CONVERGENCE OF APICAL AND BASOLATERAL ENDOCYTIC PATHWAYS FOR BETA(2)-MICROGLOBULIN IN LLC-PK1 CELLS

Polarized epithelial cells internalize molecules from both apical and basolateral (BL) plasma membrane (PM) domains via receptor-mediated endocytosis. In the kidney, low-molecular-weight proteins (LMWP) are internalized across the apical membrane of proximal tubule cells and degraded in lysosomes. Although indirect evidence suggests some uptake may occur at the BL surface, in vivo studies performed in rats suggest little if any LMWP uptake occurs at the BL surface. The studies presented here showed that native human beta(2)-microglobulin (beta(2)M) was internalized across the apical surface and followed the same intracellular pathway in the proximal tubule-derived cell line LLC-PK1 as that described in vivo. Either I-125- or gold-labeled beta(2)M (I-125-beta(2)M and gold-beta(2)M) bound specifically and reversibly to the apical surface of confluent LLC-PK1 cells. These results were qualitatively similar to previously documented in vivo results. Subsequently, using gold-beta(2)M and LLC-PK1 cells grown on porous supports, we showed that a functional uptake system for the LMWP beta(2)M was present at the BL surface. Finally, using different-size gold particles conjugated to beta(2)M applied simultaneously to the apical and BL surfaces, we observed that apical and BL endocytic routes converged in multivesicular acid phosphatase-negative endosomal structures. Taken together, these data imply that beta(2)M can be internalized across both apical and BL domains and that the two pathways converge at a multivesicular level within the endosomal pathway.