Glucose Toxicity Is Responsible for the Development of Impaired Regulation of Endogenous Glucose Production and Hepatic Glucokinase in Zucker Diabetic Fatty Rats

Abstract

The effect of restoration of normoglycemia by a novel sodium-dependent glucose transporter inhibitor (T-1095) on impaired hepatic glucose uptake was examined in 14-week-old Zucker diabetic fatty (ZDF) rats. The nontreated group exhibited persistent endogenous glucose production (EGP) despite marked hyperglycemia. Gluconeogenesis and glucose cycling (GC) were responsible for 46 and 51% of glucose-6-phosphatase (G6Pase) flux, respectively. Net incorporation of plasma glucose into hepatic glycogen was negligible. Glucokinase (GK) and its inhibitory protein, GK regulatory protein (GKRP), were colocalized in the cytoplasm of hepatocytes. At day 7 of drug administration, EGP was slightly reduced, but G6Pase flux and GC were markedly lower compared with the nontreated group. In this case, GK and GKRP were colocalized in the nuclei of hepatocytes. When plasma glucose and insulin levels were raised during a clamp, EGP was completely suppressed and GC, glycogen synthesis from plasma glucose, and the fractional contribution of plasma glucose to uridine diphosphoglucose flux were markedly increased. GK, but not GKRP, was translocated from the nucleus to the cytoplasm. Glucotoxicity may result in the blunted response of hepatic glucose flux to elevated plasma glucose and/or insulin associated with impaired regulation of GK by GKRP in ZDF rats.

Contributions of glucokinase and phosphofructokinase-2/fructose bisphosphatase-2 to the elevated glycolysis in hepatocytes from Zuckerfa/farats

“…Previous studies have shown that glucose-induced translocation of glucokinase from the nucleus to the cytoplasm is impaired in models of insulin resistance and diabetes such as the Goto-Kakizaki, OLETF (41), and Zucker diabetic fatty rats (20,21). However, the underlying mechanisms have not been determined.…”

“…Defects in glucokinase translocation have been reported in animal models of Type 2 diabetes, including the Goto-Kakizaki rat and the Otsuka Long Evans Tokushima Fatty (OLETF) rat (41) and the Zucker diabetic fatty rat (20,21). The Zucker diabetic rat had impaired glucokinase translocation during a glucose and insulin infusion (21).…”

A defect in glucose-induced dissociation of glucokinase from the regulatory protein in Zucker diabetic fatty rats in the early stage of diabetes

“…The SA (dpm/mol) of plasma [2-3 H]-and [3-3 H]glucose were determined as described previously (24). Plasma samples were deproteinized using Ba(OH)2 and ZnSO4.…”

“…In the liver, the immunostaining for GK and GKRP expression, along with the Western blot analyses of these proteins, was performed using sheep anti-rat GST-GK serum and rabbit anti-rat GST-GKRP serum, as reported previously (13,23,24). Quantitative image analysis of GK and GKRP immunofluorescence in the nucleus and cytoplasm of hepatocytes was performed using a Zeiss LSM510 confocal laser scanning microscope (13,23,24). The internal He/Ne laser and external argon-krypton laser at 543, 647, and 488 nm were used to optimally excite Cy3, Cy5, and YoPro-1 fluorescence, respectively.…”

“…[ 3 H]glucose remained intact. The liver content of uridine 5Ј-diphosphate (UDP)-glucose and UDP-galactose was obtained through two sequential chromatographic separations, and the amount of [ 3 H] radioactivity in each fraction was measured, as described previously (23,24).…”