Tristetraprolin regulates necroptosis during tonic Toll-like receptor 4 (TLR4) signaling in murine macrophages

The necrosome is a protein complex required for signaling in cells that results in necroptosis, which is also dependent on tumor necrosis factor receptor (TNF-R) signaling. TNFalpha promotes necroptosis, and its expression is facilitated by mitogen-activated protein (MAP) kinase-activated protein kinase 2 (MK2), but is inhibited by the RNA-binding protein tristetraprolin (TTP, encoded by the Zfp36 gene). We have stimulated murine macrophages from wildtype, MyD88-/-, Trif-/-, MyD88-/-Trif-/-, MK2-/-, and Zfp36-/- mice with graded doses of lipopolysaccharide (LPS) and various inhibitors to evaluate the role of various genes in Toll-like receptor 4 (TLR4)-induced necroptosis. Necrosome signaling, cytokine production and cell death were evaluated by immunoblotting, ELISA and cell death assays respectively. We observed that during TLR4 signaling, necrosome activation is mediated through the adaptor proteins MyD88 and TRIF, and this is inhibited by MK2. In the absence of MK2-mediated necrosome activation, LPS-induced TNFalpha expression was drastically reduced, but MK2-deficient cells became highly sensitive to necroptosis even at low TNFalpha levels. In contrast, during tonic TLR4 signaling, wildtype cells did not undergo necroptosis, even when MK2 was disabled. Of note, necroptosis occurred only in the absence of TTP and was mediated by the expression of TNFalpha and activation of JUN N-terminal kinase (JNK). These results reveal that TTP plays an important role in inhibiting TNFa/JNK-induced necrosome signaling and resultant cytotoxicity.