Hiroyuki Tsuda

Transgenic rats carrying human c-Ha-ras proto-oncogene (Hras128 rats) have been shown to be highly susceptible to induction of tumors. We have found an early induction of tongue tumors in Hras128 rats treated with 4-nitroquinoline 1-oxide (4NQO). 4NQO was administered to the Hras128 and wild-type Sprague-Dawley (SD) rats for 4 and 8 weeks, respectively. The experiment was terminated at 14 (Hras128 rats) and 28 (SD rats) weeks. Either during or after treatment with 4NQO, dysplastic hyperplasia, papilloma and squamous cell carcinoma were found on the tongue of both Hras128 and wild-type rats, with a higher incidence and multiplicity in Hras128 rats. Treatment of the Hras128 rats with 4NQO significantly increased cell proliferation in the tumor compared to the control rats. In the tongue tumors of the Hras128 rats, there was a significant increase in the mRNA expression levels of cyclin D1 and COX2. To examine whether this experimental system is useful for screening of the candidate agents for cancer preventive effect, nimesulide, a selective COX2 inhibitor, was tested in the present model. Nimesulide significantly decreased total multiplicity of tongue lesions compared to the control rats. Treatment of Hras128 rats with nimesulide caused a significant decrease in the levels of mRNA expression of cyclin D1 and COX2 in the tumor. Therefore, the current 4NQO-induced Hras128 rat tongue carcinogenesis model provides a simple and rapid system for investigating carcinogenesis process and evaluating the effect of possible cancer preventive agents for human tongue cancer.

Environmental lighting conditions affect circadian rhythm and carcinogenesis. The effect of diethylnitrosamine (DEN, i.p., 200 mg/kg) on carcinogenesis and circadian rhythmicity under a light-dark (LD) cycle, constant dark (DD) and constant light (LL) was analyzed in rats. After the recognition of entrainment in locomotor activity rhythm to LD cycle, animals remained under the LD cycle or were released into DD or LL. Liver carcinogenicity, measured by GST-P immunostaining, was higher under the LD cycle than under DD and LL. Two weeks after DEN injection, locomotor activity in 24 hr had increased under the LD. Circadian rhythmicity might be coupled with the carcinogenicity of DEN.

Appropriate animal models for specific diseases in man can facilitate elucidation of mechanisms underlying tumour development and allow potential interventions and therapeutic regimens to be tested in vivo before consideration for use in the human situation. In the North-east of Thailand exceptionally high levels of cholangiocellular carcinomas (CCCs) are encountered, related to infestation with Opisthorchis viverrini liver flukes. The Syrian hamster can also be infected with metacercariae of the fluke and heavy loads of parasites cause the development of cirrhotic livers. While the presence of flukes alone does not give rise to neoplasms, large yields of cholangiofibrotic lesions and CCCs can be readily induced with additional carcinogenic insult. While removal of the parasite with the antihelminthic drug Praziquantel can protect against carcinogenesis, this is dependent on the timing of the drug administration and the efficacy of application to the human situation remains to be confirmed. The available information would suggest that interest needs to be concentrated on potential chemopreventive agents which could be administered to individuals at high risk. Furthermore, understanding of the genesis of CCCs and the characteristics of preneoplastic lesions, again as assessed in the animal model, might allow novel approaches to identification of early stage cases and effective surgical intervention.

In response to the request for 'Breakthrough Questions' for 'Grand Challenges in Global Health' recently published in Nature, the Asian Pacific Organization for Cancer Prevention should focus its attention on what projects are of the highest priority for integration with its Practical Prevention Program (PPP). The most common female cancers in most of the countries of Asia are carcinoma of the breast, followed by the uterine cervix. While the incidences of breast adenocarcinomas are still generally lower than in the Western world they are rapidly increasing, and squamous cell carcinomas of the cervix are a major problem. Clearly there are many areas which would reward research. One factor which appears of major relevance in the mammary gland case is the diet, and particularly the phytoestrogens included in 'tofu', along with physical exercise. The age at which these could be operating needs to be elucidated, with reference to timing of menarche and menopause, and also breast mammographic density, another predictor of likelihood of neoplasia. In the cervix, the predominant influence is well established to be persistent infection with a high risk 'oncogenic' type of human papilloma virus (HPV). Vaccines therefore hold much promise, but a better understanding of the mechanisms underlying spontaneous clearance of both infection and cervical intraepithelial neoplasia (CIN) of different grades is also essential for optimal intervention. The roles of smoking and antioxidant intake in particular deserve emphasis. In Asia, with the considerable variation evident in both breast and cervical cancer incidence rates, as well as in cultural and other environmental factors, we are in a very favourable position to meet two specific challenges: 1). elucidation of how diet in adolescence determines susceptibility to neoplasia of the mammary glands; and 2). determination of what governs persistence of HPV infection. Realisation of these pivotal research aims, with especial emphasis on the context of the PPP, is our shared goal.

Although a great deal of concern has been raised about the hazard potential of endocrine disruptors present in the environment, the in vivo data available from both experimental and epidemiological studies suggest that the majority of those agents do not pose a risk with regard to cancer development. Indeed, naturally occurring examples such as isoflavonoids even appear to exert protective effects. Only for xenobiotics such as 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), polychlorinated biphenyls (PCBs) and tetrachloro-p-dioxin (TCDD) and special cases of phenols and phthalates is there unequivocal evidence of carcinogenicity and this appears to be directly linked to their toxicity. Thus, careful in vivo assessment is required before drawing any conclusions regarding agents capable of affecting the mammalian endocrine system.

In order to determine which areas of cancer screening are currently receiving greatest emphasis in different parts of the world a Medline search of the literature for the period 2000-2002 was performed, concentrating attention on research into all aspects of efforts for early detection of tumours, with especial attention to methodology, motivation (including awareness of utility in the general populace and in minority groups), and intervention (professional training and general education). Focus on the skin, lung, cervix, breast, ovary + endometrium, oral cavity-oesophagus, gastric, colorectal, kidney + urinary tract and prostate, demonstrated large numbers of journals to be publishing papers in the field, with 10, 33, 130, 53, 24, 21, 6, 81, 12 and 58, respectively, in the period investigated, the grand total being 259. The average numbers of papers/journal ranged from 1.0-2.4 with only 15-35% appearing in journals with wide coverage. With the exception of oral, oesophageal and gastric cancer screening, an approximately 50% contribution in all areas was made by scientists in the US, followed by Europe (31% overall,) Asia (11%) then Australasia, Central and South America and Africa (3%, 2% and 1%, respectively). Clear differences were evident with the organ regarding specific topics receiving attention, most publications concerning the lung, ovary and urological tract dealing with detection methods. With the cervix and colorectum this topic accounted for half of the papers with especial attention to the relative advantages of the PAP smear, HPV testing and direct visual acetic acid (DVA) in the one, and FOBT and endoscopy in the other. Another major focus was found to be minority attitudes to breast, prostate and cervical screening in the US, whereas only few papers were found dealing with practical intervention, targeting professionals or screenees to increase participation in screening programs. The present approach suggested a number of areas requiring more attention, not least being the need for more comprehensive reviews across organs to allow the general reader a better understanding of the overall picture, and which avenues might best reward exploration in the future.

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.