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Hormonal deinduction of tyrosine aminotransferase
Abstract:
The effects of several antagonists of glucocorticoid action on a line of hepatoma cells (HTC strain) have been studied in order to determine their mechanisms of action. The induction of tyrosine aminotransferase by dexamethasone can be partially or totally inhibited if an antagonist is added simultaneously with dexamethasone or some time later. Antagonists, even if they have as much affinity for the cytoplasmic receptor as dexamethasone, must be administered at a 100-fold excess as compared to dexamethasone. Their receptor binding kinetics are not identical to those of inducer steroids: moreover there is no correlation between relative binding affinities and anti-inducing capacities. A short contact between the cells and the antagonist is sufficient to obtain a full antagonistic effect, but the antagonist is inactive if administered and removed from the cells before induction. An interpretation if suggested, considering these results which do not find a satisfactory explanation in the classical theory of receptor action.
Insights
Glucocorticoid antagonists can block dexamethasone
Area of Science:
- Cell biology
- Biochemistry
- Pharmacology
Background:
- Glucocorticoids regulate gene expression via cytoplasmic receptors.
- Hepatoma cells (HTC strain) are a model for studying steroid hormone action.
- Tyrosine aminotransferase induction by dexamethasone is a well-characterized glucocorticoid response.
Purpose of the Study:
- To investigate the mechanism of action of glucocorticoid antagonists.
- To compare the efficacy of different antagonists in inhibiting dexamethasone-induced tyrosine aminotransferase.
- To elucidate the receptor binding kinetics and anti-inducing capacities of these antagonists.
Main Methods:
- Treatment of HTC cells with dexamethasone and various glucocorticoid antagonists.
- Measurement of tyrosine aminotransferase activity.
- Analysis of receptor binding affinity and kinetics.
- Evaluation of antagonist efficacy at different administration times and concentrations.
Main Results:
- Antagonists partially or totally inhibit dexamethasone-induced tyrosine aminotransferase.
- High concentrations (100-fold excess) of antagonists are required for significant inhibition, even with high receptor affinity.
- Receptor binding kinetics of antagonists differ from inducers, with no direct correlation between binding affinity and anti-inducing capacity.
- Short exposure to antagonists is sufficient for full effect, but pre-incubation without induction is ineffective.
Conclusions:
- The classical theory of receptor action does not fully explain the observed antagonistic effects.
- Glucocorticoid antagonists may act through mechanisms beyond simple receptor binding competition.
- Further research is needed to understand the complex interplay between antagonists, receptors, and cellular responses.