Effect of tamoxifen and α-difluoromethylornithine on clones of nitrosomethylurea-induced rat mammary tumor cells grown in soft agar culture

N-Nitrosomethylurea-induced mammary tumors were grown in a bilayer soft-agar culture system. The antiestrogen tamoxi-fen prevented formation of 50% of colonies formed in this in vitro system. Possible mediation of these antimitotic effects through the polyamine pathway was suggested by a similar inhibition of colony formation by difluoromethylornithine (DFMO), a suicide inhibitor of ornithine decarboxylase, and the lack of additivity of DFMO and tamoxifen. The cytostatic effect of DFMO was found to be dose dependent. The specificity of the DFMO effect through the polyamine pathway was indicated by the dose-dependent rescue of colony growth with exogenous administration of putrescine, the polyamine distal to the site of inhibition. A lack of alteration of colony size in proliferating clones was uniformly observed. These data indicate that the soft-agar culture technique can be successfully used to investigate the endocrine mechanisms affecting the growth of individual experimental mammary cancers. The data also suggest an important role of polyamines in mediating the growth of this mammary tumor model.