OBJECTIVE Leukotriene B4 (LTB4) is a powerful chemoattractant and pro-inflammatory mediator in several inflammatory diseases, including atherosclerosis. It acts through its two membrane receptors, BLT1 and BLT2. The aim of this study was to determine the molecular mechanism involved in the proatherogenic effect of LTB4, BLT1 and BLT2 in atherosclerosis. Moreover, we characterized the expression of 5-lipoxygenase (5-LO) pathway and LTB4 receptors in blood and plaques from patients with carotid atherosclerosis. RESULTS In cultured monocytic cells, LTB4 induced a rapid phosphorylation of mitogen-activated protein kinases (MAPKs ERK1/2 and JNK1/2) and PI3K/Akt via BLT1 and BLT2 in a pertussis toxin (PTX)-dependent mechanism (assessed via western blotting) and also increased nuclear factor-kappaB (NF-kappaB) DNA binding activity (assessed via EMSA) in a MAPK- and reactive oxygen species-dependent mechanism. Furthermore, LTB4 elicited interleukin-6, monocyte chemoattractant protein-1 and tumour necrosis factor-alpha mRNA overexpression also via BLT1 and BLT2 by a PTX- and NF-kB-dependent mechanism (assessed by real-time PCR), promoting an inflammatory environment. When compared with healthy subjects, patients with carotid atherosclerosis showed a significant increase in the expression of all the components of the 5-LO pathway and BLT1 and BLT2 mRNA (real-time PCR) in peripheral blood mononuclear cells and LTB4 plasma levels (ELISA). In these patients, an overexpression of 5-LO, leukotriene A-4 hydroxylase (LTA4-H) and BLT1 was noted in the inflammatory region of carotid plaques when compared with the fibrous cap (assessed by immunohistochemistry). CONCLUSIONS The 5-LO pathway is enhanced in patients with carotid atherosclerosis. Furthermore, its product LTB4 phosphorylates MAPKs and stimulates NF-kappaB-dependent inflammation via BLT1 and BLT2 receptors in cultured monocytic cells. The blockade of this pathway could be a novel and potential therapeutic target in atherothrombosis.