Negative feedback regulation of NF-κB-inducing kinase is proteasome-dependent but does not require cellular inhibitors of apoptosis

Non-canonical NF-κB signaling is controlled by the precise regulation of NF-κB inducing kinase (NIK) stability. NIK is constitutively ubiquitylated by cellular inhibitor of apoptosis (cIAP) proteins 1 and 2, leading to its complete proteasomal degradation in resting cells. Following stimulation, cIAP-mediated ubiquitylation of NIK ceases and NIK is stabilized, allowing for inhibitor of κB kinase (IKK)α activation and non-canonical NF-κB signaling. Non-canonical NF-κB signaling is terminated by feedback phosphorylation of NIK by IKKα that promotes NIK degradation; however, the mechanism of active NIK protein turnover remains unknown. To address this question, we established a strategy to precisely distinguish between basal degradation of newly synthesized endogenous NIK and induced active NIK in stimulated cells. Using this approach, we found that IKKα-mediated degradation of signal-induced activated NIK occurs through the proteasome. To determine whether cIAP1 or cIAP2 play a role in active NIK turnover, we utilized a Smac mimetic (GT13072), which promotes degradation of these E3 ubiquitin ligases. As expected, GT13072 stabilized NIK in resting cells. However, loss of the cIAPs did not inhibit proteasome-dependent turnover of signal-induced NIK showing that unlike the basal regulatory mechanism, active NIK turnover is independent of cIAP1 and cIAP2. Our results therefore establish that the negative feedback control of IKKα-mediated NIK turnover occurs via a novel proteasome-dependent and cIAP-independent mechanism.     

Non-canonical NF-κB signaling pathway

The non-canonical NF-κB pathway is an important arm of NF-κB signaling that predominantly targets activation of the p52/RelB NF-κB complex. This pathway depends on the inducible processing of p100, a molecule functioning as both the precursor of p52 and a RelB-specific inhibitor. A central signaling component of the non-canonical pathway is NF-κB-inducing kinase (NIK), which integrates signals from a subset of TNF receptor family members and activates a downstream kinase, IκB kinase-α (IKKα), for triggering p100 phosphorylation and processing. A unique mechanism of NIK regulation is through its fate control: the basal level of NIK is kept low by a TRAF-cIAP destruction complex and signal-induced non-canonical NF-κB signaling involves NIK stabilization. Tight control of the fate of NIK is important, since deregulated NIK accumulation is associated with lymphoid malignancies.     

TACI induces cIAP1-mediated ubiquitination of NIK by TRAF2 and TANK to limit non-canonical NF-κB signaling

B-cell-activating factor of the TNF family (BAFF) is a critical factor for B-cell survival and maturation through non-canonical nuclear factor κB (NF-κB) pathway, a NF-κB inducing kinase (NIK)-dependent pathway for the processing of NF-κB2 p100 to generate p52. While BAFF acts primarily through BAFF receptor (BAFF-R), the transmembrane activator and CAML interactor (TACI), the other receptor for BAFF, is thought to serve as a negative regulator for B-cell responses. However, how TACI regulates NF-κB2 activity is largely unknown. In this study, we showed that constitutive activation of TACI signaling suppressed BAFF-R–mediated NF-κB2 p100 processing with the up-regulation of cellular inhibitors of apoptosis 1 (cIAP1) and TNF receptor associated factor (TRAF)-associated NF-κB activator (TANK). The ubiquitination of NIK by cIAP1 was inhibited by the expression of TRAF2 with physical binding to cIAP1. TANK deficiency by small interfering RNA (siRNA) impaired TACI-dependent inhibition of NF-κB2 p100 processing. TANK also inhibited TRAF2-mediated cIAP1 inactivation. Moreover, the recruitment of TRAF2 to TACI induced the ubiquitination of NIK. Taken together, the regulation of NIK by TACI through the interaction of TANK/TRAF2/cIAP1 plays a pivotal role in the suppression of non-canonical NF-κB signaling.     

Lipopolysaccharide-induced activation of NF-κB non-canonical pathway requires BCL10 serine 138 and NIK phosphorylations

Background and aims

B-cell lymphoma/leukemia (BCL)-10 and reactive oxygen species mediate two pathways of NF-κB (RelA) activation by lipopolysaccharide (LPS) in human colonic epithelial cells. The pathway for LPS activation of RelB by the non-canonical pathway (RelB) in non-myeloid cells was not yet reported, but important for understanding the range of potential microbial LPS-induced effects in inflammatory bowel disease.

Methods

Experiments were performed in human colonic epithelial cells and in mouse embryonic fibroblasts deficient in components of the IkappaB kinase (IKK) signalosome, in order to detect mediators of the non-canonical pathway of NF-κB activation, including nuclear RelB and p52 and phospho- and total NF-κB inducing kinase (NIK). BCL10 was silenced by siRNA and effects of mutations of specific phosphorylation sites of BCL10 (Ser138Gly and Ser218Gly) were determined.

Results

By the non-canonical pathway, LPS exposure increased nuclear RelB and p52, and phospho-NIK, with no change in total NIK. Phosphorylation of BCL10 serine 138 was required for NIK phosphorylation, since mutation of this residue eliminated the increases in phospho-NIK and nuclear RelB and p52. Mutations of either serine 138 or serine 218 reduced RelA, p50, and phospho-IκBα of the canonical pathway. Effects of LPS stimulation and BCL10 silencing on NIK phosphorylation were demonstrated in confocal images.

Conclusions

LPS induces activation of both canonical and non-canonical pathways of NF-κB in human colonic epithelial cells, and the non-canonical pathway requires phosphorylations of BCL10 (serine 138) and NIK. These findings demonstrate the important role of BCL10 in mediating LPS-induced inflammation in human colonic epithelial cells and may open new avenues for therapeutic interventions.     

NIK is involved in constitutive activation of the alternative NF-κB pathway and proliferation of pancreatic cancer cells

Pancreatic cancer has one of the poorest prognoses among human neoplasms. Constitutive activation of NF-κB is frequently observed in pancreatic cancer cells and is involved in their malignancy. However, little is known about the molecular mechanism of this constitutive NF-κB activation. Here, we show that the alternative pathway is constitutively activated and NF-κB-inducing kinase (NIK), a mediator of the alternative pathway, is significantly expressed in pancreatic cancer cells. siRNA-mediated silencing of NIK expression followed by subcellular fractionation revealed that NIK is constitutively involved in the processing of p100 and nuclear transport of p52 and RelB in pancreatic cancer cells. In addition, NIK silencing significantly suppressed proliferation of pancreatic cancer cells. These results clearly indicate that NIK is involved in the constitutive activation of the alternative pathway and controls cell proliferation in pancreatic cancer cells. Therefore, NIK might be a novel target for the treatment of pancreatic cancer.     

The TRAF3 adaptor protein drives proliferation of anaplastic large cell lymphoma cells by regulating multiple signaling pathways

T cells devoid of tumor necrosis factor receptor associated factor-3 (Traf3) exhibit decreased proliferation, sensitivity to apoptosis, and an improper response to antigen challenge. We therefore hypothesized that TRAF3 is critical to the growth of malignant T cells. By suppressing TRAF3 protein in different cancerous T cells, we found that anaplastic large cell lymphoma (ALCL) cells require TRAF3 for proliferation. Since reducing TRAF3 results in aberrant activation of the noncanonical nuclear factor-κB (NF-κB) pathway, we prevented noncanonical NF-κB signaling by suppressing RelB together with TRAF3. This revealed that TRAF3 regulates proliferation independent of the noncanonical NF-κB pathway. However, suppression of NF-κB-inducing kinase (NIK) along with TRAF3 showed that high levels of NIK have a partial role in blocking cell cycle progression. Further investigation into the mechanism by which TRAF3 regulates cell division demonstrated that TRAF3 is essential for continued PI3K/AKT and JAK/STAT signaling. In addition, we found that while NIK is dispensable for controlling JAK/STAT activity, NIK is critical to regulating the PI3K/AKT pathway. Analysis of the phosphatase and tensin homolog (PTEN) showed that NIK modulates PI3K/AKT signaling by altering the localization of PTEN. Together our findings implicate TRAF3 as a positive regulator of the PI3K/AKT and JAK/STAT pathways and reveal a novel function for NIK in controlling PI3K/AKT activity. These results provide further insight into the role of TRAF3 and NIK in T cell malignancies and indicate that TRAF3 differentially governs the growth of B and T cell cancers.     

Opposing roles for TRAF1 in the alternative versus classical NF-κB pathway in T cells

T cells lacking TRAF1 hyperproliferate in response to T cell receptor signaling but have impaired signaling downstream of specific TNFR family members such as 4-1BB. Here we resolve this paradox by showing that while TRAF1 is required for maximal activation of the classical NF-κB pathway downstream of 4-1BB in primary T cells, TRAF1 also restricts the constitutive activation of NIK in anti-CD3-activated T cells. Activation of the alternative NF-κB pathway is restricted in unstimulated cells by a cIAP1/2:TRAF2:TRAF3:NIK complex. Using knockdown of NIK by siRNA we show that in activated CD8 T cells TRAF1 is also involved in this process and that constitutive activation of the alternative NF-κB pathway is responsible for costimulation independent hyperproliferation and excess cytokine production in TRAF1-deficient CD8 T cells compared with WT CD8 T cells. The T cell costimulatory molecule 4-1BB critically regulates the survival of activated and memory CD8 T cells. We demonstrate that stimulation through 4-1BB induces cIAP1-dependent TRAF3 degradation and activation of the alternative NF-κB pathway. We also show that while both TRAF1 and cIAP1 have non-redundant roles in suppressing the alternative NF-κB pathway in T cells activated in the absence of costimulation, activation of the classical NF-κB pathway downstream of 4-1BB requires TRAF1, whereas cIAP1 plays a redundant role with cIAP2. Collectively these results demonstrate that TRAF1 plays a critical role in regulating T cell activation both through restricting the costimulation independent activation of NIK in activated T cells and by promoting the 4-1BB-induced classical NF-κB pathway.     

Early embryonic lethality caused by targeted disruption of the TRAF-interacting protein (TRIP) gene

Tumor necrosis factor receptor (TNFR)-associated factors (TRAFs) are key adaptor molecules in the TNFR-signaling complexes that promote a wide variety of signaling cascades including cell proliferation, activation, differentiation, and apoptosis. TRAF-interacting protein (TRIP) is required for the inhibitory regulation of TNF-induced NF-κB signaling via the TNFR/TRAF-signaling complexes in vitro. TRIP also directly interacts with the familial cylindromatosis tumor suppressor gene (CYLD) and negatively regulates NF-κB activation in vitro. However, although there appears to be a relationship between TRIP, the TRAFs and also CYLD as modulators of NF-κB signaling in vitro, the functional role of TRIP in vivo is still unclear. To identify the role of TRIP in vivo, we have generated TRIP-deficient mice. Homozygous mouse embryos were found to die shortly after implantation due to proliferation defects and excessive cell death. These results indicate that TRIP is an essential factor during early mouse embryonic development in vivo.     

Toll-like receptor 9 signaling has anti-inflammatory effects on the early phase of Helicobacter pylori-induced gastritis

Helicobacter pylori (H. pylori)-induced immune responses in the gastric mucosa are skewed toward T helper (Th) 1 phenotype, which is characterized by predominant production of tumor necrosis factor (TNF)-α and interferon (IFN)-γ by helper T cells. Toll-like receptors (TLRs) play an essential role in mucosal defense against microbes through the recognition of bacterial molecules. Among the members of the TLR family, TLR9 recognizes bacterial unmethylated CpG DNA sites, and signal transduction of TLR9 induces production of a variety of cytokines, including type-I IFN (IFN-α/β). We investigated the expression and role of TLR9 in H. pylori-induced gastritis in mice. Expression of TLR9 mRNA in the gastric tissue increased after infection with H. pylori. TLR9 was mainly expressed in the macrophages, dendritic cells, and CD3⁺ cells in the gastric mucosa. Neutrophil infiltration and the expression levels of TNF-α and IFN-γ mRNA were higher in TLR9 knockout (KO) mice than in wild-type mice at 2 and 4 months after H. pylori inoculation. These differences in inflammatory parameters between H. pylori-infected wild-type and TLR9 KO mice disappeared 6 months after H. pylori inoculation. Expression of interleukin-4 mRNA, typical Th2 cytokine, in the gastric tissue did not differ between H. pylori-infected wild-type and TLR9 KO mice. Expression level of IFN-α/β mRNA in the TLR9 KO mice was lower than that in wild-type mice by 4 months after inoculation. Administration of IFN-α reduced H. pylori infection-induced increase in neutrophil infiltration and the expression levels of TNF-α and IFN-γ mRNA in TLR9 KO mice. Our findings suggest that TLR9 signaling plays important roles in the suppression of H. pylori-induced gastritis in the early phase via downregulation of Th1-type cytokines modulated by IFN-α.     

Genetic regulation of MUC1 expression by Helicobacter pylori in gastric cancer cells

Helicobacter pylori infection of the stomach is associated with the development of gastritis, peptic ulcers, and gastric adenocarcinomas, but the mechanisms are unknown. MUC1 is aberrantly overexpressed by more than 50% of stomach cancers, but its role in carcinogenesis remains to be defined. The current studies were undertaken to identify the genetic mechanisms regulating H. pylori-dependent MUC1 expression by gastric epithelial cells. Treatment of AGS cells with H. pylori increased MUC1 mRNA and protein levels, and augmented MUC1 gene promoter activity, compared with untreated cells. H. pylori increased binding of STAT3 and MUC1 itself to the MUC1 gene promoter within a region containing a STAT3 binding site, and decreased CpG methylation of the MUC1 promoter proximal to the STAT3 binding site, compared with untreated cells. These results suggest that H. pylori upregulates MUC1 expression in gastric cancer cells through STAT3 and CpG hypomethylation.     

Genetic host factors in Helicobacter pylori-induced carcinogenesis: Emerging new paradigms

Helicobacter Pylori is a gram negative rod shaped microaerophilic bacterium that colonizes the stomach of approximately half the world's population. Infection with c may cause chronic gastritis which via a quite well described process known as Correas cascade can progress through sequential development of atrophic gastritis, intestinal metaplasia and dysplasia to gastric cancer. H. pylori is currently the only bacterium that is classified as a class 1 carcinogen by the WHO, although the exact mechanisms by which this bacterium contributes to gastric carcinogenesis are still poorly understood. Only a minority of H. pylori-infected patients will eventually develop gastric cancer, suggesting that host factors may be important in determining the outcome of H. pylori infection. This is supported by a growing body of evidence suggesting that the host genetic background contributes to risk of H. pylori infection and gastric carcinogenesis. In particular single nucleotide polymorphisms in genes that influence bacterial handling via pattern recognition receptors appear to be involved, further strengthening the link between host risk factors, H. pylori incidence and cancer. Many of these genes influence cellular pathways leading to inflammatory signaling, inflammasome formation and autophagy. In this review we summarize known carcinogenic effects of H. pylori, and discuss recent findings that implicate host genetic pattern recognition pathways in the development of gastric cancer and their relation with H. pylori.     

TIFA has dual functions in Helicobacter pylori‐induced classical and alternative NF‐κB pathways

Helicobacter pylori infection constitutes one of the major risk factors for the development of gastric diseases including gastric cancer. The activation of nuclear factor‐kappa‐light‐chain‐enhancer of activated B cells (NF‐κB) via classical and alternative pathways is a hallmark of H. pylori infection leading to inflammation in gastric epithelial cells. Tumor necrosis factor receptor‐associated factor (TRAF)‐interacting protein with forkhead‐associated domain (TIFA) was previously suggested to trigger classical NF‐κB activation, but its role in alternative NF‐κB activation remains unexplored. Here, we identify TRAF6 and TRAF2 as binding partners of TIFA, contributing to the formation of TIFAsomes upon H. pylori infection. Importantly, the TIFA/TRAF6 interaction enables binding of TGFβ‐activated kinase 1 (TAK1), leading to the activation of classical NF‐κB signaling, while the TIFA/TRAF2 interaction causes the transient displacement of cellular inhibitor of apoptosis 1 (cIAP1) from TRAF2, and proteasomal degradation of cIAP1, to facilitate the activation of the alternative NF‐κB pathway. Our findings therefore establish a dual function of TIFA in the activation of classical and alternative NF‐κB signaling in H. pylori‐infected gastric epithelial cells.     

The Interleukin-17 Receptor B Subunit Is Essential for the Th2 Response to Helicobacter pylori,but Not for Control of Bacterial Burden

Helicobacter pylori infection leads to an inflammatory response in 100% of infected individuals. The inflammatory cells which are recruited to the gastric mucosa during infection produce several pro- and anti-inflammatory cytokines including several cytokines in the interleukin-17 family. The anti-inflammatory cytokine, interleukin 25 (IL-25, also known as IL-17E), signals through a receptor, which is a heterotrimeric receptor comprised of two IL-17 receptor A subunits and an IL-17 receptor B subunit. Previous studies in our laboratory demonstrated that IL-17RA is required to control infection with Helicobacter pylori in the mouse model. Moreover, the absence of IL-17 receptor A leads to a significant B cell infiltrate and a remarkable increase in lymphoid follicle formation in response to infection compared to infection in wild-type mice. We hypothesized that IL-25, which requires both IL-17 receptor A and IL-17 receptor B for signaling, may play a role in control of inflammation in the mouse model of Helicobacter pylori infection. IL-17 receptor B deficient mice, IL-17 receptor A deficient mice and wild-type mice were infected with Helicobacter pylori (strains SS1 and PMSS1). At several time points H. pylori- infected mice were sacrificed to investigate their ability to control infection and inflammation. Moreover, the effects of IL-17 receptor B deficiency on T helper cytokine expression and H. pylori- specific serum antibody responses were measured. IL-17 receptor B−/− mice (unlike IL-17 receptor A−/− mice) exhibited similar or modest changes in gastric colonization, inflammation, and Th1 and Th17 helper cytokine responses to wild-type mice infected with Helicobacter pylori. However, H. pylori-infected IL-17 receptor B−/− mice have reduced expression of IL-4 and lower serum IgG1 and IgG2a levels compared to infected IL-17 receptor A−/− and wild-type mice. These data indicate that signaling through the IL-17 receptor B subunit is not necessary for control of Helicobacter pylori in our model.     

Cathepsin X-deficient Gastric Epithelial Cells in Co-culture with Macrophages: CHARACTERIZATION OF CYTOKINE RESPONSE AND MIGRATION CAPABILITY AFTER HELICOBACTER PYLORI INFECTION*

Our previous studies have shown an association between Helicobacter pylori infection, the strong up-regulation of cathepsin X (CTSX, also called cathepsin Z/P), and the development of gastric cancer. In the present study, we analyzed primary and conventional gastric epithelial cell lines to establish an optimal in vitro mouse model system for the examination of H. pylori-induced overexpression of Ctsx in a functional way. Gastric epithelial cells were isolated from stomachs of wild-type C57BL6/N and Ctsx^−/− mice and compared with the gastric cancer cell line CLS103. Indirect co-cultures of epithelial cells and macrophages were infected with H. pylori strain SS1 and analyzed for the expression of cathepsins, cytokines, and adhesion factors. Cellular interactions, migration capability, and adherence of H. pylori were assessed using time-lapse video microscopy and colony-forming assays. Isolated primary cells from wild-type and transgenic mice revealed qualities and expression profiles similar to those of corresponding tissue samples. Adherence of H. pylori was significantly higher in primary compared with commercially cells. Thus, induction of cathepsins, cytokines, and adhesion proteins was detected solely in primary cells and co-cultured macrophages. Microarray and migration experiments indicated that Ctsx is involved in B/T-cell proliferation/migration and adhesion of macrophages. Primary epithelial cells from stomach of Ctsx^−/− mice represent an excellent model of H. pylori gastritis to elaborate the special functions of Ctsx in regulating the immune response to H. pylori.