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Selectivity of cell cycle regulation of glucocorticoid receptor function
1Department of Biological Sciences, University of Pittsburgh, Pennsylvania 15260.
Abstract:
The restricted expression of some genes to distinct stages of the cell cycle is often brought about through alterations in the activity and/or abundance of specific transcription factors. Many cells have been shown to be unresponsive to glucocorticoid hormone action during the G2 phase of the mammalian cell cycle, suggesting that some activities of the glucocorticoid receptor (GR), a ligand-activated transcription factor, are subjected to cell cycle control. We show here that GR insensitivity in G2 is selective, affecting receptor-mediated transactivation from a simple glucocorticoid response element, but not repression from a composite glucocorticoid response element. Since glucocorticoid-dependent down-regulation of GR protein levels is also unaffected in G2, distinct activities of the receptor that participate in this homologous down-regulation must be operating as effectively in G2-synchronized cells as in asynchronous cells. Finally, the phosphorylation state of the GR is altered in G2-synchronized cells reflecting, in part, both site-specific phosphorylation and dephosphorylation events. These results suggest that, while GR may be a target for cell cycle regulated kinases and phosphatases, the resulting changes in receptor phosphorylation have an impact only on selected GR functions.
Insights
Glucocorticoid receptor (GR) insensitivity during the G2 phase of the cell cycle selectively impacts gene transactivation, not repression or protein down-regulation. GR phosphorylation changes in G2 affect only specific GR functions.
Area of Science:
- Molecular Biology
- Cell Biology
- Endocrinology
Background:
- Gene expression is often regulated by transcription factors whose activity or abundance changes during the cell cycle.
- Cells exhibit reduced responsiveness to glucocorticoid hormones during the G2 phase of the cell cycle.
- This suggests that the glucocorticoid receptor (GR), a ligand-activated transcription factor, is subject to cell cycle control.
Purpose of the Study:
- To investigate the cell cycle-dependent regulation of glucocorticoid receptor (GR) activity.
- To determine which specific functions of the GR are affected by cell cycle progression, particularly during the G2 phase.
Main Methods:
- Synchronization of cells to the G2 phase of the cell cycle.
- Assays to measure GR-mediated transactivation from simple and composite glucocorticoid response elements.
- Analysis of GR protein levels and phosphorylation status in G2-synchronized cells.
Main Results:
- GR insensitivity in G2 is selective, impairing transactivation from simple response elements but not repression from composite elements.
- Glucocorticoid-induced down-regulation of GR protein levels remains unaffected in G2-synchronized cells.
- The phosphorylation state of GR is altered in G2, involving site-specific phosphorylation and dephosphorylation events.
Conclusions:
- Cell cycle regulation affects specific GR functions, with G2 phase impacting transactivation more than repression.
- While GR is targeted by cell cycle-regulated kinases and phosphatases, these modifications influence only selected GR activities.
- These findings highlight the intricate interplay between cell cycle control and transcription factor function.