Role of transforming growth factor-alpha-related peptides in the autocrine/paracrine control of experimental breast cancer growth in vitro by estradiol, prolactin, and progesterone.

We have recently suggested that estradiol (E2), prolactin (oPrl), and progesterone (Pg) support the growth of the hormone-responsive N-nitrosomethylurea (NMU) rat mammary tumor in soft agar through autocrine/paracrine mechanisms. To gain insight into the nature of these hormonally regulated growth factors, we tested the ability of two monoclonal antibodies (MAb-425 and 528) directed against the epidermal growth factor receptor (EGF-R) to inhibit the colony-stimulating effects of conditioned media (CM) obtained from E2, oPrl, and Pg-treated NMU rat mammary tumors. Since both MAbs are specific for human EGF-R, MCF-7 breast cancer cells grown in soft agar in the absence of serum were used as our indicator system. Both MAb-425 and 528 totally abolished the colony-stimulating effect of genuine EGF, while having no agonistic/antagonistic action when added alone. Both MAb-425 and 528 markedly inhibited the colony-stimulating effect of rat mammary tumor E2-CM in a dose-dependent fashion. MAb-425 was also found to inhibit the growth-promoting action of Pg-CM, although this effect appeared to be somewhat less consistent and pronounced than that observed with E2-CM. In contrast, the colony-stimulating effect of Prl-CM was only rarely and, usually, modestly affected by the addition of either MAb-425 or 528. Our data suggest that in the NMU mammary tumor grown in soft agar, EGF/TGF alpha-related peptides are produced upon exposure to E2 and possibly Pg but only rarely following Prl administration. The possible role of these growth factors as mediators of hormonal effects in our experimental system remains to be established.