Benzo[b]naphtho[d]thiophenes and naphthylbenzo[b]thiophenes: Their aryl hydrocarbon receptor-mediated activities and environmental presence

Abstract

Polycyclic aromatic sulfur heterocyclic compounds (PASHs) belong among ubiquitous environmental pollutants; however, their toxic effects remain poorly understood. Here, we studied the aryl hydrocarbon receptor (AhR)-mediated activity of dibenzothiophene, benzo[b]naphtho[d]thiophenes, and naphthylbenzo[b]thiophenes, as well as their presence in two types of environmental matrices: river sediments collected from both rural and urban areas, and in airborne particulate matter (PM_2.5) sampled in cities with different levels and sources of pollution. Benzo[b]naphtho[2,1-d]thiophene, benzo[b]naphtho[2,3-d]thiophene, 2,2-naphthylbenzo[b]thiophene, and 2,1-naphthylbenzo[b]thiophene were newly identified as efficient AhR agonists in both rat and human AhR-based reporter gene assays, with 2,2-naphthylbenzo[b]thiophene being the most potent compound identified in both species. Benzo[b]naphtho[1,2-d]thiophene and 3,2-naphthylbenzo[b]thiophene elicited AhR-mediated activity only in the rat liver cell model, while dibenzothiophene and 3,1-naphthylbenzo[b]thiophene were inactive in either cell type. Independently of their ability to activate the AhR, benzo[b]naphtho[1,2-d]thiophene, 2,1-naphthylbenzo[b]thiophene, 3,1-naphthylbenzo[b]thiophene, and 3,2-naphthylbenzo[b]thiophene inhibited gap junctional intercellular communication in a model of rat liver epithelial cells. Benzo[b]naphtho[d]thiophenes were dominant PASHs present in both PM_2.5 and sediment samples, with benzo[b]naphtho[2,1-d]thiophene being the most abundant one, followed by benzo[b]naphtho[2,3-d]thiophene. The levels of naphthylbenzo[b]thiophenes were mostly low or below detection limit. Benzo[b]naphtho[2,1-d]thiophene and benzo[b]naphtho[2,3-d]thiophene were identified as the most significant contributors to the AhR-mediated activity in the environmental samples evaluated in this study. Both induced nuclear translocation of the AhR, and they induced CYP1A1 expression in a time-dependent manner, suggesting that their AhR-mediated activity may depend on the rate of their intracellular metabolism. In conclusion, some PASHs could be significant contributors to the overall AhR-mediated toxicity of complex environmental samples suggesting that more attention should be paid to the potential health impacts of this group of environmental pollutants.

Keywords: AhR activity; Airborne particulate matter; Freshwater sediments; Gap junctional intercellular communication; Polycyclic aromatic sulfur heterocyclic compounds.