DNA-Dependent Protein Kinase Activity Correlates With Clinical and In Vitro Sensitivity of Chronic Lymphocytic Leukemia Lymphocytes to Nitrogen Mustards

DNA repair pathways as guardians of the genome: Therapeutic potential and possible prognostic role in hematologic neoplasms

DNA repair pathways, which are also identified as guardians of the genome, protect cells from frequent damage that can lead to DNA breaks. The most deleterious types of damage are double-strand breaks (DSBs), which are repaired by homologous recombination (HR) and non-homologous end joining (NHEJ). Single strand breaks (SSBs) can be corrected through base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR). Failure to restore DNA lesions or inappropriately repaired DNA damage culminates in genomic instability and changes in the regulation of cellular functions. Intriguingly, particular mutations and translocations are accompanied by special types of leukemia. Besides, expression patterns of certain repair genes are altered in different hematologic malignancies. Moreover, analysis of mutations in key mediators of DNA damage repair (DDR) pathways, as well as investigation of their expression and function, may provide us with emerging biomarkers of response/resistance to treatment. Therefore, defective DDR pathways can offer a rational starting point for developing DNA repair-targeted drugs. In this review, we address genetic alterations and gene/protein expression changes, as well as provide an overview of DNA repair pathways.

Targeting DNA-PK in cancer

DNA-dependent protein kinase (DNA-PK) is involved in many cellular pathways. It has a key role in the cellular response to DNA damage, in the repair of DNA double-strand break (DNA-DSBs) and as a consequence an important role in maintaining genomic integrity. In addition, DNA-PK has been shown to modulate transcription, to be involved in the development of the immune system and to protect telomeres. These pleotropic involvements and the fact that its expression is de-regulated in cancer have made DNA-PK an intriguing therapeutic target in cancer therapy, especially when combined with agents causing DNA-DSBs such as topoisomerase II inhibitors and ionizing radiation. Different small molecule inhibitors of DNA-PK have been recently synthesized and some are now being tested in clinical trials. This review discusses what is known about DNA-PK, its role in tumor biology, DNA repair and cancer therapy and critically discusses its inhibition as a potential therapeutic approach.

Targeting the DNA-PK complex: Its rationale use in cancer and HIV-1 infection

The DNA-PK complex is the major component of the predominant mechanism of DSB repair in humans. In addition, this complex is involved in many other processes such as DNA recombination, genome maintenance, apoptosis and transcription regulation. Several studies have linked the decrease of the DNA-PK activity with cancer initiation, due to defects in the repair. On another hand, higher DNA-PK expression and activity have been observed in various other tumor cells and have been linked with a decrease of the efficiency of anti-tumor drugs. It has also been shown that DNA-PK is critical for the integration of the HIV-1 DNA into the cell host genome and promotes replication and transcription of the virus. Targeting this complex makes therefore sense to treat these two pathologies. However, according to the status of HIV-1 replication (active versus latent replication) or to the tumor grade cells (initiation versus metastasis), the way to target this DNA-PK complex might be rather different. In this review, we discuss the importance of DNA-PK complex in two major pathologies i.e. HIV-1 infection and cancer, and the rationale use of therapies aiming to target this complex.

The Role of the Core Non-Homologous End Joining Factors in Carcinogenesis and Cancer

DNA double-strand breaks (DSBs) are deleterious DNA lesions that if left unrepaired or are misrepaired, potentially result in chromosomal aberrations, known drivers of carcinogenesis. Pathways that direct the repair of DSBs are traditionally believed to be guardians of the genome as they protect cells from genomic instability. The prominent DSB repair pathway in human cells is the non-homologous end joining (NHEJ) pathway, which mediates template-independent re-ligation of the broken DNA molecule and is active in all phases of the cell cycle. Its role as a guardian of the genome is supported by the fact that defects in NHEJ lead to increased sensitivity to agents that induce DSBs and an increased frequency of chromosomal aberrations. Conversely, evidence from tumors and tumor cell lines has emerged that NHEJ also promotes chromosomal aberrations and genomic instability, particularly in cells that have a defect in one of the other DSB repair pathways. Collectively, the data present a conundrum: how can a single pathway both suppress and promote carcinogenesis? In this review, we will examine NHEJ's role as both a guardian and a disruptor of the genome and explain how underlying genetic context not only dictates whether NHEJ promotes or suppresses carcinogenesis, but also how it alters the response of tumors to conventional therapeutics.

Taking a Bad Turn: Compromised DNA Damage Response in Leukemia

Genomic integrity is of outmost importance for the survival at the cellular and the organismal level and key to human health. To ensure the integrity of their DNA, cells have evolved maintenance programs collectively known as the DNA damage response. Particularly challenging for genome integrity are DNA double-strand breaks (DSB) and defects in their repair are often associated with human disease, including leukemia. Defective DSB repair may not only be disease-causing, but further contribute to poor treatment outcome and poor prognosis in leukemia. Here, we review current insight into altered DSB repair mechanisms identified in leukemia. While DSB repair is somewhat compromised in all leukemic subtypes, certain key players of DSB repair are particularly targeted: DNA-dependent protein kinase (DNA-PK) and Ku70/80 in the non-homologous end-joining pathway, as well as Rad51 and breast cancer 1/2 (BRCA1/2), key players in homologous recombination. Defects in leukemia-related DSB repair may not only arise from dysfunctional repair components, but also indirectly from mutations in key regulators of gene expression and/or chromatin structure, such as p53, the Kirsten ras oncogene (K-RAS), and isocitrate dehydrogenase 1 and 2 (IDH1/2). A detailed understanding of the basis for defective DNA damage response (DDR) mechanisms for each leukemia subtype may allow to further develop new treatment methods to improve treatment outcome and prognosis for patients.

Dual TORK/DNA-PK inhibition blocks critical signaling pathways in chronic lymphocytic leukemia

TORK/DNA-PK inhibition induces cytotoxicity and blocks signaling pathways important for CLL survival, proliferation, and drug resistance. Preliminary clinical effects of TORK/DNA-PK inhibition show 7 of 8 CLL patients with decreased lymphadenopathy.

Activation of DNA damage repair pathways in response to nitrogen mustard-induced DNA damage and toxicity in skin keratinocytes

Nitrogen mustard (NM), a structural analog of chemical warfare agent sulfur mustard (SM), forms adducts and crosslinks with DNA, RNA and proteins. Here we studied the mechanism of NM-induced skin toxicity in response to double strand breaks (DSBs) resulting in cell cycle arrest to facilitate DNA repair, as a model for developing countermeasures against vesicant-induced skin injuries. NM exposure of mouse epidermal JB6 cells decreased cell growth and caused S-phase arrest. Consistent with these biological outcomes, NM exposure also increased comet tail extent moment and the levels of DNA DSB repair molecules phospho H2A.X Ser139 and p53 Ser15 indicating NM-induced DNA DSBs. Since DNA DSB repair occurs via non homologous end joining pathway (NHEJ) or homologous recombination repair (HRR) pathways, next we studied these two pathways and noted their activation as defined by an increase in phospho- and total DNA-PK levels, and the formation of Rad51 foci, respectively. To further analyze the role of these pathways in the cellular response to NM-induced cytotoxicity, NHEJ and HRR were inhibited by DNA-PK inhibitor NU7026 and Rad51 inhibitor BO2, respectively. Inhibition of NHEJ did not sensitize cells to NM-induced decrease in cell growth and cell cycle arrest. However, inhibition of the HRR pathway caused a significant increase in cell death, and prolonged G2M arrest following NM exposure. Together, our findings, indicating that HRR is the key pathway involved in the repair of NM-induced DNA DSBs, could be useful in developing new therapeutic strategies against vesicant-induced skin injury.

Poly(ADP-ribose) polymerase inhibitor CEP-8983 synergizes with bendamustine in chronic lymphocytic leukemia cells in vitro

DNA repair aberrations and associated chromosomal instability is a feature of chronic lymphocytic leukemia (CLL). To evaluate if DNA repair insufficiencies are related to methylation changes, we examined the methylation of nine promoter regions of DNA repair proteins by bisulfide sequencing in 26 CLL primary samples and performed quantitative PCR on a subset of samples to examine BRCA1 expression. We also investigated if changes in cytogenetic or expression level of DNA repair proteins led to changes in sensitivity to a novel PARP inhibitor, CEP-8983, alone and in combination with bendamustine. No changes in promoter methylation were identified in BRCA1, BRCA2, FANC-C, FANC-F, FANC-L, ATM, MGMT, hMLH1 and H2AX except for two cases of minor BRCA1 hypermethylation. CLL samples appeared to have reduced BRCA1 mRNA expression uniformly in comparison to non-malignant lymphocytes irrespective of promoter hypermethylation. CEP-8983 displayed single agent cytotoxicity and the combination with bendamustine demonstrated synergistic cytotoxicity in the majority of CLL samples. These results were consistent across cytogenetic subgroups, including 17p deleted and previously treated patients. Our results provide rationale for further exploration of the combination of a PARP inhibitor and DNA damaging agents as a novel therapeutic strategy in CLL.

The different biological effects of single, fractionated and continuous low dose rate irradiation on CL187 colorectal cancer cells

PURPOSE

To determine the biological effectiveness of single, fractionated and continuous low dose rate irradiation on the human colorectal cancer cell line CL187 in vitro and explore the cellular mechanisms.

MATERIALS AND METHODS

The CL187 cells were exposed to radiation of 6 MV X-ray at a high dose rate of 4Gy/min and 125I seed at a low dose rate of 2.77 cGy/h. Three groups were employed: single dose radiation group (SDR), fractionated dose radiation group (FDR) by 2Gy/f and continuous low dose rate radiation group (CLDR). Four radiation doses 2, 4, 6 and 8Gy were chosen and cells without irradiation as the control. The responses of CL187 cells to distinct modes of radiation were evaluated by the colony-forming assay, cell cycle progression as well as apoptosis analysis. In addition, we detected the expression patterns of DNA-PKcs, Ku70 and Ku80 by Western blotting.

RESULTS

The relative biological effect for 125I seeds compared with 6 MV X-ray was 1.42. 48 hrs after 4Gy irradiation, the difference between proportions of cells at G2/M phase of SDR and CLDR groups were statistically significant (p = 0.026), so as the FDR and CLDR groups (p = 0.005). 48 hrs after 4Gy irradiation, the early apoptotic rate of CLDR group was remarkably higher than SDR and FDR groups (CLDR vs. SDR, p = 0.001; CLDR vs. FDR, p = 0.02), whereas the late apoptotic rate of CLDR group increased significantly compared with SDR and FDR group (CLDR vs. SDR, p = 0.004; CLDR vs. FDR, p = 0.007). Moreover, DNA-PKcs and Ku70 expression levels in CLDR-treated cells decreased compared with SDR and FDR groups.

CONCLUSIONS

Compared with the X-ray high dose rate irradiation, 125I seeds CLDR showed more effective induction of cell apoptosis and G2/M cell cycle arrest. Furthermore, 125I seeds CLDR could impair the DNA repair capability by down-regulating DNA-PKcs and Ku70 expression.

DNA-dependent protein kinase and its inhibition in support of radiotherapy

PURPOSE

Radiotherapy has been used as a treatment of almost 50% of all malignant tumors. The aim of this review is to provide a comprehensive overview of the recent knowledge in the field of molecular mechanisms of radiation-induced double-stranded breaks (DSB) repair. This paper gives particular emphasis to a key DNA repair enzyme, DNA-dependent protein kinase (DNA-PK), which plays a pivotal role in non-homologous end-joining. Furthermore, we discuss possibilities of DNA-PK inhibition and other molecular approaches employed in order to facilitate radiotherapy.

CONCLUSIONS

We have reviewed the recent studies using novel potent and selective small-molecular DNA-PK inhibitors and we conclude that targeted inhibition of DNA repair proteins like DNA-PK in cancer cells, in combination with ionizing radiation, improves the efficacy of cancer therapy while minimizing side-effects of ionizing radiation. Moreover, the recent discovery of short interfering RNA (siRNA) and signal interfering DNA (siDNA)-based therapeutics, or small peptides and RNA, shows a new opportunity of selective and safe application of biological treatment. All of these approaches are believed to contribute to more personalized anti-cancer therapy.

Waldenström's macroglobulinemia harbors a unique proteome where Ku70 is severely underexpressed as compared with other B-lymphoproliferative disorders

Waldenström's macroglobulinemia (WM) is a clonal B-cell lymphoproliferative disorder (LPD) of post-germinal center nature. Despite the fact that the precise molecular pathway(s) leading to WM remain(s) to be elucidated, a hallmark of the disease is the absence of the immunoglobulin heavy chain class switch recombination. Using two-dimensional gel electrophoresis, we compared proteomic profiles of WM cells with that of other LPDs. We were able to demonstrate that WM constitutes a unique proteomic entity as compared with chronic lymphocytic leukemia and marginal zone lymphoma. Statistical comparisons of protein expression levels revealed that a few proteins are distinctly expressed in WM in comparison with other LPDs. In particular we observed a major downregulation of the double strand repair protein Ku70 (XRCC6); confirmed at both the protein and RNA levels in an independent cohort of patients. Hence, we define a distinctive proteomic profile for WM where the downregulation of Ku70—a component of the non homologous end-joining pathway—might be relevant in disease pathophysiology.

Further characterisation of the cellular activity of the DNA-PK inhibitor, NU7441, reveals potential cross-talk with homologous recombination

PURPOSE

Inhibition of DNA repair is emerging as a new therapeutic strategy for cancer treatment. One promising target is DNA-PK, a pivotal kinase in double-strand break repair. The purpose of this study was to further characterise the activity of the DNA-PK inhibitor NU7441, giving some new insights into the biology of DNA-PK.

METHODS

We used NU7441, a potent DNA-PK inhibitor, to evaluate potential pharmacodynamic markers of DNA-PK inhibition, inhibition of DNA repair and chemo- and radio-potentiation in isogenic human cancer cells proficient (M059-Fus1) and deficient (M059 J) in DNA-PK.

RESULTS

NU7441 strongly inhibited DNA-PK in cell lines (IC(50) = 0.3 μM) but only weakly inhibited PI3 K (IC(50) = 7 μM). The only available anti-phospho-DNA-PK antibody also recognised some phosphoprotein targets of ATM. NU7441 caused doxorubicin- and IR-induced DNA DSBs (measured by γ-H2AX foci) to persist and also slightly decreased homologous recombination activity, as assessed by Rad51 foci. Chemo- and radio-potentiation were induced by NU7441 in M059-Fus-1, but not in DNA-PK-deficient M059 J cells. DNA-PK was highly expressed in a chronic lymphocytic leukaemia sample but undetectable in resting normal human lymphocytes, although it could be induced by PHA-P treatment. In K652 cells, DNA-PK expression was not related to cell cycle phase.

CONCLUSION

These data confirm NU7441 not only as a potent chemo- and radio-sensitiser clinical candidate but also as a powerful tool to study the biology of DNA-PK.

Bioengineering approaches to study multidrug resistance in tumor cells

The ability of cancer cells to become resistant to chemotherapeutic agents is a major challenge for the treatment of malignant tumors. Several strategies have emerged to attempt to inhibit chemoresistance, but the fact remains that resistance is a problem for every effective anticancer drug. The first part of this review will focus on the mechanisms of chemoresistance. It is important to understand the environmental cues, transport limitations and the cellular signaling pathways associated with chemoresistance before we can hope to effectively combat it. The second part of this review focuses on the work that needs to be done moving forward. Specifically, this section focuses on the necessity of translational research and interdisciplinary directives. It is critical that the expertise of oncologists, biologists, and engineers be brought together to attempt to tackle the problem. This discussion is from an engineering perspective, as the dialogue between engineers and other cancer researchers is the most challenging due to non-overlapping background knowledge. Chemoresistance is a complex and devastating process, meaning that we urgently need sophisticated methods to study the process of how cells become resistant.

Dual inhibition of the homologous recombinational repair and the nonhomologous end-joining repair pathways in chronic lymphocytic leukemia therapy

Resistance to chlorambucil in chronic lymphocytic leukemia (CLL) has been associated with increased DNA repair. Specifically, inhibition of either c-abl, which modulates Rad51 directed homologous recombination or DNA-PK dependent nonhomologous end joining has been shown to sensitize primary CLL lymphocytes to chlorambucil. Here we report that inhibition of c-abl can result in a compensatory increase in DNA-PK and thus inhibition of both c-abl and DNA-PK optimally sensitizes CLL lymphocytes to chlorambucil. In this paper we report a drug-induced compensatory change between two DNA repair pathways with potential therapeutic implications in CLL therapy.

Telomere dysfunction-induced foci arise with the onset of telomeric deletions and complex chromosomal aberrations in resistant chronic lymphocytic leukemia cells

In somatic cells, eroded telomeres can induce DNA double-strand break signaling, leading to a form of replicative senescence or apoptosis, both of which are barriers to tumorigenesis. However, cancer cells might display telomere dysfunctions which in conjunction with defects in DNA repair and apoptosis, enables them to circumvent these pathways. Chronic lymphocytic leukemia (CLL) cells exhibit telomere dysfunction, and a subset of these cells are resistant to DNA damage-induced apoptosis and display short telomeres. We show here that these cells exhibit significant resection of their protective telomeric 3' single-stranded overhangs and an increased number of telomere-induced foci containing gammaH2AX and 53BP1. Chromatin immunoprecipitation and immunofluorescence experiments demonstrated increased levels of telomeric Ku70 and phospho-S2056-DNA-PKcs, 2 essential components of the mammalian nonhomologous end-joining DNA repair system. Notably, these CLL cells display deletions of telomeric signals on one or 2 chromatids in parallel with 11q22 deletions, or with 13q14 deletions associated with another chromosomal aberration or with a complex karyotype. Taken together, our results indicate that a subset of CLL cells from patients with an unfavorable clinical outcome harbor a novel type of chromosomal aberration resulting from telomere dysfunction.

New molecular markers in resistant B-CLL

B-chronic lymphocytic leukemia (B-CLL) is characterized by a highly variable clinical course which has long remained a stumbling block for clinicians. This variability appears to arise from complex molecular alterations identified in malignant cells from patient subsets. Recent studies have focused in particular on identifying new molecular markers to help predict the most effective and adapted treatments. In addition to the mutation status of immunoglobulin variable heavy-chain region (IgVH) genes, which is a well-established predictive factor in B-CLL, these new markers include defects of cell factors involved in the maintenance of genome stability, such as telomere function, DNA repair, ATM and p53. Other predictive factors, such as tyrosine kinase Zap-70 and soluble factors found in patient sera, may be associated with B-cell receptor signal transduction. Interestingly, an alteration of these factors fits closely, though not strikingly, with the absence of somatic mutations in IgVH genes, suggesting that the latter may be due either to epigenetic events leading to an unstable genome or to an inherited defect in the immune response of malignant B-cells. Recent lessons from Zap-70 expression/phosphorylation suggest that some of these markers may reflect the defective pathways in B-CLL cells rather than being markers of cell malignancy per se. Furthermore, specific subsets of markers are found in patient cells resistant to treatment. Current studies on gene expression profiling and proteomic analyses should soon lead to a better understanding of how these pathways are affected, especially in multi-drug resistant B-CLL.

Activities of DNA-PK and Ku86, but not Ku70, may predict sensitivity to cisplatin in human gliomas

OBJECTIVE

This study was designed to investigate the relationship between activities of DNA-dependent protein kinase (DNA-PK), its subunits Ku86/Ku70, and sensitivities to cisplatin in human glioma samples.

METHODS

Thirty-six glioma samples from patients without prior treatment before neurosurgery were included in this study. The sensitivities to cisplatin as indicated by IC(50) (the inhibitory concentration leading to 50% cell death) were assessed by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenytetrazolium (MTT) assay; activities of DNA-PK and Ku70/Ku86 were analyzed by SigmaTECT DNA-Dependent Protein Kinase Assay System and Ku70/Ku86 DNA Repair Kit, respectively.

RESULTS

Sensitivities to cisplatin correlated with the activities of DNA-PK/Ku86, but not with the Ku70 or other clinical parameters such as age, sex of the patients, pathological gradings of the tumors, or tumor size. The levels of DNA-PK activities also associated with pathological grading and Ku86, but not with other clinical parameters. The tumors of the patients who failed to respond to cisplatin-based chemotherapy tended to display higher activity levels of DNA-PK and Ku86. Furthermore, platinum-based chemotherapy did not result in significant changes of DNA-PK/Ku activities in four matched samples before and after chemotherapy.

CONCLUSION

Pretreatment determination of DNA-PK/Ku86 activities might be helpful in identifying patients who will actually benefit from platinum-based treatment.

Endosomal translocation of CpG-oligodeoxynucleotides inhibits DNA-PKcs-dependent IL-10 production in macrophages

Synthetic oligodeoxynucleotides containing unmethylated CpG motifs (CpG-ODNs) function as powerful immune adjuvants by activating macrophages, dendritic cells, and B cells. However, the molecular recognition mechanism that initiates signaling in response to CpG-ODN has not fully been identified. We show in this study that peritoneal macrophages from SCID mice having mutations in the catalytic subunit of DNA-protein kinase (DNA-PKcs) were almost completely defective in the production of IL-10 and in ERK activation when treated with CpG-ODN. In contrast, IL-12 p70 production significantly increased. Furthermore, small interfering RNA (siRNA)-mediated knockdown of DNA-PKcs expression in the mouse monocyte/macrophage cell line RAW264.7 led to reduced IL-10 production and ERK activation by CpG-ODN. IL-10 and IL-12 p70 production, but not ERK activation, are blocked by chloroquine, an inhibitor of endosomal acidification. Endosomal translocation of CpG-ODN in a complex with cationic liposomes consisting of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) (CpG-DOTAP-liposomes) decreased IL-10 production and ERK activation, whereas the endosomal escape of CpG-ODN in a complex with cationic liposomes consisting of DOTAP and dioleyl-phosphatidylethanolamine (DOPE) (CpG-DOTAP/DOPE-liposomes) increased. In contrast, IL-12 p70 production was increased by CpG-DOTAP-liposomes and decreased by CpG-DOTAP/DOPE-liposomes. IL-10 production induced by CpG-DOTAP/DOPE-liposomes was not observed in macrophages from SCID mice. Thus, our findings suggest that DNA-PKcs in the cytoplasm play an important role in CpG-ODN-induced production of IL-10 in macrophages. In addition, DNA-PKcs-mediated production of IL-10 and IL-12 p70 can be regulated by manipulating the intracellular trafficking of CpG-ODN in macrophages.

Chlorambucil cytotoxicity in malignant B lymphocytes is synergistically increased by 2-(morpholin-4-yl)-benzo[h]chomen-4-one (NU7026)-mediated inhibition of DNA double-strand break repair via inhibition of DNA-dependent protein kinase

Chlorambucil (CLB) treatment is used in chronic lymphocytic leukemia (CLL) but resistance to CLB develops in association with accelerated repair of CLB-induced DNA damage. Phosphorylated histone H2AX (gammaH2AX) is located at DNA double-strand break (DSB) sites; furthermore, it recruits and retains damage-responsive proteins. This damage can be repaired by nonhomologous DNA end-joining (NHEJ) and/or homologous recombinational repair (HR) pathways. A key component of NHEJ is the DNA-dependent protein kinase (DNA-PK) complex. Increased DNA-PK activity is associated with resistance to CLB in CLL. We used the specific DNA-PK inhibitor 2-(morpholin-4-yl)-benzo[h]chomen-4-one (NU7026) to sensitize CLL cells to chlorambucil. Our results indicate that in a CLL cell line (I83) and in primary CLL-lymphocytes, chlorambucil plus NU7026 has synergistic cytotoxic activity at nontoxic doses of NU7026. CLB treatment results in G(2)/M phase arrest, and NU7026 increases this CLB-induced G(2)/M arrest. Moreover, a kinetic time course demonstrates that CLB-induced DNA-PK activity was inhibited by NU7026, providing direct evidence of the ability of NU7026 to inhibit DNA-PK function. DSBs, visualized as gammaH2AX, were enhanced 24 to 48 h after CLB and further increased by CLB plus NU7026, suggesting that the synergy of the combination is mediated by NU7026 inhibition of DNA-PK with subsequent inhibition of DSB repair.

Polymorphisms in DNA repair genes and therapeutic outcomes of AML patients from SWOG clinical trials

Repair of damage to DNA resulting from chemotherapy may influence drug toxicity and survival in response to treatment. We evaluated the role of polymorphisms in DNA repair genes APE1, XRCC1, ERCC1, XPD, and XRCC3 in predicting therapeutic outcomes of older adults with acute myeloid leukemia (AML) from 2 Southwest Oncology Group (SWOG) clinical trials. All patients received standard chemotherapy induction regimens. Using logistic and proportional hazards regression models, relationships between genotypes, haplotypes, and toxicities, response to induction therapy, and overall survival were evaluated. Patients with XPD Gln751C/Asp312G (‘D’) haplotype were more likely to have complete response (OR = 3.06; 95% CI, 1.44-6.70) and less likely to have resistant disease (OR = 0.32; 95%CI, 0.14-0.72) than patients with other haplotypes. ERCC1 polymorphisms were significantly associated with lung (P = .037) and metabolic (P = .041) toxicities, and patients with the XRCC3 241Met variant had reduced risk of liver toxicity (OR = 0.32; 95%CI, 0.11-0.95). Significant associations with other toxicities were also found for variant XPD genotypes/haplotypes. These data from clinical trials of older patients treated for AML indicate that variants in DNA repair pathways may have an impact on both outcomes of patients and toxicities associated with treatments. With validation of results in larger samples, these findings could lead to optimizing individual chemotherapy options.

Chronic exposure to sublethal doses of radiation mimetic Zeocin selects for clones deficient in homologous recombination

DNA double-strand breaks (DSBs) are highly toxic lesions leading to genome variability/instability. The balance between homologous recombination (HR) and non-homologous end-joining (NHEJ), two alternative DSB repair systems, is essential to ensure genome maintenance in mammalian cells. Here, we transfected CHO hamster cells with the pcDNA3.1/Zeo plasmid, and selected transfectants with Zeocin, a bleomycin analog which produces DSBs. Despite the presence of a Zeocin resistance gene in pcDNA3.1/Zeo, Zeocin induced 8-10 gamma-H2AX foci per cell. This shows that the Zeocin resistance gene failed to fully detoxify cells treated with Zeocin, and that during selection cells were submitted to a chronic sublethal DSB stress. Selected clones show decreases in both spontaneous and induced intrachromosomal HR. In contrast, in an in vitro assay, these clones show an increase in NHEJ products specific to the KU86 pathway. We selected cells, in the absence of pcDNA3.1/Zeo, with low and sublethal doses of Zeocin, producing a mean 8-10 gamma-H2AX foci per cell. Newly selected clones exhibited similar phenotypes: HR decrease accompanied by an increase in KU86-dependent NHEJ efficiency. Thus chronic exposure to sublethal numbers of DSBs selects cells whose HR versus NHEJ balance is altered. This may well have implications for radio- and chemotherapy, and for management of environmental hazards.

The DNA repair complex DNA-PK, a pharmacological target in cancer chemotherapy and radiotherapy

A line of investigation in the search for sensitizing tumor cells to chemotherapy or radiotherapy relies on the selection of DNA repair inhibitors. In the area of DNA repair mechanisms, DNA-dependent protein kinase (DNA-PK) represents a key complex. Indeed DNA-PK is involved in the non-homologous end joining (NHEJ) process that corresponds to the major activity responsible for cell survival after ionizing radiation or chemotherapeutic treatment producing DNA double strand breaks. DNA-PK belongs to the PI3-K related kinase family and specific inhibitors have been recently selected and evaluated as radio- and chemo-sensitizers. These drugs, along with other ways to inhibit the DSBs repair process, are presented and discussed.

The emerging role of DNA repair proteins as predictive, prognostic and therapeutic targets in cancer

Advanced cancer is the second leading cause of death in the western world. Chemotherapy and radiation are the two main treatment modalities currently available to improve patient outcomes, but treatment related toxicity and the emergence of resistance limit their effectiveness. Hence there is an urgent need to develop novel treatment strategies. Rapid advances in cancer biology have identified key pathways involved in the repair of DNA damage induced by chemotherapeutic agents and irradiation. Efficient DNA repair in the cancer cell is an important mechanism for therapeutic resistance. Up to 130 genes have been identified that are associated with human DNA repair. Several of these proteins are emerging as important predictive and prognostic factors in solid tumours. Inhibition of DNA repair has the potential to enhance the efficacy of currently available DNA damaging agents. In recent years, several promising drug targets have been identified and novel drugs synthesised that target specific DNA repair proteins. These agents have shown impressive anti-cancer effects in preclinical studies in combination with chemotherapy or irradiation. Their role in human cancer is now being investigated in early phase clinical trials in combination with chemotherapy. MGMT inhibitors, PARP inhibitors and methoxyamine are currently in early stages of clinical development. Innovative clinical trial designs are essential to evaluate the potential of DNA repair inhibitor in cancer therapy.

DNA end binding activity and Ku70/80 heterodimer expression in human colorectal tumor

AIM: To determine the DNA binding activity and protein levels of the Ku70/80 heterodimer, the functional mediator of the NHEJ activity, in human colorectal carcinogenesis.

METHODS: The Ku70/80 DNA-binding activity was determined by electrophoretic mobility shift assays in 20 colon adenoma and 15 colorectal cancer samples as well as matched normal colonic tissues. Nuclear and cytoplasmic protein expression was determined by immunohistochemistry and Western blot analysis.

RESULTS: A statistically significant difference was found in both adenomas and carcinomas as compared to matched normal colonic mucosa (P<0.00). However, changes in binding activity were not homogenous with approximately 50% of the tumors showing a clear increase in the binding activity, 30% displaying a modest increase and 15% showing a decrease of the activity. Tumors, with increased DNA-binding activity, also showed a statistically significant increase in Ku70 and Ku86 nuclear expression, as determined by Western blot and immunohistochemical analyses (P<0.001). Cytoplasmic protein expression was found in pathological samples, but not in normal tissues either from tumor patients or from healthy subjects.

CONCLUSION: Overall, our DNA-binding activity and protein level are consistent with a substantial activation of the NHEJ pathway in colorectal tumors. Since the NHEJ is an error prone mechanism, its abnormal activation can result in chromosomal instability and ultimately lead to tumorigenesis.

Expression of DNA repair gene Ku80 in lymphoid neoplasm

OBJECTIVES

Ku, a heterodimer of KU70 and Ku80 that binds to double-strand DNA breaks (DSBs) and activates the catalytic subunit (DNA-PKcs) when DNA is bound, is essential in DSB repair and V(D)J recombination. Ku80 is a putative tumor suppressor gene that might play an important role in drug resistance. Our aim was to determine the role of Ku80 in lymphoid malignancy.

PATIENTS AND METHODS

Competitive reverse transcription-polymerase chain reaction assays were performed and the expression levels of Ku80 were measured in normal peripheral blood mononuclear cells (n = 9) and malignant cells from 25 patients with acute lymphoblastic leukemia (ALL) (14 children, 11 adults), and chronic lymphoproliferative disorders (n = 6). The Ku80 transcripts were sequencing for the possibility of mutation.

RESULTS

No mutation or Ku80 variant at the RNA level was seen in any patient samples or in the Raji or CCRF-CEM cell lines. In Ku80 expression, 8.8-, 1.9-, and 6.2-fold mean increases were seen in adult, pediatric ALL, and chronic lymphoid malignancies compared with the control. The Ku80 was significantly higher in adult than in pediatric ALL (P = 0.02). The amount of Ku80 expression in ALL was moderately correlated with peripheral white blood cell counts, but not with Ki67 labeling index. High Ku80 expressers (higher than the mean of all patients with ALL) tended to respond poorly to therapy: Only 22% of high Ku80 expressers achieved durable complete remission compared to 62% of low expressers.

CONCLUSIONS

Our study suggests that Ku80 might contribute to generally poor prognoses in adult ALL.

DNA-PKcs, but not TLR9, is required for activation of Akt by CpG-DNA

CpG-DNA and its related synthetic CpG oligodeoxynucleotides (CpG-ODNs) play an important role in immune cell survival. It has been suggested that Akt is one of the CpG-DNA-responsive serine/threonine kinases; however, the target protein of CpG-DNA that leads to Akt activation has not been elucidated. Here, we report that ex vivo stimulation of bone marrow-derived macrophages (BMDMs) from mice lacking the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) results in defective phosphorylation and activation of Akt by CpG-DNA. Unexpectedly, loss of the Toll-like receptor 9 has a minimal effect on Akt activation in response to CpG-DNA. Further in vitro analysis using purified DNA-PK and recombinant Akt proteins reveals that DNA-PK directly induces phosphorylation and activation of Akt. In addition, in BMDMs, DNA-PKcs associates with Akt upon CpG-DNA stimulation and triggers transient nuclear translocation of Akt. Thus, our findings establish a novel role for DNA-PKcs in CpG-DNA signaling and define a CpG-DNA/DNA-PKcs/Akt pathway.

Expression of DNA-PKcs and Ku86, but not Ku70, differs between lymphoid malignancies

We have investigated the role of DNA-dependent protein kinase (DNA-PK) and related it to proliferation and maturation of different lymphoid malignancies. DNA-PK and Ki-67 protein content was investigated in tumour samples of lymphoid malignancies, obtained from patients with low- and high-grade lymphomas, acute lymphoblastic leukaemia and multiple myeloma. All patients were untreated before sampling. Normal bone marrow, reactive tonsillar tissue and ordinary lymph node tissue were used as controls. We show here that lymphoid malignancies display differences in DNA-PK protein expression. Low-grade lymphoma, appearing as chronic lymphocytic leukaemia (CLL) displayed a significantly lower frequency of cells staining positive for DNA-PKcs and Ku86, but surprisingly not for Ku70, compared with acute lymphoblastic leukaemia (ALL) cells. When material from individual CLL patients was investigated, cells from lymph nodes showed a higher frequency of positive cells with respect to all DNA-PK subunits, compared with CLL cells infiltrating the bone marrow. High-grade lymphoma lymph node samples showed an increased frequency of cells staining positive for DNA-PKcs, Ku86 and Ki-67 compared with lymph node samples from low-grade lymphoma patients. Again, no difference in the Ku70 levels between the two lymphoma entities was noted. In multiple myeloma, the frequency of cells with positive staining for DNA-PKcs was similar to that detected in ALL and high-grade lymphoma. We conclude that with the exception of multiple myeloma, expression of DNA-PK coincides with the degree of maturation of lymphoid malignancies. In low- and high-grade lymphoma, DNA-PK is associated with the proliferation rate.

A novel DNA-dependent protein kinase inhibitor, NU7026, potentiates the cytotoxicity of topoisomerase II poisons used in the treatment of leukemia

We report for the first time the use of a selective small-molecule inhibitor of DNA repair to potentiate topoisomerase II (topo II) poisons, identifying DNA-dependent protein kinase (DNA-PK) as a potential target for leukemia therapy. Topo II poisons form cleavable complexes that are processed to DNA double-strand breaks (DSBs). DNA-PK mediates nonhomologous end joining (NHEJ). Inhibition of this DSB repair pathway may sensitize cells to topo II poisons. We investigated the effects of a novel DNA-PK inhibitor, NU7026 (2-(morpholin-4-yl)-benzo[h]chomen-4-one), on the response to topo II poisons using K562 leukemia cells. NU7026 (10 microM) potentiated the growth inhibition of idarubicin, daunorubicin, doxorubicin, etoposide, amsacrine (mAMSA), and mitroxantrone with potentiation factors at 50% growth inhibition ranging from approximately 19 for mAMSA to approximately 2 for idarubicin (potentiation of etoposide was confirmed by clonogenic assay). In contrast, NU7026 did not potentiate camptothecin or cytosine arabinoside (araC). NU7026 did not affect the levels of etoposide-induced topo IIalpha or beta cleavable complexes. NU7026 alone had no effect on cell cycle distribution, but etoposide-induced accumulation in G2/M was increased by NU7026. A concentration-dependent increase in etoposide-induced DSB levels was increased by NU7026. The mechanism of NU7026 potentiation of topo II poisons involves inhibition of NHEJ and a G2/M checkpoint arrest.

Up-regulation of DNA-dependent protein kinase correlates with radiation resistance in oral squamous cell carcinoma

DNA-PK is a nuclear protein with serine/threonine kinase activity and forms a complex consisting of the DNA-PKcs and a heterodimer of Ku70 and Ku80 proteins. Recent laboratory experiments have demonstrated that the DNA-PK complex formation is one of the major pathways by which mammalian cells respond to DNA double-strand breaks induced by ionizing radiation. In this study, we evaluated the relationship between expression levels of DNA-PKcs, Ku70 and Ku80 proteins and radiation sensitivity in oral squamous cell carcinoma (OSCC) cell lines and in OSCC patients treated with preoperative radiation therapy. The OSCC cell lines greatly differed in their response to irradiation, as assessed by a standard colony formation assay. However, the expression levels of the DNA-PK complex proteins were all similar, and there was no association between the magnitude of their expression and the tumor radiation sensitivity. Expression of DNA-PK complex proteins increased after radiation treatment, and the increased values correlated with the tumor radiation resistance. Expression of DNA-PKcs and Ku70 after irradiation was increased in the surviving cells of OSCC tissues irradiated preoperatively. These results suggest that up-regulation of DNA-PK complex protein, especially DNA-PKcs, after radiation treatment correlates to radiation resistance. DNA-PKcs might be a molecular target for a novel radiation sensitization therapy of OSCC.

Topical mechlorethamine restores autoimmune-arrested follicular activity in mice with an alopecia areata-like disease by targeting infiltrated lymphocytes

Alopecia areata is an autoimmune disease targeted at hair follicles with infiltrated T lymphocytes probably playing an important role in the pathogenesis. It was reported in 1985 that mechlorethamine was effective on alopecia areata patients. This has never been confirmed since. The aims of the study were to investigate the effects of mechlorethamine on balding C3H/HeJ mice affected with an alopecia-areata-like disease and to study the underlying mechanisms. Mice were treated on half of the dorsal skin with mechlorethamine and the contralateral side was treated with the vehicle ointment. After 10 wk of mechlorethamine therapy, a full pelage of hair covered the treated side in all the mice and was maintained during the study, whereas the vehicle-treated sides showed either no change or continued hair loss. Immunohistochemistry revealed that infiltrated CD4+ and CD8+ lymphocytes were eliminated from the treated side. In vitro cell viability assay showed that lymphocytes were much more sensitive to the cytotoxic effects of mechlorethamine than skin and hair follicular cells. RNase protection assay and real-time reverse transcription polymerase chain reaction showed that tumor necrosis factor alpha/beta, interleukin-12, and interferon-gamma were inhibited by mechlorethamine upon successful treatment. Our findings support that mechlorethamine restores follicular activity by selectively targeting infiltrated lymphocytes in vivo in alopecia-areata-affected mice.

The radioprotective effect of the 24 kDa FGF-2 isoform in HeLa cells is related to an increased expression and activity of the DNA dependent protein kinase (DNA-PK) catalytic subunit

We previously reported that overexpression of the 24 kDa basic fibroblast factor (or FGF-2) isoform provides protection from the cytotoxic effect of ionizing radiation (IR). DNA double-strand breaks (DSB), the IR-induced lethal lesions, are mainly repaired in human cells by non-homologous end joining system (NHEJ). NHEJ reaction is dependent on the DNA-PK holoenzyme (composed of a regulatory sub-unit, Ku, and a catalytic sub-unit, DNA-PKcs) that assembles at sites of DNA damage. We demonstrated here that the activity of DNA-PK was increased by twofold in two independent radioresistant cell lines, HeLa 3A and CAPAN A3, over expressing the 24 kDa FGF-2. This increase was associated with an overexpression of the DNA-PKcs without modification of Ku expression or activity. This overexpression was due to an up-regulation of the DNA-PKcs gene transcription by the 24 kDa FGF-2 isoform. Finally, HeLa 3A cells exhibited the hallmarks of phenotypic changes associated with the overexpression of an active DNA-PKcs. Indeed, a faster repair rate of DSB and sensitization to IR by wortmannin was observed in these cells. Our results represent the characterization of a new mechanism of control of DNA repair and radioresistance in human tumor cells dependent on the overproduction of the 24 kDa FGF-2 isoform.

Repair of DNA interstrand crosslinks as a mechanism of clinical resistance to melphalan in multiple myeloma

Melphalan is widely used as a preparative agent in patients with multiple myeloma (MM) undergoing autologous stem cell transplantation (SCT). Although disease relapse is the major cause of death after a melphalan-conditioned autograft, the mechanism remains unclear. Melphalan produces a number of DNA adducts with the DNA interstrand crosslink (ICL) considered to be the critical cytotoxic lesion. By using a modification of the single-cell gel electrophoresis (Comet) assay, we have measured formation and repair of DNA ICL in plasma cells from melphalan- naive and melphalan-treated patients (ie, those who have relapsed after a melphalan-conditioned autologous SCT or oral melphalan therapy). Similar levels of dose-dependent DNA interstand crosslinking were observed in cells from both melphalan-naive and -treated patients. However, marked differences in ICL repair were observed: cells from naive patients showed no repair, whereas those from treated patients exhibited between 42% and 100% repair at 40 hours. In vitro sensitivity to melphalan in plasma cells was found to correlate with ICL repair. These findings suggest that ICL repair may be an important mechanism by which melphalan resistance emerges after autologous SCT or oral therapy. This mechanism may have implications for MM patients undergoing melphalan therapy.

Homologous recombinational repair vis-à-vis chlorambucil resistance in chronic lymphocytic leukemia

The objective of this study was to further define the role of homologous recombinational repair (HRR) in resistance to the nitrogen mustards in B-cell chronic lymphocytic leukemia (B-CLL). We have demonstrated previously that increased chlorambucil (CLB)-induced HsRad51 nuclear foci formation correlated with a CLB-resistant phenotype in B-CLL lymphocytes. In this report, we measured the protein levels of HsRad51 and Xrcc3 (an HsRad51 paralog) and correlated them with the in vitro CLB cytotoxicity (LD(50)) in lymphocytes from seventeen B-CLL patients. Both HsRad51 (r=0.75, P=0.0005) and Xrcc3 (r=0.52, P=0.03) protein levels correlated with the in vitro CLB LD(50). In addition, multiple linear regression analysis showed a significant correlation between Xrcc3 and Rad51 protein levels versus the CLB LD(50) (r=0.78, P=0.0014), suggesting that both proteins influence CLB cytotoxicity. Moreover, since HsRad51 expression varies in cell lines during the cell cycle, we determined proliferating cell nuclear antigen (PCNA) protein levels to assess possible differences in cell cycle progression. There was no correlation between PCNA protein levels and the CLB LD(50) (r=0.042, P=0.87) or with HsRad51/Xrcc3 protein levels. Our data suggest that HsRad51 and Xrcc3 protein expression may be predictive of the response in B-CLL patients to treatment with nitrogen mustards.

The role of DNA repair in nitrogen mustard drug resistance

The nitrogen mustards are an important class of DNA cross-linking agents, which are utilized in the treatment of many types of cancer. Unfortunately, resistance often develops in the treatment of patients and the tumor either never responds to or becomes refractory to these agents. Resistance to the nitrogen mustards in murine and human tumor cells has been reported to be secondary to alterations in (i) the transport of these agents, (ii) their reactivity, (iii) apoptosis and (iv) altered DNA repair activity. In the present review, we will discuss the role of DNA repair in nitrogen mustard resistance in cancer. The nitrogen mustards' lethality is based on the induction of DNA interstrand cross-links (ICLs). Two DNA repair pathways are known to be involved in removal of ICLs: non-homologous DNA end-joining (NHEJ) and Rad51-related homologous recombinational repair (HRR). The reports discussed here lead us to hypothesize that low NHEJ activity defines a hypersensitive state, while high NHEJ activity, along with increased HRR activity, contributes to the resistant state in chronic lymphocytic leukemia. Studies on human epithelial tumor cell lines suggest that HRR rather than NHEJ plays a role in nitrogen mustard sensitivity.

2-Chloro-2'-deoxyadenosine inhibits DNA repair synthesis and potentiates UVC cytotoxicity in chronic lymphocytic leukemia B lymphocytes

2-Chloro-2'-deoxyadenosine (CdA) is a deoxyadenosine analogue which targets enzymes involved in DNA synthesis, and hence might interfere with the resynthesis step of DNA repair. We tested this hypothesis in resting B cell chronic lymphocytic leukemia (B-CLL) lymphocytes, after firstly characterizing unscheduled DNA synthesis occurring in these cells. We observed that the spontaneous incorporation of [methyl-3H]thymidine (dThd) into DNA of B-CLL cells was not completely inhibitable by hydroxyurea (HU) which blocks DNA replication. In addition, in the presence of HU, dThd incorporation could be upregulated by UVC radiation or DNA alkylation, without re-entry of the cells into S phase. CdA was found to inhibit both spontaneous and upregulated DNA synthesis in B-CLL cells. Phosphorylation of CdA was essential to exert this effect. We finally observed a strong synergistic cytotoxicity between UV light and CdA, which was correlated with activation of caspase-3 and high molecular weight DNA fragmentation, two markers of apoptosis. Taken together, these observations indicate that in B-CLL cells CdA inhibits unscheduled DNA synthesis which represents the polymerizing step of a repair process responsive to DNA aggression. Inhibition of this process by CdA, together with a combined activation of the apoptotic proteolytic cascade by CdA and UV, may explain their synergistic cytotoxicity.

Expression and DNA binding activity of the Ku heterodimer in bladder carcinoma

BACKGROUND

The Ku protein is a tightly associated heterodimer, comprised of 70-kilodalton (kD) and 86-kD subunits, that forms the DNA-dependent protein kinase (DNA-PK) complex together with the 470-kD DNA-PKcs catalytic subunit, and is involved mainly in DNA double-strand breaks (DSBs) repair. The objective of the current study was to investigate the expression and DNA-binding activity of the Ku protein in fresh tissues from patients with bladder carcinoma and to compare it with that in nontumor tissues obtained from the same organ. Moreover, the DNA-binding activity of Ku was assessed after exposure of the tumor cells to 1 or 2 grays (Gy) of X-rays. Furthermore, the level of phosphorylated Ku was analyzed in both the nuclear and cytoplasmic compartment of normal tissue after exposure to 2 Gy of X-rays.

METHODS

The expression and DNA-binding activity of Ku protein were assessed in tumor samples from patients who all were diagnosed with transitional cell carcinoma (TCC) of the bladder using Western blot analysis and the electrophoretic mobility shift assay, respectively.

RESULTS

Enhanced Ku activity and expression were found in tumor tissue compared with normal tissue for each patient. Moreover, variations in Ku activity were found in a dose-dependent manner after the tumor cells were exposed to 1 or 2 Gy of X-rays. A decrease in phosphorylated Ku in the cytoplasm and a parallel increase in the nucleus of normal tissue cells were observed after exposure to X-rays.

CONCLUSIONS

The results of the current study suggest a possible role of Ku in regulating the DNA-PK activity of DSBs repair in bladder tumors.

In vitro evidence for homologous recombinational repair in resistance to melphalan

BACKGROUND

The generation of DNA interstrand cross-links is thought to be important in the cytotoxicity of nitrogen mustard alkylating agents, such as melphalan, which have antitumor activity. Cell lines with mutations in recombinational repair pathways are hypersensitive to nitrogen mustards. Thus, resistance to melphalan may require accelerated DNA repair by either recombinational repair mechanisms involving Rad51-related proteins (including x-ray repair cross-complementing proteins Xrcc2, Xrcc3, and Rad52) or by nonhomologous endjoining involving DNA-dependent protein kinase (DNA-PK) and Ku proteins. We investigated the role of DNA repair in melphalan resistance in epithelial tumor cell lines.

METHODS

Melphalan cytotoxicity was determined in 14 epithelial tumor cell lines by use of the sulforhodamine assay. Homologous recombinational repair involving Rad51-related proteins was investigated by determining the levels of Rad51, Rad52, and Xrcc3 proteins and the density of nuclear melphalan-induced Rad51 foci, which represent sites of homologous recombinational repair. Nonhomologous endjoining was investigated by determining the levels of Ku70 and Ku86 proteins and DNA-PK activity. Linear regression analysis was used to analyze correlations between the various protein levels, DNA-PK activity, or Rad51 foci formation and melphalan cytotoxicity. All statistical tests were two-sided.

RESULTS

Melphalan resistance was correlated with Xrcc3 levels (r =.587; P =.027) and the density of melphalan-induced Rad51 foci (r =.848; P =.008). We found no correlation between melphalan resistance and Rad51, Rad52, or Ku protein levels or DNA-PK activity.

CONCLUSION

Correlations of melphalan resistance in epithelial tumor cell lines with Xrcc3 protein levels and melphalan-induced Rad51 foci density suggest that homologous recombinational repair is involved in resistance to this nitrogen mustard.

Ku86 variant expression and function in multiple myeloma cells is associated with increased sensitivity to DNA damage

Ku is a heterodimer of Ku70 and Ku86 that binds to double-stranded DNA breaks (DSBs), activates the catalytic subunit (DNA-PKcs) when DNA is bound, and is essential in DSB repair and V(D)J recombination. Given that abnormalities in Ig gene rearrangement and DNA damage repair are hallmarks of multiple myeloma (MM) cells, we have characterized Ku expression and function in human MM cells. Tumor cells (CD38(+)CD45RA(-)) from 12 of 14 (86%) patients preferentially express a 69-kDa variant of Ku86 (Ku86v). Immunoblotting of whole cell extracts (WCE) from MM patients shows reactivity with Abs targeting Ku86 N terminus (S10B1) but no reactivity with Abs targeting Ku86 C terminus (111), suggesting that Ku86v has a truncated C terminus. EMSA confirmed a truncated C terminus in Ku86v and further demonstrated that Ku86v in MM cells had decreased Ku-DNA end binding activity. Ku86 forms complexes with DNA-PKcs and activates kinase activity, but Ku86v neither binds DNA-PKcs nor activates kinase activity. Furthermore, MM cells with Ku86v have increased sensitivity to irradiation, mitomycin C, and bleomycin compared with patient MM cells or normal bone marrow donor cells with Ku86. Therefore, this study suggests that Ku86v in MM cells may account for decreased DNA repair and increased sensitivity to radiation and chemotherapeutic agents, whereas Ku86 in MM cells confers resistance to DNA damaging agents. Coupled with a recent report that Ku86 activity correlates with resistance to radiation and chemotherapy, these results have implications for the potential role of Ku86 as a novel therapeutic target.

Potentiation of chemosensitivity in multidrug-resistant human leukemia CEM cells by inhibition of DNA-dependent protein kinase using wortmannin

DNA-dependent protein kinase (DNA-PK) is activated by DNA strand breaks and participates in DNA repair. Its regulatory subunit, Ku autoantigen, binds to DNA and recruits the catalytic subunit (DNA-PKcs). We show here a new role of DNA-PK in the development of multidrug resistance (MDR). The Ku-DNA binding activity, the levels of Ku70/Ku80 and DNA-PKcs in MDR variants, CEM/VLB(10-2), CEM/VLB(55-8) and CEM/VLB100 were higher than those in their parental drug-sensitive CEM cells in a drug resistance-dependent fashion. Also, CEM/VLB100 cells showed about 3-fold increase of DNA-PK enzyme activity as compared with CEM cells. Similar results were observed in another MDR cell line, FM3A/M mouse mammary carcinoma cells. Moreover, we observed that CEM/VLB100 cells were about 11-fold sensitive to wortmannin, which inhibits DNA-PK, compared with the CEM cells, and sensitized the MDR cells when combined with either bleomycin or vincristine, but have a little effect on CEM cells. Wortmannin was shown to inhibit DNA-PK and Ku-DNA binding activity in CEM/VLB100 cells dose dependently but had a little or no effect on their parental cells. Our results suggested that enhanced expression of DNA-PK participates in the development of MDR, and the use of DNA-PK inhibitors such as wortmannin is likely to improve the effectiveness of anticancer drugs and thus could partially overcome drug resistance in MDR cells, through its ability to inhibit Ku/DNA-PK activity.

Nitric oxide upregulates expression of DNA-PKcs to protect cells from DNA-damaging anti-tumour agents

Nitric-oxide synthase (NOS) activity has been detected in many human tumours, although its function is unclear. Here we show that exposure of cells to nitric oxide (NO) results in a 4-5-fold increase in expression of the DNA-dependent protein-kinase catalytic subunit (DNA-PKcs), one of the key enzymes involved in repairing double-stranded DNA breaks. This NO-mediated increase in enzymatically active DNA-PK not only protects cells from the toxic effects of NO, but also provides crossprotection against clinically important DNA-damaging agents, such as X-ray radiation, adriamycin, bleomycin and cisplatin. The NO-mediated increase in DNA-PKcs described here demonstrates the presence of a new and highly effective NO-mediated mechanism for DNA repair.

Repair of intermediate structures produced at DNA interstrand cross-links in Saccharomyces cerevisiae

Bifunctional alkylating agents and other drugs which produce DNA interstrand cross-links (ICLs) are among the most effective antitumor agents in clinical use. In contrast to agents which produce bulky adducts on only one strand of the DNA, the cellular mechanisms which act to eliminate DNA ICLs are still poorly understood, although nucleotide excision repair is known to play a crucial role in an early repair step. Using haploid Saccharomyces cerevisiae strains disrupted for genes central to the recombination, nonhomologous end-joining (NHEJ), and mutagenesis pathways, all these activities were found to be involved in the repair of nitrogen mustard (mechlorethamine)- and cisplatin-induced DNA ICLs, but the particular pathway employed is cell cycle dependent. Examination of whole chromosomes from treated cells using contour-clamped homogenous electric field electrophoresis revealed the intermediate in the repair of ICLs in dividing cells, which are mostly in S phase, to be double-strand breaks (DSBs). The origin of these breaks is not clear since they were still efficiently induced in nucleotide excision and base excision repair-deficient, mismatch repair-defective, rad27 and mre11 disruptant strains. In replicating cells, RAD52-dependent recombination and NHEJ both act to repair the DSBs. In contrast, few DSBs were observed in quiescent cells, and recombination therefore seems dispensable for repair. The activity of the Rev3 protein (DNA polymerase zeta) is apparently more important for the processing of intermediates in stationary-phase cells, since rev3 disruptants were more sensitive in this phase than in the exponential growth phase.

The activity of the DNA-dependent protein kinase (DNA-PK) complex is determinant in the cellular response to nitrogen mustards

The DNA-dependent protein kinase plays a critical role in mammalian DNA double strand break (DSB) repair and in specialized recombination, such as lymphoid V(D)J recombination. Its regulatory subunit Ku (dimer of the Ku70 and Ku80 protein) binds to DNA and recruits the kinase catalytic sub-unit, DNA-PKcs. We show here that three different strains deficient in either the Ku80 (xrs-6) or DNA-PKcs (V-3, scid) component of DNA-PK are markedly sensitive (3.5- to 5-fold) to a group of DNA cross-linking agents, the nitrogen mustards (NMs) (melphalan and mechlorethamine) as compared to their parental cell line. Importantly, the level of hypersensitivity to these drugs was close to the level of hypersensitivity observed for radiomimetic agents that create DSBs in DNA (bleomycin and neocarzinostatin). In addition, sensitivity to NMs was restored to the parental level in the xrs-6 cell line stably transfected with the human Ku80 gene (xrs-6/Ku80), showing unequivocally that DNA-PK is involved in this phenotype. These results indicate that a function of the whole DNA-PK protein complex is involved in the cellular response to NMs and suggest that the repair of DNA interstrand cross-links induced in DNA by NMs involved a DNA-PK dependent pathway that shares common features with DNA DSBs repair.