EXPRESSION OF GLUCOKINASE IN CULTURED HUMAN MUSCLE-CELLS CONFERS INSULIN-INDEPENDENT AND GLUCOSE CONCENTRATION-DEPENDENT INCREASES IN GLUCOSE DISPOSAL AND STORAGE

Insulin resistance, as is found in skeletal muscle of individuals with obesity and NIDDM, appears to involve a reduced capacity of the hormo ne to stimulate glucose uptake and/or phosphorylation. The glucose pho sphorylation step, as catalyzed by hexokinase II, has been described a s rate limiting for glucose disposal in muscle, but overexpression of this enzyme under control of a muscle-specific promoter in transgenic mice has had limited metabolic impact. In the current study, me invest igated in a cultured muscle model whether expression of glucokinase, w hich in contrast to hexokinase II is not inhibited by glucose-6-phosph ate (G-6-P), mould have a pronounced metabolic impact. We used a recom binant adenovirus containing the cDNA-encoding rat liver glucokinase ( AdCMV-GKL) to increase the glucose phosphorylating activity in culture d human muscle cells by fourfold. G-6-P levels increased in AdCMV-GKL- treated cells in a glucose concentration-dependent manner over the ran ge of 1-30 mmol/l, whereas the much smaller increases in G-6-P in cont rol cells mere maximal at glucose concentrations <5 mmol/l. Further ce lls expressing glucokinase accumulated 17 times more 2-deoxyglucose-6- phosphate than control cells. In AdCMV-GKL-treated cells, the time-dep endent rise in G-6-P correlated with an increase in the activity ratio of glycogen synthase. AdCMV-GKL-treated cells also exhibited a 2.5- t o 3-fold increase in glycogen content and a four- to fivefold increase in glycolytic flux, proportional to the increase in glucose phosphory lating capacity. AU of these observations mere made in the absence of insulin. Thus we concluded that expression of glucokinase in cultured human muscle cells results in proportional increases in insulin-indepe ndent glucose disposal, and that muscle glucose storage and utilizatio n becomes controlled in a glucose concentration-dependent manner in Ad CMV-GKL-treated cells. These results encourage testing whether deliver y of glucokinase to muscle in vivo has an impact on glycemic control, which could be a method for circumventing the failure of insulin to st imulate glucose uptake and/or phosphorylation in muscle normally in in sulin-resistant subjects.