Decreased DNA repair activity in bone marrow due to low expression of DNA damage repair proteins

The bone marrow (BM) is one of the organs that is sensitive to acute exposure of ionizing radiation (IR); however, the mechanism of its high sensitivity to IR remains to be elucidated. BM is differentiated into dendritic cells (DC) with granulocyte macrophage-colony stimulating factor (GM-CSF). Using this in vitro model, we studied whether radiosensitivity is distinctly regulated in undifferentiated and differentiated BM. We discovered that levels of DNA damage repair (DDR) proteins are extremely low in BM, and they are markedly increased upon differentiation to DC. Efficiency of both homologous recombination (HR)- and non-homologous end joining (NHEJ)-mediated repair of DNA double strand breaks (DSBs) is much lower in BM compared with that of DC. Consistent with this, immunofluorescent γH2AX is highly detected in BM after IR. These results indicate that increased radiosensitivity of BM is at least due to low expression of the DNA repair machinery.     

Heat exposure enhances radiosensitivity by depressing DNA-PK kinase activity during double strand break repair

Purpose: From the role of double strand DNA dependent protein kinase (DNA-PKcs) activity of non-homologous end joining (NHEJ) repair for DNA double strand breaks (DSBs), we aim to define possible associations between thermo-sensitisation and the enzyme activities in X-ray irradiated cells. Materials and methods: DNA-PKcs deficient mouse, Chinese hamster and human cultured cells were compared to the parental wild-type cells. The radiosensitivities, the number of DSBs and DNA-PKcs activities after heat-treatment were measured. Results: Both DNA-PKcs deficient cells and the wild-type cells showed increased radiosensitivities after heat-treatment. The wild-type cells have two repair processes; fast repair and slow repair. In contrast, DNA-PKcs deficient cells have only the slow repair process. The fast repair component apparently disappeared by heat-treatment in the wild-type cells. In both cell types, additional heat exposure enhanced radiosensitivities. Although DNA-PKcs activity was depressed by heat, the inactivated DNA-PKcs activity recovered during an incubation at 37?°C. DSB repair efficiency was dependent on the reactivation of DNA-PKcs activity. Conclusion: It was suggested that NHEJ is the major process used to repair X-ray-induced DSBs and utilises DNA-PKcs activity, but homologous recombination repair provides additional secondary levels of DSB repair. The thermo-sensitisation in X-ray-irradiated cells depends on the inhibition of NHEJ repair through the depression of DNA-PKcs activities.     

Synergistic radiosensitizing effect of BR101801, a specific DNA-dependent protein kinase inhibitor,in various human solid cancer cells and xenografts

DNA-dependent protein kinase (DNA-PK), an essential component of the non-homologous end-joining (NHEJ) repair pathway, plays an important role in DNA damage repair (DDR). Therefore, DNA-PK inhibition is a promising approach for overcoming radiotherapy or chemotherapy resistance in cancers. In this study, we demonstrated that BR101801, a potent DNA-PK inhibitor, acted as an effective radiosensitizer in various human solid cancer cells and an in vivo xenograft model. Overall, BR101801 strongly elevated ionizing radiation (IR)-induced genomic instability via induction of cell cycle G₂/M arrest, autophagic cell death, and impairment of DDR pathway in human solid cancer cells. Interestingly, BR101801 inhibited not only phosphorylation of DNA-PK catalytic subunit in NHEJ factors but also BRCA2 protein level in homologous recombination (HR) factors. In addition, combination BR101801 and IR suppressed tumor growth compared with IR alone by reducing phosphorylation of DNA-PK in human solid cancer xenografts. Our findings suggested that BR101801 is a selective DNA-PK inhibitor with a synergistic radiosensitizing effect in human solid cancers, providing evidence for clinical applications.     

胶质瘤DNA依赖性蛋白激酶活性与化疗药物敏感性的关系

目的 研究DNA依赖性蛋白激酶（DNA-PK）活性和脑胶质瘤化疗敏感性的关系。方法原代培养胶质瘤细胞,用MTT法检测其对6种不同抗癌药物的敏感性,以血药峰浓度（PPC）下的抑制率（IR）表示。提取同一胶质瘤组织标本中核蛋白质,用p53蛋白为特异底物的磷酸化反应检测其中的DNA-PK活性。结果不同胶质瘤组织标本中DNA-PK活性差别较大,36例组织标本中,16例的DNA-PK活性较高（相对活性≥0．40）,20例的DNA-PK活性较低（相对活性〈0．40）。对DDP、VCR敏感（IR≥50％）的肿瘤组织,DNA-PK活性低;对DDP、VCR不敏感（IR〈50％）的肿瘤组织,DNA-PK活性高（t=-3．445,P〈0．01）。同时,DNA-PK活性高的肿瘤组织,DDP、VCR体外抑制肿瘤细胞的IR值低;而DNA-PK活性低的肿瘤组织,相应的IR值高（t=-2．145,P〈0．05）。结论胶质瘤标本的DNA-PK活性与其对DDP、VCR敏感性显著相关,DNA-PK活性的高低可能是胶质瘤化疗敏感性的一个新的标记物。     

Radioresistant human lung adenocarcinoma cells that survived multiple fractions of ionizing radiation are sensitive to HSP90 inhibition

Despite the common usage of radiotherapy for the treatment of NSCLC, outcomes for these cancers when treated with ionizing radiation (IR) are still unsatisfactory. A better understanding of the mechanisms underlying resistance to IR is needed to design approaches to eliminate the radioresistant cells and prevent tumor recurrence and metastases. Using multiple fractions of IR we generated radioresistant cells from T2821 and T2851 human lung adenocarcinoma cells. The radioresistant phenotypes present in T2821/R and T2851/R cells include multiple changes in DNA repair genes and proteins expression, upregulation of EMT markers, alterations of cell cycle distribution, upregulation of PI3K/AKT signaling and elevated production of growth factors, cytokines, important for lung cancer progression, such as IL-6, PDGFB and SDF-1 (CXCL12). In addition to being radioresistant these cells were also found to be resistant to cisplatin.HSP90 is a molecular chaperone involved in stabilization and function of multiple client proteins implicated in NSCLC cell survival and radioresistance. We examined the effect of ganetespib, a novel HSP90 inhibitor, on T2821/R and T2851/R cell survival, migration and radioresistance. Our data indicates that ganetespib has cytotoxic activity against parental T2821 and T2851 cells and radioresistant T2821/R and T2851/R lung tumor cells. Ganetespib does not affect proliferation of normal human lung fibroblasts. Combining IR with ganetespib completely abrogates clonogenic survival of radioresistant cells.Our data show that HSP90 inhibition can potentiate the effect of radiotherapy and eliminate radioresistant and cisplatin -resistant residual cells, thus it may aid in reducing NSCLC tumor recurrence after fractionated radiotherapy.     

Base excision repair targets for cancer therapy

Cellular DNA repair is a frontline system that is responsible for maintaining genome integrity and thus preventing premature aging and cancer by repairing DNA lesions and strand breaks caused by endogenous and exogenous mutagens. However, it is also the principal cellular system in cancer cells that counteracts the killing effect of the major cancer treatments e.g. chemotherapy and ionizing radiation. The major goal of this review is to critically exam the base excision repair pathway and mechanisms regulating base excision repair capacity as a potential targets for improving cancer therapy.     

Polymorphisms in DNA repair genes, ionizing radiation exposure and risk of breast cancer in U.S. Radiologic technologists

High-dose ionizing radiation exposure to the breast and rare autosomal dominant genes have been linked with increased breast cancer risk, but the role of low-to-moderate doses from protracted radiation exposure in breast cancer risk and its potential modification by polymorphisms in DNA repair genes has not been previously investigated among large numbers of radiation-exposed women with detailed exposure data. Using carefully reconstructed estimates of cumulative breast doses from occupational and personal diagnostic ionizing radiation, we investigated the potential modification of radiation-related breast cancer risk by 55 candidate single nucleotide polymorphisms in 17 genes involved in base excision or DNA double-strand break repair among 859 cases and 1083 controls from the United States Radiologic Technologists (USRT) cohort. In multivariable analyses, WRN V114I (rs2230009) significantly modified the association between cumulative occupational breast dose and risk of breast cancer (adjusted for personal diagnostic exposure) (p = 0.04) and BRCA1 D652N (rs4986850), PRKDC IVS15 + 6C > T (rs1231202), PRKDC IVS34 + 39T > C (rs8178097) and PRKDC IVS31 - 634C > A (rs10109984) significantly altered the personal diagnostic radiation exposure-response relationship (adjusted for occupational dose) (p < or = 0.05). None of the remaining 50 SNPs significantly modified breast cancer radiation dose-response relationships. The USRT genetic study provided a unique opportunity to examine the joint effects of common genetic variation and ionizing radiation exposure on breast cancer risk using detailed occupational and personal diagnostic exposure data. The suggestive evidence found for modification of radiation-related breast cancer risk for 5 of the 55 SNPs evaluated requires confirmation in larger studies of women with quantified radiation breast doses in the low-to-moderate range.     

The novel HDAC inhibitor NDACI054 sensitizes human cancer cells to radiotherapy

Background and purpose

Inhibition of histone deacetylases (HDACs) has preclinically and clinically shown promise to overcome radio- and chemoresistance of tumor cells. NDACI054 is a novel HDAC inhibitor, which has been evaluated here for its effects on cell survival and radiosensitization of human tumor cell lines from different origins cultured under more physiological three-dimensional (3D), extracellular matrix (ECM)-based conditions.

Material and methods

A549 lung, DLD-1 colorectal, MiaPaCa2 pancreatic and UT-SCC15 head and neck squamous cell carcinoma cells were treated with increasing NDACI054 concentrations (0–50 nM, 24 h) either alone or in combination with X-rays (single dose, 0–6 Gy). Subsequently, 3D clonogenic cell survival, HDAC activity, histone H3 acetylation, apoptosis, residual DNA damage (γH2AX/p53BP1 foci assay 24 h post irradiation) and phosphorylation kinetics of Ataxia telangiectasia mutated (ATM), DNA-dependent protein kinase (DNA-PK), Caspase-3 and Poly(ADP-ribose)-Polymerase 1 (PARP 1) cleavage were analyzed.

Results

NDACI054 potently decreased HDAC activity with concomitant increase in acetyl-histone H3 levels, mediated significant cytotoxicity and radiosensitization. These effects were accompanied by a significant increase of residual γH2AX/p53BP1-positive foci, slightly elevated levels of Caspase-3 and PARP 1 cleavage but no induction of apoptosis.

Conclusions

Our data show potent antisurvival and radiosensitizing effects of the novel HDAC inhibitor NDACI054 encouraging further preclinical examinations on this compound for future clinical use.     

Enhanced deoxyribonuclease activity in human transformed cells and in Bloom's syndrome cells

Human hereditary diseases such as xeroderma pigmentosum, Fanconi's anemia, ataxia telangiectasia, and Bloom's syndrome are characterized by a proneness for developing cancer associated with abnormalities in the processing of DNA damage. The molecular defects responsible for predisposing human tissues to cancer are still not well understood, despite the fact that a considerable amount of work has already been done on this problem. In this paper, we show that in human tumor cell lines, in cells transformed by DNA tumor viruses, and in cells derived from certain cancer-prone disorders, the level of activity of a 42-kDa deoxyribonuclease is many times higher than in diploid untransformed control cells. This suggests that this activity is linked to, or may play a role in, malignant transformation.     

Trichostatin A enhances radiosensitivity and radiation-induced DNA damage of esophageal cancer cells

BackgroundTrichostatin A (TSA) is emerging as a potential component of anticancer therapy. In this study, we aimed to identify the radiosensitizing effects of TSA in esophageal squamous carcinoma cell lines and identify the genomic alteration of histone acetylation associated with TSA treatment.MethodsEC109 and KYSE450 cells were pretreated with TSA (0.1 µM) for 12 hours prior to irradiation, and the cell viability, flow cytometry, and comet assays were performed to analyze cell growth, cell apoptosis, and DNA damage, respectively. Chromatin immunoprecipitation sequencing (ChIP-Seq) was performed to identify the acetylation sites of histone H3 lysine 9 (H3K9), which was altered by TSA.ResultsOur data showed that TSA could sensitize esophageal cancer cells to radiation by inducing cell cycle arrest and increasing cell apoptosis. DNA damage induced by radiation was enhanced by TSA treatment. In addition, a total of 105 differential peak-related genes were found to be associated with TSA treatment, which was identified using ChIP-Seq with specific antibodies against acetylated histone H3K9.ConclusionsOur data suggest that pretreatment with TSA can enhance ionizing radiation-induced DNA damage of esophageal cancer cells, which was associated with the altered histone modification of whole genome. TSA has potential implications for clinical use in increasing the anticancer efficacy of radiation.     

Caspase-3-like activity determines the type of cell death following ionizing radiation in MOLT-4 human leukaemia cells

Caspases, a family of cysteine proteases, play a central role in the pathways leading to apoptosis. Recently, it has been reported that a broad spectrum inhibitor of caspases, the tripeptide Z-VAD-fmk, induced a switch from apoptosis to necrosis in dexamethasone-treated B lymphocytes and thymocytes. As such a cell death conversion could increase the efficiency of radiation therapy and in order to identify the caspases involved in this cell death transition, we investigated the effects of caspase-3-related proteases inhibition in irradiated MOLT-4 cells. Cells were pretreated with Ac-DEVD-CHO, an inhibitor of caspase-3-like activity, and submitted to X-rays at doses ranging from 1 to 4 Gy. Our results show that the inhibition of caspase-3-like activity prevents completely the appearance of the classical hallmarks of apoptosis such as internucleosomal DNA fragmentation or hypodiploid particles formation and partially the externalization of phosphatidylserine. However, this was not accompanied by any persistent increase in cell survival. Instead, irradiated cells treated by this inhibitor exhibited characteristics of a necrotic cell death. Therefore, functional caspase-3-subfamily not only appears as key proteases in the execution of the apoptotic process, but their activity may also influence the type of cell death following an exposure to ionizing radiation.     

Hyperthermia and irradiation of head and neck squamous cancer cells causes migratory profile changes of tumour infiltrating lymphocytes

Purpose: CD4⁺CD25⁺FoxP3⁺ regulatory T-cells (Treg) are responsible for immunoevasion mechanisms induced by cancer. Specific chemokines such as CCL22 are presumed to mediate active Treg trafficking into the tumour site. In this context, the effects of irradiation and hyperthermia of tumour cells on Treg migration and the CCL22 concentration in the tumour cell supernatants after treatment were studied. Moreover, the relationship between CCL22 concentration and Treg cell migration was also examined.

Materials and methods: Treg and CD4⁺CD25^? T-cells were isolated from human peripheral blood. Supernatants were obtained from primary cell cultures derived from head and neck carcinoma patients. Tumour cell cultures were treated with a dose of 2 Gy and hyperthermia (41.5°C) or with hyperthermia or irradiation alone. Cancer cell culture supernatants were then used for a transmigration assay.

Results: Treg and CD4⁺CD25^? T-cells showed an increased transmigration towards supernatants of hyperthermia-treated tumour cells. After combined application of hyperthermia and irradiation, Treg migration was similar to control levels, but CD4⁺CD25^? migration was still enhanced. Irradiation caused a significantly decreased Treg influx, whereas the CD4⁺CD25^? T-cell migration was not altered after the same treatment. Changes of Treg chemotaxis could be attributed to a treatment-associated escalation of the CCL22 in the tumour cell supernatants.

Conclusion: The combination of irradiation and hyperthermia is able to modify transmigration of tumour infiltrating lymphocytes beneficially and individually. In this in vitro system hyperthermia alone negatively impacts the immune response by selectively recruiting Treg, whereas hyperthermia with the addition of irradiation negates this effect.     

甲基化芯片检测电离辐射诱导人外周血淋巴细胞DNA甲基化水平的变化

目的：筛选正常人细胞基因组中电离辐射后甲基化水平发生变化的基因,为在其中寻找新型辐射损伤标志物奠定研究基础。方法：⁶⁰Co γ射线照射人外周血全血,照射剂量为0、0.5和2.0 Gy。利用Illumina 450K芯片检测外周血淋巴细胞基因组DNA甲基化水平的变化,筛选甲基化水平差异的基因,利用GO功能富集分析基因的分子功能和参与的生物学途径。结果：0.5和2.0 Gy γ射线照射下人外周血淋巴细胞基因组DNA甲基化水平显著上调的基因有1 311个,显著下调的基因286个。GO功能富集分析表明,按富集度大小排列,差异基因在生物途径水平上排在首位的是“细胞过程”（富集度=5.86）,在细胞定位水平上排在首位的是“细胞”（富集度=7.48）,在分子功能水平上排在首位的是“结合”（富集度=5.27）。进一步细分后得到差异基因显著富集在与转录调控及转录因子活性相关的功能上,与以往的研究结果认为DNA甲基化参与转录调控和基因表达相一致;甲基化水平差异基因还显著富集在核苷连接（GO：0001882）上,并在其中寻找到与DNA修复相关的基因RAD50、RAD54L和INIP,以及与维持染色体结构稳定相关的基因HIST1H4K、SMC1B。结论：正常人外周血淋巴细胞基因甲基化水平受电离辐射影响,可进一步研究将这些与DNA修复、维持染色体结构稳定相关的基因甲基化水平作为辐射损伤标志物的可行性。     

HIV-1 Tat depresses DNA-PK(CS) expression and DNA repair, and sensitizes cells to ionizing radiation

PURPOSE: There is accumulating evidence that cancer patients with human immmunodeficiency virus-1/acquired immunodeficency syndrome (HIV-1/AIDS) have more severe tissue reactions and often develop cutaneous toxic effects when subjected to radiotherapy. Here we explored the effects of the HIV-1 Tat protein on cellular responses to ionizing radiation. METHODS AND MATERIALS: Two Tat-expressing cell lines, TT2 and TE671-Tat, were derived from human rhabdomyosarcoma cells by transfecting with the HIV-1 tat gene. Radiosensitivity was determined using colony-forming ability. Gene expression was assessed by cDNA microarray and immunohybridization. The Comet assay and gamma-H2AX foci were use to detect DNA double-strand breaks (DSBs) and repair. Radiation-induced cell cycle changes were detected by flow cytometry. RESULTS: The radiosensitivity of TT2 and TE671-Tat cells was significantly increased as compared with parental TE671 cells or the control TE671-pCI cells. Tat also increased proliferation activity. The comet assay and gammaH2AX foci detection revealed a decreased capacity to repair radiation-induced DNA DSBs in Tat-expressing cells. Microarray assay demonstrated that the DNA repair gene DNA-PKcs, and cell cycle-related genes Cdc20, Cdc25C, KIF2C and CTS1 were downregulated in Tat-expressing cells. Depression of DNA-PKcs in Tat-expressing cells was further confirmed by RT-PCR and immuno-hybridization analysis. Tat-expressing cells exhibited a prolonged S phase arrest after 4 Gy gamma-irradiation, and a noticeable delay in the initiation and elimination of radiation-induced G(2)/M arrest as compared with parental cells. In addition, the G(2)/M arrest was incomplete in TT2 cells. Moreover, HIV-1 Tat resulted in a constitutive overexpression of cyclin B1 protein. CONCLUSION: HIV-1 Tat protein sensitizes cells to ionizing radiation via depressing DNA repair and dysregulating cell cycle checkpoints. These observations provide new insight into the increased tissue reactions of AIDS cancer patients to radiotherapy.     

Chk1 and DNA-PK mediate TPEN-induced DNA damage in a ROS dependent manner in human colon cancer cells

Recently, we showed that the metal chelator TPEN targets colon cancer cells through redox cycling of copper. Here, we studied the DNA damage potential of TPEN and deciphered the role of Chk1, ATM and DNA-PK in TPEN-induced toxicity in 3 human colon cancer cell lines, HCT116, SW480 and HT29. We also investigated the role of reactive oxygen species (ROS) in TPEN-induced DNA damage. TPEN reduced cell viability in a dose- and time-dependent manner. Cytotoxicity was associated with significant DNA damage and higher expression of γ-H2AX protein and activation of ATM/ATR signaling pathway. Cell death by TPEN was dependent on ROS generation as evidenced by the reversal of cell viability, and DNA damage and the abrogation of γ-H2AX levels in the presence of antioxidants. Treatment with antioxidants, however, failed to reverse cytotoxicity at high TPEN concentrations (10µM). TPEN-induced cell death was also dependent on the redox cycling of copper since the copper chelator neocuproine inhibited DNA damage and reduced pChk1, γ-H2AX, and ATM protein expression. Cell death by low TPEN concentrations, involved ATM/ATR signaling in all 3 cell lines, since pre-incubation with specific inhibitors of ATM and DNA-PK led to the recovery of cells from TPEN-induced DNA damage. In addition, siRNA silencing of Chk1, DNA-PK and ATM abrogated the expression of γ-H2AX and reversed cell death, suggesting that Chk1 and DNA-PK mediate TPEN-induced cytotoxicity in colon cancer cells. This study shows for the first time the involvement of Chk1, DNA-PK and ATM in TPEN-induced DNA damage and confirms our previous findings that ROS generation and the redox cycling of copper in response to TPEN are the main mechanisms by which this compound induces cell death in human colon cancer cells. Inhibition of ATM or DNA-PK did not reverse cytotoxicity at high TPEN concentrations that cause excessive levels of ROS and irreversible cellular damage.     

Backup pathways of NHEJ in cells of higher eukaryotes: Cell cycle dependence

DNA double-strand breaks (DSBs) induced by ionizing radiation (IR) in cells of higher eukaryotes are predominantly repaired by a pathway of non-homologous end joining (NHEJ) utilizing Ku, DNA-PKcs, DNA ligase IV, XRCC4 and XLF/Cernunnos (D-NHEJ) as central components. Work carried out in our laboratory and elsewhere shows that when this pathway is chemically or genetically compromised, cells do not shunt DSBs to homologous recombination repair (HRR) but instead use another form of NHEJ operating as a backup (B-NHEJ). Here I review our efforts to characterize this repair pathway and discuss its dependence on the cell cycle as well as on the growth conditions. I present evidence that B-NHEJ utilizes ligase III, PARP-1 and histone H1. When B-NHEJ is examined throughout the cell cycle, significantly higher activity is observed in G2 phase that cannot be attributed to HRR. Furthermore, the activity of B-NHEJ is compromised when cells enter the plateau phase of growth. Together, these observations uncover a repair pathway with unexpected biochemical constitution and interesting cell cycle and growth factor regulation. They generate a framework for investigating the mechanistic basis of HRR contribution to DSB repair.     

Effect of the combination of hyperthermia and irradiation on human colon cancer cells

The combination of hyperthermia and radiation is considered to have a synergistic effect on mammalian cells, which depends on temperature and duration of time of the hyperthermia and on the sequence in which the modalities are given. The response of cultured colon cancer cells to hyperthermia and ionizing radiation was evaluated using the flow cytometry method by measuring DNA damage. The enhanced DNA damage after combined treatment with hyperthermia and irradiation was recognized after both sequences: heat followed by radiation as well as radiation followed by heat. The results demonstrated that the sequence of radiation plus heat produced more DNA damage on colon cancer cells than did the reverse order. The clinical application of these results is discussed. © 1994 Wiley-Liss, Inc.     

Drug and radiation resistance in spheroids: cell contact and kinetics

Cells from multicellular spheroids are often more resistant than monolayers to drugs and radiation. While explanations for resistance can be based on differences in cell cycle distribution, inability of the drug to penetrate the spheroid, or the presence of hypoxic cells, these mechanisms do not adequately explain resistance to all agents. Small spheroids (containing about 25–50 cells) exposed to ionizing radiation, hyperthermia, photodynamic therapy, or topoisomerase II inhibitors, are more resistant to killing than monolayers; the close three-dimensional contact in spheroids has been implicated in this resistance. Proposed mechanisms for the contact effect include gap junctional reciprocity, cell shape mediated changes in (repair-related) gene expression, and alterations in chromatin packaging which influence DNA repair. The consequences of the contact effect are especially important for multifraction exposures. Another form of resistance can be demonstrated during repetitive treatments; regrowth resistance reflects the capacity of spheroid cells to proliferate more efficiently to compensate for cell killing.     

Involvement of nucleophosmin/B23 in the response of HeLa cells to UV irradiation.

The steady-state mRNA level of nucleophosmin/B23 in HeLa cells increased after UV irradiation. Nucleophosmin/B23 antisense transfection potentiated ultraviolet (UV)-induced cell killing. A block in G(2)/M phase, larger peak of apoptotic cells and higher caspase-3 in vitro activity were noted in nucleophosmin/B23 antisense-transfected cells compared with vector-transfected cells after UV treatment. Irradiated cells that received vector plasmid exhibited increased levels of [(3)H]thymidine incorporation due to DNA repair synthesis. In contrast, irradiated cells that received nucleophosmin/B23 antisense plasmid exhibited no such increase of [(3)H]thymidine incorporation, indicating inhibition of DNA repair. Cotransfection of cells with vector allowed repair of the damaged chloramphenicol acetyl transferase (CAT) reporter and rescue of CAT activity by host repair machinery. CAT activity in cells cotransfected with nucleophosmin/B23 antisense was less (<50%) than that of vector-transfected cells, indicating reduction of host nucleotide excision repair activity. Lower protein expressions of nucleophosmin/B23 and proliferating cell nuclear antigen (PCNA) were observed in nucleophosmin/B23 antisense-transfected cells compared with vector-transfected cells with or without UV treatment. Cotransfection of nucleophosmin/B23 antisense-transfected HeLa cells with PCNA construct made the cells less susceptible to UV-induced cell killing. Our results indicate that nucleophosmin/B23 correlates with PCNA and DNA repair capacity in cellular sensitivity to UV.