Substrate selectivity of epidermal growth factor-receptor ligand sheddases and their regulation by phorbol esters and calcium influx

ADAM8 is a negative regulator of retinal neovascularization and of the growth of heterotopically injected tumor cells in mice

ADAM8 is a member of the "a disintegrin and metalloproteinase" (ADAM) family of membrane-anchored metalloproteinases. ADAM8-deficient mice have no evident spontaneous developmental or pathological defects, and little is currently known about the role of ADAM8 in disease. Here, we investigated the contribution of ADAM8 to pathological neovascularization in mice using an oxygen-induced retinopathy (OIR) model and heterotopical injection of tumor cells. We found an increase in retinal re-vascularization but fewer neovascular tufts in the OIR model and increased growth of heterotopically injected tumor cells in Adam8-/- mice compared with wild-type controls. These results suggest that ADAM8 functions to limit both of these processes in wild-type mice. In cell-based assays, overexpression of ADAM8 increased the ectodomain shedding of several co-expressed membrane proteins with roles in angiogenesis (CD31, Tie-2, Flk-1, Flt-1, EphrinB2, EphB4, VE-cadherin, KL-1, E-selectin, and neuregulin-1beta2). Thus, dysregulated expression of ADAM8 in endothelial cells in vivo could potentially increase the processing of these and other substrate proteins. Taken together, our findings suggest that inhibiting ADAM8 could be useful for promoting re-vascularization and thereby preventing formation of neovascular tufts in proliferative retinopathies. On the other hand, blocking ADAM8 could be detrimental in the context of rapidly growing tumors.

Receptor activator of NF-kappaB (RANK) ligand induces ectodomain shedding of RANK in murine RAW264.7 macrophages

Osteoclastogenesis is a highly sophisticated process that involves a variety of membrane-bound proteins expressed in osteoblasts and osteoclast precursors. Over the past several years, proteolytic cleavage and release of the ectodomain of membrane-bound proteins, also referred to as ectodomain shedding, has emerged as an important posttranslational regulatory mechanism for modifying the function of cell surface proteins. In line with this notion, several membrane-bound molecules involved in osteoclastogenesis, including CSF-1R and receptor activator of NF-kappaB ligand (RANKL), are proteolytically cleaved and released from the cell surface. In this study, we investigated whether receptor activator of NF-kappaB (RANK), one of the most essential molecules in osteoclastogenesis, undergoes ectodomain shedding. The results showed that RANK is released in the form of a soluble monomeric protein and that TNF-alpha-converting enzyme is involved in this activity. We also identified potential cleavage sites in the juxtamembrane domain of RANK and found that rRANKL induces RANK shedding in a macrophage-like cell line RAW264.7 via TNFR-associated factor 6 and MAPK pathways. Furthermore, we found that RANKL-induced osteoclastogenesis is accelerated in TNF-alpha-converting enzyme-deficient osteoclast precursors. These observations suggest the potential involvement of ectodomain shedding in the regulation of RANK functions and may provide novel insights into the mechanisms of osteoclastogenesis.

Pathological neovascularization is reduced by inactivation of ADAM17 in endothelial cells but not in pericytes

RATIONALE

Pathological neovascularization is a critical component of diseases such as proliferative retinopathies, cancer and rheumatoid arthritis, yet much remains to be learned about the underlying causes. Previous studies showed that vascular endothelial growth factor (VEGF)-A activates the membrane-anchored metalloproteinase ADAM17 (a disintegrin and metalloproteinase 17) in endothelial cells, thereby stimulating crosstalk between VEGF receptor 2 and extracellular signal-regulated kinase. These findings raised interesting questions about the role of ADAM17 in angiogenesis and neovascularization in vivo.

OBJECTIVE

The objective of this study was to inactivate ADAM17 in endothelial cells or in pericytes to determine how this affects developmental angiogenesis, pathological retinal neovascularization and heterotopic tumor growth.

METHODS AND RESULTS

We generated animals in which floxed ADAM17 was removed by Tie2-Cre in endothelial cells, or by smooth muscle (sm) Cre in smooth muscle cells and pericytes. There were no evident developmental defects in either conditional knockout strain, but pathological retinal neovascularization and growth of heterotopically injected tumor cells was reduced in Adam17flox/flox/Tie2-Cre mice, although not in Adam17flox/flox/sm-Cre mice. Moreover, lack of ADAM17 in endothelial cells decreased ex vivo chord formation, and this could be largely restored by addition of the ADAM17 substrate HB-EGF (heparin-binding epidermal growth factor-like growth factor). Finally we found that ADAM17 is important for the VEGF receptor 2 stimulated processing of several receptors with known functions in endothelial cell biology.

CONCLUSIONS

These results provide the first evidence for a role for ADAM17 in pathological neovascularization in vivo. Because ADAM17 does not appear to be required for normal developmental angiogenesis or vascular homeostasis, it could emerge as a good target for treatment of pathological neovascularization.

An arginine stretch limits ADAM10 exit from the endoplasmic reticulum

A disintegrin and metalloproteinase 10 (ADAM10) is a type I transmembrane glycoprotein responsible for the ectodomain shedding of a number of proteins implicated in the pathogenesis of diseases ranging from cancer to Alzheimer Disease. ADAM10 is synthesized in an inactive form, which is proteolytically activated during its forward transport along the secretory pathway and at the plasma membrane. Therefore, modulation of its trafficking could provide a mechanism to finely tune its shedding activity. Here we report the identification of an endoplasmic reticulum (ER) retention motif within the ADAM10 intracellular C-terminal tail. Sequential deletion/mutagenesis analyses showed that an arginine-rich ((723)RRR) sequence was responsible for the retention of ADAM10 in the ER and its inefficient surface trafficking. Mutating the second arginine to alanine was sufficient to allow ER exit and surface expression in both heterologous cells and hippocampal neurons. As synapse-associated protein 97 (SAP97) binds ADAM10 at its cytoplasmic tail and facilitates forward ADAM10 trafficking in neurons, we tested whether SAP97 could modulate ER export. However, neither expression nor Ser-39 phosphorylation of SAP97 in heterologous cells or hippocampal neurons were sufficient to allow the ER exit of ADAM10, suggesting that other signaling pathways or alternative binding partners are responsible for ADAM10 ER exit. Together, these results identify a novel mechanism regulating the intracellular trafficking and membrane delivery of ADAM10.

Ectodomain shedding of Fcalpha receptor is mediated by ADAM10 and ADAM17

FcalphaR (CD89) plays important roles in immunoglobulin A (IgA)-mediated immune responses. Soluble forms of FcalphaR (sFcalphaR) are found in the culture supernatants of FcalphaR-expressing cells, in human serum and in the serum of FcalphaR transgenic mice, and have been suggested to be produced through a proteolytic process. However, little is known about the mechanism involved in the proteolytic release of sFcalphaR. In this study, we investigated the shedding mechanism of FcalphaR and determined the nature of the proteinase involved in FcalphaR shedding. In chemical inhibitor assays, shedding of FcalphaR was dramatically inhibited by EDTA, EGTA and a broad-spectrum metalloproteinase inhibitor, GM6001, suggesting that a metalloproteinase was responsible for FcalphaR shedding. Overexpression of dominant-negative mutants of ADAM (a disintegrin and metalloproteinase) 10 and ADAM17 markedly inhibited the production of sFcalphaR. Finally, knockdown of both endogenous ADAM10 and endogenous ADAM17 inhibited FcalphaR shedding, demonstrating that ADAM10 and ADAM17 were involved in the shedding of FcalphaR. The characterization of ADAM10 and ADAM17 as sFcalphaR-releasing enzymes provides a novel insight into the molecular mechanism of sFcalphaR production and will help in further elucidation of the physiological and pathological roles of sFcalphaR.

Active-site determinants of substrate recognition by the metalloproteinases TACE and ADAM10

The metalloproteinases TACE [tumour necrosis factor alpha-converting enzyme; also known as ADAM17 (a disintegrin and metalloproteinase 17)] and ADAM10 are the primary enzymes responsible for catalysing release of membrane-anchored proteins from the cell surface in metazoan organisms. Although the repertoire of protein substrates for these two proteases is partially overlapping, each one appears to target a subset of unique proteins in vivo. The mechanisms by which the two proteases achieve specificity for particular substrates are not completely understood. We have used peptide libraries to define the cleavage site selectivity of TACE and ADAM10. The two proteases have distinct primary sequence requirements at multiple positions surrounding the cleavage site in their substrates, which allowed us to generate peptide substrates that are highly specific for each of these proteases. The major difference between the two protease specificities maps to the P1' position (immediately downstream of the cleavage site) of the substrate. At this position, TACE is selective for smaller aliphatic residues, whereas ADAM10 can accommodate aromatic amino acids. Using mutagenesis we identified three residues in the S1' pockets of these enzymes that dramatically influence specificity for both peptide and protein substrates. Our results suggest that substrate selectivity of TACE and ADAM10 can be at least partly rationalized by specific features of their active sites.

Cytoplasmic relaxation of active Eph controls ephrin shedding by ADAM10

Novel imaging strategies reveal a conformational shift in a receptor tyrosine kinase domain that controls ligand shedding by an ADAM metalloprotease.

Release of cell surface-bound ligands by A-Disintegrin-And-Metalloprotease (ADAM) transmembrane metalloproteases is essential for signalling by cytokine, cell adhesion, and tyrosine kinase receptors. For Eph receptor ligands, it provides the switch between cell-cell adhesion and repulsion. Ligand shedding is tightly controlled by intrinsic tyrosine kinase activity, which for Eph receptors relies on the release of an inhibitory interaction of the cytoplasmic juxtamembrane segment with the kinase domain. However, a mechanism linking kinase and sheddase activities had remained elusive. We demonstrate that it is a membrane-proximal localisation of the latent kinase domain that prevents ephrin ligand shedding in trans. Fluorescence lifetime imaging microscopy and electron tomography reveal that activation extends the Eph receptor tyrosine kinase intracellular domain away from the cell membrane into a conformation that facilitates productive association with ADAM10. Accordingly, EphA3 mutants with constitutively-released kinase domains efficiently support shedding, even when their kinase is disabled. Our data suggest that this phosphorylation-activated conformational switch of EphA3 directly controls ADAM-mediated shedding.

The Eph transmembrane receptors are part of the receptor tyrosine kinase family and play important roles in communication between neighbouring cells. An Eph receptor binds to its ligand, membrane-tethered ephrin, on a neighbouring cell so as to form a stable complex and activate downstream signalling events. One such event is regulation of ADAM10, a transmembrane protease of the ADAM metalloprotease family, which provides a feedback mechanism to Eph signalling. ADAM10 is located on Eph-expressing cells and cleaves ephrin from its membrane tether on the opposite cell (through its so-called sheddase activity), thereby separating the cell-cell connection and allowing the signalling complex to internalise. In other biological contexts, activity of the ADAM metalloprotease family underlies signalling mechanisms such as oncogenic EGF-receptor transactivation, adhesion molecule shedding and cytokine/chemokine release. In general, ADAM function is enhanced when receptor tyrosine signalling is active and repressed when tyrosine kinase signalling is inhibited. However, the mechanism through which receptor tyrosine kinase signalling regulates ADAM10, have remained elusive. By combining fluorescence lifetime imaging microscopy (FLIM) and electron microscopic tomography of EphA3, we have demonstrated in live cells at molecular resolution that tyrosine phosphorylation of activated EphA3 triggers a measurable movement of the kinase domain away from the plasma membrane. Only this conformation of the EphA3 kinase domain away from the plasma membrane permits ADAM10 to come close enough to EphA3 so that it can reach its tightly EphA3-bound substrate, ephrin-A5. Our findings delineate a new regulatory concept in cell-cell communication, whereby control over proteolytic sheddase activity is provided by an activation-induced switch in the conformation of the cytoplasmic domain of a receptor tyrosine kinase, rather than by a cytosolic signalling pathway.

ADAM12 localizes with c-Src to actin-rich structures at the cell periphery and regulates Src kinase activity

ADAM12 is an active metalloprotease playing an important role in tumour progression. Human ADAM12 exists in two splice variants: a long transmembrane form, ADAM12-L, and a secreted form, ADAM12-S. The subcellular localization of ADAM12-L is tightly regulated and involves intracellular interaction partners and signalling proteins. We demonstrate here a c-Src-dependent redistribution of ADAM12-L from perinuclear areas to actin-rich Src-positive structures at the cell periphery, and identified two separate c-Src binding sites in the cytoplasmic tail of ADAM12-L that interact with the SH3 domain of c-Src with different binding affinities. The association between ADAM12-L and c-Src is transient, but greatly stabilized when the c-Src kinase activity is disrupted. In agreement with this observation, kinase-active forms of c-Src induce ADAM12-L tyrosine phosphorylation. Interestingly, ADAM12-L was also found to enhance Src kinase activity in response to external signals, such as integrin engagement. Thus, we suggest that activated c-Src binds, phosphorylates, and redistributes ADAM12-L to specific sites at the cell periphery, which may in turn promote signalling mechanisms regulating cellular processes with importance in cancer.

ADAM-mediated amphiregulin shedding and EGFR transactivation

INTRODUCTION

The ectodomain shedding of epidermal growth factor receptor (EGFR) ligands, such as amphiregulin (AREG), by ADAMs (A Disintegrin And Metalloproteases) can be stimulated by G protein-coupled receptor (GPCR) agonists. Interactions between the CXCR4 GPCR and the CXCL12 chemokine have been shown to mediate gene transcription and cellular proliferation in non-transformed and transformed prostate epithelial cells, as well as motility/invasiveness in transformed cells.

OBJECTIVES

In this report, we investigated the ability of CXCL12 to stimulate amphiregulin ectodomain shedding in non-transformed and transformed prostate epithelial cells that respond proliferatively to sub-nanomolar levels of CXCL12 and amphiregulin.

MATERIALS AND METHODS

Non-transformed N15C6 and transformed PC3 prostate epithelial cells were assessed for amphiregulin shedding, ADAM activation, Src phosphorylation and EGFR activation using ELISA, immunoblot, and immunoprecipitation techniques, and for proliferation using cell counting after stimulation with CXCL12 or vehicle.

RESULTS

The results of these studies identify CXCL12 as a novel inducer of amphiregulin ectodomain shedding and show that both basal and CXCL12-mediated amphiregulin shedding are ADAM10- and Src kinase-dependent in non-transformed N15C6 cells. In contrast, amphiregulin shedding is not amplified subsequent to stimulation with exogenous CXCL12, and is not reduced subsequent to metalloprotease- or Src kinase-inhibition, in highly aggressive PC3 prostate cancer cells. These data also show that CXCL12-mediated cellular proliferation requires EGFR transactivation in a Src- and ADAM-dependent manner in non-transformed prostate epithelial cells. However, these same mechanisms are dysfunctional in highly transformed prostate cancer cells, which secrete amphiregulin in an autocrine manner that cannot be repressed through metalloprotease- or Src kinase inhibition.

CONCLUSION

These findings show that non-transformed and transformed prostate epithelial cells may employ different mechanisms to activate EGFR ligands and thereby utilize the EGFR axis to promote cellular proliferation.

Shedding of collagen XVII/BP180 in skin depends on both ADAM10 and ADAM9

Collagen XVII is a transmembrane collagen and the major autoantigen of the autoimmune skin blistering disease bullous pemphigoid. Collagen XVII is proteolytically released from the membrane, and the pathogenic epitope harbors the cleavage site for its ectodomain shedding, suggesting that proteolysis has an important role in regulating the function of collagen XVII in skin homeostasis. Previous studies identified ADAMs 9, 10, and 17 as candidate collagen XVII sheddases and suggested that ADAM17 is a major sheddase. Here we show that ADAM17 only indirectly affects collagen XVII shedding and that ADAMs 9 and 10 are the most prominent collagen XVII sheddases in primary keratinocytes because (a) collagen XVII shedding was not stimulated by phorbol esters, known activators of ADAM17, (b) constitutive and calcium influx-stimulated shedding was sensitive to the ADAM10-selective inhibitor GI254023X and was strongly reduced in Adam10(-/-) cells, (c) there was a 55% decrease in constitutive collagen XVII ectodomain shedding from Adam9(-/-) keratinocytes, and (d) H(2)O(2) enhanced ADAM9 expression and stimulated collagen XVII shedding in skin and keratinocytes of wild type mice but not of Adam9(-/-) mice. We conclude that ADAM9 and ADAM10 can both contribute to collagen XVII shedding in skin with an enhanced relative contribution of ADAM9 in the presence of reactive oxygen species. These results provide critical new insights into the identity and regulation of the major sheddases for collagen XVII in keratinocytes and skin and have implications for the treatment of blistering diseases of the skin.

Selective use of ADAM10 and ADAM17 in activation of Notch1 signaling

Notch signaling requires a series of proteolytic cleavage events to release the Notch intracellular domain (NICD) that functions directly in signal transduction. The Notch receptor is locked down in a protease-resistant state by a negative regulatory region (NRR) that protects an ADAM (a disintegrin and metalloprotease) cleavage site. Engagement with ligand-bearing cells induces global conformational movements in Notch that unfold the NRR structure to expose the ADAM cleavage site and initiate proteolytic activation. Although both ADAM10 and ADAM17 have been reported to cleave Notch to facilitate NICD release by gamma-secretase, the relevant ADAM has remained controversial. Our study provides new insight into this conflict, as we find that although Notch1 (N1) is a substrate for both ADAM10 and ADAM17, the particular ADAM required for receptor activation is context dependent. Specifically, ADAM10 was absolutely required for N1 signaling induced by ligands, while signaling independent of ligands required ADAM17. In contrast to the strict and differential use of ADAM10 and ADAM17 in normal and dysregulated signaling, respectively, both proteases participated in signaling intrinsic to N1 mutations associated with leukemia. We propose that in addition to exposing the ADAM cleavage site, activating N1 conformational changes facilitate selective cleavage by specific proteases.

Synaptic plasticity-associated proteases and protease inhibitors in the brain linked to the processing of extracellular matrix and cell adhesion molecules

Research on the molecular and cellular basis of learning and memory has focused on the mechanisms that underlie the induction and expression of synaptic plasticity. There is increasing evidence that structural changes at the synapse are associated with synaptic plasticity and that extracellular matrix (ECM) components and cell adhesion molecules are associated with these changes. The functions of both groups of molecules can be regulated by proteolysis. In this article we review the roles of selected proteases and protease inhibitors in perisynaptic proteolysis of the ECM and synaptic adhesion proteins and the impact of proteolysis on synaptic modification and cognitive function.

Ectodomain shedding of FLT3 ligand is mediated by TNF-alpha converting enzyme

FLT3 ligand (FLT3L) has diverse roles in the hematopoietic system, which include stimulating proliferation of hematopoietic precursors and development of NK cells and dendritic cells. FLT3L is initially synthesized as a membrane-bound protein, which must be cleaved to become a soluble growth factor. However, little is known about the enzyme involved in the proteolytic release of FLT3L. In the current study, we show that shedding of FLT3L is metalloprotease-dependent, and that this proteolytic activity was abolished in fibroblasts lacking TNF-alpha converting enzyme (TACE) and could be rescued by reintroducing wild-type TACE in these cells. Moreover, we found that cells derived from the thymus of conditional TACE-deficient mice produce less FLT3L, and that serum FLT3L levels in these TACE mutant mice are significantly lower, both after LPS treatment and in the absence of such a challenge, further corroborating the relevance of TACE as FLT3L sheddase in vivo. Considering the involvements of FLT3 and FLT3L in hematopoietic malignancies and stem cell mobilization, the identification of the enzyme involved in FLT3L shedding may have important clinical implications.

Regulation of endothelial protein C receptor shedding by cytokines is mediated through differential activation of MAP kinase signaling pathways

The endothelial protein C receptor (EPCR) plays a pivotal role in coagulation, inflammation, cell proliferation, and cancer, but its activity is markedly changed by ectodomain cleavage and release as the soluble protein (sEPCR). In this study we examined the mechanisms involved in the regulation of EPCR shedding in human umbilical endothelial cells (HUVEC). Interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha), but not interferon-gamma and interleukin-6, suppressed EPCR mRNA transcription and cell-associated EPCR expression in HUVEC. The release of sEPCR induced by IL-1beta and TNF-alpha correlated with activation of p38 MAPK and c-Jun N-terminal kinase (JNK). EPCR shedding was also induced by phorbol 12-myristate 13-acetate, ionomycin, anisomycin, thiol oxidants or alkylators, thrombin, and disruptors of lipid rafts. Both basal and induced shedding of EPCR was blocked by the metalloproteinase inhibitors, TAPI-0 and GM6001, and by the reduced non-protein thiols, glutathione, dihydrolipoic acid, dithiothreitol, and N-acetyl-l-cysteine. Because other antioxidants and scavengers of reactive oxygen species failed to block the cleavage of EPCR, a direct suppression of metalloproteinase activity seems responsible for the observed effects of reduced thiols. In summary, the shedding of EPCR in HUVEC is effectively regulated by IL-1beta and TNF-alpha, and downstream by MAP kinase signaling pathways and metalloproteinases.

Ectodomain shedding of angiotensin converting enzyme 2 in human airway epithelia

Angiotensin-converting enzyme 2 (ACE2) is a terminal carboxypeptidase and the receptor for the SARS and NL63 coronaviruses (CoV). Loss of ACE2 function is implicated in severe acute respiratory syndrome (SARS) pathogenesis, but little is known about ACE2 biogenesis and activity in the airways. We report that ACE2 is shed from human airway epithelia, a site of SARS-CoV infection. The regulation of ACE2 release was investigated in polarized human airway epithelia. Constitutive generation of soluble ACE2 was inhibited by DPC 333, implicating a disintegrin and metalloprotease 17 (ADAM17). Phorbol ester, ionomycin, endotoxin, and IL-1beta and TNFalpha acutely induced ACE2 release, further supporting that ADAM17 and ADAM10 regulate ACE2 cleavage. Soluble ACE2 was enzymatically active and partially inhibited virus entry into target cells. We determined that the ACE2 cleavage site resides between amino acid 716 and the putative transmembrane domain starting at amino acid 741. To reveal structural determinants underlying ACE2 release, several mutant and chimeric ACE2 proteins were engineered. Neither the juxtamembrane stalk region, transmembrane domain, nor the cytosolic domain was needed for constitutive ACE2 release. Interestingly, a point mutation in the ACE2 ectodomain, L584A, markedly attenuated shedding. The resultant ACE2-L584A mutant trafficked to the cell membrane and facilitated SARS-CoV entry into target cells, suggesting that the ACE2 ectodomain regulates its release and that residue L584 might be part of a putative sheddase "recognition motif." Thus ACE2 must be cell associated to serve as a CoV receptor and soluble ACE2 might play a role in modifying inflammatory processes at the airway mucosal surface.

Characterization of the catalytic activity of the membrane-anchored metalloproteinase ADAM15 in cell-based assays

ADAM15 (a disintegrin and metalloproteinase 15) is a membrane-anchored metalloproteinase, which is overexpressed in several human cancers and has been implicated in pathological neovascularization and prostate cancer metastasis. Yet, little is known about the catalytic properties of ADAM15. Here, we purified soluble recombinant ADAM15 to test for its ability to cleave a library of peptide substrates. However, we found no processing of any of the peptide substrates tested here, and therefore turned to cell-based assays to characterize the catalytic properties of ADAM15. Overexpression of full-length membrane-anchored ADAM15 or the catalytically inactive ADAM15E-->A together with various membrane proteins resulted in increased release of the extracellular domain of the fibroblast growth factor receptor 2iiib (FGFR2iiib) by ADAM15, but not ADAM15E-->A. This provided a robust assay for a characterization of the catalytic properties of ADAM15 in intact cells. We found that increased expression of ADAM15 resulted in increased FGFR2iiib shedding, but that ADAM15 was not stimulated by phorbol esters or calcium ionophores, two commonly used activators of ectodomain shedding. Moreover, ADAM15-dependent processing of FGFR2iiib was inhibited by the hydroxamate-based metalloproteinase inhibitors marimastat, TAPI-2 and GM6001, and by 50 nM TIMP-3 (tissue inhibitor of metalloproteinases 3), but not by 100 nM TIMP-1, and only weakly by 100 nM TIMP-2. These results define key catalytic properties of ADAM15 in cells and its response to stimulators and inhibitors of ectodomain shedding. A cell-based assay for the catalytic activity of ADAM15 could aid in identifying compounds, which could be used to block the function of ADAM15 in pathological neovascularization and cancer.

Role of ADAMs in cancer formation and progression

The ADAMs (a disintegrin and metalloproteinase) comprise a family of multidomain transmembrane and secreted proteins. One of their best-established roles is the release of biologically important ligands, such as tumor necrosis factor-alpha, epidermal growth factor, transforming growth factor-alpha, and amphiregulin. Because these ligands have been implicated in the formation and progression of tumors, it might be expected that the specific ADAMs involved in their release would also be involved in malignancy. Consistent with this hypothesis, emerging data from model systems suggest that ADAMs, such as ADAM-9, ADAM-12, ADAM-15, and ADAM-17, are causally involved in tumor formation/progression. In human cancer, specific ADAMs are up-regulated, with levels generally correlating with parameters of tumor progression and poor outcome. In preclinical models, selective ADAM inhibitors against ADAM-10 and ADAM-17 have been shown to synergize with existing therapies in decreasing tumor growth. The ADAMs are thus a new family of potential targets for the treatment of cancer, especially malignancies that are dependent on human epidermal growth factor receptor ligands or tumor necrosis factor-alpha.

ADAM9 is involved in pathological retinal neovascularization

Pathological ocular neovascularization, caused by diabetic retinopathy, age-related macular degeneration, or retinopathy of prematurity, is a leading cause of blindness, yet much remains to be learned about its underlying causes. Here we used oxygen-induced retinopathy (OIR) and laser-induced choroidal neovascularization (CNV) to assess the contribution of the metalloprotease-disintegrin ADAM9 to ocular neovascularization in mice. Pathological neovascularization in both the OIR and CNV models was significantly reduced in Adam9(-/-) mice compared to wild-type controls. In addition, the level of ADAM9 expression was strongly increased in endothelial cells in pathological vascular tufts in the OIR model. Moreover, tumor growth from heterotopically injected B16F0 melanoma cells was reduced in Adam9(-/-) mice compared to controls. In cell-based assays, the overexpression of ADAM9 enhanced the ectodomain shedding of EphB4, Tie-2, Flk-1, CD40, VCAM, and VE-cadherin, so the enhanced expression of ADAM9 could potentially affect pathological neovascularization by increasing the shedding of these and other membrane proteins from endothelial cells. Finally, we provide the first evidence for the upregulation of ADAM9-dependent shedding by reactive oxygen species, which in turn are known to play a critical role in OIR. Collectively, these results suggest that ADAM9 could be an attractive target for the prevention of proliferative retinopathies, CNV, and cancer.

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.

Cutting edge: the metalloproteinase ADAM17/TNF-alpha-converting enzyme regulates proteolytic shedding of the MHC class I-related chain B protein

MHC class I-related chain (MIC) A/B are transmembrane proteins expressed in pathological conditions that are ligands for the activating receptor NKG2D found on cytotoxic lymphocytes. Soluble NKG2D ligands are detected in sera of patients suffering from multiple types of cancer where they are associated with reduced levels of receptor expression and compromised function of NK and CTLs. In this study, we report the identification of a metalloproteinase involved in the cleavage process of MIC; inhibition and knockdown of ADAM17/TACE blocks the shedding of these proteins. Strikingly, the recruitment of both enzyme and substrate to detergent-resistant membrane microdomains is crucial for efficient proteolysis. These findings provide a novel insight into the molecular mechanisms of MIC shedding.

ADAMs 10 and 17 represent differentially regulated components of a general shedding machinery for membrane proteins such as transforming growth factor alpha, L-selectin, and tumor necrosis factor alpha

Protein ectodomain shedding is a critical regulator of many membrane proteins, including epidermal growth factor receptor-ligands and tumor necrosis factor (TNF)-alpha, providing a strong incentive to define the responsible sheddases. Previous studies identified ADAM17 as principal sheddase for transforming growth factor (TGF)-alpha and heparin-binding epidermal growth factor, but Ca++ influx activated an additional sheddase for these epidermal growth factor receptor ligands in Adam17-/- cells. Here, we show that Ca++ influx and stimulation of the P2X7R signaling pathway activate ADAM10 as sheddase of many ADAM17 substrates in Adam17-/- fibroblasts and primary B cells. Importantly, although ADAM10 can shed all substrates of ADAM17 tested here in Adam17-/- cells, acute treatment of wild-type cells with a highly selective ADAM17 inhibitor (SP26) showed that ADAM17 is nevertheless the principal sheddase when both ADAMs 10 and 17 are present. However, chronic treatment of wild-type cells with SP26 promoted processing of ADAM17 substrates by ADAM10, thus generating conditions such as in Adam17-/- cells. These results have general implications for understanding the substrate selectivity of two major cellular sheddases, ADAMs 10 and 17.

Tetraspanins regulate ADAM10-mediated cleavage of TNF-alpha and epidermal growth factor

Several cytokines and growth factors are released by proteolytic cleavage of a membrane-anchored precursor, through the action of ADAM (a disintegrin and metalloprotease) metalloproteases. The activity of these proteases is regulated through largely unknown mechanisms. In this study we show that Ab engagement of several tetraspanins (CD9, CD81, CD82) increases epidermal growth factor and/or TNF-alpha secretion through a mechanism dependent on ADAM10. The effect of anti-tetraspanin mAb on TNF-alpha release is rapid, not relayed by intercellular signaling, and depends on an intact MEK/Erk1/2 pathway. It is also associated with a concentration of ADAM10 in tetraspanin-containing patches. We also show that a large fraction of ADAM10 associates with several tetraspanins, indicating that ADAM10 is a component of the "tetraspanin web." These data show that tetraspanins regulate the activity of ADAM10 toward several substrates, and illustrate how membrane compartmentalization by tetraspanins can control the function of cell surface proteins such as ectoproteases.

The ADAMs: signalling scissors in the tumour microenvironment

Over the last few years disintegrin metalloproteinases of the Adam (a disintegrin and metalloproteinase) family have been associated with the process of proteolytic 'shedding' of membrane-associated proteins and hence the rapid modulation of key cell signalling pathways in the tumour microenvironment. Furthermore, numerous members of the Adam family have been associated with tumorigenesis and tumour progression. The question now arises of whether pharmacological manipulation of their functions would be a useful adjunct to therapies targeting intercellular communications. To learn from the lessons of matrix metalloproteinase inhibitors as anticancer agents, there are many facets of the biological and clinical relevance of the ADAMs that need to be understood before embarking with confidence on such an approach.

The "a disintegrin and metalloprotease" (ADAM) family of sheddases: physiological and cellular functions

There is an exciting increase of evidence that members of the disintegrin and metalloprotease (ADAM) family critically regulate cell adhesion, migration, development and signalling. ADAMs are involved in "ectodomain shedding" of various cell surface proteins such as growth factors, receptors and their ligands, cytokines, and cell adhesion molecules. The regulation of these proteases is complex and still poorly understood. Studies in ADAM knockout mice revealed their partially redundant roles in angiogenesis, neurogenesis, tissue development and cancer. ADAMs usually trigger the first step in regulated intramembrane proteolysis leading to activation of intracellular signalling pathways and the release of functional soluble ectodomains.

ADAM function in embryogenesis

Cleavage of proteins inserted into the plasma membrane (shedding) is an essential process controlling many biological functions including cell signaling, cell adhesion and migration as well as proliferation and differentiation. ADAM surface metalloproteases have been shown to play an essential role in these processes. Gene inactivation during embryonic development have provided evidence of the central role of ADAM proteins in nematodes, flies, frogs, birds and mammals. The relative contribution of four subfamilies of ADAM proteins to developmental processes is the focus of this review.

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.

VEGF-A stimulates ADAM17-dependent shedding of VEGFR2 and crosstalk between VEGFR2 and ERK signaling

Vascular endothelial growth factor (VEGF)-A and the VEGF receptors are critical for regulating angiogenesis during development and homeostasis and in pathological conditions, such as cancer and proliferative retinopathies. Most effects of VEGF-A are mediated by the VEGFR2 and its coreceptor, neuropilin (NRP)-1. Here, we show that VEGFR2 is shed from cells by the metalloprotease disintegrin ADAM17, whereas NRP-1 is released by ADAM10. VEGF-A enhances VEGFR2 shedding by ADAM17 but not shedding of NRP-1 by ADAM10. VEGF-A activates ADAM17 via the extracellular signal-regulated kinase (ERK) and mitogen-activated protein kinase pathways, thereby also triggering shedding of other ADAM17 substrates, including tumor necrosis factor alpha, transforming growth factor alpha, heparin-binding epidermal growth factor-like growth factor, and Tie-2. Interestingly, an ADAM17-selective inhibitor shortens the duration of VEGF-A-stimulated ERK phosphorylation in human umbilical vein endothelial cells, providing evidence for an ADAM17-dependent crosstalk between the VEGFR2 and ERK signaling. Targeting the sheddases of VEGFR2 or NRP-1 might offer new opportunities to modulate VEGF-A signaling, an already-established target for treatment of pathological neovascularization.

Heparan sulfate regulates ADAM12 through a molecular switch mechanism

The disintegrin and metalloproteases (ADAMs) are emerging as therapeutic targets in human disease, but specific drug design is hampered by potential redundancy. Unlike other metzincins, ADAM prodomains remain bound to the mature enzyme to regulate activity. Here ADAM12, a protease that promotes tumor progression and chondrocyte proliferation in osteoarthritic cartilage, is shown to possess a prodomain/catalytic domain cationic molecular switch, regulated by exogenous heparan sulfate and heparin but also endogenous cell surface proteoglycans and the polyanion, calcium pentosan polysulfate. Sheddase functions of ADAM12 are regulated by the switch, as are proteolytic functions in placental tissue and sera of pregnant women. Moreover, human heparanase, an enzyme also linked to tumorigenesis, can promote ADAM12 sheddase activity at the cell surface through cleavage of the inhibitory heparan sulfate. These data present a novel concept that might allow targeting of ADAM12 and suggest that other ADAMs may have specific regulatory activity embedded in their prodomain and catalytic domain structures.

Tumor-associated MICA is shed by ADAM proteases

The immunoreceptor NKG2D promotes immunosurveillance of malignant cells and protects the host from tumor initiation by activating natural killer cells and costimulating CD8 T cells. NKG2D-mediated recognition of malignant cells by cytotoxic lymphocytes is enabled through the tumor-associated expression of NKG2D ligands (NKG2DL) resulting from cellular or genotoxic stress. Shedding of NKG2DL is thought to constitute a major countermechanism of tumor cells to subvert NKG2D-mediated immunosurveillance. Here, we report that the prototypical NKG2DL MICA is released by proteolytic cleavage in the stalk of the MICA ectodomain, where deletions, but not alanine substitutions, impede MICA shedding. Small compound-mediated stimulation and inhibition of MICA shedding adduced characteristics that indicated an involvement of members of the "a disintegrin and metalloproteinase" (ADAM) family. Accordingly, MICA shedding by tumor cells was inhibited by silencing of the related ADAM10 and ADAM17 proteases, which are known to promote tumor growth by releasing epidermal growth factor receptor ligands. Collectively, our data show that ADAM10 and ADAM17 are critically involved in the tumor-associated proteolytic release of soluble MICA facilitating tumor immune escape. Hence, therapeutic blockade of ADAM10 and ADAM17 seems promising for cancer treatment by targeting both growth and immune escape of tumors.

Ectodomain cleavage of the EGF ligands HB-EGF, neuregulin1-beta, and TGF-alpha is specifically triggered by different stimuli and involves different PKC isoenzymes

Metalloproteinase cleavage of transmembrane proteins (ectodomain cleavage), including the epidermal growth factor (EGF) ligands heparin-binding EGF-like growth factor (HB-EGF), neuregulin (NRG), and transforming growth factor-alpha (TGF-alpha), is important in many cellular signaling pathways and is disregulated in many diseases. It is largely unknown how physiological stimuli of ectodomain cleavage--hypertonic stress, phorbol ester, or activation of G-protein-coupled receptors [e.g., by lysophosphatidic acid (LPA)]--are molecularly connected to metalloproteinase activation. To study this question, we developed a fluorescence-activated cell sorting (FACS)- based assay that measures cleavage of EGF ligands in single living cells. EGF ligands expressed in mouse lung epithelial cells are differentially and specifically cleaved depending on the stimulus. Inhibition of protein kinase C (PKC) isoenzymes or metalloproteinase inhibition by batimastat (BB94) showed that different regulatory signals are used by different stimuli and EGF substrates, suggesting differential effects that act on the substrate, the metalloproteinase, or both. For example, hypertonic stress led to strong cleavage of HB-EGF and NRG but only moderate cleavage of TGF-alpha. HB-EGF, NRG, and TGF-alpha cleavage was not dependent on PKC, and only HB-EGF and NRG cleavage were inhibited by BB94. In contrast, phorbol 12-myristate-13-acetate (TPA) -induced cleavage of HB-EGF, NRG, and TGF-alpha was dependent on PKC and sensitive to BB94 inhibition. LPA led to significant cleavage of only NRG and TGF-alpha and was inhibited by BB94; only LPA-induced NRG cleavage required PKC. Surprisingly, specific inhibition of atypical PKCs zeta and iota [not activated by diacylglycerol (DAG) and calcium] significantly enhanced TPA-induced NRG cleavage. Employed in a high-throughput cloning strategy, our cleavage assay should allow the identification of candidate proteins involved in signal transduction of different extracellular stimuli into ectodomain cleavage.

The ADAM metalloproteinases

The ADAMs (a disintegrin and metalloproteinase) are a fascinating family of transmembrane and secreted proteins with important roles in regulating cell phenotype via their effects on cell adhesion, migration, proteolysis and signalling. Though all ADAMs contain metalloproteinase domains, in humans only 13 of the 21 genes in the family encode functional proteases, indicating that at least for the other eight members, protein–protein interactions are critical aspects of their biological functions. The functional ADAM metalloproteinases are involved in “ectodomain shedding” of diverse growth factors, cytokines, receptors and adhesion molecules. The archetypal activity is shown by ADAM-17 (tumour necrosis factor-α convertase, TACE), which is the principal protease involved in the activation of pro-TNF-α, but whose sheddase functions cover a broad range of cell surface molecules. In particular, ADAM-17 is required for generation of the active forms of Epidermal Growth Factor Receptor (EGFR) ligands, and its function is essential for the development of epithelial tissues. Several other ADAMs have important sheddase functions in particular tissue contexts. Another major family member, ADAM-10, is a principal player in signalling via the Notch and Eph/ephrin pathways. For a growing number of substrates, foremost among them being Notch, cleavage by ADAM sheddases is essential for their subsequent “regulated intramembrane proteolysis” (RIP), which generates cleaved intracellular domains that translocate to the nucleus and regulate gene transcription. Several ADAMs play roles in spermatogenesis and sperm function, potentially by effecting maturation of sperm and their adhesion and migration in the uterus. Other non-catalytic ADAMs function in the CNS via effects on guidance mechanisms. The ADAM family are thus fundamental to many control processes in development and homeostasis, and unsurprisingly they are also linked to pathological states when their functions are dysregulated, including cancer, cardiovascular disease, asthma, Alzheimer’s disease. This review will provide an overview of current knowledge of the human ADAMs, discussing their structure, function, regulation and disease involvement.

ADAM12: a potential target for the treatment of chronic wounds

Wound healing is a complex process involving multiple cellular events, including cell proliferation, migration, and tissue remodeling. A disintegrin and metalloprotease 12 (ADAM12) is a membrane-anchored metalloprotease, which has been implicated in activation-inactivation of growth factors that play an important role in wound healing, including heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) and insulin growth factor (IGF) binding proteins. Here, we report that expression of ADAM12 is fivefold upregulated in the nonhealing edge of chronic ulcers compared to healthy skin, based on microarrays of biopsies taken from five patients and from healthy controls (p = 0.013). The increase in ADAM12 expression in chronic ulcers was confirmed by quantitative real-time polymerase chain reaction (RT-PCR). Moreover, immunohistochemical analysis demonstrated a pronounced increase in the membranous and intracellular signal for ADAM12 in the epidermis of chronic wounds compared to healthy skin. These findings, coupled with our previous observations that lack of keratinocyte migration contributes to the pathogenesis of chronic ulcers, prompted us to evaluate how the absence of ADAM12 affects the migration of mouse keratinocytes. Skin explants from newborn ADAM12-/- or wild-type (WT) mice were used to quantify keratinocyte migration out of the explants over a period of 7 days. We found a statistically significant increase in the migration of ADAM12-/- keratinocytes compared to WT control (p = 0.0014) samples. Taken together, the upregulation of ADAM12 in chronic wounds and the increased migration of keratinocytes in the absence of ADAM12 suggest that ADAM12 is an important mediator of wound healing. We hypothesize that increased expression of ADAM12 in chronic wounds impairs wound healing through the inhibition of keratinocyte migration and that topical ADAM12 inhibitors may therefore prove useful for the treatment of chronic wounds.

ADAM proteases: ligand processing and modulation of the Notch pathway

ADAM metalloproteases play important roles in development and disease. One of the key functions of ADAMs is the proteolytic processing of Notch receptors and their ligands. ADAM-mediated cleavage of Notch represents the first step in regulated intramembrane proteolysis of the receptor, leading to activation of the Notch pathway. Recent reports indicate that the transmembrane Notch ligands also undergo ADAM-mediated processing in cultured cells and in vivo. The proteolytic processing of Notch ligands modulates the strength and duration of Notch signals, leads to generation of soluble intracellular domains of the ligands, and may support a bi-directional signaling between cells.

Expression, purification and insights into structure and folding of the ADAM22 pro domain

The ADAMs (a disintegrin and metalloproteases) are an important class of enzymes in the regulation of human disease. The pro domains of ADAMs are responsible for the latency and secretion of mature enzymes. Unlike other metzincins, ADAM pro domains remain bound to the mature enzyme after secretion. To understand the functions of human ADAM pro domains and to determine three-dimensional structures, we have screened promising targets for expression and purification properties when using Escherichia coli as the host. The pro domain of ADAM22 (ADAM22-P) expressed in E. coli was folded, as determined by CD and NMR spectroscopy. An ADAM22-P fragment encoding residues 26-199 could be expressed in high amounts, remained soluble above 1 mM, and was suitable for structural studies by NMR spectroscopy. CD spectroscopy and predictions suggest that the secondary structure in ADAM22-P consists of beta-strands. Furthermore, our data indicate that the pro domains of ADAMs are expressed as two subdomains. The most N-terminal subdomain (ADAM22-P(N)) was found to be susceptible to proteolysis and was required for folding stability of the second subdomain (ADAM22-P(C)).

Breast cancer-associated mutations in metalloprotease disintegrin ADAM12 interfere with the intracellular trafficking and processing of the protein

ADAM12 has recently emerged as a Candidate Cancer Gene in a comprehensive genetic analysis of human breast cancers. Three somatic mutations in ADAM12 were observed at significant frequencies in breast cancers: D301H, G479E and L792F. The first 2 of these mutations involve highly conserved residues in ADAM12, and our computational sequence analysis confirms that they may be cancer-related. We show that the corresponding mutations in mouse ADAM12 inhibit the proteolytic processing and activation of ADAM12 in NIH3T3, COS-7, CHO-K1 cells and in MCF-7 breast cancer cells. The D/H and G/E ADAM12 mutants exert a dominant-negative effect on the processing of the wild-type ADAM12. Immunofluorescence analysis and cell surface biotinylation experiments demonstrate that the D/H and G/E mutants are retained inside the cell and are not transported to the cell surface. Consequently, the D/H and G/E mutants, unlike the wild-type ADAM12, are not capable of shedding Delta-like l, a ligand for Notch receptor, at the cell surface, or of stimulating cell migration. Our results suggest that the breast cancer-associated mutations interfere with the intracellular trafficking of ADAM12 and result in loss of the functional ADAM12 at the cell surface.

The ADAM17-amphiregulin-EGFR axis in mammary development and cancer

In order to fulfill its function of producing and delivering sufficient milk to newborn mammalian offspring, the mammary gland first has to form an extensive ductal network. As in all phases of mammary development, hormonal cues elicit local intra- and inter-cellular signaling cascades that regulate ductal growth and differentiation. Among other things, ductal development requires the epidermal growth factor receptor (EGFR), its ligand amphiregulin (AREG), and the transmembrane metalloproteinase ADAM17, which can cleave and release AREG from the cell surface so that it may interact with its receptor. Tissue recombination and transplantation studies demonstrate that EGFR phosphorylation and ductal development proceed only when ADAM17 and AREG are expressed on mammary epithelial cells and EGFR is present on stromal cells, and that local administration of soluble AREG can rescue the development of ADAM17-deficient transplants. Thus proper mammary morphogenesis requires the ADAM17-mediated release of AREG from ductal epithelial cells, the subsequent activation of EGFR on stromal cells, and EGFR-dependent stromal responses that in return elicit a new set of epithelial responses, all culminating in the formation of a fully functional ductal tree. This, however, raises new issues concerning what may act upstream, downstream or in parallel with the ADAM17-AREG-EGFR axis, how it may become hijacked or corrupted during the onset and evolution of cancer, and how such ill effects may be confronted.

Proinflammatory role of amphiregulin, an epidermal growth factor family member whose expression is augmented in rheumatoid arthritis patients

BACKGROUND

The epidermal growth factor (EGF) and EGF receptor (EGFR) families play important roles in the hyperplastic growth of several tissues as well as tumor growth. Since synovial hyperplasia in rheumatoid arthritis (RA) resembles a tumor, involvement of the EGF/EGFR families in RA pathology has been implied. Although several reports have suggested that ErbB2 is the most important member of the EGFR family for the synovitis in RA, it remains unclear which members of the EGF family are involved. To clarify the EGF-like growth factors involved in the pathology of RA, we investigated the expression levels of seven major EGF-like growth factors in RA patients compared with those in osteoarthritis (OA) patients and healthy control subjects.

METHODS

The expression levels of seven EGF-like growth factors and four EGFR-like receptors were measured in mononuclear cells isolated from bone marrow and venous blood, as well as in synovial tissues, using quantitative RT-PCR. Further evidence of gene expression was obtained by ELISAs. The proinflammatory roles were assessed by the growth-promoting and cytokine-inducing effects of the corresponding recombinant proteins on cultured fibroblast-like synoviocytes (FLS).

RESULTS

Among the seven EGF-like ligands examined, only amphiregulin (AREG) was expressed at higher levels in all three RA tissues tested compared with the levels in OA tissues. The AREG protein concentration in RA synovial fluid was also higher than that in OA synovial fluid. Furthermore, recombinant human AREG stimulated FLS to proliferate and produce several proinflammatory cytokines, including angiogenic cytokines such as interleukin-8 and vascular endothelial growth factor (VEGF), in a dose-dependent manner. The VEGF mRNA levels in RA synovia and VEGF protein concentrations in RA synovial fluid were significantly higher than those in the corresponding OA samples and highly correlated with the levels of AREG.

CONCLUSION

The present findings suggest that AREG functions to stimulate synovial cells and that elevated levels of AREG may be involved in the pathogenesis of RA.

Drug insight: tumor necrosis factor-converting enzyme as a pharmaceutical target for rheumatoid arthritis

The success of agents that inhibit tumor necrosis factor (TNF), such as infliximab, adalimumab and etanercept, has led to a desire for orally available small molecules that have a better safety profile and are less costly to produce than current agents. One target for anti-TNF therapy that is currently under investigation is TNF-converting enzyme, which promotes the release of soluble TNF from its membrane-bound precursor. Inhibitors of this enzyme with drug-like properties have been made and tested in the clinic. These inhibitors include TMI-005 and BMS-561392, both of which have entered into phase II clinical trials. This article summarizes preclinical and clinical findings regarding the use of inhibitors of TNF-converting enzyme for the treatment of rheumatoid arthritis.

Membrane-anchored growth factors, the epidermal growth factor family: beyond receptor ligands

The epidermal growth factor (EGF) family and the EGF receptor (EGFR, ErbB) tyrosine kinase family have been spearheading the studies of signal transduction events that determine cell fate and behavior in vitro and in vivo. The EGFR family and their signaling pathways are giving us tremendous advantages in developing fascinating molecular target strategies for cancer therapy. Currently, two important types of EGFR inhibitors are in clinical use: neutralizing antibodies of EGFR or ErbB2, and synthetic small compounds of tyrosine kinase inhibitors designed for receptors. On the other hand, basic research of the EGF family ligands presents new challenges as membrane-anchored growth factors. All members of the EGF family have important roles in development and diseases and are shed from the plasma membrane by metalloproteases. The ectodomain shedding of the ligands has emerged as a critical component in the functional transactivation of EGFRs in interreceptor cross-talk in response to various shedding stimulants such as G-protein coupled receptor agonists, growth factors, cytokines, and various physicochemical stresses. Among the EGFR-ligands, heparin-binding EGF-like growth factor (HB-EGF) is a prominent ligand in our understanding of the pathophysiological roles of ectodomain shedding in cancer, wound healing, cardiac diseases, etc. Here we focus on ectodomain shedding of the EGF family ligands, especially HB-EGF by disintegrin and metalloproteases, which are not only key events of receptor cross talk, but also novel intercellular signaling by their carboxy-terminal fragments to regulate gene expression directly.

Cellular roles of ADAM12 in health and disease

ADAM12 belongs to the large family of ADAMs (a disintegrin and metalloproteases) and possesses extracellular metalloprotease and cell-binding functions, as well as intracellular signaling capacities. Interest in ADAM12 has increased recently because its expression is related to tumor progression and it is a potential biomarker for breast cancer. It is therefore important to understand ADAM12's functions. Many cellular roles for ADAM12 have been suggested. It is an active metalloprotease, and has been implicated in insulin-like growth factor (IGF) receptor signaling, through cleavage of IGF-binding proteins, and in epidermal growth factor receptor (EGFR) pathways, via ectodomain shedding of membrane-tethered EGFR ligands. These proteolytic events may regulate diverse cellular responses, such as altered cell differentiation, proliferation, migration, and invasion. ADAM12 may also regulate cell-cell and cell-extracellular matrix contacts through interactions with cell surface receptors - integrins and syndecans - potentially influencing the actin cytoskeleton. Moreover, ADAM12 interacts with several cytoplasmic signaling and adaptor molecules through its intracellular domain, thereby directly transmitting signals to or from the cell interior. These ADAM12-mediated cellular effects appear to be critical events in both biological and pathological processes. This review presents current knowledge on ADAM12 functions gained from in vitro and in vivo observations, describes ADAM12's role in both normal physiology and pathology, particularly in cancer, and discusses important areas for future investigation.

Heparin-binding epidermal growth factor and Src family kinases in proliferation of renal epithelial cells

Our recent studies have shown that proliferation of renal proximal tubular cells (RPTC) in the absence of growth factors requires activation of the epidermal growth factor (EGF) receptor. We sought to identify the endogenous EGF receptor ligand and investigate the mechanism(s) by which RPTC proliferate in different models. RPTC expressed both pro- and cleaved forms of heparin-binding epidermal growth factor (HB-EGF) and several metalloproteinases (MMP-2, -3, -9, and ADAM10, ADAM17) that have been reported to cleave HB-EGF. Treatment of RPTC with CRM 197, an inhibitor of HB-EGF binding to the EGF receptor, or downregulation of HB-EGF with small interfering RNA inhibited RPTC proliferation following plating. Furthermore, GM6001 (pan-MMP inhibitor), tumor-necrosis factor protease inhibitor-1 (TAPI-1; MMP and ADAM17 inhibitor), and GW280264X (ADAM10 and -17 inhibitor), but not GI254023X (ADAM10 inhibitor), attenuated the proliferation after plating. Although EGF receptor activation is required for RPTC proliferation after oxidant injury, CRM197, GM6001, and TAPI-1 did not block this response. In contrast, inhibition of Src with PP1 blocked EGF receptor activation and RPTC proliferation after oxidant injury. In addition, PP1 treatment attenuated HB-EGF-enhanced RPTC proliferation. We suggest that RPTC proliferation after plating is mediated by HB-EGF produced through an autocrine/paracrine mechanism and RPTC proliferation following oxidant injury is mediated by Src without involvement of HB-EGF.

Cell surface colony-stimulating factor 1 can be cleaved by TNF-alpha converting enzyme or endocytosed in a clathrin-dependent manner

CSF-1 is a hemopoietic growth factor, which plays an essential role in macrophage and osteoclast development. Alternative splice variants of CSF-1 are synthesized as soluble or membrane-anchored molecules, although membrane CSF-1 (mCSF-1) can be cleaved from the cell membrane to become soluble CSF-1. The activities involved in this proteolytic processing, also referred to as ectodomain shedding, remain poorly characterized. In the present study, we examined the properties of the mCSF-1 sheddase in cell-based assays. Shedding of mCSF-1 was up-regulated by phorbol ester treatment and was inhibited by the metalloprotease inhibitors GM6001 and tissue inhibitor of metalloproteases 3. Moreover, the stimulated shedding of mCSF-1 was abrogated in fibroblasts lacking the TNF-alpha converting enzyme (TACE, also known as a disintegrin and metalloprotease 17) and was rescued by expression of wild-type TACE in these cells, strongly suggesting that the stimulated shedding is TACE dependent. Additionally, we observed that mCSF-1 is predominantly localized to intracellular membrane compartments and is efficiently internalized in a clathrin-dependent manner. These results indicate that the local availability of mCSF-1 is actively regulated by ectodomain shedding and endocytosis. This mechanism may have important implications for the development and survival of monocyte lineage cells.

Different ADAMs have distinct influences on Kit ligand processing: phorbol-ester-stimulated ectodomain shedding of Kitl1 by ADAM17 is reduced by ADAM19

Kit ligand (Kitl), the ligand for the Kit receptor tyrosine kinase, plays important roles in hematopoiesis, gametogenesis and melanogenesis. Kitl is synthesized as a membrane-anchored precursor that can be processed to produce the soluble growth factor. Here, we evaluated the role of ADAM (a disintegrin and metalloprotease) metalloproteases in ectodomain shedding of Kitl. We found that both ADAM17 and ADAM19 affect Kitl1 shedding, albeit in different ways. Overexpression of ADAM19 resulted in decreased levels of Endo-H-resistant mature Kitl1, thereby reducing the amount of Kitl that is shed from cells following stimulation with phorbol esters. ADAM17 was identified as the major phorbol-ester-stimulated sheddase of Kitl1, whereas ADAMs 8, 9, 10, 12 and 15 were not required for this process. ADAM17 also emerged as the major constitutive and phorbol-ester-stimulated sheddase of Kitl2 in mouse embryonic fibroblasts. Mutagenesis of the juxtamembrane domain of Kitl2 showed no stringent sequence requirement for cleavage by ADAM17, although two nonadjacent stretches of four amino acid residues were identified that are required for Kitl2 shedding. Taken together, this study identifies a novel sheddase, ADAM17, for Kitl1 and Kitl2, and demonstrates that ADAM19 can reduce ADAM17-dependent phorbol-ester-stimulated Kitl1 ectodomain shedding.

The ADAM10 prodomain is a specific inhibitor of ADAM10 proteolytic activity and inhibits cellular shedding events

ADAM10 is a disintegrin metalloproteinase that processes amyloid precursor protein and ErbB ligands and is involved in the shedding of many type I and type II single membrane-spanning proteins. Like tumor necrosis factor-alpha-converting enzyme (TACE or ADAM17), ADAM10 is expressed as a zymogen, and removal of the prodomain results in its activation. Here we report that the recombinant mouse ADAM10 prodomain, purified from Escherichia coli, is a potent competitive inhibitor of the human ADAM10 catalytic/disintegrin domain, with a K(i) of 48 nM. Moreover, the mouse ADAM10 prodomain is a selective inhibitor as it only weakly inhibits other ADAM family proteinases in the micromolar range and does not inhibit members of the matrix metalloproteinase family under similar conditions. Mouse prodomains of TACE and ADAM8 do not inhibit their respective enzymes, indicating that ADAM10 inhibition by its prodomain is unique. In cell-based assays we show that the ADAM10 prodomain inhibits betacellulin shedding, demonstrating that it could be of potential use as a therapeutic agent to treat cancer.

Emerging roles of ADAM and ADAMTS metalloproteinases in cancer

A disintegrin and metalloproteinases (ADAMs) are a recently discovered family of proteins that share the metalloproteinase domain with matrix metalloproteinases (MMPs). Among this family, structural features distinguish the membrane-anchored ADAMs and the secreted ADAMs with thrombospondin motifs referred to as ADAMTSs. By acting on a large panel of membrane-associated and extracellular substrates, they control several cell functions such as adhesion, fusion, migration and proliferation. The current review addresses the contribution of these proteinases in the positive and negative regulation of cancer progression as mainly mediated by the regulation of growth factor activities and integrin functions.

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.