Mechanism of synergistic DNA damage induced by the hydroquinone metabolite of brominated phenolic environmental pollutants and Cu(II): Formation of DNA-Cu complex and site-specific production of hydroxyl radicals

Highlights

• •OH was found to be responsible for 2,6-DBrHQ/Cu(II)-induced DNA damage.

• First systematic investigation of the interactions between Cu(II)/Cu(I) and DNA.

• 2,6-DBrHQ/Cu(II) induce DNA damage via a site-specific mechanism.

• Site-specifically generated •OH attacks adjacent DNA sites, mainly at T, C and G.

Abstract

2,6-Dibromohydroquinone (2,6-DBrHQ) has been identified as an reactive metabolite of many brominated phenolic environmental pollutants such as tetrabromobisphenol-A (TBBPA), bromoxynil and 2,4,6-tribromophenol, and was also found as one of disinfection byproducts in drinking water. In this study, we found that the combination of 2,6-DBrHQ and Cu(II) together could induce synergistic DNA damage as measured by double strand breakage in plasmid DNA and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) formation, while either of them alone has no effect. 2,6-DBrHQ/Cu(II)-induced DNA damage could be inhibited by the Cu(I)-specific chelating agent bathocuproine disulfonate and catalase, but not by superoxide dismutase, nor by the typical hydroxyl radical (•OH) scavengers such as DMSO and mannitol. Interestingly, we found that Cu(II)/Cu(I) could be combined with DNA to form DNA-Cu(II)/Cu(I) complex by complementary application of low temperature direct ESR, circular dichroism, cyclic voltammetry and oxygen consumption methods; and the highly reactive •OH were produced synergistically by DNA-bound-Cu(I) with H₂O₂ produced by the redox reactions between 2,6-DBrHQ and Cu(II), which then immediately attack DNA in a site-specific manner as demonstrated by both fluorescent method and by ESR spin-trapping studies. Further DNA sequencing investigations provided more direct evidence that 2,6-DBrHQ/Cu(II) caused preferential cleavage at guanine, thymine and cytosine residues. Based on these data, we proposed that the synergistic DNA damage induced by 2,6-DBrHQ/Cu(II) might be due to the synergistic and site-specific production of •OH near the binding site of copper and DNA. Our findings may have broad biological and environmental implications for future research on the carcinogenic polyhalogenated phenolic compounds.

Introduction

Polyhalogenated quinoid compounds are a class of toxic intermediates that can cause acute hepatoxicity, nephrotoxicity, and carcinogenesis [1], [2]. They have also been found as reactive oxidation intermediates or products in processes used to oxidize or destroy polychlorinated aromatic pollutants in various chemical and enzymatic systems [3], [4], [5], [6], [7]. More recently, more than a dozen of polyhalogenated quinoid compounds and their hydroxylated derivatives, which are suspected to be bladder carcinogens, were identified as new chlorination disinfection byproducts (DBPs) in drinking water [8].

Tetrabromobisphenol-A (TBBPA) is the most widely used brominated flame retardant in the manufacture of industrial equipment and consumer goods in order to prevent or minimize fire damage [9], [10]. The annual global consumption of TBBPA was estimated to be more than 170,000 t [11]. Human TBBPA serum levels were measured and found to be in the range from 1 to 3.4 pmol/g lipid [12]. TBBPA has been considered to be an immune-toxicant and thyroid hormone agonist and has the potential to disrupt estrogen signaling, at least in vitro. Although these reports suggest that TBBPA induce many toxic effects, the underlying molecular mechanism remains to be clarified. It has been shown that 2,6-dibromohydroquinone (2,6-DBrHQ) is one of the major metabolites of TBBPA in rats, and the hepatotoxicity of TBBPA in rats might be due to the in vivo generation of the reactive oxygen species as a result of redox reactions involving 2,6-DBrHQ and its corresponding semiquinone radicals [13]. It should be noted that 2,6-DBrHQ was also found to be the metabolite of the widely used herbicide bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and brominated phenols such as 2,4,6-tribromophenol (TBP), which has been used as a flame retardant by itself and also as an important intermediate for other chemical synthesis [14], [15], [16].

Both 2,6-DBrHQ and its oxidation product 2,6-dibromo-1,4-benzoquinone (2,6-DBrBQ) were also identified as DBPs in simulated drinking water, and 2,6-DBrBQ was detected in 11 of the 16 drinking water samples (68.8%) at concentrations up to 37.9 ng/L [8].

Taken together, we can see that 2,6-DBrHQ is not only a common reactive metabolite or decomposition product from several typical brominated phenolic environmental pollutants with high occurrence frequency, it is also a disinfection byproduct in simulated drinking water, highlighting the need for further research.

Copper is an essential trace element for humans [17]. Besides forming the essential redox active center in a variety of metallo-proteins, copper has also been found in the nucleus and to be closely associated with the chromosomes and DNA bases, particularly guanine [18]. DNA-associated copper has been suggested to be involved in maintaining normal chromosome structure and in gene regulatory processes [19], [20], [21].

Studies have shown that transition metals, particularly copper, are capable of mediating the activation of several classes of less reactive compounds, such as hydroquinones, catechols, and dietary flavonoids and other xenobiotics by a redox Fenton-like mechanism, leading to the formation of more reactive oxygen species (ROS) [22], [23]. Among the ROS, hydroxyl radical (•OH) is considered to be the most reactive species that could damage DNA and other macromolecules. Since copper is capable of mediating activation of a variety of phenolic compounds [24], it is reasonable to propose that copper may have the potential to activate 2,6-DBrHQ via a copper-redox reaction, producing more reactive ROS and inducing more severe DNA damage.

Therefore, in this study, we plan to address the following questions. (1) Can 2,6-DBrHQ together with Cu(II) induce synergistic DNA damage; and if so, (2) what is the underlying molecular mechanism?