Polyamines and the synthesis of estradiol-regulated growth factors in rat mammary cancer in culture

We have recently provided evidence to suggest that the polyamine pathway plays an essential role in the expression of the growth-promoting effect of estradiol (E₂) regulated growth factors in the N-nitrosomethylurea (NMU) induced rat mammary tumor cultured in vitro in the soft agar clonogenic assay. To further explore the interaction between the polyamine pathway and autocrine control of tumor growth by E₂, we tested whether, in our system, polyamines play a role in the synthesis of E₂-regulated growth factors. Conditioned medium (CM) obtained from tumors treated with E₂ and the polyamine biosynthesis inhibitor α-difluoromethyl-ornithine (DFMO) (1 mM) no longer exhibited the colony-stimulating effect which was consistently observed with E₂-CM. Such growth promoting activity was restored in a dose-dependent fashion with CM obtained from tumors treated with E₂, DFMO, and increasing concentrations of spermidine (from 1 to 100 μM). Conditioned medium obtained from tumors treated with DFMO with and without spermidine in the absence of E₂ had no discernible effects on colony formation. The colony stimulating effect of the CM employed could not be accounted for by the contaminating presence in the media of E₂, DFMO, or polyamines. These results indicate that, in our system, the polyamine pathway plays an important role in the synthesis of E₂-regulated growth factors.