The retinal pigment epithelium (RPE) is a highly specialized, unique epithelial cell that interacts with photoreceptors on its apical side and with Bruch’s membrane and the choriocapillaris on its basal side. Due to vital functions that keep photoreceptors healthy, the RPE is essential for maintaining vision. With aging and the accumulated effects of environmental stresses, the RPE can become dysfunctional and die. This degeneration plays a central role in age-related macular degeneration (AMD) pathobiology, the leading cause of blindness among the elderly in western societies. Oxidative stress and inflammation have both physiological and potentially pathological roles in RPE degeneration. Given the central role of the RPE, this review will focus on the impact of oxidative stress and inflammation on the RPE with AMD pathobiology. Physiological sources of oxidative stress as well as unique sources from photo-oxidative stress, the phagocytosis of photoreceptor outer segments, and modifiable factors such as cigarette smoking and high fat diet ingestion that can convert oxidative stress into a pathological role, and the negative impact of impairing the cytoprotective roles of mitochondrial dynamics and the Nrf2 signaling system on RPE health in AMD will be discussed. Likewise, the response by the innate immune system to an inciting trigger, and the potential role of local RPE production of inflammation, as well as a potential role for damage by inflammation with chronicity if the inciting trigger is not neutralized, will be debated.

Dysregulated complement is thought to play a central role in AMD pathogenesis, but the specific mechanisms have yet to be determined. In maculas of AMD specimens, we found that the complement regulatory protein, CD59, was increased in regions of uninvolved retinal pigmented epithelium (RPE) of early AMD, but decreased in the RPE overlying drusen and in geographic atrophy, an advanced form of AMD. While CD46 immunostaining was basolaterally distributed in the RPE of unaffected controls, it was decreased in diseased areas of early AMD samples. Since oxidized low density lipoproteins (oxLDL) collect in drusen of AMD and are a known complement trigger, we treated ARPE-19 cells with oxLDL and found that cellular CD46 and CD59 proteins were decreased by 2.9-fold and 9-fold (p<0.01), respectively. OxLDLs increased complement factor B mRNA and Bb protein, but not factor D, I, or H. OxLDLs increased C3b, but not C3a, C5 or C5b-9. C5b-9 was increased by 27% (p<0.01) when medium was supplemented with human serum, which was sufficient to induce poly (ADP-ribose) polymerase cleavage, a marker of apoptosis. The decreased levels of CD46 and CD59 were in part, explained by their release in exosomal and apoptotic membranous particles. In addition, CD59 was partially degraded through activation of IRE1α. Collectively, these results suggest that a combination of impaired complement regulators results in inadequately controlled complement by the RPE in AMD that induces RPE damage.

While cigarette smoking (CS) and dysregulated complement are thought to play a central role in age-related macular degeneration (AMD), their exact roles are unknown. The aim of this study is to determine if CS activates complement and if the antioxidant transcription factor Nrf2 modulates this response. In AMD specimens, Nrf2 immunolabeling was strong in the cytoplasm with scattered nuclear labeling of macular retinal pigmented epithelial (RPE) cells that appeared normal, but was decreased and without nuclear labeling in dysmorphic cells overlying drusen, a hallmark AMD lesion. Cigarette smoke extract (CSE) induced Nrf2 nuclear translocation in RPE cells with increased antioxidant and complement gene expression. While CFH protein was not altered by CSE, cell membrane regulator proteins CD46, CD55, and CD59 were decreased, while C3a and C3b, but not iC3b, were increased compared to controls. C5b-9 was increased by CSE, but at sublytic levels only after addition of normal human serum. Nrf2-knockdown enhanced the increase of C3a and C3b from CSE, but not iC3b, C5a, or C5b-9. CSE also increased IL-1b expression and secretion after C3a generation, and was reduced by a C3aR antagonist. In contrast, the Nrf2 activator CDDO-Im restored complement gene expression in RPE cells exposed to CSE. We provide evidence of altered Nrf2 in human AMD, and that CSE induces a pro-inflammatory environment specifically by generating C3a and C3b, and Nrf2 deficiency magnifies this specific complement response.

Age-related macular degeneration (AMD) is the leading cause of blindness among the elderly. While excellent treatment has emerged for neovascular disease, treatment for early AMD is lacking due to an incomplete understanding of the early molecular events. A prominent age-related change is the accumulation of neutral lipid in normal Bruch's membrane (BrM) throughout adulthood and also disease-related BrM accumulations called basal deposits and drusen. AMD lesion formation has thus been conceptualized as sharing mechanisms with atherosclerotic plaque formation, where low-density lipoprotein (LDL) retention within the arterial wall initiates a cascade of pathologic events. However, we do not yet understand how lipoproteins contribute to AMD. This paper explores how systemic and local production of lipoproteins might contribute to the pathogenesis of AMD.