SCP-2/SCP-x gene ablation alters lipid raft domains in primary cultured mouse hepatocytes

Barbara P. Atshaves, Avery L. McIntosh, H. Ross Payne, Adalberto M. Gallegos, Kerstin K. Landrock, Nobuyo Maeda, Ann B. Kier, and Friedhelm Schroeder

Physiology and Pharmacology, TVMC, Texas A&M University, College Station, Texas 77843

Corresponding Author: fschroeder{at}cvm.tamu.edu

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 net removal of cholesterol from the body. Lipid rafts purified from hepatocyte plasma membranes by a non-detergent affinity chromatography method were: (i) Present at 33+3% of total plasma membrane protein; (ii) Enriched in key proteins of the reverse cholesterol pathway (SR-B1, ABCA-1, P-gp, 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 ABCA-1, P-gp, and SR-B1; (ii) Increase total phospholipids, while decreasing GM1 in lipid rafts; (iii) Decrease fluidity of lipid rafts, consistent with 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 which were differentially regulated by SCP-2 expression.

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