1. Top of page
  2. Abstract
  3. HER receptors family and its ligands

Members of the epidermal growth factor receptor family of receptor tyrosine kinases play a critical role in both development and oncogenesis. The latter is suggested by the frequent overexpression of HER-2, EGFR, and HER-3 in some human carcinomas, primarily breast and squamous cancer. The biological activities of the EGFR family are exerted through various ligand–receptor and receptor–receptor interactions. One receptor that plays a central role in this signaling network is HER-2/Neu, which is considered the preferred heterodimerization partner for other members of the EGFR family. The role of these receptors and their ligands in development is discussed, with particular emphasis on their ability to mediate a variety of pathways and cellular responses, including proliferation, differentiation, and apoptosis. © 2004 Wiley-Liss, Inc.

HER receptors family and its ligands

  1. Top of page
  2. Abstract
  3. HER receptors family and its ligands

The flow of information from the extracellular environment into the cell is at the core of a functional biological system. Receptor tyrosine kinases (RTKs) are primary mediators of many of these signals and thus determine whether the cell grows, differentiates, migrates, or dies. RTKs are cell surface allosteric enzymes consisting of a single transmembrane domain that separates an intracellular kinase domain from an extracellular ligand-binding domain. Ligand binding induces receptor homo- or heterodimerization which is essential for activation of the tyrosine kinase and subsequent recruitment of target proteins, in turns initiating a complex signaling cascade that leads into distinct transcriptional programs. These involve not only the proto-oncogenes fos, jun, and myc, but also a family of zinc-finger-containing transcription factors that include Sp1 and Egr1, as well as Ets family members such as GA-binding protein (GABP) (Schaeffer et al., 1998). The HER family of RTKs consists of four receptors: epidermal growth factor receptor (EGFR, also called HER-1 or erbB-1), HER-2 (also called erbB-2 or Neu), HER-3 and HER-4 (also called erbB-3 and erbB-4, respectively).

Extensive receptor–receptor interactions and the existence of a wide group of ligands underlies the enormous potential for diversification of biological messages mediated by the HER family. These peptide ligands are produced as transmembrane precursors, and the ectodomains are processed by proteolysis, which leads to shedding of soluble growth factors (Massagué and Pandiella, 1993). There are several HER-specific ligands, all sharing an EGF-like motif of 45–55 amino acids and including six cysteine residues that interact covalently to form three loops. This region is probably the most important, conferring binding specificity by which HER ligands can be divided into three groups. The first group includes EGF, amphiregulin (AR), and transforming growth factor-α (TGF-α), which bind specifically to HER-1. The second group includes betacellulin (BTC), heparin-binding EGF (HB-EGF), and epiregulin (EPR) (Yarden, 2001), which exhibit dual specificity for HER-1 and HER-4. The third group is composed of the neuregulins (NRG, also called Neu differentiation factors, NDFs, or heregulins, HRG) and includes two subgroups based on their capacity to bind HER-3 and HER-4 (NRG-1 and NRG-2) or only HER-4 (NRG-3 and NRG-4) (Zhang et al., 1997; Harari et al., 1999). NRGs, are expressed predominantly in parenchymal organs and in the embryonic central and peripheral nervous systems. The different NRG isoforms are the products of alternative splicing of a single gene (Marchionni et al., 1993).

Each of the many ligands has a different preference for stabilizing distinct receptor dimers, and each receptor dimer has a different set of tyrosine autophosphorylation sites, which serve as docking sites for specific SH2-containing proteins and recruit different combinations of signaling molecules (Di Fiore et al., 1990; Olayioye et al., 2000; Yarden, 2001). Further complexity of this system derives from the existence of a receptor that is activated only by heterodimerization with another ligand-bound member of the family: HER-2, which enhances and stabilizes dimerization, but apparently has no direct or specific ligand (Horan et al., 1995), and HER-3 (Kim et al., 1998), a receptor that can recruit novel SH2-containing proteins, but that is devoid of kinase activity itself due to substitutions of critical residues in its kinase domain (Guy et al., 1994). At least nine different homo- and heterodimers of HER proteins exist, but their formation displays a distinct hierarchy. In this network, HER-2 plays a major coordinating role, since each receptor with a specific ligand appears to prefer HER-2 as its heterodimeric partner (Tzahar et al., 1996; Graus-Porta et al., 1997). This preference is further biased upon overexpression of HER-2, as seen in many types of human cancer cells. HER-2-containing heterodimers are characterized by extremely high signaling potency because HER-2 dramatically reduces the rate of ligand dissociation, allowing strong and prolonged activation of downstream signaling pathways (Beerli et al., 1995; Graus-Porta et al., 1995). The most important intracellular pathways activated by HER receptors are those involving mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI-3K) (Olayioye et al., 2000; Yarden and Sliwkowski, 2001). HER-3, which contains six docking sites for the p85 adaptator subunit of PI-3K, efficiently couples to this pathway (Fedi et al., 1994; Prigent and Gullick, 1994). Based on observations that a major consequence of targeting overexpressed HER-2 in breast cancer cell lines is decreased PI-3K activity (Lane et al., 2000; Neve et al., 2000; Basso et al., 2002), and that constitutive tyrosine phosphorylation of HER-3 depends on the activity of overexpressed HER-2 (Alimandi et al., 1995). Holbro et al. (2003) have suggested that HER-2/HER-3 dimers are the oncogene-driven unit. On the other hand, complete inhibition of breast carcinoma proliferation by a HER-1-specific tyrosine kinase inhibitor suggests a central role for HER-1 (Campiglio et al., 2004).


  1. Top of page
  2. Abstract
  3. HER receptors family and its ligands

Expression patterns of HER receptors and their ligands, as well as targeted inactivation of components of the HER signaling network have highlighted the importance of short-range ligand–receptor interactions, especially in mid-gestation processes. Apparently, the network is involved primarily in two types of interactions: (1) mesenchyme–epithelia crosstalk and (2) neuronal effects on target cells, including muscle, astroglia, oligodendrocytes, and Schwann cells. NRG, for example, is synthesized by mesenchymal or neuronal cells and influences adjacent epithelial or non-neuronal cells, respectively, with respect to their differentiation, proliferation, and migration. This may explain the crucial role of the HER receptor family in development of the cardiovascular system, nervous system, mammary gland ,and probably others (Table 1).

Table 1. Summary of the effect of HER receptors knockout and conditional mutant animals
 HER-1−/−HER-2−/−HER-3−/−HER-4−/−HER-2 conditional mutantsHER-4 conditional mutants
  • *

    Causes of embryonic lethality.

SurvivalEmbryonic or perinatal lethalityEmbryonic day-10 lethalityEmbryonic day-13.5 lethalityEmbryonic day-10 lethality  
Cardiovascular system Aberrant cardiac development*Defective cardiac valve formation*Aberrant cardiac development*Severe dilated cardiomyopathyAberrant cardiac development
Nervous system Aberrant peripheral development*Aberrant peripheral development*Aberrant peripheral development*Aberrant developmentAberrant development
Mammary glandImpaired development   Defect in development late in gestation and early post-partumUnpaired lactation
EpitheliumEpithelial defects as wavy hair     

Cardiovascular system

An essential role for the HER receptor family in mid-gestation development was indicated by embryonic lethality of HER-2- (Lee et al., 1995), HER-4- (Gassmann et al., 1995), and NRG-deficient mice at around day-10 post-fertilization due to aberrant cardiac and peripheral nervous system development. The trabeculae, finger-like extensions of the ventricular myocardium, fail to develop in these mice and the resulting mutant heart is characterized by irregular beat, an enlarged common ventricle, and reduced blood flow. HER-3-knockout mice have less severe heart defects and consequently survive several days longer (to around embryonic day-13.5), displaying normal heart trabeculation but defective valve formation. The role of HER-4 has been further established in HER-4-knockout mice by re-expressing HER-4 under the regulatory control of the cardiac α-myosin heavy chain (αMHC) promoter (Morris et al., 1999; Tidcombe et al., 2003). Cardiac-rescued MHC-HER4 HER4−/− mice are viable, but display abnormalities in the central nervous system and mammary gland. The mid-gestation cardiac defect in HER-2 knockout animals has been circumvented through the use of Cre-Lox technology, a conditional mutagenesis approach based on the Cre DNA recombinase-mediated deletion of a DNA segment flanked on both sides by two loxP sequences. When the recombinase is expressed under an appropriate promoter, this method allows the introduction of a mutation in an inducible and tissue-specific manner. Conditional mutagenesis of the HER-2 gene in murine ventricular cardiomyocytes (Ozcelik et al., 2002) revealed development of severe dilated cardiomyopathy, with signs of cardiac dysfunction generally appearing by the second postnatal month. Based on these findings, the authors concluded that signaling from the HER-2 receptor, which is enriched in T-tubules in cardiomyocytes, is crucial for adult heart function. In light of the adverse cardiac side effects observed in breast cancer patients treated with the monoclonal anti-HER-2 antibody Trastuzumab (Slamon et al., 2001), an improved understanding of the molecular mechanisms by which HER-2 regulates heart function is especially important.

Nervous system

The NRGs exert many functions in neural development. Roles for NRG-1-HER signaling in neural development have been demonstrated in mice carrying an HER-3 null mutation (Riethmacher et al., 1997), as well as by selective mutations of NRG-1 ectodomains (Meyer et al., 1997; Wolpowitz et al., 2000), by MHC-HER-specific expression (Morris et al., 1999; Tidcombe et al., 2003), and by Cre-targeted HER-2 ablation in Schwann cells (Garratt et al., 2000). In all of these mice, peripheral motorneuron axons defasciculate as they enter the muscle mass and fail to form mature neuromuscular junctions. Aberrant cranial nerve architecture and increased numbers of large interneurons within the cerebellum have been demonstrated in MHC-HER4 HER4−/− mice, while loss of NRG-1-HER signaling led to hypoplasia of the sympathetic ganglion chain and the neural crest-derived portion of the cranial sensory ganglia (Kramer et al., 1996; Erickson et al., 1997; Riethmacher et al., 1997; Britsch et al., 1998). Moreover, the NRG-1-HER mutants were completely devoid of Schwann cells in peripheral nerves at late development stages (Woldeyesus et al., 1999; Wolpowitz et al., 2000). Thus, NRG-1 signaling through the HER-2/HER-3 heterodimer is required for normal Schwann cell development. The role of HER-2 signaling in later development of the Schwann cell lineage was also analyzed using conditional mutagenesis (Garratt et al., 2000). In that study, the Cre allele introduced the HER-2 mutation in maturing myelinating Schwann cells, allowing the initial development of Schwann cell precursors to proceed unperturbed. These conditional HER-2 mutants displayed peripheral nerve hypomyelination associated with neuropathy, a phenotype reminiscent of the pathology in patients with Charcot-Marie-Tooth disease (Benstead and Grant, 2001). Thus HER-2 is the first signaling molecule for which a role in control of Schwann cell myelination has been demonstrated in vivo. Moreover, Kim et al. (2003) have very recently demonstrated that HER-2 signaling is also critical for oligodendrocyte differentiation in vivo, to date, no data have been reported on nervous system toxicity in patients treated with anti-HER-2-targeted therapy. Nevertheless, patients long-term treated should be carefully monitored for potential side effect since a possible role for HER-2 in mature nervous system tissue cannot be excluded.

Epithelial development

In contrast to the embryonic lethality caused by HER-2 inactivation, mice carrying a naturally occurring germ-line mutation in the kinase domain of EGFR known as Waved-2 (hypomorphic allele with severely reduced catalytic activity) are completely viable and display only epithelial defects, such as a wavy hair phenotype. Mutant mice display impaired epithelial development in several organs, resulting in phenotypes ranging from peri-implantation death to live progeny with abnormalities in multiple organs, such as liver and skin depending on the genetic background. (Luetteke et al., 1994; Fowler et al., 1995).

Mammary gland

The importance of HER receptor family and ligands in human mammary carcinoma has evoked keen interest in the normal functions of these receptors in the mammary gland, an organ that undergoes considerable postnatal development. Analyses of HER family ligands in mammary development have revealed a complicated picture. RNAs encoding the majority of the HER-specific growth factors such as AR, BTC, HB-EGF, EPR, EGF, NRG1, and TGFα are all present, each with a unique temporal pattern of transcriptional regulation during the normal course of mammary development, maturation, and involution (Schroeder and Lee, 1998).

HERs play several normal non-oncogenic roles in regulating growth, differentiation, apoptosis, and/or remodeling in normal mammary glands. These receptors are differentially expressed in mammary epithelial and/or stromal cells during various stages of development (Table 2). In the mouse virgin gland, HER-1 and HER-2 colocalize in all major cell types during ductal morphogenesis, but localized differentially in the mature gland. EGFR and HER-2 are preferentially expressed in lactating ducts and alveoli, and HER-3 and HER-4 are more pronounced in alveoli (Schroeder and Lee, 1998). Interestingly, a switch from HER-3 to HER-4 expression was observed in the developing mammary gland, suggesting that the two receptors play different roles in mammary morphogenesis. Activated EGFR and HER-2 are highly expressed in extracts of mammary glands collected at puberty, suggesting a prominent role of these receptors at this stage of development, while both are expressed to only a minor extent in mammary glands in late-stage pregnancy and in lactation (Sebastian et al., 1998). By contrast, HER-3 and HER-4 are active in mammary glands mostly during pregnancy and lactation (Yang et al., 1995). With respect to the signaling pathways activated by HER-2, Niemann et al. (1998) demonstrated that formation of branched tubules relies on a pathway involving PI-3K, whereas alveolar morphogenesis requires MAPK.

Table 2. Expression of HER receptors during the different stages of mammary gland development
PubertyPresent at high levelsPresent at high levelsAbsent or present at low levelsAbsent or present at low levels
PregnancyPresentPresentPresent at high levelsPresent at high levels
LactationPresentPresentPresent at high levelsPresent at high levels

Analysis of the role of HER receptors in development of immature mammary gland have been hampered by mid-gestation lethality due to disruption of HER-2 (Lee et al., 1995), HER-3 (Riethmacher et al., 1997), and early postnatal lethality (within 3 days of birth) in HER-1−/− mice, all prior to the major transitions of mammary development (Miettinen et al., 1995). However, the prolonged survival of a fraction of HER-1−/− mice enabled the determination that postnatal ductal development was seriously impaired. These mice are characterized by a reduced proliferation of mammary epithelium and stroma, as well as a substantial loss of periductal fibroblasts. Although cardiac-rescued MHC-HER4 HER4−/− mice reach adulthood and are fertile, they show abnormal mammary lobulo-alveolar differentiation and defective lactation (Tidcombe et al., 2003).

Mammary functions of EGFR, HER-2, and HER-4 have also been assessed using cytoplasmic, truncated dominant-negative forms of the receptors, under the control of the mouse mammary tumor virus (MMTV) promoter. Transgenic animals expressing dominant-negative MMTV-driven truncated HER-2 have significant defects in mammary development late in gestation and early postpartum, with failure of alveolar expansion (Jones and Stern, 1999). Ductal development occurs in these animals, but they have lactation problems, and mammary glands early postpartum show an immature phenotype more typical of late pregnancy (Tidcombe et al., 2003). The MMTV-driven truncated HER-4 dominant-negative phenotype resembles that in mice with inactivated mammary differentiation factors Stat5s in the mammary gland (Liu et al., 1997; Teglund et al., 1998; Jones et al., 1999). Targeted recombination-mediated inactivation of HER-4, through Cre-Lox technology, has demonstrated that this receptor is an essential mediator of STAT5 signaling (Long et al., 2003). Mice with Cre-Lox deletions of both HER-4 alleles in the developing mammary gland fail to accumulate lobulo-alveoli or successfully engage lactation at parturition due, in part, to impaired epithelial proliferation. These data do not reveal any indispensable role for a particular member of the family in mammary gland development. Moreover, the cross-talk between HERs and steroid hormone receptors in mammary gland development remains to be established. It seems very likely that these two receptor types act synergistically and that inhibition of both pathways is required for complete ablation of mammary gland development. In that context, Tamoxifen, an antagonist of estrogen, shown to prevent 50% of breast carcinoma development (Radmacher and Simon, 2000), might be used in combination with anti-HER therapy to completely inhibit mammary gland development.


  1. Top of page
  2. Abstract
  3. HER receptors family and its ligands

HER-2 overexpression, occurring in 25–30% of human breast cancers, is associated to shorter time to relapse and lower overall survival (Slamon et al., 1987; Ménard et al., 1996, 1999, 2002). HER-2 oncogenic action is exerted through activation of the PI-3K pathway, which inhibits apoptosis (Sepp-Lorenzino et al., 1996; Kulik et al., 1997; Nelson and Fry, 2001). The survival signal is also normally coupled to the activation of the mitogenic signal involving MAPK pathway recruitment. Increased HER-2 expression in cancer enhances and prolongs signaling from both the PI-3K and MAPK pathways (Karunagaran et al., 1996; Olayioye et al., 2001; Neve et al., 2002), associating upregulation of this receptor to the malignant phenotype. Nevertheless, HER-2 transfection of some cell lines leads to decreased plating and cloning efficiency, decreased growth rate (Giani et al., 1998; Casalini et al., 2001), inhibition of entry into the S-phase of the cell cycle, and differentiation (Giani et al., 1998). This is consistent with the induction of differentiation (Peles et al., 1992), growth inhibition (Daly et al., 1997) and/or apoptosis induced by heregulin and some antibodies against HER receptors (Aguilar et al., 1999; Guerra-Vladusic et al., 1999, 2001; Le et al., 2001).

An important role for HER receptors in apoptosis has been recently demonstrated. Indeed, UVB-induced apoptosis of human keratinocytes occurs through HER-1 and HER-2 activation since a specific inhibitor of the receptors, DAPH, or neutralizing antibodies to either HER-1 or HER-2, protect cells from UVB-induced apoptosis (Lewis et al., 2003). Moreover, several experimental lines of evidence demonstrate that, under certain conditions, the activation of these receptors leads directly to the apoptotic death of the cell. For example experimentally induced increases in HER-1 expression levels, predictably lead to apoptosis in a variety of cell types (Hognason et al., 2001; Lehto, 2001). HER-2 receptor activation has been found to up-modulate p53 expression (Bacus et al., 1996), a major determinant of growth, differentiation, and induction of apoptosis i.e., wild-type p53 leads to apoptosis/differentiation, whereas mutated p53 leads to proliferation (Fig. 1) (Giani et al., 1998; Casalini et al., 2001). It has also been demonstrated that HRG stimulation of HER-2-overexpressing cells leads to enhanced c-Myc protein synthesis through activation of the PI3K/Akt/mTOR pathway (Galmozzi et al., 2004), which in turn could act as transcriptional factor for p53 leading to its upregulation (Fig. 2).

thumbnail image

Figure 1. A: Colony formation of HER-2-transfected cells bearing wt or mutated p53. Cells were transfected with HER2 c-DNA or with empty vector (control). After 3 weeks of selection, colonies were counted. Results are given as mean percentage (bars, SE of two separate experiments) of colony inhibition in comparison to control cells. B: Apoptosis in HER2-transfected cells. Cells bearing wt or mutated p53 were seeded in chamber slides and transiently transfected with HER2 c-DNA cloned in a vector expressing a fusion product with the N terminus of green fluorescent protein. Control is represented by cells transfected with empty vector expressing only green florescence protein. Apoptosis is expressed as mean percentage of apoptotic nuclei in green transfected cells in comparison to control cells.

Download figure to PowerPoint

thumbnail image

Figure 2. Schematic diagrammatic representation of the possible signal transduction cascades that are involved in HER receptors activation.[Color figure can be viewed in the online issue, which is available at]

Download figure to PowerPoint

The finding that HER-2 overexpression is associated with proliferation in cell lines with mutated p53 is consistent with the frequent overexpression of HER-2 in breast tumors with p53 alterations (James and Olson, 1989). However, the existence of HER-2-overexpressing, p53 wild-type tumors suggests the presence of other alterations related to the apoptotic pathway in tumor cells that allow the shift from apoptosis to proliferation related to HER-2 oversignaling (Casalini et al., 2001; Huang et al., 2002).


  1. Top of page
  2. Abstract
  3. HER receptors family and its ligands

The HER family of receptor tyrosine kinases plays a crucial role in the development of the nervous system, cardiovascular system, and the mammary gland, mediating activities that are various and often opposite: proliferation, differentiation, and apoptosis. The ability to promote different cellular responses appears to seat in a complex protein network which acts through and activates several pathways, and might underlie the multifaceted role of this receptor family in physiological cellular regulation and in carcinogenesis. The HER network is actually a powerful mechanism controlling cell fate through subtle regulation; indeed, deregulation of this network is a common finding in human cancers.

To date, it remains unclear whether HER receptor activity is crucial in driven transformation processes or whether their action is directed mainly to conferring a proliferative advantage in tumor progression. If overexpression of a particular HER member were shown to be essential for tumor existence, HER-targeted therapies would be expected to reverse transformation. However, the use of receptor tyrosine kinase inhibitors, for example, ZD1836, did not lead to improved clinical outcome of patients, in recent clinical phase III trial (Manegold, 2003). The key role of HERs in tumor development is likely exerted during the early stage of the transformation process, with their overexpression providing a proliferative advantage that allows tumor cell survival during clonal selection. This concept is supported by higher frequency of HER2 overexpression in DCIS than in more advanced breast carcinoma (Latta et al., 2002). Thus, it appears that HER-2 overexpression alone is insufficient for the maintenance and progression of the tumor, which selects further alterations for its establishment.

Further investigations of HER multiple functions are needed to improve our understanding of their physiological role in the development and function of several organs and their role in oncogenesis. Such understanding will provide the rationale basis for safe and effective HER-targeted therapies.


  1. Top of page
  2. Abstract
  3. HER receptors family and its ligands
  • Aguilar Z, Akita RW, Finn RS, Ramos BL, Pegram MD, Kabbinavar FF, Pietras RJ, Pisacane P, Sliwkowski MX, Slamon DJ. 1999. Biologic effects of heregulin/neu differentiation factor on normal and malignant human breast and ovarian epithelial cells. Oncogene 18: 60506062.
  • Alimandi M, Romano A, Curia MC, Muraro R, Fedi P, Aaronson SA, Di Fiore PP, Kraus MH. 1995. Cooperative signaling of ErbB3 and ErbB2 in neoplastic transformation and human mammary carcinomas. Oncogene 10: 18131821.
  • Bacus SS, Yarden Y, Oren M, Chin DM, Lyass L, Zelnick CR, Kazarov A, Toyofuku W, Gray-Bablin J, Beerli RR, Hynes NE, Nikiforov M, Haffner R, Gudkov A, Keyomarsi K. 1996. Neu differentiation factor (heregulin) activates a p53-dependent pathway in cancer cells. Oncogene 12: 25352547.
  • Basso AD, Solit DB, Chiosis G, Giri B, Tsichlis P, Rosen N. 2002. Akt forms an intracellular complex with heat shock protein 90 (Hsp90) and Cdc37 and is destabilized by inhibitors of Hsp90 function. J Biol Chem 277: 3985839866.
  • Beerli RR, Graus-Porta D, Woods-Cook K, Chen XM, Yarden Y, Hynes NE. 1995. Neu differentiation factor activation of ErbB-3 and ErbB- 4 is cell specific and displays a differential requirement for ErbB-2. Mol Cell Biol 15: 64966505.
  • Benstead TJ, Grant IA. 2001. Progress in clinical neurosciences: Charcot-Marie-Tooth disease and related inherited peripheral neuropathies. Can J Neurol Sci 28: 199214.
  • Britsch S, Li L, Kirchhoff S, Theuring F, Brinkmann V, Birchmeier C, Riethmacher D. 1998. The ErbB2 and ErbB3 receptors and their ligand, neuregulin-1, are essential for development of the sympathetic nervous system. Genes Dev 12: 18251836.
  • Campiglio M, Locatelli A, Olgiati C, Normanno N, Somenzi G, Viganò L, Fumagalli M, Ménard S, Gianni L. 2004. Inhibition of proliferation and induction of apoptosis in breast cancer cells by the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor ZD1839 (‘Iressa’) is independent of EGFR expression level. J Cell Physiol 198: 259268.
  • Casalini P, Botta L, Ménard S. 2001. Role of p53 in HER2-induced proliferation or apoptosis. J Biol Chem 276: 1244912453.
  • Daly JM, Jannot CB, Beerli RR, Graus-Porta D, Maurer FG, Hynes NE. 1997. Neu differentiation factor induces ErbB2 down-regulation and apoptosis of ErbB2-overexpressing breast tumor cells. Cancer Res 57: 38043811.
  • Di Fiore PP, Segatto O, Taylor W, Aaronson SA, Pierce JH. 1990. EGF receptor and erbB-2 tyrosine kinase domains confer cell specificity for mitogenic signaling. Science 248: 7983.
  • Erickson SL, O'Shea KS, Ghaboosi N, Loverro L, Frantz G, Bauer M, Lu LH, Moore MW. 1997. ErbB3 is required for normal cerebellar and cardiac development: A comparison with ErbB2-and heregulin-deficient mice. Development 124: 49995011.
  • Fedi P, Pierce JH, Di Fiore PP, Kraus MH. 1994. Efficient coupling with phosphatidylinositol 3-kinase, but not phospholipase Cgamma or GTPase-activating protein, distinguishes ErbB-3 signaling from that of other ErbB/EGFR family members. Mol Cell Biol 14: 492500.
  • Fowler KJ, Walker F, Alexander W, Hibbs ML, Nice EC, Bohmer RM, Mann GB, Thumwood C, Maglitto R, Danks JA. 1995. A mutation in the epidermal growth factor receptor in waved-2 mice has a profound effect on receptor biochemistry that results in impaired lactation. Proc Natl Acad Sci USA 92: 14651469.
  • Galmozzi E, Casalini P, Iorio MV, Casati B, Olgiati C, Ménard S. 2004. HER2 signaling enhances 5′UTR-mediated translation of c-Myc mRNA. J Cell Physiol (in press).
  • Garratt AN, Voiculescu O, Topilko P, Charnay P, Birchmeier C. 2000. A dual role of erbB2 in myelination and in expansion of the Schwann cell precursor pool [in process citation]. J Cell Biol 148: 10351046.
  • Gassmann M, Casagranda F, Orioli D, Simon H, Lai C, Klein R, Lemke G. 1995. Aberrant neural and cardiac development in mice lacking the ErbB4 neuregulin receptor. Nature 378: 390394.
  • Giani C, Casalini P, Pupa SM, De Vecchi R, Ardini E, Colnaghi MI, Giordano A, Ménard S. 1998. Increased expression of c-erbB-2 in hormone-dependent breast cancer cells inhibits cell growth and induces differentiation. Oncogene 17: 425432.
  • Graus-Porta D, Beerli RR, Hynes NE. 1995. Single-chain antibody-mediated intracellular retention of ErbB-2 impairs neu differentiation factor and epidermal growth factor signaling. Mol Cell Biol 15: 11821191.
  • Graus-Porta D, Beerli RR, Daly JM, Hynes NE. 1997. ErbB-2, the preferred heterodimerization partner of all ErbB receptor, is a mediator of lateral signaling. EMBO J 16: 16471655.
  • Guerra-Vladusic FK, Scott G, Weaver V, Vladusic EA, Tsai MS, Benz CC, Lupu R. 1999. Constitutive expression of heregulin induces apoptosis in an erbB-2 overexpressing breast cancer cell line SKBr-3. Int J Oncol 15: 883892.
  • Guerra-Vladusic FK, Vladusic EA, Tsai MS, Lupu R. 2001. Signaling molecules implicated in heregulin induction of growth arrest and apoptosis. Oncol Rep 8: 12031214.
  • Guy PM, Platko JV, Cantley LC, Cerione RA, Carraway KL III. 1994. Insect cell-expressed p180erbB3 possesses an impaired tyrosine kinase activity. Proc Natl Acad Sci USA 91: 81328136.
  • Harari D, Tzahar E, Romano J, Shelly M, Pierce JH, Andrews GC, Yarden Y. 1999. Neuregulin-4: A novel growth factor that acts through the ErbB-4 receptor tyrosine kinase. Oncogene 18: 26812689.
  • Hognason T, Chatterjee S, Vartanian T, Ratan RR, Ernewein KM, Habib AA. 2001. Epidermal growth factor receptor induced apoptosis: Potentiation by inhibition of Ras signaling. FEBS Lett 491: 915.
  • Holbro T, Beerli RR, Maurer F, Koziczak M, Barbas CF III, Hynes NE. 2003. The ErbB2/ErbB3 heterodimer functions as an oncogenic unit: ErbB2 requires ErbB3 to drive breast tumor cell proliferation. Proc Natl Acad Sci USA 100: 89338938.
  • Horan T, Wen J, Arakawa T, Liu NL, Brankow D, Hu S, Ratzkin B, Philo JS. 1995. Binding of neu differentiation factor with the extracellular domain of Her2 and Her3. J Biol Chem 270: 2460424608.
  • Huang GC, Hobbs S, Walton M, Epstein RJ. 2002. Dominant negative knockout of p53 abolishes ErbB2-dependent apoptosis and permits growth acceleration in human breast cancer cells. Br J Cancer 86: 11041109.
  • James G, Olson EN. 1989. Identification of a novel fatty acylated protein that partitions between the plasma membrane and cytosol and is deacylated in response to serum and growth factor stimulation. J Biol Chem 264: 2099821006.
  • Jones FE, Stern DF. 1999. Expression of dominant-negative ErbB2 in the mammary gland of transgenic mice reveals a role in lobuloalveolar development and lactation. Oncogene 18: 34813490.
  • Jones FE, Welte T, Fu XY, Stern DF. 1999. ErbB4 signaling in the mammary gland is required for lobuloalveolar development and stat5 activation during lactation. J Cell Biol 147: 7788.
  • Karunagaran D, Tzahar E, Beerli RR, Chen XM, Graus-Porta D, Ratzkin BJ, Seger R, Hynes NE, Yarden Y. 1996. ErbB-2 is a common auxiliary subunit of NDF and EGF receptors: Implications for breast cancer. EMBO J 15: 254264.
  • Kim HH, Vijapurkar U, Hellyer NJ, Bravo D, Koland JG. 1998. Signal transduction by epidermal growth factor and heregulin via the kinase-deficient ErbB3 protein. Biochem J 334: 189195.
  • Kim JY, Sun Q, Oglesbee M, Yoon SO. 2003. The role of ErbB2 signaling in the onset of terminal differentiation of oligodendrocytes in vivo. J Neurosci 23: 55615571.
  • Kramer R, Bucay N, Kane DJ, Martin LE, Tarpley JE, Theill LE. 1996. Neuregulins with an Ig-like domain are essential for mouse myocardial and neuronal development. Proc Natl Acad Sci USA 93: 48334838.
  • Kulik G, Klippel A, Weber MJ. 1997. Antiapoptotic signalling by the insulin-like growth factor I receptor, phosphatidylinositol 3-kinase, and Akt. Mol Cell Biol 17: 15951606.
  • Lane HA, Beuvink I, Motoyama AB, Daly JM, Neve RM, Hynes NE. 2000. ErbB2 potentiates breast tumor proliferation through modulation of p27(Kip1)-Cdk2 complex formation: Receptor overexpression does not determine growth dependency. Mol Cell Biol 20: 32103223.
  • Latta EK, Tjan S, Parkes RK, O'Malley FP. 2002. The role of HER2/neu overexpression/amplification in the progression of ductal carcinoma in situ to invasive carcinoma of the breast. Mod Pathol 15: 13181325.
  • Le XF, Marcelli M, McWatters A, Nan B, Mills GB, O'Brian CA, Bast RC, Jr. 2001. Heregulin-induced apoptosis is mediated by down-regulation of Bcl-2 and activation of caspase-7 and is potentiated by impairment of protein kinase C alpha activity. Oncogene 20: 82588269.
  • Lee K-F, Simon H, Chen H, Bates B, Hung M-C, Hauser C. 1995. Requirement for neuregulin receptor erbB2 in neural and cardiac development. Nature 378: 394398.
  • Lehto VP. 2001. EGF receptor: Which way to go? FEBS Lett 491: 13.
  • Lewis DA, Zweig B, Hurwitz SA, Spandau DF. 2003. Inhibition of erbB receptor family members protects HaCaT keratinocytes from ultraviolet-B-induced apoptosis. J Invest Dermatol 120: 483488.
  • Liu X, Robinson GW, Wagner KU, Garret L, Wynshaw-Boris A, Hennigausen L. 1997. Stata is mandatory for adult mammary gland development and lactogenesis. Genes Dev 11: 179186.
  • Long W, Wagner KU, Lloyd KC, Binart N, Shillingford JM, Hennighausen L, Jones FE. 2003. Impaired differentiation and lactational failure of Erbb4-deficient mammary glands identify ERBB4 as an obligate mediator of STAT5. Development 130: 52575268.
  • Luetteke NC, Phillips HK, Qiu TH, Copeland NG, Earp HS, Jenkins NA, Lee DC. 1994. The mouse waved-2 phenotype results from a point mutation in the EGF receptor tyrosine kinase. Genes Dev 8: 399413.
  • Manegold C. 2003. Gefitinib (Iressa, ZD 1839) for non-small cell lung cancer (NSCLC): Recent results and further strategies. Adv Exp Med Biol 532: 247252.
  • Marchionni MA, Goodearl ADJ, Chen MS, Bermingham-McDonogh O, Kirk C, Hendricks M, Danehy F, Misumi D, Sudhalter J, Kobayashi K, Wroblewski D, Lynch C, Baldassare M, Hiles I, Davis JB, Hsuan JJ, Totty NF, Otsu M, McBurney RN, Waterfield MD, Stroobant P, Gwynne D. 1993. Glial growth factors are alternatively spliced erbB2 ligands expressed in the nervous system. Nature 362: 312318.
  • Massagué J, Pandiella A. 1993. Membrane-anchored growth factors. Annu Rev Biochem 62: 515541.
  • Meyer D, Yamaai T, Garratt A, Riethmacher-Sonnenberg E, Kane D, Theill LE, Birchmeier C. 1997. Isoform-specific expression and function of neuregulin. Development 124: 35753586.
  • Miettinen PJ, Berger JE, Meneses J, Phung Y, Pedersen RA, Werb Z, Derynck R. 1995. Epithelial immaturity and multiorgan failure in mice lacking epidermal growth factor receptor. Nature 376: 337341.
  • Morris JK, Lin W, Hauser C, Marchuk Y, Getman D, Lee KF. 1999. Rescue of the cardiac defect in ErbB2 mutant mice reveals essential roles of ErbB2 in peripheral nervous system development. Neuron 23: 273283.
  • Ménard S, Casalini P, Pilotti S, Cascinelli N, Rilke F, Colnaghi MI. 1996. No additive impact on patient survival of the double alteration of p53 and c-erbB-2 in breast carcinomas. J Natl Cancer Inst 88: 10021003.
  • Ménard S, Casalini P, Tomasic G, Pilotti S, Cascinelli N, Bufalino R, Perrone F, Longhi C, Rilke F, Colnaghi MI. 1999. Pathobiologic identification of two distinct breast carcinoma subsets with diverging clinical behaviors. Breast Cancer Res Treat 55: 169177.
  • Ménard S, Balsari A, Casalini P, Tagliabue E, Campiglio M, Bufalino R, Cascinelli N. 2002. HER2-positive breast carcinomas as a particular subset with peculiar clinical behaviors. Clin Cancer Res 8: 520525.
  • Nelson JM, Fry DW. 2001. Akt, MAPK (Erk1/2), and p38 act in concert to promote apoptosis in response to ErbB receptor family inhibition. J Biol Chem 276: 1484214847.
  • Neve RM, Sutterluty H, Pullen N, Lane HA, Daly JM, Krek W, Hynes NE. 2000. Effects of oncogenic ErbB2 on G1 cell cycle regulators in breast tumour cells. Oncogene 19: 16471656.
  • Neve RM, Holbro T, Hynes NE. 2002. Distinct roles for phosphoinositide 3-kinase, mitogen-activated protein kinase and p38 MAPK in mediating cell cycle progression of breast cancer cells. Oncogene 21: 45674576.
  • Niemann C, Brinkmann V, Spitzer E, Hartmann G, Sachs M, Naundorf H, Birchmeier W. 1998. Reconstitution of mammary gland development in vitro: Requirement of c-met and c-erbB2 signaling for branching and alveolar morphogenesis [in process citation]. J Cell Biol 143: 533545.
  • Olayioye MA, Neve RM, Lane HA, Hynes NE. 2000. The ErbB signaling network: Receptor heterodimerization in development and cancer. EMBO J 19: 31593167.
  • Olayioye MA, Badache A, Daly JM, Hynes NE. 2001. An essential role for Src kinase in ErbB receptor signaling through the MAPK pathway. Exp Cell Res 267: 8187.
  • Ozcelik C, Erdmann B, Pilz B, Wettschureck N, Britsch S, Hubner N, Chien KR, Birchmeier C, Garratt AN. 2002. Conditional mutation of the ErbB2 (HER2) receptor in cardiomyocytes leads to dilated cardiomyopathy. Proc Natl Acad Sci USA 99: 88808885.
  • Peles E, Bacus SS, Koski RA, Lu HS, Wen D, Ogden SG, Ben Levy R, Yarden Y. 1992. Isolation of the neu/HER-2 stimulatory ligand: A 44 kd glycoprotein that induces differentiation of mammary tumor cells. Cell 69: 205216.
  • Prigent SA, Gullick WJ. 1994. Identification of c-erbB-3 binding sites for phosphatidylinositol 3′-kinase and SHC using an EGF receptor/c-erbB-3 chimera. EMBO J 13: 28312841.
  • Radmacher MD, Simon R. 2000. Estimation of tamoxifen's efficacy for preventing the formation and growth of breast tumors. J Natl Cancer Inst 92: 4853.
  • Riethmacher D, Sonnenberg-Riethmacher E, Brinkmann V, Yamaai T, Lewin GR, Birchmeier C. 1997. Severe neuropathies in mice with targeted mutations in the ErbB3 receptor. Nature 389: 725730.
  • Schaeffer L, Duclert N, Huchet-Dymanus M, Changeux JP. 1998. Implication of a multisubunit Ets-related transcription factor in synaptic expression of the nicotinic acetylcholine receptor. EMBO J 17: 30783090.
  • Schroeder JA, Lee DC. 1998. Dynamic expression and activation of ERBB receptors in the developing mouse mammary gland. Cell Growth Differ 9: 451464.
  • Sebastian J, Richards RG, Walker MP, Wiesen JF, Werb Z, Derynck R, Hom YK, Cunha GR, DiAugustine RP. 1998. Activation and function of the epidermal growth factor receptor and erbB-2 during mammary gland morphogenesis. Cell Growth Differ 9: 777785.
  • Sepp-Lorenzino L, Eberhard I, Ma Z, Cho C, Serve H, Liu F, Rosen N, Lupu R. 1996. Signal transduction pathways induced by heregulin in MDA-MB-453 breast cancer cells. Oncogene 12: 16791687.
  • Slamon DJ, Clark GM, Wong SG, Levin WJ, Ullrich A, McGuire WL. 1987. Human breast cancer: Correlation of relapse and survival with amplification of the HER-2/neu oncogene. Science 235: 177182.
  • Slamon DJ, Leyland-Jones B, Shak S, Fuchs H, Paton V, Bajamonde A, Fleming T, Eiermann W, Wolter J, Pegram M, Baselga J, Norton L. 2001. Use of chemotherapy plus a monoclonal antibody against HER2 for metastatic breast cancer that overexpresses HER2. N Engl J Med 344: 783792.
  • Teglund S, McKay C, Schuetz E, van Deursen JM, Stravopodis D, Wang D, Brown M, Bodner S, Grosveld G, Ihle JN. 1998. Stat5a and Stat5b proteins have essential and nonessential, or redundant, roles in cytokine responses. Cell 93: 841850.
  • Tidcombe H, Jackson-Fisher A, Mathers K, Stern DF, Gassmann M, Golding JP. 2003. Neural and mammary gland defects in ErbB4 knockout mice genetically rescued from embryonic lethality. Proc Natl Acad Sci USA 100: 82818286.
  • Tzahar E, Waterman H, Chen X, Levkowitz G, Karunagaran D, Lavi S, Ratzkin BJ, Yarden Y. 1996. A hierarchinal network of interreceptor interactions determines signal transduction by neu differentiation factor/neuregulin and epidermal growth factor. Mol Cell Biol 16: 52765287.
  • Woldeyesus MT, Britsch S, Riethmacher D, Xu L, Sonnenberg-Riethmacher E, Abou-Rebyeh F, Harvey R, Caroni P, Birchmeier C. 1999. Peripheral nervous system defects in erbB2 mutants following genetic rescue of heart development. Genes Dev 13: 25382548.
  • Wolpowitz D, Mason TB, Dietrich P, Mendelsohn M, Talmage DA, Role LW. 2000. Cysteine-rich domain isoforms of the neuregulin-1 gene are required for maintenance of peripheral synapses. Neuron 25: 7991.
  • Yang Y, Spitzer E, Meyer D, Sachs M, Niemann C, Hartmann G, Weidner KM, Birchmeier C, Birchmeier W. 1995. Sequential requirement of hepatocyte growth factor and neuregulin in the morphogenesis and differentiation of the mammary gland. J Cell Biol 131: 215226.
  • Yarden Y. 2001. The EGFR family and its ligands in human cancer. Signalling mechanisms and therapeutic opportunities. Eur J Cancer 37: S3S8.
  • Yarden Y, Sliwkowski MX. 2001. Untangling the ErbB signalling network. Nat Rev Mol Cell Biol 2: 127137.
  • Zhang D, Sliwkowski MX, Mark M, Frantz G, Akita R, Sun Y, Hillan K, Crowley C, Brush J, Godowski PJ. 1997. Neuregulin-3 (NRG3): A novel neural tissue-enriched protein that binds and activates ErbB4. Proc Natl Acad Sci USA 94: 95629567.