Mechanisms of Regulation of GLUT4 Expression
GLUT4 is the primary glucose transport protein responsible for insulin-mediated glucose uptake in mammalian tissues. It is the predominant facilitative glucose transporter isoform expressed in muscle and in adipose tissue. As little as a 2- to 3-fold increase in GLUT4 expression has been shown to markedly enhance glucose uptake in both normal and diabetic mouse models, correcting the diabetic phenotype in the latter. Modulation of GLUT4 levels is therefore an attractive molecular target for therapeutic intervention insulin-resistant states, including diabetes mellitus. A straightforward approach to enhance GLUT4 expression is to increase the transcription rate of the gene. Glut4 gene expression is transcriptionally regulated in physiologic states such insulin-deficiency and exercise, and it is likely that a pharmacological intervention can be developed to enhance glut4 gene transcription provided that a suitable system for rapid screening of these compounds is developed. To reach these goals, we must first understand the molecular basis for transcriptional regulation of the glut4 gene. Using transgenic mice, we have shown that cis-elements regulating the human glut4 promoter are located within 895 bp located immediately 5' of the transcription initiation site. This region contains two major regulatory domains, referred to as Domain I and the MEF2 domain. In transgenic mice, these elements function cooperatively to support regulated expression of a reporter gene in GLUT4-expressing tissues. The MEF2 domain binds isoforms of the Myocyte Enhancer Factor 2 (MEF2) family of transcription factors, while Domain I binds GEF (Glut4 Enhancer Factor), a novel activator of transcription recently cloned in our laboratory. We propose that both the tissue-specific and the hormonal and metabolic regulation of the GLUT4 gene are carried out through these 2 regulatory domains. The primary goal of this proposal to understand the mechanisms of regulation of tissue-specific, hormonal and metabolic regulation of glut4 gene transcription. To achieve these goals, the following aims are proposed: 1) to define the functional domains of GEF; 2) examine the interaction between GEF and MEF2 proteins in cultured cells; and 3) to determine the mechanisms by which GEF and MEF2 regulate GIut4 gene regulation in transgenic mouse models of insulin deficiency or insulin resistance.