Specific uptake of alpha-fetoprotein and albumin by rat Morris 7777 hepatoma cells.
Alpha-fetoprotein (AFP) is a major globulin of embryonic plasma and a physiological carrier of unesterified fatty acids. In the present work, we have characterized the interaction of AFP and albumin, a major serum protein of adult mammals which presents numerous biochemical analogies with AFP, with the plasma membrane of the rat Morris 7777 hepatoma cells. Time course analysis of the uptake of AFP and albumin by these cells showed a saturable profile at 4 degrees C and 37 degrees C. Saturable binding of 125I-AFP or 125I-albumin were observed when the concentration of these proteins increased (ranging from 0.3 to 4.5 microM). The Hill and Scatchard analysis revealed the existence of binding sites in the surface of hepatoma cells, with a k'd = 2.2 x 10(-6) M (2.9 x 10(6) sites/cell) in the case of AFP and a k'd = 4.5 x 10(-6) M (3.9 x 10(6) sites/cell) in the case of albumin. 125I-AFP and 125I-albumin bound to the cells were completely displaced in the presence of a 200-fold excess of unlabeled AFP or albumin, respectively, suggesting that these interactions were specific. We have observed crossed competition between AFP and albumin for their respective binding sites; no such crossed competition was observed when an excess of unlabeled transferrin was added. Pulse-chase experiments showed that about 50% of the AFP and 75% of the albumin taken up by the cells were released undegraded into the medium after 1 h. Cytochemical studies performed with covalent conjugates of AFP, albumin and transferrin with horseradish peroxidase have shown that AFP and albumin entered the cells via a vesicular system. This intracellular pathway is different from that of transferrin, a plasma protein whose internalization mediated by specific receptors via coated pits has been reported in other cells. The results presented here suggest that AFP and albumin interact with sites in the membrane of hepatoma cells, probably physically related, and then they are transported inside the cells by a mechanism different from that described for transferrin.