**10. miRNAs in the suppression of VSMC proliferation**

Certain miRNAs including miRNA-143, miRNA-145 and miRNA-26a alter VSMC phenotype by causing suppression of VSMC proliferation (Table 1). Among these miRNAs, miRNA-143 and -145 are considered master regulators of contractile phenotype by promoting contractile protein expression [32, 58, 60]. Moreover, miRNA-145 not only stimulates differentiation of adult VSMC, but also promotes differentiation of multipotent neural crest stem cells into VSMC [57]. In normal vessel walls the miRNA-143/145 cluster is lavishly expressed. However, both miRNAs are dramatically reduced not only in injured carotid arteries following angio‐ plasty [32, 58, 60, 66] but also downregulated in different cancer cell lines [67]. Further studies proved that miRNA-145 is a critical modulator of VSMC differentiation via its target gene KLF-5. Consistent with this, while the use of miRNA-145 oligonucleotide mimics upregulated the expression of VSMC differentiation marker genes such as SM α-actin, calponin, and SM-MHC, both at gene and protein levels, overexpression of KLF-5 reduced the gene expression of SM α-actin implicating a relationship between miRNA-145 and KLF-5 gene in VSMC differentiation.

Analysis of growth arrested human aortic VSMC by miRNA array screening identified upregulation of miRNA-26a in differentiated VSMC, which is associated with reduction in SMAD activity [59, 60, 68]. This miRNA is dramatically downregulated in two murine models of aneurysm.

Embryonic stem cells are known to differentiate to VSMC and one of the factors that induces VSMC differentiation is all trans retinoic acid, which in addition to regulating a wide variety of protein coding genes it also regulates expression of miRNAs that affect smooth muscle cell differentiation. It is found that expression of miRNA-10a contributes to retinoic acid-induced VSMC differentiation by negatively regulating its target histone deacetylase 4 [69]. Involve‐ ment of miRNAs in stem cell and vessel wall progenitor cell differentiation has significant implications in the pathogenesis of atherosclerosis, the response to vascular injury and vascular remodeling.
