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666

J.M. Calandria and N.G. Bazan

activity is highest in RPE cells. Consequently, the regulation of those processes is of major importance in the prevention of oxidative stress. In particular, anomalous functions of mitochondria produce abnormalities in the mitochondrial electron transport system, which are implicated as a cause of oxidative stress (Lenaz et al. 2002). Likewise, within the membranes of some epithelial cells the activity of 12-lipoxygenase (LOX) is linked with Nox-1 ROS production, a part of the NADPH oxidase complex (de Carvalho et al. 2008).

The production ROS is a double-edged sword because of their high reactivity. On one hand they are necessary to maintain several pathways, but on the other hand their synthesis needs to be strictly controlled; that is to say, after their formation there should be mechanisms that remove them or their products. In normal conditions, excess ROS are scavenged either by enzymatic methods (Superoxide dismutase, catalase, etc.) or by non-enzymatic antioxidants (Vitamin E, etc.) (Margrain et al. 2004). Therefore, RPE cells are exposed to noxious stimuli either due to their environment or to their own function, making these cells especially susceptible to oxidative stress-induced cell death.

In summary, RPE cells are involved in nutrition, intercellular and intra/intertissue communication and remodeling, scavenging of sub-products produced by retinal function, and promotion of neurotrophic signaling, all of which depends to a large extent on the preservation of their epithelium structure (see Fig. 76.1).

76.2 The Loss of RPE Cells in Retinal Degeneration

As key scavenger and filter cells in the oBRB, RPE cells maintain vital life cycles in photoreceptors and the neural retina milieu. In this sense, the slowdown of RPE cell detoxification and filtering functions not only represent a problem for the recycling of the photoreceptors, but also for the retinal pigment epithelium itself. In this sense, disappearance of RPE cells may cause subsequent photoreceptor degeneration. As it occurs with oxidative stress caused by hyperglycemia in diabetes patients, the aberrant formation of advanced glycation end-products (AGE) may be involved in the evolving deregulation of the neural retina milieu through the ablation of the oBRB, which in turn fails to clear internal toxic sub-products, such as lipofuscins capable of inducing the production of ROS in the retinal pigment epithelium (Sparrow and Boulton, 2005). AGE promotes pro-inflammatory signaling that results in nuclear factor-kinase beta (NF-kβ)-dependent gene activation (Bierhaus et al. 2001).

Recent evidence, however, suggests the starvation-dependent disappearance of photoreceptors as the initial cause of retinal pigment epithelium breakdown in retinitis pigmentosa, an inherited retinal degeneration. These findings propose that metabolic defects induce the death of photoreceptors that disrupt the structure of the RPE layer, making the neural-retina permeable to external toxic factors that act as a positive loop to enhance the death of more photoreceptors, just as if it were a house of cards (Punzo et al. 2009).

76 NPD1 Modulates the Induction of Pro-Inflammatory Signaling

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In any case, the structural or toxic disruption of the retinal pigment epithelium is of great importance for the preservation of retinal functionality, and RPE cells in particular are key elements that contribute to neural-retina homeostasis. The adult retinal pigment epithelium is a non-dividing layer of cells, and thus its protection becomes essential in the prevention of retinal degenerative diseases. This idea is supported by the impracticability of retinal pigment epithelium monolayer replacement, which makes the idea of prevention very attractive (Sheridan et al. 2009).

76.3 DHA and NPD1 Properties and Neuroprotection

DHA is a major component of the structural lipids of photoreceptor outer segment membranes and discs and is incorporated and retained with high efficiency (Bazan et al. 1993; Niemoller et al. 2009). DHA was proposed to be important in influencing the photoreceptor outer segment membranes, biophysically altering permeability fluidity and thickness that, for instance, modulates membrane-bound protein interactions (Clandinin et al. 1994; Jumpsen and Clandinin 1997). In the past, however, DHA was also attributed to biochemical actions involving the direct modulation of gene expression by inducing changes in mRNA processing, transport and stabilization (Uauy et al. 2001).

Even though DHA was proposed to be responsible for neuroprotection, it was recently demonstrated that a stereospecific oxidation derivative, neuroprotectin D1 (NPD1, 10R,17S-dihydroxy-docosa-4Z,7Z,11E,13E,15Z,19Z-hexaenoic acid), possesses the ability to promote cell survival upon stress-induced apoptotic cellular damage. RPE cells, when confronted with oxidative stress in vitro, enhance production of NPD1 as a homeostatic survival mechanism (Mukherjee et al. 2004). Although the exact biosynthetic pathway is unclear, recent evidence points to 15-lipoxygenase-1 (15-LOX-1) as being responsible for the conversion of DHA into NPD1. 15-lipoxygenase-1 catalyzes the formation of NPD1 in T-helper lymphocytes, where the silencing of 15-LOX-1 leads to reduced production of NPD1 (Ariel et al. 2005). Moreover, 15-LOX-1 knock-down cells show, in addition to decreased production of NPD1, an increased susceptibility to apoptosis, which is only rescued by the addition of exogenous NPD1 (unpublished observations).

15-LOX-1, a nonheme iron-containing dioxygenase, stereospecifically inserts oxygen in arachidonic acid, dually forming 15(S)-hydroxyeicosatetraenoic acid (HETE), 12(S)-HETE, and lipoxin A4, a product of its joint activity with 5-lipoxygenase. 15-LOX-1 also has the capability to oxygenate linoleic acid into 13-hydroxyoctadecadienoic acid (13-HODE) (Kühn et al. 1993). Human 15-LOX-1 and 12-LOX are highly homologous proteins (65% identity) encoded by different genes, and their messenger RNAs are similar (little more than 70% identity). On the other hand, 15-LOX-2 is a different lipoxygenase that shares only 39% identity with human 15-LOX-1 (Brash et al. 1997). 15-LOX-2 and human 12-LOX differ from 15-LOX-1 in the ratio of 15-HETE and 12-HETE produced from arachidonic acid. This means that they possess different selective product formation, and thus

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their activities contribute to different pools of lipid mediators. These compensatory functions of lipoxygenases do not affect the production of NPD1. In fact, when 15-LOX-1 is knocked down, this reveals only NPD1 and lipoxin A4 depletion, whereas other oxygenation products are modified, but to a lesser extent. 15-LOX is inducible by Ca++ signaling (Brinkmann et al. 1998; Walther et al. 2004); thus, it is highly possible that its activity may also be induced by the initiation of oxidative stress-mediated pro-inflammatory signaling as a way for the cell to counteract the pro-apoptotic process in the integration of antagonist pathways.

76.4NPD1 Modulates the Expression of Survival and Apoptotic-Related Proteins

NPD1 is a potent neuroprotective lipid mediator that inhibits the expression of proinflammatory genes and enhances the expression of anti-apoptotic proteins of the Bcl-2 family (Mukherjee et al. 2004; Lukiw et al. 2005). Moreover, NPD1 signaling is involved in neurotrophin-mediated RPE cell survival (Mukherjee et al. 2007a). Although the exact molecular mechanisms responsible for this decision are not yet clear, some evidence links the consequent pro-survival response of ARPE-19 cells after NPD1-pathway induction with the decrease in the activation of caspase-3. A DNA array-based human mRNA expression profiling in the central nervous system (CNS) shows that NPD1 ‘turns off’ pro-inflammatory and anti-apoptotic genes, whereas these genes elicit the opposite effect of that produced by the amyloid β peptide, Aβ42 (Lukiw et al. 2005).

In summary, NPD1 is involved in the preservation of the retinal pigment epithelium through the promotion of pro-survival signaling, at which point these cells decide whether to continue along the pro-apoptotic pathway or counteract this process by activating the pro-survival mode.

Acknowledgments Supported by NIH/NEI grant R01 EY005121, NIH/NCRR grant P20 RR016816, the Edward G. Schlieder Educational Foundation, the Eye, Ear, Nose and Throat Foundation, and the Ernest C. and Yvette C. Villere Chair for Research in Retinal Degeneration (NGB).

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