Vascular smooth muscle (SMC) phenotypic modulation, the transition from a contractile to a proliferative phenotype accompanied by neointima formation following vascular injury, plays a critical role in the development and progression of several proliferative cardiovascular diseases such as atherosclerosis, hypertension, restenosis after angioplasty or bypass, diabetic vascular complications, and transplantation arteriopathy. The regulatory mechanisms underlying SMC phenotypic modulation, however, are poorly understood. A hallmark feature of the phenotypic modulation is the down-regulation of SMC contractile genes. Platelet-derived growth factor-BB (PDGF-BB), a well-known stimulator of SMC phenotypic modulation, down- regulates SMC gene expression and stimulates SMC proliferation via posttranscriptional regulation of the related genes. The post-transcriptional mechanisms involved in SMC phenotype gene expression, however, remain largely unknown. Our exciting preliminary data indicate that the down-regulation of SMC contractile genes is caused by abnormal RNA editing of their precursor mRNAs (pre-mRNAs). This abnormal pre-mRNA editing is facilitated by adenosine deaminase acting on RNA (ADAR), which converts adenosines to inosines (A->I editing). A-to-I RNA editing of the pre-mRNA transcripts from introns or 3'-untranslated regions alters pre- mRNA splicing, leading to decreased mature mRNA levels and abnormal cellular functions. PDGF-BB induces ADAR1 while down-regulating SMC myosin heavy chain (SMMHC) and calponin (CNN). Knockdown of ADAR1 by shRNA restores PDGF-BB-blocked SMMHC and CNN expression, demonstrating that ADAR1 plays an essential role in SMC phenotype modulation. ADAR1 appears to be also important for PDGF-BB-induced SMC proliferation/survival. In vivo studies show that SMMHC and CNN pre-mRNA is accumulated when their mature mRNA is decreased in balloon-injured rat carotid arteries. Moreover, ADAR1 is highly induced in media layer SMCs initially, and neointima SMCs subsequently following the injury. Of importance, knockdown of ADAR1 dramatically inhibits injury-induced neointima formation, demonstrating a critical role of ADAR1 in vascular remodeling in vivo. These seminal findings strongly support a novel hypothesis that ADAR1/abnormal RNA editing mediates PDGF-BB-induced down-regulation of SMC contractile genes and SMC proliferation/survival, leading to SMC phenotypic modulation and vascular remodeling. Using primary culture of SMCs, in vivo rat balloon injury and mouse wire injury models combining with molecular, cellular and histological approaches, we will 1) determine the role and mechanism whereby ADAR1 modulates SMC phenotype;2) elucidate the molecular mechanisms underlying ADAR1 function in regulating SMC proliferation/survival;and 3) study the role of ADAR1 in SMC phenotypic modulation and vascular remodeling in vivo. Successful completion of these aims will unravel a novel mechanism governing SMC phenotypic modulation, which will ultimately lead to identification of novel targets for developing therapeuti agents to treat proliferative vascular diseases.
Cardiovascular disease is the #1 cause of mortality in the United States. Transition of vascular smooth muscle (SMC) from a differentiated phenotype to a proliferative state accompanied by neointima formation plays a critical role in the development of atherosclerosis, hypertension, restenosis after angioplasty or bypass, diabetic vascular complications, and transplantation arteriopathy. In this application, a combination of molecular, cellular, and genetic approaches with gain-of-function and loss-of-function studies will be used to establish a novel mechanism underlying SMC phenotypic modulation. The completion of this project will advance our understanding of the fundamental pathologic mechanisms that contribute to the progression of aforementioned proliferative vascular diseases, and most importantly, lead to identification of important novel targets for developing therapeutic agents to treat these diseases.
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