Investigation of the pathological function of PGC1B in the retinal pigment epithelium and its implications for age-related macular degeneration
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Age-Related Macular Degeneration (AMD) is a retinal eye disease that is the leading cause of blindness in those over 50 years of age throughout the developed world. Oxidative and metabolic dysfunction of the retinal pigment epithelium (RPE) has been shown to play an important role in AMD. However, the mechanism of dysfunction in the RPE is poorly understood. The peroxisome proliferator-activated receptor-gamma coactivator 1α and β (PGC1A and PGC1B) are coactivators that interact with transcription factors to regulate mitochondria metabolism. In a previous study, it was demonstrated that one of the isoforms, PGC1A, protects RPE cells from oxidative stress through the upregulation of transcription factors that regulate important antioxidant enzymes. There is experimental and clinical evidence that demonstrates that PGC1B may play a deleterious role in the RPE cell. The objective of this study is to characterize the pathological effect of PGC1B on the RPE cell. PGC1B was overexpressed in the human retinal pigment epithelium cell line (ARPE-19) and expression of the PGC1 isoforms and their main gene targets was evaluated using quantitative polymerase chain reaction (qPCR). Cell death was evaluated under basal and pro-oxidant conditions by quantification of lactate dehydrogenase (LDH) release from the RPE cell. The effect of PGC1B gain of function on the RPE pro-angiogenic function was evaluated using the choroid explant sprouting assay and by testing the proliferative, migratory, and tube formation potential of RPE-derived conditioned media on the rhesus monkey chorioretinal cell line (RF/6A). Quantitative PCR analysis showed that overexpression of PGC1B in ARPE-19 cells leads to increased mitochondrial metabolism and decreased antioxidant enzyme expression, causing oxidative stress. After treatment with H2O2, PGC1B overexpression caused ARPE-19 cells to become more susceptible to cytotoxicity. The ex vivo choroid sprouting assay demonstrated that PGC1B overexpression in RPE is pro-angiogenic. However, cell proliferation as measured by MTT and the cell migration assay provided conflicting results on the pro-angiogenic effect of PGC1B. Previous research has demonstrated that oxidative stress in the RPE cell plays a role in AMD progression. It has been demonstrated in this study that PGC1B expression leads to increased mitochondrial metabolism and repression of antioxidant enzymes needed to prevent oxidative stress and dysfunction in the RPE cell. While experiments to test the effect of PGC1B on angiogenesis provided conflicting results, a different endothelial cell model may be better suited in demonstrating the pro-angiogenic effect of PGC1B. The hope is that the information provided from this study may be used to further our understanding of AMD and lead to the development of therapeutic targets to combat the effects of AMD.