Facilitative glucose transporters: regulatory mechanisms and dysregulation in diabetes

Regulation of glucose transporter and hexokinase II expression in tissues of diabetic rats

Glucose transport and phosphorylation are decreased in muscle and adipose tissue in diabetes mellitus. The glucose transporter GLUT-4 and hexokinase II (HK II) are the main isoforms of proteins involved in glucose transport and phosphorylation in insulin-sensitive tissues, adipose tissue, skeletal muscle, and heart. The molecular mechanisms responsible for the decrease of glucose transport and phosphorylation have been studied during the first 3 days after streptozotocin (STZ) administration in adult male Wistar rats. GLUT-4 mRNA and protein and HK II mRNA and enzyme activity were measured. After the injection of STZ (30 h), GLUT-4 and HK II mRNAs were decreased to 10 +/- 1 and 20 +/- 3% that found in nondiabetic rats, respectively; they remained at these low levels for 72 h. Normalization of the blood glucose level by phlorizin infusion did not restore GLUT-4 and HK II mRNA concentrations to normal. In contrast, normalization of the blood glucose level by physiological infusion of insulin resulted in a total normalization of GLUT-4 and HK II mRNA concentrations. When insulin therapy was stopped, GLUT-4 and HK II mRNA and protein concentrations fell in 6 h to 40 and 20% of control levels, respectively. Minimal changes of GLUT-4 and HK II mRNA, and of HK II activity, were observed in skeletal muscle and heart of diabetic rats. We conclude that GLUT-4 and HK II mRNA are coordinately expressed in white adipose tissue. They are rapidly affected by an acute decrease of the plasma insulin concentrations but are not modified by hyperglycemia. In contrast, skeletal muscle and heart GLUT-4 and HK II mRNA are not greatly affected by short-term diabetes.

Effects of pH on the activity of the human red cell glucose transporter Glut 1: transport retention chromatography of D-glucose and L-glucose on immobilized Glut 1 liposomes

The facilitative glucose transporter Glut 1 from human red cells was reconstituted into liposomes that were size-fractionated and immobilized in an octyl sulfide-Sephacryl S-1000 column. D-[14C]Glucose was eluted later than L-[3H]glucose from the Glut 1 liposome column (by delta V microliters), apparently because the D-glucose was transported through the liposomes. The corresponding difference with protein-free liposomes was delta V0. The Glut 1 transport retention chromatographic effect, delta VG = delta V - delta V0, 40-50 microliters at pH 7, was nearly constant at pH 6-10 (400 mM NaCl, 23 degrees C, internal liposome volume approximately 240 microliters) but decreased steeply below pH 5 to become zero at pH 3.6. The decrease corresponded to a pKa of approximately 4.4 and was partly reversible above pH 4.7. Similarly, glucose exchange by non-immobilized freeze-thawed proteoliposomes with Glut 1 slowed down drastically as the pH was lowered from pH 5.5 to 4; and octyl glucoside-solubilized Glut 1 lost half its activity in 15 min at pH 4.5 (low ionic strength, 2 degrees C) as shown by glucose exchange determinations at pH 7.2 The results suggest that Glut 1 is inactivated at low pH upon protonation of carboxylate groups of pKa approximately 4.4-4.8. It seems likely that carboxylate groups form hydrogen bonds to transported D-glucose.

Insulin therapy normalizes GLUT1 glucose transporter mRNA but not immunoreactive transporter protein in streptozocin-diabetic rats

Previous studies have shown that the principal glucose transporter isoform within the blood-brain barrier (BBB) is GLUT1, and that GLUT1 mRNA is upregulated and immunoreactive GLUT1 protein is downregulated in rats with streptozocin (STZ)-induced experimental diabetes. The present studies investigate effects of insulin therapy on both GLUT1 mRNA and immunoreactive GLUT1 protein in brain capillaries isolated from control (CO), diabetic (DM), and insulin-treated diabetic (IRx) rats. The following variables were measured: serum glucose levels, rat brain capillary immunoreactive GLUT1 level by quantitative Western blotting, and rat brain capillary GLUT1 and actin mRNA levels by quantitative Northern blotting. Serum glucose levels were 6.4 +/- 1.2, 30.3 +/- 3.2, and 3.7 +/- 1.7 mmol/L in CO, DM, and IRx rats, respectively. Brain capillary immunoreactive GLUT1 transporter protein level was 53% +/- 13% of CO values in DM rats, and this value was unchanged with insulin treatment. GLUT1 mRNA level in rat brain was increased to 131% +/- 8% of CO values in DM rats and was 80% +/- 5% of CO values in IRx rats. In conclusion, short-term insulin therapy in rats with STZ-induced diabetes normalizes BBB GLUT1 mRNA level, but does not normalize depressed immunoreactive GLUT1 protein level.

Effect of insulin on the rates of synthesis and degradation of GLUT1 and GLUT4 glucose transporters in 3T3-L1 adipocytes

The effect of continuous insulin stimulation on the rates of turnover and on the total cellular contents of the glucose-transporter proteins GLUT1 and GLUT4 in 3T3-L1 adipocytes was investigated. Pulse-and-chase studies with [35S]methionine followed by immunoprecipitation of GLUT1 and GLUT4 with isoform-specific antibodies revealed the half-lives of these proteins to be 19 h and 50 h respectively. Inclusion of 100 nM insulin in the chase medium resulted in a decrease in the half-lives of both proteins to about 15.5 h. This effect of insulin was specific for the glucose-transporter proteins, as the average half-life of all proteins was found to be 55 h both with and without insulin stimulation. The effect of insulin on the rate of synthesis of the glucose transporters was determined by the rate of incorporation of [35S]methionine. After 24 h of insulin treatment, the rate of synthesis of GLUT1 and GLUT4 were elevated over control levels by 3.5-fold and 2-fold respectively. After 72 h of treatment under the same conditions, the rate of synthesis of GLUT1 remained elevated by 2.5-fold, whereas the GLUT4 synthesis rate was not different from control levels. Western-blot analysis of total cellular membranes revealed a 4.5-fold increase in total cellular GLUT1 content and a 50% decrease in total cellular GLUT4 after 72 h of insulin treatment. These observations suggest that the rates of synthesis and degradation of GLUT1 and GLUT4 in 3T3-L1 adipocytes are regulated independently and that these cells respond to prolonged insulin treatment by altering the metabolism of GLUT1 and GLUT4 proteins in a specific manner.

Placental glucose transporter gene expression and metabolism in the rat

In situ hybridization was used to evaluate patterns of gene expression for glucose transporters 1-4 (GT1-4) in the rat uteroplacenta from the time of implantation through term, and in vivo regional placental glucose metabolism was measured by 14C-labeled 2-deoxyglucose uptake. GT1 mRNA was highly abundant and GT3 was barely detected in the postimplantation decidual reaction. GT1 and 3 mRNAs were colocalized in the labyrinthine syncitiotrophoblast layer of the chorioallantoic placenta, which forms the membranous barrier between maternal and fetal circulations. The level of labyrinthine GT3 mRNA showed no change from midgestation through term; however, the volume of the labyrinth and hence total GT 3 gene expression increased greatly during this period. Labyrinthine GT1 mRNA levels, in contrast, showed significant diminution near term. GT1 mRNA was also localized in the placental growth plate, or junctional zone, where it was most abundant during the period of rapid placental growth and was decreased at term. Placental glucose metabolism, as reflected by steady-state 2-deoxyglucose uptake, was highest in the junctional zone during the rapid growth phase during midgestation, and decreased significantly at term, in parallel with GT1 gene expression. These findings suggest that GT1 is responsible for supplying glucose for use as a placental fuel and that GT3 is important for glucose transfer to the embryo.

Insulin down-regulates expression of the insulin-responsive glucose transporter (GLUT4) gene: effects on transcription and mRNA turnover

Insulin rapidly represses expression of the gene encoding the insulin-responsive glucose transporter (GLUT4) in 3T3-L1 mouse adipocytes. Upon exposure to the hormone the cellular level of GLUT4 mRNA falls (t1/2 approximately 2.5 hr) to 20-30% of its initial level within 10 hr. This is followed by a similar decrease in the level of GLUT4 protein. Down-regulation of GLUT4 mRNA is a result of both rapid repression of transcription of the GLUT4 gene and an increased rate of turnover of the GLUT4 message. As a consequence of prolonged exposure to insulin, 3T3-L1 adipocytes lose their capacity for acute stimulation of hexose uptake by insulin. These findings provide an explanation for the resistance of glucose uptake to insulin in adipose tissue observed in non-insulin-dependent (type 2) diabetes mellitus, particularly that associated with hyperinsulinemia and obesity.