Transcellular transport of vitamin B(12) in LLC-PK1 renal proximal tubule cells.

The transcobalamin-vitamin B(12) complex is responsible for the transport of B(12) from plasma and into the tissues. The complex is filtered in the renal glomeruli and is a high-affinity ligand for the endocytic receptor megalin expressed in the proximal tubule. This study shows by the use of the proximal tubule LLC-PK1 cell line that transcobalamin-B(12) is internalized by megalin-mediated endocytosis. After endocytosis and accumulation in endosomes, transcobalamin is degraded and the B(12) molecule is released from the cells in complex with newly synthesized proteins. The release is polarized in such a way that vitamin in the apical medium is bound to proteins with the size of haptocorrin, whereas the B(12) released at the basolateral side is complexed to two different proteins with the sizes of transcobalamin and haptocorrin. Furthermore, transcobalamin mRNA was identified by reverse transcription-PCR in LLC-PK1 cells and human and pig kidney, whereas haptocorrin mRNA was identified only in LLC-PK1 cells. The results strongly suggest that megalin located in the proximal tubule cells is important for receptor-mediated tubular reabsorption followed by transcellular transport and release of vitamin B(12) complexed to newly synthesized carrier proteins. This mechanism is likely to play a significant role in the maintenance of B(12) homeostasis by returning filtered B(12) to the pool of circulating vitamin.