The effect of acteoside on intracellular Ca(2+) mobilization and phospholipase C activity in RBL-2H3 cells stimulated by melittin.

This study was performed to investigate the effects of acteoside on various cellular functions such as, intracellular Ca(2+) mobilization, phospholipase C activity, and exocytosis induced by melittin. Melittin (0.1-1 µM) dose-dependently increased intracellular Ca(2+) mobilization in the presence of extracellular Ca(2+), but was not affected by 1 µM U73122, a specific PLC inhibitor. In the absence of extracellular Ca(2+), melittin (1 µM) did not induce a change in intracellular Ca(2+) mobilization, which suggests that melittin-induced intracellular Ca(2+) mobilization may be dependent on the influx of extracellular Ca(2+) rather than on the release of intracellular Ca(2+) storage. Acteoside (10 µM) significantly inhibited 1 µM melittin-induced Ca(2+) mobilization by 33 %. In [(3)H]inositol-labeled cells, 1 µM melittin did not increase inositol phosphate formation, but more than 5 µM melittin significantly increased inositol phosphate formation, which was significantly inhibited by acteoside. Melittin (1 µM) significantly increased histamine release from RBL 2H3 cells in the presence or absence of extracellular Ca(2+). Acteoside significantly inhibited 1-µM-melittin-induced histamine release by 74 % in the presence of extracellular Ca(2+) and by 71 % in the absence of extracellular Ca(2+). These data suggest that the inhibitory effect of acteoside on 1 µM-melittin-induced histamine release may be related to blockage of the calcium-independent pathway. Taken together, these data suggest that melittin has an influence on cellular functions such as intracellular Ca(2+) mobilization, the PLC pathway, and exocytosis via various independent signalling pathways in RBL-2H3 cells, and was significantly inhibited by acteoside.

Competitive inhibition of cytosolic Ca2+-dependent phospholipase A2 by acteoside in RBL-2H3 cells.

The aim of this study was to investigate whether acteoside isolated from Clerodendron trichotomum Thunberg may act as a selective inhibitor of phospholipase A(2) in RBL-2H3 cells. Acteoside dose-dependently inhibited 0.5 µM melittin-induced release of [(3)H]arachidonic acid, which was due to the inhibition of cytosolic Ca(2+)-dependent phospholipase A(2) (cPLA(2)) rather than secretory PLA(2) (sPLA(2)). In Dixon plots, the apparent K ( i ) value of acteoside on cPLA(2) was 5.9 µM and the inhibitory pattern appeared to be a competitive inhibitor. The above data, suggests that acteoside acts as a competitive inhibitor of cPLA(2) in RBL-2H3 cells.