LXR-activating oxysterols induce the expression of inflammatory markers in endothelial cells through LXR-independent mechanisms
Introduction
Nuclear receptors LXRα and LXRβ (Liver X receptors, NR1H3 and NR1H2) are master regulators of body cholesterol and control its intestinal absorption, reverse transport and liver metabolism [1]. Endogenous LXR agonists include 22-hydroxy, 24,25-epoxy, 25-hydroxy and 27-hydroxycholesterol (22OH, 24,25E, 25OH, 27OH). These oxysterol compounds derive from cholesterol via enzymatic and non-enzymatic reactions. The latter depend on oxidative radicals, while the enzymatic production of LXR-activating oxysterols parallels cholesterol loading [2], [3]. Recently, several synthetic LXR agonists have been developed, such as T0901317 and GW3965. The administration of these compounds to apoE- and LDR-deficient mice reduces the development and progression of atherosclerosis, owing to a more efficient reverse cholesterol transport and to a net inhibition of plaque inflammation [4], [5], [6], [7], [8], [9], [10]. LXR agonists have thus emerged as promising anti-atherosclerotic drugs.
In vivo, endothelial cells can be exposed to LXR-activating oxysterols released from atherosclerotic plaques, oxidized lipoproteins and lipid-loaded cells [3], [11]. Endothelial cells express functional LXRs and are able to up-regulate ABC transporters upon treatment with LXR agonists [12], [13]. Moreover, shear stress has been shown to activate the expression and function of endothelial LXRs [14]. However, it was also previously shown that 25OH can also augment eicosanoid release from cultured endothelial cells and increase endothelial–leukocyte interaction through the up-regulation of VCAM1 [15], [16]. Furthermore, 25OH can increase cytokine production in non-endothelial cells [17], [18]. Contradicting the anti-atherosclerotic actions of synthetic LXR agonists, LXR-activating oxysterols have thus been proposed as pro-inflammatory and atherogenic mediators. Such paradox prompts to underlying differences in mechanism and awaits further characterization. The objective of the present study was therefore to specifically and comprehensively examine the effects of LXR agonists on human endothelial cells. In particular, our specific aims were: (1) to clarify if LXR agonists can indeed modulate the expression of endothelial inflammatory markers and (2) to uncover any mechanistic dissociation between oxysterol and synthetic LXR agonists in endothelial cells.
Section snippets
Materials and methods
For detailed procedures, see Supplementary Materials and Methods.
LXRs are expressed and functional in human endothelial cells
LXRα and LXRβ mRNA and protein were detected in both HUVEC and HUAEC and showed a preferential nuclear localization (Fig. 1A and B). Treatment of HUVEC with 22OH, 24,25E, T0901317 and GW3965 lead to a significant up-regulation of LXR target gene ABCA1, indicating that endothelial LXRs can regularly transactivate and recruit the transcriptional machinery (Fig. 1C). Accordingly, the transcriptional inhibitor actinomycin D could blunt the up-regulation of ABCA1 by LXRs.
LXR agonists modulate endothelial responses to inflammatory stimuli
Opposing effects have been
Discussion
In the present work, we have shown that oxysterol LXR agonists comprehensively stimulate inflammatory pathways in endothelial cells, leading to the up-regulation of adhesion molecules (ICAM1, VCAM1, SELE), chemokines (IL8, IL1α, CCLs and CXCLs), transcription factors (EGR1, FOS) and enzymes (COX2), and to the down-regulation of eNOS. These findings are in line with previous studies, which have reported that 25OH can promote endothelial–leukocyte interaction, augment eicosanoid release from
Acknowledgments
The study was supported by grants from M.U.R. (ex 60%), Regione Piemonte (Ric. Fin. 2006 to FV and 2008 to FM), University of Torino and Compagnia di San Paolo di Torino.
References (30)
- et al.
Sterol regulators of cholesterol homeostasis and beyond: the oxysterol hypothesis revisited and revised
Prog Lipid Res
(2008) - et al.
Enzymatic reduction of oxysterols impairs LXR signaling in cultured cells and the livers of mice
Cell Metab
(2007) - et al.
T-0901317, a synthetic liver X receptor ligand, inhibits development of atherosclerosis in LDL receptor-deficient mice
FEBS Lett
(2003) - et al.
Synthetic LXR agonist attenuates plaque formation in apoE−/− mice without inducing liver steatosis and hypertriglyceridemia
J Lipid Res
(2009) - et al.
Oxysterols and atherosclerosis
Atherosclerosis
(1999) - et al.
Tocotrienols reduce 25-hydroxycholesterol-induced monocyte–endothelial cell interaction by inhibiting the surface expression of adhesion molecules
Atherosclerosis
(2005) - et al.
Molecular determinants of crosstalk between nuclear receptors and toll-like receptors
Cell
(2005) - et al.
Parallel SUMOylation-dependent pathways mediate gene- and signal-specific transrepression by LXRs and PPARgamma
Mol Cell
(2007) - et al.
T0901317 is a potent PXR ligand: implications for the biology ascribed to LXR
FEBS Lett
(2007) - et al.
T0901317 is a dual LXR/FXR agonist
Mol Genet Metab
(2004)
Liver X receptors as integrators of metabolic and inflammatory signaling
J Clin Invest
Synthetic LXR ligand inhibits the development of atherosclerosis in mice
Proc Natl Acad Sci USA
Discovery of phenyl acetic acid substituted quinolines as novel liver X receptor agonists for the treatment of atherosclerosis
J Med Chem
Ligand activation of LXR{beta} reverses atherosclerosis and cellular cholesterol overload in mice lacking LXR{alpha} and apoE
J Clin Invest
Differential anti-atherosclerotic effects in the innominate artery and aortic sinus by the liver X receptor agonist T0901317
Atherosclerosis
Cited by (59)
Liver X receptor activation mitigates oxysterol-induced dysfunction in fetoplacental endothelial cells
2024, Biochimica et Biophysica Acta - Molecular and Cell Biology of LipidsOxysterols and nuclear receptors
2019, Molecular and Cellular EndocrinologyUp-regulation of COX-2 and mPGES-1 by 27-hydroxycholesterol and 4-hydroxynonenal: A crucial role in atherosclerotic plaque instability
2018, Free Radical Biology and MedicineCitation Excerpt :Of note, LXR activation induces the expression of genes different from those stimulated by endogenous oxysterols. Therefore, although LXR-activating oxysterols might reduce inflammation, they can also act by activating opposing pathways and inducing expression of inflammation markers independently of LXRs [73]. Another beneficial effect of 27-OH is due to its ability to protect human macrophages from cholesterol overload [75] since oxysterols, by binding to LXR, might act as “cholesterol sensors”, increasing the expression of target genes associated with reverse cholesterol transport [74].
25-Hydroxycholesterol activates the expression of cholesterol 25-hydroxylase in an LXR-dependent mechanism
2018, Journal of Lipid ResearchInflammation at the blood-brain barrier: The role of liver X receptors
2017, Neurobiology of Disease27-hydroxycholesterol: A novel player in molecular carcinogenesis of breast and prostate cancer
2017, Chemistry and Physics of Lipids