IκB Kinase 2 Regulates TPL-2 Activation of Extracellular Signal-Regulated Kinases 1 and 2 by Direct Phosphorylation of TPL-2 Serine 400

Abstract

Tumor progression locus 2 (TPL-2) functions as a MEK-1/2 kinase, which is essential for Toll-like receptor 4 (TLR4) activation of extracellular signal-regulated kinase 1 and 2 (ERK-1/2) mitogen-activated protein (MAP) kinases in lipopolysaccharide (LPS)-stimulated macrophages and for inducing the production of the proinflammatory cytokines tumor necrosis factor and interleukin-1β. In unstimulated cells, association of TPL-2 with NF-κB1 p105 prevents TPL-2 phosphorylation of MEK-1/2. LPS stimulation of TPL-2 MEK-1/2 kinase activity requires TPL-2 release from p105. This is triggered by IκB kinase 2 (IKK-2) phosphorylation of the p105 PEST region, which promotes p105 ubiquitination and degradation by the proteasome. LPS activation of ERK-1/2 additionally requires transphosphorylation of TPL-2 on serine 400 in its C terminus, which controls TPL-2 signaling to ERK-1/2 independently of p105. However, the identity of the protein kinase responsible for TPL-2 serine 400 phosphorylation remained unknown. In the present study, we show that TPL-2 serine 400 phosphorylation is mediated by IKK2. The IKK complex therefore regulates two of the key regulatory steps required for TPL-2 activation of ERK-1/2, underlining the close linkage of ERK-1/2 MAP kinase activation to upregulation of NF-κB-dependent transcription.

ACKNOWLEDGMENTS

We thank P. Tsichlis (Tufts University, Boston, MA) and Thomas Jefferson University for the Map3k8^−/− mice, Philip Cohen for IKK2 inhibitor (University of Dundee, Scotland), Mike Howell and Becky Sanders (High Throughput Screening Laboratory, Cancer Research UK, London, United Kingdom) for their help with the siRNA screen, Manolis Pasparakis and Claudia Uthoff-Hachenberg (Institute for Genetics, University of Cologne) for providing BM cells from Ikk2^fl/fl LysM-Cre and Ikk2^fl/fl mice, the NIMR Photographics Department, NIMR Biological Services, and other members of the Ley laboratory for help during the course of this work.

This work was supported by the U.K. Medical Research Council (U117584209), Arthritis Research UK (grant reference 18864), and Leukemia and Lymphoma Research (grant reference 06050).