Tumor necrosis factor alpha mediates a T cell receptor-independent induction of the gene regulatory factor NF-kappa B in T lymphocytes

Targeting tumour necrosis factor-α in hypoxia and synaptic signalling

Tumour necrosis factor (TNF)-α is a pro-inflammatory cytokine, which is synthesised and released in the brain by astrocytes, microglia and neurons in response to numerous internal and external stimuli. It is involved in many physiological and pathophysiological processes such as gene transcription, cell proliferation, apoptosis, synaptic signalling and neuroprotection. The complex actions of TNF-α in the brain are under intense investigation. TNF-α has the ability to induce selective necrosis of some cells whilst sparing others and this has led researchers to discover multiple activated signalling cascades. In many human diseases including acute stroke and inflammation and those involving hypoxia, levels of TNF-α are increased throughout different brain regions. TNF-α signalling may also have several positive and negative effects on neuronal function including glutamatergic synaptic transmission and plasticity. Exogenous TNF-α may also exacerbate the neuronal response to hypoxia. This review will summarise the actions of TNF-α in the central nervous system on synaptic signalling and its effects during hypoxia.

Innate immune responses in NF-kappaB-repressing factor-deficient mice

NF-kappaB-repressing factor (NRF) is a transcriptional silencer protein that specifically counteracts the basal activity of several NF-kappaB-dependent promoters by direct binding to specific neighboring DNA sequences. In cell culture experiments, the reduction of NRF mRNA leads to a derepression of beta interferon, interleukin-8, and inducible nitric oxide synthase transcription. The X chromosome-located single-copy NRF gene is ubiquitously expressed and encodes a protein of 690 amino acids. The N-terminal part contains a nuclear localization signal, the DNA-binding domain, and the NF-kappaB-repressing domain, while the C-terminal part is responsible for double-stranded RNA binding and nucleolar localization. To study the function of NRF in a systemic context, transgenic mice lacking the NRF gene were created. Against predictions from in vitro experiments, mice with a deletion of the NRF gene are viable and have a phenotype that is indistinguishable from wild-type mice, even after challenge with different pathogens. The data hint towards an unexpected functional redundancy of NRF.

Involvement of regulatory and catalytic subunits of phosphoinositide 3-kinase in NF-kappaB activation

Hypoxia, reoxygenation, and the tyrosine phosphatase inhibitor pervanadate activate the transcription factor NF-kappaB, involving phosphorylation of its inhibitor IkappaB-alpha on tyrosine 42. This modification does not lead to degradation of IkappaB by the proteasome/ubiquitin pathway, as is seen on stimulation of cells with proinflammatory cytokines. It is currently unknown how tyrosine-phosphorylated IkappaB is removed from NF-kappaB. Here we show that p85alpha, the regulatory subunit of PI3-kinase, specifically associates through its Src homology 2 domains with tyrosine-phosphorylated IkappaB-alpha in vitro and in vivo after stimulation of T cells with pervanadate. This association could provide a mechanism by which newly tyrosine-phosphorylated IkappaB is sequestered from NF-kappaB. Another mechanism by which PI3-kinase contributed to NF-kappaB activation in response to pervanadate appeared to involve its catalytic p110 subunit. This was evident from the inhibition of pervanadate-induced NF-kappaB activation and reporter gene induction by treatment of cells with nanomolar amounts of the PI3-kinase inhibitor wortmannin. The compound had virtually no effect on tumor necrosis factor- and interleukin-1-induced NF-kappaB activities. Wortmannin did not inhibit tyrosine phosphorylation of IkappaB-alpha or alter the stability of the PI3-kinase complex but inhibited Akt kinase activation in response to pervanadate. Our data suggest that both the regulatory and the catalytic subunit of PI3-kinase play a role in NF-kappaB activation by the tyrosine phosphorylation-dependent pathway.