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C.N. Roybal, L.Y. Marmorstein, D.L. Vander Jagt, S.F. Abcouwer; Aberrant Accumulation of EFEMP1 Leads to Activation of the Unfolded Protein Response and VEGF Expression . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1648.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Choroidal neovascularization (CNV) subsequent to age–related macular degeneration (AMD) is a leading cause of visual impairment. Aberrant expression of vascular endothelial growth factor (VEGF) by the retinal pigment epithelium (RPE) is thought to cause AMD associated CNV by an unknown mechanism. We recently demonstrated that VEGF transcription is regulated by the unfolded protein response (UPR) via activating transcription factor 4 (ATF4). Two inherited early–onset macular degenerative retinal diseases Malattia Leventinese (ML) and Doyne honeycomb retinal dystrophy (DHRD) have been linked to a point mutation (R345W) in fibulin–3 (EFEMP1). EFEMP1 R345W mutant protein is poorly secreted and accumulates in the RPE cell layer of post–mortem ML retinas. This led to the hypothesis that ER aggregation of EFEMP1 R345W results in activation of the UPR, and induces VEGF expression. Methods: An RPE cell line (ARPE–19) was exposed to adenoviral vectors containing EFEMP1 wild type (Wt) and EFEMP1 R345W expression cassettes. The effect of EFEMP1 Wt and R345W expression on UPR activation and VEGF expression was tested by Northern blotting and Western blotting. Aggregation of EFEMP1 was confirmed by immunocytochemistry. VEGF trancription was measured with a VEGF promoter construct and a dual luciferase system. Results: We confirmed that EFEMP1 R345W protein aggregates in the ER and is poorly secreted compared to its Wt control. GRP78/BiP mRNA and protein upregulation demonstrated UPR activation by EFEMP1 R345W. UPR activation was accompanied by increased expression of VEGF mRNA and protein. EFEMP1 R345W increased VEGF transcription as it activated an 8.2kB VEGF promoter construct. Conclusions: These findings point to a novel mechanism by which a mutated EFEMP1 could accumulate within the ER of RPE cells activating the UPR leading to increased VEGF expression. The study suggests that the UPR could be a common pathway activated by mutated proteins associated with AMD and early–onset retinal dystrophies. Activation of the UPR could explain the end–stage choroidal neovascularization and RPE dysfunction in AMD patients.
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