When checkpoints fail

Expression of p16(INK4A) and alterations of the 9p21-23 chromosome region in non-small-cell lung carcinomas: relationship with tumor growth parameters and ploidy status

The 9p21-23 chromosome region harbors a number of known and putative tumor-suppressor genes (TSGs). The best characterized gene in this area is p16(INK4A) (CDKN2A). Alterations of its product have been observed in various malignancies, including non-small-cell lung carcinomas (NSCLCs). We earlier investigated the mechanisms underlying p16(INK4A) inactivation. In the present study, we examined, in a series of 87 NSCLCs, its relationship with the kinetic parameters [proliferation index (PI) and apoptotic index (Al)] and the ploidy status of the tumors. In addition, we extended our previous LOH analysis of the 9p21-23 region by examining flanking areas of p16(INK4A). Aberrant p16 expression was observed in 41.4% of the carcinomas. A significant association was found with increased PI (p = 0.037), but not with apoptosis. Aneuploid tumors were more frequently correlated with abnormal p16 staining (p = 0. 05). A high frequency of allelic imbalance (Alm) was noticed at the D9S161 (51.3%) and D9S157 (64.5%) loci, which lie approximately 4cM centromeric and 7cM telomeric, respectively, to CDKN2A. Abnormal p16(INK4A) expression was strongly correlated with Alm at D9S161 (p = 0.004). Allelic losses at D9S157 occurred more frequently in early stages (p = 0.018) and were significantly associated with deletions at D9S161 (p = 0.035). We conclude that, in a sub-set of NSCLCs, (i) abnormal p16 expression contributes to tumor growth mainly by increasing the proliferative activity in the initial stages of carcinogenesis; (ii) the association with aneuploidy merely reflects the impact of aberrant p16 on proliferative activity; and (iii) other putative TSGs possibly reside within the 9p21-23 region that possibly co-operate in certain cases with CDKN2A in the development of NSCLCs.

Defective embryonic neurogenesis in Ku-deficient but not DNA-dependent protein kinase catalytic subunit-deficient mice

Mammalian nonhomologous DNA end joining employs Ku70, Ku80, DNA-dependent protein kinase catalytic subunit (DNA-PKcs), XRCC4, and DNA ligase IV (Lig4). Herein, we show that Ku70 and Ku80 deficiency but not DNA-PKcs deficiency results in dramatically increased death of developing embryonic neurons in mice. The Ku-deficient phenotype is qualitatively similar to, but less severe than, that associated with XRCC4 and Lig4 deficiency. The lack of a neuronal death phenotype in DNA-PKcs-deficient embryos and the milder phenotype of Ku-deficient versus XRCC4- or Lig4-deficient embryos correlate with relative leakiness of residual end joining in these mutant backgrounds as assayed by a V(D)J recombination end joining assay. We conclude that normal development of the nervous system depends on the four evolutionarily conserved nonhomologous DNA end joining factors.

Meiotic arrest and germ cell apoptosis in androgen-binding protein transgenic mice

The fundamental role of androgen-binding protein (ABP) in spermatogenesis remains obscure after nearly 25 yr since its first characterization. In the present investigation, we used a transgenic mouse model that overexpresses rat ABP to examine the potential involvement of this protein in the regulation of processes occurring during spermatogenesis. Specifically, homozygous or heterozygous transgenic mice were analyzed in terms of spermatogenic progression, DNA fragmentation pattern, and germinal cell ploidy status. All animals homozygous for transgenic ABP exhibited an increased accumulation of primary spermatocytes and cells at metaphase with abnormal morphology and localization within the seminiferous epithelium. Analysis of DNA fragmentation by in situ techniques and agarose gel electrophoresis provided evidence for an increased occurrence of apoptosis in the transgenic animals, principally involving pachytene spermatocytes and cells at metaphase. Flow cytometric analysis of the DNA content of isolated germ cells revealed a reduction in the number of haploid cells, an increase in the number of tetraploid cells, and the appearance of a hypotetraploid cell population, consistent with degenerating primary spermatocytes. In mice heterozygous for the transgene, the effects were less prominent, and the degree to which spermatogenesis was compromised correlated with the levels of ABP messenger RNA in individual animals. The present results are interpreted to suggest that ABP can act as a modulator of spermatogenesis by regulating completion of the first meiotic division of primary spermatocytes.

Understanding factors that limit the productivity of suspension-based perfusion cultures operated at high medium renewal rates

One of the key parameters in perfusion culture is the rate of medium replacement (D). Intensifying D results in enhanced provision of nutrients, which can lead to an increase in the viable cell density (X(v)). The daily MAb production of hybridoma cells can thus be increased proportionally without modifying the bioreactor scale, provided that both viable cell yield per perfusion rate (Y(Xv/D)) and specific MAb productivity (q(MAb)) remain constant at higher D. To identify factors prone to limit productivity in perfusion, a detailed kinetic analysis was carried out on a series of cultures operated within a D range of 0.48/4.34 vvd (volumes of medium/reactor volume/day) in two different suspension-based systems. In the Celligen/vortex-flow filter system, significant reductions in Y(Xv/D) and q(MAb) resulting from the use of gas sparging were observed at D > 1.57 vvd (X(v) > 15 x 10(6) cells/mL). Through glucose supplementation, we have shown that the decrease in Y(Xv/D) encountered in presence of sparging was not resulting from increased cellular destruction or reduced cell growth, but rather from glucose limitation. Thus, increases in hydrodynamic shear stress imparted to the culture via intensification of gas sparging resulted in a gradual increase in specific glucose consumption (q(glc)) and lactate production rates (q(lac)), while no variations were observed in glutamine-consumption rates. As a result, while glutamine was the sole limiting-nutrient under non-sparging conditions, both glutamine and glucose became limiting under sparging conditions. Although a reduction in q(MAb) was observed at high-sparging rates, inhibition of MAb synthesis did not result from direct impact of bubbles, but was rather associated with elevated lactate levels (25-30 mM), resulting from shear stress-induced increases in q(lac), q(glc), and Y(lac/glc). Deleterious effects of sparging on Y(Xv/D) and q(MAb) encountered in the Celligen/vortex-flow filter system were eliminated in the sparging-free low-shear environment of the Chemap-HRI/ultrasonic filter system, allowing for the maintenance of up to 37 x 10(6) viable cells/mL. A strategy aimed at reducing requirements for sparging in large-scale perfusion cultures by way of a reduction in the oxygen demand using cellular engineering is discussed.

The controlling role of ATM in homologous recombinational repair of DNA damage

The human genetic disorder ataxia telangiectasia (A-T), caused by mutation in the ATM gene, is characterized by chromosomal instability, radiosensitivity and defective cell cycle checkpoint activation. DNA double-strand breaks (dsbs) persist in A-T cells after irradiation, but the underlying defect is unclear. To investigate ATM's interactions with dsb repair pathways, we disrupted ATM along with other genes involved in the principal, complementary dsb repair pathways of homologous recombination (HR) or non-homologous end-joining (NHEJ) in chicken DT40 cells. ATM(-/-) cells show altered kinetics of radiation-induced Rad51 and Rad54 focus formation. Ku70-deficient (NHEJ(-)) ATM(-/-) chicken DT40 cells show radiosensitivity and high radiation-induced chromosomal aberration frequencies, while Rad54-defective (HR(-)) ATM(-/-) cells show only slightly elevated aberration levels after irradiation, placing ATM and HR on the same pathway. These results reveal that ATM defects impair HR-mediated dsb repair and may link cell cycle checkpoints to HR activation.

A role for the umuDC gene products of Escherichia coli in increasing resistance to DNA damage in stationary phase by inhibiting the transition to exponential growth

The umuDC gene products, whose expression is induced by DNA-damaging treatments, have been extensively characterized for their role in SOS mutagenesis. We have recently presented evidence that supports a role for the umuDC gene products in the regulation of growth after DNA damage in exponentially growing cells, analogous to a prokaryotic DNA damage checkpoint. Our further characterization of the growth inhibition at 30 degrees C associated with constitutive expression of the umuDC gene products from a multicopy plasmid has shown that the umuDC gene products specifically inhibit the transition from stationary phase to exponential growth at the restrictive temperature of 30 degrees C and that this is correlated with a rapid inhibition of DNA synthesis. These observations led to the finding that physiologically relevant levels of the umuDC gene products, expressed from a single, SOS-regulated chromosomal copy of the operon, modulate the transition to rapid growth in E. coli cells that have experienced DNA damage while in stationary phase. This activity of the umuDC gene products is correlated with an increase in survival after UV irradiation. In a distinction from SOS mutagenesis, uncleaved UmuD together with UmuC is responsible for this activity. The umuDC-dependent increase in resistance in UV-irradiated stationary-phase cells appears to involve, at least in part, counteracting a Fis-dependent activity and thereby regulating the transition to rapid growth in cells that have experienced DNA damage. Thus, the umuDC gene products appear to increase DNA damage tolerance at least partially by regulating growth after DNA damage in both exponentially growing and stationary-phase cells.

Cell cycle delay and apoptosis are induced by high salt and urea in renal medullary cells

We investigated the effects of hyperosmolality on survival and proliferation of subconfluent cultures of mIMCD3 mouse renal collecting duct cells. High NaCl and/or urea (but not glycerol) reduces the number of viable cells, as measured with 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT). Raising osmolality from a normal level (300 mosmol/kg) to 550-1,000 mosmol/kg by adding NaCl and/or urea greatly increases the proportion of cells in the G(2)M phase of the cell cycle within 8 h, as measured by flow cytometry. Up to 600 mosmol/kg the effect is only transient, and by 12 h at 550 mosmol/kg the effect reverses and most cells are in G(1). Flow cytometry with 5-bromodeoxyuridine (BrdU) pulse-chase demonstrates that movement through the S phase of the cell cycle slows, depending on the concentrations of NaCl and/or urea, and that the duration of G(2)M increases greatly (from 2.5 h at 300 mosmol/kg to more than 16 h at the higher osmolalities). Addition of NaCl and/or urea to total osmolality of 550 mosmol/kg or more also induces apoptosis, as demonstrated by characteristic electron microscopic morphological changes, appearance of a subdiploid peak in flow cytometry, and caspase-3 activation. The number of cells with subdiploid DNA and activated caspase-3 peaks at 8-12 h. Caspase-3 activation occurs in all phases of the cell cycle, but to a disproportionate degree in G(0)/G(1) and S phases. We conclude that elevated NaCl and/or urea reduces the number of proliferating mIMCD3 cells by slowing the transit through the S phase, by cell cycle delay in the G(2)M and G(1), and by inducing apoptotic cell death.

Radiation-induced gadd45 expression correlates with clinical response to radiotherapy of cervical carcinoma

BACKGROUND

Recent work has identified a category of genes devoted to the control of genomic stability and prevention of cellular evolution. They encode components of cell cycle checkpoint, i.e., regulatory pathways committed to ordered cell cycle transition and fidelity of replicated DNA under adverse environmental conditions, such as those following exposure to genotoxic agents. Gadd45 belongs to the class II family of DNA damage-inducible (DDI) gene, and its role in DNA repair has been proved in many experimental models. The aim of our study was to correlate gadd45 radio-induction with the responsiveness to radiotherapy of cervical carcinomas, a type of cancer most commonly treated with radiotherapy alone.

METHODS

By means of a competitive polymerase chain reaction strategy, we compared in 14 patients the gene expression levels before and during external beam radiotherapy, when a dose ranging from 18 to 25 Gy was delivered to the target.

RESULTS

We found a correlation between the lack of gadd45 induction and a good clinical response to radiotherapy, in terms of both local control and disease-free survival.

CONCLUSION

Our results support the measure of the induction of gadd45, and possibly of other genes required for regulated G1-S checkpoint, as a method useful for prognostic evaluation of cervical carcinoma patients.

Human homologue of S. pombe Rad9 interacts with BCL-2/BCL-xL and promotes apoptosis

DNA damage induces apoptosis through a signalling pathway that can be suppressed by the BCL-2 protein, but the mechanism by which DNA damage does this is unknown. Here, using yeast two-hybrid and co-immunoprecipitation studies, we show that RAD9, a human protein involved in the control of a cell-cycle checkpoint, interacts with the anti-apoptotic Bcl-2-family proteins BCL-2 and BCL-x L, but not with the pro-apoptotic BAX and BAD. When overexpressed in mammalian cells, RAD9 induces apoptosis that can be blocked by BCL-2 or BCL-x L. Conversely, antisense RAD9 RNA suppresses cell death induced by methyl methanesulphonate. These findings indicate that RAD9 may have a new role in regulating apoptosis after DNA damage, in addition to its previously described checkpoint-control and other radioresistance-promoting functions.

Exploring the relationship between neutral and selective mutations in cancer

The transformation of normal cells into cancerous cells is an evolutionary process. Populations of precancerous cells reproduce, mutate, and compete for resources. Some of these mutations eventually lead to cancer. We calculate the probability of developing cancer under a set of simplifying assumptions and then elaborate these calculations, culminating in a simple simulation of the cell dynamics. The agent-based model allows us to examine the interactions of mutations critical for the development of cancer that are either evolutionarily neutral or selective. We can also examine the interaction of these mutations with a "mutator phenotype" derived from mutations that raise the mutation rate for the entire cell. The simulations suggest that there must be at least two selectively neutral mutations necessary for the development of cancer and that preventive treatments will be most effective when they increase this number. The model also suggests that selective mutations facilitate the development of cancer, so that the more selective mutations necessary for the development of cancer, the greater the chance of developing it.

Loss of heterozygosity on chromosome 10 is more extensive in primary (de novo) than in secondary glioblastomas

Glioblastomas develop de novo (primary glioblastomas) or through progression from low-grade or anaplastic astrocytoma (secondary glioblastomas). There is increasing evidence that these glioblastoma subtypes develop through different genetic pathways. Primary glioblastomas are characterized by EGFR and MDM2 amplification/overexpression, PTEN mutations, and p16 deletions, whereas secondary glioblastomas frequently contain p53 mutations. Loss of heterozygosity (LOH) on chromosome 10 (LOH#10) is the most frequent genetic alteration in glioblastomas; the involvement of tumor suppressor genes, other than PTEN, has been suggested. We carried out deletion mappings on chromosome 10, using PCR-based microsatellite analysis. LOH#10 was detected at similar frequencies in primary (8/17; 47%) and secondary glioblastomas (7/13; 54%). The majority (88%) of primary glioblastomas with LOH#10 showed LOH at all informative markers, suggesting loss of the entire chromosome 10. In contrast, secondary glioblastomas with LOH#10 showed partial or complete loss of chromosome 10q but no loss of 10p. These results are in accordance with the view that LOH on 10q is a major factor in the evolution of glioblastoma multiform as the common phenotypic end point of both genetic pathways, whereas LOH on 10p is largely restricted to the primary (de novo) glioblastoma.

Heterozygous germ line hCHK2 mutations in Li-Fraumeni syndrome

The hCHK2 gene encodes the human homolog of the yeast Cds1 and Rad53 G2 checkpoint kinases, whose activation in response to DNA damage prevents cellular entry into mitosis. Here, it is shown that heterozygous germ line mutations in hCHK2 occur in Li-Fraumeni syndrome, a highly penetrant familial cancer phenotype usually associated with inherited mutations in the TP53 gene. These observations suggest that hCHK2 is a tumor suppressor gene conferring predisposition to sarcoma, breast cancer, and brain tumors, and they also provide a link between the central role of p53 inactivation in human cancer and the well-defined G2 checkpoint in yeast.

Ultraviolet-irradiated spermatozoa activate oocytes but arrest preimplantation development after fertilization and nuclear transplantation in cattle

Artificial means of parthenogenetically activating mammalian oocytes are believed to lack an essential sperm epigenetic component required for normal development. The main goal of this study was to examine the potential of ultraviolet (UV)-irradiated sperm as a means of functionally eliminating the chromatin component of spermatozoa without affecting the ability to induce activation and support parthenogenetic development in cattle. Spermatozoa were stained with a DNA dye, exposed to various UV irradiation doses, and used to fertilize secondary oocytes. Although the percentage of pronuclei at 18 h postinsemination was similar using treated and control sperm, most oocytes fertilized by UV-irradiated sperm failed to develop beyond the 2-cell stage, suggesting that UV irradiation can functionally destroy the genomic component of spermatozoa with limited effects on the ability to induce oocyte activation. However, when oocytes activated with UV-irradiated sperm were used as hosts for nuclear transfer, developmental rates to cleavage and to blastocyst improved only marginally and remained lower than in the controls, indicating that UV-treated spermatozoa blocked development even in the presence of a diploid donor nucleus. Although DNA replication was not inhibited by UV irradiation treatment, abnormal chromatin morphology after cleavage suggests improper segregation of chromatin to daughter blastomeres during the first mitotic division. Together, these results indicate that although sperm exposed to UV can activate oocytes, a developmental block occurs at or soon after the first mitosis in parthenotes and oocytes reconstructed by nuclear transfer.

Search for in vivo somatic mutations in the mitotic checkpoint gene, hMAD1, in human lung cancers

We previously reported the presence of mitotic check-point impairment in about 40% of lung cancer cell lines. To gain an insight into the molecular basis of this impairment, we examined 49 lung cancer specimens for alterations in the hMAD1 mitotic checkpoint gene and identified a somatic, non-conservative missense mutation, which substitutes alanine (GCG) for threonine (ACG) at codon 299, together with a number of amino acid substituting, single nucleotide polymorphisms. This is the first demonstration of hMAD1 mutation in any type of human cancers. The present finding marks hMAD1 as a potential target, although with low frequency, for genetic alterations in lung cancer. Thus, further studies of hMAD1 dysfunction caused by other mechanisms appear to be warranted, as well as potential involvement of other components of the mitotic checkpoint.

Role of c-Jun N-terminal kinase 1 (JNK1) in cell cycle checkpoint activated by the protease inhibitor N-acetyl-leucinyl-leucinyl-norleucinal

The cysteine protease inhibitor N-acetyl-leucinyl-leucinyl-norleucinal (LLnL) inhibited the growth of the Calu-1 lung carcinoma cells and induced a prolonged cell cycle arrest in the S phase. c-Jun N-terminal kinases (JNKs) participate in cellular responses to mitogenic stimuli, environmental stresses, and apoptotic signals but its role in cell cycle checkpoint control has not been elucidated. In this report, we examined the role of JNK in LLnL-induced S phase checkpoint by overexpression of a dominant-negative mutant of JNK1 (JNK1-APF) in Calu-1 cells. Expression of high levels of JNK1-APF blocked the growth-inhibitory effects of LLnL and abrogated S phase arrest induced by LLnL. These results support the role of JNK in the activation of cell cycle checkpoint induced by LLnL.

The bacterial cytolethal distending toxin (CDT) triggers a G2 cell cycle checkpoint in mammalian cells without preliminary induction of DNA strand breaks

The bacterial cytolethal distending toxin (CDT) was previously shown to arrest the tumor-derived HeLa cell line in the G2-phase of the cell cycle through inactivation of CDK1, a cyclin-dependent kinase whose state of activation determines entry into mitosis. We have analysed the effects induced in HeLa cells by CDT, in comparison to those induced by etoposide, a prototype anti-tumoral agent that triggers a G2 cell cycle checkpoint by inducing DNA damage. Both CDT and etoposide inhibit cell proliferation and induces the formation of enlarged mononucleated cells blocked in G2. In both cases, CDK1 from arrested cells could be reactivated both in vitro by dephosphorylation by recombinant Cdc25B phosphatase and in vivo by caffeine. However, the cell cycle arrest triggered by CDT, unlike etoposide, did not originate from DNA strand breaks as demonstrated in the single cell gel electrophoresis assay and by the absence of slowing down of S phase in synchronized cells. Together with additional observations on synchronized HeLa cells, our results suggest that CDT triggers a G2 cell cycle checkpoint that is initiated during DNA replication and that is independent of DNA damage.

ATM: a mediator of multiple responses to genotoxic stress

The ATM protein kinase is the product of the gene responsible for the pleiotropic recessive disorder ataxia-telangiectasia. ATM-deficient cells show enhanced sensitivity and greatly reduced responses to genotoxic agents that generate DNA double strand breaks (DSBs), such as ionizing radiation and radiomimetic chemicals, but exhibit normal responses to DNA adducts and base modifications induced by other agents. Therefore, DSBs are most likely the predominant signal for the activation of ATM-mediated pathways. Identification of the ATM gene triggered extensive research aimed at elucidating the numerous functions of its large multifaceted protein product. While ATM has both nuclear and cytoplasmic functions, this review will focus on its roles in the nucleus where it plays a central role in the very early stages of damage detection and serves as a master controller of cellular responses to DSBs. By activating key regulators of multiple signal transduction pathways, ATM mediates the efficient induction of a signaling network responsible for repair of the damage, and for cellular recovery and survival.

Aurora/Ipl1p-related kinases, a new oncogenic family of mitotic serine-threonine kinases

During the past five years, a growing number of serine-threonine kinases highly homologous to the Saccharomyces cerevisiae Ipl1p kinase have been isolated in various organisms. A Drosophila melanogaster homologue, aurora, was the first to be isolated from a multicellular organism. Since then, several related kinases have been found in mammalian cells. They localise to the mitotic apparatus: in the centrosome, at the poles of the bipolar spindle or in the midbody. The kinases are necessary for completion of mitotic events such as centrosome separation, bipolar spindle assembly and chromosome segregation. Extensive research is now focusing on these proteins because the three human homologues are overexpressed in various primary cancers. Furthermore, overexpression of one of these kinases transforms cells. Because of the myriad of kinases identified, we suggest a generic name: Aurora/Ipl1p-related kinase (AIRK). We denote AIRKs with a species prefix and a number, e.g. HsAIRK1.

Role of human Cds1 (Chk2) kinase in DNA damage checkpoint and its regulation by p53

In response to DNA damage, mammalian cells adopt checkpoint regulation, by phosphorylation and stabilization of p53, to delay cell cycle progression. However, most cancer cells that lack functional p53 retain an unknown checkpoint mechanism(s) by which cells are arrested at the G(2)/M phase. Here we demonstrate that a human homolog of Cds1/Rad53 kinase (hCds1) is rapidly phosphorylated and activated in response to DNA damage not only in normal cells but in cancer cells lacking functional p53. A survey of various cancer cell lines revealed that the expression level of hCds1 mRNA is inversely related to the presence of functional p53. In addition, transfection of normal human fibroblasts with SV40 T antigen or human papilloma viruses E6 or E7 causes a marked induction of hCds1 mRNA, and the introduction of functional p53 into SV40 T antigen- and E6-, but not E7-, transfected cells decreases the hCds1 level, suggesting that p53 negatively regulates the expression of hCds1. In cells without functional ataxia telangiectasia mutated (ATM) protein, phosphorylation and activation of hCds1 were observed in response to DNA damage induced by UV but not by ionizing irradiation. These results suggest that hCds1 is activated through an ATM-dependent as well as -independent pathway and that it may complement the function of p53 in DNA damage checkpoints in mammalian cells.

A requirement for recombinational repair in Saccharomyces cerevisiae is caused by DNA replication defects of mec1 mutants

To examine the role of the RAD52 recombinational repair pathway in compensating for DNA replication defects in Saccharomyces cerevisiae, we performed a genetic screen to identify mutants that require Rad52p for viability. We isolated 10 mec1 mutations that display synthetic lethality with rad52. These mutations (designated mec1-srf for synthetic lethality with rad-fifty-two) simultaneously cause two types of phenotypes: defects in the checkpoint function of Mec1p and defects in the essential function of Mec1p. Velocity sedimentation in alkaline sucrose gradients revealed that mec1-srf mutants accumulate small single-stranded DNA synthesis intermediates, suggesting that Mec1p is required for the normal progression of DNA synthesis. sml1 suppressor mutations suppress both the accumulation of DNA synthesis intermediates and the requirement for Rad52p in mec1-srf mutants, but they do not suppress the checkpoint defect in mec1-srf mutants. Thus, it appears to be the DNA replication defects in mec1-srf mutants that cause the requirement for Rad52p. By using hydroxyurea to introduce similar DNA replication defects, we found that single-stranded DNA breaks frequently lead to double-stranded DNA breaks that are not rapidly repaired in rad52 mutants. Taken together, these data suggest that the RAD52 recombinational repair pathway is required to prevent or repair double-stranded DNA breaks caused by defective DNA replication in mec1-srf mutants.

Cell cycle checkpoints as therapeutic targets

Most human breast tumors arise from multiple genetic changes which gradually transform differentiated and growth-limited cells into highly invasive cells that are unresponsive to growth controls. The genetic evolution of normal breast cells into cancer cells is largely determined by the fidelity of DNA replication, repair, and division. Cell cycle arrest in response to DNA damage is an important part of the mechanism used to maintain genomic integrity. The control mechanisms that restrain cell cycle transition after DNA damage are known as cell cycle checkpoints. This review will focus on cell cycle checkpoint signaling pathways commonly mutated in human breast tumors and suggest how different components of these checkpoint pathways offer the potential for chemotherapeutic intervention.

Roles of p53 and caspases in the induction of cell cycle arrest and apoptosis by HIV-1 vpr

The vpr gene from the human immunodeficiency virus type-1 (HIV-1) encodes a 14-kDa protein that prevents cell proliferation by causing a block in the G(2) phase of the cell cycle. This cellular function of vpr is conserved in evolution because other primate lentiviruses, including HIV-2, SIV(mac), and SIV(agm) encode related genes that also induce G(2) arrest. After G(2) arrest, cells expressing vpr undergo apoptosis. The signaling pathways that result in vpr-induced cell cycle arrest and apoptosis have yet to be determined. The p53 tumor suppressor protein is involved in signaling pathways leading to cell cycle arrest and apoptosis in a variety of cell types. In this work, we examine the potential role of p53 in mediating cell cycle block and/or apoptosis by HIV-1 vpr and demonstrate that both phenomena occur independently of the presence and function of p53. Caspases are common mediators of apoptosis. We examined the potential role of caspases in mediating vpr-induced apoptosis by treating vpr-expressing cells with Boc-D-FMK, a broad spectrum, irreversible inhibitor of the caspase family. Boc-D-FMK significantly reduced the numbers of apoptotic cells induced by vpr. Therefore, we conclude that vpr-induced apoptosis is effected via the activation of caspases.

A model for a umuDC-dependent prokaryotic DNA damage checkpoint

The products of the Escherichia coli umuDC operon are required for translesion synthesis, the mechanistic basis of most mutagenesis caused by UV radiation and many chemicals. The UmuD protein shares homology with LexA, the repressor of SOS-regulated loci, and similarly undergoes a facilitated autodigestion on interaction with the RecA/single-stranded DNA nucleoprotein filaments formed after a cell experiences DNA damage. This cleavage, in which Ser-60 of UmuD acts as the nucleophile, produces UmuD', the form active in translesion synthesis. Expression of the noncleavable UmuD(S60A) protein and UmuC was found to increase survival after UV irradiation, despite the inability of the UmuD(S60A) protein to participate in translesion synthesis; this survival increase is uvr(+) dependent. Additional observations that expression of the UmuD(S60A) protein and UmuC delayed the resumption of DNA replication and cell growth after UV irradiation lead us to propose that the uncleaved UmuD protein and UmuC delay the resumption of DNA replication, thereby allowing nucleotide excision repair additional time to repair the damage accurately before replication is attempted. After a UV dose of 20 J/m(2), uncleaved UmuD is the predominant form for approximately 20 min, after which UmuD' becomes the predominant form, suggesting that the umuDC gene products play two distinct and temporally separated roles in DNA damage tolerance, the first in cell-cycle control and the second in translesion synthesis over unrepaired or irreparable lesions. The relationship of these observations to the eukaryotic DNA damage checkpoint is discussed.

c-Myc overexpression uncouples DNA replication from mitosis

c-myc has been shown to regulate G(1)/S transition, but a role for c-myc in other phases of the cell cycle has not been identified. Exposure of cells to colcemid activates the mitotic spindle checkpoint and arrests cells transiently in metaphase. After prolonged colcemid exposure, the cells withdraw from mitosis and enter a G(1)-like state. In contrast to cells in G(1), colcemid-arrested cells have decreased G(1) cyclin-dependent kinase activity and show hypophosphorylation of the retinoblastoma protein. We have found that overexpression of c-myc causes colcemid-treated human and rodent cells to become either apoptotic or polyploid by replicating DNA without chromosomal segregation. Although c-myc-induced polyploidy is not inhibited by wild-type p53 in immortalized murine fibroblasts, overexpression of c-myc in primary fibroblasts resulted in massive apoptosis of colcemid-treated cells. We surmise that additional genes are altered in immortalized cells to suppress the apoptotic pathway and allow c-myc-overexpressing cells to progress forward in the presence of colcemid. Our results also suggest that c-myc induces DNA rereplication in this G(1)-like state by activating CDK2 activity. These observations indicate that activation of c-myc may contribute to the genomic instability commonly found in human cancers.

Deletion of SFI1, a novel suppressor of partial Ras-cAMP pathway deficiency in the yeast Saccharomyces cerevisiae, causes G(2) arrest

When glucose is added to Saccharomyces cerevisiae cells grown into stationary phase or on non-fermentable carbon sources a rapid loss of heat stress resistance occurs. Mutants that retain high stress resistance after addition of glucose are called 'fil', for deficient in fermentation induced loss of stress resistance. Transformation of the fil1 mutant, which harbours a point mutation in adenylate cyclase, with a yeast gene library on a single copy plasmid resulted in transformants that were again stress-sensitive. One of the genes isolated in this way was a gene of previously unknown function. We have called it SFI1, for suppressor of fil1. SFI1 is an essential gene. Combination of Sfi1 and cAMP pathway mutations indicates that Sfi1 itself is not involved in the cAMP pathway. Conditional sfi1 mutants did not show enhanced heat resistance under the restrictive condition, whereas overexpression of SFI1 rendered cells heat-sensitive. Sfi1 may be a downstream target of the protein kinase A pathway, but its precise relationship with heat resistance remains unclear. Further analysis showed that Sfi1 is required for cell cycle progression, more specifically for progression through G(2)-M transition. Cells expressing SFI1 under the control of a galactose-inducible promoter arrest after addition of glucose as doublets of undivided mother and daughter cells. These doublets contain a single nucleus and lack mitotic spindles. Sfi1 shares homology with Xenopus laevis XCAP-C, a protein required for chromosome assembly. The conserved residues between these two proteins show a strong bias for charged amino acids. Hence, Sfi1 might be required for correct mitotic spindle assembly and its precise role might be in chromosome condensation. In conclusion, we have identified an essential function in the G(2)-M transition of the cell cycle for a yeast gene of previously unknown function.

Identification of frequent impairment of the mitotic checkpoint and molecular analysis of the mitotic checkpoint genes, hsMAD2 and p55CDC, in human lung cancers

The mitotic checkpoint is thought to be essential for ensuring accurate chromosome segregation by implementing mitotic delay in response to a spindle defect. To date, however, very little data has become available on the defects of the mitotic checkpoint in human cancer cells. In the present study, impaired mitotic checkpoint was found in four (44%) of nine human lung cancer cell lines. To our knowledge, this is the first demonstration of frequent impairment of the mitotic checkpoint in this leading cause of cancer deaths. As an initial step towards elucidation of the underlying mechanism, we further undertook a search for mutations in a key component of the mitotic checkpoint, known as hsMAD2, and its immediate downstream molecule, p55CDC. No such mutations were found, however, in either 21 lung cancer cell lines or 25 primary lung cancer cases, although we could identify silent polymorphisms and the transcribed and processed hsMAD2 pseudogene that was subsequently mapped at 14q21-q23. The present observations appear to warrant further investigations, such as search for alterations in other components, to better understand the molecular pathogenesis of this fatal disease, and warn against potential misinterpretation when performing mutational analyses for other cancer types based on cDNA templates.

The ataxia-telangiectasia related protein ATR mediates DNA-dependent phosphorylation of p53

Levels of the tumour suppressor protein p53 are increased in response to a variety of DNA damaging agents. DNA damage-induced phosphorylation of p53 occurs at serine-15 in vivo. Phosphorylation of p53 at serine-15 leads to a stabilization of the polypeptide by inhibiting its interaction with Mdm2, a protein that targets p53 for ubiquitin-dependent degradation. However, the mechanisms by which DNA damage is signalled to p53 remain unclear. Here, we report the identification of a novel DNA-activated protein kinase that phosphorylates p53 on serine-15. Fractionation of HeLa nuclear extracts and biochemical analyses indicate that this kinase is distinct from the DNA-dependent protein kinase (DNA-PK) and corresponds to the human cell cycle checkpoint protein ATR. Immunoprecipitation studies of recombinant ATR reveal that catalytic activity of this polypeptide is required for DNA-stimulated phosphorylation of p53 on serine-15. These data suggest that ATR may function upstream of p53 in a signal transduction cascade initiated upon DNA damage and provide a biochemical assay system for ATR activity.

An E mu-BCL-2 transgene facilitates leukaemogenesis by ionizing radiation

Clonogenic murine B cell precursors are normally ultrasensitive to apoptosis following genotoxic exposure in vitro but can be protected by expression of an E mu-BCL-2 transgene. Such exposures are likely to be mutagenic. This in turn suggests that a level of in vivo genotoxic exposure that usually has minimal pathological consequences might become leukaemogenic when damaged cells fail to abort by apoptosis. If this were to be the case, then the cell type that becomes leukaemic and the chromosomal/molecular changes that occur would also be of considerable interest. We tested this possibility by exposing E mu-BCL-2 and wild-type mice of differing ages to a single dose of X-irradiation of 1-4 Gy. Young (approximately 4-6 weeks) transgenic mice developed leukaemia at a high rate following exposure to 2 Gy but adult mice (4-6 months) did not. Exposure to 4 Gy produced leukaemia in both young and adult transgenic mice but at a higher frequency in the former. Leukaemic cell populations showed clonal rearrangements of the IGH gene but in most cases analysed had immunophenotypic features of an early B lympho-myeloid progenitor population which has not previously been recorded in radiation leukaemogenesis. Molecular cytogenetic analysis of leukaemic cells by banded karyotype and FISH revealed a consistent double abnormality: trisomy 15 plus an interstitial deletion of chromosome 4 that was confirmed by LOH analysis.

Molecular cloning and tissue-specific expression of the mutator2 gene (mu2) in Drosophila melanogaster

We present here the molecular cloning and characterization of the mutator2 (mu2) gene of Drosophila melanogaster together with further genetic analyses of its mutant phenotype. mu2 functions in oogenesis during meiotic recombination, during repair of radiation damage in mature oocytes, and in proliferating somatic cells, where mu2 mutations cause an increase in somatic recombination. Our data show that mu2 represents a novel component in the processing of double strand breaks (DSBs) in female meiosis. mu2 does not code for a DNA repair enzyme because mu2 mutants are not hypersensitive to DSB-inducing agents. We have mapped and cloned the mu2 gene and rescued the mu2 phenotype by germ-line transformation with genomic DNA fragments containing the mu2 gene. Sequencing its cDNA demonstrates that mu2 encodes a novel 139-kD protein, which is highly basic in the carboxy half and carries three nuclear localization signals and a helix-loop-helix domain. Consistent with the sex-specific mutant phenotype, the gene is expressed in ovaries but not in testes. During oogenesis its RNA is rapidly transported from the nurse cells into the oocyte where it accumulates specifically at the anterior margin. Expression is also prominent in diploid proliferating cells of larval somatic tissues. Our genetic and molecular data are consistent with the model that mu2 encodes a structural component of the oocyte nucleus. The MU2 protein may be involved in controlling chromatin structure and thus may influence the processing of DNA DSBs.

Presenilin overexpression arrests cells in the G1 phase of the cell cycle. Arrest potentiated by the Alzheimer's disease PS2(N141I)mutant

To investigate the mechanism by which presenilin (PS) overexpression induces apoptosis, we studied the effects of these proteins on cell cycle progression. Transiently transfected HeLa cells were bromodeoxyuridine (BrdU) labeled to visualize DNA synthesis by immunofluorescence and stained with propidium iodide to measure DNA content by fluorescence-activated cell sorting (FACS). BrdU labeling was decreased in cells expressing presenilin-1 (PS1), presenilin-2 (PS2), an Alzheimer's disease-associated missense mutation PS2(N141I), and the carboxyl-terminally deleted PS2 construct PS2(166aa), compared with mock and neurofilament-light (NF-L) transfected cells. Analysis of BrdU incorporation in mitotically synchronized HeLa cells suggested that cells were arresting in the G1 phase of the cell cycle, and this was confirmed by FACS analysis. Interestingly, cell cycle progression was more inhibited by the expression of PS2(N141I) compared with wild-type PS2. In addition, ATM, the gene product mutated in ataxia-telangiectasia, does not appear to be a downstream effector of PS-induced cell cycle arrest as transfection of PS constructs into an ataxia-telangiectasia cell line also resulted in cell cycle inhibition. Quantitative immunoblotting of whole-cell lysates from PS-transfected cells did not reveal increases or decreases in the steady-state levels of p21, p27, p53, pRb, or c-myc, suggesting that the presenilins mediate cell cycle arrest by mechanisms other than simple changes in the steady-state levels of these cell-cycle-related proteins.

Mouse Hus1, a homolog of the Schizosaccharomyces pombe hus1+ cell cycle checkpoint gene

Cell cycle checkpoints are regulatory mechanisms that arrest the cell cycle or initiate programmed cell death when critical events such as DNA replication fail to be completed or when DNA or spindle damage occurs. In fission yeast, cell cycle checkpoint responses to DNA replication blocks and DNA damage require the hus1+ gene. Mammalian homologs of hus1+ were recently identified, and here we report a detailed analysis of mouse Hus1. An approximately 4.2-kb full-length cDNA encoding the 32-kDa mouse Hus1 protein was isolated. The genomic structure and exon-intron boundary sequences of the gene were determined, and mouse Hus1 was found to consist of nine exons. Mouse Hus1 was mapped to the proximal end of chromosome 11 and is therefore a candidate gene for the mouse mutation germ cell deficient, which maps to the same genomic region. Finally, mouse Hus1 was found to be expressed in a variety of adult tissues and at several stages of embryonic development.

Apoptosis-resistant E1B-19K-expressing NS/0 myeloma cells exhibit increased viability and chimeric antibody productivity under perfusion culture conditions

We have shown previously that recombinant NS/0 myelomas expressing sufficient amounts of E1B-19K were resistant to apoptosis occurring in the late phase of batch culture and under stressful conditions such as cultivation in glutamine-free medium or following heat shock. However, no significant increase in monoclonal antibodies (MAb) was observed during the prolonged stationary phase of these batch cultures. Here, we show that E1B-19K can enhance cell survival and improve MAb productivity in high cell density perfusion culture. Typically, lymphoid cells grown under steady state in perfusion exhibit decreasing viabilities with concomitant accumulation of apoptotic cells. By modulating the ability of these cells to resist to induction of apoptosis in low nutrient environment, a 3-fold decrease in specific death rate from 0.22 day-1 for NS/0 control to 0.07 day-1 for E1B-19K cells was achieved, resulting in a significant improvement in cell viability throughout perfusion. E1B-19K cells at the perfusion plateau phase also exhibited a 3-fold reduction in specific growth rate concomitant with a lower percentage of S and higher percentage of G1 phase cells. This was associated with a 40% decrease in specific oxygen consumption rate, likely related to a reduction in the specific consumption rates of limiting nutrient(s). Expression of E1B-19K consequently had a significant impact on the steady-state viable cell density, allowing maintenance of 11.5 x 10(6) E1B-19K cells/mL versus 5.9 x 10(6) control NS/0 cells/mL for the same amount of fresh medium brought into the system (half a volume per day). Whereas MAb concentrations found in perfusion culture of control NS/0 myelomas were almost 3-fold higher than those found in batch culture; in the case of E1B-19K-expressing myelomas, the MAb concentration in perfusion was more than 7-fold higher than in batch. This was attributable to the 2-fold increase in viable cell plateau and to a 40% increase in the perfusion to batch ratio of specific MAb productivity (2.2-fold for E1B-19K myelomas versus 1.6-fold for NS/0 control).

Age-dependent atrophy and microgravity travel: what do they have in common?

Space travel and extending human lifespan are two of the many advances of the twentieth century. However, both of these scientific wonders exact a price for their gains; i.e. deleterious effects on normal physiological processes. For example, both old age and prolonged microgravity travel are associated with atrophy in heart, muscle, and bone. The underlying signal transduction pathways, the control mechanisms for the processes of proliferation, differentiation, and apoptosis, may prove to be similarly altered in both old age and microgravity travel. We suggest that the mechanical events involved in space travel provide a telescopic compression of lifespan changes in these tissues; if so, space travel provides an excellent opportunity to investigate how long-term degeneration occurs on Earth. With the aid of biochip technology for multi-factorial analysis, a platform can be generated to create therapeutic modalities to contain, retard, reduce, or prevent this tissue atrophy, either in space or on Earth.

RAD53, DUN1 and PDS1 define two parallel G2/M checkpoint pathways in budding yeast

Eukaryotic checkpoint genes regulate multiple cellular responses to DNA damage. In this report, we examine the roles of budding yeast genes involved in G2/M arrest and tolerance to UV exposure. A current model posits three gene classes: those encoding proteins acting on damaged DNA (e.g. RAD9 and RAD24), those transducing a signal (MEC1, RAD53 and DUN1) or those participating more directly in arrest (PDS1). Here, we define important features of the pathways subserved by those genes. MEC1, which we find is required for both establishment and maintenance of G2/M arrest, mediates this arrest through two parallel pathways. One pathway requires RAD53 and DUN1 (the 'RAD53 pathway'); the other pathway requires PDS1. Each pathway independently contributes approximately 50% to G2/M arrest, effects demonstrable after cdc13-induced damage or a double-stranded break inflicted by the HO endonuclease. Similarly, both pathways contribute independently to tolerance of UV irradiation. How the parallel pathways might interact ultimately to achieve arrest is not yet understood, but we do provide evidence that neither the RAD53 nor the PDS1 pathway appears to maintain arrest by inhibiting adaptation. Instead, we think it likely that both pathways contribute to establishing and maintaining arrest.

Molecular mechanisms of ionizing radiation-induced apoptosis

Ionizing radiation activates not only signalling pathways in the nucleus as a result of DNA damage, but also signalling pathways initiated at the level of the plasma membrane. Proteins involved in DNA damage recognition include poly(ADP ribose) polymerase (PARP), DNA-dependent protein kinase, p53 and ataxia- telangiectasia mutated (ATM). Many of these proteins are inactivated by caspases during the execution phase of apoptosis. Signalling pathways outside the nucleus involve tyrosine kinases such as stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), protein kinase C, ceramide and reactive oxygen species. Recent evidence shows that tumour cells resistant to ionizing radiation-induced apoptosis have defective ceramide signalling. How these signalling pathways converge to activate the caspases is presently unknown, although in some cell types a role for calpain has been suggested.

Aberrations of cell cycle and cell death in normal development of the chick embryo growth plate

The epiphyses of femurs from 7.5-15 day chicken embryos were studied by electron microscopy. Several forms of aberrant cell cycles were present: (1) in the perichondrium, polyploid metaphases, segmentating large (giant) cells, and mitotic catastrophe (midway between mitosis and apoptosis) were observed; (2) in the resting zone, premature chromosome condensation was found; (3) in the proliferative zone, approximately 5% of divisions were aberrant, representing most often mitosis restitution from metaphase and more seldom from the anaphase; (4) in all layers, 'dark chondrocytes' representing a premortal form of hypersecretory cells undergoing often a-mitotic nuclear segmentation were present. Many of the aberrations of cell cycle were combined with cell death. These deviations omitting or adapting the cell cycle check-points represent evidently the normal epigenetic mechanisms of development and repair. At the same time, by origin and appearances they seem very close to the loss of the growth control displayed by malignant tumours. This connection is briefly analysed in view of some current concepts of carcinogenesis.

Critical role for Atm in suppressing V(D)J recombination-driven thymic lymphoma

Chromosome translocations involving T cell receptor (TCR) loci have been found in tumors from Ataxia telangiectasia (AT) patients and in mouse Atm-/- thymoma, suggesting the involvement of V(D)J recombination in these malignancies. By introducing a RAG-1 deficiency into Atm-/- mice in the presence of a TCR transgene, we show that V(D)J recombination is critical for thymoma development in these mice. Therefore, aberrant V(D)J recombination, normally suppressed by Atm, facilitates tumorigenic events leading to cancer. Because V(D)J recombination is dispensable for lymphomagenesis upon p53 deficiency, this study also indicates that Atm and p53 function by distinct mechanisms in suppressing thymoma.

Cisplatin DNA cross-links do not inhibit S-phase and cause only a G2/M arrest in Saccharomyces cerevisiae

Cisplatin (CDDP) has been used as a DNA cross-linking agent to evaluate whether there is a specific cell cycle checkpoint response to such damage in Saccharomyces cerevisiae (S. cerevisiae). Fluorescent-activated cell sorting (FACS) analysis showed only a G2/M checkpoint, normal exit from G1 and progression through S-phase following alpha-factor arrest and CDDP treatment. Of the checkpoint mutants tested, rad9, rad17 and rad24, did not show increased sensitivity to CDDP compared to isogenic wild-type cells. However, other checkpoint mutants tested (mec1, mec3 and rad53) showed increased sensitivity to CDDP, as did controls with a defect in excision repair (rad1 and rad14) or a defect in recombination (rad51 and rad52). Thus, by survival and cell cycle kinetics, it appears that DNA cross-links do not inhibit entry into S-phase or slow DNA replication and that replication continues after cisplatin treatment in yeast.

Selective, covalent modification of beta-tubulin residue Cys-239 by T138067, an antitumor agent with in vivo efficacy against multidrug-resistant tumors

Microtubules are linear polymers of alpha- and beta-tubulin heterodimers and are the major constituents of mitotic spindles, which are essential for the separation of chromosomes during mitosis. Here we describe a synthetic compound, 2-fluoro-1-methoxy-4-pentafluorophenylsulfonamidobenzene (T138067), which covalently and selectively modifies the beta1, beta2, and beta4 isotypes of beta-tubulin at a conserved cysteine residue, thereby disrupting microtubule polymerization. Cells exposed to T138067 become altered in shape, indicating a collapse of the cytoskeleton, and show an increase in chromosomal ploidy. Subsequently, these cells undergo apoptosis. Furthermore, T138067 exhibits cytotoxicity against tumor cell lines that exhibit substantial resistance to vinblastine, paclitaxel, doxorubicin, and actinomycin D. T138067 is also equally efficacious in inhibiting the growth of sensitive and multidrug-resistant human tumor xenografts in athymic nude mice. These observations suggest that T138067 may be clinically useful for the treatment of multidrug-resistant tumors.

Association with Cdc2 and inhibition of Cdc2/Cyclin B1 kinase activity by the p53-regulated protein Gadd45

Recently Gadd45, a p53-regulated stress protein, has been implicated in the activation of a G2/M checkpoint after damage by UV radiation and alkylating agents. While inhibitory phosphorylation of Cdc2 and suppression of cyclin B1 levels are known to be involved in G2 delays after genotoxic stress, Gadd45 has now been found to directly inhibit the activity of Cdc2/Cyclin B1 complex, while it had no appreciable effect on Cdk2/ Cyclin E activity even at very high levels of Gadd45. In contrast, p21CiP1/Waf1 is an universal cdk/cyclin inhibitor and inhibited both of the cyclin complexes tested here. Gadd45 was also able to physically interact with Cdc2, but not Cyclin B1. Addition of Gadd45 to immunoprecipitated Cdc2/Cyclin B1 in vitro led to a dissociation of this complex, and thus may represent a new checkpoint mechanism whereby Cdc2/Cyclin B1 can be inhibited. With the use of an antisense approach, reduced Gadd45 expression attenuated the suppression of Cdc2/Cyclin B1 activity in UV-irradiated human cells. Taken together, these results implicate Gadd45 in the control of G2/M cell cycle progression after certain stresses.

Characterization of p53 in Chinese hamster cell lines CHO-K1, CHO-WBL, and CHL: implications for genotoxicity testing

Since the p53 gene function is critical to how a cell responds to DNA damage, we investigated the p53 status in Chinese hamster cell lines commonly used in genotoxicity tests for cytogenetic damage around the world. These included: Chinese hamster ovary K1 (CHO-K1), Chinese hamster ovary WBL (CHO-WBL), and Chinese hamster lung (CHL) cells. The results of DNA sequencing, protein analysis, and cell cycle analysis demonstrate that the CHO-K1 and CHO-WBL cell lines have mutant p53 sequence [a mutation in codon 211 in exon 6 resulting in a change from Thr (ACA) to Lys (AAA)], mutant protein (high spontaneous levels that are non-inducible after X-irradiation), and mutant function (lack of G1 checkpoint). Interestingly, the CHL cell line has a completely wild-type p53 DNA sequence. However, the CHL cells have an abnormally high spontaneous level of wild-type p53 protein expression that is not inducible after X-irradiation, yet there is some evidence of G1 delay after irradiation. The protein data suggests that p53 in CHL cells is not being regulated normally, and thus is probably not functioning normally. The mechanism leading to this abnormal regulation of p53 in CHL cells clearly does not involve mutation in the p53 gene. Overall, the CHL cell line may be similar to the CHO cell lines, in that they all appear to have abnormal p53 function. Further work is needed to determine whether the presence of spontaneously high levels of wild-type p53 in CHL cells results in a difference in response to DNA damage (quantitatively or qualitatively) compared to the p53 mutant CHO cell lines.

Growth arrest of melanoma cells is differentially regulated by contact inhibition and serum deprivation

Both growth-factor deprivation and contact inhibition suppress cell growth; however, the mechanisms by which they inhibit cell proliferation may not be identical. The function of antiproliferative genes and the induction of programmed cell death are among the potential differences between these growth-arrest mechanisms. Specifically, an inverse relation between the expression of cyclin-dependent kinase inhibitors (CDKIs) and the susceptibility to apoptosis has been reported. To test this relation, we examined the features of growth arrest in a canine melanoma cell line, TLM1. Both contact inhibition and serum deprivation halted cell-cycle progression of TLM1 cells in the G1 phase. Prolonged growth arrest of the cells without restimulation resulted in apoptosis; conversely, the cells reentered the cell cycle after release from contact inhibition or on restimulation with serum. Cell-to-cell contact, but not serum deprivation, led to the expression of p53 and p21/Waf-1. The expression of p21/Waf-1 did not prevent apoptosis. Moreover, the ectopic overexpression of CDKIs increased apoptosis. These results support the premise that growth arrest induced by contact inhibition and serum deprivation are mediated through distinct mechanisms. Furthermore, CDKIs are not universal inhibitors of apoptosis, and in some cases, they may initiate or enhance the apoptotic program.

Chemical inhibitors of cyclin-dependent kinases: insights into design from X-ray crystallographic studies

Cyclin-dependent kinases (CDKs) are a family of protein kinases that regulate progression through the eukaryotic cell cycle. Aberrant CDK activity or function is a common defect in human tumours, resulting in unrestrained cellular proliferation. X-ray crystallographic analysis of monomeric CDK2 and CDK2 complexes has revealed how phosphorylation and cyclin binding mediate enzyme activation and how this activity can be regulated by further protein association. Current research aims to improve the selectivity and/or potency of small molecule CDK inhibitors, both to develop specific probes to study the roles of the different CDK family members in coordinating cell cycle progression, and as lead molecules for the design of therapeutically useful drugs. This design process has been assisted by the availability of a number of CDK2/inhibitor structures determined using X-ray crystallography. These structures have shown that molecules related to ATP can be accommodated in the ATP-binding site in a number of orientations, utilising interactions observed between CDK2 and its natural ligand, as well as novel interactions with CDK2 residues that lie both within and outside the active site cleft. This site can also bind inhibitors that are structurally unrelated to ATP. These results suggest that it may be possible to design pharmacologically and pharmaceutically important ATP-binding site-directed ligands that act as specific and potent inhibitors of CDK activity.

Co-localization of mitochondrial and double minute DNA in the nuclei of HL-60 cells but not normal cells

In an attempt to isolate genes located on double minute (Dmin) DNA in HL-60 cells, we prepared DNA probe from purified micronuclei. Micronucleation was induced in HL-60 cells by treatment with hydroxyurea. Screening of a cDNA library unexpectedly produced a number of clones containing mitochondrial DNA (mtDNA) sequences. Here, we show that amplified mtDNA sequences were localized in nuclei and micronuclei of HL-60 and COLO 320DM cells, but not in nuclei of WI-38 normal human fibroblasts or peripheral blood T-cells. To unequivocally demonstrate the presence of mtDNA inside of nuclei and micronuclei, we obtained tomographic fluorescence in situ hybridization (FISH) images of mtDNA by confocal microscopy of consecutive sections of paraformaldehyde (PFA)-fixed material. We also located mtDNA in nuclear buds and purified micronuclei. Dmin DNA and mtDNA were always located at similar sites. The mechanisms of nuclear retention of mtDNA and Dmin DNA and the resulting influence on tumorigenesis are discussed.