RECEPTOR-MEDIATED ENDOCYTOSIS OF IMMUNOGLOBULIN LIGHT-CHAINS BY RENALPROXIMAL TUBULE CELLS

We examined the binding, endocytosis, and degradation of immunoglobuli n light chains by primary cultures from rat renal kidneys and immortal ized human proximal tubule cells. Both the association and dissociatio n of light chain were rapid and plateaued within 30 min at 4 degrees C . Up to 10(-3) M bovine serum albumin did not inhibit light chain bind ing to cells. Internalization studies with I-125-labeled kappa- and la mbda-light chains by cells using the acid wash technique showed that u p to 80% of total cell-associated binding at equilibrium (30 min) is r apidly internalized at 22 degrees C. Comparison of binding and interna lization of light chains with transferrin, a ligand known to undergo r eceptor-mediated endocytosis, showed that both ligands displayed satur able kinetics. In contrast, endocytosis of sucrose, a marker for fluid -phase endocytosis, was unsaturable and nearly 200-fold less efficient than light chain internalization. Scatchard analysis of binding exper iments done at 4 degrees C with trace I-125-labeled lambda-light chain in presence of 0 to 3.0 x 10(-3) M cold light chain revealed a single class of binding sites with a dissociation constant of 5.0 +/- 0.8 x 10(-5) and a maximal binding capacity of 1.6 +/- 0.3 x 10(-9) mol/mg c ell protein. Hypertonic medium, a maneuver which interferes with the f ormation of the clathrin lattice, reduced endocytosis of light chain s ignificantly but did not affect endocytosis of sucrose. Chloroquine an d bafilomycin A, agents that interfere with vesicular acidification, a lso significantly suppressed light chain endocytosis. Using acid preci pitation method, we observed that endocytosis of I-125-labeled lambda- light chain results in degradation by the rat renal proximal tubule ce lls. Degradation was maximum at 37 degrees C, significantly reduced at 22 degrees C, and absent at 4 degrees C. Excess light chain inhibited degradation of radiolabel, whereas excess albumin had no effect. Thes e studies document the presence of binding sites for light chains on p roximal tubule cells that mediate endocytosis of light chains by proxi mal tubule cells. The present data suggest that receptor-mediated endo cytosis of light chains leads to delivery of this ligand to degradativ e sites through acidified vesicles.