Chondrocyte activation by interleukin-1: Analysis of the synergistic properties of fibroblast growth factor and phorbol myristate acetate

Following activation, monolayers of lapine articular chondrocytes secreted into their culture media large amounts of prostaglandin E₂ (PGE₂) and the neutral metalloproteinases collagenase and gelatinase. Partially purified preparations of synovial "chondrocyte activating factors" (CAF), which contain interleukin-1 (IL-1), generally proved stronger activators of chondrocytes than recombinant, human, IL-1α (rHIL-1α) or IL-1β (rHIL-1β). The presence of synergistic cytokines within the synovial material provides one possible explanation of this discrepancy. As first reported by K. Phadke (1987, Biochem. Biophys. Res. Commun.142, 448-453) fibroblast growth factor (FGF) synergized with rHIL-1 in promoting the synthesis of neutral metalloproteinases. In our hands FGF alone did not induce neutral metalloproteinases and increased PGE₂ synthesis only modestly. However, at doses from 1 ng/ml to 1 μg/ml, FGF progressively enhanced the synthesis of PGE₂, collagenase, and gelatinase by chondrocytes responding to rHIL-1. Acidic and basic FGF synergized equally well with both rHIL-1α and rHIL-1β. Phorbol myristate acetate (PMA), but not the Ca²⁺-ionophore A23187, could substitute for FGF as a synergist. PMA alone was a poor inducer of collagenase or gelatinase but, unlike FGF, it greatly enhanced the synthesis of PGE₂ by chondrocytes. Dot-blot analyses with a cDNA probe to collagenase mRNA confirmed that partially purified synovial CAF induced collagenase mRNA more effectively than rHIL-1, with rHIL-1α being superior to rHIL-1β in this regard. The synergistic effects of both FGF and PMA upon IL-1-mediated collagenase induction were associated with increased abundance of collagenase mRNA.