Changes in sphingolipid levels induced by epidermal growth factor in osteoblastic cells. Effects of these metabolites on cytosolic calcium levels

Sphingolipids mediate a number of cellular functions in a variety of cell systems. The role they play in osteoblast signaling is yet unknown. This study investigated the effects of epidermal growth factor (EGF) on the levels of ceramide, sphingosine (SPH), and sphingosine-1-phosphate (S1P) in rat calvariae osteoblastic cells, and whether these metabolites mediated cytosolic calcium ([Ca²⁺](i)) mobilization in these cells. EGF significantly (P<0.05) increased the levels of all three sphingolipids, and the phorbol ester PMA partially inhibited these effects. SPH and S1P markedly increased [Ca²⁺](i) levels, with thapsigargin (depletes [Ca²⁺](i) pools) decreasing the response by 60%. Verapamil (calcium channel blocker) only inhibited ceramide's effects on [Ca²⁺](i). Furthermore, SPH enhanced the EGF' induced increase in [Ca²⁺](i). This study demonstrates that ceramide, SPH and S1P mediate [Ca²⁺](i) mobilization in rat calvarial osteoblastic cells, and that EGF induces changes in the levels of these metabolites with PKC playing an important role in the mechanisms regulating these events. (C) 2000 Harcourt Publishers Ltd.