Excess ribonucleotide reductase R2 subunits coordinate the S phase checkpoint to facilitate DNA damage repair and recovery from replication stress

Loss of Dnajc21 leads to cytopenia and altered nucleotide metabolism in zebrafish

Mutations in the DNAJC21 gene were recently described in Shwachman-Diamond syndrome (SDS), a bone marrow failure syndrome with high predisposition for myeloid malignancies. To study the underlying biology in hematopoiesis regulation and disease, we generated the first in vivo model of Dnajc21 deficiency using the zebrafish. Zebrafish dnajc21 mutants phenocopy key SDS patient phenotypes such as cytopenia, reduced growth, and defective protein synthesis. We show that cytopenia results from impaired hematopoietic differentiation, accumulation of DNA damage, and reduced cell proliferation. The introduction of a biallelic tp53 mutation in the dnajc21 mutants leads to the development of myelodysplastic neoplasia-like features defined by abnormal erythroid morphology and expansion of hematopoietic progenitors. Using transcriptomic and metabolomic analyses, we uncover a novel role for Dnajc21 in nucleotide metabolism. Exogenous nucleoside supplementation restores neutrophil counts, revealing an association between nucleotide imbalance and neutrophil differentiation, suggesting a novel mechanism in dnajc21-mutant SDS biology.

Inhibition of ribonucleotide reductase subunit M2 enhances the radiosensitivity of metastatic pancreatic neuroendocrine tumor

Ribonucleotide Reductase (RNR) is a rate-limiting enzyme in the production of deoxyribonucleoside triphosphates (dNTPs), which are essential substrates for DNA repair after radiation damage. We explored the radiosensitization property of RNR and investigated a selective RRM2 inhibitor, 3-AP, as a radiosensitizer in the treatment of metastatic pNETs. We investigated the role of RNR subunit, RRM2, in pancreatic neuroendocrine (pNET) cells and responses to radiation in vitro. We also evaluated the selective RRM2 subunit inhibitor, 3-AP, as a radiosensitizer to treat pNET metastases in vivo. Knockdown of RNR subunits demonstrated that RRM1 and RRM2 subunits, but not p53R3, play significant roles in cell proliferation. RRM2 inhibition activated DDR pathways through phosphorylation of ATM and DNA-PK protein kinases but not ATR. RRM2 inhibition also induced Chk1 and Chk2 phosphorylation, resulting in G1/S phase cell cycle arrest. RRM2 inhibition sensitized pNET cells to radiotherapy and induced apoptosis in vitro. In vivo, we utilized pNET subcutaneous and lung metastasis models to examine the rationale for RNR-targeted therapy and 3-AP as a radiosensitizer in treating pNETs. Combination treatment significantly increased apoptosis of BON (human pNET) xenografts and significantly reduced the burden of lung metastases. Together, our results demonstrate that selective RRM2 inhibition induced radiosensitivity of metastatic pNETs both in vitro and in vivo. Therefore, treatment with the selective RRM2 inhibitor, 3-AP, is a promising radiosensitizer in the therapeutic armamentarium for metastatic pNETs.

Identification of several African swine fever virus replication inhibitors by screening of a library of FDA-approved drugs

African swine fever (ASF) caused by African swine fever virus (ASFV) is a highly infectious and lethal swine disease. Currently, there is only one novel approved vaccine and no antiviral drugs for ASFV. In the study, a high-throughput screening of an FDA-approved drug library was performed to identify several drugs against ASFV infection in primary porcine alveolar macrophages. Triapine and cytarabine hydrochloride were identified as ASFV infection inhibitors in a dose-dependent manner. The two drugs executed their antiviral activity during the replication stage of ASFV. Furthermore, molecular docking studies showed that triapine might interact with the active center Fe2+ in the small subunit of ASFV ribonucleotide reductase while cytarabine hydrochloride metabolite might interact with three residues (Arg589, Lys593, and Lys631) of ASFV DNA polymerase to block new DNA chain extension. Taken together, our results suggest that triapine and cytarabine hydrochloride displayed significant antiviral activity against ASFV in vitro.

Schiff bases and their metal complexes to target and overcome (multidrug) resistance in cancer

Overcoming multidrug resistance (MDR) is one of the major challenges in cancer therapy. In this respect, Schiff base-related compounds (bearing a R1R2CNR3 bond) gained high interest during the past decades. Schiff bases are considered privileged ligands for various reasons, including the easiness of their preparation and the possibility to form complexes with almost all transition metal ions. Schiff bases and their metal complexes exhibit many types of biological activities and are used for the treatment and diagnosis of various diseases. Until now, 13 Schiff bases have been investigated in clinical trials for cancer treatment and hypoxia imaging. This review represents the first collection of Schiff bases and their complexes which demonstrated MDR-reversal activity. The areas of drug resistance covered in this article involve: 1) Modulation of ABC transporter function, 2) Targeting lysosomal ABCB1 overexpression, 3) Circumvention of ABC transporter-mediated drug efflux by alternative routes of drug uptake, 4) Selective activity against MDR cancer models (collateral sensitivity), 5) Targeting GSH-detoxifying systems, 6) Overcoming apoptosis resistance by inducing necrosis and paraptosis, 7) Reactivation of mutated p53, 8) Restoration of sensitivity to DNA-damaging anticancer therapy, and 9) Overcoming drug resistance through modulation of the immune system. Through this approach, we would like to draw attention to Schiff bases and their metal complexes representing highly interesting anticancer drug candidates with the ability to overcome MDR.

In silico screening identifies a novel small molecule inhibitor that counteracts PARP inhibitor resistance in ovarian cancer

Poly ADP-ribose polymerase (PARP) inhibitors are promising targeted therapy for epithelial ovarian cancer (EOC) with BRCA mutations or defective homologous recombination (HR) repair. However, reversion of BRCA mutation and restoration of HR repair in EOC lead to PARP inhibitor resistance and reduced clinical efficacy of PARP inhibitors. We have previously shown that triapine, a small molecule inhibitor of ribonucleotide reductase (RNR), impaired HR repair and sensitized HR repair-proficient EOC to PARP inhibitors. In this study, we performed in silico screening of small molecule libraries to identify novel compounds that bind to the triapine-binding pocket on the R2 subunit of RNR and inhibit RNR in EOC cells. Following experimental validation of selected top-ranking in silico hits for inhibition of dNTP and DNA synthesis, we identified, DB4, a putative RNR pocket-binding inhibitor markedly abrogated HR repair and sensitized BRCA-wild-type EOC cells to the PARP inhibitor olaparib. Furthermore, we demonstrated that the combination of DB4 and olaparib deterred the progression of BRCA-wild type EOC xenografts and significantly prolonged the survival time of tumor-bearing mice. Herein we report the discovery of a putative small molecule inhibitor of RNR and HR repair for combination with PARP inhibitors to treat PARP inhibitor-resistant and HR repair-proficient EOC.

Cadmium (II)-Induced Oxidative Stress Results in Replication Stress and Epigenetic Modifications in Root Meristem Cell Nuclei of Vicia faba

Among heavy metals, cadmium is considered one of the most toxic and dangerous environmental factors, contributing to stress by disturbing the delicate balance between production and scavenging of reactive oxygen species (ROS). To explore possible relationships and linkages between Cd(II)-induced oxidative stress and the consequent damage at the genomic level (followed by DNA replication stress), root apical meristem (RAM) cells in broad bean (V. faba) seedlings exposed to CdCl₂ treatment and to post-cadmium recovery water incubations were tested with respect to H₂O₂ production, DNA double-strand breaks (γ-phosphorylation of H2AX histones), chromatin morphology, histone H3S10 phosphorylation on serine (a marker of chromatin condensation), mitotic activity, and EdU staining (to quantify cells typical of different stages of nuclear DNA replication). In order to evaluate Cd(II)-mediated epigenetic changes involved in transcription and in the assembly of nucleosomes during the S-phase of the cell cycle, the acetylation of histone H3 on lysine 5 (H3K56Ac) was investigated by immunofluorescence. Cellular responses to cadmium (II) toxicity seem to be composed of a series of interlinked biochemical reactions, which, via generation of ROS and DNA damage-induced replication stress, ultimately activate signal factors engaged in cell cycle control pathways, DNA repair systems, and epigenetic adaptations.

Inhibiting RRM2 to enhance the anticancer activity of chemotherapy

RRM2, the small subunit of ribonucleotide reductase, is identified as a tumor promotor and therapeutic target. It is common to see the overexpression of RRM2 in chemo-resistant cancer cells and patients. RRM2 mediates the resistance of many chemotherapeutic drugs and could become the predictor for chemosensitivity and prognosis. Therefore, inhibition of RRM2 may be an effective means to enhance the anticancer activity of chemotherapy. This review tries to discuss the mechanisms of RRM2 overexpression and the role of RRM2 in resistance to chemotherapy. Additionally, we compile the studies on small interfering RNA targets RRM2, RRM2 inhibitors, kinase inhibitors, and other ways that could overcome the resistance of chemotherapy or exert synergistic anticancer activity with chemotherapy through the expression inhibition or the enzyme inactivation of RRM2.

Phase I/II multisite trial of optimally dosed clofarabine and low-dose TBI for hematopoietic cell transplantation in acute myeloid leukemia

Clofarabine is an immunosuppressive purine nucleoside analog that may have better anti-leukemic activity than fludarabine. We performed a prospective phase I/II multisite trial of clofarabine with 2 Gy total body irradiation as non-myeloablative conditioning for allogeneic hematopoietic cell transplantation in adults with acute myeloid leukemia who were unfit for more intense regimens. Our main objective was to improve the 6-month relapse rate following non-myeloablative conditioning, while maintaining historic rates of non-relapse mortality (NRM) and engraftment. Forty-four patients, 53 to 74 (median: 69) years, were treated with clofarabine at 150 to 250 mg/m² , of whom 36 were treated at the maximum protocol-specified dose. One patient developed multifactorial acute kidney injury and another developed multiorgan failure, but no other grade 3 to 5 non-hematologic toxicities were observed. All patients fully engrafted. The 6-month relapse rate was 16% (95% CI, 5%-27%) among all patients and 14% (95% CI, 3%-26%) among high-risk patients treated at the maximum dose, meeting the pre-specified primary efficacy endpoint. Overall survival was 55% (95% CI, 40%-70%) and leukemia-free survival was 52% (95% CI, 37%-67%) at 2 years. Compared to a historical high-risk cohort treated with the combination of fludarabine at 90 mg/m² and 2 Gy TBI, protocol patients treated with the clofarabine-TBI regimen had lower rates of overall mortality (HR of 0.50, 95% CI, 0.28-0.91), disease progression or death (HR 0.48, 95% CI, 0.27-0.85), and morphologic relapse (HR 0.30, 95% CI, 0.13-0.69), and comparable NRM (HR 0.85, 95% CI 0.36-2.00). The combination of clofarabine with TBI warrants further investigation in patients with high-risk AML.

Iron chelation and 2-oxoglutarate-dependent dioxygenase inhibition suppress mantle cell lymphoma's cyclin D1

The patients with mantle cell lymphoma (MCL) have translocation t(11;14) associated with cyclin D1 overexpression. We observed that iron (an essential cofactor of dioxygenases including prolyl hydroxylases [PHDs]) depletion by deferoxamine blocked MCL cells' proliferation, increased expression of DNA damage marker γH2AX, induced cell cycle arrest and decreased cyclin D1 level. Treatment of MCL cell lines with dimethyloxalylglycine, which blocks dioxygenases involving PHDs by competing with their substrate 2-oxoglutarate, leads to their decreased proliferation and the decrease of cyclin D1 level. We then postulated that loss of EGLN2/PHD1 in MCL cells may lead to down-regulation of cyclin D1 by blocking the degradation of FOXO3A, a cyclin D1 suppressor. However, the CRISPR/Cas9-based loss-of-function of EGLN2/PHD1 did not affect cyclin D1 expression and the loss of FOXO3A did not restore cyclin D1 levels after iron chelation. These data suggest that expression of cyclin D1 in MCL is not controlled by ENGL2/PHD1-FOXO3A pathway and that chelation- and 2-oxoglutarate competition-mediated down-regulation of cyclin D1 in MCL cells is driven by yet unknown mechanism involving iron- and 2-oxoglutarate-dependent dioxygenases other than PHD1. These data support further exploration of the use of iron chelation and 2-oxoglutarate-dependent dioxygenase inhibitors as a novel therapy of MCL.

Activity and electrochemical properties: iron complexes of the anticancer drug triapine and its analogs

Triapine (3-AP), is an iron-binding ligand and anticancer drug that is an inhibitor of human ribonucleotide reductase (RNR). Inhibition of RNR by 3-AP results in the depletion of dNTP precursors of DNA, thereby selectively starving fast-replicating cancer cells of nucleotides for survival. The redox-active form of 3-AP directly responsible for inhibition of RNR is the Fe(II)(3-AP)₂ complex. In this work, we synthesize 12 analogs of 3-AP, test their inhibition of RNR in vitro, and study the electronic properties of their iron complexes. The reduction and oxidation events of 3-AP iron complexes that are crucial for the inhibition of RNR are modeled with solution studies. We monitor the pH necessary to induce reduction in iron complexes of 3-AP analogs in a reducing environment, as well as the kinetics of oxidation in an oxidizing environment. The oxidation state of the complex is monitored using UV-Vis spectroscopy. Isoquinoline analogs of 3-AP favor the maintenance of the biologically active reduced complex and possess oxidation kinetics that allow redox cycling, consistent with their effective inhibition of RNR seen in our in vitro experiments. In contrast, methylation on the thiosemicarbazone secondary amine moiety of 3-AP produces analogs that form iron complexes with much higher redox potentials, that do not redox cycle, and are inactive against RNR in vitro. The catalytic subunit of human Ribonucleotide Reductase (RNR), contains a tyrosyl radical in the enzyme active site. Fe(II) complexes of 3-AP and its analogs can quench the radical and, subsequently, inactivate RNR. The potency of RNR inhibitors is highly dependent on the redox properties of the iron complexes, which can be tuned by ligand modifications. Complexes are found to be active within a narrow redox window imposed by the cellular environment.

Gemcitabine resistance mediated by ribonucleotide reductase M2 in lung squamous cell carcinoma is reversed by GW8510 through autophagy induction

Although chemotherapeutic regimen containing gemcitabine is the first-line therapy for advanced lung squamous cell carcinoma (LSCC), gemcitabine resistance remains an important clinical problem. Some studies suggest that overexpressions of ribonucleotide reductase (RNR) subunit M2 (RRM2) may be involved in gemcitabine resistance. We used a novel RRM2 inhibitor, GW8510, as a gemcitabine sensitization agent to investigate the therapeutic utility in reversing gemcitabine resistance in LSCC. Results showed that the expressions of RRM2 were increased in gemcitabine intrinsic resistant LSCC cells upon gemcitabine treatment. GW8510 not only suppressed LSCC cell survival, but also sensitized gemcitabine-resistant cells to gemcitabine through autophagy induction mediated by RRM2 down-regulation along with decrease in dNTP levels. The combination of GW8510 and gemcitabine produced a synergistic effect on killing LSCC cells. The synergism of the two agents was impeded by addition of autophagy inhibitors chloroquine (CQ) or bafilomycin A1 (Baf A1), or knockdown of the autophagy gene, Bcl-2-interacting protein 1 (BECN1). Moreover, GW8510-caused LSCC cell sensitization to gemcitabine through autophagy induction was parallel with impairment of DNA double-strand break (DSB) repair and marked increase in cell apoptosis, revealing a cross-talk between autophagy and DNA damage repair, and an interplay between autophagy and apoptosis. Finally, gemcitabine sensitization mediated by autophagy induction through GW8510-caused RRM2 down-regulation was demonstrated in vivo in gemcitabine-resistant LSCC tumor xenograft, further indicating that the sensitization is dependent on autophagy activation. In conclusion, GW8510 can reverse gemcitabine resistance in LSCC cells through RRM2 downregulation-mediated autophagy induction, and GW850 may be a promising therapeutic agent against LSCC as it combined with gemcitabine.

Triapine Radiochemotherapy in Advanced Stage Cervical Cancer

Clinical ribonucleotide reductase (RNR) inhibitors have reinvigorated enthusiasm for radiochemotherapy treatment of patients with regionally advanced stage cervical cancers. About two-thirds of patients outlive their cervical cancer (1), even though up to half of their tumors retain residual microscopic disease (2). The National Cancer Institute Cancer Therapy Evaluation Program conducted two prospective trials of triapine–cisplatin–radiation to improve upon this finding by precisely targeting cervical cancer’s overactive RNR. Triapine’s potent inactivation of RNR arrests cells at the G1/S cell cycle restriction checkpoint and enhances cisplatin–radiation cytotoxicity. In this article, we provide perspective on challenges encountered in and future potential of clinical development of a triapine–cisplatin–radiation combination for patients with regionally advanced cervical cancer. New trial results and review presented here suggest that a triapine–cisplatin–radiation combination may offer molecular cell cycle target control to maximize damage in cancers and to minimize injury to normal cells. A randomized trial now accrues patients with regionally advanced stage cervical cancer to evaluate triapine’s contribution to clinical benefit after cisplatin–radiation (clinicaltrials.gov, NCT02466971).

Profiling of ribonucleotides and deoxyribonucleotides pools in response to DNA damage and repair induced by methyl methanesulfonate in cancer and normal cells

The absolute and relative pool sizes of deoxyribonucleotides (dRNs) are essential in DNA replication fidelity, DNA damage and repair. We found in this study that although DNA damage induced by methyl methanesulfonate (MMS) seemed similar in cancer (HepG2) and normal (LO2) cells, more extensive alterations in ribonucleotides (RNs) and dRNs pools occurred in HepG2 cells indicating that HepG2 cells were more vigilant to DNA damage. After 10 h repair, RNs pools were still severely perturbed in LO2 cells. Compared to LO2 cells, deoxyribonucleotide triphosphates (dNTPs) pools in HepG2 cells elevated by more folds which could facilitate more efficient DNA repair and improve survival probability following DNA damage, although this should definitely lead to higher mutation rates. DNA repair was more efficient in HepG2 cells at S phase and it partly came to an end while DNA repair was still uncompleted in LO2 cells outside S phase. In conclusion, our results demonstrated that HepG2 and LO2 cells presented many differences in nucleotide metabolism, cell cycle checkpoints and DNA repair pathways in response to DNA damage, which could be potential targets for cancer treatment.

Triapine potentiates platinum-based combination therapy by disruption of homologous recombination repair

Background:

Platinum resistance may be attributable to inherent or acquired proficiency in homologous recombination repair (HRR) in epithelial ovarian cancer (EOC). The objective of this study was to evaluate the efficacy of the small molecule inhibitor triapine to disrupt HRR and sensitise BRCA wild-type EOC cells to platinum-based combination therapy in vitro and in vivo.

Methods:

The sensitivity of BRCA wild-type cancer cells to olaparib, cisplatin, carboplatin, doxorubicin, or etoposide in combination with triapine was evaluated by clonogenic survival assays. The effects of triapine on HRR activity in cells were measured with a DR-GFP reporter assay. The ability of triapine to enhance the effects of the carboplatin-doxil combination on EOC tumour growth delay was determined using a xenograft tumour mouse model.

Results:

Platinum resistance is associated with wild-type BRCA status. Triapine inhibits HRR activity and enhances the sensitivity of BRCA wild-type cancer cells to cisplatin, olaparib, and doxorubicin. However, sequential combination of triapine and cisplatin is necessary to achieve synergism. Moreover, triapine potentiates platinum-based combination therapy against BRCA wild-type EOC cells and produces significant delay of EOC tumour growth.

Conclusions:

Triapine promises to augment the clinical efficacy of platinum-based combination regimens for treatment of platinum-resistant EOC with wild-type BRCA and proficient HRR activity.

Novel immunosuppressive agent caerulomycin A exerts its effect by depleting cellular iron content

BACKGROUND AND PURPOSE

Recently, we have described the use of caerulomycin A (CaeA) as a potent novel immunosuppressive agent. Immunosuppressive drugs are crucial for long-term graft survival following organ transplantation and treatment of autoimmune diseases, inflammatory disorders, hypersensitivity to allergens, etc. The objective of this study was to identify cellular targets of CaeA and decipher its mechanism of action.

EXPERIMENTAL APPROACH

Jurkat cells were treated with CaeA and cellular iron content, iron uptake/release, DNA content and deoxyribonucleoside triphosphate pool determined. Activation of MAPKs; expression level of transferrin receptor 1, ferritin and cell cycle control molecules; reactive oxygen species (ROS) and cell viability were measured using Western blotting, qRT-PCR or flow cytometry.

KEY RESULTS

CaeA caused intracellular iron depletion by reducing its uptake and increasing its release by cells. CaeA caused cell cycle arrest by (i) inhibiting ribonucleotide reductase (RNR) enzyme, which catalyses the rate-limiting step in the synthesis of DNA; (ii) stimulating MAPKs signalling transduction pathways that play an important role in cell growth, proliferation and differentiation; and (iii) by targeting cell cycle control molecules such as cyclin D1, cyclin-dependent kinase 4 and p21(CIP1/WAF1) . The effect of CaeA on cell proliferation was reversible.

CONCLUSIONS AND IMPLICATIONS

CaeA exerts its immunosuppressive effect by targeting iron. The effect is reversible, which makes CaeA an attractive candidate for development as a potent immunosuppressive drug, but also indicates that iron chelation can be used as a rationale approach to selectively suppress the immune system, because compared with normal cells, rapidly proliferating cells require a higher utilization of iron.

Transcriptomic response of cowpea bruchids to N-acetylglucosamine-specific lectins

Griffonia simplicifolia lectin II (GSII) and wheat germ agglutinin (WGA) are N-acetylglucosamine-binding lectins. Previous studies demonstrated that they have anti-insect activity, a property potentially useful in pest control. To gain some insight into the insect response to dietary lectins, we performed transcriptomic analysis using the cowpea bruchid (Callosobruchus maculatus) midgut microarray platform we built. Compared to the nonnutritional cellulose treatment, dietary lectins induced more profound changes in gene expression. Ingestion of relatively high doses of lectins for 24 h resulted in alteration of gene expression involved in sugar and lipid metabolism, transport, development, defense, and stress tolerance. Metabolic genes were largely downregulated. Moreover, we observed disorganized microvilli resulting from ingestion of WGA. This morphological change is consistent with the lectin-induced changes in genes related to midgut epithelial cell repair. In addition, suboptimal nutrient conditions may serve as a stress signal to trigger senescence processes, leading to growth arrest and developmental delay.

Pegylated arginine deiminase synergistically increases the cytotoxicity of gemcitabine in human pancreatic cancer

Background

Pancreatic ductal adenocarcinoma has proven to be one of the most chemo-resistant among all solid organ malignancies. Several mechanisms of resistance have been described, though few reports of strategies to overcome this chemo-resistance have been successful in restoring sensitivity to the primary chemotherapy (gemcitabine) and enter the clinical treatment arena.

Methods

We examined the ability of cellular arginine depletion through treatment with PEG-ADI to alter in vitro and in vivo cytotoxicity of gemcitabine. The effect on levels of key regulators of gemcitabine efficacy (e.g. RRM2, hENT1, and dCK) were examined.

Results

Combination of PEG-ADI and gemcitabine substantially increases growth arrest, leading to increased tumor response in vivo. PEG-ADI is a strong inhibitor of the gemcitabine-induced overexpression of ribonucleotide reductase subunit M2 (RRM2) levels both in vivo and in vitro, which is associated with gemcitabine resistance. This mechanism is through the abrogation of the gemcitabine-mediated inhibitory effect on E2F-1 function, a transcriptional repressor of RRM2.

Conclusion

The ability to alter gemcitabine resistance in a targeted manner by inducing metabolic stress holds great promise in the treatment of advanced pancreatic cancer.

Expanding horizons in iron chelation and the treatment of cancer: role of iron in the regulation of ER stress and the epithelial-mesenchymal transition

Cancer is a major public health issue and, despite recent advances, effective clinical management remains elusive due to intra-tumoural heterogeneity and therapeutic resistance. Iron is a trace element integral to a multitude of metabolic processes, including DNA synthesis and energy transduction. Due to their generally heightened proliferative potential, cancer cells have a greater metabolic demand for iron than normal cells. As such, iron metabolism represents an important "Achilles' heel" for cancer that can be targeted by ligands that bind and sequester intracellular iron. Indeed, novel thiosemicarbazone chelators that act by a "double punch" mechanism to both bind intracellular iron and promote redox cycling reactions demonstrate marked potency and selectivity in vitro and in vivo against a range of tumours. The general mechanisms by which iron chelators selectively target tumour cells through the sequestration of intracellular iron fall into the following categories: (1) inhibition of cellular iron uptake/promotion of iron mobilisation; (2) inhibition of ribonucleotide reductase, the rate-limiting, iron-containing enzyme for DNA synthesis; (3) induction of cell cycle arrest; (4) promotion of localised and cytotoxic reactive oxygen species production by copper and iron complexes of thiosemicarbazones (e.g., Triapine(®) and Dp44mT); and (5) induction of metastasis and tumour suppressors (e.g., NDRG1 and p53, respectively). Emerging evidence indicates that chelators can further undermine the cancer phenotype via inhibiting the epithelial-mesenchymal transition that is critical for metastasis and by modulating ER stress. This review explores the "expanding horizons" for iron chelators in selectively targeting cancer cells.

Triapine disrupts CtIP-mediated homologous recombination repair and sensitizes ovarian cancer cells to PARP and topoisomerase inhibitors

PARP inhibitors exploit synthetic lethality to target epithelial ovarian cancer (EOC) with hereditary BRCA mutations and defects in homologous recombination repair (HRR). However, such an approach is limited to a small subset of EOC patients and compromised by restored HRR due to secondary mutations in BRCA genes. Here, it was demonstrated that triapine, a small-molecule inhibitor of ribonucleotide reductase, enhances the sensitivity of BRCA wild-type EOC cells to the PARP inhibitor olaparib and the topoisomerase II inhibitor etoposide. Triapine abolishes olaparib-induced BRCA1 and Rad51 foci, and disrupts the BRCA1 interaction with the Mre11-Rad50-Nbs1 (MRN) complex in BRCA1 wild-type EOC cells. It has been shown that phosphorylation of CtIP (RBBP8) is required for the interaction with BRCA1 and with MRN to promote DNA double-strand break (DSB) resection during S and G(2) phases of the cell cycle. Mechanistic studies within reveal that triapine inhibits cyclin-dependent kinase (CDK) activity and blocks olaparib-induced CtIP phosphorylation through Chk1 activation. Furthermore, triapine abrogates etoposide-induced CtIP phosphorylation and DSB resection as evidenced by marked attenuation of RPA32 phosphorylation. Concurrently, triapine obliterates etoposide-induced BRCA1 foci and sensitizes BRCA1 wild-type EOC cells to etoposide. Using a GFP-based HRR assay, it was determined that triapine suppresses HRR activity induced by an I-SceI-generated DSB. These results suggest that triapine augments the sensitivity of BRCA wild-type EOC cells to drug-induced DSBs by disrupting CtIP-mediated HRR.

IMPLICATIONS

These findings provide a strong rationale for combining triapine with PARP or topoisomerase inhibitors to target HRR-proficient EOC cells.

Bcl2 induces DNA replication stress by inhibiting ribonucleotide reductase

DNA replication stress is an inefficient DNA synthesis process that leads replication forks to progress slowly or stall. Two main factors that cause replication stress are alterations in pools of deoxyribonucleotide (dNTP) precursors required for DNA synthesis and changes in the activity of proteins required for synthesis of dNTPs. Ribonucleotide reductase (RNR), containing regulatory hRRM1 and catalytic hRRM2 subunits, is the enzyme that catalyzes the conversion of ribonucleoside diphosphates (NDP) to deoxyribonucleoside diphosphates (dNDP) and thereby provides dNTP precursors needed for the synthesis of DNA. Here, we demonstrate that either endogenous or exogenous expression of Bcl2 results in decreases in RNR activity and intracellular dNTP, retardation of DNA replication fork progression, and increased rate of fork asymmetry leading to DNA replication stress. Bcl2 colocalizes with hRRM1 and hRRM2 in the cytoplasm and directly interacts via its BH4 domain with hRRM2 but not hRRM1. Removal of the BH4 domain of Bcl2 abrogates its inhibitory effects on RNR activity, dNTP pool level, and DNA replication. Intriguingly, Bcl2 directly inhibits RNR activity by disrupting the functional hRRM1/hRRM2 complex via its BH4 domain. Our findings argue that Bcl2 reduces intracellular dNTPs by inhibiting ribonucleotide reductase activity, thereby providing insight into how Bcl2 triggers DNA replication stress.

Cyclin F-mediated degradation of ribonucleotide reductase M2 controls genome integrity and DNA repair

F-box proteins are the substrate binding subunits of SCF (Skp1-Cul1-F-box protein) ubiquitin ligase complexes. Using affinity purifications and mass spectrometry, we identified RRM2 (the ribonucleotide reductase family member 2) as an interactor of the F-box protein cyclin F. Ribonucleotide reductase (RNR) catalyzes the conversion of ribonucleotides to deoxyribonucleotides (dNTPs), which are necessary for both replicative and repair DNA synthesis. We found that, during G2, following CDK-mediated phosphorylation of Thr33, RRM2 is degraded via SCF(cyclin F) to maintain balanced dNTP pools and genome stability. After DNA damage, cyclin F is downregulated in an ATR-dependent manner to allow accumulation of RRM2. Defective elimination of cyclin F delays DNA repair and sensitizes cells to DNA damage, a phenotype that is reverted by expressing a nondegradable RRM2 mutant. In summary, we have identified a biochemical pathway that controls the abundance of dNTPs and ensures efficient DNA repair in response to genotoxic stress.

Reduced level of ribonucleotide reductase R2 subunits increases dependence on homologous recombination repair of cisplatin-induced DNA damage

Ribonucleotide reductase (RNR) catalyzes the rate-limiting step in the production of deoxyribonucleoside triphosphates (dNTPs) required for replicative and repair DNA synthesis. Mammalian RNR is a heteromeric enzyme consisting primarily of R1 and R2 subunits during the S phase of the cell cycle. We have shown previously that the presence of excess R2 subunits protects p53-deficient human colon cancer cells from cisplatin-induced DNA damage and replication stress. However, the mode of DNA repair influenced by changes in the level of the R2 subunit remained to be defined. In the present study, we demonstrated that depletion of BRCA1, an important factor of homologous recombination repair (HRR), preferentially sensitized stable R2-knockdown p53(-/-) HCT116 cells to the cytotoxicity of cisplatin and γ-H2AX induction. In accord with this finding, these R2-knockdown cells exhibited increased dependence on HRR, as evidenced by elevated levels of cisplatin-induced Rad51 foci and sister chromatid exchange frequency. Furthermore, stable knockdown of the R2 subunit also led to decreased cisplatin-induced gap-filling synthesis in nucleotide excision repair (NER) and a reduced dATP level in the G(2)/M phase of the cell cycle. These results suggest that an increased level of the R2 subunit extends the availability of dATP in the G(2)/M phase to promote the repair of NER-mediated single-strand gaps that are otherwise converted into double-strand breaks in the subsequent S phase. We propose that HRR becomes important for recovery from cisplatin-DNA lesions when the postexcision process of NER is restrained by reduced levels of the R2 subunit and dATP in p53-deficient cancer cells.

Deoxyribonucleotide metabolism in cycling and resting human fibroblasts with a missense mutation in p53R2, a subunit of ribonucleotide reductase

Ribonucleotide reduction provides deoxynucleotides for nuclear and mitochondrial (mt) DNA replication and DNA repair. In cycling mammalian cells the reaction is catalyzed by two proteins, R1 and R2. A third protein, p53R2, with the same function as R2, occurs in minute amounts. In quiescent cells, p53R2 replaces the absent R2. In humans, genetic inactivation of p53R2 causes early death with mtDNA depletion, especially in muscle. We found that cycling fibroblasts from a patient with a lethal mutation in p53R2 contained a normal amount of mtDNA and showed normal growth, ribonucleotide reduction, and deoxynucleoside triphosphate (dNTP) pools. However, when made quiescent by prolonged serum starvation the mutant cells strongly down-regulated ribonucleotide reduction, decreased their dCTP and dGTP pools, and virtually abolished the catabolism of dCTP in substrate cycles. mtDNA was not affected. Also, nuclear DNA synthesis and the cell cycle-regulated enzymes R2 and thymidine kinase 1 decreased strongly, but the mutant cell populations retained unexpectedly larger amounts of the two enzymes than the controls. This difference was probably due to their slightly larger fraction of S phase cells and therefore not induced by the absence of p53R2 activity. We conclude that loss of p53R2 affects ribonucleotide reduction only in resting cells and leads to a decrease of dNTP catabolism by substrate cycles that counterweigh the loss of anabolic activity. We speculate that this compensatory mechanism suffices to maintain mtDNA in fibroblasts but not in muscle cells with a larger content of mtDNA necessary for their high energy requirements.

Regulation and functional contribution of thymidine kinase 1 in repair of DNA damage

Cellular supply of dNTPs is essential in the DNA replication and repair processes. Here we investigated the regulation of thymidine kinase 1 (TK1) in response to DNA damage and found that genotoxic insults in tumor cells cause up-regulation and nuclear localization of TK1. During recovery from DNA damage, TK1 accumulates in p53-null cells due to a lack of mitotic proteolysis as these cells are arrested in the G(2) phase by checkpoint activation. We show that in p53-proficient cells, p21 expression in response to DNA damage prohibits G(1)/S progression, resulting in a smaller G(2) fraction and less TK1 accumulation. Thus, the p53 status of tumor cells affects the level of TK1 after DNA damage through differential cell cycle control. Furthermore, it was shown that in HCT-116 p53(-/-) cells, TK1 is dispensable for cell proliferation but crucial for dTTP supply during recovery from DNA damage, leading to better survival. Depletion of TK1 decreases the efficiency of DNA repair during recovery from DNA damage and generates more cell death. Altogether, our data suggest that more dTTP synthesis via TK1 take place after genotoxic insults in tumor cells, improving DNA repair during G(2) arrest.

Modulating radiation resistance by inhibiting ribonucleotide reductase in cancers with virally or mutationally silenced p53 protein

Therapeutic ionizing radiation damages DNA, increasing p53-regulated ribonucleotide reductase (RNR) activity required for de novo synthesis of the deoxyribonucleotide triphosphates used during DNA repair. This study investigated the pharmacological inhibition of RNR in cells of virally or mutationally silenced p53 cancer cell lines using 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, Triapine(R), NSC #663249), a chemotherapeutic radiosensitizer that equally inhibits RNR M2 and p53R2 small subunits. The effects of 3-AP on RNR inhibition and resulting radiosensitization were evaluated in cervical (CaSki, HeLa and C33-a) and colon (RKO, RKO-E6) cancer cells. 3-AP treatment significantly enhanced radiation-related cytotoxicity in cervical and colon cancer cells. 3-AP treatment significantly decreased RNR activity, caused prolonged radiation-induced DNA damage, and resulted in an extended G(1)/S-phase cell cycle arrest in all cell lines. Similar effects were observed in both RKO and RKO-E6 cells, suggesting a p53-independent mechanism of radiosensitization. We conclude that inhibition of ribonucleotide reductase by 3-AP enhances radiation-mediated cytotoxicity independent of p53 regulation by impairing repair processes that rely on deoxyribonucleotide production, thereby substantially increasing the radiation sensitivity of human cancers.

Characterization of the behavior of functional viral genomes during the early steps of human immunodeficiency virus type 1 infection

Infectious viral DNA constitutes only a small fraction of the total viral DNA produced during retroviral infection, and as such its exact behavior is largely unknown. In the present study, we characterized in detail functional viral DNA produced during the early steps of human immunodeficiency virus type 1 infection by analyzing systematically their kinetics of synthesis and integration in different target cells. In addition, we have compared the functional stability of viral nucleoprotein complexes arrested at their pre-reverse transcription state, and we have attempted to measure the kinetics of loss of capsid proteins from viral complexes through the susceptibility of the early phases of infection to cyclosporine, a known inhibitor of the interaction between viral capsid and cyclophilin A. Overall, our data suggest a model in which loss of capsid proteins from viral complexes and reverse transcription occur concomitantly and in which the susceptibility of target cells to infection results from a competition between the ability of the cellular environment to quickly destabilize viral nucleoprotein complexes and the capability of the virus to escape such targeting by engaging the reverse transcription reaction.

Implication of checkpoint kinase-dependent up-regulation of ribonucleotide reductase R2 in DNA damage response

To investigate drug mechanisms of action and identify molecular targets for the development of rational drug combinations, we conducted synthetic small interfering RNA (siRNA)-based RNAi screens to identify genes whose silencing affects anti-cancer drug responses. Silencing of RRM1 and RRM2, which encode the large and small subunits of the human ribonucleotide reductase complex, respectively, markedly enhanced the cytotoxicity of the topoisomerase I inhibitor camptothecin (CPT). Silencing of RRM2 was also found to enhance DNA damage as measured by histone gamma-H2AX. Further studies showed that CPT up-regulates both RRM1 and RRM2 mRNA and protein levels and induces the nuclear translocation of RRM2. The checkpoint kinase 1 (Chk1) was up-regulated and activated in response to CPT, and CHEK1 down-regulation by siRNA and small molecule inhibitors of Chk1 blocked RRM2 induction by CPT. CHEK1 siRNA also suppressed E2F1 up-regulation by CPT, and silencing of E2F1 suppressed the up-regulation of RRM2. Silencing of ATR or ATM and inhibition of ATM activity by KU-55933 blocked Chk1 activation and RRM2 up-regulation. This study links the known components of CPT-induced DNA damage response with proteins required for the synthesis of dNTPs and DNA repair. Specifically, we propose that upon DNA damage, Chk1 activation, mediated by ATM and ATR, up-regulates RRM2 expression through the E2F1 transcription factor. Up-regulation in RRM2 expression levels coupled with its nuclear recruitment suggests an active role for ribonucleotide reductase in the cellular response to CPT-mediated DNA damage that could potentially be exploited as a strategy for enhancing the efficacy of topoisomerase I inhibitors.

Introduction to molecular combing: genomics, DNA replication, and cancer

The sequencing of the human genome inaugurated a new era in both fundamental and applied genetics. At the same time, the emergence of new technologies for probing the genome has transformed the field of pharmaco-genetics and made personalized genomic profiling and high-throughput screening of new therapeutic agents all but a matter of routine. One of these technologies, molecular combing, has served to bridge the technical gap between the examination of gross chromosomal abnormalities and sequence-specific alterations. Molecular combing provides a new perspective on the structure and dynamics of the human genome at the whole genome and sub-chromosomal levels with a resolution ranging from a few kilobases up to a megabase and more. Originally developed to study genetic rearrangements and to map genes for positional cloning, recent advances have extended the spectrum of its applications to studying the real-time dynamics of the replication of the genome. Understanding how the genome is replicated is essential for elucidating the mechanisms that both maintain genome integrity and result in the instabilities leading to human genetic disease and cancer. In the following, we will examine recent discoveries and advances due to the application of molecular combing to new areas of research in the fields of molecular cytogenetics and cancer genomics.

Disruption of cAMP and prostaglandin E2 transport by multidrug resistance protein 4 deficiency alters cAMP-mediated signaling and nociceptive response

Multidrug resistance protein 4 (MRP4; ABCC4) is a member of the MRP/ATP-binding cassette family serving as a transmembrane transporter involved in energy-dependent efflux of anticancer/antiviral nucleotide agents and of physiological substrates, including cyclic nucleotides and prostaglandins (PGs). Phenotypic consequences of mrp4 deficiency were investigated using mrp4-knockout mice and derived immortalized mouse embryonic fibroblast (MEF) cells. Mrp4 deficiency caused decreased extracellular and increased intracellular levels of cAMP in MEF cells under normal and forskolin-stimulated conditions. Mrp4 deficiency and RNA interference-mediated mrp4 knockdown led to a pronounced reduction in extracellular PGE(2) but with no accumulation of intracellular PGE(2) in MEF cells. This result was consistent with attenuated cAMP-dependent protein kinase activity and reduced cyclooxygenase-2 (Cox-2) expression in mrp4-deficient MEF cells, suggesting that PG synthesis is restrained along with a lack of PG transport caused by mrp4 deficiency. Mice lacking mrp4 exhibited no outward phenotypes but had a decrease in plasma PGE metabolites and an increase in inflammatory pain threshold compared with wild-type mice. Collectively, these findings imply that mrp4 mediates the efflux of PGE(2) and concomitantly modulates cAMP mediated signaling for balanced PG synthesis in MEF cells. Abrogation of mrp4 affects the regulation of peripheral PG levels and consequently alters inflammatory nociceptive responses in vivo.

Clofarabine acts as radiosensitizer in vitro and in vivo by interfering with DNA damage response

PURPOSE

Combination treatment with radiotherapy and chemotherapy has emerged as the dominant form of cancer adjuvant regimens in recent years. Clofarabine, a newly approved drug for pediatric leukemia, is a second-generation purine nucleoside analogue that can block DNA synthesis and inhibit DNA repair. Therefore, we hypothesized that clofarabine could work synergistically with radiotherapy to increase the tumor cell response.

METHODS AND MATERIALS

The effects of clofarabine on radiosensitivity have been established in several tumor cell lines in vitro and in vivo using colony-forming assays and tumor xenografts. The effect of clofarabine on the DNA damage response was also studied in vitro by measuring gamma-H2AX focus formation.

RESULTS

Clonogenic survival was significantly reduced in irradiated cells treated with clofarabine, demonstrating the strong radiosensitizing effect of clofarabine. Furthermore, clofarabine displayed a radiosensitizing effect that was greater than gemcitabine or 5-fluorouracil. We also found that low doses of clofarabine can prolong the presence of radiation-induced gamma-H2AX nuclear focus formation, and high doses of clofarabine can induce DNA double-strand breaks, suggesting that clofarabine can interfere with DNA damage response pathways. In addition, clofarabine-induced radiosensitization was also established in vivo using a colorectal cancer model, DLD-1, in athymic nude mice. When combined with fractionated radiotherapy, a moderate dose of clofarabine led to a significant increase in tumor growth inhibition.

CONCLUSION

Clofarabine acts as a powerful radiosensitizer both in vitro and in vivo by interfering with the DNA damage response.

Ataxia-telangiectasia mutated kinase regulates ribonucleotide reductase and mitochondrial homeostasis

Ataxia-telangiectasia mutated (ATM) kinase orchestrates nuclear DNA damage responses but is proposed to be involved in other important and clinically relevant functions. Here, we provide evidence for what we believe are 2 novel and intertwined roles for ATM: the regulation of ribonucleotide reductase (RR), the rate-limiting enzyme in the de novo synthesis of deoxyribonucleoside triphosphates, and control of mitochondrial homeostasis. Ataxia-telangiectasia (A-T) patient fibroblasts, wild-type fibroblasts treated with the ATM inhibitor KU-55933, and cells in which RR is inhibited pharmacologically or by RNA interference (RNAi) each lead to mitochondrial DNA (mtDNA) depletion under normal growth conditions. Disruption of ATM signaling in primary A-T fibroblasts also leads to global dysregulation of the R1, R2, and p53R2 subunits of RR, abrogation of RR-dependent upregulation of mtDNA in response to ionizing radiation, high mitochondrial transcription factor A (mtTFA)/mtDNA ratios, and increased resistance to inhibitors of mitochondrial respiration and translation. Finally, there are reduced expression of the R1 subunit of RR and tissue-specific alterations of mtDNA copy number in ATM null mouse tissues, the latter being recapitulated in tissues from human A-T patients. Based on these results, we propose that disruption of RR and mitochondrial homeostasis contributes to the complex pathology of A-T and that RR genes are candidate disease loci in mtDNA-depletion syndromes.