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

Abstract

Ch 2The abbreviations used are: Ch, cholesterol(s); ROS, reactive oxygen species; LDL, low density lipoprotein; DM, diabetes mellitus; ER, endoplasmic reticulum; SREBP, sterol regulatory element-binding protein. is a major lipid component in eukaryotic cells and can attain levels as high as 50% (on a molar basis) of all lipids present in the plasma membrane of certain cells (1Koumanov K.S. Tessier C. Momchilova A.B. Rainteau D. Wolf C. Quinn P.J. Arch. Biochem. Biophys. 2005; 434: 150-158Crossref PubMed Scopus (79) Google Scholar). The pathways responsible for Ch accumulation at this site are a consequence of gene regulation largely worked out by Brown and Goldstein (2Brown M.S. Goldstein J.L. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 11041-11048Crossref PubMed Scopus (1110) Google Scholar). The very high concentration of this unique and singular lipid at the plasma membrane of cells places it in a position to encounter ROS that cross this permeability barrier. Ch can be oxidized in membranes even when polyunsaturated fatty acyl groups are not. Also, some unique products can serve as markers of certain ROS. The structure of Ch makes it susceptible to a variety of radical as well as nonradical oxidation reactions due to the 5,6-double bond and the concomitant vinylic methylene group at C-7 in the B ring. The isooctyl side chain at C-17 is a site for enzymatic oxidation largely by P450 cytochromes to form a series of biochemically active oxysterols, but this site of oxidation is typically not a target for ROS relevant to cellular biochemistry (3Smith, L. L. (1981) Cholesterol Autoxidation, Plenum Press, New YorkGoogle Scholar). Oxysterols are clearly formed in cells, but considerable evidence exists for a dietary origin of some oxysterols transported to cells by chylomicrons (4Vine D.F. Croft K.D. Beilin L.J. Mamo J.C. Lipids. 1997; 32: 887-893Crossref PubMed Scopus (62) Google Scholar). Various mechanisms, including heating of foods in air, can lead to formation of oxysterols that can be ingested. Several excellent reviews by Smith (3Smith, L. L. (1981) Cholesterol Autoxidation, Plenum Press, New YorkGoogle Scholar, 5Smith L.L. Chem. Phys. Lipids. 1987; 44: 87-125Crossref PubMed Scopus (283) Google Scholar) have covered this area, and although somewhat older literature, they are a wealth of information about Ch autoxidation. The free radical-based oxidation of Ch has been the focus of several biochemical studies (6Girotti A.W. J. Lipid Res. 1998; 39: 1529-1542Abstract Full Text Full Text PDF PubMed Google Scholar). The fairly complex mixture of oxysterols that result from free radical-mediated Ch oxidation confounds insight into the exact mechanism of formation of individual oxysterols. This is due in part to the random character of free radical oxidation as well as subsequent rearrangement of reactive intermediates. Four major products are typically observed, 7α,β-OOH-Ch, 7α,β-OH-Ch, 7-ox-Ch, and 5,6-epoxy-Ch (abbreviations and nomenclature are listed in supplemental Table 1), but many minor products, including A ring-oxidized species, are known (3Smith, L. L. (1981) Cholesterol Autoxidation, Plenum Press, New YorkGoogle Scholar, 5Smith L.L. Chem. Phys. Lipids. 1987; 44: 87-125Crossref PubMed Scopus (283) Google Scholar). The generation of highly reactive hydroxyl radicals (OH•) by various mechanisms, including H2O2 reduction by superoxide anion catalyzed by iron(II) (Fenton reaction), peroxynitrite (ONOO–) following the reaction of nitric oxide with superoxide anion, or ionizing radiation, can all eventually lead to abstraction of a C-7 allylic hydrogen atom, forming a carbon-centered radical at this site. The focus of these free radical oxidations of Ch is at C-7 because the carbon–hydrogen bond is sufficiently weak at this position (bond dissociation energy of 88 kcal/mol (7Gardner H.W. Free Radic. Biol. Med. 1989; 7: 65-86Crossref PubMed Scopus (586) Google Scholar)) that longer lived lipid peroxy radical or alkoxyl radicals derived from polyunsaturated fatty acyl lipid peroxyl radicals (LOO•) can readily abstract this hydrogen atom. The C-7-centered radical intermediate of Ch has a sufficiently long half-life to encounter a ground state molecular oxygen molecule, forming a hydroperoxy radical. This means that the propagation of free radical chemistry through the lipid bilayer, which might be initiated at some point by hydroxyl radical, can lead to the observed peroxidation of Ch. This hydroperoxy radical intermediate can abstract certain hydrogen atoms and thus propagate the free radical reaction, forming (after hydrogen abstraction) chemically stable 7α- and 7β-OOH-Ch (Fig. 1). Interestingly, recent evidence has shown that Ch hydroperoxides move much more efficiently between various cellular compartments compared with Ch itself (8Vila A. Levchenko V.V. Korytowski W. Girotti A.W. Biochemistry. 2004; 43: 12592-12605Crossref PubMed Scopus (42) Google Scholar) and thus become available for many sites with