Toll- like receptors (TLRs) have an important role in mediating inflammation, and because this process can cause different changes in physiology, TLRs induce many genes with different functions. This makes unlikely that the regulation of these genes relies only at the signalling pathway level. Authors thus hypothesized:
[...] TLR-induced genes with different biological functions should have distinct requirements for regulation. Specifically, genes encoding pro-inflammatory mediators should be transiently inactivated in tolerant macrophages to limit tissue damage. On the other hand, genes encoding antimicrobial effectors and other proteins that do not negatively affect tissue physiology should remain inducible even after repeated stimulation of TLRs to provide continuous protection from infection.First, using microarrays and real- time PCR, they found two classes of genes after their responsiveness to re-stimulation with LPS: tolerizable (T, genes not inducible in tolerant macrophages, pro- inflammatory), and non- tolerizable (NT, inducible in tolerant macrophages, antimicrobial) genes. These results, and other experiments detailed in the paper, confirmed that the induction of LPS tolerance inhibits the pro- inflammatory genes, while the other group (antimicrobial genes) remains inducible. This led to analyze the gene- specific regulation by chromatin modifications. ChIP experiments of acetylated H4 and trimethylated H3K4 showed that these chromatin modifications are lost in class T genes:
- Acetylated H4 was found in both classes of genes, but only NT genes were re- acetylated after stimulation of tolerant cells;
- H3K4 trimethylation was also found in both classes of genes, but only maintained in class NT genes following LPS stimulation;
- Exposure to trichostatin A (TSA) inhibited the expression of class T genes;
- Pargyline (an LSD1 inhibitor) treatment prevented the silencing of class T genes; and
- Both treatments (TSA, pargyline) did not affect activations via NF-kB or MAPK pathways.
Recruitment of nucleosome remodelling complexes, Brg1 and Mi-2ß, was also inhibited in class T genes -this was observed using ChIP and REA/LM-PCR experiments. Taken together, these results mean that although both groups of genes were induced by the same receptor, their regulation is gene- specific and controlled via different chromatin modifications. Authors proposed a model for this process:
Foster SL. et al. 2007 May 30. Nature [Epub ahead of print]Model for gene-specific regulation of class T and NT genes
[...] Collectively, these results indicate that gene products induced by LPS in naive macrophages differentially modify chromatin at class T and class NT promoters to silence the former and to prime the latter for their differential regulation by a second LPS stimulation.
epigenetics
