Suppression of mammary gland carcinogenesis by post-initiation treatment of rats with tamoxifen or indole-3-carbinol or their combination.

This study examined whether suppression of mammary gland carcinogenesis elicited by low doses of tamoxifen (TAM) can be enhanced by concomitant treatment of rats with indole-3-carbinol (I3C), a component of cruciferous vegetables and a dietary supplement used for its putative antiestrogenicity. Two weeks after one oral dose of 7,12-dimethylbenz[a]anthracene (DMBA) at 65 mg/kg body weight, female Sprague-Dawley rats started treatment with TAM (10 microg/rat) by subcutaneous injection, I3C (250 mg/kg body weight) by oral gavage, TAM+I3C or their respective vehicles three times per week, for up to 20 weeks. Significant increases in the median latency of malignant mammary tumors and decreases in the mean tumor mass per rat were due to TAM. Significant decreases in the mean tumor number per rat in TAM, I3C and TAM+I3C-treated rats indicated a cooperative effect of the two compounds. In both DMBA-initiated and uninitiated rats, significant increases in the ratios of liver to body weight in I3C and TAM+I3C-treated groups coincided with I3C-dependent increases of hepatic cytochrome P450 levels and activities (1A1, 1A2 and 2B1/2). The ratios of uterus to body weight decreased with the number of treatments and the decreases effected by TAM were greater than those by I3C. The levels of circulating estrone were increased in response to I3C treatment and were greater in DMBA-initiated rats than in uninitiated rats, which may contribute to the preventive effect of I3C. Chemoprevention may be accomplished through up-regulation of apoptotic enzyme (caspase) activities in the mammary gland or mammary tumors. Treatment with TAM, I3C or TAM+I3C had no effect on caspase-3&7, caspase-6, caspase-8 and caspase-9 activities in the mammary tumors or mammary gland of tumor-bearing rats or that of uninitiated rats. In the mammary gland of DMBA-initiated tumor-free rats, however, I3C treatment increased the levels of caspase-3&7 and caspase-9 activities, suggesting an I3C-mediated protective effect. Even though I3C alone is a much less effective suppressing agent of mammary carcinogenesis than TAM, I3C in combination with TAM does not weaken but may foster the benefits of chemoprevention with TAM.

Suppression of 7,12-dimethylbenz[a]anthracene-induced mammary carcinogenesis by pre-initiation treatment of rats with beta-naphthoflavone coincides with decreased levels of the carcinogen-derived DNA adducts in the mammary gland.

BACKGROUND: Mechanisms underlying prevention by beta-naphthoflavone (beta-NF) of mammary carcinogenesis initiated with 7,12-dimethylbenz[a]anthracene (DMBA) in the rat were elucidated. METHODS AND RESULTS: Treatment of female Sprague-Dawley rats with beta-NF at 40 mg/kg b.wt. for 4 days by oral gavage in corn oil before a single oral dose of DMBA (112 mg/kg b.wt.) suppressed mammary gland carcinogenesis as shown by an increase in the median latent period from 10 to 24 weeks and a 60% decrease in the multiplicity of mammary adenocarcinomas. In contrast, a 20-day treatment with beta-NF starting 3 weeks after DMBA had no significant effects on mammary tumorigenesis. The activities of phase I and phase II enzymes were examined in the liver and mammary gland 24 h after treatment of rats with beta-NF, DMBA, or beta-NF followed by DMBA as in the first bioassay. Treatment with either beta-NF or DMBA increased the hepatic activities of cytochrome P450 (CYP)1A1, 1A2, and 2B1/2, and glutathione S-transferase, and the mammary activity of CYP1A1. The activity of mammary CYP2B1/2 induced by DMBA was decreased by beta-NF. In the liver, the increase of UDP-glucuronosyl transferase (GT) activity in rats treated with beta-NF and DMBA was 2.3-fold greater than in rats treated with DMBA alone. Thus, treatment with beta-NF likely increased the rate of glucuronidation of DMBA dihydrodiols leading to carcinogen detoxification. The levels of the DMBA adducts determined by 32P-postlabeling of the mammary gland DNA were decreased in the beta-NF-pretreated rats. CONCLUSION: The beta-NF-induced increase in the hepatic UDP-GT activity and decrease in the mammary DNA-DMBA adducts occurred under the same treatment regimen that led to suppression of DMBA-induced mammary carcinogenesis.