Iron-induced Complement Dysregulation in the Retinal Pigment Epithelium: Implications for Age-Related Macular Degeneration
Age-related macular degeneration (AMD), typically manifesting as a loss of central vision in elderly persons, is a leading cause of blindness in highly developed nations. AMD is a multifactorial disease associated with aging, oxidative stress, complement dysregulation, dsRNA toxicity, among many other possible factors. The formation of extracellular deposits, termed drusen, below the retinal pigment epithelial (RPE) cell layer in the outer retina is a pathognomonic hallmark of AMD. The composition of drusen is complex, but identified elements include iron, complement components, and amyloid protein derivatives. This suggests that iron may be involved in the pathophysiology of AMD. Further support for this hypothesis comes from mice lacking ferroxidases Ceruloplasmin (Cp) and Hephaestin (Heph), which have a primary genetic defect in iron homeostasis. These mice develop some AMD-like morphological features and a telling molecular feature: activated complement component 3 (C3) fragment deposition at the basolateral aspect of the RPE (the location of drusen in AMD). In our studies, we investigated the molecular mechanisms by which C3 is up-regulated by iron in RPE cells. ERK1/2, SMAD3, and CCAAT/enhancer-binding protein-δ (C/EBP-δ) are part of a non-canonical TGF-β signaling pathway that is responsible for iron-induced C3 expression. Pharmacologic inhibition of either ERK1/2 or SMAD3 phosphorylation decreased iron-induced C3 expression levels. Knockdown of SMAD3 blocked the iron-induced up-regulation and nuclear accumulation of C/EBP-δ, a transcription factor known to promote C3 expression by binding to the basic leucine zipper (bZIP1) domain of the gene promoter. We show herein that mutation of this domain reduced iron-induced C3 promoter activity. The molecular events in the iron-C3 pathway represent therapeutic targets for AMD. To better understand the relative contribution of systemic iron and local dysregulation of iron homeostasis to RPE iron accumulation, we used Bmp6 KO mice and WT mice and found that retinal hepcidin levels are not changed, but in fact may be slightly greater in KO compared to WT mice. As such, systemic iron overload by genetic KO or intravenous supplementation in WT mice resulted in increased RPE labile iron and oxidative stress, suggesting that systemic iron overload may lead to retinal iron overload despite the presence of an intact blood retinal barrier. Systemic iron status appears to be a leading determinant of retinal iron status.
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