Comparison of the cytotoxic, pro-oxidant and pro-inflammatory characteristics of different oxysterols

Abstract

Oxidized low-density lipoproteins play important roles in the development of atherosclerosis and contain several lipid-derived, bioactive molecules which are believed to contribute to atherogenesis. Of these, some cholesterol oxidation products, referred to as oxysterols, are suspected to favor the formation of atherosclerotic plaques involving cytotoxic, pro-oxidant and pro-inflammatory processes. Ten commonly occurring oxysterols (7alpha-, 7beta-hydroxycholesterol, 7-ketocholesterol, 19-hydroxycholesterol, cholesterol-5alpha,6alpha-epoxide, cholesterol-5beta,6beta-epoxide, 22R-, 22S-, 25-, and 27-hydroxycholesterol) were studied for both their cytotoxicity and their ability to induce superoxide anion production (O2*-) and IL-8 secretion in U937 human promonocytic leukemia cells. Cytotoxic effects (phosphatidylserine externalization, loss of mitochondrial potential, increased permeability to propidium iodide, and occurrence of cells with swollen, fragmented and/or condensed nuclei) were only identified with 7beta-hydroxycholesterol, 7-ketocholesterol and cholesterol-5beta,6beta-epoxide, which also induce lysosomal destabilization associated or not associated with the formation of monodansylcadaverine-positive cytoplasmic structures. No relationship between oxysterol-induced cytotoxicity and HMG-CoA reductase activity was found. In addition, the highest O2*- overproduction quantified with hydroethidine was identified with 7beta-hydroxycholesterol, 7-ketocholesterol and cholesterol-5beta,6beta-epoxide, with cholesterol-5alpha, 6alpha-epoxide and 25-hydroxycholesterol. The highest capacity to simultaneously stimulate IL-8 secretion (quantified by ELISA and by using a multiplexed, particle-based flow cytometric assay) and enhance IL-8 mRNA levels (determined by RT-PCR) was observed with 7beta-hydroxycholesterol and 25-hydroxycholesterol. None of the effects observed for the oxysterols were detected for cholesterol. Therefore, oxysterols may have cytotoxic, oxidative, and/or inflammatory effects, or none whatsoever.

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

“…Generally, 7-ketocholesterol, 7ß-hydroxycholesterol, and 27-hydroxycholesterol induce apoptosis on various cell types, whereas 25-hydroxycholesterol has a more or less pronounced cytotoxicity depending on the cell considered (15,26). Moreover, the ability of 7-ketocholesterol to induce an apoptotic mode of cell death associated with autophagic characteristics in U937 cells and in human aortic smooth muscle cells has been reported based on increased fluorescence staining with monodansylcadaverine in dying cells (27) and on the analysis of the two forms of the LC3 protein (microtubule-associated protein 1 light chain 3) (28), respectively.…”

“…In vitro, under treatment with 7-ketocholesterol, 7ß-hydroxycholesterol, cholesterol-5ß,6ß-epoxide, cholesterol-5α,6α-epoxide, and 25-hydroxycholesterol, important oxidative processes, sometimes associated with a complex mode of cell death with some of the characteristics of apoptosis, have also been described on various cell types (15,27,32). Thus, on 7-ketocholesteroltreated U937 cells, an abrupt drop in reduced glutathione associated with an increased production of ROS preceding caspase activation has been reported (46).…”

“…When human monocytes, monocyte-derived macrophages (THP-1 cells), and porcine retinal pigment epithelial cells were exposed to a series of different oxysterols (7-ketocholesterol, 7ß-hydroxycholesterol, 24-hydroxycholesterol, 25-hydroxycholesterol, or cholestane-3ß,5α,6ß-triol), all of them had a tendency to stimulate IL-8 production, but 25-hydroxycholesterol was the most potent (15). It is worth noting that in human mononuclear cells isolated by Ficoll-Paque centrifugation, the pro-inflammatory activity of 25-hydroxycholesterol was enhanced in hypoxic conditions, and this oxysterol was also capable of potentiating LPS-induced IL-1ß secretion (51).…”

“…Indeed, 7-ketocholesterol and 7ß-hydroxycholesterol, which are major components of oxidized LDL (oxLDL), are known to contribute to the genesis of atherosclerosis (14), and are found at increased levels in atherosclerotic lesions, and in the plasma of patients with cardiovascular diseases and of subjects after a fat-rich meal (1,5,14). A number of investigations have also demonstrated that oxysterols can have cytotoxic and pro-inflammatory activities (15), and some studies have shown that these molecules can stimulate the differentiation of mesenchymal cells, monocytes, keratinocytes, lens epithelial cells, and osteoblasts (16). Therefore, in addition to atherosclerosis, it is now suggested that oxysterols might contribute to the development of numerous other degenerative diseases such as age-related macular degeneration (17), Alzheimer's disease (18), and osteoporosis (19).…”

Cholesterol, Reactive Oxygen Species, and the Formation of Biologically Active Mediators

“…Numerous studies have revealed that 7␤-OH-Ch, 7-oxo-Ch, and 5␤,6␤-epoxy-Ch are cytotoxic at relatively low concentrations (25 nM), most likely by inducing cellular apoptosis (40). The possible mechanism for induction of apoptosis in cells like smooth muscle cells may be related to the altered biophysical properties of membranes within the cells such as the ER (31).…”