Increased expression of lipocalin-type prostaglandin D2 synthase in osteoarthritic cartilage

Prostaglandin D synthase (PGDS) is responsible for the biosynthesis of PGD and J series, which have been shown to exhibit anti-inflammatory and anticatabolic effects. Two isoforms have been identified: hematopoietic- and lipocalin-type PGDS (H-PGDS and L-PGDS, respectively). The aims of this study were to investigate the expressions of H-PGDS and L-PGDS in cartilage from healthy donors and from patients with osteoarthritis (OA) and to characterize their regulation by interleukin-1-beta (IL-1beta) in cultured OA chondrocytes.

The expressions of H-PGDS and L-PGDS mRNA and protein in cartilage were analyzed by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry, respectively. Chondrocytes were stimulated with IL-1beta, and the expression of L-PGDS was evaluated by real-time RT-PCR and Western blotting. The roles of de novo protein synthesis and of the signalling pathways mitogen-activated protein kinases (MAPKs), nuclear factor-kappa-B (NF-kappaB), and Notch were evaluated using specific pharmacological inhibitors.

L-PGDS and H-PGDS mRNAs were present in both healthy and OA cartilage, with higher levels of L-PGDS than H-PGDS (> 20-fold). The levels of L-PGDS mRNA and protein were increased in OA compared with healthy cartilage. Treatment of chondrocytes with IL-1beta upregulated L-PGDS mRNA and protein expressions as well as PGD2 production in a dose- and time-dependent manner. The upregulation of L-PGDS by IL-1beta was blocked by the translational inhibitor cycloheximide, indicating that this effect is indirect, requiring de novo protein synthesis. Specific inhibitors of the MAPK p38 (SB 203580) and c-jun N-terminal kinase (JNK) (SP600125) and of the NF-kappaB (SN-50) and Notch (DAPT) signalling pathways suppressed IL-1beta-induced upregulation of L-PGDS expression. In contrast, an inhibitor of the extracellular signal-regulated kinase (ERK/MAPK) (PD98059) demonstrated no significant influence. We also found that PGD2 prevented IL-1beta-induced upregulation of L-PGDS expression.

This is the first report demonstrating increased levels of L-PGDS in OA cartilage. IL-1beta may be responsible for this upregulation through activation of the JNK and p38 MAPK and NF-kappaB signalling pathways. These data suggest that L-PGDS might have an important role in the pathophysiology of OA.