Expression and Iron-dependent Regulation of Succinate Receptor GPR91 in Retinal Pigment Epithelium

Abstract

Purpose: : Recent studies have shown that GPR91, a succinate receptor, is expressed in retinal ganglion cells and induces vascular endothelial growth factor (VEGF) expression. RPE also expresses VEGF, but whether this cell expresses GPR91 is not known. Excessive iron is also pro-angiogenic, and hemochromatosis is associated with iron overload. Therefore, we examined whether GPR91 is expressed in RPE and whether the expression is regulated by iron.

Methods: : GPR91 expression was examined by RT-PCR and immunohistochemistry. Hemochromatosis mice, cytomegalovirus (CMV) infection of mouse retina, exogenous expression of CMV-US2 in RPE, and exposure of RPE to ferric ammonium citrate (FAC) were used to examine iron-dependent regulation of GPR91 expression. Succinate-induced activation of GPR91 was monitored by bioluminescence resonance energy transfer assay. VEGF expression was quantified by qPCR.

Results: : GPR91 was expressed in RPE but only in the apical membrane. Retinal expression of GPR91 was higher in hemochromatosis (Hfe^-/-) mice than in wildtype mice. Primary RPE cells from Hfe^-/- mice had increased GPR91 expression compared to wildtype RPE cells. Iron accumulation in cells induced by CMV infection, exogenous expression of CMV-US2 or treatment with FAC increased GPR91 expression. VEGF expression in Hfe^-/- mouse retina was increased at ages <18 months, but the expression was downregulated at older ages.

Conclusions: : GPR91 is expressed in RPE with specific localization to the apical membrane, indicating that succinate in the subretinal space serves as the GPR91 agonist. Excessive iron in retina and RPE enhances GPR91 expression; however, VEGF expression does not always parallel GPR91 expression.

Keywords: retinal pigment epithelium • receptors • vascular endothelial growth factor