The pathophysiology of non-insulin-dependent diabetes mellitus (NIDDM) is complex, consisting of both abnormal insulin secretory patterns, and resistance to the actions of insulin [1][2][3][4][5][6][7][8]. While insulin resistance is a characteristic feature of many NIDDM patients, it is also observed in non-diabetic subjects [1]. In fact, it has been estimated that up to 25 % of normoglycaemic non-obese individuals are as insulin resistant as NIDDM patients [1], and it has been suggested that a significant proportion of these individuals will eventually develop diabetes if either insulin secretion declines and/or insulin resistance increases [1]. However, in most individuals, the cellular mechanisms causing human insulin resistance are not understood [3][4][5][6][7][8].Insulin regulates cellular functions by binding to the insulin receptor, a tetrameric plasma membrane glycoprotein of the receptor tyrosine-kinase family [6][7][8]. After binding, insulin activates receptor tyrosine-kinase activity, leading to the phosphorylation of key intracellular substrates, and insulin action ensues [6][7][8]. Many studies have been performed on insulin receptor tyrosine-kinase activity in NIDDM Diabetologia (1997) 40: 282-289 Increased adipose tissue PC-1 protein content, but not tumour necrosis factor-a gene expression, is associated with a reduction of both whole body insulin sensitivity and insulin receptor tyrosine-kinase activity Summary In the present study we measured PC-1 content, tumour necrosis factor (TNF)-a gene expression, and insulin stimulation of insulin receptor tyrosine-kinase activity in adipose tissue from nonobese, non-diabetic subjects. These parameters were correlated with in vivo insulin action as measured by the intravenous insulin tolerance test (K itt values). PC-1 content was negatively correlated with K itt values (r = -0.5, p = 0.04) and positively with plasma insulin levels both fasting (r = 0.58, p = 0.009) and after 120 min during oral glucose tolerance test (OGTT) (r = 0.67, p = 0.002). Moreover, adipose tissue PC-1 content was higher in relatively insulin-resistant subjects (K itt values lower than 6) than in relatively insulin-sensitive subjects (K itt values higher than 6) (525 ± 49 ng/mg protein vs 336 ± 45, respectively, p = 0.012). Adipose tissue insulin receptor tyrosinekinase activity in response to insulin was significantly lower at all insulin concentrations tested (p = 0.017, by two-way analysis of variance test) in insulin-resistant than in insulin-sensitive subjects (K itt values lower or higher than 6, respectively). In contrast to PC-1, no significant correlation was observed between adipose tissue TNF-a mRNA content and K itt values, and plasma insulin levels, both fasting and at after 120 min during OGTT. Also, no difference was observed in TNF-a mRNA content between subjects with K itt values higher or lower than 6. These studies in adipose tissue, together with our previous studies in skeletal muscle raise the possibility that PC-1, by regulating insulin receptor funct...

Decreased plasma fibrinolysis may contribute to accelerated atherothrombosis in diabetes. To observe whether hyperglycemia and hyperinsulinemia, common findings in type 2 diabetes, acutely affect plasma fibrinolysis in vivo, we evaluated plasma fibrinolysis (lysis of fibrin plates, free PAI-1 activity and t-PA activity) in the rat after a hyperglycemic euinsulinemic clamp (n=8), an euglycemic hyperinsulinemic clamp (n=7) or a saline infusion (n=15). Plasma fibrinolytic activity was sharply reduced after both the hyperglycemic and hyperinsulinemic clamps as compared to the respective controls (mean lysis areas on the fibrin plate, 139+/-21 vs. 323+/-30 mm2, p<0.001; 78+/-27 vs. 312+/-27 mm2 p<0.001, respectively). Plasma PAI-1 activity was greater after both hyperglycemic and hyperinsulinemic clamps as compared to saline infusion (6.6+/-2.6 vs. 1.6+/-0.6 IU/ml, p<0.001; 26+/-4 vs. 1.3+/-0.7 IU/ml, p<0.0001, respectively). Plasma t-PA activity was significantly reduced both after the hyperglycemic (0.36+/-0.15 vs. 2.17+/-0.18 IU/ml in controls, p<0.001) and the hyperinsulinemic (0.3+/-0.1 vs. 2.3+/-0.3 IU/ml in control, p<0.001) clamps. These data show that in vivo both acute hyperglycemia and acute hyperinsulinemia can decrease plasma fibrinolytic potential and that this is due to increased plasma PAI-1 and decreased free t-PA activities.

Protein kinase C y (PKC-y) is the PKC isoform predominantly expressed in skeletal muscle, and it is supposed to mediate many signals necessary for muscle histogenesis and homeostasis, such as TGFb, nerve-dependent signals and insulin. To study the role of PKC-y in these mechanisms we generated transgenic mice expressing a ''kinase dead'' mutant form of PKC-y (PKC-yK/R), working as ''dominant negative,'' specifically in skeletal muscle. These mice are viable and fertile, however, by the 6-7 months of age, they gain weight, mainly due to visceral fat deposition. Before the onset of obesity (4 months of age), they already show increased fasting and fed insulin levels and reduced insulin-sensitivity, as measured by ipITT, but normal glucose tolerance, as measured by ipGTT. After the 6-7 months of age, transgenic mice develop hyperinsulinemia in the fasting and fed state. The ipGTT revealed in the transgenic mice both hyperglycemia and hyperinsulinemia. At the molecular level, impaired activation of the IR/IRS/PI3K pathway and a significant decrease both in the levels and in insulin-stimulated activation of the serine/threonine kinase Akt were observed. Taken together these data demonstrate that over-expression of dominant negative PKC-y in skeletal muscle causes obesity associated to insulin resistance, as demonstrated by defective receptor and postreceptorial activation of signaling cascade.

No abstract

We tested the hypothesis that glucosamine, a putative activator of glucose toxicity in vitro through acceleration of the hexosamine pathway, may determine in vivo the two key features of glucose toxicity in diabetes, namely, peripheral insulin resistance and decreased insulin secretion. Two groups of awake rats were studied either with intraarterial administration of glucosamine (5 mumol.kg-1.min-1) or saline. Insulin secretion was determined after arginine, glucose (hyperglycaemic clamp), and arginine/glucose infusions, while insulin-mediated glucose metabolism was assessed by the euglycaemic hyperinsulinaemic clamp in combination with [3-3H]-glucose infusion. Glucosamine had no effects on arginine-induced insulin secretion both at euglycaemia and hyperglycaemia, but significantly (40-50%) impaired glucose-induced insulin secretion (both first and second phases). During euglycaemic hyperinsulinaemic clamp studies, glucosamine decreased glucose uptake by approximately 30%, affecting glycolysis (estimated from 3H2O rate of appearance) and muscle glycogen synthesis (calculated from accumulation of [3H]-glucosyl units in muscle glycogen) to a similar extent. Muscle glucose 6-phosphate concentration was markedly reduced in the glucosamine-infused rats, suggesting an impairment in glucose transport/phosphorylation. Therefore, an increase in hexosamine metabolism in vivo: 1) inhibits glucose-induced insulin secretion, and 2) reduces insulin stimulation of both glycolysis and glycogen synthesis, thereby mimicking in normal rats the major alterations due to glucose toxicity in diabetes.