探討中鏈脂肪酸透過調控肝臟細胞自噬延緩二型糖尿病與非酒精性脂肪肝之分子機制

Protective Effects and Molecular Mechanisms of Medium-Chain Fatty Acid-Regulated Hepatic Autophagy on Type 2 Diabetes and Non-Alcoholic Fatty Liver Disease

王沐恩(Mu-En Wang)

指導教授 ： 邱智賢

國立臺灣大學/生物資源暨農學院/動物科學技術學研究所/博士(2018年)

https://doi.org/10.6342/NTU201800379

摘要

由於生活作息與飲食習慣的改變，二型糖尿病（Type 2 Diabetes）與非酒精性脂肪肝病（non-alcoholic fatty liver disease, NAFLD）近年在全球的高盛行率已成為重要的臨床醫學議題。在致病機轉上，即便過去已有許多研究指出飽和脂肪酸（saturated fatty acids）相較於不飽和脂肪酸（unsaturated fatty acids）具有較高的毒性於誘發二型糖尿病與非酒精性脂肪肝，但卻鮮少有文獻對於飽和脂肪酸之碳鏈長度在其中所扮演之角色進行探究。雖然先前已有部分研究指出，中鏈脂肪酸（碳鏈長度在6至12碳之間之飽和脂肪酸）具有延緩二型糖尿病與非酒精性脂肪肝之功效，然目前對於其分子機轉仍有許多未知。為了釐清中鏈脂肪酸（medium-chain fatty acid）在保護二型糖尿病與非酒精性脂肪肝之可能機制，我們進行了一系列小鼠活體與離體細胞培養的試驗。利用以椰子油（coconut oil）部分取代高脂飼糧中豬油（lard）的方式，我們增加了小鼠高脂飼糧（high-fat diet）的中鏈脂肪酸比例，並餵飼小鼠16週。試驗結果顯示，增加飲食中的中鏈脂肪酸比例能顯著延緩高脂飼糧引發的二型糖尿病與非酒精性脂肪肝。並且，我們發現中鏈脂肪酸能夠減緩高脂飼糧引發之肝臟脂肪堆積、胰島素阻抗（insulin resistance）、細胞自噬（autophagy）抑制、內質網壓力（ER stress）以及細胞凋亡（apoptosis）。進一步利用離體培養肝臟細胞株HepG2的方式，我們發現中鏈脂肪酸能透過調控肝臟細胞自噬的方式降低長鏈脂肪酸造成之油脂堆積、胰島素阻抗以及脂毒性（lipotoxicity）。更重要的是，我們發現中鏈脂肪酸並非透過活化調控初期細胞自噬（early-stage autophagy）的AMPK/mTOR/ULK1訊息傳遞路徑，而是以降低後期細胞自噬（late-stage autophagy）負調控者Rubicon蛋白之表現量達到回復肝臟細胞自噬流（autophagic flux）之效果。綜上所述，我們在此研究中清楚地證明除了飽和程度以外，脂肪酸之碳鏈長度也在脂肪酸造成的肝臟細胞自噬功能下降與脂毒性產生中扮演重要角色。我們認為本研究提供了對於二型糖尿病與非酒精性脂肪肝致病機轉更新穎的見解，並且也可能借此幫助開發更具效果的臨床治療方法。

關鍵字

中鏈脂肪酸 ； 細胞自噬 ； 二型糖尿病 ； 非酒精性脂肪肝 ； Rubicon

並列摘要

Due to altered life and eating styles, the high prevalence of Type 2 Diabetes (T2D) and non-alcoholic fatty liver disease (NAFLD) has become an important clinical issue worldwide. Although many previous studies proposed that saturated fatty acids (SFAs) are generally more toxic than unsaturated fatty acids (UFAs) in promoting T2D and NAFLD, few of them had addressed the significance of carbon-chain length in SFA-induced lipotoxicity. Beneficial effects of medium-chain fatty acids (MCFAs, fatty acids with a carbon chain length of 6-12 carbon atoms) on T2D and NAFLD patients have been reported previously. However, the detailed molecular mechanisms of MCFA-mediated protections are still unclear. Here in our study, we used mouse and in vitro cell culture models to investigate the possible molecular mechanisms of MCFA-protected T2D and NAFLD. To confirm the protective effects of MCFAs on T2D and NAFLD, we increased the MCFA/LCFA ratio in a mouse high-fat diet (HFD) by partially replacing the lard with coconut oil, and fed mice for 16 weeks. Our results showed that increasing dietary MCFA/LCFA ratio significantly mitigated HFD-induced T2D and NAFLD in mice. In addition, we found that the increased dietary MCFAs rescued HFD-induced hepatic insulin resistance, autophagy impairment, ER stress, and apoptosis. More importantly, using HepG2 cell line we demonstrated that MCFAs cell-autonomously protected against LCFA-induced fat accumulation, insulin resistance, and lipotoxicity in hepatic cells by reactivating autophagy. It is worth noting that MCFAs rescued hepatic autophagy by mitigating Rubicon-suppressed late-stage autophagy independently of regulating AMPK/mTOR/ULK1-controlled early-stage autophagy induction. In summary, this study highlights the importance of carbon chain length in addition to saturation in fatty acid-induced hepatic steatosis, insulin resistance, and lipotoxicity during T2D and NAFLD development. In addition, our study also demonstrated that MCFAs exert their hepatoprotective effects by rescuing Rubicon-suppressed late-stage autophagy. Our study provided novel insights into the pathogenesis of T2D and NAFLD, which may help in developing alternative clinical therapies.

並列關鍵字

medium-chain fatty acids ； autophagy ； Type 2 Diabetes ； NAFLD ； Rubicon

參考文獻

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