The HER‐2/ErbB‐2 oncoprotein is overexpressed in human breast and ovarian adenocarcinomas and is clearly associated with the malignant phenotype. Although no specific ligand for this receptor has been positively identified, ErbB‐2 was shown to play a central role in a network of interactions with the related ErbB‐1, ErbB‐3 and ErbB‐4 receptors. We have selected new peptides binding to ErbB‐2 extracellular domain protein (ECD) by screening 2 newly developed constrained and unconstrained random hexapeptide phage libraries. Out of 37 phage clones, which bound specifically to ErbB‐2 ECD, we found 6 constrained and 10 linear different hexapeptide sequences. Among the latter, 5 consensus motifs, all with a common methionine and a positively charged residue at positions 1 and 3, respectively, were identified. Furthermore, 3 representative hexapeptides were fused to a coiled‐coil pentameric recombinant protein to form the so‐called peptabodies recently developed in our laboratory. The 3 peptabodies bound specifically to the ErbB‐2 ECD, as determined by enzyme‐linked immunosorbent assay and BIAcore analysis and to tumor cells overexpressing ErbB‐2, as shown by flow cytometry. Interestingly, one of the free selected linear peptides and all 3 peptabodies inhibited the proliferation of tumor cells overexpressing ErbB‐2. In conclusion, a novel type of ErbB‐2‐specific ligand is described that might complement presently available monoclonal antibodies. © 2001 Wiley‐Liss, Inc.
We have tried to develop a new model consisting of rats transplanted with syngeneic colon carcinoma PROb cells transfected with cDNA coding for the carcinoembryonic antigen (CEA), the human tumor marker most commonly used as target for MAbs. The antigenic density of the 4 CEA-expressing clones selected for a precise characterization ranged from 5 x 10(4) to 1 x 10(6) CEA molecules per cell. In all clones the CEA was shown to be attached to the membrane by a phosphatidylinositol (PI) anchor. Using a panel of radiolabeled MAbs directed against the 5 major epitopes described on the CEA molecule, we showed that all these CEA epitopes were expressed by the 4 transfectants. Southern-blot analysis showed that the entire CEA cDNA was present in the transfectants. Western-blot analysis, however, showed that the size of the CEA expressed by the 4 transfectants was slightly smaller than that of CEA produced by 2 reference human colon-carcinoma cell lines. Two clones, expressing 1 x 10(5) and 1 x 10(6) CEA molecules per cell, respectively, were grafted s.c. in nude mice and rats. Injection of radiolabeled anti-CEA F(ab')2 fragments into these animals showed specific tumor localization with the highest percentages of injected doses for the transfectants expressing the highest CEA level. When grafted into immunocompetent syngeneic BDIX rats, the CEA-expressing clones induced a strong antibody response against CEA and tumor rejections in a majority of the animals. Although the analysis of the immune response against the CEA-cDNA-transfected carcinoma cells is under investigation, the present results demonstrate that human CEA could function as a rejection antigen when transfected into rat carcinoma cells.
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