Permissive role of polyamines in the cooperative action of estrogens and insulin or insulin-like growth factor I on human breast cancer cell growth.

Polyamines have been proposed as specific mediators of estrogen action in breast cancer cells, but their exact role in this process is still controversial. As estrogens cooperatively interact with peptide growth factors in several hormonal responses, the involvement of polyamines in the synergistic effect of 17 beta-estradiol (E2) and insulin or insulin-like-growth factor I (IGF-I) on cell growth, polyamine pools, specific gene induction, and cell cycle progression was examined in estrogen-responsive MCF-7 and ZR-75-1 human breast cancer cells. Spermidine depletion induced by the ornithine decarboxylase inhibitor, alpha-difluoromethylornithine (DFMO), resulted in complete cytostasis and loss of mitogenic response to either E2 or insulin (or IGF-I). In contrast, a steroidal antiestrogen blocked the mitogenic effect of E2, but only partly interfered with the synergistic stimulation of estrogen action by insulin. Whereas antiestrogen-resistant growth in insulin-treated cells was halted by DFMO, the antiestrogen did not further inhibit growth upon prior polyamine depletion. E2 and either IGF-I or insulin induced early increases in putrescine and spermidine, but not spermine, contents in both MCF-7 and ZR-75-1 cells. Moreover, spermidine depletion and decarboxylated S-adenosylmethionine accumulation induced by DFMO required prior mitogenic stimulation by E2 and/or IGF-I. The antiestrogen alone had only a limited effect on polyamine and nucleoside pools. DFMO did not interfere with the coordinate induction of the estrogen- and growth factor-inducible pS2 messenger ribonucleic acid by E2 and insulin even after a 5-day treatment with the drug. On the other hand, DFMO depressed the cycling fraction of E2/IGF-I-stimulated MCF-7 cell population far more dramatically than the antiestrogen and to less than that noted in mitogen-deprived cells. However, in ZR-75-1 cells, which have a much lower spermidine/spermine ratio than MCF-7 cells, specific inhibition of spermine synthase selectively antagonized the effect of E2 compared with that of insulin. These data indicate that spermidine has a permissive role for macromolecular synthesis and cell cycle traverse, but does not qualify as a limiting factor in estrogen receptor-mediated events per se in breast cancer cells. Moreover, polyamine depletion is an efficient complementary strategy to block the mitogenic action of peptide growth factors, which is only partly antagonized by antiestrogens.