Epigenetic Regulation of Vascular Endothelial Biology/Pathobiology and Response to Fluid Shear Stress

Biofluids, cell mechanics and epigenetics: Flow-induced epigenetic mechanisms of endothelial gene expression

Epigenetics is the regulation of gene expression (transcription) in response to changes in the cell environment through genomic modifications that largely involve the non-coding fraction of the human genome and that cannot be attributed to modification of the primary DNA sequence. Epigenetics is dominant in establishing cell fate and positioning during programmed embryonic development. However the same pathways are used by mature postnatal and adult mammalian cells during normal physiology and are implicated in disease mechanisms. Recent research demonstrates that blood flow and pressure are cell environments that can influence transcription via epigenetic pathways. The principal epigenetic pathways are chemical modification of cytosine residues of DNA (DNA methylation) and of the amino tails of histone proteins associated with DNA in nucleosomes. They also encompass the post-transcriptional degradation of mRNA transcripts by non-coding RNAs (ncRNA). In vascular endothelium, epigenetic pathways respond to temporal and spatial variations of flow and pressure, particularly hemodynamic disturbed blood flow, with important consequences for gene expression. The biofluid environment is linked by mechanotransduction and solute transport to cardiovascular cell phenotypes via signaling pathways and epigenetic regulation for which there is an adequate interdisciplinary infrastructure with robust tools and methods available. Epigenetic mechanisms may be less familiar than acute genomic signaling to Investigators at the interface of biofluids, biomechanics and cardiovascular biology. Here we introduce a biofluids / cellular biomechanics readership to the principal epigenetic pathways and provide a contextual overview of endothelial epigenetic plasticity in the regulation of flow-responsive transcription.

Cell shape-dependent shear stress on adherent cells in a micro-physiologic system as revealed by FEM

Flow-induced shear stress on adherent cells leads to biochemical signaling and mechanical responses of the cells. To determine the flow-induced shear stress on adherent cells cultured in a micro-scaled reaction chamber, we developed a suitable finite element method model. The influence of the most important parameters-cell shape, cell density, shear modulus and fluid velocity-was investigated. Notably, the cell shape strongly influences the resulting shear stress. Long and smooth cells undergo lower shear stress than more rounded cells. Changes in the curvature of the cells lead to stress peaks and single cells experience higher shear stress values than cells of a confluent monolayer. The computational results of the fluid flow simulation were validated experimentally. We also analyzed the influence of flow-induced shear stress on the metabolic activity and shape of L929, a mouse fibroblast cell line, experimentally. The results indicate that threshold stress values for continuous flow conditions cannot be transferred to quasi static flow conditions interrupted by short fluid exchange events.

Endothelial epigenetics in biomechanical stress: disturbed flow-mediated epigenomic plasticity in vivo and in vitro

Arterial endothelial phenotype is regulated by local hemodynamic forces that are linked to regional susceptibility to atherogenesis. A complex hierarchy of transcriptional, translational, and post-translational mechanisms is greatly influenced by the characteristics of local arterial shear stress environments. We discuss the emerging role of localized disturbed blood flow on epigenetic mechanisms of endothelial responses to biomechanical stress, including transcriptional regulation by proximal promoter DNA methylation, and post-transcriptional and translational regulation of gene and protein expression by chromatin remodeling and noncoding RNA-based mechanisms. Dynamic responses to flow characteristics in vivo and in vitro include site-specific differentially methylated regions of swine and mouse endothelial methylomes, histone marks regulating chromatin conformation, microRNAs, and long noncoding RNAs. Flow-mediated epigenomic responses intersect with cis and trans factor regulation to maintain endothelial function in a shear-stressed environment and may contribute to localized endothelial dysfunctions that promote atherosusceptibility.

Role of histone deacetylases in transcription factor regulation and cell cycle modulation in endothelial cells in response to disturbed flow

Vascular endothelial cells (ECs) are exposed to different flow patterns (i.e., disturbed vs. laminar), and the associated oscillatory shear stress (OSS) or pulsatile shear stress (PSS) lead to differential responses. We investigated the roles of class I and II histone deacetylases (HDAC-1/2/3 and HDAC-5/7, respectively) in regulating NF-E2-related factor-2 (Nrf2) and Krüppel-like factor-2 (KLF2), two transcription factors governing many shear-responsive genes, and the cell cycle in ECs in response to OSS. Application of OSS (0.5 ± 4 dynes/cm(2)) to cultured ECs sustainably up-regulated class I and II HDACs and their nuclear accumulation, whereas PSS (12 ± 4 dynes/cm(2)) induced phosphorylation-dependent nuclear export of class II HDACs. En face immunohistochemical examination of rat aortic arch and experimentally stenosed abdominal aorta revealed high HDAC-2/3/5 levels in ECs in areas exposed to disturbed flow. OSS induced the association of HDAC-1/2/3 with Nrf2 and HDAC-3/5/7 with myocyte enhancer factor-2; deacetylation of these factors led to down-regulation of antioxidant gene NAD(P)H quinone oxidoreductase-1 (NQO1) and KLF2. HDAC-1/2/3- and HDAC-3/5/7-specific small interfering RNAs eliminated the OSS-induced down-regulation of NQO1 and KLF2, respectively. OSS up-regulated cyclin A and down-regulated p21(CIP1) in ECs and induced their proliferation; these effects were mediated by HDAC-1/2/3. Intraperitoneal administration of the class I-specific HDAC inhibitor valproic acid into bromodeoxyuridine (BrdU)-infused rats inhibited the increased EC uptake of BrdU at poststenotic sites. The OSS-induced HDAC signaling and EC responses are mediated by phosphatidylinositol 3-kinase/Akt. Our findings demonstrate the important roles of different groups of HDACs in regulating the oxidative, inflammatory, and proliferative responses of ECs to disturbed flow with OSS.