ABSTRACT
Cellular senescence is a hallmark of aging and has been linked to age-related diseases. Age-related macular degeneration (AMD), the most common aging-related retinal disease, is prospectively associated with retinal pigment epithelial (RPE) senescence. However, the mechanism of RPE cell senescence remains unknown. In this study, tert-butyl hydroperoxide (TBH)-induced ARPE-19 cells and D-galactose-treated C57 mice were used to examine the cause of elevated iron in RPE cell senescence. Ferric ammonium citrate (FAC)-treated ARPE-19 cells and C57 mice were used to elucidated the mechanism of iron overload-induced RPE cell senescence. Molecular biology techniques for the assessment of iron metabolism, cellular senescence, autophagy, and mitochondrial function in vivo and in vitro. We found that iron level was increased during the senescence process. Ferritin, a major iron storage protein, is negatively correlated with intracellular iron levels and cell senescence. NCOA4, a cargo receptor for ferritinophagy, mediates degradation of ferritin and contributes to iron accumulation. Besides, we found that iron overload leads to mitochondrial dysfunction. As a result, mitochondrial DNA (mtDNA) is released from damaged mitochondria to cytoplasm. Cytoplasm mtDNA activates the cGAS-STING pathway and promotes inflammatory senescence-associated secretory phenotype (SASP) and cell senescence. Meanwhile, iron chelator Deferoxamine (DFO) significantly rescues RPE senescence and retinopathy induced by FAC or D-gal in mice. Taken together, these findings imply that iron derived from NCOA4-mediated ferritinophagy causes cellular senescence via the cGAS-STING pathway. Inhibiting iron accumulation may represent a promising therapeutic approach for age-related diseases such as AMD.
ABSTRACT
Diabetic retinopathy (DR) is a serious and relatively under-recognized complication of diabetes. Müller glial cells extend throughout the retina and play vital roles in maintaining retinal homeostasis. Previous studies have demonstrated that TGR5, a member of the bile acid-activated GPCR family, could ameliorate DR. However, the role of TGR5 in regulating Müller cell function and the underlying mechanism remains to be ascertained. To address this, high glucose (HG)-treated human Müller cells and streptozotocin-treated Sprague-Dawley rats were used in the study. The IP3R1-GRP75-VDAC1 axis and mitochondrial function were assessed after TGR5 ablation or agonism. Cytosolic mitochondrial DNA (mtDNA)-mediated cGAS-STING activation was performed. The key markers of retinal vascular leakage, apoptosis, and inflammation were examined. We found that mitochondrial Ca2+ overload and mitochondrial dysfunction were alleviated by TGR5 agonist. Mechanically, TGR5 blocked the IP3R1-GRP75-VDAC1 axis mediated Ca2+ efflux from the endoplasmic reticulum into mitochondria under diabetic condition. Mitochondrial Ca2+ overload led to the opening of the mitochondrial permeability transition pore and the release of mitochondrial DNA (mtDNA) into the cytosol. Cytoplasmic mtDNA bound to cGAS and upregulated 2'3' cyclic GMP-AMP. Consequently, STING-mediated inflammatory responses were activated. TGR5 agonist prevented retinal injury, whereas knockdown of TGR5 exacerbated retinal damage in DR rats, which was rescued by the STING inhibitor. Based on the above results, we propose that TGR5 might be a novel therapeutic target for the treatment of DR.
