Effect of the tumor promoter phenobarbital on the pattern of global gene expression in liver of connexin32-wild-type and connexin32-deficient mice

Targeting gap junctional intercellular communication by hepatocarcinogenic compounds

Gap junctions in liver, as in other organs, play a critical role in tissue homeostasis. Inherently, these cellular constituents are major targets for systemic toxicity and diseases, including cancer. This review provides an overview of chemicals that compromise liver gap junctions, in particular biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. The focus in this review is placed upon the mechanistic scenarios that underlie these adverse effects. Further, the potential use of gap junctional activity as an in vitro biomarker to identify non-genotoxic hepatocarcinogenic chemicals is discussed.

Connexin-based signaling and drug-induced hepatotoxicity

Being critical mediators of liver homeostasis, connexins and their channels are frequently involved in liver toxicity. In the current paper, specific attention is paid to actions of hepatotoxic drugs on these communicative structures. In a first part, an overview is provided on the structural, regulatory and functional properties of connexin-based channels in the liver. In the second part, documented effects of acetaminophen, hypolipidemic drugs, phenobarbital and methapyriline on connexin signaling are discussed. Furthermore, the relevance of this subject for the fields of clinical and in vitro toxicology is demonstrated.

Relevance for patients: The role of connexin signaling in drug-induced hepatotoxicity may be of high clinical relevance, as it offers perspectives for the therapeutic treatment of such insults by interfering with connexin channel opening.

Dynamic metabolic change is indicative of inflammation-induced transformation of hepatic cells

The observation that prolonged inflammation plays a causative role in cancer development has been well documented. However, an incremental process that leads from healthy to malignant phenotypes has not yet been described. Experimentally induced hepatocellular carcinoma is considered one of the representative laboratory models for studying this process. Hepatic exposure to viral infection or toxic reagents leads to chronic inflammation and gradual transformation into hepatocellular carcinoma. Here we present metabolomic profiles of hepatic cells at different stages during inflammation-induced cellular transformation by N-nitrosodiethylamine. Using gas chromatography-mass spectrometry, we quantitatively assessed the changes in cellular metabolites during the transformation process in hepatitis and liver cirrhosis. Further pathway analysis of the differentially expressed metabolites showed that carbohydrate metabolism and lipid metabolism were greatly altered in hepatitis and liver cirrhosis, respectively. Additionally, the enhanced inflammation in cirrhosis was associated with a shift from carbohydrate metabolism to lipid and amino acid metabolism. Among the differentially expressed metabolites found in diseased mouse livers, d-glucose and d-mannitol showed the most significant changes, highlighting them as potential early-diagnostic biomarkers of hepatocellular carcinoma development. Taken together, these investigations into the dynamic metabolic changes that occur during the precancerous stages of hepatocellular carcinoma add to and refine understanding of how chronic inflammation ultimately leads to cancer. Furthermore, the findings set the stage for identifying metabolites that may serve as early-diagnostic indicators of these unfolding events.

Preparation of archival formalin-fixed paraffin-embedded mouse liver samples for use with the Agilent gene expression microarray platform

INTRODUCTION

Tissue samples are routinely formalin-fixed and paraffin-embedded (FFPE) for long term preservation. Gene expression analysis of archival FFPE tissues may advance knowledge of the molecular perturbations contributing to disease. However, formalin causes extensive degradation of RNA.

METHODS

We compared RNA quality/yield from FFPE samples using six commercial FFPE RNA extraction kits. In addition we compared four DNA microarray protocols for the Agilent 8×60K platform using 16year old FFPE mouse liver samples treated with phenobarbital or vehicle.

RESULTS

Despite low quality RNA, archival phenobarbital samples exhibited strong induction of the positive control genes Cyp2b9 and Cyp2b10 by quantitative real-time PCR (qPCR). We tested one- and two-color microarray designs and evaluated the effects of increasing the amount of hybridized cDNA. Canonical gene responders to phenobarbital were measurably induced under each experimental condition. Increasing the amount of labeled cDNA did not improve the overall signal intensity. One-color experiments yielded larger fold changes than two-color and the number of differentially expressed genes varied between protocols. Gene expression changes were validated by qPCR and literature searches. Individual protocols exhibited high rates of false positives; however, pathway analysis revealed that nine of the top ten canonical pathways were consistent across experiments. Genes that were differentially expressed in more than one experiment were more likely to be validated. Thus, we recommend that experiments on FFPE samples be done in duplicate to reduce false positives.

DISCUSSION

In this analysis of archival FFPE samples we were able to identify pathways that are consistent with phenobarbital's mechanism of action. Therefore, we conclude that FFPE samples can be used for meaningful microarray gene expression analyses.

Gap junctional intercellular communication as a target for liver toxicity and carcinogenicity

Direct communication between hepatocytes, mediated by gap junctions, constitutes a major regulatory platform in the control of liver homeostasis, ranging from hepatocellular proliferation to hepatocyte cell death. Inherent to this pivotal task, gap junction functionality is frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity and carcinogenicity. In the present paper, the deleterious effects of a number of chemical and biological toxic compounds on hepatic gap junctions are discussed, including environmental pollutants, biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. Particular attention is paid to the molecular mechanisms that underlie the abrogation of gap junction functionality. Since hepatic gap junctions are specifically targeted by tumor promoters and epigenetic carcinogens, both in vivo and in vitro, inhibition of gap junction functionality is considered as a suitable indicator for the detection of nongenotoxic hepatocarcinogenicity.

Phenobarbital elicits unique, early changes in the expression of hepatic genes that affect critical pathways in tumor-prone B6C3F1 mice

At 2 and 4 weeks following treatment with phenobarbital (PB), the classical nongenotoxic rodent liver carcinogen, we elucidated unique gene expression changes (both induction and repression) in liver tumor-susceptible B6C3F1 mice, as compared with the relatively resistant C57BL/6. Based on their cancer-related roles, we believe that altered expression of at least some of these genes might underlie PB-induced liver tumorigenesis. Putative constitutive active/androstane (CAR) response elements (CAREs), a subset of PB response elements, were present within multiple genes whose expression was uniquely altered in the B6C3F1 mice, suggesting a role for CAR in their regulation. Additionally, three DNA methyltransferase genes (Dnmt1, Dnmt3a, and Dnmt3b) were repressed uniquely in the tumor-prone B6C3F1 mice, and all possess putative CAREs, providing a potential direct link between PB and expression of key genes that regulate DNA methylation status. Previously, we demonstrated that PB-elicited unique regions of altered methylation (RAMs) in B6C3F1 mice, as compared with the relatively resistant C57BL/6, at 2 and 4 weeks, and annotation of the regions harboring these changes revealed 51 genes. This is extended by the current study, which employed RNA isolated from the same liver tissue used in the earlier investigations. Genes elucidated from both the methylation and expression analyses are involved in identical processes/pathways (e.g., cell cycle, apoptosis, angiogenesis, epithelial-mesenchymal cell transition, invasion/metastasis, and mitogen-activated protein kinase, transforming growth factor-beta, and Wnt signaling). Therefore, these changes might represent very early events that directly contribute to PB-induced tumorigenesis. It is instructive to consider the possibility that, in a hypothesis-driven fashion, these genes are initial candidates that could be utilized to develop a biomarker "fingerprint" of early exposure to PB and PB-like compounds.

Differential expression of genes associated with cell proliferation and apoptosis induced by okadaic acid during the transformation process of BALB/c 3T3 cells

Okadaic acid (OA) is a tumor promoter in two-stage carcinogenesis experiments. Nevertheless, the effects of OA on cell transformation, cell proliferation and apoptosis vary widely, and the molecular events underlying these effects of OA are not well understood. In the present study, we examined the promoting activity and the associated effects on cell growth and apoptosis mediated by OA in BALB/c 3T3 cells, and evaluated alterations of gene transcriptional expression by microarray analysis. The promoting activity of OA was estimated by a two-stage transformation assay, in which cells were treated first with a low dose of the initiator N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and then with OA for 14 days. It showed that OA, at concentrations of 7.8-31.3 ng/ml, enhanced the transformation of MNNG-treated cells. In the promotion phase, cells exposed to OA (7.8 ng/ml) grew slowly for the first 2 days and subsequently died. As determined by Hoechst 33342 fluorescent dye and Annexin-V/PI dual-colored flow cytometry, OA induced morphologically apoptotic cells and increased the percentage of early apoptotic cells. The gene expression profile induced by OA at five time points in the promotion phase was determined by use of a specific mouse toxicological microarray containing 1796 clones, and a total of 177 differentially expressed genes were identified. By gene ontology analysis, 31 of these were determined to be functionally involved with cell growth and/or maintenance. In this group, numerous genes associated with the cell proliferation and cell cycle progression were down-regulated at early and/or middle time points. Among these was a subset of genes associated with apoptosis, in which Bnip3, Cycs, Casp3 and Bag1 genes are involved in the mitochondrial pathway of apoptosis. Ier3, Mdm2 and Bnip3 genes may be p53 targets. Furthermore, real-time PCR confirmed the expression changes of five genes selected at random from the differentially expressed genes. We conclude that OA induces cell growth inhibition and apoptosis in the two-stage, MNNG-initiated transformation of BALB/c 3T3 cells. The results of gene expression profile analysis imply that multiple molecular pathways are involved in OA-induced proliferation inhibition and apoptosis. Mitochondrial and p53-associated apoptotic pathways also may contribute to OA-induced apoptosis.

Zonal gene expression in mouse liver resembles expression patterns of Ha-ras and beta-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 beta-catenin- and 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 beta-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 Ras- and beta-catenin-dependent signaling pathways.

Ablation of gap junctional communication in hepatocytes of transgenic mice does not lead to disrupted cellular homeostasis or increased spontaneous tumourigenesis

Gap junctions between murine hepatocytes are composed of two subunit proteins, connexin26 (Cx26) and connexin32 (Cx32). Previously, we found increased formation of chemically induced liver tumours but no increase in spontaneous development of preneoplastic hepatic foci in mice that lacked Cx32 and expressed decreased amounts of Cx26. In order to clarify this tumour-suppressive effect and to overcome embryonic lethality of constitutive Cx26-deficient mice, cell type-specific targeting of the Cx26 gene was performed. Mice with loxP-flanked Cx26 coding DNA were crossed with mice expressing the Cre recombinase exclusively in hepatocytes. Progeny mice lacking Cx26 in the liver were viable and fertile with no obvious signs of phenotypic alterations. To generate mice that totally lack gap junctional intercellular coupling, these mice were crossed with constitutive Cx32-deficient mice. We found no increase in spontaneously induced liver tumour formation in Cx26 and double deficient Cx26/Cx32 mice. Occasionally, double deficient livers exhibited morphological alterations, like amyloidosis, and a slightly increased basal proliferation rate of hepatocytes. Although the absence of gap junction channels led to altered expression of adhesion-related proteins like E-cadherin and actin, microarray analyses of total liver transcripts yielded only few differences between Cx26-deficient and double deficient livers compared to control samples. Our results suggest that total lack of gap junctional communication due to hepatocytic ablation of Cx26 and Cx32 does not drastically alter basal hepatocytic function and does not lead to increased spontaneous liver tumour formation.

Genotype-phenotype relationships in hepatocellular tumors from mice and man

Experimentally induced liver tumors in mice harbor activating mutations in either Catnb (beta-catenin) or Ha-ras, according to the carcinogenic treatment. We have now investigated by microarray analysis the gene expression profiles in tumors of the two genotypes. In total, 364 genes or expressed sequences with aberrant expression relative to normal liver were identified, but only 30 of these demonstrated unidirectional changes in both tumor types. Several functional clusters were identified that involve changes in amino acid utilization and ammonia disposition in Catnb-mutated tumors as opposed to alterations in lipid and cholesterol metabolism in Ha-ras-mutated tumors. Moreover, several genes coding for inhibitory molecules within the Wnt-signaling pathway were upregulated in Catnb-mutated tumors, suggesting induction of a negative feedback loop, whereas Ha-ras-mutated tumors showed alterations in the expression of several genes functional in monomeric G-protein signaling. We conclude that mouse hepatoma cells adopt different evolutionary strategies that allow for their selective outgrowth under variable environmental conditions. Human hepatocellular cancers (HCC) lack RAS mutations but are frequently mutated in CTNNB1, the human Catnb ortholog. The set of genes aberrantly expressed in Catnb-mutated mouse tumors was used to screen, by expression profiling, for dysregulation of orthologous genes within a panel of 25 HCCs, of which 10 were CTNNB1-mutated. HCCs with activated beta-catenin displayed a gene expression profile that was similar to Catnb-mutated mouse tumors but distinct from the other human HCCs. In conclusion, expression fingerprints may be used for diagnostic purposes and potential new therapeutic intervention strategies. Supplementary material for this article can be found on the HEPATOLOGY website (http://www.interscience.wiley.com/jpages/0270-9139/suppmat/index/html).