Homozygous targeted disruption of the mouse Caspase 8 (Casp8) gene was found to be lethal in utero. The Caspase 8 null embryos exhibited impaired heart muscle development and congested accumulation of erythrocytes. Recovery of hematopoietic colony-forming cells from the embryos was very low. In fibroblast strains derived from these embryos, the TNF receptors, Fas/Apo1, and DR3 were able to activate the Jun N-terminal kinase and to trigger IkappaB alpha phosphorylation and degradation. They failed, however, to induce cell death, while doing so effectively in wild-type fibroblasts. These findings indicate that Caspase 8 plays a necessary and nonredundant role in death induction by several receptors of the TNF/NGF family and serves a vital role in embryonal development.

R.Winzen and M.Kracht contributed equally to this workStabilization of mRNAs contributes to the strong and rapid induction of genes in the inflammatory response. The signaling mechanisms involved were investigated using a tetracycline-controlled expression system to determine the half-lives of interleukin (IL)-6 and IL-8 mRNAs. Transcript stability was low in untreated HeLa cells, but increased in cells expressing a constitutively active form of the MAP kinase kinase kinase MEKK1. Destabilization and signal-induced stabilization was transferred to the stable β-globin mRNA by a 161-nucleotide fragment of IL-8 mRNA which contains an AU-rich region, as well as by defined AU-rich elements (AREs) of the c-fos and GM-CSF mRNAs. Of the different MEKK1-activated signaling pathways, no significant effects on mRNA degradation were observed for the SAPK/JNK, extracellular regulated kinase and NF-κB pathways. Selective activation of the p38 MAP kinase (⍧SAPK2) pathway by MAP kinase kinase 6 induced mRNA stabilization. A dominant-negative mutant of p38 MAP kinase interfered with MEKK1 and also IL-1-induced stabilization. Furthermore, an active form of the p38 MAP kinase-activated protein kinase (MAPKAP K2 or MK2) induced mRNA stabilization, whereas a negative interfering MK2 mutant interfered with MAP kinase kinase 6-induced stabilization. These findings indicate that the p38 MAP kinase pathway contributes to cytokine/stress-induced gene expression by stabilizing mRNAs through an MK2-dependent, ARE-targeted mechanism.

Humoral autoimmunity paralleled by the accumulation of follicular helper T cells (T(FH) cells) is linked to mutation of the gene encoding the RNA-binding protein roquin-1. Here we found that T cells lacking roquin caused pathology in the lung and accumulated as cells of the T(H)17 subset of helper T cells in the lungs. Roquin inhibited T(H)17 cell differentiation and acted together with the endoribonuclease regnase-1 to repress target mRNA encoding the T(H)17 cell-promoting factors IL-6, ICOS, c-Rel, IRF4, IκBNS and IκBζ. This cooperation required binding of RNA by roquin and the nuclease activity of regnase-1. Upon recognition of antigen by the T cell antigen receptor (TCR), roquin and regnase-1 proteins were cleaved by the paracaspase MALT1. Thus, this pathway acts as a 'rheostat' by translating TCR signal strength via graded inactivation of post-transcriptional repressors and differential derepression of targets to enhance T(H)17 differentiation.

We demonstrate that lipopolysaccharide-induced tumor necrosis factor (TNF) biosynthesis becomes independent of MAPKAP kinase 2 (MK2) when the AU-rich element (ARE) of the TNF gene is deleted. In spleen cells and macrophages where TNF biosynthesis is restored as a result of this deletion, interleukin (IL)-6 biosynthesis is still dependent on MK2. In MK2-deficient macrophages the half-life of IL-6 mRNA is reduced more than 10-fold, whereas the half-life of TNF mRNA is only weakly decreased. It is shown that the stability of a reporter mRNA carrying the AU-rich 3-untranslated region (3-UTR) of IL-6 is increased by MK2. The data provide in vivo evidence that the AU-rich 3-UTRs of TNF and IL-6 are downstream to MK2 signaling and make MK2 an essential component of mechanisms that regulate biosynthesis of IL-6 at the levels of mRNA stability, and of TNF mainly through TNF-ARE-dependent translational control.

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