Posttranscriptional regulatory mechanisms control TNFα expression through AU-rich elements in the 3′UTR of its mRNA. This is mediated through Erk and p38 MAP kinase signaling, although the mechanisms involved remain poorly understood. Here, we show that the MAP kinase signal-integrating kinases (Mnks), which are activated by both these pathways, regulate TNFα expression in T cells via the 3′UTR. A selective Mnk inhibitor or siRNA-mediated knockdown of Mnk1 inhibits TNFα production in T cells, whereas Mnk1 overexpression enhances expression of a reporter construct containing the TNFα 3′UTR. We identify ARE binding proteins that are Mnk substrates, such as hnRNP A1, which they phosphorylate at two sites in vitro. hnRNP A1 is phosphorylated in response to T cell activation, and this is blocked by Mnk inhibition. Moreover, Mnk-mediated phosphorylation decreases binding of hnRNP A1 to TNFα-ARE in vitro or TNFα-mRNA in vivo. Therefore, Mnks are novel players in cytokine regulation and potential new targets for anti-inflammatory therapy.
Present Address: Department of Biochemistry and Molecular Biology, University of British Columbia, 2350 Health Sciences Mall, Vancouver, British Columbia, V6T 1Z3, Canada.
