山苦瓜作為雌激素相關受體α調節劑調控小鼠骨骼肌粒線體之活性

Abstract

近代國人平均壽命逐年增加，然而卻因年輕世代的生育率日漸低迷，使得人口結構愈趨失衡，高齡化社會遂成為我國的重要議題。在許多影響老年人生活品質的因素中，肌少症造成的危害最為直接且嚴重。因此，如何維持年長者的肌肉質量與功能，是科學界急需解決的課題。在眾多可能的治療方法中，雌激素相關受體α(ERRα)被視為最有希望的藥物目標之一。當該受體被活化時，可以提升骨骼肌中的粒線體增殖與活性，進而提升肌肉功能。由於山苦瓜過去被證實能夠提升小鼠骨骼肌中的粒腺體生合成指標基因表現，且其中重要的活性物質葫蘆烷型三萜類與可能是ERRα內源性配體的膽固醇有相似結構，因此本研究的假說為山苦瓜能夠藉由調節ERRα的活性以提升骨骼肌粒線體的增生與功能。 本研究設計了兩階段的實驗以驗證假說。在第一部分中，我們採用了轉錄活化實驗測試山苦瓜的乙酸乙酯萃取物是否能調節ERRα的活性。結果顯示，試驗採用的五種山苦瓜樣品皆能有效降低ERRα的轉錄活性。進一步以三種山苦瓜的活性成分檢視後，顯示共軛亞麻油酸具有最佳的調節效果。於第二部分的實驗，我們將山苦瓜凍乾粉末加入了高脂飼料中，檢視以飲食誘發肥胖的C57BL/6J小鼠骨骼肌中ERRα及其下游基因的表現。最終確認小鼠攝食山苦瓜P81品系後，比目魚肌中的粒腺體動態平衡指標mitofusin 2 mRNA表現顯著提升。 總結以上，本研究結果支持山苦瓜內含成分可以調節雌激素相關受體α活性的假說，並且在肥胖小鼠模式中能提升部分肌肉的mitofusin 2表現，顯示可能具有維持粒線體功能與健康的潛力。

The aging of our population has become one of the major concerns in Taiwan. Sarcopenia, the degenerative loss of skeletal muscle mass, quality and strength associated with aging, is one of the most important causes of functional decline and loss of independence in the elderly. Maintaining the mass and functions of skeletal muscles is hence a key issue for healthy aging. Estrogen-related receptor α, ERRα, is required for the activation of mitochondrial genes, and known to regulate genes of mitochondrial biogenesis and energy metabolism. The essential role of ERRα in skeletal muscle fitness and regeneration has been demonstrated. Previously, wild bitter gourds (WBG) were found to elevate mitochondrial biogenesis markers in skeletal muscles in mice. As some active compounds in WBG shows some structural similarity to the endogenous ligand of ERRα, we hypothesize that WBG can promote mitochondrial activity by regulating ERRα. A cell-based ERRα transactivation assay and an animal experiment were conducted to test this hypothesis. ERRα ligand binding domain was first cloned and then used to develop the cell-based transactivation assay. In this assay, XCT790, a synthetic inverse agonist, activated ERRα at low concentrations, but showed antagonism at higher ones. Genistein, a reported ERRα ligand, dose-dependently inhibited transactivation. A total of five samples, including four cultivars, 1758, P81, 55M and H4, and the product of P81 hydrolyzed at 50℃, P81_50, were tested in this assay. The ethyl acetate extracts of the WBGs also showed dose-dependently inhibitions which were to a greater extent than that of genistein. All the five samples exhibited comparable effects. Among the three active constituents of WBG, α-eleostearic acid, a conjugated linolenic acid showed the highest activity. In the mice study, groups of C57BL/6J mice were fed the high fat diet supplemented with 4% of lyophilized WBG powder for four weeks. The mRNA gene expressions of ERRα and its downstream genes in the skeletal muscles were analyzed. Among these, the mitochondrial dynamics marker, mitofusin 2, which contains ERRα response element within its promoter, was significantly up-regulated in the soleus muscle of mice fed the P81 cultivar of WBG. In conclusion, this study provided preliminary evidence that constituents of WBG might regulate ERRα activity, and promote mitofusin 2 mRNA expression in soleus muscle of high-fat diet fed mice. These results suggest that WBG might have health benefit through the modulation of ERRα.