Background  

Osteoarthritis (OA) is a disorder that causes pain and degeneration of the joint over a chronic time course. Chondrocytes in OA play important roles in maintaining the homeostasis of the joint while they produce many cytokines and pathological mediators, including interleukin-1β (IL-1β), cyclooxygenases (COX), and prostaglandin E₂ (PGE₂). To elucidate the mechanisms of pain due to OA, the pathway of PGE₂ synthesis was analyzed using cells derived from chondrocytes obtained from patients with OA.

Methods  

Chondrocytes were isolated from cartilage samples obtained at the time of joint replacement surgery from patients with OA. The chondrocytes at the second passage were cultured with or without IL-1β, dexamethasone (DEX), or COX inhibitors such as NS-398, meloxicam, and indomethacin. Reverse transcription-polymerase chain reaction and Western blotting analysis were performed to study the levels of mRNA and protein, respectively. An enzyme-linked immunosorbent assay was performed to investigate the translocation of nuclear factor-κB (NF-κB) to the nucleus, and Western blotting analysis was performed to study the phosphorylation of mitogen-activated protein kinases.

Results  

IL-1β markedly enhanced the expression of COX-2 and microsomal prostaglandin E synthase-1 (mPGES-1) at both the mRNA and protein levels. The up-regulation was suppressed by DEX or COX inhibitors. IL-1β strongly increased the translocation of NF-κB to the nucleus and the phosphorylation of extracellular-signal-regulated kinase, p38, and c-Jun amino-terminal kinase; but the up-regulation was not inhibited by DEX or COX inhibitors. Interestingly, in a dose-dependent manner, PGE₂ recovered mPGES-1 expression from suppression by DEX, whereas it did not restore the expression of COX-2 in the presence of DEX and IL-1β.

Conclusions  

These results suggested that in cells derived from OA chondrocytes different mechanisms of regulation exist between mPGES-1 and COX-2, and the expression of mPGES-1 was, at least partially, regulated through the autocrine positive feedback by PGE₂.