GLUCOSE-6-PHOSPHATE PRODUCED BY GLUCOKINASE, BUT NOT HEXOKINASE-I, PROMOTES THE ACTIVATION OF HEPATIC GLYCOGEN-SYNTHASE

In a previous study (O'Doherty, R. Ri., Lehman, D. L., Seoane, J., Gom ez-Foix, A. M., Guinovart, J. J., and New-gard, C.B. (1996) J. Biol. C hem. 271, 20524-20530), we demonstrated that adenovirus-mediated overe xpression of glucokinase but not hexokinase I has a potent enhancing e ffect on glycogen synthesis in primary hepatocytes. In an effort to un derstand the underlying mechanism of this differential effect of the t wo hexokinase isoforms, we have investigated changes in key intracellu lar metabolites and the activation state of glycogen synthase in cells treated with recombinant adenoviruses expressing the liver isoform of glucokinase (AdCMV-GKL) or hexokinase I (AdCMV-HKI). Glucose 6-phosph ate (Glu-6-P) levels are elevated from approximately 1.5 nmol/mg prote in Do 8-10 nmol/mg protein in both AdCMV-GKL- and AdCMV-HKI-treated he patocytes as glucose is raised from 1 to 5 mM, levels four times highe r than those in untreated cells. In AdCMV-GKL-treated cells, Glu-6-P c ontinues to accumulate at glucose levels greater than 5 mM, reaching a maximum of 120 nmol/mg protein in cells incubated at 25 mM glucose, a value 10 and 50 times greater than the maximal levels achieved in AdC MV-HKI-treated and untreated cells, respectively. In parallel with the changes observed in Glu-6-P levels, increases in UDP-Glc in AdCMV-HKI - and AdCMV-GKL-treated cells were most pronounced at low (1-5 mM) and high (25 mM) glucose levels, respectively. Despite the significant in creases in Glu-6-P and UDP-Glc achieved in AdCMV-HKI-treated cells, on ly AdCMV-GKL-treated cells exhibited increases in glycogen synthase ac tivity ratio and translocation of the enzyme from a soluble to a parti culate form relative to untreated control cells. We conclude that Glu- 6-P produced by overexpressed glucokinase is glycogenic because it eff ectively promotes activation of glycogen synthase. Glu-6-P produced by overexpressed hexokinase, in contrast, appears to be unable to exert the same regulatory effects, probably due to the different subcellular distribution of the two glucose-phosphorylating enzymes.