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 rena l glomeruli and is a high-affinity ligand for the endocytic receptor megali n expressed in the proximal tubule. This study shows by the use of the prox imal tubule LLC-PK1 cell line that transcobalamin B-12 is internalized by m egalin-mediated endocytosis. After endocytosis and accumulation in endosome s, transcobalamin is degraded and the B-12 molecule is released from the ce lls 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 the pool of circulating vitamin. rel eased at the basolateral side is complexed to two different proteins with t he sizes of transcobalamin and haptocorrin. Furthermore, transcobalamin mRN A was identified by reverse transcription-PCR in LLC-PK1 cells and human an d 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 t ranscellular transport and release of vitamin B-12 complexed to newly synth esized carrier proteins. This mechanism is likely to play a significant rol e in the maintenance of B-12 homeostasis by returning filtered B-12 to the pool of circulating vitamin.