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Posttranscriptional regulation is important for tumor necrosis factor alpha (TNF-␣) expression in monocytes and macrophages, and an AU-rich element (ARE) in the 3 untranslated region (UTR) of TNF-␣ mRNA is implicated in control of its translation and mRNA stability. Regulation of mRNA turnover is thought to be mediated by trans-acting proteins, which bind the ARE and stabilize or destabilize the transcript. However, with the exception of the destabilizing factor tristetraprolin, the identity and function of the proteins binding the TNF-␣ mRNA ARE have not been established. To identify other proteins involved in the posttranscriptional control of TNF-␣, the subcellular location of TNF-␣ mRNA was determined in the macrophage-like cell line RAW 264.7. TNF-␣ mRNA was located in the pellet following centrifugation of cytoplasm at 100,000 ؋ g (P100 fraction). This fraction also contained proteins which formed two distinct ARE-specific complexes with the TNF-␣ mRNA 3 UTR in electrophoretic mobility shift assays (EMSAs). A protein present in these two complexes was purified and identified by peptide mass mapping and tandem mass spectrometry as HuR. In EMSAs both complexes were supershifted by an anti-HuR antibody, while Western blotting also demonstrated the presence of HuR in the P100 extract. A HeLa cell tetracycline-regulated reporter system was used to determine the effect of HuR on mRNA stability. In this system, overexpression of HuR resulted in stabilization of an otherwise unstable reporter-mRNA containing the TNF-␣ ARE. These results demonstrate that the TNF-␣ ARE is a target of the mRNA-stabilizing factor HuR.

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

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