Inhibition of intestinal tumor development in rat multi-organ carcinogenesis and aberrant crypt foci in rat colon carcinogenesis by 22-oxa-calcitriol, a synthetic analogue of 1α,25-dihydroxyvitamin D3

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Abstract
The modifying effects of 22-oxa-calcitriol (OCT), a synthetic analog of 1α,25-dihydroxyvitamin D3, were assessed in a multi-organ carcinogenesis model using male F344 rats initially treated with five kinds of carcinogens. In experiment 1 the rats were given OCT intraperitoneally at doses of 30 μg/kg (25 rats) or 3 μg/kg (25 rats), three times a week for 24 weeks after initial carcinogen exposure over 4 weeks and a 2 week non-treatment period. Twenty-two rats received the five carcinogens and were given the vehicle intraperitoneally as a control. A further group of 10 rats was given the 30 μg/kg dose of OCT without prior carcinogen application. At the end of the total observation period of 30 weeks the carcinoma incidence in the small intestine of rats given 30 μg/kg OCT after carcinogen treatment was 0%. This incidence was significantly smaller when compared with the control group. The incidence of large intestine carcinomas in the 30 μg/kg OCT group showed a tendency to decrease. The numbers of small and large intestinal carcinomas per rat were also significantly lower in the group given 30 μg/kg OCT than after 3 μg/kg OCT or carcinogens alone. Attention was, therefore, focused on colon carcinogenesis and in experiment 2 30 μg/kg OCT administered six times a week to rats for 8 weeks after the last injection of N,N'-dimethylhydrazine (DMH) exposure. OCT significantly reduced the formation of DMH-induced aberrant crypt foci, considered to be putative preneoplastic lesions. In experiment 3 30 μg/kg OCT was administered six times a week to rats for 4 weeks without prior carcinogen treatment. The proliferating cell nuclear antigen labeling index for the colonic epithelium of rats given 30 μg/kg OCT was decreased. Ornithine decarboxylase and spermidine/spermine N1-acetyltransferase activities in colonic epithelium, assayed as indicators of cell proliferation, were not significantly decreased as compared with control group values. Furthermore, vitamin D receptors in colonic epithelium were not significantly increased. Thus the present study indicates that OCT can exert inhibitory effects on tumor development in the small and large intestines, although the mechanism is unclear.

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