Although reverse cholesterol transport from peripheral cell types is mediated through plasma membrane microdomains termed lipid rafts, almost nothing is known regarding the existence, protein/lipid composition, or structure of these putative domains in liver hepatocytes, cells responsible for the net removal of cholesterol from the body. Lipid rafts purified from hepatocyte plasma membranes by a nondetergent affinity chromatography method were: i) present at 33 +/- 3% of total plasma membrane protein; ii) enriched in key proteins of the reverse cholesterol pathway [scavenger receptor class B type I (SR-B1), ABCA1, P-glycoprotein (P-gp), sterol carrier protein-2 (SCP-2)]; iii) devoid of caveolin-1; iv) enriched in cholesterol, sphingomyelin, GM1, and phospholipids low in polyunsaturated fatty acid and double bond index; and v) exhibited an intermediate liquid-ordered lipid phase with significant transbilayer fluidity gradient. Ablation of the gene encoding SCP-2 significantly altered lipid rafts to: i) increase the proportion of lipid rafts present, thereby increasing raft total content of ABCA1, P-gp, and SR-B1; ii) increase total phospholipids while decreasing GM1 in lipid rafts; iii) decrease the fluidity of lipid rafts, consistent with the increased intermediate liquid-ordered phase; and iv) abolish the lipid raft transbilayer fluidity gradient. Thus, despite the absence of caveolin-1 in liver hepatocytes, lipid rafts represented nearly one-third of the mouse hepatocyte plasma membrane proteins and displayed unique protein, lipid, and biophysical properties that were differentially regulated by SCP-2 expression.
