Autophagy is induced by raptor degradation via the ubiquitin/proteasome system in granular corneal dystrophy type 2.

Abstract

Granular corneal dystrophy type 2 (GCD2) is an autosomal dominant disorder that is caused by a point mutation in transforming growth factor-β-induced gene-h3 (TGFBI), which encodes transforming growth factor-β-induced protein (TGFBIp). Recently, we found that the autophagic clearance of mutant-TGFBIp is delayed in GCD2 corneal fibroblasts; however, any potential correlation between mutant-TGFBIp turnover and autophagy-lysosome pathway remains unknown. Here, we report that mutant-TGFBIp is accumulated and that autophagy, a key clearance pathway for mutant-TGFBIp, is induced in primary cultured GCD2 homozygous (HO) and wild-type (WT) corneal fibroblasts that express exogenously introduced mutant-TGFBIp. Mutant-TGFBI colocalized with LC3-enriched cytosolic vesicles and cathepsin D in primary cultured GCD2 corneal fibroblasts. We also observed reduced levels of raptor (regulatory-associated protein of the mammalian target of rapamycin [mTOR]) in GCD2 corneal fibroblasts and WT corneal fibroblasts expressing mutant-TGFBIp. Strikingly, treatment with MG132, a ubiquitin/proteasome system inhibitor, significantly increased the levels of both total and ubiquitinated raptor in GCD2 corneal fibroblasts. The levels of the autophagy marker LC3-II were also increased in WT corneal fibroblasts that were treated with shRNA against raptor. However, mutant-TGFBIp accumulated in autophagosomes or/and lysosomes in spite of the significant activation of basal autophagy in GCD2 corneal fibroblasts. These results suggest that an insufficient autophagy-lysosome pathway might be responsible for the intracellular accumulation of mutant-TGFBIp during the pathogenesis of GCD2.

• Authors: Seung-Il Choi, Yong-Sun Maeng, Kyu Seo Kim, Tae-Im Kim, Eung Kweon Kim
• Journal: Biochemical and biophysical research communications (Biochem Biophys Res Commun) Vol. 450 Issue 4 Pg. 1505-11 (Aug 08 2014) ISSN: 1090-2104 [Electronic] United States
• PMID: 25044116 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)
• Copyright: Copyright © 2014 Elsevier Inc. All rights reserved.

Chemical References

• Adaptor Proteins, Signal Transducing
• RPTOR protein, human
• Regulatory-Associated Protein of mTOR
• Ubiquitin
• Proteasome Endopeptidase Complex

Topics

• Adaptor Proteins, Signal Transducing (metabolism)
• Autophagy
• Corneal Dystrophies, Hereditary (enzymology, metabolism)
• Humans
• Proteasome Endopeptidase Complex (metabolism)
• Proteolysis
• Regulatory-Associated Protein of mTOR
• Ubiquitin (metabolism)