Heparan sulfate is essential to amphiregulin-induced mitogenic signaling by the epidermal growth factor receptor

Heparan sulfate regulates amphiregulin signaling towards reparative lung mesenchymal cells during influenza A infection

Amphiregulin (Areg), a growth factor produced by regulatory T (Treg) cells to facilitate tissue repair/regeneration, contains a heparan sulfate (HS) binding domain. How HS, a highly sulfated glycan subtype that alters growth factor signaling, influences Areg repair/regeneration functions is unclear. Here we report that inhibition of HS in various cell lines and primary lung mesenchymal cells (LMC) qualitatively alters downstream signaling and highlights the existence of HS-dependent vs. -independent Areg transcriptional signatures. Utilizing a panel of cell lines with targeted deletions in HS synthesis-related genes, we found that the presence of the glypican family of heparan sulfate proteoglycans is critical for Areg signaling and confirmed this dependency in primary LMC by siRNA-mediated knockdown. Furthermore, in the context of influenza A (IAV) infection in vivo , we found that an Areg-responsive subset of reparative LMC upregulate glypican-4 and HS. Conditional deletion of HS primarily within this LMC subset resulted in reduced blood oxygen saturation following infection with IAV, with no changes in viral load. Finally, we found that co-culture of HS-knockout LMC with IAV-induced Treg cells results in reduced LMC responses. Collectively, this study reveals the essentiality of HS on a specific lung mesenchymal population as a mediator of Treg cell-derived Areg reparative signaling during IAV infection.

Amphiregulin in infectious diseases: Role, mechanism, and potential therapeutic targets

Amphiregulin (AREG) serves as a ligand for the epidermal growth factor receptor (EGFR) and is involved in vital biological functions, including inflammatory responses, tissue regeneration, and immune system function. Upon interaction with the EGFR, AREG initiates a series of signaling cascades necessary for several physiological activities, such as metabolism, cell cycle regulation, and cellular proliferation. Recent findings have provided evidence for the substantial role of AREG in maintaining the equilibrium of homeostasis in damaged tissues and preserving epithelial cell structure in the context of viral infections affecting the lungs. The development of resistance to influenza virus infection depends on the presence of type 1 cytokine responses. Following the eradication of the pathogen, the lungs are subsequently colonized by several cell types that are linked with type 2 immune responses. These cells contribute to the process of repairing and resolving the tissue injury and inflammation caused by infections. Following influenza infection, the activation of AREG promotes the regeneration of bronchial epithelial cells, enhancing the tissue's structural integrity and increasing the survival rate of infected mice. In the same manner, mice afflicted with influenza experience rapid mortality due to a subsequent bacterial infection in the pulmonary region when both bacterial and viral infections manifest concurrently inside the same host. The involvement of AREG in bacterial infections has been demonstrated. The gene AREG experiences increased transcriptional activity inside host cells in response to bacterial infections caused by pathogens such as Escherichia coli and Neisseria gonorrhea. In addition, AREG has been extensively studied as a mitogenic stimulus in epithelial cell layers. Consequently, it is regarded as a prospective contender that might potentially contribute to the observed epithelial cell reactions in helminth infection. Consistent with this finding, mice that lack the AREG gene exhibit a delay in the eradication of the intestinal parasite Trichuris muris. The observed delay is associated with a reduction in the proliferation rate of colonic epithelial cells compared to the infected animals in the control group. The aforementioned findings indicate that AREG plays a pivotal role in facilitating the activation of defensive mechanisms inside the epithelial cells of the intestinal tissue. The precise cellular sources of AREG in this specific context have not yet been determined. However, it is evident that the increased proliferation of the epithelial cell layer in infected mice is reliant on CD4+ T cells. The significance of this finding lies in its demonstration of the crucial role played by the interaction between immunological and epithelial cells in regulating the AREG-EGFR pathway. Additional research is necessary to delve into the cellular origins and signaling mechanisms that govern the synthesis of AREG and its tissue-protective properties, independent of infection.

Regulation of the alveolar regenerative niche by amphiregulin-producing regulatory T cells

Following respiratory viral infection, regeneration of the epithelial barrier is required to preserve lung function and prevent secondary infections. Lung regulatory T (Treg) cells are critical for maintaining blood oxygenation following influenza virus infection through production of the EGFR ligand amphiregulin (Areg); however, how Treg cells engage with progenitors within the alveolar niche is unknown. Here, we describe local interactions between Treg cells and an Areg-responsive population of Col14a1+EGFR+ lung mesenchymal cells that mediate type II alveolar epithelial (AT2) cell-mediated regeneration following influenza virus infection. We propose a mechanism whereby Treg cells are deployed to sites of damage and provide pro-survival cues that support mesenchymal programming of the alveolar niche. In the absence of fibroblast EGFR signaling, we observe impaired AT2 proliferation and disrupted lung remodeling following viral clearance, uncovering a crucial immune/mesenchymal/epithelial network that guides alveolar regeneration.

Suppression of heparan sulfation re-sensitizes YAP1-driven melanoma to MAPK pathway inhibitors

Accumulating evidence identifies non-genetic mechanisms substantially contributing to drug resistance in cancer patients. Preclinical and clinical data implicate the transcriptional co-activators YAP1 and its paralog TAZ in resistance to multiple targeted therapies, highlighting the strong need for therapeutic strategies overcoming YAP1/TAZ-mediated resistance across tumor entities. Here, we show particularly high YAP1/TAZ activity in MITF^low/AXL^high melanomas characterized by resistance to MAPK pathway inhibition and broad receptor tyrosine kinase activity. To uncover genetic dependencies of melanoma cells with high YAP1/TAZ activity, we used a genome-wide CRISPR/Cas9 functional screen and identified SLC35B2, the 3′-phosphoadenosine-5′-phosphosulfate transporter of the Golgi apparatus, as an essential gene for YAP1/TAZ-driven drug resistance. SLC35B2 expression correlates with tumor progression, and its loss decreases heparan sulfate expression, reduces receptor tyrosine kinase activity, and sensitizes resistant melanoma cells to BRAF inhibition in vitro and in vivo. Thus, targeting heparan sulfation via SLC35B2 represents a novel approach for breaking receptor tyrosine kinase-mediated resistance to MAPK pathway inhibitors.

Notch4 signaling limits regulatory T-cell-mediated tissue repair and promotes severe lung inflammation in viral infections

A cardinal feature of COVID-19 is lung inflammation and respiratory failure. In a prospective multi-country cohort of COVID-19 patients, we found that increased Notch4 expression on circulating regulatory T (Treg) cells was associated with disease severity, predicted mortality, and declined upon recovery. Deletion of Notch4 in Treg cells or therapy with anti-Notch4 antibodies in conventional and humanized mice normalized the dysregulated innate immunity and rescued disease morbidity and mortality induced by a synthetic analog of viral RNA or by influenza H1N1 virus. Mechanistically, Notch4 suppressed the induction by interleukin-18 of amphiregulin, a cytokine necessary for tissue repair. Protection by Notch4 inhibition was recapitulated by therapy with Amphiregulin and, reciprocally, abrogated by its antagonism. Amphiregulin declined in COVID-19 subjects as a function of disease severity and Notch4 expression. Thus, Notch4 expression on Treg cells dynamically restrains amphiregulin-dependent tissue repair to promote severe lung inflammation, with therapeutic implications for COVID-19 and related infections.

Synthetic heparan sulfate ligands for vascular endothelial growth factor to modulate angiogenesis

We report the discovery of a potential heparan sulfate (HS) ligand to target several growth factors using 13 unique HS tetrasaccharide ligands. By employing an HS microarray and SPR, we deciphered the crucial structure-binding relationship of these glycans with the growth factors BMP2, VEGF₁₆₅, HB-EGF, and FGF2. Notably, GlcNHAc(6-O-SO₃^-)-IdoA(2-O-SO₃^-) (HT-2,6S-NAc) tetrasaccharide showed strong binding with the VEGF₁₆₅ growth factor. In vitro vascular endothelial cell proliferation, migration and angiogenesis was inhibited in the presence of VEGF₁₆₅ and HT-2,6S-NAc or HT-6S-NAc, revealing the potential therapeutic role of these synthetic HS ligands.

A systems perspective of heterocellular signaling

Signal exchange between different cell types is essential for development and function of multicellular organisms, and its dysregulation is causal in many diseases. Unfortunately, most cell-signaling work has employed single cell types grown under conditions unrelated to their native context. Recent technical developments have started to provide the tools needed to follow signaling between multiple cell types, but gaps in the information they provide have limited their usefulness in building realistic models of heterocellular signaling. Currently, only targeted assays have the necessary sensitivity, selectivity, and spatial resolution to usefully probe heterocellular signaling processes, but these are best used to test specific, mechanistic models. Decades of systems biology research with monocultures has provided a solid foundation for building models of heterocellular signaling, but current models lack a realistic description of regulated proteolysis and the feedback processes triggered within and between cells. Identification and understanding of key regulatory processes in the extracellular environment and of recursive signaling patterns between cells will be essential to building predictive models of heterocellular systems.

SMOC Binds to Pro-EGF, but Does Not Induce Erk Phosphorylation via the EGFR

In an attempt to identify the cell-associated protein(s) through which SMOC (Secreted Modular Calcium binding protein) induces mitogen-activated protein kinase (MAPK) signaling, the epidermal growth factor receptor (EGFR) became a candidate. However, although in 32D/EGFR cells, the EGFR was phosphorylated in the presence of a commercially available human SMOC-1 (hSMOC-1), only minimal phosphorylation was observed in the presence of Xenopus SMOC-1 (XSMOC-1) or human SMOC-2. Analysis of the commercial hSMOC-1 product demonstrated the presence of pro-EGF as an impurity. When the pro-EGF was removed, only minimal EGFR activation was observed, indicating that SMOC does not signal primarily through EGFR and its receptor remains unidentified. Investigation of SMOC/pro-EGF binding affinity revealed a strong interaction that does not require the C-terminal extracellular calcium-binding (EC) domain of SMOC or the EGF domain of pro-EGF. SMOC does not appear to potentiate or inhibit MAPK signaling in response to pro-EGF, but the interaction could provide a mechanism for retaining soluble pro-EGF at the cell surface.

Emerging functions of amphiregulin in orchestrating immunity, inflammation, and tissue repair

Type 2 inflammatory responses can be elicited by diverse stimuli, including toxins, venoms, allergens, and infectious agents, and play critical roles in resistance and tolerance associated with infection, wound healing, tissue repair, and tumor development. Emerging data suggest that in addition to characteristic type 2-associated cytokines, the epidermal growth factor (EGF)-like molecule Amphiregulin (AREG) might be a critical component of type 2-mediated resistance and tolerance. Notably, numerous studies demonstrate that in addition to the established role of epithelial- and mesenchymal-derived AREG, multiple leukocyte populations including mast cells, basophils, group 2 innate lymphoid cells (ILC2s), and a subset of tissue-resident regulatory CD4(+) T cells can express AREG. In this review, we discuss recent advances in our understanding of the AREG-EGF receptor pathway and its involvement in infection and inflammation and propose a model for the function of this pathway in the context of resistance and tissue tolerance.

Interactions of signaling proteins, growth factors and other proteins with heparan sulfate: mechanisms and mysteries

Heparan sulfate (HS) is a component of cell surface and matrix-associated proteoglycans (HSPGs) that, collectively, play crucial roles in many physiologic processes including cell differentiation, organ morphogenesis and cancer. A key function of HS is to bind and interact with signaling proteins, growth factors, plasma proteins, immune-modulators and other factors. In doing so, the HS chains and HSPGs are able to regulate protein distribution, bio-availability and action on target cells and can also serve as cell surface co-receptors, facilitating ligand-receptor interactions. These proteins contain an HS/heparin-binding domain (HBD) that mediates their association and contacts with HS. HBDs are highly diverse in sequence and predicted structure, contain clusters of basic amino acids (Lys and Arg) and possess an overall net positive charge, most often within a consensus Cardin-Weintraub (CW) motif. Interestingly, other domains and residues are now known to influence protein-HS interactions, as well as interactions with other glycosaminoglycans, such as chondroitin sulfate. In this review, we provide a description and analysis of HBDs in proteins including amphiregulin, fibroblast growth factor family members, heparanase, sclerostin and hedgehog protein family members. We discuss HBD structural and functional features and important roles carried out by other protein domains, and also provide novel conformational insights into the diversity of CW motifs present in Sonic, Indian and Desert hedgehogs. Finally, we review progress in understanding the pathogenesis of a rare pediatric skeletal disorder, Hereditary Multiple Exostoses (HME), characterized by HS deficiency and cartilage tumor formation. Advances in understanding protein-HS interactions will have broad implications for basic biology and translational medicine as well as for the development of HS-based therapeutics.

Oncogenic signaling in amphiregulin and EGFR-expressing PTEN-null human breast cancer

A subset of triple negative breast cancer (TNBC) is characterized by overexpression of the epidermal growth factor receptor (EGFR) and loss of PTEN, and patients with these determinants have a poor prognosis. We used cell line models of EGFR‐positive/PTEN null TNBC to elucidate the signaling networks that drive the malignant features of these cells and cause resistance to EGFR inhibitors. In these cells, amphiregulin (AREG)‐mediated activation of EGFR results in up‐regulation of fibronectin (FN1), which is known to be a mediator of invasive capacity via interaction with integrin β1. EGFR activity in this PTEN null background also results in Wnt/beta‐catenin signaling and activation of NF‐κB. In addition, AKT is constitutively phosphorylated in these cells and is resistant to gefitinib. Expression profiling demonstrated that AREG‐activated EGFR regulates gene expression differently than EGF‐activated EGFR, and functional analysis via genome‐scale shRNA screening identified a set of genes, including PLK1 and BIRC5, that are essential for survival of SUM‐149 cells, but are uncoupled from EGFR signaling. Thus, our results demonstrate that in cells with constitutive EGFR activation and PTEN loss, critical survival genes are uncoupled from regulation by EGFR, which likely mediates resistance to EGFR inhibitors.

Activation of EGFR by AREG alters signaling and gene expression compared to EGF.

Activation of EGFR by AREG reduces mTORC1 pathway expression and phosphorylation.

EGF‐positive, PTEN‐null TNBC cells are poised for Wnt/beta‐catenin signaling.

Wnt/beta‐catenin activity occurs in a subset of cells and is enhanced in mammospheres.

Regulation of growth/survival genes is uncoupled from EGFR in PTEN‐null TNBC cells.

Amphiregulin

Amphiregulin (AREG) is a ligand of the epidermal growth factor receptor (EGFR), a widely expressed transmembrane tyrosine kinase. AREG is synthesized as a membrane-anchored precursor protein that can engage in juxtacrine signaling on adjacent cells. Alternatively, after proteolytic processing by cell membrane proteases, mainly TACE/ADAM17, AREG is secreted and behaves as an autocrine or paracrine factor. AREG gene expression and release is induced by a plethora of stimuli including inflammatory lipids, cytokines, hormones, growth factors and xenobiotics. Through EGFR binding AREG activates major intracellular signaling cascades governing cell survival, proliferation and motility. Physiologically, AREG plays an important role in the development and maturation of mammary glands, bone tissue and oocytes. Chronic elevation of AREG expression is increasingly associated with different pathological conditions, mostly of inflammatory and/or neoplastic nature. Here we review the essential aspects of AREG structure, function and regulation, discuss the basis for its differential role within the EGFR family of ligands, and identify emerging aspects in AREG research with translational potential.

The heparin-binding domain of HB-EGF mediates localization to sites of cell-cell contact and prevents HB-EGF proteolytic release

Heparin-binding EGF-like growth factor (HB-EGF) is a ligand for EGF receptor (EGFR) and possesses the ability to signal in juxtacrine, autocrine and/or paracrine mode, with these alternatives being governed by the degree of proteolytic release of the ligand. Although the spatial range of diffusion of released HB-EGF is restricted by binding heparan-sulfate proteoglycans (HSPGs) in the extracellular matrix and/or cellular glycocalyx, ascertaining mechanisms governing non-released HB-EGF localization is also important for understanding its effects. We have employed a new method for independently tracking the localization of the extracellular EGF-like domain of HB-EGF and the cytoplasmic C-terminus. A striking observation was the absence of the HB-EGF transmembrane pro-form from the leading edge of COS-7 cells in a wound-closure assay; instead, this protein localized in regions of cell-cell contact. A battery of detailed experiments found that this localization derives from a trans interaction between extracellular HSPGs and the HB-EGF heparin-binding domain, and that disruption of this interaction leads to increased release of soluble ligand and a switch in cell phenotype from juxtacrine-induced growth inhibition to autocrine-induced proliferation. Our results indicate that extracellular HSPGs serve to sequester the transmembrane pro-form of HB-EGF at the point of cell-cell contact, and that this plays a role in governing the balance between juxtacrine versus autocrine and paracrine signaling.

A truncated soluble epidermal growth factor receptor-Fc fusion ligand trap displays anti-tumour activity in vivo

A number of therapeutic strategies including small molecule tyrosine kinase inhibitors and monoclonal antibodies have been developed to target the epidermal growth factor receptor (EGFR) signalling axis for the treatment of cancer. To date, the focus of therapeutic intervention has been the EGFR itself. In the current study, we have assembled and expressed in mammalian cells a soluble, EGFR ligand trap comprising the first 501 amino acids of the mature EGFR sequence fused in-frame with a human IgG Fc domain. The fusion protein, designated sEGFR501.Fc, was secreted as a 220 kDa disulphide-linked homodimer that exhibited high affinity (0.4-8 nM) in competition assays for a number of EGFR ligands including EGF and transforming growth factor-alpha (TGF-alpha). sEGFR501.Fc inhibited EGF-stimulated tyrosine phosphorylation of the EGFR of the lung cancer cell lines A549 and H1437, and inhibited and blocked the proliferation of H1437 cells. Administration of sEGFR501.Fc to mice bearing human tumour xenografts derived from A431 (epidermoid carcinoma) and DU145 (androgen-independent prostate cancer) tumour cell lines resulted in modest retardation of tumour growth. These results provide proof-in-principle that using high affinity soluble receptors is a viable method for inhibiting multi-ligand systems, and the impetus to optimize this approach and develop reagents with greater affinity and broader specificity.

Amphiregulin as a novel target for breast cancer therapy

Amphiregulin, an EGF family growth factor, binds and activates the epidermal growth factor receptor (EGFR or ErbB1). Activation of the EGFR by amphiregulin can occur through autocrine, paracrine and juxtacrine mechanisms. Amphiregulin plays a role in several biological processes including nerve regeneration, blastocyst implantation, and bone formation. Amphiregulin also plays an important role in mammary duct formation as well as the outgrowth and branching of several other human tissues such as the lung, kidney and prostate. This effect is most likely due to the induction of genes involved in invasion and migration such as cytokines and matrix metalloproteases. Clinical studies have suggested that amphiregulin also plays a role in human breast cancer progression and its expression has been associated with aggressive disease. Therefore, amphiregulin may be a novel and effective target for the treatment of breast cancer and could represent an alternative to targeting the EGFR.

Amphiregulin: role in mammary gland development and breast cancer

Extensive epithelial cell proliferation underlies the ductal morphogenesis of puberty that generates the mammary tree that will eventually fill the fat pad. This estrogen-dependent process is believed to be essentially dependent on locally produced growth factors that act in a paracrine fashion. EGF-like growth factor ligands, acting through EGF receptors are some of the principal promoters of pubertal ductal morphogenesis. Amphiregulin is the most abundant EGF-like growth factor in the pubertal mammary gland. Its gene is transcriptionally regulated by ERalpha, and recent evidence identifies it as a key mediator of the estrogen-driven epithelial cell proliferation of puberty: The pubertal deficiency in mammary gland ductal morphogenesis in ERalpha, amphiregulin, and EGFR knockout mice phenocopy each other. As a prognostic indicator in human breast cancer, amphiregulin indicates an outcome identical to that predicted by ERalpha presence. Despite this, a range of studies both on preneoplastic human breast tissue and on cell culture based models of breast cancer, suggest a possibly significant role for amphiregulin in driving human breast cancer progression. Here we summarise our current understanding of amphiregulin's contribution to mammary gland development and breast cancer progression.

The binding of human betacellulin to heparin, heparan sulfate and related polysaccharides

Recombinant human betacellulin binds strongly to heparin, requiring of the order of 0.8 M NaCl for its elution from a heparin affinity matrix. This is in complete contrast to the prototypic member of its cytokine superfamily, epidermal growth factor, which fails to bind to the column at physiological pH and strength. We used a well-established heparin binding ELISA to demonstrate that fucoidan and a highly sulfated variant of heparan sulfate compete strongly for heparin binding. Low sulfated heparan sulfates and also chondroitin sulfates are weaker competitors. Moreover, although competitive activity is reduced by selective desulfation, residual binding to extensively desulfated heparin remains. Even carboxyl reduction followed by extensive desulfation does not completely remove activity. We further demonstrate that both hyaluronic acid and the E. coli capsular polysaccharide K5, both of which are unsulfated polysaccharides with unbranched chains of alternating N-acetylglucosamine linked beta(1-4) to glucuronic acid, are also capable of a limited degree of competition with heparin. Heparin protects betacellulin from proteolysis by LysC, but K5 polysaccharide does not. Betacellulin possesses a prominent cluster of basic residues, which is likely to constitute a binding site for sulfated polysaccharides, but the binding of nonsulfated polysaccharides may take place at a different site.

Amphiregulin: A new growth factor in hepatocarcinogenesis

Amphiregulin (AR) is a member of the epidermal growth factor family and a ligand of the epidermal growth factor receptor (EGFR). As other ligands of the EGFR, AR is synthesized as a precursor that is shed from the plasma membrane by metalloproteases. Hyperactive autocrine loops involving AR production have been described in a variety of tumors, and this growth factor is thought to play a non-redundant role in cancer development. AR expression is not detected in the normal liver, however it is readily induced during acute liver injury and behaves as a potent pro-regenerative and survival factor. Increased AR expression is also detected in human chronic liver injury (liver cirrhosis), which is considered a pre-neoplastic condition. Recent evidences suggest that AR can play a unique role in liver tumorigenesis and in the maintenance of the neoplastic phenotype of hepatocarcinoma cells. In this review, we summarize some aspects of AR patho-biology and the rationale behind its definition as a novel target in hepatocarcinoma therapy.

Depolarization-induced, glutamate receptor–mediated, and transactivation-dependent extracellular-signal regulated kinase phosphorylation in cultured cerebellar granule neurons

Depolarization of 7-8-day-old mouse cerebellar granule neurons in primary cultures, a glutamatergic preparation, by elevation of the extracellular potassium ion concentration ([K+]e) to 45 mM induces an increase of phosphorylation of extracellular-signal regulated kinase 1 and 2 (ERK1/2) at two time periods: 20 min and 60 min after the [K+]e increase. This effect can be mimicked by 5 min of exposure to 50 microM glutamate, suggesting that ERK1/2 phosphorylation in response to the depolarization is brought about by the resulting glutamate release. This concept is supported by the observation that the K+ -mediated stimulation of phosphorylation at both times is inhibited by MK-801, an NMDA antagonist, and by CNQX, an AMPA/kainate antagonist. These antagonists also inhibit the response to glutamate. Both increases in ERK1/2 phosphorylation are also inhibited by GM 6001 (a metalloproteinase inhibitor, preventing 'shedding' of growth factors), by AG 1478 (a receptor tyrosine kinase inhibitor, preventing epidermal growth factor [EGF] receptor activation), and also partly by heparin (inactivating heparin-binding epidermal growth factor [HB-EGF]), suggesting transactivation of epidermal growth factor receptors (EGFR). Transactivation is an intracellular/extracellular signal transduction pathway in which release from receptor- or depolarization-stimulated cells of EGFR ligand(s) (including HB-EGF), catalyzed by a metalloproteinase, stimulates receptor tyrosine kinases on the same (an autocrine effect) or adjacent (a paracrine effect) cells. The expression of HB-EGF as well as of transforming growth factor-alpha (TGF-alpha), two of the EGFR ligands, in the cells was confirmed by reverse transcription polymerase chain reaction, and the only partial inhibition by heparin suggests that both of these EGFR agonists are involved. Such a transactivation may play a major role in glutamate-mediated signaling and plasticity.

Loss of HSulf-1 expression enhances autocrine signaling mediated by amphiregulin in breast cancer

Heparan sulfate (HS) glycosaminoglycans are the oligosaccharide chains of heparan sulfate proteoglycans. The sulfation of HS glycosaminoglycan residues is required for its interaction with various heparin-binding growth factors to promote their biological activities to activate their high affinity receptor tyrosine kinases. We have identified HS glycosaminoglycan-6-O-endosulfatase HSulf-1 as a down-regulated gene in ovarian, breast, and several other cancer cell lines. Here we have shown that HSulf-1 inhibits autocrine activation of the EGFR-ERK (epidermal growth factor receptor-extracellular signal-regulated kinase) pathway induced by serum withdrawal in MDA-MB-468 breast cancer cells. Short hairpin RNA-mediated down-regulation of HSulf-1 in HSulf-1 clonal lines of MDA-MB-468 led to a significant increase in autocrine activation of ERK compared with vector only control. The autocrine signaling was also inhibited with neutralization antibodies against amphiregulin and HB-EGF, the heparin-binding growth factor family of the EGF superfamily. Furthermore, HSulf-1-mediated inhibition of autocrine signaling was associated with reduced cyclin D1 levels, leading to decreased S phase fraction and increased G(2)-M fraction, as well as increased cell death. Finally, evaluation of HSulf-1 expression levels in primary invasive breast tumors by RNA in situ hybridization indicated that HSulf-1 is down-regulated in the majority (60%) of tumors, with a predominant association with lobular histology. These data suggest a potential role of HSulf-1 down-regulation in mammary carcinogenesis.

Heparan sulphate proteoglycans are essential for the myeloma cell growth activity of EGF-family ligands in multiple myeloma

The epidermal growth factor (EGF)/EGF-receptor (ErbB1-4) family is involved in the biology of multiple myeloma (MM). In particular, ErbB-specific inhibitors induce strong apoptosis of myeloma cells (MMC) in vitro. To delineate the contribution of the 10 EGF-family ligands to the pathogenesis of MM, we have assessed their expression and biological activity. Comparing Affymetrix DNA-microarray-expression-profiles of CD138-purified plasma-cells from 65 MM-patients and 7 normal individuals to those of plasmablasts and B-cells, we found 5/10 EGF-family genes to be expressed in MMC. Neuregulin-2 and neuregulin-3 were expressed by MMC only, while neuregulin-1, amphiregulin and transforming growth factor-alpha were expressed by both MMC and normal plasma-cells. Using real-time polymerase chain reaction, we found HB-EGF, amphiregulin, neuregulin-1 and epiregulin to be expressed by cells from the bone marrow-environment. Only the EGF-members able to bind heparan-sulphate proteoglycans (HSPGs) - neuregulin-1, amphiregulin, HB-EGF - promote the growth of MMC. Those ligands strongly bind MMC through HSPGs. The binding and the MMC growth activity was abrogated by heparitinase, heparin or deletion of the HS-binding domain. The number of HS-binding EGF ligand molecules bound to MMC was higher than 10(5) molecules/cell and paralleled that of syndecan-1. Syndecan-1, the main HSPG present on MM cells, likely concentrates high levels of HS-binding-EGF-ligands at the cell membrane and facilitates ErbB-activation. Altogether, our data further identify EGF-signalling as promising target for MM-therapy.

Myristoylated Naked2 escorts transforming growth factor alpha to the basolateral plasma membrane of polarized epithelial cells

The epidermal growth factor receptor ligands transforming growth factor alpha (TGF alpha) and amphiregulin are delivered to the basolateral surface of polarized epithelial cells where they are cleaved by TACE/ADAM17. Basolateral sorting information resides in their cytoplasmic tail domains, but tail-interacting proteins required for basolateral trafficking have not been identified. Naked (NKD)1 and NKD2 are mammalian homologs of Drosophila Naked Cuticle, which negatively regulates canonical Wnt signaling by binding Dishevelled. We present evidence that NKD2, but not NKD1, binds to basolateral sorting motifs in the cytoplasmic tail of TGF alpha. Processing and cell-surface delivery of TGF alpha are accelerated in NKD2-overexpressing Madin-Darby canine kidney cells. NKD2 is myristoylated on glycine, the second residue. On expression of myristoylation-defective (G2A) NKD2, neither NKD2 nor TGF alpha appears at the basolateral plasma membrane of polarized Madin-Darby canine kidney cells; however, membrane staining for TGF alpha is restored on silencing expression of this mutant NKD2. Amphiregulin does not interact with NKD2 and retains its basolateral localization in G2A-NKD2-expressing cells, as do Na(+), K(+) ATPase alpha 1 and E-cadherin. These data identify an unexpected function for NKD2, i.e., myristoylation-dependent escort of TGF alpha to the basolateral plasma membrane of polarized epithelial cells.

Heparin-derived disaccharides modulate proliferation and Erb-B2-mediated signal transduction in colon cancer cell lines

Organ-specific extracellular matrix (ECM) determines metastasis formation by regulating tumor cell proliferation. Hepatocyte-derived ECM enhances proliferation of colon cancer cell lines by increasing expression of tyrosine kinase receptors of the erb-B family. The active components in the ECM are the heparan sulfates, which are highly heterogeneous in their chemistry and size. We determined the effect of heparan sulfate disaccharides, of defined chemistry and present in high amounts in the liver heparan sulfate chains, on the proliferation of colon cancer cell lines and investigated the mechanism involved. The low-metastatic cell line KM12 was stimulated to proliferate by a highly sulfated disaccharide, found in the highest amounts in hepatocyte-derived heparan sulfate. Growth of the highly metastatic cell line KM12SM was inhibited by the second most common disaccharide in hepatocyte-derived heparan sulfate. The effect of both disaccharides was not accompanied by changes in the expression of erb-B1, erb-B2, erb-B3 or heregulin-alpha. We determined whether the disaccharides modified the signal-transduction pathways mediated by the erb-B receptors. The erb-B2-specific tyrosine kinase inhibitor AG825 abolished the enhancement of KM12 cell proliferation by the stimulatory disaccharide. This disaccharide increased tyrosine phosphorylation of erb-B1 and erb-B2 receptors, effects that were abolished by AG825. Moreover, the disaccharide caused increased expression of cyclin D1 and of activated MAP kinase, again reduced in the presence of the inhibitor AG825. The growth-inhibitory disaccharide reduced phosphorylation of erb-B1, but not of erb-B2, receptors in KM12SM cells. In conclusion, not only hepatocyte-derived heparan sulfate but also disaccharide molecules derived from heparan sulfate can affect colon cancer cell proliferation. Their effect is mediated by modulation of the erb-B signal transduction.

Antisense expression for amphiregulin suppresses tumorigenicity of a transformed human breast epithelial cell line

The epidermal growth factor (EFG) family of receptors and their respective ligands play a major role in breast cancer progression and are the targets of new therapeutic approaches. Following immortalization with SV40 T antigen of normal human breast epithelial cells, a transformed variant cell line (NS2T2A1) was selected for its increased tumorigenicity in nude mice. This cell line was shown to have a higher expression of EGF receptors (EGFR) and amphiregulin (AR) when compared to their normal counterparts or less aggressive transformed cells. Dual staining of EGFR and AR was observed in 50-60% of NS2T2A1 cells, while 30-40% cells expressed AR only. To explore the potential tumorigenic role of AR, a 1.1 kb AR cDNA in an antisense orientation was transfected in NS2T2A1 cells. Three clones, selected by hygromycin B, expressed AR antisense RNA (AR AS1, AR AS2 and AR AS3 cell lines) in which AR protein expression was reduced (ranging from about 50 to < 5%). The anchorage-independent growth of AR AS cell lines was reduced to levels ranging from 32.4-6.8% relative to the control cell line transfected with the vector alone. The clones expressing AR antisense RNA showed a reversion of the malignant phenotype when injected in nude mice, since a significant reduction of tumor intake was observed coincident with a significant tumor mass reduction (> 96%). Moreover, intra-tumoral vascularization decreased significantly in tumors derived from AR AS cells (26.7, 70.7 and 50.4% of control). These in vitro and in vivo data reveal the oncogenic nature of AR in transformed breast epithelial cells and imply a role for AR in tumor angiogenesis.

Metalloprotease-mediated ligand release regulates autocrine signaling through the epidermal growth factor receptor

Ligands that activate the epidermal growth factor receptor (EGFR) are synthesized as membrane-anchored precursors that appear to be proteolytically released by members of the ADAM family of metalloproteases. Because membrane-anchored EGFR ligands are thought to be biologically active, the role of ligand release in the regulation of EGFR signaling is unclear. To investigate this question, we used metalloprotease inhibitors to block EGFR ligand release from human mammary epithelial cells. These cells express both transforming growth factor alpha and amphiregulin and require autocrine signaling through the EGFR for proliferation and migration. We found that metalloprotease inhibitors reduced cell proliferation in direct proportion to their effect on transforming growth factor alpha release. Metalloprotease inhibitors also reduced growth of EGF-responsive tumorigenic cell lines and were synergistic with the inhibitory effects of antagonistic EGFR antibodies. Blocking release of EGFR ligands also strongly inhibited autocrine activation of the EGFR and reduced both the rate and persistence of cell migration. The effects of metalloprotease inhibitors could be reversed by either adding exogenous EGF or by expressing an artificial gene for EGF that lacked a membrane-anchoring domain. Our results indicate that soluble rather than membrane-anchored forms of the ligands mediate most of the biological effects of EGFR ligands. Metalloprotease inhibitors have shown promise in preventing spread of metastatic disease. Many of their antimetastatic effects could be the result of their ability to inhibit autocrine signaling through the EGFR.

A differential requirement for the COOH-terminal region of the epidermal growth factor (EGF) receptor in amphiregulin and EGF mitogenic signaling

The epidermal growth factor receptor (EGFR) mediates the actions of a family of bioactive peptides that include epidermal growth factor (EGF) and amphiregulin (AR). Here we have studied AR and EGF mitogenic signaling in EGFR-devoid NR6 fibroblasts that ectopically express either wild type EGFR (WT) or a truncated EGFR that lacks the three major sites of autophosphorylation (c'1000). COOH-terminal truncation of the EGFR significantly impairs the ability of AR to (i) stimulate DNA synthesis, (ii) elicit Elk-1 transactivation, and (iii) generate sustained enzymatic activation of mitogen-activated protein kinase. EGFR truncation had no significant effect on AR binding to receptor but did result in defective GRB2 adaptor function. In contrast, EGFR truncation did not impair EGF mitogenic signaling, and in c'1000 cells EGF was able to stimulate the association of ErbB2 with GRB2 and SHC. Elk-1 transactivation was monitored when either ErbB2 or a truncated dominant-negative ErbB2 mutant (ErbB2-(1-813)) was overexpressed in cells. Overexpression of full-length ErbB2 resulted in a strong constitutive transactivation of Elk-1 in c'1000 but only slightly stimulated Elk-1 in WT or parental NR6 cells. Conversely, overexpression of ErbB2-(1-813) inhibited EGF-stimulated Elk-1 transactivation in c'1000 but not in WT cells. Thus, the cytoplasmic tail of the EGFR plays a critical role in AR mitogenic signaling but is dispensable for EGF, since EGF-activated truncated EGFRs can signal through ErbB2.

Syndecan-1 expression inhibits myoblast differentiation through a basic fibroblast growth factor-dependent mechanism

Expression of syndecan-1, a cell-surface heparan sulfate proteoglycan, is down-regulated during skeletal muscle differentiation (Larraín, J., Cizmeci-Smith, G., Troncoso, V., Stahl, R. C., Carey, D. J., and Brandan, E. (1997) J. Biol. Chem. 272, 18418-18424). We examined the role of syndecan-1 in basic fibroblast growth factor (bFGF)-dependent inhibition of myogenesis. C2C12 myoblasts were stably transfected with an expression plasmid containing the rat syndecan-1 coding region cDNA. Constitutive syndecan-1 expression resulted in a strongly diminished capacity of the transfected clones to differentiate and to express skeletal muscle-specific markers such as fusion, creatine kinase, and myosin. The expression of myogenin, a master transcription factor for muscle differentiation, was also reduced and delayed. Analysis of the induction of a myogenin promoter-driven reporter revealed that syndecan-1 expression resulted in a 6-7-fold increase in sensitivity to bFGF-dependent inhibition of myogenin expression. Transfecting the cells with a plasmid containing myogenin cDNA reversed the inhibition of myogenin transcriptional activation and myosin expression in syndecan-1-transfected cells; however, cell fusion was not observed. These results demonstrate that syndecan-1 expression enhances cell responsiveness to bFGF and inhibits myoblast fusion and suggest that muscle terminal differentiation is regulated by syndecan-1 expression.

Hepatocyte extracellular matrix modulates expression of growth factors and growth factor receptors in human colon cancer cells

We investigated the role of hepatocyte extracellular matrix (ECM) on the growth of human colon cancer cell lines. We cultured four cell lines with different liver-colonizing potential on ECM derived from primary rat hepatocyte cultures. We investigated the effect of ECM on cell proliferation, clonal growth, and expression of growth factors and growth factor receptors. The highly metastatic cells showed better clonal growth and produced larger colonies on ECM. The proliferation of all colon cancer cell lines was enhanced on hepatocyte ECM, yet inhibited on fibroblast ECM. Screening of autocrine growth factors and receptors showed that the cells expressed growth factors and receptors of the EGF family: EGF receptor, erb-B2, amphiregulin, and cripto. The expression of cripto mRNA, but not of amphiregulin, was induced in KM12SM cells grown on ECM. All colon cancer cell lines grown on ECM showed increased expression of erb-B2. The effect of ECM on erb-B2 expression was mediated by the heparin chains of heparin proteoglycan. ECM from hepatocytes grown in the presence of nitrophenyl-beta-D-xylopyrannoside or sodium chlorate, which prevent formation of heparin proteoglycan, as well as ECM treated with heparinase, had no effect on erb-B2 expression. Our studies suggest a role for liver ECM as a determinant of colon cancer metastasis. Liver ECM acts, in part, via induction of members of the EGF family of growth factors and their receptors.

Autocrine regulation of keratinocytes: the emerging role of heparin-binding, epidermal growth factor-related growth factors

Although originally conceived as a basis for malignant cell growth, autocrine signaling networks are currently known to be activated during tissue repair and with in vitro cultivation. In human epidermal keratinocytes, activation of the epidermal growth factor receptor by cognate ligands mediates the majority of the autonomous replicative capacity of these cells and is necessary to inhibit differentiation and apoptosis. The importance of heparin-binding growth factors in activation of this receptor was first suggested by the strong anti-proliferative effects of soluble heparin-like molecules on keratinocyte growth. This and related evidence led to the identification of amphiregulin as a major autocrine factor for keratinocytes. The binding of amphiregulin and its homolog, heparin-binding epidermal growth factor-like growth factor, to the receptor is potentially amplified by autoinduction and cross-signaling through epidermal growth factor-related polypeptides and by transmodulation of other ErbB-family receptors (HER-2, -3, -4) in cells expressing these receptors. Heparan sulfate proteoglycans and the tetraspanin family of membrane-associated proteins appear to act as cofactors in amphiregulin-driven mitogenesis mediated by the epidermal growth factor receptor, but amphiregulin's immunolocalization to keratinocyte nuclei and to filopodia may indicate other potentially novel effects. Following from the observation that amphiregulin is overexpressed in lesional psoriatic epidermis, the importance of amphiregulin in hyperproliferative skin diseases has been further supported by recent studies of the targeted expression of a transgene encoding keratin 14 promoter-driven human amphiregulin to the basal epidermis of mice. Founder transgenic mice displayed a morphologic and microscopic cutaneous phenotype that shares characteristics with psoriasis. Pharmacologic regulation of amphiregulin's expression and receptor signaling may eventually prove to be an effective strategy in the treatment of hyperproliferative skin diseases.

Extracellular matrix modulates expression of growth factors and growth-factor receptors in liver-colonizing colon-cancer cell lines

Site-specific metastasis is determined by the extracellular matrix (ECM) of the colonized organ. We have shown that hepatocyte-derived ECM stimulated proliferation of colon-cancer cells via induction of autocrine growth factors and their receptors. The ECM component responsible was heparin proteoglycan. We therefore investigated the effect of exogenously added heparin on colon cell lines of varying liver-colonizing potential. The cells were grown on typical liver matrix components, such as fibronectin and collagens type I and IV. We assessed the effect of these matrix components on clonal growth, proliferation and expression of autocrine growth factors and their receptors. The clonal growth of the KM12 cells was not affected by heparin, while the other cell lines were inhibited by heparin. Cell proliferation in weakly metastatic KM12, but not in strongly metastatic KM12SM, was inhibited by heparin on plastic. Weakly metastatic LS174T, but not strongly metastatic LiM6, was inhibited by heparin on fibronectin. Expression of erb-B2 was also differently modulated by heparin in weakly metastatic vs. highly metastatic cells. In weakly metastatic cells, heparin reduced erb-B2 levels when cells were on plastic and fibronectin, while in strongly metastatic cells, erb-B2 was induced by heparin. In all 4 cell lines, mRNA for cripto was induced by heparin when the cells were grown on fibronectin. In KM12SM cells, amphiregulin was induced by heparin in cells on fibronectin and collagen IV. We show that soluble heparin, similar in its carbohydrate chemistry to liver heparin proteoglycan, regulates the growth of colon-cancer cells. This effect depends on other matrix components found in the liver and is mediated in part by EGF family members.

Cell surface ectodomain cleavage of human amphiregulin precursor is sensitive to a metalloprotease inhibitor. Release of a predominant N-glycosylated 43-kDa soluble form

Biosynthesis and processing of amphiregulin (AR) have been investigated in human colorectal (HCA-7, Caco-2) and mammary (MCF-7) cancer cell lines, as well as in Madin-Darby canine kidney cells stably expressing various human AR precursor (pro-AR) forms. Both cells expressing endogenous and transfected AR produce multiple cellular and soluble forms of AR with an N-glycosylated 50-kDa pro-AR form being predominant. Our results demonstrate that sequential proteolytic cleavage within the ectodomain of the 50-kDa pro-AR form leads to release of a predominant N-glycosylated 43-kDa soluble AR, as well as the appearance of other cellular and soluble AR forms. Cell surface biotinylation studies using a C-terminal epitope-tagged pro-AR indicate that all cell surface forms are membrane-anchored and support that AR is released by ectodomain cleavage of pro-AR at the plasma membrane. We also show that pro-AR ectodomain cleavage is a regulated process, which can be stimulated by phorbol 12-myristate 13-acetate and inhibited by the metalloprotease inhibitor, batimastat. In addition, we provide evidence that high molecular mass AR forms may retain the full-length N-terminal pro-region, which may influence the biological activities of these forms.

In vivo analysis of Argos structure-function. Sequence requirements for inhibition of the Drosophila epidermal growth factor receptor

The Drosophila Argos protein is the only known extracellular inhibitor of the epidermal growth factor receptor (EGFR). It is structurally related to the activating ligands, in that it is a secreted protein with a single epidermal growth factor (EGF) domain. To understand the mechanism of Argos inhibition, we have investigated which regions of the protein are essential. A series of deletions were made and tested in vivo; furthermore, by analyzing chimeric proteins between Argos and the activating ligand, Spitz (a transforming growth factor-alpha-like factor), we have examined what makes one inhibitory and the other activating. Our results reveal that Argos has structural requirements that differ from all known EGFR activating ligands; domains flanking the EGF domain are essential for its function. We have also defined the important regions of the atypical Argos EGF domain. The extended B-loop is necessary, whereas the C-loop can be replaced with the equivalent Spitz region without substantially affecting Argos function. Comparison of the argos genes from Drosophila melanogaster and the housefly, Musca domestica, supports our structure-function analysis. These studies are a prerequisite for understanding how Argos inhibits the Drosophila EGFR and provide a basis for designing mammalian EGFR inhibitors.

Topography of amphiregulin expression in cultured human keratinocytes: colocalization with the epidermal growth factor receptor and CD44

Much of the autonomous growth of cultured keratinocytes is attributable to the signaling of amphiregulin, a heparin-binding autocrine growth factor, through the epidermal growth factor receptor. Emerging evidence suggests, moreover, that the membrane proteoglycan, CD44, is a cofactor for the interaction of heparin-binding ligands with their receptors. This model was evaluated by characterizing the patterns of the immunolabeled molecules in cultured human neonatal keratinocytes, to test the hypothesis that involvement in a common function results in coordinate segregation within or on the cell. The molecules were localized by double immunofluorescence labeling to detect amphiregulin and either the epidermal growth factor receptor or CD44, and the immunostained products were imaged by scanning laser confocal microscopy. Both amphiregulin and the epidermal growth factor receptor segregated to a perinuclear distribution and to intercellular contacts. In addition, amphiregulin localized to the outer leading edge of colonies and focally to intranuclear sites. Metabolic blockade of proteoglycan sulfation with sodium chlorate inhibited growth of the cells and concurrently enhanced the nuclear, but decreased the outer leading edge, labeling for amphiregulin. There was no nuclear or perimeter labeling for the epidermal growth factor receptor. Cultures co-immunolabeled for CD44 and amphiregulin exhibited variable perinuclear staining for both, but otherwise CD44 was distributed to intercellular contacts. The intercellular localizations of CD44 with amphiregulin and of amphiregulin with the epidermal growth factor receptor were strongly concordant. These data are consistent with a concerted function at intercellular contacts, where cytokine signaling is mediated via receptor binding and possibly regulated by the CD44 proteoglycan as cofactor. The intranuclear and perimeter labeling of amphiregulin, however, suggests that this cytokine has additional functions, both in the nucleus and as a matrix receptor.

Engineered mutants of HGF/SF with reduced binding to heparan sulphate proteoglycans, decreased clearance and enhanced activity in vivo

BACKGROUND

Although a number of growth factors bind cell-surface heparan sulphate proteoglycans (HSPGs), the role of this interaction is unclear except for fibroblast growth factor which requires HSPG binding for signalling. Hepatocyte growth factor/scatter factor (HGF/SF) plays important roles in mammalian development and tissue regeneration and acts on target cells through a specific receptor tyrosine kinase encoded by the c-met proto-oncogene. This factor also binds HSPGs with high affinity, but conflicting data have been reported on the role of HSPG binding in HGF/SF signalling.

RESULTS

To map the binding sites for HSPG and the Met receptor in HGF/SF, we have engineered a number of HGF/SF mutants in which several clusters of solvent-accessible residues in the hairpin structure of the amino-terminal domain or in kringle 2 have been replaced. Two of the mutants (HP1 and HP2) showed greatly decreased (more than 50-fold) affinity for heparin and HSPGs but retained full mitogenic and motogenic activities on target cells in culture. Furthermore, when compared with wild-type HGF/SF, the HP1 mutant exhibited a delayed clearance from the blood, higher tissue levels and a higher induction of DNA synthesis in normal, adult murine liver.

CONCLUSIONS

These results establish the following: the binding sites in HGF/SF for Met and for HSPGs can be dissociated by protein engineering; high-affinity binding of HGF/SF to HSPGs is not essential for signalling; one role of HSPG binding in the HGF/SF system appears to be sequestration and degradation of the growth factor; and HGF/SF mutants with decreased affinity for HSPGs exhibit enhanced activity in vivo.

Epidermal growth factor receptor (EGFR) and EGFR mutations, function and possible role in clinical trials

The epidermal growth factor receptor (EGFR) is a growth factor receptor that induces cell differentiation and proliferation upon activation through the binding of one of its ligands. The receptor is located at the cell surface, where the binding of a ligand activates a tyrosine kinase in the intracellular region of the receptor. This tyrosine kinase phosphorylates a number of intracellular substrates that activates pathways leading to cell growth, DNA synthesis and the expression of oncogenes such as fos and jun. EGFR is thought to be involved the development of cancer, as the EGFR gene is often amplified, and/or mutated in cancer cells. In this review we will focus on: (I) the structure and function of EGFR, (II) implications of receptor/ligand coexpression and EGFR mutations or overexpression, (III) its effect on cancer cells, (IV) the development of the malignant phenotype and (V) the clinical aspects of therapeutic targeting of EGFR.

Differential effects of EGF and amphiregulin on adhesion molecule expression and migration of colon carcinoma cells

Epidermal growth factor (EGF) is a potent morphogen affecting cell shape and motility through regulation of adhesive interactions. We have characterized the morphological effects of EGF on GP2d and GP5d colon carcinoma cell lines and have compared the ability of the heparin-binding EGF receptor ligand amphiregulin (AR) to elicit the same effects. EGF induced a marked epithelial-mesenchymal transition in both cell lines. This effect was evident at 7 pM EGF and was associated with a reduction in cellular adherens junctions and diminished cell-cell contact; it was also associated with an increase in expression of alpha2-integrin as well as enhanced adhesion to the substratum and cell spreading. These changes in adhesion molecule expression were accompanied by enhanced migration on collagen. Blockade of cell growth with mitomycin C did not prevent the EGF-induced morphological change, showing that the mitogenic and morphogenic responses of the GP cells were separable. The phosphatidyl inositol (PI) 3-kinase inhibitor wortmannin inhibited basal proliferation but had no effect on the EGF-induced morphological change, further suggesting that the PI 3-kinase pathway was not involved in the morphogenic response of these cells. Amphiregulin stimulated proliferation of both cell lines, but could only elicit a modest morphological change if used at considerably higher doses or if growth was blocked with mitomycin C. In cells treated with 55 nM AR, alpha2-integrin expression was slightly increased; however, unlike the EGF case, adherens junctions remained intact. These differences in the ability of EGF and amphiregulin to affect cellular adhesion and migration may be significant factors influencing normal and tumor cell behavior.

Expression and function of amphiregulin during murine preimplantation development

Amphiregulin (Ar) is an EGF receptor ligand that functions to modulate the growth of both normal and malignant epithelial cells. We asked whether mouse preimplantation embryos express Ar, and if so, what the function of Ar is during preimplantation development. We used RT-PCR to show expression of Ar mRNA in mouse blastocysts, and using a polyclonal anti-Ar antibody and indirect immunofluorescence, we detected the presence of Ar protein in morula- and blastocyst-stage embryos. Ar protein was present in both the cytoplasm and nucleus in both morulae- and blastocyst-stage embryos, which is similar to Ar distribution in other cell types. Embryos cultured in Ar developed into blastocysts more quickly and also exhibited increased cell numbers compared to control embryos. In addition, 4-cell stage embryos cultured in an antisense Ar phosphorothioate-modified oligodeoxynucleotide (S-oligo) for 48 hr exhibited slower rates of blastocyst formation and reduced embryo cell numbers compared to embryos exposed to a random control S-oligo. TGF-alpha significantly improved blastocyst formation, but not cell numbers, for embryos cultured in the antisense Ar S-oligo. From these observations, we propose that Ar may function as an autocrine growth factor for mouse preimplantation embryos by promoting blastocyst formation and embryo cell number. We also propose that blastocyst formation is stimulated by Ar and TGF-alpha, while Ar appears to exert a greater stimulatory effect on cell proliferation than does TGF-alpha in these embryos.

Characterization of a novel amphiregulin-related molecule in 12-O-tetradecanoylphorbol-13-acetate-treated breast cancer cells

Amphiregulin (AR) can be induced at the mRNA level by 17-beta-estradiol (E2) or the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). This study compares the effects of TPA and E2 on the regulation of processing of AR isoforms and on subcellular localization in human MCF-7 breast cancer cells. AR was localized in the nucleus of MCF-7 cells after E2 treatment, whereas it was predominantly secreted after TPA treatment. AR isoforms of 28, 18, and 10 kDa and an additional species of approximately 55-60 kDa were detected in the cellular conditioned media after TPA stimulation. Expression of this unusual AR isoform was inhibited by protein kinase C (PKC) inhibitors such as bryostatin or H-7. The biochemical properties of this isoform are consistent with it being an N-linked glycosylated form of the AR precursor that contains unprocessed mannose residues. The size of this large isoform is reduced to approximately 40 kDa after treating the TPA-induced MCF-7 cells with tunicamycin or treating the conditioned media of such cells with N-glycosidase F or with endoglycosidase H. Moreover, this isoform is able to blind several lectins with specificity for mannose residues. The 55-60 kDa glycosylated AR isoform, like lower Mr AR isoforms, is able to bind to heparin and to stimulate the growth of MCF-10A cells by interacting with the EGF receptor. These data suggest that TPA activation of PKC may be involved in post-translational modifications of AR, such as glycosylation, and in alteration of its subcellular routing to predominantly a secretory pathway.

Suppression of syndecan-1 expression in endothelial cells by tumor necrosis factor-alpha

Syndecan-1 is a cell surface proteoglycan that binds extracellular matrix components and modulates the activity of heparin-binding growth factors. The expression of syndecan-1 is modified during development, carcinogenesis, and tissue regeneration. During cutaneous wound healing, syndecan-1 expression is transiently induced in newly-formed capillaries of granulation tissue as well as in proliferating keratinocytes. To study the mechanisms underlying this regulation we investigated the effects of several growth factors/cytokines on syndecan-1 expression in two human cell lines: EA.hy 926 endothelial cells and HaCaT keratinocytes. None of these factors significantly altered syndecan-1 mRNA expression in cultured keratinocytes, but when given to endothelial cells, tumor necrosis factor-alpha (TNF-alpha) specifically and dose-dependently suppressed syndecan-1 expression at both mRNA and protein levels. TNF-alpha reduced the amount of syndecan-1 protein in EA.hy 926 cells in both the presence and absence of serum and, at the same time, induced the expression of intercellular adhesion molecule-1 (ICAM-1). The suppressive effect of TNF-alpha on endothelial syndecan-1 expression was reproducible in in vivo experiments in which TNF-alpha-coated beads were administered directly to healing skin wounds of mice. Data supporting these findings were further obtained by injecting TNF-alpha into an experimental rat granulation tissue model. In this tissue TNF-alpha suppressed syndecan-1 mRNA expression by approximately 80%. These results indicate that TNF-alpha is capable of down-regulating syndecan-1 expression in endothelial cells both in vitro and in vivo and suggest that similar mechanisms may be responsible for the changes in syndecan-1 expression observed during various regenerative, developmental, and malignant processes.

COOH-terminal extended recombinant amphiregulin with bioactivity comparable with naturally derived growth factor

The mature secreted form of the epidermal growth factor (EGF) receptor ligand amphiregulin (AR) is reported to be an 84-amino acid residue polypeptide, which is generated by proteolytic processing of a 252-amino acid precursor. This form of recombinant AR (rAR84) and two forms with COOH-terminal extensions corresponding to sequences from the AR precursor (rAR87 and rAR92) were expressed at high levels in Escherichia coli, oxidized to the correct disulfide arrangement, and purified to homogeneity. rAR84 competed poorly for binding of radiolabeled EGF to the EGF receptor and had little ability to stimulate growth of Balb/c/3T3 cells. In striking contrast, rAR87 and rAR92 possessed 42- and 20-fold greater receptor binding activity and 55- and 14-fold greater bioactivity, respectively. Furthermore, addition of the COOH-terminal four amino acids from transforming growth factor alpha to the COOH terminus of rAR84 improved the activity of rAR84 by 100- and 1000-fold, respectively, in these assays. rAR87 was found to have approximately 32% of the specific activity of natural AR from MCF-7 cells when compared in two different bioassays. These findings strongly suggest that the 84-amino acid sequence is not the correct structure of the naturally occurring secreted form of AR and that natural AR contains additional amino acid residues at the COOH-terminal end.

The insulin-like growth factor and epidermal growth factor families in mammary cell growth in ruminants: action and interaction with hormones

Selective breeding and improved management have had major effects in increasing peak milk yields but relatively little effect on lactation persistency. In ruminants, cell loss appears to be largely responsible for the decline in milk yield. Little is known about the longevity of individual cells, but, in lactating dairy cows, few epithelial cells are in the S phase (DNA synthesis) of the cell cycle. The IGF and epidermal growth factor families are direct mitogens, stimulating DNA synthesis in cultures of ruminant mammary epithelial cells. Receptors that mediate the effects of these growth factors, the type 1 IGF receptor and the epidermal growth factor receptor, respectively, are present at similar levels in membranes prepared from the mammary glands of nonpregnant and pregnant sheep. Binding capacity falls by parturition and remains low during lactation. These findings suggest that the drive to mammary development in pregnancy comes from control of growth factors, and, in the case of IGF, modulating binding proteins, a control exerted by hormones, which, in general, are not themselves mitogens. A paracrine or autocrine mode of action and, therefore, local growth factor synthesis, are more likely to be important than systemic concentrations of growth factor. Stimulatory growth factors produced locally by the mammary gland include IGF-I, IGF-II, transforming growth factor-alpha, and amphiregulin. More information is needed on the control of stimulatory and inhibitory growth factors and on how growth factors control the cell cycle. Knowledge of these processes could result in strategies to improve lactation persistency by increasing secretory cell renewal or reducing cell loss during lactation.

Amphiregulin in lung branching morphogenesis: interaction with heparan sulfate proteoglycan modulates cell proliferation

Epithelial and mesenchymal cells isolated from mouse embryonic lungs synthesized and responded to amphiregulin (AR) in a different fashion. Mesenchymal cells produced and deposited 3- to 4-fold more AR than epithelial cells, proliferated in the presence of exogenous AR, and their spontaneous growth was blocked by up to 85% by anti-AR antibodies. In contrast, epithelial cells exhibited a broad response to this growth regulator factor depending on whether they were supplemented with extracellular matrix (ECM) and whether this ECM was of epithelial or mesenchymal origin. AR-treated epithelial cells proliferated by up to 3-fold in the presence of mesenchymal-deposited ECM, remained unchanged in the presence of epithelial-deposited ECM, and decreased in their proliferation rate below controls in the absence of ECM supplementation. This effect was abolished by treatment with the glycosaminoglycan-degrading enzymes heparinase and heparitinase suggesting the specific involvement of heparan sulfate proteoglycan (HSPG) in AR-mediated cell proliferation. In whole lung explants, branching morphogenesis was inhibited by antibodies against the AR heparan sulfate binding site and stimulated by exogenous AR. Since during development, epithelial cells are in contact with mesenchymal ECM at the tips of the growing buds and alongside the basement membrane, focal variations in the proportion of epithelial and mesenchymal HSPG will focally affect epithelial proliferation rates. Therefore, AR-HSPG interaction may underlie the process of branching morphogenesis by inducing differential cell proliferation.

Targeting the epidermal growth factor receptor for therapy of carcinomas

As a group, the carcinomas represent a substantial proportion of all human malignancies, but, with relatively few exceptions, current treatments are ineffective. Modification of existing chemotherapeutic agents has not led to significant improvements in the survival of carcinoma patients, and development of new therapeutic strategies is imperative. It is now becoming apparent that activation of the epidermal growth factor receptor (EGF-R) has much wider implications than a straightforward stimulation of cell division. The pleiotropic effects of EGF-R signalling may influence tumour behaviour and the response of carcinomas to treatment; these are important considerations for the development of new therapies that aim to exploit the expression or modulate the function of the EGF-R in these tumours.

STAT activation by epidermal growth factor (EGF) and amphiregulin. Requirement for the EGF receptor kinase but not for tyrosine phosphorylation sites or JAK1

The epidermal growth factor (EGF) receptor activates several signaling cascades in response to the ligands EGF and amphiregulin (AR). One of these signaling events involves the tyrosine phosphorylation of STATs (signal transducers and activators of transcription), a process believed to require the activation of a tyrosine kinase of the JAK family. In this report we demonstrate that EGF- and AR-induced STAT activation requires the intrinsic kinase activity of the receptor but not the presence of Jak1. We show that both wild type (WT) and truncated EGF receptors lacking all autophosphorylation sites activate STAT 1, 3, and 5 in response to either EGF or AR. Furthermore, relative to cells expressing WT receptor, ligand-induced tyrosine phosphorylation of the STATs was enhanced in cells expressing only the truncated receptor. These results provide the first evidence that (i) EGF receptor-mediated STAT activation occurs in a Jak1-independent manner, (ii) the intrinsic tyrosine kinase activity of the receptor is essential for STAT activation, and (iii) tyrosine phosphorylation sites within the EGF receptor are not required for STAT activation.

Epidermal growth factor-related peptides activate distinct subsets of ErbB receptors and differ in their biological activities

Numerous epidermal growth factor (EGF)-related peptide binding members of the ErbB family of receptor tyrosine kinases have been described. While several EGF agonists bind and activate ErbB-1/EGF receptor, neu differentiation factor (NDF) functions as a ligand for ErbB-3 and ErbB-4. However, it is currently unknown which specific subsets of ErbB receptors become activated in response to each of these ligands. The present study addresses this issue using the T47D breast tumor cell line, which expresses moderate levels of all the presently known ErbB receptors. We show that all the EGF agonists, but not NDF, stimulated tyrosine phosphorylation of ErbB-1. In contrast, all the EGF-related factors except amphiregulin were able to induce tyrosine phosphorylation of ErbB-2. The ability to induce tyrosine phosphorylation of ErbB-3 varied dramatically among the different EGF-related peptides. While EGF, transforming growth factor (TGF)-alpha, and amphiregulin only had a moderate effect, NDF dramatically increased the ErbB-3 phosphotyrosine content. Most notably, heparin binding EGF-related growth factor (HB-EGF) and betacellulin (BTC) were more effective than other EGF agonists. Consequently, only NDF, HB-EGF, and BTC significantly stimulated association of phosphatidylinositol kinase activity with ErbB-3. Among the EGF agonists, HB-EGF induced a low level of ErbB-4 tyrosine phosphorylation, while BTC was as efficient as NDF in activating ErbB-4. The BTC activation of ErbB-4 appears to be independent of ErbB-1, as shown by pretreatment of cells with an antibody that inhibits binding of EGF agonists to ErbB-1. As a result of the differential activation of ErbB receptors, most of the EGF-related growth factors had distinguishable biological activities on cultured mammary epithelial cell lines.

Induction of anchorage-independent growth by amphiregulin

We have previously shown that the epidermal growth factor receptor (EGFR) ligand, amphiregulin (AR) exhibits low potency as a result of its C-terminal truncation. This led us to investigate whether its inability to promote anchorage-independent growth (AIG) of normal cells arose because of its compromised interaction with EGFR. Wild type AR(1-84) was tested in AIG and mitogenesis assays using NRK-49F or NR6/HER fibroblasts. In contrast to NR6/HER cells, the response of NRK-49F fibroblasts to AR was much lower than expected. As the effect of AR was heparin-insensitive, contributions from heparan sulphate proteoglycan interactions could not explain the differing sensitivities of the cells. Comparison of the effects of AR on two additional cell lines indicated that low EGFR number correlated with AR insensitivity: this suggested that the low potency of AR precluded activation of sufficient receptors to elicit a response. Consistent with this proposal, a modified form of AR (AR[1-90(leu86)]) with enhanced potency was able to induce AIG of NRK-49F fibroblasts. Thus, the ability of AR to promote AIG is determined both by ligand potency and the EGFR complement of cells.