Laminar shear stress acts as a switch to regulate divergent functions of NF-kappaB in endothelial cells

Attenuation of retinal endothelial cell migration and capillary morphogenesis in the absence of bcl-2

Apoptosis plays a critical role during development and in the maintenance of the vascular system. B-cell leukemia lymphoma 2 (bcl-2) protects endothelial cells (EC) from apoptosis in response to a variety of stimuli. Previous work from this laboratory demonstrated attenuation of postnatal retinal vascular development and retinal neovascularization during oxygen-induced ischemic retinopathy in bcl-2-deficient (bcl-2-/-) mice. To gain further insight into the function of bcl-2 in the endothelium, we isolated retinal EC from bcl-2+/+ and bcl-2-/- mice. Retinal EC lacking bcl-2 demonstrated reduced cell migration, tenascin-C expression, and adhesion to vitronectin and fibronectin. The bcl-2-/- retinal EC also failed to undergo capillary morphogenesis in Matrigel. In addition, using an ex vivo angiogenesis assay, we observed reduced sprouting from aortic rings grown in culture from bcl-2-/- mice compared with bcl-2+/+ mice. Furthermore, reexpression of bcl-2 was sufficient to restore migration and capillary morphogenesis defects observed in bcl-2-/- retinal EC. Mechanistically, bcl-2-/- cells expressed significantly less endothelial nitric oxide synthase, an important downstream effecter of proangiogenic signaling. This may be attributed to increased oxidative stress in the absence of bcl-2. In fact, incubation of retinal EC or aortic rings from bcl-2-/- mice with the antioxidant N-acetylcysteine rescued their capillary morphogenesis and sprouting defects. Thus, bcl-2-mediated cellular functions play important roles not only in survival but also in proangiogenic phenotype of EC with a significant impact on vascular development and angiogenesis.

Activation of p300 histone acetyltransferase activity is an early endothelial response to laminar shear stress and is essential for stimulation of endothelial nitric-oxide synthase mRNA transcription

Previous studies have shown that the acute stimulation of endothelial nitric-oxide synthase (eNOS) mRNA transcription by laminar shear stress is dependent on nuclear factor κ B (NFκB) subunits p50 and p65 binding to a shear stress response element (SSRE) in the human eNOS promoter and that mutation of the SSRE abrogates the shear-stimulated increase in eNOS promoter activity. In the present study, we found that although shear markedly increased eNOS mRNA, the increase in nuclear translocation of p50 and p65 caused by shear was only 2-fold, suggesting that shear has additional effects on NFκB cofactor activity beyond nuclear translocation. Chromatin immunoprecipitation assays showed that virtually no p50 or p65 was bound to the eNOS promoter at base line but that shear increased the binding of these subunits to the human eNOS SSRE by 10- to 20-fold. Co-immunoprecipitation studies demonstrated during the first 30 min of shear p300 bound to p65. Shear also increased p300 histone acetyltransferase (HAT) activity by 2.5-fold and increased acetylation of p65. The increase in eNOS mRNA caused by shear was completely blocked by pharmacological inhibition of p300/HAT activity with curcumin or by p300 small interfering RNA. Chromatin immunoprecipitation assays also showed that shear stimulated acetylation of histones 3 and 4 at the region of the eNOS promoter SSRE and extended 3′ toward the eNOS coding region. This was associated with opening of chromatin at the SSRE. In conclusion, these studies reveal a previously unknown role of p300/HAT activation as a very early response to shear that is essential for increasing eNOS mRNA levels.

KLF2-dependent, shear stress-induced expression of CD59: a novel cytoprotective mechanism against complement-mediated injury in the vasculature

Complement activation may predispose to vascular injury and atherogenesis. The atheroprotective actions of unidirectional laminar shear stress led us to explore its influence on endothelial cell expression of complement inhibitory proteins CD59 and decay-accelerating factor. Human umbilical vein and aortic endothelial cells were exposed to laminar shear stress (12 dynes/cm(2)) or disturbed flow (+/- 5 dynes/cm(2) at 1Hz) in a parallel plate flow chamber. Laminar shear induced a flow rate-dependent increase in steady-state CD59 mRNA, reaching 4-fold at 12 dynes/cm(2). Following 24-48 h of laminar shear stress, cell surface expression of CD59 was up-regulated by 100%, whereas decay-accelerating factor expression was unchanged. The increase in CD59 following laminar shear was functionally significant, reducing C9 deposition and complement-mediated lysis of flow-conditioned endothelial cells by 50%. Although CD59 induction was independent of PI3-K, ERK1/2 and nitric oxide, an RNA interference approach demonstrated dependence upon an ERK5/KLF2 signaling pathway. In contrast to laminar shear stress, disturbed flow failed to induce endothelial cell CD59 protein expression. Likewise, CD59 expression on vascular endothelium was significantly higher in atheroresistant regions of the murine aorta exposed to unidirectional laminar shear stress, when compared with atheroprone areas exposed to disturbed flow. We propose that up-regulation of CD59 via ERK5/KLF2 activation leads to endothelial resistance to complement-mediated injury and protects from atherogenesis in regions of laminar shear stress.

Low shear stress-induced interleukin-8 mRNA expression in endothelial cells is mechanotransduced by integrins and the cytoskeleton

Shear stress is thought to be important in maintaining vascular homeostasis and regulating vascular remodeling. The authors have previously shown that low shear stress increases interleukin (IL)-8 gene expression in endothelial cells. However, the detailed events that contribute to the regulation of this expression remain to be identified. In this study, the authors examined whether the shear stress-induced IL-8 mRNA expression in endothelial cells is mechanotransduced by integrins and the cytoskeleton. Exposure of endothelial cells to low shear stress (4.2 dyne/cm(2)) rapidly increased the expression of IL-8 mRNA. The induced IL-8 mRNA expression was inhibited by GRGDNP, which blocked the binding of integrins to the extracellular matrix. Additionally, this increase was attenuated by both anti-alpha(v)beta(3) integrin and anti-beta(1) integrin antibodies. Treatment of endothelial cells with either low shear stress or cytochalasin D resulted in the disruption of the cytoskeleton. Following the disruption of filamentous F-actin, the IL-8 mRNA expression in endothelial cells also increased, indicating that the shear stress-induced IL-8 mRNA expression may be mediated by the disruption of actin fibers. Taken together, these data suggest that integrins and the actin cytoskeleton play important roles in regulating the shear stress-induced IL-8 gene expression. Supplementary materials are available for the Materials and Methods section of this article. Go to the publisher's online of edition of Endothelium for this free supplemental resource.

NF-kappaB suppression by the deubiquitinating enzyme Cezanne: a novel negative feedback loop in pro-inflammatory signaling

Transcription factors belonging to the NF-kappaB family regulate inflammation by inducing pro-inflammatory molecules (e.g. interleukin (IL)-8) in response to cytokines (e.g. tumor necrosis factor (TNF) alpha, IL-1) or other stimuli. Several negative regulators of NF-kappaB, including the ubiquitin-editing enzyme A20, participate in the resolution of inflammatory responses. We report that Cezanne, a member of the A20 family of the deubiquitinating cysteine proteases, can be induced by TNFalpha in cultured cells. Silencing of endogenous Cezanne using small interfering RNA led to elevated NF-kappaB luciferase reporter gene activity and enhanced expression of IL-8 transcripts in TNFalpha-treated cells. Thus we conclude that endogenous Cezanne can attenuate NF-kappaB activation and the induction of pro-inflammatory transcripts in response to TNF receptor (TNFR) signaling. Overexpression studies revealed that Cezanne suppressed NF-kappaB nuclear translocation and transcriptional activity by targeting the TNFR signaling pathway at the level of the IkappaB kinase complex or upstream from it. These effects were not observed in a form of Cezanne that was mutated at the catalytic cysteine residue (Cys209), indicating that the deubiquitinating activity of Cezanne is essential for NF-kappaB regulation. Finally, we demonstrate that Cezanne can be recruited to activated TNFRs where it suppresses the build-up of polyubiquitinated RIP1 signal adapter proteins. Thus we conclude that Cezanne forms a novel negative feedback loop in pro-inflammatory signaling and that it suppresses NF-kappaB activation by targeting RIP1 signaling intermediaries for deubiquitination.