Fetuin/α2-HS glycoprotein is a transforming growth factor-β type II receptor mimic and cytokine antagonist

Abstract

The serum glycoprotein fetuin is expressed during embryogenesis in multiple tissues including limb buds and has been shown to promote bone remodeling and stimulate cell proliferation in vitro. In this report, we demonstrate that fetuin antagonizes the antiproliferative action of transforming growth factor-β1 (TGF-β1) in cell cultures. Surface plasmon resonance measurements show that fetuin binds directly to TGF-β1 and TGF- β2 and with greater affinity to the TGF-β-related bone morphogenetic proteins (BMP-2, BMP-4, and BMP-6). In a competitive enzyme-linked immunosorbent assay, fetuin blocked binding of TGF-β1 to the extracellular domain of TGF-β receptor type II (TβRII), one of the primary TGF-β- binding receptors. A comparison of fetuin and TβRII shows homology in an 18- 19-amino acid sequence, which we have designated TGF-β receptor II homology 1 domain (TRH1). Since the TRH1 sequence is known to form a disulfide loop in fetuin, cyclized TRH1 peptides from both fetuin and TβRII were chemically synthesized and tested for cytokine binding activity. Cyclized TRH1 peptide from TβRII bound to TGF-β1 with greater affinity than to BMP-2, while the cyclized TRH1 peptide from fetuin bound preferentially to BMP-2. Finally, fetuin or neutralizing anti-TGF-β antibodies blocked osteogenesis and deposition of calcium-containing matrix in cultures of dexamethasone-treated rat bone marrow cells. In summary, these experiments define the TRH1 peptide loop as a cytokine-binding domain in both TβRII and fetuin and suggest that fetuin is a natural antagonist of TGF-β and BMP activities.