Regional determinants of arterial endothelial phenotype dominate the impact of gender or short-term exposure to a high-fat diet

Investigating Effects of Fluid Shear Stress on Lymphatic Endothelial Cells

Recent studies using in vivo models have characterized lymph flow and demonstrated that lymph flow plays a key role in the later stages of lymphatic vascular development, including vascular remodeling, to create a hierarchical collecting vessel network and lymphatic valves (Sweet et al., J Clin Invest 125, 2995-3007, 2015). However, mechanistic insights into the response of lymphatic endothelial cells to fluid flow are difficult to obtain from in vivo studies because of the small size of lymphatic vessels and the technical challenge of lymphatic endothelial cell isolation. On the other hand, in vitro experiments can be tailored to isolate and test specific mechanotransduction pathways more cleanly than conditions in vivo. To measure in vitro the cellular response to flow, cultured primary lymphatic endothelial cells can be exposed to highly specific fluid forces like those believed to exist in vivo. Such in vitro studies have recently helped identify FOXC2 and GATA2 as important transcriptional regulators of lymphatic function during valve formation that are regulated by lymph flow dynamics. This chapter discusses the methods used to expose primary lymphatic endothelial cells (LECs) to lymph fluid dynamics and the relationship of these in vitro studies to in vivo lymphatic biology.

High-fat diet increases and the polyphenol, S17834, decreases acetylation of the sirtuin-1-dependent lysine-382 on p53 and apoptotic signaling in atherosclerotic lesion-prone aortic endothelium of normal mice

Our purpose was to determine if high-fat diet and treatment with a polyphenol regulate the acetylation of lysine-382 of p53, the site regulated by sirtuin-1, and apoptosis in the endothelium of the atherosclerotic lesion-prone mouse aortic arch. In cultured endothelial cells, 2 atherogenic stimuli, hydrogen peroxide and tumor necrosis factor-α, increased the acetylation of p53 lysine-382, and caspase-3 cleavage, an indicator of apoptotic signaling. The polyphenol, S17834, significantly prevented these changes. In low-density lipoprotein receptor-deficient mice, a high-fat diet increased, and treatment with S17834 attenuated early atherosclerotic lesions on the lesser curvature of the aortic arch. In wild-type C57BL6 mice fed the same diet, no atherosclerotic lesions were observed in this lesion-prone area, but p53 acetylation and caspase-3 cleavage increased in the endothelium. In high-fat fed mice, S17834 increased sirtuin-1 protein in the lesion-prone endothelium and prevented both the increase in p53 acetylation and caspase-3 cleavage without affecting blood lipids. These results indicate that high-fat diet increases and S17834 decreases the acetylation of p53 in lesion-prone aortic endothelial cells of normal mice independently of blood lipids, suggesting that the polyphenol may regulate endothelial cell p53 acetylation and apoptosis via local actions.

Coronary artery endothelial transcriptome in vivo: identification of endoplasmic reticulum stress and enhanced reactive oxygen species by gene connectivity network analysis

BACKGROUND

Endothelial function is central to the localization of atherosclerosis. The in vivo endothelial phenotypic footprints of arterial bed identity and site-specific atherosusceptibility are addressed.

METHODS AND RESULTS

Ninety-eight endothelial cell samples from 13 discrete coronary and noncoronary arterial regions of varying susceptibilities to atherosclerosis were isolated from 76 normal swine. Transcript profiles were analyzed to determine the steady-state in vivo endothelial phenotypes. An unsupervised systems biology approach using weighted gene coexpression networks showed highly correlated endothelial genes. Connectivity network analysis identified 19 gene modules, 12 of which showed significant association with circulatory bed classification. Differential expression of 1300 genes between coronary and noncoronary artery endothelium suggested distinct coronary endothelial phenotypes, with highest significance expressed in gene modules enriched for biological functions related to endoplasmic reticulum (ER) stress and unfolded protein binding, regulation of transcription and translation, and redox homeostasis. Furthermore, within coronary arteries, comparison of endothelial transcript profiles of susceptible proximal regions to protected distal regions suggested the presence of ER stress conditions in susceptible sites. Accumulation of reactive oxygen species throughout coronary endothelium was greater than in noncoronary endothelium consistent with coronary artery ER stress and lower endothelial expression of antioxidant genes in coronary arteries.

CONCLUSIONS

Gene connectivity analyses discriminated between coronary and noncoronary endothelial transcript profiles and identified differential transcript levels associated with increased ER and oxidative stress in coronary arteries consistent with enhanced susceptibility to atherosclerosis.

Discovery of novel mechanosensitive genes in vivo using mouse carotid artery endothelium exposed to disturbed flow

Recently, we showed that disturbed flow caused by a partial ligation of mouse carotid artery rapidly induces atherosclerosis. Here, we identified mechanosensitive genes in vivo through a genome-wide microarray study using mouse endothelial RNAs isolated from the flow-disturbed left and the undisturbed right common carotid artery. We found 62 and 523 genes that changed significantly by 12 hours and 48 hours after ligation, respectively. The results were validated by quantitative polymerase chain reaction for 44 of 46 tested genes. This array study discovered numerous novel mechanosensitive genes, including Lmo4, klk10, and dhh, while confirming well-known ones, such as Klf2, eNOS, and BMP4. Four genes were further validated for protein, including LMO4, which showed higher expression in mouse aortic arch and in human coronary endothelium in an asymmetric pattern. Comparison of in vivo, ex vivo, and in vitro endothelial gene expression profiles indicates that numerous in vivo mechanosensitive genes appear to be lost or dysregulated during culture. Gene ontology analyses show that disturbed flow regulates genes involved in cell proliferation and morphology by 12 hours, followed by inflammatory and immune responses by 48 hours. Determining the functional importance of these novel mechanosensitive genes may provide important insights into understanding vascular biology and atherosclerosis.

Endothelial heterogeneity associated with regional athero-susceptibility and adaptation to disturbed blood flow in vivo

Endothelial phenotypic heterogeneity plays an important role in the susceptibility of the cardiovascular system to disease. Arteries and heart valves are susceptible to chronic inflammatory disease in regions of blood flow disturbance that implicates hemodynamic forces and transport characteristics as prominent influences on endothelial phenotype. By combining in vivo high-throughput genomics (discovery science) and in vitro mechanistic approaches (reductionist science), we present endothelial patho-susceptibility as an imbalance of multiple interrelated pathways that sensitize the cells to pathological change. The recently identified association of endoplasmic reticulum stress with endothelium in regions of flow disturbance is outlined as an important example of susceptible phenotype linked to proinflammatory and oxidative stress pathways.

Prelesional arterial endothelial phenotypes in hypercholesterolemia: universal ABCA1 upregulation contrasts with region-specific gene expression in vivo

Atherosclerosis originates as focal arterial lesions having a predictable distribution to regions of bifurcations, branches, and inner curvatures where blood flow characteristics are complex. Distinct endothelial phenotypes correlate with regional hemodynamics. We propose that systemic risk factors modify regional endothelial phenotype to influence focal susceptibility to atherosclerosis. Transcript profiles of freshly isolated endothelial cells from three atherosusceptible and three atheroprotected arterial regions in adult swine were analyzed to determine the initial prelesional effects of hypercholesterolemia on endothelial phenotypes in vivo. Cholesterol efflux transporter ATP-binding cassette transporter A1 (ABCA1) was upregulated at all sites in response to short-term high-fat diet. Proinflammatory and antioxidative endothelial gene expression profiles were induced in atherosusceptible and atheroprotected regions, respectively. However, markers for endoplasmic reticulum stress, a signature of susceptible endothelial phenotype, were not further enhanced by brief hypercholesterolemia. Both region-specific and ubiquitous (ABCA1) phenotype changes were identified as early prelesional responses of the endothelium to hypercholesterolemia.

Chronic endoplasmic reticulum stress activates unfolded protein response in arterial endothelium in regions of susceptibility to atherosclerosis

RATIONALE

Endothelial function and dysfunction are central to the focal origin and regional development of atherosclerosis; however, an in vivo endothelial phenotypic footprint of susceptibility to atherosclerosis preceding pathological change remains elusive.

OBJECTIVE

To conduct a comparative multi-site genomics study of arterial endothelial phenotype in atherosusceptible and atheroprotected regions.

METHODS AND RESULTS

Transcript profiles of freshly isolated endothelial cells from 7 discrete arterial regions in normal swine were analyzed to determine the steady state in vivo endothelial phenotypes in regions of varying susceptibilities to atherosclerosis. The most abundant common feature of the endothelium of all atherosusceptible regions was the upregulation of genes associated with endoplasmic reticulum (ER) stress. The unfolded protein response pathway, induced by ER stress, was therefore investigated in detail in endothelium of the atherosusceptible aortic arch and was found to be partially activated. ER transmembrane signal transducers IRE1alpha and ATF6alpha and their downstream effectors, but not PERK, were activated concomitant with a higher transcript expression of protein folding enzymes and chaperones, indicative of ER stress in vivo.

CONCLUSIONS

The findings demonstrate the prevalence of chronic endothelial ER stress and activated unfolded protein response in vivo at atherosusceptible arterial sites. We propose that chronic localized biological stress is linked to spatial susceptibility of the endothelium to the initiation of atherosclerosis.

Laminar shear stress regulates endothelial kinin B1 receptor expression and function: potential implication in atherogenesis

OBJECTIVE

The proinflammatory phenotype induced by low laminar shear stress (LSS) is implicated in atherogenesis. The kinin B1 receptor (B1R), known to be induced by inflammatory stimuli, exerts many proinflammatory effects including vasodilatation and leukocyte recruitment. We investigated whether low LSS is a stimulus for endothelial B1R expression and function.

METHODS AND RESULTS

Human and mouse atherosclerotic plaques expressed high level of B1R mRNA and protein. In addition, B1R expression was upregulated in the aortic arch (low LSS region) of ApoE(-/-) mice fed a high-fat diet compared to vascular regions of high LSS and animals fed normal chow. Of interest, a greater expression of B1R was noticed in endothelial cells from regions of low LSS in aortic arch of ApoE(-/-) mice. B1R was also upregulated in human umbilical vein endothelial cells (HUVECs) exposed to low LSS (0 to 2 dyn/cm(2)) compared to physiological LSS (6 to 10 dyn/cm(2)): an effect similarly evident in murine vascular tissue perfused ex vivo. Functionally, B1R activation increased prostaglandin and CXCL5 expression in cells exposed to low, but not physiological, LSS. IL-1beta and ox-LDL induced B1R expression and function in HUVECs, a response substantially enhanced under low LSS conditions and inhibited by blockade of NFkappaB activation.

CONCLUSIONS

Herein, we show that LSS is a major determinant of functional B1R expression in endothelium. Furthermore, whereas physiological high LSS is a powerful repressor of this inflammatory receptor, low LSS occurring [corrected] at sites of atheroma is associated with substantial upregulation, identifying this receptor as a potential therapeutic target.

Endothelial primary cilia in areas of disturbed flow are at the base of atherosclerosis

Atherosclerosis develops in the arterial system at sites of low as well as low and oscillating shear stress. Previously, we demonstrated a shear-related distribution of ciliated endothelial cells in the embryonic cardiovascular system and postulated that the primary cilium is a component of the shear stress sensor, functioning as a signal amplifier. This shear-related distribution is reminiscent of the atherosclerotic predilection sites. Thus, we determined whether a link exists between location and frequency of endothelial primary cilia and atherogenesis. We analyzed endothelial ciliation of the adult aortic arch and common carotid arteries of wild type C57BL/6 and apolipoprotein-E-deficient mice. Primary cilia are located at the atherosclerotic predilection sites, where flow is disturbed, in wild type mice and they occur on and around atherosclerotic lesions in apolipoprotein-E-deficient mice, which have significantly more primary cilia in the aortic arch than wild type mice. In addition, common carotid arteries were challenged for shear stress by application of a restrictive cast, resulting in the presence of primary cilia only at sites of induced low and disturbed shear. In conclusion, these data relate the presence of endothelial primary cilia to regions of atherogenesis, where they increase in number under hyperlipidemia-induced lesion formation. Experimentally induced flow disturbance leads to induction of primary cilia, and subsequently to atherogenesis, which suggests a role for primary cilia in endothelial activation and dysfunction.