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.     

NF-κB pathway controls mitochondrial dynamics

The Optic atrophy 1 protein (OPA1) is a key element in the dynamics and morphology of mitochondria. We demonstrated that the absence of IκB kinase-α, which is a key element of the nonclassical NF-κB pathway, has an impact on the mitochondrial network morphology and OPA1 expression. In contrast, the absence of NF-κB essential modulator (NEMO) or IκB kinase-β, both of which are essential for the canonical NF-κB pathway, has no impact on mitochondrial dynamics. Whereas Parkin has been reported to positively regulate the expression of OPA1 through NEMO, herein we found that PARK2 overexpression did not modify the expression of OPA1. PARK2 expression reduced the levels of Bax, and it prevented stress-induced cell death only in Bak-deficient mouse embryonic fibroblast cells. Collectively, our results point out a role of the nonclassical NF-κB pathway in the regulation of mitochondrial dynamics and OPA1 expression.Mitochondria perform multiple functions that are critical to the maintenance of cellular homeostasis. Mitochondrial dysfunctions have been linked to the development of degenerative diseases and aging. Damaged mitochondria are removed by mitophagy, a process partially regulated by the PARK2-encoded E3 ubiquitin ligase (Parkin) in a PTEN-induced putative protein kinase 1 (PINK1)-dependent manner.^{1, 2, 3, 4} During mitophagy, the phosphorylation of mitofusin (Mfn) 2 by PINK1 has been suggested to induce the recruitment of Parkin to the mitochondria in cardiomyocytes.⁵ However, previous groups have shown that that Mfn 1 and 2 are dispensable for Parkin-dependent mitophagy in fibroblasts, whereas the Parkin-dependent degradation of these proteins may impair fusion of damaged mitochondria with the healthy network.^{6, 7, 8} PINK1 and Parkin thus act as a quality control machinery on the outer mitochondrial membrane (OMM) to preserve mitochondrial integrity through the ubiquitination of OMM proteins.^{9, 10} Moreover, through its E3 ubiquitin ligase activity,^{11, 12} Parkin was reported to bind to the linear ubiquitin chain assembly complex (LUBAC) and to increase the ubiquitination of NF-κB essential modulator (NEMO),¹³ a component of the classical NF-κB signaling pathway.¹⁴ Müller–Rischart et al. also proposed that Parkin positively regulates the expression of the mitochondrial guanosine triphosphatase Optic atrophy 1 protein (OPA1) through linear ubiquitination of NEMO.¹³ OPA1 is a regulator of mitochondrial inner membrane fusion and cristae remodeling.^{15, 16, 17} A defect in OPA1 expression is associated with mitochondrial network fragmentation and enhanced sensitivity of the cells to undergo apoptosis by promoting cytochrome c release from the mitochondria.^{18, 19, 20} Because NEMO-deficient mouse embryonic fibroblast (MEF) cells display a normal mitochondrial network morphology, we decided to re-examine the role of Parkin in regulating OPA1 expression through the NF-κB signaling pathway.     

Akt/Nox2/NF-κB signaling pathway is involved in Tat-induced HIV-1 long terminal repeat (LTR) transactivation

Human immunodeficiency virus (HIV) regulatory protein Tat has pro-oxidant property, which might contribute to Tat-induced long terminal repeat region (LTR) transactivation. However, the intracellular mechanisms whereby Tat triggers ROS production, and the relationship between Tat-induced ROS production and LTR transactivation, are still subject to debate. The present study was undertaken to evaluate the specific effects of Tat on nicotinamide adenine denucleotide phosphate (NADPH) oxidase in MAGI cells, and to determine the specific role of NADPH oxidase in Tat-induced LTR transactivation. Application of Tat to MAGI cells caused increases in ROS formation that were prevented by both pharmacologic NADPH oxidase inhibitors and by siRNA Nox2, but not by other inhibitors of pro-oxidant enzymes or siRNA Nox4. Furthermore, inhibition of NADPH oxidase by both pharmacologic NADPH oxidase inhibitors and by siRNA Nox2 attenuated Tat-induced p65 phosphorylation and IKK phosphorylation. Phosphatidylinositol 3-kinase/Akt signaling pathway was involved in Tat-induced NADPH oxidase stimulation. Finally, NADPH oxidase inhibitors or Nox2 siRNA, but not control siRNA, inhibited Tat-induced LTR transactivation. Tat-induced HIV-1 LTR transactivation was inhibited in wortmannin or LY294002 treated cells compared to control cells. Together, these data describe a specific and biologically significant signaling component of the MAGI cells response to Tat, and suggest the PI3K/Akt signaling pathway might originate in part with Tat-induced activation of NADPH oxidase and LTR transactivation.     

VIP alleviates sepsis-induced cognitive dysfunction as the TLR-4/NF-κB signaling pathway is inhibited in the hippocampus of rats

Journal of Molecular Histology - Cognitive dysfunction caused by sepsis-associated encephalopathy (SAE) is still poorly understood. It is reported that vasoactive intestinal peptide (VIP) exerts...     

Microtubule-mediated NF-κB activation in the TNF-α signaling pathway

The microtubule cytoskeleton is known to play a role in cell structure and serve as a scaffold for a variety of active molecules in processes as diverse as motility and cell division. The literature on the role of microtubules in signal transduction, however, is marked by inconsistencies. We have investigated a well-studied signaling pathway, TNF-α-induced NF-κB activation, and found a connection between the stability of microtubules and the regulation of NF-κB signaling in C2C12 myotubes. When microtubules are stabilized by paclitaxel (taxol), there is a strong induction of NF-κB even in the absence of TNF-α . Although there was no additive effect of taxol and TNF-α on NF-κB activity suggesting a shared mechanism of activation, taxol strongly induced the NF-κB reporter in the presence of a TNF receptor (TNFR) blocking antibody while TNF-α did not. Both TNF-α and taxol induce the degradation of endogenous IκBα and either taxol or TNF-α induction of NF-κB activity was blocked by inhibitors of NF-κB acting at different sites in the signaling pathway. Both TNF-α and taxol strongly induce known NF-κB chemokine target genes. On the other hand, if microtubules are destabilized by colchicine, then the induction of NF-κB by TNF-α or taxol is greatly reduced. Taken together, we surmise that the activity of microtubules is at the level of the TNFR intracellular domain. This phenomenon may indicate a new level of signaling organization in cell biology, actively created by the state of the cytoskeleton, and has ramifications for therapies where microtubule regulating drugs are used.     

Herbal melanin activates TLR4/NF-κB signaling pathway

Expression of many pro-inflammatory cytokines is controlled by the NF-κB signaling pathway. NF-κB is induced by LPS through activation of TLR4. Melanins extracted from fungal, plant and human sources modulate cytokine production and activate NF-κB pathway. We showed that a herbal melanin (HM) from Nigella sativa L. modulates cytokine production and suggested it as a ligand for TLR4. In this study we investigated the possibility that the HM-induced cytokine production is via an NF-κB signaling pathway. We found that HM induced the degradation of IκBα, a key step in the activation of NF-κB. Moreover, addition of IκB kinase (IKK) specific inhibitors effectively inhibited the observed HM-induced production of IL-8 and IL-6 by TLR4-transfected HEK293 cells and THP-1 cells. Our results have also shown that HM induced cleavage of caspase 8, and that this cleavage was partially abrogated by IKK inhibitors. We suggest that HM can modulate the inflammatory response by inducing IL-8 and IL-6 production via TLR4-dependent activation of the NF-κB signaling pathway.     

IAPs: a central element in the NF-κB activating signaling pathway

The function of IAP has long been limited to an inhibition of apoptosis through their capacity to bind some caspases. Since the expression of these proteins is altered in some tumor samples, IAPs are targets for anticancer therapy and many small molecules have been designed for their capacity to inhibit IAP-caspase interaction. Unexpectedly, these molecules appeared to significantly affect NF-κB activation. In this review, we will discuss the central role of cIAP1, cIAP2 and XIAP in the regulation of NF-κB activating signaling pathways.     

TLR2–MyD88–NF-κB pathway is involved in tubulointerstitial inflammation caused by proteinuria

Proteinuria is an important risk factor for chronic kidney diseases (CKD). Several studies have suggested that proteinuria initiates tubulointerstitial inflammation, while the mechanisms have not been fully understood. In this study, we hypothesized whether the activation of the TLR2–MyD88–NF-κB pathway is involved in tubulointerstitial inflammation induced by proteinuria. We observed expression of TLR2, MyD88, NF-κB, as well as TNF-α and IL-6 detected by immunohistostaining, Western blotting and real-time PCR in albumin-overloaded (AO) nephropathy rats. In vitro, we observed these markers in HK-2 cells stimulated by albumin. We used TLR2 siRNA or the NF-κB inhibitor BAY 11-7082 to observe the influence of TNF-α and IL-6 expression caused by albumin overload. Finally, we studied these markers in non-IgA mesangioproliferative glomerulonephritis (MsPGN) patients with different levels of proteinuria. It was demonstrated that expression of TLR2, MyD88 and NF-κB were significantly increased in AO rats and in non-IgA MsPGN patients with high levels of proteinuria, and TNF-α and IL-6 expressions were increased after NF-κB activation. Furthermore, TNF-α and IL-6 expression was positively correlated with the level of proteinuria. Albumin-overload induced TNF-α and IL-6 secretions by the TLR2–MyD88–NF-κB pathway activation, which could be attenuated by the TLR2 siRNA or BAY 11-7082 in HK-2 cells. In summary, we demonstrated that proteinuria may exhibit an endogenous danger-associated molecular pattern (DAMP) that induces tubulointerstitial inflammation via the TLR2–MyD88–NF-κB pathway activation.     

Nuclear factor-κB signaling pathway is involved in phospholipase Cε-regulated proliferation in human renal cell carcinoma cells

Glioblastoma is the most aggressive cerebral gliomas. Despite advances in therapies, the prognosis is still very poor. Therefore, novel therapeutic strategies are required. As a proteasome inhibitor, bortezomib has shown its efficacy as an active antitumor agent against a variety of tumors. However, inhibition of proteasome activity leads to cell death and also induces cell autophagy, and due to the dual roles of autophagy in the survival and death of tumor cells, the effect of inhibition of autophagy on glioblastoma cells remains to be explored. We therefore assessed whether bortezomib is capable of inducing autophagy, and investigated the antitumor effect of bortezomib combined with autophagy inhibitors on human glioblastoma U251 and U87 cells. Cell viability was measured by MTT assay. The expressions of autophagy and apoptosis-related proteins were determined by Western blot analysis. U251 and U87 cells proliferation was inhibited in a dose-dependent manner. Both apoptosis and autophagy induced by bortezomib were observed in human glioblastoma U87 and U251 cells. However, when U251 and U87 cells were co-treated with bortezomib and autophagy inhibitors 3-MA or Atg7 siRNA, the autophagy inhibitors blocked the autophagy in the cells and resulted in a further inhibition of cell proliferation and a further increase in cell apoptosis as compared with that treated with bortezomib alone. These findings indicated that combination of bortezomib and autophagy inhibitors may shed new light on glioblastoma treatment.     

Chlorogenic acid inhibits proliferation in human hepatoma cells by suppressing noncanonical NF-κB signaling pathway and triggering mitochondrial apoptosis

Chlorogenic acid (CGA), a phenylpropanoid derived from Eucommia ulmoides Oliver, has been shown to exhibit potent cytotoxic and anti-proliferative activities against several human cancers. However, the effects of CGA on hepatocellular carcinoma (HCC) and the underlying mechanisms have not been intensively studied. In this study, the CGA treatment effects on the viability of human hepatoma cells were investigated by MTT assay. Our data showed that CGA could dose-dependently inhibit the activity of human hepatoma cells Hep-G2 and Huh-7, but did not affect the activity and growth of normal human hepatocyte QSG-7701. The genes and pathways influenced by CGA treatment were explored by RNA sequencing and bioinformatics analysis, which identified 323 differentially expressed genes (DEGs) involved in multiple pharmacological signaling pathways such as MAPK, NF-κB, apoptosis and TGF-β signaling pathways. Further analyses by real-time quantitative PCR, Western blot and flow cytometry revealed that CGA effectually suppressed the noncanonical NF-κB signaling pathway, meanwhile it activated the mitochondrial apoptosis of HCC by upregulation of the BH3-only protein Bcl-2 binding component 3 (BBC3). Our findings demonstrated the potential of CGA in suppressing human hepatoma cells and provided a new insight into the anti-cancer mechanism of CGA.

     

PKCβ/NF-κB pathway in diabetic atrial remodeling

Journal of Physiology and Biochemistry - Atrial remodeling in diabetes is partially attributed to NF-κB/TGF-β signal transduction pathway activation. We examined whether the...     

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.     

Correlation between oxidative stress and NF-κB signaling pathway in the obesity-asthma mice

Molecular Biology Reports - In this study, a mice model of obesity-asthma was established. We investigated the correlation between oxidative stress and NF-κB signaling pathway in the lung...     

Astragalus polysaccharides alleviates LPS-induced inflammation via the NF-κB/MAPK signaling pathway

Early weaning usually causes intestinal disorders, enteritis, and diarrhea in young animals and human infants. Astragalus polysaccharides (APS) possesses anti-inflammatory activity. To study the anti-inflammatory mechanisms of APS and its potential effects on intestinal health, we performed an RNA sequencing (RNA-seq) study in lipopolysaccharide (LPS)-stimulated porcine intestinal epithelial cells (IPEC-J2) in vitro. In addition, LPS-stimulated BALB/c mice were used to study the effects of APS on intestinal inflammation in vivo. The results from the RNA-seq analysis show that there were 107, 756, and 5 differentially expressed genes in the control versus LPS, LPS versus LPS+APS, and control versus LPS+APS comparison groups, respectively. The results of Kyoto Encyclopedia of Genes and Genomes enrichment analysis indicated that the mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) signaling pathways play significant roles in the regulation of inflammatory factors and chemokine expression by APS. Further verification of the above two pathways by using western blot and immunofluorescence analysis revealed that the gene expression levels of the phosphorylated p38 MAPK, ERK1/2, and NF-κB p65 were inhibited by APS, while the expression of IκB-α protein was significantly increased (p < .05), indicating that APS inhibits the production of inflammatory factors and chemokines by the inhibition of activation of the MAPK and NF-κB inflammatory pathways induced by LPS stimulation. Animal experiments further demonstrated that prefeeding APS in BALB/c mice can alleviate the expression of the jejunal inflammatory factors interleukin 6 (IL-6), IL-Iβ, and tumor necrosis factor-α induced by LPS stimulation and improve jejunal villus morphology.     

Deubiquitinases in the regulation of NF-κB signaling

Nuclear factor-kappa B (NF-κB) is a critical regulator of multiple biological functions including innate and adaptive immunity and cell survival. Activation of NF-κB is tightly regulated to preclude chronic signaling that may lead to persistent inflammation and cancer. Ubiquitination of key signaling molecules by E3 ubiquitin ligases has emerged as an important regulatory mechanism for NF-κB signaling. Deubiquitinases (DUBs) counteract E3 ligases and therefore play a prominent role in the downregulation of NF-κB signaling and homeostasis. Understanding the mechanisms of NF-κB downregulation by specific DUBs such as A20 and CYLD may provide therapeutic opportunities for the treatment of chronic inflammatory diseases and cancer.