TNF‐<i>α</i> Antagonizes the Effect of Leptin on Insulin Secretion through FOXO1‐Dependent Transcriptional Suppression of LepRb in INS‐1 Cells
Yang Zhang, Weidong Jin, Dongyun Zhang, Changhai Lin, Haiyan He, Fengxin Xie, Lixia Gan, Weiling Fu, Lixiang Wu, Yongzhong Wu
Abstract
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 β cell dysfunction. However, molecular mechanisms for the association remained elusive. In this study, we first found that both leptin (10 nM) and TNF‐ α (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‐ α attenuated the inhibiting effect of leptin on GSIS of INS‐1E cells. Similarly, we found that TNF‐ α 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‐ α 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‐ α . Overexpression of constitutively active FOXO1 markedly aggravated the LepRb reduction by TNF‐ α treatment of INS‐1E cells, and the endogenous FOXO1 knockdown abolished the effect of TNF‐ α 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‐ α ‐induced LepRb downregulated in pancreatic β cells and demonstrate that transcriptional reduction of FOXO1 might be the primary mechanism underlying TNF‐ α promoting INS‐1E leptin resistance and β cell dysfunction. Conclusions . Our current studies based on INS‐1E cells in vitro indicate that the inflammatory factor TNF‐ α 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 β cells, and FOXO1 may be a novel target for treating β cell dysfunction in obesity‐induced hyperinsulinemia and T2DM.