The role of Nuclear Respiratory Factor‐1 in regulating GLUT4 expression

Overexpression of NRF‐1 in transgenic mice has a dramatic increase in GLUT4 and myocyte enhancing factor (MEF)‐2 (Baar et al., 2003). The increase in GLUT4 was surprising since NRF‐1 has no binding sites on the Glut4 gene; hence the question is how does NRF‐1 regulate GLUT4? Our hypothesis is that GLUT4 expression is a secondary effect of NRF‐1 on the Mef2a gene, which has NRF‐1 binding sites. The aim of this study was to investigate the molecular mechanism leading to GLUT4 upregulation by NRF‐1. C2C12 cells were used to develop a tetracycline based gene expression system (Tet‐On) to form a double stable cell line which overexpresses NRF‐1 upon activation by Doxycycline. Our results show that NRF‐1 overexpression increases the binding of NRF‐1 to the Mef2a and δAlas promoters and MEF2A to Glut4 promoter. Furthermore NRF‐1 overexpression resulted in increase expressions for MEF2A, GLUT4 and δALAS proteins. Using small interference RNA directed against MEF2A, overexpression of NRF‐1 resulted in downregulation of GLUT4 protein expression. From these results we could conclude that NRF‐1 overexpression increases GLUT4 via Mef2a binding resulting in increased MEF2A which in turn increases its binding to the Glut4 gene resulting in GLUT4 expression. The parallel increases in mitochondrial and GLUT4 expression are essential in alleviating metabolic disease such as type II diabetes.