Membrane protein secretases

Multiple factors determine the selection of the ectodomain cleavage site of human cell surface macrophage colony-stimulating factor

Human cell surface macrophage colony-stimulating factor (CSF-1256, M-CSF alpha) is converted to a soluble growth factor by a regulated proteolytic cleavage process at amino acid residues 157-159. We have previously shown that multiple factors specified by the juxtamembrane region determine the cleavage efficiency [Deng, P., Rettenmier, C. W., and Pattengale, P. K. (1996) J. Biol. Chem. 271, 16338-16343]. In the present paper, we studied the effect of various deletion, insertion, and substitution mutations at or near the cleavage site on both the number and size of cleaved CSF-1(256) products to identify the mechanisms by which the cleavage sites are selected. Deletion of regions 161-162 or 163-165, C-terminal to the cleavage site, as well as deletion of region 150-156, N-terminal to the cleavage site, each yielded a single cleavage product that was smaller than that derived from the wild type (WT). In these experiments cleavage apparently occurred at a specific distance from the transmembrane domain. Insertion of three additional residues between the normal cleavage site and the transmembrane domain yielded one major product that was the same size as the processed form of WT CSF-1(256). In this case the selection of the cleavage site was apparently determined by the amino acid sequence of the juxtamembrane region rather than by the distance from the transmembrane domain. Other amino acid substitutions at the cleavage site caused changes in cleavage site selection, providing additional evidence for the role of amino acid sequence in cleavage site selection. Finally, a comparison of cleavage site selection in the presence and absence of tunicamycin treatment showed that N-glycosylation of certain mutant forms of CSF-1(256) sterically interfered with protease accessibility, which in turn had an effect on the selection of the site used for cleavage. Taken together, these results indicate that cleavage site selection is determined by the amino acid sequence of the juxtamembrane region, the distance of the site from the transmembrane domain, and steric accessibility of the protease.

Neural crest-specific and general expression of distinct metalloprotease-disintegrins in early Xenopus laevis development

Metalloprotease-disintegrins are a family of membrane-anchored glycoproteins that have been implicated in diverse cellular processes, including fertilization and myoblast fusion, release of TNFalpha from the plasma membrane, and neurogenesis. Here we report the cloning of cDNAs encoding three full-length (xMDC9, xMDC11b, and xMDC13), and one partial (xMDC11a) metalloprotease-disintegrin from the amphibian Xenopus laevis, and the analysis of their expression during early X. laevis development and in adult tissues. The most notable finding was the highly localized and specific expression pattern of xmdc11a at the tailbud stage in the cranial neural crest and in a subset of neural tube cells in the trunk region. In contrast, expression of the closely related xmdc11b was not detectable during the early stages of X. laevis development, and remained low in the adult tissues examined here. Distinct expression patterns were also observed for two other highly related X. laevis genes, xmdc13 and adam13 (Alfandari et al., 1997). While adam13 is expressed in the somitic mesoderm and in neural crest cells, but not in adult testis, xmdc13 expression is low and ubiquitous in the developing embryo, but is clearly present in adult testis. Finally, xmdc9, the putative orthologue of human and mouse mdc9, was found at all stages of development, and in all tissues examined, suggesting a function that may be utilized by most or all cells. The noteworthy features of these four xmdc genes and the implications of their distinct spatial and temporal expression patterns are discussed.

An aggrecan-degrading activity associated with chondrocyte membranes

The breakdown of aggrecan in cartilage is, in part, mediated by an enzyme named aggrecanase that cleaves within the interglobular domain of the molecule between a glutamic residue and an alanine residue. Although the enzyme cleavage site has been identified, the identity, characteristics and localization of this enzyme remain unclear. We have demonstrated that membranes isolated from stimulated chondrocytes are able to generate aggrecan fragments that are labelled by an antibody that recognizes the new N-terminus formed by aggrecanase activity. It was further shown that the membrane activity was a metalloproteinase but was not inhibited by the naturally occurring matrix metalloproteinase (MMP) inhibitors, TIMPs 1 and 2. These results show that an aggrecanase activity is associated with the membranes of the chondrocytes and is a metalloproteinase, but might not be a member of the MMP family.

An essential role for ectodomain shedding in mammalian development

The ectodomains of numerous proteins are released from cells by proteolysis to yield soluble intercellular regulators. The responsible protease, tumor necrosis factor-alpha converting enzyme (TACE), has been identified only in the case when tumor necrosis factor-alpha (TNFalpha) is released. Analyses of cells lacking this metalloproteinase-disintegrin revealed an expanded role for TACE in the processing of other cell surface proteins, including a TNF receptor, the L-selectin adhesion molecule, and transforming growth factor-alpha (TGFalpha). The phenotype of mice lacking TACE suggests an essential role for soluble TGFalpha in normal development and emphasizes the importance of protein ectodomain shedding in vivo.

Phorbol ester-induced juxtamembrane cleavage of angiotensin-converting enzyme is not inhibited by a stalk containing intrachain disulfides

Specialized proteases, referred to as sheddases, secretases, or membrane-protein-solubilizing proteases (MPSPs), solubilize the extracellular domains of diverse membrane proteins by catalyzing a specific cleavage in the juxtamembrane stalk regions of such proteins. A representative MPSP (tumor necrosis factor-alpha convertase) was cloned recently and shown to be a disintegrin metalloprotease that is inhibited by peptide hydroxamates including the compound TAPI. Substrate determinants that specify cleavage by MPSPs remain incompletely characterized, but may include the physicochemical properties of the stalk or unidentified recognition motifs in the stalk or the extracellular domain. We constructed a mutant angiotensin-converting enzyme (ACE) in which the stalk has been replaced with an epidermal growth factor (EGF)-like domain (ACE-JMEGF), to test the hypothesis that MPSP cleavage requires an open, comparatively unfolded or extended stalk. Wild-type ACE is a type I transmembrane (TM) ectoprotein that is efficiently solubilized by a typical MPSP activity. We found that ACE-JMEGF was solubilized inefficiently and accumulated in a cell-associated form on transfected Chinese hamster ovary (CHO) cells; cleavage was stimulated by phorbol ester and inhibited by TAPI, features typical of MPSP activity. Determination of the C-terminus of soluble ACE-JMEGF revealed that, surprisingly, cleavage occurred at a Gly-Phe bond between the fifth and sixth cysteines within the third disulfide loop of the EGF-like domain. Reduction of intact CHO cells with tributylphosphine resulted in the rapid release of ACE-JMEGF (but not wild-type ACE) into the medium, suggesting that a proportion of membrane-bound ACE-JMEGF is cleaved but remains cell-associated via disulfide tethering. The mechanism for the release of ACE-JMEGF in the absence of chemical reduction is unclear. We conclude that the presence of a compact, disulfide-bridged domain does not per se inhibit cleavage by an MPSP activity, but ectodomain release is prevented by disulfide tethering to the TM domain.

Processing of the Alzheimer's disease amyloid precursor protein in Pichia pastoris: immunodetection of alpha-, beta-, and gamma-secretase products

betaA4 (Abeta) amyloid peptide, a major component of Alzheimer's disease (AD) plaques, is a proteolytic product of the amyloid precursor protein (APP). Endoproteases, termed beta- and gamma-secretase, release respectively the N- and C-termini of the peptide. APP default secretion involves cleavage within the betaA4 domain by alpha-secretase. To study the conservation of APP processing in lower eukaryotes, the yeast Pichia pastoris was transfected with human APP695 cDNA. In addition to the full-length integral transmembrane protein found in the cell lysate, soluble/secreted APP (sAPP) was detected in the culture medium. Most sAPP comprised the N-terminal moiety of betaA4 and corresponds to sAPPalpha, the product of alpha-secretase. The culture medium also contained minor secreted forms detected by a monoclonal antibody specific for sAPPbeta (the ectodomain released by beta-secretase cleavage). Analysis of the cell lysates with specific antibodies also detected membrane-associated C-terminal fragments corresponding to the products of alpha and beta cleavages. Moreover, immunoprecipitation of the culture medium with three antibodies directed at distinct epitopes of the betaA4 domain yielded a 4 kDa product with the same electrophoretic mobility as betaA4 synthetic peptide. These results suggest that the alpha-, beta-, and gamma-secretase cleavages are conserved in yeast and that P. pastoris may offer an alternative to mammalian cells to identify the proteases involved in the generation of AD betaA4 amyloid.

Cloning and expression of a novel, tissue specifically expressed member of the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase family

We report the cloning and expression of the fifth member of the mammalian UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGaNTase) family. Degenerate polymerase chain reaction amplification and hybridization screening of a rat sublingual gland (RSLG) cDNA library were used to identify a novel isoform termed ppGaNTase-T5. Conceptual translation of the cDNA reveals a uniquely long stem region not observed for other members of this enzyme family. Recombinant proteins expressed transiently in COS7 cells displayed transferase activity in vitro. Relative activity and substrate preferences of ppGaNTase-T5 were compared with previously identified isoforms (ppGaNTase-T1, -T3, and -T4); ppGaNTase-T5 and -T4 glycosylated a restricted subset of peptides whereas ppGaNTase-T1 and -T3 glycosylated a broader range of substrates. Northern blot analysis revealed that ppGaNTase-T5 is expressed in a highly tissue-specific manner; abundant expression was seen in the RSLG, with lesser amounts of message in the stomach, small intestine, and colon. Therefore, the pattern of expression of ppGaNTase-T5 is the most restricted of all isoforms examined thus far. The identification of this novel isoform underscores the diversity and complexity of the family of genes controlling O-linked glycosylation.

Two forms of collagen XVII in keratinocytes. A full-length transmembrane protein and a soluble ectodomain

The cDNA sequence of human collagen XVII predicts an unusual type II transmembrane protein, but a biochemical characterization of this structure has not been accomplished yet. Using domain-specific antibodies against recombinant collagen XVII fragments, we identified two molecular forms of the collagen in human skin and epithelial cells. Full-length collagen XVII appeared as a homotrimeric transmembrane molecule of three 180-kDa alpha1(XVII) chains. The globular intracellular domain was disulfide-linked, and the N-glycosylated extracellular domain of three 120-kDa polypeptides was triple-helical at physiological temperatures. A second, soluble form of collagen XVII in keratinocyte culture media was recognized with antibodies to the ectodomain, but not the endodomain. The soluble form exhibited molecular properties of the collagen XVII ectodomain: a triple-helical, N-glycosylated molecule of three 120-kDa polypeptides. Northern blot analysis with probes spanning either the distal 5'or the distal 3' end of the collagen XVII cDNA revealed an identical 6-kb mRNA, suggesting that both the 180- and 120-kDa polypeptides were translated from the same mRNA, and that the 120-kDa polypeptide was generated post-translationally. In concert, keratinocytes harboring a homozygous nonsense mutation in the COL17A1 gene synthesized neither the 180-kDa alpha1(XVII) chain nor the 120-kDa polypeptide. Finally, treatment of normal keratinocytes with a synthetic inhibitor of furin proprotein convertases, decanoyl-RVKR-chloromethyl ketone, prevented the generation of the 120-kDa polypeptide. These data strongly suggest that the soluble 120-kDa polypeptide represents a specifically cleaved ectodomain of collagen XVII, generated through furin-mediated proteolytic processing. Thus, collagen XVII is not only an unusual type II transmembrane collagen, but the first collagen with a specifically processed, soluble triple-helical ectodomain.

ADAMs: focus on the protease domain

ADAMs are proteins containing a disintegrin and metalloproteinase domain. Several important insights were provided in the past year regarding ADAM metalloproteinases. ADAM 10 was implicated in the Notch signaling pathway. ADAM 17 was shown to be the long sought after tumor necrosis factor-alpha convertase and the crystal structure of its metalloproteinase domain was determined.

Pro-tumor necrosis factor-alpha processing activity is tightly controlled by a component that does not affect notch processing

The extracellular domain of a heterogeneous group of transmembrane proteins can be proteolytically released from the cell surface, a process known as protein ectodomain shedding. Despite the biomedical importance of several substrates of the shedding system, such as the beta-amyloid precursor protein (betaAPP), little is known about the regulation of protein ectodomain shedding, and the only protease known to be involved is the metalloprotease disintegrin, tumor necrosis factor-alpha converting enzyme (TACE). Here, we show that previously described pro-transforming growth factor-alpha shedding-defective cell mutants (M2 cells), known to be defective in ectodomain shedding of several molecules, that include betaAPP, fail to shed the ectodomain of pro-TNF-alpha. The target of the mutation is a component required for TACE activity, since transfection of TACE into M2 cells has no effect on the shedding of pro-TNF-alpha and somatic cell fusions between M2 cells and TACE null cells recover the ability to shed pro-TNF-alpha, pro-transforming growth factor-alpha, and betaAPP. Furthermore, we show that TACE is also necessary for the shedding of betaAPP since TACE null cells show defective betaAPP shedding. Biochemical evidence shows that the component that controls TACE is different from protein kinase C, the only known activator of protein ectodomain shedding, and that this component does not affect biosynthesis or processing of TACE or other metalloprotease disintegrins. The component mutated in M2 cells is likely to control only a subset of metalloprotease disintegrins involved in regulated ectodomain shedding, since Notch processing, a process known to be dependent on the activity of another metalloprotease disintegrin, Kuzbanian, is normal in M2 cells.

TNF-alpha converting enzyme (TACE) is inhibited by TIMP-3

TNF-alpha converting enzyme (TACE; ADAM-17) is a membrane-bound disintegrin metalloproteinase that processes the membrane-associated cytokine proTNF-alpha to a soluble form. Because of its putative involvement in inflammatory diseases, TACE represents a significant target for the design of specific synthetic inhibitors as therapeutic agents. In order to study its inhibition by tissue inhibitors of metalloproteinases (TIMPs) and synthetic inhibitors of metalloproteinases, the catalytic domain of mouse TACE (rTACE) was overexpressed as a soluble Ig fusion protein from NS0 cells. rTACE was found to be well inhibited by peptide hydroxamate inhibitors as well as by TIMP-3 but not by TIMP-1, -2 and -4. These results suggest that TIMP-3, unlike the other TIMPs, may be important in the modulation of pathological events in which TNF-alpha secretion is involved.

Tyrosine phosphorylation and proteolysis. Pervanadate-induced, metalloprotease-dependent cleavage of the ErbB-4 receptor and amphiregulin

Enhancement of tyrosine phosphorylation in cells by the application of pervanadate, an extremely potent phosphotyrosine phosphatase inhibitor, provokes the rapid metalloprotease-dependent cleavage of ErbB-4, a transmembrane receptor tyrosine kinase. The pervanadate-induced proteolysis occurs in NIH 3T3 cells expressing transfected human ErbB-4 and in several cell lines that express endogenous ErbB-4. One product of this proteolytic event is a membrane-anchored molecule of approximately 80 kDa, which is heavily tyrosine phosphorylated and which possesses tyrosine kinase catalytic activity toward an exogenous substrate in vitro. This response to pervanadate is not dependent on protein kinase C activation, which has previously been demonstrated to also activate ErbB-4 cleavage. Hence, the pervanadate and 12-O-tetradecanoylphorbol-13-acetate-induced proteolytic cleavage of ErbB-4 seem to proceed by different mechanisms, although both require metalloprotease activity. Moreover, pervanadate activation of ErbB-4 cleavage, but not that of 12-O-tetradecanoylphorbol-13-acetate , is blocked by the oxygen radical scavenger pyrrolidine dithiocarbomate. A second phosphotyrosine phosphatase inhibitor, phenylarsine oxide, also stimulates a similar cleavage of ErbB-4 but, unlike pervanadate, is not sensitive to pyrrolidine dithiocarbomate. Last, pervanadate is shown to stimulate the proteolytic cell surface processing of a second and unrelated transmembrane molecule: the precursor for amphiregulin, an epidermal growth factor-related molecule. Amphiregulin cleavage by pervanadate occurred in the absence of a cytoplasmic domain and tyrosine phosphorylation of this substrate.

CD23 (FcepsilonRII) release from cell membranes is mediated by a membrane-bound metalloprotease

CD23 (low-affinity IgE receptor, FcepsilonRII) is expressed as a Type II extracellular protein on a variety of cells such as B-cells, monocytes and macrophages and is cleaved from the cell surface to generate several distinct fragments. The expression of CD23 on the cell surface as well as the generation of soluble fragments of CD23 has been shown to be involved in the regulation of IgE synthesis. Here we report that the release of CD23 from the cell surface is mediated by a metalloprotease. An assay utilizing purified CD23 and an neo-epitope antibody specific for one of the known cleavage products is described and used to demonstrate unambiguously the cleavage of CD23 by a distinct protease. Characterization of the mechanism of CD23 processing shows that the protease exists as an integral membrane protein with a functional molecular mass of approx. 63 kDa as determined by gel-filtration chromatography. The CD23-cleaving activity found in enriched plasma membranes from RPMI 8866 cells is inhibited by the metalloprotease inhibitors 1, 10-phenanthroline and imidazole and by the matrix metalloprotease inhibitor batimastat, but not by inhibitors of cysteine proteases, serine proteases or acid proteases. The same or a similar activity that cleaves CD23 to the known 33 kDa fragment and is inhibited by batimastat is present in diverse cell types such as unstimulated fibroblasts and monocytic cell lines not expressing CD23, as well as in the Epstein-Barr virus-transformed B-cell line, RPMI 8866, which constitutively expresses CD23.

Proteome and proteomics: new technologies, new concepts, and new words

The goal of proteomics is a comprehensive, quantitative description of protein expression and its changes under the influence of biological perturbations such as disease or drug treatment. Quantitative analysis of protein expression data obtained by high-throughput methods has led us to define the concept of "regulatory homology" and use it to begin to elucidate the basic structure of gene expression control in vivo. Such investigations lay the groundwork for construction of comprehensive databases of mechanisms (cataloguing possible biological outcomes), the next logical step after the soon to be completed cataloguing of genes and gene products. Mechanism databases provide a roadmap towards effective therapeutic intervention that is more direct than that offered by conventional genomics approaches.

Glycoprotein-inspired materials promote the proteolytic release of cell surface L-selectin

The proteolytic release, or shedding, of a cell surface protein can serve a regulatory role; the process liberates a soluble form of the protein into circulation while downregulating its cell surface concentration. The characteristics that render a protein susceptible to proteolytic cleavage are not known. We hypothesized that the clustering of a protein at the cell surface might target it for proteolysis. To test this hypothesis, we synthesized molecules that display multiple copies of sulfated galactose residues, termed neoglycopolymers, that are designed to mimic natural ligands for the cell adhesion protein L-selectin. We found that treatment of human neutrophils with the neoglycopolymers resulted in a dose-dependent loss of L-selectin from the cell surface, while monovalent compounds and unsulfated neoglycopolymers had no effect. Because L-selectin is an important mediator in the inflammatory response, such compounds could lead to novel antiinflammatory drugs. Moreover, molecules that control receptor shedding can be used to alter cellular responsiveness to specific ligands or to promote responses at distal sites; consequently, these results have broad implications for regulating the location and presentation of important biomolecules.

Surface coats and secretory trafficking in African trypanosomes

Recent advances in transfection technology have been exploited to address fundamental questions relating to secretory trafficking in African trypanosomes. Targeted gene disruptions and ectopic expression of the major stage-specific surface proteins have provided unexpected insights into both the function and assembly of the essential parasite surface coats. A growing list of novel secretory cargo molecules, as well as advances in the characterization of trypanosomal secretory machinery, provide a unique model system for the study of eukaryotic secretory processes.

The secretases that cleave angiotensin converting enzyme and the amyloid precursor protein are distinct from tumour necrosis factor-alpha convertase

Angiotensin converting enzyme (ACE) and the Alzheimer's amyloid precursor protein are cleaved from the membrane by zinc metalloproteinases termed ACE secretase and alpha-secretase, respectively. Tumour necrosis factor-alpha (TNF-alpha) convertase (ADAM 17) is a recently identified member of the adamalysin family of mammalian zinc metalloproteinases that is involved in the production of TNF-alpha and possibly in the cleavage of other membrane proteins. Using two different cell-free assays we were unable to detect significant cleavage and secretion of ACE by TNF-alpha convertase. In addition, there was a different effect of three hydroxamic acid-based inhibitors (batimastat, compound 1 and compound 4) towards TNF-alpha convertase as compared to ACE secretase and alpha-secretase. Thus TNF-alpha convertase would appear to be distinct from, but possibly related to, the secretases that cleave ACE and the amyloid precursor protein.

Amyloid precursor protein processing in sterol regulatory element-binding protein site 2 protease-deficient Chinese hamster ovary cells

Amyloid peptides of 39-43 amino acids (Abeta) are the major constituents of amyloid plaques present in the brains of Alzheimer's (AD) patients. Proteolytic processing of the amyloid precursor protein (APP) by the yet unidentified beta- and gamma-secretases leads to the generation of the amyloidogenic Abeta peptides. Recent data suggest that all of the known mutations leading to early onset familial AD alter the processing of APP such that increased amounts of the 42-amino acid form of Abeta are generated by a gamma-secretase activity. Identification of the beta- and/or gamma-secretases is a major goal of current AD research, as they are prime targets for therapeutic intervention in AD. It has been suggested that the sterol regulatory element-binding protein site 2 protease (S2P) may be identical to the long sought gamma-secretase. We have directly tested this hypothesis using over-expression of the S2P cDNA in cells expressing APP and by characterizing APP processing in mutant Chinese hamster ovary cells that are deficient in S2P activity and expression. The data demonstrate that S2P does not play an essential role in the generation or secretion of Abeta peptides from cells, thus it is unlikely to be a gamma-secretase.

Human myeloma cells shed the interleukin-6 receptor: inhibition by tissue inhibitor of metalloproteinase-3 and a hydroxamate-based metalloproteinase inhibitor

Interleukin-6 (IL-6) is the major growth factor for human myeloma cells, exerting its effect through the IL-6 receptor (IL-6R). A soluble form of IL-6R (sIL-6R) has been identified, which increases the sensitivity of myeloma cells to IL-6. In patients with multiple myeloma (MM), serum concentrations of sIL-6R are elevated and associated with poor prognosis. The present study was undertaken to determine whether proteolytic cleavage of IL-6R could contribute to sIL-6R release from human myeloma cells, and also to identify the class of proteinase responsible for this event. Human myeloma cell lines were shown to express IL-6R upon their surface and also to release sIL-6R into culture supernatants. In addition, phorbol 12-myristate 13-acetate (PMA) stimulated a loss of IL-6R from the cell surface, with a corresponding increase in the concentration of sIL-6R in the supernatant. Inhibitors of serine and cysteine proteinases, and tissue inhibitor of metalloproteinase (TIMP) -1 and TIMP-2, were shown to have no effect on the magnitude of sIL-6R release. In contrast, TIMP-3 and a hydroxamate-based metalloproteinase inhibitor (BB-94), inhibited both constitutive and PMA-induced release of sIL-6R. Myeloma cells freshly isolated from the bone marrow of a patient with MM were also shown to express IL-6R upon their surface, and to shed this receptor in response to PMA. These data demonstrate that increased proteolytic cleavage of IL-6R, mediated by a non-matrix-type metalloproteinase, is likely to contribute to the elevated concentrations of sIL-6R found in the serum of patients with MM. Inhibition of sIL-6R release by hydroxamate-based metalloproteinase inhibitors may represent a novel therapeutic approach to the treatment of MM.

Cleavage of beta-catenin and plakoglobin and shedding of VE-cadherin during endothelial apoptosis: evidence for a role for caspases and metalloproteinases

Growth factor deprivation of endothelial cells induces apoptosis, which is characterized by membrane blebbing, cell rounding, and subsequent loss of cell-matrix and cell-cell contacts. In this study, we show that initiation of endothelial apoptosis correlates with cleavage and disassembly of intracellular and extracellular components of adherens junctions. beta-Catenin and plakoglobin, which form intracellular links between vascular endothelial cadherin (VE-cadherin) and actin-binding alpha-catenin in adherens junctions, are cleaved in apoptotic cells. In vitro incubations of cell lysates and immunoprecipitates with recombinant caspases indicate that CPP32 and Mch2 are involved, possibly by initiating proteolytic processing. Cleaved beta-catenin from lysates of apoptotic cells does not bind to endogenous alpha-catenin, whereas plakoglobin retains its binding capacity. The extracellular portion of the adherens junctions is also altered during apoptosis because VE-cadherin, which mediates endothelial cell-cell interactions, dramatically decreases on the surface of cells. An extracellular fragment of VE-cadherin can be detected in the conditioned medium, and this "shedding" of VE-cadherin can be blocked by an inhibitor of metalloproteinases. Thus, cleavage of beta-catenin and plakoglobin and shedding of VE-cadherin may act in concert to disrupt structural and signaling properties of adherens junctions and may actively interrupt extracellular signals required for endothelial cell survival.

Down-Regulation of CXCR2 Expression on Human Polymorphonuclear Leukocytes by TNF-α

TNF-α is implicated in the initiation of cytokine cascades in various inflammatory settings. To assess the interactions of multiple cytokines at the level of inflammatory effector cells, we examined the effects of TNF-α on the expression of two IL-8Rs (CXCR1 and CXCR2) on polymorphonuclear leukocytes (PMNs). TNF-α decreased the surface expression of CXCR2 in a dose- and time-dependent manner. In contrast, CXCR1 expression was not affected by TNF-α. The release of CXCR2 into the supernatant of TNF-α-treated PMNs was detected by immunoblotting and immuno-slot-blot analyses, suggesting that the down-regulation of CXCR2 was caused mainly by shedding from the cell surface. The CXCR2 down-regulation was inhibited by PMSF and aprotinin, supporting the hypothesis that the shedding was mediated by serine protease(s). The intracellular Ca2+ mobilization and chemotaxis in response to IL-8 were suppressed by the pretreatment of PMNs with TNF-α, indicating that the decrease in CXCR2 was reflected in the decreased functional responses to IL-8. In contrast, the O2− release, which is mediated by CXCR1, was not suppressed by TNF-α. The treatment of whole blood with TNF-α also caused a significant reduction in CXCR2 and markedly suppressed intracellular Ca2+ mobilization and chemotaxis in response to IL-8, while enhancing the O2− release. These findings suggest that TNF-α down-regulates CXCR2 expression on PMNs and modulates IL-8-induced biologic responses, leading to the intravascular retention of PMNs with an enhanced production of reactive oxygen metabolites.

The metallo-disintegrin ADAM10 (MADM) from bovine kidney has type IV collagenase activity in vitro

The metallo-disintegrins (ADAMs) are a family of mammalian proteins with significant amino acid sequence identity and a domain organisation similar to the snake venom metalloproteinases (reprolysins). They have been implicated in a wide variety of processes such as cell-cell and cell matrix adhesion and proteolysis of the extracellular matrix in a wide variety of cell types. They may also be involved in events such as the processing of plasma membrane proteins, proteolysis in the secretory pathway and pro-cytokine conversion processes. Due to the close relationship of the ADAM proteins with snake venom enzymes which have been demonstrated to be type IV collagenases, we investigated whether purified bovine ADAM10 could cleave basement membrane type IV collagen. We show here that ADAM10 purified from bovine kidney can cleave a basement membrane collagen type IV preparation as assessed by SDS-PAGE analysis and novel epitope recognition with a specific antibody to type IV collagen. The demonstration that a metallo-disintegrin displays a type IV collagenase activity may be relevant to tumour metastasis and may have general relevance to extracellular re-modeling in renal pathology and a variety of other pathological states where compromise of the basement membrane is involved.

Shedding of CD44 from PMA-differentiated U-937 cells is enhanced by treatment with mineral particles

In this report, we show that enhanced shedding of CD44 might contribute to the down-regulation of this receptor observed after phagocytosis of MnO2 particles by PMA-differentiated U-937. The apparent Mr of the soluble CD44 detected in culture supernatants was slightly lower than that of the membrane form suggesting that shedding resulted from proteolytic cleavage. Increased shedding of CD44 was also noted with other mineral particles (chrysotile and DQ12) but to a lower extent whereas some (TiO2 and amosite) had no effect on this process. These results indicate that shedding enhancement was particle-specific rather than a general consequence of phagocytosis. The ability of the particles to enhance CD44 shedding was not directly dependent on their cytotoxic potency. Different patterns of reactivity were noted with CD11b, suggesting that the underlying mechanisms are specific.

Crystal structure of the catalytic domain of human tumor necrosis factor-alpha-converting enzyme

Tumor necrosis factor-alpha (TNFalpha) is a cytokine that induces protective inflammatory reactions and kills tumor cells but also causes severe damage when produced in excess, as in rheumatoid arthritis and septic shock. Soluble TNFalpha is released from its membrane-bound precursor by a membrane-anchored proteinase, recently identified as a multidomain metalloproteinase called TNFalpha-converting enzyme or TACE. We have cocrystallized the catalytic domain of TACE with a hydroxamic acid inhibitor and have solved its 2.0 A crystal structure. This structure reveals a polypeptide fold and a catalytic zinc environment resembling that of the snake venom metalloproteinases, identifying TACE as a member of the adamalysin/ADAM family. However, a number of large insertion loops generate unique surface features. The pro-TNFalpha cleavage site fits to the active site of TACE but seems also to be determined by its position relative to the base of the compact trimeric TNFalpha cone. The active-site cleft of TACE shares properties with the matrix metalloproteinases but exhibits unique features such as a deep S3' pocket merging with the S1' specificity pocket below the surface. The structure thus opens a different approach toward the design of specific synthetic TACE inhibitors, which could act as effective therapeutic agents in vivo to modulate TNFalpha-induced pathophysiological effects, and might also help to control related shedding processes.

Transforming the cell surface through proteolysis

Protein shedding, or the proteolytic cleavage of a protein from the surface of a cell, is emerging as an important mechanism in the regulation of cellular activity but it is poorly understood. Growing evidence suggesting that protein shedding and protein function are closely linked may lead to new strategies for the treatment of a wide range of diseases.

Alzheimer's amyloid precursor protein alpha-secretase is inhibited by hydroxamic acid-based zinc metalloprotease inhibitors: similarities to the angiotensin converting enzyme secretase

The 4 kDa beta-amyloid peptide that forms the amyloid fibrils in the brain parenchyma of Alzheimer's disease patients is derived from the larger integral membrane protein, the amyloid precursor protein. In the nonamyloidogenic pathway, alpha-secretase cleaves the amyloid precursor protein within the beta-amyloid domain, releasing an extracellular portion and thereby preventing deposition of the intact amyloidogenic peptide. The release of the amyloid precursor protein from both SH-SY5Y and IMR-32 neuronal cells by alpha-secretase was blocked by batimastat and other related synthetic hydroxamic acid-based zinc metalloprotease inhibitors, but not by the structurally unrelated zinc metalloprotease inhibitors enalaprilat and phosphoramidon. Batimastat inhibited the release of the amyloid precursor protein from both cell lines with an I50 value of 3 microM. Removal of the thienothiomethyl substituent adjacent to the hydroxamic acid moiety or the substitution of the P2' substituent decreased the inhibitory potency of batimastat toward alpha-secretase. In the SH-SY5Y cells, both the basal and the carbachol-stimulated release of the amyloid precursor protein were blocked by batimastat. In contrast, neither the level of full-length amyloid precursor protein nor its cleavage by beta-secretase were inhibited by any of the zinc metalloprotease inhibitors examined. In transfected IMR-32 cells, the release of both the amyloid precursor protein and angiotensin converting enzyme was inhibited by batimastat, marimastat, and BB2116 with I50 values in the low micromolar range, while batimastat and BB2116 inhibited the release of both proteins from HUVECs. The profile of inhibition of alpha-secretase by batimastat and structurally related compounds is identical with that observed with the angiotensin converting enzyme secretase suggesting that the two are closely related zinc metalloproteases.

Purification and spectroscopic characterization of beta-amyloid precursor protein from porcine brains

Soluble and membrane-bound isoforms of beta-amyloid protein precursor (APP) of Alzheimer's disease were extracted and purified from porcine brains. At least three types of soluble APP and membrane-bound APP with different molecular masses, ranging from 86 kDa to 116 kDa, were obtained. CD and infrared spectroscopies were used to determine the overall secondary-structure content of APP. The infrared spectra of soluble and membrane-bound APP (in dry and hydrated states) were similar in the amide-I and amide-II regions, suggesting that the overall secondary structures of the soluble and membrane isoforms were roughly identical. The amide-I band is composed of at least five component bands, located at 1694, 1674, 1652, 1637 and 1618 cm(-1) for soluble APP, and located at 1687, 1674, 1651, 1637 and 1614-1606 cm(-1) for membrane-bound APP, as evidenced by their respective second-derivative infrared spectra. The 1651-1652-cm(-1) band was associated with alpha-helix structures, while two types of beta-sheet structures are evidenced by two characteristic pairs of component bands. The 1674-cm(-1) and 1637-cm(-1) bands for soluble APP and membrane-bound APP were tentatively associated to beta-sheet structures. The second pair of bands, located at 1694 cm(-1) and at 1618 cm(-1) for soluble APP and at 1687 cm(-1) and 1614-1606 cm(-1) for membrane-bound APP, were associated with intermolecular beta-sheet structures or aggregated strands, as confirmed by heat denaturation. CD spectra indicated the presence of alpha-helix structures in soluble and membrane-bound APP. The secondary-structure content, estimated from CD spectra, was about 40-45% alpha-helix and 15-20% beta-sheet structures for soluble and membrane-bound APP.

Human amyloid precursor-like protein 1--cDNA cloning, ectopic expression in COS-7 cells and identification of soluble forms in the cerebrospinal fluid

Amyloid precursor-like protein 1 (APLP1) represents an integral membrane type 1 protein of unknown function which was originally cloned from a mouse cDNA library on the basis of sequence similarity with the Alzheimer's amyloid precursor protein (APP). Here we report on the molecular cloning and expression of the human APLP1 (hAPLP1). hAPLP1 consists of 650 amino acids, displays 89% identity on the amino acid level to its mouse homologue and has a calculated molecular mass of 72 kDa. hAPLP1 synthesized in a cell-free system displays an apparent molecular mass of approximately 80 kDa in SDS-containing gels and becomes N-glycosylated when the in vitro translation is performed in the presence of microsomes. The hAPLP1 cDNA was also expressed ectopically in COS-7 cells and the protein expression was analyzed by immunoprecipitation and western blotting. We have demonstrated that hAPLP1 represents a novel glycoprotein which carries both N- and O-linked glycans. Moreover, hAPLP1 undergoes limited proteolysis which results in the secretion of the carboxy-terminal truncated molecule into the cells conditioned medium. Examination of cells transfected with hAPLP1 cDNA by confocal laser microscopy reveals an intense perinuclear and Golgi staining, a pattern resembling the subcellular distribution of APP. Using a novel hAPLP1-specific antiserum, we identified soluble hAPLP1 in the human cerebrospinal fluid, which suggests that secretion of hAPLP1 from brain cells also takes place in vivo.

Normoxic lung ischemia/reperfusion accelerates shedding of angiotensin converting enzyme from the pulmonary endothelium

Normoxic lung ischemia/reperfusion (I/R) leads to oxidative injury of the pulmonary tissue. We analyzed angiotensin-converting enzyme (ACE) in perfused rat lungs upon I/R in order to assess the endothelial injury produced. I/R led to a time-dependent increase in ACE activity in the perfusate, from 145+/-14 mU to 252+/-1 mU, and to reduction of ACE activity in the lung tissue homogenate, from 29.7+/-2.3 U to 22.7+/-1.7 U. About 80% of ACE activity in control and I/R rat lungs was associated with an aqueous phase of extracted perfusates, thus indicating that I/R accelerates shedding of the hydrophilic form of ACE from the plasma membrane. To specifically assess ACE localized on the luminal surface of the pulmonary endothelium, we perfused rat lungs with a radiolabeled monoclonal antibody (mAb) to ACE (anti-ACE mAb 9B9). Pulmonary uptake of mAb 9B9 with I/R was reduced from 32.1+/-1.7% to 24.8+/-0.9%. In contrast, I/R led to a marked increase in the pulmonary uptake of nonspecific [125I]IgG, from 0.17+/-0.02% to 0.67+/-0.04%. Lung wet weight was equal to 0.78+/-0.08% of body weight in the I/R group versus 0.57+/-0.02% at the control level. The observed increase in [125I]IgG uptake and wet lung weight indicate that I/R causes an increase in lung vascular permeability. These results indicate that normoxic lung I/R induces injury to the pulmonary vascular endothelium.

Angiotensin-converting enzyme secretase is inhibited by zinc metalloprotease inhibitors and requires its substrate to be inserted in a lipid bilayer

Mammalian angiotensin-converting enzyme (ACE; EC 3.4.15.1) is one of several proteins that exist in both membrane-bound and soluble forms as a result of a post-translational proteolytic processing event. For ACE we have previously identified a metalloprotease (secretase) responsible for this proteolytic cleavage. The effect of a range of structurally related zinc metalloprotease inhibitors on the activity of the secretase has been examined. Batimastat (BB94) was the most potent inhibitor of the secretase in pig kidney microvillar membranes, displaying an IC50 of 0.47 microM, whereas TAPI-2 was slightly less potent (IC50 18 microM). Removal of the thienothiomethyl substituent adjacent to the hydroxamic acid moiety or the substitution of the P2' substituent decreased the inhibitory potency of batimastat towards the secretase. Several other non-hydroxamate-based collagenase inhibitors were without inhibitory effect on the secretase, indicating that ACE secretase is a novel zinc metalloprotease that is realted to, but distinct from, the matrix metalloproteases. The full-length amphipathic form of ACE was labelled selectively with 3-trifluoromethyl-3-(m-[125I]iodophenyl)diazirine in the membrane-spanning hydrophobic region. Although trypsin was able to cleave the hydrophobic anchoring domain from the bulk of the protein, there was no cleavage of full-length ACE by a Triton X-100-solubilized pig kidney secretase preparation when the substrate was in detergent solution. In contrast, the Triton X-100-solubilized secretase preparation released ACE from pig intestinal microvillar membranes, which lack endogenous secretase activity, and cleaved the purified amphipathic form of ACE when it was incorporated into artificial lipid vesicles. Thus the secretase has an absolute requirement for its substrate to be inserted in a lipid bilayer, a factor that might have implications for the development of cell-free assays for other membrane protein secretases. ACE secretase could be solubilized from the membrane with Triton-X-100 and CHAPS, but not with n-octyl beta-D-glucopyranoside. Furthermore trypsin could release the secretase from the membrane, implying that like its substrate, ACE, it too is a stalked integral membrane protein.

Involvement of a metalloprotease in the shedding of human neutrophil Fc gammaRIIIB

Fc gammaRIIIb is a glycosylphosphatidylinositol(GPI)-anchored, low-affinity IgG receptor, expressed exclusively on human neutrophils. Upon activation or apoptosis of neutrophils, Fc gammaRIIIb is shed from the cell surface, but the enzyme(s) responsible for this process is (are) still unknown. Recently, metalloproteases have been suggested to mediate the shedding of cell surface proteins such as L-selectin and TNF-alpha. Using hydroxamic acid-based inhibitors of this class of proteases (BB-3103, Ro31-9790), we have observed a clear inhibitory effect on Fc gammaRIIIb shedding after PMA stimulation of neutrophils or induction of apoptosis. These inhibitors did not affect PMA-induced degranulation or superoxide generation.

Apical enrichment of human EGF precursor in Madin-Darby canine kidney cells involves preferential basolateral ectodomain cleavage sensitive to a metalloprotease inhibitor

EGF precursor (proEGF) is a member of the family of membrane-anchored EGF-like growth factors that bind with high affinity to the epidermal growth factor receptor (EGFR). In contrast to human transforming growth factor-alpha precursor (proTGFalpha), which is sorted basolaterally in Madin-Darby canine kidney (MDCK) cells (Dempsey, P., and R. Coffey, 1994. J. Biol. Chem. 269:16878-16889), we now demonstrate that human proEGF overexpressed in MDCK cells is found predominantly at the apical membrane domain under steady-state conditions. Nascent proEGF (185 kD) is not sorted but is delivered equally to the apical and basolateral membranes, where it is proteolytically cleaved within its ectodomain to release a soluble 170-kD EGF form into the medium. Unlike the fate of TGFalpha in MDCK cells, the soluble 170-kD EGF species accumulates in the medium, does not interact with the EGFR, and is not processed to the mature 6-kD peptide. We show that the rate of ectodomain cleavage of 185-kD proEGF is fourfold greater at the basolateral surface than at the apical surface and is sensitive to a metalloprotease inhibitor, batimastat. Batimastat dramatically inhibited the release of soluble 170-kD EGF into the apical and basal medium by 7 and 60%, respectively, and caused a concordant increase in the expression of 185-kD proEGF at the apical and basolateral cell surfaces of 150 and 280%, respectively. We propose that preferential ectodomain cleavage at the basolateral surface contributes to apical domain localization of 185-kD proEGF in MDCK cells, and this provides a novel mechanism to achieve a polarized distribution of cell surface membrane proteins under steady-state conditions. In addition, differences in disposition of EGF and TGFalpha in polarized epithelial cells offer a new conceptual framework to consider the actions of these polypeptide growth factors.

Expression of members of the novel membrane linked metalloproteinase family ADAM in cells derived from a range of haematological malignancies

ADAMs (A disintegrin and metalloproteinase) are a recently discovered family of proteins with significant primary sequence similarity to the reprolysin family of snake venomases. These ADAMs closest known homologues are the type III reprolysin enzymes which have been demonstrated to be, among other things potent type IV collagenases. ADAMs are putative membrane linked proteins with several domains including a metalloproteinase domain, a potential integrin binding domain, a cysteine rich sequence and an EGF like sequence. They have been implicated in a wide variety of functions including basement membrane degradation and cell-cell and cell-matrix interactions. We have used RT-PCR and Northern blotting to characterise the expression of members of this family in cells derived from a variety of haematological malignancies including leukaemia (HL60 and Jurkat), erythroleukaemia (K562), lymphoma (U937 and Cupillo) and myeloma (U266B1). We find clear expression of four members of this novel family of proteins but note differences in the expression levels of each member. The ADAMs known as MADM (ADAM10), MCMP (ADAM12, MDC9) and Metargidin (ADAM15) which all possess potentially active metalloproteinase domains are expressed in all these cell types to significant levels. The putative tumour suppressor gene MDC (ADAM11) is expressed at very low levels in all cells examined. As ADAMs may have both potential metalloproteinase activity and adhesive domains we wish to explore the role of these proteins with regard to pathophysiology of haematological malignancy such as egression of leukaemic cells from the bone marrow.