Potential involvement of BMP receptor type IB activation in a synergistic effect of chondrogenic promotion between rhTGFβ3 and rhGDF5 or rhBMP7 in human mesenchymal stem cells

Chondrogenic promotion by rhGDF5 with or without rhTGF beta 3 was studied in pellet culture of human mesenchymal stem cells (HMSCs). A synergy between rhGDF5 and rhTGF beta 3 was observed in promoting chondrogenesis. rhBMP2, rhBMP6, rhBMP7 and rhTGF beta 1 were further tested and showed the same effect. To explore the mechanism, the expression of TGF beta type I and II receptors, ALK5, ALK2, ALK3, ALK6, TGF beta RII, BMPRII, ActRII was studied. ALK6 showed increase by the rhTGF beta 1 or rhTGF beta 3 treatment. ALK6 protein expression also showed increase by rhTGF beta 3. rhTGF beta 1/rhTGF beta 3 induced ALK6 up-regulation was inhibited by SD-208, a TGF beta type I receptor inhibitor. Chondrogenesis by rhTGF beta I/rhTGF beta 3 or the combination between rhTGF beta 1/rhTGF beta 3 and rhGDF5 also was diminished by SD-208. SMAD1/5/8 phosphorylation in nascent human mesenchymal stem cells (HMSCs) was stimulated weakly by rhGDF5 but strongly by rhBMP7. The rhGDF5 stimulated SMAD1/5/8 phosphorylation was enhanced by rhTGF beta 1/rhTGF beta 3 but inhibited by SD-208. The rhBMP7 stimulated SMAD1/5/8 phosphorylation did not show influence by rhTGF beta 3 and SD208. Our results indicated the potential involvement of ALK6 activation by rhTGF beta s in the synergy between rhTGF beta s and rhBMPs.