Human acidic fibroblast growth factor overexpressed in insect cells is not secreted into the medium

We have overexpressed human acidic fibroblast growth factor (aFGF) in Spodoptera frugiperda Sf9 cells from a cDNA clone under the control of the promoter of the polyhedrin gene of the baculovirus Autographa californica nuclear polyhedrosis virus. A 16.5-kD product was made in recombinant virus-infected cells that specifically reacted in immunoblots with various antibodies prepared against aFGF. Recombinant aFGF was mitogenic for BHK21 cells and its activity was stimulated by heparin. The mechanism of release of FGF from mammalian cells is unknown. Both acidic and basic FGF lack classical amino-terminal signal sequences for secretion, and they are very inefficiently released from cells. Sf9 cells infected with the recombinant virus produced 10-20 mg aFGF/10(9) cells, corresponding to about 10-20 pg/cell. Despite this high level of expression, only about 0.5 and 1.3% of the total aFGF was found in the culture medium at 48 and 72 hr postinfection, respectively. This indicates that aFGF is not actively secreted out of the cells either via the normal exocytic pathway or directly through the plasma membrane in this heterologous cell system.