Modification of liver tumor development in rodents

Epigenetic mechanisms of chemical carcinogenesis

Chemically induced cancer is a multi-step process involving damage to the genome initially followed by clonal expansion of the DNA damaged cell eventually leading to a neoplasm. Chemical carcinogens have been shown to impact at all of the stages of the tumorigenesis process. It has become apparent that chemical and physical agents that induce cancer may do so through several different cellular and molecular mechanisms. Epigenetic (nongenotoxic) chemical carcinogens are those agents that function to induce tumor formation by mechanisms exclusive of direct modification or damage to DNA. These agents appear to modulate cell growth and cell death and exhibit dose response relationships between exposure and tumor formation. The exact and/or exclusive mechanisms by which these agents function have not been established, however, changes in cell growth regulation and gene expression are important to tumor formation. This review focuses on several potential mechanisms and cellular processes that may be involved in nongenotoxic chemical carcinogenesis.

Monograph: reassessment of human cancer risk of aldrin/dieldrin

In 1987, the US Environmental Protection Agency (EPA) classified aldrin and dieldrin as category B2 carcinogens, i.e. probable human carcinogens, based largely on the increase in liver tumors in mice fed either organochlorine insecticide. At that date, the relevant epidemiology was deemed inadequate to influence the cancer risk assessment. More time has now elapsed since early exposures of manufacturing workers to aldrin/dieldrin; therefore, updated epidemiological data possess more power to detect exposure-related differences in cancer risk and mortality. Also, recent experimental studies provide a plausible mode of action to explain the mouse specificity of dieldrin-induced hepatocarcinogenesis and call into question the relevance of this activity to human cancer risk. This monograph places this new information within the historic and current perspectives of human cancer risk assessment, including EPA's 1996 Proposed Guidelines for Carcinogen Risk Assessment. Updated epidemiological studies of manufacturing workers in which lifetime exposures to aldrin/dieldrin have been quantified do not indicate increased mortality or cancer risk. In fact, at the middle range of exposures, there is evidence of a decrease in both mortality from all causes and cancer. Recent experimental studies indicate that dieldrin-induced hepatocarcinogenesis in mice occurs through a nongenotoxic mode of action, in which the slow oxidative metabolism of dieldrin is accompanied by an increased production of reactive oxygen species, depletion of hepatic antioxidant defenses (particularly alpha-tocopherol), and peroxidation of liver lipids. Dieldrin-induced oxidative stress or its sequelae apparently result in modulation of gene expression that favors expansion of initiated mouse, but not rat, liver cells; thus, dieldrin acts as a nongenotoxic promoter/accelerator of background liver tumorigenesis in the mouse. Within the framework of EPA's Proposed Guidelines for Carcinogen Risk Assessment, it is proposed that the most appropriate cancer risk descriptor for aldrin/dieldrin, relating to the mouse liver tumor response, is 'not likely a human carcinogen', a descriptor consistent with the example of phenobarbital cited by EPA.

Emerging issues in mouse liver carcinogenesis

The mouse liver is the primary target site for carcinogenesis of more than 200 chemicals (including pesticides, food additives, pharmaceuticals, and industrial intermediates) tested in long-term toxicity safety assessment assays. Mouse liver tumors develop through defined morphological stages (similar to those found in other species) whether their origin is of undetermined etiology (spontaneous) or induced by chemicals. The morphologic type of hepatocytes in the various stages of hepatocarcinogenesis is sometimes associated with the specific inducing agent. Liver tumors developing in toxic livers often have more benign appearances and may progress to carcinomas at a slower rate than tumors developing in histologically normal livers. Specific tumors, dependent on the inducing chemical, may regress under defined protocols. Genotoxic and nongenotoxic mouse hepatocarcinogens each may induce tumors of either high malignant or low malignant potential. Liver tumors with specific H-ras oncogene mutations may appear morphologically and biologically similar to those without proven ras mutations. Thus, distinguishing mechanism of carcinogenesis by liver tumor morphology and mutation spectra may be difficult. Additionally, the presence of liver tumors with a morphology and a ras oncogene mutation spectrum characteristic of spontaneous tumors in histologically normal livers of mice exposed to a nongenotoxic test chemical may indicate promotion of spontaneous hepatocarcinogenesis by one of several potential mechanisms. Further research into the mechanisms responsible for the increased incidences of liver tumors in mice exposed to test chemicals could enhance human cancer risk assessments.

Immunohistochemical analysis of dichloroacetic acid (DCA)-induced hepatocarcinogenesis in male Fischer (F344) rats

We examined the incidence of proliferative lesions, hyperplastic nodules and altered hepatic foci, in male F344 rat liver, to determine their preneoplastic potential during dichloroacetic acid (DCA)-induced hepatocarcinogenesis. Immunohistochemical and image analysis methods were used to detect the expression of 6 histochemical markers of neoplastic cells; p21 ras, p39 c-jun, p55 c-fos, aldehyde dehydrogenase (ALDH), glutathione s-transferase (GST-p), and alpha fetoprotein (AFP) during DCA-induced hepatocarcinogenesis. Our results were consistent with our previous data and suggested that the hyperplastic nodules, rather than altered hepatic foci, is a putative preneoplastic lesion during DCA-induced hepatocarcinogenesis in the male F344 rat.

The stem cells of the liver ? a selective review

The current status of the much-debated question of the still-hypothetical stem cells of the liver is reviewed, with an emphasis on their role in hepatocarcinogenesis. The widely held view of the primacy of the hepatocyte, notably of the mononuclear diploid type, in this process--the "hepatocytic theory"--has been compared with variants of the "stem cell hypothesis" based on the "non-parenchymal epithelial cells" of the liver--the "oval" or biliary ductular cells, the "nondescript periductular" cells and the "primitive" bipotential epithelial cells. An attempt has been made to concentrate mainly on the more recent publications, in an effort to balance the conflicting opinions expressed by comparing results obtained by the newer procedures currently in use. Despite some interesting and relevant findings it appears that the evidence in favour of the stem-cell hypothesis is still circumstantial and that the hepatocytic theory has not been invalidated. Presumably the question of the hepatic stem cells will be answered when the riddle of hepatocarcinogenesis has been solved.