Short Communication Zonal Gene Expression in Mouse Liver Resembles Expression Patterns of Ha-ras and -Catenin Mutated Hepatomas

Hepatocytes of the periportal and perivenous zones of the liver lobule differ in their levels and activities of various enzymes and other proteins. We have recently suggested that -cateninand Ras-dependent signaling pathways play an important role in the regulation of perivenous and periportal gene expression profiles. This hypothesis was primarily based on similarities in zonal differences in gene expression of hepatocytes from normal liver with gene expression patterns of liver tumors: several proteins and mRNAs preferentially expressed in periportal hepatocytes were often overexpressed in Ha-ras mutated mouse liver tumors, whereas perivenous markers were overexpressed in Ctnnb1 (encoding -catenin) mutated tumors. We have now extended this work by use of data from two previously conducted microarray analyses aimed to analyze 1) global gene expression patterns of Ha-ras and Ctnnb1 mutated mouse liver tumors and 2) transcriptome differences between periportal and perivenous mouse hepatocytes. By comparison of the datasets, 134 genes or expressed sequences were identified that were present in both datasets. Gene expression patterns in perivenous hepatocytes and Ctnnb1 mutated hepatoma cells were strongly correlated: 96.5% of the genes present in both datasets were regulated in the same direction. In analogy, expression of 74.1% of the genes deregulated in Ha-ras mutated tumors was correlated with the respective expression patterns in periportal hepatocytes. These findings favor the hypothesis that gene expression patterns in periportal and perivenous hepatocytes are regulated, at least in part, by Rasand -catenin-dependent signaling pathways. Based on the location of the blood vessels, the terminal branches of the portal and the hepatic (central) veins, and the direction of the blood flow, the individual liver lobule can be subdivided into an upstream “periportal” and a downstream “perivenous” (pericentral) region. Hepatocytes located in either of the two regions have different, often complementary functions, as indicated by differences in the content and activity of several metabolic key enzymes (for review see Jungermann and Katz, 1989; Gebhardt, 1992). This so-called “zonation” of metabolism in the liver is also of particular impact on the metabolism of pharmaceuticals and xenobiotics, since most of the enzymes involved in the metabolism of such compounds show distinct differences in basal as well as inducible expression between perivenous and periportal hepatocytes, with the main detoxification enzymes of both phase I and phase II of xenobiotic metabolism being located preferentially in perivenous hepatocytes (for reviews on zonation of xenobiotic metabolism see Jungermann and Katz, 1989; Oinonen and Lindros, 1998). As a consequence, various exogenous compounds lead to a preferential damage of hepatocytes located in either the perivenous or periportal area of the liver lobule (for review see Lindros, 1997). We have recently developed a hypothesis to explain the mechanisms that govern zonal differences in gene expression in liver (Hailfinger et al., 2006). According to our hypothesis, two opposing signals, each gradual in nature, dictate the gene expression patterns of hepatocytes within the different areas of the liver lobule. We postulated that a -catenin-activating signal, which is probably delivered by endothelial cells of the central veins, and a second opposing signal, presumably generated by blood-borne molecules activating a Rasdependent signaling pathway, trigger perivenousand periportal-specific mRNA expression profiles. This concept was deduced from the observation of striking similarities in the expression of selected “marker” genes in Ctnnb1 (encoding -catenin) and Ha-ras mutated hepatoma cells with their corresponding expression patterns in perivenous and periportal hepatocytes, respectively. We have now extended our previous study and compared the transcriptome of periportal and perivenous hepatocyte subpopulations with that of Ha-ras and Ctnnb1 mutated mouse liver tumors. Materials and Methods Datasets on global mRNA expression patterns in mouse liver tumors mutated in either Ha-ras or Ctnnb1 and datasets on global mRNA expression patterns in perivenous and periportal mouse hepatocytes were available to us from previous experiments conducted in our laboratory (Stahl et al., 2005a; Braeuning et al., 2006). In these studies, gene expression profiles had been analyzed by use of the Affymetrix GeneChip MOE-430A (Affymetrix, Santa Clara, CA) containing approximately 22,600 probe sets including more than 14,000 well characterized mouse genes. Since the criteria for statistical analysis of expression data varied in the two studies, we reevaluated the tumor datasets using as discriminators a log2 expression ratio 1 (equivalent to 2-fold change) and a threshold of 0.1 for the false discovery rate-adjusted p values to allow a direct comparison with our data on zonal differences in gene expression in normal liver (for further details see Stahl et al., 2005a,b; Braeuning et al., 2006). The former cutoff was chosen because expression differences smaller than 2-fold are very difficult to detect by quantitative reverse transcription-polymerase chain reaction, which was used in the previous studies for verification of the microarray results. The latter was chosen to keep the expected proportion of false positives below 10%. This study was supported by the Deutsche Krebshilfe (Grant 106356). Article, publication date, and citation information can be found at http://dmd.aspetjournals. org. doi:10.1124/dmd.106. 013656. □S The online version of this article (available at http://dmd.aspetjournals.org) contains supplemental material. 0090-9556/07/3504-503–507$20.00 DRUG METABOLISM AND DISPOSITION Vol. 35, No. 4 Copyright © 2007 by The American Society for Pharmacology and Experimental Therapeutics 13656/3186302 DMD 35:503–507, 2007 Printed in U.S.A. 503 http://dmd.aspetjournals. org/content/suppl/2007/01/12/dmd.106. 013656.DC1 Supplemental material to this article can be found at: at A PE T Jornals on July 9, 2018 dm d.aspurnals. org D ow nladed from