TGF beta 1 signaling and stimulation of osteoadherin in human odontoblasts in vitro.

Transforming growth factor beta 1 (TGF beta 1) is generally considered to be a potent inducer of dentin formation. In order to further assess this role, we studied the influence of this factor in human dental pulp cells on the expression of osteoadherin (OSAD), a newly described proteoglycan found in bone and dentin and suspected to play a role in mineralization events. We performed TGF beta 1 stimulation both in cultures of human tooth thick slices including mature odontoblasts and in pulp explant cultures giving rise to early secretory odontoblasts or pulpal fibroblasts. We first showed by immunohistochemistry that molecules involved in TGF beta 1 signal transduction, that is, membrane receptors T beta RI and T beta RII and intracellular proteins SMAD-2, SMAD-3, and SMAD-4, were present in human dental cells in vivo and were all maintained after culture of thick-sliced teeth in cells undergoing TGF beta 1 stimulation. In this culture system, OSAD synthesis was increased in mature odontoblasts close to the TGF beta 1 delivery system. In explant cultures, semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) analysis indicated that the growth factor stimulated OSAD gene expression in early secretory odontoblasts and in pulpal fibroblasts. Taken together, these results indicate that OSAD expression is stimulated by TGF beta 1 in pulpal fibroblasts and in early secretory and mature odontoblasts. We suggest that TGF beta 1 in this way could control the organization and the mineralization of the extracellular matrix deposited by these cells during dentin formation.