Department of Toxicology, University of Cagliari, Cagliari, Italy.
Hepatology. 2011 Jun;53(6):2086-96. doi: 10.1002/hep.24289. Epub 2011 May 13.
The Hippo kinase cascade, a growth-suppressive pathway that ultimately antagonizes the transcriptional coactivator Yes-associated protein (YAP), has been shown in transgenic animals to orchestrate organ size regulation. The purpose of this study was to determine whether in non-genetically modified mice (1) the Hippo pathway is involved in the regulation of adaptive liver enlargement caused by the mitogen 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), an agonist of constitutive androstane receptor and (2) a dysregulation of this pathway occurs during the development of chemically induced hepatocellular carcinoma (HCC). We show that liver enlargement caused by TCPOBOP was associated with an increase of YAP protein levels that paralleled the increase in 2-bromodeoxyuridine incorporation. Interestingly, when a second dose of TCPOBOP was given to mice with enlarged livers, no further increases in liver mass or YAP protein levels were observed, suggesting that the Hippo pathway prevents further growth of the hyperplastic liver. Viral-mediated exogenous expression of active YAP in mouse livers was able to partially overcome the block of hepatocyte proliferation. We also show that HCCs developed in mice given diethylnitrosamine and then subjected to repeated treatments with TCPOBOP had increased levels of YAP that were associated with down-regulation of microRNA 375, which is known to control YAP expression, and with enhanced levels of alpha-fetoprotein and connective tissue growth factor, two target genes of YAP.
These results suggest that the Hippo pathway regulates adaptive liver enlargement and is probably inactivated in initiated cells that escape the suppressive constrain exerted on the surrounding normal tissue, thus allowing clonal expansion to HCC.
Hippo 激酶级联反应是一种生长抑制途径，最终拮抗转录共激活因子 Yes 相关蛋白（YAP），已在转基因动物中表明可协调器官大小调节。本研究的目的是确定非遗传修饰小鼠中：（1）Hippo 途径是否参与由丝裂原 1,4-双[2-（3,5-二氯吡啶氧基）]苯（TCPOBOP）引起的适应性肝增大的调节，TCPOBOP 是组成型雄烷受体激动剂；（2）在化学诱导性肝细胞癌（HCC）的发展过程中，该途径是否失调。我们表明，TCPOBOP 引起的肝增大与 YAP 蛋白水平的增加有关，该增加与 2-溴脱氧尿苷掺入的增加平行。有趣的是，当第二次给予 TCPOBOP 剂量给肝肿大的小鼠时，肝质量或 YAP 蛋白水平没有进一步增加，表明 Hippo 途径可防止肝肥大进一步生长。在小鼠肝脏中病毒介导的外源性表达活性 YAP 能够部分克服肝细胞增殖的阻断。我们还表明，在给予二乙基亚硝胺然后反复用 TCPOBOP 处理的小鼠中发展的 HCC 具有增加的 YAP 水平，这与 microRNA 375 的下调有关，microRNA 375 已知控制 YAP 表达，并且与α-胎蛋白和结缔组织生长因子的水平增强有关，YAP 的两个靶基因。
这些结果表明 Hippo 途径调节适应性肝增大，并且可能在逃避抑制对周围正常组织施加的抑制约束的起始细胞中失活，从而允许克隆扩张到 HCC。
