Mdm2 as a sensitive and mechanistically informative marker for genotoxicity induced by benzo[a]pyrene and dibenzo[a,l]pyrene

Lung cancer associated with combustion particles and fine particulate matter (PM2.5) - The roles of polycyclic aromatic hydrocarbons (PAHs) and the aryl hydrocarbon receptor (AhR)

Air pollution is the leading cause of lung cancer after tobacco smoking, contributing to 20% of all lung cancer deaths. Increased risk associated with living near trafficked roads, occupational exposure to diesel exhaust, indoor coal combustion and cigarette smoking, suggest that combustion components in ambient fine particulate matter (PM2.5), such as polycyclic aromatic hydrocarbons (PAHs), may be central drivers of lung cancer. Activation of the aryl hydrocarbon receptor (AhR) induces expression of xenobiotic-metabolizing enzymes (XMEs) and increase PAH metabolism, formation of reactive metabolites, oxidative stress, DNA damage and mutagenesis. Lung cancer tissues from smokers and workers exposed to high combustion PM levels contain mutagenic signatures derived from PAHs. However, recent findings suggest that ambient air PM2.5 exposure primarily induces lung cancer development through tumor promotion of cells harboring naturally acquired oncogenic mutations, thus lacking typical PAH-induced mutations. On this background, we discuss the role of AhR and PAHs in lung cancer development caused by air pollution focusing on the tumor promoting properties including metabolism, immune system, cell proliferation and survival, tumor microenvironment, cell-to-cell communication, tumor growth and metastasis. We suggest that the dichotomy in lung cancer patterns observed between smoking and outdoor air PM2.5 represent the two ends of a dose-response continuum of combustion PM exposure, where tumor promotion in the peripheral lung appears to be the driving factor at the relatively low-dose exposures from ambient air PM2.5, whereas genotoxicity in the central airways becomes increasingly more important at the higher combustion PM levels encountered through smoking and occupational exposure.

Influence of selected anti-cancer drugs on the induction of DNA double-strand breaks and changes in gene expression in human hepatoma HepG2 cells

In chemotherapy, various anti-cancer drugs with different mechanisms of action are used and may represent different risk of undesirable delayed side effects in treated patients as well as in occupationally exposed populations. The aim of the present study was to evaluate genotoxic potential of four widely used anti-cancer drugs with different mechanisms of action: 5-fluorouracil (5-FU), cisplatin (CDDP) and etoposide (ET) that cause cell death by targeting DNA function and imatinib mesylate (IM) that inhibits targeted protein kinases in cancer cells in an experimental model with human hepatoma HepG2 cells. After 24 h of exposure all four anti-cancer drugs at non-cytotoxic concentrations induced significant increase in formation of DNA double strand breaks (DSBs), with IM being the least effective. The analysis of the changes in the expression of genes involved in the response to DNA damage (CDKN1A, GADD45A, MDM2), apoptosis (BAX, BCL2) and oncogenesis (MYC, JUN) showed that 5-FU, CDDP and ET upregulated the genes involved in DNA damage response, while the anti-apoptotic gene BCL2 and oncogene MYC were downregulated. On the contrary, IM did not change the mRNA level of the studied genes, showing different mechanism of action that probably does not involve direct interaction with DNA processing. Genotoxic effects of the tested anti-cancer drugs were observed at their therapeutic concentrations that may consequently lead to increased risk for development of delayed adverse effects in patients. In addition, considering the genotoxic mechanism of action of 5-FU, CDDP and ET an increased risk can also not be excluded in occupationally exposed populations. The results also indicate that exposure to 5-FU, CDDP and ET represent a higher risk for delayed effects such as cancer, reproductive effects and heritable disease than exposure to IM.

The modulating effect of ATM, ATR, DNA-PK inhibitors on the cytotoxicity and genotoxicity of benzo[a]pyrene in human hepatocellular cancer cell line HepG2

The effect of inhibitors of phosphatidylinositol-3-kinase-related kinases (PIKK): ataxia-telangiectasia mutated (ATM), ATM- and Rad3-related (ATR) and DNA-dependent protein kinase (DNA-PK) on response of HepG2 human liver cancer cells to benzo[a]pyrene (BaP) was investigated. PIKK inhibitors: KU55933 (5 μM), NU7026 (10 μM) or caffeine (1 and 2mM) when used as single agents or in combinations (KU55933/NU7026 and caffeine/NU7026) did not significantly influence the BaP (3 μM) cytotoxicity (MTT reduction test). BaP induced a weak proapoptotic effect which was moderately enhanced by both inhibitor combinations. HepG2 cells exposed to BaP showed a strong S-phase arrest which was considerably diminished by both inhibitor combinations. The DNA damage (comet assay) induced after continuous 24h exposure to BaP was significantly diminished by both inhibitor combinations. Weak induction of reactive oxygen species by BaP was observed, which was not modulated by the inhibitor combinations. Similarly, no modulation of the glutathione levels was observed.

The influence of ATM, ATR, DNA-PK inhibitors on the cytotoxic and genotoxic effects of dibenzo[def,p]chrysene on human hepatocellular cancer cell line HepG2

The effect of inhibitors of phosphatidylinositol-3-kinase related kinases (PIKK): ataxia-telangiectasia mutated (ATM), ATM- and Rad3-related (ATR) and DNA-dependent protein kinase (DNA-PK) on the response of HepG2 human liver cancer cells to dibenzo[def,p]chrysene (DBC) was investigated. High cytotoxicity of DBC (IC50=0.1μM) was observed after 72h incubation. PIKK inhibitors: KU55933 (5μM), NU7026 (10μM) or caffeine (1 and 2mM) when used alone did not significantly influence the cytotoxicity. However, two combinations: KU55933/NU7026 and caffeine/NU7026 significantly increased HepG2 viability (by 25%) after treatment with DBC at 0.5μM. The cytoprotective effect was confirmed by cell cycle and apoptosis/necrosis analysis. DNA damage level after exposure to DBC assessed by comet assay (single strand breaks) showed a long persistence and significant decrease after incubation of the cells in the presence the inhibitors (the combination of KU55933+NU7026 showed the strongest effect). Weak induction of reactive oxygen species (ROS) by DBC (0.5μM) was observed. Although, KU55933 and NU7026 when used alone did not increase ROS levels in the cells, their combination induced the ROS increase and moderately enhanced ROS generation by DBC. We propose a mechanism how cells with damaged DNA after exposure to DBC and under the condition of PIKK inhibition, may be at higher risk of undergoing malignant transformation.

Induction of human UDP-Glucuronosyltransferase 2B7 gene expression by cytotoxic anticancer drugs in liver cancer HepG2 cells

We recently reported induction of UGT2B7 by its substrate epirubicin, a cytotoxic anthracycline anticancer drug, via activation of p53 and subsequent recruitment of p53 to the UGT2B7 promoter in hepatocellular carcinoma HepG2 cells. Using the same HepG2 model cell line, the present study assessed the possibility of a similar induction of UGT2B7 by several other cytotoxic drugs. We first demonstrated by reverse transcriptase quantitative real-time polymerase chain reaction that, as observed with epirubicin, nine cytotoxic drugs including three anthracyclines (doxorubicin, daunorubicin, and idarubicin) and six nonanthracyclines (mitomycin C, 5-fluorouracil, camptothecin, 7-ethyl-10-hydroxycamptothecin, topotecan, and etoposide) significantly increased UGT2B7 mRNA levels. To investigate a potential involvement of p53 in this induction, we conducted further experiments with four of the nine drugs (doxorubicin, daunorubicin, idarubicin, and mitomycin C). The cytotoxic drugs studied increased p53 and UGT2B7 protein levels. Knockdown of p53 expression by small interfering RNA reduced cytotoxic drug-induced UGT2B7 expression. Luciferase reporter assays showed activation of the UGT2B7 promoter by cytotoxic drugs via a previously reported p53 site. Finally, chromatin immunoprecipitation assays demonstrated p53 recruitment to the UGT2B7 p53 site upon exposure to mitomycin C, the most potent UGT2B7 inducer among the nine tested drugs. Taken together, these results provide further evidence supporting UGT2B7 as a p53 target gene. The cytotoxic drug-induced UGT2B7 activity in target liver cancer cells or possibly in normal liver cells may affect the therapeutic efficacy of co-administered cytotoxic drugs (e.g., epirubicin) and noncytotoxic drugs (e.g., morphine), which are UGT2B7 substrates.

DNp73 exerts function in metastasis initiation by disconnecting the inhibitory role of EPLIN on IGF1R-AKT/STAT3 signaling

Dissemination of cancer cells from primary tumors is the key event in metastasis, but specific determinants are widely unknown. Here, we show that DNp73, an inhibitor of the p53 tumor suppressor family, drives migration and invasion of nonmetastatic melanoma cells. Knockdown of endogenous DNp73 reduces this behavior in highly metastatic cell lines. Tumor xenografts expressing DNp73 show a higher ability to invade and metastasize, while growth remains unaffected. DNp73 facilitates an EMT-like phenotype with loss of E-cadherin and Slug upregulation. We provide mechanistic insight toward regulation of LIMA1/EPLIN by p73/DNp73 and demonstrate a direct link between the DNp73-EPLIN axis and IGF1R-AKT/STAT3 activation. These findings establish initiation of the invasion-metastasis cascade via EPLIN-dependent IGF1R regulation as major activity of DNp73.

Recent advances in particulate matter and nanoparticle toxicology: a review of the in vivo and in vitro studies

Epidemiological and clinical studies have linked exposure to particulate matter (PM) to adverse health effects, which may be registered as increased mortality and morbidity from various cardiopulmonary diseases. Despite the evidence relating PM to health effects, the physiological, cellular, and molecular mechanisms causing such effects are still not fully characterized. Two main approaches are used to elucidate the mechanisms of toxicity. One is the use of in vivo experimental models, where various effects of PM on respiratory, cardiovascular, and nervous systems can be evaluated. To more closely examine the molecular and cellular mechanisms behind the different physiological effects, the use of various in vitro models has proven to be valuable. In the present review, we discuss the current advances on the toxicology of particulate matter and nanoparticles based on these techniques.

Expression of glutathione S-transferase M2 in stage I/II non-small cell lung cancer and alleviation of DNA damage exposure to benzo[a]pyrene

Glutathione S-transferases (GSTs) are a family of inducible enzymes that are important in carcinogen detoxification. GST-Mu class is showing the high activity towards most polycyclic aromatic hydrocarbon (PAH) epoxide. Our objective is to clarify the expression of GST-M2 in non-small-cell lung carcinoma (NSCLC) patients and to determine the role of GST-M2 in protecting against DNA damage. We detected changes in GST-M2 expression at mRNA levels with a panel of lung cell lines and clinical samples of malignant and paired adjacent non-malignant tissues from 50 patients with stage I or II non-small-cell lung carcinoma using real-time RT-PCR. Comet assay and gamma-H2AX were used to clarify whether DNA damaged was protected by GST-M2. Our data demonstrate that the expression of GST-M2 in tumor tissues is significantly lower than in paired adjacent non-malignant tissues (p=0.016). Loss of GST-M2 is closely associated with age, gender, T value, N value and cell differentiation (p<0.05) in early stage I/II patients. Downregulation of GST-M2 is mediated through aberrant hypermethylation in lung cancer cell lines. Protection against B[a]P-induced DNA damage by GST-M2 in lung cancer cells was detected by Comet assay and gamma-H2AX. In conclusion, DNA hypermethylation altered and reduced GST-M2 expression that resulted in susceptible to benzo[a]pyrene (B[a]P) induced DNA damage. It implies that GST-M2 reduction occurs prior to tumorigenesis.

Exposure of HepG2 cells to low levels of PAH-containing extracts from contaminated soils results in unpredictable genotoxic stress responses

Contaminated soil is a serious environmental problem, constituting a risk to humans and the environment. Polycyclic aromatic hydrocarbons (PAHs) are often present at contaminated sites. However, risk levels are difficult to estimate because of the complexity of contaminants present. Here, we compare cellular effects of extracts from contaminated soils collected at six industrial settings in Sweden. Chemical analysis showed that all soils contained complex mixtures of PAHs and oxy-PAHs. Western blotting and immunocytochemistry were used to investigate DNA damage signaling in HepG2 cells exposed to extracts from these soils. The effects on phosphorylated Mdm2, p53, Erk, H2AX, 53BP1, and Chk2, cell cycle regulating proteins (cyclin D1 and p21), and cell proliferation were compared. We found that most soil extracts induced phosphorylation of Mdm2 at the 2A10 epitope at low concentrations. This is in line with previous studies suggesting that this endpoint reflects readily repaired DNA-damage. However, we found concentration- and time-dependent gammaH2AX and 53BP1 responses that were sustained for 48 hr. These endpoints may reflect the presence of different types of persistent DNA-damage. High concentrations of soil extracts decreased cyclin D1 and increased p21 response, indicating cell cycle arrest. Phosphorylation of Mdm2 at Ser166, which attenuates the p53 response and is induced by many tumor promoters, was induced in a time-dependent manner and was associated with Erk phosphorylation. Taken together, the PAH extracts elicited unpredictable signaling responses that differed between samples. More polar compounds, i.e., oxy-PAHs, also contributed to the complexity.