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Ser/Thr protein phosphatase type 5 (PP5) is a negative regulator of glucocorticoid receptor-mediated growth arrest
1Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile 36688, USA.
Abstract:
Ligand-induced glucocorticoid receptor (GR) activation has recently been linked to the inhibition of cell proliferation via the transcriptional induction of p21(WAF1/Cip1), which functions as a universal inhibitor of cyclin-dependent protein kinases. Herein, we identify a Ser/Thr protein phosphatase (PP5) that promotes cellular proliferation by inhibiting both glucocorticoid and p53-mediated signaling pathways leading to p21(WAF1/Cip1)-mediated growth arrest. The suppression of PP5 expression (1) markedly increases the association of GR with its cognate DNA-binding sequence, (2) induces GR transcriptional activity without the addition of hormone, and (3) increases dexamethasone-mediated induction of GR reporter activity to a level that is approximately 10 times greater than the maximal response obtainable in the presence of PP5. PP5 has no apparent effect on the binding of hormone to the GR, and dexamethasone-mediated growth arrest correlates with an increase in p53 phosphorylation. Comparative studies in p53-wild-type, p53-defective, and p53-deficient cell lines indicate that either (1) p53 participates in GR-mediated induction of p21(WAF1/Cip1), with the hyperphosphorylation of basal p53 induced by glucocorticoids sufficient for the propagation of an antiproliferative response when PP5 expression is inhibited, or (2) PP5 acts where p53-mediated and GR-induced signaling networks converge to regulate the transcriptional induction of p21(WAF1/Cip1). Thus, aberrant PP5 expression may have an additive effect on the development of human cancers by promoting cell proliferation via the inhibition of a GR-induced antiproliferative signaling cascade, and facilitating neoplastic transformation via the inhibition of a growth-arresting p53-mediated response that guards against genomic instability.
Insights
Protein phosphatase 5 (PP5) promotes cell proliferation by inhibiting glucocorticoid receptor (GR) and p53 signaling. Suppressing PP5 enhances GR activity and p21 induction, blocking cancer cell growth.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Glucocorticoid receptor (GR) activation inhibits cell proliferation by inducing p21(WAF1/Cip1), a cyclin-dependent kinase inhibitor.
- The role of protein phosphatases in regulating GR and p53 signaling pathways is not fully understood.
Purpose of the Study:
- To identify and characterize a novel Ser/Thr protein phosphatase (PP5) involved in cell proliferation.
- To elucidate the mechanism by which PP5 regulates glucocorticoid and p53-mediated signaling pathways leading to p21(WAF1/Cip1) induction.
Main Methods:
- Cell culture and manipulation of PP5 expression (e.g., suppression).
- Assays for GR DNA-binding activity, transcriptional activity, and hormone-binding.
- Western blot analysis to assess p53 phosphorylation.
- Reporter gene assays to measure GR activity.
- Comparative studies in cell lines with varying p53 status.
Main Results:
- PP5 promotes cellular proliferation by inhibiting GR and p53 signaling pathways that induce p21(WAF1/Cip1).
- Suppression of PP5 expression significantly enhances GR DNA-binding and transcriptional activity, leading to increased p21(WAF1/Cip1) induction.
- Dexamethasone-induced growth arrest correlates with increased p53 phosphorylation upon PP5 inhibition.
- PP5 inhibition potentiates GR-mediated antiproliferative responses and p53-mediated growth arrest.
Conclusions:
- PP5 is a key regulator of cell proliferation by negatively impacting GR and p53 signaling.
- Aberrant PP5 expression may contribute to cancer development by promoting cell proliferation and inhibiting tumor-suppressive pathways.
- Targeting PP5 could be a therapeutic strategy for cancers involving dysregulated GR or p53 signaling.