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Tonic suppression of adrenal AP-1 activity by glucocorticoids
1Physiology Unit, School of Molecular and Medical Biosciences, MOMED, Cardiff University of Wales, UK.
Abstract:
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.
Insights
Glucocorticoid depletion in rats increases adrenal AP-1 activity, primarily by upregulating JunB expression in the adrenal medulla. This suggests a mechanism where glucocorticoids tonically suppress JunB, potentially adapting to hormone level changes.
Area of Science:
- Endocrinology
- Molecular Biology
- Cellular Signaling
Background:
- The AP-1 transcription factor, a complex of Fos and Jun proteins, interacts with nuclear receptors like the glucocorticoid receptor (GR).
- These interactions integrate cellular signaling pathways, but in vivo mechanisms are not well understood.
- The adrenal gland presents a unique context with high local glucocorticoid levels and constitutive AP-1 activity.
Purpose of the Study:
- To investigate in vivo interactions between GR and AP-1 signaling in the rat adrenal gland.
- To characterize the molecular mechanisms underlying AP-1 regulation by glucocorticoids in this context.
- To explore the physiological relevance of these interactions as an adaptive response.
Main Methods:
- Pharmacological inhibition of glucocorticoid production using metyrapone.
- Assessment of AP-1 activity via DNA binding assays.
- Analysis of jun gene expression using in situ hybridization and immunoblotting.
- Treatment with the glucocorticoid agonist dexamethasone to reverse effects.
Main Results:
- Metyrapone-induced glucocorticoid depletion caused a 5-fold increase in adrenal AP-1 activity.
- This increase was reversed by dexamethasone and localized to the adrenal medulla.
- Glucocorticoid depletion led to increased expression of junB mRNA and nuclear JunB protein in the adrenal medulla.
- c-Jun and JunD were basal AP-1 components, but JunB changes underpinned the metyrapone-induced activity increase.
Conclusions:
- Glucocorticoids exert tonic suppression on adrenal medullary JunB expression.
- This provides evidence for a nuclear mechanism by which the adrenal gland adapts to fluctuating glucocorticoid levels.
- The findings highlight a novel interaction between steroid hormone signaling and transcription factor regulation in the adrenal gland.