Amino acid control of the human glyceraldehyde 3-phosphate dehydrogenase gene transcription in hepatocyte.

Glutamine (Gln) is the most potent of the amino acids (AAs) that regulate liver anabolism, and its effect is similar to that of insulin in peripheral tissues. However, the influence of AAs on regulation of metabolic enzyme-encoding genes is not known at the molecular level in liver. We now report that Gln and some essential AAs activate the human GAPDH gene that codes for GAPDH, a central enzyme of glycolysis and a target for insulin regulation. In HepG2 cells, Gln upregulated the GAPDH mRNA level, and this effect was additive to that of insulin. Transient transfection of GAPDH promoter/cat constructs demonstrated that a gene-specific and insulin-independent transcriptional step is involved in the Gln responsiveness of GAPDH. Transfected HepG2 cells challenged with various AAs, Gln metabolites or inhibitors of Gln metabolism showed that the Gln-induced effect is similar to that of some essential AAs and that Gln metabolism is a necessary step for GAPDH activation. Deletion mutants and site-directed mutagenesis of the GAPDH promoter indicated that the Gln responsiveness is mediated by a sequence that is distinct from insulin-responsive elements and from positively acting elements previously described in this promoter. This motif located at -126/-118 clearly differs from AA-responsive elements recently identified in other genes. Electromobility shift assay and supershifts showed that the transcription factors bound to the Gln-responsive element in the GAPDH promoter are C/EBPalpha and -delta. This finding is consistent with the role of C/EBP family members in controlling the hepatic expression of genes involved in nutrient metabolism.