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 known concerning the effects of these cytokines on growth plate chondrocytes. In this study, we examined the effect of IL-1α, IL-1β, and IL-6 on growth plate chondrocyte proliferation, differentiation, and matrix production as a function of cell maturation and examined the ability of these cells to produce IL-1α and IL-1β. Confluent fourth passage cultures of rat costochondral resting zone and growth zone chondrocytes were treated with 0- 100 ng/ml of IL-1α, IL-1β, or IL-6 for 24 h and then assayed for [³H]- thymidine incorporation, alkaline phosphatase specific activity, [³⁵S]- sulfate incorporation, and percent collagen production. Neutralizing polyclonal antibodies were used to confirm the specificity of response to each cytokine. Treatment of resting zone cells with IL-1α produced a significant, dose-dependent decrease in [³H]-thymidine incorporation, while similarly treated growth zone cells were unaffected by treatment with this cytokine. IL-1α also stimulated alkaline phosphatase specific activity and inhibited [³⁵S]-sulfate incorporation by resting zone chondrocytes, but had no affect on growth zone chondrocytes. When collagen production was examined, it was observed that IL-1α had a stimulatory affect on growth zone cells but no affect on resting zone cells. When the effect of IL-1β was examined, it was observed that this cytokine inhibited [³H]-thymidine incorporation by resting zone cells and stimulated isotope incorporation in growth zone cells. IL-1β also stimulated alkaline phosphatase specific activity and inhibited [³⁵S]-sulfate incorporation by resting zone chondrocytes but had no affect on growth zone chondrocytes. In contrast to IL-1α, IL-1β stimulated collagen production by resting zone cells but not growth zone cells. IL-6 had no affect on any of the parameters measured in either cell type. When cytokine production was measured, it was found that IL-1β was produced by both cell types, while IL-1β was produced only by resting zone cells. Resting zone cells secreted both IL-1α and IL-1β into the media, but 75% of the total cytokine produced by these cells was retained in the cell layer. In contrast, growth zone cells did not secrete measurable IL-1α into the media. These results suggest that IL-1α and IL-1β target resting zone cells, inducing them to differentiate and acquire a phenotype characteristic of the more mature growth zone cells. Moreover, resting zone chondrocytes produce both IL-1α and IL-1β, suggesting the possibility of an autocrine effect of these cytokines on the cells.