Inhibin binding sites and proteins in pituitary, gonadal, adrenal and bonecells

Activin signals via complexes of type I (50-55 kDa) and II (70- 75 kDa) act ivin receptors. but the mechanism of inhibin action is unclear. Proposed mo dels range from an anti-activin action at the type II activin receptor to i ndependent actions involving putative inhibin receptors. Two membrane-embed ded proteoglycans. betaglycan and p120, have recently been implicated in in hibin binding. but neither appears to be a signalling receptor. The present studies on primary cultures of rat pituitary and adrenal cells, and severa l murine and human cell lines were undertaken to characterise inhibin bindi ng to its physiological targets. High affinity binding of inhibin to the pr imary cultures and several of the cell lines, like that previously describe d for ovine pituitary cells. was saturable and reversible. Scatchard analys is revealed two classes of binding sites (K-d of 40-400 and 500-5000 pM. re spectively). Affinity labelling identified [I-125]inhibin binding proteins with apparent molecular weights of 41, 74, 114 and > 170 kDa in all cell ty pes that displayed high affinity, high capacity binding of inhibin. Additio nal labelling of a 124 kDa species was evident in gonadal TM3 and TM4 cell lines. In several cases, activin (greater than or equal to 20 nM) competed poorly or not at all for binding to these proteins. The 74, 114 and > 170 k Da inhibin binding proteins in TM3 and TM4 cells were immunoprecipitated by an anti-betaglycan antiserum. These three proteins correspond in size to t he activin receptor type II and the core protein and glycosylated forms of betaglycan, respectively, that have been proposed to mediate anti-activin a ctions of inhibin, but the identity of the 74 kDa species is yet to be conf irmed. Studies of [I-125]inhibin binding kinetics and competition for affin ity labelling of individual binding proteins in several cell lines suggest these three species and the 41 and 124 kDa proteins form a high affinity in hibin binding complex. In summary, common patterns of inhibin binding and a ffinity labelling were observed in inhibin target cells. Novel inhibin bind ing proteins of around 41 and 124 kDa were implicated in the high affinity binding of inhibin to cells from several sources. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.