**3. Epigenomic Regulation in Hypertensive Vasculature**

While numerous studies have outlined the epigenetic mechanisms in pulmonary hypertension (reviewed in [56–59]), the epigenomic regulation in systemic hypertension remains largely undescribed. Due to the well-known involvement of the renin-angiotensin-aldosterone system (RAAS) system on arterial pressure regulation, the effects of epigenomic regulation of the RAAS system have been extensively tested in animal models of systemic hypertension [4]. For example, it has been shown that the angiotensin 1α receptor (AT1aR), encoded by *Atgr1α*, is significantly increased in spontaneously hypertensive rats (SHR) compared to its counterpart, the Wistar-Kyoto rats (WKY), which may be responsible for the increased BP in SHR [6]. Bisulfite sequencing further revealed that the *Atgr1*α promoter is hypomethylated at its CpG islands in SHR at 20 weeks compared to WKY rats, implicating that the methylation of *Atgr1α* CpG islands reduces its expression, leading to normotensive BP [6]. In addition, hypomethylation of the promoter regions of the angiotensin II type 1β receptor (AT1bR) gene, (*Atgr1β*), in the adrenal glands of the maternal low protein rat exhibited hypertension in response to salt intake [7]. A similar effect was seen in another study in mice where maternal protein deficiency during pregnancy reduced methylation of promoter regions of the angiotensin I converting enzyme gene (*Ace-1*), which is responsible for converting angiotensin I to the active angiotensin II, eventually leading to hypertension in offspring [8]. In human cell lines, the luciferase activity of the *ACE-1* promoter/reporter constructs of somatic ACE (sACE) was inhibited by DNA methylation and subsequent inhibition of DNA methylation and/or histone deacetylation by 5-aza-cytidine injections in rats restored sACE expression in the lung and liver, highlighting the epigenetic regulation of sACE in hypertension [9].

The hydroxysteroid dehydrogenase-11β2 enzyme (HSD11B2) is responsible for degrading cortisol to biologically inert cortisone. Cortisol can be found in the blood at concentrations 2-3 orders of magnitude higher than aldosterone, the key mineralocorticoid in the RAAS [60]. Although cortisol and aldosterone bind mineralocorticoid receptors with similar affinity, degradation of cortisol to cortisone by the HSD11B2 enzyme in mineralocorticoid target tissues ensures that aldosterone is able to bind to the mineralocorticoid receptors [61]. In normal situations, aldosterone regulates sodium reabsorption due to the inactivation of cortisol to cortisone by the 11β HSD enzyme. Thus cortisol only has effects in the absence of this enzyme. Hypermethylation of the *HSD11B2* gene promoter impairs HSD11B2-mediated degradation of cortisol to cortisone, leading to an altered tetrahydrocortisol (THF) to tetrahydrocortisone (THE) ratio [10,62]. A high concentration of cortisol in mineralocorticoid target tissues and enables cortisol to regulate sodium reabsorption by the kidney, and ultimately arterial pressure. The hypermethylation of the HSD11B2 promoter may also contribute to the development of

apparent mineralocorticoid excess (AME) [63]. This presents a case where both genetic and epigenetic regulation may be present.

Interestingly, in addition to the reports of the regulation of DNA methylation on RAAS, SHR aortas also showed higher enrichment of H3Ac and H3K4me3, while enrichment of H3K9me2 was reduced on the angiotensin-converting enzyme 1 (*ACE1*) promoter [19]. Thus, both the regulation of DNA methylation and histone modification exist on RAAS system.

Despite the importance of epigenetic regulation in the RAAS system on arterial pressure regulation, the blood vessels are the end-effect targets of RAAS and the function of blood vessels, particular the arteries, play a determinant role in the control of BP. Thus, in addition to the epigenetic regulation in the RAAS system described above, we will outline the recent studies on the epigenomic regulation in vascular tissue and cells, focusing on DNA methylation and histone modification in hypertension.
