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The mitogen-activated protein kinase (MAPK) p38/MAPK-activated protein kinase 2 (MK2) signaling pathway plays an important role in the posttranscriptional regulation of tumor necrosis factor (TNF), which is dependent on the adenine/uridine-rich element (ARE) in the 3 untranslated region of TNF mRNA. After lipopolysaccharide (LPS) stimulation, MK2-deficient macrophages show a 90% reduction in TNF production compared to the wild type. Tristetraprolin (TTP), a protein induced by LPS, binds ARE and destabilizes TNF mRNA. Accordingly, macrophages lacking TTP produce large amounts of TNF. Here, we generated MK2/TTP double knockout mice and show that, after LPS stimulation, bone marrow-derived macrophages produce TNF mRNA and protein levels comparable to those of TTP knockout cells, indicating that in the regulation of TNF biosynthesis TTP is genetically downstream of MK2. In addition, we show that MK2 is essential for the stabilization of TTP mRNA, and phosphorylation by MK2 leads to increased TTP protein stability but reduced ARE affinity. These data suggest that MK2 inhibits the mRNA destabilizing activity of TTP and, in parallel, codegradation of TTP together, with the target mRNA resulting in increased cellular levels of TTP.

p38 mitogen-activated protein kinase (MAPK) is activated by inflammatory stimuli such as bacterial lipopolysaccharide (LPS), interleukin-1, and tumor necrosis factor. We have previously shown that the pyridinyl imidazole SB 203580, which inhibits it, blocks the interleukin-1 induction of cyclooxygenase-2 (COX-2) and matrix metalloproteinase 1 and 3 mRNAs in fibroblasts. Here we explore the role of p38 MAPK in the response of human monocytes to LPS. 0.1 M SB 203580 significantly inhibited the LPS induction of COX-2 and tumor necrosis factor protein and mRNAs. The activity of MAPKactivated protein kinase-2 (a substrate of p38 MAPK) in the cells was commensurately reduced. Some isoforms of c-jun N-terminal kinase (which is also activated by LPS) are sensitive to SB 203580; the inhibitor had little effect on monocyte c-jun N-terminal kinases up to 2 M. We investigated the mechanism of inhibition of COX-2 induction. Transcription (measured by a nuclear run-on assay) was 60% inhibited by SB 203580 (2 M). Importantly, we found that p38 MAPK was essential for stabilizing COX-2 mRNA: when cells stimulated for 4 h with LPS were treated with actinomycin D, COX-2 mRNA decayed slowly. Treatment of stimulated cells with 2 M SB 203580 caused a rapid disappearance of COX-2 mRNA, even with actinomycin D present. We conclude p38 MAPK plays a role in the transcription and stabilization of COX-2 mRNA.p38 mitogen-activated protein kinase (MAPK) 1 is a member of the MAPK family and is activated by the inflammatory cytokines interleukin-1 (IL-1) and tumor necrosis factor (TNF), by bacterial lipopolysaccharide (LPS), and by a range of cellular stresses (1-5). Although originally characterized as a stress or inflammatory kinase, it is likely to have diverse functions because it is also activated in platelets by thrombin and collagen (6) and in T cells upon activation by various stimuli (7) and is constitutively active in liver (8, 9). Little is known about the physiological functions it controls. One substrate is MAPKactivated protein kinase-2 (MAPKAPK-2) (10, 11), which in turn phosphorylates the small heat shock protein hsp27 (12) and the cAMP-response element binding protein (13). Other putative targets are the MAPK integrating kinase (14, 15) and the transcription factors CHOP (16), myocyte enhancer factor 2C (17), and activating transcription factor 2 (4).Besides the original p38 MAPK (called ␣), a closely similar ␤ form has been described (18) as well as two more distantly related enzymes that also contain the TGY motif: stress-activated protein kinase 3 (or p38␥) (19 -21) and stress-activated protein kinase 4 (or p38␦) (22-24). The p38␣ and p38␤ MAPKs are inhibited by a class of pyridinyl imidazole compounds of which the best characterized is SB 203580 (11). These were first identified as inhibitors of TNF (and IL-1) production by LPS-activated monocytes (25) and were later shown to inhibit p38 MAPK (5, 11). The pyridinyl imidazoles inhibited TNF (and IL-1) protein production with relatively little effect on the levels of mRNA ...

The p38 mitogen-activated protein kinase (MAPK) signaling pathway, acting through the downstream kinase MK2, regulates the stability of many proinflammatory mRNAs that contain adenosine/uridine-rich elements (AREs). It is thought to do this by modulating the expression or activity of ARE-binding proteins that regulate mRNA turnover. MK2 phosphorylates the ARE-binding and mRNA-destabilizing protein tristetraprolin (TTP) at serines 52 and 178. Here we show that the p38 MAPK pathway regulates the subcellular localization and stability of TTP protein. A p38 MAPK inhibitor causes rapid dephosphorylation of TTP, relocalization from the cytoplasm to the nucleus, and degradation by the 20S/26S proteasome. Hence, continuous activity of the p38 MAPK pathway is required to maintain the phosphorylation status, cytoplasmic localization, and stability of TTP protein. The regulation of both subcellular localization and protein stability is dependent on MK2 and on the integrity of serines 52 and 178. Furthermore, the extracellular signal-regulated kinase (ERK) pathway synergizes with the p38 MAPK pathway to regulate both stability and localization of TTP. This effect is independent of kinases that are known to be synergistically activated by ERK and p38 MAPK. We present a model for the actions of TTP and the p38 MAPK pathway during distinct phases of the inflammatory response.The tandem zinc finger protein tristetraprolin (TTP; also known as Nup475, Tis11, or Zfp36) (23,26,40,46,62) is expressed in activated monocytic cells (13, 47) and T lymphocytes (49, 51). It functions to regulate the expression of tumor necrosis factor ␣ (TNF-␣) by binding to a conserved adenosine/uridine-rich element (ARE) within the 3Ј-untranslated region of TNF-␣ mRNA (13,31,32,36,47). TTP promotes both mRNA deadenylation and 3Ј to 5Ј degradation of the mRNA body (35, 37-39), consistent with its ability to recruit several factors involved in these processes (14,25,39,45). The pivotal role of TTP in the regulation of TNF-␣ is illustrated by the proinflammatory phenotype of a TTP Ϫ/Ϫ mouse strain, in which chronic overexpression of TNF-␣ by macrophages results in severe polyarthritis and cachexia (11,13,57). TTP has also been implicated in the posttranscriptional regulation of granulocyte-macrophage colony-stimulating factor (12), interleukin-2 (51), cyclooxygenase 2 (COX-2) (50), and inducible nitric oxide synthase (24). It may also regulate its own expression by binding to an ARE in the 3Ј untranslated region of TTP mRNA (60). The minimum binding site of TTP is the nonameric sequence UUAUUUAUU (2,3,38,65), and it is likely that additional posttranscriptional targets of TTP containing this sequence remain to be identified.The p38 mitogen-activated protein kinase (MAPK) and its downstream kinase MK2 play a central role in the posttranscriptional regulation of inflammatory gene expression in myeloid and other cells (5, 16, 20-22, 33, 34, 54). We and others have therefore investigated interactions of the p38 MAPK pathway with TTP. In a mouse macrophage-like...

The translation of tumour necrosis factor K K (TNFK K) mRNA is regulated by the stress-activated protein kinase p38, which also controls the stability of several pro-inflammatory mRNAs. The regulation of TNFK K gene expression in a mouse macrophage cell line RAW264.7 was re-examined using an inhibitor of stress-activated protein kinases. Stimulation of these cells with bacterial lipopolysaccharide resulted in stabilisation of TNFK K mRNA, which was reversed by specific inhibition of p38. An adenosine/uridine-rich element from the TNFK K 3P P untranslated region conferred p38-sensitive decay in a tetracyclineregulated mRNA stability assay. Therefore the p38 pathway also controls TNFK K mRNA turnover. ß

The stability of cyclooxygenase 2 (Cox-2) mRNA is regulated positively by proinflammatory stimuli acting through mitogen-activated protein kinase (MAPK) p38 and negatively by anti-inflammatory glucocorticoids such as dexamethasone. A tetracycline-regulated reporter system was used to investigate mechanisms of regulation of Cox-2 mRNA stability. Dexamethasone was found to destabilize ␤-globin-Cox-2 reporter mRNAs by inhibiting p38. This inhibition occurred at the level of p38 itself: stabilization of reporter mRNA by a kinase upstream of p38 was blocked by dexamethasone, while stabilization by a kinase downstream of p38 was insensitive to dexamethasone. Inhibition of p38 activity by dexamethasone was observed in a variety of cell types treated with different activating stimuli. Furthermore, inhibition of p38 was antagonized by the antiglucocorticoid RU486 and was delayed and actinomycin D sensitive, suggesting that ongoing glucocorticoid receptor-dependent transcription is required.

Tristetraprolin (TTP) is an mRNA-destabilizing protein that negatively regulates the expression of proinflammatory mediators such as tumor necrosis factor ␣, granulocyte/macrophage colony-stimulating factor, and cyclooxygenase 2. Here we investigate the regulation of TTP expression in the mouse macrophage cell line RAW264.7. We show that TTP mRNA is expressed in a biphasic manner following stimulation of cells with lipopolysaccharide and that the second phase of expression, like the first, is dependent on mitogen-activated protein kinase (MAPK) p38. MAPK p38 acts through a downstream kinase to stabilize TTP mRNA, and this stabilization is mediated by an adenosine/uridine-rich region at the 3-end of the TTP 3-untranslated region. Hence TTP is post-transcriptionally regulated in a similar manner to several proinflammatory genes. We also demonstrate that TTP is able to bind to its own 3-untranslated region and negatively regulate its own expression, forming a feedback loop to limit expression levels.

The mechanism by which p38 mitogen-activated protein kinase (MAPK) regulates the induction of cyclooxygenase (COX)-2 by interleukin-1 (IL-1) has been investigated in HeLa cells. SB 203580, an inhibitor of p38 MAPK, in the range 0.1^1 W WM inhibited IL-1-stimulated PGE 2 (but not arachidonic acid) release and this was associated with inhibition of induction of COX-2 protein and mRNA. IL-1 stimulated COX-2 transcription in HeLa cells about 2-fold as judged by both reporter gene and nuclear run-on assays. The inhibitor had no significant effect on this. However, in cells previously stimulated with IL-1 it caused rapid destabilisation of COX-2 mRNA independently of on-going transcription. The results suggest a novel function for p38 MAPK in the regulation of mRNA stability.z 1998 Federation of European Biochemical Societies.