MicroRNA and vascular remodelling in acute vascular injury and pulmonary vascular remodelling

Abstract

Vascular remodelling is an integral pathological process central to a number of cardiovascular diseases. The complex interplay between distinct cell populations in the vessel wall following vascular injury leads to inflammation, cellular dysfunction, pro-growth signals in the smooth muscle cell (SMC) compartment, and the acquisition of a synthetic phenotype. Although the signals for vascular remodelling are diverse in different pathological contexts, SMC proliferation and migration are consistently observed. It is therefore critical to elucidate key mechanisms central to these processes. MicroRNAs (miRNAs) are small non-coding sequences of RNA that have the capacity to regulate many genes, pathways, and complex biological networks within cells, acting either alone or in concert with one another. In diseases such as cancer and cardiac disease, the role of miRNA in disease pathogenesis has been documented in detail. In contrast, despite a great deal of interest in miRNA, relatively few studies have directly assessed the role of miRNA in vascular remodelling. The potential for modulation of miRNA to achieve therapeutic benefits in this setting is attractive. Here, we focus on the role of miRNA in vascular inflammation and remodelling associated with acute vascular injury (vein graft disease, angioplasty restenosis, and in-stent restenosis) as well as in vascular remodelling associated with the development of pulmonary arterial hypertension.

Figure 1

Basic schematic of acute vessel wall injury. Vascular injury results in endothelial damage and platelet activation leading to the release of PDGF and expression of P-selectin on the luminal surface. In addition to endothelial dysfunction, mechanical forces (due to surgery or blood pressure) elicit medial damage resulting in the release of bFGF and activation of proteases. During the course of neointimal formation, inflammatory cells release a cytokine which up-regulate proteases and chemokines resulting in further macrophage (or leucocyte) infiltration. This results in further SMC activation, proliferation, migration, and secretion of extracellular matrix.

Figure 2

The miRNA biogenesis pathway. miRNAs are encoded in the genome and transcribed by RNA Pol II which generates long pri-miRNAs which are 5′-capped and 3′-polyadenylated similarly to mRNAs. miRNAs are expressed from intergenic regions or from within genes. In the nucleus, pri-miRNAs undergo first cropping by Drosha to form ∼60–100 nt pre-miRNAs. Pre-miRNAs are exported to the cytoplasm by exportin V and undergo second cropping by Dicer to form the mature miRNA/miRNA* duplex of ∼22 nt. After miRNA/miRNA* duplex is assembled into the RISC complex, miRNA/miRNA* duplex is separated and only miRNA strand stably associates with the RISC complex. The RISC complex loaded with miRNA then directs translational inhibition or promoting degradation of mRNAs which contain partially complementary miRNA recognition sequence often located in the 3′UTR. Adapted from Davis-Dusenbery and Hata.

Figure 3

Vascular injury and miRNA regulation. A summary illustrating some of the mechanisms whereby specific miRNA molecules interact with known transcription factors to activate or repress SMC-specific marker genes and pathways of proliferation. Expression of virtually all SMC marker genes is dependent on one or more CArG elements within their promoter. Serum response factor (SRF) activates genes involved in SMC differentiation and proliferation by recruiting a number of co-activators such as myocardin and a number of co-repressors such as Kruppel transcription factors (KLF-4 and -5) and the est-1 domain containing proteins to CArG elements in the promoter. MiR-24 and miR-221/222 up-regulate KLF-4 and -5, which results in a down-regulation of myocardin resulting in inhibition of signalling. Note that miR-145 antagonizes these effects. The middle boxed (hatched region) demonstrates that KLF-4 can inhibit myocardin signalling following binding to both the SRF and directly binding to G/C rich region next to the CArG box preventing binding of SRF to the CArG elements resulting in reduced SMC gene expression. Activation of the PDGF receptor increases miR-24 level and inhibits up-regulation of miR-21 in response to TGF-β and BMP, preventing subsequent up-regulation of SMC differentiation marker gene expression. The far left panel demonstrates that activation of est-1 proteins (such as ElK-1, SAP-1, and -2) following phosphorylation by ERK-1/2 increases their affinity for SRF leading to displacement of myocardin resulting in decreased SMC gene expression. Note that miR-143 can inhibit Elk-1 activation which would prevent down-regulation of SMC marker genes. The circular hatched region illustrates changes in actin treadmilling following growth factor stimulation or injury. Growth factor activation inhibits F-actin polymerization, resulting in increased free G-actin. This signalling event is transduced to the nuclease resulting in nuclear export of MRTF-A (and/or MRTF-B) and disruption of SRF/MRTF complexes, leading to down-regulation of genes encoding SMC contractile proteins. Pathways where miRNAs induce a positive effect are depicted with filled arrows and negative effects are depicted with hashed arrows.

Figure 4

TGF-β- and BMP-dependent activation of pri- to pre-miR-21 processing. Upon BMP4 or TGF-β stimulation, R-Smads translocate to the nucleus, associate with the Drosha complex, and facilitate processing of pri- to pre-miR-21 by the Drosha complex, which leads to elevated expression of mature miR-21. miR-21 targets various mRNAs, including PDCD4 mRNA. Adapted from Davis-Dusenbery and Hata.