Prostaglandin E2 induces interleukin-6 expression in human chondrocytes via cAMP/protein kinase A- and phosphatidylinositol 3-kinase-dependent NF-κB activation

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Abstract
Elevated levels of prostaglandin (PG)E2 and interleukin (IL)-6 have been reported in the cartilage and synovial fluid from patients with arthritic disorders. PGE2 regulates IL-6 production in numerous different cells including macrophages and synovial fibroblasts. Although PGE2 stimulates IL-6 expression in human chondrocytes, the underlying signaling pathway of this process has yet to be delineated. Here, we investigate the mechanism of IL-6 induction in human T/C-28a2 chondrocytes treated with exogenously added PGE2. PGE2 induces IL-6 mRNA and protein expression via a cAMP-dependent pathway, reaching maximal levels after 60 min of stimulation before declining to baseline levels at 6 h. Forskolin, an adenylyl cyclase activator, also stimulates IL-6 expression in human chondrocytes in a dose- and time-dependent fashion. Inhibition of downstream effectors of cAMP activity such as protein kinase A (PKA) or phosphatidylinositol 3 kinase (PI3K) blocks PGE2- and forskolin-induced IL-6 upregulation. Simultaneous inhibition of PKA and PI3K reduces IL-6 expression in stimulated chondrocytes well below the basal levels of untreated cells. Gel shift, supershift, and chromatin immunoprecipitation assays reveal the activation and binding of the nuclear factor (NF)-κB p65 subunit to the IL-6 promoter, which is markedly suppressed by selective PI3K or PKA pharmacological inhibitors. p65 knockdown completely abrogates IL-6 mRNA synthesis in PGE2- and forskolin-primed chondrocytes. Cumulatively, our data show that PGE2 and forskolin induce IL-6 expression in human chondrocytes via cAMP/PKA and PI3K-dependent pathways, which in turn regulate the activation and binding of p65 to the IL-6 promoter.