ATP-binding cassette transporter G2 mediates the efflux of phototoxins on the luminal membrane of retinal capillary endothelial cells

Purpose: The purpose of this study was to clarify the localization and function of the ATP-binding cassette transporter G2 (ABCG2; BCRP/MXR/ABCP) in retinal capillary endothelial cells, which form the inner blood-retinal barrier, as an efflux transport system. Methods: The expression was determined by reverse transcriptase polymerase chain reaction and Western blotting. The localization was identified by immunostaining. The transport function of ABCG2 was measured by flow cytometry. Results: Western blotting indicated that ABCG2 was expressed as a glycosylated disulfide-linked complex in the mouse retina and in peripheral tissues, including liver, kidney, and small intestine. Double immunolabeling of ABCG2 and glucose transporter 1 suggested that ABCG2 was localized on the luminal membrane of mouse retinal capillary endothelial cells. ABCG2 mRNA and protein were found to be expressed in a conditionally immortalized rat retinal capillary endothelial cell line, TR-iBRB, and rat retina. Treatment with Ko143, an ABCG2 inhibitor, restored the accumulation of pheophorbide a and protoporphyrin IX in TR-iBRB cells. Conclusion: ABCG2 is expressed on the luminal membrane of retinal capillary endothelial cells, where ABCG2 acts as the efflux transporter for photosensitive toxins such as pheophorbide a and protoporphyrin IX. ABCG2 could play an important role at the inner blood-retinal barrier in restricting the distribution of phototoxins and xenobiotics in retinal tissue.