Direct activation of TACE-mediated ectodomain shedding by p38 MAP kinase regulates EGF receptor-dependent cell proliferation. Mol Cell. 2010;37:551-566. [PMID: 20188673 DOI: 10.1016/j.molcel.2010.01.034]

Molecular Pathways: Receptor Ectodomain Shedding in Treatment, Resistance, and Monitoring of Cancer

Proteases known as sheddases cleave the extracellular domains of their substrates from the cell surface. The A Disintegrin and Metalloproteinases ADAM10 and ADAM17 are among the most prominent sheddases, being widely expressed in many tissues, frequently overexpressed in cancer, and promiscuously cleaving diverse substrates. It is increasingly clear that the proteolytic shedding of transmembrane receptors impacts pathophysiology and drug response. Receptor substrates of sheddases include the cytokine receptors TNFR1 and IL6R; the Notch receptors; type-I and -III TGFβ receptors; receptor tyrosine kinases (RTK) such as HER2, HER4, and VEGFR2; and, in particular, MET and TAM-family RTKs AXL and Mer (MerTK). Activation of receptor shedding by mechanical cues, hypoxia, radiation, and phosphosignaling offers insight into mechanisms of drug resistance. This particularly holds for kinase inhibitors targeting BRAF (such as vemurafenib and dabrafenib) and MEK (such as trametinib and cobimetinib), along with direct sheddase inhibitors. Receptor proteolysis can be detected in patient fluids and is especially relevant in melanoma, glioblastoma, lung cancer, and triple-negative breast cancer where RTK substrates, MAPK signaling, and ADAMs are frequently dysregulated. Translatable strategies to exploit receptor shedding include combination kinase inhibitor regimens, recombinant decoy receptors based on endogenous counterparts, and, potentially, immunotherapy. Clin Cancer Res; 23(3); 623-9. ©2016 AACR.

Growth factor and co-receptor release by structural regulation of substrate metalloprotease accessibility

Release of cytokines, growth factors and other life-essential molecules from precursors by a-disintegrin-and-metalloproteases (ADAMs) is regulated with high substrate-specificity. We hypothesized that this is achieved by cleavage-regulatory intracellular-domain (ICD)-modifications of the precursors. We show here that cleavage-stimuli-induced specific ICD-modifications cause structural substrate changes that enhance ectodomain sensitivity of neuregulin-1 (NRG1; epidermal-growth-factor) or CD44 (receptor-tyrosine-kinase (RTK) co-receptor) to chymotrypsin/trypsin or soluble ADAM. This inside-out signal transfer required substrate homodimerization and was prevented by cleavage-inhibitory ICD-mutations. In chimeras, regulation could be conferred to a foreign ectodomain, suggesting a common higher-order structure. We predict that substrate-specific protease-accessibility-regulation controls release of numerous ADAM substrates.

Epidermal growth factor receptor signalling regulates granulocyte-macrophage colony-stimulating factor production by airway epithelial cells and established allergic airway disease

BACKGROUND

Airway epithelial cells (AEC) are increasingly recognized as a major signalling centre in the pathogenesis of allergic asthma. A previous study demonstrated that epithelial growth factor receptor (EGFR) signalling in AEC regulated key features of allergic airway disease. However, it is unclear what mediators are regulated by EGFR signalling in AEC, although the production of the pro-inflammatory cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) is EGFR dependent in keratinocytes.

OBJECTIVES

To determine whether EGFR signalling regulates GM-CSF production by human AEC downstream of the clinically relevant mediators house dust mite (HDM) and interleukin (IL)-17A and in a mouse model of established allergic asthma.

METHODS

EGFR inhibitors were used to determine whether EGFR signalling regulates GM-CSF production by cultured human AEC in response to HDM and IL-17A. The roles of EGFR ligands, p38 mitogen-activated protein kinase (MAPK) and tumour necrosis factor-alpha (TNF-α) converting enzyme (TACE) were also assessed. To determine whether EGFR regulates GM-CSF as well as key asthma characteristics in vivo, mice were chronically exposed to HDM to establish allergic airway disease and then treated with the EGFR inhibitor Erlotinib.

RESULTS

EGFR inhibition reduced HDM and IL-17A induced GM-CSF production in a dose-dependent manner in cultured human AEC. GM-CSF production also required amphiregulin, p38 MAPK signalling and protease/TACE activity. In mice with established allergic airway disease, EGFR inhibition reduced levels of GM-CSF and TNF-α, as well as airway hyperreactivity, cellular inflammation, smooth muscle thickening and goblet cell metaplasia without changes in IgE and Th1, Th2 and Th17 cytokines.

CONCLUSIONS AND CLINICAL RELEVANCE

Results link HDM, IL-17A, amphiregulin, EGFR and GM-CSF in a mechanistic pathway in AEC and demonstrate that EGFR regulates GM-CSF production and the severity of established disease in a clinically relevant asthma model. These results identify the EGFR→GM-CSF axis as a target for therapeutic development.

A Disintegrin and Metalloprotease 17 in the Cardiovascular and Central Nervous Systems

ADAM17 is a metalloprotease and disintegrin that lodges in the plasmatic membrane of several cell types and is able to cleave a wide variety of cell surface proteins. It is somatically expressed in mammalian organisms and its proteolytic action influences several physiological and pathological processes. This review focuses on the structure of ADAM17, its signaling in the cardiovascular system and its participation in certain disorders involving the heart, blood vessels, and neural regulation of autonomic and cardiovascular modulation.

Control of ADAM17 activity by regulation of its cellular localisation

An important, irreversible step in many signalling pathways is the shedding of membrane-anchored proteins. A Disintegrin And Metalloproteinase (ADAM) 17 is one of the major sheddases involved in a variety of physiological and pathophysiological processes including regeneration, differentiation, and cancer progression. This central role in signalling implies that ADAM17 activity has to be tightly regulated, including at the level of localisation. Most mature ADAM17 is localised intracellularly, with only a small amount at the cell surface. We found that ADAM17 is constitutively internalised by clathrin-coated pits and that physiological stimulators such as GPCR ligands induce ADAM17-mediated shedding, but do not alter the cell-surface abundance of the protease. In contrast, the PKC-activating phorbol ester PMA, often used as a strong inducer of ADAM17, causes not only proteolysis by ADAM17 but also a rapid increase of the mature protease at the cell surface. This is followed by internalisation and subsequent degradation of the protease. Eventually, this leads to a substantial downregulation of mature ADAM17. Our results therefore imply that physiological activation of ADAM17 does not rely on its relocalisation, but that PMA-induced PKC activity drastically dysregulates the localisation of ADAM17.

Role of the ACE2/Angiotensin 1-7 Axis of the Renin-Angiotensin System in Heart Failure

Heart failure (HF) remains the most common cause of death and disability, and a major economic burden, in industrialized nations. Physiological, pharmacological, and clinical studies have demonstrated that activation of the renin-angiotensin system is a key mediator of HF progression. Angiotensin-converting enzyme 2 (ACE2), a homolog of ACE, is a monocarboxypeptidase that converts angiotensin II into angiotensin 1-7 (Ang 1-7) which, by virtue of its actions on the Mas receptor, opposes the molecular and cellular effects of angiotensin II. ACE2 is widely expressed in cardiomyocytes, cardiofibroblasts, and coronary endothelial cells. Recent preclinical translational studies confirmed a critical counter-regulatory role of ACE2/Ang 1-7 axis on the activated renin-angiotensin system that results in HF with preserved ejection fraction. Although loss of ACE2 enhances susceptibility to HF, increasing ACE2 level prevents and reverses the HF phenotype. ACE2 and Ang 1-7 have emerged as a key protective pathway against HF with reduced and preserved ejection fraction. Recombinant human ACE2 has been tested in phase I and II clinical trials without adverse effects while lowering and increasing plasma angiotensin II and Ang 1-7 levels, respectively. This review discusses the transcriptional and post-transcriptional regulation of ACE2 and the role of the ACE2/Ang 1-7 axis in cardiac physiology and in the pathophysiology of HF. The pharmacological and therapeutic potential of enhancing ACE2/Ang 1-7 action as a novel therapy for HF is highlighted.

PPM1A silences cytosolic RNA sensing and antiviral defense through direct dephosphorylation of MAVS and TBK1

Cytosolic RNA sensing is a prerequisite for initiation of innate immune response against RNA viral pathogens. Signaling through RIG-I (retinoic acid-inducible gene I)-like receptors (RLRs) to TBK1 (Tank-binding kinase 1)/IKKε (IκB kinase ε) kinases is transduced by mitochondria-associated MAVS (mitochondrial antiviral signaling protein). However, the precise mechanism of how MAVS-mediated TBK1/IKKε activation is strictly controlled still remains obscure. We reported that protein phosphatase magnesium-dependent 1A (PPM1A; also known as PP2Cα), depending on its catalytic ability, dampened the RLR-IRF3 (interferon regulatory factor 3) axis to silence cytosolic RNA sensing signaling. We demonstrated that PPM1A was an inherent partner of the TBK1/IKKε complex, targeted both MAVS and TBK1/IKKε for dephosphorylation, and thus disrupted MAVS-driven formation of signaling complex. Conversely, a high level of MAVS can dissociate the TBK1/PPM1A complex to override PPM1A-mediated inhibition. Loss of PPM1A through gene ablation in human embryonic kidney 293 cells and mouse primary macrophages enabled robustly enhanced antiviral responses. Consequently, Ppm1a(-/-) mice resisted to RNA virus attack, and transgenic zebrafish expressing PPM1A displayed profoundly increased RNA virus vulnerability. These findings identify PPM1A as the first known phosphatase of MAVS and elucidate the physiological function of PPM1A in antiviral immunity on whole animals.

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.

Mst1 shuts off cytosolic antiviral defense through IRF3 phosphorylation

Here, Meng et al. investigated how interferon regulatory factor 3 (IRF3) activation, a key signal mediator/transcriptional factor of the antiviral-sensing pathway, is regulated. They demonstrate that Mst1, a stress response kinase, represses cytosolic antiviral sensing and defense through the repression of RNA virus-induced activation of TBK1 and interference with the IRF3 homodimerization and chromatin binding via direct phosphorylation of IRF3 Thr253 and Thr75 residues.

Cytosolic RNA/DNA sensing elicits primary defense against viral pathogens. Interferon regulatory factor 3 (IRF3), a key signal mediator/transcriptional factor of the antiviral-sensing pathway, is indispensible for interferon production and antiviral defense. However, how the status of IRF3 activation is controlled remains elusive. Through a functional screen of the human kinome, we found that mammalian sterile 20-like kinase 1 (Mst1), but not Mst2, profoundly inhibited cytosolic nucleic acid sensing. Mst1 associated with IRF3 and directly phosphorylated IRF3 at Thr75 and Thr253. This Mst1-mediated phosphorylation abolished activated IRF3 homodimerization, its occupancy on chromatin, and subsequent IRF3-mediated transcriptional responses. In addition, Mst1 also impeded virus-induced activation of TANK-binding kinase 1 (TBK1), further attenuating IRF3 activation. As a result, Mst1 depletion or ablation enabled an enhanced antiviral response and defense in cells and mice. Therefore, the identification of Mst1 as a novel physiological negative regulator of IRF3 activation provides mechanistic insights into innate antiviral defense and potential antiviral prevention strategies.

The ADAMs family of proteases as targets for the treatment of cancer

The ADAMs (a disintegrin and metalloproteases) are transmembrane multidomain proteins implicated in multiple biological processes including proteolysis, cell adhesion, cell fusion, cell proliferation and cell migration. Of these varied activities, the best studied is their role in proteolysis. However, of the 22 ADAMs believed to be functional in humans, only approximately a half possess matrix metalloproteinase (MMP)-like protease activity. In contrast to MMPs which are mostly implicated in the degradation of extracellular matrix proteins, the main ADAM substrates are the ectodomains of type I and type II transmembrane proteins. These include growth factor/cytokine precursors, growth factor/cytokine receptors and adhesion proteins. Recently, several different ADAMs, especially ADAM17, have been shown to play a role in the development and progression of multiple cancer types. Consistent with this role in cancer, targeting ADAM17 with either low molecular weight inhibitors or monoclonal antibodies was shown to have anti-cancer activity in multiple preclinical systems. Although early phase clinical trials have shown no serious side effects with a dual ADAM10/17 low molecular weight inhibitor, the consequences of long-term treatment with these agents is unknown. Furthermore, efficacy in clinical trials remains to be shown.

Endogenous Transmembrane TNF-Alpha Protects Against Premature Senescence in Endothelial Colony Forming Cells

RATIONALE

Transmembrane tumor necrosis factor-α (tmTNF-α) is the prime ligand for TNF receptor 2, which has been shown to mediate angiogenic and blood vessel repair activities in mice. We have previously reported that the angiogenic potential of highly proliferative endothelial colony-forming cells (ECFCs) can be explained by the absence of senescent cells, which in mature endothelial cells occupy >30% of the population, and that exposure to a chronic inflammatory environment induced premature, telomere-independent senescence in ECFCs.

OBJECTIVE

The goal of this study was to determine the role of tmTNF-α in the proliferation of ECFCs.

METHODS AND RESULTS

Here, we show that tmTNF-α expression on ECFCs selects for higher proliferative potential and when removed from the cell surface promotes ECFC senescence. Moreover, the induction of premature senescence by chronic inflammatory conditions is blocked by inhibition of tmTNF-α cleavage. Indeed, the mechanism of chronic inflammation-induced premature senescence involves an abrogation of tmTNF/TNF receptor 2 signaling. This process is mediated by activation of the tmTNF cleavage metalloprotease TNF-α-converting enzyme via p38 MAP kinase activation and its concurrent export to the cell surface by means of increased iRhom2 expression.

CONCLUSIONS

Thus, we conclude that tmTNF-α on the surface of highly proliferative ECFCs plays an important role in the regulation of their proliferative capacity.

Reduced Proteolytic Shedding of Receptor Tyrosine Kinases Is a Post-Translational Mechanism of Kinase Inhibitor Resistance

Kinase inhibitor resistance often involves upregulation of poorly understood "bypass" signaling pathways. Here, we show that extracellular proteomic adaptation is one path to bypass signaling and drug resistance. Proteolytic shedding of surface receptors, which can provide negative feedback on signaling activity, is blocked by kinase inhibitor treatment and enhances bypass signaling. In particular, MEK inhibition broadly decreases shedding of multiple receptor tyrosine kinases (RTK), including HER4, MET, and most prominently AXL, an ADAM10 and ADAM17 substrate, thus increasing surface RTK levels and mitogenic signaling. Progression-free survival of patients with melanoma treated with clinical BRAF/MEK inhibitors inversely correlates with RTK shedding reduction following treatment, as measured noninvasively in blood plasma. Disrupting protease inhibition by neutralizing TIMP1 improves MAPK inhibitor efficacy, and combined MAPK/AXL inhibition synergistically reduces tumor growth and metastasis in xenograft models. Altogether, extracellular proteomic rewiring through reduced RTK shedding represents a surprising mechanism for bypass signaling in cancer drug resistance.

SIGNIFICANCE

Genetic, epigenetic, and gene expression alterations often fail to explain adaptive drug resistance in cancer. This work presents a novel post-translational mechanism of such resistance: Kinase inhibitors, particularly targeting MAPK signaling, increase tumor cell surface receptor levels due to widely reduced proteolysis, allowing tumor signaling to circumvent intended drug action.

Genomic characterization of esophageal squamous cell carcinoma: Insights from next-generation sequencing

Two major types of cancer occur in the esophagus: squamous cell carcinoma, which is associated with chronic smoking and alcohol consumption, and adenocarcinoma, which typically arises in gastric reflux-associated Barrett's esophagus. Although there is increasing incidence of esophageal adenocarcinoma in Western counties, esophageal squamous cell carcinoma (ESCC) accounts for most esophageal malignancies in East Asia, including China and Japan. Technological advances allowing for massively parallel, high-throughput next-generation sequencing (NGS) of DNA have enabled comprehensive characterization of somatic mutations in large numbers of tumor samples. Recently, several studies were published in which whole exome or whole genome sequencing was performed in ESCC tumors and compared with matched normal DNA. Mutations were validated in several genes, including in TP53, CDKN2A, FAT1, NOTCH1, PIK3CA, KMT2D and NFE2L2, which had been previously implicated in ESCC. Several new recurrent alterations have also been identified in ESCC. Combining the clinicopathological characteristics of patients with information obtained from NGS studies may lead to the development of effective diagnostic and therapeutic approaches for ESCC. As this research becomes more prominent, it is important that gastroenterologist become familiar with the various NGS technologies and the results generated using these methods. In the present study, we describe recent research approaches using NGS in ESCC.

Helicobacter pylori-elicited induction in gastric mucosal matrix metalloproteinase-9 (MMP-9) release involves ERK-dependent cPLA2 activation and its recruitment to the membrane-localized Rac1/p38 complex

Matrix metalloproteinases (MMPs) are a family of endopeptidases implicated in a wide rage of degenerative and inflammatory diseases, including Helicobacter pylori-associated gastritis, and gastric and duodenal ulcer. As gastric mucosal inflammatory responses to H. pylori are characterized by the rise in MMP-9 production, as well as the induction in mitogen-activated protein kinase (MAPK) and Rac1 activation, we investigated the role of Rac1/MAPK in the processes associated with the release of MMP-9. We show that H. pylori LPS-elicited induction in gastric mucosal MMP-9 release is associated with MAPK, ERK and p38 activation, and occurs with the involvement of Rac1 and cytosolic phospholipase A2 (cPLA2). Further, we demonstrate that the LPS-induced MMP-9 release requires ERK-mediated phosphorylation of cPLA2 on Ser(505) that is essential for its membrane localization with Rac1, and that this process necessitates p38 participation. Moreover, we reveal that the activation and membrane translocation of p38 to the Rac1-GTP complex plays a pivotal role in cPLA2-dependent enhancement in MMP-9 release. Hence, our findings provide a strong evidence for the role of ERK/cPLA2 and Rac1/p38/cPLA2 cascade in H. pylori LPS-induced up-regulation in gastric mucosal MMP-9 release.

Release of Matrix Metalloproteinases-2 and 9 by S-Nitrosylated Caveolin-1 Contributes to Degradation of Extracellular Matrix in tPA-Treated Hypoxic Endothelial Cells

Intracranial hemorrhage remains the most feared complication in tissue plasminogen activator (tPA) thrombolysis for ischemic stroke. However, the underlying molecular mechanisms are still poorly elucidated. In this study, we reported an important role of caveolin-1 (Cav-1) s-nitrosylation in matrix metalloproteinase (MMP)-2 and 9 secretion from tPA-treated ischemic endothelial cells. Brain vascular endothelial cells (bEND3) were exposed to oxygen-glucose deprivation (OGD) for 2 h before adding recombinant human tPA for 6 h. This treatment induced a significant increase of MMP2 and 9 in the media of bEND3 cells and a simultaneous degradation of fibronectin and laminin β-1, the two main components of extracellular matrix (ECM). Inhibition of MMP2 and 9 with SB-3CT completely blocked the degradation of fibronectin and laminin β-1. ODG+tPA treatment led to Cav-1 shedding from bEND3 cells into the media. Notably, OGD triggered nitric oxide (NO) production and S-nitrosylationof Cav-1 (SNCav-1). Meanwhile tPA induced activation of ERK signal pathway and stimulates the secretion of SNCav-1. Pretreatment of bEND3 cells with C-PTIO (a NO scavenger) or U0126 (a specific ERK inhibitor) significantly reduced OGD-induced S-nitrosylation of Cav-1 in cells and blocked the secretion of Cav-1 and MMP2 and 9 into the media as well as the degradation of fibronectin and laminin β-1 in OGD and tPA-treated cells. These data indicate that OGD-triggered Cav-1 S-nitrosylation interacts with tPA-induced ERK activation to augment MMP2 and 9 secretion and subsequent ECM degradation, which may account for the exacerbation of ischemic blood brain barrier damage following tPA thrombolysis for ischemic stroke.

Regulation of TGF-β Receptors

In cells responding to extracellular polypeptide ligands, regulatory mechanisms at the level of cell surface receptors are increasingly seen to define the nature of the ligand-induced signaling responses. Processes that govern the levels of receptors at the plasma membrane, including posttranslational modifications, are crucial to ensure receptor function and specify the downstream signals. Indeed, extracellular posttranslational modifications of the receptors help define stability and ligand binding, while intracellular modifications mediate interactions with signaling mediators and accessory proteins that help define the nature of the signaling response. The use of various molecular biology and biochemistry techniques, based on chemical crosslinking, e.g., biotin or radioactive labeling, immunofluorescence to label membrane receptors and flow cytometry, allows for quantification of changes of cell surface receptor presentation. Here, we discuss recent progress in our understanding of the regulation of TGF-β receptors, i.e., the type I (TβRI) and type II (TβRII) TGF-β receptors, and describe basic methods to identify and quantify TGF-β cell surface receptors.

Helicobacter pylori-induced gastric mucosal TGF-α ectodomain shedding and EGFR transactivation involves Rac1/p38 MAPK-dependent TACE activation

Infection of gastric mucosa by H. pylori triggers a pattern of inflammatory responses characterized by the rise in proinflammatory cytokine production, up-regulation in mitogen-activated protein kinase (MAPK) cascade, and the induction in epidermal growth factor receptor (EGFR) activation. In this study, we report on the role of MAPK/p38 and Rac1 in the regulation of H. pylori LPS-induced TGF-α ectodomain shedding and EGFR transactivation. We show that stimulation of gastric mucosal cells with the LPS, reflected in p38 phosphorylation, guanine nucleotide exchange factor Dock180 activation and the rise in Rac1-GTP level, is accompanied by the activation of membrane-associated metalloprotease, (TACE) also known as ADAM17, responsible for soluble TGF-α release. Further, we reveal that the LPS-induced TGF-α shedding and EGFR transactivation involves the TACE activation through phosphorylation by p38 that requires Rac1 participation. Moreover, we demonstrate that up-regulation in H. pylori LPS-elicited Rac1-GTP membrane translocation plays a pivotal role in recruitment of the activated p38 to the membrane for TACE activation through phosphorylation on Thr(735). Taken together, our findings provide strong evidence as to the essential function of Rac1 in TACE activation, TGF-α ectodomain shedding, and the EGFR transactivation.

Upregulation of a disintegrin and metalloprotease 8 is associated with progression and prognosis of patients with gastric cancer

A disintegrin and metalloprotease 8 (ADAM8) is involved in the tumorigenesis of several types of solid tumors. However, its exact role in gastric cancer (GC) remains unclear. The aim of this study was to evaluate the clinical significance of ADAM8 in GC and to explore its biological effects on gastric carcinogenesis. In this study, quantitative reverse transcription-polymerase chain reaction, Western blotting, and immunohistochemical staining analysis revealed that ADAM8 messenger RNA expression was significantly upregulated in GC tissues compared with noncancerous tissues (P = 0.004), and that positive ADAM8 expression is much more common in tumor tissues compared with normal tissues (P < 0.001) and is correlated with T stage (P = 0.036), N stage (P = 0.048), vessel invasion (P = 0.002), and a shorter patient overall survival (P = 0.024). In vitro assay indicated that ADAM8 overexpression promoted cell growth and increased migration and invasion abilities by decreasing the p-p38/p-extracellular regulated protein kinases (p-ERK) ratio. In conclusion, ADAM8 promotes GC cell proliferation and invasion, and its expression is positively correlated with poor survival, indicating that it might be a promising target in GC therapy.

A systematic study of modulation of ADAM-mediated ectodomain shedding by site-specific O-glycosylation

Regulated shedding of the ectodomain of cell membrane proteins by proteases is a common process that releases the extracellular domain from the cell and activates cell signaling. Ectodomain shedding occurs in the immediate extracellular juxtamembrane region, which is also where O-glycosylation is often found and examples of crosstalk between shedding and O-glycosylation have been reported. Here, we systematically investigated the potential of site-specific O-glycosylation mediated by distinct polypeptide GalNAc-transferase (GalNAc-T) isoforms to coregulate ectodomain shedding mediated by the A Disintegrin And Metalloproteinase (ADAM) subfamily of proteases and in particular ADAM17. We analyzed 25 membrane proteins that are known to undergo ADAM17 shedding and where the processing sites included Ser/Thr residues within ± 4 residues that could represent O-glycosites. We used in vitro GalNAc-T enzyme and ADAM cleavage assays to demonstrate that shedding of at least 12 of these proteins are potentially coregulated by O-glycosylation. Using TNF-α as an example, we confirmed that shedding mediated by ADAM17 is coregulated by O-glycosylation controlled by the GalNAc-T2 isoform both ex vivo in isogenic cell models and in vivo in mouse Galnt2 knockouts. The study provides compelling evidence for a wider role of site-specific O-glycosylation in ectodomain shedding.

ADAM Proteases and Gastrointestinal Function

A disintegrin and metalloproteinases (ADAMs) are a family of cell surface proteases that regulate diverse cellular functions, including cell adhesion, migration, cellular signaling, and proteolysis. Proteolytically active ADAMs are responsible for ectodomain shedding of membrane-associated proteins. ADAMs rapidly modulate key cell signaling pathways in response to changes in the extracellular environment (e.g., inflammation) and play a central role in coordinating intercellular communication within the local microenvironment. ADAM10 and ADAM17 are the most studied members of the ADAM family in the gastrointestinal tract. ADAMs regulate many cellular processes associated with intestinal development, cell fate specification, and the maintenance of intestinal stem cell/progenitor populations. Several signaling pathway molecules that undergo ectodomain shedding by ADAMs [e.g., ligands and receptors from epidermal growth factor receptor (EGFR)/ErbB and tumor necrosis factor α (TNFα) receptor (TNFR) families] help drive and control intestinal inflammation and injury/repair responses. Dysregulation of these processes through aberrant ADAM expression or sustained ADAM activity is linked to chronic inflammation, inflammation-associated cancer, and tumorigenesis.

Serum amyloid A inhibits RANKL-induced osteoclast formation

When mouse bone marrow-derived macrophages were stimulated with serum amyloid A (SAA), which is a major acute-phase protein, there was strong inhibition of osteoclast formation induced by the receptor activator of nuclear factor kappaB ligand. SAA not only markedly blocked the expression of several osteoclast-associated genes (TNF receptor-associated factor 6 and osteoclast-associated receptor) but also strongly induced the expression of negative regulators (MafB and interferon regulatory factor 8). Moreover, SAA decreased c-fms expression on the cell surface via shedding of the c-fms extracellular domain. SAA also restrained the fusion of osteoclast precursors by blocking intracellular ATP release. This inhibitory response of SAA is not mediated by the well-known SAA receptors (formyl peptide receptor 2, Toll-like receptor 2 (TLR2) or TLR4). These findings provide insight into a novel inhibitory role of SAA in osteoclastogenesis and suggest that SAA is an important endogenous modulator that regulates bone homeostasis.

A metal-based tumour necrosis factor-alpha converting enzyme inhibitor

We report herein a novel iridium(III) complex 1 as an antitumour necrosis factor agent and the first metal-based inhibitor of TACE enzymatic activity. Complex 1 inhibited TNF-α secretion and p38 phosphorylation in human monocytic THP-1 cells.

ADAMs family and relatives in cardiovascular physiology and pathology

A disintegrin and metalloproteinases (ADAMs) are a family of membrane-bound proteases. ADAM-TSs (ADAMs with thrombospondin domains) are a close relative of ADAMs that are present in soluble form in the extracellular space. Dysregulated production or function of these enzymes has been associated with pathologies such as cancer, asthma, Alzheimer's and cardiovascular diseases. ADAMs contribute to angiogenesis, hypertrophy and apoptosis in a stimulus- and cell type-dependent manner. Among the ADAMs identified so far (34 in mouse, 21 in human), ADAMs 8, 9, 10, 12, 17 and 19 have been shown to be involved in cardiovascular development or cardiomyopathies; and among the 19 ADAM-TSs, ADAM-TS1, 5, 7 and 9 are important in development of the cardiovascular system, while ADAM-TS13 can contribute to vascular disorders. Meanwhile, there remain a number of ADAMs and ADAM-TSs whose function in the cardiovascular system has not been yet explored. The current knowledge about the role of ADAMs and ADAM-TSs in the cardiovascular pathologies is still quite limited. The most detailed studies have been performed in other cell types (e.g. cancer cells) and organs (nervous system) which can provide valuable insight into the potential functions of ADAMs and ADAM-TSs, their mechanism of action and therapeutic potentials in cardiomyopathies. Here, we review what is currently known about the structure and function of ADAMs and ADAM-TSs, and their roles in development, physiology and pathology of the cardiovascular system.

Chlamydia-infected cells shed Gp96 to prevent chlamydial re-infection

Chlamydia trachomatis is an obligate intracellular human pathogen with a biphasic developmental life cycle. The infectious elementary bodies (EBs) enter a host cell where they transform into reticulate bodies (RBs) that use cellular metabolites to multiply. Re-infection of an infected cell during the replicative phase of chlamydial development may prevent formation of infectious EBs, interrupting the infectious cycle. Here, we report that Glucose Regulated Protein 96 (Gp96), a chaperone for cell surface receptors, binds to and facilitates adherence and entry of C. trachomatis. Gp96 expression was increased early in infection in a MAP kinase-dependent way, thereby increasing chlamydial adherence and invasion. Gp96 co-precipitated with Protein Disulphide Isomerase (PDI), known to be involved in chlamydial host cell entry. During the replicative phase, Gp96 was depleted from infected cells and shed into the supernatant by activation of metalloproteinase TACE (ADAM17). Loss of Gp96 also reduced the activity of PDI on the cell surface. Reduced surface display of Gp96 prevented chlamydial re-infection in a TACE-dependent manner in cell lines but also in primary cells derived from human fimbriae, the natural site of chlamydial infection. Our data suggest a role of infection-induced Gp96 shedding in the protection of the chlamydial replicative niche.

α-Lipoic acid reduces neurogenic hypertension by blunting oxidative stress-mediated increase in ADAM17

We previously reported that type 2 angiotensin-converting enzyme (ACE2) compensatory activity is impaired by the disintegrin and metalloprotease 17 (ADAM17), and lack of ACE2 is associated with oxidative stress in neurogenic hypertension. To investigate the relationship between ADAM17 and oxidative stress, Neuro2A cells were treated with ANG II (100 nM) 24 h after vehicle or α-lipoic acid (LA, 500 μM). ADAM17 expression was increased by ANG II (120.5 ± 9.1 vs. 100.2 ± 0.8%, P < 0.05) and decreased after LA (69.0 ± 0.3 vs. 120.5 ± 9.1%, P < 0.05). In another set of experiments, LA reduced ADAM17 (92.9 ± 5.3 vs. 100.0 ± 11.2%, P < 0.05) following its overexpression. Moreover, ADAM17 activity was reduced by LA in ADAM17-overexpressing cells [109.5 ± 19.8 vs. 158.0 ± 20.0 fluorescence units (FU)·min(-1)·μg protein(-1), P < 0.05], in which ADAM17 overexpression increased oxidative stress (114.1 ± 2.5 vs. 101.0 ± 1.0%, P < 0.05). Conversely, LA-treated cells attenuated ADAM17 overexpression-induced oxidative stress (76.0 ± 9.1 vs. 114.1 ± 2.5%, P < 0.05). In deoxycorticosterone acetate (DOCA)-salt hypertensive mice, a model in which ADAM17 expression and activity are increased, hypertension was blunted by pretreatment with LA (119.0 ± 2.4 vs. 131.4 ± 2.2 mmHg, P < 0.05). In addition, LA improved dysautonomia and baroreflex sensitivity. Furthermore, LA blunted the increase in NADPH oxidase subunit expression, as well as the increase in ADAM17 and decrease in ACE2 activity in the hypothalamus of DOCA-salt hypertensive mice. Taken together, these data suggest that LA might preserve ACE2 compensatory activity by breaking the feedforward cycle between ADAM17 and oxidative stress, resulting in a reduction of neurogenic hypertension.

Shedding of TNF receptor 2 by effector CD8⁺ T cells by ADAM17 is important for regulating TNF-α availability during influenza infection

Elevated levels of solTNFR2 are observed in a variety of human pathophysiological conditions but regulation of TNFR2 levels during disease is not well understood. We found that solTNFR2 levels were increased following influenza infection or live-attenuated influenza virus challenge in mice and humans, respectively. As influenza-specific CD8(+) T cells up-regulated expression of TNFR2 after infection in mice, we hypothesized that CD8(+) T cells contributed, in part, to solTNFR2 production after influenza infection and were interested in the mechanisms by which CD8(+) T cells regulate TNFR2 shedding. Activation of these cells by TCR stimulation resulted in enhanced shedding of TNFR2 that required actin remodeling and lipid raft formation and was dependent on MAPK/ERK signaling. Furthermore, we identified ADAM17 as the protease responsible for TNFR2 shedding by CD8(+) T cells, with ADAM17 and TNFR2 required in "cis" for shedding to occur. We observed similar activation thresholds for TNF-α expression and TNFR2 shedding, suggesting that solTNFR2 functioned, in part, to regulate solTNF-α levels. Production of solTNFR2 by activated CD8(+) T cells reduced the availability of solTNF-α released by these cells, and TNFR2 blockade during influenza infection in mice enhanced the levels of solTNF-α, supporting this hypothesis. Taken together, this study identifies critical cellular mechanisms regulating TNFR2 shedding on CD8(+) T cells and demonstrates that TNFR2 contributes, in part, to the regulation of TNF-α levels during infection.

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

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

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

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

A Monoclonal Antibody to ADAM17 Inhibits Tumor Growth by Inhibiting EGFR and Non-EGFR-Mediated Pathways

ADAM17 is the primary sheddase for HER pathway ligands. We report the discovery of a potent and specific ADAM17 inhibitory antibody, MEDI3622, which induces tumor regression or stasis in many EGFR-dependent tumor models. The inhibitory activity of MEDI3622 correlated with EGFR activity both in a series of tumor models across several indications as well in as a focused set of head and neck patient-derived xenograft models. The antitumor activity of MEDI3622 was superior to that of EGFR/HER pathway inhibitors in the OE21 esophageal model and the COLO205 colorectal model suggesting additional activity outside of the EGFR pathway. Combination of MEDI3622 and cetuximab in the OE21 model was additive and eradicated tumors. Proteomics analysis revealed novel ADAM17 substrates that function outside of the HER pathways and may contribute toward the antitumor activity of the monoclonal antibody.

High-throughput protease activity cytometry reveals dose-dependent heterogeneity in PMA-mediated ADAM17 activation

As key components of autocrine signaling, pericellular proteases, a disintegrin and metalloproteinases (ADAMs) in particular, are known to impact the microenvironment of individual cells and have significant implications in various pathological situations including cancer, inflammatory and vascular diseases. There is great incentive to develop a high-throughput platform for single-cell measurement of pericellular protease activity, as it is essential for studying the heterogeneity of protease response and the corresponding cell behavioral consequences. In this work, we developed a microfluidic platform to simultaneously monitor protease activity of many single cells in a time-dependent manner. This platform isolates individual microwells rapidly on demand and thus allows single-cell activity measurement of both cell-surface and secreted proteases by confining individual cells with diffusive FRET-based substrates. With this platform, we observed dose-dependent heterogeneous protease activation of HepG2 cells treated with phorbol 12-myristate 13-acetate (PMA). To study the temporal behavior of PMA-induced protease response, we monitored the pericellular protease activity of the same single cells during three different time periods and revealed the diversity in the dynamic patterns of single-cell protease activity profile upon PMA stimulation. The unique temporal information of single-cell protease response can help unveil the complicated functional role of pericellular proteases.

A biosensor for the activity of the "sheddase" TACE (ADAM17) reveals novel and cell type-specific mechanisms of TACE activation

Diverse environmental conditions stimulate protein "shedding" from the cell surface through proteolytic cleavage. The protease TACE [tumor necrosis factor-α (TNFα)--converting enzyme, encoded by ADAM17] mediates protein shedding, thereby regulating the maturation and release of various extracellular substrates, such as growth factors and cytokines, that induce diverse cellular responses. We developed a FRET (fluorescence resonance energy transfer)-based biosensor called TSen that quantitatively reports the kinetics of TACE activity in live cells. In combination with chemical biology approaches, we used TSen to probe the dependence of TACE activation on the induction of the kinases p38 and ERK (extracellular signal-regulated kinase) in various epithelial cell lines. Using TSen, we found that disruption of the actin cytoskeleton in keratinocytes induced rapid and robust TSen cleavage and the accumulation of TACE at the plasma membrane. Cytoskeletal disruption also increased the cleavage of endogenous TACE substrates, including transforming growth factor-α. Thus, TSen is a useful tool for unraveling the mechanisms underlying the spatiotemporal activation of TACE in live cells.

Tumor Suppressor NF2 Blocks Cellular Migration by Inhibiting Ectodomain Cleavage of CD44

Ectodomain cleavage (shedding) of transmembrane proteins by metalloproteases (MMP) generates numerous essential signaling molecules, but its regulation is not totally understood. CD44, a cleaved transmembrane glycoprotein, exerts both antiproliferative or tumor-promoting functions, but whether proteolysis is required for this is not certain. CD44-mediated contact inhibition and cellular proliferation are regulated by counteracting CD44 C-terminal interacting proteins, the tumor suppressor protein merlin (NF2) and ERM proteins (ezrin, radixin, moesin). We show here that activation or overexpression of constitutively active merlin or downregulation of ERMs inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced [as well as serum, hepatocyte growth factor (HGF), or platelet-derived growth factor (PDGF)] CD44 cleavage by the metalloprotease ADAM10, whereas overexpressed ERM proteins promoted cleavage. Merlin- and ERM-modulated Ras or Rac activity was not required for this function. However, latrunculin (an actin-disrupting toxin) or an ezrin mutant which is unable to link CD44 to actin, inhibited CD44 cleavage, identifying a cytoskeletal C-terminal link as essential for induced CD44 cleavage. Cellular migration, an important tumor property, depended on CD44 and its cleavage and was inhibited by merlin. These data reveal a novel function of merlin and suggest that CD44 cleavage products play a tumor-promoting role. Neuregulin, an EGF ligand released by ADAM17 from its pro-form NRG1, is predominantly involved in regulating cellular differentiation. In contrast to CD44, release of neuregulin from its pro-form was not regulated by merlin or ERM proteins. Disruption of the actin cytoskeleton however, also inhibited NRG1 cleavage. This current study presents one of the first examples of substrate-selective cleavage regulation.

IMPLICATIONS

Investigating transmembrane protein cleavage and their regulatory pathways have provided new molecular insight into their important role in cancer formation and possible treatment.

Network dynamics determine the autocrine and paracrine signaling functions of TNF

A hallmark of the inflammatory response to pathogen exposure is the production of tumor necrosis factor (TNF) that coordinates innate and adaptive immune responses by functioning in an autocrine or paracrine manner. Numerous molecular mechanisms contributing to TNF production have been identified, but how they function together in macrophages remains unclear. Here, we pursued an iterative systems biology approach to develop a quantitative understanding of the regulatory modules that control TNF mRNA synthesis and processing, mRNA half-life and translation, and protein processing and secretion. By linking the resulting model of TNF production to models of the TLR-, the TNFR-, and the NFκB signaling modules, we were able to study TNF's functions during the inflammatory response to diverse TLR agonists. Contrary to expectation, we predicted and then experimentally confirmed that in response to lipopolysaccaride, TNF does not have an autocrine function in amplifying the NFκB response, although it plays a potent paracrine role in neighboring cells. However, in response to CpG DNA, autocrine TNF extends the duration of NFκB activity and shapes CpG-induced gene expression programs. Our systems biology approach revealed that network dynamics of MyD88 and TRIF signaling and of cytokine production and response govern the stimulus-specific autocrine and paracrine functions of TNF.

Membrane-enabled dimerization of the intrinsically disordered cytoplasmic domain of ADAM10

Intrinsically disordered protein regions are widely distributed in the cytoplasmic domains of many transmembrane receptors. The cytoplasmic domain of a disintegrin and metalloprotease (ADAM)10, a transmembrane metalloprotease mediating ectodomain shedding of diverse membrane proteins, was recently suggested to mediate the homodimerization of ADAM10. Here we show that a recombinant cytoplasmic domain of ADAM10 (A10Cp) is unstructured as judged by its susceptibility to limited trypsin digestion and its circular dichroism spectrum. In comparison, recombinant transmembrane-cytoplasmic domain of ADAM10 (A10TmCp) reconstituted in dodecylphosphocholine (DPC) micelles exhibits much greater resistance to trypsin digestion, with its cytoplasmic domain taking on a significant ordered structure. FRET analysis demonstrates that, although A10Cp remains monomeric, A10TmCp forms a tight homodimer (K(d) ∼ 7 nM) in DPC micelles. Phospholipid-conjugated A10Cp dose-dependently inhibits formation of A10TmCp homodimer, whereas A10Cp achieves only limited inhibition. Placing the transmembrane and cytoplasmic domains of ADAM10, but not the transmembrane domain alone, in their native orientation in the inner membrane of Escherichia coli produces specific and strong dimerization signal in the AraC-based transcriptional reporter assay. A chimeric construct containing the otherwise monomeric transmembrane domain of L-selectin and the cytoplasmic domain of ADAM10 produces a similar dimerization signal. Overall, these results demonstrate that a transmembrane domain imparts a stable structure to the adjacent and intrinsically disordered cytoplasmic domain of ADAM10 to form a homodimer in the membrane. This finding advances our understanding of the regulatory mechanism of ADAMs and has general implications for membrane-protein interactions in the process of transmembrane signaling.

Embelin inhibits TNF-α converting enzyme and cancer cell metastasis: molecular dynamics and experimental evidence

Background

Embelin, a quinone derivative, is found in the fruits of Embelia ribes Burm (Myrsinaceae). It has been shown to have a variety of therapeutic potentials including anthelmintic, anti-tumor, anti-diabetic, anti-bacterial and anti-inflammation. Inflammation is an immunological response to external harmful stimuli and is regulated by an endogenous pyrogen and pleiotropic pro-inflammatory cytokine, tumor necrosis factor alpha (TNF-α). TNF-α production has been implicated in a variety of other human pathologies including neurodegeneration and cancer. Several studies have shown that the anti-inflammatory activity of embelin is mediated by reduction in TNF-α. The latter is synthesized as a membrane anchored protein (pro-TNF-α); the soluble component of pro-TNF-α is then released into the extracellular space by the action of a protease called TNF-α converting enzyme (TACE). TACE, hence, has been proposed as a therapeutic target for inflammation and cancer.

Methods

We used molecular docking and experimental approaches to investigate the docking potential and molecular effects of embelin to TACE and human cancer cell characteristics, respectively.

Results

We demonstrate that embelin is a potential inhibitor of TACE. Furthermore, in vitro studies revealed that it inhibits malignant properties of cancer cells through inactivation of metastatic signaling molecules including MMPs, VEGF and hnRNP-K in breast cancer cells.

Conclusion

Based on the molecular dynamics and experimental data, embelin is proposed as a natural anti-inflammatory and anticancer drug.

Sphingosine-1-phosphate promotes expansion of cancer stem cells via S1PR3 by a ligand-independent Notch activation

Many tumours originate from cancer stem cells (CSCs), which is a small population of cells that display stem cell properties. However, the molecular mechanisms that regulate CSC frequency remain poorly understood. Here, using microarray screening in aldehyde dehydrogenase (ALDH)-positive CSC model, we identify a fundamental role for a lipid mediator sphingosine-1-phosphate (S1P) in CSC expansion. Stimulation with S1P enhances ALDH-positive CSCs via S1P receptor 3 (S1PR3) and subsequent Notch activation. CSCs overexpressing sphingosine kinase 1 (SphK1), an S1P-producing enzyme, show increased ability to develop tumours in nude mice, compared with parent cells or CSCs. Tumorigenicity of CSCs overexpressing SphK1 is inhibited by S1PR3 knockdown or S1PR3 antagonist. Breast cancer patient-derived mammospheres contain SphK1(+)/ALDH1(+) cells or S1PR3(+)/ALDH1(+) cells. Our findings provide new insights into the lipid-mediated regulation of CSCs via Notch signalling, and rationale for targeting S1PR3 in cancer.

A1 adenosine receptor-stimulated exocytosis in bladder umbrella cells requires phosphorylation of ADAM17 Ser-811 and EGF receptor transactivation

The role of phosphorylation in ADAM17-dependent shedding is controversial. We show that the A₁ adenosine receptor stimulates exocytosis in umbrella cells by a pathway that requires phosphorylation of ADAM17–Ser-811, followed by HB-EGF shedding and EGF receptor transactivation. Preventing ADAM17 phosphorylation blocks these downstream events.

Despite the importance of ADAM17-dependent cleavage in normal biology and disease, the physiological cues that trigger its activity, the effector pathways that promote its function, and the mechanisms that control its activity, particularly the role of phosphorylation, remain unresolved. Using native bladder epithelium, in some cases transduced with adenoviruses encoding small interfering RNA, we observe that stimulation of apically localized A₁ adenosine receptors (A₁ARs) triggers a G_i-G_βγ-phospholipase C-protein kinase C (PKC) cascade that promotes ADAM17-dependent HB-EGF cleavage, EGFR transactivation, and apical exocytosis. We further show that the cytoplasmic tail of rat ADAM17 contains a conserved serine residue at position 811, which resides in a canonical PKC phosphorylation site, and is phosphorylated in response to A₁AR activation. Preventing this phosphorylation event by expression of a nonphosphorylatable ADAM17^S811A mutant or expression of a tail-minus construct inhibits A₁AR-stimulated, ADAM17-dependent HB-EGF cleavage. Furthermore, expression of ADAM17^S811A in bladder tissues impairs A₁AR-induced apical exocytosis. We conclude that adenosine-stimulated exocytosis requires PKC- and ADAM17-dependent EGFR transactivation and that the function of ADAM17 in this pathway depends on the phosphorylation state of Ser-811 in its cytoplasmic domain.

Hog1: 20 years of discovery and impact

The protein kinase Hog1 (high osmolarity glycerol 1) was discovered 20 years ago, being revealed as a central signaling mediator during osmoregulation in the budding yeast Saccharomyces cerevisiae. Homologs of Hog1 exist in all evaluated eukaryotic organisms, and this kinase plays a central role in cellular responses to external stresses and stimuli. Here, we highlight the mechanism by which cells sense changes in extracellular osmolarity, the method by which Hog1 regulates cellular adaptation, and the impacts of the Hog1 pathway upon cellular growth and morphology. Studies that have addressed these issues reveal the influence of the Hog1 signaling pathway on diverse cellular processes.

Differential expression and localization of ADAM10 and ADAM17 during rat spermatogenesis suggest a role in germ cell differentiation

Background

Extracellular metolloproteases have been implied in different process such as cell death, differentiation and migration. Membrane-bound metalloproteases of the ADAM family shed the extracellular domain of many cytokines and receptor controlling auto and para/juxtacrine cell signaling in different tissues. ADAM17 and ADAM10 are two members of this family surface metalloproteases involved in germ cell apoptosis during the first wave of spermatogenesis in the rat, but they have other signaling functions in somatic tissues.

Results

In an attempt to further study these two enzymes, we describe the presence and localization in adult male rats. Results showed that both enzymes are detected in germ and Sertoli cells during all the stages of spermatogenesis. Interestingly their protein levels and cell surface localization in adult rats were stage-specific, suggesting activation of these enzymes at particular events of rat spermatogenesis.

Conclusions

Therefore, these results show that ADAM10 and ADAM17 protein levels and subcellular (cell surface) localization are regulated during rat spermatogenesis.

Taiwan cobra phospholipase A2 suppresses ERK-mediated ADAM17 maturation, thus reducing secreted TNF-α production in human leukemia U937 cells

The goal of this study was to explore the signaling pathway regulating the processing of proADAM17 into ADAM17 in Taiwan cobra phospholipase A2 (PLA2)-treated human leukemia U937 cells. PLA2 induced reactive oxygen species (ROS)-elicited p38 MAPK activation and ERK inactivation in U937 cells. Catalytically inactive bromophenacylated PLA2 (BPB-PLA2) and PLA2 mutants evoked Ca(2+)-mediated p38 MAPK activation, and the level of phosphorylated ERK remained unchanged. PLA2 treatment reduced mature ADAM17 expression and secreted TNF-α (sTNF-α) production. Co-treatment of SB202190 (p38 MAPK inhibitor) and catalytically inactive PLA2 increased ERK phosphorylation, ADAM17 maturation and sTNF-α production. Nevertheless, mRNA levels of ADAM17 and TNF-α were insignificantly altered after PLA2 and SB202190/BPB-PLA2 treatment. ADAM17 activity assay and knock-down of ADAM17 revealed that ADAM17 was involved in sTNF-α production. Restoration of ERK activation increased the processing of proADAM17 into ADAM17 in PLA2-treated cells, while inactivation of ERK reduced ADAM17 maturation in untreated and SB202190/BPB-PLA2-treated cells. Removal of cell surface heparan sulfate abrogated PLA2 and SB202190/BPB-PLA2 effect on ADAM17 maturation. Taken together, the present data reveal that PLA2 suppresses ERK-mediated ADAM17 maturation, thus reducing sTNF-α production in U937 cells. Moreover, the binding with heparan sulfate is crucial for the PLA2 effect.

Accelerated receptor shedding inhibits kidney injury molecule-1 (KIM-1)-mediated efferocytosis

Efficient clearance of apoptotic cells (efferocytosis) prevents inflammation and permits repair following tissue injury. Kidney injury molecule-1 (KIM-1) is a receptor for phosphatidylserine, an "eat-me" signal exposed on the surface of apoptotic cells that marks them for phagocytic clearance. KIM-1 is upregulated on proximal tubule epithelial cells (PTECs) during ischemic acute kidney injury (AKI), enabling efferocytosis by surviving PTECs. KIM-1 is spontaneously cleaved at its ectodomain region to generate a soluble fragment that serves a sensitive and specific biomarker for AKI, but the biological relevance of KIM-1 shedding is unknown. Here, we sought to determine how KIM-1 shedding might regulate efferocytosis. Using cells that endogenously and exogenously express KIM-1, we found that hydrogen peroxide-mediated oxidative injury or PMA treatment accelerated KIM-1 shedding in a dose-dependent manner. KIM-1 shedding was also accelerated when apoptotic cells were added. Accelerated shedding or the presence of excess soluble KIM-1 in the extracellular milieu significantly inhibited efferocytosis. We also identified that TNF-α-converting enzyme (TACE or ADAM17) mediates both the spontaneous and PMA-accelerated shedding of KIM-1. While accelerated shedding inhibited efferocytosis, we found that spontaneous KIM-1 cleavage does not affect the phagocytic efficiency of PTECs. Our results suggest that KIM-1 shedding is accelerated by worsening cellular injury, and excess soluble KIM-1 competitively inhibits efferocytosis. These findings may be important in AKI when there is severe cellular injury.

p75 neurotrophin receptor cleavage by α- and γ-secretases is required for neurotrophin-mediated proliferation of brain tumor-initiating cells

Malignant gliomas are highly invasive, proliferative, and resistant to treatment. Previously, we have shown that p75 neurotrophin receptor (p75NTR) is a novel mediator of invasion of human glioma cells. However, the role of p75NTR in glioma proliferation is unknown. Here we used brain tumor-initiating cells (BTICs) and show that BTICs express neurotrophin receptors (p75NTR, TrkA, TrkB, and TrkC) and their ligands (NGF, brain-derived neurotrophic factor, and neurotrophin 3) and secrete NGF. Down-regulation of p75NTR significantly decreased proliferation of BTICs. Conversely, exogenouous NGF stimulated BTIC proliferation through α- and γ-secretase-mediated p75NTR cleavage and release of its intracellular domain (ICD). In contrast, overexpression of the p75NTR ICD induced proliferation. Interestingly, inhibition of Trk signaling blocked NGF-stimulated BTIC proliferation and p75NTR cleavage, indicating a role of Trk in p75NTR signaling. Further, blocking p75NTR cleavage attenuated Akt activation in BTICs, suggesting role of Akt in p75NTR-mediated proliferation. We also found that p75NTR, α-secretases, and the four subunits of the γ-secretase enzyme were elevated in glioblastoma multiformes patients. Importantly, the ICD of p75NTR was commonly found in malignant glioma patient specimens, suggesting that the receptor is activated and cleaved in patient tumors. These results suggest that p75NTR proteolysis is required for BTIC proliferation and is a novel potential clinical target.