Oenocytoid cell lysis to release prophenoloxidase is induced by eicosanoid via protein kinase C

Eicosanoids mediate insect cellular immune responses, which depend largely on phenoloxidase (PO) activity. In plasma, PO is activated by the proteolytic cleavage of proPO, which is stored in oenocytoids, a specific hemocyte type, of the beet armyworm, Spodoptera exigua. Eicosanoids induce an acute cell lysis of oenocytoids, which releases proPO into the plasma. We investigated an intracellular signal pathway following a functional interaction of eicosanoid(s) to a putative membrane receptor. U-73122 (a specific inhibitor of phospholipase C) inhibited oenocytoid lysis of S. exigua significantly after bacterial infection. We concluded that oenocytoid lysis required a certain level of calcium ion because EGTA (a calcium chelator) treatment inhibited cell lysis. Two protein kinase C (PKC) inhibitors (staurosporine and calphostin C) significantly inhibited the oenocytoid lysis. Oenocytoid lysis was likely induced by Na⁺ entry and subsequent osmotic shock because juvenile hormone analog, pyriproxyfen, which activates Na⁺-K⁺ ATPase and induces subsequent cell shrinkage, antagonized the effect of eicosaniod on cell lysis. Furthermore, ouabain (a specific Na⁺ pump inhibitor) significantly inhibited oenocytoid lysis. These results suggest that eicosanoid mediates oenocytoid lysis by activating the intracellular PKC pathway.