| Summary |
The purpose of this study was to test the hypothesis that calcium/calmodulin activated protein kinase (CaMK) and/or calcineurin regulate GLUT4 gene expression in response to altered cellular calcium and to learn ifthis regulation was mediated by myocyte enhancer factor 2 (MEF2) and/or GLUT4 enhancer factor (GEF). To investigate the ability of CaMK and calcineurin to regulate GLUT4, we measured their effect on GLUT4 expression in states where endogenous CaMK and calcineurin activity levels would differ from the basal state. Transgenic mice were chronically exercised and CaMK was shown to activate GLUT4 expression to a similar extent as exercise. There was no further increase in GLUT4 after exercise in CaMK-transgenic mice, suggesting that CaMK activated the same stimulatory pathway as exercise. Nuclear and total MEF2 content was increased both by CaMK and exercise. GEF was not affected by exercise, but was increased in response to CaMKTV expression. There was no effect of exercise or of CaMKIV expression on the MEF2 corepressor, histone deacetylase 5 (HDAC5). CaMKIV overexpression was not able to maintain GLUT4 content in denervated muscle. We proposed that constitutively-active CaMK and/or calcineurin expression would sustain GLUT4 expression if decreased signaling through those enzymes is the cause for the denervation-induced decrease in GLUT4. To determine the mechanism of decreased GLUT4 expression in denervated muscle, assessments were made of MEF2 and GEF binding to the GLUT4 promoter and of their protein content. Denervation decreased GEF binding to the promoter and the content of GEF in the nucleus, but there was no change in either MEF2 binding or MEF2 protein content. To determine the denervation responsive element(s) ofthe GLUT4 promoter, mice expressing a reporter gene driven by different lengths of GLUT4 promoter were denervated. Several different promoter/reporter gene constructs were analyzed, and by measuring reporter gene expression in their denervated muscle, we concluded that the denervation responsive element must be in the basal promoter region. |
