A NOVEL ROLE OF FOLLISTATIN, AN ACTIVIN-BINDING PROTEIN, IN THE INHIBITION OF ACTIVIN ACTION IN RAT PITUITARY-CELLS - ENDOCYTOTIC DEGRADATION OF ACTIN AND ITS ACCELERATION BY FOLLISTATIN ASSOCIATED WITH CELL-SURFACE HEPARAN-SULFATE

There are two types of the activin-binding protein follistatin (BS), B 8-288 and FS-315. These result from alternative splicing of mRNA. FS-2 88 exhibits high affinity for cell-surface heparan sulfate proteoglyca ns, whereas FS-315 shows low affinity. To understand the physiological role of cell associated FS, we investigated the binding of activin to cell-associated FS and its behavior on the cell surface using primary cultured rat pituitary cells. Affinity cross-linking experiments usin g I-I25-activin A demonstrated that activin bound to rat pituitary cel ls via PS as well as to their receptors on the cell surface. FS-288 pr omoted the binding of activin A to the cell surface more markedly than FS-315, When the cells were incubated with I-125-activin A in the pre sence of B8-288, significant degradation of activin A was observed, an d this was dependent on the FS-288 concentration. This activin degrada tion was abolished by heparan sulfate, chloroquine, and several lysoso mal enzyme inhibitors. Moreover, FS-288 stimulated cellular uptake of activin A, whereas chloroquine suppressed lysosomal degradation follow ing internalization, as demonstrated by microscopic autoradiography. T hese results suggest that cell-associated FS-288 accelerates the uptak e of activin A into pituitary cells, leading to increased degradation by lysosomal enzymes, and thus plays a role in the activin clearance s ystem.