Transforming growth factor‐beta and fibroblast growth factor act synergistically to inhibit collagen II synthesis through a mechanism involving regulatory DNA sequences

Transforming growth factor-beta (TGF-beta) and fibroblast growth factor (FGF) are two growth factors that will modulate chondrocyte growth and matrix synthesis. Here we report that these two growth factors act in a synergistic fashion to suppress the synthesis of type II collagen by embryonic chicken sternal chondrocytes. Treatment of chondrocytes with 20 ng/ml TGF-beta or 100 ng/ml FGF (acidic or basic) results in a 60-70% suppression of expression of the pro alpha 1 chain of type II collagen. By comparison, when chondrocytes are exposed to a combination of 1 ng/ml TGF-beta and 10 ng/ml FGF, a complete suppression of type II collagen synthesis was observed. Epidermal growth factor (EGF), platelet-derived growth factor (PDGF), or insulin-like growth factor-1 (IGF-1) produce no suppression of synthesis either individually or in combination with TGF-beta. The decreased expression of the protein results from a decrease in the steady-state level of the mRNA transcript coding for type II procollagen, as indicated by a northern analysis. Finally, chondrocytes transfected with a plasmid carrying the CAT gene driven by the collagen II promoter/enhancer sequence displayed high levels of CAT activity when cultured in control media, but treatment of the cells with a combination of the two growth factors resulted in a dramatic reduction of CAT activity, indicating diminished promoter activity. These results suggest that both TGF-beta and FGF can down-regulate transcription of the collagen II gene through regulatory DNA sequences in the promoter and/or enhancer region. In addition, the finding of synergy suggests that these two growth factors may act through different pathways.