Expression and Regulation of Inducible IκB Kinase (IKK-i) in Human Fibroblast-like Synoviocytes

IκB kinase (IKK) plays a key role in the regulation of nuclear factor κB (NF-κB). We previously demonstrated the expression of two kinases, IKK1 and IKK2, in fibroblast-like synoviocytes (FLS) and determined their functional consequences for inflammatory gene expression in vitro and in vivo. Recently, a novel inducible IκB kinase has been described, namely, IKK-i or IKK-ϵ, which is functionally and structurally distinct from constitutively expressed IKK1 and IKK2. Therefore, we investigated the expression and regulation of this novel kinase in FLS from patients with rheumatoid arthritis and osteoarthritis. Interestingly, constitutive gene expression and protein expression were observed in all cell lines examined. TNFα stimulation for 24 h increased IKK-i expression 7.2 ± 1.8-fold in FLS (P < 0.02). IL-1 also significantly increased IKK-i gene expression. Time course experiments demonstrated that IKK-i gene expression increased within 3 h of TNFα stimulation and persisted for at least 24 h. Dose–response studies showed that as little as 1 ng/ml of TNFα increased IKK-i gene expression. Constitutive IKK-1 gene expression was also noted in rheumatoid arthritis, osteoarthritis, and normal synovium. This is the first report demonstrating constitutive expression and cytokine regulation of this novel kinase in primary human synovial cells.