A novel binding site for a synthetic progestagen in breast cancer cells

A novel, high-affinity saturable binding site for the synthetic 19-nor testosterone progestagen, 17 alpha-ethinyl-13 beta-ethyl 17 beta-hydroxy-4,15-oestradiene-3-one (gestodene) has been detected using a sensitive affinity chromatography technique. This binding site is present in a range of malignant breast-derived cells lines, regardless of the presence of oestrogen and progesterone receptors, but is absent from endometrial carcinoma cells that contain both oestrogen and progesterone receptors. Competition studies show that this binding is not attributable to the receptors for the progestagens, androgens, glucocorticoids or mineralocorticoids. Cytosolic gestodene binding is refractory to competition with oestradiol but nuclear gestodene binding is completely abolished by oestradiol. The binding of oestradiol to the oestrogen receptor is reduced 40-50% by competition with gestodene. Non-dissociating polyacrylamide gel electrophoresis and size-exclusion high performance liquid chromatography reveal that this binding activity is associated with a protein of mean molecular mass 47 +/- 9 kDa. Ligand binding studies with a range of other cell lines indicates that this binding site appears to be specific to breast cancer cells. These data show the presence of a partly oestrogen competable novel binding protein in breast cancer cells which does not appear to be due to any of the conventional steroid receptors.