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J Biol Chem. 2015 May 8;290(19):11918-34. doi: 10.1074/jbc.M115.645903. Epub 2015 Mar 23.

Iron-induced Local Complement Component 3 (C3) Up-regulation via Non-canonical Transforming Growth Factor (TGF)-β Signaling in the Retinal Pigment Epithelium.

Author information

1
From the F. M. Kirby Center for Molecular Ophthalmology, Scheie Eye Institute.
2
Penn Molecular Profiling Facility, and.
3
Department of Pharmacology, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, Pennsylvania 19104.
4
From the F. M. Kirby Center for Molecular Ophthalmology, Scheie Eye Institute, jdunaief@mail.med.upenn.edu.

Abstract

Dysregulation of iron homeostasis may be a pathogenic factor in age-related macular degeneration (AMD). Meanwhile, the formation of complement-containing deposits under the retinal pigment epithelial (RPE) cell layer is a pathognomonic feature of AMD. In this study, we investigated the molecular mechanisms by which complement component 3 (C3), a central protein in the complement cascade, is up-regulated by iron in RPE cells. Modulation of TGF-β signaling, involving ERK1/2, SMAD3, and CCAAT/enhancer-binding protein-δ, is responsible for iron-induced C3 expression. The differential effects of spatially distinct SMAD3 phosphorylation sites at the linker region and at the C terminus determined the up-regulation of C3. 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 CCAAT/enhancer-binding protein-δ, a transcription factor that has been shown previously to bind the basic leucine zipper 1 domain in the C3 promoter. We show herein that mutation of this domain reduced iron-induced C3 promoter activity. In vivo studies support our in vitro finding of iron-induced C3 up-regulation. Mice with a mosaic pattern of RPE-specific iron overload demonstrated co-localization of iron-induced ferritin and C3d deposits. Humans with aceruloplasminemia causing RPE iron overload had increased RPE C3d deposition. The molecular events in the iron-C3 pathway represent therapeutic targets for AMD or other diseases exacerbated by iron-induced local complement dysregulation.

KEYWORDS:

AMD; C3; CCAAT/Enhancer-binding Protein (C/EBP); Extracellular Signal-regulated Kinase (ERK); Iron; RPE; SMAD3; Signaling; Transforming Growth Factor β (TGF-β)

PMID:
25802332
PMCID:
PMC4424331
DOI:
10.1074/jbc.M115.645903
[Indexed for MEDLINE]
Free PMC Article

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