Differential regulation of two distinct glucose transporter species expressed in 3T3-L1 adipocytes: effect of chronic insulin and tolbutamide treatment.

Abstract

The HepG2-type glucose transporter (HepG2-GT) is expressed in 3T3-L1 fibroblasts and adipocytes. In contrast, the acutely insulin-regulatable glucose transporter (IRGT) is expressed only in the adipocytes. In the present study, the expression of the IRGT was shown to increase in parallel with the acquisition of acutely insulin-stimulated glucose uptake during differentiation of these cells, whereas the level of the HepG2-GT decreased during the course of differentiation in parallel with a decline in basal glucose uptake. We examined the effects of chronic insulin and tolbutamide treatment on glucose transporter activity in conjunction with the expression of these two glucose transporter species in 3T3-Ll adipocytes. Treatment of adipocytes with insulin, tolbutamide, or both agents in combination increased 2-deoxyglucose uptake, HepG2-GT protein, and HepG2-GT mRNA levels in parallel. The effect of combined insulin/tolbutamide administration on these three parameters was greater than the effect of either treatment alone. In contrast, these treatments either had no significant effect or decreased levels of IRGT protein and mRNA. We conclude that chronic treatment of 3T3-Ll adipocytes with insulin or tolbutamide increases glucose uptake primarily by means of a selective increase in the expression of the HepG2-GT. We suggest that part of the in vivo hypoglycemic effect of insulin and sulfonylureas may involve an increased expression of the HepG2-GT.Muscle and fat represent a major depot for insulin-stimulated whole-body glucose disposal (1, 2). Glucose transport across the plasma membrane of these insulin-sensitive tissues appears to occur by means of at least two distinct facilitated diffusion transport proteins. The major species, termed the insulin-regulatable glucose transporter (IRGT), is expressed exclusively in fat and muscle tissues (3, 4). The HepG2-type glucose transporter (HepG2-GT) (5), the predominant species expressed in mammalian brain, erythrocytes, and several other tissues, appears to be expressed at low levels in fat and muscle (3,(6)(7)(8). Most of the enhancement of glucose transport observed in response to acute insulin stimulation in fat and muscle is believed to occur by means of the rapid translocation of glucose transporters from an intracellular pool to the plasma membrane (9-12). Both the IRGT and the HepG2-GT undergo insulin-dependent translocation in rat and 3T3-L1 adipocytes (3,13,14), and thus both presumably contribute to the acute increase in glucose transport observed in the presence of the hormone. However, an investigation of the long-term regulatory effect ofinsulin on these two distinct glucose transport systems has not been reported. Some studies on insulin resistance have indirectly suggested a negative effect of chronic insulin administration on glucose disposal in humans (15), although claims to the contrary have also been made (16,17). Most animal studies, however, have indicated that insulin generally exerts a positive regulatory effect, presumably by raising the p...

Development of insulin-responsive glucose uptake and GLUT4 expression in differentiating human adipocyte precursor cells

“…3À5 In our experiments, the amount of GLUT4 protein increased from very low levels by more than 16-fold, which is in the same magnitude as ®ndings reported by other groups in rodent cell lines. 6±8 It has been demonstrated in 3T3 L1 cells that basal rates of 2-deoxy-glucose uptake are higher in growing, than in con¯uent, cells and newly developed fat 8 The decline in basal glucose uptake during the course of differentiation was found to occur in parallel with the decrease of GLUT1 level, 6 although in one of these studies the decline was only moderate. 7 In our study, the marked decrease in basal glucose uptake was due to a dramatic reduction in the number of GLUT1 transporters to almost undetectable levels.…”

GLUT4Gene Regulation and Manipulation

“…Interestingly, chronic exposure of 3T3-L1 in vitro cells or adipose tissue in vivo to insulin has differential effects on GLUT4 gene expression. Animals chronically treated with insulin show increased GLUT4 mRNA in adipose tissue (12,13) whereas chronic insulin treatment of 3T3-L1 adipocytes has resulted in either no change or in a marked reduction in GLUT4 mRNA levels (14,15). The different responses of GLUT4 mRNA to chronic insulin treatment in vivo and in vitro suggest GLUT4 mRNA does not respond directly to insulin action on adipose tissue.…”

Glucosamine-induced Insulin Resistance in 3T3-L1 Adipocytes Is Caused by Depletion of Intracellular ATP

“…2-Deoxyglucose Uptake Measurements-[ 3 H]2-Deoxyglucose uptake was measured as described previously (22). After the 2.5-h preincubation described above, cells were washed three times with Krebs-Ringer phosphate and then treated or not treated for 20 min at 37°C with 1 M insulin.…”

“…Treatment of 3T3-L1 Adipocytes-3T3-L1 fibroblasts were grown to confluence and 48 h later subjected to differentiation as described previously (22). 3T3-L1 adipocytes were used 10 -14 days after differentiation.…”