TPL2-mediated activation of ERK1 and ERK2 regulates the processing of pre-TNFα in LPS-stimulated macrophages

Activation of the TPL2-MKK1/2-ERK1/2 signalling pathway is essential for lipopolysaccharide (LPS)-stimulated production of TNFα in macrophages. Here, we demonstrate that, unexpectedly, TPL2-deficient or MKK1-inhibited macrophages produce near normal levels of pre-TNFα when TLR2, TLR4 and TLR6 are activated by their respective agonists, but fail to secrete TNFα. We show that LPS stimulates the appearance of pre-TNFα at the cell surface and that this is prevented by inhibition of MAPK kinases 1 and 2 (MKK1/2) or in TPL2-deficient macrophages. However, the transport of pre-TNFα from the Golgi to the plasma membrane is unaffected by inhibition of the TPL2-MKK1/2-ERK1/ 2 pathway. Finally, we show that TACE, the protease that cleaves pre-TNFα to secreted TNFα, is phosphorylated by ERK1 and ERK2 (ERK1/2) at Thr735 in LPS-stimulated macrophages. Therefore, although TACE activity per se is not required for the LPS-stimulated cell surface expression of pre-TNFα, the phosphorylation of this protease might contribute to, or be required for, the cell surface expression of the pre-TNFα-TACE complex.