Constitutive proteolysis of the ErbB-4 receptor tyrosine kinase by a unique, sequential mechanism

The Yin and Yang of ERBB4: Tumor Suppressor and Oncoprotein

ERBB4 (HER4) is a member of the ERBB family of receptor tyrosine kinases, a family that includes the epidermal growth factor receptor (EGFR/ERBB1/HER1), ERBB2 (Neu/HER2), and ERBB3 (HER3). EGFR and ERBB2 are oncoproteins and validated targets for therapeutic intervention in a variety of solid tumors. In contrast, the role that ERBB4 plays in human malignancies is ambiguous. Thus, here we review the literature regarding ERBB4 function in human malignancies. We review the mechanisms of ERBB4 signaling with an emphasis on mechanisms of signaling specificity. In the context of this signaling specificity, we discuss the hypothesis that ERBB4 appears to function as a tumor suppressor protein and as an oncoprotein. Next, we review the literature that describes the role of ERBB4 in tumors of the bladder, liver, prostate, brain, colon, stomach, lung, bone, ovary, thyroid, hematopoietic tissues, pancreas, breast, skin, head, and neck. Whenever possible, we discuss the possibility that ERBB4 mutants function as biomarkers in these tumors. Finally, we discuss the potential roles of ERBB4 mutants in the staging of human tumors and how ERBB4 function may dictate the treatment of human tumors. SIGNIFICANCE STATEMENT: This articles reviews ERBB4 function in the context of the mechanistic model that ERBB4 homodimers function as tumor suppressors, whereas ERBB4-EGFR or ERBB4-ERBB2 heterodimers act as oncogenes. Thus, this review serves as a mechanistic framework for clinicians and scientists to consider the role of ERBB4 and ERBB4 mutants in staging and treating human tumors.

Mechanisms of Receptor Tyrosine-Protein Kinase ErbB-3 (ERBB3) Action in Human Neoplasia

It is well established that the epidermal growth factor (EGF) receptor, receptor tyrosine-protein kinase erbB-2 (ERBB2)/human EGF receptor 2 (HER2), and, to a lesser extent, ERBB4/HER4, promote the pathogenesis of many types of human cancers. In contrast, the role that ERBB3/HER3, the fourth member of the ERBB family of receptor tyrosine kinases, plays in these diseases is poorly understood and, until recently, underappreciated. In large part, this was because early structural and functional studies suggested that ERBB3 had little, if any, intrinsic tyrosine kinase activity and, thus, was unlikely to be an important therapeutic target. Since then, however, numerous publications have demonstrated an important role for ERBB3 in carcinogenesis, metastasis, and acquired drug resistance. Furthermore, somatic ERBB3 mutations are frequently encountered in many types of human cancers. Dysregulation of ERBB3 trafficking as well as cooperation with other receptor tyrosine kinases further enhance ERBB3's role in tumorigenesis and drug resistance. As a result of these advances in our understanding of the structure and biochemistry of ERBB3, and a growing focus on the development of precision and combinatorial therapeutic regimens, ERBB3 is increasingly considered to be an important therapeutic target in human cancers. In this review, we discuss the unique structural and functional features of ERBB3 and how this information is being used to develop effective new therapeutic agents that target ERBB3 in human cancers.

ErbB4 promotes malignant peripheral nerve sheath tumor pathogenesis via Ras-independent mechanisms

BACKGROUND

We have found that erbB receptor tyrosine kinases drive Ras hyperactivation and growth in NF1-null malignant peripheral nerve sheath tumors (MPNSTs). However, MPNSTs variably express multiple erbB receptors with distinct functional characteristics and it is not clear which of these receptors drive MPNST pathogenesis. Here, we test the hypothesis that altered erbB4 expression promotes MPNST pathogenesis by uniquely activating key cytoplasmic signaling cascades.

METHODS

ErbB4 expression was assessed using immunohistochemistry, immunocytochemistry, immunoblotting and real-time PCR. To define erbB4 functions, we generated mice that develop MPNSTs with floxed Erbb4 alleles (P0-GGFβ3;Trp53+/-;Erbb4flox/flox mice) and ablated Erbb4 in these tumors. MPNST cell proliferation and survival was assessed using 3H-thymidine incorporation, MTT assays, Real-Time Glo and cell count assays. Control and Erbb4-null MPNST cells were orthotopically xenografted in immunodeficient mice and the growth, proliferation (Ki67 labeling), apoptosis (TUNEL labeling) and angiogenesis of these grafts was analyzed. Antibody arrays querying cytoplasmic kinases were used to identify erbB4-responsive kinases. Pharmacologic or genetic inhibition was used to identify erbB4-responsive kinases that drive proliferation.

RESULTS

Aberrant erbB4 expression was evident in 25/30 surgically resected human MPNSTs and in MPNSTs from genetically engineered mouse models (P0-GGFβ3 and P0-GGFβ3;Trp53+/- mice); multiple erbB4 splice variants that differ in their ability to activate PI3 kinase and nuclear signaling were present in MPNST-derived cell lines. Erbb4-null MPNST cells demonstrated decreased proliferation and survival and altered morphology relative to non-ablated controls. Orthotopic allografts of Erbb4-null cells were significantly smaller than controls, with reduced proliferation, survival and vascularization. ERBB4 knockdown in human MPNST cells similarly inhibited DNA synthesis and viability. Although we have previously shown that broad-spectrum erbB inhibitors inhibit Ras activation, Erbb4 ablation did not affect Ras activation, suggesting that erbB4 drives neoplasia via non-Ras dependent pathways. An analysis of 43 candidate kinases identified multiple NRG1β-responsive and erbB4-dependent signaling cascades including the PI3K, WNK1, STAT3, STAT5 and phospholipase-Cγ pathways. Although WNK1 inhibition did not alter proliferation, inhibition of STAT3, STAT5 and phospholipase-Cγ markedly reduced proliferation.

CONCLUSIONS

ErbB4 promotes MPNST growth by activating key non-Ras dependent signaling cascades including the STAT3, STAT5 and phospholipase-Cγ pathways. ErbB4 and its effector pathways are thus potentially useful therapeutic targets in MPNSTs.

Resolving the conformational dynamics of ErbB growth factor receptor dimers

The combinatorial dimerization of the ErbB growth factor receptors (ErbB¹- ErbB⁴) are critical for their function. Here, we have characterized the conformational dynamics of ErbB transmembrane homo-dimers and hetero-dimers by using a coarse-grain simulation framework. All dimers, except ErbB^4-4 and ErbB^1-4, exhibit at least two conformations. The reported NMR structures correspond to one of these conformations, representing the N-terminal active state in ErbB^1-1 (RH2), ErbB^2-2 (RH1) and ErbB^4-4 (RH) homo-dimers and the LH dimer in ErbB^3-3 homo-dimer, validating the computational approach. Further, we predict a right-handed ErbB^3-3 dimer conformer that warrants experimental testing. The five hetero-dimers that have not yet been experimentally resolved display prominent right-handed dimers associating by the SmXXXSm motif. Our results provide insights into the constitutive signaling of ErbB⁴ after cleavage of the extracellular region. The presence of the inactive-like dimer conformers leading to symmetric kinase domains gives clues on the autoinhibition of the receptor dimers. The dimer states characterized here represent an important step towards understanding the combinatorial cross associations in the ErbB family.

Post-translational modification of the interferon-gamma receptor alters its stability and signaling

The IFN gamma receptor 1 (IFNGR1) binds IFN-γ and activates gene transcription pathways crucial for controlling bacterial and viral infections. Although decreases in IFNGR1 surface levels have been demonstrated to inhibit IFN-γ signaling, little is known regarding the molecular mechanisms controlling receptor stability. Here, we show in epithelial and monocytic cell lines that IFNGR1 displays K48 polyubiquitination, is proteasomally degraded, and harbors three ubiquitin acceptor sites at K277, K279, and K285. Inhibition of glycogen synthase kinase 3 beta (GSK3β) destabilized IFNGR1 while overexpression of GSK3β increased receptor stability. We identified critical serine and threonine residues juxtaposed to ubiquitin acceptor sites that impacted IFNGR1 stability. In CRISPR-Cas9 IFNGR1 generated knockout cell lines, cellular expression of IFNGR1 plasmids encoding ubiquitin acceptor site mutations demonstrated significantly impaired STAT1 phosphorylation and decreased STAT1-dependent gene induction. Thus, IFNGR1 undergoes rapid site-specific polyubiquitination, a process modulated by GSK3β. Ubiquitination appears to be necessary for efficient IFNGR1-dependent gamma gene induction and represents a relatively uncharacterized regulatory mechanism for this receptor.

A structural perspective on the regulation of the epidermal growth factor receptor

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that plays a critical role in the pathogenesis of many cancers. The structure of intact forms of this receptor has yet to be determined, but intense investigations of fragments of the receptor have provided a detailed view of its activation mechanism, which we review here. Ligand binding converts the receptor to a dimeric form, in which contacts are restricted to the receptor itself, allowing heterodimerization of the four EGFR family members without direct ligand involvement. Activation of the receptor depends on the formation of an asymmetric dimer of kinase domains, in which one kinase domain allosterically activates the other. Coupling between the extracellular and intracellular domains may involve a switch between alternative crossings of the transmembrane helices, which form dimeric structures. We also discuss how receptor regulation is compromised by oncogenic mutations and the structural basis for negative cooperativity in ligand binding.

An iron-regulated and glycosylation-dependent proteasomal degradation pathway for the plasma membrane metal transporter ZIP14

Protein degradation is instrumental in regulating cellular function. Plasma membrane proteins targeted for degradation are internalized and sorted to multivesicular bodies, which fuse with lysosomes, where they are degraded. ZIP14 is a newly identified iron transporter with multitransmembrane domains. In an attempt to dissect the molecular mechanisms by which iron regulates ZIP14 levels, we found that ZIP14 is endocytosed, extracted from membranes, deglycosylated, and degraded by proteasomes. This pathway did not depend on the retrograde trafficking to the endoplasmic reticulum and thus did not involve the well-defined endoplasmic reticulum-associated protein degradation pathway. Iron inhibited membrane extraction of internalized ZIP14, resulting in higher steady-state levels of ZIP14. Asparagine-linked (N-linked) glycosylation of ZIP14, particularly the glycosylation at N102, was required for efficient membrane extraction of ZIP14 and therefore is necessary for its iron sensitivity. These findings highlight the importance of proteasomes in the degradation of endocytosed plasma membrane proteins.

Deletion of ErbB4 accelerates polycystic kidney disease progression in cpk mice

ErbB4 is highly expressed in the cystic kidneys with polycystic kidney diseases. To investigate its potential role in cystogenesis, cpk mice carrying a heart-rescued ErbB4 deletion were generated. Accelerated cyst progression and renal function deterioration were noted as early as 10 days postnatally in cpk mice with ErbB4 deletion compared to cpk mice, as indicated by increased cystic index, higher kidney weight to body weight ratios and elevated BUN levels. No apparent defects in renal development were noted with ErbB4 deletion itself. Increased cell proliferation was predominately seen in the cortex of cystic kidneys with or without ErbB4 deletion. However, there was significantly more cell proliferation in the cyst-lining epithelial cells in cpk mice with ErbB4 deletion. TUNEL staining localized apoptotic cells mainly to the renal medulla. There were significantly more apoptotic cells in the cyst-lining epithelial cells in ErbB4-deleted cpk kidneys, with decreased levels of cyclin D1, increased levels of p21, p27 and cleaved caspase 3. Thus, lack of ErbB4 may contribute to elevated cell proliferation and unbalanced cell apoptosis, resulting in accelerated cyst formation and early renal function deterioration. These studies suggest that the high level of ErbB4 expression seen in cpk mice may exert relative cytoprotective effects in renal epithelia.

Dissociated presenilin-1 and TACE processing of ErbB4 in lung alveolar type II cell differentiation

Neuregulin (NRG) stimulation of ErbB4 signaling is important for type II cell surfactant synthesis. ErbB4 may mediate gene expression via a non-canonical pathway involving enzymatic cleavage releasing its intracellular domain (4ICD) for nuclear trafficking and gene regulation. The accepted model for release of 4ICD is consecutive cleavage by Tumor necrosis factor alpha Converting Enzyme (TACE) and γ-secretase enzymes. Here, we show that 4ICD mediates surfactant synthesis and its release by γ-secretase is not dependent on previous TACE cleavage. We used siRNA to silence Presenilin-1 (PSEN-1) expression in a mouse lung type II epithelial cell line (MLE12 cells), and both siRNA knockdown and chemical inhibition of TACE. Knockdown of PSEN-1 significantly decreased baseline and NRG-stimulated surfactant phospholipid synthesis, expression of the surfactant proteins SP-B and SP-C, as well as 4ICD levels, with no change in ErbB4 ectodomain shedding. Neither siRNA knockdown nor chemical inhibition of TACE inhibited 4ICD release or surfactant synthesis. PSEN-1 cleavage of ErbB4 for non-canonical signaling through 4ICD release does not require prior cleavage by TACE.

Emerging role of rhomboid family proteins in mammalian biology and disease

From proteases that cleave peptide bonds in the plane of the membrane, rhomboids have evolved into a heterogeneous superfamily with a wide range of different mechanistic properties. In mammals 14 family members have been annotated based on a shared conserved membrane-integral rhomboid core domain, including intramembrane serine proteases and diverse proteolytically inactive homologues. While the function of rhomboid proteases is the proteolytic release of membrane-tethered factors, rhomboid pseudoproteases including iRhoms and derlins interact with their clients without cleaving them. It has become evident that specific recognition of membrane protein substrates and clients by the rhomboid fold reflects a spectrum of cellular functions ranging from growth factor activation, trafficking control to membrane protein degradation. This review summarizes recent progress on rhomboid family proteins in the mammalian secretory pathway and raises the question whether they can be seen as new drug targets for inflammatory diseases and cancer. This article is part of a special issue entitled: Intramembrane Proteases.

Regulated intramembrane cleavage of the EGF receptor

Following the addition of EGF or ionomycin to A431 cells, protease activity mediates cleavage of the EGF receptor producing a 60 kDa fragment that includes the intracellular domain (ICD). This fragment is located in both membrane and nuclear fractions. On the basis of sensitivity to chemical inhibitors and overexpression of cDNAs, the rhomboid intramembrane proteases, not γ-secretase proteases, are identified as responsible for the cleavage event. Agonist-initiated cleavage occurs slowly over 3-24 h. Inhibition of calpain protease activity significantly increased the detectable level of ICD fragment.

WWP1: a versatile ubiquitin E3 ligase in signaling and diseases

WW domain-containing E3 ubiquitin protein ligase 1 (WWP1) is a multifunction protein containing an N-terminal C2 domain, four tandem WW domains for substrate binding, and a C-terminal catalytic HECT domain for ubiquitin transferring. WWP1 has been suggested to function as the E3 ligase for several PY motif-containing proteins, such as Smad2, KLF5, p63, ErbB4/HER4, RUNX2, JunB, RNF11, SPG20, and Gag, as well as several non-PY motif containing proteins, such as TβR1, Smad4, KLF2, and EPS15. WWP1 regulates a variety of cellular biological processes including protein trafficking and degradation, signaling, transcription, and viral budding. WWP1 has been implicated in several diseases, such as cancers, infectious diseases, neurological diseases, and aging. In this review article, we extensively summarize the current knowledge of WWP1 with special emphasis on the roles and action of mechanism of WWP1 in signaling and human diseases.

Sequential proteolytic processing of an interferon-alpha receptor subunit by TNF-alpha converting enzyme and presenilins

It is well established that interferons trigger tyrosine-kinase-dependent signaling via JAK kinases and STAT transcription factors. However, we have observed both IFNaR2 receptor cleavage and functional activity of the liberated intracellular domain (ICD), suggesting that interferon-alpha (IFN-alpha) can also signal via regulated intramembrane proteolysis (RIP), an evolutionarily conserved mechanism of receptor-mediated signaling. Sequential cleavage of the receptor ectodomain and transmembrane domain is a hallmark of the most common class of RIP. To investigate the mechanisms of IFNaR2 RIP signaling, we examined IFNaR2 cleavage by TNF-alpha converting enzyme (TACE) and presenilin proteases. We tracked the fate of epitope-tagged and fusion variants of IFNaR2 in cells expressing wild-type, mutant, or null versions of TACE and presenilins 1 and 2. Cleavage and subcellular location were determined by immunoblot, fluoresence microscopy, and reporter assays. We found that both TACE and presenilin 1/2 cleave IFNaR2, in a sequential manner that allows the ICD to move to the nucleus. TACE cleavage was induced by IFN-alpha but was not consistently required for the anti-proliferative effects of IFN-alpha. In conclusion, IFNaR2 is cleaved by TACE and Presenilin 1/2, suggesting that interferons signal by both kinase and RIP-mediated pathways.

Control of interneuron dendritic growth through NRG1/erbB4-mediated kalirin-7 disinhibition

Neuregulin 1 (NRG1) is a secreted trophic factor that activates the postsynaptic erbB4 receptor tyrosine kinase. Both NRG1 and erbB4 have been repeatedly associated with schizophrenia, but their downstream targets are not well characterized. ErbB4 is highly abundant in interneurons, and NRG1-mediated erbB4 activation has been shown to modulate interneuron function, but the role for NRG1-erbB4 signaling in regulating interneuron dendritic growth is not well understood. Here we show that NRG1/erbB4 promote the growth of dendrites in mature interneurons through kalirin, a major dendritic Rac1-GEF. Recent studies have shown associations of the KALRN gene with schizophrenia. Our data point to an essential role of phosphorylation in kalirin-7's C terminus as the critical site for these effects. As reduced interneuron dendrite length occurs in schizophrenia, understanding how NRG1-erbB4 signaling modulates interneuron dendritic morphogenesis might shed light on disease-related alterations in cortical circuits.

Ligand activation leads to regulated intramembrane proteolysis of fibroblast growth factor receptor 3

FGFR3 is implicated in several human diseases. Following activation and endocytosis, FGFR3 undergoes sequential ectodomain and intramembrane cleavages to generate a soluble cytoplasmic fragment that can translocate to the nucleus.

Fibroblast growth factor receptor 3 (FGFR3) is a major negative regulator of bone growth that inhibits the proliferation and differentiation of growth plate chondrocytes. Activating mutations of its c isoform cause dwarfism in humans; somatic mutations can drive oncogenic transformation in multiple myeloma and bladder cancer. How these distinct activities arise is not clear. FGFR3 was previously shown to undergo proteolytic cleavage in the bovine rib growth plate, but this was not explored further. Here, we show that FGF1 induces regulated intramembrane proteolysis (RIP) of FGFR3. The ectodomain is proteolytically cleaved (S1) in response to ligand-induced receptor activation, but unlike most RIP target proteins, it requires endocytosis and does not involve a metalloproteinase. S1 cleavage generates a C-terminal domain fragment that initially remains anchored in the membrane, is phosphorylated, and is spatially distinct from the intact receptor. Ectodomain cleavage is followed by intramembrane cleavage (S2) to generate a soluble intracellular domain that is released into the cytosol and can translocate to the nucleus. We identify the S1 cleavage site and show that γ-secretase mediates the S2 cleavage event. In this way we demonstrate a mechanism for the nuclear localization of FGFR3 in response to ligand activation, which may occur in both development and disease.

A nuclear variant of ErbB3 receptor tyrosine kinase regulates ezrin distribution and Schwann cell myelination

Reciprocal interactions between glia and neurons are essential for the proper organization and function of the nervous system. Recently, the interaction between ErbB receptors (ErbB2 and ErbB3) on the surface of Schwann cells and neuronal Neuregulin-1 (NRG1) has emerged as the pivotal signal that controls Schwann cell development, association with axons, and myelination. To understand the function of NRG1-ErbB2/3 signaling axis in adult Schwann cell biology, we are studying the specific role of ErbB3 receptor tyrosine kinase (RTK) since it is the receptor for NRG1 on the surface of Schwann cells. Here, we show that alternative transcription initiation results in the formation of a nuclear variant of ErbB3 (nuc-ErbB3) in rat primary Schwann cells. nuc-ErbB3 possesses a functional nuclear localization signal sequence and binds to chromatin. Using chromatin immunoprecipitation (ChIP)-chip arrays, we identified the promoters that associate with nuc-ErbB3 and clustered the active promoters in Schwann cell gene expression. nuc-ErbB3 regulates the transcriptional activity of ezrin and HMGB1 promoters, whereas inhibition of nuc-ErbB3 expression results in reduced myelination and altered distribution of ezrin in the nodes of Ranvier. Finally, we reveal that NRG1 regulates the translation of nuc-ErbB3 in rat Schwann cells. For the first time, to our knowledge, we show that alternative transcription initiation from a gene that encodes a RTK is capable to generate a protein variant of the receptor with a distinct role in molecular and cellular regulation. We propose a new concept for the molecular regulation of myelination through the expression and distinct role of nuc-ErbB3.

Ligand-stimulated VEGFR2 signaling is regulated by co-ordinated trafficking and proteolysis

Vascular endothelial growth factor A (VEGF-A)-induced signaling through VEGF receptor 2 (VEGFR2) regulates both physiological and pathological angiogenesis in mammals. However, the temporal and spatial mechanism underlying VEGFR2-mediated intracellular signaling is not clear. Here, we define a pathway for VEGFR2 trafficking and proteolysis that regulates VEGF-A-stimulated signaling and endothelial cell migration. Ligand-stimulated VEGFR2 activation and ubiquitination preceded proteolysis and cytoplasmic domain removal associated with endosomes. A soluble VEGFR2 cytoplasmic domain fragment displayed tyrosine phosphorylation and activation of downstream intracellular signaling. Perturbation of endocytosis by the depletion of either clathrin heavy chain or an ESCRT-0 subunit caused differential effects on ligand-stimulated VEGFR2 proteolysis and signaling. This novel VEGFR2 proteolysis was blocked by the inhibitors of 26S proteasome activity. Inhibition of proteasome activity prolonged VEGF-A-induced intracellular signaling to c-Akt and endothelial nitric oxide synthase (eNOS). VEGF-A-stimulated endothelial cell migration was dependent on VEGFR2 and VEGFR tyrosine kinase activity. Inhibition of proteasome activity in this assay stimulated VEGF-A-mediated endothelial cell migration. VEGFR2 endocytosis, ubiquitination and proteolysis could also be stimulated by a protein kinase C-dependent pathway. Thus, removal of the VEGFR2 carboxyl terminus linked to phosphorylation, ubiquitination and trafficking is necessary for VEGF-stimulated endothelial signaling and cell migration.

ErbB4 regulates the timely progression of late fetal lung development

The ErbB4 receptor has an important function in fetal lung maturation. Deletion of ErbB4 leads to alveolar hypoplasia and hyperreactive airways similar to the changes in bronchopulmonary dysplasia (BPD). BPD is a chronic pulmonary disorder affecting premature infants as a consequence of lung immaturity, lung damage, and abnormal repair. We hypothesized that proper ErbB4 function is needed for the timely progression of fetal lung development. An ErbB4 transgenic cardiac rescue mouse model was used to study the effect of ErbB4 deletion on fetal lung structure, surfactant protein (SP) expression, and synthesis, and inflammation. Morphometric analyses revealed a delayed structural development with a significant decrease in saccular size at E18 and more pronounced changes at E17, keeping these lungs in the canalicular stage. SP-B mRNA expression was significantly down regulated at E17 with a subsequent decrease in SP-B protein expression at E18. SP-D protein expression was significantly decreased at E18. Surfactant phospholipid synthesis was significantly decreased on both days, and secretion was down regulated at E18. We conclude that pulmonary ErbB4 deletion results in a structural and functional delay in fetal lung development, indicating a crucial regulatory role of ErbB4 in the timely progression of fetal lung development.

MUC1 is a substrate for gamma-secretase

Understanding the underlying mechanisms by which a normal cell avoids the oncogenic potential of MUC1 signaling requires further definition of the pathways by which the MUC1 cytoplasmic tail is processed in both normal and tumor-derived cells. In the present study we describe the processing pathway initiated by TACE/ADAM17 cleavage of MUC1. Utilizing the human uterine epithelial cell line, HES, derived from normal endometrium, we show that endogenous full length MUC1 undergoes regulated intramembranous proteolysis mediated by presenillin-dependent gamma-secretase. Cytokine-stimulated HES cells exposed to gamma-secretase inhibitors accumulated a membrane-associated 15 kDa fragment of the MUC1 C-terminal subunit (CTF15). Inhibitors of TACE/ADAM17-mediated shedding inhibited accumulation of MUC1-CTF15 and MUC1 ectodomain release to a similar extent consistent with MUC1-CTF15 being a product of TACE/ADAM17 action. Reduction of catalytically active gamma-secretase complex by nicastrin siRNA treatment also resulted in CTF15 accumulation. Furthermore, mature nicastrin, the substrate receptor for gamma-secretase, co-immunoprecipitated with CTF15 in the presence of gamma-secretase inhibitors indicating the formation of CTF15: nicastrin complexes. MUC1-CTF15 accumulation in response to gamma-secretase inhibition was demonstrated in both normal and tumor-derived cells from humans and mice indicating that this processing pathway exists in many cell contexts. We did not detect products of MUC1 cleavage by gamma-secretase in the presence of various proteasomal inhibitors indicating that subsequent degradation is either non-proteasomal or extremely efficient. We suggest that this efficient pathway attenuates potential signaling mediated by cytoplasmic tail fragments.

SUMOylation mediates the nuclear translocation and signaling of the IGF-1 receptor

The insulin-like growth factor 1 receptor (IGF-1R) plays crucial roles in developmental and cancer biology. Most of its biological effects have been ascribed to its tyrosine kinase activity, which propagates signaling through the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways. Here, we report that IGF-1 promotes the modification of IGF-1R by small ubiquitin-like modifier protein-1 (SUMO-1) and its translocation to the nucleus. Nuclear IGF-1R associated with enhancer-like elements and increased transcription in reporter assays. The SUMOylation sites of IGF-1R were identified as three evolutionarily conserved lysine residues-Lys(1025), Lys(1100), and Lys(1120)-in the beta subunit of the receptor. Mutation of these SUMO-1 sites abolished the ability of IGF-1R to translocate to the nucleus and activate transcription but did not alter its kinase-dependent signaling. Thus, we demonstrate a SUMOylation-mediated mechanism of IGF-1R signaling that has potential implications for gene regulation.

MMP-9 controls Schwann cell proliferation and phenotypic remodeling via IGF-1 and ErbB receptor-mediated activation of MEK/ERK pathway

Phenotypic remodeling of Schwann cells is required to ensure successful regeneration of damaged peripheral axons. After nerve damage, Schwann cells produce an over 100-fold increase in metalloproteinase-9 (MMP-9), and therapy with an MMP inhibitor increases the number of resident (but not infiltrating) cells in injured nerve. Here, we demonstrate that MMP-9 regulates proliferation and trophic signaling of Schwann cells. Using in vivo BrdU incorporation studies of axotomized sciatic nerves of MMP-9-/- mice, we found increased Schwann cell mitosis in regenerating (proximal) stump relative to wild-type mice. Treatment of cultured primary Schwann cells with recombinant MMP-9 suppressed their growth, mitogenic activity, and produced a dose-dependent, biphasic, and selective activation of ERK1/2, but not JNK and p38 MAPK. MMP-9 induced ERK1/2 signaling in both undifferentiated and differentiated (using dbcAMP) Schwann cells. Using inhibitors to MEK and trophic tyrosine kinase receptors, we established that MMP-9 regulates Ras/Raf/MEK-ERK pathways through IGF-1, ErbB, and PDGF receptors. We also report on the early changes of MMP-9 mRNA expression (within 24 h) after axotomy. These studies establish that MMP-9 controls critical trophic signal transduction pathways and phenotypic remodeling of Schwann cells.

Bidirectional signaling of ErbB and Eph receptors at synapses

Synapse development and remodeling are regulated by a plethora of molecules such as receptor tyrosine kinases (RTKs), a family of cell surface receptors that play critical roles in neural development. Two families of RTKs implicated in synaptic functions, ErbBs and Ephs, share similar characteristics in terms of exhibiting forward and reverse signaling. In this review, we will discuss the latest advances in the functions of ErbBs and Ephs at the synapse, including dendritic spine morphogenesis, synapse formation and maturation, and synaptic transmission and plasticity. In addition to signaling at interneuronal synapses, communication between neuron and glia is increasingly implicated in the control of synaptic functions. Studies on RTKs and their cognate ligands in glial cells enhance our understanding on the nature of 'tripartite synapse'. Implications of these signaling events in human diseases will be discussed.

Deletion of the ectodomain unleashes the transforming, invasive, and tumorigenic potential of the MET oncogene

The c-MET proto-oncogene, encoding the p190 hepatocyte growth factor tyrosine kinase receptor, can acquire oncogenic potential by multiple mechanisms, such as gene rearrangement, amplification and overexpression, point mutation, and ectopic expression, all resulting in its constitutive activation. Hepatocyte growth factor receptor truncated forms are generated by post-translational cleavage: p140 and p130 lack the kinase domain and are inactive. Their C-terminal remnant fragments are generally undetectable in normal cells, but a membrane-associated truncated form is recognized by anti-C-terminus antibodies in some human tumors, suggesting that a hepatocyte growth factor receptor lacking the ectodomain, but retaining the transmembrane and intracellular domains (Met-EC-), could acquire oncogenic properties. Herein we show that NIH-3T3 cells transduced with MET-EC- expressed a membrane-associated constitutively tyrosine-phosphorylated 60-kDa protein and, similarly to NIH-3T3 cells expressing the cytosolic oncoprotein Tpr-Met, showed activated extracellular regulated kinase 1/2 mitogen-activated protein kinase and Akt downstream transducers. Compared to control NIH-3T3 cells, NIH-3T3-Met-EC- cells grew faster and showed anchorage-independent growth and invasive properties in all aspects similar to cells expressing the transforming TPR-MET. Nude female mice injected subcutaneously with NIH-3T3-Met-EC- cells developed visible tumors, displaying the typical morphology of carcinomas with polygonal cells, in contrast to sarcomas with spindle-shaped cells induced by the injection of NIH-3T3-Tpr-Met cells. It is suggested that the different subcellular localization of the oncoproteins, more than differences in signal transduction, could be responsible for the tumor phenotype. All together, these data show that deletion of the ectodomain activates the hepatocyte growth factor receptor and its downstream signaling pathways, unleashing its transforming, invasive, and tumorigenic potential.

The E3 ubiquitin ligase WWP1 selectively targets HER4 and its proteolytically derived signaling isoforms for degradation

In general, epidermal growth factor receptor family members stimulate cell proliferation. In contrast, at least one HER4 isoform, JM-a/Cyt1, inhibits cell growth after undergoing a two-step proteolytic cleavage that first produces a membrane-anchored 80-kDa fragment (m80(HER4)) and subsequently liberates a soluble 80-kDa fragment, s80(HER4). Here we report that s80(HER4) Cyt1 action increased the expression of WWP1 (for WW domain-containing protein 1), an E3 ubiquitin ligase, but not other members of the Nedd4 E3 ligase family. The HER4 Cyt1 isoform contains three proline-rich tyrosine (PY) WW binding motifs, while Cyt2 has only two. WWP1 binds to all three Cyt1 PY motifs; the interaction with PY2 found exclusively in Cyt1 was strongest. WWP1 ubiquitinated and caused the degradation of HER4 but not of EGFR, HER2, or HER3. The HER4-WWP1 interaction also accelerated WWP1 degradation. Membrane HER4 (full length and m80(HER4), the product of the first proteolytic cleavage) were the preferred targets of WWP1, correlating with the membrane localization of WWP1. Conversely s80(HER4), a poorer WWP1 substrate, was found in the cell nucleus, while WWP1 was not. Deletion of the C2 membrane association domain of WWP1 allowed more efficient s80(HER4) degradation, suggesting that WWP1 is normally part of a membrane complex that regulates HER4 membrane species levels, with a predilection for the growth-inhibitory Cyt1 isoform. Finally, WWP1 expression diminished HER4 biologic activity in MCF-7 cells. We previously showed that nuclear s80(HER4) is ubiquitinated and degraded by the anaphase-promoting complex, suggesting that HER4 ubiquitination within specific cellular compartments helps regulate the unique HER4 signaling capabilities.

HER2 oncogenic function escapes EGFR tyrosine kinase inhibitors via activation of alternative HER receptors in breast cancer cells

BACKGROUND

The response rate to EGFR tyrosine kinase inhibitors (TKIs) may be poor and unpredictable in cancer patients with EGFR expression itself being an inadequate response indicator. There is limited understanding of the mechanisms underlying this resistance. Furthermore, although TKIs suppress the growth of HER2-overexpressing breast tumor cells, they do not fully inhibit HER2 oncogenic function at physiological doses.

METHODOLOGY AND PRINCIPAL FINDINGS

Here we have provided a molecular mechanism of how HER2 oncogenic function escapes TKIs' inhibition via alternative HER receptor activation as a result of autocrine ligand release. Using both Förster Resonance Energy Transfer (FRET) which monitors in situ HER receptor phosphorylation as well as classical biochemical analysis, we have shown that the specific tyrosine kinase inhibitors (TKIs) of EGFR, AG1478 and Iressa (Gefitinib) decreased EGFR and HER3 phosphorylation through the inhibition of EGFR/HER3 dimerization. Consequent to this, we demonstrate that cleavage of HER4 and dimerization of HER4/HER2 occur together with reactivation of HER3 via HER2/HER3, leading to persistent HER2 phosphorylation in the now resistant, surviving cells. These drug treatment-induced processes were found to be mediated by the release of ligands including heregulin and betacellulin that activate HER3 and HER4 via HER2. Whereas an anti-betacellulin antibody in combination with Iressa increased the anti-proliferative effect in resistant cells, ligands such as heregulin and betacellulin rendered sensitive SKBR3 cells resistant to Iressa.

CONCLUSIONS AND SIGNIFICANCE

These results demonstrate the role of drug-induced autocrine events leading to the activation of alternative HER receptors in maintaining HER2 phosphorylation and in mediating resistance to EGFR tyrosine kinase inhibitors (TKIs) in breast cancer cells, and hence specify treatment opportunities to overcome resistance in patients.

Specific developmental reductions in subventricular zone ErbB1 and ErbB4 mRNA in the human brain

The primate postnatal subventricular zone (SVZ) lies under the ventrolateral borders of the lateral ventricles as a discrete region of cells with gliogenic and neurogenic capacity regulated by ErbB receptors. However, the specific role of each ErbB subtype in SVZ cell development remains unclear, particularly in the human brain. The postnatal spatial and temporal expression profile of ErbB subtypes in the human brain may provide valuable insight into their distinct functions in the SVZ following birth. Hence, we examined the expression profile of ErbB1, ErbB2, ErbB3 and ErbB4 mRNA in the SVZ of human postmortem brains from neonates, infants, toddlers, school age subjects, adolescents, young adults and adults using in situ hybridization. SVZ transcript levels of ErbB1 and ErbB4 were highest in neonates and diminished with age. SVZ ErbB4 mRNA quantities significantly decreased by >85% to almost undetectable levels after the first year of life, while SVZ ErbB1 transcript levels displayed more gradual reductions, stabilizing to approximately 30-40% of neonate levels after the age of 5 years. In the neonate and infant SVZ, ErbB4 mRNA was localized to cell clusters resembling migratory neuroblast aggregates whereas ErbB1 mRNA was expressed in cells along but not within these clusters. ErbB2 mRNA appeared to be constantly expressed in the human SVZ at all postnatal ages as opposed to ErbB3 transcripts, which were not detected in the human SVZ at any age following birth. These findings suggest that ErbB1 and ErbB4 may play more salient roles than ErbB2 and ErbB3 in mediating early postnatal neurodevelopmental events. In addition, ErbB1- and ErbB4-immunoreactive cells and fibers were extensive throughout the human infant SVZ, but did not appear to overlap with PSA-NCAM-immunopositive clusters. The restriction of robust SVZ ErbB4 expression to neonate and infant age groups may indicate that SVZ-derived ErbB4-dependent postnatal neuronal development is most extensive within a narrow time frame early after birth.

Met, the hepatocyte growth factor receptor, localizes to the nucleus in cells at low density

Some breast cancer cases in our previous immunohistochemical studies show Met expression in the nucleus. Given nuclear localization of other receptor tyrosine kinases, we proceeded to investigate Met. Nuclear Met is seen in numerous cell lines and in germinal regions of many tissues using four unique antibodies. Cell fractionation reveals a 60-kDa band recognized by COOH-terminal Met antibodies that is present independent of hepatocyte growth factor treatment. Green fluorescent protein (GFP) fusion proteins of the cytoplasmic domain of Met transfected into HEK293 cells are found in the nucleus whereas the full-length Met-GFP fusion is membranous. Further deletions of the Met-GFP fusions identify a region of the juxtamembrane domain required for nuclear translocation. In a CaCo2 cell line model for epithelial maturation, we find that Met is initially nuclear, and then becomes membranous, after confluence. This work suggests processing of the Met receptor, analogous to ErbB4, resulting in the release of the cytoplasmic domain and its translocation to the nucleus in cells at low density.

ErbB4 isoforms selectively regulate growth factor induced Madin-Darby canine kidney cell tubulogenesis

ErbB4, a member of the epidermal growth factor (EGF) receptor family that can be activated by heregulin beta1 and heparin binding (HB)-EGF, is expressed as alternatively spliced isoforms characterized by variant extracellular juxtamembrane (JM) and intracellular cytoplasmic (CYT) domains. ErbB4 plays a critical role in cardiac and neural development. We demonstrated that ErbB4 is expressed in the ureteric buds and developing tubules of embryonic rat kidney and in collecting ducts in adult. The predominant isoforms expressed in kidney are JM-a and CYT-2. In ErbB4-transfected MDCK II cells, basal cell proliferation and hepatocyte growth factor (HGF)-induced tubule formation were decreased by all four isoforms. Only JM-a/CYT-2 cells formed tubules upon HB-EGF stimulation. ErbB4 was activated by both HRG-beta1 and HB-EGF stimulation; however, compared with HRG-beta1, HB-EGF induced phosphorylation of the 80-kDa cytoplasmic cleavage fragment of the JM-a/CYT-2 isoform. HB-EGF also induced early activation of ERK1/2 in JM-a/CYT-2 cells and promoted nuclear translocation of the JM-a/CYT-2 cytoplasmic tail. In summary, our data indicate that JM-a/CYT-2, the ErbB4 isoform that is proteinase cleavable but does not contain a PI3K-binding domain in its cytoplasmic tail, mediates important functions in renal epithelial cells in response to HB-EGF.

Regulated proteolytic processing of Tie1 modulates ligand responsiveness of the receptor-tyrosine kinase Tie2

Regulated ectodomain shedding followed by intramembrane proteolysis has recently been recognized as important in cell signaling and for degradation of several type I transmembrane proteins. The receptor-tyrosine kinase Tie1 is known to undergo ectodomain cleavage generating a membrane-tethered endodomain. Here we show Tie1 is a substrate for regulated intramembrane proteolysis. After Tie1 ectodomain cleavage the newly formed 45-kDa endodomain undergoes additional proteolytic processing mediated by gamma-secretase to generate an amino-terminal-truncated 42-kDa fragment that is subsequently degraded by proteasomal activity. This sequential processing occurs constitutively and is stimulated by phorbol ester and vascular endothelial growth factor. To assess the biological significance of regulated Tie1 processing, we analyzed its effects on angiopoietin signaling. Activation of ectodomain cleavage causes loss of phosphorylated Tie1 holoreceptor and generation of phosphorylated receptor fragments in the presence of cartilage oligomeric protein angiopoietin 1. A key function of gamma-secretase is in preventing accumulation of these phosphorylated fragments. We also find that regulated Tie1 processing modulates ligand responsiveness of the Tie-1-associated receptor Tie2. Activation of Tie1 ectodomain cleavage increases cartilage oligomeric protein angiopoietin 1 activation of Tie2. This correlates with increased ability of Tie2 to bind ligand after shedding of the Tie1 extracellular domain. A similar enhancement of ligand activation of Tie2 is seen when Tie1 expression is suppressed by RNA interference. Together these data indicate that Tie1, via its extracellular domain, limits the ability of ligand to bind and activate Tie2. Furthermore the data suggest that regulated processing of Tie1 may be an important mechanism for controlling signaling by Tie2.

Matrix metalloproteinase 7 mediates mammary epithelial cell tumorigenesis through the ErbB4 receptor

To delineate the role of matrix metalloproteinase 7 (MMP7) in mammary tumorigenesis, MMP7 was expressed in the normal murine mammary gland cell line, c57MG. MMP7 markedly enhanced the growth rate of the c57MG cells in three-dimensional culture and promoted tumor formation in vivo. Subsequent investigation showed that MMP7 (a) up-regulated ErbB4 receptor levels, (b) solubilized the ErbB4 receptor cognate ligand heparin-bound epidermal growth factor, and (c) mediated the proteolytic processing of ErbB4 to yield a soluble intracellular domain (ICD) that localized to the cytoplasm and the nucleus. Furthermore, overexpression of the ErbB4 ICD in the c57MG cell line recapitulated the proliferative effects of MMP7 in vitro and in vivo. These data indicate a novel mechanism for mammary epithelial cell transformation by MMP7.

Bone microenvironment and androgen status modulate subcellular localization of ErbB3 in prostate cancer cells

ErbB-3, an ErbB receptor tyrosine kinase, has been implicated in the pathogenesis of several malignancies, including prostate cancer. We found that ErbB-3 expression was up-regulated in prostate cancer cells within lymph node and bone metastases. Despite being a plasma membrane protein, ErbB-3 was also detected in the nuclei of the prostate cancer cells in the metastatic specimens. Because most metastatic specimens were from men who had undergone androgen ablation, we examined the primary tumors from patients who have undergone hormone deprivation therapy and found that a significant fraction of these specimens showed nuclear localization of ErbB3. We thus assessed the effect of androgens and the bone microenvironment on the nuclear translocation of ErbB-3 by using xenograft tumor models generated from bone-derived prostate cancer cell lines, MDA PCa 2b, and PC-3. In subcutaneous tumors, ErbB-3 was predominantly in the membrane/cytoplasm; however, it was present in the nuclei of the tumor cells in the femur. Castration of mice bearing subcutaneous MDA PCa 2b tumors induced a transient nuclear translocation of ErbB-3, with relocalization to the membrane/cytoplasm upon tumor recurrence. These findings suggest that the bone microenvironment and androgen status influence the subcellular localization of ErbB-3 in prostate cancer cells. We speculate that nuclear localization of ErbB-3 may aid prostate cancer cell survival during androgen ablation and progression of prostate cancer in bone.

The E3 ligase Aip4/Itch ubiquitinates and targets ErbB‐4 for degradation

The ErbB-4 receptors are unique in the EGFR/ErbB family for the ability to associate with WW domain-containing proteins. To identify new ligands of the cytoplasmic tail of ErbB-4, we panned a brain cDNA phage library with ErbB-4 peptides containing sequence motifs corresponding to putative docking sites for class-I WW domains. This approach led to identification of AIP4/Itch, a member of the Nedd4-like family of E3 ubiquitin protein ligases, as a protein that specifically interacts with and ubiquitinates ErbB-4 in vivo. Interaction with the ErbB-4 receptors occurs via the WW domains of AIP4/Itch. Functional analyses demonstrate that AIP4/Itch is recruited to the ErbB-4 receptor to promote its polyubiquitination and degradation, thereby regulating stability of the receptor and access of receptor intracellular domains to the nuclear compartment. These findings expand our understanding of the mechanisms contributing to the integrity of the ErbB signaling network and mechanistically link the cellular ubiquitination pathway of AIP4/Itch to the ErbB-4 receptor.

Regulation of ErbB-4 endocytosis by neuregulin in GABAergic hippocampal interneurons

Neuregulin (NRG)/ErbB receptor signaling pathways have recently been implicated in the reversal of long-term potentiation at hippocampal glutamatergic synapses. Moreover, polymorphisms in NRG-1 and ErbB-4 genes have been linked to an increased risk for developing schizophrenia. ErbB-4 is highly expressed at glutamatergic synapses where it binds to PSD-95 via its carboxyl terminal T-V-V sequence. Here we investigated the expression, localization and trafficking of ErbB-4 in cultured hippocampal neurons by immunocytochemistry, surface protein biotinylation, and live labeling of native receptors. We show that neuronal ErbB-4 is detected at its highest levels in GABAergic interneurons, as observed in vivo. ErbB-4 immunoreactivity precedes PSD-95 expression, with ErbB-4 cluster initially forming in the absence of, but later associating with, PSD-95-positive puncta. By surface protein biotinylation, the fraction of ErbB-4 receptors on the plasma membrane increases from 30% to 65% between 6 and 16 days in vitro (DIV). Interestingly, 30 min of NRG stimulation triggers measurable ErbB-4 receptor internalization at DIV 16, despite increased colocalization with PSD-95. We also investigated the role of TNFalpha-converting enzyme (TACE)-mediated receptor processing in regulating ErbB-4 surface expression. We found that the cleavage-resistant JM-b isoform accounts for 80% of all ErbB-4 transcripts in cultured hippocampal neurons. Receptor stimulation or treatment with phorbol esters does not induce detectable ErbB-4 processing, indicating that neurons mostly rely on endocytosis of the intact receptor to regulate ErbB-4 surface expression. These results enhance our understanding of the regulation of ErbB-4--mediated signaling at glutamatergic synapses.

Disease-associated intronic variants in the ErbB4 gene are related to altered ErbB4 splice-variant expression in the brain in schizophrenia

The neuregulin 1 (NRG1) receptor, ErbB4, has been identified as a potential risk gene for schizophrenia. HER4/ErbB4 is a receptor tyrosine kinase whose transcript undergoes alternative splicing in the brain. Exon 16 encodes isoforms containing a metalloprotease cleavable extracellular domain (JM-a), exon 15 for a cleavage resistant form (JM-b) and exon 26 for a cytoplasmic domain (CYT-1) with a phosphotidylinositol-3 kinase (PI3K) binding site. Disease-associated variants in the ErbB4 gene are intronic and implicate altered splicing of the gene. We examined ErbB4 splice-variant gene expression in the hippocampus and dorsolateral prefrontal cortex (DLPFC) in schizophrenia using qPCR and investigated whether expression levels are associated with previously reported genomic risk variants in ErbB4 in a large cohort of human brains. In the DLPFC, we confirmed previous observations, in a separate cohort, that mRNA for ErbB4 splice isoforms containing exon 16 (JM-a) and exon 26 (CYT-1) are significantly elevated in patients with schizophrenia. A main effect of genotype was observed in the DLPFC and hippocampus at a single risk SNP located in intron 12 (rs4673628) on isoforms containing exon 16 (JM-a). We also found that three intronic risk SNPs (rs7598440, rs707284, rs839523) and a core-risk haplotype surrounding exon 3 are strongly associated with elevated expression of splice variants containing exon 26 (CYT-1). These findings suggest that dysregulation of splice-variant specific expression of ErbB4 in the brain underlies the genetic association of the gene with schizophrenia and that the NRG1/ErbB4 signaling pathway may be an important genetic network involved in the pathogenesis of the disease.

Shedding of a soluble form of BMP receptor-IB controls bone cell responses to BMP

Bone morphogenetic proteins (BMP) are members of the transforming growth factor beta (TGF-beta) superfamily and are involved in a wide variety of biological processes, including osteoblast differentiation and bone healing. The activities of the BMP are mediated by signal transduction via three BMP receptors (BMPR-IA, -IB and -II), which are thus essential for the biological actions of the BMP. Although the precise mechanisms which control the BMPR are not yet known, it is possible that post-translational regulation of these cell surface antigens by shedding could modulate their expression and thereby at least partly determine the response of the cells to the BMP. To test this possibility, the present study has examined whether soluble forms of the BMPR are produced by shedding from primary human bone cells in vitro. The results showed that human bone cells expressed both mRNA transcripts and antigens corresponding to BMPR-IA, -IB and -II. Incubation of the cells with phorbol 12-myristate 13-acetate (PMA), a potent inducer of proteolytic shedding, resulted in a pronounced decrease in cell surface expression of all three BMPR and, concurrently, the presence of "soluble" forms of these antigens in culture supernatants. Moreover, PMA treatment significantly reduced the level of BMP-2-induced Smad1/5 phosphorylation, a major early activation step in signal transduction initiated by BMP/BMPR interaction. It is notable that, while treatment of bone cells with interleukin-1beta (IL-1beta) also reduced the level of surface BMPR-IB, this inflammatory cytokine had no effect on BMPR-IA or -II levels, hence only the soluble form of BMPR-IB was detected. Furthermore, in addition to down-regulating BMP-2-induced Smad1/5 phosphorylation, IL-1beta also caused a reduction in the level of BMP-2-induced alkaline phosphatase activity and osteocalcin expression, both closely associated with bone cell differentiation. In conclusion, our study has provided evidence, for the first time, that BMPR can be modulated at the cell surface by the shedding of a soluble form of the antigen, resulting in a markedly diminished response to BMP-2 in vitro.

(Make) stick and cut loose--disintegrin metalloproteases in development and disease

"A disintegrin and metalloprotease" (ADAM) proteases form a still growing family of about 40 type 1 transmembrane proteins. They are defined by a common modular ectodomain architecture that combines cell deadhesion/adhesion and fusion motifs (disintegrin and cysteine-rich domains), with a Zn-protease domain capped by a large prodomain. Their ectodomain thus strikingly resembles snake venom disintegrin proteases, which by combined integrin blocking and extracellular proteolysis, can cause extensive tissue damage after snake bites. A surprisingly large proportion (13 ADAMs) is exclusively expressed in the male gonads, and only a minority can be found throughout all tissues. As predicted by their amino acid sequence, a major proportion of this family has not maintained a functional protease domain, most probably rendering them into pure adhesion and/or fusion proteins. For most ADAMs, the respective key function has remained elusive. Despite their overall conserved ectodomain structure, ADAMs appear to be subdivided into those with a predominant role in direct adhesion (e.g., ADAMs 1, 2, and 3) and those mainly acting as proteases (e.g., ADAMs 10 and 17). Only for a few of them are functions of more than one domain documented (e.g., ADAM9 in cell fusion and proteolysis). Several ADAMs exist in both membrane-resident and secreted isoforms; the functional significance of this dichotomy is in most cases still unclear. Knockout phenotypes have been informative only in a few cases (ADAMs 1, 2, 10, 12, 15, 17, and 19) and are mainly related to their protease function. A common denominator of ADAM-mediated proteolysis is the ectodomain shedding of a broad spectrum of substrates, including paracrine growth factors like epidermal growth factor receptor (EGFR) ligands, cell adhesion molecules like CD44 or cadherins, and the initiation of regulated intramembrane proteolysis (RIP), whereby the transmembrane fragment of the respective substrate is further cleaved by an intramembrane cleaving protease to release an intracellular domain acting as a nuclear transcription regulator. Most ADAMs feature a significant overlap of substrate specificities, explaining why an inactivation of individual ADAMs only rarely causes major phenotypes.

Metalloproteinase/Presenilin1 processing of ephrinB regulates EphB-induced Src phosphorylation and signaling

Bidirectional signaling triggered by interacting ephrinB receptors (EphB) and ephrinB ligands is crucial for development and function of the vascular and nervous systems. A signaling cascade triggered by this interaction involves activation of Src kinase and phosphorylation of ephrinB. The mechanism, however, by which EphB activates Src in the ephrinB-expressing cells is unknown. Here we show that EphB stimulates a metalloproteinase cleavage of ephrinB2, producing a carboxy-terminal fragment that is further processed by PS1/gamma-secretase to produce intracellular peptide ephrinB2/CTF2. This peptide binds Src and inhibits its association with inhibitory kinase Csk, allowing autophosphorylation of Src at residue tyr418. EphrinB2/CTF2-activated Src phosphorylates ephrinB2 and inhibits its processing by gamma-secretase. These data show that the PS1/gamma-secretase system controls Src activation and ephrinB phosphorylation by regulating production of Src activator ephrinB2/CTF2. Accordingly, gamma-secretase inhibitors prevented the EphB-induced sprouting of endothelial cells and the recruitment of Grb4 to ephrinB. PS1 FAD and gamma-secretase dominant-negative mutants inhibited the EphB-induced cleavage of ephrinB2 and Src autophosphorylation, raising the possibility that FAD mutants interfere with the functions of Src and ephrinB2 in the CNS.

The involvement of ErbB4 with schizophrenia: association and expression studies

Neuregulin 1 (NRG1) has been found to be associated with schizophrenia in several populations. Consistently, mutant mice heterozygous for either NRG1 or its receptor, ErbB4, show a behavioral phenotype that overlaps with mouse models for schizophrenia. These observations raised the hypothesis that impaired NRG1-ErbB4 signaling may contribute to schizophrenia susceptibility. Nineteen SNPs encompassing the ErbB4 gene were selected from the HapMap database and genotyped in genomic DNA isolated from 59 Ashkenazi schizophrenia patients and 130 matched controls. Expression analysis of ErbB4 splice variants was performed on postmortem DLPFC samples obtained from Caucasian patients and controls by real-time PCR. We found a highly significant difference between patient and control groups in three SNPs from one linkage disequilibrium (LD) block both in allele (P = 0.013, 0.0045, 0.0049) and genotype frequencies (P = 0.00013, 0.000021, 0.00018), as well as a risk haplotype (P = 0.00044). Expression analysis indicated that the CYT-1 isoform is overexpressed in patients (P = 0.047) and that juxtamembrane (JM)-a displays a similar trend (P = 0.081). This study provides a direct link between ErbB4 and the disease. We propose that NRG1 and its receptor ErbB4 are components of a biological pathway, involved in the pathophysiology of schizophrenia.

Differential expression of neuregulins and their receptors in the olfactory bulb layers of the developing mouse

Neuregulins (NRGs), and their cognate receptors (ErbBs), play essential roles in numerous aspects of neural development and adult synaptic plasticity. The goal of this study was to investigate the developmental expression profiles of these molecules during the olfactory bulb (OB) maturation. The OB is a highly organized structure with cell types and synaptic connections segregated into discrete anatomical layers. We employed a novel approach by combining single-layer microdissection at different development ages, with isoform-specific semi-quantitative RT-PCR and Western blotting to monitor layer-specific developmental profiles of these molecules and alternate splice variants. Layer and age specific regulation was observed for the ErbB4 splice variants JMa/JMb and NRG-1-beta1/beta2 forms. With the exception of the outermost (nerve) layer, ErbB4-JMb and NRG-1-beta1 are expressed throughout the OB and their expressions decrease in the adult age in most layers. In contrast both ErbB4-JMa and NRG-1-beta2 are highly expressed in the granule cell layer in the early postnatal OB. This early postnatal expression correlates with the dramatic change from radial glia to astrocytes and appearance of the bulk of granule cells occurring at this developmental stage.

Expression of the ErbB4 receptor causes reversal regulation of PP2A in the Shc signal transduction pathway in human cancer cells

Expression of ErbB4 receptor is correlated with the incidence of non-metastatic types of human cancers, whereas the overexpression of other ErbB receptor families (ErbB1/EGFR, ErbB2 and ErbB3) is correlated to the formation of metastatic tumors. However, the molecular mechanism underlying this phenomenon has been unclear. Earlier, we demonstrated that okadaic acid (OA), an inhibitor of a serine/threonine phosphatase PP2A, stimulated the growth hormone-induced ERK phosphorylation in the wild type Chinese hamster ovary (CHO) cells and the cells expressing ErbB1 receptor, but suppressed ERK activation in CHO cells that express ErbB4 receptor. PP2A had been understood as a negative regulator of the growth hormone-stimulated signal transduction pathways, however, this observation suggested that expression of ErbB4 receptor reversed the regulation of PP2A in the ErbB4 signalling pathway. In this study, we found that OA suppressed phosphorylation of Shc at Tyr317, therefore it down-regulated ERK phosphorylation in the ErbB4 expressing CHO cells. Accordingly, basal PP2A contributed to the phosphorylation of Shc Tyr317 in ErbB4 expressing CHO cells, nevertheless it had been reported that PP2A negatively regulates Shc tyrosine phosphorylation in the EGF- or IGF-I-induced signalling pathways. By testing OA for human cancer cell lines that express different types of ErbB receptors, we found that ErbB4 receptor expression was accompanied with positive regulation of PP2A for phosphorylation of Shc Tyr317 and its downstream ERK phosphorylation in MCF-7 and SK-OV-3 cell lines, but not in LNCaP and PC-3 cells. Thus, PP2A regulates the ERK activity in a cell-specific manner, and it is speculated that distinct regulation of PP2A in the ErbB4 receptor signalling pathway may cause a difference in progression of cancer phenotypes.

The ErbB-4 s80 intracellular domain is a constitutively active tyrosine kinase

The ErbB-4 receptor tyrosine kinase homo- and heterodimerizes following heregulin binding, which provokes increased levels of tyrosine autophosphorylation. Unique to the ErbB family, ErbB-4 is then proteolytically cleaved by alpha- and gamma-secretase to produce an 80 kDa intracellular domain (s80 ICD) fragment. This fragment is found in both the cytoplasm and nucleus of many normal and cancer cells and can interact with transcription factors in the cytoplasm and nucleus. Since the s80 ICD lacks ectodomain sequences known to play a major role in dimerization of ErbB family members, we asked whether the s80 ICD is an active tyrosine kinase. Here, we demonstrate that the s80 ICD is a constitutively active tyrosine kinase and can form homodimers. The s80 ICD is autophosphorylated in cells and can phosphorylate an exogenous substrate in vitro. Also, the s80 ICD can coassociate and dimers are detected by chemical crosslinking. This is the first example of constitutive kinase activation and dimerization totally within the cytoplasmic domain of an ErbB receptor and suggests that the s80 ICD may function to phosphorylate substrates in the cytoplasm or nucleus.

Bone morphogenetic protein receptors and bone morphogenetic protein signaling are controlled by tumor necrosis factor-α in human bone cells

Bone morphogenetic proteins (BMP) stimulate osteoblast differentiation by signal transduction via three BMP receptors (BMPR-IA, -IB and -II), whereas the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) has been shown to suppress osteoblast differentiation. Although the mechanisms which regulate the BMPR are not yet known, it is possible that they may be negatively controlled by TNF-alpha, thereby inhibiting BMP-induced osteoblast differentiation. To test this hypothesis, we have examined the effects of TNF-alpha on BMPR-IA, -IB and -II expression and the functional consequences of this cytokine on BMPR-mediated functions in human bone cells. The results showed that although TNF-alpha down-regulated BMPR-IA and -II transcripts, it increased the level of BMPR-IB mRNA via a MAPK-dependent pathway. In marked contrast, however, TNF-alpha nevertheless caused marked down-regulation of the expression of the BMPR-IB surface antigen specifically. Moreover, the cytokine-induced decrease in BMPR-IB expression was found to be associated with the concurrent presence of a 'soluble' form of this antigen in supernatants of TNF-alpha-treated cultures. Furthermore, the TNF-alpha-induced loss of BMPR-IB was found to ablate BMP-2-stimulated bone cell functions, including phosphorylation of Smad1/5/8, alkaline phosphatase activity and osteocalcin expression. In conclusion, our study has provided evidence, for the first time, that BMPR can be differentially modulated by TNF-alpha at both the post-transcriptional and post-translational levels, with the TNF-alpha-induced shedding of the BMPR-IB antigen associated with a significantly diminished response to BMP-2 in vitro.

Proteolytic cleavage and phosphorylation of a tumor-associated ErbB4 isoform promote ligand-independent survival and cancer cell growth

The ErbB1 and ErbB2 receptors are oncogenes with therapeutic significance in human cancer, whereas the transforming potential of the related ErbB4 receptor has remained controversial. Here, we have addressed whether four alternatively spliced ErbB4 isoforms differ in regulating cellular responses relevant for tumor growth. We show that the two tumor necrosis factor-alpha converting enzyme (TACE)-cleavable ErbB4 isoforms (the juxtamembrane [JM]-a isoforms) were overexpressed in a subset of primary human breast cancers together with TACE. The overexpression of the JM-a cytoplasmic (CYT)-2 ErbB4 isoform promoted ErbB4 phosphorylation, survival of interleukin-3-dependent cells, and proliferation of breast cancer cells even in the absence of ligand stimulation, whereas activation of the other three ErbB4 isoforms required ligand stimulation. Ligand-independent cellular responses to ErbB4 JM-a CYT-2 overexpression were regulated by both tyrosine kinase activity and a two-step proteolytic generation of an intracellular receptor fragment involving first a TACE-like proteinase, followed by gamma-secretase activity. These data suggest a novel transforming mechanism for the ErbB4 receptor in human breast cancer that is 1) specific for a single receptor isoform and 2) depends on proteinase cleavage and kinase activity but not ligand activation of the receptor.

ADAM proteases, ErbB pathways and cancer

A disintegrin and metalloproteases (ADAMs) are zinc-dependent trans-membrane metalloproteases that shed the extracellular domains of membrane-bound growth factors, cytokines and receptors. Key functions of ADAMs have emerged in ErbB signalling pathways as being sheddases for multiple ErbB ligands. As the ErbB pathway is a validated target for anti-cancer drugs, the upstream activators of ErbB ligands, their sheddases, now enter the spotlight as new drug targets in the ErbB pathway. ADAMs are involved not only in tumour cell proliferation but also in angiogenesis and metastasis. Therefore, strategies targeting ADAMs might be an important complement to existing anti-ErbB approaches.