KLF2 suppresses TGF-β signaling in endothelium through induction of Smad7 and inhibition of AP-1

OBJECTIVE - The flow-responsive Kruppel-like factor 2 (KLF2) is crucial for maintaining endothelial cell quiescence. Here, we describe its detailed effects on transforming growth factor-β (TGF-β) signaling, which normally has proatherogenic effects on endothelium. METHODS AND RESULTS - In-depth analysis of genome-wide expression data shows that prolonged lentiviral-mediated overexpression of KLF2 in human umbilical vein endothelial cells (HUVECs) diminishes the expression of a large panel of established TGF-β-inducible genes. Both baseline and TGF-β-induced expression levels of plasminogen activator inhibitor 1 (PAI-1) and thrombospondin-1 are greatly diminished by KLF2. Using a combination of ectopic expression, small interfering RNA-mediated knockdown, and promoter activity assays, we show that KLF2 partly inhibits the phosphorylation and subsequent nuclear accumulation of Smad2, thereby suppressing the TGF-β-induced Smad4-mediated transcriptional activity. This is achieved through TGF-β-independent induction of inhibitory Smad7. Additionally, a full inhibition of TGF-β signaling is functionally achieved through a simultaneous suppression of activator protein 1 (AP-1), which is an essential cofactor for TGF-β-dependent transcription of many genes. CONCLUSIONS - The concerted mechanism by which KLF2 inhibits TGF-β signaling through induction of inhibitory Smad7 and attenuation of AP-1 activity provides a novel mechanism by which KLF2 contributes to sustaining a quiescent, atheroprotective status of vascular endothelium

Angiogenic Monocytes : Another Colorful Blow to Endothelial Progenitors

This Commentary provides perspective on a related article by Sun-Jin Kim and coworkers (Am J Pathol: 172 AJP08-0819), who assess the contribution of bone marrow-derived cells to tumor angiogenesis in a physiologic, non-myeloablative setting and conclude that the actual angiogenic cell type incorporated in the newly formed vessels is actually monocytes/macrophages

Endothelial KLF2 Links Local Arterial Shear Stress Levels to the Expression of Vascular Tone-Regulating Genes

Lung Krüppel-like factor (LKLF/KLF2) is an endothelial transcription factor that is crucially involved in murine vasculogenesis and is specifically regulated by flow in vitro. We now show a relation to local flow variations in the adult human vasculature: decreased LKLF expression was noted at the aorta bifurcations to the iliac and carotid arteries, coinciding with neointima formation. The direct involvement of shear stress in the in vivo expression of LKLF was determined independently by in situ hybridization and laser microbeam microdissection/reverse transcriptase-polymerase chain reaction in a murine carotid artery collar model, in which a 4- to 30-fold induction of LKLF occurred at the high-shear sites. Dissection of the biomechanics of LKLF regulation in vitro demonstrated that steady flow and pulsatile flow induced basal LKLF expression 15- and 36-fold at shear stresses greater than ∼5 dyne/cm(2), whereas cyclic stretch had no effect. Prolonged LKLF induction in the absence of flow changed the expression of angiotensin-converting enzyme, endothelin-1, adrenomedullin, and endothelial nitric oxide synthase to levels similar to those observed under prolonged flow. LKLF repression by siRNA suppressed the flow response of endothelin-1, adrenomedullin, and endothelial nitric oxide synthase (P < 0.05). Thus, we demonstrate that endothelial LKLF is regulated by flow in vivo and is a transcriptional regulator of several endothelial genes that control vascular tone in response to flow