Arsenic exposure and its health effects and risk of cancer in developing countries: micronutrients as host defence.

Arsenic (As) is a ubiquitous metalloid found in several forms in food and the environment, such as the soil, air and water. The predominant form is inorganic arsenic in drinking water, which is both highly toxic and carcinogenic and rapidly bioavailable. As is currently one of the most important environmental global contaminants and toxicants, particularly in the developing countries. For decades, very large populations have been and are currently still exposed to inorganic As through geogenically contaminated drinking water. An increased incidence of disease mediated by this toxicant is the consequence of long-term exposure. In humans, chronic ingestion of inorganic arsenic (> 500 mg/L As) has been associated with cardiovascular, nervous, hepatic and renal diseases and diabetes mellitus as well as cancer of the skin, bladder, lung, liver and prostate. Contrary to the earlier view that methylated compounds are innocuous, the methylated metabolites are now recognized to be both toxic and carcinogenic, possibly due to genotoxicity, inhibition of antioxidative enzyme functions, or other mechanisms. As inhibits indirectly sulfhydryl containing enzymes and interferes with cellular metabolism. Effects involve such phenomena as cytotoxicity, genotoxicity and inhibition of enzymes with antioxidant function. These are all related to nutritional factors directly or indirectly. Nutritional studies both in experimental and epidemiological studies provide convincing evidence that nutritional intervention, including chemoprevention, offers a pragmatic approach to mitigate the health effects of arsenic exposure, particularly cancer, in the relatively resource-poor developing countries. Nutritional intervention, especially with micronutrients, many of which are antioxidants and share the same pathway with As, appears a host defence against the health effects of arsenic contamination in developing countries and should be embraced as it is pragmatic and inexpensive.

Lack of promoting effects of phenobarbital at low dose on diethylnitrosamine-induced hepatocarcinogenesis in TGF-alpha transgenic mice.

Phenobarbital (PB), a rodent non-genotoxic carcinogen, showed hormesis, biphasic effects on rat liver carcinogenesis. To test the hypothesis that the hormesis earlier observed for PB induced hepatocarcinogenesis might also exist in the TGF-alpha transgenic mice model, one which is highly susceptible to carcinogenesis, the carcinogenic or promotion effects of a wide range of phenobarbital (PB) concentrations were investigated. Two weeks after a single i.p. dose of 5 mg /kg bw of diethylnitrosamine (DEN) to 15 day old mice, animals were treated with diet containing PB at doses of 0, 2, 15 or 500 ppm. The incidence and multiplicity of tumors, including hepatocellular adenomas and carcinomas, were significantly increased by the high dose of PB, but no significant difference among the groups receiving 2 and 15 ppm for liver tumors when compared to DEN alone group. The proliferating cell nuclear antigen indices for liver tumors and surrounding hepatocytes in high dose PB treated mice were significantly increased, but no change was noted at the lower doses. The total cytochrome P450 content in the liver was also elevated by 500 ppm of PB, while hepatic 8-OHdG levels demonstrated no significant change. In conclusion, PB at high dose enhances DEN-induced hepatocarcinogenesis in TGF-alpha transgenic mice, but low doses lack any significant effects. One possible mechanism of phenobarbital carcinogenicity might be influenced by cytochrome P450 system exhibiting a strong promoting activity for liver of mice.

Inhibition of rat urinary bladder carcinogenesis by the antiangiogenic drug TNP-470.

Potential inhibitory effects of the antiangiogenic drug TNP-470 on rat urinary bladder carcinogenesis were investigated in F344 male rats initiated with 0.05% BBN in the drinking water for 8 weeks. Group 1 was then continuously treated with TNP-470 by subcutaneous injection using osmotic minipump until the end of the experiment; group 2 served as the control with only initiation. The incidences and multiplicities of papillomas and carcinomas in the TNP-470-treated group were significantly decreased compared to the control group values along with the tumor vascular density. In conclusion, TNP-470 can inhibit rat urinary bladder carcinogenesis, presumably through its effects on angiogenesis.

Effects of cessation of alcohol exposure on rat hepatocarcinogenesis.

Cessation of long-term alcohol exposure is reported to enhance rat hepatocarcinogenesis. The purpose of the present study was to assess this possibility using glutathione-S transferase placental form (GST-P) positive foci as end point lesions. All rats were treated with a single i.p. injection of diethylnitrosamine (DEN) (200 mg/kg body weight) and then given a MF pellet diet for 2 weeks. Thereafter, the animals were maintained on: alcohol liquid diet in which 36% of total calories were provided by alcohol (5% Al diet) for 6 weeks (group 1); control liquid diet (C diet) for 6 weeks (group 2); 5% Al diet for 6 weeks and subsequently C diet for 4 weeks (group 3); 5% Al diet for 10 weeks (group 4); or C diet for 10 weeks (group 5). All rats were subjected to two thirds partial hepatectomy at 3 weeks after DEN injection. The number and area of GST-P positive foci per cm2 of liver tissue were slightly increased in group 1 compared to the group 2 and significantly elevated in the group 4 compared to group 5. However, numbers in group 3 were significantly lower in group 4 and similar to the group 5 values. PCNA positive cells in the GST-P positive foci in the group 1 and group 4 were significantly increased as compared with respective controls (groups 2 and 5, respectively), while indices in the group 3 were again similar to values for group 5. Cessation of short-term alcohol administration thus had no promoting effects on development of GST-P foci, suggesting that the duration of alcohol treatment may be important. The results also imply the existence of a cumulative exposure time or dose threshold for alcohol if promoting effects of cessation are to be seen on rat hepatocarcinogenesis.

Inhibition of azoxymethane-induced colon carcinogenesis in rats due to JTE-522, a selective cyclooxygenase-2 inhibitor.

Prostaglandin E2, which is produced by cyclooxygenase (COX) during arachidonic acid metabolism, is considered to be related to colon carcinogenesis and selective COX-2 inhibitors may be effective for chemoprevention without the adverse side effects of non-selective, nonsteroid anti-inflammatory drugs. Therefore, the influence of JTE-522 (4-(4-cyclohexyl-2-methyloxazol-5-yl)-2-fluorobenzensulfonamide), a selective COX-2 inhibitor, was examined in azoxymethane(AOM)-induced rat colon carcinogenesis. A total of 40 male F344 rats were randomly divided into two groups. Group 1 received diet containing 0.015% JTE-522 and group 2 the normal diet without supplement as a control group; one week later, all rats were administered axozymethane (AOM) s.c. at a dose of 15 mg/kg body weight once a week for 3 successive weeks. At the termination of the experiment (30 weeks after the start), the multiplicity of colon cancer in group 1 was significantly less than that of group 2. The proliferating cell nuclear antigen (PCNA) indices for non-neoplastic cells of the colon mucosa in group 1 were also lower. These data thus suggest that JTE-522 has chemopreventive potential against colon carcinogenesis with decrease of mucosal cell proliferation in rats.

Appropriate models and an understanding of carcinogenic mechanisms--requirements for hazard risk assessment.

Given the immense variety of compounds developed for introduction into the human environment, appropriate carcinogen risk assessment is essential. One of the responsible international bodies recognized as providing a lead in this endeavour is the International Agency for Research on Cancer (IARC), primarily through the Monographs on the Evaluation of Carcinogenic Risks to Humans. However, serious allegations have recently been made that industry now has undue influence on the decisions of the IARC Workshops as to category assignment, especially concerning down-grading of risk. The contention is that too much stress is placed on mechanistic considerations which have not been sufficiently validated. Since avoidance of carcinogens in our environment is clearly of prime importance to cancer prevention, open discussion of how they should be identified is of essential significance to the APOCP. Clearly, decisions should be based solely on scientific evidence and there should be no place for politics or polemic. We have therefore looked, in what we hope is a dispassionate fashion, at the arguments offered in the recent literature, while admitting to a bias towards taking into account all the available knowledge on mechanisms of action of carcinogens and modulating agents. As scientists, generation of an understanding of this area is one of the main reasons why we receive our salaries. To blindly argue that carcinogenicity, for example at high dose in one strain of experimental animal, necessarily implies human risk at normal levels of exposure is obviously untenable. At the same time, precipitous conclusions regarding species-specific mechanisms must naturally be avoided. Both academic and industrial researchers need to apply a balanced judgement and to simply imply that any association with industrial concerns is likely to lead to irresponsible behaviour to the detriment of public health is not tenable. With regard to regulatory decision making, we should be concentrating more attention on mechanisms, rather than less, especially in light of recent findings pointing to hormesis at low doses of carcinogens, which will inevitably generate heated discussion and the charge of bias in favour of industry. The onus is on all members of the scientific community to impartially view all the epidemiological and experimental data which are available in decision-making.

No inhibition of urinary bladder carcinogenesis in rats with intravesical instillation of alpha-galactosylceramide.

The immunostimulatory a-galactosylceramide, KRN 7000 ((2S,3S,4R)-1-O-(a-D-galactopyranosyl)-2-(N-hexacosnoylamino)-1,3,4-octadecatrienol), may be anticipated to have antitumor activity in vivo apart from any direct toxicity to cancer cells. In this experiment, inhibition of rat bladder carcinogenesis by intravesically instillated KRN7000 was investigated. Male Fischer 344 rats, 6-weeks-old at the start, were divided into 4 groups, all first receiving the carcinogen, 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine, in their drinking water for 12 weeks. Then groups 1 and 2 respectively were administered 500 and 50 mg/kg of KRN7000 intravesically once weekly for 17 weeks. Group 3 similarly received only 0.3 micro/l of saline (vehicle control). Group 4 did not undergo bladder catheterization. On macroscopic examination at 30 weeks, multiplicities and sizes of bladder tumors in the KRN 7000 high and low-dose groups were not significantly different from those of the vehicle control group. Histologic examination confirmed no significant variation in incidences of carcinomas or preneoplastic lesions in the bladder among groups 1 to 4. Thus the results indicate that intravesical instillation of KRN7000 does not inhibit bladder carcinogenesis in rats.