A Combination of Hypoxia and Lipopolysaccharide Activates Tristetraprolin to Destabilize Proinflammatory mRNAs such as Tumor Necrosis Factor-α

Inflammation is often accompanied by hypoxia because of the high oxygen consumption of invading bacteria and immune cells. During resolution of inflammation, the formation of inflammatory mediators such as tumor necrosis factor-α (TNF-α), which is produced by macrophages, needs to be terminated. We show in RAW264.7 macrophages that TNF-α mRNA as well as intracellular and secreted TNF-α protein levels are reduced after prolonged incubations with lipopolysaccharide (LPS) under hypoxic conditions. The decrease in TNF-α was mediated by destabilization of TNF-α mRNA via a 3′-untranslated region-dependent mechanism. Specifically, the RNA-binding protein tristetraprolin (TTP) increased at mRNA and protein levels after 16-hour incubations with LPS under hypoxia. Interestingly, TTP accumulated in a dephosphorylated and active form, and this accumulation was attributable to reduced p38 mitogen-activated protein kinase activity under these conditions. Knockdown of TTP by small interfering RNA abolished destabilization of TNF-α mRNA. Prolonged incubations with LPS under hypoxia also reduced mRNA amounts and stability of other proinflammatory mediators such as macrophage inflammatory protein-2, interleukin-6, and granulocyte macrophage colony-stimulating factor. Therefore, we propose that hypoxia plays a key role during resolution of inflammation by activating posttranscriptional, TTP-dependent regulatory mechanisms.