OVEREXPRESSION OF MUSCLE GLYCOGEN-PHOSPHORYLASE IN CULTURED HUMAN MUSCLE-FIBERS CAUSES INCREASED GLUCOSE CONSUMPTION AND NONOXIDATIVE DISPOSAL

The effect of increased expression of glycogen phosphorylase on glucos e metabolism inhuman muscle was examined in primary cultured fibers tr ansduced with recombinant adenovirus AdCMV-MGP encoding muscle glycoge n phosphorylase, Increments of 20-fold in total enzyme activity and of 14-fold of the active form of the enzyme were associated with a 30% r eduction in basal glycogen levels, Total glycogen synthase activity wa s doubled in AdCMV-MGP-transduced cells even though the activity ratio was decreased, Incubation with forskolin, which inactivated glycogen synthase and activated glycogen phosphorylase, induced greater net gly cogenolysis in engineered cells, In unstimulated fibers, lactate produ ction was three times higher in AdCMV-MGP fibers as compared with cont rols, despite similar rates of glycogenolysis. In transduced fibers in cubated with 2-deoxyglucose, the level: of 2-deoxyglucose 6-phosphate was about 8-fold elevated over the control even though hexokinase acti vity was unmodified in AdCMV-MGP fibers, Overexpression of glycogen ph osphorylase also led to enhancement of [U-C-14]glucose incorporation i nto glycogen, lactate, and lipid. Accordingly, determination of lipid cell content revealed that engineered cells were accumulating lipids, Furthermore, (CO2)-C-14 formation from [U-C-14]glucose was 1.6-fold hi gher, whereas (CO2)-C-14 formation from [6-C-14]glucose was unmodified , in AdCMV-MGP fibers, Our data show that in human skeletal muscle cel ls in culture, the increase in glycogen phosphorylase activity is able to up-regulate glycogen synthase activity indicating the enhancement of glycogen turnover, We suggest that the increase in glycogen phospho rylase and, thereby, in glycogen metabolism, is sufficient to enhance glucose uptake in the muscle cell, Glucose taken up by engineered musc le cells is essentially disposed of through nonoxidative metabolism an d converted into lactate and lipid.