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Posttranscriptional gene regulation by microRNAs (miRNAs) is important for many aspects of development, homeostasis, and disease. Here, we show that reduction of endothelial miRNAs by cell-specific inactivation of Dicer, the terminal endonuclease responsible for the generation of miRNAs, reduces postnatal angiogenic response to a variety of stimuli, including exogenous VEGF, tumors, limb ischemia, and wound healing. Furthermore, VEGF regulated the expression of several miRNAs, including the upregulation of components of the c-Myc oncogenic cluster miR-17-92. Transfection of endothelial cells with components of the miR-17-92 cluster, induced by VEGF treatment, rescued the induced expression of thrombospondin-1 and the defect in endothelial cell proliferation and morphogenesis initiated by the loss of Dicer. Thus, endothelial miRNAs regulate postnatal angiogenesis and VEGF induces the expression of miRNAs implicated in the regulation of an integrated angiogenic response.endothelium ͉ VEGF M icroRNAs (miRNAs) are short (Ϸ22 nt) noncoding RNAs derived from long primary transcripts through sequential processing by the enzymes Drosha and Dicer. Dicer-generated miRNAs are incorporated into the RNA-induced silencing complex that mediates miRNA-dependent translational suppression or in some instances cleavage of respective mRNA targets or translational activation (1, 2). The significance of miRNAs in mammalian biology has been dissected by Dicer gene disruption in mice. Mutant and disrupted Dicer alleles caused embryonic lethality associated with a loss of pluripotent stem cells (3) and defective blood vessel formation (4). Tissue-specific inactivation of Dicer has led to the conclusion that Dicer is essential for several processes, for example, limb, lung, and skin morphogenesis, the maintenance of hair follicles, T cell development/ differentiation, and neuronal survival (5-11).The growth of blood vessels is essential for organ growth and tissue repair. During adulthood, most blood vessels remain quiescent to fulfill their main function of conducting nutritive blood flow to organs; however, during pathological events such as tissue ischemia, inflammation, and tumor progression, endothelial cells (ECs) become activated and angiogenesis ensues to provide conduits for blood flow (12). An imbalance in the growth of blood vessels contributes to the pathogenesis of numerous disorders (13), and the growth of vessels is a complex process, requiring a finely tuned balance between numerous stimulatory and inhibitory signals (14). VEGF has been identified as a central mediator of angiogenesis (15). We (16) and others (17) have recently shown that reduction of miRNA levels via Dicer silencing strongly impacts EC functions in vitro, suggesting a critical role for miRNAs in angiogenesis. The role of Dicer-regulated miRNAs in ovarian angiogenesis is suggested by data obtained in mice expressing a global hypomorphic Dicer1 allele, where female mice are infertile because of corpus luteum insufficiency and defective ovarian angiogenesis...

Summary Maintenance of normal endothelial function is critical to various aspects of blood vessel function but its regulation is poorly understood. In this study we show that disruption of baseline FGF signaling to the endothelium leads to a dramatic reduction in let-7 miRNA levels that in turns increases expression of TGFβ ligands and receptors and activation of TGFβ signaling leading to endothelial-to-mesenchymal transition (Endo-MT). We further find that Endo-MT is an important driver of neointima formation in a murine transplant arteriopathy model and in rejecting human transplants lesions. The decline in endothelial FGF signaling input is due to the appearance of an FGF resistance state that is characterized by inflammation-dependent reduction in expression and activation of key components of the FGF signaling cascade. These results establish FGF signaling as a critical factor in maintenance of endothelial homeostasis and point to an unexpected role of Endo-MT in vascular pathology.