Bioaccumulation and transfer characteristics of dechlorane plus in human adipose tissue and blood stream and the underlying mechanisms

Science of the Total Environment 700 (2020) 134391

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Bioaccumulation and transfer characteristics of dechlorane plus in human adipose tissue and blood stream and the underlying mechanisms Jun-Fa Yin a,b, Ji-Fang-Tong Li a,b, Xing-Hong Li a,b,⇑, You-Lin Yang c, Zhan-Fen Qin a,b a State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center of Eco-Environmental Sciences, Chinese Academy of Sciences, PO Box 2871, 18 Shuangqing Road, Haidian District, Beijing 100085, PR China b University of the Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, PR China c The First People’s Hospital of Wenling, 333 Chuang’annan Road, Chengxi Street, Taizhou 317500, Zhejiang Province, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The levels of DP and adipose-serum

partitioning relationship are reported.  DP partitioning among human tissues

is related to lipid-driven mechanism.  Stereo-selective behavior of DP is not

found between serum and adipose tissue.  Internal mono-dechlorinated potential of anti-DP might not be predominant in humans.

a r t i c l e

i n f o

Article history: Received 5 July 2019 Received in revised form 7 September 2019 Accepted 9 September 2019 Available online 8 October 2019 Keywords: Dechlorane plus Adipose tissue Serum Lipids

a b s t r a c t In this study, bioaccumulation and transfer characteristics of dechlorane plus (DP) were examined between human adipose tissue and matched maternal serum, and the possible transfer mechanism between tissues was further discussed. The median level of total DP was 971 pg g 1 wet weight (ww) and 1.22 ng g 1 lipid weight (lw) in adipose tissue, respectively, and was 34.7 pg g 1 ww and 3.98 ng g 1 lw for serum, respectively. DP wet levels’ positive association with fat contents of five types of human tissues indicated that DP distribution might be related to lipid-driven mechanism. However, the lipidadjusted adipose-serum partitioning ratios were estimated to be 0.35 for syn-DP and 0.35 for anti-DP, accordingly, which implied that the DP distribution between serum and adipose tissues, was not only regulated by the tissue lipid contents. Both the internal mono-dechlorination of anti-DP, and stereoselective behavior of DP isomers were not found in DP transfer from blood to adipose tissue. The marginal positive relationship was observed between serum levels and apolipoprotein A concentrations (p = 0.095 for total DP and 0.045 for syn-DP), and neither association was found between serum levels and thyroid hormone concentrations (THs). To our best knowledge, this is the first report about the accumulation relationship of DP between human adipose tissue and blood stream with the corresponding distribution-related mechanism. Ó 2019 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center of Eco-Environmental Sciences, Chinese Academy of Sciences, PO Box 2871, 18 Shuangqing Road, Haidian District, Beijing 100085, PR China. E-mail address: [email protected] (X.-H. Li). https://doi.org/10.1016/j.scitotenv.2019.134391 0048-9697/Ó 2019 Elsevier B.V. All rights reserved.

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1. Introduction Dechlorane plus (DP) has been widely used as the flame retardants in various materials for almost 50 years. At present, DP has been listed as a high production volume (HPV) chemical in the United States and Canada (Sverko et al., 2010; 2011), and has been considered as a possible alternative for decabromodiphenyl ether (BDE-209) by the European Commission (2011), suggesting its extended application potentials in the future. However, DP is now viewed as a global contaminant because of its potentials for long-range atmospheric transport, and to bioaccumulation. Therefore, DP has been classified into Candidate List of Substances of Very High Concern by European Chemicals Agencies (2018). The monitoring of DP in fish and other wild life showed that it could be accumulated to a high level within biological tissue (Sverko et al., 2011; Xian et al., 2011). Furthermore, a dozen recent studies revealed that exposure to DP induced genotoxicity, cytotoxicity and negative physiological effects (Baron et al., 2016; Chen et al., 2017; Crump et al., 2011; Dou et al., 2015; Liang et al., 2014; Zhang et al., 2014). For example, DP demonstrated genotoxicity in marine bivalves (Baron et al., 2016) and developmental neurobehavioral toxicity in embryo-larval zebrafish (Chen et al., 2017). To date, some classical persistent organic pollutants (POPs), such as polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs), have been proven with strong links between human exposure and adverse effects, but there is a paucity of information on DP, partly ascribing to the scarce academic attention paid to human exposure (Ben et al., 2013; Ren et al., 2009; Siddique et al., 2012). More studies on human exposure to DP shall be required in order to thoroughly assess its health risks, which would in turn determine whether DP could be used extendedly in the future. The assessment of human exposure to lipophilic chemicals is based mainly on the determination of concentration in matrices derived from human body. Adipose tissue is commonly the preferred biological specimen for assessing the human exposure to lipophilic chemicals (i.e. PBDEs and PCBs) (Frederiksen et al., 2009; Kimbrough, 1995; Lv et al., 2015), PCDD/Fs (Srogi, 2008) and OCPs (Kutz et al., 1991)) since these chemicals’ residual levels in adipose tissue can reflect the steady-state concentrations and integrated levels of accumulation over time (Johnson-Restrepo et al., 2005). However, insightful knowledge is still unavailable on the accumulation potential of DP in human adipose tissue. In organisms, the lipid-adjusted tissue concentration is generally used to represent the body burden of lipophilic chemicals since the lipid-normalized tissue levels in equilibrium are assumed to be similar with each other. In humans, the specific partitioning relationships of DP have been proven between maternal serum and breast milk, as well as among maternal serum, placenta and cord serum in our previous reports (Ben et al., 2013; 2014), in which DP distribution was in favor of maternal serum lipids, rather than keeping uniform among tissue lipids. The background knowledge mentioned above manifested a great possibility for the accumulation of DP in human adipose tissue and its specific partitioning between serum and adipose tissues. Furthermore, the particular pattern meant that DP distribution in humans might be underlying a different mechanism from those classic lipophilic compounds. These pointed out the necessity for a thorough investigation in order to fully understand the human health risk of DP. In the present study, the levels of DP were measured in sixtyfour matched adipose tissue and serum samples from women volunteers living in Wenling, China. The long-term cumulative potentials of DP in humans were evaluated by measuring DP levels in adipose tissue, and partitioning characteristics were further exam-

ined between adipose tissue and the matched serum. Effect of tissue lipids was discussed to comprehend the mechanism of DP tissue distribution. These results will be helpful to fully understand the toxico-kinetics and the human health risk of DP. 2. Materials and methods 2.1. Chemicals and materials Stock solutions including syn-DP (100 mg mL 1), anti-DP (100 mg mL 1), 13C10-labelled syn-DP (100 mg mL 1), 13C10-labelled anti-DP (100 mg mL 1), 13C12-labelled PCB-208 (40 mg mL 1) were obtained from Cambridge isotope laboratories Inc. (Andover, USA). Stock solutions of anti-[DP-2Cl] (50 mg mL 1), anti-[DP-1Cl] (50 mg mL 1) were purchased from Wellington Laboratories Inc. (Guelph, ON, Canada). 2.2. Sample collection Samples were collected from pregnant women living in Wenling, China. More detailed description of the sampling area (Ben et al., 2013), sampling method and personal information (Lv et al., 2015), has been previously published (Fig. SI-1 and Table SI-1 of supporting information). In brief, Wenling’s natural environment has been heavily contaminated by a number of ewaste-related chemicals, such as PCBs, PBDEs and DP (Ni et al., 2010). Sixty-four pairs of abdominal subcutaneous adipose tissue and serum samples from the same women, were sampled in a Wenling local hospital. The donors were healthy general population resided in Wenling for more than 5 years, without professional history on e-waste recycling operation. Data about DP distribution among maternal serum, matched placenta and cord serum (Ben et al., 2014), and between maternal serum and matched breast milk (Ben et al., 2013), were also used in order to discuss the impacts of tissue lipids on DP distribution across human tissues. Notably, triplet of maternal serum, matched placenta and cord serum used in this study were collected between July 2010 and July 2011 (Ben et al., 2014). Both maternal serum and matched breast milk referred to in this study were collected from July 2010 to March 2011 (Ben et al., 2013). Despite the different sampling times and samples between this study and those two previous reports (Ben et al., 2013; 2014), the donors came from the same region and possessed similar characteristics. 2.3. Sample extraction, clean-up, and analysis The procedures on sample extraction and clean-up have been described in our previous study (Lv et al., 2015). In brief, Adipose tissue samples (0.5 g), after spiking the surrogate standards (13C10 labelled syn-DP and 13C10 labelled anti-DP), were extracted ultrasonically with MtBE:Hex:DCM (1:1:1, V:V:V) with three times repeats. The combined extract was cleaned up in a multi-layer silica gel chromatography column, and was washed by the mixture of hexane and dichloromethane. The serum samples (5 mL), after spiking the surrogate standards, were denatured with HCl and isopropanol, then ultrasonically extracted by the mixture solution of MtBE and Hexane with three times repeats. The combined extracts were washed with KCl solution, dried with Na2SO4, and then concentrated. The concentrated extract was cleaned up in a chromatography column as mentioned above. The final effluence of either adipose tissue or serum was concentrated to 50 lL and the known amount of the injection internal standard (13C12-labelled PCB-208) was added prior to the GC/MS analysis. The quantification of syn-DP, anti-DP, anti-[DP-2Cl] and anti-[DP-1Cl] was performed

J.-F. Yin et al. / Science of the Total Environment 700 (2020) 134391

using an Agilent 6890 GC, coupled with 5973 MSD in electron capture negative ionization (ECNI) mode. The separation procedure of chromatography and the monitored ion fragments of chemicals have been described by Ben et al. (2013; 2014). Determination for serum thyroid hormones (THs), lipoproteins, total cholesterol and triglyceride was conducted in a local hospital (The First People’s Hospital of Wenling). In brief, levels of THs, including thyroid stimulating hormone (TSH), total triiodothyronine (TT3), total thyroxine (TT4), free triiodothyronine (FT3), and free thyroxine (FT4), were measured by the chemiluminescent microparticle immunoassays (Siemens, Advia Centaur XP). Levels of lipid-related proteins (HDL-C, LDL-C, apolipoprotein A, apolipoprotein B and lipoprotein A), total cholesterol and triglyceride in serum samples were determined by colorimetry, immunoturbidimetry (Siemens, Advia 2400). As described in Lv et al.’s report (Lv et al., 2015), adipose lipids were determined using a gravimetric method, and serum lipids were calculated using the serum concentrations of triglycerides and total cholesterol in terms of formula (Phillips et al., 1989). Although determination method for serum lipids differed from that for adipose fat, researches had proven that the enzymatic determination of serum lipids could provide reliable results by comparison gravimetric method (Akins et al., 1989; Bergonzi et al., 2009). The average lipid concentration was 6.0 g L 1 for serum and 71.4% for adipose tissue, respectively. The personal information of donors and their infants, levels of thyroid hormones (THs), lipoproteins, total cholesterol and triglyceride in serum from some donors are listed in Tables SI-1 and SI-2 of supporting information, respectively. 2.4. Quality control Quality control samples (including solvent blank, procedure blank and matrix blank) were analyzed in each batch. Bovine serum and pig fat were used as the matrix blank of the serum sample and the adipose tissue. No target compounds were detected in blank samples. The average recoveries of DP and its de-chlorinated analogues in the matrix spiked blanks were 72–97%, respectively. The recovery of the analytical method was monitored using the surrogate standards (13C10 labelled syn-DP and 13C10 labelled antiDP). Recoveries of surrogate standards were in the range of 69– 108% for the adipose tissue, and were in the range of 64–102% for the serum samples, respectively. The method detection limits (MDLs) were defined as three times the standard deviation (SD) of the concentration of the target compounds spiked into matrix blank samples (4 pg g 1 for bovine serum and 40 pg g 1 for pig fat). MDLs were in the range of 3.7– 9.1 pg g 1 ww for DP and its two dechlorinated analogs in the adipose tissue. For serum, MDLs were in the range of 0.6–1.5 pg g 1 ww for DP and its two dechlorinated analogs. 2.5. Data analysis All statistical analyses were performed using SPSS 23.0 software (SPSS Inc., Chicago, IL, USA) and least-squares linear regressions were fitted using OriginPro8 SR0 software (www.OriginLab.com). The Kolmogorov–Smirnov test was used to test continuous variables for the normal distribution. The paired-samples t-test was used to P test the difference of anti-DP/ DPs in adipose tissue and serum. Wilcoxon t-test was used for examining the difference of anti[DP-1Cl]/anti-DP ratios in the maternal serum and adipose tissue. The relationship of compounds’ concentrations between adipose tissue and serum was conducted using Spearman correlation analysis. The partitioning of compounds in adipose tissue and serum, and the relationship of the tissue levels and lipid contents, were fitted by least-squares linear regressions. One-way ANOVA was used to test the discrepancy of the lipid-adjusted ratios across tissues. A

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p-value < 0.05 was considered to indicate statistical significance. For samples with concentrations below MDL, zero was used for the calculations. 3. Results and discussion 3.1. Concentrations and body burden of DP and its dechlorinated analogues In the present study, detection frequencies of syn-DP and antiDP were 100% in adipose tissue, and in serum were 95% and 100% respectively. For the dechlorinated product of DP, anti-[DP1Cl], was detected in 88% adipose tissue and 41% maternal serum, accordingly. No anti-[DP-2Cl] was detected in any samples. This chemical, anti-[DP-2Cl], has been found in the environmental sample, and in the photolysis product of DP, but so far not been reported in animals or humans with the exception of one report as mentioned in two literature reviews (Sverko et al., 2011; Xian et al., 2011). Our result, together with those from the published literatures, indicated that DP and its mono-dechlorinated analogues might be prone to accumulations in organisms (including humans), but the case was not for anti-[DP-2Cl]. The concentrations of DP and its mono-dechlorinated products based on both wet weight (ww) and lipid weight (lw), are displayed in Table 1 for adipose tissue and the matched serum. Although anti-[DP-1Cl] was detected only in 26 serum samples here, its concentration is still displayed in order to discuss the internal metabolism potentials of DP below. In the present study, the median level of total DP was 971 pg g 1 wet weight (ww) in adipose tissue, and 34.7 pg g 1 ww in serum, respectively. The wet-weight levels in adipose tissues were about 30-times higher than those in the serum samples. The much higher wet weight concentration of DP in the adipose tissue compared with serum, resembled the characteristics of other POPs and lipophilic compounds, e.g. dioxin, PCBs, PBDEs, OCPs (Haddad et al., 2000; Lv et al., 2015). To date, a few reports on DP body burden in humans have been generally expressed as lipid-adjusted concentrations. Here, lipidadjusted DP levels (lw) in this study was used for comparison with other publications. The serum concentration of total DP (sum of synDP and anti-DP), varied from 0.410 to 252 ng g 1 lipid weight (lw) with the median value as 3.98 ng g 1 lw. Serum DP median levels in our samples were much lower than those in the e-waste recycling workers (Chen et al., 2015; Ren et al., 2009; Yan et al., 2012), but lied in the middel of those in the population living in/nearby the contamination source (Ben et al., 2014; Norrgran Engdahl et al., 2017), and on the top of those in the general population (Brasseur et al., 2014; Fromme et al., 2015; Qiao et al., 2018; Yan et al., 2012) (Table SI-3). For adipose tissue, the concentration of total DP, ranged from 0.234 to 75.3 ng g 1 lipid weight (lw) with the median value as 1.22 ng g 1 lw. The levels in adipose tissue was not horizontally compared since there were no available data from other population. Some literatures presumed that the pollutant levels in blood stream mainly represent the results from the recent exposure, while those in adipose tissue show the outcome of the long-term exposure (Archibeque-Engle et al., 1997; Arrebola et al., 2012; Crinnion, 2009). If so, we therefore concluded that the frequent detection and the existing levels of DP in blood stream and adipose tissue, indicated that evaluating DP risk to humans should not only take the acute impact into consideration, but also the long-term exposure detriment. More proofs are needed to prove the assumption. 3.2. Distribution of DP between serum and adipose tissue Generally, the body burden of certain compounds in humans could be reflected by tissue levels. However, as seen in Table 1,

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Table 1 Dechlorane plus and its mono-dechlorinated analogue in adipose tissues and matched serum collected from the e-waste recycling area in Wenling, China (n = 64). Pollutant

Adipose tissue syn-DP anti-[DP-1Cl] anti-DP P DPs Serum syn-DP anti-[DP-1Cl] anti-DP P DPs

Frequency%

lipid-basis level (ng g

1

, lw)

wet-basis level (pg g

1

, ww)

Mean ± SD

Min.

Median

Max.

Mean ± SD

Min.

Median

Max.

100 88 100

1.04 ± 1.94 0.100 ± 0.129 3.56 ± 9.08 4.69 ± 10.9

0.0483 nd 0.179 0.234

0.316 0.0494 0.915 1.22

10.3 0.673 65.2 75.3

802 ± 1580 76.2 ± 104 2770 ± 7580 3570 ± 8990

389 nd 141 184

273 37.6 712 971

8522 572 55,354 639,071

95 41 100

5.42 ± 21.6 0.563 ± 0.696 8.48 ± 15.2 13.9 ± 34.5

nd nd 0.410 0.410

1.00 0.329 2.96 3.98

167 3.29 82.4 252

41.9 ± 171 3.74 ± 3.88 63.2 ± 107 105 ± 264

nd nd 3.97 3.97

8.56 2.42 23.9 34.7

1334 16.9 657 2007

nd:
the lipid-adjusted DP levels in serum (3.98 ng g 1 lw) showed more dominance when compared with those in adipose tissue (1.22 ng g 1 lw). This meant there existed great discrepancy when using lipid-adjusted levels of different tissues to estimate the DP body burden. It was observed that in this study, strong associations for individual compound between adipose tissue and the matched serum were found, either for ww or lw (p < 0.001 for syn-DP and anti-DP), indicating their high coherences across tissues. Due to high association, distribution relationship could be fitted through DP levels found in the two sample types. In order to reduce the impact from the outlier, the outlying data were dealt with the similar method reported by Lv et al. (2015). In brief, adipose-serum partitioning ratio was first calculated as concentrations in the adipose tissue divided by the maternal serum. The ratio was eliminated when the value was not within the three standard deviations from the overall average. The remaining data were transferred in a logarithmic scale and fitted by the least-square liner model. Then, the outcome of the liner model and the predicted adipose-serum ratio of two DP isomers could be obtained, as shown in Table 2 and Fig. SI-2. R2 values for syn-DP and anti-DP were over 0.6, and p values were significantly < 0.001, suggesting that the models were effective to describe the relationship of DP distribution between blood and adipose tissue. To our best knowledge, this was the first report about the accumulation relationship of DP between human adipose tissue and blood stream. The established relationship between adipose tissue and serum provided an effective tool to reduce the errors for estimating the human body burden when only one tissue was measured (Peterson et al., 2016). Besides DP, some studies revealed data gap of lipophilic chemicals among tissue lipids, especially for certain high halogenated lipophilic chemicals, such as PCB-209 and BDE-209, whose lipidadjusted levels in human breast milk (Mannetje et al., 2012) or adipose tissue (Lv et al., 2015) were significantly lower relative to serum levels. Wan et al. (2013) observed the distinctively different distribution of BDE-209 from those of less-brominated BDEs in the

Chinese sturgeon. Hence when lipid-adjusted tissue levels of high halogenated lipophilic chemicals were used to reflect the body burden, it should be regarded with caution due to the great data gap among tissues. Additionally, the result further emphasized the importance of enhancing the studies on tissue-distribution of high halogenated lipophilic chemicals. 3.3. Effect of tissue lipids on DP partitioning As for DP, it has high octanol-water partition coefficient (log Kow = 9.3 for DP technical product), and is considered as a possible ‘‘lipid-binding compounds”(Peng et al., 2012). Nevertheless, no direct evidence is so far provided to verify this assumption. Our previous academic work about the transfer of DP among maternal serum and paired placenta and cord serum (Ben et al., 2014), and between maternal serum and matched breast milk (Ben et al., 2013), together with this study, provided an opportunity to assess the impact from tissue lipids on DP partitioning. It was worth noting that the paired/matched samples came from different batches (footnote in Table 3), but distribution relationship of DP between serum and paired/matched tissues (lipid-adjusted partitioning ratio of DP) could be obtained in these studies. The partitioning ratio represents the relative levels between tissues in equilibrium and is not affected by the sample batches. Therefore, in this study, influence of tissue lipids on DP partitioning was discussed based on the tissue/maternal serum ratios obtained. Among human tissues (including maternal serum, placenta, cord serum, breast milk and adipose tissue), the lipid-adjusted tissue/maternal serum ratios of two DP isomers and tissue lipid contents could be acquired, as listed in Table 3. Noticeably, the listed partitioning ratios (Table 3) were values of matched tissue levels (lw) relative to maternal serum levels (lw), thereby DP level (lw) in matched tissue could be estimated if maternal serum level (lw) was known. Here, given that the level in the maternal serum was considered as 1.0 ng g 1 lw, DP level (lw) in matched tissue could be calculated by partitioning ratio

Table 2 Liner parameters of individual compound in log-transformed adipose tissue levels and matched serum levels, and predicted adipose/serum ratio. Compound Lipid-weight syn-DP anti-DP Wet weight syn-DP anti-DP

Na

Correlation coefficient (r2)

54 57

0.70 0.64

54 53

0.74 0.76

Intercept (log(b)) 0.45 0.46 1.47 1.46

adipose/serum ratio (10

b

)

p value

0.35 0.35

<0.001 <0.001

29.5 28.8

<0.001 <0.001

The equation below was used to assess the relationship of DP levels in adipose tissue and serum (Lv et al., 2015): Log(adipose) = log(serum) + log(b) . . ... . ...(1). Adipose/serum ratio for individual compound expressed as: adipose/serum = log(b). . ... . ...(2).

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J.-F. Yin et al. / Science of the Total Environment 700 (2020) 134391 Table 3 Lipid-adjusted partitioning ratio of DP between human tissues, and DP tissue concentration relative to maternal serum concentration (wet weight or dry weight, pg g

* ** ***

****

1

).

Ratio between tissues

syn-DP

anti-DP

Tissue concentration relative to maternal serum***

syn-DP

anti-DP

Lipid content (%)

Maternal serum/Maternal serum Cord serum/Maternal serum (Ben et al., 2014) Breast milk/Maternal serum (Ben et al., 2013) Placental tissue*/Maternal serum (Ben et al., 2014) Adipose tissue/Maternal serum**

1.00 0.45 0.43 0.27 0.36

1.00 0.35 0.47 0.30 0.35

Maternal serum Cord serum Breast milk Placental tissue* Adipose tissue

6.00 1.04 6.88 16.8 257

6.00 0.805 7.52 18.7 250

0.600**** 0.230 1.60 6.24 71.4

Lipid content in placental tissue was detected based on the basis of dry weight, and other tissue was detected based on the basis of wet weight. This study. Given that DP level in maternal serum was considered as 1 ng g 1 lw, DP level in matched tissue (lw) was obtained by partitioning ratio multiplying by serum level (lw). Then, wet weight/dry weight levels could be obtained by DP level in the corresponding tissue (lw) multiplying by the tissue lipid content and factor 1000. Lipid content of maternal serum was considered as 0.6%, which was average value in references (Ben et al., 2013; Ben et al., 2014) and this study. The maternal serum and matched adipose tissue in this study were collected in June and December 2011. Triplet of maternal serum, matched placenta and cord serum were collected between July 2010 and July 2011 (Ben et al., 2014). Both maternal serum and matched breast milk were collected from July 2010 to March 2011 (Ben et al., 2013).

multiplying by DP serum level (1.0 ng g 1 lw). However, since lipid-adjusted levels of DP were used for the partitioning ratios between two tissues, they should be transformed as the wetweight/dry-weight levels when the impact of lipid pool on tissue distribution was discussed. Table 3 lists the relative DP tissue levels in wet or dry weight, which was calculated by lipidadjusted tissue level multiplying by tissue lipid content. Log transformation of DP tissue levels relative to maternal serum levels (wet or dry weight) and the corresponding lipid content is plotted in Fig. 1 for syn-DP and Fig. SI-3 of the Supporting Information for anti-DP. As shown in Figs. 1 and SI-3, DP wet/dry weight level showed positive association with the tissue lipid content (R2 = 0.94, p < 0.05 for two DP isomers), indicating levels in different tissues strongly corresponded to the lipid content of tissues. This significantly positive association proffered direct proof for lipid-driven mechanism on DP distribution in humans. As a result, the 30-fold higher DP wet-weight levels found in the adipose tissue might be related to its higher lipid content with comparison to serum. However, the lipid-adjusted ratios in Table 3 were significantly different (p < 0.01, one-way ANOVA), showing that DP partitioning across tissues was not entirely subjected to the tissue lipid contents. In addition, in the same population with this present study,

Lv et al. (2015) had reported the lipid-adjusted adipose-serum ratios of major PCB/PBDE congeners. Among them, of high lipophilic chemicals, such as CB-153 and BDE-153, the ratios generally fluctuated around 1.0. Nevertheless, the ratio of DP found in this study, was only about one third, although its lipophilicity (LogKow value) was almost the highest (OxyChem, 2010). This comparison indicated that DP distribution mechanism between adipose tissue and serum might be different from those of CB-153 and BDE-153. Relative lipid contents between tissues would be the sole mechanistic determinant of the adipose tissue-blood partition coefficients of highly lipophilic organic chemicals unless these chemicals were significantly bound to biological macromolecules (Haddad et al., 2000). Under this condition, lipid-adjusted adipose tissue-blood ratio should be equal to one (Haddad et al., 2000). In this study, those ratios of DP (Table 3) were significantly different from one (p < 0.05 for all paired/matched samples), showing a high probability of DP interaction with tissue biological macromolecules. Furthermore, the ratios (Table 3) were only one half or one third, suggesting that interaction with certain serum macromolecular compositions (e.g. serum lipoproteins and human serum albumin), might exert a more important impact on its tissue distribution relative to other listed tissue compositions. In all, the results presented the evidence that the tissue distribution of DP in humans was greatly associated with the tissue lipid contents and tissue compositions. More studies including in-vivo and in-vitro experiments, are required to identify the effect of tissue compositions, especially serum compositions on DP distribution. 3.4. Stereo-selective behavior of DP between tissues

Fig. 1. Relationship of the log-transformed syn-DP tissue concentration relative to maternal serum (wet weight or dry weight) and the log-transformed lipid content in human tissues. syn-DP level (wet weight or dry weight) in tissue was calculated by partitioning ratio multiplying by tissue lipid content and factor 1000 if DP level in maternal serum was presumed as 1 ng g 1 lw. * Lipid content in placental tissue was detected based on the basis of dry weight, and other tissue was detected based on the basis of wet weight. CS: cord serum; MS: maternal serum; BM: breast milk; PT: placental tissue; AT: adipose tissue.

P The fraction of anti-DP (anti-DP/ DPs, fanti), is generally used to examine the stereo-selective behavior of two DP isomers among tissues/organs of organisms. In this study, the median value of fanti was 0.74 in human adipose tissue (0.72–0.77 from the 1st to the 3rd quartiles), and 0.73 (0.71–0.77 from the 1st to the 3rd quartiles) in the serum sample. Assuming the occurrence of stereoselective behavior of DP, a significant difference in fanti values between serum and adipose tissue is expected. However, fanti values found between the two tissues were similar to each other, and the significant difference was not observed by a pairedsamples t-test (p > 0.05). The similar fanti values between the adipose tissue and matched serum, indicated that stereo-selective behavior of DP isomers did not occur or occurred less obviously in the human blood-adipose tissue circulatory process. However, stereo-selective behavior of two DP isomers have been demonstrated to varying degrees in certain tissues/organs of organisms. For example, the stereo-selective behavior of two DP isomers occurred during the transfer process from mother to fetus, with a lower fanti in cord blood and a higher fanti in placenta

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relative to maternal blood (Ben et al., 2014). Additionally, clear preferential distribution of anti-DP has also been observed in hepatic and brain tissues compared to other tissues/organs (e.g. muscle) in chicken (Zheng et al., 2014). Placenta, liver and brain are all active organs, and those stereo-selective behaviors may be ascribed to enzyme-mediated biotransformation and/or specific binding to certain proteins, and/or the protective blood-brain/ placental barrier (Ben et al., 2014; Zheng et al., 2014). Adipose tissue is considered more often as a storage organ for lipophilic compounds, functioning differently from either the liver, brain or placenta, which might contribute to the non-stereo-selective results in the circulation of adipose tissue and blood stream in this study. Similar to serum and adipose tissue, non-stereoselective behavior was also discovered in DP transfer between serum and breast milk in our previous study (Ben et al., 2013). Recently, Chen et al. (2019) reported that human serum albumin (HSA) might prefer combination with anti-DP, rather than syn-DP by BIA core (Biomolecular Interaction Analysis) system based on Surface Plasmon Resonance (SPR) technology. It appeared that the stereoselective binding to HSA might not be a reasonable explanation for non-stereoselective behavior of two DP isomers between serum and adipose tissue/breast milk. The interaction with other blood compositions other than HSA, could together contribute to the non-stereoselective results although this is not identified. 3.5. Internal mono-dechlorinated potential of anti-DP Additionally, the ratio of anti-[DP-1Cl] to anti-DP level was used to explore the internal mono-dechlorinated capacity of anti-DP among tissues/organs in organisms (Zhang et al., 2011). The anti[DP-1Cl]/anti-DP ratios would be significantly different between both adipose tissue and maternal serum if the internal monodechlorinated biotransformation of anti-DP occurred. In this study, the median value of anti-[DP-1Cl]/anti-DP was 0.040 (0.030–0.055 from the 1st to the 3rd quartiles) in adipose tissue and 0.030 in serum (0.023–0.048 from the 1st to the 3rd quartiles), respectively. Furthermore, it was found that the anti-[DP-1Cl]/anti-DP values in adipose tissue were not significantly different from those in the matched serum (p > 0.05, n = 25, by the paired samples t-test). The similar anti-[DP-1Cl]/anti-DP values, at least to a certain degree, suggested that internal mono-dechlorination of anti-DP via metabolism/degradation, was not predominant in the human blood-adipose tissue circulatory process. Ben et al. (2013) also reported that the anti-[DP-1Cl]/anti-DP values in serum did not differ significantly from those in matched breast milk. However, since only both anti-[DP-1Cl] and anti-[DP-2Cl] were measured, and the detected levels of anti-[DP-1Cl] was low and the matched sample sizes were relatively small in this study, together with the limited types of human samples and the absent environment-related samples (e.g. food and dust), more research efforts were required to evaluate the tissue dechlorinated potential of DP and source of dechlorinated products in humans. 3.6. Relationship with lipid compositions and thyroid hormone in serum As mentioned above, tissue distribution of DP was likely associated with the lipid compositions to a great degree. Here, the relationship between DP levels and some lipid compositions in serum were further explored, although these parameters could be obtained only from 44 subjects (Table SI-2). The experimental results showed that serum DP levels possessed almost no association with all lipid compositions listed with the exception of apolipoprotein A. The apolipoprotein A exhibited significant positive association with serum syn-DP (p = 0.046, n = 44) and marginal

association with total DP (p = 0.095, n = 44). Since interaction of chemicals with certain serum components (e.g. proteins) might be related with endocrine function and disease process (Joseph, 1994; Ljunggren et al., 2014) as well as transfer across tissues (Chen et al., 2019; Peng et al., 2012), the observed marginal positive relationship between serum level and apolipoprotein A might shed light upon DP-related health effect and its transfer mechanism in humans. Although it was reported that DP concentrations were likely associated with thyroid hormone levels (Ben et al., 2014), insignificant relationship was found between serum DP levels and matched THs concentrations in this study. Further studies are required to fully investigate this issue due to the limited sample size. 3.7. Conclusions In summary, this study revealed the relative distribution of DP between adipose tissue and maternal serum, which suggested apparent difference when compared to common lipophilic compounds. Both the tissue lipid contents and tissue compositions might together significantly influence DP tissue distribution. The stereo-selective behaviors of syn-DP and anti-DP did not occur in the blood-adipose tissue circulatory process, and the internal metabolism/degradation to remove one or two chlorine atoms of anti-DP molecular was neither predominant. Both DP’s possible binding to certain blood compositions, and the marginal positive relationship between serum and apolipoprotein A, reminded that close concerns should be paid to DP-related blood effects in the future. Declaration of Competing Interest The authors declare no actual or potential conflicts of interest. Acknowledgment We gratefully acknowledge the contribution from the volunteer donors and medical staff in this study. This work was funded by the National Natural Science Foundation of China (21477157 and 21777185) and the Ministry of Science and Technology of China (2016YFA0203102). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2019.134391. References Agency, E.C. Inclusion of substances of very high concern in the Candidate List for eventual inclusion in Annex XIV. https://echa.europa.eu/documents/10162/ 50ad6004-912d-483f-533a-e5bd294cee22. 2018. Akins, J.R., Waldrep, K., Bernert, J.T., 1989. The estimation of total serum-lipids by a completely enzymatic summation method. Clin. Chim. Acta 184, 219–226. Archibeque-Engle, S.L., Tessari, J.D., Winn, D.T., Keefe, T.J., Nett, T.M., Zheng, T., 1997. Comparison of organochlorine pesticide and polychlorinated biphenyl residues in human breast adipose tissue and serum. J. Toxicol. Environ. Health 52, 285– 293. Arrebola, J.P., Cuellar, M., Claure, E., Quevedo, M., Antelo, S.R., Mutch, E., Ramirez, E., Fernandez, M.F., Olea, N., Mercado, L.A., 2012. Concentrations of organochlorine pesticides and polychlorinated biphenyls in human serum and adipose tissue from Bolivia. Environ. Res. 112, 40–47. Baron, E., Dissanayake, A., Vila-Cano, J., Crowther, C., Readman, J.W., Jha, A.N., Eljarrat, E., Barcelo, D., 2016. Evaluation of the Genotoxic and Physiological Effects of Decabromodiphenyl Ether (BDE-209) and Dechlorane Plus (DP) Flame Retardants in Marine Mussels (Mytilus galloprovincialis). Environ. Sci. Technol. 50, 2700–2708. Ben, Y.J., Li, X.H., Yang, Y.L., Li, L., Di, J.P., Wang, W.Y., Zhou, R.F., Xiao, K., Zheng, M.Y., Tian, Y., Xu, X.B., 2013. Dechlorane Plus and its dechlorinated analogs from an ewaste recycling center in maternal serum and breast milk of women in Wenling. China. Environ Pollut 173, 176–181.

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