抑制成年小鼠SLC13A3的表現降低脂肪累積及胰島素阻抗之研究

Abstract

長壽基因Indy (I’m Not Dead Yet)對於代謝調控的重要性最早是透過使用果蠅和線蟲的研究所揭露。Indy基因會轉譯出在細胞膜上負責克氏循環二羧酸和三羧酸運送的通道蛋白，若在果蠅中將這個基因突變可以降低果蠅的體脂肪含量、類胰島素蛋白以及過氧化物質的產生。除此之外，以Indy在哺乳類中被稱為Solute carrier family 13, member 5 (SLC13A5)的同源基因為目標的基因剔除小鼠，也被發現能夠抵抗高脂飲食所造成的肥胖、脂肪肝和胰島素阻抗，另外有些已發表的文獻也指出患有第二型糖尿病和非酒精性脂肪肝的肥胖病人具有較高的SLC13A5表現量。 然而，Indy的另外一個被稱為Solute carrier family 13, member 3 (SLC13A3)的哺乳類同源基因，雖然也能夠運送多種克氏循環的中間產物，但是它在哺乳類中的功能卻還沒有被完整地研究。根據目前對於SLC13A3的研究成果，我們發現透過傳統性剔除模式使小鼠喪失SLC13A3的功能可以透過增強小鼠的能量代謝和減少血液、肝臟以及白色脂肪組織中三酸甘油脂的堆積來避免小鼠肥胖，除此之外，我們也發現缺乏SLC13A3基因表現的小鼠具有較好的胰島素敏感性以及較低的肝臟中脂肪合成相關基因表現，而對於小鼠的認知功能則是沒有顯著的影響。由於SLC13A3也會在小鼠的胎盤中表現，使用傳統性剔除模式使小鼠喪失SLC13A3的功能可能會影響胚胎時期母體與胎兒之間的養分交換，進而影響胎兒的發育，因此在這項研究中，我們利用誘導性DNA重組酶系統讓小鼠發育成熟之後才使其失去SLC13A3的功能，以確認SLC13A3在成年小鼠的代謝調控中扮演的角色，我們發現在誘發全身性SLC13A3基因剔除之後，小鼠的體增重和脂肪累積顯著減少，我們也觀察到誘發全身性SLC13A3基因剔除小鼠具有較高的能量代謝、較輕的肝臟和脂肪組織以及較好的葡萄糖恆定和胰島素敏感性，而在認知功能方面則沒有顯著的影響。大部分的研究成果都和我們先前的發現一致，說明了SLC13A3是未來對科學家研究如何預防肥胖和非酒精性脂肪肝疾病的一個具有潛力的目標基因。

The importance of the life-extending gene, Indy (I’m Not Dead Yet) in metabolic regulation was first revealed through studies in Drosophila melanogaster and Caenorhabditis elegans. Indy gene encodes di and tri-carboxylate transporter of Krebs cycle intermediates on the plasma membrane, and mutating this gene in D. melanogaster reduced body fat content, insulin-like proteins and reactive oxygen species production. Besides, knockout-mouse model of the mammalian homologue of Indy, which is called Solute carrier family 13, member 5 (SLC13A5), was reported to be protected from HFD induced obesity, fatty liver and insulin resistance, and some published studies had also shown that obese humans with type 2 diabetes and non-alcoholic fatty liver disease have increased levels of SLC13A5. However, another mammalian homologue of Indy called Solute carrier family 13, member 3 (SLC13A3), which could transport a wide range of intermediates in tricarboxylate cycle, has not been fully investigated for its function. According to the results of the current research on SLC13A3, we found that the conventional knockout of SLC13A3 prevented mice from obesity through increased energy expenditure and reduced accumulation of triglyceride in serum, liver and adipose tissue. Besides, we also found better insulin sensitivity and downregulation of the lipogenesis-associated genes in the liver of SLC13A3 knockout mice, but there was no significant change in their cognitive function. Because SLC13A3 is also expressed in the placenta of mice and may affect embryonic development, in this study, we induced systemic deletion of SLC13A3 after they were fully developed with inducible Cre recombinase system to confirm the function of this gene in adult mice. We found that the body weight gain and fat mass of mice were decreased after induced systemic deletion of SLC13A3, and we also observed higher energy expenditure, lower weight of liver and white adipose tissue, as well as better glucose homeostasis and insulin sensitivity in mice after induced systemic deletion of SLC13A3. However, there was no significant difference in cognitive function. Most of the results are consistent with our previous findings and show that SLC13A3 may be a potential target for scientists to investigate how to prevent obesity and non-alcoholic fatty liver disease (NAFLD) in the future.