Triphenyl phosphate (TPP) is an organic phosphorus flame retardant (OPFRs), which has been polluting the environment worldwide as one of the new type flame retardants. Meanwhile, the “old” type flame retardants, such as polybrominated diphenyl ethers (PBDEs), have been prohibited or restricted for use, production and transportation, due to their persistence, long-range atmospheric migration, and highly toxic effects on various organisms [1]. Therefore, organophosphorus esters (OPFRs) have emerged as substitutes for PBDEs, owing to their excellent flame retardant effect, preferred physical and chemical properties, and relatively low cost. Now, OPFRs have widely been used in the world, their annual market capacity, for example in 2013, was estimated as 620,000 tons, accounting for 30% of global flame retardants; their production in China amounted to 100,000 tons in 2011, and continues to increase at an annual rate of 15% [2]. As for TPP, its annual production/use amounted to 20,000–30,000 tonnes in Europe in 2000, and 4500–22700 tonnes in the U.S. in 2004 [3] (relevant data in the recent decade are lacking). TPP is also environmentally persistent due to its physical and chemical stability, high hydrophobicity, and easy adsorption to particles [4]. TPP are ubiquitously present in the environment, including the water, air, soil [3,[5], [6], [7]], and many kinds of food [8].

TPP has also been present in various organisms, including the human. For example, TPP was detected in the serum sampled from the residents of Shandong Province, China, in 2011 and 2015 at 26–43 and 25–41 ng/g lipid, respectively [9]. Another study investigated the daily intake of OPFRs by some residents in Hebei Province, China, in which TPP was detectable in 70% of the whole blood samples, with the highest and median level being 1.24 ng/mL (3.87 nM) and 0.23 ng/mL (0.72 nM), respectively, accounting for about 20% of the total OPFRs detected [10]. Occupational and other high level exposure to TPP may lead to extraordinarily elevated burdens in the body, as indicated in a recent study, in the hair samples from the dismantling workers and a population residing in a former e-waste recycling area in north Guangdong Province, China, the concentration of TPP was 11.9–286 (median 34.2) and 0–437 (median 80.9) ng/g in the dismantling workers and local residents (some of them having experienced private E-waste recycling at home), respectively [11]. Moreover, TPP has also been detected in human placental samples collected from residents in some areas in Western China, which amounted from 0 (N.D.) to 112 (averaged 15.1) ng/g lipid [12]. Like other OPFRs, TPP exposes the human body through multiple pathways, including ingestion of contaminated foods, inhalation of tainted dust, and skin contact with TPP-containing products [13]. Children appear to have particularly high intakes of TPP, for example, the median estimated daily intakes (EDIs) of TPP in the 0 ∼ 5-year old children residing in Nanchang, a central Chinese city, was 267 ng/kg b.w./day [14], and that in the 14 – 15-year old adolescents in Belgium was 197.7 ng/kg b.w./day [15].

TPP is toxic on multiple systems, causing neuro- and developmental toxicity, and endocrine disrupting effects [[16], [17], [18], [19], [20]]. Moreover, epidemiological studies indicate that TPP may increase the risk of gastrointestinal cancer, cervical cancer and prostate cancer [[21], [22], [23]]. Meanwhile, TPP has been observed to cause DNA breaks, cytotoxicity, and increased reactive oxygen species generation in various human cell lines, including the human hepatoma (HepG2), human lung cancer (A549), and human colon cancer (Caco-2) cell line (An et al., 2016). Moreover, by using simulated molecular docking method human CYP1A2 and CYP2E1 seem to be capable of catalyzing the biotransformation of TPP, and TPP was metabolized by human hepatic microsomes [rich in xenobiotic metabolizing enzymes, such as cytochrome P450s (CYPs)] into its diester derivative, dipentyl phthalate (DPHP), and mono- and dihydroxylated metabolites [24]. Other phase-I metabolites formed by co-incubation of TPP with both human hepatic S9 mix and microsomes included those resulting from both hydroxylation and O-dealkylation, presumably catalyzed by some CYP enzymes [25]. The biotransformation of TPP and its metabolokinetics have recently been studied using mouse hepatic microsomes, which indicates the involvement of mouse CYP enzymes, primarily CYP2E1, in the metabolism of TPP, based on the influence of disulfiram, a CYP2E1 inhibitor [26]. Existing reports are mainly focused on (and limited to) the metabolism of TPP and the related metabolites, without linkage to the influence of metabolism on TPP toxicity. We speculated that TPP might be activated by some human CYP enzymes, presumably human CYP1A2 and CYP2E1, to form DNA/chromosome-damaging metabolites.

The present study was aimed on exploring the impact of TPP metabolism on its genotoxicity, in particular, the potential of human CYP1A2 and 2E1 in metabolically activating TPP was clarified. We made use of a human hepatoma (C3A) cell line which endogenously express CYP enzymes relatively abundantly, its parental line HepG2 whose endogenous expression of CYPs was unfavored but it is well responsive to nuclear receptor/CYP inducers. In addition, Chinese hamster (V79)-derived cells genetically engineered for the expression of individual human CYP and sulfotransferase (SULT) enzymes were also employed to study the role of relevant CYPs/SULT1A1 in TPP-induced genotoxicity.