Prostaglandin catabolism inSpodoptera exigua, a lepidopteran insect

ABSTRACT Several prostaglandins (PGs) and PG-synthesizing enzymes have been identified from insects. PGs can mediate cellular and humoral immune responses. However, uncontrolled and prolonged immune responses might have adverse effects on survival. PG catabolism in insects has not been reported. Here, using a transcriptomic analysis, we predicted two PG-degrading enzymes, PG dehydrogenase ( SePGDH ) and PG reductase ( SePGR ), in Spodoptera exigua , a lepidopteran insect. SePGDH and SePGR expression levels were upregulated after immune challenge. However, their expression peaks occurred after those of PG biosynthesis genes such as PGE 2 synthase or PGD 2 synthase. Indeed, SePGDH and SePGR expression levels were upregulated after injection with PGE 2 or PGD 2 . In contrast, such upregulated expression was not detected after injection with leukotriene B 4 , an eicosanoid inflammatory mediator. RNA interference (RNAi) using double-stranded RNAs specific to SePGDH or SePGR suppressed their expression levels. The RNAi treatment resulted in an excessive and fatal melanization of larvae even after a non-pathogenic bacterial infection. Phenoloxidase (PO) activity mediating the melanization in larval plasma was induced by bacterial challenge or PGE 2 injection. Although the induced PO activity decreased after 8 h in control, larvae treated with dsRNAs specific to PG-degrading enzyme genes kept the high PO activities for a longer period compared to control larvae. These results suggest that SePGDH and SePGR are responsible for PG degradation at a late phase of immune responses.