it is well known that patients with type 2 diabetes mellitus are at increased risk of cardiovascular ( cv ) disease . elevated plasma glucose levels lead to increased cv risk combined with associated comorbidities , such as obesity , hypertension and dyslipidemia . incretinbased therapy , such as glucagonlike peptide1 ( glp1 ) receptor agonists ( ra ) and dipeptidyl peptidase4 inhibitors ( dpp4i ) , have been widely used in clinical practice . incretinbased therapy is expected to provide cv benefits because of its amelioration of fasting and postprandial hyperglycemia without hypoglycemia or bodyweight gain1 , 2 . there have been a number of animal studies showing that glp1ra ( glp1 receptor agonists ) or dpp4i exert an antiatherosclerotic effect in hypercholestetolemic mice6 , 7 , 8 , 9 , 10 , 11 , 12 . because these mice are not diabetic , direct prevention of the effect of glp1ra or dpp4i on atherosclerosis beyond glucose has been implicated . unlike animal studies , recent clinical trials have failed to show a favorable effect of dpp4i on the suppression of cv outcomes in type 2 diabetic patients13 , 14 , 15 . however , the majority of subjects enrolled in these trials already had cv diseases and were being treated with cardioprotective drugs , such as statins . additionally , the observation periods were only a few years , which make the evaluation of the antiatherogenic potential of dpp4 inhibitors difficult , especially in the early stages of atherosclerosis . in the present minireview , we introduce representative animal studies , including our own study , implicating the favorable effects of glp1 , glucosedependent insulinotropic polypepide ( gip ) and dpp4i on atherosclerosis . arakawa et  al.16 reported for the first time that exendin(ex)4 , a glp1ra , reduced monocyte adhesion to the endothelium of aorta and suppressed atherosclerotic lesions in apolipoprotein e knockout ( apoe 
 ) mice . our group subsequently reported that chronic infusion of native glp1 significantly suppressed atherosclerotic lesions and macrophage infiltration in the aortic wall in apoe 
  mice17 . these effects were cancelled by coinfusions with a specific antagonist for glp1 receptors , including exendin ( 939 ; ex9 ) . surprisingly , the infusion of native gip also suppressed the development of atherosclerotic lesions in apoe 
  mice as potently as glp1 infusion17 , 18 . this suppression was completely reversed by coinfusions with a specific antagonist for gip receptors , pro(3)gip . these results suggest that both native incretins confer an antiatherosclerotic effect through their own receptors . gaspari et  al.19 reported that a glp1ra , liraglutide , inhibited the progression of early onset , lowburden atherosclerotic disease in the apoe 
  mouse mode , whereas no significant effect of liraglutide on the progression of late onset , highburden atherosclerotic disease was observed . that study might reasonably explain why incretinbased therapy failed to prevent secondary events in diabetic patients premorbid for cv diseases . given that glp1 and gip both have antiatherosclerotic effects , increases in both active glp1 and gip by dpp4i might synergistically suppress the development of atherosclerosis . indeed , a number of publications have shown that dpp4i powerfully suppresses the development of atherosclerotic lesions in apoe 
  mice6 , 7 , 8 , 9 , 10 , 11 . endogenous glp1 levels are increased by just two to threefold by dpp4i ; however , the suppressive effect of dpp4i on atherosclerotic lesions was comparable with that of liraglutide , providing a much higher concentration of glp1 in the blood20 . therefore , increased gip might participate in the antiatherogenic effects of dpp4i . it remains unknown , however , whether this antiatherogenic property of dpp4i can be credited to higher endogenous levels of the active incretins glp1 and gip . we attempted to determine whether the antiatherogenic property of dpp4 inhibitor is derived from increased levels of the endogenous active incretins glp1 and gip or from other mechanisms . to achieve this , we administered glp1 and gip receptor antagonists to apoe 
  mice that were simultaneously treated with dpp4i , and examined how an incretin receptor blockade attenuated the antiatherosclerotic effect of the dpp4 inhibitor . the two receptor blockers in combination completely abolished the antiatherosclerotic effect of vildagliptin in nondiabetic mice10 . we subsequently attempted the same experiment in streptozotocininduced diabetic apoe 
  mice showing furtherprogressed atherosclerotic lesions . unlike nondiabetic apoe 
  mice , infusions with dual antagonists incompletely abolish the suppressive effect of vildagliptin on atherosclerosis , implying that vildagliptin partly confers an antiatherogenic effect beyond that from the incretins in the diabetic animals10 . we estimated that incretindependent and incretinindependent mechanisms equally contributed to the antiatherosclerotic effect of vildagliptin in the diabetic apoe 
  mice . as we will mention later , much attention has been given to the new hypothesis that dpp4is confer antiatherogenic effects by blocking the proinflammatory and proatherogenic properties of dpp4/cd26 . we speculate that vascular inflammation is largely enhanced in diabetic apoe 
  mice ; thus , vildagliptin could show remarkable antiinflammatory effects , particularly in diabetic mice , beyond the action of incretins . as previously mentioned , the antiatherogenic effects of glp1 , gip and dpp4 inhibitors are supported by in  vivo and in  vitro studies , and these findings have raised a new question as to whether incretins and dpp4 inhibitors can protect against other vascular diseases . restenosis is an exaggerated healing process that arises after arterial interventions , such as percutaneous coronary angioplasty with or without stenting , and leads to decreased blood flow by luminal narrowing21 , ultimately requiring an additional intervention . the development of restenosis is mainly attributed to the hyperplasia of regenerated intima , which is known as neointima , and consists of vascular smooth muscle cells and extracellular matrix . although the pathology of neointimal hyperplasia is different from that of atherosclerotic plaque formation , there are a number of similarities in their mechanisms . among the mechanisms involved , the migration and proliferation of vascular smooth muscle cells ( vsmcs ) have been shown to play an important role22 . because glp1 and gip receptors are expressed abundantly in vsmcs , researchers have focused on the effects of incretins and dpp4 inhibitors on restenosis . denudation of the endothelium and apoptosis of medial vsmcs by the mechanical stress of arterial interventions is the initiating event in neointimal formation . in preclinical restenosis models , these events are induced by withdrawing an inflated balloon catheter from the rat carotid artery or by inserting a guidewire into the mouse femoral artery23 , 24 . these models are useful to evaluate therapeutic effects on restenosis because of their resemblance to the pathological characteristics of restenosis in humans . in wildtype mice , exendin4(ex4 ) , a glp1 receptor agonist , reduces neointimal hyperplasia after femoral artery wire injury without affecting the metabolic parameters , including glucose tolerability25 , 26 . similarly , ex4 showed a protective effect against neointimal hyperplasia with reduced vsmc proliferation in a rat model of restenosis27 . furthermore , in  vitro studies show supportive data that ex4 suppressed rat vsmc proliferation stimulated by plateletderived growth factor . this effect of ex4 was completely canceled by the coadministration of a protein kinase a ( pka ) inhibitor26 , suggesting that the glp1 receptorcamppka pathway is involved in the suppression of neointimal hyperplasia . in addition , the effect of dpp4is was examined in otsuka longevans tokushima fatty rats , which is a model of type 2 diabetes with insulin resistance29 . in that study , sitagliptin significantly improved glucose tolerability and suppressed neointimal hyperplasia after a carotid artery balloon injury in a dosedependent manner . this effect was accompanied with reduced cell proliferation and increased cell apoptosis in the neointima . furthermore , sitagliptin reduced vsmc proliferation and migration in  vitro , showing that the inhibition of dpp4 activity directly affects vsmcs , independent of the increased activity of incretins or other bioactive peptides . gip treatment significantly suppressed neointimal hyperplasia accompanied with reduced cell proliferation in the neointima compared with vehicle . in accordance with the in  vivo findings , gip suppressed vsmc proliferation stimulated by plateletderived growth factor in  vitro , showing the possibility that the suppression of neointimal hyperplasia is attributed to the direct effects of gip on vsmcs . those studies and our data show that an incretinbased therapy would be a novel and beneficial therapeutic approach for the treatment of arterial restenosis . rabbits are bigger than rodents , and several earlier publications have shown that hypercholesterolemic rabbits develop severe atherosclerotic lesions in their coronary arteries30 , 31 . we examined the effect of anagliptin , a dpp4i , on atherosclerosis development in the aorta and coronary arteries in highcholesterol dietfed rabbits . dietary cholesterol intake markedly increased the serum total cholesterol levels , and the most striking increase was observed in a major lipoprotein , very lowdensity lipoprotein . anagliptin slightly decreased both total cholesterol and very lowdensity lipoprotein cholesterol , as well as the cholesterol absorption markers sitosterol and campesterol , although not significantly . anagliptin remarkably suppressed aortic atherosclerosis , which was comparable with the suppression observed in apoe 
  mice . atherosclerotic lesions were clearly observed in the coronary arteries , where the mean intimamedia area was enlarged and intimal formation developed . ( smooth muscle actin)positive and macrophagepositive areas in the coronary arteries were remarkably suppressed after anagliptin treatment . the aortic lesion ratio and the coronary intima area were correlated with each other , and each lesion correlated with total cholesterol in whole cholesterolfed rabbits . gene expression of the proinflammatory cytokines , tumor necrosis factor ( tnf)and interleukin ( il)6 in the carotid arteries were markedly reduced , and vascular dpp4 activity was reduced after anagliptin treatment . because the rabbits were not diabetic and anagliptin treatment did not affect glucose metabolism , the antiatherosclerotic effect of anagliptin could be attributed to the pleiotropic effect of dpp4is beyond their glucoselowering effect . vittone et  al.32 reported that sitagliptin treatment did not reduce the atherosclerotic lesion area in apoe 
  mice , although macrophage infiltration into the plaque was significantly suppressed . ta et  al.6 reported that alogliptin suppressed aortic plaque formation in diabetic apoe 
  mice , but not in nondiabetic apoe 
  mice . these results might be based on the technical difficulty of measuring vessel lesions in small animals . the merit of using rabbits for atherosclerosis models is that atherosclerotic lesions in the aorta are easily visible without special staining . this simple approach for the identification of atherosclerotic lesions convinced us of a powerful antiatherosclerotic effect of the dpp4i . nader et  al.33 recently reported that sitagliptin suppressed atherosclerotic lesion development in 0.5% cholesterol and 1% methioninefed rabbits , which is in good agreement with our results , and suggests that the antiatherosclerotic effect of dpp4i is applicable to animals that are bigger than rodents . atherosclerosis is a chronic inflammatory disease in the vasculature , and monocytes / macrophages play a crucial role in atherogenesis . it has been hypothesized that dpp4 , which is known as cd26 , is an inflammatory cytokine that is involved in atherosclerosis ; hence , dpp4i might suppress the progression of atherosclerosis by inhibiting cd26mediated inflammation34 , 35 , 36 , 37 . ikeda et  al.38 showed that soluble cd26 enhanced the expression of tnfa and il6 messenger ribonucleic acid and protein in thp1 human monocytes in this context , the antiinflammatory effect of dpp4i would be totally incretinindependent . incretins exert their actions through their own receptors in various tissues , including pancreatic islets , and cyclic adenosine monophosphate ( camp ) acts as a second messenger of incretin receptors39 . panjwani et  al.40 reported that classical glp1 receptors were not present in mouse macrophages , and doubted the antiatherogenic effect of glp1 receptor agonists . we showed that the glp1 receptor and gip receptor genes were present in mouse and human monocytes , but were dramatically downregulated when differentiated to macrophages or foam cells10 , 17 , 30 . thus far , it remains to be elucidated whether incretins exert antiinflammatory actions through their own receptors in monocytes . in u937 human monocytes , the active form of the incretin receptor agonists , ex4 , gip(142 ) , and d(ala2)gip , elicited camp generation , whereas the inactive form , glp1(936)amide and gip ( 342 ) , did not , suggesting that the incretins ' signal enters monocytes through classical incretin receptors . 8bromocamp ( a camp mimetic ) or forskolin ( a camp stimulator ) attenuated lipopolysaccharide ( lps)induced il6 gene expression . ex4 or gip significantly suppressed the expression of il1 and il6 that was stimulated by lps . the suppression with incretins were completely reversed by incretin receptor antagonists ( ex9 or [ pro3]gip ) , an adenylate cyclase inhibitor ( sq22,536 ) or pka inhibitor . therefore , the antiinflammatory effect of incretins was mediated through the incretin receptors  camp  pka pathway . teneligliptin , a dpp4i , substantially suppressed the lpsinduced gene expression of il1 , il6 and tnf without affecting cell viability or proliferation . these results suggest that glp1 and gip have a similar antiinflammatory effect in human monocytes in a camp / pkadependentmanner , whereas dpp4i exerts an antiinflammatory effect independent of incretins . we compared the antiinflammatory effect between gip , ex4 and teneligliptin at the same concentration . teneligliptin more strongly suppressed the lpsinduced gene expression of inflammatory molecules than incretins without affecting cell proliferation . similar to dpp4i , the anticd26 antibody or cd26 small inferring ribonucleic acid remarkably suppressed proinflammatory cytokine expression . these results suggest that the antiinflammatory effect of teneligliptin is mediated by the inactivation of cd26 . the mechanisms of the antiinflammatory effect of incretinrelated agents still remains largely unknown ; however , experimental data have shown that glp1 or dpp4 inhibitors could shift the polarization profile of macrophages from the m1 phenotype towards the m2 phenotype41 , 42 . the development of atherosclerosis is influenced by abnormalities in cellular cholesterol homeostasis , which are shown by the subendothelial accumulation of lipidladen macrophage foam cells . the accumulated foam cells in the subendothelial space create a lipidrich plaque , which is the initial process of atherosclerosis43 . foam cell formation is regulated by several factors , including scavenger receptors , such as cd3644 ; acylcoenzyme a : cholesterol acyltransferase 1 , a ratelimiting enzyme for the esterification of cholesterol46 ; and freecholesterol efflux , which is mediated by adenosine triphosphatebinding cassette transporter a145 . we focused on the effect of gip with regard to the development of aortic atherosclerotic lesions and macrophage foam cell formation , as well as related molecules in mice . foam cell formation was determined by the incorporation of ( 3h)oleate into cholesteryloleate , which was stimulated by oxidizedlowdensity lipoprotein . we previously reported that the infusions of glp1 and gip for 4  weeks significantly suppressed foam cell formation in apo enull mice17 . similar to apo e null mice , gip infusion for 4  weeks significantly suppressed foam cell formation in c57bl/6 mice . we carried out a similar experiment using giprknockout mice that were a kind gift from professor yamada from akita university . as expected , the suppressive effect of gip on foam cell formation was not observed in the gipr ( gip receptors)ko mice . glp1 and gip significantly stimulated adenosine triphosphatebinding cassette transporter a1 expression , and suppressed acylcoenzyme a : cholesterol acyltransferase 1 and cd 36 expression in human monocytederived macrophages20 . recently , dai et  al.47 reported that dpp4i directly repress foam cell formation through the inhibition of cd36 and a scavenger receptor , lox1 . we confirmed the direct suppressive effect of dpp4i on foam cell formation in mouse peritoneal macrophages obtained from db / db diabetic mice . therefore , it is highly likely that the antiinflammatory property of incretinrelated agents is associated with the suppressive effect on macrophage foam cell formation . figure  1 shows the possible mechanisms of the antiatherosclerotic properties of incretinrelated agents based on animal studies . glp1 , gip and dpp4 inhibitors all confer antiatherosclerotic effects in experimental animals through the suppression of inflammation and foam cell formation in monocytes / macrophages .
abstractwe reported that native incretins , liraglutide and dipeptidyl peptidase4 inhibitors ( dpp4i ) all confer an antiatherosclerotic effect in apolipoprotein enull ( apoe 
 / ) mice . 
 we confirmed the antiatherogenic property of incretinrelated agents in the mouse wire injury model , in which the neointimal formation in the femoral artery is remarkably suppressed . 
 furthermore , we showed that dpp4i substantially suppresses plaque formation in coronary arteries with a marked reduction in the accumulation of macrophages in cholesterolfed rabbits . 
 dpp4i showed an antiatherosclerotic effect in apoe 
 / mice mainly through the actions of glucagonlike peptide1 and glucosedependent insulinotropic polypepide . 
 however , the dual incretin receptor antagonists partially attenuated the suppressive effect of dpp4i on atherosclerosis in diabetic apoe 
 / mice , suggesting an incretinindependent mechanism . exendin4 and glucosedependent insulinotropic polypepide elicited cyclic adenosine monophosphate generation , and suppressed the lipopolysaccharideinduced gene expression of inflammatory molecules , such as interleukin1 , interleukin6 and tumor necrosis factor , in u937 human monocytes . 
 this suppressive effect , however , was attenuated by an inhibitor of adenylate cyclase and mimicked by 8bromocyclic adenosine monophosphate or forskolin . dpp4i 
 substantially suppressed the lipopolysaccharideinduced expression of inflammatory cytokines without affecting cyclic adenosine monophosphate generation or cell proliferation . 
 dpp4i more strongly suppressed the lipopolysaccharideinduced gene expression of inflammatory molecules than incretins , most likely through inactivation of cd26 . 
 glucagonlike peptide1 and glucosedependent insulinotropic polypepide suppressed oxidized lowdensity lipoproteininduced macrophage foam cell formation in a receptordependent manner , which was associated with the downregulation of acylcoenzyme a cholesterol acyltransferase1 and cd36 , as well as the upregulation of adenosine triphosphatebinding cassette transporter a1 . 
 our studies strongly suggest that incretinrelated agents have favorable effects on macrophagedriven atherosclerosis in experimental animals .