Distinct roles of sphingosine kinases 1 and 2 in human mast-cell functions

Abstract

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...

Sphingosine Kinase1 Is Pivotal for FcεRI-Mediated Mast Cell Signaling and Functional Responses In Vitro and In Vivo

“…Recently, it has been suggested that FcRI, activates both SphK1 and SphK2 to drive different responses in mouse and human mast cells (15,16). However, this is in contradiction with previous reports showing that SphK1 is activated by FcRI, and mediates proinflammatory responses, in rat, mouse, and human mast cells (11,13).…”

“…In contrast, a recent study, using mast cells-derived from SphK1 or SphK2 knockout (KO) mice, suggests that SphK2 is the isoform used by FcRI to mediate degranulation and cytokine and eicosanoids production (15). In contrast, a more recent study using small interfering RNA (siRNA) to silence the SPHK genes in human mast cells, shows that SphK1, but not SphK2, plays a critical role in Ag-induced degranulation migration and cytokine and eicosanoids production, with SphK2 playing a lesser role and only for the partial secretion of TNF-␣ (16). Thus, given the differences between silencing SPHK genes in wild-type cells and those cells where the kinases were deleted in the preimplantation embryo, it is possible that the differences observed are due to a compensatory mechanism during embryonic development of the KO mice and that SphK1 may indeed also be required for mouse mast cell degranulation and for the release of other proinflammatory mediators from murine mast cells in vitro and in vivo.…”

Bioactive Sphingolipids in Response to Chemotherapy: A Scope on Leukemias

“…Furthermore, S1P appears to be a promising target for the treatment of rheumatoid arthritis (as reviewed in [119]) and asthma [120]. Interestingly, in a study of murine collagen-induced arthritis model, two isoenzymes responsible for the production of S1P (sphingosine kinase 1 and sphingosine kinase 2) were shown to have distinct cellular functions [121].This surprising finding was also confirmed by other groups in different models [122][123][124]. In one of those studies, SphK1 and SphK2 were shown to have opposing functions for the regulation of ceramide biosynthesis [124].…”

Subcellular distribution of FTY720 and FTY720-phosphate in immune cells – another aspect of Fingolimod action relevant for therapeutic application

“…It is known from the literature that a deficiency of both sphingosine kinases, leads to embryonic lethality. However, no obvious phenotype is known in single knockout mice, indicating that apart from specific exceptions (Oskeritzian et al, 2008;Price et al, 2013) one enzyme may substitute the other (Alemany et al, 2007). The increased concentration of FTY720-P in the cytosol of the SphK1-deficient cells may thus be the result of a compensatory increase of SphK2 activity.…”