Activation of ErbB4 by the bifunctional epidermal growth factor family hormone epiregulin is regulated by ErbB2

Wound-induced HB-EGF ectodomain shedding and EGFR activation in corneal epithelial cells

PURPOSE

Epithelial wound healing is, at least in part, mediated in an autocrine fashion by epidermal growth factor (EGF) receptor (EGFR)-ligand interactions. This study sought to identify the endogenous EGFR ligand and the mechanism by which it is generated in response to wounding in cultured porcine corneas and human corneal epithelial cells.

METHODS

Epithelial debridement wounds in cultured porcine corneas and scratch wounds in an epithelial monolayer of SV40-immortalized human corneal epithelial (THCE) cells were allowed to heal in the presence of tyrphostin AG1478 (an EGFR inhibitor), GM6001 (a matrix metalloproteinase [MMP] inhibitor), or CRM197 (a diphtheria toxin mutant), with or without HB-EGF. The activation of EGFR and extracellular signal-regulated kinase (ERK) was analyzed by immunoprecipitation using EGFR antibodies and Western blot analysis with phosphotyrosine antibody. Wound induced HB-EGF shedding was assessed by isolation of secreted HB-EGF from wounded THCE cells and by measuring the release of alkaline phosphatase (AP) in THCE stable cell lines expressing HB-EGF-AP.

RESULTS

In THCE cells, wound-induced EGFR phosphorylation and ERK activation. In both organ and cell culture models, epithelial wounds were healed in basal media and inhibition of EGFR activation by AG1478 blocked wound closure with or without exogenously added HB-EGF. GM6001 delayed wound closure. Its effects diminished in the presence of exogenous EGF or HB-EGF, suggesting that the MMP inhibitor primarily blocks the release of EGFR ligands. CRM197, a highly specific antagonist of HB-EGF, impaired epithelial wound closure, suggesting that HB-EGF is an endogenous ligand released on epithelial wounding. Consistent with the effects on epithelial migration, all inhibitors as well as HB-EGF function-blocking antibodies retarded wound-induced EGFR phosphorylation in cultured THCE cells. The release of HB-EGF in response to wounding was demonstrated by the fact that heparin-binding proteins isolated from wounded, but not control, THCE-conditioned medium stimulated EGFR and ERK phosphorylation and by the expression of HB-EGF-AP in THCE cells, in which wounding induced the release of AP activity in an MMP-inhibitor-sensitive manner.

CONCLUSIONS

HB-EGF released on wounding acts as an autocrine-paracrine EGFR ligand. HB-EGF shedding and EGFR activation represent a critical event during corneal epithelial wound healing, suggesting a possible manipulation of wound healing during the early phases.

Five carboxyl-terminal residues of neuregulin2 are critical for stimulation of signaling by the ErbB4 receptor tyrosine kinase

The neuregulins (NRGs) are members of the epidermal growth factor (EGF) family of peptide growth factors. These hormones are agonists for the ErbB family of receptor tyrosine kinases, a family that includes the epidermal growth factor receptor (EGFR/ErbB1), ErbB2/Neu/HER2, ErbB3/HER3, and ErbB4/HER4. We recently observed that the EGF family hormone NRG2beta is a potent agonist for ErbB4. In contrast, NRG2alpha, a splicing isoform of the same gene that encodes NRG2beta, is a poor ErbB4 agonist. We hypothesized that carboxyl-terminal residues of NRG2beta are critical for stimulation of ErbB4 tyrosine phosphorylation and coupling to downstream signaling events. Here, we demonstrate that the substitution of a lysine residue for Phe45 in NRG2beta results in reduced ligand potency. We also demonstrate that substitution of a phenylalanine for Lys45 in NRG2alpha results in increased ligand potency. Finally, analyses of the gain-of-function NRG2alpha Chg5 mutant demonstrate that Gln43, Met47, Asn49, and Phe50 regulate ligand efficacy. Thus, these data indicate that carboxyl-terminal residues of NRG2beta are critical for activation of ErbB4 signaling. Moreover, these NRG2alpha and NRG2beta mutants reveal new insights into models for ligand-induced ErbB family receptor tyrosine phosphorylation and coupling to downstream signaling events.

Solution structure of epiregulin and the effect of its C-terminal domain for receptor binding affinity

Epiregulin (EPR), a novel member of epidermal growth factor (EGF) family, is a ligand for ErbB-1 and ErbB-4 receptors. The binding affinity of EPR for the receptors is lower than those of other EGF-family ligands. The solution structure of EPR was determined using two-dimensional nuclear magnetic resonance spectroscopy. The secondary structure in the C-terminal domain of EPR is different from other EGF-family ligands because of the lack of hydrogen bonds. The structural difference in the C-terminal domain may provide an explanation for the reduced binding affinity of EPR to the ErbB receptors.

The SRC homology 2 domain of Rin1 mediates its binding to the epidermal growth factor receptor and regulates receptor endocytosis

Activated epidermal growth factor receptors (EGFRs) recruit intracellular proteins that mediate receptor signaling and endocytic trafficking. Rin1, a multifunctional protein, has been shown to regulate EGFR internalization (1). Here we show that EGF stimulation induces a specific, rapid, and transient membrane recruitment of Rin1 and that recruitment is dependent on the Src homology 2 (SH2) domain of Rin1. Immunoprecipitation of EGFR is accompanied by co-immunoprecipitation of Rin1 in a time- and ligand-dependent manner. Association of Rin1 and specifically the SH2 domain of Rin1 with the EGFR was dependent on tyrosine phosphorylation of the intracellular domain of the EGFR. The recruitment of Rin1, observed by light microscopy, indicated that although initially cytosolic, Rin1 was recruited to both plasma membrane and endosomes following EGF addition. Moreover, the expression of the SH2 domain of Rin1 substantially impaired the internalization of EGF without affecting internalization of transferrin. Finally, we found that Rin1 co-immunoprecipitated with a number of tyrosine kinase receptors but not with cargo endocytic receptors. These results indicate that Rin1 provides a link via its SH2 domain between activated tyrosine kinase receptors and the endocytic pathway through the recruitment and activation of Rab5a.

Epiregulin is more potent than EGF or TGFalpha in promoting in vitro wound closure due to enhanced ERK/MAPK activation

Epiregulin (EPR) is a broad specificity EGF family member that activates ErbB1 and ErbB4 homodimers and all possible heterodimeric ErbB complexes. We have previously shown that topical EPR enhances the repair of murine excisional wounds. The purpose of this study was to determine whether EPR was more effective than EGF or TGFalpha in promoting in vitro wound closure and to compare the EPR induced signal transduction pathways with those activated by EGF and TGFalpha. Normal human epidermal keratinocytes or A431 cells were scratch wounded and treated for 24 h with varying doses of EPR, EGF or TGFalpha. Five-fold lower doses of EPR were significantly better than EGF or TGFalpha in stimulating in vitro wound closure. Mitomycin-c reduced EPR induced wound closure by 59%, versus a 9% and 25% decrease in EGF and TGFalpha induced closure. The ERK/MAPK inhibitor PD-98059 decreased EPR induced wound closure by 88%. By contrast, the PLC inhibitor U-73122, only reduced the EPR induced response by 21%. Immunoblot analysis revealed that 2 nM EPR stimulated a six-fold increase in p-ERK1/2, whereas 10 nM EGF or TGFalpha stimulated only a 3- and 2.5-fold increase in p-ERK1/2. When compared with EGF or TGFalpha, EPR is a more potent and more effective inducer of in vitro wound closure due to its ability to promote significantly greater ERK/MAPK activation.

Neuregulin isoforms exhibit distinct patterns of ErbB family receptor activation

During the last decade, several novel members of the Epidermal Growth Factor family of peptide growth factors have been identified. Most prominent among these are the Neuregulins or Heregulins. To date, four different Neuregulin genes have been identified (Neuregulin1-4) and several different splicing isoforms have been identified for at least two of these genes (Neuregulin1 and Neuregulin2). While Neuregulin1 isoforms have been extensively studied, comparatively little is known about Neuregulin3, Neuregulin4, or the Neuregulin2 isoforms. Indeed, there has been no systematic comparison of the activities of these molecules. Here we demonstrate that Neuregulin2alpha and Neuregulin2beta stimulate ErbB3 tyrosine phosphorylation and coupling to biological responses. In contrast, Neuregulin3 and Neuregulin4 fail to activate ErbB3 signaling. Furthermore, Neuregulin2beta, but not Neuregulin2alpha, stimulates ErbB4 tyrosine phosphorylation and coupling to biological responses. Finally, both Neuregulin3 and Neuregulin4 stimulate modest amounts of ErbB4 tyrosine phosphorylation. However, whereas Neuregulin3 stimulates a modest amount of ErbB4 coupling to biological responses, Neuregulin4 fails to stimulate ErbB4 coupling to biological responses. This suggests that there are qualitative as well as quantitative differences in ErbB family receptor activation by Neuregulin isoforms.

ErbB2 and ErbB3 do not quantitatively modulate ligand-induced ErbB4 tyrosine phosphorylation

The ErbB family of receptor tyrosine kinases consists of four members: the epidermal growth factor receptor (EGFR/ErbB1), ErbB2/HER2/Neu, ErbB3/HER3, and ErbB4/HER4. ErbB2 is an "orphan" for which there is no naturally occurring, soluble ligand. ErbB3 lacks tyrosine kinase activity. Thus, we hypothesized that ErbB2 enhances ligand-induced ErbB family receptor signalling through mass action. In contrast, we hypothesized that ErbB3 reduces ligand-induced ErbB family receptor signalling by forming receptor heterodimers that cannot undergo bidirectional cross-phosphorylation. We tested these hypotheses using three cell lines that express equal levels of ErbB4. One expresses ErbB4 alone, the second expresses ErbB2 and ErbB4, and the third expresses ErbB3 and ErbB4. We treated the cells with the ErbB4 ligands betacellulin (BTC) and neuregulin1beta (NRG1 beta) and assayed ErbB4 tyrosine phosphorylation. ErbB2 and ErbB3 do not affect the amount of ligand-induced ErbB4 tyrosine phosphorylation. We will discuss these findings within the context of a model for ErbB receptor signalling.

Neuregulin-1 activates the JAK-STAT pathway and regulates lung epithelial cell proliferation

Neuregulin-1 (NRG-1) is part of a family of proteins whose members are structurally related to epidermal growth factor. NRG-1 induces cell proliferation through a high-affinity receptor complex composed of a heterodimer of human epidermal growth factor-like receptor (HER) 2 and 3. In this study, we show that NRG-1 activates the Janus kinases (JAK) and signal transducer and activator of transcription proteins (STAT). NRG-1 induced a rapid and transient increase in tyrosine phosphorylation of TYK2 and JAK3, but not JAK1 or JAK2, and induced STAT3 and STAT5 tyrosine phosphorylation. Upon phosphorylation, STAT3 translocated to the nucleus within 1 h. Activation of the JAK-STAT pathway was dependent on HER2/HER3 heterodimerization and was necessary for NRG-1-induced proliferation. Inhibition of HER2's ability to dimerize using the HER2-specific antibody 2C4 completely blocked NRG-1-induced JAK3, TYK2, STAT3, and STAT5 tyrosine phosphorylation. Blocking the JAK-STAT pathway with a specific JAK-STAT pathway inhibitor, AG490, inhibited NRG-1-induced JAK and STAT phosphorylation and cell proliferation. These data suggest that NRG-1 activates the JAK-STAT signal transduction pathway through its high-affinity receptor, the HER2/HER3 heterodimer. This pathway plays an important role in NRG-1-stimulated proliferation of pulmonary epithelial cells.

Solution structure of betacellulin, a new member of EGF-family ligands

The solution structure of the EGF-like domain of betacellulin (BTCe), a newly discovered member of the epidermal growth factor (EGF) family, has been determined using two-dimensional nuclear magnetic resonance spectroscopy. This is the first report to identify the solution structure of the EGF-family ligand monomers that interact with both ErbB-1 and ErbB-4. The solution structure of BTCe was calculated using 538 NMR-derived restraints. The overall structure of BTCe was stabilized by three disulfide bonds, a hydrophobic core, and 23 hydrogen bonds. It appears that BTCe is comprised of five beta-strands and one short 3(10) helical turn. The secondary structural elements of BTCe are basically similar to those of the other EGF-family proteins, except that several significant variations of the structural properties were found. It is suggested that the structural variations between BTCe and the other EGF-family ligands may affect the specific receptor-recognition properties of EGF-family ligands.

Epidermal growth factor contains both positive and negative determinants for interaction with ErbB-2/ErbB-3 heterodimers

Epidermal growth factor (EGF) and transforming growth factor (TGF)-alpha are potent activators of the ErbB-1 receptor, but, unlike TGF-alpha, EGF is also a weak activator of ErbB-2/ErbB-3 heterodimers. To understand the specificity of EGF-like growth factors for binding to distinct ErbB members, we used EGF/TGF-alpha chimeras to examine the requirements for ErbB-2/ErbB-3 activation. Here we show that in contrast to these two wild-type ligands, distinct EGF/TGF-alpha chimeras are potent activators of ErbB-2/ErbB-3 heterodimers. On the basis of differences in the potency of these various chimeras, specific residues in the linear N-terminal region and the so-called B-loop of these ligands were identified to be involved in interaction with ErbB-2/ErbB-3. A chimera consisting of human EGF sequences with the linear N-terminal region of human TGF-alpha was found to be almost as potent as the natural ligand neuregulin (NRG)-1beta in activating 32D cells expressing ErbB-2/ErbB-3 and human breast cancer cells. Binding studies revealed that this chimera, designated T1E, has high affinity for ErbB-2/ErbB-3 heterodimers, but not for ErbB-3 alone. Subsequent exchange studies revealed that introduction of both His2 and Phe3 into the linear N-terminal region was already sufficient to make EGF a potent activator of ErbB-2/ErbB-3 heterodimers, indicating that these two amino acids contribute positively to this receptor binding. Analysis of the B-loop revealed that Leu26 in EGF facilitates interaction with ErbB-2/ErbB-3 heterodimers, while the equivalent Glu residue in TGF-alpha impairs binding. Since all EGF/TGF-alpha chimeras tested have maintained high binding affinity for ErbB-1, it is concluded that the diversity of the ErbB signaling network is determined by specific amino acids that facilitate binding to one receptor member, in addition to residues that impede binding to other ErbB family members.

ErbB-4: a receptor tyrosine kinase

ErbB-4 is a receptor tyrosine kinase that is activated by the binding of specific growth factors to its ectodomain. In addition to the initiation of signal transduction pathways that direct cell responses, such as proliferation or differentiation, this receptor is subject to ligand-dependent trafficking events. The signal transduction events are controlled by ligand-dependent activation of the receptor tyrosine kinase activity, which results in receptor autophosphorylation and the tyrosine phosphorylation of other cellular proteins. The trafficking events include migration into and out of membrane microdomains, entry into internalization pathways and endocytosis, plus proteolytic fragmentation.

Her4 mediates ligand-dependent antiproliferative and differentiation responses in human breast cancer cells

The function of the epidermal growth factor receptor (EGFR) family member HER4 remains unclear because its activating ligand, heregulin, results in either proliferation or differentiation. This variable response may stem from the range of signals generated by HER4 homodimers versus heterodimeric complexes with other EGFR family members. The ratio of homo- and heterodimeric complexes may be influenced both by a cell's EGFR family member expression profile and by the ligand or even ligand isoform used. To define the role of HER4 in mediating antiproliferative and differentiation responses, human breast cancer cell lines were screened for responses to heregulin. Only cells that expressed HER4 exhibited heregulin-dependent antiproliferative responses. In-depth studies of one line, SUM44, demonstrated that the antiproliferative and differentiation responses correlated with HER4 activation and were abolished by stable expression of a kinase-inactive HER4. HB-EGF, a HER4-specific ligand in this EGFR-negative cell line, also induced an antiproliferative response. Moreover, introduction and stable expression of HER4 in HER4-negative SUM102 cells resulted in the acquisition of a heregulin-dependent antiproliferative response, associated with increases in markers of differentiation. The role of HER2 in these heregulin-dependent responses was examined through elimination of cell surface HER2 signaling by stable expression of a single-chain anti-HER2 antibody that sequestered HER2 in the endoplasmic reticulum. In the cell lines with either endogenously (SUM44) or exogenously (SUM102) expressed HER4, elimination of HER2 did not alter HER4-dependent decreases in cell growth. These results suggest that HER4 is both necessary and sufficient to trigger an antiproliferative response in human breast cancer cells.

Src family kinases and HER2 interactions in human breast cancer cell growth and survival

Evidence from murine fibroblast models and human breast cancer cells indicates that c-Src and human EGF receptor (HER1) synergize to enhance neoplastic growth of mammary epithelial cells. To investigate whether interactions between c-Src and other HER family members may also play a role in breast tumor progression, we characterized 13 human breast carcinoma cell lines and 13 tumor samples for expression of HER family members and c-Src and examined a subset of the cell lines for Src-dependent, heregulin (HRG)-augmented, anchorage-dependent and independent growth. By immunoblotting, we found that all cell lines overexpressed one or more HER family member, and 60% overexpressed c-Src. Seventy-five per cent of the tumor tissues overexpressed HER2, while 64% overexpressed c-Src. Colony formation in soft agar was enhanced by HRG in three of five cell lines tested, a response that correlated with the presence of a c-Src/HER2 heterocomplex. This result suggests that HRG may act through both HER2 and c-Src to facilitate anchorage-independent growth. In contrast, HRG had little effect on anchorage-dependent growth in any of the cell lines tested. PP1, a Src family kinase inhibitor, reduced or ablated HRG-dependent and independent soft agar growth or anchorage dependent growth, and triggered apoptosis in all cell lines tested. The apoptotic effect of PP1 could be partially or completely reversed by HRG, depending on the cell line. These results suggest that while Src family kinases may cooperate with HRG to promote the survival and growth of human breast tumor cells, they also function independently of HER2/HRG in these processes.

Coexpression patterns of EGFR, HER2, HER3 and HER4 in non-melanoma skin cancer

The receptor tyrosine kinases (RTKs) epidermal growth factor receptor (EGFR), HER2, HER3 and HER4 are involved in the pathogenesis of multiple human malignant neoplasias. However, their role in the carcinogenesis of basal cell carcinomas (BCC) and squamous cell carcinomas (SCC) remains to be elucidated. In order to further define the role of these RTKs, 56 human skin tissue samples of normal skin, BCC and SCC were studied by conventional and differential and quantitative reverse transcriptase-polymerase chain reaction (rtPCR). EGFR and HER3 were predominantly expressed in the BCCs and SCCs, while HER2 was ubiquitously expressed. HER4 was not expressed in any sample. Since in vitro studies have provided compelling evidence that heterodimer formation of these receptors are associated with different signal transduction processes, coexpression patterns might be decisive for the induction and maintenance of a malignant phenotype. These results confirm this concept: isolated HER2 expression and EGFR/HER2 were predominantly found in normal skin, while HER2/HER3 and the triple expression of EGFR/HER2/HER3 were seen more frequently in the BCCs and SCCs compared with normal skin (50% and 40% compared with 26%, respectively). The activation of HER3, in addition to EGFR and HER2, might therefore be associated with the malignant phenotype. However, due to the small numbers in this study, further confirmation of the patterns is needed.

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.

An anti-oncogenic role for decorin. Down-regulation of ErbB2 leads to growth suppression and cytodifferentiation of mammary carcinoma cells

The leucine-rich proteoglycan decorin interacts with the epidermal growth factor receptor and triggers a signaling pathway that leads to growth suppression. We find that decorin causes a functional inactivation of the oncogenic ErbB2 protein in breast carcinoma cells. Upon de novo expression of decorin, the ErbB2 protein is reduced by approximately 40%, whereas its degree of tyrosyl phosphorylation is almost completely abrogated. Both co-culture experiments or experiments with recombinant decorin demonstrate an initial induction of ErbB2 tyrosine kinase, followed by a profound and long-lasting down-regulation of its activity. This leads to growth inhibition and cytodifferentiation of mammary tumor cells and a concurrent suppression of their tumorigenic potential in vivo. These decorin-mediated effects appear to involve the activation of ErbB4, which in turn would block the phosphorylation of heterodimers containing either ErbB2 or ErbB3. These results provide an explanation for the heightened decorin levels around invasive carcinomas and suggest that decorin may function as a natural antagonist of neoplastic cells enriched in ErbB2.

Unique carboxyl-terminal sequences of wild type and alternatively spliced variant forms of transforming growth factor-alpha precursors mediate specific interactions with ErbB4 and ErbB2

We have previously reported that the human transforming growth factor-alpha (TGF-alpha) gene encodes three forms of TGF-alpha precursors, designated wild type (WT), variant I (VaI), and variant II (VaII), derived from alternative splicing. The two carboxyl-terminal valine residues of WT are replaced by 5 (GCRLY) or 4 (ATLG) amino acids in VaI or VaII, respectively. When overexpressed in Chinese hamster ovary (CHO) cells, VaI and ValI, but not WT, support autonomous growth. We detected tyrosine phosphorylation of ErbB2 in the absence of serum, in CHO cells expressing WT, VaI, or VaII, but not in mock transfectants. These observations prompted us to investigate possible interactions between the ErbBs and the TGF-alpha precursors in CHO cells. All TGF-alpha precursors were found to co-immunoprecipitate with the ErbBs, but with different specificity. WT co-immunoprecipitated with ErbB4, but not with ErbB1, ErbB2, or ErbB3. VaI and VaII co-immunoprecipitated with ErbB2, but not with ErbB1, ErbB3, or ErbB4. Confocal fluorescent microscopy analysis demonstrated that WT, VaI, and VaII all distribute equally to the cell surface while, as expected, a WT mutant lacking the two C-terminal valine residues does not. Point and deletion mutants involving the unique carboxyl-terminal residues of WT, VaI and VaII, indicated that the interactions between the three TGF-alpha precursors and the ErbBs were mediated by their carboxyl-terminal regions, which constitute distinct protein-binding motifs. A chimera of the intracellular domain of WT TGF-alpha linked to exogenous transmembrane and extracellular domains retained both the cell surface distribution and the specific interaction with ErbB4 of full-length WT, confirming that this interaction is mediated by the C-terminus of the TGF-alpha precursor. While interactions of WT and variant TGF-alpha with the ErbBs all result in ErbB2 activation, they produce different biological consequences, suggesting that the various TGF-alpha precursors differentially modulate ErbB signaling.

Epiregulin, a novel member of the epidermal growth factor family, is an autocrine growth factor in normal human keratinocytes

Epiregulin is a new member of the epidermal growth factor (EGF) family purified from conditioned medium of NIH-3T3 clone T7. Some EGF family growth factors play essential roles in human keratinocytes in an autocrine manner. We show here that epiregulin is another autocrine growth factor for human keratinocytes. Epiregulin stimulated human keratinocyte proliferation under both subconfluent and confluent culture conditions in the absence of exogenous EGF family growth factors. Immunoprecipitation of [(35)S]methionine-labeled conditioned medium revealed a 5-kDa band corresponding to epiregulin. Northern blot analysis detected a 4. 8-kilobase transcript of epiregulin, and the addition of epiregulin up-regulated epiregulin mRNA synthesis. Furthermore, an anti-epiregulin blocking antibody reduced DNA synthesis by 25%. Epiregulin up-regulated the mRNA levels of heparin-binding EGF-like growth factor (HB-EGF), amphiregulin, and TGF-alpha. In turn, the addition of EGF, HB-EGF, amphiregulin, and TGF-alpha increased epiregulin mRNA levels. These results demonstrate that epiregulin acts as an autocrine growth factor in human epidermal keratinocytes and is part of auto- and cross-induction mechanisms involving HB-EGF, amphiregulin, and TGF-alpha. The mRNA expression profile resulting from induction of differentiation with high calcium and fetal calf serum revealed the differential expression of epiregulin, HB-EGF, amphiregulin, and TGF-alpha in keratinocytes. This indicates that these four growth factors have distinct, non-redundant biological functions.

Glial-neuronal interactions in the neuroendocrine control of mammalian puberty: facilitatory effects of gonadal steroids

It is now clear that astroglial cells actively contribute to both the generation and flow of information within the central nervous system. In the hypothalamus, astrocytes regulate the secretory activity of neuroendocrine neurons. A small subset of these neurons secrete luteinizing hormone-releasing hormone (LHRH), a neuropeptide essential for sexual development and adult reproductive function. Astrocytes stimulate LHRH secretion via cell-cell signaling mechanisms involving growth factors recognized by receptors with either serine/threonine or tyrosine kinase activity. Two members of the epidermal growth factor (EGF) family and their respective tyrosine kinase receptors appear to play key roles in this regulatory process. Transforming growth factor-alpha (TGFalpha) and its distant congeners, the neuregulins (NRGs), are produced in hypothalamic astrocytes. They stimulate LHRH secretion indirectly, via activation of erbB-1/erbB-2 and erbB-4/erbB-2 receptor complexes also located on astrocytes. Activation of these receptors leads to release of prostaglandin E(2) (PGE(2)), which then binds to specific receptors on LHRH neurons to elicit LHRH secretion. Gonadal steroids facilitate this glia-to-neuron communication process by acting at three different steps along the signaling pathway. They (a) increase astrocytic gene expression of at least one of the EGF-related ligands (TGFalpha), (b) increase expression of at least two of the receptors (erbB-4 and erbB-2), and (c) enhance the LHRH response to PGE(2) by up-regulating in LHRH neurons the expression of specific PGE(2) receptor isoforms. Focal overexpression of TGFalpha in either the median eminence or preoptic area of the hypothalamus accelerates puberty. Conversely, blockade of either TGFalpha or NRG hypothalamic actions delays the process. Thus, both TGFalpha and NRGs appear to be physiological components of the central neuroendocrine mechanism controlling the initiation of female puberty. By facilitating growth factor signaling pathways in the hypothalamus, ovarian steroids accelerate the pace and progression of the pubertal process.

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

Overexpression of the receptor tyrosine kinase ErbB2/HER2/Neu (ErbB2) occurs in 15-40% of human breast cancers. To determine the function of ErbB2 signaling during normal mouse mammary gland development, we expressed a carboxyl-terminal truncated dominant negative allele of ErbB2 (ErbB2deltaIC) in the developing mouse mammary gland. Despite ErbB2deltaIC expression within mammary glands of pubescent virgin and pregnant mice, a phenotype was not observed until late in pregnancy. At 1 day post-partum, lactationally active, distended lobuloalveoli failed to form. This phenotype was exaggerated in multiparous females expressing ErbB2deltaIC. Immunohistochemical staining for ErbB2deltaIC revealed a concordance between high levels of ErbB2deltaIC protein expression and the absence of lactational products within the lumens of ErbB2deltaIC stained lobuloalveoli. These results demonstrate that ErbB2 signaling is required for proper mammary development and lactation at parturition.

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.

Binding specificities and affinities of egf domains for ErbB receptors

ErbB receptor activation is a complex process and is dependent upon the type and number of receptors expressed on a given cell. Previous studies with defined combinations of ErbB receptors expressed in mammalian cells have helped elucidate specific biological responses for many of the recognized gene products that serve as ligands for these receptors. However, no study has examined the binding of these ligands in a defined experimental system. To address this issue, the relative binding affinities of the egf domains of eleven ErbB ligands were measured on six ErbB receptor combinations using a soluble receptor-ligand binding format. The ErbB2/4 heterodimer was shown to bind all ligands tested with moderate to very high affinity. In contrast, ErbB3 showed much more restrictive ligand binding specificity and measurable binding was observed only with heregulin, neuregulin2beta, epiregulin and the synthetic heregulin/egf chimera, biregulin. These studies also revealed that ErbB2 preferentially enhances ligand binding to ErbB3 or ErbB4 and to a lesser degree to ErbB1.

Epiregulin is a potent vascular smooth muscle cell-derived mitogen induced by angiotensin II, endothelin-1, and thrombin

Vasoactive GTP-binding protein-coupled receptor agonists such as angiotensin II (AII), endothelin-1 (ET-1), and alpha-thrombin (alpha-Thr) have been reported to indirectly stimulate vascular smooth muscle cell (VSMC) proliferation by regulating the expression of one or more autocrine growth factors. Using ion-exchange, gel-filtration, and reverse-phase chromatographic purification methods, we isolated a major mitogenic protein present in AII-stimulated rat aortic smooth muscle (RASM) cell conditioned medium. Twenty N-terminal amino acids of the purified peptide were identified, and they had 75% amino acid sequence identity with mouse epiregulin, an epidermal growth factor (EGF)-related growth factor. We cloned the cDNA for rat epiregulin to determine its pattern of expression in G-protein-coupled receptor agonist-stimulated cells and confirm its activity as a mitogen. After treatment of RASM cells with AII, ET-1, or alpha-Thr for 1 h, induction of two epiregulin transcripts was observed, including a 4.8-kb transcript and a novel transcript of approximately 1.2 kb. Recombinant rat epiregulin was strongly mitogenic for RASM cells, stimulating DNA synthesis to levels similar to those induced by serum or platelet-derived growth factor and approximately 3-fold above that observed with saturating concentrations of EGF. In addition, epiregulin caused rapid EGF receptor activation in RASM cells. However, relative levels of EGF receptor tyrosine phosphorylation stimulated by epiregulin were less than those induced by EGF or betacellulin. Taken together, these results indicate that epiregulin is a potent VSMC-secreted mitogen, induced in common by AII, ET-1, and alpha-Thr, that may contribute to VSMC proliferation and vascular remodeling stimulated by vasoactive agonists.

erbB genes in the mouse uterus: cell-specific signaling by epidermal growth factor (EGF) family of growth factors during implantation

We previously described spatiotemporal expression of various epidermal growth factor (EGF)-like ligands and receptor subtypes, ErbB1 and ErbB2, during the peri-implantation period. To better understand the roles of these ligands and their possible signaling schemes in implantation, it is important to define the status of all the ligands and receptor subtypes in the uterus/embryo. No information is available about uterine and embryonic status of ErbB3 or ErbB4 during implantation. We cloned mouse erbB3 and erbB4 cDNAs and examined their expression and bioactivity in the peri-implantation uterus (days 1-8). Two erbB3 (cytoplasmic and extracellular) and three erbB4 (two cytoplasmic and one extracellular) clones were generated. Both forms of the erbB3 clone showed similar transcript profiles, while different transcript profiles were obtained with erbB4 clones. The steady-state levels of erbB3 and erbB4 mRNAs in whole uterine poly(A)+ RNA samples showed little changes during the peri-implantation period, while their unique cell-specific accumulation was noted. erbB3 is predominantly expressed in the epithelial cells, although decidual and embryonic cells also accumulate this mRNA. In contrast, the erbB4 mRNA is primarily expressed in the submyometrial stroma and myometrial connective tissues during this period. Additionally, the extracellular form of the erbB4 clone detected signals in a subpopulation of stromal cells. Autophosphorylation and immunoprecipitation studies provided evidence that uterine ErbB3 and ErbB4 are biologically active. This study provides a comprehensive analysis of possible ligand-receptor signaling schemes for EGF-like ligands in implantation.