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Updated: Aug 7, 2026
Analysis of Cell Cycle Position in Mammalian Cells
Published on: January 21, 2012
Glucocorticoid receptor-mediated cell cycle arrest is achieved through distinct cell-specific transcriptional
I Rogatsky1, J M Trowbridge, M J Garabedian
1Department of Microbiology and The Kaplan Cancer Center, New York University Medical Center, New York 10016, USA.
Abstract:
Glucocorticoids inhibit proliferation of many cell types, but the events leading from the activated glucocorticoid receptor (GR) to growth arrest are not understood. Ectopic expression and activation of GR in human osteosarcoma cell lines U2OS and SAOS2, which lack endogenous receptors, result in a G1 cell cycle arrest. GR activation in U2OS cells represses expression of the cyclin-dependent kinases (CDKs) CDK4 and CDK6 as well as their regulatory partner, cyclin D3, leading to hypophosphorylation of the retinoblastoma protein (Rb). We also demonstrate a ligand-dependent reduction in the expression of E2F-1 and c-Myc, transcription factors involved in the G1-to-S-phase transition. Mitogen-activated protein kinase, CDK2, cyclin E, and the CDK inhibitors (CDIs) p27 and p21 are unaffected by receptor activation in U2OS cells. The receptor's N-terminal transcriptional activation domain is not required for growth arrest in U2OS cells. In Rb-deficient SAOS2 cells, however, the expression of p27 and p21 is induced upon receptor activation. Remarkably, in SAOS2 cells that express a GR deletion derivative lacking the N-terminal transcriptional activation domain, induction of CDI expression is abolished and the cells fail to undergo ligand-dependent cell cycle arrest. Similarly, murine S49 lymphoma cells, which, like SAOS2 cells, lack Rb, require the N-terminal activation domain for growth arrest and induce CDI expression upon GR activation. These cell-type-specific differences in receptor domains and cellular targets linking GR activation to cell cycle machinery suggest two distinct regulatory mechanisms of GR-mediated cell cycle arrest: one involving transcriptional repression of G1 cyclins and CDKs and the other involving enhanced transcription of CDIs by the activated receptor.
Insights
Glucocorticoid receptor (GR) activation halts cell growth by repressing cell cycle genes or inducing cell cycle inhibitors, depending on the cell type. These findings reveal distinct mechanisms for GR-mediated growth arrest.
Area of Science:
- Molecular Biology
- Cell Biology
- Endocrinology
Background:
- Glucocorticoids are known to inhibit cell proliferation, but the precise molecular events linking activated glucocorticoid receptor (GR) to growth arrest remain unclear.
- Understanding these mechanisms is crucial for developing targeted therapies, particularly in cancer where uncontrolled cell proliferation is a hallmark.
Purpose of the Study:
- To elucidate the molecular mechanisms by which activated glucocorticoid receptor (GR) induces cell cycle arrest in different human cancer cell lines.
- To investigate the role of specific GR domains and cellular targets in mediating ligand-dependent growth inhibition.
Main Methods:
- Ectopic expression and activation of GR in human osteosarcoma cell lines (U2OS and SAOS2) lacking endogenous receptors.
- Analysis of cell cycle progression, expression of key cell cycle regulatory proteins (cyclins, cyclin-dependent kinases, retinoblastoma protein, E2F-1, c-Myc, CDK inhibitors).
- Utilized GR deletion mutants to assess the role of the N-terminal transcriptional activation domain.
Main Results:
- GR activation in U2OS cells (Rb-positive) repressed CDK4, CDK6, cyclin D3, E2F-1, and c-Myc, leading to G1 arrest.
- In Rb-deficient SAOS2 cells, GR activation induced expression of CDK inhibitors p27 and p21, causing G1 arrest.
- The N-terminal transcriptional activation domain of GR was essential for growth arrest and CDI induction in Rb-deficient cells (SAOS2 and S49).
Conclusions:
- GR-mediated cell cycle arrest involves at least two distinct mechanisms: repression of G1 cyclins/CDKs or induction of CDK inhibitors.
- The specific mechanism is cell-type-dependent, influenced by factors such as retinoblastoma protein status and the requirement for GR's N-terminal domain.
- These findings provide critical insights into the complex regulation of cell proliferation by glucocorticoids.
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