Regulation of Tyrosine α-Ketoglutarate Transaminase in Rat Liver

The repression and derepression of hepatic tyrosine aminotransferase by carcinogens

Like hydrocortisone, a single carcinogenic dose of dimethylnitrosamine (50 mg/kg) initiates the induction cycle for hepatic tyrosine aminotransferase in adrenalectomized rats. However, following this initial induction in the presence of dimethylnitrosamine, the enzyme becomes refractory to reinduction by known inducers. The administration of thioacetamide to either adrenalectomized or intact rats leads to an immediate and progressive loss of inducibility by hydrocortisone, dibutyrylcyclic AMP or dimethylnitrosamine. Although the thioacetamide-induced repression was not reversed even up to 10 weekds after the cessation of treatment, it was reversed after the induction of liver regeneration. Both the carcinogen-mediated induction and repression of tyrosine aminotransferase appears to occur by mechanisms which do not involve the corticosteroid-binding proteins which normally mediate the induction by glucocorticoids.

Enhancement of phenotypic expression in cultured malignant liver epithelial cells by a complex medium

The ability of two media identically supplemented with serum to maintain functional and structural differential properties of cultured malignant liver epithelial cells was compared. Reuber H35 hepatoma cells grown in a complex medium, Medium E, possessed the gluconeogenic enzyme phosphoenolpyruvate carboxykinase and bile canaliculi. When grown in Eagle's Minimal Essential Medium, the activity of phosphoenolpyruvate carboxykinase was reduced and bile canaliculi were not formed. This decrease in properties was restored when cells maintained in Minimal Essential Medium was returned to Medium E. It was concluded that the enhanced phenotypic expression was probably due to enrichment of the nutrient environment.

Variation in tyrosine aminotrasferase induction in HTC cell clones

Examination of tyrosine aminotransferase induction in many HTC cell subclones revealed a wide and unstable distribution of inducibility. The instability of this phenotype cannot easily be explained by classical mutation rates. These observations may be important in interpreting certain cell fusion experiments.