2022 Volume 4 Issue 2 Pages 37-39
Although intake of ω-3 fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are well known to be beneficial for some cardiovascular and inflammatory diseases, their action mechanism remains elusive. Recently, we revealed that a EPA-metabolite 5,6-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid (5,6-DiHETE) represents strong anti-inflammatory reaction in animal body. We originally found that 5,6-DiHETE is produced rich in mouse inflamed colon tissue in the healing phase of colitis. Administration of 5,6-DiHETE could suppress histamine-induced vascular hyperpermeability by inhibiting intracellular Ca2+ increase in vascular endothelial cells. Following studies revealed that these bioactivities of 5,6-DiHETE is derived from antagonistic reaction against transient receptor potential vanilloid 4 (TRPV4) channels. In vivo, intraperitoneal or oral administration of 5,6-DiHETE promoted healing of mouse colitis. Interestingly, this novel anti-inflammatory lipid metabolite is contained rich in blue-back fish intestines. In summary, 5,6-DiHETE is an EPA-derived anti-inflammatory lipid mediator which promote healing of inflammation. It could be a new therapeutic tool as a fish-derived supplement.
We found that 5,6-DiHETE is a novel anti-inflammatory lipid mediator produced in inflamed colon tissue. This is an EPA-metabolite which attenuates vascular hyper-permeability upon inflammation by inhibiting TRPV4 channel, and its administration represents therapeutic reactions against colitis of mice. Since 5,6-DiHETE is contained rich in fish intestine, it can be a new therapeutic tool against various inflammatory diseases.
Lipid mediators is a class of bioactive lipids produced locally in response to extracellular stimuli. For example, prostaglandin (PG) and leukotriene (LT) are representative lipid mediators which induce inflammation [1]. It is widely known that they are involved in the onset and/or exacerbation of various inflammatory diseases. Especially, ω-6 fatty acids including arachidonic acids are metabolized into pro-inflammatory lipid mediators including PGE2 and LTB4. On the other hand, there have been several studies showing that intake of ω-3 fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), abundantly contained in fish oil, provide beneficial effects to our body such as lowing blood pressure and inhibition of inflammation. There have been reports suggesting its mechanism. For example, intake EPA and DHA increase plasma membrane fluidity [2], and a DHA derivative resolvin D1 suppresses neutrophil infiltration and promotes phagocytosis of macrophage [3]. However, the mechanism of anti-inflammatory effects of ω-3 polyunsaturated fatty acids (PUFAs) and their derivatives remains largely unclear. We have attempted to reveal how ω-3 PUFAs-derivatives modulate inflammatory reactions and found a novel anti-inflammatory EPA-derivative named 5,6-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid (5,6-DiHETE). This review introduces the anti-inflammatory activities and therapeutic potential of this newly discovered lipid mediator, 5,6-DiHETE (The information is summarized in Fig. 1).
Graphical illustration of the anti-inflammatory bioactivities of 5,6-DiHETE.
We generated dextran sodium sulfate (DSS)-induced colitis model of mice and comprehensively analyzed the lipid mediator productions in the colon tissue using liquid chromatograph-mass spectrometry (LC-MS/MS). Oral administration of 2% DSS in drinking water in mice for 4 days occurred body weight loss, diarrhea, and severe intestinal inflammation by day 7, and gradually recovered by day 18. In lipid analysis using LC-MS/MS, the productions of many lipid mediators especially ω-6 fatty acids derivatives were increased according to the exacerbation of colitis and then decreased during recovery phase. These results are consistent with the fact that ω-6 fatty acids derivatives including PGs and LTs represent pro-inflammatory reactions. In contrast to these lipid mediators, we found some lipid mediators which represent unique production profiles. Among them, the tissue concentration of 5,6-DiHETE, a cytochrome P450 (CYP)-metabolite of EPA, in colon was dramatically increased in the recovery phase of colitis [4].
We next attempted to reveal the bioactivity of 5,6-DiHETE on the function of blood vessel, an important player in inflammation. Dilation and increased permeability of vessels induce fluid leakage and leukocytes migration into peripheral tissues, which result in edema and aggregation of inflammatory cells. In a mouse ear, administration of 400 µg histamine induced vascular hyperpermeability and dilation. Pretreatment of 0.1 µg 5,6-DiHETE significantly inhibited the histamine-induced inflammation. In vitro experiment using isolated mouse aorta, 1 µM 5,6-DiHETE attenuated acetylcholine (1 µM)-induced vascular dilation. In human umbilical vein endothelial cells (HUVECs), 0.1 µM 5,6-DiHETE suppressed 10 µM histamine-induced barrier disruption, calcium influx, and the production of nitric oxide, a vasorelaxant factor [5]. These were the first results to elucidate the anti-inflammatory bioactivity of 5,6-DiHETE.
We then investigated the mechanism of anti-inflammatory activity of 5,6-DiHETE. Transient receptor potential vanilloid 4 (TRPV4) is a non-selective cation channel and a multi-stimulatory receptor activated by osmotic pressure, mechanical stimuli, and temperatures around 27–35°C. Since TRPV4 is reported to (i) be expressed in vascular endothelial cells and involved in vascular hyperpermeability, (ii) contribute to the aggravation of DSS colitis [6,7,8], we hypothesized that TRPV4 could be a point of action of 5,6-DiHETE. Indeed, our following study showed that 1 µM of 5,6-DiHETE abolished TRPV4 agonist (GSK1016790A, 50 nM)-induced disruption of endothelial cell-barrier formation in HUVECs. And using TRPV4-overexpressing cells, pretreatment of 1 µM of 5,6-DiHETE significantly suppressed calcium influx induced by 10 nM of GSK1016790A [9]. These suggest 5,6-DiHETE inhibits TRPV4-mediated signaling in cells.
We next assessed therapeutic effects of 5,6-DiHETE on colitis. As shown above, 4 days administration of 2% DSS solution induced severe colitis in mice. Intraperitoneal administration of 5,6-DiHETE (50 µg/kg/day, during days 9–13) to DSS colitis mice promoted recovery from weight loss and diarrhea symptoms. Neutrophil infiltration and edema in the colon tissue were also ameliorated [9]. Furthermore, oral administration (150 or 600 µg/kg/day, during days 9–13) suppressed diarrhea and colon inflammation of DSS-induced colitis [10].
To sum up, the lipid mediator 5,6-DiHETE increases in the colon during the recovery phase of colonic inflammation, inhibits the vascular hyperpermeability at least via TRPV4, and bring therapeutic effects on the DSS colitis mice.
Considering clinical application, we sought for foodstuff containing rich 5,6-DiHETE. Because ω-3 fatty acids such as EPA and DHA are abundantly contained in fish oil, we hypothesized 5,6-DiHETE is also rich contained in fish. We measured the concentration of 5,6-DiHETE in muscle, bone, heart, liver, and intestine of 3 species of blue back fish. As a result, intestine contained the highest levels of 5,6-DiHETE compared with other organs. And especially intestine of sardine most contained 5,6-DiHETE in much higher level than EPA and DHA [11]. This suggests that daily consumption of such fish ingredients could exert therapeutic effect for inflammatory diseases. However, it is still unknown which types of CYP produces 5,6-DiHETE from EPA. We would like to investigate how 5,6-DiHETE is produced and the way to apply 5,6-DiHETE to supplements or drugs for the future.
The authors declare no conflict of interest related to this review.
This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (20H05678, to Takahisa Murata), and by the Kobayashi Foundation, Terumo Life Science Foundation, and Shimadzu Science Foundation (to Takahisa Murata).
