A novel role of follistatin, an activin-binding protein, in the inhibition of activin action in rat pituitary cells. Endocytotic degradation of activin and its acceleration by follistatin associated with cell-surface heparan sulfate.
There are two types of the activin-binding protein follistatin (FS), FS-288 and FS-315. These result from alternative splicing of mRNA. FS-288 exhibits high affinity for cell-surface heparan sulfate proteoglycans, 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 using 125I-activin A demonstrated that activin bound to rat pituitary cells via FS as well as to their receptors on the cell surface. FS-288 promoted the binding of activin A to the cell surface more markedly than FS-315. When the cells were incubated with 125I-activin A in the presence of FS-288, significant degradation of activin A was observed, and this was dependent on the FS-288 concentration. This activin degradation was abolished by heparan sulfate, chloroquine, and several lysosomal enzyme inhibitors. Moreover, FS-288 stimulated cellular uptake of activin A, whereas chloroquine suppressed lysosomal degradation following internalization, as demonstrated by microscopic autoradiography. These results suggest that cell-associated FS-288 accelerates the uptake of activin A into pituitary cells, leading to increased degradation by lysosomal enzymes, and thus plays a role in the activin clearance system.
Hashimoto
O
O
Institute for Enzyme Research, University of Tokushima, Kuramoto, Tokushima 770, Japan.
Nakamura
T
T
Shoji
H
H
Shimasaki
S
S
Hayashi
Y
Y
Sugino
H
H
D029404:Activin Receptors; D028341:Activins; D017398:Alternative Splicing; D000818:Animals; D019556:COS Cells; D002454:Cell Differentiation; D004705:Endocytosis; D005260:Female; D005640:Follicle Stimulating Hormone / Q000037:antagonists & inhibitors*; D038681:Follistatin; D006023:Glycoproteins / Q000235:genetics / Q000378:metabolism*; D006133:Growth Substances / Q000378:metabolism*; D006497:Heparan Sulfate / Q000378:metabolism*; D006801:Humans; D007265:Inhibins / Q000037:antagonists & inhibitors*; D008970:Molecular Weight; D010902:Pituitary Gland / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D017978:Receptors, Growth Factor / Q000378:metabolism; D013552:Swine
D029404:Activin Receptors; D028341:Activins; D005640:Follicle Stimulating Hormone; D038681:Follistatin; D006023:Glycoproteins; D006133:Growth Substances; D006497:Heparan Sulfate; D007265:Inhibins; D017346:Protein Serine-Threonine Kinases; D012333:RNA, Messenger; D017978:Receptors, Growth Factor