The first report of SphK signalling in immune cells was in fact based on results derived from mast cells and was published in 1996 2. In this report, the authors demonstrated that FcεRI stimulation rapidly induces SphK activity and raises S1P levels in the rat mast cell line RBL. Moreover, an SphK inhibitor (dihydrosphingosine, DHS) inhibited the Ca2+ response triggered by FcεRI stimulation 2. Later on, it was shown that in human bone marrow-derived mast cells, SphK1 is critical for calcium responses and degranulation triggered by FcεRI 3 – a process that is defined by two distinct waves of Ca2+ responses. The first one (fast and transient) is dependent on SphK1, and the second, slower Ca2+ wave is dependent on phospholipase-Cγ1 and responsible for the influx of external Ca2+. The initial mast cell degranulation in this system is totally and exclusively dependent on SphK1 activity. These findings position SphK1 on par with phospholipase-Cγ, i.e. as an additional major enzyme creating a “second messenger”, making this part of the sphingolipid pathway equivalent to the diacylglycerol and inositol-1,4,5-trisphosphate generation in the glycerolipid pathway. Furthermore, some proinflammatory molecules secreted by mast cells after FcεRI stimulation, including leukotrien C4 and TNF, are completely inhibited by high intracellular levels of sphingosine, suggesting that SphK not only generates a second messenger but “removes” an inhibitory one. Therefore, it was postulated that a (lipid) rheostat composed of sphingosine and S1P, together with SphK, act as a kind of permissive switch and are involved in the fine tuning of allergic susceptibility of mast cells by FcεRI engagement 33. Later studies explained how the intracellular activation of SphK and the extracellular functions of S1P cooperate in the activation of mast cells 34. Activation of SphK, by FcεRI stimulation, leads to S1P secretion, which acts through its receptors S1P1 and S1P2 present on human and rodent mast cells, to amplify the responses observed during FcεRI activation 34. Experimentally, this was demonstrated through inhibition of SphK1, which blocked FcεRI-mediated internalization of S1P1 and S1P2 receptors and reduced degranulation and chemotaxis 34. The authors postulated that the S1P1 receptor is important for cytoskeletal rearrangements and migration of mast cells toward antigen, but dispensable for degranulation. In contrast, the S1P2 receptor appeared to be required for degranulation and inhibited migration toward antigen 34. More recently, it was postulated that both SphK1 and SphK2 are activated by FcεRI in mouse mast cells 5, 35, 36. Of interest, it was recently shown that the SPHK1 gene is one of the first genes to be activated during IgE-sensitization of human mast cells, and that SphK1 mRNA is further increased during FcεRI stimulation 37.
The role of each specific SphK isoform in mediating mast cell responses and signalling via FcεRI stimulation remains controversial. Initially, it was shown that SphK1 is essential for calcium signals and degranulation in human and rat mast cells 3, 4; however, a recent study suggested that SphK1 was not required for degranulation or for cytokine or eicosanoid production in mast cells from SphK1−/− mice 35. In fact, it was SphK2 that was determined to be responsible for in vitro activation of mast cell responses 35. In contrast, another recent study 36 demonstrated that SphK1 but not SphK2 is critical for FcεRI-induced degranulation, migration, and CCL2 secretion. Interestingly, in the same study, both isoenzymes were required for efficient TNF-α secretion, suggesting that SphK1 and SphK2 may have distinct signalling cascades triggered by FcεRI/Ag in mast cells. Consistent with this idea, recent work has shown that in mouse mast cells, FcεRI stimulation primarily activates SphK1, which is downstream of PLD1 and PKCα 38. Moreover, blocking this pathway at various points, including blockade of SphK1, inhibited the FcεRI-triggered initial rise in cytosolic calcium, degranulation, NF-κB activation, and the production of various cytokines and eicosanoids 38. Figure 2 shows a schematic representation of the FcεRI-triggered pathways described in this paragraph.
Figure 2. Blockade of SphK1 inhibits FcεRI-mediated mast cell responses. (A) Normally, FcεRI couples to PKCα, PLD1 and SphK1 to trigger the initial raise in cytosolic calcium responsible for triggering degranulation, NF-κB activation, and cytokine and eicosanoids secretion. (B) Blockade of SphK1 by either siRNA-gene silencing or pharmacological inhibitors, blocked the FcεRI-triggered SphK1-dependent calcium signal, and the key mast cell responses are blocked.
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