In the present study, dietary ARA supplementation significantly increased ARA content in the colon in a dose-dependent manner, but did not affect colonic inflammation in a DSS-induced murine colitis model. These results suggest that ARA intake does not affect colitis. In the previous studies, there have been reported that ARA contents increased colitis and bowel disease [20, 21] and they suggested that ARA exacerbates colitis. On the other hands, Ramakers et al.  reported that an ARA-enriched diet increased colonic ARA content in a DSS-induced murine colitis model, but did not result in more colonic inflammation as compared with fish oil- and oleic acid (OA) -enriched diets. It is interesting that the same group reported that ARA but not EPA and OA activates NF-kappaB and elevates ICAM-1 expression in Caco-2 cells . Therefore, we carefully adjusted the experimental conditions of the colitis model, in which both suppression and exacerbation of colitis symptoms can be detected (Additional file 5: Supplemental Table S2).
Ramakers et al.  noted that intake of ARA ethyl ester improved body weight loss and diarrhea in the DSS-induced mouse colitis model. The experimental conditions of colitis induction and the severity of the colitis appear to be very similar. The main differences between the two studies were the lipids in the diets, and the chemical form and dose of ARA. With regard to lipids in diets, we adjusted the amounts of total lipids, total saturated fatty acids (S), total monounsaturated fatty acids (M), total polyunsaturated fatty acids (P), total n-6 fatty acids, and total n-3 fatty acids to similar among the groups. Therefore, the ratios of S:M:P and n-6:n-3 were automatically adjusted (approximately 1:1:1 and 2:1, respectively) in all the diets. On the other hand, the amounts of S, M, P, n-6 and n-3, and the ratios of S:M:P and n-6:n-3 were different among the groups in the previous study. This difference in diet may have resulted in the differences in the results of the two studies, as the ratios of S:M:P and n-6:n-3 affect systemic status.
The second possibility is the chemical form and dose of ARA. These differ in the two studies (present study: ARA-enriched triacylglycerol and 0.305% ARA equivalent in the diet (ARA(H) group); previous study: ARA ethyl ester and 0.78% ARA equivalent in the diet), and may have affected the increase in ARA composition in colon (present study: 12% (control group) and 18% (ARA(H) group); previous study: 15.8% (control (OA)) and 23.6% (AA group)). This might have led to the difference in the results of the two studies. However, the increases in ARA composition in the colon are sufficiently large in both studies. The present study also confirms an ARA dose-dependent increase in ARA composition in the colon in colitis. Taken together, the results of these studies suggest that dietary ARA does not exacerbate colitis in this model, and that the protective effects of dietary ARA on the colitis model are relatively small, if they exist.
This is the first study to demonstrate the relationship between ARA composition and ARA-derived lipid mediators in a colitis model. Several ARA-derived lipid mediators are known to be related to colitis. PGE2 has a protective role as a proliferative factor in the recovery of damaged colonic epithelial tissues. The reason why NSAID administration is avoided in colitis patients is that NSAIDs inhibit PGE2 production and mucosal maintenance [5–7, 23]. It is also reported that administration of PGE2 or its analogues can alleviate colitis symptoms [24–27]. On the other hand, PGE2 is known to be proinflammatory for various types of acute inflammation, and may accelerate the inflammatory process by activating macrophages or fibroblasts . Another ARA-derived lipid mediator, LXA4, is produced by 15- and 5-lipoxygenases and is clarified to have an anti-inflammatory role . In the colitis model, it was reported that LXA4 analogue administration can protect this model . Therefore, it is uncertain whether the increase in ARA content in the colon influences the contents of ARA-derived lipid mediators in the colon or colitis severity. Surprisingly, the present study revealed that the increase in ARA composition did not influence PGE2 content in the colon under the conditions used in the colitis model. On the other hand, LXA4 content in the colon increased with ARA dose. The increase in LXA4 contents was approximately 3.3 pg/mg colon weight in the ARA(H) group. Based on the observation that the severity of colonic inflammation was unchanged in the ARA(H) group, the increase in LXA4 contents observed in the present study may be not sufficient to show a marked anti-inflammatory effect in this model. Formation of higher concentrations of LXA4 could result in anti-inflammatory effects, as high doses of LXA4 analogues in drinking water (10 μg/ml 15-epi-16-parafluoro-LXA4) were able to reduce the severity of DSS-induced colitis .
It should be recognized that we quantified ARA-derived lipid mediators only at the final time point. DSS treatment continued for 7 days, whereas various physiological or pathological responses to the DSS treatment began at the first or second day. Further studies are necessary to evaluate the responses of lipid mediators at earlier stages, and to clarify the roles of lipid mediators in colitis development and recovery.
We clarified that the amounts of two ARA-derived lipid mediators, PGE2 and LXA4, show the different responses to increases in the ARA composition of colon phospholipids. This indicates that the amounts of ARA-derived lipid mediators are not determined simply by ARA composition in the colon. The reasons are unclear, but may be due to separate control of the synthetic pathways for PGE2 and LXA4. In fact, gene expression of COX-1 and COX-2, which is related to PGE2 synthesis, was unchanged by dietary ARA, whereas that of 12/15-LOX and 5-LOX, which are related to LXA4 synthesis, was increased by dietary ARA. The changes in gene expression are consistent with the changes of PGE2 and LXA4 contents. However, it is uncertain whether the changes in the expression of these genes actually contribute to the changes in PGE2 and LXA4 contents. It should also be noted that we quantified ARA-derived lipid mediators only at the final time point, similarly to the quantification of lipid mediators. Time courses of their expression are expected to be clarified in a future study.
DHA, EPA or fish oil was reported to be effective against colitis in both animal and clinical studies [29–33], while other studies failed to show any benefit [34–36]. In the present study, fish oil administration significantly increased the compositions of DHA and EPA in the colon, but did not affect inflammatory parameters other than hemoccult score at day 6 and spleen weight. The present data are consistent with the previous study; DHA composition in the colon of the DHA and CON groups was 9.8% and 6.7%, respectively, in the present study, and that of the fish oil and control groups was 9.8% and 7.1%, respectively, in the previous study . Colitis symptoms were not clearly improved in either study. The DHA increase in the colon may be insufficient for improving colitis. Also, adjustment of the ratios of S:M:P and n-6:n-3 may be another reason that fish oil showed no effect. We shifted the balance of fatty acid composition in the colon by administration of ARA or DHA, but colitis severity was unchanged. This suggests that dietary PUFAs may not be a critical factor in the DSS-induced colitis model. Concerning PGE2 and LXA4 levels, either level was not affected significantly by DHA administration, but tended to be low in the DHA group. The tendency of low PGE2 and LXA4 levels is reasonable because n-3 PUFAs compete with ARA metabolisms. Further studies were needed to clarify the effect of DHA on PGE2 and LXA4 levels in colitis models.
Leukotrienes are also known as proinflammatory mediators. Synthesis of LTB4, one of the major leukotrienes, was shown to be enhanced in the colon of the patients with inflammatory bowel disease [30, 37, 38]. In the present study, LTB4 contents in the colon were not different between the groups, and ARA or DHA administration did not affect LTB4 contents. These results suggest that fatty acid composition of the colon has little impact on LTB4 contents in the colon under the conditions. However, LTB4 contents in the DHA group appear to be low, and further studies are expected to reveal the relation of DHA administration and LTB4 contents in colitis.
Dietary ARA intakes in the ARA(L), ARA(M) and ARA(H) groups were estimated to be approximately 75, 150 and 305 mg/kg/day, respectively, on the assumption that mice consumed about 10% of their body weight daily. In humans, the average of ARA intake from daily foods is approximately 150–200 mg ARA per day . As compared to the ARA intake of humans, the ARA doses used in the present study are markedly higher. The results in the present study may thus be regarded as those under excess administration.