Objective-Osteopontin (OPN), a noncollagenous adhesive protein, is implicated in atherosclerosis, in which macrophages within atherosclerotic plaques express OPN. However, it is not known whether the elevated OPN expression is a cause or result of atherosclerosis. Methods and Results-We generated mice that lacked OPN and crossed them with apolipoprotein (apo) E-deficient mice and analyzed these mice with a mixed C57BL/6ϫ129 background after 36 weeks on a normal chow diet. In female mice, OP ϩ/Ϫ E Ϫ/Ϫ and OP Ϫ/Ϫ E Ϫ/Ϫ mice had significantly smaller atherosclerotic and inflammatory lesions compared with OP ϩ/ϩ E Ϫ/Ϫ mice, and that was reflected by smaller area of MOMA-2-positive staining. In male mice, however, there was no significant difference in the atherosclerosis lesion areas among 3 genotypes. In both OP Ϫ/Ϫ E Ϫ/Ϫ and OP ϩ/ϩ E Ϫ/Ϫ mice, typical atherosclerotic lesions were detected, which include necrotic core, foamy cell collections, and cholesterol clefts. However, we found that vascular mineral-deposited areas in 60-week-old male OP Ϫ/Ϫ E Ϫ/Ϫ mice were significantly increased compared with those in OP ϩ/ϩ E Ϫ/Ϫ male mice. Key Words: osteopontin Ⅲ atherosclerosis Ⅲ macrophage Ⅲ calcification Ⅲ lipid metabolism A therosclerosis is characterized as a chronic inflammatory process of the vessel wall. Atherosclerosis is initiated by the infiltration of monocytes and T-lymphocytes into activated endothelium, followed by their migration into the intima and subsequent lipid accumulation within macrophages. In late stages of atherosclerosis, calcification is a common advanced complication. Osteopontin (OPN), a noncollagenous adhesive protein, was first found at sites of dystrophic calcification and is synthesized at high levels by macrophages in calcified aortic valves and atherosclerotic plaques. 1 The expression of OPN protein was detected in not only macrophages but also vascular smooth muscle cells within atherosclerotic lesion. [1][2][3][4][5] In addition, it was shown that vascular smooth muscle cells during the proliferative and migratory phase, but not the quiescent and contractile phase, expressed OPN in a model of balloon catheter injury of rat carotid artery. 2 More importantly, Liaw et al 6 reported that neutralizing antibodies directed against OPN inhibited rat carotid neointimal thickening after endothelial denudation. Taken together, these results suggested that OPN can play a pivotal role in the early stage of atherosclerosis, including proliferation and migration of smooth muscle cells as well as at the late stage of atherosclerosis, characterized by calcified atheromatous plaque formation.
Conclusions-TheseTo more directly address the question of whether OPN initiates the development of atherosclerotic lesion and plays a role in calcification, we took advantage of OPN-deficient mice we had recently generated. 7 We crossed them with apolipoprotein (apo) E-deficient mice and made OPN and apoE double-deficient mice. In mice deficient for OPN gene expression, OPN mRNA was not detected in any organs. ...