Lack of elevated liver carcinogenicity of aminophenylnorharman in p53-deficient mice

Stem cells and hepatocellular carcinoma

Stem cells are a class of special embryonic or adult cells that are able to self-renew and undergo multi-directional differentiation. Studies have shown that stem cells have selective tropism toward tumor tissue. Previous studies have shown that hepatic stem cells play an important role in hepatocarcinogenesis by participating in regulation of cell growth and differentiation. However, some other studies demonstrated that stem cells could inhibit cell growth in hepatocellular carcinoma. Elucidation of relationship between stem cells and hepatocellular carcinoma could provide new clues to the pathogenesis of hepatocellular carcinoma and help develop new therapeutic strategies for the disease.

Tumor-initiating potency of a novel heterocyclic amine, aminophenylnorharman in mouse colonic carcinogenesis model

A novel heterocyclic amine, 9-(4'-aminophenyl)-9H-pyrido[3,4-b]indole (aminophenylnorharman, APNH), which is formed from nonmutagenic 9H-pyrido[3,4-b]indole (norharman) and aniline, is mutagenic to bacteria and mammalian cells and potently carcinogenic in rats. APNH is detected in human urine samples, suggesting that humans are continuously exposed to APNH. In the present study, (32)P-postlabelin analysis revealed that the levels of APNH-DNA adduct 24 hr after the treatment with APNH (1, 5 and 20 mg/kg body weight) in male ICR mice were increased in a dose-dependent manner in the colon and liver. Based on these findings, we determined the tumor-initiating potency of APNH in an inflammation-related and two-stage mouse colon carcinogenesis model. Male Crj: CD-1 (ICR) mice were given a single intragastric administration (1, 2, 5 or 10 mg/kg body weight) of APNH and subsequent 1-week oral exposure to dextran sodium sulfate (DSS, 2% in drinking water). Treatment with APNH and DSS resulted in numerous colon tumor development: the incidence and multiplicity of the tumors were the highest in the mice received 10 mg/kg body weight of APNH and followed by DSS. Development of colon tumors was dose-dependent of APNH. Seven of 9 (77.8%) colonic adenocarcinomas developed in mice treated with APNH (10 mg/kg body weight) and DSS had beta-catenin gene mutations at codons 32 and 37, being predominantly transversion. These findings indicate that APNH has an initiating activity in inflamed mouse colon and the APNH-DNA adduct formation correlates with its tumorigenic potential.

Organ-dependent susceptibility of p53 knockout mice to 2-amino-3-methylimidazo[4,5-f]quinoline (IQ)

p53 knockout mice are now being frequently used to identify the carcinogenic potential of chemicals, thus it is important to precisely assess the susceptibility of the animals to various test chemicals. In the present study the susceptibility of p53 nullizygous((-/-)), heterozygous((+/-)), and wild-type((+/+)) mice to 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) was investigated. Mice of all three genotypes were first fed a diet containing 100 or 300 p.p.m. IQ for 15 weeks in a short-term experiment. p53((+/-)) and ((+/+)) mice were then treated with IQ for 40 weeks and maintained without further treatment for an additional 12 weeks in the long-term experiment. In the forestomach, the incidence of squamous cell hyperplasia was significantly higher in p53((-/-)) than in ((+/-)) and ((+/+)) mice at 15 weeks and higher in ((+/-)) mice than ((+/+)) mice with long-term IQ treatment, indicating an elevated susceptibility of p53 knockout mice. In contrast, in the liver, various hepatocellular lesions developed mainly in female mice with long-term IQ exposure but no significant differences were evident between p53 knockout and wild-type mice, indicating a lack of elevated susceptibility in the knockout animals. Furthermore, polymerase chain reaction-single strand conformation polymorphism and sequencing analysis revealed relatively high (13/30) and low (1/15) incidences of p53 mutations (exons 5-8) in squamous cell hyperplasia and hepatocellular tumors, respectively. These results clearly indicate that the susceptibility of p53 knockout mice is organ-dependent, coinciding to some extent with the likelihood of p53 gene alteration in the induced tumors.

Origin of hepatocellular carcinoma: role of stem cells

The question of whether hepatocellular carcinoma (HCC) arises from the differentiation block of stem cells or dedifferentiation of mature cells remains controversial. Recently, researchers suggested that HCC may originate from the transdifferentiation of bone marrow cells. Interestingly, there are four levels of cells in the hepatic stem cell lineage: bone marrow cells, hepato-pancreas stem cells, oval cells and hepatocytes. Hematopoietic stem cells and the liver are known to have a close relationship in early development. Bone marrow stem cells could differentiate into oval cells, which could differentiate into hepatocytes and duct cells. The development of pancreatic and liver buds in embryogenesis suggests the existence of a common progenitor cell to both the pancreas and liver. Cellular events during hepatocarcinogenesis illustrate that HCC may arise from cells at various stages of differentiation in the hepatic stem cell lineage.

Organ-specific susceptibility of p53 knockout mice to N-bis(2-hydroxypropyl)nitrosamine carcinogenesis

To elucidate which is the major determinant of susceptibility of p53 deficient mice, the carcinogen or the target organ, N-bis(2-hydroxypropyl)nitrosamine was administered to induce tumors in multi-organs. In a 15-week experiment, the incidences of both lung and hepatic vascular tumors were found to be significantly higher in p53 nullizygous (-/-) than in heterozygous (+/-) and wild-type (+/+) mice, indicating universal susceptibility of p53 (-/-) mice. In a 40-week experiment, p53 (+/-) mice showed increased susceptibility only with regard to vascular tumors, coinciding with significantly more frequent (60%) p53 gene mutations, in comparison with lung tumors with their low mutation rate (10.8%) (P<0.005). These results indicate that the target organ may be a more important factor than the carcinogen in determining susceptibility of p53 (+/-) mice.