29 Connexins New genes in atherosclerosis

REVIEW ARTICLEConnexins:New genes in atherosclerosisCHRISTOS E.CHADJICHRISTOS &BRENDA R.KWAKDivision of Cardiology,Department of Internal Medicine,Geneva University Hospitals,Geneva,SwitzerlandAbstractAtherosclerosis,the main cause of death and disability in adult populations of industrialized societies,is a multifactorial progressive process involving a variety of pathogenic mechanisms.Our current view on the pathogenesis of the disease implies complex patterns of interactions between a dysfunctional endothelium,leukocytes,and activated smooth muscle cells in which cytokines and growth factors are known to play a crucial role.Apart from paracrine cell-to-cell signalling,a role for gap junction-mediated intercellular communication in the development of the disease has been recently suggested.Gap junction channels result from the docking of two hemichannels or connexons,formed by the hexameric assembly of connexins,and directly connect the cytoplasm of adjacent cells.In this review,we summarize existing evidence implicating connexins in atherosclerosis.Indeed,the expression pattern of vascular connexins is altered during atherosclerotic plaque formation.In addition,changes in connexin expression or gap junctional communication have been observed in vascular cells in vitro by disturbances in blood flow,cholesterol,inflammatory cytokines,and growth factors.Furthermore,genetically modifying connexin expression affects the course of the atherosclerotic process in mouse models of the disease.Finally,the involvement of connexins in treatment of atherosclerotic disease will be discussed.Key words:Atherosclerosis,connexin,endothelium,gap junction,leukocytes,smooth muscle cells,vascular wallPathogenesis of atherosclerosisAtherosclerosis leads as cause of death and disability in the Western world.This progressive disease is characterized by the accumulation of lipids,T lymphocytes and macrophages as well as fibrous elements in large and medium-sized arteries.Numerous observations in human and animals have led to the formulation of the ‘response-to-injury’hypothesis of atherosclerosis (1,2).This hypothesis emphasizes endothelium dysfunction as the initiat-ing step of the disease.Possible causes of endothe-lium dysfunction leading to atherosclerosis include elevated low-density lipoprotein (LDL),obesity,free radicals (caused by cigarette smoking,for example),hypertension,diabetes,and infectious micro-organisms (3–5).As a response to these harmful agents,the injured endothelium displays increased expression of a variety of cell adhesion molecules and secretes chemoattractants to recruit specific leukocytes (6).However,despite a given patient’s risk factor profile or type of animal model,the early lipid-rich lesions of atherosclerosis show a markedly non-random pattern of distribution within the arterial vasculature.Atherosclerotic plaques often form at branch points of arteries (7),which are regions of disturbed blood flow,suggesting that local fluid dynamic factors may contribute to the focal distribution of lesions.Indeed,various in vitro and in vivo experiments have shown flow-induced changes in the endothelial expression of pro-inflammatory chemokines and adhesion molecules (8–10).Leukocyte recruitment in the early phases of atherosclerosis involves mostly monocytes.However,T lymphocytes are also implicated in the early development of the disease (11).After adhering to the dysfunctional endothelium,monocytes trans-migrate between intact endothelial cells (ECs)to infiltrate into the arterial intima.Once in the intima,monocytes propagate and mature under the influ-ence of cytokines,chemokines,and growth factors secreted by themselves and other atheroma-associated cells.In addition,the induced expression of scavenger receptors allows macrophages toCorrespondence:Brenda R.Kwak PhD,Division of Cardiology,Geneva University Hospitals,Foundation for Medical Research,64avenue de la Roseraie,1211Geneva 4,Switzerland.Fax:+4122-382-72-45.E-mail:Brenda.KwakChanson@medecine.unige.ch Annals of Medicine .2007;39:402–411ISSN 0785-3890print/ISSN 1365-2060online #2007Taylor &Francis A n n M e d D o w n l o a d e d f r o m i n f o r m a h e a l t h c a r e .c o m b y S h a n h a i S e c o n d M e d i c a l U n i v o n 01/09/11F o r p e r s o n a l u s e o n l y .accumulate lipids within their cytoplasm and even-tually progress to the arterial foam cells,a hall-mark of the atherosclerotic lesion.These foam cells along with the T cells represent the fatty streak known as the earliest form of atherosclerotic plaque.The continued inflammatory response and accu-mulation of lipids collaborate with other events to promote atherosclerotic plaque growth and even-tually rupture (4,5).During the growing phase,medial smooth muscle cells (SMCs)migrate to the top of the intima where they proliferate and generate extra-cellular matrix components.The SMCs and matrix molecules coalesce to structure a strong fibrous cap that covers the original atherosclerotic lesion.Although this adds to the size of the plaque,it also seals the plaque off safely from the blood and reduces the possibility of rupture.As this cap matures some of the foam cells underneath die and lipids are released,thus forming the necrotic or lipid core of the advanced atherosclerotic plaque.Finally,the fibrous cap of a plaque might rupture,triggering thrombus formation at the site of the lesion.Rupture of an atherosclerotic plaque is the primary cause of sudden cardiac death,accounting for 60%of sudden deaths with thrombosis (12).In human,plaques that are most likely to break possess a thinned cap,a large lipid core,and many inflammatory cells (5,6,11,12).This plaque pheno-type is partially dependent on the macrophage activity.Macrophage foam cells produce reactive oxygen species and pro-inflammatory cytokines that intensify the local inflammatory response and further induce macrophage proliferation and lipid uptake.In addition,the activated macrophages produce matrixmetalloproteinases that can degrade the extra-cellular matrix (ECM)components thus deteriorat-ing the plaque’s fibrous cap and increasing its likelihood of rupture (13).Gap junctions in the vascular wallAtherosclerotic lesion formation involves complex patterns of interaction between inflammatory cells and normal cellular elements of the arterial wall,in which cytokines,chemokines,and growth factors are known to play a critical role.Aside from these paracrine mechanisms,another form of intercellular communication involves gap junctions.Gap junctions are composed of intercellular channels that allow the direct exchange of ions,small metabolites,and other second messenger molecules between adjacent cells,this way synchro-nizing responses in multicellular organisms (14).This type of intercellular communication permits rapidly co-ordinated activities,such as contraction of cardiac and smooth muscle,and transmission of neuronal signals at electrical synapses.In addition,gap junctional intercellular communication (GJIC)plays a role in slower physiological processes,such as cell growth and development.Molecular cloning studies have demonstrated that gap junction chan-nels are formed by members of a family of related proteins called connexins (Cx)in vertebrates.There are more than 20different Cx types in the human and mouse genomes (15).The commonly used nomenclature distinguishes Cx by their molecular mass deduced from their respective sequences.Full gap junction channels are complex in that they spanAbbreviations ADP adenosine diphosphate apoB apolipoprotein B apoE apolipoprotein EATP adenosine triphosphate Cx connexinECs endothelial cells ECM extra-cellular matrixGJICgap junctional intercellular communicationHMG-CoA 3-hydroxy-3-methylglutaryl-CoA IL interleukin LDL low-density lipoprotein LDLR low-density lipoprotein receptor LPS lipopolysaccharide SMCs smooth muscle cells TNF tumour necrosis factorKey messagesNThe development of atherosclerosis,a chronic immuno-inflammatory disease,critically depends on co-ordinated interactions between circulating blood cells and cells that reside within the arterial wall.NGap junction channels,formed by connexins,allow the direct exchange of ions and small molecules between cells in contact thus co-ordinating physiological processes such as cell growth and differentiation.NSignificant changes have been observed in the pattern of vascular connexin expression during atherosclerotic plaque formation.Recent studies on connexin-deficient mouse models for atherosclerosis have revealed key roles for these proteins in the development of the disease.Connexins in atherosclerosis403A n n M e d D o w n l o a d e d f r o m i n f o r m a h e a l t h c a r e .c o m b y S h a n h a i S e c o n d M e d i c a l U n i v o n 01/09/11F o r p e r s o n a l u s e o n l y .two plasma membranes.One gap junction channel results from the docking of two hemichannels or connexons.Each connexon is assembled from six Cx proteins.Recent findings suggest that hemichannels themselves can also open under both physiological and pathological conditions,and this activity may participate in a number of cellular processes (16,17).As a consequence of the molecular diversity of the Cx family,connexons and gap junction channels may be assembled from different Cx.Each type of connexin-made channel has unique inherent gating properties or permeabilities to various mole-cules and ions.Historically,gap junctions have been described as relatively non-selective and permeable to a wide range of molecules up to ,1kD.However,experiments carefully examining the electrophysiological properties of gap junction channels between cells expressing different Cx have revealed that channels made of different Cx exhibit unique conductances and permeabilities to ions and fluorescent dyes (18,19).Also endogenous metabo-lites,such as adenosine diphosphate (ADP),adeno-sine triphosphate (ATP),glutathione,and glutamate,have been shown to permeate gap junction channels between living cells (20).Moreover,the Cx composition of gap junction channels appeared to determine selectivity among second messengers (21).Four connexins,Cx37,Cx40,Cx43,and Cx45,have been described in the vascular wall,a tissue that contains EC-EC,SMC-SMC and EC-SMC gap junctions (22–24).Although Cx expression profiles have not yet been completely described for all parts of the vascular tree,it has become apparent that it is not uniform in all blood vessels (22).In addition,differences in Cx expression have been reported in some vessels,like coronary arteries,when comparing different species (25).In general,ECs express Cx37and Cx40,whereas SMCs express Cx43and Cx45.Cx43has also been found in a subset of ECs near branch points of arteries and in capillaries (26,27).Additional expression of Cx37or Cx40in SMCs has been observed in small elastic or resistance arteries or during development (28,29).The recent availability of connexin-specific tools,such as dominant negative connexins or connexin-mimetic blocking peptides,has boosted our know-ledge about the physiological function of vascular gap junctions.Thus,GJIC plays a significant role in the conduction of vasomotor responses along arterioles (23,24,30).In addition,ECs are induced to migrate during the process of new capillary sprout formation and during repair of the endothelial lining after injury in large vessels,a process that seems dependent on temporary switches in Cx expression (31).More recently,Cx43,Cx40,Cx37,and Cx45gene-targeted mice have been created,each display-ing a different vascular phenotype.From these knock-out mice,it appeared that Cx40is required for normal transmission of endothelium-dependent vasodilator responses (32).In addition,Cx40-deficient mice appeared hypertensive,pointing to a possible role of this Cx in the regulation of blood pressure (32,33).Interestingly,EC-specific deletion of Cx43seems to cause hypotension in mice (34).However,this observation remains to be confirmed as similar mice that were developed by another laboratory do not display a vascular phenotype (27).The complete deletion of Cx45causes abnormalities in vascular development and mouse embryos die early between days 9.5and 10.5(35).Although the deletion of Cx37leads to female infertility because the mice do not ovulate,these animals do not show an obvious vascular phenotype (36,37).In contrast to the single knock-out animals,mice lacking both Cx37and Cx40display severe vascular abnormalities and die perinatally (38).An overview of the different Cx-deficient mice and their vascular phenotype is illustrated in Table I.Gap junctions and atherosclerosisCx are dynamic proteins with half-lives ranging from 1to 5hours,indicating that gap junction channels are fully exchanged several times per day (14).This may provide a mechanism to regulate direct cyto-plasmic cross-talk between cells under normal or pathological conditions.Over the last 15years,thereTable I.Cx-deficient mice and their associated vascular phenotype.Cx-deficient miceVascular phenotypeReferences Cx372/2normal(36,37)Cx402/2irregular arteriolar vasomotion,hypertension (32,33)EC-specific deletion of Cx40normal(86)Cx402/2Cx372/2lethal (P0–P1),severe vascular abnormalities(38)Cx432/2lethal (P0),swelling and blockage of right ventricular outflow tract (72)EC-specific deletion of Cx43Normal/hypotension(27)/(34)Cx452/2lethal (E9.5–10.5),defective vascular development(35)404 C.E.Chadjichristos &B.R.KwakA n n M e d D o w n l o a d e d f r o m i n f o r m a h e a l t h c a r e .c o m b y S h a n h a i S e c o n d M e d i c a l U n i v o n 01/09/11F o r p e r s o n a l u s e o n l y .has been increasing support that Cx might partici-pate in the process of atherogenesis.Thus,signifi-cant changes in the pattern of vascular Cx expression have been observed during atherosclerotic plaque formation.Furthermore,atherosclerotic risk factors acting on the endothelium,inflammation,and SMC activation/proliferation have been shown to affect Cx expression or GJIC.It suggests that Cx may play a role in the pathogenesis,but it does not establish causality between these proteins and atherosclerosis.More extensive research on the vascular Cx has been conducted in recent years,to further investigate this association.For this purpose,mouse models have been created by interbreeding Cx-deficient mice and atherosclerosis-prone mice (LDLR 2/2mice or apo-lipoprotein E-deficient (ApoE 2/2)mice).Altered connexin expression during atherogenesis In human coronary atherosclerosis,Cx43expression in intimal SMCs is increased at early stages of the disease but reduced in advanced atheroma (39).Hypercholesterolaemia-induced atherosclerosis in the rabbit arterial wall resulted in Cx43expression associated with macrophage foam cells and,compar-able with the human situation,reduced levels of Cx43between intimal SMCs were observed in advanced lesions (40).A similar temporal pattern of Cx43expression in intimal SMCs and macro-phage foam cells was observed during atherogenesis in LDLR 2/2mice fed a high-fat diet for 0,6,10,or 14weeks (41).Although Cx43is mostly absent in aortic endothelium of healthy LDLR 2/2mice,this protein appeared in ECs at the shoulder region of advanced atherosclerotic plaques,a localization known to experience turbulent blood flow.However,the endothelium covering advanced ather-osclerotic plaques no longer expressed Cx37or Cx40.A similar observation has been reported in a study in C57BL/6mice (42).Cx37and Cx40were dramatically reduced when these mice were fed a high-cholesterol diet for several months.In addition,Cx37expression has been reported in macrophages in early and late atheroma (41).This protein also appeared in medial SMCs beneath advanced atherosclerotic lesions (41).Of note,similar Cx37expression patterns were observed in advanced atherosclerotic plaques in human carotid artery.Initiation of atherosclerosisEndothelium dysfunction,the initiating step of atherosclerosis,is frequently observed with hyper-cholesterolaemia and mostly in arterial regionsexperiencing disturbed flow patterns.As described above,gap junctions in arterial endothe-lium in situ are known to consist mainly of Cx40and Cx37.An increasing number of reports reveal,however,that Cx43is moderately expressed or absent in quiescent endothelia,but is induced under conditions associated with endothelium dysfunction.Indeed,Cx43was found to be abun-dant in endothelia localized at the downstream edge of the ostia of branching vessels and at flow dividers,regions that experience turbulent shear stress (26).In addition,various in vitro studies have shown a positive correlation between Cx43expres-sion and mechanical load or disturbed flow patterns (43–45).In a recent study,we have systematically investigated the effects of pulsed unidirectional and oscillatory flows in combination with different levels of hydrostatic pressure and circumferential stretch on the expression of Cx43using an in vitro perfusion system (46).We observed that endothelial Cx43expression significantly increased in response to oscillatory shear stress within 4hours,with the highest levels (up to 3-fold)reached after 24hours.In contrast,increasing hydrostatic pressure from 100to 150mmHg did not affect Cx43expression levels.Interestingly,4%cyclic circumferential stretch combined with high unidirectional shear induced only a 1.5-fold increase in Cx43expression,whereas the same cyclic circumferential stretch combined with oscillatory shear stress increased Cx43by almost 7-fold.Taken together,these studies suggest a causal relation between haemodynamic conditions and endothelial Cx43expression,which may be significant to focal vulnerability to atherosclerosis.Interestingly,it has been demonstrated that gap junction assembly between cultured Novikoff hepatoma cells can be increased by cholesterol supplementation (47),LDL,and apolipoprotein B (apoB)treatments (48).Such enhancement of gap junction assembly seems dependent on stimula-tion of Cx trafficking from intercellular stores (49).In addition to its effects on assembly,increasing cellular cholesterol content affects the biophysical properties of Cx43gap junction channels.Indeed,neonatal cardiomyocytes were protected against heptanol-induced closure of Cx43gap junction channels upon increased cellular cholesterol content (50).Whether cholesterol/LDL exerts simi-lar effects on Cx43in ECs remains to be investi-gated.In this context,it is worth mentioning that lipoprotein-derived phospholipid oxidation products upregulate Cx43,downregulate Cx37,and do not affect Cx40expression in the endothe-lium of murine carotid arteries in vivo and EC cultures (51).Connexins in atherosclerosis405A n n M e d D o w n l o a d e d f r o m i n f o r m a h e a l t h c a r e .c o m b y S h a n h a i S e c o n d M e d i c a l U n i v o n 01/09/11F o r p e r s o n a l u s e o n l y .Progression of the diseaseLeukocyte recruitment in the early phases of atherosclerosis involves monocytes and T lympho-cytes (11).There is evidence in the literature,although mainly circumstantial,for a role of GJIC in the inflammatory response.Thus,altered Cx expression and/or GJIC has been described in a number of inflammatory conditions in vivo ,such as the ischaemic heart,and after injection of endotoxin or lipopolysaccharide (LPS)in liver or heart (see for reviews:(52,53)).More specifically,reduced GJIC has been observed with the pro-inflammatory mediators LPS,tumour necrosis factor a (TNF-a ),interleukin 1a (IL-1a )or Il-1b in in vitro studies on ECs (54,55)and SMCs (56).Moreover,rapid closure of myoendothelial gap junctions has been reported in a co-culture of human ECs and SMCs that were exposed to TNF-a (57).Most of these rapid changes in GJIC might be explained by actions of the pro-inflammatory mediators on gating and permeability properties of the gap junction channels involved.The possibility that GJIC may play a role in the modulation of the inflammatory response has been subject of a number of in vitro studies in recent years.Thus,various laboratories have tested the possibility that gap junctions are involved in leukocyte transmi-gration in vitro with somewhat conflicting outcomes.In general,transmigration of leukocytes across endothelial or epithelial monolayers was assessed in these studies in the presence of connexin-mimetic peptides or gap junction channel-blockers.The results revealed that transmigration of neutrophils was increased or not affected under those conditions (53,58,59),transmigration of monocytes was decreased (60),but that these treatments had only modest effects on lymphocytes (61).The differences in the in vitro cell systems used are most likely responsible for the variability in results between laboratories.Beyond doubt,in vivo studies on Cx-deficient mice will provide a better defined and more physiological setting to address whether Cx are involved in the inflammatory response.As described before,the expression profile of Cx37is altered in mouse and human atherosclerotic plaques.Notably,Cx37is no longer found in the endothelium covering advanced plaques but detected in macrophages recruited to the lesions (41,62).Subsequently,we investigated the effect of Cx37deletion (Cx372/2)on atherogenesis in ApoE 2/2mice,an animal model for the disease (63).We induced atherosclerosis in Cx37+/+ApoE 2/2and Cx372/2ApoE 2/2mice by feeding them a high-cholesterol diet and demonstrated that atherosclero-tic lesion development was accelerated in micelacking Cx37,both in the descending aortas and aortic sinuses.Knowing that recruitment of leuko-cytes is a prerequisite for atherosclerotic lesion formation,we tested whether the increased athero-sclerosis in Cx372/2ApoE 2/2mice was caused by accelerated monocyte migration across the endothe-lium.Thus,criss-cross in vivo adoptive transfer assays revealed that the presence of Cx37in monocyte/macrophages but not in the endothelium contributed to the recruitment of these leukocytes to atherosclerotic lesions.To explore whether Cx37could control the earliest step of the leukocyte recruitment paradigm,we then compared adhesion of Cx37+/+ApoE 2/2and Cx372/2ApoE 2/2monocyte/macrophages to an activated mouse EC monolayer.Indeed,an increased number of Cx37-deficient monocytes or macrophages adhered.Because indi-vidual monocytes and macrophages cannot form gap junctions,we speculated that cell adhesion may be regulated by the activity of Cx37hemichannels.We further demonstrated in primary monocytes,macro-phages,and a macrophage cell line transfected with Cx37cDNA that Cx37hemichannel activity inhib-ited leukocyte adhesion.This anti-adhesive effect was mediated by ATP release into the extra-cellular space.Thus,Cx37hemichannels control the initia-tion of atherosclerotic plaque development by modulating the autocrine ATP-dependent regula-tion of monocyte adhesion.A genetic polymorphism in the human Cx37gene (C1019T)has been reported as a potential prog-nostic marker for atherosclerotic plaque develop-ment and myocardial infarction (64–68).This specific polymorphism results in a non-conservative amino acid change (serine to proline)in the intracellular C-terminus of the Cx37protein.In this context,the observation that expression of Cx37-S or Cx37-P by transfection of a macrophage cell line revealed differential adhesiveness to substrates is of particular importance (63).These observations suggests that the C1019T polymorphism of Cx37generates hemichannels with different properties in terms of ATP transport and provide a potential mechanism by which Cx37may function as a prognostic marker for atherosclerosis.Plaque ruptureThe continued inflammatory response and accumu-lation of lipids work together with other events to promote atherosclerotic plaque growth and even-tually rupture (4,5).During the growing phase,medial SMCs migrate to the top of the intima where they multiply and produce components of the ECM.The SMCs and matrix molecules coalesce to form a406 C.E.Chadjichristos &B.R.KwakA n n M e d D o w n l o a d e d f r o m i n f o r m a h e a l t h c a r e .c o m b y S h a n h a i S e c o n d M e d i c a l U n i v o n 01/09/11F o r p e r s o n a l u s e o n l y .strong fibrous cap that covers the original athero-sclerotic site.Migration and proliferation of SMCs as well as synthesis of ECM by these cells commonly involve phenotypic transformation of SMCs from the differentiated contractile state to the activated synthetic state.The major gap junction protein expressed in vascular SMCs is Cx43.Interestingly,the synthetic SMC phenotype is known to express considerably higher levels of this protein in vitro as compared to contractile SMCs (69,70),suggesting that the level of Cx43expression critically depends on SMC phenotype.Furthermore,increased Cx43expression between intimal SMC is observed in vivo in early atherosclerotic lesions in human and mice (39,41).These increased Cx43expression levels between intimal SMCs declined with progression of the lesion (39).Besides the changes in Cx43expression in SMCs,the expression of this protein is increased in other cell types during atherogenesis.Indeed,relatively high expression levels of Cx43are observed in macrophage foam cells (41,71)and in ECs covering the shoulder region of atherosclerotic lesions (41).To elucidate whether these changes in Cx43expression participate in the development of atherosclerotic plaques in vivo ,we have intercrossed atherosclerosis-susceptible LDLR 2/2mice with mice heterozygous for a Cx43null mutation (Cx43+/2mice).Indeed,the Cx43knock-out mutation (Cx432/2)in mice is lethal because of cardiac malformations (72),and the amount of Cx43is reduced by half in Cx43+/2mice (73).Male LDLR 2/2mice,10weeks old,with normal (Cx43+/+LDLR 2/2)or reduced (Cx43+/2LDLR 2/)Cx43expression were fed a cholesterol-rich diet for 14weeks.The progression of atherosclerosis was reduced by about 50%both at the level of the aortic roots and in the thoraco-abdominal aortas of Cx43+/2LDLR 2/2mice in comparison to control littermates (74).Of note,both groups of mice showed similar increases in serum cholesterol and triglycerides as well as body weight.Interestingly,our results also revealed that the composition of atherosclerotic plaques in Cx43+/2LDLR 2/2mice was strikingly different from control mice.Lesions of Cx43+/2LDLR 2/2mice had smaller lipid-cores and fewer macrophages,whereas leukocyte counts in peripheral blood were similar for both groups.In addition,the lesions had thicker fibrous caps containing more SMCs and interstitial collagen.The content of SMCs versus macrophages,the extent of collagen within the lesion,and the size of the lipid core are related to the vulnerability of human atherosclerotic lesions to rupture,an event which might lead to acute myocardial infarction.Thus,targeting Cx43may favour potential plaquestabilizing processes rather than affecting plaque size alone.The mechanism by which reduced Cx43ultimately leads to this dual benefit remains to be identified (75).Possible role for connexins in treatment of atherosclerotic disease StatinsInhibitors of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)reductase (called statins)lower plasma choles-terol in human and reduce atherosclerosis-related morbidity and mortality (76,77).Because several in vitro studies have identified numerous pleiotropic effects of statins on vascular cells that could modulate atherogenesis and plaque rupture via mechanisms other than lowering cholesterol (78),we evaluated the effects of statins on gap junctions in primary human vascular cells (75).We observed that various types of statins dose-dependently inhibited Cx43expression.Of note,the lower concentrations of simvastatin (0.4–0.08m M)used in our study are within the range of expected tissue levels derived from prescribed pharmacological dosages.The effect of the statins on Cx43expression was abolished in the presence of L-mevalonate,indicating that inhibition of HMG-CoA reductase is responsible for the reduc-tion in Cx43.The statin-induced reduction in Cx43expression was associated with a marked reduction in GJIC.The clinical benefits of lipid lowering with statins are often attributed to changes in atherosclero-tic plaque composition leading to plaque stability (79).Interestingly,statins do not reduce plasma lipid levels in mice due to compensatory upregulation of HMG-CoA reductase.Consequently,possible beneficial effects of statin treatment on the composition of mouse atherosclerotic plaques can be interpreted without this confounding variable.We observed that statin treatment reduced Cx43expression in athero-sclerotic lesions in LDLR 2/2mice.Interestingly,statin-treated mice displayed beneficial changes in plaque morphology that were similar to the ones observed in Cx43+/2LDLR 2/2mice (74).In another study,it has been shown that Cx37and Cx40expression was decreased in mouse aorta during long-term hyperlipidaemia,a major risk factor of atherosclerosis.Interestingly,simvastatin has been shown to reverse the hyperlipidaemia-induced decrease of endothelial Cx37in mice,thus possibly yielding an additional connexin-mediated benefice to this treatment (42).Percutaneous coronary interventionA consequence of coronary atherosclerosis is ischemic heart disease.In addition to coronaryConnexins in atherosclerosis407A n n M e d D o w n l o a d e d f r o m i n f o r m a h e a l t h c a r e .c o m b y S h a n h a i S e c o n d M e d i c a l U n i v o n 01/09/11F o r p e r s o n a l u s e o n l y .。