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Amplification of PPM1D in human tumors abrogates p53 tumor-suppressor activity
Dmitry V Bulavin1, Oleg N Demidov, Shin'ichi Saito
1Gene Response Section, Bethesda, Maryland 20892, USA.
Abstract:
Expression of oncogenic Ras in primary human cells activates p53, thereby protecting cells from transformation. We show that in Ras-expressing IMR-90 cells, p53 is phosphorylated at Ser33 and Ser46 by the p38 mitogen-activated protein kinase (MAPK). Activity of p38 MAPK is regulated by the p53-inducible phosphatase PPM1D, creating a potential feedback loop. Expression of oncogenic Ras suppresses PPM1D mRNA induction, leaving p53 phosphorylated at Ser33 and Ser46 and in an active state. Retrovirus-mediated overexpression of PPM1D reduced p53 phosphorylation at these sites, abrogated Ras-induced apoptosis and partially rescued cells from cell-cycle arrest. Inactivation of p38 MAPK (the product of Mapk14) in vivo by gene targeting or by PPM1D overexpression expedited tumor formation after injection of mouse embryo fibroblasts (MEFs) expressing E1A+Ras into nude mice. The gene encoding PPM1D (PPM1D, at 17q22/q23) is amplified in human breast-tumor cell lines and in approximately 11% of primary breast tumors, most of which harbor wildtype p53. These findings suggest that inactivation of the p38 MAPK through PPM1D overexpression resulting from PPM1D amplification contributes to the development of human cancers by suppressing p53 activation.
Insights
Oncogenic Ras activates p53 via p38 MAPK, but PPM1D overexpression suppresses this, promoting cancer. PPM1D amplification in breast tumors suggests a role in human cancer development by inhibiting p53.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- Oncogenic Ras expression typically activates p53, a tumor suppressor, preventing cellular transformation.
- p53 phosphorylation at Ser33 and Ser46 is crucial for its activation.
- p38 MAPK (mitogen-activated protein kinase) phosphorylates p53 at these sites.
Purpose of the Study:
- To investigate the role of p38 MAPK and PPM1D in Ras-induced p53 activation.
- To determine if PPM1D regulates p38 MAPK activity in the context of oncogenic Ras.
- To explore the implications of PPM1D dysregulation in human cancer.
Main Methods:
- Utilized Ras-expressing human IMR-90 cells and mouse embryo fibroblasts (MEFs).
- Employed retrovirus-mediated overexpression of PPM1D.
- Assessed p53 phosphorylation, apoptosis, cell-cycle arrest, and tumor formation in vivo.
Main Results:
- Oncogenic Ras suppresses PPM1D mRNA induction, maintaining p53 phosphorylation and activation.
- PPM1D overexpression reduced p53 phosphorylation, abrogated Ras-induced apoptosis, and partially rescued cell-cycle arrest.
- Inactivation of p38 MAPK via PPM1D overexpression expedited tumor formation in mice.
- PPM1D gene amplification was observed in human breast tumor cell lines and primary tumors.
Conclusions:
- PPM1D acts as a negative regulator of p38 MAPK, forming a feedback loop that impacts p53 activity.
- PPM1D overexpression, driven by gene amplification, contributes to human cancer development by suppressing p53 activation.
- Targeting the PPM1D-p38 MAPK-p53 axis may offer therapeutic strategies for cancers with PPM1D alterations.