Temperature sensitivity for conformation is an intrinsic property of wild-type p53

Molecular and Circulating Biomarkers in Patients with Glioblastoma

Glioblastoma is the most aggressive malignant tumor of the central nervous system with a low survival rate. The difficulty of obtaining this tumor material represents a major limitation, making the real-time monitoring of tumor progression difficult, especially in the events of recurrence or resistance to treatment. The identification of characteristic biomarkers is indispensable for an accurate diagnosis, the rigorous follow-up of patients, and the development of new personalized treatments. Liquid biopsy, as a minimally invasive procedure, holds promise in this regard. The purpose of this paper is to summarize the current literature regarding the identification of molecular and circulating glioblastoma biomarkers and the importance of their integration as a valuable tool to improve patient care.

Melatonin: Regulation of Biomolecular Condensates in Neurodegenerative Disorders

Biomolecular condensates are membraneless organelles (MLOs) that form dynamic, chemically distinct subcellular compartments organizing macromolecules such as proteins, RNA, and DNA in unicellular prokaryotic bacteria and complex eukaryotic cells. Separated from surrounding environments, MLOs in the nucleoplasm, cytoplasm, and mitochondria assemble by liquid-liquid phase separation (LLPS) into transient, non-static, liquid-like droplets that regulate essential molecular functions. LLPS is primarily controlled by post-translational modifications (PTMs) that fine-tune the balance between attractive and repulsive charge states and/or binding motifs of proteins. Aberrant phase separation due to dysregulated membrane lipid rafts and/or PTMs, as well as the absence of adequate hydrotropic small molecules such as ATP, or the presence of specific RNA proteins can cause pathological protein aggregation in neurodegenerative disorders. Melatonin may exert a dominant influence over phase separation in biomolecular condensates by optimizing membrane and MLO interdependent reactions through stabilizing lipid raft domains, reducing line tension, and maintaining negative membrane curvature and fluidity. As a potent antioxidant, melatonin protects cardiolipin and other membrane lipids from peroxidation cascades, supporting protein trafficking, signaling, ion channel activities, and ATPase functionality during condensate coacervation or dissolution. Melatonin may even control condensate LLPS through PTM and balance mRNA- and RNA-binding protein composition by regulating N6-methyladenosine (m6A) modifications. There is currently a lack of pharmaceuticals targeting neurodegenerative disorders via the regulation of phase separation. The potential of melatonin in the modulation of biomolecular condensate in the attenuation of aberrant condensate aggregation in neurodegenerative disorders is discussed in this review.

Selective functional inhibition of a tumor-derived p53 mutant by cytosolic chaperones identified using split-YFP in budding yeast

Life requires the oligomerization of individual proteins into higher-order assemblies. In order to form functional oligomers, monomers must adopt appropriate 3D structures. Molecular chaperones transiently bind nascent or misfolded proteins to promote proper folding. Single missense mutations frequently cause disease by perturbing folding despite chaperone engagement. A misfolded mutant capable of oligomerizing with wild-type proteins can dominantly poison oligomer function. We previously found evidence that human-disease-linked mutations in Saccharomyces cerevisiae septin proteins slow folding and attract chaperones, resulting in a kinetic delay in oligomerization that prevents the mutant from interfering with wild-type function. Here, we build upon our septin studies to develop a new approach for identifying chaperone interactions in living cells, and use it to expand our understanding of chaperone involvement, kinetic folding delays, and oligomerization in the recessive behavior of tumor-derived mutants of the tumor suppressor p53. We find evidence of increased binding of several cytosolic chaperones to a recessive, misfolding-prone mutant, p53(V272M). Similar to our septin results, chaperone overexpression inhibits the function of p53(V272M) with minimal effect on the wild type. Unlike mutant septins, p53(V272M) is not kinetically delayed under conditions in which it is functional. Instead, it interacts with wild-type p53 but this interaction is temperature sensitive. At high temperatures or upon chaperone overexpression, p53(V272M) is excluded from the nucleus and cannot function or perturb wild-type function. Hsp90 inhibition liberates mutant p53 to enter the nucleus. These findings provide new insights into the effects of missense mutations.

Understanding p53 functions through p53 antibodies

TP53 is the most frequently mutated gene across all cancer types. Our understanding of its functions has evolved since its discovery four decades ago. Initially thought to be an oncogene, it was later realized to be a critical tumour suppressor. A significant amount of our knowledge about p53 functions have come from the use of antibodies against its various forms. The early anti-p53 antibodies contributed to the recognition of p53 accumulation as a common feature of cancer cells and to our understanding of p53 DNA-binding and transcription activities. They led to the concept that conformational changes can facilitate p53’s activity as a growth inhibitory protein. The ensuing p53 conformational-specific antibodies further underlined p53’s conformational flexibility, collectively forming the basis for current efforts to generate therapeutic molecules capable of altering the conformation of mutant p53. A subsequent barrage of antibodies against post-translational modifications on p53 has clarified p53’s roles further, especially with respect to the mechanistic details and context-dependence of its activity. More recently, the generation of p53 mutation-specific antibodies have highlighted the possibility to go beyond the general framework of our comprehension of mutant p53—and promises to provide insights into the specific properties of individual p53 mutants. This review summarizes our current knowledge of p53 functions derived through the major classes of anti-p53 antibodies, which could be a paradigm for understanding other molecular events in health and disease.

Liquid-like droplet formation by tumor suppressor p53 induced by multivalent electrostatic interactions between two disordered domains

Early in vivo studies demonstrated the involvement of a tumor-suppressing transcription factor, p53, into cellular droplets such as Cajal and promyelocytic leukemia protein bodies, suggesting that the liquid-liquid phase separation (LLPS) might be involved in the cellular functions of p53. To examine this possibility, we conducted extensive investigations on the droplet formation of p53 in vitro. First, p53 itself was found to form liquid-like droplets at neutral and slightly acidic pH and at low salt concentrations. Truncated p53 mutants modulated droplet formation, suggesting the importance of multivalent electrostatic interactions among the N-terminal and C-terminal domains. Second, FRET efficiency measurements for the dimer mutants of p53 revealed that distances between the core domains and between the C-terminal domains were modulated in an opposite manner within the droplets. Third, the molecular crowding agents were found to promote droplet formation, whereas ssDNA, dsDNA, and ATP, to suppress it. Finally, the p53 mutant mimicking posttranslational phosphorylation did not form the droplets. We conclude that p53 itself has a potential to form droplets that can be controlled by cellular molecules and by posttranslational modifications, suggesting that LLPS might be involved in p53 function.

TP53 promoter methylation in primary glioblastoma: relationship with TP53 mRNA and protein expression and mutation status

Reduced expression of TP53 by promoter methylation has been reported in several neoplasms. It remains unclear whether TP53 promoter methylation is associated with reduced transcriptional and protein expression in glioblastoma (GB). The aim of our work was to study the impact of TP53 methylation and mutations on TP53 mRNA level and protein expression in 42 molecularly characterized primary GB tumors. We also evaluate the impact of all molecular alterations on the overall patient survival. The frequency of TP53 promoter methylation was found in 21.4%. To the best of our knowledge, this is the first report showing such high frequency of TP53 promoter methylation in primary GB. There was no relation between TP53 promoter methylation and TP53 mRNA level (p=0.5722) and between TP53 promoter methylation and TP53 protein expression (p=0.2045). No significant associations were found between TP53 mRNA expression and mutation of TP53 gene (p=0.9076). However, significant association between TP53 mutation and TP53 protein expression was found (p=0.0016). Our data suggest that in primary GB TP53 promoter methylation does not play a role in silencing of TP53 transcriptional and protein expression and is probably regulated by other genetic and epigenetic mechanisms associated with genes involved in the TP53 pathway.

Hsp90 prevents interaction between CHIP and HERG proteins to facilitate maturation of wild-type and mutant HERG proteins

AIMS

We examined the role of Hsp90 in expression and maturation of wild-type (WT) and mutant ether-a-go-go related gene (HERG) proteins by using Hsp90 inhibitors, geldanamycin (GA) and radicicol, and Hsp90 overexpression.

METHODS AND RESULTS

The proteins were expressed in HEK293 cells or collected from HL-1 mouse cardiomyocytes, and analysed by western blotting, immunoprecipitation, immunofluorescence, and whole-cell patch-clamp techniques. GA and radicicol suppressed maturation of HERG-FLAG proteins and increased their immature forms. Co-expression of Hsp90 counteracted the effects of Hsp90 inhibitors and suppressed ubiquitination of HERG proteins. Overexpressed Hsp90 also inhibited the binding of endogenous C-terminus of Hsp70-interacting protein (CHIP) to HERG-FLAG proteins. Hsp90-induced increase of functional HERG proteins was verified by their increased expression on the cell surface and enhanced HERG channel currents. CHIP overexpression decreased both mature and immature forms of HERG-FLAG proteins in cells treated with GA. Hsp90 facilitated maturation of endogenous ERG proteins, whereas CHIP decreased both forms of ERG proteins in HL-1 cells. Mutant HERG proteins harbouring disease-causing missense mutations were mainly in the immature form and had a higher binding capacity to CHIP than the WT; Hsp90 overexpression suppressed this association. Overexpressed Hsp90 increased the mature form of HERG(1122fs/147) proteins, reduced its ubiquitinated form, increased its immunoreactivity in the endoplasmic reticulum and on the plasma membrane, and increased the mutant-mediated membrane current. CHIP overexpression decreased the immature form of HERG(1122fs/147) proteins.

CONCLUSION

Enhancement of HERG protein expression through Hsp90 inhibition of CHIP binding might be a novel therapeutic strategy for long QT syndrome 2 caused by trafficking abnormalities of HERG proteins.

Effects of temperature on the p53-DNA binding interactions and their dynamical behavior: comparing the wild type to the R248Q mutant

Background

The protein p53 plays an active role in the regulation of cell cycle. In about half of human cancers, the protein is inactivated by mutations located primarily in its DNA-binding domain. Interestingly, a number of these mutations possess temperature-induced DNA-binding characteristics. A striking example is the mutation of Arg248 into glutamine or tryptophan. These mutants are defective for binding to DNA at 310 K although they have been shown to bind specifically to several p53 response elements at sub-physiological temperatures (298–306 K).

Methodology/Principal Findings

This important experimental finding motivated us to examine the effects of temperature on the structure and configuration of R248Q mutant and compare it to the wild type protein. Our aim is to determine how and where structural changes of mutant variants take place due to temperature changes. To answer these questions, we compared the mutant to the wild-type proteins from two different aspects. First, we investigated the systems at the atomistic level through their DNA-binding affinity, hydrogen bond networks and spatial distribution of water molecules. Next, we assessed changes in their long-lived conformational motions at the coarse-grained level through the collective dynamics of their side-chain and backbone atoms separately.

Conclusions

The experimentally observed effect of temperature on the DNA-binding properties of p53 is reproduced. Analysis of atomistic and coarse-grained data reveal that changes in binding are determined by a few key residues and provide a rationale for the mutant-loss of binding at physiological temperatures. The findings can potentially enable a rescue strategy for the mutant structure.

Cancer biomarker discovery: the entropic hallmark

Background

It is a commonly accepted belief that cancer cells modify their transcriptional state during the progression of the disease. We propose that the progression of cancer cells towards malignant phenotypes can be efficiently tracked using high-throughput technologies that follow the gradual changes observed in the gene expression profiles by employing Shannon's mathematical theory of communication. Methods based on Information Theory can then quantify the divergence of cancer cells' transcriptional profiles from those of normally appearing cells of the originating tissues. The relevance of the proposed methods can be evaluated using microarray datasets available in the public domain but the method is in principle applicable to other high-throughput methods.

Methodology/Principal Findings

Using melanoma and prostate cancer datasets we illustrate how it is possible to employ Shannon Entropy and the Jensen-Shannon divergence to trace the transcriptional changes progression of the disease. We establish how the variations of these two measures correlate with established biomarkers of cancer progression. The Information Theory measures allow us to identify novel biomarkers for both progressive and relatively more sudden transcriptional changes leading to malignant phenotypes. At the same time, the methodology was able to validate a large number of genes and processes that seem to be implicated in the progression of melanoma and prostate cancer.

Conclusions/Significance

We thus present a quantitative guiding rule, a new unifying hallmark of cancer: the cancer cell's transcriptome changes lead to measurable observed transitions of Normalized Shannon Entropy values (as measured by high-througput technologies). At the same time, tumor cells increment their divergence from the normal tissue profile increasing their disorder via creation of states that we might not directly measure. This unifying hallmark allows, via the the Jensen-Shannon divergence, to identify the arrow of time of the processes from the gene expression profiles, and helps to map the phenotypical and molecular hallmarks of specific cancer subtypes. The deep mathematical basis of the approach allows us to suggest that this principle is, hopefully, of general applicability for other diseases.

Influence of magnesium ion on the binding of p53 DNA-binding domain to DNA-response elements

Site-specific recognition and DNA-binding activity of p53 are crucial for its tumour suppressor function. Previous reports have shown that metal ions can affect the specific recognition and DNA-binding activity of p53DBD. Here we firstly report that magnesium ion can bind to the protein and influence its DNA-binding activity. To elucidate the nature and the effect of metal ions in the reaction chemistry, we utilized endogenous tryptophan fluorescence to quantitate the interaction between p53DBD and metal ions. The K(a) value for the binding of Mg(2+) to the protein is 1.88 x 10(3) M(-1). Analysis of the CD data clearly suggested that the binding of magnesium ion induced a subtle conformational change rather than a radical modification of the overall protein architecture. Based on the results of electrophoretic mobility shift assays and fluorescence experiments, we concluded that the binding of Mg(2+) significantly stimulated the binding of the protein to DNA in a sequence-independent manner, which differed from that of zinc ions in a sequence-specific manner. Based on these results and the fact that Mg(2+) exists at relatively high concentration in the cell, we propose that Mg(2+) is one of potential factors to affect or regulate the transactivation of p53.

CHIP chaperones wild type p53 tumor suppressor protein

Wild type p53 exists in a constant state of equilibrium between wild type and mutant conformation and undergoes conformational changes at elevated temperature. We have demonstrated that the co-chaperone CHIP (carboxyl terminus of Hsp70-interacting protein), which suppressed aggregation of several misfolded substrates and induced the proteasomal degradation of both wild type and mutant p53, physically interacts with the amino terminus of WT53 and prevented it from irreversible thermal inactivation. CHIP preferentially binds to the p53 mutant phenotype and restored the DNA binding activity of heat-denatured p53 in an ATP-independent manner. In cells under elevated temperatures that contained a higher level of p53 mutant phenotype, CHIP restored the native-like conformation of p53 in the presence of geldanamycin, whereas CHIP-small interfering RNA considerably increased the mutant form. Further, under elevated temperatures, the levels of CHIP and p53 were higher in nucleus, and chromatin immunoprecipitation shows the presence of p53 and CHIP together upon the DNA binding site in the p21 and p53 promoters. We propose that CHIP might be a direct chaperone of wild type p53 that helps p53 in maintaining wild type conformation under physiological condition as well as help resurrect p53 mutant phenotype into a folded native state under stress condition.

The influence of thermo-chemotherapy on bladder tumours: an immunohistochemical analysis

To study the influence of microwave induced thermo-chemotherapy on high-grade urothelial cell carcinomas. Five groups of each three patients were formed of whom initial biopsies and cystectomy samples were collected. Patients were treated 2 days prior to cystectomy with mitomycin-C (group 1), hyperthermia (group 2) or thermo-chemotherapy (group 3). Group 4 patients had been treated with a cycle of six thermo-chemotherapy treatments prior to cystectomy and group 5 patients served as control (no treatment). Tumour samples were stained with Haematoxylin and Eosin, monoclonal antibody Ki-67 and the monoclonal antibody p53. In six out of the nine patients treated with hyperthermia a decrease in proliferation activity in the tumour was found. Seven out of nine patients treated with hyperthermia showed a decrease in p53 activity. A decrease in proliferation activity and p53 activity illustrate the potential role of thermo-chemotherapy as a promising intravesical treatment.

Characterisation of the interface between nucleophosmin (NPM) and p53: potential role in p53 stabilisation

We have used surface plasmon resonance to quantify the kinetics and stoichiometry of the interaction between p53 and nucleophosmin (NPM). Domains characterising the interface between the two proteins were identified by chemical cross-linking, proteolytic digestion and mass spectrometry based peptide mapping. We show that the C-terminal domain of NPM (residues 242-269) interacts with two regions of p53 (residues 175-196 and residues 343-363) which belong, respectively, to the DNA binding domain and the tetramerisation domain. Potential biological consequences of such interactions are discussed.

Progenitor cell injury after irradiation to the developing brain can be modulated by mild hypothermia or hyperthermia

Ionizing radiation induced acute cell death in the dentate gyrus subgranular zone (SGZ) and the subventricular zone (SVZ). Hypomyelination was also observed. The effects of mild hypothermia and hyperthermia for 4 h after irradiation (IR) were studied in postnatal day 9 rats. One hemisphere was irradiated with a single dose of 8 Gy and animals were randomized to normothermia (rectal temperature 36 degrees C for 4 h), hypothermia (32 degrees C for 4 h) or hyperthermia (39 degrees C for 4 h). Cellular injury, e.g. chromatin condensation and nitrotyrosine formation, appeared to proceed faster when the body temperature was higher. Caspase-3 activation was more pronounced in the hyperthermia group and nuclear translocation of p53 was less pronounced in the hypothermia group 6 h after IR. In the SVZ the loss of nestin-positive progenitors was more pronounced (48%) and the size was smaller (45%) in the hyperthermia group 7 days post-IR. Myelination was not different after hypo- or hyperthermia. This is the first report to demonstrate that hypothermia may be beneficial and that hyperthermia may aggravate the adverse side-effects after radiation therapy to the developing brain.

The chaperone-associated ubiquitin ligase CHIP is able to target p53 for proteasomal degradation

The cellular level of the tumor suppressor p53 is tightly regulated through induced degradation via the ubiquitin/proteasome system. The ubiquitin ligase Mdm2 plays a pivotal role in stimulating p53 turnover. However, recently additional ubiquitin ligases have been identified that participate in the degradation of the tumor suppressor. Apparently, multiple degradation pathways are employed to ensure proper destruction of p53. Here we show that the chaperone-associated ubiquitin ligase CHIP is able to induce the proteasomal degradation of p53. CHIP-induced degradation was observed for mutant p53, which was previously shown to associate with the chaperones Hsc70 and Hsp90, and for the wild-type form of the tumor suppressor. Our data reveal that mutant and wild-type p53 transiently associate with molecular chaperones and can be diverted onto a degradation pathway through this association.

Effect of hyperthermia on the cytotoxicity of 4 chemotherapeutic agents currently used for the treatment of transitional cell carcinoma of the bladder: an in vitro study

PURPOSE

Hyperthermia combined with chemotherapy is not a novel cancer treatment. However, the working mechanism of this combination therapy is not fully understood. In the current in vitro study we investigated the differences in cytotoxicity of 4 chemotherapeutic agents at 37C or 43C.

MATERIALS AND METHODS

The human transitional cell carcinoma cell lines used were RT4, RT112, 253J and T24. Cells were seeded in 96-well microtiter plates. After 24 hours cells were treated for 60 minutes with increasing concentrations of mitomycin C, epirubicin, gemcitabine and EO9 at a temperature of 37C or 43C. After treatment cells were rinsed 3 times and left for 24 hours in the incubator at 37C. The influence of chemotherapy and temperature on cell survival was determined by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide) assay.

RESULTS

Decreased cell proliferation with increasing concentrations of chemotherapeutic agents was demonstrated. EO9 proved to be the most potent agent at each temperature. Hyperthermia alone did not demonstrate decreased cell proliferation. However, a synergistic effect on decreased cell proliferation was demonstrated in all cell lines and chemotherapeutic agents used, although each had a maximum at a different chemotherapy concentration and to a different extent. Synergism was most obvious in cell lines treated with low dose epirubicin.

CONCLUSIONS

Synergism with hyperthermia and chemotherapy was clearly demonstrated for epirubicin, EO9, mitomycin C and to a lesser extent gemcitabine. Hyperthermia alone did not cause decreased cell proliferation. Synergism was most prominent with low drug doses and the most potent drug used in this in vitro study was EO9.

The Effect of Hyperthermia on Mitomycin-C Induced Cytotoxicity in Four Human Bladder Cancer Cell Lines

INTRODUCTION

Hyperthermia and mitomycin-C (MMC) have given very encouraging results in several clinical studies for the treatment of superficial transitional cell carcinoma of the bladder. However, a synergistic effect of hyperthermia and MMC on the decrease of cell proliferation has never been demonstrated accurately in vitro. We investigated the effect of MMC versus MMC combined with hyperthermia on the cytotoxicity in four human bladder cancer cell lines.

MATERIAL AND METHODS

The RT112, RT4, 253J and T24 human bladder cancer cell lines were seeded in 96-well microtiter plates at 2.0 x 10(4) cells per well and were left to attach for 24 hours. The cells were then treated for 60 minutes with MMC concentrations ranging from 0 to 400 microg/ml at a temperature of 37 degrees C or 43 degrees C. After treatment cells were rinsed three times with culture medium and left for 24 hours in the incubator. Dimethyl thiazolyl tetrazolium (MTT) solution was added and after 4 hours of incubation the MTT containing media was aspired from all wells and 100 microl of dimethyl sulfoxide was added to each well. A spectrum analyses was performed at 595 nm light wavelength.

RESULTS

A decrease of cell proliferation after treatment with increasing concentrations MMC was demonstrated. Hyperthermia has a synergistic effect on the decrease of cell proliferation by different concentrations MMC. In the cells treated without MMC no significant difference in the extent of cell killing at 37 degrees C and 43 degrees C was observed. Furthermore, no difference was observed between cells with a p53 protein mutation (RT112 and T24) or without a p53 protein mutation (253J and RT4).

CONCLUSION

A clear synergistic effect of MMC and hyperthermia has been demonstrated in four human bladder cancer cell lines.

The European flounder (Platichthys flesus) TP53 functions as a temperature-sensitive transcription factor which inhibits cell growth in yeast

Numerous studies focus on biological roles of the TP53 tumor suppressor gene in mammals but little is known about the actual function of TP53 in lower vertebrates. In this study, we used an in vivo functional assay in yeast to address the transactivation capacity of the flounder TP53 protein. We showed that the flounder TP53 acts as a sequence-specific transcription factor which is able to transactivate various human promoters containing a p53-responsive element (RE). This transcriptional activity was completely abrogated in the Val147Glu TP53 mutant previously identified in two flounder hepatic hyperplasia. In addition, we showed that the wild-type (wt) flounder TP53 but not the Val147Glu mutant inhibits cell growth when expressed in yeast. We finally reported that transcription regulation and growth inhibition by the wild-type flounder TP53 is temperature-dependent. The flounder TP53 optimal temperature appeared lower than those reported for the Xenopus and human homologues.

Therapeutic potential of iron chelators in cancer therapy

The success of DFO at markedly inhibiting the growth of aggressive tumors such as neuroblastoma and leukemia justifies interest in the development of chelators as anti-neoplastic agents. This is emphasized by the fact that DFO has suboptimal properties, namely poor membrane permeability and a very short serum half-life. More recently, the thiosemicarbazone chelator, Triapine, has entered a phase I clinical trial again confirming the potential of these compounds. Further studies examining the effects of chelators on neoplastic cells will not only be valuable in terms of identifing novel anti-cancer agents, but will also provide new information on the role of Fe in cell cycle control.

Change of conformation of the DNA-binding domain of p53 is the only key element for binding of and interference with p73

Xenopus p53 has biological and biochemical properties similar to those of human p53, except for optimal temperature. The frog protein is fully active at 30 degrees C and inactive at 37 degrees C, leading to a temperature-sensitive behavior similar to that of the human mutant p53Ala(143) and the murine mutant p53Val(135). Using hybrid proteins between human and Xenopus expressed from artificial p53 minigenes, we have been able to demonstrate that change of conformation of the DNA-binding domain is the major determinant of this heat sensitivity. It has been reported that some human tumor-derived p53 mutants can engage in a physical association with p73, thus inhibiting its transactivating properties. The mechanism of this association remains to be elucidated. The nature of the mutant p53 that can engage in this association also remains controversial. Using the unique opportunity of the temperature sensitivity of Xenopus p53, we demonstrate that binding of and interference with p73 require a change of conformation in the p53 protein. This interaction occurs through the DNA-binding domain of p53 only when it is in a denatured state. These results reinforce the notion that mutant p53 with a conformational change can act as a down-regulator of the p73 pathway in human cancer and could confer a selective advantage to the tumor.

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.

p53 contains large unstructured regions in its native state

The human tumor suppressor protein p53 is understood only to some extent on a structural level. We performed a comprehensive biochemical and biophysical structure-function analysis of p53 full-length protein and p53 fragments. The analysis showed that p53 and the fragments investigated form stable functional units. Full-length p53 and the tetrameric fragment N93p53 (residues 93-393) are, however, destabilized significantly compared to the monomeric core domain (residues 94-312) and the monomeric fragment p53C312 (residues 1-312). At the physiological temperature of 37 degrees C and in the absence of modifications or stabilizing partners, wild-type p53 is more than 50% unfolded correlating with a 75% loss in DNA-binding activity. Furthermore the analysis of CD spectra revealed that full-length p53 contains large unstructured regions in its N and C-terminal parts. Our results indicate that full-length p53 is a modular protein consisting of defined structured and unstructured regions. We propose that p53 belongs to the growing family of loosely folded or partially unstructured native proteins. The lack of a rigid structure combined with the low overall stability may allow the physiological interaction of p53 with a multitude of partner proteins and the regulation of its turnover.

Effects of hyperthermia on p53 protein expression and activity

Although p53 responses after DNA damage have been investigated extensively, p53 responses after heat shock, which exerts cytotoxic action by mechanisms other than direct induction of DNA damage, are less well characterized. We investigated, therefore, the effect of hyperthermic exposures on the levels and DNA-binding activity of p53. Experiments were carried out with U2OS and ML-1 cells, known to express wild-type p53 protein. Although heating at 41 degrees C for up to 6 h had only a small effect on p53 levels or DNA binding activity, exposure to temperatures between 42.5 and 45.5 degrees C caused an immediate decrease in protein levels that was associated with a reduction in DNA binding activity. This observation is compatible with a high lability of p53 to heat shock, or heat sensitivity of the pathway regulating p53 levels in non-stressed cells. When cells were heated to 42.5 degrees C and returned to normal temperatures, a strong p53 response associated with an increase in protein levels and DNA binding activity was observed, suggesting the production of p53-inducing cellular damage. At higher temperatures, however, this response was compromised in an exposure-time-dependent manner. The increase in DNA binding activity was more heat sensitive than the increase in p53 levels and was inhibited at lower temperatures and shorter exposure times. Thus, the pathway of p53 activation is itself heat sensitive and compromised at high levels of exposure. Compared to p53 activation after exposure to ionizing radiation, heat-induced activation is rapid and short lived. When cells were exposed to combined heat and radiation, the response observed approximated that of cells exposed to heat alone. Wortmannin at 10 microM inhibited p53 activation for up to 2 h after heat shock suggesting the involvement of wortmannin-sensitive kinases, such as DNA-PK and ATM. Heat shock causes phosphorylation of p53 at Serine-15, but there is no correlation between phosphorylation at this site and activation of the protein. The results in aggregate indicate p53 activation in the absence of DNA damage by a heat-sensitive mechanism operating with faster kinetics than radiation-induced p53 activation. The former response may induce pathways preventing other stimuli from activating p53, as heat-induced activation of p53 is dominant over activation of p53 by DNA damage in combined-treatment experiments. These observations suggest means for abrogating p53 induction after DNA damage with the purpose of potentiating response and enhancing cell killing.

Restoration of wild-type conformation and activity of a temperature-sensitive mutant of p53 (p53(V272M)) by the cytoprotective aminothiol WR1065 in the esophageal cancer cell line TE-1

The aminothiol WR1065, the active metabolite of the cytoprotector amifostine, exerts its antimutagenic effects through free-radical scavenging and other unknown mechanisms. In an earlier report, we showed that WR1065 activates wild-type p53 in MCF-7 cells, leading to p53-dependent arrest in the G(1) phase of the cell cycle. To determine whether WR1065 activates p53 by modulating protein conformation, we analyzed its effects on p53 conformation and activity in the esophageal cancer cell line TE-1. This cell line contains a mutation in codon 272 of p53 (p53(V272M), with methionine instead of a valine), conferring temperature-sensitive properties to the p53 protein. At the nonpermissive temperature (37 degrees C), p53(V272M) adopts the mutant p53 conformation (nonreactive with the antibody PAb1620), does not bind specifically to DNA, and is not activated in response to DNA-damaging treatment. However, treatment with 0.5-4 mM WR1065 partially restored wild-type conformation at 37 degrees C, stimulated DNA binding activity, and increased the expression of p53 target genes WAF-1, GADD45, and MDM2, leading to cell-cycle arrest in G(1). These results suggest that WR1065 activates p53 through a mechanism distinct from DNA-damage signaling, which involves modulation of p53 protein conformation.

Protein-DNA binding correlates with structural thermostability for the full-length human p53 protein

Full-length p53 protein purified from Escherichia coli in the unmodified, "latent" form was examined by several methods to correlate thermal stability of structure with functional DNA binding. Structure prediction algorithms indicate that the majority of beta-sheet structure occurs in the p53 core DNA binding domain. Circular dichroism spectra demonstrate that the intact protein is surprisingly stable with a midpoint for the irreversible unfolding transition at approximately 73 degrees C. Significant beta-sheet structural signal remains even to 100 degrees C. The persistent beta-sheet CD signal correlates with significant DNA binding (K(d) approximately nM range) to temperatures as high as 50 degrees C. These data confirm the ability of the DNA binding domain in the full-length "latent" protein to bind consensus dsDNA targets effectively in the absence of activators over a broad temperature range. In addition, we demonstrate that Ab1620 reactivity is not directly correlated with the functional activity of the full-length protein since loss of this epitope occurs at temperatures at which significant specific DNA binding can still be measured.

Binding of p53 and its core domain to supercoiled DNA

We have compared the binding of human full-length p53 protein (p53; expressed in bacteria and insects) and its isolated core domain (p53CD, amino acids 94-312; expressed in bacteria) to negatively supercoiled (sc) DNA using gel electrophoresis and immunoblotting. Significant differences were observed; p53CD produced a relatively small and continuous retardation of scDNA, in contrast to the ladder of distinct bands formed by p53 in agarose gels. The ladder produced by full-length protein expressed in bacteria (p53b) was similar to that observed earlier with protein expressed in insect cells (p53i). Competition between scDNAs and their linearized (lin) forms showed a preference for scDNAs by both p53 and p53CD, but the ratios characterizing the distribution of the protein between sc and lin pBluescript DNAs were substantially higher for p53 (sc/lin > 60 in p53b) than for p53CD (sc/lin approximately 4). Strong binding of p53 to scDNA lacking the p53 consensus sequence may represent a new p53-binding mode, which we tentatively denote supercoil-selective (SCS) binding. This binding requires both the C-terminal domain and the core domain. Targets of this binding may include: (a) DNA segments defined both by the nucleotide sequence and local topology, and/or (b) strand crossings and/or bending. The binding preference of p53CD for scDNA may be due to the known nonspecific binding to internal single-stranded regions in scDNA (absent in relaxed DNA molecules) and/or to SCS binding albeit with reduced affinity due to the absence of contributions from other p53 domains.

Metalloregulation of the tumor suppressor protein p53: zinc mediates the renaturation of p53 after exposure to metal chelators in vitro and in intact cells

The tumor suppressor p53 is a transcription factor which binds DNA through a structurally complex domain stabilized by a zinc atom. Zinc chelation disrupts the architecture of this domain, inducing the protein to adopt an immunological phenotype identical to that of many mutant forms of p53. In this report, we used 65Zn to show that incorporation of zinc within the protein was required for folding in the 'wild-type' conformation capable of specific DNA-binding. Using a cellular assay, we show that addition of extracellular zinc at concentrations within the physiological range (5 microM) was required for renaturation and reactivation of wild-type p53. Among other divalent metals tested (Cd2+, Cu2+, Co2+, Fe2+ and Ni2+), only Co2+ at 125 microM had a similar effect. Recombinant metallothionein (MT), a metal chelator protein, was found to modulate p53 conformation in vitro. In cultured cells, overexpression of MT by transfection could modulate p53 transcriptional activity. Taken together, these results suggest that zinc binding plays a regulatory role in the control of p53 folding and DNA-binding activity.

Redox signalling and transition metals in the control of the p53 pathway

The p53 tumour suppressor protein exerts multiple, antiproliferative effects in response to genotoxic exposures. Reactive oxygen intermediates (ROI) play several distinct roles in the p53 pathway. First, they are important activators of p53 through their capacity to induce DNA strand breaks. Second, they regulate the DNA-binding activity of p53 by modulating the redox status of a critical set of cysteines in the DNA-binding domain, which are also involved in the coordination of zinc. Third, they play a role in the signalling pathways regulated by p53, as several genes encoding redox effectors are transcriptionally controlled by p53. In this review, we summarize the evidence for the involvement of ROI at these three levels. Emphasis is placed on the role of metals and ROI as potential regulators of p53 protein conformation and functions, and on the putative toxicological consequences of such a regulation.

Effect of p53 protein redox states on binding to supercoiled and linear DNA

The binding of p53 to its DNA consensus sequence is modulated by the redox state of the protein in vitro. We have shown previously that reduced wild-type p53 binds strongly to supercoiled DNA (scDNA) regardless of the presence or absence of p53CON. Here we compare the effects of oxidation of p53 by azodicarboxylic acid bis[dimethylamide] (diamide) and other agents on p53 binding to p53CON and to scDNA. Oxidation decreases the binding of p53 to scDNA; however, under conditions where binding to p53CON in a DNA fragment is completely abolished, some residual binding to scDNA is still observed. Increasing the concentration of oxidized p53 confers minimal changes in p53 binding to both scDNA and p53CON. Reduction of the oxidized protein by dithiothreitol neither restores its binding to DNA nor to p53CON in DNA fragments. In the presence of excess zinc ions, oxidation of p53 is, however, reversible. We conclude that the irreversibility of p53 oxidation is due, at least in part, to the removal of intrinsic zinc from its position in the DNA binding domain accompanied by a conformational change of the p53 molecule after oxidation of the three cysteines to which the zinc ion is coordinated in the reduced protein.

Direct involvement of p53 in the base excision repair pathway of the DNA repair machinery

The p53 tumor suppressor that plays a central role in the cellular response to genotoxic stress was suggested to be associated with the DNA repair machinery which mostly involves nucleotide excision repair (NER). In the present study we show for the first time that p53 is also directly involved in base excision repair (BER). These experiments were performed with p53 temperature-sensitive (ts) mutants that were previously studied in in vivo experimental models. We report here that p53 ts mutants can also acquire wild-type activity under in vitro conditions. Using ts mutants of murine and human origin, it was observed that cell extracts overexpressing p53 exhibited an augmented BER activity measured in an in vitro assay. Depletion of p53 from the nuclear extracts abolished this enhanced activity. Together, this suggests that p53 is involved in more than one DNA repair pathway.

The N terminus of the murine p53 tumour suppressor is an independent regulatory domain affecting activation and thermostability

The contribution of each of the structural domains of p53 to its function has been discussed widely in the literature. Crystallographic studies have revealed much about the structure of the core DNA binding domain, but as it has not been possible to use this approach for the intact protein, the effect of the domains flanking the core must be investigated by more indirect techniques. In this study a series of truncated murine p53 proteins has been investigated for DNA binding activity at 4 degrees C and 37 degrees C, transcriptional activation, and tumour suppression activity. Full-length p53, and truncations lacking the N terminus, purified from a baculovirus expression system all show latency for DNA binding; that is, they must be activated to bind by association with a C-terminal antibody such as PAb421. This demonstrates that latency for DNA binding is independent of the N terminus. Truncations lacking the C-terminal oligomerisation domain, and the isolated core domain, can only be activated to bind DNA and PAb1620 (an antibody recognising the wild-type conformation of the core domain) in the presence of cross-linking antibodies, while murine core only binds to DNA in the presence of PAb1620. An analysis of the thermostability of DNA binding revealed that antibodies that bind the N terminus of p53 could protect the protein against loss of activity at 37 degrees C. C-terminal antibodies, however, were ineffective unless the N-terminal 37 amino acid residues were absent. The N terminus may retain some secondary structure, since it is the main contributor to the anomalous migration in SDS-polyacrylamide gels. Our results suggest that the N terminus has a destabilising effect that influences conformation of p53 at 37 degrees C, so cellular proteins binding to the N terminus in vivo may modulate p53 conformation and stability. The effects on thermostability are also direct evidence showing that antibodies binding to N-terminal deletions create a conformational change in the rest of the molecule. In addition, longer deletions of the C terminus reduce the ability of p53 to transactivate target genes and inactivate tumour suppression activity, while truncations of the N terminus retain partial tumour suppression activity. Our results clearly show participation of both the N and C termini in the regulation of all the functions of p53 at 37 degrees C, indicating that distinct, independent domains interact with each other within, the flexible structure of p53.

Cooperation of p53 loss of function and v-Ha-ras in transformation of mouse keratinocyte cell lines

We previously demonstrated that after transduction with the v-Ha-ras oncogene and grafting onto nude mouse hosts, primary epidermal keratinocytes with a null mutation in the p53 gene form tumors with increased growth rates and predisposition to malignant conversion relative to p53 wild-type keratinocytes (Weinberg WC, et al., Cancer Res 54:5584-5592, 1994). To further explore the cooperation between p53 loss of function and activation of the ras oncogene, cell lines were established from the normal epidermises of newborn and adult p53-null mice, and parallel subclones were reconstituted with the p53val135 temperature-sensitive mutant. Reconstituted lines C, G, N, and V demonstrated functional p53 transcriptional activator activity at the wild-type-permissive temperature of 32 degrees C, compared with the hygromycin-selected control line X and parental p53-null lines NHK4 and AK1b. Hygromycin-selected subclones, but not the parental lines, made normal skin in vivo; all cell lines made carcinomas after introduction of v-Ha-ras, independent of p53 status. These cell lines were compared in vitro at 32 degrees C to maximize the amount of p53val135 in the wild-type conformation. Expression of v-Ha-ras did not consistently alter p53-mediated transcriptional activity, suggesting tat ras acts downstream or independently of p53. No correlation was observed between p53-mediated transcriptional activity and in vitro growth rates, colony formation after exposure to ultraviolet light, or suppression by normal neighboring keratinocytes. However, keratinocyte cell lines devoid of p53 and expressing v-Ha-ras formed colonies in soft agar; this was blocked at 32 degrees C in all cell lines reconstituted with p53val135. These keratinocyte lines provide a model for exploring the role of p53 and the interaction of p53 and ras in keratinocyte transformation.

HSP70 binding sites in the tumor suppressor protein p53

Mutations within conserved regions of the tumor suppressor protein, p53, result in oncogenic forms of the protein with altered tertiary structures. In most cases, the mutant p53 proteins are selectively recognized and bound by members of the HSP70 family of molecular chaperones, but the binding site(s) in p53 for these chaperones have not been clearly defined. We have screened a library of overlapping biotinylated peptides, spanning the entire human p53 sequence, for binding to the HSP70 proteins, Hsc70 and DnaK. We show that most of the high affinity binding sites for these proteins map to secondary structure elements, particularly beta-strands, in the hydrophobic core of the central DNA binding domain, where the majority of oncogenic p53 mutations are found. Although peptides corresponding to the C-terminal region of p53 also contain potential binding sites, p53 proteins with C-terminal deletions are capable of binding to Hsc70, indicating that this region is not required for complex formation. We propose that mutations in the p53 protein alter the tertiary structure of the central DNA binding domain, thus exposing high affinity HSP70 binding sites that are cryptic in the wild-type molecule.

A comparison of p53 and p16 expression in human tumor cells treated with hyperthermia or ionizing radiation

To assess the potential relationship between p53 and p16 proteins in the cellular response to stress, we have examined the levels of these proteins in a series of human tumor cell lines after treatment with either ionizing radiation or hyperthermia. We found that cells with abnormal radiation-induced G1 arrest (non-functional p53) had significantly higher constitutive levels of p16 than cells showing a normal G1 arrest (functional p53). Time-course experiments were done to test the effect of gamma-irradiation on intracellular levels of p16. The pattern of changes in p16 response was similar in all cell lines studied, and p16 expression was not related to cellular sensitivity to radiation or to the level of p53 induction after treatment. We also provide evidence that short-term exposure to high temperature causes p53 accumulation. Hyperthermia-induced p53 accumulation was greatest in those cells exhibiting the highest radiation-induced p53 accumulation, suggesting a possible relationship between p53 induction after these 2 different stresses. p16 synthesis was also induced in different cell lines after heat treatment, and this response was independent of p53 functionality. When we compared the level of p16 expression with the extent of G0/G1 arrest induced by heat, a linear correlation was found, raising the possibility that p16 may be involved in the control of cell cycle progression in response to heat treatment.

Regulation of mutant p53 temperature-sensitive DNA binding

We have examined in detail the DNA binding properties of several immunopurified tumor-derived mutant p53 proteins (Val-143 --> Ala, Arg-175 --> His, Arg-248 --> Trp, Arg-249 --> Ser, and Arg-273 --> His). While all mutants were defective for binding to DNA at 37 ;C, each bound specifically to several cognate p53 binding sites at sub-physiological temperatures (25-33 ;C), and several mutants activated transcription from a p53-responsive promoter at 26 degrees C in transfected H1299 cells. Heating mutant p53 proteins at 37 degrees C irreversibly destroyed their ability to subsequently bind at 25 degrees C. However, several different monoclonal antibodies that each share the ability to recognize an epitope encompassing amino acids 46-55 markedly stabilized binding by mutant p53 proteins at 37 degrees C. Both intact antibody and FAb fragments allowed mutant p53 to bind to DNA. By contrast, antibodies that recognize epitopes located elsewhere within p53 stabilized mutant p53 binding significantly less effectively. Our data show that the major hot-spot p53 mutants have the intrinsic ability to bind to DNA and that a unique region within the N terminus of p53 may be critical for rescuing them from loss of binding at physiological temperatures. This suggests the possibility of developing small molecules that can stabilize mutant p53 proteins under physiological conditions.

Mutant conformation of p53 translated in vitro or in vivo requires functional HSP90

The p53 mutant, 143ala, was translated in vitro in either rabbit reticulocyte lysate (RRL) or wheat germ extract (WGE). In RRL, p53-143ala protein of both mutant and wild-type conformation, as detected immunologically with conformation-specific antibodies, was translated. The chaperone protein HSP90, present in RRL, was found to coprecipitate only with the mutated conformation of p53. Geldanamycin, shown previously to bind to HSP90 and destabilize its association with other proteins, decreased the amount of immunologically detectable mutated p53 and increased the amount of detectable wild-type protein, without affecting the total translation of p53. When translated in WGE, known to contain functionally deficient HSP90, p53-143ala produced p53 protein, which was not recognized by a mutated conformation-specific antibody. In contrast, the synthesis of conformationally detectable wild-type p53 in this system was not compromised. Reconstitution of HSP90 function in WGE permitted synthesis of conformationally detectable mutated p53, and this was abrogated by geldanamycin. Finally, when p53-143ala was stably tansfected into yeast engineered to be defective for HSP90 function, conformational recognition of mutated p53 was impaired. When stable transfectants of p53-143ala were prepared in yeast expressing wild-type HSP90, conformational recognition of mutated p53 was antagonized by macbecin I, a geldanamycin analog also known to bind HSP90. Taken together, these data demonstrate a role for HSP90 in the achievement and/or stabilization of the mutated conformation of p53-143ala. Furthermore, we show that the mutated conformation of p53 can be pharmacologically antagonized by drugs targeting HSP90.

Redox regulation of transcriptional activators

Transcription factors/activators are a group of proteins that bind to specific consensus sequences (cis elements) in the promoter regions of downstream target/effector genes and transactivate or repress effector gene expression. The up- or downregulation of effector genes will ultimately lead to many biological changes such as proliferation, growth suppression, differentiation, or senescence. Transcription factors are subject to transcriptional and posttranslational regulation. This review will focus on the redox (reduction/oxidation) regulation of transcription factors/activators with emphasis on p53, AP-1, and NF-kappa B. The redox regulation of transcriptional activators occurs through highly conserved cysteine residues in the DNA binding domains of these proteins. In vitro studies have shown that reducing environments increase, while oxidizing conditions inhibit sequence-specific DNA binding of these transcriptional activators. When intact cells have been used for study, a more complex regulation has been observed. Reduction/oxidation can either up- or downregulate DNA binding and/or transactivation activities in transcriptional activator-dependent as well as cell type-dependent manners. In general, reductants decrease p53 and NF-kappa B activities but dramatically activate AP-1 activity. Oxidants, on the other hand, greatly activate NF-kappa B activity. Furthermore, redox-induced biochemical alterations sometimes lead to change in the biological functions of these proteins. Therefore, differential regulation of these transcriptional activators, which in turn, regulate many target/effector genes, may provide an additional mechanism by which small antioxidant molecules play protective roles in anticancer and antiaging processes. Better understanding of the mechanism of redox regulation, particularly in vivo, will have an important impact on drug discovery for chemoprevention and therapy of human disease such as cancer.