Development and characterization of monoclonal antibodies to a specific domain of human estrogen receptor.

We have synthesized three peptides with amino acid sequences identical to those spanning amino acids 201-215, 231-245, and 247-261 of the human estrogen receptor (hER). These peptides were conjugated to keyhole limpet hemocyanin and used as immunogens to develop monoclonal antibodies (MoAbs) to hER. Antibody responses were only elicited by the peptide with amino acid sequence 247-261. Splenocytes from immunized mice were used for hybridoma production. Of the seven MoAbs that recognized the native (functional) form of the ER, four (MoAbs 16, 33, 114, and 213) recognized the ER with high affinity, as demonstrated by the increased sedimentation coefficient of the antibody-complexed ER in sucrose density gradients. Antibodies 318, 35, and 36 bound to ER with low affinity since they immunoprecipitated ER, but the ER-antibody complex appeared to dissociate on sucrose density gradients. The high-affinity MoAbs appear to be site-specific since the peptide competed effectively for binding of the receptor by the antibody. The fact that they reacted with ER from human breast cancer and calf, rat, and mouse uterine tissues suggests that this epitope of the receptor is conserved in these species. Although the DNA-binding region appears to be conserved among the various steroid receptors, these MoAbs did not recognize the native forms of progesterone, androgen, or glucocorticoid receptors. These MoAbs bound to the KCl-activated 4S ER and heat-transformed 5S ER, suggesting that the antibody-binding site is accessible in the monomeric and dimeric forms of ER. The antibodies did not recognize the untransformed 8S ER in the presence of molybdate and without KCl, suggesting that the antibody-binding site in the oligomeric form of ER is inaccessible. The fact that the antibodies did bind to the unoccupied 4S ER was demonstrated by the data obtained with sucrose density gradient analysis followed by postlabeling of ER with [3H]estradiol. The antibodies bound to ERs with high affinity (KD = 0.4 to 1.8 nM). At a fixed concentration of antibody, ERs ranging from 20 to 1,000 fmol were detectable. These MoAbs did not inhibit nuclear or DNA binding of ER in vitro. This can be attributed to the dissociation of the antibodies from ER when the latter interacts with its acceptor site. These results demonstrate the development of site-specific MoAbs to the native form of the hER using synthetic peptides as immunogens.