Role of benzo[alpha]pyrene in generation of clustered DNA damage in human breast tissue

Notch-3 affects chemoresistance in colorectal cancer via DNA base excision repair enzymes

Colon cancer is the second leading cause of cancer death. With over 153,000 new CRC cases predicted, it is the third most commonly diagnosed cancer. Early detection can lead to curative surgical intervention, but recurrent and late metastatic disease is frequently treated with chemotherapeutic options based on induction of DNA damage. Understanding mechanism(s) that regulate DNA damage repair within colon tumor cells is essential to developing effective therapeutic strategies. The Notch signaling pathway is known to participate in normal colon development and we have recently described a pathway by which Notch-1, Notch-3 and Smad may regulated EMT and stem-like properties in colon tumor cells, promoting tumorigenesis. Little is known about how Notch may regulate drug resistance. In this study, we used shRNA to generate colon tumor cells with loss of Notch-3 expression. These cells exhibited reduced expression of the base-excision repair proteins PARP1 and APE1, along with increased sensitivity to ara-c and cisplatin. These data point to a pathway in which Notch-3 signaling can regulate DNA repair within colon tumor cells and suggests that targeting Notch-3 may be an effective approach to rendering colon tumors sensitive to chemotherapeutic drugs.

A potential requirement for Smad3 phosphorylation in Notch-mediated EMT in colon cancer

Colorectal cancer (CRC) remains a challenging disease to treat due to several factors including stemness and epithelial to mesenchymal transition (EMT). Dysfunctional signaling pathways such as Notch and TGF-β contribute to these phenomena. We previously found that cells expressing constitutively active Notch1 also had increased expression of Smad3, an important member of the TGF-β signaling pathway. We hypothesized that Smad3, mediates the Notch-induced stemness and EMT observed in CRC cells. The human colorectal carcinoma cell line HCT-116, stably transduced with constitutively active Notch-1 (ICN) or a GFP-vector control was treated with different combinations of TGF-β1, DAPT (a Notch inhibitor), or SIS3 (a Smad3 inhibitor). Western blot analysis was performed to determine the effects of Smad3 stimulation and inhibition on Notch and potential downstream EMT-related targets, CD44, Slug and Snail. Smad3 inhibition induced a decrease in Notch1 and Notch3 receptor expression and effectively inhibited CD44, Slug, and Snail expression. Colosphere forming ability was also reduced in cells with inhibited Smad3. These results indicate a key role of TGF-β signaling in Notch1-induced tumorigenesis, and suggest a potential use for Smad3 inhibitors in combination with Notch1 inhibitors that are already in use for CRC treatments.

Diallyl Sulfide Attenuation of Carcinogenesis in Mammary Epithelial Cells through the Inhibition of ROS Formation, and DNA Strand Breaks

Garlic has long been used medicinally for many diseases, including cancer. One of the active garlic components is diallyl sulfide (DAS), which prevents carcinogenesis and reduces the incidence rate of several cancers. In this study, non-cancerous MCF-10A cells were used as a model to investigate the effect of DAS on Benzo (a)pyrene (BaP)-induced cellular carcinogenesis. The cells were evaluated based on changes in proliferation, cell cycle arrest, the formation of peroxides, 8-hydroxy-2-deoxyguanosine (8-OHdG) levels, the generation of DNA strand breaks, and DNA Polymerase β (Pol β) expression. The results obtained indicate that when co-treated with BaP, DAS inhibited BaP-induced cell proliferation (p < 0.05) to levels similar to the negative control. BaP treatment results in a two-fold increase in the accumulation of cells in the G2/M-phase of the cell cycle, which is restored to baseline levels, similar to untreated cells and vehicle-treated cells, when pretreated with 6 μM and 60 μM DAS, respectively. Co-treatment with DAS (60 μM and 600 μM) inhibited BaP-induced reactive oxygen species (ROS) formation by 132% and 133%, respectively, as determined by the accumulation of H₂O₂ in the extracellular medium and an increase in 8-OHdG levels of treated cells. All DAS concentrations inhibited BaP-induced DNA strand breaks through co-treatment and pre-treatment methods at all time points evaluated. Co-Treatment with 60 μM DAS increased DNA Pol β expression in response to BaP-induced lipid peroxidation and oxidative DNA damage. These results indicate that DAS effectively inhibited BaP-induced cell proliferation, cell cycle transitions, ROS, and DNA damage in an MCF-10A cell line. These results provide more experimental evidence for garlic's antitumor abilities and corroborate many epidemiological studies regarding the association between the increased intake of garlic and the reduced risk of several types of cancer.

Carvedilol suppresses malignant proliferation of mammary epithelial cells through inhibition of the ROS‑mediated PI3K/AKT signaling pathway

Reactive oxygen species (ROS) cause oncogenic mutations through direct interaction with DNA. Carvedilol (CAR) exhibits antioxidative activity, and pre-clinical studies have identified that CAR may prevent malignant transformation in certain carcinogenic models. This suggests that CAR may be a potential agent in cancer prevention. In the present study, non-cancerous MCF-10A cells were used as a model to investigate the chemopreventive effect of CAR on benzo(a)pyrene (BaP)-induced cellular carcinogenesis. It was identified that CAR had the ability to eliminate BaP-induced ROS production and subsequent DNA damage. CAR/BaP activated the ROS-mediated phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)^Thr308 signaling pathway, whereas the effectors of the PI3K/AKT signaling pathway, murine double minute 2 (MDM2) and p53^Ser15, served important functions in the BaP/CAR-mediated MCF10A cellular transformation. The results of the present study indicated that CAR may be a novel chemopreventive agent, notably in the prevention of estrogen receptor-negative breast cancer. The antioxidant effects of CAR may contribute to its chemopreventive activity.

Filling the gap between chemical carcinogenesis and the hallmarks of cancer: A temporal perspective

BACKGROUND

Cancer is believed to arise through the perturbation of pathways and the order of pathway perturbation events can enhance understanding and evaluation of carcinogenicity. This order has not been examined so far, and this study aimed to fill this gap by attempting to gather evidence on the potential temporal sequence of events in carcinogenesis.

DESIGN

The methodology followed was to discuss first the temporal sequence of hallmarks of cancer from the point of view of pathological specimens of cancer (essentially branched mutations) and then to consider the hallmarks of cancer that one well-known carcinogen, benzo(a)pyrene, can modify.

RESULTS

Even though the sequential order of driving genetic alterations can vary between and within tumours, the main cancer pathways affected are almost ubiquitous and follow a generally common sequence: resisting cell death, insensitivity to antigrowth signals, sustained proliferation, deregulated energetics, replicative immortality and activation of invasion and metastasis. The first 3 hallmarks can be regarded as almost simultaneous while angiogenesis and avoiding immune destruction are perhaps the only hallmarks with a varying position in the above sequence.

CONCLUSIONS

Our review of hallmarks of cancer and their temporal sequence, based on mutational spectra in biopsies from different cancer sites, allowed us to propose a hypothetical temporal sequence of the hallmarks. This sequence can add molecular support to the evaluation of an agent as a carcinogen as it can be used as a conceptual framework for organising and evaluating the strength of existing evidence.

Notch-1 promotes stemness and epithelial to mesenchymal transition in colorectal cancer

Colorectal cancer (CRC) is the third leading cause of cancer death in the United States, resulting in an average of 50,000 deaths per year. Surgery and combination chemotherapy comprise current treatment strategies. However, curative options are limited if surgery and chemotherapy are unsuccessful. Several studies have indicated that CRC aggressiveness and potential for metastatic spread are associated with the acquisition of stem cell like properties. The Notch-1 receptor and its cognate signaling pathway is well known for controlling cell fate decisions and stem-cell phenotypes. Alterations in Notch receptors and Notch signaling has been reported for some colon cancers. Herein, we examine a potential role for Notch-1 signaling in CRC. In CRC patient samples, Notch-1 expression was increased in colon tumor tissue as compared with normal colon tissue. Retroviral transduction of constitutively active Notch-1 (ICN1) into the colon tumor cell line HCT-116 resulted in increased expression of the EMT/stemness associated proteins CD44, Slug, Smad-3, and induction of Jagged-1 expression. These changes in ICN1 expressing cells were accompanied by increased migration and increased anchorage independent growth by 2.5-fold and 23%, respectively. Experiments with the pan-Notch inhibitor DAPT, and soluble Jagged-1-Fc protein provided evidence that Notch-1 signaling activates CD44, Slug, and Smad-3 via a cascade of other Notch-receptors through induction of Jagged-1 expression. These data indicate a key role for Notch signaling in the phenotype of CRC and suggest that targeting of Notch signaling may be of therapeutic value in colon cancers.

Nonhomologous DNA end joining and chromosome aberrations in human embryonic lung fibroblasts treated with environmental pollutants

In order to evaluate the ability of a representative polycyclic aromatic hydrocarbon (PAH) and PAH-containing complex mixtures to induce double strand DNA breaks (DSBs) and repair of damaged DNA in human embryonic lung fibroblasts (HEL12469 cells), we investigated the effect of benzo[a]pyrene (B[a]P) and extractable organic matter (EOM) from ambient air particles <2.5μm (PM2.5) on nonhomologous DNA end joining (NHEJ) and induction of stable chromosome aberrations (CAs). PM2.5 was collected in winter and summer 2011 in two Czech cities differing in levels and sources of air pollutants. The cells were treated for 24h with the following concentrations of tested chemicals: B[a]P: 1μM, 10μM, 25μM; EOMs: 1μg/ml, 10μg/ml, 25μg/ml. We tested several endpoints representing key steps leading from DSBs to the formation of CAs including histone H2AX phosphorylation, levels of proteins Ku70, Ku80 and XRCC4 participating in NHEJ, in vitro ligation activity of nuclear extracts of the HEL12469 cells and the frequency of stable CAs assessed by whole chromosome painting of chromosomes 1, 2, 4, 5, 7 and 17 using fluorescence in situ hybridization. Our results show that 25μM of B[a]P and most of the tested doses of EOMs induced DSBs as indicated by H2AX phosphorylation. DNA damage was accompanied by induction of XRCC4 expression and an increased frequency of CAs. Translocations most frequently affected chromosome 7. We observed only a weak induction of Ku70/80 expression as well as ligation activity of nuclear extracts. In summary, our data suggest the induction of DSBs and NHEJ after treatment of human embryonic lung fibroblasts with B[a]P and complex mixtures containing PAHs.

Oxidative DNA damage is involved in ochratoxin A-induced G2 arrest through ataxia telangiectasia-mutated (ATM) pathways in human gastric epithelium GES-1 cells in vitro

Ochratoxin A (OTA), one of the most abundant mycotoxin food contaminants, is classified as "possibly carcinogenic to humans." Our previous study showed that OTA could induce a G2 arrest in immortalized human gastric epithelium cells (GES-1). To explore the putative roles of oxidative DNA damage and the ataxia telangiectasia-mutated (ATM) pathways on the OTA-induced G2 arrest, the current study systematically evaluated the roles of reactive oxygen species (ROS) production, DNA damage, and ATM-dependent pathway activation on the OTA-induced G2 phase arrest in GES-1 cells. The results showed that OTA exposure elevated intracellular ROS production, which directly induced DNA damage and increased the levels of 8-OHdG and DNA double-strand breaks (DSBs). In addition, it was found that OTA treatment induced the phosphorylation of the ATM protein, as well as its downstream molecules Chk2 and p53, in response to DNA DSBs. Inhibition of ATM by the pharmacological inhibitor caffeine or siRNA effectively prevented the activation of ATM-dependent pathways and rescued the G2 arrest elicited by OTA. Finally, pretreatment with the antioxidant N-acetyl-L-cysteine (NAC) reduced the OTA-induced DNA DSBs, ATM phosphorylation, and G2 arrest. In conclusion, the results of this study suggested that OTA-induced oxidative DNA damage triggered the ATM-dependent pathways, which ultimately elicited a G2 arrest in GES-1 cells.

The attenuation of early benzo(a)pyrene-induced carcinogenic insults by diallyl disulfide (DADS) in MCF-10A cells

Diallyl disulfide (DADS), a garlic organosulfur compound, has been researched as a cancer prevention agent; however, the role of DADS in the suppression of cancer initiation in nonneoplastic cells has not been elucidated. To evaluate DADS inhibition of early carcinogenic events, MCF-10A cells were pretreated (PreTx) with DADS followed by the ubiquitous carcinogen benzo(a)pyrene (BaP), or cotreated (CoTx) with DADS and BaP for up to 24 h. The cells were evaluated for changes in cell viability/proliferation, cell cycle, induction of peroxide formation, and DNA damage. BaP induced a statistically significant increase in cell proliferation at 6 h, which was attenuated with DADS CoTx. PreTx with 6 and 60 μM of DADS inhibited BaP-induced G2/M arrest by 68% and 78%, respectively. DADS, regardless of concentration or method, inhibited BaP-induced extracellular aqueous peroxide formation within 24 h. DADS attenuated BaP-induced DNA single-strand breaks at all time points through both DADS Pre- and CoTx, with significant inhibition for all treatments sustained after 6 h. DADS was effective in inhibiting BaP-induced cell proliferation, cell cycle transitions, reactive oxygen species, and DNA damage in a normal cell line, and thus may inhibit environmentally induced breast cancer initiation.

High molecular weight hyaluronan decreases oxidative DNA damage induced by EDTA in human corneal epithelial cells

PURPOSE

To investigate the toxic effects of ethylenediaminetetraacetic acid disodium salt (EDTA), a corneal penetration enhancer in topical ophthalmic formulations, on DNA in human corneal epithelial cells (HCEs), and to investigate whether the effect induced by EDTA can be inhibited by high molecular weight hyaluronan (HA).

METHODS

Cells were exposed to EDTA in concentrations ranging from 0.00001 to 0.01% for 60 min, or 30 min high molecular weight HA pretreatment followed by EDTA treatment. The cell viability was measured by the MTT test. Cell apoptosis was determined with annexin V staining by flow cytometry. The DNA single- and double-strand breaks of HCEs were examined by alkaline comet assay and by immunofluorescence microscope detection of the phosphorylated form of histone variant H2AX (γH2AX) foci, respectively. Reactive oxygen species (ROS) production was assessed by the fluorescent probe, 2', 7'-dichlorodihydrofluorescein diacetate.

RESULTS

EDTA exhibited no adverse effect on cell viability and did not induce cell apoptosis in human corneal epithelial cells at concentrations lower than 0.01%. However, a significant increase of DNA single- and double-strand breaks was observed in a dose-dependent manner with all the concentrations of EDTA tested in HCEs. In addition, EDTA treatment led to elevated ROS generation. Moreover, 30 min preincubation with high molecular weight HA significantly decreased EDTA-induced ROS generation and DNA damage.

CONCLUSIONS

EDTA could induce DNA damage in HCEs, probably through oxidative stress. Furthermore, high molecular weight HA was an effective protective agent that had antioxidant properties and decreased DNA damage induced by EDTA.

Diallyl trisulfide as an inhibitor of benzo(a)pyrene-induced precancerous carcinogenesis in MCF-10A cells

Diallyl trisulfide (DATS) is a garlic organosulfide that is toxic to cancer cells, however, little is known about its effect in the initiation phase of carcinogenesis. We sought to determine whether DATS could inhibit the carcinogen, benzo(a)pyrene (BaP), from inducing precancerous activity, in vitro. MCF-10A cells were either pre-treated (PreTx) or concurrently treated (CoTx) with 1 μM BaP, and 6 or 60 μM DATS for up to 24h. The DATS 6 and 60 μM CoTx inhibited BaP-induced cell proliferation by an average of 71.1% and 120.8%, respectively, at 6h. The 60 μM DATS pretreatment decreased BaP-induced G2/M cell cycle transition by 127%, and reduced the increase in cells in the S-phase by 42%; whereas 60 μM DATS CoTx induced a 177% increase in cells in G1. DATS effectively inhibited (P<0.001) BaP-induced peroxide formation by at least 54%, which may have prevented the formation of BaP-induced DNA strand breaks. In this study, we reveal mechanisms involved in DATS inhibition of BaP-induced carcinogenesis, including inhibition of cell proliferation, regulation of cell cycle, attenuation of ROS formation, and inhibition of DNA damage. At the doses evaluated, DATS appears to be an effective attenuator of BaP-induced breast carcinogenesis, in vitro.