Eicosanoids mediate hemolymph oxidative and antioxidative response in larvae of Galleria mellonella L

Antioxidant enzymes play a major role in the defense against pro-oxidative effects of xenobiotics and pro-oxidant plant allelochemicals in insects. We posed the hypothesis that eicosanoids also mediate antioxidant enzymatic defense reactions to pro-oxidant challenge. To test this idea, we reared first-instar larvae of Galleria mellonella (L.) with the lypoxygenase inhibitor, esculetin (0.001%), the phospholipase A(2) inhibitor, dexamethasone (0.001%) and the dual inhibitor of cyclooxygenase and lipoxygenase, phenidone (0.1%) to seventh-instars. Newly ecdysed seventh-instars were then fed on artificial diet containing 0.05% xanthotoxin (XA) for 2 days. Treating seventh-instar larvae of G. mellonella with XA induced lipid peroxidation and protein carbonylation as evident from the increased content of malondialdehyde (MDA) and protein carbonyls respectively, and antioxidative enzymatic response in a dose-dependent manner. High dietary XA concentrations (0.005 and 0.1%) were associated with increasing MDA and carbonyl content (by 3-fold) and antioxidant enzyme activities, superoxide dismutase (SOD) (by 3-fold) and catalase (CAT) (by 4-fold), and glutathione-dependent enzymes, glutathione S-transferase (GST) (by 15-fold) and glutathione peroxidase (GPx) (by 7-fold). Relative to control, eicosanoid biosynthesis inhibitors (EBIs) esculetin, dexamethasone and phenidone also resulted in impaired MDA content and antioxidant enzyme activities. However, carbonyl content did not differ between control- and EBIs-feeding larvae. Finally, MDA and carbonyl content, and antioxidant enzymes SOD, GST and GPx activities exhibited an incremental increase while CAT activity was decreased in the experimental larvae that had been reared on media amended with esculetin, dexamethasone and phenidone and then challenged with our standard XA challenge dose. Two of the markers indicated that significantly higher levels of oxidative stress were produced in the hemolymph tissue of larvae fed diets supplemented with EBIs and then challenged with XA. This oxidative stress was associated with elicited antioxidative responses by increasing SOD, GST and GPx and decreasing CAT activities in hemolymph. We infer from these findings that eicosanoids mediate insect antioxidant enzymatic responses to dietary pro-oxidants.