The Tyrosine Kinase Syk Regulates TPL2 Activation Signals

Philip N Tsichlis,Aristides G Eliopoulos,Santasabuj Das

doi:10.1074/jbc.m506790200

Philip N Tsichlis, Aristides G Eliopoulos + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m506790200

Copy DOI

Abstract

Tpl2/Cot is a serine/threonine kinase that plays a key physiological role in the regulation of immune responses to pro-inflammatory stimuli, including tumor necrosis factor-alpha (TNF-alpha). TNF-alpha stimulates the JNK, ERK, and p38 mitogen-activated protein kinases and the NF-kappaB pathway by recruiting RIP1 and TRAF2 to the TNF receptor 1. Here we showed that Tpl2 activation by TNF-alpha signals depends on the integrity of the Tpl2-interacting proteins RIP1 and TRAF2, which are required for the engagement of the ERK mitogen-activated protein kinase pathway. However, neither RIP1 nor TRAF2 overexpression was sufficient to activate Tpl2 and ERK. We also showed that Tpl2 activation by TNF-alpha depends on a tyrosine kinase activity that is detected in TNF-alpha-stimulated cells. Based on both genetic and biochemical evidence, we concluded that in a variety of cell types, Syk is the tyrosine kinase that plays an important role in the activation of Tpl2 upstream of ERK. These data therefore dissect the TNF receptor 1 proximal events that regulate Tpl2 and ERK and highlight a role for RIP1, TRAF2, and Syk in this pathway.

Highlights

Despite the profound biological effects of overexpressed Tpl2, Tpl2 knock-out (Tpl2Ϫ/Ϫ) mice are viable and have no obvious phenotypic defects (10)
We found that ERK phosphorylation induced by TNF-␣ is severely impaired in Tpl2Ϫ/Ϫ macrophages and that the ERK activation signals are induced by triggering the p55 TNF receptor 1 (TNFR1) (12)
The binding of soluble TNF-␣ to TNFR1 induces the recruitment of receptor-interacting protein-1 (RIP1) and TNF receptor-associated factor-2 (TRAF2) to the cytoplasmic C terminus of TNFR1 (23), which results in the transduction of signals that regulate the MAPK pathways and NF-␬B (24 –26)

Summary

Introduction

Despite the profound biological effects of overexpressed Tpl, Tpl knock-out (Tpl2Ϫ/Ϫ) mice are viable and have no obvious phenotypic defects (10). Fibroblasts isolated from Tpl2Ϫ/Ϫ mice demonstrate additional defects in NF-␬B transactivation and JNK signal transduction in response to TNF stimulation (13). Taken together, these findings may provide an explanation to the reported protection of Tpl knock-out mice from TNF-␣-induced inflammatory bowel disease (14). The inactivation of the IKK␤ catalytic subunit of the IKK signaling complex results in impaired NF-␬B signaling and in diminished activation of Tpl and ERK in response to TNF-␣ or LPS (18, 19). The binding of soluble TNF-␣ to TNFR1 induces the recruitment of receptor-interacting protein-1 (RIP1) and TNF receptor-associated factor-2 (TRAF2) to the cytoplasmic C terminus of TNFR1 (23), which results in the transduction of signals that regulate the MAPK pathways and NF-␬B (24 –26). We provide biochemical and genetic data that demonstrate that the activation of the Tpl2/MEK/ERK cascade by TNF-␣ depends on the tyrosine kinase Syk

Methods

Results

Conclusion