Heregulin stimulates mitogenesis and phosphatidylinositol 3-kinase in mouse fibroblasts transfected with erbB2/neu and erbB3

Induction or suppression of expression of cytochrome C oxidase subunit II by heregulin beta 1 in human mammary epithelial cells is dependent on the levels of ErbB2 expression

The ErbB family of receptor kinases is composed of four members: epidermal growth factor receptor (EGFR/ErbB1), ErbB2/neu, ErbB3, and ErbB4. Amplification of the ErbB2/neu is found in about 30% of breast cancer patients and is associated with a poor prognosis. Heregulin (HRG) activates the ErbB2 via induction of heterodimerization with ErbB3 and ErbB4 receptors. With suppression subtractive hybridization, we demonstrated that the expression of cytochrome c oxidase subunit II (COXII) is HRG-responsive. Two nontransformed human mammary epithelial cell lines, the HB2 and the HB2(ErbB2) (the HB2 engineered to overexpress ErbB2), displayed an opposite response to HRG-mediated regulation. HRG upregulated mRNA expression of COXII in the HB2 cells, but suppressed COXII expression in the HB2(ErbB2) cells. A human breast cancer cell line (T47D), which expresses ErbB2 at a level similar to that of the HB2 cells, also responded to HRG by increasing COXII mRNA levels. Therefore, HRG regulation of COXII expression depends on the levels of ErbB2 expression. Furthermore, the expression of COXII was inversely correlated to the levels of ErbB2, i.e., the cells overexpressing ErbB2 exhibited lower COXII levels. HRG-evoked signal transduction differed between the cells with normal ErbB expression and the cells overexpressing ErbB2. The activation of both ERK and PI3-K was essential for HRG regulation of COXII, i.e., blockage of either pathway eliminated HRG-mediated alteration. This is the first report demonstrating that the expression of mitochondria-encoded COXII is HRG-responsive. The levels of ErbB2 expression are decisive for the diverse biological activities of HRG.

PI3K is required for proliferation and migration of human pulmonary vascular smooth muscle cells

Human vascular smooth muscle cell proliferation and migration contribute to vascular remodeling in pulmonary hypertension and atherosclerosis. The precise mechanisms that regulate structural remodeling of the vessel wall remain unknown. This study tests the hypothesis that phosphatidylinositol 3-kinase (PI3K) activation is both necessary and sufficient to mediate human pulmonary vascular smooth muscle (PVSM) cell proliferation and migration. Microinjection of human PVSM cells with a dominant-negative class IA PI3K inhibited platelet-derived growth factor (PDGF)-induced DNA synthesis by 65% (P < 0.001; chi(2) analysis) compared with cells microinjected with control plasmid, whereas microinjection of cells with a constitutively active class IA PI3K (p110*-CA) was sufficient to induce DNA synthesis (mitotic index of p110*-CA-microinjected cells was 15% vs. 3% in control cells; P < 0.01). Transfection of PVSM cells with p110*-CA was also sufficient to promote human PVSM cell migration. In parallel experiments, stimulation of human PVSM cells with PDGF induced PI3K-dependent activation of Akt, p70 S6 kinase, and ribosomal protein S6 but not mitogen-activated protein kinase. PDGF-induced proliferation and migration was inhibited by LY-294002. These results demonstrate that PI3K signaling is both necessary and sufficient to mediate human PVSM cell proliferation and migration and suggest that the activation of PI3K may play an important role in vascular remodeling.

Expression of heregulin and ErbB/Her receptors in adult chinchilla cochlear and vestibular sensory epithelium

Immunolabeling of heregulin, a growth factor that enhances cell proliferation in damaged utricles, and one of its binding receptors, ErbB-2, has been briefly described in the P3 rat cochlea and utricle [Zheng et al. (1999) J. Neurocytol. 28, 901-912]. However, little is known about the distribution of heregulin and its three binding receptors in adult animals. Here we describe the immunolabeling patterns for heregulin, ErbB-2, ErbB-3 and ErB-4 in the cochlea, spiral ganglion, utricle and saccule of the adult chinchilla using confocal microscopy. Heregulin immunolabeling was intense along the apical pole of Deiters cells and Hensen cells and along the membrane of supporting cells of the utricle and saccule; light immunolabeling was present in the outer layer of the spiral prominence and cytoplasm of spiral ganglion neurons. In the cochlea, intense to moderate ErbB-2 immunolabeling was evident in the cytoplasm of pillar cells, outer hair cells (OHCs), border cells, stria vascularis and spiral ligament; moderate ErbB-2 immunolabeling was present in the cytoplasm of the hair cell and supporting cell layers of the utricle and saccule. In the cochlea, light ErbB-3 immunolabeling was present in the inner hair cells, OHCs, marginal and intermediate cell layers of the stria vascularis and spiral ganglion neurons; moderate ErbB-3 immunolabeling was present in the cytoplasm of hair cells and supporting cells of the utricle and saccule. In the cochlea, utricle and saccule, ErbB-4 immunolabeling was intense in the nuclei and light to moderate in the cytoplasm and membrane of sensory cells and supporting cells. These results suggest that heregulin acting through ErbB receptors and various receptor complexes may play an important role in cell proliferation and survival in the cochlea and vestibular system.

Phosphorylation on tyrosine-15 of p34(Cdc2) by ErbB2 inhibits p34(Cdc2) activation and is involved in resistance to taxol-induced apoptosis

ErbB2 overexpression confers resistance to taxol-induced apoptosis by inhibiting p34(Cdc2) activation. One mechanism is via ErbB2-mediated upregulation of p21(Cip1), which inhibits Cdc2. Here, we report that the inhibitory phosphorylation on Cdc2 tyrosine (Y)15 (Cdc2-Y15-p) is elevated in ErbB2-overexpressing breast cancer cells and primary tumors. ErbB2 binds to and colocalizes with cyclin B-Cdc2 complexes and phosphorylates Cdc2-Y15. The ErbB2 kinase domain is sufficient to directly phosphorylate Cdc2-Y15. Increased Cdc2-Y15-p in ErbB2-overexpressing cells corresponds with delayed M phase entry. Expressing a nonphosphorylatable mutant of Cdc2 renders cells more sensitive to taxol-induced apoptosis. Thus, ErbB2 membrane RTK can confer resistance to taxol-induced apoptosis by directly phosphorylating Cdc2.

Nerve growth factor activates kinases that phosphorylate atypical protein kinase C

Activation of atypical protein kinase C by nerve growth factor (NGF) involves phosphorylation. In order to identify kinases that regulate atypical PKC (aPKC), we surveyed PC12 cell lysates for protein kinases that are activated by NGF and which could phosphorylate aPKC. Employing an in-gel kinase assay where aPKC-zeta was copolymerized within the gel matrix as a substrate, three kinases, pp175, pp87 and pp60, were identified as enzymes that phosphorylated aPKC. Phosphorylation of aPKC by these three kinases coincided with NGF-induced activation of the enzyme. Each kinase possessed a unique subcellular distribution pattern and could be activated by either ceramide or H(2)0(2), second messengers that mimic NGF signaling events. Upstream, pp175 and pp60 lie in a ras pathway, whereas pp87 lies in a pathway dependent upon src. Altogether, these findings reveal that the aPKCs are subject to regulation by a novel group of kinases.

An RBCC protein implicated in maintenance of steady-state neuregulin receptor levels

Despite numerous recent advances in our understanding of the molecular mechanisms underlying receptor tyrosine kinase down-regulation and degradation in response to growth factor binding, relatively little is known about ligand-independent receptor tyrosine kinase degradation mechanisms. In a screen for proteins that might regulate the trafficking or localization of the ErbB3 receptor, we have identified a tripartite or RBCC (RING, B-box, coiled-coil) protein that interacts with the cytoplasmic tail of the receptor in an activation-independent manner. We have named this protein Nrdp1 for neuregulin receptor degradation protein-1. Northern blotting reveals ubiquitous distribution of Nrdp1 in human adult tissues, but message is particularly prominent in heart, brain, and skeletal muscle. Nrdp1 interacts specifically with the neuregulin receptors ErbB3 and ErbB4 and not with epidermal growth factor receptor or ErbB2. When coexpressed in COS7 cells, Nrdp1 mediates the redistribution of ErbB3 from the cell surface to intracellular compartments and induces the suppression of ErbB3 and ErbB4 receptor levels but not epidermal growth factor receptor or ErbB2 levels. A putative dominant-negative form of Nrdp1 potentiates neuregulin-stimulated Erk1/2 activity in transfected MCF7 breast tumor cells. Our observations suggest that Nrdp1 may act to regulate steady-state cell surface neuregulin receptor levels, thereby influencing the efficiency of neuregulin signaling.

Targeting retroviral vector infection to cells that express heregulin receptors using a TVA-heregulin bridge protein

Previously we have shown that it is possible to target retroviral vectors to cells using avian sarcoma and leukosis virus (ASLV) receptor-ligand and receptor-single-chain antibody bridge proteins (now designated as GATEs for guided adaptors for targeted entry). In this report we were interested in determining whether this approach can be used to deliver retroviral vectors specifically to cells that express heregulin receptors. Heregulin receptors are attractive targets for retroviral vector-based gene delivery protocols since they are often overexpressed on the surfaces of cancer cells. To explore this possibility, the TVA-herbeta1 protein was generated, consisting of the extracellular domain of the TVA receptor for ASLV-A fused to the EGF-like region of heregulin beta1. TVA-herbeta1 bound specifically to cells that express heregulin receptors, rendering them susceptible to efficient and specific infection by subgroup A ASLV vectors. In addition, these activities of TVA-herbeta1 were abrogated specifically in the presence of another bridge protein that contained the same ligand domain. These data confirm that the GATE protein TVA-herbeta1 mediates targeted retroviral infection via cell surface heregulin receptors.

The transmembrane heregulin precursor is functionally active

A variety of eucaryotic polypeptide growth factors are synthesized as transmembrane precursors. Many of these precursors are released from plasma membranes by proteolytic cleavage and converted into soluble mature proteins. A number of studies, however, indicate that bound growth factor precursors can be biologically active, suggesting a role for these membrane-associated ligands in cell-cell communication. Secreted heregulin is a 45-kDa growth factor with homology to epidermal growth factor. This growth factor binds directly to HER-3 and HER-4 and activates heterodimeric receptor complexes composed of the type I receptor tyrosine kinases, i.e. HER-1, HER-2, HER-3, and HER-4. Heregulin was originally detected in the conditioned medium of the human breast cancer cell line MDA-MB-231 and purified based on its ability to stimulate phosphorylation of p185(HER-2/neu). In the current study, the biologic activity of plasma membrane-anchored heregulin was evaluated in human breast cells. Transmembrane heregulin binds to cells expressing p180(HER-3), induces p185(HER-2/neu) phosphorylation, and increases DNA synthesis in cells overexpressing the HER-2/neu gene product. In addition, when cells containing heregulin receptors are co-cultured with heregulin-producing cells, specific in vivo associations are observed. This study demonstrates that transmembrane heregulin is functionally active and suggest it is capable of playing a role in cell-cell communication and subsequent signal transduction in vivo.

Heregulin-dependent activation of phosphoinositide 3-kinase and Akt via the ErbB2/ErbB3 co-receptor

The ErbB2/ErbB3 heregulin co-receptor has been shown to couple to phosphoinositide (PI) 3-kinase in a heregulin-dependent manner. The recruitment and activation of PI 3-kinase by this co-receptor is presumed to occur via its interaction with phosphorylated Tyr-Xaa-Xaa-Met (YXXM) motifs occurring in the ErbB3 C terminus. In this study, mutant ErbB3 receptor proteins expressed in COS7 cells were used to investigate PI 3-kinase-dependent signaling pathways activated by the ErbB2/ErbB3 co-receptor. We observed that a mutant ErbB3 protein with each of its six YXXM motifs containing a Tyr --> Phe substitution was unable to bind either the p85 regulatory or p110 catalytic subunit of PI 3-kinase. However, restoration of a single YXXM motif was sufficient to mediate association with the PI 3-kinase holoenzyme, although at a lower level than wild-type ErbB3. When ErbB3 YXXM motifs were restored in pairs, evidence for cooperativity between two, those incorporating Tyr-1273 and Tyr-1286, was observed. Interestingly, we have shown that an apparent association of PI 3-kinase activity with ErbB2/Neu was due to the residual presence of ErbB3 in ErbB2 immunoprecipitates. The necessity of ErbB3 association with PI 3-kinase for downstream signaling to the effector kinase Akt was also investigated. Here, the heregulin-dependent translocation of Akt to the plasma membrane and its subsequent activation was observed in intact NIH-3T3 fibroblasts. Recruitment of PI 3-kinase to ErbB3 was required for both activities, and it appeared that ErbB2 activation alone was not sufficient to activate PI 3-kinase signaling in these cells.

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.

A novel role of neuregulin in skeletal muscle. Neuregulin stimulates glucose uptake, glucose transporter translocation, and transporter expression in muscle cells

Neuregulins regulate the expression of acetylcholine receptor genes and induce development of the neuromuscular junction in muscle. In studying whether neuregulins regulate glucose uptake in muscle, we analyzed the effect of a recombinant neuregulin, (r)heregulin-beta1-(177-244) (HRG), on L6E9 muscle cells, which express the neuregulin receptors ErbB2 and ErbB3. L6E9 responded acutely to HRG by a time- and concentration-dependent stimulation of 2-deoxyglucose uptake. HRG-induced stimulation of glucose transport was additive to the effect of insulin. The acute stimulation of the glucose transport induced by HRG was a consequence of the translocation of GLUT4, GLUT1, and GLUT3 glucose carriers to the cell surface. The effect of HRG on glucose transport was dependent on phosphatidylinositol 3-kinase activity. HRG also stimulated glucose transport in the incubated soleus muscle and was additive to the effect of insulin. Chronic exposure of L6E9 cells to HRG potentiated myogenic differentiation, and under these conditions, glucose transport was also stimulated. The activation of glucose transport after chronic HRG exposure was due to enhanced cell content of GLUT1 and GLUT3 and to increased abundance of these carriers at the plasma membrane. However, under these conditions, GLUT4 expression was markedly down-regulated. Muscle denervation is associated with GLUT1 induction and GLUT4 repression. In this connection, muscle denervation caused a marked increase in the content of ErbB2 and ErbB3 receptors, which occurred in the absence of alterations in neuregulin mRNA levels. This fact suggests that neuregulins regulate glucose transporter expression in denervated muscle. We conclude that neuregulins regulate glucose uptake in L6E9 muscle cells by mechanisms involving the recruitment of glucose transporters to the cell surface and modulation of their expression. Neuregulins may also participate in the adaptations in glucose transport that take place in the muscle fiber after denervation.

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.

Grb2 downregulation leads to Akt inactivation in heregulin-stimulated and ErbB2-overexpressing breast cancer cells

ErbB2 can be activated by its own overexpression or be transactivated by the heregulin polypeptide growth factor. Activation of ErbB2 leads to breast cancer cell proliferation, presumably by inducing the activation of extracellular signal-regulated kinases 1,2 (Erk1,2) and Akt. We have previously reported that the growth factor receptor bound protein-2 (Grb2) is required for the proliferation of ErbB2-overexpressing breast cancer cells. We investigated here whether Grb2 protein plays a role in heregulin-stimulated proliferation. Grb2 protein inhibition led to growth inhibition of heregulin-stimulated breast cancer cells, but not Erk1,2 inactivation. These findings are similar to our earlier observations in ErbB2-overexpressing cells. Since Akt can also be activated by heregulin, the effects of Grb2 inhibition on Akt were examined. Akt was inactivated following Grb2 downregulation in heregulin-stimulated breast cancer cells. We then examined the effects of Grb2 downregulation on Akt in ErbB2-overexpressing cells in the absence of heregulin. Similar to heregulin-stimulated cells, Grb2 inhibition also led to Akt inactivation in ErbB2-overexpressing breast cancer cells. Our results indicate that the activation of ErbB2 by heregulin or by its overexpression requires Grb2 to stimulate the Akt pathway to propagate mitogenic signals.

Ligand discrimination in signaling through an ErbB4 receptor homodimer

The epidermal growth factor (EGF)-like family of growth factors elicits cellular responses by stimulating the dimerization, autophosphorylation, and tyrosine kinase activities of the ErbB family of receptor tyrosine kinases. Although several different EGF-like ligands are capable of binding to a single ErbB family member, it is generally thought that the biological and biochemical responses of a single receptor dimer to different ligands are indistinguishable. To test whether an ErbB receptor dimer is capable of discriminating among ligands we have examined the effect of four EGF-like growth factors on signaling through the ErbB4 receptor homodimer in CEM/HER4 cells, a transfected human T cell line ectopically expressing ErbB4 in an ErbB-null background. Despite stimulating similar levels of gross receptor tyrosine phosphorylation, the EGF-like growth factors betacellulin, neuregulin-1beta, neuregulin-2beta, and neuregulin-3 exhibited different biological potencies in a cellular growth assay. Moreover, the different ligands induced different patterns of recruitment of intracellular signaling proteins to the activated receptor and induced differential usage of intracellular kinase signaling cascades. Finally, two-dimensional phosphopeptide mapping of ligand-stimulated ErbB4 revealed that the different growth factors induce different patterns of receptor tyrosine phosphorylation. These results indicate that ErbB4 activation by growth factors is not generic and suggest that individual ErbB receptors can discriminate between different EGF-like ligands within the context of a single receptor dimer. More generally, our observations significantly modify our understanding of signaling through receptor tyrosine kinases and point to a number of possible models for ligand-mediated signal diversification.

Activation of m3 muscarinic receptors induces rapid tyrosine phosphorylation of p125(FAK), p130(cas), and paxillin in rat pancreatic acini

Tyrosine phosphorylation plays a key role in transmembrane and cytoplasmic signal transduction mechanisms stimulated by oncogenes, integrins, growth factors, neuropeptides, and bioactive lipids. Moreover, recent studies show that stimulation of odd-numbered muscarinic receptors increases the tyrosine phosphorylation of several proteins in different cellular types. The present study was aimed at examining whether activation of m3 muscarinic receptors in rat pancreatic acini evokes tyrosine phosphorylation of p125(FAK), and its substrates, p130(cas) and paxillin. Results show that stimulation of pancreatic acini with carbachol resulted in a rapid and transient increase in tyrosine phosphorylation of p125(FAK), p130(cas), and paxillin. Tyrosine phosphorylation of these proteins occurred in a time- and concentration-dependent manner. Simultaneous blockage of both PKC activation and increases in [Ca(2+)](i) partially decreased p125(FAK), p130(cas), and paxillin tyrosine phosphorylation stimulated by carbachol. Pretreatment of pancreatic acini with Clostridium botulinum C3 transferase, which specifically inactivates p21(rho), partially inhibited carbachol-induced p125(FAK), p130(cas), and paxillin tyrosine phosphorylation. In contrast, this treatment had no effect on amylase release stimulated by carbachol. Cytochalasin D, which disrupts actin microfilaments network, completely inhibited carbachol stimulated tyrosine phosphorylation of these proteins without having significant effects in carbachol-stimulated amylase secretion. These results dissociate tyrosine phosphorylation of p125(FAK), p130(cas), and paxillin from amylase secretion after m3 muscarinic receptors occupation in rat pancreatic acini. Taken together, these data suggest that (a) activation of m3 muscarinic receptors in rat pancreatic acini increases tyrosine phosphorylation of p125(FAK) and its substrates, p130(cas) and paxillin by diacylglycerol-activated PKC- and calcium- dependent, and independent pathways, (b) these responses require activation of p21(rho) and an intact actin cytoskeleton, and (c) p125(FAK), p130(cas), and paxillin are unlikely related to secretion in rat pancreatic acinar cells.

Cooperative signaling between alpha(6)beta(4) integrin and ErbB-2 receptor is required to promote phosphatidylinositol 3-kinase-dependent invasion

We previously demonstrated that beta(4) integrin subunit overexpression increases in vitro invasiveness of NIH3T3 cells that have been transformed by ErbB-2 oncogene. We used this model to identify domains within the large beta(4) cytoplasmic domain that are involved in the interaction of alpha(6)beta(4) with ErbB-2, invasion, and phosphatidylinositol 3-kinase (PI3K) activation. For this purpose, we expressed deletion mutants of beta(4) that lacked either all or portions of the beta(4) cytoplasmic domain in NIH3T3/ErbB-2 cells. We also used an ecto-domain mutant in which most of the extracellular domain of beta(4) was replaced with a c-Myc tag. These transfectants were examined for their ability to invade Matrigel and their ability to activate PI3K, as well as for the ability of alpha(6)beta(4) to co-immunoprecipitate with ErbB-2. The results obtained revealed that a region of the beta(4) cytoplasmic domain between amino acids 854 and 1183 is critical for the ability of alpha(6)beta(4) integrin to increase invasion. Interestingly, the extracellular domain of beta(4) is not necessary for alpha(6)beta(4) to stimulate invasion. The association of alpha(6)beta(4) with ErbB-2 is dependent upon the beta(4) cytoplasmic domain and can occur in the absence of alpha(6)beta(4) heterodimerization. Finally, we observed strong activation of PI3K with beta(4) wild type and with those beta(4) deletion mutants that were able to stimulate invasion upon the expression in NIH3T3/ErbB-2 cells. In conclusion, our results establish that there is cooperation between alpha(6)beta(4) and ErbB-2 in promoting PI3K-dependent invasion and implicate a specific region of the beta(4) cytoplasmic domain (amino acids 854-1183) in this event.

Neuregulin-1 and human epidermal growth factor receptors 2 and 3 play a role in human lung development in vitro

The human epidermal growth factor receptor (HER) family consists of four distinct receptors: HER1 (epidermal growth factor receptor), HER2, HER3, and HER4. Their specific activating ligands are collectively known as neuregulins (NRG). We hypothesized that one member of the NRG family, NRG-1, and the HER family would play a role in fetal lung development. To test this hypothesis, we defined NRG-1 and HER gene expression in mid-trimester human fetal lung tissue. HER2 and HER3 messenger RNA and protein were detected in the fetal lung, but HER4 expression was not detected. Immunohistochemical staining of fetal lung tissue localized HER2 and HER3 protein to the developing lung epithelium. NRG-1 expression was not found in freshly isolated human fetal lung, but it was observed in fetal lung explants after 2 d of explant culture. Immunohistochemistry of cultured human fetal lung explants revealed that NRG-1 protein was also expressed in pulmonary epithelial cells. Exposing human fetal lung to recombinant NRG-1 activated the HER receptor complex as measured by approximately 4-fold increases in receptor phosphotyrosine content. In addition, NRG-1 increased explant epithelial cell volume density approximately 2-fold (P < 0. 03); increased epithelial cell proliferation approximately 2-fold, as determined by bromodeoxyuridine labeling (P = 0.002); and reduced surfactant protein-A (SP-A) levels by 53% (P < 0.05). These data are consistent with an autocrine regulatory process mediated by NRG-1 activation of HER2/HER3 heterodimers expressed on developing human fetal lung epithelial cells. Receptor activation results in increased lung epithelial cell proliferation and volume density, and decreased SP-A production, a marker of type II pneumocyte differentiation.

Signal transduction in mammary tumorigenesis: a transgenic perspective

A number of genes have been implicated in breast cancer development, yet few have been demonstrated to play causative roles in mammary tumor formation. The advent of transgenic mouse and embryonic stem cell technologies now permits manipulation of the mouse genome in such a way as to temporally and spatially control a gene product's expression. Thus, the basic researcher now can directly assess the involvement of particular genes in tumorigenesis and disease progression and, in the process, to develop mouse models of human genetic disease. The utility of such technologies is emphasized in transgenic mice expressing genes thought to play important roles in the initiation and progression of mammary carcinomas. As these transgenic strains have been the subject of several reviews, here we focus on two mouse mammary tumor models, Polyomavirus middle T antigen and the Neu/ErbB-2 receptor tyrosine kinase, which are most amenable to study specific signaling pathways in process of mammary tumorigenesis.

Molecular requirements for the effect of neuregulin on cell spreading, motility and colony organization

Neuregulin can trigger morphogenetic signals in cells both in vivo and in culture through the activation of receptors from the ErbB family. We have ectopically expressed various ErbB-receptors in 32D myeloid cells lacking endogenous ErbB-proteins, and in CHO cells, which express only ErbB-2. We show here that activation of ErbB-3/ErbB-2 heterodimeric receptors triggers PI3-kinase-dependent lamellipodia formation and spreading, while individual ErbB-receptor homodimers as well as ErbB-3/ErbB-1 heterodimers are much less effective. CHO cells expressing ErB-3/ErbB-2 together with N-cadherin, an adhesion receptor, form epithelioid colonies. Neuregulin activates cell motility leading to transition of these colonies into ring-shaped multicellular arrays, similar to those induced by neuregulin in epithelial cells of different types (Chausovsky et al., 1998). This process requires both PI3-kinase and MAP kinase kinase activity and depends on coordinated changes in the actin- and microtubule-based cytoskeleton. Transactivation of ErbB-2 is not sufficient for the activation of cell motility and ring formation, and the C-terminal domain of ErbB-3 bearing the docking sites for the p85 subunit of PI3-kinase is essential for these morphogenetic effects. Thus, ErbB-3 in conjunction with ErbB-2 mediates, via its C-terminal domain, cytoskeletal and adhesion alterations which activate cell spreading and motility, leading to the formation of complex structures such as multicellular rings. Oncogene (2000) 19, 878 - 888.

Role of HER2 gene overexpression in breast carcinoma

The HER2 proto-oncogene encodes a transmembrane glycoprotein of 185 kDa (p185(HER2)) with intrinsic tyrosine kinase activity. Amplification of the HER2 gene and overexpression of its product induce cell transformation. Numerous studies have demonstrated the prognostic relevance of p185(HER2), which is overexpressed in 10% to 40% of human breast tumors. Recent data suggest that p185(HER2) is a ligand orphan receptor that amplifies the signal provided by other receptors of the HER family by heterodimerizing with them. Ligand-dependent activation of HER1, HER3, and HER4 by EGF or heregulin results in heterodimerization and, thereby, HER2 activation. HER2 overexpression is associated with breast cancer patient responsiveness to doxorubicin, to cyclophosphamide, methotrexate, and fluorouracil (CMF), and to paclitaxel, whereas tamoxifen was found to be ineffective and even detrimental in patients with HER2-positive tumors. In vitro analyses have shown that the role of HER2 overexpression in determining the sensitivity of cancer cells to drugs is complex, and molecules involved in its signaling pathway are probably the actual protagonists of the sensitivity to drugs. The association of HER2 overexpression with human tumors, its extracellular accessibility, as well as its involvement in tumor aggressiveness are all factors that make this receptor an appropriate target for tumor-specific therapies. A number of approaches are being investigated as possible therapeutic strategies that target HER2: (1) growth inhibitory antibodies, which can be used alone or in combination with standard chemotherapeutics; (2) tyrosine kinase inhibitors (TKI), which have been developed in an effort to block receptor activity because phosphorylation is the key event leading to activation and initiation of the signaling pathway; and (3) active immunotherapy, because the HER2 oncoprotein is immunogenic in some breast carcinoma patients.

Interaction of the PA2G4 (EBP1) protein with ErbB-3 and regulation of this binding by heregulin

The processes by which ErbB-3, an inactive tyrosine kinase, exerts its biological effects are poorly understood. Using the yeast two-hybrid system, we have isolated an ErbB-3 binding protein (Ebp1) that interacts with the juxtamembrane domain of ErbB-3. This protein is identical to that predicted to be encoded for by the human PA2G4 gene. Ebp1 is the human homologue of a previously identified cell cycle-regulated mouse protein p38-2G4. Two transcripts of ebp1 mRNA (1.7 and 2.2 kb) were detected in several normal human organs. The interaction of Ebp1 with ErbB-3 was examined in vitro and in vivo. The first 15 amino acids of the juxtamembrane domain of ErbB-3 were essential for Ebp1 binding in vitro. Treatment of AU565 cells with the ErbB-3 ligand heregulin resulted in dissociation of Ebp1 from ErbB-3. Ebp1 translocated from the cytoplasm into the nucleus following heregulin stimulation. These findings suggest that Ebp1 may be a downstream member of an ErbB-3-regulated signal transduction pathway.

ErbB2 and ErbB3 receptors mediate inhibition of calcium-dependent chloride secretion in colonic epithelial cells

We have previously demonstrated that epidermal growth factor (EGF) inhibits calcium-dependent chloride secretion via a mechanism involving stimulation of phosphatidylinositol 3-kinase (PI3-K). The muscarinic agonist of chloride secretion, carbachol (CCh), also stimulates an antisecretory pathway that involves transactivation of the EGF receptor (EGFR) but does not involve PI3-K. Here, we have examined if ErbB receptors, other than the EGFR, have a role in regulation of colonic secretion and if differential effects on ErbB receptor activation may explain the ability of the EGFR to propagate diverse signaling pathways in response to EGF versus CCh. Basolateral, but not apical, addition of the ErbB3/ErbB4 ligand alpha-heregulin (HRG; 1-100 ng/ml) inhibited secretory responses to CCh (100 microM) across voltage-clamped T(84) epithelial cells. Immunoprecipitation/Western blot studies revealed that HRG (100 ng/ml) stimulated tyrosine phosphorylation and dimerization of ErbB3 and ErbB2, but had no effect on phosphorylation of the EGFR. HRG also stimulated recruitment of the p85 subunit of PI3-K to ErbB3/ErbB2 receptor dimers, while the PI3-K inhibitor, wortmannin (50 nM), completely reversed the inhibitory effect of HRG on CCh-stimulated secretion. Further studies revealed that, while both EGF (100 ng/ml) and CCh (100 microM) stimulated phosphorylation of the EGFR, only EGF stimulated phosphorylation of ErbB2, and neither stimulated ErbB3 phosphorylation. EGF, but not CCh, stimulated the formation of EGFR/ErbB2 receptor dimers and the recruitment of p85 to ErbB2. We conclude that ErbB2 and ErbB3 are expressed in T(84) cells and are functionally coupled to inhibition of calcium-dependent chloride secretion. Differential dimerization with other ErbB family members may underlie the ability of the EGFR to propagate diverse inhibitory signals in response to activation by EGF or transactivation by CCh.

Biologic effects of heregulin/neu differentiation factor on normal and malignant human breast and ovarian epithelial cells

The heregulins are a family of ligands with ability to induce phosphorylation of the p185HER-2/neu receptor. Various investigators have reported a variety of responses of mouse and human breast and ovarian cells to this family of ligands including growth stimulation, growth inhibition, apoptosis and induction of differentiation in cells expressing the HER-2/neu receptor. Some of the disparity in the literature has been attributed to variations in the cell lines studied, ligand dose applied, methodologies utilized or model system evaluated (i.e. in vitro or in vivo). To evaluate the effects of heregulin on normal and malignant human breast and ovarian epithelial cells expressing known levels of the HER-2/neu receptor, this report presents the use of several different assays, performed both in vitro and in vivo, in vitro proliferation assays, direct cell counts, clonogenicity under anchorage-dependent and anchorage-independent conditions, as well as the in vivo effects of heregulin on human cells growing in nude mice to address heregulin activity. Using a total of five different biologic assays in nine different cell lines, across two different epithelia and over a one log heregulin dose range, we obtained results that clearly indicate a growth-stimulatory role for this ligand in human breast and ovarian epithelial cells. We find no evidence that heregulin has any growth-inhibitory effects in human epithelial cells. We also quantitated the amount of each member of the type I receptor tyrosine kinase family (RTK I, i.e. HER-1, HER-2, HER-3 and HER-4) in the cell lines employed and correlated this to their respective heregulin responses. These data demonstrate that HER-2/neu overexpression itself affects the expression of other RTK I members and that cells expressing the highest levels of HER-2/neu have the greatest response to HRG.

Essential roles of c-JUN and c-JUN N-terminal kinase (JNK) in neuregulin-increased expression of the acetylcholine receptor epsilon-subunit

Neuregulin is a neural factor implicated in upregulation of acetylcholine receptor (AChR) synthesis at the neuromuscular junction. Previous studies have demonstrated that the extracellular signal-regulated kinase (ERK) subgroup of MAP kinases is required for neuregulin-induced AChR gene expression. We report here that the neuregulin-mediated increase in AChR epsilon-subunit mRNA was a delayed response in C2C12 muscle cells. Neuregulin induced expression of immediate early genes c-jun and c-fos, which followed and depended on the ERK activation. Treatment of muscle cells with cycloheximide to inhibit c-JUN synthesis at the protein level and suppression of c-JUN function by a dominant-negative mutant blocked neuregulin-induced expression of the epsilon-subunit gene, indicating an essential role of c-JUN in neuregulin signaling. Furthermore, neuregulin activated c-JUN N-terminal kinase (JNK) in C2C12 muscle cells. Blockade of JNK activation by overexpressing dominant-negative MKK4 inhibited epsilon-promoter activation. Moreover, overexpression of the JNK dominant-negative mutant inhibited neuregulin-mediated expression of the epsilon-transgene and endogenous epsilon-mRNA. Taken together, our results demonstrate important roles of c-JUN and JNK in neuregulin-mediated expression of the AChR epsilon-subunit gene and suggest that neuregulin activates multiple signaling cascades that converge to regulate AChR epsilon-subunit gene expression.

The biology of human epidermal growth factor receptor 2

Our understanding of the normal signaling mechanisms and functions of human epidermal growth factor receptor 2 (HER2) and other members of the HER family, namely epidermal growth factor receptor, HER3, and HER4, is growing rapidly. Activation of these receptors results in a diverse array of signals through the formation of homodimeric and heterodimeric receptor complexes; HER2 is the preferred dimerization partner for the other HERs. These oligomeric receptor complexes activate distinct signaling pathways, such as the Ras-MAPK and PI3-kinase pathways. These, in turn, affect various cellular processes. Recent gene deletion experiments in mice point to an important role for HER2 in cardiac and neural development, and evidence from other studies indicates that HER2 is involved in normal breast growth and development. Thus, HER2 is a key component of a complex signaling network that plays a critical role in the regulation of tissue development, growth, and differentiation.

Functional analysis of H-Ryk, an atypical member of the receptor tyrosine kinase family

H-Ryk is an atypical receptor tyrosine kinase which differs from other members of this family at a number of conserved residues in the activation and nucleotide binding domains. Using a chimeric receptor approach, we demonstrate that H-Ryk has impaired catalytic activity. Despite the receptor's inability to undergo autophosphorylation or phosphorylate substrates, we demonstrate that ligand stimulation of the chimeric receptor results in activation of the mitogen-activated protein kinase pathway. The ability to transduce signals is abolished by mutation of the invariant lysine (K334A) in subdomain II of H-Ryk. Further, by in vitro mutagenesis, we show that the amino acid substitutions in the activation domain of H-Ryk account for the loss of catalytic activity. In addition to the essential aspartate residue, either phenylalanine or glycine is required in the activation domain to maintain proper conformation of the catalytic domain and thus ensure receptor autophosphorylation. Homology modelling of the catalytic domain of H-Ryk provides a rationale for these findings. Thus, the signalling properties of H-Ryk are divergent from those of other classical receptor tyrosine kinases.

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.

Heregulin regulation of Akt/protein kinase B in breast cancer cells

In the present studies, we demonstrate that heregulin is a potent and rapid activator of the serine/threonine kinase called Akt in the MCF-7 breast cancer cell line but not in 3 other breast cancer cell lines (T47D, HBL-100, and MDA-231). The extent of activation of Akt in the 4 cell lines correlated with the ability of heregulin to activate phosphatidylinositol 3-kinase and inhibition of the kinase blocked Akt activation. A monoclonal antibody to HER2 inhibited the ability of heregulin to activate Akt in the MCF-7 cells. BT474, a breast cancer cell line which overexpresses HER2, had high basal Akt enzymatic activity. This high basal activity was lowered when cells were pre-incubated with an anti-HER2 monoclonal antibody which is used to treat breast cancer patients. Our results indicate that heregulin is a potent activator of Akt and that overexpression of HER2 in breast cancers could also lead to activation of Akt.

Schwann cell survival mediated by the signaling phospholipid lysophosphatidic acid

Lysophosphatidic acid (LPA) is a bioactive phospholipid with properties of an extracellular growth factor for many cell lines, including those derived from neuroblastomas. However, the relevance of LPA signaling to the normal, developing nervous system is unknown, in part reflecting the previous unavailability of cloned receptor genes. Recent studies of the first such gene, encoding the G protein-coupled receptor LPA1/VZG-1 (lysophospholipid receptor A1/ventricular zone gene-1), revealed a major locus of expression in oligodendrocytes and Schwann cells (SCs) during development, suggesting an influence of LPA on these myelinating cells. Here we report that LPA (>/=10 nM) is a potent survival factor for cultured neonatal SCs, with survival activity equaling the maximal effect of neuregulin, the major peptide SC survival factor. LPA activates a pharmacologically defined signaling pathway in SCs, involving Gi and phosphoinositide 3-kinase. Moreover, LPA's effect depends on Akt, a downstream kinase that can mediate phosphoinositide 3-kinase-dependent survival, as demonstrated by both Western blot and transfection analyses. Overexpression of functional epitope-tagged LPA1/VZG-1 protein decreases SC apoptosis in response to serum withdrawal. These data demonstrate a role for extracellular LPA and its receptor LPA1/VZG-1 in SC survival and, more broadly, implicate G protein-coupled receptor-mediated lysophospholipid signaling as a significant mechanism in neural development.

ERK MAP kinase activation is required for acetylcholine receptor inducing activity-induced increase in all five acetylcholine receptor subunit mRNAs as well as synapse-specific expression of acetylcholine receptor epsilon-transgene

The AChR is a pentamer of four different subunits in a stoichiometry of alpha2betagammadelta in embryonic and alpha2betaepsilondelta in adult animals. Transcription of AChR subunit genes is most active in synaptic nuclei in adult skeletal muscle cells, and is regulated by neural factors such as ARIA. We report here that ARIA up-regulated specifically the expression of all five AChR subunits in C2C12 cells. The mRNA level of erbB2, erbB3, rapsyn, MuSK, SHP-2 and beta-actin remained unchanged in response to ARIA stimulation in C2C12 cells. The ARIA-induced increase in AChR subunit expression in C2C12 cells was inhibited by the erbB kinase inhibitor tyrphostin AG1478 and the MEK inhibitor PD98059, but not by the PI3 kinase inhibitor wortmannin, suggesting an important role of the erbB protein tyrosine kinases and MAP kinase in the regulation of the expression of the five different AChR subunits. To determine the signaling pathways in vivo, we studied the expression of reporter genes driven by the epsilon-promoter in injected muscles. The in vivo expression of the epsilon-transgene was inhibited by co-expression of dominant negative mutants of key components in the MAP kinase pathway including ras, raf and MEK, but not the dominant negative mutant of PI3 kinase. These results suggest that ERK MAP kinase activation is required for ARIA-induced increase in all five AChR subunit mRNAs as well as synapse-specific expression of AChR epsilon-transgene.

Differential association of phosphatidylinositol-5-phosphate 4-kinase with the EGF/ErbB family of receptors

Phosphatidylinositol-5-phosphate 4-kinase (PIP4K) is required for the production of phosphoinositol-4,5-hisphosphate (PIP2), which has been closely associated with growth factor signalling. Here we have tested the possibility that phosphoinositide kinases may be take part in signal transduction through interactions with the epidermal growth factor (EGF) receptor and the ErbB family of tyrosine kinase receptors. Interactions of the Type IIbeta isoform of PIP4K were observed with the EGF receptor family members in a number of diverse cell lines, including A431, PC12 and MCF7 cells but not with the N6F TrkA receptor. Co-immunoprecipitation experiments indicate that PIP4K interacts with not only the EGF receptor, but also selectively with members of the ErbB tyrosine kinase family. These results demonstrate another enzyme substrate for EGF receptors that facilitates the production of phosphoinositides at the cell membrane.

An intramembrane modulator of the ErbB2 receptor tyrosine kinase that potentiates neuregulin signaling

The ErbB2 receptor tyrosine kinase plays a critical role in a variety of developmental processes, and its aberrant activation may contribute to the progression of some breast and ovarian tumors. ASGP2, a transmembrane glycoprotein found on the surface of the highly metastatic ascites 13762 rat mammary adenocarcinoma cell line, is constitutively associated with ErbB2 in these cells and in mammary tissue from pregnant rats. Expression studies indicate that ASGP2 interacts directly and specifically with ErbB2 through one of its epidermal growth factor-like domains and that the co-expression of the two proteins in the same cell dramatically facilitates their direct stable interaction. Ectopic expression of ASGP2 in human melanoma tumor cells potentiates the response of endogenous ErbB2 to the neuregulin-1 growth factor. These observations point to a novel intramembrane mechanism for the modulation of receptor tyrosine kinase activity.

GGF/neuregulin induces a phenotypic reversion of oligodendrocytes

We have previously shown that glial growth factor (GGF), a member of the neuregulin (NRG) family of growth factors, is a mitogen and survival factor for oligodendrocyte progenitors in cell culture and blocks their differentiation at the pro-oligodendrocyte stage (P. D. Canoll et al., 1996, Neuron 17, 229-243). We now show that GGF is able to induce differentiated oligodendrocytes to undergo a phenotypic reversion characterized by loss of MBP expression, reexpression of the intermediate filament protein nestin, reorganization of the actin cytoskeleton, and a dramatic reduction in the number of processes per cell. TUNEL analysis demonstrates that GGF is not cytotoxic for mature oligodendrocytes, but rather enhances their survival. GGF also induces the rapid activation of the PI 3-kinase and MAP kinase signaling pathways. These results further support a role for the NRGs in promoting the proliferation and survival of and inhibiting the differentiation of cells in the oligodendrocyte lineage and demonstrate that oligodendrocytes that differentiate in culture retain a substantial degree of phenotypic plasticity.

Regulation of cyclooxygenase-2 pathway by HER2 receptor

Emerging lines of evidence suggest that in addition to growth factors, the process of colorectal tumorigenesis may also be driven by the upregulation of the inducible form of cyclooxygenase-2 (COX-2), an enzyme responsible for the conversion of arachidonic acid to PGEs. The present study was undertaken to investigate the expression and activation of the HER family members, and to explore the regulation of COX-2 expression by the HER2 pathway in human colorectal cancer cells. Here, we report that human colorectal cancer cell lines express abundant levels of HER2 and HER3 receptors, and are growth-stimulated by recombinant neu-differentiation factor-beta 1 (NDF). NDF-treatment of colorectal cancer cells was accompanied by increased tyrosine phosphorylation and heterodimerization of HER3 with HER2. In addition, we demonstrated that HER2 and HER3 receptors in colorectal cancer cells are constitutively phosphorylated on tyrosine residues and form heterodimeric complexes in the absence of exogenous NDF. Inhibition of HER2/HER3 signaling by an anti-HER3 mAb against the ligand binding site resulted in a decrease in the levels of constitutively activated HER2/ HER3 heterodimers, and the unexpected reduction of COX-2 expression. Activation of the HER2/HER3 pathway by NDF induced the activation of COX-2 promoter, expression of COX-2 mRNA, COX-2 protein and accumulation of prostaglandin E2 in the culture medium. Finally, we demonstrated that NDF promotes the ability of colorectal cancer cells to survive in an extracellular matrix milieu, such as Matrigel, and also to invade through a 8 microm porous membrane. These biological activities of NDF and its stimulation of cell proliferation are blocked by a specific inhibitor of COX-2. Taken together, our findings provide the first biochemical evidence of a possible role of the COX-2 pathway in the mitogenic action of NDF in colorectal cancer cells where it may be constitutively upregulated due to the autocrine/paracrine activation of HER2/ HER3 heterodimers.

Morphogenetic effects of neuregulin (neu differentiation factor) in cultured epithelial cells

Neuregulin, or neu differentiation factor, induces cell proliferation or differentiation through interaction with members of the ErbB family of receptor tyrosine kinases. We report that neuregulin can also induce profound morphogenic responses in cultured epithelial cells of different origins. These effects include scattering of small epithelial islands and rearrangement of larger cell islands into ordered ring-shaped arrays with internal lumens. The ring-forming cells are interconnected by cadherin- and beta-catenin-containing adherens junctions. In confluent cultures, neuregulin treatment induces formation of circular lumenlike gaps in the monolayer. Both cell scattering and ring formation are accompanied by a marked increase in cell motility that is independent of hepatocyte growth factor/scatter factor and its receptor (c-Met). Affinity-labeling experiments implied that a combination of ErbB-2 with ErbB-3 mediates the morphogenic signal of neuregulin in gastric cells. Indeed, a similar morphogenic effect could be reconstituted in nonresponsive cells by coexpression of ErbB-2 and -3. We conclude that a heterodimer between the kinase-defective neuregulin receptor, ErbB-3, and the coreceptor, ErbB-2, mediates the morphogenetic action of neuregulin.

Differential signaling by the epidermal growth factor-like growth factors neuregulin-1 and neuregulin-2

The neuregulins comprise a subfamily of epidermal growth factor (EGF)-like growth factors that elicit diverse cellular responses by activating members of the ErbB family of receptor tyrosine kinases. Although neuregulin-1 and neuregulin-2 are both binding ligands for the ErbB3 and ErbB4 receptors, they exhibit distinct biological activities depending on cellular context. In MDA-MB-468 human mammary tumor cells, neuregulin-2beta (NRG2beta) inhibits cell growth, whereas neuregulin-1beta (NRG1beta) does not. In these cells, NRG2beta appears to preferentially act through the EGF receptor, stimulating receptor tyrosine phosphorylation and the recruitment of phospholipase C-gamma, Cbl, SHP2, and Shc to that receptor. NRG1beta preferentially acts through ErbB3 in these cells by stimulating the tyrosine phosphorylation and recruitment of phosphatidylinositol 3-kinase and Shc to that receptor. In MDA-MB-453 cells, both NRG1beta and NRG2beta stimulate the tyrosine phosphorylation of the ErbB2 and ErbB3 receptors to similar extents, but only NRG1beta potently stimulates morphological changes consistent with their differentiation. The profiles of SH2 domain-containing proteins that are efficiently recruited to activated receptors differ for the two factors. These observations indicate that despite their overlapping receptor specificity, the neuregulins exhibit distinct biological and biochemical properties. Since both of these cell lines express only two of the known ErbB receptors, our results imply that EGF-like ligands might elicit differential signaling within the context of a single receptor heterodimer.

The sevenfold way of PKC regulation

Protein kinase C (PKC) is a family of enzymes that are physiologically activated by 1,2-diacylglycerol (DAG) and other lipids. To date, 11 different isozymes, alpha, betaI, betaII, gamma, delta, epsilon, nu, lambda(iota), mu, theta and zeta, have been identified. On the basis of their structure and activators, they can be divided into three groups, two of which are activated by DAG or its surrogate, phorbol 12-myristate 13-acetate (PMA). PKC isozymes are remarkably different in number and prevalence in different cell lines and tissues. When activated, the isozymes bind to membrane phospholipids or to receptors that are located in and anchor the enzymes in a subcellular compartment. Some PKCs may also be activated in their soluble form. These enzymes phosphorylate serine and threonine residues on protein substrates, perhaps the best known of which are the myristoylated, alanine-rich C kinase substrate and nuclear lamins A, B and C. The enzymes clearly play a role in signal transduction, and, because of the importance of PMA as a tumor promoter, they are thought to affect some aspect of cell cycling. How PKC takes part in the regulation of cell transformation, growth, differentiation, ruffling, vesicle trafficking and gene expression, however, is largely unknown.

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.

Mutation of a Shc binding site tyrosine residue in ErbB3/HER3 blocks heregulin-dependent activation of mitogen-activated protein kinase

The ErbB2 and ErbB3 proteins together constitute a functional coreceptor for heregulin (neuregulin). Heregulin stimulates the phosphorylation of both coreceptor constituents and initiates a variety of other signaling events, which include phosphorylation of the Shc protein. The role of Shc in heregulin-stimulated signal transduction through the ErbB2.ErbB3 coreceptor was investigated here. Heregulin was found to promote ErbB3/Shc association in NIH-3T3 cells expressing endogenous ErbB2 and recombinant ErbB3. A mutant ErbB3 protein was generated in which Tyr-1325 in a consensus Shc phosphotyrosine-binding domain recognition site was mutated to Phe (ErbB3-Y/F). This mutation abolished the association of Shc with ErbB3 and blocked the activation of mitogen-activated protein kinase by heregulin. Whereas heregulin induced mitogenesis in NIH-3T3 cells transfected with wild-type ErbB3 cDNA, this mitogenic response was markedly attenuated in NIH-3T3 cells transfected with the ErbB3-Y/F cDNA. These results showed a specific interaction of Shc with the ErbB3 receptor protein and demonstrated the importance of this interaction in the activation of mitogenic responses by the ErbB2. ErbB3 heregulin coreceptor complex.

Heregulin-dependent autocrine loop regulates growth of K-ras but not erbB-2 transformed rat thyroid epithelial cells

The EGF-like family of proteins, such as epidermal growth factor (EGF), transforming growth factor alpha (TGFalpha), amphiregulin (AR), betacellulin (BTC), cripto-1 (CR-1), and heregulin (HRG), plays an important role in the pathogenesis of several human carcinomas as autocrine growth factors. Differentiation and proliferation of rat thyroid cells in culture (FRTL-5 cells) are regulated by thyrotropin (TSH); withdrawal of TSH from culture medium produces growth arrest, whereas its addition to quiescent cells stimulates cell entry into S phase. Instead, transformed thyroid cell lines as FRTL-5H2 cell line, overexpressing erbB-2, Kimol cells, transformed by the wild-type K-ras and A6 clone, transformed by a temperature sensitive K-ras mutant, can grow without addition of TSH to the culture medium. In order to identify whether EGF-like growth factors and corresponding receptors (erbB-2, erbB-3, and erbB-4) could be involved in the autonomous growth of these transformed rat thyroid epithelial cells, Northern blot for mRNA analysis and Western blot for protein expression were performed. In contrast to normal control FRTL-5 cells, both K-ras and erbB-2-transformed cells expressed elevated levels of erbB-2 receptor. Moreover, both K-ras transformed cells, Kimol and A6 cells, but no FRTL-5H2 cells, were found able to express also high levels of erbB-4 receptor and HRG/NDF ligand. Treatment of K-ras transformed thyroid cells with neutralizing antibody against HRG/NDF reduced by 50% cell proliferation. These data indicate that unlike the erbB-2 overexpressing FRTL-5 cells, in K-ras rat thyroid epithelial cells, the growth factor heregulin signals through the heterodimer erbB-2/erbB-4 receptors in an autocrine fashion.

ErbB3 (HER3) interaction with the p85 regulatory subunit of phosphoinositide 3-kinase

ErbB3 (HER3), a unique member of the ErbB receptor family, lacks intrinsic protein tyrosine kinase activity and contains six Tyr-Xaa-Xaa-Met (YXXM) consensus binding sites for the SH2 domains of the p85 regulatory subunit of phosphoinositide 3-kinase. ErbB3 also has a proline-rich sequence that forms a consensus binding site for the SH3 domain of p85. Here we have investigated the interacting domains of ErbB3 and p85 by a unique application of the yeast two-hybrid system. A chimaeric ErbB3 molecule containing the epidermal growth factor receptor protein tyrosine kinase domain was developed so that the C-terminal domain of ErbB3 could become phosphorylated in the yeast system. We also generated several ErbB3 deletion and Tyr-->Phe site-specific mutants, and observed that a single ErbB3 YXXM motif was necessary and sufficient for the association of ErbB3 with p85. The incorporation of multiple YXXM motifs into the ErbB3 C-terminus enabled a stronger ErbB3/p85 interaction. The proline-rich region of ErbB3 was not necessary for interaction with p85. However, either deletion or mutation of the p85 SH3 domain decreased the observed ErbB3/p85 association. Additionally an ErbB3/p85 SH3 domain interaction was detected by an assay in vitro. These results were consistent with a model in which pairs of phosphorylated ErbB3 YXXM motifs co-operate in binding to the tandem SH2 domains of p85. Although a contributing role for the p85 SH3 domain was suggested, the N- and C-terminal SH2 domains seemed to be primarily responsible for the high-affinity association of p85 and ErbB3.

Development of salivary gland cell lines for studies of signaling and physiology

We developed and characterized an immortalized rat parotid cell line to use in salivary gland studies. The cells were immortalized by retroviral transduction of SV40 large T antigen into isolated parotid cells. Using immunocytochemical techniques, we found that the immortalized epithelial cells were ductal, rather than acinar, in nature. Cells were grown under coculture conditions with lethally irradiated NIH3T3 cells. One cell line, which was designated RPG1/SV40 cells (for rat parotid gland 1/SV40 transformant), was selected for characterization. These cells formed a sheet epithelium with tight junctions and a measurable transepithelial resistance. RPG1/SV40 cells responded to muscarinic receptor (carbachol) and/or P2 purinoceptor (ATP and UTP) stimuli with increases in the following: (1) intracellular free-calcium concentration ([Ca2+]i); (2) the short-circuit current (ISC) across the epithelium; (3) the tyrosine phosphorylation of PKC delta; and (4) MAP kinase activity. Thus, the cells appear to be useful for a wide range of studies involving physiology, biochemistry, and signal transduction approaches.

Changes in heregulin beta1 (HRGbeta1) signaling after inhibition of ErbB-2 expression in a human breast cancer cell line

Specific biological responses to the erbB3-erbB4 ligand heregulin (HRG) have been postulated to be due to the formation of heterodimers of those receptors with erbB2. To test the role that erbB2 plays in the response to HRG in a human breast carcinoma cell line, antisense oriented erbB2 was stably transfected into AU565 cells. In the absence of HRG, inhibition of erbB2 expression slowed cell growth, leading to accumulation of cells in the G2/M phase, and suppressed colony growth in soft agar. Low concentrations of HRG induced cell proliferation in both the erbB2-nonexpressing cells and the parental AU565 cells. In contrast, high concentrations of HRG failed to induce differentiation of the erbB2-nonexpressing cells as compared with the parental cells. ErbB3 expression was significantly decreased in the erbB2 nonexpressing cells. ErbB3 was constitutively tyrosine phosphorylated in both the parental AU565 cells and in the erbB2 nonexpressing cells. HRG further increased tyrosine phosphorylation of erbB3 with a maximum response at 1 ng/ml of HRG in erbB2 nonexpressing cells, as compared with 10 ng/ml of HRG in AU565 cells. This finding suggested that the biochemical responsiveness of erbB3 to HRG was changed, but not abrogated, by inhibition of erbB2 expression. These results suggest that inhibition of erbB2 expression modulates, but does not abolish, HRG mediated signal transduction pathways in a human breast cancer cell line.

In vivo analysis of Argos structure-function. Sequence requirements for inhibition of the Drosophila epidermal growth factor receptor

The Drosophila Argos protein is the only known extracellular inhibitor of the epidermal growth factor receptor (EGFR). It is structurally related to the activating ligands, in that it is a secreted protein with a single epidermal growth factor (EGF) domain. To understand the mechanism of Argos inhibition, we have investigated which regions of the protein are essential. A series of deletions were made and tested in vivo; furthermore, by analyzing chimeric proteins between Argos and the activating ligand, Spitz (a transforming growth factor-alpha-like factor), we have examined what makes one inhibitory and the other activating. Our results reveal that Argos has structural requirements that differ from all known EGFR activating ligands; domains flanking the EGF domain are essential for its function. We have also defined the important regions of the atypical Argos EGF domain. The extended B-loop is necessary, whereas the C-loop can be replaced with the equivalent Spitz region without substantially affecting Argos function. Comparison of the argos genes from Drosophila melanogaster and the housefly, Musca domestica, supports our structure-function analysis. These studies are a prerequisite for understanding how Argos inhibits the Drosophila EGFR and provide a basis for designing mammalian EGFR inhibitors.

Phosphatidylinositol 3-kinase mediates epidermal growth factor-induced activation of the c-Jun N-terminal kinase signaling pathway

The signaling events which mediate activation of c-Jun N-terminal kinase (JNK) are not yet well characterized. To broaden our understanding of upstream mediators which link extracellular signals to the JNK pathway, we investigated the role of phosphatidylinositol (PI) 3-kinase in epidermal growth factor (EGF)-mediated JNK activation. In this report we demonstrate that a dominant negative form of PI 3-kinase as well as the inhibitor wortmannin blocks EGF-induced JNK activation dramatically. However, wortmannin does not have an effect on JNK activation induced by UV irradiation or osmotic shock. In addition, a membrane-targeted, constitutively active PI 3-kinase (p110beta) was shown to produce in vivo products and to activate JNK, while a kinase-mutated form of this protein showed no activation. On the basis of these experiments, we propose that PI 3-kinase activity plays a role in EGF-induced JNK activation in these cells.

Epidermal growth factor and betacellulin mediate signal transduction through co-expressed ErbB2 and ErbB3 receptors

Interleukin-3 (IL-3)-dependent murine 32D cells do not detectably express epidermal growth factor receptors (EGFRs) and do not proliferate in response to EGF, heregulin (HRG) or other known EGF-like ligands. Here, we report that EGF specifically binds to and can be crosslinked to 32D transfectants co-expressing ErbB2 and ErbB3 (32D.E2/E3), but not to transfectants expressing either ErbB2 or ErbB3 individually. [125I]EGF-crosslinked species detected in 32D. E2/E3 cells were displaced by HRG and betacellulin (BTC) but not by other EGF-like ligands that were analyzed. EGF, BTC and HRG also induced receptor tyrosine phosphorylation, activation of downstream signaling molecules and proliferation of 32D.E2/E3 cells. 32D transfectants were also generated which expressed an ErbB3-EGFR chimera alone (32D.E3-E1) or in combination with ErbB2 (32D. E2/E3-E1). While HRG stimulation of 32D.E3-E1 cells resulted in DNA synthesis and receptor phosphorylation, EGF and BTC were inactive. However, EGF and BTC were as effective as HRG in mediating signaling when ErbB2 was co-expressed with the chimera in the 32D.E2/E3-E1 transfectant. These results provide evidence that ErbB2/ErbB3 binding sites for EGF and BTC are formed by a previously undescribed mechanism that requires co-expression of two distinct receptors. Additional data utilizing MDA MB134 human breast carcinoma cells, which naturally express ErbB2 and ErbB3 in the absence of EGFRs, supported the results obtained employing 32D cells and suggest that EGF and BTC may contribute to the progression of carcinomas that co-express ErbB2 and ErbB3.

Roles of ErbB-3 and ErbB-4 in the physiology and pathology of the mammary gland

ErbB-3 and ErbB-4 are the most recently discovered and least characterized of the class I tyrosine kinase receptors. ErbB-3 is noteworthy for its low tyrosine kinase activity, suggesting that it may function more as an adaptor in signaling than as a kinase. Heregulin serves as a ligand for both receptors. A primary mechanism of heregulin action involves heterodimerization of its targeted receptors with other members of the class I family to promote cross-phosphorylation and cellular responses. Betacellulin also acts as a ligand for ErbB-4 to stimulate its kinase activity in both homo- and hetero-dimers. A new ligand (ASGP-2) for ErbB-2 has been discovered which operates by an intramembrane mechanism and may be able to modulate external ligand-dependent ErbB-3 or ErbB-4 heterodimeric interactions with ErbB-2. Heterodimerization stimulated by the ligands is a key feature of mitogenic signaling in mammary epithelial cells and tumors. Characterization of the signaling pathways for these receptors is still incomplete, but phosphatidylinositol 3-kinase and SHC have been implicated. Heregulin synthesized by the mesenchyme has been implicated in mammary development, modulated by systemic hormones. Observations on cultured mammary cells and mammary tumors have suggested linkages of ErbB-3 and ErbB-4 to proliferation and differentiation, respectively, but further work is needed to establish their definitive roles.

Heregulin-stimulated acetylcholine receptor gene expression in muscle: requirement for MAP kinase and evidence for a parallel inhibitory pathway independent of electrical activity

Binding of heregulin (HRG) to its receptor, ErbB3, results in a dimerization with ErbB2/neu and activation of their intrinsic tyrosine kinases, initiating a cascade of events resulting in the stimulation of acetylcholine receptor (AChR) genes in muscle. Here we have examined the signalling downstream of the HRG receptor. We show that phosphatidylinositol 3'-kinase (PI3K) and SHC bind to the HRG-activated ErbB3 in myotubes. Subsequently, p70S6 kinase (p70S6k), and MAP kinase ERK2 and thereby p90rsk are activated. However, inhibition of PI3K and p70S6k by wortmannin and rapamycin, respectively, failed to antagonize AChR alpha-subunit gene expression stimulated by HRG, despite the fact that the activities of the kinases were inhibited. In contrast, these inhibitors elevated AChR alpha-subunit mRNA levels, by themselves, independently of muscle electrical activity. On the other hand, the 17mer antisense oligonucleotide, EAS1, caused a specific depletion of ERK2 and eliminated the ability of HRG to stimulate AChR alpha-subunit gene expression. These results indicate that HRG stimulates expression of AChR genes via ERK2 activation, and provide a physiological example of neurotrophic factor-associated repression of AChR genes by stimulation of p70S6k activity which may contribute to the expression of adult type AChR genes at the neuromuscular junction.