Halogenated Phenolic Contaminants Inhibit the In Vitro Activity of the Thyroid-Regulating Deiodinases in Human Liver

Abstract

Halogenated contaminants, particularly brominated flame retardants, disrupt circulating levels of thyroid hormones (THs), potentially affecting growth and development. Disruption may be mediated by impacts on deiodinase (DI) activity, which regulate the levels of active hormones available to bind to nuclear receptors. The goal of this study was to develop a mass spectrometry-based method for measuring the activity of DIs in human liver microsomes and to examine the effect of halogenated phenolic contaminants on DI activity. Thyroxine (T4) and reverse triiodothyronine (rT3) deiodination kinetics were measured by incubating pooled human liver microsomes with T4 or rT3 and monitoring the production of T3, rT3, 3,3'-diiodothyronine, and 3-monoiodothyronine by liquid chromatography tandem mass spectrometry. Using this method, we examined the effects of several halogenated contaminants, including 2,2',4,4',5-pentabromodiphenyl ether (BDE 99), several hydroxylated polybrominated diphenyl ethers (OH-BDEs), tribromophenol, tetrabromobisphenol A, and triclosan, on DI activity. The Michaelis constants (K(M)) of rT3 and T4 deiodination were determined to be 3.2 ± 0.7 and 17.3 ± 2.3μM. The V(max) was 160 ± 5.8 and 2.8 ± 0.10 pmol/min.mg protein, respectively. All studied contaminants inhibited DI activity in a dose-response manner, with the exception of BDE 99 and two OH-BDEs. 5'-Hydroxy 2,2',4,4',5-pentabromodiphenyl ether was found to be the most potent inhibitor of DI activity, and phenolic structures containing iodine were generally more potent inhibitors of DI activity relative to brominated, chlorinated, and fluorinated analogues. This study suggests that some halogenated phenolics, including current use compounds such as plastic monomers, flame retardants, and their metabolites, may disrupt TH homeostasis through the inhibition of DI activity in vivo.

PBDE flame retardants

“…Table 3 provides a summary of studies in fish conducted to date. Similar to results in fish, in vivo rodent studies and cell-based assays have reported PBDE-induced disruption of thyroid homeostasis, including declines in circulating THs [144][145][146] ; altered expression and activity of TH metabolizing enzymes, 147,148 and competitive binding with plasma transporters. 149 Human epidemiology studies have shown associations between altered plasma concentrations of THs in adults and PBDE levels in serum and dust.…”

“…As discussed, altered mRNA expression and enzymatic activity of some Dio isoforms has been observed in fish exposed to BDE-209 61,62,155 and 6-OH-BDE-47, 153 as well as in rodents exposed to PentaBDE 148 and human microsomes incubated with 5′-OH-BDE-99 and 2,4,6-TBP. 147 However, it remains unclear whether PBDEs (or OH-BDEs) can bind directly to Dio enzymes or whether they may elicit other allosteric effects that affect the capacity of Dios to mediate TH regulation.…”

New insights into the structure and mechanism of iodothyronine deiodinases

“…An example for a potent Dio inhibitor from these studies is the 5 0 -OH derivative of the flame retardant BDE-99 ( Fig. 7; Butt et al 2011). As in most of these studies, inhibition of cellular lysates rather than isolated Dio isoforms was studied, and the structure-activity relationship was restricted to a smaller number of compounds, since they aimed at the identification of effects of nutrients and xenobiotics on TH metabolism and function.…”

Iodotyrosine Deiodinase, a Novel Target of Environmental Halogenated Chemicals for Disruption of the Thyroid Hormone System in Mammals

“…24,25) The activity and gene expression of thyroid hormone metabolic enzymes, SULTs, UGTs and Ds, are affected by hydroxylated metabolites of PCBs and PBDEs. [26][27][28][29] The chemical structures of several agrichemicals, antiparasitics, pharmaceuticals and food colorants resemble those of thyroid hormones. Among them, triclosan, a chlorinated phenolic antibacterial agent, decreases serum T4 concentration in male juvenile rats.…”

“…Interestingly, the structural requirements of PCBs and PBDEs for thyroid hormone-disrupting action (e.g., affinity for thyroid hormone receptor and TTR) and for inhibition of SULTs and Ds also include hydroxylation and halogenation on a phenyl ring. 21,22,24,26,28,29) Therefore, a halogenated phenolic ring seems to be a critical structural requirement for thyroid hormone disruption, including IYD inhibition, by environmental chemicals.…”