*3.3. miRNAs Involved in VSMCs and Other Cells in Hypertension*

Understanding the role of vascular smooth muscle cells (VSMC) and the factors that influence their function is important for clarity on the pathogenesis and treatment of hypertension. Within the blood vessels, there is a complex interplay of neurotransmitters, circulating hormones and endothelium-derived factors for vasodilation and vasoconstriction. VSMC influences the vascular tone and regulates blood pressure, vascular resistance and tissue perfusion. Various antihypertensive agents have been designed to target the VSMCs (such as the ACE inhibitor, calcium channel blockers) [86]. miRNAs have been found to be involved in the function of VSMC, the development of arterial stiffness and the progression to hypertension [87]. The miRNA miR-21, which has been known to regulate arterial fibrosis, was reported to have a correlation with improvement in arterial stiffness [88,89]. A study with 95 essential hypertensive patients that underwent antihypertensive treatment showed a negative correlation between miR-21 and the pulse wave velocity readings [63]. A study involving 89 individuals, of which 60 had essential hypertension and 29 were normotensive [64], showed that lower miR-143, miR-145 and miR-133, but higher miR-21 and miR-1 were found in peripheral blood mononuclear cells from the hypertensive group, compared to the normotensives. Negative correlation of the diastolic blood

pressure (DBP) was found with miR-143, miR-145 and miR-21, but there was a positive correlation with miR-133. Interestingly, the miR-145 expression level was also overexpressed in 22 human atherosclerotic plaques (15 hypertensive and seven control) [90]. Furthermore, miR-145 was reported to have dual role in its binding to TGFβ receptor II (TGFBR2) [65]. The modulation of TGFβ receptor 2 signaling affects the downstream expression of the matrix genes in VSMC [65]. The same group that investigated the hypertensive cohort also compared the expression of miR-9 and miR-126 in peripheral blood mononuclear cells between the hypertensive and normotensive group [66]. There was a significant lower expression found in both miR-9 and miR-126 in the hypertensive group, and their expression level showed positive correlation with the 24-h mean pulse pressure [66]. The expression level of miR-9 showed a positive correlation with the left ventricular mass index. Another study reported that both miR-126 and miR-223 are involved in regulating vascular inflammation by repressing vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1), respectively [67,68]. The VSMC proliferation has also been found to be influenced by the level of miR-34b. Through qPCR, in silico analysis and the luciferase assay, miR-34b was found to target cyclin-dependent kinase 6 (CDK6), which controls cell cycle progression and proliferation [69].

Recently, a number of miRNAs has been detected to be differentially expressed in essential hypertensive individuals, compared to the healthy individuals. Increased expression of circulating miR-29a/b/c and miR-510 by qPCR was found in hypertensive individuals, compared to the normotensive individuals [70,71]. Another group has reported two sets of data of the upregulation of miRNAs: let-7 and miR-92a, in correlation with the increase in carotid intima-media thickness (CMIT), compared to the normal CMIT [72,73]. This shows that the miRNA levels could reveal the development of subclinical atherosclerosis with the thickening of the CMIT. Thus, here is another piece of evidence for the possibilities of miRNAs to be used as biomarker and in this case for the detection of end-organ damage in hypertension.

A summary of the miRNAs associated with essential hypertension is further tabulated based on species (see Table 3), and the essential miRNAs with their targets are presented in Figure 1.

**Figure 1.** The miRNAs involved in essential hypertension, their association with the RAAS, EC and VSMC and their known target genes. The big central arrows indicate a system/cell type's influence on another and on the increase in blood pressure (BP). Small arrows in the tables indicate the upregulated (up arrow) or downregulated (down arrow) miRNA expression. Red droplets represent the biomarker potential from detectability in blood samples. EC: endothelial cells; RAAS: renin-angiotensin aldosterone system; VSMC: vascular smooth muscle cells.


