Plausibility of Daphnia magna as an alternative experimental model to evaluate effects on eicosanoid synthesis

Eicosanoids play important roles in inflammation, allergy, fever, and immune responses. In the eicosanoid pathway, cyclooxygenase (COX) catalyzes the conversion of arachidonic acid to prostaglandins and is a crucial target of nonsteroidal anti-inflammatory drugs (NSAIDs). Thus, toxicological studies on the eicosanoid pathway are important for drug discovery and the evaluation of adverse health outcomes due to environmental contaminants. However, experimental models are limited owing to concerns regarding ethical standards. Thus, new alternative models for evaluating toxic effects on the eicosanoid pathway must be developed. To this end, we adopted an invertebrate species, Daphnia magna, as an alternative model. D. magna was exposed to ibuprofen, a major NSAID, for 6 and 24 h. Transcription of eicosanoid-related genes (pla2, cox, pgd synthase, pgd2r2, ltb4dh, and lox) was analyzed by qPCR, eicosanoids (arachidonic acid, prostaglandin F2, dihydroxy prostaglandin F2, and 5-hydroxyeicosatetraenoate) were quantified by multiple reaction monitoring, and enzyme-linked immunosorbent assay was used to determine protein levels of arachidonic acid and prostaglandin E2 (PGE2). After 6 h of exposure, transcription of the pla2 and cox genes was downregulated. In addition, the whole-body level of arachidonic acid, an upstream of COX pathway, increased by over 1.5-fold. The levels of PGE2, a downstream of COX pathway, decreased after 24 h of exposure. According to our results, it is expected that the eicosanoid pathway might be conserved in D. magna, at least partially. This indicates the plausibility of D. magna as an alternative model for the screening of new drugs or chemical toxicity.