Regulation by glucagon of serine:pyruvate/alanine:glyoxylate aminotransferase gene expression in cultured hepatocytes

Summary

In rat liver, serine:pyruvate/alanine:glyoxylate aminotransferase (SPT/AGT) locates in both mitochondria and peroxisomes, and only the mitochondrial enzyme is markedly induced by glucagon. In the rat, a single SPT/AGT gene locates on chromosome 9q34-q36, and there are two transcription initiation sites in the first exon. Transcription from the upstream and downstream start sites generate mRNAs for the precursor of mitochondrial SPT/AGT (mitochondrial SPT/AGT-mRNA) and peroxisomal SPT/AGT-niRNA, respectively. In the present study, the effect of glucagon on SPT/AGT gene expression were studied in primary cultured rat hepatocytes. Hepatocytes were first precultured for 16–18 h under serum- and hormone-free conditions. Glucagon and/or other effectors were then added, and the culture was continued as required. Under these conditions, the addition of glucagon caused a remarkable increase in the cellular level of mitochondrial SPT/AGT-mRNA via the cAMP-protein kinase A signaling pathway. A run-on transcription assay revealed that activation of transcription is responsible for the increase in mitochondrial SPT/AGT-mRNA. Notable features of the activation of SPT/AGT gene transcription by glucagon or cAMP are inhibition by cycloheximide and a slow response taking as long as 90 min for the maximum rate of transcription, suggesting that a protein synthesis is involved in this process. It appears that the rat SPT/AGT gene is a dual promoter gene. In this case, a remarkable feature of this gene is that the alternative usage of the two promoters eventually determines the alternative organelle localization of the expression product.