Isolation of plasma membranes from the bovine retinal pigment epithelium

Tamoxifen induces changes in the lipid composition of the retinal pigment epithelium cell line D407

Tamoxifen, the antioestrogenic drug prescribed for long-term, low-dose therapy of breast cancer, induces retinopathy. This study evaluates the effects of tamoxifen on the human retinal pigment epithelial cell line D407, attempting to identify the underlying mechanisms on tamoxifen-induced retinopathy and the involvement of cellular membranes in the cytotoxic action mechanism. We demonstrate that the tamoxifen-induced decrease in the cell growth of the D407 cell line results from pyknosis and cell cycle arrest rather than from necrosis. Furthermore, D407 cells influence the lipid composition of both plasma membrane and intracellular membranes in response to tamoxifen. Tamoxifen increases the physical order of the lipid bilayer. We observed a compensatory decrease in the cholesterol content of the plasma membrane which results in an increase of the plasma membrane fluidity. In intracellular membranes the phosphatidylcholine content is reduced to 50% of the controls. This reduction may be related to the formation of a second messenger via phospholipase pathway and sustained activation of protein kinase C. Since increased plasma membrane fluidity as well as sustained activation of protein kinase C influence the rod outer segments binding and/or ingestion by retinal pigment epithelial cells, our results suggest that membrane-mediated pathways contribute to the tamoxifen-induced retinopathy.

Carotenoid incorporation into natural membranes from artificial carriers: liposomes and beta-cyclodextrins

Liposomes and beta-cyclodextrin (beta-CD) have been used as carriers for the incorporation of three dietary carotenoids (beta-carotene (BC), lutein (LUT) and canthaxanthin (CTX)) into plasma, mitochondrial, microsomal and nuclear membrane fractions from pig liver cells or the retinal epithelial cell line D407. The uptake dynamics of the carotenoids from the carriers to the organelle membranes and their incorporation yield (IY) was followed by incubations at pH 7.4 for up to 3 h. The mean IYs saturated between 0.1 and 0.9 after 10-30 min of incubation, depending on membrane characteristics (cholesterol to phospholipid ratio) and carotenoid specificity. Mitochondrial membranes (more fluid) favour the incorporation of BC (non-polar), while plasma membranes (more rigid) facilitate the incorporation of lutein, the most polar carotenoid. A high susceptibility of BC to degradation in the microsomal suspension was observed by parallel incubations with/without 2,6-di-t-buthyl-p-cresol (BHT) as antioxidant additive. The beta-CD carrier showed to be more effective for the incorporation of lutein while BC was incorporated equally into natural membranes either from liposomes or from cyclodextrins. The presence of cytosol in the incubation mixture had no significant effects on the carotenoid incorporations.

Distribution of 11-cis LRAT, 11-cis RD and 11-cis REH in bovine retinal pigment epithelium membranes

Our recent finding of the co-localization of 11-cis retinyl esters and 11-cis retinyl ester hydrolase (11-cis REH) activity in bovine retinal pigment epithelium (RPE) plasma membrane (PM) has led us to explore the possibility that the PM may provide 11-cis retinal for rhodopsin regeneration. In the RPE, visual chromophore is synthesized via a membrane associated 11-cis retinol dehydrogenase (11-cis RD). Accordingly, bovine RPE membranes enriched with either endoplasmic reticulum (ER) or plasma membrane (PM) enzyme markers were prepared and assayed for visual cycle enzyme activities. Pronounced 11-cis RD activity was associated with both ER- and PM-enriched membrane fractions. In contrast, 11-cis REH activity was mostly recovered in PM-enriched fractions while LRAT activity was found only in ER-enriched membranes. The finding that both 11-cis retinol and 11-cis retinal can be produced at the PM of the bovine RPE strongly suggests that 11-cis retinyl esters at this subcellular locale serve as a precursor of visual chromophore for pigment regeneration.

Lipids of frog retinal pigment epithelium: comparison with rod outer segments, retina, plasma and red blood cells

The glycerolipid and fatty acid compositions of frog retinal pigment epithelium (RPE) were determined and compared with rod outer segments (ROS), retina, plasma, and red blood cells (RBC). The glycerolipid class composition of RPE was similar to RBC and ROS or retina, with phosphatidylcholine and phosphatidylethanolamine being the major components. The fatty acid composition of RPE differed substantially from that of plasma or RBC; the former contained much higher levels of C-20 and C-22 polyunsaturated fatty acids (PUFAs), such as 20:4n-6 and 22:6n-3, but less C-18 mono-, dienoic, and trienoic acids. The difference between RPE and ROS or retina with respect to fatty acid profile was also dramatic; RPE had relatively less 22:6n-3, but more 20:4n-6 and 18:2n-6, than ROS or retina. These results suggest that frog RPE cells may selectively take up C-20 and C-22 PUFAs from the circulation, but preferentially deliver 22:6n-3 to the ROS and retina. Fatty acid analyses show that 20:4n-6 and 22:6n-3 were unevenly distributed among RPE glycerolipids; phosphatidic acid, diglyceride, triglyceride, and phosphatidylserine are relatively more enriched in 22:6n-3 compared with 20:4n-6. This information might imply that these two PUFAs are metabolized differently inside the frog RPE cells.

The occurrence of glutathione-insulin transhydrogenase (protein-disulfide interchange enzyme) in the lens

Glutathione-insulin transhydrogenase (GIT, thiol:protein-disulfide isomerase/oxidoreductase, E.C. 5.3.4.1/1.8.4.2) catalyzes via sulfhydryldisulfide interchange, the scission as well as formation of disulfide bonds in many diverse proteins. Using insulin as a substrate, the lens epithelial layer of cows, rats and rabbits was found to contain GIT activity. The enzyme's activity is activated by GSH and inhibited by N-ethylmaleimide. Subcellular distribution of bovine lens epithelial homogenates showed that the majority of GIT activity is located in the insoluble fraction (10,000 g pellet) and in the high molecular weight fraction (60,000 g pellet). Lens epithelial extracts were subjected to SDS-PAGE followed by Western blot, and probed with a polyclonal antibody to rat liver GIT, or with either of two monoclonal antibodies directed against different epitopes of the enzyme. Lens epithelium was found to contain two forms of GIT, one with the same molecular weight as the purified enzyme (Mr 56Kd), and a second having an Mr of 67Kd. Immunoblots using polyclonal antibodies revealed an additional major immunoreactive band of 32Kd in the cow lens epithelial layer as well as in the isolated cortical and nuclear portions. Rat lenses showed no immunoreactive 32Kd band. Using a bovine cortical/nuclear fraction the 32Kd reactivity was found to be associated with the beta H-crystallin fraction, but the extract failed to show GIT activity with the insulin substrate. This suggests that beta H-crystallin may share a common epitope with GIT.

Retinal pigment epithelial cell plasma membrane: a monoclonal antibody study

The plasma membranes of retinal pigment epithelial cells are highly specialized organelles with multiple functions including nutritional and metabolic support of the photoreceptor cells. Using purified bovine retinal epithelial cell plasma membranes as antigen, we produced two monoclonal antibodies, MAbD1-C6 and MAbD1-C8, that cross react with the plasma membranes from bovine, rat and human retinal pigment epithelial cells. In radioimmunoassay both MAbD1-C6 and MAbD1-C8 had similar affinities for bovine plasma membranes. Both monoclonal antibodies identified a protein of 72 Kd with an apparent subunit of 32-35 Kd. The protein was localized to the cell surface of human and bovine retinal pigment epithelium by immunocytohistochemistry. In the normal eye the antigen identified by the monoclonal antibodies was strongly associated with the retinal pigment epithelium and weakly associated with lens tissue. Using either monoclonal antibody, components of purified bovine or rat retinal pigment epithelial plasma membranes were precipitated from solution. Based on these results, we conclude that both monoclonal antibodies are closely related and that they may be useful for the isolation and study of retinal pigment epithelial cell structure.

Effect of light history on rod outer-segment membrane composition in the rat

Albino rats were born and raised through 12 weeks of age in 12L:12D regimes of 5, 300- or 800-lx illuminance. Upon killing, the animals' retinas were examined for the following: (1) rhodopsin of whole retina and isolated rod outer-segment membrane; (2) retinal morphology, including outer segment length and outer nuclear layer area; and (3) outer-segment membrane lipid biochemistry. The three groups of animals show significant differences with respect to one another for nearly every parameter measured. Rod outer-segment membranes of rats raised in dim cyclic light (5 lx) have high rhodopsin packing densities, high levels of polyunsaturated fatty acids, and low cholesterol levels in comparison with animals raised in brighter illuminances (300- or 800 lx). The mole ratio of phospholipid to rhodopsin in the outer-segment membrane of rats raised in 5-lx cyclic light is only 43% of that of rats raised in 800-lx cyclic light. The difference between these two groups of animals for docosahexaenoic acid is greater than three times, with dim light-reared animals showing higher levels. These rats (5 lx-reared) have less cholesterol in their photoreceptor outer segments, 6.6 mol% compared with 19.7 mol% for animals from the 800-lx regime. In all cases, rats from the intermediate rearing illuminance (300 lx) exhibit intermediate membrane composition values. It is likely that these differences in membrane composition illustrate a profound effect of light history on photoreceptor function.

Surface modification of retinal pigment epithelial cells: effects on phagocytosis and glycoprotein composition

Proteases have been used as a tool to investigate the role of cell-surface molecules of cultured retinal pigment epithelial cells (RPE) in the phagocytosis of rod outer segments (ROS). Proteolytic digestion of RPE cells by pronase, thermolysin and Staphylococcus aureus V8 protease (V8 protease) inhibited the phagocytosis of ROS without affecting the viability of the RPE cells. A particular feature of RPE cell proteolysis was that those macromolecules responsible for ROS ingestion were susceptible, while those macromolecules that mediated ROS binding were resistant to cleavage by all three proteases. By taking advantage of this phenomenon, ROS were used as affinity particles to obtain a plasma membrane-enriched fraction of RPE cells before and after proteolytic digestion. All three proteases partially or completely removed several glycoproteins from the cell surfaces. Removal of these glycoproteins was correlated with a loss in phagocytic ability by RPE cells. Two high-molecular-weight (MW) glycoproteins of MWs 160,000 and 214,000 were consistently removed by all proteases tested. Protease-treated RPE cells restored their phagocytic capabilities and normal glycoprotein composition within 24 hr after proteolytic treatment. These data suggest that glycoproteins located on the surfaces of RPE cells may be involved in mediating the phagocytosis of ROS by these cells.

Isolation and characterization of plasma membrane proteins of cultured human retinal pigment epithelium

Two-dimensional gel electrophoresis (2-D) was used to resolve the plasma membrane proteins from cultured human retinal pigment epithelial cells. The cells were metabolically labeled either with [35S]methionine to reveal proteins in general or with [3H]glucosamine or [3H]fucose which are more specific for glycoprotein visualization. The cell surface proteins were also iodinated, using the lactoperoxidase--glucose oxidase technique. These labeled membranes were separate into plasma membrane-enriched fractions by subjecting the water-shocked postnuclear supernatant to a discontinuous sucrose-density gradient. The five resulting membrane fractions were assayed for protein, RNA (microsomes), galactosyltransferase (Golgi membranes), 5'-nucleotidase (plasma membranes), and succinate dehydrogenase (mitochondrial membranes) and were examined by electron microscopy. The plasma membranes were enriched with minimal contamination at the 0.6-0.85 M (F2) and 0.85-1.0 M (F3) sucrose interfaces based on these biochemical and morphological criteria. Examination of 2-D autoradiographic profiles of F2 and F3 showed that approximately 180 proteins or protein subunits had incorporated [35S]methionine. Certain proteins were also labeled by [3H]glucosamine and [3H]fucose, and surface-labeled by iodination. This was especially true of 17 different high-molecular-weight (43-139 X 10(3) MW) very acidic glycoproteins which formed a constellation of spots. These glycoproteins, as well as others, were also seen in the whole-cell acidic glucosamine-labeled 2-D profiles, where about 150 proteins were detected. A total of 39 proteins were catalogued, of which 34 were detectable in the plasma membrane-enriched fractions. The results show that the use of subcellular fractionation, specific precursors, and labeling techniques aids in the detection and characterization of minor proteins in 2-D gels.

Lipids of the retinal pigment epithelium in RCS dystrophic and normal rats

The lipid composition of retinal pigment epithelial cells was determined for normal cells which have full phagocytic ability and for a genetic variant with impaired phagocytic function. Retinal pigment epithelial cells from 9-14-day-old congenic strains of normal (RCS-rdy+) and dystrophic (RCS-rdy/rdy) rats were separated from intact retinas and homogenized in 0.08 M Tris base, pH 7.4. The lipids were extracted using 2:1 chloroform--methanol. Fatty-acid methyl esters identified by gas chromatography were: 16:0, 17:0, 18:0, 18:1, 18:2 omega 6, 20:0, 20:2, 22:0, 20:4 omega 6, 22:4, 22:5, 22:6 omega 3. Major fatty acids for both normal and dystrophic cells were: 16:0, 18:0, 20:4 omega 6, 22:6 omega 3. One- and two-dimensional thin-layer chromatography was used to determine phospholipid composition of pigment epithelial cells at two different age groups. The relative amount of phosphatidylethanolamine was significantly higher in dystrophic RPE cells compared with normal cells (20.7% for 9-11-day-old and 17.3% for 12-14-day-old dystrophic rats). Cells from normal animals contained a higher level of phosphatidylethanolamine in the older age group whereas RPE cells from dystrophic animals contained a lower level of phosphatidylcholine in the older group. Anomalous phospholipid composition of dystrophic pigment epithelial cells may be associated with a change in cellular membranes and a defect in the cellular processes involved in phagocytic function.