Transforming growth factor-alpha and beta-amyloid precursor protein share a secretory mechanism

New insights into the function and pathophysiology of the ectodomain sheddase A Disintegrin And Metalloproteinase 10 (ADAM10)

The 'A Disintegrin And Metalloproteinase 10' (ADAM10) has gained considerable attention due to its discovery as an 'α-secretase' involved in the nonamyloidogenic processing of the amyloid precursor protein, thereby possibly preventing the excessive generation of the amyloid beta peptide, which is associated with the pathogenesis of Alzheimer's disease. ADAM10 was found to exert many additional functions, cleaving about 100 different membrane proteins. ADAM10 is involved in many pathophysiological conditions, ranging from cancer and autoimmune disorders to neurodegeneration and inflammation. ADAM10 cleaves its substrates close to the plasma membrane, a process referred to as ectodomain shedding. This is a central step in the modulation of the functions of cell adhesion proteins and cell surface receptors. ADAM10 activity is controlled by transcriptional and post-translational events. The interaction of ADAM10 with tetraspanins and the way they functionally and structurally depend on each other is another topic of interest. In this review, we will summarize findings on how ADAM10 is regulated and what is known about the biology of the protease. We will focus on novel aspects of the molecular biology and pathophysiology of ADAM10 that were previously poorly covered, such as the role of ADAM10 on extracellular vesicles, its contribution to virus entry, and its involvement in cardiac disease, cancer, inflammation, and immune regulation. ADAM10 has emerged as a regulator controlling cell surface proteins during development and in adult life. Its involvement in disease states suggests that ADAM10 may be exploited as a therapeutic target to treat conditions associated with a dysfunctional proteolytic activity.

Glycosaminoglycan modifications of betaglycan regulate ectodomain shedding to fine-tune TGF-β signaling responses in ovarian cancer

In pathologies including cancer, aberrant Transforming Growth Factor-β (TGF-β) signaling exerts profound tumor intrinsic and extrinsic consequences. Intense clinical endeavors are underway to target this pathway. Central to the success of these interventions is pinpointing factors that decisively modulate the TGF-β responses. Betaglycan/type III TGF-β receptor (TβRIII), is an established co-receptor for the TGF-β superfamily known to bind directly to TGF-βs 1-3 and inhibin A/B. Betaglycan can be membrane-bound and also undergo ectodomain cleavage to produce soluble-betaglycan that can sequester its ligands. Its extracellular domain undergoes heparan sulfate and chondroitin sulfate glycosaminoglycan modifications, transforming betaglycan into a proteoglycan. We report the unexpected discovery that the heparan sulfate glycosaminoglycan chains on betaglycan are critical for the ectodomain shedding. In the absence of such glycosaminoglycan chains betaglycan is not shed, a feature indispensable for the ability of betaglycan to suppress TGF-β signaling and the cells' responses to exogenous TGF-β ligands. Using unbiased transcriptomics, we identified TIMP3 as a key inhibitor of betaglycan shedding thereby influencing TGF-β signaling. Our results bear significant clinical relevance as modified betaglycan is present in the ascites of patients with ovarian cancer and can serve as a marker for predicting patient outcomes and TGF-β signaling responses. These studies are the first to demonstrate a unique reliance on the glycosaminoglycan chains of betaglycan for shedding and influence on TGF-β signaling responses. Dysregulated shedding of TGF-β receptors plays a vital role in determining the response and availability of TGF-βs', which is crucial for prognostic predictions and understanding of TGF-β signaling dynamics.

Heparan sulfate modifications of betaglycan promote TIMP3-dependent ectodomain shedding to fine-tune TGF-β signaling

In pathologies such as cancer, aberrant Transforming Growth Factor-β (TGF-β) signaling exerts profound tumor intrinsic and extrinsic consequences. Intense clinical endeavors are underway to target this pivotal pathway. Central to the success of these interventions is pinpointing factors that decisively modulate the TGF-β responses. Betaglycan/type III TGF-β receptor (TβRIII), is an established co-receptor for the TGF-β superfamily known to bind directly to TGF-βs 1-3 and inhibin A/B. While betaglycan can be membrane-bound, it can also undergo ectodomain cleavage to produce soluble-betaglycan that can sequester its ligands. The extracellular domain of betaglycan undergoes heparan sulfate and chondroitin sulfate glycosaminoglycan modifications, transforming betaglycan into a proteoglycan. Here we report the unexpected discovery that the heparan sulfate modifications are critical for the ectodomain shedding of betaglycan. In the absence of such modifications, betaglycan is not shed. Such shedding is indispensable for the ability of betaglycan to suppress TGF-β signaling and the cells' responses to exogenous TGF-β ligands. Using unbiased transcriptomics, we identified TIMP3 as a key regulator of betaglycan shedding and thereby TGF-β signaling. Our results bear significant clinical relevance as modified betaglycan is present in the ascites of patients with ovarian cancer and can serve as a marker for predicting patient outcomes and TGF-β signaling responses. These studies are the first to demonstrate a unique reliance on the glycosaminoglycan modifications of betaglycan for shedding and influence on TGF-β signaling responses. Dysregulated shedding of TGF-β receptors plays a vital role in determining the response and availability of TGF-βs', which is crucial for prognostic predictions and understanding of TGF-β signaling dynamics.

Secretome and Tunneling Nanotubes: A Multilevel Network for Long Range Intercellular Communication between Endothelial Cells and Distant Cells

As a cellular interface between the blood and tissues, the endothelial cell (EC) monolayer is involved in the control of key functions including vascular tone, permeability and homeostasis, leucocyte trafficking and hemostasis. EC regulatory functions require long-distance communications between ECs, circulating hematopoietic cells and other vascular cells for efficient adjusting thrombosis, angiogenesis, inflammation, infection and immunity. This intercellular crosstalk operates through the extracellular space and is orchestrated in part by the secretory pathway and the exocytosis of Weibel Palade Bodies (WPBs), secretory granules and extracellular vesicles (EVs). WPBs and secretory granules allow both immediate release and regulated exocytosis of messengers such as cytokines, chemokines, extracellular membrane proteins, coagulation or growth factors. The ectodomain shedding of transmembrane protein further provide the release of both receptor and ligands with key regulatory activities on target cells. Thin tubular membranous channels termed tunneling nanotubes (TNTs) may also connect EC with distant cells. EVs, in particular exosomes, and TNTs may contain and transfer different biomolecules (e.g., signaling mediators, proteins, lipids, and microRNAs) or pathogens and have emerged as a major triggers of horizontal intercellular transfer of information.

DEEPSMP: A deep learning model for predicting the ectodomain shedding events of membrane proteins

Membrane proteins play essential roles in modern medicine. In recent studies, some membrane proteins involved in ectodomain shedding events have been reported as the potential drug targets and biomarkers of some serious diseases. However, there are few effective tools for identifying the shedding event of membrane proteins. So, it is necessary to design an effective tool for predicting shedding event of membrane proteins. In this study, we design an end-to-end prediction model using deep neural networks with long short-term memory (LSTM) units and attention mechanism, to predict the ectodomain shedding events of membrane proteins only by sequence information. Firstly, the evolutional profiles are encoded from original sequences of these proteins by Position-Specific Iterated BLAST (PSI-BLAST) on Uniref50 database. Then, the LSTM units which contain memory cells are used to hold information from past inputs to the network and the attention mechanism is applied to detect sorting signals in proteins regardless of their position in the sequence. Finally, a fully connected dense layer and a softmax layer are used to obtain the final prediction results. Additionally, we also try to reduce overfitting of the model by using dropout, L2 regularization, and bagging ensemble learning in the model training process. In order to ensure the fairness of performance comparison, firstly we use cross validation process on training dataset obtained from an existing paper. The average accuracy and area under a receiver operating characteristic curve (AUC) of five-fold cross-validation are 81.19% and 0.835 using our proposed model, compared to 75% and 0.78 by a previously published tool, respectively. To better validate the performance of the proposed model, we also evaluate the performance of the proposed model on independent test dataset. The accuracy, sensitivity, and specificity are 83.14%, 84.08%, and 81.63% using our proposed model, compared to 70.20%, 71.97%, and 67.35% by the existing model. The experimental results validate that the proposed model can be regarded as a general tool for predicting ectodomain shedding events of membrane proteins. The pipeline of the model and prediction results can be accessed at the following URL: http://www.csbg-jlu.info/DeepSMP/.

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.

Characterization of the catalytic properties of the membrane-anchored metalloproteinase ADAM9 in cell-based assays

ADAM9 (A Disintegrin And Metalloprotease 9) is a membrane-anchored metalloproteinase that has been implicated in pathological retinal neovascularization and in tumor progression. ADAM9 has constitutive catalytic activity in both biochemical and cell-based assays and can cleave several membrane proteins, including epidermal growth factor and Ephrin receptor B4; yet little is currently known about the catalytic properties of ADAM9 and its post-translational regulation and inhibitor profile in cell-based assays. To address this question, we monitored processing of the membrane-anchored Ephrin receptor B4 (EphB4) by co-expressing ADAM9, with the catalytically inactive ADAM9 E > A mutant serving as a negative control. We found that ADAM9-dependent shedding of EphB4 was not stimulated by three commonly employed activators of ADAM-dependent ectodomain shedding: phorbol esters, pervanadate or calcium ionophores. With respect to the inhibitor profile, we found that ADAM9 was inhibited by the hydroxamate-based metalloprotease inhibitors marimastat, TAPI-2, BB94, GM6001 and GW280264X, and by 10 nM of the tissue inhibitor of metalloproteinases (TIMP)-3, but not by up to 20 nM of TIMP-1 or -2. Additionally, we screened a non-hydroxamate small-molecule library for novel ADAM9 inhibitors and identified four compounds that selectively inhibited ADAM9-dependent proteolysis over ADAM10- or ADAM17-dependent processing. Taken together, the present study provides new information about the molecular fingerprint of ADAM9 in cell-based assays by showing that it is not stimulated by strong activators of ectodomain shedding and by defining a characteristic inhibitor profile. The identification of novel non-hydroxamate inhibitors of ADAM9 could provide the basis for designing more selective compounds that block the contribution of ADAM9 to pathological neovascularization and cancer.

SheddomeDB: the ectodomain shedding database for membrane-bound shed markers

BACKGROUND

A number of membrane-anchored proteins are known to be released from cell surface via ectodomain shedding. The cleavage and release of membrane proteins has been shown to modulate various cellular processes and disease pathologies. Numerous studies revealed that cell membrane molecules of diverse functional groups are subjected to proteolytic cleavage, and the released soluble form of proteins may modulate various signaling processes. Therefore, in addition to the secreted protein markers that undergo secretion through the secretory pathway, the shed membrane proteins may comprise an additional resource of noninvasive and accessible biomarkers. In this context, identifying the membrane-bound proteins that will be shed has become important in the discovery of clinically noninvasive biomarkers. Nevertheless, a data repository for biological and clinical researchers to review the shedding information, which is experimentally validated, for membrane-bound protein shed markers is still lacking.

RESULTS

In this study, the database SheddomeDB was developed to integrate publicly available data of the shed membrane proteins. A comprehensive literature survey was performed to collect the membrane proteins that were verified to be cleaved or released in the supernatant by immunological-based validation experiments. From 436 studies on shedding, 401 validated shed membrane proteins were included, among which 199 shed membrane proteins have not been annotated or validated yet by existing cleavage databases. SheddomeDB attempted to provide a comprehensive shedding report, including the regulation of shedding machinery and the related function or diseases involved in the shedding events. In addition, our published tool ShedP was embedded into SheddomeDB to support researchers for predicting the shedding event on unknown or unrecorded membrane proteins.

CONCLUSIONS

To the best of our knowledge, SheddomeDB is the first database for the identification of experimentally validated shed membrane proteins and currently may provide the most number of membrane proteins for reviewing the shedding information. The database included membrane-bound shed markers associated with numerous cellular processes and diseases, and some of these markers are potential novel markers because they are not annotated or validated yet in other databases. SheddomeDB may provide a useful resource for discovering membrane-bound shed markers. The interactive web of SheddomeDB is publicly available at http://bal.ym.edu.tw/SheddomeDB/ .

Functional Differences of Very-Low-Density Lipoprotein Receptor Splice Variants in Regulating Wnt Signaling

The very-low-density lipoprotein receptor (VLDLR) negatively regulates Wnt signaling. VLDLR has two major alternative splice variants, VLDLRI and VLDLRII, but their biological significance and distinction are unknown. Here we found that most tissues expressed both VLDLRI and VLDLRII, while the retina expressed only VLDLRII. The shed soluble VLDLR extracellular domain (sVLDLR-N) was detected in the conditioned medium of retinal pigment epithelial cells, interphotoreceptor matrix, and mouse plasma, indicating that ectodomain shedding of VLDLR occurs endogenously. VLDLRII displayed a higher ectodomain shedding rate and a more potent inhibitory effect on Wnt signaling than VLDLRI in vitro and in vivo O-glycosylation, which is present in VLDLRI but not VLDLRII, determined the differential ectodomain shedding rates. Moreover, the release of sVLDLR-N was inhibited by a metalloproteinase inhibitor, TAPI-1, while it was promoted by phorbol 12-myristate 13-acetate (PMA). In addition, sVLDLR-N shedding was suppressed under hypoxia. Further, plasma levels of sVLDLR-N were reduced in both type 1 and type 2 diabetic mouse models. We concluded that VLDLRI and VLDLRII had differential roles in regulating Wnt signaling and that decreased plasma levels of sVLDLR-N may contribute to Wnt signaling activation in diabetic complications. Our study reveals a novel mechanism for intercellular regulation of Wnt signaling through VLDLR ectodomain shedding.

Role of ADAM17 in the non-cell autonomous effects of oncogene-induced senescence

Introduction

Cellular senescence is a terminal cell proliferation arrest that can be triggered by oncogenes. One of the traits of oncogene-induced senescence (OIS) is the so-called senescence-associated secretory phenotype or senescence secretome. Depending on the context, the non-cell autonomous effects of OIS may vary from tumor suppression to promotion of metastasis. Despite being such a physiological and pathologically relevant effector, the mechanisms of generation of the senescence secretome are largely unknown.

Methods

We analyzed by label-free proteomics the secretome of p95HER2-induced senescent cells and compared the levels of the membrane-anchored proteins with their transcript levels. Then, protein and RNA levels of ADAM17 were evaluated by using Western blot and reverse transcription-polymerase chain reaction, its localization by using biotin labeling and immunofluorescence, and its activity by using alkaline phosphatase-tagged substrates. The p95HER2-expressing cell lines, senescent MCF7 and proliferating MCF10A, were analyzed to study ADAM17 regulation. Finally, we knocked down ADAM17 to determine its contribution to the senescence-associated secretome. The effect of this secretome was evaluated in migration assays in vitro and in nude mice by assessing the metastatic ability of orthotopically co-injected non-senescent cells.

Results

Using breast cancer cells expressing p95HER2, a constitutively active fragment of the proto-oncogene HER2 that induces OIS, we show that the extracellular domains of a variety of membrane-bound proteins form part of the senescence secretome. We determine that these proteins are regulated transcriptionally and, in addition, that their shedding is limited by the protease ADAM17. The activity of the sheddase is constrained, at least in part, by the accumulation of cellular cholesterol. The blockade of ADAM17 abrogates several prometastatic effects of the p95HER2-induced senescence secretome, both in vitro and in vivo.

Conclusions

Considering these findings, we conclude that ectodomain shedding is tightly regulated in oncogene-induced senescent cells by integrating transcription of the shedding substrates with limiting ADAM17 activity. The remaining activity of ADAM17 contributes to the non-cell autonomous protumorigenic effects of p95HER2-induced senescent cells. Because ADAM17 is druggable, these results represent an approximation to the pharmacological regulation of the senescence secretome.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-015-0619-7) contains supplementary material, which is available to authorized users.

The sorting protein PACS-2 promotes ErbB signalling by regulating recycling of the metalloproteinase ADAM17

The metalloproteinase ADAM17 activates ErbB signalling by releasing ligands from the cell surface, a key step underlying epithelial development, growth and tumour progression. However, mechanisms acutely controlling ADAM17 cell-surface availability to modulate the extent of ErbB ligand release are poorly understood. Here, through a functional genome-wide siRNA screen, we identify the sorting protein PACS-2 as a regulator of ADAM17 trafficking and ErbB signalling. PACS-2 loss reduces ADAM17 cell-surface levels and ADAM17-dependent ErbB ligand shedding, without apparent effects on related proteases. PACS-2 co-localizes with ADAM17 on early endosomes and PACS-2 knockdown decreases the recycling and stability of internalized ADAM17. Hence, PACS-2 sustains ADAM17 cell-surface activity by diverting ADAM17 away from degradative pathways. Interestingly, Pacs2-deficient mice display significantly reduced levels of phosphorylated EGFR and intestinal proliferation. We suggest that this mechanism controlling ADAM17 cell-surface availability and EGFR signalling may play a role in intestinal homeostasis, with potential implications for cancer biology.

Inside-out Regulation of Ectodomain Cleavage of Cluster-of-Differentiation-44 (CD44) and of Neuregulin-1 Requires Substrate Dimerization

Ectodomain shedding of transmembrane precursor proteins generates numerous life-essential molecules, such as epidermal growth factor receptor ligands. This cleavage not only releases the regulatory growth factor, but it is also the required first step for the subsequent processing by γ-secretase and the release of gene regulatory intracellular fragments. Signaling within the cell modifies the cytoplasmic tails of substrates, a step important in starting the specific and regulated cleavage of a large number of studied substrates. Ectodomain cleavage occurs, however, on the outside of the plasma membrane and is carried out by membrane-bound metalloproteases. How the intracellular domain modification communicates with the ectodomain of the substrate to allow for cleavage to occur is unknown. Here, we show that homodimerization of a cluster-of-differentiation-44 or of pro-neuregulin-1 monomers represents an essential pre-condition for their regulated ectodomain cleavage. Both substrates are associated with their respective metalloproteases under both basal or cleavage-stimulated conditions. These interactions only turn productive by specific intracellular signal-induced intracellular domain modifications of the substrates, which in turn regulate metalloprotease access to the substrates' ectodomain and cleavage. We propose that substrate intracellular domain modification induces a relative rotation or other positional change of the dimerization partners that allow metalloprotease cleavage in the extracellular space. Our findings fill an important gap in understanding substrate-specific inside-out signal transfer along cleaved transmembrane proteins and suggest that substrate dimerization (homo- or possibly heterodimerization) might represent a general principle in ectodomain shedding.

Microfabricated collagen tracks facilitate single cell metastatic invasion in 3D

While the mechanisms employed by metastatic cancer cells to migrate remain poorly understood, it has been widely accepted that metastatic cancer cells can invade the tumor stroma by degrading the extracellular matrix (ECM) with matrix metalloproteinases (MMPs). Although MMP inhibitors showed early promise in preventing metastasis in animal models, they have largely failed clinically. Recently, studies have shown that some cancer cells can use proteolysis to mechanically rearrange their ECM to form tube-like "microtracks" which other cells can follow without using MMPs themselves. We speculate that this mode of migration in the secondary cells may be one example of migration which can occur without endogenous protease activity in the secondary cells. Here we present a technique to study this migration in a 3D, collagen-based environment which mimics the size and topography of the tracks produced by proteolytically active cancer cells. Using time-lapse phase-contrast microscopy, we find that these microtracks permit the rapid and persistent migration of noninvasive MCF10A mammary epithelial cells, which are unable to otherwise migrate in 3D collagen. Additionally, while highly metastatic MDAMB231 breast cancer cells are able to invade a 3D collagen matrix, seeding within the patterned microtracks induced significantly increased cell migration speed, which was not decreased by pharmacological MMP inhibition. Together, these data suggest that microtracks within a 3D ECM may facilitate the migration of cells in an MMP-independent fashion, and may reveal novel insight into the clinical challenges facing MMP inhibitors.

Dominant-negative effect of truncated mannose 6-phosphate/insulin-like growth factor II receptor species in cancer

Oligomerization of the mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) is important for optimal ligand binding and internalization. M6P/IGF2R is a tumor suppressor gene that exhibits loss of heterozygosity and is mutated in several cancers. We tested the potential dominant-negative effects of two cancer-associated mutations that truncate M6P/IGF2R in ectodomain repeats 9 and 14. Our hypothesis was that co-expression of the truncated receptors with the wild-type/endogenous full-length M6P/IGF2R would interfere with M6P/IGF2R function by heterodimer interference. Immunoprecipitation confirmed formation of heterodimeric complexes between full-length M6P/IGF2Rs and the truncated receptors, termed Rep9F and Rep14F. Remarkably, increasing expression of either Rep9F or Rep14F provoked decreased levels of full-length M6P/IGF2Rs in both cell lysates and plasma membranes, indicating a dominant-negative effect on receptor availability. Loss of full-length M6P/IGF2R was not due to increased proteasomal or lysosomal degradation, but instead arose from increased proteolytic cleavage of cell-surface M6P/IGF2Rs, resulting in ectodomain release, by a mechanism that was inhibited by metal ion chelators. These data suggest that M6P/IGF2R truncation mutants may contribute to the cancer phenotype by decreasing the availability of full-length M6P/IGF2Rs to perform tumor-suppressive functions such as binding/internalization of receptor ligands such as insulin-like growth factor II.

Interleukin-1 stimulates ADAM17 through a mechanism independent of its cytoplasmic domain or phosphorylation at threonine 735

ADAM17 (a disintegrin and metalloproteinase) is a membrane-anchored metalloproteinase that regulates the release of EGFR-ligands, TNFα and other membrane proteins from cells. ADAM17 can be rapidly activated by a variety of signaling pathways, yet little is known about the underlying mechanism. Several studies have demonstrated that the cytoplasmic domain of ADAM17 is not required for its rapid activation by a variety of stimuli, including phorbol esters, tyrosine kinases and some G-protein coupled receptors. However, phosphorylation of cytoplasmic residue T735 was recently reported as a crucial step for activation of ADAM17 by IL-1β and by the p38 MAP-kinase pathway. One possible mechanism to reconcile these results would be that T735 has an inhibitory role and that it must be phosphorylated as a pre-requisite for the activation of ADAM17, which would then proceed via a mechanism that is independent of its cytoplasmic domain. To test this hypothesis, we performed rescue experiments of Adam17−/− cells with wild type and mutant forms of ADAM17. However, these experiments showed that an inactivating mutation (T735A) or an activating mutation (T735D) of cytoplasmic residue T735 or the removal of the cytoplasmic domain of ADAM17 did not significantly affect the stimulation of ADAM17 by IL-1β or by activation of MAP-kinase with anisomycin. Moreover, we found that the MAP-kinase inhibitor SB203580 blocked activation of cytoplasmic tail-deficient ADAM17 and of the T735A mutant by IL-1β or by anisomycin, providing further support for a model in which the activation mechanism of ADAM17 does not rely on its cytoplasmic domain or phosphorylation of T735.

Shedding of syndecan-1 from human hepatocytes alters very low density lipoprotein clearance

We recently showed that the heparan sulfate proteoglycan syndecan-1 mediates hepatic clearance of triglyceride-rich lipoproteins in mice based on systemic deletion of syndecan-1 and hepatocyte-specific inactivation of sulfotransferases involved in heparan sulfate biosynthesis. Here, we show that syndecan-1 expressed on primary human hepatocytes and Hep3B human hepatoma cells can mediate binding and uptake of very low density lipoprotein (VLDL). Syndecan-1 also undergoes spontaneous shedding from primary human and murine hepatocytes and Hep3B cells. In human cells, phorbol myristic acid induces syndecan-1 shedding, resulting in accumulation of syndecan-1 ectodomains in the medium. Shedding occurs through a protein kinase C-dependent activation of ADAM17 (a disintegrin and metalloproteinase 17). Phorbol myristic acid stimulation significantly decreases DiD (1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate)-VLDL binding to cells, and shed syndecan-1 ectodomains bind to VLDL. Although mouse hepatocytes appear resistant to induced shedding in vitro, injection of lipopolysaccharide into mice results in loss of hepatic syndecan-1, accumulation of ectodomains in the plasma, impaired VLDL catabolism, and hypertriglyceridemia.

CONCLUSION

These findings suggest that syndecan-1 mediates hepatic VLDL turnover in humans as well as in mice and that shedding might contribute to hypertriglyceridemia in patients with sepsis.

Identification and biology of α-secretase

'Secretase' is a generic term coined more than 20 years ago to refer to a group of proteases responsible for the cleavage of a vast number of membrane proteins. These endoproteolytic events result in the extracellular or intracellular release of soluble metabolites associated with a broad range of intrinsic physiological functions. α-Secretase refers to the activity targeting the amyloid precursor protein (APP) and generating sAPPα, a soluble extracellular fragment potentially associated with neurotrophic and neuroprotective functions. Several proteases from the a disintegrin and metalloproteinase (ADAM) family, including ADAM10 and ADAM17, have been directly or indirectly associated with the constitutive and regulated α-secretase activities. Recent evidence in primary neuronal cultures indicates that ADAM10 may represent the genuine constitutive α-secretase. Mainly because α-secretase cleaves APP within the sequence of Aβ, the core component of the cerebral amyloid plaques in Alzheimer's disease, α-secretase activation is considered to be of therapeutic value. In this article, we will provide a historical perspective on the characterization of α-secretase and review the recent literature on the identification and biology of the current α-secretase candidates.

Release of membrane-associated L-dopa decarboxylase from human cells

L-Dopa Decarboxylase is a pyridoxal 5-phosphate (PLP)-dependent enzyme that catalyses the decarboxylation of L-Dopa to dopamine. In this study, we investigated the cellular topology of the active human enzyme. Fractionation of membranes from human cell lines, of neural and non-neural origin, by temperature-induced phase separation in Triton X-114 resulted in the detection of DDC molecules in all separation phases. Solubilization of membrane-associated DDC was observed in a pH and time-dependent manner and was affected by divalent cations and protease inhibitors, suggesting the involvement of a possible release mechanism. The study of the biological properties and function of the solubilization phenomenon described here, as well as, the study of the membrane-associated enzyme could provide us with new information about the participation of the human L-Dopa decarboxylase in physiological and aberrant processes.

The "A Disintegrin And Metalloproteases" ADAM10 and ADAM17: novel drug targets with therapeutic potential?

Proteolytic ectodomain release, a process known as "shedding", has been recognised as a key mechanism for regulating the function of a diversity of cell surface proteins. A Disintegrin And Metalloproteinases (ADAMs) have emerged as the major proteinase family that mediates ectodomain shedding. Dysregulation of ectodomain shedding is associated with autoimmune and cardiovascular diseases, neurodegeneration, infection, inflammation and cancer. Therefore, ADAMs are increasingly regarded as attractive targets for novel therapies. ADAM10 and its close relative ADAM17 (TNF-alpha converting enzyme (TACE)) have been studied in particular in the context of ectodomain shedding and have been demonstrated as key molecules in most of the shedding events characterised to date. Whereas the level of expression of ADAM10 may be of importance in cancer and neurodegenerative disorders, ADAM17 mainly coordinates pro- and anti-inflammatory activities during immune response. Despite the high therapeutical potential of ADAM inhibition, all clinical trials using broad-spectrum metalloprotease inhibitors have failed so far. This review will cover the emerging roles of both ADAM10 and ADAM17 in the regulation of major physiological and developmental pathways and will discuss the suitability of specifically modulating the activities of both proteases as a feasible way to inhibit inflammatory states, cancer and neurodegeneration.

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.

ADAM17 is regulated by a rapid and reversible mechanism that controls access to its catalytic site

Protein ectodomain shedding is crucial for cell-cell interactions because it controls the bioavailability of soluble tumor necrosis factor-α (TNFα) and ligands of the epidermal growth factor (EGF) receptor, and the release of many other membrane proteins. Various stimuli can rapidly trigger ectodomain shedding, yet much remains to be learned about the identity of the enzymes that respond to these stimuli and the mechanisms underlying their activation. Here, we demonstrate that the membrane-anchored metalloproteinase ADAM17, but not ADAM10, is the sheddase that rapidly responds to the physiological signaling pathways stimulated by thrombin, EGF, lysophosphatidic acid and TNFα. Stimulation of ADAM17 is swift and quickly reversible, and does not depend on removal of its inhibitory pro-domain by pro-protein convertases, or on dissociation of an endogenous inhibitor, TIMP3. Moreover, activation of ADAM17 by physiological stimuli requires its transmembrane domain, but not its cytoplasmic domain, arguing against inside-out signaling via cytoplasmic phosphorylation as the underlying mechanism. Finally, experiments with the tight binding hydroxamate inhibitor DPC333, used here to probe the accessibility of the active site of ADAM17, demonstrate that this inhibitor can quickly bind to ADAM17 in stimulated, but not quiescent cells. These findings support the concept that activation of ADAM17 involves a rapid and reversible exposure of its catalytic site.

Roles for the type III TGF-beta receptor in human cancer

Transforming growth factor beta (TGF-beta) superfamily ligands have important roles in regulating cellular homeostasis, embryonic development, differentiation, proliferation, immune surveillance, angiogenesis, motility, and apoptosis in a cell type and context specific manner. TGF-beta superfamily signaling pathways also have diverse roles in human cancer, functioning to either suppress or promote cancer progression. The TGF-beta superfamily co-receptor, the type III TGF-beta receptor (TbetaRIII, also known as betaglycan) mediates TGF-beta superfamily ligand dependent as well as ligand independent signaling to both Smad and non-Smad signaling pathways. Loss of TbetaRIII expression during cancer progression and direct effects of TbetaRIII on regulating cell migration, invasion, proliferation, and angiogenesis support a role for TbetaRIII as a suppressor of cancer progression and/or as a metastasis suppressor. Defining the physiological function and mechanism of TbetaRIII action and alterations in TbetaRIII function during cancer progression should enable more effective targeting of TbetaRIII and TbetaRIII mediated functions for the diagnosis and treatment of human cancer.

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.

Neurotrophins induce neuregulin release through protein kinase Cdelta activation

Proper, graded communication between different cell types is essential for normal development and function. In the nervous system, heart, and for some cancer cells, part of this communication requires signaling by soluble and membrane-bound factors produced by the NRG1 gene. We have previously shown that glial-derived neurotrophic factors activate a rapid, localized release of soluble neuregulin from neuronal axons that can, in turn promote proper axoglial development (Esper, R. M., and Loeb, J. A. (2004) J. Neurosci. 24, 6218-6227). Here we elucidate the mechanism of this localized, regulated release by implicating the delta isoform of protein kinase C (PKC). Blocking the PKC delta isoform with either rottlerin, a selective antagonist, or small interference RNA blocks the regulated release of neuregulin from both transfected cells and primary neuronal cultures. PKC activation also leads to the rapid phosphorylation of the pro-NRG1 cytoplasmic tail on serine residues adjacent to the membrane-spanning segment, that, when mutated markedly reduce the rate of NRG1 activity release. These findings implicate this specific PKC isoform as an important factor for the cleavage and neurotrophin-regulated release of soluble NRG1 forms that have important effects in nervous system development and disease.

Selective and specific regulation of ectodomain shedding of angiotensin-converting enzyme 2 by tumor necrosis factor alpha-converting enzyme

Angiotensin-converting enzyme 2 (ACE2) is a newly identified regulator of the renin-angiotensin system. This type I membrane-anchored protein has a catalytically active ectodomain that undergoes shedding. Tumor necrosis factor alpha-converting enzyme (TACE) has been shown to be involved in ACE2 shedding. Although pathophysiological significance of ACE2 shedding has been suggested, regulation of this process by TACE is not clearly defined. We characterized TACE-mediated constitutive ectodomain shedding of ACE2 using wild-type Chinese Hamster Ovary (WT-CHO), the TACE-mutant M2 (M2-CHO) cells, and EC-4 and EC-2 cells that are fibroblasts from wild-type and TACE-null mice, respectively. ACE2 was constitutively cleaved to release two distinct major soluble forms. The deglycosylated molecular masses of the larger (LSF) and smaller soluble form (SSF) were approximately 80 and 70 kDa, respectively. These forms had equivalent enzyme activities. Reduced shedding for the LSF from M2-CHO and EC-2 cells when compared with WT-CHO and EC-4 cells, respectively, was noted. TACE reconstitution in EC-2 cells expressing ACE2 resulted in increase in LSF but not SSF release, demonstrating a main role of TACE in LSF release and distinct regulations of release of the two soluble forms. Deletions of the juxtamembrane region of ACE2 reduced LSF release in CHO cell lines, whereas it abolished TACE-induced shedding in EC-2 cells. Analysis of TACE structural domains confirmed that the active site in the catalytic domain is essential for ACE2 shedding but that noncatalytic domains also play additional roles. These results demonstrate selective and specific regulation of constitutive shedding of ACE2 by TACE.

The role of protease activity in ErbB biology

Proteases are now recognized as having an active role in a variety of processes aside from their recognized metabolic role in protein degradation. Within the ErbB system of ligands and receptors, proteases are known to be necessary for the generation of soluble ligands from transmembrane precursors and for the processing of the ErbB4 receptor, such that its intracellular domain is translocated to the nucleus. There are two protease activities involved in the events: proteases that cleave within the ectodomain of ligand (or receptor) and proteases that cleave the substrate within the transmembrane domain. The former are the ADAM proteases and the latter are the gamma-secretase complex and the rhomboid proteases. This review discusses the roles of each of these protease systems within the ErbB system.

DLG1/SAP97 modulates transforming growth factor alpha bioavailability

TGFalpha and its receptor EGFR participate in the development of a wide range of tumors including gliomas, the main adult primary brain tumors. TGFalpha soluble form results from the cleavage by the metalloprotease TACE/ADAM17 of the extracellular part of its transmembrane precursor, pro-TGFalpha. To gain insights into the mechanisms underlying TGFalpha bioavailability, a yeast two-hybrid screen was performed to identify proteins interacting with pro-TGFalpha intracellular domain (ICD). DLG1/SAP97 (Discs Large Gene 1 or Synapse Associated Protein 97) was found to interact with both pro-TGFalpha and TACE ICDs through distinct PDZ domains. An in vivo pro-TGFalpha-DLG1-TACE complex was detected in U251 glioma cells and in gliomas-derived tumor initiating cells. Interaction between DLG1 and TACE diminished in response to stimulations promoting pro-TGFalpha shedding. Manipulation of DLG1 levels revealed dual actions of DLG1 on pro-TGFalpha shedding, favoring approximation of pro-TGFalpha and TACE, while limiting TACE full shedding activity. These results show that DLG1 participates in the control of TGFalpha bioavailability through its dynamic interaction with the growth factor precursor and TACE.

Substitution of methionine 435 with leucine, isoleucine, and serine in tumor necrosis factor alpha converting enzyme inactivates ectodomain shedding activity

Tumor necrosis factor alpha (TNF-alpha) converting enzyme (TACE) is a zinc metalloprotease that has emerged as a general sheddase, which is responsible for ectodomain release of numerous membrane proteins, including the proinflammatory cytokine TNF-alpha, the leukocyte adhesin L-selectin and epidermal growth factor receptor ligand-transforming growth factor alpha (TGF-alpha), and related family members. Structurally, TACE belongs to a large clan of proteases, designated the metzincins, because TACE possesses a conserved methionine (Met435), frequently referred to as the met-turn residue, in its active site. A vital role of this residue in the function of TACE is supported by the fact that cells expressing the M435I TACE variant are defective in ectodomain shedding. However, the importance of Met435 in TACE appears to be uncertain, since another metzincin, matrix metalloprotease-2, has been found to be enzymatically fully active with either leucine or serine in place of its met-turn residue. We constructed TACE mutants with leucine or serine in place of Met435 to further examine the role of the met-turn residue in TACE-mediated ectodomain cleavage. Similar to the M435I TACE mutant, both the M435L and M435S constructs are defective in cleaving transmembrane TNF-alpha, TGF-alpha, and L-selectin. Comparative modeling and dynamic computation detected structural perturbations, which resulted in higher energy, in the catalytic zinc complexes of the Met435 TACE mutants compared with that in the wild-type enzyme. Thus, Met435 serves to maintain the stability of the catalytic center of TACE for the hydrolysis of peptide bonds in substrates.

Characterisation of endothelin converting enzyme-1 shedding from endothelial cells

The aim of this study was to determine if endothelin converting enzyme-1 (ECE-1) like other members of this metalloprotease family undergoes ectodomain shedding. The release/shedding of catalytically active ECE-1 was measured by monitoring the fluorescence resulting from the cleavage of a specific quenched fluorescent substrate. Catalytically active ECE-1 was detected in the media of human umbilical vein endothelial cells, and was confirmed by mass spectrometry based assays. Specificity of cleavage was confirmed by using both narrow and broad specificity inhibitors. In conclusion we demonstrate and characterize for the first time, ECE-1 shedding from the surface of endothelial cells.

IL-13-induced proliferation of airway epithelial cells: mediation by intracellular growth factor mobilization and ADAM17

BACKGROUND

The pleiotrophic cytokine interleukin (IL)-13 features prominently in allergic and inflammatory diseases. In allergic asthma, IL-13 is well established as an inducer of airway inflammation and tissue remodeling. We demonstrated previously that IL-13 induces release of transforming growth factor-alpha (TGFalpha) from human bronchial epithelial cells, with proliferation of these cells mediated by the autocrine/paracrine action of this growth factor. TGFalpha exists as an integral membrane protein and requires proteolytic processing to its mature form, with a disintegrin and metalloproteinase (ADAM)17 responsible for this processing in a variety of tissues.

METHODS

In this study, normal human bronchial epithelial (NHBE) cells grown in air/liquid interface (ALI) culture were used to examine the mechanisms whereby IL-13 induces release of TGFalpha and cellular proliferation. Inhibitors and antisense RNA were used to examine the role of ADAM17 in these processes, while IL-13-induced changes in the intracellular expression of TGFalpha and ADAM17 were visualized by confocal microscopy.

RESULTS

IL-13 was found to induce proliferation of NHBE cells, and release of TGFalpha, in an ADAM17-dependent manner; however, this IL-13-induced proliferation did not appear to result solely from ADAM17 activation. Rather, IL-13 induced a change in the location of TGFalpha expression from intracellular to apical regions of the NHBE cells. The apical region was also found to be a site of significant ADAM17 expression, even prior to IL-13 stimulation.

CONCLUSION

Results from this study indicate that ADAM17 mediates IL-13-induced proliferation and TGFalpha shedding in NHBE cells. Furthermore, they provide the first example wherein a cytokine (IL-13) induces a change in the intracellular expression pattern of a growth factor, apparently inducing redistribution of intracellular stores of TGFalpha to the apical region of NHBE cells where expression of ADAM17 is prominent. Thus, IL-13-induced, ADAM17-mediated release of TGFalpha, and subsequent epithelial cell proliferation, could contribute to the epithelial hypertrophy, as well as other features, associated with airway remodeling in allergic asthma.

Metalloproteases regulate T-cell proliferation and effector function via LAG-3

Tight control of T-cell proliferation and effector function is essential to ensure an effective but appropriate immune response. Here, we reveal that this is controlled by the metalloprotease-mediated cleavage of LAG-3, a negative regulatory protein expressed by all activated T cells. We show that LAG-3 cleavage is mediated by two transmembrane metalloproteases, ADAM10 and ADAM17, with the activity of both modulated by two distinct T-cell receptor (TCR) signaling-dependent mechanisms. ADAM10 mediates constitutive LAG-3 cleavage but increases approximately 12-fold following T-cell activation, whereas LAG-3 shedding by ADAM17 is induced by TCR signaling in a PKCtheta-dependent manner. LAG-3 must be cleaved from the cell surface to allow for normal T-cell activation as noncleavable LAG-3 mutants prevented proliferation and cytokine production. Lastly, ADAM10 knockdown reduced wild-type but not LAG-3(-/-) T-cell proliferation. These data demonstrate that LAG-3 must be cleaved to allow efficient T-cell proliferation and cytokine production and establish a novel paradigm in which T-cell expansion and function are regulated by metalloprotease cleavage with LAG-3 as its sole molecular target.

The generation and function of soluble apoE receptors in the CNS

More than a decade has passed since apolipoprotein E4 (APOE-ε4) was identified as a primary risk factor for Alzheimer 's disease (AD), yet researchers are even now struggling to understand how the apolipoprotein system integrates into the puzzle of AD etiology. The specific pathological actions of apoE4, methods of modulating apolipoprotein E4-associated risk, and possible roles of apoE in normal synaptic function are still being debated. These critical questions will never be fully answered without a complete understanding of the life cycle of the apolipoprotein receptors that mediate the uptake, signaling, and degradation of apoE. The present review will focus on apoE receptors as modulators of apoE actions and, in particular, explore the functions of soluble apoE receptors, a field almost entirely overlooked until now.

Ectodomain shedding of preadipocyte factor 1 (Pref-1) by tumor necrosis factor alpha converting enzyme (TACE) and inhibition of adipocyte differentiation

Preadipocyte factor 1 (Pref-1), an epidermal growth factor repeat containing transmembrane protein found in the preadipocytes, inhibits adipocyte differentiation in vitro and in vivo. Here, we examined the processing of membrane form of Pref-1A to release the 50-kDa soluble form that inhibits adipocyte differentiation. The ectodomain cleavage of Pref-1 is markedly enhanced by phorbol 12-myristate 13-acetate in a dose- and time-dependent manner. The basal and stimulated cleavage is inhibited by the broad metalloproteinase inhibitor GM6001, a fact that suggests that cleavage of membrane Pref-1A is dependent on a metalloproteinase. Next, we showed that release of soluble Pref-1A is inhibited by TAPI-0 and by a tissue inhibitor of metalloproteinase-3, TIMP-3, that can inhibit tumor necrosis factor alpha converting enzyme (TACE), but not by TIMP-1 or TIMP-2. On the other hand, overexpression of TACE increases Pref-1 cleavage to produce the 50-kDa soluble form. Furthermore, this cleavage was not detected in cells with TACE mutation or with TACE small interfering RNA. TACE-mediated shedding of Pref-1 ectodomain inhibits adipocyte differentiation of 3T3-L1 cells and in Pref-1-null mouse embryo fibroblasts transduced with Pref-1A. Identification of TACE as the major protease responsible for conversion of membrane-bound Pref-1 to the biologically active diffusible form provides a new insight into Pref-1 function in adipocyte differentiation.

Biosynthesis of tumorigenic HER2 C-terminal fragments by alternative initiation of translation

The overactivation of the HERs, a family of tyrosine kinase receptors, leads to the development of cancer. Although the canonical view contemplates HER receptors restricted to the secretory and endocytic pathways, full-length HER1, HER2 and HER3 have been detected in the nucleoplasm. Furthermore, limited proteolysis of HER4 generates nuclear C-terminal fragments (CTFs). Using cells expressing a panel of deletion and point mutants, here we show that HER2 CTFs are generated by alternative initiation of translation from methionines located near the transmembrane domain of the full-length molecule. In vitro and in vivo, HER2 CTFs are found in the cytoplasm and nucleus. Expression of HER2 CTFs to levels similar to those found in human tumors induces the growth of breast cancer xenografts in nude mice. Tumors dependent on CTFs are sensitive to inhibitors of the kinase activity but do not respond to therapeutic antibodies against HER2. Thus, the kinase domain seems necessary for the activity of HER2 CTFs and the presence of these HER2 fragments could account for the resistance to treatment with antibodies.

Soluble Axl is generated by ADAM10-dependent cleavage and associates with Gas6 in mouse serum

Axl receptor tyrosine kinase exists as a transmembrane protein and as a soluble molecule. We show that constitutive and phorbol 12-myristate 13-acetate-induced generation of soluble Axl (sAxl) involves the activity of disintegrin-like metalloproteinase 10 (ADAM10). Spontaneous and inducible Axl cleavage was inhibited by the broad-spectrum metalloproteinase inhibitor GM6001 and by hydroxamate GW280264X, which is capable of blocking ADAM10 and ADAM17. Furthermore, murine fibroblasts deficient in ADAM10 expression exhibited a significant reduction in constitutive and inducible Axl shedding, whereas reconstitution of ADAM10 restored sAxl production, suggesting that ADAM10-mediated proteolysis constitutes a major mechanism for sAxl generation in mice. Partially overlapping 14-amino-acid stretch deletions in the membrane-proximal region of Axl dramatically affected sAxl generation, indicating that these regions are involved in regulating the access of the protease to the cleavage site. Importantly, relatively high circulating levels of sAxl are present in mouse sera in a heterocomplex with Axl ligand Gas6. Conversely, two other family members, Tyro3 and Mer, were not detected in mouse sera and conditioned medium. sAxl is constitutively released by murine primary cells such as dendritic and transformed cell lines. Upon immobilization, sAxl promoted cell migration and induced the phosphorylation of Axl and phosphatidylinositol 3-kinase. Thus, ADAM10-mediated generation of sAxl might play an important role in diverse biological processes.

Up-regulated expression of ADAM17 in human colon carcinoma: co-expression with EGFR in neoplastic and endothelial cells

The ADAM17 metalloproteinase (a disintegrin and metalloprotease 17) controls epidermal growth factor receptor (EGFR) activation through regulated shedding of EGFR ligands. With the advent of new therapeutic options targeting EGFR signalling in colon carcinoma, it was decided to determine ADAM17 status in relation to clinico-pathological parameters and EGFR status. To this end, a series of 39 colon carcinomas were analysed. Immunohistochemistry and immunofluorescence were used to localize ADAM17, EGFR, and the activated forms of EGFR. The activated form of ADAM17 was assessed in primary cancers and colon cell lines by immunoblotting. ADAM17 and EGFR mRNA levels were assessed by quantitative RT-PCR. Chromogenic in situ hybridization (CISH) was used to quantify the HER1 gene. ADAM17 was strongly expressed in all tumours, by both neoplastic and endothelial cells. It was expressed both as a pro- and as an active form in tumours and colonic cancer cell lines. ADAM17 mRNA was up-regulated in 90% of colon carcinomas relative to the paired normal mucosa, whatever the tumour grade or stage. When present, activated EGFR was co-expressed with ADAM17 by colon carcinomas, although at a variable level among tumour cells, and by endothelial cells. EGFR mRNA was overexpressed in 77% of colon carcinomas compared with the paired normal mucosa. One case showed high-level amplification of HER1. In conclusion, this study is the first demonstration that ADAM17 is overexpressed in human primary colon carcinoma, whatever the tumour stage and differentiation and whatever the level of EGFR expression. Its co-expression with EGFR, in both neoplastic and endothelial cells, suggests a role for ADAM17 in tumour growth and angiogenesis.

Detection of transforming growth factor-alpha and epidermal growth factor receptor mRNA and immunohistochemical localization of the corresponding proteins in the canine uterus during the estrous cycle

Uterine expression of the epidermal growth factor (EGF) family of growth factors has not been studied in the dog. The present study looks at the presence of mRNA transcripts and immunohistochemical localization for transforming growth factor-alpha (TGF-alpha), which is the potent EGF family member, and for EGF receptor (EGF-R) in the canine uterus during the estrous cycle. The reverse transcriptase-polymerase chain reaction together with sequencing of the products confirmed the presence of their mRNA transcripts in the endometrium throughout the estrous cycle. Immunohistochemical analysis found clear positive staining for TGF-alpha and EGF-R in the luminal and glandular epithelia at proestrus and estrus. Immunoreactivity decreased at the early stage of diestrus. In the mid stage of diestrus, clear staining for TGF-alpha was again found in the glands of the luminal region, and staining for EGF-R was observed in all glands. Very little staining was seen at anestrus for either TGF-alpha or EGF-R. These results suggest that TGF-alpha expressed in the uterus may be involved in regulating growth, differentiation and regression in the endometrial epithelial cells during the estrous cycle in the dog.

Tumor necrosis factor-alpha convertase (ADAM17) mediates regulated ectodomain shedding of the severe-acute respiratory syndrome-coronavirus (SARS-CoV) receptor, angiotensin-converting enzyme-2 (ACE2)

Angiotensin-converting enzyme-2 (ACE2) is a critical regulator of heart function and a cellular receptor for the causative agent of severe-acute respiratory syndrome (SARS), SARS-CoV (coronavirus). ACE2 is a type I transmembrane protein, with an extracellular N-terminal domain containing the active site and a short intracellular C-terminal tail. A soluble form of ACE2, lacking its cytosolic and transmembrane domains, has been shown to block binding of the SARS-CoV spike protein to its receptor. In this study, we examined the ability of ACE2 to undergo proteolytic shedding and investigated the mechanisms responsible for this shedding event. We demonstrated that ACE2, heterologously expressed in HEK293 cells and endogenously expressed in Huh7 cells, undergoes metalloproteinase-mediated, phorbol ester-inducible ectodomain shedding. By using inhibitors with differing potency toward different members of the ADAM (a disintegrin and metalloproteinase) family of proteases, we identified ADAM17 as a candidate mediator of stimulated ACE2 shedding. Furthermore, ablation of ADAM17 expression using specific small interfering RNA duplexes reduced regulated ACE2 shedding, whereas overexpression of ADAM17 significantly increased shedding. Taken together, these data provided direct evidence for the involvement of ADAM17 in the regulated ectodomain shedding of ACE2. The identification of ADAM17 as the protease responsible for ACE2 shedding may provide new insight into the physiological roles of ACE2.

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.

Extracellular proteases in atherosclerosis and restenosis

Extracellular proteolysis plays a key role in many pathophysiologic processes including cancer, inflammatory diseases, and cardiovascular conditions such as atherosclerosis and restenosis. Whereas matrix metalloproteinases are their best known member, many others are becoming better known. The extracellular proteases are a complex and heterogeneous superfamily of enzymes. They include metalloproteinases (matrix metalloproteinases, adamalysins, or pappalysins), serine proteases (elastase, coagulation factors, plasmin, tissue plasminogen activator, urokinase plasminogen activator), and the cysteine proteases (such cathepsins). In addition to their matrix degradation capabilities, they have other less well known biologic functions that include angiogenesis, growth factor bioavailability, cytokine modulation, receptor shedding, enhancing cell migration, proliferation, invasion, and apoptosis. This review discusses extracellular proteases relevant to the vasculature, their classification and function, and how protease disorders contribute to arterial plaque growth, including chronic atherosclerosis, acute coronary syndromes, restenosis, and vascular remodeling. These broad extracellular protease functions make them potentially interesting therapeutic targets.

Regulation of ApoE receptor proteolysis by ligand binding

Two brain receptors for apolipoprotein E (apoE), the apoE receptor 2 (apoEr2) and the very low density lipoprotein receptor (VLDLr), are important in neuronal migration during development. Using a series of tagged constructs, we found that these two receptors undergo extracellular cleavages to release secreted forms of the receptors and intramembranous cleavages to degrade the C-terminal fragments. Release of the extracellular domains was blocked by an inhibitor of metalloproteinases and increased by PMA. Accumulation of the intracellular domain was increased by the presence of an inhibitor of gamma-secretase. ApoE binding to these receptors caused increased release of the extracellular domain and accumulation of the intracellular domains; similar increases in receptor proteolytic domains were observed from endogenous apoEr2 after treatment of primary neurons or rat hippocampus with an apoE-derived peptide. The human apoE isoforms promoted proteolysis to different degrees, with apoE2 resulting in a greater accumulation of the C-terminal fragments of apoEr2 compared with apoE4. These effects of apoE on receptor proteolysis were mediated by the ligand binding domain of the receptor. Similar promotion of receptor cleavage was seen with two other ligands, reelin and activated alpha2-macroglobulin. We suggest that signaling promoted by these receptors depends in part on these regulated proteolytic events.

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.

Modulation of statin-activated shedding of Alzheimer APP ectodomain by ROCK

BACKGROUND

Statins are widely used cholesterol-lowering drugs that act by inhibiting HMGCoA reductase, the rate-limiting enzyme in cholesterol biosynthesis. Recent evidence suggests that statin use may be associated with a decreased risk for Alzheimer disease, although the mechanisms underlying this apparent risk reduction are poorly understood. One popular hypothesis for statin action is related to the drugs' ability to activate alpha-secretase-type shedding of the alpha-secretase-cleaved soluble Alzheimer amyloid precursor protein ectodomain (sAPP(alpha)). Statins also inhibit the isoprenoid pathway, thereby modulating the activities of the Rho family of small GTPases-Rho A, B, and C-as well as the activities of Rac and cdc42. Rho proteins, in turn, exert many of their effects via Rho-associated protein kinases (ROCKs). Several cell-surface molecules are substrates for activated alpha-secretase-type ectodomain shedding, and regulation of shedding typically occurs via activation of protein kinase C or extracellular-signal-regulated protein kinases, or via inactivation of protein phosphatase 1 or 2A. However, the possibility that these enzymes play a role in statin-stimulated shedding has been excluded, leading us to investigate whether the Rho/ROCK1 protein phosphorylation pathway might be involved.

METHODS AND FINDINGS

We found that both atorvastatin and simvastatin stimulated sAPP(alpha) shedding from a neuroblastoma cell line via a subcellular mechanism apparently located upstream of endocytosis. A farnesyl transferase inhibitor also increased sAPP(alpha) shedding, as did a dominant negative form of ROCK1. Most conclusively, a constitutively active ROCK1 molecule inhibited statin-stimulated sAPP(alpha) shedding.

CONCLUSION

Together, these data suggest that statins exert their effects on shedding of sAPP(alpha) from cultured cells, at least in part, by modulation of the isoprenoid pathway and ROCK1.

Natural Soluble Interleukin-15Rα Is Generated by Cleavage That Involves the Tumor Necrosis Factor-α-converting Enzyme (TACE/ADAM17)

This study shows that the high affinity alpha-chain of the interleukin (IL)-15 receptor exists not only in membrane-anchored but also in soluble form. Soluble IL-15Ralpha (sIL-15Ralpha) can be detected in mouse sera and cell-conditioned media by enzyme-linked immunosorbent assay and by immunoprecipitation and Western blotting. This protein has a molecular mass of about 30 kDa because of the presence of a single N-glycosylation site, which is reduced to 26 kDa after N-glycosidase treatment. Transmembrane IL-15Ralpha is constitutively converted into its soluble form by proteolytic cleavage that involves tumor necrosis factor-alpha-converting enzyme (TACE), and this process is further enhanced by phorbol 12-myristate 13-acetate (PMA) stimulation. The hydroxamate GW280264X, which is capable of blocking TACE and the closely related disintegrin-like metalloproteinase 10 (ADAM10), effectively inhibited both spontaneous and PMA-inducible cleavage of IL-15Ralpha, whereas GI254023X, which preferentially blocks ADAM10, was ineffective. Overexpression of TACE but not ADAM10 in COS-7 cells enhanced the constitutive and PMA-inducible cleavage of IL-15Ralpha. Moreover, murine fibroblasts deficient in TACE but not ADAM10 expression exhibited a significant reduction in the spontaneous and inducible IL-15Ralpha shedding, whereas a reconstitution of TACE in these cells restored the release of sIL-15Ralpha, thereby suggesting that TACE-mediated proteolysis may represent a major mechanism for sIL-15Ralpha generation in mice. The existence of natural sIL-15Ralpha offers novel insights into the complex biology of IL-15 and envisages a new level for therapeutic intervention.

Loss of ectodomain shedding due to mutations in the metalloprotease and cysteine-rich/disintegrin domains of the tumor necrosis factor-alpha converting enzyme (TACE)

Tumor necrosis factor-alpha converting enzyme (TACE), a multidomain protease essential for development and disease, releases the ectodomains from many transmembrane proteins in a regulated fashion. To understand the mechanism underlying the regulation of TACE activity, we sought to identify the cause of ectodomain shedding deficiencies in two mutated CHO sublines designated M1 and M2. Transfection of expression vectors for human and mouse TACE restored ectodomain shedding of TNF-alpha and TGF-alpha, suggesting that defects in the TACE gene contribute to the phenotype of M1 and M2 cells. The overall levels of endogenous TACE forms in M1 cells were significantly lower than those found in their parental cells, whereas only TACE zymogen, but not its mature form, was detectable in M2 cells. Molecular analyses suggested that M1 cells contained only one expressible TACE allele encoding an M435I point mutation in the catalytic center of the protease, and M2 cells produced two TACE variants with distinct point mutations in the catalytic domain (C225Y) and the cysteinerich/disintegrin domain (C600Y). Overexpression of the C225Y and C600Y TACE by transient transfection largely compensated for maturation defects in the variants but failed to restore TNF-alpha and TGF-alpha release in the shedding-defective CHO cell lines and fibroblasts derived from TACE-null mouse embryo. Further mutagenesis and functional analyses demonstrated that Cys(600) was absolutely essential for ectodomain shedding, suggesting that Cys(600), similar to Cys(225), participates in disulfide bonding, which is critical for both the processing and catalysis of TACE.