Insect prostaglandins and other eicosanoids: From molecular to physiological actions

Abstract In this essay we provide a comprehensive update on the biology and molecular biology of prostaglandins (PGs) and other eicosanoids in insects. Phospholipase A2 (PLA2) is the first biochemical step in eicosanoid biosynthesis. Cellular and secretory PLA2 types, similar to those of vertebrates, have been identified and demonstrated to act in insect eicosanoid biosynthesis. However, eicosanoid biosynthetic pathways in insects are not identical to those of vertebrates. The relatively low concentrations of arachidonic acid (AA) in phospholipids of insect tissues suggest that PLA2 may hydrolyse linoleic acid as a precursor of eicosanoid biosynthesis and its subsequent desaturation and elongation leads to AA. Unlike vertebrates, AA is not oxidized by cyclooxygenase, but by a specific peroxidase called peroxinectin to produce a prostaglandin (PG) precursor, PGH2, which is then isomerized into PGE2 by PGE2 synthase. PGs and other eicosanoids mediate insect reproduction such as oocyte development and egg-laying behaviour, trehalose metabolism, fluid secretion, and immunity. They also interact with insect cytokine and other immune mediators to propagate non-self recognition signals to immune effector tissues. In the cross-talks, eicosanoids act as downstream signals because any intervention of eicosanoid signalling leads to substantial immunosuppression. Because host immunosuppression favours pathogens, some entomopathogens exploit a PLA2 inhibitory strategy to express their virulence. We propose PGs and other eicosanoids as central signalling systems in insect biology.