Endocytic uptake, transport, and catabolism of proteins by epithelial cells

Adsorptive and/or receptor-mediated endocytosis of proteins is a universal cell property, which is highly expressed in epithelial cells. Some absorbed proteins are transported intact across cells and in this manner subserve specialized functions such as the transference of immunity from mother to child. Mainly, however, absorbed proteins are transported to lysosomes, where they undergo complete hydrolysis to amino acids. This process is essential for the homeostasis of circulating proteins. This brief review considers the intracellular pathways taken by endocytosed proteins and the quantitative aspects and determinants of protein uptake and catabolism by epithelial cells. The topics to be briefly discussed are initial internalization sites, transport organelles (endosomes), and lysosomal and nonlysosomal pathways of transport; intracellular sorting of internalized proteins, membranes, and receptors; kinetics and selectivity of renal cell uptake of low-molecular-weight proteins and proteohormones; receptor-mediated endocytosis of larger proteins (e.g., glycoproteins) by hepatocytes; and lysosomal catabolism of absorbed proteins and its dependence on protein load and endosomal-lysosomal pH and function. The perspectives of the field and some of the outstanding unsolved problems are briefly discussed.