Induction of DNA damage in human urothelial cells by the brominated flame retardant 2,2-bis(bromomethyl)-1,3-propanediol: role of oxidative stress

Cellular and physiological mechanisms of halogenated and organophosphorus flame retardant toxicity

Flame retardants (FRs) are chemical substances used to inhibit the spread of fire in numerous industrial applications, and their abundance in modern manufactured products in the indoor and outdoor environment leads to extensive direct and food chain exposure of humans. Although once considered relatively non-toxic, FRs are demonstrated by recent literature to have disruptive effects on many biological processes, including signaling pathways, genome stability, reproduction, and immune system function. This review provides a summary of research investigating the impact of major groups of FRs, including halogenated and organophosphorus FRs, on animals and humans in vitro and/or in vivo. We put in focus those studies that explained or referenced the modes of FR action at the level of cells, tissues and organs. Since FRs are highly hydrophobic chemicals, their biophysical and biochemical modes of action usually involve lipophilic interactions, e.g. with biological membranes or elements of signaling pathways. We present selected toxicological information about these molecular actions to show how they can lead to damaging membrane integrity, damaging DNA and compromising its repair, changing gene expression, and cell cycle as well as accelerating cell death. Moreover, we indicate how this translates to deleterious bioactivity of FRs at the physiological level, with disruption of hormonal action, dysregulation of metabolism, adverse effects on male and female reproduction as well as alteration of normal pattern of immunity. Concentrating on these subjects, we make clear both the advances in knowledge in recent years and the remaining gaps in our understanding, especially at the mechanistic level.

Oxidative debromination of 2,2-bis(bromomethyl)-1,3-propanediol by UV/persulfate process and corresponding formation of brominated by-products

This study investigated the oxidative debromination of 2,2-bis(bromomethyl)-1,3-propanediol (BBMP), a widely used brominated flame retardant, and the corresponding formation of brominated by-products by the UV/persulfate process. The debromination of BBMP by the UV/persulfate process was primarily driven by sulfate radicals (SO₄^-) at pHs 4.0-6.0 and hydroxyl radicals (HO) at pHs 9.0-12.0. The debromination rate increased with increasing pH from 4.0 to 9.0 and remained the same at pHs 9.0 and 12.0. Bromate was formed through the oxidation of bromide released from BBMP mainly by SO₄^-, with free bromine as a key intermediate. Bromate formation increased with increasing pH from 4.0 to 6.0, while it remarkably decreased with increasing pH from 6.0 to 12.0. This was mainly due to the transformation of SO₄^- to HO and also the quenching of bromine atoms that were the key intermediate for the formation of free bromine, by hydroxyl ions at the alkaline pH. In addition, the oxidative debromination of BBMP resulted in a significant decrease in the concentrations of total organic bromine, but the formation of brominated acetic acids and unknown brominated organic by-products. The concentrations of brominated organic by-products firstly increased and then decreased with prolonged reaction time. Also, the formation of brominated organic by-products and genotoxicity at pH 9.0 were much lower than that at pH 6.0. In this study, we propose that the UV/persulfate process under mildly alkaline conditions not only debrominates BBMP efficiently but also eliminates the formation of bromate and brominated organic by-products and genotoxicity.

Functional genomic assessment of 2, 2-bis (bromomethyl)-1, 3-propanediol induced cytotoxicity in a single-gene knockout library of E. coli

Functional gene fingerprinting of chemicals could be used to understand the direct gene-chemical interaction in the process of toxification from a genome-wide scale. 2, 2-bis (bromomethyl)-1, 3-propanediol (BMP) is a brominated flame retardant with widespread production but with very limited toxicological data. Here the cytotoxicity of BMP was assessed by Escherichia coli (E. coli) functional genome-wide knockout mutants screening and the underlying molecular mechanism was investigated. The median inhibition concentration (IC50) of BMP was 1.608 ± 0.078 mg/ml after 24 h exposure. 119 initial, including 66 sensitive and 53 resistant single gene mutants, were identified by a full library screening of BMP at the concentration of IC50. The resistant genes were significantly enriched in nucleobase-containing compound biosynthetic process (GO: 0034654) by gene ontology (GO) biological process analyses, which suggested that the pathway of DNA repair is a critical cellular process in the survival of cells exposed to BMP. Meanwhile, function annotation of all BMP responsive genes suggested the mechanism of BMP was associated with DNA damage, oxidative stress and cellular transmembrane transport process. Many genes were exclusively responsive to BMP comparing with other chemicals that has been assessed by E. coli mutant screening approach, which indicated that BMP has a distinct mode of toxic action. Overall, the functional genomic screening approach presented here provides a great tool to assess the cellular toxicological mechanism of environmental chemicals.

Classification and toxicity mechanisms of novel flame retardants (NFRs) based on whole genome expression profiling

Recently some novel alternative flame retardants (NFRs), which have been widely applied to meet demands for mandated flame retardation of products, have been detected in various matrices of the environment. However, knowledge on toxic effects and associated molecular mechanisms of these chemicals was limited. Here, toxic mechanisms of action of six NFRs, bis (2-ethylhexyl) phosphate (BEHP), chlorendic acid (Het acid), 2,2-bis (bromomethyl)-1,3-propanediol (BMP), tris (2-butoxyethyl) phosphate (TBEP), triethyl phosphate (TEP), tributyl phosphate (TBP) were investigated by use of a library containing ∼1820 modified green fluorescent protein (GFP) expressing promoter reporter vectors constructed from Escherichia coli K12(E.coli). BEHP, Het acid, BMP, TBEP, TEP, TBP inhibited growth of E. coli with 4 h 10%-inhibition concentrations of 53.0-3102.3 μM. A total of 119, 44, 26, 131, 62, 103 genes out of 336 genes selected during preliminary screening were significantly altered with fold-changes greater than 1.5 by BEHP, Het acid, BMP, TBEP, TEP and TBP, respectively. GO analyses of responsive genes suggested that RNA and primary metabolism process were involved in molecular mechanisms of toxicity. Chemical clustering based on expression of 62 multi-responsive genes showed that BEHP, TBP and TBEP were grouped together, which is consistent with similarity of their chemical structures, especially for BEHP and TBP. Clustering by molecular descriptors and molecular activity by use of the multivariate classification system ToxCast was consistent with that by profiles of multi-responsive genes. The results of this study demonstrated the utility of the E. coli, whole-cell assay for determining mechanisms of toxic action of chemicals.

Exposure of Female Rats to an Environmentally Relevant Mixture of Brominated Flame Retardants Targets the Ovary, Affecting Folliculogenesis and Steroidogenesis

Brominated flame retardants (BFRs) are incorporated into various consumer products to prevent flame propagation. These compounds leach into the domestic environment, resulting in chronic exposure and contamination. Pregnancy failure is associated with high levels of BFRs in human follicular fluid, raising serious questions regarding their impact on female reproductive health. The goal of this study is to elucidate the effects of an environmentally relevant BFR mixture on female rat ovarian functions (i.e., folliculogenesis and steroidogenesis). A BFR dietary mixture formulated to mimic the relative BFR congener levels in North American house dust was administered to adult female Sprague-Dawley rats from 2 to 3 wk before mating until Gestational Day 20; these diets were designed to deliver nominal doses of 0, 0.06, 20, or 60 mg/kg/day of the BFR mixture. Exposure to BFRs triggered an approximately 50% increase in the numbers of preantral and antral follicles and an enlargement of the antral follicles in the ovaries of the dams. A significant reduction in the expression of catalase, an antioxidant enzyme, and downregulation of the expression of insulin-like factor 3 (Insl3) and 17alpha-hydroxylase (Cyp17a1) were observed in the ovary. In addition, BFR exposure affected steroidogenesis; we observed a significant decrease in circulating 17-hydroxypregnenolone and an increase in testosterone concentrations in BFR-exposed dams. Thus, BFRs target ovarian function in the rat, adversely affecting both folliculogenesis and steroidogenesis.

New exposure biomarkers as tools for breast cancer epidemiology, biomonitoring, and prevention: a systematic approach based on animal evidence

BACKGROUND

Exposure to chemicals that cause rodent mammary gland tumors is common, but few studies have evaluated potential breast cancer risks of these chemicals in humans.

OBJECTIVE

The goal of this review was to identify and bring together the needed tools to facilitate the measurement of biomarkers of exposure to potential breast carcinogens in breast cancer studies and biomonitoring.

METHODS

We conducted a structured literature search to identify measurement methods for exposure biomarkers for 102 chemicals that cause rodent mammary tumors. To evaluate concordance, we compared human and animal evidence for agents identified as plausibly linked to breast cancer in major reviews. To facilitate future application of exposure biomarkers, we compiled information about relevant cohort studies.

RESULTS

Exposure biomarkers have been developed for nearly three-quarters of these rodent mammary carcinogens. Analytical methods have been published for 73 of the chemicals. Some of the remaining chemicals could be measured using modified versions of existing methods for related chemicals. In humans, biomarkers of exposure have been measured for 62 chemicals, and for 45 in a nonoccupationally exposed population. The Centers for Disease Control and Prevention has measured 23 in the U.S. population. Seventy-five of the rodent mammary carcinogens fall into 17 groups, based on exposure potential, carcinogenicity, and structural similarity. Carcinogenicity in humans and rodents is generally consistent, although comparisons are limited because few agents have been studied in humans. We identified 44 cohort studies, with a total of > 3.5 million women enrolled, that have recorded breast cancer incidence and stored biological samples.

CONCLUSIONS

Exposure measurement methods and cohort study resources are available to expand biomonitoring and epidemiology related to breast cancer etiology and prevention.

Piperine causes G1 phase cell cycle arrest and apoptosis in melanoma cells through checkpoint kinase-1 activation

In this study, we determined the cytotoxic effects of piperine, a major constituent of black and long pepper in melanoma cells. Piperine treatment inhibited the growth of SK MEL 28 and B16 F0 cells in a dose and time-dependent manner. The growth inhibitory effects of piperine were mediated by cell cycle arrest of both the cell lines in G1 phase. The G1 arrest by piperine correlated with the down-regulation of cyclin D1 and induction of p21. Furthermore, this growth arrest by piperine treatment was associated with DNA damage as indicated by phosphorylation of H2AX at Ser139, activation of ataxia telangiectasia and rad3-related protein (ATR) and checkpoint kinase 1 (Chk1). Pretreatment with AZD 7762, a Chk1 inhibitor not only abrogated the activation of Chk1 but also piperine mediated G1 arrest. Similarly, transfection of cells with Chk1 siRNA completely protected the cells from G1 arrest induced by piperine. Piperine treatment caused down-regulation of E2F1 and phosphorylation of retinoblastoma protein (Rb). Apoptosis induced by piperine was associated with down-regulation of XIAP, Bid (full length) and cleavage of Caspase-3 and PARP. Furthermore, our results showed that piperine treatment generated ROS in melanoma cells. Blocking ROS by tiron protected the cells from piperine mediated cell cycle arrest and apoptosis. These results suggest that piperine mediated ROS played a critical role in inducing DNA damage and activation of Chk1 leading to G1 cell cycle arrest and apoptosis.

Comparison of 2,2-bis(bromomethyl)-1,3-propanediol induced genotoxicity in UROtsa cells and primary rat hepatocytes: relevance of metabolism and oxidative stress

2,2-Bis(bromomethyl)-1,3-propanediol (BMP) is a brominated flame retardant used in urethane foams and polyester resins. In a two year dietary study, BMP caused neoplastic lesions at multiple sites including the urinary bladder of both rats and mice. However, liver was not a target tissue. We previously reported that BMP elicited oxidative DNA damage in a human uroepithelial cell line (UROtsa). The present in vitro study investigated the susceptibility of target (UROtsa cells) and non-target cells (primary rat hepatocytes) to BMP-induced genotoxicity. In contrast to hepatocytes, BMP exhibited greater genotoxic potential in UROtsa cells as evidenced by the concentration dependent increase in DNA strand breaks and DNA binding. Total content of intracellular GSH quantified in UROtsa cells (2.7±1.0nmol/mg protein) was 4 fold lower than that in hepatocytes (10.7±0.3nmol/mg protein). HPLC analysis indicated BMP was not metabolized and/or consumed in UROtsa cells at any of the concentrations tested (10-250μM) but was extensively converted to a mono-glucuronide in hepatocytes. These results demonstrate that a target cell line such as UROtsa cells are more susceptible to BMP-induced DNA damage when compared to non-target cells. This increased susceptibility may relate to the deficiency of antioxidant and/or metabolic capabilities in UROtsa cells.

Visfatin affects redox adaptative responses and proliferation in Me45 human malignant melanoma cells: an in vitro study

Visfatin has recently been established as a novel adipokine that is predominantly expressed in subcutaneous and visceral fat. Only few studies have investigated the effect of visfatin on prostate, breast, ovarian cancer as well as on astrocytoma cell biology. There have been no previous studies on antioxidative enzyme activities, proliferation processes or levels of DNA damage in malignant melanoma cells in response to visfatin stimulation. Here, we report that visfatin increases activity of selected antioxidative enzymes (SOD, CAT, GSH-Px) in culture supernatants of Me45 human malignant melanoma cells. Our findings suggest that visfatin triggers a redox adaptation response, leading to an upregulation of antioxidant capacity along with decreased levels of the lipid peroxidation process in Me45 melanoma cells. Moreover, visfatin led to a significantly increased proliferation rate in the study using the [(3)H]thymidine incorporation method. Unlike insulin, visfatin-induced melanoma cell proliferation is not mediated by an insulin receptor. Better understanding of the role of visfatin in melanoma redox states may provide sound insight into the association between obesity-related fat adipokines and the antioxidant defense system in vitro in melanoma progression.

Heat shock protein 70 (Hsp70) inhibits oxidative phosphorylation and compensates ATP balance through enhanced glycolytic activity

To address possible effects of heat shock protein 70 (Hsp70) on energy metabolism, we established a cell line expressing different levels of Hsp70 and evaluated changes in glucose and lactate metabolites, as well as ATP levels accordingly. In addition, activities of enzymes involved in glycolysis [phosphofructokinase (PFK) and lactate dehydrogenase (LDH)], Krebs cycle [citric synthase (CS)], and oxidative phosphorylation {NADH dehydrogenase [complex I (CI)] and ubiquinol:cytochrome-c reductase [complex III (CIII)]} were analyzed. The results show that both glucose consumption and lactate excretion were elevated significantly in cells expressing increased levels of Hsp70. Simultaneously, the activities of glycolytic enzymes PFK and LDH were increased markedly in cells overexpressing Hsp70. Activities of enzymes CI and CIII, both involved in oxidative phosphorylation, decreased upon increased expression of Hsp70. These findings were supported by nonsignificant reductions of CS activities in cells that overexpressed Hsp70, whereas intracellular ATP levels remained constant over a wide range of Hsp70 expression. In conclusion, overexpression of Hsp70 in HeLa cells results in downregulation of oxidative phosphorylation, in particular, multiprotein CIII, the main source of reactive oxygen species. In exchange, upregulation of the glycolytic pathway compensates for the homeostasis of cellular ATP supply.