Endocrine Abstracts

Background: Cortisol has a suppressive effect on corticotroph proliferation. In Cushing’s disease (CD), the relative resistance to glucocorticoid (GC) action may favor corticotroph tumor formation and autonomous ACTH secretion.

Objective: To analyze the consequences of long-term GC exposure on corticotroph tumor cell proliferation and to evaluate the modification of gene expression attributable to prolonged GC treatment.

Methods: Mouse AtT20 corticotroph pituitary tumor cells were used. Cells were treated continuously with dexamethasone (DEX) for 10-weeks at a dose of 10nM which is supraphysiological and achievable in the case of CD. Subsequently, cells were cultured in DEX-free medium for up to 5-weeks (washout period). AtT20 cells cultured only in DEX-free medium served as control. After 1- and 5-week of washout, dose-response experiments were performed to determine cell sensitivity to DEX. Quantitative PCR was performed on control cells and cells previously treated with DEX at 1- and 5-week washout periods (DEX-treated cells). mRNA analysis was performed at 72-hr of DEX (100 nM).

Results: DEX inhibited the proliferation of control AtT20 cells in a dose-dependent manner. In long-term DEX-treated cells, DEX failed to inhibit cell growth despite 1- and 5-week washout period. No differences were observed between control and DEX-treated cells regarding DEX-induced effects on mRNA expression of the glucocorticoid receptor, Pomc and Fkbp5. However, DEX induced expression of the proapoptotic Bid gene in control cells but had no effect in DEX-treated cells. DEX did not affect Bad gene expression in control cells but suppressed its expression in DEX-treated cells. For cell cycle related genes, DEX induced a strong upregulation of Cdkn1a, a cell cycle inhibitory gene, in control cells, whereas this induction was lower in DEX-treated cells.

Conclusion: Long-term treatment of corticotroph tumor cells with GC results in cells resistant to the growth inhibition by GC, despite an unchanged sensitivity of genes directly related to GR activation indicating differential effects on (long-term) gene expression. Continued exposure of GC may thus superimpose adaptive changes, in which acute GC challenge elicit a different pattern of gene expression less committed to cell growth inhibition, such as lower expression of proapoptotic genes and less induction of the cell cycle inhibitor CDKN1a.