Regulation of osteogenesis by fetuin

Osteoporosis is a common problem of aging and results from a failure of hom eostatic mechanisms to regulate osteogenesis and mineralization, Bovine and human forms of fetuin glycoprotein bind to the transforming growth factor (TGF)-beta/BMP (bone morphogenic protein) cytokines and block their osteoge nic activity in cell culture assays (Demetriou, M., Binkert, C,, Sukhu, B., Tenenbaum, H. C., and Dennis, J. W. (1996) J, Biol. Chem, 271, 12755-12761 ). Fetuin is a prominent serum glycoprotein and a major noncollagenous comp onent of mineralized bone in mammals. In this study, we show that recombina nt fetuin and native serum protein have similar potency as inhibitors of os teogenesis in dexamethasone-treated rat bone marrow cell cultures (dex-RBMC ). Recombinant bovine fetuin also bound to TGF-beta 1 and BMP-2 in vitro wi th kinetics similar to native fetuin, Although TGF-beta 1 is required for o steogenesis in dex-RBMC, the cytokine also inhibited osteogenesis at concen trations greater than or equal to 10 pM. Titration of fetuin or anti-TGF-be ta 1 antibodies into the bone marrow cultures in the presence of 10 pM TGF- beta 1 restored osteogenesis, whereas titrations of the same reagents into cultures with 0.3 pM added TGF-beta 1 were inhibitory, confirming the bipha sic nature of the TGF-beta 1 response. Suppression of osteogenesis by both TGF-beta 1 and the antagonist proteins required their presence within the f irst 6 days of culture, well before mineralization at 10-12 days. Northern analysis showed that both fetuin and high dose TGF-beta 1 suppressed expres sion of the bone-associated transcripts alkaline phosphatase, osteopontin, collagen type I, and bone sialoprotein. The suppression of osteogenesis by fetuin and by high dose TGF-beta 1 was accompanied by the differentiation o f an alternate cell lineage with adipocyte characteristics. In summary, the biphasic osteogenic response to TGF-beta 1 suggests that overlapping gradi ents of TGF-beta/EMP cytokines and fetuin regulate osteogenesis in remodeli ng bone.