Tonic suppression of adrenal AP-1 activity by glucocorticoids.

The AP-1 transcription factor is a variable complex of Fos and Jun nuclear phosphoproteins that is induced in many cell types. AP-1 interacts with transcription factors of different classes, including the nuclear steroid hormone receptors, an interaction that is often mutually antagonistic and thereby serves to integrate different cellular signalling events. In addition to direct, molecular interactions between AP-1 and glucocorticoid receptor (GR), there is also evidence that the two signalling pathways may interact at different levels, but in vivo interactions of this nature have not been well characterized. We have investigated a unique cellular context for GR/AP-1 interactions, namely in the adrenal gland of the rat where the production of glucocorticoids leads to extremely high local levels of glucocorticoids, and where high constitutive AP-1 activity (as determined by in vitro DNA binding activity) has been demonstrated. We have now shown that depletion of glucocorticoid production in rats with the 11-beta-hydroxylase inhibitor, metyrapone, results in increased adrenal AP-1 activity. The demonstrated 5-fold increase is reversed by prior treatment with the glucocorticoid agonist, dexamethasone, and is largely localized to the adrenal medullary region. Further experiments have shown that c-Jun and JunD are the principal components of adrenal AP-1 in the basal state, but a change in jun-B expression appears to underly the metyrapone-induced increase in AP-1 activity. In situ hybridization analysis has shown that glucocorticoid depletion is associated with a dramatic increase in adrenal medullary junB mRNA, and using immunoblotting we have demonstrated a similar increase in nuclear levels of both the 43 kD JunB protein, and an associated phosphorylated JunB. Our use of a pharmacological intervention to demonstrate tonic suppression of adrenal medullary JunB expression by glucocorticoids has provided evidence of a nuclear mechanism that may have physiological relevance as an adaptive response to fluctuating levels of glucocorticoids.