Natural carotenoids and cancer

Author

H Nishino

Proceeding

6th Asian Congress Of Nutrition: Nutritional Challanged & Frontiers Towards Year 2000, Kuala Lumpur, Malaysia.

Date

16/9/1991

Keyword

natural product, carotenoids, green and yellow vegetables, biological activity, anticancer.

Abstract

Epidemiological investigations have shown that cancer risk is inversely related to the consumption of green and yellow vegetables. β-Carotene which is known to have the highest potency to serve as pro-vitamin A and which is present in abundance in green and yellow vegetables, has been proposed to be one of the key principles to prevent cancer. In fact, p-carotene was proved experimentally to prevent carcinogenesis induced by chemicals or viruses.β-Carotene is one of the most widespread natural carotenoids, but it is often associated with other carotenoids, such as α-carotene, α-carotene, lycopene and so on. Therefore, it is of interest to investigate the biological activity of these various kinds of carotenoids more extensively. Recently, we found that palm oil-derived natural carotene which consists of 60%β-carotene, 30% α-carotene and 10% others, remarkably suppressed the promotional stage of carcinogenesis. In this context, we compared the anti-tumour-promoting activity of α-carotene with that of  β-carotene. Since the lung is suggested epidemiologically to be a possible target organ for carotenoids to suppress carcinogenesis, it is of particular interest to investigate the effect of these carotenoids on the lung carcinogenesis. In this study, we examined the effect of α- and β-carotene on the tumour-promoting action of glycerol in 4NQC-initiated mice. The animals used were 6-week-old ddY male mice. Initiation of carcinogenesis was performed by a single subcutaneous injection of 0.3 mg of 4NQO on the first experimental day. Glycerol, a tumour promoter, was dissolved in water and 10% solution was given as drinking water ad libitum from the beginning of experimental week 5. α- and β-carotene (at the concentration of 0.05%) or vehicle was mixed in drinking water. Mice were killed at week 30 by cervical dislocation. At autopsy, the lungs were fixed via intratracheal instillation of 10% formaldehyde. After separation of each pulmonary lobe, the number of induced tumours was counted under a microscope. Administration of a-carotene resulted in the decrease of the mean number of tumours per mouse to about 23% of the control group (p < 0.01, Student's t-test). p-Carotene also showed the tendency to suppress the lung tumour formation, but the effect was not statistically significant. Thus, it is apparent that α-carotene has more effective anti-tumour-promoting activity than β-carotene in lung carcinogenesis. The higher potency of α-carotene than β-carotene to suppress tumour promotion was confirmed by other experimental system of two-stage carcinogenesis in vivo; i.e., α-carotene was proved to have stronger effect than p-carotene to suppress the promoting activity of TPA on skin carcinogenesis in DMBA-initiated mice. Since the vitamin A activity of  α-carotene is about half of that of β-carotene, it is unlikely that the anti-carcinogenic effect of these carotenoids reflects their vitamin A activity. The mechanism of the anti-tumour-promoting effect of  α- and β-carotene remains to be elucidated. From these results, the more extensive investigations of biological activity of not only β-carotene but also of α-carotene and other kinds of carotenoids seem to be important to evaluate the anti-carcinogenic effect of natural carotenoids in daily foods, which might play significant role to reduce the incidence of human cancer and might be useful for the purpose of cancer chemoprevention.