Blocking ERK-1/2 reduces tumor necrosis factor alpha-induced interleukin-18 bioactivity in rheumatoid arthritis synovial fibroblasts by induction of interleukin-18 binding protein A.
OBJECTIVE
To examine the mechanism of regulation of interleukin-18 (IL-18) bioactivity by IL-18 binding protein (IL-18BP) induction.


METHODS
Levels of IL-18 and IL-18BPa in synovial fluid samples from patients with osteoarthritis (OA) or rheumatoid arthritis (RA) were determined by enzyme-linked immunosorbent assays (ELISAs), followed by calculation of free IL-18. IL-18 and IL-18BPa synthesis in RA synovial fibroblasts that had been treated with proinflammatory and antiinflammatory cytokines were assessed by quantitative real-time polymerase chain reaction and ELISA, respectively, followed by IL-18 bioactivity determination using KG-1 cells. Chemical signaling inhibitors were used for determination of the signal transduction pathways involved in IL-18BPa/IL-18 regulation. Tumor necrosis factor alpha (TNFalpha)-induced caspase 1 activity was determined by a colorimetric assay.


RESULTS
IL-18BPa was lower in RA synovial fluid than in OA synovial fluid (P < 0.05; n = 8), and free IL-18 was higher in RA synovial fluid than in OA synovial fluid. TNFalpha induced RA synovial fibroblast IL-18BPa and IL-18 in a time-dependent manner (P < 0.05). Evaluation of signaling pathways suggested that TNFalpha induced IL-18 production through the ERK-1/2, protein kinase Cdelta (PKCdelta), and Src pathways, whereas IL-18BPa synthesis was mediated through the NFkappaB, PKC, Src, and JNK pathways. Furthermore, addition of exogenous IL-18BPa-Fc reduced the RA synovial fibroblast phosphorylation of ERK-1/2 induced by TNFalpha.


CONCLUSIONS
These results suggest that IL-18BPa reduces IL-18 bioactivity induced by TNFalpha, by regulating the ERK-1/2 pathway in RA synovial fibroblasts. Targeting IL-18 bioactivity by induction or addition of IL-18BPa may provide another therapeutic option in the management of RA.
University of Michigan, Ann Arbor, MI 48109-2200, USA.
Hubert
H
Marotte
Salahuddin
S
Ahmed
Jeffrey H
JH
Ruth
Alisa E
AE
Koch
D001172:Arthritis, Rheumatoid / Q000378:metabolism*; D004797:Enzyme-Linked Immunosorbent Assay; D048049:Extracellular Signal-Regulated MAP Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000032:analysis / Q000502:physiology*; D020382:Interleukin-18 / Q000032:analysis / Q000502:physiology*; D010003:Osteoarthritis / Q000378:metabolism; D016133:Polymerase Chain Reaction; D015398:Signal Transduction; D013582:Synovial Fluid / Q000737:chemistry; D014409:Tumor Necrosis Factor-alpha / Q000502:physiology*
D036341:Intercellular Signaling Peptides and Proteins; D020382:Interleukin-18; D014409:Tumor Necrosis Factor-alpha; C118370:interleukin-18 binding protein; D048049:Extracellular Signal-Regulated MAP Kinases