Interleukin-6 Synthesis in Human Chondrocytes Is Regulated via the Antagonistic Actions of Prostaglandin (PG)E2 and 15-deoxy-Δ12,14-PGJ2

BackgroundElevated levels of interleukin-6 (IL-6), prostaglandin (PG)E2, PGD2 and its dehydration end product 15-deoxy-Δ12,14-PGJ2 (15d-PGJ2) have been detected in joint synovial fluids from patients with rheumatoid arthritis (RA). PGE2 directly stimulates IL-6 production in human articular chondrocytes. However, the effects of PGD2 and 15d-PGJ2 in the absence or presence of PGE2 on IL-6 synthesis in human chondrocytes have yet to be determined. It is believed that dysregulated overproduction of IL-6 is responsible for the systemic inflammatory manifestations and abnormal laboratory findings in RA patients. Methodology/Principal FindingsUsing the T/C-28a2 chondrocyte cell line as a model system, we report that exogenous PGE2 and PGD2/15d-PGJ2 exert antagonistic effects on IL-6 synthesis in human T/C-28a2 chondrocytes. Using a synthesis of sophisticated molecular biology techniques, we determined that PGE2 stimulates Toll-like receptor 4 (TLR4) synthesis, which is in turn responsible for the activation of the ERK1/2, PI3K/Akt and PKA/CREB pathways that phosphorylate the NF-κB p65 subunit leading to NF-κB activation. Binding of the activated NF-κB p65 subunit to IL-6 promoter induces IL-6 synthesis in human T/C28a2 chondrocytes. PGD2 or 15d-PGJ2 concurrently downregulates TLR4 and upregulates caveolin-1, which in turn inhibit the PGE2-dependent ERK1/2, PI3-K and PKA activation, and ultimately with NF-κB-dependent IL-6 synthesis in chondrocytes. Conclusions/SignificanceWe have delineated the signaling cascade by which PGE2 and PGD2/15d-PGJ2 exert opposing effects on IL-6 synthesis in human chondrocytes. Elucidation of the molecular pathway of IL-6 synthesis and secretion by chondrocytes will provide insights for developing strategies to reduce inflammation and pain in RA patients.