Modulation of cultured chicken growth plate chondrocytes by transforming growth factor‐β1 and basic fibroblast growth factor

Expression of several cellular and matrix proteins which increase significantly during the maturation of growth plate cartilage has been shown to be affected by various endocrine and autocrine factors. In the studies reported here, transforming growth factor‐β (TGF‐β1) and basic fibroblast growth factor (bFGF) were administered to primary cultures of avian growth plate chondrocytes at pre‐ or post‐confluent stages to study the interplay that occurs between these factors in modulating chondrocytic phenotype. Added continuously to pre‐confluent chondrocytes, TGF‐β1 stimulated the cells to produce abundant extracellular matrix and multilayered cell growth; cell morphology was altered to a more spherical configuration. These effects were generally mimicked by bFGF, but cell shape was not affected. Administered together with TGF‐β1, bFGF caused additive stimulation of protein synthesis, and alkaline phosphatase (AP) activity was markedly, but transiently enhanced. During this pre‐confluent stage, TGF‐β1 also increased fibronectin secretion into the culture medium. Added to post‐confluent cells, TGF‐β1 alone caused a dosage‐dependent suppression of AP activity, but bFGF alone did not. Under these conditions, TGF‐β1 and bFGF had little effect on general protein synthesis, but TGF‐β1 alone caused large, dosage‐dependent increases in synthesis of fibronectin, and to some extent type II and X collagens. Given together with bFGF, TGF‐β1 synergistically increased secretion of fibronectin. These findings reveal that regulation of phenotypic expression in maturing growth plate chondrocytes involves complex interactions between growth factors that are determined by timing, level, continuity, and length of exposure.