Blocking NF-κB: an inflammatory issue

Abstract

The nuclear factor (NF)-κB is considered the master regulator of inflammatory responses. Studies in mouse models have established this transcription factor as an important mediator of many inflammatory disease states, including pulmonary diseases such as acute lung injury and acute respiratory distress syndrome. Endothelial cells provide the first barrier for leukocytes migrating to the inflamed sites and hence offer an attractive cellular context for targeting NF-κB for treatment of these diseases. However, recent studies showing that NF-κB also plays an important role in resolution phase of inflammation and in tissue repair and homeostasis have challenged the view of therapeutic inhibition of NF-κB. This article reviews the regulation of NF-κB in the context of endothelial cell signaling and provides a perspective on why "dampening" rather than "abolishing" NF-κB activation may be a safe and effective treatment strategy for inflammation-associated pulmonary and other inflammatory diseases.

Figure 1.

(A) Inflammation is usually self-limiting; it is activated in response to injury or infection and resolves with elimination of the invading pathogen or healing of the injured tissue. However, in many disease conditions, inflammation becomes excessive, fails to resolve, and exerts detrimental effects on the host tissues, leading to (B) many inflammatory disease states. (C) Therapeutic strategy for inflammatory diseases (blocking versus dampening inflammation). The key to an effective treatment of inflammatory diseases may lie selectively targeting the detrimental inflammation. ALI/ARDS = acute lung injury/acute respiratory distress syndrome; BPD = bronchoalveolar dysplasia; COPD = chronic obstructive pulmonary disease.

Figure 2.

Nuclear factor (NF)-κB activation pathways. The canonical pathway is activated by proinflammatory cytokines (TNF-α, IL-1β), G-protein coupled receptors (GPCR) (thrombin), Toll-like receptors (TLR) (LPS), and many other stimuli, including viruses and antigen receptors. This pathway is dependent on the activation of IKKβ, which phosphorylates IκBα at Ser³² and Ser³⁶, leading to its ubiquitylation and degradation by the 26 proteasome. NF-κB complexes, typically a heterodimer of p50 and RelA/p65, migrate to the nucleus and activate expression of multiple inflammatory, innate immune, and antiapoptotic genes. The noncanonical pathway is induced by cytokines such as lymphotoxin-β receptor (LTβR), B-cell–activating factor (BAFF), CD40L, and LPS and requires NF-κB–inducing kinase (NIK)-dependent activation of IKKα. Activated IKKα phosphorylates p100 NF-κB subunit for its proteasome-dependent processing to p52, generating p52/RelB heterodimers for translocation to the nucleus. The nuclear p52/RelB activates distinct transcriptional programs, mainly the ones involved in the development and maintenance of secondary lymphoid organs.

Figure 3.

Thrombin regulation of NF-κB signaling in endothelial cells. Ligation of protease-activated receptor-1 (PAR-1) by thrombin causes activation of Gα_q and dissociation of G_βγ complex, which in turn lead to parallel activation of PKCδ- and PI3 kinase and the downstream kinase Akt-dependent pathways. These pathways converge to stimulate IKK, which in turn promotes RelA/p65 homodimer binding to the promoter of target genes secondary to phosphorylation and degradation of IκBα. PKCδ also engages p38 MAP kinase to induce phosphorylation and thereby transcriptional capacity of RelA/p65. The resultant expression of proinflammatory genes such as ICAM-1 promotes adhesion and transendothelial migration of leukocytes.

Figure 4.

TNF-α regulation of NF-κB signaling in endothelial cells. Activation of TNF receptor (TNFr) stimulates PI3Kγ, which in turn activates PKCζ. Activated PKCζ triggers the activation of NADPH oxidase, resulting in ROS generation, which in turn induces RelA/p65 homodimer binding to the promoter of target genes secondary to phosphorylation and degradation of IκBα. Additionally, PKCζ phosphorylates and thereby increases the transcriptional activity of the RelA/p65. The expression of proinflammatory genes such as ICAM-1 by this mechanism mediates adhesion and transendothelial migration of leukocytes.

Figure 5.

Regulation of thrombin-induced RelA/p65 nuclear translocation by the Rho-actin pathway. Activation of RhoA/ROCK in endothelial cells after thrombin stimulation of PAR-1 leads to two signaling events; one involves activation of IKK, and the other causes phosphorylation and thereby inactivation of cofilin-1, an actin-binding protein that promotes actin depolymerization. Activated IKK catalyzes phosphorylation and thereby degradation of IκBα to mediate the release of RelA/p65. Inactivation of cofilin results in dynamic reorganization of the actin cytoskeleton. These events mediate the interaction of the released RelA/p65 with actin, and this interaction in turn promotes, by an undetermined mechanism, the nuclear import of RelA/p65. The nuclear RelA/p65 binds to the promoter and induces the expression of target genes such as ICAM-1.

Figure 6.

An effective and safe therapeutic strategy for inflammation-associated diseases may lie in the identification of target(s) whose inhibition may lead to “dampening” rather than complete blockade of NF-κB signaling. (A) Direct inhibition of the receptor or NF-κB may lead to impaired host defense response, tissue repair, and homeostasis. (B) Inhibition of a given arm of signaling to NF-κB (alone or in combination) may allow appropriate inhibition of inflammation such that the host defense and repair mechanisms are least affected.