A 60-KD PROTEIN MEDIATES THE BINDING OF TRANSFORMING GROWTH-FACTOR-BETA TO CELL-SURFACE AND EXTRACELLULAR-MATRIX PROTEOGLYCANS

The biological activity of many cytokines is regulated by binding prot eins present at the cell surface, in extracellular matrices or in solu ble phase. We describe here a TGF-beta binding protein that is both an extracellular matrix and a cell surface protein. When intact extracel lular matrices of HEP-G2 cells were affinity cross-linked with I-125-T GF-beta1, two major binding components were seen: a 250-kD, proteoglyc an-like molecule, presumed to be betaglycan, and a 60-kD protein. The 60-kD TGF-beta-binding protein was also present at the cell surface. I t could be released from the cell surface by treating cells with high salt, heparin, chondroitin sulfate, heparitinase, or chondroitinase, i ndicating that it is bound to heparan sulfate and chondroitin sulfate proteoglycans. The 60-kD protein bound TGF-beta1 with an apparent diss ociation constant of 1.6 nM, and there were 30,000 binding sites per c ell at the cell surface. In addition to the HEP-G2 cells and another h epatoma cell line, the 60-kD protein was also found in a human colon c arcinoma (HT-29) cell line but not in rat kidney (NRK-49F) or human fi broblast (HUT-12) cell lines. The 60-kD protein could be extracted fro m cells containing it and transferred to the surface of previously neg ative cells. The 60-kD protein may serve to regulate the binding of TG F-beta to its signal transducing receptors by targeting TGF-beta to ap propriate locations in the microenvironment of cells.