Expression of an insulin-responsive glucose transporter (GLUT4) minigene in transgenic mice: effect of exercise and role in glucose homeostasis.

The effects of a GLUT4 mini-transgene (containing 7 kb of 5' flanking and 1 kb of 3' flanking sequence and all exons and introns of the GLUT4 gene as well as a small foreign DNA tag) and of exercise training on expression of GLUT4 and glycemic control in mice were investigated. Transgenic mice harboring the minigene expressed < or = 2-fold the normal level of GLUT4 mRNA and protein in skeletal (gastrocnemius) muscle and adipose tissue. This modest tissue-specific increase in GLUT4 expression led to an unexpectedly rapid blood glucose clearance rate following oral glucose administration. In nontransgenic animals exercise caused a 1.5-fold increase in expression of GLUT4 mRNA and protein as well as a significant improvement of glycemic control. In transgenic animals harboring the minigene exercise increased expression of GLUT4 mRNA and protein derived from the minigene and endogenous gene and led to a further improvement of glycemic control. These findings indicate that the cis-regulatory element(s) controlling exercise-induced expression of the GLUT4 gene is located within the nucleotide sequence encompassed by the GLUT4 minigene. The fact that glycemic control is markedly improved by a relatively low level of expression of GLUT4 caused by the transfected minigene and is further enhanced by exercise in transgenic animals demonstrates that GLUT4 plays a pivotal role in glucose homeostasis in vivo. Of the effectors--i.e., cAMP, insulin, and arachidonic acid--known to down-regulate expression of GLUT4 by 3T3-L1 adipocytes in culture, only the decline in circulating arachidonate level in vivo correlated with up-regulation of GLUT4 caused by exercise.