不飽和脂肪酸降低鴨脂肪酸延長酶與去飽和酶之基因表現

Abstract

二十二碳六烯酸 (docosahexaenoic acid, DHA) 為人體必需的 n-3 不飽和脂肪酸，其在人體生理機能上扮演極重要的角色，特別對冠狀動脈疾病的預防、膽固醇和三酸甘油酯的防禦等均有正面作用。不飽和脂肪酸必須由食物攝取或經由其前驅物代謝合成。α-亞麻酸 (α-linolenic acid, ALA) 是n-3 不飽和脂肪酸中的一員，哺乳動物無法自行合成，其可經由一系列的碳鏈延長 (elongation) 與去飽和作用 (desaturation)，將 ALA 轉換成較長碳鏈的二十碳五烯酸 (Eicosapentaenoic acid, EPA) 和DHA。在肝臟中，長鏈脂肪酸延長酶 (elongase, Elovl) 與長鏈脂肪酸脫飽和酶 (delta-desaturase) 在代謝轉換中扮演重要角色，主要參與此轉換的合成酶包含了Elovl2、Elovl5、delta-desaturase5 (D5D) 與delta-desaturase6 (D6D)。然而，此轉換系統目前在家禽尚未釐清，故本試驗的目的為探討不飽和脂肪酸對鴨的Elovl2、Elovl5、D5D 與D6D 等基因表現的調控。 體外試驗部分利用肝癌細胞 (LMH) 與肝臟初代培養系統處理不同脂肪酸與化過氧化小體促進劑 (PPAR-α agonist, WY14643)。細胞試驗分爲對照組、WY14643、DHA、ALA 與棕櫚酸 (palmitic acid, PA) 四個處理組別。實驗結果顯示 PA 不具調控上述基因的能力，而 DHA 與 ALA 處理組則可降低 Elovl5、D5D 與D6D 的基因表現。此外，WY14643 能顯著提高 Elovl5、D5D 與 D6D 的基因表現，而 Elovl2 皆不受上述任何處理組的調控。這些結果證實高劑量的不飽和脂肪酸具有抑制 Elovl5、D5D 與D6D 的基因表現，進而產生負回饋作用。 活體試驗部分利用北京鴨與番鴨作爲動物模式，餵飼控制組飼糧與試驗飼糧三週。試驗飼糧分別含有1.5% 與 3.0% 的亞麻仁油 (linseed oil, LO)。結果顯示經三週餵飼後各組間的體重無顯著差異。添加3.0% 的LO能提高北京鴨肝臟上述基因，但對番鴨則無相似的影響。 根據上述結果顯示，體外試驗證實不飽和脂肪酸對於家禽上述基因的調控與哺乳類相似，在北京鴨活體試驗中也證明這些基因會受LO的調控。不過調控的幅度並不大，如果可以增加其影響幅度，則具有經濟效益。

N-3 polyunsaturated fatty acids (PUFA) can only be obtained directly from the diet or converted from their precursor α-linolenic acid (ALA, 18:3n-3) through a series of chain-elongation and desaturation processes, involving Δ6-desaturase (D6D), Δ5-desaturase (D5D), elongase-2 (Elovl2), elongase-5 (Elovl5) and acyl-CoA oxidase in the liver. While many studies have examined the regulation of these enzymes in mammals, very little is known about their gene expressions in avian species, especially in ducks. Therefore, the purpose of this study was to link fatty acid supplementation to the gene expression of elongase and desaturase in duck liver and primary hepatocytes. The chicken hepatocellular carcinoma cell line (LMH) and primary liver cells from 7- to 9-day-old unsexed Pekin ducks were treated with PPAR-α agonist WY14643 or PUFAs, including docosahexaenoic acid (DHA) and ALA, and saturated fatty acid palmatic acid (PA). We found that in both systems the hepatic Elovl5, D5D and D6D mRNAs, but not Elovl2, were significantly induced by WY14643, suggesting that PPAR-α induces long-chain PUFA synthesis. By contrast, DHA and ALA treatments significantly reduced the expression of genes tested, except Elovl2, which suggests a negative feedback mechanism existing for the biosynthesis of PUFA. No significant change was found with the PA treatments. We also performed in vivo studies on 7-day-old unsexed ducklings of Pekin and Muscovy, fed with diet supplemented with linseed oil (LO, 1.5% and 3.0%) for 3 weeks. Our results showed that Elovl2, Elovl5 and D5D expressions (as analyzed by RT-qPCR) in liver were slightly, but not significantly, increased by the treatments in Pekin. In addition, Elovl2 was increased by LO in Muscovy. By contrast, Elovl5, D5D and D6D were reduced by 1.5% LO, yet increased by 3.0%, in Muscovy. In conclusion, we demonstrated here that n-3 PUFA down-regulates the expression of Elovl5, D5D and D6D in duck liver cells, despite not quite consistent for the in vivo experiments. The underlying mechanism remains to be elucidated in the future.