Reduced levels of mutagen processing potential in the Schistosoma japonicum-infected mouse liver

Schistosomiasis and signet ring cell carcinoma of the rectum

A definitive link between Schistosoma hematobium infection and squamous cell carcinoma of the bladder has been identified. A weaker association between S japonicum infection and colorectal neoplasia has been proposed, although reports are limited to case reports, a case series, and epidemiologic studies. Virtually all cases presented in the literature describe intestinal-type adenocarcinoma occurring in association with S japonicum. We here describe a 40-year-old male Filipino patient with signet ring cell carcinoma of the rectum and evidence of infection by S japonicum.

The role of helminth infections in carcinogenesis

This review examines the significant literature on the role of helminth infections in carcinogenesis. Both parasitic infections and cancer have complex natural histories and long latent periods during which numerous exogenous and endogenous factors interact to obfuscate causality. Although only two helminths, Schistosoma haematobium and Opisthorchis viverrini, have been proven to be definitely carcinogenic to humans, others have been implicated in facilitating malignant transformation. The known mechanisms of helminth-induced cancer include chronic inflammation, modulation of the host immune system, inhibition of intracellular communication, disruption of proliferation-antiproliferation pathways, induction of genomic instability and stimulation of malignant stem cell progeny. Approximately 16% of all cancer cases worldwide are attributable to pathogenic agents, including schistosomes and liver flukes. This equates to 1,375,000 preventable cancer deaths per year. Means to reduce the incidence of helminth-associated malignancies are discussed.

Mutagenicity evaluation of Schistosoma spp. extracts by the umu-test and V79/HGPRT gene mutation assay

Schistosomiasis has been suspected of being a risk factor for various types of cancers for sometime, e.g., bladder cancer, colorectal cancer and hepatic cancer. Among them, the etiological relationship between urinary schistosomiasis and bladder cancer is now widely accepted. However, mechanisms of the carcinogenesis are still unclear. Here, we tested the mutagenicity of the parasite extracts by the umu-test and hypoxanthine guanine phosphoribosyltransferase (HGPRT) gene mutation assay, which both overcome disadvantages of the Ames plate assay. Adult worm extracts and egg extracts of Schistosoma haematobium and Schistosoma mansoni were tested. Under our experimental conditions, neither worm nor egg extracts were shown to have any mutagenicity in both tests even in the presence of S9 mix. Our results suggest that there is very little possibility of immediate gene mutation due to the parasite-derived substances in schistosomiasis-related carcinogenesis.

Alteration of drug biotransformation and elimination during infection and inflammation

During infection or inflammation, the expression of cytochrome P450 and its dependent biotransformation pathways are modified. This results in a change in the capacity of the liver to handle drugs and in alterations in the production and elimination of endogenous substances throughout the body. The majority of the CYP isoforms are modified at pre-translational steps in protein synthesis, and, in most cases, cytokines are involved as mediators of the response. Recent information suggests that inflammatory responses that are localized to the CNS cause a loss of CYP within the brain. This is accompanied by a parallel down-regulation of CYP in peripheral organs that is mediated by a signaling pathway between the brain and periphery. This review covers the loss that occurs in the major mammalian CYP families in response to infection/inflammation and the mediator pathways that are key to this response.

Retention of a mutagen, 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2), in the liver of mice infected with Schistosoma japonicum

Regarding the mechanism underlying the suspected enhancement of hepatic cancers among Schistosoma japonicum-infected humans, we hypothesized that mutagen exposures in the livers of patients may be enhanced due to the parasitic infection. To explore this possibility, we have done a model experiment using mice and a carcinogenic mutagen, 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2). Mice infected with Schistosoma japonicum were intravenously administered Trp-P-2, and the mutagenic activities of the mouse serum and of the liver tissue extracts, which were observable during the 6-h period after the administration, were investigated. The level of serum indirect mutagenicity, which probably reflected the amount of unmetabolized Trp-P-2, was higher in the infected animals than in uninfected control animals. Direct mutagenicity in the serum, on the other hand, was higher in the control animals than in the infected mice. Furthermore, the liver tissue extracts from infected mice showed higher indirect-mutagenicity than those from the controls. These data suggest that the infection results in a decreased metabolism and an increased retention of Trp-P-2 in the liver. Consistent with this phenomenon, pigments in the liver formed by the schistosome infection were found to be an efficient adsorbent for Trp-P-2. Thus, the possibility exists that these pigments, which contain hematin as a major constituent, may function as a reservoir for the mutagen, thereby prolonging the exposure period of the liver to the mutagen.

Implications for the involvement of the immune system in parasite-associated cancers

Biological factors can be carcinogenic risk factors in humans and in animals. Numerous theories have been developed to explain the causal link between biologically-associated disease and the ensuing neoplasia. In this paper we discuss the merits of one of these theories, the possible association between the mammalian inflammatory response and cancer.

Parasite infection and cancer: with special emphasis on Schistosoma japonicum infections (Trematoda). A review

This article contains a review of current knowledge on the association of parasite infections and cancer formation, especially that of Schistosoma japonicum (Trematoda) in man and experimental animals. The association of S. haematobium infection and bladder cancer is well known and documented. However, S. japonicum infection has also been reported to be associated with cancer, in this case hepatocellular carcinoma and/or colorectal cancer. Pathological records and analyses have shown a correlation between this infection and cancer, and pathohistological descriptions have been numerous, together with clinical case reports. Epidemiological analyses have been conducted in China and Japan and support a role of S. japonicum infection as one of the risk factors in cancer formation, along with others, such as hepatitis virus infection and alcoholic intake. Experimental results have also shown that cancer appears early and in larger numbers in experimentally infected animals given a known carcinogen. In spite of these positive end-point associations, the mechanism of schistosome-mediated enhancement of carcinogenesis is obscure. A suggestive observation is that in S. japonicum-infected mice carcinogen-metabolizing hepatic activity including P-450 was decreased so that an administered carcinogen persisted for a longer period than in uninfected mice. Further studies, both epidemiological and experimental, are needed to firmly establish the relationship between schistosome infection and cancer.

Modified metabolism of a carcinogen, 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2), by liver S9 from Schistosoma japonicum-infected mice

Schistosoma japonicum infection has been associated with an increased incidence of liver and colorectal cancers in humans. To explore the mechanisms underlying this association, we investigated the carcinogen-metabolizing properties of liver S9 preparations from S. japonicum-infected mice and compared them with those of S9 from uninfected animals. When the carcinogen 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) was incubated with these S9s and the products were analyzed by high-performance liquid chromatography, we observed that the S9 from infected mice had a lower ability to convert Trp-P-2 into 3-hydroxyamino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2(NHOH)), an activated form of promutagenic Trp-P-2, than the S9 from uninfected mice. We found that both of these S9 preparations have a high ability to reduce Trp-P-2(NHOH) into Trp-P-2; however, the infected-mouse S9 showed a significantly greater reducing power than the control S9. This difference appears to be responsible for the observed lower mutagen-activating potential of the infected mouse S9. These results suggest that hepatic enzyme activities of S. japonicum-infected mice are quantitatively different from those of normal mice.

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.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 18. Review of the genotoxicity and carcinogenicity of antischistosomal drugs; is there a case for a study of mutation epidemiology? Report of a task group on mutagenic antischistosomals

One of the interests of ICPEMC is to identify situations in which the possible induction of inherited defects in man by mutagen exposure could actually be studied. The large-scale use of mutagenic drugs in field programmes against schistosomiasis, mainly during the 1970's, was considered a possible case. An ICPEMC task group approached the problem by (1) updating the genetic toxicology data base for antischistosomal drugs, and (2) reviewing possible study areas. Expertise was combined from genetic toxicology, mutation epidemiology and tropical medicine. It was considered that: (a) if any, hycanthone would be the most appropriate candidate drug for study; (b) it would be virtually impossible to meet the basic requirements of an appropriate mutation epidemiology study, in endemic countries; (c) as more defined genetic endpoints would be selected (e.g. sentinel phenotypes) the required large sample sizes would seem prohibitive, since documentation on past programmes is limited and local demography would render the reliable tracking of substantial numbers of offspring of treated persons an almost impossible task; (d) in most endemic countries proper diagnosis and registration of inherited defects is largely lacking; (e) the problems encountered in demonstrating inherited effects in humans after heavy or chronic exposure to established animal mutagens such as ionizing radiation and cancer chemotherapy, in combination with the ambiguous nature of the animal germ cell data with hycanthone, do not particularly warrant large expectations; (f) since non-mutagenic antischistosomal drugs are now in use, the problem is academic and of low priority in the endemic countries whose medical and research resources are often limited. Thus, studying offspring of hycanthone-treated people to demonstrate the mutagenic potential of the drug in man is not a viable enterprise.