Metabolic impact of glucokinase overexpression in liver: lowering of blood glucose in fed rats is accompanied by hyperlipidemia.

Abstract

The balance between hepatic glucose uptake and production is perturbed in both major forms of diabetes. It has been suggested that pharmacologic or genetic methods for enhancing glucokinase (GK) enzymatic activity in liver might be a means of increasing glucose disposal and lowering blood glucose in diabetic patients. To better evaluate this possibility, we used a recombinant adenovirus containing the cDNA encoding GK (AdCMV-GKL) to achieve overexpression of the enzyme at different levels in liver of normal rats. In a first set of experiments, in rats fasted for 18 h, AdCMV-GKL infusion caused a 211% increase in hepatic GK activity relative to animals infused with a control virus (AdCMV-betaGAL). AdCMV-GKL-treated fasted rats exhibited no significant changes in circulating glucose, free fatty acids (FFAs), lactate, beta-hydroxybutyrate, or insulin levels relative to controls, whereas triglyceride (TG) levels were slightly increased (53%). In a second set of studies, in rats fed ad libitum, GK was overexpressed in liver by 3- and 6.4-fold. Animals with the lower degree of GK overexpression exhibited no significant changes in circulating glucose, FFAs, insulin, TG, or lactate levels relative to controls that received a virus encoding a catalytically inactive mutant GK (AdCMV-GK203), but did show a modest increase in lactate (58%) relative to AdCMV-betaGAL-infused controls. In contrast, the higher level of GK overexpression caused a 38% decrease in blood glucose levels and a 67% decrease in circulating insulin levels relative to AdCMV-GK203-infused animals. The decline in glucose levels was accompanied by a 190% increase in circulating TG and a 310% increase in circulating FFAs; total plasma cholesterol was unaffected. Finally, fasted animals treated with AdCMV-GKL had 5.4 times as much liver glycogen as AdCMV-betaGAL-treated controls; no significant increases in liver glycogen were observed at either level of GK overexpression in ad libitum-fed rats relative to fed controls. In sum, levels of hepatic GK overexpression associated with a decline in blood glucose are accompanied by equally dramatic increases in FFAs and TG, raising concerns about manipulation of liver GK activity as a viable strategy for treatment of diabetes.

Improved metabolic disorders of insulin receptor-deficient mice by transgenic overexpression of glucokinase in the liver

“…The transgenic mice showed improved glucose tolerance and mild hypoglycaemia under starved conditions as reported for other transgenic mouse lines with hepatic overexpression of GK [11,12,14]. The observed increases in serum triglyceride and lactate concentrations in our transgenic line have similarly been reported in other rodents with overexpression of liver GK achieved either by transgenesis or by adenoviral vector-mediated gene transfer [12,13,15].…”

“…Moreover, mice carrying liver-specific GK gene disruption showed decreased hepatic glucose uptake and were moderately hyperglycaemic [26]. The validity of the GK approach was questioned in a subsequent study which showed, using adenoviral vector-mediated GK gene transfer in rats, that the amount of GK overexpression in the liver required to modulate hyperglycaemia in Type II diabetes might have the adverse effect of exacerbating hyperlipidaemia associated with the disease [13].…”

“…It was thus proposed that increasing GK gene expression or enzyme activity in the liver in a constitutive manner could be a means for normalizing hyperglycaemia resulting from insulin deficiency or defective insulin signalling. To evaluate the effect of GK overexpression on hepatic function and fuel homeostasis under various physiological conditions, several groups have generated rodent models overexpressing GK in the liver either by transgenesis or adenovirus-mediated transfer [11,12,13,14]. These transgenic mice showed lower fasting blood glucose concentrations, improved glucose tolerance and increased glycogen synthesis in the liver as compared to wild-type (wt) mice.…”

Long-term overexpression of glucokinase in the liver of transgenic mice leads to insulin resistance

“…However, high levels of adenovirus-mediated GK overexpression decrease blood glucose concentrations with concomitant increases in circulating free fatty acids and triglycerides [11]. Moreover, an increase in GK activity has been described in obese-diabetic subjects [12] and in obese hyperinsulinaemic (fa/fa) Zucker rats [13], suggesting that hepatic GK overexpression could have diabetogenic effects.…”

Dietary coconut kernel protein beneficially modulates NFκB and RAGE expression in streptozotocin induced diabetes in rats

“…Hexokinase that catalyses the conversion of glucose to glucose-6-phosphate plays a central role in the maintenance of glucose homeostasis. In the liver, hexokinase is an important regulatory enzyme in the oxidation of glucose (O'Doherty et al 1999). Being an insulindependent enzyme, the hepatic hexokinase activity of diabetic rats is almost entirely inhibited or inactivated due to the absence of insulin (Gupta et al 1999), since insulin increases hepatic glycolysis by increasing the activity and amount of several key enzymes.…”