p53 inhibits DNA replication in vitro in a DNA-binding-dependent manner

Mapping the structural and dynamical features of multiple p53 DNA binding domains: insights into loop 1 intrinsic dynamics

The transcription factor p53 regulates cellular integrity in response to stress. p53 is mutated in more than half of cancerous cells, with a majority of the mutations localized to the DNA binding domain (DBD). In order to map the structural and dynamical features of the DBD, we carried out multiple copy molecular dynamics simulations (totaling 0.8 μs). Simulations show the loop 1 to be the most dynamic element among the DNA-contacting loops (loops 1-3). Loop 1 occupies two major conformational states: extended and recessed; the former but not the latter displays correlations in atomic fluctuations with those of loop 2 (~24 Å apart). Since loop 1 binds to the major groove whereas loop 2 binds to the minor groove of DNA, our results begin to provide some insight into the possible mechanism underpinning the cooperative nature of DBD binding to DNA. We propose (1) a novel mechanism underlying the dynamics of loop 1 and the possible tread-milling of p53 on DNA and (2) possible mutations on loop 1 residues to restore the transcriptional activity of an oncogenic mutation at a distant site.

Crystal structure of SV40 large T-antigen bound to p53: interplay between a viral oncoprotein and a cellular tumor suppressor

The transformation potential of Simian Virus 40 depends on the activities of large T-antigen (LTag), which interacts with several cellular tumor suppressors including the important "guardian" of the genome, p53. Inhibition of p53 function by LTag is necessary for both efficient viral replication and cellular transformation. We determined the crystal structure of LTag in complex with p53. The structure reveals an unexpected hexameric complex of LTag binding six p53 monomers. Structure-guided mutagenesis of LTag and p53 residues supported the p53-LTag interface defined by the complex structure. The structure also shows that LTag binding induces dramatic conformational changes at the DNA-binding area of p53, which is achieved partially through an unusual "methionine switch" within p53. In the complex structure, LTag occupies the whole p53 DNA-binding surface and likely interferes with formation of a functional p53 tetramer. In addition, we showed that p53 inhibited LTag helicase function through direct complex formation.

Wild-type p53 stimulates homologous recombination upon sequence-specific binding to the ribosomal gene cluster repeat

p53 plays a central role in the maintenance of the genome integrity, both as a gatekeeper and a caretaker. Sequence-specific recognition of DNA is underlying the ability of p53 to transcriptionally transactivate target genes during checkpoint control and to regulate DNA replication at the TGCCT repeat from the ribosomal gene cluster (RGC). In contrast, suppression of recombination by p53 has been observed with nonconsensus DNA sequences. In this study, we discovered that wild-type p53 stimulates homologous recombination adjacent to the RGC repeat, whereas downregulation is seen with a mutated version thereof and with a microsatellite repeat sequence. Analysis of the causes possibly underlying the enhancement of homologous recombination revealed that p53 binding to the RGC element delays DNA synthesis. This was demonstrated after integration of the corresponding DNA fragments into our Simian virus 40-based model system, which was used to study recombination on replicating minichromosomes. Differently, with plasmid-based substrates, p53 did not stimulate recombination at the RGC sequence. Thus, in combination with our previous findings, p53 may promote homologous recombination by two separate mechanisms involving either molecular interactions with topoisomerase I or/and by specific binding to certain genomic regions, thereby causing replication fork stalling and recombination.

The papillomavirus E8-E2C protein represses DNA replication from extrachromosomal origins

Carcinogenic DNA viruses such as high-risk human papillomaviruses (HPV) and Epstein-Barr-Virus (EBV) replicate during persistent infections as low-copy-number plasmids. EBV DNA replication is restricted by host cell replication licensing mechanisms. In contrast, copy number control of HPV genomes is not under cellular control but involves the viral sequence-specific DNA-binding E2 activator and E8-E2C repressor proteins. Analysis of HPV31 mutant genomes revealed that residues outside of the DNA-binding/dimerization domain of E8-E2C limit viral DNA replication, indicating that binding site competition or heterodimerization among E2 and E8-E2C proteins does not contribute to copy number control. Domain swap experiments demonstrated that the amino-terminal 21 amino acids of E8-E2C represent a novel, transferable DNA replication repressor domain, whose activity requires conserved lysine and tryptophan residues. Furthermore, E8-E2C (1-21)-GAL4 fusion proteins inhibited the replication of the plasmid origin of replication of EBV, suggesting that E8-E2C functions as a general replication repressor of extrachromosomal origins. This finding could be important for the development of novel therapies against persistent DNA tumor virus infections.

Replication of damaged DNA in vitro is blocked by p53

The tumor suppressor protein p53 may have other roles and functions in addition to its well-documented ability to serve as a sequence-specific transcriptional activator in response to DNA damage. We showed previously that p53 can block the replication of polyomavirus origin-containing DNA (Py ori-DNA) in vitro when p53 binding sites are present on the late side of the Py ori. Here we have both further extended these observations and have also examined whether p53 might be able to bind directly to and inhibit the replication of damaged DNA. We found that p53 strongly inhibits replication of gamma-irradiated Py ori-DNA and such inhibition requires both the central DNA binding domain and the extreme C-terminus of the p53 protein. An endogenous p53 binding site lies within the Py origin and is required for the ability of p53 to block initiation of replication from gamma-irradiated Py ori-DNA, suggesting the possibility of DNA looping caused by p53 binding both non-specifically to sites of DNA damage and specifically to the endogenous site in the polyomavirus origin. Our results thus suggest the possibility that under some circumstances p53 might serve as a direct regulator of DNA replication and suggest as well an additional function for cooperation between its two autonomous DNA binding domains.

Inhibition of polyomavirus ori-dependent DNA replication by mSin3B

When tethered in cis to DNA, the transcriptional corepressor mSin3B inhibits polyomavirus (Py) ori-dependent DNA replication in vivo. Histone deacetylases (HDACs) appear not to be involved, since tethering class I and class II HDACs in cis does not inhibit replication and treating the cells with trichostatin A does not specifically relieve inhibition by mSin3B. However, the mSin3B L59P mutation that impairs mSin3B interaction with N-CoR/SMRT abrogates inhibition of replication, suggesting the involvement of N-CoR/SMRT. Py large T antigen interacts with mSin3B, suggesting an HDAC-independent mechanism by which mSin3B inhibits DNA replication.

Induction and bypass of p53 during productive infection by polyomavirus

Lytic infection by polyomavirus leads to elevated levels of p53 and induction of p53 target genes p21Cip1/WAF1 (p21) and BAX. This is seen both in polyomavirus-infected primary mouse cell cultures and in kidney tissue of infected mice. Stabilization of p53 and induction of a p53 response are accompanied by phosphorylation of p53 on serine 18, mimicking a DNA damage response. Stabilization of p53 does not depend on p19Arf interaction with mdm2. Cells infected by a mutant virus defective in binding pRb and in inducing G(1)-to-S progression show a greatly diminished p53 response. However, cells infected by wild-type virus and blocked from entering S phase by addition of mimosine still show a p53 response. These results suggest a role of E2F target genes in inducing a p53 response. Polyomavirus large T antigen coprecipitates with p53 phosphorylated on serine 18 and also with p21Cip1/WAF1. Implications of these and other findings on possible mechanisms of induction and override of p53 functions during productive infection by polyomavirus are discussed.

Wild-type p53 inhibits replication-associated homologous recombination

In mammalian cells homologous recombination is stimulated, when the replication fork stalls at DNA breaks or unrepaired lesions. The tumor suppressor p53 downregulates homologous recombination independently of its transcriptional transactivation function and has been linked to enzymes of DNA recombination and replication. To study recombination with respect to replication, we utilized a SV40 virus based assay, to follow the synchronous events after primate cell infection. gamma-ray treatment at different times after viral entry unveiled an increase of interchromosomal exchange frequencies, when the damage was introduced during DNA synthesis. Elevated recombination frequencies were fully suppressed by p53. With respect to the downregulation of spontaneous recombination, we noticed a requirement for active p53 molecules, when replication started. After a transient treatment with replication inhibitors, we observed inhibition of the drug induced recombination by p53, particularly for the elongation inhibitor aphidicolin. Consequently, we propose that p53 is a surveillance factor of homologous recombination at replication forks, when they stall as a consequence of endogenous or of exogenously introduced damage.

Estrogen up-regulation of p53 gene expression in MCF-7 breast cancer cells is mediated by calmodulin kinase IV-dependent activation of a nuclear factor kappaB/CCAAT-binding transcription factor-1 complex

This study investigates the mechanism of hormonal regulation of p53 gene expression in MCF-7 human breast cancer cells. 17beta-Estradiol (E2) induced a 2-fold increase in p53 mRNA levels and a 2- to 3-fold increase in p53 protein. Analysis of the p53 gene promoter has identified a minimal E2-responsive region at -106 to -40, and mutation/deletion analysis of the promoter showed that motifs that bind CCAAT-binding transcription factor-1 (CTF-1) and nuclear factor kappaB (NFkappaB) proteins are required for hormone responsiveness. The p65 subunit of NFkappaB was identified in both nuclear and cytosolic fractions of untreated MCF-7 cells; however, formation of the nuclear NFkappaB complex was E2 independent. Hormonal activation of constructs containing p53 promoter inserts (-106 to -40) and the GAL4-p65 fusion proteins was inhibited by the intracellular Ca2+ ion chelator EGTA-AM and Ca2+/calmodulin-dependent protein kinase (CaMK) inhibitor KN-93. Constitutively active CaMKIV but not CaMKI activated p65, and treatment of MCF-7 cells with E2 induced phosphorylation of CaMKIV but not CaMKI. The results indicate that hormonal activation of p53 though nongenomic pathways was CaMKIV-dependent and involved cooperative p65-CTF-1 interactions.

Yph1p, an ORC-interacting protein: potential links between cell proliferation control, DNA replication, and ribosome biogenesis

Immunoprecipitation of the origin recognition complex (ORC) from yeast extracts identified Yph1p, an essential protein containing a BRCT domain. Two Yph1p complexes were characterized. Besides ORC, MCM proteins, cell-cycle regulatory proteins, checkpoint proteins, 60S ribosomal proteins, and preribosome particle proteins were found to be associated with Yph1p. Yph1p is predominantly nucleolar and is required for 60S ribosomal subunit biogenesis and possibly for translation on polysomes. Proliferating cells depleted of Yph1p arrest in G(1) or G(2), with no cells in S phase, or significantly delay S phase progression after release from a hydroxyurea arrest. Yph1p levels decline as cells commit to exit the cell cycle, and levels vary depending on energy source. Yph1p may link cell proliferation control to DNA replication, ribosome biogenesis, and translation on polysomes.

DNA damage and cell cycle checkpoints in hyperoxic lung injury: braking to facilitate repair

The beneficial use of supplemental oxygen therapies to increase arterial blood oxygen levels and reduce tissue hypoxia is offset by the knowledge that it injures and kills cells, resulting in increased morbidity and mortality. Although many studies have focused on understanding how hyperoxia kills cells, recent findings reveal that it also inhibits proliferation through activation of cell cycle checkpoints rather than through overt cytotoxicity. Cell cycle checkpoints are thought to be protective because they allow additional time for injured cells to repair damaged DNA and other essential molecules. During recovery in room air, the lung undergoes a burst of proliferation to replace injured and dead cells. Failure to terminate this proliferation has been associated with fibrosis. These observations suggest that growth-suppressive signals, which inhibit proliferation of injured cells and terminate proliferation when tissue repair has been completed, may play an important role in the pulmonary response to hyperoxia. Because DNA replication is coupled with DNA repair, activation of cell cycle checkpoints during hyperoxia may be a mechanism by which cells protect themselves from oxidant genotoxic stress. This review examines the effect of hyperoxia on DNA integrity, pulmonary cell proliferation, and cell cycle checkpoints activated by DNA damage.

Cell-specific modulation of papovavirus replication by tumor suppressor protein p53

Small DNA tumor viruses like human papillomaviruses, simian virus 40, and adenoviruses modulate the activity of cellular tumor suppressor proteins p53 and/or pRB. These viruses replicate as nuclear multicopy extrachromosomal elements during the S phase of the cell cycle, and it has been suggested that inactivation of p53 and pRb is necessary for directing the cells to the S phase. Mouse polyomavirus (Py), however, modulates only the pRB protein activity without any obvious interference with the action of p53. We show here that Py replication was not suppressed by the p53 protein indeed in all tested different mouse cell lines. In addition, E1- and E2-dependent papillomavirus origin replication was insensitive to the action of p53 in mouse cells. We show that in hamster (Chinese hamster ovary) or human (osteosarcoma 143) cell lines the replication of both Py and papillomavirus origins was efficiently blocked by p53. The block of Py replication in human and hamster cells is not caused by the downregulation of large T-antigen expression. The deletion analysis of the p53 protein shows that the RPA binding, proline-rich regulatory, DNA-binding, and oligomerization domains are necessary for p53 action in both replication systems. These results indicate that in mouse cells the p53 protein could be inactive for the suppression of papovavirus replication.

The role of the E6-p53 interaction in the molecular pathogenesis of HPV

Human papillomaviruses (HPVs) are associated with a number of clinical conditions, of which the most serious is cervical carcinoma. The E6 protein of the oncogenic, mucosal-specific HPV types has been shown to complex with p53 and, as a result, target it for rapid proteasome-mediated degradation. As a consequence, p53's growth-arrest and apoptosis-inducing activities are abrogated. Since p53 is frequently wild type in cervical cancers, unlike other cancers in which it is often mutated, the notion has arisen that E6's activity with respect to p53 is equivalent to an inactivating mutation of p53. In addition, several studies have shown that the pathways both upstream and downstream of p53 are intact in cervical cancers; this suggests the potential importance of the E6 - p53 interaction for therapeutic intervention. However, like all viral oncoproteins, E6 is a multifunctional protein and a plethora of other cellular targets has been identified. Indeed, E6's interactions with some of these additional targets appear to be equally important in the pathogenesis of HPV, and may also represent valid targets for therapeutic intervention.

Nuclear import of p53 during Xenopus laevis early development in relation to DNA replication and DNA repair

The role of p53 in transcriptional activation of genes involved in cell cycle progression is well established. However, the wide range of functions attributed to this gene suggests that some of them might be unrelated to transcription. Here we investigated p53 localization and recruitment to chromatin during Xenopus early development when 12 rapid cell cycles occur without transcription of the genome. We show that after fertilization, part of the large store of p53 previously stored in the cytoplasm of the oocyte is imported into the nucleus. This import was further analyzed in relation with DNA replication and DNA repair using cell-free systems from Xenopus eggs. Formation of a nuclear lamina envelope is necessary for the import of p53 into the nucleus. p53 associates both with decondensed DNA and the nuclear lamina envelope, but no colocalization with prereplication or replication complexes is observed. We show that UV- or gamma-damaged nuclei recruit p53 as well as replication protein A (RPA) in large common foci. Together, these data suggest that p53 plays a role in the regulation of the accelerated S phases that occur during Xenopus early development, in a manner that does not rely on its transcription-mediated activity.

The retinoblastoma protein alters the phosphorylation state of polyomavirus large T antigen in murine cell extracts and inhibits polyomavirus origin DNA replication

The retinoblastoma tumor suppressor protein (pRb) can associate with the transforming proteins of several DNA tumor viruses, including the large T antigen encoded by polyomavirus (Py T Ag). Although pRb function is critical for regulating progression from G1 to S phase, a role for pRb in S phase has not been demonstrated or excluded. To identify a potential effect of pRb on DNA replication, pRb protein was added to reaction mixtures containing Py T Ag, Py origin-containing DNA (Py ori-DNA), and murine FM3A cell extracts. We found that pRb strongly represses Py ori-DNA replication in vitro. Unexpectedly, however, this inhibition only partially depends on the interaction of pRb with Py T Ag, since a mutant Py T Ag (dl141) lacking the pRb interaction region was also significantly inhibited by pRb. This result suggests that pRb interferes with or alters one or more components of the murine cell replication extract. Furthermore, the ability of Py T Ag to be phosphorylated in such extracts is markedly reduced in the presence of pRb. Since cyclin-dependent kinase (CDK) phosphorylation of Py T Ag is required for its replication function, we hypothesize that pRb interferes with this phosphorylation event. Indeed, the S-phase CDK complex (cyclin A-CDK2), which phosphorylates both pRb and Py T Ag, alleviates inhibition caused by pRb. Moreover, hyperphosphorylated pRb is incapable of inhibiting replication of Py ori-DNA in vitro. We propose a new requirement for maintaining pRb phosphorylation in S phase, namely, to prevent deleterious effects on the cellular replication machinery.

Molecular epidemiology of human cancer

A challenging goal of molecular epidemiology is to identify an individual's risk of cancer. Molecular epidemiology integrates molecular biology, in vitro and in vivo laboratory models, biochemistry and epidemiology to infer individual cancer risk. Molecular dosimetry of carcinogen exposure is an important facet of molecular epidemiology and cancer risk assessment. Carcinogen macromolecular adduct levels, cytogenetic alterations and somatic cell mutations can be measured to determine the biologically effective doses of carcinogens. Molecular epidemiology also explores host cancer susceptibilities, such as carcinogen metabolism, DNA repair, and epigenetic and genetic alterations in tumor suppressor genes. p53 is a prototype tumor suppressor gene and is well suited for analysis of mutational spectrum in human cancer. The analyses of germ line and somatic mutation spectra of the p53 tumor suppressor gene provide important clues for cancer risk assessment in molecular epidemiology. For example, characteristic p53 mutation spectra have been associated with: dietary aflatoxin B1 exposure and hepatocellular carcinoma; sunlight exposure and skin carcinoma; and cigarette smoking and lung cancer. The mutation spectrum also reveals those p53 mutants that provide cells with a selective clonal expansion advantage during the multistep process of carcinogenesis. The p53 gene encodes a multifunctional protein involved in the cellular response to stress including DNA damage and hypoxia. Certain p53 mutants lose tumor suppressor activity and gain oncogenic activity, which is one explanation for the commonality of p53 mutations in human cancer. Molecular epidemiological results can be evaluated for causation by inference of the Bradford-Hill criteria, i.e., strength of association (consistency, specificity and temporality) and biological plausibility, which utilizes the "weight of the evidence principle."

p53 protein is a suppressor of papillomavirus DNA amplificational replication

p53 protein was able to block human and bovine papillomavirus DNA amplificational replication while not interfering with Epstein-Barr virus oriP once-per-cell cycle replication. Oligomerization, intact DNA-binding, replication protein A-binding, and proline-rich domains of the p53 protein were essential for efficient inhibition, while the N-terminal transcriptional activation and C-terminal regulatory domains were dispensable for the suppressor activity of the p53 protein. The inhibition of replication was caused neither by the downregulation of expression of the E1 and E2 proteins nor by cell cycle block or apoptosis. Our data suggest that the intrinsic activity of p53 to suppress amplificational replication of the papillomavirus origin may have an important role in the virus life cycle and in virus-cell interactions.

Gene expression during amphibian limb regeneration

Limb regeneration in adult urodeles is an important phenomenon that poses fundamental questions both in biology and in medicine. In this review, we focus on recent advances in the characterization of the regeneration blastema at cellular and molecular levels and on the current understanding of the molecular basis of limb regeneration and its relationship to development. In particular, we discuss (i) the spatiotemporal distribution of genes and gene products in the mesenchyme and wound epidermis of the regenerating limb, (ii) how growth is controlled in the regeneration blastema, and (iii) molecules that are likely to be involved in patterning the regenerating limb such as homeobox genes and retinoids.

Identification of positive and negative regulatory regions involved in regulating expression of the human cytomegalovirus UL94 late promoter: role of IE2-86 and cellular p53 in mediating negative regulatory function

The human cytomegalovirus (HCMV) UL94 gene product is a herpesvirus-common virion protein that is expressed with true late kinetics. To identify the important cis- and trans-acting factors which contribute to UL94 transcriptional regulation, we have cloned, sequenced, and analyzed UL94 promoter function by transient transfection analysis. Transfection of UL94 promoter-reporter gene constructs into permissive human fibroblasts or U373(MG) cells indicated that promoter activity was detected following infection with HCMV. Point mutations within a TATA-like element located upstream of the RNA start site significantly reduced UL94 promoter activity. Deletion mutagenesis of the promoter indicated that a positive regulatory element (PRE) was likely to exist downstream of the UL94 mRNA start site, while a negative regulatory element (NRE) was present upstream of the TATA box. At late times of infection, the PRE appeared to have a dominant effect over the NRE to stimulate maximum levels of UL94 promoter activity, while at earlier times of infection, no activity associated with the PRE could be detected. The NRE, however, appeared to cause constitutive down-regulation of UL94 promoter activity. Binding sites for the cellular p53 protein located within the NRE appeared to contribute to NRE function, and NRE function could be recapitulated in cotransfection assays by concomitant expression of p53 and HCMV IE2-86 protein. Our results suggest a novel mechanism by which the cellular protein p53, which is involved in both transcriptional regulation and progression of cellular DNA synthesis, plays a central role in the regulation of a viral promoter which is not activated prior the onset of viral DNA replication.

Assembly of complete, functionally active herpes simplex virus DNA replication compartments and recruitment of associated viral and cellular proteins in transient cotransfection assays

Early during the herpes simplex virus (HSV) lytic cycle or in the presence of DNA synthesis inhibitors, core viral replication machinery proteins accumulate in intranuclear speckled punctate prereplicative foci, some of which colocalize with numerous sites of host cellular DNA synthesis initiation known as replisomes. At later times, in the absence of inhibitors, several globular or large irregularly shaped replication compartments are formed; these compartments also contain progeny viral DNA and incorporate the IE175(ICP4) transcription factor together with several cellular proteins involved in DNA replication and repair. In this study, we demonstrate that several forms of both prereplication foci and active viral replication compartments that display an appearance similar to that of the compartments in HSV-infected cells can be successfully assembled in transient assays in DNA-transfected cells receiving genes encoding all seven essential HSV replication fork proteins together with oriS target plasmid DNA. Furthermore, bromodeoxyuridine (BrdU)-pulse-labeled DNA synthesis initiation sites colocalized with the HSV single-stranded DNA-binding protein (SSB) in these replication compartments, implying that active viral DNA replication may be occurring. The assembly of complete HSV replication compartments and incorporation of BrdU were both abolished by treatment with phosphonoacetic acid (PAA) and by omission of any one of the seven viral replication proteins, UL5, UL8, UL9, UL42, UL52, SSB, and Pol, that are essential for viral DNA replication. Consistent with the fact that both HSV IE175 and IE63(ICP27) localize within replication compartments in HSV-infected cells, the assembled HSV replication compartments were also able to recruit both of these essential regulatory proteins. Blocking viral DNA synthesis with PAA, but not omission of oriS, prevented the association of IE175 with prereplication structures. The assembled HSV replication compartments also redistributed cotransfected cellular p53 into the viral replication compartments. However, the other two HSV immediate-early nuclear proteins IE110(ICP0) and IE68(ICP22) did not enter the replication compartments in either infected or transfected cells.

Complex formation between p53 and replication protein A inhibits the sequence-specific DNA binding of p53 and is regulated by single-stranded DNA

Human replication protein A (RP-A) (also known as human single-stranded DNA binding protein, or HSSB) is a multisubunit complex involved in both DNA replication and repair. Potentially important to both these functions, it is also capable of complex formation with the tumor suppressor protein p53. Here we show that although p53 is unable to prevent RP-A from associating with a range of single-stranded DNAs in solution, RP-A is able to strongly inhibit p53 from functioning as a sequence-specific DNA binding protein when the two proteins are complexed. This inhibition, in turn, can be regulated by the presence of various lengths of single-stranded DNAs, as RP-A, when bound to these single-stranded DNAs, is unable to interact with p53. Interestingly, the lengths of single-stranded DNA capable of relieving complex formation between the two proteins represent forms that might be introduced through repair and replicative events. Increasing p53 concentrations can also overcome the inhibition by steady-state levels of RP-A, potentially mimicking cellular points of balance. Finally, it has been shown previously that p53 can itself be stimulated for site-specific DNA binding when complexed through the C terminus with short single strands of DNA, and here we show that p53 stays bound to these short strands even after binding a physiologically relevant site. These results identify a potential dual role for single-stranded DNA in the regulation of DNA binding by p53 and give insights into the p53 response to DNA damage.

Redox state regulates binding of p53 to sequence-specific DNA, but not to non-specific or mismatched DNA

Redox modulation of wild-type p53 plays a role in sequence-specific DNA binding in vitro . Reduction produces a DNA-binding form of the protein while oxidation produces a non-DNA-binding form. Primer extension analysis reveals that increasing concentrations of reduced p53 result in enhanced protection of the consensus sequence, while increasing concentrations of oxidized p53 confer minimal protection of the consensus sequence. DNA binding by oxidized p53 is, therefore, not sequence-specific. In contrast, there is no observable difference in the binding of oxidized p53 and reduced p53 to double-stranded non-specific or mismatched DNA in gel mobility shift assays. Both forms of p53 bind equally well, suggesting that redox modulation of p53 does not play a role in its binding to non-specific or mismatched DNA. In view of the in vitro evidence that redox state influences the sequence-specific DNA-binding of p53, we have examined the effect of oxidative stress on the in vivo ability of p53 to bind to and transactivate PG13-CAT, a reporter construct containing multiple copies of the p53 consensus binding site linked to the chloramphenicol acetyltransferase gene. Hydrogen peroxide treatment of cells cotransfected with p53 results in a marked decrease in CAT activity, suggesting that oxidation of p53 decreases the ability of the protein to bind to consensus DNA and transactivate target genes in vivo.

Apoptosis in rheumatoid arthritis: p53 overexpression in rheumatoid arthritis synovium

DNA damage induces p53 tumor suppressor gene expression and protein production, which in turn facilitates DNA repair or apoptosis. Wild-type p53 protein has a short half-life, so it is rarely detected in non-neoplastic tissue. Because DNA fragmentation is abundant in the intimal lining in rheumatoid arthritis (RA) synovial tissue (ST) using in situ end-labeling (Firestein GS, Yeo M, Zvaifler NJ: Apoptosis in rheumatoid arthritis synovium. J Clin Invest 1995, 96:1631-1638), we assessed ST p53 expression. Immunohistochemical analysis of fixed RA synovium using antibody PAb 1801 showed prominent p53 staining in the cytoplasm and nuclei of intimal lining cells. Noninflammatory and osteoarthritis (OA) ST had significantly less p53 in the lining. These data were confirmed by Western blot analysis of ST extracts, with abundant p53 found in RA compared with OA. p53 expression in cultured fibroblast-like synoviocytes (FLS) was then examined. Flow cytometry on permeabilized cells showed that RA FLS constitutively express p53 protein. Western blots showed that RA FLS expressed significantly more p53 than either OA FLS or dermal fibroblasts. Immunohistochemistry of FLS cultured in chamber slides localized the p53 to the cytoplasm of most resting FLS, with nuclear staining in only 10.7 +/- 2.4%. Exposure to hydrogen peroxide for increased nuclear staining to 70.7 +/- 12.8% after 8 hours (P = 0.003). These data indicate that p53 is overexpressed in RA ST in the intimal lining, which is the primary site of DNA damage, and is constitutively expressed by FLS.

Analysis of genomic instability in Li-Fraumeni fibroblasts with germline p53 mutations

Germline p53 mutations are frequently observed in the normal DNA of cancer-prone patients with Li-Fraumeni syndrome (LFS). Fibroblasts from LFS patients develop chromosomal aberrations, loss of cell cycle control, and spontaneous immortalization. We transfected four different mutant p53 genes into human skin fibroblasts from normal donors with two copies of wild-type p53 (p53(wt/wt)). Each mutant p53 expression-plasmid induced genomic instability equivalent to that seen in LFS cells. To test the role of wild-type and mutant p53 alleles in DNA replication and fidelity in LFS cells, we analysed the replication of the SV40-based shuttle vector pZ189 in four types of cells. We used p53(wt/mut) and p53(mut/-) LFS fibroblasts, and p53(-/-) non-LFS cells. Replication of pZ189 in vivo was significantly reduced by the presence of a p53(wt) allele. To show that this was not just due to inhibition of the function of T-antigen in SV40-based replication, we constructed a shuttle vector, pZ402, that contains a mutation in SV40 T-antigen which blocks its ability to interact with p53. Replication of pZ402 in LFS cells was also reduced by the presence of p53(wt), indicating that p53 can inhibit replication by interacting with proteins within the cellular replication machinery. Replicative errors in this shuttle vector are detected as mutations in a marker gene, supF. In addition to supF mutations, we observed deletion of a portion of the SV40 T-antigen gene in 100% of replicated plasmid pZ189 mutants (supF-) from the p53(wt/mut) fibroblasts and in 88% of the supF mutants from the p53(mut/-) (amino acid 175 arg to his) LFS cells. In one cell strain of immortal LFS cells, P53(mut/-) , containing a p53 frameshift mutation at amino acid 184, pZ189 replication yielded very few of these deleted shuttle vector plasmids (15%). These large deletions were not detected in plasmids replicated in p53(-/-) non-LFS cells, Saos-2 cells. Replicated plasmids with a normal supF gene were never found to have this large deletion regardless of the cell from which they were derived. Because the supF gene is not in the same region of the shuttle vector as the T-antigen gene it appears that second, independent gene deletions are frequent when replicative errors in supF occur in cells with a mutant p53. We conclude, therefore, that p53(wt/mut) LFS cells contain an activity that promotes mutations. Such an activity, which is likely to be due to the p53(mut), could result in the high rate of chromosomal instability and allelic loss of the wild-type p53 observed as these cells spontaneously immortalize.

p53: puzzle and paradigm

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