**9. miRNAs in the mediation of VSMC proliferation**

Some miRNAs, such as miR-21, miRNA-221, miRNA-222 and 146a are demonstrated to promote VSMC proliferation in balloon-injured rat carotid arteries and cultured rat VSMC by silencing their target proteins (Table 1). Among these, miRNA-21 is the first miRNA that is recognized to regulate VSMC growth and survival by silencing phosphatase and tensin homolog (PTEN), a tumor suppressor protein and increasing B-cell lymphoma 2 (Bcl-2), which increased VSMC proliferation and survival [32,59-62]. Interestingly, this same miRNA is shown to regulate features of both proliferative and contractile phenotype by separate mechanisms. Through the regulation of processing of the miRNA-21 primary transcript to the mature miRNA -21 transcript, transforming growth factor-β (TGF-β) and bone morphogenetic proteins (BMPs) increased the miRNA-21. This increased miRNA-21 is shown to promote VSMC differentiation by upregulating VSMC restricted contractile proteins by silencing programmed cell death 4, a tumor suppressor protein [63].

Other miRNAs that stimulated proliferative phenotype include miRNA-221 and -222. Their proliferative effect on VSMC is mediated through silencing of their target proteins, p27kip1 and p57kip2, respectively, both of which are negative regulators of cell cycle progression [32, 64]. miRNA-146a is shown to directly target Krupple-like factor-4 (KLF-4) and promote VSMC proliferation in cultured rat VSMC and vascular neointimal hyperplasia [32, 60, 65]. KLF-4 and miRNA-146a appear to exhibit a feedback relationship regulating each other's expression. While miRNA-146a inhibits KLF-4 expression by targeting the 3'-UTR region of KLF-4, KLF-4 inhibits miRNA-146a at the transcriptional level. KLF-5, another member of KLF family promoted the transcription of miRNA-146a. It appears these molecules form a regulatory control to appropriately modulate VSMC proliferation [32, 60].

normal individuals and in patients with coronary artery disease revealed circulating levels of angiogenesis-related miRNA-126 and miRNA-92a, the inflammation-associated miRNA-155; and VSMC-enriched miRNA-145 and miRNA-17 are significantly reduced in patients with coronary artery disease compared to normal individuals [51]. Whereas cardiac muscleenriched miRNAs, miRNA-133a and -208a are elevated in patients with disease. These observations suggest that circulating miRNAs can be used as biomarkers for diagnosis of

MicroRNAome of Vascular Smooth Muscle Cells: Potential for MicroRNA-Based Vascular Therapies

http://dx.doi.org/10.5772/54636

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Although roles of various miRNAs and their participation in biological processes have been recognized in various cultured cells or animal models, and expression profiles of circulating miRNAs in patients of cardiovascular diseases [50, 51], involvement of miRNAs in human atherosclerotic plaques has received little attention. However, one of the recent studies investigated miRNA/mRNA expression profiles of human atherosclerotic plaques from peripheral arteries in comparison to nonatherosclerotic left internal thoracic arteries to determine the relationship between miRNA/mRNA expression profiles and biological processes in atherosclerosis [70]. Results of this study revealed significant amounts of miRNA-21,-34a, -146a, -146b-5p, and -210 expressions in atherosclerotic lesions. Consis‐ tent with this there was downregulation of several predicted targets of these miRNAs in atherosclerotic plaques. According to the combination of miRNA/mRNA profiles and bioinformatic analysis, nine KEGG pathways including immunodeficiency, metabolism, p53 and cell proliferation signaling pathways enriched with predicted targets were significantly upregulated. On the contrary, VSMC contraction and purine metabolism were

Role of miRNAs in restenosis is mainly studied using the common rat carotid artery balloon injury animal model. miRNA profiles in the carotid artery is determined by using miRNA arrays [62]. One of the miRNA that was aberrantly overexpressed in injury-induced neointimal lesions is miRNA-21. miRNA-21 promotes VSMC proliferation and inhibits apoptosis of VSMC by directly targeting PTEN and programmed cell death 4, respectively [32]. Similarly miRNA-221 and -222, which are encoded by a gene cluster on X chromosome, share the same seed sequence, identical targets and similar functions were upregulated in balloon-injured carotid arteries. Consistent with their upregulation, their target genes, p27kip1 and p57kip2 were downregulated [32, 64]. Additionally, miRNA-143 and miRNA-145 that promote VSMC differentiation and expressed highly in vascular tissue, were significantly reduced in apoli‐ poprotein E knockout mice where vascular injury was induced by hypercholesterolemic diet [71]. Cooperatively, both miRNA-143 and miRNA-145 target a network of transcription factors such as Elk1, KLF-4 and myocardin to stimulate differentiation and inhibit proliferation of

**12. miRNAs in atherosclerosis and neointimal hyperplasia**

cardiovascular diseases.

downregulated.

**13. miRNAs in restenosis**
