Oxysterol mixtures prevent proapoptotic effects of 7‐ ketocholesterol in macrophages: implications for proatherogenic gene modulation

Abstract

Oxysterols are common components of oxidized low-density lipoprotein and accumulate in the core of fibrotic plaques as a mixture of cholesterol and cholesteryl ester oxidation products. The proapoptotic effects of a biologically representative mixture of oxysterols was compared with equimolar amounts of 7-ketocholesterol and unoxidized cholesterol. The oxysterol mixture in a concentration range actually detectable in hypercholesterolemic patients did not stimulate programmed cell death in cultivated murine macrophages. Unoxidized cholesterol also produced no effect. By contrast, when given alone, 7-ketocholesterol strongly stimulated the mitochondrial pathway of apoptosis with cytochrome c release, caspase-9 activation, and eventually caspase-3 activation. Subsequent experiments showed that when 7-ketocholesterol was administered to cells together with another oxysterol, namely 7betaOH-cholesterol, the strong proapoptotic effect of 7-ketocholesterol was markedly attenuated. As regards the mechanism underlying this quenching, we found that the combined oxysterol treatment counteracted the ability of 7-ketocholesterol, when administered alone, to strongly up-regulate the steady-state levels of reactive oxygen species (ROS) without interfering with sterol uptake. Furthermore, this increase in intracellular ROS appeared to be responsible for the up-regulation of proapoptotic factor, p21, after treatment with 7-ketocholesterol but not in cells challenged with the oxysterol mixture. Competition among oxysterols, apparently at the level of NADPH oxidase, diminishes the ROS induction and direct toxicity that is evoked by specific oxysterols. As a consequence, a more subtle gene modulation by oxysterols becomes facilitated in vascular cells.

Side effects of oxysterols: cytotoxicity, oxidation, inflammation, and phospholipidosis

“…Thus, when 7ß-hydroxycholesterol is combined with cholesterol-5ß,6ß-epoxide, the decrease in the glutathione level in U937 cells is significantly greater than with 7ß-hydroxycholesterol alone (48). Subsequent experiments showed that when 7-ketocholesterol was administered to cells together with another oxysterol, namely 7ß-hydroxycholesterol, the generation of ROS in J774-A1 murine macrophages was markedly attenuated (42).…”

Expression and localization of sterol 27-hydroxylase (CYP27A1) in monkey retina

“…The attenuation of 7kCh cytotoxicity by 27OH7kCh has not been previously reported although a protective effect has been previously demonstrated by co-incubation of 7kCh with other oxysterols (Biasi et al, 2004); the authors suspect that the attenuation effect is due to competition of oxysterols for NADPH oxidase (Biasi et al, 2004). Analysis of plasma and model membranes by X-ray diffraction (Phillips, et al, 2001), however, suggests that the mechanism for 7kCh toxicity may be due to crystal formation in the plasma membrane.…”

“…The brain, in contrast with the retina synthesizes essentially all of its own cholesterol and uses cholesterol 24-hydroxylase (CYP46A1) to generate 24S-hydroxycholesterol which is excreted into circulation in order to maintain cholesterol homeostasis (Björkem and Meaney, 2004; Diestschy and Turley, 2004, reviews). The brain has also been shown to uptake circulating 27OHCh (which can readily cross blood-brain barrier, Heverin, et al, 2005) where it is suspected of regulating cholesterol-sensitive genes for the purpose of maintain cholesterol homeostasis (Heverin et al, 2005).The attenuation of 7kCh cytotoxicity by 27OH7kCh has not been previously reported although a protective effect has been previously demonstrated by co-incubation of 7kCh with other oxysterols (Biasi et al, 2004); the authors suspect that the attenuation effect is due to (Biasi et al, 2004). Analysis of plasma and model membranes by X-ray diffraction (Phillips, et al, 2001), however, suggests that the mechanism for 7kCh toxicity may be due to crystal formation in the plasma membrane.…”

“…2) must be viewed with caution. The delivery of 7kCh to cells in ethanol (Biasi et al, 2004) or in 2-hydroxypropyl β-cyclodextrin (Fig. 2) is unlikely to be physiologically relevant.…”

Different apoptotic mechanisms are involved in the antiproliferative effects of 7β-hydroxysitosterol and 7β-hydroxycholesterol in human colon cancer cells

“…21,22 It was reported that they cause cellular damage in cultured macrophage-derived cell lines 23 and that hydroxysitosterol induces apoptosis of U937 cells, accompanied by a reduction of cellular glutathione. 24 Furthermore, numerous studies reported atherogenic 25,26 and cytotoxic properties [27][28][29][30] of cholesterol oxides, leading to apoptosis or necrosis in various cell types. 24,[31][32][33] They may also exert their effects by lowering cholesterol availability for cell membrane formation by inhibiting 3-hydroxy-methylglutaryl coenzyme A reductase (HMG-CoA reductase), a key enzyme in cholesterol synthesis.…”