Abstract
The effect of increased expression of glycogen phosphorylase on glucose metabolism in human muscle was examined in primary cultured fibers transduced with recombinant adenovirus AdCMV-MGP encoding muscle glycogen 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% reduction in basal glycogen levels. Total glycogen synthase activity was 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 glycogenolysis in engineered cells. In unstimulated fibers, lactate production was three times higher in AdCMV-MGP fibers as compared with controls, despite similar rates of glycogenolysis. In transduced fibers incubated with 2-deoxyglucose, the level of 2-deoxyglucose 6-phosphate was about 8-fold elevated over the control even though hexokinase activity was unmodified in AdCMV-MGP fibers. Overexpression of glycogen phosphorylase also led to enhancement of [U-14C]glucose incorporation into glycogen, lactate, and lipid. Accordingly, determination of lipid cell content revealed that engineered cells were accumulating lipids. Furthermore, 14CO2 formation from [U-14C]glucose was 1.6-fold higher, whereas 14CO2 formation from [6-14C]glucose was unmodified, in AdCMV-MGP fibers. Our data show that in human skeletal muscle cells 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 phosphorylase and, thereby, in glycogen metabolism, is sufficient to enhance glucose uptake in the muscle cell. Glucose taken up by engineered muscle cells is essentially disposed of through nonoxidative metabolism and converted into lactate and lipid.
Highlights
Of Ca2ϩ and an increase in cyclic AMP, respectively, which in turn lead to the phosphorylation and activation of glycogen phosphorylase by phosphorylase kinase [2]
An increment of about 2-fold was detected in AdCMVMGP cells, which led to activity ratio values equivalent to those measured in control cells after forskolin challenge, indicating that almost all of the phosphorylase was in the active form after forskolin treatment
Depletion of glycogen was similar in control and AdCMV-MGP cells, the rate of net lactate production was much higher in engineered cells, as was the incorporation of radioactivity from 14C-glucose into lactate
Summary
Of Ca2ϩ and an increase in cyclic AMP, respectively, which in turn lead to the phosphorylation and activation of glycogen phosphorylase by phosphorylase kinase [2]. We have used adenoviruses bearing the rabbit muscle glycogen phosphorylase cDNA to increase the expression of the enzyme in human myotubes in culture This approach has allowed us to evaluate the contribution of phosphorylase to the regulation of glucose metabolism in human muscle fibers and the repercussion of the stimulation of the glycogenolytic process. Our data show that human muscle fibers overexpressing glycogen phosphorylase show a higher capacity for glucose uptake and metabolism through nonoxidative glycolysis and lipid synthesis. These results may be related to the physiological mechanism by which muscle glucose disposal is increased during and after exercise
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