ErbB3 is involved in activation of phosphatidylinositol 3-kinase by epidermal growth factor. Mol Cell Biol. 1994;14:3550-3558. [PMID: 7515147 DOI: 10.1128/mcb.14.6.3550]

Effects of in vivo heregulin beta1 treatment in wild-type and ErbB gene-targeted mice depend on receptor levels and pregnancy

Mice heterozygous (+/-) for either heregulin (HRG), ErbB2, or ErbB3 were created by gene targeting, resulting in the loss of one functional gene copy and an associated decrease in targeted protein. We examined the in vivo activity of recombinant HRG peptide, rHRG beta1 (amino acids 177 to 241), in the three heterozygous mouse lines and in wild-type (WT) mice, both pregnant and nonpregnant. Nonpregnant WT and HRG(+/-) mice of both sexes were sensitive to rHRG beta1 treatment as evidenced by a high mortality rate associated with abdominal enlargement and parietal cell loss. However, pregnant WT mice and ErbB2 and ErbB3 heterozygous mice treated with rHRG beta1 were less affected, with significantly lower mortality rates and a less severe abdominal phenotype. Histological analysis revealed extensive breast ductal hyperplasia in females of all genotypes after rHRG beta1 treatment. Hyperplasia of other epithelial tissues such as the pancreas and intestine and the growth of cardiac nerve bundles were also observed, independent of sex.

Mechanisms of action of flavopiridol

Flavopiridol inhibits phosphokinases. Its activity is strongest on cyclin dependent kinases (cdk-1, -2, -4, -6, -7) and less on receptor tyrosine kinases (EGFR), receptor associates tyrosine kinases (pp60 Src) and on signal transducing kinases (PKC and Erk-1). Although the inhibiting activity of flavopiridol is strongest for cdk, the cytotoxic activity of flavopiridol is not limited to cycling cells. Resting cells are also killed. This fact suggests that inhibition of cdks involved in the control of cell cycle is not the only mechanism of action. Inhibition of cdk's with additional functions (i.e. involved in the control of transcription or function of proteins that do not control cell cycle) may contribute to the antitumoral effect. Moreover, direct and indirect inhibition of receptor activation (EGFR) and/or a direct inhibition of kinases (pp60 Src, PKC, Erk-1) involved in the signal transduction pathway could play a role in the antiproliferative activity of flavopiridol. From pharmacokinetic data in patients it can be concluded that the inhibitory activity (IC50) of flavopiridol on these kinases is in the range of concentrations that might be achieved intracellularly after systemic application of non-toxic doses of flavopiridol. However, no in situ data from flavopiridol treated cells have been published yet that prove that by inhibition of EGFR, pp60 Src, PKC and/or Erk-1 (in addition to inhibition of cdk's) flavopiridol is able to induce apoptosis. Thus many questions regarding the detailed mechanism of antitumoral action of flavopiridol are still open. For the design of protocols for future clinical studies this review covers the essential information available on the mechanism of antitumoral activity of flavopiridol. The characteristics of this antitumoral activity include: High rate of apoptosis, especially in leukemic cells; synergy with the antitumoral activity of many cytostatics; independence of its efficacy on pRb, p53 and Bcl-2 expression; lack of interference with the most frequent multidrug resistance proteins (P-glycoprotein and MRP-190); and a strong antiangiogenic activity. Based on these pharmacological data it can be concluded that flavopiridol could be therapeutically active in tumor patients: independent on the genetic status of their tumors or leukemias (i.e. mutations of the pRb and/or p53, amplification of bcl-2); in spite of drug resistance of their tumors induced by first line treatment (and caused by enhanced expression of multidrug resistance proteins); in combination with conventional chemotherapeutics preferentially given prior to flavopiridol; and due to a complex mechanism involving cytotoxicity on cycling and on resting tumor cells, apoptosis and antiangiogenic activity. In consequence, flavopiridol is a highly attractive, new antitumoral compound and deserves further elucidation of its clinical potency.

ErbB transmembrane tyrosine kinase receptors are differentially expressed throughout the adult rat central nervous system

The neuregulin (NRG) family of growth and differentiation factors and their erbB receptors contribute importantly to the development of the nervous system, but their distribution and function in the adult brain are poorly understood. The present study showed that erbB2, erbB3, and erbB4 transcripts and protein are distributed throughout all areas of adult rat brain. These three receptors were differentially expressed in neurons and glia. Some neurons expressed only a subset of erbB kinases, whereas other neurons expressed all three erbB receptors but sequestered each of these polypeptides into distinct cellular compartments. In synapse-rich regions, erbB immunoreactivity appeared as punctate-, axon-, and/or dendrite-associated staining, suggesting that NRGs are involved in the formation and maintenance of synapses in adult brain. ErbB labeling also was present in neuronal soma, indicating that NRGs act at sites in addition to the synapse. Glia in adult brain also differentially expressed erbB3 and erbB4. Approximately half of the erbB3 labeling in white matter was associated with S100beta+/glial fibrillary acidic protein negative macroglia (i.e., oligodendrocytes or glial fibrillary acidic protein negative astrocytes). In contrast, macroglia in gray matter did not express erbB3. The remaining erbB3 immunoreactivity in white matter and erbB4 glial staining seemed to be associated with microglia. These results showed that erbB receptors are expressed widely in adult rat brain and that each erbB receptor subtype has a distinct distribution. The differential distributions of erbB receptors in neurons and glia and the known functional differences between these kinases suggest that NRGs have distinct effects on these cells. The continued expression of NRGs and their erbB receptors in mature brain also implies that these molecules perform important functions in the brain throughout life.

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.

Molecular mechanisms underlying endocytosis and sorting of ErbB receptor tyrosine kinases

The major process that regulates the amplitude and kinetics of signal transduction by tyrosine kinase receptors is endocytic removal of active ligand-receptor complexes from the cell surface, and their subsequent sorting to degradation or to recycling. Using the ErbB family of receptor tyrosine kinases we exemplify the diversity of the down regulation process, and concentrate on two sorting steps whose molecular details are emerging. These are the Eps15-mediated sorting to clathrin-coated regions of the plasma membrane and the c-Cbl-mediated targeting of receptors to lysosomal degradation. Like in yeast cells, sorting involves not only protein phosphorylation but also conjugation of ubiquitin molecules. The involvement of other molecules is reviewed and recent observations that challenge the negative regulatory role of endocytosis are described. Finally, we discuss the relevance of receptor down regulation to cancer therapy.

Five isoforms of the phosphatidylinositol 3-kinase regulatory subunit exhibit different associations with receptor tyrosine kinases and their tyrosine phosphorylations

There are five isoforms of the regulatory subunit for the heterodimeric type of phosphatidylinositol 3-kinase. These five regulatory subunit isoforms were overexpressed using an adenovirus transfection system, and their own tyrosine phosphorylations and associations with various tyrosine kinase receptors were investigated. When overexpressed in CHO-PDGFR cells, the associations of these regulatory subunit isoforms with the platelet-derived growth factor receptor were similar. However, when overexpressed in CHO-IR cells, p55gamma exhibited a significantly lower ability to bind with IRS-1 upon insulin stimulation, as compared with other regulatory subunit isoforms. Furthermore, p55alpha and p55gamma were found to be tyrosine-phosphorylated. Finally, interestingly, when overexpressed in CHO-EGFR cells or A431 cells and stimulated with epidermal growth factor (EGF), phosphorylated EGF receptor was detected in p85alpha, p85beta and p50alpha immunoprecipitates, but not in p55alpha and p55gamma immunoprecipitates. In addition, EGF-induced tyrosine phosphorylation was observed in p85alpha, p85beta, p55alpha and p55gamma, but not in p50alpha, immunoprecipitates. Thus, each regulatory subunit exhibits specific responses regarding both the association with tyrosine-phosphorylated substrates and its own tyrosine phosphorylation. These results suggest that each isoform possesses specific roles in signal transduction, based on its individual tyrosine kinase receptor.

Involvement of insulin receptor substrates in epidermal growth factor induced activation of phosphatidylinositol 3-kinase in rat hepatocyte primary culture

Short-term incubation of adult rat hepatocytes with epidermal growth factor (EGF) caused tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and IRS-2 when the cells had been submitted to primary culture from 1-18 h. Tyrosine-phosphorylated IRS-1 and IRS-2 bound to the regulatory subunit (p85) of phosphatidylinositol (PtdIns) 3-kinase, thereby activating the enzymic activity. Tyrosine phosphorylation of the IRSs and activation of PtdIns 3-kinase in 3 h cultured hepatocytes both proceeded similarly to the same actions of insulin; the activation was rapid and transient, with peak values at 15-30 s and with similar EC(50)s in the nM range in both cases. A possible involvement of insulin receptors in these insulin-like actions of EGF was excluded by the following three lines of evidence. Insulin caused tyrosine phosphorylation of the insulin receptor beta-subunit but EGF did not. In contrast, the EGF receptor was phosphorylated by EGF, but the insulin receptor was not. The actions of EGF, but not those of insulin, were inhibited by AG1478, a selective inhibitor of EGF receptor tyrosine kinase. Cultured hepatocytes exposed to insulin or insulin-like growth factor-I (IGF-I) for a short period responded to the subsequent addition of EGF, whereas EGF-treated cells responded to insulin. The cells, however, displayed receptor desensitization under the same conditions, that is, no response was observed upon repeated addition of the same agonist, EGF, insulin or IGF-I. Thus, the EGF receptor-initiated signalling was mediated by PtdIns 3-kinase associated with tyrosine-phosphorylated IRSs in short-term cultured rat hepatocytes.

Deceiving appearances: signaling by "dead" and "fractured" receptor protein-tyrosine kinases

The mechanisms by which most receptor protein-tyrosine kinases (RTKs) transmit signals are now well established. Binding of ligand results in the dimerization of receptor monomers followed by transphosphorylation of tyrosine residues within the cytoplasmic domains of the receptors. This tidy picture has, however, some strange characters lurking around the edges. Cases have now been identified in which RTKs lack kinase activity, but, despite being "dead" appear to have roles in signal transduction. Even stranger are the cases in which genes encoding RTKs produce protein products consisting of only a portion of the kinase domain. At least one such "fractured" RTK appears to be involved in signal transduction. Here we describe how these strange molecules might function and discuss the questions associated with their evolution. BioEssays 23:69-76, 2001.

Epidermal growth factor-induced phosphatidylinositol 3-kinase activation and DNA synthesis. Identification of Grb2-associated binder 2 as the major mediator in rat hepatocytes

In previous work we showed that the phosphatidylinositol 3-kinase (PI3-kinase), not the mitogen-activated protein kinase, pathway is necessary and sufficient to account for insulin- and epidermal growth factor (EGF)-induced DNA synthesis in rat hepatocytes. Here, using a dominant-negative p85, we confirmed the key role of EGF-induced PI3-kinase activation and sought to identify the mechanism by which this is effected. Our results show that EGF activates PI3-kinase with a time course similar to that of the association of p85 with three principal phosphotyrosine proteins (i. e. PY180, PY105, and PY52). We demonstrated that each formed a distinct p85-associated complex. PY180 and PY52 each constituted about 10% of EGF-activated PI3-kinase, whereas PY105 was responsible for 80%. PY105 associated with Grb2 and SHP-2, and although it behaved like Gab1, none of the latter was detected in rat liver. We therefore cloned a cDNA from rat liver, which was found to be 95% homologous to the mouse Grb2-associated binder 2 (Gab2) cDNA sequence. Using a specific Gab2 antibody, we demonstrated its expression in and association with p85, SHP-2, and Grb2 upon EGF treatment of rat hepatocytes. Gab2 accounted for most if not all of the PY105 species, since immunoprecipitation of Gab2 with specific antibodies demonstrated parallel immunodepletion of Gab2 and PY105 from the residual supernatants. We also found that the PI3-kinase activity associated with Gab2 was totally abolished by dominant negative p85. Thus, Gab2 appears to be the principal EGF-induced PY protein recruiting and activating PI3-kinase and mitogenesis.

Expression of the BRK tyrosine kinase in mammary epithelial cells enhances the coupling of EGF signalling to PI 3-kinase and Akt, via erbB3 phosphorylation

A high proportion of human breast cancers, in contrast with normal mammary tissue, express the intracellular tyrosine kinase BRK. BRK expression enhances the mitogenic response of mammary epithelial cells to epidermal growth factor, and conferment of a proliferative advantage through this mechanism may account for the frequent elevation of BRK expression in tumours. Here we report that BRK expression in mammary epithelial cells, at pathologically relevant levels, results in an enhanced phosphorylation of the epidermal growth factor receptor-related receptor erbB3 in response to epidermal growth factor. As a consequence, erbB3 recruits increased levels of phosphoinositide 3-kinase, and this is associated with a potentiated activation of Akt. This effect of BRK on the regulation of phosphoinositide 3-kinase and Akt activity may account for BRK's ability to enhance mammary cell mitogenesis, and raises the possibility that breast tumours expressing BRK may acquire a resistance to pro-apoptotic signals.

Enhanced drug resistance in cells coexpressing ErbB2 with EGF receptor or ErbB3

Overexpression of ErbB2 has been found in approximately 25-30% of human breast cancers and has been shown to render the cancer cells more resistant to chemotherapy. However, it is not clear whether ErbB2 overexpression renders the cells more resistant to specific anti-cancer drugs or renders the cells more resistant to a broad range of anti-cancer drugs. It is not clear how the function of ErbB2 in drug resistance is related to expression and activation of the other ErbB receptors. In this communication, we showed that several breast cancer cell lines including BT20, BT474, MCF-7, MDA-MB-453, and SKBR-3 cells had a similar pattern of resistance to a broad range of anti-cancer drugs including 5-Fluorouracil, Cytoxan, Doxorubincin, Taxol, and Vinorelbin, suggesting a mechanism of multidrug resistance. High expression of P-glycoprotein and the ErbB receptors contribute to drug resistance of these breast cancer cells; however, overexpression of ErbB2 alone is not a major factor in determining drug resistance. To further determine the role of the ErbB receptors in drug resistance, we selected various NIH 3T3 cell lines that specifically expressed EGF receptor (EGFR), ErbB2, ErbB3, EGFR/ErbB2, EGFR/ErbB3, or ErbB2/ErbB3. A cytotoxicity assay showed that expression of ErbB2 alone did not significantly enhance drug resistance, whereas coexpression of either EGFR or ErbB3 with ErbB2 significantly enhanced drug resistance. Moreover, ErbB2 was highly phosphorylated in NIH 3T3 cells that coexpress ErbB2 with either EGFR or ErbB3, but not in NIH 3T3 cells that express ErbB2 alone. Together, our results suggest that coexpression of EGFR or ErbB3 with ErbB2 induces high phosphorylation of ErbB2 and renders the cells more resistant to various anti-cancer drugs.

Akt/protein kinase B isoforms are differentially regulated by epidermal growth factor stimulation

Overexpression of epidermal growth factor receptor (EGFR) in certain cancers is well established. There is growing evidence that epidermal growth factor (EGF) activates Akt/protein kinase B (PKB) in a phosphoinositide 3-OH kinase (PI3K)-dependent manner, but it is not yet clear which Akt isoforms are involved in this signal transduction pathway. We investigated the functional regulation of three Akt isoforms, Akt1/PKBalpha, Akt2/PKBbeta, and Akt3/PKBgamma, in esophageal cancer cells where EGFR is frequently overexpressed. Upon EGF simulation, phosphorylation of Akt1 at the Ser-473 residue was remarkably induced. This result was corroborated by in vitro Akt kinase assays using glycogen synthase kinase 3beta as the substrate. PI3K inhibitors, wortmannin or LY294002, significantly blocked the Akt kinase activity induced by EGF. Akt2 activity was evaluated by electrophoretic mobility shift assays. Robust activation of Akt2 by EGF was observed in some cell lines in a PI3K-dependent manner. EGF-induced Akt3 activation was demonstrated by Ser-472 phosphorylation of Akt3 but in a restrictive fashion. In aggregate, EGF-mediated activation of Akt isoforms is overlapping and distinctive. The mechanism by which EGFR recruits the PI3K/Akt pathway was in part differentially regulated at the level of Ras but independent of heterodimerization of EGFR with either ErbB2 or ErbB3 based upon functional dissection of pathways in esophageal cancer cell lines.

Phosphorylation and nuclear exclusion of the forkhead transcription factor FKHR after epidermal growth factor treatment in human breast cancer cells

Akt, when activated by IGF/insulin, can phosphorylate forkhead transcription factors. We undertook this study to determine whether epidermal growth factor (EGF) treatment could produce a signaling cascade resulting in phosphorylation of the forkhead transcription factor FKHR in a breast cancer cell line, MDA-MB-231. After establishing ErbB1, cbl, PI3 kinase and Akt were activated in EGF treated MDA-MB-231, we determined by immunoblot with FKHR antiserum that the electrophoretic mobility of FKHR was retarded after EGF treatment. This mobility retardation was reversible by treatment with alkaline phosphatase, and immunoblot with phospho-Ser256 FKHR antibody further confirmed phosphorylation on an Akt consensus site after EGF treatment. EGF stimulated FKHR phosphorylation was blocked by the PI3 kinase inhibitor LY294002, and the ErbB1 inhibitor AG1478. FKHR immunoblotting after purification of nuclear and cytoplasmic proteins showed that EGF induced a simultaneous increase of FKHR in the cytoplasm and decrease in the nucleus. This finding was confirmed by immunofluorescence staining. Treatment of cells with pharmacological inhibitors of PI3 kinase or ErbB1 blocked this effect. Thus, these results demonstrate the phosphorylation and nuclear exclusion of FKHR after EGF treatment by a PI3 kinase dependent mechanism, and represent the first report of growth factor regulation of endogenous FKHR localization.

Heregulin-beta is especially potent in activating phosphatidylinositol 3-kinase in nontransformed human mammary epithelial cells

The neu differentiation factors/heregulins (HRGs) comprise a family of polypeptide growth factors that activate p185(erbB-2) through direct binding to either erbB-3 or erbB-4 receptor tyrosine kinases. We have previously shown that HRG-beta is mitogenic for various human mammary epithelial cell lines that coexpress c-erbB-2 and c-erbB-3. Phosphatidylinositol 3-kinase (PI3K) is activated by p185(erbB-2) /erbB-3 heterodimers in cells stimulated by HRG, and PI3K is constitutively activated by p185(erbB-2) /erbB-3 in breast carcinoma cells that overexpress c-erbB-2. To better understand the relative abilities of HRGs, epidermal growth factor (EGF), or insulin to activate PI3K under normal physiological conditions, we compared the levels of recruitment of the 85-kDa regulatory subunit of PI3K when activated by the type I (erbB) or type II [insulin-like growth factor (IGF)] receptor tyrosine kinases in two different nontransformed human mammary epithelial cell lines. The nontransformed H16N-2 cells isolated from normal tissue express EGFR, p185(erbB-2), and erbB-3, and are highly responsive to the mitogenic effects of HRG-beta as well as to the combination of EGF and insulin in serum-free culture. We measured the stoichiometry of p85 recruited by tyrosine-phosphorylated proteins induced in H16N-2 cells by either the alpha or the beta isoform of HRG. HRG-beta was greater than 10-fold more potent in inducing p85 recruitment than was the less biologically active HRG-alpha isoform. HRG-beta was also a more potent inducer of p85 recruited by tyrosine-phosphorylated proteins than was either EGF, insulin, or EGF and insulin combined. Furthermore, erbB-3 principally mediated the direct recruitment of p85 in cells stimulated by HRG or EGF, indicating that, in addition to the high-level activation of PI3K by p185(erbB-2) / erbB-3, EGFR/erbB-3 heterodimer interaction is essential for the weak but significant level of PI3K activated by EGF in cells that express normal EGFR levels. Studies using the PI3K inhibitor wortmannin also indicated that PI3K activation was required for the proliferation of H16N-2 cells induced by either HRG-beta or EGF and insulin in serum-free culture. Finally, HRG-beta was also an especially potent inducer of PI3K in the nontransformed MCF-10A cells, which were derived spontaneously from normal reduction mammoplasty tissue. These data show, for the first time, a side-by-side quantitative comparison of the relative degree of PI3K activated by different growth factors in nontransformed growth factor-dependent cells under precisely defined conditions in culture.

Mammary gland neoplasia: insights from transgenic mouse models

Current theories of breast cancer progression have been greatly influenced by the development and refinement of mouse transgenic and gene targeting technologies. Early transgenic mouse models confirmed the involvement of oncogenes, previously implicated in human breast cancer, by establishing a causal relationship between overexpression or activation of these genes and mammary tumorigenesis. More recently, the importance of genes located at sites of loss of heterozygosity in human breast cancer have been examined in mice by their targeted disruption via homologous recombination. The union of these two approaches allows the generation of complex animal models that more accurately reflect the multistep nature of human breast cancer. This review will examine how the study of transgenic mice has increased our understanding of the molecular events responsible for oncogenic transformation of the mammary gland. BioEssays 22:554-563, 2000.

Activation of phosphatidylinositol 3-kinase by a complex of p59fyn and the receptor tyrosine kinase Xmrk is involved in malignant transformation of pigment cells

Malignant melanoma in the fish Xiphophorus is induced by overexpression of the Xmrk-oncogene, encoding a subclass I receptor tyrosine kinase. The mutationally activated Xmrk protein triggers constitutive mitogenic signalling in fish melanoma cells. In recent studies we showed that in melanoma cells phosphatidylinositol (PtdIns) 3-kinase, as well as p59fyn, has elevated levels of kinase activity. Both bind directly to different phosphotyrosine residues in the Xmrk receptor C-terminus through their SH2 domains. To analyse the mechanism of regulation of these Xmrk-associated kinases in melanoma we characterized the protein-protein interactions between PtdIns 3-kinase, p59fyn and the Xmrk receptor in detail. A ternary complex in which the p85 subunit of PtdIns 3-kinase is associated with p59fyn as well as with Xmrk was identified. Contrary to complexes described for other receptors, the adaptor protein p120Cbl was not involved in these interactions. Thus, we describe here a new mechanism of activation of PtdIns 3-kinase by a receptor of the epidermal growth factor receptor family in which p59fyn acts as an adaptor as well as an activator of PtdIns 3-kinase. Activation of PtdIns 3-kinase activity by fyn was also found in vivo. The fact that this was only detectable in highly transformed Xmrk overexpressing melanomas but not in benign lesions points to the essential role of the Xmrk receptor in this mechanism of regulation.

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

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

Cross-talk between epidermal growth factor receptor and c-Met signal pathways in transformed cells

In rat liver epithelial cells constitutively expressing transforming growth factor alpha (TGFalpha), c-Met is constitutively phosphorylated in the absence of its ligand, hepatocyte growth factor. We proposed that TGFalpha and the autocrine activation of its receptor, epidermal growth factor receptor (EGFR), leads to phosphorylation and activation of c-Met. We found that there is constitutive c-Met phosphorylation in human hepatoma cell lines and the human epidermoid carcinoma cell line, A431 which express TGFalpha, but not in normal human hepatocytes. Constitutive c-Met phosphorylation in A431, HepG2, AKN-1, and HuH6 cells was inhibited by neutralizing antibodies against TGFalpha and/or EGFR. Exposure to exogenous TGFalpha or EGF increased the phosphorylation of c-Met in the human epidermoid carcinoma cell line, A431. The increase of c-Met phosphorylation by TGFalpha in A431 cells was inhibited by neutralizing antibodies against TGFalpha and/or EGFR and by the EGFR-specific inhibitor tyrphostin AG1478. These results indicate that constitutive c-Met phosphorylation, and the increase of c-Met phosphorylation by TGFalpha or EGF, in tumor cell lines is the result of the activation via EGFR. We found that c-Met in tumor cells co-immunoprecipitates with EGFR regardless of the existence of their ligands in tumor cells, but not in normal human hepatocytes. We conclude that c-Met associates with EGFR in tumor cells, and this association facilitates the phosphorylation of c-Met in the absence of hepatocyte growth factor. This cross-talk between c-Met and EGFR may have significant implications for altered growth control in tumorigenesis.

Kinase inhibitors in cancer therapy: a look ahead

The most essential kinases involved in cell membrane receptor activation, signal transduction and cell cycle control or programmed cell death and their interconnections are reviewed. In tumours, the genes of many of those kinases are mutated or amplified or the proteins are overexpressed. The use of key kinases offers the possibility to screen in vitro for synthetic small molecule kinase inhibitors. In view of the many interconnections of cellular kinases, their role in preventing or inducing programmed cell death and the possibility that a considerable number of signal transducing proteins are still unknown, cellular test systems are recommended in which the respective key kinase or one of its main partner molecules are overexpressed.

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.

E5 oncoprotein mutants activate phosphoinositide 3-kinase independently of platelet-derived growth factor receptor activation

The E5 oncoprotein of bovine papillomavirus type 1 is a Golgi-resident, 44-amino acid polypeptide that can transform fibroblast cell lines by activating endogenous platelet-derived growth factor receptor beta (PDGF-R). However, the recent discovery of E5 mutants that exhibit strong transforming activity but minimal PDGF-R tyrosine phosphorylation indicates that E5 can potentially use additional signal transduction pathway(s) to transform cells. We now show that two classes of E5 mutants, despite poorly activating the PDGF-R, induce tyrosine phosphorylation and activation of phosphoinositide 3-kinase (PI 3-K) and that this activation is resistant to a selective inhibitor of PDGF-R kinase activity, tyrphostin AG1296. Consistent with this independence from PDGF-R signaling, the E5 mutants fail to induce significant cell proliferation in the absence of PDGF, unlike wild-type E5 or the sis oncoprotein. Despite differences in growth factor requirements, however, both wild-type E5 and mutant E5 cell lines form colonies in agarose. Interestingly, activation of PI 3-K occurs without concomitant activation of the ras-dependent mitogen-activated protein kinase pathway. The known ability of constitutively activated PI 3-K to induce anchorage-independent cell proliferation suggests a mechanism by which the mutant E5 proteins transform cells.

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.

Identification of an interaction between the angiotensin II receptor sub-type AT2 and the ErbB3 receptor, a member of the epidermal growth factor receptor family

To identify the proteins that interact and mediate angiotensin II receptor AT2-specific signaling, a random peptide library was screened by yeast-based Two-Hybrid protein-protein interaction assay technique. A peptide that shared significant homology with the amino acids located between the residues Gly-Xaa-Gly-Xaa-Xaa-Gly721 and Lys742, the residues predicted to be important for ATP binding of the ErbB3 and ErbB2 receptors, was identified to be interacting with the AT2 receptor. The interaction between the human ErbB3 receptor and the AT2 receptor was further confirmed using the cytoplasmic domain (amino acids 671-782) of the human ErbB3 receptor. Moreover, an AT2 receptor peptide that spans the amino acids 226-363, (spans the third ICL and carboxy terminal domain) could also interact with the AT2 receptor in a yeast Two-Hybrid protein-protein interaction assay. Studies using mutated and chimeric AT2 receptors showed that replacing the third intracellular loop (ICL) of the AT2 receptor with that of the AT1 abolishes the interaction between the ErbB3 and the AT2 in yeast Two-Hybrid protein-protein interaction assay. Thus the interaction between the AT2 receptor and the ErbB3 receptor seems to require the region spanning the third ICL and carboxy terminus of the AT2 receptor. Since the third ICL of the AT2 receptor is essential for exerting its inhibitory effects on cell growth, possible involvement of this region in the interaction with the cytoplasmic domain of the ErbB3 receptor suggests a novel signaling mechanism for the AT2 receptor mediated inhibition of cell growth. Furthermore, since both the AT2 and the ErbB3 receptors are expressed during fetal development, we propose that the existence of direct interaction between these two receptors may play a role in the regulation of growth during the initial stages of development.

KFC, a Ste20-like kinase with mitogenic potential and capability to activate the SAPK/JNK pathway

The Sterile-20 (Ste20) family of serine-threonine kinases has been implicated in the activation of the stress-activated protein kinase pathways. However, the physiological role has remained ambiguous for most of the investigated mammalian Ste20's. Here we report the cloning of a novel Ste20-like kinase, from chicken embryo fibroblast (CEF) cells, which we have named KFC, for Kinase From Chicken. The 898 amino acid full-length KFC protein contains an amino-terminal kinase domain, an adjacent downstream serine-rich region, and a C-terminal tail containing a coiled-coil domain. Here we show that the coiled-coil domain of KFC negatively regulates the intrinsic kinase activity. We have also identified a splice variant of KFC in which there is a 207 nucleotide in-frame deletion. This deletion of 69 amino acids encompasses the serine-rich region. These two isoforms, called KFCL, for full-length, and KFCS for spliced (or short) form, not only differ in structure, but also in biological properties. Stable CEF cells overexpressing KFCL, but not KFCS, have a significant increase in growth rate when compared to parental cells. This mitogenic effect is the first such reported for this family of kinases. Finally, we found that KFC, when activated by truncation of the regulatory C-terminus, has a specific activation of the stress-activated protein kinase (SAPK/JNK) pathway.

Tyrosine kinase signalling in breast cancer: tyrosine kinase-mediated signal transduction in transgenic mouse models of human breast cancer

The ability of growth factors and their cognate receptors to induce mammary epithelial proliferation and differentiation is dependent on their ability to activate a number of specific signal transduction pathways. Aberrant expression of specific receptor tyrosine kinases (RTKs) has been implicated in the genesis of a significant proportion of sporadic human breast cancers. Indeed, mammary epithelial expression of activated RTKs such as ErbB2/neu in transgenic mice has resulted in the efficient induction of metastatic mammary tumours. Although it is clear from these studies that activation these growth factor receptor signalling cascades are directly involved in mammary tumour progression, the precise interaction of each of these signalling pathways in mammary tumourigenesis and metastasis remains to be elucidated. The present review focuses on the role of several specific signalling pathways that have been implicated as important components in RTK-mediated signal transduction. In particular, it focuses on two well characterized transgenic breast cancer models that carry the polyomavirus middle T(PyV mT) and neu oncogenes.

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

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

An epidermal growth factor receptor/Gab1 signaling pathway is required for activation of phosphoinositide 3-kinase by lysophosphatidic acid

Phosphoinositide 3-kinase (PI3K) has been shown to play an essential role in G protein-induced signaling even in non-myeloid cells where few agonists of G protein-coupled receptors are known to activate PI3K. We have identified adherent cell lines where lysophosphatidic acid (LPA) strongly and rapidly activates the accumulation of PI3K lipid products. The process is not modified by expression of a kinase-dead mutant of the Gbetagamma-responsive PI3K p110gamma. In contrast, it is inhibited by genistein or expression of a dominant negative mutant of p85 and potentiated by overexpressing wild-type p110alpha or -beta but not -gamma. By using a specific chemical inhibitor of the epidermal growth factor receptor (EGFR) and expression of a dominant negative mutant, we have observed that recruitment of p85/p110 PI3Ks occurs through transactivation of the EGFR by LPA and downstream mobilization of the docking protein Gab1 that associates with p85 upon LPA stimulation. Finally, we show that LPA cannot activate PI3K in cell lines lacking the EGFR/Gab1 pathway, including cells that transactivate the PDGF receptor. Altogether, these results demonstrate that activation of PI3K by LPA is conditioned by the ability of LPA to transactivate an EGFR/Gab1 signaling pathway.

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

BACKGROUND & AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Ionizing radiation stimulates existing signal transduction pathways involving the activation of epidermal growth factor receptor and ERBB-3, and changes of intracellular calcium in A431 human squamous carcinoma cells

Previous studies demonstrated that ionizing radiation activates the epidermal growth factor receptor (EGFR), as measured by Tyr autophosphorylation, and induces transient increases in cytosolic free [Ca2+], [Ca2+]f. The mechanistic linkage between these events has been investigated in A431 squamous carcinoma cells with the EGFR Tyr kinase inhibitor, AG1478. EGFR autophosphorylation induced by radiation at doses of 0.5-5 Gy or EGF concentrations of 1-10 ng/ml is inhibited by >75% at 100 nM AG1478. Activation of EGFR enhances IP3 production as a result of phospholipase C (PLC) activation. At the doses used, radiation stimulates Tyr phosphorylation of both, PLCgamma and erbB-3, and also mediates the association between erbB-3 and PLCgamma not previously described. The increased erbB-3 Tyr phosphorylation is to a significant extent due to transactivation by EGFR as >70% of radiation- and EGF-induced erbB-3 Tyr phosphorylation is inhibited by AG 1478. The radiation-induced changes in [Ca2+]f are dependent upon EGFR, erbB-3 and PLCgamma activation since radiation stimulated IP3 formation and Ca2+ oscillations are inhibited by AG1478, the PLCgamma inhibitor U73122 or neutralizing antibody against an extracellular epitope of erbB-3. These results demonstrate that radiation induces qualitatively and quantitatively similar responses to EGF in stimulation of the plasma membrane-associated receptor Tyr kinases and immediate downstream effectors, such as PLCgamma and Ca2+.

PI3-kinase is involved in mitogenic signaling by the oncogenic receptor tyrosine kinase Xiphophorus melanoma receptor kinase in fish melanoma

Overexpression of the mutationally activated receptor tyrosine kinase Xiphophorus melanoma receptor kinase (Xmrk) initiates formation of hereditary malignant melanoma in the fish Xiphophorus. In melanoma as well as in a melanoma-derived cell line (PSM) this receptor is highly activated resulting in constitutive Xmrk-mediated mitogenic signaling. In order to analyze mitogenic signaling triggered by Xmrk a possible involvement of phosphatidylinositol 3 (PI3)-kinase in Xmrk signal transduction was examined. Constitutive binding of the p85 adapter subunit of PI3-kinase to the Xmrk receptor was detected in PSM melanoma cells. Further analyses in BHK cells expressing a Xmrk chimera (HER-mrk) showed that p85 association with the intracellular part of Xmrk was dependent on autophosphorylation of the receptor. In vitro binding studies revealed that the interaction is mediated mainly through the N-terminal SH2 domain of p85 which directly binds to a sequence motif around phosphorylated Tyr-983 in the Xmrk carboxy-terminus. In accordance with recruitment of p85 by Xmrk in PSM cells, the PI3-kinase downstream target Akt was found to be highly phosphorylated on Ser-473, indicating efficient PI3-kinase signaling in melanoma cells. PI3-kinase activation was also detected in Xiphophorus melanoma. Moreover, malignant melanomas exhibited an increased level of PI3-kinase activity which was about three times higher than that in benign pigmented lesions. Inhibition of PI3-kinase activity in PSM melanoma cells by both Wortmannin and LY294002 blocked entry into S-phase. Together these data demonstrate that PI3-kinase is a substrate of the oncogenic Xmrk receptor and plays a significant role in mitogenic signaling of melanoma cells and the formation of malignant melanoma in Xiphophorus.

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.

Intracellular movement of green fluorescent protein-tagged phosphatidylinositol 3-kinase in response to growth factor receptor signaling

Phosphatidylinositol 3-kinase (PI 3-kinase) is a lipid kinase which has been implicated in mitogenesis, protein trafficking, inhibition of apoptosis, and integrin and actin functions. Here we show using a green fluorescent protein-tagged p85 subunit that phosphatidylinositol 3-kinase is distributed throughout the cytoplasm and is localized to focal adhesion complexes in resting NIH-3T3, A431, and MCF-7 cells. Ligand stimulation of an epidermal growth factor receptor/c-erbB-3 chimera expressed in these cells results in a redistribution of p85 to the cell membrane which is independent of the catalytic activity of the enzyme and the integrity of the actin cytoskeleton. The movement is, however, dependent on the phosphorylation status of the erbB-3 chimera. Using rhodamine-labeled epidermal growth factor we show that the phosphatidylinositol 3-kinase and the receptors colocalize in discrete patches on the cell surface. Low concentrations of ligand cause patching only at the periphery of the cells, whereas at high concentrations patches were seen over the whole cell surface. Using green fluorescent protein-tagged fragments of p85 we show that binding to the receptor requires the NH(2)-terminal part of the protein as well as its SH2 domains.

erbB-2 overexpression in human mammary epithelial cells confers growth factor independence

Previously, we demonstrated that human breast cancer cells with progressively elevated levels of constitutively tyrosine phosphorylated erbB-2 are independent of growth factors required by normal human mammary epithelial (HME) cells for proliferation in serum-free medium. To determine whether erbB-2 overexpression alone is sufficient to confer the growth factor-independence phenotype in HME cells, the spontaneously immortalized MCF-10A cell line and the HPV-16-immortalized H16N2 cell line were infected with the bicistronic retroviral vector pTPerbB-2 and tested for their ability to grow in the absence of specific factors. Selection of infected cells in G418-containing medium resulted in moderate levels of erbB-2 overexpression in approximately 40% of cells. The subpopulation of erbB-2 overexpressing cells could be selected for by culturing the cells in medium devoid of insulin. When MCF-10A or H16N2 cells were infected with pTPerbB-2 and directly selected in growth factor-deficient medium over long periods of time, populations of both cell lines emerged that expressed levels of erbB-2 protein equivalent to levels expressed by breast cancer cells with an erbB-2 gene amplification. Furthermore, overexpressed p185(erbB-2) was constitutively tyrosine phosphorylated in these cells. The levels of tyrosine phosphorylated p185(erbB-2) differed in the two recipient lines, with H16N2-erbB-2 cells having higher levels of activated receptor than MCF-10AerbB-2 cells. Furthermore, only the H16N2-erbB-2 cells were independent of both insulin and epidermal growth factor for growth in serum-free medium. Overexpression of erbB-2 also resulted in progressively increasing levels of tyrosine-phorphorylated erbB-3, without any significant changes in p180(erbB-3) levels. These studies demonstrate a direct relationship between the level of expression and activation of p185(erbB-2) and the requirements of HME cells for insulin-like and epidermal growth factor-like growth factors. The results also suggest that genetic alterations present in breast cancer cells, or mediated by HPV-16-induced alterations in pRb and p53, can influence the expression level and activation status of erbB-2 as well as erbB-3 and, in turn, their degree of growth factor independence.

Growth factors and cytokines in pancreatic carcinogenesis

Pancreatic cancer is a deadly disease challenging basic and clinical researchers alike in characterizing its pathobiology and finding better treatment options. A number of molecular alterations including gene mutations such as k-ras, p53, and Smad4 and aberrant expression of a variety of genes have been identified in recent years. This review focuses on two families of growth factors and growth factor receptors which are representative for the molecular alterations observed in pancreatic cancer: the transforming growth factor-beta superfamily of serine-threonine kinase receptors and their ligands, which usually act as negative growth regulators, and the epidermal growth factor receptor family and their ligands, which have the potential to act as growth promoters in pancreatic cancer. In addition, we will discuss the role of the cytokines TNF-alpha, IFN-gamma, and IL-6 and its effects on pancreatic cancer cell proliferation in vitro and in vivo. Pancreatic cancer cell biology consists of complex interactions of various factors, and a better understanding of the molecular pathogenesis of this disorder might lead to better treatment strategies in the near future.

ErbB receptor-induced activation of stat transcription factors is mediated by Src tyrosine kinases

Epidermal growth factor (EGF) binding to its receptor, ErbB1, triggers various signal transduction pathways, one of which leads to the activation of signal transducer and activator of transcription (Stat) factors. The mechanism underlying ErbB1-induced Stat activation and whether Stats are downstream targets of other ErbB receptors have not been explored. In this report we show that ErbB2, ErbB3, and ErbB4 do not potentiate Stat5 phosphorylation by EGF. However, neu differentiation factor-induced heterodimers of ErbB2 and ErbB4 activated Stat5. In A431 cells, Stat1, Stat3, and Stat5, were constitutively complexed with ErbB1 and rapidly phosphorylated on tyrosine in response to EGF. Neither mutation of the conserved tyrosine residue (Tyr694) nor inactivation of the Stat5a SH2 domain disrupted this association. However, an intact SH2 domain was necessary for EGF-induced Stat5a phosphorylation. In contrast to prolactin, which induced only Tyr694 phosphorylation of Stat5a, EGF promoted phosphorylation on Tyr694 and additional tyrosine residue(s). Janus kinases (Jaks) were also constitutively associated with ErbB receptors and were phosphorylated in response to EGF-related ligands. However, we provide evidence that EGF- and neu differentiation factor-induced Stat activation are dependent on Src but not Jak kinases. Upon EGF stimulation, c-Src was rapidly recruited to Stat/ErbB receptor complexes. Pharmacological Src kinase inhibitors and a dominant negative c-Src ablated both Stat and Jak tyrosine phosphorylation. However, dominant negative Jaks did not affect EGF-induced Stat phosphorylation. Taken together, the experiments establish two independent roles for Src kinases: (i) key molecules in ErbB receptor-mediated Stat signaling and (ii) potential upstream regulators of Jak kinases.

Cellular aspects of trophic actions in the nervous system

During the past three decades the number of molecules exhibiting trophic actions in the brain has increased drastically. These molecules promote and/or control proliferation, differentiation, migration, and survival (sometimes even the death) of their target cells. In this review a comprehensive overview of small diffusible factors showing trophic actions in the central nervous system (CNS) is given. The factors discussed are neurotrophins, epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, insulin-like growth factors, ciliary neurotrophic factor and related molecules, glial-derived growth factor and related molecules, transforming growth factor-beta and related molecules, neurotransmitters, and hormones. All factors are discussed with respect to their trophic actions, their expression patterns in the brain, and molecular aspects of their receptors and intracellular signaling pathways. It becomes evident that there does not exist "the" trophic factor in the CNS but rather a multitude of them interacting with each other in a complicated network of trophic actions forming and maintaining the adult nervous system.

Expression of c-erbB receptors and ligands in human bronchial mucosa

The epidermal growth factor receptor (EGFR, c-erbB1) plays a pivotal role in maintenance and repair of epithelial tissues; however, little is known about coexpression of c-erbB receptors and their ligands in human bronchial epithelium. We therefore analyzed the expression of these molecules in cultured bronchial epithelial cells and normal bronchial mucosa, using reverse transcription-polymerase chain reaction (RT- PCR), flow cytometry, and immunohistochemistry. Messenger RNA (mRNA) encoding EGFR, c-erbB2, and c-erbB3, but not c-erbB4, was detected in primary cultures of human bronchial epithelial cells, as well as in the human bronchial epithelial-derived cell lines H292 and 16HBE 14o-. Transcripts encoding epidermal growth factor (EGF), heparin binding epidermal growth factor (HB-EGF), transforming growth factor-alpha (TGF-alpha), and amphiregulin (AR) were also detected, and expression of the three receptors and four ligands was confirmed by immunocytochemical staining of the cultured cells. Immunohistochemical analysis of resin- or paraffin-embedded sections from surgical specimens of bronchial mucosa revealed strong membrane staining for EGFR within the bronchial epithelium; this was particularly evident between basal cells and the basal aspect of columnar cells. The patterns of staining for c-erbB2 and c-erbB3 in the bronchial epithelium were similar to those for EGFR. Immunostaining for EGF, TGF-alpha, AR, HB- EGF, and betacellulin (BTC) was intense in the submucosal glands; with the exception of BTC, EGFR ligand immunoreactivity was also observed in the bronchial epithelium, where it paralleled EGFR staining. Colocalization of c-erbB receptors and ligands demonstrates the potential for productive c-erbB receptor interactions in bronchial epithelium. Further study of these interactions may help to define their role in maintenance and repair of the bronchial epithelium.

Endocytosis deficiency of epidermal growth factor (EGF) receptor-ErbB2 heterodimers in response to EGF stimulation

Epidermal growth factor (EGF) stimulates the homodimerization of EGF receptor (EGFR) and the heterodimerization of EGFR and ErbB2. The EGFR homodimers are quickly endocytosed after EGF stimulation as a means of down-regulation. However, the results from experiments on the ability of ErbB2 to undergo ligand-induced endocytosis are very controversial. It is unclear how the EGFR-ErbB2 heterodimers might behave. In this research, we showed by subcellular fractionation, immunoprecipitation, Western blotting, indirect immunofluorescence, and microinjection that, in the four breast cancer cell lines MDA453, SKBR3, BT474, and BT20, the EGFR-ErbB2 heterodimerization levels were positively correlated with the ratio of ErbB2/EGFR expression levels. ErbB2 was not endocytosed in response to EGF stimulation. Moreover, in MDA453, SKBR3, and BT474 cells, which have very high levels of EGFR-ErbB2 heterodimerization, EGF-induced EGFR endocytosis was greatly inhibited compared with that in BT20 cells, which have a very low level of EGFR-ErbB2 heterodimerization. Microinjection of an ErbB2 expression plasmid into BT20 cells significantly inhibited EGF-stimulated EGFR endocytosis. Coexpression of ErbB2 with EGFR in 293T cells also significantly inhibited EGF-stimulated EGFR endocytosis. EGF did not stimulate the endocytosis of ectopically expressed ErbB2 in BT20 and 293T cells. These results indicate that ErbB2 and the EGFR-ErbB2 heterodimers are impaired in EGF-induced endocytosis. Moreover, when expressed in BT20 cells by microinjection, a chimeric receptor composed of the ErbB2 extracellular domain and the EGFR intracellular domain underwent normal endocytosis in response to EGF, and this chimera did not block EGF-induced EGFR endocytosis. Thus, the endocytosis deficiency of ErbB2 is due to the sequence of its intracellular domain.

Characterization of a naturally occurring ErbB4 isoform that does not bind or activate phosphatidyl inositol 3-kinase

Receptor tyrosine kinases regulate cell behavior by activating specific signal transduction cascades. Epidermal growth factor (EGF) receptor tyrosine kinases include ErbB1, ErbB2, ErbB3 and ErbB4. ErbB4 is a tyrosine kinase receptor that binds neuregulins (NRG) and several other EGF family members. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis identified two isoforms of ErbB4 that differed in their cytoplasmic domain sequences. Specifically, RT-PCR using primers flanking the putative phosphatidyl inositol 3-kinase (PI3-K) binding site of ErbB4 generated two specific bands when human and mouse heart and kidney tissues were analysed. Cloning and sequencing of these RT-PCR products revealed that one of the ErbB4 isoforms (ErbB4 CYT-2) lacked a 16 amino acid sequence including a putative PI3-K binding site, that was present in the other isoform (ErbB4 CYT-1). RT-PCR analysis of mouse tissues suggested that the expression of ErbB4 CYT-1 and ErbB4 CYT-2 was tissue-specific. Heart, breast and abdominal aorta expressed predominantly ErbB4 CYT-1 whereas neural tissues and kidney expressed predominantly ErbB4 CYT-2. To ascertain whether the absence of the putative PI3-K binding site in ErbB4 CYT-2 also resulted in the loss of PI3-K activity, NIH3T3 cell lines overexpressing ErbB4 CYT-1 or ErbB4 CYT-2 were produced. NRG-1 bound to and stimulated equivalent tyrosine phosphorylation of both isoforms. However, unlike ErbB4 CYT-1, the ErbB4 CYT-2 isoform was unable to bind the p85 subunit of PI3-K and to stimulate PI3-K activity in these cells. Furthermore, tyrosine phosphorylation of p85 or association of PI3-K activity with phosphotyrosine was not induced in NRG-1 treated cells expressing ErbB4 CYT-2, indicating that this isoform was incapable of activating PI3-K even indirectly. It was concluded that a novel naturally occurring ErbB4 isoform exists with a deletion of the cytoplasmic domain sequence required for the activation of the PI3-K intracellular signal transduction pathway and that this is the only PI3-K binding site in ErbB4.

Increased mitogenicity of an alphabeta heterodimeric PDGF receptor complex correlates with lack of RasGAP binding

The different platelet-derived growth factor (PDGF) isoforms cause activation of their alpha and beta protein tyrosine kinase receptors through dimerization. Homodimerization as well as heterodimerization of receptors occur. It has been shown previously that the heterodimeric receptor complex mediates a stronger mitogenic response than either of the homodimeric complexes. In this report, we show that in cells expressing both PDGF alpha- and beta-receptors, stimulation with PDGF-AB, which leads to preferential heterodimer formation, leads to a very low degree of phosphorylation of Tyr771 in the beta-receptor. In contrast, Tyr771 is phosphorylated in a homodimeric complex of beta-receptors. Phosphorylated Tyr771 is a binding site for RasGAP; an analogous site is not present in the alpha-receptor, which lacks the ability to associate with RasGAP. The lowered phosphorylation of Tyr771 in the heterodimeric receptor complex correlates with lowered association with RasGAP, as well as with a more efficient activation of Ras and MAP kinase, which is consistent with the increased mitogenicity elicited by PDGF-AB, compared to PDGF-AA or PDGF-BB.

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.

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.

EGF activates ErbB-2 and stimulates phosphatidylinositol 3-kinase in human airway smooth muscle cells

The epidermal growth factor (EGF)-receptor (EGFR) family includes four homologous transmembrane receptor protein tyrosine kinases, EGFR, ErbB-2, ErbB-3, and ErbB-4. This receptor family plays a pivotal role in regulating cell proliferation, differentiation, and transformation. Ligand-induced activation of these receptors results in formation of homo- and heterodimers, which undergo transphosphorylation and transactivation. The aim of this study was to characterize the role of EGFR family members in signaling EGF-induced human airway smooth muscle (HASM) cell proliferation. Here, we show that EGF stimulates activation of EGFR and transactivation of ErbB-2 in quiescent HASM cells. Phosphatidylinositol (PI) 3-kinase, a critical signaling enzyme that modulates HASM cell growth, is preferentially associated with ErbB-2, and EGF-stimulated transactivation of ErbB-2 induces PI 3-kinase activation. ErbB-3 and ErbB-4 are present in HASM cells; however, EGF has no effect on their activation. Betacellulin, a ligand for EGFR, ErbB-3, and ErbB-4, induced DNA synthesis of HASM cells and stimulated signaling through the activation of EGFR and ErbB-2 but not of ErbB-3 and ErbB-4. Heregulin, a specific ligand for ErbB-3 and ErbB-4, did not effect DNA synthesis and did not activate its specific receptors. These data suggest that EGF imparts signals that involve activation of ErbB-2 and are associated with ErbB-2 PI 3-kinase activation. Despite the mRNA and protein expression of all members of the EGFR family, ligand-stimulated signaling induced activation of EGFR and ErbB-2 but not of ErbB-3 and ErbB-4.

Formation of distinct signalling complexes involving phosphatidylinositol 3-kinase activity with stimulation of epidermal growth factor or insulin-like growth factor-I in human skin fibroblasts

We recently described a better correlation of DNA synthesis with phosphatidylinositol (PI) 3-kinase than with mitogen-activated protein (MAP) kinase stimulated by insulin-like growth factor (IGF)-1 or epidermal growth factor (EGF) in human skin fibroblasts (Takahashi et al., 1997, Endocrinology 138:741-750). IGF-I-induced PI 3-kinase activation is generally mediated via insulin receptor substrate (IRS)-1, but EGF-induced PI 3-kinase activation is mediated by various signalling molecules such as ErbB3 and c-Cbl in different cells. We therefore investigated the mechanism regulating PI 3-kinase in human skin fibroblasts by comparing complexes involving PI 3-kinase when stimulated by IGF-I or EGF and found that p115 and p105, which were tyrosine-phosphorylated by EGF stimulation and associated with SHP-2, were also associated with the p85 subunit of PI 3-kinase by EGF. Anti-SHP-2 and anti-p85 subunits of PI 3-kinase antibodies did not coprecipitate tyrosine-phosphorylated EGF receptor or ErbB3; in addition, p115 and p105 appeared to be distinct from tyrosine-phosphorylated c-Cbl. Thus, tyrosine-phosphorylated p115 and p105 may provide a novel platform recruiting p85, which may simultaneously bind to SHP-2. In contrast, tyrosine phosphorylation of p115 or p 105 was undetectable by immunoblot with IGF-I stimulation, and PI 3-kinase activity was mediated via IRS-1 phosphorylated with IGF-I stimulation, little of which was associated with SHP-2. Thus, EGF and IGF-I cause formation of a distinct signalling complex which associates with p85 subunit of PI 3-kinase.