p53-oriented cancer therapies: current progress

Resistance and gain-of-resistance phenotypes in cancers harboring wild-type p53

Chemotherapy is the bedrock for the clinical management of cancer, and the tumor suppressor p53 has a central role in this therapeutic modality. This protein facilitates favorable antitumor drug response through a variety of key cellular functions, including cell cycle arrest, senescence, and apoptosis. These functions essentially cease once p53 becomes mutated, as occurs in ∼50% of cancers, and some p53 mutants even exhibit gain-of-function effects, which lead to greater drug resistance. However, it is becoming increasingly evident that resistance is also seen in cancers harboring wild-type p53. In this review, we discuss how wild-type p53 is inactivated to render cells resistant to antitumor drugs. This may occur through various mechanisms, including an increase in proteasomal degradation, defects in post-translational modification, and downstream defects in p53 target genes. We also consider evidence that the resistance seen in wild-type p53 cancers can be substantially greater than that seen in mutant p53 cancers, and this poses a far greater challenge for efforts to design strategies that increase drug response in resistant cancers already primed with wild-type p53. Because the mechanisms contributing to this wild-type p53 "gain-of-resistance" phenotype are largely unknown, a concerted research effort is needed to identify the underlying basis for the occurrence of this phenotype and, in parallel, to explore the possibility that the phenotype may be a product of wild-type p53 gain-of-function effects. Such studies are essential to lay the foundation for a rational therapeutic approach in the treatment of resistant wild-type p53 cancers.

Adenovirally mediated p53 overexpression diversely influence the cell cycle of HEp-2 and CAL 27 cell lines upon cisplatin and methotrexate treatment

PURPOSE

p53 gene plays a crucial role in the response to therapy. Since it is inactivated in the majority of human cancers, it is strongly believed that the p53 mutations confer resistance to therapeutics. In this paper we analyzed the influence of two mechanistically diverse antitumor agents--cisplatin and methotrexate on the proliferation and cell cycle of two head and neck squamous cancer cell lines HEp-2 (wild type p53 gene, but HPV 18/E6-inactivated protein) and CAL 27 (mutated p53 gene), along with the influence of adenovirally mediated p53 overexpression in modulation of cisplatin and methoterexate effects, whereby subtoxic vector/compound concentrations were employed.

METHODS

p53 gene was introduced into tumor cells using adenoviral vector (AdCMV-p53). The cell cycle perturbations were measured by two parameter flow cytometry. The expression of p53, p21(WAF1/CIP1) and cyclin B1 proteins was examined using immunocytochemistry and western blot methods.

RESULTS

In CAL 27 cells overexpression of p53 completely abrogated high S phase content observed in methotrexate-treated cells into a G1 and slight G2 arrest, while it sustained G2 arrest of the cells treated with cisplatin, along with the reduction of DNA synthesis and cyclin B1 expression. On the other hand, in HEp-2 cell line p53 overexpression slightly slowed down the progression through S phase in cells treated with methotrexate, decreased the cyclin B1 expression only after 24 h, and failed to sustain the G2 arrest after treatment with cisplatin alone. Instead, it increased the population of S phase cells that were not actively synthesizing DNA, sustained cyclin B1 expression and allowed the G2 cells to progress through mitosis.

CONCLUSIONS

This study demonstrates that adenovirally mediated p53 overexpression at sub-cytotoxic levels enhanced the activity of low doses of cisplatin and methotrexate in HEp-2 and CAL 27 cells through changes in the cell cycle. However, the mechanisms of these effects differ depending on the genetic context and on the chemotherapeutics' modality of action.

Model-driven approaches for in vitro combination therapy using ONYX-015 replicating oncolytic adenovirus

Replicating genetically modified adenoviruses have shown promise as a new treatment approach against cancer. Recombinant adenoviruses replicate only in cancer cells which contain certain mutations, such as the loss of functional p53, as is the case in the virus ONYX-015. The successful entry of the viral particle into target cells is strongly dependent on the presence of the main receptor for adenovirus, the coxsackie- and adenovirus receptor (CAR). This receptor is frequently down-regulated in highly malignant cells, rendering this population less vulnerable to viral attack. It has been shown that the use of MEK inhibitors can up-regulate CAR expression, resulting in enhanced adenovirus entry into the cells. However, inhibition of MEK results in G1 cell cycle arrest, rendering infected cells temporarily unable to produce virus. This forces a tradeoff. While drug mediated up-regulation of CAR enhances virus entry into cancer cells, the consequent cell cycle arrest inhibits production of new virus particles and the replication of the virus. Optimal control-based schedules of MEK inhibitor application should increase the efficacy of this treatment, maximizing the overall tumor toxicity by exploiting the dynamics of CAR expression and viral production. We introduce a mathematical model of these dynamics and show simple optimal control based strategies which motivate this approach.

Nanoparticle-mediated wild-type p53 gene delivery results in sustained antiproliferative activity in breast cancer cells

Gene expression with nonviral vectors is usually transient and lasts for only a few days. Therefore, repeated injection of the expression vector is required to maintain a therapeutic protein concentration in the target tissue. Biodegradable nanoparticles (approximately 200 nm diameter) formulated using a biocompatible polymer, poly(D,L-lactide-co-glycolide) (PLGA), have the potential for sustained gene delivery. Our hypothesis is that nanoparticle-mediated gene delivery would result in sustained gene expression, and hence better efficacy with a therapeutic gene. In this study, we have determined the antiproliferative activity of wild-type (wt) p53 gene-loaded nanoparticles in a breast cancer cell line. Nanoparticles containing plasmid DNA were formulated using a multiple-emulsion-solvent evaporation technique. To understand the mechanism of sustained gene expression with nanoparticles, we monitored the intracellular trafficking of both the nanoparticles and the nanoparticle-entrapped DNA, and also determined p53 mRNA levels over a period of time. Cells transfected with wt-p53 DNA-loaded nanoparticles demonstrated a sustained and significantly greater antiproliferative effect than those with naked wt-p53 DNA or wt-p53 DNA complexed with a commercially available transfecting agent (Lipofectamine). Cells transfected with wt-p53 DNA-loaded nanoparticles demonstrated sustained p53 mRNA levels compared to cells which were transfected with naked wt-p53 DNA or the wt-p53 DNA-Lipofectamine complex, thus explaining the sustained antiproliferative activity of nanoparticles. Studies with fluorescently labeled DNA using confocal microscopy and quantitative analyses using a microplate reader demonstrated sustained intracellular localization of DNA with nanoparticles, suggesting the slow release of DNA from nanoparticles localized inside the cells. Cells which were transfected with naked DNA demonstrated transient intracellular DNA retention. In conclusion, nanoparticle-mediated wt-p53 gene delivery results in sustained antiproliferative activity, which could be therapeutically beneficial in cancer treatment.

The prosurvival activity of p53 protects cells from UV-induced apoptosis by inhibiting c-Jun NH2-terminal kinase activity and mitochondrial death signaling

The cytoprotective function of p53 recently has been exploited as a therapeutic advantage for cancer prevention; agents activating the prosurvival activity of p53 are shown to prevent UV-induced damages. To explore the mechanisms of p53-mediated protection from UV-induced apoptosis, we have established stable clones of H1299 lung carcinoma cells expressing a temperature-sensitive p53 mutant, tsp53(V143A). At the permissive temperature of 32 degrees C, the tsp53(V143A)-expressing cells were arrested in G(1) phase without the occurrence of apoptosis; consistent with this is the preferential induction of genes related to growth arrest and DNA damage repair. Previous expression of functional tsp53(V143A) for > or =18 hours inhibited the release of proapoptotic molecules from mitochondria and protected the cells from UV-induced apoptosis; moreover, it suppressed the activation of c-Jun NH(2)-terminal kinase (JNK) signaling and relieved the effect of UV on p53 target gene activation. p53 associated with JNK and inhibited its kinase activity. Using the p53-null H1299 cells, we showed that inhibition of JNK blocked the UV-elicited mitochondrial death signaling and caspase activation. Our results suggest that the ability of p53 to bind and inactivate JNK, together with the activation of the p53 target genes related to cell cycle arrest and DNA damage repair, is responsible for its protection of cells against UV-induced apoptosis.

Clinical applications of genetics in sporadic cancers

OBJECTIVES

To describe how genetic information shapes our understanding of carcinogenesis; how genetic information influences recommendations for cancer screening, prevention, diagnosis, and treatment; and how genetic information may affect the prognosis of patients with cancer and the monitoring of anticancer treatment.

DATA SOURCES

Medical and nursing literature, textbooks, Internet websites.

CONCLUSION

The Human Genome Project has taken oncology one step further toward accurate diagnosis and treatment of many forms of cancer. There are many genetic traits that can be associated with increased cancer risk, diagnosis, and selection of treatments.

IMPLICATIONS FOR NURSING PRACTICE

Oncology nursing practice is directly affected by the developments of medical genetics. The information gained can be used by nurses at all stages of the cancer continuum when administering these new therapies.

Gene therapy for thyroid cancer: current status and future prospects

Despite multimodality treatment for thyroid cancer, including surgical resection, radioiodine therapy, thyrotropin (TSH)-suppressive thyroxine treatment, and chemotherapy/radiotherapy, survival rates have not improved over the last decades. Therefore, development and evaluation of novel treatment strategies, including gene therapy, are urgently needed. A variety of gene therapy approaches have been evaluated for the treatment of follicular cell-derived and medullary thyroid cancer, including corrective gene therapy (p53 restoration, expression of a dominant negative RET mutant), cytoreductive gene therapy (suicide gene/prodrug strategy herpes simplex virus-thymidine kinase [HSV-tk]/ganciclovir, antiangiogenic therapy with endostatin) and immunomodulatory gene therapy (expression of interleukin (IL)-2 and IL-12). Furthermore, cloning of the sodium iodide symporter (NIS) gene has paved the way for the development of a novel cytoreductive gene therapy strategy based on NIS gene transfer followed by the application of radioiodine therapy ((131)I). NIS gene delivery into medullary and follicular cell-derived thyroid cancer cells has been shown to be capable of establishing or restoring radioiodine accumulation and might therefore represent an effective therapy for medullary and dedifferentiated thyroid tumors that lack iodide accumulating activity. The data summarized in this review article clearly demonstrate that the currently available strategies represent potentially curative novel therapeutic approaches for future gene therapy of thyroid cancer. The combination of different therapeutic genes has been demonstrated to be very useful to enhance therapeutic efficacy and seems to have a promising role at least as part of a multimodality approach for advanced thyroid cancer.

A global suppressor motif for p53 cancer mutants

The transcription factor and tumor suppressor protein p53 is frequently inactivated in human cancers. In many cases, p53 gene mutations result in high levels of inactive, full-length p53 protein with one amino acid change in the core domain that recognizes p53 DNA-binding sites. The ability to endow function to mutated p53 proteins would dramatically improve cancer therapy, because it would reactivate a central apoptotic pathway. By using genetic strategies and p53 assays in yeast and mammalian cells, we identified a global suppressor motif involving codons 235, 239, and 240. These intragenic suppressor mutations, either alone or in combination, restored function to 16 of 30 of the most common p53 cancer mutants tested. The 235-239-240 suppressor motif establishes that manipulation of a small region of the core domain is sufficient to activate a large number of p53 cancer mutants. Understanding the structural basis of the rescue mechanism will allow the pursuit of small compounds able to achieve a similar stabilization of p53 cancer mutants.

p21WAF1/CIP1 is more effective than p53 in growth suppression of mouse renal carcinoma cell line Renca in vitro and in vivo

PURPOSE

Although there are many controversial reports about the effect of p53 and p21(WAF1/CIP1) overexpression in different human tumor cells, the p53 gene is shown to be a more effective candidate for cancer gene therapy because of its more pronounced ability to induce apoptosis. In the present study, we present the effect of p53 and p21(WAF1/CIP1) overexpression on mouse renal carcinoma cells in vitro and in vivo.

METHODS

p53 and p21(WAF1/CIP1) genes were introduced into Renca cells using adenoviral vectors (Ad5CMV-p53 and Ad5CMV-p21). The induction of apoptosis was measured using Annexin V assay and DNA fragmentation analysis. The expression of proteins was examined using immunocytochemistry and Western blot methods. The ability of adenoviral vectors to inhibit tumorigenicity of Renca cells, as well as the growth of pre-established tumors was measured.

RESULTS

In vitro growth assays revealed higher growth suppression after Ad5CMV-p21 infection. Although both vectors induced apoptosis, Ad5CMV-p53 was slightly more efficient. In vivo studies in Balb/c mice, demonstrated that tumorigenicity was completely suppressed by Ad5CMV-p21. Besides this, Ad5CMV-p21 significantly inhibited the growth of established tumors, while Ad5CMV-p53 did not.

CONCLUSIONS

These data suggest that p21(WAF1/CIP1) is a more potent growth suppressor than p53 of mouse tumor cells Renca. The divergent responses of tumor cells to p21(WAF1/CIP1) overexpression could be due to various networks that differ between species.

Endogenous p21WAF1/CIP1 status predicts the response of human tumor cells to wild-type p53 and p21WAF1/CIP1 overexpression

Expression of exogenous wild-type (wt) p53 protein can suppress the growth and/or induce apoptosis in different tumor cells. The effect of exogenous p21(WAF1/CIP1) expression is more controversial: while it can induce apoptosis in some cells, it can protect against p53-mediated apoptosis in others. We used adenoviral vectors to introduce p53 and p21(WAF1/CIP1) genes into human tumor cell lines with different p53 and/or p21(WAF1/CIP1) status. The cell growth inhibition and the induction of apoptosis were measured. Overexpression of wt p53 induced more efficient growth inhibition and apoptosis in SW 620 (mutant p53) and HeLa (inactivated p53 protein) than in MCF-7 (wt p53) and CaCo-2 cell line, which was the most resistant to p53 overexpression despite the p53 mutation. Unlike HeLa and SW 620 cells, the basal p21 protein level was readily detected in CaCo-2 and MCF-7 cells. Overexpression of p21(WAF1/CIP1) gene induced somewhat less pronounced growth inhibition of all cell lines tested, but it also induced apoptosis in HeLa and SW 620 cells. These results suggest that the basal, but not the inducible, levels of p21(WAF1/CIP1) protein in tumor cells could protect from p53-mediated apoptosis. On the other hand, overexpression of p21(WAF1/CIP1) gene itself can induce apoptosis in cells with no basal p21(WAF1/CIP1) protein level. Possible mechanisms of the differential response to these genes are discussed.

7-Hydroxystaurosporine (UCN-01) preferentially sensitizes cells with a disrupted TP53 to gamma radiation in lung cancer cell lines

Mutations in TP53 occur in more than 50% of the lung cancer patients and are associated with an increased resistance to chemotherapy and radiotherapy. The human lung adenocarcinoma cell lines A549 and LXSN contain a wild-type TP53 and were growth arrested at both the G(1)- and G(2)-phase checkpoints after irradiation. However, a TP53-disrupted cell line, E6, was arrested only at the G(2)-phase checkpoint. UCN-01 (7-hydroxystaurosporine), a CHEK1 inhibitor that abrogates the G(2) block, has been reported to enhance radiation toxicity in human lymphoma and colon cancer cell lines. In this study, UCN-01 preferentially enhanced the radiosensitivity of the TP53-disrupted E6 cells compared to the TP53 wild-type cells. This effect was more pronounced in cells synchronized in early G(1) phase, where the E6 cells showed a higher resistance to radiation in the absence of drug. These results indicate that the combination of UCN-01 and radiation can more specifically target resistant TP53 mutated cancer cells and spare TP53 wild-type normal cells.

Mutations in apoptosis genes: a pathogenetic factor for human disease

Cell death by apoptosis is exerted by the coordinated action of many different gene products. Mutations in some of them, acting at different levels in the apoptosis process, have been identified as cause or contributing factor for human diseases. Defects in the transmembrane tumor necrosis factor receptor 1 (TNF-R1) lead to the development of familial periodic fever syndromes. Mutations in the homologous receptor Fas (also named CD95; Apo-1) are observed in malignant lymphomas, solid tumors and the autoimmune lymphoproliferative syndrome type I (ALPS I). A mutation in the ligand for Fas (Fas ligand; CD95 ligand, Apo-1 ligand), which induces apoptosis upon binding to Fas, was described in a patient with systemic lupus erythematodes and lymphadenopathy. Perforin, an other cytotoxic protein employed by T- and NK-cells for target cell killing, is mutated in chromosome 10 linked cases of familial hemophagocytic lymphohistiocytosis. Caspase 10, a representative of the caspase family of proteases, which plays a central role in the execution of apoptosis, is defect in autoimmune lymphoproliferative syndrome type II (ALPS II). The intracellular pro-apoptotic molecule bcl-10 is frequently mutated in mucosa-associated lymphoid tissue (MALT) lymphomas and various non-hematologic malignancies. The p53, an executioner of DNA damage triggered apoptosis, and Bax, a pro-apoptotic molecule with the ability to perturb mitochondrial membrane integrity, are frequently mutated in malignant neoplasms. Anti-apoptotic proteins like bcl-2, cellular-inhibitor of apoptosis protein 2 (c-IAP2) and neuronal apoptosis inhibitory protein 1 (NAIP1) are often altered in follicular lymphomas, MALT lymphomas and spinal muscular atrophy (SMA), respectively. This article reviews the current knowledge on mutations of apoptosis genes involved in the pathogenesis of human diseases and summarises the gradual transformation of discoveries in apoptosis research into benefits for the clinical management of diseases.

Novel human p53 mutations that are toxic to yeast can enhance transactivation of specific promoters and reactivate tumor p53 mutants

Since highly expressed human p53 can inhibit human and yeast cell growth, we predicted that p53 mutants could be generated with increased growth inhibition of the yeast Saccharomyces cerevisiae and that these would be useful for characterizing p53 functions and tumor p53 mutants. A random mutagenesis screen led to the isolation of mutations in the DNA binding domain that result in p53 being lethal even at moderate expression levels in yeast. Three independent mutants had an alanine change at the evolutionary invariant V122 in the L1 loop. The other toxic mutations affected codons 277 (C277R, C277W) and 279 (G279R). This latter amino acid change was also reported in tumors, while all the other mutations are novel. A recently developed rheostatable GALI promoter system that provides graded increases in expression of p53 was used to examine the transactivation function of the toxic mutations when expression was greatly reduced and cells were viable. At low expression levels the toxic mutants lacked transactivation from a 3xRGC responsive element (RE). Surprisingly some exhibited enhanced transactivation with p21 and bax REs. The V122A mutant was able to re-activate transactivation of various p53 tumor mutants and retained growth inhibition when co-expressed with dominant-negative tumor mutations. Upon expression in human Saos-2 cells the V122A p53 mutant caused growth suppression, was capable of transactivation and exhibited higher than wild type activity with the bax promoter in luciferase assays. A non-functional p53 tumor mutant was partially reactivated by V122A for both transactivation and growth suppression. Thus, the screen for toxic p53 mutants in yeast can identify novel p53 variants that may be useful in dissecting p53 regulated cellular responses and in developing p53-based cancer therapies.

Transcriptional activation of TRADD mediates p53-independent radiation-induced apoptosis of glioma cells

Survival of patients with Glioblastoma Multiforme (GM), a highly malignant brain tumor, remains poor despite concerted efforts to improve therapy. The median survival of patients with GM has remained approximately 1 year regardless of the therapeutic approach. Since radiation therapy is the most effective adjuvant therapy for GM and nearly half of GM tumors harbor p53 mutations, we sought to identify genes that mediate p53-independent apoptosis of GM cells in response to ionizing radiation. Using broad-scale gene expression analysis we found that following radiation treatment, TRADD expression was induced in a uniquely radiosensitive GM cell line but not in radioresistant GM cell lines. TRADD over-expression killed GM cells and activated NF-kappa B. We found that blocking the TRADD-mediated pathway using a dominant-negative mutant of FADD (FADD-DN) enhanced radiation resistance of GM cells, as reflected in both susceptibility to apoptosis and clonogenic survival following irradiation. Conversely, stable expression of exogenous TRADD enhanced radiation-induced apoptosis of GM cell lines, reflecting the biological significance of TRADD regulation in p53-independent apoptosis. These findings generate interest in utilizing TRADD in gene therapy for GM tumors, particularly in light of its dual function of directly inducing rapid apoptosis and sensitizing GM cells to standard anti-neoplastic therapy.

Key targets for the execution of radiation-induced tumor cell apoptosis: the role of p53 and caspases

In many human hematologic and solid malignancies, intrinsic or acquired treatment resistance remains a major obstacle for successful cancer therapy. The molecular understanding of how tumor cells respond to chemotherapy and ionizing radiation is rapidly evolving. Induction of programmed cell death, apoptosis, is one important strategy for successful cancer therapy. This has been shown convincingly for oncogene-transformed normal cells as well as tumor cells of lymphoid origin. However, the relevance of apoptosis in solid human malignancies is less clear. Loss of apoptosis might be linked to specific mutations in the often tissue-specific apoptotic pathways due to aberrations in the stress-related signal transduction cascades. Restoration of a dysfunctional apoptotic program in cancer tissue where apoptosis has been identified as an important mechanism for tissue homeostasis is one rational approach for innovative cancer therapy. In this review, we focus on the relevance of the tumor suppressor p53 for apoptosis-induction and successful cancer therapy outlining the importance of an intact caspase machinery for apoptosis execution. Strategies are discussed to overcome treatment resistance and a high apoptotic threshold in human malignancies where apoptosis is the dominant mode of cell death and the status of p53 is an important determinant for apoptosis induction.

Human fibulin-4: analysis of its biosynthetic processing and mRNA expression in normal and tumour tissues

Here, we report the identification of a human orthologue of fibulin-4, along with analysis of its biosynthetic processing and mRNA expression levels in normal and tumour tissues. Comparative sequence analysis of fibulin-4 cDNAs revealed apparent polymorphisms in the signal sequence that could account for previously reported inefficient secretion in fibulin-4 transfectants. In vitro translation of fibulin-4 mRNA revealed the presence of full-length and truncated polypeptides, the latter apparently generated from an alternative translation initiation site. Since this polypeptide failed to incorporate into endoplasmic reticulum membrane preparations, it was concluded that it lacked a signal sequence and thus could represent an intracellular form of fibulin-4. Using fluorescence in situ hybridisation analysis, the human fibulin-4 gene was localised to chromosome 11q13, this region being syntenic to portions of mouse chromosomes 7 and 19. Considering the fact that translocations, amplifications and other rearrangements of the 11q13 region are associated with a variety of human cancers, the expression of human fibulin-4 was evaluated in a series of colon tumours. Reverse transcription-polymerase chain reaction analysis of RNA from paired human colon tumour and adjacent normal tissue biopsies showed that a significant proportion of tumours had approximately 2-7-fold increases in the level of fibulin-4 mRNA expression. Taken together, results reported here suggest that an intracellular form of fibulin-4 protein may exist and that dysregulated expression of the fibulin-4 gene is associated with human colon tumourigenesis.

p53 mutants exhibiting enhanced transcriptional activation and altered promoter selectivity are revealed using a sensitive, yeast-based functional assay

Changes in promoter specificity and binding affinity that may be associated with p53 mutations or post-translational modifications are useful in understanding p53 structure/function relationships and categorizing tumor mutations. We have exploited variable expression of human p53 in yeast to identify mutants with novel phenotypes that would correspond to altered promoter selectivity and affinity. The p53 cDNA regions coding for the DNA binding and tetramerization domains were subjected to random PCR mutagenesis and were cloned directly by recombination in yeast into a vector with a GAL1 promoter whose level of expression could be easily varied. p53 variants exhibiting higher than wild type levels of transactivation (supertrans) for the RGC responsive element were identified at low level of p53 protein expression. All the p53 mutants obtained with this screen were located in the DNA binding domain. Two out of 17 supertrans mutants have been found in tumors. Six mutations were in the L1 loop region between amino acids 115 and 124. The transactivation potential of a panel of supertrans p53 mutants on different promoters was evaluated using the p53 responsive elements, RGC, PIG3, p21 and bax. Although all mutants retained some activity with all promoters, we found different patterns of induction based on strength and promoter specificity. In particular none of the mutants was supertrans for the p21 responsive element. Interestingly, further analysis in yeast showed that the transactivation function could be retained even in the presence of dominant-negative p53 tumor mutations that could inhibit wild type p53. Five mutants were also characterized in human cells in terms of growth suppression and transactivation of various promoters. These novel supertrans p53 mutants may be useful in studies aimed at dissecting p53 downstream pathways, understanding specific interactions between p53 and the DNA, and could replace wild type p53 in cancer gene therapy protocols. The approach may also prove useful in identifying p53 tumor mutations.

Anchorage-independent growth of p53-knockout dermal fibroblasts is reversed by wild-type p53

BACKGROUND

p53 is a 393-residue nuclear phosphoprotein. Mutation of p53 occurs in over half of all human cancers and thus is a crucial step in the process of cell transformation and tumorigenesis. Since tumorigenesis is a multistep process, it generally requires the mutation of certain key oncogenes and/or tumor-suppressor genes. Using p53-deficient mice, we can investigate the p53-dependent mechanisms leading to tumorigenesis.

OBJECTIVE

To examine the unique anchorage-independent growth characteristics of dermal fibroblasts isolated from p53-deficient mice.

METHODS

The growth characteristics of highly confluent cultured dermal fibroblasts from wild-type (p53+/+) and p53-deficient (p53-/-) mice were compared by DNA fragmentation assay, colony formation in soft agar, and overexpression of a wild-type p53 transgene in p53-deficient cells.

RESULTS

p53-/- fibroblasts have a growth rate dramatically higher than p53+/+ cells and detach from plastic cultureware at high density. The detachment of p53-/- cells is not due to apoptosis. Furthermore, these cells have the capacity to grow in soft agar-a hallmark of cell transformation-and this anchorage-independent growth can be reversed by the introduction of a wild-type p53 transgene.

CONCLUSION

Dermal fibroblasts isolated from p53-deficient mice show anchorage-independent growth. Therefore, the absence of p53 is sufficient for the initiation of cell transformation in this cell type and establishes this model system as an excellent tool to dissect the molecular steps involved in oncogenesis.

Tp53 gene therapy: a key to modulating resistance to anticancer therapies?

Abnormalities in the p53 tumor suppressor have been identified in over 60% of human cancers. The status of p53 within tumor cells has been proposed to be one of the major determinants of the response to anticancer therapies. In this review we examine the relationship between functional p53 and sensitivity, or resistance, to chemotherapy and radiotherapy. We also discuss the potential of current gene-therapy approaches to restore functional p53 to tumors as a means of modulating the effects of radiation and chemotherapy.

Does p53 status influence tumor response to anticancer therapies?

Abnormalities in the tumor suppressor gene p53 have been identified in over 60% of human cancers. Since it plays such a pivotal role in cell growth regulation and apoptosis, the status of the p53 gene has been proposed as one of the major determinants of a tumor's response to anticancer therapies. In this review we examine the relationship between functional p53 and sensitivity/resistance to both chemotherapy and radiotherapy, and discuss the potential use of some of the current gene therapy approaches to restore functional p53 to tumors as a means of modulating the effects of radiation and chemotherapy.

Microbial pathogens and apoptotic anticancer therapy

In spite of tremendous efforts to control cancer, the mortality associated with this disease has been increasing in developed countries in the recent decades. Inadequate efficiency of existing therapeutic regimens and the rise of multi-drug resistant cancer cells are the main factors which require a broadening of investigations into novel anticancer strategies. Enhancement of apoptosis in tumours has been suggested recently as a new anticancer strategy. It targets the disruption of equilibrium between cell proliferation and cell death in tumours and suggests to restore it through the use of pharmacological agents or genetic approaches. Apoptotic therapy has attracted many groups of investigators and several companies have entered the race to develop the first generation of apoptotic anticancer agents. The review discusses the role that pathogenic microorganisms may have as the source of agents for apoptotic therapy.

Loss-of-function genetics in mammalian cells: the p53 tumor suppressor model

Using an improved system for the functional identification of active antisense fragments, we have isolated antisense fragments which inactivate the p53 tumour suppressor gene. These antisense fragments map in two small regions between nt 350 and 700 and nt 800 and 950 of the coding sequence. These antisense fragments appear to act by inhibition of p53 mRNA translation both in vivo and in vitro. Expression of these antisense fragments overcame the p53-induced growth arrest in a cell line which expresses a thermolabile mutant of p53 and extended the in vitro lifespan of primary mouse embryonic fibroblasts. Continued expression of the p53 antisense fragment contributed to immortalisation of primary mouse fibroblasts. Subsequent elimination of the antisense fragment in these immortalised cells led to restoration of p53 expression and growth arrest, indicating that immortal cells continuously require inactivation of p53. Expression of MDM2 or SV40 large T antigen, but not E7 nor oncogenic ras, overcomes the arrest induced by restoration of p53 expression. Functional inactivation of both p21 and bax (by overexpression of Bcl2), but not either alone, allowed some bypass of p53-induced growth arrest, indicating that multiple transcriptional targets of p53 may mediate its antiproliferative action. The ability to conditionally inactivate and subsequently restore normal gene function may be extremely valuable for genetic analysis of genes for which loss-of-function is involved in specific phenotypes.

MBP1: a novel mutant p53-specific protein partner with oncogenic properties

Using a yeast two-hybrid screening strategy with a common tumour-derived p53 mutant as bait, we identified several mutant p53-interacting partners including the known proteins wild-type (wt) p53, hUBC9 and GBP/PIAS1. In addition, a novel protein partner was identified which we have termed MBP1, for Mutant p53-Binding Protein 1. MBP1 is a new member of the emerging fibulin gene family, which currently comprises fibulin-1, fibulin-2 and S1-5. Expression of MBP1 mRNA is differentially regulated both temporally during development of the mouse embryo and in a tissue-specific manner within the adult. Specific interaction between MBP1 and mutant p53 was illustrated by both two-hybrid analysis in yeast and co-immunoprecipitation in mammalian cells. MBP1 displayed the following order of binding specificity towards different p53 forms: H175 > G281 > H273 > or = W248>wt p53. Thus, MBP1 appears to bind preferentially to p53 mutants of the 'structural' rather than 'contact' class, reflecting a potential bias towards those mutants having a significant alteration in conformation from that assumed by wt p53. We propose that MBP1 is the product of a candidate oncogene as rates of both neoplastic transformation and tumour cell growth were shown to be significantly enhanced when the protein is ectopically overexpressed. Furthermore, MBP1 may play a role in determining if a 'gain of function' effect is seen with certain p53 mutants.