Hyperglycemia inhibits ascorbic acid (VitC) mass efflux by Glucose 6 phosphate (G6P) mediated and m-TOR independent translation of a repressor protein

Authors

• AN Henning
• HJ Alter
• N Dashdorj
• V De Giorgi

Abstract

Compared to healthy people, people with diabetes have lower VitC concentrations. VitC is normally reabsorbed by kidney tubules. People with diabetes abnormally lose VitC in urine, independent of glycosuria. We hypothesized that hyperglycemia disrupts VitC efflux from polarized cells that mediate VitC absorption and release in intestine, liver, and kidney. We chose human hepatocytes because if our hypothesis is correct, clinical testing can follow: hepatic VitC sequestration with hyperglycemia may lower plasma VitC concentrations. Cultured hepatocytes were incubated with VitC to achieve physiologic internal VitC concentrations. D-Glucose (2-25 mM) progressively inhibited VitC efflux from hepatocytes as measured by extracellular mass of VitC released. Glucose did not effect VitC uptake into hepatocytes, nor SVCT1 (VitC transporter)-mediated VitC uptake in Xenopus oocytes microinjected with SVCT1 cRNA. In hepatocytes, D-glucose inhibited VitC efflux, but L-glucose and 3-o-methyl D-glucose did not, indicating inhibition required glucose uptake and metabolism. Inhibiting G6P metabolic progression to glycolysis, glycogenesis, and the pentose shunt potentiated glucose inhibition of efflux, indicating glucose-mediated VitC efflux occurred via G6P. As predicted, G6P was increased by inhibiting glycolysis. Chemical or genetically-transient inhibition of protein synthesis abolished inhibitory effects of glucose on VitC efflux, but rapamycin (mTOR inhibitor) had no effect. Together, these data indicate that with hyperglycemia, VitC efflux from polarized epithelial cells was inhibited via G6P-mediated m-TOR independent translation of an as yet unidentified repressor protein. We predict that hyperglycemia in diabetes will cause hepatic sequestration and decreasing plasma VitC concentrations during hyperglycemia. To test this possibility, clinical experiments are underway.

Scientific Focus Area: Cell Biology

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