Zhiyong Lin

Integration of flow-dependent endothelial phenotypes by Kruppel-like factor 2

In the face of systemic risk factors, certain regions of the arterial vasculature remain relatively resistant to the development of atherosclerotic lesions. The biomechanically distinct environments in these arterial geometries exert a protective influence via certain key functions of the endothelial lining; however, the mechanisms underlying the coordinated regulation of specific mechano-activated transcriptional programs leading to distinct endothelial functional phenotypes have remained elusive. Here, we show that the transcription factor Kruppel-like factor 2 (KLF2) is selectively induced in endothelial cells exposed to a biomechanical stimulus characteristic of atheroprotected regions of the human carotid and that this flow-mediated increase in expression occurs via a MEK5/ERK5/MEF2 signaling pathway. Overexpression and silencing of KLF2 in the context of flow, combined with findings from genome-wide analyses of gene expression, demonstrate that the induction of KLF2 results in the orchestrated regulation of endothelial transcriptional programs controlling inflammation, thrombosis/hemostasis, vascular tone, and blood vessel development. Our data also indicate that KLF2 expression globally modulates IL-1beta-mediated endothelial activation. KLF2 therefore serves as a mechano-activated transcription factor important in the integration of multiple endothelial functions associated with regions of the arterial vasculature that are relatively resistant to atherogenesis.

KLF2 Is a Novel Transcriptional Regulator of Endothelial Proinflammatory Activation

The vascular endothelium is a critical regulator of vascular function. Diverse stimuli such as proinflammatory cytokines and hemodynamic forces modulate endothelial phenotype and thereby impact on the development of vascular disease states. Therefore, identification of the regulatory factors that mediate the effects of these stimuli on endothelial function is of considerable interest. Transcriptional profiling studies identified the Kruppel-like factor (KLF)2 as being inhibited by the inflammatory cytokine interleukin-1β and induced by laminar shear stress in cultured human umbilical vein endothelial cells. Overexpression of KLF2 in umbilical vein endothelial cells robustly induced endothelial nitric oxide synthase expression and total enzymatic activity. In addition, KLF2 overexpression potently inhibited the induction of vascular cell adhesion molecule-1 and endothelial adhesion molecule E-selectin in response to various proinflammatory cytokines. Consistent with these observations, in vitro flow assays demonstrate that T cell attachment and rolling are markedly attenuated in endothelial monolayers transduced with KLF2. Finally, our studies implicate recruitment by KLF2 of the transcriptional coactivator cyclic AMP response element–binding protein (CBP/p300) as a unifying mechanism for these various effects. These data implicate KLF2 as a novel regulator of endothelial activation in response to proinflammatory stimuli.

Kruppel-Like Factor 2 (KLF2) Regulates Endothelial Thrombotic Function

The vascular endothelium maintains blood fluidity by inhibiting blood coagulation, inhibiting platelet aggregation, and promoting fibrinolysis. Endothelial cells lose these nonthrombogenic properties on exposure to proinflammatory stimuli. We recently identified the Kruppel-like factor KLF2 as a novel regulator of endothelial proinflammatory activation. Here it is found that KLF2 differentially regulates key factors involved in maintaining an antithrombotic endothelial surface. Overexpression of KLF2 strongly induced thrombomodulin (TM) and endothelial nitric oxide synthase (eNOS) expression and reduced plasminogen activator inhibitor-1 (PAI-1) expression. Furthermore, overexpression of KLF2 inhibited the cytokine-mediated induction of tissue factor (TF). In contrast, siRNA mediated knockdown of KLF2 reduced antithrombotic gene expression while inducing the expression of pro-coagulant factors. The functional importance of KLF2 was verified by in vitro clotting assays. By comparison to control infected cells, KLF2 overexpression increased blood clotting time as well as flow rates under basal and inflammatory conditions. In contrast, siRNA-mediated knockdown of KLF2 reduced blood clotting time and flow rates. These observations identify KLF2 as a novel transcriptional regulator of endothelial thrombotic function. The full text of this article is available online at http://circres.ahajournals.org.

Kruppel-like Factor 4 Regulates Endothelial Inflammation

The vascular endothelium plays a critical role in vascular homeostasis. Inflammatory cytokines and non-laminar blood flow induce endothelial dysfunction and confer a pro-adhesive and pro-thrombotic phenotype. Therefore, identification of factors that mediate the effects of these stimuli on endothelial function is of considerable interest. Kruppel-like factor 4 expression has been documented in endothelial cells, but a function has not been described. In this communication we describe the expression in vitro and in vivo of Kruppel-like factor 4 in human and mouse endothelial cells. Furthermore, we demonstrate that endothelial Kruppel-like factor 4 is induced by pro-inflammatory stimuli and shear stress. Overexpression of Kruppel-like factor 4 induces expression of multiple anti-inflammatory and anti-thrombotic factors including endothelial nitric-oxide synthase and thrombomodulin, whereas knockdown of Kruppellike factor 4 leads to enhancement of tumor necrosis factor α-induced vascular cell adhesion molecule-1 and tissue factor expression. The functional importance of Kruppel-like factor 4 is verified by demonstrating that Kruppel-like factor 4 expression markedly decreases inflammatory cell adhesion to the endothelial surface and prolongs clotting time under inflammatory states. Kruppel-like factor 4 differentially regulates the promoter activity of pro- and anti-inflammatory genes in a manner consistent with its anti-inflammatory function. These data implicate Kruppel-like factor 4 as a novel regulator of endothelial activation in response to pro-inflammatory stimuli.

Kruppel-like factor 15 is a regulator of cardiomyocyte hypertrophy

Cardiac hypertrophy is a common response to injury and hemodynamic stress and an important harbinger of heart failure and death. Herein, we identify the Kruppel-like factor 15 (KLF15) as an inhibitor of cardiac hypertrophy. Myocardial expression of KLF15 is reduced in rodent models of hypertrophy and in biopsy samples from patients with pressure-overload induced by chronic valvular aortic stenosis. Overexpression of KLF15 in neonatal rat ventricular cardiomyocytes inhibits cell size, protein synthesis and hypertrophic gene expression. KLF15-null mice are viable but, in response to pressure overload, develop an eccentric form of cardiac hypertrophy characterized by increased heart weight, exaggerated expression of hypertrophic genes, left ventricular cavity dilatation with increased myocyte size, and reduced left ventricular systolic function. Mechanistically, a combination of promoter analyses and gel-shift studies suggest that KLF15 can inhibit GATA4 and myocyte enhancer factor 2 function. These studies identify KLF15 as part of a heretofore unrecognized pathway regulating the cardiac response to hemodynamic stress.

Tumor necrosis factor alpha-mediated reduction of KLF2 is due to inhibition of MEF2 by NF-κB and histone deacetylases

ABSTRACT Activation of the endothelium by inflammatory cytokines is a key event in the pathogenesis of vascular disease states. Proinflammatory cytokines repress the expression of KLF2, a recently identified transcriptional inhibitor of the cytokine-mediated activation of endothelial cells. In this study the molecular basis for the cytokine-mediated inhibition of KLF2 is elucidated. Tumor necrosis factor alpha (TNF-α) potently inhibited KLF2 expression. This effect was completely abrogated by a constitutively active form of IκBα, as well as treatment with trichostatin A, implicating a role for the NF-κB pathway and histone deacetylases. Overexpression studies coupled with observations with p50/p65 null cells support an essential role for p65. A combination of promoter deletion and mutational analyses, chromatin immunoprecipitation assays, and coimmunoprecipitation studies indicates that p65 and histone deacetylases 4 cooperate to inhibit the ability of MEF2 factors to induce the KLF2 promoter. These studies identify a novel mechanism by which TNF-α can inhibit endothelial gene expression. Furthermore, the inhibition of MEF2 function by p65 and HDAC4 has implications for other cellular systems where these factors are operative.

Hemizygous deficiency of Krüppel-like factor 2 augments experimental atherosclerosis.

Krüppel-like factor (KLF)2 is a central regulator of endothelial and monocyte/macrophage gene expression and function in vitro. Although the composite effects of KLF2 in these 2 cell types predict that it likely inhibits vascular inflammation, the role of KLF2 in this process in vivo is uncharacterized. In this study, we provide evidence that hemizygous deficiency of KLF2 increased diet-induced atherosclerosis in apolipoprotein E–deficient mice. Our studies highlight an important role for KLF2 in primary macrophage foam cell formation via the potential regulation of the key lipid binding protein adipocyte protein 2/fatty acid–binding protein 4. These novel observations establish that KLF2 is an atheroprotective factor.

Inhibition of Vascular Permeability Factor/Vascular Endothelial Growth Factor-mediated Angiogenesis by the Kruppel-like Factor KLF2

The Kruppel-like factor KLF2 was recently identified as a novel regulator of endothelial pro-inflammatory and pro-thrombotic function. Here it is shown that overexpression of KLF2 potently inhibits vascular permeability factor/vascular endothelial growth factor (VEGF-A)-mediated angiogenesis and tissue edema in the nude ear mouse model of angiogenesis. In vitro, KLF2 expression retards VEGF-mediated calcium flux, proliferation and induction of pro-inflammatory factors in endothelial cells. This effect is due to a potent inhibition of VEGFR2/KDR expression and promoter activity. These observations identify KLF2 as a regulator of VEGFR2/KDR and provide a foundation for novel approaches to regulate angiogenesis.

Kruppel-like Factor 2 Inhibits Hypoxia-inducible Factor 1α Expression and Function in the Endothelium

Hypoxia-inducible factor 1 (HIF-1) is a central regulator of the hypoxic response in many cell types. In endothelial cells, HIF-1 induces the expression of key proangiogenic factors to promote angiogenesis. Recent studies have identified Kruppel-like factor 2 (KLF2) as a potent inhibitor of angiogenesis. However, the role of KLF2 in regulating HIF-1 expression and function has not been evaluated. KLF2 expression was induced acutely by hypoxia in endothelial cells. Adenoviral overexpression of KLF2 inhibited hypoxia-induced expression of HIF-1α and its target genes such as interleukin 8, angiopoietin-2, and vascular endothelial growth factor in endothelial cells. Conversely, knockdown of KLF2 increased expression of HIF-1α and its targets. Furthermore, KLF2 inhibited hypoxia-induced endothelial tube formation, whereas endothelial cells from mice with haploinsufficiency of KLF2 showed increased tube formation in response to hypoxia. Consistent with this ex vivo observation, KLF2 heterozygous mice showed increased microvessel density in the brain. Mechanistically, KLF2 promoted HIF-1α degradation in a von Hippel-Lindau protein-independent but proteasome-dependent manner. Finally, KLF2 disrupted the interaction between HIF-1α and its chaperone Hsp90, suggesting that KLF2 promotes degradation of HIF-1α by affecting its folding and maturation. These observations identify KLF2 as a novel inhibitor of HIF-1α expression and function. Therefore, KLF2 may be a target for modulating the angiogenic response in disease states.

Kruppel-Like Factor 2 Regulates Endothelial Barrier Function

Objective—A central function of the endothelium is to serve as a selective barrier that regulates fluid and solute exchange. Although perturbation of barrier function can contribute to numerous disease states, our understanding of the molecular mechanisms regulating this aspect of endothelial biology remains incompletely understood. Accumulating evidence implicates the Kruppel-like factor 2 (KLF2) as a key regulator of endothelial function. However, its role in vascular barrier function is unknown. Methods and Results—To assess the role of KLF2 in vascular barrier function in vivo, we measured the leakage of Evans blue dye into interstitial tissues of the mouse ear after treatment with mustard oil. By comparison with KLF2+/+ mice, KLF2+/− mice exhibited a significantly higher degree of vascular leak. In accordance with our in vivo observation, adenoviral overexpression of KLF2 in human umbilical vein endothelial cells strongly attenuated the increase of endothelial leakage by thrombin and H2O2 as measured by fluorescein isothiocyanate dextrans (FITC-dextran) passage. Conversely, KLF2 deficiency in human umbilical vein endothelial cells and primary endothelial cells derived from KLF2+/− mice exhibited a marked increase in thrombin and H2O2-induced permeability. Mechanistically, our studies indicate that KLF2 confers barrier-protection via differential effects on the expression of key junction protein occludin and modification of a signaling molecule (myosin light chain) that regulate endothelial barrier integrity. Conclusion—These observations identify KLF2 as a novel transcriptional regulator of vascular barrier function.