TPL2 meets p38^MAPK: emergence of a novel positive feedback loop in inflammation

The activation of p38^MAPK by Toll-like receptor signalling is essential for the inflammatory response of innate immunity due to its role in post-transcriptional regulation of TNFα and cytokine biosynthesis. p38^MAPK activation proceeds by the upstream MAP2Ks, MAPK kinase (MKK)3/6 as well as MKK4, which in turn are substrates for MAP3Ks, such as TGFβ-activated protein kinase-1 (TAK1). In contrast, TPL2 has been described as an exclusive MAP3K of MKK1/2-triggering activation of the classical ERKs, ERK1/2. In the recent issue of the Biochemical Journal, Pattison et al. report their screening for TPL2 substrates in LPS-stimulated macrophages and the identification of MKK3/6. Using catalytic-dead TPL2 (Map3k8^D270A/D270A) knockin macrophages, they demonstrated that activation of MKK3/6 by TPL2 significantly contributes to LPS-dependent TNFα biosynthesis and is also essential for TNF-receptor 1 signalling. Hence, a new signalling pathway from TAK1 via IκB kinase, p105 NFκB and TPL2 to MKK3/6 and p38^MAPK is established in macrophages. Taking into account that some isoforms of p38^MAPK are necessary for maintaining functional steady-state levels of TPL2, a positive feedback loop in inflammation emerges.