Effect of transforming growth factor-beta on DNA synthesis by growth plate chondrocytes: modulation by factors present in serum.

Chondrocytes in the growth plate undergo rapid proliferation during the process of enchondral ossification. The factors that regulate this proliferative activity have not been defined although several local autocrine or paracrine growth factors have recently been discovered in cartilage. Transforming growth factor-beta 1 (TGF-beta) is an important regulator of cell metabolism and growth and is present in cartilage. The present study was designed to examine the influence of TGF-beta on DNA synthesis in chick epiphyseal chondrocytes. Chondrocytes were plated in serum-free (BSA-supplemented) medium or medium containing 10% FBS, and after 24 hours in monolayer culture, were treated with TGF-beta in identical medium. A 24 hour incubation with TGF-beta caused a biphasic dose-dependent increase in thymidine incorporation. The effect was greatly increased in serum-containing cultures where a maximal 20-fold increase in thymidine incorporation occurred compared with the 2 1/2-fold maximal increase obtained in serum-free cultures. The effect was present by 12 hours and maximal between 0.3 and 1.0 ng/ml of TGF-beta. Higher concentrations of TGF-beta were less stimulatory. The serum enhancement was dependent upon the simultaneous presence of TGF-beta and serum factors and was abolished when chondrocytes were plated and exposed to TGF-beta in medium containing dialyzed FBS (12-14 kD MW membrane). The nature of the uptake and incorporation of thymidine into DNA by individual chondrocytes appeared to be the same in both TGF-beta-treated and control cultures as the apparent Kms were similar. The present study indicates that TGF-beta increases DNA synthesis by growth plate chondrocytes. The effect is enhanced by factors present in serum.