SUMMARY Inflammatory bowel disease is an important risk factor for colorectal cancer. We show that sphingosine-1-phosphate (S1P) produced by upregulation of sphingosine kinase 1 (SphK1) links chronic intestinal inflammation to colitis-associated cancer (CAC) and both are exacerbated by deletion of Sphk2. S1P is essential for production of the multifunctional NF-κB-regulated cytokine IL-6, persistent activation of the transcription factor STAT3, and consequent upregulation of the S1P receptor, S1PR1. The pro-drug FTY720 decreased SphK1 and S1PR1 expression and eliminated the NF-κB/IL-6/STAT3 amplification cascade and development of CAC even in Sphk2−/− mice and may be useful in treating colon cancer in individuals with ulcerative colitis. Thus, the SphK1/S1P/S1PR1 axis is at the nexus between NF-κB and STAT3 and connects chronic inflammation and CAC.

The potent lipid mediator sphingosine-1-phosphate (S1P) regulates diverse physiological processes by binding to 5 specific GPCRs, although it also has intracellular targets. Here, we demonstrate that S1P, produced in the mitochondria mainly by sphingosine kinase 2 (SphK2), binds with high affinity and specificity to prohibitin 2 (PHB2), a highly conserved protein that regulates mitochondrial assembly and function. In contrast, S1P did not bind to the closely related protein PHB1, which forms large, multimeric complexes with PHB2. In mitochondria from SphK2-null mice, a new aberrant band of cytochrome-c oxidase was detected by blue native PAGE, and interaction between subunit IV of cytochrome-c oxidase and PHB2 was greatly reduced. Moreover, depletion of SphK2 or PHB2 led to a dysfunction in mitochondrial respiration through cytochrome-c oxidase. Our data point to a new action of S1P in mitochondria and suggest that interaction of S1P with homomeric PHB2 is important for cytochrome-c oxidase assembly and mitochondrial respiration.

Sphingosine-1-phosphate (S1P) is now emerging as a potent lipid mediator produced by mast cells that contributes to inflammatory and allergic responses. In contrast to its weak effect on degranulation of murine mast cells, S1P potently induced degranulation of the human LAD2 mast-cell line and cord blood-derived human mast cells (hMCs). S1P also stimulated production and secretion of cytokines, TNF-␣ and IL-6, and markedly enhanced secretion of a chemokine, CCL2/MCP-1, important modulators of inflammation. S1P is produced in mast cells by the 2 sphingosine kinases, SphK1 and SphK2. SphK1 but not SphK2 plays a critical role in IgE/Ag-induced degranulation, migration toward antigen, and CCL2 secretion from hMCs, as determined by specifically down-regulating their expression. However, both isoenzymes were required for efficient TNF-␣ secretion. Taken together, our data suggest that differential formation of S1P by SphK1 and SphK2 has distinct and important actions in hMCs. IntroductionMast cells (MCs) play pivotal roles in immediate-type and inflammatory allergic reactions initiated by cross-linking with antigen (Ag) of the antigen-specific immunoglobulin E (IgE) on their cell-surface high-affinity receptors for IgE (Fc⑀RI). MC activation leads to release of preformed mediators, such as histamine, localized in specialized granules, and the de novo synthesis and secretion of a plethora of cytokines, chemokines, eicosanoids, and the bioactive sphingolipid metabolite, sphingosine-1-phosphate (S1P). The combined actions of these mediators trigger symptoms associated with allergy and propagate inflammatory responses. S1P levels are elevated in the bronchoalveolar lavage (BAL) fluid of asthmatics after Ag challenge, suggesting that secretion of S1P by MCs is of relevance in inflammatory responses and asthma. 1 S1P is a ligand for 5 G protein-coupled receptors (GPCRs), designated S1P 1-5 , through which it exerts many of its actions. 2,3 Indeed, S1P is secreted by activated MCs, 4,5 and studies with rodent MCs have shown that this S1P is able to rapidly bind and activate its receptors, S1P 1 and S1P 2 , in an autocrine manner. S1P 1 induces cytoskeletal rearrangements, leading to the movement of MCs toward an Ag gradient, whereas activation of S1P 2 is critical for degranulation. 4 In rodent MCs and human bone marrow-derived mast cells (BMMCs), aggregation of Fc⑀RI leads to activation of both isoforms of sphingosine kinase, SphK1 and SphK2, the enzymes that produce S1P from sphingosine. 4,6-8 Down-regulation of SphK1 but not SphK2 in these MCs inhibits calcium mobilization, degranulation, and migration induced by Ag. 4,6,7 In human BMMCs and murine MCs, SphK1 translocates to the plasma membrane within minutes of Fc⑀RI clustering. 4,6 SphK1 interacts with the protein tyrosine kinases, Src kinase members Lyn and Fyn (Fc⑀RI proximal kinases that initiate the signaling events following cross-linking of this receptor), but not Src or other tyrosine kinases, such as Syk. Following interaction with Lyn, SphK1 is recruited to me...

Systemic exacerbation of allergic responses, in which mast cells play a critical role, results in life-threatening anaphylactic shock. Sphingosine-1–phosphate (S1P), a ligand for a family of G protein–coupled receptors, is a new addition to the repertoire of bioactive lipids secreted by activated mast cells. Yet little is known of its role in human mast cell functions and in anaphylaxis. We show that S1P2 receptors play a critical role in regulating human mast cell functions, including degranulation and cytokine and chemokine release. Immunoglobulin E–triggered anaphylactic responses, including elevation of circulating histamine and associated pulmonary edema in mice, were significantly attenuated by the S1P2 antagonist JTE-013 and in S1P2-deficient mice, in contrast to anaphylaxis induced by administration of histamine or platelet-activating factor. Hence, S1P and S1P2 on mast cells are determinants of systemic anaphylaxis and associated pulmonary edema and might be beneficial targets for anaphylaxis attenuation and prophylaxis.

Although interleukin-1 (IL-1) induces expression of interferon regulatory factor 1 (IRF1), its roles in immune and inflammatory responses and mechanisms of activation remain elusive. Here, we show that IRF1 is essential for IL-1-induced expression of chemokines CXCL10 and CCL5 that recruit mononuclear cells into sites of sterile inflammation. Newly synthesized IRF1 acquires K63-linked polyubiquitylation mediated by cellular inhibitor of apoptosis 2 (cIAP2), which is enhanced by the bioactive lipid sphingosine-1 phosphate (S1P). In response to IL-1, cIAP2 and sphingosine kinase 1, the enzyme that generates S1P, form a complex with IRF1, which leads to its activation. Thus, IL-1 triggers a hitherto unknown signaling cascade that controls induction of IRF1-dependent genes important for sterile inflammation.