Role of transforming growth factor-α-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/TGFα-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.