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NJ Philp, H Yoon, D Wang; Expression and Polarity of Monocarboxylate Transporters in Human Retinal Pigment Epithelium . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2428.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To identify the monocarboxylate transporters (MCTs)expressed in human retinal pigment epithelium (RPE) in situ and in ARPE-19 cells. Methods: MCT expression in human RPE and ARPE-19 cells was determined using reverse transcription-polymerase chain reaction (RT-PCR) with isoform specific primers. Immunohistochemical localization of MCTs in human donor eyes and ARPE-19 cells was performed using isoform specific peptide antibodies. Specificity of antibodies was determined by Western blot analysis. Results: MCT1 and MCT3 were amplified by RT-PCR from RPE-choroid complex and differentiated ARPE-19 cells. While most cells express MCT1, we previously showed in mouse that MCT3 is preferentially expressed by the RPE. Immunofluorescence microscopy of adult human donor eye revealed a polarized distribution of MCTs in the RPE. MCT1 antibody labeled the apical membrane of the RPE while labeling with MCT3 antibodies was restricted to the basolateral surface. Similarly, immuno-labeling of sections through differentiated ARPE-19 cell cultures showed that MCT1 was polarized to the apical membrane. There was no detectable MCT3 labeling in ARPE-19 cells even though the transcript was expressed. ARPE-19 cells expressed MCT4, a MCT isoform closely related to MCT3. Immunohistochemical labeling of ARPE-19 cells with antibodies specific for MCT4 demonstrated selective labeling of the basolateral membrane. While the RPE cells express two MCT isoforms, only one glucose transporter is expressed, GLUT1. GLUT1 antibody labeled the apical and basolateral membranes of human RPE and ARPE-19 cells. Conclusion: Monocarboxylate transporters (MCTs) are a family of highly homologous membrane proteins that mediate the 1:1 transport of a proton and a lactate ion. Lactate is both an end product and a substrate of energy metabolism in the retina. The expression two distinct MCT isoforms in RPE is consistent with a role for the RPE in regulating lactate levels in the outer retina. The coordinated activities of MCT1 in the apical membrane and MCT3 in the basolateral membrane could control transepithelial movement of lactate.
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