Abstract
Objective
To determine the effects of hypoxia and reoxygenation on the metabolism of chondrocytes and their response to interleukin‐1β (IL‐1β). The study included activation of hypoxia‐inducible factor 1 (HIF‐1), NF‐κB, and activator protein 1 (AP‐1) transcription factors, expression of matrix components and metalloproteases and transforming growth factor β (TGFβ) and TGFβ receptors, and production of nitric oxide (NO) and prostaglandin E~2~ (PGE~2~).
Methods
Bovine articular chondrocytes (BACs) were cultured to confluency in either 5% O~2~ (hypoxia) or 21% O~2~ (normoxia) in media supplemented with 10% fetal calf serum (FCS). BACs were preincubated for 18 hours in media with 1% FCS only and then incubated for 24 hours in the presence of IL‐1β. For reoxygenation experiments, cells were treated in the same way in 5% O~2~, except that cultures were transferred to normal atmospheric conditions and used after 4 hours for RNA extraction or after 30 minutes for cytoplasmic or nuclear protein extraction.
Results
In hypoxic and reoxygenated chondrocytes, we observed strong DNA binding of HIF‐1. IL‐1β‐induced DNA binding of NF‐κB and AP‐1 was significantly higher in hypoxic and reoxygenated cultures than in normoxia. Greater activation of the MAPKs was also observed with IL‐1β treatment in hypoxia compared with normoxia. Steady‐state levels of type II collagen and aggrecan core protein messenger RNA (mRNA) were decreased by IL‐1β in all instances. Matrix metalloprotease 1 (MMP‐1) and MMP‐3 mRNA were increased by IL‐1β in normoxia and hypoxia, whereas only MMP‐3 mRNA was enhanced in reoxygenated cultures. The MMP‐2 mRNA level was not significantly affected by IL‐1β in normoxia or hypoxia, whereas it was enhanced in reoxygenated cultures. MMP‐9 mRNA was dramatically decreased by IL‐1β only in low oxygen tension. Tissue inhibitor of metalloproteinases 1 (TIMP‐1) message was significantly enhanced by the cytokine in most instances, whereas TIMP‐2 message was markedly decreased by IL‐1β in reoxygenated cultures. Stimulation of TGFβ1 expression by IL‐1β was observed only in normal atmospheric conditions. One of the more striking findings of the study was the greater stimulating effect of IL‐1β on NO production observed in hypoxia, which was much higher than in normoxia, whereas the reverse was observed for IL‐1β–induced PGE~2~ production.
Conclusion
Oxygen level and reoxygenation stress significantly modulate gene expression and the response of articular chondrocytes to cytokines such as IL‐1β. In hypoxic conditions, which mimic the in vivo condition of cartilage, the effects of IL‐1β on both synthesis and degradative processes are significantly different from those in normoxia, conditions that are unlikely encountered by chondrocytes in a normal state. In low oxygen tension, high IL‐1β–induced NO production is associated with a significant decrease in PGE~2~ synthesis. These data should influence our concept of the role of oxygen in the pathophysiology of joint disease and may help define the best conditions in which to develop bioartificial cartilage.