Ipriflavone down-regulates endothelin receptor levels during differentiation of rat calvarial osteoblast-like cells

Ipriflavone (7-isopropoxy-3-phenyl-4H-1-benzopyran-4-one) is a synthetic fl avonoid that has been shown to stimulate the activity of osteoblasts, We sh ow here that ipriflavone also promotes the deposition of calcium and the fo rmation of mineralized nodules by newborn rat calvarial osteoblast-like (RO B) cells as well as the activity of alkaline phosphatase. We reported previ ously that endothelin-1 inhibits the differentiation of ROB cells [Y, Hirum a et al. (1998) J. Cardiovasc. Pharmacol. 31, S521-S523], Therefore, we exa mined the effects of ipriflavone on the expression of endothelin receptors in ROB cells by polymerase chain reaction-Southern blot analysis and in bin ding assays with I-125-labeled endothelin-1. Ipriflavone reduced levels of endothelin ETA receptors (to 48% of the control level) in ROB cells around day 7 in our standard cultures, while it had no apparent effect on the expr ession of the mRNA for the endothelin ETA receptor. By contrast, treatment with 10(-7) M. endothelin-1 on days 6 through 9 alone suppressed mineraliza tion by ROB cells, Ipriflavone also reduced the ability of endothelin-1 to inhibit mineralization by ROB cells. These results suggest that the acceler ation of osteoblastic differentiation by ipriflavone might be due, at least in part, to a time-specific down-regulation of endothelin receptors.