Braeuning Albert, Menzel Moritz, Kleinschnitz Eva-Maria, Harada Naomoto, Tamai Yoshitaka, Köhle Christoph, Buchmann Albrecht, Schwarz Michael
Institute of Pharmacology und Toxicology, Department of Toxicology, University of Tuebingen, Germany.
FEBS J. 2007 Sep;274(18):4766-77. doi: 10.1111/j.1742-4658.2007.06002.x. Epub 2007 Aug 14.
Hepatocytes of the periportal and perivenous zones of the liver lobule show marked differences in the contents and activities of many enzymes and other proteins. Previous studies from our and other groups have pointed towards an important role of beta-catenin-dependent signaling in the regulation of expression of genes encoding proteins with preferential perivenous localization, whereas, in contrast, signaling through Ras-dependent pathway(s) may induce a 'periportal' phenotype. We have now conducted a series of experiments to further investigate this hypothesis. In transgenic mice with scattered expression of an activated Ha-ras (Ha-ras(G12V)) mutant in liver, expression of the perivenous markers glutamine synthetase and two cytochrome P450 isoforms was completely abolished in those hepatocytes demonstrating constitutively activated extracellular signal-regulated kinase activity, even though they were located directly adjacent to central veins. Similarly, incubation of primary hepatocytes or hepatoma cells with increasing amounts of serum caused a concentration-dependent attenuation of expression of perivenous marker mRNAs, whereas the expression of periportal markers was increased. The inhibitory effect of high amounts of serum on the expression of perivenous markers was also observed if their expression was stimulated by activation of beta-catenin signaling, and comparable inhibitory effects were seen in cells stably transfected with a T-cell factor/lymphoid-enhancing factor-driven luciferase reporter. Epidermal growth factor could partly mimic serum effects in hepatoma cells, and its effect could be blocked by an inhibitor of extracellular signal-regulated kinase activity. These data suggest that activation of the Ras/mitogen-activated protein kinase (extracellular signal-regulated kinase) pathway favors periportal gene expression while simultaneously antagonizing a perivenous phenotype of hepatocytes.
肝小叶汇管区和中央静脉周围区的肝细胞在许多酶及其他蛋白质的含量和活性方面存在显著差异。我们团队以及其他团队之前的研究表明，β-连环蛋白依赖性信号传导在调控优先定位于中央静脉周围的蛋白质编码基因的表达中发挥重要作用，而与之相反，通过Ras依赖性途径的信号传导可能诱导出“汇管区”表型。我们现在进行了一系列实验以进一步探究这一假说。在肝脏中散在表达活化型Ha-ras（Ha-ras(G12V)）突变体的转基因小鼠中，即使那些肝细胞紧邻中央静脉，但在那些显示出持续活化的细胞外信号调节激酶活性的肝细胞中，中央静脉周围标志物谷氨酰胺合成酶和两种细胞色素P450同工型的表达完全被消除。同样，用越来越多的血清孵育原代肝细胞或肝癌细胞会导致中央静脉周围标志物mRNA表达呈浓度依赖性减弱，而汇管区标志物的表达则增加。如果通过激活β-连环蛋白信号来刺激中央静脉周围标志物的表达，也会观察到高浓度血清对其表达的抑制作用，并且在稳定转染了T细胞因子/淋巴细胞增强因子驱动的荧光素酶报告基因的细胞中也能看到类似的抑制作用。表皮生长因子可部分模拟血清对肝癌细胞的作用，并且其作用可被细胞外信号调节激酶活性抑制剂阻断。这些数据表明，Ras/丝裂原活化蛋白激酶（细胞外信号调节激酶）途径的激活有利于汇管区基因表达，同时拮抗肝细胞的中央静脉周围表型。
