Hormone, growth factor, and vitamin handling by proximal tubule cells.

The proximal tubule in the kidney is a main organ for the metabolic clearance of small peptide hormones, and is intensively involved in reabsorption, storage and homeostatic regulation of different vitamins. Recently, the endocytic membrane protein megalin has been shown to bind several smaller peptides, including insulin and epidermal growth factor, as well as to bind and mediate uptake of transcobalamin-B12, attracting new attention on the cellular mechanisms involved in these processes.