编者按：肝纤维化研究是2017年美国肝病研究学会（AASLD）年会的一大主题，多项具有突破性意义的纤维化研究在肝细胞生物学特别兴趣小组（SIG）专题中报）。其中，部分研究团队介绍了之前从未提出过的新数据，描述了一些分子的新作用，即危险信号。这些细胞内的危险相关分子或损伤相关分子模式（DAMPs）一旦释放，就会刺激免疫系统作出相应应答。美国哈佛大学医学院Yury Popov教授及其他研究团队首次在AASLD上提出：这些危险分子不仅在免疫应答中发挥作用，而且可直接促进肝纤维化形成。

 

“mito-DAMPS（或线粒体衍生危险信号）研究虽然是一个非常早期的项目，但必定会带来新的干预靶点和治疗契机，在肝纤维化和肝脏疾病研究领域具有奠基意义”，美国哈佛大学医学院Yury Popov教授在接受《国际肝病》记者采访时指出。会上，Popov教授介绍了“Mitochondria-derived DAMPs and Fibrosis”。采访中，Popov教授细述了团队的最新发现和下一步研究计划，并分享了选择mito-DAMPS作为研究突破点的主要原因，他认为此项基础研究成果可向临床诊疗转化，用于非酒精性脂肪性肝病（NAFLD）的无创诊断。

 

mito-DAMPS在肝纤维化中发挥什么作用？

 

Popov教授：我们研究小组正着手定义mito-DAMPS（或线粒体衍生危险信号）在肝纤维化形成中的作用。目前已知这些信号在机体遭遇严重创伤时可引起败血症样综合征。此次，研究小组首次揭露，mito-DAMPS来源于受损肝细胞，在经细胞凋亡和巨噬细胞的吞噬作用后，仍可长时间存在而不被清除；如果受损的肝细胞在组织中长时间存在，则会释放出这些危险分子直接作用于星状细胞，进而促使纤维化形成。这是前所未知的重要发现，翻开了肝纤维化研究的新篇章，这可归功于我的中国同事的大胆探索。

 

研究的下一步是发现哪些受体发挥关键作用，明确来自肝细胞的信号转导机制。目前已知线粒体DNA（mtDNA）是一种非常特殊的分子，与细菌DNA有很多相似之处，这可能就是其能够引起肝脏不良反应的潜在原因。mtDNA的这一特点缩小了未来研究的范围，有助于明确治疗靶点和受体，也这是我们目前正在进行的工作。

 

以mito-DAMPS为切入点探索肝纤维化仍是一个具有挑战性的特定研究项目。从定义看，mito-DAMPS是一个复杂的分子组合，特征尚不明确。但通过团队合作，我们已经缩小了研究范围。就治疗转化而言，现阶段的数据意味着肝纤维化治疗正开辟一个新世界，未来可能会带来许多创新性的治疗靶点。所以，虽然挑战巨大，但同时潜在的回报也是不可估量的。

 

该项基础研究对临床诊疗有何影响？

 

Popov教授：就治疗靶点而言，我们将关注两大方向：①干扰mito-DAMP的信号转导通路；②限制mito-DAMPs或其他DAMPs暴露，通过增强巨噬细胞的吞噬活性以清除产生DAMPs的凋亡细胞，进而减少可促进纤维化形成的DAMPs产生。

 

想实现以上这两个治疗目标尚需时日，但现阶段我们可以尝试将mtDNA与诊断联系起来。我们发现，NAFLD患者与正常患者的血清mtDNA水平存在显著差异，即血清mtDNA浓度平均升高了10倍。初步研究显示，mtDNA在区别疾病状态与非疾病状态方面具有很好的鉴别力。

 

下一步，我们将完善这项工作，看看是否可以非侵入性地评估脂肪肝患者的纤维化程度、纤维化活性和NASH进展。

 

为何选择线粒体来源的DAMPs作为肝纤维化研究切入点？

 

Popov教授：有两个原因。首先，细胞释放的危险分子来自线粒体、细胞核及其他细胞器，而线粒体是一种非常有效的信号来源。线粒体是非常特殊的细胞器，来自二十亿年前的细菌，是细胞能量生成的场所。线粒体的许多组分仅在线粒体内可见，如mtDNA。来自线粒体的危险信号基本上模仿了机体逃避细菌病原体的所有信号。所以，线粒体是一种非常有效的信号来源，这是我们对线粒体衍生危险信号感兴趣的主要原因。

 

其次，肝脏线粒体可能是危险信号的重要来源。肝脏是机体的重要代谢器官，是人体内线粒体最多的场所之一，肝细胞中体积比最大的细胞器是线粒体（约25%）。肝脏的线粒体如此丰富，使得它们很可能成为这些危险信号的重要来源。这是很久以前发起这项工作的假设理由，也是我们选择在肝脏，尤其是肝细胞中进行研究的一个非常特殊的原因。

 

Hepatology Digest: Can you outline the important points of your liver fibrosis study?

 

Dr Popov: This is a big theme at this AASLD. On Friday, there was a Special Interest Group session on liver cell biology that reported on several studies that we believe are of ground-breaking importance to the field of fibrosis. Several groups reported new data that has not been presented before that describes a new role for some molecules, the so-called danger signals. These are danger-associated molecules or DAMPs (damage-associated molecular patterns) that are intracellular molecules, and once released, alert the immune system to respond. For the first time at AASLD, we and other groups have implicated these danger molecules not only in the immune response, but also as direct drivers of fibrogenesis. That has potential fundamental implications for the field of liver fibrosis and liver disease. Although it is a very early stage project, it will definitely yield new targets and new opportunities for therapeutic intervention.

 

Our group’s study was actually the first presentation at this meeting on Friday. We set out to define the role of the mito-DAMPS or mitochondria-derived danger signals, which were already known to cause a sepsis-like syndrome in massive trauma. But for the first time, our team has shown that these mito-DAMPS are derived from hepatocytes that are injured, and these are not cleared for a long time by apoptosis and phagocytosis by macrophages. If these hepatocytes persist for a long time in the tissue, they release the danger molecules that directly act on stellate cells and stimulate fibrosis. This is something that has never been shown before and was completely unknown. This is probably a new chapter in fibrosis research as we know it. It is a very challenging project, and credit goes to my Chinese colleagues for daring to address it. The next step in the research is to discover what receptors are responsible. At this moment, we know that mitochondrial DNA is a very special molecule that bears a lot of similarities with a bacterial component, which is probably why it is so potent in stimulating the adverse hepatological response. That narrows down the area for future research, and will help to identify future therapeutic targets and the receptors involved. That is something we are currently working on.

 

So far, it has been a challenging project. This particular area of research is challenging by definition. This is a complex mix of molecules that have previously been poorly characterized. So the challenges are great, but the potential rewards are great. We have shown through our collective work that these challenges can be addressed and can be solved. Some things are clear now. I think we have narrowed down our area of research. In terms of therapeutic translation, at this stage these new data mean there is a new field that is opening up for studies in liver fibrosis. It does not mean we will immediately come up with a therapeutic target, but it definitely opens a big area of research that can potentially yield many targets in the future that are innovative and never considered before. The next challenge will be to identify the molecular mechanism that is transducing that signal emanating from the hepatocytes.

 

Hepatology Digest: What are the implications of this basic research to clinical cases? What are the future study directions?

 

Dr Popov: In terms of therapeutic targets, there are two major areas we will be focusing on. One is the molecular pathways that transduce the signal from mito-DAMPs. The other big topic would be trying to limit exposure to these mito-DAMPs or other DAMPs by enhancing the phagocytic activity of macrophages, which scavenge these dead cells that produce the DAMPs and therefore limit exposure to these fibrogenic DAMPs. These therapeutic targets are some way down the road yet. What we can envision immediately from the results of this work is associated with diagnostics. While we were working on our project, it became obvious that the mitochondrial DNA in serum, for example, is elevated up to ten-fold in patients with NASH and NAFLD. It appears from initial studies that this has substantial discriminating power between the disease and non-disease state. The next step would be to refine that work and see if we can non-invasively determine the extent of fibrosis or fibrogenic activity or presence of NASH in those patients with fatty liver and NAFLD. That is something that is literally around the corner and ongoing. I expect there are more immediate consequences to follow from our current work.

 

Hepatology Digest: What prompted you to study mito-DAMPs in liver fibrosis?

 

Dr Popov: There are two reasons. Firstly, there are many danger molecules that are released from a cell. Some of them are from mitochondria; some of them are from the nucleus; and then other components of the cell. The mitochondria though are quite distinct in their function and their origins. All the danger signals basically mimic the signals our bodies avoid from bacterial pathogens. Mitochondria are very special organelles in that they were derived from bacteria two billion years ago. They originate from bacteria that were engulfed by our ancestor cells and were adapted as a symbiont, as a powerhouse to be exploited for energy. That is the reason that mitochondria have many components that are not found anywhere in the body outside of a mitochondria. They are very similar to bacteria. Mitochondrial DNA is one of these examples. Cardiolipin is a bacteria-specific phospholipid only found in the mitochondria in the human body. And there are other components that are also typical to bacteria. So the mitochondria are a source of extremely potent signals that mimic bacteria more closely than any other cellular component. That is the main reason to be interested in mitochondria-derived danger signals. The second one is a realization that because the liver has such an important metabolic function in the body, it is one of the places that probably has the most mitochondria in our bodies. The liver is extremely rich in mitochondria. Twenty-five percent of each hepatocyte is mitochondria. Mitochondria represent the largest organelle by volume in the liver, which makes them potentially a rich source of these danger signals. That is a very special reason why they should be studied in the liver and they are significant especially for the hepatocyte. They were the hypothetical reasons that initiated this work a long time ago.