**2. S1P biosynthesis, metabolism and secretion**

All cells are able to generate S1P during the normal physiologic metabolism of sphingolipids (Hla, 2004). Generally, systemic and local concentrations of S1P are tightly regulated by the balance between its synthesis and degradation via three enzyme families, including SphKs (SphK1 and SphK2), SPPs (SPP1 and SPP2), and SPL (Fig. 1). SphKs generate S1P through phosphorylation of its precursor, sphingosine, while SPPs reversibly convert S1P back to sphingosine by removing the phosphate (Xia et al., 2000). Irreversible degradation of S1P is carried out by a single enzyme SPL, which cleaves S1P into ethanolamine phosphate and hexadecenal, representing the last step in the sphingolipid degradation pathway (Xia et al., 2000).

Fig. 1. S1P metabolism pathway. S1P homeostasis is tightly regulated by the balance between its synthesis and degradation via three enzyme families: (1) sphingosine kinases (SphK1 and SphK2), which generate S1P through phosphorylation of its precursor, sphingosine, (2) S1P phosphatases (SPP1 and SPP2), which reversibly convert S1P back to sphingosine, and (3) S1P lyase (SPL), which irreversibly degrades S1P to generate ethanolamine phosphate and hexadecenal.

SphK is represented by two different isoforms (SphK1 and SphK2). Although both SphK1 and SphK2 can phosphorylate sphingosine, SphK1 produces the majority of S1P in cells (Kohama et al., 1998). SphKs can be activated by a large variety of agonists, including growth factors (such as platelet-derived growth factor, vascular endothelial growth factor, epidermal growth factor and hepatocyte growth factor), cytokines (such as TNF-), steroid hormones (such as estradiol), and GPCR ligands (such as lysophosphatidic acid (LPA) and S1P) (reviewed in (Alvarez et al., 2007; Spiegel and Milstien, 2003; Taha et al., 2006)). Following cellular activation by these agonists, SphK1 can be activated through: 1)

RA. We also discuss the potential therapeutic benefit of modulators of S1P metabolism and S1P receptors, including SphK/SPL inhibitors, FTY-720, S1P receptor antagonists, and anti-

All cells are able to generate S1P during the normal physiologic metabolism of sphingolipids (Hla, 2004). Generally, systemic and local concentrations of S1P are tightly regulated by the balance between its synthesis and degradation via three enzyme families, including SphKs (SphK1 and SphK2), SPPs (SPP1 and SPP2), and SPL (Fig. 1). SphKs generate S1P through phosphorylation of its precursor, sphingosine, while SPPs reversibly convert S1P back to sphingosine by removing the phosphate (Xia et al., 2000). Irreversible degradation of S1P is carried out by a single enzyme SPL, which cleaves S1P into ethanolamine phosphate and hexadecenal, representing the last step in the

Fig. 1. S1P metabolism pathway. S1P homeostasis is tightly regulated by the balance between its synthesis and degradation via three enzyme families: (1) sphingosine kinases (SphK1 and SphK2), which generate S1P through phosphorylation of its precursor, sphingosine, (2) S1P phosphatases (SPP1 and SPP2), which reversibly convert S1P back to

sphingosine, and (3) S1P lyase (SPL), which irreversibly degrades S1P to generate

SphK is represented by two different isoforms (SphK1 and SphK2). Although both SphK1 and SphK2 can phosphorylate sphingosine, SphK1 produces the majority of S1P in cells (Kohama et al., 1998). SphKs can be activated by a large variety of agonists, including growth factors (such as platelet-derived growth factor, vascular endothelial growth factor, epidermal growth factor and hepatocyte growth factor), cytokines (such as TNF-), steroid hormones (such as estradiol), and GPCR ligands (such as lysophosphatidic acid (LPA) and S1P) (reviewed in (Alvarez et al., 2007; Spiegel and Milstien, 2003; Taha et al., 2006)). Following cellular activation by these agonists, SphK1 can be activated through: 1)

S1P monoclonal antibodies, in the treatment of RA.

**2. S1P biosynthesis, metabolism and secretion** 

sphingolipid degradation pathway (Xia et al., 2000).

ethanolamine phosphate and hexadecenal.

phosphorylation and translocation to the plasma membrane, 2) interaction with acidic phospholipids such as phosphatidic acid produced by phospholipase D (Delon et al., 2004), and 3) possible association with other proteins. All or parts of these mechanisms may be required for full activation of SphK1 (Takabe et al., 2008). Phosphorylation of SphK1 leads to its translocation to the plasma membrane where its substrate sphingosine is located, resulting in the production of S1P (Johnson et al., 2002; Pitson et al., 2005). S1P, in turn, activates specific S1P receptors present on the surface of the same cell or on nearby cells in autocrine and/or paracrine manners (Alvarez et al., 2007). The "inside-out" export of intracellularly generated S1P is crucial for many S1P functions (Takabe et al., 2008).

The mechanism by which S1P is exported from the inside to the outside of cells after synthesis is not fully understood. Several lines of evidence suggest the involvement of the ATP-binding cassette (ABC) family of transporters in S1P secretion (Kobayashi et al., 2006; Mitra et al., 2006; Sato et al., 2007). Mitra et al. (2006) revealed that the release of S1P from mast cells is regulated by ABCC1. Similarly, the ABCA1 transporter is critical for the release of S1P from astrocytes (Sato et al., 2007). In breast cancer the export of S1P mediated by ABCC1 and ABCG2 transporters is stimulated by estrogen receptor- (Takabe et al., 2010). Altogether, these studies suggest that members of the family of ABC transporters may be important for export of S1P out of cells.
