alpha-Fetoprotein-mediated transfer of arachidonic acid into cultured cloned cells derived from a rat rhabdomyosarcoma.

alpha-Fetoprotein (AFP) is a major constituent of embryonal plasma and a physiological carrier of free fatty acids. The purpose of the present work was to study the mechanism by which fatty acids bound to AFP are transferred to living cells. Radiolabeled rat AFP and arachidonic acid were used to follow up the uptake and metabolism of both the protein and the fatty acid by rhabdomyosarcoma cells isolated from a nickel-induced rat tumor. Time course uptake of AFP and arachidonic acid by these cells exhibited a saturable profile at both 4 and 37 degrees C. A diffusible nonsaturable uptake of arachidonic acid was observed in experiments at both 4 and 37 degrees C with preparations of fixed AFP content and increased molar amounts of arachidonic acid (up to 8-fold molar excess). On the contrary, saturable binding and uptake of fatty acid and protein were evidenced when the molar ratio of arachidonic to AFP was fixed at 0.5, and the concentration of both increased simultaneously. This suggests that, under physiological conditions (low fatty acid to AFP ratio), the uptake of arachidonic acid by the tumor cells is regulated by the protein. Fatty acid distribution in cell lipids after 2 and 24 h of culture at 37 degrees C, in the presence of arachidonic acid bound to AFP, revealed that this fatty acid was mainly incorporated in cell phospholipids. At 4 degrees C, however, the totality of cell-associated arachidonic acid was in the unesterified form. Pulse-chase experiments showed that about 25 and 40% of the AFP initially taken up by cells were released undegraded after 6 and 60 min, respectively. Under the same conditions, nearly all the arachidonic acid remained in the cells. Taken together, these facts suggest a two-receptor model for the physiological uptake of fatty acids. The AFP binds to an AFP receptor and the fatty acid is then removed and transported inside the cell by a specific fatty acid-binding protein.