Direct involvement of the tumor suppressor p53 in nucleotide excision repair

Effects of in vivo treatment of mice with sulforaphane on repair of DNA pyridyloxylbutylation

The phytochemical sulforaphane (SF) has gained interest for its apparent association with reduced cancer risk and other cytoprotective properties, at least some of which are attributed to activation of the transcription factor Nrf2. Repair of bulky DNA adducts is important for mitigating carcinogenesis from exogenous DNA damaging agents, but it is unknown whether in vivo treatment with SF affects adduct repair. At 12 h following a single oral dose of 100 mg/kg SF, an almost doubling in activity for repair of pyridyloxobutylated DNA was observed in CD-1 mouse liver nuclear extracts, but not in lung extracts. This change at 12 h in repair activity was preceded by the induction of Nrf2-regulated genes but not accompanied by changes in levels of the specific nucleotide excision repair (NER) proteins XPC, XPA, XPB and p53 or in binding of hepatic XPC, XPA and XPB to damaged DNA. SF also did not significantly alter histone deacetylase activity as measured by acetylated histone H3 levels, or stimulate formation of γ-H2A.X, a marker of DNA damage. A significant reduction in oxidative DNA damage, as measured by 8-OHdG (a biomarker of oxidative DNA damage), was observed only in DNA from the lungs of SF-treated mice 3 h post-dosing. These results suggest that the ability of SF to increase bulky adduct repair activity is organ-selective and is consistent with activation of the Nrf2 signaling pathway.

Overexpression of PCNA Attenuates Oxidative Stress-Caused Delay of Gap-Filling during Repair of UV-Induced DNA Damage

UVC irradiation-caused DNA lesions are repaired in mammalian cells solely by nucleotide excision repair (NER), which consists of sequential events including initial damage recognition, dual incision of damage site, gap-filling, and ligation. We have previously shown that gap-filling during the repair of UV-induced DNA lesions may be delayed by a subsequent treatment of oxidants or prooxidants such as hydrogen peroxide, flavonoids, and colcemid. We considered the delay as a result of competition for limiting protein/enzyme factor(s) during repair synthesis between NER and base excision repair (BER) induced by the oxidative chemicals. In this report, using colcemid as oxidative stress inducer, we showed that colcemid-caused delay of gap-filling during the repair of UV-induced DNA lesions was attenuated by overexpression of PCNA but not ligase-I. PCNA knockdown, as expected, delayed the gap-filling of NER but also impaired the repair of oxidative DNA damage. Fen-1 knockdown, however, did not affect the repair of oxidative DNA damage, suggesting repair of oxidative DNA damage is not of long patch BER. Furthermore, overexpression of XRCC1 delayed the gap-filling, and presumably increase of XRCC1 pulls PCNA away from gap-filling of NER for BER, consistent with our hypothesis that delay of gap-filling of NER attributes the competition between NER and BER.

Loss of the N-terminal methyltransferase NRMT1 increases sensitivity to DNA damage and promotes mammary oncogenesis

Though discovered over four decades ago, the function of N-terminal methylation has mostly remained a mystery. Our discovery of the first mammalian N-terminal methyltransferase, NRMT1, has led to the discovery of many new functions for N-terminal methylation, including regulation of DNA/protein interactions, accurate mitotic division, and nucleotide excision repair (NER). Here we test whether NRMT1 is also important for DNA double-strand break (DSB) repair, and given its previously known roles in cell cycle regulation and the DNA damage response, assay if NRMT1 is acting as a tumor suppressor. We find that NRMT1 knockdown significantly enhances the sensitivity of breast cancer cell lines to both etoposide treatment and γ-irradiation, as well as, increases proliferation rate, invasive potential, anchorage-independent growth, xenograft tumor size, and tamoxifen sensitivity. Interestingly, this positions NRMT1 as a tumor suppressor protein involved in multiple DNA repair pathways, and indicates, similar to BRCA1 and BRCA2, its loss may result in tumors with enhanced sensitivity to diverse DNA damaging chemotherapeutics.

PHD1 regulates p53-mediated colorectal cancer chemoresistance

Overcoming resistance to chemotherapy is a major challenge in colorectal cancer (CRC) treatment, especially since the underlying molecular mechanisms remain unclear. We show that silencing of the prolyl hydroxylase domain protein PHD1, but not PHD2 or PHD3, prevents p53 activation upon chemotherapy in different CRC cell lines, thereby inhibiting DNA repair and favoring cell death. Mechanistically, PHD1 activity reinforces p53 binding to p38α kinase in a hydroxylation-dependent manner. Following p53-p38α interaction and chemotherapeutic damage, p53 can be phosphorylated at serine 15 and thus activated. Active p53 allows nucleotide excision repair by interacting with the DNA helicase XPB, thereby protecting from chemotherapy-induced apoptosis. In accord with this observation, PHD1 knockdown greatly sensitizes CRC to 5-FU in mice. We propose that PHD1 is part of the resistance machinery in CRC, supporting rational drug design of PHD1-specific inhibitors and their use in combination with chemotherapy.

Efficient one-pot synthesis of trans-Pt(ii)(salicylaldimine)(4-picoline)Cl complexes: effective agents for enhanced expression of p53 tumor suppressor genes

One-pot synthesizedtrans-Pt(ii)(salicylaldimine)(4-picoline)Cl complexes showed promisingin vitrocytotoxicity in MCF-7 and A549 cancer cell lines.

Up-regulation of nucleotide excision repair in mouse lung and liver following chronic exposure to aflatoxin B₁ and its dependence on p53 genotype

Aflatoxin B₁(AFB₁) is biotransformed in vivo into an epoxide metabolite that forms DNA adducts that may induce cancer if not repaired. p53 is a tumor suppressor gene implicated in the regulation of global nucleotide excision repair (NER). Male heterozygous p53 knockout (B6.129-Trp53(tm1Brd)N5, Taconic) and wild-type mice were exposed to 0, 0.2 or 1.0 ppm AFB₁ for 26 weeks. NER activity was assessed with an in vitro assay, using AFB₁-epoxide adducted plasmid DNA as a substrate. For wild-type mice, repair of AFB₁-N7-Gua adducts was 124% and 96% greater in lung extracts from mice exposed to 0.2 ppm and 1.0 ppm AFB₁respectively, and 224% greater in liver extracts from mice exposed to 0.2 ppm AFB₁( p<0.05). In heterozygous p53 knockout mice, repair of AFB₁-N7-Gua was only 45% greater in lung extracts from mice exposed to 0.2 ppm AFB₁ (p<0.05), and no effect was observed in lung extracts from mice treated with 1.0 ppm AFB₁or in liver extracts from mice treated with either AFB₁concentration. p53 genotype did not affect basal levels of repair. AFB₁exposure did not alter repair of AFB₁-derived formamidopyrimidine adducts in lung or liver extracts of either mouse genotype nor did it affect XPA or XPB protein levels. In summary, chronic exposure to AFB₁increased NER activity in wild-type mice, and this response was diminished in heterozygous p53 knockout mice, indicating that loss of one allele of p53 limits the ability of NER to be up-regulated in response to DNA damage.

Is the oxidative DNA damage level of human lymphocyte correlated with the antioxidant capacity of serum or the base excision repair activity of lymphocyte?

A random screening of human blood samples from 24 individuals of nonsmoker was conducted to examine the correlation between the oxidative DNA damage level of lymphocytes and the antioxidant capacity of serum or the base excision repair (BER) activity of lymphocytes. The oxidative DNA damage level was measured with comet assay containing Fpg/Endo III cleavage, and the BER activity was estimated with a modified comet assay including nuclear extract of lymphocytes for enzymatic cleavage. Antioxidant capacity was determined with trolox equivalent antioxidant capacity assay. We found that though the endogenous DNA oxidation levels varied among the individuals, each individual level appeared to be steady for at least 1 month. Our results indicate that the oxidative DNA damage level is insignificantly or weakly correlated with antioxidant capacity or BER activity, respectively. However, lymphocytes from carriers of Helicobacter pylori (HP) or Hepatitis B virus (HBV) tend to give higher levels of oxidative DNA damage (P < 0.05). Though sera of this group of individuals show no particular tendency with reduced antioxidant capacity, the respective BER activities of lymphocytes are lower in average (P < 0.05). Thus, reduction of repair activity may be associated with the genotoxic effect of HP or HBV infection.

Contrasting effects of Krüppel-like factor 4 on X-ray-induced double-strand and single-strand DNA breaks in mouse astrocytes

Astrocytes, the most common cell type in the brain, play a principal role in the repair of damaged brain tissues during external radiotherapy of brain tumours. As a downstream gene of p53, the effects of Krüppel-like factor 4 (KLF4) in response to X-ray-induced DNA damage in astrocytes are unclear. In the present study, KLF4 expression was upregulated after the exposure of astrocytes isolated from the murine brain. Inhibition of KLF4 expression using lentiviral transduction produced less double-strand DNA breaks (DSB) determined by a neutral comet assay and flow cytometric analysis of phosphorylated histone family 2A variant and more single-strand DNA breaks (SSB) determined by a basic comet assay when the astrocytes were exposed to 4 Gy of X-ray radiation. These data suggest that radiation exposure of the tissues around brain tumour during radiation therapy causes KLF4 overexpression in astrocytes, which induces more DSB and reduces SSB. This causes the adverse effects of radiation therapy in the treatment of brain tumours.

Aqueous extracts of the edible Gracilaria tenuistipitata are protective against H₂O₂-induced DNA damage, growth inhibition, and cell cycle arrest

Potential antioxidant properties of an aqueous extract of the edible red seaweed Gracilaria tenuistipitata (AEGT) against oxidative DNA damage were evaluated. The AEGT revealed several antioxidant molecules, including phenolics, flavonoids and ascorbic acid. In a cell-free assay, the extract exhibited 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity that significantly reduced H₂O₂-induced plasmid DNA breaks in a dose-response manner (P < 0.001). The AEGT also suppressed H₂O₂-induced oxidative DNA damage in H1299 cells by reducing the percentage of damaged DNA in a dose-response manner (P < 0.001) as measured by a modified alkaline comet-nuclear extract (comet-NE) assay. The MTT assay results showed that AEGT confers significant protection against H₂O₂-induced cytotoxicity and that AEGT itself is not cytotoxic (P < 0.001). Moreover, H₂O₂-induced cell cycle G2/M arrest was significantly released when cells were co-treated with different concentrations of AEGT (P < 0.001). Taken together, these findings suggest that edible red algae Gracilaria water extract can prevent H₂O₂-induced oxidative DNA damage and its related cellular responses.

Differences in nucleotide excision repair capacity between newly diagnosed colorectal cancer patients and healthy controls

Alteration of DNA integrity is a potential cause of cancer and it is assumed that reduced DNA repair capacity and accumulation of DNA damage may represent intermediate markers in carcinogenesis. In this case-control study, DNA damage and nucleotide excision repair capacity (NER-DRC) were assessed in association with sporadic colorectal cancer (CRC). Both parameters were quantified by comet assay in blood cells of 70 untreated incident patients and 70 age-matched healthy controls. mRNA expression and polymorphisms in relevant NER genes were concurrently analyzed. The aim of this study was to characterize incident CRC patients for NER-DRC and to clarify possible relations between investigated variables. Comet assay and mRNA expression analysis showed that CRC patients differ in repair capacity as compared to controls. Patients had a lower NER-DRC and simultaneously they exhibited higher endogenous DNA damage (for both P < 0.001). Accumulation of DNA damage and decreasing NER-DRC behaved as independent modulating parameters strongly associated with CRC. Expression levels of 6 out of 9 studied genes differed between groups (P ≤ 0.001), but none of them was related to DRC or to any of the studied NER polymorphisms. However, in patients only, XPC Ala499Val modulated expression levels of XPC, XPB and XPD gene, whereas XPC Lys939Gln was associated with XPA expression level in controls (for all P < 0.05). This study provides evidence on altered DRC and DNA damage levels in sporadic CRC and proposes the relevance of the NER pathway in this malignancy. Further, alterations in a complex multigene process like DNA repair may be better characterized by functional quantification of repair capacity than by quantification of individual genes transcripts or gene variants alone.

(-)-Anonaine induces DNA damage and inhibits growth and migration of human lung carcinoma h1299 cells

The anticancer effects of (-)-anonaine were investigated in this current study. (-)-Anonaine at concentration ranges of 50-200 μM exhibited significant inhibition to cell growth and migration activities on human lung cancer H1299 cells at 24 h, albeit cell cycle analyses showed that (-)-anonaine at the above concentration ranges did not cause any significant changes in cell-cycle distributions. Significant nuclear damages of H1299 cells were observed with 10-200 μM (-)-anonaine treatment in a comet assay, whereas higher concentrations (6 and 30 mM) of (-)-anonaine concentrations were required to cause DNA damages in an in vitro plasmid cleavage assay. In summary, our results demonstrated that (-)-anonaine exhibited dose-dependent antiproliferatory, antimigratory, and DNA-damaging effects on H1299 cells. We inferred that (-)-anonaine can cause cell-cycle arrest and DNA damage to hamper the physiological behavior of cancer cells at 72 h, and therefore, it can be useful as one of the potential herbal supplements for chemoprevention of human lung cancer.

Obtusilactone A and (-)-sesamin induce apoptosis in human lung cancer cells by inhibiting mitochondrial Lon protease and activating DNA damage checkpoints

Several compounds from Cinnamomum kotoense show anticancer activities. However, the detailed mechanisms of most compounds from C. kotoense remain unknown. In this study, we investigated the anticancer activity of obtusilactone A (OA) and (-)-sesamin in lung cancer. Our results show that human Lon is upregulated in non-small-cell lung cancer (NSCLC) cell lines, and downregulation of Lon triggers caspase-3 mediated apoptosis. Through enzyme-based screening, we identified two small-molecule compounds, obtusilactone A (OA) and (-)-sesamin from C. kotoense, as potent Lon protease inhibitors. Obtusilactone A and (-)-sesamin interact with Ser855 and Lys898 residues in the active site of the Lon protease according to molecular docking analysis. Thus, we suggest that cancer cytotoxicity of the compounds is partly due to the inhibitory effects on Lon protease. In addition, the compounds are able to cause DNA double-strand breaks and activate checkpoints. Treatment with OA and (-)-sesamin induced p53-independent DNA damage responses in NSCLC cells, including G(1) /S checkpoint activation and apoptosis, as evidenced by phosphorylation of checkpoint proteins (H2AX, Nbs1, and Chk2), caspase-3 cleavage, and sub-G(1) accumulation. In conclusion, OA and (-)-sesamin act as both inhibitors of human mitochondrial Lon protease and DNA damage agents to activate the DNA damage checkpoints as well induce apoptosis in NSCLC cells. These dual functions open a bright avenue to develop more selective chemotherapy agents to overcome chemoresistance and sensitize cancer cells to other chemotherapeutics.

Absence of p53-dependent apoptosis leads to UV radiation hypersensitivity, enhanced immunosuppression and cellular senescence

Genotoxic stress triggers the p53 tumor suppressor network to activate cellular responses that lead to cell cycle arrest, DNA repair, apoptosis or senescence. This network functions mainly through transactivation of different downstream targets, including cell cycle inhibitor p21, which is required for short-term cell cycle arrest or long-term cellular senescence, or proapoptotic genes such as p53 upregulated modulator of apoptosis (PUMA) and Noxa. However, the mechanism that switches from cell cycle arrest to apoptosis is still unknown. In this study, we found that mice harboring a hypomorphic mutant p53, R172P, a mutation that abrogates p53-mediated apoptosis while keeping cell cycle control mostly intact, are more susceptible to ultraviolet-B (UVB)-induced skin damage, inflammation and immunosuppression than wild-type mice. p53(R172P) embryonic fibroblasts (MEFs) are hypersensitive to UVB and prematurely senesce after UVB exposure, in stark contrast to wild-type MEFs, which undergo apoptosis. However, these mutant cells are able to repair UV-induced DNA lesions, indicating that the UV hypersensitive phenotype results from the subsequent damage response. Mutant MEFs show an induction of p53 and p21 after UVR, while wild-type MEFs additionally induce PUMA and Noxa. Importantly, p53(R172P) MEFs failed to downregulate anti-apoptotic protein Bcl-2, which has been shown to play an important role in p53-dependent apoptosis. Taken together, these data demonstrate that in the absence of p53-mediated apoptosis, cells undergo cellular senescence to prevent genomic instability. Our results also indicate that p53-dependent apoptosis may play an active role in balancing cellular growth.

The oncogenic phosphatase WIP1 negatively regulates nucleotide excision repair

Nucleotide excision repair (NER) is the only mechanism in humans to repair UV-induced DNA lesions such as pyrimidine (6-4) pyrimidone photoproducts and cyclobutane pyrimidine dimers (CPDs). In response to UV damage, the ataxia telangiectasia mutated and Rad3-related (ATR) kinase phosphorylates and activates several downstream effector proteins, such as p53 and XPA, to arrest cell cycle progression, stimulate DNA repair, or initiate apoptosis. However, following the completion of DNA repair, there must be active mechanisms that restore the cell to a prestressed homeostatic state. An important part of this recovery must include a process to reduce p53 and NER activity as well as to remove repair protein complexes from the DNA damage sites. Since activation of the damage response occurs in part through phosphorylation, phosphatases are obvious candidates as homeostatic regulators of the DNA damage and repair responses. Therefore, we investigated whether the serine/threonine wild-type p53-induced phosphatase 1 (WIP1/PPM1D) might regulate NER. WIP1 overexpression inhibits the kinetics of NER and CPD repair, whereas WIP1 depletion enhances NER kinetics and CPD repair. This NER suppression is dependent on WIP1 phosphatase activity, as phosphatase-dead WIP1 mutants failed to inhibit NER. Moreover, WIP1 suppresses the kinetics of UV-induced damage repair largely through effects on NER, as XPD-deficient cells are not further suppressed in repairing UV damage by overexpressed WIP1. Wip1 null mice quickly repair their CPD and undergo less UV-induced apoptosis than their wild-type counterparts. In vitro phosphatase assays identify XPA and XPC as two potential WIP1 targets in the NER pathway. Thus WIP1 may suppress NER kinetics by dephosphorylating and inactivating XPA and XPC and other NER proteins and regulators after UV-induced DNA damage is repaired.

4beta-Hydroxywithanolide E from Physalis peruviana (golden berry) inhibits growth of human lung cancer cells through DNA damage, apoptosis and G2/M arrest

Background

The crude extract of the fruit bearing plant, Physalis peruviana (golden berry), demonstrated anti-hepatoma and anti-inflammatory activities. However, the cellular mechanism involved in this process is still unknown.

Methods

Herein, we isolated the main pure compound, 4β-Hydroxywithanolide (4βHWE) derived from golden berries, and investigated its antiproliferative effect on a human lung cancer cell line (H1299) using survival, cell cycle, and apoptosis analyses. An alkaline comet-nuclear extract (NE) assay was used to evaluate the DNA damage due to the drug.

Results

It was shown that DNA damage was significantly induced by 1, 5, and 10 μg/mL 4βHWE for 2 h in a dose-dependent manner (p < 0.005). A trypan blue exclusion assay showed that the proliferation of cells was inhibited by 4βHWE in both dose- and time-dependent manners (p < 0.05 and 0.001 for 24 and 48 h, respectively). The half maximal inhibitory concentrations (IC₅₀) of 4βHWE in H1299 cells for 24 and 48 h were 0.6 and 0.71 μg/mL, respectively, suggesting it could be a potential therapeutic agent against lung cancer. In a flow cytometric analysis, 4βHWE produced cell cycle perturbation in the form of sub-G₁ accumulation and slight arrest at the G₂/M phase with 1 μg/mL for 12 and 24 h, respectively. Using flow cytometric and annexin V/propidium iodide immunofluorescence double-staining techniques, these phenomena were proven to be apoptosis and complete G₂/M arrest for H1299 cells treated with 5 μg/mL for 24 h.

Conclusions

In this study, we demonstrated that golden berry-derived 4βHWE is a potential DNA-damaging and chemotherapeutic agent against lung cancer.

Fern plant-derived protoapigenone leads to DNA damage, apoptosis, and G(2)/m arrest in lung cancer cell line H1299

Protoapigenone, isolated from the native fern plant Thelypteris torresiana, has anticancer activity against some cancer cells. However, the toxicological mechanism for protoapigenone is still unknown. Here, we investigated the anticancer effect of protoapigenone on human lung cancer cell lines. The comet assay showed that DNA damage induced by protoapigenone is dose-dependent. Trypan blue exclusion showed that the cell killing by protoapigenone is both time and dose dependent. The IC(50) of protoapigenone for 12, 24, and 48 h in H1299 cells is 6.11, 2.74, and 1.49 microM, respectively. Flow cytometry showed cell cycle perturbation such as sub-G(1) accumulation (at 1.57 microM for 48 h and at 3.57 microM for 12 and 24 h) and G(2)/M arrest (at 3.57 microM for 12 and 24 h) for protoapigenone. The sub-G(1) accumulation phenomena in the 3.57 microM for 24 h sample were shown to be apoptosis using Annexin V-immunofluorescence/propidium iodide staining. These results suggest protoapigenone is a potential chemotherapeutic agent for lung cancers.

In vivo treatment with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) induces organ-specific alterations in in vitro repair of DNA pyridyloxobutylation

To investigate the mechanisms responsible for inter-organ differences in susceptibility to 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK)-induced carcinogenesis, the objectives were to compare DNA repair activities of extracts from mouse lung and liver towards NNK-induced pyridyloxobutyl (POB) damage to plasmid DNA, and to determine if and the mechanism by which in vivo NNK treatment of mice alters DNA repair. Repair activity of POB adducts was three times greater in mouse liver than in mouse lung (P<0.05). Repair activities of lung extracts from mice 4 or 24 h post-NNK treatment were 30-45% those of control (P<0.05). Conversely, POB adduct repair was 2-3 times higher in liver extracts from NNK treated mice than in controls (4 h, 24 h, P<0.05). NNK treatment also decreased incision of POB adducts by 92% (4 h, P<0.05) in lung and increased incision by 169% (24 h, P<0.05) in liver. NNK decreased immunoreactive levels of the incision protein RPA in lung (P<0.05) 4 h post-treatment but increased immunoreactive lung RPA and XPB after 24 h (P<0.05). In liver, levels of immunoreactive proteins, XPA, XPB and ERCC1 were increased after NNK treatment (24 h, P<0.05). Binding of XPA and XPB from liver extracts to POB adducts increased following NNK treatment, while binding of XPA and XPB from lung decreased (4 h, 24 h). These results suggest that lower incision activity of nucleotide excision repair and NNK-mediated alterations in levels and activities of key incision proteins contribute to the relative susceptibility of mouse lung to NNK-induced carcinogenesis.

Histone deacetylase inhibitor sodium butyrate enhances the cell killing effect of psoralen plus UVA by attenuating nucleotide excision repair

The use of histone deacetylase inhibitors (HDACI), a promising new class of antineoplastic agents, in combination with cytotoxic agents, such as ionizing radiation and anticancer drugs, has been attracting attention. In this study, we found that sodium butyrate (SB), a widely studied HDACI, remarkably enhanced the cell killing effect of psoralen plus UVA (PUVA) in several cancer cell lines, including skin melanoma. Although a single treatment with PUVA or SB did not greatly affect cell survival, combined treatment with SB and PUVA induced marked apoptosis within 24 hours. The SB-induced augmentation of the cell killing effect was more dramatic in combination with PUVA than with anticancer drugs. The number of double-strand breaks that formed during the repair of PUVA-induced interstrand cross-links (ICL) in chromosomal DNA was significantly reduced in SB-pretreated cells, suggesting that the ability to repair ICL was attenuated by SB. In addition, the incorporation of bromodeoxyuridine and the formation of repair foci of proliferating cell nuclear antigen after PUVA treatment, associated with nucleotide excision repair (NER) in the removal of ICL, were not observed in SB-pretreated cells. Furthermore, the repair kinetics of UV-induced cyclobutane pyrimidine dimers (well-known photolesions repaired by NER) were much slower in SB-pretreated cells than in untreated cells. These results indicated that the enhanced cell killing effect of PUVA by SB was attributable to an attenuated ability to repair DNA and, especially, dysfunctional NER.