Oxysterols and related sterols. Chemical, biochemical and biological aspects

Cholesterol and its congeners phytosterols – which are widely present in animals, plants and/or micro-organisms, are parent compounds of oxygenated forms namely oxysterols and oxyphytosterols that are produced either from auto-oxidation or enzymatically [1,2]. Depending on the concentration, these molecules have potential beneficial or detrimental effects at cellular level. Experiments on various cell types from different species have revealed that oxysterols are cytotoxic at relatively high concentrations. They are important in several diseases, including cardiovascular diseases and neurodegenerative diseases, osteoporosis, and some forms of cancer [3–6]. In these diseases, oxysterols are often found at increased concentrations in the tissues and in various biological fluids of patients [7]. Moreover, whatever the cell type considered, several oxysterols are able to induce oxidative stress, inflammation and/or to trigger different forms of cell death (apoptosis, necrosis, necroptosis, autophagy) [1,8]. However, it is now emerging that low concentrations of oxysterols have specific biological activities [9]. It is expected that a better knowledge on oxysterols effects on cell membranes, cytoplasmic and nuclear receptors capable to modulate gene expression, will contribute to identify new pharmacological targets. Oxyphytosterols, which are mainly C28 and C29 carbon steroid alcohols with a methyl or ethyl chain at C24, are increasingly recognized for their biological activity that could eventually counteract the, putative, beneficial effects of phytosterols given as cholesterol-lowering food supplements.