The role of PARP1 in the DNA damage response and its application in tumor therapy

Targeting the DNA Damage Response for Cancer Therapy

Over the course of long-term evolution, cells have developed intricate defense mechanisms in response to DNA damage; these mechanisms play a pivotal role in maintaining genomic stability. Defects in the DNA damage response pathways can give rise to various diseases, including cancer. The DNA damage response (DDR) system is instrumental in safeguarding genomic stability. The accumulation of DNA damage and the weakening of DDR function both promote the initiation and progression of tumors. Simultaneously, they offer opportunities and targets for cancer therapeutics. This article primarily elucidates the DNA damage repair pathways and the progress made in targeting key proteins within these pathways for cancer treatment. Among them, poly (ADP-ribose) polymerase 1 (PARP1) plays a crucial role in DDR, and inhibitors targeting PARP1 have garnered extensive attention in anticancer research. By delving into the realms of DNA damage and repair, we aspire to explore more precise and effective strategies for cancer therapy and to seek novel avenues for intervention.

A Review of PARP-1 Inhibitors: Assessing Emerging Prospects and Tailoring Therapeutic Strategies

Eukaryotic organisms contain an enzyme family called poly (ADP-ribose) polymerases (PARPs), which is responsible for the poly (ADP-ribosylation) of DNA-binding proteins. PARPs are members of the cell signaling enzyme class. PARP-1, the most common isoform of the PARP family, is responsible for more than 90% of the tasks carried out by the PARP family as a whole. A superfamily consisting of 18 PARPs has been found. In order to synthesize polymers of ADP-ribose (PAR) and nicotinamide, the DNA damage nick monitor PARP-1 requires NAD+ as a substrate. The capability of PARP-1 activation to boost the transcription of proinflammatory genes, its ability to deplete cellular energy pools, which leads to cell malfunction and necrosis, and its involvement as a component in the process of DNA repair are the three consequences of PARP-1 activation that are of particular significance in the process of developing new drugs. As a result, the pharmacological reduction of PARP-1 may result in an increase in the cytotoxicity toward cancer cells.

Phenethyl isothiocyanate and dasatinib combination synergistically reduces hepatocellular carcinoma growth via cell cycle arrest and oxeiptosis

Introduction: Hepatocellular carcinoma (HCC) is the most common type of liver cancer, which is among the most lethal tumours. Combination therapy exploits multiple drugs to target key pathways synergistically to reduce tumour growth. Isothiocyanates have been shown to possess anticancer potential and to complement the anticancer activity of other compounds. This study aimed to investigate the potential of phenethyl isothiocyanate (PEITC) to synergise with dasatinib, improving its anticancer potential in HCC. Methods: MTT, 3D spheroids and clonogenic assays were used to assess the combination anti-tumour effect in vitro, whereas a murine syngeneic model was employed to evaluate the combination efficacy in vivo. DCFDA staining was employed to evaluate the production of reactive oxygen species (ROS), while flow cytometry and Western blot assays were used to elucidate the molecular mechanism of the synergistic activiy. Results: PEITC and dasatinib combination exhibited a synergistic effect in vitro and in vivo. The combination induced DNA damage and oxidative stress through the production of ROS, which led to the formation of a premature CDK1/Cyclin B1 complex associated with induction of mitotic catastrophe. Furthermore, ROS activated oxeiptosis, a caspase-independent form of programmed cell death. Conclusion: PEITC showed to enhance dasatinib action in treating HCC with increased production of ROS that induced cell cycle arrest followed by mitotic catastrophe, and to induce oxeiptosis. These results highlight the role that ITCs may have in cancer therapy as a complement of clinically approved chemotherapeutic drugs.

Expression of NORAD correlates with breast cancer aggressiveness and protects breast cancer cells from chemotherapy

The recently discovered human lncRNA NORAD is induced after DNA damage in a p53-dependent manner. It plays a critical role in the maintenance of genomic stability through interaction with Pumilio proteins, limiting the repression of their target mRNAs. Therefore, NORAD inactivation causes chromosomal instability and aneuploidy, which contributes to the accumulation of genetic abnormalities and tumorigenesis. NORAD has been detected in several types of cancer, including breast cancer, which is the most frequently diagnosed and the second-leading cause of cancer death in women. In the present study, we confirmed upregulated NORAD expression levels in a set of human epithelial breast cancer cell lines (MDA-MB-231, MDA-MB-436, and MDA-MB-468), which belong to the most aggressive subtypes (triple-negative breast cancer). These results are in line with previous data showing that high NORAD expression levels in basal-like tumors were associated with poor prognosis. Here, we demonstrate that NORAD downregulation sensitizes triple-negative breast cancer cells to chemotherapy, through a potential accumulation of genomic aberrations and an impaired capacity to signal DNA damage. These results show that NORAD may represent an unexploited neoadjuvant therapeutic target for chemotherapy-unresponsive breast cancer.

An institutional review of treatment outcomes in extraosseous Ewing's sarcoma- the largest Asian experience

PURPOSE

To analyze the treatment strategies, outcomes and factors impacting these outcomes in extraosseous ewings sarcoma (EOES).

MATERIAL AND METHODS

A search of the hospital database yielded a total of 109 EOES patients registered in last 10 years out of which 25 patients were excluded from analysis due to incomplete medical records. Demographic and clinical characteristics were reported using descriptive statistics. Overall survival (OS) was taken from the time of diagnosis to death. Patients who were alive or lost to follow up were censored from the survival analysis. A total of 12 clinical and treatment related variables were taken into univariate analysis and those showing significance or a trend towards significance were taken up for multivariate analysis. Further a cluster analysis was done in a quest to find a subgroup which would have a better survival outcome as compared to other clusters. SPSS version 23 was used for statistical analysis.

RESULTS

Chest wall (n=26), lower extremity (n=22) and paraspinal area (n=14) were the common sites involved. Localised swelling (n=43) was the most common presenting symptom and the median time to presentation was 2 months. Overall survival of the entire cohort at 5 years was 52 percent. Stage at presentation had a significant impact (P value<0.001) on estimated median OS (localised 70 months versus 36 months in metastatic stage). Cluster analysis showed that, patients with localised stage at presentation, good response to chemotherapy, negative resection margin and no adjuvant RT had a median survival of 69 months.

CONCLUSION

Judicious use of trimodality treatment in EOES yields optimal results and it also adds significantly onto the scarce literature on this subtype of sarcoma.

BMN673 Is a PARP Inhibitor with Unique Radiosensitizing Properties: Mechanisms and Potential in Radiation Therapy

BMN673 is a relatively new PARP inhibitor (PARPi) that exhibits superior efficacy in vitro compared to olaparib and other clinically relevant PARPi. BMN673, similar to most clinical PARPi, inhibits the catalytic activities of PARP-1 and PARP-2 and shows impressive anticancer potential as monotherapy in several pre-clinical and clinical studies. Tumor resistance to PARPi poses a significant challenge in the clinic. Thus, combining PARPi with other treatment modalities, such as radiotherapy (RT), is being actively pursued to overcome such resistance. However, the modest to intermediate radiosensitization exerted by olaparib, rucaparib, and veliparib, limits the rationale and the scope of such combinations. The recently reported strong radiosensitizing potential of BMN673 forecasts a paradigm shift on this front. Evidence accumulates that BMN673 may radiosensitize via unique mechanisms causing profound shifts in the balance among DNA double-strand break (DSB) repair pathways. According to one of the emerging models, BMN673 strongly inhibits classical non-homologous end-joining (c-NHEJ) and increases reciprocally and profoundly DSB end-resection, enhancing error-prone DSB processing that robustly potentiates cell killing. In this review, we outline and summarize the work that helped to formulate this model of BMN673 action on DSB repair, analyze the causes of radiosensitization and discuss its potential as a radiosensitizer in the clinic. Finally, we highlight strategies for combining BMN673 with other inhibitors of DNA damage response for further improvements.

Recent advances in cancer therapy using PARP inhibitors

When DNA repair is inadequate it increases the chances of the genome becoming unstable and it undergoes a malignant mutation. The deficiency of DNA repair PARP proteins may be leveraged for cancer therapy by increasing genomic instability and causing massive DNA damage in cancer cells. DNA repair components are under increased demand in cancer cells because of the continuous replication of DNA. The oncogenic loss of BRCA and an inefficient DNA repair led to cancer cells being dependent on particular DNA repair pathways, like the Poly (ADP-ribose) polymerase pathway. Breast cancer gene 1 and 2 plays a crucial role in DNA repair and genome integrity explaining how BRCA1 and BRCA2 mutations raise the menace of cancer. PARP inhibitors inhibit the base exclusion repair pathway, resulting in the buildup of unrepaired single strand breaks, which cause inflated replication forks in the S phase and subsequently the development of damaging double stranded breaks. Cells having BRCA mutations are unable to repair DNA breaks, leading to apoptosis and eventually death of cancer cells. Numerous indicators, such as a lack of homologous recombination and a high degree of replication pressure, indicate that this therapy will be very effective. Combining PARP inhibitors with chemotherapy, an immune checkpoint inhibitor, and a targeted drug is an effective strategy for combating PARP inhibitors resistance. Several PARP-based combination approaches are in preclinical and clinical development. Various clinical trials are successfully completed and some are undergoing to evaluate the efficacy of these molecules. This review will describe the current views and clinical updates on PARP inhibitors.

Chaperone-mediated autophagy attenuates H2 O2 -induced cardiomyocyte apoptosis by targeting poly (ADP-ribose) polymerase 1 (PARP1) for lysosomal degradation

Poly (ADP-ribose) polymerase 1 (PARP1) is a typical representative of the PARP enzyme family and is mainly related to DNA repair, gene transcription regulation, inflammation, and oxidative stress. Studies have found that PARP1 is involved in the pathophysiological processes of a variety of cardiovascular diseases. Chaperone-mediated autophagy (CMA) is involved in the molecular regulation of various diseases, including cardiovascular diseases, and plays a critical role in maintaining intracellular metabolism balance. However, the link between PARP1 and CMA in cardiomyocytes remains unclear. Therefore, the aims of this study were to investigate whether CMA is involved in PARP1 regulation and to further clarify the specific molecular mechanisms. Earle's balanced salt solution (EBSS)-induced activation of autophagy reduced PARP1 expression, whereas the autophagy lysosomal inhibitor CQ had the opposite effect. Correspondingly, treatment with the autophagy inhibitor 3-methyladenine did not abolish the autophagy-inducing effects of EBSS. Additionally, PARP1 binds to heat shock cognate protein 70 and lysosome-associated membrane protein 2A (LAMP2A). Moreover, adenovirus-mediated LAMP2A overexpression to activate the CMA signaling pathway in cardiomyocytes reduces PARP1 (cleaved) expression and further decreases cardiomyocyte apoptosis caused by oxidative stress. In contrast, downregulation of LAMP2A increased PARP1 (cleaved) expression and the degree of apoptosis. More importantly, we report that appropriate concentrations of H₂ O₂ triggered the nuclear translocation of PARP1, which subsequently promoted the degradation of PARP1 through the CMA pathway. In summary, our data are the first to reveal that CMA targeted PARP1 for lysosomal degradation in cardiomyocytes, which ultimately inhibited apoptosis by promoting the degradation of the PARP1 protein.

Genotoxic Mechanism of Action of TBBPA, TBBPS and Selected Bromophenols in Human Peripheral Blood Mononuclear Cells

Bromophenolic flame retardants (BFRs) are a large group of synthetic substances used in the industry in order to reduce the flammability of synthetic materials used in electrical and electronic devices, textiles, furniture and other everyday products. The presence of BFRs has been documented in the environment, food, drinking water, inhaled dust and the human body. Due to the widespread exposure of the general population to BFRs and insufficient knowledge on their toxic action, including genotoxic potential, we have compared the effect of tetrabromobisphenol A (TBBPA), tetrabromobisphenol S (TBBPS), 2,4,6,-tribromophenol (2,4,6-TBP) and pentabromophenol (PBP) on DNA damage in human peripheral blood mononuclear cells (PBMCs) (playing a crucial role in the immune system) as well as examined underlying mechanism of action of these substances. The cells were incubated for 24 h with studied compounds in the concentrations ranging from 0.01 to 10 µg/mL. The study has shown that examined BFRs induced single and, to a lesser extent, double strand-breaks formation and caused oxidative damage to pyrimidines, and particularly to purines in the incubated cells. PBMCs efficiently repaired the DNA strand-breaks induced by BFRs, but they were unable to remove completely damaged DNA (except cells treated with TBBPS). The greatest changes in the above-mentioned parameters were observed in cells incubated with TBBPA, while the smallest in PBMCs treated with TBBPS. The results have also revealed that tested compounds do not form adducts with DNA in PBMCs, while the observed changes were the most probably induced by indirect DNA-damaging agents, such as ROS and other reactive species.

Novel Targeted Therapeutic Strategies for Ewing Sarcoma

Simple Summary

Ewing sarcoma is an uncommon cancer that arises in mesenchymal tissues and represents the second most widespread malignant bone neoplasm after osteosarcoma in children. Therapy has increased the 5-year survival rate in the last 40 years, although the recurrence rate has remained high. There is an immediate and unmet need for the development of novel Ewing sarcoma therapies. We offer new prospective targets for the therapy of Ewing sarcoma. The EWSR1/FLI1 fusion protein, which is identified in 85–90% of Ewing sarcoma tumors, and its direct targets are given special focus in this study. Experimantal therapy that targets multiple signaling pathways activated during ES progression, alone or in combination with existing regimens, may become the new standard of care for Ewing sarcoma patients, improving patient survival.

Abstract

Ewing sarcoma (ES) is an uncommon cancer that arises in mesenchymal tissues and represents the second most widespread malignant bone neoplasm after osteosarcoma in children. Amplifications in genomic, proteomic, and metabolism are characteristics of sarcoma, and targeting altered cancer cell molecular processes has been proposed as the latest promising strategy to fight cancer. Recent technological advancements have elucidated some of the underlying oncogenic characteristics of Ewing sarcoma. Offering new insights into the physiological basis for this phenomenon, our current review examines the dynamics of ES signaling as it related to both ES and the microenvironment by integrating genomic and proteomic analyses. An extensive survey of the literature was performed to compile the findings. We have also highlighted recent and ongoing studies integrating metabolomics and genomics aimed at better understanding the complex interactions as to how ES adapts to changing biochemical changes within the tumor microenvironment.

Analysis of Adaptive Olaparib Resistance Effects on Cisplatin Sensitivity in Triple Negative Breast Cancer Cells

Poly-(ADP)-ribose polymerase inhibitors (PARPi) and platinum-based drugs are promising therapies for triple negative breast cancers (TNBC) with BRCA1 or BRCA2 loss. PARPi(s) show better efficacies when combined with platinum-based therapy, however, acquisition of PARPi resistance has been linked with co-resistance to platinum-based drugs. Here, we show that TNBCs with constitutively hyperactivated PARP-1 display greater tolerances for the PARPi olaparib and cisplatin, and respond synergistically to olaparib/cisplatin combinations with increased cytotoxicity. Regardless of BRCA1 and PARP-1 activity status, upon gaining olaparib resistance (OlaR), OlaR MDA-MB-468 (BRCA1 wild-type) and SUM1315 (BRCA1 mutant) TNBC cells retain cisplatin sensitivities of their isogenic parental counterparts. OlaR TNBC cells express decreased levels of PARP-1 and Pol η, a translesion-synthesis polymerase important in platinum-induced interstrand crosslink repair. Although native RAD51 recombinase levels are unaffected, anti-RAD51 immunoreactive low molecular weight sbands are exclusively detected in OlaR cells. Despite normal BRCA1, RAD51 foci formation/recruitment to double-strand breaks are impaired in OlaR MDA-MB-468 cells, suggesting homologous-recombination impairment. RNA-seq and pathway analysis of cisplatin-affected genes revealed enrichment of G2/M cell cycle regulation and DNA repair pathways in parental and OlaR MDA-MB-468 cells whereas parental and OlaR SUM1315 cells showed enrichment of inflammatory stress response pathways associated with TNFR1/2, TWEAK and IL-17 signaling. These data show that TNBC models with wild type versus mutant BRCA1 exhibit differences in CDDP-induced cellular response pathways, however, the CDDP-induced signaling responses remain stable across the isogenic models of OlaR from the same lineage. These data also show that adaptive OlaR does not automatically promote cisplatin resistance, implicating the potential benefit of platinum-based therapy for OlaR TNBCs.

Bioinformatic Analysis of the Nicotinamide Binding Site in Poly(ADP-Ribose) Polymerase Family Proteins

Simple Summary

The PARP family consists of 17 proteins, and some of them are responsible for cancer cells’ viability. Much attention is therefore given to the search for chemical compounds with the ability to suppress distinct PARP family members (for example, PARP-5a and 5b). Here, we present the results of a family-wide bioinformatic analysis of an important functional region in the PARP structure and describe factors that can guide the design of highly selective compounds.

Abstract

The PARP family consists of 17 members with diverse functions, including those related to cancer cells’ viability. Several PARP inhibitors are of great interest as innovative anticancer drugs, but they have low selectivity towards distinct PARP family members and exert serious adverse effects. We describe a family-wide study of the nicotinamide (NA) binding site, an important functional region in the PARP structure, using comparative bioinformatic analysis and molecular modeling. Mutations in the NA site and D-loop mobility around the NA site were identified as factors that can guide the design of selective PARP inhibitors. Our findings are of particular importance for the development of novel tankyrase (PARPs 5a and 5b) inhibitors for cancer therapy.

Genotoxic risk assessment and mechanism of DNA damage induced by phthalates and their metabolites in human peripheral blood mononuclear cells

The human genome is persistently exposed to damage caused by xenobiotics, therefore the assessment of genotoxicity of substances having a direct contact with humans is of importance. Phthalates are commonly used in industrial applications. Widespread exposure to phthalates has been evidenced by their presence in human body fluids. We have assessed the genotoxic potential of selected phthalates and mechanism of their action in human peripheral blood mononuclear cells (PBMCs). Studied cells were incubated with di-n-butyl phthalate (DBP), butylbenzyl phthalate (BBP) and their metabolites: mono-n-butylphthalate (MBP), mono-benzylphthalate (MBzP) in the concentrations range of 0.1-10 µg/mL for 24 h. Analyzed compounds induced DNA single and double strand-breaks (DBP and BBP ≥ 0.5 µg/mL, MBP and MBzP ≥ 1 µg/mL) and more strongly oxidized purines than pyrimidines. None of the compounds examined was capable of creating adducts with DNA. All studied phthalates caused an increase of total ROS level, while hydroxyl radical was generated mostly by DBP and BBP. PBMCs exposed to DBP and BBP could not completely repair DNA strand-breaks during 120 min of postincubation, in opposite to damage caused by their metabolites, MBP and MBzP. We have concluded that parent phthalates: DBP and BBP caused more pronounced DNA damage compared to their metabolites.

New insight into the significance of KLF4 PARylation in genome stability, carcinogenesis, and therapy

KLF4 plays a critical role in determining cell fate responding to various stresses or oncogenic signaling. Here, we demonstrated that KLF4 is tightly regulated by poly(ADP‐ribosyl)ation (PARylation). We revealed the subcellular compartmentation for KLF4 is orchestrated by PARP1‐mediated PARylation. We identified that PARylation of KLF4 is critical to govern KLF4 transcriptional activity through recruiting KLF4 from soluble nucleus to the chromatin. We mapped molecular motifs on KLF4 and PARP1 that facilitate their interaction and unveiled the pivotal role of the PBZ domain YYR motif (Y430, Y451 and R452) on KLF4 in enabling PARP1‐mediated PARylation of KLF4. Disruption of KLF4 PARylation results in failure in DNA damage response. Depletion of KLF4 by RNA interference or interference with PARP1 function by KLF4^YYR/AAA (a PARylation‐deficient mutant) significantly sensitizes breast cancer cells to PARP inhibitors. We further demonstrated the role of KLF4 in modulating homologous recombination through regulating BRCA1 transcription. Our work points to the synergism between KLF4 and PARP1 in tumorigenesis and cancer therapy, which provides a potential new therapeutic strategy for killing BRCA1‐proficient triple‐negative breast cancer cells.

Morin Hydrate Sensitizes Hepatoma Cells and Xenograft Tumor towards Cisplatin by Downregulating PARP-1-HMGB1 Mediated Autophagy

The cross-talk between apoptosis and autophagy influences anticancer drug sensitivity and cellular death in various cancer cell lines. However, the fundamental mechanisms behind this phenomenon are still unidentified. We demonstrated anti-cancerous role of cisplatin (CP) and morin hydrate (Mh) as an individual and/or in combination (CP-Mh) in hepatoma cells and tumor model. Exposure of CP resulted in the production of intracellular reactive oxygen species (ROS)-mediated cellular vacuolization, expansion of mitochondria membrane and activation of endoplasmic reticulum (ER)-stress. Consequently, Cyt c translocation led to the increase of Bax/Bcl-2 ratio, which simultaneously triggered caspase-mediated cellular apoptosis. In addition, CP-induced PARP-1 activation led to ADP-ribosylation of HMGB1, which consequently developed autophagy as evident by the LC3I/II ratio. Chemically-induced inhibition of autophagy marked by increased cell death signified a protective role of autophagy against CP treatment. CP-Mh abrogates the PARP-1 expression and significantly reduced HMGB1-cytoplasmic translocation with subsequent inhibition of the HMGB1-Beclin1 complex formation. In the absence of PARP-1, a reduced HMGB1 mediated autophagy was observed followed by induced caspase-dependent apoptosis. To confirm the role of PARP-1-HMGB1 signaling in autophagy, we used the PARP-1 inhibitor, 4-amino-1,8-naphthalimide (ANI), HMGB1 inhibitor, ethyl pyruvate (EP), autophagy inhibitors, 3-methyl adenine (3-MA) and bafilomycin (baf) and small interfering RNAs (siRNA) to target Atg5 in combination of CP and Mh. Exposure to these inhibitors enhanced the sensitivity of HepG2 cells to CP. Collectively, our findings indicate that CP-Mh in combination served as a prominent regulator of autophagy and significant inducer of apoptosis that maintains a homeostatic balance towards HepG2 cells and the subcutaneous tumor model.

Molecular Insights Into Di(2-Picolyl) Amine-Induced Cytotoxicity and Apoptosis in Human Kidney (HEK293) Cells

Di(2-picolyl) amine (DPA) is a pyridine derivative known to chelate metal ions and thus has potential anticancer properties; however, its effect on normal cells remains unchartered necessitating further research. This study, therefore, investigated the mechanistic effects of DPA-induced cytotoxicity and apoptosis in the HEK293 cell line. Methods required that an half the maximum inhibition concentration (IC₅₀) was derived using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. Analyses aimed to assess oxidative stress, membrane damage, and DNA fragmentation by means of biochemical assays were performed. Luminometry analysis was carried out to understand the mechanism of apoptosis induction by determining the levels of adenosine triphosphate (ATP) and the activities of caspase-8, -9, and -3/7. Western blotting was used to ascertain the expression of apoptotic and stress-related proteins. An IC₅₀ of 1,079 µM DPA was obtained. Antioxidant effect correlated with a minimum increase in reactive oxygen species induced lipid peroxidation. The increase in initiator caspase-8 and -9 and executioner caspase-3/7 activities by DPA-induced apoptosis albeit prompting a decline in the levels of ATP. Furthermore, DPA brought about the following consequences on HEK293 cells: markedly elevated tail lengths of the comets, poly (ADP-ribose) polymerase 1 cleavage, and apoptotic body formation observed in the late stages. The cytotoxic effects of DPA in HEK293 cells may be mediated by induction of apoptosis via the caspase-dependent mechanism.

Functional characterization of full-length BARD1 strengthens its role as a tumor suppressor in neuroblastoma

BARD1 is associated with the development of high-risk neuroblastoma patients. Particularly, the expression of full length (FL) isoform, FL BARD1, correlates to high-risk neuroblastoma development and its inhibition is sufficient to induce neuroblastoma cells towards a worst phenotype. Here we have investigated the mechanisms of FL BARD1 in neuroblastoma cell lines depleted for FL BARD1 expression. We have shown that FL BARD1 expression protects the cells from spontaneous DNA damage and from damage accumulated after irradiation. We demonstrated a role for FL BARD1 as tumor suppressor to prevent unscheduled mitotic entry of DNA damaged cells and to lead to death cells that have bypassed cell cycle checkpoints. FL BARD1-depleted cells that have survived to checkpoints acquire features of aggressiveness. Overall, our results show that FL BARD1 may defend cells against cancer and prevent malignant transformation of cells.

Selective small molecule PARG inhibitor causes replication fork stalling and cancer cell death

Poly(ADP-ribose)ylation (PARylation) by PAR polymerase 1 (PARP1) and PARylation removal by poly(ADP-ribose) glycohydrolase (PARG) critically regulate DNA damage responses; yet, conflicting reports obscure PARG biology and its impact on cancer cell resistance to PARP1 inhibitors. Here, we found that PARG expression is upregulated in many cancers. We employed chemical library screening to identify and optimize methylxanthine derivatives as selective bioavailable PARG inhibitors. Multiple crystal structures reveal how substituent positions on the methylxanthine core dictate binding modes and inducible-complementarity with a PARG-specific tyrosine clasp and arginine switch, supporting inhibitor specificity and a competitive inhibition mechanism. Cell-based assays show selective PARG inhibition and PARP1 hyperPARylation. Moreover, our PARG inhibitor sensitizes cells to radiation-induced DNA damage, suppresses replication fork progression and impedes cancer cell survival. In PARP inhibitor-resistant A172 glioblastoma cells, our PARG inhibitor shows comparable killing to Nedaplatin, providing further proof-of-concept that selectively inhibiting PARG can impair cancer cell survival.

The Role of PARPs in Inflammation-and Metabolic-Related Diseases: Molecular Mechanisms and Beyond

Poly(ADP-ribosyl)ation (PARylation) is an essential post-translational modification catalyzed by poly(ADP-ribose) polymerase (PARP) enzymes. Poly(ADP-ribose) polymerase 1 (PARP1) is a well-characterized member of the PARP family. PARP1 plays a crucial role in multiple biological processes and PARP1 activation contributes to the development of various inflammatory and malignant disorders, including lung inflammatory disorders, cardiovascular disease, ovarian cancer, breast cancer, and diabetes. In this review, we will focus on the role and molecular mechanisms of PARPs enzymes in inflammation- and metabolic-related diseases. Specifically, we discuss the molecular mechanisms and signaling pathways that PARP1 is associated with in the regulation of pathogenesis. Recently, increasing evidence suggests that PARP inhibition is a promising strategy for intervention of some diseases. Thus, our in-depth understanding of the mechanism of how PARPs are activated and how their signaling downstream effecters can provide more potential therapeutic targets for the treatment of the related diseases in the future is crucial.

Finding solutions for agricultural wastes: antioxidant and antitumor properties of pomegranate Akko peel extracts and β-glucan recovery

Pomegranate peels are the major by-products of the processing of pomegranate juice. Scientific research studies have shown that pomegranate peels are an extraordinary source of bioactive compounds, most of which can be converted into value added products. From this point of view, the present study was carried out with the aim of providing a solid basis for the use of whole pomegranate peels (Akko variety) as a source of nutraceutical compounds, such as β-glucans. Moreover, acetone (S1) and methanolic (S2) extracts, obtained in the preliminary stage of the β-glucan isolation procedure, have been tested for their antioxidant and antitumor activities. The total phenolic content and phenolic profile of S1 and S2 were determined. S1 and S2 exhibited a significant DPPH scavenging activity, with an IC50 value for S1 1.5-fold lower than that for the standard Trolox. Both S1 and S2 extracts possess in vitro ROS scavenging activity toward 3T3-L1 murine fibroblasts and Hek-293 human embryonic kidney epithelial cells and antiproliferative activity towards human breast MCF-7 and uterine HeLa cancer cells.

Poly(ADP-ribose) polymerase 1 accelerates vascular calcification by upregulating Runx2

Vascular calcification is highly prevalent in end-stage renal diseases and is predictive of cardiovascular events and mortality. Poly(ADP-ribose) polymerase 1 (PARP1) inhibition or deletion is vasoprotective in several disease models. Here we show that PARP activity is increased in radial artery samples from patients with chronic renal failure, in arteries from uraemic rats, and in calcified vascular smooth muscle cells (VSMCs) in vitro. PARP1 deficiency blocks, whereas PARP1 overexpression exacerbates, the transdifferentiation of VSMCs from a contractile to an osteogenic phenotype, the expression of mineralization-regulating proteins, and calcium deposition. PARP1 promotes Runx2 expression, and Runx2 deficiency offsets the pro-calcifying effects of PARP1. Activated PARP1 suppresses miRNA-204 expression via the IL-6/STAT3 pathway and thus relieves the repression of its target, Runx2, resulting in increased Runx2 protein. Together, these results suggest that PARP1 counteracts vascular calcification and that therapeutic agents that influence PARP1 activity may be of benefit to treat vascular calcification.

Vascular calcification is a hallmark of end stage renal disease. Here, Cheng et al. show that poly(ADP-ribose) polymerase (PARP) activity is increased in calcified arteries in patients and uremic rats, and that PARP1 promotes vascular calcification by suppressing miR-204 expression via IL-6/STAT3 signaling, thus relieving repression of the osteogenic regulator Runx2.

Polybrominated Diphenyl Ethers Quinone Induced Parthanatos-like Cell Death through a Reactive Oxygen Species-Associated Poly(ADP-ribose) Polymerase 1 Signaling

Polybrominated diphenyl ethers (PBDEs) are emerging organic environmental pollutants, which were accused of various toxic effects. Here, we studied the role of a potential PBDEs quinone metabolite, PBDEQ, on cytotoxicity, oxidative DNA damage, and the alterations of signal cascade in HeLa cells. PBDEQ exposure leads to reactive oxygen species (ROS) accumulation, mitochondrial membrane potential (MMP) loss, lactate dehydrogenase (LDH) release, increasing terminal transferase-mediated dUTP-biotin nick end labeling (TUNEL) positive foci, and the elevation of apoptosis rate. Furthermore, we showed PBDEQ exposure result in increased DNA migration, micronucleus frequency, and the promotion of 8-OHdG and phosphorylation of histone H2AX (γ-H2AX) levels. Mechanism study indicated that PBDEQ caused poly(ADP-ribose) polymerase 1 (PARP-1) activation and apoptosis-inducing factor (AIF) nuclear translocation. All together, these results confirmed the occurrence of parthanatos-like cell death upon PBDEQ exposure.

Biallelic Mutations in ADPRHL2, Encoding ADP-Ribosylhydrolase 3, Lead to a Degenerative Pediatric Stress-Induced Epileptic Ataxia Syndrome

ADP-ribosylation, the addition of poly-ADP ribose (PAR) onto proteins, is a response signal to cellular challenges, such as excitotoxicity or oxidative stress. This process is catalyzed by a group of enzymes referred to as poly(ADP-ribose) polymerases (PARPs). Because the accumulation of proteins with this modification results in cell death, its negative regulation restores cellular homeostasis: a process mediated by poly-ADP ribose glycohydrolases (PARGs) and ADP-ribosylhydrolase proteins (ARHs). Using linkage analysis and exome or genome sequencing, we identified recessive inactivating mutations in ADPRHL2 in six families. Affected individuals exhibited a pediatric-onset neurodegenerative disorder with progressive brain atrophy, developmental regression, and seizures in association with periods of stress, such as infections. Loss of the Drosophila paralog Parg showed lethality in response to oxidative challenge that was rescued by human ADPRHL2, suggesting functional conservation. Pharmacological inhibition of PARP also rescued the phenotype, suggesting the possibility of postnatal treatment for this genetic condition.

The CHK1 inhibitor SRA737 synergizes with PARP1 inhibitors to kill carcinoma cells

Inhibitors of PARP1 are approved therapeutic agents in ovarian carcinomas. We determined whether the novel clinically relevant CHK1 inhibitor SRA737 interacted with PARP1 inhibitors to kill carcinoma cells. In multiple mammary and ovarian cancer lines SRA737 synergized with the PARP1 inhibitors olaparib and niraparib to cause cell death. The [SRA737 + niraparib] drug combination activated an ATM-AMPK-ULK1-mTOR pathway which resulted in the formation of autophagosomes, temporally followed by autolysosome formation. Phosphorylation of ULK1 S317 was essential for kinase activation against ATG13. The drug combination elevated eIF2α phosphorylation which was causal at increasing Beclin1 and ATG5 expression, reducing MCL-1 and BCL-XL levels, and causing CD95 activation. Knock down of CD95, eIF2α, ATM, AMPKα, ULK1, Beclin1 or ATG5 reduced drug combination lethality. Blockade of either caspase 9 function or that of AIF each partially prevented cell death. Expression of activated mTOR or of c-FLIP-s or of BCL-XL reduced cell killing. In vivo, SRA737 and niraparib interacted in an additive fashion to suppress the growth of mammary tumors. Multiplex analyses revealed that drug combination treated tumors had reduced their plasma levels of sERBB1, sERBB2, sVEGFR1, sVEGFR2, sIL-6R, HGF, PDGFAB/BB and CXCL16 and enhanced the levels of CCL26, IL-8 and MIF. Surviving tumors had activated ERK1/2 and AKT. This finding argues that IL-8/ERK/AKT signaling may be an evolutionary survival response to [SRA737 + niraparib].

PARP-1 Overexpression as an Independent Prognostic Factor in Adult Non-M3 Acute Myeloid Leukemia

AIMS

Poly (ADP-ribose) polymerase-1 (PARP-1) plays an important role in the repair of damaged DNA and has prognostic significance in a variety of human malignancies. However, little is known about its expression levels and clinical implication in patients with acute myeloid leukemia (AML).

MATERIALS AND METHODS

Quantitative reverse transcription-polymerase chain reaction was done to evaluate PARP-1 expression levels in the bone marrow of 65 patients with non-M3 AML and 54 healthy counterparts. The correlation of PARP-1 expression with clinicopathological features of non-M3 AML patients was also analyzed.

RESULTS

Non-M3 AML patients have higher PARP-1 expression than the healthy controls (p < 0.01). Patients with adverse cytogenetic risk have higher PARP-1 expression than other cytogenetic risk groups (p = 0.004). The PARP-1 median expression level divided AML patients into PARP-1 low-expressed and PARP-1 high-expressed groups. High expression levels of PARP-1 were associated with worse overall survival (OS) (p = 0.01) and relapse-free survival (RFS) (p = 0.005). Moreover, multivariate analysis revealed that high PARP-1 expression was an independent risk factor for both OS and RFS.

CONCLUSIONS

Our results suggest that PARP-1 overexpression may define an important risk factor in non-M3 AML patients and PARP-1 is a potential therapeutic target for AML treatment.

OGG1-initiated base excision repair exacerbates oxidative stress-induced parthanatos

Oxidative stress-induced DNA damage has been well acknowledged as a major cause leading to cell death, which is etiologically linked to ischemic injury and degenerative alterations. The most common oxidation product of DNA is base lesion 8-oxo-7,8-dihydroguanine (8-oxoG), which is repaired by 8-oxoG glycosylase1 (OGG1)-initiated baseexcision repair (BER) pathway (OGG1-BER); however, the role of OGG1-BER in oxidative stress-induced cell death is poorly investigated. DNA strand breaks and apurinic/apyrimidinic (AP) sites are effective substrates to activate DNA damage sensor poly(ADP-ribose) polymerase 1 (PARP1). Overactivation of PARP1 is associated with apoptosis-inducing factor (AIF)-mediated and caspase-independent cell death (parthanatos). We hypothesized that after an excessive oxidative insult, OGG1-BER-generated strand breaks result in hyperactivation of PARP1 and consequently cell death. To test, wild type, knockout, siRNA-depleted MEFs and neuroblastoma cells, or those expressing repair-deficient OGG1 mutants were oxidatively stressed and the role of OGG1 was examined. Results showed that OGG1-BER further increases the levels of ROS-induced DNA damage by generating repair intermediates, leading to PARP1 overactivation and cell death. Cells lacking or expressing repair-deficient OGG1 showed lower levels of DNA strand lesions, PARP1 activation, and nuclear translocation of apoptosis-inducing factor, resulting in the increased resistance to ROS-induced parthanatos. These results suggested that OGG1 guards genome integrity through either lesion repair or elimination of cells with malignant potential, to maintain the homeostasis of the host, which might depend on the magnitude of guanine oxidation.

NLRX1 Modulates Immunometabolic Mechanisms Controlling the Host-Gut Microbiota Interactions during Inflammatory Bowel Disease

Interactions among the gut microbiome, dysregulated immune responses, and genetic factors contribute to the pathogenesis of inflammatory bowel disease (IBD). Nlrx1^−/− mice have exacerbated disease severity, colonic lesions, and increased inflammatory markers. Global transcriptomic analyses demonstrate enhanced mucosal antimicrobial defense response, chemokine and cytokine expression, and epithelial cell metabolism in colitic Nlrx1^−/− mice compared to wild-type (WT) mice. Cell-specificity studies using cre-lox mice demonstrate that the loss of NLRX1 in intestinal epithelial cells (IEC) recapitulate the increased sensitivity to DSS colitis observed in whole body Nlrx1^−/− mice. Further, organoid cultures of Nlrx1^−/− and WT epithelial cells confirm the altered patterns of proliferation, amino acid metabolism, and tight junction expression. These differences in IEC behavior can impact the composition of the microbiome. Microbiome analyses demonstrate that colitogenic bacterial taxa such as Veillonella and Clostridiales are increased in abundance in Nlrx1^−/− mice and in WT mice co-housed with Nlrx1^−/− mice. The transfer of an Nlrx1^−/−-associated gut microbiome through co-housing worsens disease in WT mice confirming the contributions of the microbiome to the Nlrx1^−/− phenotype. To validate NLRX1 effects on IEC metabolism mediate gut–microbiome interactions, restoration of WT glutamine metabolic profiles through either exogenous glutamine supplementation or administration of 6-diazo-5-oxo-l-norleucine abrogates differences in inflammation, microbiome, and overall disease severity in Nlrx1^−/− mice. The influence NLRX1 deficiency on SIRT1-mediated effects is identified to be an upstream controller of the Nlrx1^−/− phenotype in intestinal epithelial cell function and metabolism. The altered IEC function and metabolisms leads to changes in barrier permeability and microbiome interactions, in turn, promoting greater translocation and inflammation and resulting in an increased disease severity. In conclusion, NLRX1 is an immunoregulatory molecule and a candidate modulator of the interplay between mucosal inflammation, metabolism, and the gut microbiome during IBD.

RBM45 competes with HDAC1 for binding to FUS in response to DNA damage

DNA damage response (DDR) is essential for genome stability and human health. Recently, several RNA binding proteins (RBPs), including fused-in-sarcoma (FUS), have been found unexpectedly to modulate this process. The role of FUS in DDR is closely linked to the pathogenesis of amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease that affects nerve cells in the brain and the spinal cord. Given that RBM45 is also an ALS-associated RBP, we wondered whether RBM45 plays any function during this process. Here, we report that RBM45 can be recruited to laser microirradiation-induced DNA damage sites in a PAR- and FUS-dependent manner, but in a RNA-independent fashion. Depletion of RBM45 leads to abnormal DDR signaling and decreased efficiency in DNA double-stranded break repair. Interestingly, RBM45 is found to compete with histone deacetylase 1 (HDAC1) for binding to FUS, thereby regulating the recruitment of HDAC1 to DNA damage sites. A common familial ALS-associated FUS mutation (FUS-R521C) is revealed to prefer to cooperate with RBM45 than HDAC1. Our findings suggest that RBM45 is a key regulator in FUS-related DDR signaling whose dysfunction may contribute to the pathogenesis of ALS.

Boosting the effects of hyperthermia-based anticancer treatments by HSP90 inhibition

Hyperthermia - application of supra-physiological temperatures to cells, tissues or organs - is a pleiotropic treatment that affects most aspects of cellular metabolism, but its effects on DNA are of special interest in the context of cancer research and treatment. Hyperthermia inhibits repair of various DNA lesions, including double-strand breaks (DSBs), making it a powerful radio- and chemosensitizer, with proven clinical efficacy in therapy of various types of cancer, including tumors of head and neck, bladder, breast and cervix. Among the challenges for hyperthermia-based therapies are the transient character of its effects, the technical difficulties in maintaining uniformly elevated tumor temperature and the acquisition of thermotolerance. Approaches to reduce or eliminate these challenges could simplify the application of hyperthermia, boost its efficacy and improve treatment outcomes. Here we show that a single, short treatment with a relatively low dose of HSP90 inhibitor Ganetespib potentiates cytotoxic as well as radio- and chemosensitizing effects of hyperthermia and reduces thermotolerance in cervix cancer cell lines. Ganetespib alone, applied at this low dose, has virtually no effect on survival of non-heated cells. Our results thus suggest that HSP90 inhibition can be a safe, simple and efficient approach to improving hyperthermia treatment efficacy and reducing thermotolerance, paving the way for in vivo studies.

Discovery of potent 2,4-difluoro-linker poly(ADP-ribose) polymerase 1 inhibitors with enhanced water solubility and in vivo anticancer efficacy

Poly (ADP-ribose) polymerase 1 (PARP1) is overexpressed in a variety of cancers, especially in breast and ovarian cancers; tumor cells that are deficient in breast cancer gene 1/2 (BRCA1/2) are highly sensitive to PARP1 inhibition. In this study, we identified a series of 2,4-difluorophenyl-linker analogs (15-55) derived from olaparib as novel PARP1 inhibitors. Four potent analogs 17, 43, 47, and 50 (IC₅₀=2.2-4.4 nmol/L) effectively inhibited the proliferation of Chinese hamster lung fibroblast V-C8 cells (IC₅₀=3.2-37.6 nmol/L) in vitro, and showed specificity toward BRCA-deficient cells (SI=40-510). The corresponding hydrochloride salts 56 and 57 (based on 43 and 47) were highly water soluble in pH=1.0 buffered salt solutions (1628.2 μg/mL, 2652.5 μg/mL). In a BRCA1-mutated xenograft model, oral administration of compound 56 (30 mg·kg^-1·d^-1, for 21 d) exhibited more prominent tumor growth inhibition (96.6%) compared with the same dose of olaparib (56.3%); in a BRCA2-mutated xenograft model, oral administration of analog 43 (10 mg·kg^-1·d^-1, for 28 d) significantly inhibited tumor growth (69.0%) and had no negative effects on the body weights. Additionally, compound 56 exhibited good oral bioavailability (F=32.2%), similar to that of olaparib (F=45.4%). Furthermore, the free base 43 of the hydrochloride salt 56 exhibited minimal hERG inhibition activity (IC₅₀=6.64 μmol/L). Collectively, these data demonstrate that compound 56 may be an excellent drug candidate for the treatment of cancer, particularly BRCA-deficient tumors.

PARPi potentiates with current conventional therapy in MLL leukemia

Acute myeloid leukemias driven by MLL fusion proteins are commonly associated with poor prognosis and refractory treatment. Here, we provide evidence that olaparib can potentiate sensitivity of MLL leukemia cells to conventional chemotherapy and DNMT inhibitors offering new potential therapeutic strategies for MLL rearranged leukemias Using the primary mouse leukemia cells and human MLL-AF9 leukemic cell line, we demonstrate that treatment of MLL-AF9 leukemic cells with DNMT inhibitors or chemotherapies in combination with olaparib results in significant reduction in colony formation or cell growth while the single agent treatments had little impacts. Combining olaparib with DNMT inhibitor induce cell cycle block and apoptosis. Furthermore, we observe a significant increase in proportion of cells with >10 γH2AX DNA damage foci and double stranded breaks when treated with DNMT inhibitors or chemotherapy agents in combination with olaparib, thus providing possible mechanistic insights for the synergism.

Matrix Screen Identifies Synergistic Combination of PARP Inhibitors and Nicotinamide Phosphoribosyltransferase (NAMPT) Inhibitors in Ewing Sarcoma

Purpose: Although many cancers are showing remarkable responses to targeted therapies, pediatric sarcomas, including Ewing sarcoma, remain recalcitrant. To broaden the therapeutic landscape, we explored the in vitro response of Ewing sarcoma cell lines against a large collection of investigational and approved drugs to identify candidate combinations.Experimental Design: Drugs displaying activity as single agents were evaluated in combinatorial (matrix) format to identify highly active, synergistic drug combinations, and combinations were subsequently validated in multiple cell lines using various agents from each class. Comprehensive metabolomic and proteomic profiling was performed to better understand the mechanism underlying the synergy. Xenograft experiments were performed to determine efficacy and in vivo mechanism.Results: Several promising candidates emerged, including the combination of small-molecule PARP and nicotinamide phosphoribosyltransferase (NAMPT) inhibitors, a rational combination as NAMPTis block the rate-limiting enzyme in the production of nicotinamide adenine dinucleotide (NAD⁺), a necessary substrate of PARP. Mechanistic drivers of the synergistic cell killing phenotype of these combined drugs included depletion of NMN and NAD⁺, diminished PAR activity, increased DNA damage, and apoptosis. Combination PARPis and NAMPTis in vivo resulted in tumor regression, delayed disease progression, and increased survival.Conclusions: These studies highlight the potential of these drugs as a possible therapeutic option in treating patients with Ewing sarcoma. Clin Cancer Res; 23(23); 7301-11. ©2017 AACR.

Synthetic lethality in malignant pleural mesothelioma with PARP1 inhibition

Malignant pleural mesotheliomas (MPM) are most often surgically unresectable, and they respond poorly to current chemotherapy and radiation therapy. Between 23 and 64% of malignant pleural mesothelioma have somatic inactivating mutations in the BAP1 gene. BAP1 is a homologous recombination (HR) DNA repair component found in the BRCA1/BARD1 complex. Similar to BRCA1/2 deficient cancers, mutation in the BAP1 gene leads to a deficient HR pathway and increases the reliance on other DNA repair pathways. We hypothesized that BAP1-mutant MPM would require PARP1 for survival, similar to the BRCA1/2 mutant breast and ovarian cancers. Therefore, we used the clinical PARP1 inhibitors niraparib and olaparib to assess whether they could induce synthetic lethality in MPM. Surprisingly, we found that all MPM cell lines examined, regardless of BAP1 status, were addicted to PARP1-mediated DNA repair for survival. We found that niraparib and olaparib exposure markedly decreased clonal survival in multiple MPM cell lines, with and without BAP1 mutations. This clonal cell death may be due to the extensive replication fork collapse and genomic instability that PARP1 inhibition induces in MPM cells. The requirement of MPM cells for PARP1 suggests that they may generally arise from defects in HR DNA repair. More importantly, these data demonstrate that the PARP1 inhibitors could be effective in the treatment of MPM, for which little effective therapy exists.

miR-342 overexpression results in a synthetic lethal phenotype in BRCA1-mutant HCC1937 breast cancer cells

Expression of miR-342 has been strongly correlated with estrogen receptor (ER) status in breast cancer, where it is highest in ER-positive and lowest in triple-negative tumors. We investigated the effects of miR-342 transfection in the triple-negative breast cancer cell lines MDA-MB-231 and HCC1937, the latter carrying a germ-line BRCA1 mutation. Reconstitution of miR-342 led to caspase-dependent induction of apoptosis only in HCC1937 cells, while overexpression of wild-type BRCA1 in HCC1937 cells counteracted miR-342-mediated induction of apoptosis, suggesting that miR-342 overexpression and the lack of functional BRCA1 result in a synthetic lethal phenotype. Moreover, siRNA-mediated depletion of BRCA1 in MDA-MB-231 cells expressing the wild-type protein led to apoptosis upon transfection with miR-342. Using an in silico approach and a luciferase reporter system, we identified and functionally validated the Baculoviral IAP repeat-containing 6 gene (BIRC6), which encodes the anti-apoptotic factor Apollon/BRUCE, as a target of miR-342. In our model, BIRC6 likely acts as a determinant of the miRNA-dependent induction of apoptosis in BRCA1-mutant HCC1937 cells. Together, our findings suggest a tumor-suppressive function of miR-342 that could be exploited in the treatment of a subset of BRCA1-mutant hereditary breast cancers.

MiR-335 regulates the chemo-radioresistance of small cell lung cancer cells by targeting PARP-1

The role of miR-335 in the regulation of chemosensitivity and radiosensitivity of small cell lung cancer (SCLC) was investigated. miR-335 was significantly downregulated in multi-drug-resistant SCLC H69AR and H446DDP cells compared with parental cells as detected by qRT-PCR. Then, we demonstrated the negative correlation between miR-335 expression and the chemo-radiosensitivity of SCLC cells, including cell proliferation, cell clonality and cell apoptosis. In addition, miR-335 overexpression inhibited cell migration in vitro and tumor growth in vivo, whereas inhibition of miR-335 promoted cell migration and tumor growth. The underlying mechanism was further studied. Poly [ADP-ribose] polymerase 1 (PARP-1) was identified as a direct target gene of miR-335 in SCLC by bioinformatics analysis and validated via luciferase reporter assay. Overexpression of miR-335 decreased the expression of PARP-1 mRNA and protein, and NF-κB protein levels were correspondingly downregulated, thus regulating the chemo-radiosensitivity of SCLC. Taken together, these findings indicate that miR-335 may serve as a critical regulator of chemo-radiotherapy resistance in SCLC and a new potential therapeutic target.

5-Fluorouracil-induced RNA stress engages a TRAIL-DISC-dependent apoptosis axis facilitated by p53

Despite recent advances in targeted therapeutics, administration of 5-fluorouracil (5-FU) remains a common clinical strategy for post-surgical treatment of solid tumors. Although it has been proposed that RNA metabolism is disturbed by 5-FU treatment, the key cytotoxic response is believed to be enzymatic inhibition of thymidylate synthase resulting in nucleotide pool disproportions. An operating p53 tumor suppressor signaling network is in many cases essential for the efficiency of chemotherapy, and malfunctions within this system remain a clinical obstacle. Since the fate of chemotherapy-insensitive tumor cells is rarely described, we performed a comparative analysis of 5-FU toxicity in p53-deficient cells and conclude that p53 acts as a facilitator rather than a gatekeeper of cell death. Although p53 can act as a regulator of several cellular stress responses, no rerouting of cell death mode was observed in absence of the tumor suppressor. Thus, the final death outcome of 5-FU-treated p53^−/− cells is demonstrated to be caspase-dependent, but due to a slow pace, accumulation of mitochondrial reactive oxygen species contributes to necrotic characteristics. The oligomerization status of the p53 target gene DR5 is determined as a significant limiting factor for the initiation of caspase activity in an intracellular TRAIL-dependent manner. Using several experimental approaches, we further conclude that RNA- rather than DNA-related stress follows by caspase activation irrespectively of p53 status. A distinct 5-FU-induced stress mechanism is thereby functionally connected to a successive and discrete cell death signaling pathway. Finally, we provide evidence that silencing of PARP-1 function may be an approach to specifically target p53-deficient cells in 5-FU combinatorial treatment strategies. Together, our results disclose details of impaired cell death signaling engaged as a consequence of 5-FU chemotherapy. Obtained data will contribute to the comprehension of factors restraining 5-FU efficiency, and by excluding DNA as the main stress target in some cell types they propose alternatives to currently used and suggested synergistic treatment regimens.

Novel poly (ADP-ribose) polymerase inhibitor, AZD2281, enhances radiosensitivity of both normoxic and hypoxic esophageal squamous cancer cells

Radiotherapy plays an important role in the treatment of esophageal squamous cell carcinoma (ESCC). However, the outcome of radiotherapy in ESCC remains unsatisfactory because esophageal squamous cancer cells, particularly those under hypoxic condition, exhibit radioresistance. The aim of this study was to determine whether or not AZD2281, a potent poly (ADP-ribose) polymerase (PARP) inhibitor, could enhance the radiation sensitivity of two ESCC cell lines, namely ECA109 and TE13. The radiosensitizing effect of AZD2281 was evaluated on the basis of cell death, clonogenic survival and tumor xenograft progression. AZD2281 alone was slightly toxic to ESCC cell lines. Apoptosis was increased and clonogenic survival was decreased in both cell lines when AZD2281 was combined with ionizing radiation (IR) under normoxic condition. AZD2281 enhanced IR-induced apoptosis to a more significant level under chronic hypoxic condition (0.2% O(2), 48 hour) than under normoxic condition. AZD2281 also slightly enhanced clonogenic cell death under chronic hypoxic condition compared with that under normoxic condition. This result could be associated with increased radiation-induced DNA double-strand breaks (DSB), decreased DSB repair and increased apoptosis of ESCC cells. Furthermore, homologous recombination (HR) protein Rad51 expression and focus formation were decreased in ESCC cells exposed to moderate chronic hypoxic condition (0.2% O(2), 48 hour); this result indicated that chronic hypoxic ESCC cells were HR deficient, possibly causing contextual synthetic lethality with PARP inhibitor in radiation sensitization. AZD2281 was also a radiation sensitizer in ESCC tumor xenograft models. Hence, in vitro and in vivo findings provide evidence that AZD2281 potently sensitizes ESCC cells to X-ray irradiation. The selective cell killing of HR-defective hypoxic cells contributes to radiosensitization by PARP inhibitor in ESCC cells under hypoxic condition.

The role of NF-κB in PARP-inhibitor-mediated sensitization and detoxification of arsenic trioxide in hepatocellular carcinoma cells

The therapeutic efficacy of arsenic trioxide (ATO) for treatments of solid tumors is restricted by its drug resistance and chemotoxicity. In this study, we investigated ATO sensitization and detoxification effect of the Poly (ADP ribose) polymerase-1 (PARP-1) inhibitor 4-Amino-1,8-naphthalimide (4AN) in the hepatocellular carcinoma cell line HepG2. We firstly reported that ATO treatment induced the activation of Nuclear factor of κB (NF-κB) and its downstream anti-apoptosis and pro-inflammatory effectors in a PARP-1-dependent manner and thus conferred HepG2 cells with ATO resistance and toxicity. 4AN significantly suppressed the ATO-induced NF-κB activation, which promotes the apoptotic response and alleviates the inflammatory reaction induced by ATO, resulting in sensitization and detoxification against ATO. We also demonstrated that the ATO-induced activation of PARP-1 and NF-κB was closely associated with the oxidative DNA damage mediated by the generated reactive oxygen species (ROS). Furthermore, the attenuation of ATO-induced ROS and the resulting oxidative DNA damage by N-acetyl-L-cysteine (NAC), a potent antioxidant, significantly reduced the activation of PARP-1 and NF-κB in ATO-treated cells. Our study provides novel insights into the mechanism of the PARP-1-mediated NF-κB signaling pathway in ATO resistance and toxicity in anticancer treatments. This study also highlights the application potential of PARP-1 inhibitors in ATO-based anti-cancer treatments and in prevention of NF-κB-mediated therapeutic resistance and toxicity.

Curcumin induces apoptosis in p53-null Hep3B cells through a TAp73/DNp73-dependent pathway

Curcumin has anticancer functions in various tumors. It has been shown to induce apoptosis through p53-dependent pathways. p73 gene is a member of the p53 family which encodes both a tumor suppressor (transactivation-competent p73 (TAp73)) and a putative oncogene (dominant-negative p73 (DNp73)); the former shares similarity with the tumor suppressor p53, and the latter behaves as dominant-negative proteins that interfere with the activity of TAp73. To understand the p73-dependent mechanisms that are engaged during curcumin-induced apoptosis, we established a p73 overexpression cell models using p53-deficient Hep3B cells (Hep3B(TAp73/DNp73)). Our results demonstrated that curcumin at concentrations of 40 and 80 μM induced DNA damage, increased TAp73/DNp73 ratio, and also led to apoptosis in the Hep3B(TAp73/DNp73) cells. The apoptotic cell death was concurrent with the loss of mitochondrial membrane potential; release of cytochrome c from mitochondria; and the cleavage of caspase 9, caspase 3, and poly(ADP-ribose) polymerase (PARP). These results demonstrated a p73-dependent mechanism for curcumin-induced apoptosis that involves the mitochondria-mediated pathway.

A new arylbenzofuran derivative functions as an anti-tumour agent by inducing DNA damage and inhibiting PARP activity

We previously reported that 7-hydroxy-5, 4'-dimethoxy-2-arylbenzofuran (HDAB) purified from Livistona chinensis is a key active agent. The present study investigated the function and molecular mechanism of HDAB. HDAB treatment of cervical cancer cells resulted in S phase arrest and apoptosis, together with cyclin A2 and CDK2 upregulation. Cyclin A2 siRNA and a CDK inhibitor efficiently relieved S phase arrest but increased the apoptosis rate. Mechanistic studies revealed that HDAB treatment significantly increased DNA strand breaks in an alkaline comet assay and induced ATM, CHK1, CHK2 and H2A.X phosphorylation. Wortmannin (a broad inhibitor of PIKKs) and CGK733 (a specific ATM inhibitor), but not LY294002 (a phosphatidylinositol 3-kinase inhibitor) or NU7026 (a DNA-PK specific inhibitor), prevented H2A.X phosphorylation and γH2A.X-positive foci formation in the nuclei, reversed S phase arrest and promoted the HDAB-induced apoptosis, suggesting that HDAB is a DNA damaging agent that can activate the ATM-dependent DNA repair response, thereby contributing to cell cycle arrest. In addition, molecular docking and in vitro activity assays revealed that HDAB can correctly dock into the hydrophobic pocket of PARP-1 and suppress PARP-1 ADP-ribosylation activity. Thus, the results indicated that HDAB can function as an anti-cancer agent by inducing DNA damage and inhibiting PARP activity.

The Combination of the PARP Inhibitor Rucaparib and 5FU Is an Effective Strategy for Treating Acute Leukemias

The existing treatments to cure acute leukemias seem to be nonspecific and suboptimal for most patients, drawing attention to the need of new therapeutic strategies. In the last decade the anticancer potential of poly ADP-ribose polymerase (PARP) inhibitors became apparent and now several PARP inhibitors are being developed to treat various malignancies. So far, the usage of PARP inhibitors has been mainly focused on the treatment of solid tumors and not too much about their efficacy on leukemias is known. In this study we test, for the first time on leukemic cells, a combined therapy that associates the conventional chemotherapeutic agent fluorouracil (5FU), used as a source of DNA damage, and a PARP inhibitor, rucaparib. We demonstrate the efficacy and the specificity of this combined therapy in killing both acute myeloid leukemia and acute lymphoid leukemia cells in vitro and in vivo. We clearly show that the inhibition of DNA repair induced by rucaparib is synthetic lethal with the DNA damage caused by 5FU in leukemic cells. Therefore, we propose a new therapeutic strategy able to enhance the cytotoxic effect of DNA-damaging agents in leukemia cells via inhibiting the repair of damaged DNA.

Combined inhibition of Wee1 and PARP1/2 for radiosensitization in pancreatic cancer

PURPOSE

While the addition of radiation to chemotherapy improves survival in patients with locally advanced pancreatic cancer, more effective therapies are urgently needed. Thus, we investigated the radiosensitizing efficacy of the novel drug combination of Wee1 and PARP1/2 inhibitors (AZD1775 and olaparib, respectively) in pancreatic cancer.

EXPERIMENTAL DESIGN

Radiosensitization of AsPC-1 or MiaPaCa-2 human pancreatic cancer cells was assessed by clonogenic survival and tumor growth assays. Mechanistically, the effects of AZD1775, olaparib, and radiation on cell cycle, DNA damage (γH2AX), and homologous recombination repair (HRR) were determined.

RESULTS

Treatment of AsPC-1 and MiaPaCa-2 cells with either AZD1775 or olaparib caused modest radiosensitization, whereas treatment with the combination significantly increased radiosensitization. Radiosensitization by the combination of AZD1775 and olaparib was associated with G2 checkpoint abrogation and persistent DNA damage. In addition, AZD1775 inhibited HRR activity and prevented radiation-induced Rad51 focus formation. Finally, in vivo, in MiaPaCa-2-derived xenografts, olaparib did not radiosensitize, whereas AZD1775 produced moderate, yet significant, radiosensitization (P < 0.05). Importantly, the combination of AZD1775 and olaparib produced highly significant radiosensitization (P < 0.0001) evidenced by a 13-day delay in tumor volume doubling (vs. radiation alone) and complete eradication of 20% of tumors.

CONCLUSIONS

Taken together, these results demonstrate the efficacy of combined inhibition of Wee1 and PARP inhibitors for radiosensitizing pancreatic cancers and support the model that Wee1 inhibition sensitizes cells to PARP inhibitor-mediated radiosensitization through inhibition of HRR and abrogation of the G2 checkpoint, ultimately resulting in unrepaired, lethal DNA damage and radiosensitization. Clin Cancer Res; 20(19); 5085-96. ©2014 AACR.

Histones and their modifications in ovarian cancer - drivers of disease and therapeutic targets

Epithelial ovarian cancer has the highest mortality of the gynecological malignancies. High grade serous epithelial ovarian cancer (SEOC) is the most common subtype, with the majority of women presenting with advanced disease where 5-year survival is around 25%. Platinum-based chemotherapy in combination with paclitaxel remains the most effective treatment despite platinum therapies being introduced almost 40 years ago. Advances in molecular medicine are underpinning new strategies for the treatment of cancer. Major advances have been made by international initiatives to sequence cancer genomes. For SEOC, with the exception of TP53 that is mutated in virtually 100% of these tumors, there is no other gene mutated at high frequency. There is extensive copy number variation, as well as changes in methylation patterns that will influence gene expression. To date, the role of histones and their post-translational modifications in ovarian cancer is a relatively understudied field. Post-translational histone modifications play major roles in gene expression as they direct the configuration of chromatin and so access by transcription factors. Histone modifications include methylation, acetylation, and monoubiquitination, with involvement of enzymes including histone methyltransferases, histone acetyltransferases/deacetylases, and ubiquitin ligases/deubiquitinases, respectively. Complexes such as the Polycomb repressive complex also play roles in the control of histone modifications and more recently roles for long non-coding RNA and microRNAs are emerging. Epigenomic-based therapies targeting histone modifications are being developed and offer new approaches for the treatment of ovarian cancer. Here, we discuss histone modifications and their aberrant regulation in malignancy and specifically in ovarian cancer. We review current and upcoming histone-based therapies that have the potential to inform and improve treatment strategies for women with ovarian cancer.

Targeting poly(ADP-ribose) polymerase and the c-Myb-regulated DNA damage response pathway in castration-resistant prostate cancer

Androgen deprivation is the standard treatment for advanced prostate cancer (PCa), but most patients ultimately develop resistance and tumor recurrence. We found that MYB is transcriptionally activated by androgen deprivation therapy or genetic silencing of the androgen receptor (AR). MYB silencing inhibited PCa growth in culture and xenografts in mice. Microarray data revealed that c-Myb and AR shared a subset of target genes that encode DNA damage response (DDR) proteins, suggesting that c-Myb may supplant AR as the dominant regulator of their common DDR target genes in AR inhibition-resistant or AR-negative PCa. Gene signatures including AR, MYB, and their common DDR-associated target genes positively correlated with metastasis, castration resistance, tumor recurrence, and decreased survival in PCa patients. In culture and in xenograft-bearing mice, a combination strategy involving the knockdown of MYB, BRCA1, or TOPBP1 or the abrogation of cell cycle checkpoint arrest with AZD7762, an inhibitor of the checkpoint kinase Chk1, increased the cytotoxicity of the poly[adenosine 5'-diphosphate (ADP)-ribose] polymerase (PARP) inhibitor olaparib in PCa cells. Our results reveal new mechanism-based therapeutic approaches for PCa by targeting PARP and the DDR pathway involving c-Myb, TopBP1, ataxia telangiectasia mutated- and Rad3-related (ATR), and Chk1.

E1B and E4 oncoproteins of adenovirus antagonize the effect of apoptosis inducing factor

Adenovirus inundates the productively infected cell with linear, double-stranded DNA and an abundance of single-stranded DNA. The cellular response to this stimulus is antagonized by the adenoviral E1B and E4 early genes. A mutant group C adenovirus that fails to express the E1B-55K and E4orf3 genes is unable to suppress the DNA-damage response. Cells infected with this double-mutant virus display significant morphological heterogeneity at late times of infection and frequently contain fragmented nuclei. Nuclear fragmentation was due to the translocation of apoptosis inducing factor (AIF) from the mitochondria into the nucleus. The release of AIF was dependent on active poly(ADP-ribose) polymerase-1 (PARP-1), which appeared to be activated by viral DNA replication. Nuclear fragmentation did not occur in AIF-deficient cells or in cells treated with a PARP-1 inhibitor. The E1B-55K or E4orf3 proteins independently prevented nuclear fragmentation subsequent to PARP-1 activation, possibly by altering the intracellular distribution of PAR-modified proteins.

The histone methyltransferase SMYD2 methylates PARP1 and promotes poly(ADP-ribosyl)ation activity in cancer cells

Poly(ADP-ribose) polymerase-1 (PARP1) catalyzes the poly(ADP-ribosyl)ation of protein acceptors using NAD(+) as the substrate is now considered as an important target for development of anticancer therapy. PARP1 is known to be post-translationally modified in various ways including phosphorylation and ubiquitination, but the physiological role of PARP1 methylation is not well understood. Herein we demonstrated that the histone methyltransferase SMYD2, which plays critical roles in human carcinogenesis, mono-methylated PARP1. We confirmed lysine 528 to be a target of SMYD2-dependent PARP1 methylation by LC-MS/MS and Edman Degradation analyses. Importantly, methylated PARP1 revealed enhanced poly(ADP-ribose) formation after oxidative stress, and positively regulated the poly(ADP-ribosyl)ation activity of PARP1. Hence, our study unveils a novel mechanism of PARP1 in human cancer through its methylation by SMYD2.

DNA repair pathways and their therapeutic potential in lung cancer

SUMMARY: 

Lung cancer is the leading cause of cancer-related mortality. According to WHO, 1.37 million deaths occur globally each year as a result of this disease. More than 70% of these cases are associated with prior tobacco consumption and/or cigarette smoking, suggesting a direct causal relationship. The development and progression of lung cancer and other malignancies involves the loss of genetic stability, resulting in acquisition of cumulative genetic changes; this affords the cell increased malignant potential. As such, an understanding of the mechanisms through which these events may occur will potentially allow for development of new anticancer therapies. This review will address the association between lung cancer and genetic instability, with a central focus on genetic mutations in the DNA damage repair pathways. In addition, we will discuss the potential clinical exploitation of these pathways, both in terms of biomarker staging, as well as through direct therapeutic targeting.