MicroRNAs (miRNAs) are small non-coding RNAs that participate in the spatiotemporal regulation of messenger RNA and protein synthesis. Aberrant miRNA expression leads to developmental abnormalities and diseases, such as cardiovascular disorders and cancer; however, the stimuli and processes regulating miRNA biogenesis are largely unknown. The transforming growth factor beta (TGF-beta) and bone morphogenetic protein (BMP) family of growth factors orchestrates fundamental biological processes in development and in the homeostasis of adult tissues, including the vasculature. Here we show that induction of a contractile phenotype in human vascular smooth muscle cells by TGF-beta and BMPs is mediated by miR-21. miR-21 downregulates PDCD4 (programmed cell death 4), which in turn acts as a negative regulator of smooth muscle contractile genes. Surprisingly, TGF-beta and BMP signalling promotes a rapid increase in expression of mature miR-21 through a post-transcriptional step, promoting the processing of primary transcripts of miR-21 (pri-miR-21) into precursor miR-21 (pre-miR-21) by the DROSHA (also known as RNASEN) complex. TGF-beta- and BMP-specific SMAD signal transducers are recruited to pri-miR-21 in a complex with the RNA helicase p68 (also known as DDX5), a component of the DROSHA microprocessor complex. The shared cofactor SMAD4 is not required for this process. Thus, regulation of miRNA biogenesis by ligand-specific SMAD proteins is critical for control of the vascular smooth muscle cell phenotype and potentially for SMAD4-independent responses mediated by the TGF-beta and BMP signalling pathways.

Summary The signal transducers of the Transforming Growth Factor β (TGFβ)/Bone Morphogenetic Protein (BMP), the Smads, promote the expression of a subset of miRNAs by facilitating the cleavage reaction by Drosha. The mechanism that limits Smad-mediated processing to a selective group of miRNAs remained hitherto unexplored. In this study, we expand the number of TGFβ/BMP-regulated miRNAs (T/B-miRs) to 20. Interestingly, a majority of T/B-miRs contain a consensus sequence (R-SBE) within the stem region of the primary transcripts of T/B-miRs (pri-T/B-miRs). Here, we demonstrate that Smads directly bind the R-SBE. Mutation of the R-SBE abrogates TGFβ/BMP-induced recruitment of Smads, Drosha, and DGCR8 to pri-T/B-miRs, and impairs their processing, while introduction of R-SBE to unregulated pri-miRNAs is sufficient to recruit Smads and allow regulation by TGFβ/BMP. Thus, Smads are multifunctional proteins which modulate gene expression transcriptionally through DNA binding, and post-transcriptionally by pri-miRNA binding and regulation of miRNA processing.

The platelet-derived growth factor (PDGF) signaling pathway is a critical regulator of animal development and homeostasis. Activation of the PDGF pathway leads to neointimal proliferative responses to artery injury; it promotes a switch of vascular smooth muscle cells (vSMC) to a less contractile phenotype by inhibiting the SMC-specific gene expression and increasing the rate of proliferation and migration. The molecular mechanism for these pleiotropic effects of PDGFs has not been fully described. Here, we identify the microRNA-221 (miR-221), a small noncoding RNA, as a modulator of the phenotypic change of vSMCs in response to PDGF signaling. We demonstrate that miR-221 is transcriptionally induced upon PDGF treatment in primary vSMCs, leading to down-regulation of the targets c-Kit and p27Kip1. Down-regulation of p27Kip1 by miR-221 is critical for PDGF-mediated induction of cell proliferation. Additionally, decreased c-Kit causes inhibition of SMC-specific contractile gene transcription by reducing the expression of Myocardin (Myocd), a potent SMC-specific nuclear coactivator. Our study demonstrates that PDGF signaling, by modulating the expression of miR-221, regulates two critical determinants of the vSMC phenotype; they are SMC gene expression and cell proliferation.Unlike many terminally differentiated cells, smooth muscle cells (SMCs) 3 can switch between differentiated and dedifferentiated phenotypes in response to changes in the local environment (2). In response to vascular injury, quiescent contractile vSMCs reduce the expression of SMC-specific genes such as ␣-smooth muscle actin (SMA), smooth muscle calponin (CNN), SM22␣ (SM22), and smooth muscle myosin heavy chain, increase proliferation, and synthesize collagens and matrix metalloproteinases (2, 9). Although this phenotype switch is believed to be essential for repair of vascular injury, deregulation of this process also plays a role in the pathogenesis of various human diseases, including atherosclerosis, hypertension, asthma, and cancer. Therefore, a complete understanding of the molecular mechanisms of vSMC phenotype regulation is essential for treatment or prevention of vascular disorders.PDGFs potently mediate the vSMC phenotype switch from a differentiated, contractile state to a dedifferentiated, synthetic state by repressing SMC marker gene expression as well as promoting vSMC proliferation and migration (2, 10). Consistent with the effect of PDGF on vSMC, up-regulation of molecules in the PDGF signaling pathway is found in various cardiovascular disorders and vascular injuries, including atherosclerosis and restenosis (1). PDGF released from platelets and endothelial cells at sites of vascular injury is thought to be a contributing factor to atherosclerosis (11). Inhibition of PDGF signaling by the PDGF receptor (PDGFR) kinase inhibitor imatinib mesylate (Gleevec) demonstrated a major protective effect on atherosclerosis development (1). It is of note that hyperactivation of the PDGFR pathway has been implicated in the pathogenesis of id...