UC San Diego

The disparate effects of different flow patterns play important roles in regulating the functions of vascular endothelial cells (ECs). The focal nature of atherosclerotic plaque formation reveals that the pulsatile flow (PS) along the straight part of arterial tree exerts athero-protective effects on the ECs, while the disturbed flow characterized as the oscillatory flow (OS) found at the arterial bifurcation induces pro- atherogenic responses. MicroRNAs (miRNAs) can regulate many vascular functions, but their roles in regulating endothelial responses to different types of fluid shear stresses remain unexplored. Using a genome-wide microarray approach, I profiled miRNA and mRNA expression levels in human ECs under 24-h PS and OS flows, as well as static control. Bioinformatics analyses of mRNA profiles indicate that PS flow predominantly regulates EC growth arrest genes, while OS flow primarily modulates EC inflammatory genes. Functional and miRNA studies show that PS flow suppresses EC proliferation through the effects of miR-23b on cell cycle machinery, and that OS causes sustained expression of miR-21 to contribute to pro-inflammatory responses. Further mechanistic studies indicate that the PS flow-induced miR-23b modulates the integrity and activity of CDK-activating kinase (CAK) by directly targeting the cyclin H (CCNH) expression post- transcriptionally, which in turn suppresses the transcriptions and activities of CDK2, CDK4, as well as RNA polymerase II (Pol II). The reductions in basal transcriptions and cell cycle regulatory proteins result in the hypo-phosphorylation state of retinoblastoma (Rb) gene and protein, and hence EC growth arrest. These results reveal that the miR-23b/CAK/Pol II pathway is differentially regulated by PS and OS flows to result in their opposite effects on EC proliferation. Such a flow pattern-dependent correlation is validated in studies in vivo on rat carotid artery stenosis and ligation: the flow disturbance/reduction induced in the carotid artery in vivo led to a lower expression of miR-23b and a higher EC proliferation, in agreement with the studies on cultured ECs in vitro. These findings provide the functional understanding of the role of miRNAs in vascular biology, and also provide directions for the development of novel approaches for the diagnosis, treatment, and prevention of atherosclerotic diseases