Acute glucose intolerance in insulinoma cells with unbalanced overexpression of glucokinase

The INS-r3-GK27 insulinoma cells are endowed with artificially inducible glucokinase under control of the reverse tetracycline-dependent transcriptional activator. Moderate induction of glucokinase has been shown to result in proportionate increases in glycolytic flux and in potentiation of glucose effects on insulin secretion and pyruvate kinase gene expression. In cells with 20-fold overexpression of glucokinase, however, glucose activation of secretion and gene expression was severely impaired. Measurements of the glycolytic flux in cells with 7- and 21-fold increases in glucokinase activity and determination of the flux control coefficient of this enzyme showed that control of glycolysis at the glucokinase step was lost in the cells at the higher level of overexpression. Challenging the cells with glucose above 6 mM resulted in massive accumulation of glucose 6-phosphate and caused a rapid and sustained depletion of cellular ATP, in contrast with the glucose-induced rise in ATP in cells with wild-type glucokinase levels. Loss of cell viability ensued upon prolonged culture in high glucose. In summary, in insulinoma beta cells strongly overexpressing glucokinase, an imbalance between glucose phosphorylation and turnover of glucose 6-phosphate resulted in acute glucose intolerance due to trapping of cellular orthophosphate in dead-end product and severe paralysis of energy metabolism.