Endocytosis of Peptide Hormones and Other Ligands

ENDOCYTOSIS refers to the uptake of materials into cells from the external environment by means of vesicles derived from invaginations of the cell surface. Several types of endocytosis have been distinguished (1). Phagocytosis (cell eating) describes the uptake of large particulate matter; whereas pinocytosis (cell drinking) refers to the internalization of fluid and small particles. Although the term endocytosis in its general sense subsumes phagocytototic behavior, it has frequently been used interchangeably with pinocytosis, a universal activity of living cells. The terms adsorptive and fluid phase endocytosis (pinocytosis) distinguish between the uptake of macromolecules bound to the cell surface (and eventually the inner membrane of vesicles) and the uptake of solute in the extracellular fluid. The former is a device for both selecting and concentrating specific solutes for internalization by virtue of their binding to cell surface receptors. Receptor-mediated endocytosis has now been described for a diversity of substances including lipoproteins, glycoproteins, hydrolytic enzymes, toxins, hormones, and growth factors. In this review we focus on the receptor-mediated endocytosis of polypeptide hormones and growth factors but shall selectively refer to instructive observations on other substances as well.