Biochemical characterization of different conformational states of the Sf9 cell-purified p53His175 mutant protein

Pharmacological reactivation of p53 as a strategy to treat cancer

It has been confirmed through studies using the technique of unbiased sequencing that the TP53 tumour suppressor is the most frequently inactivated gene in cancer. This finding, together with results from earlier studies, provides compelling evidence for the idea that p53 ablation is required for the development and maintenance of tumours. Genetic reconstitution of the function of p53 leads to the suppression of established tumours as shown in mouse models. This strongly supports the notion that p53 reactivation by small molecules could provide an efficient strategy to treat cancer. In this review, we summarize recent advances in the development of small molecules that restore the function of mutant p53 by different mechanisms, including stabilization of its folding by Apr-246, which is currently being tested in a Phase II clinical trial. We discuss several classes of compounds that reactivate wild-type p53, such as Mdm2 inhibitors, which are currently undergoing clinical testing, MdmX inhibitors and molecules targeting factors upstream of Mdm2/X or p53 itself. Finally, we consider the clinical applications of compounds targeting p53 and the p53 pathway.

[Recent advances in mutant p53 and novel personalized strategies for cancer therapy]

Protein p53 is the most intensively studied tumor suppressor protein. Recent studies keep revealing its new function in metabolism and reproduction. At the same time, it is also found that varieties of p53 mutant gained new function in promoting tumorigenesis. These studies provide the basis for understanding the personalized gain of function of p53 mu-tants and help us searching for the new strategies for reactivation of wild-type p53 and correction of the function of p53 mutants. The personalized treatment targeting different p53 mutants will be the focus for cancer treatment. Here, we re-viewed the discovered gain of function of some p53 mutants and the molecular strategies for reactivating wild type p53 function: by use of small molecules or polypeptides to reactivate the wild type function of p53 mutants in tumor cells; by exogenous expression of wild type p53 carried by recombinant adenovirus in tumor cells; and by inhibition the interaction between p53 and mdm2 to stabilize wild type p53 proteins. Further study of variety of p53 point mutations farcilitates de-signing more effectively personalized strategies in the cancer therapy.

Reactivation of mutant p53: molecular mechanisms and therapeutic potential

The p53 tumor suppressor gene is the most frequently mutated gene in cancer. Most p53 mutations are missense point mutations that cluster in the DNA-binding core domain. This results in distortion of core domain folding and disruption of DNA binding and transcriptional transactivation of p53 target genes. Structural studies have demonstrated that mutant p53 core domain unfolding is not irreversible. Mutant p53 is expressed at high levels in many tumors. Therefore, mutant p53 is a promising target for novel cancer therapy. Mutant p53 reactivation will restore p53-dependent apoptosis, resulting in efficient removal of tumor cells. A number of strategies for targeting mutant p53 have been designed, including peptides and small molecules that restore the active conformation and DNA binding to mutant p53 and induce p53-dependent suppression of tumor cell growth in vitro and in vivo. This opens possibilities for the clinical application of mutant p53 reactivation in the treatment of cancer.

On the mechanism of sequence-specific DNA-dependent acetylation of p53: the acetylation motif is exposed upon DNA binding

P53 acetylation requires p300-docking to two contiguous sites in the activation domain that in turn mediates DNA-dependent acetylation of the tetramer. In an attempt to further define the mechanism of DNA-dependent acetylation of p53, an in vitro system has been reconstituted with distinct p53 isoforms and has been used to reveal conformational constraints on p53 acetylation. Two native p53 tetrameric isoforms purified from Sf9 cells differing by the extent of phosphorylation within the C-terminal acetylation site are both acetylated in a sequence-specific DNA-dependent manner. By contrast, p53 purified from an Escherichia coli expression system is in a largely denatured conformation and its acetylation is DNA-independent. Heating native p53 to destroy the folded structure restores DNA-independent acetylation similar to that seen with bacterially expressed p53. There are at least two sites of conformational flexibility in the p53 tetramer: the first in the flexible S10 beta-sheet within the MDM2 ubiquitination sequence and the second in the C-terminal regulatory domain. We analysed therefore whether DNA-dependent acetylation correlated with conformational changes in either of these two regions. DNA-dependent acetylation of p53 is maintained in a dose-dependent manner by low concentrations of consensus site DNA under conditions where flexibility in the S10 beta-sheet region is maintained. Oligonucleotide DNAs that promote acetylation stimulate the binding of monoclonal antibodies PAb421 and ICA-9; two antibodies whose contiguous epitopes overlap the C-terminal acetylation motif. By contrast, bent oligonucleotide DNAs that conceal both the S10 beta-sheet from binding of the monoclonal antibody DO-12 and attenuate binding of the monoclonal antibody PAb421 can preclude acetylation. These data suggest that, in the absence of DNA, the acetylation motif of p53 is in a cryptic state, but after DNA binding, allosteric effects mediate an exposure of the acetylation motif to allow DNA-dependent acetylation of the tetramer.

The proline repeat domain of p53 binds directly to the transcriptional coactivator p300 and allosterically controls DNA-dependent acetylation of p53

The transcription coactivator p300 cannot acetylate native p53 tetramers, thus revealing intrinsic conformational constraints on p300-catalyzed acetylation. Consensus site DNA is an allosteric effector that promotes acetylation of p53, suggesting that p300 has an undefined conformationally flexible interface within the p53 tetramer. To identify such conformationally responsive p300-binding sites, p300 was subjected to peptide selection from a phage-peptide display library, a technique that can define novel protein-protein interfaces. The enriched p300-binding peptides contained a proline repeat (PXXP/PXPXP) motif, and five proline repeat motifs actually reside within the p53 transactivation domain, suggesting that this region of p53 may harbor the second p300 contact site. p300 binds in vitro to PXXP-containing peptides derived from the proline repeat domain, and PXXP-containing peptides inhibit sequence-specific DNA-dependent acetylation of p53, indicating that p300 docking to both the LXXLL and contiguous PXXP motif in p53 is required for p53 acetylation. Deletion of the proline repeat motif of p53 prevents DNA-dependent acetylation of p53 by occluding p300 from the p53-DNA complex. Sequence-specific DNA places an absolute requirement for the proline repeat domain to drive p53 acetylation in vivo. Chromatin immunoprecipitation was used to show that the proline repeat deletion mutant p53 is bound to the p21 promoter in vivo, but it is not acetylated, indicating that proline-directed acetylation of p53 is a post-DNA binding event. The PXXP repeat expands the basic interface of a p300-targeted transactivation domain, and proline-directed acetylation of p53 at promoters indicates that p300-mediated acetylation can be highly constrained by substrate conformation in vivo.

Restoration of the tumor suppressor function to mutant p53 by a low-molecular-weight compound

The tumor suppressor p53 inhibits tumor growth primarily through its ability to induce apoptosis. Mutations in p53 occur in at least 50% of human tumors. We hypothesized that reactivation of mutant p53 in such tumors should trigger massive apoptosis and eliminate the tumor cells. To test this, we screened a library of low-molecular-weight compounds in order to identify compounds that can restore wild-type function to mutant p53. We found one compound capable of inducing apoptosis in human tumor cells through restoration of the transcriptional transactivation function to mutant p53. This molecule, named PRIMA-1, restored sequence-specific DNA binding and the active conformation to mutant p53 proteins in vitro and in living cells. PRIMA-1 rescued both DNA contact and structural p53 mutants. In vivo studies in mice revealed an antitumor effect with no apparent toxicity. This molecule may serve as a lead compound for the development of anticancer drugs targeting mutant p53.

Rescuing the function of mutant p53

One protein--p53--plays nemesis to most cancers by condemning damaged cells to death or quarantining them for repair. But the activity of p53 relies on its intact native conformation, which can be lost following mutation of a single nucleotide. With thousands of such mutations identified in patients, how can a future cancer drug buttress this fragile protein structure and restore the cell's natural defence?

The 'wildtype' conformation of p53: epitope mapping using hybrid proteins

The function of p53 correlates with its 'wildtype' conformation, specifically recognized by antibodies PAb1620 and PAb246, and many cancer-associated mutations cause loss of this conformation. The epitopes of these antibodies were identified using hybrid p53 proteins created by a new method. Plasmids carrying homologous genes cut at appropriate sites recombined efficiently when transformed into RecE(+) E. coli. PAb1620 and PAb246 recognize mouse but not chicken p53; we created mouse-chicken hybrids of the p53 core domain and tested antibody reactivity. PAb246 binding mapped to residues 201-212, while PAb1620 required both residues 145-157 and 201-212 (human p53 numbering used throughout). An alanine-scan showed that the key residues for PAb246 and PAb1620 are completely distinct: PAb246 recognizes residues 202-204 (Tyr-Pro-Glu) while PAb1620 recognizes residues Arg156, Leu206, Arg209, and Gln/Asn210, the last two residues being essential. Both antibody epitopes are far from the p53 interface with DNA, but near the epitope of the 'mutant' conformation antibody PAb240. These epitope locations may help in dissecting the interactions of p53, including those with E6/E6-AP and in its DNA-bound state.

Strategies for manipulating the p53 pathway in the treatment of human cancer

Human cancer progression is driven in part by the mutation of oncogenes and tumour-suppressor genes which, under selective environmental pressures, give rise to evolving populations of biochemically altered cells with enhanced tumorigenic and metastatic potential. Given that human cancers are biologically and pathologically quite distinct, it has been quite surprising that a common event, perturbation of the p53 pathway, occurs in most if not all types of human cancers. The central role of p53 as a tumour-suppressor protein has fuelled interest in defining its mechanism of function and regulation, determining how its inactivation facilitates cancer progression, and exploring the possibility of restoring p53 function for therapeutic benefit. This review will highlight the key biochemical properties of p53 protein that affect its tumour-suppressor function and the experimental strategies that have been developed for the re-activation of the p53 pathway in cancers.