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Functional interactions between PI3-kinase and NF-κB signaling pathways promote joint destruction in rheumatoid arthritis
  1. L M Hartkamp,
  2. A M Grabiec,
  3. J Ludikhuize,
  4. I E van Es,
  5. M E Sanders,
  6. P P Tak,
  7. K A Reedquist
  1. Division of Clinical Immunology and Rheumatology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands

Abstract

Background and objectives Rheumatoid arthritis (RA) synovium is characterised by hyperplasia of fibroblast-like synoviocytes (FLS), which display properties of transformed cells. Regulation of proto-oncogenic phosphatidylinositol 3-kinase (PI3-K) signaling by components of the NF-κB signal transduction pathway has emerged as a candidate molecular mechanism linking inflammation to cancer. Conversely, activation of PI3-K pathways in genetically engineered mice, via reducing expression of PTEN or FoxO transcription factors or transgenically expressing constitutively active PKB, can drive NF-κB-mediated autoimmunity. Here we examined potential functional relationships between PI3-K and NF-κB signaling in RA FLS and synovial tissue.

Materials and methods Expression and/or phosphorylation of IKKβ, IkBα, PTEN, a negative regulator of PI3-kinase, PKB and FoxO1, downstream targets of PI3-kinase, was detected by immunohistochemistry combined with digital image analysis in synovial tissue from 15 disease-modifying antirheumatic drug (DMARD)-naïve RA patients. Biopsies were obtained at baseline, and x-rays made of hands and feet at baseline and at 2 years follow-up. At 2 years, patients were classified as having non-erosive or erosive disease (Sharp van der Heijde score > 2). Effects of the pan-PI3-K inhibitor LY294002 and inhibitors specific for α/β, γ and λ isoforms of PI3-K on RA FLS IL-1β-induced IkBα phosphorylation and NF-κB p65 activation were assessed by immunoblotting and ELISA, respectively.

Results In vivo we observed a strong correlation between expression of IKKβ and phosphorylation of IκBα (r = 0.83, p < 0.005) in RA synovial tissue, but IKKβ did not downregulate PTEN or FoxO protein expression, as predicted by tumour models. Instead, we observed a strong positive correlation between PKB activation, phosphorylation of FoxO1 (r = 0.90, p < 0.0001) and activation of NF-κB signalling (r = 0.56, p < 0.05), and NF-κB signaling was elevated in patients with accelerated progressive disease (p < 0.05). In vitro, sustained phosphorylation of IkBα and activation of NF-κB in RA FLS by IL-1β stimulation required PI3-K activity.

Conclusions Our studies provide evidence that PI3-K signaling pathways may contribute to joint destruction in RA through enhancement of NF-κB-dependent gene transcription and that PI3-kinase inhibitors may be effective in preventing disease progression.

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