Inability of insulin to activate liver glycogen transferase D phosphatase in the diabetic pancreatectomized dog

Immunomodulatory and Antidiabetic Effects of a New Herbal Preparation (HemoHIM) on Streptozotocin-Induced Diabetic Mice

HemoHIM (a new herbal preparation of three edible herbs: Angelica gigas Nakai, Cnidium officinale Makino, and Paeonia japonica Miyabe) was developed to protect immune, hematopoietic, and self-renewal tissues against radiation. This study determined whether or not HemoHIM could alter hyperglycemia and the immune response in diabetic mice. Both nondiabetic and diabetic mice were orally administered HemoHIM (100 mg/kg) once a day for 4 weeks. Diabetes was induced by single injection of streptozotocin (STZ, 200 mg/kg, i.p.). In diabetic mice, HemoHIM effectively improved hyperglycemia and glucose tolerance compared to the diabetic control group as well as elevated plasma insulin levels with preservation of insulin staining in pancreatic β-cells. HemoHIM treatment restored thymus weight, white blood cells, lymphocyte numbers, and splenic lymphocyte populations (CD4⁺ T and CD8⁺ T), which were reduced in diabetic mice, as well as IFN-γ production in response to Con A stimulation. These results indicate that HemoHIM may have potential as a glucose-lowering and immunomodulatory agent by enhancing the immune function of pancreatic β-cells in STZ-induced diabetic mice.

Antioxidant, anti-diabetic and renal protective properties of Stevia rebaudiana

BACKGROUND

Stevia rebaudiana Bertoni has been used for the treatment of diabetes in, for example, Brazil, although a positive effect on antidiabetic and its complications has not been unequivocally demonstrated. This herb also has numerous therapeutic properties which have been proven safe and effective over hundreds of years. Streptozotocin is a potential source of oxidative stress that induces genotoxicity.

OBJECTIVE

We studied the effects of stevia leaves and its extracted polyphenols and fiber on streptozotocin induced diabetic rats. We hypothesize that supplementation of polyphenols extract from stevia to the diet causes a reduction in diabetes and its complications.

DESIGN/METHODS

Eighty Wistar rats were randomly divided into 8 groups; a standard control diet was supplemented with either stevia whole leaves powder (4.0%) or polyphenols or fiber extracted from stevia separately and fed for one month. Streptozotocin (60 mg/kg body weight, i.p) was injected to the diabetic groups on the 31st day. Several indices were analyzed to assess the modulation of the streptozotocin induced oxidative stress, toxicity and blood glucose levels by stevia.

RESULTS

The results showed a reduction of blood glucose, ALT and AST, and increment of insulin level in the stevia whole leaves powder and extracted polyphenols fed rats compared to control diabetic group. Its feeding also reduced the MDA concentration in liver and improved its antioxidant status through antioxidant enzymes. Glucose tolerance and insulin sensitivity were improved by their feeding. Streptozotocin was also found to induce kidney damage as evidenced by decreased glomerular filtration rate; this change was however alleviated in the stevia leaves and extracted polyphenol fed groups.

CONCLUSION

The results suggested that stevia leaves do have a significant role in alleviating liver and kidney damage in the STZ-diabetic rats besides its hypoglycemic effect. It might be adequate to conclude that stevia leaves could protect rats against streptozotocin induced diabetes, reduce the risk of oxidative stress and ameliorate liver and kidney damage.

Beneficial effects of ginger (Zingiber officinale) on carbohydrate metabolism in streptozotocin-induced diabetic rats

Zingiber officinale (ZO), commonly known as ginger, has been traditionally used in the treatment of diabetes mellitus. Several studies have reported the hypoglycaemic properties of ginger in animal models. The present study evaluated the antihyperglycaemic effect of its aqueous extract administered orally (daily) in three different doses (100, 300, 500 mg/kg body weight) for a period of 30 d to streptozotocin (STZ)-induced diabetic rats. A dose-dependent antihyperglycaemic effect revealed a decrease of plasma glucose levels by 38 and 68 % on the 15th and 30th day, respectively, after the rats were given 500 mg/kg. The 500 mg/kg ZO significantly (P<0·05) decreased kidney weight (% body weight) in ZO-treated diabetic rats v. control rats, although the decrease in liver weight (% body weight) was not statistically significant. Kidney glycogen content increased significantly (P<0·05) while liver and skeletal muscle glycogen content decreased significantly (P<0·05) in diabetic controls v. normal controls. ZO (500 mg/kg) also significantly decreased kidney glycogen (P<0·05) and increased liver and skeletal muscle glycogen in STZ-diabetic rats when compared to diabetic controls. Activities of glucokinase, phosphofructokinase and pyruvate kinase in diabetic controls were decreased by 94, 53 and 61 %, respectively, when compared to normal controls; and ZO significantly increased (P<0·05) those enzymes' activities in STZ-diabetic rats. Therefore, the present study showed that ginger is a potential phytomedicine for the treatment of diabetes through its effects on the activities of glycolytic enzymes.

Attenuation of Helicteres isora L. bark extracts on streptozotocin-induced alterations in glycogen and carbohydrate metabolism in albino rats

The present study was undertaken to assess the effect of Helicteres isora L. on four important enzymes of carbohydrate metabolism (glucokinase [GK], hexokinase [HK] phosphofructokinase [PFK] and fructose-1, 6-bisphosphatase [FBP]) along with glycogen content of insulin-dependent (skeletal muscle and liver) and insulin-independent tissues (kidneys and brain) in streptozotocin (STZ; 60 mg/kg)-induced model of diabetes for 30 days. Administration of bark extracts (100, 200 mg/kg) for 30 days led to decrease in plasma glucose levels by approximately 9.60% and 22.04% and 19.18% and 33.93% on 15th and 30th day, respectively, of the experiment. Liver and two-kidney weight expressed as percentage of body weight significantly increased in diabetics (P < 0.05) versus normal controls. Renal glycogen content increased by 10 folds while hepatic and skeletal muscle glycogen content decreased by 75% and 68% in diabetic controls versus controls. H. isora did not affect glycogen content in any tissue. The decreased activities of PFK, GK, FBP and HK in diabetic controls were 40%, 50%, 50% and 60% and bark extract of H. isora partially corrected this alteration. The efficacy of the bark extract was comparable with Tolbutamide, a well-known hypoglycemic drug.

Modulatory effects of resveratrol on attenuating the key enzymes activities of carbohydrate metabolism in streptozotocin-nicotinamide-induced diabetic rats

Resveratrol, a ubiquitous stress-induced phytoalexin, has demonstrated a wide variety of biological activities which make it a good candidate for the treatment of diabetes mellitus. The present study was aimed to evaluate its therapeutic potential by assaying the activities of key enzymes of carbohydrate metabolism in streptozotocin-nicotinamide-induced diabetic rats. The daily oral treatment of resveratrol (5 mg/kg body weight) to diabetic rats for 30 days demonstrated a significant (p<0.05) decline in blood glucose and glycosylated hemoglobin levels and a significant (p<0.05) increase in plasma insulin level. The altered activities of the key enzymes of carbohydrate metabolism such as hexokinase, pyruvate kinase, lactate dehydrogenase, glucose-6-phosphatase, fructose-1,6-bisphosphatase, glucose-6-phosphate dehydrogenase, glycogen synthase and glycogen phosphorylase in liver and kidney tissues of diabetic rats were significantly (p<0.05) reverted to near normal levels by the administration of resveratrol. Further, resveratrol administration to diabetic rats improved hepatic glycogen content suggesting the antihyperglycemic potential of resveratrol in diabetic rats. The obtained results were compared with glyclazide, a standard oral hypoglycemic drug. Thus, the modulatory effects of resveratrol on attenuating these enzymes activities afford a promise for widespread use for treatment of diabetes in the future.

Insulin and the stimulation of glycogen synthesis. The road from glycogen structure to glycogen synthase to cyclic AMP-dependent protein kinase to insulin mediators

The enhanced phosphorylations via cAMP, Ca2+ mobilization, and diacyl glycerol formation via the activation of the respective kinases is now classical. The decreased phosphorylation via inhibition of adenylate cyclase via the alpha adrenergic receptor is also becoming understood. What the insulin studies on the control of glycogen synthesis have taught us is that the rate limiting enzyme glycogen synthase is regulated by multiple covalent phosphorylation in an elegant but complex manner. The overall pattern of dephosphorylation is influenced by effecting both phosphatase and kinase activities in a set of interrelated mechanisms. In the presence of glucose, in muscle, fat, and liver under physiological conditions G-6-P acts as a signal to stimulate the phosphatase. An additional stimulation could occur via a novel insulin phosphatase stimulatory mediator. The phosphatase is also stimulated by at least three covalent mechanisms involving altered phosphorylation state. In one there is a decreased phosphorylation of the phosphatase inhibitor 1 potentially related to decreased cAMP-dependent protein kinase activity. In the second, there is decreased phosphorylation of the deinhibitor also potentially related to decreased cAMP-dependent protein kinase phosphorylation. In the third, an increased activity of casein kinase 2 could activate the ATP-Mg dependent phosphatase by an increased phosphorylation of phosphatase inhibitor 2 (modulatory subunit). In the liver, allosteric control of the phosphatase by G-6-P and nucleotides is of great importance. Insulin also stimulates the phosphatase in long-term experiments via increased protein synthesis. It is clear that future work will be required to determine which species of the various classes of phosphatases are regulated in short-term and long-term regulation by insulin. In terms of kinases, the effects of insulin to inactivate and desensitize the cAMP-dependent protein kinase are established. The molecular mechanisms of this effect remain to be worked out. The enhanced activity of MAP and S-6 kinase would appear to be part of a cascade of reactions perhaps originating in the autophosphorylation and activation of the insulin receptor tyrosine kinase. The mechanism of the short-term activation of casein kinase 2 remains to be elucidated. A cAMP-dependent protein kinase inhibitory mediator, which also inhibits adenylate cyclase is an important element in the regulation of kinase and adenylate cyclase activity by insulin. Its physiological significance must be established in the future, in terms of its control of glycogen synthase activation by insulin. Clearly this kinase inhibitor as well as the phosphatase stimulator are potential regulators of glycogen synthase activity by insulin.

Ethanolic extract of Ocimum sanctum leaves partially attenuates streptozotocin-induced alterations in glycogen content and carbohydrate metabolism in rats

Ocimum sanctum (OS) has been mentioned in Indian system of traditional medicine to be of value in the treatment of diabetes mellitus. We have previously shown that OS shows a dose-dependent hypoglycemic effect and prevented rise in plasma glucose in normal rats. It also showed significant antihyperglycemic effect in STZ-induced diabetes. The present study was undertaken to assess the effect of OS on three important enzymes of carbohydrate metabolism [glucokinase (GK) (EC 2.7.1.2), hexokinase (HK) (EC 2.7.1.1) and phosphofructokinase (PFK) (EC 2.7.1.11)] along with glycogen content of insulin-dependent (skeletal muscle and liver) and insulin-independent tissues (kidneys and brain) in STZ (65 mg/kg) induced model of diabetes for 30 days. Administration of OS extract 200mg/kg for 30 days led to decrease in plasma glucose levels by approximately 9.06 and 26.4% on 15th and 30th day of the experiment. Liver and two-kidney weight expressed as percentage of body weight significantly increased in diabetics (P<0.0005) versus normal controls. OS significantly decreased renal (P<0.0005) but not liver weight. Renal glycogen content increased by over 10 folds while hepatic and skeletal muscle glycogen content decreased by 75 and 68% in diabetic controls versus controls. OS did not affect glycogen content in any tissue. Activity of HK, GK and PFK in diabetic controls was 35, 50 and 60% of the controls and OS partially corrected this alteration.

Effect of T. foenumgraecum on glycogen content of tissues and the key enzymes of carbohydrate metabolism

The Indian traditional system of medicine prescribed plant therapies for diseases including diabetes mellitus called madhumeh in Sanskrit. One such plant mentioned in Ayurveda is Trigonella foenumgraecum (FG). In the present study, FG (1g/kg PO) was assessed for its effect on glycogen levels of insulin dependent (skeletal muscle and liver), insulin independent tissues (kidneys and brain) and enzymes such as glucokinase (GK), hexokinase (HK), and phosphofructokinase (PFK). Administration of FG led to decrease in blood glucose levels by 14.4 and 46.64% on 15th and 30th day of the experiment. Liver and 2-kidney weight expressed as percentage of body weight was significantly increased in diabetics (P<0.0005) versus normal controls and this alteration in the renal weight (P<0.0005) but not liver weight was normalized by feeding of FG. Renal glycogen content increased by over 10 folds while hepatic and skeletal muscle glycogen content decreased by 75 and 68% in diabetic controls versus controls and these alteration in glycogen content was partly prevented by FG. Activity of HK, GK and PFK in diabetic controls was 35, 50 and 60% of the controls and FG partially corrected this alteration in PFK, HK and GK.

Influence of long-term diabetes on liver glycogen metabolism in the rat

Diabetes acutely impairs the ability of the liver to synthesize glycogen. However, the effect of chronic diabetes on the glycogenic function of the liver is not known. We measured hepatic glycogen contents in streptozotocin (STZ)-diabetic rats 3 weeks or 9 months after the induction of diabetes, in the fed state and following a 24-hour fast. In the fed state, liver glycogen levels were markedly decreased in short-term diabetic animals (5.8 +/- 2.0 v 33.9 +/- 2.3 mg/g, P less than .001), but not in long-term diabetic rats (18.3 +/- 4.4 v 20.7 +/- 1.3 mg/g, P = NS) as compared with age-matched nondiabetic animals, despite comparable hyperglycemia (portal plasma glucose levels of 424 +/- 21 and 449 +/- 24 mg/100 mL, short- and long-term diabetics, respectively). In the fasted state, on the other hand, liver glycogen was depleted in acute diabetes (4.5 +/- 2.2 mg/g v 1.9 +/- 0.5 of control rats), but significantly increased in chronic diabetes (10.1 +/- 3.1 v 0.2 +/- 0.03 mg/g, P less than .001). The latter finding was confirmed by electron-microscopical examination of liver cells. Furthermore, the percentage of hepatic glycogen synthase in the active form (synthase a) was lower than normal in short-term diabetic rats and in old nondiabetic rats. In long-term diabetic animals, on the other hand, synthase a was significantly higher than in old controls (P less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Increase in liver protein phosphatase-1 in spontaneously diabetic Chinese hamsters

Two broad-specifically protein phosphatases, termed protein phosphatase-1 (PrP-1) and protein phosphatase-2A (PrP-2A), accounting for all the hepatic activity regulating glycogen phosphorylase, were measured in spontaneously diabetic Chinese hamsters exhibiting persistent glycosuria. When compared with genetically related inbred sublines free of glycosuria, diabetic animals demonstrated approximately 25% increase in PrP-1 activity measured either in crude tissue extracts or in cytosols fractionated by ion-exchange chromatography. No significant alteration in total PrP-2A activity was observed in the diabetic animals. These findings indicate that a specific change in hepatic PrP-1 is associated with genetically acquired diabetes in Chinese hamsters. In contrast to reported data using animals with experimentally induced diabetes mellitus, hepatic PrP-1 was increased in the spontaneously diabetic Chinese hamsters. The data suggests that distinct alterations in PrP-1 and associated metabolic consequences are exhibited by different types of diabetes.

Insulin-induced increases in the activity of the spontaneously active and ATP.Mg-dependent forms of phosphatase-1 in alloxan-diabetic rat liver

Liver supernatant from normal and alloxan-diabetic rats was fractionated by DEAE-cellulose chromatography and the separated phosphoprotein phosphatase fractions were assayed with [32P]histone f2b, [32P]phosphorylase a and [32P]phosphorylase kinase as substrates. In diabetic rat liver, one of the phosphatase fractions found in the normal liver was significantly reduced. This fraction was identified as a mixture of the spontaneously active form and the ATP . Mg-dependent form of phosphoprotein phosphatase-1 (Fc) based on sensitivity to inhibitor-2, substrate specificity, and the fact that it could be activated 42-70% by glycogen synthase kinase-3 in the presence of ATP . Mg. Further analysis of this fraction showed that liver cytosol from diabetic rats contained 62-79% lower spontaneously active phosphatase-1 activity and 40-51% lower combined spontaneously active and ATP . Mg-dependent protein phosphatase-1 (Fc) activity. Insulin administration increased the spontaneously active and the ATP . Mg-dependent protein phosphatase-1 activities approximately 45% and 36%, respectively, in alloxan-diabetic rats. These data imply that the lower levels of spontaneously active phosphatase-1 activity in diabetic rat liver cannot be explained by presuming phosphatase-1 to have been present as Fc, the inactive form. Moreover, insulin restored the total activity of the spontaneously active and activatable forms of phosphatase-1 to those present in normal liver implying that both forms of phosphatase-1 activity are under hormonal control.

Regulation of hepatic glycogen metabolism: effects of diabetes, insulin infusion, and pancreatic islet transplantation

Insulin-deficient diabetes mellitus results in diminished capacity of the liver to accumulate glycogen. One site of metabolic lesion in the diabetic liver is at the level of the synthase-activating enzyme, synthase phosphatase. This activity is progressively diminished with increasing severity of chemically induced diabetes in both soluble and smooth endoplasmic reticulum (SER) associated subfractions. Insulin administration via an implanted miniosmotic pump or via intrahepatic islet transplantation increased synthase phosphatase activity, particularly in SER. Hepatic glycogen synthesis and accumulation was enhanced as well. The data support a role for insulin in maintenance of the ability of the liver to synthesize and accumulate glycogen mediated either directly or indirectly through SER-synthase phosphatase activity.

Effects of insulin-glucagon interactions on glycogenolysis and protein kinase activity in rat hepatocytes

The effects of insulin and glucagon on cAMP accumulation, protein kinase activation, and glycogenolysis were investigated in isolated rat hepatocytes. Glucagon (0.01 nM to 10 micro M) increased the activation state of protein kinase and the rate of glucose accumulation. Addition of 1.0 nM insulin to cells preincubated with 0.1 nM glucagon attenuated the rate of glucose accumulation, but did not alter the protein kinase activity ratio. Addition of 0.1 nM glucagon to cells preincubated with 1.0 nM insulin caused a rapid activation of protein kinase; however, glycogenolysis was not immediately affected. These effects were enhanced with pharmacological concentrations of glucagon and insulin. These data indicate that the degree of protein kinase activation does not always correlate temporally or quantitatively with rates of glycogenolysis in liver cells exposed to insulin and glucagon.

Adverse effects of fructose in perfused livers of diabetic rats

Livers isolated from both fed normal and alloxan diabetic rats were perfused for 30 min using Krebs-Henseleit bicarbonate blood buffer medium followed by 10 min flow-through infusions with either 5 mM or 28 mM fructose concentrations. In livers of normal and diabetic rats, both 5 mM and 28 mM fructose concentrations produced an elevation in tissue cyclic AMP levels, activation of glycogen phosphorylase, increased protein kinase activity, decreased tissue ATP levels, large increases in tissue fructose-1-phosphate, and variable effects upon glycogen synthase. These results are consistent with previously reported cyclic AMP mediated activation of glycogen phosphorylase by fructose via protein kinase in normal rat liver. In addition, both 5 mM and 28 mM fructose infusion resulted in large decreases in normal and diabetic synthase phosphatase activity. Therefore, these results in both normal and diabetic livers are inconsistent with a direct beneficial effect of fructose in the isolated perfused rat liver.

Stimulation of muscle glycogen synthase phosphatase by polyamines

The naturally-occurring polyamines were found to stimulate glycogen synthase phosphatase from rat skeletal muscles. The sequence of effectiveness in the stimulation was spermine greater than spermidine greater than putrescine. It was shown that the spermine-sensitive phosphatase was present primarily in the soluble fraction of the muscle extract. In the presence of spermine, the phosphatase activity can be further stimulated by Mn2+; however, a lower Mn2+ concentration is required for the activation of the enzyme in comparison with that in the absence of spermine. Kinetic studies indicated that activation of glycogen synthase phosphatase by spermine was achieved by an increase in its V without significant alteration in the Km, suggesting that spermine directly stimulated the catalytic efficiencly of the phosphatase enzyme.

Glucose activation of liver glycogen synthase. Insulin-mediated restoration of glucose effect in diabetic rats is blocked by protein synthesis inhibitor

The loss of glucose regulation of glycogen synthase in perfused livers from diabetic rats was associated with a substantial reduction in synthase phosphatase activity. Treatment of diabetic rats with insulin alone resulted in total restoration of the glucose effect and synthase phosphatase activity, while simultaneous treatment with cycloheximide severely reduced the hormonal effect. Although treatment of normal rats with cycloheximide had no effect on glucose activation of synthase, it did result in severe depletion of liver glycogen, increased liver glycogen phosphorylase activity, and elevation of liver adenosine 3',5'-monophosphate (cyclic AMP), but without elevation of liver protein kinase activity. Simultaneous treatment of alloxan-diabetic rats with insulin and cycloheximide resulted in reduction of total liver glycogen, increased phosphorylase activity, a reduction in the ability of insulin to lower hepatic cyclic AMP, and a further reduction of protein kinase activity. In summary, the effect of insulin treatment of diabetic rats to restore glucose regulation of hepatic glycogen synthase probably involves synthesis of new protein, and the data remain consistent with the hypothesis that the defect may be due to a diabetes-related deficiency in a specific synthase phosphatase and/or alteration of the synthase molecule itself.

Hormonal control of glycogen metabolism

In summary, we have presented evidence which relates to the action pathway of hormonal control of glycogen metabolism. In the case of insulin, there are changes demonstrable in the cyclic AMP-dependent protein kinase and also in the phosphoprotein phosphatase, under conditions where no direct relationship to either cyclic AMP or cyclic GMP levels are measurable. Therefore, a new unknown intermediate or second messenger system is again proposed. An insulin-generated labile compound(s) which inhibits the protein kinase has been discovered. This may function as an intermediate. Finally, the fact that the glycogen synthase system clearly differs from phosphorylase in its regulation by covalent phosphorylation is discussed. Synthase is now accepted as a multiply phosphorylated subunit, in contrast to phosphorylase which is singly phosphorylated. The inherent theoretical advantages of multiple phosphorylation over single phosphorylation are considered. The advantages of a multistate over a two-state model of enzyme interconversion are mentioned. The importance of the multiple phosphorylations interacting in a nonlinear manner with the control by cellular metabolites is in the explanation of how a small change in covalent phosphorylation signalled by a hormone can be translated in the cell milieu into a much larger change in rate.

Regulation of synthase phosphatase and phosphorylase phosphatase in rat liver

Using substrates purified from liver, the apparent Km values of synthase phosphatase ([UDPglucose--glycogen glucosyltransferase-D]phosphohydrolase, EC 3.1.3.42) and phosphorylase phosphatase (phosphorylase a phosphohydrolase, EC 3.1.3.17) were found to be 0.7 and 60 units/ml respectively. The maximal velocity of phosphorylase phosphatase was more than a 100 times that of synthase phosphatase. In adrenalectomized, fasted animals there was a complete loss of synthase phosphatase but only a slight decrease in phosphorylase phosphatase when activity was measured using endogenous substrates in a concentrated liver extract. When assayed under optimal conditions with purified substrates, both activities were present but had decreased to very low levels. Mixing experiments indicated that synthase D present in the extract of adrenalectomized fasted animals was altered such that it was no longer a substrate for synthase phosphatase from normal rats. Phosphorylase a substrate on the other hand was unaltered and readily converted. When glucose was given in vivo, no change in percent of synthase in the I form was seen in adrenalectomized rats but the percent of phosphorylase in the a form was reduced. Precipitation of protein from an extract of normal fed rats with ethanol produced a large activation of phosphorylase phosphatase activity with no corresponding increase in synthase phosphatase activity. Despite the low phosphorylase phosphatase present in extracts of adrenalectomized fasted animals, ethanol precipitation increased activity to the same high level as obtained in the normal fed rats. Synthase phosphatase and phosphorylase phosphatase activities were also decreased in normal fasted, diabetic fed and fasted, and adrenalectomized fed rats. Both enzymes recovered in the same manner temporally after oral glucose administration to adrenalectomized, fasted rats. These results suggest an integrated regulatory mechanism for the two phosphatase.