Role of the GLUT 2 Glucose Transporter in the Response of the L-type Pyruvate Kinase Gene to Glucose in Liver-derived Cells*

Abstract

Twenty-six different hepatoma cell lines established from cancer-prone transgenic mice exhibited a close correlation between expression of the GLUT 2 glucose transporter and activation of the L-type pyruvate kinase (L-PK) gene by glucose, as judged by Northern blot analyses and transient transfection assays. The L-PK gene and a transfected L-PK construct were silent in GLUT 2(+) cells and active in GLUT 2(−) cells cultured in glucose-free medium. Transfection of GLUT 2(−) cells with a GLUT 2 expression vector restored the inducibility of the L-PK promoter by glucose, mainly by suppressing the glucose-independent activity of this promoter. Culture of GLUT 2(−) cells, in which the L-PK gene is constitutively expressed, in a culture medium using fructose as fuel selected GLUT 2(+) clones in which the L-PK gene responded to glucose.

The expression of the L-PK gene in GLUT 2(−) cells cultured in the absence of glucose was correlated with a high intracellular glucose 6-phosphate (Glu-6-P) concentration while under similar culture conditions Glu-6-P concentration was very low in GLUT 2(+) cells. Consequently, a role of GLUT 2 in the glucose responsiveness of glucose-sensitive genes in cultured hepatoma cells could be to allow for Glu-6-P depletion under gluconeogenic culture conditions. In the absence of GLUT 2, glucose endogeneously produced might be unable to be exported from the cells and would be phosphorylated again to Glu-6-P by constitutively expressed hexokinase isoforms, continuously generating the glycolytic intermediates active on the L-PK gene transcription.