Dysregulation of ErbB Receptor Trafficking and Signaling in Demyelinating Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is the most common inherited peripheral neuropathy with the majority of cases involving demyelination of peripheral nerves. The pathogenic mechanisms of demyelinating CMT remain unclear, and no effective therapy currently exists for this disease. The discovery that mutations in different genes can cause a similar phenotype of demyelinating peripheral neuropathy raises the possibility that there may be convergent mechanisms leading to demyelinating CMT pathogenesis. Increasing evidence indicates that ErbB receptor-mediated signaling plays a major role in the control of Schwann cell-axon communication and myelination in the peripheral nervous system. Recent studies reveal that several demyelinating CMT-linked proteins are novel regulators of endocytic trafficking and/or phosphoinositide metabolism that may affect ErbB receptor signaling. Emerging data have begun to suggest that dysregulation of ErbB receptor trafficking and signaling in Schwann cells may represent a common pathogenic mechanism in multiple subtypes of demyelinating CMT. In this review, we focus on the roles of ErbB receptor trafficking and signaling in regulation of peripheral nerve myelination and discuss the emerging evidence supporting the potential involvement of altered ErbB receptor trafficking and signaling in demyelinating CMT pathogenesis and the possibility of modulating these trafficking and signaling processes for treating demyelinating peripheral neuropathy.     

Constitutively active ErbB4 and ErbB2 mutants exhibit distinct biological activities.

ErbB4 is a member of the epidermal growth factor receptor(EGFR) family of tyrosine kinases, which includes EGFR/ErbB1, ErbB2/HER2/Neu, and ErbB3/HER3. These receptors play important roles both in normal development and in neoplasia. For example, deregulated signaling by ErbB1 and ErbB2 is observed in many human malignancies. In contrast, the roles that ErbB4 plays in tumorigenesis and normal biological processes have not been clearly defined. To identify the biological responses that are coupled to ErbB4, we have constructed three constitutively active ErbB4 mutants. Unlike a constitutively active ErbB2 mutant, the ErbB4 mutants are not coupled to increased cell proliferation, loss of contact inhibition, or anchorage independence in a rodent fibroblast cell line. This suggests that ErbB2 and ErbB4 may play distinct roles in tumorigenesis in vivo.     

Essential role for Rac in heregulin beta1 mitogenic signaling: a mechanism that involves epidermal growth factor receptor and is independent of ErbB4

Heregulins are a family of ligands for the ErbB3/ErbB4 receptors that play important roles in breast cancer cell proliferation and tumorigenesis. Limited information is available on the contribution of Rho GTPases to heregulin-mediated signaling. In breast cancer cells, heregulin beta1 (HRG) causes a strong activation of Rac; however, it does so with striking differences in kinetics compared to epidermal growth factor, which signals through ErbB1 (epidermal growth factor receptor [EGFR]). Using specific ErbB receptor inhibitors and depletion of receptors by RNA interference (RNAi), we established that, surprisingly, activation of Rac by HRG is mediated not only by ErbB3 and ErbB2 but also by transactivation of EGFR, and it is independent of ErbB4. Similar receptor requirements are observed for HRG-induced actin cytoskeleton reorganization and mitogenic activity via extracellular signal-regulated kinase (ERK). HRG-induced Rac activation was phosphatidylinositol 3-kinase dependent and Src independent. Furthermore, inactivation of Rac by expression of the Rac GTPase-activating protein beta2-chimerin inhibited HRG-induced ERK activation, mitogenicity, and migration in breast cancer cells. HRG mitogenic activity was also impaired by depletion of Rac1 using RNAi. Our studies established that Rac is a critical mediator of HRG mitogenic signaling in breast cancer cells and highlight additional levels of complexity for ErbB receptor coupling to downstream effectors that control aberrant proliferation and transformation.     

Molecular modeling of nearly full-length ErbB2 receptor

Members of the epidermal growth factor receptor family play important roles in various cellular processes, both in physiological and in pathological conditions. Dimerization and autophosphorylation of these receptor tyrosine kinases are key events of signal transduction. Details of the molecular events of the signaling are not entirely known. To facilitate the understanding of receptor structure and function at the molecular level, a molecular model was built for the nearly full-length ErbB2 dimer. Modeling was based on the x-ray or nuclear-magnetic resonance structures of extracellular, transmembrane, and intracellular domains. The extracellular domain was positioned above the cell membrane based on the distance determined from experimentally measured fluorescence resonance energy transfer. Favorable dimerization interactions are predicted for the extracellular, transmembrane, and protein kinase domains in the model of a nearly full-length dimer of ErbB2, which may act in a coordinated fashion in ErbB2 homodimerization, and also in heterodimers of ErbB2 with other members of the ErbB family.     

ErbB and HB-EGF signaling in heart development and function

The epidermal growth factor (EGF)-ErbB signaling network is composed of multiple ligands of the EGF family and four tyrosine kinase receptors of the ErbB family. In higher vertebrates, these four receptors bind a multitude of ligands. Ligand binding induces the formation of various homo- and heterodimers of ErbB, potentially providing for a high degree of signal diversity. ErbB receptors and their ligands are expressed in a variety of tissues throughout development. Recent advances in gene targeting strategies in mice have revealed that the EGF-ErbB signaling network has fundamental roles in development, proliferation, differentiation, and homeostasis in mammals. The heparin-binding EGF-like growth factor (HB-EGF) is a member of the EGF family of growth factors that binds to and activates the EGF receptor (EGFR/ErbB1) and ErbB4. Recent studies using several mutant mice lacking HB-EGF expression have revealed that HB-EGF has a critical role in normal heart function and in normal cardiac valve formation in conjunction with ErbB receptors. HB-EGF signaling through ErbB2 is essential for the maintenance of homeostasis in the adult heart, whereas HB-EGF signaling through EGFR is required during cardiac valve development. In this review, we introduce and discuss the role of ErbB receptors in heart function and development, focusing on the physiological function of HB-EGF in these processes.     

Endocytic downregulation of ErbB receptors: mechanisms and relevance in cancer

ErbB receptors (EGFR (ErbB1), ErbB2, ErbB3, and ErbB4) are important regulators of normal growth and differentiation, and they are involved in the pathogenesis of cancer. Following ligand binding and receptor activation, EGFR is endocytosed and transported to lysosomes where the receptor is degraded. This downregulation of EGFR is a complex and tightly regulated process. The functions of ErbB2, ErbB3, and ErbB4 are also regulated by endocytosis to some extent, although the current knowledge of these processes is sparse. Impaired endocytic downregulation of signaling receptors is frequently associated with cancer, since it can lead to increased and uncontrolled receptor signaling. In this review we describe the current knowledge of ErbB receptor endocytic downregulation. In addition, we outline how ErbB receptors can escape endocytic downregulation in cancer, and we discuss how targeted anti-cancer therapy may induce endocytic downregulation of ErbB receptors.     

ErbB receptors and cell polarity: New pathways and paradigms for understanding cell migration and invasion

The ErbB family of receptor tyrosine kinases is involved in initiation and progression of a number of human cancers, and receptor activation or overexpression correlates with poor patient survival. Research over the past two decades has elucidated the molecular mechanisms underlying ErbB-induced tumorigenesis, which has resulted in the development of effective targeted therapies. ErbB-induced signal transduction cascades regulate a wide variety of cell processes, including cell proliferation, apoptosis, cell polarity, migration and invasion. Within tumors, disruption of these core processes, through cooperative oncogenic lesions, results in aggressive, metastatic disease. This review will focus on the ErbB signaling networks that regulate migration and invasion and identify a potential role for cell polarity pathways during cancer progression.     

The ErbB kinase domain: structural perspectives into kinase activation and inhibition

Epidermal growth factor receptor (EGFR) and its family members, ErbB2, ErbB3 and ErbB4, are receptor tyrosine kinases which send signals into the cell to regulate many critical processes including development, tissue homeostasis, and tumorigenesis. Central to the signaling of these receptors is their intracellular kinase domain, which is activated by ligand-induced dimerization of the receptor and phosphorylates several tyrosine residues in the C-terminal tail. The phosphorylated tail then recruits other signaling molecules and relays the signal to downstream pathways. A model of the autoinhibition, activation and feedback inhibition mechanisms for the ErbB kinase domain has emerged from a number of recent structural studies. Meanwhile, recent clinical studies have revealed the relationship between specific ErbB kinase mutations and the responsiveness to kinase inhibitor drugs. We will review these regulation mechanisms of the ErbB kinase domain, and discuss the binding specificity of kinase inhibitors and the effects of kinase domain mutations found in cancer patients from a structural perspective.     

ErbB signaling in cardiac development and disease

The ErbB family of receptor tyrosine kinases (RTKs) is a family of receptors that allow cells to interact with the extracellular environment and transduce signals to the nucleus that promote differentiation, migration and proliferation necessary for proper heart morphogenesis and function. This review focuses on the role of the ErbB family of receptor tyrosine kinases, and their importance in proper heart morphogenesis, as well as their role in maintenance and function of the adult heart. Studies from transgenic mouse models have shown the importance of ErbB receptors in heart development, and provide insight into potential future therapeutic targets to help reduce congenital heart defect (CHD) mortality rates and prevent disease in adults. Cancer therapeutics have also shed light to the ErbB receptors and signaling network, as undesired side effects have demonstrated their importance in adult cardiomyocytes and prevention of cardiomyopathies. This review will discuss ErbB receptor tyrosine kinases (RTK) in heart development and disease including valve formation and partitioning of a four-chambered heart as well as cardiotoxicity when ErbB signaling is attenuated in adults.     

The ErbB receptors and their role in cancer progression

The involvement of the ErbB receptor tyrosine kinases in human cancer, as well as their essential role in a variety of physiological events during normal development, have motivated the interest in this receptor family. Approaches taken to block the activity of ErbB receptors in cancer cells have not only proven that they drive in vitro tumor cell proliferation, but have also become clinically relevant for targeting tumors with deregulated ErbB signaling. The mechanisms and downstream effectors through which the ErbB receptors influence processes linked to malignant development, including proliferation, cell survival, angiogenesis, migration, and invasion, are, however, only now becoming apparent. Our particular emphasis in this review will be on how ErbB receptors, in particular ErbB1 and ErbB2, contribute to processes linked to cancer progression. Importantly, in keeping with the emerging theme that ErbB receptors do not function in isolation, we will focus on receptor cooperativity, i.e., ErbB1 cooperates with other classes of receptors, and the ligand-less ErbB2 functions as a heterodimer with other ErbBs.     

以ErbB受体为靶向的融合蛋白抗肿瘤治疗研究进展

ErbB受体介导的信号转导途径对外胚层来源的肿瘤的增殖、浸润、转移、及血管新生起促进作用。因此,出现了针对ErbB受体信号转导途径各环节的不同的抗肿瘤治疗策略,包括抗ErbB受体的单克隆抗体、酪氨酸激酶抑制剂等,而ErbB受体介导的融合蛋白因其高选择性引起人们的重视。主要探讨了具有抗肿瘤作用的以ErbB受体介导的融合蛋白在组成和细胞毒性方面的研究进展。     

Energetics of ErbB1 Transmembrane Domain Dimerization in Lipid Bilayers

One of the most extensively studied receptor tyrosine kinases is EGFR/ErbB1. Although our knowledge of the role of the extracellular domains and ligands in ErbB1 activation has increased dramatically based on solved domain structures, the exact mechanism of signal transduction across the membrane remains unknown. The transmembrane domains are expected to play an important role in the dimerization process, but the contribution of ErbB1 TM domain to dimer stability is not known, with published results contradicting one another. We address this controversy by showing that ErbB1 TM domain dimerizes in lipid bilayers and by calculating its contribution to stability as −2.5 kcal/mol. The stability calculations use two different methods based on Förster resonance energy transfer, which give the same result. The ErbB1 TM domain contribution to stability exceeds the change in receptor tyrosine kinases dimerization propensities that can convert normal signaling processes into pathogenic processes, and is thus likely important for biological function.     

Transmembrane helix-helix interactions involved in ErbB receptor signaling

Among the many transmembrane receptor classes, the receptor tyrosine kinases represent an important superfamily, involved in many cellular processes like embryogenesis, development and cell division. Deregulation and dysfunctions of these receptors can lead to various forms of cancer and other diseases. Mostly, only fragmented knowledge exists about functioning of the entire receptors, and many studies have been performed on isolated receptor domains. In this review we focus on the function of the ErbB family of receptor tyrosine kinases with a special emphasis on the role of the transmembrane domain and on the mechanisms underlying regulated and deregulated signaling. Many general aspects of ErbB receptor structure and function have been analyzed and described. All human ErbBs appear to form homo- and heterodimers within cellular membranes and the single transmembrane domain of the receptors is involved in dimerization. Additionally, only defined structures of the transmembrane helix dimer allows signaling of ErbB receptors.Key words: ErbB, EGFR, receptor, receptor-tyrosine kinase, transmembrane proteins, signaling, helix-helix interaction     

Rapid phospho-turnover by receptor tyrosine kinases impacts downstream signaling and drug binding

Epidermal growth factor receptors (ErbB1-4) are oncogenic receptor tyrosine kinases (RTKs) that regulate diverse cellular processes. In this study, we combine measurement and mathematical modeling to quantify phospho-turnover at ErbB receptors in human cells and to determine the consequences for signaling and drug binding. We find that phosphotyrosine residues on ErbB1 have half-lives of a few seconds and therefore turn over 100-1000 times in the course of a typical immediate-early response to ligand. Rapid phospho-turnover is also observed for EGF-activated ErbB2 and ErbB3, unrelated RTKs, and multiple intracellular adaptor proteins and signaling kinases. Thus, the complexes formed on the cytoplasmic tail of active receptors and the downstream signaling kinases they control are highly dynamic and antagonized by potent phosphatases. We develop a kinetic scheme for binding of anti-ErbB1 drugs to receptors and show that rapid phospho-turnover significantly impacts their mechanisms of action.     

Lipid rafts in neuregulin signaling at synapses

Neuregulins are a family of EGF domain-containing factors that play an important role in development. In the nervous system, they promote glial differentiation, induce neurotransmitter receptor expression, and regulate synaptic plasticity. Recent studies indicate that ErbB protein tyrosine kinases, neuregulin receptors, translocate to lipid raft microdomains in the plasma membrane in response to neuregulin. Localization of ErbB proteins in lipid rafts appeared to be necessary for neuregulin signaling and regulation of synaptic plasticity. We will review recent studies of lipid rafts and neuregulin function and discuss possible roles of lipid rafts in compartmentalized neuregulin signaling and translocation of ErbB proteins to synapses.     

Coupled Stochastic Spatial and Non-Spatial Simulations of ErbB1 Signaling Pathways Demonstrate the Importance of Spatial Organization in Signal Transduction

Background

The ErbB family of receptors activates intracellular signaling pathways that control cellular proliferation, growth, differentiation and apoptosis. Given these central roles, it is not surprising that overexpression of the ErbB receptors is often associated with carcinogenesis. Therefore, extensive laboratory studies have been devoted to understanding the signaling events associated with ErbB activation.

Methodology/Principal Findings

Systems biology has contributed significantly to our current understanding of ErbB signaling networks. However, although computational models have grown in complexity over the years, little work has been done to consider the spatial-temporal dynamics of receptor interactions and to evaluate how spatial organization of membrane receptors influences signaling transduction. Herein, we explore the impact of spatial organization of the epidermal growth factor receptor (ErbB1/EGFR) on the initiation of downstream signaling. We describe the development of an algorithm that couples a spatial stochastic model of membrane receptors with a nonspatial stochastic model of the reactions and interactions in the cytosol. This novel algorithm provides a computationally efficient method to evaluate the effects of spatial heterogeneity on the coupling of receptors to cytosolic signaling partners.

Conclusions/Significance

Mathematical models of signal transduction rarely consider the contributions of spatial organization due to high computational costs. A hybrid stochastic approach simplifies analyses of the spatio-temporal aspects of cell signaling and, as an example, demonstrates that receptor clustering contributes significantly to the efficiency of signal propagation from ligand-engaged growth factor receptors.     

The Achilles Heel of ErbB-2/HER2: Regulation by the Hsp90 Chaperone Machine and Potential for Pharmacological Intervention

Signal transduction mediated by ErbB/HER receptor tyrosine kinases is crucial for the development and maintenance of epithelial tissues, and aberrant signaling is frequently associated with malignancies of epithelial origin. This review focuses on the roles played by the Hsp90 chaperone machinery in the regulation of signaling through the ErbB/HER network, and discusses potential therapeutic strategies that disrupt chaperone functions. Hsp90 and its associated co-chaperones regulate ErbB signal transduction through multiple mechanisms. The chaperone system controls the stability of the nascent forms of both ErbB-1 (EGF-receptor) and ErbB-2/HER2, while regulation of the mature form is restricted to ErbB-2. Regulation by the Hsp90 complex extends to downstream effectors of ErbB signaling, namely Raf-1, Pdk-1 and Akt/PKB. Disrupting the function of Hsp90 results in the degradation of both the receptors and their effectors, thereby inhibiting tumor cell growth. The importance of an Hsp90-recognition motif located within the kinase domain of ErbB-2 is discussed, as well as a direct role for Hsp90 in regulating tyrosine kinase activity. In light of recent observations, we emphasize the ability of specific tyrosine kinase inhibitors to selectively target ErbB-2 to the chaperone-mediated degradation pathway. ErbB-specific drugs are already used to treat cancers, and clinical trials are underway for additional compounds that intercept ErbB signaling, including drugs that target Hsp90. Hence, the dependence of ErbB-2 upon Hsp90 reveals an Achilles heel, which opens a window of opportunity for combating cancers driven by the ErbB/HER signaling network.     

The achilles heel of ErbB-2/HER2: regulation by the Hsp90 chaperone machine and potential for pharmacological intervention

Signal transduction mediated by ErbB/HER receptor tyrosine kinases is crucial for the development and maintenance of epithelial tissues, and aberrant signaling is frequently associated with malignancies of epithelial origin. This review focuses on the roles played by the Hsp90 chaperone machinery in the regulation of signaling through the ErbB/HER network, and discusses potential therapeutic strategies that disrupt chaperone functions. Hsp90 and its associated cochaperones regulate ErbB signal transduction through multiple mechanisms. The chaperone system controls the stability of the nascent forms of both ErbB-1 (EGF-receptor) and ErbB-2/HER2, while regulation of the mature form is restricted to ErbB-2. Regulation by the Hsp90 complex extends to downstream effectors of ErbB signaling, namely Raf-1, Pdk-1 and Akt/PKB. Disrupting the function of Hsp90 results in the degradation of both the receptors and their effectors, thereby inhibiting tumor cell growth. The importance of an Hsp90-recognition motif located within the kinase domain of ErbB-2 is discussed, as well as a direct role for Hsp90 in regulating tyrosine kinase activity. In light of recent observations, we emphasize the ability of specific tyrosine kinase inhibitors to selectively target ErbB-2 to the chaperone-mediated degradation pathway. ErbB-specific drugs are already used to treat cancers, and clinical trials are underway for additional compounds that intercept ErbB signaling, including drugs that target Hsp90. Hence, the dependence of ErbB-2 upon Hsp90 reveals an Achilles heel, which opens a window of opportunity for combating cancers driven by the ErbB/HER signaling network.