Claspin operates downstream of TopBP1 to direct ATR signaling towards Chk1 activation

TopBP1 and Claspin are adaptor proteins that facilitate phosphorylation of Chk1 by the ATR kinase in response to genotoxic stress. Despite their established requirement for Chk1 activation, the exact way in which TopBP1 and Claspin control Chk1 phosphorylation remains unclear. We show that TopBP1 tightly colocalizes with ATR in distinct nuclear subcompartments generated by DNA damage. Although depletion of TopBP1 by RNA interference (RNAi) strongly impaired phosphorylation of multiple ATR targets, including Chk1, Nbs1, Smc1, and H2AX, it did not interfere with ATR assembly at the sites of DNA damage. These findings challenge the current concept of ATR activation by recruitment to damaged DNA. In contrast, Claspin, like Chk1, remained distributed throughout the nucleus both before and after DNA damage. Consistently, the RNAi-mediated ablation of Claspin selectively abrogated ATR's ability to phosphorylate Chk1 but not other ATR targets. In addition, downregulation of Claspin mimicked Chk1 inactivation by inducing spontaneous DNA damage. Finally, we show that TopBP1 is required for the DNA damage-induced interaction between Claspin and Chk1. Together, these results suggest that while TopBP1 is a general regulator of ATR, Claspin operates downstream of TopBP1 to selectively regulate the Chk1-controlled branch of the genotoxic stress response.

Structure meets function—Centrosomes, genome maintenance and the DNA damage response

Centrosomes are cytoplasmic organelles playing a fundamental role in organizing both the interphase cytoskeleton and the bipolar mitotic spindle. In addition, the centrosome has recently come into focus as part of the network that integrates cell cycle arrest and repair signals in response to genotoxic stress--the DNA damage response. One important mediator of this response, the checkpoint kinase Chk1, has been shown to negatively regulate the G(2)/M transition via its centrosomal localization. Moreover, there is growing evidence that a centrosome inactivation checkpoint exists, which utilizes DNA damage-induced centrosome fragmentation or amplification to provoke a "mitotic catastrophe" and eliminate damaged cells. Candidate regulators of this centrosomal checkpoint include the checkpoint kinase Chk2 and its upstream regulators ATM and ATR. In addition, a growing number of other proteins have been implicated in centrosomal regulation of the DNA damage response, e.g. the tumor suppressor p53, the breast cancer susceptibility gene product BRCA1 and mitotic regulators such as Aurora A, Nek2 and the Polo-like kinases Plk1 and Plk3. However, many missing links and discrepancies between different model systems remain.

Chromatin relaxation in response to DNA double-strand breaks is modulated by a novel ATM- and KAP-1 dependent pathway

The cellular DNA-damage response is a signaling network that is vigorously activated by cytotoxic DNA lesions, such as double-strand breaks (DSBs). The DSB response is mobilized by the nuclear protein kinase ATM, which modulates this process by phosphorylating key players in these pathways. A long-standing question in this field is whether DSB formation affects chromatin condensation. Here, we show that DSB formation is followed by ATM-dependent chromatin relaxation. ATM's effector in this pathway is the protein KRAB-associated protein (KAP-1, also known as TIF1beta, KRIP-1 or TRIM28), previously known as a corepressor of gene transcription. In response to DSB induction, KAP-1 is phosphorylated in an ATM-dependent manner on Ser 824. KAP-1 is phosphorylated exclusively at the damage sites, from which phosphorylated KAP-1 spreads rapidly throughout the chromatin. Ablation of the phosphorylation site of KAP-1 leads to loss of DSB-induced chromatin decondensation and renders the cells hypersensitive to DSB-inducing agents. Knocking down KAP-1, or mimicking a constitutive phosphorylation of this protein, leads to constitutive chromatin relaxation. These results suggest that chromatin relaxation is a fundamental pathway in the DNA-damage response and identify its primary mediators.

Cellular responses to DNA damage: Current state of the field and review of the 52nd Benzon Symposium

The response of cells to DNA damage is critical for maintaining genomic integrity and for preventing cancer development. Current advances in our understanding of the response of cells to DNA double-strand breaks and to stalled DNA replication forks and the relationship of these responses to human disease were discussed at the 52nd Benzon Symposium in Denmark, Copenhagen. Here we review the novel findings that were presented at this Symposium and the current state of the field.

Spatial organization of the mammalian genome surveillance machinery in response to DNA strand breaks

We show that DNA double-strand breaks (DSBs) induce complex subcompartmentalization of genome surveillance regulators. Chromatin marked by gamma-H2AX is occupied by ataxia telangiectasia-mutated (ATM) kinase, Mdc1, and 53BP1. In contrast, repair factors (Rad51, Rad52, BRCA2, and FANCD2), ATM and Rad-3-related (ATR) cascade (ATR, ATR interacting protein, and replication protein A), and the DNA clamp (Rad17 and -9) accumulate in subchromatin microcompartments delineated by single-stranded DNA (ssDNA). BRCA1 and the Mre11-Rad50-Nbs1 complex interact with both of these compartments. Importantly, some core DSB regulators do not form cytologically discernible foci. These are further subclassified to proteins that connect DSBs with the rest of the nucleus (Chk1 and -2), that assemble at unprocessed DSBs (DNA-PK/Ku70), and that exist on chromatin as preassembled complexes but become locally modified after DNA damage (Smc1/Smc3). Finally, checkpoint effectors such as p53 and Cdc25A do not accumulate at DSBs at all. We propose that subclassification of DSB regulators according to their residence sites provides a useful framework for understanding their involvement in diverse processes of genome surveillance.

‘Blind’ antibiotic treatment targeting Chlamydia is not effective in patients with MALT lymphoma of the ocular adnexa

BACKGROUND

Recent results have implicated Chlamydia, especially Chlamydia psittaci, in the development of ocular adnexal lymphoma in the large majority of patients. We present our experience with ex-juvantibus antibiotic treatment in patients diagnosed with MALT lymphoma of the ocular adnexa.

PATIENTS AND METHODS

A retrospective analysis identified a total of 11 patients (six female, five male) with MALT-lymphoma of the ocular adnexa who were given doxycyclin 200 mg p.o. daily over 3 weeks. Patients were tested also for autoimmune conditions, Helicobacter status and hepatitis along with assessment of MALT-lymphoma specific genetic changes.

RESULTS

After a median follow-up of 9 months, none of the patients responded to 'blind' antibiotic treatment with doxycyclin. Only one patient with bilateral conjunctival lymphoma related a short lasting subjective improvement, but was referred to alternative therapy due to progression and worsening symptoms after 6 months.

CONCLUSIONS

In this uncontrolled series, no effect of 'blind' antibiotic treatment with doxycyclin could be found in our patients with MALT lymphoma of the ocular adnexa. These results are in contrast to other series and suggest a potential geographic difference in the role of Chlamydia in ocular adnexal lymphoma. Thus, antibiotic therapy without prior testing for Chlamydia should be discouraged.

Fine-needle aspiration biopsy in the diagnostic of the tumors and non-neoplastic lesions of salivary glands

OBJECTIVES

Fine-needle aspiration biopsy (FNAB) has not been part of the basic algorithm of preoperative evaluation of tumors of the major salivary glands in the Czech Republic. Most opponents of salivary gland FNAB consider it unnecessary. This paper documents the utility of the punction aspiration biopsy in the diagnosis of these lesions.

METHODS

Between January 1998, and December 2003, 136 patients with clinically significant masses of the salivary glands were evaluated using FNAB. The parotid gland was involved in 68 patients, the submandibular gland in 58 patients and the region of the sublingual gland in 10 cases. In the retrospective analysis, the preoperative cytological findings were correlated with the postoperative histopathological evaluation.

RESULTS

Histopathological evaluation revealed 107 benign lesions (79%), of which 68 were non-neoplastic, and 39 were benign neoplasms, and 19 malignant tumors (21%). The cytological specimens were found to be non-diagnostic in 15 (11%) cases, true-positive in 68 (50%), true-negative in 40 (29.4%), false-positive in 1 (0.73%) and false-negative in 12 (8.8%) cases in detecting tumors. The sensitivity, specificity and diagnostic accurary aspiration cytology were 85.0%, 97.5% and 89.2%, respectively. We recorded complications in only one case (0.7%), a subcutaneous haematoma.

CONCLUSION

Our results document the utility of the FNAB in the diagnosis of salivary gland masses. FNAB does not substitute other diagnostic methods or as an adjunct to sound clinical judgement. Together with the clinical and ultrasonographical evaluations it should belong to the basic algorithm of diagnosis in salivary gland lesions (Tab. 3, Ref. 18).

ATM- and cell cycle-dependent regulation of ATR in response to DNA double-strand breaks

It is generally thought that the DNA-damage checkpoint kinases, ataxia-telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR), work independently of one another. Here, we show that ATM and the nuclease activity of meiotic recombination 11 (Mre11) are required for the processing of DNA double-strand breaks (DSBs) to generate the replication protein A (RPA)-coated ssDNA that is needed for ATR recruitment and the subsequent phosphorylation and activation of Chk1. Moreover, we show that efficient ATM-dependent ATR activation in response to DSBs is restricted to the S and G2 cell cycle phases and requires CDK kinase activity. Thus, in response to DSBs, ATR activation is regulated by ATM in a cell-cycle dependent manner.

Dynamic assembly and sustained retention of 53BP1 at the sites of DNA damage are controlled by Mdc1/NFBD1

53BP1 is a key component of the genome surveillance network activated by DNA double strand breaks (DSBs). Despite its known accumulation at the DSB sites, the spatiotemporal aspects of 53BP1 interaction with DSBs and the role of other DSB regulators in this process remain unclear. Here, we used real-time microscopy to study the DSB-induced redistribution of 53BP1 in living cells. We show that within minutes after DNA damage, 53BP1 becomes progressively, yet transiently, immobilized around the DSB-flanking chromatin. Quantitative imaging of single cells revealed that the assembly of 53BP1 at DSBs significantly lagged behind Mdc1/NFBD1, another DSB-interacting checkpoint mediator. Furthermore, short interfering RNA-mediated ablation of Mdc1/NFBD1 drastically impaired 53BP1 redistribution to DSBs and triggered premature dissociation of 53BP1 from these regions. Collectively, these in vivo measurements identify Mdc1/NFBD1 as a key upstream determinant of 53BP1's interaction with DSBs from its dynamic assembly at the DSB sites through sustained retention within the DSB-flanking chromatin up to the recovery from the checkpoint.

Imaging of protein movement induced by chromosomal breakage: tiny 'local' lesions pose great 'global' challenges

Interruption of chromosomal integrity by DNA double-strand breaks (DSBs) causes a major threat to genomic stability. Despite tremendous progress in understanding the genetic and biochemical aspects of DSB-induced genome surveillance and repair mechanisms, little is known about organization of these molecular pathways in space and time. Here, we outline the key spatio-temporal problems associated with DSBs and focus on the imaging approaches to visualize the dynamics of DSB-induced responses in mammalian cells. We delineate benefits and limitations of these assays and highlight the key recent discoveries where live microscopy provided unprecedented insights into how cells defend themselves against genome-destabilizing effects of DNA damage.

Inhibition of human Chk1 causes increased initiation of DNA replication, phosphorylation of ATR targets, and DNA breakage

Human checkpoint kinase 1 (Chk1) is an essential kinase required to preserve genome stability. Here, we show that Chk1 inhibition by two distinct drugs, UCN-01 and CEP-3891, or by Chk1 small interfering RNA (siRNA) leads to phosphorylation of ATR targets. Chk1-inhibition triggered rapid, pan-nuclear phosphorylation of histone H2AX, p53, Smc1, replication protein A, and Chk1 itself in human S-phase cells. These phosphorylations were inhibited by ATR siRNA and caffeine, but they occurred independently of ATM. Chk1 inhibition also caused an increased initiation of DNA replication, which was accompanied by increased amounts of nonextractable RPA protein, formation of single-stranded DNA, and induction of DNA strand breaks. Moreover, these responses were prevented by siRNA-mediated downregulation of Cdk2 or the replication initiation protein Cdc45, or by addition of the CDK inhibitor roscovitine. We propose that Chk1 is required during normal S phase to avoid aberrantly increased initiation of DNA replication, thereby protecting against DNA breakage. These results may help explain why Chk1 is an essential kinase and should be taken into account when drugs to inhibit this kinase are considered for use in cancer treatment.

ATM Activation in Normal Human Tissues and Testicular Cancer

The ATM kinase is a tumor suppressor and key regulator of biological responses to DNA damage. Cultured cells respond to genotoxic insults that induce DNA double-strand breaks by prompt activation of ATM through its autophosphorylation on serine 1981. However, whether ATM-S1981 becomes phosphorylated in vivo, for example during physiological processes that generate DSBs, is unknown. Here we produced phospho-specific monoclonal antibodies against S1981-phosphorylated ATM (pS-ATM), and applied them to immunohistochemical analyses of a wide range of normal human tissues and testicular tumors. Our data show that regardless of proliferation and differentiation, most human tissues contain only the S1981-nonphosphorylated, inactive form of ATM. In contrast, nuclear staining for pS-ATM was detected in subsets of bone-marrow lymphocytes and primary spermatocytes in the adult testes, cell types in which DSBs are generated during physiological V(D)J recombination and meiotic recombination, respectively. Among testicular germ-cell tumors, an aberrant constitutive pS-ATM was observed especially in embryonal carcinomas, less in seminomas, and only modestly in teratomas and the pre-invasive carcinoma-in-situ stage. Compared with pS-ATM, phosphorylated histone H2AX (gammaH2AX), another DNA damage marker and ATM substrate, was detected in a higher proportion of cancer cells, and also in normal fetal gonocytes, and a wider range of adult spermatocyte differentiation stages. Collectively, our results strongly support the physiological relevance of the recently proposed model of ATM autoactivation, and provide further evidence for constitutive activation of the DNA damage machinery during cancer development. The new tools characterized here should facilitate monitoring of ATM activation in clinical specimens, and help develop future treatment strategies.

DNA damage response as a candidate anti-cancer barrier in early human tumorigenesis

During the evolution of cancer, the incipient tumour experiences 'oncogenic stress', which evokes a counter-response to eliminate such hazardous cells. However, the nature of this stress remains elusive, as does the inducible anti-cancer barrier that elicits growth arrest or cell death. Here we show that in clinical specimens from different stages of human tumours of the urinary bladder, breast, lung and colon, the early precursor lesions (but not normal tissues) commonly express markers of an activated DNA damage response. These include phosphorylated kinases ATM and Chk2, and phosphorylated histone H2AX and p53. Similar checkpoint responses were induced in cultured cells upon expression of different oncogenes that deregulate DNA replication. Together with genetic analyses, including a genome-wide assessment of allelic imbalances, our data indicate that early in tumorigenesis (before genomic instability and malignant conversion), human cells activate an ATR/ATM-regulated DNA damage response network that delays or prevents cancer. Mutations compromising this checkpoint, including defects in the ATM-Chk2-p53 pathway, might allow cell proliferation, survival, increased genomic instability and tumour progression.

Human Asf1 regulates the flow of S phase histones during replicational stress

Maintenance of chromosomal integrity requires tight coordination of histone biosynthesis with DNA replication. Here, we show that extracts from human cells exposed to replication stress display an increased capacity to support replication-coupled chromatin assembly. While in unperturbed S phase, hAsf1 existed in equilibrium between an active form and an inactive histone-free pool, replication stress mobilized the majority of hAsf1 into an active multichaperone complex together with histones. This active multichaperone complex was limiting for chromatin assembly in S phase extracts, and hAsf1 was required for the enhanced assembly activity in cells exposed to replication stress. Consistently, siRNA-mediated knockdown of hAsf1 impaired the kinetics of S phase progression. Together, these data suggest that hAsf1 provides the cells with a buffering system for histone excess generated in response to stalled replication and explains how mammalian cells maintain a critical "active" histone pool available for deposition during recovery from replication stresses.

Inhibition of Chk1 by CEP-3891 accelerates mitotic nuclear fragmentation in response to ionizing Radiation

The human checkpoint kinase Chk1 has been suggested as a target for cancer treatment. Here, we show that a new inhibitor of Chk1 kinase, CEP-3891, efficiently abrogates both the ionizing radiation (IR)-induced S and G(2) checkpoints. When the checkpoints were abrogated by CEP-3891, the majority (64%) of cells showed fragmented nuclei at 24 hours after IR (6 Gy). The formation of nuclear fragmentation in IR-treated human cancer cells was directly visualized by time-lapse video microscopy of U2-OS cells expressing a green fluorescent protein-tagged histone H2B protein. Nuclear fragmentation occurred as a result of defective chromosome segregation when irradiated cells entered their first mitosis, either prematurely without S and G(2) checkpoint arrest in the presence of CEP-3891 or after a prolonged S and G(2) checkpoint arrest in the absence of CEP-3891. The nuclear fragmentation was clearly distinguishable from apoptosis because caspase activity and nuclear condensation were not induced. Finally, CEP-3891 not only accelerated IR-induced nuclear fragmentation, it also increased the overall cell killing after IR as measured in clonogenic survival assays. These results demonstrate that transient Chk1 inhibition by CEP-3891 allows premature mitotic entry of irradiated cells, thereby leading to accelerated onset of mitotic nuclear fragmentation and increased cell death.

Correlation of CHEK2 protein expression and c.1100delC mutation status with tumor characteristics among unselected breast cancer patients

The CHEK2 kinase is a tumor suppressor whose activation in response to DNA double-strand breaks contributes to cell cycle arrest or apoptosis. The c.1100delC mutation is associated with familial breast cancer, and tumors from mutation carriers show reduced or absent CHEK2 protein expression. We have here studied CHEK2 protein expression by immunohistochemistry on a tissue microarray of 611 unselected breast tumors and also evaluated the tumor characteristics among 1,297 unselected breast cancer patients defined for the c.1100delC germ line mutation status (2.5% carrier frequency). CHEK2 protein expression was reduced in 21.1% of the unselected breast cancers studied. Tumors with reduced CHEK2 expression had more often larger primary tumor size (pT3-4; nominal significance p = 0.002) compared to tumors with normal staining. A similar trend for larger tumor size was seen among the 37 breast tumors from c.1100delC germ line mutation carriers. Tumors from c.1100delC mutation carriers were of higher grade than those of noncarriers (nominal significance p = 0.02). The c.1100delC germ line mutation also associated strongly with bilateral breast cancer. No significant correlation was seen between CHEK2 status and hormone receptor status, histology, lymph node status, or overall survival.

Differential impact of diverse anticancer chemotherapeutics on the Cdc25A-degradation checkpoint pathway

When exposed to DNA-damaging insults such as ionizing radiation (IR) or ultraviolet light (UV), mammalian cells activate checkpoint pathways to halt cell cycle progression or induce cell death. Here we examined the ability of five commonly used anticancer drugs with different mechanisms of action to activate the Chk1/Chk2-Cdc25A-CDK2/cyclin E cell cycle checkpoint pathway, previously shown to be induced by IR or UV. Whereas exposure of human cells to topoisomerase inhibitors camptothecin, etoposide, or adriamycin resulted in rapid (within 1 h) activation of the pathway including degradation of the Cdc25A phosphatase and inhibition of cyclin E/CDK2 kinase activity, taxol failed to activate this checkpoint even after a prolonged treatment. Unexpectedly, although the alkylating agent cisplatin also induced degradation of Cdc25A (albeit delayed, after 8-12 h), cyclin E/CDK2 activity was elevated and DNA synthesis continued, a phenomena that correlated with increased E2F1 protein levels and consequently enhanced expression of cyclin E. These results reveal a differential impact of various classes of anticancer chemotherapeutics on the Cdc25A-degradation pathway, and indicate that the kinetics of checkpoint induction, and the relative balance of key components within the DNA damage response network may dictate whether the treated cells arrest their cell cycle progression.

Alternative splicing and mutation status of CHEK2 in stage III breast cancer

The DNA damage checkpoint kinase, CHK2, promotes growth arrest or apoptosis through phosphorylating targets such as Cdc25A, Cdc25C, BRCA1, and p53. Both germline and somatic loss-of-function CHEK2 mutations occur in human tumours, the former linked to the Li-Fraumeni syndrome, and the latter found in diverse types of sporadic malignancies. Here we examined the status of CHK2 by genetic and immunohistochemical analyses in 53 breast carcinomas previously characterized for TP53 status. We identified two CHEK2 mutants, 470T>C (Ile157Thr), and a novel mutation, 1368insA leading to a premature stop codon in exon 13. The truncated protein encoded by CHEK2 carrying the 1368insA was stable yet mislocalized to the cytoplasm in tumour sections and when ectopically expressed in cultured cells. Unexpectedly, we found CHEK2 to be subject to extensive alternative splicing, with some 90 splice variants detected in our tumour series. While all cancers expressed normal-length CHEK2 mRNA together with the spliced transcripts, we demonstrate and/or predict some of these splice variants to lack CHK2 function and/or localize aberrantly. We conclude that cytoplasmic sequestration may represent a novel mechanism to disable CHK2, and propose to further explore the significance of the complex splicing patterns of this tumour suppressor gene in oncogenesis.

Aberrations of the Chk2 tumour suppressor in advanced urinary bladder cancer

Checkpoint kinase 2 (Chk2) is a tumour suppressor and signal transducer in genome integrity checkpoints that coordinate cell-cycle progression with DNA repair or cell death in response to DNA damage. Defects of Chk2 occur in subsets of diverse sporadic malignancies and predispose to several types of hereditary carcinomas. However, the status of Chk2 in tumours of the urinary bladder remains unknown. Here, we report that among 58 advanced (grade T2-T4) human bladder carcinomas, immunohistochemical analysis revealed tumour-specific reduction or lack of Chk2 protein in 6 (10.3%) cases. Genetic analysis of the latter subset showed that a Chk2-negative carcinoma #668 harboured a truncating mutation 1100delC, in one Chk2 allele and loss of the corresponding second allele. The 1100delC mutation was also found in the germ line of this patient. Sequencing of TP53 in tumour #668 identified two missense mutations. Furthermore, the vast majority of the tumours showed 'unscheduled' activatory phosphorylation on Thr68 of Chk2 in the absence of any DNA-damaging treatment. Our results indicate that the otherwise dormant DNA damage signal transducer Chk2 is aberrantly and constitutively activated in invasive urinary bladder carcinomas, and that such likely proapoptotic checkpoint signalling can be disabled by inactivation of Chk2 and/or p53 tumour suppressors in subsets of these tumours.

Checking on DNA damage in S phase

The precise replication of the genome and the continuous surveillance of its integrity are essential for survival and the avoidance of various diseases. Cells respond to DNA damage by activating a complex network of the so-called checkpoint pathways to delay their cell-cycle progression and repair the defects. In this review we integrate findings on the emerging mechanisms of activation, the signalling pathways and the spatio-temporal organization of the intra-S-phase DNA-damage checkpoint and its impact on the cell-cycle machinery, and discuss its biological significance.

Watching the DNA repair ensemble dance

Repair of damaged DNA is a dynamic process that requires careful orchestration of a multitude of enzymes, adaptor proteins, and chromatin constituents. In this issue of Cell, Lisby et al. (2004) provide a visual glimpse into how the diverse signaling and repair machines are organized in space and time around the deadliest genetic lesions--the DNA double-strand breaks.

Variable frequencies of MALT lymphoma-associated genetic aberrations in MALT lymphomas of different sites

Although several recurrent genetic aberrations are known to occur in MALT lymphoma, no comprehensive study on the most prevalent MALT lymphoma-associated genetic aberrations is available. We therefore screened 252 primary MALT lymphomas for translocations t(11;18)(q21;q21), t(14;18)(q32;q21), and t(1;14)(p22;q32), and trisomies 3 and 18. The above-listed translocations occurred mutually exclusively and were detected overall in 13.5, 10.8, and 1.6% of the cases; trisomy 3 and/or 18 occurred in 42.1%. The frequency at which the translocations occurred varied markedly with the primary site of disease. The t(11;18)(q21;q21) was mainly detected in pulmonary and gastric tumors, whereas the t(14;18)(q32;q21) was most commonly found in lesions of the ocular adnexa/orbit, skin, and salivary glands. Trisomies 3 and 18 each occurred most frequently in intestinal and salivary gland MALT lymphomas. Our results demonstrate that the three translocations and trisomies 3 and 18 occur at markedly variable frequencies in MALT lymphoma of different sites.

Checking out the centrosome

Centrosomes consist of a pair of barrel-shaped microtubule assemblies called centrioles, surrounded by a pericentriolar matrix. The only well-characterized functions of centrosomes is to recognize both interphase microtubule arrays responsible for cell polarity and the mitotic spindle, which mediates the strictly bipolar separations of chromosomes. In addition to these established functions it has been speculated that centrosomes might be involved in several different cell cycle regulatory events like entry into mitosis, cytokinesis, G(1)/S transition and monitoring of DNA damage. These assumptions are mainly based on a rapidly growing list of centrosome-associated regulatory proteins such as p53, Brca1, Chk1, Chk2, TopBP1, Aurora-A, Plk1, cyclin B1, and Cdk1. However, only very few direct links between their localization to the centrosome and specific cellular functions have been unraveled until recently. This review will focus on recent advances in the understanding of the role of centrosomes as integrators of positive and negative pathways for mitotic entry.

Loss of Geminin induces rereplication in the presence of functional p53

Strict regulation of DNA replication is essential to ensure proper duplication and segregation of chromosomes during the cell cycle, as its deregulation can lead to genomic instability and cancer. Thus, eukaryotic organisms have evolved multiple mechanisms to restrict DNA replication to once per cell cycle. Here, we show that inactivation of Geminin, an inhibitor of origin licensing, leads to rereplication in human normal and tumor cells within the same cell cycle. We found a CHK1-dependent checkpoint to be activated in rereplicating cells accompanied by formation of gammaH2AX and RAD51 nuclear foci. Abrogation of the checkpoint leads to abortive mitosis and death of rereplicated cells. In addition, we demonstrate that the induction of rereplication is dependent on the replication initiation factors CDT1 and CDC6, and independent of the functional status of p53. These data show that Geminin is required for maintaining genomic stability in human cells.

Differentiation-induced radioresistance in muscle cells

DNA damage induces cell cycle arrest and DNA repair or apoptosis in proliferating cells. Terminally differentiated cells are permanently withdrawn from the cell cycle and partly resistant to apoptosis. To investigate the effects of genotoxic agents in postmitotic cells, we compared DNA damage-activated responses in mouse and human proliferating myoblasts and their differentiated counterparts, the myotubes. DNA double-strand breaks caused by ionizing radiation (IR) induced rapid activating autophosphorylation of ataxia-teleangiectasia-mutated (ATM), phosphorylation of histone H2AX, recruitment of repair-associated proteins MRE11 and Nbs1, and activation of Chk2 in both myoblasts and myotubes. However, IR-activated, ATM-mediated phosphorylation of p53 at serine 15 (human) or 18 (mouse) [Ser15(h)/18(m)], and apoptosis occurred in myoblasts but was impaired in myotubes. This phosphorylation could be enforced in myotubes by the anthracycline derivative doxorubicin, leading to selective activation of proapoptotic genes. Unexpectedly, the abundance of autophosphorylated ATM was indistinguishable after exposure of myotubes to IR (10 Gy) or doxorubicin (1 microM/24 h) despite efficient phosphorylation of p53 Ser15(h)/18(m), and apoptosis occurred only in response to doxorubicin. These results suggest that radioresistance in myotubes might reflect a differentiation-associated, pathway-selective blockade of DNA damage signaling downstream of ATM. This mechanism appears to preserve IR-induced activation of the ATM-H2AX-MRE11/Rad50/Nbs1 lesion processing and repair pathway yet restrain ATM-p53-mediated apoptosis, thereby contributing to life-long maintenance of differentiated muscle tissues.

CHEK2 variant I157T may be associated with increased breast cancer risk

Cell cycle checkpoint kinase 2 (CHEK2) is a transducer of cellular responses to DNA damage. The CHEK2 1100delC has previously been shown to be a low-penetrance breast cancer susceptibility allele. We have evaluated the role of another CHEK2 variant, I157T in the FHA domain of the gene, for association with breast cancer. I157T was found at a significantly higher frequency in the population-based series of breast cancer patients (77/1035, 7.4%, odds ratio [OR] = 1.43, 95% confidence interval [CI] = 1.06-1.95, p = 0.021) than among population controls (100/1885, 5.3%). The frequency in the familial breast cancer patients was not elevated (28/507, 5.5%, OR = 1.04, 95% CI = 0.68-1.61). The I157T protein, that undermines cellular responses to ionizing radiation and shows deficiency in substrate recognition in vivo, was expressed at normal level in tumor tissues as well as in cultured cells. The I157T protein was stable and it dimerized with the wild-type CHEK2 co-expressed in human cells. These functional properties of the I157T protein suggest that this variant may have negative effect on the pool of normal CHEK2 protein in heterozygous carrier cells by formation of heterodimers with wild-type CHEK2. The I157T variant may be associated with breast cancer risk, but the risk is lower than for 1100delC.

Centrosome-associated Chk1 prevents premature activation of cyclin-B-Cdk1 kinase

Entry into mitosis occurs after activation of Cdk1, resulting in chromosome condensation in the nucleus and centrosome separation, as well as increased microtubule nucleation activity in the cytoplasm. The active cyclin-B1-Cdk1 complex first appears at the centrosome, suggesting that the centrosome may facilitate the activation of mitotic regulators required for the commitment of cells to mitosis. However, the signalling pathways involved in controlling the initial activation of Cdk1 at the centrosome remain largely unknown. Here, we show that human Chk1 kinase localizes to interphase, but not mitotic, centrosomes. Chemical inhibition of Chk1 resulted in premature centrosome separation and activation of centrosome-associated Cdk1. Forced immobilization of kinase-inactive Chk1 to centrosomes also resulted in premature Cdk1 activation. Conversely, under such conditions wild-type Chk1 impaired activation of centrosome-associated Cdk1, thereby resulting in DNA endoreplication and centrosome amplification. Activation of centrosomal Cdk1 in late prophase seemed to be mediated by cytoplasmic Cdc25B, whose activity is controlled by centrosome-associated Chk1. These results suggest that centrosome-associated Chk1 shields centrosomal Cdk1 from unscheduled activation by cytoplasmic Cdc25B, thereby contributing to proper timing of the initial steps of cell division, including mitotic spindle formation.

Mammalian cell cycle checkpoints: signalling pathways and their organization in space and time

The major mission of the cell division cycle is a faithful and complete duplication of the genome followed by an equal partitioning of chromosomes to subsequent cell generations. In this review, we discuss the advances in our understanding of how mammalian cells control the fidelity of these fundamental processes when exposed to diverse genotoxic insults. We focus on the most recent insights into the molecular pathways that link the sites of DNA lesions with the cell cycle machinery in specific phases of the cell cycle. We also highlight the potential of a new technology allowing direct visualization of molecular interactions and redistribution of checkpoint proteins in live cell nuclei, and document the emerging significance of live-cell imaging for elucidation of the spatio-temporal organization of the DNA damage response network.

ATR, Claspin and the Rad9-Rad1-Hus1 complex regulate Chk1 and Cdc25A in the absence of DNA damage

The ATR and Chk1 kinases are essential to maintain genomic integrity. ATR, with Claspin and the Rad9-Rad1-Hus1 complex, activates Chk1 after DNA damage. Chk1-mediated phosphorylation of the Cdc25A phosphatase is required for the mammalian S-phase checkpoint. Here, we show that during physiological S phase the regulation of the Chk1-Cdc25A pathway depends on ATR, Claspin, Rad9, and Hus1. Human cells with chemically or genetically ablated ATR showed inhibition of Chk1-dependent phosphorylation of Cdc25A, and they accumulated Cdc25A without external DNA damage. Furthermore, siRNA-mediated depletion of Claspin, Rad9 and Hus1 also stabilized Cdc25A. ATR ablation also inhibited the activatory phosphorylation of Chk1 on serine 345. Thus, the ATR-Chk1-Cdc25A pathway represents an integral part of physiological S-phase progression, and interference with this mechanism undermines viability of somatic mammalian cells. DNA damage further activates and switches this pathway from its constitutively operating "surveillance mode" compatible with DNA replication into an "emergency" checkpoint response.

Mdc1 couples DNA double-strand break recognition by Nbs1 with its H2AX-dependent chromatin retention

Mdc1/NFBD1 controls cellular responses to DNA damage, in part via interacting with the Mre11-Rad50-Nbs1 complex that is involved in the recognition, signalling, and repair of DNA double-strand breaks (DSBs). Here, we show that in live human cells, the transient interaction of Nbs1 with DSBs and its phosphorylation by ATM are Mdc1-independent. However, ablation of Mdc1 by siRNA or mutation of the Nbs1's FHA domain required for Mdc1 binding reduced the affinity of Nbs1 for DSB-flanking chromatin and caused aberrant pan-nuclear dispersal of Nbs1. This occurred despite normal phosphorylation of H2AX, indicating that lack of Mdc1 does not impair this DSB-induced chromatin change, but rather precludes the sustained engagement of Nbs1 with these regions. Mdc1 (but not Nbs1) became partially immobilized to chromatin after DSB generation, and siRNA-mediated depletion of H2AX prevented such relocalization of Mdc1 and uncoupled Nbs1 from DSB-flanking chromatin. Our data suggest that Mdc1 functions as an H2AX-dependent interaction platform enabling a switch from transient, Mdc1-independent recruitment of Nbs1 to DSBs towards sustained, Mdc1-dependent interactions with the surrounding chromosomal microenvironment.

Distinct functional domains of Nbs1 modulate the timing and magnitude of ATM activation after low doses of ionizing radiation

The ATM kinase is a tumour suppressor and a key activator of genome integrity checkpoints in mammalian cells exposed to ionizing radiation (IR) and other insults that elicit DNA double-strand breaks (DSBs). In response to IR, autophosphorylation on serine 1981 causes dissociation of ATM dimers and initiates cellular ATM kinase activity. Here, we show that the kinetics and magnitude of ATM Ser1981 phosphorylation after exposure of human fibroblasts to low doses (2 Gy) of IR are altered in cells deficient in Nbs1, a substrate of ATM and a component of the MRN (Mre11-Rad50-Nbs1) complex involved in processing/repair of DSBs and ATM-dependent cell cycle checkpoints. Timely phosphorylation of both ATM Ser1981 and the ATM substrate Smc1 after IR were rescued via retrovirally mediated reconstitution of Nbs1-deficient cells by wild-type Nbs1 or mutants of Nbs1 defective in the FHA domain or nonphosphorylatable by ATM, but not by Nbs1 lacking the Mre11-interaction domain. Our data indicate that apart from its role downstream of ATM in the DNA damage checkpoint network, the MRN complex serves also as a modulator/amplifier of ATM activity. Although not absolutely required for ATM activation, the MRN nuclease complex may help reach the threshold activity of ATM necessary for optimal genome maintenance and prevention of cancer.

Distinct modes of deregulation of the proto-oncogenic Cdc25A phosphatase in human breast cancer cell lines

The rapid cell cycle arrest in response to DNA damage is mediated by degradation of the Cdc25A phosphatase, a proto-oncogene whose mRNA is frequently overexpressed in human tumours. Here, we study the occurrence and mechanisms of Cdc25A deregulation in human breast cancer cell lines. We demonstrate aberrantly elevated Cdc25A protein abundance and phosphatase activity in eight out of 15 cell lines, in some cases resulting in abrogation of the Cdc25A-mediated checkpoint response to ionizing radiation (IR), and this defect correlated with hypersensitivity to IR. Furthermore, we present evidence that deregulation of Cdc25A occurs predominantly on the post-transcriptional level, as overabundant Cdc25A protein was usually not accompanied by adequate mRNA overexpression. Instead, we demonstrate that aberrantly enhanced protein stability is an important mechanism underlying Cdc25A overabundance in a subset of breast cancer cell lines. Given the frequency of this mechanism, we propose that the DNA integrity checkpoint controlling Cdc25A protein stability might be a common target for deregulation in breast cancer.

Deregulation of the RB pathway in human testicular germ cell tumours

Deregulation of the RB pathway is shared by most human malignancies. Components upstream of the retinoblastoma tumour suppressor (pRB), namely the INK4 family of cyclin-dependent kinase (CDK) inhibitors, the D-type cyclins, their partner kinases CDK4/CDK6, and pRB as their critical substrate, are differentially targeted in diverse types of cancer. An 'unorthodox' spectrum of defects within this cascade occurs in testicular germ cell tumours (TGCTs), including silencing of pRB transcription, overexpression of cyclin D2, and loss of p18INK4c. To improve understanding of the role of this pathway in spermatogenesis, and its subversion in TGCTs, we examined immunohistochemical expression patterns of CDK4, p16INK4a, p15INK4b, and pRB, and established an in situ assay for cyclin D-mediated phosphorylation of serine795, a phosphorylation event critical for neutralization of pRB's growth-restraining ability. pRB was expressed throughout adult spermatogenesis and was detectable in teratomas, but was absent or grossly reduced in carcinoma in situ (CIS) and most seminomas and embryonal carcinomas. Unexpectedly, we also found that pRB was absent from fetal human gonocytes, the candidate target cell for all types of TGCTs. Thus, rather than a tumorigenesis-promoting loss of pRB, the lack of pRB in TGCTs likely reflects its developmental control. Widespread expression of p15INK4b, found in normal testes, was preserved in TGCTs. In contrast, p16INK4a was lost or reduced in large subsets of TGCTs. CDK4 was expressed in normal spermatogonia, CIS, and invasive TGCTs, as was serine795-phosphorylated pRB. Our data on expression of pRB support the plausible origin of TGCTs from fetal gonocytes, and the serine795 phosphorylation demonstrates that the cyclin D-dependent kinases are active, and neutralize pRB in spermatogonia and in those TGCTs that express pRB. We hope that this study will inspire further immunohistochemical applications of phosphospecific antibodies in pathology, and examination of the RB pathway defects in relation to curability of TGCTs.

Integrating proteomic and functional genomic technologies in discovery-driven translational breast cancer research

The application of state-of-the-art proteomics and functional genomics technologies to the study of cancer is rapidly shifting toward the analysis of clinically relevant samples derived from patients, as the ultimate aim of translational research is to bring basic discoveries closer to the bedside. Here we describe the essence of a long-term initiative undertaken by The Danish Centre for Translational Breast Cancer Research and currently underway for cancer biomarker discovery using fresh tissue biopsies and bio-fluids. The Centre is a virtual hub that brings together scientists working in various areas of basic cancer research such as cell cycle control, invasion and micro-environmental alterations, apoptosis, cell signaling, and immunology, with clinicians (oncologists, surgeons), pathologists, and epidemiologists, with the aim of understanding the molecular mechanisms underlying breast cancer progression and ultimately of improving patient survival and quality of life. The unifying concept behind our approach is the use of various experimental paradigms for the prospective analysis of clinically relevant samples obtained from the same patient, along with the systematic integration of the biological and clinical data.

Chk1 and Chk2 kinases in checkpoint control and cancer

Accumulation of mutations and chromosomal aberrations is one of the hallmarks of cancer cells. This enhanced genetic instability is fueled by defects in the genome maintenance mechanisms including DNA repair and cell cycle checkpoint pathways. Here, we discuss the emerging roles of the mammalian Chk1 and Chk2 kinases as key signal transducers within the complex network of genome integrity checkpoints, as candidate tumor suppressors disrupted in sporadic as well as some hereditary malignancies and as potential targets of new anticancer therapies.

Ataxia-telangiectasia-mutated (ATM) and NBS1-dependent phosphorylation of Chk1 on Ser-317 in response to ionizing radiation

In mammals, the ATM (ataxia-telangiectasia-mutated) and ATR (ATM and Rad3-related) protein kinases function as critical regulators of the cellular DNA damage response. The checkpoint functions of ATR and ATM are mediated, in part, by a pair of checkpoint effector kinases termed Chk1 and Chk2. In mammalian cells, evidence has been presented that Chk1 is devoted to the ATR signaling pathway and is modified by ATR in response to replication inhibition and UV-induced damage, whereas Chk2 functions primarily through ATM in response to ionizing radiation (IR), suggesting that Chk2 and Chk1 might have evolved to channel the DNA damage signal from ATM and ATR, respectively. We demonstrate here that the ATR-Chk1 and ATM-Chk2 pathways are not parallel branches of the DNA damage response pathway but instead show a high degree of cross-talk and connectivity. ATM does in fact signal to Chk1 in response to IR. Phosphorylation of Chk1 on Ser-317 in response to IR is ATM-dependent. We also show that functional NBS1 is required for phosphorylation of Chk1, indicating that NBS1 might facilitate the access of Chk1 to ATM at the sites of DNA damage. Abrogation of Chk1 expression by RNA interference resulted in defects in IR-induced S and G(2)/M phase checkpoints; however, the overexpression of phosphorylation site mutant (S317A, S345A or S317A/S345A double mutant) Chk1 failed to interfere with these checkpoints. Surprisingly, the kinase-dead Chk1 (D130A) also failed to abrogate the S and G(2) checkpoint through any obvious dominant negative effect toward endogenous Chk1. Therefore, further studies will be required to assess the contribution made by phosphorylation events to Chk1 regulation. Overall, the data presented in the study challenge the model in which Chk1 only functions downstream from ATR and indicate that ATM does signal to Chk1. In addition, this study also demonstrates that Chk1 is essential for IR-induced inhibition of DNA synthesis and the G(2)/M checkpoint.

Human Tousled like kinases are targeted by an ATM- and Chk1-dependent DNA damage checkpoint

All eukaryotes respond to DNA damage by modulation of diverse cellular processes to preserve genomic integrity and ensure survival. Here we identify mammalian Tousled like kinases (Tlks) as a novel target of the DNA damage checkpoint. During S-phase progression, when Tlks are maximally active, generation of DNA double-strand breaks (DSBs) leads to rapid and transient inhibition of Tlk activity. Experiments with chemical inhibitors, genetic models and gene targeting through RNA interference demonstrate that this response to DSBs requires ATM and Chk1 function. Chk1 phosphorylates Tlk1 on serine 695 (S695) in vitro, and this UCN-01- and caffeine-sensitive site is phosphorylated in vivo in response to DNA damage. Substitution of S695 to alanine impaired efficient downregulation of Tlk1 after DNA damage. These findings identify an unprecedented functional co- operation between ATM and Chk1 in propagation of a checkpoint response during S phase and suggest that, through transient inhibition of Tlk kinases, the ATM-Chk1-Tlk pathway may regulate processes involved in chromatin assembly.

Distinct spatiotemporal dynamics of mammalian checkpoint regulators induced by DNA damage

Cell cycle checkpoints are signal transduction pathways activated after DNA damage to protect genomic integrity. Dynamic spatiotemporal coordination is a vital, but poorly understood aspect, of these checkpoints. Here, we provide evidence for a strikingly different behaviour of Chk2 versus Nbs1, key mediators of the ataxia-telangiecatesia-mutated (ATM)-controlled checkpoint pathways induced by DNA double-strand breaks (DSBs). In live human cells with DSBs restricted to small sub-nuclear areas, Nbs1 was rapidly recruited to the damaged regions and underwent a dynamic exchange in the close vicinity of the DSB sites. In contrast, Chk2 continued to rapidly move throughout the entire nucleus, irrespective of DNA damage and including the DSB-free areas. Although phosphorylation of Chk2 by ATM occurred exclusively at the DSB sites, forced immobilization of Chk2 to spatially restricted, DSB-containing nuclear areas impaired its stimulating effect on p53-dependent transcription. These results unravel a dynamic nature of Nbs1 interaction with DSB lesions and identify Chk2 as a candidate transmitter of the checkpoint signal, allowing for a coordinated pan-nuclear response to focal DNA damage.

Chk1 regulates the S phase checkpoint by coupling the physiological turnover and ionizing radiation-induced accelerated proteolysis of Cdc25A

Chk1 kinase coordinates cell cycle progression and preserves genome integrity. Here, we show that chemical or genetic ablation of human Chk1 triggered supraphysiological accumulation of the S phase-promoting Cdc25A phosphatase, prevented ionizing radiation (IR)-induced degradation of Cdc25A, and caused radioresistant DNA synthesis (RDS). The basal turnover of Cdc25A operating in unperturbed S phase required Chk1-dependent phosphorylation of serines 123, 178, 278, and 292. IR-induced acceleration of Cdc25A proteolysis correlated with increased phosphate incorporation into these residues generated by a combined action of Chk1 and Chk2 kinases. Finally, phosphorylation of Chk1 by ATM was required to fully accelerate the IR-induced degradation of Cdc25A. Our results provide evidence that the mammalian S phase checkpoint functions via amplification of physiologically operating, Chk1-dependent mechanisms.

Deregulation of the G1/S-phase control in human testicular germ cell tumours

Deregulated cell cycle and defective genome-integrity checkpoints are among the hallmarks of cancer. Here we summarize our recent studies of key components of the GI/S machinery in normal human spermatogenesis, and their abnormalities in testicular germ cell tumours (TGCTs), with special emphasis on carcinoma in situ lesions (CIS). Our combined immunohistochemical and immunoblotting analyses of normal human adult and fetal testes, CIS, seminomas, embryonal carcinomas, and teratomas, revealed an 'unorthodox' spectrum of defects within the so-called RB pathway in TGCTs. The early aberrations included lack of expression of the retinoblastoma tumour suppressor (pRB) and the CDK inhibitor pl9ink4d, and overexpression of cyclin D2. Progression from CIS to invasive TGCTswas associated with loss of another two CDK inhibitors and tumour suppressors: pl6ink4a and pl8ink4c. We also found the lack of pRB and pl9ink4d in fetal gonocytes, the candidate target cell for all types of TGCTs. These findings, together with the status of the Chk2-p53 DNA-integrity checkpoint, are considered in relation to the origin, biology and pathogenesis of TGCTs, and potential implications of the GI/S defects for the curability of these tumours.

Tracheostomy in critically ill patients

The authors made a retrospective analysis of results and complications of elective tracheostomies performed by percutaneous dilatational technique (PDT) as well as standard surgical procedure (ST) in critically ill patients in the ICU. The indication for tracheostomy was to facilitate long-term mechanical ventilation, to facilitate cleaning and management of the airway and to maintain upper airway patency. During a 5-year period there were 495 elective tracheostomies performed in the ICU setting, in 209 females and 286 males. From these, 408 were standard (82.4%) and 87 percutaneous dilatational tracheostomies (17.6%). Mean age of patients with tracheostomy was 63.3 years (range 17-93 years) and the mean duration of endotracheal intubation before tracheostomy was 7 days (range from 13 minutes to 21 days). During the monitored period 144 patients (29.0%) were decannulated, out of which 34 patients (23.6%) had PDT and 110 patients (76.4%) ST. A total of 265 patients (53.5%) with tracheostomy died and 86 patients (17.3%) had the tracheostomy cannule in place at the study conclusion. Perioperative complications totaled 14 (2.8%), the most serious being one cardiac arrest and death (0.4%) both in ST as well as in PDT groups. Early postoperative complications totaled 46 (9.2%). Late postoperative complications totaled 7 (1.4%). Percutaneous dilatational tracheostomy is an alternative method to standard surgical tracheostomy in critically ill patients in the ICUs. Standard surgical tracheostomy is an irreplaceable procedure in patients with complex anatomic condition or in high-risk patients. (Tab. 3, Ref. 11).

The pRb-related protein p130 is regulated by phosphorylation-dependent proteolysis via the protein-ubiquitin ligase SCF(Skp2)

p130 is a tumor suppressor of the pocket protein family whose expression is posttranscriptionally regulated and largely G0 restricted. The mechanism of down-regulation of p130 expression in proliferating cells was investigated. Our results indicate that the decline of p130 expression as G0 cells reenter the cell cycle is due to a decrease in protein stability. The enhancement of p130 turnover in late G1 and S phase compared with G0 and early G1 phase was dependent on Cdk4/6-specific phosphorylation of p130 on Serine 672, and independent of Cdk2 activity. The activity of the ubiquitin ligase complex Skp1-Cul1/Cdc53-F-box protein Skp2 (SCF(Skp2)) and the proteasome were necessary for p130 degradation. In vitro, recombinant Skp2 was able to bind hyperphosphorylated but not dephosphorylated p130. Furthermore, in vitro polyubiquitination of p130 by SCF(Skp2) was specifically dependent on phosphorylation of p130 on Serine 672. Thus, like the Cdk inhibitor p27(Kip1), p130 turnover is regulated by Cdk-dependent G1 phosphorylation leading to ubiquitin-dependent proteolysis.

Regulation of G(2)/M events by Cdc25A through phosphorylation-dependent modulation of its stability

DNA replication in higher eukaryotes requires activation of a Cdk2 kinase by Cdc25A, a labile phosphatase subject to further destabilization upon genotoxic stress. We describe a distinct, markedly stable form of Cdc25A, which plays a previously unrecognized role in mitosis. Mitotic stabilization of Cdc25A reflects its phosphorylation on Ser17 and Ser115 by cyclin B-Cdk1, modifications required to uncouple Cdc25A from its ubiquitin-proteasome-mediated turnover. Cdc25A binds and activates cyclin B-Cdk1, accelerates cell division when overexpressed, and its downregulation by RNA interference (RNAi) delays mitotic entry. DNA damage-induced G(2) arrest, in contrast, is accompanied by proteasome-dependent destruction of Cdc25A, and ectopic Cdc25A abrogates the G(2) checkpoint. Thus, phosphorylation-mediated switches among three differentially stable forms ensure distinct thresholds, and thereby distinct roles for Cdc25A in multiple cell cycle transitions and checkpoints.

E2F activity is essential for survival of Myc-overexpressing human cancer cells

Effective cell cycle completion requires both Myc and E2F activities. However, whether these two activities interact to regulate cell survival remains to be tested. Here we have analysed survival of inducible c-Myc-overexpressing cell lines derived from U2OS human osteosarcoma cells, which carry wild-type pRb and p53 and are deficient for p16 and ARF expression. Induced U2OS-Myc cells neither underwent apoptosis spontaneously nor upon reconstitution of the ARF-p53 axis and/or serum-starvation. However, they died massively when concomitantly exposed to inhibitors of E2F activity, including a constitutively active pRb (RbDeltacdk) mutant, p16, a stable p27 (p27T187A) mutant, a dominant-negative (dn) CDK2, or dnDP-1. Similar apoptotic effect was observed upon down-modulation of endogenous E2Fs through overexpression of E2F binding site oligonucleotides in U2OS-Myc cells, upon expression of RbDeltacdk or dnDP-1 in the Myc-amplified HL-60 (ARF-; p53-) human leukemia cells, and upon co-transfection of Myc and RbDeltacdk in SAOS-2 (ARF+; p53-) human osteosarcoma cells but not in human primary fibroblasts. Consistent with these results, a dnp53 mutant did not abrogate the Myc-induced apoptotic phenotype, which instead strictly depended on caspase-3-like proteases and on Myc transcriptional activity. Our data indicate that in contrast to normal cells, Myc-overexpressing human cancer cells need E2F activity for their survival, regardless of their ARF and p53 status, a notion that may have important implications for antineoplastic treatment strategies.

Distinct phosphorylation events regulate p130- and p107-mediated repression of E2F-4

The "pocket proteins" pRb (retinoblastoma tumor suppressor protein), p107, and p130 regulate cell proliferation via phosphorylation-sensitive interactions with E2F transcription factors and other proteins. We previously identified 22 in vivo phosphorylation sites in human p130, including three sites selectively targeted by cyclin D-Cdk4(6) kinases. Here we assessed the effects of alanine substitution at the individual or combined Cdk4(6)-specific sites in p130, compared with homologous sites in p107 (Thr(369)/Ser(650)/Ser(964)). In U-2-OS cells, the triple p107(DeltaCdk4)* mutant strongly inhibited E2F-4 activity and imposed a G(1) arrest resistant to cyclin D1 coexpression. In contrast, the p130(DeltaCdk4) mutant still responded to cyclin D1, suggesting the existence of additional phosphorylation sites critical for E2F-4 regulation. Extensive mutagenesis, sensitive E2F reporter assays, and cell cycle analyses allowed the identification of six such residues (serines 413, 639, 662, 1044, 1080, and 1112) that, in addition to the Cdk4-specific sites, are necessary and sufficient for the regulation of E2F-4 and the cell cycle by p130. Surprisingly, 12 of the in vivo phosphorylation sites seem dispensable for E2F regulation and probably modulate other functions of p130. These results further elucidate the complex regulation of p130 and provide a molecular mechanism to explain the differential control of p107 and p130 by cyclin-dependent kinases.