Epigenetic Signals: The lipid sphingosine-1-phosphate (S1P) is a signaling molecule that binds to receptors on the cell surface to initiate biochemical changes that control a range of biological processes from growth and survival to immune reactions. Hait et al. (p. 1254 ) report that S1P can also function by direct binding to the nuclear enzymes, histone deacetylases (HDACs) 1 and 2. The enzyme that generates S1P, sphingosine kinase 2 (ShpK2) is present in the nucleus in complexes with HDAC1 and HDAC2. Generation of S1P and its binding to HDACs inhibited deacetylation of histone. Such histone modification is an epigenetic mechanism that controls gene transcription. Thus, generation of S1P in the nucleus appears to be a signaling mechanism by which cells can control gene expression in response to various stimuli.

TRAF2 (tumour-necrosis factor receptor-associated factor 2), a component of the NF-κB activation pathway important in inflammatory, anti-apoptotic, and immune processes, is shown to be a target of sphingosine kinase 1, one of the isoenzymes that generates the pro-survival lipid mediator sphingosine-1-phosphate inside cells. In addition, sphingosine kinase 1 is revealed as the 'missing' cofactor for TRAF2 E3 ubiquitin ligase activity. These findings provide a mechanistic explanation for the numerous observations of the importance of SphK1 in inflammatory, anti-apoptotic and immune processes. Engagement of the tumour-necrosis factor (TNF) receptor results in the assembly of multi-component signalling complexes by adaptor proteins that include TNF receptor-associated factor 2 (TRAF2). Genetic evidence indicates that TRAF2 is needed for the polyubiquitination of receptor interacting protein 1 (RIP1), but direct evidence has been lacking. Here it is shown that the lipid sphingosine-1-phosphate is a co-factor needed for this ubiquitination activity of TRAF2. Tumour-necrosis factor (TNF) receptor-associated factor 2 (TRAF2) is a key component in NF-κB signalling triggered by TNF-α1,2. Genetic evidence indicates that TRAF2 is necessary for the polyubiquitination of receptor interacting protein 1 (RIP1)3 that then serves as a platform for recruitment and stimulation of IκB kinase, leading to activation of the transcription factor NF-κB. Although TRAF2 is a RING domain ubiquitin ligase, direct evidence that TRAF2 catalyses the ubiquitination of RIP1 is lacking. TRAF2 binds to sphingosine kinase 1 (SphK1)4, one of the isoenzymes that generates the pro-survival lipid mediator sphingosine-1-phosphate (S1P) inside cells. Here we show that SphK1 and the production of S1P is necessary for lysine-63-linked polyubiquitination of RIP1, phosphorylation of IκB kinase and IκBα, and IκBα degradation, leading to NF-κB activation. These responses were mediated by intracellular S1P independently of its cell surface G-protein-coupled receptors. S1P specifically binds to TRAF2 at the amino-terminal RING domain and stimulates its E3 ligase activity. S1P, but not dihydro-S1P, markedly increased recombinant TRAF2-catalysed lysine-63-linked, but not lysine-48-linked, polyubiquitination of RIP1 in vitro in the presence of the ubiquitin conjugating enzymes (E2) UbcH13 or UbcH5a. Our data show that TRAF2 is a novel intracellular target of S1P, and that S1P is the missing cofactor for TRAF2 E3 ubiquitin ligase activity, indicating a new paradigm for the regulation of lysine-63-linked polyubiquitination. These results also highlight the key role of SphK1 and its product S1P in TNF-α signalling and the canonical NF-κB activation pathway important in inflammatory, antiapoptotic and immune processes.

Signal transducer and activator of transcription‐3 (STAT‐3) is one of six members of a family of transcription factors. It was discovered almost 15 years ago as an acute‐phase response factor. This factor has now been associated with inflammation, cellular transformation, survival, proliferation, invasion, angiogenesis, and metastasis of cancer. Various types of carcinogens, radiation, viruses, growth factors, oncogenes, and inflammatory cytokines have been found to activate STAT‐3. STAT‐3 is constitutively active in most tumor cells but not in normal cells. Phosphorylation of STAT‐3 at tyrosine 705 leads to its dimerization, nuclear translocation, DNA binding, and gene transcription. The phosphorylation of STAT‐3 at serine 727 may regulate its activity negatively or positively. STAT‐3 regulates the expression of genes that mediate survival (survivin, bcl‐xl, mcl‐1, cellular FLICE‐like inhibitory protein), proliferation (c‐fos, c‐myc, cyclin D1), invasion (matrix metalloproteinase‐2), and angiogenesis (vascular endothelial growth factor). STAT‐3 activation has also been associated with both chemoresistance and radioresistance. STAT‐3 mediates these effects through its collaboration with various other transcription factors, including nuclear factor‐κB, hypoxia‐inducible factor‐1, and peroxisome proliferator activated receptor‐γ. Because of its critical role in tumorigenesis, inhibitors of this factor's activation are being sought for both prevention and therapy of cancer. This has led to identification of small peptides, oligonucleotides, and small molecules as potential STAT‐3 inhibitors. Several of these small molecules are chemopreventive agents derived from plants. This review discusses the intimate relationship between STAT‐3, inflammation, and cancer in more detail.