Mitogen-activated kinases in psoriatic arthritis: differences in activation and distinct regulation by etanercept therapy

Mitogen-activated protein kinases (MAPKs) are important effector enzymes of different molecular signaling cascades with critical roles in inflammation, in particular in the induction and downstream effects of tumor necrosis factor (TNF) alpha and IL-1. Inhibition of the MAPK signaling cascades has been identified as a potential therapeutic target in chronic arthritis. Little is known about the activation and extent of MAPK signaling in the synovium of patients with psoriatic arthritis. We studied phosphorylation of the three common MAPKs (p38, extracellular regulating kinase [ERK] and C-jun-N-terminal kinase [JNK]) in psoriatic arthritis and the effect of anti-TNF therapy on their activation.

Synovial biopsies were obtained by needle arthroscopy in nine patients with active psoriatic arthritis in a clinically involved knee before the initiation of etanercept (Enbrel^©; Wyeth Pharmaceuticals, Louvain-la-Neuve, Belgium). All patients were included in a 6-month open-label clinical trial of 25 mg etanercept two times a week [1]. Follow-up biopsies were taken in the same knee after 6 months. Six biopsies were used for histological examination. Frozen sections were stained with H&E or used for immunofluorescense. Sections were fixed with acetone and stained with goat-anti-human phosphorylated ERK, mouse-anti-human phosphorylated p38 or mouse-anti-human phosphorylated JNK (all from Santa Cruz Technologies, Santa Cruz, USA). Cy2-conjugated donkey anti-goat or Cy3-conjugated goat anti-mouse antibodies (Jackson Immunoresearch, West Grove, USA) were used as secondary antibodies. Negative controls were performed with non-specific IgG. Severity of synovitis was assessed with a composite score based on a blinded semi-quantitative score (0–3) of four parameters (lining layer thickness, sublining vascularity, inflammatory cell infiltration and presence of lymphoid aggregates). Activation of MAPKs was quantified with Image J digital analysis software. The area of fluorescent staining due to antibody-linked fluorochrome emission was normalized to cell number based on area of emission by DAPI nuclear staining. For statistical analysis non-parametric Wilcoxon test for paired samples was used.

Activated p38 was present in both lining and sublining layers. In the sublining layer, positive cells were found in inflammatory infiltrates, in perivascular zones and in the endothelium. Activated ERK was present in the sublining layer only, both in mononuclear cell infiltrates and perivascularly. Activation of JNK was recognized in cells of the lining layer, in some sublining cell infiltrates and the endothelium. The composite histological severity score was significantly lower after etanercept treatment (median score 4 versus 1; P < 0.02). Etanercept therapy resulted in a significant decrease in ERK (median normalized fluorescent area 0.096 versus 0.04; P < 0.03) and JNK (median normalized fluorescent area 0.91 versus 0.34; P < 0.03) but not in p38 activation (median normalized fluorescent area 0.04 versus 0.07; P > 0.07).

Different MAPKs are activated in psoriatic arthritis synovium. The activation pattern suggests distinct roles for specific MAPKs in synovitis. The activation pattern of p38 and pERK strongly mimics the pattern described in rheumatoid arthritis [2]. Activation of JNK in the lining layer of rheumatois arthritis synovium has not been reported. Etanercept therapy had a significant effect on JNK and ERK but not on p38 activation. These data suggest that other, probably TNF-independent, signals activate p38 in psoriasis arthritis synovium.