GLUT4 Overexpression in db/db Mice Dose-Dependently Ameliorates Diabetes But Is Not a Lifelong Cure

GLUT4 Overexpression in db/db Mice Dose-Dependently Ameliorates Diabetes But Is Not a Lifelong Cure Joseph T. Brozinick, Jr. 1 , Scott C. McCoid 2 , Thomas H. Reynolds 1 , Nancy A. Nardone 2 , Diane M. Hargrove 2 , Ralph W. Stevenson 2 , Samuel W. Cushman 1 and E. Michael Gibbs 2 1 Experimental Diabetes, Metabolism and Nutrition Section, DB/NIDDK, National Institutes of Health, Bethesda, Maryland, and the 2 Department of Cardiovascular and Metabolic Diseases, Pfizer Central Research, Groton, Connecticut Abstract We previously reported that overexpression of GLUT4 in lean, nondiabetic C57BL/KsJ- lepr db/+ ( db/+ ) mice resulted in improved glucose tolerance associated with increased basal and insulin-stimulated glucose transport in isolated skeletal muscle. We used the diabetic ( db/db ) litter mates of these mice to examine the effects of GLUT4 overexpression on in vivo glucose utilization and on in vitro glucose transport and GLUT4 translocation in diabetic mice. We examined in vivo glucose disposal by oral glucose challenge and hyperinsulinemic-hyperglycemic clamps. We also evaluated the in vitro relationship between glucose transport activity and cell surface GLUT4 levels as assessed by photolabeling with the membrane-impermeant reagent 2- N -(4-(1-azi-2,2,2-trifluoroethyl)benzoyl)-1,3-bis( d -mannose-4-yloxy)-2-propylamine in extensor digitorum longus (EDL) muscles. All parameters were examined as functions of animal age and the level of GLUT4 overexpression. In young mice (age 10–12 weeks), both lower (two- to threefold) and higher (four- to fivefold) levels of GLUT4 overexpression were associated with improved glucose tolerance compared to age-matched nontransgenic (NTG) mice. However, glucose tolerance deteriorated with age in db/db mice, although less rapidly in transgenic mice expressing the higher level of GLUT4. Glucose infusion rates during hyperinsulinemic-hyperglycemic clamps were increased with GLUT4 overexpression, compared with NTG mice in both lower and higher levels of GLUT4 overexpression, even in the older mice. Surprisingly, isolated EDL muscles from diabetic db/db mice did not exhibit alterations in either basal or insulin-stimulated glucose transport activity or cell surface GLUT4 compared to nondiabetic db/+ mice. Furthermore, both GLUT4 overexpression levels and animal age are associated with increased basal and insulin-stimulated glucose transport activities and cell surface GLUT4. However, the observed increased glucose transpor