Sphingosine-1-phosphate, FTY720, and sphingosine-1-phosphate receptors in the pathobiology of acute lung injury

Abstract

Acute lung injury (ALI) attributable to sepsis or mechanical ventilation and subacute lung injury because of ionizing radiation (RILI) share profound increases in vascular permeability as a key element and a common pathway driving increased morbidity and mortality. Unfortunately, despite advances in the understanding of lung pathophysiology, specific therapies do not yet exist for the treatment of ALI or RILI, or for the alleviation of unremitting pulmonary leakage, which serves as a defining feature of the illness. A critical need exists for new mechanistic insights that can lead to novel strategies, biomarkers, and therapies to reduce lung injury. Sphingosine 1-phosphate (S1P) is a naturally occurring bioactive sphingolipid that acts extracellularly via its G protein-coupled S1P1-5 as well as intracellularly on various targets. S1P-mediated cellular responses are regulated by the synthesis of S1P, catalyzed by sphingosine kinases 1 and 2, and by the degradation of S1P mediated by lipid phosphate phosphatases, S1P phosphatases, and S1P lyase. We and others have demonstrated that S1P is a potent angiogenic factor that enhances lung endothelial cell integrity and an inhibitor of vascular permeability and alveolar flooding in preclinical animal models of ALI. In addition to S1P, S1P analogues such as 2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol (FTY720), FTY720 phosphate, and FTY720 phosphonates offer therapeutic potential in murine models of lung injury. This translational review summarizes the roles of S1P, S1P analogues, S1P-metabolizing enzymes, and S1P receptors in the pathophysiology of lung injury, with particular emphasis on the development of potential novel biomarkers and S1P-based therapies for ALI and RILI.

Figure 1.

Metabolism of sphingolipids in mammalian cells. Key enzymatic steps in the biosynthesis and degradation of sphingoid bases and recycling of trans–2-hexadecenal and ethanolamine phosphate from sphingolipids into glycerophospholipids are summarized. SPT, serine palmitoyltransferase; SMase, sphingomyelinase; S1P, sphingosine 1–phosphate; SphK, sphingosine kinase; SPP, S1P phosphatases; LPP, lipid phosphate phosphatase; CoA, coenzyme A; CDP, cytidine 5'-diphosphate.

Figure 2.

Intracellular generation and catabolism of S1P. The ceramide generated by the agonist-dependent hydrolysis of sphingomyelin (SM) by SMase is converted to sphingosine by ceramidases. Sphingosine is phosphorylated by SphK1 and/or SphK2 to S1P, which is transported outside the cell or catabolized to trans–2-hexadecenal and ethanolamine phosphate by S1P lyase. SMase, sphingomyelinase; S1P, sphingosine 1–phosphate.

Figure 3.

Regulation of endothelial barrier function by S1P. S1P binding to G protein–coupled S1P1 activates Rac1 and induces a series of signaling cascades, including cytoskeletal reorganization, the assembly of adherens junction and tight junction proteins, and the formation of focal adhesions that act together to enhance endothelial barrier function. However, cleavage of the protease-activated receptor (PAR-1) by thrombin induces actin stress fiber formation, and disrupts the assembly of adherens junctions and tight junction and focal adhesion proteins, resulting in barrier disruption. LIM, LIM kinase; PAK, p21-activated kinase–1; ZO, zona occludins; JAM, junctional adhesion molecule; PXN, paxillin; GIT, G protein–coupled receptor kinase–interacting protein; FAK, focal adhesion kinase; MLCK, myosin light chain kinase; cat, catenin; Src, Rous sarcoma oncogene cellular honolog; Rac1, Ras related C3 botulinum toxin 1.

Figure 4.

Structures of 2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol (FTY720) and FTY720 analogues. The chemical structures of FTY720, the (R) and (S) stereoisomers of FTY720 phosphate, FTY720 phosphonate, and FTY720 vinylphosphonate are illustrated.

Figure 5.

Comparative signaling pathways involved in endothelial cell barrier enhancement by FTY720 and FTY720-phosphate (P). Both FTY720 (left panel) and FTY720-P (right panel) potently increase endothelial cell (EC) barrier function in vitro when concentrations of less than 10 μM are used (higher concentrations or prolonged stimulation over hours to days may disrupt barrier function), but multiple aspects differ in the signaling pathways involved. FTY720-P rapidly induces a series of events similar to the actions of S1P to enhance barrier function, including S1P₁ ligation, Gi-coupled signaling, lipid raft membrane platforms, increased intracellular Ca²⁺, Rac1 activation, and dynamic actin changes, producing increased cortical actin linked to adherens junction and focal adhesion complex formation and stabilization. However, FTY720-P also induces the ubiquitination and subsequent proteosomal degradation of barrier-promoting S1P₁, eventually leading to increased permeability after prolonged exposure. EC barrier enhancement by FTY720 is slower in onset and may involve an alternative, but not yet identified, G protein–coupled receptor (GPCR) in addition to S1P₁. Similar to FTY720-P, FTY720-induced barrier enhancement requires Gi and lipid raft–coupled signaling. However, no significant Ca²⁺ increase is observed in pulmonary ECs, nor does dramatic cytoskeletal rearrangement or cortical actin formation occur during the timeframe associated with maximal barrier effects. Tyrosine kinase activity is involved, and recent work indicates that c-Abl and FAK signaling are necessary for optimal barrier enhancement. Focal adhesion complexes also appear to participate in this process after FTY720. EC, endothelial cell; Ub, ubiquitination; PXN, paxillin; FAK, focal adhesion kinase; GIT, G protein–coupled receptor kinase interactor–1; Tyr, tyrosine; c-Abl, Abelson tyrosine kinase.

Figure 6.

Potential role of S1P receptors in cellular and biological processes. Extracellular S1P signals via G protein–coupled S1P receptors, and regulates a number of cellular and biological processes such as barrier integrity, barrier disruption, inflammation, migration, and angiogenesis in mammalian cells. S1PR, S1P receptor.

Figure 7.

Comparative signaling pathways involved in endothelial barrier integrity and dysfunction by S1P via S1P1 and S1P3. S1P enhances endothelial barrier functions that include S1P1 ligation, Gi-coupled signaling, lipid raft membrane platforms, increased intracellular Ca²⁺, Rac1 activation, Tiam 1, PAK1 and PI3K recruitment to lipid rafts, and dynamic actin changes, producing increased cortical actin, which is linked to adherens junction and focal adhesion complex formation and stabilization. However, the ligation of S1P to S1P₃ enhances RhoGEF recruitment to lipid rafts, and Rho activation leads to cytoskeletal reorganization, decreased cortical actin, and barrier dysfunction. S1P, sphingosine 1–phosphate; Tiam 1, T-cell lymphoma invasion and metastasis factor–1; PAK, p21–activated kinase; GEF, guanine nucleotide exchange factor; MYPT1, myosin phosphatase–targeting subunit; PI3K, phosphatidylinositol 3–kinase.