The ErbB family includes four homologous transmembrane tyrosine kinases. Whereas ErbB-1 binds to the epidermal growth factor (EGF), both ErbB-3 and ErbB-4 bind to the Neu differentiation factors (NDFs, or neuregulins), and ErbB-2, the most oncogenic family member, is an orphan receptor whose function is still unknown. Because previous lines of evidence indicated the existence of interreceptor interactions, we used ectopic expression of individual ErbB proteins and their combinations to analyze the details of receptor cross talks. We show that 8 of 10 possible homo-and heterodimeric complexes of ErbB proteins can be hierarchically induced by ligand binding. Although ErbB-2 binds neither ligand, even in a heterodimeric receptor complex, it is the preferred heterodimer partner of the three other members, and it favors interaction with ErbB-3. Selective receptor overexpression in human tumor cells appears to bias the hierarchical relationships. The ordered network is reflected in receptor transphosphorylation, ErbB-2-mediated enhancement of ligand affinities, and remarkable potentiation of mitogenesis by a coexpressed ErbB-2. The observed superior ability of ErbB-2 to form heterodimers, in conjunction with its uniquely high basal tyrosine kinase activity, may explain why ErbB-2 overexpression is associated with poor prognosis.Polypeptide growth factors regulate cellular growth by binding to surface receptors with intrinsic tyrosine kinase activity (14,54,61). These receptor tyrosine kinases constitute a family of related proteins that have been classified into subgroups on the basis of their structural homology. The receptor for epidermal growth factor (EGF), also called ErbB-1 or HER-1, is the prototype of the type I subfamily, which includes three additional members: ErbB-2/Neu, ErbB-3, and ErbB-4. Whereas ErbB-1 binds multiple distinct ligands that share the EGFlike motif (33), all of the known ligands of ErbB-3 and ErbB-4 are isoforms of the Neu differentiation factor (NDF, or neuregulin) (8,41,53), and no completely characterized ligand binds to ErbB-2 (12). Nevertheless, ErbB-2 has been implicated more than other transmembrane tyrosine kinases in cancer development (21, 50). Overexpression of this protein occurs in a significant fraction of breast and ovarian carcinomas, and it correlates with reduced patient survival (18,26,44,45). Because an oncogenic point mutation in the rat homolog of ErbB-2 mimics ligand binding (2, 59), and the ErbB-2 kinase can be stimulated by a heterologous ligand in the context of chimeric receptors (4, 30, 31), it is believed that a still unknown ligand directly binds to this orphan receptor.All members of the ErbB family are characterized by extracellular domains with two cysteine-rich sequences, and a cytoplasmic tyrosine kinase domain flanked by large hydrophilic tails, that display sequence heterogeneity and carry several tyrosine autophosphorylation sites (40, 42). The latter serve as docking sites for various cytoplasmic signaling proteins that share a 100-amino-acid-long dom...

Overexpression of the erbB‐2 gene contributes to aggressive behavior of various human adenocarcinomas, including breast cancer, through an unknown molecular mechanism. The erbB‐2‐encoded protein is a member of the ErbB family of growth factor receptors, but no direct ligand of ErbB‐2 has been reported. We show that in various cells ErbB‐2 can form heterodimers with both EGF receptor (ErbB‐1) and NDF receptors (ErbB‐3 and ErbB‐4), suggesting that it may affect the action of heterologous ligands without the involvement of a direct ErbB‐2 ligand. This possibility was addressed in breast cancer cells through either overexpression of ErbB‐2 or by blocking its delivery to the cell surface by means of an endoplasmic reticulum‐trapped antibody. We report that ErbB‐2 overexpression enhanced binding affinities to both EGF and NDF, through deceleration of ligand dissociation rates. Likewise, removal of ErbB‐2 from the cell surface almost completely abolished ligand binding by accelerating dissociation of both growth factors. The kinetic effects resulted in enhancement and prolongation of the stimulation of two major cytoplasmic signaling pathways, namely: MAP kinase (ERK) and c‐Jun kinase (SAPK), by either ligand. Our results imply that ErbB‐2 is a pan‐ErbB subunit of the high affinity heterodimeric receptors for NDF and EGF. Therefore, the oncogenic action of ErbB‐2 in human cancers may be due to its ability to potentiate in trans growth factor signaling.