Xenorhabdus nematophilus inhibits p-bromophenacyl bromide (BPB)-Sensitive PLA(2) of Spodoptera exigua

Xenorhabdus nematophilus is a Gram-negative symbiotic bacterium of the entomopathogenic nematode, Steinernema carpocapsae. The bacteria delivered into the insect hemocoel by the nematodes cause immunodepression of the target insects to protect host nematodes and themselves from the cellular immune reaction. Previous reports suggest that the immunodepression is caused by inhibition of the eicosanoid pathway that is known to be critically important to mediate cellular immunity. This study focused on the inhibitory effect of X. nematophilus on PLA(2) activity of Spodoptera exigua. The PLA(2) activity was functionally associated with the activation cascade of pro phenol oxidase (pPO). Dexamethasone (DEX), a specific PLA(2) inhibitor, inhibited pPO activation completely at the higher doses of approximate to2.4 muM in vitro condition. The inhibitory effect of DEX was reversed by the addition of arachidonic acid, the catalytic product of PLA(2). By means of this in vitro PLA(2) inhibitor assay system, two different PLA(2) inhibitors were used to compare their inhibitory effects on the hemolymph PLA(2) of S. exigua. p-Bromophenacyl bromide (BPB), a specific inhibitor of secretary PLA(2) (sPLA(2)), significantly inhibited pPO activation, but methylarochidonyl fluorophosphates (MAFP), a specific inhibitor of cytosolic PLA(2) (cPLA(2)), did not show any inhibitory effect. BPB also inhibited pPO activation of the plasma, though much higher PO activation and its inhibition by BPB was found in the hemocytes. Growth medium of X. nematophilus at the stationary phase had a PLA(2) inhibitory effect. Via the in vitro PLA(2) inhibitor assay, it was shown that the ethyl ether extract of the medium contained significant PLA(2) inhibitor activity. These results indicate that X. nematophilus produces and secretes PLA(2) inhibitor, which acts on BPB-susceptible PLA(2) of S. exigua. Arch. Insect Biochem. Physiol. 54:134-142, 2003. (C) 2003 Wiley-Liss, Inc.