2001 年 51 巻 4 号 p. 501-509
Genistein, a soybean-derived isoflavone with an inhibitory effect on protein tyrosine kinases (PTKs), has been shown to suppress osteoclastic bone resorption. To clarify the mechanisms underlying this action, we investigated the effects of genistein on inward rectifier K+ current (IKir) in rat osteoclasts by using the whole-cell patch-clamp technique. Extracellularly applied genistein inhibited IKir in a concentration-dependent manner. Physiologically attainable concentrations of genistein inhibited IKir. IC50 values obtained 5 and 10 min after the application of genistein were 54 and 27 μM, respectively. The removal of genistein partially restored the current. Daidzein, an isoflavone without PTK-inhibiting activity, also showed a weak inhibitory effect on IKir, but genistin had no effect. Other PTK inhibitors, tyrphostin A25, tyrphostin B42, and tyrphostin B46, inhibited IKir, whereas herbimycin A and lavendustin A were without effect. The inactive tyrphostin, A1, showed a similar inhibitory effect as tyrphostin A25. The tyrosine phosphatase inhibitor, orthovanadate, did not affect the inhibitory potency of genistein on IKir. The inhibitory action of genistein was unaffected by changing intracellular Ca2+ concentration ([Ca2+]i) or by pretreatment of the cell with GDPβS, Rp-cAMPS, okadaic acid, or staurosporine. Therefore the inhibition of IKir by genistein does not depend on PTK inhibition, involvement of changes in [Ca2+]i, or secondary interaction with protein kinase A or protein kinase C. Genistein-induced inhibition of IKir would cause membrane depolarization, elevation of [Ca2+]i, and inhibition of osteoclastic bone resorption.
