Novel gain of function activity of p53 mutants: activation of the dUTPase gene expression leading to resistance to 5-fluorouracil

Drug resistance, predictive markers and pharmacogenomics in colorectal cancer

Resistance to chemotherapy limits the effectiveness of current cancer therapies, including those used to treat colorectal cancer, which is the second most common cause of cancer death in Europe and the United States. 5-Fluorouracil-based chemotherapy regimens are the standard treatment for colorectal cancer in both the adjuvant and advanced disease settings. Drug resistance is thought to cause treatment failure in over 90% of patients with metastatic cancer, while drug resistant micrometastic tumour cells may also reduce the impact of adjuvant chemotherapy treatment. The identification of panels of biomarkers that not only identify those patients most likely to benefit from chemotherapy treatment, but also which chemotherapies to use, would be a major advance. In this review, we describe molecular mechanisms of drug resistance that may be relevant to colorectal cancer. We also describe the results of predictive biomarker studies in this disease. Finally, we discuss how pharmacogenomics and other high through-put technologies may impact on the clinical management of colorectal cancer in the future.

Predicting the oncogenicity of missense mutations reported in the International Agency for Cancer Research (IARC) mutation database on p53

Many mutation databases, comprising thousands of reported mutations, are available. Often the clinical significance of the reported mutations is unknown. In this study we developed an algorithm that allows prediction of the clinical significance of missense mutations reported in a mutation database. Nonsense mutations are used as a referent group for this assessment. We used the International Association for Research on Cancer (IARC) mutation database on TP53 to implement the algorithm. First, on the basis of published data [Nachman MW, Crowell SL. 2000. Genetics 156:297-304], we ascribed mutation rates to every single nucleotide substitution (SNS) in the core domain of the TP53 gene. Second, for every possible SNS we computed the expected number of missense mutations, under the assumption that missense mutations are as oncogenic as nonsense ones. The natural logarithm of the ratio of the observed to the expected number of missense mutations (LR) was used as a quantitative measure of oncogenicity (i.e., the ability of a mutation to produce cancer). We estimated the relative oncogenicity of all missense mutations reported in the IARC p53 mutation database, and constructed a profile of oncogenicity of the missense mutations along the DNA-binding region of p53.

Signs of positive selection of somatic mutations in human cancers detected by EST sequence analysis

BACKGROUND

Carcinogenesis typically involves multiple somatic mutations in caretaker (DNA repair) and gatekeeper (tumor suppressors and oncogenes) genes. Analysis of mutation spectra of the tumor suppressor that is most commonly mutated in human cancers, p53, unexpectedly suggested that somatic evolution of the p53 gene during tumorigenesis is dominated by positive selection for gain of function. This conclusion is supported by accumulating experimental evidence of evolution of new functions of p53 in tumors. These findings prompted a genome-wide analysis of possible positive selection during tumor evolution.

METHODS

A comprehensive analysis of probable somatic mutations in the sequences of Expressed Sequence Tags (ESTs) from malignant tumors and normal tissues was performed in order to access the prevalence of positive selection in cancer evolution. For each EST, the numbers of synonymous and non-synonymous substitutions were calculated. In order to identify genes with a signature of positive selection in cancers, these numbers were compared to: i) expected numbers and ii) the numbers for the respective genes in the ESTs from normal tissues.

RESULTS

We identified 112 genes with a signature of positive selection in cancers, i.e., a significantly elevated ratio of non-synonymous to synonymous substitutions, in tumors as compared to 37 such genes in an approximately equal-sized EST collection from normal tissues. A substantial fraction of the tumor-specific positive-selection candidates have experimentally demonstrated or strongly predicted links to cancer.

CONCLUSION

The results of EST analysis should be interpreted with extreme caution given the noise introduced by sequencing errors and undetected polymorphisms. Furthermore, an inherent limitation of EST analysis is that multiple mutations amenable to statistical analysis can be detected only in relatively highly expressed genes. Nevertheless, the present results suggest that positive selection might affect a substantial number of genes during tumorigenic somatic evolution.

HAMLET triggers apoptosis but tumor cell death is independent of caspases, Bcl-2 and p53

HAMLET (Human alpha-lactalbumin Made Lethal to Tumor cells) triggers selective tumor cell death in vitro and limits tumor progression in vivo. Dying cells show features of apoptosis but it is not clear if the apoptotic response explains tumor cell death. This study examined the contribution of apoptosis to cell death in response to HAMLET. Apoptotic changes like caspase activation, phosphatidyl serine externalization, chromatin condensation were detected in HAMLET-treated tumor cells, but caspase inhibition or Bcl-2 over-expression did not prolong cell survival and the caspase response was Bcl-2 independent. HAMLET translocates to the nuclei and binds directly to chromatin, but the death response was unrelated to the p53 status of the tumor cells. p53 deletions or gain of function mutations did not influence the HAMLET sensitivity of tumor cells. Chromatin condensation was partly caspase dependent, but apoptosis-like marginalization of chromatin was also observed. The results show that tumor cell death in response to HAMLET is independent of caspases, p53 and Bcl-2 even though HAMLET activates an apoptotic response. The use of other cell death pathways allows HAMLET to successfully circumvent fundamental anti-apoptotic strategies that are present in many tumor cells.

Mutational hotspots in the TP53 gene and, possibly, other tumor suppressors evolve by positive selection

Background

The mutation spectra of the TP53 gene and other tumor suppressors contain multiple hotspots, i.e., sites of non-random, frequent mutation in tumors and/or the germline. The origin of the hotspots remains unclear, the general view being that they represent highly mutable nucleotide contexts which likely reflect effects of different endogenous and exogenous factors shaping the mutation process in specific tissues. The origin of hotspots is of major importance because it has been suggested that mutable contexts could be used to infer mechanisms of mutagenesis contributing to tumorigenesis.

Results

Here we apply three independent tests, accounting for non-uniform base compositions in synonymous and non-synonymous sites, to test whether the hotspots emerge via selection or due to mutational bias. All three tests consistently indicate that the hotspots in the TP53 gene evolve, primarily, via positive selection. The results were robust to the elimination of the highly mutable CpG dinucleotides. By contrast, only one, the least conservative test reveals the signature of positive selection in BRCA1, BRCA2, and p16. Elucidation of the origin of the hotspots in these genes requires more data on somatic mutations in tumors.

Conclusion

The results of this analysis seem to indicate that positive selection for gain-of-function in tumor suppressor genes is an important aspect of tumorigenesis, blurring the distinction between tumor suppressors and oncogenes.

Reviewers

This article was reviewed by Sandor Pongor, Christopher Lee and Mikhail Blagosklonny.

Linking uracil base excision repair and 5-fluorouracil toxicity in yeast

5-fluorouracil (5-FU) is a widely used anticancer drug that disrupts pyrimidine nucleotide pool balances and leads to uracil incorporation in DNA, which is then recognized and removed by the uracil base excision repair (BER) pathway. Using complementary biochemical and genetic approaches we have examined the role of uracil BER in the cell killing mechanism of 5-FU. A yeast strain lacking the enzyme uracil DNA glycosylase (Ung1), which excises uracil from the DNA backbone leaving an abasic site, showed significant protection against the toxic effects of 5-FU, a G1/S cell cycle arrest phenotype, and accumulated massive amounts of U/A base pairs in its genome (approximately 4% of T/A pairs were now U/A). A strain lacking the major abasic site endonuclease of Saccharomyces cerevisiae (Apn1) showed significantly increased sensitivity to 5-FU with G2/M arrest. Thus, efficient processing of abasic sites by this enzyme is protective against the toxic effects of 5-FU. However, contrary to expectations, the Apn1 deficient strain did not accumulate intact abasic sites, indicating that another repair pathway attempts to process these sites in the absence Apn1, but that this process has catastrophic effects on genome integrity. These findings suggest that new strategies for chemical intervention targeting BER could enhance the effectiveness of this widely used anticancer drug.

Effect of p53 Status and STAT1 on Chemotherapy-Induced, Fas-Mediated Apoptosis in Colorectal Cancer

We investigated the role of p53 and the signal transducer and activator of transcription 1 (STAT1) in regulating Fas-mediated apoptosis in response to chemotherapies used to treat colorectal cancer. We found that 5-fluorouracil (5-FU) and oxaliplatin only sensitized p53 wild-type (WT) colorectal cancer cell lines to Fas-mediated apoptosis. In contrast, irinotecan (CPT-11) and tomudex sensitized p53 WT, mutant, and null cells to Fas-mediated cell death. Furthermore, CPT-11 and tomudex, but not 5-FU or oxaliplatin, up-regulated Fas cell surface expression in a p53-independent manner. In addition, increased Fas cell surface expression in p53 mutant and null cell lines in response to CPT-11 and tomudex was accompanied by only a slight increase in total Fas mRNA and protein expression, suggesting that these agents trigger p53-independent trafficking of Fas to the plasma membrane. Treatment with CPT-11 or tomudex induced STAT1 phosphorylation (Ser727) in the p53-null HCT116 cell line but not the p53 WT cell line. Furthermore, STAT1-targeted small interfering RNA (siRNA) inhibited up-regulation of Fas cell surface expression in response to CPT-11 and tomudex in these cells. However, we found no evidence of altered Fas gene expression following siRNA-mediated down-regulation of STAT1 in drug-treated cells. This suggests that STAT1 regulates expression of gene(s) involved in cell surface trafficking of Fas in response to CPT-11 or tomudex. We conclude that CPT-11 and tomudex may be more effective than 5-FU and oxaliplatin in the treatment of p53 mutant colorectal cancer tumors by sensitizing them to Fas-mediated apoptosis in a STAT1-dependent manner.

Transcriptional activities of mutant p53: when mutations are more than a loss

The dominant oncogenic properties of mutant p53 have been recognized as a phenomenon associated with tumor progression a long time ago, even before it was realized that the major function of wild type p53 is that of a tumor suppressor. Recent advances in this fascinating area in tumor cell biology reveal that the community of mutant p53 proteins is comprised of proteins that are extremely diverse both structurally and functionally, and elicit a multitude of cellular responses that not only are entirely distinct from those mediated by wild type p53, but also vary among different mutant p53 proteins. Aberrant regulation of transcription is one of the mechanisms underlying the ability of some mutant p53 proteins to act as oncogenic factors. Systematic analyses of the transcriptional activities of mutant p53 suggest that not the loss of transcriptional activity as such, but alterations of target DNA selectivity may be the driving force of mutant p53 specific transcription underlying the growth-promoting effects of mutant p53. This article focuses on mechanistic aspects of mutp53 "gain-of-function" with the emphasis on possible mechanisms underlying transcriptional activation by mutp53.

Evaluation of the combined effect of p53 codon 72 polymorphism and hotspot mutations in response to anticancer drugs

Mutations in p53 are common events during carcinogenesis and have been suggested to affect sensitivity to chemotherapy. Recently, the common polymorphism at codon 72, resulting in either an arginine (72R) or proline (72P) residue, was shown to differentially affect the response to anticancer drugs. Here, we have generated isogenic lung cancer cell lines to evaluate the effect of six p53 hotspot mutations (R175H, G245S, R248W, R249S, R273H, and R282W) in conjunction with the codon 72 polymorphism, for their response to a variety of anticancer drugs, either alone or in combination. The data indicate that 72R mutations do not confer general resistance to cisplatin, etoposide, gemcitabine, vinblastine, and taxol. For doxorubicin, cells expressing 249-72R were more resistant than the 249-72P cells. Combined treatment with cisplatin + etoposide resulted in an additive effect in cells expressing most 72R and 72P mutations, except for the 175-72R cells which were refractory to combined treatment. However, combined treatment with cisplatin + gemcitabine resulted in the absence of an additive effect in cells expressing the 273-72R and 282-72R mutants, unlike their 72P counterparts. Nonetheless, all p53 mutants (72R or 72P) equally inhibited p73-mediated transcriptional activity in lung cancer cells, suggesting that the selective resistance conferred by some 72R mutants to certain drugs is probably due to other p73-independent effects of these mutants. Together, the data show that the status of codon 72 polymorphism and p53 mutations can be used as a means for prediction of treatment response, although variables for each cancer type requires detailed evaluation.

Gain of function of a p53 hot spot mutation in a mouse model of Li-Fraumeni syndrome

Individuals with Li-Fraumeni syndrome carry inherited mutations in the p53 tumor suppressor gene and are predisposed to tumor development. To examine the mechanistic nature of these p53 missense mutations, we generated mice harboring a G-to-A substitution at nucleotide 515 of p53 (p53+/515A) corresponding to the p53R175H hot spot mutation in human cancers. Although p53+/515A mice display a similar tumor spectrum and survival curve as p53+/- mice, tumors from p53+/515A mice metastasized with high frequency. Correspondingly, the embryonic fibroblasts from the p53515A/515A mutant mice displayed enhanced cell proliferation, DNA synthesis, and transformation potential. The disruption of p63 and p73 in p53-/- cells increased transformation capacity and reinitiated DNA synthesis to levels observed in p53515A/515A cells. Additionally, p63 and p73 were functionally inactivated in p53515A cells. These results provide in vivo validation for the gain-of-function properties of certain p53 missense mutations and suggest a mechanistic basis for these phenotypes.

Molecular mechanisms of drug resistance

Resistance to chemotherapy limits the effectiveness of anti-cancer drug treatment. Tumours may be intrinsically drug-resistant or develop resistance to chemotherapy during treatment. Acquired resistance is a particular problem, as tumours not only become resistant to the drugs originally used to treat them, but may also become cross-resistant to other drugs with different mechanisms of action. Resistance to chemotherapy is believed to cause treatment failure in over 90% of patients with metastatic cancer, and resistant micrometastic tumour cells may also reduce the effectiveness of chemotherapy in the adjuvant setting. Clearly, if drug resistance could be overcome, the impact on survival would be highly significant. This review focuses on molecular mechanisms of drug resistance that operate to reduce drug sensitivity in cancer cells. Drug resistance can occur at many levels, including increased drug efflux, drug inactivation, alterations in drug target, processing of drug-induced damage, and evasion of apoptosis. Advances in DNA microarray and proteomic technology, and the ongoing development of new targeted therapies have opened up new opportunities to combat drug resistance. We are now able to characterize the signalling pathways involved in regulating tumour cell response to chemotherapy more completely than ever before. This will facilitate the future development of rational combined chemotherapy regimens, in which the newer targeted therapies are used in combination with cytotoxic drugs to enhance chemotherapy activity. The ability to predict response to chemotherapy and to modulate this response with targeted therapies will permit selection of the best treatment for individual patients.

Immunohistochemical expression of 14-3-3 sigma protein in various histological subtypes of uterine cervical cancers

14-3-3 sigma (sigma) has been a major G2/M checkpoint control gene and has demonstrated that its inactivation in various cancers occurs mostly by epigenetic hypermethylation, not by genetic change. In order to confirm 14-3-3sigma protein expression together with p16 and p53 in cervical cancers, immunohistochemistry was performed using various histological subtypes of cervical cancers and dysplasia. Strong and diffuse immunoreactivity for 14-3-3sigma was uniformly observed in all the cervical dysplasia (17/17) and squamous cell carcinomas (29/29) including human papillomavirus (HPV)-negative cases. Even in adenosquamous carcinomas and adenocarcinomas of the cervix, immunohistochemical expression of 14-3-3sigma was shown with relatively high frequency (13/15, 87% and 22/27, 81%). In the in situ hybridization study, mRNA of 14-3-3sigma was expressed in six of eight immunohistochemical-negative cases. Therefore, the undetectable expression of 14-3-3sigma protein in cervical cancers might, at least in part, be due to a proteolysis not epigenetic hypermethylation. It is of interest that cancers without 14-3-3sigma expression were predominantly those lacking HPV DNA, and that there were no cases with concomitant inactivation of 14-3-3sigma and p16 in the present study. These observations are consistent with the hypothesis that inactivation of either 14-3-3sigma or p16 has an effect equivalent to the expression of E6 and E7 oncoproteins of HPV.

Combination of 5-fluorouracil and N1,N11-diethylnorspermine markedly activates spermidine/spermine N1-acetyltransferase expression, depletes polyamines, and synergistically induces apoptosis in colon carcinoma cells

The thymidylate synthase inhibitor 5-fluorouracil (5-FU) is used widely for chemotherapy of colorectal carcinoma. Recent studies showed that 5-FU affects polyamine metabolism in colon carcinoma cells. We therefore examined whether combinations of 5-FU with drugs that specifically target polyamine metabolism, i.e. N1,N11-diethylnorspermine (DENSPM) or alpha-difluoromethylornithine (DFMO), have synergistic effects in killing HCT116 colon carcinoma cells with wild-type or absent p53. Our results showed that simultaneous 5-FU and DENSPM, a spermine analogue, synergistically increased transcript levels of the polyamine catabolism enzyme spermidine/spermine N1-acetyltransferase, depleted spermine and spermidine, increased acetylated spermidine, and produced synergistic tumor cell apoptosis in both p53 wild-type and p53-null variants. By contrast, simultaneous combination of 5-FU with DFMO, an inhibitor of the polyamine biosynthetic enzyme ornithine decarboxylase, depleted putrescine but did not produce synergistic cell killing. Some pre-treatment and post-treatment regimens of DENSPM and DFMO were antagonistic to 5-FU depending on cellular p53 status. Protein and transcriptome expression analysis showed that combined 5-FU and DENSPM treatment activated caspase 9, but not caspase 3, and significantly suppressed NADH dehydrogenases and cytochrome c oxidases, consistent with the observed increase in hydrogen peroxide, loss of mitochondrial membrane potential, and release of cytochrome c. Our findings demonstrate the importance of the polyamine pathway in 5-FU effects and suggest that the combination of 5-FU with DENSPM has potential for development as therapy for colorectal carcinoma.

No anti-apoptotic effects of single copies of mutant p53 genes in drug-treated tumor cells

Some mutant forms of the p53 tumor suppressor have been documented to exert novel oncogenic functions including the increase of tumorigenicity, metastatic potential, genomic instability and therapy resistance of tumor cells. The latter has been suggested to be caused, primarily, by inhibition of apoptosis and, in part, through the activation of genes by mutant p53 whose products can counteract drug activities. Recently described in this context was the dUTPase, which may confer resistance to fluoropyrimidine drugs such as 5-fluorouracil (5-FU). We report here findings that call in question the existence of a direct anti-apoptotic effect of mutant p53. Wild-type p53-negative human fibroblasts, and Saos-2, H1299 and HCT116 tumor cells, treated with adriamycin, etoposide, cisplatin or 5-FU, failed to show apoptosis resistance when retrovirally bulk-infected to express the p53 mutants 175H or 273H at levels observed in naturally mutant p53-producing tumor cells. Furthermore, dUTPase gene expression was not stimulated by mutant p53, but instead by cellular events that involve DNA synthesis. We interpret the combined available data to suggest that much of the anti-apoptotic effect of mutant p53 is indirect and secondary to DNA-damaging and/or repair-interfering effects of these proteins.

Structural Insights into the Catalytic Mechanism of Phosphate Ester Hydrolysis by dUTPase

dUTPase is essential to keep uracil out of DNA. Crystal structures of substrate (dUTP and alpha,beta-imino-dUTP) and product complexes of wild type and mutant dUTPases were determined to reveal how an enzyme responsible for DNA integrity functions. A kinetic analysis of wild type and mutant dUTPases was performed to obtain relevant mechanistic information in solution. Substrate hydrolysis is shown to be initiated via in-line nucleophile attack of a water molecule oriented by an activating conserved aspartate residue. Substrate binding in a catalytically competent conformation is achieved by (i) multiple interactions of the triphosphate moiety with catalysis-assisting Mg2+, (ii) a concerted motion of residues from three conserved enzyme motifs as compared with the apoenzyme, and (iii) an intricate hydrogen-bonding network that includes several water molecules in the active site. Results provide an understanding for the catalytic role of conserved residues in dUTPases.

Mutation hotspots in the p53 gene in tumors of different origin: correlation with evolutionary conservation and signs of positive selection

We present a classification analysis of the mutation spectra of the p53 gene and construct maps of hotspots for the germline (Li-Fraumein syndrome), different types of tumors and their derived cell lines. While spectra from solid tumors share common hotspots with the germline spectrum, they also contain unique sets of somatic hotspots that are not observed in the germline. All these hotspots correspond to amino acid replacements in the DNA-binding interface of p53. The mutation spectra of lymphomas and cell lines derived from lymphomas and lung cancers contained few hotspots compared to solid tumors. Thus, the distribution of hotspots in the p53 gene appears to depend on the tumor type and cell growth conditions; this specificity is missed by the bulk hotspot analysis. A negative correlation was detected between the amino acid replacement propensity in tumors and evolutionary variability: the hotspots are located in the positions that are highly conserved in p53 and its paralogs, p63 and p73. In all the mutation spectra, substitutions leading to amino acid replacements strongly dominate over silent substitutions, indicating that functional sites evolving under strong purifying selection are subject to intensive positive selection in p53-dependent tumors. These results are compatible with the gain-of-function concept of the role of p53 in tumorigenesis.

p53 mutations are common in human papillomavirus type 38-positive non-melanoma skin cancers

In cervical cells, the E6 protein of the oncogenic human papillomavirus (HPV) types inactivates p53, promoting its degradation. Consequently, mutations of the p53 gene are rarely seen in these cancers. Our recent data indicate that the cutaneous HPV38 is involved in skin carcinogenesis. In this study, we have determined the presence of HPV38 and the status of p53 gene in 32 non-melanoma skin cancers. We found that p53 gene is frequently mutated in HPV38-positive skin cancers and that HPV38 E6 does not promote p53 degradation. Thus, different mechanisms appear to be involved in the development of HPV-positive cervical and skin cancers.

Discrimination of single amino acid mutations of the p53 protein by means of deterministic singularities of recurrence quantification analysis

p53 is mutated in roughly 50% of all human tumors, predominantly in the DNA-binding domain codons. Structural, biochemical, and functional studies have reported that the different p53 mutants possess a broad range of behaviors that include the elimination of the tumor-suppression function of wild-type protein, the acquisition of dominant-negative function over the wild-type form, and the establishment of gain-of-function activities. The contribution of each of these types of mutations to tumor progression, grade of malignancy, and response to anticancer treatments has been so far analyzed only for a few "hot-spots." In an attempt to identify new approaches to systematically characterize the complete spectrum of p53 mutations, we applied recurrence quantification analysis (RQA), a non-linear signal analysis technique, to p53 primary structure. Moving from the study of the p53 hydrophobicity pattern, which revealed important similarities with the singular deterministic structuring of prions, we could statistically discriminate, on a pure amino acid sequence basis, between experimentally characterized DNA-contact defective and conformational p53 mutants with a very high percentage of success. This result indicates that RQA is a mathematical tool particularly advantageous for the development of a database of p53 mutations that integrates epidemiological data with structural and functional categorizations.

Developmental regulation of dUTPase in Drosophila melanogaster

dUTPase prevents uracil incorporation into DNA by strict regulation of the cellular dUTP:dTTP ratio. Lack of the enzyme initiates thymineless cell death, prompting studies on enzyme regulation. We investigated expression pattern and localization of Drosophila dUTPase. Similarly to human, two isoforms of the fly enzyme were identified at both mRNA and protein levels. During larval stages, a drastic decrease of dUTPase expression was demonstrated at the protein level. In contrast, dUTPase mRNAs display constitutive character throughout development. A putative nuclear localization signal was identified in one of the two isoforms. However, immunohistochemistry of ovaries and embryos did not show a clear correlation between the presence of this signal and subcellular localization of the protein, suggesting that the latter may be perturbed by additional factors. Results are in agreement with a multilevel regulation of dUTPase in the Drosophila proteome, possibly involving several interacting protein partners of the enzyme. Using independent approaches, the existence of such macromolecular partners was verified.

Altered active site flexibility and a structural metal-binding site in eukaryotic dUTPase: kinetic characterization, folding, and crystallographic studies of the homotrimeric Drosophila enzyme

dUTPase is responsible for preventive DNA repair via exclusion of uracil. Developmental regulation of the Drosophila enzyme is suggested to be involved in thymine-less apoptosis. Here we show that in addition to conserved dUTPase sequence motifs, the gene of Drosophila enzyme codes for a unique Ala-Pro-rich segment. Kinetic and structural analyses of the recombinant protein and a truncation mutant show that the Ala-Pro segment is flexible and has no regulatory role in vitro. The homotrimer enzyme unfolds reversibly as a trimeric entity with a melting temperature of 54 degrees C, 23 degrees C lower than Escherichia coli dUTPase. In contrast to the bacterial enzyme, Mg(2+) binding modulates conformation of fly dUTPase, as identified by spectroscopy and by increment in melting temperature. A single well folded, but inactive, homotrimeric core domain is generated through three distinct steps of limited trypsinolysis. In fly, but not in bacterial dUTPase, binding of the product dUMP induces protection against proteolysis at the tryptic site reflecting formation of the catalytically competent closed conformer. Crystallographic analysis argues for the presence of a stable monomer of Drosophila dUTPase in crystal phase. The significant differences between prototypes of eukaryotic and prokaryotic dUTPases with respect to conformational flexibility of the active site, substrate specificity, metal ion binding, and oligomerization in the crystal phase are consistent with alteration of the catalytic mechanism and hydropathy of subunit interfaces.

p53 family members and chemoresistance in cancer: what we know and what we need to know

Resistance to chemotherapy remains a major obstacle to the successful management of many human cancers. Numerous genetic and epigenetic changes in the cancer cell may contribute to drug resistance. However, with the recognition of important roles for both p53 and its more recently described paralog p73 in mediating the activity of anti-cancer drugs, there has been increasing recognition that cellular resistance to such agents can and does arise through failure of p53 family member signalling. Abrogation of function in p53 occurs through point mutations which abolish DNA binding or by dominant negative inhibition by variants of p73 lacking the N-terminal transactivation domain. Conversely, the function of full-length transactivation-competent (TA)p73 variants expressing the transactivation domain is itself subject to trans-dominant inhibition by certain p53 mutants and, in some cancers, by transcriptional silencing. The specificity of target gene activation by p53 and TAp73 is modulated by transcriptional co-activators which thereby act as response modifiers. Changes in the activity/expression of co-activators may therefore cause an altered cellular response to p53/p73 activation. Despite these advances in understanding how cells respond to DNA damage in vitro, and how this is affected by molecular genetic changes which affect p53 family member signalling, the contribution of these to in vivo drug resistance has not been definitively established. Our major task now is to determine how these changes operate individually and collectively in vivo to produce the phenotype of clinical drug resistance, and how we can translate this knowledge into clinically useful strategies to improve the outcome of chemotherapy.

Elevated levels of prostate-specific antigen (PSA) in prostate cancer cells expressing mutant p53 is associated with tumor metastasis

The underlying basis for rising levels of prostate-specific antigen (PSA) in prostate cancer is not fully understood, but attention has turned to the possibility that loss of normal p53 function might be directly involved. We have investigated the relationship between p53 function and PSA expression using in vitro and in vivo approaches. Three prostate cancer-derived p53 mutants (F134L, M237L, R273H) were introduced into LNCaP prostate cancer cells and stable transfectants established. Expression of mutant p53 was demonstrated by Western blot analysis, inactivation of wtp53 function, and a loss of p53-dependent responses to DNA damage induced by UV-irradiation and cisplatin. Levels of PSA mRNA and secreted protein were determined by RT-PCR and Western blotting, respectively. Serine protease activity was assessed using an esterase assay. In vivo effects of mutant p53 expression were examined after orthotopic implantation into prostates of nude mice. Expression of all p53 mutants was associated with elevated PSA mRNA and secreted PSA protein. In a representative line, mutant p53 was also associated with increased PSA protease-like activity compared with a control line expressing wildtype p53. Overall PSA levels, and PSA levels in serum from mice bearing tumors derived from cells expressing mutant p53, were increased compared with levels in mice bearing tumors derived from control cells. In addition, the tumors derived from cells with mutant p53 had increased vascularization and induced lymph node metastases. These data provide in vitro and in vivo support for the notion that p53 mutations directly contribute to increased levels of serum PSA, and are associated with more aggressive tumors.

Tumor suppressor p53 and its homologue p73alpha affect cell migration

The p53 tumor suppressor plays a central role in the negative control of growth and survival of abnormal cells. Previously we demonstrated that in addition to these functions, p53 expression affects cell morphology and lamellar activity of the cell edge (Alexandrova, A., Ivanov, A., Chumakov, P. M., Kopnin, P. B., and Vasiliev, J. M. (2000) Oncogene 19, 5826-5830). In the present work we studied the effects of p53 and its homologue p73alpha on cell migration. We found that loss of p53 function correlated with decreased cell migration that was analyzed by in vitro wound closure test and Boyden chamber assay. The decreased motility of p53-deficient cells was observed in different cell contexts: human foreskin fibroblasts (BJ), human colon and lung carcinoma cell lines (HCT116 and H1299, respectively), as well as mouse normal fibroblasts from lung and spleen, peritoneal macrophages, and keratinocytes. On the other hand, overexpression of the p53 family member p73alpha stimulated cell migration. Changes in cell migration correlated directly with transcription activation induced by p53 or p73alpha. Noteworthy, p53 modulated cell motility in the absence of stress. The effect of p53 and p73alpha on cell migration was mediated through the activity of the phosphatidylinositol 3-kinase/Rac1 pathway. This p53/p73 function was mainly associated with some modulation of intracellular signaling rather than with stimulation of production of secreted motogenic factors. The identified novel activity of the p53 family members might be involved in regulation of embryogenesis, wound healing, or inflammatory response.

Resistance to DNA-damaging agents is discordant from experimental metastatic capacity in MEF ras-transformants-expressing gain of function MTp53

Tumor cells can acquire aggressive phenotypes secondary to the loss of expression of the wild-type p53 (WTp53) protein or by the gain of function for selected mutant p53 (MTp53) proteins. However, it is unclear as to whether the development of aggressive phenotypes is inter-related. Herein we report the radiosensitivity, chemosensitivity, and in vivo growth characteristics of isogenic p53(-/-) MEF ras-transformants that variably express an MTp53 protein. Initial experiments revealed significant clonal heterogeneity with respect to cellular sensitivity to DNA-damaging agents (i.e. ionizing radiation, ultraviolet radiation, cis-platinum, and methotrexate) within subclones of a pre-existing p53(-/-) MEF cell population. Moreover, this differential sensitivity was also observed within subclones of p53(-/-) MEF cells transformed with an activated ras allele, suggesting that secondary genetic events and clonal selection, but not cellular transformation per se, may drive the resistance patterns for certain null-p53 tumors. In contrast, uniform resistance was observed following the additional transfection of an MTp53 allele (MTp53pro193) into p53(-/-) MEF transformants and p53(-/-) DP-16 Friend erythroleukemia cells, consistent with a gain of MTp53 function for this allele. Relative tumor growth rate and experimental metastatic ability was not enhanced by MTp53pro193 expression. Our results support the concept that gain of MTp53pro193 function leads to the selection of dominant clones, which may exhibit cellular resistance following cancer therapy.