Induction by peroxisome proliferators and triiodothyronine of serine:pyruvate/alanine:glyoxylate aminotransferase of rat liver

The role of Sp1 and AP-2 in basal and protein kinase A--induced expression of mitochondrial serine:pyruvate aminotransferase in hepatocytes

Transcription of mitochondrial serine:pyruvate aminotransferase (SPT) mRNA (SPTm-mRNA) in rat liver is unique in that it occurs from the upstream site of the two transcription start sites within the first exon of the SPT gene and is selectively enhanced by cAMP via the protein kinase A (PKA) signaling pathway. In this study, we identified the DNA elements and nuclear factors responsible for the basal and PKA-induced activities of the upstream promoter. By using a luciferase reporter assay with HepG2 cells, DNase I footprinting analysis, and gel shift experiments, we identified the binding sites for Sp1 and AP-2 within the regions -125 to -89 and -14 to +10, respectively. Mutational analyses indicated that these regions are essential for the transcription factor binding and the SPT promoter activity. Expression of AP-2 caused a marked increase in the basal promoter activity to about the same level as that achieved by PKA. On the other hand, both the basal and PKA-induced activities were elevated by overexpression of Sp1, its effect on PKA-induced activity being more pronounced with coexpression of CBP and repressed by E1A oncoprotein. These results suggest that AP-2 and Sp1 regulate basal promoter activity, and Sp1 is also involved in PKA-mediated expression of the rat SPT gene in concert with the transcriptional coactivator CBP.

Troglitazone inhibits glutamine metabolism in rat mesangial cells

Troglitazone is a peroxisome proliferator-activated receptor-gamma agonist that has been shown to halt mesangium expansion in experimental models of type 2 diabetes mellitus and to act directly on rat mesangial cells. Because glutamine serves as the precursor for cellular biosynthetic processes, we asked whether troglitazone would inhibit mesangial cell glutamine metabolism under these conditions. Confluent monolayers of rat mesangial cells were incubated in RPMI medium in the presence of troglitazone or vehicle (DMSO). Troglitazone effected a dose-dependent reduction in glutamine utilization and in alanine formation, associated with a decrease in monolayer collagen-glycosaminoglycan content. Despite the reduced glutamine uptake, ammonium formation did not decrease, consistent with increased glutamate flux through the deamination pathway. Assayable activity of the alanine aminotransferase decreased by 63%, whereas assayable glutamate dehydrogenase remained unchanged. In control monolayers, the sum of ammonium plus alanine plus glutamate nitrogen released accounted for <75% of the glutamine nitrogen uptake. In troglitazone-treated monolayers, all of the glutamine nitrogen taken up could be accounted for as ammonium nitrogen released into the medium. These results are consonant with troglitazone reducing glutamine metabolism and specifically the transamination pathway in rat mesangial cells associated with a reduction in collagen-glycosaminoglycan content.