A p53-mediated DNA damage response limits reprogramming to ensure iPS cell genomic integrity

The reprogramming of differentiated cells to pluripotent cells (induced pluripotent stem (iPS) cells) is known to be an inefficient process. We recently reported that cells with short telomeres cannot be reprogrammed to iPS cells despite their normal proliferation rates, probably reflecting the existence of 'reprogramming barriers' that abort the reprogramming of cells with uncapped telomeres. Here we show that p53 (also known as Trp53 in mice and TP53 in humans) is critically involved in preventing the reprogramming of cells carrying various types of DNA damage, including short telomeres, DNA repair deficiencies, or exogenously inflicted DNA damage. Reprogramming in the presence of pre-existing, but tolerated, DNA damage is aborted by the activation of a DNA damage response and p53-dependent apoptosis. Abrogation of p53 allows efficient reprogramming in the face of DNA damage and the generation of iPS cells carrying persistent DNA damage and chromosomal aberrations. These observations indicate that during reprogramming cells increase their intolerance to different types of DNA damage and that p53 is critical in preventing the generation of human and mouse pluripotent cells from suboptimal parental cells.

Up-regulation of the Notch ligand Delta-like 4 inhibits VEGF-induced endothelial cell function

Delta-like 4 (Dll4), a membrane-bound ligand for Notch1 and Notch4, is selectively expressed in the developing endothelium and in some tumor endothelium, and it is induced by vascular endothelial growth factor (VEGF)-A and hypoxia. Gene targeting studies have shown that Dll4 is required for normal embryonic vascular remodeling, but the mechanisms underlying Dll4 regulatory functions are currently not defined. In this study, we generated primary human endothelial cells that overexpress Dll4 protein to study Dll4 function and mechanism of action. Human umbilical vein endothelial cells retrovirally transduced with Dll4 displayed reduced proliferative and migratory responses selectively to VEGF-A. Expression of VEGF receptor-2, the principal signaling receptor for VEGF-A in endothelial cells, and coreceptor neuropilin-1 was significantly decreased in Dll4-transduced endothelial cells. Consistent with Dll4 signaling through Notch, expression of HEY2, one of the transcription factors that mediates Notch function, was significantly induced in Dll4-overexpressing endothelial cells. The gamma-secretase inhibitor L-685458 significantly reconstituted endothelial cell proliferation inhibited by immobilized extracellular Dll4 and reconstituted VEGFR2 expression in Dll4-overexpressing endothelial cells. These results identify the Notch ligand Dll4 as a selective inhibitor of VEGF-A biologic activities down-regulating 2 VEGF receptors expressed on endothelial cells and raise the possibility that Dll4 may be exploited therapeutically to modulate angiogenesis.

Global chromatin compaction limits the strength of the DNA damage response

In response to DNA damage, chromatin undergoes a global decondensation process that has been proposed to facilitate genome surveillance. However, the impact that chromatin compaction has on the DNA damage response (DDR) has not directly been tested and thus remains speculative. We apply two independent approaches (one based on murine embryonic stem cells with reduced amounts of the linker histone H1 and the second making use of histone deacetylase inhibitors) to show that the strength of the DDR is amplified in the context of “open” chromatin. H1-depleted cells are hyperresistant to DNA damage and present hypersensitive checkpoints, phenotypes that we show are explained by an increase in the amount of signaling generated at each DNA break. Furthermore, the decrease in H1 leads to a general increase in telomere length, an as of yet unrecognized role for H1 in the regulation of chromosome structure. We propose that slight differences in the epigenetic configuration might account for the cell-to-cell variation in the strength of the DDR observed when groups of cells are challenged with DNA breaks.

ATM regulates ATR chromatin loading in response to DNA double-strand breaks

DNA double-strand breaks (DSBs) are among the most deleterious lesions that can challenge genomic integrity. Concomitant to the repair of the breaks, a rapid signaling cascade must be coordinated at the lesion site that leads to the activation of cell cycle checkpoints and/or apoptosis. In this context, ataxia telangiectasia mutated (ATM) and ATM and Rad-3-related (ATR) protein kinases are the earliest signaling molecules that are known to initiate the transduction cascade at damage sites. The current model places ATM and ATR in separate molecular routes that orchestrate distinct pathways of the checkpoint responses. Whereas ATM signals DSBs arising from ionizing radiation (IR) through a Chk2-dependent pathway, ATR is activated in a variety of replication-linked DSBs and leads to activation of the checkpoints in a Chk1 kinase-dependent manner. However, activation of the G2/M checkpoint in response to IR escapes this accepted paradigm because it is dependent on both ATM and ATR but independent of Chk2. Our data provides an explanation for this observation and places ATM activity upstream of ATR recruitment to IR-damaged chromatin. These data provide experimental evidence of an active cross talk between ATM and ATR signaling pathways in response to DNA damage.

Oncogenic stress sensitizes murine cancers to hypomorphic suppression of ATR

Oncogenic Ras and p53 loss-of-function mutations are common in many advanced sporadic malignancies and together predict a limited responsiveness to conventional chemotherapy. Notably, studies in cultured cells have indicated that each of these genetic alterations creates a selective sensitivity to ataxia telangiectasia and Rad3-related (ATR) pathway inhibition. Here, we describe a genetic system to conditionally reduce ATR expression to 10% of normal levels in adult mice to compare the impact of this suppression on normal tissues and cancers in vivo. Hypomorphic suppression of ATR minimally affected normal bone marrow and intestinal homeostasis, indicating that this level of ATR expression was sufficient for highly proliferative adult tissues. In contrast, hypomorphic ATR reduction potently inhibited the growth of both p53-deficient fibrosarcomas expressing H-rasG12V and acute myeloid leukemias (AMLs) driven by MLL-ENL and N-rasG12D. Notably, DNA damage increased in a greater-than-additive fashion upon combining ATR suppression with oncogenic stress (H-rasG12V, K-rasG12D, or c-Myc overexpression), indicating that this cooperative genome-destabilizing interaction may contribute to tumor selectivity in vivo. This toxic interaction between ATR suppression and oncogenic stress occurred without regard to p53 status. These studies define a level of ATR pathway inhibition in which the growth of malignancies harboring oncogenic mutations can be suppressed with minimal impact on normal tissue homeostasis, highlighting ATR inhibition as a promising therapeutic strategy.

ATR signaling can drive cells into senescence in the absence of DNA breaks

The ATR kinase is a key transducer of "replicative stress," the type of genomic damage that has been postulated to be induced by oncogenes. Here we describe a cellular system in which we can unleash ATR activity at will, in the absence of any actual damage or additional signaling pathways triggered by DNA breaks. We demonstrate that activating ATR is sufficient to promote cell cycle arrest and, if persistent, triggers p53-dependent but Ink4a/ARF-independent senescence. Moreover, we show that an ectopic activation of ATR leads to a G1/S arrest in ATM-/- cells, providing the first evidence of functional complementation of ATM deficiency by ATR. Our system provides a novel platform for the study of the specific functions of ATR signaling and adds evidence for the tumor-suppressive potential of the DNA damage response.

Mutation of E2F2 in mice causes enhanced T lymphocyte proliferation, leading to the development of autoimmunity

E2Fs are important regulators of proliferation, differentiation, and apoptosis. Here we characterize the phenotype of mice deficient in E2F2. We show that E2F2 is required for immunologic self-tolerance. E2F2(-/-) mice develop late-onset autoimmune features, characterized by widespread inflammatory infiltrates, glomerular immunocomplex deposition, and anti-nuclear antibodies. E2F2-deficient T lymphocytes exhibit enhanced TCR-stimulated proliferation and a lower activation threshold, leading to the accumulation of a population of autoreactive effector/memory T lymphocytes, which appear to be responsible for causing autoimmunity in E2F2-deficient mice. Finally, we provide support for a model to explain E2F2's unexpected role as a suppressor of T lymphocyte proliferation. Rather than functioning as a transcriptional activator, E2F2 appears to function as a transcriptional repressor of genes required for normal S phase entry, particularly E2F1.

Neuropilin-1 regulates attachment in human endothelial cells independently of vascular endothelial growth factor receptor-2

Neuropilin-1 (NRP-1) is a type 1 membrane protein that binds the axon guidance factors belonging to the class-3 semaforin family. In endothelial cells, NRP-1 serves as a co-receptor for vascular endothelial growth factor (VEGF) and regulates VEGF receptor 2 (VEGFR-2)-dependent angiogenesis. Although gene-targeting studies documenting embryonic lethality in NRP-1 null mice have demonstrated a critical role for NRP-1 in vascular development, the activities of NRP-1 in mature endothelial cells have been incompletely defined. Using RNA interference-mediated silencing of NRP-1 or VEGFR-2 in primary human endothelial cells, we confirm that NRP-1 modulates VEGFR-2 signaling-dependent mitogenic functions of VEGF. Importantly, we now show that NRP-1 regulates endothelial cell adhesion to extracellular matrix proteins independently of VEGFR-2. Based on its dual role as an enhancer of VEGF activity and a mediator of endothelial cell adhesiveness described here, NRP-1 emerges as a promising molecular target for the development of antiangiogenic drugs.

BRCA1 Haploinsufficiency Is Masked by RNF168-Mediated Chromatin Ubiquitylation

BRCA1 functions at two distinct steps during homologous recombination (HR). Initially, it promotes DNA end resection, and subsequently it recruits the PALB2 and BRCA2 mediator complex, which stabilizes RAD51-DNA nucleoprotein filaments. Loss of 53BP1 rescues the HR defect in BRCA1-deficient cells by increasing resection, suggesting that BRCA1's downstream role in RAD51 loading is dispensable when 53BP1 is absent. Here we show that the E3 ubiquitin ligase RNF168, in addition to its canonical role in inhibiting end resection, acts in a redundant manner with BRCA1 to load PALB2 onto damaged DNA. Loss of RNF168 negates the synthetic rescue of BRCA1 deficiency by 53BP1 deletion, and it predisposes BRCA1 heterozygous mice to cancer. BRCA1^+/-RNF168^-/- cells lack RAD51 foci and are hypersensitive to PARP inhibitor, whereas forced targeting of PALB2 to DNA breaks in mutant cells circumvents BRCA1 haploinsufficiency. Inhibiting the chromatin ubiquitin pathway may, therefore, be a synthetic lethality strategy for BRCA1-deficient cancers.

Efficacy of ATR inhibitors as single agents in Ewing sarcoma

Ewing sarcomas (ES) are pediatric bone tumors that arise from a driver translocation, most frequently EWS/FLI1. Current ES treatment involves DNA damaging agents, yet the basis for the sensitivity to these therapies remains unknown. Oncogene-induced replication stress (RS) is a known source of endogenous DNA damage in cancer, which is suppressed by ATR and CHK1 kinases. We here show that ES suffer from high endogenous levels of RS, rendering them particularly dependent on the ATR pathway. Accordingly, two independent ATR inhibitors show in vitro toxicity in ES cell lines as well as in vivo efficacy in ES xenografts as single agents. Expression of EWS/FLI1 or EWS/ERG oncogenic translocations sensitizes non-ES cells to ATR inhibitors. Our data shed light onto the sensitivity of ES to genotoxic agents, and identify ATR inhibitors as a potential therapy for Ewing Sarcomas.

ATR suppresses telomere fragility and recombination but is dispensable for elongation of short telomeres by telomerase

Telomere shortening caused by incomplete DNA replication is balanced by telomerase-mediated telomere extension, with evidence indicating that the shortest telomeres are preferred substrates in primary cells. Critically short telomeres are detected by the cellular DNA damage response (DDR) system. In budding yeast, the important DDR kinase Tel1 (homologue of ATM [ataxia telangiectasia mutated]) is vital for telomerase recruitment to short telomeres, but mammalian ATM is dispensable for this function. We asked whether closely related ATR (ATM and Rad3 related) kinase, which is important for preventing replicative stress and chromosomal breakage at common fragile sites, might instead fulfill this role. The newly created ATR-deficient Seckel mouse strain was used to examine the function of ATR in telomerase recruitment and telomere function. Telomeres were recently found to resemble fragile sites, and we show in this study that ATR has an important role in the suppression of telomere fragility and recombination. We also find that wild-type ATR levels are important to protect short telomeres from chromosomal fusions but do not appear essential for telomerase recruitment to short telomeres in primary mouse embryonic fibroblasts from the ATR-deficient Seckel mouse model. These results reveal a previously unnoticed role for mammalian ATR in telomere protection and stability.

Immunoreactivity for the group III metabotropic glutamate receptor subtype mGluR4a in the superficial laminae of the rat spinal dorsal horn

Studies indicate that metabotropic glutamate receptors (mGluRs) may play a role in spinal sensory transmission. We examined the cellular and subcellular distribution of the mGluR subtype 4a in spinal tissue by means of a specific antiserum and immunocytochemical techniques for light and electron microscopy. A dense plexus of mGluR4a-immunoreactive elements was seen in the dorsal horn, with an apparent accumulation in lamina II. The immunostaining was composed of sparse immunoreactive fibres and punctate elements. No perikaryal staining was seen. Immunostaining for mGluR4a was detected in small to medium-sized cells but not in large cells in dorsal root ganglia. At the electron microscopic level, superficial dorsal horn laminae demonstrated numerous immunoreactive vesicle-containing profiles. Labelling was present in the cytoplasmic matrix, but accretion of immunoreaction product to presynaptic specialisations was commonly observed. Axolemmal labelling was confirmed by using a preembedding immunogold technique, which revealed distinctive deposits of gold immunoparticles along presynaptic thickenings with an average centre-to-centre distance of 41 nm (41.145 +/- 13.59). Immunoreactive terminals often formed synaptic contacts with dendritic profiles immunonegative for mGluR4a. Immunonegative dendritic profiles were observed in apposition to both mGluR4a-immunoreactive and immunonegative terminals. Diffuse immunoperoxidase reaction product was also detected in dendritic profiles, some of which were contacted by mGluR4a-immunoreactive endings, but only occasionally were they observed to accumulate immunoreaction product along the postsynaptic density. Terminals immunoreactive for mGluR4a also formed axosomatic contacts. The present results reveal that mGluR4a subserves a complex spinal circuitry to which the primary afferent system seems to be a major contributor.

Targeting the kinase activities of ATR and ATM exhibits antitumoral activity in mouse models of MLL-rearranged AML

Among the various subtypes of acute myeloid leukemia (AML), those with chromosomal rearrangements of the MLL oncogene (AML-MLL) have a poor prognosis. AML-MLL tumor cells are resistant to current genotoxic therapies because of an attenuated response by p53, a protein that induces cell cycle arrest and apoptosis in response to DNA damage. In addition to chemicals that damage DNA, efforts have focused on targeting DNA repair enzymes as a general chemotherapeutic approach to cancer treatment. Here, we found that inhibition of the kinase ATR, which is the primary sensor of DNA replication stress, induced chromosomal breakage and death of mouse AML(MLL) cells (with an MLL-ENL fusion and a constitutively active N-RAS independently of p53. Moreover, ATR inhibition as a single agent exhibited antitumoral activity, both reducing tumor burden after establishment and preventing tumors from growing, in an immunocompetent allograft mouse model of AML(MLL) and in xenografts of a human AML-MLL cell line. We also found that inhibition of ATM, a kinase that senses DNA double-strand breaks, also promoted the survival of the AML(MLL) mice. Collectively, these data indicated that ATR or ATM inhibition represent potential therapeutic strategies for the treatment of AML, especially MLL-driven leukemias.

Derivation of Endothelial Cells from CD34−Umbilical Cord Blood

CD34 is a transmembrane glycoprotein constitutively expressed on endothelial cells and hematopoietic stem cells. Use of CD34-recognizing antibodies has helped in the identification and isolation of CD34+ endothelial precursors from embryonic and adult tissues. However, CD34-null mice display no vascular abnormalities, demonstrating that CD34 antigen expression is not required for normal vascular development. Here we show that a CD34- cell population that includes endothelial cell precursors can be isolated from cord blood. In the presence of angiogenic factors, these cells mature to express the endothelial cell markers vascular endothelial-cadherin, vascular endothelial growth factor receptor-1 and -2, Tie-1 and -2 (tyrosine kinase with immunoglobulin and epidermal growth factor homology domains), von Willebrand factor, and CD31 while maintaining their CD34- status, and can be expanded in vitro for over 20 passages. Moreover, in functional studies, these cells can undergo extracellular matrix-dependent morphogenic changes into capillary-like tubular structures. When transplanted into immunodeficient mice in conjunction with tumor cells or with the proangiogenic factor basic fibroblast growth factor, these cells can form functional microvessels arising along with host blood cells. These studies provide strong evidence for the existence of CD34- endothelial cell precursors in cord blood and suggest the use of ex vivo-expanded cord blood CD34- cells as a unique tool for the investigation of postnatal lineage diversification.

Increased Rrm2 gene dosage reduces fragile site breakage and prolongs survival of ATR mutant mice

In S. cerevisiae, deletion of the checkpoint kinase Mec1 (ATR) is viable upon mutations that increase the activity of the ribonucleotide reductase (RNR) complex. Lopez-Contreras et al. show that cells from mice carrying extra alleles of the RNR regulatory subunit RRM2 present supraphysiological RNR activity and reduced chromosomal breakage at fragile sites. Increased Rrm2 gene dosage also extends the life span of ATR mutant mice.

In Saccharomyces cerevisiae, absence of the checkpoint kinase Mec1 (ATR) is viable upon mutations that increase the activity of the ribonucleotide reductase (RNR) complex. Whether this pathway is conserved in mammals remains unknown. Here we show that cells from mice carrying extra alleles of the RNR regulatory subunit RRM2 (Rrm2^TG) present supraphysiological RNR activity and reduced chromosomal breakage at fragile sites. Moreover, increased Rrm2 gene dosage significantly extends the life span of ATR mutant mice. Our study reveals the first genetic condition in mammals that reduces fragile site expression and alleviates the severity of a progeroid disease by increasing RNR activity.

NSMCE2 suppresses cancer and aging in mice independently of its SUMO ligase activity

The SMC5/6 complex is the least understood of SMC complexes. In yeast, smc5/6 mutants phenocopy mutations in sgs1, the BLM ortholog that is deficient in Bloom's syndrome (BS). We here show that NSMCE2 (Mms21, in Saccharomyces cerevisiae), an essential SUMO ligase of the SMC5/6 complex, suppresses cancer and aging in mice. Surprisingly, a mutation that compromises NSMCE2-dependent SUMOylation does not have a detectable impact on murine lifespan. In contrast, NSMCE2 deletion in adult mice leads to pathologies resembling those found in patients of BS. Moreover, and whereas NSMCE2 deletion does not have a detectable impact on DNA replication, NSMCE2-deficient cells also present the cellular hallmarks of BS such as increased recombination rates and an accumulation of micronuclei. Despite the similarities, NSMCE2 and BLM foci do not colocalize and concomitant deletion of Blm and Nsmce2 in B lymphocytes further increases recombination rates and is synthetic lethal due to severe chromosome mis-segregation. Our work reveals that SUMO- and BLM-independent activities of NSMCE2 limit recombination and facilitate segregation; functions of the SMC5/6 complex that are necessary to prevent cancer and aging in mice.

Limited role of murine ATM in oncogene-induced senescence and p53-dependent tumor suppression

Recent studies in human fibroblasts have provided a new general paradigm of tumor suppression according to which oncogenic signaling produces DNA damage and this, in turn, results in ATM/p53-dependent cellular senescence. Here, we have tested this model in a variety of murine experimental systems. Overexpression of oncogenic Ras in murine fibroblasts efficiently induced senescence but this occurred in the absence of detectable DNA damage signaling, thus suggesting a fundamental difference between human and murine cells. Moreover, lung adenomas initiated by endogenous levels of oncogenic K-Ras presented abundant senescent cells, but undetectable DNA damage signaling. Accordingly, K-Ras-driven adenomas were also senescent in Atm-null mice, and the tumorigenic progression of these lesions was only modestly accelerated by Atm-deficiency. Finally, we have examined chemically-induced fibrosarcomas, which possess a persistently activated DNA damage response and are highly sensitive to the activity of p53. We found that the absence of Atm favored genomic instability in the resulting tumors, but did not affect the persistent DNA damage response and did not impair p53-dependent tumor suppression. All together, we conclude that oncogene-induced senescence in mice may occur in the absence of a detectable DNA damage response. Regarding murine Atm, our data suggest that it plays a minor role in oncogene-induced senescence or in p53-dependent tumor suppression, being its tumor suppressive activity probably limited to the maintenance of genomic stability.

DPSS yellow-green 561-nm lasers for improved fluorochrome detection by flow cytometry

INTRODUCTION

Blue-green 488-nm laser sources are widespread in flow cytometry but suffer some drawbacks for cell analysis, including their excitation of endogenous proteins (resulting in high cellular autofluorescence) and their less-than-optimal coincidence with the excitation maxima of commonly used fluorochromes, including the phycoerythrins (PE). Longer wavelength lasers such as green helium-neons and, more recently, diode-pumped solid state (DPSS) 532-nm sources have previously been employed to overcome these difficulties and improve overall sensitivity for PE. In this study, we evaluate an even longer wavelength DPSS 561-nm for its ability to improve PE and DsRed fluorescent protein detection sensitivity.

METHODS

A DPSS 561-nm laser emitting at 10 mW was mounted onto a BD LSR II. Mouse thymoma cells labeled with cell surface marker antibodies conjugated to the R- and B-forms of PE were analyzed and compared with conventional 488-nm excitation using the same bandpass filters and signal travel distances. A similar analysis was carried out with cell lines expressing the red fluorescent protein DsRed, several green-yellow excited low molecular weight fluorochromes, and a rhodamine-based caspase substrate. Additionally, cells labeled with PE and co-labeled with fluorescein or simultaneously expressing green fluorescent protein (GFP) were analyzed to determine if PE excitation at 561 nm with simultaneous fluorescein/GFP detection was feasible.

RESULTS

The DPSS 561-nm laser gave a several-fold improvement in the fluorochrome to autofluorescence ratios between PE-labeled cells and unlabeled controls. Analysis of cells expressing the fluorescent protein DsRed with the DPSS 561-nm source gave a 6-7-fold improvement in sensitivity over 488-nm excitation, and gave excellent excitation of yellow-green excited fluorochromes and rhodamine-based physiological probes. Yellow-green laser light also caused virtually no impingement on the spatially separated fluorescein/GFP detector, a significant problem with green laser sources, and also allowed simultaneous analysis of GFP and PE with virtually no signal overlap or requirement for color compensation.

CONCLUSIONS

DPSS 561-nm laser excitation gave significantly improved sensitivity for both PE-labeled and DsRed expressing cells, with little contamination of a typical fluorescein/GFP detector.

Intradiscal injection of oxygen-ozone gas mixture for the treatment of cervical disc herniations

For disc herniations the use of open surgical approaches is reduced since new percutaneous methods allowing shrinkage of the disc and improvement of the radicular function are gaining interest. Studies on the spontaneous disappearance of disc fragments have demonstrated autoimmune responses with a chronic inflammatory reaction. Also radicular pain has been shown to be mostly due to biochemical mechanisms. Researchers in different fields surprisingly noticed that a brief, calculated, oxidative stress by ozone administration may correct a persistent imbalance due to excessive, chronic oxidative injury. Oxygen-ozone gas injection in painful patients has a dramatic effect on clinical symptoms. On these bases the intradiscal injection of oxygen-ozone gas has been conceived. We report the treatment on a series of patients affected by cervical disc pathology, treated by intradiscal injection of oxygen-ozone gas mixture. The effects both on pain and on radicular dysfunction are impressive. The morphological effect of the treatment was also evaluated by pathological examination.

Diabetes and exocrine pancreatic insufficiency in E2F1/E2F2 double-mutant mice

E2F transcription factors are thought to be key regulators of cell growth control. Here we use mutant mouse strains to investigate the function of E2F1 and E2F2 in vivo. E2F1/E2F2 compound-mutant mice develop nonautoimmune insulin-deficient diabetes and exocrine pancreatic dysfunction characterized by endocrine and exocrine cell dysplasia, a reduction in the number and size of acini and islets, and their replacement by ductal structures and adipose tissue. Mutant pancreatic cells exhibit increased rates of DNA replication but also of apoptosis, resulting in severe pancreatic atrophy. The expression of genes involved in DNA replication and cell cycle control was upregulated in the E2F1/E2F2 compound-mutant pancreas, suggesting that their expression is repressed by E2F1/E2F2 activities and that the inappropriate cell cycle found in the mutant pancreas is likely the result of the deregulated expression of these genes. Interestingly, the expression of ductal cell and adipocyte differentiation marker genes was also upregulated, whereas expression of pancreatic cell marker genes were downregulated. These results suggest that E2F1/E2F2 activity negatively controls growth of mature pancreatic cells and is necessary for the maintenance of differentiated pancreatic phenotypes in the adult.

Activation of ARF by oncogenic stress in mouse fibroblasts is independent of E2F1 and E2F2

The ARF tumour suppressor protein (p14(ARF) in human and p19(ARF) in mouse) is a major mediator of the activation of p53 in response to oncogenic stress. Little is known about the signalling pathways connecting oncogenic stimuli to the activation of ARF. Regulation of ARF occurs primarily at the transcriptional level and several modulators of ARF transcription have been identified. Notably, ectopic expression of E2F1 upregulates ARF transcriptionally, and both E2F1 and ARF have been implicated in apoptosis and cell-cycle arrest. We have used primary mouse fibroblasts deficient for E2F1, E2F2, or both to determine the possible role of these E2F proteins as upstream regulators of ARF in response to oncogenic stimuli and other stresses. In particular, we have studied the effects of oncogenic Ras and the viral oncoprotein E1A on ARF levels, neoplastic transformation, and sensitization to apoptosis. We have also examined the behaviour of the E2F-deficient MEFs with respect to immortalization and sensitivity to DNA damage. None of the ARF-mediated responses that we have analysed is significantly affected in E2F1(-/-), E2F2(-/-) or E2F1/2(-/-) MEFs, and ARF is upregulated normally in all cases. Taken together, our results indicate that the activation of ARF in response to oncogenic stress can occur by E2F1- and E2F2-independent mechanisms. This challenges previous suggestions implicating E2F factors as key mediators in the activation of ARF by oncogenic stress.

A Chemical Screen Identifies Compounds Capable of Selecting for Haploidy in Mammalian Cells

The recent availability of somatic haploid cell lines has provided a unique tool for genetic studies in mammals. However, the percentage of haploid cells rapidly decreases in these cell lines, which we recently showed is due to their overgrowth by diploid cells present in the cultures. Based on this property, we have now performed a phenotypic chemical screen in human haploid HAP1 cells aiming to identify compounds that facilitate the maintenance of haploid cells. Our top hit was 10-Deacetyl-baccatin-III (DAB), a chemical precursor in the synthesis of Taxol, which selects for haploid cells in HAP1 and mouse haploid embryonic stem cultures. Interestingly, DAB also enriches for diploid cells in mixed cultures of diploid and tetraploid cells, including in the colon cancer cell line DLD-1, revealing a general strategy for selecting cells with lower ploidy in mixed populations of mammalian cells.

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Mammalian haploid cell cultures become progressively enriched in diploid cells

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DAB, a precursor of Taxol, facilitates the maintenance of haploidy

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DAB selects for cells with lower ploidy in mixed cultures of mammalian cells

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Statins accelerate the gradual loss of haploid cells in culture

INK4a/ARF limits the expansion of cells suffering from replication stress

Replication stress (RS) is a source of DNA damage that has been linked to cancer and aging, which is suppressed by the ATR kinase. In mice, reduced ATR levels in a model of the ATR-Seckel syndrome lead to RS and accelerated aging. Similarly, ATR-Seckel embryonic fibroblasts (MEF) accumulate RS and undergo cellular senescence. We previously showed that senescence of ATR-Seckel MEF cannot be rescued by p53-deletion. Here, we show that the genetic ablation of the INK4a/Arf locus fully rescues senescence on ATR mutant MEF, but also that induced by other conditions that generate RS such as low doses of hydroxyurea or ATR inhibitors. In addition, we show that a persistent exposure to RS leads to increased levels of INK4a/Arf products, revealing that INK4a/ARF behaves as a bona fide RS checkpoint. Our data reveal an unknown role for INK4a/ARF in limiting the expansion of cells suffering from persistent replication stress, linking this well-known tumor suppressor to the maintenance of genomic integrity.