Growth factor involvement in the multihormonal regulation of MCF-7 breast cancer cell growth in soft agar

The hormone dependency of the MCF-7 breast cancer cell line, while extensively tested in liquid culture, has not been previously evaluated under conditions of anchorage-independent growth in serum-free media. Using the soft agar clonogenic assay, we demonstrate that physiologically relevant concentrations of estradiol (E₂), progesterone (Pg), and prolactin (PRL) similarly stimulated MCF-7 cell colony formation in the absence of serum. Addition of an anti-insulin-like growth factor-I (IGF-I) antibody inhibited E₂- and Pg-stimulated growth, while PRL action was not affected. Similar results were obtained with an anti-IGF-I receptor antibody, except that its inhibitory effect on Pg-induced colony formation was modest and not statistically significant. Administration of either an anti-transforming growth factor-α (TGF-α) antibody or an anti-epidermal growth factor (EGF) receptor antibody similarly inhibited E₂-stimulated MCF-7 cell growth in soft agar, while neither antibody influenced Pg or PRL effects. Addition of TGF-β₁, -β₂, -β₃ similarly suppressed MCF-7 cell colony formation in a dose dependent manner to a degree comparable to that observed with 4-OH-tamoxifen (4-OH-T). Furthermore, the growth inhibitory effect of 4-OH-T was completely reversed by an anti-TGF-β antibody. We conclude that IGFs and TGF-α are important mediators of E₂-stimulated MCF-7 cell growth in soft agar. IGFs may also be playing a role in Pg action, while neither growth factor is involved in PRL-stimulated colony formation. Finally, TGF-β appears to be an important mediator of antiestrogen-induced inhibition of tumor growth.