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In vivo effects of vanadium on protein kinase B and key gluconeogenic enzymes in animal models of diabetes : comparison with insulin Marzban, Lucy

Abstract

Insulin resistance is a major characteristic of both Type 1 and Type 2 diabetes. However, despite numerous studies during the past decade the mechanism of insulin resistance is still not clear. Protein kinase B (PKB) has been proposed to be an intermediate protein kinase in the insulin signaling pathway by which insulin controls glucose disposal via stimulation of glycogen synthesis and glucose uptake in insulin sensitive tissues as well as hepatic glucose output. Hence, PKB may play a potential role in the development of insulin resistance. Vanadium compounds are known to have insulin mimetic/enhancing effects both in vitro and in vivo and therefore, are candidates for oral therapy in diabetes. Bis(maltolato)oxovanadium (IV) (BMOV) is an organic vanadium compound which corrects hyperglycemia in streptozotocin (STZ)-diabetic rats and lowers the elevated plasma insulin levels in fatty Zucker rats. In this study, we investigated 1) the association between PKB activity and insulin resistance 2) the in vivo effects of insulin and chronic BMOV treatment on PKB activity in the skeletal muscle and liver of two animal models of diabetes: STZ-diabetic Wistar rats, an animal model of poorly controlled Type 1 diabetes and fatty Zucker rats, an animal model that represents several characteristics of Type 2 diabetes. Animals were treated with BMOV in the drinking water (0.75-1 mg/ml) for 3 (or 8) weeks and sacrificed with or without insulin injection. Insulin (5 U/kg, i.v.) increased PKBα activity more than 10-fold and PKBβ activity more than 3-fold in both animal models. Despite the development of insulin resistance, PKBα activity was not impaired in STZ-diabetic rats up to 9 weeks of diabetes, thus excluding a role for PKBα in the development of insulin resistance in Type 1 diabetes. In contrast, insulin-induced PKBα (but not PKBβ) activity was markedly reduced in the skeletal muscle (fatty: 7-fold vs. lean: 14-fold), and significantly increased in the liver (fatty: 15.7-fold vs. lean: 7.6-fold) of fatty Zucker rats. These observations indicated an association between altered insulin-stimulated PKB activity and insulin resistance in this model. Comparison of basal PKBα activity in Zucker fatty and Zucker diabetic fatty rats indicated that basal enzyme activity was not affected by diabetes. BMOV, at doses sufficient to normalize fasting plasma glucose in STZ-diabetic rats and decrease plasma insulin levels in fatty Zucker rats had no detectable effect on basal or insulin-induced PKBα or PKBβ activities, indicating that the glucoregulatory effects of BMOV are independent of PKB activity in vivo. These findings led to the notion that BMOV may have more selective targets in the metabolic pathways. One potential pathway could be gluconeogenesis in the liver, since elevated hepatic glucose output is shown to be responsible for the fasting hyperglycemia in both types of diabetes. Hence, we tested the hypothesis that in vivo effects of vanadium may be mediated by changes in key gluconeogenic enzymes and inhibition of hepatic glucose output. STZ-diabetic rats were treated with BMOV in the drinking water (0.75-1 mg/ml) for 4 weeks or, for comparison, with insulin implants (4 U/day) for the final week of study. Phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase), two key gluconeogenic enzymes, were measured in the liver and kidney, the main sites of endogenous glucose production. Treatment of STZ-diabetic rats with BMOV led to normalization of PEPCK enzyme activity and mRNA, and G6Pase mRNA levels in both liver and kidney, indicating that hypoglycemic effects of BMOV in STZ-diabetic rats are at least partially mediated by its direct and/or indirect effects on PEPCK and G6Pase mRNA expression. BMOV had no detectable effect on the expression of PEPCK and G6Pase in either liver or kidney in the non-diabetic rats. Furthermore, insulin treatment of STZ-diabetic rats restored the elevated mRNA levels of PEPCK and G6Pase in both tissues. In summary, results of this study demonstrated that: 1) In STZ-diabetic rats, both basal and insulin-stimulated PKBα activity were normal up to 9 weeks of diabetes, thus excluding a role for PKB in the development of insulin resistance in Type 1 diabetes. 2) In fatty Zucker rats, insulin-induced activation of PKBα (but not PKBβ) was markedly altered in the skeletal muscle and liver, indicating an association between PKBα activity and insulin resistance in this model. 3) Changes in PKBα activity were tissue specific implying that different mechanisms may be involved in the regulation of PKB. 4) The hypoglycemic effects of BMOV were at least partially mediated by its direct and/or indirect effects on PEPCK and G6Pase mRNA levels in STZ-diabetic rats via a PKB independent pathway.

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