Regulation of the expression of some genes for enzymes of glutathione metabolism in hepatotoxicity and hepatocarcinogenesis

Kernel-based hierarchical structural component models for pathway analysis

MOTIVATION

Pathway analyses have led to more insight into the underlying biological functions related to the phenotype of interest in various types of omics data. Pathway-based statistical approaches have been actively developed, but most of them do not consider correlations among pathways. Because it is well known that there are quite a few biomarkers that overlap between pathways, these approaches may provide misleading results. In addition, most pathway-based approaches tend to assume that biomarkers within a pathway have linear associations with the phenotype of interest, even though the relationships are more complex.

RESULTS

To model complex effects including nonlinear effects, we propose a new approach, Hierarchical structural CoMponent analysis using Kernel (HisCoM-Kernel). The proposed method models nonlinear associations between biomarkers and phenotype by extending the kernel machine regression and analyzes entire pathways simultaneously by using the biomarker-pathway hierarchical structure. HisCoM-Kernel is a flexible model that can be applied to various omics data. It was successfully applied to three omics datasets generated by different technologies. Our simulation studies showed that HisCoM-Kernel provided higher statistical power than other existing pathway-based methods in all datasets. The application of HisCoM-Kernel to three types of omics dataset showed its superior performance compared to existing methods in identifying more biologically meaningful pathways, including those reported in previous studies.

AVAILABILITY AND IMPLEMENTATION

Freely available at http://statgen.snu.ac.kr/software/HisCom-Kernel/.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: focus on the cancer hallmark of tumor angiogenesis

One of the important 'hallmarks' of cancer is angiogenesis, which is the process of formation of new blood vessels that are necessary for tumor expansion, invasion and metastasis. Under normal physiological conditions, angiogenesis is well balanced and controlled by endogenous proangiogenic factors and antiangiogenic factors. However, factors produced by cancer cells, cancer stem cells and other cell types in the tumor stroma can disrupt the balance so that the tumor microenvironment favors tumor angiogenesis. These factors include vascular endothelial growth factor, endothelial tissue factor and other membrane bound receptors that mediate multiple intracellular signaling pathways that contribute to tumor angiogenesis. Though environmental exposures to certain chemicals have been found to initiate and promote tumor development, the role of these exposures (particularly to low doses of multiple substances), is largely unknown in relation to tumor angiogenesis. This review summarizes the evidence of the role of environmental chemical bioactivity and exposure in tumor angiogenesis and carcinogenesis. We identify a number of ubiquitous (prototypical) chemicals with disruptive potential that may warrant further investigation given their selectivity for high-throughput screening assay targets associated with proangiogenic pathways. We also consider the cross-hallmark relationships of a number of important angiogenic pathway targets with other cancer hallmarks and we make recommendations for future research. Understanding of the role of low-dose exposure of chemicals with disruptive potential could help us refine our approach to cancer risk assessment, and may ultimately aid in preventing cancer by reducing or eliminating exposures to synergistic mixtures of chemicals with carcinogenic potential.

Bile duct-bound growth of precursor cells of preneoplastic foci inducible in the initiation stage of rat chemical hepatocarcinogenesis by 2-acetylaminofluorene

BACKGROUND

We previously detected precursor cell populations of preneoplastic foci, GST-P(+)/GGT(-) and GST-P(+)/GGT(+) minifoci, in rat liver in the initiation stage of chemical hepatocarcinogenesis, where GST-P and GGT represent glutathione S-transferase P-form and gamma-glutamyltranspeptidase, respectively.

METHODS

Sprague-Dawley male rats were fed a basal diet containing 2-acetylaminofluorene (0.02%) over 16 weeks. Precursor cells were detected by our sensitive staining method for GGT activity and immunocytochemical staining for GST-P.

RESULTS

GST-P(+)/GGT(-) single cells were overproduced maximally in the animal liver after the 6 weeks followed by a gradual growth of GST-P(+)/GGT(-) and GST-P(+)/GGT(+) minifoci, which were bound to bile ducts and ductules. GGT was expressed within GST-P(+) minifoci gradually with time forming GGT(+) lane-like structures. The bile duct binding and lane-like structure formation were prominent especially when minifoci-bearing rats were subjected to two-thirds partial hepatectomy.

CONCLUSIONS

A variety of precursor minifoci were noted to be selectively bound to bile ducts and ductules in rat liver, which may be of physiologic significance in excretion of carcinogens during initiation.

Effect of selenium on N-nitrosodiethylamine-induced multistage hepatocarcinogenesis with reference to lipid peroxidation and enzymic antioxidants

We have studied the relationship between antioxidant and anticancer properties of selenium (Se) in multistage hepatocarcinogenesis induced by N-nitrosodiethylamine (DEN). In this study we have observed an increased level of lipid peroxide (LPO) products and decreased antioxidant enzyme activities (superoxide dismutase and catalase) in hepatoma and surrounding liver tissues of cancer-bearing animals. Selenium (Se) was supplemented either before initiation or during initiation and selection/promotion phases of hepatocarcinogenesis and was found to be effective in altering hepatic lipid peroxidation and antioxidant enzyme activities to a statistically significant level measured either in the hepatoma or in the surrounding liver tissues. These alterations inclined towards normal in a time-dependent manner on selenium supplementation. Furthermore, increased levels of lipid peroxidation and decreased levels of antioxidants (superoxide dismutase and catalase) were also observed in distant organs of cancer-bearing rats other than the tumour-bearing site. These alterations are brought back to normal levels upon Se treatment. Our results confirm the fact that Se is particularly protective in limiting the action of DEN by its antioxidant property.

Expression of 27-kD heat-shock protein isoforms in human neoplastic and nonneoplastic liver tissues

Previous study of rat liver during chemically induced hepatocarcinogenesis has shown that expression of isoforms of the 27-kD heat-shock protein was greater in neoplastic nodules and in hepatocellular carcinoma than in control livers. In this study, various human neoplastic and nonneoplastic liver tissues were investigated with electrophoresis after amino acid labeling to evaluate the expression of 27-kD heat-shock protein isoforms. This revealed that human liver contains 27-kD proteins that are recognized by a polyclonal antibody raised against human 27-kD heat-shock protein. Basal levels of fluorographical and immunostaining intensity of the 27-kD heat-shock protein spots (respectively, after [3H]leucine or 32P incorporation or as checked with a specific human 27-kD heat-shock protein antibody) were higher in hepatomas than in non-tumorous liver. Phosphorylation patterns of the 27-kD heat-shock protein isoforms were, however, similar in hepatocellular carcinoma and in uninvolved surrounding liver. Heat inducibility of the 27-kD heat-shock protein, tested in one case of liver cell adenoma and in the surrounding liver, was also preserved in both tissues. The role of the overexpression of 27-kD heat-shock protein in neoplastic liver tissues remains unknown. We propose, as a working hypothesis, that it is related to the resistant phenotype acquired by some tumors during malignant progression.

Enzyme induction by drugs and toxins

Enzyme induction by drugs mostly concerns those enzymes involved in drug metabolism: cytochromes P-450, UDP-glucuronosyltransferases, glutathione S-transferases, gamma-glutamyltransferases and epoxide hydrolases. A large variety of molecular forms exists, but not all of them are inducible (e.g. the inducible cytochromes P-450 in man are members of family IA, IIA, IIC, IIE, IIIA). Induction is most common in the liver, but also occurs in other organs (lung, placenta, lymphocytes). Over the past 20 years a relatively small number of drugs and environmental chemicals have been identified as enzyme inducers, perhaps fewer than early studies suggested. Information on inducing properties must be obtained as early as possible during the development of a new drug and made available to clinicians and clinical chemists when the drug is marketed. The main consequences of enzyme induction are changes in pharmacokinetics of the drug itself or of an associated drug. Much progress has been made in methods to identify these inducers.

Glutathione transferases and cancer

The glutathione transferases, a family of multifunctional proteins, catalyze the glutathione conjugation reaction with electrophilic compounds biotransformed from xenobiotics, including carcinogens. In preneoplastic cells as well as neoplastic cells, specific molecular forms of glutathione transferase are known to be expressed and have been known to participate in the mechanisms of their resistance to drugs. In this article, following a brief description of recently identified molecular forms, we review new findings regarding the respective molecular forms involved in carcinogenesis and anticancer drug resistance, with particular emphasis on Pi class forms in preneoplastic tissues. The rat Pi class form, GST-P (GST 7-7), is strongly expressed not only in hepatic foci and hepatomas, but also in initiated cells that occur at the very early stages of chemical hepatocarcinogenesis, and is regarded as one of the most reliable markers for preneoplastic lesions in the rat liver. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-responsive element-like sequences have been identified in upstream regions of the GST-P gene, and oncogene products c-jun and c-fos are suggested to activate the gene. The Pi-class forms possess unique enzymatic properties, including broad substrate specificity, glutathione peroxidase activity toward lipid hydroperoxides, low sensitivity to organic anion inhibitors, and high sensitivity to active oxygen species. The possible functions of Pi class glutathione transferases in neoplastic tissues and drug-resistant cells are discussed.

Naturally-occurring age-dependent glutathione S-transferase pi immunoreactive hepatocytes in aging female F344 rat liver as potential promotable targets for non-genotoxic carcinogens

Naturally occurring basophilic foci (focal hepatocellular proliferative lesions) (FHPL) in the livers of aging female F344/NCr rats could not be promoted to grow or progress into tumors after phenobarbital (PB) exposure. Instead, PB induced new unique eosinophilic hepatocellular foci and adenomas much quicker in old rats than in young rats. We now report that these foci are immunoreactive for glutathione S-transferase pi (GSTP) and that they appear to arise from some naturally occurring single and double GSTP-reactive cells and foci which occur spontaneously in the liver of aging F344 rats in an age-related fashion. PB and other nongenotoxic chemicals may act as "carcinogens" by promoting the growth (clonal expansion) of some of these putative spontaneously-initiated cells and foci into tumors.

Critical parameters in the quantitation of the stages of initiation, promotion, and progression in one model of hepatocarcinogenesis in the rat

Critical parameters in the quantitation of altered hepatic foci (AHF) developing during multistage hepatocarcinogenesis in the rat include: 1) the enumeration of AHF induced by test agents as well as those AHF occurring spontaneously in livers of untreated animals; 2) the volume percentage or fraction of the liver occupied by all AHF as a reflection of the total number of altered cells within the liver and the degree of tumor promotion which has occurred; and 3) the phenotype of individual AHF as determined by multiple markers with serial sections. These parameters, especially the number of AHF, should be corrected by the presence of spontaneous AHF which increase with the age of the animal, more so in males than females. While accurate estimation of the background level of spontaneous AHF can be important in demonstrating that a carcinogenic agent does not possess the ability to increase the numbers of AHF above the background level, a better method to distinguish the effectiveness and relative potencies of agents as initiators or promoters is reviewed. The relative effectiveness of four different markers--gamma-glutamyltranspeptidase (GGT), a placental form of glutathione S-transferase (GST), canalicular ATPase, and glucose 6-phosphatase (G6Pase)--was described for the chemicals C.I. Solvent Yellow 14 and chlorendic acid as promoting agents in males and females. C.I. Solvent Yellow 14 is a more effective promoting agent in females than males, and AHF exhibit extremely low numbers scored by GGT. On the other hand, the numbers of AHF present in livers of male rats promoted by this agent are more than twice those seen in livers of female animals, possibly owing to the effectiveness of this agent as an initiator in the male but not the female. Very few AHF, especially in the male, are scored by GGT during chlorendic acid promotion. The distribution of phenotypes with these markers also differs in the spontaneous AHF appearing in the livers of animals fed 0.05% phenobarbital on either a crude NIH-07 or AIN-76 purified diet. Such studies emphasize the extreme dependence of the promoting stage of hepatocarcinogenesis on environmental factors of sex, diet, and the molecular nature of the promoting agent itself. The hallmark of the final stage of progression in the development of hepatocellular carcinomas is aneuploidy, which may be reflected by phenotypic heterogeneity within individual AHF, termed foci-in-foci. The implications of such quantitative analyses during hepatocarcinogenesis induced by specific agents in relation to the specific action of the agent at one or more of the stages of hepatocarcinogenesis are discussed.