Impact of DNA repair pathways on the cytotoxicity of piperlongumine in chicken DT40 cell-lines

The mutagenic consequences of defective DNA repair

Multiple separate repair mechanisms safeguard the genome against various types of DNA damage, and their failure can increase the rate of spontaneous mutagenesis. The malfunction of distinct repair mechanisms leads to genomic instability through different mutagenic processes. For example, defective mismatch repair causes high base substitution rates and microsatellite instability, whereas homologous recombination deficiency is characteristically associated with deletions and chromosome instability. This review presents a comprehensive collection of all mutagenic phenotypes associated with the loss of each DNA repair mechanism, drawing on data from a variety of model organisms and mutagenesis assays, and placing greatest emphasis on systematic analyses of human cancer datasets. We describe the latest theories on the mechanism of each mutagenic process, often explained by reliance on an alternative repair pathway or the error-prone replication of unrepaired, damaged DNA. Aided by the concept of mutational signatures, the genomic phenotypes can be used in cancer diagnosis to identify defective DNA repair pathways.

The Antitumor Activity of Piplartine: A Review

Cancer is a worldwide health problem with high mortality in children and adults, making searching for novel bioactive compounds with potential use in cancer treatment essential. Piplartine, also known as piperlongumine, is an alkamide isolated from Piper longum Linn, with relevant therapeutic potential. Therefore, this review covered research on the antitumor activity of piplartine, and the studies reported herein confirm the antitumor properties of piplartine and highlight its possible application as an anticancer agent against various types of tumors. The evidence found serves as a reference for advancing mechanistic research on this metabolite and preparing synthetic derivatives or analogs with better antitumor activity in order to develop new drug candidates.

Decrypting the interaction pattern of Piperlongumine with calf thymus DNA and dodecamer d(CGCGAATTCGCG)2 B-DNA: Biophysical and molecular docking analysis

The mechanisms of interaction of DNA with pharmacological molecules are critical to understanding their therapeutic actions on physiological systems. Piperlongumine is widely studied for its anticancer potential. Multi-spectrometry, calorimetry and in silico studies were employed to study the interaction of piperlongumine and calf thymus DNA. UV-Vis spectroscopy illustrated a hyperchromic pattern in spectra of the calf thymus DNA-piperlongumine complex, while fluorescent quenching was observed in emission spectral studies. Competitive displacement assay demonstrated higher displacement and binding constant for DNA-rhodamine B complex by piperlongumine than DNA-methylene blue complex. Differential scanning calorimetry presented non-significant changes in melting temperature and molecular docking presented the precise interaction site of piperlongumine with calf thymus DNA at minor groove. Further, piperlongumine treatment did not result in pBluescript KS plasmid DNA cleavage as revealed from the DNA topology assay. All these experiments confirmed the binding of piperlongumine with DNA through minor groove binding mode.

Dichloroacetate Radiosensitizes Hypoxic Breast Cancer Cells

Mitochondrial metabolism is an attractive target for cancer therapy. Reprogramming metabolic pathways can potentially sensitize tumors with limited treatment options, such as triple-negative breast cancer (TNBC), to chemo- and/or radiotherapy. Dichloroacetate (DCA) is a specific inhibitor of the pyruvate dehydrogenase kinase (PDK), which leads to enhanced reactive oxygen species (ROS) production. ROS are the primary effector molecules of radiation and an increase hereof will enhance the radioresponse. In this study, we evaluated the effects of DCA and radiotherapy on two TNBC cell lines, namely EMT6 and 4T1, under aerobic and hypoxic conditions. As expected, DCA treatment decreased phosphorylated pyruvate dehydrogenase (PDH) and lowered both extracellular acidification rate (ECAR) and lactate production. Remarkably, DCA treatment led to a significant increase in ROS production (up to 15-fold) in hypoxic cancer cells but not in aerobic cells. Consistently, DCA radiosensitized hypoxic tumor cells and 3D spheroids while leaving the intrinsic radiosensitivity of the tumor cells unchanged. Our results suggest that although described as an oxidative phosphorylation (OXPHOS)-promoting drug, DCA can also increase hypoxic radioresponses. This study therefore paves the way for the targeting of mitochondrial metabolism of hypoxic cancer cells, in particular to combat radioresistance.

Piperlongumine induces ROS mediated cell death and synergizes paclitaxel in human intestinal cancer cells

Piperlongumine (PL), a herbal drug extracted from long pepper (Piper longum L), is known for its anti-inflammatory and anti-cancer properties. Although, its anti-cancer potential has been evaluated in cancer models like breast, pancreatic, gastric, hepatocellular and lung carcinoma, there is no report on its bio-activity evaluation in intestinal cancers. Here, we report the anti-neoplastic potential of PL against human intestinal carcinoma in-vitro and its possible mechanisms of action. Cytotoxicity studies demonstrate that PL inhibits cell proliferation of INT-407 and HCT-116 cells in a concentration and time-dependent manner. Also, PL elevated the levels of intracellular reactive oxygen species, which may lead to lethal oxidative stress, mitochondrial dysfunction, and nuclear fragmentation. Remarkably, P53, P21, BAX, and SMAD4 were significantly upregulated after PL treatment whereas; BCL2 and SURVIVIN were down-regulated. Moreover, the combination study also shows the synergistic effect of PL with the current chemotherapeutic drug paclitaxel. These findings suggest that PL possesses anti-neoplastic properties in intestinal cancer cells.

Participation of TDP1 in the repair of formaldehyde-induced DNA-protein cross-links in chicken DT40 cells

Proteins are covalently trapped on DNA to form DNA-protein cross-links (DPCs) when cells are exposed to DNA-damaging agents. Aldehyde compounds produce common types of DPCs that contain proteins in an undisrupted DNA strand. Tyrosyl-DNA phosphodiesterase 1 (TDP1) repairs topoisomerase 1 (TOPO1) that is trapped at the 3’-end of DNA. In the present study, we examined the contribution of TDP1 to the repair of formaldehyde-induced DPCs using a reverse genetic strategy with chicken DT40 cells. The results obtained showed that cells deficient in TDP1 were sensitive to formaldehyde. The removal of formaldehyde-induced DPCs was slower in tdp1-deficient cells than in wild type cells. We also found that formaldehyde did not produce trapped TOPO1, indicating that trapped TOPO1 was not a primary cytotoxic DNA lesion that was generated by formaldehyde and repaired by TDP1. The formaldehyde treatment resulted in the accumulation of chromosomal breakages that were more prominent in tdp1-deficient cells than in wild type cells. Therefore, TDP1 plays a critical role in the repair of formaldehyde-induced DPCs that are distinct from trapped TOPO1.

CTDP1 regulates breast cancer survival and DNA repair through BRCT-specific interactions with FANCI

BRCA1 C-terminal domains are found in a specialized group of 23 proteins that function in the DNA damage response to protect genomic integrity. C-terminal domain phosphatase 1 (CTDP1) is the only phosphatase with a BRCA1 C-terminal domain in the human proteome, yet direct participation in the DNA damage response has not been reported. Examination of the CTDP1 BRCA1 C-terminal domain-specific protein interaction network revealed 103 high confidence interactions enriched in DNA damage response proteins, including FANCA and FANCI that are central to the Fanconi anemia DNA repair pathway necessary for the resolution of DNA interstrand crosslink damage. CTDP1 expression promotes DNA damage-induced FANCA and FANCD2 foci formation and enhances homologous recombination repair efficiency. CTDP1 was found to regulate multiple aspects of FANCI activity, including chromatin localization, interaction with γ-H2AX, and SQ motif phosphorylations. Knockdown of CTDP1 increases MCF-10A sensitivity to DNA interstrand crosslinks and double-strand breaks, but not ultraviolet radiation. In addition, CTDP1 knockdown impairs in vitro and in vivo growth of breast cancer cell lines. These results elucidate the molecular functions of CTDP1 in Fanconi anemia interstrand crosslink repair and identify this protein as a potential target for breast cancer therapy.

Piperlongumine increases sensitivity of colorectal cancer cells to radiation: Involvement of ROS production via dual inhibition of glutathione and thioredoxin systems

Piperlongumine (PL), naturally synthesized in long pepper, is known to selectively kill tumor cells via perturbation of reactive oxygen species (ROS) homeostasis. ROS are the primary effector molecules of radiation, and increase of ROS production by pharmacological modulation is known to enhance radioresponse. We therefore investigated the radiosensitizing effect of PL in colorectal cancer cells (CT26 and DLD-1) and CT26 tumor-bearing mice. Firstly, we found that PL induced excessive production of ROS due to depletion of glutathione and inhibition of thioredoxin reductase. Secondly, PL enhanced both the intrinsic and hypoxic radiosensitivity of tumor cells, linked to ROS-mediated increase of DNA damage, G2/M cell cycle arrest, and inhibition of cellular respiration. Finally, the radiosensitizing effect of PL was verified in vivo. PL improved the tumor response to both single and fractionated radiation, resulting in a significant increase of survival rate of tumor-bearing mice, while it was ineffective on its own. In line with in vitro findings, enhanced radioresponse is associated with inhibition of antioxidant systems. In conclusion, our results suggest that PL could be a potential radiosensitizer in colorectal cancer.

Determination of genotoxic potential by comparison of structurally related azo dyes using DNA repair-deficient DT40 mutant panels

Azo dyes, including Sudan I, Orange II and Orange G, are industrial dyes that are assumed to have genotoxic potential. However, neither the type of DNA damage induced nor the structural features responsible for toxicity have been determined. We used a panel of DNA-repair-pathway-deficient mutants generated from chicken DT40 cells to evaluate the ability of these azo dyes to induce DNA damage and to identify the type of DNA damage induced. We compared the structurally related azo dyes Sudan I, Orange II and Orange G to identify the structural features responsible for genotoxicity. Compared with wild type cells, the double-strand break repair defective RAD54^-/-/KU70^-/- cells were significantly more sensitive to Sudan I, but not to Orange II or Orange G. The quantum-chemical calculations revealed that Sudan I, but not Orange II or Orange G, has a complete planar aromatic ring structure. These suggest that the planar feature of Sudan I is critical to the inducing of double-strand breaks. In summary, we used a DNA-repair mutant panel in combination with quantum-chemical calculations to provide a clue to the chemical structure responsible for genotoxicity.

Synthesis and Antileukemic Activities of Piperlongumine and HDAC Inhibitor Hybrids against Acute Myeloid Leukemia Cells

Synergistic-to-additive antileukemic interactions of piperlongumine (PL) and HDAC inhibitor (HDACi) SAHA (Vorinostat) provide a compelling rationale to construct PL-HDACi hybrids, such as 1-58, which recapitulated the synergism between the parental compounds in high-risk and chemoresistant AML cells. Both PL and HDACi components, either in combination or in hybrid molecules, are essential for inducing significant DNA damage and apoptosis. Introducing C2-chloro substituent to 1-58 yielded 3-35 with increased cytotoxicity but decreased selectivity in noncancerous MCF-10A cells; eliminating C7-C8 olefin of PL obtained 3-31/3-98 scaffolds which were still more active than PL or SAHA in AML and were well-tolerated by MCF-10A cells. The HDACi function was crucial for modulating expression of DNA repair and apoptosis-related proteins. Collectively, PL and SAHA hybrids are potent, multifunctional anti-AML agents, acting in part, by interfering cellular GSH defense, suppressing expression of DNA repair and pro-survival proteins, and inducing expression of pro-apoptotic proteins.

Targeted Inhibition of Glutamine-Dependent Glutathione Metabolism Overcomes Death Resistance Induced by Chronic Cycling Hypoxia

AIMS

Tumor hypoxia is a major biological factor causing poor patient outcome. Evidence is increasing that improved protection against reactive oxygen species (ROS) participates in therapy resistance of chronically hypoxic cancer cells. We aimed at characterizing the relevance of improved ROS defense for radiation resistance of cancer cells with tolerance to cycling anoxia/re-oxygenation stress ("anoxia-tolerant") and at designing rational treatment strategies for overcoming the resulting therapy resistance by targeting the underlying mechanisms identified in an in vitro model.

RESULTS

We demonstrate that chronic exposure of NCH-H460 lung adenocarcinoma, DU145 prostate cancer, and T98G glioblastoma cells to cycling anoxia/re-oxygenation stress induced upregulation of the aspartate-aminotransferase glutamic-oxaloacetic transaminase (GOT1), particularly in RAS-driven anoxia-tolerant NCI-H460 cells. Altered glutamine utilization of the anoxia-tolerant cancer cells contributed to the observed decrease in cellular ROS levels, the increase in cellular glutathione levels, and improved cell survival on ROS-inducing treatments, including exposure to ionizing radiation. Importantly, targeting glutamine-dependent antioxidant capacity or glutathione metabolism allowed us to hit anoxia-tolerant cancer cells and to overcome their increased resistance to radiation-induced cell death. Targeting glutathione metabolism by Piperlongumine also improved the radiation response of anoxia-tolerant NCI-H460 cells in vivo.

INNOVATION

Improved antioxidant capacity downstream of up-regulated GOT1-expression is a characteristic of anoxia-tolerant cancer cells and is predictive for a specific vulnerability to inhibition of glutamine utilization or glutathione metabolism, respectively.

CONCLUSION

Unraveling the molecular alterations underlying improved ROS defense of anoxia-tolerant cancer cells allows the design of rational strategies for overcoming radiation resistance caused by tumor cell heterogeneity in hypoxic tumors. Antioxid. Redox Signal. 25, 89-107.

Cytotoxic and genotoxic profiles of benzo[a]pyrene and N-nitrosodimethylamine demonstrated using DNA repair deficient DT40 cells with metabolic activation

Benzo[a]pyrene and N-nitrosodimethylamine are major genotoxic compounds present in cigarette smoke, food and oil. To examine the type(s) of DNA damage induced by these compounds, we used a panel of DNA-repair-pathway-deficient mutants generated from chicken DT40 cells and achieved metabolic activation of the test compounds by including rat liver S9 mix. Consistent with expections, benzo[a]pyrene and N-nitrosodimethylamine require metabolicactivation to become genotoxic. The REV3(-/-) mutant cell line exhibited the highest sensitivity, in terms of increased cytotoxicity, to the both compounds after metabolic activation consistent with the known ability of these two compounds to induce DNA adducts. Strikingly, we found that the RAD54(-/-)/KU70(-/-) cell line, a mutant defective in the repair of double-strand breaks, is sensitive to benzo[a]pyrene, suggesting that this compound also induces strand breaks in these cells. In this study we combined a previously employed method, metabolic activation by S9 mix, with the use of a DNA-repair mutant panel, thereby broadening the range of compounds that can be screened for potential genotoxicity.

Enzymatic Activity Assays for Base Excision Repair Enzymes in Cell Extracts from Vertebrate Cells

We previously reported enzymatic activity assays for the base excision repair (BER) enzymes DNA polymerase β (pol β), aprataxin (APTX), and flap endonuclease 1 (FEN1) in cell extracts from Saccharomyces cerevisiae (Çağlayan and Wilson, 2014). Here, we describe a method to prepare cell extracts from vertebrate cells to investigate these enzymatic activities for the processing of the 5'-adenylated-sugar phosphate-containing BER intermediate. This new protocol complements our previous publication. The cell lines used are wild-type and APTX-deficient human lymphoblast cells from an Ataxia with Oculomotor Apraxia Type 1 (AOA1) disease patient, wild-type and APTX-null DT40 chicken B cells, and mouse embryonic fibroblast (MEF) cells. This protocol is a quick and efficient way to make vertebrate cell extracts without using commercial kits.