Direct phosphorylation of NF-kappaB1 p105 by the IkappaB kinase complex on serine 927 is essential for signal-induced p105 proteolysis

The p105 precursor protein of NF-kappaB1 acts as an NF-kappaB inhibitory protein, retaining associated Rel subunits in the cytoplasm of unstimulated cells. Tumor necrosis factor alpha (TNFalpha) and interleukin-1alpha (IL-1alpha) stimulate p105 degradation, releasing associated Rel subunits to translocate into the nucleus. By using knockout embryonic fibroblasts, it was first established that the IkappaB kinase (IKK) complex is essential for these pro-inflammatory cytokines to trigger efficiently p105 degradation. The p105 PEST domain contains a motif (Asp-Ser(927)-Gly-Val-Glu-Thr), related to the IKK target sequence in IkappaBalpha, which is conserved between human, mouse, rat, and chicken p105. Analysis of a panel of human p105 mutants in which serine/threonine residues within and adjacent to this motif were individually changed to alanine established that only serine 927 is essential for p105 proteolysis triggered by IKK2 overexpression. This residue is also required for TNFalpha and IL-1alpha to stimulate p105 degradation. By using a specific anti-phosphopeptide antibody, it was confirmed that IKK2 overexpression induces serine 927 phosphorylation of co-transfected p105 and that endogenous p105 is also rapidly phosphorylated on this residue after TNFalpha or IL-1alpha stimulation. In vitro kinase assays with purified proteins demonstrated that both IKK1 and IKK2 can directly phosphorylate p105 on serine 927. Together these experiments indicate that the IKK complex regulates the signal-induced proteolysis of NF-kappaB1 p105 by direct phosphorylation of serine 927 in its PEST domain.     

Lipopolysaccharide activation of the TPL-2/MEK/extracellular signal-regulated kinase mitogen-activated protein kinase cascade is regulated by IkappaB kinase-induced proteolysis of NF-kappaB1 p105

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.     

Shared pathways of IkappaB kinase-induced SCF(betaTrCP)-mediated ubiquitination and degradation for the NF-kappaB precursor p105 and IkappaBalpha

p105 (NFKB1) acts in a dual way as a cytoplasmic IkappaB molecule and as the source of the NF-kappaB p50 subunit upon processing. p105 can form various heterodimers with other NF-kappaB subunits, including its own processing product, p50, and these complexes are signal responsive. Signaling through the IkappaB kinase (IKK) complex invokes p105 degradation and p50 homodimer formation, involving p105 phosphorylation at a C-terminal destruction box. We show here that IKKbeta phosphorylation of p105 is direct and does not require kinases downstream of IKK. p105 contains an IKK docking site located in a death domain, which is separate from the substrate site. The substrate residues were identified as serines 923 and 927, the latter of which was previously assumed to be a threonine. S927 is part of a conserved DSGPsi motif and is functionally most critical. The region containing both serines is homologous to the N-terminal destruction box of IkappaBalpha, -beta, and -epsilon. Upon phosphorylation by IKK, p105 attracts the SCF E3 ubiquitin ligase substrate recognition molecules betaTrCP1 and betaTrCP2, resulting in polyubiquitination and complete degradation by the proteasome. However, processing of p105 is independent of IKK signaling. In line with this and as a physiologically relevant model, lipopolysaccharide (LPS) induced degradation of endogenous p105 and p50 homodimer formation, but not processing in pre-B cells. In mutant pre-B cells lacking IKKgamma, processing was unaffected, but LPS-induced p105 degradation was abolished. Thus, a functional endogenous IKK complex is required for signal-induced p105 degradation but not for processing.     

Regulation of I(kappa)B kinase complex by phosphorylation of (gamma)-binding domain of I(kappa)B kinase (beta) by Polo-like kinase 1

IkappaB kinase (IKK) complex is a key regulator of NF-kappaB pathways. Signal-induced interaction of the IKKgamma (NEMO) subunit with the C-terminal IKKgamma/NEMO-binding domain (gammaBD) of IKKbeta is an essential interaction for IKK regulation. Underlying regulatory mechanism(s) of this interaction are not known. Phosphorylation of gammaBD has been suggested to play a regulatory role for IKK activation. However, a kinase that phosphorylates gammaBD has not been identified. In this study, we used a C-terminal fragment of IKKbeta as substrate and purified Polo-like kinase 1 (Plk1) from HeLa cell extracts by standard chromatography as a gammaBD kinase. Plk1 phosphorylates serines 733, 740, and 750 in the gammaBD of IKKbeta in vitro. Phosphorylating gammaBD with Plk1 decreased its affinity for IKKgamma in pulldown assay. We generated phosphoantibodies against serine 740 and showed that gammaBD is phosphorylated in vivo. Expressing a constitutively active Plk1 in mammalian cells reduced tumor necrosis factor (TNF)-induced IKK activation, resulting in decreased phosphorylation of endogenous IkappaBalpha and reduced NF-kappaB activation. To activate endogenous Plk1, cells were treated with nocodazole, which reduced TNF-induced IKK activation, and increased the phosphorylation of gammaBD. Knocking down Plk1 in mammalian cells restored TNF-induced IKK activation in nocodazole-treated cells. Activation of Plk1 inhibited TNF-induced expression of cyclin D1. In cells in which Plk1 was knocked down, TNFalpha increased expression of cyclin D1 and the proportion of cells in the S phase of the cell cycle. Taken together, this study shows that phosphorylation regulates the interaction of gammaBD of IKKbeta with IKKgamma and therefore plays a critical role for IKK activation. Moreover, we identify Plk1 as a gammaBD kinase, which negatively regulates TNF-induced IKK activation and cyclin D1 expression, thereby affecting cell cycle regulation. Untimely activation of cyclin D1 by TNFalpha can provide a potential mechanism for an involvement of TNFalpha in inflammation-induced cancer.     

Functionally distinct serine phosphorylation sites of p36, the cellular substrate of retroviral protein kinase; differential inhibition of reassociation with p11.

P36 was originally defined as the major cytoplasmic target of retrovirally coded tyrosine-kinases. While recently much has been learned about its biochemistry, the functional importance of its tyrosine and serine phosphorylation has not been approached. As p36 is now understood as a multi-ligand protein its in vitro phosphorylation by three different serine/threonine kinases was followed. Monomeric p36 is a much better substrate than the complex containing two copies each of p36 and p11 (protein I). All p36 phosphorylation sites occur within the amino-terminal 29 residues specifically released by mild proteolysis. As this region harbors an important interaction site for p11 the reduced phosphorylation of p36 in the protein I complex results most likely from a lowered accessibility. Phosphorylation of p36 is serine specific. Reconstitution experiments define at least two functionally distinct sites. One product of protein kinase C reconstitutes with p11 to protein I, while this complex formation normal for p36 is observed neither for the second phosphorylation product nor for the derivatives resulting from phosphorylation by calmodulin or cAMP dependent kinases. The results lend direct support to the hypothesis that phosphorylation of p36 can modulate one of its molecular functions. Obvious implications for other Ca2+-dependent lipid binding proteins are discussed.     

Hydrogen peroxide activates IkappaB kinases through phosphorylation of serine residues in the activation loops

The cellular redox state regulates nuclear factor-kappaB (NF-kappaB) signaling systems. We investigated the effects of H2O2 on inhibitor of NF-kappaB (IkappaB) kinases (IKKalpha and IKKbeta), which phosphorylate IkappaB leading to its degradation and NF-kappaB activation. Tumor necrosis factor (TNF) stimulation increased IKK activity within 10 min, and then IKK activity decreased gradually within 30 min in HeLa cells. Stimulation of the cells with H2O2 induced a slight activation of IKK within 30 min. Furthermore, co-stimulation with TNF suppressed the downregulation of IKK and sustained the activation for more than 30 min. H2O2 also markedly activated IKK in cells that were pretreated with TNF or phorbol myristate acetate. Electrophoretic mobility shift assay revealed that H2O2 enhanced TNF-induced NF-kappaB activation. Studies using IKK mutants and an antibody against phosphorylated IKK proteins revealed that phosphorylation of serine residues, Ser180 of IKKalpha and Ser181 of IKKbeta, in the activation loops was essential for the H2O2-mediated activation of IKK. H2O2-induced activation of IKKalpha and IKKbeta was reduced by IKKbeta and IKKalpha kinase-negative mutants, respectively, indicating that IKKalpha and IKKbeta were stimulated by H2O2 in an interdependent manner. These results suggest that oxidative radical stress has stimulatory effects on NF-kappaB through the activation of IKK, which is mediated by the phosphorylation of serine residues in the activation loops.     

IkappaB kinases phosphorylate NF-kappaB p65 subunit on serine 536 in the transactivation domain.

Recent investigations have elucidated the cytokine-induced NF-kappaB activation pathway. IkappaB kinase (IKK) phosphorylates inhibitors of NF-kappaB (IkappaBs). The phosphorylation targets them for rapid degradation through a ubiquitin-proteasome pathway, allowing the nuclear translocation of NF-kappaB. We have examined the possibility that IKK can phosphorylate the p65 NF-kappaB subunit as well as IkappaB in the cytokine-induced NF-kappaB activation. In the cytoplasm of HeLa cells, the p65 subunit was rapidly phosphorylated in response to TNF-alpha in a time dependent manner similar to IkappaB phosphorylation. In vitro phosphorylation with GST-fused p65 showed that a p65 phosphorylating activity was present in the cytoplasmic fraction and the target residue was Ser-536 in the carboxyl-terminal transactivation domain. The endogenous IKK complex, overexpressed IKKs, and recombinant IKKbeta efficiently phosphorylated the same Ser residue of p65 in vitro. The major phosphorylation site in vivo was also Ser-536. Furthermore, activation of IKKs by NF-kappaB-inducing kinase induced phosphorylation of p65 in vivo. Our finding, together with previous observations, suggests dual roles for IKK complex in the regulation of NF-kappaB.IkappaB complex.     

ERKs and p38 kinases mediate ultraviolet B-induced phosphorylation of histone H3 at serine 10

Histone H3 is the core protein of the nucleosome. Phosphorylation of H3 involves immediate early gene expression, chromatin remodeling, and chromosome condensation during mitosis. Very recently, Rsk2 or MSK1 kinase-mediated phosphorylation of H3 at serine 10 was reported. In the present study, we show that both ERKs and p38 kinase may mediate ultraviolet B-induced phosphorylation of H3 at serine 10. PD 98059, a MEK1 inhibitor, and SB 202190, a p38 kinase inhibitor, efficiently inhibited ultraviolet B-induced phosphorylation of H3. Phosphorylation of H3 was also inhibited in cells expressing dominant negative mutant (DNM) ERK2 and DNM p38 kinase. In contrast, no inhibition of H3 phosphorylation in Jnk1 or Jnk2 knockout cells (Jnk1(-/-) or Jnk2(-/-)) and cells expressing DNM JNK1 was observed. More importantly, incubation of active ERK2 or p38 kinase with H3 protein resulted in phosphorylation of H3 at serine 10 in vitro. These results suggest that ERK and p38 kinase are at least two important mediators of phosphorylation of H3 at serine 10.     

Inactivation of Bcl-2 through IκB kinase (IKK)-dependent phosphorylation mediates apoptosis upon exposure to 4-hydroxynonenal (HNE)

Apoptosis of macrophage foam cells loaded with modified/oxidized lipids is implicated in destabilization of advanced atherosclerotic plaques in humans. Concentration of HNE, main aldehydic product of plasma LDL peroxidation, elevates in atherosclerotic lesions as well as in cultured cells under oxidative stress. Although this reactive aldehyde has been shown to promote apoptosis with the involvement of p38 MAPK and JNK in various mammalian cell lines, roles of B-cell lymphoma 2 (Bcl-2) family proteins remain to be deciphered. We demonstrated that HNE-induced apoptosis was accompanied by concurrent downregulations of antiapoptotic Bcl-x(L) and Mcl-1 as well as upregulation of proapoptotic Bak. Furthermore, phoshorylation of Bcl-2 at Thr56, Ser70, and probably more phosphorylation sites located on N-terminal loop domain associated with HNE-induced apoptosis in both U937 and HeLa cells while ectopic expression of a phospho-defective Bcl-2 mutant significantly attenuated apoptosis. In parallel to this, HNE treatment caused release of proapoptotic Bax from Bcl-2. Pharmacological inhbition of IKK inhibited HNE-induced Bcl-2 phosphorylation. Similarly, silencing IKKα and -β both ended up with abrogation of Bcl-2 phosphorylation along with attenuation of apoptosis. Moreover, both IKKα and -β coimmunoprecipitated with Bcl-2 and in vitro kinase assay proved the ability of IKK to phosphorylate Bcl-2. In view of these findings and considering HNE inhibits DNA-binding activity of nuclear factor-κB (NF-κB) through prevention of IκB phosphorylation/ubiquitination/proteolysis, IKK appears to directly interfere with Bcl-2 activity through phosphorylation in HNE-mediated apoptosis independent of NF-κB signaling.