Senescence and tumour clearance is triggered by p53 restoration in murine liver carcinomas

Distinct roles for p107 and p130 in Rb-independent cellular senescence

Telomere attrition, DNA damage and constitutive mitogenic signaling can all trigger cellular senescence in normal cells and serve as a defense against tumor progression. Cancer cells may circumvent this cellular defense by acquiring genetic mutations in checkpoint proteins responsible for regulating permanent cell cycle arrest. A small family of tumor suppressor genes encoding the retinoblastoma susceptibility protein family (Rb, p107, p130) exerts a partially redundant control of entry into S phase of DNA replication and cellular proliferation. Here we report that activation of the p53-dependent DNA damage response has been found to accelerate senescence in human prostate cancer cells lacking a functional Rb protein. This novel form of irradiation-induced premature cellular senescence reinforces the notion that other Rb family members may compensate for loss of Rb protein in the DNA damage response pathway. Consistent with this hypothesis, depletion of p107 potently inhibits the irradiation-induced senescence observed in DU145 cells. In contrast, p130 depletion triggers a robust and unexpected form of premature senescence in unirradiated cells. The dominant effect of depleting both p107 and p130, in the absence of Rb, was a complete blockade of irradiation-induced cellular senescence. Onset of the p107-dependent senescence was temporally associated with p53-mediated stabilization of the cyclin-dependent kinase inhibitor p27 and decreases in c-myc and cks1 expression. These results indicate that p107 is required for initiation of accelerated cellular senescence in the absence of Rb and introduces the concept that p130 may be required to prevent the onset of terminal growth arrest in unstimulated prostate cancer cells lacking a functional Rb allele.

Secreting tumor suppression

Cellular senescence limits the proliferative capacity of damaged cells and thereby acts as an intrinsic mechanism of tumor suppression. In this issue, Wajapeyee et al. (2008) identify insulin growth factor binding protein 7 (IGFBP7) as a secreted factor that mediates senescence induced by oncogenic BRAF in normal melanocytes. In addition, IGFBP7 triggers apoptosis in cells that have progressed to melanoma, suggesting a new approach for melanoma treatment.

Temporal activation of p53 by a specific MDM2 inhibitor is selectively toxic to tumors and leads to complete tumor growth inhibition

We have designed MI-219 as a potent, highly selective and orally active small-molecule inhibitor of the MDM2-p53 interaction. MI-219 binds to human MDM2 with a K(i) value of 5 nM and is 10,000-fold selective for MDM2 over MDMX. It disrupts the MDM2-p53 interaction and activates the p53 pathway in cells with wild-type p53, which leads to induction of cell cycle arrest in all cells and selective apoptosis in tumor cells. MI-219 stimulates rapid but transient p53 activation in established tumor xenograft tissues, resulting in inhibition of cell proliferation, induction of apoptosis, and complete tumor growth inhibition. MI-219 activates p53 in normal tissues with minimal p53 accumulation and is not toxic to animals. MI-219 warrants clinical investigation as a new agent for cancer treatment.

Integrative oncogenomic approaches for accelerated cancer-gene discovery

PURPOSE OF REVIEW

The completion of the human genome project has enabled several new technologies for studying cancer genetics and cancer genomes. However, genomic instability and heterogeneity of human tumors impedes a straightforward cataloging of cancer genes and possible therapeutic targets. Strategies enabling the distinction of causal genetic alterations from bystander genomic noise are needed and should significantly speed up the process of cancer-gene discovery.

RECENT FINDINGS

A series of recent papers described the development of integrative oncogenomic approaches based on innovative cancer mouse models and how these can be used to speed up the discovery of new cancer genes. In the presented studies, spontaneously acquired genetic alterations in mouse tumors of defined genetic origin are used to filter/prioritize relevant lesions from complex human cancer genomes. As will be discussed in this review, a great advantage of this approach is that pinpointed candidate genes can be functionally validated in the right genetic context in vivo, which significantly increases confidence for later therapeutic development efforts.

SUMMARY

The discussed approaches hold great promise to speed up the process of cancer-gene discovery and should be considered to complement time-consuming and costly endeavors like the Cancer Genome Project.

CP-31398 restores mutant p53 tumor suppressor function and inhibits UVB-induced skin carcinogenesis in mice

Mutations in the tumor suppressor p53 are detectable in over 50% of all human malignancies. Mutant p53 protein is incapable of transactivating its downstream target genes that are required for DNA repair and apoptosis. Chronic exposure to UVB induces p53 mutations and is carcinogenic in both murine and human skin. CP-31398, a styrylquinazoline compound, restores the tumor suppressor functions of mutant forms of p53 in tumor cells. However, its effectiveness in vivo remains unclear. Here, we demonstrate that CP-31398 blocked UVB-induced skin carcinogenesis and was associated with increases in p53, p21, and BclXs. CP-31398 downregulated Bcl2, proliferating nuclear cell antigen, and cyclin D1. Activation of caspase-3 and cleavage of poly (ADP-ribose) polymerase also occurred in both tumor and perilesional skin following treatment. CP-31398 induced the expression of p53-dependent target proteins, and this was followed by apoptosis in UVB-irradiated wild-type mice but not in their p53-deficient littermates. Similar effects were observed in human skin carcinoma A431 cells expressing mutant p53. In addition, CP-31398 induced mitochondrial translocation of p53, leading to changes in mitochondrial membrane permeability pore transition (MPT) and consequent cytochrome c release in these cells. Blocking MPT diminished p53 translocation and apoptosis. These studies indicate that reconstituting p53 tumor suppressor functions in vivo by small molecular weight compounds may block the pathogenesis and progression of skin cancer.

Senescence: the good the bad and the dysfunctional

Nearly 50 years have elapsed since Hayflick challenged the dogma that individual human cells were immortal by demonstrating that after a predictable number of cellular divisions, normal human fibroblasts eventually entered a state of permanent growth arrest [Hayflick L: The limited in vitro lifetime of human diploid cell strains. Exp Cell Res 1965, 37:614-636.; Hayflick L, Moorhead PS: The serial cultivation of human diploid cell strains. Exp Cell Res 1961, 25:585-621]. This growth arrest, referred to as senescence, was hypothesized to function as a tumor suppressive mechanism, capable of limiting the replicative capacity of an incipient tumor cell. While originally met with skepticism, the existence of senescence and its importance as a tumor suppressive mechanism is now accepted. Here, we highlight this work and introduce studies that indicate that while senescent cells themselves cannot produce a neoplasia, they possess the ability to promote the growth of nearby preneoplastic cells and in this way may contribute to age-related increases in tumor incidences. This added level of complexity suggests that senescence functions as a biological 'double edged sword.'

Immunological aspects of cancer chemotherapy

Accumulating evidence indicates that the innate and adaptive immune systems make a crucial contribution to the antitumour effects of conventional chemotherapy-based and radiotherapy-based cancer treatments. Moreover, the molecular and cellular bases of the immunogenicity of cell death that is induced by cytotoxic agents are being progressively unravelled, challenging the guidelines that currently govern the development of anticancer drugs. Here, we review the immunological aspects of conventional cancer treatments and propose that future successes in the fight against cancer will rely on the development and clinical application of combined chemo- and immunotherapies.

Molecular mechanisms of cellular senescence and immortalization of human cells

Cellular senescence was originally described as a phenomenon observed in cultured human cells. Accumulating lines of evidence now indicate that the same processes also take place in vivo, suggesting important implications for tumor development. Telomere shortening is the most well-established cause of cellular senescence that can be induced by many other intrinsic and extrinsic factors. The retinoblastoma susceptibility gene product is a convergent target that is downstream of these factors. p53, p38MAPK and cyclin-dependent kinase inhibitors p16INK4a (p16) and p21CIP1 (p21) are key mediators. As most stresses that induce cellular senescence are also known causes of cancer, a common strategy might be applied to the development of cancer chemopreventive agents and anti-ageing drugs.

A novel p53 rescue compound induces p53-dependent growth arrest and sensitises glioma cells to Apo2L/TRAIL-induced apoptosis

Reactivation of mutant p53 in tumours is a promising strategy for cancer therapy. Here we characterise the novel p53 rescue compound P53R3 that restores sequence-specific DNA binding of the endogenously expressed p53(R175H) and p53(R273H) mutants in gel-shift assays. Overexpression of the paradigmatic p53 mutants p53(R175H), p53(R248W) and p53(R273H) in the p53 null glioma cell line LN-308 reveals that P53R3 induces p53-dependent antiproliferative effects with much higher specificity and over a wider range of concentrations than the previously described p53 rescue drug p53 reactivation and induction of massive apoptosis (PRIMA-1). Furthermore, P53R3 enhances recruitment of endogenous p53 to several target promoters in glioma cells bearing mutant (T98G) and wild-type (LNT-229) p53 and induces mRNA expression of numerous p53 target genes in a p53-dependent manner. Interestingly, P53R3 strongly enhances the mRNA, total protein and cell surface expression of the death receptor death receptor 5 (DR5) whereas CD95 and TNF receptor 1 levels are unaffected. Accordingly, P53R3 does not sensitise for CD95 ligand- or tumour necrosis factor alpha-induced cell death, but displays synergy with Apo2L.0 in 9 of 12 glioma cell lines. Both DR5 surface induction and synergy with Apo2L.0 are sensitive to siRNA-mediated downregulation of p53. Thus this new p53 rescue compound may open up novel perspectives for the treatment of cancers currently considered resistant to the therapeutic induction of apoptosis.

Many roads lead to oncogene-induced senescence

Oncogene-induced senescence is a mechanism of tumor suppression that restricts the progression of benign tumors. Important advances have been made toward elucidating the mechanisms that regulate this response; however, there is presently no unified model that integrates all current findings. DNA damage, replicative stress, reactive oxygen species, heterochromatin formation and negative feedback signaling networks have all been proposed to play an integral role in promoting senescence in response to various oncogenic insults. In all cases, these signals have been shown to function through Rb and p53, but utilize different intermediaries. Thus, it appears that senescence is not triggered by a single, linear series of events, but instead is regulated by a complex signaling network. Accordingly, multiple proteins may cooperate to establish a senescence response, but the limiting signal(s) may be dictated by the initiating genetic alteration and/or tissue type. This review will focus on integrating current models and will highlight data that provide new insight into the signals that function to suppress human tumor development.

Updates on p53: modulation of p53 degradation as a therapeutic approach

The p53 pathway is aberrant in most human tumours with over 50% expressing mutant p53 proteins. The pathway is critically controlled by protein degradation. Here, we discuss the latest developments in the search for small molecules that can modulate p53 pathway protein stability and restore p53 activity for cancer therapy.

Hepatic IGFBP1 is a prosurvival factor that binds to BAK, protects the liver from apoptosis, and antagonizes the proapoptotic actions of p53 at mitochondria

Liver is generally refractory to apoptosis induced by the p53 tumor suppressor protein, but the molecular basis remains poorly understood. Here we show that p53 transcriptional activation leads to enhanced expression of hepatic IGFBP1 (insulin-like growth factor-binding protein-1). Exhibiting a previously unanticipated role, a portion of intracellular IGFBP1 protein localizes to mitochondria where it binds to the proapoptotic protein BAK and hinders BAK activation and apoptosis induction. Interestingly, in many cells and tissues p53 also has a direct apoptotic function at mitochondria that includes BAK binding and activation. When IGFBP1 is in a complex with BAK, formation of a proapoptotic p53/BAK complex and apoptosis induction are impaired, both in cultured cells and in liver. In contrast, livers of IGFBP1-deficient mice exhibit spontaneous apoptosis that is accompanied by p53 mitochondrial accumulation and evidence of BAK oligomerization. These data support the importance of BAK as a mediator of p53's mitochondrial function. The results also identify IGFBP1 as a negative regulator of the BAK-dependent pathway of apoptosis, whose expression integrates the transcriptional and mitochondrial functions of the p53 tumor suppressor protein.

Implications of cellular senescence in tissue damage response, tumor suppression, and stem cell biology

Cellular senescence is characterized by an irreversible cell cycle arrest that, when bypassed by mutation, contributes to cellular immortalization. Activated oncogenes induce a hyperproliferative response, which might be one of the senescence cues. We have found that expression of such an oncogene, Akt, causes senescence in primary mouse hepatoblasts in vitro. Additionally, AKT-driven tumors undergo senescence in vivo following p53 reactivation and show signs of differentiation. In another in vivo system, i.e., liver fibrosis, hyperproliferative signaling through AKT might be a driving force of the senescence in activated hepatic stellate cells. Senescent cells up-regulate and secrete molecules that, on the one hand, can reinforce the arrest and, on the other hand, can signal to an innate immune system to clear the senescent cells. The mechanisms governing senescence and immortalization are overlapping with those regulating self-renewal and differentiation. These respective control mechanisms, or their disregulation, are involved in multiple pathological conditions including fibrosis, wound healing, and cancer. Understanding extracellular cues that regulate these processes may enable new therapies for these conditions.

Beta-catenin induces immortalization of melanocytes by suppressing p16INK4a expression and cooperates with N-Ras in melanoma development

Tumor progression is a multistep process in which proproliferation mutations must be accompanied by suppression of senescence. In melanoma, proproliferative signals are provided by activating mutations in NRAS and BRAF, whereas senescence is bypassed by inactivation of the p16(Ink4a) gene. Melanomas also frequently exhibit constitutive activation of the Wnt/beta-catenin pathway that is presumed to induce proliferation, as it does in carcinomas. We show here that, contrary to expectations, stabilized beta-catenin reduces the number of melanoblasts in vivo and immortalizes primary skin melanocytes by silencing the p16(Ink4a) promoter. Significantly, in a novel mouse model for melanoma, stabilized beta-catenin bypasses the requirement for p16(Ink4a) mutations and, together with an activated N-Ras oncogene, leads to melanoma with high penetrance and short latency. The results reveal that synergy between the Wnt and mitogen-activated protein (MAP) kinase pathways may represent an important mechanism underpinning the genesis of melanoma, a highly aggressive and increasingly common disease.

Telomerase, senescence and ageing

Telomeres serve to camouflage chromosome ends from the DNA damage response machinery. Telomerase activity is required to maintain telomeres. One consequence of telomere dysfunction is cellular senescence, a permanent growth arrest state. We review the key regulators of cellular senescence and recent in vivo evidence which supports p53-dependent senescence induced by short telomeres as a potent tumor suppressor pathway. The in vivo link between cellular senescence and tumor regression is also discussed. The relationship between short telomere length and ageing or disease states in various cells of the body is increasingly reported. Paradoxically, the introduction of telomerase is proposed as a method to combat ageing via cell therapy and a possible method to regenerate tissue, while telomerase inhibition and telomere shortening is suggested as a possible therapy to defeat cancers with intact p53. Researchers thus face the challenge of understanding the complex processes which regulate the potential benefits of both telomerase inhibition and activation.

[Cellular ageing, ageing and cancer]

Cellular division is essential for the survival of the multicellular organisms which contain renewable tissues. However, cellular division also puts the organisms in danger to develop any types of cancers. Cellular senescence has emerged in part as a tumor suppressor mechanism. Here we discuss the function and regulation of the tumor suppressor proteins INK4a/ARF in connection with replicative senescence, cancer and aging. double dagger.

Chemosensitization of head and neck cancer cells by PUMA

Head and neck squamous cell carcinoma (HNSCC) ranks the eighth most common cancer worldwide. The patients often present with advanced disease, which responds poorly to chemoradiation therapy. PUMA is a BH3-only Bcl-2 family protein and a p53 target that is required for apoptosis induced by p53 and various chemotherapeutic agents. In this study, we found that PUMA induction by chemotherapeutic agents is abrogated in most HNSCC cell lines. Adenoviral gene delivery of PUMA induced apoptosis and chemosensitization more potently than did adenoviral delivery of p53 in HNSCC cells. Finally, we showed that PUMA suppressed the growth of HNSCC xenograft tumors and sensitized them to cisplatin through induction of apoptosis. Our data suggest that absence of PUMA activation in HNSCC cells contributes to chemoresistance and that gene therapy with PUMA might be an efficient substitute for p53 to enhance the responses of HNSCC cells to chemotherapy.

Tumour suppressors in liver carcinogenesis

The circuitous cell signalling pathways of hepatocytes comprise several factors that operate to downgrade or even interrupt the transmission of a given signal. These down-regulating influences are essential to keep cell proliferation and cell survival in check and if impaired, can alter a delicate balance in favour of cell proliferation. Each signalling pathway that has been implicated in carcinogenesis is influenced by both oncogenic factors that promote tumour growth when activated as well as tumour suppressor proteins that have to be impaired to favour tumour growth. This summary of the Tumour Suppressors in Liver Carcinogenesis Symposium held at the 2007 EASL Annual Meeting discusses four pathways with pre-eminent tumour suppressor activity, each involved in hepatocarcinogenesis: p53, mTOR, beta-catenin and hedgehog.

Molecular characteristics of immunogenic cancer cell death

Apoptotic cell death is initiated by a morphologically homogenous entity that was considered to be non-immunogenic and non-inflammatory in nature. However, recent advances suggest that apoptosis, under certain circumstances, can be immunogenic. In particular, some characteristics of the plasma membrane, acquired at preapoptotic stage, can cause immune effectors to recognize and attack preapoptotic tumor cells. The signals that mediate the immunogenicity of tumor cells involve elements of the DNA damage response (such as ataxia telangiectasia mutated and p53 activation), elements of the endoplasmic reticulum stress response (such as eukaryotic initiation factor 2alpha phosphorylation), as well as elements of the apoptotic response (such as caspase activation). Depending on the signal-transduction pathway, tumor cells responding to chemotherapy or radiotherapy can express 'danger' and 'eat me' signals on the cell surface (such as NKG2D ligands, heat-shock proteins and calreticulin) or can secrete/release immunostimulatory factors (such as cytokines and high-mobility group box 1) to stimulate innate immune effectors. Likewise, the precise sequence of such events influences the 'decision' of the immune system to mount a cognate response or not. We therefore anticipate that the comprehension of the mechanisms governing the immunogenicity of cell death will have a profound impact on the design of anticancer therapies.

Decoding dangerous death: how cytotoxic chemotherapy invokes inflammation, immunity or nothing at all

Chemotherapy and immunotherapy can be either synergistic or antagonistic modalities in the treatment of cancer. Cytotoxic chemotherapy not only affects the tumor but also targets dividing lymphocytes, the very cells that are required to develop an immune response. For this reason, chemo- and immunotherapy have been seen as antagonistic. However, cell death can be immunogenic and the way in which chemotherapeutic drug kills a tumor cell is likely to be an important determinant of how that dying cell interacts with the immune system and whether the interaction will lead to an immune response. When a cell dies as the result of infection, the immune system responds rapidly and the system of Toll-like receptors (TLR) plays a key role in this process. In this review, we will briefly summarize the intracellular signaling pathways that link TLR ligation with immune activation and we will address the questions where and how TLRs recognize their targets.

Tumor stress, cell death and the ensuing immune response

A cornucopia of physiological and pathological circumstances including anticancer chemotherapy and radiotherapy can induce cell death. However, the immunological consequences of tumor cell demise have remained largely elusive. The paradigm opposing 'apoptosis versus necrosis' as to their respective immunogenicity does not currently hold to predict long-term immunity. Moreover, the notion that tumor cells may be 'stressed' before death to be recognized by immune cells deserves to be underlined. 'Eat-me', 'danger' and 'killing' signals released by stressed tumor under the pressure of cytotoxic compounds may serve as links between the chemotherapy-elicited response of tumor cells and subsequent immune responses. This review will summarize the state-of-the-art of cancer immunity and describe how tumor cell death dictates the links between innate and acquired immunity.

Breaking news: high-speed race ends in arrest--how oncogenes induce senescence

Oncogene activation in normal cells induces a permanent proliferative arrest known as cellular senescence. This phenomenon restrains the expansion of cells that bear an activated oncogene and acts as a powerful tumor-suppressive process. Although the full molecular mechanisms are still being elucidated, it has been observed recently that some oncogenes alter the DNA-replication process and cause DNA-damage accumulation. DNA-damage checkpoint-response activation together with the increased appearance of heterochromatin formation that leads to transcriptional silencing of proliferative genes are, presently, the two main mechanisms known that establish and maintain oncogene-induced senescence. Here, we discuss the most recent advancements in understanding the molecular and cellular mechanisms that control cellular senescence caused by oncogene activation and their impact on cancer studies.

Models, mechanisms and clinical evidence for cancer dormancy

Patients with cancer can develop recurrent metastatic disease with latency periods that range from years even to decades. This pause can be explained by cancer dormancy, a stage in cancer progression in which residual disease is present but remains asymptomatic. Cancer dormancy is poorly understood, resulting in major shortcomings in our understanding of the full complexity of the disease. Here, I review experimental and clinical evidence that supports the existence of various mechanisms of cancer dormancy including angiogenic dormancy, cellular dormancy (G0-G1 arrest) and immunosurveillance. The advances in this field provide an emerging picture of how cancer dormancy can ensue and how it could be therapeutically targeted.

Targeted therapy for Kaposi's sarcoma and Kaposi's sarcoma-associated herpesvirus

PURPOSE OF REVIEW

To summarize major recent findings on the biology of human herpesvirus-8, i.e. Kaposi's sarcoma-associated herpesvirus, and the implications of these findings for Kaposi's sarcoma treatment.

RECENT FINDINGS

Although reduced in incidence in developed countries since the introduction of highly active antiretroviral therapy, Kaposi's sarcoma incidence is still markedly increased in HIV-infected patients in resource-rich areas of the world and is a major complication among HIV-infected individuals in sub-Saharan Africa. The Akt/mammalian target of rapamycin pathway has emerged as a major driving force in Kaposi's sarcoma. In addition, the roles of p53, the Kaposi's sarcoma-associated herpesvirus viral cyclin and nuclear factor-kappaB in the development and progression of Kaposi's sarcoma are being further clarified, and therapeutic agents are being developed that may target these pathogenetic mechanisms. New Kaposi's sarcoma treatments should be considered that target the molecular interface between virus and host.

SUMMARY

The growing knowledge of Kaposi's sarcoma biology provides multiple opportunities for rational targeted therapies. Further research is needed to better understand the mechanisms by which Kaposi's sarcoma develops and to develop therapeutic strategies that prevent resistance to treatment.

Cellular senescence: when bad things happen to good cells

Cells continually experience stress and damage from exogenous and endogenous sources, and their responses range from complete recovery to cell death. Proliferating cells can initiate an additional response by adopting a state of permanent cell-cycle arrest that is termed cellular senescence. Understanding the causes and consequences of cellular senescence has provided novel insights into how cells react to stress, especially genotoxic stress, and how this cellular response can affect complex organismal processes such as the development of cancer and ageing.

Shaping genetic alterations in human cancer: the p53 mutation paradigm

p53 mutations are found in 50% of human cancers. Molecular epidemiology has shown strong correlations between the spectrum of p53 mutations and exposure to exogenous carcinogens. This spectrum is influenced quantitatively and qualitatively by various upstream genetic filters that modulate carcinogen activation, detoxification, and/or DNA repair. In this review, we will discuss how other factors such as tissue specificity, SNP of genes associated with the p53 pathway, other genetic alterations, or p53 mutant heterogeneity can act as a second set of downstream filters that also have a profound impact on the spectrum of p53 mutations.

Targeting ubiquitin specific proteases for drug discovery

Deregulation of the ubiquitin-proteasome system has been implicated in the pathogenesis of many human diseases, including cancer, neurodegenerative disorders and viral diseases. The recent approval of the proteasome inhibitor bortezomib (Velcade) for the treatment of multiple myeloma and mantle cell lymphoma establishes this system as a valid target for cancer treatment. A promising alternative to targeting the proteasome itself would be to interact at the level of the upstream, ubiquitin conjugation/deconjugation system to generate more specific, less toxic anticancer agents. Ubiquitin specific proteases (USP) are de-ubiquitinating enzymes which remove ubiquitin from specific protein substrates and allow protein salvage from proteasome degradation, regulation of protein localization or activation. Due to their protease activity and their involvement in several pathologies, USPs are emerging as potential target sites for pharmacological interference in the ubiquitin regulatory machinery. We will review here this class of enzymes from target validation to small molecule drug discovery.

Cellular senescence in cancer and aging

Cellular senescence, a state of irreversible growth arrest, can be triggered by multiple mechanisms including telomere shortening, the epigenetic derepression of the INK4a/ARF locus, and DNA damage. Together these mechanisms limit excessive or aberrant cellular proliferation, and so the state of senescence protects against the development of cancer. Recent evidence suggests that cellular senescence also may be involved in aging.

Maximizing mouse cancer models

Animal models of cancer provide an alternative means to determine the causes of and treatments for malignancy, thus representing a resource of immense potential for cancer medicine. The sophistication of modelling cancer in mice has increased to the extent that investigators can both observe and manipulate a complex disease process in a manner impossible to perform in patients. However, owing to limitations in model design and technology development, and the surprising underuse of existing models, only now are we realising the full potential of mouse models of cancer and what new approaches are needed to derive the maximum value for cancer patients from this investment.

Cancer and ageing: convergent and divergent mechanisms

Cancer and ageing are both fuelled by the accumulation of cellular damage. Consequently, those mechanisms that protect cells from damage simultaneously provide protection against cancer and ageing. By contrast, cancer and longevity require a durable cell proliferation potential and, therefore, those mechanisms that limit indefinite proliferation provide cancer protection but favour ageing. The overall balance between these convergent and divergent mechanisms guarantees fitness and a cancer-free life until late adulthood for most individuals.

Utilizing RNA interference to enhance cancer drug discovery

With the development of RNA interference (RNAi) libraries, systematic and cost-effective genome-wide loss-of-function screens can now be carried out with the aim of assessing the role of specific genes in neoplastic phenotypes, and the rapid identification of novel drug targets. Here, we discuss the existing applications of RNAi in cancer drug discovery and highlight areas in this process that may benefit from this technology in the future.

p53 Aerosol Formulation with Low Toxicity and High Efficiency for Early Lung Cancer Treatment

PURPOSE

To develop an optimal nonviral aerosol formulation for locoregional treatment of early lung cancer.

EXPERIMENTAL DESIGN

The formulation was made of polylysine/protamine combination (AND) as the carrier and p53 gene (p53sm) as therapeutic agent. To estimate the aerosol deposition, the aerodynamic size of the AND-p53sm was measured with extrusion-precipitation method. To accurately determine the dose, the aerosol efficiency in mice was measured with a fluorescent dye. The transfection efficiency and DNA protection function of the aerosolized formulation in cultured cells and mouse lungs were detected with reporter gene assays and/or reverse transcription-PCR. The preclinical safety and efficacy of AND-p53sm were studied in healthy mice and mice bearing orthotopic human non-small-cell lung cancer (NSCLC) xenograft.

RESULTS

After aerosolization, AND is 3- to 17-fold more effective than commonly used PEI or cationic lipid formulations in transfecting the NSCLC cells (relative light units, 1,494 versus 534 and 86; P < 0.003). Aerodynamic size of AND-p53sm ranged 0.2 to 3 mum is the optimal aerosol droplets for deposition in the entire human respiratory tract. Significant gene expression was detected in the lungs of mice given aerosolized AND-p53sm and AND-luciferase. Aerosolized AND-p53sm significantly prolonged the life of mice bearing orthotopic human NSCLC xenografts, and it was more effective than an optimal i.v. cisplatin chemotherapy (increased life span, 93% versus 25%; P = 0.014). Inhalation of AND produced low and reversible pulmonary toxicity and no systemic toxicity.

CONCLUSIONS

This optimal formulation is suitable for delivering biological materials to human lung with aerosol administration. This therapeutic strategy is an option for patients with early lung cancer and bronchoalveolar carcinoma.

Radiosensitization of prostate cancer by priming the wild-type p53-dependent cellular senescence pathway

A dramatic stage-migration in diagnosis of prostate cancer has led to earlier detection of clinically localized carcinoma and an increased use of radiation therapy. The p53 protein responds to irradiation-induced DNA damage by removing critically damaged cells from the proliferative pool. This review will focus on the dominant role that p53-dependent cellular senescence, rather than cell death, plays in determining the radiosensitivity of human prostate cancer cells in vitro. The finding that senescence is a primary mechanism of tumor regression indicates that p53 activators or downstream effectors may prove effective in radiosensitizing some carcinoma of the prostate.

The common biology of cancer and ageing

At first glance, cancer and ageing would seem to be unlikely bedfellows. Yet the origins for this improbable union can actually be traced back to a sequence of tragic--and some say unethical--events that unfolded more than half a century ago. Here we review the series of key observations that has led to a complex but growing convergence between our understanding of the biology of ageing and the mechanisms that underlie cancer.

Induction of p53-Dependent Senescence by the MDM2 Antagonist Nutlin-3a in Mouse Cells of Fibroblast Origin

Cellular senescence is emerging as an important in vivo anticancer response elicited by multiple stresses, including currently used chemotherapeutic drugs. Nutlin-3a is a recently discovered small-molecule antagonist of the p53-destabilizing protein murine double minute-2 (MDM2) that induces cell cycle arrest and apoptosis in cancer cells with functional p53. Here, we report that nutlin-3a induces cellular senescence in murine primary fibroblasts, oncogenically transformed fibroblasts, and fibrosarcoma cell lines. No evidence of drug-induced apoptosis was observed in any case. Nutlin-induced senescence was strictly dependent on the presence of functional p53 as revealed by the fact that cells lacking p53 were completely insensitive to the drug, whereas cells lacking the tumor suppressor alternative reading frame product of the CDKN2A locus underwent irreversible cell cycle arrest. Interestingly, irreversibility was achieved in neoplastic cells faster than in their corresponding parental primary cells, suggesting that nutlin-3a and oncogenic signaling cooperate in activating p53. Our current results suggest that senescence could be a major cellular outcome of cancer therapy by antagonists of the p53-MDM2 interaction, such as nutlin-3a.

Cellular senescence is an important mechanism of tumor regression upon c-Myc inactivation

Oncogene-induced senescence is an important mechanism by which normal cells are restrained from malignant transformation. Here we report that the suppression of the c-Myc (MYC) oncogene induces cellular senescence in diverse tumor types including lymphoma, osteosarcoma, and hepatocellular carcinoma. MYC inactivation was associated with prototypical markers of senescence, including acidic beta-gal staining, induction of p16INK4a, and p15INK4b expression. Moreover, MYC inactivation induced global changes in chromatin structure associated with the marked reduction of histone H4 acetylation and increased histone H3 K9 methylation. Osteosarcomas engineered to be deficient in p16INK4a or Rb exhibited impaired senescence and failed to exhibit sustained tumor regression upon MYC inactivation. Similarly, only after lymphomas were repaired for p53 expression did MYC inactivation induce robust senescence and sustained tumor regression. The pharmacologic inhibition of signaling pathways implicated in oncogene-induced senescence including ATM/ATR and MAPK did not prevent senescence associated with MYC inactivation. Our results suggest that cellular senescence programs remain latently functional, even in established tumors, and can become reactivated, serving as a critical mechanism of oncogene addiction associated with MYC inactivation.

HIPK2 knock-down compromises tumor cell efficiency to repair damaged DNA

Homeodomain Interacting Protein Kinase-2 (HIPK2) is a protein with many functions and a modulator of p53 oncosuppressor functions. TP53 is the "guardian of the genome" thus, is the most critical tumor suppressor gene product that inhibits malignant transformation. P53R2 gene is directly induced by p53 in response to DNA damage and is involved in the p53 checkpoint for repairing damaged DNA to block genome instability. Here we wanted to explore the involvement of HIPK2 in damaged-DNA repair by regulating p53-induced p53R2 gene. We show that, induction of p53R2 expression, p53 recruitment onto p53R2 promoter, and its transcriptional activation was strongly impaired by HIPK2 knock-down, in response to drug. The failure of p53-induced p53R2 activation markedly compromised damaged-DNA repair efficiency. Finally, overexpression of exogenous p53 overcame the inability of endogenous p53 to activate p53R2-luc promoter in HIPK2 depleted cells. These data suggest that HIPK2 is involved in damaged-DNA repair taking part in restraining tumor progression, at least in part depending on p53 regulation.

Indole-3-carbinol - induced growth inhibition can be converted to a cytotoxic response in the presence of TPA+Ca(2+) in squamous cell carcinoma cell lines

We examined the possibility that I3C, when combined with a differentiation stimulus (TPA+CaCl(2)), would sensitise SCC cells to a differentiation stimulus. We report that I3C induces a profound growth inhibition in SCC cells that is dissimilar to the growth inhibition required to initiate differentiation. Moreover, we report that I3C, when combined with TPA+CaCl(2) treatment, induces a loss of colony forming ability that was differentiation and senescence - independent but was due to delayed cytotoxicity. This study shows that I3C in combination with a PKC activator+Ca(2+) may be a useful therapeutic strategy for treating oral SCC.

How important are post-translational modifications in p53 for selectivity in target-gene transcription and tumour suppression?

A number of elegant studies exploring the consequences of expression of various mutant forms of p53 in mice have been published over the last years. The results and conclusions drawn from these studies often contradict results previously obtained in biochemical assays and cell biology studies, questioning their relevance for p53 function in vivo. Owing to the multitude of post-translational modifications imposed on p53, however, the in vivo validation of their relevance for proper protein function and tumour suppression is constantly lagging behind new biochemical discoveries. Nevertheless, mouse genetics presents again its enormous power. Despite being relatively slow and tedious, it has become indispensable for researchers to sort out the wheat from the chaff in an endless sea of publications on p53.

Tissue-specific and reversible RNA interference in transgenic mice

Genetically engineered mice provide powerful tools for understanding mammalian gene function. These models traditionally rely on gene overexpression from transgenes or targeted, irreversible gene mutation. By adapting the tetracycline (tet)-responsive system previously used for gene overexpression, we have developed a simple transgenic system to reversibly control endogenous gene expression using RNA interference (RNAi) in mice. Transgenic mice harboring a tet-responsive RNA polymerase II promoter driving a microRNA-based short hairpin RNA targeting the tumor suppressor Trp53 reversibly express short hairpin RNA when crossed with existing mouse strains expressing general or tissue-specific 'tet-on' or 'tet-off' transactivators. Reversible Trp53 knockdown can be achieved in several tissues, and restoring Trp53 expression in lymphomas whose development is promoted by Trp53 knockdown leads to tumor regression. By leaving the target gene unaltered, this approach permits tissue-specific, reversible regulation of endogenous gene expression in vivo, with potential broad application in basic biology and drug target validation.

A microRNA component of the p53 tumour suppressor network

A global decrease in microRNA (miRNA) levels is often observed in human cancers, indicating that small RNAs may have an intrinsic function in tumour suppression. To identify miRNA components of tumour suppressor pathways, we compared miRNA expression profiles of wild-type and p53-deficient cells. Here we describe a family of miRNAs, miR-34a-c, whose expression reflected p53 status. Genes encoding miRNAs in the miR-34 family are direct transcriptional targets of p53, whose induction by DNA damage and oncogenic stress depends on p53 both in vitro and in vivo. Ectopic expression of miR-34 induces cell cycle arrest in both primary and tumour-derived cell lines, which is consistent with the observed ability of miR-34 to downregulate a programme of genes promoting cell cycle progression. The p53 network suppresses tumour formation through the coordinated activation of multiple transcriptional targets, and miR-34 may act in concert with other effectors to inhibit inappropriate cell proliferation.