On the basis mainly of pharmacological experiments, the p38α MAP kinase isoform has been established as an important regulator of immune and inflammatory responses. However, the role of the related p38γ and p38δ kinases has remained unclear. Here, we show that deletion of p38γ and p38δ impaired the innate immune response to lipopolysaccharide (LPS), a Toll-like receptor 4 (TLR4) ligand, by blocking the extracellular signal-regulated kinase 1/2 (ERK1/2) activation in macrophages and dendritic cells. p38γ and p38δ were necessary to maintain steady-state levels of tumor progression locus 2 (TPL2), the MKK kinase that mediates ERK1/2 activation after TLR4 stimulation. TNFα, IL-1β, and IL-10 production were reduced in LPS-stimulated macrophages from p38γ/δ-null mice, whereas IL-12 and IFNβ production increased, in accordance with the known effects of TPL2/ERK1/2 signaling on the induction of these cytokines. Furthermore, p38γ/δ-deficient mice were less sensitive than controls to LPS-induced septic shock, showing lower TNFα and IL-1β levels after challenge. Together, our results establish p38γ and p38δ as key components in innate immune responses.

The immune-response gene 1 (IRG1) plays a key role in anti-pathogen defense, as deletion of Irg1 in mice causes severe defects in response to bacterial and viral infection, and decreased survival 1, 2 . IRG1 transcription is rapidly induced by pathogen infection and in ammatory conditions primarily in cells of myeloid lineage 3 . IRG1 encodes a mitochondrial metabolic enzyme, aconitate decarboxylase 1 (ACOD1), that catalyzes the decarboxylation of cis-aconitate to produce the anti-in ammatory metabolite itaconic acid (ITA) 4 . Several molecular processes are affected by ITA, including succinate dehydrogenase (SDH) inhibition 5 , resulting in succinate accumulation and metabolic reprogramming 6,7 , and alkylation of protein cysteine residues, inducing the electrophilic stress response mediated by NRF2 and IκBζ 8, 9 and impairing aerobic glycolysis 10 . However, the mechanisms by which ITA exerts its profound antiin ammatory effect still remains to be fully elucidated. Here, we show that ITA is a potent inhibitor of the TET family DNA dioxygenases, which catalyze the conversion of 5-methylcytosine (5mC) to 5hydroxymethylcytosine (5hmC) during the process of active DNA demethylation. ITA binds to the same site of α-ketoglutarate (α-KG) in TET2, inhibiting its catalytic activity. Lipopolysaccharides (LPS) treatment, which induces Irg1 expression and ITA accumulation, inhibits Tet activity in macrophages. Transcriptome analysis reveals TET2 is a major target of ITA in suppressing LPS-induced genes, including those regulated by NF-κB and STAT signaling pathways. In vivo, ITA decreases 5hmC, reduces LPS-induced acute pulmonary edema and lung and liver injury, and protects mice against lethal endotoxaemia in a manner that is dependent on the catalytic activity of Tet2. Our study thus identi es ITA as an immune modulatory metabolite that selectively inhibits TET enzymes to dampen the in ammatory response. MainDeletion of the Irg1 gene or treatment with cell permeable ITA alters the transcriptional signature in response to LPS 2 . We speculated that ITA may impact epigenetics to in uence gene expression, and therefore, we determined the effect of Irg1 expression and ITA accumulation on global histone and DNA de/methylation in transfected HEK293T cells (Extended Data Fig. 1a). We found that ectopic expression of either wild-type or catalytic inactive mutant Irg1 had little effect on mono-, di-, and trimethylation of all ve histone H3 lysine residues (Extended Data Fig. 1b, 1c). In contrast, expression of wild-type Irg1, but not the catalytic inactive mutant, dramatically reduced Tet2-mediated global 5hmC in cells (Fig. 1a and Extended Data Fig. 1d-e). Like α-KG, which is a crucial co-substrate for the activity of TET2, ITA is also a dicarboxylic acid containing a 4-or 5-carboxylate that, in the case of α-KG, forms hydrogen and ionic bonds with H1416, R1896, and S1898 in TET2 11 . Of note, α-KG binds to Fe(II) in a bidentate manner via its C-1 carboxylate and C-2 keto groups, which are lacking in ITA. This raises the possibi...

The role of MYC in regulating p53 stability as a function of increased ribosome biogenesis is controversial. On the one hand, it was suggested that MYC drives the overexpression of ribosomal proteins (RP)L5 and RPL11, which bind and inhibit HDM2, stabilizing p53. On the other, it has been proposed that increased ribosome biogenesis leads the consumption of RPL5/RPL11 into nascent ribosomes, reducing p53 levels and enhancing tumorigenesis. Here, we show that the components that make up the recently described impaired ribosome biogenesis checkpoint (IRBC) complex, RPL5, RPL11, and 5S rRNA, are reduced following MYC silencing. This leads to a rapid reduction in p53 protein half-life in an HDM2-dependent manner. In contrast, MYC induction leads to increased ribosome biogenesis and p53 protein stabilization. Unexpectedly, there is no change in free RPL5/RPL11 levels, but there is a striking increase in IRBC complex bound to HDM2. Our data support a cell-intrinsic tumor-suppressor response to MYC expression, which is presently being exploited to treat cancer. Significance: Oncogenic MYC induces the impaired ribosome biogenesis checkpoint, which could be potentially targeted for cancer treatment.

<p>Supplementary experiments demonstrate that equivalent results to those obtained in RKO cells are also obtained in HCT 116 cells, that the RPL5 antibody precipitation of the IRBC complex is specific and that ribosome biogenesis is upregulated in MYC induced cells.</p>