Inhibition of pro-inflammatory cytokine production by the dual p38/JNK2 inhibitor BIRB796 correlates with the inhibition of p38 signaling

The characterization of the potent p38 inhibitor BIRB796 as a dual inhibitor of p38/Jun N-terminal kinases (JNK) mitogen-activated protein kinases (EC 2.7.11.24) has complicated the interpretation of its reported anti-inflammatory activity. To better understand the contribution of JNK2 inhibition to the anti-inflammatory activities of BIRB796, we explored the relationship between the effects of BIRB796 and analogues on cytokine production and on cellular p38 and JNK signaling. We determined the binding affinity for BIRB796 and structural analogues to p38α and JNK2 and characterized compound 2 as a p38 inhibitor that binds to p38α with an affinity equivalent to BIRB796 but does not bind to any of the JNK isoforms. High-content imaging enabled us to show that the inhibition of p38 signaling by BIRB796 and analogues correlates with the ability of these compounds to inhibit the lipopolysaccharide (LPS)-induced TNF-α production in THP-1 monocytes. This finding was extended to cytokine release by disease-relevant human primary cells: to the production of TNF-α by peripheral blood mononuclear cells, and of IL-8 by neutrophils. Furthermore, BIRB796 and compound 2 inhibited the production of TNF-α in THP-1 monocytes and the IL-12/IL-18-induced production of interferon-γ in human T-cells with similar potencies. In contrast, cellular JNK signaling in response to cytokines or stress stimuli was only weakly inhibited by BIRB796 and analogues and not affected by compound 2. In summary, our data suggest that p38 inhibition alone is sufficient to completely suppress cytokine production and that the added inhibition of JNK2 does not significantly contribute to the effects of BIRB796 on cytokine production.