Abstract
Abstract: :
Purpose: The retinal pigment epithelium (RPE) expresses two proton–coupled monocarboxylate transporters (MCTs) that regulate the transepithelial transport of lactate, H+ and water from the retinal to the choroid. MCT1 is expressed in the apical membrane of the RPE and MCT3 is polarized to the basolateral membrane. MCT1 has been shown to specifically associate with CD147 suggesting that MCTs are heterodimers composed of a catalytic subunit non–covalently associated with the type I glycoprotein. A role for CD147 in proper expression of MCT3 in the plasma membrane was suggested by studies from my laboratory. We found that the expression of both MCT1 and MCT3 was greatly diminished in the RPE of the CD147 null (bsg–/–) mouse. In the present studies we tested the hypothesis that MCT3 specifically associates with CD147 and regulates its expression in the plasma membrane. Methods: Human MCT3 was stably expressed in the Fisher rat thyroid cell line (FRT–hMCT3). RT–PCR, immunofluorecence, Western blot analysis and cell surface biotinylation were used to assess MCT3, MCT1 and CD147 expression in FRT and FRT–hMCT3 cell lines. Results: The FRT cell line is able to develop a polarized phenotype when grown on plastic or filters. In FRT cells stably expressing hMCT3, the protein was detected in the basolateral membrane, co–localizing with the endogenous MCT1 and CD147 proteins. Over–expression of MCT3 in FRT cells did not alter the level of endogenously expressed MCT1 but increased the expression of CD147 in the plasma membrane. Additionally, both MCT1 and MCT3 were found to co–immunoprecipitate with CD147. Conclusions: The results of these studies establish that CD147 associates with MCT3 demonstrating that CD147 is capable of forming heterodimers with multiple MCTs in the same cell. The results support our hypothesis that in the RPE, both MCT1 and MCT3 require association with CD147 for expression in the plasma membrane.
Keywords: retinal pigment epithelium • pump/barrier function • metabolism