Differential expression of tyrosine aminotransferase by glucocorticoids and insulin

The differential response of the tyrosine aminotransferase (TAT) gene to glucocorticoids and insulin in HTC cells and cell clones derived from Reuber H35 cells (FaO and Fu5.5) have been analyzed by nuclear run-on assay. It has been previously shown that clones of cells from HTC and Reuber H35 cell lines, exhibit different sensitivities for the induction of TAT mRNA and enzyme activity. The purpose of the present study was to determine whether this difference in TAT expression between hepatocytes and hepatoma cell lines occurs at the level of TAT gene transcription or mRNA stability. A study of the TAT mRNA accumulation in all cell types showed that TAT mRNA in the Reuber H35 cell clones and hepatocytes was synthesized at a higher rate than in HTC cells. However, dexamethasone induction of alpha 1 AGP mRNA and glutamine synthetase was comparable to glucocorticoid bound receptors. In addition, cycloheximide decreased the rate at which induced levels of TAT mRNA were degraded. We also show that a heterologous fusion gene constructed from 3.0 kilobases (kb) 5' to the transcription initiation site of the rat TAT gene and the bacterial chloramphenicol acetyltransferase gene (CAT) responds similarly to dexamethasone in Fu5.5 and HTC cells as determined by transient transfection assay; and insulin inhibits dexamethasone mediated transcription in Reuber H35 cells and primary adult hepatocytes. These data indicate that DNA sequences involved in the differential response of the TAT gene to hormone treatments between HTC and Reuber H35 cell lines are not located in the first 3.0 kb fragment.