Arachidonic acid for loading induced prostacyclin and prostaglandin E(2) release from osteoblasts and osteocytes is derived from the activities of different forms of phospholipase A(2).

Abstract

Mechanical loading of bone stimulates resident bone cells to produce prostacyclin (PGI(2)) and prostaglandin (PG)E(2) by a mechanism that can be differentially regulated by ion channel blockers. We have investigated differences in the loading-related PG production mechanisms in rat ulnae explants loaded ex vivo. Loading and aluminium fluoride (AlF(3), a nonselective activator of G-proteins) both increased PGI(2) and PGE(2) release into culture medium. Pertussis toxin (PTX) blocked loading-related release of PGE(2), but not PGI(2), while isotetrandrine, an inhibitor of G-protein-mediated activation of phospholipase (PL)A(2), abolished the loading-related release of both PGs. This suggests both PTX-sensitive and -insensitive G-protein-dependent, PLA(2)-mediated mechanisms for loading-related PG production. Blockade of secretory (s)PLA(2) activity prevented loading-related release of PGE(2) and PGI(2), whereas inhibition of cytosolic (c)PLA(2) activity blocked loading-related release of PGE(2) alone. cPLA(2) was localized immuno-cytochemically to osteoblasts, but not to osteocytes. sPLA(2) was localized to osteocytes and osteoblasts. Exogenous type-IA sPLA(2) and type-IB sPLA(2) stimulated significant increases in PGE(2) and PGI(2) release. PTX reduced the release of both PGs stimulated by type IA PLA(2), but not type IB. Furthermore, inhibition of protein kinase C (PKC) activity blocked loading-related release of PGE(2), but not that of PGI(2). These data suggest that loading-related release of PGI(2) and PGE(2) utilizes arachidonic acid derived from the activity of different PLA(2)s. In osteocytes and osteoblasts, arachidonic acid for PGI(2) synthesis is liberated by PTX-insensitive G-protein-dependent sPLA(2) alone. In osteoblasts, arachidonic acid for PGE(2) synthesis is released by PTX-sensitive, G-protein-dependent, cPLA(2)-mediated activity, which also requires upstream sPLA(2) and PKC activities.