The transcription factor NF-κB and human disease

Beginning with its discovery in 1986 and continuing through the present, the transcription factor NF-kB has attracted widespread interest based on its unusual regulation, the variety of stimuli that activate it, the diverse genes and biological responses that it controls, the striking evolutionary conservation of structure and function among family members, and its apparent involvement in a variety of human diseases (Table 1). Importantly, and consistent with the last point, NF-kB has been shown to be the target of several anti-inflammatory and anticancer drugs

Control of oncogenesis and cancer therapy resistance by the transcription factor NF-κB

The abilities of NF-κB to promote cell proliferation, suppress apoptosis, promote cell migration, and suppress differentiation apparently have been co-opted by cellular and viral oncoproteins to promote oncogenesis (Figure 2). Direct evidence, using both in vitro and in vivo models, indicates that NF-κB is required for oncogenesis, probably at multiple levels. NF-κB likely plays an important role in the early events of oncogenesis, possibly functioning primarily in protecting against transformation-associated apoptosis. In most late-stage tumor cells, classic NF-κB (the p50-p65 heterodimer) is clearly not the only survival factor, because its inhibition does not induce apoptosis in many of these tumor cells. This observation suggests that other events have occurred to upregulate NF-κB-independent cell survival pathways. However, clearly some cancer cells depend on NF-κB for their survival. NF-κB also can contribute to cell progression by transcriptionally upregulating cyclin D1 with corresponding hyperphosphorylation of the tumor suppressor protein Rb. The induction of NF-κB-controlled proliferation may correlate with loss of differentiation in certain settings (47), which may promote oncogenesis. NF-κB is known to regulate certain genes associated with metastasis, such as matrix metalloproteinase 9, tissue plasminogen activator, and ICAM-1. Thus, a more relevant role for NF-κB in later-stage oncogenesis may be to promote metastasis and angiogenesis. Although many tumor cells display some level of constitutive nuclear NF-κB, higher levels of NF-κB and the transcriptional potential of NF-κB can be further enhanced in response to certain types of chemotherapy. Consistent with this, inhibition of NF-κB in parallel with certain (but apparently not all) chemotherapy treatments strongly enhances the apoptotic potential of the chemotherapy. This observation indicates that NF-κB plays an important role in inducible chemoresistance and establishes NF-κB inhibition as a new adjuvant approach in chemotherapy

The NF-κB Pathway and Cancer Stem Cells

The NF-κB transcription factor pathway is a crucial regulator of inflammation and immune responses. Additionally, aberrant NF-κB signaling has been identified in many types of cancer. Downstream of key oncogenic pathways, such as RAS, BCR-ABL, and Her2, NF-κB regulates transcription of target genes that promote cell survival and proliferation, inhibit apoptosis, and mediate invasion and metastasis. The cancer stem cell model posits that a subset of tumor cells (cancer stem cells) drive tumor initiation, exhibit resistance to treatment, and promote recurrence and metastasis. This review examines the evidence for a role for NF-κB signaling in cancer stem cell biology

Positive and negative regulation of NF-κB by COX-2. Roles of different prostaglandins

The prostaglandin H synthases (PGHS) catalyze the conversion of arachidonic acid to prostaglandin H2, the committed step in prostanoid synthesis. Two forms of PGHS exist, PGHS-1 (COX-1) and PGHS-2 (COX-2). The gene encoding the latter form is known to be inducible by a number of stimuli including several inflammatory mediators. Recent evidence indicates that the inducible cyclooxygenase may have both pro- and anti-inflammatory properties through the generation of different prostaglandins. Previous reports indicate that the transcription factor NF-κB can function upstream of COX-2 to control transcription of this gene and that the cyclopentenone prostaglandins can inhibit NF-κB activation via the inhibition of the IκB kinase. Thus, it is suggested that cyclopentenones feed back to inhibit continued nuclear accumulation of NF-κB. In this report we demonstrate COX-2 expression inhibits nuclear translocation of NF-κB, and we confirm that the cyclopentenone prostaglandins inhibit NF-κB. In addition, we show that prostaglandin E2 and its analogs promote the inherent transcriptional activity of the p65/RelA subunit of NF-κB in a manner independent of induced nuclear accumulation. Consistent with this evidence, prostaglandin E2 strongly synergizes with the inflammatory cytokine tumor necrosis factor-α to promote NF-κB-dependent transcription and gene expression. The data provide a molecular rationale to explain both the pro-and anti-inflammatory nature of COX-2

IKK/Nuclear Factor-kappaB and Oncogenesis: Roles in tumor-initiating cells and in the tumor microenvironment

The IKK/nuclear factor-kappaB pathway (NF-κB) is critical in proper immune function, cell survival, apoptosis, cellular proliferation, synaptic plasticity, and even memory. While NF-κB is crucial for both innate and adaptive immunity, defective regulation of this master transcriptional regulator is seen in a variety of diseases including autoimmune disease, neurodegenerative disease, and, important to this review, cancer. While NF-κB functions in cancer to promote a number of critical oncogenic functions, here we discuss the importance of the NF-κB signaling pathway in contributing to cancer through promotion of the tumor microenvironment and through maintenance/expansion of tumor-initiating cells, processes that appear to be functionally interrelated

IKK promotes cytokine-induced and cancer-associated AMPK activity and attenuates phenformin-induced cell death in LKB1-deficient cells

The 5′ AMP-activated protein kinase (AMPK) is an energy sensor that is activated upon phosphorylation of Thr172 in its activation loop by the kinase LKB1, CAMKK2, or TAK1. TAK1-dependent AMPK phosphorylation of Thr172 is less well characterized than phosphorylation of this site by LKB1 or CAMKK2. An important target of TAK1 is IB kinase (IKK), which controls the activation of the transcription factor NF-B. We tested the hypothesis that IKK acted downstream of TAK1 to activate AMPK by phosphorylating Thr172. IKK was required for the phosphorylation of Thr172 in AMPK in response to treatment with the inflammatory cytokine IL-1 or TNF- or upon TAK1 overexpression. In addition, IKK regulated basal AMPK Thr172 phosphorylation in several cancer cell types independently of TAK1, indicating that other modes of IKK activation could stimulate AMPK. We found that IKK directly phosphorylated AMPK at Thr172 independently of the tumor suppressor LKB1 or energy stress. Accordingly, in LKB1-deficient cells, IKK inhibition reduced AMPK Thr172 phosphorylation in response to the mitochondrial inhibitor phenformin. This response led to enhanced apoptosis and suggests that IKK inhibition in combination with phenformin could be used clinically to treat patients with LKB1-deficient cancers

Apoptosis promotes a caspase-induced amino-terminal truncation of IκBα that functions as a stable inhibitor of NF-kB

Caspases are cell death cysteine proteases that are activated upon the induction of the apoptotic program and cleave target proteins in a sequence- specific manner to promote cell death. Recently, Barkett et al. (Barkett, M., Xue, D., Horvitz, H. R., and Gilmore, T. D. (1997) J. BioL Chem. 272, 29419- 29422) have shown that IκBα, the inhibitory subunit of the transcription factor NF-κB, can be cleaved by caspase-3 in vitro at a site that potentially produces a dominant inhibitory form of IκBα. The involvement of NF-κB in the inhibition of cell death led us to ask whether apoptotic stimuli would induce the caspase-mediated cleavage of IκBα in vivo. In this study, we show that apoptosis leads to the caspase-mediated amino-terminal truncation of IκBα (δN-IκBα). Our data show that δN-IκBα can bind NF- κB, suppress NF-κB activation, and sensitize cells to death. Since activated NF-κB plays a role in the inhibition of cell death, these data suggest that caspase-mediated cleavage of IκBα may be a mechanism to suppress NF-κB and its associated antiapoptotic activity

Activation of nuclear factor-κB-dependent transcription by tumor necrosis factor-α is mediated through phosphorylation of RelA/p65 on serine 529

Nuclear factor-κB (NF-κB) is an essential transcription factor in the control of expression of genes involved in immune and inflammatory responses. In unstimulated cells, NF-κB complexes are sequestered in the cytoplasm through interactions with IκBα and other IκB proteins. Extracellular stimuli that activate NF-κB, such as tumor necrosis factor α (TNFα), cause rapid phosphorylation of IκBα at serines 32 and 36. The inducible phosphorylation of IκBα is followed by its ubiquitination and degradation, allowing NF-κB complexes to translocate into the nucleus and to activate gene expression. Previously, it has been shown that TNFα as well as other stimuli also lead to the phosphorylation of the RelA/p65 subunit of NF-κB. In this report, we demonstrate that the TNFα-induced phosphorylation of the RelA/p65 subunit occurs on serine 529, which is in the C-terminal (TA1) transactivation domain. Accordingly, the TNFα-induced phosphorylation of Rel/p65 increases NF-κB transcriptional activity but does not affect nuclear translocation or DNA binding affinity

Selective Effects of Thioridazine on Self-Renewal of Basal-Like Breast Cancer Cells

Several recent publications demonstrated that DRD2-targeting antipsychotics such as thioridazine induce proliferation arrest and apoptosis in diverse cancer cell types including those derived from brain, lung, colon, and breast. While most studies show that 10–20 µM thioridazine leads to reduced proliferation or increased apoptosis, here we show that lower doses of thioridazine (1–2 µM) target the self-renewal of basal-like breast cancer cells, but not breast cancer cells of other subtypes. We also show that all breast cancer cell lines tested express DRD2 mRNA and protein, regardless of thioridazine sensitivity. Further, DRD2 stimulation with quinpirole, a DRD2 agonist, promotes self-renewal, even in cell lines in which thioridazine does not inhibit self-renewal. This suggests that DRD2 is capable of promoting self-renewal in these cell lines, but that it is not active. Further, we show that dopamine can be detected in human and mouse breast tumor samples. This observation suggests that dopamine receptors may be activated in breast cancers, and is the first time to our knowledge that dopamine has been directly detected in human breast tumors, which could inform future investigation into DRD2 as a therapeutic target for breast cancer

Expanding the View of IKK: New Substrates and New Biology

The inhibitor of kappa B kinase (IKK) family consists of IKKα, IKKβ, and the IKK-related kinases TBK1 and IKKε. These kinases are considered master regulators of inflammation and innate immunity via their control of the transcription factors NF-κB, IRF3, and IRF7. Novel phosphorylated substrates have been attributed to these kinases, a subset of which is not directly related to either inflammation or innate immunity. These findings have greatly expanded the perspectives on the biological activities of these kinases. In this review we highlight some of the novel substrates for this kinase family and discuss the biological implications of these phosphorylation events