Translocation of FGF2 to the cell surface without release into conditionedmedia

Like most cells in culture, stably transfected COS-1 cells (CF18) that cons titutively overexpress basic fibroblast growth factor (FGF2) do not release the growth factor into conditioned media. Yet. when cells were biotinylate d, 30% of the total cell-associated immunoreactive FGF2 was detected on the cell surface. Under similar conditions, up to 70% of the total immunoreact ive FGF2 in transfected endothelial cells (MAE ZIP) or untransfected rat (C 6) and human (U87MG) glioblastoma cell lines was detected on their cell sur face. When peripheral plasma membrane proteins were removed from the cell s urface with 0.1 M sodium carbonate, the amount of exported FGF2 was signifi cantly reduced, whereas cell viability was unaffected. FGF2 then reappeared on the cell surface in a time-dependent manner; Ouabain, a cardenolide pre viously shown to inhibit the export of FGF2 from transiently transfected CO S-1 cells, blocked the appearance of FGF2 onto the surface of transfected C F18 cells and MAE ZIP cells but had no detectable effect on C6 and U87MG ce lls. The observation that the translocation of FGF2 onto the cell surface i s dissociated from its release into conditioned medium is consistent with F GF2's being rarely found in biological fluids but always cell associated an d in the extracellular matrix. The findings point to a role played by the p rotein export pathway in controlling FGF2 activity and the normal physiolog ical function that this growth Factor plays in cell growth and differentiat ion. The widely accepted presumption that the absence of FGF2 in conditione d media reflects its inability to exit the cell needs to be reevaluated. (C ) 2000 Wiley-Liss, Inc.