The environmental carcinogen benzo[a]pyrene induces expression of monocyte-chemoattractant protein-1 in vascular tissue: a possible role in atherogenesis.

Exposure to carcinogenic polycyclic aromatic hydrocarbons (PAHs) has been implicated in the aetiology of atherosclerosis. Previously we showed that chronic exposure of ApoE-/- mice to the prototype PAH benzo[a]pyrene (B[a]P) causes enhanced progression of atherosclerosis, which was characterised by an increased inflammatory cell content in the atherosclerotic plaques. The aim of the present study was to evaluate the effect of B[a]P on vascular expression of monocyte-chemoattractant protein 1 (MCP-1), which is a crucial molecule promoting the recruitment of monocytes into atherosclerotic lesions. We hypothesised that B[a]P-induced expression of MCP-1 is mediated through aryl hydrocarbon receptor (AhR) activation. Initially we performed in vivo studies showing that acute treatment with B[a]P induces MCP-1 gene expression in aortic tissue of ApoE-/- mice. These observations could be confirmed by in vitro studies with human endothelial cells (RF24 cell line and primary HUVEC), showing a dose- and time-dependent increase in MCP-1 expression upon exposure to B[a]P. This was paralleled by an induction of cytochrome P450 1A1 and 1B1, indicating Ah receptor activation. No increased gene expression (MCP-1, CYP1A1 and 1B1) was found upon incubation with the structural isomer benzo[e]pyrene, which is a weak AhR agonist. Moreover, B[a]P-induced MCP-1 gene and protein expression was inhibited by co-treatment with the AhR antagonist alpha-naphthoflavone. In addition to its effect on basal gene expression, we showed that B[a]P significantly enhanced TNFalpha-induced expression of MCP-1. We were unable to block B[a]P-induced MCP-1 expression by antioxidant treatment. In contrast, we found that addition of N-acetylcysteine or vitamin C enhanced transcription of MCP-1 by B[a]P. In conclusion, our studies revealed potent vascular pro-inflammatory effects of B[a]P, as evidenced by AhR-mediated induction of MCP-1. These observations further contribute to the concept that induction of inflammation is a crucial process in PAH-enhanced atherogenesis.