Expression and localization of GPR109A (PUMA-G/HM74A) mRNA and protein in mammalian retinal pigment epithelium

Purpose: GPR109A has been identified as a G-protein-coupled receptor for niacin. β-hydroxybutyrate (β-HB) is a physiologic ligand for the receptor. β-HB, the predominate ketone body in circulation, is an important energy source for neurons, including retinal neurons, under various physiologic and pathologic conditions. The identification of GPR109A as the receptor for β-HB suggests additional, hitherto unknown, functions for this metabolite. The circulating levels of β-HB increase in diabetes. Since retinopathy is a serious complication associated with diabetes, we investigated GPR109A expression in retina and in different retinal cell types to determine if the receptor may have a role in the pathophysiology of diabetic retinopathy. Methods: RT-PCR, fluorescent in situ hybridization, and immunofluorescent techniques were used to analyze GPR109A expression in mouse retina and in three transformed retinal cell lines: ARPE-19 (RPE), RGC-5 (ganglion), and rMC-1 (Müller). Activation of GPR109A by niacin and β-HB was demonstrated in ARPE-19 cells by cAMP assay. Results: Studies conducted using mouse retinal tissues demonstrated that GPR109A is expressed in retina with its expression restricted to RPE, where it differentially polarizes to the basolateral membrane. These results were confirmed with cell lines, which demonstrated GPR109A expression in ARPE-19, but not in rMC-1 and RGC-5 cells. Primary cultures of mouse RPE also showed robust expression of GPR109A. cAMP assay demonstrated that GPR109A expressed in RPE is functional. Conclusions: These data represent the first report on GPR109A expression in retina. The exclusive expression of GPR109A in RPE basolateral membrane, which has access to β-HB in blood, may have biologic importance in diabetic retinopathy.