Cholesterol distribution in plasma membranes of β1 integrin-expressing and β1 integrin-deficient fibroblasts

The effect of integrin receptors on the level and transmembrane localization of cholesterol molecules was investigated in β1 integrin-expressing (β1) and β1 integrin-deficient (β1 null) cells. We found that the content of specific raft components-cholesterol, sphingomyelin, and caveolin-was increased in integrin-expressing cells. Integrin presence affected as well the transmembrane distribution of cholesterol-a higher percent was found in the plasma membrane outer monolayer of β1 compared to β1 null cells. Sphingomyelin depletion reduced the presence of cholesterol in the outer membrane monolayer of both cell lines, but the differences in cholesterol asymmetry, observed between β1 and β1 null cells before sphingomyelinase treatment were preserved. These findings implied that integrin receptors affected the non-random transmembrane distribution of cholesterol. Finally, a higher percent of detergent-resistant membranes was obtained from β1 integrin-expressing cells, suggesting that the presence of these receptors in the membranes influenced the formation and/or stabilization of lipid raft domains.