Lack of mutations of the p53 tumor suppressor gene in hepatocellular carcinomas induced in rats by a peroxisome proliferator

Central role of PPARalpha in the mechanism of action of hepatocarcinogenic peroxisome proliferators

Peroxisome proliferators (PP) are a large class of structurally dissimilar chemicals. These chemicals have diverse effects in rodents and humans, including regulation of lipid metabolism, growth promotion, and induction of hepatocarcinogenesis. Most, if not all, effects of PP are mediated by three members of the nuclear receptor superfamily called PP-activated receptors (PPAR). In this review, we discuss the evidence that PPARalpha, the predominant PPAR in the, liver is involved in the growth promoting and hepatocarcinogenic effects of PP.

Hepatic expression of acute-phase protein genes during carcinogenesis induced by peroxisome proliferators

Concern exists regarding peroxisome proliferator (PP) xenobiotic exposure because many PPs are potent hepatocarcinogens in rodents. The mechanism of carcinogenicity induced by PPs is atypical compared with those of other hepatocarcinogens in that the former appears to involve alterations in expression of PP-activated receptor (PPAR) target genes rather than direct mutagenicity. To begin to identify some of these genes, we used differential display to compare mRNA expression between hepatic adenomas and adjacent non-tumor liver from rats fed the potent PP Wy-14643 (WY) for 78 wk. Here, we report increased expression of the acute-phase protein (APP) gene alpha-1 antitrypsin (AT) and decreased expression of alpha2-urinary globulin in the tumors. Similar changes were seen in hepatic adenomas induced by a diethylnitrosamine and phenobarbital protocol, indicating a lack of specificity for PP-induced tumors. Additional APP genes, including ceruloplasmin, haptoglobin, beta-fibrinogen, and alpha1-acid glycoprotein were also upregulated in WY-induced tumors but were downregulated in the livers of rats administered a different PP for 13 wk. Mice treated with either WY or di(2-ethylhexyl) phthalate for 3 wk had decreased hepatic AT expression but increased expression of ceruloplasmin and haptoglobin. PPARalpha-null mice showed no hepatic APP gene alteration after PP treatment but had higher basal expression than did wild-type controls. We conclude that PPARalpha activation by several different PPs leads to dysregulation of hepatic APP gene expression in rats and mice. This dysregulation may indicate alterations in cytokine signaling networks regulating both APP gene expression and hepatocellular proliferation.

Highly metastatic hepatocellular carcinomas induced in male F344 rats treated with N-nitrosomorpholine in combination with other hepatocarcinogens show a high incidence of p53 gene mutations along with altered mRNA expression of tumor-related genes

The carcinogenic and metastatic processes are thought to consist of a sequence of steps, and animal models featuring highly metastatic lesions are clearly necessary to allow analysis of the whole process of transformation from preneoplastic changes to high grade metastatic tumors, and to access effectiveness of therapeutic treatments of advanced cancers in vivo. The purpose of the present study was to establish a model and to screen for reported genetic alterations in induced lesions. In the present study, it was confirmed that lung metastasis of hepatocellular carcinomas (HCCs) induced in male F344 rats by N-nitrosomorpholine (NNM), given in the drinking water at a dose of 120 ppm for 24 weeks, was significantly enhanced by additional carcinogenic pretreatments and that a single i.p. injection of 100 mg/kg body weight N-diethylnitrosamine (DEN) alone was sufficient for that purpose. Molecular biological analyses of the induced lesions revealed point mutations in the p53 gene in 60.9% of HCCs, and elevated expression of mRNAs for p53, c-myc, c-fos, TGF-alpha, TGF-beta1, alpha-fetoprotein, GST-P, and GGT, and decreased mRNA expression of EGF and EGFR in HCCs when compared to controls. No obvious association of gene alterations with metastatic potential of primary tumors was found except for an increase in the incidence of p53 mutations. Since the process of metastasis is thought to be sequential and selective, further comparative analysis of metastatic and primary lesions should clarify the mechanisms involved in the multi-step process of metastasis.

Studies of oncogene activation and tumor suppressor gene inactivation in normal and neoplastic rodent tissue

Emerging short-term bioassays for chemically-induced carcinogenesis are dependent for their relevance to human risk assessment on the degree of coincidence of human and rodent tumor pathways. Since these pathways do not always converge, these new tests may have a number of unanticipated pitfalls. Models of liver and renal tumors are described. The results from Rb and p53 tumor suppressor gene transgenic animals are compared to human tumor syndromes. The question of mutagenic and epigenetic fingerprints of chemicals versus the cell-specific selection of spontaneous mutations is debated. Examples of specific pitfalls, such as the recently discovered Helicobacter hepaticus promoted liver tumors in mice are presented. The rat pseudogenes for p53 and the rare role of p53 in most important rodent tumor models other than epithelial tumors present experimental quandaries. The differential effects of carcinogens during various stages of rodent perinatal and adult development are also discussed. It is concluded that the pathways of both animal models and their human counterparts should be better identified so that realistic endpoint markers can be chosen for human carcinogenic risk assessment.

Mutagenic nitroarenes, diesel emissions, particulate-induced mutations and cancer: an essay on cancer-causation by a moving target

Initial analyses of the lung tumors seen in rats exposed for their lifetime to elevated levels of the emissions of diesel engines suggested that they were due to powerful mutagens and carcinogens (PAHs, nitro PAHS) absorbed onto the diesel particles. However, further studies showed that carcinogenicity occurred only under conditions that resulted in impaired lung clearance ('overloading') leading to inflammatory reactions and other pathologic sequelae. These observations together with the findings that carbon black, a model for diesel particles devoid of organic mutagens and carcinogens, also induced lung cancers under conditions of overloading led to the suggestion that the cancers resulted from a non-genotoxic mechanism. However, the further finding that inert particulate carcinogens devoid of organics, induce mutations has led to a re-evaluation of the role of mutations in lung carcinogenesis caused by particles and the relevance of the rat model to humans. This is especially timely as epidemiological studies suggest that humans may develop lung cancers following occupational exposure to diesel emission by a mechanism unlikely to involve lung overloading. Finally, the recent recognition that environmental PM-10 (respiratory size particles) may be responsible for a significant portion of human morbidity and mortality, ensures that the health effect of diesel emissions will continue to receive scrutiny as they contribute to the PM-10 load.

The potential for the use of cell proliferation and oncogene expression as intermediate markers during liver carcinogenesis

Intense research using animal models has indicated that chemically-induced rat liver cancer proceeds through multiple, distinct stages that can be characterised morphologically and biochemically. Primary human liver cancer, with hepatitis B and other environmental factors such as poor nutrition and food contaminating mycotoxins as contributing etiological factors, is one of the major causes of cancer deaths in African, Asian and some Western countries. Recent advances in surgical and diagnostic techniques have also allowed the identification of potential morphological precursors of primary human liver cancer, and suggested a model consistent with the concepts of initiation--promotion--progression as in the rat. The expression of proliferating cell nuclear antigen (PCNA), silver-staining nucleolar organiser regions (AgNOR), oncogenes and the tumor suppressor gene p53 in preneoplastic and neoplastic lesions of rat and human livers is presently reviewed. This undertaking is an attempt to evaluate whether the current knowledge regarding molecular mechanisms of carcinogenesis is sufficient to permit the use of these molecular parameters as 'intermediate' markers in studies of risk assessment and cancer prevention, without having to resort to tumor appearance as an end-point.

Molecular aspects of chemical carcinogenesis: the roles of oncogenes and tumour suppressor genes

The observation that oncogenes are frequently activated in human tumours raises the question of whether these genes are involved in chemical carcinogenesis. H-ras activation is probably an initiating event in mouse skin and rat mammary gland systems. The H-ras oncogene is also important in mouse liver tumours; in mouse lung the K-ras gene is commonly activated. In both, the mutations observed are usually those predicted from the adduct-forming properties of the carcinogen. Among non-ras oncogenes, only raf and neu have been detected in experimental tumours. Tumour suppressor genes are frequently inactivated in human tumours. Searches for such phenomena in animal tumours have generally had disappointing results. p53 and Rb gene alterations are rarely observed in chemically-induced tumours. The reason may be that unknown tumour suppressor genes are involved in animal tumour development. Several novel genes have been identified using animal tumour susceptibility models. Thus, ras genes are important in chemical carcinogenesis, but as the methodology for studying other genes improves, their roles will be seen in perspective.

Low frequency of retinoblastoma gene alterations in rat hepatocellular carcinomas

Abnormalities of the retinoblastoma (Rb) gene have been reported in some human cancers, including hepatocellular carcinomas (HCCs). We examined by Southern blotting the status of the Rb gene in HCCs induced in rats in four experimental models. A low frequency of Rb gene alterations, detected as novel hybridizing bands unique to each tumor, was observed. Expression of the Rb protein product was examined in the HCCs and in seven established rat hepatoma cell lines studied. It appears, therefore, that alterations in the structure or expression of the Rb gene do occur but probably do not contribute in a major way to hepatocarcinogenesis in the rat.