Specification of DNA binding activity of NF-kappaB proteins

Disruption of IkappaB kinase (IKK)-mediated RelA serine 536 phosphorylation sensitizes human multiple myeloma cells to histone deacetylase (HDAC) inhibitors

Post-translational modifications of RelA play an important role in regulation of NF-κB activation. We previously demonstrated that in malignant hematopoietic cells, histone deacetylase inhibitors (HDACIs) induced RelA hyperacetylation and NF-κB activation, attenuating lethality. We now present evidence that IκB kinase (IKK) β-mediated RelA Ser-536 phosphorylation plays a significant functional role in promoting RelA acetylation, inducing NF-κB activation, and limiting HDACI lethality in human multiple myeloma (MM) cells. Immunoblot profiling revealed that although basal RelA phosphorylation varied in MM cells, Ser-536 phosphorylation correlated with IKK activity. Exposure to the pan-HDACIs vorinostat or LBH-589 induced phosphorylation of IKKα/β (Ser-180/Ser-181) and RelA (Ser-536) in MM cells, including cells expressing an IκBα "super-repressor," accompanied by increased RelA nuclear translocation, acetylation, DNA binding, and transactivation activity. These events were substantially blocked by either pan-IKK or IKKβ-selective inhibitors, resulting in marked apoptosis. Consistent with these events, inhibitory peptides targeting either the NF-κB essential modulator (NEMO) binding domain for IKK complex formation or RelA phosphorylation sites also significantly increased HDACI lethality. Moreover, IKKβ knockdown by shRNA prevented Ser-536 phosphorylation and significantly enhanced HDACI susceptibility. Finally, introduction of a nonphosphorylatable RelA mutant S536A, which failed to undergo acetylation in response to HDACIs, impaired NF-κB activation and increased cell death. These findings indicate that HDACIs induce Ser-536 phosphorylation of the NF-κB subunit RelA through an IKKβ-dependent mechanism, an action that is functionally involved in activation of the cytoprotective NF-κB signaling cascade primarily through facilitation of RelA acetylation rather than nuclear translocation.

Genomic analysis reveals a novel nuclear factor-κB (NF-κB)-binding site in Alu-repetitive elements

The transcription factor NF-κB is a critical regulator of immune responses. To determine how NF-κB builds transcriptional control networks, we need to obtain a topographic map of the factor bound to the genome and correlate it with global gene expression. We used a ChIP cloning technique and identified novel NF-κB target genes in response to virus infection. We discovered that most of the NF-κB-bound genomic sites deviate from the consensus and are located away from conventional promoter regions. Remarkably, we identified a novel abundant NF-κB-binding site residing in specialized Alu-repetitive elements having the potential for long range transcription regulation, thus suggesting that in addition to its known role, NF-κB has a primate-specific function and a role in human evolution. By combining these data with global gene expression profiling of virus-infected cells, we found that most of the sites bound by NF-κB in the human genome do not correlate with changes in gene expression of the nearby genes and they do not appear to function in the context of synthetic promoters. These results demonstrate that repetitive elements interspersed in the human genome function as common target sites for transcription factors and may play an important role in expanding the repertoire of binding sites to engage new genes into regulatory networks.

Midazolam suppresses interleukin-1β-induced interleukin-6 release from rat glial cells

Background

Peripheral-type benzodiazepine receptor (PBR) expression levels are low in normal human brain, but their levels increase in inflammation, brain injury, neurodegenerative states and gliomas. It has been reported that PBR functions as an immunomodulator. The mechanisms of action of midazolam, a benzodiazepine, in the immune system in the CNS remain to be fully elucidated. We previously reported that interleukin (IL)-1β stimulates IL-6 synthesis from rat C6 glioma cells and that IL-1β induces phosphorylation of inhibitory kappa B (IκB), p38 mitogen-activated protein (MAP) kinase, stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase 1/2, and signal transducer and activator of transcription (STAT)3. It has been shown that p38 MAP kinase is involved in IL-1β-induced IL-6 release from these cells. In the present study, we investigated the effect of midazolam on IL-1β-induced IL-6 release from C6 cells, and the mechanisms of this effect.

Methods

Cultured C6 cells were stimulated by IL-1β. IL-6 release from C6 cells was measured using an enzyme-linked immunosorbent assay, and phosphorylation of IκB, the MAP kinase superfamily, and STAT3 was analyzed by Western blotting.

Results

Midazolam, but not propofol, inhibited IL-1β-stimulated IL-6 release from C6 cells. The IL-1β-stimulated levels of IL-6 were suppressed by wedelolactone (an inhibitor of IκB kinase), SP600125 (an inhibitor of SAPK/JNK), and JAK inhibitor I (an inhibitor of JAK 1, 2 and 3). However, IL-6 levels were not affected by PD98059 (an inhibitor of MEK1/2). Midazolam markedly suppressed IL-1β-stimulated STAT3 phosphorylation without affecting the phosphorylation of p38 MAP kinase, SAPK/JNK or IκB.

Conclusion

These results strongly suggest that midazolam inhibits IL-1β-induced IL-6 release in rat C6 glioma cells via suppression of STAT3 activation. Midazolam may affect immune system function in the CNS.

IKKβ phosphorylation regulates RPS3 nuclear translocation and NF-κB function during infection with Escherichia coli strain O157:H7

NF-κB is a major gene regulator in immune responses and ribosomal protein S3 (RPS3) is an NF-κB subunit that directs specific gene transcription. However, it is unknown how RPS3 nuclear translocation is regulated. Here we report that IKKβ phosphorylation of serine 209 (S209) was crucial for RPS3 nuclear localization in response to activating stimuli. Moreover, the foodborne pathogen Escherichia coli O157:H7 virulence protein NleH1 specifically inhibited RPS3 S209 phosphorylation and blocked RPS3 function, thereby promoting bacterial colonization and diarrhea but decreasing mortality in a gnotobiotic piglet infection model. Thus, the IKKβ-dependent modification of a specific amino acid in RPS3 promotes specific NF-κB functions that underlie the molecular pathogenetic mechanisms of E. coli O157:H7.

C/EBPα and C/EBPα oncoproteins regulate nfkb1 and displace histone deacetylases from NF-κB p50 homodimers to induce NF-κB target genes

Mutated CEBPA defines a subgroup of acute myeloid leukemia (AML). We have previously shown that C/EBPα or its AML mutants synergize with NF-κB p50 to activate antiapoptotic genes, including BCL2 and FLIP. Furthermore, p50 binds and activates the CEBPA gene in myeloid cells. We now report that C/EBPα or C/EBPα leucine zipper AML mutants bind in vivo to the nfkb1 (p50) promoter and induce its expression even in the presence of cycloheximide. Induction of p50 by C/EBPα depends on 2 conserved κB sites in the nfkb1 promoter. C/EBPα did not induce p65 expression. Thus, C/EBPα and p50 reciprocally regulate each other's expression, establishing a positive feedback relationship. Although p50 homodimers inhibit transcription, C/EBPα and p50 synergistically activate antiapoptotic genes. ChIP analysis showed that C/EBPα diminishes the occupation of histone deacetylase 1 (HDAC1) or HDAC3 on the endogenous FLIP promoter but not in mice lacking p50. Coimmunoprecipitation confirmed that C/EBPα, its AML variants, or C/EBPβ disrupt interaction between p50 and HDACs dependent on the C/EBP basic region. These findings suggest that C/EBPs displace HDACs from p50 homodimers bound to antiapoptotic genes, contributing to NF-κB dysregulation in leukemia, and that the C/EBPα:p50 complex is a potential therapeutic target.

NF-kappaB in renal inflammation

The NF-kappaB family of transcription factors regulates the induction and resolution of inflammation. Two main pathways, classical and alternative, control the nuclear translocation of NF-kappaB. Classical NF-kappaB activation is usually a rapid and transient response to a wide range of stimuli whose main effector is RelA/p50. The alternative NF-kappaB pathway is a more delayed response to a smaller range of stimuli resulting in DNA binding of RelB/p52 complexes. Additional complexity in this system involves the posttranslational modification of NF-kappaB proteins and an ever-increasing range of co-activators, co-repressors, and NF-kappaB complex proteins. Collectively, NF-kappaB regulates the expression of numerous genes that play a key role in the inflammatory response during human and experimental kidney injury. Multiple stimuli activate NF-kappaB through the classical pathway in somatic renal cells, and noncanonical pathway activation by TWEAK occurs in acute kidney injury. Under most test conditions, specific NF-kappaB inhibitors tend to reduce inflammation in experimental kidney injury but not always. Although many drugs in current use clinically influence NF-kappaB activation, there are no data regarding specific NF-kappaB inhibition in human kidney disease.

The nuclear signaling of NF-kappaB: current knowledge, new insights, and future perspectives

The nuclear factor-kappa B (NF-kappaB) transcription factor plays a critical role in diverse cellular processes associated with proliferation, cell death, development, as well as innate and adaptive immune responses. NF-kappaB is normally sequestered in the cytoplasm by a family of inhibitory proteins known as inhibitors of NF-kappaB (IkappaBs). The signal pathways leading to the liberation and nuclear accumulation of NF-kappaB, which can be activated by a wide variety of stimuli, have been extensively studied in the past two decades. After gaining access to the nucleus, NF-kappaB must be actively regulated to execute its fundamental function as a transcription factor. Recent studies have highlighted the importance of nuclear signaling in the regulation of NF-kappaB transcriptional activity. A non-Rel subunit of NF-kappaB, ribosomal protein S3 (RPS3), and numerous other nuclear regulators of NF-kappaB, including Akirin, Nurr1, SIRT6, and others, have recently been identified, unveiling novel and exciting layers of regulatory specificity for NF-kappaB in the nucleus. Further insights into the nuclear events that govern NF-kappaB function will deepen our understanding of the elegant control of its transcriptional activity and better inform the potential rational design of therapeutics for NF-kappaB-associated diseases.