TNFR signaling: ubiquitin-conjugated TRAFfic signals control stop-and-go for MAPK signaling complexes
Michael Karin1, Ewen Gallagher
1Laboratory of Gene Regulation and Signal Transduction, Department of Pharmacology, School of Medicine, University of California San Diego, La Jolla, CA 92093-0723, USA. Karinoffice@ucsd.edu
Abstract:
Nearly two decades after the initial cloning and identification of the founding father of the tumor necrosis factor receptor (TNFR) family, much has been learned about the mechanisms by which these receptors signal to critical transcription factors and other targets that regulate gene expression and cellular physiology. Mitogen-activated protein kinases (MAPKs) and inhibitor of nuclear factor (NF)-kappaB (I kappaB) kinases (IKKs) were identified early on as the upstream kinases responsible for activation of activator-protein 1 (AP-1) and NF-kappaB, respectively, and later on for their ability to control life-or-death decisions in TNF-stimulated cells. Both of these critical pathways are regulated at the level of MAPK kinase kinases (MAP3Ks), after which point they diverge. Recent work, however, illustrates that protein ubiquitination cascades play a critical initiating role in TNFR signaling and account for spatial and temporal separation of IKK and MAPK signaling cascades and thereby determine biological specificity and outcome. Cellular inhibitors of apoptosis (cIAPs) 1 and 2 are ubiquitin (Ub) ligases (E3s) that mediate canonical Lys48-linked ubiquitination of TNFR-associated factor 3 (TRAF3), marking it for subsequent degradation by the proteasome. TRAF3 degradation releases the brake on TRAF2/6:MAP3K signaling complexes responsible for MAPK activation, leading to their translocation from the cytoplasmic segment of the receptor to the cytosol where they initiate MAPK phosphorylation and activation. By contrast, IKK activation proceeds considerably faster than MAPK activation, takes place at the receptor, and is independent of cIAP1/2 activity and TRAF3 degradation. This arrangement may be important for ensuring the proper delivery of NF-kappaB-dependent survival signals and conversion of JNK-promoted death signals to proliferative ones.
Insights
Protein ubiquitination initiates tumor necrosis factor receptor (TNFR) signaling, controlling activator-protein 1 (AP-1) and nuclear factor-kappaB (NF-kappaB) pathway activation and cellular fate. This process ensures distinct signaling outcomes for cell survival and proliferation.
Area of Science:
- Cellular Biology
- Molecular Signaling
- Immunology
Background:
- The tumor necrosis factor receptor (TNFR) superfamily plays crucial roles in regulating gene expression, cellular physiology, and life-or-death decisions.
- Mitogen-activated protein kinases (MAPKs) and inhibitor of nuclear factor-kappaB (NF-kappaB) kinases (IKKs) are key upstream kinases activated by TNFRs, controlling activator-protein 1 (AP-1) and NF-kappaB pathways.
- While MAP3Ks regulate both pathways, their precise temporal and spatial separation in response to TNFR stimulation remains an area of active investigation.
Purpose of the Study:
- To elucidate the role of protein ubiquitination in initiating and regulating TNFR signaling pathways.
- To understand how ubiquitination cascades contribute to the distinct activation kinetics and biological outcomes of MAPK and IKK signaling.
- To investigate the specific mechanisms by which cellular inhibitors of apoptosis (cIAPs) influence TNFR-associated factor 3 (TRAF3) stability and downstream signaling.
Main Methods:
- The study likely involves biochemical assays to analyze protein ubiquitination, degradation, and phosphorylation.
- Techniques such as Western blotting, immunoprecipitation, and proteasome inhibition assays may be employed.
- Investigating the interactions between TNFR-associated proteins, ubiquitin ligases, and signaling kinases.
Main Results:
- Protein ubiquitination cascades critically initiate TNFR signaling, dictating the spatial and temporal separation of IKK and MAPK pathways.
- Cellular inhibitors of apoptosis (cIAPs) mediate Lys48-linked ubiquitination of TRAF3, targeting it for proteasomal degradation.
- TRAF3 degradation is essential for releasing the inhibition on TRAF2/6:MAP3K complexes, thereby enabling MAPK activation, while IKK activation occurs rapidly at the receptor independently of cIAP1/2 and TRAF3 degradation.
Conclusions:
- Ubiquitination is a pivotal regulatory mechanism in TNFR signaling, ensuring distinct biological outcomes.
- The differential regulation of MAPK and IKK pathways by ubiquitination allows for the appropriate cellular response, balancing survival and death signals.
- This intricate signaling network highlights the importance of precise temporal and spatial control in TNFR-mediated cellular processes.
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