Metalloprotease-mediated GH receptor proteolysis and GHBP shedding. Determination of extracellular domain stem region cleavage site

GHR signalling: Receptor activation and degradation mechanisms

Growth hormone (GH) actions via initiating cell signalling through the GH receptor (GHR) are important for many physiological processes, in addition to its well-known role in regulating growth. The activation of JAK-STAT signalling by GH is well characterized, however knowledge on GH activation of SRC family kinases (SFKs) is still limited. In this review we summarise the collective knowledge on the activation, regulation, and downstream signalling of GHR. We highlight studies on GH activation of SFKs and the important outcome of this signalling pathway with a focus on the different degradation mechanisms that can regulate GHR availability since this is an area that warrants further study considering its role in tumour progression.

Classical and novel GH receptor signaling pathways

In this review, I summarize historical and recent features of the classical pathways activated by growth hormone (GH) through the cell surface GH receptor (GHR). GHR is a cytokine receptor superfamily member that signals by activating the non-receptor tyrosine kinase, JAK2, and members of the Src family kinases. Activation of the GHR engages STATs, PI3K, and ERK pathways, among others, and details of these now-classical pathways are presented. Modulating elements, including the SOCS proteins, phosphatases, and regulated GHR metalloproteolysis, are discussed. In addition, a novel physical and functional interaction of GHR with IGF-1R is summarized and discussed in terms of its mechanisms, consequences, and physiological and therapeutic implications.

iGHBP: Computational identification of growth hormone binding proteins from sequences using extremely randomised tree

A soluble carrier growth hormone binding protein (GHBP) that can selectively and non-covalently interact with growth hormone, thereby acting as a modulator or inhibitor of growth hormone signalling. Accurate identification of the GHBP from a given protein sequence also provides important clues for understanding cell growth and cellular mechanisms. In the postgenomic era, there has been an abundance of protein sequence data garnered, hence it is crucial to develop an automated computational method which enables fast and accurate identification of putative GHBPs within a vast number of candidate proteins. In this study, we describe a novel machine-learning-based predictor called iGHBP for the identification of GHBP. In order to predict GHBP from a given protein sequence, we trained an extremely randomised tree with an optimal feature set that was obtained from a combination of dipeptide composition and amino acid index values by applying a two-step feature selection protocol. During cross-validation analysis, iGHBP achieved an accuracy of 84.9%, which was ~7% higher than the control extremely randomised tree predictor trained with all features, thus demonstrating the effectiveness of our feature selection protocol. Furthermore, when objectively evaluated on an independent data set, our proposed iGHBP method displayed superior performance compared to the existing method. Additionally, a user-friendly web server that implements the proposed iGHBP has been established and is available at http://thegleelab.org/iGHBP.

Disrupting Hedgehog Cardin-Weintraub sequence and positioning changes cellular differentiation and compartmentalization in vivo

Metazoan Hedgehog (Hh) morphogens are essential regulators of growth and patterning at significant distances from their source, despite being produced as N-terminally palmitoylated and C-terminally cholesteroylated proteins, which firmly tethers them to the outer plasma membrane leaflet of producing cells and limits their spread. One mechanism to overcome this limitation is proteolytic processing of both lipidated terminal peptides, called shedding, but molecular target site requirements for effective Hh shedding remained undefined. In this work, by using Drosophila melanogaster as a model, we show that mutagenesis of the N-terminal Cardin-Weintraub (CW) motif inactivates recombinant Hh proteins to variable degrees and, if overexpressed in the same compartment, converts them into suppressors of endogenous Hh function. In vivo, additional removal of N-palmitate membrane anchors largely restored endogenous Hh function, supporting the hypothesis that proteolytic CW processing controls Hh solubilization. Importantly, we also observed that CW repositioning impairs anterior/posterior compartmental boundary maintenance in the third instar wing disc. This demonstrates that Hh shedding not only controls the differentiation of anterior cells, but also maintains the sharp physical segregation between these receiving cells and posterior Hh-producing cells.

Alternative splicing of ALCAM enables tunable regulation of cell-cell adhesion through differential proteolysis

While many adhesion receptors are known to influence tumor progression, the mechanisms by which they dynamically regulate cell-cell adhesion remain elusive. We previously identified Activated Leukocyte Cell Adhesion Molecule (ALCAM) as a clinically relevant driver of metastasis and hypothesized that a tunable mechanism of ectodomain shedding regulates its contribution to dissemination. To test this hypothesis, we examined an under-explored ALCAM splice variant (ALCAM-Iso2) and demonstrated that loss of the membrane-proximal region of ALCAM (exon 13) increased metastasis four-fold. Mechanistic studies identified a novel MMP14-dependent membrane distal cleavage site in ALCAM-Iso2, which mediated a ten-fold increase in shedding, thereby decreasing cellular cohesion. Importantly, the loss of cohesion is not limited to the cell capable of shedding because the released extracellular domain diminished cohesion of non-shedding cells through disruption of ALCAM-ALCAM interactions. ALCAM-Iso2-dominated expression in bladder cancer tissue, compared to normal bladder, further emphasizes that ALCAM alternative splicing may contribute to clinical disease progression. The requirement for both the loss of exon 13 and the gain of metalloprotease activity suggests that ALCAM shedding and concomitant regulation of tumor cell adhesion is a locally tunable process.

ADAM17 is the main sheddase for the generation of human triggering receptor expressed in myeloid cells (hTREM2) ectodomain and cleaves TREM2 after Histidine 157

Triggering receptor expressed in myeloid cells (TREM2) is a member of the immunoglobulin superfamily and is expressed in macrophages, dendritic cells, microglia, and osteoclasts. TREM2 plays a role in phagocytosis, regulates release of cytokine, contributes to microglia maintenance, and its ectodomain is shed from the cell surface. Here, the question was addressed at which position sheddases cleave TREM2 and what are the proteases involved in this process. Using both pharmacological and genetic approaches we report that the main protease contributing to the release of TREM2 ectodomain is ADAM17, (a disintegrin and metalloproteinase domain containing protein, also called TACE, TNFα converting enzyme) while ADAM10 plays a minor role. Complementary biochemical experiments reveal that cleavage occurs between histidine 157 and serine 158. Shedding is not altered for the R47H-mutated TREM2 protein that confers an increased risk for the development of Alzheimers disease. These findings reveal a link between shedding of TREM2 and its regulation during inflammatory conditions or chronic neurodegenerative disease like AD in which activity or expression of sheddases might be altered.

Stress-Related Mitogen-Activated Protein Kinases Stimulate the Accumulation of Small Molecules and Proteins in Arabidopsis thaliana Root Exudates

A delicate balance in cellular signaling is required for plants to respond to microorganisms or to changes in their environment. Mitogen-activated protein kinase (MAPK) cascades are one of the signaling modules that mediate transduction of extracellular microbial signals into appropriate cellular responses. Here, we employ a transgenic system that simulates activation of two pathogen/stress-responsive MAPKs to study release of metabolites and proteins into root exudates. The premise is based on our previous proteomics study that suggests upregulation of secretory processes in this transgenic system. An advantage of this experimental set-up is the direct focus on MAPK-regulated processes without the confounding complications of other signaling pathways activated by exposure to microbes or microbial molecules. Using non-targeted metabolomics and proteomics studies, we show that MAPK activation can indeed drive the appearance of dipeptides, defense-related metabolites and proteins in root apoplastic fluid. However, the relative levels of other compounds in the exudates were decreased. This points to a bidirectional control of metabolite and protein release into the apoplast. The putative roles for some of the identified apoplastic metabolites and proteins are discussed with respect to possible antimicrobial/defense or allelopathic properties. Overall, our findings demonstrate that sustained activation of MAPKs alters the composition of apoplastic root metabolites and proteins, presumably to influence the plant-microbe interactions in the rhizosphere. The reported metabolomics and proteomics data are available via Metabolights (Identifier: MTBLS441) and ProteomeXchange (Identifier: PXD006328), respectively.

Growth Hormone Resistance-Special Focus on Inflammatory Bowel Disease

Growth hormone (GH) plays major anabolic and catabolic roles in the body and is important for regulating several aspects of growth. During an inflammatory process, cells may develop a state of GH resistance during which their response to GH stimulation is limited. In this review, we will emphasize specific mechanisms governing the formation of GH resistance in the active phase of inflammatory bowel disease. The specific molecular effects mediated through individual inflammatory mediators and processes will be highlighted to provide an overview of the transcriptional, translational and post-translational inflammation-mediated impacts on the GH receptor (GHR) along with the impacts on GH-induced intracellular signaling. We also will review GH’s effects on mucosal healing and immune cells in the context of experimental colitis, human inflammatory bowel disease and in patients with short bowel syndrome.

TIMP3 Modulates GHR Abundance and GH Sensitivity

GH receptor (GHR) binds GH at the cell surface via its extracellular domain and initiates intracellular signal transduction, resulting in important anabolic and metabolic actions. GH signaling is subject to dynamic regulation, which in part is exerted by modulation of cell surface GHR levels. Constitutive and inducible metalloprotease-mediated cleavage of GHR regulate GHR abundance and thereby modulate GH action. We previously demonstrated that GHR proteolysis is catalyzed by the TNF-α converting enzyme (TACE; ADAM17). Tissue inhibitors of metalloproteases-3 (TIMP3) is a natural specific inhibitor of TACE, although mechanisms underlying this inhibition are not yet fully understood. In the current study, we use two model cell lines to examine the relationships between cellular TACE, TIMP3 expression, GHR metalloproteolysis, and GH sensitivity. These two cell lines exhibited markedly different sensitivity to inducible GHR proteolysis, which correlated directly to their relative levels of mature TACE vs unprocessed TACE precursor and indirectly to their levels of cellular TIMP3. Our results implicate TIMP3 as a modulator of cell surface GHR abundance and the ability of GH to promote cellular signaling; these modulatory effects may be conferred by endogenous TIMP3 expression as well as exogenous TIMP3 exposure. Furthermore, our analysis suggests that TIMP3, in addition to regulating the activity of TACE, may also modulate the maturation of TACE, thereby affecting the abundance of the active form of the enzyme.

Growth hormone binding protein - physiological and analytical aspects

A significant proportion of total circulating growth hormone (GH) is bound to a high affinity growth hormone binding protein (GHBP). Several low affinity binding proteins have also been described. Significant differences between species exist with respect to origin and regulation of GHBP, but generally it resembles the extracellular domain of the GH receptor. Concentrations are associated with GH status, body composition and other factors. Although the clinical relevance of GHBP is not fully understood it is suggested that concentrations indirectly reflect GH receptor status. This is supported by cases of Laron's syndrome where a molecular defect in the extracellular domain of the GH receptor is associated with low or unmeasurable GHBP concentrations. Methods to measure GHBP have evolved from chromatographic, activity based procedures to direct immunoassays. In clinical practice, measurement of GHBP can be helpful to differentiate between GH deficiency and GH insensitivity, particularly if GHBP is absent.

Identification of cleavage sites leading to the shed form of the anti-aging protein klotho

Membrane protein shedding is a critical step in many normal and pathological processes. The anti-aging protein klotho (KL), mainly expressed in kidney and brain, is secreted into the serum and CSF, respectively. KL is proteolytically released, or shed, from the cell surface by ADAM10 and ADAM17, which are the α-secretases that also cleave the amyloid precursor protein and other proteins. The transmembrane KL is a coreceptor with the FGF receptor for FGF23, whereas the shed form acts as a circulating hormone. However, the precise cleavage sites in KL are unknown. KL contains two major cleavage sites: one close to the juxtamembrane region and another between the KL1 and KL2 domains. We identified the cleavage site involved in KL release by mutating potential sheddase(s) recognition sequences and examining the production of the KL extracellular fragments in transfected COS-7 cells. Deletion of amino acids T958 and L959 results in a 50–60% reduction in KL shedding, and an additional P954E mutation results in further reduction of KL shedding by 70–80%. Deletion of amino acids 954–962 resulted in a 94% reduction in KL shedding. This mutant also had moderately decreased cell surface expression, yet had overall similar subcellular localization as that of WT KL, as demonstrated by immunofluorescence. Cleavage-resistant mutants could function as a FGFR coreceptor for FGF23, but they lost activity as a soluble form of KL in proliferation and transcriptional reporter assays. Cleavage between the KL1 and KL2 domains is dependent on juxtamembrane cleavage. Our results shed light onto mechanisms underlying KL release from the cell membrane and provide a target for potential pharmacologic interventions aimed at regulating KL secretion.

Rosiglitazone treatment of type 2 diabetic db/db mice attenuates urinary albumin and angiotensin converting enzyme 2 excretion

Alterations within the renal renin angiotensin system play a pivotal role in the development and progression of cardiovascular and renal disease. Angiotensin converting enzyme 2 (ACE2) is highly expressed in renal tubules and has been shown to be renoprotective in diabetes. The protease, a disintegrin and metalloprotease (ADAM) 17, is involved in the ectodomain shedding of several transmembrane proteins including ACE2. Renal ACE2 and ADAM17 were significantly increased in db/db mice compared to controls. We investigated the effect of the insulin sensitizer, rosiglitazone, on albuminuria, renal ADAM17 protein expression and ACE2 shedding in db/db diabetic mice. Rosiglitazone treatment of db/db mice normalized hyperglycemia, attenuated renal injury and decreased urinary ACE2 and renal ADAM17 protein expression. Urinary excreted ACE2 is enzymatically active. Western blot analysis of urinary ACE2 demonstrated two prominent immunoreactive bands at approximately 70 & 90 kDa. The predominant immunoreactive band is approximately 20 kDa shorter than the one demonstrated for kidney lysate, indicating possible ectodomain shedding of active renal ACE2 in the urine. Therefore, it is tempting to speculate that renoprotection of rosiglitazone could be partially mediated via downregulation of renal ADAM17 and ACE2 shedding. In addition, there was a positive correlation between blood glucose, urinary albumin, plasma glucagon, and triglyceride levels with urinary ACE2 excretion. In conclusion, urinary ACE2 could be used as a sensitive biomarker of diabetic nephropathy and for monitoring the effectiveness of renoprotective medication.

Growth hormone-induced JAK2 signaling and GH receptor down-regulation: role of GH receptor intracellular domain tyrosine residues

GH receptor (GHR) mediates important somatogenic and metabolic effects of GH. A thorough understanding of GH action requires intimate knowledge of GHR activation mechanisms, as well as determinants of GH-induced receptor down-regulation. We previously demonstrated that a GHR mutant in which all intracellular tyrosine residues were changed to phenylalanine was defective in its ability to activate signal transducer and activator of transcription (STAT)5 and deficient in GH-induced down-regulation, but able to allow GH-induced Janus family of tyrosine kinase 2 (JAK2) activation. We now further characterize the signaling and trafficking characteristics of this receptor mutant. We find that the mutant receptor's extracellular domain conformation and its interaction with GH are indistinguishable from the wild-type receptor. Yet the mutant differs greatly from the wild-type in that GH-induced JAK2 activation is augmented and far more persistent in cells bearing the mutant receptor. Notably, unlike STAT5 tyrosine phosphorylation, GH-induced STAT1 tyrosine phosphorylation is retained and augmented in mutant GHR-expressing cells. The defective receptor down-regulation and persistent JAK2 activation of the mutant receptor do not depend on the sustained presence of GH or on the cell's ability to carry out new protein synthesis. Mutant receptors that exhibit resistance to GH-induced down-regulation are enriched in the disulfide-linked form of the receptor, which reflects the receptor's activated conformation. Furthermore, acute GH-induced internalization, a proximal step in down-regulation, is markedly impaired in the mutant receptor compared to the wild-type receptor. These findings are discussed in the context of determinants and mechanisms of regulation of GHR down-regulation.

Juxtamembrane shedding of Plasmodium falciparum AMA1 is sequence independent and essential, and helps evade invasion-inhibitory antibodies

The malarial life cycle involves repeated rounds of intraerythrocytic replication interspersed by host cell rupture which releases merozoites that rapidly invade fresh erythrocytes. Apical membrane antigen-1 (AMA1) is a merozoite protein that plays a critical role in invasion. Antibodies against AMA1 prevent invasion and can protect against malaria in vivo, so AMA1 is of interest as a malaria vaccine candidate. AMA1 is efficiently shed from the invading parasite surface, predominantly through juxtamembrane cleavage by a membrane-bound protease called SUB2, but also by limited intramembrane cleavage. We have investigated the structural requirements for shedding of Plasmodium falciparum AMA1 (PfAMA1), and the consequences of its inhibition. Mutagenesis of the intramembrane cleavage site by targeted homologous recombination abolished intramembrane cleavage with no effect on parasite viability in vitro. Examination of PfSUB2-mediated shedding of episomally-expressed PfAMA1 revealed that the position of cleavage is determined primarily by its distance from the parasite membrane. Certain mutations at the PfSUB2 cleavage site block shedding, and parasites expressing these non-cleavable forms of PfAMA1 on a background of expression of the wild type gene invade and replicate normally in vitro. The non-cleavable PfAMA1 is also functional in invasion. However – in contrast to the intramembrane cleavage site - mutations that block PfSUB2-mediated shedding could not be stably introduced into the genomic pfama1 locus, indicating that some shedding of PfAMA1 by PfSUB2 is essential. Remarkably, parasites expressing shedding-resistant forms of PfAMA1 exhibit enhanced sensitivity to antibody-mediated inhibition of invasion. Drugs that inhibit PfSUB2 activity should block parasite replication and may also enhance the efficacy of vaccines based on AMA1 and other merozoite surface proteins.

The malaria parasite invades red blood cells. During invasion several parasite proteins, including a vaccine candidate called PfAMA1, are clipped from the parasite surface. Most of this clipping is performed by an enzyme called PfSUB2, but some also occurs through intramembrane cleavage. The function of this shedding is unknown. We have examined the requirements for shedding of PfAMA1, and the effects of mutations that block shedding. Mutations that block intramembrane cleavage have no effect on the parasite. We then show that PfSUB2 does not recognise a specific amino acid sequence in PfAMA1, but rather cleaves it at a position determined primarily by its distance from the parasite membrane. Certain mutations at the PfSUB2 cleavage site prevent shedding, and parasites expressing non-cleavable PfAMA1 along with unmodified PfAMA1 grow normally. However, these mutations cannot be introduced into the parasite's genome, showing that some shedding by PfSUB2 is essential for parasite survival. Parasites expressing shedding-resistant mutants of PfAMA1 show enhanced sensitivity to invasion-inhibitory antibodies, suggesting that shedding of surface proteins during invasion helps the parasite to evade potentially protective antibodies. Drugs that inhibit PfSUB2 may prevent disease and enhance the efficacy of vaccines based on PfAMA1.

Inhibitory GH receptor extracellular domain monoclonal antibodies: three-dimensional epitope mapping

GH receptor (GHR) mediates the anabolic and metabolic effects of GH. We previously characterized a monoclonal antibody (anti-GHR(ext-mAb)) that reacts with subdomain 2 of the rabbit GHR extracellular domain (ECD) and is a conformation-specific inhibitor of GH signaling in cells bearing rabbit or human GHR. Notably, this antibody has little effect on GH binding and also inhibits inducible metalloproteolysis of the GHR that occurs in the perimembranous ECD stem region. In the current study, we demonstrate that anti-GHR(ext-mAb) inhibits GH-dependent cellular proliferation and also inhibits hepatic GH signaling in vivo in mice that adenovirally express rabbit GHR, as assessed with our noninvasive bioluminescence hepatic signaling assay. A separate monoclonal antibody (anti-GHR(mAb 18.24)) is a sister clone of anti-GHR(ext-mAb). Here, we demonstrate that anti-GHR(mAb 18.24) also inhibits rabbit and human GHR signaling and inducible receptor proteolysis. Further, we use a random PCR-generated mutagenic expression system to map the three-dimensional epitopes in the rabbit GHR ECD for both anti-GHR(ext-mAb) and anti-GHR(mAb 18.24). We find that each of the two antibodies has similar, but nonidentical, discontinuous epitopes that include regions of subdomain 2 encompassing the dimerization interface. These results have fundamental implications for understanding the role of the dimerization interface and subdomain 2 in GHR activation and regulated GHR metalloproteolysis and may inform development of therapeutics that target GHR.

Ectodomain shedding and autocleavage of the cardiac membrane protease corin

Corin is a cardiac membrane protease that activates natriuretic peptides. It is unknown how corin function is regulated. Recently, soluble corin was detected in human plasma, suggesting that corin may be shed from cardiomyocytes. Here we examined soluble corin production and activity and determined the proteolytic enzymes responsible for corin cleavage. We expressed human corin in HEK 293 cells and detected three soluble fragments of ∼180, ∼160, and ∼100 kDa, respectively, in the cultured medium by Western blot analysis. All three fragments were derived from activated corin molecules. Similar results were obtained in HL-1 cardiomyocytes. Using protease inhibitors, ionomycin and phorbol myristate acetate stimulation, small interfering RNA knockdown, and site-directed mutagenesis, we found that ADAM10 was primarily responsible for shedding corin in its juxtamembrane region to release the ∼180-kDa fragment, corresponding to the near-entire extracellular region. In contrast, the ∼160- and ∼100-kDa fragments were from corin autocleavage at Arg-164 in frizzled 1 domain and Arg-427 in LDL receptor 5 domain, respectively. In functional studies, the ∼180-kDa fragment activated atrial natriuretic peptide, whereas the ∼160- and ∼100-kDa fragments did not. Our data indicate that ADAM-mediated shedding and corin autocleavage are important mechanisms regulating corin function and preventing excessive, potentially hazardous, proteolytic activities in the heart.

ADAM-17: the enzyme that does it all

This review focuses on the role of ADAM-17 in disease. Since its debut as the tumor necrosis factor converting enzyme (TACE), ADAM-17 has been reported to be an indispensible regulator of almost every cellular event from proliferation to migration. The central role of ADAM-17 in cell regulation is rooted in its diverse array of substrates: cytokines, growth factors, and their receptors as well as adhesion molecules are activated or inactivated by their cleavage with ADAM-17. It is therefore not surprising that ADAM-17 is implicated in numerous human diseases including cancer, heart disease, diabetes, rheumatoid arthritis, kidney fibrosis, Alzheimer's disease, and is a promising target for future treatments. The specific role of ADAM-17 in the pathophysiology of these diseases is very complex and depends on the cellular context. To exploit the therapeutic potential of ADAM-17, it is important to understand how its activity is regulated and how specific organs and cells can be targeted to inactivate or activate the enzyme.

Ectodomain shedding of interleukin-2 receptor beta and generation of an intracellular functional fragment

Interleukin-2 (IL-2) regulates different functions of various lymphoid cell subsets. These are mediated by its binding to the IL-2 receptor (IL-2R) composed of three subunits (IL2-Ralpha, -beta, and -gamma(c)). IL-2Rbeta is responsible for the activation of several signaling pathways. Ectodomain shedding of membrane receptors is thought to be an important mechanism for down-regulation of cell surface receptor abundance but is also emerging as a mechanism that cell membrane-associated molecules require for proper action in vivo. Here, we demonstrate that IL-2Rbeta is cleaved in cell lines of different origin, including T cells, generating an intracellular 37-kDa fragment (37beta ic) that comprises the full intracellular C-terminal and transmembrane domains. Ectodomain shedding of IL-2Rbeta decreases in a mutant deleted of the juxtamembrane region, where cleavage is predicted to occur, and is inhibited by tissue inhibitor of metalloproteases-3. 37Beta ic is tyrosine-phosphorylated and associates with STAT-5, a canonic signal transducer of IL-2R. Finally, lymphoid cell transfection with a truncated form of IL-2Rbeta mimicking 37beta ic increases their proliferation. These data indicate that IL-2Rbeta is subject to ectodomain shedding generating an intracellular fragment biologically functional, because (i) it is phosphorylated, (ii) it associates with STAT5A, and (iii) it increases cell proliferation.

Unexpected tolerance of alpha-cleavage of the prion protein to sequence variations

The cellular form of the prion protein, PrP^C, undergoes extensive proteolysis at the α site (109K↓H110). Expression of non-cleavable PrP^C mutants in transgenic mice correlates with neurotoxicity, suggesting that α-cleavage is important for PrP^C physiology. To gain insights into the mechanisms of α-cleavage, we generated a library of PrP^C mutants with mutations in the region neighbouring the α-cleavage site. The prevalence of C1, the carboxy adduct of α-cleavage, was determined for each mutant. In cell lines of disparate origin, C1 prevalence was unaffected by variations in charge and hydrophobicity of the region neighbouring the α-cleavage site, and by substitutions of the residues in the palindrome that flanks this site. Instead, α-cleavage was size-dependently impaired by deletions within the domain 106–119. Almost no cleavage was observed upon full deletion of this domain. These results suggest that α-cleavage is executed by an α-PrPase whose activity, despite surprisingly limited sequence specificity, is dependent on the size of the central region of PrP^C.

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.

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

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

SRC family kinases accelerate prolactin receptor internalization, modulating trafficking and signaling in breast cancer cells

Despite the growing body of evidence supporting prolactin (PRL) actions in human breast cancer, little is known regarding PRL regulation of its own receptor in these cells. Ligand-initiated endocytosis is a key process in the regulation of receptor availability and signaling cascades that may lead to oncogenic actions. Although exposure to exogenous PRL accelerates degradation of the long isoform of the PRL receptor (lPRLR), neither the signals initiated by PRL that lead to lPRLR internalization and subsequent down-regulation, nor the relationship to downstream pathways are understood in breast cancer cells. In this study, we showed that PRL-induced down-regulation of the lPRLR was reduced by inhibition of src family kinases (SFKs), but not Janus kinase 2, in MCF-7 cells. Inhibition of SFKs also resulted in accumulation of a PRL-induced PRLR fragment containing the extracellular domain, which appeared to be generated from newly synthesized PRLR. lPRLR was constitutively associated with SFKs in lipid rafts. PRL-induced SFK activation led to recruitment of the guanosine triphosphatase, dynamin-2, to an internalization complex, resulting in endocytosis. Inhibition of endocytosis by small interfering RNA-mediated knockdown of dynamin-2 blocked PRL-induced down-regulation of lPRLR, confirming that internalization is essential for this process. Endocytosis also was required for optimal phosphorylation of ERK1/2 and Akt, but not for Janus kinase 2 or signal transducer and activator of transcription 5, indicating that internalization selectively modulates signaling cascades. Together, these data indicate that SFKs are key mediators of ligand-initiated lPRLR internalization, down-regulation, and signal transduction in breast cancer cells, and underscore the importance of target cell context in receptor trafficking and signal transduction.

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.

Modulation of growth hormone receptor abundance and function: roles for the ubiquitin-proteasome system

Growth hormone plays an important role in regulating numerous functions in vertebrates. Several pathways that negatively regulate the magnitude and duration of its signaling (including expression of tyrosine phosphatases, SOCS and PIAS proteins) are shared between signaling induced by growth hormone itself and by other cytokines. Here we overview downregulation of the growth hormone receptor as the most specific and potent mechanism of restricting cellular responses to growth hormone and analyze the role of several proteolytic systems and, specifically, ubiquitin-dependent pathways in this regulation.

Endotoxin-induced proteolytic reduction in hepatic growth hormone (GH) receptor: a novel mechanism for GH insensitivity

GH is an important anabolic hormone. We previously demonstrated in cell culture that the cell surface GH receptor (GHR) is susceptible to inducible metalloproteolytic cleavage that yields the shed receptor extracellular domain (called GH binding protein) and renders the cells desensitized to subsequent GH stimulation. Sepsis and inflammatory states are associated with hepatic desensitization to GH, although disparate mechanisms have been postulated in various animal models. Using C3H/HeJ mice, we now demonstrate that administration of lipopolysaccharide (LPS) causes marked hepatic desensitization to GH, assessed by monitoring signal transducer and activator of transcription 5 tyrosine phosphorylation and nuclear accumulation and with a novel noninvasive bioluminescence imaging system to track in vivo hepatic GH signaling serially in individual mice. This endotoxin-induced desensitization was accompanied by marked loss of hepatic GHR, which was not explained by changes in GHR mRNA abundance. Furthermore, we observe that LPS causes GH-binding protein shedding of a hepatically expressed wild-type GHR but not a GHR with a mutation in the metalloprotease cleavage site. These data suggest that in this model system, LPS-induced desensitization to GH is associated with proteolytic GHR cleavage. These data are the first to demonstrate inducible in vivo GHR proteolysis and suggest this is a mechanism to regulate GH sensitivity and its anabolic effects during sepsis or inflammation.

A growth hormone receptor mutation impairs growth hormone autofeedback signaling in pituitary tumors

Pituitary tumors are a diverse group of neoplasms that are classified based on clinical manifestations, hormone excess, and histomorphologic features. Those that cause growth hormone (GH) excess and acromegaly are subdivided into morphologic variants that have not yet been shown to have pathogenetic significance or predictive value for therapy and outcome. Here, we identify a selective somatic histidine-to-leucine substitution in codon 49 of the extracellular domain of the GH receptor (GHR) in a morphologic subtype of human GH-producing pituitary tumors that is characterized by the presence of cytoskeletal aggresomes. This GHR mutation significantly impairs glycosylation-mediated receptor processing, maturation, ligand binding, and signaling. Pharmacologic GH antagonism recapitulates the morphologic phenotype of pituitary tumors from which this mutation was identified, inducing the formation of cytoskeletal keratin aggresomes. This novel GHR mutation provides evidence for impaired hormone autofeedback in the pathogenesis of these pituitary tumors. It explains the lack of responsiveness to somatostatin analogue therapy of this tumor type, in contrast to the exquisite sensitivity of tumors that lack aggresomes, and has therapeutic implications for the safety of GH antagonism as a therapeutic modality in acromegaly.

Autocrine growth hormone: effects on growth hormone receptor trafficking and signaling

GH and GH receptor are expressed in many extrapituitary tissues, permitting autocrine/paracrine activity. Autocrine GH has regulatory functions in embryonic development and cellular differentiation and proliferation and is reported to be involved in the development and metastasis of tumor cells. To understand the principles of transport and signaling of autocrine GH and GH receptor, we used a model system to express both proteins in the same cell. Our experiments show that GH binds the GH receptor immediately after synthesis in the endoplasmic reticulum and facilitates maturation of GH receptor. The hormone-receptor complexes arrive at the cell surface where exogenously added GH is unable to bind these receptors. Autocrine GH activates the GH receptors, but signal transduction occurs only after exiting the endoplasmic reticulum. This model study explains why autocrine GH-producing cells may be insensitive for GH (antagonist) treatment and clarifies autocrine signaling events.

ADAM proteins in the brain

ADAM proteins are a family of metalloproteinases with a disintegrin domain. They have proteolytic as well as adhesive functions and can be involved in cell fusion events. Some ADAM proteins are expressed in a highly tissue restricted fashion, whereas others are expressed quite ubiquitously. In the brain, ADAM proteins have a role in neural development, axon guidance and inflammatory responses. Although there may be some functional overlap, homozygous deletion of some ADAM genes in mice can have fatal consequences. The expression and possible role of ADAM proteins in the brain will be discussed.

Role of the growth hormone (GH) receptor transmembrane domain in receptor predimerization and GH-induced activation

The GH receptor (GHR) mediates GH effects by activating the GHR-associated cytoplasmic tyrosine kinase, Janus kinase 2. Recent studies indicate that GHRs exist as dimers independently of GH binding. Some authors suggest that receptor predimerization is mediated by the transmembrane domain (TMD) and that GH binding initiates signaling by triggering changes in the orientation of the two GHRs within the dimer. In this study, we investigate the role of GHR TMD in GH-independent receptor dimerization and ligand-induced activation. We prepared a GHR mutant, GHR(LDLR), in which the TMD is replaced with the TMD of the human low-density lipoprotein receptor (LDLR). The resultant chimera has a TMD two residues shorter than the native GHR TMD; thus, in addition to possessing a different TMD, the altered GHR(LDLR) TMD helical register may change positions of the GHR extracellular domain (ECD) and intracellular domain relative to the TMD when compared with the wild-type (WT) receptor. When each was coexpressed with an intracellular domain-truncated GHR mutant, GHR(1-274-Myc), both WT GHR and GHR(LDLR) were specifically coprecipitated with GHR(1-274-Myc), indicating that the GHR TMD was not required for GHR heterodimerization with GHR(1-274-Myc). We further examined the contribution of the so-called "dimerization interface," a GHR ECD region that is critical for GH-induced signaling, to receptor predimerization. Coimmunoprecipitation experiments with either WT GHR, a dimerization interface mutant (GHR-H150D), or a control mutant (GHR-T147D) with GHR(1-274-Myc) showed dramatically reduced coprecipitation of GHR-H150D with GHR(1-274-Myc) when compared with WT GHR or GHR-T147K. This result suggests that, in contrast to some recent models, the dimerization interface contributes to GHR predimerization. We also compared WT GHR with GHR(LDLR) and GHR(LDLRDelta4) (a chimera in which the LDLR TMD has an internal deletion of four residues) with regard to response to GH stimulation. Although the chimeras had similar GH dose responses and time courses for signaling as WT GHR, they were markedly less sensitive to inhibition of signaling by a conformation-sensitive GHR ECD monoclonal antibody. Further, the chimeras were much less sensitive to inducible metalloprotease cleavage than was WT GHR, implying that the ECD conformations of the chimera receptors differ from WT GHR. Collectively, our data indicate that the composition and/or length of the TMD affect some aspects of GHR function, but do not affect receptor predimerization or GH-induced GHR activation. Further, they suggest that the GHR ECD-TMD is more flexible than previously thought in terms of the ability to achieve the active conformation in response to GH.

Differential regulation of membrane associated-growth hormone binding protein (MA-GHBP) and growth hormone receptor (GHR) expression by growth hormone (GH) in mouse liver

Growth hormone (GH) binding to GH receptor (GHR) is the initial step that leads to the physiological functions of the hormone. Proteolytical cleavage of the GHR in humans and rabbits and alternative processing of the GHR transcript in rodents generates circulating growth hormone binding protein (GHBP). Moreover, other GHR truncated forms that result from alternative processing of the GHR mRNA transcript have been described. These GHR short forms are inserted in the plasma membrane but they are unable to transduce the signal. In rodents, membrane associated-GHBP (MA-GHBP), which accounts for a significant proportion of liver GH binding capacity, represents the main GHR short form found in membranes, and may therefore function as a negative form of the receptor. In the present study, GHR and MA-GHBP content in liver were analyzed using mutant and transgenic mice expressing different concentrations of growth hormone to evaluate the correlation between GH levels, body weight (BW), GHR and MA-GHBP expression. It was found that GH deficiency was associated with diminished BW, GHR and MA-GHBP expression, while increased GH concentration led to increased BW, GHR and MA-GHBP expression, but MA-GHBP upregulation was more pronounced than the observed increase in GHR expression. Since GHR and MA-GHBP both contribute to liver GH binding capacity, GH-induced enrichment of the dominant negative form would represent a compensatory mechanism triggered by high levels of the hormone. This attempt to attenuate the effects of supraphysiological concentrations of GH may be critical to reduce or prevent their plausible damaging effects on the organism.

Soluble Interleukin IL-15Ralpha is generated by alternative splicing or proteolytic cleavage and forms functional complexes with IL-15

Interleukin 15 (IL-15) is a pleiotropic cytokine that is hardly detectable in biological fluids. Here, we show that IL-15 forms functional heterocomplexes with soluble high affinity IL-15 receptor alpha (IL-15Ralpha) chain in mouse serum and cell-conditioned medium, which prevents IL-15 detection by ELISA. We also demonstrate that two soluble IL-15Ralpha (sIL-15Ralpha) sushi domain isoforms are generated through a novel alternative splicing mechanism within the IL-15Ralpha gene. These isoforms potentiate IL-15 action by promoting the IL-15-mediated proliferation of the CTLL cell line and interferon gamma production by murine NK cells, which suggests a role in IL-15 transpresentation. Conversely, a full-length sIL-15Ralpha ectodomain released by tumor necrosis factor-alpha-converting enzyme (TACE)-dependent proteolysis inhibits IL-15 activity. Thus, a dual mechanism of sIL-15Ralpha generation exists in mice, giving rise to polypeptides with distinct properties, which regulate IL-15 function.

A soluble form of the Mer receptor tyrosine kinase inhibits macrophage clearance of apoptotic cells and platelet aggregation

Membrane-bound receptors generate soluble ligand-binding domains either by proteolytic cleavage of the extracellular domain or alternative mRNA splicing yielding a secreted protein. Mertk (Mer) is in a receptor tyrosine kinase family with Axl and Tyro-3, and all 3 receptors share the Gas6 ligand. Mer regulates macrophage activation, promotes apoptotic cell engulfment, and supports platelet aggregation and clot stability in vivo. We have found that the membrane-bound Mer protein is cleaved in the extracellular domain via a metalloproteinase. The cleavage results in the production of a soluble Mer protein released in a constitutive manner from cultured cells. Significant amounts of the soluble Mer protein were also detected in human plasma, suggesting its physiologic relevance. Cleavage of Mer was enhanced by treatment with LPS and PMA and was specifically inhibited by a tumor necrosis factor alpha-converting enzyme metalloproteinase inhibitor. As a decoy receptor for Gas6, soluble Mer prevented Gas6-mediated stimulation of membrane-bound Mer. The inhibition of Gas6 activity by soluble Mer led to defective macrophage-mediated engulfment of apoptotic cells. Furthermore, soluble Mer decreased platelet aggregation in vitro and prevented fatal collagen/epinephrine-induced thromboembolism in mice, suggesting a potential therapeutic use for soluble Mer in the treatment of clotting disorders.

Differential regulation of soluble and membrane CD40L proteins in T cells

CD40 ligand is an important immunoregulatory protein expressed by T cells. This protein exists as two isoforms, a membrane glycoprotein and a truncated soluble form. Here we demonstrate that membrane and soluble CD40L (sCD40L) are differentially regulated depending upon the activation stimulus. In T cell receptor activated cells, both membrane and sCD40L proteins are expressed and CD28 costimulation further increases their expression. The dissection of TCR generated signals into calcium and PKC-dependent pathways demonstrates that calcium is sufficient to induce membrane CD40L yet insufficient for sCD40L. In contrast, sCD40L is preferentially induced by PKC. Moreover, sCD40L production is blocked by Zn(2+)-dependent metalloproteinase inhibitors while membrane CD40L is concurrently increased. This profile suggests the potential involvement of the ADAM-10 protease which was subsequently shown to cleave membrane CD40L to generate sCD40L. Given the role of sCD40L in numerous disease pathologies and its ability to activate proximal and distal immune responses, the regulated cleavage of CD40L may likely contribute to disease mechanisms.

Protein interacting with C-kinase 1/protein kinase Calpha-mediated endocytosis converts netrin-1-mediated repulsion to attraction

In vertebrates, the receptor families deleted in colorectal cancer (DCC) and UNC5 mediate responses to the bifunctional guidance cue netrin-1. DCC mediates attraction, whereas a complex of DCC and UNC5 mediates repulsion. Thus, a primary determinant of the responsiveness of an axon to netrin-1 is the presence or absence of UNC5 family members on the cell surface. Currently, little is known about the role of receptor trafficking in regulating neuronal responses to netrin-1. We show that protein interacting with C-kinase 1 (PICK1) recruits activated protein kinase Calpha (PKCalpha) to MycUNC5A at the plasma membrane, stimulating its endocytosis. We identify two PKCalpha phosphorylation sites at serines 408 and 587, as well as dileucine internalization motifs, which are required for this endocytosis. We find that PKCalpha-stimulated internalization of UNC5A alters the functional response of developing hippocampal axons to netrin-1, preventing UNC5A-mediated growth cone collapse and converting netrin-1-stimulated chemorepulsion to attraction. To address whether this conversion in axonal response occurs in neurons expressing endogenous levels of UNC5, we show that mouse cerebellar granule axons exhibit chemorepulsion in a netrin-1 gradient and that this chemorepulsion is converted to chemoattraction after PKCalpha activation. We demonstrate that this repulsion depends on UNC5A because Unc5a-/- axons are not repelled and show this conversion depends on PICK1 because PICK1-/- axons are not converted to chemoattraction after PKCalpha activation. Together, these data provide a potential mechanism to explain how developing neurons alter their responsiveness to netrin-1 at intermediate choice points as they navigate to their targets.

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

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

Tmem27: a cleaved and shed plasma membrane protein that stimulates pancreatic beta cell proliferation

The signals and molecular mechanisms that regulate the replication of terminally differentiated beta cells are unknown. Here, we report the identification and characterization of transmembrane protein 27 (Tmem27, collectrin) in pancreatic beta cells. Expression of Tmem27 is reduced in Tcf1(-/-) mice and is increased in islets of mouse models with hypertrophy of the endocrine pancreas. Tmem27 forms dimers and its extracellular domain is glycosylated, cleaved and shed from the plasma membrane of beta cells. This cleavage process is beta cell specific and does not occur in other cell types. Overexpression of full-length Tmem27, but not the truncated or soluble protein, leads to increased thymidine incorporation, whereas silencing of Tmem27 using RNAi results in a reduction of cell replication. Furthermore, transgenic mice with increased expression of Tmem27 in pancreatic beta cells exhibit increased beta cell mass. Our results identify a pancreatic beta cell transmembrane protein that regulates cell growth of pancreatic islets.

Janus kinase 2 enhances the stability of the mature growth hormone receptor

The abundance of surface GH receptor (GHR) is an important determinant of cellular GH sensitivity and is regulated at both transcriptional and posttranscriptional levels. In previous studies of GHR-expressing Janus kinase 2 (JAK2)-deficient human fibrosarcoma cells (gamma2A-GHR), we demonstrated that stable transfection with JAK2 resulted in increased steady-state levels of mature GHR (endoH-resistant; relative molecular mass, 115-140 kDa) relative to precursor GHR (endoH-sensitive; relative molecular mass, 100 kDa). We now examine further the effects of JAK2 on GHR trafficking by comparing gamma2A-GHR to gamma2A-GHR cells stably reconstituted with JAK2 (C14 cells). In the presence of JAK2, GHR surface expression was increased, as assessed by surface biotinylation, 125I-labeled human GH cell surface binding, and immunofluorescence microscopy assays. Although the absence of JAK2 precluded GH-stimulated signaling, GH-induced GHR disulfide linkage (a proxy for the GH-induced conformational changes in the GHR dimer) proceeded independent of JAK2 expression, indicating that the earliest steps in GH-induced GHR triggering are not prevented by the absence of JAK2. RNA interference-mediated knockdown of JAK2 in C14 cells resulted in a decreased mature to precursor ratio, supporting a primary role for JAK2 either in enhancing GHR biogenesis or dampening mature GHR degradation. To address these potential mechanisms, metabolic pulse-chase labeling experiments and experiments in which the fate of previously synthesized GHR was followed by anti-GHR immunoblotting after cycloheximide treatment (cycloheximide chase experiments) were performed. These indicated that the presence of JAK2 conferred modest enhancement (1.3- to 1.5-fold) in GHR maturation but substantially prolonged the t1/2 of the mature GHR, suggesting a predominant effect on mature GHR stability. Cycloheximide chase experiments with metalloprotease, proteasome, and lysosome inhibitors indicated that the enhanced stability of mature GHR conferred by JAK2 is not related to effects on constitutive receptor metalloproteolysis but rather is a result of reduced constitutive endosomal/lysosomal degradation of the mature GHR. These results are discussed in the context of emerging information on how JAK-family members modulate surface expression of other cytokine receptors.

Prolactin and growth hormone signaling

Prolactin (PRL) and growth hormone (GH) act by way of their receptors as either hormones (systemically) or cytokines (locally). The Jak2/Stat5 pathway is the principal route by which PRL/GH activate target genes. The availability of knockout mice for each member of this signaling cascade has provided opportunities to understand their unique interactions. Jak2 is important in alternative signal transduction schema such as the MAP kinase and PI3K/Akt pathways. The putative Jak2/RUSH pathway is based on the fact that RUSH mediates the ability of PRL to augment progesterone-dependent gene transcription. New evidence shows that suppressors, regulators, and degraders control Jak2/Stat5. This review focuses on the most recent advances in the field of PRL/GH signal transduction.

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

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

Negative regulation of growth hormone receptor signaling

GH has been of significant scientific interest for decades because of its capacity to dramatically change physiological growth parameters. Furthermore, GH interacts with a range of other hormonal pathways and is an established pharmacological agent for which novel therapeutical applications can be foreseen. It is easy to see the requirement for a number of postreceptor mechanisms to regulate and control target tissue sensitivity to this versatile hormone. In recent years, some of the components that take part in the down-regulatory mechanism targeting the activated GH receptor (GHR) have been defined, and the physiological significance of some of these key components has begun to be characterized. Down-regulation of the GHR is achieved through a complex mechanism that involves rapid ubiquitin-dependent endocytosis of the receptor, the action of tyrosine phosphatases, and the degradation by the proteasome. The suppressors of cytokine signaling (SOCS) protein family, particularly SOCS2, plays an important role in regulating GH actions. The aim of this review is to summarize collected knowledge, including very recent findings, regarding the intracellular mechanisms responsible for the GHR signaling down-regulation. Insights into these mechanisms can be of relevance to several aspects of GH research. It can help to understand growth-related disease conditions, to explain GH resistance, and may be used to develop pharmaceuticals that enhance some the beneficial actions of endogenously secreted GH in a tissue-specific manner.

A new release on life: emerging concepts in proteolysis and parasite invasion

Cell invasion by apicomplexan pathogens such as the malaria parasite and Toxoplasma is accompanied by extensive proteolysis of zoite surface proteins (ZSPs) required for attachment and penetration. Although there is still little known about the proteases involved, a conceptual framework is emerging for the roles of proteolysis in cell invasion. Primary processing of ZSPs, which includes the trimming of terminal peptides or segmentation into multiple fragments, is proposed to activate these adhesive ligands for tight binding to host receptors. Secondary processing, which occurs during penetration, results in the shedding of ZSPs by one of two mechanistically distinct ways, shaving or capping. Resident surface proteins are typically shaved from the surface whereas adhesive ligands mobilized from intracellular secretory vesicles are capped to the posterior end of the parasite before being shed during the final steps of penetration. Intriguingly, recent studies have revealed that ZSPs can be released either by being cleaved adjacent to the membrane anchor or actually within the membrane itself. Mounting evidence suggests that intramembrane cleavage is catalysed by one or more integral membrane serine proteases of the Rhomboid family and we propose that several malaria adhesive ligands may be potential substrates for these enzymes. We also discuss the evidence that the key reason for ZSP shedding during invasion is to break the connection between parasite surface ligands and host receptors. The sequential proteolytic events associated with invasion by pathogenic protozoa may represent vulnerable pathways for the future development of synergistic anti-protozoal therapies.

Growth Hormone Receptor Is a Target for Presenilin-dependent γ-Secretase Cleavage

Growth hormone receptor (GHR) is a cytokine receptor superfamily member that binds growth hormone (GH) via its extracellular domain and signals via interaction of its cytoplasmic domain with JAK2 and other signaling molecules. GHR is a target for inducible metalloprotease-mediated cleavage in its perimembranous extracellular domain, a process that liberates the extracellular domain as the soluble GH-binding protein and leaves behind a cell-associated GHR remnant protein containing the transmembrane and cytoplasmic domains. GHR metalloproteolysis can be catalyzed by tumor necrosis factor-alpha-converting enzyme (ADAM-17) and is associated with down-modulation of GH signaling. We now study the fate of the GHR remnant protein. By anti-GHR cytoplasmic domain immunoblotting, we observed that the remnant induced in response to phorbol ester or platelet-derived growth factor has a reliable pattern of appearance and disappearance in both mouse preadipocytes endogenously expressing GHR and transfected fibroblasts expressing rabbit GHR. Lactacystin, a specific proteasome inhibitor, did not appreciably change the time course of remnant appearance or clearance but allowed detection of the GHR stub, a receptor fragment slightly smaller than the remnant but containing the C terminus of the remnant (receptor cytoplasmic domain). In contrast, MG132, another (less specific) proteasome inhibitor, strongly inhibited remnant clearance and prevented stub appearance. Inhibitors of gamma-secretase, an aspartyl protease, also prevented the appearance of the stub, even in the presence of lactacystin, and concomitantly inhibited remnant clearance in the same fashion as MG132. In addition, mouse embryonic fibroblasts derived from presenilin 1 and 2 (PS1/2) knockouts recapitulated the gamma-secretase inhibitor studies, as compared with their littermate controls (PS1/2 wild type). Confocal microscopy indicated that the GHR cytoplasmic domain became localized to the nucleus in a fashion dependent on PS1/2 activity. These data indicate that the GHR is subject to sequential proteolysis by metalloprotease and gamma-secretase activities and may suggest GH-independent roles for the GHR.

Regulation of platelet membrane levels of glycoprotein VI by a platelet-derived metalloproteinase

Thrombosis can be initiated when activated platelets adhere to injured blood vessels via the interaction of subendothelial collagen with its platelet receptor, glycoprotein (GP) VI. Here we observed that incubation of platelets with convulxin, collagen, or collagen-related peptide (CRP) resulted in GPVI signaling-dependent loss of surface GPVI and the appearance of an approximately 55-kDa soluble fragment of GPVI as revealed by immunoblotting. Ethylenediaminetetraacetic acid (EDTA) or GM6001 (a metalloproteinase inhibitor with broad specificity) prevented this loss. In other receptor systems, calmodulin binding to membrane-proximal cytoplasmic sequences regulates metalloproteinase-mediated ectodomain shedding. In this regard, we have previously shown that calmodulin binds to a positively charged, membrane-proximal sequence within the cytoplasmic tail of GPVI. Incubation of platelets with calmodulin inhibitor W7 (150 μM) resulted in a time-dependent loss of GPVI from the platelet surface. Both EDTA and GM6001 prevented this loss. Surface plasmon resonance demonstrated that W7 specifically blocked the association of calmodulin with an immobilized synthetic peptide corresponding to the calmodulin-binding sequence of GPVI. These findings suggest that disruption of calmodulin binding to receptor cytoplasmic tails by agonist binding to the receptor triggers metalloproteinase-mediated loss of GPVI from the platelet surface. This process may represent a potential mechanism to regulate GPVI-dependent platelet adhesion.

Estrogen regulation of growth hormone action

GH plays a pivotal role in regulating body growth and development, which is modulated by sex steroids. A close interplay between estrogen and GH leads to attainment of gender-specific body composition during puberty. The physiological basis of the interaction is not well understood. Most previous studies have focused on the effects of estrogen on GH secretion. There is also strong evidence that estrogen modulates GH action independent of secretion. Oral but not transdermal administration of estrogen impairs the metabolic action of GH in the liver, causing a fall in IGF-I production and fat oxidation. This results in a loss of lean tissue and a gain of body fat in postmenopausal women and an impairment of GH effect in hypopituitary women on GH replacement. The negative metabolic sequelae are potentially important because of the widespread use of oral estrogen and estrogen-related compounds. Estrogen affects GH action at the level of receptor expression and signaling. More recently, estrogen has been shown to inhibit Janus kinase/signal transducer and activator of transcription signaling by GH via the induction of suppressor of cytokine signaling-2, a protein inhibitor for cytokine signaling. This represents a novel paradigm of steroid regulation of cytokine receptors and is likely to have significance for a diverse range of cytokine function.

Increased expression of ADAM family members in human breast cancer and breast cancer cell lines

PURPOSE

ADAMs (A Disintegrin and Metalloprotease) are multifunctional, membrane-bound cell surface glycoproteins, which have numerous functions in cell growth, differentiation, and motility. We wished to investigate the expression of ADAM 9, 10, 12, 15, and in human breast cancer.

METHODS

Expression of ADAMs was determined in breast cancer specimens and the corresponding non-neoplastic breast tissue from 24 patients, and in the MCF-7 and MDA-MB 453 breast cancer cell lines via quantitative RT-PCR and immunohistochemistry. The effects of anti-ADAM antibodies on cell proliferation were assessed by measuring DNA-synthesis.

RESULTS

Breast cancer tissue samples showed increased mRNA expression of ADAM 9, 12, and 17, whereas ADAM 10 and 15 were not differently expressed. Protein expression was studied by immunohistochemistry. All ADAMs were expressed in MCF-7 and MDA-MB453 cell lines, with the highest expression levels being observed for ADAM 9, 12, and 17. Application of anti-ADAM 15 and anti-ADAM 17 antibodies significantly inhibited the proliferation of both MCF-7 and MDA-MB453 breast cancer cell lines. In contrast, the growth of MCF-7 cells appeared to be stimulated by the administration of anti-ADAM 12 antibody.

CONCLUSION

The results of this study suggest that ADAMs are differentially expressed in human breast cancer and are capable of modulating tumour cell growth.

ADAM17 mRNA expression and pathological features of hepatocellular carcinoma

AIM: To study the expression of a disintegrin and metalloproteinase 17 (ADAM17) mRNA in hepatocellular carcinoma (HCC) and to evaluate the relationship between ADAM17 mRNA expression and clinicopathological features of HCC.

METHODS: Hepatocellular carcinomas (HCC) from 31 cases were divided into small HCC (SHCC), nodular HCC (NHCC) and solitary large HCC (SLHCC) according to tumor diameter and the number of nodes. ADAM17 mRNA expressions were compared among those groups by means of semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). The relationship between ADAM17 mRNA expression level and clinicopathological features of HCC was evaluated.

RESULTS: NHCC had lower differentiation and was more frequently of microvascular invasion (10/12) than SHCC (3/11) and SLHCC (3/8) (P < 0.05), but no statistical difference was observed between SHCC and SLHCC comparing their clinicopathological features. ADAM17 mRNA expression was detected in 77.4% (24/31) of HCC tissues and was significantly higher than that in paired non-cancerous liver tissues in which only 35.5% (11/31) of the samples were detected of the expression (P < 0.05). The expression of ADAM17 mRNA was much higher in NHCC than in SHCC and SLHCC (P < 0.05), while no significant difference was discovered between SHCC and SLHCC. The quantities of ADAM17 mRNA were significantly higher in poorly differentiated HCC than in well or moderately differentiated HCC, but no statistical difference was found concerning liver cirrhosis, tumor capsule formation or microvascular invasion of the cancer.

CONCLUSION: The increased expression of ADAM17 may play a key role in the development of HCC. The expression levels of ADAM17 mRNA varied among different pathological types of HCC. Lower mRNA expression of ADAM17 mRNA in SLHCC may be associated with the better molecular pathological features of SLHCC.