In Silico Identification of Molecular Interactions of the Emerging Contaminant Octyl Methoxycinnamate (OMC) on HPT Axis: Implications for Humans and Zebrafish.

Background/

Objectives

Thyroid hormones (THs) regulate almost all physiological processes in vertebrates via specific mechanisms exercised spatiotemporally throughout the lifespan. The TH signalling can be impaired by thyroid-disrupting chemicals (TDCs) capable of disrupting the hypothalamic-pituitary-thyroid (HPT) axis. Octyl methoxycinnamate (OMC) (also designated octinoxate), one of the most widely used ultraviolet (UV) filters, has emerged as an environmental contaminant and has raised significant concerns recently due to its disruptive effects as TDC on humans and animals. Although the disruption of TH homeostasis has been reported, its exact modes of action (MoA) remain largely unknown. Our study aimed to provide a comparative information on the molecular interactions of OMC on TH signalling in humans and zebrafish.

Methods

In silico approaches were performed comparing OMC with endogenous thyroid hormone T3 and the anti-thyroid drug propylthiouracil (PTU).

Results

Our findings suggested a key role of OMC on the corticotrophin-releasing hormone receptor (crhr2), thyrotropin receptor (TSHR/tshr), and thyroid nuclear receptors (TR/tr-α and -β). At the hypothalamic level, a favourable binding of OMC to zebrafish crhr2 was found, involving ALA86, CYS44, HIS89, ILE63, ILE64, LEU92, PRO87, PRO88, SER48, and THR47. At the pituitary level, OMC was bound to human TSHR by the amino acid residues ASN590, GLU506, ILE583, ILE640, LEU570, MET572, PRO571, SER505, TYR667, VAL502, VAL586, ALA644, LEU587, MET637, SER641, and TYR582 and to zebrafish tsrh by ASN589, ILE639, MET636, ILE582, LEU569, LEU586, VAL501, and VAL585. Concerning nuclear receptors, OMC showed a more favourable binding energy of T3, involving the shared residues PHE218 and MET259 with T3 in both species. For human TRβ, OMC shared T3 with residues ILE 275, ILE276, LEU346, PHE269, PHE272, THR273, ALA279, ASN331, HIS435, LEU330, MET310, MET313, and PHE455. No similar residues were obtained for zebrafish trβ compared with the humans.

Conclusions

Overall, the action of OMC seems to agree with primary hypothyroidism (anti-thyroid action) mimicking the T3 hormone. This investigation demonstrates that OMC acts as a potential TDC and provides new insights into its disruptive action on the HPT axis.