Thermodynamic stability of wild-type and mutant p53 core domain

Transcriptional regulation by p53: one protein, many possibilities

The p53 tumor suppressor protein is a DNA sequence-specific transcriptional regulator that, in response to various forms of cellular stress, controls the expression of numerous genes involved in cellular outcomes including among others, cell cycle arrest and cell death. Two key features of the p53 protein are required for its transcriptional activities: its ability to recognize and bind specific DNA sequences and to recruit both general and specialized transcriptional co-regulators. In fact, multiple interactions with co-activators and co-repressors as well as with the components of the general transcriptional machinery allow p53 to either promote or inhibit transcription of different target genes. This review focuses on some of the salient features of the interactions of p53 with DNA and with factors that regulate transcription. We discuss as well the complexities of the functional domains of p53 with respect to these interactions.

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.

Zn2+-Dependent Misfolding of the p53 DNA Binding Domain

The DNA binding domain (DBD) of p53 folds by a complex mechanism that involves parallel pathways and multiple intermediates, both on- and off-pathway. This heterogeneity renders DBD particularly susceptible to misfolding and aggregation. The origins of parallel folding mechanisms are not well understood. DBD folding heterogeneity may be caused by the presence of the single bound Zn2+. To test that hypothesis, we carried out kinetic folding studies of DBD in its Zn2+-free form (apoDBD) and in the presence of various concentrations of free Zn2+ and the Zn2+-nitrilotriacetate (NTA) complex. Folding kinetics of apoDBD and DBD are similar, although apoDBD folds faster than DBD at some urea concentrations. The principle consequence of Zn2+ removal is to accelerate unfolding and simplify it from two exponential phases to one. Metal binding interactions are therefore not responsible for the observed complexity of the folding reaction. A slight stoichiometric excess of free Zn2+ arrests folding and traps the protein in a misfolded state in which Zn2+ is bound to nonphysiological ligands. Folding can be rescued by providing metal ions in the form of the NTA-Zn2+ complex, which simultaneously protects against misligation and provides a source of Zn2+ for regenerating the functional protein. This chemical metallochaperone strategy may be an effective means for improving folding efficiency of other metal binding proteins. The findings suggest that, in vivo, DBD must fold in an environment where free Zn2+ concentration is low and its bioavailability is carefully regulated by cellular metallochaperones.

p53 gene in treatment of hepatic carcinoma: status quo

Hepatocellular carcinoma (HCC) is one of the 10 most common cancers worldwide. There is no ideal treatment for HCC yet and many researchers are trying to improve the effects of treatment by changing therapeutic strategies. As the majority of human cancers seem to exhibit either abnormal p53 gene or disrupted p53 gene activation pathways, intervention to restore wild-type p53 (wt-p53) activities is an attractive anti-cancer therapy including HCC. Abnormalities of p53 are also considered a predisposition factor for hepatocarcinogenesis. p53 is frequently mutated in HCC. Most HCCs have defects in the p53-mediated apoptotic pathway although they carry wt-p53. High expression of p53 in vivo may exert therapeutic effects on HCC in two aspects: (1) High expression of exogenous p53 protein induces apoptosis of tumor cells by inhibiting proliferation of cells through several biologic pathways and (2) Exogenous p53 renders HCC more sensitive to some chemotherapeutic agents. Several approaches have been designed for the treatment of HCC via the p53 pathway by restoring the tumor suppression function from inactivation, rescuing the mutated p53 gene from instability, or delivering therapeutic exogenous p53. Products with p53 status as the target have been studied extensively in vitro and in vivo. This review elaborates some therapeutic mechanisms and advances in using recombinant human adenovirus p53 and oncolytic virus products for the treatment of HCC.

Structural biology of the tumor suppressor p53 and cancer-associated mutants

The tumor suppressor protein p53 is a transcription factor that plays a key role in the prevention of cancer development. In response to oncogenic or other stresses, the p53 protein is activated and regulates the expression of a variety of target genes, resulting in cell cycle arrest, senescence, or apoptosis. Mutation of the p53 gene is the most common genetic alteration in human cancer, affecting more than 50% of human tumors. Most of these mutations inactivate the DNA-binding domain of the protein. In this chapter, we describe the structure of the wild-type p53 protein and present structural and functional data that provide the molecular basis for understanding the effects of common cancer mutations. Further, we assess novel therapeutic strategies that aim to rescue the function of p53 cancer mutants.

Restoring wild-type conformation and DNA-binding activity of mutant p53 is insufficient for restoration of transcriptional activity

Most human tumors contain inactivated p53 protein, either by mutations and/or functional deactivation. Restoration of wild-type p53 function could be one of the key tools in new anticancer therapy. Using an electromobility shift assay, we investigated the effect of temperature on DNA binding of wild-type and mutant p53 proteins. We showed that analysis of the DNA-binding capacity of mutant p53 proteins is complicated by the temperature at which the assay is performed. Furthermore, neither ability to bind to DNA nor conformational analysis accurately defines the transcriptional activity of human tumor-derived p53 mutant proteins. That some mutants can bind DNA and adopt a wild-type conformation in vitro, but are transcriptionally inactive in vivo, points to the involvement of cellular factors required for transactivation. Therefore, the common use of purified proteins and in vitro determinations of DNA binding and conformation are not the best indicators of the functional properties of mutant p53.

Structural assessment of single amino acid mutations: application to TP53 function

Single amino acid substitution is the type of protein alteration most related to human diseases. Current studies seek primarily to distinguish neutral mutations from harmful ones. Very few methods offer an explanation of the final prediction result in terms of the probable structural or functional effect on the protein. In this study, we describe the use of three novel parameters to identify experimentally-verified critical residues of the TP53 protein (p53). The first two parameters make use of a surface clustering method to calculate the protein surface area of highly conserved regions or regions with high nonlocal atomic interaction energy (ANOLEA) score. These parameters help identify important functional regions on the surface of a protein. The last parameter involves the use of a new method for pseudobinding free-energy estimation to specifically probe the importance of residue side-chains to the stability of protein fold. A decision tree was designed to optimally combine these three parameters. The result was compared to the functional data stored in the International Agency for Research on Cancer (IARC) TP53 mutation database. The final prediction achieved a prediction accuracy of 70% and a Matthews correlation coefficient of 0.45. It also showed a high specificity of 91.8%. Mutations in the 85 correctly identified important residues represented 81.7% of the total mutations recorded in the database. In addition, the method was able to correctly assign a probable functional or structural role to the residues. Such information could be critical for the interpretation and prediction of the effect of missense mutations, as it not only provided the fundamental explanation of the observed effect, but also helped design the most appropriate laboratory experiment to verify the prediction results.

p53—A Natural Cancer Killer: Structural Insights and Therapeutic Concepts

Every single day, the DNA of each cell in the human body is mutated thousands of times, even in absence of oncogenes or extreme radiation. Many of these mutations could lead to cancer and, finally, death. To fight this, multicellular organisms have evolved an efficient control system with the tumor-suppressor protein p53 as the central element. An intact p53 network ensures that DNA damage is detected early on. The importance of p53 for preventing cancer is highlighted by the fact that p53 is inactivated in more than 50 % of all human tumors. Thus, for good reason, p53 is one of the most intensively studied proteins. Despite the great effort that has been made to characterize this protein, the complex function and the structural properties of p53 are still only partially known. This review highlights basic concepts and recent progress in understanding the structure and regulation of p53, focusing on emerging new mechanistic and therapeutic concepts.

Folding and misfolding mechanisms of the p53 DNA binding domain at physiological temperature

p53 modulates a large number of cellular response pathways and is critical for the prevention of cancer. Wild-type p53, as well as tumorigenic mutants, exhibits the singular property of spontaneously losing DNA binding activity at 37 degrees C. To understand the molecular basis for this effect, we examine the folding mechanism of the p53 DNA binding domain (DBD) at elevated temperatures. Folding kinetics do not change appreciably from 5 degrees C to 35 degrees C. DBD therefore folds by the same two-channel mechanism at physiological temperature as it does at 10 degrees C. Unfolding rates, however, accelerate by 10,000-fold. Elevated temperatures thus dramatically increase the frequency of cycling between folded and unfolded states. The results suggest that function is lost because a fraction of molecules become trapped in misfolded conformations with each folding-unfolding cycle. In addition, at 37 degrees C, the equilibrium stabilities of the off-pathway species are predicted to rival that of the native state, particularly in the case of destabilized mutants. We propose that it is the presence of these misfolded species, which can aggregate in vitro and may be degraded in the cell, that leads to p53 inactivation.

Effects of common cancer mutations on stability and DNA binding of full-length p53 compared with isolated core domains

Common cancer mutations of p53 tend either to lower the stability or distort the core domain of the protein or weaken its DNA binding affinity. We have previously analyzed in vitro the effects of mutations on the core domain of p53. Here, we extend those measurements to full-length p53, using either the wild-type protein or a biologically active superstable construct that is more amenable to accurate biophysical measurements to assess the possibilities of rescuing different types of mutations by anticancer drugs. The tetrameric full-length proteins had similar apparent melting temperatures to those of the individual domains, and the structural mutations lowered the melting temperature by similar amounts. The thermodynamic stability of tetrameric p53 is thus dictated by its core domain. We determined that the common contact mutation R273H weakened binding to the gadd45 recognition sequence by approximately 700-1000 times. Many mutants that have lowered melting temperatures should be good drug targets, although the common R273H mutant binds response elements too weakly for simple rescue.

Predicting the transactivation activity of p53 missense mutants using a four-body potential score derived from Delaunay tessellations

We describe a novel statistical scoring method based on a computational geometry approach to predict the functional impact (transactivation activity) of missense mutations in the DNA-binding domain (DBD) of the tumor suppressor TP53, which is the most frequently mutated gene in human cancer. Residual scores (RS) for each residue were calculated to reflect differences in the compositional preferences of four nearest-neighbor residues between mutant and wild-type proteins. The RS were then combined into a residual score profile (RSP) representing the RS values for all 194 residues in the DBD. Mutants were grouped into functional categories based on their transactivation activities experimentally measured in yeast functional assays using p53-response elements from eight different promoters. While these functional categories showed significant differences in average RS, the latter lacked resolution power to predict the transactivation activities of individual mutants. In contrast, using decision tree models, we found that the RSP predicted transactivation with an accuracy varying between 64.2% and 78.5% depending on the promoter. Lastly, we used the best model to predict the functional outcome of all missense mutants in the DBD of p53 and compared the predictions with their frequency of occurrence in human cancers. We found that mutants predicted as functional (F) accounted for approximately 14% of all missense mutants found in cancers, while mutants predicted as nonfunctional (NF) represented approximately 86% of the mutants. These results show that this computational approach provides a fast and reliable method for predicting the functional impact of p53 mutants associated with cancer.

Characterization of the native and fibrillar conformation of the human Nalpha-acetyltransferase ARD1

ARD1 (arrest-defective protein 1), together with NAT1 (N-acetyltransferase protein 1), is part of the major N(alpha)-acetyltransferase complex in eukaryotes responsible for alpha-acetylation of proteins and peptides. Protein acetylation has been implicated in gene expression regulation and protein-protein interaction. We characterized the native folded and misfolded conformation of hARD1. Structural characterization of native hARD1 using size exclusion chromatography, circular dichroism, and fluorescence spectroscopy shows the protein consists of a compact globular region comprising two thirds of the protein and a flexible unstructured C terminus. In addition, hARD1 forms protofilaments under physiological conditions of pH and temperature, as judged by electron microscopy and staining with the dyes Congo red and thioflavin T. The process is accelerated by thermal denaturation and high protein concentrations. Limited proteolysis of aggregated hARD1 revealed a resistant fragment whose sequence matched a region contained within the acetyl transferase domain.

Functional analysis and molecular modeling show a preserved wild-type activity of p53C238Y

In human tumors, p53 is often disabled by mutations in its DNA-binding domain and is thus inactive as a transcription factor. Alternatively, MDM2 gene amplification or up-regulation represents a mechanism of p53 wild-type inactivation, mainly reported in soft tissue sarcomas. In a previous TP53 analysis carried out on sporadic and NF1-related malignant peripheral nerve sheath tumors, in two cases, we observed the occurrence of C238Y missense mutation, leading to p53 stabilization unexpectedly coupled with immunophenotypic MDM2 overexpression. To investigate this TP53 missense mutation not yet functionally characterized in mammalian cell, we did MDM2 Southern blot and p53(C238Y)/MDM2 biochemical and functional analyses followed by molecular modeling. The results showed a lack of MDM2 gene amplification, evidence of p53-MDM2 protein complexes, and presence of a p53 that retains the ability to become phosphorylated on Ser15 and to induce the transcription of p21(waf1). Additional molecular modeling data highlighted the structural similarities between p53(C238Y) and wild-type p53, further supporting that the p53(C238Y) mutant still retains functional wild-type p53 properties.

Comparison of the human and worm p53 structures suggests a way for enhancing stability

Maintaining the native conformation is essential for the proper function of tumor suppressor protein p53. However, p53 is a low-stability protein that can easily lose its function upon structural perturbations such as those resulting from missense mutations, leading to the development of cancer. Therefore, it is important to develop strategies to design stable p53 which still maintains its normal function. Here, we compare the stabilities of the human and worm p53 core domains using molecular dynamics simulations. We find that the worm p53 is significantly more stable than the human form. Detailed analysis of the structural fluctuations shows that the stability difference lies in the peripheral structural motifs that contrast in their structural features and flexibility. The most dramatic difference in stability originates from loop L1, from the turn between helix H1 and beta-strand S5, and from the turn that connects beta-strands S7 and S8. Structural analysis shows significant differences for these motifs between the two proteins. Loop L1 lacks secondary structure, and the turns between helix H1 and strand S5 and between strands S7 and S8 are much longer in the human form p53. On the basis of these differences, we designed a mutant by shortening the turn between strands S7 and S8 to enhance the stability. Surprisingly, this mutant was very stable when probed by molecular dynamics simulations. In addition, the stabilization was not localized in the turn region. Loop L1 was also significantly stabilized. Our results show that stabilizing peripheral structural motifs can greatly enhance the stability of the p53 core domain and therefore is likely to be a viable alternative in the development of stable p53. In addition, loop- or turn-related mutants with different stabilities may also be used to probe the relationship between function, a particular structural motif, and its flexibility.

Functional census of mutation sequence spaces: the example of p53 cancer rescue mutants

Many biomedical problems relate to mutant functional properties across a sequence space of interest, e.g., flu, cancer, and HIV. Detailed knowledge of mutant properties and function improves medical treatment and prevention. A functional census of p53 cancer rescue mutants would aid the search for cancer treatments from p53 mutant rescue. We devised a general methodology for conducting a functional census of a mutation sequence space by choosing informative mutants early. The methodology was tested in a double-blind predictive test on the functional rescue property of 71 novel putative p53 cancer rescue mutants iteratively predicted in sets of three (24 iterations). The first double-blind 15-point moving accuracy was 47 percent and the last was 86 percent; r = 0.01 before an epiphanic 16th iteration and r = 0.92 afterward. Useful mutants were chosen early (overall r = 0.80). Code and data are freely available (http://www.igb.uci.edu/research/research.html, corresponding authors: R.H.L. for computation and R.K.B. for biology).

Solution structure of p53 core domain: structural basis for its instability

The 25-kDa core domain of the tumor suppressor p53 is inherently unstable and melts at just above body temperature, which makes it susceptible to oncogenic mutations that inactivate it by lowering its stability. We determined its structure in solution using state-of-the-art isotopic labeling techniques and NMR spectroscopy to complement its crystal structure. The structure was very similar to that in the crystal but far more mobile than expected. Importantly, we were able to analyze by NMR the structural environment of several buried polar groups, which indicated structural reasons for the instability. NMR spectroscopy, with its ability to detect protons, located buried hydroxyl and sulfhydryl groups that form suboptimal hydrogen-bond networks. We mutated one such buried pair, Tyr-236 and Thr-253 to Phe-236 and Ile-253 (as found in the paralogs p63 and p73), and stabilized p53 by 1.6 kcal/mol. We also detected differences in the conformation of a mobile loop that might reflect the existence of physiologically relevant alternative conformations. The effects of temperature on the dynamics of aromatic residues indicated that the protein also experiences several dynamic processes that might be related to the presence of alternative hydrogen-bond patterns in the protein interior. p53 appears to have evolved to be dynamic and unstable.

Change of the Protein p53 Electrochemical Signal According to its Structural Form – Quick and Sensitive Distinguishing of Native, Denatured, and Aggregated Form of the “Guardian of the Genome”

Presence of mutated and/or structurally modified (e.g., denatured, aggregated) protein p53 form is associated with several disorders such as Alzheimer's disease, Parkinson's disease, prion diseases, and many types of tumours. The aim of this work was to distinguish native, denatured and aggregated form of full-length p53 by flow injection analysis coupled with electrochemical detector (FIA-ED). Firstly FIA-ED method used for protein native form determination was optimized (detection limit 45.8 amol per 5 mul injection; 3 x S/N). In addition the technique was applied to identify p53 structural forms (denatured and aggregated). It was found out that denatured form provides about three times higher electrochemical response (protein structure unfolding, approach of more electroactive centers - aminoacid residues - towards electrode surface) in comparison with native form. On the other hand, aggregated form offers lower response (steric eclipse of electroactive protein parts) when compared with the signal of native form. The obtained data show that we are not only able to sensitively determine native, denatured, and aggregated structural forms of p53 protein but also to distinguish them.

Functional analysis of disease-causing mutations in human UDP-galactose 4-epimerase

UDP-galactose 4-epimerase (GALE, EC 5.1.3.2) catalyses the interconversion of UDP-glucose and UDP-galactose. Point mutations in this enzyme are associated with the genetic disease, type III galactosemia, which exists in two forms - a milder, or peripheral, form and a more severe, or generalized, form. Recombinant wild-type GALE, and nine disease-causing mutations, have all been expressed in, and purified from, Escherichia coli in soluble, active forms. Two of the mutations (N34S and G319E) display essentially wild-type kinetics. The remainder (G90E, V94M, D103G, L183P, K257R, L313M and R335H) are all impaired in turnover number (k cat) and specificity constant (k cat/Km), with G90E and V94M (which is associated with the generalized form of galactosemia) being the most affected. None of the mutations results in a greater than threefold change in the Michaelis constant (Km). Protein-protein crosslinking suggests that none of the mutants are impaired in homodimer formation. The L183P mutation suffers from severe proteolytic degradation during expression and purification. N34S, G90E and D103G all show increased susceptibility to digestion in limited proteolysis experiments. Therefore, it is suggested that reduced catalytic efficiency and increased proteolytic susceptibility of GALE are causative factors in type III galactosemia. Furthermore, there is an approximate correlation between the severity of these defects in the protein structure and function, and the symptoms observed in patients.

Kinetic partitioning during folding of the p53 DNA binding domain

The DNA-binding domain (DBD) of wild-type p53 loses DNA binding activity spontaneously at 37 degrees C in vitro, despite being thermodynamically stable at this temperature. We test the hypothesis that this property is due to kinetic misfolding of DBD. Interrupted folding experiments and chevron analysis show that native molecules are formed via four tracks (a-d) under strongly native conditions. Folding half-lives of tracks a-d are 7.8 seconds, 50 seconds, 5.3 minutes and more than five hours, respectively, in 0.3M urea (10 degrees C). Approximately equal fractions of molecules fold through each track in zero denaturant, but above 2.0M urea approximately 90% fold via track c. A kinetic mechanism consisting of two parallel folding channels (fast and slow) is proposed. Each channel populates an on-pathway intermediate that can misfold to form an aggregation-prone, dead-end species. Track a represents direct folding through the fast channel. Track b proceeds through the fast channel but via the off-pathway state. Track c corresponds to folding via the slow channel, primarily through the off-pathway state. Track d proceeds by way of an even slower, uncharacterized route. We postulate that activity loss is caused by partitioning to the slower tracks, and that structural unfolding limits this process. In support of this view, tumorigenic hot-spot mutants G245S, R249S and R282Q accelerate unfolding rates but have no affect on folding kinetics. We suggest that these and other destabilizing mutants facilitate loss of p53 function by causing DBD to cycle unusually rapidly between folded and unfolded states. A significant fraction of DBD molecules become effectively trapped in a non-functional state with each unfolding-folding cycle.

In the quest for stable rescuing mutants of p53: computational mutagenesis of flexible loop L1

p53 is a protein with marginal stability. Its transcriptional functions are often inactivated by single missense mutations, shown to be associated with half of all human cancers. Here, we aim to design stable functional p53 mutants. We target loop L1, one of the most mobile structural motifs in the p53 core domain (p53C). Specifically, we selected Ser116 in the middle of loop L1 and mutated it to 14 other amino acids. All resulting mutants were subjected to molecular dynamics simulations, revealing a wide spectrum of stabilities. Among these, mutant S116M displayed a remarkable stability, with a structural deviation comparable to that of the experimental quadruple mutant M133L/V203A/N239Y/N268D that is thermodynamically more stable than that of the wild type by 2.6 kcal/mol. Structural analysis showed that the high stability of the S116M mutant was indeed due to the preservation of the p53C loop L1 conformation and the reduction of mobility in that region. The differential stabilities conferred by the single mutations are rationalized based on the geometries and chemical properties of the side chains introduced into this site. Linearity (i.e., nonbranched), moderate size, and balanced hydrophobic and hydrophilic properties of the side chain are crucial to the stabilizing effect of the residue substitutions.

PDBSite: a database of the 3D structure of protein functional sites

The PDBSite database provides comprehensive structural and functional information on various protein sites (post-translational modification, catalytic active, organic and inorganic ligand binding, protein-protein, protein-DNA and protein-RNA interactions) in the Protein Data Bank (PDB). The PDBSite is available online at http://wwwmgs.bionet.nsc.ru/mgs/gnw/pdbsite/. It consists of functional sites extracted from PDB using the SITE records and of an additional set containing the protein interaction sites inferred from the contact residues in heterocomplexes. The PDBSite was set up by automated processing of the PDB. The PDBSite database can be queried through the functional description and the structural characteristics of the site and its environment. The PDBSite is integrated with the PDBSiteScan tool allowing structural comparisons of a protein against the functional sites. The PDBSite enables the recognition of functional sites in protein tertiary structures, providing annotation of function through structure. The PDBSite is updated after each new PDB release.

Structures of p53 cancer mutants and mechanism of rescue by second-site suppressor mutations

We have solved the crystal structures of three oncogenic mutants of the core domain of the human tumor suppressor p53. The mutations were introduced into a stabilized variant. The cancer hot spot mutation R273H simply removes an arginine involved in DNA binding without causing structural distortions in neighboring residues. In contrast, the "structural" oncogenic mutations H168R and R249S induce substantial structural perturbation around the mutation site in the L2 and L3 loops, respectively. H168R is a specific intragenic suppressor mutation for R249S. When both cancer mutations are combined in the same molecule, Arg(168) mimics the role of Arg(249) in wild type, and the wild type conformation is largely restored in both loops. Our structural and biophysical data provide compelling evidence for the mechanism of rescue of mutant p53 by intragenic suppressor mutations and reveal features by which proteins can adapt to deleterious mutations.

Binding of Rad51 and other peptide sequences to a promiscuous, highly electrostatic binding site in p53

Homologous recombination is repressed by the binding of p53 to Rad51. We identified by fluorescence and NMR spectroscopy that peptides corresponding to residues 179-190 of Rad51 bind to the core domain of p53 in a site that overlaps with its specific DNA binding site. The p53 site is quite promiscuous, since it also binds peptides derived from 53BP1, 53BP2, Hif-1alpha, and BCL-X(L) in overlapping regions. Binding is mediated mainly by a strong, nonspecific, electrostatic component and is fine tuned by specific interactions. Competition of the different proteins with each other and with specific DNA for a single site in p53 could be a factor in regulation of its activity.

Hsp90 Chaperones Wild-type p53 Tumor Suppressor Protein

Immortalized human fibroblasts were used to investigate the putative interactions of the Hsp90 molecular chaperone with the wild-type p53 tumor suppressor protein. We show that geldanamycin or radicicol, specific inhibitors of Hsp90, diminish specific wild-type p53 binding to the p21 promoter sequence. Consequently, these inhibitors decrease p21 mRNA levels, which lead to a reduction in cellular p21/Waf1 protein, known to induce cell cycle arrest. In control experiments, we show that neither geldanamycin nor radicicol affect p53 mRNA levels. A minor decrease in p53 protein level following the treatment of human fibroblasts with the inhibitors suggests the potential involvement of Hsp90 in the stabilization of wild-type p53. To support our in vivo findings, we used a reconstituted system with highly purified recombinant proteins to examine the effects of Hsp90 on wild-type p53 binding to the p21 promoter sequence. The human recombinant Hsp90 alpha-isoform as well as bovine brain Hsp90 were purified to homogeneity. Both of these molecular chaperones displayed ATPase activity and the ability to refold heat-inactivated luciferase in a geldanamycin- and radicicol-sensitive manner, suggesting that post-translational modifications are not involved in the modulation of Hsp90alpha activity. We show that the incubation of recombinant p53 at 37 degrees C decreases the level of its wild-type conformation and strongly inhibits the in vitro binding of p53 to the p21 promoter sequence. Interestingly, Hsp90 in an ATP-dependent manner can positively modulate p53 DNA binding after incubation at physiological temperature of 37 degrees C. Other recombinant human chaperones from Hsp70 and Hsp40 families were not able to efficiently substitute Hsp90 in this reaction. Consistent with our in vivo results, geldanamycin can suppress Hsp90 ability to regulate in vitro p53 DNA binding to the promoter sequence. In summary, the results presented in this article state that chaperone activity of Hsp90 is important for the transcriptional activity of genotypically wild-type p53.

Comparison of BRCT domains of BRCA1 and 53BP1: a biophysical analysis

53BP1 interacts with the DNA-binding core domain of the tumor suppressor p53 and enhances p53-mediated transcriptional activation. The p53-binding region of 53BP1 maps to the C-terminal BRCT domains, which are homologous to those found in the breast cancer protein BRCA1 and in other proteins involved in DNA repair. Here we compare the thermodynamic behavior of the BRCT domains of 53BP1 and BRCA1 and examine their ability to interact with the p53 core domain. The free energies of unfolding are of similar magnitude, although slightly higher for 53BP1-BRCT, and both populate an aggregation-prone partly folded intermediate. Interaction studies performed in vitro by analytical size-exclusion chromatography, analytical ultracentrifugation, and isothermal titration calorimetry reveal that 53BP1-BRCT interacts with p53 with a K(d) in the low micromolar range. Despite their homology with 53BP1-BRCT domains, the BRCT domains of BRCA1 did not bind p53 with any detectable affinity. In summary, although other studies have indicated that the BRCT domains of both BRCA1 and 53BP1 interact with p53 core domain, the quantitative biophysical measurements performed here indicate that only 53BP1 can bind. Although both proteins may be involved in the same DNA repair pathways, our study indicates that a direct role in p53 function is unique to 53BP1.

CP-31398 Restores DNA-binding Activity to Mutant p53 in Vitro but Does Not Affect p53 Homologs p63 and p73

The p53 protein plays a major role in the maintenance of genome stability in mammalian cells. Mutations of p53 occur in over 50% of all cancers and are indicative of highly aggressive cancers that are hard to treat. Recently, there has been a high degree of interest in therapeutic approaches to restore growth suppression functions to mutant p53. Several compounds have been reported to restore wild type function to mutant p53. One such compound, CP-31398, has been shown effective in vivo, but questions have arisen to whether it actually affects p53. Here we show that mutant p53, isolated from cells treated with CP-31398, is capable of binding to p53 response elements in vitro. We also show the compound restores DNA-binding activity to mutant p53 in cells as determined by a chromatin immunoprecipitation assay. In addition, using purified p53 core domain from two different hotspot mutants (R273H and R249S), we show that CP-31398 can restore DNA-binding activity in a dose-dependent manner. Using a quantitative DNA binding assay, we also show that CP-31398 increases significantly the amount of mutant p53 that binds to cognate DNA (B(max)) and its affinity (K(d)) for DNA. The compound, however, does not affect the affinity (K(d) value) of wild type p53 for DNA and only increases B(max) slightly. In a similar assay PRIMA1 does not have any effect on p53 core DNA-binding activity. We also show that CP-31398 had no effect on the DNA-binding activity of p53 homologs p63 and p73.

Reversible aggregation plays a crucial role on the folding landscape of p53 core domain

The role of tumor suppressor protein p53 in cell cycle control depends on its flexible and partially unstructured conformation, which makes it crucial to understand its folding landscape. Here we report an intermediate structure of the core domain of the tumor suppressor protein p53 (p53C) during equilibrium and kinetic folding/unfolding transitions induced by guanidinium chloride. This partially folded structure was undetectable when investigated by intrinsic fluorescence. Indeed, the fluorescence data showed a simple two-state transition. On the other hand, analysis of far ultraviolet circular dichroism in 1.0 M guanidinium chloride demonstrated a high content of secondary structure, and the use of an extrinsic fluorescent probe, 4,4'-dianilino-1,1' binaphthyl-5,5'-disulfonic acid, indicated an increase in exposure of the hydrophobic core at 1 M guanidinium chloride. This partially folded conformation of p53C was plagued by aggregation, as suggested by one-dimensional NMR and demonstrated by light-scattering and gel-filtration chromatography. Dissociation by high pressure of these aggregates reveals the reversibility of the process and that the aggregates have water-excluded cavities. Kinetic measurements show that the intermediate formed in a parallel reaction between unfolded and folded structures and that it is under fine energetic control. They are not only crucial to the folding pathway of p53C but may explain as well the vulnerability of p53C to undergo departure of the native to an inactive state, which makes the cell susceptible to malignant transformation.

Structural distortion of p53 by the mutation R249S and its rescue by a designed peptide: implications for "mutant conformation"

Missense mutations in the DNA-binding core domain of the tumour suppressor protein p53 are frequent in cancer. Many of them result in loss of native structure. The mutation R249S is one of the six most common cancer-associated p53 mutations ("hot-spots"). As it is highly frequent in hepatocellular carcinoma, its rescue is an important therapeutic target. We have used NMR techniques to study the structural effects of the R249S mutation. The overall fold of the core domain is retained in R249S, and it does not take up a denatured "mutant conformation". However, the beta-sandwich had increased flexibility and, according to changes in chemical shift, there was local distortion throughout the DNA-binding interface. It is likely that the R249S mutation resulted in an ensemble of native and native-like conformations in a dynamic equilibrium. The peptide FL-CDB3 that was designed to rescue mutants of p53 by binding specifically to its native structure was found to revert the chemical shifts of R249S back towards the wild-type values and so reverse the structural effects of mutation. We discuss the implications for a rescue strategy and also for the analysis of antibody-binding data.

Ras Protects Rb Family Null Fibroblasts from Cell Death

The retinoblastoma protein (Rb) controls cell proliferation, differentiation, and senescence and provides an essential tumor suppressive function that cells must eliminate to attain unlimited proliferative potential. Elimination of the Rb pathway also results in apoptosis, however, thereby providing an efficient surveillance mechanism to sense the loss of Rb. To become tumorigenic cells must thus overcome not only Rb function but also the apoptotic response caused by the loss of Rb function. We show that oncogenic Ras (RasV12) potently blocks cell death in Rb family member knockout mouse embryo fibroblasts (TKO cells). Activation of phosphatidylinositol 3-kinase and Raf by oncogenic Ras mediated this protection, implying that multiple Ras effector pathways are required, in concert, for this pro-survival signal. Although activation of Raf by selective Ras mutants protected TKO cells from cell death, pharmacologic inhibition of MEK had little effect on RasV12 protection, suggesting that a Raf-dependent, MEK-independent pathway was important for this effect. We show that this Raf-dependent protection occurred through activation of c-Jun and thus AP-1 activation. These observations could account for the dependence of Ras transformation on c-Jun activity and for the roles of AP-1 in oncogenesis. Our results support the concept of two oncogenic events cooperating to achieve a balance between immortalization and survival.

The von Hippel-Lindau tumor suppressor protein is a molten globule under native conditions: implications for its physiological activities

The von-Hippel Lindau tumor suppressor protein (pVHL) is conserved throughout evolution, as its homologues are found in organisms ranging from mammals to the Drosophila melanogaster and Anopheles gambiae insects and the Caenorhabditis elegans nematode. Although the physiological role of pVHL is not fully understood, it has been shown to interact with a large number of unrelated proteins and was suggested to play a role in protein degradation as an E3 ubiquitin ligase component in the ubiquitin pathway. To gain insight into the molecular basis of pVHL activity, we analyzed its folding and stability in solution under physiologically relevant conditions. Dynamic light-scattering and gel filtration chromatography of the purified pVHL clearly indicated that the Stokes radius of the protein is larger than what would be expected from its crystal structure. However, under these conditions, the protein shows a clear secondary structure as determined by far-UV circular dichroism. Yet, the near-UV CD experiments show an absence of a tertiary structure. Upon the addition of urea, even at very low concentrations, the protein unfolds in a non-reversible manner, leading to the formation of amorphous aggregates. Furthermore, a large increase in fluorescence (>50-fold) is observed upon the addition of pVHL into a solution containing 8-anilino-1-naphthalene sulfonic acid. We therefore conclude that, under native conditions, the non-bound pVHL has a molten globule configuration with marginal stability. Although molten globular structures can be induced in many proteins under extreme conditions, this is one of the few reported cases of such a structure under the physiological conditions of pH, ionic strength, and temperature. The significance of the pVHL structural properties is being discussed in the context of its physiological activities.

In vitro folding and characterization of the p53 DNA binding domain

The transcription factor p53 acts as major tumor suppressor and is inactivated by mutation in more than 50% of all human tumors. We have established an efficient procedure for the in vitro folding and purification of the p53 DNA binding domain (p53DBD) using a modified factorial matrix approach that supplies large amounts of homogeneous (isotope-labeled) p53DBD for application in biochemical, crystallographic and NMR spectroscopic studies. We further show with biophysical methods that in vitro folded p53DBD is fully functional and that its conformation is identical to that obtained from the soluble fraction.

Tumorigenic mutations in VHL disrupt folding in vivo by interfering with chaperonin binding

The eukaryotic chaperonin TRiC/CCT mediates folding of an essential subset of newly synthesized proteins, including the tumor suppressor VHL. Here we show that chaperonin binding is specified by two short hydrophobic beta strands in VHL that, upon folding, become buried within the native structure. These TRiC binding determinants are disrupted by tumor-causing point mutations that interfere with chaperonin association and lead to misfolding. Strikingly, while unable to fold correctly in vivo, some of these VHL mutants can reach the native state when refolded in a chaperonin-independent manner. The specificity of TRiC/CCT for extended hydrophobic beta strands may help explain its role in folding aggregation-prone polypeptides. Our findings reveal a class of disease-causing mutations that inactivate protein function by disrupting chaperone-mediated folding in vivo.

Conversion of wild-type p53 core domain into a conformation that mimics a hot-spot mutant

The wild-type p53 protein can be driven into a conformation corresponding to that adopted by structural mutant forms by heterodimerization with a mutant subunit. To seek partially folded states of the wild-type p53 core domain (p53C) we used high hydrostatic pressure (HP) and subzero temperatures. Aggregation of the protein was observed in parallel with its pressure denaturation at 25 and 37 degrees C. However, when HP experiments were performed at 4 degrees C, the extent of denaturation and aggregation was significantly less pronounced. On the other hand, subzero temperatures under pressure led to cold denaturation and yielded a non-aggregated, alternative conformation of p53C. Nuclear magnetic resonance (1H15N-NMR) data showed that the alternative p53C conformation resembled that of the hot-spot oncogenic mutant R248Q. This alternative state was as susceptible to denaturation and aggregation as the mutant R248Q when subjected to HP at 25 degrees C. Together these data demonstrate that wild-type p53C adopts an alternative conformation with a mutant-like stability, consistent with the dominant-negative effect caused by many mutants. This alternative conformation is likely related to inactive forms that appear in vivo, usually driven by interaction with mutant proteins. Therefore, it can be a valuable target in the search for ways to interfere with protein misfolding and hence to prevent tumor development.

Rescue of mutants of the tumor suppressor p53 in cancer cells by a designed peptide

We designed a series of nine-residue peptides that bound to a defined site on the tumor suppressor p53 and stabilized it against denaturation. To test whether the peptides could act as chaperones and rescue the tumor-suppressing function of oncogenic mutants of p53 in living cells, we treated human tumor cells with the fluorescein-labeled peptide Fl-CDB3 (fluorescent derivative of CDB3). Before treatment, the mutant p53 in the cell was predominantly denatured. Fl-CDB3 was taken up into the cytoplasm and nucleus and induced a substantial up-regulation of wild-type p53 protein and representative mutants. The mutants, His-273 and His-175 p53, adopted the active conformation, with a dramatic decrease in the fraction of denatured protein. In all cases, there was p53-dependent induction of expression of the p53 target genes mdm2, gadd45, and p21, accompanied by p53-dependent partial restoration of apoptosis. Fl-CDB3 sensitized cancer cells that carried wild-type p53 to p53-dependent gamma-radiation-induced apoptosis. Although Fl-CDB3 did not elicit a full biological response, it did bind to and rescue p53 in cells and so can serve as a lead for the development of novel drugs for anticancer therapy.

Crystal structure of a superstable mutant of human p53 core domain. Insights into the mechanism of rescuing oncogenic mutations

Most of the cancer-associated mutations in the tumor suppressor p53 map to its DNA-binding core domain. Many of them inactivate p53 by decreasing its thermodynamic stability. We have previously designed the superstable quadruple mutant M133L/V203A/N239Y/N268D containing the second-site suppressor mutations N239Y and N268D, which specifically restore activity and stability in several oncogenic mutants. Here we present the x-ray structure of this quadruple mutant at 1.9 A resolution, which was solved in a new crystal form in the absence of DNA. This structure reveals that the four point mutations cause only small local structural changes, whereas the overall structure of the central beta-sandwich and the DNA-binding surface is conserved. The suppressor mutation N268D results in an altered hydrogen bond pattern connecting strands S1 and S10, thus bridging the two sheets of the beta-sandwich scaffold in an energetically more favorable way. The second suppressor mutation N239Y, which is located in close proximity to the DNA-binding surface in loop L3, seems to reduce the plasticity of the structure in large parts of loop L3 as indicated by decreased crystallographic temperature factors. The same is observed for residues in the vicinity of the N268D substitution. This increase in rigidity provides the structural basis for the increase in thermostability and an understanding how N268D and N239Y rescue some of the common cancer mutants.

Mutagenesis of the Runt domain defines two energetic hot spots for heterodimerization with the core binding factor beta subunit

Core-binding factors (CBFs) are a small family of heterodimeric transcription factors that play critical roles in several developmental pathways and in human disease. Mutations in CBF genes are found in leukemias, bone disorders, and gastric cancers. CBFs consist of a DNA-binding CBF alpha subunit (Runx1, Runx2, or Runx3) and a non-DNA-binding CBF beta subunit. CBF alpha binds DNA in a sequence-specific manner, whereas CBF beta enhances DNA binding by CBF alpha. Both DNA binding and heterodimerization with CBF beta are mediated by a single domain in the CBF alpha subunits known as the "Runt domain." We analyzed the energetic contribution of amino acids in the Runx1 Runt domain to heterodimerization with CBF beta. We identified two energetic "hot spots" that were also found in a similar analysis of CBF beta (Tang, Y.-Y., Shi, J., Zhang, L., Davis, A., Bravo, J., Warren, A. J., Speck, N. A., and Bushweller, J. H. (2000) J. Biol. Chem. 275, 39579-39588). The importance of the hot spot residues for Runx1 function was demonstrated in in vivo transient transfection assays. These data refine the structural analyses and further our understanding of the Runx1-CBF beta interface.

Energetic contribution of residues in the Runx1 Runt domain to DNA binding

Core-binding factors (CBFs) are a small family of heterodimeric transcription factors that play critical roles in hematopoiesis and in the development of bone, stomach epithelium, and proprioceptive neurons. Mutations in CBF genes are found in leukemias, bone disorders, and gastric cancer. CBFs consist of a DNA-binding CBF alpha subunit and a non-DNA-binding CBF beta subunit. DNA binding and heterodimerization with CBF beta are mediated by the Runt domain in CBF alpha. Here we report an alanine-scanning mutagenesis study of the Runt domain that targeted amino acids identified by structural studies to reside at the DNA or CBF beta interface, as well as amino acids mutated in human disease. We determined the energy contributed by each of the DNA-contacting residues in the Runt domain to DNA binding both in the absence and presence of CBF beta. We propose mechanisms by which mutations in the Runt domain found in hematopoietic and bone disorders affect its affinity for DNA.

Fibrillar aggregates of the tumor suppressor p53 core domain

Alzheimer's disease, Parkinson's disease, cystic fibrosis, prion diseases, and many types of cancer are considered to be protein conformation diseases. Most of them are also known as amyloidogenic diseases due to the occurrence of pathological accumulation of insoluble aggregates with fibrillar conformation. Some neuroblastomas, carcinomas, and myelomas show an abnormal accumulation of the wild-type tumor suppressor protein p53 either in the cytoplasm or in the nucleus of the cell. Here we show that the wild-type p53 core domain (p53C) can form fibrillar aggregates after mild perturbation. Gentle denaturation of p53C by pressure induces fibrillar aggregates, as shown by electron and atomic force microscopies, by binding of thioflavin T, and by circular dichroism. On the other hand, heat denaturation produced granular-shaped aggregates. Annular aggregates similar to those found in the early aggregation stages of alpha-synuclein and amyloid-beta were also observed by atomic force microscopy immediately after pressure treatment. Annular and fibrillar aggregates of p53C were toxic to cells, as shown by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] reduction assay. Interestingly, the hot-spot mutant R248Q underwent similar aggregation behavior when perturbed by pressure or high temperature. Fibrillar aggregates of p53C contribute to the loss of function of p53 and seed the accumulation of conformationally altered protein in some cancerous cells.

Kinetic instability of p53 core domain mutants: implications for rescue by small molecules

Oncogenic mutations in the tumor suppressor protein p53 are found mainly in its DNA-binding core domain. Many of these mutants are thermodynamically unstable at body temperature. Here we show that these mutants also denature within minutes at 37 degrees C. The half-life (t(1/2)) of the unfolding of wild-type p53 core domain was 9 min. Hot spot mutants denatured more rapidly with increasing thermodynamic instability. The highly destabilized mutant I195T had a t(1/2) of less than 1 min. The wild-type p53-(94-360) construct, containing the core and tetramerization domains, was more stable, with t(1/2) = 37 min at 37 degrees C, similar to full-length p53. After unfolding, the denatured proteins aggregated, the rate increasing with higher concentrations of protein. A derivative of the p53-stabilizing peptide CDB3 significantly slowed down the unfolding rate of the p53 core domain. Drugs such as CDB3, which rescue the conformation of unstable mutants of p53, have to act during or immediately after biosynthesis. They should maintain the mutant protein in a folded conformation and prevent its aggregation, allowing it enough time to reach the nucleus and bind its sequence-specific target DNA or the p53 binding proteins that will stabilize it.

Nuclear localization of Y-box factor YB1 requires wild-type p53

Nuclear localization and high levels of the Y-box binding protein YB1 appear to be important indicators of drug resistance and tumor prognosis. YB1 also interacts with the p53 tumor suppressor protein. In this paper, we explore a role for p53 in the nuclear localization of YB1. We report that various genotoxic stresses induce nuclear localization of YB1 in a small proportion of treated cells, but only in cells with wild-type p53. We go on to show directly that functional p53 is required for YB1 to translocate to the nucleus. Tumor-associated p53 mutants however are attenuated for YB1 nuclear localization as are mutants mutated in the proline-rich domain of p53. These data link the DNA-damage response of p53 to YB1 nuclear translocation. In addition, we find that YB1 inhibits p53-induced cell death and its ability to trans-activate promoters of genes involved in cell death signaling. Together these data suggest that some forms of p53 cause YB1 to accumulate in the nucleus, which in turn inhibits p53 activity. These results provide a possible explanation for the correlation of nuclear YB1 with drug resistance and poor prognosis in some tumor types, and for the first time implicate p53 in the process of nuclear translocation.

Drug discovery and p53

In the past two decades, the identification of commonly mutated oncogenes and tumour suppressor genes has driven an unprecedented growth in our understanding of the genetic basis of human cancer. Although oncogenes can clearly serve as classically defined drug targets whose inactivation by small molecules could place a brake on cancer cell proliferation, the restoration of mutated tumour suppressor gene activity by small molecules might appear on the surface to be unrealistic. However, there is a growing realization that many eukaryotic regulatory proteins are partially unfolded and such intrinsically disordered proteins acquire a folded structure after binding to their biological target. Molecular characterization of the p53 protein has shown that its conformational flexibility and intrinsic thermodynamic instability provide a foundation from which its conformation can be quickly post-translationally modified.

The tumor-selective over-expression of the human Hsp70 gene is attributed to the aberrant controls at both initiation and elongation levels of transcription

The tumor selective over-expression of the human Hsp70 gene has been well documented in human tumors, linked to the poor prognosis, being refractory to chemo- and radio-therapies as well as the advanced stage of tumorous lesions in particular. However, both the nature and details of aberrations in the control of the Hsp70 expression in tumor remain enigmatic. By comparing various upstream segments of the Hsp70 gene for each's ability to drive the luciferase reporter genes in the context of the tumor cell lines varying in their p53 status and an immortal normal liver cell line, we demonstrated in a great detail the defects in the control mechanisms at the both initiation and elongation levels of transcription being instrumental to the tumor selective profile of its expression. Our data should not only offer new insights into our understanding of the tumor specific over-expression of the human Hsp70 gene, but also paved the way for the rational utilization of the tumor selective mechanism with the Hsp70 at the central stage for targeting the therapeutic gene expression to human tumors.

Thermodynamics of a designed protein catenane

Topological linking of proteins is a new approach for stabilizing and controlling the oligomerization state of proteins that fold in an interwined manner. The recent design of a backbone cyclized protein catenane based on the p53tet domain suggested that topological cross-linking provided increased stability against thermal and chemical denaturation. However, the tetrameric structure complicated detailed biophysical analysis of this protein. Here, we describe the design, synthesis and thermodynamic characterization of a protein catenane based on a dimeric mutant of the p53tet domain (M340E/L344K). The formation of the catenane proceeded efficiently, and the overall structure and oligomerization of the domain was not affected by the formation of the topological link. Unfolding and refolding of the catenane was consistent with a two-state process. The topological link stabilized the dimer against thermal and chemical denaturation considerably, raising the apparent melting temperature by 59 degrees C and the midpoint of denaturation by 4.5M GuHCl at a concentration of 50 microM. The formation of the topological link increased the resistance of the dimer to proteolysis. However, the m value decreased by 1.7kcalmol(-1)M(-1), suggesting a decrease in accessible surface area in the unfolded state. This implies that the stabilization from the topological link is largely due to a destabilization of the unfolded state, similar to other cross-links in proteins. Topological linking therefore provides a powerful and orthogonal tool for the stabilization of peptide and protein oligomers.

Structure, function, and aggregation of the zinc-free form of the p53 DNA binding domain

The p53 DNA binding domain (DBD) contains a single bound zinc ion that is essential for activity. Zinc remains bound to wild-type DBD at temperatures below 30 degrees C; however, it rapidly dissociates at physiological temperature. The resulting zinc-free protein (apoDBD) is folded and stable. NMR spectra reveal that the DNA binding surface is altered in the absence of Zn(2+). Fluorescence anisotropy studies show that Zn(2+) removal abolishes site-specific DNA binding activity, although full nonspecific DNA binding affinity is retained. Surprisingly, the majority of tumorigenic mutations that destabilize DBD do not appreciably destabilize apoDBD. The R175H mutation instead substantially accelerates the rate of Zn(2+) loss. A considerable fraction of cellular p53 may therefore exist in the folded zinc-free form, especially when tumorigenic mutations are present. ApoDBD appears to promote aggregation of zinc-bound DBD via a nucleation-growth process. These data provide an explanation for the dominant negative phenotype exhibited by many mutations. Through a combination of induced p53 aggregation and diminished site-specific DNA binding activity, Zn(2+) loss may represent a significant inactivation pathway for p53 in the cell.

Phosphorylation and hsp90 binding mediate heat shock stabilization of p53

The p53 tumor suppressor is stabilized and activated by diverse stress signals. In this study, we investigated the mechanism of p53 activation by heat shock. We found that heat shock inhibited p53 ubiquitination and caused accumulation of p53 at the post-transcriptional level. Heat shock induced phosphorylation of p53 at serine 15 in an ATM kinase-dependent fashion, which may contribute partially to heat-induced p53 accumulation. However, p53 accumulation also occurred after heat shock in ATM-deficient cells. Heat shock induced conformational change of wild type p53 and binding to hsp90. Inhibition of hsp90-p53 interaction by geldanamycin prevented p53 accumulation partially in ATM-wild type cells and completely in ATM-deficient cells. Therefore, phosphorylation and interaction with hsp90 both contribute to stabilization of p53 after heat shock.

Lung-specific expression of human mutant p53-273H is associated with a high frequency of lung adenocarcinoma in transgenic mice

To investigate the tumorigenic potential of mutant p53 when selectively expressed in lung tissue, a transgenic mouse model was developed in which a mutant form of p53 (p53-273H) was placed under the transcriptional control of the lung-specific human surfactant protein C (SP-C) promoter. Two founder mice were identified, and a line of SP-C/p53-273H transgenic mice was established from one of the founders. Human p53-273H protein was detected specifically in lung tissue from transgenic mice. Malignant tumors, which were histologically characterized as adenocarcinomas, were observed in transgenic mice, with the earliest onset documented at 4 months of age. To further evaluate incidence and onset of tumor formation, transgenic mice (n=113) were sacrificed at age intervals ranging from 4-15 months. At 13-15 months of age, transgenic mice were significantly more likely to have lung tumors at necropsy than age-matched non-transgenic littermates (9 out of 39 (23%) versus 2 out of 35 (5.7%), chi(2) test, P=0.036). The SP-C/p53-273H transgenic mice described here thus represent a genetically defined model with which to study the role of p53 mutations in lung tumorigenesis, as well as the potential complementary contributions of other genetic alterations or environmental carcinogens to lung tumor development.