亞洲特異性ALDH2突變造成肥胖與胰島素阻抗性的分子機轉

Abstract

東亞地區，有過40％的東亞人群攜帶ALDH2特有的錯義點突變(Glu487Lys)，使其酵素在異合子酵素活性失去60-80%，同合子酵素活性失去約90%，此基因突變為全球盛行率最高之單一基因疾病(占~8%全球人口)。 ALDH2 (acetaldehyde dehydrogenase 2,mitochondria)是一粒線體內的酵素，代謝乙醛為乙酸。除此之外，ALDH2還會代謝多種有害醛類，包括人體的中間代謝、腸道細菌發酵、環境當中煙霧、香菸、各種塑料以及氧化壓力引發細胞脂肪過氧化而產生的有害醛類，如4-hydroxynonenal（4-HNE），而這些有害醛類會與蛋白質產生共價鍵修飾而改變其功能。 ALDH2特有的錯義性點突變(Glu487Lys)在東亞人全基因組掃描中被發現與肥胖與相關表現型有關，我們以模擬人類突變的ALDH2 *2/*2基因嵌入鼠為模型，發現ALDH2 *2/*2基因嵌入鼠在餵食高脂高糖的飲食下，相較於對照小鼠，體重明顯上升，胰島素阻抗性增加並伴隨脂肪肝與脂肪細胞肥厚。 在這項研究中，我們探討ALDH2 *2/*2基因嵌入鼠產生肥胖的原因，發現此小鼠能量消耗減少，起因推測可能與進食後產熱降低相關；另外觀察到嵌入鼠的棕色脂肪組織明顯變小。這些現象顯示此嵌入鼠可能因棕色脂肪組織功能減少、產熱降低，而產生肥胖。 我們進一步去探討分子機制，發現4-HNE會降低棕色脂肪細胞的脂肪酸氧化功能。利用LC-MS/MS，我們發現4-HNE會與脂肪酸氧化的酵素以及電子傳遞鏈上的酵素結合，綜上所述，推測ALDH2 *2/*2基因嵌入鼠會因為有害醛類累積，影響脂肪酸氧化與粒線體功能而造成小鼠肥胖。

In East Asia, approximately 40% of the East Asian population carries an inactivating missense Glu487Lys mutation of ALDH2 (SNP671) gene. The ALDH2 enzymatic activity reduces to ~60-80 % in heterozygotes and ~90 % in homozygotes of mutants. It is the most prevalent monogenetic disease in the world (~8% global population). ALDH2 (acetaldehyde dehydrogenase 2, mitochondria) is the primary enzyme responsible for metabolizing toxic acetaldehydes in mitochondria. ALDH2 also metabolizes a variety of toxic aldehydes, including acetaldehyde from intermediate metabolism, intestinal bacterial fermentation, environmental smog, cigarette smoke, various plastics and toxic aldehydes generated from lipid peroxidation by oxidative stress, such as 4-hydroxynonenal (4-HNE). These aldehydes modify proteins by forming covalently bond crosslinks, thereby altering their biological functions. Several recent large-scale meta-analysis of genome-wide association studies in East Asian identified ALDH2 Glu487Lys polymorphism is significantly associated with obesity and related metabolic phenotypes. In our study, we generated Aldh2 *2/*2 knock-in mice mimicking the human mutation as a model. We found that Aldh2 *2/*2 knock-in mice are prone to develop obesity, insulin resistance, fatty liver, and adipocyte hypertrophy on high-fat high sucrose diet as compared to controls. We found that the Aldh2 *2/*2 knock-in mice had significantly lower energy expenditure than controls. This reduction in energy expenditure may result from reduced diet-induced thermogenesis. In particular, the brown adipose tissue was markedly smaller in Aldh2 *2/*2 knock-in mice. This finding suggests the reduced thermogenesis of Aldh2 *2/*2 knock-in mice may result from impaired brown adipose tissue function. We further explored the molecular mechanism by which the knock-in mice have reduced brown adipose tissue function. We found 4-hydroxynonenal (4-HNE) inhibits fatty acid beta-oxidation in brown adipose tissue. Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we found that 4-HNE modifies enzymes involved in fatty acid beta-oxidation and the electron transfer chain. In summary, we found that Aldh2 *2/*2 knock-in mice are prone to develop obesity, insulin resistance, and fatty liver due to reduced thermogenesis, which may be related to impaired development of brown adipose tissue and toxic aldehydes-mediated suppression of fatty acid oxidation and mitochondrial function.