Mitochondrial MDM2 Regulates Respiratory Complex I Activity Independently of p53.
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ABSTRACT:
SUBMITTER: Arena G
PROVIDER: S-EPMC8215449 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:In unstressed cells, the tumor suppressor p53 is maintained at low levels by ubiquitin-mediated proteolysis mainly through Mdm2. In response to DNA damage, p53 is stabilized and becomes activated to turn on transcriptional programs that are essential for cell cycle arrest and apoptosis. Activation of p53 leads to accumulation of Mdm2 protein, a direct transcriptional target of p53. It is not understood how p53 is protected from degradation when Mdm2 is up-regulated. Here we report that p53 stabilization in the late phase after ionizing radiation correlates with active ubiquitination. We found that an E3 ubiquitin ligase RFWD3 (RNF201/FLJ10520) forms a complex with Mdm2 and p53 to synergistically ubiquitinate p53 and is required to stabilize p53 in the late response to DNA damage. This process is regulated by the DNA damage checkpoint, because RFWD3 is phosphorylated by ATM/ATR kinases and the phosphorylation mutant fails to stimulate p53 ubiquitination. In vitro experiments suggest that RFWD3 is a p53 E3 ubiquitin ligase and that RFWD3-Mdm2 complex restricts the polyubiquitination of p53 by Mdm2. Our study identifies RFWD3 as a positive regulator of p53 stability when the G(1) cell cycle checkpoint is activated and provides an explanation for how p53 is protected from degradation in the presence of high levels of Mdm2.
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Project description:The MDM2 N-terminal domain can bind to the transactivation domain of p53 and downregulate its ability to activate transcription. It was found that binding with p53 stabilizes the MDM2 N-terminal domain. Thus the coupling between binding and folding is essential in the normal functional interactions between p53 and MDM2. We have performed explicit-solvent molecular dynamics simulations (MD) for both bound MDM2(N) and apo-MDM2 to study the interdependence of binding and folding in the p53-MDM2 complex. Kinetic analysis of high-temperature MD simulations shows that both bound MDM2(N) and apo-MDM2 unfold via a two-state process. Both kinetics and free energy landscape analyses indicate that bound MDM2 unfolds in the order of p53 unbinding, tertiary unfolding, and finally secondary structure unfolding. Our data show that the unfolding pathways are different between bound MDM2(N) and apo-MDM2: the unfolding order of unstable helices and tertiary contacts is reversed. Transition state analysis shows that the transition state of bound MDM2 is more nativelike and more heterogeneous than that of apo-MDM2. The predicted Phi-values suggest that the stable helices are more nativelike than other regions in both bound MDM2(N) and apo-MDM2. Within the stable helices, helix II in bound MDM2 is more nativelike than that in apo-MDM2. However, helix I and IV in bound MDM2 are less nativelike than those in apo-MDM2.
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Project description:MDM2 is a key regulator of the p53 tumor suppressor acting primarily as an E3 ubiquitin ligase to promote its degradation. MDM2 also inhibits p53 transcriptional activity by recruiting histone deacetylase and corepressors to p53. Here, we show that immunopurified MDM2 complexes have significant histone H3-K9 methyltransferase activity. The histone methyltransferases SUV39H1 and EHMT1 bind specifically to MDM2 but not to its homolog MDMX. MDM2 mediates formation of p53-SUV39H1/EHMT1 complex capable of methylating H3-K9 in vitro and on p53 target promoters in vivo. Furthermore, MDM2 promotes EHMT1-mediated p53 methylation at K373. Knockdown of SUV39H1 and EHMT1 increases p53 activity during stress response without affecting p53 levels, whereas their overexpression inhibits p53 in an MDM2-dependent manner. The p53 activator ARF inhibits SUV39H1 and EHMT1 binding to MDM2 and reduces MDM2-associated methyltransferase activity. These results suggest that MDM2-dependent recruitment of methyltransferases is a novel mechanism of p53 regulation through methylation of both p53 itself and histone H3 at target promoters.
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Project description:The expression level of the tumor suppressor p53 is controlled by the E3 ubiquitin ligase MDM2 with a regulatory feedback loop, which allows p53 to upregulate its inhibitor MDM2. In this manuscript we demonstrated that p90RSK binds and phosphorylates MDM2 on serine 166 both in vitro and in vivo by kinase assay, immunoblot, and co-immunoprecipitation assay; this phosphorylation increases the stability of MDM2 which in turn binds p53, ubiquitinating it and promoting its degradation by proteasome. A pharmacological inhibitor of p90RSK, BI-D1870, decreases MDM2 phosphorylation, and restores p53 function, which in turn transcriptionally increases the expression of cell cycle inhibitor p21 and of pro-apoptotic protein Bax and downregulates the anti-apoptotic protein Bcl-2, causing a block of cell proliferation, measured by a BrdU assay and growth curve, and promoting apoptosis, measured by a TUNEL assay. Finally, an immunohistochemistry evaluation of primary thyroid tumors, in which p90RSK is very active, confirms MDM2 stabilization mediated by p90RSK phosphorylation.
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Project description:Under normal, non-stressed conditions, intracellular p53 is continually ubiquitinated by MDM2 and targeted for degradation. However, in response to severe genotoxic stress, p53 protein levels are markedly increased and apoptotic cell death is triggered. Inhibiting the ubiquitination of p53 under conditions where DNA damage has occurred is therefore crucial for preventing the development of cancer, because if cells with severely damaged genomes are not removed from the population, uncontrolled growth can result. However, questions remain about the cellular mechanisms underlying the regulation of p53 stability. In this study, we show that p53-inducible gene 3 (PIG3), which is a transcriptional target of p53, regulates p53 stability. Overexpression of PIG3 stabilized both endogenous and transfected wild-type p53, whereas a knockdown of PIG3 lead to a reduction in both endogenous and UV-induced p53 levels in p53-proficient human cancer cells. Using both in vivo and in vitro ubiquitination assays, we found that PIG3 suppressed both ubiquitination- and MDM2-dependent proteasomal degradation of p53. Notably, we demonstrate that PIG3 interacts directly with MDM2 and promoted MDM2 ubiquitination. Moreover, elimination of endogenous PIG3 in p53-proficient HCT116 cells decreased p53 phosphorylation in response to UV irradiation. These results suggest an important role for PIG3 in regulating intracellular p53 levels through the inhibition of p53 ubiquitination.
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Project description:p53 is a critical coordinator of a wide range of stress responses. To facilitate a rapid response to stress, p53 is produced constitutively but is negatively regulated by MDM2. MDM2 can inhibit p53 in multiple independent ways: by binding to its transcription activation domain, inhibiting p53 acetylation, promoting nuclear export, and probably most importantly by promoting proteasomal degradation of p53. The latter is achieved via MDM2's E3 ubiquitin ligase activity harbored within the MDM2 RING finger domain. We have discovered that MTBP promotes MDM2-mediated ubiquitination and degradation of p53 and also MDM2 stabilization in an MDM2 RING finger-dependent manner. Moreover, using small interfering RNA to down-regulate endogenous MTBP in unstressed cells, we have found that MTBP significantly contributes to MDM2-mediated regulation of p53 levels and activity. However, following exposure of cells to UV, but not gamma-irradiation, MTBP is destabilized as part of the coordinated cellular response. Our findings suggest that MTBP differentially regulates the E3 ubiquitin ligase activity of MDM2 towards two of its most critical targets (itself and p53) and in doing so significantly contributes to MDM2-dependent p53 homeostasis in unstressed cells.
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