12/15-lipoxygenase (12/15-LOX) is an enzyme, which oxidizes polyunsaturated fatty acids, particularly omega-6 and −3 fatty acids, to generate a number of bioactive lipid metabolites. A large number of studies have revealed the importance of 12/15-LOX role in oxidative and inflammatory responses. The in vitro studies have demonstrated the ability of 12/15-LOX metabolites in the expression of various genes and production of cytokine related to inflammation and resolution of inflammation. The studies with the use of knockout and transgenic animals for 12/15-LOX have further shown its involvement in the pathogenesis of a variety of human diseases, including cardiovascular, renal, neurological and metabolic disorders. This review summarizes our current knowledge on the role of 12/15-LOX in inflammation and various human diseases.

VSMC3 migration from media to intima plays a determinant role in atherosclerosis and restenosis (1-3). Arachidonic acid (AA) and its oxygenated metabolites, collectively known as eicosanoids, are involved in the maintenance of vascular tone (4, 5). Lipoxygenases (LOXs) are non-heme iron dioxygenases that stereospecifically introduce molecular oxygen into polyunsaturated fatty acids, resulting in the formation of hydroperoxyeicosatetraenoic acids, which are subsequently converted to hydroxyeicosatetraenoic acids (HETEs) (6). Two 15-LOXs, namely, 15-LOX1 and 15-LOX2, have been shown to be expressed in humans (7,8). Both enzymes metabolize linoleic acid to 13(S)-hydroperoxyoctadecadienoic acid and AA to 15(S)-hydroperoxyeicosatetraenoic acid preferentially (9, 10). In regard to their tissue distribution, whereas 15-LOX1 shows a narrow cell-specific expression, including human reticulocytes and airway epithelial cells, 15-LOX2 appears to be expressed in epithelial cell types in cornea, lung, prostate, and skin (11). The studies from our laboratory showed that both 15-LOX1 and 15-LOX2 are expressed in human retinal microvascular endothelial cells (12). Although the presence of 15-LOX2 in VSMC has yet to be reported, these cells express 15-LOX1 (also known as 12/15-LOX in murines) and, when exposed to AA, produce both 15(S)-HETE and 12(S)-HETE (13,14). It is also known that 15-LOX1 and 12/15-LOX are involved in the oxidation of low density lipoprotein, a contributing factor in the pathogenesis of atherosclerosis (15, 16). Furthermore, using either transgenic or knock-out mice models, a number of studies have demonstrated that 15-LOX1 and its murine ortholog 12/15-LOX play a role in atherosclerosis and restenosis (17)(18)(19). In addition, human atheroma homogenates upon incubation with AA converted it mainly to .In recent years LOX products of polyunsaturated fatty acids have also been shown to be potent chemoattractants for resi-* This work was supported, in whole or in part, by National Institutes of Health Grant HL064165 (NHLBI; to G. N. R.

It has been demonstrated that 15-lipoxygenase (15-LO) plays a role in atherogenesis, but the underlying mechanisms were unclear. Therefore, the purpose of the present study is to explore the mechanisms of 15-LO role in atherogenesis. 15(S)-hydroxyeicosatetraenoic acid [15(S)-HETE], the major 15-LO-dependent metabolite of arachidonic acid (AA), stimulated the production of reactive oxygen species (ROS) by monocytes through the xanthine oxidase–mediated activation of NADPH oxidase, which led to the Syk-, Pyk2-, mitogen-activated protein kinase (MAPK)–, and cyclic adenosine monophosphate response element–binding protein (CREB)–dependent production of the pro-inflammatory cytokine interleukin-17A (IL-17A). In addition, this pathway was required for the 15(S)-HETE–dependent migration and adhesion of monocytes to endothelial cells. Consistent with these observations, we found that peritoneal macrophages from ApoE−/− mice fed a high-fat diet (a mouse model of atherosclerosis) exhibited increased xanthine oxidase and NADPH oxidase activities, ROS production, phosphorylation of Syk, Pyk2, MAPK, and CREB, and enhanced IL-17A production compared to those from ApoE−/−:12/15-LO−/− mice. These events correlated with increased lipid deposits and numbers of monocytes and macrophages in the aortic arches of these mice, which resulted in atherosclerotic plaque formation. Together, these observations suggest that 15(S)-HETE exacerbates atherogenesis by enhancing CREB-dependent IL-17A production.

Background: Tight junctions play an essential role in the maintenance of endothelial barrier function. Results: 15(S)-HETE stimulated ZO-1 phosphorylation at Thr-770/772 residues in PKC⑀-dependent MEK1-ERK1/2 activation disrupting endothelial tight junctions and barrier function. Conclusion: PKC⑀ by interrupting ZO-1 and occludin interactions plays a role in 15(S)-HETE-induced endothelial TJ disruption. Significance: 12/15-Lipoxygenase appears to be a crucial player in the modulation of endothelial barrier permeability.

In understanding the mechanisms of cholesterol in the pathogenesis of atherosclerosis, previous studies from other laboratories have demonstrated that cholesterol crystals (CC) induce scavenger receptor CD36 expression and NLRP3-mediated inflammasome formation. In elucidating the mechanisms by which CC could enhance CD36 expression and foam cell formation, here we report that CC via NADPH and xanthine oxidases-mediated ROS production activates BTK, a non-receptor tyrosine kinase. In addition, CC induce p300 tyrosine phosphorylation and activation in a BTK-dependent manner, which in turn, leads to STAT1 acetylation and its interaction with PPARγ in CD36 expression, oxLDL uptake and foam cell formation. Furthermore, p300, STAT1 and PPARγ bound to a STAT binding site at −107 nt in CD36 promoter and enhanced its activity in ROS and BTK-dependent manner. Disruption of this STAT binding site by site-directed mutagenesis abolished CC-induced CD36 promoter activity. Together these results reveal for the first time that CC via producing ROS and activating BTK causes p300-mediated STAT1 acetylation and its interaction with PPARγ in CD36 expression, oxLDL uptake and foam cell formation.