and the survival and apoptosis of insulin cells. Further studies showed that TNF-alpha blocks leptin pathway by reducing the expression of leptin receptor (LepRb, also called OBRb) and inhibiting the activation of STAT3, a key molecule involved in the leptin signaling pathway in INS-1E cells. Besides, the downregulated expression of phosphorylated FOXO1 was found to be involved in the possible mechanism of TNF-alpha. Overexpression of constitutively active FOXO1 markedly aggravated the LepRb reduction by TNF-alpha treatment of INS-1E cells, and the endogenous FOXO1 knockdown abolished the effect of TNF-alpha on INS-1E cells. Furthermore, we have proved that FOXO1 could directly bind to the promoter of LepRb as a negative transcription regulator. Taken together, the results of this study reveal that TNF-alpha-induced LepRb downregulated in pancreatic beta cells and demonstrate that transcriptional reduction of FOXO1 might be the primary mechanism underlying TNF-alpha promoting INS-1E leptin resistance and beta cell dysfunction. Conclusions. Our current studies based on INS-1E cells in vitro indicate that the inflammatory factor TNF-alpha plays an important role in the development of INS-1E leptin resistance and glucose metabolism disorders, probably through FOXO1-induced transcription reduction of LepRb promoter in pancreatic beta cells, and FOXO1 may be a novel target for treating beta cell dysfunction in obesity-induced hyperinsulinemia and T2DM.