Showing papers by "Subbiah Pugazhenthi published in 1995"

In vivo effects of vanadate on hepatic glycogen metabolizing and lipogenic enzymes in insulin-dependent and insulin-resistant diabetic animals.

Abstract: The insulin-mimetic action of vanadate is well established but the exact mechanism by which it exerts this effect is still not clearly understood. The role of insulin in the regulation of hepatic glycogen metabolizing and lipogenic enzymes is well known. In our study, we have, therefore, examined the effects of vanadate on these hepatic enzymes using four different models of diabetic and insulin-resistant animals. Vanadate normalized the blood glucose levels in all animal models. In streptozotocin-induced diabetic rats, the amount of liver glycogen and the activities of the active-form of glycogen synthase, both active and inactive-forms of phosphorylase, and lipogenic enzymes like glucose 6-phosphate dehydrogenase and malic enzyme were decreased and vanadate treatment normalized all of these to near normal levels. The other three animal models (db/db mouse, sucrose-fed rats and fa/fa obese Zucker rats) were characterized by hyperinsulinemia, hypertriglyceridemia, increases in activities of lipogenic enzymes, and marginal changes in glycogen metabolizing enzymes. Vanadate treatment brought all of these values towards normal levels. It should be noted that vanadate shows differential effects in the modulation of lipogenic enzymes activities in type I and type II diabetic animals. It increases the activities of lipogenic enzymes in streptozotocin-induced diabetic animals and prevents the elevation of activities of these enzymes in hyperinsulinemic animals. The insulin-stimulated phosphorylation of insulin receptor β subunit and its tyrosine kinase activity was increased in streptozotocin-induced diabetic rats after treatment with vanadate. Our results support the view that insulin receptor is one of the sites involved in the insulin-mimetic actions of vanadate.

Regulation of glycogen synthase activation in isolated hepatocytes.

Abstract: Glycogen synthase, the regulatory enzyme of glycogen synthesis undergoes multisite phosphorylation leading to its inactivation The kinases responsible for this covalent modification (ex cAMP-dependent protein kinase, protein kinase C and glycogen synthase kinase-3 ) are controlled by the second messengers generated by different hormones The isolated hepatocytes has been used as one of the experimental models for studying this complex regulatory process Inactivation of glycogen synthase by glucagon and vasopressin has been shown to be accompanied with incorporation of phosphate into the enzyme protein Insulin has been shown to activate glycogen synthase by inhibition of kinases and activation of synthase phosphatase Glycogen synthase is activated by several gluconeogenic substrates, in addition to glucose Studies in hepatocytes with activators and inhibitors of protein kinase C show that this enzyme negatively controls glycogen synthase The differential effects of the phosphatase inhibitors, calyculin A and okadaic acid in liver cells provide supporting evidence that protein phosphatase type-1 plays a major role in the regulation of glycogen synthase Hepatocytes isolated from diabetic rats of both types (insulin- dependent and non-insulin-dependent) mimic the defective glycogen synthase activation seen in vivo

Decrease in protein tyrosine phosphatase activities in vanadate-treated obese Zucker (fa/fa) rat liver.

Abstract: The inhibitory action of vanadate towards protein tyrosine phosphatase (PTPase) has been considered as a probable mechanism by which it exerts insulin-like effects. In this study, we have examined thein vivo effects of vanadate on PTPases in the liver of obese Zucker rats, a genetic animal model for obesity and type II diabetes. These animals were characterized by hyperinsulinemia and mild hyperglycemia. The number of insulin receptors were significantly (p<0.01) decreased in liver. After chronic administration of vanadate in obese rats, 80% decrease in the plasma levels of insulin was observed. The insulin receptor numbers were significantly (p<0.01) higher in vanadate-treated obese rats as compared to the untreated ones. The hepatic PTPase activities in cytosolic and particulate fractions, with phosphorylated poly glu:tyr (4∶1) and the insulin receptor peptide (residues 1142–1153) as substrates, increased in obese rats. In vanadate-treated obese rat livers, the PTPase activities in both subcellular fractions with these substrates decreased significantly (p<0.001). The decreases in PTPase activities from these groups of rats were further supported by chromatography on a Mono Q column. These data support the view that inhibition of PTPases plays a role in the insulin-mimetic action of vanadate.

Vanadate induces normolipidemia and a reduction in the levels of hepatic lipogenic enzymes in obese Zucker rat

Abstract: The effects of vanadate administration on the plasma lipids and hepatic lipogenic enzymes were investigated in Zucker (fa/fa) rat, a model for obesity and non insulin-dependent diabetes. These animals were administered sodium orthovanadate through drinking water for a period of four months. The plasma levels of insulin, triacylglycerols and total cholesterol were significantly (p<0.001) elevated in untreated obese control rats as compared to the lean animals. In the livers of obese rats, the number of insulin receptors decreased by 60% and the activities of lipogenic enzymes acetyl-CoA carboxylase and ATP-citrate lyase increased by 4.7- and 5.6-folds, respectively. The messenger RNA for ATP-citrate lyase as measured by Northern blot analysis showed a parallel increase in obese control rats. Treatment of these rats with vanadate caused 56–77% decreases in the plasma levels of insulin, triacylglycerols and total cholesterol. The insulin receptor numbers in vanadate-treated obese rats increased (119%) compared to levels in untreated obese animals. The elevated activities of acetyl-CoA carboxylase and ATP-citrate lyase observed in livers of obese rats were significantly reduced by vanadate. The messenger RNA for ATP-citrate lyase also decreased in vanadate-treated obese rats back to the lean control levels. This study demonstrates that vanadate exerts potent actions on lipid metabolism in diabetic animals in addition to the recognized effects on glucose homeostasis.

Glycosylated proteins and serum lipid profile in complicated and uncomplicated niddm patients

Abstract: In the present study, 58 diabetic patients (type II) comprising uncomplicated (27) and complicated (31) cases along with 38 healthy matched controls were investigated for glycosylated hemoglobin (HbA1), glycosylated serum proteins (GSP) and serum lipid profile viz. triacylglycerol, total cholesterol, HDL-cholesterol (HDL-C), VLDL-C, LDL-C and their ratios. HbA1 and GSP were significantly elevated in diabetic group as a whole when compared with controls (p 0.05). Follow-up studies showed that while GSP levels reflect variations in blood glucose over a shorter period of 15 days, those of HbA1 do so for about 3 months. HbA1 correlated with fasting blood glucose (p 0.05) even after accounting for sex in both the groups. Interestingly TC:HDL-C was elevated significantly (p<0.01) in the above groups. These findings seem to suggest that at the levels of hyperglycemia, hypercholesterolemia and hypertriacylglycerolemia observed, some patients are prone to develop complications earlier.

In vivo modulation of N-myristoyltransferase activity by orthovanadate.

Abstract: N-Myristoyltransferase (NMT) catalyses the transfer of myristate from myristoyl-CoA to the NH2-terminal glycine residue of several proteins and are important in signal transduction. STZ-induced diabetes (an animal model for insulin-dependent diabetes mellitus, IDDM) resulted in a 2-fold increase in rat liver NMT activity as compared with control animals. In obese Zucker (fa/fa) rats (an animal model for non-insulin dependent diabetes mellitus, NIDDM) there was a∼4.7-fold lower liver particulate NMT activity as compared with the control lean rat livers. Administration of sodium orthovanadate to the diabetic rats normalised liver NMT activity. These results would indicate that the rat liver particulate N-myristoyltransferase activity appears to be inversely proportional to the level of plasma insulin, implicating insulin in the control of N-myristoylation.