PURPOSE: Growth factors stimulate protein phosphorylation resulting in transmission of mitogenic signals. In breast cancer, protein kinases and their substrate proteins are importnat in cell proliferation and phathogenesis. Polymine is known as a mediator of stimuli-induced proliferation in many cell systems. In the present study, we report the importance of polyamines in protein phosphorylation in MCF-7 human breast cancer cells. MATERIALS AND METGODS: Protein phosphorylation study was done by incubating cells in the DMEM containing [gamma-(32)P]-ATP. Quantitation of phosphorylation was analysed by fluorescene image analyzer. Tyrosine phosphorylation was detected by anti-phosphotyrosine antibody. Shc was detected by radioimmunoprecipitation and Western blotting. RESULTS: E2, TGF-alpha, and EGF enhanced the protein phosphorylation in very similar pattern. Among those proteins, 67 kDa protein was most strongly phosphorylated. But the most prominent tyrosine phosphoprotein was 52 kDa protein. DFMO at 5 mM strongly inhibited the phosphorylation of the most proteins. Externally added polyamine could recover the inhibitory effect of DFMO in protein phosphorylation. Among the 5 major tyrosine phosphoproteins, 52 and 46 kDa proteins appeared to be Shc proteins. CONCLUSION: Polyamines modulate signal transduction in relation with estrogen receptor and EGF receptor through multiple steps of protein phosphorylations. Tyrosine phosphorylation of Shc proteins were most significantly influenced by polyamines in growth factor-stimulate breast cancer cell proliferation.