Chemoprevention of mammary carcinogenesis in a transgenic mouse model by α-difluoromethylornithine (DFMO) in the diet is associated with decreased cyclin D1 activity

Mechanisms underlying the chemopreventive effect of difluoromethylornithine (DFMO) on the development of mammary cancer were investigated utilizing the whey acidic protein promoter-T antigen transgenic mouse model of breast cancer progression. Mice were exposed to four different doses of DFMO in the diet (3.5, 4.9, 7.0 and 10g/kg diet). Tumor latency was increased in a dose-dependent manner. DFMO at the highest dose significantly delayed tumor onset (131 days as compared to 109 days in control unexposed mice, P = 0.018). Analyses of preneoplastic mammary tissue collected 1 month after DFMO treatment demonstrated that DFMO (10g/kg diet) significantly increased the ratio of apoptotic to proliferative indices (P = 0.013) and significantly reduced the percentage of cells demonstrating nuclear localized cyclin D1 (P = 0.013). Nuclear localizations of p27, p21 and Stat5a were not affected. Inhibitory effects of DFMO on cell growth and survival were lost as the cells progressed to cancer. In conclusion, the chemopreventive effects of DFMO on mammary cancer progression were mediated by changes in both apoptosis and cell proliferation in preneoplastic cells. Alterations in cyclin D1 activity in preneoplastic cells could represent an early biomarker of chemopreventive action and are consistent with a mechanistic role for cyclin D1 in progression of mammary cancer.