Abstract
The transcription factor MafA is a key regulator of insulin gene expression and maturation of islet β cells. Despite its importance, the regulatory mechanism of MafA gene expression is still unclear. To identify the transcriptional regulators of MafA, we examined various transcription factors, which are potentially involved in β cell differentiation. An adenovirus-mediated overexpression study clearly demonstrated that Onecut1 suppresses the promoter activity of MafA through the Foxa2-binding cis-element on the MafA enhancer region (named area A). However, ChIP analysis showed that Foxa2 but not Onecut1 could directly bind to area A. Furthermore, overexpression of Onecut1 inhibited the binding of Foxa2 onto area A upon ChIP analysis. Importantly, insertion of a mutation in the Foxa2-binding site of area A significantly decreased the promoter activity of MafA. These findings suggest that Onecut1 suppresses MafA gene expression through the Foxa2-binding site. In the mouse pancreas, MafA expression was first detected at the latest stage of β cell differentiation and was scarcely observed in Onecut1-positive cells during pancreas development. In addition, Onecut1 expression was significantly increased in the islets of diabetic db/db mice, whereas MafA expression was markedly decreased. The improved glucose levels of db/db mice with insulin injections significantly reduced Onecut1 expression and rescued the reduction of MafA expression. These in vivo experiments also suggest that Onecut1 is a negative regulator of MafA gene expression. This study implicates the novel role of Onecut1 in the control of normal β cell differentiation and its involvement in β cell dysfunction under diabetic conditions by suppressing MafA gene expression.
Highlights
Opportune expression of MafA is essential for the development and function of pancreatic  cells
Excessive Expression of Onecut1 or Foxa2 Reduces MafA Gene Expression—We first evaluated various transcription factors involved in pancreas development as candidates for the transcriptional regulator of MafA gene expression
Consistent with its importance in islet  cells, MafA is expressed only in insulin-producing cells during pancreas development that are destined to populate the islet  cells, which is an expression pattern not found for any other characterized transcription factors
Summary
Opportune expression of MafA is essential for the development and function of pancreatic  cells. MafA can be detected only in insulin-positive cells during the pre- and postnatal period, both Pdx and Neurod are expressed in various cell types in the pancreas [11,12,13,14,15,16] These findings suggested that MafA is one of the key regulators of insulin gene expression, and its own expression is precisely regulated by other transcription factors involved in pancreas and islet development. In the islet  cells, Foxa binds to the conserved cis-acting elements of the MafA and Pdx genes, and inserting of a mutation in its binding site reduced the transcription activity of these genes [28, 29] These findings suggest that Foxa positively regulates MafA and Pdx gene expression and plays a key role in  cell function. These findings suggest a novel role of Onecut in the regulation of normal  cell differentiation and function through MafA gene expression
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