Ординатура / Офтальмология / Английские материалы / Handbook of Nutrition and Ophthalmology_Semba_2007

Fig. 3. Diagram of basal laminar deposits.

Fig. 4. Geographic atrophy. (Courtesy of Jay M. Haynie.)

4.2.1. DRY AGE-RELATED MACULAR DEGENERATION (GEOGRAPHIC ATROPY)

Dry age-related macular degeneration is characterized by sharply delineated round or oval areas of hypopigmentation or depigmentation in which there is an apparent absence of retinal pigment epithelium and areas in which choroidal vessels are more visible than in surrounding areas (Fig. 4).

4.2.2. WET AGE-RELATED MACULAR DEGENERATION

(NEOVASCULAR, EXUDATIVE, OR DISCIFORM)

Wet age-related macular degeneration is characterized by any of the following: subretinal or sub-retinal pigment epithelium neovascular membrane(s), detachment(s) of the retinal pigment epithelium, epiretinal, intraretinal, subretinal, or sub-retinal pigment epithelium scar or glial tissue or fibrin-like deposits, subretinal hemorrhages, and hard exudates within the macula and not related to other retinal vascular disease (13) (Fig. 5).

Fig. 5. Neovascular age-related macular degeneration. (Courtesy of Egbert Saavedra.)

5. PATHOPHYSIOLOGY

5.1. Pathological Features of Age-Related Macular Degeneration

Age-related changes in Bruch’s membrane include an increase in thickness from childhood to adult life (101,102), and an accumulation of debris on both sides of the elastic layer, a change that begins to occur in the second decade and is common by 20–60 yr of age (103). The main source of the debris appears to be the retinal pigment epithelium (103). Basal laminar deposits are thought to consist of abnormal, undigested material from the retinal pigment epithelium (104). Drusen appear to form by budding or evagination of a portion of retinal pigment epithlium into the subpigment epithelial space, followed by degeneration and disintegration of the basement membrane of the budded portion and deposition of this vesicular, granular, tubular, and linear material in the space external to the retinal pigment epithelium (104) (Fig. 6). Choroidal perfusion on fluorescein angiography also appears to show diffuse thickening of Bruch’s membrane (105). With increasing age, there is also an increase in lipofuscin granules in the retinal pigment epithelium (106). Normal aging is also associated with an increase in major histocompatibility complex (MHC) class II immunoreactivity in retinal vascular elements, and a further increase in MHC-II immunoreactivity was associated with incipient age-related macular degeneration (107). A granulomatous reaction to Bruch’s membrane has been described in agerelated macular degeneration (108). There appears to be a continuum of pathological changes

Fig. 6. Formation of drusen (Reprinted from ref. 104, with permission of Elsevier.)

in age-related macular degeneration, and disturbances of retinal pigment epithelium pigmentation, drusen, thickening of Bruch’s membrane, and formation of basal laminar deposits are associated with loss of photoreceptor outer segments and atrophy of the choriocapillaris (109). Visual impairment and blindness can follow the loss of photoreceptors from the macula.

5.2. Oxidative Stress

5.2.1. OVERVIEW

The current hypothesis under widespread investigation is that oxidative stress contributes to the pathogenesis of age-related macular degeneration (110–117). Oxygen is required for energy-producing intracellular reactions, but in the process of oxidation, reactive oxygen species can be produced. Antioxidants, such as carotenoids, vitamin C, α-tocopherol, and bilirubin are thought to balance prooxidants such as reactive oxygen species. In addition, cells have enzyme systems with antioxidant activity such as superoxide dismutase, catalase, and glutathione peroxidase. Oxidative stress is used to describe the condition in which there is an imbalance due to a relative deficiency of antioxidants. With oxidative stress, reactive oxygen species can cause damage to DNA, lipids, proteins, and carbohydrates, and cell damage and tissue destruction may result. Reactive oxygen species, such as superoxide anion (O2•~), hydroxyl radical (OH•), hydrogen peroxide (H2O2), and singlet oxygen (1O2) can be generated through various processes in the retina including mitochondrial respiration, phagocytosis of rod and cone outer segments by the retinal pigment epithelium, reaction of blue light with lipofuscin, reaction of light with endogenous porphyric photosensitizers in the choroid, xanthine oxidase, NADPH-depen- dent oxidase system, auto-oxidation of catecholamines, and prostaglandin H2 synthase (114). There are at least five major reasons why the retina may be subject to an extremely high degree of oxidative stress: (1) the retina is well vascularized and has a greater degree of oxygen consumption than any other tissue, (2) there is a high level of exposure to cumulative irradiation, (3) photoreceptor outer segments are rich in polyunsaturated fatty acids, which can readily be oxidized, (4) the retina contains photosensitizers, and (5) phagocytosis of photoreceptor outer segments by the retinal pigment epithelium produces reactive oxygen intermediates (117). In vitro studies with human retinas suggest that lipid peroxidation is greatest in the macular region and that lipid peroxidation increases in the human retina with age (118). Some antioxidants and antioxidant enzyme systems that are thought to protect the retina from increased oxidative stress are shown in Table 3.

5.2.2. ANTIOXIDANT ENZYME SYSTEMS IN THE RETINA

Enzyme systems in the retina that function in reducing reactive oxygen species include glutathione peroxidase, catalase, and superoxide dismutase. Glutathione acts as an antioxidant by reducing peroxides in a reaction catalyzed by glutathione peroxidase, a sele- nium-dependent enzyme. Glutathione is found in high concentrations in the retina and retinal pigment epithelium (119). Exogenous glutathione was found to protect culture human retinal pigment epithelial cells from oxidative injury (120). These in vitro studies suggested that glutathione and its amino acid precursors could protect retinal pigment epithelium from oxidative injury. Retinal pigment epithelium appears to synthesize glutathione directly from amino acid precursors (121). Individuals with age-related macular degeneration were found to have significantly lower plasma glutathione concentrations compared with age-matched healthy controls (122). In addition, plasma glutathione concentrations appeared to decrease with increasing age (122).

Catalase is an iron-dependent enzyme that dismutates hydrogen peroxide to water and molecular oxygen, protecting tissues against oxidative damage. The retinal pigment epithelium contains extremely high levels of catalase activity (123), with a level of activity

182 Handbook of Nutrition and Ophthalmology

Table 3

Antioxidants and Antioxidant Enzymes in the Human Retina and Their Putative Functions

that is six times higher than that found in other ocular tissues (124). In a study of donor eyes of adults 50 to 90 yr of age, catalase activity in the retinal pigment epithelium decreased with age and with the presence of macular degeneration (124). In vitro studies demonstrate that zinc can induce catalase expression in cultured fetal human retinal pigment epithelial cells (125), suggesting a potential mechanism by which zinc status might influence catalase activity in vivo. In a primate model of age-related macular degeneration, lower catalase activity and markedly lower zinc concentrations were found in affected retinas compared with control retinas (126).

Two forms of superoxide dismutase are found in human retinal pigment epithelium, a copper-zinc superoxide dismutase and a manganese superoxide dismutase (127). Manganese superoxide dismutase may play a potential role in protecting mitochondria from oxidative damage (127). Two other enzymes that may potentially play a role in the pathogenesis of age-related macular degeneration are heme oxygenase-1 and -2 (128). These enzymes convert heme, a pro-oxidant, to biliverdin. Biliverdin is converted to bilirubin, a strong antioxidant, by bilverdin reductase. In an immunohistochemical study of 21 eyes obtained postmortem from donors aged 42 to 94 yr, copper and zinc superoxide dismutase activity in cytoplasm and lysosomes from macular retinal pigment epithelial cells increased with age, whereas catalase immunoreactivity decreased with age (128). Heme-oxygenase-1 and heme-oxygenase-2 reactivity were significantly higher in macular retinal pigment epithelial cells from eyes with neovascular age-related macular degeneration, suggesting that these two enzymes are upregulated in age-related macular degeneration (128).

Oxidative stress in the retina may ultimately induce apoptosis in retinal pigment epithelial cells. Increased apoptosis was found in cultured human retinal pigment epithelial cells that were exposed to a chemical oxidant, t-butylhydroperoxide (129). Mitochondria play an important role in regulating signal transduction in apoptosis, and an early change preceding apoptosis is a decreased in inner transmembrane potential (130). Mitochondrial membrane potential was altered by t-butylhydroperoxide in these studies, suggesting that the oxidant induced apoptosis in retinal pigment epithelial cells as a consequence of changes induced in mitochrondria (114,129). Antioxidant enzyme activity in red blood cells does not appear to correlate with age-related macular degeneration (131), which

suggests that the disease severity of age-related macular degeneration might relate more closely to localized oxidative stress in the retina, rather than biomarkers of oxidative stress in peripheral blood.

5.2.3. EFFECTS OF PHOTIC IRRADIATION

Photochemical injury to the retina has been described in rats (132,133) and primates (134–136) exposed to visible light. The retinal pigment epithelium appears to be the most susceptible to light-induced damage (133). Blue light of 441 nm wavelength (which does not induce an appreciable temperature rise in the retina), was sufficient to induce retinal damage in the primate after 1000 s of exposure (136). The primary lesion occurs in the retinal pigment epithelium and results in hypopigmentation (135). Increased lipid hydroperoxide has been found in rod outersegments exposed to light (137). Photic injury to the retina was also found in rhesus monkeys that were exposed to the light of an indirect ophthalmoscope (138). The damage was more severe in the perifoveal zone compared to the foveal area, which is consistent with the idea that macular pigment protects the foveal region from photic injury (138). Carotenoid pigments have been hypothesized to protect the eye against photo-oxidative stress (139). The role of carotenoids in the retina is presented under Subheading 5.3.9.

5.2.4. PHOTOSENSITIZATION

Photoactive compounds in erythrocytes, such as protoporphyrin IX, a precursor molecule to hemoglobin, have been proposed to play a role in the pathogenesis of age-related macular degeneration (140). On exposure to light, protoporphyrin IX generates superoxide anion and singlet oxygen, and these reactive oxygen species could potentially damage vascular endothelium of the choriocapillaris, Bruch’s membrane, and the retinal pigment epithelium (140). In a mouse model of protoporphyria, exposure to blue light was associated with a time and light-dependent increase in choriocapillary and subretinal pigmental epithelium basal laminar-like deposits (141). A model using liposomes has also been used to examine the effect of visible light on photosensitizers (142). When carotenoids were incorporated into a model using liposomal membranes, there was less lipid peroxidation and lysosomal lysis (142).

5.2.5. LIPOFUSCIN

Lipofuscin, a heterogeneous material composed of lipids, proteins, and different fluorescent compounds, accumulates within the retinal pigment epithelium with aging (143). Photoreceptor loss in the human retina has been associated with increasing lipofuscin accumulation (144). The age-related increase in lipofuscin may be an important mechanism in the pathogenesis of age-related macular degeneration. Lipofuscin appears as yellow-brown refractile granules, and these “aging pigments” are thought to be due to the accumulation of lysosomal residual bodies containing the end products of photoreceptor outer segment phagocytosis (145,146). Some of the fluorophores in retinal pigment epithelium appear to be metabolites of vitamin A (147–149). The fluorescence of lipofuscin granules increases with age (146). Lipofuscin may act as a sensitizer for the generation of reactive oxygen species, as singlet oxygen, superoxide anion, and hydrogen peroxide, can be produced on exposure to blue light (150–153). In an in vitro study, cultured human retinal pigment epithelium were fed lipofuscin granules, and on subsequent light exposure, severe damage was seen in these retinal pigment epithelial cells compared to control cells (154). Dietary

restriction of vitamin A (147) or caloric restriction (147) has been shown to reduce lipofuscin accumulation in rats.

5.2.6. VASCULAR ENDOTHELIAL GROWTH FACTOR

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen and an inducer of angiogenesis (156). VEGF has been shown to play a role in retinal neovascularization (157), and the expression of VEGF is upregulated by hypoxia and oxidative stress through hypoxia-inducible factor (HIF)-1α, which binds to the VEGF-A promoter and induces transcription and through nuclear factor (NF)-κB, a transcription factor that is induced by redox balance (158).

5.3. Carotenoids

5.3.1. INTRODUCTION

Carotenoids are a group of pigments found in the plant and animal kingdoms that vary across the spectrum from yellow, orange, and red to violet in color. Fruits and vegetables are rich plant sources of carotenoids. There are more than 600 carotenoids found in nature, of which about 50 have been identified in the human diet (159) and 34 have been described in human serum (160). The major dietary carotenoids are α-carotene, β-carotene, β-cryp- toxanthin, lycopene, lutein, and zeaxanthin. Of these, α-carotene, β-carotene, and β-cryp- toxanthin can be converted into retinol and thus, have been termed provitamin A carotenoids. Lycopene, lutein, and zeaxanthin do not have vitamin A activity and are referred to as nonprovitamin A carotenoids. Recently, there has been a great deal of interest in carotenoids because epidemiological studies have shown associations between high intakes of fruits and vegetables and lower incidence of some cancers, decreased risk of cardiovascular disease, and reduced risk of age-related macular degeneration (161,162).

5.3.2. HISTORICAL BACKGROUND

Some of the major carotenoids were first isolated in the 19th century and early 20th century, and the advent of chromatography helped to accelerate scientific understanding of these substances. In 1831, Heinrich Wilhelm Ferdinand Wackenroder (1798–1854), an analytical chemist at the Pharmaceutical Institute in Jena, discovered carotin in the root of the carrot (163). William Christopher Zeise (1789–1847) conducted further investigations on carotin in Copenhagen and gave it an empirical formula of C5H8 (164). In his studies, August Husemann proposed that carotin contained oxygen (165), and in 1861, A. Arnaud established that carotin is a hydrocarbon (166). In 1869, Johann Ludwig Wilhelm Thudichum (1829–1901), a chemist at St. Thomas’s Hospital in London, found that parts of plants and animals contain a yellow crystallizable substance, which he named “luteine” (167). A dark red pigment, later identified as lycopene, was isolated from Tamus communis in 1873 (168) and from tomatoes by the French botanist Pierre-Marie-Alexis Millardet (1838–1902) in 1875 (169). C. A. Schunck showed that the red pigment isolated from tomatoes, which he termed lycopene, has a different absorption spectrum than carotene (170). In 1907, the correct formula C40H56 was assigned to carotene by Richard Willstätter (1873–1942), and 3 yr later, Willstätter determined that lycopene, with the formula C40H56, is an isomer of carotene. Lutein was isolated from egg yolk in 1912 by Willstätter and Escher (171).

The term carotenoids was originally proposed in 1911 by Mikhail Semenovich Tswett (or Tsvett) (1872–1919), a botanist in Warsaw who pioneered the chromatographic analysis of plant pigments (172,173). By 1922, six carotenoids had been crystallized and analyzed (carotene, lycopene, xanthophyll, lutein, fucoxanthin, and rhodoxanthin) (174). In 1929, a new carotenoid, zeaxanthin, was isolated from maize by Karrer and associates (175,176), and its chemical structure was described in 1931–1932 (177,178). The conversion of β-carotene to vitamin A was demonstrated in 1930 (179). Paul Karrer (1889–1971), a Swiss chemist, elucidated the structures of vitamin A and β-carotene (180,181), two scientific accomplishments for which he received the Nobel Prize in chemistry in 1937.

5.3.3. BIOCHEMISTRY OF THE CAROTENOIDS

Carotenoids are characterized by a polyisoprenoid structure, a long conjugated chain of double bonds known as the polyene chain, and near symmetry around a central double bond (182). In general, well known trivial names based on the source from which the carotenoid was isolated—such as lycopene and zeaxanthin—are used instead of the structural name based on accepted chemical nomenclature (183). The polyene chain consists of a central, long system of alternating double and single bonds, and in this conjugated system, the π-electrons are effectively delocalized over the length of the chain (182). Dietary carotenoids that are the most common in human plasma are either carotenes, 40-carbon hydrocarbons, or xanthophylls (oxocarotenoids), 40-carbon hydroxylated compounds. Carotenes include α-carotene, β-carotene, and lycopene, and xanthophylls include lutein, zeaxanthin, and β-cryptoxanthin (Fig. 7). Carotene consists of polyenes with carbon and hydrogen only, whereas xanthophylls consist of oxygenated polyenes. The all-trans isomer is the most common and stable form of carotenoids found in foods, but cis isomers exist and may also be produced by heating, as in cooking (159).

Carotenoids are hydrophobic molecules, and thus, carotenoids interact with lipophilic elements of the cell, such as the lipid membrane bilayer. Carotenoids are commonly located within cell membranes, and the location of specific carotenoids within the membrane structure depends on the chemical structure of the carotenoid. The physical properties of carotenoids include the absorption of visible light, the ability to play a role in singlet-singlet energy transfer, and the ability to quench singlet oxygen (184). The long conjugated double bond system of carotenoids allows the carotenoids to absorb light, and the absorption of visible light depends on their specific chemical structure. The absorption of light energy produces a transition π → π* in which one of bonding π-electrons of the polyene chain is promoted to a previously unoccupied π* antibonding orbital (182). The π-electrons are delocalized over the polyene chain, and the energy that is needed to produce the transition of π → π* is small and corresponds to light in the visible spectrum of 400–500 nm (182). Lutein and zeaxanthin are yellow carotenoids that absorb blue light. β-carotene appears orange, and lycopene absorbs light at longer wavelengths and appears red. In photosynthesis, carotenoids act as antenna pigments, absorb light, and transfer this energy to chlorophylls (185).

The carotenoids have distinctive photochemical properties related to having two lowlying electronic excited singlet states (186). The strong absorption of light in the visible region has been attributed to the transition from the ground state S0 to the second singlet excited state S2 (182). Carotenoids can also accept excitation energy from highly reactive singlet oxygen, 1O2, and this allows carotenoids to protect against damage caused by a

Fig. 7. Structures of major dietary carotenoids.

combination of light and oxygen (187,188). Singlet oxygen is highly reactive and can damage DNA and lipids. The reaction with singlet oxygen generates a triplet excited carotenoid:

1O2 + carotenoid → 3O2 + 3carotenoid*

The triplet excited carotenoid then dissipates the energy harmlessly through rotational and vibrational interactions to recover the ground state:

3carotenoid* → carotenoid + thermal energy

Thus, carotenoids can serve to deactivate potentially harmful 1O2 (188). The ability of carotenoids to protect against photosensitization depends on the number of conjugated double bonds (189). Carotenoids can also quench peroxyl radicals (190) and can inhibit lipid peroxidation (191). The carotenoids were the first singlet oxygen quenchers to be characterized and are among the most effective quenchers known (192). Of the major dietary carotenoids in humans, lycopene appears to have the best singlet oxygen quenching ability (193).

5.3.4. DIETARY SOURCES OF CAROTENOIDS

The three major dietary carotenoids in the US diet are β-carotene, lutein, and lycopene (194). Vegetables and fruits such as carrots, spinach, collard greens, apricots, and canta-

Table 4

Lutein and Zeaxanthin Concentrations in Some Foods (Mole %)

loupe are rich in β-carotene. Tomatos are a rich source of lycopene. Lutein and zeaxanthin, which accumulate in the human macula, are found in high concentrations in food sources such as egg yolk, corn, orange juice, honeydew melon, and orange pepper (195) (Table 4). The carotenoid content of normal US diet is 1.3–3 mg/d of lutein and zeaxanthin combined and about 2.5–3.5 mg/d of β-carotene (194,196). Most studies of the carotenoid composition of foods provide data on lutein and zeaxanthin together or lutein alone (197), as special laboratory techniques are needed to provide separation of lutein and zeaxanthin peaks in high performance liquid chromatography analyses. The human diet is dominated by one stereoisomer of lutein, 3R,3'R,6'R)-β,ε-carotene-3,3'-diol, and one stereoisomer of zeaxanthin, the 3R,3'R stereoisomer (85). The ratio of lutein to zeaxanthin in the human diet ranges from 7:1 to 4:1 (85). Although in the US diet, lutein dominates over zeaxanthin, in some parts of the world where the corn is the main dietary staple, zeaxanthin may potentially dominate over lutein. Fresh spinach and corn meal may contain small amounts of 13-cis-lutein and 13-cis-zeaxanthin, and these isomers may be found in human plasma (198).

5.3.5. ABSORPTION, STORAGE, AND METABOLISM OF CAROTENOIDS

The absorption of carotenoids depends on several factors, including the matrix within the fruit or vegetable, the physical processing of the foods during cooking and preparation, and the amount of fat consumed with the meal. The amount of carotenoids that is absorbed may vary widely, with greater than 50% absorption of carotenoids in palm oil or pharmacological preparations to as low as 1–2% with raw carrots (199). The bioavailability of carotenoids in foods is increased both by cooking and by decreasing the particle size of the food through slicing, chopping, or blending. Heating is thought to denature the protein in protein-pigment complexes in plant tissues and allow the release of carotenoids (199). More prolonged heating or higher temperatures in cooking may convert many of the carotenoids with all-trans configuration to cis isomers (200). After foods containing carotenoids are ingested, the carotenoids are incorporated into micelles within