Studies on the autocrine activation of a synovial cell line

The lapine synovial cell line HIG‐82 secretes factors that activate cultures of articular chondrocytes. We showed that these “chrondrocyte‐activating factors” (CAF) also activate quiescent cultures of HIG‐82 cells in an autocrine fashion. After exposure to partially purified preparations of CAF, HIG‐82 cells increased their synthesis of prostaglandin E₂ (PGE₂) and the neutral proteinases collagenase, gelatinase, and stromelysin. CAF also induced their own synthesis. Both PGE₂ synthesis and endogenous production of CAF started to increase between 1 and 3 h after treatment of cells with exogenous CAF, but the neutral proteolytic activity of the conditioned medium took ∼ 12 h to increase. Induction of neutral proteinases by CAF was inversely related to the degree of cell confluency. Whereas their induction by phorbol myristate acetate (PMA) was independent of this parameter. Both CAF and PMA provoked morphologic changes in subconfluent cultures of HIG‐82 cells. Although the intracellular concentration of free Ca²⁺ increased rapidly in response to CAF, the results of experiments with calcium channel blockers and ionophores failed to support a role for Ca²⁺ fluxes in induction of neutral proteinases. In similar types of experiments, no evidence could be found to implicate fluxes in cyclic AMP or cyclic GMP in the induction of collagenase, gelatinase, or stromelysin. Because PMA is such a strong inducer of these enzymes, protein kinase C may be involved in signal transduction, but further work is needed to determine whether this is so.