MDM2 inhibits PCAF (p300/CREB-binding protein-associated factor)-mediated p53 acetylation

Role of the Histone Acetyl Transferase Tip60 in the p53 Pathway

The histone acetyl transferase Tip60 (HTATIP) shares many properties with the tumor suppressor p53 (TP53). Both proteins are involved in the cellular response to DNA damage, are subjected to proteasomal digestion following Mdm2-mediated ubiquitination, and accumulate after UV irradiation. We found here that knock-down of Tip60 affects the p53-dependent response following actinomycin D treatment, most likely because it inhibits p21 (CDKN1A) accumulation. Moreover, Tip60 is required for p53 to activate the endogenous p21 promoter, suggesting that it functions as a p53 co-activator. However, we also found that knock-down of Tip60 increases the turnover rate of p53 under normal growth conditions. Tip60 interferes with Mdm2-mediated degradation of p53, probably because it affects its subcellular localization. Taken together, our results suggest that Tip60 plays a double role in the p53 pathway: under normal growth conditions, Tip60 contributes to maintain a basal pool of p53 by interfering with its degradation; following DNA damage, Tip60 functions as p53 co-activator. That these two distinct roles are linked during the p53-dependent response is an attractive hypothesis.

Ribosomal protein L23 activates p53 by inhibiting MDM2 function in response to ribosomal perturbation but not to translation inhibition

The p53-MDM2 feedback loop is vital for cell growth control and is subjected to multiple regulations in response to various stress signals. Here we report another regulator of this loop. Using an immunoaffinity method, we purified an MDM2-associated protein complex that contains the ribosomal protein L23. L23 interacted with MDM2, forming a complex independent of the 80S ribosome and polysome. The interaction of L23 with MDM2 was enhanced by treatment with actinomycin D but not by gamma-irradiation, leading to p53 activation. This activation was inhibited by small interfering RNA against L23. Ectopic expression of L23 reduced MDM2-mediated p53 ubiquitination and also induced p53 activity and G(1) arrest in p53-proficient U2OS cells but not in p53-deficient Saos-2 cells. These results reveal that L23 is another regulator of the p53-MDM2 feedback regulation.

Inhibition of MDM2-mediated p53 ubiquitination and degradation by ribosomal protein L5

The oncoprotein MDM2 associates with ribosomal proteins L5, L11, and L23. Both L11 and L23 have been shown to activate p53 by inhibiting MDM2-mediated p53 suppression. Here we have shown that L5 also activates p53. Overexpression of L5 stabilized ectopic p53 in H1299 cells and endogenous p53 in U2OS cells. Consequently, L5 enhanced p53 transcriptional activity and induced p53-dependent G1 cell cycle arrest. Furthermore, like L11 and L23, L5 also remarkably inhibited MDM2-mediated p53 ubiquitination. The interaction of L5 with MDM2 was also enhanced by treatment with a low dose of actinomycin D. Actinomycin D-induced p53 was inhibited by small interference RNA against L5. By reciprocal co-immunoprecipitation, we further showed that there were at least two MDM2-ribosomal protein complexes in cells: MDM2-L5-L11-L23 and p53-MDM2-L5-L11-L23. We propose that the MDM2-L5-L11-L23 complex functions to inhibit MDM2-mediated p53 ubiquitination and thus activates p53.

Ubiquitin ligases and the immune response

Ubiquitin (Ub)-protein conjugation represents a novel means of posttranscriptional modification in a proteolysis-dependent or -independent manner. E3 Ub ligases play a key role in governing the cascade of Ub transfer reactions by recognizing and catalyzing Ub conjugation to specific protein substrates. The E3s, which can be generally classified into HECT-type and RING-type families, are involved in the regulation of many aspects of the immune system, including the development, activation, and differentiation of lymphocytes, T cell-tolerance induction, antigen presentation, immune evasion, and virus budding. E3-promoted ubiquitination affects a wide array of biological processes, such as receptor downmodulation, signal transduction, protein processing or translocation, protein-protein interaction, and gene transcription, in addition to proteasome-mediated degradation. Deficiency or mutation of some of the E3s like Cbl, Cbl-b, or Itch, causes abnormal immune responses such as autoimmunity, malignancy, and inflammation. This review discusses our current understanding of E3 Ub ligases in both innate and adaptive immunity. Such knowledge may facilitate the development of novel therapeutic approaches for immunological diseases.

HCMV IE2-mediated inhibition of HAT activity downregulates p53 function

Targeting of cellular histone acetyltransferases (HATs) by viral proteins is important in the development of virus-associated diseases. The immediate-early 2 protein (IE2) of human cytomegalovirus (HCMV) binds to the tumor suppressor, p53, and inactivates its functions by unknown mechanisms. Here, we show that IE2 binds to the HAT domain of the p53 coactivators, p300 and CREB-binding protein (CBP), and blocks their acetyltransferase activity on both histones and p53. The minimal HAT inactivation region on IE2 involves the N-terminal 98 amino acids. The in vivo DNA binding of p53 and local histone acetylation on p53-dependent promoters are all reduced by IE2, but not by mutant IE2 proteins that lack the HAT inhibition region. Furthermore, the p53 acetylation site mutant, K320/373/382R, retains both DNA binding and promoter transactivation activity in vivo and these effects are repressed by IE2 as well. Together with the finding that only wild-type IE2 exerts an antiapoptotic effect, our results suggest that HCMV IE2 downregulates p53-dependent gene activation by inhibiting p300/CBP-mediated local histone acetylation and that IE2 may have oncogenic activity.

MDM2 Mediates p300/CREB-binding Protein-associated Factor Ubiquitination and Degradation

We recently reported that MDM2, a negative feedback regulator of the tumor suppressor p53, inhibits p300/CREB-binding protein-associated factor (PCAF)-mediated p53 acetylation. Our further study showed that MDM2 also regulates the stability of PCAF. MDM2 ubiquitinated PCAF in vitro and in cells. PCAF ubiquitination occurred at the N terminus and in the nucleus, as the nuclear localization signal sequence-deletion mutant of MDM2, which localized in the cytoplasm and degraded p53, was unable to degrade nuclear PCAF. Restriction of PCAF in the nucleus by leptomycin B did not affect MDM2-mediated PCAF degradation. Consistently, overexpression of MDM2 in p53 null cells caused the reduction of the protein level of PCAF, but not the mRNA level. Conversely, PCAF levels were higher in MDM2-deficient mouse p53(-/-)/mdm2(-/-) embryonic fibroblast (MEF) cells than that in MDM2-containing MEF cells. Furthermore, MDM2 reduced the half-life of PCAF by 50%. These results demonstrate that MDM2 regulates the stability of PCAF by ubiquitinating and degrading this protein.

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.

Inhibition of p53 degradation by Mdm2 acetylation

Mdm2 is a RING finger E3 ubiquitin ligase, which promotes ubiquitination and proteasomal degradation of the p53 tumor suppressor protein. Acetylation of p53 regulates p53's transcriptional activity and inhibits Mdm2-mediated p53 ubiquitination and degradation. We now report that Mdm2 is also a target for acetylation. Mdm2 is acetylated in vitro by CREB-binding protein (CBP) and to a lesser extent by p300, but not by p300/CPB-associated factor. Acetylation occurs primarily within the RING finger domain of Mdm2. In vivo acetylation of Mdm2 was detected easily with CBP but not p300. Efficient in vivo acetylation required the preservation of the RING finger. An Mdm2 mutant (K466/467Q) mimicking acetylation is impaired in its ability to promote p53 ubiquitination, as well as Mdm2 autoubiquitination. Moreover, K466/467Q is defective in promoting p53 degradation in living cells. We thus suggest that acetyltransferases may modulate cellular p53 activity not only by modifying p53, but also by inactivating Mdm2.

The diverse superfamily of lysine acetyltransferases and their roles in leukemia and other diseases

Acetylation of the epsilon-amino group of lysine residues, or N(epsilon)-lysine acetylation, is an important post-translational modification known to occur in histones, transcription factors and other proteins. Since 1995, dozens of proteins have been discovered to possess intrinsic lysine acetyltransferase activity. Although most of these enzymes were first identified as histone acetyltransferases and then tested for activities towards other proteins, acetyltransferases only modifying non-histone proteins have also been identified. Lysine acetyltransferases form different groups, three of which are Gcn5/PCAF, p300/CBP and MYST proteins. While members of the former two groups mainly function as transcriptional co-activators, emerging evidence suggests that MYST proteins, such as Esa1, Sas2, MOF, TIP60, MOZ and MORF, have diverse roles in various nuclear processes. Aberrant lysine acetylation has been implicated in oncogenesis. The genes for p300, CBP, MOZ and MORF are rearranged in recurrent leukemia-associated chromosomal abnormalities. Consistent with their roles in leukemogenesis, these acetyltransferases interact with Runx1 (or AML1), one of the most frequent targets of chromosomal translocations in leukemia. Therefore, the diverse superfamily of lysine acetyltransferases executes an acetylation program that is important for different cellular processes and perturbation of such a program may cause the development of cancer and other diseases.

Defective p53 Post-translational Modification Required for Wild Type p53 Inactivation in Malignant Epithelial Cells with mdm2 Gene Amplification

Mdm2 gene amplification occurs in benign and chemotherapy-responsive malignant tumors with wtp53 genes as well as in breast and epithelial cancers. Mdm2 amplification in benign tumors suggests that it is not sufficient for p53 inactivation in cancer, implying that other defects in the p53 pathway are required for malignancy. We investigated mechanisms of wtp53 protein inactivation in malignant conversion of epithelial cells by comparing clonally related initiated cells with their derivative cancerous cells that have mdm2 amplification. Deficiencies in p53 accumulation and activities in response to DNA damage were not due simply to Mdm2 destabilization of p53 protein, but to continued association of DNA-bound p53 with Mdm2 protein and lack of binding and acetylation by p300 protein. The aberrant interactions were not because of mdm2 amplification alone, because DNA-bound p53 protein from initiated cells failed to bind ectopically expressed Mdm2 or endogenous overexpressed Mdm2 from cancerous cells. Phosphorylations of endogenous p53 at Ser18, -23, or -37 were insufficient to dissociate Mdm2, because each was induced by UV in cancerous cells. Interestingly, phospho-mimic p53-T21E did dissociate the Mdm2 protein from DNA-bound p53 and recovered p300 binding and p21 induction in the cancerous cells. Thus wtp53 in malignant cells with mdm2 amplification can be inactivated by continued association of DNA-bound p53 protein with Mdm2 and failure of p300 binding and acetylation, coupled with a defect in p53 phosphorylation at Thr21. These findings suggest therapeutic strategies that address both p53/Mdm2 interaction and associated p53 protein defects in human tumors that have amplified mdm2 genes.

Human immunodeficiency virus type-1 Tat/co-activator acetyltransferase interactions inhibit p53Lys-320 acetylation and p53-responsive transcription

Patients with AIDS are at increased risk for developing various neoplasms, including Hodgkin's and non-Hodgkin's lymphomas, Kaposi's sarcomas, and anal-rectal carcinomas, suggestive that human immunodeficiency virus type-1 infection might promote establishment of AIDS-related cancers. Tat, the viral trans-activator, can be endocytosed by uninfected cells and has been shown to inhibit p53 functions, providing a candidate mechanism through which the human immunodeficiency virus type-1 might contribute to malignant transformation. Because Tat has been shown to interact with histone acetyltransferase domains of p300/cAMP-responsive element-binding protein (CREB)-binding protein and p300/CREB-binding protein-associated factor, we have investigated whether Tat might alter p53 acetylation and tumor suppressor-responsive transcription. Here, we demonstrate that both Tat and p53 co-localize with p300/CREB-binding protein-associated factor and p300 in nuclei of IMR-32 human neuroblastoma cells and in PC-12 pheochromocytoma cells. Further, p53 trans-activation of the 14-3-3varsigma promoter was markedly repressed by Tat-histone acetyltransferase interactions, and p53 acetylation by p300/CREB-binding protein-associated factor on residue Lys(320) was diminished as a result of Tat-histone acetyltransferase binding in vivo and in vitro. Tat also inhibited p53 acetylation by p300 in a dosage-dependent manner in vitro. Finally, HIV-1-infected Molt-4 cells displayed reduced p53 acetylation on lysines 320 and 373 in response to UV irradiation. Our results allude to a mechanism whereby the human immunodeficiency virus type-1 trans-activator might impair tumor suppressor functions in immune/neuronal-derived cells, thus favoring the establishment of neoplasia during AIDS.

The acetylase activity of p300 is dispensable for MDM2 stabilization

It has been shown that p300 binds to MDM2 and leads to down-regulation of the p53 function. However, it remains unclear whether the acetylase activity of p300 is necessary for regulating MDM2 stability. In this study, we address this issue. First, p300 did not acetylate MDM2 in solution and in cells. Second, overexpression of p300 in cells increased the level of the MDM2 protein but not its mRNA. Similarly, the acetylase-defective p300 AT2 mutant stabilized the MDM2 protein as well. Consistently, the deacetylase inhibitor, trichostatin A, did not significantly affect the half-life of the endogenous MDM2 protein, whereas p300 enhanced the half-life of MDM2. Finally, both wild type and acetylase-defective mutant p300 proteins associated with MDM2 in nuclear body-like structures where MDM2 might be protected from proteasomal degradation. Thus, these results suggest that p300 appears to stabilize MDM2 by retaining this protein in a specific nuclear structure rather than by acetylating it.

Tumor p53 status and response to topoisomerase II inhibitors

It is thought that when tumor cells are treated with anticancer drugs, they die through the apoptotic pathway and that cell resistance to cancer chemotherapy is mainly a resistance to apoptosis commitment. p53 is not functional in nearly half of the tumors examined and because of its involvement (directly or through its target genes) in the apoptotic pathway, drug resistance to chemotherapy has been largely attributed to the status of this "tumor suppressor protein". Topoisomerase II (topo II) inhibitors are widely used not only as single agents, but also in the majority of combination treatment protocols for hematologic malignancies and solid tumors. The relationship between p53 and topo II raises many questions about basic regulatory, biochemical, structural and functional characteristics that could be different in cells in different tissues, and most importantly, between different tumor cell types and their normal tissue counterpart. Understanding these relationships may lead to strategies for chemotherapy optimization and further precision targeting of tumor cells in order to avoid drug resistance and thereby chemotherapy failure.