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Growth factors enable cells to escape irradiation-induced death (apoptosis). One important family of growth factors share an epidermal growth factor motif, and all bind to ErbB transmembrane receptors. In response to growth factor ligands, ErbB receptor tyrosine kinases induce a variety of cellular responses, including proliferation, differentiation and motility. Signal transduction pathways are initiated upon ligand-induced receptor homo- or heterodimerization and activation of tyrosine kinase activity. The complement of induced signaling pathways, as well as their magnitude and duration, determines the biological outcome of signaling, and in turn, is regulated by the identity of the ligand and the receptor composition. Recent insights into the structural basis for receptor dimerization, as provided by crystallographic analysis, are described, as is the differential activation of signaling pathways and downregulatory mechanisms. Further, dysregulation of the ErbB network is implicated in a variety of human cancers, and the nature of aberrant signaling through ErbB proteins, as well as current therapeutic approaches, are discussed, highlighting the role of the highly oncogenic ErbB-2 molecule.

ErbB-2 (also called HER2/neu) and ErbB-3 are closely related to the epidermal growth factor receptor (EGFR/ErbB-1), but unlike EGFR, ErbB-2 is a ligandless receptor, whereas ErbB-3 lacks tyrosine kinase activity. Hence, both ErbB-2 and ErbB-3 are active only in the context of ErbB heterodimers, and ErbB-2. ErbB-3 heterodimers, which are driven by neuregulin ligands, are the most prevalent and potent complexes. These stringently controlled heterodimers are repeatedly employed throughout embryonic development and dictate the establishment of several cell lineages through mesenchyme-epithelial inductive processes and the interactions of neurons with muscle, glia, and Schwann cells. Likewise, the potent combination of signaling pathways engaged by the heterodimers drives an aggressive phenotype of tumors of secretory epithelia, including breast and lung cancers. This review highlights recent structural insights into the mechanism of ligand-induced heterodimer formation, and concentrates on signaling pathways employed by ErbB-2 and ErbB-3 in normal and in malignant cells.