Involvement of Hus1 in the chain elongation step of DNA replication after exposure to camptothecin or ionizing radiation

Role of N-acetyl cysteine and acetyl-l-carnitine combination treatment on DNA-damage-related genes induced by radiation in HEI-OC1 cells

PURPOSE

The aim of the present study was to evaluate the effect of acetyl-l-carnitine (ALC) and N-acetyl cysteine (NAC) on ionizing radiation (IR)-induced cytotoxicity and change in DNA damage-related genes in House Ear Institute-Organ of Corti 1 (HEI-OC1) cells.

METHODS

HEI-OC1 cells were irradiated with 5 Gy radiation and treated by eight combinations of NAC and/or ALC: control, NAC, ALC, IR, NAC + IR, ALC + NAC, ALC + IR, and ALC + NAC + IR. Cell viability, apoptotic cell death, and DNA damage were measured at the 72nd hour. Eighty-four IR-induced DNA-damage-related genes were determined by RT-PCR gene array and >10-fold changes were considered significant.

RESULTS

IR decreased cell viability by about 50% at 72 hours of incubation. In particular, the ALC and/or NAC combination before IR protected the HEI-OC1 cells (p < .05). Single and combination treatment prior to IR led to lower apoptotic cell death (p < .05). There was a significant lower DNA damage in ALC + NAC + IR group compared to IR group (p < .05). Expressions of Brca2, Xpc, Mlh3, Rad51, Xrcc2, Hus1, Rad9a, Cdkn1a, Gadd45a which are the DNA-repair genes were found to be significantly higher in NAC + ALC + IR group than those in individual treatment of ALC or NAC.

CONCLUSIONS

ALC and/or NAC treatment prior to IR led to higher cell viability and lower apoptotic cell damage compared to the IR group. The results of the study show that the ALC + NAC combination treatment inhibits DNA damage and induces DNA-repair genes to repair radiation damage, and this combination treatment is more effective against radiation-induced DNA damage than NAC or ALC therapy individually.

Replication fork slowing and stalling are distinct, checkpoint-independent consequences of replicating damaged DNA

In response to DNA damage during S phase, cells slow DNA replication. This slowing is orchestrated by the intra-S checkpoint and involves inhibition of origin firing and reduction of replication fork speed. Slowing of replication allows for tolerance of DNA damage and suppresses genomic instability. Although the mechanisms of origin inhibition by the intra-S checkpoint are understood, major questions remain about how the checkpoint regulates replication forks: Does the checkpoint regulate the rate of fork progression? Does the checkpoint affect all forks, or only those encountering damage? Does the checkpoint facilitate the replication of polymerase-blocking lesions? To address these questions, we have analyzed the checkpoint in the fission yeast Schizosaccharomyces pombe using a single-molecule DNA combing assay, which allows us to unambiguously separate the contribution of origin and fork regulation towards replication slowing, and allows us to investigate the behavior of individual forks. Moreover, we have interrogated the role of forks interacting with individual sites of damage by using three damaging agents-MMS, 4NQO and bleomycin-that cause similar levels of replication slowing with very different frequency of DNA lesions. We find that the checkpoint slows replication by inhibiting origin firing, but not by decreasing fork rates. However, the checkpoint appears to facilitate replication of damaged templates, allowing forks to more quickly pass lesions. Finally, using a novel analytic approach, we rigorously identify fork stalling events in our combing data and show that they play a previously unappreciated role in shaping replication kinetics in response to DNA damage.

Effective intra-S checkpoint responses to UVC in primary human melanocytes and melanoma cell lines

The objective of this study was to assess potential functional attenuation or inactivation of the intra-S checkpoint during melanoma development. Proliferating cultures of skin melanocytes, fibroblasts, and melanoma cell lines were exposed to increasing fluences of UVC and intra-S checkpoint responses were quantified. Melanocytes displayed stereotypic intra-S checkpoint responses to UVC qualitatively and quantitatively equivalent to those previously demonstrated in skin fibroblasts. In comparison with fibroblasts, primary melanocytes displayed reduced UVC-induced inhibition of DNA strand growth and enhanced degradation of p21Waf1 after UVC, suggestive of enhanced bypass of UVC-induced DNA photoproducts. All nine melanoma cell lines examined, including those with activating mutations in BRAF or NRAS oncogenes, also displayed proficiency in activation of the intra-S checkpoint in response to UVC irradiation. The results indicate that bypass of oncogene-induced senescence during melanoma development was not associated with inactivation of the intra-S checkpoint response to UVC-induced DNA replication stress.

DNA damage checkpoint responses in the S phase of synchronized diploid human fibroblasts

We investigated the hypothesis that the strength of the activation of the intra-S DNA damage checkpoint varies within the S phase. Synchronized diploid human fibroblasts were exposed to either 0 or 2.5 J m⁻² UVC in early, mid- and late-S phase. The endpoints measured were the following: (1) radio-resistant DNA synthesis (RDS), (2) induction of Chk1 phosphorylation, (3) initiation of new replicons and (4) length of replication tracks synthesized after irradiation. RDS analysis showed that global DNA synthesis was inhibited by approximately the same extent (30 ± 12%), regardless of when during S phase the fibroblasts were exposed to UVC. Western blot analysis revealed that the UVC-induced phosphorylation of checkpoint kinase 1 (Chk1) on serine 345 was high in early and mid S but 10-fold lower in late S. DNA fiber immunostaining studies indicated that the replication fork displacement rate decreased in irradiated cells at the three time points examined; however, replicon initiation was inhibited strongly in early and mid S, but this response was attenuated in late S. These results suggest that the intra-S checkpoint activated by UVC-induced DNA damage is not as robust toward the end of S phase in its inhibition of the latest firing origins in human fibroblasts.

Checkpoint regulation of replication forks: global or local?

Cell-cycle checkpoints are generally global in nature: one unattached kinetochore prevents the segregation of all chromosomes; stalled replication forks inhibit late origin firing throughout the genome. A potential exception to this rule is the regulation of replication fork progression by the S-phase DNA damage checkpoint. In this case, it is possible that the checkpoint is global, and it slows all replication forks in the genome. However, it is also possible that the checkpoint acts locally at sites of DNA damage, and only slows those forks that encounter DNA damage. Whether the checkpoint regulates forks globally or locally has important mechanistic implications for how replication forks deal with damaged DNA during S-phase.

Conditional inactivation of the DNA damage response gene Hus1 in mouse testis reveals separable roles for components of the RAD9-RAD1-HUS1 complex in meiotic chromosome maintenance

The RAD9-RAD1-HUS1 (9-1-1) complex is a heterotrimeric PCNA-like clamp that responds to DNA damage in somatic cells by promoting DNA repair as well as ATR-dependent DNA damage checkpoint signaling. In yeast, worms, and flies, the 9-1-1 complex is also required for meiotic checkpoint function and efficient completion of meiotic recombination; however, since Rad9, Rad1, and Hus1 are essential genes in mammals, little is known about their functions in mammalian germ cells. In this study, we assessed the meiotic functions of 9-1-1 by analyzing mice with germ cell-specific deletion of Hus1 as well as by examining the localization of RAD9 and RAD1 on meiotic chromosomes during prophase I. Hus1 loss in testicular germ cells resulted in meiotic defects, germ cell depletion, and severely compromised fertility. Hus1-deficient primary spermatocytes exhibited persistent autosomal γH2AX and RAD51 staining indicative of unrepaired meiotic DSBs, synapsis defects, an extended XY body domain often encompassing partial or whole autosomes, and an increase in structural chromosome abnormalities such as end-to-end X chromosome-autosome fusions and ruptures in the synaptonemal complex. Most of these aberrations persisted in diplotene-stage spermatocytes. Consistent with a role for the 9-1-1 complex in meiotic DSB repair, RAD9 localized to punctate, RAD51-containing foci on meiotic chromosomes in a Hus1-dependent manner. Interestingly, RAD1 had a broader distribution that only partially overlapped with RAD9, and localization of both RAD1 and the ATR activator TOPBP1 to the XY body and to unsynapsed autosomes was intact in Hus1 conditional knockouts. We conclude that mammalian HUS1 acts as a component of the canonical 9-1-1 complex during meiotic prophase I to promote DSB repair and further propose that RAD1 and TOPBP1 respond to unsynapsed chromatin through an alternative mechanism that does not require RAD9 or HUS1.

WRN helicase regulates the ATR-CHK1-induced S-phase checkpoint pathway in response to topoisomerase-I-DNA covalent complexes

Checkpoints are cellular surveillance and signaling pathways that coordinate the response to DNA damage and replicative stress. Consequently, failure of cellular checkpoints increases susceptibility to DNA damage and can lead to profound genome instability. This study examines the role of a human RECQ helicase, WRN, in checkpoint activation in response to DNA damage. Mutations in WRN lead to genomic instability and the premature aging condition Werner syndrome. Here, the role of WRN in a DNA-damage-induced checkpoint was analyzed in U-2 OS (WRN wild type) and isogenic cells stably expressing WRN-targeted shRNA (WRN knockdown). The results of our studies suggest that WRN has a crucial role in inducing an S-phase checkpoint in cells exposed to the topoisomerase I inhibitor campthothecin (CPT), but not in cells exposed to hydroxyurea. Intriguingly, WRN decreases the rate of replication fork elongation, increases the accumulation of ssDNA and stimulates phosphorylation of CHK1, which releases CHK1 from chromatin in CPT-treated cells. Importantly, knockdown of WRN expression abolished or delayed all these processes in response to CPT. Together, our results strongly suggest an essential regulatory role for WRN in controlling the ATR-CHK1-mediated S-phase checkpoint in CPT-treated cells.

Targeted deletion of mouse Rad1 leads to deficient cellular DNA damage responses

The Rad1 gene is evolutionarily conserved from yeast to human. The fission yeast Schizosaccharomyces pombe Rad1 ortholog promotes cell survival against DNA damage and is required for G(2)/M checkpoint activation. In this study, mouse embryonic stem (ES) cells with a targeted deletion of Mrad1, the mouse ortholog of this gene, were created to evaluate its function in mammalian cells. Mrad1 (-/-) ES cells were highly sensitive to ultraviolet-light (UV light), hydroxyurea (HU) and gamma rays, and were defective in G(2)/M as well as S/M checkpoints. These data indicate that Mrad1 is required for repairing DNA lesions induced by UV-light, HU and gamma rays, and for mediating G(2)/M and S/M checkpoint controls. We further demonstrated that Mrad1 plays an important role in homologous recombination repair (HRR) in ES cells, but a minor HRR role in differentiated mouse cells.

The human intra-S checkpoint response to UVC-induced DNA damage

The intra-S checkpoint response to 254 nm light (UVC)-induced DNA damage appears to have dual functions to slow the rate of DNA synthesis and stabilize replication forks that become stalled at sites of UVC-induced photoproducts in DNA. These functions should provide more time for repair of damaged DNA before its replication and thereby reduce the frequencies of mutations and chromosomal aberrations in surviving cells. This review tries to summarize the history of discovery of the checkpoint, the current state of understanding of the biological features of intra-S checkpoint signaling and its mechanisms of action with a focus primarily on intra-S checkpoint responses in human cells. The differences in the intra-S checkpoint responses to UVC and ionizing radiation-induced DNA damage are emphasized. Evidence that [6-4]pyrimidine-pyrimidone photoproducts in DNA trigger the response is discussed and the relationships between cellular responses to UVC and the molecular dose of UVC-induced DNA damage are briefly summarized. The role of the intra-S checkpoint response in protecting against solar radiation carcinogenesis remains to be determined.

PARP-1 ensures regulation of replication fork progression by homologous recombination on damaged DNA

Poly-ADP ribose polymerase 1 (PARP-1) is activated by DNA damage and has been implicated in the repair of single-strand breaks (SSBs). Involvement of PARP-1 in other DNA damage responses remains controversial. In this study, we show that PARP-1 is required for replication fork slowing on damaged DNA. Fork progression in PARP-1^−/− DT40 cells is not slowed down even in the presence of DNA damage induced by the topoisomerase I inhibitor camptothecin (CPT). Mammalian cells treated with a PARP inhibitor or PARP-1–specific small interfering RNAs show similar results. The expression of human PARP-1 restores fork slowing in PARP-1^−/− DT40 cells. PARP-1 affects SSB repair, homologous recombination (HR), and nonhomologous end joining; therefore, we analyzed the effect of CPT on DT40 clones deficient in these pathways. We find that fork slowing is correlated with the proficiency of HR-mediated repair. Our data support the presence of a novel checkpoint pathway in which the initiation of HR but not DNA damage delays the fork progression.

Integrating S-phase checkpoint signaling with trans-lesion synthesis of bulky DNA adducts

Bulky adducts are DNA lesions generated in response to environmental agents including benzo[a]pyrene (a combustion product) and solar ultraviolet radiation. Error-prone replication of adducted DNA can cause mutations, which may result in cancer. To minimize the detrimental effects of bulky adducts and other DNA lesions, S-phase checkpoint mechanisms sense DNA damage and integrate DNA repair with ongoing DNA replication. The essential protein kinase Chk1 mediates the S-phase checkpoint, inhibiting initiation of new DNA synthesis and promoting stabilization and recovery of stalled replication forks. Here we review the mechanisms by which Chk1 is activated in response to bulky adducts and potential mechanisms by which Chk1 signaling inhibits the initiation stage of DNA synthesis. Additionally, we discuss mechanisms by which Chk1 signaling facilitates bypass of bulky lesions by specialized Y-family DNA polymerases, thereby attenuating checkpoint signaling and allowing resumption of normal cell cycle progression.

The intra-S-phase checkpoint affects both DNA replication initiation and elongation: single-cell and -DNA fiber analyses

To investigate the contribution of DNA replication initiation and elongation to the intra-S-phase checkpoint, we examined cells treated with the specific topoisomerase I inhibitor camptothecin. Camptothecin is a potent anticancer agent producing well-characterized replication-mediated DNA double-strand breaks through the collision of replication forks with topoisomerase I cleavage complexes. After a short dose of camptothecin in human colon carcinoma HT29 cells, DNA replication was inhibited rapidly and did not recover for several hours following drug removal. That inhibition occurred preferentially in late-S-phase, compared to early-S-phase, cells and was due to both an inhibition of initiation and elongation, as determined by pulse-labeling nucleotide incorporation in replication foci and DNA fibers. DNA replication was actively inhibited by checkpoint activation since 7-hydroxystaurosporine (UCN-01), the specific Chk1 inhibitor CHIR-124, or transfection with small interfering RNA targeting Chk1 restored both initiation and elongation. Abrogation of the checkpoint markedly enhanced camptothecin-induced DNA damage at replication sites where histone gamma-H2AX colocalized with replication foci. Together, our study demonstrates that the intra-S-phase checkpoint is exerted by Chk1 not only upon replication initiation but also upon DNA elongation.

The human Tim/Tipin complex coordinates an Intra-S checkpoint response to UV that slows replication fork displacement

UV-induced DNA damage stalls DNA replication forks and activates the intra-S checkpoint to inhibit replicon initiation. In response to stalled replication forks, ATR phosphorylates and activates the transducer kinase Chk1 through interactions with the mediator proteins TopBP1, Claspin, and Timeless (Tim). Murine Tim recently was shown to form a complex with Tim-interacting protein (Tipin), and a similar complex was shown to exist in human cells. Knockdown of Tipin using small interfering RNA reduced the expression of Tim and reversed the intra-S checkpoint response to UVC. Tipin interacted with replication protein A (RPA) and RPA-coated DNA, and RPA promoted the loading of Tipin onto RPA-free DNA. Immunofluorescence analysis of spread DNA fibers showed that treating HeLa cells with 2.5 J/m(2) UVC not only inhibited the initiation of new replicons but also reduced the rate of chain elongation at active replication forks. The depletion of Tim and Tipin reversed the UV-induced inhibition of replicon initiation but affected the rate of DNA synthesis at replication forks in different ways. In undamaged cells depleted of Tim, the apparent rate of replication fork progression was 52% of the control. In contrast, Tipin depletion had little or no effect on fork progression in unirradiated cells but significantly attenuated the UV-induced inhibition of DNA chain elongation. Together, these findings indicate that the Tim-Tipin complex mediates the UV-induced intra-S checkpoint, Tim is needed to maintain DNA replication fork movement in the absence of damage, Tipin interacts with RPA on DNA and, in UV-damaged cells, Tipin slows DNA chain elongation in active replicons.

Functional interactions of DNA topoisomerases with a human replication origin

The human DNA replication origin, located in the lamin B2 gene, interacts with the DNA topoisomerases I and II in a cell cycle-modulated manner. The topoisomerases interact in vivo and in vitro with precise bonds ahead of the start sites of bidirectional replication, within the pre-replicative complex region; topoisomerase I is bound in M, early G1 and G1/S border and topoisomerase II in M and the middle of G1. The Orc2 protein competes for the same sites of the origin bound by either topoisomerase in different moments of the cell cycle; furthermore, it interacts on the DNA with topoisomerase II during the assembly of the pre-replicative complex and with DNA-bound topoisomerase I at the G1/S border. Inhibition of topoisomerase I activity abolishes origin firing. Thus, the two topoisomerases are closely associated with the replicative complexes, and DNA topology plays an essential functional role in origin activation.

Genome maintenance defects in cultured cells and mice following partial inactivation of the essential cell cycle checkpoint gene Hus1

Cell cycle checkpoints are evolutionarily conserved signaling pathways that uphold genomic integrity. Complete inactivation of the mouse checkpoint gene Hus1 results in chromosomal instability, genotoxin hypersensitivity, and embryonic lethality. To determine the functional consequences of partial Hus1 impairment, we generated an allelic series in which Hus1 expression was incrementally reduced by combining a hypomorphic Hus1 allele, Hus1(neo), with either wild-type or null (Hus1(Delta1)) alleles. Primary Hus1(neo/Delta1) embryonic fibroblasts exhibited spontaneous chromosomal abnormalities and underwent premature senescence, while higher Hus1 expression in Hus1(neo/neo) cells allowed for normal proliferation. Antioxidant treatment almost fully suppressed premature senescence in Hus1(neo/Delta1) cultures, suggesting a critical role for Hus1 in oxidative stress responses. Treatment of Hus1(neo/neo) and Hus1(neo/Delta1) cells with the DNA adducting agent benzo(a)pyrene dihydrodriol epoxide resulted in a loss of cell viability that was associated with S-phase DNA damage checkpoint failure. Likewise, the DNA polymerase inhibitor aphidicolin triggered increased cell death, chromosomal aberrations, and H2AX phosphorylation, a marker for double-stranded DNA breaks, in Hus1(neo/neo) and Hus1(neo/Delta1) cultures compared to controls. Despite these pronounced genome maintenance defects in cultured Hus1(neo/Delta1) and Hus1(neo/neo) cells, mice of the same genotypes were born at expected frequencies and appeared grossly normal. A significant increase in micronucleus formation was observed in peripheral blood cells from Hus1(neo/Delta1) mice, but reduced Hus1 expression did not cause an elevated predisposition to spontaneous tumor development or accelerate tumorigenesis in p53-deficient mice. These results identify differential effects of altered Hus1 gene dosage on genome maintenance during in vitro culture, genotoxic stress responses, embryonic development, and adult homeostasis.

Increased common fragile site expression, cell proliferation defects, and apoptosis following conditional inactivation of mouse Hus1 in primary cultured cells

Targeted disruption of the mouse Hus1 cell cycle checkpoint gene results in embryonic lethality and proliferative arrest in cultured cells. To investigate the essential functions of Hus1, we developed a system for the regulated inactivation of mouse Hus1 in primary fibroblasts. Inactivation of a loxP site-flanked conditional Hus1 allele by using a cre-expressing adenovirus resulted in reduced cell doubling, cell cycle alterations, and increased apoptosis. These phenotypes were associated with a significantly increased frequency of gross chromosomal abnormalities and an S-phase-specific accumulation of phosphorylated histone H2AX, an indicator of double-stranded DNA breaks. To determine whether these chromosomal abnormalities occurred randomly or at specific genomic regions, we assessed the stability of common fragile sites, chromosomal loci that are prone to breakage in cells undergoing replication stress. Hus1 was found to be essential for fragile site stability, because spontaneous chromosomal abnormalities occurred preferentially at common fragile sites upon conditional Hus1 inactivation. Although p53 levels increased after Hus1 loss, deletion of p53 failed to rescue the cell-doubling defect or increased apoptosis in conditional Hus1 knockout cells. In summary, we propose that Hus1 loss leads to chromosomal instability during DNA replication, triggering increased apoptosis and impaired proliferation through p53-independent mechanisms.

The dispersal of replication proteins after Etoposide treatment requires the cooperation of Nbs1 with the ataxia telangiectasia Rad3-related/Chk1 pathway

In mammalian cells, DNA replication takes place in functional subnuclear compartments, called replication factories, where replicative factors accumulate. The distribution pattern of replication factories is diagnostic of the different moments (early, mid, and late) of the S phase. This dynamic organization is affected by different agents that induce cell cycle checkpoint activation via DNA damage or stalling of replication forks. Here, we explore the cell response to etoposide, an anticancer drug belonging to the topoisomerase II poisons. Etoposide does not induce an immediate block of DNA synthesis and progressively affects the distribution of replication proteins in S phase. First, it triggers the formation of large nuclear foci that contain the single-strand DNA binding protein replication protein A (RPA), suggesting that lesions produced by the drug are processed into extended single-stranded regions. These RPA foci colocalize with DNA replicated at the beginning of the treatment. Etoposide also triggers the dispersal of replicative proteins, proliferating cell nuclear antigen and DNA ligase I, from replication factories. This event requires the activity of the ataxia telangiectasia Rad3-related (ATR) checkpoint kinase. By comparing the effect of the drug in cell lines defective in different DNA repair and checkpoint pathways, we show that, along with the downstream kinase Chk1, the Nbs1 protein, mutated in the Nijmegen breakage syndrome, is also relevant for this response and for ATR-dependent phosphorylation. Finally, our analysis evidences a critical role of Nbs1 in the etoposide-induced inhibition of DNA replication in early S phase.

Chromosome transfer experiments link regions on chromosome 7 to radiation resistance in human glioblastoma multiforme

Glioblastoma multiforme (GM) is the most lethal form of brain tumor, with a median survival of approximately 1 year. Treatment options are limited. Radiation therapy is a common form of treatment, but many tumors are resistant. In earlier studies, we found that gain of chromosome 7 is associated with radiation resistance in human primary GM. In this study, we extend that result to a model system in which we transferred chromosome 7 to recipient cells and confirmed radiation resistance as a function of chromosome 7 gain. We identified three candidate regions on chromosome 7 that conferred radiation resistance in our model system.

The effect of Hus1 on ionizing radiation sensitivity is associated with homologous recombination repair but is independent of nonhomologous end-joining

Mammalian Hus1 plays an important role in maintaining genomic integrity. Cells lacking mouse Hus1 are hypersensitive to DNA damage inducers including UV and camptothecin (CPT). By using clonogenic assay, we show here that Hus1 deficient mouse cells are hypersensitive to ionizing radiation (IR) compared with their Hus1-positive counterparts. However, these cells show similar induction levels and similar rejoining rates of DNA double strand breaks (DSBs) following IR, indicating that the effect of Hus1 on cell radiosensitivity is independent of nonhomologous end-joining (NHEJ). By combining an I-SceI-induced-DNA DSBs system and a siRNA approach, we also show that knocking down Hus1 decreases the efficiency of homologous recombination repair (HRR), which is associated with the cellular sensitivity to IR-induced killing. Together, these results indicate that the role of Hus1 affecting the sensitivity of cells to IR-induced killing is independent of NHEJ but might be linked to HRR.

Genomic profiling of restraint stress-induced alterations in mouse T lymphocytes

DNA damage-, DNA repair-, and apoptosis-related gene expression in CD3(+) T lymphocytes of BALB/c mice subjected to 2-h restraint stress were compared to that in CD3(+)T lymphocytes from control mice. Using targeted cDNA arrays, significant increases in expression of genes serving as sensors of DNA damage, including MSH genes and RAD53, were observed. GADD45g, a gene responsible for regulating cell cycle arrest and apoptosis, was significantly induced; as was Pura, a gene involved in cell proliferation. These data suggest that, at the molecular level, stress activates genes responsible for priming the T cell to either undergo apoptosis or proliferation.

Bacillus clausii effect on gene expression pattern in small bowel mucosa using DNA microarray analysis

BACKGROUND

Probiotics are widely used for the cure or prevention of several clinical conditions. However, clinical decisions need to be substantiated by an analysis of the complex bacteria-host interplay in the intestinal lumen.

AIMS

To identify the gene expression pattern induced by Bacillus clausii in the intestinal mucosa of healthy individuals.

METHODS

Six male patients (mean age 38+/-5 years) affected by endoscopically confirmed mild oesophagitis were treated for one month with esomeprazole, and were randomly selected to receive or not B. clausii (groups I and II, respectively). Duodenal biopsies were taken pre and post-treatment to identify the modification of gene expression, using the GeneChip Human U133A array. To validate the microarray analysis, real-time reverse transcriptase-polymerase chain reaction (PCR) of five target genes was performed.

RESULTS

After B. clausii administration, a total of 158 and 265 genes were up and downregulated, respectively. Quantitative PCR confirmed the microarray data. B. clausii mainly affected the expression of genes involved in immune response and inflammation, apoptosis and cell growth, cell differentiation, cell-cell signalling, cell adhesion, signal transcription and transduction.

CONCLUSIONS

Our data represent the first global analysis of B. clausii effects on the gene expression profile in normal intestine, and provide the basis to identify the mechanisms by which these agents interact with the host and exert their beneficial effects. Future studies are needed to clarify the B. clausii-induced gene pattern in the clinical disorders in which probiotics have proved to be effective.

The role of checkpoint kinase 1 in sensitivity to topoisomerase I poisons

Agents that target topoisomerase I are widely utilized to treat human cancer. Previous studies have indicated that both the ataxia telangiectasia mutated (ATM)/checkpoint kinase (Chk) 2 and ATM- and Rad 3-related (ATR)/Chk1 checkpoint pathways are activated after treatment with these agents. The relative contributions of these two pathways to survival of cells after treatment with topoisomerase I poisons are currently unknown. To address this issue, we assessed the roles of ATR, Chk1, ATM, and Chk2 in cells treated with the topoisomerase I poisons camptothecin and 7-ethyl-10-hydroxycamptothecin (SN-38), the active metabolite of irinotecan. Colony forming assays demonstrated that down-regulation of ATR or Chk1 sensitized cells to SN-38 and camptothecin. In contrast, ATM and Chk2 had minimal effect of sensitivity to SN-38 or camptothecin. Additional experiments demonstrated that the Hsp90 inhibitor 17-allylamino-17-demethoxygeldanamycin, which down-regulates Chk1, also sensitized a variety of human carcinoma cell lines to SN-38. Collectively, these results show that the ATR/Chk1 pathway plays a predominant role in the response to topoisomerase I inhibitors in carcinoma cells and identify a potential approach for enhancing the efficacy of these drugs.

cAMP-mediated inhibition of DNA replication and S phase progression: involvement of Rb, p21Cip1, and PCNA

cAMP exerts an antiproliferative effect on a number of cell types including lymphocytes. This effect of cAMP is proposed to be mediated by its ability to inhibit G1/S transition. In this report, we provide evidence for a new mechanism whereby cAMP might inhibit cellular proliferation. We show that elevation of intracellular levels of cAMP inhibits DNA replication and arrests the cells in S phase. The cAMP-induced inhibition of DNA synthesis was associated with the increased binding of p21Cip1 to Cdk2-cyclin complexes, inhibition of Cdk2 kinase activity, dephosphorylation of Rb, and dissociation of PCNA from chromatin in S phase cells. The ability of cAMP to inhibit DNA replication and trigger release of PCNA from chromatin required Rb and p21Cip1 proteins, since both processes were only marginally affected by increased levels of cAMP in Rb-/- and p21Cip1-/- 3T3 fibroblasts. Importantly, the implications of cAMP-induced inhibition of DNA synthesis in cancer treatment was demonstrated by the ability of cAMP to reduce apoptosis induced by S phase-specific cytotoxic drugs. Taken together, these results demonstrate a novel role for cAMP in regulation of DNA synthesis and support a model in which activation of cAMP-dependent signaling protects cells from the effect of S phase-specific antitumor agents.