Three secretory rates in human hepatoma cells

It is presently unknown what factors regulate the rate of intracellular transport of secretory proteins. The human hepatoma cell line Hep G2 is highly differentiated and secretes many of the proteins characteristic of normal hepatocytes. The secretion kinetics of nine proteins by Hep G2 cells in culture was investigated using pulse-chase techniques and immunoisolation of proteins with monospecific antibodies. Our results show that the export of nine proteins falls into three discrete kinetic classes: (i) a rapidly secreted class with an intracellular retention half-time of 30-40 min (albumin, fibronectin, alpha-fetoprotein and alpha 1-proteinase inhibitor), (ii) an intermediate secreted class with a half-time of 75-80 min (ceruloplasmin, alpha 2-macroglobulin and plasminogen), (iii) and a slowly secreted class with an intracellular retention half-time of 110-120 min (fibrinogen and transferrin). Our findings that there are three distinct kinetic classes of secretory proteins strongly suggests that intracellular transport is selective and that proteins of the same secretory class share structural features which influence their rate of export.