TRANSCRIPTIONAL REGULATION OF THE GENE FOR GLUCOSE-TRANSPORTER GLUT4 IN SKELETAL-MUSCLE - EFFECTS OF DIABETES AND FASTING

GLUT4 glucose transporter protein and mRNA levels in rat skeletal muscle are decreased with streptozotocin (STZ)-induced diabetes and increased by fasting, indicating that GLUT4 expression may be regulated at the pretranslational level. The purpose of the present study was to determine whether GLUT4 is subject to transcriptional regulation in skeletal muscle under the altered metabolic conditions of diabetes and fasting. Nuclei were isolated from red and white portions of the quadriceps and gastrocnemius/plantaris muscles of control, 7-day STZ-diabetic, and 3-day fasted rats. STZ-induced diabetes resulted in a 35% reduction in GLUT4 transcription in red skeletal muscle and thus accounted for a major portion of the corresponding 50% reduction in GLUT4 mRNA observed in red skeletal muscle. STZ-induced diabetes had no significant effect on GLUT4 transcription or mRNA in white skeletal muscle. Fasting, however, significantly increased both GLUT4 transcription (2.2-fold) and mRNA (2.9-fold) in white skeletal muscle with no change detected for either parameter in red skeletal muscle. The nearly 2-fold higher steady-state GLUT4 mRNA in red versus white skeletal muscle of control rats was not associated with any difference in basal transcription. These findings demonstrate that expression of the GLUT4 glucose transporter protein in skeletal muscle is subject to regulation in vivo at the level of transcription of the GLUT4 gene. In addition, GLUT4 transcription is regulated in a fiber type-specific manner in response to the metabolic challenges elicited by STZ-induced diabetes and fasting.