TGF-beta and PDGF act synergistically in affecting the growth of human osteoblast-enriched cultures

Transforming growth factor beta (TGF-beta) is a polypeptide found in high concentrations in bone and is produced by and acts on primary adult human derived osteoblast-enriched cultures (PHO cells). Receptors for TGF-beta are present on PHO cells and TGF-beta is mitogenic for these cells. Results of these studies in conjunction with those of others suggest that TGF-beta may have an important therapeutic role in orthopaedic surgery; however, with respect to its mitogenic actions, further studies were needed to establish whether TGF-beta was acting directly to stimulate the growth of PHO cells. TGF-beta has been found in other systems to act as an indirect mitogen, stimulating growth via secretion of another growth factor, platelet-derived growth factor (PDGF). In an effort to determine whether the TGF-beta growth stimulation was mediated directly or indirectly, we have examined the growth stimulation of PHO cells by PDGF alone and in combination with TGF-beta. These studies revealed that TGF-beta in combination with either PDGF-AA or BB led to stimulation greater than that observed with either growth factor alone. TGF-beta in combination with PDGF-BB led to a synergistic stimulatory response while that observed with the AA isoform was more nearly additive. Further studies demonstrated that TGF-beta was capable of up-regulating the protein levels of the PDGF alpha (alpha) receptor within thirty minutes of TGF-beta pretreatment. Thus, TGF-beta appears to have both direct and indirect mechanisms of action as a mitogen in the PHO system. Finally, we showed that both the positive and negative alkaline phosphatase staining PHO cells were responsive to the mitogenic effects of both growth factor singly and in combination.