军事医学科学院两篇Cell Research：独特的细胞自食

　　

   人的肿瘤组织是一个动态的进化体，但肿瘤进化的细胞学机制并不清楚。来自军事医学科学院生物工程研究所的孙强博士通过与美国纽约纪念斯隆-凯特琳癌症研究中心的Michael Overholtzer博士合作研究，发现细胞自食（cell cannibalism）的一种机制——entosis能够介导肿瘤细胞间发生竞争（cell competition），清除良性细胞克隆，保留恶性细胞克隆，从而促进克隆选择和肿瘤进化。

   很长时间以来，病理学家们在多种人肿瘤组织中都发现一种很独特的细胞结构，就是一个形态上没有明显异常的活细胞存在于另外一个细胞中（Cell-in-cell structure）（图1），这种现象被称为“细胞自食”（Cell Cannibalism），即“细胞吃细胞”的现象。

   与我们熟知的吞噬（Phagocytosis）不同的是，“细胞自食”情况下，被吞入的是一个活细胞，执行吞入功能的细胞一般是肿瘤细胞。而吞噬情况下，被吞入细胞是死细胞，执行吞入功能的一般是各种巨噬细胞。“细胞自食”有别于目前人们广为熟知的细胞自噬（Autophagy），自噬过程中，细胞包裹清除的是细胞浆的物质（包括变性的细胞器，蛋白等），其特点是所有事件发生在一个细胞内部。

   根据被吞入与吞入细胞的相对身份，肿瘤组织中细胞自食大体可以分为两种：同种自食（homotypic cannibal）和异种自食（heterotypic cannibal）。异种自食是指肿瘤细胞“吞入”非肿瘤细胞，目前报道的非肿瘤细胞包括红细胞、NK细胞、T细胞、嗜中性粒细胞等造血系来源的细胞，该种现象有时也被称为Emperipolesis [4]；同种自食则是指一个肿瘤细胞吞入相邻的肿瘤细胞。这种现象在包括乳腺癌、胰腺癌、肺癌、膀胱以及肝癌等多种上皮来源的肿瘤中都有报道。虽然细胞自食现象在文献中被报道了一个多世纪[1]，但肿瘤细胞自食的生物学意义一直争论不清。

   孙强博士等人的研究发现，细胞自食的一种机制——Entosis实际上是一种肿瘤细胞间竞争（Cell competition）机制，通过该机制，具有特定促癌突变的恶性肿瘤细胞能够不断“吞入”并杀死正常细胞或良性肿瘤细胞，从而促使肿瘤组织中恶性细胞克隆得以保留和扩增（clonal selection），肿瘤不断恶性演化。有趣的是抑癌基因E-cadherin能够介导这一竞争性行为的发生，加速肿瘤细胞群体内部的淘汰过程，其结果是暂时性的减缓肿瘤组织的生长。

   而细胞内的相对Rho GTPase活性是调控这种选择性清除行为的关键因素，研究发现相对于正常细胞或者良性肿瘤细胞而言，恶性肿瘤细胞具有较低的Rho GTPase活性，因此细胞表面刚性（stiffness）较弱，容易变形并且吞入Rho GTPase活性较高的正常细胞或良性肿瘤细胞。一些致癌突变如KrasV12能够通过抑制Rho GTPase活性赋予恶性肿瘤细胞竞争中的“赢家”身份。

   这一发现不仅揭示了促进人类肿瘤恶化的一种全新细胞学机制，同时也回答了长期以来人们对于细胞自食在肿瘤中生物学功能的争论。此外，研究结果更新了人们对于癌基因以及抑癌基因功能和作用方式的传统认识，为肿瘤的研究以及临床治疗提供了新视角、新思路。

原文摘要：

1. Sun Q, Luo T, Ren Y, et al. Competition between human cells by entosis. Cell Res 2014; 24: 1299-1310.
Competition between human cells by entosis
Human carcinomas are comprised of complex mixtures of tumor cells that are known to compete indirectly for nutrients and growth factors. Whether tumor cells could also compete directly, for example by elimination of rivals, is not known. Here we show that human cells can directly compete by a mechanism of engulfment called entosis. By entosis, cells are engulfed, or cannibalized while alive, and subsequently undergo cell death. We find that the identity of engulfing (“winner”) and engulfed (“loser”) cells is dictated by mechanical deformability controlled by RhoA and actomyosin, where tumor cells with high deformability preferentially engulf and outcompete neighboring cells with low deformability in heterogeneous populations. We further find that activated Kras and Rac signaling impart winner status to cells by downregulating contractile myosin, allowing for the internalization of neighboring cells that eventually undergo cell death. Finally, we compute the energy landscape of cell-in-cell formation, demonstrating that a mechanical differential between winner and loser cells is required for entosis to proceed. These data define a mechanism of competition in mammalian cells that occurs in human tumors.

2. Sun Q, Cibas ES, Huang H, Hodgson L, and Overholtzer M. Induction of entosis by epithelial cadherin expression. Cell Res 2014; 24: 1288-1298.

Induction of entosis by epithelial cadherin expression
Cell engulfment typically targets dead or dying cells for clearance from metazoan tissues. However, recent evidence demonstrates that live cells can also be targeted and that engulfment can cause cell death. Entosis is one mechanism proposed to mediate the engulfment and killing of live tumor cells by their neighbors, an activity often referred to as cell cannibalism. Here we report that the expression of exogenous epithelial cadherin proteins (E- or P-cadherin) in human breast tumor cells lacking endogenous expression of epithelial cadherins induces entosis and inhibits transformed growth. Entosis induced by cadherin expression is associated with the polarized distribution of Rho and Rho-kinase (ROCK) activity within entotic cells, which is dependent on p190A RhoGAP activity. ROCK inhibition or downregulation of p190A RhoGAP expression reduces entosis and increases the transformed growth of epithelial cadherin-expressing tumor cells. These data define new cell systems for the study of entosis, and identify entosis as a mechanism of cell cannibalism that is induced by the establishment of epithelial adhesion and inhibits transformed growth.