Modifying effect of aging on chemical carcinogenesis. A review

Cancer and aging: symposium of the 27th annual meeting of the Japanese society for biomedical gerontology, Tokyo

Although many hypotheses have been proposed to explain the strong link between aging and cancer, the exact mechanisms responsible for the increased frequency of occurrence of cancer with advancing age have not been fully defined. Recent evidence indicates that malregulation of the apoptotic process may be involved in some aging process as well as in the development of cancer. Although it is still under debate how apoptosis is expressed during aging in vivo, this phenomenon is an important factor in unwinding the complicated mechanisms that link cancer and aging. In this review, we report on the discussion at the symposium of the 27th annual meeting of the Japanese society for biomedical gerontology, regarding recent findings from aging and carcinogenesis studies using animal models, the characteristics of cancer in patients with Werner's syndrome, the epigenetic changes in human cancers and aging, and the characteristics of human cancers in the elderly. It was concluded that apoptosis plays a role in the aging process and carcinogenesis in vivo, likely as an inherent protective mechanism against various kinds of damages to genes/chromosomes.

Effects of age on endometrial carcinogenesis induced by concurrent oral administration of ethylenethiourea and sodium nitrite in mice

Aging effects on the susceptibility to chemical endometrial carcinogenesis were investigated in ICR female mice. The animals were divided into 3 groups of different ages: 1 month (young), 6 months (middle), and 12 months (old) at initiation of treatment. They received weekly oral administration of mixture of ETU (100 mg/kg body weight) and sodium nitrite (70 mg/kg body weight) for 6 months followed by a withdrawal period of 3 months. All animals were subjected to histopathology. The incidence of endometrial adenocarcinomas was highest in the middle age group (8/20), secondary in the old age group (4/20), and lowest in the young group (1/20). The incidence of atypical glandular hyperplasia, a precursor lesion of the tumor, was also higher in the middle age group. The endometrial adenocarcinomas showed morphological similarities among all age groups and the nuclei of tumor cells lost almost all staining reactivity to estrogen receptors. The labeling indices with bromodeoxyuridine (BrdU) were notably higher in the old age group than in the young and middle age groups. A further investigation on the aging process of female genital organs in control mice revealed that their senility seemed to be preceded by the formation of ovarian cysts which first appeared at 6 months of age with a concomitant elevation of plasma 17 beta-estradiol level. These results indicate that the susceptibility of the mouse endometrium to the carcinogenic effects of N-nitroso ETU could be closely linked with the stage of aging process of the genital organs and it appears to be most susceptible when initiated at around 6 months of age. However, the mitotic activity of neoplastic endometrial glandular cells seems to be higher in older mice than younger ones.

Influence of aging on multi-organ carcinogenesis in rats induced by N-methyl-N-nitrosourea

The effects of aging on the multi-organ carcinogenesis induced in rats by N-methyl-N-nitrosourea (MNU), a direct carcinogen which does not need metabolic activation to exert carcinogenicity, were examined in male F344 rats. In the first experiment, rats at 6, 52, and 98 weeks of age were treated with MNU (20 mg/kg body weight, i.p.) twice weekly for 6 weeks and then maintained without any further treatment for 24 weeks in the case of young and middle-aged rats and for 18 weeks in the case of the old rats. In young rats, malignant lymphomas, particularly thymic types, were observed at significantly high incidence. A striking result in the middle-aged rats was the significantly higher incidence of adenocarcinomas in the small intestine than in young or old animals. The induction of proliferative and neoplastic lesions of the large intestine also tended to be increased in middle-aged rats. In addition, epithelial hyperplasia of the tongue, but not the forestomach, occurred at the highest incidence in the middle-aged group. There were no differences in the induction of epithelial lesions in the urinary bladder among the groups. In a second experiment, investigation of DNA synthesis in the tongue, small and large intestines, urinary bladder and lymph nodes did reveal significant increases or tendency for increase in the MNU-treated groups, but without differences with age. In contrast, the thymus of young rats showed significantly increased incorporation of BrdU label after administration of MNU, whereas it was markedly reduced in middle-aged rats. In a third experiment, O6-methyldeoxyguanine (O6-medG) DNA adduct formation was immunohistochemically detected in various organs including the thymus, forestomach, and small intestine without any differences with age. Thus, the results demonstrated that while the target organs of MNU are modified by the age of the animals, levels of DNA synthesis and O6-medG DNA adduct formation in most cases can not explain the observed differences in carcinogenic susceptibility.

The potential relationships between aging and cancer

The potential relationships between aging and cancer have received considerable attention in the scientific literature in recent years. While it is clear that the rates of most types of cancer increase with advancing age and that both the processes of aging and those of cancer are time dependent, an unequivocal relationship between the etiology of cancers and the mechanistic processes of aging has yet to be established. This article discusses the potential causal relationships between the processes of aging and the etiologies of most cancers.

Age-dependent induction of preneoplastic liver cell foci by 2-acetylaminofluorene, phenobarbital and acetaminophen in F344 rats initially treated with diethylnitrosamine

Effects of age on the induction of glutathione S-transferase placental form (GST-P)-positive hepatic foci in rats were examined using a medium-term liver bioassay system (for carcinogens). F344 male rats aged 6, 26 and 46 weeks were initially given a single intraperitoneal injection of diethylnitrosamine (DEN, 200 mg/kg) and, beginning 2 weeks later, received 0.02% 2-acetylaminofluorene (2-AAF), 0.05% phenobarbital (PB) or 1.3% acetaminophen (AAP) in the diet for 6 weeks. All animals were subjected to two-thirds hepatectomy 3 weeks after the DEN injection and were killed at week 8. Quantitative analysis of GST-P-positive foci revealed significantly (P less than 0.001) increased induction over control levels in terms of both numbers and areas for 2-AAF at all ages (6, 26 and 46 weeks), but especially in the 6-week-old case. In the PB- and AAP-treated groups, the respective enhancing and inhibitory influences were most pronounced in the animals aged 6 weeks, and were less marked in older rats. Thus, the response of F344 rats to the modifying effects of chemicals was age-dependent, the conclusion being drawn that young rats are more susceptible and therefore more appropriate for assessment of carcinogenic, promoting and inhibitory effects of chemicals.

Effects of age on multiple organ carcinogenesis induced by 3,2'-dimethyl-4-aminobiphenyl in rats, with particular reference to the prostate

The effects of age on multi-organ carcinogenesis induced by 3,2'-dimethyl-4-aminobiphenyl (DMAB) in male F344 rats were examined. Groups of 5-, 35-, and 65-week-old animals were given 4 weekly sc injections of DMAB at a dose of 200 or 150 mg/kg body weight. Prostate carcinomas were induced in 8 to 19% of rats treated, no significant differences in the incidence between different ages being observed. Tumors in the small intestine, skin, pancreas and peritoneum, however, developed more frequently in young than in old animals, whereas higher incidences of testis, preputial and mammary gland lesions were found in the 35- and/or 65-week-old groups. Colon and Zymbal gland carcinogenesis did not reveal any age dependence.

Age related variations of hepatocarcinogenic effect of azo dye (3'-MDAB) as linked to the level of hepatocyte polyploidization

Four-week-old CBA mice fed a diet containing the hepatocarcinogenic azo dye 3'-MDAB showed a rapid polyploidization of hepatocytes, a sharp increase of two liver-specific acid soluble non-histone proteins (LSP 1 and 2) and induction of hepatomas between 44 and 52 weeks of the regimen. More mature 18-week-old mice fed the same diet did not develop induced hepatocarcinogenesis after 55 weeks of the regimen. Interruption of the azo dye regimen showed that the increase of LSP 1 and 2 was reversible, whereas the carcinogenic effect and polyploidization were irreversible. Sprague-Dawley rats were more sensitive to the carcinogenic effect of the azo dye regimen. It is suggested that the higher resistance of older mice to the carcinogenic effect could be linked to the higher level of hepatocyte polyploidization and that the increase of LSP 1 and 2 is relevant to the toxic effect of the azo dye.

Rapid development of hepatocellular neoplasms in aging male C3H/HeNCr mice given phenobarbital

One hundred and nineteen male C3H/HeNCr mice, 12 months of age, with spontaneous preneoplastic and neoplastic hepatocellular lesions were given phenobarbital (PB) at 500 ppm in drinking water. Groups of 9-10 mice were sacrificed after 12, 24 and 36 weeks of PB exposure. Identical numbers of untreated controls were used. A group of 6-week-old C3H/HeNCr mice were also given PB and sacrificed at 12, 24 or 36 weeks. In aging mice, PB exposure significantly increased the number of gross tumors or microscopic foci, adenomas or carcinomas per liver at all time periods, especially unique eosinophilic proliferative lesions, while young mice did not develop any focal proliferative lesions by 36 weeks. These findings suggest that in aging mice a fraction of the hepatocyte population (normal, spontaneously-initiated or preneoplastic) is more highly susceptible to phenobarbital 'carcinogenesis' than are hepatocytes of younger mice.

Cancer and aging

It is widely accepted that the incidence of cancer increases with aging and many hypotheses have been put forward to explain this association. A review of the literature, however, shows that the relationship between cancer and aging has to be reappraised. Although the incidence of most cancers increases during the second half of the animal life span, only with a few does it increase progressively with senescence in humans as well as in animals. As a matter of fact in many cases the incidence does not vary, levels off or even decreases in old individuals. Late developing tumors also seem in many instances to progress more slowly. Thus it seems that the incidence of cancer is related to age rather than to aging and that some conditions may be created during senescence which oppose the development of some cancers.

Methionine-cysteine deficiency and alkylation of DNA in liver, kidney and lung of mice administered dimethylnitrosamine

The effect of methionine-cysteine deficiency on the methylation of DNA purines by dimethylnitrosamine metabolites was studied in subadult and adult mice. In liver, no dietary effect on the specific methylation of 7-methylguanine was observed, while that of 3-methyladenine decreased in the adult animals. The specific methylation of guanine in the 0(6)-position and the ratio of 0(6)-methylguanine to 7-methylguanine increased significantly after methionine-cysteine deficiency. Methylation in kidney decreased in subadult but increased in adult mice. In lung, the amount of 7-methylguanine was significantly elevated after methionine-cysteine deficiency in both the subadult and adult mice. The results demonstrate an increase in the specific methylation in liver of guanine in the 0(6)-position by the methionine-cysteine deficient diet, together with differences in the methylation pattern between organs of the two age groups.

The value and significance of life span and scheduled termination data in long-term toxicity and carcinogenesis studies

Consideration is given to the age association of lesions, the duration of long-term toxicity and carcinogenesis studies, and to the value and significance of including scheduled termination in such long-term studies. There is now enough evidence that age and cancer are associated. It is argued that the increase in incidence of lesions with age has such disadvantages that extension of the duration of the long-term toxicity and carcinogenicity study beyond 2 years is not warranted in most cases. Incorporation of scheduled termination in long-term studies gives more insight into the biologic behavior of toxic lesions and cancer and may enable one to make a distinction between "incidental" and "fatal" lesions. This distinction may be important for the statistical evaluation of data from chemical carcinogenesis studies.

Carcinogenesis and aging

Figure 2 illustrates a suggested mechanism of carcinogenesis. This scheme takes into account the effect of carcinogens at different integration levels: subcellular, tissue, and organism. Any of these levels may be age dependent. Age-associated changes in the activity of enzymes responsible for activation and inactivation of carcinogens, and variations in concentrations of lipids and proteins contributing to the transport of carcinogenic agents into cells, may play an important role in the modifying effect of age on carcinogenesis. The effects of age-associated changes in DNA repair need clarification. However, they are thought to exert a permissive influence on the age-associated rise in tumor incidence. It seems that proliferative activity of target tissues is the important modifying factor of carcinogenesis. Age-related changes of regulation at tissue and organism levels are also powerful factors in carcinogenesis modification. Age-dependent changes in the neuroendocrine system provide conditions for metabolic immunodepression and promotion of carcinogenesis. On the other hand, carcinogens per se (especially chemical and radiological) may intensify aging processes in the organism. Normalization, by drugs, of age-associated shifts requiring synthetic and energetic changes of a transformed tumor cells, and of immunological shifts, may exert both antitumor and geroprotective effects.

Carcinogenesis and aging. IV. Effect of low-molecular-weight factors of thymus, pineal gland and anterior hypothalamus on immunity, tumor incidence and life span of C3H/Sn mice

The low-molecular-weight polypeptide factors were obtained from bovine thymus (TF), pineal gland (PF) and anterior hypothalamus (AHF). Both TF and PF administration enhanced the rejection of skin allograft and stimulated the immunological response to sheep erythrocytes in adult CBA mice. Treatment of CBA mice with AHF increased the graft survival and inhibited antibody formation to sheep erythrocytes. Chronic TF or PF administration decreased spontaneous tumor development and prolonged the life span of female C3H/Sn mice. Administration of AHF failed to influence the life span and the tumor incidence of female C3H/Sn mice. The role of immunity and hormonometabolic shifts in mechanisms of both aging and the age-associated increase in cancer incidence are discussed.