Localization of CD9, an enhancer protein for proheparin-binding epidermal growth factor-like growth factor, in human atherosclerotic plaques: possible involvement of juxtacrine growth mechanism on smooth muscle cell proliferation.

Heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF), a member of the EGF family, has a potent mitogenic activity for vascular smooth muscle cells (SMCs). We previously reported that HB-EGF is involved in atherogenesis of human aorta and coronary arteries. ProHB-EGF (the membrane-anchored form of HB-EGF) has also been demonstrated to possess a mitogenic activity, which is approximately 30-fold increased when coexpressed with CD9 in mouse L cells. Thus, in the process of atherogenesis, CD9 may be involved in the proliferation of SMCs. We immunohistochemically investigated the localization of CD9 and proHB-EGF in the human aorta and coronary arteries. In normal aorta and coronary arteries, CD9 immunostaining was virtually negative, whereas proHB-EGF immunostaining was positive, especially in the arteries of babies. In contrast, in atherosclerotic lesions, some intimal SMCs were strongly positive for CD9 and proHB-EGF immunostaining. The juxtacrine growth activities of human aortic SMCs were inhibited in vitro by adding neutralization antibodies for CD9 or adding the specific inhibitor of HB-EGF. Besides, coexpressed CD9 and proHB-EGF cells markedly incorporated [(3)H]thymidine into the SMCs. CD9 is localized immunohistochemically in the SMCs of the atherosclerotic aorta and coronary arteries. CD9, when coexpressed with proHB-EGF, enhances proHB-EGF activities for SMC growth in a so-called juxtacrine manner in vitro and may be involved in atherogenesis.