Structural insights into the down-regulation of overexpressed p185 her2/neu protein of transformed cells by the antibody chA21

Abstract

p185her2/neu belongs to the ErbB receptor tyrosine kinase family, which has been associated with human breast, ovarian, and lung cancers. Targeted therapies employing ectodomain-specific p185her2/neu monoclonal antibodies (mAbs) have demonstrated clinical efficacy for breast cancer. Our previous studies have shown that p185her2/neu mAbs are able to disable the kinase activity of homomeric and heteromeric kinase complexes and induce the conversion of the malignant to normal phenotype. We previously developed a chimeric antibody chA21 that specifically inhibits the growth of p185her2/neu-overexpressing cancer cells in vitro and in vivo. Herein, we report the crystal structure of the single-chain Fv of chA21 in complex with an N-terminal fragment of p185her2/neu, which reveals that chA21 binds a region opposite to the dimerization interface, indicating that chA21 does not directly disrupt the dimerization. In contrast, the bivalent chA21 leads to internalization and down-regulation of p185her2/neu. We propose a structure-based model in which chA21 cross-links two p185 her2/neu molecules on separate homo- or heterodimers to form a large oligomer in the cell membrane. This model reveals a mechanism for mAbs to drive the receptors into the internalization/degradation path from the inactive hypophosphorylated tetramers formed dynamically by active dimers during a "physiologic process." © 2011 by The American Society for Biochemistry and Molecular Biology, Inc.