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The Glycosphingolipid, Lactosylceramide, Regulates ß₁-Integrin Clustering and Endocytosis

Department of Biochemistry and Molecular Biology, Thoracic Diseases Research Unit, and Mayo Clinic Cancer Center, Mayo Clinic College of Medicine, Rochester, Minnesota

Requests for reprints: Richard E. Pagano, Mayo Clinic and Foundation, Stabile 8, 200 First Street, Southwest, Rochester, MN 55905-0001. Phone: 507-284-8754; Fax: 507-266-4413; E-mail: pagano.richard{at}mayo.edu.

Glycosphingolipids are known to play roles in integrin-mediated cell adhesion and migration; however, the mechanisms by which glycosphingolipids affect integrins are unknown. Here, we show that addition of the glycosphingolipid, C8-lactosylceramide (C8-LacCer), or free cholesterol to human fibroblasts at 10°C causes the formation of glycosphingolipid-enriched plasma membrane domains as shown by visualizing a fluorescent glycosphingolipid probe, BODIPY-LacCer, incorporated into the plasma membrane of living cells. Addition of C8-LacCer or cholesterol to cells initiated the clustering of ß₁-integrins within these glycosphingolipid-enriched domains and the activation of the ß₁-integrins as assessed using a HUTS antibody that only binds activated integrin. On warming to 37°C, ß₁-integrins were rapidly internalized via caveolar endocytosis in cells treated with C8-LacCer or cholesterol, whereas little ß₁-integrin was endocytosed in untreated fibroblasts. Incubation of cells with C8-LacCer or cholesterol followed by warm-up caused src activation, a reorganization of the actin cytoskeleton, translocation of RhoA GTPase away from the plasma membrane as visualized using total internal reflection fluorescence microscopy, and transient cell detachment. These studies show that LacCer can regulate integrin function both by modulating integrin clustering in microdomains and by regulating integrin endocytosis via caveolae. Our findings suggest the possibility that aberrant levels of glycosphingolipids found in cancer cells may influence cell attachment events by direct effects on integrin clustering and internalization.

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