Transmembrane TGF-alpha precursors activate EGF/TGF-alpha receptors

TGF-α/EGFR signaling promotes lipopolysaccharide-induced abnormal elastin deposition and alveolar simplification

Bronchopulmonary dysplasia (BPD) is characterized by shortened secondary septa and fewer, larger alveoli. Elastin deposition to the distal tips of the secondary septa is critical for elongation of the secondary septa. Alveolar myofibroblasts, which are thought to migrate to the septal tips during alveolarization, are mainly responsible for elastin production and deposition. Antenatal exposure to inflammation induces abnormal elastin deposition, thereby increasing the risk of developing BPD. Here, we found that lipopolysaccharide (LPS) significantly increased the expression of transforming growth factor-α (TGF-α) in an LPS-induced rat model of BPD and in LPS-treated human pulmonary epithelial cells (BEAS-2B). In addition, in vitro experiments suggested that LPS upregulated TGF-α expression via toll-like receptor 4 (TLR4)/tumor necrosis factor α-converting enzyme (TACE) signaling. Increased TGF-α levels via its receptor epidermal growth factor receptor (EGFR)-induced lysyl oxidase (LOX) overactivation and cell division cycle 42 (Cdc42) activity inhibition of myofibroblasts. Similarly, in vivo LOX overactivation and inhibition of Cdc42 activity were observed in the lungs of LPS-exposed pups. LOX overactivation led to abnormal elastin deposition, and inhibition of Cdc42 activity disturbed the directional migration of myofibroblasts and disrupted elastin localization. Most importantly, the EGFR inhibitor erlotinib partially rescued LOX overactivation and Cdc42 activity inhibition, and improved elastin deposition and alveolar development in antenatal LPS-treated rats. Taken together, our data suggest that TGF-α/EGFR signaling is critically involved in the regulation of elastin deposition and represents a novel therapeutic target.

ADAM17: An Emerging Therapeutic Target for Lung Cancer

Lung cancer is the leading cause of cancer-related mortality, which histologically is classified into small-cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). NSCLC accounts for approximately 85% of all lung cancer diagnoses, with the majority of patients presenting with lung adenocarcinoma (LAC). KRAS mutations are a major driver of LAC, and are closely related to cigarette smoking, unlike mutations in the epidermal growth factor receptor (EGFR) which arise in never-smokers. Although the past two decades have seen fundamental progress in the treatment and diagnosis of NSCLC, NSCLC still is predominantly diagnosed at an advanced stage when therapeutic interventions are mostly palliative. A disintegrin and metalloproteinase 17 (ADAM17), also known as tumour necrosis factor-α (TNFα)-converting enzyme (TACE), is responsible for the protease-driven shedding of more than 70 membrane-tethered cytokines, growth factors and cell surface receptors. Among these, the soluble interleukin-6 receptor (sIL-6R), which drives pro-inflammatory and pro-tumourigenic IL-6 trans-signaling, along with several EGFR family ligands, are the best characterised. This large repertoire of substrates processed by ADAM17 places it as a pivotal orchestrator of a myriad of physiological and pathological processes associated with the initiation and/or progression of cancer, such as cell proliferation, survival, regeneration, differentiation and inflammation. In this review, we discuss recent research implicating ADAM17 as a key player in the development of LAC, and highlight the potential of ADAM17 inhibition as a promising therapeutic strategy to tackle this deadly malignancy.

Dynamics of Axl Receptor Shedding in Hepatocellular Carcinoma and Its Implication for Theranostics

Signaling of the receptor tyrosine kinase Axl and its ligand Gas6 is crucially involved in the development of liver fibrosis and hepatocellular carcinoma (HCC) by activation of hepatic stellate cells and modulation of hepatocyte differentiation. Shedding of Axl’s ectodomain leads to the release of soluble Axl (sAxl), which is increased in advanced fibrosis and in early-to-late stage HCC in the presence and absence of cirrhosis. Here, we focus on the dynamics of Axl receptor shedding and delineate possible scenarios how Axl signaling might act as driver of fibrosis progression and HCC development. Based on experimental and clinical data, we discuss the consequences of modifying Axl signaling by sAxl cleavage, as well as cellular strategies to escape from antagonizing effects of Axl shedding by the involvement of the hepatic microenvironment. We emphasize a correlation between free Gas6 and free sAxl levels favoring abundant Gas6/Axl signaling in advanced fibrosis and HCC. The raised scenario provides a solid basis for theranostics allowing the use of sAxl as an accurate diagnostic biomarker of liver cirrhosis and HCC, as well as Axl receptor signaling for therapeutic intervention in stratified HCC patients.

Co-culture Activation of MAP Kinase in Drosophila S2 Cells

Intercellular communication often involves phosphorylation of signal transduction proteins, including mitogen-activated protein kinases (MAPKs). Immunological detection of phosphorylated MAPK can be used to monitor signaling in vivo, identify novel pathway components, and assess ligand activity. In this chapter, I describe a cell co-culture method to assess activity of cell-bound extracellular ligands that result in phosphorylation of the ERK (extracellular signal-regulated kinase) MAPK in Drosophila. This protocol may be adaptable to other pathways and/or model systems.

Tooth agenesis and orofacial clefting: genetic brothers in arms?

Tooth agenesis and orofacial clefts represent the most common developmental anomalies and their co-occurrence is often reported in patients as well in animal models. The aim of the present systematic review is to thoroughly investigate the current literature (PubMed, EMBASE) to identify the genes and genomic loci contributing to syndromic or non-syndromic co-occurrence of tooth agenesis and orofacial clefts, to gain insight into the molecular mechanisms underlying their dual involvement in the development of teeth and facial primordia. Altogether, 84 articles including phenotype and genotype description provided 9 genomic loci and 26 gene candidates underlying the co-occurrence of the two congenital defects: MSX1, PAX9, IRF6, TP63, KMT2D, KDM6A, SATB2, TBX22, TGFα, TGFβ3, TGFβR1, TGFβR2, FGF8, FGFR1, KISS1R, WNT3, WNT5A, CDH1, CHD7, AXIN2, TWIST1, BCOR, OFD1, PTCH1, PITX2, and PVRL1. The molecular pathways, cellular functions, tissue-specific expression and disease association were investigated using publicly accessible databases (EntrezGene, UniProt, OMIM). The Gene Ontology terms of the biological processes mediated by the candidate genes were used to cluster them using the GOTermMapper (Lewis-Sigler Institute, Princeton University), speculating on six super-clusters: (a) anatomical development, (b) cell division, growth and motility, (c) cell metabolism and catabolism, (d) cell transport, (e) cell structure organization and (f) organ/system-specific processes. This review aims to increase the knowledge on the mechanisms underlying the co-occurrence of tooth agenesis and orofacial clefts, to pave the way for improving targeted (prenatal) molecular diagnosis and finally to reflect on therapeutic or ultimately preventive strategies for these disabling conditions in the future.

TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with loss of a C-peptide region. The potential pathophysiological importance of TSHR cleavage into A- and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling.

Withdrawn: TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with lossofaC-peptideregion. The potential pathophysiological importance of TSHR cleavage into A-and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling. (Endocrine Reviews 37: 114-134, 2016).

Illuminating breast cancer invasion: diverse roles for cell-cell interactions

Metastasis begins when tumors invade into surrounding tissues. In breast cancer, the study of cell interactions has provided fundamental insights into this complex process. Powerful intravital and 3D organoid culture systems have emerged that enable biologists to model the complexity of cell interactions during cancer invasion in real-time. Recent studies utilizing these techniques reveal distinct mechanisms through which multiple cancer cell and stromal cell subpopulations interact, including paracrine signaling, direct cell-cell adhesion, and remodeling of the extracellular matrix. Three cell interaction mechanisms have emerged to explain how breast tumors become invasive: epithelial-mesenchymal transition, collective invasion, and the macrophage-tumor cell feedback loop. Future work is needed to distinguish whether these mechanisms are mutually exclusive or whether they cooperate to drive metastasis.

From wavy hair to naked proteins: the role of transforming growth factor alpha in health and disease

Since its discovery in 1978 and cloning in 1984, transforming growth factor-alpha (TGF-α, TGFA) has been one of the most extensively studied EGF receptor (EGFR) ligands. In this review, we provide a historical perspective on TGFA-related studies, highlighting what we consider important advances related to its function in normal and disease states.

Trafficking of the EGFR ligand Spitz regulates its signaling activity in polarized tissues

Epidermal growth factor receptor (EGFR) ligands undergo a complex series of processing events during their maturation to active signaling proteins. Like its mammalian homologs, the predominant Drosophila EGFR ligand Spitz is produced as a transmembrane pro-protein. In the secretory pathway, Spitz is cleaved within its transmembrane domain to release the extracellular signaling domain. This domain is modified with an N-terminal palmitate group that tethers it to the plasma membrane. We found that the pro-protein can reach the cell surface in the absence of proteolysis, but that it fails to activate the EGFR. To address why the transmembrane pro-protein is inactive, whereas membrane association through the palmitate group promotes activity, we generated a panel of chimeric constructs containing the Spitz extracellular region fused to exogenous transmembrane proteins. Although the orientation of the EGF domain and its distance from the plasma membrane varies in these chimeras, they are all active in vivo. Thus, tethering Spitz to the membrane via a transmembrane domain at either terminus does not prevent activity. Conversely, removing the N-terminal palmitate group from the C-terminally tethered pro-protein does not render it active. Furthermore, we show that the Spitz transmembrane pro-protein can activate the EGFR in a tissue culture assay, indicating that its failure to signal in vivo is not due to structural features. In polarized imaginal disc cells, unprocessed Spitz pro-protein localizes to apical puncta, whereas the active chimeric Spitz constructs are basolaterally localized. Taken together, our data support the model that localized trafficking of the pro-protein restricts its ability to activate the receptor in polarized tissues.

A comparison of the behavioral and anatomical outcomes in sub-acute and chronic spinal cord injury models following treatment with human mesenchymal precursor cell transplantation and recombinant decorin. Exp Neurol. 2013;248:343-359. [PMID: 23867131 DOI: 10.1016/j.expneurol.2013.06.018]

This study assessed the potential of highly purified (Stro-1(+)) human mesenchymal precursor cells (hMPCs) in combination with the anti-scarring protein decorin to repair the injured spinal cord (SC). Donor hMPCs isolated from spinal cord injury (SCI) patients were transplanted into athymic rats as a suspension graft, alone or after previous treatment with, core (decorin(core)) and proteoglycan (decorin(pro)) isoforms of purified human recombinant decorin. Decorin was delivered via mini-osmotic pumps for 14 days following sub-acute (7 day) or chronic (1 month) SCI. hMPCs were delivered to the spinal cord at 3 weeks or 6 weeks after the initial injury at T9 level. Behavioral and anatomical analysis in this study showed statistically significant improvement in functional recovery, tissue sparing and cyst volume reduction following hMPC therapy. The combination of decorin infusion followed by hMPC therapy did not improve these measured outcomes over the use of cell therapy alone, in either sub-acute or chronic SCI regimes. However, decorin infusion did improve tissue sparing, reduce spinal tissue cavitation and increase transplanted cell survivability as compared to controls. Immunohistochemical analysis of spinal cord sections revealed differences in glial, neuronal and extracellular matrix molecule expression within each experimental group. hMPC transplanted spinal cords showed the increased presence of serotonergic (5-HT) and sensory (CGRP) axonal growth within and surrounding transplanted hMPCs for up to 2 months; however, no evidence of hMPC transdifferentiation into neuronal or glial phenotypes. The number of hMPCs was dramatically reduced overall, and no transplanted cells were detected at 8 weeks post-injection using lentiviral GFP labeling and human nuclear antigen antibody labeling. The presence of recombinant decorin in the cell transplantation regimes delayed in part the loss of donor cells, with small numbers remaining at 2 months after transplantation. In vitro co-culture experiments with embryonic dorsal root ganglion explants revealed the growth promoting properties of hMPCs. Decorin did not increase axonal outgrowth from that achieved by hMPCs. We provide evidence for the first time that (Stro-1(+)) hMPCs provide: i) an advantageous source of allografts for stem cell transplantation for sub-acute and chronic spinal cord therapy, and (ii) a positive host microenvironment that promotes tissue sparing/repair that subsequently improves behavioral outcomes after SCI. This was not measurably improved by recombinant decorin treatment, but does provide important information for the future development and potential use of decorin in contusive SCI therapy.

DNA-mediated assembly of protein heterodimers on membrane surfaces

We present a method based on self-assembling oligonucleotides to anchor proteins to a supported membrane surface. This anchoring method allows control of the surface density of multiple proteins. By incorporating additional recognition sequences into the DNA linkers, defined heterodimers can be produced upon the addition of a heterospecific DNA cross-linking strand. Characterization by fluorescence cross-correlation spectroscopy (FCCS) confirmed lateral mobility and the formation of specific heterodimers. We further demonstrate that proteins linked in this manner as either monomers or dimers can form functional interfaces with living cells.

ADAM17 transactivates EGFR signaling during embryonic eyelid closure

PURPOSE

During mammalian embryonic eyelid closure ADAM17 has been proposed to play a role as a transactivator of epidermal growth factor receptor (EGFR) signaling by shedding membrane bound EGFR ligands. However, ADAM17 also sheds numerous other ligands, thus implicating ADAM17 in additional molecular pathways. The goal of this study was to experimentally establish the role of ADAM17 and determine ADAM17-mediated pathways essential for the embryonic eyelid closure.

METHODS

Wild-type (WT) and woe mice, carrying a hypomorphic mutation in Adam17, were evaluated using H&E and scanning electron microscopy. Expressions of ADAM17, EGFR, and the phosphorylated form EGFR-P were evaluated using immunohistochemistry. BrdU and TUNEL assays were used to evaluate cell proliferation and apoptosis, respectively. In vitro scratch assays of primary cultures were used to evaluate cell migration. Clinical and histologic analyses established if the hypermorphic Egfr(Dsk5) allele can rescue the woe embryonic eyelid closure.

RESULTS

woe mice exhibited a failure to develop the leading edge of the eyelid and consequently failure of the embryonic eyelid closure. Expression of ADAM17 was identified in the eyelid epithelium in the cells of the leading edge. ADAM17 is essential for epithelial cell migration, but does not play a role in proliferation and apoptosis. EGFR was expressed in both WT and woe eyelid epithelium, but the phosphorylated EGFR-P form was detected only in WT. The Egfr(Dsk5) allele rescued woe eyelid closure defects, but also rescued woe anterior segment defects and the absence of meibomian glands.

CONCLUSIONS

We provide in vivo genetic evidence that the role of ADAM17 during embryonic eyelid closure is to transactivate EGFR signaling.

Effect of heparin-binding EGF-like growth factor and amphiregulin on the MAP kinase-induced production of vascular endothelial growth factor by human granulosa cells

The function of granulosa cells is regulated by various hormones and growth factors. Our aim is to clarify the regulation of vascular endothelial growth factor (VEGF) production induced by heparin-binding EGF-like growth factor (HB-EGF) and amphiregulin (AR) in a human granulosa cell line, KGN. KGN cells were cultured and incubated for 24 h with HB-EGF and AR. The levels of VEGF in the culture media were measured by an enzyme-linked immunosorbent assay. The activation of MAP kinase in KGN cells was detected by Western blot analysis. VEGF production was significantly increased by HB-EGF or AR alone in a dose-dependent manner, whereas it was decreased by AG1478 or U0126. The MAP kinase activity was increased by treatment with HB-EGF or AR. The results suggested that VEGF is induced by HB-EGF and AR through mechanisms involving MAP kinase. The increase in VEGF may contribute to neovascularization, which in turn would promote various ovulation phenomena as well as follicular growth.

Differential contribution to neuroendocrine tumorigenesis of parallel egfr signaling in cancer cells and pericytes

Factors associated with tumor sensitivity to epidermal growth factor receptor (EGFR) inhibitors in the context of wild-type EGFR remain elusive. This study investigates the mechanistic basis of responsiveness to EGFR inhibitors in the RIP1-Tag2 (RT2) mouse model of pancreatic neuroendocrine tumorigenesis (PNET). Upon treatment of RT2 mice with EGFR inhibitors, PNET tumors harboring wild-type, nonamplified alleles of Egfr grow at a markedly reduced rate and display a significant increase in tumor cell apoptosis, as well as reduced neovascularization. The authors identify Tgf-α and Hb-egf as key limiting mediators of separable pathological functions of Egfr in neuroendocrine tumor progression: Tgf-α mutant tumors present with an elevated apoptotic index, whereas Hb-egf mutant lesions exhibit decreased angiogenic switching and neovascularization. This study not only associates Tgf-α and Hb-egf expression with wild-type Egfr oncogenicity but also ascribes the proangiogenic activity of Egfr in this tumor model to a novel mesenchymal Hb-egf/Egfr signaling axis, whereby endothelial and pericyte-derived Hb-egf activates Egfr specifically in tumor-associated perivascular cells, leading to increased pericyte coverage of the tumor endothelium and enhanced angiogenesis.

Bace1 processing of NRG1 type III produces a myelin-inducing signal but is not essential for the stimulation of myelination

Myelin sheath thickness is precisely adjusted to axon caliber, and in the peripheral nervous system, neuregulin 1 (NRG1) type III is a key regulator of this process. It has been proposed that the protease BACE1 activates NRG1 dependent myelination. Here, we characterize the predicted product of BACE1-mediated NRG1 type III processing in transgenic mice. Neuronal overexpression of a NRG1 type III-variant, designed to mimic prior cleavage in the juxtamembrane stalk region, induces hypermyelination in vivo and is sufficient to restore myelination of NRG1 type III-deficient neurons. This observation implies that the NRG1 cytoplasmic domain is dispensable and that processed NRG1 type III is sufficient for all steps of myelination. Surprisingly, transgenic neuronal overexpression of full-length NRG1 type III promotes hypermyelination also in BACE1 null mutant mice. Moreover, NRG1 processing is impaired but not abolished in BACE1 null mutants. Thus, BACE1 is not essential for the activation of NRG1 type III to promote myelination. Taken together, these findings suggest that multiple neuronal proteases collectively regulate NRG1 processing. © 2011 Wiley Periodicals, Inc.

Molecular and cellular mechanisms of ectodomain shedding

The extracellular domain of several membrane-anchored proteins is released from the cell surface as soluble proteins through a regulated proteolytic mechanism called ectodomain shedding. Cells use ectodomain shedding to actively regulate the expression and function of surface molecules, and modulate a wide variety of cellular and physiological processes. Ectodomain shedding rapidly converts membrane-associated proteins into soluble effectors and, at the same time, rapidly reduces the level of cell surface expression. For some proteins, ectodomain shedding is also a prerequisite for intramembrane proteolysis, which liberates the cytoplasmic domain of the affected molecule and associated signaling factors to regulate transcription. Ectodomain shedding is a process that is highly regulated by specific agonists, antagonists, and intracellular signaling pathways. Moreover, only about 2% of cell surface proteins are released from the surface by ectodomain shedding, indicating that cells selectively shed their protein ectodomains. This review will describe the molecular and cellular mechanisms of ectodomain shedding, and discuss its major functions in lung development and disease.

Mitochondrial reactive oxygen species mediate GPCR-induced TACE/ADAM17-dependent transforming growth factor-alpha shedding

Epidermal growth factor receptor (EGFR) activation by GPCRs regulates many important biological processes. ADAM metalloprotease activity has been implicated as a key step in transactivation, yet the regulatory mechanisms are not fully understood. Here, we investigate the regulation of transforming growth factor-alpha (TGF-alpha) shedding by reactive oxygen species (ROS) through the ATP-dependent activation of the P2Y family of GPCRs. We report that ATP stimulates TGF-alpha proteolysis with concomitant EGFR activation and that this process requires TACE/ADAM17 activity in both murine fibroblasts and CHO cells. ATP-induced TGF-alpha shedding required calcium and was independent of Src family kinases and PKC and MAPK signaling. Moreover, ATP-induced TGF-alpha shedding was completely inhibited by scavengers of ROS, whereas calcium-stimulated shedding was partially inhibited by ROS scavenging. Hydrogen peroxide restored TGF-alpha shedding after calcium chelation. Importantly, we also found that ATP-induced shedding was independent of the cytoplasmic NADPH oxidase complex. Instead, mitochondrial ROS production increased in response to ATP and mitochondrial oxidative complex activity was required to activate TACE-dependent shedding. These results reveal an essential role for mitochondrial ROS in regulating GPCR-induced growth factor shedding.

Rho/ROCK and MEK/ERK activation by transforming growth factor-alpha induces articular cartilage degradation

Identification and characterization of therapeutic targets for joint conditions, such as osteoarthritis (OA), is exceedingly important for addressing the increasing burden of disease. Transforming growth factor-alpha (TGFalpha) is upregulated by articular chondrocytes in experimentally induced and human OA. To test the potential involvement of TGFalpha, which is an activator of epidermal growth factor receptor (EGFR) signaling, in joint degeneration and to identify signaling mechanisms mediating articular chondrocyte responses to TGFalpha, rat chondrocytes and osteochondral explants were treated with TGFalpha and various inhibitors of intracellular signaling pathways. Stimulation of EGFR signaling in articular chondrocytes by TGFalpha resulted in the activation of RhoA/ROCK (Rho kinase), MEK (MAPK/ERK kinase)/ERK (extracellular-signal-regulated kinase), PI3K (phosphoinositide 3-kinase) and p38 MAPK (mitogen-activated protein kinase) pathways. Modification of the chondrocyte actin cytoskeleton was stimulated by TGFalpha, but inhibition of only Rho or ROCK activation prevented morphological changes. TGFalpha suppressed expression of anabolic genes including Sox9, type II collagen and aggrecan, which were rescued only by inhibiting MEK/ERK activation. Furthermore, catabolic factor upregulation by TGFalpha was prevented by ROCK and p38 MAPK inhibition, including matrix metalloproteinase-13 and tumor necrosis factor-alpha, which are well known to contribute to cartilage digestion in OA. To assess the ability of TGFalpha to stimulate degradation of mature articular cartilage, type II collagen and aggrecan cleavage fragments were analyzed in rat osteochondral explants exposed to exogenous TGFalpha. Normal articular cartilage contained low levels of both cleavage fragments, but high levels were observed in the cartilage treated with TGFalpha. Selective inhibition of MEK/ERK and Rho/ROCK activation greatly reduced or completely prevented excess type II collagen and aggrecan degradation in response to TGFalpha. These data suggest that TGFalpha is a strong stimulator of cartilage degradation and that Rho/ROCK and MEK/ERK signaling have critical roles in mediating these effects.

Epidermal growth factor variations in amniotic membrane used for ex vivo tissue constructs

INTRODUCTION

The amniotic membrane (AM) is used for engineering ex vivo tissue constructs used in ocular surface reconstruction. Epidermal growth factor (EGF) content of the AM is believed to play a key role in supporting corneal epithelial cell expansion on AM. This study investigated EGF content in AM in relation to intra- and inter-donor variations and the effect of processing and preservation (handling).

METHODS

Fifteen human AM, both fresh and handled, were analyzed for EGF gene and protein expression by real-time polymerase chain reaction and ELISA, respectively.

RESULTS

EGF gene expression was predominantly seen in the AM epithelium (p<0.01). Similarly, EGF protein too was predominantly seen in the epithelial layer (p<0.01) for fresh and handled samples. EGF protein content varied between membranes (inter-donor) and at different sites within the same membrane (intra-donor). The highest EGF protein concentration was noted in the AM apical and mid-region epithelium. Significant EGF protein loss (p<0.01) was observed after handling.

CONCLUSION

There is a considerable variation in EGF content between and within donors. This is further affected by handling of the AM. Such variations could affect the clinical efficacy of tissue constructs. Current use of AM for ex vivo expansion of epithelial cells is not standardized and remains an area of concern.

The epidermal growth factor receptor ligands at a glance

The epidermal growth factor receptor (EGFR) regulates key processes of cell biology, including proliferation, survival, and differentiation during development, tissue homeostasis, and tumorigenesis. Canonical EGFR activation involves the binding of seven peptide growth factors. These ligands are synthesized as transmembrane proteins comprising an N-terminal extension, the EGF module, a short juxtamembrane stalk, a hydrophobic transmembrane domain, and a carboxy-terminal fragment. The central structural and functional feature is the EGF module, a sequence containing six cysteines in a conserved spacement which is responsible for binding to the EGFR. While the membrane-anchored peptide can be biologically active by juxtacrine signaling, in most cases the EGF module is proteolytically cleaved (a process termed ectodomain shedding) to release the soluble growth factor, which may act in an endocrine, paracrine, or autocrine fashion. This review summarizes the structural and functional properties of these fascinating molecules and presents selected examples to illustrate their roles in development, physiology, and pathology.

Activation of the epidermal growth factor receptor (EGFR) by a novel metalloprotease pathway

Neutrophil Elastase (NE) is a pro-inflammatory protease present at higher than normal levels in the lung during inflammatory disease. NE regulates IL-8 production from airway epithelial cells and can activate both EGFR and TLR4. TACE/ADAM17 has been reported to trans-activate EGFR in response to NE. Here, using 16HBE14o-human bronchial epithelial cells we demonstrate a new mechanism by which NE regulates both of these events. A high molecular weight soluble metalloprotease activity detectable only in supernatants from NE-treated cells by gelatin and casein zymography was confirmed to be meprin alpha by Western immunoblotting. In vitro studies demonstrated the ability of NE to activate meprin alpha, which in turn could release soluble TGFalpha and induce IL-8 production from 16HBE14o- cells. These effects were abrogated by actinonin, a specific meprin inhibitor. NE-induced IL-8 expression was also inhibited by meprin alpha siRNA. Immunoprecipitation studies detected EGFR/TLR4 complexes in NE-stimulated cells overexpressing these receptors. Confocal studies confirmed colocalization of EGFR and TLR4 in 16HBE14o- cells stimulated with meprin alpha. NFkappaB was also activated via MyD88 in these cells by meprin alpha. In bronchoalveolar lavage fluid from NE knock-out mice infected intra-tracheally with Pseudomonas aeruginosa meprin alpha was significantly decreased compared with control mice, and was significantly increased and correlated with NE activity, in bronchoalveolar lavage fluid from individuals with cystic fibrosis but not healthy controls. The data describe a previously unidentified lung metalloprotease meprin alpha, and its role in NE-induced EGFR and TLR4 activation and IL-8 production.

Association of tetraspanin CD9 with transmembrane TGF{alpha} confers alterations in cell-surface presentation of TGF{alpha} and cytoskeletal organization

Ligand presentation is a major determinant of receptor activation. The epidermal growth factor receptor (EGFR), a tyrosine kinase receptor, is activated by growth factors of the transforming growth factor alpha (TGFalpha) family. The tetraspanin CD9 interacts with transmembrane TGFalpha and decreases its ectodomain shedding to release soluble TGFalpha. Here we report that CD9 has a role in the maturation of transmembrane TGFalpha and its stabilization at the cell surface, and in the cell-surface distribution in polarized epithelial cells. Furthermore, coexpression of CD9 and TGFalpha confers changes in cytoskeletal organization with a decrease in actin stress fibers and focal adhesions, and changes in RhoA and Rac1 GTPase activity. These alterations are reversed by blocking EGFR signaling. Finally, we demonstrate changes in cell adhesion and migration resulting from coexpression of TGFalpha with CD9. These results provide insight into the role of CD9 in the presentation of TGFalpha in epithelial and carcinoma cells, whose physiology is driven by ligand-induced EGFR activation.

TACE/ADAM-17: a component of the epidermal growth factor receptor axis and a promising therapeutic target in colorectal cancer

PURPOSE

Activation of the epidermal growth factor receptor (EGFR) requires cell surface cleavage of EGFR ligands, uptake of soluble ligand by the receptor, and initiation of EGFR tyrosine kinase activity. We define these collective events as the EGFR axis. Transforming growth factor-alpha (TGF-alpha) and amphiregulin are two EGFR ligands that are delivered preferentially to the basolateral surface of polarized epithelial cells where the EGFR resides. TACE/ADAM-17 (tumor necrosis factor-alpha converting enzyme/a disintegrin and metalloprotease) has been implicated in ectodomain cleavage of TGF-alpha and amphiregulin.

EXPERIMENTAL DESIGN

Using a human polarizing colorectal cancer (CRC) cell line, HCA-7, and a tissue array of normal colonic mucosa and primary and metastatic CRC, we determined the intracellular localization of TACE and the effects of EGFR axis inhibition in CRC.

RESULTS

Herein, we show that TACE is localized to the basolateral plasma membrane of polarized HCA-7 cells. TACE is overexpressed in primary and metastatic CRC tumors compared with normal colonic mucosa; the intensity of its immunoreactivity is inversely correlated with that of TGF-alpha and amphiregulin. Pharmacologic blockade of HCA-7 cells with an EGFR monoclonal antibody, a selective EGFR tyrosine kinase inhibitor, and a selective TACE inhibitor results in concentration-dependent decreases in cell proliferation and active, phosphorylated mitogen-activated protein kinase. Combining suboptimal concentrations of these agents results in cooperative growth inhibition, increased apoptosis, and reduced mitogen-activated protein kinase pathway activation. Furthermore, an EGFR tyrosine kinase-resistant clone of HCA-7 cells is growth-inhibited by combined monoclonal antibody and TACE inhibition.

CONCLUSION

These results implicate TACE as a promising target of EGFR axis inhibition in CRC.

Transforming growth factor alpha suppression of articular chondrocyte phenotype and Sox9 expression in a rat model of osteoarthritis

OBJECTIVE

To define the roles of transforming growth factor alpha (TGFalpha) in cartilage degradation.

METHODS

Primary rat articular chondrocytes and articular osteochondral explants were cultured with TGFalpha to assess the effects of TGFalpha on chondrocyte physiology and phenotype.

RESULTS

TGFalpha altered chondrocyte morphology through reorganization of the actin cytoskeleton and formation of stress fibers. Expression of anabolic genes, including aggrecan, type II collagen, and cartilage link protein, was reduced in response to TGFalpha. Proliferation of chondrocytes and formation of articular chondrocyte clusters was stimulated by TGFalpha. Expression of matrix metalloproteinase 13 and cathepsin C was increased by TGFalpha. We demonstrated the down-regulation of Sox9 messenger RNA and protein levels by TGFalpha. This was associated with reduced levels of phosphorylated and total SOX9 in cartilage explants upon TGFalpha treatment. In contrast, another growth factor identified in our microarrays, Kitl, had no effects on the chondrocyte parameters tested. To examine correlations between the increased levels of TGFalpha in experimental knee osteoarthritis (OA) with the levels of TGFalpha in humans with knee OA, a microarray analysis of mRNA from 13 normal and 12 late-stage OA cartilage samples was performed. Seven OA samples showed TGFA mRNA levels similar to those in the normal controls, but expression was markedly increased in the other 5 OA samples. These data confirm that TGFA transcript levels are increased in a subset of patients with OA.

CONCLUSION

This study adds TGFalpha to the list of dysregulated cytokines present in degrading cartilage in OA. Since TGFalpha inhibits articular chondrocyte anabolic capacity, increases catabolic factors, and contributes to the development of chondrocyte clusters, TGFalpha may be a potential target for therapeutic strategies in the treatment of OA.

Betacellulin regulates hair follicle development and hair cycle induction and enhances angiogenesis in wounded skin

Betacellulin (BTC) belongs to the EGF family, whose members play important roles in skin morphogenesis, homeostasis, and repair. However, the role of BTC in skin biology is still unknown. We employed transgenic mice overexpressing BTC ubiquitously to study its role in skin physiology. Immunohistochemistry revealed increased levels of BTC especially in the hair follicles and in the epidermis of transgenic animals. Expression of key markers of epithelial differentiation was unaltered, but keratinocyte proliferation was significantly increased. At post-natal day 1 (P1), transgenic mice displayed a significant retardation of hair follicle morphogenesis. At P17, when most follicles in control mice had initiated hair follicle cycling and had already entered into their first late catagen or telogen phase, all follicles of transgenic mice were still at the mid- to late catagen phases, indicating retarded initiation of hair follicle cycling. Healing of full-thickness excisional wounds and bursting strength of incisional wounds were similar in control and transgenic mice. However, an increase in the area covered by blood vessels at the wound site was detected in transgenic animals. These results provide evidence for a role of BTC in the regulation of epidermal homeostasis, hair follicle morphogenesis and cycling, and wound angiogenesis.

Cornichon regulates transport and secretion of TGFα-related proteins in metazoan cells

Cornichon proteins are structurally related transmembrane proteins that have been studied in and Drosophila and yeast. In Drosophila, Cornichon (Cni) is involved in embryo polarization by the TGFα-related Gurken. In yeast, the Cni-related Erv14 is required for axial budding. A cargo receptor function has been proposed for Erv14 and Cni. Four mammalian Cni-like sequences have been identified. We carried out parallel functional analyses of the human Cni ortholog CNIH and Drosophila Cni in the processing and presentation of TGFα family proteins. Human CNIH complements the loss of Erv14 in yeast. Human CNIH and Drosophila Cni are primarily localized in the endoplasmic reticulum and associate with immature TGFα family proteins. Alterations of cornichon expression result in changes in transport, processing and secretion of TGFα proteins. In particular, increased cornichon expression retains TGFα proteins in the endoplasmic reticulum, whereas cornichon is required for their transport and secretion. Thus, cornichon proteins represent a functionally conserved protein family that acts in the selective transport and maturation of TGFα family proteins.

Mitogenic activity and signaling mechanism of 2-(14,15- epoxyeicosatrienoyl)glycerol, a novel cytochrome p450 arachidonate metabolite

Arachidonic acid is an essential constituent of cell membranes that is esterified to the sn-2 position of glycerophospholipids and is released from selected phospholipid pools by tightly regulated phospholipase cleavage. Metabolism of the released arachidonic acid by the cytochrome P450 enzyme system (cP450) generates biologically active compounds, including epoxyeicosatrienoic acids (EETs) and hydroxyeicosatetraenoic acids. Here we report that 2-(14,15-epoxyeicosatrienoyl)glycerol (2-14,15-EG), a novel cP450 arachidonate metabolite produced in the kidney, is a potent mitogen for renal proximal tubule cells. This effect is mediated by activation of tumor necrosis factor alpha-converting enzyme (ADAM17), which cleaves membrane-bound transforming growth factor alpha (proTGF-alpha) and releases soluble TGF-alpha as a ligand that binds and activates epidermal growth factor receptor (EGFR). The present studies additionally demonstrate that the structurally related 14,15-EET stimulates release of soluble heparin-binding EGF-like growth factor as an EGFR ligand by activation of ADAM9, another member of the ADAM family. Thus, in addition to the characterization of 2-14,15-EG's mitogenic activity and signaling mechanism, our study provides the first example that two structurally related biologically active lipid mediators can activate different metalloproteinases and release different EGFR ligands in the same cell type to activate EGFR and stimulate cell proliferation.

The extracellular linker of pro-neuregulin-alpha2c is required for efficient sorting and juxtacrine function

The neuregulins (NRGs) play important roles in animal physiology, and their disregulation has been linked to diseases such as cancer or schizophrenia. The NRGs may be produced as transmembrane proteins (proNRGs), even though they lack an N-terminal signal sequence. This raises the question of how NRGs are sorted to the plasma membrane. It is also unclear whether in their transmembrane state, the NRGs are biologically active. During studies aimed at solving these questions, we found that deletion of the extracellular juxtamembrane region termed the linker, decreased cell surface exposure of the mutant proNRG(DeltaLinker), and caused its entrapment at the cis-Golgi. We also found that cell surface-exposed transmembrane NRG forms retain biological activity. Thus, a mutant whose cleavage is impaired but is correctly sorted to the plasma membrane activated ErbB receptors in trans and also stimulated proliferation. Because the linker is implicated in surface sorting and the regulation of the cleavage of transmembrane NRGs, our data indicate that this region exerts multiple important roles in the physiology of NRGs.

Targeting ADAM-mediated ligand cleavage to inhibit HER3 and EGFR pathways in non-small cell lung cancer

We describe here the existence of a heregulin-HER3 autocrine loop, and the contribution of heregulin-dependent, HER2-mediated HER3 activation to gefitinib insensitivity in non-small cell lung cancer (NSCLC). ADAM17 protein, a major ErbB ligand sheddase, is upregulated in NSCLC and is required not only for heregulin-dependent HER3 signaling, but also for EGFR ligand-dependent signaling in NSCLC cell lines. A selective ADAM inhibitor, INCB3619, prevents the processing and activation of multiple ErbB ligands, including heregulin. In addition, INCB3619 inhibits gefitinib-resistant HER3 signaling and enhances gefitinib inhibition of EGFR signaling in NSCLC. These results show that ADAM inhibition affects multiple ErbB pathways in NSCLC and thus offers an excellent opportunity for pharmacological intervention, either alone or in combination with other drugs.

Control of ErbB signaling through metalloprotease mediated ectodomain shedding of EGF-like factors

Epidermal growth factor (EGF)-like proteins comprise a group of structurally similar growth factors, which contain a conserved six-cysteine residue motif called the EGF-domain. EGF-like factors are synthesized as transmembrane precursors, which can undergo proteolytic cleavage at the cell surface to release a mature soluble ectodomain; a process often referred to as "ectodomain shedding". Ectodomain shedding of EGF-like factors has been linked to multiple zinc-binding metalloproteases of the matrix metalloprotease (MMP) and a disintegrin and metalloprotease (ADAM) families. Shedding can be activated by a variety of pharmacological and physiological stimuli and these activation events have been linked to the enhancement of metalloprotease activity, possibly via the action of intracellular signaling modules. Once shed from the cell surface, EGF-like factors bind to a family of four cell surface receptors named ErbB-1, -2, -3 and -4. Heterodimerization or homodimerization of these receptors following ligand binding drives intracellular signal transduction cascades, which eventuate in diverse cell fates including proliferation, differentiation, migration and inhibition of apoptosis. In addition to its role in driving normal developmental processes, a wealth of evidence now exists showing that de-regulated ErbB signaling is associated with the formation of tumors in a variety of tissues and that ectodomain shedding of EGF-like factors plays a critical event in this process. Thus, knowledge of the molecular mechanisms by which EGF-like factors are shed from the cell surface and the nature of the proteases and cellular signals that govern this process is crucial to understanding ErbB receptor signaling and potentially also in the development of novel cancer therapeutics targeting the ErbB pathway. This review focuses on the structure and function of EGF-like factors, and the mechanisms that govern the shedding of these transmembrane molecules from the cell surface.

Metalloprotease-dependent amphiregulin release mediates tumor necrosis factor-α-induced IL-8 secretion in the human airway epithelial cell line NCI-H292

Tumor necrosis factor-alpha (TNF-alpha) is a potent multifunctional cytokine that plays a central role in the pathogenesis of many inflammatory diseases. Interleukin-8 (IL-8) is a principle neutrophil chemoattractant and activator in humans. The alveolar macrophage-derived TNF-alpha initiates lung inflammation through its ability to stimulate IL-8 synthesis in airway epithelial cells. Since recent studies demonstrated that the stimulation of epidermal growth factor receptor (EGFR) could induce IL-8 secretion, the involvement of EGFR in TNF-alpha-induced IL-8 secretion in airway epithelium-like NCI-H292 cells was investigated in this study. TNF-alpha and epidermal growth factor (EGF) stimulated IL-8 secretion in a time- and concentration-dependent manner. Inhibition of the EGFR by either an anti-EGFR neutralizing antibody or by its specific inhibitor AG1478 (1 microM) blocked TNF-alpha-induced IL-8 secretion. In addition, TNF-alpha stimulated tyrosine phosphorylation of the EGFR within 5 min after stimulation. Further, TNF-alpha-induced IL-8 secretion was completely inhibited by the neutralizing antibody against amphiregulin (AR), an EGFR ligand, suggesting that TNF-alpha-induced IL-8 secretion was mediated by the AR-EGFR pathway. Furthermore, TNF-alpha stimulated the release of AR in a concentration-dependent manner. Finally, both AR and IL-8 release-induced by TNF-alpha were eliminated by pretreatment with either GM6001, a broad-spectrum inhibitor for metalloprotease, or TAPI-1, relatively selective inhibitor for TNF-alpha converting enzyme (TACE). These findings indicate that metalloprotease-mediated AR shedding and subsequent activation of EGFR play a critical role in TNF-alpha-induced IL-8 secretion from the human airway epithelium-like NCI-H292 cells, and that TACE is one of the most possible candidates for metalloprotease responsible for TNF-alpha-induced AR shedding.

Human airway trypsin-like protease induces amphiregulin release through a mechanism involving protease-activated receptor-2-mediated ERK activation and TNF alpha-converting enzyme activity in airway epithelial cells

Human airway trypsin-like protease (HAT), a serine protease found in the sputum of patients with chronic airway diseases, is an agonist of protease-activated receptor-2 (PAR-2). Previous results have shown that HAT enhances the release of amphiregulin (AR); further, it causes MUC5AC gene expression through the AR-epidermal growth factor receptor pathway in the airway epithelial cell line NCI-H292. In this study, the mechanisms by which HAT-induced AR release can occur were investigated. HAT-induced AR gene expression was mediated by extracellular signal-regulated kinase (ERK) pathway, as pretreatment of cells with ERK pathway inhibitor eliminated the effect of HAT on AR mRNA. Both HAT and PAR-2 agonist peptide (PAR-2 AP) induced ERK phosphorylation; further, desensitization of PAR-2 with a brief exposure of cells to PAR-2 AP resulted in inhibition of HAT-induced ERK phosphorylation, suggesting that HAT activates ERK through PAR-2. Moreover, PAR-2 AP induced AR gene expression subsequent to protein production in the cellular fraction through the ERK pathway indicating that PAR-2-mediated activation of ERK is essential for HAT-induced AR production. However, in contrast to HAT, PAR-2 AP could not cause AR release into extracellular space; it appears that activation of PAR-2 is not sufficient for HAT-induced AR release. Finally, HAT-induced AR release was eliminated by blockade of tumour necrosis factor alpha-converting enzyme (TACE) by the TAPI-1 and RNA interference, suggesting that TACE activity is necessary for HAT-induced AR release. These observations show that HAT induces AR production through the PAR-2 mediated ERK pathway, and then causes AR release by a TACE-dependent mechanism.

N-terminal cleavage of proTGFalpha occurs at the cell surface by a TACE-independent activity

ProTGFalpha (transforming growth factor alpha precursor) maturation and conversion into soluble TGFalpha is a complex process that involves three proteolytic steps. One, that occurs co-translationally, eliminates the signal sequence. Another, occurring at the juxta-membrane domain, solubilizes TGFalpha. A third cleavage removes the N-terminal extension of proTGFalpha. This latter step has been poorly studied, mainly because of the rapid kinetics of this cleavage. In the present study, we have designed a strategy to analyse several aspects regarding this N-terminal cleavage. In vivo treatment with the hydroxamate-based metalloprotease inhibitors BB3103 or TAPI-2 (tumour necrosis factor-alpha protease inhibitor 2) reversibly induced accumulation of forms of proTGFalpha that included the N-terminal extension. N-terminal shedding was rapid, and occurred at the cell surface. However, the machinery responsible for the N-terminal cleavage was inactive in other cellular sites, such as the endoplasmic reticulum. Experiments of proTGFalpha expression and maturation in cells deficient in TACE (tumour-necrosis-factor-alpha-converting enzyme) activity indicated that this protease was dispensable for N-terminal processing of proTGFalpha in vivo, but was required for regulated cleavage at the C-terminus. These findings indicate that TACE is not involved in N-terminal processing of proTGFalpha, and suggest differences in the machineries that control the cleavage at both ends of TGFalpha within its precursor.

Targeting Ligand Cleavage to Inhibit the ErbB Pathway in Cancer

ADAMs (a disintegrin and metalloproteases) are zinc-dependent transmembrane metalloproteases that shed the extracellular domains of membrane-bound growth factors, cytokines, and receptors. Recently, ADAMs have emerged in ErbB signaling pathways as sheddases or multiple ErbB ligands. As the ErbB pathway is a validated target for anticancer drugs, upstream activators of ErbB ligands, their sheddases, become new drug targets in the ErbB pathway. We have identified selective small molecule inhibitors of ADAM proteases that block shedding and activation of multiple ErbB ligands, and we are planning to test the compounds in the clinic.

G-protein-coupled receptor signaling and the EGF network in endocrine systems

The epidermal growth factor (EGF) network is composed of a complex array of growth factors synthesized as precursors and expressed on the cell surface. These latent growth factors are activated by cleavage and shedding from the cell surface and act by binding to various homo- and hetero-dimers of the EGF receptors (ErbBs). Although the exact molecular steps are poorly understood, ligand binding to G-protein-coupled receptors as diverse as the beta-adrenoceptors or the lysophosphatidic acid receptors leads to shedding of EGF growth factors and activation of EGF receptors. Recent observations from the pituitary and in the ovary are providing new insight into the role of this network in endocrine systems.

Autocrine regulation of human urothelial cell proliferation and migration during regenerative responses in vitro

Regeneration of the urothelium is rapid and effective in order to maintain a barrier to urine following tissue injury. Whereas normal human urothelial (NHU) cells are mitotically quiescent and G0 arrested in situ, they rapidly enter the cell cycle upon seeding in primary culture and show reversible growth arrest at confluency. We have used this as a model to investigate the role of EGF receptor signaling in urothelial regeneration and wound-healing. Transcripts for HER-1, HER-2, and HER-3 were expressed by quiescent human urothelium in situ. Expression of HER-1 was upregulated in proliferating cultures, whereas HER-2 and HER-3 were more associated with a growth-arrested phenotype. NHU cells could be propagated in the absence of exogenous EGF, but autocrine signaling through HER-1 via the MAPK and PI3-kinase pathways was essential for proliferation and migration during urothelial wound repair. HB-EGF was expressed by urothelium in situ and HB-EGF, epiregulin, TGF-alpha, and amphiregulin were expressed by proliferating NHU cells. Urothelial wound repair in vitro was attenuated by neutralizing antibodies against HER-1 ligands, particularly amphiregulin. By contrast, the same ligands applied exogenously promoted migration, but inhibited proliferation, implying that HER-1 ligands provoke differential effects in NHU cells depending upon whether they are presented as soluble or juxtacrine ligands. We conclude that proliferation and migration during wound healing in NHU cells are mediated through an EGFR autocrine signalling loop and our results implicate amphiregulin as a key mediator.

Role of the epidermal growth factor network in ovarian follicles

The LH surge causes major remodeling of the ovarian follicle in preparation for the ovulatory process. These changes include reprogramming of granulosa cells to differentiate into luteal cells, changes in cumulus cell secretory properties, and oocyte maturation. This review summarizes published data in support of the concept that LH stimulation of ovarian follicles involves activation of a local epidermal growth factor (EGF) network. A model describing this property of LH signaling and its branching to other signaling modules is discussed. According to this model, LH activation of mural granulosa cells stimulates cAMP signaling, which, in turn, induces the expression of the EGF-like growth factors epiregulin, amphiregulin, and betacellulin. These growth factors function by activating EGF receptors in either an autocrine/juxtacrine fashion within the mural layer, or they diffuse to act on cumulus cells. Activation of EGF receptor signaling in cumulus cells, together with cAMP priming, triggers oocyte nuclear maturation and acquisition of developmental competence as well as cumulus expansion. This model has important implications for ovarian physiology and for the development of new strategies for the pharmacological control of ovulation and for gamete maturation in vitro.

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.

The membrane-anchoring domain of epidermal growth factor receptor ligands dictates their ability to operate in juxtacrine mode

All ligands of the epidermal growth factor (EGF) receptor (EGFR) are synthesized as membrane-anchored precursors. Previous work has suggested that some ligands, such as EGF, must be proteolytically released to be active, whereas others, such as heparin-binding EGF-like growth factor (HB-EGF) can function while still anchored to the membrane (i.e., juxtacrine signaling). To explore the structural basis for these differences in ligand activity, we engineered a series of membrane-anchored ligands in which the core, receptor-binding domain of EGF was combined with different domains of both EGF and HB-EGF. We found that ligands having the N-terminal extension of EGF could not bind to the EGFR, even when released from the membrane. Ligands lacking an N-terminal extension, but possessing the membrane-anchoring domain of EGF, still required proteolytic release for activity, whereas ligands with the membrane-anchoring domain of HB-EGF could elicit full biological activity while still membrane anchored. Ligands containing the HB-EGF membrane anchor, but lacking an N-terminal extension, activated EGFR during their transit through the Golgi apparatus. However, cell-mixing experiments and fluorescence resonance energy transfer studies showed that juxtacrine signaling typically occurred in trans at the cell surface, at points of cell-cell contact. Our data suggest that the membrane-anchoring domain of ligands selectively controls their ability to participate in juxtacrine signaling and thus, only a subclass of EGFR ligands can act in a juxtacrine mode.

Proteolytic processing of TGFα redirects its mitogenic activity: the membrane-anchored form is autocrine, the secreted form is paracrine

Wild-type transforming growth factor alpha (TGFalpha) expression in lactotrope cells in the pituitary gland led to lactotrope-specific pituitary hyperplasia and adenomata. To indicate whether the EGF receptor is involved in this TGFalpha-mediated phenotype, we bred TGFalpha mice with mice expressing the cytoplasmic truncated-EGF receptor (EGFR-tr), which is dominant-negative in other models. These bitransgenic mice developed pituitary pathology despite expression of the dominant-negative receptor. To further characterize this observation, we generated two lineages of transgenic mice that overexpress mutant forms of TGFalpha: a processed soluble form (s TGFalpha) and a cytoplasmic-deleted form (TGFalphaDeltaC). While sTGFalpha expression in lactotrope cells failed to induce autocrine lactotrope hyperplasia, the pituitary became very enlarged due to proliferation of neighboring interstitial cells. In contrast, the TGFalphaDeltaC mice did not develop a phenotype, although the mRNA and protein were present in the pituitary and this form of TGFalpha was confirmed to be biologically active and targeted properly to the plasma membrane of cultured CHO cells. The results suggest that the cytoplasmic domain of TGFalpha is required for autocrine parenchymal tumor formation in the pituitary gland. This signal cannot be inhibited by the EGFR-tr. Conversely, the released form of TGFalpha appears to have primarily paracrine activity.

Identification of MAGI-3 as a transforming growth factor-alpha tail binding protein

The cytoplasmic domain of the transforming growth factor-alpha precursor (proTGFalpha) contains a C-terminal PSD-95/SAP90, Discs Large, and Zona Occludens-1 (PDZ) recognition motif (TVV). By yeast two-hybrid screening of a mouse embryo library, we have found that a third member of a family of PDZ-containing proteins, membrane associated guanylate kinase inverted-3 (MAGI-3), binds to TGFalpha's TVV. MAGI-3 is widely expressed in multiple mouse tissues, including brain. Immunolocalization showed that MAGI-3 and TGFalpha were colocalized in neurons in the cortex and dentate gyrus, as well as in ependymal cells and some astrocytes. In vitro, proTGFalpha bound the PDZ-1 domain of MAGI-3 and MAGI-2, but not MAGI-1. MAGI-3 and the 17-kDa cell surface form of proTGFalpha interact transiently in MDCK cells stably transfected with both MAGI-3 and human proTGFalpha cDNAs. MAGI-3 and wild-type proTGFalpha colocalize at the cell surface. In contrast, MAGI-3 forms a stable complex with membrane-fixed TGFalpha early in the secretory pathway and interacts with immature and cell surface forms of membrane-fixed TGFalpha. Overexpression of MAGI-3 resulted in increased levels of TGFalpha in the basolateral medium of polarized MDCK cells, suggesting that MAGI-3 has a role in efficient trafficking of TGFalpha to the cell surface in polarized epithelial cells.

ADAMs: key components in EGFR signalling and development

ADAM (a disintegrin and metalloprotease) proteins are membrane-anchored metalloproteases that process and shed the ectodomains of membrane-anchored growth factors, cytokines and receptors. ADAMs also have essential roles in fertilization, angiogenesis, neurogenesis, heart development and cancer. Research on ADAMs and their role in protein ectodomain shedding is emerging as a fertile ground for gathering new insights into the functional regulation of membrane proteins.

ADAM10 Mediates Ectodomain Shedding of the Betacellulin Precursor Activated by p-Aminophenylmercuric Acetate and Extracellular Calcium Influx

Betacellulin belongs to the family of epidermal growth factor-like growth factors that are expressed as transmembrane precursors and undergo proteolytic ectodomain shedding to release a soluble mature growth factor. In this study, we investigated the ectodomain shedding of the betacellulin precursor (pro-BTC) in conditionally immortalized wild-type (WT) and ADAM-deficient cell lines. Sequential ectodomain cleavage of the predominant cell-surface 40-kDa form of pro-BTC generated a major (26-28 kDa) and two minor (20 and 15 kDa) soluble forms and a cellular remnant lacking the ectodomain (12 kDa). Pro-BTC shedding was activated by calcium ionophore (A23187) and by the metalloprotease activator p-aminophenylmercuric acetate (APMA), but not by phorbol esters. Culturing cells in calcium-free medium or with the protein kinase Cdelta inhibitor rottlerin, but not with broad-based protein kinase C inhibitors, blocked A23187-activated pro-BTC shedding. These same treatments were without effect for constitutive and APMA-induced cleavage events. All pro-BTC shedding was blocked by treatment with a broad-spectrum metalloprotease inhibitor (GM6001). In addition, constitutive and activated pro-BTC shedding was differentially blocked by TIMP-1 or TIMP-3, but was insensitive to treatment with TIMP-2. Pro-BTC shedding was functional in cells from ADAM17- and ADAM9-deficient mice and in cells overexpressing WT or catalytically inactive ADAM17. In contrast, overexpression of WT ADAM10 enhanced constitutive and activated shedding of pro-BTC, whereas overexpression of catalytically inactive ADAM10 reduced shedding. These results demonstrate, for the first time, activated pro-BTC shedding in response to extracellular calcium influx and APMA and provide evidence that ADAM10 mediates constitutive and activated pro-BTC shedding.

Characterization of a human Rhomboid homolog, p100hRho/RHBDF1, which interacts with TGF-α family ligands

The activity of the TGF-alpha-like ligand Spitz in Drosophila depends on Rhomboid, a seven-transmembrane spanning protein that resides in the Golgi and acts as a serine protease to cleave Spitz, thereby releasing the soluble ligand. Several rhomboids in Drosophila have been implicated in the processing of TGF-alpha-like ligands, and consequent EGF receptor activation. The larger number of TGF-alpha-like ligands in vertebrates raises the possibility that they too might be subject to regulation by rhomboid-like proteins. We present the cDNA cloning and polypeptide sequence of an atypically long human rhomboid, which, based on the absence of critical residues for serine protease activity, is not predicted to act as a serine protease. We examined its tissue distribution, in comparison with TGF-alpha and the TGF-alpha-related protein HB-EGF, and the EGF/TGF-alpha receptor, in mouse embryo. This rhomboid, named p100(hRho) or RHBDF1, is a seven-transmembrane protein with a long N-terminal cytoplasmic extension that comprises half of the polypeptide sequence, and is found in the endoplasmic reticulum and Golgi, but not on the cell surface. It is expressed as two forms with different lengths, forms dimers and interacts with TGF-alpha ligands through a luminal interaction with the EGF core ectodomain. Finally, we evaluated the function of p100(hRho)/RHBDF1 in Drosophila, demonstrating that the short, but not the full-length form has functional activity. The characterization of this protein extends our understanding of the rhomboid family of regulatory proteins.

Distinct roles for ADAM10 and ADAM17 in ectodomain shedding of six EGFR ligands

All ligands of the epidermal growth factor receptor (EGFR), which has important roles in development and disease, are released from the membrane by proteases. In several instances, ectodomain release is critical for activation of EGFR ligands, highlighting the importance of identifying EGFR ligand sheddases. Here, we uncovered the sheddases for six EGFR ligands using mouse embryonic cells lacking candidate-releasing enzymes (a disintegrin and metalloprotease [ADAM] 9, 10, 12, 15, 17, and 19). ADAM10 emerged as the main sheddase of EGF and betacellulin, and ADAM17 as the major convertase of epiregulin, transforming growth factor α, amphiregulin, and heparin-binding EGF-like growth factor in these cells. Analysis of adam9/12/15/17^−/− knockout mice corroborated the essential role of adam17^−/− in activating the EGFR in vivo. This comprehensive evaluation of EGFR ligand shedding in a defined experimental system demonstrates that ADAMs have critical roles in releasing all EGFR ligands tested here. Identification of EGFR ligand sheddases is a crucial step toward understanding the mechanism underlying ectodomain release, and has implications for designing novel inhibitors of EGFR-dependent tumors.