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 2M) 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 125I- or gold-labeled beta 2M (125I-beta 2M and gold-beta 2M) 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 2M and LLC-PK1 cells grown on porous supports, we showed that a functional uptake system for the LMWP beta 2M was present at the BL surface. Finally, using different-size gold particles conjugated to beta 2M 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 2M can be internalized across both apical and BL domains and that the two pathways converge at a multivesicular level within the endosomal pathway.