The Roles And Mechanisms Of MiR-24 In Pancreatic β-Cell Impairment

Diabetes is a systemic metabolic syndrome due to the absolute or relative deficiency of insulin, the main characteristic of which is chronic hyperglycemia. At present, it is believed that the dysfunction of pancreatic β cells is the main cause of type 2 diabetes mellitus. With the occurrence of excess nutrition and obesity, free fatty acid levels in human plasma are significantly increased. Lipotoxity is an important pathological factor leading to the failure of pancreatic β cells, but the mechanisms of the impairment have not been fully elucidated.In this study, miRNA-24(miR-24) was highly expressed in pancreatic β cells and further upregulated in islets from obesity related diabetic mice (db/db and HFD mice). Saturated fatty acid palmitate treatment, in order to mimic the high blood lipid status in vivo, could also increase the expression of miR-24 in human primary pancreatic islets and a mouse β cell line MIN6 cells. Overexpression of miR-24 inhibited β-cell proliferation and insulin secretion. MiR-24 mediated inhibition of cell proliferation was dependent on the decrease of Cdk4 and Ccnd3 protein levels. To better explain the mechanisms of the phenomenon above, our study employed the Affymetrix gene chip technology. MRNA microarray revealed 351 genes were downregulated at least 1.5-fold by miR-24 overexpression directly or indirectly. Gene ontology analysis for the 351 downregulated genes by DAVID Bioinformatics Resources 6.7 demonstrated that upregulation of miR-24 was relevant to cell cycle and insulin secretion in response to external stimuli, which also supported the phenotype caused by miR-24 overexpression observed above. Using bioinformatic analysis combined with luciferase-based promoter activity assays and quantitative real-time PCR assays, we identified multiple direct targets of miR-24, including 4 MODY genes, such as Hnfla and Neurodl. Silencing either of Hnfla and Neurodl mimicked the cellular phenotype caused by miR-24 overexpression, whereas overexpression of Hnfla or Neurodl could reverse β-cell proliferation inhibition and GSIS dysfunction caused by miR-24 overexpression. Therefore, Hnfla and Neurodl played leading roles in the effects of miR-24. It was also found that inhibition of miR-24 expression in islets obtained from HFD-fed mice could significantly restore GSIS. In addition, hyperglycemia and hydrogen peroxide could also increase miR-24 expression and downregulate miR-24 targets Neurod1 and Hnfla.In conclusion, this study first reveal the role of the miR-24/MODY regulatory pathway in pancreatic β-cell dysfunction in type 2 diabetes. This study will be helpful to further clarify the molecular mechanisms of pancreatic β-cell impairment in type 2 diabetes, especially in the high-fat status, and provide a new theoretical basis for the prevention and treatment of diabetes.