At once harmful and beneficial: the dual properties of NF-κBSawsan Youssef & Lawrence Steinman

Transcription factor NF-κB is key in both the injury and repair of damaged tissues. Astrocytes use the non-canonical NF-κB activation pathway to modulate brain inflammation in experimental autoimmune encephalomyelitis.

Sawsan Youssef and Lawrence Steinman are in

the Department of Neurology and Neurological

Sciences and the Interdepartmental Program

in Immunology, Stanford University, Stanford,

California 94305, USA.

e-mail: steinman@stanford.edu

Perhaps one of the most notable examples of a duality in the function of a single

molecule in all of biology is exemplified by nuclear factor-κB (NF-κB)1,2. The Nobel lau-reate in chemistry, Roald Hoffmann, has writ-ten (http://www.npr.org/templates/story/story.php?storyId=5519776) that “molecules fight categorization. They are poised along several polarities. Harm and benefit is one.” . NF-κB has contradictory functions, with a ‘Janus-like’ character. NF-κB either promotes harm-ful inflammation or guides the regeneration required to repair inflamed tissues, including brain2. Molecules like the cytokine TNF, which can activate NF-κB (Fig. 1), are also endowed with this same dual capacity to promote either harm or benefit by triggering inflammation or regeneration1,2. Now van Loo and colleagues have analyzed how regulatory components of the NF-κB pathway modulate autoimmune inflammation in the brain using their collection of mouse models with targeted knockout of genes encoding those critical molecules3. They have demonstrated that the so-called ‘canoni-cal’ pathway for the activation of NF-κB has a key pathogenic function in the development of inflammation in experimental autoimmune encephalomyelitis (EAE), a useful model for research into diseases like multiple sclerosis. The names and aliases of the participants in this canonical pathway are complex and var-ied, but the pathway comprises the inhibitor of

NF-κB (IκB) kinase 2 (IKK2; also called IKKβ) and NF-κB essential modulator (NEMO; also called IKKγ; Fig. 1).

NF-κΒ was first identified as a transcription factor restricted to cells of the immune system that specifically bound to a DNA motif 4 later identified as a Rel homology domain found in the enhancer of the immunoglobulin κ light-chain gene. After two decades of research, it is now known this family of NF-κB and Rel transcription factors is found in all cell types. These transcription factors are activated by diverse stimuli and are regulated differentially depending on cell type5. The NF-κB factors have considerable diversity of functions in regulating immune and inflammatory responses; devel-opment; cellular growth and repair processes; oncogenesis; and cell death5. NF-κB complexes are activated in several disease processes, includ-ing many cancers, many types of autoimmunity and many forms of neurodegeneration. The NF-κB pathways are thus interesting as poten-tial targets for drugs to treat these diverse conditions.

NF-κB family consists of the following five subunits: RelA (also called p65), RelB, c-Rel, NF-κΒ1 (p50) and NF-κΒ2 (p52). The last two are synthesized as large precursors of 105 kilodaltons (p105) and 100 kilodaltons (p100), respectively, and are partially proteolyzed by the 26S proteasome, resulting in the mature units6. Hetero- or homodimers of those subunits are then translocated into the nucleus to activate target genes. NF-κB activity is bifurcated into the NF-κB1 ‘canonical’ pathway and the NF-κB2 ‘alternative’ pathway6 (Fig. 1). The canoni-cal pathway is activated by many stimuli, such as microbial lipopolysaccharide and cytokines such as tumor necrosis factor and interleukin 1,

resulting in translocation of the NF-κB1 (p50-p65) dimer into the nucleus and activation of inflammatory and cell survival responses7. The alternative pathway is activated by a few stimuli, such as lymphotoxin-β, CD40 ligand and B cell–activating factor, to release the active NF-κB2 (p52-RelB) heterodimer into the nucleus. This alternative pathway is involved in the development of secondary lymphoid organs, B cell maturation and the adaptive immune response8.

The diversity of NF-κB complexes and the variety of mechanisms for their activation dem-onstrate the intricate regulation of NF-κB activity. One key regulatory mechanism is the sequestra-tion of NF-κB complexes in the cytoplasm by interaction with the inhibitory IκB proteins5,6. NF-κB complexes are rapidly activated by means of an ‘upstream’ Janus kinase that phosphory-lates and thereby releases the inhibitory sub-unit, which is degraded in the 26S proteasome5. In the canonical pathway, phosphorylation of IκBα is critical for activation of the NF-κΒ1 complex. That activity is orchestrated by a cas-cade of kinases, including a complex consisting of IKK1 (IKKα), IKK2 (IKKβ) and NEMO5. In the alternative pathway, the slower de novo synthesis of NF-κB-inducing kinase, the first kinase in the cascade, is the rate-limiting step in the activation5. NF-κB kinase then activates the IKK1-IKK1 homodimeric complex that in turn phosphorylates and partially degrades the NF-κB2 (p100-RelB) complex into active NF-κB2 (p52-RelB).

Mechanistically, NF-κB contributes to the pathogenesis of autoimmune diseases such as EAE by acting as a potent proinflammatory regulator in cells of the immune system9. Here, van Loo and colleagues address the importanc e

kept tightly in check by multiple repressive mechanisms that also facilitate the domi-nance of TH1 or TH2 cells.

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of the canonical pathway in the development of EAE3. They show that astrocytes, of neuro-nal-ectodermal origin, are the key cell type for the modulation of brain inflammation. They used the Nes-Cre system to block the expres-sion of either the catalytic subunits IKK1 and IKK2 or the regulatory NEMO subunit in astrocytes. Only in IKK2- and NEMO-knockout mice, but not in IKK1-knockout mice, was there a substantial reduction in the severity of paralytic disease and in the extent of inflammation in the brain. The key cell mediating this effect was, unexpectedly, the astrocyte rather than microglial cells, perivas-cular dendritic cells, macrophages or T cells. Each of those other cell types has been shown

to be important in controlling inflammatory destruction in EAE10.

The activation of NF-κB depends on many post-translational modifications in key mol-ecules, which either directly influence the cascade or indirectly modulate the cascade through the regulation of intersecting path-ways. These post-translational modifica-tions include isoprenylation of the GTPases Ras and Rho, whose well described pathways impinge on NF-κB, and the ubiquitination of IKK2, which directly modulates NF-κB (Fig. 1). There is considerable crosstalk between cyclooxygenase-mediated prostaglan-din metabolism and NF-κB. Aspirin blocks IκB phosphorylation11, whereas statins inhibit

the canonical NF-κB pathway, resulting in the attenuation of EAE and other autoimmune diseases1,12. Regulation of NF-κB therefore can be achieved to a certain extent by means of widely used drugs, including statins, aspi-rin and glucocorticoids1,11. The development of drugs targeting the ubiquitin system and the proteasome should increase the reper-toire of classes of therapeutics that influence NF-κB13. The conundrum, however, for Janus-like molecules such as NF-κB is that although they are pathogenic in inflammation, they may be necessary for regeneration and for the termination of autoimmunity through the apoptotic elimination of self-reactive T cells1,5,6. Whether targeting NF-κB directly will ‘tip this balance’ to physiological amounts remains a challenge.

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Chen, Y. J. Immunol. 163, 2937–2943 (1999).10. Platten, M. & Steinman, L. Nat. Med. 11, 252–253

(2005).11. Lee, J.I. & Burckart, G. J. Clin. Pharmacol. 38, 981–993

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Figure 1 Potential NF-κB signaling pathways in the central nervous system. Several types of stimuli can activate the canonical NF-κB pathway (NF-κB1 (p50-p65); left) or the alternative NF-κB pathway (NF-κB2 (p52-RelB); right). These pathways are active in neurons and in glial cells such as astrocytes, which reside in the central nervous system. These pathways can modulate survival and death signals. Small-molecule inhibitors such as aspirin, the proteosome inhibitor bortezomib and statins can indirectly inhibit these cascades by interrupting pathways that intersect with the NF-κB cascades. Dashed arrows indicate pathways yet to be demonstrated in the central nervous system. P inside a red star indicates phosphorylation. TNF, tumor necrosis factor; IL-1R, interleukin 1 receptor; LT, lymphotoxin; LPS, lipopolysaccharide; CD40L, CD40 ligand; NMDA, N-methyl-D-aspartate; TNFR, tumor necrosis factor receptor; TLR, Toll-like receptor; LTR, lymphotoxin receptor; MEKK3, mitogen-activated protein kinase kinase kinase 3; TAK1,transforming growth factor-β–activated kinase 1; Rac, GTP-binding Rho family protein; Erk, extracellular signal–regulated kinase; Ub, polyubiquitination site; COX-2, cyclooxygenase 2; iNOS, inducible nitric oxide synthase; MHC, major histocompatibility complex; BCL, B cell lymphoma; IFN-β, interferon-β.

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