Effects of the antioxidants dithiothreitol and vitamin E on phospholipid metabolism in isolated rod outer segments

Antioxidant activity of retinol, glutathione, and taurine in bovine photoreceptor cell membranes

The antioxidant activities of compounds endogenous to mammalian rod outer segments (ROS) were investigated in vitro by measuring the oxidative loss of polyunsaturated fatty acids (PUFA's) from the membranes of intact ROS and from liposomes made from ROS phospholipids (PL) to which lipid soluble compounds had been added. The membranes were exposed to the water-soluble oxidant 2, 2'-azobis(2-amidinopropane) dihydrochloride (AAPH). Retinol protected PUFA's in ROS liposome PL's, whereas retinaldehyde promoted lipid peroxidation. When isolated ROS were stimulated to produce endogenous retinol, PUFA loss was inhibited by up to 17%. These findings suggest an antioxidant function for the enzymatic reduction of retinaldehyde to retinol during the visual cycle. Water-soluble antioxidants, taurine and reduced glutathione (GSH), were investigated individually and in combination with retinol in ROS PL liposomes. GSH protected PUFA's in ROS PL liposomes. Taurine alone showed little antioxidant activity, but in combination with retinol it protected lipids twice as much as retinol alone. These results support previous findings that taurine protects ROS lipids during exposure to cyclic light.

Lipid metabolism in photoreceptor membranes: regulation and mechanisms

Lipid metabolism in photoreceptor rod outer segments has attracted considerable attention because of its importance in providing the appropriate environment for supporting an efficient phototransduction mechanism. Recent studies suggest that lipid metabolism in these membranes is involved in the generation of second messengers and in signal transduction mechanisms. Phospholipid turnover is tightly regulated by phosphorylation-dephosphorylation reactions and light, and provides, in turn, with molecules capable of activating protein kinases and cellular processes such as membrane fusion or light-adaptation. These findings suggest that photoreceptor membrane lipids are more than just important structural components of the visual cell rod outer segment.

CD36 participates in the phagocytosis of rod outer segments by retinal pigment epithelium

Mechanisms of phagocytosis are complex and incompletely understood. The retinal pigment epithelium provides an ideal system to study the specific aspects of phagocytosis since an important function of this cell is the ingestion of packets of membranous discs that are normally discarded at the apical ends of rod and cone cells during outer segment renewal. Here we provide evidence that rod outer segment phagocytosis by retinal pigment epithelium is mediated by CD36, a transmembrane glycoprotein which has been previously characterized on hematopoietic cells as a receptor for apoptotic neutrophils and oxidized low density lipoprotein. Immunocytochemical staining with monoclonal and polyclonal antibodies demonstrated CD36 expression by both human and rat retinal pigment epithelium in transverse cryostat sections of normal retina and in primary cultured cells. By western blot analysis of retinal pigment epithelial cell lysates, polyclonal and monoclonal antibodies to CD36 recognized an 88 kDa protein which comigrated with platelet CD36. Furthermore, the synthesis of CD36 mRNA by retinal pigment epithelium was confirmed by reverse transcriptase-PCR using specific CD36 oligonucleotides. The addition of CD36 antibodies to cultured retinal pigment epithelial cells reduced the binding and internalization of 125I-labeled rod outer segments by 60%. Immunofluorescence confocal microscopy confirmed that outer segment uptake was significantly diminished by an antibody to CD36. Moreover, we found that transfection of a human melanoma cell line with CD36 cDNA enabled these cells to bind and internalize isolated photoreceptor outer segments as seen by double immunofluorescent staining for surface bound and total cell-associated rod outer segments, and by measurement of cell-associated 125I-labeled rod outer segments. We conclude that the multifunctional scavenger receptor CD36 participates in the clearance of photoreceptor outer segments by retinal pigment epithelium and thus, participates in the visual process.

Lipid mediators in the rat retina: light exposure and trauma elicit leukotriene B4 release in vitro

Light exposure not only elicits a visual response but may also alter functional and structural characteristics of the retina. Furthermore, light exposure can lead to reversible or irreversible lesions of photoreceptors and pigment epithelium. Previous studies in our laboratory have shown that light liberates arachidonic acid from retinal membrane phospholipids mainly by activating the phospholipase A2. In this study we show that light and trauma elicit the synthesis of leukotriene B4 in the isolated rat retina in vitro. Male albino rats were dark adapted for 36 h, isolated retinae were taken, incubated and exposed a) either to darkness or to 5,000 lux of cool white fluorescent light for 5, 10 or 15 min at 37 degrees C, b) either to darkness or to 5,000 lux of cool white fluorescent light for 15 min at 0 degrees C or c) either to darkness or to 5,000 lux of cool white fluorescent light for 15 min at 37 degrees C with a 5-lipoxygenase inhibitor (zileuton). Eicosanoids were extracted and leukotriene B4 levels were determined by radioimmunoassay. Removal of retinae and incubation in darkness caused a significant rise in leukotriene B4 levels with increasing incubation time. This rise was further augmented significantly after light exposure. The leukotriene B4 levels obtained when incubating the retinae either at 0 degree C or with the lipoxygenase inhibitor zileuton as well as the high specificity of the radioimmunoassay indicate that the light- and trauma-elicited synthesis of leukotriene B4 is mediated by activating the 5-lipoxygenase. Leukotriene B4 may be involved, at least in part, in the pathogenesis of retinal diseases including light damage. Curr. Eye Res. 14: 1001-1008, 1995.

Failure of vitamin E to protect cultured human arterial smooth muscle cells against oxysterol-induced cytotoxicity

OBJECTIVE

The cytotoxicity of oxysterols including 7 alpha-hydroxycholesterol (7 alpha OHC), 7 beta-hydroxycholesterol (7 beta OHC), cholesterol 5 alpha,6 alpha-epoxide (alpha epoxyC), cholesterol 5 beta,6 beta-epoxide (beta epoxyC), 7-ketocholesterol (7ketoC), 26-hydroxycholesterol (26OHC), cholesterol-3 beta,5 alpha,6 beta-triol (TriolC) and the possible protecting effect of vitamin E on 26OHC-induced cytotoxicity were investigated in smooth muscle cells isolated from the arteries of human umbilical cords.

METHODS

To study the cytotoxicity of oxysterols, the cells were incubated with each oxysterol at a level of 10 micrograms/ml from 24 to 120 hours, then 45Ca++ uptake, cytosolic free Ca++ level, [3H]thymidine incorporation, total DNA content and viable cell number were measured. Cholesterol was used as a control. For tracing the possible origin of cytotoxicity of 26OHC, cholesterol, phospholipid and 26OHC content in the membrane were investigated from 24 to 72 hours. For determining whether antioxidants had a protective effect against the cytotoxicity of 26OHC, vitamin E and butylated hydroxytoluene (BHT) were used.

RESULTS

The results indicated that the oxysterols elevated 45Ca++ uptake and cytosolic free Ca++ level, but diminished [3H]thymidine incorporation, total DNA content and viable cell number. 26OHC lowered the cholesterol content of the membrane and incorporated into the membrane after 24 hours of the incubation, but did not alter the total phospholipid content of the membrane until 72 hours. Neither vitamin E or BHT significantly protected the cells from the 26OHC-induced alterations.

CONCLUSION

We suggest that the cytotoxicity of oxysterols, which might result in an alteration in Ca++ ion flow into the cell by decreasing cholesterol content and incorporating oxysterol itself into the membranes, could not be protected by vitamin E.

N-acetyl cysteine is an early but also a late preventive agent against carbon tetrachloride-induced liver necrosis

N-Acetyl cysteine (NAC) treatment 30 min before or 6 or 10 h after carbon tetrachloride (CCl4) administration significantly prevented the liver necrosis produced by the hepatotoxin at 24 h. NAC pretreatment was able to partially decrease the covalent binding of CCl4 reactive metabolites at 1 and 3 h of poisoning and, to a small extent, the concentration of CCl4 reaching the liver at 3 h. NAC also diminished partially the CCl4-promoted increases in lipid peroxidation at 3 h, but had an enhancing effect of its own of small intensity. Results suggest that early and late protective effects of NAC might be attributable to its prior conversion to cysteine and glutathione.

Antioxidative stress therapy with dithiothreitol tetraacetate. I. Protection against carbon tetrachloride induced liver necrosis

Dithiothreitol (DTT) is known to prevent or even reverse several deleterious effects of radiation or of chemical agents operating via free radical and oxidative stress. However, its use has been hampered by its chemical instability and toxic properties. In this work, we synthesized and characterized dithiothreitol tetraacetate (DTT-Ac) which is less toxic and chemically stable, and we provided GLC/MS evidence that it is able to rapidly generate fully deacetylated DTT in liver after its administration to rats. Treatment with DTT-Ac simultaneously with CCl4 or 3 h after the hepatotoxin was able to significantly prevent the CCl4-induced liver necrosis at 24 h after poisoning. DTT-Ac administration was able to significantly reduce the intensity of the covalent binding of CCl4 reactive metabolites to microsomal lipids (CB), but it did not prevent the CCl4-induced initiation of a lipid peroxidation (LP) process as evidenced by diene hyperconjugation of microsomal lipids. Results suggest that DTT-Ac protective effects might be due to its in vivo conversion to DTT which in turn would decrease the intensity of CB via different potential mechanisms to be explored. Protection cannot be attributed to decreases in levels of CCl4 reaching the liver or to chain breaking effects on LP.

Properties and function of the ocular melanin--a photobiophysical view

This paper reviews the biosynthesis and physicochemical properties of the ocular melanin. Age-related changes of melanin granules and the corresponding formation of lipofuscin pigments in the retinal pigment epithelium (RPE) are also described. Adverse photoreactions of the eye and, in particular, light-induced damage to the RPE-retina are reviewed in relation to the ocular pigmentation. A hypothesis on the photoprotective role of the RPE melanin is presented that is based on the ability of the cellular melanin to bind redoxactive metal ions. Since bound-to-melanin metal ions are expected to be less damaging to the pigment cells, it is proposed that sequestration of heavy metal ions by the RPE melanin is an efficient detoxifying mechanism. It is postulated that oxidative degradation of RPE melanin may lower its metal-binding capability and decrease its anti-oxidant efficiency. Cellular and environmental factors that may contribute to possible oxidative damage of the RPE melanin are discussed in connection with the etiology of age-related macular degeneration.

Carotenoids in the retina--a review of their possible role in preventing or limiting damage caused by light and oxygen

Two of the circa 600 naturally occurring carotenoids, zeaxanthin and lutein, the major carotenoids of maize and melon respectively, are the constituents of the macula lutea, the yellow spot in the macula, the central part of the retina in primates and humans. Of the circa ten carotenoids found in the blood these two are specifically concentrated in this area, which is responsible for sharp and detailed vision. This paper reviews the ideas that this concentration of dietary carotenoids in the macula is not accidental, but that their presence may prevent or limit damage due to their physicochemical properties and their capability to quench oxygen free radicals and singlet oxygen, which are generated in the retina as a consequence of the simultaneous presence of light and oxygen. Additionally, in vitro and in vivo animal experiments are reviewed as well as observational and epidemiological data in humans. These show that there is enough circumstantial evidence for a protective role of carotenoids in the retina to justify further research. Some emphasis will be put on age-related macular degeneration (AMD), a multifactorial degenerative retinal disease for which the exposure to light and thus photochemical damage has been suggested as one of the etiological factors. Recent attempts at nutritional intervention in this condition will also be reviewed.