Recent Highlights of ATVB Arterial Smooth Muscle Valerie Z. Wall, Karin E. Bornfeldt
T
he prime function of the arterial smooth muscle cell (SMC) in adult individuals is to contract and relax, thereby regulating blood flow to target tissues. However, in several vascular diseases, arterial SMCs in the adult vessel undergo major changes in structure and function. For example, SMCs can take on properties that allow them to proliferate and migrate, to promote calcification, to undergo apoptosis, and to orchestrate other cell types to take on different properties. These altered SMCs are often viewed as vascular troublemakers. However, these responses can also be seen as an attempt of the SMC to mend a diseased blood vessel. The plasticity and diversity of SMC populations and the presence of SMC progenitor cells contribute to the growing complexity of SMC biology. In the past couple of years, several new mechanisms that allow SMCs to proliferate, migrate, promote calcification, and communicate with and respond to other cell types have been discovered. Much of this work has been published in ATVB, and some of that body of work is highlighted in this article. We emphasize areas that have generated substantial interest, including novel mechanisms governing SMC phenotypic switching, SMC progenitor cells, noncoding RNAs as regulators of SMC function and phenotype, novel mechanisms of cross talk between SMCs and other cells and differences in regulation of SMCs and other vascular cells.
Plasticity of the Arterial SMC The arterial SMC exhibits an impressive potential for plasticity. SMCs have been known to exist in different phenotypic states for decades, and the switch from a quiescent contractile phenotype to a synthetic proliferative phenotype is thought to play an important role in cardiovascular disease (for a historical perspective1). Thus, switching to a synthetic phenotype is generally thought to enable SMCs to migrate and proliferate in atherosclerotic lesions,2 thereby forming a fibrous cap covering a core of macrophages and other immune cells, and in injured vessels after angioplasty. However, additional lineagetracing studies are needed to address the origin of intimal lesion cells.2 Conversion of an early fatty streak atherosclerotic lesion to a fibrotic lesion is thought to cause permanent nonreversible changes in the artery wall. In that sense, SMC phenotypic switching can be considered to be detrimental. From the Departments of Pathology (V.Z.W., K.E.B.) and Medicine, Division of Metabolism, Endocrinology and Nutrition (K.E.B.), Diabetes and Obesity Center of Excellence, University of Washington School of Medicine, Seattle. Correspondence to Karin E. Bornfeldt, PhD, Departments of Medicine and Pathology, Diabetes and Obesity Center of Excellence, 850 Republican St, University of Washington, Seattle, WA 98109. E-mail [email protected] (Arterioscler Thromb Vasc Biol. 2014;34:2175-2179.) © 2014 American Heart Association, Inc. Arterioscler Thromb Vasc Biol is available at http://atvb.ahajournals.org DOI: 10.1161/ATVBAHA.114.304441
However, at later stages of atherogenesis, fibrotic lesions are often more stable than acellular necrotic lesions, making them less likely to cause clinical symptoms. In such lesions, SMC phenotypic switching and survival could be considered beneficial.3 Different SMC phenotypes overlap and probably exist as a continuum in vivo. Recent research on intracellular regulators of SMC phenotypic switching has focused on the role of kinases and downstream transcriptional regulators controlling expression of contractile proteins and proteins associated with the cytoskeleton in SMCs. These proteins, including smooth muscle α-actin, smooth muscle myosin heavy chain, calponin, and smooth muscle 22α, are often used as markers of the contractile SMC phenotype. It is now known that myocardin, a transcriptional coactivator of serum response factor (SRF), is a master regulator of the SMC contractile phenotype and is required for the expression of these contractile proteins.4 The Hippo kinase pathway, which received its name from a Drosophila mutant with tissue overgrowth, was recently shown to control SMC phenotypic switching, perhaps through modulating the myocardin–SRF axis’ effects on gene expression. Yes-associated protein (YAP) is phosphorylated and inhibited by the Hippo kinase pathway. Phosphorylated YAP remains in the cytosol and is thereby prevented to mediate its effects on repressing gene expression of smooth muscle α-actin, smooth muscle myosin heavy chain, calponin, and smooth muscle 22α. YAP is induced in SMCs transitioning from a contractile to a synthetic proliferative phenotype in injured rat carotid arteries, YAP expression promotes SMC proliferation and migration, and deletion of YAP results in protection against neointimal formation after vascular injury.5 The findings that YAP interacts with myocardin and disrupts SRF binding and downstream effects on SMC contractile protein expression tie these observations into the myocardin–SRF axis.6 Another kinase recently discovered to regulate SMC phenotypic switching is p38 MAPKα (mitogen-activated protein kinase; Mapk14). Knockdown of Mapk14, like YAP, results in increased expression of contractile proteins in SMCs.7 Interestingly, MAPK14 and YAP might both act, at least in part, within the myocardin–SRF pathway. MAPK14 acts through the myocardin paralog myocardin-related transcription factor A. However, the exact relationship between YAP and MAPK14 remains to be clarified. There are many known extracellular factors that can activate the Hippo pathway and p38 MAPKα. One likely regulator is cell–cell or cell–matrix contact, which in turn modulates cytoskeletal organization. Cell adherence is crucial for SMC survival and function, and mice with SMC-targeted deletion of the β1 integrin exhibit a loss of SMCs.8 Other recent interest on extracellular regulators of SMC phenotypic switching include bioactive lipids and enzymes involved in regulation of bioactive lipid levels, such as lipid phosphate phosphatase
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015 2175
2176 Arterioscler Thromb Vasc Biol October 2014 3, which degrades lysophospholipids9 and the saturated fatty acid palmitate.10 SMCs can also take on properties of other cell types. For example, SMCs in atherosclerotic lesions have long been known to be able to accumulate cholesterol and thereby resemble macrophage foam cells. As much as 50% of oil red O–positive foam cells in human coronary artery lesions have been identified as SMCs, based on their expression of smooth muscle α-actin.11 Recently, beige adipocytes have also been identified as having a smooth muscle–like origin.12 Furthermore, during the process leading to vascular calcification, SMCs largely lose their expression of smooth muscle α-actin and smooth muscle 22α and gain expression of bone-forming proteins through a process, at least in part, dependent on the sodium-dependent phosphate cotransporters Pit-1 and Pit-2, which act through redundant mechanisms to promote calcification13 and through tumor necrosis factor–related protein-3,14 osteoprotegerin,15 and angiotensin II.16 To complicate the identity of SMCs further, the origin of SMCs in the vasculature is diverse, and different arteries or segments of arteries contain SMCs of different developmental origin.17 In the mouse, neural crest-derived SMCs are found in the aortic arch and arteries branching off of the arch, such as the brachiocephalic artery often used for studies of atherosclerosis in mouse models. SMCs in the thoracic aorta are derived from somites, whereas SMCs in the abdominal aorta are derived primarily from splanchnic mesoderm.17 In addition, other lineages exist in other arteries, and stem cells and mesangioblasts also contribute to the composition of SMCs in different arteries. Although the phenotypic switching described above seems to be common to all SMCs regardless of origin, these SMCs respond differently to various stimuli.17 Building on this information, a recent study demonstrated that regional phenotypic variability of SMCs contributes to their regulation of nuclear factor-κB activity in response to tumor necrosis factor-α stimulation.18,19 These interesting findings might explain the differences in atherosclerosis susceptibility of different arteries under conditions in which systemic inflammation is elevated. Another research area currently receiving considerable attention is that of SMC progenitor cells. These progenitor cells are now known to exist in the adult blood vessel wall20 and their differentiation into cells expressing SMC proteins is regulated by specific signaling pathways and transcription factors,20 as well as mechanical force.21 Interestingly, sirolimus, an inhibitor of the mammalian target of rapamycin has recently been shown to stimulate SMC progenitor cell differentiation, which might explain the recurrence of coronary artery restenosis in some patients who received sirolimuseluting stents.22 In addition, endothelial cells and macrophages have been described to direct mesenchymal stem cells toward SMC fates.23,24 Therefore, loss of functional endothelial cells or accumulation of lesion macrophages could affect the differentiation of SMCs from progenitor cells. The plasticity of the SMCs suggests that new treatment strategies for cardiovascular disease could target these processes.
Noncoding RNA and Micro-RNA as Regulators of SMC Phenotype Noncoding RNAs are divided into 2 groups: the short (processed transcript length, 200 nucleotides) noncoding RNAs. Several recent articles published in ATVB support the concept of noncoding RNAs as important regulators of SMC proliferation, migration, and calcification. For example, identification of long noncoding RNAs in human vasculature by RNA sequencing25 identified Smooth muscle and Endothelial cell-enriched migration/differentiation-associated long NonCoding RNA as a previously unannotated long noncoding RNA expressed in human coronary artery SMCs. By several in vitro studies, Smooth muscle and Endothelial cell-enriched migration/differentiation-associated long NonCoding RNA was shown to maintain SMCs in a contractile phenotype by maintaining expression of myocardin and SMC contractile protein genes.25 Conversely, myocardin was demonstrated to regulate the SMC response to injury, in part, through the micro-RNAs, miR-24 and miR-29a,26 and the expression of myocardin in injured blood vessels prevented neointimal thickening by reducing SMC proliferation and migration. Another micro-RNA that inhibits SMC proliferation in response to estrogen receptor-α activation in SMCs is miR-203.27 The ability of estrogen to induce miR-203 was shown to be because of the transcription factors, Zeb-1 and AP-1. Furthermore, in a study using a rat model of metabolic syndrome, miR-145 delivered to the arterial wall by adenovirus was identified as being able to maintain the contractile phenotype of SMCs after injury.28 Another micro-RNA, miR663, was also recently found to regulate SMC phenotypic switching and neointimal formation.29 This micro-RNA is downregulated in SMCs stimulated by platelet-derived growth factor (PDGF), and its overexpression resulted in upregulation of SMC markers of the contractile phenotype and suppression of neointimal formation after injury in mice.29 Other microRNAs promote SMC proliferation, as has recently been shown to be the case for miR-130a in pulmonary SMCs.30 Micro-RNAs have also been identified as regulators of SMC calcification. Thus, miR-29 mediates vascular calcification through upregulation of ADAMTS-7 (a disintegrin and metalloproteinase with thrombospondin motifs-7) in rat SMCs.31 Another concept that has received much interest recently is that micro-RNAs can be released from cells and travel to other sites in circulation bound to particles, such as lipoproteins.32 Thus, it is possible that systemic changes in levels of micro-RNAs might influence SMC phenotype and response to local and systemic factors. It is also plausible that lipid particles might see a use as delivery tools of therapeutic noncoding RNAs.
Cross Talk Among SMCs and Other Cell Types SMCs are continuously responding to signals derived from other cells and dysfunction of neighboring cells can lead to maladaptive SMC proliferation, migration, calcification, or apoptosis. Conversely, SMCs can signal to nearby SMCs or other cell types to direct the status of these neighboring cells. Much of the recent work on cross talk between SMCs and
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
Wall and Bornfeldt Recent Highlights of ATVB: Arterial Smooth Muscle 2177 other cell types in the vascular wall has been accomplished through the use of ex vivo coculture experiments, which can directly test the outcome of secreted factors from one cell type on another. In vivo models of cell-type–specific knockout mice are often done in conjunction with these studies to support the relevance of these findings. Such coculture experiments recently suggested that matrix metalloproteinase 13 (MMP13), a protease that cleaves membrane-associated and extracellular proteins and is secreted by, for example, classically activated macrophages and probably also by lesion macrophages, promotes migration of SMCs after aortic endothelial denudation in endothelial nitric oxide (Nos3)–deficient mice.33 MMP13 acts as the predominant interstitial collagenase in mouse atherosclerotic lesions, and mice deficient in both Mmp13 and apolipoprotein E (Apoe) exhibit increased SMC accumulation in lesions of atherosclerosis, as well as increased intraplaque collagen when compared with Apoe-/- control mice.34 Chemokines released from other cell types also affect SMCs. For instance, a recent perivascular adipose tissue (PVAT) transplantation study using PVAT from wild-type or monocyte chemoattractant protein 1 (CCL2)–deficient donor mice identified the PVAT as a driver of neointimal SMC accumulation through secreted monocyte chemoattractant protein 1.35 This work supports other recent findings in which monocyte chemoattractant protein 1, a known chemoattractant for monocytes, contributes to SMC proliferation.36 PVAT has 10- to 40-fold greater expression of monocyte chemoattractant protein 1 when compared with other depots of adipose tissue. Therefore, expanded PVAT as a result of risk factors, such as consumption of a high-fat diet, seems to contribute to neointimal SMC accumulation directly.35 Beyond being responsive to signals from other cell types, SMCs can actively signal through paracrine and autocrine mechanisms to neighboring SMCs, creating a feed-forward loop and amplifying signals from their environment and, in disease models, deregulated migration, proliferation, and neointimal thickening can occur. SMCs produce chemokines that promote migration and recruitment of additional SMCs and remodeling of the vessel wall. Recently, the cytokines interleukin-6 and CXCL10 were found to promote SMC migration in this manner. Media from SMCs treated with a toll-like receptor 2 agonist promote migration of other SMCs in an interleukin-6–dependent manner,37 and deficiency in CXCL10 secretion from SMCs impairs SMC chemotaxis in a gradient of fetal calf serum.38 Similarly, MMP8 is released by SMCs and is correlated with atherosclerotic lesion progression.39 In addition to its known role in leukocyte trafficking,40 MMP8 was recently found to promote maturation of ADAM10 (a disintegrin and metalloproteinase domain-containing protein 10) in other SMCs by cleavage of its propeptide domain. Mature ADAM10 induces SMC migration and proliferation through the cadherin-β-catenin pathway.41 Other interactions are more complex. For instance, serum aldosterone is increased after endothelial injury. Alderosterone signals to mineralocorticoid receptors on the SMCs leading to increased placental growth factor secretion and increased expression of its receptor, vascular endothelial growth factor receptor 1. Placental growth factor signals to vascular endothelial growth factor
receptor 1 leading to SMC proliferation and vascular remodeling.42 Beyond neighboring SMC-amplifying signals, cellular cross talk among multiple cell types can occur, activating these other cell types and enhancing proliferative behavior in SMCs. SMCs have been shown to activate monocytes,43 macrophages,44 and dendritic cells45 in this manner through release of soluble growth factors and cytokines. In turn, SMCs receive migration and proliferation signals from these neighboring cell types. Finally, SMCs actively signal to other cell types to promote these cells to aid in vascular remodeling. Coculture experiments demonstrate that SMCs signal to endothelial cells both by direct contact and through paracrine interactions to modulate endothelial cell expression of the adhesion molecule VCAM1 (vascular cell adhesion molecule-1) and endothelial cell permeability through phosphorylation of β-catenin.46 Undoubtedly, we have just begun to uncover the autocrine and paracrine cross talk between SMCs, neighboring SMCs, and other cell types.
Emerging Potential Drug Targets in SMCs With the many roles that SMCs have in normal and diseased vessels they represent an exciting area for therapies that target vascular disease. Chemokines secreted by SMCs and other vascular cells are potentially promising therapeutic candidates. Biological therapies can be developed to target these chemokines before reaching their SMC target. Another area that has received continued attention is therapies to prevent restenosis after angioplasty. Life-saving surgeries, such as balloon-angioplasty and placement of stents, are used to restore blood flow in atherosclerotic arteries. However, endothelial cell damage inevitably occurs with these surgeries, promoting SMC proliferation and migration in an attempt to heal these new wounds. In turn, this narrows the artery and creates a vicious cycle. Drug-eluting stents are now being used to help control SMC proliferation after some of these surgeries. The drugs currently in use are antiproliferative agents as sirolimus or paxitaxel (taxol, a drug originally isolated from the Pacific Yew, inhibits SMC proliferation by microtubule stabilization). These drugs inhibit not only SMC proliferation but also endothelial cell proliferation, slowing the repair of the initial endothelial cell damage and leaving the patients at greater risk for thrombosis. Drug targets that can distinguish between SMC and endothelial cell proliferation are in need. As our understanding of the mechanisms of phenotype switching in SMCs deepens, specific targeting of these cells will be more attainable. We are seeing some recent progress in this area of research. In SMCs, CTP synthase 1 is increased after the stimulation of SMCs with PDGF. This enzyme catalyzes the CTP/pyrimidine biosynthesis required for cell proliferation. Blockade of CTP synthase 1 (by an inhibitor or shRNA) prevents in vitro proliferation of SMCs in response to PDGF and blocks in vivo neointimal formation after vascular injury but has less effect on endothelial cells.47 PDGF also increases the mitochondrial membrane potential, a phenomenon observed in vessel injury models. Treatment with the mitochondrial pyruvate dehydrogenase kinase inhibitor drug dichloroacetate, or knockdown of pyruvate dehydrogenase kinase isoform
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
2178 Arterioscler Thromb Vasc Biol October 2014 2, reduces mitochondrial membrane potential through reduced association between hexokinase 2 and voltage-dependent anion channels. This negates the proliferative and antiapoptotic effects of PDGF treatment and reduces neointimal formation in models of vessel injury. Dichloroacetate does not inhibit endothelial cell migration or re-endothelialization, making it an interesting novel drug candidate.48 In addition, overexpression and knockdown studies have identified miR203 as a micro-RNA with distinct effects on SMCs and endothelial cells. Although miR-203 slows SMC proliferation, it has no effect on endothelial cell growth.27 Targets that aid endothelial cell recovery without promoting SMC growth may also be good therapy options for restenosis. Recent research on Apoe−/− mice that lack the CXCL12 receptor CXCR4 indicates CXCR4’s requirement for normal proliferation and recruitment of endothelial cells to heal the endothelium after wire injury.49 The toll-like receptor 2/6 ligand, MALP-2, is another interesting agent in vascular injury models. Injection of MALP-2 2 hours after arterial injury in mice promoted re-endothelialization and inhibited neointimal hyperplasia. In vitro, MALP-2 led to enhanced endothelial cell proliferation but not that of SMCs in an artificial woundhealing test.50 However, myelopoiesis induced by MALP-2 therapy might be a detrimental side effect. Other potential new drug target strategies might involve delivering micro-RNAs by lipid particles to target SMCs, or perhaps to boost SMC progenitor cells to strengthen unstable atherosclerotic lesions with an increased risk of rupture. As we learn more about the plasticity and versatility of the SMC, new treatment strategies for vascular diseases are likely to be uncovered.
Summary The arterial SMC is a highly plastic cell type that plays crucial roles in normal physiology and several vascular diseases. The SMC’s ability to undergo phenotypic switching, the mosaicism of SMC origin in the vascular system, and the presence of SMC progenitor cells all contribute to the versatility and complexity of SMC responses. Recent research highlights the importance of noncoding RNAs and cross talk among SMCs and other cell types in vascular biology. The next few years are likely to see many more exciting discoveries related to this fascinating cell type.
Sources of Funding Research in K.E. Bornfeldt’s laboratory is supported by the National Institutes of Health grants R01HL062887, P01HL092969, R01HL097365, and P30DK017047 and by a Grant-in-Aid from the American Heart Association (14GRNT20410033). V.Z. Wall is supported by the Samuel and Althea Fellowship in Diabetes Research through the University of Washington’s Diabetes Research Center (P30DK017047).
Disclosures None.
References 1. Campbell JH, Campbell GR. Smooth muscle phenotypic modulation–a personal experience. Arterioscler Thromb Vasc Biol. 2012;32:1784–1789.
2. Gomez D, Owens GK. Smooth muscle cell phenotypic switching in atherosclerosis. Cardiovasc Res. 2012;95:156–164. 3. Clarke MC, Littlewood TD, Figg N, Maguire JJ, Davenport AP, Goddard M, Bennett MR. Chronic apoptosis of vascular smooth muscle cells accelerates atherosclerosis and promotes calcification and medial degeneration. Circ Res. 2008;102:1529–1538. 4. Wang Z, Wang DZ, Pipes GC, Olson EN. Myocardin is a master regulator of smooth muscle gene expression. Proc Natl Acad Sci U S A. 2003;100:7129–7134. 5. Wang X, Hu G, Gao X, Wang Y, Zhang W, Harmon EY, Zhi X, Xu Z, Lennartz MR, Barroso M, Trebak M, Chen C, Zhou J. The induction of yes-associated protein expression after arterial injury is crucial for smooth muscle phenotypic modulation and neointima formation. Arterioscler Thromb Vasc Biol. 2012;32:2662–2669. 6. Xie C, Guo Y, Zhu T, Zhang J, Ma PX, Chen YE. Yap1 protein regulates vascular smooth muscle cell phenotypic switch by interaction with myocardin. J Biol Chem. 2012;287:14598–14605. 7. Long X, Cowan SL, Miano JM. Mitogen-activated protein kinase 14 is a novel negative regulatory switch for the vascular smooth muscle cell contractile gene program. Arterioscler Thromb Vasc Biol. 2013;33:378–386. 8. Turlo KA, Scapa J, Bagher P, Jones AW, Feil R, Korthuis RJ, Segal SS, Iruela-Arispe ML. β1-integrin is essential for vasoregulation and smooth muscle survival in vivo. Arterioscler Thromb Vasc Biol. 2013;33:2325–2335. 9. Panchatcharam M, Miriyala S, Salous A, Wheeler J, Dong A, Mueller P, Sunkara M, Escalante-Alcalde D, Morris AJ, Smyth SS. Lipid phosphate phosphatase 3 negatively regulates smooth muscle cell phenotypic modulation to limit intimal hyperplasia. Arterioscler Thromb Vasc Biol. 2013;33:52–59. 10. Shen H, Eguchi K, Kono N, Fujiu K, Matsumoto S, Shibata M, Oishi-Tanaka Y, Komuro I, Arai H, Nagai R, Manabe I. Saturated fatty acid palmitate aggravates neointima formation by promoting smooth muscle phenotypic modulation. Arterioscler Thromb Vasc Biol. 2013;33:2596–2607. 11. Allahverdian S, Chehroudi AC, McManus BM, Abraham T, Francis GA. Contribution of intimal smooth muscle cells to cholesterol accumulation and macrophage-like cells in human atherosclerosis. Circulation. 2014;129:1551–1559. 12. Long JZ, Svensson KJ, Tsai L, Zeng X, Roh HC, Kong X, Rao RR, Lou J, Lokurkar I, Baur W, Castellot JJ Jr, Rosen ED, Spiegelman BM. A smooth muscle-like origin for beige adipocytes. Cell Metab. 2014;19:810–820. 13. Crouthamel MH, Lau WL, Leaf EM, Chavkin NW, Wallingford MC, Peterson DF, Li X, Liu Y, Chin MT, Levi M, Giachelli CM. Sodiumdependent phosphate cotransporters and phosphate-induced calcification of vascular smooth muscle cells: redundant roles for PiT-1 and PiT-2. Arterioscler Thromb Vasc Biol. 2013;33:2625–2632. 14. Zhou YB, Zhang J, Peng DQ, Chang JR, Cai Y, Yu YR, Jia MZ, Wu W, Guan YF, Tang CS, Qi YF. Peroxisome proliferator-activated receptor γ ligands retard cultured vascular smooth muscle cells calcification induced by high glucose. Cell Biochem Biophys. 2013;66:421–429. 15. Callegari A, Coons ML, Ricks JL, Yang HL, Gross TS, Huber P, Rosenfeld ME, Scatena M. Bone marrow- or vessel wall-derived osteoprotegerin is sufficient to reduce atherosclerotic lesion size and vascular calcification. Arterioscler Thromb Vasc Biol. 2013;33:2491–2500. 16. Osako MK, Nakagami H, Shimamura M, Koriyama H, Nakagami F, Shimizu H, Miyake T, Yoshizumi M, Rakugi H, Morishita R. Cross-talk of receptor activator of nuclear factor-κB ligand signaling with renin-angiotensin system in vascular calcification. Arterioscler Thromb Vasc Biol. 2013;33:1287–1296. 17. Majesky MW. Developmental basis of vascular smooth muscle diversity. Arterioscler Thromb Vasc Biol. 2007;27:1248–1258. 18. Trigueros-Motos L, González-Granado JM, Cheung C, Fernández P, Sánchez-Cabo F, Dopazo A, Sinha S, Andrés V. Embryological-origindependent differences in homeobox expression in adult aorta: role in regional phenotypic variability and regulation of NF-κB activity. Arterioscler Thromb Vasc Biol. 2013;33:1248–1256. 19. Awgulewitsch A, Majesky MW. Interpreting inflammation: smooth muscle positional identity and nuclear factor-κB signaling. Arterioscler Thromb Vasc Biol. 2013;33:1113–1115. 20. Majesky MW, Dong XR, Regan JN, Hoglund VJ. Vascular smooth muscle progenitor cells: building and repairing blood vessels. Circ Res. 2011;108:365–377. 21. Potter CMF, Lao KH, Zeng L, Xu Q. Role of biomechanical forces in stem cell vascular lineage differentiation. Arterioscler Thromb Vasc Biol. 2014;34:2186–2192.
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
Wall and Bornfeldt Recent Highlights of ATVB: Arterial Smooth Muscle 2179 22. Wong MM, Winkler B, Karamariti E, Wang X, Yu B, Simpson R, Chen T, Margariti A, Xu Q. Sirolimus stimulates vascular stem/progenitor cell migration and differentiation into smooth muscle cells via epidermal growth factor receptor/extracellular signal-regulated kinase/β-catenin signaling pathway. Arterioscler Thromb Vasc Biol. 2013;33:2397–2406. 23. Lin CH, Lilly B. Endothelial cells direct mesenchymal stem cells toward a smooth muscle cell fate [published ahead of print July 14, 2014]. Stem Cells Dev. doi:10.1089/scd.2014.0163. http://online.liebertpub.com/doi/ abs/10.1089/scd.2014.0163. 24. Lee MJ, Kim MY, Heo SC, Kwon YW, Kim YM, Do EK, Park JH, Lee JS, Han J, Kim JH. Macrophages regulate smooth muscle differentiation of mesenchymal stem cells via a prostaglandin F2α-mediated paracrine mechanism. Arterioscler Thromb Vasc Biol. 2012;32:2733–2740. 25. Bell RD, Long X, Lin M, Bergmann JH, Nanda V, Cowan SL, Zhou Q, Han Y, Spector DL, Zheng D, Miano JM. Identification and initial functional characterization of a human vascular cell-enriched long noncoding RNA. Arterioscler Thromb Vasc Biol. 2014;34:1249–1259. 26. Talasila A, Yu H, Ackers-Johnson M, Bot M, van Berkel T, Bennett MR, Bot I, Sinha S. Myocardin regulates vascular response to injury through miR-24/-29a and platelet-derived growth factor receptor-β. Arterioscler Thromb Vasc Biol. 2013;33:2355–2365. 27. Zhao J, Imbrie GA, Baur WE, Iyer LK, Aronovitz MJ, Kershaw TB, Haselmann GM, Lu Q, Karas RH. Estrogen receptor-mediated regulation of microRNA inhibits proliferation of vascular smooth muscle cells. Arterioscler Thromb Vasc Biol. 2013;33:257–265. 28. Hutcheson R, Terry R, Chaplin J, Smith E, Musiyenko A, Russell JC, Lincoln T, Rocic P. MicroRNA-145 restores contractile vascular smooth muscle phenotype and coronary collateral growth in the metabolic syndrome. Arterioscler Thromb Vasc Biol. 2013;33:727–736. 29. Li P, Zhu N, Yi B, Wang N, Chen M, You X, Zhao X, Solomides CC, Qin Y, Sun J. MicroRNA-663 regulates human vascular smooth muscle cell phenotypic switch and vascular neointimal formation. Circ Res. 2013;113:1117–1127. 30. Bertero T, Lu Y, Annis S, et al. Systems-level regulation of microrna networks by mir-130/301 promotes pulmonary hypertension. J Clin Invest. 2014;124:3514–3528. 31. Du Y, Gao C, Liu Z, Wang L, Liu B, He F, Zhang T, Wang Y, Wang X, Xu M, Luo GZ, Zhu Y, Xu Q, Wang X, Kong W. Upregulation of a disintegrin and metalloproteinase with thrombospondin motifs-7 by miR-29 repression mediates vascular smooth muscle calcification. Arterioscler Thromb Vasc Biol. 2012;32:2580–2588. 32. Wagner J, Riwanto M, Besler C, Knau A, Fichtlscherer S, Röxe T, Zeiher AM, Landmesser U, Dimmeler S. Characterization of levels and cellular transfer of circulating lipoprotein-bound microRNAs. Arterioscler Thromb Vasc Biol. 2013;33:1392–1400. 33. Lavin B, Gómez M, Pello OM, Castejon B, Piedras MJ, Saura M, Zaragoza C. Nitric oxide prevents aortic neointimal hyperplasia by controlling macrophage polarization. Arterioscler Thromb Vasc Biol. 2014;34:1739–1746. 34. Quillard T, Araújo HA, Franck G, Tesmenitsky Y, Libby P. Matrix metalloproteinase-13 predominates over matrix metalloproteinase-8 as the functional interstitial collagenase in mouse atheromata. Arterioscler Thromb Vasc Biol. 2014;34:1179–1186. 35. Manka D, Chatterjee TK, Stoll LL, Basford JE, Konaniah ES, Srinivasan R, Bogdanov VY, Tang Y, Blomkalns AL, Hui DY, Weintraub NL. Transplanted perivascular adipose tissue accelerates injury-induced neointimal hyperplasia: role of monocyte chemoattractant protein-1. Arterioscler Thromb Vasc Biol. 2014;34:1723–1730. 36. Schepers A, Eefting D, Bonta PI, Grimbergen JM, de Vries MR, van Weel V, de Vries CJ, Egashira K, van Bockel JH, Quax PH. Anti-MCP-1 gene therapy inhibits vascular smooth muscle cells proliferation and attenuates
vein graft thickening both in vitro and in vivo. Arterioscler Thromb Vasc Biol. 2006;26:2063–2069. 37. Lee GL, Chang YW, Wu JY, Wu ML, Wu KK, Yet SF, Kuo CC. TLR 2 induces vascular smooth muscle cell migration through cAMP response element-binding protein-mediated interleukin-6 production. Arterioscler Thromb Vasc Biol. 2012;32:2751–2760. 38. van den Borne P, Haverslag RT, Brandt MM, Cheng C, Duckers HJ, Quax PH, Hoefer IE, Pasterkamp G, de Kleijn DP. Absence of chemokine (C-x-C motif) ligand 10 diminishes perfusion recovery after local arterial occlusion in mice. Arterioscler Thromb Vasc Biol. 2014;34:594–602. 39. Tuomainen AM, Nyyssönen K, Laukkanen JA, Tervahartiala T, Tuomainen TP, Salonen JT, Sorsa T, Pussinen PJ. Serum matrix metalloproteinase-8 concentrations are associated with cardiovascular outcome in men. Arterioscler Thromb Vasc Biol. 2007;27:2722–2728. 40. Laxton RC, Hu Y, Duchene J, et al. A role of matrix metalloproteinase-8 in atherosclerosis. Circ Res. 2009;105:921–929. 41. Xiao Q, Zhang F, Grassia G, Hu Y, Zhang Z, Xing Q, Yin X, Maddaluno M, Drung B, Schmidt B, Maffia P, Ialenti A, Mayr M, Xu Q, Ye S. Matrix metalloproteinase-8 promotes vascular smooth muscle cell proliferation and neointima formation. Arterioscler Thromb Vasc Biol. 2014;34:90–98. 42. Pruthi D, McCurley A, Aronovitz M, Galayda C, Karumanchi SA, Jaffe IZ. Aldosterone promotes vascular remodeling by direct effects on smooth muscle cell mineralocorticoid receptors. Arterioscler Thromb Vasc Biol. 2014;34:355–364. 43. Li P, Li YL, Li ZY, Wu YN, Zhang CC, A X, Wang CX, Shi HT, Hui MZ, Xie B, Ahmed M, Du J. Cross talk between vascular smooth muscle cells and monocytes through interleukin-1beta/interleukin-18 signaling promotes vein graft thickening. Arterioscler Thromb Vasc Biol. 2014;34:2001–2011. 44. Ostriker A, Horita HN, Poczobutt J, Weiser-Evans MC, Nemenoff RA. Vascular smooth muscle cell-derived transforming growth factor-β promotes maturation of activated, neointima lesion-like macrophages. Arterioscler Thromb Vasc Biol. 2014;34:877–886. 45. Paccosi S, Musilli C, Caporale R, Gelli AM, Guasti D, Clemente AM, Torcia MG, Filippelli A, Romagnoli P, Parenti A. Stimulatory interactions between human coronary smooth muscle cells and dendritic cells. PLoS One. 2014;9:e99652. 46. Chang SF, Chen LJ, Lee PL, Lee DY, Chien S, Chiu JJ. Different modes of endothelial-smooth muscle cell interaction elicit differential β-catenin phosphorylations and endothelial functions. Proc Natl Acad Sci U S A. 2014;111:1855–1860. 47. Tang R, Cui XB, Wang JN, Chen SY. CTP synthase 1, a smooth muscle-sensitive therapeutic target for effective vascular repair. Arterioscler Thromb Vasc Biol. 2013;33:2336–2344. 48. Deuse T, Hua X, Wang D, et al. Dichloroacetate prevents restenosis in preclinical animal models of vessel injury. Nature. 2014;509:641–644. 49. Noels H, Zhou B, Tilstam PV, et al. Deficiency of endothelial cxcr4 reduces reendothelialization and enhances neointimal hyperplasia after vascular injury in atherosclerosis-prone mice. Arterioscler Thromb Vasc Biol. 2014;34:1209–1220. 50. Grote K, Sonnenschein K, Kapopara PR, Hillmer A, Grothusen C, Salguero G, Kotlarz D, Schuett H, Bavendiek U, Schieffer B. Toll-like receptor 2/6 agonist macrophage-activating lipopeptide-2 promotes reendothelialization and inhibits neointima formation after vascular injury. Arterioscler Thromb Vasc Biol. 2013;33:2097–2104. Key Words: atherosclerosis ◼ microRNAs ◼ myocytes, smooth muscle ◼ neointima
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
Arterial Smooth Muscle Valerie Z. Wall and Karin E. Bornfeldt Arterioscler Thromb Vasc Biol. 2014;34:2175-2179 doi: 10.1161/ATVBAHA.114.304441 Arteriosclerosis, Thrombosis, and Vascular Biology is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231 Copyright © 2014 American Heart Association, Inc. All rights reserved. Print ISSN: 1079-5642. Online ISSN: 1524-4636
The online version of this article, along with updated information and services, is located on the World Wide Web at: http://atvb.ahajournals.org/content/34/10/2175
Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally published in Arteriosclerosis, Thrombosis, and Vascular Biology can be obtained via RightsLink, a service of the Copyright Clearance Center, not the Editorial Office. Once the online version of the published article for which permission is being requested is located, click Request Permissions in the middle column of the Web page under Services. Further information about this process is available in the Permissions and Rights Question and Answer document. Reprints: Information about reprints can be found online at: http://www.lww.com/reprints Subscriptions: Information about subscribing to Arteriosclerosis, Thrombosis, and Vascular Biology is online at: http://atvb.ahajournals.org//subscriptions/
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
T
he prime function of the arterial smooth muscle cell (SMC) in adult individuals is to contract and relax, thereby regulating blood flow to target tissues. However, in several vascular diseases, arterial SMCs in the adult vessel undergo major changes in structure and function. For example, SMCs can take on properties that allow them to proliferate and migrate, to promote calcification, to undergo apoptosis, and to orchestrate other cell types to take on different properties. These altered SMCs are often viewed as vascular troublemakers. However, these responses can also be seen as an attempt of the SMC to mend a diseased blood vessel. The plasticity and diversity of SMC populations and the presence of SMC progenitor cells contribute to the growing complexity of SMC biology. In the past couple of years, several new mechanisms that allow SMCs to proliferate, migrate, promote calcification, and communicate with and respond to other cell types have been discovered. Much of this work has been published in ATVB, and some of that body of work is highlighted in this article. We emphasize areas that have generated substantial interest, including novel mechanisms governing SMC phenotypic switching, SMC progenitor cells, noncoding RNAs as regulators of SMC function and phenotype, novel mechanisms of cross talk between SMCs and other cells and differences in regulation of SMCs and other vascular cells.
Plasticity of the Arterial SMC The arterial SMC exhibits an impressive potential for plasticity. SMCs have been known to exist in different phenotypic states for decades, and the switch from a quiescent contractile phenotype to a synthetic proliferative phenotype is thought to play an important role in cardiovascular disease (for a historical perspective1). Thus, switching to a synthetic phenotype is generally thought to enable SMCs to migrate and proliferate in atherosclerotic lesions,2 thereby forming a fibrous cap covering a core of macrophages and other immune cells, and in injured vessels after angioplasty. However, additional lineagetracing studies are needed to address the origin of intimal lesion cells.2 Conversion of an early fatty streak atherosclerotic lesion to a fibrotic lesion is thought to cause permanent nonreversible changes in the artery wall. In that sense, SMC phenotypic switching can be considered to be detrimental. From the Departments of Pathology (V.Z.W., K.E.B.) and Medicine, Division of Metabolism, Endocrinology and Nutrition (K.E.B.), Diabetes and Obesity Center of Excellence, University of Washington School of Medicine, Seattle. Correspondence to Karin E. Bornfeldt, PhD, Departments of Medicine and Pathology, Diabetes and Obesity Center of Excellence, 850 Republican St, University of Washington, Seattle, WA 98109. E-mail [email protected] (Arterioscler Thromb Vasc Biol. 2014;34:2175-2179.) © 2014 American Heart Association, Inc. Arterioscler Thromb Vasc Biol is available at http://atvb.ahajournals.org DOI: 10.1161/ATVBAHA.114.304441
However, at later stages of atherogenesis, fibrotic lesions are often more stable than acellular necrotic lesions, making them less likely to cause clinical symptoms. In such lesions, SMC phenotypic switching and survival could be considered beneficial.3 Different SMC phenotypes overlap and probably exist as a continuum in vivo. Recent research on intracellular regulators of SMC phenotypic switching has focused on the role of kinases and downstream transcriptional regulators controlling expression of contractile proteins and proteins associated with the cytoskeleton in SMCs. These proteins, including smooth muscle α-actin, smooth muscle myosin heavy chain, calponin, and smooth muscle 22α, are often used as markers of the contractile SMC phenotype. It is now known that myocardin, a transcriptional coactivator of serum response factor (SRF), is a master regulator of the SMC contractile phenotype and is required for the expression of these contractile proteins.4 The Hippo kinase pathway, which received its name from a Drosophila mutant with tissue overgrowth, was recently shown to control SMC phenotypic switching, perhaps through modulating the myocardin–SRF axis’ effects on gene expression. Yes-associated protein (YAP) is phosphorylated and inhibited by the Hippo kinase pathway. Phosphorylated YAP remains in the cytosol and is thereby prevented to mediate its effects on repressing gene expression of smooth muscle α-actin, smooth muscle myosin heavy chain, calponin, and smooth muscle 22α. YAP is induced in SMCs transitioning from a contractile to a synthetic proliferative phenotype in injured rat carotid arteries, YAP expression promotes SMC proliferation and migration, and deletion of YAP results in protection against neointimal formation after vascular injury.5 The findings that YAP interacts with myocardin and disrupts SRF binding and downstream effects on SMC contractile protein expression tie these observations into the myocardin–SRF axis.6 Another kinase recently discovered to regulate SMC phenotypic switching is p38 MAPKα (mitogen-activated protein kinase; Mapk14). Knockdown of Mapk14, like YAP, results in increased expression of contractile proteins in SMCs.7 Interestingly, MAPK14 and YAP might both act, at least in part, within the myocardin–SRF pathway. MAPK14 acts through the myocardin paralog myocardin-related transcription factor A. However, the exact relationship between YAP and MAPK14 remains to be clarified. There are many known extracellular factors that can activate the Hippo pathway and p38 MAPKα. One likely regulator is cell–cell or cell–matrix contact, which in turn modulates cytoskeletal organization. Cell adherence is crucial for SMC survival and function, and mice with SMC-targeted deletion of the β1 integrin exhibit a loss of SMCs.8 Other recent interest on extracellular regulators of SMC phenotypic switching include bioactive lipids and enzymes involved in regulation of bioactive lipid levels, such as lipid phosphate phosphatase
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015 2175
2176 Arterioscler Thromb Vasc Biol October 2014 3, which degrades lysophospholipids9 and the saturated fatty acid palmitate.10 SMCs can also take on properties of other cell types. For example, SMCs in atherosclerotic lesions have long been known to be able to accumulate cholesterol and thereby resemble macrophage foam cells. As much as 50% of oil red O–positive foam cells in human coronary artery lesions have been identified as SMCs, based on their expression of smooth muscle α-actin.11 Recently, beige adipocytes have also been identified as having a smooth muscle–like origin.12 Furthermore, during the process leading to vascular calcification, SMCs largely lose their expression of smooth muscle α-actin and smooth muscle 22α and gain expression of bone-forming proteins through a process, at least in part, dependent on the sodium-dependent phosphate cotransporters Pit-1 and Pit-2, which act through redundant mechanisms to promote calcification13 and through tumor necrosis factor–related protein-3,14 osteoprotegerin,15 and angiotensin II.16 To complicate the identity of SMCs further, the origin of SMCs in the vasculature is diverse, and different arteries or segments of arteries contain SMCs of different developmental origin.17 In the mouse, neural crest-derived SMCs are found in the aortic arch and arteries branching off of the arch, such as the brachiocephalic artery often used for studies of atherosclerosis in mouse models. SMCs in the thoracic aorta are derived from somites, whereas SMCs in the abdominal aorta are derived primarily from splanchnic mesoderm.17 In addition, other lineages exist in other arteries, and stem cells and mesangioblasts also contribute to the composition of SMCs in different arteries. Although the phenotypic switching described above seems to be common to all SMCs regardless of origin, these SMCs respond differently to various stimuli.17 Building on this information, a recent study demonstrated that regional phenotypic variability of SMCs contributes to their regulation of nuclear factor-κB activity in response to tumor necrosis factor-α stimulation.18,19 These interesting findings might explain the differences in atherosclerosis susceptibility of different arteries under conditions in which systemic inflammation is elevated. Another research area currently receiving considerable attention is that of SMC progenitor cells. These progenitor cells are now known to exist in the adult blood vessel wall20 and their differentiation into cells expressing SMC proteins is regulated by specific signaling pathways and transcription factors,20 as well as mechanical force.21 Interestingly, sirolimus, an inhibitor of the mammalian target of rapamycin has recently been shown to stimulate SMC progenitor cell differentiation, which might explain the recurrence of coronary artery restenosis in some patients who received sirolimuseluting stents.22 In addition, endothelial cells and macrophages have been described to direct mesenchymal stem cells toward SMC fates.23,24 Therefore, loss of functional endothelial cells or accumulation of lesion macrophages could affect the differentiation of SMCs from progenitor cells. The plasticity of the SMCs suggests that new treatment strategies for cardiovascular disease could target these processes.
Noncoding RNA and Micro-RNA as Regulators of SMC Phenotype Noncoding RNAs are divided into 2 groups: the short (processed transcript length, 200 nucleotides) noncoding RNAs. Several recent articles published in ATVB support the concept of noncoding RNAs as important regulators of SMC proliferation, migration, and calcification. For example, identification of long noncoding RNAs in human vasculature by RNA sequencing25 identified Smooth muscle and Endothelial cell-enriched migration/differentiation-associated long NonCoding RNA as a previously unannotated long noncoding RNA expressed in human coronary artery SMCs. By several in vitro studies, Smooth muscle and Endothelial cell-enriched migration/differentiation-associated long NonCoding RNA was shown to maintain SMCs in a contractile phenotype by maintaining expression of myocardin and SMC contractile protein genes.25 Conversely, myocardin was demonstrated to regulate the SMC response to injury, in part, through the micro-RNAs, miR-24 and miR-29a,26 and the expression of myocardin in injured blood vessels prevented neointimal thickening by reducing SMC proliferation and migration. Another micro-RNA that inhibits SMC proliferation in response to estrogen receptor-α activation in SMCs is miR-203.27 The ability of estrogen to induce miR-203 was shown to be because of the transcription factors, Zeb-1 and AP-1. Furthermore, in a study using a rat model of metabolic syndrome, miR-145 delivered to the arterial wall by adenovirus was identified as being able to maintain the contractile phenotype of SMCs after injury.28 Another micro-RNA, miR663, was also recently found to regulate SMC phenotypic switching and neointimal formation.29 This micro-RNA is downregulated in SMCs stimulated by platelet-derived growth factor (PDGF), and its overexpression resulted in upregulation of SMC markers of the contractile phenotype and suppression of neointimal formation after injury in mice.29 Other microRNAs promote SMC proliferation, as has recently been shown to be the case for miR-130a in pulmonary SMCs.30 Micro-RNAs have also been identified as regulators of SMC calcification. Thus, miR-29 mediates vascular calcification through upregulation of ADAMTS-7 (a disintegrin and metalloproteinase with thrombospondin motifs-7) in rat SMCs.31 Another concept that has received much interest recently is that micro-RNAs can be released from cells and travel to other sites in circulation bound to particles, such as lipoproteins.32 Thus, it is possible that systemic changes in levels of micro-RNAs might influence SMC phenotype and response to local and systemic factors. It is also plausible that lipid particles might see a use as delivery tools of therapeutic noncoding RNAs.
Cross Talk Among SMCs and Other Cell Types SMCs are continuously responding to signals derived from other cells and dysfunction of neighboring cells can lead to maladaptive SMC proliferation, migration, calcification, or apoptosis. Conversely, SMCs can signal to nearby SMCs or other cell types to direct the status of these neighboring cells. Much of the recent work on cross talk between SMCs and
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
Wall and Bornfeldt Recent Highlights of ATVB: Arterial Smooth Muscle 2177 other cell types in the vascular wall has been accomplished through the use of ex vivo coculture experiments, which can directly test the outcome of secreted factors from one cell type on another. In vivo models of cell-type–specific knockout mice are often done in conjunction with these studies to support the relevance of these findings. Such coculture experiments recently suggested that matrix metalloproteinase 13 (MMP13), a protease that cleaves membrane-associated and extracellular proteins and is secreted by, for example, classically activated macrophages and probably also by lesion macrophages, promotes migration of SMCs after aortic endothelial denudation in endothelial nitric oxide (Nos3)–deficient mice.33 MMP13 acts as the predominant interstitial collagenase in mouse atherosclerotic lesions, and mice deficient in both Mmp13 and apolipoprotein E (Apoe) exhibit increased SMC accumulation in lesions of atherosclerosis, as well as increased intraplaque collagen when compared with Apoe-/- control mice.34 Chemokines released from other cell types also affect SMCs. For instance, a recent perivascular adipose tissue (PVAT) transplantation study using PVAT from wild-type or monocyte chemoattractant protein 1 (CCL2)–deficient donor mice identified the PVAT as a driver of neointimal SMC accumulation through secreted monocyte chemoattractant protein 1.35 This work supports other recent findings in which monocyte chemoattractant protein 1, a known chemoattractant for monocytes, contributes to SMC proliferation.36 PVAT has 10- to 40-fold greater expression of monocyte chemoattractant protein 1 when compared with other depots of adipose tissue. Therefore, expanded PVAT as a result of risk factors, such as consumption of a high-fat diet, seems to contribute to neointimal SMC accumulation directly.35 Beyond being responsive to signals from other cell types, SMCs can actively signal through paracrine and autocrine mechanisms to neighboring SMCs, creating a feed-forward loop and amplifying signals from their environment and, in disease models, deregulated migration, proliferation, and neointimal thickening can occur. SMCs produce chemokines that promote migration and recruitment of additional SMCs and remodeling of the vessel wall. Recently, the cytokines interleukin-6 and CXCL10 were found to promote SMC migration in this manner. Media from SMCs treated with a toll-like receptor 2 agonist promote migration of other SMCs in an interleukin-6–dependent manner,37 and deficiency in CXCL10 secretion from SMCs impairs SMC chemotaxis in a gradient of fetal calf serum.38 Similarly, MMP8 is released by SMCs and is correlated with atherosclerotic lesion progression.39 In addition to its known role in leukocyte trafficking,40 MMP8 was recently found to promote maturation of ADAM10 (a disintegrin and metalloproteinase domain-containing protein 10) in other SMCs by cleavage of its propeptide domain. Mature ADAM10 induces SMC migration and proliferation through the cadherin-β-catenin pathway.41 Other interactions are more complex. For instance, serum aldosterone is increased after endothelial injury. Alderosterone signals to mineralocorticoid receptors on the SMCs leading to increased placental growth factor secretion and increased expression of its receptor, vascular endothelial growth factor receptor 1. Placental growth factor signals to vascular endothelial growth factor
receptor 1 leading to SMC proliferation and vascular remodeling.42 Beyond neighboring SMC-amplifying signals, cellular cross talk among multiple cell types can occur, activating these other cell types and enhancing proliferative behavior in SMCs. SMCs have been shown to activate monocytes,43 macrophages,44 and dendritic cells45 in this manner through release of soluble growth factors and cytokines. In turn, SMCs receive migration and proliferation signals from these neighboring cell types. Finally, SMCs actively signal to other cell types to promote these cells to aid in vascular remodeling. Coculture experiments demonstrate that SMCs signal to endothelial cells both by direct contact and through paracrine interactions to modulate endothelial cell expression of the adhesion molecule VCAM1 (vascular cell adhesion molecule-1) and endothelial cell permeability through phosphorylation of β-catenin.46 Undoubtedly, we have just begun to uncover the autocrine and paracrine cross talk between SMCs, neighboring SMCs, and other cell types.
Emerging Potential Drug Targets in SMCs With the many roles that SMCs have in normal and diseased vessels they represent an exciting area for therapies that target vascular disease. Chemokines secreted by SMCs and other vascular cells are potentially promising therapeutic candidates. Biological therapies can be developed to target these chemokines before reaching their SMC target. Another area that has received continued attention is therapies to prevent restenosis after angioplasty. Life-saving surgeries, such as balloon-angioplasty and placement of stents, are used to restore blood flow in atherosclerotic arteries. However, endothelial cell damage inevitably occurs with these surgeries, promoting SMC proliferation and migration in an attempt to heal these new wounds. In turn, this narrows the artery and creates a vicious cycle. Drug-eluting stents are now being used to help control SMC proliferation after some of these surgeries. The drugs currently in use are antiproliferative agents as sirolimus or paxitaxel (taxol, a drug originally isolated from the Pacific Yew, inhibits SMC proliferation by microtubule stabilization). These drugs inhibit not only SMC proliferation but also endothelial cell proliferation, slowing the repair of the initial endothelial cell damage and leaving the patients at greater risk for thrombosis. Drug targets that can distinguish between SMC and endothelial cell proliferation are in need. As our understanding of the mechanisms of phenotype switching in SMCs deepens, specific targeting of these cells will be more attainable. We are seeing some recent progress in this area of research. In SMCs, CTP synthase 1 is increased after the stimulation of SMCs with PDGF. This enzyme catalyzes the CTP/pyrimidine biosynthesis required for cell proliferation. Blockade of CTP synthase 1 (by an inhibitor or shRNA) prevents in vitro proliferation of SMCs in response to PDGF and blocks in vivo neointimal formation after vascular injury but has less effect on endothelial cells.47 PDGF also increases the mitochondrial membrane potential, a phenomenon observed in vessel injury models. Treatment with the mitochondrial pyruvate dehydrogenase kinase inhibitor drug dichloroacetate, or knockdown of pyruvate dehydrogenase kinase isoform
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
2178 Arterioscler Thromb Vasc Biol October 2014 2, reduces mitochondrial membrane potential through reduced association between hexokinase 2 and voltage-dependent anion channels. This negates the proliferative and antiapoptotic effects of PDGF treatment and reduces neointimal formation in models of vessel injury. Dichloroacetate does not inhibit endothelial cell migration or re-endothelialization, making it an interesting novel drug candidate.48 In addition, overexpression and knockdown studies have identified miR203 as a micro-RNA with distinct effects on SMCs and endothelial cells. Although miR-203 slows SMC proliferation, it has no effect on endothelial cell growth.27 Targets that aid endothelial cell recovery without promoting SMC growth may also be good therapy options for restenosis. Recent research on Apoe−/− mice that lack the CXCL12 receptor CXCR4 indicates CXCR4’s requirement for normal proliferation and recruitment of endothelial cells to heal the endothelium after wire injury.49 The toll-like receptor 2/6 ligand, MALP-2, is another interesting agent in vascular injury models. Injection of MALP-2 2 hours after arterial injury in mice promoted re-endothelialization and inhibited neointimal hyperplasia. In vitro, MALP-2 led to enhanced endothelial cell proliferation but not that of SMCs in an artificial woundhealing test.50 However, myelopoiesis induced by MALP-2 therapy might be a detrimental side effect. Other potential new drug target strategies might involve delivering micro-RNAs by lipid particles to target SMCs, or perhaps to boost SMC progenitor cells to strengthen unstable atherosclerotic lesions with an increased risk of rupture. As we learn more about the plasticity and versatility of the SMC, new treatment strategies for vascular diseases are likely to be uncovered.
Summary The arterial SMC is a highly plastic cell type that plays crucial roles in normal physiology and several vascular diseases. The SMC’s ability to undergo phenotypic switching, the mosaicism of SMC origin in the vascular system, and the presence of SMC progenitor cells all contribute to the versatility and complexity of SMC responses. Recent research highlights the importance of noncoding RNAs and cross talk among SMCs and other cell types in vascular biology. The next few years are likely to see many more exciting discoveries related to this fascinating cell type.
Sources of Funding Research in K.E. Bornfeldt’s laboratory is supported by the National Institutes of Health grants R01HL062887, P01HL092969, R01HL097365, and P30DK017047 and by a Grant-in-Aid from the American Heart Association (14GRNT20410033). V.Z. Wall is supported by the Samuel and Althea Fellowship in Diabetes Research through the University of Washington’s Diabetes Research Center (P30DK017047).
Disclosures None.
References 1. Campbell JH, Campbell GR. Smooth muscle phenotypic modulation–a personal experience. Arterioscler Thromb Vasc Biol. 2012;32:1784–1789.
2. Gomez D, Owens GK. Smooth muscle cell phenotypic switching in atherosclerosis. Cardiovasc Res. 2012;95:156–164. 3. Clarke MC, Littlewood TD, Figg N, Maguire JJ, Davenport AP, Goddard M, Bennett MR. Chronic apoptosis of vascular smooth muscle cells accelerates atherosclerosis and promotes calcification and medial degeneration. Circ Res. 2008;102:1529–1538. 4. Wang Z, Wang DZ, Pipes GC, Olson EN. Myocardin is a master regulator of smooth muscle gene expression. Proc Natl Acad Sci U S A. 2003;100:7129–7134. 5. Wang X, Hu G, Gao X, Wang Y, Zhang W, Harmon EY, Zhi X, Xu Z, Lennartz MR, Barroso M, Trebak M, Chen C, Zhou J. The induction of yes-associated protein expression after arterial injury is crucial for smooth muscle phenotypic modulation and neointima formation. Arterioscler Thromb Vasc Biol. 2012;32:2662–2669. 6. Xie C, Guo Y, Zhu T, Zhang J, Ma PX, Chen YE. Yap1 protein regulates vascular smooth muscle cell phenotypic switch by interaction with myocardin. J Biol Chem. 2012;287:14598–14605. 7. Long X, Cowan SL, Miano JM. Mitogen-activated protein kinase 14 is a novel negative regulatory switch for the vascular smooth muscle cell contractile gene program. Arterioscler Thromb Vasc Biol. 2013;33:378–386. 8. Turlo KA, Scapa J, Bagher P, Jones AW, Feil R, Korthuis RJ, Segal SS, Iruela-Arispe ML. β1-integrin is essential for vasoregulation and smooth muscle survival in vivo. Arterioscler Thromb Vasc Biol. 2013;33:2325–2335. 9. Panchatcharam M, Miriyala S, Salous A, Wheeler J, Dong A, Mueller P, Sunkara M, Escalante-Alcalde D, Morris AJ, Smyth SS. Lipid phosphate phosphatase 3 negatively regulates smooth muscle cell phenotypic modulation to limit intimal hyperplasia. Arterioscler Thromb Vasc Biol. 2013;33:52–59. 10. Shen H, Eguchi K, Kono N, Fujiu K, Matsumoto S, Shibata M, Oishi-Tanaka Y, Komuro I, Arai H, Nagai R, Manabe I. Saturated fatty acid palmitate aggravates neointima formation by promoting smooth muscle phenotypic modulation. Arterioscler Thromb Vasc Biol. 2013;33:2596–2607. 11. Allahverdian S, Chehroudi AC, McManus BM, Abraham T, Francis GA. Contribution of intimal smooth muscle cells to cholesterol accumulation and macrophage-like cells in human atherosclerosis. Circulation. 2014;129:1551–1559. 12. Long JZ, Svensson KJ, Tsai L, Zeng X, Roh HC, Kong X, Rao RR, Lou J, Lokurkar I, Baur W, Castellot JJ Jr, Rosen ED, Spiegelman BM. A smooth muscle-like origin for beige adipocytes. Cell Metab. 2014;19:810–820. 13. Crouthamel MH, Lau WL, Leaf EM, Chavkin NW, Wallingford MC, Peterson DF, Li X, Liu Y, Chin MT, Levi M, Giachelli CM. Sodiumdependent phosphate cotransporters and phosphate-induced calcification of vascular smooth muscle cells: redundant roles for PiT-1 and PiT-2. Arterioscler Thromb Vasc Biol. 2013;33:2625–2632. 14. Zhou YB, Zhang J, Peng DQ, Chang JR, Cai Y, Yu YR, Jia MZ, Wu W, Guan YF, Tang CS, Qi YF. Peroxisome proliferator-activated receptor γ ligands retard cultured vascular smooth muscle cells calcification induced by high glucose. Cell Biochem Biophys. 2013;66:421–429. 15. Callegari A, Coons ML, Ricks JL, Yang HL, Gross TS, Huber P, Rosenfeld ME, Scatena M. Bone marrow- or vessel wall-derived osteoprotegerin is sufficient to reduce atherosclerotic lesion size and vascular calcification. Arterioscler Thromb Vasc Biol. 2013;33:2491–2500. 16. Osako MK, Nakagami H, Shimamura M, Koriyama H, Nakagami F, Shimizu H, Miyake T, Yoshizumi M, Rakugi H, Morishita R. Cross-talk of receptor activator of nuclear factor-κB ligand signaling with renin-angiotensin system in vascular calcification. Arterioscler Thromb Vasc Biol. 2013;33:1287–1296. 17. Majesky MW. Developmental basis of vascular smooth muscle diversity. Arterioscler Thromb Vasc Biol. 2007;27:1248–1258. 18. Trigueros-Motos L, González-Granado JM, Cheung C, Fernández P, Sánchez-Cabo F, Dopazo A, Sinha S, Andrés V. Embryological-origindependent differences in homeobox expression in adult aorta: role in regional phenotypic variability and regulation of NF-κB activity. Arterioscler Thromb Vasc Biol. 2013;33:1248–1256. 19. Awgulewitsch A, Majesky MW. Interpreting inflammation: smooth muscle positional identity and nuclear factor-κB signaling. Arterioscler Thromb Vasc Biol. 2013;33:1113–1115. 20. Majesky MW, Dong XR, Regan JN, Hoglund VJ. Vascular smooth muscle progenitor cells: building and repairing blood vessels. Circ Res. 2011;108:365–377. 21. Potter CMF, Lao KH, Zeng L, Xu Q. Role of biomechanical forces in stem cell vascular lineage differentiation. Arterioscler Thromb Vasc Biol. 2014;34:2186–2192.
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
Wall and Bornfeldt Recent Highlights of ATVB: Arterial Smooth Muscle 2179 22. Wong MM, Winkler B, Karamariti E, Wang X, Yu B, Simpson R, Chen T, Margariti A, Xu Q. Sirolimus stimulates vascular stem/progenitor cell migration and differentiation into smooth muscle cells via epidermal growth factor receptor/extracellular signal-regulated kinase/β-catenin signaling pathway. Arterioscler Thromb Vasc Biol. 2013;33:2397–2406. 23. Lin CH, Lilly B. Endothelial cells direct mesenchymal stem cells toward a smooth muscle cell fate [published ahead of print July 14, 2014]. Stem Cells Dev. doi:10.1089/scd.2014.0163. http://online.liebertpub.com/doi/ abs/10.1089/scd.2014.0163. 24. Lee MJ, Kim MY, Heo SC, Kwon YW, Kim YM, Do EK, Park JH, Lee JS, Han J, Kim JH. Macrophages regulate smooth muscle differentiation of mesenchymal stem cells via a prostaglandin F2α-mediated paracrine mechanism. Arterioscler Thromb Vasc Biol. 2012;32:2733–2740. 25. Bell RD, Long X, Lin M, Bergmann JH, Nanda V, Cowan SL, Zhou Q, Han Y, Spector DL, Zheng D, Miano JM. Identification and initial functional characterization of a human vascular cell-enriched long noncoding RNA. Arterioscler Thromb Vasc Biol. 2014;34:1249–1259. 26. Talasila A, Yu H, Ackers-Johnson M, Bot M, van Berkel T, Bennett MR, Bot I, Sinha S. Myocardin regulates vascular response to injury through miR-24/-29a and platelet-derived growth factor receptor-β. Arterioscler Thromb Vasc Biol. 2013;33:2355–2365. 27. Zhao J, Imbrie GA, Baur WE, Iyer LK, Aronovitz MJ, Kershaw TB, Haselmann GM, Lu Q, Karas RH. Estrogen receptor-mediated regulation of microRNA inhibits proliferation of vascular smooth muscle cells. Arterioscler Thromb Vasc Biol. 2013;33:257–265. 28. Hutcheson R, Terry R, Chaplin J, Smith E, Musiyenko A, Russell JC, Lincoln T, Rocic P. MicroRNA-145 restores contractile vascular smooth muscle phenotype and coronary collateral growth in the metabolic syndrome. Arterioscler Thromb Vasc Biol. 2013;33:727–736. 29. Li P, Zhu N, Yi B, Wang N, Chen M, You X, Zhao X, Solomides CC, Qin Y, Sun J. MicroRNA-663 regulates human vascular smooth muscle cell phenotypic switch and vascular neointimal formation. Circ Res. 2013;113:1117–1127. 30. Bertero T, Lu Y, Annis S, et al. Systems-level regulation of microrna networks by mir-130/301 promotes pulmonary hypertension. J Clin Invest. 2014;124:3514–3528. 31. Du Y, Gao C, Liu Z, Wang L, Liu B, He F, Zhang T, Wang Y, Wang X, Xu M, Luo GZ, Zhu Y, Xu Q, Wang X, Kong W. Upregulation of a disintegrin and metalloproteinase with thrombospondin motifs-7 by miR-29 repression mediates vascular smooth muscle calcification. Arterioscler Thromb Vasc Biol. 2012;32:2580–2588. 32. Wagner J, Riwanto M, Besler C, Knau A, Fichtlscherer S, Röxe T, Zeiher AM, Landmesser U, Dimmeler S. Characterization of levels and cellular transfer of circulating lipoprotein-bound microRNAs. Arterioscler Thromb Vasc Biol. 2013;33:1392–1400. 33. Lavin B, Gómez M, Pello OM, Castejon B, Piedras MJ, Saura M, Zaragoza C. Nitric oxide prevents aortic neointimal hyperplasia by controlling macrophage polarization. Arterioscler Thromb Vasc Biol. 2014;34:1739–1746. 34. Quillard T, Araújo HA, Franck G, Tesmenitsky Y, Libby P. Matrix metalloproteinase-13 predominates over matrix metalloproteinase-8 as the functional interstitial collagenase in mouse atheromata. Arterioscler Thromb Vasc Biol. 2014;34:1179–1186. 35. Manka D, Chatterjee TK, Stoll LL, Basford JE, Konaniah ES, Srinivasan R, Bogdanov VY, Tang Y, Blomkalns AL, Hui DY, Weintraub NL. Transplanted perivascular adipose tissue accelerates injury-induced neointimal hyperplasia: role of monocyte chemoattractant protein-1. Arterioscler Thromb Vasc Biol. 2014;34:1723–1730. 36. Schepers A, Eefting D, Bonta PI, Grimbergen JM, de Vries MR, van Weel V, de Vries CJ, Egashira K, van Bockel JH, Quax PH. Anti-MCP-1 gene therapy inhibits vascular smooth muscle cells proliferation and attenuates
vein graft thickening both in vitro and in vivo. Arterioscler Thromb Vasc Biol. 2006;26:2063–2069. 37. Lee GL, Chang YW, Wu JY, Wu ML, Wu KK, Yet SF, Kuo CC. TLR 2 induces vascular smooth muscle cell migration through cAMP response element-binding protein-mediated interleukin-6 production. Arterioscler Thromb Vasc Biol. 2012;32:2751–2760. 38. van den Borne P, Haverslag RT, Brandt MM, Cheng C, Duckers HJ, Quax PH, Hoefer IE, Pasterkamp G, de Kleijn DP. Absence of chemokine (C-x-C motif) ligand 10 diminishes perfusion recovery after local arterial occlusion in mice. Arterioscler Thromb Vasc Biol. 2014;34:594–602. 39. Tuomainen AM, Nyyssönen K, Laukkanen JA, Tervahartiala T, Tuomainen TP, Salonen JT, Sorsa T, Pussinen PJ. Serum matrix metalloproteinase-8 concentrations are associated with cardiovascular outcome in men. Arterioscler Thromb Vasc Biol. 2007;27:2722–2728. 40. Laxton RC, Hu Y, Duchene J, et al. A role of matrix metalloproteinase-8 in atherosclerosis. Circ Res. 2009;105:921–929. 41. Xiao Q, Zhang F, Grassia G, Hu Y, Zhang Z, Xing Q, Yin X, Maddaluno M, Drung B, Schmidt B, Maffia P, Ialenti A, Mayr M, Xu Q, Ye S. Matrix metalloproteinase-8 promotes vascular smooth muscle cell proliferation and neointima formation. Arterioscler Thromb Vasc Biol. 2014;34:90–98. 42. Pruthi D, McCurley A, Aronovitz M, Galayda C, Karumanchi SA, Jaffe IZ. Aldosterone promotes vascular remodeling by direct effects on smooth muscle cell mineralocorticoid receptors. Arterioscler Thromb Vasc Biol. 2014;34:355–364. 43. Li P, Li YL, Li ZY, Wu YN, Zhang CC, A X, Wang CX, Shi HT, Hui MZ, Xie B, Ahmed M, Du J. Cross talk between vascular smooth muscle cells and monocytes through interleukin-1beta/interleukin-18 signaling promotes vein graft thickening. Arterioscler Thromb Vasc Biol. 2014;34:2001–2011. 44. Ostriker A, Horita HN, Poczobutt J, Weiser-Evans MC, Nemenoff RA. Vascular smooth muscle cell-derived transforming growth factor-β promotes maturation of activated, neointima lesion-like macrophages. Arterioscler Thromb Vasc Biol. 2014;34:877–886. 45. Paccosi S, Musilli C, Caporale R, Gelli AM, Guasti D, Clemente AM, Torcia MG, Filippelli A, Romagnoli P, Parenti A. Stimulatory interactions between human coronary smooth muscle cells and dendritic cells. PLoS One. 2014;9:e99652. 46. Chang SF, Chen LJ, Lee PL, Lee DY, Chien S, Chiu JJ. Different modes of endothelial-smooth muscle cell interaction elicit differential β-catenin phosphorylations and endothelial functions. Proc Natl Acad Sci U S A. 2014;111:1855–1860. 47. Tang R, Cui XB, Wang JN, Chen SY. CTP synthase 1, a smooth muscle-sensitive therapeutic target for effective vascular repair. Arterioscler Thromb Vasc Biol. 2013;33:2336–2344. 48. Deuse T, Hua X, Wang D, et al. Dichloroacetate prevents restenosis in preclinical animal models of vessel injury. Nature. 2014;509:641–644. 49. Noels H, Zhou B, Tilstam PV, et al. Deficiency of endothelial cxcr4 reduces reendothelialization and enhances neointimal hyperplasia after vascular injury in atherosclerosis-prone mice. Arterioscler Thromb Vasc Biol. 2014;34:1209–1220. 50. Grote K, Sonnenschein K, Kapopara PR, Hillmer A, Grothusen C, Salguero G, Kotlarz D, Schuett H, Bavendiek U, Schieffer B. Toll-like receptor 2/6 agonist macrophage-activating lipopeptide-2 promotes reendothelialization and inhibits neointima formation after vascular injury. Arterioscler Thromb Vasc Biol. 2013;33:2097–2104. Key Words: atherosclerosis ◼ microRNAs ◼ myocytes, smooth muscle ◼ neointima
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
Arterial Smooth Muscle Valerie Z. Wall and Karin E. Bornfeldt Arterioscler Thromb Vasc Biol. 2014;34:2175-2179 doi: 10.1161/ATVBAHA.114.304441 Arteriosclerosis, Thrombosis, and Vascular Biology is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231 Copyright © 2014 American Heart Association, Inc. All rights reserved. Print ISSN: 1079-5642. Online ISSN: 1524-4636
The online version of this article, along with updated information and services, is located on the World Wide Web at: http://atvb.ahajournals.org/content/34/10/2175
Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally published in Arteriosclerosis, Thrombosis, and Vascular Biology can be obtained via RightsLink, a service of the Copyright Clearance Center, not the Editorial Office. Once the online version of the published article for which permission is being requested is located, click Request Permissions in the middle column of the Web page under Services. Further information about this process is available in the Permissions and Rights Question and Answer document. Reprints: Information about reprints can be found online at: http://www.lww.com/reprints Subscriptions: Information about subscribing to Arteriosclerosis, Thrombosis, and Vascular Biology is online at: http://atvb.ahajournals.org//subscriptions/
Downloaded from http://atvb.ahajournals.org/ by guest on March 11, 2015
