αvβ5 Integrin/FAK/PGC-1α Pathway Confers Protective Effects on Retinal Pigment Epithelium

Purpose To elucidate the mechanism of the induction of peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1 alpha) by photoreceptor outer segments (POS) and its effects on retinal pigment epithelium (RPE). Methods PGC-1 alpha upregulation by POS was confirmed in ARPE-19 cells and in RPE ex vivo. To elucidate the mechanism, siRNAs against beta 5 integrin, CD36, Mer tyrosine kinase (MerTK), and Atg5, blocking antibodies against CD36 and MerTK, and a specific inhibitor for focal adhesion kinase (FAK) were used. We examined the effect of POS-induced PGC-1 alpha upregulation on the levels of reactive oxygen species (ROS), mitochondrial biogenesis, senescence-associated beta-galactosidase (SA-beta-gal) after H2O2 treatment, and lysosomal activity. Lysosomal activity was evaluated through transcriptional factor EB and its target genes, and the activity of cathepsin D. Lipid metabolism after POS treatment was assessed using Oil Red O and BODIPY C11. RPE phenotypes of PGC-1 alpha-deficient mice were examined. Results POS-induced PGC-1 alpha upregulation was suppressed by siRNA against beta 5 integrin and a FAK inhibitor. siRNAs and blocking antibodies against CD36 and MerTK enhanced the effect of POS on PGC-1 alpha. The upregulation of PGC-1 alpha increased the levels of mRNA for antioxidant enzymes and stimulated mitochondrial biogenesis, decreased ROS levels, and reduced SA-beta-gal staining in H2O2-treated ARPE-19 cells. PGC-1 alpha was critical for lysosomal activity and lipid metabolism after POS treatment. PGC-1 alpha-deficient mice demonstrated an accumulation of type 2 lysosomes in RPE, thickening of Bruch's membrane, and poor choriocapillaris vasculature. Conclusions The binding, but not the internalization of POS confers protective effects on RPE cells through the alpha v beta 5 integrin/FAK/PGC-1 alpha pathway.