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.
Ikemoto
S
S
Division of Clinical Nutrition, National Institute of Health and Nutrition, Tokyo, Japan.
Thompson
K S
KS
Itakura
H
H
Lane
M D
MD
Ezaki
O
O
D000818:Animals; D016718:Arachidonic Acid / Q000097:blood; D000242:Cyclic AMP / Q000032:analysis; D005260:Female; D018628:Gene Dosage; D005786:Gene Expression Regulation; D005947:Glucose / Q000378:metabolism*; D005951:Glucose Tolerance Test; D051275:Glucose Transporter Type 4; D006706:Homeostasis / Q000502:physiology*; D007328:Insulin / Q000097:blood; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D009004:Monosaccharide Transport Proteins / Q000096:biosynthesis* / Q000235:genetics; D009124:Muscle Proteins; D009928:Organ Specificity; D010805:Physical Conditioning, Animal / Q000502:physiology*; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics
D051275:Glucose Transporter Type 4; D007328:Insulin; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; D012333:RNA, Messenger; C495693:Slc2a4 protein, mouse; D016718:Arachidonic Acid; D000242:Cyclic AMP; D005947:Glucose