Functional synergism in the carbohydrate-induced activation of liver-type pyruvate kinase gene expression

Hepatic expression of the liver-type pyruvate kinase (L-PK) gene is induced at the transcriptional level by increased carbohydrate metabolism in the rat. The carbohydrate response of the L-PK gene requires sequences from -171 to -124, which encompass adjacent major late transcription factor (MLTF)-like and hepatic nuclear factor (HNF)-4 binding sites. Neither site alone is capable of conferring a response, prompting us to explore the mechanism of synergy between the MLTF-like factor and HNF-4. Spacing requirements between the two factor binding sites were tested by generating a series of mutations that altered the distance between these sites. Surprisingly, all of the constructs with spacing mutations were capable of responding to elevated glucose when introduced into primary hepatocytes. Thus the glucose response does not depend on the rigid phasing of the MLTF-like and HNF-4 factors, suggesting that the factors binding to these two sites do not interact directly with each other. Substitution or inversion of the PK HNF-4 site abrogated the response to glucose and also significantly suppressed the promoter activity under non-inducing conditions. We conclude that the MLTF-like factor and HNF-4 co-operate functionally to maintain the basal activity, as well as the carbohydrate responsiveness, of the L-PK gene. A mechanism other than co-operative DNA binding is responsible for the synergism.