Iron antagonism of DICER1 promotes NLRP3 inflammasome priming due to enhanced Alu RNA stability

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Abstract

Purpose

Excessive free iron induces retinal toxicity in several human diseases and has been linked to age-related macular degeneration. Iron toxicity is widely attributed to its ability to catalyze hydroxyl radical formation through Fenton’s reaction. Iron was recently described as an inhibitor of canonical DICER1 enzymatic activity through sequestration of the cofactor poly(C)-binding protein 2 (PCBP2). We sought to determine whether iron similarly impaired non-canonical DICER1 activity to clear cytotoxic Alu RNA in the retinal pigemented epithelium.

Methods

Iron overload was induced in human ARPE-19 cells by supplementing culture media with ferric ammonium citrate for 72 hours and in wild-type mice by subretinal injection. Alu, B1 and B2 RNA abundance was measured by northern blotting. Alu RNA/DICER1/PCBP2 binding was assessed by immunoprecipitation, western blotting and LC-MS. Alu RNA cleavage efficiency was evaluated using synthetic in vitro transcribed Alu RNA and recombinant human DICER1 and PCBP2. Inflammasome priming was assessed by quantifying NLRP3 mRNA via qRT-PCR.

Results

Iron overload in cells and mouse retina induces robust accumulation of Alu transcripts as well as the rodent homologs B1 and B2 RNAs. Iron overload impairs Alu RNA clearance by RPE cells, independently of DICER1 mRNA abundance. Alu RNA binds to the iron sensitive co-factor PCBP2, which enhances DICER1-mediated Alu RNA cleavage. Iron overload or siRNAs targeting PCBP2 prime the NRLP3 inflammasome, which can be prevented by antisense-mediated Alu RNA antagonism.

Conclusion

These data suggest that Alu RNA induced inflammasome signaling could contribute to retinal iron toxicity in addition catalytic free radical generation.

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