MUC1 synthetic peptide inhibition of intercellular adhesion molecule-1 andMUC1 binding requires six tandem repeats

We reported recently that breast cancer-associated MUC1 is a ligand for int ercellular adhesion molecule-1 (ICAM-1; L, H. Regimbald et at, Cancer Res., 56: 4244-4249, 1996). We report here the results of a competitive indirect binding assay to detect the molecular requirements for binding between ICA M-1 and MUC1. The assay involved inhibition of the binding of recombinant h uman ICAM-1 to a murine breast adenocarcinoma cell line transfected with hu man MUC1. The addition of a Library of human MUC1 synthetic peptides rangin g from 9 to 24 amino acids (aa) showed minimal or no inhibition. However, a 120-aa peptide that corresponds to six tandem repeats of the human mucin M UC1 was as effective an inhibitor as purified tumor MUC1 and MUC1 epitope ( PDTR-PAP)-specific antibody (B27.29). We conclude that the number of MUC1 t andem repeats necessary for an ordered tertiary structure (D. Fontenot et a l., Cancer Res., 53: 5386-5394, 1993) is also important for ICAM-1 recognit ion. These findings are similar to those described recently for MUC1 induct ion of T-cell anergy (B, Agrawal et at, Nat, Med,, 4: 43-49, 1998). This su ggests that the anergy induction by MUC1 may be due to ICAM-1 binding by MU C1.