PT  - JOURNAL ARTICLE
AU  - Hoon-Suk Cha
AU  - David L. Boyle
AU  - Tomoyuki Inoue
AU  - Reineke Schoot
AU  - Paul P. Tak
AU  - Polly Pine
AU  - Gary S. Firestein
TI  - A Novel Spleen Tyrosine Kinase Inhibitor Blocks c-Jun N-Terminal Kinase-Mediated Gene Expression in Synoviocytes
AID  - 10.1124/jpet.105.097436
DP  - 2006 May 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 571--578
VI  - 317
IP  - 2
4099  - http://jpet.aspetjournals.org/content/317/2/571.short
4100  - http://jpet.aspetjournals.org/content/317/2/571.full
SO  - J Pharmacol Exp Ther2006 May 01; 317
AB  - Spleen tyrosine kinase (Syk) is a key regulator of cell signaling induced by cytokines or Fc receptor engagement. However, the role of Syk in rheumatoid arthritis (RA) is not known yet. We investigated the pathways activated by Syk in tumor necrosis factor-α (TNFα)-stimulated fibroblast-like synoviocytes (FLS) using the novel Syk inhibitor N4-(2,2-dimethyl-3-oxo-4H-pyrid[1,4]oxazin-6-yl)-5-fluoro-N2-(3,4,5-trimethoxyphenyl)-2,4-pyrimidinediamine (R406). Using immunohistochemistry, Syk was detected in RA synovial tissue (ST), primarily in the synovial intimal lining. Western blot analysis demonstrated significantly greater amounts of phospho-Syk expression in RA ST compared with osteoarthritis ST. The kinase was expressed and functionally activated by TNFα in FLS and was blocked by R406. Western blot analysis demonstrated that Syk inhibition by R406 markedly suppressed TNFα-induced c-Jun N-terminal kinase (JNK) phosphorylation in FLS, with a modest decrease in extracellular signal-regulated kinase phosphorylation. Surprisingly, p38 activation was not affected by R406. The Syk inhibitor also decreased TNFα-induced mitogen-activated protein kinase kinase (MKK) 4 phosphorylation but not MKK3 and MKK6 phosphorylation, which is consistent with its selective sparing of p38. The connection between Syk and JNK was confirmed by demonstrating decreased phospho-c-Jun protein expression and complete inhibition of JNK function in R406-treated cells. R406 also suppressed downstream actions of JNK, as determined by activator protein 1 binding, as well as matrix metalloproteinase 3 gene expression. These data demonstrate that Syk activation plays an essential role in TNFα-induced cytokine and matrix metalloproteinase production in RA FLS, especially by suppressing activation of the JNK pathway. The American Society for Pharmacology and Experimental Therapeutics