ANTIPROLIFERATIVE EFFECT OF ESCULETIN ON VASCULAR SMOOTH-MUSCLE CELLS- POSSIBLE ROLES OF SIGNAL-TRANSDUCTION PATHWAYS

The effect of esculetin, a coumarin derivative with lipoxygenase inhib itor activity, on the proliferation response of cultured rabbit vascul ar smooth muscle cells was studied. Proliferation response was determi ned by the uptake of tritiated thymidine. Esculetin (10(-5)-10(-4) M) dose dependently inhibited the enhanced proliferation stimulated by 5% fetal calf serum. The structure-activity relationship of esculetin an d eight other coumarin derivatives indicates that two adjacent phenoli c hydroxyl groups at the C-6 and C-7 positions in the coumarin skeleto n are necessary for the potent antiproliferative effect. The antiproli ferative effects of other lipoxygenase inhibitors, 5,8,11,14-eicosatet raynoic acid (ETYA) and ketoconazole, were comparable to the effect of esculetin. However, esculetin exhibited the greatest maximal suppress ion. The enhanced releases of 12-hydroxyeicosatetraenoic acid (12-HETE ), prostaglandin E2 and 6-keto-prostaglandin F1alpha in the culture me dium of smooth muscle cells stimulated by 5% fetal calf serum were sig nificantly reduced by esculetin. Furthermore, the fetal calf serum-sti mulated protein tyrosine kinase activity was reduced by esculetin (10( -5)-10(-4) M) in a dose-dependent manner. In contrast, the protein kin ase C activity stimulated by phorbol-12-myristate-13-acetate was not a ffected by esculetin (10(-6)-10(-4) M). These results suggest that the antiproliferative effect of esculetin on vascular smooth muscle cells may be partly mediated through inhibition of protein tyrosine kinase and modulated by inhibition of lipoxygenase.