Transcriptional regulation by glucose in the liver.
Numerous hepatic and adipocytic genes are transcriptionally controlled by glucose and insulin. It is the case, for example, of the pyruvate kinase L (L-PK) gene in the liver and of the spot 14 gene in adipocytes, coding for proteic factors of glycolysis and lipogenesis, respectively. At the hepatic level, the role of insulin is mainly to stimulate the synthesis of glucokinase, needed for phosphorylation of glucose to glucose 6-phosphate. An efficient regulation of the L-PK gene by glucose also needs the synthesis of the glucose transporter (Glut2): in its absence, transcription of the gene is independent of the presence of glucose in the medium. The role of Glut2 can be to enhance the depletion of gluconeogenic cells into glucose-6-phosphate (G6-P) when cultivated without glucose. G6-P seems to act by one of its metabolites in the pentose phosphate pathway, probably a pentose phosphate, maybe xylulose 5-phosphate. The active metabolites of this pathway could control the activity of protein kinase and protein phosphatase cascades, leading to a modification of the phosphorylation state of the glucose response complex. This complex is assembled by so-called glucose/carbohydrate response elements (GIRE, ChoRE) that are composed of E boxes of the CACGTG type, more or less modified, forming a palindrome whose both parts are separated by five bases. These sequences are able to bind USF1 and USF2 proteins, which seem to be necessary to the glucose response. However, the binding of USF proteins to the GIRE of the L-PK gene, appreciated by in vivo footprints, is not modulated by nutritional conditions. Therefore, these USF proteins could interact with different partners which are targets of regulating cues: transcription factors bound in the immediate vicinity of the glucose response complex, notably the HNF4 factor, and, maybe, other proteins interacting with the USF factors assembled to the GIRE. The actually ongoing experiments try to appreciate the nature and the role of these partners, and to evaluate the metabolic response of mice whose USF genes were disabled by homologous recombination.