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, whe re it is believed to play a role in arterial pathology. We hypothesize that direct membrane effects independent of receptor binding may mediate its bi ological activity. To test this, small-angle x-ray diffraction approaches w ere used to examine the interactions of 7-keto with other membrane componen ts in well-defined lipid vesicles and in murine aortic smooth muscle cell m embranes. These data were compared with the interactions of 25-hydroxychole sterol (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 Angstrom from the bilayer center). Additionally, there was an incre ase in electron density associated with the upper acyl chain region (+/- 9- 21 Angstrom) corresponding to the bilayer location of the steroid nucleus o f 7-keto. In contrast, 25-OHC did not appear to intercalate into the membra ne hydrocarbon core and did not form separate domains. Cells grown in the p resence 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 Angstrom greater than measured with cholesterol. Domains were formed with in 4 h and persisted up to 72 h, after which cells showed signs of declinin g viability. We conclude that 7-keto is found in a membrane location distin ct from cholesterol, does not condense phospholipids as efficiently as chol esterol and is able to self-associate into discrete intrabilayer domains. W hile 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 lesion s. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.