Specific inhibition of Xenorhabdus hominickii, an entomopathogenic bacterium, against different types of host insect phospholipase A2

Phospholipase A2 (PLA2) hydrolyzes ester bond of phospholipids at the sn-2 position to release free fatty acid and lysophospholipids. Some PLA2s preferentially release arachidonic acid which is subsequently oxygenated into eicosanoids to mediate immune responses in insects. Xenorhabdus hominickii is an entomopathogenic bacterium that can suppress insect immunity by inhibiting PLA2 activity. However, little is known about target PLA2 types inhibited by X. hominickii. Therefore, the objective of this study was to determine PLA2 types in the host insect, Spodoptera exigua using specific inhibitors. All developmental stages of S. exigua possessed significant PLA2 activities, with late larval stages showing relatively higher PLA2 activities. In different larval tissues, hemocytes had higher PLA2 activities than fat body, gut, or epidermis. Various developmental and tissue extracts exhibited differential susceptibilities to three different PLA2 inhibitors. Late larva-to-adult stages were highly susceptible to all three different types of PLA2 inhibitors. In contrast, extracts from egg and young larval stages were not susceptible to secretory PLA2 (sPLA2) or calcium-independent cellular PLA2 (iPLA2) inhibitors, although they were susceptible to a calcium-dependent cellular PLA2 (cPLA2) inhibitor in a dose-dependent manner. Different tissues of fifth instars exhibited variation in susceptibility to inhibitors, with epidermal tissue being sensitive to cPLA2 inhibitor only while other tissues were sensitive to all three types of inhibitors. Bacterial challenge with heat-killed X. hominickii significantly increased PLA2 activity. However, live bacteria suppressed the induction of PLA2 activity. An organic extract of X. hominickii-culture broth inhibited the susceptibility of S. exigua to sPLA2- and iPLA2- specific inhibitors, but not to cPLA2-specific inhibitor. Oxindole, a component of the organic extract, exhibited an inhibitory pattern similar to the organic extract. Taken together, our results indicate that S. exigua possesses different PLA2 types and that X. hominickii can inhibit PLA2s susceptible to sPLA2- and iPLA2- specific inhibitors.