CARBOHYDRATE REGULATION OF HEPATIC GENE-EXPRESSION - EVIDENCE AGAINSTA ROLE FOR THE UPSTREAM STIMULATORY FACTOR

Hepatic expression of the genes encoding L-type pyruvate kinase (L-PR) and S-14 is induced in rats upon feeding them a high carbohydrate, lo w fat diet. A carbohydrate response element (ChoRE) containing two CAC GTG-type E boxes has been mapped in the 5'-flanking region of both of these genes. The nature of the ChoRE suggests that a member of the bas ic/helix-loop-helix/leucine zipper family of proteins may be responsib le for mediating the response to carbohydrate. Indeed, the upstream st imulatory factor (USF), a ubiquitous basic/helix-loop-helix/leucine zi pper protein, is present in hepatic nuclear extracts and binds to the ChoREs of L-PK and S-14 in vitro. We have conducted experiments to det ermine whether USF is involved in the carbohydrate-mediated regulation of L-PH and S-14. For this purpose, dominant negative forms of USF th at are capable of heterodimerizing with endogenous USF but not of bind ing to DNA were expressed in primary hepatocytes. Expression of these forms did not block either S-14 Or L-PK induction by glucose. In addit ion, we have constructed mutant ChoREs that retain their carbohydrate responsiveness but have lost the ability to bind USF. Together, these data suggest that USF is not the carbohydrate-responsive factor that s timulates S-14 and L-PK expression and that a distinct hepatic factor is likely to be responsible for the transcriptional response.