ACTIVIN-A BINDING AND BIOCHEMICAL EFFECTS IN OSTEOBLAST-ENRICHED CULTURES FROM FETAL-RAT PARIETAL BONE

Actin, a disulfide-linked polypeptide dimer first isolated from gonadal tissue extracts, has amino acid sequence and structural homology with transforming growth factor-beta (TGF-beta). Along with other activities, TGF-beta regulates replication and differentiation and interacts with a defined set of binding sites on isolated bone cells. To determine if activin shares these properties, recombinant human actin-A (A-chain homodimer) was examined in osteoblast-enriched cultures obtained from fetal-rat parietal bone. After 23 h of treatment, 60 to 6,000 pM activin-A increased the rate of [H-3]thymidine incorporation into DNA 1.5- to 4.0-fold, and at 600 to 6,000 pM, it enhanced the rate of [H-3]proline incorporation into collagen and noncollagen protein by up to 1.7-fold. Like earlier studies with TGF-beta in primary osteoblast-enriched cultures, the stimulatory effects of activin-A on DNA and protein synthesis were opposed by parathyroid hormone, and the influence of activin-A on collagen synthesis was independent of cell replication. Binding studies with I-125-activin-A indicated approximately 8,000 high-affinity (K(d) = 0.4 nM) and 300,000 low-affinity (K(d) = 40 to 50 nM) binding sites per cell. Polyacrylamide gel analysis revealed I-124-activin-A-binding complexes of M(r) > 200,000 and 73,000 which did not appear to correspond to primary TGF-beta-binding sites. These results indicate that activin-A produces TGF-beta-like effects in bone and that some of these effects may be mediated, at least in part, by distinct activin receptors on bone cells.