Studies on the Functions of Sphingosine Kinase and Sphingosine 1-Phosphate during Inflammation

The lysophospholipid sphingosine 1-phosphate (S1P) has important roles in regulating vascular physiology, vascular development, and immune function. However, the contribution of S1P to inflammation remains relatively unknown. To better understand the role of S1P in inflammation, we conducted models of inflammation using transgenic mice deficient in S1P receptors or S1P generating enzymes. ^ S1P, through Sphk1, is suggested to function as an intracellular second messenger for inflammatory mediators including formyl peptide (fMLP) and complement fragment C5a. We show that Sphk1 knockout mice exhibit normal inflammatory cell recruitment during thioglycollate-induced peritonitis and that sphingosine kinase 1-null neutrophils respond normally to formyl peptide. In the collagen-induced arthritis model of rheumatoid arthritis, sphingosine kinase 1 knockout mice develop arthritis with normal incidence and severity. ^ Despite the physiologic importance of S1P, the source of S1P in the blood stream is unknown. Using bone marrow chimeras and Sphk deficient mice we demonstrate that Sphk expression in both the hematopoietic and non-hematopoietic compartments contribute to plasma S1P levels in mice. In addition, we find that adenoviral gene delivery of Sphk1 to the liver can restore the reduced plasma S1P levels in Sphk1 deficient mice.^ Despite its roles in vascular biology, the potential role of S1P in atherosclerosis remains unknown. Using Apoe-/- and Ldllr-/- models of atherosclerosis we demonstrate that hematopoietic expression of is required for atherosclerotic plaque development. ^ S1P receptors regulate the trafficking of lymphocytes, but S1P\u27s influence on macrophage trafficking remains unknown. Here we demonstrate that S1P 2R-deficient mice have enhanced macrophage recruitment during thioglycollate peritonitis and that S1P2R inhibits macrophage migration in vitro. We also identify the signaling mechanisms used by S1P 2R in macrophage, involving the second messenger cyclic AMP and inhibition of Akt phosphorylation.^ Together, these results establish dispute claims that Sphk1 and S1P function as second messengers during acute inflammation. At the same time, they establish that S1P2R regulates both atherosclerosis and macrophage trafficking.