**7. Evaluation of essential role of miRNAs in VSMC**

Because activity of Dicer is essential for the miRNA processing, loss of Dicer activity should result in global loss of miRNAs. Importance of miRNAs for VSMC development and biolo‐ gy can be validated by knocking out the miRNA processing enzyme Dicer in VSMC. To demonstrate the importance of miRNAs in VSMC development and function, a smooth muscle restricted -Dicer knockout model, SM-Dicer KO mice, is investigated recently [59,61]. Outcomes of the study indicate deletion of Dicer causes embryonic lethality due to de‐ creased VSMC proliferation and differentiation resulting in thinner vessel walls, impaired contractility and hemorrhage as well as reduced expression of VSMC-specific genes and proteins [59, 61]. Overall, these observations suggest that Dicer-generated miRNAs are cru‐ cial for normal VSMC development, differentiation and contractile function.

factors and other gene products and multiple pathways that are still vaguely understood. Recently several reports have demonstrated the involvement of miRNA-mediated gene silencing in the regulation of VSMC proliferation, migration and differentiation in normal vascular development and in vascular pathologies. A list of a few selected miRNAs that regulate VSMC proliferation and differentiation in cell cultures and animal models with angioplasty is shown in Table 1 along with factors that regulate miRNA expression, their validated target proteins and function of the target proteins. While some of these miRNAs

**Regulator Target Proteins Cellular Functions of Target**

Bcl2

KLF5

p27kip1,

**miRNA -146a** KLF5 KLF4 Increase proliferation 64 **miRNA 26a** Serum deprivation Smad 1, Smad 4 Decrease proliferation 68

Myocardin PDGFR, Elk1 Decrease 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

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,

**Proteins**

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

apoptosis

p57kip2 Increase proliferation <sup>63</sup>

Increase proliferation,

stimulate differentiation

stimulate differentiation 58,65

Decrease proliferation

**References**

62

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

153

58

promote VSMC proliferation, others stimulate differentiation.

**Inducer/**

**miRNA 21** Vascular injury PTEN,

PDGF

**miRNA 145** SRF/ Myocardin CamKIIδ, KLF4,

**Table 1.** miRNAs regulating vascular smooth muscle cell phenotype

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

programmed cell death 4, a tumor suppressor protein [63].

**microRNA**

**miRNA 221/222** Injury,

**miRNA 143** p53, SRF/

**Figure 1. Biogenesis of miRNA and gene silencing pathway.** The miRNA synthesis starts in the nucleus where primiRNA transcript is cleaved by Drosha/ DGCR8 to form ~60-100 nucleotides long hairpin loop pre-miRNA. Pre-miRNA is then transported to cytoplasm through the mediation of Exportin5 and Ran-GTP6 where it is further processed by RNase activity of Dicer to ~22 nucleotides mature miRNA duplex. The miRNA duplex then loads onto Ago in the RISC complex and undergoes strand separation. The guide strand of the miRNA mediates gene silencing by degrading the target mRNA or interfering with translational process. The passenger strand gets degraded.
