To provide insight into the mechanisms whereby sustained tumour necrosis factor (TNF) stimulation promotes chronic inflammatory responses, we have examined TNF-R signalling pathways in murine T-cell hybridomas. Prolonged TNF signalling attenuates p55 TNF-R (TNF-RI)-induced JNK activation through mechanisms that are NF-κB dependent and that target receptor proximal pathways, since arsenite-induced JNK activation is preserved. In contrast, phosphorylation and degradation of IkB alpha, combined with nuclear translocation and gel shift assays, demonstrate that prolonged TNF stimulation promotes chronic NF-κB activation. However, acute and chronic TNF signals are qualitatively distinct. In chronic TNF-treated T cells degradation of IkB alpha and nuclear translocation of RelA (p65) are not as pronounced as that in cells following acute stimulation, while translocation of NF-κB1 (p50) is enhanced. Gel shift assays demonstrate consistent differences in the binding of Rel family proteins to DNA, since acute TNF stimulation induces formation of a dominant DNA binding complex comprising p65:p50 heterodimers (complex I), while the binding capacity of a second complex (complex II), which may include p50:p50 homodimers, is increased in nuclear extracts of chronic TNF-stimulated cells. Preliminary data indicate that chronic but not acute TNF stimulation also induces RelB nuclear translocation and DNA binding. A direct comparison of gene expression profiles in control, acute and chronic TNF-stimulated T cells suggests that differences in TNF-R signalling translate to distinct gene fingerprints. By interrogation of extended fragments of genomic sequence from 66 TNF-induced genes, we have mapped the DNA binding consensus sequence sites for different transcription factors. For the classical NF-κB DNA binding motif GGGRNNYYCC our analysis revealed multiple kB 'hits' in 55/66 TNF-induced genes. Variation in kB motifs was observed across gene clusters. Our results have uncovered differences between acute and chronic TNF signals that may arise through variation in signal input, specific requirements for Rel family dimers, divergence in kB consensus sequences and differences in affinities of protein-DNA binding.
This work was supported by The Wellcome Trust and by arc.