We have shown that the 12/15-lipoxygenase (12/15-LO) product 12S-hydroxyeicosatetraenoic acid increases monocyte adhesion to human endothelial cells (EC) in vitro. Recent studies have implicated 12/15-LO in mediating atherosclerosis in mice. We generated transgenic mice on a C57BL/6J (B6) background that modestly overexpressed the murine 12/15-LO gene (designated LOTG). LOTG mice had 2.5-fold elevations in levels of 12S-hydroxyeicosatetraenoic acid and a 2-fold increase in expression of 12/15-LO protein in vivo. These mice developed spontaneous aortic fatty streak lesions on a chow diet. Thus, we examined effects of 12/15-LO expression on early events leading to atherosclerosis in these mice. We found that, under basal unstimulated conditions, LOTG EC bound more monocytes than B6 control EC (18 ؎ 2 versus 7 ؎ 1 monocytes/field, respectively; p < 0.0001). Inhibition of 12/15-LO activity in LOTG EC using a 12/15-LO ribozyme completely blocked monocyte adhesion in LOTG mice. Thus, 12/15-LO activity is required for monocyte/EC adhesion in the vessel wall. Expression of ICAM-1 in aortic endothelia of LOTG mice was increased severalfold. VCAM-1 expression was not changed. In a series of blocking studies, antibodies to ␣ 4 and  2 integrins in WEHI monocytes blocked monocyte adhesion to both LOTG and B6 control EC. Inhibition of ICAM-1, VCAM-1, and connecting segment-1 fibronectin in EC significantly reduced adhesion of WEHI monocytes to LOTG EC. In summary, these data indicate that EC from LOTG mice are "pre-activated" to bind monocytes. Monocyte adhesion in LOTG mice is mediated through  2 integrin and ICAM-1 interactions as well as through VLA-4 and connecting segment-1 fibronectin/ VCAM-1 interactions. Thus, 12/15-LO mediates monocyte/EC interactions in the vessel wall in atherogenesis at least in part through molecular regulation of expression of endothelial adhesion molecules.Murine 12/15-lipoxygenase (12/15-LO) 1 incorporates molecular oxygen in a stereospecific manner into arachidonic and linoleic acids to generate 12S-and 15S-hydroxyeicosatetraenoic acids (12S-HETE) and 13S-hydroxyoctadecadienoic acid (13S-HODE) (1-3). Murine 12/15-LO is similar biochemically and structurally to the porcine leukocyte-type 12-LO and human 15-LO enzymes (2-4). There also exists a murine platelet 12-LO, which utilizes arachidonic acid solely as a substrate to generate 12S-HETE (4, 5).The exact biologic functions of 12/15-LO are unknown. However, considerable evidence exists to support a role for 12/ 15-LO in promoting both diabetes and atherosclerosis (6 -9). Nadler and co-workers (6) have shown that mice deficient in 12/15-LO are protected from development of low dose streptozotocin-induced diabetes. We have recently shown that diabetic db/db mice produce significant quantities of 12S-HETE in vivo (10). Importantly, using a catalytic ribozyme to inactivate 12/ 15-LO mRNA, we have shown that disruption of 12/15-LO mRNA in diabetic db/db mice blocks monocyte adhesion (10). Striking evidence for the role of 12/15-LO in atherogenes...