Transforming growth factor-β1: Induction of bone morphogenetic protein genes expression during endochondral bone formation in the baboon, and synergistic interaction with osteogenic protein-1 (BMP-7)

Abstract

Certain members of the bone morphogenetic protein (BMP) and transforming growth factor-β (TGF-ß) families are inducers of endochondral bone formation in vivo. TGF-ßs, however, do not initiate bone formation when implanted in heterotopic (extraskeletal) sites of rodents. Here we show that platelet-derived porcine TGF-ß1 (pTGF-ß1) induces endochondral bone in heterotopic sites of the baboon (Papio ursinus) at doses of 5 μg per 100 mg of guanidinium-inactivated collagenous bone matrix as carrier, with an inductive efficiency comparable to 5 and 25 μg of recombinant osteogenic protein-1 (hOP-1, BMP-7), a well characterized inducer of bone formation. We further demonstrate that pTGF-β1 and hOP-1 interact synergistically to induce large ossicles in the rectus abdominis of the primate as evaluated by key parameters of bone formation on day 14 and 30. Tissue generated on day 30 by 5 μg pTGF-ß1 or 25 μg hOP-1 induced comparable expression levels of OP-1, BMP-3 and type IV collagen mRNA transcripts, whereas TGF-β1 and type II collagen expression was 2 to 3 fold higher in pTGF-ß1-treated implants, as determined by Northern analysis. In ossicles generated by 25 μg hOP-1 in combination with relatively low doses of pTGF-ß1 (0.5, 1.5 and 5 μg), type II collagen expression increased in a pTGF-ß1 dose-dependent manner, whilst type IV collagen was synergistically upregulated with a 3 to 4 fold increase compared to ossicles generated by a single application of 5 μg pTGF-ß1 or 25 μg hOP-1. Morphogen combinations (5 μg pTGF-β1 with 20 μg hOP-1, and 5 and 15 μg pTGF-β1 with 100 μg hOP-1 per g of collagenous matrix as carrier) induced exuberant tissue formation and greater amounts of osteoid than hOP-1 alone when implanted in calvarial defects of the baboon as evaluated on day 30 and 90, with displacement of the temporalis muscle above the defects. Since a single application of TGF-β1 in the primate did not induce bone formation in calvarial defects, whilst it induces endochondral bone differentiation in heterotopic sites, our data indicate that the bone inductive activity of TGF-β1 is site and tissue specific. mRNA expression of multiple members of the TGF-β superfamily suggests complex autocrine and paracrine activities of the ligands and different signalling pathways on responding cells during the cascade of endochondral bone formation in the primate. The present findings may provide the basis for synergistic molecular therapeutics for cartilage and bone regeneration in clinical contexts.