Inhibition of transforming growth factor β production by nitric oxide- treated chondrocytes

Objective. Nitric oxide (NO) is generated copiously by articular chondrocytes activated by interleukin-1β (IL-1β). If NO production is blocked, much of the IL-1β inhibition of proteoglycan synthesis is prevented. We tested the hypothesis that this inhibitory effect of NO on proteoglycan synthesis is secondary to changes in chondrocyte transforming growth factor (TGFβ). Methods. Monolayer, primary cultures of lapine articular chondrocytes and cartilage slices were studied. NO production was determined as nitrite accumulation in the medium. TGFβ bioactivity in chondrocyte- and cartilage-conditioned medium (CM) was measured with the mink lung epithelial cell bioassay. Proteoglycan synthesis was measured as the incorporation of ³⁵S-sodium sulfate into macromolecules separated from unincorporated label by gel filtration on PD-10 columns. Results. IL-1β increased active TGFβ in chondrocyte CM by 12 hours; by 24 hours, significant increases in both active and latent TGFβ were detectable. N(G)- monomethyl-L-arginine (L-NMA) potentiated the increase in total TGFβ without affecting the early TGFβ activation. IL-1β stimulated a NO-independent, transient increase in TGFβ3 at 24 hours; however, TGFβ1 was not changed. When NO synthesis was inhibited with L-NMA, IL-1β increased CM concentrations of TGFβ1 from 24-72 hours of culture. L-arginine (10 mM) reversed the inhibitory effect of L-NMA on NO production and blocked the increases in TGFβ1. Anti-TGFβ antibody prevented the restoration of proteoglycan synthesis by chondrocytes exposed to IL-1β + L-NMA, confirming that NO inhibition of TGFβ1 in IL-1β-treated chondrocytes effected, in part, the decreased proteoglycan synthesis. Furthermore, the increase in TGFβ and proteoglycan synthesis seen with L-NMA was reversed by the NO donor S-nitroso-N-acetylpenicillamide. Similar results were seen with cartilage slices in organ culture. The autocrine increase in CM TGFβ1 levels following prior exposure to TGFβ1 was also blocked by NO. Conclusion. NO can modulate proteoglycan synthesis indirectly by decreasing the production of TGFβ1 by chondrocytes exposed to IL-1β. It prevents autocrine-stimulated increases in TGFβ1, thus potentially diminishing the anabolic effects of this cytokine in chondrocytes.