Effects of omega-3 fatty acids docosahexaenoic acid and eicosapentaenoic acid and their metabolites in acute inflammation

High levels of omega-3 polyunsaturated fatty acids (n-3 PUFAs) have been associated with a reduced risk of coronary heart disease. It has been found that these compounds unfold potent anti-inflammatory and pro-resolving effects as precursor molecules of a newly discovered set of lipid mediators, called resolvins and protectins/neuroprotectins. Numerous studies have reported strong attenuation of inflammation and promotion of inflammation resolution through these n-3 mediators. In order to establish an in vitro model of these anti-inflammatory and pro-resolving properties of n-3 mediators, we decided to analyze the effect of n-3 PUFAs and resolvin precursor metabolites 17-hydroxydocosahexaenoic acid (17-HDHA) and 18- hydroxyeicosapentaenoic acid (18-HEPE) in the RAW 264.7 murine macrophage cell line. We found that incubation with the n-3 metabolites 17-HDHA and 18-HEPE led to a decrease of lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNF-α) secretion, supporting the inflammation dampening effect of these compounds. Moreover, we detected enhanced phagocytic activity of these macrophages when exposed to the n-3 PUFAs docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) as well as 17-HDHA or 18-HEPE, arguing for a pro-resolution role of these compounds. Next we carried out an animal model using transgenic fat-1 mice with endogenously high levels of n-3 PUFAs. 24 hours after induction of peritonitis with intra-peritoneal injection of zymosan A, exudates were collected by means of peritoneal lavage. Subsequently, cellular composition of exudates was analyzed in Wright-Giemsa smears, demonstrating no significant differences between fat-1 and wild type (WT) mice. This was reflected in similar systemic TNF-α levels for both groups. While these findings argue against a protective effect of n-3 PUFA in this context, several factors might have disguised a potential protective effect in this setting, such as (1) wrong timeframe of analysis, (2) dietary differences between the two groups of mice, or (3) wrong analysis parameters (e.g. no assessment of phagocytic activity of macrophages in both groups). In contrast to the unequivocal findings with regard to the cell numbers of the peritoneal exudates, we found significantly increased levels of protective n-3 PUFA metabolites 17-HDHA and 18-HEPE in livers of fat-1 mice with zymosan peritonitis as compared to the WT group, arguing for an increased formation of protective n-3 PUFA lipid mediators in fat-1 mice.