Fas signaling induces raft coalescence that is blocked by cholesterol depletion in human RPE cells undergoing apoptosis.

PURPOSE

To investigate whether the signaling events occurring in Fas-mediated apoptosis alter raft membrane formation in human RPE cells.

METHODS

Formation of lipid rafts in cultured human retinal pigment epithelial cells (ARPE-19) was studied by confocal microscopy, with fluorescein-labeled cholera toxin subunit B binding protein (BODIPY)-labeled ganglioside GM1 lipid after Fas-L induction of apoptosis. Apoptosis was assessed by fluorescein-labeled annexin V detection of phosphatidylserine externalization and quadrant analysis with flow cytometry. Membrane rafts were localized into membrane vesicles by passing BODIPY-labeled GM1 RPE cells through a 2-microm-pore polycarbonate membrane using an extruder device. The labeled fractions, containing vesicles enriched in GM1, were detected by flow cytometry and then analyzed for the presence of Fas protein.

RESULTS

Differential punctate staining of membrane rafts was demonstrated in normal and FasL-induced apoptotic human ARPE-19 cells in culture by confocal microscopy, using cholera toxin B and GM1 labeling of extruded vesicles. The lipid raft-associated vesicles were derived by plasma membrane dissociation, via a newly developed whole-cell extrusion technique that produced 2-microm vesicles with both GM1 lipid and Fas protein abundance enriched in a subpopulation of the membrane-derived vesicles. The amount of Fas protein in the vesicles containing raft domains markedly increased in FasL-treated cells. Treatment of human ARPE 19 cells with methyl beta-cyclodextrin after FasL induction of apoptosis resulted in cellular cholesterol depletion and markedly reduced the incidence of Fas-receptor localization in GM1 rafts.

CONCLUSIONS

Human ARPE-19 cells in culture contain membrane rafts with apoptotic signaling effectors uniformly distributed in the native state. The cells stimulated to undergo apoptosis appear to use membrane rafts in the death-signaling process by mobilization of rafts to localized regions of the membrane that are now enriched with apoptotic signaling effectors. Fas signaling induces apoptotic raft formation that results in polar condensation, or capping, of the rafts in the late stages of apoptosis. A novel extrusion technique is described that allows localization and enrichment of rafts into membrane vesicles, which can be assayed by flow cytometry. Cholesterol depletion, after Fas ligand activation of apoptosis, reduced raft formation in cells induced to undergo apoptosis. Therapeutic implications for the treatment of retinal disorders are discussed.