Therapeutic Potential Of Targeting Sphingosine-1-Phosuphate and Sphingosine Kinases In Multiple Myeloma

Multiple myeloma (MM) is one of the common hematological malignancies and is a uniformly fatal disorder of B cells characterized by accumulation of abnormal plasma cells in the bone marrow.Clinical progression of patients with MM is improved with the proteasome inhibitor (PI) (e.g. bortezomib) and the immunomodulatory drugs (IMiDs) such as thalidomide and lenalidomide. Although PI and IMiDs have considerably changed the treatment paradigm of MM, many patients show disease relapse due to developing into drug resistance of MM cells. Since the prognosis remains poor for patients with refractory disease, the new therapeutic strategies are required to treat against these patients. Sphingosine-1-phosphate (S1P) is a potent bioactive sphingolipid. Two isoforms of sphingosine kinases (SphKs), SK1 and SK2, catalyze the formation of the S1P in mammalian cells. SphKs have also been shown to be up-regulated in the variety of cancer types. SphKs/S1P/S1P receptor (S1PR) axis is involved in multiple biological processes. It has been reported that S1P is involved in cell proliferation, angiogenesis and inflammation. S1P is also involved in cancer progression including cell transformation, oncogenesis and cell survival in hematological malignancies such as multiple myeloma. Therefore, S1P and SphKs may present attractive targets for MM treatment. One of the S1P analog, fingolimod (FTY720), which is an orally active immunomodulatory drug, is developed for the treatment of multiple sclerosis. SKI-I, which is a non-lipid pan-SphK inhibitor and ABC294640, selective inhibitor of SK2, are currently investigated in a pivotal phase 1 clinical trial against solid tumors. In this study, we investigated the efficacy of fingolimod, SKI-I, and ABC294640 by using the MM cell lines, RPMI8226, MM1.S and MM1.R. 72 hours treatment of fingolimod exhibited cell growth inhibition of MM cell lines in a dose dependent manner. Treatment of SKI-I and ABC294640 also exhibited cell growth inhibition in a dose dependent manner. Since S1P is the ligand for a family of five G-protein-coupled receptors with distinct signaling pathways that regulate angiogenesis and chemotaxis, we next evaluated the chemotactic response of human umbilical vein endothelial cells (HUVEC). We found that 4 hours treatment of S1P significantly induced the migration of HUVECs compared to control medium. Treatment of HUVECs with fingolimod inhibited S1P-stimulated chemotaxis in a dose dependent manner. We also found that S1P-induced chemotaxis was abolished by the SKI-I and ABC294640. These results suggest that intracellular SK1 and SK2 may play the important role in S1P induced chemotaxis of HUVEC. We next investigated the S1P concentrations in MM patient by enzyme-linked immune sorbent assay (ELISA), because S1P is a potent tumorigenic growth factor that is likely released from tumor cells. We found that serum concentrations of S1P were significantly higher in patient with MM compared with normal samples. The average S1P levels of MM and normal control are 1503.431 and 1103.38 (p <0.05). We also found that conditioned medium from MM cell line had chemotactic activity for HUVECs. These results implicate that S1P may be a novel biomarker for early stage of MM and that S1P is an important bioactive sphingolipid involved in angiogenesis. In this study, we also demonstrate that fingolimod, SKI-I and ABC294640 have potent preclinical anti-tumor activity in MM. These agents possibly inhibit angiogenesis with relation to MM cell growth and offer unique opportunities for novel therapeutic strategies for the treatment of multiple myeloma.