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Tyrosine phosphorylation of Mdm2 by c-Abl: implications for p53 regulation
Zehavit Goldberg1, Ronit Vogt Sionov, Michael Berger
1Lautenberg Center for General and Tumor Immunology, The Hebrew University Hadassah Medical School, Jerusalem 91120, Israel.
Abstract:
The p53 tumor suppressor is inhibited and destabilized by Mdm2. However, under stress conditions, this downregulation is relieved, allowing the accumulation of biologically active p53. Recently we showed that c-Abl is important for p53 activation under stress conditions. In response to DNA damage, c-Abl protects p53 by neutralizing the inhibitory effects of Mdm2. In this study we ask whether this neutralization involves a direct interplay between c-Abl and Mdm2, and what is the contribution of the c-Abl kinase activity? We demonstrate that the kinase activity of c-Abl is required for maintaining the basal levels of p53 expression and for achieving maximal accumulation of p53 in response to DNA damage. Importantly, c-Abl binds and phosphorylates Mdm2 in vivo and in vitro. We characterize Hdm2 (human Mdm2) phosphorylation at Tyr394. Substitution of Tyr394 by Phe394 enhances the ability of Mdm2 to promote p53 degradation and to inhibit its transcriptional and apoptotic activities. Our results suggest that phosphorylation of Mdm2 by c-Abl impairs the inhibition of p53 by Mdm2, hence defining a novel mechanism by which c-Abl activates p53.
Insights
The c-Abl kinase phosphorylates Mdm2, impairing its ability to inhibit the p53 tumor suppressor. This phosphorylation is crucial for maintaining p53 levels and activation during DNA damage responses.
Area of Science:
- Oncology
- Molecular Biology
- Cellular Stress Response
Background:
- The p53 tumor suppressor is regulated by Mdm2, which inhibits and destabilizes it.
- Stress conditions relieve Mdm2-mediated inhibition, allowing p53 accumulation and activation.
- c-Abl kinase plays a role in p53 activation under stress by counteracting Mdm2.
Purpose of the Study:
- To investigate the direct interaction between c-Abl and Mdm2.
- To determine the role of c-Abl kinase activity in p53 regulation by Mdm2.
- To elucidate the mechanism by which c-Abl activates p53.
Main Methods:
- In vivo and in vitro binding assays to study c-Abl and Mdm2 interaction.
- Kinase assays to assess the role of c-Abl activity.
- Site-directed mutagenesis to analyze Mdm2 phosphorylation at Tyr394.
Main Results:
- c-Abl kinase activity is essential for basal p53 levels and maximal p53 accumulation after DNA damage.
- c-Abl directly binds and phosphorylates Mdm2.
- Phosphorylation of Mdm2 at Tyr394 by c-Abl inhibits Mdm2's ability to degrade p53 and suppress its activity.
Conclusions:
- c-Abl activates p53 by phosphorylating Mdm2, thereby reducing Mdm2's inhibitory effect.
- This phosphorylation of Mdm2 at Tyr394 represents a novel mechanism for p53 activation.
- The c-Abl/Mdm2/p53 axis is a critical pathway in cellular stress response and tumor suppression.