Induction of endochondral bone formation by recombinant human transforminggrowth factor-beta 2 in the baboon (Papio ursinus)

Members of the transforming growth factor-beta (TGF-beta) superfamily, the bone morphogenetic and osteogenic proteins (BMPs/OPs) but not the TGF-beta proteins themselves, induce endochondral bone formation in vivo, when impla nted in extraskeletal heterotopic sites of rodents. Here we show that recom binant human TGF-beta 2 (hTGF-beta 2) induces endochondral bone formation 3 0 days after implantation in heterotopic intramuscular sites of the baboon (Papio ursinus) at doses of 1, 5 and 25 mu g per 100 mg of guanidinium-inac tivated collagenous bone matrix as carrier. On day 90 there was generation of large radiopaque and corticalized intramuscular ossicles. Five and 25 mu g hTGF-beta 2 induced large ossicles in the rectus abdominis of the primat e as evaluated by key parameters of bone formation, including generated tis sue area, mineralized bone and osteoid volumes, and tissue alkaline phospha tase activity. On day 30 and 90 after healing, hTGF-beta 2 also induced bon e formation when implanted in the rectus abdominis in conjunction with a si ntered porous hydroxyapatite as carrier. mRNA expression in tissues from he terotopic specimens showed OP-1 (BMP-7) and BMP-3 transcripts in low abunda nce and with a linear dose-dependent increase both in collagenous matrix an d hydroxyapatite samples. Type IV collagen mRNA expression, a marker of ang iogenesis, was stronger in collagenous than hydroxyapatite samples. Growth and differentiation factor-10 (GDF-10)mRNA transcripts were expressed in os sicles with a distinctly chondrogenic phase, but its expression was greater in ossicles generated in porous hydroxyapatites, in which bone formation i s not via a chondrogenic phase, but is rather intramembranous, without expr ession of type II collagen mRNA. In the same animals, however, 10 and 100 m u g of the recombinant morphogen delivered by identical carriers (collageno us matrix and sintered hydroxyapatite) failed to heal calvarial defects. Th us in the primate, TGF-beta s themselves are inducers of endochondral bone formation, although the present data strongly indicate that the bone induct ive activity of hTGF-beta 2 is site and tissue specific, since a single app lication of hTGF-beta 2, or hTGF-beta 1 in previously published experiments , did not induce bone in calvarial defects, but did induce endochondral bon e differentiation in heterotopic sites.