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

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.

• Authors: Yafeng Li, Delu Song, Ying Song, Liangliang Zhao, Natalie Wolkow, John W Tobias, Wenchao Song, Joshua L Dunaief
• Journal: The Journal of biological chemistry (J Biol Chem) Vol. 290 Issue 19 Pg. 11918-34 (May 08 2015) ISSN: 1083-351X [Electronic] United States
• PMID: 25802332 (Publication Type: Journal Article, Research Support, N.I.H., Extramural, Research Support, Non-U.S. Gov't)
• Copyright: © 2015 by The American Society for Biochemistry and Molecular Biology, Inc.

Chemical References

• Complement C3
• Ligands
• SMAD3 protein, human
• Smad3 Protein
• Smad3 protein, mouse
• TGFB1 protein, human
• Tgfb1 protein, mouse
• Transforming Growth Factor beta1
• CCAAT-Enhancer-Binding Protein-delta
• Iron

Topics

• Animals
• CCAAT-Enhancer-Binding Protein-delta (metabolism)
• Cell Line
• Complement C3 (metabolism)
• Gene Expression Regulation
• Gene Knockdown Techniques
• Humans
• Iron (chemistry)
• Iron Overload
• Ligands
• Macular Degeneration (metabolism)
• Male
• Mice
• Mice, Inbred C57BL
• Mice, Transgenic
• Middle Aged
• Retinal Pigment Epithelium (metabolism)
• Signal Transduction
• Smad3 Protein (metabolism)
• Transforming Growth Factor beta1 (metabolism)
• Up-Regulation