contributed equally to this workThe tumor suppressor p53 is activated in response to many types of cellular and environmental insults via mechanisms involving post-translational modi®cation.Here we demonstrate that, unlike phosphorylation, p53 invariably undergoes acetylation in cells exposed to a variety of stress-inducing agents including hypoxia, anti-metabolites, nuclear export inhibitor and actinomycin D treatment. In vivo, p53 acetylation is mediated by the p300 and CBP acetyltransferases. Overexpression of either p300 or CBP, but not an acetyltransferase-de®cient mutant, ef®ciently induces speci®c p53 acetylation. In contrast, MDM2, a negative regulator of p53, actively suppresses p300/CBPmediated p53 acetylation in vivo and in vitro. This inhibitory activity of MDM2 on p53 acetylation is in turn abrogated by tumor suppressor p19 ARF , indicating that regulation of acetylation is a central target of the p53±MDM2±p19 ARF feedback loop. Functionally, inhibition of deacetylation promotes p53 stability, suggesting that acetylation plays a positive role in the accumulation of p53 protein in stress response. Our results provide evidence that p300/CBP-mediated acetylation may be a universal and critical modi®-cation for p53 function. Keywords: acetylation/CBP/MDM2/p300/p53 IntroductionThe tumor suppressor p53 plays a critical role in human cancer formation. In response to a variety of stress signals, often associated with the progression of neoplastic diseases, p53 becomes activated and induces cell cycle arrest and/or programmed cell death (apoptosis). By eliminating damaged and potentially dangerous cells that might otherwise become cancerous, p53 suppresses tumor formation. In unstressed cells, p53 is latent and is maintained at low levels by targeted degradation mediated by its negative regulator, MDM2 (reviewed in Freedman et al., 1999). The critical role of MDM2 in regulating p53 is best illustrated by a study carried out in mice where inactivation of p53 was shown to completely rescue the embryonic lethality caused by the loss of MDM2 function (Montes de Oca Luna et al., 1995). MDM2 counteracts p53 tumor suppressor activity by physically binding to p53 and suppressing its transcriptional activity. MDM2 also functions as the p53 ubiquitin ligase and triggers its degradation (reviewed in Freedman et al., 1999). This latter activity requires the Ring ®nger domain located at the C-terminus of MDM2 (Fang et al., 2000), and may also involve the acetyltransferase p300, which binds both MDM2 and p53 (Grossman et al., 1998). Therefore, MDM2 negatively regulates p53 by at least two independent mechanisms.The activation and stabilization of p53 are thought to be mediated by speci®c protein modi®cations, with phosphorylation being the major focus of earlier studies (reviewed in Giaccia and Kastan, 1998;Appella and Anderson, 2000). Although the exact functions of speci®c phosphorylation events remain controversial, evidence indicates that they probably contribute to both the stabilization and activation of p53. For ex...

The tumor suppressor p53 is stabilized and activated in response to cellular stress through post-translational modi®cations including acetylation. p300/CBPmediated acetylation of p53 is negatively regulated by MDM2. Here we show that MDM2 can promote p53 deacetylation by recruiting a complex containing HDAC1. The HDAC1 complex binds MDM2 in a p53-independent manner and deacetylates p53 at all known acetylated lysines in vivo. Ectopic expression of a dominant-negative HDAC1 mutant restores p53 acetylation in the presence of MDM2, whereas wildtype HDAC1 and MDM2 deacetylate p53 synergistically. Fibroblasts overexpressing a dominant negative HDAC1 mutant display enhanced DNA damageinduced p53 acetylation, increased levels of p53 and a more pronounced induction of p21 and MDM2. These results indicate that acetylation promotes p53 stability and function. As the acetylated p53 lysine residues overlap with those that are ubiquitylated, our results suggest that one major function of p53 acetylation is to promote p53 stability by preventing MDM2-dependent ubiquitylation, while recruitment of HDAC1 by MDM2 promotes p53 degradation by removing these acetyl groups.

Background: Parkin is proposed to maintain mitochondrial QC through promoting mitophagy. Results: Under moderate mitochondrial stress conditions, parkin stimulates mitochondrial fusion instead of mitophagy by catalyzing K63-linked ubiquitination and inactivating ␣-synuclein. Conclusion: Parkin, PINK1, and ␣-synuclein form a regulatory circuit to regulate mitochondrial stress response. Significance: This study provides a physiological context to functionally connect key PARK genes in the pathogenesis of Parkinson disease.