Zhi-Wei Li

NF-κB in cancer: from innocent bystander to major culprit

Nuclear factor of κB (NF-κB) is a sequence-specific transcription factor that is known to be involved in the inflammatory and innate immune responses. Although the importance of NF-κB in immunity is undisputed, recent evidence indicates that NF-κB and the signalling pathways that are involved in its activation are also important for tumour development. NF-κB should therefore receive as much attention from cancer researchers as it has already from immunologists.

IKKβ Links Inflammation and Tumorigenesis in a Mouse Model of Colitis-Associated Cancer

A link between inflammation and cancer has long been suspected, but its molecular nature remained ill defined. A key player in inflammation is transcription factor NF-kappaB whose activity is triggered in response to infectious agents and proinflammatory cytokines via the IkappaB kinase (IKK) complex. Using a colitis-associated cancer model, we show that although deletion of IKKbeta in intestinal epithelial cells does not decrease inflammation, it leads to a dramatic decrease in tumor incidence without affecting tumor size. This is linked to increased epithelial apoptosis during tumor promotion. Deleting IKKbeta in myeloid cells, however, results in a significant decrease in tumor size. This deletion diminishes expression of proinflammatory cytokines that may serve as tumor growth factors, without affecting apoptosis. Thus, specific inactivation of the IKK/NF-kappaB pathway in two different cell types can attenuate formation of inflammation-associated tumors. In addition to suppressing apoptosis in advanced tumors, IKKbeta may link inflammation to cancer.

IKK-beta links inflammation to obesity-induced insulin resistance.

Inflammation may underlie the metabolic disorders of insulin resistance and type 2 diabetes. IκB kinase β (IKK-β, encoded by Ikbkb) is a central coordinator of inflammatory responses through activation of NF-κB. To understand the role of IKK-β in insulin resistance, we used mice lacking this enzyme in hepatocytes (IkbkbΔhep) or myeloid cells (IkbkbΔmye). IkbkbΔhep mice retain liver insulin responsiveness, but develop insulin resistance in muscle and fat in response to high fat diet, obesity or aging. In contrast, IkbkbΔmye mice retain global insulin sensitivity and are protected from insulin resistance. Thus, IKK-β acts locally in liver and systemically in myeloid cells, where NF-κB activation induces inflammatory mediators that cause insulin resistance. These findings demonstrate the importance of liver cell IKK-β in hepatic insulin resistance and the central role of myeloid cells in development of systemic insulin resistance. We suggest that inhibition of IKK-β, especially in myeloid cells, may be used to treat insulin resistance.

The Lymphotoxin-β Receptor Induces Different Patterns of Gene Expression via Two NF-κB Pathways

The lymphotoxin-beta receptor (LTbetaR) plays critical roles in inflammation and lymphoid organogenesis through activation of NF-kappaB. In addition to activation of the classical NF-kappaB, ligation of this receptor induces the processing of the cytosolic NF-kappaB2/p100 precursor to yield the mature p52 subunit, followed by translocation of p52 to the nucleus. This activation of NF-kappaB2 requires NIK and IKKalpha, while NEMO/IKKgamma is dispensable for p100 processing. IKKbeta-dependent activation of canonical NF-kappaB is required for the expression but not processing of p100 and for the expression of proinflammatory molecules including VCAM-1, MIP-1beta, and MIP-2 in response to LTbetaR ligation. In contrast, IKKalpha controls the induction by LTbetaR ligation of chemokines and cytokines involved in lymphoid organogenesis, including SLC, BLC, ELC, SDF1, and BAFF.

Inhibition of JNK activation through NF-κB target genes

The proinflammatory cytokine tumour necrosis factor-α (TNF-α) regulates immune responses, inflammation and programmed cell death (apoptosis). The ultimate fate of a cell exposed to TNF-α is determined by signal integration between its different effectors, including IκB kinase (IKK), c-Jun N-terminal protein kinase (JNK) and caspases. Activation of caspases is required for apoptotic cell death, whereas IKK activation inhibits apoptosis through the transcription factor NF-κB, whose target genes include caspase inhibitors. JNK activates the transcription factor c-Jun/AP-1, as well as other targets. However, the role of JNK activation in apoptosis induced by TNF-α is less clear. It is unknown whether any crosstalk occurs between IKK and JNK, and, if so, how it affects TNF-α-induced apoptosis. We investigated this using murine embryonic fibroblasts that are deficient in either the IKKβ catalytic subunit of the IKK complex or the RelA/p65 subunit of NF-κB. Here we show that in addition to inhibiting caspases, the IKK/NF-κB pathway negatively modulates TNF-α-mediated JNK activation, partly through NF-κB-induced X-chromosome-linked inhibitor of apoptosis (XIAP). This negative crosstalk, which is specific to TNF-α signalling and does not affect JNK activation by interleukin-1 (IL-1), contributes to inhibition of apoptosis.

Prevention of fat-induced insulin resistance by salicylate

Insulin resistance is a major factor in the pathogenesis of type 2 diabetes and may involve fat-induced activation of a serine kinase cascade involving IKK-beta. To test this hypothesis, we first examined insulin action and signaling in awake rats during hyperinsulinemic-euglycemic clamps after a lipid infusion with or without pretreatment with salicylate, a known inhibitor of IKK-beta. Whole-body glucose uptake and metabolism were estimated using [3-(3)H]glucose infusion, and glucose uptake in individual tissues was estimated using [1-(14)C]2-deoxyglucose injection during the clamp. Here we show that lipid infusion decreased insulin-stimulated glucose uptake and activation of IRS-1-associated PI 3-kinase in skeletal muscle but that salicylate pretreatment prevented these lipid-induced effects. To examine the mechanism of salicylate action, we studied the effects of lipid infusion on insulin action and signaling during the clamp in awake mice lacking IKK-beta. Unlike the response in wild-type mice, IKK-beta knockout mice did not exhibit altered skeletal muscle insulin signaling and action following lipid infusion. In summary, high-dose salicylate and inactivation of IKK-beta prevent fat-induced insulin resistance in skeletal muscle by blocking fat-induced defects in insulin signaling and action and represent a potentially novel class of therapeutic agents for type 2 diabetes.

The two faces of IKK and NF-kappaB inhibition: prevention of systemic inflammation but increased local injury following intestinal ischemia-reperfusion.

We studied the role of NF-κB in acute inflammation caused by gut ischemia-reperfusion through selective ablation of IκB kinase (IKK)-β, the catalytic subunit of IKK that is essential for NF-κB activation. Ablation of IKK-β in enterocytes prevented the systemic inflammatory response, which culminates in multiple organ dysfunction syndrome (MODS) that is normally triggered by gut ischemia-reperfusion. IKK-β removal from enterocytes, however, also resulted in severe apoptotic damage to the reperfused intestinal mucosa. These results show the dual function of the NF-κB system, which is responsible for both tissue protection and systemic inflammation, and underscore the caution that should be exerted in using NF-κB and IKK inhibitors.

JNK2 and IKKβ Are Required for Activating the Innate Response to Viral Infection

Viral infection or double-stranded (ds) RNA induce interferons (IFN) and other cytokines. Transcription factors mediating IFN induction are known, but the signaling pathways that regulate them are less clear. We now describe two such pathways. The first pathway leading to NF-kappaB depends on the dsRNA-responsive protein kinase (PKR), which in turn activates IKB kinase (IKK) through the IKKbeta subunit. The second viral-and dsRNA-responsive pathway is PKR independent and involves Jun kinase (JNK) activation leading to stimulation of AP-1. Both IKKbeta and JNK2 are essential for efficient induction of type I IFN and other cytokines in response to viral infection or dsRNA. This study establishes a general role for these kinases in activation of innate immune responses.

Interplay of IKK/NF-κB signaling in macrophages and myofibers promotes muscle degeneration in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal X-linked disorder associated with dystrophin deficiency that results in chronic inflammation and severe skeletal muscle degeneration. In DMD mouse models and patients, we find that IkappaB kinase/NF-kappaB (IKK/NF-kappaB) signaling is persistently elevated in immune cells and regenerative muscle fibers. Ablation of 1 allele of the p65 subunit of NF-kappaB was sufficient to improve pathology in mdx mice, a model of DMD. In addition, conditional deletion of IKKbeta in mdx mice elucidated that NF-kappaB functions in activated macrophages to promote inflammation and muscle necrosis and in skeletal muscle fibers to limit regeneration through the inhibition of muscle progenitor cells. Furthermore, specific pharmacological inhibition of IKK resulted in improved pathology and muscle function in mdx mice. Collectively, these results underscore the critical role of NF-kappaB in the progression of muscular dystrophy and suggest the IKK/NF-kappaB signaling pathway as a potential therapeutic target for DMD.

IKKβ Is Required for Prevention of Apoptosis Mediated by Cell-Bound but Not by Circulating TNFα

Abstract IκB kinase β (IKKβ) is required for NF-κB activation and suppression of TNFα-mediated liver apoptosis. To investigate how IKKβ suppresses apoptosis, we generated hepatocyte-specific Ikkβ knockout mice, Ikkβ Δhep , which exhibit little residual NF- κB activity but are healthy with normal liver function. Unexpectedly, Ikkβ Δhep mice are slightly more sensitive than controls to LPS-induced liver apoptosis but are highly susceptible to liver destruction following concanavalin A (ConA)-induced T cell activation. Unlike LPS, a potent inducer of circulating TNFα, ConA exerts cytotoxic effects through cell-bound TNFα, which activates type 1 and 2 TNF receptors (TNFR). While TNFR2 does not contribute to NF-κB activation, it is important for ConA-induced JNK activation, which is augmented by the absence of IKKβ. Using JNK-deficient mice we show that JNK is required for ConA-induced liver damage. Thus, the antiapoptotic function of IKKβ, which is most critical in situations that involve cell-bound TNFα, is mediated partially through attenuation of JNK activity.