PRESENCE OF A SPECIFIC ANTIESTROGEN BINDING-SITE ON HUMAN FOLLICULAR THYROID-CARCINOMA CELL-LINE (UCLA RO 82 W-1) - INHIBITION BY AN ENDOGENOUS LIGAND PRESENT IN HUMAN SERUM

A receptor for antiestrogens, distinct from the estrogen receptor, has been identified in several tissues including the MCF-7 breast cancer cell line. Estrogen receptors have also been found in normal and patho logical thyroid tissue homogenates. We demonstrate the presence of an antiestrogen binding site (AEBS) on a pure human follicular thyroid ca rcinoma cell line (UCLA RO 82 W-1) using a H-3-tamoxifen (H-3-TAM) bin ding assay. The binding of H-3-TAM to the AEBS was determined after pr eincubation (30 min) of the cells with excess 17 beta-estradiol (2 mu mol/L). Specific and saturable binding of H-3-TAM to the cells was obs erved. Displacement of the tracer from its binding site was dose depen dent. Scatchard analysis revealed a dissociation constant (K-d) of 73 nmol/L, indicating a binding site with moderate affinity and capacity (72 pmol/l0(6) cells). Using this assay we were also able to demonstra te the presence of an endogenous ligand for the AEBS in ethanol extrac ts of human serum. Cell growth and H-3-thymidine incorporation by the follicular thyroid carcinoma cells were inhibited when the cells were exposed to TAM (1.5 mu mol/L). In conclusion, TAM is able to bind to a specific receptor on this follicular thyroid carcinoma cell line, and a natural circulating ligand present in ethanol extracts of human ser um interferes with its binding.