E3 Ubiquitin Ligase MDM2 Research Articles

Phospholipase D elevates the level of MDM2 and suppresses DNA damage-induced increases in p53.

Phospholipase D (PLD) has been reported to generate survival signals that prevent apoptosis induced by serum withdrawal. We have now found that elevated expression of PLD also suppresses DNA damage-induced apoptosis. Since DNA damage-induced apoptosis is often mediated by p53, we examined the effect of elevated PLD expression on the regulation of p53 stabilization. We report here that PLD suppresses DNA damage-induced increases in p53 stabilization in cells where PLD has been shown to provide a survival signal. Elevated expression of PLD also led to increased expression of the p53 E3 ubiquitin ligase MDM2 and increased turnover of p53. PLD1-stimulated increases in MDM2 expression and suppression of p53 activation were blocked by inhibition of mTOR and the mitogen-activated protein kinase pathway. Although PLD did not activate the phosphatidylinositol 3-kinase (PI3K)/Akt survival pathway activate the basal levels of PI3K activity were partially required for PLD1-induced increases in MDM2. These data provide evidence that survival signals generated by PLD involve suppression of the p53 response pathway.

Accelerated MDM2 auto-degradation induced by DNA-damage kinases is required for p53 activation.

p53 activation prevents the proliferation of genetically unstable cells. Conversely, p53 antagonism by its transcriptional target, the E3 ubiquitin ligase MDM2, is critical for the viability of unstressed, cycling cells. We demonstrate that MDM2 induces the degradation of p53 in both the nucleus and the cytoplasm. As p53 and MDM2 accumulate in the nuclei of stressed cells, we investigated mechanisms enabling p53 activation despite the high MDM2 levels generated during a DNA-damage response. We show that DNA damage destabilized MDM2 by a mechanism involving damage-activated kinases and MDM2 auto-ubiquitination. p53 was stable and transcriptionally active when MDM2 was unstable, but became unstable and inactive as the damage response waned and MDM2 stabilized. Importantly, blocking MDM2 destabilization in DNA-damaged cells prevented p53 target gene activation. Our data reveal that controlled MDM2 degradation is an important new step in p53 regulation.

BRCA1 interacts with dominant negative SWI/SNF enzymes without affecting homologous recombination or radiation-induced gene activation of p21 or Mdm2.

BRCA1 is a tumor suppressor gene linked to familial breast and ovarian cancer. The BRCA1 protein has been implicated in a diverse set of cellular functions, including activation of gene expression by the p53 tumor suppressor and control of homologous recombination (HR) during DNA repair. Prior reports have demonstrated that BRCA1 can exist in cells in a complex with the BRG1-based SWI/SNF ATP-dependent chromatin remodeling enzymes and that SWI/SNF components contribute to p53-mediated gene activation. To investigate the link between SWI/SNF function and BRCA1 mediated effects on p53-mediated gene activation and on mechanisms of homologous recombination, we have utilized mammalian cells that inducibly express an ATPase-deficient, dominant negative SWI/SNF enzymes. Mutant SWI/SNF ATPases retain the ability to interact with BRCA1 in cells. We report that expression of dominant negative SWI/SNF enzymes does not affect p53-mediated induction of the p21 cyclin dependent kinase inhibitor or the Mdm2 E3 ubiquitin ligase that regulates p53 in cells exposed to UV or gamma irradiation. Similarly, integration of a reporter that monitors homologous recombination by gene conversion into these cells demonstrated no change in the recombination rate in the absence of functional SWI/SNF enzyme. We conclude that the SWI/SNF chromatin remodeling enzymes may contribute to but are not required for these processes.

Fateful Dose of Mdm2

The p53 tumor suppressor pathway is frequently inactivated in human cancer. In response to stress, the cellular levels of p53 protein rise, which can lead either to cell growth arrest or apoptosis. In normal cells, p53 levels are kept low through rapid protein turnover. This process is tightly regulated by the E3 ubiquitin ligase Mdm2, but experiments aimed at dissecting the mechanistic details have yielded conflicting results. Li et al. now show that the fate of p53 is determined by the level of Mdm2 activity in the cell. Low levels of Mdm2 promote monoubiquitination and nuclear export of p53, whereas high levels of Mdm2 promote its polyubiquitination and degradation in the nucleus. M. Li, C. L. Brooks, F. Wu-Baer, D. Chen, R. Baer, W. Gu, Mono- versus polyubiquitination: Differential control of p53 fate by Mdm2. Science 302 , 1972-1975 (2003). [Abstract] [Full Text]

Nucleotide Binding by the MDM2 RING Domain Facilitates Arf-Independent MDM2 Nucleolar Localization

The RING domain of Mdm2 contains a conserved Walker A or P loop motif that is a characteristic of nucleotide binding proteins. We found that Mdm2 binds adenine-containing nucleotides preferentially and that nucleotide binding leads to a conformational change in the Mdm2 C terminus. Although nucleotide binding is not required for Mdm2 E3 ubiquitin ligase activity, we show that nucleotide binding-defective P loop mutants are impaired in p14ARF-independent nucleolar localization both in vivo and in vitro. Consistent with this, ATP-bound Mdm2 is preferentially localized to the nucleolus. Indeed, we identify a unique amino acid substitution in the P loop motif (K454A) that uncouples nucleolar localization and E3 ubiquitin ligase activity of Mdm2 and leads to upregulation of the E3 activity both in human cells and in Caenorhabditis elegans. We propose that nucleotide binding-facilitated nucleolar localization of Mdm2 is an evolutionarily conserved regulator of Mdm2 activity.

Ubiquitin Exposes the Nuclear Export Sequence

The mechanism by which the E3 ubiquitin ligase MDM2 promotes the nuclear export of the tumor suppressor and regulator of cell proliferation p53 remains controversial. The ubiquitin ligase activity of MDM2 is required to promote p53 nuclear export. One model is that ubiquitination disrupts a p53 oligomer, thus exposing the nuclear export signal (NES). Another model is that ubiquitination promotes the degradation of p53, which results in decreased formation of p53 oligomers and allows exposure of the NES. Gu et al. used chimeras between p53 and p73 (a related protein that is restricted to the nucleus by the presence of MDM2) to define the mechanism by which MDM2 promotes the nuclear export of p53. Chimeras with the p53 NES fused to p73 were localized to the cytoplasm, suggesting that in the context of p73, the NES is exposed and not "hidden" as it is in p53 in the absence of MDM2. In order to restore nuclear localization, the chimera had to have the entire COOH-terminus of p53. However, this protein did not redistribute to the cytoplasm in response to MDM2 expression. A third domain was required to restore MDM2-mediated nuclear export: the DNA binding domain. All of these protein chimeras were capable of interacting with MDM2 and of forming tetramers, suggesting that disruption of tetramers does not account for nuclear export of p53. Gu et al. and Lohrum et al. both provide evidence that the ubiquination of p53, and not simply binding to MDM2, promotes MDM2-mediated nuclear export of p53. Both groups found that mutation of the six lysines in the COOH-terminus disrupted MDM2-dependent nuclear export. Gu et al. also found that mutation of one lysine in the domain between the DNA binding domain and the oligomerization domain resulted in a cytoplasmically localized p53 that was independent of the presence of MDM2, suggesting that this region is essential for maintaining the structure that controls the accessibility of the NES. Thus, control of p53 distribution is complex and most likely based on conformational changes induced by ubiquitination that exposes the NES. J. Gu, L. Nie, D. Wiederschain, Z.-M. Yuan, Identification of p53 sequence elements that are required for MDM2-mediated nuclear export. Mol. Cell. Biol. 21 , 8533-8546 (2001). [Abstract] [Full Text] M. A. Lohrum, D. B. Woods, R. L. Ludwig, É. Bálint, K. H. Vousden, C-terminal ubiquitination of p53 contributes to nuclear export. Mol. Cell. Biol. 21 , 8521-8532 (2001). [Abstract] [Full Text]

Regulation of receptor fate by ubiquitination of activated beta 2-adrenergic receptor and beta-arrestin.

Although trafficking and degradation of several membrane proteins are regulated by ubiquitination catalyzed by E3 ubiquitin ligases, there has been little evidence connecting ubiquitination with regulation of mammalian G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR) function. Agonist stimulation of endogenous or transfected beta2-adrenergic receptors (beta2ARs) led to rapid ubiquitination of both the receptors and the receptor regulatory protein, beta-arrestin. Moreover, proteasome inhibitors reduced receptor internalization and degradation, thus implicating a role for the ubiquitination machinery in the trafficking of the beta2AR. Receptor ubiquitination required beta-arrestin, which bound to the E3 ubiquitin ligase Mdm2. Abrogation of beta-arrestin ubiquitination, either by expression in Mdm2-null cells or by dominant-negative forms of Mdm2 lacking E3 ligase activity, inhibited receptor internalization with marginal effects on receptor degradation. However, a beta2AR mutant lacking lysine residues, which was not ubiquitinated, was internalized normally but was degraded ineffectively. These findings delineate an adapter role of beta-arrestin in mediating the ubiquitination of the beta2AR and indicate that ubiquitination of the receptor and of beta-arrestin have distinct and obligatory roles in the trafficking and degradation of this prototypic GPCR.

Cooperative signals governing ARF-mdm2 interaction and nucleolar localization of the complex.

The ARF tumor suppressor protein stabilizes p53 by antagonizing its negative regulator, Mdm2 (Hdm2 in humans). Both mouse p19(ARF) and human p14(ARF) bind to the central region of Mdm2 (residues 210 to 304), a segment that does not overlap with its N-terminal p53-binding domain, nuclear import or export signals, or C-terminal RING domain required for Mdm2 E3 ubiquitin ligase activity. The N-terminal 37 amino acids of mouse p19(ARF) are necessary and sufficient for binding to Mdm2, localization of Mdm2 to nucleoli, and p53-dependent cell cycle arrest. Although a nucleolar localization signal (NrLS) maps within a different segment (residues 82 to 101) of the human p14(ARF) protein, binding to Mdm2 and nucleolar import of ARF-Mdm2 complexes are both required for cell cycle arrest induced by either the mouse or human ARF proteins. Because many codons of mouse ARF mRNA are not recognized by the most abundant bacterial tRNAs, we synthesized ARF minigenes containing preferred bacterial codons. Using bacterially produced ARF polypeptides and chemically synthesized peptides conjugated to Sepharose, residues 1 to 14 and 26 to 37 of mouse p19(ARF) were found to interact independently and cooperatively with Mdm2, while residues 15 to 25 were dispensable for binding. Paradoxically, residues 26 to 37 of mouse p19(ARF) are also essential for ARF nucleolar localization in the absence of Mdm2. However, the mobilization of the p19(ARF)-Mdm2 complex into nucleoli also requires a cryptic NrLS within the Mdm2 C-terminal RING domain. The Mdm2 NrLS is unmasked upon ARF binding, and its deletion prevents import of the ARF-Mdm2 complex into nucleoli. Collectively, the results suggest that ARF binding to Mdm2 induces a conformational change that facilitates nucleolar import of the ARF-Mdm2 complex and p53-dependent cell cycle arrest. Hence, the ARF-Mdm2 interaction can be viewed as bidirectional, with each protein being capable of regulating the subnuclear localization of the other.