Interaction of strain and interleukin-1 in articular cartilage: effects onproteoglycan synthesis in chondrocytes

In temporomandibular joint disorders, the release of proinflammatory cytoki nes such as interleukin-1 (IL-1) initiates an inflammatory process disrupti ng cartilage homeostasis, ultimately leading to cartilage destruction. Addi tionally, mechanical stimuli affect articular chondrocyte metabolism. While articular chondrocytes generate nitric oxide (NO) in the presence of IL-1 proteoglycan synthesis is consecutively suppressed. The purpose of this stu dy was to assess the effects of proinflammatory cytokines and mechanical st rain in the form of cyclic tensile stretch on proteoglycan synthesis in cho ndrocytes, as compared to the NO competitive inhibitor L-N-monomethyl argin ine (LMA), and to assess whether this effect is secondarily related to the activity of growth factors such as transforming growth factor beta (TGF-bet a). Lapine articular chondrocytes were exposed to one of four different tre atment regimens: no cyclic tensile stretch, IL-1, cyclic tensile stretch, o r IL-1 plus cyclic tensile stretch. NO production was determined as medium nitrite accumulation. TGF-beta -bioactivity in chondrocyte conditioned medi um was measured with the mink-lung epithelial cell bioassay. Proteoglycan s ynthesis was measured as the incorporation of (35)-[S]- sodium sulfate into macromolecules separated from unincorporated label by gel filtration on PD -10 columns. In resting chondrocyte cultures, only baseline levels of NO we re measured and the application of stretch for 24 h did not affect NO produ ction. Addition of IL-1 provoked a large increase in NO synthesis which was abrogated in the presence of LMA. Application of stretch decreased the IL- 1 induced NO synthesis, but did not modify the effect of LMA (being a compe titive inhibitor of the inducible NO synthase) inhibiting IL-1 induced NO p roduction. Glucosaminoglycan production was noted as proteoglycan synthesis showing almost no effect of cyclic stretch alone in comparison to the cont rol condition, which correlates with the missing NO production in control a nd stretch conditions. Addition of IL-1 strongly inhibited proteoglycan syn thesis, which was partly restored in the presence of LMA. However, cyclic s tretch acted as a stronger restorer of proteoglycan synthesis in IL-1 treat ed conditions in the absence, and even more in the presence, of LMA. It was concluded that motion in the form of cyclic tensile stretch is a remarkabl e anti-inflammatory stimulus reversing the IL-1 induced suppression of prot eoglycan synthesis in chondrocytes. These findings have therapeutic implica tions for the treatment of temporomandibular joint disorders, supporting ea rly onset of postoperative and post-traumatic continuous passive motion the rapy.