Zucker diabetic fatty (ZDF) rats, an animal model of human type 2 diabetes associated with obesity, have a blunted response to hyperglycemia with a failure to suppress net hepatic glucose production and increase glucose hepatic flux to glycogen associated with a failure to accelerate glucokinase (GK) catalyzed glucose phosphorylation. The ensuing hyperglycemia is thought to sustain elevated expression of hepatic lipogenic enzymes and lipogenesis, resulting in hepatosteatosis and hyperlipidemia. We identified various impairments of GK regulation as ZDF rats progress through different stages of severity of diabetes. During an early stage of diabetes, characterized by excessive postprandial hyperglycemia and hyperinsulinemia, GK resides with its inhibitory protein (GKRP) in the nucleus and fails to translocate to the cytoplasm where its activity would promote glucose phosphorylation. During a middle stage of diabetes, characterized by prolonged and marked hyperglycemia and hyperinsulinemia, glucose phosphorylation is impaired despite continuous GK residency in the cytoplasm. During a late stage of diabetes, characterized by prolonged and extreme hyperglycemia and euinsulinemia, GK expression is markedly decreased by an altered posttranscriptional process. Preventing large excursions of postprandial hyperglycemia as well as prolonged postabsorptive hyperglycemia reverses these impairments. To explore the mechanisms responsible for each type of altered GK regulation and how GK impacts hepatic lipogenic flux, we propose three aims. First, determine mechanisms underlying the impairments in GK activity during chronic hyperglycemia in ZDF rats. Specifically, we will address during an early stage of diabetes whether glucotoxicity alters the binding affinity between GK and GKRP and/or GK and 6-phosphofructose-2-kinase/fructose-2, 6-phosphatase (a cytoplasmic binding protein partner) by altering the phosphorylation status of these proteins. Additionally, we will examine the role of the polyol pathway and metabolites in glucotoxicity-induced reduction of GK protein via a decrease in the synthesis and/or increase in the degradation rate of the enzyme. Second, explore a strategy of normalizing expression of functional GK during the late stage of diabetes. We will examine whether low dose insulin therapy in the combination with reduction of glucotoxicity will maintain GK protein levels and normal hepatic glucose flux. Third, we will determine the mechanism by which hepatic lipogenesis is sustained even when glucose phosphorylation by GK is impaired. We will assess the effects of changing hepatic glucose flux by chronically altering glycemia without aggressive insulin therapy and/or inhibiting the polyol pathway on hepatic lipogenic gene expression and lipogenic flux. To execute these aims we will use a combination of glucose and pancreatic clamp techniques to trace hepatic glucose and lipid fluxes in conscious ZDF rats. Understanding how chronic hyperglycemia impairs the action of glucose and/or insulin on the liver in terms of glucose and lipid metabolism will provide the basis for improved treatment of type 2 diabetes.
The liver is an important organ for the maintenance of normal blood glucose and lipid, an impairment of the liver to switch from a producer of glucose and convert glucose to lipid contributes to the elevated blood glucose and lipid associated with many of the complications of type 2 diabetes. We found that inappropriate regulation within the liver of a key metabolic enzyme, glucokinase, is caused by elevated blood glucose, which can lead to a further elevation of blood glucose. We proposed to determine in a diabetic animal model of type 2 diabetes the reasons for impaired glucokinase activity, mechanism by which improved glycemic control reverses this impairment, a role of impaired glucokinase for unsuitable lipid production from liver, and the potential for diabetic complications.
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