Interleukin 6 Stimulates Endothelial Binding and Transport of High-Density Lipoprotein Through Induction of Endothelial Lipase

Abstract

Objective-In the reverse cholesterol transport pathway, high-density lipoprotein (HDL) passes the endothelial cell barrier by mechanisms involving the scavenger receptor class B type I and the ATP-binding cassette G1. However, little is known on how inflammation influences this transendothelial transport. Approach and Results-On stimulation with interleukin-6, cultivated primary endothelial cells showed increased binding and transport of 125 I-HDL without changing the expression of scavenger receptor class B type I and ATP-binding cassette G1. Therefore, we analyzed the involvement of endothelial lipase (EL), a known HDL-binding protein expressed by endothelial cells. Here, we show an increased EL expression after interleukin-6 stimulation. Moreover, using pharmacological inhibitors or RNA interference against EL, we demonstrated its participation in HDL binding and transport through the endothelium. Furthermore, adenovirus-mediated transfection of endothelial cells with either catalytically active or nonactive EL revealed that EL facilitates the endothelial binding and transport by both bridging and lipolysis of HDL. EL was also found responsible for the reduction of HDL particle size occurring during the specific transport through a monolayer of endothelial cells. Finally, pharmacological inhibition of EL reversed the inducing effect of interleukin-6 on HDL binding and transport.Conclusions-Interleukin-6 stimulates the translocation of HDL through the endothelium, the first step in reverse cholesterol transport pathway, by enhancing EL expression. In addition, we demonstrated the role of EL in the transendothelial transport of HDL. [13][14][15][16] Moreover, several groups demonstrated the upregulation of EL on inflammatory stimulation both in vitro and in vivo. [17][18][19][20] In this study, we provide evidence that EL participates in endothelial binding, cell association, and transport of HDL. Materials and MethodsMaterials and Methods are available in the online-only Supplement. Results IL-6 Induces HDL Binding, Cell Association, and TransportTo investigate whether the interaction of HDL with endothelial cells is changed by the inflammatory cytokine IL-6, we stimulated aortic endothelial cells with IL-6. The interaction of HDL with the endothelial cells was characterized as binding at 4°C, cell association and transport at 37°C. IL-6 induced HDL binding to endothelial cells in a dose-and timedependent manner ( Figure IA and IB in the online-only Data Supplement). HDL binding was significantly increased after incubation with 1-or 10-ng/mL IL-6. Because of the maximal effects seen, all subsequent experiments were performed on cells that were stimulated without 0-or 10-ng/mL IL-6 for 24 hours. After 24 hours of IL-6 stimulation, specific binding was induced by 200±67% compared with not stimulated cells (100±28%; Figure 1A). Conversely, the specific cellularassociated HDL and the transported HDL were also increased by 144±21% and 178±39%, respectively, compared with not stimulated cells (100±17%; Figure 1B a...

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“…The results of this study further suggest that the binding of lipid-free apoA-I to endothelial cell surface β-ATPase facilitates the uptake and transport of lipidated apoA-I and mature HDLs by enhancing β-ATPase-mediated hydrolysis of extracellular ATP into ADP and inducing consecutive activation of the purinergic P2Y12 receptor (Cavelier et al 2012). More recently, transendothelial transport of HDL particles was found to be modulated by endothelial lipase both through its enzymatic lipolytic activity and its ability to bridge the binding of lipoproteins to the endothelial surface (Robert et al 2013). Initiation of an atherosclerosis-related inflammatory phenotype after stimulation of vascular endothelial cells with interleukin-6 significantly increased the binding, cell association, and transport of HDLs, which was linked mechanistically to the increase in endothelial lipase expression in response to interleukin-6 (Robert et al 2013).…”

“…More recently, transendothelial transport of HDL particles was found to be modulated by endothelial lipase both through its enzymatic lipolytic activity and its ability to bridge the binding of lipoproteins to the endothelial surface (Robert et al 2013). Initiation of an atherosclerosis-related inflammatory phenotype after stimulation of vascular endothelial cells with interleukin-6 significantly increased the binding, cell association, and transport of HDLs, which was linked mechanistically to the increase in endothelial lipase expression in response to interleukin-6 (Robert et al 2013). …”