PARP-1 inhibition-induced activation of PI-3-kinase-Akt pathway promotes resistance to taxol

PARP1 inhibition protects mice against Japanese encephalitis virus infection

Japanese encephalitis (JE) is a vector-borne viral disease that causes acute encephalitis in children. Although vaccines have been developed against the JE virus (JEV), no effective antiviral therapy exists. Our study shows that inhibition of poly(ADP-ribose) polymerase 1 (PARP1), an NAD+-dependent (poly-ADP) ribosyl transferase, protects against JEV infection. Interestingly, PARP1 is critical for JEV pathogenesis in Neuro-2a cells and mice. Small molecular inhibitors of PARP1, olaparib, and 3-aminobenzamide (3-AB) significantly reduce clinical signs and viral load in the serum and brains of mice and improve survival. PARP1 inhibition confers protection against JEV infection by inhibiting autophagy. Mechanistically, upon JEV infection, PARP1 PARylates AKT and negatively affects its phosphorylation. In addition, PARP1 transcriptionally upregulates PTEN, the PIP3 phosphatase, negatively regulating AKT. PARP1-mediated AKT inactivation promotes autophagy and JEV pathogenesis by increasing the FoxO activity. Thus, our findings demonstrate PARP1 as a potential mediator of JEV pathogenesis that can be effectively targeted for treating JE.

KP372-1-Induced AKT Hyperactivation Blocks DNA Repair to Synergize With PARP Inhibitor Rucaparib via Inhibiting FOXO3a/GADD45α Pathway

Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) have exhibited great promise in the treatment of tumors with homologous recombination (HR) deficiency, however, PARPi resistance, which ultimately recovers DNA repair and cell progress, has become an enormous clinical challenge. Recently, KP372-1 was identified as a novel potential anticancer agent that targeted the redox enzyme, NAD(P)H:quinone oxidoreductase 1 (NQO1), to induce extensive reactive oxygen species (ROS) generation that amplified DNA damage, leading to cancer cell death. To overcome PARPi resistance and expand its therapeutic utility, we investigated whether a combination therapy of a sublethal dose of KP372-1 with a nontoxic dose of PARPi rucaparib would synergize and enhance lethality in NQO1 over-expressing cancers. We reported that the combination treatment of KP372-1 and rucaparib induced a transient and dramatic AKT hyperactivation that inhibited DNA repair by regulating FOXO3a/GADD45α pathway, which enhanced PARPi lethality and overcame PARPi resistance. We further found that PARP inhibition blocked KP372-1-induced PARP1 hyperactivation to reverse NAD⁺/ATP loss that promoted Ca²⁺-dependent autophagy and apoptosis. Moreover, pretreatment of cells with BAPTA-AM, a cytosolic Ca²⁺ chelator, dramatically rescued KP372-1- or combination treatment-induced lethality and significantly suppressed PAR formation and γH2AX activation. Finally, we demonstrated that this combination therapy enhanced accumulation of both agents in mouse tumor tissues and synergistically suppressed tumor growth in orthotopic pancreatic and non-small-cell lung cancer xenograft models. Together, our study provides novel preclinical evidence for new combination therapy in NQO1⁺ solid tumors that may broaden the clinical utility of PARPi.

Protective Effect of Minocycline Hydrochloride on the Mouse Embryonic Development Against Suboptimal Environment

Numerous studies have reported how inner cell mass (ICM) and trophectoderm (TE) was determined during the process of early mouse embryonic development from zygotes into organized blastocysts, however, multiple mysteries still remain. It is noteworthy that pluripotent stem cells (PSCs), which are derived from embryos at different developmental stages, have identical developmental potential and molecular characteristics to their counterpart embryos. Advances of PSCs research may provide us a distinctive perspective of deciphering embryonic development mechanism. Minocycline hydrochloride (MiH), a critical component for maintaining medium of novel type of extended pluripotent stem cells, which possesses developmental potential similar to both ICM and TE, can be substituted with genetic disruption of Parp1 in our previous study. Though Parp1-deficient mouse ESCs are more susceptible to differentiate into trophoblast derivatives, what role of MiH plays in mouse preimplantation embryonic development is still a subject of concern. Here, by incubating mouse zygotes in a medium containing MiH till 100 h after fertilization, we found that MiH could slow down embryonic developmental kinetics during cleavage stage without impairing blastocyst formation potential. Olaparib and Talazoparib, two FDA approved PARP1 inhibitors, exhibited similar effects on mouse embryos, indicating the aforementioned effects of MiH were through inhibiting of PARP1. Besides, we showed an embryonic protective role of MiH against suboptimal environment including long term exposure to external environment and H2O2 treatment, which could mimic inevitable manipulation during embryo culture procedures in clinical IVF laboratory. To our knowledge, it is not only for the first time to study MiH in the field of embryo development, but also for the first time to propose MiH as a protective supplement for embryo culture, giving the way to more studies on exploring the multiple molecular mechanisms on embryonic development that might be useful in assisted reproductive technology.

Molecular hybrids: A five-year survey on structures of multiple targeted hybrids of protein kinase inhibitors for cancer therapy

Protein kinases have grown over the past few years as a crucial target for different cancer types. With the multifactorial nature of cancer, and the fast development of drug resistance for conventional chemotherapeutics, a strategy for designing multi-target agents was suggested to potentially increase drug efficacy, minimize side effects and retain the proper pharmacokinetic properties. Kinase inhibitors were used extensively in such strategy. Different kinase inhibitor agents which target EGFR, VEGFR, c-Met, CDK, PDK and other targets were merged into hybrids with conventional chemotherapeutics such as tubulin polymerization and topoisomerase inhibitors. Other hybrids were designed gathering kinase inhibitors with targeted cancer therapy such as HDAC, PARP, HSP 90 inhibitors. Nitric oxide donor molecules were also merged with kinase inhibitors for cancer therapy. The current review presents the hybrids designed in the past five years discussing their design principles, results and highlights their future perspectives.

Interplay between ADP-ribosyltransferases and essential cell signaling pathways controls cellular responses

Signaling cascades provide integrative and interactive frameworks that allow the cell to respond to signals from its environment and/or from within the cell itself. The dynamic regulation of mammalian cell signaling pathways is often modulated by cascades of protein post-translational modifications (PTMs). ADP-ribosylation is a PTM that is catalyzed by ADP-ribosyltransferases and manifests as mono- (MARylation) or poly- (PARylation) ADP-ribosylation depending on the addition of one or multiple ADP-ribose units to protein substrates. ADP-ribosylation has recently emerged as an important cell regulator that impacts a plethora of cellular processes, including many intracellular signaling events. Here, we provide an overview of the interplay between the intracellular diphtheria toxin-like ADP-ribosyltransferase (ARTD) family members and five selected signaling pathways (including NF-κB, JAK/STAT, Wnt-β-catenin, MAPK, PI3K/AKT), which are frequently described to control or to be controlled by ADP-ribosyltransferases and how these interactions impact the cellular responses.

The PARP Enzyme Family and the Hallmarks of Cancer Part 1. Cell Intrinsic Hallmarks

The 17-member poly (ADP-ribose) polymerase enzyme family, also known as the ADP-ribosyl transferase diphtheria toxin-like (ARTD) enzyme family, contains DNA damage-responsive and nonresponsive members. Only PARP1, 2, 5a, and 5b are capable of modifying their targets with poly ADP-ribose (PAR) polymers; the other PARP family members function as mono-ADP-ribosyl transferases. In the last decade, PARP1 has taken center stage in oncology treatments. New PARP inhibitors (PARPi) have been introduced for the targeted treatment of breast cancer 1 or 2 (BRCA1/2)-deficient ovarian and breast cancers, and this novel therapy represents the prototype of the synthetic lethality paradigm. Much less attention has been paid to other PARPs and their potential roles in cancer biology. In this review, we summarize the roles played by all PARP enzyme family members in six intrinsic hallmarks of cancer: uncontrolled proliferation, evasion of growth suppressors, cell death resistance, genome instability, reprogrammed energy metabolism, and escape from replicative senescence. In a companion paper, we will discuss the roles of PARP enzymes in cancer hallmarks related to cancer-host interactions, including angiogenesis, invasion and metastasis, evasion of the anticancer immune response, and tumor-promoting inflammation. While PARP1 is clearly involved in all ten cancer hallmarks, an increasing body of evidence supports the role of other PARPs in modifying these cancer hallmarks (e.g., PARP5a and 5b in replicative immortality and PARP2 in cancer metabolism). We also highlight controversies, open questions, and discuss prospects of recent developments related to the wide range of roles played by PARPs in cancer biology. Some of the summarized findings may explain resistance to PARPi therapy or highlight novel biological roles of PARPs that can be therapeutically exploited in novel anticancer treatment paradigms.

The PTEN Conundrum: How to Target PTEN-Deficient Prostate Cancer

Loss of the tumor suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN), which negatively regulates the PI3K-AKT-mTOR pathway, is strongly linked to advanced prostate cancer progression and poor clinical outcome. Accordingly, several therapeutic approaches are currently being explored to combat PTEN-deficient tumors. These include classical inhibition of the PI3K-AKT-mTOR signaling network, as well as new approaches that restore PTEN function, or target PTEN regulation of chromosome stability, DNA damage repair and the tumor microenvironment. While targeting PTEN-deficient prostate cancer remains a clinical challenge, new advances in the field of precision medicine indicate that PTEN loss provides a valuable biomarker to stratify prostate cancer patients for treatments, which may improve overall outcome. Here, we discuss the clinical implications of PTEN loss in the management of prostate cancer and review recent therapeutic advances in targeting PTEN-deficient prostate cancer. Deepening our understanding of how PTEN loss contributes to prostate cancer growth and therapeutic resistance will inform the design of future clinical studies and precision-medicine strategies that will ultimately improve patient care.

Inhibition of ELF3 confers synthetic lethality of PARP inhibitor in non-small cell lung cancer

BACKGROUND

E74 Like ETS Transcription Factor 3 (ELF3) functions as a transcriptional factor to regulate non-small cell lung cancer (NSCLC) differentiation and progression. Poly(ADP-ribose) polymerase (PARP) inhibitors demonstrate anti-tumor effect in NSCLC. This study aimed to investigate whether ELF3 confers synthetic lethal with PARP inhibitor in NSCLC.

MATERIALS AND METHODS

The sensitivity of PARP inhibitor, Olaparib, to different NSCLC cell lines was determined by half maximal inhibitory concentration (IC₅₀). Expression of ELF3 in NSCLC cell lines was evaluated by western blot. The effects of ELF3 on cytotoxicity of Olaparib to NSCLC were investigated by MTT (3-(4,5- di methyl thiazol -2-yl)-2,5-di phenyl tetrazolium bromide) and colony formation assays. The underlying mechanism involved in synthetic lethality with ELF3 and PARP inhibitors in NSCLC were detected by immunofluorescence and Western blot.

RESULTS

ELF3 was up-regulated in NSCLC cell lines exhibiting resistance to PARP inhibitor, Olaparib. Knock down of ELF3 decreased the sensitivity and enhanced cytotoxicity of Olaparib to NSCLC cells. Moreover, knock down of ELF3 increased S139 phosphorylated histone H2AX (γH2AX), and inhibited homologous recombination activity via down-regulation of DNA repair protein RAD51 homolog 1 (RAD51), thus showing deficiency in DNA damage repair. Over-expression of ELF3 could up-regulate phosphorylation of AKT (Protein kinase B), while knock down of ELF3 regulated homologous recombination-mediated DNA repair via down-regulation of phosphorylation of AKT.

CONCLUSION

Knock down of ELF3 revealed homologous recombination deficiency via AKT signaling pathway, and synthetic lethality with ELF3 inhibition and PARP inhibitor indicated the clinical significance of PARP inhibitor in ELF3-deficient NSCLC.

Role of ABT888, a Novel Poly(ADP-Ribose) Polymerase (PARP) Inhibitor in Countering Autophagy and Apoptotic Processes Associated to Spinal Cord Injury

Accidents are the cause of some 50 deaths per 100,000 population each year; some 3% of these are from traumatic spinal cord injury (SCI), a damage that causes temporary or permanent motor deficits, often leading to permanent neurological alterations. The activation of poly(ADP-ribose) polymerase (PARP) as DNA damage response, together with autophagy and apoptosis processes contributes to the secondary injury processes seen after SCI. Thus, in the present study, a mouse compression model of SCI was used to determine whether the treatment with ABT888, as PARP-1/2 inhibitor, could restore the neuronal damage induced by SCI. Mice were orally administered with ABT888 (at a dose of 25 mg/kg) 1 h and 6 h after SCI induction. Histological analysis, myeloperoxidase (MPO) activity, and Basso Mouse scale (BMS) were performed. The expression of autophagy-related proteins and apoptosis-inducing factors was quantified in the cytosolic fraction from spinal cord tissue collected after 24 h after SCI. TUNEL assay was performed in SCI-tissues 24 h after damage. ABT888 treatment significantly reduced histological damage and neutrophilic infiltration, improving motor skills. PARP-1/2 inhibition by ABT888 slowed cell death, decreasing autophagy-activation proteins. These results showed that ABT888, inhibiting PARP-1/2 activity, through a reduction in the apoptosis-autophagy machinery, plays a protective role after SCI, suggesting a new insight into the potential application of ABT888 as novel candidate in SCI therapies.

Mitochondrial Protection by PARP Inhibition

Inhibitors of the nuclear DNA damage sensor and signalling enzyme poly(ADP-ribose) polymerase (PARP) have recently been introduced in the therapy of cancers deficient in double-strand DNA break repair systems, and ongoing clinical trials aim to extend their use from other forms of cancer non-responsive to conventional treatments. Additionally, PARP inhibitors were suggested to be repurposed for oxidative stress-associated non-oncological diseases resulting in a devastating outcome, or requiring acute treatment. Their well-documented mitochondria- and cytoprotective effects form the basis of PARP inhibitors’ therapeutic use for non-oncological diseases, yet can limit their efficacy in the treatment of cancers. A better understanding of the processes involved in their protective effects may improve the PARP inhibitors’ therapeutic potential in the non-oncological indications. To this end, we endeavoured to summarise the basic features regarding mitochondrial structure and function, review the major PARP activation-induced cellular processes leading to mitochondrial damage, and discuss the role of PARP inhibition-mediated mitochondrial protection in several oxidative stress-associated diseases.

Role of Akt Activation in PARP Inhibitor Resistance in Cancer

Poly(ADP-ribose) polymerase (PARP) inhibitors have recently been introduced in the therapy of several types of cancers not responding to conventional treatments. However, de novo and acquired PARP inhibitor resistance is a significant limiting factor in the clinical therapy, and the underlying mechanisms are not fully understood. Activity of the cytoprotective phosphatidylinositol-3 kinase (PI3K)-Akt pathway is often increased in human cancer that could result from mutation, expressional change, or amplification of upstream growth-related factor signaling elements or elements of the Akt pathway itself. However, PARP-inhibitor-induced activation of the cytoprotective PI3K-Akt pathway is overlooked, although it likely contributes to the development of PARP inhibitor resistance. Here, we briefly summarize the biological role of the PI3K-Akt pathway. Next, we overview the significance of the PARP-Akt interplay in shock, inflammation, cardiac and cerebral reperfusion, and cancer. We also discuss a recently discovered molecular mechanism that explains how PARP inhibition induces Akt activation and may account for apoptosis resistance and mitochondrial protection in oxidative stress and in cancer.

Puerarin protects against high-fat high-sucrose diet-induced non-alcoholic fatty liver disease by modulating PARP-1/PI3K/AKT signaling pathway and facilitating mitochondrial homeostasis

As yet, there was no effective pharmacological therapy approved for non-alcoholic fatty liver disease (NAFLD). Here, we aimed to evaluate the therapeutic potential of puerarin against NAFLD and explored the underlying mechanisms. C57BL/6J mice were fed with a high-fat high-sucrose (HFHS) diet with or without puerarin coadministration intragastrically. The levels of hepatocellular injury, steatosis, fibrosis, and mitochondrial and metabolism alteration were detected. First, puerarin ameliorated histopathologic abnormalities due to HFHS. We observed a marked increase in hepatic lipid content, inflammation, and fibrosis level, which were attenuated by puerarin. Possible mechanisms were related to puerarin-mediated activation of PI3K/AKT pathway and further improvement in fatty acid metabolism. Puerarin restored the NAD+ content and beneficially affected the hepatic mitochondrial function, which attenuated HFHS-induced steatosis and metabolic disturbances. Finally, hepatic PARP-1 was activated due to excessive fat intake. Puerarin attenuated the PARP-1 expression in HFHS-fed mice, and PJ34, the PARP inhibitor, could mimic these protections of puerarin. However, pharmacological inhibition of PI3K disabled the protection of puerarin or PJ34 toward NAD+ refilling and mitochondrial homeostasis. In conclusion, our findings indicated that puerarin could be a promising and practical therapeutic strategy in NAFLD through modulating PARP-1/PI3K/AKT signaling pathway and further facilitating mitochondrial function.

PARP Inhibitors in Ovarian Cancer: The Route to "Ithaca"

Poly (ADP-ribose) polymerase (PARP) inhibitors are a novel class of therapeutic agents that target tumors with deficiencies in the homologous recombination DNA repair pathway. Genomic instability characterizes high-grade serous ovarian cancer (HGSOC), with one half of all tumors displaying defects in the important DNA repair pathway of homologous recombination. Early studies have shown significant efficacy for PARP inhibitors in patients with germline breast related cancer antigens 1 and 2 (BRCA1/2) mutations. It has also become evident that BRCA wild-type patients with other defects in the homologous recombination repair pathway benefit from this treatment. Companion homologous recombination deficiency (HRD) scores are being developed to guide the selection of patients that are most likely to benefit from PARP inhibition. The choice of which PARP inhibitor is mainly based upon the number of prior therapies and the presence of a BRCA mutation or HRD. The identification of patients most likely to benefit from PARP inhibitor therapy in view of HRD and other biomarker assessments is still challenging. The aim of this review is to describe the current evidence for PARP inhibitors in ovarian cancer, their mechanism of action, and the outstanding issues, including the rate of long-term toxicities and the evolution of resistance.

PARP inhibition induces Akt-mediated cytoprotective effects through the formation of a mitochondria-targeted phospho-ATM-NEMO-Akt-mTOR signalosome

PURPOSE

The cytoprotective effect of poly(ADP-ribose) polymerase 1 (PARP1) inhibition is well documented in various cell types subjected to oxidative stress. Previously, we have demonstrated that PARP1 inhibition activates Akt, and showed that this response plays a critical role in the maintenance of mitochondrial integrity and in cell survival. However, it has not yet been defined how nuclear PARP1 signals to cytoplasmic Akt.

METHODS

WRL 68, HeLa and MCF7 cells were grown in culture. Oxidative stress was induced with hydrogen peroxide. PARP was inhibited with the PARP inhibitor PJ34. ATM, mTOR- and NEMO were silenced using specific siRNAs. Cell viability assays were based on the MTT assay. PARP-ATM pulldown experiments were conducted; each protein was visualized by Western blotting. Immunoprecipitation of ATM, phospho-ATM and NEMO was performed from cytoplasmic and mitochondrial cell fractions and proteins were detected by Western blotting. In some experiments, a continually active Akt construct was introduced. Nuclear to cytoplasmic and mitochondrial translocation of phospho-Akt was visualized by confocal microscopy.

RESULTS

Here we present evidence for a PARP1 mediated, PARylation-dependent interaction between ATM and NEMO, which is responsible for the cytoplasmic transport of phosphorylated (thus, activated) ATM kinase. In turn, the cytoplasmic p-ATM and NEMO forms complex with mTOR and Akt, yielding the phospho-ATM-NEMO-Akt-mTOR signalosome, which is responsible for the PARP-inhibition induced Akt activation. The phospho-ATM-NEMO-Akt-mTOR signalosome localizes to the mitochondria and is essential for the PARP-inhibition-mediated cytoprotective effects in oxidatively stressed cells. When the formation of the signalosome is prevented, the cytoprotective effects diminish, but cells can be rescued by constantly active Akt1, further confirming the critical role of Akt activation in cytoprotection.

CONCLUSIONS

Taken together, the data presented in the current paper are consistent with the hypothesis that PARP inhibition suppresses the PARylation of ATM, which, in turn, forms an ATM-NEMO complex, which exits the nucleus, and combines in the cytosol with mTOR and Act, resulting in Act phosphorylation (i.e. activation), which, in turn, produces the cytoprotective action via the induction of Akt-mediated survival pathways. This mechanism can be important in the protective effect of PARP inhibitor in various diseases associated with oxidative stress. Moreover, disruption of the formation or action of the phospho-ATM-NEMO-Akt-mTOR signalosome may offer potential future experimental therapeutic checkpoints.

Poly(adenosine diphosphate-ribose) polymerase as therapeutic target: lessons learned from its inhibitors

Poly(ADP-ribose) polymerases are a family of DNA-dependent nuclear enzymes catalyzing the transfer of ADP-ribose moieties from cellular nicotinamide-adenine-dinucleotide to a variety of target proteins. Although they have been considered as resident nuclear elements of the DNA repair machinery, recent works revealed a more intricate physiologic role of poly(ADP-ribose) polymerases with numerous extranuclear activities. Indeed, poly(ADP-ribose) polymerases participate in fundamental cellular processes like chromatin remodelling, transcription or regulation of the cell-cycle. These new insight into the physiologic roles of poly(ADP-ribose) polymerases widens the range of human pathologies in which pharmacologic inhibition of these enzymes might have a therapeutic potential. Here, we overview our current knowledge on extranuclear functions of poly(ADP-ribose) polymerases with a particular focus on the mitochondrial ones and discuss potential fields of future clinical applications.

PARP inhibition protects mitochondria and reduces ROS production via PARP-1-ATF4-MKP-1-MAPK retrograde pathway

Oxidative stress induces DNA breaks and PARP-1 activation which initiates mitochondrial reactive oxygen species (ROS) production and cell death through pathways not yet identified. Here, we show the mechanism by which PARP-1 influences these processes via PARylation of activating transcription factor-4 (ATF4) responsible for MAP kinase phosphatase-1 (MKP-1) expression and thereby regulates MAP kinases. PARP inhibitor, or silencing, of PARP induced MKP-1 expression by ATF4-dependent way, and inactivated JNK and p38 MAP kinases. Additionally, it induced ATF4 expression and binding to cAMP-response element (CRE) leading to MKP-1 expression and the inactivation of MAP kinases. In contrast, PARP-1 activation induced the PARylation of ATF4 and reduced its binding to CRE sequence in vitro. CHIP-qPCR analysis showed that PARP inhibitor increased the ATF4 occupancy at the initiation site of MKP-1. In oxidative stress, PARP inhibition reduced ROS-induced cell death, suppressed mitochondrial ROS production and protected mitochondrial membrane potential on an ATF4 and MKP-1 dependent way. Basically identical results were obtained in WRL-68, A-549 and T24/83 human cell lines indicating that the aforementioned mechanism can be universal. Here, we provide the first description of PARP-1-ATF4-MKP-1-JNK/p38 MAPK retrograde pathway, which is responsible for the regulation of mitochondrial integrity, ROS production and cell death in oxidative stress, and may represent a new mechanism of PARP in cancer therapy since cancer stem cells development is JNK-dependent.

Metabolic roles of poly(ADP-ribose) polymerases

Poly(ADP-ribosyl)ation (PARylation) is an evolutionarily conserved reaction that had been associated with numerous cellular processes such as DNA repair, protein turnover, inflammatory regulation, aging or metabolic regulation. The metabolic regulatory tasks of poly(ADP-ribose) polymerases (PARPs) are complex, it is based on the regulation of metabolic transcription factors (e.g. SIRT1, nuclear receptors, SREBPs) and certain cellular energy sensors. PARP over-activation can cause damage to mitochondrial terminal oxidation, while the inhibition of PARP-1 or PARP-2 can induce mitochondrial oxidation by enhancing the mitotropic tone of gene transcription and signal transduction. These PARP-mediated processes impact on higher order metabolic regulation that modulates lipid metabolism, circadian oscillations and insulin secretion and signaling. PARP-1, PARP-2 and PARP-7 are related to metabolic diseases such as diabetes, alcoholic and non-alcoholic fatty liver disease (AFLD, NAFLD), or on a broader perspective to Warburg metabolism in cancer or the metabolic diseases accompanying aging.

Spatane diterpinoid from the brown algae, Stoechospermum marginatum induces apoptosis via ROS induced mitochondrial mediated caspase dependent pathway in murine B16F10 melanoma cells

Spatane diterpinoids isolated from the brown marine algae Stoechospermum marginatum were known to have cytotoxic effects in human cancerous cell lines and murine melanoma cells; the underling apoptotic mechanism of diterpinoids still remains unclear so far. Thus, in the present study, the apoptotic mechanism of a spatane diterpinoid, 5(R), 19-diacetoxy-15,18(R and S), dihydro spata-13, 16(E)-diene (DDSD) was investigated mainly in B16F10 melanoma cells because they were most susceptible to DDSD than THP1, U937, COLO205, and HL60 cells. The treatment of B6F10 cells with DDSD resulted in morphological alterations, nuclear condensation, and DNA fragmentation, which leads to cell growth inhibition in a concentration-dependent manner. Data indicate that DDSD induced the generation of ROS, consequentially caused alteration in Bax/Bcl-2 ratio that disrupted the inner mitochondrial transmembrane potential (ΔΨm) resulting in cytochrome c redistribution to the cytoplasm and activation of caspase-mediated apoptotic pathway. Flow cytometric analysis clearly indicated that the DDSD inducing phosphatidylserine externalization and mediated "S-phase" arrest in cell cycle. In addition, results also found that DDSD induced apoptosis through deregulating PI3K/AKT signaling pathway. The anti-tumor activity of DDSD was evaluated in C57BL/6 mice bearing B16F10 melanoma. It effectively inhibited tumor growth (volume and weight) in a dose dependent manner, yet without apparent toxic effects. Morphology and apoptotic status of tumor tissues in the treated mice were assessed by microscopy and TUNEL assay, respectively. Our study shows a therapeutic potential of DDSD for the treatment of malignant melanoma and a new source of anticancer drugs. © 2016 Wiley Periodicals, Inc.

MK-2206 sensitizes BRCA-deficient epithelial ovarian adenocarcinoma to cisplatin and olaparib

BACKGROUND

Platinum resistance is a major obstacle in the treatment of epithelial ovarian cancer (EOC). Activation of the AKT pathway promotes platinum resistance while inhibition of AKT sensitizes chemoresistant cells. Patients with BRCA mutant EOC, and thus a defect in the homologous recombination (HR) repair pathway, demonstrate greater clinical response to platinum and olaparib therapy than patients with BRCA wild-type EOC. MK-2206, an allosteric inhibitor of AKT phosphorylation, sensitizes a variety of cell types to various anticancer agents and is currently undergoing phase II trials as monotherapy for platinum-resistant ovarian, fallopian tube, and peritoneal cancer. This study examines the differential effects of AKT inhibition with cisplatin and olaparib therapy in BRCA1/2-deficient versus wild-type EOC.

METHODS

PEO1, a chemosensitive BRCA2-mutant serous ovarian adenocarcinoma, and PEO4, a reverted BRCA2-proficient line from the same patient after the development of chemotherapeutic resistance, were primarily used for the study. In PEO1, MK-2206 demonstrated moderate to strong synergism with cisplatin and olaparib at all doses, while demonstrating antagonism at all doses in PEO4.

RESULTS

Baseline phospho-AKT activity in untreated cells was upregulated in both BRCA1- and 2-deficient cell lines. MK-2206 prevented cisplatin- and olaparib-induced AKT activation in the BRCA2-deficient PEO1 cells. We propose that BRCA-deficient EOC cells upregulate baseline AKT activity to enhance survival in the absence of HR. Higher AKT activity is also required to withstand cytotoxic agent-induced DNA damage, leading to strong synergism between MK-2206 and cisplatin or olaparib therapy in BRCA-deficient cells.

CONCLUSIONS

MK-2206 shows promise as a chemosensitization agent in BRCA-deficient EOC and merits clinical investigation in this patient population.

Effects for Sequential Treatment of siAkt and Paclitaxel on Gastric Cancer Cell Lines

Real-time screening of cellular response on the drugs could provide valuable insights for the early detection of therapeutic efficiency and the evaluation of disease progression. Cancer cells have the ability to vary widely in response to stress in a manner to adjust the signaling pathway to promote the survival or having a resistance to stimulation. Cell-based label-free technologies using electronic impedance sensor have strategies for constructing the signature profiles of each cells. To achieve exquisite sensitivity to substantially change of live-cell response have an important role that predict the potential of therapeutic effects. In this study, we use an impedance-based real-time cell analysis system to investigate dynamic phenotypes of cells described as a cellular index value. We show that gastric cancer cells generated characteristic kinetic patterns that corresponded to the treatment order of therapeutics. The kinetic feature of the cells offers insightful information that cannot be acquired from a conventional single end-point assay. Furthermore, we employ a 'sequential treatment strategy' to increase cytotoxic effects with minimizing the use of chemotherapeutics. Specifically, treatment of paclitaxel (PTX) after down-regulating Akt gene expression using RNAi reduces the cell proliferation and increases apoptosis. We propose that the sequential treatment may exhibit more effective approach rather than traditional combination therapy. Moreover, the dynamic monitoring of cell-drug interaction enables us to obtain a better understanding of the temporal effects in vitro.

Poly(ADP-ribose) polymerases as modulators of mitochondrial activity

Mitochondria are essential in cellular stress responses. Mitochondrial output to environmental stress is a major factor in metabolic adaptation and is regulated by a complex network of energy and nutrient sensing proteins. Activation of poly(ADP-ribose) polymerases (PARPs) has been known to impair mitochondrial function; however, our view of PARP-mediated mitochondrial dysfunction and injury has only recently fundamentally evolved. In this review, we examine our current understanding of PARP-elicited mitochondrial damage, PARP-mediated signal transduction pathways, transcription factors that interact with PARPs and govern mitochondrial biogenesis, as well as mitochondrial diseases that are mediated by PARPs. With PARP activation emerging as a common underlying mechanism in numerous pathologies, a better understanding the role of various PARPs in mitochondrial regulation may help open new therapeutic avenues.

The role of p38 signaling and poly(ADP-ribosyl)ation-induced metabolic collapse in the osteogenic differentiation-coupled cell death pathway

Osteogenic differentiation is a multistep process regulated by a diverse set of morphogenic and transcription factors. Previously we identified endogenous hydrogen peroxide-induced poly(ADP-ribose) polymerase-1 (PARP1) activation as a mediator of osteodifferentiation and associated cell death. Here we set out to investigate whether or not activation of PARP1 is dependent on DNA breaks and how PARP1 mediates cell death during osteodifferentiation of mesenchymal stem cells and SAOS-2 cells. Here we show that the MAP kinases p38, JNK, and ERK1/2 become activated during the differentiation process. However, only p38 activation depended both on hydrogen peroxide production and on PARP1 activation as the hydrogen peroxide decomposing enzyme catalase, the PARP inhibitor PJ34, and the silencing of PARP1 suppressed p38 activation. Inhibition of p38 suppressed cell death and inhibited osteogenic differentiation (calcium deposition, alkaline phosphatase activity, and marker gene expression) providing further support for the close coupling of osteodifferentiation and cell death. Metabolic collapse appears to be central in the hydrogen peroxide-PARP1-p38 pathway as silencing PARP1 or inhibition of p38 prevented differentiation-associated loss of cellular NAD, inhibition of mitochondrial respiration, and glycolytic activity. We also provide evidence that endogenous hydrogen peroxide produced by the differentiating cells is sufficient to cause detectable DNA breakage. Moreover, p38 translocates from the cytoplasm to the nucleus where it interacts and colocalizes with PARP1 as detected by immunoprecipitation and immunofluorescence, respectively. In summary, hydrogen peroxide-induced PARP1 activation leads to p38 activation and this pathway is required both for the successful completion of the differentiation process and for the associated cell death.

PI3K/AKT/mTOR pathway inhibitors: the ideal combination partners for breast cancer therapies?

Activation of the phosphatidylinositol 3-kinase (PI3K)/AKT/mTOR pathway is common in breast cancer. PI3K pathway activation has been associated with tumor growth and progression, and thus is a promising target for breast cancer therapy. Agents targeting the PI3K pathway can restore sensitivity to standard breast cancer therapies, including endocrine, HER2-targeted agents and chemotherapy, by suppressing PI3K pathway activation, which is central to the development of therapeutic resistance. The combination of endocrine therapy and PI3K pathway (mTOR) inhibition has proven clinical benefit, and novel combination strategies involving PI3K pathway inhibitors and other investigational targeted therapies are now being explored clinically in patients with breast cancer.

Beyond DNA Repair: Additional Functions of PARP-1 in Cancer

Poly(ADP-ribose) polymerases (PARPs) are DNA-dependent nuclear enzymes that transfer negatively charged ADP-ribose moieties from cellular nicotinamide-adenine-dinucleotide (NAD(+)) to a variety of protein substrates, altering protein-protein and protein-DNA interactions. The most studied of these enzymes is poly(ADP-ribose) polymerase-1 (PARP-1), which is an excellent therapeutic target in cancer due to its pivotal role in the DNA damage response. Clinical studies have shown susceptibility to PARP inhibitors in DNA repair defective cancers with only mild adverse side effects. Interestingly, additional studies are emerging which demonstrate a role for this therapy in DNA repair proficient tumors through a variety of mechanisms. In this review, we will discuss additional functions of PARP-1 - including regulation of inflammatory mediators, cellular energetics and death pathways, gene transcription, sex hormone- and ERK-mediated signaling, and mitosis - and the role these PARP-1-mediated processes play in oncogenesis, cancer progression, and the development of therapeutic resistance. As PARP-1 can act in both a pro- and anti-tumor manner depending on the context, it is important to consider the global effects of this protein in determining when, and how, to best use PARP inhibitors in anticancer therapy.

Momordin Ic induces HepG2 cell apoptosis through MAPK and PI3K/Akt-mediated mitochondrial pathways

Momordin Ic is a natural triterpenoid saponin enriched in various Chinese and Japanese natural medicines such as the fruit of Kochia scoparia (L.) Schrad. So far, there is little scientific evidence for momordin Ic with regard to the anti-tumor activities. The aim of this work was to elucidate the anti-tumor effect of momordin Ic and the signal transduction pathways involved. We found that momordin Ic induced apoptosis in human hepatocellular carcinoma HepG2 cells, which were supported by DNA fragmentation, caspase-3 activation and PARP cleavage. Meanwhile, momordin Ic triggered reactive oxygen species (ROS) production together with collapse of mitochondrial membrane potential, cytochrome c release, down-regulation of Bcl-2 and up-regulation of Bax expression. The activation of p38 and JNK, inactivation of Erk1/2 and Akt were also demonstrated. Although ROS production rather than NO was stimulated, the expression of iNOS and HO-1 were altered after momordin Ic treatment for 4 h. Furthermore, the cytochrome c release, caspase-3 activation, Bax/Bcl-2 expression and PARP cleavage were promoted with LY294002 and U0126 intervention but were blocked by SB203580, SP600125, PI3K activator, NAC and 1,400 W pretreatment, demonstrating the mitochondrial disruption. Furthermore, momordin Ic combination with NAC influenced MAPK, PI3K/Akt and HO-1, iNOS pathways, MAPK and PI3K/Akt pathways also regulated the expression of HO-1 and iNOS. These results indicated that momordin Ic induced apoptosis through oxidative stress-regulated mitochondrial dysfunction involving the MAPK and PI3K-mediated iNOS and HO-1 pathways. Thus, momordin Ic might represent a potential source of anticancer candidate.

Inhibition of PARP1 by small interfering RNA enhances docetaxel activity against human prostate cancer PC3 cells

Though poly(ADP-ribose) polymerase 1 (PARP1) inhibitors have benefits in combination with radiotherapy in prostate cancers, few is known about the exactly role and underlying mechanism of PARP1 in combination with chemotherapy agents. Here our data revealed that inhibition of PARP1 by small interfering RNA (siRNA) could enhance docetaxel's activity against PC3 cells, which is associated with an accelerate repression of EGF/Akt/FOXO1 signaling pathway. Our results provide a novel role of PARP1 in transcription regulation of EGFR/Akt/FOXO1 signaling pathway and indicate that PARP1 siRNA combined with docetaxel can be an innovative treatment strategy to potentially improve outcomes in CRPC patients.

Towards the small and the beautiful: a small dibromotyrosine derivative from Pseudoceratina sp. sponge exhibits potent apoptotic effect through targeting IKK/NFκB signaling pathway

A dibromotyrosine derivative, (1'R,5'S,6'S)-2-(3',5'-dibromo-1',6'-dihydroxy-4'-oxocyclohex-2'-enyl) acetonitrile (DT), was isolated from the sponge Pseudoceratina sp., and was found to exhibit a significant cytotoxic activity against leukemia K562 cells. Despite the large number of the isolated bromotyrosine derivatives, studies focusing on their biological mechanism of action are scarce. In the current study we designed a set of experiments to reveal the underlying mechanism of DT cytotoxic activity against K562 cells. First, the results of MTT cytotoxic and the annexin V-FITC/PI apoptotic assays, indicated that the DT cytotoxic activity is mediated through induction of apoptosis. This effect was also supported by caspases-3 and -9 activation as well as PARP cleavage. DT induced generation of reactive oxygen species (ROS) and the disruption of mitochondrial membrane potential (MMP) as indicated by flow cytometric assay. The involvement of ROS generation in the apoptotic activity of DT was further corroborated by the pretreatment of K562 cells with N-acetyl-cysteine (NAC), a ROS scavenger, which prevented apoptosis and the disruption of MMP induced by DT. Results of cell-free system assay suggested that DT can act as a topoisomerase II catalytic inhibitor, unlike the clinical anticancer drug, etoposide, which acts as a topoisomerase poison. Additionally, we found that DT treatment can block IKK/NFκB pathway and activate PI3K/Akt pathway. These findings suggest that the cytotoxic effect of DT is associated with mitochondrial dysfunction-dependent apoptosis which is mediated through oxidative stress. Therefore, DT represents an interesting reference point for the development of new cytotoxic agent targeting IKK/NFκB pathway.

PARP1 inhibition affects pleural mesothelioma cell viability and uncouples AKT/mTOR axis via SIRT1

Malignant Pleural Mesothelioma (MMe) is a rare but increasingly prevalent, highly aggressive cancer with poor prognosis. The aetiology of MMe is essentially a function of previous exposure to asbestos fibres, which are considered to be an early-stage carcinogen. Asbestos is toxic to human mesothelial cells (HMCs), that activate the nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP1) to repair DNA. The targeting of PARP1 is showing considerable potential for delivering selective tumour cell kill while sparing normal cells, and offers a scientifically rational clinical application. We investigated PARP1 expression in normal mesothelial and MMe tissues samples. Immunohistochemical analysis revealed low PARP1 staining in peritumoural mesothelium. As opposite, a progressive increase in epithelioid and in the most aggressive sarcomatoid MMe tissues was evident. In MMe cell lines, we correlated increased PARP1 expression to sensitivity to its inhibitor CO-338 and demonstrated that CO-338 significantly reduced cell viability as single agent and was synergistic with cis-platin. Interestingly, we described a new correlation between PARP1 and the AKT/mTOR axis regulated by SIRT1. SIRT1 has a role in the modulation of AKT activation and PARP1 has been described to be a gatekeeper for SIRT1 activity by limiting NAD+ availability. Here, we firstly demonstrate an inverse correlation between AKT acetylation and phosphorylation modulated by SIRT1 in MMe cells treated with CO-338. In conclusion, this study demonstrates that PARP1 overexpression defines increased responsiveness to its inhibition, then these results imply that a substantial fraction of patients could be candidates for therapy with PARP inhibitors.

The Sound of Silence: RNAi in Poly (ADP-Ribose) Research

Poly(ADP-ribosyl)-ation is a nonprotein posttranslational modification of proteins and plays an integral part in cell physiology and pathology. The metabolism of poly(ADP-ribose) (PAR) is regulated by its synthesis by poly(ADP-ribose) polymerases (PARPs) and on the catabolic side by poly(ADP-ribose) glycohydrolase (PARG). PARPs convert NAD+ molecules into PAR chains that interact covalently or noncovalently with target proteins and thereby modify their structure and functions. PAR synthesis is activated when PARP1 and PARP2 bind to DNA breaks and these two enzymes account for almost all PAR formation after genotoxic stress. PARG cleaves PAR molecules into free PAR and finally ADP-ribose (ADPR) moieties, both acting as messengers in cellular stress signaling. In this review, we discuss the potential of RNAi to manipulate the levels of PARPs and PARG, and consequently those of PAR and ADPR, and compare the results with those obtained after genetic or chemical disruption.

Intratumoral injection of taxol in vivo suppresses A549 tumor showing cytoplasmic vacuolization

Based on our recent in vitro studies, this report was designed to explore the mechanism by which high concentration of taxol (70 µM) induced paraptosis-like cell death in human lung carcinoma (A549) cells, and to evaluate the therapeutic efficacy of taxol using A549 tumor-bearing mice in vivo. Exposure of cells to taxol induced time-dependent cytotoxicity and cytoplasmic vacuolization without the involvement of Bax, Bak, Mcl-1, Bcl-XL, and caspase-3. Although taxol treatment induced activating transcription factor 6 (ATF6) cleavage indicative of endoplasmic reticulum (ER) stress, silencing ATF6 by shATF6 did not prevent taxol-induced both cytotoxcity and cytoplasmic vacuolization, suggesting that taxol-induced cytoplasmic vacuolization and cell death were not due to ER stress. Moreover, taxol-treated cells did not show DNA fragmentation and loss of mitochondrial membrane potential, the typical characteristics of apoptosis. In addition, taxol-induced cytoplasmic vacuolization did not show the cellular lysis, the characteristics of oncosis, and positive of β-galactosidase, the characteristic of senescence, indicating that taxol induced paraptosis-like cell death is neither oncosis nor senescence. Moreover, our in vivo data showed that intratumoral injection of taxol (50 mg/kg) in A549 tumor xenograft mice on day 1 and day 19 potently suppressed tumor growth showing significant ER vacuolization without toxicity. In conclusion, high concentration of taxol exhibits a significant anticancer activity by inducing paraptosis-like cell death in vitro and in vivo, without significant toxicity, suggesting a promising therapeutic strategy for apoptosis-resistance cancer by inducing ER vacuolization.

Protective effect of the poly(ADP-ribose) polymerase inhibitor PJ34 on mitochondrial depolarization-mediated cell death in hepatocellular carcinoma cells involves attenuation of c-Jun N-terminal kinase-2 and protein kinase B/Akt activation

Background

2,4-Dimethoxyphenyl-E-4-arylidene-3-isochromanone (IK11) was previously described to induce apoptotic death of A431 tumor cells. In this report, we investigated the molecular action of IK11 in the HepG2 human hepatocellular carcinoma cell line to increase our knowledge of the role of poly (ADP-ribose)-polymerase (PARP), protein kinase B/Akt and mitogen activated protein kinase (MAPK) activation in the survival and death of tumor cells and to highlight the possible role of PARP-inhibitors in co-treatments with different cytotoxic agents in cancer therapy.

Results

We found that sublethal concentrations of IK11 prevented proliferation, migration and entry of the cells into their G2 phase. At higher concentrations, IK11 induced reactive oxygen species (ROS) production, mitochondrial membrane depolarization, activation of c-Jun N-terminal kinase 2 (JNK2), and substantial loss of HepG2 cells. ROS production appeared marginal in mediating the cytotoxicity of IK11 since N-acetyl cysteine was unable to prevent it. However, the PARP inhibitor PJ34, although not a ROS scavenger, strongly inhibited both IK11-induced ROS production and cell death. JNK2 activation seemed to be a major mediator of the effect of IK11 since inhibition of JNK resulted in a substantial cytoprotection while inhibitors of the other kinases failed to do so. Inhibition of Akt slightly diminished the effect of IK11, while the JNK and Akt inhibitor and ROS scavenger trans-resveratrol completely protected against it.

Conclusions

These results indicate significant involvement of PARP, a marginal role of ROS and a pro-apoptotic role of Akt in this system, and raise attention to a novel mechanism that should be considered when cancer therapy is augmented with PARP-inhibition, namely the cytoprotection by inhibition of JNK2.

Poly(ADP-ribose) polymerase inhibition improves erectile function by activation of nitric oxide/cyclic guanosine monophosphate pathway in diabetic rats

INTRODUCTION

Endothelial dysfunction-induced abnormalities of the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signaling pathway in the corpus cavernosum are thought to be the main factors involved in the pathogenesis of diabetes-induced erectile dysfunction (ED). Recent studies have shown that the poly(adenosine diphosphate ribose) polymerase (PARP) pathway plays a critical role in diabetic endothelial dysfunction.

AIM

  The aim of this study is to determine whether activation of the PARP pathway is involved in diabetic cavernosal endothelial dysfunction and abnormalities of the NO/cGMP pathway.

METHODS

Male Sprague-Dawley rats were randomly divided into three groups: age-matched controls, diabetic controls (DM), and the 3-aminobenzamide (3-AB, a PARP inhibitor)-treated diabetic group (DM+3-AB). Diabetes was induced by intraperitoneal injection of streptozotocin. Eight weeks after inducing diabetes, the DM+3-AB group was treated with 3-AB for 4 weeks.

MAIN OUTCOME MEASURES

Erectile function was assessed at 12 weeks after inducing diabetes by stimulating the cavernous nerve. Expression of poly(ADP-ribose), protein kinase B (Akt), phospho-Akt, endothelial nitric oxide synthase (eNOS), phospho-eNOS, and neuronal nitric oxide synthase (nNOS) were evaluated by Western blot. Cavernous NO generation and cGMP levels were also determined.

RESULTS

The DM group showed impaired erectile function and significantly increased PARP activity. Expression of total eNOS and nNOS, phospho-Akt, and eNOS decreased significantly in the DM group compared with those in the control group. In addition, cavernous NO generation and cGMP levels decreased significantly in the DM group compared with those in the control group. Treatment with 3-AB restored erectile function and significantly reversed all molecular alterations except decreased nNOS expression.

CONCLUSION

Overactivation of the PARP pathway in the corpus cavernosum of diabetic rats was involved in cavernosal endothelial dysfunction and abnormalities of the NO/cGMP pathway resulting in ED. These findings may be applied to develop novel therapies for patients with diabetic ED.

Poly(ADP-ribose) polymerase 1 is involved in glucose toxicity through SIRT1 modulation in HepG2 hepatocytes

Accelerated glucose metabolism leads to oxidative stress and DNA damage in cells; these effects are related to glucose toxicity. The precise mechanisms of glucose toxicity are still unclear. The aim of this work was to investigate the mechanism of poly(ADP-ribose) polymerase 1 (PARP1), which is a DNA repair enzyme activated by high-glucose-induced oxidative stress, and its effect on glucose toxicity in HepG2 hepatocytes. HepG2 cells were cultured under normal (5.5 mM) or high (30 mM) glucose conditions for 4 days. PJ34, which is an inhibitor of PARP1, was used to determine the downstream effects of PARP1 activation. PARP1 activity in 30 mM-glucose-treated cells was more than that in 5.5 mM-glucose-treated cells, and the activity correlated with the increase in ROS generation and DNA damage. PJ34 suppressed PARP1 activation and prevented the high-glucose-induced suppression of SIRT1 and AMP-activated protein kinase (AMPK) activity, which was similar to its effect on the restoration of intracellular nicotinamide adenine dinucleotide (NAD) content. Further, the phosphorylation of insulin receptor was attenuated in response to insulin stimulation under high glucose conditions, and PJ34 could reverse this effect. The results of transfection of HepG2 cells with PARP1 small interfering RNA were similar to those obtained by treatment of the cells with PARP1 inhibitor PJ34. These data suggest that high-glucose-induced PARP1 activation might play a role in glucose toxicity by down-regulating SIRT1 and AMPK activity through NAD depletion and resulting in insulin insensitivity.

Poly(ADP-ribose) polymerase-1 inhibition: preclinical and clinical development of synthetic lethality

The hereditary forms of breast cancer identified by BRCA1 and BRCA2 genes have a defect in homologous DNA repair and demonstrate a dependence on alternate DNA repair processes by base excision repair, which requires poly(ADP-ribose) polymerase 1 (PARP-1). siRNA and deletion mutations demonstrate that interference with PARP-1 function results in enhanced cell death when the malignancy has a defect in homologous recombination. These findings resulted in a plethora of agents in clinical trials that interfere with DNA repair, and these agents offer the potential of being more selective in their effects than classic chemotherapeutic drugs. An electronic search of the National Library of Medicine for published articles written in English used the terms "PARP inhibitors" and "breast cancer" to find prospective, retrospective and review articles. Additional searches were done for articles dealing with mechanism of action. A total of 152 articles dealing with breast cancer and PARP inhibition were identified. PARP inhibition not only affects nonhomologous repair, but also has several other nongenomic functions. Mutational resistance to these agents was seen in preclinical studies. To date, PARP-1 inhibitors were shown to enhance cytotoxic effects of some chemotherapy agents. This new class of agents may offer more therapeutic specificity by exploiting a DNA repair defect seen in some human tumors with initial clinical trials demonstrating antitumor activity. Although PARP inhibitors may offer a therapeutic option for selected malignancies, the long-term effects of these agents have not yet been defined.

Regulation of MKP-1 expression and MAPK activation by PARP-1 in oxidative stress: a new mechanism for the cytoplasmic effect of PARP-1 activation

Previously, it was suggested that the release of nuclearly formed ADP-ribose polymers or ADP-ribosylated proteins could be responsible for the cytosolic and mitochondrial effects of poly(ADP-ribose) polymerase (PARP)-1 activation in oxidative stress. In this report, we provide a novel alternative mechanism. We found that reactive oxygen species-activated PARP-1 regulated the activation of JNK and p38 mitogen-activated protein kinases (MAPKs) because inhibition of PARP-1 by pharmacons, small interfering RNA silencing of PARP-1 expression, or the transdominant expression of enzymatically inactive PARP-1 resulted in the inactivation of these MAPKs. This regulation was achieved by increased expression and enlarged cytoplasmic localization of MAPK phosphatase-1 (MKP-1) upon PARP-1 inhibition in oxidative stress because changes in MKP-1 expression were reflected in the phosphorylation states of JNK and p38. Furthermore, we found that in MKP-1-silenced cells, PARP inhibition was unable to exert its protective effect, indicating the pivotal roles of JNK and p38 in mediating the oxidative-stress-induced cell death as well as that of increased MKP-1 expression in mediating the protective effect of PARP inhibition. We suggest that regulation of a protein that can directly influence cytoplasmic signaling cascades at the expression level represents a novel mechanism for the cytoplasmic action of PARP-1 inhibition.

Making the best of PARP inhibitors in ovarian cancer

Drugs that inhibit the enzyme poly(ADP-ribose)polymerase (PARP) are showing considerable promise for the treatment of cancers that have mutations in the BRCA1 or BRCA2 tumor suppressors. This therapeutic approach exploits a synthetic lethal strategy to target the specific DNA repair pathway in these tumors. High-grade ovarian cancers have a generally poor prognosis, and accumulating evidence suggests that mutations in BRCA1 or BRCA2, or silencing of BRCA1 by promoter methylation, may be common in this disease. Here, we consider how the potential benefit of PARP inhibitors might be maximized in ovarian cancer. We suggest that it will be crucial to explore novel therapeutic trial strategies and drug combinations, and incorporate robust biomarkers predictive of response if these drugs are to reach their full potential.