Biosynthetic pathway of arachidonic acid in Spodoptera exigua in response to bacterial challenge

Eicosanoids play crucial roles in mediating insect immune responses. In vertebrates, phospholipase A₂ (PLA₂) releases arachidonic acid (AA) from phospholipids (PLs) for biosynthesis of various eicosanoids. However, little AA is found in PLs of lepidopteran insects. Spodoptera exigua, a lepidopteran insect, is known to use eicosanoids to mediate immunity. Although AA was not detected in PLs of hemocytes and fat body (two immune tissues) of naïve larvae, it was detected at small but significant level after bacterial infection, suggesting induction of AA biosynthesis for immunity. Based on a mammalian AA biosynthetic pathway, this study hypothesizes that AA is synthesized from C18 polyunsaturated fatty acid (PUFA) precursor by subsequent desaturation and elongation reactions because PLs of S. exigua larvae are rich in linoleic acid. After inhibiting PLA₂ activity to prevent release of free fatty acids, different PUFA precursors were injected to S. exigua larvae followed by assessment of eicosanoid-mediated cellular immune response. ω-6 PUFAs were effective in inducing immune response whereas α-linolenic acid (an ω-3 PUFA) was not. Several fatty acyl desaturases (SeDESs) have been predicted from S. exigua transcriptomes. Specific inhibitors against Δ5 or Δ6 DESs inhibited eicosanoid-mediated immune responses. Furthermore, RNA interference (RNAi) specific to Δ5 or Δ6 DES genes significantly suppressed eicosanoid-mediated immune responses. Four very long chain fatty acid elongase genes (SeEloV-A ∼ SeEloV-D) were predicted. Among respective RNAi treatments of these genes, only one RNAi treatment specific to type 5 elongase (SeEloV-B) suppressed eicosanoid-mediated immune response. These results suggest that S. exigua larvae can synthesize AA from linoleic acid via Δ5- and Δ6-desaturations by SeDESs along with chain elongation by SeEloV-B. Finally, this study showed significant fitness cost of uncontrolled AA biosynthesis. AA injection alone without bacterial challenge significantly induced both cellular and humoral immune responses. This unnecessary energy expense due to free AA resulted in reduced pupal size and decreased adult egg production. The detrimental effect of free AA explains physiological significance of little AA content in lepidopteran insects except for life-or-death situation such as pathogen infection.