ET(A) receptor mediates the signaling of endothelin-1 in osteoblast-like cells

We previously reported that endothelin-1 (ET-1) stimulates phosphatidylcholine-hydrolyzing phospholipase D independently of phosphoinositide hgdrolysis in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the characteristics of the receptors mediating ET-1-induced intracellular signaling pathway in MC3T3-E1 cells. Cyclo-D-Trp-D-Asp-Pro-D-Val-Leu (BQ123), a selective ET(A) receptor antagonist, significantly inhibited the ET-1-induced formation of inositol phosphates in a dose-dependent manner in the range between 22 nmol/L (IC₅₀) and 2.2 μmol/L (IC₅₀ x 100). On the contrary, N-cis-2,6- dimethylpiperidinocarbonyl-L-γMeLeu-D-Trp(COOMe)-D-Nle-ONa (BQ788), a selective ET(B) receptor antagonist, had no effect on the ET-1-induced formation of inositol phosphates in the range between 1.2 nmol/L (IC₅₀) and 120 nmol/L (IC₅₀ x 100), BQ123 significantly suppressed the ET-1-induced formation of choline dose-dependently, however, BQ788 did not affect the choline formation, BQ123 also inhibited the ET-1-induced release of arachidonic acid, but BQ788 had little effect. The results strongly suggest that ET(A) receptor mediates the three intracellular signaling pathways of ET-1: (1) phosphoinositide hydrolysis by phospholipase C; (2) phosphatidylcholine hydrolysis by phospholipase D; and (3) arachidonic acid release in osteoblast-like cells.