Fetuin/α2-HS Glycoprotein Is a Transforming Growth Factor-β Type II Receptor Mimic and Cytokine Antagonist

doi:10.1074/jbc.271.22.12755

Fetuin/α2-HS Glycoprotein Is a Transforming Growth Factor-β Type II Receptor Mimic and Cytokine Antagonist*

From the ^‡ Samuel Lunenfeld Research Institute, Mt. Sinai Hospital, 600 University Avenue, Toronto, Ontario M5G 1X5, Canada the ^‡ Department of Medical Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada, and the
^¶ Medical Research Council Group in Periodontal Physiology, Faculty of Dentistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada

^∥
A Senior Research Scientist of the National Cancer Institute of Canada.
To whom correspondence should be addressed. Tel.: 416-586-8233; Fax: 416-586-8588; E-mail: Dennis{at}MSHRI.ON.CA

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 GF-β eceptor II omology 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.

Footnotes

↵^§ Supported by an MRC studentship.
↵* This work was supported in part by research grants from the National Science and Engineering Research Council of Canada, National Cancer Institute of Canada, and Medical Research Council (MRC) of Canada. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

TGF-β

transforming growth factor-β

TβRI and TβRII

TGF-β

receptor type I and II

respectively

BMP

bone morphogenetic protein

BSA

bovine serum albumin

HPLC

high pressure liquid chromatography

ELISA

enzyme-linked immunosorbent assay

ActRII

activin receptor type II.
↵² J. Wrana, personal communication.
↵³ M. Demetriou, C. Binkert, B. Sukhu, H. C. Tenenbaum, and J. W. Dennis, unpublished data.
- Received June 30, 1995.
- Revision received March 11, 1996.