Development of atherosclerosis in osteopontin transgenic mice

Osteopontin (OPN), a noncollagenous adhesive protein, may possibly be implicated in atherosclerosis, in which macrophages and activated T lymphocytes could have higher OPN levels within the atherosclerotic plaques. However, it is not known whether a higher OPN level is a cause or a result of atherosclerosis or whether it has a promoting or inhibitory effect on atherosclerosis. To clarify the role of OPN in atherosclerosis, we developed a transgenic mouse (OPN-TG) in which the exogenous OPN gene was designed to be expressed by hematopoietic cells, expressing OPN, which carried the immunoglobulin enhancer (Emu)/SV40 promoter. In OPN-TG, the expression of exogenously transfected OPN RNA was found in lymphoid organs, such as the thymus and spleen, and the kidney. In the present study, OPN-TG mice were assigned into two groups, an atherogenic diet group (15% fat, 1.25% cholesterol) for 3 months or a standard diet group (4% fat), and both groups were compared with wild-type C57BL/6 mice to investigate the relationship between osteopontin and the atherosclerotic lesion. In wild-type mice, OPN mRNA was detected in kidney, but not in lymphoid tissues. In both OPN-TG and wild-type mice fed with control diets, atherosclerotic lesions were not found in the aortic sinus or the thoracic and abdominal aorta. In both OPN-TG and wild-type mice fed with atherogenic diets, a high incidence of atherosclerotic lesions was noted in the aortic sinus. The atherosclerotic lesions were significantly larger in OPN-TG as compared with those in control littermate mice (size: 33.8% +/- 23.4% vs. 10.9% +/- 20.4%, respectively, P < 0.05). Activated foamy macrophages within atherosclerotic plaque in OPN-TG expressed a considerably larger amount of OPN compared with such macrophages in control mice. The OPN protein detected in the atherosclerotic lesions was not due to the deposition of serum OPN, but mainly due to in situ production by the infiltrating macrophages. Thus, these results suggest that OPN is atherogenic and that macrophages expressing OPN can be easily activated and thus promote atheromatous lesions if a high fat diet is consumed.