MAP kinase subtypes and Akt regulate diosgenin-induced apoptosis of rheumatoid synovial cells in association with COX-2 expression and prostanoid production

Abstract

In the present study, we investigated the signalling pathways involved in diosgenin-induced apoptosis in human rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS) in vitro with particular interest on Akt and MAPKs activation in relation to arachidonic acid metabolism via COX-2 pathway. MAPK activation was measured by ELISA quantification in diosgenin-treated human RA FLS. Expression of Akt and phospho-Akt was analyzed by Western blot analysis. Nuclear factor-κB (NF-κB) translocation was evaluated by electromobility shift assay. The prostanoid production (COX-2 activity) was measured by quantitative ELISA. Diosgenininduced apoptosis in the presence of MAPK or Akt inhibitors was detected by a quantitative determination of DNA fragmentation. Treatment of human RA FLS with 40 μM diosgenin caused an activation of p38 and JNK and an inhibition of ERK phosphorylation. Akt and NF-κB are potentially required for diosgenin-induced apoptosis in human RA FLS because 40 μM diosgenin abrogated Akt phosphorylation which correlated with an inhibition of NF-κB nuclear translocation. SB203580 and SP600125 (p38 and JNK inhibitors) reduced diosgenin-induced DNA fragmentation whereas U0126 and LY294002 (MEK and PI3 kinase/Akt inhibitors) caused an amplification of proapoptotic effect of diosgenin. Diosgenin increased COX-2 activity resulting in PGE2 and 6-keto-PGF1α overproduction in human RA FLS. All MAPK inhibitors markedly reduced diosgenin-induced PGE2 and 6-keto-PGF1α synthesis except for SP600125 on 6-keto-PGF 1α production. These results provide, for the first time, strong evidence that a combined association implicating a MEK inhibitor (U0126) and diosgenin is the most effective in inducing very strong apoptosis with down-regulation of COX-2 expression and activity in human RA FLS.