IPG (INOSITOLPHOSPHATE GLYCAN) AS A CELLULAR SIGNAL FOR TGF-BETA-1 MODULATION OF CHONDROCYTE CELL-CYCLE

The knowledge of transforming growth factor (TGF)-beta receptors has g reatly progressed in the recent years. TGF-beta receptors type f and I I have been implicated in the modulation of cell proliferation, wherea s type III (betaglycan) may act as a component presenting TGF-beta to its signaling receptors. In addition, four other proteins that bind TG F-beta1 or TGF-beta2 have been recently identified in some cell lines, three being anchored to the membrane through a glycosylphosphatidylin ositol (GPI). Despite this knowledge, the molecular mechanism of signa l transduction through the TGF-beta receptors remain an enigma. TGF-be ta family does not signal via any of the classical pathways. As GPI an chors of membrane proteins have been implicated in the transduction of some hormonal effects, we investigated the putative role of GPI in si gnaling the TGF-beta effects on the proliferation of rabbit articular chondrocytes (RAC). We previously showed that TGF-beta1 increased DNA replication rate of RAC, with a recruitment of cells in G2/M followed by a subsequent mitosis wave. Here, we find that the factor causes spe cific GPI hydrolysis, with correlated increase of inositolphosphate gl ycan (IPG). This effect was specifically inhibited by antibodies that bind TGF-beta1. Using [H-3]-inositol labeling and Triton X-114 extract ion, we demonstrate that a hydrophobic material from the membrane is c leaved by treatment of cell cultures with phosphatidylinositol specifi c phospholipase C (PI-PLC) or by exposure to TGF-beta, supporting that a PI-anchored molecule gives rise to IPG by TGF-beta-induced hydrolys is. The biological relevance of this hydrolysis was demonstrated by th e enhancing effect of purified IPG on the DNA synthesis rate, which mi micked the TGF-beta action. These results demonstrate that IPG could b e an early messenger in the cellular signaling that mediates the effec t of TGF-beta on RAC growth.