1,25-DIHYDROXYVITAMIN-D-3 INDUCES ARACHIDONATE MOBILIZATION IN EMBRYONIC CHICK MYOBLASTS

1,25-Dihydroxyvitamin D-3 (1,25(OH)2D3) which activates the phospholipase C (PLC)-protein kinase C (PKC) signalling pathway, induces within 1 min a dose-dependent (10(-11)-10(-7) M) increase in the release of [H-3]arachidonic acid ([H-3]AA) from prelabeled embryonic chick myoblasts. The response is dependent on extracellular calcium, since it is suppressed by EGTA and nifedipine, a Ca2+-channel blocker, and is mimicked by the calcium ionophore A23187. 1,25(OH)2 D3-induced release of [H-3]AA is not affected by neomycin (0.5 mM), an inhibitor of phosphoinositide hydrolysis. 12-o-tetradecanoylphorbol-13-acetate (TPA), a PKC activator, induces an extracellular Ca2+-independent release of [H-3]AA and amplifies the release of AA stimulated by 1,25(OH)2D3. 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (H7), a PKC inhibitor, markedly suppressed TPA as well as 1,25(OH)2D3-induced [H-3]AA release. Down-regulation of cellular PKC abolishes the effect of the phorbol ester, and partially inhibits 1,25(OH)2D3-induced [H-3]AA release. Temporally correlated with AA liberation, the hormone increases the formation of lysophosphatidylcholine (lysoPC) and lysophosphatidylethanolamine (lysoPE) and decreases the cellular content of PC and PE. These results indicate that part of AA release by 1,25(OH)2D3 derives from PLA2 activation and that the effects of the hormone are mediated by PKC in a mode independent of phosphoinositide hydrolysis by PLC.