Differential effects of tumor necrosis factor-α and CD40L on NF-κB inhibitory proteins IκBα, β and ε and on the induction of the Jun amino-terminal kinase pathway in Ramos Burkitt lymphoma cellsNF

Interaction between the CD40 ligand and its cognate receptor is known to affect various aspects of B-cell biology. Less is known about the biological consequences of B-cell signaling through tumor necrosis factor alpha (TNF-α) and its two receptors. We have used Ramos germinal center (GC)-derived Burkitt's lymphoma (BL) cells as a model system to compare some of the early signaling events of TNF-α and CD40L on the NF-κB and c-Jun amino-terminal kinase (JNK) pathways. We have previously found that both TNF-α and CD40L induced enhanced cell aggregation, adherence and modified cell surface morphology of Ramos cells. In the present report, it was found that treatment with either TNF-α or CD40L resulted in a rapid degradation (within 15 min) of IκBα, followed by a recovery period lasting up to a few hours. The level of IκBβ, another inhibitory molecule of the NF-κB pathway, also decreased following treatment with CD40L or TNF-α. However, whereas CD40L induced a rapid drop without significant recovery within 2 h, TNF-α caused a slow and gradual decline of IκBβ. In addition, treatment with CD40L resulted in a gradual and modest decline of up to 60% of the level of IκBε within 2 h, whereas a much smaller decline was seen with TNF-α (approx. 20%) Our results thus show that in Ramos cells, TNF-α and CD40L have common, as well as differential, signaling effects on the IκBα, IκBβ and IκBε, which form inhibitory complex(es) with the NF-κB cytosolic proteins. We also found that CD40L, but not TNF-α activates the JNK pathway through transient phosphorylation of its threonine183/tyrosine185 residues. As expected, c-Jun, which is known to be a substrate of JNK, was also phosphorylated at serine residue 73 by treatment with CD40L, but not by TNF-α.