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Glucocorticoid negative feedback in sheep corticotrophs: a comparison with AtT-20 corticotroph tumor cells
1Department of Physiology and Pharmacology, College of Veterinary Medicine, Auburn University, Alabama 36849.
Abstract:
Early glucocorticoid feedback in sheep anterior pituitary (AP) cells was compared and contrasted with that in mouse pituitary tumor AtT-20 cells. Dexamethasone (DEX) inhibited corticotropin-releasing hormone (CRH)-stimulated adrenocorticotropin (ACTH) release in a concentration- and time-dependent manner with similar potency amongst cell types. This inhibition was mediated through type II glucocorticoid receptors and required the synthesis of new protein. However, stimulation of protein kinase C with phorbol 12-myristate 13-acetate (PMA) resulted in greater ACTH release and greater inhibition by DEX in sheep AP cells. In contrast to sheep AP cells, AtT-20 cells were insensitive to glucocorticoids when secretion was stimulated by KCl depolarization or the voltage-dependent calcium channel agonist, maitotoxin (MTX). In both cell types, CRH-, KCl-, and MTX-stimulated ACTH release was inhibited by the calcium channel blocker, nifedipine (NIF). Whereas NIF also inhibited PMA-induced ACTH secretion in AtT-20 cells, it did not in sheep AP cells. These data demonstrate that early glucocorticoid feedback is operative in sheep corticotrophs and that AtT-20 cells appear to serve as an appropriate mechanistic model for aspects of negative feedback when the CRH-protein kinase A pathway is activated but may not be appropriate when ACTH secretion is activated via other intracellular signaling pathways.
Insights
Early glucocorticoid feedback in sheep anterior pituitary cells is similar to mouse AtT-20 cells, but sheep cells show greater inhibition with protein kinase C activation. AtT-20 cells are not ideal models for all ACTH secretion pathways.
Area of Science:
- Endocrinology
- Cell Biology
- Neuroscience
Background:
- Glucocorticoids regulate the hypothalamic-pituitary-adrenal (HPA) axis through negative feedback.
- Anterior pituitary (AP) cells are key targets for glucocorticoid feedback.
- AtT-20 mouse pituitary tumor cells are a common model for studying ACTH secretion and feedback.
Purpose of the Study:
- To compare early glucocorticoid feedback mechanisms in sheep AP cells and AtT-20 cells.
- To investigate the differential effects of various secretagogues on glucocorticoid feedback in these cell types.
- To determine the suitability of AtT-20 cells as a model for glucocorticoid feedback under diverse signaling conditions.
Main Methods:
- Cells were treated with dexamethasone (DEX), corticotropin-releasing hormone (CRH), phorbol 12-myristate 13-acetate (PMA), KCl, and maitotoxin (MTX).
- Adrenocorticotropin (ACTH) release was measured.
- The effects of the calcium channel blocker nifedipine (NIF) were assessed.
Main Results:
- Dexamethasone inhibited CRH-stimulated ACTH release similarly in both cell types via type II glucocorticoid receptors, requiring new protein synthesis.
- Sheep AP cells exhibited greater ACTH release and DEX inhibition when stimulated by PMA (protein kinase C activator) compared to AtT-20 cells.
- AtT-20 cells were insensitive to DEX inhibition when ACTH secretion was stimulated by KCl or MTX, unlike sheep AP cells.
Conclusions:
- Early glucocorticoid feedback is functional in sheep corticotrophs.
- AtT-20 cells serve as a valid model for glucocorticoid feedback involving the CRH-protein kinase A pathway.
- AtT-20 cells may not accurately reflect glucocorticoid feedback mechanisms when ACTH secretion is stimulated by alternative intracellular pathways, such as those involving calcium channels.