The effects of streptozotocin diabetes on the activities of rat liver glycosyltransferases

Serum level of sialic acid (SA) and carbohydrate-deficient transferrin (CDT) in type 2 diabetes mellitus with microvascular complications

Sialic acid (SA) is responsible for the composition of different isoforms of transferrin and is reported to be a marker of microvascular complications in type 2 diabetes mellitus. Therefore, we explored the serum concentration of SA, and the less sialylated isoforms of transferrin, termed carbohydrate-deficient transferrin (CDT), in relation to the presence of microvascular complications in type 2 diabetes mellitus. We studied 21 patients with type 2 diabetes with microangiopathy and 22 patients without complications who were hospitalized at a diabetic clinic. The prevalence of microvascular complications was based on clinical history, fundoscopy, and laboratory tests. Blood samples were taken for measurements of SA, CDT, total transferrin, glucose, HbA1c, fibrinogen, C-reactive protein (CRP), and indicators of renal dysfunction (i.e., creatinine, urea, albumin excretion rate (AER), and glomerular filtration rate (GFR)). A rise in serum SA and a decrease in CDT concentrations were observed in both diabetic groups with and without complications, and there were no differences between the two groups of patients. There was a statistically significant correlation between serum SA and CDT in diabetic subjects with microvascular complications, but not in patients without such complications. This proves that the serum changes in CDT and SA levels in the course of type 2 diabetes mellitus are associated with each other in the presence of microangiopathy.

Increased expression of protein C-mannosylation in the aortic vessels of diabetic Zucker rats

C-Mannosylation is a novel type of glycosylation in proteins. There are several examples of proteins in which the specific motif Trp-X-X-Trp is mannosylated at the first Trp to produce C-mannosylated Trp (CMW). Although C-mannosylation modifies Trp-X-X-Trp, predicted to be a functional motif of various integral proteins such as cytokine receptors, the physiological or pathological relevance of C-mannosylation in the cell is still not known. In this study, to characterize C-mannosylation in biological samples, we generated specific polyclonal antibodies against CMW by using a chemically synthesized CMW as an antigen. Using the antibody, we investigated the effect of hyperglycemic conditions on protein C-mannosylation in cultured cells and diabetic Zucker fatty rats. We found that protein C-mannosylation was increased in macrophage-like RAW264.7 cells under hyperglycemic conditions compared to low-glucose conditions. Furthermore, C-mannosylation was increased in the aortic vessel wall of Zucker fatty rats. Thrombospondin-1 was identified as a protein modified with C-mannosylation, and its expression was also increased in the aortic tissues of Zucker fatty rats. These results indicate that C-mannosylation is increased in specific tissues or cell types under hyperglycemic conditions, suggesting a pathological role for the increased C-mannosylation in the development of diabetic complications.

Dolichol-mediated enhanced protein N-glycosylation in experimental diabetes--a possible additional deleterious effect of hyperglycemia

In liver cells from diabetic rats, an increased incorporation of labeled glucosamine into cellular and secretory proteins was found, when related to the incorporation of labeled leucine. This increased N-glycosylation was present in the face of decreased synthesis of hepatic cellular and secretory proteins evident from reduced leucine incorporation and diminished glycosyltransferase activity. To elucidate the mechanisms involved we incubated isolated hepatocytes with two N-glycosylation inhibitors: tunicamycin and 2-deoxyglucose. Tunicamycin exerted a marked inhibitory effect on the incorporation rate of labeled glucosamine into proteins in liver cells from diabetic rats, while 2-deoxyglucose had a negligible effect on this process in these cells. These diverse effects might be explained by the fact that tunicamycin acts through strong association with the enzyme catalyzing the first step in glycoprotein synthesis, namely, the transfer of UDP-GlcNAc to dolichol-P (indicating noncompetitive inhibition). This enzyme is reduced in liver cells from diabetic animals. On the other hand, 2-deoxyglucose exerts its effect by being attached to dolichol-P, preventing further elongation of oligosaccharide chain on the protein backbone. This latter effect might be eliminated by excess dolichol-P (indicating competitive inhibition). The dolichol content in liver extract from diabetic rats was about 2.5-fold higher compared with nondiabetic rats (51.6 micrograms/g versus 20.6 micrograms/g wet liver weight). These two lines of evidence confirm the notion that the enhanced enzymatic glycosylation in liver from diabetic animals is maintained by an increased hepatic dolichol concentration, which is most probably related to the hyperglycemia. Thus, the dolichol-N-glycosylation pathway may represent another detrimental aspect of hyperglycemia and may operate by dolichol mass action rather than through glycosylating enzyme activity.

Fluidity of rat liver Golgi membranes in streptozotocin diabetes. A spin label study

The mobility of 5-doxylstearic acid spin label (5-SASL) in the intact rat liver Golgi membranes of streptozotocin diabetes was studied as a function of free blood sugar level and temperature. During development of diabetes, indicated by the increase of the free blood sugar level, the membrane fluidity measured in the physiological temperature range (1) does not change in comparison with control in light diabetes, (2) decreases significantly in advanced diabetes and (3) again increases to the control level in heavy diabetes (the free blood sugar levels being 200-250 mg/100 ml, 250-350 mg/100 ml and greater than 350 mg/100 ml, respectively). The development of streptozotocin diabetes is accompanied by significant changes in lipid composition of liver Golgi membranes as also shown in our previous observations. The measurements of motion of 5-SASL in Golgi membranes as well as in vesicles, made from commercially available lipids of composition close to the liver Golgi membranes, show that a decrease of cholesterol contents is the main factor which induces the increase membrane fluidity. We suggest that in the heavy diabetes the hemostatic regulation in the lipid composition leads to minimization of alterations in membrane fluidity to obtain comparatively normal activity of certain membrane enzymes.

Free fatty acid inhibition of the insulin induction of glucose-6-phosphate dehydrogenase in rat hepatocyte monolayers

Rat hepatocytes in monolayer culture were utilized to determine if the decrease in glucose-6-phosphate dehydrogenase (G6PD) activity resulting from the ingestion of fat can be mimicked by the addition of fatty acids to a chemically, hormonally defined medium. G6PD activity in cultured hepatocytes was induced several-fold by insulin. Dexamethasone or T3 did not amplify the insulin induction of G6PD. Glucose alone increased G6PD activity in cultured hepatocytes from fasted donors by nearly 500%. Insulin in combination with glucose induced G6PD an additional two-fold. The increase in G6PD activity caused by glucose was greater in hepatocytes isolated from 72 hr-fasted rats as compared to fed donor rats. Such a response was reminiscent of the "overshoot" phenomenon in which G6PD activity is induced well above the normal level by fasting-refeeding rats a high glucose diet. Addition of linoleate to the medium resulted in a significant suppression of insulin's ability to induce G6PD, but linoleate had no effect on the induction of G6PD activity by glucose alone. A shift to the right in the insulin-response curve for the induction of G6PD also was detected for the induction of malic enzyme and acetyl-CoA carboxylase. Arachidonate (0.25 mM) was a significantly more effective inhibitor of the insulin action than linoleate was. Apparently rat hepatocytes in monolayer culture can be utilized as a model to investigate the molecular mechanism by which fatty acids inhibit the production of lipogenic enzymes. In part, this mechanism of fatty acid inhibition involves desensitization of hepatocytes to the lipogenic action of insulin.

Alpha 1 acid glycoprotein in streptozotocin diabetic rats. Some aspects of sugar moiety structure and metabolism

The pure alpha 1 acid glycoprotein (AGP) preparation from streptozotocin diabetic rat sera showed diminished sialic acid content and lower ratios of galactose to mannose and galactose to fucose. In vivo incorporation of 14C-leucine and 3H-galactose into AGP of control and diabetic animals was studied 30, 60 and 120 min after administration of the radioisotopic precursors. The changed 14C/3H ratio in AGP of diabetic rats suggested disturbed glycosylation of AGP in this disease. Similar activity of UDP-galactose 4-epimerase in control and diabetic rat liver was found suggesting that this enzyme has no influence on the decreased level of serum protein bound galactose.

Diabetes-induced functional and structural changes in insulin receptors from rat skeletal muscle

The effect of diabetes on the structure and function of insulin receptors was studied in rats 7 d after streptozotocin injection, using solubilized, partially purified receptors from rat hindlimb muscles. Diabetes increased the number of insulin receptors per gram of muscle 60-70% without apparent change in insulin binding affinity. Incubation of receptors at 4 degrees C with [gamma-32P]ATP and insulin resulted in dose-dependent autophosphorylation of the beta-subunit on tyrosine residues; receptors from diabetic rats showed decreased base-line phosphorylation, as well as a decrease in autophosphorylation at maximally stimulating insulin concentrations. These receptors also showed diminished exogenous substrate kinase activity using histone H2b and angiotensin II as phosphoacceptors. The electrophoretic mobility (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) of a subpopulation of beta-subunits derived from diabetics was slightly decreased; differences in electrophoretic mobility between control- and diabetic-derived beta-subunits were enhanced by generating fragments by partial Staphylococcus aureus V8 protease digestion. Endoglycosidase-H or neuraminidase treatment increased the electrophoretic mobility of beta-subunits in both groups, but only neuraminidase appeared to decrease or abolish differences in electrophoretic mobility between controls and diabetics, suggesting that excess sialilation may account, in part, for the altered mobility of diabetic derived beta-subunits. All structural and functional alterations in insulin receptors were prevented by treating diabetic rats with insulin for 60 h. Peripheral insulin resistance associated with insulinopenic diabetes may be related to modifications in insulin receptor structure, resulting in impaired signal transmission.