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Organophosphate esters (OPEs), a major group of organophosphorus flame retardants, are regarded as emerging environmental contaminants of health concern. Amino acid decarboxylases catalyze the conversion of amino acids into polyamines that are essential for cell proliferation, hypertrophy and tissue growth. In this paper, inhibitory effect of twelve OPEs with aromatic, alkyl or chlorinated alkyl substituents on the activity of lysine decarboxylase (LDC) was assessed quantitatively with an economic and label-free fluorescence sensor and cell assay. The sensor comprises a macrocyclic host (cucurbit[7]uril) and a fluorescent dye (acridine orange) reporter. The twelve OPEs were found to vary in their capacity to inhibit LDC activity. Alkyl group substituted OPEs had no inhibitory effect. By contrast, six OPEs substituted with aromatic or chlorinated alkyl groups inhibited LDC activity significantly with IC 50 ranging from 1.32 μM to 9.07 μM. Among them, the inhibitory effect of tri- m -cresyl phosphate (TCrP) was even more effective as an inhibitor than guanosine 5′-diphosphate-3′-diphosphate (ppGpp) (1.60 μM), an LDC natural inhibitor in vivo . Moreover, at non-cytotoxic concentrations, these six OPEs showed perceptible inhibitory effects on LDC activity in PC12 living cells, and led to a marked loss in the cadaverine content. Molecular docking analysis of the LDC/OPE complexes revealed that different binding modes contribute to the difference in their inhibitory effect. Our finding suggested that LDC, as a new potential biological target of OPEs, might be implicated in toxicological and pathogenic mechanism of OPEs. [ABSTRACT FROM AUTHOR]

Organophosphate flame retardants (OPFRs), as alternatives of brominated flame retardants, can cause neurodevelopmental effects similar to organophosphate pesticides. However, the molecular mechanisms underlying the toxicity remain elusive. O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) regulates numerous neural processes through the O-GlcNAcylation modification of nuclear and cytoplasmic proteins. In this study, we aimed to investigate the molecular mechanisms accounting for the developmental neurotoxicity of OPFRs by identifying potential targets of OPFRs and the attendant effects. Twelve OPFRs were evaluated for inhibition of OGT activity using an electrochemical biosensor. Their potency differed with substituent groups. The alkyl group substituted OPFRs had no inhibitory effect. Instead, the six OPFRs substituted with aromatic or chlorinated alkyl groups inhibited OGT activity significantly, with tri-m-cresyl phosphate (TCrP) being the strongest. The six OPFRs (0–100 μM exposure) also inhibited OGT activity in PC12 cells and decreased protein O-GlcNAcylation level. Inhibition of OGT by OPFRs might be involved in the subsequent toxic effects, including intracellular reactive oxygen species (ROS), calcium level, as well as cell proliferation and autophagy. Molecular docking of the OGT/OPFR complexes provided rationales for the difference in their structure-dependent inhibition potency. Our findings may provide a new biological target of OPFRs in their neurotoxicological actions, which might be a major molecular mechanism of OPFRs developmental neurotoxicity. [ABSTRACT FROM AUTHOR]

Perfluorinated compounds (PFCs) have been shown to disrupt lipid metabolism and even induce cancer in rodents through activation of peroxisome proliferator-activated receptors (PPARs). Lines of evidence showed that PPARα was activated by PFCs. However, the information on the binding interactions between PPARγ and PFCs and subsequent alteration of PPARγ activity is still limited and sometimes inconsistent. In the present study, in vitro binding of 16 PFCs to human PPARγ ligand binding domain ( h PPARγ-LBD) and their activity on the receptor in cells were investigated. The results showed that the binding affinity was strongly dependent on their carbon number and functional group. For the eleven perfluorinated carboxylic acids (PFCAs), the binding affinity increased with their carbon number from 4 to 11, and then decreased slightly. The binding affinity of the three perfluorinated sulfonic acids (PFSAs) was stronger than their PFCA counterparts. No binding was detected for the two fluorotelomer alcohols (FTOHs). Circular dichroim spectroscopy showed that PFC binding induced distinctive structural change of the receptor. In dual luciferase reporter assays using transiently transfected Hep G2 cells, PFCs acted as h PPARγ agonists, and their potency correlated with their binding affinity with h PPARγ-LBD. Molecular docking showed that PFCs with different chain length bind with the receptor in different geometry, which may contribute to their differences in binding affinity and transcriptional activity. [ABSTRACT FROM AUTHOR]

Perfluoroalkyl substances (PFASs) have been shown to cause abnormal levels of thyroid hormones (THs) in experimental animals, but the molecular mechanism is poorly understood. Here, a fluorescence displacement assay was used to determine the binding affinities of 16 PFASs with two major TH transport proteins, transthyretin (TTR) and thyroxine-binding globulin (TBG). Most of the tested PFASs bound TTR with relative potency (RP) values of 3 × 10 −4 to 0.24 when compared with that of the natural ligand thyroxine, whereas fluorotelomer alcohols did not bind. Only perfluorotridecanoic acid and perfluorotetradecanoic acid bound TBG, with RP values of 2 × 10 −4 when compared with that of thyroxine. Based on these results, it was estimated that displacement of T4 from TTR by perfluorooctane sulfonate and perfluorooctanoic acids would be significant for the occupationally exposed workers but not the general population. Structure-binding analysis revealed that PFASs with a medium chain length and a sulfonate acid group are optimal for TTR binding, and PFASs with lengths longer than 12 carbons are optimal for TBG binding. Three mutant proteins were prepared to examine crucial residues involved in the binding of PFASs to TH transport proteins. TTR with a K15G mutation and TBG with either a R378G or R381G mutation showed decreased binding affinity to PFASs, indicating that these residues play key roles in the interaction with the compounds. Molecular docking showed that the PFASs bind to TTR with their acid group forming a hydrogen bond with K15 and the hydrophobic chain towards the interior. PFASs were modeled to bind TBG with their acid group forming a hydrogen bond with R381 and the hydrophobic chain extending towards R378. The findings aid our understanding of the behavior and toxicity of PFASs on the thyroid hormone system. [ABSTRACT FROM AUTHOR]

Abstract: Polybrominated diphenyl ethers (PBDEs) have been shown to disrupt thyroid hormone (TH) functions in experimental animals, and one of the proposed disruption mechanisms is direct binding of hydroxylated PBDE (OH-PBDE) to TH receptors (TRs). However, previous data on TH receptor binding and TH activity of OH-PBDEs were very limited and sometimes inconsistent. In the present paper, we examined the binding potency of ten OH-PBDEs with different degrees of bromination to TR using a fluorescence competitive binding assay. The results showed that the ten OH-PBDEs bound to TR with potency that correlated to their bromination level. We further examined their effect on TR using a coactivator binding assay and GH3 cell proliferation assay. Different TR activities of OH-PBDEs were observed depending on their degree of bromination. Four low-brominated OH-PBDEs (2′-OH-BDE-28, 3′-OH-BDE-28, 5-OH-BDE-47, 6-OH-BDE-47) were found to be TR agonists, which recruited the coactivator peptide and enhanced GH3 cell proliferation. However, three high-brominated OH-PBDEs (3-OH-BDE-100, 3′-OH-BDE-154, 4-OH-BDE-188) were tested to be antagonists. Molecular docking was employed to simulate the interactions of OH-PBDEs with TR and identify the structural determinants for TR binding and activity. According to the docking results, low-brominated OH-PBDEs, which are weak binders but TR agonists, bind with TR at the inner side of its binding pocket, whereas high-brominated compounds, which are potent binders but TR antagonists, reside at the outer region. These results indicate that OH-PBDEs have different activities on TR (agonistic or antagonistic), possibly due to their different binding geometries with the receptor. [Copyright &y& Elsevier]