Activation of acyl-CoA: cholesterol acyltransferase in rat liver microsomes by 25-hydroxycholesterol

Activation of LXRs using the synthetic agonist GW3965 represses the production of pro-inflammatory cytokines by murine mast cells

BACKGROUND

The activation of liver X receptor (LXR) α or LXRβ negatively regulates the expression of pro-inflammatory genes in mammalian cells. We recently reported that 25-hydroxycholesterol, a representative LXR-activating oxysterol, suppresses IL-6 production in mouse mast cells (MCs) following its engagement of the high-affinity IgE receptor (FcεRI). This finding suggests that murine MCs express functional LXRs; however, the mechanisms underlying the LXR-dependent repression of the MC-mediated production of pro-inflammatory cytokines, including IL-6, are poorly understood. Therefore, we employed the synthetic LXR ligand GW3965 to examine the functions of LXRα and LXRβ in the production of pro-inflammatory cytokines by murine bone marrow-derived MCs (BMMCs).

METHODS

We prepared BMMCs from wild-type (WT), LXRα(-/-), and LXRα/β(-/-) mice. Each group of BMMCs was pretreated with GW3965 and then stimulated with IgE+antigen (Ag) or lipopolysaccharide (LPS). Cytokine production was then analyzed using specific ELISA kits.

RESULTS

The activation of LXRs by GW3965 significantly attenuated the production of IL-1α and IL-1β, but not of IL-6, in the WT and LXRα(-/-) BMMCs stimulated with IgE+Ag. However, GW3965 treatment decreased the production of IL-1α, IL-1β, and IL-6 in WT and LXRα(-/-) BMMCs upon stimulation with LPS, while the GW3965-mediated suppression of cytokine production was nearly absent from the LXRα/β(-/-) BMMCs.

CONCLUSIONS

These findings demonstrate, for the first time, that the activation of LXRs by GW3965 attenuates the antigen- or LPS-induced production of pro-inflammatory cytokines, such as IL-1α and IL-1β, in murine MCs and that LXRβ plays an important role in the LXR-mediated repression of cytokine production.

Oxysterols in cancer cell proliferation and death

Oxysterols have been shown to interfere with proliferation and cause the death of many cancer cell types, such as leukaemia, glioblastoma, colon, breast and prostate cancer cells, while they have little or no effect on senescent cells. The mechanisms by which oxysterols may influence proliferation are manifold: they control the transcription and the turnover of the key enzyme in cholesterol synthesis, 3-hydroxy-3-methylglutaryl CoA reductase, by binding to Insig-1, Insig-2 and liver X receptors. Oxysterols are thought to be generated in proportion to the rate of cholesterol synthesis. Although there is no consensus about the mechanism by which these oxysterols are generated in vivo, it clearly has to be ubiquitous. The 25- and the 27-cholesterol hydroxylases, present in almost all tissues, are possible candidates. Cholesterol uptake from lipoproteins, intracellular vesicle transport and lipid transfer are also modified by oxysterols. Oxysterols interfere with ERK, hedgehog and wnt pathways of proliferation and differentiation. When administered in vitro to cancer cell lines, oxysterols invariably both slow down proliferation and provoke cell death. Perhaps is it sufficient to stop proliferation of a cancer to provoke its eradication. Therefore, the two facets of oxysterol action that seem important for cancer treatment, cytostaticity and cytotoxicity, will be discussed.

Liver X receptors and immune regulation

Recent studies suggest that homeostasis of lipid metabolism is crucial for the function of various immune cells. Oxygenated derivatives of cholesterol (oxysterols) are well-known regulators of lipid metabolism and have diverse functions, such as inhibition of cholesterol synthesis, efflux of intracellular cholesterol, synthesis of cholesterol esters, and activation of liver X receptors (LXRs). In this review, we introduce novel roles of the oxysterol receptors LXRs in the immune system, including regulation of inflammatory responses, T cell expansion, immunoglobulin production, and antitumor responses. We also discuss lipid-mediated signaling as a potential target for treatment of immune diseases.

Genetic background of cholesterol gallstone disease

Cholesterol gallstone formation is a multifactorial process involving a multitude of metabolic pathways. The primary pathogenic factor is hypersecretion of free cholesterol into bile. For people living in the Western Hemisphere, this is almost a normal condition, certainly in the elderly, which explains the very high incidence of gallstone disease. It is probably because the multifactorial background genes responsible for the high incidence have not yet been identified, despite the fact that genetic factors clearly play a role. Analysis of the many pathways involved in biliary cholesterol secretion reveals many potential candidates and considering the progress in unraveling the regulatory mechanisms of the responsible genes, identification of the primary gallstone genes will be successful in the near future.

7-Ketocholesterol forms crystalline domains in model membranes and murine aortic smooth muscle cells

7-Ketocholesterol (7-keto) is one of the major oxygenated products found in oxidized low-density lipoproteins (LDL) and in atherosclerotic plaque, where it is believed to play a role in arterial pathology. We hypothesize that direct membrane effects independent of receptor binding may mediate its biological activity. To test this, small-angle x-ray diffraction approaches were used to examine the interactions of 7-keto with other membrane components in well-defined lipid vesicles and in murine aortic smooth muscle cell membranes. These data were compared with the interactions of 25-hydroxycholesterol (25-OHC) and cholesterol. Replacement of cholesterol with 7-keto in lipid vesicles produced distinct changes in membrane structure, including a marked increase in molecular volume associated with the hydrocarbon core (+/-0-8 A from the bilayer center). Additionally, there was an increase in electron density associated with the upper acyl chain region (+/-9-21 A), corresponding to the bilayer location of the steroid nucleus of 7-keto. In contrast, 25-OHC did not appear to intercalate into the membrane hydrocarbon core and did not form separate domains. Cells grown in the presence of the 7-keto developed extracellular crystals concomitant with the formation of membrane domains having a unit cell periodicity of 35.4 or 1.4 A greater than measured with cholesterol. Domains were formed within 4 h and persisted up to 72 h, after which cells showed signs of declining viability. We conclude that 7-keto is found in a membrane location distinct from cholesterol, does not condense phospholipids as efficiently as cholesterol and is able to self-associate into discrete intrabilayer domains. While these domains may decrease its cytotoxicity by inducing the formation of sterol crystals in smooth muscle cells, they may, in a broader capacity, contribute to the sterol crystals found in advanced atherosclerotic lesions.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.