ACAT2

▎药明康德内容团队编辑  随着大众健康意识的提升，人们对摄入食物的成分会更加谨慎。当一些成分与疾病风险联系上之后，大家可能会对其谈之色变，胆固醇就是其一。实际上，胆固醇属于类脂的一种，参与着许多细胞生命活动。胆固醇在细胞膜上可以与周围的脂质相互作用，调节着细胞膜的流动性和通透性。同时，胆固醇也可以通过各种途径进行转化，参与其他代谢途径。因此，正常的胆固醇摄入对人体功能是必需的。图片来源：123RF之所以胆固醇有着“坏名声”，还是与当下高发的高血脂症有关，其中胆固醇代谢异常会导致高胆固醇血症。据世界卫生组织官网数据，全球每年有三分之一的缺血性心脏病都与高胆固醇有关。我们除了饮食控制，也亟需一项控制胆固醇的新策略。在近期《科学》杂志上，加州大学洛杉矶分校的研究者提出了从源头控制胆固醇的思路，在肠道中阻止胆固醇进入血液。正常情况下，食物进入肠道后，膳食胆固醇会被肠道表面一排像小刷子的肠细胞摄取。在肠细胞中，胆固醇会被加工成乳糜颗粒状，然后被释放到血液，随着循环进入到各个组织。如果能阻断胆固醇进入和加工的过程，就从根本上解决了胆固醇累积的问题。具体有哪些环节可以供研究者下手呢？根据论文介绍，游离的胆固醇在通过肠道细胞质膜时，需要借由NPC1L1蛋白的帮助。而一旦胆固醇进入细胞内部，它就会被转移到内质网上进行后续加工，这一过程中，一种名为ACAT2的酶会参与胆固醇的酯化。上述这些都是已知的，但有一条关键途径却仍属于空白，当胆固醇进入细胞后，它是怎样跑到内质网去的？▲研究示意图（图片来源：参考资料[2]）他们将目光放在了Aster蛋白家族上，原因是Aster蛋白是结合胆固醇的好手，并且还能够帮助胆固醇在膜与膜之间移动。实验中，作者会给小鼠服用一些胆固醇，并检测它们肠道细胞中的Aster蛋白是否产生变化。显微镜下，他们看到当胆固醇被NPC1L1介导进细胞内部后，Aster-B蛋白迅速聚集，并移动到了肠细胞的边界上。作者进一步构建了一批基因缺陷小鼠，敲除了Aster-B和Aaster-C蛋白的编码基因，尽管这时仍存在NPC1L1，但小鼠对胆固醇的摄取吸收率已经大大降低了。▲Aster蛋白调节着膳食胆固醇的吸收（图片来源：参考资料[2]）在观察中，作者发现缺乏Aster蛋白，胆固醇很难移动到内质网，并且内质网后续加工的乳糜颗粒中也基本不含胆固醇。这也提供了一种可以阻止胆固醇吸收的新靶点。现有的一种药物ezetimibe可以靶向NPC1L1，因此可以降低血液中的胆固醇水平，而在作者的测试实验中，他们还发现了一类名为Al-3d的小分子，它们可以直接与Aster-B与Aster-C蛋白结合，可以进一步抑制胆固醇的吸收。研究者指出，这些新发现和新分子可以让我们更高效地阻断膳食胆固醇吸收，成为一种预防和治疗高胆固醇血症的新方法。参考资料：[1] Researchers identify previously unknown step in cholesterol absorption in the gut. Retrieved November 10, 2023 from https://medicalxpress.com/news/2023-11-previously-unknown-cholesterol-absorption-gut.html[2] Alessandra Ferrari et al, Aster-dependent nonvesicular transport facilitates dietary cholesterol uptake, Science (2023). DOI: 10.1126/science.adf0966.▼细胞和基因疗法科学讲座点击图片立即观看本文来自药明康德内容微信团队，欢迎转发到朋友圈，谢绝转载到其他平台。如有开设白名单需求，请在“学术经纬”公众号主页回复“转载”获取转载须知。其他合作需求，请联系wuxi_media@wuxiapptec.com。免责声明：药明康德内容团队专注介绍全球生物医药健康研究进展。本文仅作信息交流之目的，文中观点不代表药明康德立场，亦不代表药明康德支持或反对文中观点。本文也不是治疗方案推荐。如需获得治疗方案指导，请前往正规医院就诊。更多推荐点个“在看”再走吧~

*仅供医学专业人士阅读参考肠道是人体吸收消化营养成分的最大功臣，不过有时候，奇点糕也是希望它不要这么勤劳，少吸收点也没啥真的，比如说食物里的油啊，和油啊，和油啊……什么的。肠道吸收食物中的胆固醇，对血液胆固醇水平有重要影响。比如用来治疗高胆固醇血症的依折麦布就是以肠道为靶点，抑制胆固醇的吸收。近期，加州大学洛杉矶分校科研团队在《科学》杂志发表论文，研究者们找到了小肠细胞内胆固醇转运的重要一环，Aster蛋白。Aster蛋白对于小肠细胞中从质膜运输到内质网的这条“胆固醇快线”至关重要，使用小分子药物AI-3d抑制Aster蛋白，可以减少小鼠膳食胆固醇的吸收，实现随意吃油但不高血脂的美妙设想。论文题图首先，我们来大致了解一下胆固醇进入人体后的旅程。小肠是膳食胆固醇吸收的“守门人”。小肠上皮细胞中的NPC1L1是胆固醇的关键转运蛋白，负责把胆固醇从肠腔搬运到肠上皮细胞的质膜（PM）中。随后，胆固醇从质膜移动到内质网（ER），在内质网被ACAT2酯化为胆固醇酯（CE），随后才能被包装为乳糜微粒，再被释放到循环中，去往其他组织和细胞。NPC1L1就是降胆固醇药依折麦布（ezetimibe，EZ）的主要作用靶点——不过这个药最开始被认为主要是ACAT2的抑制剂——依折麦布治疗可以让小鼠免于饮食诱导的高胆固醇血症和动脉粥样硬化。小肠上皮细胞吸收消化胆固醇的机制在整个吸收消化的过程中，胆固醇是如何从质膜移动到内质网的，其实我们还不是很了解。研究者们想到的是一组蛋白，Aster。Aster蛋白有A/B/C三种，可结合胆固醇并促进其在膜间运动。此前有研究发现Aster-B在肾上腺皮质中、以及Aster-C在肝脏中对胆固醇运输和合成的作用。转录组分析显示，小鼠小肠中Aster-B/C表达较为丰富，Aster-A表达较少。为了验证猜想，研究者使用CRISPR-Cas9在Aster-B基因（Gramd1b）中插入了标签，以便观察在摄入胆固醇前后，小鼠小肠中Aster蛋白的动向。免疫组化分析显示，Aster-B在小肠绒毛中富集、隐窝中表达较少。给予胆固醇之后，可以观察到Aster-B被招募到质膜中。给予胆固醇后，小肠上皮细胞刷状边缘可见Aster-B富集研究者敲除了小鼠中的Aster-B/C。虽然敲除小鼠小肠组织学表现很正常，但是在给小鼠灌胃胆固醇2小时后，可以很明显地观察到小肠近端吸收胆固醇明显减少了，小鼠血浆和肝脏的胆固醇水平都大幅度降低了。进一步分析发现，小鼠小肠细胞中胆固醇酯生成减少，同时观察到SREBP2转录激活，这意味着重新合成胆固醇通路的激活。显然，Aster蛋白在胆固醇吸收中有重要的作用。敲除Aster-B/C后胆固醇吸收显著减少研究者给Aster-B/C敲除小鼠喂食高胆固醇饮食（HC）21天后，相较对照组，小鼠体重略有下降，空腹血浆胆固醇水平和肝脏胆固醇水平降低；仅敲除肠道Aster蛋白，小鼠同样有较低的血浆胆固醇水平，但体重减轻不显著。大口吃胆固醇，但不会因此高血脂，简直是太梦幻了。有趣的是，研究们分析了Aster蛋白的结构，发现Aster-B/C居然还能结合依折麦布。在1.6Å下可见，Aster-C形成了一个空腔结构，刚好可以装下依折麦布。Aster-B/C可结合依折麦布不过，实际进行了实验后，研究者发现，依折麦布主要还是作用于NPC1L1而非Aster蛋白。NPC1L1将胆固醇从肠腔转运到小肠上皮细胞质膜中，质膜胆固醇增加招募内质网驻留的Aster蛋白，形成质膜和内质网的临时接触点，这就构成了一条胆固醇转运的快车道。随后Aster蛋白促进胆固醇向内质网的非囊泡运动，并在ACAT2作用下酯化为胆固醇酯。当NPC1L1被依折麦布抑制，质膜中的胆固醇减少，Aster蛋白向质膜的移动自然也就不会发生了。再对照这张图捋一捋机制可以说，NPC1L1和Aster蛋白在胆固醇转运中作用是连续的，但并不重复，这支持Aster蛋白成为一个新的可靶向位点。所以我们有药用吗？研究者们之前发现了一种小分子AI-3d，能够有效抑制Aster-A/B/C。在小鼠中进行实验发现，使用AI-3d预处理能够有效减少对膳食胆固醇的吸收，效果和依折麦布差不太多。写到这里给奇点糕也整不会了，没啥别的好说的，让肠道不吸油的药，快点再来一个吧！参考资料：[1]Alessandra Ferrari et al. ,Aster-dependent nonvesicular transport facilitates dietary cholesterol uptake.Science382,eadf0966(2023).DOI:10.1126/science.adf0966本文作者丨代丝雨

UCLA researchers have described a previously unknown step in the complex process by which dietary cholesterol is processed in the intestines before being released into the bloodstream -- potentially revealing a new pathway to target in cholesterol treatment. UCLA researchers have described a previously unknown step in the complex process by which dietary cholesterol is processed in the intestines before being released into the bloodstream -- potentially revealing a new pathway to target in cholesterol treatment. Although an existing drug and statins impact part of the process, an experimental drug being studied in UCLA research labs appears to specifically target the newfound pathway, possibly adding a new approach to the cholesterol management toolbox. "Our results show that certain proteins in the Aster family play a critical role in moving cholesterol through the absorption and uptake process," said Dr. Peter Tontonoz, a UCLA professor and researcher in Pathology and Laboratory Medicine and Biological Chemistry, senior author of an article in Science. "The Aster pathway appears to be a potentially attractive target for limiting intestinal cholesterol absorption and reducing levels of plasma cholesterol." Cholesterol from food is absorbed by cells that line the inner surface of the intestines -- enterocytes -- where it is processed into droplets that eventually reach the bloodstream. But this journey involves a multistep process. Free cholesterol is drawn into the cell's plasma membrane -- the external boundary of the cell -- by a protein called NPC1L1. It then must move to another membranous network in the cell called the endoplasmic reticulum. This is where an enzyme called ACAT2 prepares the cholesterol for packaging and transport, in a process called esterification. Scientists have not known how cholesterol finds its way into the endoplasmic reticulum to be acted upon by ACAT2. In this study, the researchers investigated the Aster protein family, which is known to bind cholesterol and help it move from one membrane to another. "How cholesterol that enters the cell through NPC1L1 reaches the endoplasmic reticulum for esterification and regulation of cholesterol synthesis has been a longstanding mystery," Tontonoz said. "We solve that mystery by showing that two members of the Aster protein family -- Aster-B and -C -- provide the link between NPC1L1 and ACAT2. By attaching to the plasma membrane, these proteins facilitate cholesterol transport to the endoplasmic reticulum." It was previously understood that NPC1L1 was a key player, but this study breaks the process into distinct steps. When NPC1L1 pulls cholesterol from the intestine into the enterocyte, it triggers the recruitment of the Aster proteins. "We show that NPC1L1 and Asters play sequential, nonredundant roles in the delivery of dietary cholesterol from the intestinal lumen to the enterocyte ER (endoplasmic reticulum)," the authors write. Additionally, the researchers found that the Aster proteins may provide a new treatment target for the control of cholesterol levels. When the actions of Aster-B and -C are blocked, cellular cholesterol stores are reduced, and the processing of cholesterol is impaired. An existing drug, ezetimibe, targets NPC1L1, inhibits the activity of ACAT2, and reduces cholesterol absorption. It sometimes is used in combination with statins to reduce blood cholesterol. But ezetimibe doesn't bind sufficiently to Asters to prevent all cholesterol transport. The research team, however, has identified an experimental drug -- a small molecule called AI-3d -- that potently inhibits Aster-A, -B, and -C. In a series of studies in both mice and human cells, the researchers found that the drug inhibits cholesterol absorption by directly targeting the effects of the Asters.