MicroRNA-30d Induces Insulin Transcription Factor MafA and Insulin Production by Targeting Mitogen-activated Protein 4 Kinase 4 (MAP4K4) in Pancreatic β-Cells

MicroRNAs (miRNAs) represent small noncoding RNAs that play a role in many diseases, including diabetes. miRNAs target genes important for pancreas development, beta-cell proliferation, insulin secretion, and exocytosis. Previously, we documented that microRNA-30d (miR-30d), one of miRNAs up-regulated by glucose, induces insulin gene expression in pancreatic beta-cells. Here, we found that the induction of insulin production by overexpression of miR-30d is associated with increased expression of MafA, a beta-cell-specific transcription factor. Of interest, overexpression of miR-30d prevented the reduction in both MafA and insulin receptor substrate 2 (IRS2) with TNF-alpha exposure. Moreover, we identified that mitogen-activated protein 4 kinase 4 (MAP4K4), a TNF-alpha-activated kinase, is a direct target of miR-30d. Overexpression of miR-30d protected beta-cells against TNF-alpha suppression on both insulin transcription and insulin secretion through the down-regulation of MAP4K4 by the miR-30d. Adecrease of miR-30d expression was observed in the islets of diabetic db/db mice, in which MAP4K4 expression level was elevated. Our data support the notion that miR-30d plays multiple roles in activating insulin transcription and protecting beta-cell functions from impaired by proinflammatory cytokines and underscore the concept that miR-30d may represent a novel pharmacological target for diabetes intervention.