The p38 MAP kinase inhibitor SB203580 enhances nuclear factor‐kappa B transcriptional activity by a non‐specific effect upon the ERK pathway

In the present study we investigated a possible role for the p38 mitogen-activated protein (MAP) kinase pathway in mediating nuclear factor-kappa B (NF-kappaB) transcriptional activity in the erythroleukaemic cell line TF-1. TF-1 cells stimulated with the phosphatase inhibitor okadaic acid (OA) demonstrated enhanced NF-kappaB and GAL4p65-regulated transcriptional activity which was associated with elevated p38 phosphorylation. However, pretreatment with the p38 MAPK specific inhibitor SB203580 (1 microM) or overexpression of kinase-deficient mutants of MKK3 or MKK6 did not affect OA-enhanced NF-kappaB transcriptional potency, as determined in transient transfection assays. In fact, 5 and 10 microM SB203580 enhanced rather than inhibited NF-kappaB-mediated promoter activity by 2 fold, which was independent of phosphorylation of the p65 subunit. The SB203580-mediated increase in NF-kappaB transcriptional activity was associated with enhanced phosphorylation of extracellular signal-regulated kinase (ERK)1/2 and c-Jun N-terminal kinase (JNK), but not p38 kinase. Overexpression of kinase-deficient mutants belonging to the ERK1/2, JNK, and p38 pathways showed that only dominant-negative Raf-1 abrogated SB203580-enhanced NF-kappaB activity. This would implicate the involvement of the ERK1/2 pathway in the enhancing effects of SB203580 on NF-kappaB-mediated gene transcription. This study demonstrates that the p38 MAP kinase pathway is not involved in the OA-induced activation of NF-kappaB. SB203580 at higher concentrations activates the ERK pathway, which subsequently enhances NF-kappaB transcriptional activity.