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Repression of glucocorticoid receptor gene transcription by c-Jun
A L Cabral1, A N Hays, P R Housley
1Ludwig Institute for Cancer Research, 01509-900, São Paulo, Brazil.
Abstract:
The regulation of glucocorticoid receptor gene expression by members of the AP-1 family was examined in glucocorticoid-free NIH3T3 cells transfected with the human glucocorticoid receptor gene promoter driving expression of a CAT reporter gene. c-Jun inhibited the promoter activity by 80% and JunB by 30%, whereas c-Fos and JunD had no inhibitory effect. Electrophoretic mobility shift assays showed that c-Jun is unable to efficiently interact with the AP-1-like site present in the human glucocorticoid receptor promoter. Moreover, c-Jun was still able to repress promoter mutants in which the region containing the AP-1-like site was deleted. NIH3T3 cell clones overexpressing c-Jun exhibited lower glucocorticoid receptor mRNA levels, which suggests that the murine glucocorticoid receptor gene can also be regulated by AP-1. These results provide a new mechanism for cross-talk between the glucocorticoid receptor and the AP-1 family of transcription factors in the absence of glucocorticoid ligands.
Insights
The AP-1 transcription factor c-Jun represses glucocorticoid receptor gene expression, even without glucocorticoids. This reveals a novel cross-talk mechanism between these crucial cellular signaling pathways.
Area of Science:
- Molecular Biology
- Gene Regulation
- Transcription Factors
Background:
- Glucocorticoid receptor (GR) is a key regulator of cellular processes.
- AP-1 family proteins are transcription factors involved in various cellular responses.
- Understanding the interplay between GR and AP-1 is crucial for cellular signaling.
Purpose of the Study:
- To investigate the role of AP-1 family members in regulating glucocorticoid receptor gene expression.
- To elucidate the mechanism of interaction between c-Jun and the glucocorticoid receptor promoter.
Main Methods:
- Transfection of NIH3T3 cells with human GR promoter-CAT reporter gene.
- Assessing promoter activity changes upon expression of different AP-1 members.
- Electrophoretic mobility shift assays (EMSA) to study protein-DNA interactions.
- Analysis of GR mRNA levels in cells overexpressing c-Jun.
Main Results:
- c-Jun significantly inhibited GR promoter activity (80%), while JunB showed a moderate inhibition (30%).
- c-Fos and JunD had no significant effect on GR promoter activity.
- c-Jun demonstrated an inability to efficiently bind to the AP-1-like site in the GR promoter.
- c-Jun repressed promoter activity even when the AP-1-like site was deleted, indicating an indirect mechanism.
- Overexpression of c-Jun led to reduced glucocorticoid receptor mRNA levels in murine cells.
Conclusions:
- c-Jun acts as a repressor of glucocorticoid receptor gene expression.
- The repressive mechanism of c-Jun on GR is independent of direct binding to the canonical AP-1 site.
- This study uncovers a novel pathway for cross-talk between the glucocorticoid receptor and AP-1 transcription factors in the absence of glucocorticoid ligands.