Rearrangements of the cellular p53 gene in erythroleukaemic cells transformed by Friend virus

A comprehensive study of p53 protein

The protein p53 has been extensively investigated since it was found 43 years ago and has become a "guardian of the genome" that regulates the division of cells by preventing the growth of cells and dividing them, that is, inhibits the development of tumors. Initial proof of protein existence by researchers in the mid-1970s was found by altering and regulating the SV40 big T antigen termed the A protein. Researchers demonstrated how viruses play a role in cancer by employing viruses' ability to create T-antigens complex with viral tumors, which was discovered in 1979 following a viral analysis and cancer analog research. Researchers later in the year 1989 explained that in Murine Friend, a virus-caused erythroleukemia, commonly found that p53 was inactivated to suggest that p53 could be a "tumor suppressor gene." The TP53 gene, encoding p53, is one of human cancer's most frequently altered genes. The protein-regulated biological functions of all p53s include cell cycles, apoptosis, senescence, metabolism of the DNA, angiogenesis, cell differentiation, and immunological response. We tried to unfold the history of the p53 protein, which was discovered long back in 1979, that is, 43 years of research on p53, and how p53's function has been developed through time in this article.

p53: From Fundamental Biology to Clinical Applications in Cancer

Simple Summary

p53 tumour suppressor gene is the most altered in cancer. Several decades of research have established that it is of pivotal importance in prompting neoplastic phenomena, including cancer initiation and progression. However, it has crucial functions for cellular life. Knowledge and awareness about these multifaceted properties should be part of the cultural background of all scientists. In this review, we describe and discuss the multifaceted roles of p53, from its discovery to clinical applications in cancer therapy.

Abstract

p53 tumour suppressor gene is our major barrier against neoplastic transformation. It is involved in many cellular functions, including cell cycle arrest, senescence, DNA repair, apoptosis, autophagy, cell metabolism, ferroptosis, immune system regulation, generation of reactive oxygen species, mitochondrial function, global regulation of gene expression, miRNAs, etc. Its crucial importance is denounced by the high percentage of amino acid sequence identity between very different species (Homo sapiens, Drosophila melanogaster, Rattus norvegicus, Danio rerio, Canis lupus familiaris, Gekko japonicus). Many of its activities allowed life on Earth (e.g., repair from radiation-induced DNA damage) and directly contribute to its tumour suppressor function. In this review, we provide paramount information on p53, from its discovery, which is an interesting paradigm of science evolution, to potential clinical applications in anti-cancer treatment. The description of the fundamental biology of p53 is enriched by specific information on the structure and function of the protein as well by tumour/host evolutionistic perspectives of its role.

Isolation of a Human Betaretrovirus from Patients with Primary Biliary Cholangitis

A human betaretrovirus (HBRV) has been linked with the autoimmune liver disease, primary biliary cholangitis (PBC), and various cancers, including breast cancer and lymphoma. HBRV is closely related to the mouse mammary tumor virus, and represents the only exogenous betaretrovirus characterized in humans to date. Evidence of infection in patients with PBC has been demonstrated through the identification of proviral integration sites in lymphoid tissue, the major reservoir of infection, as well as biliary epithelium, which is the site of the disease process. Accordingly, we tested the hypothesis that patients with PBC harbor a transmissible betaretrovirus by co-cultivation of PBC patients’ lymph node homogenates with the HS578T breast cancer line. Because of the low level of HBRV replication, betaretrovirus producing cells were subcloned to optimize viral isolation and production. Evidence of infection was provided by electron microscopy, RT-PCR, in situ hybridization, cloning of the HBRV proviral genome and demonstration of more than 3400 integration sites. Further evidence of viral transmissibility was demonstrated by infection of biliary epithelial cells. While HBRV did not show a preference for integration proximal to specific genomic features, analyses of common insertion sites revealed evidence of integration proximal to cancer associated genes. These studies demonstrate the isolation of HBRV with features similar to mouse mammary tumor virus and confirm that patients with PBC display evidence of a transmissible viral infection.

Ganglioside GD2: a novel therapeutic target in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a heterogeneous disease characterized by lack of hormone receptor expression and is known for high rates of recurrence, distant metastases, and poor clinical outcomes. TNBC cells lack targetable receptors; hence, there is an urgent need for targetable markers for the disease. Breast cancer stem-like cells (BCSCs) are a fraction of cells in primary tumors that are associated with tumorigenesis, metastasis, and resistance to chemotherapy. Targeting BCSCs is thus an effective strategy for preventing cancer metastatic spread and sensitizing tumors to chemotherapy. The CD44^hi CD24^lo phenotype is a well-established phenotype for identification of BCSCs, but CD44 and CD24 are not targetable markers owing to their expression in normal tissues. The ganglioside GD2 has been shown to be upregulated in primary TNBC tumors compared with normal breast tissue and has been shown to identify BCSCs. In this review, we discuss GD2 as a BCSC- and tumor-specific marker in TNBC; epithelial-to-mesenchymal transition and the signaling pathways that are upstream and downstream of GD2 and the role of these pathways in tumorigenesis and metastasis in TNBC; direct and indirect approaches for targeting GD2; and ongoing clinical trials and treatments directed against GD2 as well as future directions for these strategies.

Shifting the paradigms for tumor suppression: lessons from the p53 field

The TP53 gene continues to hold distinction as the most frequently mutated gene in cancer. Since its discovery in 1979, hundreds of research groups have devoted their efforts toward understanding why this gene is so frequently selected against by tumors, with the hopes of harnessing this information toward improved therapy of cancer. The result is that this protein has been meticulously analyzed in tumor and normal cells, resulting in over one hundred thousand publications, with an average of five thousand papers published on p53 every year for the past decade. The journey toward understanding p53 function has been anything but straightforward; in fact, the field is notable for the numerous times that established paradigms not only have been shifted, but in fact have been shattered or reversed. In this review, we will discuss the manuscripts, or series of manuscripts, that have most radically changed our thinking about how this tumor suppressor functions, and we will delve into the emerging challenges for the future in this important area of research. It is hoped that this review will serve as a useful historical reference for those interested in p53, and a useful lesson on the need to be flexible in the face of established paradigms.

Genomic Instability in Circulating Tumor Cells

Simple Summary

In this review, we focus on recent advances in the detection and quantification of tumor cell heterogeneity and genomic instability of CTCs and the contribution of chromosome instability studies to genetic heterogeneity in CTCs at the single-CTC level.

Abstract

Circulating tumor cells (CTCs) can promote distant metastases and can be obtained through minimally invasive liquid biopsy for clinical assessment in cancer patients. Having both genomic heterogeneity and instability as common features, the genetic characterization of CTCs can serve as a powerful tool for a better understanding of the molecular changes occurring at tumor initiation and during tumor progression/metastasis. In this review, we will highlight recent advances in the detection and quantification of tumor cell heterogeneity and genomic instability in CTCs. We will focus on the contribution of chromosome instability studies to genetic heterogeneity in CTCs at the single-CTC level by discussing data from different cancer subtypes and their impact on diagnosis and precision medicine.

Length and secondary structure of the 5' non-coding regions of mouse p53 mRNA transcripts - mouse as a model organism for p53 gene expression studies

Transcription initiation sites of Trp53 gene in mice were determined using the 5'RACE method. Based on sequence alignment of the 5'-terminal regions of p53 mRNA in mammals, the site for the most abundant transcript turned out to be essentially identical with that determined for human TP53 gene and slightly differed for the longest transcripts, in mice and humans. Secondary structures of the 5' -terminal regions of the shorter, most abundant and the longest mouse transcripts were determined in vitro and the shorter transcript was also mapped in transfected mouse cells. For the first time, secondary structure models of the 5' terminus of two mouse p53 mRNAs were proposed. Comparing these models with the conservativeness of the nucleotide sequence of the 5'-terminal region of mRNA in mouse and other mammals, the possible function of the selected structural domains of this region was discussed. To elucidate the translation mechanisms, the two studied mRNAs were translated in the presence of an increasing concentration of the cap analog. For the longest transcript, the data suggested that IRES element(s) was/were involved in translation initiation. Additionally, changes in p53 synthesis under genotoxic and endoplasmic reticulum stress conditions in mouse cells were analyzed.

p53 Isoforms and Their Implications in Cancer

In this review we focus on the major isoforms of the tumor-suppressor protein p53, dysfunction of which often leads to cancer. Mutations of the TP53 gene, particularly in the DNA binding domain, have been regarded as the main cause for p53 inactivation. However, recent reports demonstrating abundance of p53 isoforms, especially the N-terminally truncated ones, in the cancerous tissues suggest their involvement in carcinogenesis. These isoforms are ∆40p53, ∆133p53, and ∆160p53 (the names indicate their respective N-terminal truncation). Due to the lack of structural and functional characterizations the modes of action of the p53 isoforms are still unclear. Owing to the deletions in the functional domains, these isoforms can either be defective in DNA binding or more susceptive to altered ‘responsive elements’ than p53. Furthermore, they may exert a ‘dominant negative effect’ or induce more aggressive cancer by the ‘gain of function’. One possible mechanism of p53 inactivation can be through tetramerization with the ∆133p53 and ∆160p53 isoforms—both lacking part of the DNA binding domain. A recent report and unpublished data from our laboratory also suggest that these isoforms may inactivate p53 by fast aggregation—possibly due to ectopic overexpression. We further discuss the evolutionary significance of the p53 isoforms.

Uncovering the role of p53 splice variants in human malignancy: a clinical perspective

Thirty-five years of research on p53 gave rise to more than 68,000 articles and reviews, but did not allow the uncovering of all the mysteries that this major tumor suppressor holds. How p53 handles the different signals to decide the appropriate cell fate in response to a stress and its implication in tumorigenesis and cancer progression remains unclear. Nevertheless, the uncovering of p53 isoforms has opened new perspectives in the cancer research field. Indeed, the human TP53 gene encodes not only one but at least twelve p53 protein isoforms, which are produced in normal tissues through alternative initiation of translation, usage of alternative promoters, and alternative splicing. In recent years, it became obvious that the different p53 isoforms play an important role in regulating cell fate in response to different stresses in normal cells by differentially regulating gene expression. In cancer cells, abnormal expression of p53 isoforms contributes actively to cancer formation and progression, regardless of TP53 mutation status. They can also be associated with response to treatment, depending on the cell context. The determination of p53 isoform expression and p53 mutation status helps to define different subtypes within a particular cancer type, which would have different responses to treatment. Thus, the understanding of the regulation of p53 isoform expression and their biological activities in relation to the cellular context would constitute an important step toward the improvement of the diagnostic, prognostic, and predictive values of p53 in cancer treatment. This review aims to summarize the involvement of p53 isoforms in cancer and to highlight novel potential therapeutic targets.

The function, mechanisms, and role of the genes PTEN and TP53 and the effects of asbestos in the development of malignant mesothelioma: a review focused on the genes' molecular mechanisms

The malignant mesothelioma is an aggressive form of cancer with a mean survival rate of less than a year. Moreover, environmental exposure to minerals is an important factor in the development of malignant mesothelioma (MM), especially the mineral asbestos, which has a well-documented role in MM, and more recently, the mineral erionite has been proven to be a strong carcinogenic inducer of MM. In addition, the virus simian virus 40 has been implicated as a co-carcinogenic player in MM. However, the molecular mechanisms involved in the pathogenesis of this cancer are still not fully understood. Indeed, it is known that several genes are altered or mutated in MM, among those are p16(INK4A), p14(ARF), and neurofibromatosis type II. Furthermore, TP53 has been reported to be mutated in the majority of the cancers; however, in MM, it is very uncommon mutations in this gene. Also, the PTEN gene has been shown to play an important role in endometrial cancer and glioblastoma, although the role of PTEN in MM has yet to be established. Taken altogether, this review focuses on the historical aspects, molecular mechanisms, interaction with other genes and proteins, and the role of these genes in MM. Lastly, this review questions the cancer theory of the two hits because the functions of both PTEN and TP53 are not fully explained by this theory.

Self-renewal of leukemia stem cells in Friend virus-induced erythroleukemia requires proviral insertional activation of Spi1 and hedgehog signaling but not mutation of p53

Friend virus induces erythroleukemia through a characteristic two-stage progression. The prevailing model proposes that during the initial, polyclonal stage of disease most of the infected cells terminally differentiate, resulting in acute erythrocytosis. In the late stage of disease, a clonal leukemia develops through the acquisition of new mutations--proviral insertional activation of Spi1/Pu.1 and mutation of p53. Previous work from our laboratory demonstrated that Friend virus activates the bone morphogenic protein 4 (BMP4)-dependent stress erythropoiesis pathway, which leads to the rapid expansion of stress erythroid progenitors, which are the targets for Friend virus in the spleen. We recently showed that stress erythroid progenitors have intrinsic self-renewal ability and therefore could function as leukemia stem cells (LSCs) when infected with Friend virus. Here, we show that the two stages of Friend virus-induced disease are caused by infection of distinct stress progenitor populations in the spleen. The development of leukemia relies on the ability of the virus to hijack the intrinsic self-renewal capability of stress erythroid progenitors leading to the generation of LSCs. Two signals are required for the self-renewal of Friend virus LSCs proviral insertional activation of Spi1/Pu.1 and Hedgehog-dependent signaling. Surprisingly, mutation of p53 is not observed in LSCs. These data establish a new model for Friend virus-induced erythroleukemia and demonstrate the utility of Friend virus as a model system to study LSC self-renewal.

p53, Stem Cells, and Reprogramming: Tumor Suppression beyond Guarding the Genome

p53 is well recognized as a potent tumor suppressor. In its classic role, p53 responds to genotoxic insults by inducing cell cycle exit or programmed cell death to limit the propagation of cells with corrupted genomes. p53 is also implicated in a variety of other cellular processes in which its involvement is less well understood including self-renewal, differentiation, and reprogramming. These activities represent an emerging area of intense interest for cancer biologists, as they provide potential mechanistic links between p53 loss and the stem cell-like cellular plasticity that has been suggested to contribute to tumor cell heterogeneity and to drive tumor progression. Despite accumulating evidence linking p53 loss to stem-like phenotypes in cancer, it is not yet understood how p53 contributes to acquisition of "stemness" at the molecular level. Whether and how stem-like cells confer survival advantages to propagate the tumor also remain to be resolved. Furthermore, although it seems reasonable that the combination of p53 deficiency and the stem-like state could contribute to the genesis of cancers that are refractory to treatment, direct linkages and mechanistic underpinnings remain under investigation. Here, we discuss recent findings supporting the connection between p53 loss and the emergence of tumor cells bearing functional and molecular similarities to stem cells. We address several potential molecular and cellular mechanisms that may contribute to this link, and we discuss implications of these findings for the way we think about cancer progression.

Biological functions of p53 isoforms through evolution: lessons from animal and cellular models

The TP53 tumour-suppressor gene is expressed as several protein isoforms generated by different mechanisms, including use of alternative promoters, splicing sites and translational initiation sites, that are conserved through evolution and within the TP53 homologues, TP63 and TP73. Although first described in the eighties, the importance of p53 isoforms in regulating the suppressive functions of p53 has only become evident in the last 10 years, by analogy with observations that p63 and p73 isoforms appeared indispensable to fully understand the biological functions of TP63 and TP73. This review summarizes recent advances in the field of 'p53 isoforms', including new data on p63 and p73 isoforms. Details of the alternative mechanisms that produce p53 isoforms and cis- and trans-regulators identified are provided. The main focus is on their biological functions (apoptosis, cell cycle, aging and so on) in cellular and animal models, including mouse, zebrafish and Drosophila. Finally, the deregulation of p53 isoform expression in human cancers is reviewed. Based on these latest results, several developments are expected in the future: the identification of drugs modulating p53 isoform expression; the generation of animal models and the evaluation of the use of p53 isoform as biomarkers in human cancers.

Insertional oncogenesis by non-acute retroviruses: implications for gene therapy

Retroviruses cause cancers in a variety of animals and humans. Research on retroviruses has provided important insights into mechanisms of oncogenesis in humans, including the discovery of viral oncogenes and cellular proto-oncogenes. The subject of this review is the mechanisms by which retroviruses that do not carry oncogenes (non-acute retroviruses) cause cancers. The common theme is that these tumors result from insertional activation of cellular proto-oncogenes by integration of viral DNA. Early research on insertional activation of proto-oncogenes in virus-induced tumors is reviewed. Research on non-acute retroviruses has led to the discovery of new proto-oncogenes through searches for common insertion sites (CISs) in virus-induced tumors. Cooperation between different proto-oncogenes in development of tumors has been elucidated through the study of retrovirus-induced tumors, and retroviral infection of genetically susceptible mice (retroviral tagging) has been used to identify cellular proto-oncogenes active in specific oncogenic pathways. The pace of proto-oncogene discovery has been accelerated by technical advances including PCR cloning of viral integration sites, the availability of the mouse genome sequence, and high throughput DNA sequencing. Insertional activation has proven to be a significant risk in gene therapy trials to correct genetic defects with retroviral vectors. Studies on non-acute retroviral oncogenesis provide insight into the potential risks, and the mechanisms of oncogenesis.

Friend Spleen Focus-Forming Virus Activates the Tyrosine Kinase sf-Stk and the Transcription Factor PU.1 to Cause a Multi-Stage Erythroleukemia in Mice

Hematological malignancies in humans typically involve two types of genetic changes: those that promote hematopoietic cell proliferation and survival (often the result of activation of tyrosine kinases) and those that impair hematopoietic cell differentiation (often the result of changes in transcription factors). The multi-stage erythroleukemia induced in mice by Friend spleen focus-forming virus (SFFV) is an excellent animal model for studying the molecular basis for both of these changes. Significant progress has been made in understanding the molecular basis for the multi-stage erythroleukemia induced by Friend SFFV. In the first stage of leukemia, the envelope protein encoded by SFFV interacts with and activates the erythropoietin (Epo) receptor and the receptor tyrosine kinase sf-Stk in erythroid cells, causing their Epo-independent proliferation, differentiation and survival. In the second stage, SFFV integration into the Sfpi1 locus activates the myeloid transcription factor PU.1, blocking erythroid cell differentiation, and in conjunction with the loss of p53 tumor suppressor activity, results in the outgrowth of malignant cells. In this review, we discuss the current level of understanding of how SFFV alters the growth and differentiation of erythroid cells and results in the development of erythroleukemia. Our knowledge of how SFFV causes erythroleukemia in mice may give us clues as to how the highly related human retrovirus XMRV causes malignancies in humans.

p53 functions and cell lines: have we learned the lessons from the past?

p53 has a determinant role in cancer prevention and is among the most studied proteins in the world. The majority of studies devoted to this protein are carried out in cell lines because they are easy to use and have naturally emerged as the main research tool in laboratories. However, the p53 pathway is commonly deregulated in cancer cells, from which the experimental cell lines are generally derived. The fact that the pathway is deregulated challenges the relevance of using cancer-derived cell lines to study wild-type p53 activities, or, in a broader sense, to study any normal cellular process. In the present article, we identify and discuss a number of limitations of cell lines using examples related to p53. Finally, we point out the general limitations of cell lines and propose solutions as alternatives to these cells.

GATA-1 directly regulates p21 gene expression during erythroid differentiation

Lineage-determination transcription factors coordinate cell differentiation and proliferation by controlling the synthesis of lineage-specific gene products as well as cell cycle regulators. GATA-1 is a master regulator of erythropoiesis. Its role in regulating erythroid-specific genes has been extensively studied, whereas its role in controlling genes that regulate cell proliferation is less understood. Ectopic expression of GATA-1 in erythroleukemia cells releases the block to their differentiation and leads to terminal cell division. An early event in reprogramming the erythroleukemia cells is induction of the cyclin-dependent kinase inhibitor p21. Remarkably, ectopic expression of p21 also induces the erythroleukemia cells to differentiate. We now report that GATA-1 directly regulates transcription of the p21 gene in both erythroleukemia cells and normal erythroid progenitors. Using reporter, electrophoretic mobility shift, and chromatin immunoprecipitation assays, we show that GATA-1 stimulates p21 gene transcription by binding to consensus binding sites in the upstream region of the p21 gene promoter. This activity is also dependent on a binding site for Sp1/KLF-like factors near the transcription start site. Our findings indicate that p21 is a crucial downstream gene target and effector of GATA-1 during red blood cell terminal differentiation.

Tumor suppression by p53: making cells senescent

Cellular senescence is a permanent cell cycle arrest and a potent tumor suppression mechanism. The p53 tumor suppressor is a sequence-specific transcription factor and acts as a central hub sensing various stress signals and activating an array of target genes to induce cell cycle arrest, apoptosis, and senescence. Recent reports showed that restoration of p53 induces premature senescence and tumor regression in mice with hepatocarcinomas or sarcomas. Thus, p53-mediated senescence is capable of eliminating cancer cells in vivo. p63 and p73, two homologues of p53, have similar function in cell cycle arrest and apoptosis. However, the role of p63 and p73 in cellular senescence is elusive. In this review, we will discuss how p53 regulates senescence and future studies about p53 family members in senescence.

p53 Research: the past thirty years and the next thirty years

Thirty years of research on the p53 family of genes has generated almost fifty thousand publications. The first of these papers detected the p53 protein associated with a viral oncogene product in transformed cells and tumors and focused the field on cancer biology. Subsequent manuscripts have shown a wide variety of functions for the p53 family of genes and their proteins. These proteins are involved in reproduction, genomic repair, fidelity and recombination, the regulation of metabolic processes, longevity, surveillance of the stability of development, the production of stem cells and changes in epigenetic marks, the development of the nervous system (p73), the immune system (p73) and skin (p63), as well as the better known roles for the family in tumor suppression. The p53 family of genes has been found in the modern day ancestors of organisms with over one billion years of evolutionary history where they play a role in germ-line fidelity over that time span. As the body plan of the vertebrates emerged with the regeneration of tissues by stem cells over a lifetime, the p53 gene and its protein were adapted to be a tumor suppressor of somatic stem and progenitor cells complementing its' past functions in the germ line. Because the p53 family of genes has played a role in germ-line fidelity and preservation of the species, even in times of stress, these genes have been under constant selection pressure to change and adapt to new situations. This has given rise to this diversity of functions all working to preserve homeostatic processes that permit growth and reproduction in a world that is constantly challenging the fidelity of information transfer at each generation. The p53 family of gene products has influenced the rates of evolutionary change, just as evolutionary changes have altered the p53 family and its functions.

Genetically engineered mouse models in cancer research

Mouse models of human cancer have played a vital role in understanding tumorigenesis and answering experimental questions that other systems cannot address. Advances continue to be made that allow better understanding of the mechanisms of tumor development, and therefore the identification of better therapeutic and diagnostic strategies. We review major advances that have been made in modeling cancer in the mouse and specific areas of research that have been explored with mouse models. For example, although there are differences between mice and humans, new models are able to more accurately model sporadic human cancers by specifically controlling timing and location of mutations, even within single cells. As hypotheses are developed in human and cell culture systems, engineered mice provide the most tractable and accurate test of their validity in vivo. For example, largely through the use of these models, the microenvironment has been established to play a critical role in tumorigenesis, since tumor development and the interaction with surrounding stroma can be studied as both evolve. These mouse models have specifically fueled our understanding of cancer initiation, immune system roles, tumor angiogenesis, invasion, and metastasis, and the relevance of molecular diversity observed among human cancers. Currently, these models are being designed to facilitate in vivo imaging to track both primary and metastatic tumor development from much earlier stages than previously possible. Finally, the approaches developed in this field to achieve basic understanding are emerging as effective tools to guide much needed development of treatment strategies, diagnostic strategies, and patient stratification strategies in clinical research.

Activation of the N-terminally truncated form of the Stk receptor tyrosine kinase Sf-Stk by Friend virus-encoded gp55 is mediated by cysteine residues in the ecotropic domain of gp55 and the extracellular domain of Sf-Stk

Friend virus induces an erythroleukemia in susceptible mice that is initiated by the interaction of the Friend virus-encoded glycoprotein gp55 with the erythropoietin (Epo) receptor and the product of the host Fv2 gene, a naturally occurring truncated form of the Stk receptor tyrosine kinase (Sf-Stk). We have previously demonstrated that the activation of Sf-Stk, recruitment of a Grb2/Gab2/Stat3 signaling complex, and induction of Pu.1 expression by Stat3 are required for the development of the early stage of Friend disease both in vitro and in vivo. Here we demonstrate that the interaction of gp55 with Sf-Stk is dependent on cysteine residues in the ecotropic domain of gp55 and the extracellular domain of Sf-Stk. Point mutation of these cysteine residues or deletion of these domains inhibits the ability of gp55 to interact with Sf-Stk, resulting in the inability of these proteins to promote the Epo-independent growth of erythroid progenitor cells. We also demonstrate that the interaction of gp55 with Sf-Stk does not promote dimerization of Sf-Stk but results in enhanced phosphorylation of Sf-Stk and the relocalization of Sf-Stk from the cytosol to the plasma membrane. Finally, we demonstrate that a constitutively active form of Sf-Stk (Sf-StkM330T), as well as its human counterpart, Sf-Ron, promotes Epo-independent colony formation in the absence of gp55 and that this response is also dependent on the cysteines in the extracellular domains of Sf-StkM330T and Sf-Ron. These data suggest that the cysteines in the extracellular domains of Sf-Stk and Sf-Ron may also mediate the interaction of these truncated receptors with other cellular factors that regulate their ability to promote cytokine-independent growth.

Restoration of wild-type p53 function in human tumors: strategies for efficient cancer therapy

The p53 tumor suppressor gene is mutated in around 50% of all human tumors. Most mutations inactivate p53's specific DNA binding, resulting in failure to activate transcription of p53 target genes. As a consequence, mutant p53 is unable to trigger a p53-dependent biological response, that is cell cycle arrest and apoptosis. Many tumors express high levels of nonfunctional mutant p53. Several strategies for restoration of wild-type p53 function in tumors have been designed. Wild-type p53 reconstitution by adenovirus-mediated gene transfer has shown antitumor efficacy in clinical trials. Screening of chemical libraries has allowed identification of small molecules that reactivate mutant p53 and trigger mutant p53-dependent apoptosis. These novel strategies raise hopes for more efficient cancer therapy.

The first 30 years of p53: growing ever more complex

Thirty years ago p53 was discovered as a cellular partner of simian virus 40 large T-antigen, the oncoprotein of this tumour virus. The first decade of p53 research saw the cloning of p53 DNA and the realization that p53 is not an oncogene but a tumour suppressor that is very frequently mutated in human cancer. In the second decade of research, the function of p53 was uncovered: it is a transcription factor induced by stress, which can promote cell cycle arrest, apoptosis and senescence. In the third decade after its discovery new functions of this protein were revealed, including the regulation of metabolic pathways and cytokines that are required for embryo implantation. The fourth decade of research may see new p53-based drugs to treat cancer. What is next is anybody's guess.

30 years and a long way into p53 research

Among the 278,092 publications indexed into PubMed in 1979, a handful of articles stand out as the foundation of one of the most profound forays into the molecular basis of carcinogenesis: the discovery of the p53 tumour-suppressor protein. In the 30 years since then, understanding of p53 has progressed from obscure oncogene to key tumour-suppressor gene with clinical potential. Yet, p53 research has not followed a straight course. In this Historical Review, we describe how the 1979 discovery has shaped our view of the molecular basis of cancer, and identify some crucial steps ahead to transfer the wealth of knowledge accumulated on p53 into applications to cancer prevention and treatment.

Multi-stage Friend murine erythroleukemia: molecular insights into oncogenic cooperation

The Friend virus SFFV (Spleen Focus Forming Virus) provokes an acute erythroblastosis in susceptible strains of mice that progresses to overt erythroleukemia by a multi-step process. For virologists, the Friend virus-induced disease has provided deep insights into the host mechanisms influencing susceptibility to retroviral infection and viremia. These insights have contributed to the understanding of HIV and other human retroviral infections. For cell biologists and oncologists, this leukemia has been a powerful experimental model to identify critical oncogenes involved in a multi-stage process, to understand the contribution of host genes to cancer development, and to investigate the mechanisms leading to cell growth autonomy. This model also provided an example of oncogenic reversion since Friend tumor cells can reinitiate their erythroid differentiation program when exposed in vitro to some chemical inducers. This review highlights recent findings demonstrating that the leukemic progression depends on the cooperation of at least two oncogenic events, one interfering with differentiation and one conferring a proliferative advantage. The Friend model of leukemia progression recapitulates the two phases of human acute myeloid leukemia (AML). Coupling of insights from studies on the Friend erythroleukemia with knowledge on AML might allow a better understanding of the molecular mechanisms involved in the evolution of leukemia in mice and men.

Cellular and organismal ageing: Role of the p53 tumor suppressor protein in the induction of transient and terminal senescence

In recent years, an impact of the p53 tumor suppressor protein in the processes of cellular and organismal ageing became evident. First hints were found in model organisms like Saccharomyces cerevisiae, Caenorhabditis elegans, and Drosophila melanogaster where a clear connection between ageing phenotypes and pathways that are regulated by p53, were found. Interestingly, pathways that are central to the ageing process are usually also involved in energy metabolism and are highly conserved throughout evolution. This also supports the long known empiric finding that caloric restriction has a positive impact on the life span of a wide variety of organisms. Within the last years, on the molecular level, an involvement of the insulin-like growth factor and of the histone deacetylase SRIT1 could be shown. Insight on the impact of p53 on ageing at the organismal level came from mice expressing aberrant forms of the p53 protein. Obviously, the balance of the full length p53 protein and of the shorter p44/DeltaNp53 isomer bear a strong impact on ageing. The shorter isoform regulates full length p53 and in cases where there is too much of the longer isoform, this leads to elevated apoptosis resulting in decreased tumor incidence but also in premature ageing due to exhaustion of the renewal potential. Therefore, modulating the expression of the truncated p53 isoform accordingly, might lead to increased health-span and elevated life-span.

Friend virus utilizes the BMP4-dependent stress erythropoiesis pathway to induce erythroleukemia

More than 50 years of genetic analysis has identified a number of host genes that are required for the expansion of infected cells during the progression of Friend-virus-induced erythroleukemia. In this report, we show that Friend virus induces the bone morphogenetic protein 4 (BMP4)-dependent stress erythropoiesis pathway in the spleen, which rapidly amplifies target cells, propagating their infection and resulting in acute splenomegaly. This mechanism mimics the response to acute anemia, in which BMP4 expressed in the spleen drives the expansion of a specialized population of stress erythroid progenitors. Previously we demonstrated that these progenitors, termed stress BFU-E, are targets for Friend virus in the spleen (A. Subramanian, H. E. Teal, P. H. Correll, and R. F. Paulson, J. Virol. 79:14586-14594, 2005). Here, we extend those findings by showing that Friend virus infects two distinct populations of bone marrow cells. One population, when infected, differentiates into mature erythrocytes in an Epo-independent manner, while a second population migrates to the spleen after infection, where it induces BMP4 expression and acts as a reservoir of virus. The activation of the stress erythropoiesis pathway in the spleen by Friend virus results in the rapid expansion of stress BFU-E, providing abundant target cells for viral infection. These observations suggest a novel mechanism by which a virus induces a stress response pathway that amplifies target cells for the virus, leading to acute expansion of infected cells.

Regenerative capacity of neural precursors in the adult mammalian brain is under the control of p53

The question of whether or not stem cell loss drives aging in the brain has not been fully resolved. Here, we used mice over-expressing the short isoform of p53 (DeltaNp53 or p44) as a model of aging to gain insight into the cellular mechanisms underlying age-related functional deficits in the brain. By BrdU labeling, we observed an accelerated decline in the number of subventricular zone proliferating cells with age in p44Tg mice compared to mice with normal p53 expression. A 2-3-fold reduction in the number of slowly dividing stem cells was evident in the subventricular zone of 9-12-month-old p44Tg mice, but not in younger p44Tg mice or in normal mice. Consequently, the supply of new olfactory bulb neurons was also reduced. The number and size of neurospheres generated from subventricular zone cells from p44Tg mice was significantly reduced, and cells derived from these neurospheres had limited self-renewal and amplification capacities. At the cellular level, p44 lengthened the cell cycle and affected cell cycle reentry properties, evident by an increased proportion of cells in G0. At the functional level, p44 expression resulted in impaired olfactory discrimination in 15-16-month-old mice. This phenotype is driven by constitutive activation of p53 and constitutive expression of p21(Cip1/waf1) in neural stem cells. Our results demonstrate that p53 plays a crucial role in the maintenance of the regenerative capacity of the brain by regulating the proliferation of stem and progenitor cells.

Dissemination of Acinetobacter baumannii clones with OXA-23 Carbapenemase in Colombian hospitals

During 2005, 66 carbapenem-resistant isolates of Acinetobacter baumannii were collected from seven tertiary-care hospitals participating in a nationwide surveillance network in Colombia. The isolates were multidrug resistant and produced the carbapenemases OXA-23 and OXA-51. Forty-five belonged to four clones while 21 were unique pulsotypes. One clone was present in two hospitals within one city, while another had spread between two hospitals in different cities. Blood, secretions, and abdominal fluids were the most frequent sites of isolation. This is the first description of widespread dissemination of OXA-23 in South America.

Transcription regulation by mutant p53

In addition to the loss of wild-type p53 activity, a high percentage of tumor cells accumulate mutant p53 protein isoforms. Whereas the hallmark of the wild-type p53 is its tumor suppressor activities, tumor-associated mutant p53 proteins acquire novel functions enabling them to promote a large spectrum of cancer phenotypes. During the last years, it became clear that tumor-associated mutant p53 proteins are not only distinct from the wild-type p53, but they also represent a heterogeneous population of proteins with a variety of structure-function features. One of the major mechanisms underlying mutant p53 gain of function is the ability to regulate gene expression. Although a large number of specific target genes were identified, the molecular basis for this regulation is not fully elucidated. This review describes the present knowledge about the transcriptional activities of mutant p53 and the mechanisms that might underlie its target gene specificity.

Intensity-based signal separation algorithm for accurate quantification of clustered centrosomes in tissue sections

Centrosomes are small organelles that organize the mitotic spindle during cell division and are also involved in cell shape and polarity. Within epithelial tumors, such as breast cancer, and some hematological tumors, centrosome abnormalities (CAs) are common, occur early in disease etiology, and correlate with chromosomal instability and disease stage. In situ quantification of CA by optical microscopy is hampered by overlap and clustering of these organelles, which appear as focal structures. CA has been frequently associated with Tp53 status in premalignant lesions and tumors. Here the authors described an approach to accurately quantify centrosome frequencies in tissue sections and tumors, independently of background or noise levels. Applying simple optical rules in nondeconvolved conventional 3D images of stained tissue sections, the authors showed that they could evaluate more accurately and rapidly centrosome frequencies than with traditional investigator-based visual analysis or threshold-based techniques. The resulting population-based frequency of centrosomes per nucleus could then be used to approximate the proportion of cells with CA in that same population. This was done by taking into account baseline centrosome amplification and proliferation rates measured in the tissue. Using this technique, the authors showed that 20-30% of cells have amplified centrosomes in Tp53 null mammary tumors.

DeltaNp63 plays an anti-apoptotic role in ventral bladder development

The bladder, the largest smooth-muscle organ in the human body, is responsible for urine storage and micturition. P63, a homolog of the p53 tumor-suppressor gene, is essential for the development of all stratified epithelia, including the bladder urothelium. The N-terminal truncated isoform of p63, DeltaNp63, is known to have anti-apoptotic characteristics. We have established that DeltaNp63 is not only the predominant isoform expressed throughout the bladder, but is also preferentially expressed in the ventral bladder urothelium during early development. We observed a host of ventral defects in p63-/- embryos, including the absence of the abdominal and ventral bladder walls. This number of ventral defects is identical to bladder exstrophy, a congenital anomaly exhibited in human neonates. In the absence of p63, the ventral urothelium was neither committed nor differentiated, whereas the dorsal urothelium was both committed and differentiated. Furthermore, in p63-/- bladders, apoptosis in the ventral urothelium was significantly increased. This was accompanied by the upregulation of mitochondrial apoptotic mediators Bax and Apaf1, and concurrent upregulation of p53. Overexpression of DeltaNp63gamma and DeltaNp63beta in p63-/- bladder primary cell cultures resulted in a rescue, evidenced by significantly reduced expressions of Bax and Apaf1. We conclude that DeltaNp63 plays a crucial anti-apoptotic role in normal bladder development.

Mutation of the Lyn tyrosine kinase delays the progression of Friend virus induced erythroleukemia without affecting susceptibility

During the initial phase of Friend virus (FV) induced erythroleukemia, the interaction between the viral envelope glycoprotein gp55, the Erythropoietin receptor (EpoR) and the naturally occurring truncated version of the Mst1r receptor tyrosine kinase, called Sf-Stk, drives the polyclonal expansion of infected progenitors in an erythropoietin independent manner. Sf-Stk provides signals that cooperate with EpoR signals to effect expansion of erythroid progenitors. The latter phase of disease is characterized by a clonal expansion of transformed leukemic cells causing an acute erythroleukemia in mice. Signaling by Sf-Stk and EpoR mediated by gp55 renders erythroid progenitors Epo independent through the activation of the EpoR downstream pathways such as PI3K, MAPK and JAK/STAT. Previous work has shown that Src family kinases also play an important role in erythropoiesis. In particular, mutation of Src and Lyn can affect erythropoiesis. In this report we analyze the role of the Lyn tyrosine kinase in the pathogenesis of Friend virus. We demonstrate that during FV infection of primary erythroblasts, Lyn is not required for expansion of viral targets. Lyn deficient bone marrow and spleen cells are able to form Epo independent FV colonies in vitro. In vivo infection of Lyn deficient animals also results in a massive splenomegaly characteristic of the virus. However, we observe differences in the pathogenesis of Friend erythroleukemia in Lyn-/- mice. Lyn-/- mice infected with the polycythemia inducing strain of FV, FVP, do not develop polycythemia suggesting that Lyn-/- infected erythroblasts have a defect in terminal differentiation. Furthermore, the expansion of transformed cells in the spleen is reduced in Lyn-/- mice. Our data show that Lyn signals are not required for susceptibility to Friend erythroleukemia, but Lyn plays a role in later events, the terminal differentiation of infected cells and the expansion of transformed cells.

The presence of a short form of p53 in chicken lymphoblastoid cell lines during apoptosis

To examine the roles of a short form of p53 in the regulation of apoptosis in chicken lymphoblastoid tumor cell lines derived from Marek's disease (MD) and avian leukosis (AL), the expressions of the p53 proteins were analyzed in these cell lines in which apoptosis was chemically induced. In MSB1-O derived from MD, the expression of a 40 kDa protein of p53 was decreased and that of a 32 kDa protein, a short form of p53, was increased during apoptosis induced by actinomycin D. In 1104B1 derived from AL, the expressions of 42 and 32 kDa of p53 were increased during the apoptosis. The short form of p53 was undetectable in these cell lines when apoptosis was blocked by the pretreatment with endonuclease inhibitor, Zn2+, protease inhibitors, TPCK and TLCK, or caspase inhibitor, Z-VAD-FMK. In the transcriptional level, the expressions of bcl-2 and IAP were decreased in these cell lines during actinomycin D-induced apoptosis, but no change was detected in the expression level of p53. These results suggest that, in these chicken tumors, the short form of p53 could play a role in the initiation of apoptosis induced by the chemotherapeutic compound, and that the regulation of its expression may be important for the maintenance of transformation status.

The Involvement of MAPK Signaling Pathways in Determining the Cellular Response to p53 Activation

The effect of ERK, p38, and JNK signaling on p53-dependent apoptosis and cell cycle arrest was investigated using a Friend murine erythroleukemia virus (FVP)-transformed cell line that expresses a temperature-sensitive p53 allele, DP16.1/p53ts. In response to p53 activation at 32 degrees C, DP16.1/p53ts cells undergo p53-dependent G(1) cell cycle arrest and apoptosis. As a result of viral transformation, these cells express the spleen focus forming env-related glycoprotein gp55, which can bind to the erythropoietin receptor (EPO-R) and mimics many aspects of EPO-induced EPO-R signaling. We demonstrate that ERK, p38 and JNK mitogen-activated protein kinases (MAPKs) are constitutively active in DP16.1/p53ts cells. Constitutive MEK activity contributes to p53-dependent apoptosis and phosphorylation of p53 on serine residue 15. The pro-apoptotic effect of this MAPK kinase signal likely reflects an aberrant Ras proliferative signal arising from FVP-induced viral transformation. Inhibition of MEK alters the p53-dependent cellular response of DP16.1/p53ts from apoptosis to G(1) cell cycle arrest, with a concomitant increase in p21(WAF1), suggesting that the Ras/MEK pathway may influence the cellular response to p53 activation. p38 and JNK activity in DP16.1/p53ts cells is anti-apoptotic and capable of limiting p53-dependent apoptosis at 32 degrees C. Moreover, JNK facilitates p53 protein turnover, which could account for the enhanced apoptotic effects of inhibiting this MAPK pathway in DP16.1/p53ts cells. Overall, these data show that intrinsic MAPK signaling pathways, active in transformed cells, can both positively and negatively influence p53-dependent apoptosis, and illustrate their potential to affect cancer therapies aimed at reconstituting or activating p53 function.

Assessing p53 in clinical contexts: unlearned lessons and new perspectives

There is compelling evidence for the central role of the p53 pathway in human neoplasia but, despite an enormous literature, the clinical utility of assessing this pathway remains ambiguous. Even simple questions about the assessment of p53 status in clinical samples remain unanswered and the literature is confusing and often contradictory. The p53 pathway is certainly complicated and the biochemical mechanisms for regulating the function of p53 and its downstream consequences are rabbinical in complexity. This perspective considers this complexity and the reasons why establishing the true utility of clinical assessment of p53 has proven to be so difficult. Indeed, recent observations regarding the existence of alternate splice variants of p53, the complexity of p53 regulation, and the existence of allelic variants of p53 and its regulators with distinct functionality makes the situation even more complex. Problems with the available assays are considered and the need to consider an array of methodological issues is emphasized. Newer strategies including analysis of the expression of downstream targets of p53 and the use of threshold strategies to measure p53 protein may provide more robust measures of the p53 pathway in clinical settings, perhaps coupled with cheap sequencing-based approaches for mutation (and polymorphism) detection. However, progress will only be made if these methodological issues are resolved and robust assays are performed in the context of appropriately powered studies in clinical trial settings.

Resistance to friend virus-induced erythroleukemia in W/W(v) mice is caused by a spleen-specific defect which results in a severe reduction in target cells and a lack of Sf-Stk expression

The characteristic progression and specificity of Friend virus for the erythroid lineage have allowed for the identification of a number of host-encoded loci that are required for disease progression. Several of these loci, including the Friend virus susceptibility gene 2 (Fv2), dominant white spotting gene (W), and Steel gene (Sl), regulate the initial polyclonal expansion of infected erythroid progenitor cells. W and Sl encode the Kit receptor tyrosine kinase and its ligand, stem cell factor, respectively. W mutant mice are severely anemic, and earlier work suggested that this defect in erythroid differentiation is the cause for the resistance to Friend virus-induced erythroleukemia. Here we show that in bone marrow, W/W(v) mice have near normal numbers of target cells and the initial infection of bone marrow occurs normally in vivo. In contrast, spleen cells from W/W(v) mice infected both in vitro and in vivo with Friend virus failed to give rise to erythropoietin-independent colonies at any time following Friend virus infection, suggesting that mutation of the Kit receptor specifically affects target cells in the spleen, rendering the mutant mice resistant to the development of Friend virus-induced erythroleukemia. In addition, we show that the Kit+ pathogenic targets of Friend virus in the spleen are distinct from the pathogenic targets in bone marrow and this population of spleen target cells is markedly decreased in W/W(v) mice and these cells fail to express Sf-Stk. These results also underscore the unique nature of the spleen microenvironment in its role in supporting the progression of acute leukemia in Friend virus-infected mice.

GRB2-mediated recruitment of GAB2, but not GAB1, to SF-STK supports the expansion of Friend virus-infected erythroid progenitor cells

Friend virus induces the development of erythroleukemia in mice through the interaction of a viral glycoprotein, gp55, with a truncated form of the Stk receptor tyrosine kinase, short form-Stk (Sf-Stk), and the EpoR. We have shown previously that the ability of Sf-Stk to participate in the transformation of Friend virus-infected cells requires the kinase activity and Grb2-binding site of Sf-Stk. Here we show that Grb2 heterozygous mice exhibit decreased susceptibility to Friend erythroleukemia and that expansion of erythroid progenitors in response to infection requires the C-terminal SH3 domain of Grb2. A fusion protein in which the Grb2-binding site in Sf-Stk is replaced by Gab2, supports the growth of progenitors from mice lacking Sf-Stk, whereas a Sf-Stk/Gab1 fusion protein does not. Gab2 is expressed in spleens from Friend virus-infected mice, co-immunoprecipitates with Sf-Stk and is tyrosine phosphorylated in the presence of Sf-Stk. Mice with a targeted deletion in Gab2 are less susceptible to Friend erythroleukemia and the expansion of erythroid progenitor cells in response to infection can be rescued by expression of Gab2, but not Gab1. Taken together, these data indicate that a Sf-Stk/Grb2/Gab2 complex mediates the growth of primary erythroid progenitor cells in response to Friend virus.

Retroviral insertional mutagenesis: past, present and future

Retroviral insertion mutagenesis screens in mice are powerful tools for efficient identification of oncogenic mutations in an in vivo setting. Many oncogenes identified in these screens have also been shown to play a causal role in the development of human cancers. Sequencing and annotation of the mouse genome, along with recent improvements in insertion site cloning has greatly facilitated identification of oncogenic events in retrovirus-induced tumours. In this review, we discuss the features of retroviral insertion mutagenesis screens, covering the mechanisms by which retroviral insertions mutate cellular genes, the practical aspects of insertion site cloning, the identification and analysis of common insertion sites, and finally we address the potential for use of somatic insertional mutagens in the study of nonhaematopoietic and nonmammary tumour types.