Total hepatic Mg 2+ content decreases by >25% in animals maintained for two weeks on Mg 2+ deficient diet, and results in a >25% increase in glucose 6-phosphatase (G6Pase) activity in isolated liver microsomes in the absence of significant changed in enzyme expression. Incubation of Mg 2+ -deficient microsomes in the presence of 1mM external Mg 2+ returned G6Pase activity to levels measured in microsomes from animals on normal Mg 2+ diet. EDTA addition dynamically reversed the Mg 2+ effect. The effect of Mg 2+ or EDTA persisted in taurocholic acid permeabilized microsomes. An increase in G6Pase activity was also observed in liver microsomes from rats starved overnight, which presented a ~15% decrease in hepatic Mg 2+ content. In this model, G6Pase activity increased to a lesser extent than in Mg 2+ -deficient microsomes, but it could still be dynamically modulated by addition of Mg 2+ or EDTA. Our results indicate that 1) hepatic Mg 2+ content rapidly decreases following starvation or exposure to deficient diet, and 2) the loss of Mg 2+ stimulates G6P transport and hydrolysis as a possible compensatory mechanism to enhance intrahepatic glucose availability. The Mg 2+ effect appears to take place at the level of the substrate binding site of the G6Pase enzymatic complex or the surrounding phospholipid environment.

We have reported that Mg2+ dynamically regulates glucose 6-phosphate entry into the endoplasmic reticulum and its hydrolysis by the glucose 6-phosphatase in liver cells. In the present study, we report that by modulating glucose 6-phosphate entry into the endoplasmic reticulum of HepG2 cells, Mg2+ also regulates the oxidation of this substrate via hexose 6-phosphate dehydrogenase (H6PD). This regulatory effect is dynamic as glucose 6-phosphate entry and oxidation can be rapidly down-regulated by the addition of exogenous Mg2+. In addition, HepG2 cells growing in low Mg2+ show a marked increase in hexose 6-phosphate dehydrogenase mRNA and protein expression. Metabolically, these effects on hexose 6-phosphate dehydrogenase are important as this enzyme increases intra-reticular NADPH production, which favors fatty acid and cholesterol synthesis. Similar effects of Mg2+ were observed in HL-60 cells. These and previously published results suggest that in an hepatocyte culture model changes in cytoplasmic Mg2+ content regulates glucose 6-phosphate utilization via glucose 6 phosphatase and hexose-6 phosphate dehydrogenase in alternative to glycolysis and glycogen synthesis. This alternative regulation might be of relevance in the transition from fed to fasted state.

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