contributed equally to this work TIA-1 and TIAR are related proteins that bind to an AU-rich element (ARE) in the 3¢ untranslated region of tumor necrosis factor alpha (TNF-a) transcripts. To determine the functional signi®cance of this interaction, we used homologous recombination to produce mutant mice lacking TIA-1. Although lipopolysaccharide (LPS)-stimulated macrophages derived from wild-type and TIA-1 ±/± mice express similar amounts of TNF-a transcripts, macrophages lacking TIA-1 produce signi®cantly more TNF-a protein than wild-type controls. The half-life of TNF-a transcripts is similar in wild-type and TIA-1 ±/± macrophages, indicating that TIA-1 does not regulate transcript stability. Rather, the absence of TIA-1 signi®cantly increases the proportion of TNF-a transcripts that associate with polysomes, suggesting that TIA-1 normally functions as a translational silencer. TIA-1 does not appear to regulate the production of interleukin 1b, granulocyte±macrophage colony-stimulating factor or interferon g, indicating that its effects are, at least partially, transcript speci®c. Mice lacking TIA-1 are hypersensitive to the toxic effects of LPS, indicating that this translational control pathway may regulate the organismal response to microbial stress.

Objective. Heat-shock protein 90 (Hsp90) is critical in the intracellular signaling pathways that promote inflammatory cytokine production. Geldanamycin (GD) is a benzoquinone ansamycin that inhibits the function of Hsp90. GD inhibits the production of tumor necrosis factor ␣ (TNF␣) in activated macrophages and suppresses the progression of adjuvant-induced arthritis and experimental allergic encephalomyelitis in rodents. GD has been used to investigate the mechanisms by which Hsp90 regulates inflammatory cytokine production.Methods. The macrophage cell line RAW264.7 (or primary peritoneal macrophages) was activated with lipopolysaccharide in the absence or presence of GD. The effect of GD on the transcription, stability, and translation of inflammatory cytokine messenger RNA (mRNA) was determined using nuclear run-on assays, mRNA decay assays, and sucrose gradient polysome profiles, respectively.Results. Our data revealed that GD potently inhibits the production of TNF␣, interleukin-6 (IL-6), and IL-1␤ in activated macrophages. Although GD did not significantly reduce the transcription of inflammatory cytokine mRNA, it significantly decreased the stability of these transcripts. Polysome profiles indicated that GD also inhibited the translation of TNF␣ and IL-6 transcripts. These effects may be due, in part, to inhibition of p38 mitogen-activated protein kinase, a kinase known to regulate the stability and translation of inflammatory cytokine transcripts.Conclusion. These results indicate that the function of Hsp90 is important in the posttranscriptional control of inflammatory cytokine production.

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