Polarized secretion of apoA-I and apoA-II by transfected MDCK cells

• Published in: Journal of lipid research 1995-03, Vol.36 (3), p.407-413
• Main Authors: Remaley, A T, Hoeg, J M
• Format: Article
• Language: English
• Subjects: Animals; Apolipoprotein A-I - genetics; Apolipoprotein A-I - metabolism; Apolipoprotein A-II - genetics; Apolipoprotein A-II - metabolism; Cell Line; Cell Polarity; Centrifugation, Density Gradient; Dogs; Kidney; Transfection
• ISSN: 0022-2275
• DOI: 10.1016/S0022-2275(20)39875-8

Apolipoproteins (apo) are secreted preferentially from the basolateral surface of hepatocytes and enterocytes. The polarized secretion of proteins is either mediated by a protein-dependent sorting signal or by a cell-dependent default pathway. In order to determine the mechanism for the polarized secretion of apolipoproteins, we examined the secretion of apoA-I and apoA-II in transfected Madin-Darby canine kidney (MDCK) cells. Transfected MDCK cells and Caco-2 cells were grown as a polarized monolayer on tissue culture inserts, which separate an upper apical compartment from the lower basolateral compartment, and the secretion of apoA-I and apoA-II into the apical and basolateral compartments was quantitated by immunoprecipitation. Caco-2 cells almost exclusively secreted apoA-I and apoA-II basolaterally, with an apical to basolateral ratio of 18:82 for apoA-I, and 11:89 for apoA-II. In contrast, transfected MDCK cells secreted significant amounts of apoA-I and apoA-II into both compartments, but with a bias toward apical secretion and an apical to basolateral ratio of 66:34 and 68:32, respectively. The polarized secretion of MDCK cells was not due to transcytosis, diffusion, or differential recovery. As assessed by density gradient ultracentrifugation, apoA-I and apoA-II secreted from either the apical or basolateral surface were relatively lipid-poor. Overall, these results suggest that the polarized secretion of apoA-I and apoA-II does not occur by a protein-dependent sorting signal, but by a cell-dependent default pathway that leads to preferential basolateral secretion by Caco-2 cells and both apical and basolateral secretion in MDCK cells, but with a bias toward apical secretion.