Lipopolysaccharide Activation of the TPL-2/MEK/Extracellular Signal-Regulated Kinase Mitogen-Activated Protein Kinase Cascade Is Regulated by I B Kinase-Induced Proteolysis of NF- B1 p105

National Institute for Medical Research, Division of Immune Cell Biology, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom.
Molecular and Cellular Biology (Impact Factor: 4.78). 12/2004; 24(21):9658-67. DOI: 10.1128/MCB.24.21.9658-9667.2004
Source: PubMed

ABSTRACT The MEK kinase TPL-2 (also known as Cot) is required for lipopolysaccharide (LPS) activation of the extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase cascade in macrophages and consequent upregulation of genes involved in innate immune responses. In resting cells, TPL-2 forms a stoichiometric complex with NF-kappaB1 p105, which negatively regulates its MEK kinase activity. Here, it is shown that lipopolysaccharide (LPS) stimulation of primary macrophages causes the release of both long and short forms of TPL-2 from p105 and that TPL-2 MEK kinase activity is restricted to this p105-free pool. Activation of TPL-2, MEK, and ERK by LPS is also demonstrated to require proteasome-mediated proteolysis. p105 is known to be proteolysed by the proteasome following stimulus-induced phosphorylation of two serines in its PEST region by the IkappaB kinase (IKK) complex. Expression of a p105 point mutant, which is not susceptible to signal-induced proteolysis, in RAW264.7 macrophages impairs LPS-induced release of TPL-2 from p105 and its subsequent activation of MEK. Furthermore, expression of wild-type but not mutant p105 reconstitutes LPS stimulation of MEK and ERK phosphorylation in primary NF-kappaB1-deficient macrophages. Consistently, pharmacological blockade of IKK inhibits LPS-induced release of TPL-2 from p105 and TPL-2 activation. These data show that IKK-induced p105 proteolysis is essential for LPS activation of TPL-2, thus revealing a novel function of IKK in the regulation of the ERK MAP kinase cascade.

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    • "IKKb can phosphorylate and inhibit the activity of the transcription factor FOXO3a (Hu et al., 2004), a factor that inhibits proliferation of lung myofibroblasts (McGowan and McCoy, 2013). IKKb also activates the pro-proliferative MAP kinase pathway by inducing p105 processing, thereby removing p105-mediated inhibition of the MAPK pathway (Beinke et al., 2004). In addition, IKKb modulates the mRNA stability of transcripts containing AU-rich element (ARE) motifs (Gringhuis et al., 2005), thus potentially influencing the expression of numerous cytokine, chemokine , and growth factors, many of which contain ARE motifs. "
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    • "Interaction with TNIP-2 and NFκB1 p105 stabilizes TPL2, while the interaction with NFκB1 p105 also prevents TPL2 activity towards MKK1/MKK2 [6]. Activation of TPL2 requires phosphorylation and degradation of NFκB1 p105 by the IkB Kinase (IKK) complex [7], releasing TPL2 that will autophosphorylate on Thr290 [8] and auto- or transphosphorylate on Ser400 [9]. Once activated TPL2 phosphorylates MKK1/MKK2 direct upstream activators of ERK1/ERK2. "
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    • "Although previous studies in the immune system have described the involvement of GITR in activating NF-kB and Erk (Esparza et al., 2006), to our knowledge this is the first report of an inverse regulation of these signalling pathways mediated by GITR. Interestingly, Erk and NF-kB signalling have been shown to be regulated by tumour progression locus 2 (TPL2), which is a serine threonine kinase that under homeostatic conditions is stoichiometrically associated with the NF-kB component p105, the precursor of p50 (Waterfield et al., 2003; Beinke et al., 2004). "
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