FOLLISTATINS NEUTRALIZE ACTIVIN BIOACTIVITY BY INHIBITION OF ACTIVIN-BINDING TO ITS TYPE-II RECEPTORS

Follistatin is an activin-binding protein, which inhibits activin bioa ctivity in several biological systems. In the present study it is demo nstrated that preincubation of iodinated activin A with follistatin, p urified from porcine follicular fluid, completely abolished the bindin g of activin to activin type IIA, IIB2 and IIB3 receptors, and consequ ently to activin type IB receptor, transiently transfected in COS cell s. Binding of activin A to membrane proteins on the activin-responsive P19 embryonal carcinoma cells was also prevented by this follistatin preparation. The same results were obtained with a carboxy-terminally truncated form of follistatin (F5-288), which is only present in minor amounts in the purified follistatin preparation. Since FS-288 has a h igh affinity for heparan sulfate proteoglycans on the cell surface, we tested whether membrane-bound FS-288 presents activin A to the differ ent activin receptors, thereby facilitating activin binding. FS-288 di d bind to the cell surface of transfected COS cells, but inhibited the binding of activin A to its receptors IIA, IIB2 and IIB4. Furthermore , after addition of FS-288 to K562 erythroleukemia cells, the total bi nding of activin via cell surface-bound FS-288 was increased, whereas the binding of activin A to activin type II and type I receptors prese nt on these cells was inhibited. These findings reveal that different forms of follistatin can neutralize activin bioactivity by interferenc e with binding of activin to all known activin type II receptors, rath er than that they inhibit the binding of the type I receptor to the ac tivin/activin type II receptor complex. In addition, our studies indic ate that cell surface-associated follistatin cannot present ligand to signalling receptors.