Introduction

To examine the physiological role of the GLUT4/muscle-fat specific facilitative glucose transporter in regulating glucose homeostasis, we have generated transgenic mice expressing high levels of this protein in an appropriate tissue-specific manner. Examination of two independent founder lines demonstrated that high-level expression of GLUT4 protein resulted in a marked reduction of fasting glucose levels (=70 mg/dl) compared to wild-type mice ("'130 mg/dl). Surprisingly, 30 min following an oral glucose challenge the GLUT4 transgenic mice had only a slight elevation in plasma glucose levels (Q"90 mg/dl), whereas wild-type mice displayed a typical 2-to 3-fold increase (==250-300 mg/dl). In parallel to the changes in plasma glucose, insulin levels were --2-fold lower in the transgenic mice compared to the wild-type mice. Furthermore, isolated adipocytes from the GLUT4 transgenic mice had increased basal glucose uptake and subcellular fractionation indicated elevated levels of cell surface-associated GLUT4 protein. Consistent with these results, in situ immunocytochemical localization of GLUT4 protein in adipocytes and cardiac myocytes indicated a marked increase in plasma membrane-associated GLUT4 protein in the basal state. Taken together these data demonstrate that increased expression of the human GLUT4 gene in vivo results in a constitutively high level of cell surface GLUT4 protein expression and more efficient metabolic control over fluctuations in plasma glucose concentrations.The GLUT4/muscle-fat glucose transporter is one member of the facilitative glucose transporter super-gene family that is specifically expressed in muscle and adipose tissues (1, 2). In contrast to the other glucose transporter isoforms, GLUT4 contains specific amino acid targeting sequences (3-5) responsible for its localization to unique intracellular vesicular compartments found in adipocytes and muscle cells (6)(7)(8)(9)(10)(11). In response to acute insulin stimulation, these preformed GLUT4-containing vesicles rapidly translocate to the plasma membrane in a GTP-dependent process, resulting in a large increase in plasma membrane-associated GLUT4 protein (9-17).In contrast to this acute pathway of insulin action, catabolic states such as fasting and non-insulin-dependent diabetes are directly associated with a marked resistance of adipose and muscle tissue to insulin-stimulated glucose uptake (18)(19)(20). Recently, several studies have suggested that a decrease in GLUT4 expression may be the initial cause of insulin resistance in adipose tissue, which contributes to the maintenance ofinsulin resistance in muscle (21)(22)(23)(24)(25)(26)(27)(28)(29). Since the pathophysiological mechanisms responsible for insulin resistance are poorly understood, we have recently generated transgenic mice expressing high levels of the human GLUT4The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indi...

No 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 transport activity in older db/db mice was not accompanied by an equivalent increase in cell surface GLUT4 compared to younger animals. Thus, although in vivo glucose tolerance is improved with GLUT4 overexpression in young animals, it deteriorates with age; in contrast, insulin responsiveness as assessed by the clamp technique remains improved with GLUT4 overexpression, as does in vitro insulin action. In summary, despite an impairment in whole-body glucose tolerance, skeletal muscle of the old transgenic GLUT4 db/db mice is still insulin responsive in vitro and in vivo. Diabetes 50:593-600, 2001

Marked overexpression of the glucose transporter GLUT4 in skeletal muscle membrane fractions of GLUT4 transgenic (TG) mice is accompanied by disproportionately small increases in basal and insulin-stimulated glucose transport activity. Thus we have assessed cell surface GLUT4 by photolabelling with the membrane-impermeant reagent 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1, 3-bis(D-mannos-4-yloxy)-2-propylamine (ATB-BMPA) and measured the corresponding glucose transport activity using 2-deoxyglucose in isolated extensor digitorum longus (EDL) muscles from non-transgenic (NTG) and GLUT4 TG mice in the absence and presence of 13.3 nM (2000 mu units/ml) insulin, without or with hypoxia as a model of muscle contraction. TG mice displayed elevated rates of glucose transport activity under basal and insulin-stimulated conditions, and in the presence of insulin plus hypoxia, compared with NTG mice. Photoaffinity labelling of cell surface GLUT4 indicated corresponding elevations in plasma membrane GLUT4 in the basal and insulin-stimulated states, and with insulin plus hypoxia, but no difference in cell surface GLUT4 during hypoxia stimulation. Subcellular fractionation of hindlimb muscles confirmed the previously observed 3-fold overexpression of GLUT4 in the TG compared with the NTG mice. These results suggest that: (1) alterations in glucose transport activity which occur with GLUT4 overexpression in EDL muscles are directly related to cell surface GLUT4 content, regardless of the levels observed in the corresponding subcellular membrane fractions, (2) while overexpression of GLUT4 influences both basal and insulin-stimulated glucose transport activity, the response to hypoxia/ contraction-stimulated glucose transport is unchanged, and (3) subcellular fractionation provides little insight into the subcellular trafficking of GLUT4, and whatever relationship is demonstrated in EDL muscles from NTG mice is disrupted on GLUT4 overexpression.