atherosclerosis represents a major factor of coronary heart disease characterized by the formation of fat - laden plaque in large and medium - sized vessels . during atherogenesis
, the endothelial layer of the vessels is constantly confronted with a range of stress signals .
elevated levels of circulating oxidized low - density lipoprotein ( ox - ldl ) , increased turbulent blood flow , and excessive inflammation all contribute to endothelial injury .
regardless of the nature of the stress cue , the transcriptome of vascular endothelial cells is profoundly altered. for instance , down - regulation of enos transcription and simultaneous up - regulation of et-1 transcription in endothelial cells precedes the impairment of vasodilation and rhythmic vessel tone .
transcriptional activation of adhesion molecules , on the other hand , enables circulating leukocytes to attach to the endothelium and establish a pro - inflammatory microenvironment .
therefore , elucidation of the mechanisms underlying these characteristic transcriptional events will potentially further our understanding of atherogenesis and yield druggable targets for the intervention and prevention of atherosclerosis .
recent advances in the transcriptional regulation of atherosclerosis suggest a growing involvement of the epigenetic machinery , .
this mini - review is a modest attempt to summarize the current state of research on the epigenetic regulation of endothelial disorder in atherosclerosis and to provide an outlook .
unlike prokaryotic organisms , eukaryotic transcription takes place on nucleosome - wrapped chromatin . in order for the basic transcription machinery to be recruited to promoter region and initiate transcription
the epigenetic machinery , composed of histone modifying enzymes , dna modifying enzymes , chromatin remodeling proteins , non - coding regulatory small rnas , and histone variants , serves as an intricate regulatory layer for eukaryotic transcription by altering chromatin structure . due to space constraints
, we only give a brief overview of histone modifications and chromatin remodeling , which will be the focus of discussion in this review .
the n - terminal tails of core histones , h3 and h4 in particular , can be post - translationally modified . the term 
histone code  was coined to correlate a specific set of histone modifications to a predictable transcriptional outcome .
though a subject of constant controversy and debate , it is generally believed that acetylation of histones h3 and h4 surrounding the promoter region is synonymous with transcriptional activation . whereas methylation of histone h3 lysine 4 ( h3k4 ) may herald activation , h3k9 methylation often marks repressed chromatin . in order for sequence - specific transcription factors and the basic transcription machinery to access the dna and initiate transcription ,
this is achieved by moving the nucleosomes along the dna at the expense of atp hydrolysis .
initially identified and characterized in yeast , , the chromatin remodeling proteins represent a most conserved branch of the epigenetic machinery during evolution .
the mammalian chromatin remodeling complex is a multi - subunit mega - protein conglomerate that contains a catalytic component .
brahma related gene 1 ( brg1 ) and brahma ( brm ) are the best studied chromatin remodeling proteins with atpase activity .
brg1 and brm have been known to participate in both transcriptional activation and repression depending on the specific chromatin environment and the transcription factors they interact with .
the n - terminal tails of core histones , h3 and h4 in particular , can be post - translationally modified . the term  histone code  was coined to correlate a specific set of histone modifications to a predictable transcriptional outcome .
though a subject of constant controversy and debate , it is generally believed that acetylation of histones h3 and h4 surrounding the promoter region is synonymous with transcriptional activation . whereas methylation of histone h3 lysine 4 ( h3k4 ) may herald activation , h3k9 methylation often marks repressed chromatin .
in order for sequence - specific transcription factors and the basic transcription machinery to access the dna and initiate transcription , the chromatin has to be unwound and loosened .
this is achieved by moving the nucleosomes along the dna at the expense of atp hydrolysis .
initially identified and characterized in yeast , , the chromatin remodeling proteins represent a most conserved branch of the epigenetic machinery during evolution .
the mammalian chromatin remodeling complex is a multi - subunit mega - protein conglomerate that contains a catalytic component .
brahma related gene 1 ( brg1 ) and brahma ( brm ) are the best studied chromatin remodeling proteins with atpase activity .
brg1 and brm have been known to participate in both transcriptional activation and repression depending on the specific chromatin environment and the transcription factors they interact with .
under physiological conditions , the vessel wall is free from the attachment of circulating leukocytes . under certain pro - atherogenic conditions , such as turbulent shear stress , oxidative stress , intermittent hypoxia , and excessive nutrition , endothelial cells up
- regulate the transcription of adhesion molecules ( cam ) including icams , vcams , and selectins , which consequently allow a much stronger interaction between leukocytes and the endothelium perpetuating a pro - inflammatory niche , .
diabetes is one of the leading causes for vasculopathies including atherosclerosis . a seminal study by brownlee and colleagues examined the effect of transient hyperglycemia on endothelial function . of great intrigue
, these authors have found that exposure to high glucose ( hg ) for a short period time ( 16 hours ) induced a prolonged activation of vcam-1 in bovine aortic endothelial cells even when these cells were switched to and maintained in low glucose ( lg ) for additional 6 days , a phenomenon dubbed as  metabolic memory  .
chromatin immunoprecipitation ( chip ) revealed an uptick of h4k4 monomethylation on the proximal promoter of the p65 gene mediated by the histone methyltransferase set7/9 .
transient hg , these authors propose , leaves an epigenetic dent on the p65 gene such that even in the absence of the original stimulus endothelial dysfunction will sustain .
circulating oxldl presents a major risk to atherosclerosis in part by promoting the expression of adhesion molecules .
have recently reported a novel epigenetic mechanism underlying the induction of icam-1 in human endothelial cells .
oxldl activated the transcriptional modulator mrtf - a , which in turn was recruited to the icam-1 promoter by p65 and synergistically stimulated icam-1 transcription with oxldl .
depletion of mrtf - a erased h3/h4 acetylation and h3k4 dimethylation but restored h3k9 trimethylation on the icam-1 promoter . since mrtf - a is known to engage the epigenetic machinery in regulating transcription within the vasculature , it is conceivable that mrtf - a may serve as the critical link of endothelial injury bringing histone modifying enzymes to the chromatin .
alternatively , kim et al . propose that the regulatory subunit of the napdh oxidase complex p66shc mediates the induction of icam-1 by ldl .
ldl induced histone acetylation but inhibited dna methylation of the p66shc promoter to up - regulate the transcription of p66shc . of note , oxldl has been reported to elicit epigenetic changes on a host of gene promoters in endothelial cells , although a genome - wide survey is lacking .
chronic hypoxia has emerged as an independent risk factor of atherosclerosis . as a result of intermittent low oxygen supply ,
the transcriptome of endothelial cells undergoes marked changes that include an increase in the transcription rate of adhesion molecules .
our laboratory has recently uncovered a potential role for brg1 and brm in hypoxia induced endothelium - leukocyte interaction .
expression of brg1 and brm in vitro was up - regulated in cultured endothelial cells exposed to 1% o2 and in vivo in pulmonary arteries isolated from mice kept in a low - oxygen chamber for 4 weeks .
introduction of brg1 and brm into a brg1/brm - negative cell line ( sw-13 ) greatly potentiated hypoxia - induced cam transactivation whereas silencing of brg1/brm in endothelial cells crippled the effect of hypoxia .
brg1 and brm formed a dynamic interaction with p65 on the cam promoters where p65 recruits brg1/brm and brg1/brm reciprocally stabilizes p65 .
brg1/brm influenced cam transactivation by altering histone h3/h4 acetylation and h3k4 methylation creating a friendly chromatin structure for the basic transcription machinery .
more important , endothelial - specific targeting of brg1 and brm normalized cam expression and attenuated hypoxia induced leukocyte adhesion in mice . of intrigue ,
a similar strategy also alleviated the development of atherosclerotic lesions in apoe mice , indicating that brg1 and brm might be able to orchestrate endothelial injury in response to a range of different pro - inflammatory stimuli ( unpublished observation ) .
endothelium derived no plays a critical role in maintaining vascular integrity , the disruption of which contributes to atherogenesis .
oxldl decreases acetylation of h3 and h4 and dimethylation of h3k4 while simultaneously increasing h3k9 trimethylation surrounding the enos promoter consistent with the repression of enos transcription in endothelial cells .
again , mrtf - a appears to be the key coordinator of these epigenetic changes .
in addition , there is a decrease of enos expression in mice deficient in lsd1 , an h3k4/k9 demethylase , highlighting the role of histone methylation in fine - tuning enos transcription .
fish et al . conducted a comprehensive survey of histone modifications on the enos promoter region in endothelial cells challenged with anoxia . in the proximal region of the enos promoter ( -166/-26 ) ,
h3/h4 acetylation and h3k4 dimethylation declined as early as 1 hour following exposure to low oxygen .
in contrast , these signature changes were not observed on the distal enos promoter ( -891/-797 and -488/-398 ) .
a closer examination revealed that acetylation levels of specific lysine residues fluctuated with distinct patterns .
for instance , 1 hour after hypoxia , only h3k14 and h4k5 acetylation plummeted significantly , which was joined by a decrease in h3k9/h4k8/h4k12 acetylation at 2 hours with h4k16 acetylation unaltered .
interestingly , histone h2a.z was evicted from the proximal enos promoter during hypoxia thereby creating a closed chromatin conformation and rendering a repressed transcription state .
consistently , a recent study has correlated decreased enos expression with high levels of dna methylation on the proximal enos promoter in patients with obstructive sleep apnea ( osa ) , a typical pathology of hypoxia . paradoxically , enos expression has been observed to increase , rather than decrease , in endothelial cells in response to hypoxia probably as means of compensation .
the up - regulation of enos transcription is accompanied by increased h3 and h4 acetylation across the enos promoter region ( -4501/+23 ) .
therefore , while it remains debatable how enos transcription responds to hypoxia , suffice it to say that a specific epigenetic code is intimately associated with the transcription status .
the vessel wall , particularly the endothelial layer , is subject to the pressure caused by various hemodynamic forces .
whereas laminar shear stress ( lss ) is considered atheroprotective and stimulates enos transcription , turbulent blood flow creates shear stress that damages the vascular endothelium especially at the sites of arterial branches . among the detrimental effects exerted by pro - atherogenic shear stress
are accelerated turnover of endothelial cells , increased adhesion of leukocytes , accumulation of reactive oxygen species , and decreased synthesis of no stemming from enos repression. illi et al .
have reported that lss increased global h3 and h4 acetylation levels in endothelial cells through activating histone acetyltransferases ( hats ) .
in addition , lss augmented h3 and h4 acetylation on c - jun and c - fos promoters .
since c - jun / c - fos is considered essential for enos transactivation , increase ap-1 activity likely explains elevated enos transcription in response to lss .
impaired vessel relaxation during atherogenesis is also rooted in the enhanced expression of endothelin , a potent vasoconstrictor .
accumulating evidence has provided a clear link between histone modification and et-1 transactivation . in diabetic rats , increased et-1 release was accompanied by an up - regulation of the histone acetyltransferase p300 .
silencing of p300 completely abolished et-1 activation by high glucose thought it remained unclear whether p300 acted through histone or non - histone factors .
have demonstrated that h4 acetylation was increased on the et-1 promoter surrounding a conserved p65 binding element in endothelial cells treated with two pro - inflammatory stimuli , tnf- and ifn- , suggesting a potential role for a hat like p300 . in rats with intrauterine growth retardation ( iugr ) where hypoxia plays a determining role , h3k9/k18 and h4 acetylation increased on the et-1 promoter .
recently , our investigation has led to the identification of an mrtf - a - centered epigenetic complex on the et-1 promoter in response to hypoxia . under hypoxic conditions ,
mrtf - a interacted with and was recruited by the sequence specific transcription factor srf to the proximal et-1 promoter ( -81/+150 ) . upon the joining of brg1/brm , this epigenetic complex altered histone acetylation and h3k4 methylation to facilitate the binding of rna polymerase ii thereby activating et-1 transcription .
the turn - of - the - century saw a boom in research on epigenetics with the initiation of several epigenomics projects .
relying on high - fidelity chromatin immunoprecipitation ( chip ) coupled with high - throughput sequencing techniques , these projects aim to decode the epigenetic information bringing insights for the prevention and intervention of human diseases .
several exciting findings have provided clues for such basic biological events as adipogenesis , macrophage activation , and cell cycle progression .
so far , there has been a lack of effort in deciphering the epigenetic code on a genomewide scale in any of the major cardiovascular diseases including atherosclerosis .
since single - gene based epigenetic analysis tends to give very limited and often biased knowledge to the understanding of atherogenesis , long considered a multifactorial disease , basic scientists and clinicians like will benefit from an undertaking that unveils a comprehensive picture of epigenetic regulation of endothelial injury in atherosclerosis .
 atherosclerosis is a progressive human pathology that encompasses several stages of development . 
 endothelial dysfunction represents an early sign of lesion within the vasculature . 
 a number of risk factors for atherosclerosis , including hyperlipidemia , diabetes , and hypertension , target the vascular endothelium by re - programming its transcriptome . 
 these profound alterations taking place on the chromatin rely on the interplay between sequence specific transcription factors and the epigenetic machinery . the epigenetic machinery , in turn , tailor individual transcription events key to atherogenesis to intrinsic and extrinsic insults dictating the development of atherosclerotic lesions . 
 this review summarizes our current understanding of the involvement of the epigenetic machinery in endothelial injury during atherogenesis . 