Recombinant Transforming Growth Factor-β1 Induces Endochondral Bone in the Baboon and Synergizes with Recombinant Osteogenic Protein-1 (Bone Morphogenetic Protein-7) to Initiate Rapid Bone Formation

Abstract

Several members of the bone morphogenetic protein (BMP) and transforming growth factor-beta (TGF-beta) families are molecular regulators of cartilage and bone regeneration, although their actual roles and combined interactions in skeletal repair are poorly understood. The presence of several molecular forms suggests multiple functions in vivo as well as synergistic interactions during both embryonic bone development and regeneration of cartilage and bone in postfetal life. Here we show for the first time that recombinant human transforming growth factor-beta1 (TGF-beta1) induces endochondral bone formation in extraskeletal sites of adult baboons. We also show that TGF-beta1 and recombinant human osteogenic protein-1 (OP-1, bone morphogenetic protein-7) synergize in inducing large ossicles in extraskeletal sites of the primate as early as 15 days after implantation. A single application of OP-1, in conjunction with an insoluble collagenous matrix as carrier (5, 25, and 125 microg/100 mg of carrier matrix) induced bone differentiation in the rectus abdominis of the baboon. This level of tissue induction was raised several-fold by the simultaneous addition of comparatively low doses of TGF-beta1 (0.5, 1.5, and 5 microg), which by itself induces bone formation in the rectus abdominis at doses of 5 microg/100 mg of carrier matrix. Combinations of OP-1 and TGF-beta1 yielded a 2- to 3-fold increase in cross-sectional area of the newly generated ossicles, with markedly elevated key parameters of bone formation, and corticalization of the newly formed bone by day 15, culminating in bone marrow generation by day 30. The tissue generated by the combined application of OP-1 and TGF-beta1 showed distinct morphological differences when compared with OP-1-treated specimens, with large zones of endochondral development and extensive bone marrow formation. At the doses tested, synergy was optimal at a ratio of 1:20 by weight of TGF-beta1 and OP-1, respectively. These results provide evidence for a novel function of TGF-beta1 in the primate and the scientific basis for synergistic molecular therapeutics for the rapid regeneration of cartilage and bone.