Search

During macroautophagy, conjugation of ATG12 to ATG5 is essential for LC3 lipidation and autophagosome formation. Additionally, ATG12 has ATG5-independent functions in diverse processes including mitochondrial fusion and mitochondrial-dependent apoptosis. In this study, we investigated the regulation of free ATG12. In stark contrast to the stable ATG12-ATG5 conjugate, we find that free ATG12 is highly unstable and rapidly degraded in a proteasome-dependent manner. Surprisingly, ATG12, itself a ubiquitin-like protein, is directly ubiquitinated and this promotes its proteasomal degradation. As a functional consequence of its turnover, accumulation of free ATG12 contributes to proteasome inhibitor-mediated apoptosis, a finding that may be clinically important given the use of proteasome inhibitors as anticancer agents. Collectively, our results reveal a novel interconnection between autophagy, proteasome activity, and cell death mediated by the ubiquitin-like properties of ATG12.

GFP and luciferase are used extensively as markers both in vitro and in vivo although both have limitations. The utility of GFP fluorescence is restricted by high background signal and poor tissue penetrance. Luciferase throughput is limited in vitro by the requirement for cell lysis, while in vivo, luciferase readout is complicated by the need for substrate injection and the dependence on endogenous ATP. Here we show that near-infrared fluorescent protein in combination with widely available near-infrared scanners overcomes these obstacles and allows for the accurate determination of cell number in vitro and tumor growth in vivo in a high-throughput manner and at negligible per-well costs. This system represents a significant advance in tracking cell proliferation in tissue culture as well as in animals, with widespread applications in cell biology.

Mdm2 can mediate p53 ubiquitylation and degradation either in the form of the Mdm2 homodimer or Mdm2/MdmX heterodimer. The ubiquitin ligase activity of these complexes resides mainly in their respective RING finger domains and also requires adjacent C-terminal tails. So far, structural studies have failed to show significant differences between Mdm2 RING homodimers and Mdm2/MdmX RING heterodimers. Here, we report that not only the primary amino acid sequence, but also the length of the C-terminal tail of Mdm2 is highly conserved through evolution and plays an important role in Mdm2 activity toward p53. Mdm2 mutants with extended C termini do not ubiquitylate p53 despite being capable of forming Mdm2 homodimers through both RING-acidic domain and RING-RING interactions. All extended mutants also retained the ability to interact with MdmX, and this interaction led to reactivation of their E3 ubiquitin ligase activity. In contrast, only a subset of extended Mdm2 mutants was activated by the interaction with Mdm2 RING domain, suggesting that Mdm2 homodimers and Mdm2/MdmX heterodimers may not be structurally and functionally fully equivalent.

The tumor suppressor p53 is extensively regulated by post-translational modification, including modification by the small ubiquitin-related modifier SUMO. We show here that MDM2, previously shown to promote ubiquitin, Nedd8 and SUMO-1 modification of p53, can also enhance conjugation of endogenous SUMO-2/3 to p53. Sumoylation activity requires p53-MDM2 binding but does not depend on an intact RING finger. Both ARF and L11 can promote SUMO-2/3 conjugation of p53. However, unlike the previously described SUMO-1 conjugation of p53 by an MDM2-ARF complex, this activity does not depend on the ability of MDM2 to relocalize to the nucleolus. Interestingly, the SUMO consensus is not conserved in mouse p53, which is therefore not modified by SUMO-2/3. Finally, we show that conjugation of SUMO-2/3 to p53 correlates with a reduction of both activation and repression of a subset of p53-target genes.

The intricate regulation of cell survival and cell death is critical for the existence of both normal and transformed cells. Two factors central to these processes are p53 and NFkappaB, with both factors having ascribed roles in both promoting and repressing cell death. Not surprisingly, a number of studies have previously reported interplay between p53 and NFkappaB. The mechanistic basis behind these observations, however, is currently incomplete. We report here further insights into this interplay using a system where blockade of NFkappaB inhibits cell death from p53, but at the same time sensitizes cells to death by TNFalpha. We found in agreement with a recent report showing that NFkappaB is required for the efficient activation of the BH3-only protein Noxa by the p53 family member p73, that p53's ability to induce Noxa is also impeded by inhibition of NFkappaB. In contrast to the regulation by p73, however, blockade of NFkappaB downstream of p53 decreases Noxa protein levels without effects on Noxa mRNA. Our further analysis of the effects of NFkappaB inhibition on p53 target gene expression revealed that while most target genes analysed where unaffected by blockade of NFkappaB, the p53-mediated induction of the pro-apoptotic gene p53AIP1 was significantly dependent on NFkappaB. These studies therefore add further insight into the complex relationship of p53 and NFkappaB. In addition, since both Noxa and p53AIP1 have been shown to be important components of p53-mediated cell death responses, these findings may also indicate critical points where NFkappaB plays a pro-apoptotic role downstream of p53.

The tumour suppressor p53 has been shown to be modified at its C-terminus with ubiquitin and the ubiquitin-like proteins SUMO and NEDD8. Whereas monoubiquitination of p53 is strongly associated with nuclear export, the effects of sumoylation and neddylation remain unclear. In this study we have generated p53-Ub, p53-SUMO-1 and p53-NEDD8 fusion proteins as models for the effect of these modifications on the localization and function of p53. As shown before, the ubiquitin fusion clearly drives nuclear export of p53 and we now find that this is also partially the case for a SUMO-1 fusion, which does not localise to PML bodies. In contrast a NEDD8 fusion has little effect on nuclear export, and mutating NEDD8 to more closely resemble ubiquitin did not restore nuclear export. Despite their differing subcellular localization, we find that both p53-ubiquitin and p53-NEDD8 retain similar transcriptional activity and both induce apoptosis at a similar level to unfused p53. The p53-ubiquitin fusion protein is potentially a good model for studying the role of p53 outside the nucleus. However, in our experiments we find that the export of p53 from the nucleus is not sufficient to activate its cytoplasmic apoptotic function which may depend on the ability to deubiquitinate cytoplasmic p53.

While wild-type p53 is normally a rapidly degraded protein, mutant forms of p53 are stabilized and accumulate to high levels in tumor cells. In this study, we show that mutant and wild-type p53 proteins are ubiquitinated and degraded through overlapping but distinct pathways. While Mdm2 can drive the degradation of both mutant and wild-type p53, our data suggest that the ability of Mdm2 to function as a ubiquitin ligase is less important in the degradation of mutant p53, which is heavily ubiquitinated in an Mdm2-independent manner. Our initial attempts to identify ubiquitin ligases that are responsible for the ubiquitination of mutant p53 have suggested a role for the chaperone-associated ubiquitin ligase CHIP (C terminus of Hsc70-interacting protein), although other unidentified ubiquitin ligases also appear to contribute. The contribution of Mdm2 to the degradation of mutant p53 may reflect the ability of Mdm2 to deliver the ubiquitinated mutant p53 to the proteasome.

The stability of the p53 protein is regulated by Mdm2. By acting as an E3 ubiquitin ligase, Mdm2 directs the ubiquitylation of p53 and its subsequent degradation by the 26S proteasome. In contrast, the Mdmx protein, although structurally similar to Mdm2, cannot ubiquitylate or degrade p53 in vivo. To ascertain which domains determine this functional difference between Mdm2 and Mdmx and consequently are essential for p53 ubiquitylation and degradation, we generated Mdm2-Mdmx chimeric constructs. Here we show that, in addition to a fully functional Mdm2 RING finger, an internal domain of Mdm2 (residues 202 to 302) is essential for p53 ubiquitylation. Strikingly, the function of this domain can be fulfilled in trans, indicating that the RING domain and this internal region perform distinct activities in the ubiquitylation of p53.

The growth inhibitory functions of p53 are controlled in unstressed cells by rapid degradation of the p53 protein. One of the principal regulators of p53 stability is MDM2, a RING finger protein that functions as an E3 ligase to ubiquitinate p53. MDM2 promotes p53 nuclear export, and in this study, we show that ubiquitination of the C terminus of p53 by MDM2 contributes to the efficient export of p53 from the nucleus to the cytoplasm. In contrast, MDM2 did not promote nuclear export of the p53-related protein, p73. p53 nuclear export was enhanced by overexpression of the export receptor CRM1, although no significant relocalization of MDM2 was seen in response to CRM1. However, nuclear export driven by CRM1 overexpression did not result in the degradation of p53, and nuclear export was not essential for p53 degradation. These results indicate that MDM2 mediated ubiquitination of p53 contributes to both nuclear export and degradation of p53 but that these activities are not absolutely dependent on each other.

The p53 tumor suppressor is activated by many diverse stress signals through mechanisms that result in stabilization and accumulation of the p53 protein. p53 is normally degraded through the proteasome following interaction with MDM2, which both functions as a ubiquitin ligase for p53 and shuttles to the cytoplasm, where p53 degradation occurs. Stabilization of p53 in response to stress is associated with inhibition of MDM2-mediated degradation, which has been associated with phosphorylation of p53 in response to DNA damage or activation of ARF. In this study we show distinct responses, as measured by phosphorylation, transcriptional activity, and subcellular localization, of p53 stabilized by different activating signals. Although normal cells and wild-type p53-expressing tumor cells showed similar responses to actinomycin D and camptothecin treatment, the transcriptional activity of stabilized p53 induced by deferoxamine mesylate, which mimics hypoxia, in normal cells was lost in all three tumor cell lines tested. Our results show that multiple pathways exist to stabilize p53 in response to different forms of stress, and they may involve down-regulation of MDM2 expression or regulation of the subcellular localization of p53 or MDM2. Loss of any one of these pathways may predispose cells to malignant transformation, although reactivation of p53 might be achieved through alternative pathways that remain functional in these tumor cells.

The p53 tumor suppressor protein can be phosphorylated at several sites within the N- and C-terminal domains, and several protein kinases have been shown to phosphorylate p53 in vitro. In this study, we examined the activity of p53 proteins with combined mutations at all of the reported N-terminal phosphorylation sites (p53N-term), all of the C-terminal phosphorylation sites (p53C-term), or all of the phosphorylation sites together (p53N/C-term). Each of these mutant proteins retained transcriptional transactivation functions, indicating that phosphorylation is not essential for this activity of p53, although a subtle contribution of the C-terminal phosphorylation sites to the activation of expression of the endogenous p21(Waf1/Cip1)-encoding gene was detected. Mutation of the phosphorylation sites to alanine did not affect the sensitivity of p53 to binding to or degradation by Mdm2, although alteration of residues 15 and 37 to aspartic acid, which could mimic phosphorylation, resulted in a slight resistance to Mdm2-mediated degradation, consistent with recent reports that phosphorylation at these sites inhibits the p53-Mdm2 interaction. However, expression of the phosphorylation site mutant proteins in both wild-type p53-expressing and p53-null lines showed that all of the mutant proteins retained the ability to be stabilized following DNA damage. This indicates that phosphorylation is not essential for DNA damage-induced stabilization of p53, although phosphorylation could clearly contribute to p53 stabilization under some conditions.

The stability of the p53 tumor suppressor protein is regulated by interaction with Mdm2, the product of a p53-inducible gene. Mdm2-targeted degradation of p53 depends on the interaction between the two proteins and is mediated by the proteasome. We show here that in addition to the N-terminal Mdm2 binding domain, the C terminus of p53 participates in the ability of p53 to be degraded by Mdm2. In contrast, alterations in the central DNA binding domain of p53, which change the conformation of the p53 protein, do not abrogate the sensitivity of the protein to Mdm2-mediated degradation. The importance of the C-terminal oligomerization domain to Mdm2-targeted degradation of p53 is likely to reflect the importance of oligomerization of the full-length p53 protein for interaction with Mdm2, as previously shown in vitro. Interestingly, the extreme C-terminal region of p53, outside the oligomerization domain, was also shown to be necessary for efficient degradation, and deletion of this region stabilized the protein without abrogating its ability to bind to Mdm2. Mdm2-resistant p53 mutants were not further stabilized following DNA damage, supporting a role for Mdm2 as the principal regulator of p53 stability in cells. The extreme C terminus of the p53 protein has previously been shown to contain several regulatory elements, raising the possibility that either allosteric regulation of p53 by this domain or interaction between this region and a third protein plays a role in determining the sensitivity of p53 to Mdm2-directed degradation.

Suppression of tumor cell growth by p53 results from the activation of both apoptosis and cell cycle arrest, functions which have been shown to be separable activities of p53. We have characterized a series of p53 mutants with amino acid substitutions at residue 175 and show that these mutants fall into one of three classes: class I, which is essentially wild type for apoptotic and cell cycle arrest functions; class II, which retains cell cycle arrest activity but is impaired in the induction of apoptosis; and class III, which is defective in both activities. Several residue 175 mutants which retain cell cycle arrest function have been detected in cancers, and we show that these have lost apoptotic function. Furthermore, several class II mutants have been found to be temperature sensitive for apoptotic activity while showing constitutive cell cycle arrest function. Taken together, these mutants comprise an excellent system with which to investigate the biochemical nature of p53-mediated apoptosis, the function of principal importance in tumor suppression. All of the mutants that showed loss of apoptotic function also showed defects in the activation of promoters from the potential apoptotic targets Bax and the insulin-like growth factor-binding protein 3 gene (IGF-BP3), and a correlation between full apoptotic activity and activation of both of these promoters was also seen with the temperature-sensitive mutants. However, a role for additional apoptotic activities of p53 was suggested by the observation that some mutants retained significant apoptotic function despite being impaired in the activation of Bax- and IGF-BP3-derived promoters. In contrast to the case of transcriptional activation, a perfect correlation between transcriptional repression of the c-fos promoter and the ability to induce apoptosis was seen, although the observation that Bax expression induced a similar repression of transcription from this promoter suggests that this may be a consequence, rather than a cause, of apoptotic death.

The p53 tumor suppressor protein is activated in cells in response to DNA damage and prevents the replication of cells sustaining genetic damage by inducing a cell cycle arrest or apoptosis. Activation of p53 is accompanied by stabilization of the protein, resulting in accumulation to high levels within the cell. p53 is normally degraded through the proteasome following ubiquitination, although the mechanisms which regulate this proteolysis in normal cells and how the p53 protein becomes stabilized following DNA damage are not well understood. We show here that p53 can also be a substrate for cleavage by the calcium-activated neutral protease, calpain, and that a preferential site for calpain cleavage exists within the N terminus of the p53 protein. Treatment of cells expressing wild-type p53 with an inhibitor of calpain resulted in the stabilization of the p53 protein. By contrast, in vitro or in vivo degradation mediated by human papillomavirus E6 protein was unaffected by the calpain inhibitor, indicating that the stabilization did not result from inhibition of the proteasome. These results suggest that calpain cleavage plays a role in regulating p53 stability.

The p53 tumor suppressor protein is a sequence-specific transcriptional activator, a function which contributes to cell cycle arrest and apoptosis induced by p53 in appropriate cell types. Analysis of a series of p53 point mutants has revealed the potential for selective loss of the ability to transactivate some, but not all, cellular p53-responsive promoters. p53 175P and p53 181L are tumor-derived p53 point mutants which were previously characterized as transcriptionally active. Both mutants retained the ability to activate expression of the cyclin-dependent kinase inhibitor p2lcip1/waf1, and this activity correlated with the ability to induce a G1 cell cycle arrest. However, an extension of this survey to include other p53 targets showed that p53 175P was defective in the activation of p53-responsive sequences derived from the bax promoter and the insulin-like growth factor-binding protein 3 gene (IGF-BP3) promoter, while p53 181L showed loss of the ability to activate a promoter containing IGF-BP3 box B sequences. Failure to activate transcription was also reflected in the reduced ability of the mutants to bind the p53-responsive DNA sequences present in these promoters. These specific defects in transcriptional activation correlated with the impaired apoptotic function displayed by these mutants, and the results suggest that activation of cell cycle arrest genes by p53 can be separated from activation of genes with a role in mediating the p53 apoptotic response. The cellular response to p53 activation may therefore depend, at least in part, on which group of p53-responsive genes become transcriptionally activated.