The immunomodulator FTY720 is phosphorylated and released from platelets

doi:10.1016/j.ejphar.2007.04.053

European Journal of Pharmacology

Volume 568, Issues 1–3, 30 July 2007, Pages 106-111

https://doi.org/10.1016/j.ejphar.2007.04.053 Get rights and content

Abstract

The novel immunomodulator FTY720 causes lymphocytes from peripheral blood to accumulate in lymphoid tissues. In vivo, FTY720 is phosphorylated to FTY720-P, which binds to the sphingosine 1-phosphate receptor S1P₁. So far, it has been unclear where FTY720-P is produced. We demonstrate that platelets efficiently convert FTY720 to FTY720-P and release it into the extracellular space. This release is mostly independent of platelet activation, but is slightly increased upon thrombin stimulation. These results suggest that platelets are a major source of plasma FTY720-P, and that FTY720-P release is mediated by two different transporters.

Introduction

The sphingolipid metabolite sphingosine 1-phosphate (S1P) is a bioactive lipid molecule that regulates several cellular processes (proliferation, regulation of cell migration, actin cytoskeletal rearrangement, and adherens junction assembly) through binding to its cell surface receptors (Kihara et al., 2007, Sanchez and Hla, 2004). To date, five mammalian S1P receptors (S1P₁/Edg1, S1P₂/Edg5, S1P₃/Edg3, S1P₄/Edg6, S1P₅/Edg8) have been identified.

S1P is abundant in plasma and is physiologically important, especially in the vascular and immune systems and through its binding to S1P₁. The importance of S1P in the immune system has been demonstrated using a novel immunomodulating agent, FTY720 (fingolimod; 2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol). FTY720, an effective agent against transplantation rejection and autoimmune diseases, does not impair lymphocyte activation, proliferation, or effector function, but rather reduces circulating lymphocyte numbers (especially those of mature T cells) (Budde et al., 2006). Treatment causes accumulation of lymphocytes in secondary lymphoid organs such as lymph nodes and Peyer's patches (Chiba et al., 1998). FTY720 is phosphorylated in vivo by sphingosine kinase and binds to all S1P receptors except S1P₂ (Brinkmann et al., 2002, Mandala et al., 2002).

S1P₁ is highly expressed in mature T cells and endothelial cells, and is the most important S1P receptor in the immune system. S1P and the S1P₁ receptor have pivotal functions in the egress of lymphocytes from secondary lymphoid organs and thymus (Allende et al., 2004, Matloubian et al., 2004). Two models have been proposed for the molecular mechanism of FTY720-induced peripheral lymphopenia (Rosen and Goetzl, 2005). In the first, phosphorylated FTY720 (FTY720-P) causes prolonged internalization of S1P₁ on lymphocytes. Since S1P is a potent chemoattractant for lymphocytes, loss of cell surface S1P₁ results in reduced responsiveness to the S1P gradient between lymphoid organs and plasma. In the second model, FTY720-P binds to S1P₁ on endothelial cells of the lymphoid organs and stimulates adherens junction assembly. As a result, lymphocytes become unable to pass through the cell junctions, and accumulate in the lymphoid organs.

Two major blood cells, platelets and erythrocytes, provide plasma S1P (Hänel et al., 2007, Ito et al., 2007, Yatomi et al., 1997a, Yatomi et al., 1997b). Most, but not all, of the S1P release from platelets is dependent on stimuli such as thrombin and protein kinase C activation (Yatomi et al., 2000, Yatomi et al., 1997b), whereas erythrocytes release S1P constitutively (Ito et al., 2007, Yang et al., 1999). In contrast, it has been unclear where FTY720-P is produced. Therefore, we tested the involvement of plasma and related cells (endothelial cells, platelets, and erythrocytes) in the production of FTY720-P. We found that only platelets can produce, store, and release FTY720-P into plasma. This release was largely independent of stimuli, in contrast to S1P release.

Section snippets

Preparation of erythrocytes, platelets, and plasma

Wistar ST rats (7–14 weeks old, male) were anesthetized with diethyl ether and pentobarbital (50 mg/kg rat weight), and whole blood was collected from their hearts. This study was carried out in accordance with the Declaration of Helsinki and/or with the guide for the Committee on the Care and Use of Laboratory Animals of Hokkaido University.

Erythrocytes, platelets, and plasma were prepared as described elsewhere (Ito et al., 2007, Yang et al., 1999). Blood (∼ 10 ml) was collected in 2 ml

Platelets produce and release FTY720-P

We first investigated the ability of two major blood cells, platelets and erythrocytes, to produce and release FTY720-P. After platelets and erythrocytes were incubated with FTY720, cells and medium were separated by centrifugation. FTY720 and FTY720-P from each fraction were measured by HPLC. Almost no FTY720-P was detected in the erythrocyte fraction or its incubation medium (Fig. 1A). In contrast, FTY720-P was detected both in the platelet fraction and the incubation medium. About 60% of the

Discussion

FTY720 is a prodrug, and its conversion to FTY720-P is necessary for its immunomodulating effects. Conversion occurs not only at the time of administration but also constitutively during its circulation, since FTY720 exhibits a cycle of phosphorylation/dephosphorylation (Brinkmann et al., 2002, Kharel et al., 2005). However, it has been unclear where FTY720-P is produced. We have demonstrated here that platelets have the ability to produce FTY720-P and supply it to the plasma.

FTY720-P is

Acknowledgements

We thank Dr. F. Nara (Sankyo Co., Tokyo, Japan) for providing FTY720 and Dr. E.A. Sweeney for scientific editing of the manuscript. A.K. is supported by a Grant-in-Aid for Young Scientists (A) (17687011) from the Ministry of Education, Culture, Sports, Sciences and Technology of Japan.

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The immunomodulator FTY720 is phosphorylated and released from platelets

Abstract

Introduction

Section snippets

Preparation of erythrocytes, platelets, and plasma

Platelets produce and release FTY720-P

Discussion

Acknowledgements

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