Growth inhibition of MCF-7 cells by estrogen is dependent upon a serum factor

MCF-7 cell growth is normally dependent upon estrogen, but if cells are maintained in serum-free medium estrogen inhibits cell growth with an IC50 of about 1 nM. Cells adapted to serum-free medium have approximately the same levels of estrogen receptor mRNA as control cells that are stimulated by estrogen. We have identified two serum components, one estrogen dependent and one not, that appear to be responsible for the inhibition of growth by estrogen. Our observations are consistent with the presence of a plasma membrane receptor which negatively regulates MCF-7 cell growth, and that can be inhibited by a serum protein that binds estrogen.