Mechanisms regulating GLUT4 glucose transporter expression and glucose transport in skeletal muscle.

Skeletal muscle is a major glucose-utilizing tissue in the absorptive state and the major glucose transporter expressed in muscle in adulthood is GLUT4. GLUT4 expression is exquisitely regulated in muscle and this seems important in the regulation of insulin-stimulated glucose uptake by this tissues. Thus, muscle GLUT4 overexpression in transgenic animals ameliorates insulin resistance associated with obesity or diabetes. Recent information indicates that glut4 gene transcription is regulated by a number of factors in skeletal muscle that include MEF2, MyoD myogenic proteins, thyroid hormone receptors, Kruppel-like factor KLF15, NF1, Olf-1/Early B cell factor and GEF/HDBP1. In addition, studies in vivo indicate that under normal conditions the activity of the muscle-specific GLUT4 enhancer is low in adult skeletal muscle compared with the maximal potential activity that it can attain at high levels of the MRF transcription factors, MEF2, and TRalpha1. This finding indicates that glut4 transcription may be greatly up-regulated via activation of this enhancer through an increase in the levels of expression or activity of these transcription factors. Understanding the molecular basis of the expression of glut4 will be useful for the appropriate therapeutic design of treatments for insulin-resistant states. The nature of the intracellular signals that mediate the stimulation of glucose transport in response to insulin or exercise is also reviewed.