MAPK8 and rheumatoid arthritis: Further, when RA FLS were stimulated with four signal molecules' inhibitors including SP600125, SB203580, FR180204, and Ikk-16, which inhibited JNK, P38 MAPK, ERK1/2, and NF-κB activation, respectively, the phosphorylation of P38, NF-κB, ERK1/2, and JNK was suppressed apparently (Figure 3(a)).