ARF promotes MDM2 degradation and stabilizes p53: ARF-INK4a locus deletion impairs both the Rb and p53 tumor suppression pathways

Telomeres, stem cells, senescence, and cancer

Mammalian aging occurs in part because of a decline in the restorative capacity of tissue stem cells. These self-renewing cells are rendered malignant by a small number of oncogenic mutations, and overlapping tumor suppressor mechanisms (e.g., p16(INK4a)-Rb, ARF-p53, and the telomere) have evolved to ward against this possibility. These beneficial antitumor pathways, however, appear also to limit the stem cell life span, thereby contributing to aging.

Replicating Adenoviruses in Cancer Therapy

The potential use of adenoviruses in therapy against cancer has evoked a rapidly moving field of research. Unlike conventional gene therapy vectors, oncolytic adenoviruses retain the ability to replicate. However, replication is restricted as much as possible to tumor cells, with the aim of eliminating these cells through viral cytotoxicity. The two key issues are to improve the efficiency of virus replication and cell killing while ensuring the specificity of these activities for tumor cells. Wild-type adenoviruses as such may already be usable for cancer therapy. Strategies to further improve efficiency and specificity include the partial or complete removal of viral genes. The idea is that functions carried out by the corresponding gene products are not required for replication in tumor cells, but are needed in normal cells. Accordingly, the removal of genes encoding E1B-55 kDa or E1B-19 kDa, or the mutation of E1A may improve the selective killing of tumor cells. On the other hand, the overexpression of the adenovirus death protein (ADP) may enhance viral spread and oncolytic efficiency. Other strategies to improve the specific oncolytic activity of replicating adenoviruses have been pursued. For instance, some promoters are active specifically in tumor cells, and these promoters were introduced into the viral genome, to regulate essential viral genes. Moreover, replicating viruses were engineered to express toxic proteins or drug converters. A number of these viruses have been tested successfully using tumor xenografts in nude mice as a model system. An oncolytic adenovirus lacking the E1B-55 kDa gene product, termed dl1520 or ONYX015, was injected into squamous cell carcinomas of head and neck in phase II clinical trials, and the results were encouraging when chemotherapy was applied in parallel. In the future, further progress might be achieved on the level of virus constructs, but also by refining and adjusting simultaneous conventional therapies, and by standardizing the assessment of the clinical outcome. Recent progress has been made towards the use of replicating virus constructs in cancer therapy. The goal of these developments is to remove cancerous cells from patients with the help of viruses that selectively replicate in these cells. These viruses are generally termed oncolytic viruses. Some convenient properties of adenovirus make this virus particularly useful for this purpose. It infects a large number of human cell types, especially epithelial cells, which give rise to the vast majority of human malignancies. It can be grown easily and to high titers, and the creation of virus recombinants is well established. Finally, a large body of basic research has already been carried out on this virus, facilitating its manipulation. Various approaches to use adenovirus as a cancer drug have been reviewed (Alemany et al. 1999a, 2000; Curiel 2000; Galanis et al. 2001b; Gromeier 2001; Heise and Kirn 2000; Kirn 2000a; Kirn et al. 2001; Kirn and McCormick 1996; Smith and Chiocca 2000; Sunamura 2000; Wells 2000; Wodarz 2001). The aim of this chapter is to provide an integrated overview of these strategies.

P14ARF gene alterations in human hepatocellular carcinoma

INTRODUCTION

The molecular status of the p14(ARF) gene has not been fully elucidated in hepatocellular carcinoma (HCC). This study was performed to determine genetic and epigenetic alterations in the p14(ARF) tumor suppressor gene and their effect on HCC progression.

METHODS

The status of p14 was evaluated in 117 HCC tumoral nodules and 110 corresponding non-tumor tissues by loss of heterozygosity at the 9p21-22 region, homozygous deletions, single strand conformation polymorphism-polymerase chain reaction mutational analysis and methylation-specific polymerase chain reaction.

RESULTS

The most frequent inactivation mechanism was hypermethylation of the promoter region, which was found in 41.9% of tumor samples and in 19.1% of non-tumor samples. Loss of heterozygosity at the 9p21 region was detected in 27.3% and 10% of tumor and non-tumor tissues, respectively. Homozygous deletions and mutations were less common events in hepatocarcinogenesis. We found 5.9% of the tumor cases with exon 2 homozygous deletions and 3.4% of the cases with mutations. We described a silent mutation in codon 42 of exon 1beta for the first time. No association was found between inactivation of p14(ARF) and clinicopathological characteristics or prognosis.

CONCLUSION

We can conclude that p14(ARF) is frequently and early altered in HCC, being the main cause of inactivation promoter hypermethylation. Our results suggest that the p14(ARF) gene plays an important role in the pathogenesis of hepatocellular carcinoma.

The CDKN2A database: Integrating allelic variants with evolution, structure, function, and disease association

In this report, we introduce the CDKN2A Database, an online database of germline and somatic variants of the CDKN2A tumor suppressor gene recorded in human disease through the year 2002, annotated with evolutionary, structural, and functional information. The CDKN2A Database improves upon existing resources by: 1) including both somatic mutations and germline variants, thereby adding the perspective of somatic cell carcinogenesis to that of hereditary cancer predisposition; 2) including information that assists with the interpretation of allelic variants, such as other primary data (sequences, structures, alignments, functional measurements, and literature references) and annotations (extensive text, figures, and a tree-based phylogenetic classification); and 3) providing the information in a format that allows a user to either download the database or to easily manipulate it online. We describe the database structure, content, current uses, and potential implications (http://biodesktop.uvm.edu/perl/p16).

Transition to an irreversible state of senescence in HeLa cells arrested by repression of HPV E6 and E7 genes

Inhibition of human papillomavirus (HPV) E6 and E7 transcription by means of the E2 protein of bovine papillomavirus 1 (BPV1) has been shown to induce acute growth arrest in HPV-positive cervical carcinoma cells. This state of arrest is marked by the expression of senescence phenotypes including SA beta-Gal activity and lipofuscin accumulation. In this study, we examined the reversibility of these phenotypes by exogenously expressing the E6 and E7 genes into HeLa cells growth-arrested by the depletion of E6/E7. Re-expression of E7 (but not E6) in 2 days following E2 transduction induced the cells to resume growth. The proliferating cells manifested the phenotype of untreated HeLa cells, suggesting that E7 is the major factor responsible for the continued proliferation and the suppression of the senescence phenotype in cervical carcinoma cells. However, E7 in 5 days following E2 transduction did not prevent HeLa cells from entering the senescent state, indicating that the arrested state becomes irreversible. Our results suggest that, upon depletion of the viral oncoproteins, a senescent state is irreversibly induced in HeLa cells after a period of commitment. The status and cellular location of certain factors involved in signal transduction and cell cycle control was altered as well along with this irreversibility transition.

Control of human PIRH2 protein stability: involvement of TIP60 and the proteosome

Murine PIRH2 (mPIRH2) was recently identified as a RING finger-containing ubiquitin-protein isopeptide ligase that interacts with both p53 and the human androgen receptor. mpirh2 is a p53-responsive gene that is up-regulated by UV, and mPIRH2 protein has the capacity to polyubiquitylate p53, perhaps leading to p53 destruction. mpirh2 therefore has properties similar to those of the oncogene mdm2. Here, we have identified human PIRH2 (hPIRH2) as a TIP60-interacting protein. To investigate its regulation, we characterized hPIRH2 in parallel with hPIRH2 variants possessing mutations of conserved RING finger residues. We observed that wild-type hPIRH2 is an unstable protein with a short half-life and is a target for RING domain-dependent proteasomal degradation. Accordingly, we found that hPIRH2 was ubiquitylated in cells. The TIP60-hPIRH2 association appeared to regulate hPIRH2 stability; coexpression of TIP60 enhanced hPIRH2 protein stability and altered hPIRH2 subcellular localization. These results suggest that hPIRH2 activities can be controlled, at the post-translational level, in multiple ways.

Activated Kras and Ink4a/Arf deficiency cooperate to produce metastatic pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma ranks among the most lethal of human malignancies. Here, we assess the cooperative interactions of two signature mutations in mice engineered to sustain pancreas-specific Cre-mediated activation of a mutant Kras allele (KrasG12D) and deletion of a conditional Ink4a/Arf tumor suppressor allele. The phenotypic impact of KrasG12D alone was limited primarily to the development of focal premalignant ductal lesions, termed pancreatic intraepithelial neoplasias (PanINs), whereas the sole inactivation of Ink4a/Arf failed to produce any neoplastic lesions in the pancreas. In combination, KrasG12D expression and Ink4a/Arf deficiency resulted in an earlier appearance of PanIN lesions and these neoplasms progressed rapidly to highly invasive and metastatic cancers, resulting in death in all cases by 11 weeks. The evolution of these tumors bears striking resemblance to the human disease, possessing a proliferative stromal component and ductal lesions with a propensity to advance to a poorly differentiated state. These findings in the mouse provide experimental support for the widely accepted model of human pancreatic adenocarcinoma in which activated KRAS serves to initiate PanIN lesions, and the INK4A/ARF tumor suppressors function to constrain the malignant conversion of these PanIN lesions into lethal ductal adenocarcinoma. This faithful mouse model may permit the systematic analysis of genetic lesions implicated in the human disease and serve as a platform for the identification of early disease markers and for the efficient testing of novel therapies.

Ribosomal protein L11 negatively regulates oncoprotein MDM2 and mediates a p53-dependent ribosomal-stress checkpoint pathway

The gene encoding p53 mediates a major tumor suppression pathway that is frequently altered in human cancers. p53 function is kept at a low level during normal cell growth and is activated in response to various cellular stresses. The MDM2 oncoprotein plays a key role in negatively regulating p53 activity by either direct repression of p53 transactivation activity in the nucleus or promotion of p53 degradation in the cytoplasm. DNA damage and oncogenic insults, the two best-characterized p53-dependent checkpoint pathways, both activate p53 through inhibition of MDM2. Here we report that the human homologue of MDM2, HDM2, binds to ribosomal protein L11. L11 binds a central region in HDM2 that is distinct from the ARF binding site. We show that the functional consequence of L11-HDM2 association, like that with ARF, results in the prevention of HDM2-mediated p53 ubiquitination and degradation, subsequently restoring p53-mediated transactivation, accumulating p21 protein levels, and inducing a p53-dependent cell cycle arrest by canceling the inhibitory function of HDM2. Interference with ribosomal biogenesis by a low concentration of actinomycin D is associated with an increased L11-HDM2 interaction and subsequent p53 stabilization. We suggest that L11 functions as a negative regulator of HDM2 and that there might exist in vivo an L11-HDM2-p53 pathway for monitoring ribosomal integrity.

Functional and physical interaction of the human ARF tumor suppressor with Tat-binding protein-1

The p14ARF tumor suppressor is a key regulator of cellular proliferation, frequently inactivated in human cancer, whose mode of action is currently not completely understood. We report here that the so-called human immunodeficiency virus Tat-binding protein-1 (TBP-1), a component of the 19 S regulatory subunit of the proteasome 26 S, also involved in transcriptional regulation and with a supposed role in the control of cell proliferation, specifically interacts with ARF, both in yeast and mammalian cells. We present evidence that the overexpression of TBP-1 in various cell lines results in a sharp increase of both transfected and endogenous ARF protein levels. Moreover, this effect depends on the binding between the two proteins and, at least in part, is exerted at the post-translational level. We also show that the ARF increase following TBP-1 overexpression results in an increase in p53 protein levels and activity. Finally, our data underline a clear involvement of TBP-1 in the control of cell proliferation.

Two isoforms of otubain 1 regulate T cell anergy via GRAIL

The active ubiquitin E3 ligase GRAIL is crucial in the induction of CD4 T cell anergy. Here we show that GRAIL is associated with and regulated by two isoforms of the ubiquitin-specific protease otubain 1. In lethally irradiated mice reconstituted with bone marrow cells from T cell receptor-transgenic mice retrovirally transduced to express the genes encoding these proteases, otubain 1-expressing cells contained negligible amounts of endogenous GRAIL, proliferated well and produced large amounts of interleukin 2 after antigenic stimulation. In contrast, cells expressing the alternatively spliced isoform, otubain 1 alternative reading frame 1, contained large amounts of endogenous GRAIL and were functionally anergic, and they proliferated poorly and produced undetectable interleukin 2 when stimulated in a similar way. Thus, these two proteins have opposing epistatic functions in controlling the stability of GRAIL expression and the resultant anergy phenotype in T cells.

p19ARF-induced p53-independent apoptosis largely occurs through BAX

Combined disruption of the ARF gene and the p53 gene causes mouse predisposition to tumors of a wider variety and at a higher frequency than disruption of the p53 gene, indicating that the ARF gene has p53-independent anti-tumor function in addition to p53-dependent function. Coincidentally with this notion, ectopic expression of the p19(ARF) induces apoptosis for wild-type mouse embryo fibroblasts which have been immortalized by introduction of the SV40 virus genome (SV40-MEFs). The protein expression levels of p53, p21(Cip1), and Bax were not upregulated by ectopic expression of p19(ARF) in SV40-MEFs, indicating that expression of p19(ARF) induced apoptosis through p53-independent pathways in this system. Ectopic expression of p19(ARF) induced prominent apoptosis even in SV40-Bak-/-MEFs. In contrast, expression of p19(ARF) induced only a very low grade of apoptosis in Bax-/- or Bax-/-/Bak-/-SV40-MEFs. Remarkable attenuation of p19(ARF)-induced apoptosis by disruption of the Bax gene thus leads to the conclusion that Bax plays a major role in p53-independent apoptosis induced by p19(ARF).

Real-time imaging of transcriptional activation in live cells reveals rapid up-regulation of the cyclin-dependent kinase inhibitor gene CDKN1A in replicative cellular senescence

Cellular replicative senescence is a permanent growth arrest state that can be triggered by telomere shortening. The cyclin-dependent kinase (Cdk) inhibitor p21(CIP1/WAF1) (p21), encoded by the CDKN1A gene, is a critical cell cycle regulator whose expression increases as cells approach senescence. Although the pathways responsible for its up-regulation are not well understood, compelling evidence indicates that the upstream triggering event is telomere dysfunction. Studies of replicative senescence have been complicated by the asynchrony of its onset, which is caused by the continuous and stochastic variability in individual cell lifespans. In fact, the actual entry into senescence has never been observed in a single unperturbed cell. We report here a new in vitro human model system that allows entry into senescence to be monitored in real-time in individual viable cells. We used homologous recombination to generate non-immortalized fibroblast cells with the enhanced yellow fluorescence protein (EYFP) gene knocked into one CDKN1A gene copy, allowing promoter activity to be visualized as fluorescence intensity. Gamma irradiation, DNA-damaging drugs, expression of p14(ARF) or oncogenic Ras, and replicative exhaustion all resulted in elevated EYFP expression, demonstrating its proper control by physiological signalling circuits. Analysis by time-lapse microscopy of cultures approaching replicative senescence revealed that p21 levels rise abruptly in individual aging cells and remain elevated for extended periods of time.

Polycomb CBX7 has a unifying role in cellular lifespan

In contrast to cancer cells and embryonic stem cells, the lifespan of primary human cells is finite. After a defined number of population doublings, cells enter in an irreversible growth-arrested state termed replicative senescence. Mutations of genes involved in immortalization can contribute to cancer. In a genetic screen for cDNAs bypassing replicative senescence of normal human prostate epithelial cells (HPrEC), we identified CBX7, a gene that encodes a Polycomb protein, as shown by sequence homology, its interaction with Ring1 and its localization to nuclear Polycomb bodies. CBX7 extends the lifespan of a wide range of normal human cells and immortalizes mouse fibroblasts by downregulating expression of the Ink4a/Arf locus. CBX7 does not inter-function or colocalize with Bmi1, and both can exert their actions independently of each other as shown by reverse genetics. CBX7 expression is downregulated during replicative senescence and its ablation by short-hairpin RNA (shRNA) treatment inhibited growth of normal cells though induction of the Ink4a/Arf locus. Taken together, these data show that CBX7 controls cellular lifespan through regulation of both the p16(Ink4a)/Rb and the Arf/p53 pathways.

14-3-3 sigma positively regulates p53 and suppresses tumor growth

The 14-3-3 sigma (sigma) protein, a negative regulator of the cell cycle, is a human mammary epithelium-specific marker that is downregulated in transformed mammary carcinoma cells. It has also been identified as a p53-inducible gene product involved in cell cycle checkpoint control after DNA damage. Although 14-3-3 sigma is linked to p53-regulated cell cycle checkpoint control, detailed mechanisms of how cell cycle regulation occurs remain unclear. Decreased expression of 14-3-3 sigma was recently reported in several types of carcinomas, further suggesting that the negative regulatory role of 14-3-3 sigma in the cell cycle is compromised during tumorigenesis. However, this possible tumor-suppressive role of 14-3-3 sigma has not yet been characterized. Here, we studied the link between 14-3-3 sigma activities and p53 regulation. We found that 14-3-3 sigma interacted with p53 in response to the DNA-damaging agent adriamycin. Importantly, 14-3-3 sigma expression led to stabilized expression of p53. In studying the molecular mechanism of this increased stabilization of p53, we found that 14-3-3 sigma antagonized the biological functions of Mdm2 by blocking Mdm2-mediated p53 ubiquitination and nuclear export. In addition, we found that 14-3-3 sigma facilitated the oligomerization of p53 and enhanced p53's transcriptional activity. As a target gene of p53, 14-3-3 sigma appears to have a positive feedback effect on p53 activity. Significantly, we also showed that overexpression of 14-3-3 sigma inhibited oncogene-activated tumorigenicity in a tetracycline-regulated 14-3-3 sigma system. These results defined an important p53 regulatory loop and suggested that 14-3-3 sigma expression can be considered for therapeutic intervention in cancers.

Efficient growth inhibition of HPV 16 E6-expressing cells by an adenovirus-expressing p53 homologue p73β

Tumor suppressor p53 functions are downregulated in most cervical cancers, because the product of human papilloma virus (HPV) oncogene E6 binds to and inactivates p53 by promoting its degradation. p73, a p53 homologue, is similar to p53 in structure and function but yet not degraded by HPV E6 gene product. In this study, we have developed a replication-deficient recombinant adenovirus, which expresses p73beta (Ad-p73). Infection of human cancer cells with Ad-p73 results in several fold increase of p73beta levels as well as its known target genes like p21(WAF1/CIP1). Ad-p73-infected cells showed reduced cellular DNA synthesis, arrest in G1 phase of cell cycle and induction of apoptosis. Ad-p73 inhibited the growth of cancer cells of different types. More importantly, Ad-p73 inhibited the growth of cell lines carrying HPV E6 gene, which was introduced by stable integration, more efficiently in comparison to an Ad-p53. Furthermore, Ad-p73 also inhibited the growth of HeLa cells, a cell line derived from cervical cancer, very efficiently. The ability of Ad-p73 to inhibit the growth of HPV E6-expressing cells and HeLa cells correlated with the stable expression of functional p73 in the presence of E6. These results suggest that Ad-p73 could be used as a potential gene therapy agent against cervical cancer.

Prevalence of aberrant methylation of p14ARF over p16INK4a in some human primary tumors

The INK4a/ARF locus encodes two unrelated tumor suppressor proteins, p16INK4a and p14ARF, which participate in the two main cell-cycle control pathways, p16-Rb and p14-p53. Methylation of CpG promoter islands has been described as a mechanism of gene silencing. Exon 1 of the p16INK4a gene and the p14ARF promoter gene reside within CpG islands. Therefore, both can become methylated de novo and silenced. It has recently been proposed that the methylation changes in certain genes could be used as molecular markers for the detection of almost all forms of human cancer. Here, we analyzed concomitantly in each tumor sample and normal tissue the methylation status of p16INK4a and p14ARF by methylation-specific PCR (MSP) in 100 breast, 95 colon and 27 bladder carcinomas. A series of clinicopathological parameter were obtained from the medical records of the patients, p14ARF showed a higher rate of hypermethylation than p16INK4a in all three tumor types. p16INK4a and p14ARF aberrant methylation was significantly correlated with poor prognosis clinicopathological parameters of the three tumor types. We conclude that both p16INKa and p14ARF hypermethylation may be involved in breast, colon and bladder carcinogenesis, with special emphasis on the role of the lesser studied p14ARF gene, and that tumors with aberrant methylation in the two genes were associated with worse prognosis.

Changes in apoptosis-related pathways in acute myelocytic leukemia

Expression analysis of apoptotic genes was performed for 15 patients with acute myelocytic leukemia (AML) at the time of diagnosis to identify genes and signaling pathways involved in the regulation of cell survival and apoptosis during leukemogenesis. cDNA array analysis revealed 34 genes whose expression was significantly different compared to others. Tumor suppressor genes TP53 and CDKN2A were downregulated and protooncogenes JUN and GRB10 were upregulated. Furthermore, several cellular signaling pathways acting either in cell cycle regulation or in apoptosis were altered. Deregulation was found in pathways that contribute to genomic stability (by downregulation of either TP53 or CSE1L and by upregulation of GADD45A) and regulate cell cycle progression (by downregulation of CDKN2A and upregulation of RBBP4, CDC37, and NEDD5). Alterations at the transcriptional level were identified, namely, upregulation of JUN and E2F5. Abnormalities were observed in the regulation of the caspases through upregulation of CASP8 and by altered expression of BCL2-related pathway. Extrinsic apoptotic signals mediated by IGFs were deregulated and the glutathione detoxification pathway was downregulated. These findings provide insight into the regulation of balance between apoptosis and cell proliferation signals, and suggest that these genes and pathways may have an important role in the pathogenesis of AML.

Cooperativity of p19ARF, Mdm2, and p53 in murine tumorigenesis

The p19ARF gene product responds to oncogenic stresses by interfering with the inhibitory effects of Mdm2 on p53, thus enhancing p53 activity and its antiproliferative functions. The absence of p19ARF in the mouse leads to early tumor susceptibility, presumably in part due to decreased p53 activity. To examine the tumorigenic cooperativity of p19ARF, Mdm2, and p53 in vivo, p19ARF-deficient mice were crossed first to p53-deficient mice and then to Mdm2 transgenic mice. The progeny were monitored for tumors. Cooperativity between p19ARF and p53 deficiencies in accelerating tumor formation was observed for most genotypes except p53-/- p19ARF-/- mice. p53-/- p19ARF-/- mice had a tumor incidence similar to p53-/- mice. In this context, tumor suppression by ARF appears to be primarily p53 dependent. The majority of the p19ARF+/- tumors deleted the wildtype p19ARF allele, in agreement with the previous studies, suggesting that p19ARF is a classic 'two hit' tumor suppressor. In a p53+/- background, however, all p19ARF+/- tumors retained a wildtype ARF allele and most also retained wildtype p53. In the second cross between p19ARF-deficient and Mdm2 transgenic mice, cooperativity in tumor incidence between Mdm2 overexpression and ARF deficiency was observed, consistent with the role of p19ARF in negatively regulating Mdm2 activity. These experiments further demonstrate in vivo the inter-relationships of the p19ARF-Mdm2-p53 signaling axis in tumor suppression.

Genetic and epigenetic alterations as hallmarks of the intricate road to cancer

Despite the clonal origin of most tumors, their tremendous heterogeneity suggests that cancer progression springs from the combined forces of both genetic and epigenetic events, which produce variant clonal populations, together with the selective pressures of the microenvironment, which promote growth and, perhaps, dissemination of variants with a specific set of characteristics. Although the importance of genetic mutations in cancer has long been recognized, the role of epigenetic events has been suggested more recently. This review focuses on the genetic and epigenetic molecular mechanisms involved in cancer onset and progression, and discusses the possibility of new strategies in the development of anticancer treatments.

Specificity in the activation and control of transcription factor E2F-dependent apoptosis

Previous work has demonstrated a role for the E2F1 gene product in signaling apoptosis, both as a result of the deregulation of the Rb/E2F pathway as well as in response to DNA damage. We now show that the ability of cells to suppress the apoptotic potential of E2F1, as might occur during the course of normal cellular proliferation, requires the action of the Ras-phosphoinositide 3-kinase-Akt signaling pathway. In addition, we also identify a domain within the E2F1 protein, previously termed the marked-box domain, that is essential for the apoptotic activity of E2F1 and that distinguishes the E2F1 protein from E2F3. We also show that the E2F1-marked-box domain is essential for the induction of both p53 and p73 accumulation. Importantly, a role for the marked-box domain in the specificity of E2F1-mediated apoptosis coincides with recent work demonstrating a role for this domain in achieving specificity in the activation of transcription. We conclude that the unique capacity of E2F1 to trigger apoptosis reflects a specificity of transcriptional activation potential, and that this role for E2F1 is regulated through the action of the Akt protein kinase.

Nucleocytoplasmic shuttling of p53 is essential for MDM2-mediated cytoplasmic degradation but not ubiquitination

As a shuttling protein, p53 is constantly transported through the nuclear pore complex. p53 nucleocytoplasmic transport is carried out by a bipartite nuclear localization signal (NLS) located at its C-terminal domain and two nuclear export signals (NES) located in its N- and C-terminal regions, respectively. The role of nucleocytoplasmic shuttling in p53 ubiquitination and degradation has been a subject of debate. Here we show that the two basic amino acid groups in the p53 bipartite NLS function collaboratively to import p53. Mutations disrupting individual amino acids in the NLS, although causing accumulation of p53 in the cytoplasm to various degrees, reduce but do not eliminate the NLS activity, and these mutants remain sensitive to MDM2 degradation. However, disrupting both parts of the bipartite NLS completely blocks p53 from entering the nucleus and causes p53 to become resistant to MDM2-mediated degradation. Similarly, mutations disrupting four conserved hydrophobic amino acids in the p53 C-terminal NES block p53 export and prohibit it from MDM2 degradation. We also show that colocalization of a nonshuttling p53 with MDM2 either in the nucleus or in the cytoplasm is sufficient for MDM2-induced p53 polyubiquitination but not degradation. Our data provide new insight into the mechanism and regulation of p53 nucleocytoplasmic shuttling and degradation.

Using mice to decipher the molecular genetics of brain tumors

The past decade has dramatically increased our knowledge of genetic and molecular alterations in human central nervous system tumors. Important as these alterations are for the molecular classification of tumors, their actual roles in tumorigenesis and tumor progression have long remained obscure. Lately, several mouse brain tumor models have been developed that use different gene modification strategies to replicate mutations seen in the human counterpart. These genetic models will allow discrimination between mutations that are causally related to tumor formation and mutations that are a result of tumor progression. These models also provide histologically and genetically accurate models for preclinical testing and will perhaps help us identify novel targets for therapies aimed at the mechanistic cause of the disease. We present here a review of current models, with a focus on gliomas and medulloblastomas.

Independence of the prognostic value of tumor suppressor protein expression in ovarian adenocarcinomas: A multivariate analysis of expression of p53, retinoblastoma, and related proteins

Accurate estimation of prognosis of ovarian cancer is difficult. For this report, in a group of 73 patients with ovarian adenocarcinomas, clinical factors and protein expression status of p53, retinoblastoma (Rb), and related proteins were evaluated for potential prognostic values. Clinical factors included FIGO stage, age, histopathologic type, and protein expression of p53, Rb, MDM2, p14ARF, p21WAF(1)/CIP(1) was determined by an immunohistochemical technique. Univariate Cox proportional hazard regression analysis was used to determine the significant prognostic value of FIGO stage (P < 0.0001), p53 status (0.0021), and patient age (P = 0.0255), and we report here, for the first time, the significant (P = 0.0072) prognostic value of Rb status. Histopathologic type and MDM2, p14ARF, p21WAF(1)/CIP(1) status did not show any prognostic value. To examine further the independence of prognostic values, we next applied multivariate analysis: We found that FIGO stage (P < 0.0001) and p53 status (P = 0.0108) were independent prognostic factors, while age and Rb status were not. Independence of prognostic value of p53 has heretofore been controversial, but we found a definite independent prognostic value for p53 status in ovarian adenocarcinomas. We also found that selection of appropriate antibodies for immunohistochemistry was essential to obtain significant results. We used five kinds of antibodies for p53 immunolocalization, and correlation with prognosis was obtained by three of these with different grades of statistical significance.

Alterations of RB1, p53 and Wnt pathways in hepatocellular carcinomas associated with hepatitis C, hepatitis B and alcoholic liver cirrhosis

Major etiologic factors associated with human hepatocellular carcinomas (HCCs) include infection with hepatitis C (HCV) and hepatitis B virus (HBV), excess alcohol intake and aflatoxin B(1) exposure. While the G-->T p53 mutation at codon 249 has been identified as a genetic hallmark of HCC caused by aflatoxin B(1), the genetic profile associated with other etiologic factors appears to be less distinctive. In our study, we screened HCCs resulting from HCV infection (51 cases), HBV infection (26 cases) or excess alcohol intake (23 cases) for alterations in genes involved in the RB1 pathway (p16(INK4a), p15(INK4b), RB1, CDK4 and cyclin D1), the p53 pathway (p53, p14(ARF) and MDM2) and the Wnt pathway (beta-catenin, APC). Alterations of the RB1 pathway, mainly p16(INK4a) methylation, loss of RB1 expression and cyclin D1 amplification, were most common (69-100% of cases). There was a significant correlation between loss of RB1 expression and RB1 methylation. All 24 HCCs with RB1 promoter methylation lacked RB1 expression, while none of the 67 cases with RB1 expression exhibited RB1 methylation (p < 0.0001), suggesting that promoter methylation is a major mechanism of loss of RB1 expression in HCCs. Alterations of the p53 pathway consisted mostly of p53 mutations or p14(ARF) promoter methylation (20-48%). Mutations of the p53 gene were found at a similar frequency (13-15%) in all etiologic groups, without any consistent base change or hot spot. Mutations of beta-catenin were found in 13-31% of cases, while no APC mutations were detected in any of the HCCs analyzed. With the exception of only 3 of 39 cases (8%), cyclin D1 amplification and beta-catenin mutations were mutually exclusive, supporting the view that cyclin D1 is a target of the Wnt signaling pathway. Overall, the RB1, p53 and Wnt pathways were commonly affected in HCCs of different etiology, probably reflecting common pathogenetic mechanisms, i.e., chronic liver injury and cirrhosis, but tumors associated with alcoholism had more frequent alterations in the RB1 and p53 pathways than those caused by HCV infection.

The persistence of senescence

Senescence is a potent anticancer mechanism, representing a barrier that most, if not all, would-be tumor cells must traverse on their path to malignant transformation. In this Perspective, I discuss two recent publications (1, 2) that deal with the durability of senescence. These findings are of interest not only to those who study aging, but to those who study cancer as well.

Myc and E2F1 induce p53 through p14ARF-independent mechanisms in human fibroblasts

p19ARF is induced in response to oncogene activation or during cellular senescence in mouse embryo fibroblasts, triggering p53-dependent and p53-independent cell cycle arrest and apoptosis. We have studied the involvement of human p14ARF as a regulator of p53 activity in normal human skin fibroblasts (NHFs) or WI38 lung embryonic fibroblasts expressing conditional Myc or E2F1 estrogen receptor fusion proteins. Both Myc and E2F1 activation rapidly induced p53 phosphorylation at Ser-15, p53 protein accumulation, and upregulation of the p53 target genes MDM2 and p21. Activation of E2F1 induced p14ARF mRNA and protein levels. In contrast, Myc activation did not induce any significant increase in p14ARF mRNA or protein levels in neither NHFs nor WI38 fibroblasts within 48 h. Myc and E2F1 induced p53 and cell cycle arrest even after silencing of p14ARF using short-interfering RNA. Treatment with the ATM/ATR kinase inhibitor caffeine prevented p53 accumulation upon activation of Myc or E2F1. Our results indicate that p53 phosphorylation, but not p14ARF, plays a major role for the induction of p53 in response to Myc and E2F1 activation in normal human fibroblasts.

The genetics of malignant melanoma: lessons from mouse and man

Therapeutic resistance and proclivity for metastasis are hallmarks of malignant melanoma. Genetic, epidemiological and genomic investigations are uncovering the spectrum of stereotypical mutations that are associated with melanoma and how these mutations relate to risk factors such as ultraviolet exposure. The ability to validate the pathogenetic relevance of these mutations in the mouse, coupled with advances in rational drug design, has generated optimism for the development of effective prevention programmes, diagnostic measures and targeted therapeutics in the near future.

Genetic alterations of p16INK4a and p14ARF genes in human bladder cancer

PURPOSE

The growth suppressive genes p16INK4a and p14ARF located on the 9p21 gene cluster have active roles in the Rb and p53 growth control pathways, respectively. p16INK4a is a cyclin dependent kinase inhibitor functioning upstream of Rb. p14ARF restrains cell growth by abrogating Mdm2 inhibition of p53 activity, thereby, facilitating p53 mediated cell cycle arrest and apoptosis. To elucidate specific targets and aberrations affecting the 9p21 chromosomal region in bladder cancer tumorigenesis alterations in the p16INK4a and p14ARF genes were analyzed.

MATERIALS AND METHODS

A total of 53 transitional cell carcinomas from 44 patients with bladder cancer were collected. Genetic alterations of p16INK4a and p14ARF genes were analyzed by Southern hybridization, polymerase chain reaction (PCR)-single strand conformational polymorphism analysis and methylation specific PCR. In addition, mRNA expression status was detected by reverse transcriptase-PCR.

RESULTS

Homozygous deletion of p16INK4a and p14ARF genes was observed in 23% (12 of 53 samples) and 43% (23 of 53), respectively. Most deletions occurred exclusively on the E1 beta-p14ARF region. Concomitant deletion of p16INK4a and p14ARF genes was found in only 2 samples. One mutation was detected in exon 2 of p14ARF plus p16INK4a genes. Aberrant methylation of p16INK4a gene was found in 60% (24 of 40 tumors). However, no p14ARF gene methylation was detected in any case. The result of comparative reverse transcriptase-PCR showed that suppressed mRNA expression correlated with genetic alterations of p14ARF and p16INK4a genes in most tumor samples examined.

CONCLUSIONS

Results indicate that p14ARF is a primary target of homozygous deletion, whereas p16INK4a is the hot spot of hypermethylation on the 9p21 region in bladder cancer. The frequent inactivation of the p14ARF and p16INK4a genes may be an important mechanism for the dysfunction of p53 and Rb growth regulatory pathways during bladder cancer development.

Melanoma cells can tolerate high levels of transcriptionally active endogenous p53 but are sensitive to retrovirus-transduced p53

Malignant melanomas are frequently characterized by elevated levels of wild-type p53, suggesting that p53 function could be suppressed by a mechanism different from p53 mutation. We analysed the functionality of the p53-signaling pathway in a panel of seven human melanoma cell lines consisting of one p53-deficient line, two lines with mutant p53, and four lines expressing wild-type p53. Only lines with wild-type p53 were characterized by elevated levels of endogenous p21, high activity of p53-responsive reporters and accumulation of p53 in response to genotoxic stress, common properties of functional p53. The presence of wild-type p53 was associated with depletion or loss of p14ARF and p16 expression. The levels of p33ING1b and p24ING1c, two major products of Ing1 locus and putative coregulators of p53, were elevated in all cell lines tested; however, ectopic expression of either ING1 isoform had no effect on cell proliferation. All lines retained expression of Apaf-1, and all but one remained sensitive to ectopic expression of retrovirus-transduced p53. Our data indicate that regardless of abnormally high levels of p53 in melanomas, their p53 remains competent in transactivation of its targets, and, if highly overexpressed, capable of growth inhibition. Hence, the p53 pathway in malignant melanomas can be considered for pharmacological targeting and anticancer gene therapy.

EBV-encoded EBNA-5 associates with P14ARF in extranucleolar inclusions and prolongs the survival of P14ARF-expressing cells

Epstein-Barr virus (EBV) carrying lymphoblastoid cells of normal origin express the full program of all 9 virus-encoded, growth transformation associated proteins. They have an intact p53 pathway as a rule. This raises the question of whether any of the viral proteins impair the pathway functionally. Using a yeast 2-hybrid system, we have shown that EBNA-5 but not the other EBNAs interacts with the p14ARF protein, a regulator of the p53 pathway. The interaction was confirmed in vitro using a GST pull-down assay. Moreover, expression of EBNA-5 increased the survival of p14ARF-transfected cells. EBV infection of resting B cells induced the expression of p14ARF mRNA without increased level of the protein. A fraction of the p14ARF localized to the nucleoli but the bulk of the protein accumulated in nuclear but extranucleolar inclusions. Formation of the extranucleolar inclusions led to complete relocalization of EBNA-5 from nucleoplasm to these structures. The inclusions also contained p53 and HDM2, and were surrounded by PML bodies and proteasomes, which suggests that these inclusions could be targets for proteasome dependent protein degradation.

Genetic epidemiology of melanoma

Melanoma incidence has risen in many Caucasians populations over the last 20 years and research on the potential environmental and genetic risk factors has led to some interesting new findings but also to many more questions. The relationship between melanoma and ultraviolet radiation is complex and this area of research is controversial especially regarding the use of sunbeds and sunscreens. In terms of genetic factors, the discovery of two genes CDKN2A and CDK4 has been a great advance with more understanding of melanocyte biology in relation to defects in senescence. For phenotypic risk factors such as fair skin and high numbers of naevi, the role of genetic factors is clearly evident but these traits are complex and the discovery of genes involved in skin pigmentation and naevi formation is not an easy task. Research on the MC1R gene has not only shown the importance of this gene in hair and skin pigmentation but also in senescence and immunity. Functional studies involving CDKN2A and MC1R are leading to important new findings. There is also some hope regarding the use of micro-arrays in helping to dissect many genetic events in melanoma. The collection of large datasets including family, twin and case-control studies as well as tumour banks with collaborations between countries will hopefully lead to more discoveries. For the primary and secondary prevention of this tumour, efforts need to be sustained in public health campaigns on sun exposure and the recognition of individuals at high risk.

Aberrant methylation of multiple genes in neuroblastic tumours. relationship with MYCN amplification and allelic status at 1p

Aberrant hypermethylation occurs in tumour cell CpG islands and is an important pathway for the repression of gene transcription in cancers. We investigated aberrant hypermethylation of 11 genes by methylation-specific polymerase chain reaction (PCR), after treatment of the DNA with bisulphite, and correlated the findings with MYCN amplification and allelic status at 1p in a series of 44 neuroblastic tumours. This tumour series includes five ganglioneuromas (G), one ganglioneuroblastoma (GN) and 38 neuroblastomas (six stage 1 tumours; five stage 2 tumours; six stage 3 cases; 19 stage 4 tumours, and two stage 4S cases). Aberrant methylation of at least one of the 11 genes studied was detected in 95% (42 of 44) of the cases. The frequencies of aberrant methylation were: 64% for thrombospondin-1 (THBS1); 30% for tissue inhibitor of metalloproteinase 3 (TIMP-3); 27% for O6-methylguanine-DNA methyltransferase (MGMT); 25% for p73; 18% for RB1; 14% for death-associated protein kinase (DAPK), p14ARF, p16INK4a and caspase 8, and 0% for TP53 and glutathione S-transferase P1 (GSTP1). No aberrant methylation was observed in four control normal tissue samples (brain and adrenal medulla). MYCN amplification was found in 11 cases (all stage 4 neuroblastomas), whereas allelic loss at 1p was identified in 16 samples (13 stage 4 and two stage 3 neuroblastomas, and one ganglioneuroma). All but one case with caspase 8 methylation also displayed MYCN amplification. Our results suggest that promoter hypermethylation is a frequent epigenetic event in the tumorigenesis of neuroblastic tumours, but no specific pattern of hypermethylated genes could be demonstrated.

Regulation of HDM2 activity by the ribosomal protein L11

The HDM2 protein plays an important role in regulating the stability and function of the p53 tumor suppressor protein. In this report, we show that the ribosomal protein L11 can interact with HDM2 and inhibit HDM2 function, thus leading to the stabilization and activation of p53. The inhibition of HDM2 activity by L11 shows some similarity to the previously described activity of ARF, and expression of either ARF or L11 can induce a p53 response. Enhancement of the interaction between endogenous L11 and HDM2 following treatment of cells with low levels of actinomycin-D suggests that the HDM2/L11 interaction represents a novel pathway for p53 stabilization in response to perturbations in ribosome biogenesis.

Involvement of the INK4a/Arf gene locus in senescence

The INK4a/ARF locus encodes two proteins whose expression limits cellular proliferation. Whilst the biochemical activities of the two proteins appear very different, they both converge on regulating the retinoblastoma and p53 tumour suppressor pathways. Neither protein is required for normal development, but lack of either predisposes to the development of malignancy. Both proteins have also been implicated in the establishment of senescence states in response to a variety of stresses, signalling imbalances and telomere shortening. The INK4a/Arf regulatory circuits appear to be partially redundant and show evidence of rapid evolution. Especially intriguing are the large number of biological differences documented between mice and man. We review here the brief history of INK4a/Arf and explore possible links with organismal aging and the evolution of longevity.

The cell cycle: a review of regulation, deregulation and therapeutic targets in cancer

The cell cycle is controlled by numerous mechanisms ensuring correct cell division. This review will focus on these mechanisms, i.e. regulation of cyclin-dependent kinases (CDK) by cyclins, CDK inhibitors and phosphorylating events. The quality checkpoints activated after DNA damage are also discussed. The complexity of the regulation of the cell cycle is also reflected in the different alterations leading to aberrant cell proliferation and development of cancer. Consequently, targeting the cell cycle in general and CDK in particular presents unique opportunities for drug discovery. This review provides an overview of deregulation of the cell cycle in cancer. Different families of known CDK inhibitors acting by ATP competition are also discussed. Currently, at least three compounds with CDK inhibitory activity (flavopiridol, UCN-01, roscovitine) have entered clinical trials.

Oligomerization of the human ARF tumor suppressor and its response to oxidative stress

The tumor suppressor ARF plays an important role as an inhibitor of the Mdm2-mediated degradation of p53. Here we demonstrate that human ARF (p14ARF) can form homo-oligomers. The stability of the oligomers is favored by oxidizing agents in a reversible fashion and involves all three cysteine residues in p14ARF. Furthermore, the effect of p14ARF in clonogenic assays is moderately but reproducibly increased by the mutation of its cysteine residues. We also observed that altering the amino terminus of p14ARF resulted in the appearance of remarkably stable oligomers. This indicates that the amino terminus of p14ARF interferes with the ability of the protein to form multimeric complexes. These observations suggest that p14ARF activity may be linked to its oligomerization status and sensitive to the redox status of the cell.

Response of Ewing tumor cells to forced and activated p53 expression

The EWS-FLI1 transcription factor is consistently expressed in 85% of Ewing tumors (EFT). In heterologous cells, EWS-FLI1 induces p53-dependent cell cycle arrest or apoptosis. It has been speculated that the p53 tumor suppressor pathway may be generally compromised in EFT despite only rare p53 mutations. In order to test for functional integrity of this pathway, we have investigated a series of EFT cell lines that differ from each other with respect to their endogenous p53 and INK4A gene status for their response to ectopic p53 expression and to stimulation of endogenous p53 activity by X-ray treatment. Significant interindividual and intratumoral variations in the apoptotic propensity of EFT cell lines to transient expression of ectopic p53 were observed, which was independent of the level of p53 expression. In cell lines with a low apoptotic incidence, apoptosis was delayed and the surviving fraction showed a prolonged growth arrest. Complete resistance to p53-induced apoptosis in two cell lines established from the same patient was associated with a high BCL2/BAX ratio and low levels of APAF1. Sensitivity to X-rays showed a trend towards a higher apoptotic rate in wild-type (wt) p53 expressing than in p53 mutant cells. However, one wt p53-expressing EFT cell line was completely refractory to irradiation-stimulated cell death despite high apoptotic responsiveness to ectopic p53. No difference in Ser15 phosphorylation and the transcriptional activation of p53 targets was observed in wt p53 EFT cell lines irrespective of the induction of cell death or growth arrest. All together, our results demonstrate that despite significant variability in the outcome, cell death or cell cycle arrest, the p53 downstream pathway and the DNA damage signaling pathway are functionally intact in EFT.

p16/cyclin-dependent kinase inhibitor 2A deficiency in human melanocyte senescence, apoptosis, and immortalization: possible implications for melanoma progression

BACKGROUND

The melanoma susceptibility locus cyclin-dependent kinase inhibitor 2A encodes two unrelated cell growth inhibitors, p16 and alternative reading frame (ARF). In fibroblasts, both proteins are implicated in cellular senescence, a key barrier to tumor development. The p16 coding sequence is more often mutated in melanoma families than is the ARF sequence. To investigate the role of p16 in melanocytes, we assessed aspects of growth, apoptosis, and immortalization in melanocytes cultured from two melanoma patients, both of whom had two inactive p16 alleles but functional ARF.

METHODS

Growth and senescence were evaluated by cumulative population-doubling curves, and apoptosis by terminal deoxytransferase labeling. Expression of p53 and p21, which are associated with fibroblast senescence, was assessed by immunoblotting. Amphotropic retroviruses were used to transfer exogenous gene sequences into the melanocytes.

RESULTS

Both melanocyte cultures showed high rates of apoptosis, which were reduced when the cells were grown in the presence of keratinocyte feeder cells or human stem cell factor plus endothelin 1. With these growth factors, both cultures proliferated for 45-55 net population doublings, markedly longer than the maximum of 10 net population doublings of normal adult human melanocytes in similar media, indicating impaired senescence. One of the cultures developed chromosomal aberrations, with numerous dicentric chromosomes at senescence, consistent with telomere dysfunction. p53 and p21 levels were not elevated in senescent normal melanocytes but were elevated in senescent p16-deficient melanocytes. Interference with p53 function by transfer of human papillomavirus 16-E6 further extended the lifespan of p16-deficient melanocytes. Human telomerase reverse transcriptase was sufficient to immortalize both these cell strains but not normal melanocytes.

CONCLUSION

Normal senescence in human melanocytes requires p16 activity. p53 contributes to a delayed form of senescence that requires telomere shortening, in p16-deficient melanocytes. These findings provide some basis for the role of p16 in melanoma susceptibility.

The INK4a/ARF locus and melanoma

Inactivation of the INK4a/ARF (or CDKN2a) locus is a common and critical genetic event in the development of human and mouse melanoma. This locus engages the Rb and p53 tumor suppressor pathways through its capacity to encode two distinct gene products, p16(INK4a) and p14(ARF). This review highlights the body of evidence supporting a role for both p16(INK4a) and p14(ARF) in the suppression of melanoma, and speculates as to why this locus is preferentially targeted in this tumor type. In addition, the potential importance of these two pathways in mediating UV-induced melanoma genesis will be addressed via genetic and molecular evidence in the mouse.

Bmi-1 is required for maintenance of adult self-renewing haematopoietic stem cells

A central issue in stem cell biology is to understand the mechanisms that regulate the self-renewal of haematopoietic stem cells (HSCs), which are required for haematopoiesis to persist for the lifetime of the animal. We found that adult and fetal mouse and adult human HSCs express the proto-oncogene Bmi-1. The number of HSCs in the fetal liver of Bmi-1-/- mice was normal. In postnatal Bmi-1-/- mice, the number of HSCs was markedly reduced. Transplanted fetal liver and bone marrow cells obtained from Bmi-1-/- mice were able to contribute only transiently to haematopoiesis. There was no detectable self-renewal of adult HSCs, indicating a cell autonomous defect in Bmi-1-/- mice. A gene expression analysis revealed that the expression of stem cell associated genes, cell survival genes, transcription factors, and genes modulating proliferation including p16Ink4a and p19Arf was altered in bone marrow cells of the Bmi-1-/- mice. Expression of p16Ink4a and p19Arf in normal HSCs resulted in proliferative arrest and p53-dependent cell death, respectively. Our results indicate that Bmi-1 is essential for the generation of self-renewing adult HSCs.

Suprabasal p53 immunostaining in premalignant endobronchial lesions in combination with histology is associated with bronchial cancer

Endobronchial carcinoma develops through a continuum of morphologically recognizable pre-neoplastic changes. At present, no marker has been identified that can reliably predict the biological behavior of these lesions. Endobronchial lesions (n=39) sampled from patients (n=20) without clinically overt lung cancer, were analyzed by immunohistochemistry (IHC) for abnormal expression regarding the p53 protein, i.e. suprabasal p53 expression. Clear suprabasal p53 immunostaining was found in two (12%) of the hyperplastic or squamous metaplastic lesions, in one (10%) of the mildly or moderately dysplastic lesions and in nine (75%) of the severely dysplastic or carcinoma in situ (CIS) lesions. Suprabasal p53 immunostaining was found significantly more frequent in severe dysplasia or CIS (P<0.01). Of 17 patients follow-up data were available. After a median follow up of 7 months (range 2-37 months), six patients presented with bronchial carcinoma within the same lobe or bronchial spur where biopsies had been taken. Four of these patients revealed suprabasal p53 immunostaining in the biopsies obtained from the sites of future cancer. In three patients biopsies were obtained from future cancer sites as well as from distant sites in the ipsilateral lung. Suprabasal p53 immunostaining was found exclusively at future cancer sites of these patients (P=0.02). Suprabasal p53 immunostaining in addition to histology improved the specificity and the positive predictive value for bronchial carcinoma development in the same lobe or bronchial spur, compared with histology alone. These results suggest that suprabasal p53 immunostaining is associated with bronchial cancer and might have additive value to predict the biological behavior of pre-neoplastic endobronchial lesions in the population at risk of bronchial cancer.

Supramolecular complex formation between Rad6 and proteins of the p53 pathway during DNA damage-induced response

The HR6A and -B genes, homologues of the yeast Rad6 gene, encode ubiquitin-conjugating enzymes that are required for postreplication repair of DNA and damage-induced mutagenesis. Using surface plasmon resonance, we show here that HR6 protein (referred as Rad6) physically interacts with p53. Analysis of proteins coimmunoprecipitated with Rad6 antibody from metabolically labeled normal MCF10A human breast epithelial cells not only confirmed Rad6-p53 interactions in vivo but also demonstrated for the first time that exposure of MCF10A cells to cisplatin or adriamycin (ADR) induces recruitment of p14ARF into Rad6-p53 complexes. Further analysis of ADR-induced p53 response showed that stable Rad6-p53-p14ARF complex formation is associated with a parallel increase and decrease in monoubiquitinated and polyubiquitinated p53, respectively, and arrest in G(2)/M phase of the cell cycle. Interestingly, the ADR-induced suppression of p53 polyubiquitination correlated with a corresponding decline in intact Hdm2 protein levels. Treatment of MCF10A cells with MG132, a 26S proteasome inhibitor, effectively stabilized monoubiquitinated p53 and rescued ADR-induced downregulation of Hdm2. These data suggest that ADR-induced degradation of Hdm2 occurs via the ubiquitin-proteasome pathway. Rad6 is present in both the cytoplasmic and nuclear compartments of normal MCF10A cells, although in response to DNA damage it is predominantly found in the nucleus colocalizing with ubiquitinated p53, whereas Hdm2 is undetectable. Consistent with in vivo data, results from in vitro ubiquitination assays show that Rad6 mediates addition of one (mono-) to two (multimono-) ubiquitin molecules on p53 and that inclusion of Mdm2 is essential for its polyubiquitination. The data presented in the present study suggest that Rad6-p53-p14ARF complex formation and p53 ubiquitin modification are important damage-induced responses that perhaps determine the fidelity of DNA postreplication repair.

Dissecting the contribution of p16(INK4A) and the Rb family to the Ras transformed phenotype

Although oncogenic Ras commonly contributes to the development of cancer, in normal primary cells it induces cell cycle arrest rather than transformation. Here we analyze the additional genetic changes required for Ras to promote cell cycle progression rather than arrest. We show that loss of p53 is sufficient for oncogenic Ras to stimulate proliferation in the absence of extrinsic mitogens in attached cells. However, surprisingly, we find that p53 loss is not sufficient for Ras to overcome anchorage dependence or contact inhibition. In contrast, expression of simian virus 40 (SV40) large T antigen (LT) allows Ras to overcome these additional cell cycle controls. Mutational analysis of SV40 LT shows that this action of SV40 LT depends on its ability to inactivate the retinoblastoma (Rb) family of proteins, in concert with the loss of p53. Importantly, we show that inactivation of the Rb family of proteins can be mimicked by loss of the cyclin-dependent kinase inhibitor p16(INK4A). p16(INK4A) is commonly lost in human tumors, but its contribution to the transformed phenotype is unknown. We demonstrate here a role for p16(INK4A) in the loss of cell cycle controls required for tumorigenesis and show how accumulating genetic changes cooperate and contribute to the transformed phenotype.

Consistent inactivation of p19(Arf) but not p15(Ink4b) in murine myeloid cells transformed in vivo by deregulated c-Myc

Cyclin-dependent kinase inhibitors p16(INK4a) and p15(INK4b), encoded by the CDKN2A and B loci, play an important role in negative regulation of the cell cycle. Furthermore, p19(ARF) also encoded by the CDKN2A locus, has been shown to regulate positively the p53 pathway leading to growth arrest and apoptosis. All three genes have been inactivated in human tumors. In myeloid cells, p15(INK4b) mRNA is upregulated during cytokine-induced differentiation and/or growth arrest, and hypermethylation of the p15(INK4b) gene promoter region is a common event in acute myeloid leukemia. In the present study, we examined murine monocyte/macrophage tumors with deregulated c-myc for evidence of Ink4 gene inactivation. p15(Ink4b) mRNA and protein were detected in the majority of leukemias, and p16(Ink4a) mRNA and protein were highly expressed in two of them. pRb was in a hypophosphorylated state in most of the neoplasms indicating that the Cdk inhibitors that were expressed in the cells were functional. The observed expression of p15(Ink4b) is inconsistent with their proliferation state, although it might be expected to be expressed owing to the maturity of the cells. These data suggest, therefore, that deregulated c-Myc bypasses the pRb restriction point and cell cycle arrest in these tumors. An examination of p19(Arf) exons revealed deletions of the gene in up to 94% of the tumors. Since this gene shares exon 2 with p16(Ink4a), it is often difficult to determine which gene is the relevant tumor suppressor. However, the loss of only the p19(Arf)-specific exon 1 beta was observed in a tumor that had normal p16(Ink4a) protein expression. In addition, the p19(Arf)-specific exon was deleted in another tumor that expressed a functional chimeric protein, p15Ex1-p16Ex2-3; it was demonstrated here that this fusion protein is capable of inducing G1 arrest. These data overall supports the hypothesis that the critical inactivation event in these hematopoietic neoplasms is elimination of p19(Arf), and not Ink4 function.

IGF-1 activates p21 to inhibit UV-induced cell death

The insulin-like growth factor-1 (IGF-1) and its downstream effector Akt have been documented as survival factors in response to a variety of stress signals. In this study, we show that IGF-1 activates p21 protein expression in a p53-dependent manner. Inhibition of PI-3 kinase or ectopic expression of a dominant-negative Akt blocks the effect of IGF-1 on the upregulation of p21 expression. In addition, IGF-1 prevents the UV irradiation-mediated suppression of p21 and MDM2 expression. Furthermore, p21 is important for IGF-1-mediated cell survival upon UV irradiation. Taken together, these data indicate that IGF-1 may activate p21 in executing its survival function upon genotoxic insults.

Mdm2 haplo-insufficiency profoundly inhibits Myc-induced lymphomagenesis

Mdm2 harnesses the p53 tumor suppressor, yet loss of one Mdm2 allele in Mdm2(+/-) mice has heretofore not been shown to impair tumor development. Here we report that Mdm2 haplo-insufficiency profoundly suppresses lymphomagenesis in E micro -myc transgenic mice. Mdm2(+/-)E micro -myc transgenics had greatly protracted rates of B cell lymphoma development with life spans twice that of wild-type transgenic littermates. Im paired lymphoma development was associated with drastic reductions in peripheral B cell numbers in Mdm2(+/-)E micro -myc transgenics, and primary pre-B cells from Mdm2(+/-)E micro -myc transgenics and Mdm2(+/-) littermates were extremely susceptible to spontaneous apoptosis. Loss of p53 rescued all of the effects of Mdm2 haplo-insufficiency, indicating they were p53 dependent. Furthermore, half of the lymphomas that ultimately emerged in Mdm2(+/-)E micro -myc transgenics harbored inactivating mutations in p53, and the majority overcame haplo-insufficiency by overexpressing Mdm2. These results support the concept that Mdm2 functions are rate limiting in lymphomagenesis and that targeting Mdm2 will enhance p53-mediated apoptosis, compromising tumor development and/or maintenance.

The t(8;21) fusion protein contacts co-repressors and histone deacetylases to repress the transcription of the p14ARF tumor suppressor

The t(8;21) is one of the most frequent chromosomal translocations associated with acute leukemia. The translocation fuses the DNA binding domain of AML1 to nearly all of the ETO co-repressor. ETO associates with the mSin3 and N-CoR co-repressors as well as histone deacetylases 1, 2, and 3. Although this is one of the most frequent chromosomal translocations in acute leukemia, accounting for 10-15% of the cases of acute myeloid leukemia (AML), the direct targets for transcriptional regulation that stimulate leukemogenesis are unknown. We found that AML1-ETO repressed the promoter of p14(ARF) tumor suppressor in transient transfection assays and reduced endogenous levels of p14(ARF) expression in multiple cell types. Chromatin immunoprecipitation assays demonstrated that AML1-ETO bound to the p14(ARF) promoter. In acute myeloid leukemia samples containing the t(8;21), levels of p14(ARF) mRNA were markedly lower when compared to other acute myeloid leukemias. Therefore, p14(ARF) is a direct transcriptional target of AML1-ETO.