Membrane-Proximal Domain of a Disintegrin and Metalloprotease-17 Represents the Putative Molecular Switch of Its Shedding Activity Operated by Protein-disulfide Isomerase

An activated form of ADAM10 is tumor selective and regulates cancer stem-like cells and tumor growth

The transmembrane metalloprotease ADAM10 sheds a range of cell surface proteins, including ligands and receptors of the Notch, Eph, and erbB families, thereby activating signaling pathways critical for tumor initiation and maintenance. ADAM10 is thus a promising therapeutic target. Although widely expressed, its activity is normally tightly regulated. We now report prevalence of an active form of ADAM10 in tumors compared with normal tissues, in mouse models and humans, identified by our conformation-specific antibody mAb 8C7. Structure/function experiments indicate mAb 8C7 binds an active conformation dependent on disulfide isomerization and oxidative conditions, common in tumors. Moreover, this active ADAM10 form marks cancer stem-like cells with active Notch signaling, known to mediate chemoresistance. Importantly, specific targeting of active ADAM10 with 8C7 inhibits Notch activity and tumor growth in mouse models, particularly regrowth after chemotherapy. Our results indicate targeted inhibition of active ADAM10 as a potential therapy for ADAM10-dependent tumor development and drug resistance.

Considerations on inhibition approaches for proinflammatory functions of ADAM proteases

Proteases of the disintegrin and metalloproteinase (ADAM) family mediate the proteolytic shedding of various surface molecules including cytokine precursors, adhesion molecules, growth factors, and receptors. Within the vasculature ADAM10 and ADAM17 regulate endothelial permeability, transendothelial leukocyte migration, and the adhesion of leukocytes and platelets. In vivo studies show that both proteases are implicated in several inflammatory pathologies, for example, edema formation, leukocyte infiltration, and thrombosis. However, both proteases also contribute to developmental and regenerative processes. Thus, although ADAMs can be regarded as valuable drug targets in many aspects, the danger of severe side effects is clearly visible. To circumvent these side effects, traditional inhibition approaches have to be improved to target ADAMs at the right time in the right place. Moreover, the inhibitors need to be more selective for the target protease and if possible also for the substrate. Antibodies recognizing the active conformation of ADAMs or small molecules blocking exosites of ADAM proteases may represent inhibitors with the desired selectivities.

Mesenchymal Stem Cells Shed Amphiregulin at the Surface of Lung Carcinoma Cells in a Juxtacrine Manner

Solid tumors comprise cancer cells and different supportive stromal cells, including mesenchymal stem cells (MSCs), which have recently been shown to enhance tumor growth and metastasis. We provide new mechanistic insights into how bone marrow (BM)–derived MSCs co-injected with Lewis lung carcinoma cells promote tumor growth and metastasis in mice. The proinvasive effect of BM-MSCs exerted on tumor cells relies on an unprecedented juxtacrine action of BM-MSC, leading to the trans-shedding of amphiregulin (AREG) from the tumor cell membrane by tumor necrosis factor-α–converting enzyme carried by the BM-MSC plasma membrane. The released soluble AREG activates cancer cells and promotes their invasiveness. This novel concept is supported by the exploitation of different 2D and 3D culture systems and by pharmacological approaches using a tumor necrosis factor-α–converting enzyme inhibitor and AREG-blocking antibodies. Altogether, we here assign a new function to BM-MSC in tumor progression and establish an uncovered link between AREG and BM-MSC.

Regulation of A disintegrin and metalloproteinase (ADAM) family sheddases ADAM10 and ADAM17: The emerging role of tetraspanins and rhomboids

A disintegrin and metalloprotease (ADAM) 10 and ADAM17 are ubiquitous transmembrane “molecular scissors” which proteolytically cleave, or shed, the extracellular regions of other transmembrane proteins. ADAM10 is essential for development because it cleaves Notch proteins to induce Notch signaling and regulate cell fate decisions. ADAM17 is regarded as a first line of defense against injury and infection, by releasing tumor necrosis factor α (TNFα) to promote inflammation and epidermal growth factor (EGF) receptor ligands to maintain epidermal barrier function. However, the regulation of ADAM10 and ADAM17 trafficking and activation are not fully understood. This review will describe how the TspanC8 subgroup of tetraspanins (Tspan5, 10, 14, 15, 17, and 33) and the iRhom subgroup of protease-inactive rhomboids (iRhom1 and 2) have emerged as important regulators of ADAM10 and ADAM17, respectively. In particular, they are required for the enzymatic maturation and trafficking to the cell surface of the ADAMs, and there is evidence that different TspanC8s and iRhoms target the ADAMs to distinct substrates. The TspanC8s and iRhoms have not been studied functionally on platelets. On these cells, ADAM10 is the principal sheddase for the platelet collagen receptor GPVI, and the regulatory TspanC8s are Tspan14, 15, and 33, as determined from proteomic data. Platelet ADAM17 is the sheddase for the von Willebrand factor (vWF) receptor GPIb, and iRhom2 is the only iRhom that is expressed. Induced shedding of either GPVI or GPIb has therapeutic potential, since inhibition of either receptor is regarded as a promising anti-thrombotic therapy. Targeting of Tspan14, 15, or 33 to activate platelet ADAM10, or iRhom2 to activate ADAM17, may enable such an approach to be realized, without the toxic side effects of activating the ADAMs on every cell in the body.

ADAM and ADAMTS Family Proteins and Snake Venom Metalloproteinases: A Structural Overview

A disintegrin and metalloproteinase (ADAM) family proteins constitute a major class of membrane-anchored multidomain proteinases that are responsible for the shedding of cell-surface protein ectodomains, including the latent forms of growth factors, cytokines, receptors and other molecules. Snake venom metalloproteinases (SVMPs) are major components in most viper venoms. SVMPs are primarily responsible for hemorrhagic activity and may also interfere with the hemostatic system in envenomed animals. SVMPs are phylogenetically most closely related to ADAMs and, together with ADAMs and related ADAM with thrombospondin motifs (ADAMTS) family proteinases, constitute adamalysins/reprolysins or the M12B clan (MEROPS database) of metalloproteinases. Although the catalytic domain structure is topologically similar to that of other metalloproteinases such as matrix metalloproteinases, the M12B proteinases have a modular structure with multiple non-catalytic ancillary domains that are not found in other proteinases. Notably, crystallographic studies revealed that, in addition to the conserved metalloproteinase domain, M12B members share a hallmark cysteine-rich domain designated as the “ADAM_CR” domain. Despite their name, ADAMTSs lack disintegrin-like structures and instead comprise two ADAM_CR domains. This review highlights the current state of our knowledge on the three-dimensional structures of M12B proteinases, focusing on their unique domains that may collaboratively participate in directing these proteinases to specific substrates.

Phosphatidylserine exposure is required for ADAM17 sheddase function

ADAM17, a prominent member of the 'Disintegrin and Metalloproteinase' (ADAM) family, controls vital cellular functions through cleavage of transmembrane substrates. Here we present evidence that surface exposure of phosphatidylserine (PS) is pivotal for ADAM17 to exert sheddase activity. PS exposure is tightly coupled to substrate shedding provoked by diverse ADAM17 activators. PS dependency is demonstrated in the following: (a) in Raji cells undergoing apoptosis; (b) in mutant PSA-3 cells with manipulatable PS content; and (c) in Scott syndrome lymphocytes genetically defunct in their capacity to externalize PS in response to intracellular Ca(2+) elevation. Soluble phosphorylserine but not phosphorylcholine inhibits substrate cleavage. The isolated membrane proximal domain (MPD) of ADAM17 binds to PS but not to phosphatidylcholine liposomes. A cationic PS-binding motif is identified in this domain, replacement of which abrogates liposome-binding and renders the protease incapable of cleaving its substrates in cells. We speculate that surface-exposed PS directs the protease to its targets where it then executes its shedding function.

The role of proteases in regulating Eph/ephrin signaling

Proteases regulate a myriad of cell functions, both in normal and disease states. In addition to protein turnover, they regulate a range of signaling processes, including those mediated by Eph receptors and their ephrin ligands. A variety of proteases is reported to directly cleave Ephs and/or ephrins under different conditions, to promote receptor and/or ligand shedding, and regulate receptor/ligand internalisation and signaling. They also cleave other adhesion proteins in response to Eph-ephrin interactions, to indirectly facilitate Eph-mediated functions. Proteases thus contribute to Eph/ephrin mediated changes in cell-cell and cell-matrix interactions, in cell morphology and in cell migration and invasion, in a manner which appears to be tightly regulated by, and co-ordinated with, Eph signaling. This review summarizes the current literature describing the function and regulation of protease activities during Eph/ephrin-mediated cell signaling.

Distinct Intracellular Domain Substrate Modifications Selectively Regulate Ectodomain Cleavage of NRG1 or CD44

Ectodomain cleavage by A-disintegrin and -metalloproteases (ADAMs) releases many important biologically active substrates and is therefore tightly controlled. Part of the regulation occurs on the level of the enzymes and affects their cell surface abundance and catalytic activity. ADAM-dependent proteolysis occurs outside the plasma membrane but is mostly controlled by intracellular signals. However, the intracellular domains (ICDs) of ADAM10 and -17 can be removed without consequences for induced cleavage, and so far it is unclear how intracellular signals address cleavage. We therefore explored whether substrates themselves could be chosen for proteolysis via ICD modification. We report here that CD44 (ADAM10 substrate), a receptor tyrosine kinase (RTK) coreceptor required for cellular migration, and pro-NRG1 (ADAM17 substrate), which releases the epidermal growth factor (EGF) ligand neuregulin required for axonal outgrowth and myelination, are indeed posttranslationally modified at their ICDs. Tetradecanoyl phorbol acetate (TPA)-induced CD44 cleavage requires dephosphorylation of ICD serine 291, while induced neuregulin release depends on the phosphorylation of several NRG1-ICD serines, in part mediated by protein kinase Cδ (PKCδ). Downregulation of PKCδ inhibits neuregulin release and reduces ex vivo neurite outgrowth and myelination of trigeminal ganglion explants. Our results suggest that specific selection among numerous substrates of a given ADAM is determined by ICD modification of the substrate.

Cell surface protein disulfide isomerase regulates natriuretic peptide generation of cyclic guanosine monophosphate

Rationale

The family of natriuretic peptides (NPs), including atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP), and C-type natriuretic peptide (CNP), exert important and diverse actions for cardiovascular and renal homeostasis. The autocrine and paracrine functions of the NPs are primarily mediated through the cellular membrane bound guanylyl cyclase-linked receptors GC-A (NPR-A) and GC-B (NPR-B). As the ligands and receptors each contain disulfide bonds, a regulatory role for the cell surface protein disulfide isomerase (PDI) was investigated.

Objective

We utilized complementary in vitro and in vivo models to determine the potential role of PDI in regulating the ability of the NPs to generate its second messenger, cyclic guanosine monophosphate.

Methods and Results

Inhibition of PDI attenuated the ability of ANP, BNP and CNP to generate cGMP in human mesangial cells (HMCs), human umbilical vein endothelial cells (HUVECs), and human aortic smooth muscle cells (HASMCs), each of which were shown to express PDI. In LLC-PK1 cells, where PDI expression was undetectable by immunoblotting, PDI inhibition had a minimal effect on cGMP generation. Addition of PDI to cultured LLC-PK1 cells increased intracellular cGMP generation mediated by ANP. Inhibition of PDI in vivo attenuated NP-mediated generation of cGMP by ANP. Surface Plasmon Resonance demonstrated modest and differential binding of the natriuretic peptides with immobilized PDI in a cell free system. However, PDI was shown to co-localize on the surface of cells with GC-A and GC-B by co-immunoprecpitation and immunohistochemistry.

Conclusion

These data demonstrate for the first time that cell surface PDI expression and function regulate the capacity of natriuretic peptides to generate cGMP through interaction with their receptors.

ERAD and how viruses exploit it

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a universally important process among eukaryotic cells. ERAD is necessary to preserve cell integrity since the accumulation of defective proteins results in diseases associated with neurological dysfunction, cancer, and infections. This process involves recognition of misfolded or misassembled proteins that have been translated in association with ER membranes. Recognition of ERAD substrates leads to their extraction through the ER membrane (retrotranslocation or dislocation), ubiquitination, and destruction by cytosolic proteasomes. This review focuses on ERAD and its components as well as how viruses use this process to promote their replication and to avoid the immune response.

Interleukin-6 and its receptors: a highly regulated and dynamic system

Interleukin-6 (IL-6) is a multifunctional cytokine with well-defined pro- and anti-inflammatory properties. Although only small amounts in the picogram range can be detected in healthy humans, IL-6 expression is highly and transiently up-regulated in nearly all pathophysiological states. IL-6 induces intracellular signaling pathways after binding to its membrane-bound receptor (IL-6R), which is only expressed on hepatocytes and certain subpopulations of leukocytes (classic signaling). Transduction of the signal is mediated by the membrane-bound β-receptor glycoprotein 130 (gp130). In a second pathway, named trans-signaling, IL-6 binds to soluble forms of the IL-6R (sIL-6R), and this agonistic IL-6/sIL-6R complexes can in principle activate all cells due to the uniform expression of gp130. Importantly, several soluble forms of gp130 (sgp130) are found in the human blood, which are considered to be the natural inhibitors of IL-6 trans-signaling. Most pro-inflammatory roles of IL-6 have been attributed to the trans-signaling pathway, whereas anti-inflammatory and regenerative signaling, including the anti-bacterial acute phase response of the liver, is mediated by IL-6 classic signaling. In this simplistic view, only a minority of cell types expresses the IL-6R and is therefore responsive for IL-6 classic signaling, whereas gp130 is ubiquitously expressed throughout the human body. However, several reports point towards a much more complex situation. A plethora of factors, including proteases, cytokines, chemical drugs, and intracellular signaling pathways, are able to modulate the cellular expression of the membrane-bound and soluble forms of IL-6R and gp130. In this review, we summarize current knowledge of regulatory mechanisms that control and regulate the dynamic expression of IL-6 and its two receptors.

A disintegrin and metalloprotease 17 dynamic interaction sequence, the sweet tooth for the human interleukin 6 receptor

A disintegrin and metalloprotease 17 (ADAM17) is a major sheddase involved in the regulation of a wide range of biological processes. Key substrates of ADAM17 are the IL-6 receptor (IL-6R) and TNF-α. The extracellular region of ADAM17 consists of a prodomain, a catalytic domain, a disintegrin domain, and a membrane-proximal domain as well as a small stalk region. This study demonstrates that this juxtamembrane segment is highly conserved, α-helical, and involved in IL-6R binding. This process is regulated by the structure of the preceding membrane-proximal domain, which acts as molecular switch of ADAM17 activity operated by a protein-disulfide isomerase. Hence, we have termed the conserved stalk region "Conserved ADAM seventeen dynamic interaction sequence" (CANDIS). Finally, we identified the region in IL-6R that binds to CANDIS. In contrast to the type I transmembrane proteins, the IL-6R, and IL-1RII, CANDIS does not bind the type II transmembrane protein TNF-α, demonstrating fundamental differences in the respective shedding by ADAM17.

Interleukin-6: from basic biology to selective blockade of pro-inflammatory activities

Cytokines receptors exist in membrane bound and soluble form. A soluble form of the human IL-6R is generated by limited proteolysis and alternative splicing. The complex of IL-6 and soluble IL-6R stimulates target cells not stimulated by IL-6 alone, since they do not express the membrane bound IL-6R. We have named this process trans-signaling. Soluble gp130 is the natural inhibitor of IL-6/soluble IL-6R complex responses. Recombinant soluble gp130 protein is a molecular tool to discriminate between gp130 responses via membrane bound and soluble IL-6R responses. Neutralizing monoclonal antibodies for global blockade of IL-6 signaling and the sgp130Fc protein for selective blockade of IL-6 trans-signaling have been used in several animal models of human diseases. Using the sgp130Fc protein or sgp130Fc transgenic mice we demonstrate in models of inflammatory bowel disease, peritonitis, rheumatoid arthritis, atherosclerosis pancreatitis, colon cancer, ovarian cancer and pancreatic cancer, that IL-6 trans-signaling via the soluble IL-6R is the crucial step in the development and the progression of the disease. Therefore, sgp130Fc is a novel therapeutic agent for the treatment of chronic inflammatory diseases and cancer and it undergoes phase I clinical trials as an anti-inflammatory drug since June 2013.