TGFβ inhibits LPS-induced chemokine mRNA stabilization

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Abstract
The mechanisms involved in anti-inflammatory action of transforming growth factor β (TGFβ) have been examined by evaluating its effect on chemokine gene expression in mouse macrophages. Lipopolysaccharide (LPS)–stimulated expression of the CXC chemokines KC and MIP-2 was selectively reduced by TGFβ in a time- and protein synthesis–dependent process. While TGFβ had a modest effect on transcription of the KC and MIP-2 mRNAs as measured by nuclear run-on, it had no effect on LPS-stimulated luciferase expression driven by the KC promoter nor on the activation of nuclear factor κB (NFκB) DNA-binding activity and transactivation function. Interestingly, KC mRNA levels were markedly reduced by TGFβ treatment in cells transfected with KC genomic or cDNA constructs driven from either the KC or cytomegalovirus (CMV) promoters, demonstrating the importance of sequences within the mature mRNA and suggesting that suppression may involve a posttranscriptional mechanism. In support of this possibility, LPS stimulation prolonged the half-life of KC mRNA and this stabilization response was blocked in cells treated with TGFβ. Examination of KC mRNA expressed under control of a tetracycline-responsive promoter demonstrated that TGFβ prevented stabilization of KC mRNA, in response to LPS but did not alter KC mRNA half-life directly. KC mRNA stabilization by LPS was dependent on activation of p38 mitogen-activated protein kinase (MAPK) activity, and TGFβ treatment inhibited p38 MAPK activation. These findings support the hypothesis that TGFβ-mediated suppression of chemokine gene expression involves antagonism of LPS-stimulated KC mRNA stabilization via inhibition of p38 MAPK.