title: Susceptibility of rheumatoid arthritis synovial fibroblasts to FasL- and TRAIL-induced apoptosis is cell cycle-dependent.
abstract: BACKGROUND
  The rheumatoid arthritis (RA) synovium is characterised by the presence of an aggressive population of activated synovial fibroblasts (RASFs) that are prominently involved in the destruction of articular cartilage and bone. Accumulating evidence suggests that RASFs are relatively resistant to Fas-ligand (FasL)-induced apoptosis, but the data concerning tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) have been conflicting. Here, we hypothesise that the susceptibility of RASFs to receptor-mediated apoptosis depends on the proliferation status of these cells and therefore analysed the cell cycle dependency of FasL- and TRAIL-induced programmed cell death of RASFs in vitro.
  
  
  METHODS
  Synovial fibroblasts were isolated from patients with RA by enzymatic digestion and cultured under standard conditions. Cell cycle analysis was performed using flow cytometry and staining with propidium iodide. RASFs were synchronised or arrested in various phases of the cell cycle with 0.5 mM hydroxyurea or 2.5 microg/ml nocodazol and with foetal calf serum-free insulin-transferrin-sodium selenite supplemented medium. Apoptosis was induced by stimulation with 100 ng/ml FasL or 100 ng/ml TRAIL over 18 hours. The apoptotic response was measured using the Apo-ONE Homogenous Caspase-3/7 Assay (Promega GmbH, Mannheim, Germany) and the Cell Death Detection (ELISAPlus) (enzyme-linked immunosorbent assay) (Roche Diagnostics GmbH, Mannheim, Germany). Staurosporin-treated cells (1 microg/ml) served as a positive control. Expression of Fas and TRAIL receptors (TRAILR1-4) was determined by fluorescence-activated cell sorting analysis.
  
  
  RESULTS
  Freshly isolated RASFs showed only low proliferation in vitro, and the rate decreased further over time, particularly when RASFs became confluent. RASFs expressed Fas, TRAIL receptor-1, and TRAIL receptor-2, and the expression levels were independent of the cell cycle. However, the proliferation rate significantly influenced the susceptibility to FasL- and TRAIL-induced apoptosis. Specifically, proliferating RASFs were less sensitive to FasL- and TRAIL-induced apoptosis than RASFs with a decreased proliferation rate. Furthermore, RASFs that were synchronised in S phase or G2/M phase were less sensitive to TRAIL-induced apoptosis than synchronised RASFs in G0/G1 phase.
  
  
  CONCLUSIONS
  Our data indicate that the susceptibility of RASFs to FasL- and TRAIL-induced apoptosis depends on the cell cycle. These results may explain some conflicting data on the ability of RASFs to undergo FasL- and TRAIL-mediated cell death and suggest that strategies to sensitise RASFs to apoptosis may include the targeting of cell cycle-regulating genes.
authors.affiliation: Institute of Experimental Musculoskeletal Medicine, University Hospital Muenster, Domagkstr 3, Muenster 48149, Germany. noreen.pundt@ukmuenster.de
authors.forename: Noreen
authors.initials: N
authors.lastname: Pundt
authors.forename: Marvin A
authors.initials: MA
authors.lastname: Peters
authors.forename: Christina
authors.initials: C
authors.lastname: Wunrau
authors.forename: Simon
authors.initials: S
authors.lastname: Strietholt
authors.forename: Carsten
authors.initials: C
authors.lastname: Fehrmann
authors.forename: Katja
authors.initials: K
authors.lastname: Neugebauer
authors.forename: Christine
authors.initials: C
authors.lastname: Seyfert
authors.forename: Frans
authors.initials: F
authors.lastname: van Valen
authors.forename: Thomas
authors.initials: T
authors.lastname: Pap
authors.forename: Ingmar
authors.initials: I
authors.lastname: Meinecke
mesh_terms: D017209:Apoptosis / Q000502:physiology*; D001172:Arthritis, Rheumatoid / Q000378:metabolism / Q000473:pathology*; D002453:Cell Cycle / Q000502:physiology*; D049109:Cell Proliferation; D053222:Fas Ligand Protein / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism / Q000473:pathology*; D005434:Flow Cytometry; D006801:Humans; D013583:Synovial Membrane / Q000378:metabolism / Q000473:pathology*; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000378:metabolism
chemical_list: C505661:FASLG protein, human; D053222:Fas Ligand Protein; D053221:TNF-Related Apoptosis-Inducing Ligand