**5. Smoking and age-related macular degeneration**

It has been postulated that environmental oxidants are frequently implicated in RPE injury and may contribute to deposit formation. Cigarette smoking is the strongest environmental risk factor for all forms of AMD, even in people exposed to passive smoking [63-66]. People who have smoked at least 100 cigarettes (lifetime) have approximately triple risk of developing AMD compared to individuals who have never smoked. Current smokers and heavy smokers have even higher AMD risk.

Cigarette smoke is comprised of a gas and tar phase. Each phase contains both inorganic and organic free radicals, including ROS, epoxides, peroxides, NO, nitrogen dioxide, peroxynitrite, peroxynitrates and various other free radicals [67]. Moreover, cigarette smoke contains a high concentration of potent oxidants such as acrolein [68-70], dioxin [71], Benzo(a)Pyrene [72], cadmium, [73-75] quinones, nicotine, and NO [76-79]. Many of them demonstrated to be toxic to ocular tissue affecting the eye through oxidative damage and abnormal vascularization. Although the pathogenesis of AMD and the mechanism of action of smoking on the eye are not fully understood [80], the risk of developing AMD is likely to involve more than one mechanism.

of the alternative pathway of complement activation [121]. Smoking seems to alter the C3 component of the complement and to reduce the efficacy by which it binds to complement

Cigarette Smoking and Hypertension Two Risk Factors for Age-Related Macular Degeneration

http://dx.doi.org/10.5772/53958

47

The association between smoke and ApoE has also been investigated. Smokers are known to have higher serum cholesterol and LDL levels, a possible risk factor for AMD development [102]. Impaired cholesterol metabolism in particular ApoE isoforms carriers could be further enhanced among smokers. Moreover, smoking is associated with decreased endothelial NO production, while ApoE-4 genotype has been reported to increase NO synthesis, relative to ApoE2 and ApoE3 [103]. Thus, the presence of the ApoE2 protein in smokers may reduce endothelial NO levels in cell tissues. Given that the normal function of NO in the retina is to neutralize circulating oxidized lipids, its decreased availability may promote oxidative

As mentioned above, quinones, acrolein, BenzoPyrenes, and cadmium can damage the eye through oxidative processes. It has been demonstrated that hydroquinone the most abundant quinone in cigarette smoke causes oxidative damage, apoptosis, increases lipid peroxidation and mitochondrial superoxide production, and decreases intracellular glutathione in the RPE cells [104]. Acrolein, a product of lipid peroxidation in vivo, is a mitochondrial toxicant in RPE cells, which induces oxidative mitochondrial dysfunction and oxidative damage to proteins and DNA causing loss of cell viability [70-72]. Whereas Benzo(a)Pyrene causes mtDNA damage, alteration in the lysosomal activity, complement activation in the aged RPE, and upregulates expression of complement pathway components such as C3a, C5, C5b-9, and CFH in the RPE/choroid contributing to drusen formation [105,106]. Cadmium (Cd), is another toxic which is concentrated in the body through cigarette smoke. Cd has been demonstrated to be a potent inflammatory, an oxidant agent, and accumulates with aging, effects which are linked to AMD [77, 107]. In vitro studies have revealed Cd as an important potential factor in RPE

Cigarette smoke also induces angiogenesis promoting CNV and progression to neovascular AMD. Nicotine is one of the major components of the cigarette and has been demonstrated to enhance the generation of radical oxygen species producing oxidative stress [108-110] and promote the generation of pro-inflammatory responses leading to chronic inflammation in smokers [111]. Futhermore, nicotine is responsible for the mitogenesis of endothelial and smooth muscle cells [112] regulating abnormal responses to vascular injury [113] and endo‐ thelial functions [114]. We and other previously showed that nicotine by its direct action on nicotinic receptors promoted angiogenesis and increased size and severity of CNV in a mouse model [112, 114]. Moreover, it has been shown that nicotine targets the retinal microvasculature by reducing the apoptotic rate of vascular endothelial cells and inducing the formation of new capillarity, mediated at least in part by release of vascular endothelial growth factor (VEGF) [113], which is one of the key growth factors involved in neovascularization and implicated in the most severe form of AMD [115-118]. Nicotine has also the ability to stimulate the growth of smooth muscle cells after oxidative injury, leading to the development of CNV in the retina. Moreover other cigarette smoke components such as dioxin induce expression and release of

factor H (CFH).

damage to the RPE cells.

cell death associated retinal disease [76].

VEGF by RPE cells promoting CNV [104, 119].

Smoking habit appears to be related to the long-term incidence and progression of AMD [81]. Plausible biological mechanisms such as direct oxidation, depletion of antioxidant protection (e.g. decreases plasma vitamin C and carotenoids), immune system activation, atherosclerotic vascular changes, induction of hypoxia and alteration of choroidal blood flow [82,83], support the involvement of smoking in the etiology of AMD [84]. Interestingly, the Age-related Eye Disease Study (AREDS) [85] found that reduction in plasma glutathione and cysteine oxidation correlated with benefit from anti-oxidant treatment for intermediate AMD [86]. Collectively, these studies, along with the studies on cigarette smoking, implicate oxidative stress as a mechanism of AMD. Genetic variations such as a susceptibility locus in or near the hypothetical LOC387715 gene were associated with AMD [87,88]. It has been reported that this locus encodes a mitochondrial protein, raising suspicion for a role of the oxidative defense response in this disease. This locus is associated with smoking, and the combination of the LOC387715 polymorphism and smoking confers a higher risk for AMD than either factor alone [89,90]. Moreover, chemical in cigarette smoke modifies oxidative‐ ly docosohexanoic acid (DHA), the most abundant fatty acid in photoreceptor tips, to carboxyethylpyrrole (CEP) [91] and other lipids which have been identified which "tag" oxidatively oxidized damaged photoreceptors in AMD [92]. In the RPE, multiple proteins isolated from lipofuscin are also oxidatively damaged by cigarette smoke compounds including malondialdehyde, 4-hydroxynonenal and advanced glycation end products (AGE) [93,94]. Interestingly, AGEs accumulate in BrM including basal deposits and drusen, and CEP adducts appear in drusen isolated from AMD samples [95,96].The ability to defend against oxidative stress by upregulating the anti-oxidant defense response is likely to be a pivotal event that mediates the initiation and progression of AMD. The molecular damage from oxidative modification illustrated here, suggests that the anti-oxidant response in the macula at some point, becomes unable to neutralize oxidative stress.

In addition to direct oxidative damage to tissue, oxidative free radicals from cigarette smoke can modulate the immune-inflammatory system in part, through enhanced expression of proinflammatory genes, as reviewed in Biswas and Rahman (2009) [97]. The discovery of poly‐ morphisms in several complement factors with AMD susceptibility points toward a specific role for complement mediated inflammation in the pathophysiology of AMD [98,99]. AMD has been associated with local inflammatory responses in the RPE/choroid [100]. In previous works, it was demonstrated that the aged RPE/choroid becomes immunologically active [101] due to increased expression of complement components. Smoking also influences the function of the alternative pathway of complement activation [121]. Smoking seems to alter the C3 component of the complement and to reduce the efficacy by which it binds to complement factor H (CFH).

Cigarette smoke is comprised of a gas and tar phase. Each phase contains both inorganic and organic free radicals, including ROS, epoxides, peroxides, NO, nitrogen dioxide, peroxynitrite, peroxynitrates and various other free radicals [67]. Moreover, cigarette smoke contains a high concentration of potent oxidants such as acrolein [68-70], dioxin [71], Benzo(a)Pyrene [72], cadmium, [73-75] quinones, nicotine, and NO [76-79]. Many of them demonstrated to be toxic to ocular tissue affecting the eye through oxidative damage and abnormal vascularization. Although the pathogenesis of AMD and the mechanism of action of smoking on the eye are not fully understood [80], the risk of developing AMD is likely to involve more than one

46 Age-Related Macular Degeneration - Etiology, Diagnosis and Management - A Glance at the Future

Smoking habit appears to be related to the long-term incidence and progression of AMD [81]. Plausible biological mechanisms such as direct oxidation, depletion of antioxidant protection (e.g. decreases plasma vitamin C and carotenoids), immune system activation, atherosclerotic vascular changes, induction of hypoxia and alteration of choroidal blood flow [82,83], support the involvement of smoking in the etiology of AMD [84]. Interestingly, the Age-related Eye Disease Study (AREDS) [85] found that reduction in plasma glutathione and cysteine oxidation correlated with benefit from anti-oxidant treatment for intermediate AMD [86]. Collectively, these studies, along with the studies on cigarette smoking, implicate oxidative stress as a mechanism of AMD. Genetic variations such as a susceptibility locus in or near the hypothetical LOC387715 gene were associated with AMD [87,88]. It has been reported that this locus encodes a mitochondrial protein, raising suspicion for a role of the oxidative defense response in this disease. This locus is associated with smoking, and the combination of the LOC387715 polymorphism and smoking confers a higher risk for AMD than either factor alone [89,90]. Moreover, chemical in cigarette smoke modifies oxidative‐ ly docosohexanoic acid (DHA), the most abundant fatty acid in photoreceptor tips, to carboxyethylpyrrole (CEP) [91] and other lipids which have been identified which "tag" oxidatively oxidized damaged photoreceptors in AMD [92]. In the RPE, multiple proteins isolated from lipofuscin are also oxidatively damaged by cigarette smoke compounds including malondialdehyde, 4-hydroxynonenal and advanced glycation end products (AGE) [93,94]. Interestingly, AGEs accumulate in BrM including basal deposits and drusen, and CEP adducts appear in drusen isolated from AMD samples [95,96].The ability to defend against oxidative stress by upregulating the anti-oxidant defense response is likely to be a pivotal event that mediates the initiation and progression of AMD. The molecular damage from oxidative modification illustrated here, suggests that the anti-oxidant response in the

macula at some point, becomes unable to neutralize oxidative stress.

In addition to direct oxidative damage to tissue, oxidative free radicals from cigarette smoke can modulate the immune-inflammatory system in part, through enhanced expression of proinflammatory genes, as reviewed in Biswas and Rahman (2009) [97]. The discovery of poly‐ morphisms in several complement factors with AMD susceptibility points toward a specific role for complement mediated inflammation in the pathophysiology of AMD [98,99]. AMD has been associated with local inflammatory responses in the RPE/choroid [100]. In previous works, it was demonstrated that the aged RPE/choroid becomes immunologically active [101] due to increased expression of complement components. Smoking also influences the function

mechanism.

The association between smoke and ApoE has also been investigated. Smokers are known to have higher serum cholesterol and LDL levels, a possible risk factor for AMD development [102]. Impaired cholesterol metabolism in particular ApoE isoforms carriers could be further enhanced among smokers. Moreover, smoking is associated with decreased endothelial NO production, while ApoE-4 genotype has been reported to increase NO synthesis, relative to ApoE2 and ApoE3 [103]. Thus, the presence of the ApoE2 protein in smokers may reduce endothelial NO levels in cell tissues. Given that the normal function of NO in the retina is to neutralize circulating oxidized lipids, its decreased availability may promote oxidative damage to the RPE cells.

As mentioned above, quinones, acrolein, BenzoPyrenes, and cadmium can damage the eye through oxidative processes. It has been demonstrated that hydroquinone the most abundant quinone in cigarette smoke causes oxidative damage, apoptosis, increases lipid peroxidation and mitochondrial superoxide production, and decreases intracellular glutathione in the RPE cells [104]. Acrolein, a product of lipid peroxidation in vivo, is a mitochondrial toxicant in RPE cells, which induces oxidative mitochondrial dysfunction and oxidative damage to proteins and DNA causing loss of cell viability [70-72]. Whereas Benzo(a)Pyrene causes mtDNA damage, alteration in the lysosomal activity, complement activation in the aged RPE, and upregulates expression of complement pathway components such as C3a, C5, C5b-9, and CFH in the RPE/choroid contributing to drusen formation [105,106]. Cadmium (Cd), is another toxic which is concentrated in the body through cigarette smoke. Cd has been demonstrated to be a potent inflammatory, an oxidant agent, and accumulates with aging, effects which are linked to AMD [77, 107]. In vitro studies have revealed Cd as an important potential factor in RPE cell death associated retinal disease [76].

Cigarette smoke also induces angiogenesis promoting CNV and progression to neovascular AMD. Nicotine is one of the major components of the cigarette and has been demonstrated to enhance the generation of radical oxygen species producing oxidative stress [108-110] and promote the generation of pro-inflammatory responses leading to chronic inflammation in smokers [111]. Futhermore, nicotine is responsible for the mitogenesis of endothelial and smooth muscle cells [112] regulating abnormal responses to vascular injury [113] and endo‐ thelial functions [114]. We and other previously showed that nicotine by its direct action on nicotinic receptors promoted angiogenesis and increased size and severity of CNV in a mouse model [112, 114]. Moreover, it has been shown that nicotine targets the retinal microvasculature by reducing the apoptotic rate of vascular endothelial cells and inducing the formation of new capillarity, mediated at least in part by release of vascular endothelial growth factor (VEGF) [113], which is one of the key growth factors involved in neovascularization and implicated in the most severe form of AMD [115-118]. Nicotine has also the ability to stimulate the growth of smooth muscle cells after oxidative injury, leading to the development of CNV in the retina. Moreover other cigarette smoke components such as dioxin induce expression and release of VEGF by RPE cells promoting CNV [104, 119].
