Evidence obtained in human breast cancer cell lines in culture suggests that estradiol stimulates the synthesis of secretory proteins which may, in turn, mediate its mitogenic effect. We questioned whether a similar mechanism could mediate the growth-promoting effect of PRL in the N-nitrosomethylurea-induced rat mammary tumor grown in soft agar, where PRL exerts a dose-dependent colony-stimulating effect. Conditioned medium obtained from PRL-treated tumors, but not from control tumors, was found to exert a significant dosedependent colony-stimulating effect when added to N-nitrosomethylurea-induced mammary tumors plated in soft agar under serum-free medium conditions. The growth-promoting action of conditioned medium from PRL-treated tumors was abolished by pretreatment with heat, trypsin, and Concanavalin-A, suggesting the possible glycoprotein nature of the oPRL-induced growth factor(s). These results provide support for the novel hypothesis that estradiol and PRL may support the growth of hormoneresponsive breast cancer through a similar mechanism.
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