Tiludronate inhibits interleukin-6 synthesis in osteoblasts: inhibition of phospholipase D activation in MC3T3-E1 cells.

Abstract

In previous studies, we have reported that PGF2alpha stimulates phosphoinositide hydrolysis by phospholipase C and phosphatidylcholine hydrolysis by phospholipase D through heterotrimeric GTP-binding protein in osteoblast-like MC3T3-E1 cells, and that PGF2alpha and PGE1 induce interleukin-6 (IL-6) synthesis via activation of protein kinase C and protein kinase A, respectively. In the present study, we investigated the effect of tiludronate, a bisphosphonate known to inhibit bone resorption, on the PGF2alpha- and PGE1-induced IL-6 synthesis in these cells. Tiludronate significantly suppressed the PGF2alpha-induced IL-6 secretion in a dose-dependent manner in the range between 0.1 and 30 microM. However, the IL-6 secretion induced by PGE1 or (Bu)2cAMP was hardly affected by tiludronate. The choline formation induced by PGF2alpha was reduced by tiludronate dose-dependently in the range between 0.1 and 30 microM. On the contrary, tiludronate had no effect on PGF2alpha-induced formation of inositol phosphates. Tiludronate suppressed the choline formation induced by NaF, known as an activator of heterotrimeric GTP-binding protein. However, tiludronate had little effect on the formation of choline induced by TPA, a protein kinase C activator. Tiludronate significantly inhibited the NaF-induced IL-6 secretion in human osteoblastic osteosarcoma Saos-2 cells. These results strongly suggest that tiludronate inhibits PGF2alpha-induced IL-6 synthesis via suppression of phosphatidylcholine-hydrolyzing phospholipase D activation in osteoblasts, and that the inhibitory effect is exerted at the point between heterotrimeric GTP-binding protein and phospholipase D.