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Dissociation between the magnitude of nuclear binding and the biopotency of glucocorticoids
Abstract:
After 5 days of incubation with 10 nM tritiated dexamethasone, the amount of glucocorticoid displaceably-bound by the ACTH-secreting, glucocorticoid-responsive AtT-20 mouse pituitary tumor cell decreases by 70-80%. This decrease takes place in both the cytosolic and nuclear fractions. Although one might predict that there should be a decrease in agonist efficacy associated with this loss of nuclear binding, there was not; long-term incubations with dexamethasone revealed that the AtT-20 cells showed no tendency to resume secreting ACTH. Hence, although there was a dramatic decrease in the amount of nuclear-bound steroid, there was not a resulting decrease in the steroid's biopotency. These results suggest that the agonist-induced decrease in receptor content is either a means of removing redundant nuclear receptors, or a new, previously unrecognized, step in the mechanism of glucocorticoid hormone action.
Insights
Glucocorticoid hormone action involves a decrease in cellular receptors, yet biopotency remains. This suggests a novel mechanism for hormone action or removal of excess receptors.
Area of Science:
- Endocrinology
- Molecular Pharmacology
- Cell Biology
Background:
- Glucocorticoids are critical hormones regulating numerous physiological processes.
- Glucocorticoid receptors mediate hormone action, influencing gene expression and cellular function.
- Downregulation of hormone receptors is a known cellular response to prolonged ligand exposure.
Purpose of the Study:
- To investigate the effect of prolonged dexamethasone incubation on glucocorticoid receptor levels and biopotency in AtT-20 cells.
- To determine if a decrease in nuclear-bound glucocorticoid receptors impacts agonist efficacy.
- To elucidate potential mechanisms underlying agonist-induced receptor downregulation.
Main Methods:
- Incubation of AtT-20 mouse pituitary tumor cells with tritiated dexamethasone.
- Quantification of displaceable-bound glucocorticoid in cytosolic and nuclear cell fractions.
- Assessment of ACTH secretion and cellular response to long-term dexamethasone treatment.
Main Results:
- A 70-80% decrease in displaceable-bound glucocorticoids was observed after 5 days of incubation with dexamethasone.
- This receptor downregulation occurred in both cytosolic and nuclear fractions.
- Despite the significant loss of nuclear-bound steroid, dexamethasone retained its biopotency, with no resumption of ACTH secretion.
Conclusions:
- Agonist-induced decrease in glucocorticoid receptor content does not necessarily lead to a loss of biopotency.
- This phenomenon may represent a mechanism for removing redundant nuclear receptors.
- The findings suggest a previously unrecognized step in the mechanism of glucocorticoid hormone action.