Signaling Mechanisms of Anti-Proliferation by Sphingosine 1-Phoshate(S1P) on Acute Promyelocytic Leukemia(APL) Cell Lines.

There is abundant evidence that S1P can function as a second messenger important for regulation of calcium homeostasis, cell growth, and suppression of apoptosis. In many cases, the intracellular level of S1P and ceramide, another important sphingolipid metabolite associated with cell death and cell growth arrest, coordinately determine cell fate. N, N-dimethylsphingosine(DMS) potentiates TNF- and FasL- induced apoptosis in the human acute leukemia jurkat, U937 and HL-60 cell line, which suggests that combining DMS with cytotoxic drugs might be a useful chemotherapeutic approach. In this study, we investigated the role of S1P on proliferation, survival and its signaling mechanisms on the NB4 cell, the human acute promyelocytic leukemia cell line, which has chromosomal abnormality, t(15:17). NB4 cells were exposed to S1P at various concentrations (1, 5, 10 μM) for 48 hours, and then the cell growth was determined by MTT assay. We studied whether NB4 cells have S1P receptors or not. To investigate the signaling mechanisms of S1P on the NB4 cell, we used MAP kinase inhibitors and a PI3K inhibitor including PD98059, U0126 (specific ERK pathway inhibitor), SB203580 (specific p38 pathway inhibitor), and LY294002 (specific PI3K pathway inhibitor). We also examined the effect of pertussis toxin (PTX), a Gi protein inhibitor, on the proliferation of NB4 cells after S1P treatment.

RT-PCR analysis revealed a putative S1P receptor, Edg-3 mRNA expressed in NB4 cells. In the MTT assay, S1P inhibited cell proliferation in a dose-dependent manner. At 10 μM, NB4 cell proliferation was most inhibited. In addition, we found that the inhibitory effect of S1P on the NB4 cell proliferation was inhibited by PD98059, U0126, and PTX. However, SB203580 and LY294002 had not caused an inhibitory effect of S1P on the NB4 cell proliferation.

In contrast to previous knowledge that S1P increases the survival of leukemic cells because apoptosis is prevented by caspase inactivation, our data suggests that S1P inhibits NB4 cell proliferation, through ERK pathway activation, not p38 nor JNK pathways. In addition, the pertussis toxin(PTX)-sensitive GTP-binding protein-coupled receptor, Edg-3, seems to be involved in the antiproliferative signaling of S1P to the NB4 cell.

Understanding the signaling mechanisms of anti-proliferation by S1P can uncover new therapeutic targets for APL and the results of this study warrant further investigation on synergism of S1P with known therapeutic agent, ATRA and Arsenic trioxide on APL cells.