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, where it is believed to play a role in arterial pathology. We hypothesize that direct membrane effects independent of receptor binding may mediate its biological activity. To test this, small-angle x-ray diffraction approaches were used to examine the interactions of 7-keto with other membrane components in well-defined lipid vesicles and in murine aortic smooth muscle cell membranes. These data were compared with the interactions of 25-hydroxycholesterol (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 A from the bilayer center). Additionally, there was an increase in electron density associated with the upper acyl chain region (+/-9-21 A), corresponding to the bilayer location of the steroid nucleus of 7-keto. In contrast, 25-OHC did not appear to intercalate into the membrane hydrocarbon core and did not form separate domains. Cells grown in the presence 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 A greater than measured with cholesterol. Domains were formed within 4 h and persisted up to 72 h, after which cells showed signs of declining viability. We conclude that 7-keto is found in a membrane location distinct from cholesterol, does not condense phospholipids as efficiently as cholesterol and is able to self-associate into discrete intrabilayer domains. While 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 lesions.
Phillips
J E
JE
Membrane Biophysics Laboratory, Department of Medicine, Division of Cardiology, MCP Hahnemann University, Allegheny Campus, 320 East North Avenue, Pittsburgh, PA 15212-4772, USA.
Geng
Y J
YJ
Mason
R P
RP
D000818:Animals; D001011:Aorta / Q000378:metabolism*; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism; D003460:Crystallization; D006888:Hydroxycholesterols / Q000378:metabolism; D007653:Ketocholesterols / Q000378:metabolism*; D008567:Membranes, Artificial; D051379:Mice; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D014961:X-Ray Diffraction
D006888:Hydroxycholesterols; D007653:Ketocholesterols; D008567:Membranes, Artificial; C007997:25-hydroxycholesterol; D002784:Cholesterol; C003001:7-ketocholesterol