Cloning and functional characterization of human SMCT2 (SLC5A12) and expression pattern of the transporter in kidney

Recently, we cloned two Na⁺-coupled lactate transporters from mouse kidney, a high-affinity transporter (SMCT1 or slc5a8) and a low-affinity transporter (SMCT2 or slc5a12). Here we report on the cloning and functional characterization of human SMCT2 (SLC5A12) and compare the immunolocalization patterns of slc5a12 and slc5a8 in mouse kidney. The human SMCT2 cDNA codes for a protein consisting of 618 amino acids. When expressed in mammalian cells or Xenopus oocytes, human SMCT2 mediates Na⁺-coupled transport of lactate, pyruvate and nicotinate. The affinities of the transporter for these substrates are lower than those reported for human SMCT1. Several non-steroidal anti-inflammatory drugs inhibit human SMCT2-mediated nicotinate transport, suggesting that NSAIDs interact with the transporter as they do with human SMCT1. Immunofluorescence microscopy of mouse kidney sections with an antibody specific for SMCT2 shows that the transporter is expressed predominantly in the cortex. Similar studies with an anti-SMCT1 antibody demonstrate that SMCT1 is also expressed mostly in the cortex. Dual-labeling of SMCT1 and SMCT2 with 4F2hc (CD98), a marker for basolateral membrane of proximal tubular cells in the S1 and S2 segments of the nephron, shows that both SMCT1 and SMCT2 are expressed in the apical membrane of the tubular cells. These studies also show that while SMCT2 is broadly expressed along the entire length of the proximal tubule (S1/S2/S3 segments), the expression of SMCT1 is mostly limited to the S3 segment. These studies suggest that the low-affinity transporter SMCT2 initiates lactate absorption in the early parts of the proximal tubule followed by the participation of the high-affinity transporter SMCT1 in the latter parts of the proximal tubule.