A Hierarchical Network of Interreceptor Interactions Determines Signal Transduction by Neu Differentiation Factor/Neuregulin and Epidermal Growth Factor

Abstract

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...

Epiregulin binds to epidermal growth factor receptor and ErbB-4 and induces tryosine phosphorylation of epidermal growth factor receptor, ErbB-2, ErbB-3 and ErbB-4

“…Epiregulin-induced stimulation of tyrosine phosphorylation of ErbB-2 was weaker than that of EGF in SK-BR-3 and T47D cells, and HRG-a in T47D and MDA-MD-453 cells. NDF-induced heterodimerization of the receptors in cells that ectopically expressed ErbB receptors showed that the interaction between ErbB-3 and ErbB-2 is more extensive than the cross-talk between ErbB-4 and ErbB-2 (Tzahar et al, 1996). On the other hand, epiregulin bound directly to ErbB-4 but not ErbB-3 in four cell lines examined in the present study.…”

Expression of dominant-negative ErbB2 in the mammary gland of transgenic mice reveals a role in lobuloalveolar development and lactation

“…ErbB2 is the preferred heterodimer partner of activated EGFR family members (Graus-Porta et al, 1997;Tzahar et al, 1996), and, when mutated, can inactivate other ErbB-family receptors (Qian et al, 1994). Therefore, we cannot rule out the possibility that in addition to inhibiting ErbB2 signaling, ErbB2DIC inactivates signaling by other members of the EGFR-family.…”

“…This is mediated by induction of EGFR/ErbB2 heterodimers (Spivak-Kroizman et al, 1992;Wada et al, 1990). Although each ligand-activated EGFR-family member can form a signaling dimer with most other members of this receptor family, ErbB2 is the preferred heterodimer partner (Graus-Porta et al, 1997;Tzahar et al, 1996). Moreover, ligand stimulated receptors compete for dimer formation with ErbB2 Karunagaran et al, 1996).…”

c-erbB3 and c-erbB4 expression is a feature of the endocrine responsive phenotype in clinical breast cancer

“…Both c-erbB3 and c-erbB4 bind a range of ligands of which the splice variants of neuregulin (heregulin, neu di erentiation factor) have received particular attention (Riese et al, 1995). Despite the absence of a direct ligand, transphosphorylation of c-erbB2 can be induced following heterodimerisation to ligand-bound c-erbB3 or c-erbB4 (Tzahar et al, 1996). c-erbB2 is the preferred partner for heterodimerization and when co-expressed with either EGFR or c-erbB3, synergistically enhances their mitogenic and transforming activity (Tzahar et al, 1996).…”

“…Despite the absence of a direct ligand, transphosphorylation of c-erbB2 can be induced following heterodimerisation to ligand-bound c-erbB3 or c-erbB4 (Tzahar et al, 1996). c-erbB2 is the preferred partner for heterodimerization and when co-expressed with either EGFR or c-erbB3, synergistically enhances their mitogenic and transforming activity (Tzahar et al, 1996). Along with di erences in ligand binding each member of this sub-group has a di erent intrinsic tyrosine kinase activity i.e.…”