Aryl hydrocarbon receptor activation-mediated vascular toxicity of ambient fine particulate matter: contribution of polycyclic aromatic hydrocarbons and osteopontin as a biomarker

Background: Exposure to ambient fine particulate matter (PM_2.5) is associated with vascular diseases. Polycyclic aromatic hydrocarbons (PAHs) in PM_2.5 are highly hazardous; however, the contribution of PM_2.5-bound PAHs to PM_2.5-associated vascular diseases remains unclear. The ToxCast high-throughput in vitro screening database indicates that some PM_2.5-bound PAHs activate the Aryl Hydrocarbon Receptor (AhR). The present study investigated whether the AhR pathway is involved in the mechanism of PM_2.5-induced vascular toxicity, identified the PAH in PM_2.5 that was the major contributor of AhR activation, and identified a biomarker for vascular toxicity of PM_2.5-bound PAHs.

Results: Treatment of vascular smooth muscle cells (VMSCs) with an AhR antagonist inhibited the PM_2.5-induced increase in the cell migration ability; NF-κB activity; and expression of Cytochrome P450 1A1 (CYP1A1), 1B1 (CYP1B1), interleukin-6 (IL-6), and osteopontin (OPN). Most PM_2.5-bound PAHs were extracted into the organic fraction, which drastically enhanced VSMC migration and increased mRNA levels of CYP1A1, CYP1B1, IL-6, and OPN. However, the inorganic fraction of PM_2.5 moderately enhanced VSMC migration and only increased IL-6 mRNA levels. PM_2.5 increased IL-6 secretion through NF-κB activation; however, PM_2.5 and its organic extract increased OPN secretion in a CYP1B1-dependent manner. Inhibiting CYP1B1 activity and silencing OPN expression prevented the increase in VSMC migration ability caused by PM_2.5 and its organic extract. The AhR activation potencies of seven PM_2.5-bound PAHs, reported in the ToxCast database, were strongly correlated with their capabilities of enhancing the migration ability of VSMCs. Benzo(k)fluoranthene (BkF) contributed the most to the AhR agonistic activity of ambient PM_2.5-bound PAHs. The association between PM_2.5-induced vascular toxicity, AhR activity, and OPN secretion was further verified in mice; PM_2.5-induced intimal hyperplasia in pulmonary small arteries and OPN secretion were alleviated in mice with low AhR affinity. Finally, urinary concentrations of 1-hydroxypyrene, a major PAH metabolite, were positively correlated with plasma OPN levels in healthy humans.

Conclusions: The present study offers in vitro, animal, and human evidences supporting the importance of AhR activation for PM_2.5-induced vascular toxicities and that BkF was the major contributor of AhR activation. OPN is an AhR-dependent biomarker of PM_2.5-induced vascular toxicity. The AhR activation potency may be applied in the risk assessment of vascular toxicity in PAH mixtures.