EVIDENCE THAT INTERLEUKIN-1, BUT NOT INTERLEUKIN-6, AFFECTS COSTOCHONDRAL CHONDROCYTE PROLIFERATION, DIFFERENTIATION, AND MATRIX SYNTHESIS THROUGH AN AUTOCRINE PATHWAY

Although the effects of interleukin-1 (IL-1) and interleukin-6 (IL-6) on articular cartilage chondrocytes have been reported, little is know n concerning the effects of these cytokines on growth plate chondrocyt es, In this study, we examined the effect of IL-1 alpha, IL-1 beta, an d IL-6 on growth plate chondrocyte proliferation, differentiation, and matrix production as a function of cell maturation and examined the a bility of these cells to produce IL-1 alpha and IL-1 beta. Confluent f ourth passage cultures of rat costochondral resting zone and growth zo ne chondrocytes were treated with 0-100 ng/ml of IL-1 alpha IL-1 beta, or IL-6 for 24 h and then assayed for [H-3]-thymidine incorporation, alkaline phosphatase specific activity, [S-35]-sulfate incorporation, and percent collagen production. Neutralizing polyclonal antibodies we re used to confirm the specificity of response to each cytokine. Treat ment of resting zone cells,vith IL-1 alpha produced a significant, dos e-dependent decrease in [H-3]-thymidine incorporation, while similarly treated growth zone cells were unaffected by treatment with this cyto kine, IL-1 alpha also stimulated alkaline phosphatase specific activit y and inhibited [S-35]-sulfate incorporation by resting zone chondrocy tes, but had no affect on growth zone chondrocytes. When collagen prod uction was examined, it was observed that IL-1 alpha had a stimulatory affect on growth zone cells but no affect on resting zone cells. When the effect of IL-1 beta was examined, it,vas observed that this cytok ine inhibited [H-3]-thymidine incorporation by resting zone cells and stimulated isotope incorporation in growth zone cells, IL-1 beta also stimulated alkaline phosphatase specific activity and inhibited [S-35] -sulfate incorporation by resting zone chondrocytes but had no affect on growth zone chondrocytes, In contrast to IL-1 alpha, IL-1 beta stim ulated collagen production by resting zone cells but not growth zone c ells, IL-6 had no affect on any of the parameters measured in either c ell type. When cytokine production was measured, it was found that IL- 1 alpha was produced by both cell types, while IL-1 beta was produced only by resting zone cells. Resting zone cells secreted both IL-1 alph a and IL-1 beta into the media, but 75% of the total cytokine produced by these cells was retained in the cell layer. In contrast, growth zo ne cells did not secrete measurable IL-1 alpha into the media. These r esults suggest that IL-1 alpha and IL-1 beta target resting zone cells , inducing them to differentiate and acquire a phenotype characteristi c of the more mature growth zone cells. Moreover, resting zone chondro cytes produce both IL-1 alpha and IL-1 beta, suggesting the possibilit y of an autocrine effect of these cytokines on the cells.