Specificity within the EGF family/ErbB receptor family signaling network

Recent years have witnessed tremendous growth in the epidermal growth factor (EGF) family of peptide growth factors and the ErbB family of tyrosine kinases, the receptors for these factors. Accompanying this growth has been an increased appreciation for the roles these molecules play in tumorigenesis and in regulating cell proliferation and differentiation during development. Consequently, a significant question has been how diverse biological responses are specified by these hormones and receptors. Here we discuss several characteristics of hormone-receptor interactions and receptor coupling that contribute to specificity: 1) a single EGF family hormone can bind multiple receptors; 2) a single ErbB family receptor can bind multiple hormones; 3) there are three distinct functional groups of EGF family hormones; 4) EGF family hormones can activate receptors in trans, and this heterodimerization diversifies biological responses; 5) ErbB3 requires a receptor partner for signaling; and 6) ErbB family receptors differentially couple to signaling pathways and biological responses. BioEssays 20:41–48, 1998. © 1998 John Wiley & Sons, Inc.     

Signaling via vascular endothelial growth factor receptors

Angiogenesis, or development of blood vessels from preexisting vasculature, has important functions under both normal and pathophysiological conditions. Vascular endothelial growth factor receptors 1-3, also known as flt-1, KDR, and flt-4, are endothelial cell-specific receptor tyrosine kinases which serve as key mediators of the angiogenic responses. The review focuses on the signaling pathways that are initiated from these receptors and the recently identified VEGF coreceptor neuroplilin-1.     

Alphavbeta3 integrin associates with activated insulin and PDGFbeta receptors and potentiates the biological activity of PDGF.

Integrin-mediated cell attachment modulates growth responses and growth factors regulate cell attachment. Moreover, both cell attachment to extracellular matrix and mitogenic signaling by growth factors are necessary for the proliferation of most types of normal cells, suggesting that integrin and growth factor receptor signaling pathways meet at some downstream point. We report here that a small, highly tyrosine-phosphorylated fraction of PDGFbeta and insulin receptors co-immunoprecipitates with the alphavbeta3 integrin from cells. The integrin association requires growth factor stimulation of the receptors. Several signaling molecules that are known to be associated with activated growth factor receptors were present in the alphavbeta3 integrin complexes. Mitogenicity and chemotaxis induced by PDGF-BB were enhanced in cells plated on the alphavbeta3 ligand vitronectin compared with cells plated on the beta1 integrin ligand collagen. Thus, the engagement of the alphavbeta3 integrin in cell-matrix interactions appears to coordinate an intense response to growth factors, helping to explain the importance of this integrin for tissue regeneration, angiogenesis and tumor metastasis.     

Muscarinic M3 and epidermal growth factor receptors activate mutually inhibitory signaling cascades in human neuroblastoma SH-SY5Y cells

Regulatory interactions among individual receptor-coupled signal transduction systems are critically important for establishing cellular responses in the face of multiple stimuli. In this study, potential regulatory interactions between signal transduction systems activated by growth factor receptors and by G-protein-coupled receptors were examined using human neuroblastoma SH-SY5Y cells which express endogenous epidermal growth factor (EGF) and muscarinic M3 receptors. Activation of muscarinic receptors with carbachol was found to inhibit EGF-induced signaling, including tyrosine phosphorylation of the adaptor protein Cbl and of the EGF receptor, and complex formation between Shc proteins and the EGF receptor and Grb2. Protein kinase C, which is activated by muscarinic M3 receptors, mediated this inhibitory cross-talk. Activation of EGF receptors was found to inhibit muscarinic receptor-induced tyrosine phosphorylation of focal adhesion kinase and paxillin. Reactive oxygen species, which are formed as components of the EGF signaling cascade, mediated this inhibitory cross-talk. These mutual inhibitory interactions demonstrate novel mechanisms for neuronal integration of multiple signals generated by activation of receptors by neurotransmitters and growth factors.     

Distinctive features of Trk neurotrophin receptor transactivation by G protein-coupled receptors.

Ligands for G protein-coupled receptors (GPCR) are capable of activating mitogenic receptor tyrosine kinases, in addition to the mitogen-activated protein (MAP) kinase signaling pathway and classic G protein-dependent signaling pathways involving adenylyl cyclase and phospholipase. For example, receptors for epidermal growth factor (EGF), insulin-like growth-1 and platelet-derived growth factor and can be transactivated through G protein-coupled receptors. Neurotrophins, such as NGF, BDNF and NT-3 also utilize receptor tyrosine kinases, namely TrkA, TrkB and TrkC. Recently, it has been shown that activation of Trk receptor tyrosine kinases can also occur via a G protein-coupled receptor mechanism, without involvement of neurotrophins. Adenosine and adenosine agonists can activate Trk receptor phosphorylation specifically through the seven transmembrane spanning adenosine 2A (A2A) receptor. Several features of Trk receptor transactivation are noteworthy and differ significantly from other transactivation events. Trk receptor transactivation is slower and results in a selective increase in activated Akt. Unlike the biological actions of other tyrosine kinase receptors, increased Trk receptor activity by adenosine resulted in increased cell survival. This article will discuss potential mechanisms by which adenosine can activate trophic responses through Trk tyrosine kinase receptors.     

Mechanisms of transforming growth factor-beta receptor endocytosis and intracellular sorting differ between fibroblasts and epithelial cells

Transforming growth factor-betas (TGF-beta) are multifunctional proteins capable of either stimulating or inhibiting mitosis, depending on the cell type. These diverse cellular responses are caused by stimulating a single receptor complex composed of type I and type II receptors. Using a chimeric receptor model where the granulocyte/monocyte colony-stimulating factor receptor ligand binding domains are fused to the transmembrane and cytoplasmic signaling domains of the TGF-beta type I and II receptors, we wished to describe the role(s) of specific amino acid residues in regulating ligand-mediated endocytosis and signaling in fibroblasts and epithelial cells. Specific point mutations were introduced at Y182, T200, and Y249 of the type I receptor and K277 and P525 of the type II receptor. Mutation of either Y182 or Y249, residues within two putative consensus tyrosine-based internalization motifs, had no effect on endocytosis or signaling. This is in contrast to mutation of T200 to valine, which resulted in ablation of signaling in both cell types, while only abolishing receptor down-regulation in fibroblasts. Moreover, in the absence of ligand, both fibroblasts and epithelial cells constitutively internalize and recycle the TGF-beta receptor complex back to the plasma membrane. The data indicate fundamental differences between mesenchymal and epithelial cells in endocytic sorting and suggest that ligand binding diverts heteromeric receptors from the default recycling pool to a pathway mediating receptor down-regulation and signaling.     

Epidermal growth factor receptor signal transactivation

Cross-communication between heterologous signaling systems and the epidermal growth factor receptor (EGFR) has been shown to be critical for a variety of biological responses: EGFR transactivation when G-protein-coupled receptors (GPCRs) are stimulated represents the paradigm of an interreceptor network that is dependent on G-proteins, kinases, metalloproteases, and growth factor precursors. Investigating the mechanism of this process will help expand our knowledge of physiological regulatory mechanisms and diverse pathophysiological disorders.     

EGF receptor.

The receptor for the epidermal growth factor (EGF) and related ligands (EGFR), the prototypal member of the superfamily of receptors with intrinsic tyrosine kinase activity, is widely expressed on many cell types, including epithelial and mesenchymal lineages. Upon activation by at least five genetically distinct ligands (including EGF, transforming growth factor-alpha (TGF alpha) and heparin-binding EGF (HB-EGF)), the intrinsic kinase is activated and EGFR tyrosyl-phosphorylates itself and numerous intermediary effector molecules, including closely-related c-erbB receptor family members. This initiates myriad signaling pathways, some of which attenuate receptor signaling. The integrated biological responses to EGFR signaling are pleiotropic including mitogenesis or apoptosis, enhanced cell motility, protein secretion, and differentiation or dedifferentiation. In addition to being implicated in organ morphogenesis, maintenance and repair, upregulated EGFR signaling has been correlated in a wide variety of tumors with progression to invasion and metastasis. Thus, EGFR and its downstream signaling molecules' are targets for therapeutic interventions in wound repair and cancer.     

A new model for growth factor activation: type II receptors compete with the prodomain for BMP-7

Bone morphogenetic proteins (BMPs) are morphogens with long-range signaling activities. BMP-7 is secreted as a stable complex consisting of a growth factor noncovalently associated with two propeptides. In other transforming growth factor-β-like growth factor complexes, the prodomain (pd) confers latency to the complex. However, we detected no difference in signaling capabilities between the growth factor and the BMP-7 complex in multiple in vitro bioactivity assays. Biochemical and biophysical methods elucidated the interaction between the BMP-7 complex and the extracellular domains of its type I and type II receptors. Results showed that type II receptors, such as BMP receptor II, activin receptor IIA, and activin receptor IIB, competed with the pd for binding to the growth factor and displaced the pd from the complex. In contrast, type I receptors interacted with the complex without displacing the pd. These studies suggest a new model for growth factor activation in which proteases or other extracellular molecules are not required and provide a molecular mechanism consistent with a role for BMP receptors in the establishment of early morphogen gradients.     

Paradoxical biological effects of overexpressed insulin-like growth factor-1 receptors in chinese hamster ovary cells

One major approach to the study of growth factor receptor action has been to overexpress wild-type or mutant receptors in cultured cells and to evaluate biological responses to exogenous ligand. Studies of this type with insulin and insulin-like growth factor-I (IGF-I) receptors often use Chinese hamster ovary (CHO) cells. We have compared the effect of receptor overexpression in CHO cells and in NIH-3T3 fibroblasts in order to assess the suitability of CHO cells for studies of this nature and the contribution of cell type-specific factors to those responses generally assayed. Overexpression of IGF-I receptors in NIH-3T3 cells resulted in increased sensitivity and maximal responsiveness of thymidine incorporation, 2-deoxyglucose uptake, and phosphatidylinositol-3 (PI3) kinase activation to IGF-I stimulation. In CHO cells, on the other hand, overexpression of either IGF-I or insulin receptors increased the sensitivity of thymidine incorporation to ligand, but maximal responsiveness was unchanged or decreased. Overexpression of the insulin receptor increased sensitivity of glucose uptake and the maximal response of PI3 kinase activation to insulin. Overexpression of the IGF-I receptor did not affect sensitivity or maximal responsiveness of glucose uptake or PI3 kinase activation to IGF-I. These data suggest that IGF-I and insulin signal pathways may differ in CHO cells, and that there may even be divergent IGF-I signaling pathways for short vs. long-term effects. Whether this is a result of differences in the number of endogenous receptors, hybrid receptor formation, or defects in post-receptor signaling, the use of CHO cells to assess receptor function must be approached with caution. © Wiley-Liss, Inc.     

CD95 death receptor and epidermal growth factor receptor (EGFR) in liver cell apoptosis and regeneration

Recent evidence suggests that signaling pathways towards cell proliferation and cell death are much more interconnected than previously thought. Whereas not only death receptors such as CD95 (Fas, APO-1) can couple to both, cell death and proliferation, also growth factor receptors such as the epidermal growth factor receptor (EGFR) are involved in these opposing kinds of cell fate. EGFR is briefly discussed as a growth factor receptor involved in liver cell proliferation during liver regeneration. Then the role of EGFR in activating CD95 death receptor in liver parenchymal cells (PC) and hepatic stellate cells (HSC), which represent a liver stem/progenitor cell compartment, is described summarizing different ways of CD95- and EGFR-dependent signaling in the liver. Here, depending on the hepatic cell type (PC vs. HSC) and the respective signaling context (sustained vs. transient JNK activation) CD95-/EGFR-mediated signaling ends up in either liver cell apoptosis or cell proliferation.     

Adhesion events in angiogenesis.

Recent work from several laboratories indicates that the coordination of endothelial cell adhesion events with growth factor receptor inputs regulates endothelial cell responses during angiogenesis. Analyses of the signaling pathways downstream of integrins, cadherins and growth-factor receptors are providing an insight into the molecular basis of known anti-angiogenic strategies, as well as into the design of novel therapies.     

Cross-talk between RON receptor tyrosine kinase and other transmembrane receptors

RON is a transmembrane receptor tyrosine kinase that mediates biological activities of Macrophage Stimulating Protein (MSP). MSP is a multifunctional factor regulating cell adhesion, motility, growth and survival. MSP binding to RON causes receptor tyrosine phosphorylation leading to up-regulation of RON catalytic activity and subsequent activation of downstream signaling molecules. Recent studies show that RON is spatially and functionally associated with other transmembrane molecules including adhesion receptors integrins and cadherins, and cytokine and growth factor receptors IL-3 betac, EPOR and MET. For example, MSP-induced cell shape change is mediated via RON-activated IL-3 betac receptor. Activation of integrins causes MSP-independent RON phosphorylation, and the integrin/RON collaboration regulates cell survival. Thus, RON can be activated without MSP by ligand stimulation of RON-associated receptors, and MSP-activated RON can cause ligand-independent activation of RON-associated receptors. As a result of the receptor cross-activation RON-specific pathways become a part of a signal transduction network of other receptors, and conversely signaling pathways activated by other receptors can be used by RON. This receptor collaboration extends the spectrum of cellular responses generated by MSP and by putative ligands of RON-associated receptors. However signaling pathways involved in the receptor cross-talk and underlying activation mechanisms remain to be investigated. The purpose of this review is to summarize data and to discuss a role of cross-talk between RON and other transmembrane receptors.     

The F-BAR Protein PACSIN2 Regulates Epidermal Growth Factor Receptor Internalization

Signaling via growth factor receptors, including the epidermal growth factor (EGF) receptor, is key to various cellular processes, such as proliferation, cell survival, and cell migration. In a variety of human diseases such as cancer, aberrant expression and activation of growth factor receptors can lead to disturbed signaling. Intracellular trafficking is crucial for proper signaling of growth factor receptors. As a result, the level of cell surface expression of growth factor receptors is an important determinant for the outcome of downstream signaling. BAR domain-containing proteins represent an important family of proteins that regulate membrane dynamics. In this study, we identify a novel role for the F-BAR protein PACSIN2 in the regulation of EGF receptor signaling. We show that internalized EGF as well as the (activated) EGF receptor translocated to PACSIN2-positive endosomes. Furthermore, loss of PACSIN2 increased plasma membrane expression of the EGF receptor in resting cells and increased EGF-induced phosphorylation of the EGF receptor. As a consequence, EGF-induced activation of Erk and Akt as well as cell proliferation were enhanced in PACSIN2-depleted cells. In conclusion, this study identifies a novel role for the F-BAR-domain protein PACSIN2 in regulating EGF receptor surface levels and EGF-induced downstream signaling.     

PDGF beta-receptor stimulates tyrosine phosphorylation of GAP and association of GAP with a signaling complex

Platelet-derived growth factor (PDGF) stimulated the tyrosine phosphorylation of the GTPase activating protein (GAP) in 3T3 cells and in CHO cells expressing wild-type PDGF receptors, but not in several CHO cell lines expressing mutant receptors defective in transmitting mitogenic signals. Following PDGF treatment of cells, GAP physically associated with the PDGF receptor and with Raf-1, phospholipase c-gamma, and PI-3 kinase, suggesting that PDGF induced the formation of complexes of signaling molecules. The association of GAP with the PDGF receptor and the phosphorylation of GAP with the PDGF receptor and the phosphorylation of GAP were reconstituted in vitro using purified protein and in insect cells expressing murine PDGF receptor and human GAP. However, in cells transformed by activated c-Ha-ras, which are defective in certain responses to PDGF, GAP failed to associate with the PDGF receptor or increase its phosphotyrosine content in response to PDGF. The association of GAP with ligand-activated PDGF receptors may directly link PDGF and ras signaling pathways.     

Hypoxia increases Sca-1/CD44 co-expression in murine mesenchymal stem cells and enhances their adipogenic differentiation potential

The emergence of multicellular animals could only take place once evolution had produced molecular mechanisms for cell adhesion and communication. Today, all metazoans express integrin-type adhesion receptors and receptors for growth factors. Integrins recognize extracellular matrix proteins and respective receptors on other cells and, following ligand binding, can activate the same cellular signaling pathways that are regulated by growth factor receptors. Recent reports have indicated that the two receptor systems also collaborate in many other ways. Here, we review the present information concerning the role of integrins as assisting growth factor receptors and the interplay between the receptors in cell signaling and in the orchestration of receptor recycling.