cDNA Cloning of MCT2, a Second Monocarboxylate Transporter Expressed in Different Cells than MCT1

Abstract

Low stringency screening of a cDNA library from hamster liver yielded a cDNA encoding MCT2, a monocarboxylate transporter that is 60% identical to hamster MCT1, the first monocarboxylate transporter to be isolated. The functional properties of the two MCTs were compared by expression in Sf9 insect cells using recombinant baculovirus vectors. Like MCT1, MCT2 transported pyruvate and lactate. The two transporters were sensitive to inhibition by phloretin and by alpha-cyano-4-hydroxycinnamate. MCT1, but not MCT2, was sensitive to organomercurial thiol reagents such as p-chloromercuribenzoic acid. Immunoblotting and immunofluorescence studies revealed a strikingly different tissue distribution of the two MCTs. MCT1 was present in erythrocytes and on the basolateral surfaces of intestinal epithelial cells. MCT2 was not detectable in these tissues, but it was abundant on the surface of hepatocytes. In the stomach, MCT1 was present on the basolateral surfaces of epithelial cells; in contrast, MCT2 was expressed on parietal cells of the oxyntic gland. In the kidney, MCT1 was present on the basolateral surfaces of epithelial cells in proximal tubules, whereas MCT2 was restricted to the collecting ducts. MCT1 was expressed on sperm heads in the testis and proximal epididymis. In the distal epididymis, it disappeared from sperm and appeared on the microvillar surface of the lining epithelium. In contrast, MCT2 was present on sperm tails throughout the epididymis and not on the epithelium. Both transporters were expressed in mitochondria-rich (oxidative) skeletal muscle fibers and cardiac myocytes. These findings suggest that MCT1 and MCT2 are adapted to play different roles in monocarboxylate transport in different cells of the body.