Posttranscriptional control in the steroid-mediated induction of hepatic tyrosine transaminase

Effect of cytosol on transport of RNA in vitro during cardiac hypertrophy

An enhanced RNA-transport activity was observed in vitro from nuclei obtained from animals with cardiac hypertrophy as compared with that of sham-operated controls. The 100 000 g supernatants obtained from hypertrophic hearts stimulated the RNA transport from nuclei of sham-operated controls, and this stimulation was maximum with 40% supernatant. Ca2+- and nucleic acid-dependent ATPase and alkaline phosphatase activities, which may be involved in an energy-dependent transport, were high in nuclei from hypertrophic hearts, and the nuclei of sham-operated animals showed higher activities of these enzymes after incubation with supernatant from hypertrophic hearts, which stimulates the RNA transport in vitro from nuclei of sham-operated animals.

Age-related changes in inducible mouse liver enzymes: ornithine decarboxylase and tyrosine aminotransferase

The time required to induce two inducible hepatic enzymes, ornithine decarboxylase (ODC) and tyrosine aminotransferase (TAT), by growth hormone and dexamethasone, respectively, increases with age. Specific activity at the peak of induction is the same for all ages. On the other hand, for basal TAT the specific activity per unit of TAT antigen was found to decrease considerably with age. The half-life of ODC was determined after cycloheximide administration. The apparent half-life at the peak of ODC induction increases from 15 minutes in 3-4-month-old mice to 30 minutes in 24-month-old animals. Loss of efficiency in the protein degradation system is implicated in this phenomenon as no apparent differences could be observed in the susceptibility of ODC and TAT from young or old mice to chymotrypsin. ODC and TAT are activated by temperatures of up to 37 degrees C and 50 degrees C, respectively. ODC is inactivated at 50 degrees C while TAT is inactivated at 76 degrees C. "Young" ODC and TAT are more readily activated or inactivated by heating than the "old" enzymes.

Translation of mRNA from rat-liver polysomes into tyrosine aminotransferase and tryptophan oxygenase in a protein-synthesizing system from wheat germ. Effects of cortisol on the translatable levels of mRNA for these two enzymes

Messenger RNA was isolated from rat liver polysomes by phenol/chloroform extraction and subsequent oligo(dT)-cellulose chromatography. The mRNA was translated in a protein-synthesizing system in vitro derived from wheat germ. The system was optimized in respect to Mg2+ and K+. The presence of spermidine or spermine is necessary for the synthesis of polypeptides having molecular weights of over 20 000. In the absence of the bases only small molecular weight products are formed. The amount of protein synthesized is linearly dependent on the amount of mRNA added up to concentrations of 80 mug mRNA/ml. The synthesis of tyrosine aminotransferase and tryptophan oxygenase in the system in vitro has been demonstrated by specific immunoprecipitation and sodium-dodecylsulfate polyacrylamide gel electrophoresis of the precipitate with enzyme proteins as marker. The amount of specific product formed is linearly dependent on the amount of mRNA present. The amount of translatable tyrosine aminotransferase mRNA and tryptophan oxygenase mRNA increases after administration of hydrocortisone to adrenalectomized rats. At low doses of hormone (2 mg/100 g body weight) maximal values are observed at 4 h, control levels being reached at 6-8 h after hormone application. With higher doses of hydrocortisone (20 mg/100 g body weight) maximal values are attained at 6 h, tending to control levels 14 h after treatment. The enzyme activity curves are parallel to the mRNA curves, the peak of enzyme activity occurring 2 h after the peak of mRNA activity.

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.