Role of Disintegrin and Metalloproteases (ADAMs) in Cardiovascular Physiology

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 ADAMs and CVD review

Abstract

The family of a disintegrin and metalloproteases (ADAMs) are key mediators of cell signaling by ectodomain shedding of various growth factors, cytokines, receptors and adhesion molecules at the cellular membrane.  ADAMs regulate cell proliferation, cell growth, inflammation, and many other cellular processes.  ADAM17, the most extensively studied ADAM family member, is also known as tumor necrosis factor-α converting enzyme (TACE).

ADAM-mediated shedding of cytokines such as tumor necrosis factor-α orchestrates immune responses and inflammatory cascades.  ADAMs also mediate the shedding of growth factors causing cell growth and proliferation by activation of growth factor receptors such as the epidermal growth factor receptor.  Thus, increased ADAM-mediated shedding of cytokines and growth factors together likely contributes to the pathogenesis of cardiovascular diseases such as atherosclerosis, hypertension or ischemia.  Because they are involved in several pathogenic cellular processes, ADAMs can be a potential therapeutic target in these diseases.  In this review we will focuse on the role of ADAMs in cardiovascular physiology and pathophysiology.  The main aim of this review is to stimulate new interest in this area by highlighting remarkable evidence.

Introduction

A disintegrin and metalloprotease (ADAM) proteins belong to a type 1 transmembrane, Zn2+-dependent protease superfamily.  ADAMs work as key mediators of cell signaling by ectodomain shedding of various growth factors, cytokines, receptors and adhesion molecules at the cellular membrane.  The variety of substrate proteins susceptible to ADAM-dependent cleavage enable ADAMs to regulate cell proliferation, cell growth, inflammation, and other cellular processes.  Among the ADAM family, ADAM17, also known as tumor necrosis factor-α (TNFα) converting enzyme (TACE), is the most thoroughly studied protein.  ADAM17 was first purified and cloned in 1997 as a metalloproteinase that specifically cleaved precursor TNFα [Moss, 1997 #181;Black, 1997 #518].  These findings completely changed the significance of ADAMs from mere adhesion molecules to important regulators of cell signaling.

ADAM17 consists of an N-terminal signal sequence, a prodomain, a catalytic domain with a typical HEXXHXXGXXH sequence, a disintegrin domain, a membrane proximal domain, a transmembrane domain, and a cytoplasmic tail [Black, 1998 #519;Blobel, 1997 #520].  ADAM17 exists as a multimer at the cell membrane and this multimerisation is mediated by an epidermal growth factor (EGF)-like domain [Lorenzen, 2011 #493].  The maturation of ADAM17 proenzyme requires furin-dependent processing at either a canonical proprotein convertase cleavage site at the boundary between the prodomain and catalytic domain [Schlondorff, 2000 #361] or an upstream PC cleavage site [Wong, 2015 #360].  These cleavages are thought to be essential for subsequent activation of ADAM17.  ADAM17 is expressed very broadly in somatic tissues and a variety of growth factors, cytokines, receptors, and adhesion molecules have been revealed as substrates of ADAM17 in either in vivo or in vitro studies (Table 1).  After shedding, some of the cleaved substrates can bind to their receptors on the same cell (autocrine), local cells (paracrine), or non-local cells by transport through blood (endocrine) [Wiley, 1998 #256;Borrell-Pages, 2003 #521].  In this manner ADAM17-mediated shedding of cytokines such as TNFα may orchestrate the immune system for inflammatory responses.

The shedding of EGF receptor (EGFR) ligands is an important process since EGFR is an essential tyrosine kinase receptor in the development or various diseases including malignancy.  The role of EGFR in cancer is widely studied, however, recent evidence has demonstrated the importance of EGFR on cardiovascular physiology.  Specifically, G protein coupled receptor (GPCR)-mediated EGFR transactivation has been recognized as a key point of control governing cardiovascular outcomes [Forrester, 2016 #547].  GPCR activation causes initial heterotrimeric G protein dissociation. Ligand specific intermediates such as intracellular Ca2+ and reactive oxygen species (ROS) are elevated, followed by ADAM17 activation and shedding of EGFR ligands [Elliott, 2013 #15;George, 2013 #188;Ohtsu, 2006 #187].  The cytoplasmic tail of EGFR ligands is recognized as a site of protein interactions which mediates several intra- and intercellular phenomena including ligand trafficking to the cell surface, signal transduction, and gene expression [Kinugasa, 2007 #258;Nanba, 2003 #257;Hieda, 2008 #259].  Ectodomain shedding of EGFR ligands by ADAM17 can initiate bidirectional signaling events with released growth factor and free shed remnant.  Taken together, ADAM17-mediated shedding of growth factors causes cell growth or proliferation by transactivation of the growth factor receptors including EGFR.

Increased ADAM17-mediated shedding likely contributes to the progression of various cardiovascular diseases such as atherosclerosis via both EGFR transactivation and inflammation.  Therefore, ADAM17 is a potential therapeutic target in these diseases.  The role of ADAM17 in cancer and autoimmune diseases is well documented [Scheller, 2011 #8;Pruessmeyer, 2009 #7;Lisi, 2014 #2]; here, we focus on the role of ADAM17 in cardiovascular pathophysiology.  This review also includes a discussion of other ADAM family proteins which share cell specific distribution, the HEXXXHXXGXXH motif that is required for proteolytic activity, and, therefore, function, with ADAM17.  Notably, most substrates can be cleaved by a variety of ADAM family members, and this seemingly nonspecific relationship between substrates and ADAMs makes the physiology of ADAMs more complicated and confusing.  The main aim of this review is to rejuvenate interest in ADAM research by highlighting remarkable evidence.

Overview

In addition to ADAM17, ADAM8, 9, 10,12,15,19, 28 and 33 are expressed on various cells including endothelial cells, smooth muscle cells, or leukocyte, and they also have proteolytic activity.  Among these ADAM families, ADAM10 is most broadly expressed and has a close relation to ADAM17 in its structure and function.  Therefore, ADAM10 is recognized as another important shedding proteinase which mediates various signal transduction (Tatsuo to add Ref).  In this section we highlight the role of these ADAMs in cardiovascular pathophysiology.

Lessons from genetically modified animal models

Mice with global genetic deletion of ADAM8 [Kelly, 2005 #463], ADAM9 [Weskamp, 2002 #464], ADAM15 [Horiuchi, 2003 #471], and ADAM33 [Chen, 2006 #465] are viable and do not show an obvious phenotype under normal conditions.  Mice lacking ADAM8 [Guaiquil, 2010 #466],  ADAM9 [Guaiquil, 2009 #440], and ADAM15 [Horiuchi, 2003 #471] have  reduced retinal neovascularization in an experimental retinopathy model.

ADAM10 [Hartmann, 2002 #468] and ADAM19 [Zhou, 2004 #472;Kurohara, 2004 #473] knockout mice die before or shortly after birth, respectively.  ADAM10 deletion results in defects in the cardiovascular system. Endothelial cell specific silencing of the ADAM10 gene results in multiple cardiac and vascular defects similar to Notch1 mutants [Zhang, 2010 #428;Glomski, 2011 #470], suggesting that the Notch signaling pathway is key in ADAM10-mediated cardiovascular development.  Alabi et al reported that Notch1 and Notch4 control the development of several organ-specific vascular beds in an ADAM10-dependent manner [Alabi, 2016 #548].  Moreover, collecting duct-specific ADAM10 knockout mice show defects in urine concentration, polyuria, and hydronephrosis with reduction of Notch activity in the collecting duct epithelium [Guo, 2015 #528].  Bone marrow transplantation from myeloid-specific ADAM10 knockout mice to low density lipoprotein (LDL) receptor knockout mice does not affect atherosclerotic plaque size but increases plaque collagen content, indicating that myeloid ADAM10 modulate plaque stability [van der Vorst, 2015 #532].

ADAM12 has been implicated in cardiac hypertrophy.  Genetic knockdown of matrix metalloproteinase(MMP)-7 attenuates angiotensin II-induced myocardial ADAM12 overexpression, hypertension and cardiac hypertrophy, demonstrating the importance of MMP-7/ADAM12 signaling axis in hypertensive cardiac disorders [Wang, 2009 #474].

ADAM17 in Development

Global ADAM17 knockout mice die shortly after birth with defects in the aortic, pulmonic, and tricuspid valves of the heart [Horiuchi, 2007 #209].  Similarly, mice lacking the Zn2+ binding domain of ADAM17 (ADAM17 Δzn/ Δzn), which results in deactivating metalloproteinase activity, die shortly after birth [Peschon, 1998 #182].  ADAM17 Δzn/ Δzn embryos present defective cardiac valvulogenesis [Jackson, 2003 #364], abnormal vascular beds, and internal hemorrhages [Canault, 2010 #362].  The waved with open eyes (woe) mouse is a model of syntenic human ocular disorders. Woe is a hypomorphic mutation in ADAM17 in which only small amount of functional ADAM17 is produced.  These animals survive into adulthood despite having enlarged hearts and defects in the semilunar cardiac valves [Hassemer, 2010 #376].  These results demonstrate the essential role of ADAM17 in cardiac and vascular developments.  ADAM17 also plays a role in cardiac development. Mice with endothelial cell-specific ADAM17 deletion have cardiac valve enlargement during embryogenesis and progressive cardiomegaly and pronounced systolic dysfunction as adults [Wilson, 2013 #306].

ANGIOGENESIS and VASCULAR REMODELING

ADAM17 in endothelial cell physiology; angiogenesis

Both genetic variation at Tgfbm3 and pharmacological inhibition of ADAM17 modulate postnatal circulating endothelial progenitor cell (CEPC) numbers through transforming growth factor (TGF) beta R1 activity, suggesting that variant ADAM17 is an innate modifier of adult angiogenesis since CEPC numbers correlate with angiogenic potential [Kawasaki, 2014 #378].  Mice with conditional ADAM17 inactivation in smooth muscle cells (Adam17 flox/flox/sm-Cre mice) show no clear changes in angiogenesis [Weskamp, 2010 #14].  In contrast, mice with conditional inactivation of ADAM17 in endothelial cells (Adam17 flox/flox/Tie2-Cre mice) show significantly reduced pathological neovascularization and have no obvious defects in developmental angiogenesis [Weskamp, 2010 #14].  Similarly, endothelial-specific ADAM17 knockdown is reported to reduce collateral circulation formation [Lucitti, 2012 #136].  These results indicate that endothelial ADAM17 plays an essential role in neovascularization in vivo.  Additionally, pharmacological inhibition of ADAM17 prevented retinal neovascularization in a model of ishchemia-induced angiogenesis [Chikaraishi, 2009 #365].

ADAM17 regulates angiogenesis through modulation of endothelial cell proliferation, network formation, and MMP-2 activation [Gooz, 2009 #29;Kwak, 2009 #348].  Vascular endothelial growth factor (VEGF)-A and the receptor VEGFR2 are essential for angiogenesis and VEGFR2 is known to coordinate endothelial cell migration, capillary formation, and vascular permeability [Olsson, 2006 #304].  VEGF-A activates ADAM17 via the extracellular signal-regulated kinase (ERK)/mitogen-activated protein (MAP) kinase pathway and stimulates ADAM17-mediated shedding of VEGFR2 and subsequent crosstalk between VEGFR2 and ERK signaling [Swendeman, 2008 #135].  VEGF-A/VEGFR2 and fibroblast growth factor 7 (FGF7)/FGF receptor 2-IIIb (FGFR2b) have been shown to cause migration of human umbilical vein endothelial cells and epithelial cells, respectively.  These migrations depend on EGFR/ERK1/2 signaling and ADAM17-mediated heparin binding (HB)-EGF shedding [Maretzky, 2011 #336].

Upstream regulation of ADAM17 may be critical to angiogenesis.  Cdc42, a Ras-related GTPase, demonstrates an inhibitory effect on ADAM17-dependent VEGFR shedding. Mice with endothelial cell-specific deletion of Cdc42 do not survive past the angiogenic stage of embryo development and showed increased ADAM17 dependent shedding of VEFGR2. Inhibition of ADAM17 or overexpression of VEGFR2 in Cdc42-deficient cultured endothelial cells protects cells from apoptosis.  These results highlight the complications associated with ADAM17 overexpression on vascular development [Jin, 2013 #305].

Cleavage of cell surface substrates by ADAM17 has downstream signaling consequences. Flt, a surface expressed VEGF receptor, consists of a homodimer or heterodimer with VEGFR.  ADAM17-mediated ectodomain shedding of Flt antagonizes VEGF when Flt is co-expressed with VEGFR by competitive inhibition in an autocrine manner [Raikwar, 2014 #326].  Stimulation of cultured human endothelial cells with interleukin (IL)-6 and interferon gamma increases shedding of neuregulin and ADAM17 specific inhibition blocked this phenomenon.  This neuregulin shedding is speculated to have an important role in endothelial cell signaling pathway, contributing inflammation-associated angiogenesis [Kalinowski, 2010 #138]. F inally, ADAM17 inhibition enhances the expression of thrombospondin1 (TSP1), an anti-angiogenic factor, and overexpression of ADAM17 down-regulates TSP1 in ECs, suggesting that ADAM17 positively regulates angiogenesis by its negative feedback of TSP1 [Caolo, 2015 #530].  Together, these studies demonstrate the balance of endothelial ADAM17 function required for angiogenesis.

ADAM17 and VSMCs; proliferation / migration

In cultured vascular smooth muscle cells (VSMCs), angiotensin II stimulation increases ADAM17 phosphorylation [Elliott, 2013 #15], protein expression, mRNA, and promoter activity [Obama, 2015 #11]. Activation of ADAM17 via its tyrosine phosphorylation contributes to HB-EGF shedding and subsequent growth promoting signals induced by angiotensin II [Elliott, 2013 #15]. Research using VSMCs permanently over-expressing a catalytically inactive dominant-negative mutant of ADAM17 suggests that ADAM17 is essential for EGFR transactivation induced by angiotensin II [Ohtsu, 2006 #13]. Plasma kallikrein stimulates ADAM17 activity via activation of G protein-coupled protease-activated receptors (PARs) 1 and 2. This results in EGFR transactivation and the release of shed substrates such as amphiregulin and TNF alpha independent of bradykinin in VSMCs [Abdallah, 2010 #353].

While some substrates are cleaved by ADAM17 in lipid raft-independent ways [Parr-Sturgess, 2010 #200], it is well established that mature ADAM17 is associated with lipid rafts [Gil, 2007 #344;Tellier, 2006 #345;Tellier, 2008 #346]. Caveolae, a subset of lipid rafts, are cholesterol rich membrane microdomains recognized as important signaling platforms facilitating temporal and spatial localization of signal transduction. Many signaling proteins in the ADAM17-mediated EGFR transactivation pathway, including EGFR and GPCRs such as angiotensin type 1 receptor, G proteins, Src family kinases and ADAM17, are localized to caveolae [Ushio-Fukai, 2006 #350;Gratton, 2004 #351;Takaguri, 2011 #352]. Angiotensin II-induced transactivation of EGFR relies on ADAM17 compartmentalization in caveolae [Takaguri, 2011 #352]. Caveolin 1, a major structural protein of caveolae, is required for TGF beta-mediated ADAM17 activation via phosphorylation of Src and nicotinamide adenine dinucleotide phosphate-oxidase (NADPH) oxidase (NOX) 1-mediated ROS production [Moreno-Caceres, 2016 #507]. Silencing of caveolin-1 in cultured VSMCs can prevent angiotensin II-induced ADAM17 induction and activation [Takayanagi, 2014 #265]. High density lipoprotein (HDL) treatment increases the shedding of TNF and TNF receptors in the non-raft region of the membrane via reduction of ADAM17 content of the lipid raft [Tellier, 2008 #346]. Furthermore, LDL treatment causes rapid shedding of ADAM17 substrate resulting in cell proliferation, cell migration, and endothelial permeability via modulation of membrane fluidity by free fatty acid (FFA) [Reiss, 2011 #347]. Since LDL accumulation in the arterial wall triggers atherosclerosis, ADAM17 activation by FFA released from LDL facilitates atherogenesis by enhancing monocyte transmigration and VSMC proliferation.

ADAM17 and vascular remodeling in vivo

The ADAM17 gene is recognized as a candidate of atherosclerosis susceptibility. ADAM17 mRNA expression and activity is increased in association with atherosclerosis resistance in LDL receptor deficient mice [Holdt, 2008 #270]. In fact, ADAM17 is identified as a central gene associated with angiotensin II-induced AAA in genome-wide transcriptional profiling [Spin, 2011 #268]. Canault et al reported that in high-fat diet fed ApoE knockout mice, ADAM17 is highly expressed in aortic atherosclerotic lesions and suggested that ADAM17 may contribute to the elevated levels of circulating soluble TNF alpha receptors [Canault, 2006 #538].

After a balloon angioplasty, neointimal cells were strongly positive for ADAM17 immunostaining. In a dominant-negative ADAM17 adenovirus treated carotid artery, the intimal hyperplasia was markedly inhibited. These results suggest that ADAM17 activation is involved in neointimal hyperplasia after vascular injury [Takaguri, 2011 #16].

ADAM17 gene silencing by injecting shRNA into the abdominal aortic plaque is reported to enhance plaque stability and improve vascular positive remodeling via attenuation of local inflammation, neovascularization and MMP activation, and enhancement of collagen production  [Zhao, 2015 #269]. In vascular ADAM17 deficient mice or mice treated with human cross-reactive ADAM17 inhibitory antibody, angiotensin II-induced vascular medial hypertrophy and perivascular fibrosis were prevented independent of blood pressure alteration [Takayanagi, 2016 #548].

ADAM17 modulator, regarding EGFR ligands shedding

There are numerous activators of ADAM17-dependent ectodomain EGFR ligand shedding. Platelet-derived growth factor receptor beta stimulation is reported to activate ADAM17 shedding of TNF alpha or TGF alpha and subsequently initiate EGFR signaling pathways [Mendelson, 2010 #282]. The sorting protein phosphofurin acidic cluster sorting protein (PACS)-2 co-localizes with ADAM17 on early endosomes. Loss of PACS-2 from the PACS-2/ADAM17 endosomal couplet results in decreased ADAM17 recycling, stability upon endosomal internalization, cell surface expression, and EGFR ligand shedding [Dombernowsky, 2015 #291]. TNF alpha induces ADAM17 and Src dependent EGFR activation and initiates the ERK/GEF-H1/RhoA signaling pathway, suggesting a mechanistic link between inflammatory and proliferative pathophysiology [Kakiashvili, 2011 #323]. Nox4 is reported to increase ADAM17 expression and HB-EGF release in a ROS-dependent manner [Zeng, 2013 #408].

p38 MAP kinase and Src kinase are reported to activate ADAM17 via interaction with the cytoplasmic domain of ADAM17 which increases ADAM17-mediated shedding of TGF alpha family ligands and activates EGFR signaling [Xu, 2010 #104;Gooz, 2006 #331;Swendeman, 2008 #135]. p38 MAP kinase and ERK-induced ADAM17 activation is based on ADAM17 phosphorylation at Thr735  [Kommaddi, 2011 #332]. Translocation of ADAM17 from the endoplasmic reticulum to the cell surface was found to be dependent on Thr735 phosphorylation [Soond, 2005 #343]. ADAM17 -/- cells infected with either an inactivating mutation (T735A) or an activating mutation (T735D) of Thr735, and the removal of cytoplasmic domain of ADAM17 show no significant difference in activation following IL-1 beta stimulation. MAP-kinase inhibitor can block the activation of cytoplasmic tail deficient ADAM17 or T735A mutant, suggesting that IL-1 stimulates ADAM17 through a mechanism independent of its cytoplasmic domain or Thr735 [Hall, 2012 #494].

Phorbol 12-myristate 13-acetate (PMA), a known activator of the protein kinase C (PKC) pathway, induces ADAM17-mediated HB-EGF shedding and EGFR transactivation. PKC and ADAM17 dependent HB-EGF shedding is triggered by apically localized A1 adenosine receptor (A1ARs) stimulation [Prakasam, 2014 #325]. Such ADAM17-mediated shedding induced by PMA relies on a PKC alpha-dependent signal, acting together with two pathways involving PKC delta and ERK activity, respectively [Kveiborg, 2011 #324]. PKC isoforms are reported to interact with ERK and Lyn [Liu, 2010 #333;Yin, 2009 #334]. However, the phosphorylation of the intracellular domain of ADAM17 may not be essential to ADAM17 activation. Studies with an ADAM17 chimeric construct showed that PMA-induced ADAM17 activation was dependent upon the transmembrane domain but not the intracellular domain [Le Gall, 2010 #335;Maretzky, 2011 #336;Reddy, 2000 #337]. Similarly, pro-TNF alpha cleavage does not depend on cytosolic phosphorylation or interaction [Schwarz, 2013 #498].

Notably, in angiotensin II-induced EGFR ligands shedding, regulatory proteins such as PKC alpha, PKC-regulated protein phosphatase 1 inhibitor 14D (PPP1R14D), and PKC delta affect the shedding of some of ADAM17 substrates without significant effect on protease activity [Dang, 2013 #328]. These results indicate that an increase in ADAM17 activity is not necessary to induce shedding of substrate. PKC alpha and PPP1R14D act on ADAM17-mediated shedding of TGF beta, HB-EGF and amphiregulin, and PKC delta is required for ADAM17-mediated shedding of neureguline [Dang, 2013 #328].

Conformational changes in ADAM17 also affect ADAM17 activity. Changes in the redox environment like phorbol ester modulation of mitochondrial ROS enhances ADAM17 activity and the inactivation of thiol isomerases, specifically protein disulfide isomerase (PDI), is reported as key player. PDI regulates ADAM17 activity by making a conformational change in ADAM17 from an active “open form” to an inactive “closed form” [Willems, 2010 #23]. The non-catalytic domains of ADAM17 are reported to regulate ADAM17 activity via steric hindrance [Stawikowska, 2013 #79].

There are also several inhibitors of ADAM17. Alpha5 beta1 integrin binds to ADAM17 and inhibits ADAM17-mediated HB-EGF shedding [Gooz, 2012 #338]. Thioredoxin-1 (Trx-1) is reported to interact with the cytoplasmic domain of ADAM17 and negatively regulate ADAM17 activity [Aragao, 2012 #496].

Some microRNAs including miR-122 [Tsai, 2009 #74], miR-145 [Doberstein, 2013 #546], and miR-152 [Wu, 2014 #500] target ADAM17. Research with human umbilical vein endothelial cells showed miR-152 could reduce ADAM17-mediated TNF precursor shedding and inhibit cell proliferation and migration by targeting ADAM17 [Wu, 2014 #500]. Recently, inactive rhomboid protein (iRhom) 2 was identified as a key protein which controls the maturation and function of ADAM17 [McIlwain, 2012 #285;Maretzky, 2013 #286;Siggs, 2012 #287;Adrain, 2012 #515]. iRhom2 is recognized as a key regulator of EGFR signaling by controlling the activation and substrate selectivity of ADAM17-dependent shedding [Maretzky, 2013 #286]. In iRhom 1/2 -/- tissues, there is a lack of ADAM17 maturation and strongly reduced EGFR activation [Li, 2015 #289].

ATHEROSCLEROSIS AND ANEURYSMS

ADAM17 and abdominal aortic aneurysm (AAA) / Thoracic aortic aneurysm (TAA)

ADAM17 expression is increased in experimental mouse models of AAA, TAA, and pathogenic vascular remodeling [Geng, 2010 #457]. Temporal and systemic deletion of ADAM17 prevents AAA development, and, notably, attenuates inflammation [Kaneko, 2011 #384]. Caveolae are cholesterol rich membrane microdomains, thought of as important signaling platforms. Caveolin 1 is major structural protein of caveolae. Enhanced ADAM17 expression and EGFR phosphorylation in experimental AAA are markedly attenuated in caveolin 1 knockout mice, suggesting ADAM17 compartmentalization in caveolae [Takayanagi, 2014 #265].

 

ADAM17 in atherosclerosis, vasculitis, and AAA in human

ADAM17 expression is reported in human atherosclerotic plaques [Canault, 2006 #538]. Microparticles isolated from human atherosclerotic plaques are shown to carry active ADAM17 on their surfaces. These microparticles enhance the shedding of TNF, TNF receptor 1 (TNFR1), and endothelial protein C receptor (EPCR) at endothelial cells, indicating ADAM17-positive microparticles could regulate the inflammatory balance in culprit lesions [Canault, 2007 #539]. Moreover, the ADAM17 at advanced human atherosclerotic lesions is in its catalytically active form and ADAM17-expressing cells are co-localized with CD68-positive cells of monocytic origin [Oksala, 2009 #31]. These results suggest a role for ADAM17 in monocyte homing, migration, and proliferation in human atherosclerotic lesions.

Increased ADAM17 concentration found in plasma samples from patients with active proteinase-3 positive antineutrophil cytoplasmic autoantibodies-associated vasculitis might account for the associated vascular complications [Bertram, 2015 #263]. In human AAA samples obtained during surgical repair, ADAM17 was overexpressed in aortic the wall compared to normal aortae [Kaneko, 2011 #384;Kaneko, 2011 #384]. Enzymatically active ADAM10 and ADAM17 carried on membrane microvesicles in the intraluminal thrombus of human AAA implicate persistent ADAM activity as a key contributor to disease progression [Folkesson, 2015 #535].

Local ADAM17 expression and mRNA abundance were quantified in human AAA. ADAM17 protein and mRNA expression were higher in the transition zone between the lesion and non-dilated aorta than in the mid-portion of the lesion (insert figure). Furthermore, ADAM17 was expressed in CD68-positive macrophages in the media and adventitia obtained from the transition zone in AAA [Satoh, 2004 #407]. In addition, an analysis of the ADAM17 promoter polymorphism rs12692386 has revealed a positive association with AAA formation [Li, 2014 #502]. Together, ADAM17 is important in the pathogenesis of AAA and may be a useful therapeutic target or biomarker for AAA susceptibility.

INFLAMMATION

ADAM17 and Vascular inflammation in vivo

Mice with reduced ADAM17 levels in all tissues (ADAM17 ex/ex) have impaired shedding of EGFR ligands, defective epithelial cell regeneration, and breakdown of the intestinal barrier. This phenotype substantially increases susceptibility to inflammation in dextran sulfate sodium colitis [Chalaris, 2010 #375]. In vivo efferocytosis and associated anti-inflammatory effects are enhanced in ADAM17 ex/ex mice, suggesting that ADAM17 modulates apoptotic cell phagocytosis [Driscoll, 2013 #28].

When exposed to endotoxin, endothelial ADAM17 knockout mice have decreased vascular permeability and leukocyte recruitment. The ADAM17 deficient endothelial cells also released junctional adhesion molecule 1 (JAM-A), CX3CL1, TNF alpha, and IL-6, [Dreymueller, 2012 #142].

Leukocytic ADAM17 plays a role in multiple models inflammation. Mice lacking ADAM17 on hematopoietic cells have increased resistance to endotoxin infection [Long, 2010 #380]. The same mouse model is protected against acute lung inflammation by decreasing intra-alveolar neutrophil levels and the shedding of IL-6R, L-selectin, and TNF alpha [Arndt, 2011 #379] in conditional ADAM17 null mice with an ADAM17 deficiency in all leukocytes. ADAM17 inactivation in myeloid cells or temporal inactivation at 6 week reduces endotoxin shock lethality by preventing increase of serum TNF alpha levels in mice [Horiuchi, 2007 #209]. When ADAM17 function is antagonized with an ADAM17 recombinant prodomain in a mouse model of lipopolyssacharaide-induced endotoxemia, the release of sTNF alpha is decreased and the inflammatory response to endotoxin is ameliorated [Li, 2009 #366].

Tissue inhibitor of metalloproteinase 3 (Timp3) is an inhibitor of ADAM17 and MMPs. Mice with both heterozygous knockdown of Timp3 and insulin receptor heterozygosity (Insr+/-) develop glucose intolerance, insulin resistance, and vascular inflammation [Federici, 2005 #367]. In the face of twenty week high fat diet challenge, mice with macrophage Timp3 overexpression were protected from insulin resistance, adipose tissue inflammation, and non-alcoholic fatty liver [Menghini, 2012 #385]. Further, high fat diet fed mice transgenic for Timp3 overexpression and deficient in the low-density lipoprotein receptor were partially protected from atherosclerosis [Casagrande, 2012 #44]. These results indicate a pro-inflammatory role for ADAM17 in in vivo models of inflammation and are supported by reports which demonstrate that systemic inflammation is promoted in mice with deficient in the ADAM17 inhibitor Timp3 [Smookler, 2006 #368;Gill, 2010 #369;Mohammed, 2004 #370;Mahmoodi, 2005 #371].

ADAM17 and Vascular inflammation in vitro

 

Atherosclerosis is accelerated by chronic inflammation. Macrophages and monocytes are recognized as contributors to the inflammatory component of atherogenesis [Gerhardt, 2015 #275]. ADAM17 has been shown to down-regulate macrophage activation by shedding colony-stimulating factor receptor [Rovida, 2001 #381] which may influence atherogenesis [Rajavashisth, 1998 #382] and renal tubule repair after acute kidney injury [Zhang, 2012 #383]. Monocyte ADAM17, but not endothelial ADAM17, facilitates the completion of trans-endothelial migration by accelerating the rate of diapedesis [Tsubota, 2013 #274]. L-selectin shedding is reported to regulate morphological changes in leukocytes and monocytes during transmigration. Inhibition of L-selectin shedding by an ADAM17 inhibitor significantly increased pseudopodial extensions in transmigrating monocytes [Rzeniewicz, 2015 #276]. However, ADAM17-null monocytes are reported to show no acceleration of infiltration into the peritoneal cavity in spite of elevated cell-surface L-selectin level, meanwhile ADAM17-null neutrophils showed increased initial recruitment [Tang, 2011 #311].

Adhesion molecules such as vascular cell adhesion protein 1 (VCAM-1), intercellular adhesion molecule 1 (ICAM-1), and L-selectin regulate leukocyte recruitment, a key step in atherogenesis [Davies, 1993 #313;Cybulsky, 2001 #314;Eriksson, 2001 #315;Galkina, 2006 #316]. ADAM17 cleaves these adhesion molecules and regulates inflammation. ADAM17-mediated shedding of VCAM-1 produces a soluble form of VCAM-1 [Garton, 2003 #205]. Circulating VCAM-1 can be a marker of atherosclerotic lesions in diabetic patients with atherosclerosis [Otsuki, 1997 #317].

Ectodomain shedding of ICAM-1 is ADAM17-dependent; pharmacological or genetic inhibition of ADAM17 can block ICAM-1 shedding, resulting in up-regulation of cell adhesive function [Tsakadze, 2006 #197]. Similar to soluble VCAM-1, circulating ICAM-1 is also reported to serve as a molecular marker for atherosclerosis [Hwang, 1997 #318].

Analyses of ADAM17-defficient cells impaired in L-selectin shedding have demonstrated both ADAM17-dependent [Peschon, 1998 #182] and independent [Walcheck, 2003 #321] L-selectin shedding. Rapid ADAM17-mediated L-selectin shedding enhances leukocyte accumulation on the vascular wall [Walcheck, 1996 #319], and constitutive shedding of soluble L-selectin facilitates leukocyte-endothelium interaction [Schleiffenbaum, 1992 #320]. CD44, a glycoprotein which promotes cell adhesion and migration, recruits inflammatory cells into the vessel wall and activates vascular cells under atherogenic conditions. ADAM17 and ADAM10 terminate CD44 cell matrix adhesion by independent signaling pathways [Nagano, 2004 #430].

Stimulation of the thromboxane A2 receptor (TP) induces rapid ADAM17-mediated shedding of cell surface CX3CL1, a key factor in recruiting monocytes. Shedding of CX3CL1 results in recruitment of leukocytes to vascular inflammatory sites and enhanced adhesion once recruited [Tole, 2010 #277]. Similarly, PMA rapidly enhances the shedding of CX3CL1 by ADAM17 [Garton, 2001 #193]. Treatment with PMA is also known to enhance various ADAM17-mediated shedding including interleukin-6 receptor (IL6R) [Mullberg, 1993 #312], VCAM-1 [Garton, 2003 #205], and ICAM-1[Tsakadze, 2006 #197].

Cell adhesion molecules critical to cell junctions are also susceptible to cleavage by ADAM17, granting ADAM17 regulation of vascular permeability. JAM-A is known to promote monocyte [Woodfin, 2007 #280;Khandoga, 2005 #281] and endothelial cell [Cooke, 2006 #278;Naik, 2006 #279] migration which facilitates both vascular inflammation and angiogenesis. At sites of vascular inflammation, ADAM17-mediated shedding of JAM-A may interfere in leukocyte infiltration of extravascular tissues [Koenen, 2009 #143]. Taken together, this data reveals ADAM17 as a critical mediator of local and systemic inflammatory action.

ADAM17 modulators, regarding vascular inflammation

Many factors regulate ADAM17. Conserved ADAM-seventeeN Dynamic Interaction Sequence (CANDIS) is a short juxtamembrane segment of 17 amino acid residues. CANDIS is known to be involved in substrate recognition and may also regulate ADAM17 shedding activity by interacting with lipid bilayers [Dusterhoft, 2015 #505]. Site-specific O-glycosylation mediated by distinct polypeptide GalNAc-transferase isoforms is also reported to widely modulate ADAM mediated shedding [Goth, 2015 #506]. Nitric oxide (NO) activates ADAM17 by nitrosylation of the inhibitory motif of the ADAM17 prodomain [Zhang, 2000 #307].

Neutrophilic L-selectin ligation by endothelial E-selectin activates neutrophils and causes the redistribution and co-clustering of ADAM17 and L-selectin to the neutrophilic uropod. The local shift in ADAM17 density on the cell suface modulates leukocyte rolling, arrest, and extravascular infiltration [Schaff, 2008 #93]. Ceramide 1-phosphate (C1P) produced by ceramide kinase (CERK) binds ADAM17 with high affinity and inhibits TNF alpha shedding via modulation of ADAM17 activity [Lamour, 2011 #492]. Oxidative stress [Wang, 2009 #146], hypoxia, and ER stress [Rzymski, 2012 #65] activate ADAM17 and cause shedding of pro-inflammatory agents in feed-forward mechanisms. Apoptosis is also reported to increase ADAM17-mediated IL-6 receptor and L-selectin shedding [Chalaris, 2007 #242;Marin, 2002 #308;Walcheck, 2006 #309].

Interestingly, neutrophil ADAM17 stimulation relies on different signal transduction pathways depending on conditions. Fas signaling activation of ADAM17 involves caspase-8, Bid, and mitochondrial ROS, whereas ADAM17 stimulation during neutrophil activation involves p38 MAPK and ERK [Wang, 2011 #490]. Nardilysin (N-arginine dibasic convertase) enhances ADAM17 activity and TNF alpha shedding [Hiraoka, 2008 #322]. The scavenger receptor CD163 is regarded as a surrogate marker of TNF alpha released from macrophages, and in macrophages subjected to inflammatory stimuli, ADAM17-mediated shedding of CD163 is reported to be up-regulated [Etzerodt, 2010 #195]. Polo-like kinase 2 (PLK2) phosphorylates ADAM17 which promotes pro-TNF alpha and TNF receptors shedding.  PLK2 expression is up-regulated under inflammatory conditions and ADAM17-mediated proteolytic events [Schwarz, 2014 #303].

GENETICS

ADAM17 SNPs and cardiovascular risks

ADAM17 single nucleotide polymorphisms (SNPs) associate with the presence of pulmonary arteriovenous malformations in hereditary hemorrhagic telangiectasia 1, indicating genetic variations in ADAM17 can promote TGF beta-regulated vascular diseases [Kawasaki, 2014 #378]. ADAM17 SNPs are also reported to contribute to obesity risk [Junyent, 2010 #38], soluble TNF plasma levels, and the risk of cardiovascular death [Morange, 2008 #39]. Better understanding the pathological effects of ADAM17 SNPs may enable highly specific approaches for cardiovascular disease treatments.

KIDNEY AND RENAL CELLS

ADAM17 and kidney diseases

In mice, two months of angiotensin II infusion causes increased expression of ADAM17 in the kidney. ADAM17, which was redistributed to the apical membrane, increased TGF alpha shedding and EGFR transactivation. Consequently, these mice develop glomerulosclerosis, tubular atrophy, and interstitial fibrosis [Lautrette, 2005 #386]. These pathological outcomes were blunted by the specific inhibition of ADAM17 [Lautrette, 2005 #386].

The db/db mouse, a model of obesity and diabetes in which mice lack leptin receptors, has increased angiotensin 1 converting enzyme (ACE) 2 and ADAM17 in the kidney. Treatment with the thiazolidine rosiglitazone decreased both ACE2 and ADAM17 while also improving hyperglycemia and renal injury [Chodavarapu, 2013 #497]. Marked induction of ADAM17 in interstitial fibrosis and tubular atrophy is observed in an ischemia-reperfusion model in the rat kidney [Mulder, 2012 #389]. Similarly, fibrosis after an ischemia-reperfusion injury or unilateral ureteral obstruction is attenuated in ADAM17 hypomorphic mice and mice with inducible knock out of ADAM17 in proximal tubule [Kefaloyianni, 2016 #550], suggesting ADAM17 as a therapeutic target for renal fibrosis.

The OVE26 mouse is a model of type 1 diabetes characterized by overexpression of calmodulin and insufficient insulin production. ADAM17 activity is elevated in these mice, however, pharmacological inhibition of ADAM17 decreases the production of type IV collagen, Nox4 and NADPH oxidase activity. These data suggest that hyperglycemia increases extracellular matrix protein expression in kidney cortex via ADAM17 activation, and enhanced oxidative stress via Nox activation [Ford, 2013 #339]. In the Akita mouse, another model of type 1 diabetes, ADAM17 and ACE2 protein expression is increased. Insulin treatment of Akita mice attenuates the expression of renal ACE2 and ADAM17 [Salem, 2014 #387]. Finally, in STZ-diabetic mice, Src kinase inhibitors ameliorated ADAM17 activation in the cortical kidney, albuminuria, ERK and EGFR phosphorylation, glomerular collagen accumulation, and podocyte loss [Taniguchi, 2013 #341]. These studies demonstrate ADAM17 plays a significant role in the type 1 diabetic pathology.

In various human renal diseases, ADAM17 is strongly induced in podocytes, proximal tubules, and peritubular capillaries, and renal ADAM17 expression is significantly associated with glomerular and interstitial injury or renal function [Melenhorst, 2009 #416]. Patients with acute kidney injury or chronic kidney disease have high soluble amphiregulin in their urine and both ADAM17 and amphiregulin expression are strongly correlated with markers of fibrosis in kidney biopsies [Kefaloyianni, 2016 #550].

Polycystic kidney disease (PKD) is a genetic disorder leading to the formation of cysts in kidneys. The study of animal models of autosomal recessive PKD has revealed that ADAM17 expression is increased in the collecting duct epithelial cells of the cystic kidney. Activation of ADAM17 induces constitutive shedding of HB-EGF, amphiregulin, and TGF alpha, resulting in EGFR/MAPK/ERK pathway activation [Beck Gooz, 2014 #342].

 

ADAM17 in renal cells

In mesangial cells, angiotensin II mediates fibronectin expression via autocrine effects on HB-EGF and TGF beta [Uchiyama-Tanaka, 2001 #355]. TGF beta also mediates fibronectin expression via EGFR transactivation [Uchiyama-Tanaka, 2002 #356]. TGF beta induction by high-glucose stimulation is suggested to require ADAM17-mediated EGFR transactivation in mesangial cells [Uttarwar, 2011 #354]. High glucose induces ADAM17 transcriptional up-regulation in mesangial cells and augments activity through hypoxia inducible factor (HIF)-1 alpha [Li, 2015 #64]. Src kinase inhibition is reported to block high glucose-induced EGFR transactivation and collagen synthesis, indicating Src as upstream of ADAM17 [Taniguchi, 2013 #357]. The stimulation of 5-hydroxy-tryptamin (5-HT) 2A receptor [Gooz, 2006 #331] or bradykinin B2 receptor [Dey, 2010 #358] are also reported to transactivate EGFR via ADAM17-mediated ligand shedding. Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) causes ADAM17-dependent HB-EGF shedding EGFR transactivation in proximal tubular cells [Rayego-Mateos, 2013 #359]. Insert figure.

ADAM17 is also revealed to be responsible for alpha-cut of klotho cleavage in renal cells, not beta-cut of klotho [van Loon, 2015 #533]. Finally, ADAM17-mediated kidney injury molecule-1 (KIM-1) shedding is accelerated by worsening cellular injury and soluble KIM-1 competitively inhibits efficient clearance of apoptotic cells, suggesting the role of ADAM17 in acute kidney injury[Gandhi, 2014 #501].

METABOLIC

ADAM17 and metabolic disorders

In mice, high fat diet consumption increases body weight, adiposity, systolic blood pressure, fasting blood glucose, and fasting lipid levels while it decreases adiponectin levels. Temporal systemic ADAM17 deletion (TaceMx1) in mice attenuated these changes [Kaneko, 2011 #45]. In addition, increased macrophage infiltration and the expression of TNF alpha and monocyte chemoattractant protein-1 in epididymal adipose tissue induced by high fat diet are also attenuated in TaceMx1, suggesting that ADAM17 is an important mediator in the development of obesity-induced metabolic and inflammatory disorders [Kaneko, 2011 #45]. ADAM17 +/- mice are partially protected from obesity and insulin resistance compared with wild type mice [Serino, 2007 #42].

Beyond genetic manipulation, exercise and pharmacological intervention may improve the ADAM17 phenotype and subsequent metabolic outcomes. Moreover, exercise training alone and in combination with metformin decrease ADAM17 protein in the kidney and subsequent ACE2 shedding in db/db mice [Somineni, 2014 #414]. In rats, high fat/high sucrose diet did not alter the neuregulin/ErbB pathway directly, however, exercise training and returning obese rats to a normal diet increased ADAM17 protein levels and decreased TIMP3 protein levels in skeletal muscle. This resulted in increased cleavage of neuregulin 1, activating ErbB4 [Ennequin, 2015 #409]. These results suggest diet and exercise intervention can improve energy metabolism through ADAM17 modulation.

Pharmacological interventions may also provide a new strategy for combatting metabolic diseases. ADAM17 inhibitors have been shown to improve insulin sensitivity in fructose-fed rats [Togashi, 2002 #391] or high fat diet-fed mice[de Meijer, 2011 #393].

CARDIAC DISORDERS

ADAM17 and cardiovascular pathophysiology

Overview

ADAM17 is expressed in various cells including endothelial cells, VSMCs, fibroblasts, and monocytes. Previous investigations of ADAM17 in cardiovascular pathophysiology have revealed ADAM17 to be a highly regulated controller of disease progression. The influence of ADAM17 on transcription via TNF alpha induction of nuclear factor kappa B (NFkB) is well established. A recent study demonstrated, however, that ADAM17 does not act solely on the transcriptional level. Transgenic mice overexpressing ADAM17 did not experience enhanced TNF alpha shedding [Yoda, 2013 #388]. In this section, we highlight in vivo and in vitro findings regarding the role of ADAM17 and the regulation of ADAM17 in cardiovascular pathophysiology. Moreover, we also review the clinical studies investigating the role of ADAM17 in human cardiovascular diseases.

ADAM17 and cardiac diseases in vivo

In a two-week angiotensin II infusion model of vascular remodeling, ADAM17 expression and EGFR activation were enhanced in the cardiac vasculature in a HIF-1 alpha dependent manner [Obama, 2015 #11]. Inhibition of EGFR activation or endoplasmic reticulum stress significantly reduced the increased ADAM17 expression, demonstrating that angiotensin II induces vascular remodeling by EGFR activation and ER stress via a signaling mechanism involving ADAM17 induction [Takayanagi, 2015 #264]. Furthermore, treatment with ADAM17 small-interfering RNA to inhibit ADAM17 expression can prevent angiotensin II-induced cardiac hypertrophy and fibrosis. This prevention is also accompanied by a reduction in biomarkers associated with angiotensin II-induced cardiac hypertrophy and fibrosis including BMP, alpha-skeletal actin, beta- MHC, type I collagen, type II collagen, fibronectin [Wang, 2009 #108], and MMP-2 [Odenbach, 2011 #114]. Pressure overload induced upregulation of integrin beta1 is enhanced in the ADAM17 knockdown mouse. However, hypertrophy induced by a subpressor dose of angiotensin II is not affected by cardiac ADAM17 knockdown, suggesting that ADAM17 has a protective function in pressure-overload cardiomyopathy via cleavage of integrin beta1 [Fan, 2016 #549].

Left ventricular expression of ADAM17 and Nox4 are up-regulated in an abdominal artery coarctation model of cardiac hypertrophy, [Zeng, 2013 #408]. Cardiac dysfunction and hypertrophy induced by transverse aortic constriction are enhanced in cardiomyocyte-specific ADAM17 knockdown mice, offering more evidence that ADAM17 may be protective under pathological conditions in the heart.

In ischemic mouse hearts, activation of lectin-like oxidized LDL receptor-1 (LOX-1) plays a role in inflammation, apoptosis, and collagen signaling. Expression of ADAM10 and ADAM17 are increased in ischemic hearts. Genetic deletion of LOX-1 improves the ischemic heart phenotype and reduces ADAM10 and ADAM17 activity  [Lu, 2012 #491]. In an experimental rat model of pulmonary hypertension-induced right ventricular remodeling, the expression of ADAM15 and ADAM17 was up-regulated in the right ventricle. ADAM15/ADAM17 expression and right ventricular fibrosis were attenuated following estrogen therapy [Nadadur, 2012 #495].

In a model of myocardial infarction, mice with cardiomyocyte-specific ADAM17 knockdown had, in comparison to control mice, higher rates of cardiac rupture, more severe left ventricular dilation, suppressed ejection fraction, and compromised survival [Fan, 2015 #504]. Activation and expression of VEGFR2 were reduced in the infarcted myocardium in ADAM17 knockdown mice, highlighting the role of cardiomyocyte ADAM17 in recovery after myocardial infarction via modulating VEGFR2 transcription and angiogenesis. A similar myocardial infarction experimental model showed that ADAM17 expression, along with decreased TIMP-3 and increased TNF alpha expression within one week after acute myocardial infarction, was associated with cardiac remodeling [Zheng, 2016 #509]. These data clearly indicate the critical role of ADAM17 in cardiac disease.

The deoxycorticosterone acetate (DOCA)-salt model of neurogenic hypertension has revealed that DOCA-salt treatment enhances ADAM17 expression and activity in the hypothalamus and significantly reduces ACE2 expression and activity in brain. ACE2 activity is increased in the cerebrospinal fluid, resulting in increased blood pressure, hypothalamic angiotensin II levels, inflammation, impaired baroreflex sensitivity and autonomic dysfunction. Knockdown of ADAM17 in the brain blunted the development of hypertension and restored ACE2 activity and baroreflex function, indicating that ADAM17-mediated shedding of ACE2 contributes to the development of neurogenic hypertension [Xia, 2013 #405].

The role of ADAM17 in human cardiac diseases

Systemic levels of ADAM17 and TNF alpha are higher in acute myocardial infarction (AMI) patients compared to patients with stable angina. ADAM17 is highly expressed at the site of ruptured plaques in AMI patients. This local ADAM17 expression is independently and significantly correlated with adverse cardiac events in the follow up period [Satoh, 2008 #262]. Peripheral blood mononuclear cells obtained from AMI patients exhibited high ADAM17 levels under spontaneous and PMA-stimulated conditions compared to normal subjects. These high ADAM17 levels were correlated with in-hospital complications [Shimoda, 2005 #411]. Moreover, a score evaluated from ADAM17 circulating substrates (sICAM-1, sVCAM-1, sIL6R, and sTNFR1) is reported to be able to predict recurring major cardiovascular events [Rizza, 2015 #503]. ADAM17 SNPs also contribute to the risk of Kawasaki disease and involved are in secondary coronary artery lesions via the TGF beta/SMAD3 signaling pathway [Peng, 2016 #508].

Other ADAMs in cardiovascular pathophysiology

ADAMs in human cardiovascular disease

In human atherosclerotic lesions ADAM10 is expressed and its expression is associated with plaque progression and neovascularization [Donners, 2010 #423]. Increased ADAM10 expression accompanies decreased N-cadherin when apoptosis increases [Musumeci, 2014 #529]. This association between ADAM10 and vascular remodeling is further supported by some animal models. A study with CaCl2-induced TAA showed that ADAM10 expression was significantly increased in the intimal and medial layers of a TAA [Geng, 2010 #457]. Another study using diabetic minipigs showed that ADAM10 expression was increased in vascular segments obtained from coronary artery restenosis, implicating the role of ADAM10 in neointimal formation [Yang, 2013 #523]. ADAM9 and ADAM15 are also expressed in human atherosclerotic lesions [Al-Fakhri, 2003 #458], co-localized with CD68-positive cells of monocytic origin in the plaques [Oksala, 2009 #31]. ADAM8 and ADAM15 are highly expressed in the medial layer in patients with ascending aortic dissection compared to that in patients with dilatation of the ascending aorta [Levula, 2011 #485].

In high graded carotid artery lesions, macrophages and smooth muscle cells are positive for ADAM8, ADAM10, ADAM12, ADAM15, and ADAM17. The luminal surface of endothelial cells are positive for ADAM15, and neovessels are positive for ADAM12 [Pelisek, 2012 #512]. ADAM33 is expressed in smooth muscle cells and inflammatory cells within atherosclerotic lesions [Holloway, 2010 #455]. Moreover, ADAM33 SNPs are reported to correlate with the extent of atherosclerosis in coronary artery disease patients [Holloway, 2010 #455] and cardiovascular mortality [Figarska, 2013 #456]. Collectively, ADAMs contribute to arterial physiology and progression of atherosclerosis.

The risk alleles of ADAM8 SNPs are associated with elevated serum soluble ADAM8 and the risk of myocardial infarction in two independent cohorts [Raitoharju, 2011 #488]. This result is supported by an animal study investigating the correlation between ADAM8 expression and myocardial infarction which showed that myocardial infarction induced an early increase in remote ADAM8 expression in the rat heart (unclear) [Vuohelainen, 2011 #489].

ADAM19 has been implicated as a key contributor to the development of renal pro-fibrotic and pro-inflammatory processes. Increased expression of ADAM19 in mesangial cells, proximal tubules, and peritubular capillaries is associated with glomerular damage, interstitial fibrosis, and declining renal function [Melenhorst, 2006 #460]. A study with renal transplant patients showed that ADAM19 mRNA was significantly higher in chronic allograft nephropathy and patients with acute rejection exhibited elevated ADAM19 expression in the renal endothelium [Melenhorst, 2006 #454].

ADAM28 expression in blood mononuclear cells significantly correlate with parameters of metabolic syndrome including body mass index and relative fat, suggesting the role of ADAM28 in human metabolic conditions [Jowett, 2012 #481].

The role of ADAMs in cardiovascular pathophysiology

ADAM10 exhibits strong influence over cardiovascular phenotypes by cleaving a large variety of proteins. ADAM10 can affect inflammation by cleaving CD44 [Nagano, 2004 #430], CX3CL1 [Hundhausen, 2003 #420], CXCL16 [Abel, 2004 #514], IL6R [Matthews, 2003 #417], RAGE [Raucci, 2008 #418], and TNF alpha [Hikita, 2009 #419]. ADAM10 may promote or inhibit angiogenesis by cleaving JAM-A [Koenen, 2009 #143], Notch [Zhang, 2010 #428], NRP-1 [Swendeman, 2008 #135], VEGFRII [Donners, 2010 #423] and VE-cadherin [Schulz, 2008 #422]. It affects cell proliferation or migration by cleaving beta cellulin [Sahin, 2004 #424;Sanderson, 2005 #425], and HB-EGF [Ohtsu, 2006 #13]. It affects collagen turnover by cleaving DDR1 [Shitomi, 2015 #429]. It affects apoptosis by cleaving RANKL [Hikita, 2006 #426]. It affects blood pressure by cleaving corin [Jiang, 2011 #427] (Table2). Furthermore, ADAM10 can affect acute kidney injury by cleaving meprin A, a membrane-associated metalloproteinase in proximal tubules, since meprin A is one of the key players in acute kidney injury [Herzog, 2014 #524]. This paragraph should be turned into a table.

ADAM family proteins play notable roles in cardiovascular pathophysiology by mediating inflammation, angiogenesis, cell proliferation, and cell migration (Table2). As mentioned previously, most substrates can be cleaved by multiple ADAMs. The interaction between each ADAM and its substrates may vary depending upon anatomical location, cell type, cellular location, andthe pathophysiological conditions. For example, ADAM10-mediated Notch shedding is ligand-dependent, whereas ADAM17-mediated Notch shedding is ligand-independent [Bozkulak, 2009 #453]. L1 and CD44 are cleaved by ADAM17 at the cell surface and soluble forms are released into the extracellular space, whereas they are cleaved by ADAM10 in endosomes and soluble forms are released from the cell as a component of exosomes [Stoeck, 2006 #191]. Neuregulin, cleaved by ADAM17 at cell surface, is cleaved in the Golgi apparatus by ADAM19 [Yokozeki, 2007 #173]. Because the relationships between ADAM proteins and their substrates are so diverse, utilitzing ADAM proteins as therapeutic targets is challenging and requires more investigation.

ADAMs can also serve in non-proteolytic manner. ADAM15 regulates endothelial permeability and neutrophil migration by promoting Src/ERK1/2 signaling in a protease activity-independent manner [Sun, 2010 #483], resulting in atherosclerosis [Sun, 2012 #486]. ADAM28 is reported to bind to C1q, attenuating C1q-induced cell death [Miyamae, 2016 #478]. It can bind cell adhesion molecules such as P-selectin glycoprotein ligand 1 to promote leukocyte rolling and adhesion to endothelial cells [Shimoda, 2007 #480], or integrin alpha 4 beta 1 enhancing alpha 4 beta 1-dependent cell adhesion to VCAM1, regulating spatial and temporal transendothelial migration of immune cells [McGinn, 2011 #477].

Therapeutic Potential

Specific inhibitors which selectively target ADAM17 and do not inhibit other metalloproteinases have been synthesized in recent years [Bahia, 2010 #402;DasGupta, 2009 #132]. Using animal models, ADAM17 inhibition has ameliorated autoimmune disorders such as rheumatoid arthritis [Moss, 2008 #90], cardiovascular disorders such as renal fibrosis [Lautrette, 2005 #386;Mulder, 2012 #389], intestinal reperfusion injury [Souza, 2007 #394], and polycystic kidney disease [Dell, 2001 #395;Sweeney, 2003 #396]. Genetically modulation of ADAM17 also indicates the positive potential of ADAM17 inhibition in inflammatory states such as septic shock [Chalaris, 2010 #375;Long, 2010 #380;Horiuchi, 2007 #209]. In spite of these promising in vivo results, pre-clinical trials and clinical trials using ADAM17 inhibitors had to be discontinued due to hepatotoxicity [Moss, 2008 #90] or lack of efficacy [Thabet, 2006 #403]. The failure of these trials may be due to the fact that ADAM17 inhibition disrupts normal physiological homeostasis. ADAM17 deficient mice die shortly after birth because of a variety of defects [Horiuchi, 2007 #209] but mice with reduced ADAM17 level in all tissues (ADAM17 ex/ex) survive, albeit with substantially increased susceptibility to inflammation [Chalaris, 2010 #375]. This indicates that there may be a desirable range for ADAM17 activity that inhibitors may be designed to target. Importantly, the multitude of ADAM17 regulators should also be considered as potential therapeutic targets. For example, iRhom2 is an essential determinant of ADAM17-dependent substrate shedding that may be selected for  selective inactivation of ADAM17 [Issuree, 2013 #404].

Another approach is to analyze and utilize ADAM protein-modulating effects of existing approved drugs. Aspirin is widely used for the thrombosis prevention in the coronary and cerebral arteries. Aspirin at high concentrations is reported to induce ADAM17-mediated shedding of glycoprotein b alpha and GPV [Aktas, 2005 #329]. Non-steroidal anti-inflammatory drugs with diphenylamine structure cause a reduction in the neutrophil intracellular ATP concentration which influences ADAM17-dependent L-selectin shedding at the cell surface [Gomez-Gaviro, 2002 #330]. 1,25-dihydroxyvitamin D, the hormonal form of vitamin D, has potential anti-inflammatory, anti-atherosclerotic effects. It is widely used to treat patients with chronic kidney disease because 1,25-dihydroxyvitamin D is proven to significantly ameliorate both secondary hyperparathyroidism and patient survival via renal and cardiovascular protective effects [Teng, 2005 #412]. 1,25-dihydroxyvitamin D is reported to inhibit ADAM17 expression through the induction of C/EBP beta [Arcidiacono, 2015 #415] and prevent ADAM17/TNF alpha-mediated secondary hyperparathyroidism, fibrotic and inflammatory lesions in the renal parenchyma, and systemic inflammation [Dusso, 2010 #413]. Finally, 1,25-dihydroxyvitamin D is also reported to cause ADAM10-dependent TNF alpha shedding in VSMCs [Yang, 2015 #531]. These agents regulating ADAM protein activity could be considered for novel therapeutic approaches if the mechanisms are further clarified.

Concluding remarks

Given that ADAMs are ubiquitously expressed in somatic cells and that they act upon a wide range of substrates, the ADAM family of proteins, ADAM17 in particular, have important, intricate roles in cell signaling. The accumulation of research in this area steadily shed light on the role of ADAM17 and other ADAMs on cardiovascular diseases. While ADAMs are essential to normal development and cardiovascular homeostasis, excess activation of ADAMs promotes inflammatory action and cardiovascular disorders. ADAM17 inhibition is thought to be promising therapeutic target for cardiovascular and renal diseases because of its effects on key pathogenic processes. We hope further research based on the evidence highlighted in this review can help to elucidate ADAM protein contributions to cardiovascular pathophysiology and identify the therapeutic potential of ADAM protein targeting.

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