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Glucocorticoid increases rat apolipoprotein A-I promoter activity
A H Taylor1, J Raymond, J M Dionne
1Department of Medicine, University of Calgary, Alberta, Canada.
Journal of Lipid Research
|October 1, 1996
Summary
Glucocorticoids like dexamethasone boost apolipoprotein A-I (apoA-I) production in rats by influencing gene expression. This occurs indirectly through a specific DNA region, site B, rather than direct receptor binding.
Area of Science:
- Molecular Biology
- Endocrinology
- Gene Regulation
Background:
- Glucocorticoids are known to influence the levels of apolipoprotein A-I (apoA-I).
- The precise molecular mechanisms underlying this regulation remain largely unelucidated.
- Understanding this pathway is crucial for metabolic and cardiovascular research.
Purpose of the Study:
- To investigate the mechanism by which glucocorticoids, specifically dexamethasone, regulate apolipoprotein A-I gene expression in rats.
- To identify the specific DNA elements and nuclear factors involved in this glucocorticoid-mediated transcriptional control.
Main Methods:
- In vivo administration of dexamethasone to rats followed by measurement of serum apoA-I, hepatic mRNA, and gene transcription.
- Transient transfection assays using the rat apoA-I promoter in hepatoma cells.
- Analysis of DNA-protein interactions using nuclear extracts from dexamethasone-treated cells.
- Site-directed mutagenesis of specific DNA sequences within the apoA-I promoter.
Main Results:
- Dexamethasone administration significantly increased serum apoA-I protein, hepatic mRNA, and gene transcription rates.
- Transfection studies identified a cis-acting regulatory element, site B, crucial for dexamethasone's effect.
- Dexamethasone enhanced the binding of nuclear factors to site B, but site B lacks a consensus glucocorticoid receptor motif.
- Site B contains a direct repeat, and mutations abolished the dexamethasone response, indicating an indirect mechanism.
- An adjacent sequence, site S, amplified the transcriptional activity of site B in response to dexamethasone.
Conclusions:
- Dexamethasone upregulates rat apolipoprotein A-I gene expression through an indirect mechanism involving site B.
- The regulation involves specific nuclear factors binding to site B, which operates via an indirect glucocorticoid action.
- Site S acts as an amplifier for site B-mediated transcriptional regulation by dexamethasone.