DNA Damage Repair Proteins Research Articles

DNA Conformational Distribution and Dynamics during Search and Recognition of Damaged Sites by DNA Repair Protein RAD4/XPC

DNA damage repair proteins need to scan and identify DNA lesions in genomic DNA. In mammalian nucleotide excision repair (NER), lesion recognition is carried out by the Xeroderma Pigmentosum C protein complex (XPC). Rad4, a yeast XPC ortholog, recognizes DNA lesions by inserting a β-hairpin and flipping out damaged nucleotides away from the protein. Rad4, covalently tethered to undamaged DNA, flips out even normal nucleotides to form the same ‘recognition’ structure. Thus, lesion discrimination must depend on differences in the kinetics of recognition rather than structural differences in the stable states of the bound complexes. In this ‘kinetic gating’ mechanism, the probability that a freely diffusing Rad4 flips out nucleotides competes kinetically with Rad4 diffusing away from that site. Using laser temperature-jump spectroscopy, we have measured Rad4-induced nucleotide-flipping rates from within a 3-bp mismatch (model) lesion, for both freely diffusing and tethered Rad4, using 2-aminopurine (2AP) as a probe. Our results reveal 2-5 fold slower relaxation rates for 2AP-monitored nucleotide-flipping dynamics when Rad4 is tethered at the mismatch site compared to when it is freely diffusing, consistent with the notion of competing pathways if the protein can diffuse away from a lesion site. Additionally, we have combined the unique sensitivity of picoseconds-resolved fluorescence lifetime measurements with novel tricyclic cytosine analogs incorporated in DNA (FRET pair: tCo-tCnitro) as probes of DNA twisting/unwinding fluctuations. While the mismatched and matched DNA exhibited similar average FRET, suggesting similar average DNA conformations, the lifetime data reveal multiple distinct conformations for mismatched DNA in contrast to the single dominant conformation seen for matched DNA. These results indicate enhanced unwinding fluctuations in the vicinity of a mismatch “lesion”, perhaps signaling “distress” to a protein searching for damaged sites.

Cells and Stripes: A novel quantitative photo-manipulation technique.

Laser micro-irradiation is a technology widely used in the DNA damage response, checkpoint signaling, chromatin remodeling and related research fields, to assess chromatin modifications and recruitment of diverse DNA damage sensors, mediators and repair proteins to sites of DNA lesions. While this approach has aided numerous discoveries related to cell biology, maintenance of genome integrity, aging and cancer, it has so far been limited by a tedious manual definition of laser-irradiated subcellular regions, with the ensuing restriction to only a small number of cells treated and analyzed in a single experiment. Here, we present an improved and versatile alternative to the micro-irradiation approach: Quantitative analysis of photo-manipulated samples using innovative settings of standard laser-scanning microscopes. Up to 200 cells are simultaneously exposed to a laser beam in a defined pattern of collinear rays. The induced striation pattern is then automatically evaluated by a simple algorithm, which provides a quantitative assessment of various laser-induced phenotypes in live or fixed cells. Overall, this new approach represents a more robust alternative to existing techniques, and provides a versatile tool for a wide range of applications in biomedicine.

Impact of Microsatellite Instability in Signet-Ring Cell and Mucinous Components in Patients With Colorectal Carcinoma.

Impact of Microsatellite Instability in Signet-Ring Cell and Mucinous Components in Patients With Colorectal Carcinoma.

Model systems to unravel the molecular mechanisms of heavy metal tolerance in the ericoid mycorrhizal symbiosis.

Ericoid mycorrhizal plants dominate in harsh environments where nutrient-poor, acidic soil conditions result in a higher availability of potentially toxic metals. Although metal-tolerant plant species and ecotypes are known in the Ericaceae, metal tolerance in these plants has been mainly attributed to their association with ericoid mycorrhizal fungi. The mechanisms underlying plant protection by the fungal symbiont are poorly understood, whereas some insights have been achieved regarding the molecular mechanisms of heavy metal tolerance in the fungal symbiont. This review will briefly introduce the general features of heavy metal tolerance in mycorrhizal fungi and will then focus on the use of "omics" approaches and heterologous expression in model organisms to reveal the molecular bases of fungal response to heavy metals. Functional complementation in Saccharomyces cerevisiae has allowed the identification of several ericoid mycorrhizal fungi genes (i.e., antioxidant enzymes, metal transporters, and DNA damage repair proteins) that may contribute to metal tolerance in a metal-tolerant ericoid Oidiodendron maius isolate. Although a powerful system, the use of the yeast complementation assay to study metal tolerance in mycorrhizal symbioses has limitations. Thus, O. maius has been developed as a model system to study heavy metal tolerance mechanisms in mycorrhizal fungi, thanks to its high metal tolerance, easy handling and in vitro mycorrhization, stable genetic transformation, genomics, transcriptomic and proteomic resources.

Expression Levels of DNA Damage Repair Proteins Are Associated With Overall Survival in Platinum-Treated Advanced Urothelial Carcinoma

BackgroundCombination platinum chemotherapy is standard first-line therapy for metastatic urothelial carcinoma (mUC). Defining the platinum response biomarkers for patients with mUC could establish personalize medicine and provide insights into mUC biology. Although DNA repair mechanisms have been hypothesized to mediate the platinum response, we sought to analyze whether increased expression of DNA damage genes would correlate with worse overall survival (OS) in patients with mUC. Patients and MethodsWe retrospectively identified a clinically annotated cohort of patients with mUC, who had been treated with first-line platinum combination chemotherapy. A tissue microarray was constructed from formalin-fixed paraffin-embedded tissue from the primary tumor before treatment. Immunohistochemical analysis of the following DNA repair proteins was performed: ERCC1, RAD51, BRCA1/2, PAR, and PARP-1. Nuclear and cytoplasmic expression was analyzed using multispectral imaging. Nuclear staining was used for the survival analysis. Cox regression analysis was used to evaluate the associations between the percentage of positive nuclear staining and OS in multivariable analysis, controlling for known prognostic variables. ResultsIn a cohort of 104 patients with mUC, a greater percentage of nuclear staining of ERCC1 (hazard ratio [HR], 2.7; 95% confidence interval [CI], 1.5-4.9; P = .0007), RAD51 (HR, 5.6; 95% CI, 1.7-18.3; P = .005), and PAR (HR, 2.2; 95% CI, 1.1-4.4; P = .026) was associated with worse OS. BRCA1, BRCA2, and PARP-1 expression was not associated with OS (P = .76, P = .38, and P = .09, respectively). A greater percentage of combined ERCC1 and RAD51 nuclear staining was strongly associated with worse OS (P = .005). ConclusionA high percentage of nuclear staining of ERCC1, RAD51, and PAR, assessed by immunohistochemistry, correlated with worse OS for patients with mUC treated with first-line platinum combination chemotherapy, supporting the evidence of the DNA repair pathways' role in the prognosis of mUC. We also report new evidence that RAD51 and PAR might play a role in the platinum response. Additional prospective studies are required to determine the prognostic or predictive nature of these biomarkers in mUC.

The DNA damage repair protein Ku70 regulates tumor cell and hepatic carcinogenesis by interacting with FOXO4

The capability for DNA double-strand breaks (DSBs) repair is crucial for chromatin dramatic changes and DNA damage in normal and tumor cells. We have investigated the clinicopathological significance of DNA repair gene Ku70 expression in hepatocellular carcinoma. We demonstrated that Ku70 expression was significantly increased in HCC, and the high expression levels were significantly correlated with gender, maximal tumor size, HBsAg status, tumor nodule number, distant metastasis and Ki-67 expression by clinicopathological analysis. The Kaplan–Meier survival curves revealed that increasing Ku70 expression was associated with poor prognosis in HCC patients. Ku70 expression was an independent prognostic marker of overall HCC patient survival in a multivariate analysis. In addition, through serum starvation and refeeding, we found that Ku70 was lowly expressed in serum-starved Huh7 and HepG2 HCC cells, and was progressively increased after serum-additioning. Furthermore, knockdown of Ku70 inhibited cell proliferation accompanying an increase in p27kip1 levels through interacting with FOXO4. These findings provide a rational framework for the progression of HCC and could be a potential molecular therapy by targeting the Ku70–FOXO4 interaction.

Lin28A enhances chemosensitivity of colon cancer cells to 5-FU by promoting apoptosis in a let-7 independent manner.

RNA-binding protein Lin28A is frequently over-expressed in human malignant tumors and is associated with tumor advance and poor prognosis. However, the expression pattern and functions of Lin28A in colon cancer are unknown. In this study, we detected the expression of Lin28A in colon cancer patients and tested the effect of Lin28A on the chemotherapeutic sensitivity of colon cancer cells to 5-fluorouracil (5-FU). As expected, we showed that Lin28A is up-regulated in 73.3% of colon cancer patients. However, to our surprise, we found that oncogenic protein Lin28A-enforced expression in colon cancer cells enhanced the chemosensitivity of cancer cells to 5-FU via promoting the cell apoptosis. Further mechanisms study revealed that the effect of Lin28A increasing chemosensitivity of cancer cells is in a let-7 independent manner, but which is associated with decreasing the expression of DNA damage repair protein H2AX. Conclusively, here we reported an unexpected function of Lin28A, which may shed lights on fully understanding the physiological and pathological roles of this oncogene.

Increased sensitivity to platinum drugs of cancer cells with acquired resistance to trabectedin.

Background:In order to investigate the mechanisms of acquired resistance to trabectedin, trabectedin-resistant human myxoid liposarcoma (402-91/T) and ovarian carcinoma (A2780/T) cell lines were derived and characterised in vitro and in vivo.Methods:Resistant cell lines were obtained by repeated exposures to trabectedin. Characterisation was performed by evaluating drug sensitivity, cell cycle perturbations, DNA damage and DNA repair protein expression. In vivo experiments were performed on A2780 and A2780/T xenografts.Results:402-91/T and A2780/T cells were six-fold resistant to trabectedin compared with parental cells. Resistant cells were found to be hypersensitive to UV light and did not express specific proteins involved in the nucleotide excision repair (NER) pathway: XPF and ERCC1 in 402-91/T and XPG in A2780/T. NER deficiency in trabectedin-resistant cells was associated with the absence of a G2/M arrest induced by trabectedin and with enhanced sensitivity (two-fold) to platinum drugs. In A2780/T, this collateral sensitivity, confirmed in vivo, was associated with an increased formation of DNA interstrand crosslinks.Conclusions:Our finding that resistance to trabectedin is associated with the loss of NER function, with a consequent increased sensitivity to platinum drugs, provides the rational for sequential use of these drugs in patients who have acquired resistance to trabectedin.

The impact of CDK9 on radiosensitivity, DNA damage repair and cell cycling of HNSCC cancer cells.

Cyclin-dependent kinase 9 (CDK9), mainly involved in regulation of transcription, has recently been shown to impact on cell cycling and DNA repair. Despite the fact that CDK9 has been proposed as potential cancer target, it remains largely elusive whether CDK9 targeting alters tumor cell radiosensitivity. Five human head and neck squamous cell carcinoma (HNSCC) cell lines (SAS, FaDu, HSC4, Cal33, UTSCC5) as well as SAS cells stably transfected with CDK9-EGFP-N1 plasmid or empty vector controls were used. Upon either CDK9 small interfering RNA knockdown or treatment with a pan-CDK inhibitor (ZK304709), colony formation, DNA double strand breaks (DSBs), apoptosis, cell cycling, and expression and phosphorylation of major cell cycle and DNA damage repair proteins were examined. While CDK9 overexpression mediated radioprotection, CDK9 depletion clearly enhanced the radiosensitivity of HNSCC cells without an induction of apoptosis. While the cell cycle and cell cycle proteins were significantly modulated by CDK9 depletion, no further alterations in these parameters were observed after combined CDK9 knockdown with irradiation. ZK304709 showed concentration-dependent cytotoxicity but failed to radiosensitize HNSCC cells. Our findings suggest a potential role of CDK9 in the radiation response of HNSCC cells. Additional studies are warranted to clarify the usefulness to target CDK9 in the clinic.

Senataxin controls meiotic silencing through ATR activation and chromatin remodeling.

Senataxin, defective in ataxia oculomotor apraxia type 2, protects the genome by facilitating the resolution of RNA–DNA hybrids (R-loops) and other aspects of RNA processing. Disruption of this gene in mice causes failure of meiotic recombination and defective meiotic sex chromosome inactivation, leading to male infertility. Here we provide evidence that the disruption of Setx leads to reduced SUMOylation and disruption of protein localization across the XY body during meiosis. We demonstrate that senataxin and other DNA damage repair proteins, including ataxia telangiectasia and Rad3-related protein-interacting partner, are SUMOylated, and a marked downregulation of both ataxia telangiectasia and Rad3-related protein-interacting partner and TopBP1 leading to defective activation and signaling through ataxia telangiectasia and Rad3-related protein occurs in the absence of senataxin. Furthermore, chromodomain helicase DNA-binding protein 4, a component of the nucleosome remodeling and deacetylase chromatin remodeler that interacts with both ataxia telangiectasia and Rad3-related protein and senataxin was not recruited efficiently to the XY body, triggering altered histone acetylation and chromatin conformation in Setx−/− pachytene-staged spermatocytes. These results demonstrate that senataxin has a critical role in ataxia telangiectasia and Rad3-related protein- and chromodomain helicase DNA-binding protein 4-mediated transcriptional silencing and chromatin remodeling during meiosis providing greater insight into its critical role in gene regulation to protect against neurodegeneration.

Downregulation of BCRP and anti-apoptotic proteins by proadifen (SKF-525A) is responsible for the enhanced mitoxantrone accumulation and toxicity in mitoxantrone-resistant human promyelocytic leukemia cells.

Multidrug resistance caused by the overexpression of ABC transporter proteins in cancer cells remains a major obstacle limiting chemotherapy efficacy. Drugs inhibiting these transporters have been shown to increase the anti-proliferative properties of chemotherapeutics. As we previously described, proadifen, a P450 monooxygenase inhibitor, might also be able to inhibit some ABC transporters, including breast cancer resistance protein (BCRP). Because mitoxantrone (MTX) is a strong BCRP substrate and is often used in the treatment of leukemia, we investigated the effect of 24 h proadifen pre-treatment on the cytotoxicity of MTX in leukemic cell lines that are sensitive to MTX (HL-60) and MTX-resistant ABCG2-overexpressing subclone (cBCRP). We show for the first time that proadifen is able to enhance the cytotoxic properties of MTX in cBCRP cells, particularly through the inhibition of BCRP expression and activity. This proadifen-MTX synergism was also mediated by the inhibition of various cellular proteins engaged in apoptosis, including Mc-1, Bcl-xL, survivin and activation of procaspase-3. Proadifen also decreased the expression of γH2AX, which is involved in the recruitment of reparation proteins. Moreover, the inhibition of DNA damage repair proteins Ku86 and B23 after proadifen treatment indicate a possible role of proadifen in DNA repair blockage, thus suppressing the reparation rate of MTX-induced DSBs.

Adenovirus-mediated FIR demonstrated TP53-independent cell-killing effect and enhanced antitumor activity of carbon-ion beams.

Combination therapy of carbon-ion beam with the far upstream element-binding protein (FBP)-interacting repressor, FIR, which interferes with DNA damage repair proteins, was proposed as an approach for esophageal cancer treatment with low side effects regardless of TP53 status. In vivo therapeutic antitumor efficacy of replication-defective adenovirus (E1 and E3 deleted adenovirus serotype 5) encoding human FIR cDNA (Ad-FIR) was demonstrated in the tumor xenograft model of human esophageal squamous cancer cells, TE-2. Bleomycin (BLM) is an anticancer agent that introduces DNA breaks. The authors reported that Ad-FIR involved in the BLM-induced DNA damage repair response and thus applicable for other DNA damaging agents. To examine the effect of Ad-FIR on DNA damage repair, BLM, X-ray and carbon-ion irradiation were used as DNA damaging agents. The biological effects of high linear energy transfer (LET) radiotherapy used with carbon-ion irradiation are more expansive than low-LET conventional radiotherapy, such as X-rays or γ rays. High LET radiotherapy is suitable for the local control of tumors because of its high relative biological effectiveness. Ad-FIR enhanced BLM-induced DNA damage indicated by γH2AX in vitro. BLM treatment increased endogenous nuclear FIR expression in TE-2 cells, and P27Kip1 expression was suppressed by TP53 siRNA and BLM treatment. Further, Ad-FIRΔexon2, a dominant-negative form of FIR that lacks exon2 transcriptional repression domain, decreased Ku86 expression. The combination of Ad-FIR and BLM in TP53 siRNA increased DNA damage. Additionally, Ad-FIR showed synergistic cell toxicity with X-ray in vitro and significantly increased the antitumor efficacy of carbon-ion irradiation in the xenograft mouse model of TE-2 cells (P=0.03, Mann-Whitney's U-test) and was synergistic with the sensitization enhancement ratio (SER) value of 1.15. Therefore, Ad-FIR increased the cell-killing activity of the carbon-ion beam that avoids late-phase severe adverse effects independently of the TP53 status in vitro. Our findings indicated the feasibility of the combination of Ad-FIR with DNA damaging agents for future esophageal cancer treatment.

Abstract 4490: Selinexor (KPT-330) radio-sensitizes non-small cell lung cancer cells in vitro and in vivo

Abstract The primary nuclear export protein, Exportin 1 (XPO1/CRM1), is overexpressed in most cancers and this overexpression is frequently correlated with poor prognosis. Selective Inhibitors of Nuclear Export (SINE) compounds are a family of small-molecule bioavailable drugs that bind covalently to XPO1 to inhibit nuclear export. This results in nuclear retention of major tumor suppressor proteins, such as p53, pRB, FOXO3A and other critical proteins in cancer biology, which leads to selective cancer cell death. Selinexor is the most advanced SINE with >500 hematological and solid tumor cancer patients treated to date in a number of Phase I/II clinical trials. Blocking XPO1 function also blocks nuclear transport of key DNA damage repair (DDR) proteins and we therefore hypothesized that combination of selinexor with radiation therapy (RT) would lead to synergistic anti-tumor activity. Radiation-chemotherapy combination is an established therapeutic option for patients with medically inoperable non-small cell lung cancer (NSCLC), but its use is limited due to tolerability and adverse effects. Moreover, this treatment strategy typically has short-term benefits and the patients relapse with progressive disease. Here we report on studies designed to test whether selinexor could interfere with DDR and synergize with radiation to enhance anti-tumor potency relative to either treatment alone. Cell cycle analysis and clonogenic assays were conducted in the presence or absence of selinexor in combination with escalating doses of RT on the radiation-sensitive and radiation-insensitive H1299 and A549 cell lines, respectively. Interestingly, Selinexor-treatment induced G1 cell cycle arrest in the p53-deficient cell line H1299 and a prominent G2 cell cycle arrest in p53-wild type A549 cells 24 hrs post treatment. Furthermore, clonogenic assays revealed that selinexor + RT treatment act synergistically to produce a dose-dependent growth inhibition in both cell lines. In vivo combination treatment in the A549 NSCLC xenograft model with low doses of selinexor and RT displayed a strong synergistic effect for reduction of tumor volume. In addition, microscopic and immunohistochemical analysis of the resected tumors showed an overall reduction in tumor cell numbers, increased fibrosis and induction of apoptosis in the selinexor-RT treated tumors compared to controls. Finally, mechanistic studies revealed that selinexor does not induce DNA damage like RT, but downregulates CHK1, RAD51 and other DDR proteins. Together, our results suggest that selinexor treatment sensitizes cells to RT by preventing single-stranded DNA break repair via downregulation of DDR protein expression. In contrast, selinexor induces cell cycle arrest at G1 in p53 deficient H1299 cells, allowing DNA damage accumulation and induction of apoptosis. These results provide a rationale basis for combining selinexor with RT in clinical trials studies. Citation Format: Tami Rashal, Sivan Elloul, Marsha Crochiere, Trinayan Kashyap, William Senapedis, Ryan George, Sharon Friedlander, Maya Ilouze, Yosef Landesman, Robert Carlson, Nir Peled, Michael Kauffman, Sharon Shacham, Yaacov Lawrence. Selinexor (KPT-330) radio-sensitizes non-small cell lung cancer cells in vitro and in vivo. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4490. doi:10.1158/1538-7445.AM2015-4490

KMT Set7/9 affects genotoxic stress response via the Mdm2 axis.

Genotoxic stress inflicted by anti-cancer drugs causes DNA breaks and genome instability. DNA double strand breaks induced by irradiation or pharmacological inhibition of Topoisomerase II activate ATM (ataxia-telangiectasia-mutated) kinase signalling pathway that in turn triggers cell cycle arrest and DNA repair. ATM-dependent gamma-phosphorylation of histone H2Ax and other histone modifications, including ubiquitnylation, promote exchange of histones and recruitment of DNA damage response (DDR) and repair proteins. Signal transduction pathways, besides DDR itself, also control expression of genes whose products cause cell cycle arrest and/or apoptosis thus ultimately affecting the sensitivity of cells to genotoxic stress. In this study, using a number of experimental approaches we provide evidence that lysine-specific methyltransferase (KMT) Set7/9 affects DDR and DNA repair, at least in part, by regulating the expression of an E3 ubiquitin ligase, Mdm2. Furthermore, we show that Set7/9 physically interacts with Mdm2. Several cancer cell lines with inverse expression of Set7/9 and Mdm2 displayed diminished survival in response to genotoxic stress. These findings are signified by our bioinformatics studies suggesting that the unleashed expression of Mdm2 in cancer patients with diminished expression of Set7/9 is associated with poor survival outcome.

Nuclear ULK1 promotes cell death in response to oxidative stress through PARP1.

Reactive oxygen species (ROS) may cause cellular damage and oxidative stress-induced cell death. Autophagy, an evolutionarily conserved intracellular catabolic process, is executed by autophagy (ATG) proteins, including the autophagy initiation kinase Unc-51-like kinase (ULK1)/ATG1. Although autophagy has been implicated to have both cytoprotective and cytotoxic roles in the response to ROS, the role of individual ATG proteins, including ULK1, remains poorly characterized. In this study, we demonstrate that ULK1 sensitizes cells to necrotic cell death induced by hydrogen peroxide (H2O2). Moreover, we demonstrate that ULK1 localizes to the nucleus and regulates the activity of the DNA damage repair protein poly (ADP-ribose) polymerase 1 (PARP1) in a kinase-dependent manner. By enhancing PARP1 activity, ULK1 contributes to ATP depletion and death of H2O2-treated cells. Our study provides the first evidence of an autophagy-independent prodeath role for nuclear ULK1 in response to ROS-induced damage. On the basis of our data, we propose that the subcellular distribution of ULK1 has an important role in deciding whether a cell lives or dies on exposure to adverse environmental or intracellular conditions.

Functional Characterization of Long Noncoding RNA Lnc_bc060912 in Human Lung Carcinoma Cells.

Long noncoding RNAs (lncRNAs) are pervasively transcribed in the human genome. Recent studies suggest that the involvement of lncRNAs in human diseases could be far more prevalent than previously appreciated. Here we have identified a lncRNA termed Lnc_bc060912 whose expression is increased in human lung and other tumors. Lnc_bc060912 is 1.2 kb in length and is composed of two exons. The expression of Lnc_bc060912 was repressed by p53. Lnc_bc060912 suppressed cell apoptosis. Using a recently developed method for RNA-pulldown with formaldehyde cross-linking, we found that Lnc_bc060912 interacted with the two DNA damage repair proteins PARP1 and NPM1. Together, these results suggest that Lnc_bc060912, via PARP1 and NPM1, affects cell apoptosis and may play important roles in tumorigenesis and cancer progression.

SUMOylation of DNA damage response and repair proteins

SUMOylation of DNA damage response and repair proteins

Protein expression of DNA damage repair proteins dictates response to topoisomerase and PARP inhibitors in triple-negative breast cancer.

Patients with metastatic triple-negative breast cancer (TNBC) have a poor prognosis. New approaches for the treatment of TNBC are needed to improve patient survival. The concept of synthetic lethality, brought about by inactivating complementary DNA repair pathways, has been proposed as a promising therapeutic option for these tumors. The TNBC tumor type has been associated with BRCA mutations, and inhibitors of Poly (ADP-ribose) polymerase (PARP), a family of proteins that facilitates DNA repair, have been shown to effectively kill BRCA defective tumors by preventing cells from repairing DNA damage, leading to a loss of cell viability and clonogenic survival. Here we present preclinical efficacy results of combining the PARP inhibitor, ABT-888, with CPT-11, a topoisomerase I inhibitor. CPT-11 binds to topoisomerase I at the replication fork, creating a bulky adduct that is recognized as damaged DNA. When DNA damage was stimulated with CPT-11, protein expression of the nucleotide excision repair enzyme ERCC1 inversely correlated with cell viability, but not clonogenic survival. However, 4 out of the 6 TNBC cells were synergistically responsive by cell viability and 5 out of the 6 TNBC cells were synergistically responsive by clonogenic survival to the combination of ABT-888 and CPT-11. In vivo, the BRCA mutant cell line MX-1 treated with CPT-11 alone demonstrated significant decreased tumor growth; this decrease was enhanced further with the addition of ABT-888. Decrease in tumor growth correlated with an increase in double strand DNA breaks as measured by γ-H2AX phosphorylation. In summary, inhibiting two arms of the DNA repair pathway simultaneously in TNBC cell lines, independent of BRCA mutation status, resulted in un-repairable DNA damage and subsequent cell death.

Two fission yeast high mobility group box proteins in the maintenance of genomic integrity following doxorubicin insult

Drug resistance is a challenge in chemotherapy, and, to date, there has been little resolution as to how it is induced. We previously isolated a host of doxorubicin resistance (DXR) genes in fission yeast and here we investigate the regulation of this resistance through two high mobility group (HMG) motif-containing DXR proteins, Nht1 and Hap2. The concurrent deletion of nht1 and hap2 did not confer cumulative sensitivity to doxorubicin, indicating that these factors cooperate closely in similar epistatic groups. We show that doxorubicin treatment resulted in the subcellular reorganization of Rhp54, a homologous recombination-dependent DNA damage repair protein. The disruption of either nht1 or hap2 attenuated Rhp54-foci formation, suggesting that these factors modulate the repair of doxorubicin-induced DNA lesions via the recruitment of homologous recombination machinery. Epistatic analyses further confirmed that Nht1 and Hap2 act in similar functional groups with complexes related to DSB repair but act synergistically with factors that regulate transcription and chromosome segregation. Overall, this work shows the molecular crosstalk coordinated by HMG proteins in conferring doxorubicin resistance in fission yeast.

The ATM inhibitor KU55933 sensitizes radioresistant bladder cancer cells with DAB2IP gene defect

Purpose: Our preliminary results showed that differentially expressed in ovarian cancer-2/disabled homolog 2 (DOC-2/DAB2) interactive protein (DAB2IP), a putative tumor suppressor gene, is down-regulated in bladder cancer (BCa) with aggressive phenotypes. In this study, we investigated how DAB2IP knockdown influenced BCa cell response to ionizing radiation (IR) and discussed possible ways to enhance cell radiosensitivity.Methods and materials: The small interfering RNA (siRNA) system was implemented to inhibit endogenous DAB2IP expression in two human BCa cell lines, T24 and 5637. Cell sensitivity to IR alone or combined treatment was measured by a colony formation assay (CFA). Western blot was used to determine the phosphorylation levels of ataxia-telangiectasia mutated (ATM), catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs) and related DNA damage repair (DDR) proteins. Immunofluorescence as well as a flow cytometry assay were employed to detect DNA double-strand break (DSB) repair and cell cycle distribution, respectively.Results: DAB2IP-knockdown of BCa cells (i.e., siDAB2IP) exhibit increased clonogenic survival in response to IR compared with control cells (i.e., siCON) expressing an endogenous level of DAB2IP. The mechanism in siDAB2IP cells could be explained by elevated ATM expression and activation, increased S phase cell distribution as well as faster DSB repair kinetics. 2-morpholin-4-yl-6-thianthren-1-yl-pyran-4-one (KU55933) significantly sensitized siDAB2IP cells to IR due to inhibition of the phosphorylation of ATM and its downstream targets following IR and slower DSB repair kinetics.Conclusions: Loss of DAB2IP expression in BCa cells signifies their radioresistance. KU55933, which suppresses ATM phosphorylation upon irradiation, could be applied in the radiotherapy of BCa patients with a DAB2IP gene defect.