The relationship between human epidermal growth-like factor receptor expression and cellular transformation in NIH3T3 cells

Targeting epidermal growth factor receptor in head and neck cancer

It is well known that growth factors play an important role in normal cell proliferation by means of stimulation of growth factor receptors located on the surface of cells. Tumor cells express high levels of growth factor receptors that can theoretically serve as therapeutic targets in cancer treatment. Tyrosine kinase (type 1) growth factor receptors include the family of erbB receptors. The most extensively studied receptor in the erbB family is the human epidermal growth factor receptor (EGFR), also known as erbB1. Studies have shown that overexpression of EGFR is involved in the development and progression of head and neck squamous cell carcinoma (HNSCC). Blocking this receptor in HNSCC cell lines and animal models inhibits tumor growth. Strategies have been developed to target EGFR, including monoclonal antibodies, tyrosine kinase-specific inhibitors, ligand-linked immunotoxins, and antisense approaches. Laboratory studies and clinical trials are under way to explore the safety and efficacy of these various approaches in a variety of cancers, including HNSCC. Preliminary results from early phase clinical trials are encouraging and may lead to the incorporation of these EGFR targeting strategies into the management of HNSCC.

Differential regulation of tumor angiogenesis by distinct ErbB homo- and heterodimers

Interactions between cancer cells and their microenvironment are critical for the development and progression of solid tumors. This study is the first to examine the role of all members of the ErbB tyrosine kinase receptors (epidermal growth factor receptor [EGFR], ErbB-2, ErbB-3, or ErbB-4), expressed singly or as paired receptor combinations, in the regulation of angiogenesis both in vitro and in vivo. Comparison of all receptor combinations reveals that EGFR/ErbB-2 and ErbB-2/ErbB-3 heterodimers are the most potent inducers of vascular endothelial growth factor (VEGF) mRNA expression compared with EGFR/ErbB-3, EGFR/ErbB-4, ErbB-2/ErbB-4, and ErbB-3/ErbB-4. Immunohistochemistry of tumor xenografts overexpressing these heterodimers shows increased VEGF expression and remarkably enhanced vascularity. Enhanced VEGF expression is associated with increased VEGF transcription. Deletional analysis reveals that ErbB-mediated transcriptional up-regulation of VEGF involves a hypoxia-inducible factor 1-independent responsive region located between nucleotides -88 to -66 of the VEGF promoter. Mutational analysis reveals that the Sp-1 and AP-2 transcription factor binding elements within this region are required for up-regulation of VEGF by heregulin beta1 and that this up-regulation is dependent on the activity of extracellular signal-related protein kinases. These results emphasize the biological implications of cell signaling diversity among members of the ErbB receptor family in regulation of the tumor microenvironment.

c-Src-dependent transcriptional activation of TFII-I

TFII-I is a multifunctional transcription factor that is also involved in signal transduction. Here we show that TFII-I undergoes a c-Src-dependent tyrosine phosphorylation on tyrosine residues 248 and 611 and translocates to the nucleus in response to growth factor signaling. Tyrosine-phosphorylated nuclear TFII-I activates a stably integrated c-fos reporter gene. Withdrawal of signal leads to diminution of nuclear TFII-I, suggesting that the signal-dependent translocation is reversible. Antibodies against either TFII-I or c-Src abrogate growth factor-stimulated activation of c-fos. Consistent with the notion that tyrosine phosphorylation of TFII-I is required for its transcriptional activity, phosphorylation-deficient mutants of TFII-I fail to activate the c-fos promoter. These data demonstrate that TFII-I, through a Src-dependent mechanism, reversibly translocates from the cytoplasm to the nucleus, leading to the transcriptional activation of growth-regulated genes.

Ligand-independent oncogenic transformation by the EGF receptor requires kinase domain catalytic activity

The retroviral oncogene S3-v-erbB is a transduced, truncated form of the avian EGF (ErbB-1) receptor. Infection of avian fibroblasts with a retroviral vector expressing S3-v-ErbB results in ligand-independent cell transformation, which is accompanied by the assembly of a transformation-specific phosphoprotein signaling complex and anchorage-independent cell growth. It previously had been reported, using lysine-721 mutants (K721), that kinase domain function was required for ErbB-mediated cell transformation. However, since these initial reports, several studies using aspartate-813 mutants (D813) have demonstrated the ability of kinase-impaired ErbB receptors to induce mitogenic signal transduction pathways and cell transformation in a ligand-dependent manner. To determine the necessity of ErbB receptor kinase domain catalytic activity in ligand-independent cell transformation, we created S3-v-ErbB-K(-), a kinase-impaired oncoprotein constructed by replacing aspartate-813 with alanine (D813A). Subcellular routing as well as cell surface membrane and nuclear localization of the S3-v-ErbB-K(-) mutant receptor were unaffected by impairment of kinase activity. In contrast, avian fibroblasts expressing S3-v-ErbB-K(-) do not form the characteristic transformation-specific phosphoprotein complex, or induce soft agar colony growth in vitro. These results suggest that in contrast to ligand-dependent oncogenic signaling, ligand-independent cell transformation by a constitutively activated mutant form of the EGF receptor requires receptor kinase catalytic activity. In addition, these results demonstrate that phosphorylation and assembly of downstream signaling complexes require tyrosine phosphorylation events that are directly mediated by oncogenic forms of the EGF receptor.

CD44 associates with EGFR and erbB2 in metastasizing mammary carcinoma cells

Type I receptor tyrosine kinases, including the epidermal growth factor receptor (EGFR) and erbB2, have been implicated in mammary carcinoma growth and metastasis. Recent evidence suggests that type I receptor signaling may be mediated by the CD44 family of transmembrane glycoproteins that also have been implicated in mammary tumor progression. Here, the authors tested whether CD44, EGFR, and erbB2 interacted and colocalized with one another in four mammary carcinoma cell lines (MCF-7, MDA-MB-231, MDA-MB-435, and MDA-MB-436) and in cytology samples obtained from patients with metastatic breast cancer. CD44 constitutively colocalized and coimmunoprecipitated with erbB2 and EGFR in all four mammary carcinoma cell lines. CD44 also colocalized with erbB2 and EGFR in all cytology samples expressing erbB2. CD44 colocalized with EGFR in cells from only 1 of 16 erbB2-negative cytology samples. These data indicate that CD44-EGFR-erbB2 protein complexes occur in a high proportion of metastatic mammary carcinomas and suggest that CD44-type I receptor colocalization may be a novel prognostic marker for aggressive mammary cancers.

Comparison of the biochemical and kinetic properties of the type 1 receptor tyrosine kinase intracellular domains. Demonstration of differential sensitivity to kinase inhibitors

Epidermal growth factor receptor (EGFR), ErbB-2, and ErbB-4 are members of the type 1 receptor tyrosine kinase family. Overexpression of these receptors, especially ErbB-2 and EGFR, has been implicated in multiple forms of cancer. Inhibitors of EGFR tyrosine kinase activity are being evaluated clinically for cancer therapy. The potency and selectivity of these inhibitors may affect the efficacy and toxicity of therapy. Here we describe the expression, purification, and biochemical comparison of EGFR, ErbB-2, and ErbB-4 intracellular domains. Despite their high degree of sequence homology, the three enzymes have significantly different catalytic properties and substrate kinetics. For example, the catalytic activity of ErbB-2 is less stable than that of EGFR. ErbB-2 uses ATP-Mg as a substrate inefficiently compared with EGFR and ErbB-4. The three enzymes have very similar substrate preferences for three optimized peptide substrates, but differences in substrate synergies were observed. We have used the biochemical and kinetic parameters determined from these studies to develop an assay system that accurately measures inhibitor potency and selectivity between the type 1 receptor family. We report that the selectivity profile of molecules in the 4-anilinoquinazoline series can be modified through specific aniline substitutions. Moreover, these compounds have activity in whole cells that reflect the potency and selectivity of target inhibition determined with this assay system.

Estrogenic action of β-HCH through activation of c-Neu in MCF-7 breast carcinoma cells

β-HCH is known to be a poor agonist for the estrogen receptor (ER), and yet it has been shown to act like an estrogen in stimulating foci formation in MCF-7 cells. We investigated the reason for such an action of β-HCH, using a rat prolactin-luciferase reporter system transfected to MCF-7 cells. We found that the presence of c-Neu (erbB2), ER and ERE is needed for β-HCH to act estrogenic at the transcription activation level in this cell line. We compared the action of β-HCH to that of EGF which is known to act estrogenic without being an agonist for ER in this cell and found that their action patterns are quite similar, the only difference being that the former action is blocked by an antibody against c-Neu and the latter by both c-Neu and EGF receptor antibody. We concluded that β-HCH's estrogenic action in this cell model is mediated through "ligand-independent activation of ER pathway".

Growth factor-specific signaling pathway stimulation and gene expression mediated by ErbB receptors

The mechanisms by which receptor tyrosine kinases (RTKs) utilize intracellular signaling pathways to direct gene expression and cellular response remain unclear. A current question is whether different RTKs within a single cell target similar or different sets of genes. In this study we have used the ErbB receptor network to explore the relationship between RTK activation and gene expression. We profiled growth factor-stimulated signaling pathway usage and broad gene expression patterns in two human mammary tumor cell lines expressing different complements of ErbB receptors. Although the growth factors epidermal growth factor (EGF) and neuregulin (NRG) 1 similarly stimulated Erk1/2 in MDA-MB-361 cells, EGF acting through an EGF receptor/ErbB2 heterodimer preferentially stimulated protein kinase C, and NRG1beta acting through an ErbB2/ErbB3 heterodimer preferentially stimulated Akt. The two growth factors regulated partially overlapping yet distinct sets of genes in these cells. In MDA-MB-453 cells, NRG1beta acting through an ErbB2/ErbB3 heterodimer stimulated prolonged signaling of all pathways examined relative to NRG2beta acting through the same heterodimeric receptor species. Surprisingly, NRG1beta and NRG2beta also regulated partially overlapping but distinct sets of genes in these cells. These results demonstrate that the activation of different RTKs, or activation of the same RTKs with different ligands, can lead to distinct profiles of gene regulation within a single cell type. Our observations also suggest that the identity and kinetics of signaling pathway usage by RTKs may play a role in the selection of regulated genes.

Targeting growth factor receptors: integration of novel therapeutics in the management of head and neck cancer

Tyrosine kinase (type 1) growth factor receptors include the erbB family. These cell surface receptors were discovered in the context of cellular transformation and have subsequently been found to be overexpressed in many types of human cancer. Cumulative evidence suggests that upregulation of the most well-characterized receptor, erbB1, also known as the epidermal growth factor receptor, plays a significant role in the development and progression of head and neck squamous cell carcinoma. A variety of strategies have been developed that specifically target epidermal growth factor receptor, including monoclonal antibodies, ligand-linked immunotoxins, tyrosine kinase inhibitors, and antisense approaches. Epidermal growth factor receptor blockade in head and neck squamous cell carcinoma cell lines and preclinical animal models inhibits cell proliferation and tumor growth. Clinical trials are under way to test the safety and efficacy of many of these targeting strategies in head and neck squamous cell carcinoma patients. Encouraging preliminary results combining an epidermal growth factor receptor targeting approaches with chemotherapy or radiotherapy suggest that interference with this growth factor receptor may enhance antitumor efficacy of standard therapies. As erbB family member interactions and downstream signaling pathways are elucidated in head and neck squamous cell carcinoma, specific targeting strategies may become incorporated into standard treatment approaches.

Molecular mechanisms underlying ErbB2/HER2 action in breast cancer

Overexpression of ErbB2, a receptor-like tyrosine kinase, is shared by several types of human carcinomas. In breast tumors the extent of overexpression has a prognostic value, thus identifying the oncoprotein as a target for therapeutic strategies. Already, antibodies to ErbB2 are used in combination with chemotherapy in the treatment of metastasizing breast cancer. The mechanisms underlying the oncogenic action of ErbB2 involve a complex network in which ErbB2 acts as a ligand-less signaling subunit of three other receptors that directly bind a large repertoire of stroma-derived growth factors. The major partners of ErbB2 in carcinomas are ErbB1 (also called EGFR) and ErbB3, a kinase-defective receptor whose potent mitogenic action is activated in the context of heterodimeric complexes. Why ErbB2-containing heterodimers are relatively oncopotent is a function of a number of processes. Apparently, these heterodimers evade normal inactivation processes, by decreasing the rate of ligand dissociation, internalizing relatively slowly and avoiding the degradative pathway by returning to the cell surface. On the other hand, the heterodimers strongly recruit survival and mitogenic pathways such as the mitogen-activated protein kinases and the phosphatidylinositol 3-kinase. Hyper-activated signaling through the ErbB-signaling network results in dysregulation of the cell cycle homeostatic machinery, with upregulation of active cyclin-D/CDK complexes. Recent data indicate that cell cycle regulators are also linked to chemoresistance in ErbB2-dependent breast carcinoma. Together with D-type cyclins, it seems that the CDK inhibitor p21waf1 plays an important role in evasion from apoptosis. These recent findings herald a preliminary understanding of the output layer which connects elevated ErbB-signaling to oncogenesis and chemoresistance.

Blockade of the epidermal growth factor receptor tyrosine kinase suppresses tumorigenesis in MMTV/Neu + MMTV/TGF-alpha bigenic mice

Overexpression of ErbB-2/Neu has been causally associated with mammary epithelial transformation. Here we report that blockade of the epidermal growth factor receptor (EGFR) kinase with AG-1478 markedly delays breast tumor formation in mouse mammary tumor virus (MMTV)/Neu + MMTV/transforming growth factor alpha bigenic mice. This delay was associated with inhibition of EGFR and Neu signaling, reduction of cyclin-dependent kinase 2 (Cdk2) and mitogen-activated protein kinase (MAPK) activities and cyclin D1, and an increase in the levels of the Cdk inhibitor p27(Kip1). In addition, BrdUrd incorporation into tumor cell nuclei was prevented with no signs of tumor cell apoptosis. These observations prompted us to investigate the stability of p27. Recombinant p27 was degraded rapidly in vitro by untreated but not by AG-1478-treated tumor lysates. Proteasome depletion of the tumor lysates, addition of the specific MEK1/2 inhibitor U-0126, or a T187A mutation in recombinant p27 all prevented p27 degradation. Cdk2 and MAPK precipitates from untreated tumor lysates phosphorylated recombinant wild-type p27 but not the T187A mutant in vitro. Cdk2 and MAPK precipitates from AG-1478-treated tumors were unable to phosphorylate p27 in vitro. These data suggest that increased signaling by ErbB receptors up-regulates MAPK activity, which, in turn, phosphorylates and destabilizes p27, thus contributing to dysregulated cell cycle progression.

Complexity of signal transduction mediated by ErbB2: clues to the potential of receptor-targeted cancer therapy

The erbB2 oncogene belongs to the type I trans-membrane tyrosine kinase family of receptors. Its medical importance stems from its widespread over-expression in breast cancer. This review will focus on the signal transduction through this protein, and explains how the overexpression of erbB2 may result in poor prognosis of breast cancer, and finally it will summerize our current understanding about the therapeutic potential of receptor-targeted therapy in breast cancer. ErbB2 does not have any known ligand which is able to bind to it with high affinity. However the kinase activity of erbB2 can be activated without any ligand, if it is overexpressed, and by heteroassociation with other members of the erbB family (erbB1 or epidermal growth factor receptor, erbB3 and erbB4). This interaction substantially increases the efficiency and diversity of signal transduction through these receptor complexes. In addition, erbB2 forms large scale receptor clusters containing hundreds of proteins. These receptor islands may take part in recruiting cytosolic factors which relay the signal towards the nucleus or the cytoplasm. Overexpression of erbB2 was linked to higher transforming activity, increased metastatic potential, angiogenesis and drug resistence of breast tumor in laboratory experiments. As a corollary of these properties, erbB2 amplification is generally thought to be associated with a poor prognosis in breast cancer patients. These early findings lead to the development of antibodies that down-regulate erbB2. Such a therapeutic approach has already been found effective in experimental tumor models and in clinical trials as well. Further understanding of the importance of erbB2 and growth factor receptors in the transformation of normal cells to malignant ones may once give us a chance to cure erbB2 over-expressing breast cancer.

A natural ErbB4 isoform that does not activate phosphoinositide 3-kinase mediates proliferation but not survival or chemotaxis

ErbB4 is a member of the epidermal growth factor receptor (ErbB) family that mediates cellular responses activated by neuregulins (NRG) and other epidermal growth factor-like growth factors. Two naturally occurring ErbB4 isoforms, ErbB4 CYT-1 and ErbB4 CYT-2, have previously been identified. Unlike ErbB4 CYT-1, ErbB4 CYT-2 lacks a phosphoinositide 3-kinase (PI3-K)-binding site and is incapable of activating PI3-K. We have now examined the consequences of the inability of this isoform to activate PI3-K on cell proliferation, survival, and chemotaxis in response to NRG-1beta: (i) NRG-1beta stimulated proliferation of cells expressing either ErbB4 CYT-1 or ErbB4 CYT-2. Consistent with the mitogenic responsiveness, analysis of downstream signaling showed that Shc and MAPK were phosphorylated after stimulating either isoform with NRG-1beta. (ii) NRG-1beta protected cells expressing ErbB4 CYT-1 but not cells expressing ErbB4 CYT-2 from starvation-induced apoptosis as measured by effects on cell number and 4', 6-diamidino-2-phenylindole staining. Furthermore, in cells expressing ErbB4 CYT-2, Akt, a protein kinase that mediates cell survival, was not phosphorylated. (iii) NRG-1beta stimulated chemotaxis and membrane ruffling in cells expressing ErbB4 CYT-1 but not in cells expressing ErbB4 CYT-2. In summary, ErbB4 CYT-2 can mediate proliferation but not chemotaxis or survival. These results suggest a novel mechanism by which cellular responses such as chemotaxis and survival may be regulated by the expression of alternative receptor-tyrosine kinase isoforms that differ in their coupling to PI3-K signaling.

Receptor tyrosine kinase and p16/CDKN2 expression in a case of tripe palms associated with non-small-cell lung cancer

BACKGROUND

Tripe palms is a descriptive term for a cutaneous paraneoplastic keratoderma. Tripe palms are frequently associated with gastric and pulmonary carcinoma. The pathogenetic mechanism remains unknown.

OBJECTIVE

To determine the influence of receptor tyrosine kinases, which are both expressed in pulmonary carcinomas and in human skin, we performed expression studies on epidermal growth factor receptor (EGFR), HER2, HER3 in a skin sample of tripe palms obtained from a patient with non-small-cell lung cancer with lymph node involvement. Two months after diagnosis, the patient had developed palmoplantar 'tripe palms'. Additionally, the expression of SRC, c-myc and p16/ CDKN2 were studied.

METHOD

Conventional reverse-transcription polymerase chain reaction was performed on a tissue sample of tripe palms.

RESULTS

Weak expression of HER2 and of p16/CDKN2 was found. EGFR, HER3, c-myc and SRC were not expressed.

CONCLUSION

Receptor tyrosine kinases of subclass I, the tyrosine kinase SRC and the oncogene c-myc play no major role in the pathogenesis of this case of tripe palms.

Acceleration of trophoblast differentiation by heparin-binding EGF-like growth factor is dependent on the stage-specific activation of calcium influx by ErbB receptors in developing mouse blastocysts

Heparin-binding EGF-like growth factor (HB-EGF) is expressed in the mouse endometrial epithelium during implantation exclusively at sites apposed to embryos and accelerates the development of cultured blastocysts, suggesting that it may regulate peri-implantation development in utero. We have examined the influence of HB-EGF on mouse trophoblast differentiation in vitro and the associated intracellular signaling pathways. HB-EGF both induced intracellular Ca2+ signaling and accelerated trophoblast development to an adhesion-competent stage, but only late on gestation day 4 after ErbB4, a receptor for HB-EGF, translocated from the cytoplasm to the apical surface of trophoblast cells. The acceleration of blastocyst differentiation by HB-EGF was attenuated after inhibition of protein tyrosine kinase activity or removal of surface heparan sulfate, as expected. Chelation of intracellular Ca2+ blocked the ability of HB-EGF to accelerate development, as did inhibitors of protein kinase C or calmodulin. The absence of any effect by a phospholipase C inhibitor and the requirement for extracellular Ca2+ suggested that the accrued free cytoplasmic Ca2+ did not originate from inositol phosphate-sensitive intracellular stores, but through Ca2+ influx. Indeed, N-type Ca2+ channel blockers specifically inhibited the ability of HB-EGF to both induce Ca2+ signaling and accelerate trophoblast development. We conclude that HB-EGF accelerates the differentiation of trophoblast cells to an adhesion-competent stage by inducing Ca2+ influx, which activates calmodulin and protein kinase C. An upstream role for ErbB4 in this pathway is implicated by the timing of its translocation to the trophoblast surface.

The role of the epidermal growth factor receptor family in mammary tumorigenesis and metastasis

A number of receptor systems have been implicated to play an important role in the development and progression of many human cancers. The epidermal growth factor (EGF) receptor tyrosine kinase family has been found to consistently play a leading role in tumor progression. Indeed, in human breast cancer cases the prognosis of a patient is inversely correlated with the overexpression and/or amplification of this receptor family. Furthermore, downstream signaling components such as the Src kinases, PI3'K, and the Ras pathway display evidence of deregulation that can accelerate tumor progression. The transgenic mouse system has been ideal in elucidating the biological significance of this receptor family in mammary tumorigenesis. Molecular events involved in mammary tumorigenesis such as ligand binding, receptor dimerization, and the activation of downstream pathways have been addressed using this system. Although there are many molecular steps that appear to drive each stage of tumor development, the EGF receptor family appears to play a causal role in the progression to a transformed phenotype.

c-erbB-2/EGFR as dominant heterodimerization partners determine a motogenic phenotype in human breast cancer cells

Separate mechanisms for oncogenesis and metastasis have been postulated. We show here that prolonged and invasive cell migration, a key mechanism in cancer metastasis, is linked to c-erbB-2 signaling. Cell lines with c-erbB-2 and EGFR expression and transphosphorylation activity display a high transendothelial invasiveness in an endothelial-extracellular matrix model mimicking a capillary vessel wall in vitro. Tyrosine-phosphorylated c-erbB-2 receptors and EGFR are localized predominantly in areas of the cell with high membrane extension activity. On the molecular level, there is a subtle cross talk between the transmembrane signaling molecule c-erbB-2 and the actin cytoskeleton at multiple levels, including the generation of the second messenger PIP2 and the mobilization of the actin-regulatory protein gelsolin. Our data strongly suggest that c-erbB-2, especially in a heterodimer with EGFR, is closely involved in signaling pathways, inducing alterations in cell morphology that are required for a human breast cancer cell to become motile and conceivably metastatic.

Expression of heregulin alpha, erbB2, and erbB3 and their influences on proliferation of gastric epithelial cells

BACKGROUND & AIMS

Heregulins (HRGs) regulate the proliferation and differentiation of various cell types. However, very little is known about their function in the gastrointestinal tract. The aim of this study was to investigate the role of HRGs on gastrointestinal cells.

METHODS

We examined the expression of erbB receptors and HRG-alpha in human gastric cancer cell lines, rat gastrointestinal epithelial cells, and human gastric fibroblasts by Western blot analysis or reverse-transcription polymerase chain reaction. Receptor phosphorylation and heterodimerization induced by HRG-alpha were detected by Western blot analysis. We also evaluated the in vitro effects of HRG-alpha on cell proliferation and restitution.

RESULTS

Cancer cell lines and rat epithelial cells expressed erbB2 and erbB3, but protein expression of erbB4 was not detected. HRG-alpha was detected only in gastric fibroblasts. HRG-alpha activated tyrosine phosphorylation of epidermal growth factor receptor (EGFR), erbB2, and erbB3 and induced not only erbB3/erbB2 but also erbB3/EGFR and erbB2/EGFR heterodimer formation in MKN-28 cancer cells. Simultaneous cultivation of MKN-28 cells with gastric fibroblasts resulted in tyrosine phosphorylation of erbB3 in MKN-28 cells. HRG-alpha also stimulated proliferation of MKN-28 cells and gastric epithelial cells.

CONCLUSIONS

The data suggest that HRG-alpha may affect epithelial cell proliferation through mesenchymal-epithelial interaction in the gastric mucosa.

Stimulation of beta1-integrin function by epidermal growth factor and heregulin-beta has distinct requirements for erbB2 but a similar dependence on phosphoinositide 3-OH kinase

Integrins and growth factor receptors are important participants in cellular adhesion and migration. The EGF receptor (EGFR) family of tyrosine kinases and the beta1-integrin adhesion receptors are of particular interest, given the implication for their involvement in the initiation and progression of tumorigenesis. We used adhesion and chemotaxis assays to further elucidate the relationship between these two families of transmembrane signaling molecules. Specifically, we examined integrin-mediated adhesive and migratory characteristics of the metastatic breast carcinoma cell line MDA-MB-435 in response to stimulation with growth factors that bind to and activate the EGFR or erbB3 in these cells. Although ligand engagement of the EGFR stimulated modest beta1-dependent increases in cell adhesion and motility, heregulin-beta (HRGbeta) binding to the erbB3 receptor initiated rapid and potent induction of breast carcinoma cell adhesion and migration and required dimerization of erbB3 with erbB2. Pharmacologic inhibitors of phosphoinositide 3-OH kinase (PI 3-K) or transient expression of dominant negative forms of PI 3-K inhibited both EGF- and HRGbeta-mediated adhesion and potently blocked HRGbeta- and EGF-induced cell motility. Our results illustrate the critical role of PI 3-K activity in signaling pathways initiated by the EGFR or erbB3 to up-regulate beta1-integrin function.

The C-terminus of the kinase-defective neuregulin receptor ErbB-3 confers mitogenic superiority and dictates endocytic routing

Signaling by the epidermal growth factor (EGF) family and the neuregulin group of ligands is mediated by four ErbB receptor tyrosine kinases, that form homo- and heterodimeric complexes. Paradoxically, the neuregulin receptor ErbB-3 is devoid of catalytic activity, but its heterodimerization with other ErbBs, particularly the ligand-less ErbB-2 oncoprotein of carcinomas, reconstitutes superior mitogenic and transforming activities. To understand the underlying mechanism we constructed a chimeric EGF-receptor (ErbB-1) whose autophosphorylation C-terminal domain was replaced by the corresponding portion of ErbB-3. Consistent with the possibility that this domain recruits a relatively potent signaling pathway(s), the mitogenic signals generated by the recombinant fusion protein were superior to those generated by ErbB-1 homodimers and comparable to the proliferative activity of ErbB-2/ErbB-3 heterodimers. Upon ligand binding, the chimeric receptor recruited an ErbB-3-specific repertoire of signaling proteins, including Shc and the phosphatidylinositol 3-kinase, but excluding the ErbB-1-specific substrate, phospholipase Cgamma1. Unlike ErbB-1, which is destined to lysosomal degradation through a mechanism that includes recruitment of c-Cbl and receptor poly-ubiquitination, the C-terminal tail of ErbB-3 shunted the chimeric protein to the ErbB-3-characteristic recycling pathway. These observations attribute the mitogenic superiority of ErbB-3 to its C-terminal tail and imply that the flanking kinase domain has lost catalytic activity in order to restrain the relatively potent signaling capability of the C-terminus.

Neuregulin-4: a novel growth factor that acts through the ErbB-4 receptor tyrosine kinase

The ErbB/HER family of receptor tyrosine kinases consists of four receptors that bind a large number of growth factor ligands sharing an epidermal growth factor- (EGF)-like motif. Whereas ErbB-1 binds seven different ligands whose prototype is EGF, the three families of neuregulins (NRGs) activate ErbB-3 and/or ErbB-4. Here we characterize a fourth neuregulin, NRG-4, that acts through ErbB-4. The predicted pro-NRG-4 is a transmembrane protein carrying a unique EGF-like motif and a short cytoplasmic domain. A synthetic peptide encompassing the full-length EGF-like domain can induce growth of interleukin-dependent cells ectopically expressing ErbB-4, but not cells expressing the other three ErbB proteins or their combinations. Consistent with specificity to ErbB-4, NRG-4 can displace an ErbB-4-bound NRG-1 and can activate signaling downstream of this receptor. Expression of NRG-4 mRNA was detected in the adult pancreas and weakly in muscle; other tissues displayed no detectable NRG-4 mRNA. The primary structure and the pattern of expression of NRG-4, together with the strict specificity of this growth factor to ErbB-4, suggest a physiological role distinct from that of the known ErbB ligands.

The ErbB-2/HER2 oncoprotein of human carcinomas may function solely as a shared coreceptor for multiple stroma-derived growth factors

The erbB-2/HER2 oncogene is overexpressed in a significant fraction of human carcinomas of the breast, ovary, and lung in a manner that correlates with poor prognosis. Although the encoded protein resembles several receptors for growth factors, no high affinity ligand of ErbB-2 has so far been fully characterized. However, several lines of evidence have raised the possibility that ErbB-2 can augment signal transduction initiated by binding of certain growth factors to their direct receptors. Here, we contrasted these two models of ErbB-2 function: First, examination of a large series of epidermal growth factor (EGF)-like ligands and neuregulins, including virus-encoded ligands as well as related motifs derived from the precursor of EGF, failed to detect interactions with ErbB-2 when this protein was singly expressed. Second, by using antibodies that block inter-ErbB interactions and cells devoid of surface ErbB-2, we learned that signaling by all ligands examined, except those derived from the precursor of EGF, was enhanced by the oncoprotein. These results imply that ErbB-2 evolved as a shared receptor subunit of all ErbB-specific growth factors. Thus, oncogenicity of ErbB-2 in human epithelia may not rely on the existence of a specific ligand but rather on its ability to act as a coreceptor for multiple stroma-derived growth factors.

Elevated expression of activated forms of Neu/ErbB-2 and ErbB-3 are involved in the induction of mammary tumors in transgenic mice: implications for human breast cancer

To assess the importance of Neu activation during mammary tumorigenesis, altered receptors harboring in-frame deletions within the extracellular domain were expressed in transgenic mice. Females from several independent lines develop multiple mammary tumors that frequently metastasize to the lung. Tumor progression in these strains was associated with elevated levels of tyrosine-phosphorylated Neu and ErbB-3. Consistent with these observations, a survey of primary human breast tumors revealed frequent co-expression of both erbB-2 and erbB-3 transcripts. The ability of altered Neu receptors to induce mammary tumorigenesis in transgenic mice prompted us to examine whether similar mutations occurred in ErbB-2 during human breast cancer progression. Interestingly, an alternatively spliced form of erbB-2, closely resembling spontaneous activated forms of neu, was detected in human breast tumors. The ErbB-2 receptor encoded by this novel transcript harbors an in-frame deletion of 16 amino acids in the extracellular domain and can transform Rat-1 fibroblasts. Together, these observations argue that co-expression of ErbB-2 and ErbB-3 may play a critical role in the induction of human breast tumors, and raise the possibility that activating mutations in the ErbB-2 receptor may also contribute to this process.

Cripto-1 indirectly stimulates the tyrosine phosphorylation of erb B-4 through a novel receptor

Cripto-1 (CR-1) is a recently discovered protein of the epidermal growth factor family that fails to directly bind to any of the four known erb B type 1 receptor tyrosine kinases. The present study demonstrates that CR-1 indirectly induces tyrosine phosphorylation of erb B-4 but not of the epidermal growth factor-related receptors erb B-2 and erb B-3 in different mouse and human mammary epithelial cell lines. In addition, down-regulation of erb B-4 in NMuMG mouse mammary epithelial cells and in T47D human breast cancer cells, using an anti-erb B-4 blocking antibody or a hammerhead ribozyme vector targeted to erb B-4 mRNA, impairs the ability of CR-1 to fully activate mitogen-activated protein kinase. Finally, chemical cross-linking of 125I-CR-1 to mouse and human mammary epithelial cell membranes results in the labeling of two specific bands with a molecular weight of 130 and 60 kDa, suggesting that the CR-1 receptor represents a novel receptor structurally unrelated to any of the known type I receptor tyrosine kinases. In conclusion, these data demonstrate that CR-1, upon binding to an unknown receptor, can enhance the tyrosine kinase activity of erb B-4 and that a functional erb B-4 receptor is required for CR-1-induced MAPK activation.

Contributions of the epidermal growth factor receptor to keratinocyte motility

The epidermal growth factor (EGF) receptor plays a central role in numerous aspects of keratinocyte biology. In normal epidermis, the EGF receptor is important for autocrine growth of this renewing tissue, suppression of terminal differentiation, promotion of cell survival, and regulation of cell migration during epidermal morphogenesis and wound healing. In wounded skin, the EGF receptor is transiently up-regulated and is an important contributor to the proliferative and migratory aspects of wound reepithelialization. In keratinocytic carcinomas, aberrant expression or activation of the EGF receptor is common and has been proposed to play a role in tumor progression. Many cellular processes such as altered cell adhesion, expression of matrix degrading proteinases, and cell migration are common to keratinocytes during wound healing and in metastatic tumors. The EGF receptor is able to regulate each of these cellular functions and we propose that transient and dynamic elevation of EGF receptor during wound healing, or constitutive overexpression in tumors, provides an important contribution to the migratory and invasive potential of keratinocytes.

Pathogenic poxviruses reveal viral strategies to exploit the ErbB signaling network

Virulence of poxviruses, the causative agents of smallpox, depends on virus-encoded growth factors related to the mammalian epidermal growth factor (EGF). Here we report that the growth factors of Shope fibroma virus, Myxoma virus and vaccinia virus (SFGF, MGF and VGF) display unique patterns of specificity to ErbB receptor tyrosine kinases; whereas SFGF is a broad-specificity ligand, VGF binds primarily to ErbB-1 homodimers, and the exclusive receptor for MGF is a heterodimer comprised of ErbB-2 and ErbB-3. In spite of 10- to 1000-fold lower binding affinity to their respective receptors, the viral ligands are mitogenically equivalent or even more potent than their mammalian counterparts. This remarkable enhancement of cell growth is due to attenuation of receptor degradation and ubiquitination, which leads to sustained signal transduction. Our results imply that signal potentiation and precise targeting to specific receptor combinations contribute to cell transformation at sites of poxvirus infection, and they underscore the importance of the often ignored low-affinity ligand-receptor interactions.

ErbB tyrosine kinases and the two neuregulin families constitute a ligand-receptor network

The recently isolated second family of neuregulins, NRG2, shares its primary receptors, ErbB-3 and ErbB-4, and induction of mammary cell differentiation with NRG1 isoforms, suggesting functional redundancy of the two growth factor families. To address this possibility, we analyzed receptor specificity of NRGs by using an engineered cellular system. The activity of isoform-specific but partly overlapping patterns of specificities that collectively activate all eight ligand-stimulatable ErbB dimers was revealed. Specifically, NRG2-alpha [corrected], like NRG1-beta [corrected], emerges as a narrow-specificity ligand, whereas NRG2-beta [corrected] is a pan-ErbB ligand that binds with different affinities to all receptor combinations, including those containing ErbB-1, but excluding homodimers of ErbB-2. The latter protein, however, displayed cooperativity with the direct NRG receptors. Apparently, signaling by all NRGs is funneled through the mitogen-activated protein kinase (MAPK). However, the duration and potency of MAPK activation depend on the identity of the stimulatory ligand-receptor ternary complex. We conclude that the NRG-ErbB network represents a complex and nonredundant machinery developed for fine-tuning of signal transduction.

ErbB-1 and ErbB-2 acquire distinct signaling properties dependent upon their dimerization partner

The different epidermal growth factor (EGF)-related peptides elicit a diverse array of biological responses as the result of their ability to activate distinct subsets of ErbB receptor dimers, leading to the recruitment of different intracellular signaling networks. To specifically examine dimerization-dependent modulation of receptor signaling, we constructed NIH 3T3 cell lines expressing ErbB-1 and ErbB-2 singly and in pairwise combinations with each other ErbB family member. This model system allowed the comparison of EGF-activated ErbB-1 with ErbB-1 activated by Neu differentiation factor (NDF)-induced heterodimerization with ErbB-4. In both cases, ErbB-1 coupled to the adaptor protein Shc, but only when activated by EGF was it able to interact with Grb2. Compared to the rapid internalization of EGF-activated ErbB-1, NDF-activated ErbB-1 showed delayed internalization characteristics. Furthermore, the p85 subunit of phosphatidylinositol kinase (PI3-K) associated with EGF-activated ErbB-1 in a biphasic manner, whereas association with ErbB-1 transactivated by ErbB-4 was monophasic. The signaling properties of ErbB-2 following heterodimerization with the other ErbB receptors or homodimerization induced by point mutation or monoclonal antibody treatment were also analyzed. ErbB-2 binding to peptides containing the Src homology 2 domain of Grb2 or p85 and the phosphotyrosine binding domain of Shc varied according to the mode of receptor activation. Finally, tryptic phosphopeptide mapping of both ErbB-1 and ErbB-2 revealed that receptor phosphorylation is dependent on the dimerization partner. Differential receptor phosphorylation may, therefore, be the basis for the differences in the signaling properties observed.

Signal transduction by epidermal growth factor and heregulin via the kinase-deficient ErbB3 protein

The role of protein tyrosine kinase activity in ErbB3-mediated signal transduction was investigated. ErbB3 was phosphorylated in vivo in response to either heregulin (HRG) in cells expressing both ErbB3 and ErbB2, or epidermal growth factor (EGF) in cells expressing both ErbB3 and EGF receptor. A recombinant receptor protein (ErbB3-K/M, in which K/M stands for Lys-->Met amino acid substitution) containing an inactivating mutation in the putative ATP-binding site was also phosphorylated in response to HRG and EGF. Both the wild-type ErbB3 and mutant ErbB3-K/M proteins transduced signals to phosphatidylinositol 3-kinase, Shc and mitogen-activated protein kinases. Separate kinase-inactivating mutations in the EGF receptor and ErbB2 proteins abolished ErbB3 phosphorylation and signal transduction activated by EGF and HRG respectively. Hence the protein tyrosine kinase activity necessary for growth factor signalling via the ErbB3 protein seems to be provided by coexpressed EGF and ErbB2 receptor proteins.

Differential endocytic routing of homo- and hetero-dimeric ErbB tyrosine kinases confers signaling superiority to receptor heterodimers

Both homo- and hetero-dimers of ErbB receptor tyrosine kinases mediate signaling by a large group of epidermal growth factor (EGF)-like ligands. However, some ligands are more potent than others, although they bind to the same direct receptor. In addition, signaling by receptor heterodimers is superior to homodimers. We addressed the mechanism underlying these two features of signal tuning by using three ligands: EGF; transforming growth factor alpha (TGFalpha); and their chimera, denoted E4T, which act on cells singly expressing ErbB-1 as a weak, a strong, and a very strong agonist, respectively. Co-expression of ErbB-2, a developmentally important co-receptor whose expression is frequently elevated in human cancers, specifically potentiated EGF signaling to the level achieved by TGFalpha, an effect that was partially mimicked by ErbB-3. Analysis of the mechanism underlying this trans-potentiation implied that EGF-driven homodimers of ErbB-1 are destined for intracellular degradation, whereas the corresponding heterodimers with ErbB-2 or with ErbB-3, dissociate in the early endosome. As a consequence, in the presence of either co-receptor, ErbB-1 is recycled to the cell surface and its signaling is enhanced. This latter route is followed by TGFalpha-driven homodimers of ErbB-1, and also by E4T-bound receptors, whose signaling is further enhanced by repeated cycles of binding and dissociation from the receptors. We conclude that alternative endocytic routes of homo- and hetero-dimeric receptor complexes may contribute to tuning and diversification of signal transduction. In addition, the ability of ErbB-2 to shunt ligand-activated receptors to recycling may explain, in part, its oncogenic potential.

Alternative intracellular routing of ErbB receptors may determine signaling potency

The ErbB signaling module consists of four receptor tyrosine kinases and several dozen ligands that activate specific homo- and heterodimeric complexes of ErbB proteins. Combinatorial ligand/receptor/effector interactions allow large potential for signal diversification. Here we addressed the possibility that turn-off mechanisms enhance the diversification potential. Concentrating on ErbB-1 and two of its ligands, epidermal growth factor (EGF) and transforming growth factor alpha (TGF-alpha), and the Neu differentiation factor (NDF/neuregulin) and one of its receptors, ErbB-3, we show that ligand binding variably accelerates endocytosis of the respective ligand-receptor complex. However, unlike the EGF-activated ErbB-1, which is destined primarily to degradation in lysosomes, NDF and TGF-alpha direct their receptors to recycling, probably because these ligands dissociate from their receptors earlier along the endocytic pathway. In the case of NDF, structural, as well as biochemical, analyses imply that ligand degradation occurs at a relatively late endosomal stage. Attenuation of receptor down-regulation by this mechanism apparently confers to both NDF and TGF-alpha more potent and prolonged signaling activity. In conclusion, alternative endocytic trafficking of ligand-ErbB complexes may tune and diversify signal transduction by EGF family ligands.

The ErbB-2/HER2 oncogenic receptor of adenocarcinomas: from orphanhood to multiple stromal ligands

Extensive clinical and biochemical evidence implicates ErbB-2, a transmembrane tyrosine kinase related to growth factor receptors, in the development, metastasis, and resistance to therapy of multiple, common human carcinomas. Previous attempts to uncover an ErbB-2-specific ligand led to isolation of the neuregulin (NRG) family, but these ligands, like all other growth factors with an EGF-like motif, only indirectly active ErbB-2. On the other hand, biochemical and genetic evidence suggest a non-autonomous function of ErbB-2 in an interactive ErbB signaling network. Accordingly, the oncoprotein acts as a shared signaling subunit of primary growth factor receptors. By stabilizing heterodimers with other ErbB proteins, ErbB-2 prolongs and enhances signal transduction by a large group of stroma-derived growth factors. Furthermore, we have proposed a model in which all ErbB-2 ligands are bivalent and bind to ErbB-2 with low affinity, following high affinity binding to a primary receptor with which ErbB-2 is heterodimerized. Thus the presence of ErbB-2 in relevant ErbB heterodimeric structures on the surfaces of certain epithelial tumor cells can amplify signals arising from the binding of stromal ErbB ligands. This effect, in turn, may promote the growth of carcinoma cells.

Specificity within the EGF family/ErbB receptor family signaling network

Recent years have witnessed tremendous growth in the epidermal growth factor (EGF) family of peptide growth factors and the ErbB family of tyrosine kinases, the receptors for these factors. Accompanying this growth has been an increased appreciation for the roles these molecules play in tumorigenesis and in regulating cell proliferation and differentiation during development. Consequently, a significant question has been how diverse biological responses are specified by these hormones and receptors. Here we discuss several characteristics of hormone-receptor interactions and receptor coupling that contribute to specificity: 1) a single EGF family hormone can bind multiple receptors; 2) a single ErbB family receptor can bind multiple hormones; 3) there are three distinct functional groups of EGF family hormones; 4) EGF family hormones can activate receptors in trans, and this heterodimerization diversifies biological responses; 5) ErbB3 requires a receptor partner for signaling; and 6) ErbB family receptors differentially couple to signaling pathways and biological responses.

Bivalence of EGF-like ligands drives the ErbB signaling network

Signaling by epidermal growth factor (EGF)-like ligands is mediated by an interactive network of four ErbB receptor tyrosine kinases, whose mechanism of ligand-induced dimerization is unknown. We contrasted two existing models: a conformation-driven activation of a receptor-intrinsic dimerization site and a ligand bivalence model. Analysis of a Neu differentiation factor (NDF)-induced heterodimer between ErbB-3 and ErbB-2 favors a bivalence model; the ligand simultaneously binds both ErbB-3 and ErbB-2, but, due to low-affinity of the second binding event, ligand bivalence drives dimerization only when the receptors are membrane anchored. Results obtained with a chimera and isoforms of NDF/neuregulin predict that each terminus of the ligand molecule contains a distinct binding site. The C-terminal low-affinity site has broad specificity, but it prefers interaction with ErbB-2, an oncogenic protein acting as a promiscuous low-affinity subunit of the three primary receptors. Thus, ligand bivalence enables signal diversification through selective recruitment of homo- and heterodimers of ErbB receptors, and it may explain oncogenicity of erbB-2/HER2.