Efficient coupling with phosphatidylinositol 3-kinase, but not phospholipase C gamma or GTPase-activating protein, distinguishes ErbB-3 signaling from that of other ErbB/EGFR family members

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

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

Cross-talk between phorbol ester-mediated signaling and tyrosine kinase proto-oncogenes. I. Activation of protein kinase C stimulates tyrosine phosphorylation and activation of ErbB2 and ErbB3

The tumor-promoting phorbol ester, phorbol 12-myristate 13-acetate (PMA), acutely stimulates the tyrosine phosphorylation of proteins of approximately 190, 120, and 70 kDa in the well differentiated Fao rat hepatoma cell line. This phosphorylation is dependent on protein kinase C (PKC) and is abolished by down-regulation of PKC or pretreatment with a PKC inhibitor. Purification of the 190-kDa tyrosine-phosphorylated protein revealed that it consists of both ErbB2 and ErbB3. Following PMA-induced tyrosine phosphorylation, ErbB2 and ErbB3 were able to associate with the SH2 domains of several signaling proteins including the p85alpha subunit of phosphatidylinositol 3-kinase, Syp, and Grb2. The 120-kDa protein phosphorylated in response to PMA consists of at least two proteins: focal adhesion kinase that exhibits a minor increase in tyrosine phosphorylation following treatment with PMA, and a major 120-kDa tyrosine-phosphorylated species in PMA-stimulated Fao cells which as yet is unidentified. Similarly, the 70-kDa tyrosine-phosphorylated protein also appears to represent more than one protein, including paxillin and a second protein of similar mobility which appears to be the major tyrosine phosphorylation in response to PMA. Both ErbB2 and paxillin also exhibit reduced migration on SDS-polyacrylamide gel electrophoresis following PMA treatment, suggesting that they are also phosphorylated on serine/threonine residues. The mobility shift of both of these proteins is abolished by treatment with inhibitors of PKC or mitogen-activated protein kinase/extracellular signal-related kinase kinase. These results suggest a novel mechanism of cross-talk between the serine/threonine kinase PKC and tyrosine phosphorylation pathways. The activation of ErbB2 and ErbB3 that is initiated by PMA may contribute to the tumor promoting activity of these compounds.

Cross-talk between phorbol ester-mediated signaling and tyrosine kinase proto-oncogenes. II. Comparison of phorbol ester and sphingomyelinase-induced phosphorylation of ErbB2 and ErbB3

In the accompanying paper (Emkey, R., and Kahn, C. R. (1997) J. Biol. Chem. 272, 31172-31181), we demonstrated that phorbol 12-myristate 13-acetate (PMA) treatment of Fao cells induces tyrosine phosphorylation of several proteins including ErbB2 and ErbB3. In the present study we show that sphingomyelinase also results in the enhanced tyrosine phosphorylation of ErbB2 and ErbB3 in these cells. In contrast to activation by PMA, the sphingomyelinase-induced phosphorylation of these proteins is independent of protein kinase C. However, both agents stimulate tyrosine phosphorylation of the kinase Pyk2 suggesting that it may be involved in the PMA and sphingomyelinase activation of these ErbB proto-oncogenes. Insulin plays a negative regulatory role in the ligand and non-ligand-induced phosphorylation of the ErbB proto-oncogenes via two mechanisms. Prolonged insulin treatment resulted in decreased expression of both ErbB2 and ErbB3. Insulin also appears to negatively regulate the protein tyrosine kinase responsible for phosphorylating ErbB2 in PMA-stimulated cells. The former effect of insulin was relieved by treatment with inhibitors of phosphatidylinositol 3-kinase. The similarities in PMA and sphingomyelinase-induced effects and the negative regulatory role of insulin suggest a mechanism by which multiple ligands can synergize with or protect against the tumorigenic effects of phorbol esters.

Heat shock activates c-Src tyrosine kinases and phosphatidylinositol 3-kinase in NIH3T3 fibroblasts

There is increasing evidence that cellular responses to stress are in part regulated by protein kinases, although specific mechanisms are not well defined. The purpose of these experiments was to investigate potential upstream signaling events activated during heat shock in NIH3T3 fibroblasts. Experiments were designed to ask whether heat shock activates p60 c-Src tyrosine kinase or phosphatidylinositol 3-kinase (PI 3-kinase). Using in vitro protein kinase activity assays, it was demonstrated that heat shock stimulates c-Src and PI 3-kinase activity in a time-dependent manner. Also, there was increased PI 3-kinase activity in anti-phosphotyrosine and anti-c-Src immunoprecipitated immunocomplexes from heated cells. Heat shock activated mitogen-activated protein kinase (MAPK) and p70 S6 kinase (S6K) in these cells. The role of PI 3-kinase in regulating heat shock activation of MAPK and p70 S6K was investigated using wortmannin, a specific pharmacological inhibitor of PI 3-kinase. The results demonstrated that wortmannin inhibited heat shock activation of p70 S6K but only partially inhibited heat activation of MAPK. A dominant negative Raf mutant inhibited activation of MAPK by heat shock but did not inhibit heat shock stimulation of p70 S6K. Genistein, a tyrosine kinase inhibitor, and suramin, a growth factor receptor inhibitor, both inhibited heat shock stimulation of MAPK activity and tyrosine phosphorylation of MAPK. Furthermore, a selective epidermal growth factor receptor (EGFR) inhibitor, tryphostin AG1478, and a dominant negative EGFR mutant also inhibited heat shock activation of MAPK. Heat shock induced EGFR phosphorylation. These results suggest that early upstream signaling events in response to heat stress may involve activation of PI 3-kinase and tyrosine kinases, such as c-Src, and a growth factor receptor, such as EGFR; activation of important downstream pathways, such as MAPK and p70 S6K, occur by divergent signaling mechanisms similar to growth factor stimulation.

Association of csk-homologous kinase (CHK) (formerly MATK) with HER-2/ErbB-2 in breast cancer cells

Protein-tyrosine kinases, such as HER-2/ErbB-2, have been specifically linked to breast cancer. The Csk-homologous kinase (CHK), formerly MATK, is a tyrosine kinase that contains the Src homology 2 and 3 (SH2 and SH3) domains and demonstrates homology ( approximately 50%) to the Csk tyrosine kinase. Like Csk, CHK is able to phosphorylate and inactivate Src family kinases. In this report, we investigated whether CHK is expressed in breast cancer tissues and whether it participates in the ErbB-2 signaling pathway in T47D and MCF-7 breast cancer cell lines. Immunostaining of the CHK protein in breast tissues demonstrated that primary invasive ductal carcinomas, stage II (13 of 15 cases) and stage I (8 of 15 cases), expressed the CHK protein, while this protein was not detected in the adjacent normal tissues from the same patients. To study the role of CHK in the ErbB-2 signaling pathway, glutathione S-transferase fusion proteins containing the SH2 and SH3 domains of CHK were generated. CHK-SH2 and CHK-SH3-SH2, but not CHK-SH3 or CHK-NH2-SH3, precipitated the tyrosine-phosphorylated ErbB-2 upon stimulation with heregulin. EGF or interleukin-6 stimulation of T47D cells failed to induce CHK-SH2 association with ErbB-2, the EGF-receptor, or the interleukin-6 receptor. In vivo association of the tyrosine-phosphorylated ErbB-2 with CHK was observed in co-immunoprecipitation studies with anti-CHK antibodies. EGF-R, ErbB-3, and ErbB-4 were not detected in the CHK immunoprecipitates or in the precipitates of the GST-SH2 fusion proteins of CHK, suggesting that the association of CHK with ErbB-2 upon heregulin stimulation is receptor-specific (ErbB-2) and ligand-specific (heregulin). These results indicate that CHK might participate in signaling in breast cancer cells by associating, via its SH2 domain, with ErbB-2 following heregulin stimulation.

ARIA: a neuromuscular junction neuregulin

Motor neurons influence the expression and the distribution of acetylcholine receptors in skeletal muscle. Molecules that mediate this carefully choreographed interaction have recently been identified. One of them, ARIA, is a polypeptide purified from chicken brain on the basis of its ability to stimulate the synthesis of muscle acetylcholine receptors. The predicted amino acid sequence suggests that ARIA is synthesized as a transmembrane precursor protein and that it is a member of a family of ligands that activate receptor tyrosine kinases related to the epidermal growth factor receptor. Certain features of the ligand family (the neuregulins) and their receptors (erbBs) are reviewed. Evidence that ARIA plays an important role at developing and mature neuromuscularjunctions is discussed.

HER-2/neu signal transduction in human breast and ovarian cancer

The HER-2/neu proto-oncogene encodes a 185 kDa transmembrane receptor tyrosine kinase with significant sequence homology to other members of the class I receptor tyrosine kinase family. The HER-2/neu gene is amplified and/or overexpressed in 25%-30% of human breast and ovarian cancers, and overexpression of the receptor is associated with poor prognosis. Tyrosine phosphorylation and activation of the HER-2 receptor lead to activation of specific signal transduction pathways in breast and ovarian cancer cells, including the ras/MAP kinase cascade, phosphatidylinositol 3-kinase, and phospholipase C-gamma. HER-2/neu signal transduction pathways ultimately converge on the cell nucleus, where the expression of diverse genes is induced after activation of the receptor. A more complete understanding of HER-2/neu signal transduction pathways may allow the development of specific therapeutics for the treatment of those human breast and ovarian cancers containing this alteration.

The epidermal growth factor receptor couples transforming growth factor-alpha, heparin-binding epidermal growth factor-like factor, and amphiregulin to Neu, ErbB-3, and ErbB-4

The epidermal growth factor (EGF) family hormones amphiregulin (AR), transforming growth factor-alpha (TGF-alpha), and heparin-binding EGF-like growth factor (HB-EGF) are thought to play significant roles in the genesis or progression of a number of human malignancies. However, the ability of these ligands to activate all four erbB family receptors has not been evaluated. Therefore, we have assessed the stimulation of erbB family receptor tyrosine phosphorylation by these hormones in a panel of mouse Ba/F3 cell lines expressing the four erbB family receptors, singly and in pairwise combinations. We also measured the stimulation of interleukin-3-independent survival or proliferation in this panel of Ba/F3 cell lines to compare the patterns of erbB family receptor coupling to physiologic responses induced by these peptides. EGF, TGF-alpha, AR, and HB-EGF all stimulated qualitatively similar patterns of erbB family receptor tyrosine phosphorylation and coupling to physiologic responses. Therefore, EGF, TGF-alpha, AR, and HB-EGF are functionally identical in this model system and behave differently from the EGF family hormones betacellulin and neuregulins.

Transformation of NIH 3T3 cells by HER3 or HER4 receptors requires the presence of HER1 or HER2

Members of the epidermal growth factor receptor (EGFR) subfamily of receptor protein tyrosine kinases have been implicated in the pathogenesis of various malignancies. The ability of one EGFR subfamily member to influence, or function synergistically with, another is likely to be a general feature of these receptors. To assess the role of receptor heterodimerization, we analyzed the ability of Neu differentiation factor (NDF) to induce cell growth and transformation of NIH 3T3 cells transfected with different combinations of the EGFR subfamily of receptors. NDF induced mitogenesis, but not transformation, of cells expressing either HER3 or HER4 alone. However, NDF-induced cell transformation was observed when either HER1 or HER2 was coexpressed with HER3 or HER4. In analogous receptor phosphorylation experiments, NDF-induced transphosphorylation appears to be correlated with synergistic transformation of NIH 3T3 cells. Interestingly, transphosphorylation between HER1 and HER4 can be stimulated by either EGF or NDF.

Ligand-induced dimerization of growth factor receptors: variations on the theme

Growth, differentiation, apoptosis and movement of cells are regulated, in part, by polypeptide growth factors, or cytokines, which exert their effects by binding to cell surface receptors on the target cells. Recent observations have indicated that a common mechanism of activation of several classes of such receptors is ligand-induced dimerization or oligomerization of the receptors.

Diversification of Neu differentiation factor and epidermal growth factor signaling by combinatorial receptor interactions

The ErbB family includes two receptors, ErbB-1 and ErbB-3, that respectively bind to epidermal growth factor and Neu differentiation factor, and an orphan receptor, ErbB-2. Unlike ErbB-1 and ErbB-2, the intrinsic tyrosine kinase of ErbB-3 is catalytically impaired. By using interleukin-3-dependent cells that ectopically express the three ErbB proteins or their combinations, we found that ErbB-3 is devoid of any biological activity but both ErbB-1 and ErbB-2 can reconstitute its extremely potent mitogenic activity. Transactivation of ErbB-3 correlates with heterodimer formation and is reflected in receptor phosphorylation and the transregulation of ligand affinity. Inter-receptor interactions enable graded proliferative and survival signals: heterodimers are more potent than homodimers, and ErbB-3-containing complexes, especially the ErbB-2/ErbB-3 heterodimer, are more active than ErbB-1 complexes. Nevertheless, ErbB-1 signaling displays dominance over ErbB-3 when the two receptors are coexpressed. Although all receptor combinations activate the mitogen-activated protein kinases ERK and c-Jun kinase, they differ in their rate of endocytosis and in coupling to intervening signaling proteins. It is conceivable that combinatorial receptor interactions diversify signal transduction and confer double regulation, in cis and in trans, of the superior mitogenic activity of the kinase-defective ErbB-3.

Involvement of ErbB2 in the signaling pathway leading to cell cycle progression from a truncated epidermal growth factor receptor lacking the C-terminal autophosphorylation sites

To investigate the mechanisms underlying the enhanced mitogenic activity of the truncated epidermal growth factor receptor (EGFR) lacking the C-terminal autophosphorylation sites (Delta973-EGFR), we studied the intracellular signaling pathways in NR6 cells expressing human wild type EGFR and Delta973-EGFR. Microinjection of dominant/negative p21ras(N17) completely inhibited EGF-induced DNA synthesis in both cell types. EGF stimulated Shc phosphorylation as well as the formation of wild type EGFR.Shc complexes. In contrast, EGF stimulated Shc phosphorylation without formation of Delta973-EGFR.Shc complexes. Tyrosine-phosphorylated Shc formed complexes with Grb2.Sos, and microinjection of anti-Shc antibody and Shc-SH2 GST fusion protein inhibited EGF stimulation of DNA synthesis in both cell lines. EGF markedly increased ErbB2 tyrosine phosphorylation in wild type EGFR cells. In Delta973-EGFR cells, ErbB2 was tyrosine phosphorylated in the basal state and EGFR stimulated further phosphorylation of ErbB2. In addition to ErbB2, additional proteins were tyrosine phosphorylated in Delta973-EGFR cells, mostly in the molecular mass range of 120 170 kDa. Taken together with our findings indicating coupling of ErbB2 to Shc, these data suggest the importance of an alternative signaling pathway in Delta973-EGFR cells mediated by the formation of heterodimeric structures between the truncated EGFR and ErbB2, followed by coupling through Shc to Grb2.Sos and the p21ras pathway, ultimately leading to mitogenesis.

Epidermal growth factor-related peptides activate distinct subsets of ErbB receptors and differ in their biological activities

Numerous epidermal growth factor (EGF)-related peptide binding members of the ErbB family of receptor tyrosine kinases have been described. While several EGF agonists bind and activate ErbB-1/EGF receptor, neu differentiation factor (NDF) functions as a ligand for ErbB-3 and ErbB-4. However, it is currently unknown which specific subsets of ErbB receptors become activated in response to each of these ligands. The present study addresses this issue using the T47D breast tumor cell line, which expresses moderate levels of all the presently known ErbB receptors. We show that all the EGF agonists, but not NDF, stimulated tyrosine phosphorylation of ErbB-1. In contrast, all the EGF-related factors except amphiregulin were able to induce tyrosine phosphorylation of ErbB-2. The ability to induce tyrosine phosphorylation of ErbB-3 varied dramatically among the different EGF-related peptides. While EGF, transforming growth factor (TGF)-alpha, and amphiregulin only had a moderate effect, NDF dramatically increased the ErbB-3 phosphotyrosine content. Most notably, heparin binding EGF-related growth factor (HB-EGF) and betacellulin (BTC) were more effective than other EGF agonists. Consequently, only NDF, HB-EGF, and BTC significantly stimulated association of phosphatidylinositol kinase activity with ErbB-3. Among the EGF agonists, HB-EGF induced a low level of ErbB-4 tyrosine phosphorylation, while BTC was as efficient as NDF in activating ErbB-4. The BTC activation of ErbB-4 appears to be independent of ErbB-1, as shown by pretreatment of cells with an antibody that inhibits binding of EGF agonists to ErbB-1. As a result of the differential activation of ErbB receptors, most of the EGF-related growth factors had distinguishable biological activities on cultured mammary epithelial cell lines.

Insulin-like growth factor and epidermal growth factor independence in human mammary carcinoma cells with c-erbB-2 gene amplification and progressively elevated levels of tyrosine-phosphorylated p185erbB-2

Growth factor-independent proliferation is an essential aspect of the transformation process. To study the influence of c-erbB-2 overexpression on the autonomous growth of human mammary cancer cells, we used a series of non-neoplastic and neoplastic human mammary epithelial cell lines isolated from a patient with intraductal and invasive ductal carcinoma of the breast. The non-neoplastic cell line, H16N-2, which expresses a normal level (single gene copy) of c-erbB-2, was used for comparison with the neoplastic cell lines. Both the metastatic tumor cell lines, 21MT-1 and 21 MT-2, showed equivalent amplification of the c-erbB-2 gene; however, 21MT-1 cells showed a higher level of c-erbB-2 overexpression. Therefore, the H16N-2, 21MT-2, and 21MT-1 cell series forms a distinct gradient of progressively increasing c-erbB-2 gene expression. Furthermore, the overexpression of c-erbB-2 in the 21MT cell lines was concordant with increases in the constitutive tyrosine kinase activity of p185erb-2 measured in the absence of exogenous growth factors in culture. Normal mammary epithelial cells require both insulin-like growth factor (IGF)-l (or supraphysiological concentrations of insulin) and epidermal growth factor (EGF) to proliferate under serum-free conditions in culture. By contrast, 21MT-2 cells showed a reduced requirement for IGF but still required EGF to proliferate. 21MT-1 cells did not require either insulin or EGF to proliferate. Therefore, the progressive increases in constitutive p185erbB-2, tyrosine kinase activity in the 21MT-2 and 21MT-1 cell lines was directly correlated with IGF independence and combined IGF and EGF independence under defined conditions in culture. Experiments using conditioned media and anti-IGF-1 receptor and anti-EGF receptor neutralizing antibodies showed that the growth-factor independence of the tumor cells did not involve detectable IGF- or EGF-like autocrine activity expressed by the 21MT cells. Furthermore, neu differentiation factor/heregulin, a ligand that indirectly activates p185erbB-2 by direct binding to erbB-3 receptors, potently stimulated the proliferation of the growth factor-dependent H16N-2 cells (which expressed c-erbB-2 and c-erbB-3 but not c-erbB-4) in the absence of both IGF and EGF. Thus, HRG-induced mitogenesis mimicked the autonomous growth seen in the 21MT cells that have the highest level of constitutive p185erbB-2 activation. These data support the hypothesis that the constitutive activation of p185erbB-2 in human mammary carcinoma cells causes growth-factor independence by directly activating multiple signal-transduction pathways that substitute for both IGF and EGF during proliferation.

HER4-mediated biological and biochemical properties in NIH 3T3 cells. Evidence for HER1-HER4 heterodimers

The EGF receptor family of tyrosine kinase growth factor receptors is expressed in a variety of cell types and has been implicated in the progression of certain human adenocarcinomas. The most recent addition to this family of receptors, HER4, was expressed in NIH 3T3 cells to determine its biological and biochemical characteristics. Cells expressing HER4 were responsive to heregulin beta2 as demonstrated by an increase in HER4 tyrosine phosphorylation and ability to form foci on a cell monolayer. HER4 exhibited in vitro kinase activity and was able to phosphorylate the regulatory subunit of phosphatidylinositol 3-kinase and SHC. Peptide competition studies identified tyrosine 1056 of HER4 as the phosphatidylinositol 3-kinase binding site and tyrosines 1188 and 1242 as two potential SHC binding sites. Interestingly, transfection of HER4 into NIH 3T3 cells conferred responsiveness to EGF with respect to colony formation in soft agar. It was also found that in response to heregulin beta2, endogenous murine HER1 or transfected human HER1 became phosphorylated when HER4 was present. This demonstrates that HER1 and HER4 can exist in a heterodimer complex and likely activate each other by transphosphorylation.

All ErbB receptors other than the epidermal growth factor receptor are endocytosis impaired

Four transmembrane tyrosine kinases constitute the ErbB receptor family: the epidermal growth factor (EGF) receptor, ErbB-2, ErbB-3, and ErbB-4. We have measured the endocytic capacities of all four members of the EGF receptor family, including ErbB-3 and ErbB-4, which have not been described previously. EGF-responsive chimeric receptors containing the EGF receptor extracellular domain and different ErbB cytoplasmic domains (EGFR/ErbB) have been employed. The capacity of these growth factor-receptor complexes to mediate 125I-EGF internalization, receptor down-regulation, receptor degradation, and receptor co-immunoprecipitation with AP-2 was assayed. In contrast to the EGF receptor, all EGFR/ErbB receptors show impaired ligand-induced rapid internalization, down-regulation, degradation, and AP-2 association. Also, we have analyzed the heregulin-responsive wild-type ErbB-4 receptor, which does not mediate the rapid internalization of 125I-heregulin, demonstrates no heregulin-regulated receptor degradation, and fails to form association complexes with AP-2. Despite the substantial differences in ligand-induced receptor trafficking between the EGF and ErbB-4 receptors, EGF and heregulin have equivalent capacities to stimulate DNA synthesis in quiescent cells. These results show that the ligand-dependent down-regulation mechanism of the EGF receptor, surprisingly, is not a property of any other known ErbB receptor family member. Since endocytosis is thought to be an attenuation mechanism for growth factor-receptor complexes, these data imply that substantial differences in attenuation mechanisms exist within one family of structurally related receptors.

EGF or PDGF receptors activate atypical PKClambda through phosphatidylinositol 3-kinase

Overexpression of a TPA-insensitive PKC member, an atypical protein kinase C (aPKClambda), results in an enhancement of the transcriptional activation of TPA response element (TRE) in cells stimulated with epidermal growth factor (EGF) or platelet-derived growth factor (PDGF). EGF or PDGF also caused a transient increase in the in vivo phosphorylation level and a change in the intracellular localization of aPKClambda from the nucleus to the cytosol, indicating the activation of aPKClambda in response to this growth factor stimulation. These immediate signal-dependent changes in aKPClambda were observed for a PDGF receptor add-back mutant (Y40/51) that possesses only two of the five major autophosphorylation sites and binds PI3-kinase, and were inhibited by wortmannin, an inhibitor of PI3-kinase. Furthermore, an N-terminal fragment of the catalytic subunit of PI3-kinase, p110alpha, inhibited aPKClambda-dependent activation of TRE in Y40/51 cells stimulated with PDGF. Overexpression of p110alpha resulted in an enhancement of TRE expression in response to PDGF and the regulatory domain of aPKClambda inhibited this TRE activation in Y40/51 cells. These results provide the first in vivo evidence supporting the presence of a novel signalling pathway from receptor tyrosine kinases to aPKClambda through PI3-kinase.

Cloning of the rat ErbB3 cDNA and characterization of the recombinant protein

Three cDNA fragments that encoded all but the extreme N terminus of the rat ErbB3 protein were cloned by low-stringency screening of a rat liver cDNA library with a human ERBB3 probe. The remaining 5'-end of the cDNA was generated by a reverse transcription-polymerase chain reaction method, and a single full-length rat ErbB3 cDNA was assembled. A comparison of the deduced amino acid (aa) sequences of human and rat ErbB3 was made, and the effects of certain aa substitutions in the putative protein tyrosine kinase domain were considered. The rat ErbB3 cDNA was subsequently expressed in cultured NIH-3T3 mouse fibroblasts, in which a high level of approx. 180-kDa recombinant ErbB3 (re-ErbB3) was generated. The rat re-ErbB3 produced in transfected fibroblasts was responsive to the polypeptide, heregulin, a known ligand for ErbB3. Challenge of transfected fibroblasts with heregulin stimulated the phosphorylation of rat re-ErbB3 on Tyr residues and promoted its association with the p85 subunit of phosphatidylinositol 3-kinase. Together, these results indicate that a fully functional rat ErbB3 cDNA has been isolated, and that fibroblast cells expressing this cDNA will be suitable for investigations of the signal transduction mechanism of ErbB3.

Neuregulins and their receptors

The recent identification of an activator for the ErbB2/Neu receptor has uncovered a new family of polypeptide growth factors that undoubtedly play a major role in the regulation of neuronal growth and differentiation. These factors, called the neuregulins, are expressed in neural and mesenchymal tissues, and activate members of the epidermal growth factor family of receptor tyrosine kinases. The identification and characterization of the neuregulins and their receptors will facilitate the dissection of the biochemical pathways regulating nervous system development.

Heregulin-stimulated signaling in rat pheochromocytoma cells. Evidence for ErbB3 interactions with Neu/ErbB2 and p85

We have reported that overexpression of Neu leads to heregulin-stimulated neurite outgrowth and the tyrosine-phosphorylation of Neu and other cellular proteins in PC12 cells. Considering that Neu/ErbB2 alone is not able to functionally couple to heregulin, we looked for the possible involvement of ErbB3 in these neurite outgrowth and tyrosine phosphorylation responses. We found that heregulin stimulates the tyrosine phosphorylation of endogenous ErbB3 protein in PC12 cells and that this phosphorylation, like that of Neu, is greatly enhanced in cells that overexpress Neu. Furthermore, overexpression of ErbB3 in PC12 cells led to heregulin-stimulated neurite extension. In addition to becoming tyrosine-phosphorylated, Neu/ErbB2 and ErbB3 associate with each other, and each associates with the 85-kDa regulatory subunit (p85) of phosphatidylinositol 3-kinase in a heregulin-dependent manner. Thus, Neu/ErbB2 and ErbB3 appear to cooperate to mediate the heregulin signal in PC12 cells. Like heregulin, epidermal growth factor (EGF) also stimulate the tyrosine phosphorylation of both Neu and ErbB3. However, there are clear differences between the EGF- and heregulin-stimulated phosphorylations of ErbB3. In the heregulin response, two tyrosine-phosphorylated forms of ErbB3 are detected. Of these, only the more quickly migrating form (on SDS-polyacrylamide gel electrophoresis) is found to be associated with Neu, whereas the other, more slowly migrating form is uniquely capable of forming stable complexes with p85. In the EGF response, at least two tyrosine-phosphorylated forms of ErbB3 are detected, but these phosphoproteins have distinctly lower apparent molecular weights compared with the heregulin-stimulated ErbB3 phosphoproteins and do not complex with p85. Thus the formation of a stable ErbB3-p85 complex in PC12 cells is a unique outcome of heregulin signaling that correlates with the differences in cell morphology induced by the activated EGF receptor and the Neu tyrosine kinase.

Phosphatidylinositol 3-kinase activity is required at a postendocytic step in platelet-derived growth factor receptor trafficking

We have previously reported that platelet-derived growth factor (PDGF) receptor mutants that lack high affinity binding sites for phosphatidylinositol 3-kinase (PI 3-kinase) fail to concentrate in juxtanuclear vesicular structures after activation with PDGF. We have now identified the point in the endocytic pathway at which PI 3-kinase binding sites are required. Receptor internalization from the plasma membrane, measured as the acquisition of acid resistance of prebound 125I-PDGF, was only slightly decreased in cells expressing a PDGF receptor mutant (F5) lacking PI 3-kinase, GTPase-activating protein (GAP), phospholipase C gamma, and Syp binding sites but not expressing mutants where any of these individual sites were restored nor expressing a mutant lacking exclusively PI 3-kinase binding sites. In contrast, the extent of down-regulation of PDGF binding sites from the cell surface after prolonged incubation with PDGF as well as the degradation of [35S]methionine-labeled receptor were markedly reduced in cells expressing the F5 mutant, mutants restored in GAP, phospholipase C gamma, or Syp binding sites or expressing the mutant exclusively lacking PI 3-kinase binding sites but not in cells expressing the mutant where PI 3-kinase binding sites were restored. Inhibition of PI 3-kinase activity with wortmannin caused a dramatic decrease in the rates of down-regulation and degradation of wild-type receptors. These results suggest that PI 3-kinase binding sites are not required for internalization of PDGF receptor but are required to divert the PDGF receptor to a degradative pathway. Furthermore, the requirement for PI 3-kinase binding sites on the receptor appears to be due to a requirement for PI 3-kinase catalytic activity.

Mitogenic activity of neu differentiation factor/heregulin mimics that of epidermal growth factor and insulin-like growth factor-I in human mammary epithelial cells

Recently, a family of growth factors has been described that activates erbB-2 receptors. These factors, known as the neu differentiation factors (NDF) or heregulins (HRG), induce tyrosine phosphorylation of erbB-2 receptors as a result of their direct interaction with either erbB-3 or erbB-4 receptors. Although it is known that expression of erbB-2 receptors has relevance in human breast cancer progression, how erbB-2, -3 and -4 receptors regulate mammary epithelial cell proliferation is not known. Therefore, experiments were carried out to study the mitogenic activity of NDF/HRG on the human mammary epithelial cell line MCF-10A which can be cultured continuously under serum-free conditions. MCF-10A cells, like primary cultures of normal human mammary epithelial cells, express an absolute requirement for exogenous epidermal growth factor (EGF) and insulinlike growth factor I (IGF-I) for growth. The results of these experiments indicate that NDF/HRG can induce tyrosine phosphorylation of p185erbB-2 in MCF-10A cells and is mitogenic for these cells. This is consistent with the coexpression of erbB-2 and erbB-3 mRNA that we have observed in MCF-10A cells. In addition, we found that NDF/HRG can substitute for either EGF or IGF-I to stimulate proliferation of these cells. The ability to substitute for both EGF and IGF-I is a unique property of NDF/HRG and is not shared by other members of the EGF or IGF family of growth factors, nor by other factors that we have studied. A striking isoform specificity was also observed which indicated that the beta-isoforms of NDF/HRG were greater than ten times more mitogenic than the alpha-isoforms. We also examined the mitogenic activity of NDF/HRG on MCF-10A cells that overexpress the erbB-2 receptor as a result of infection with a retroviral vector containing the human c-erbB-2 gene (MCF-10AerbB-2 cells). These studies indicated that MCF-10AerbB-2 cells have increased sensitivity to the mitogenic effects of NDF/HRG and that these cells are responsive to the alpha-isoforms of NDF/HRG at physiological concentrations. Thus, NDF/HRG is a dual specificity growth factor for human mammary epithelial cells, and the responsiveness of the cells to NDF/HRG is influenced by the level of expression of erbB-2 receptors.

An incomplete program of cellular tyrosine phosphorylations induced by kinase-defective epidermal growth factor receptors

Although signaling by the epidermal growth factor (EGF) receptor is thought to be dependent on receptor tyrosine kinase activity, it is clear that mitogen-activated protein (MAP) kinase can be activated by receptors lacking kinase activity. Since analysis of the signaling pathways used by kinase-defective receptors could reveal otherwise masked capabilities, we examined in detail the tyrosine phosphorylations and enzymes of the MAP kinase pathway induced by kinase-defective EGF receptors. Following EGF stimulation of B82L cells expressing a kinase-defective EGF receptor mutant (K721M), we found that ERK2 and ERK1 MAP kinases, as well as MEK1 and MEK2 were all activated, and SHC became prominently tyrosine-phosphorylated. By contrast, kinase-defective receptors failed to induce detectable phosphorylations of GAP (GTPase-activating protein), p62, JAK1, or p91STAT1, all of which were robustly phosphorylated by wild-type receptors. These data demonstrate that kinase-defective receptors induce several protein tyrosine phosphorylations, but that these represent only a subset of those seen with wild-type receptors. This suggests that kinase-defective receptors activate a heterologous tyrosine kinase with a specificity different from the EGF receptor. We found that kinase-defective receptors induced ErbB2/c-Neu enzymatic activation and ErbB2/c-Neu binding to SHC at a level even greater than that induced by wild-type receptors. Thus, heterodimerization with and activation of endogenous ErbB2/c-Neu is a possible mechanism by which kinase-defective receptors stimulate the MAP kinase pathway.