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 GLUT 4 gene as well as a small foreign DNA tag) and of exercise training on expression of GLUT4 and glycemic control in mice were investigated. T ransgenic mice harboring the minigene expressed less than or equal to 2-fold the normal level of GLUT4 mRNA and protein in skeletal (gastroc nemius) muscle and adipose tissue. This modest tissue-specific increas e in GLUT4 expression led to an unexpectedly rapid blood glucose clear ance rate following oral glucose administration. In nontransgenic anim als exercise caused a 1.5-fold increase in expression of GLUT4 mRNA an d protein as well as a significant improvement of glycemic control. In transgenic animals harboring the minigene exercise increased expressi on of GLUT4 mRNA and protein derived from the minigene and endogenous gene and led to a further improvement of glycemic control. These findi ngs indicate that the cis-regulatory element(s) controlling exercise-i nduced expression of the GLUT4 gene is located within the nucleotide s equence encompassed by the GLUT4 minigene. The fact that glycemic cont rol is markedly improved by a relatively low level of expression of GL UT4 caused by the transfected minigene and is further enhanced by exer cise in transgenic animals demonstrates that GLUT4 plays a pivotal rol e in glucose homeostasis in vivo. Of the effectors-i.e., cAMP, insulin , and arachidonic acid-known to down-regulate expression of GLUT4 by 3 T3-L1 adipocytes in culture, only the decline in circulating arachidon ate level in vivo correlated with upregulation of GLUT4 caused by exer cise.