Glucose-stimulated insulin secretion, glucose transport, glucose phosphorylation and glucose utilization have been characterized in the insulinoma cell line MIN6, which is derived from a transgenic mouse expressing the large T-antigen of SV40 in pancreatic beta cells. Glucose-stimulated insulin secretion occurred progressively from 5 mmol/l glucose, reached the maximal level approximately seven-fold above the basal level at 25 mmol/l, and remained at this level up to 50 mmol/l. Glucose transport was very rapid with the half-maximal uptake of 3-O-methyl-D-glucose being reached within 15 s at 22 degrees C. Glucose phosphorylating activity in the cell homogenate was due mainly to glucokinase; the Vmax value of glucokinase activity was estimated to be 255 +/- 37 nmol.h-1.mg protein-1, constituting approximately 80% of total phosphorylating activity, whereas hexokinase activity constituted less than 20%. MIN6 cells exhibited mainly the high Km component of glucose utilization with a Vmax of 289 +/- 18 nmol.h-1.mg protein-1. Thus, glucose utilization quantitatively and qualitatively reflected glucose phosphorylation in MIN6 cells. In contrast, MIN7 cells, which exhibited only a small increase in insulin secretion in response to glucose, had 4.7-fold greater hexokinase activity than MIN6 cells with a comparable activity of glucokinase. These characteristics of MIN6 cells are very similar to those of isolated islets, indicating that this cell line is an appropriate model for studying the mechanism of glucose-stimulated insulin secretion in pancreatic beta cells.

Abstract-Angiotensin II (AII) is involved in the pathogenesis of both hypertension and insulin resistance, though few studies have examined the relationship between the two. We therefore investigated the effects of chronic AII infusion on blood pressure and insulin sensitivity in rats fed a normal (0.3% NaCl) or high-salt (8% NaCl) diet. AII infusion for 12 days significantly elevated blood pressure and significant insulin resistance, assessed by a hyperinsulinemic-euglycemic clamp study and glucose uptake into isolated muscle and adipocytes. High-salt loading exacerbated the effects of AII infusion significantly. Despite the insulin resistance, insulin-induced tyrosine phosphorylation of the insulin receptor and insulin receptor substrates, activation of phosphatidylinositol (PI) 3-kinase, and phosphorylation of Akt were all enhanced by AII infusion. Subsequently, to investigate whether oxidative stress induced by AII contributes to insulin resistance, the membrane-permeable superoxide dismutase mimetic, tempol, was administered to AII-infused rats. Chronic AII infusion induced an accumulated plasma cholesterylester hydroperoxide levels, indicating the increased oxidative stress, whereas the treatment with tempol normalized plasma cholesterylester hydroperoxide levels in AII-infused rats. In addition, the treatment with tempol normalized insulin resistance in AII-infused rats, shown as a decreased glucose infusion rate in the hyperinsulinemic euglycemic clamp study and a decreased insulin-induced glucose uptake into isolated skeletal muscle, as well as enhanced insulin-induced PI 3-kinase activation to those in the control rats. These results strongly suggest that AII-induced insulin resistance cannot be attributed to impairment of early insulin-signaling steps and that increased oxidative stress, possibly through impaired insulin signaling located downstream from PI 3-kinase activation, is involved in AII-induced insulin resistance. Key Words: angiotensin II Ⅲ insulin resistance Ⅲ oxidative stress Ⅲ glucose clamp technique Ⅲ sodium Ⅲ kinase S everal lines of evidence point to an association between hypertension and insulin resistance, 1,2 eg, hypertensive individuals are more likely to become diabetic than normotensive ones. 3 It is therefore notable that angiotensin II (AII) is reportedly involved in the development of both hypertension and insulin resistance, 4 -7 and agents that inhibit the action of AII, ie, angiotensin-converting enzyme inhibitors and type 1 AII (AT1) receptor antagonists, not only reduce blood pressure but also restore insulin sensitivity. 8 -14 It has been suggested that crosstalk between AII-and insulinsignaling pathways underlies AII-induced insulin resistance. According to that model, AII induces tyrosine phosphorylation of insulin receptor substrate (IRS)-1 by Janus kinase 2 (JAK2) associated with the AT1 receptor, thereby attenuating insulin-induced activation of phosphatidylinositol (PI) 3-kinase associated with IRS-1, which in turn diminishes insulin sensitivity. 15,16 However, ...

T-1095A and T-1095 are synthetic agents derived from phlorizin, a specific inhibitor of Na+-glucose cotransporters (SGLTs). Unlike phlorizin, T-1095 is absorbed into the circulation via oral administration, is metabolized to the active form, T-1095A, and suppresses the activity of SGLTs in the kidney. Orally administered T-1095 increases urinary glucose excretion in diabetic animals, thereby decreasing blood glucose levels. Indeed, the postprandial hyperglycemia after a meal load was shown to be suppressed by this compound in streptozotocin (STZ)-induced diabetic rats. With long-term T-1095 treatment, both blood glucose and HbA1c levels were reduced in STZ-induced diabetic rats and yellow KK mice. In addition, there was amelioration of abnormal carbohydrate metabolism, i.e., hyperinsulinemia and hypertriglyceridemia, and of the development of microalbuminuria, in yellow KK mice. Thus, T-1095 may be a useful antidiabetic drug, providing a novel therapeutic approach for diabetes.

Resistin is a hormone secreted by adipocytes that acts on skeletal muscle myocytes, hepatocytes, and adipocytes themselves, reducing their sensitivity to insulin. In the present study, we investigated how the expression of resistin is affected by glucose and by mediators known to affect insulin sensitivity, including insulin, dexamethasone, tumor necrosis factor-␣ (TNF-␣), epinephrine, and somatropin. We found that resistin expression in 3T3-L1 adipocytes was significantly upregulated by high glucose concentrations and was suppressed by insulin. Dexamethasone increased expression of both resistin mRNA and protein 2.5-to 3.5-fold in 3T3-L1 adipocytes and by ϳ70% in white adipose tissue from mice. In contrast, treatment with troglitazone, a thiazolidinedione antihyperglycemic agent, or TNF-␣ suppressed resistin expression by ϳ80%. Epinephrine and somatropin were both moderately inhibitory, reducing expression of both the transcript and the protein by 30 -50% in 3T3-L1 adipocytes. Taken together, these data make it clear that resistin expression is regulated by a variety of hormones and that cytokines are related to glucose metabolism. Furthermore, they suggest that these factors affect insulin sensitivity and fat tissue mass in part by altering the expression and eventual secretion of resistin from adipose cells.