The regulation of vascular smooth muscle cell phenotype by microRNAs

Vascular smooth muscle cells (vSMCs), unlike the majority of differentiated cell types, undergo the unique process of phenotypic modulation. This process involves the alteration of the cellular program between a quiescent, differentiated state and a proliferative, dedifferentiated state. The process of vSMC phenotypic switching underlies vascular remodeling, and deregulation of this process contributes to various vascular disorders including idiopathic pulmonary arterial hypertension (IPAH) and atherosclerosis. Despite the critical role of vSMC phenotype switching in both normal and disease states, much remains unknown about the mechanisms that control of vSMC phenotype. Numerous cellular stimuli contribute to the regulation of vSMC switching. In particular, Platelet Derived Growth Factor (PDGF) promotes vSMC dedifferentiation, while members of the Transforming Growth Factor-beta (TGF-beta) superfamily promote vSMC differentiation.;In this study, we investigate the role of a recently-described class of regulatory molecules, termed microRNAs (miRNAs) in the regulation of vSMC switching in response to the PDGF and TGF-beta signaling pathways. miRNAs are short (21-24nt), noncoding RNAs which post-transcriptionally regulate the expression of target genes through imperfect binding to the 3'UTR. We identify miR-221 as a target of PDGF signaling that mediates multiple aspects of vSMC dedifferentiation through the repression of p27Kip1 and c-Kit. Additionally, we identify miR-21 as a target of TGF-beta superfamily ligands which promotes vSMC differentiation through the repression of a novel repressor of vSMC differentiation, PDCD4.;Mature miRNAs are generated through the concerted action of two RNAse type III enzymes, Drosha and Dicer. However, the mechanisms that regulate miRNA expression are poorly understood. In this study, we identify a novel mechanism of post-transcriptional miRNA regulation mediated by the downstream signal transducers of the TGF-beta signaling cascade, the Smads. The Smads are well known transcription factors that, upon nuclear translocation, bind to specific sequence elements within the promoters of target genes to regulate transcription. In addition to this role, in this study we show that the nuclear accumulation of Smads promotes the processing of a sub-set of microRNAs through the interaction with the Drosha microprocessor complex. Finally, we identify a conserved sequence motif which is necessary and sufficient for Smad-mediated miRNA processing.