Sphingolipid Signaling

National Institute Of Mental Health

Investigators

Linked publications, trials & patents

Abstract

Sphingosine-1-phosphate (S1P) is a potent lipid mediator that regulates many vital biological processes, including cell growth, death, and differentiation. S1P has been shown to play important roles in normal and patho-physiological processes, including cancer, asthma, allergic responses, hearing, and development of the cardiovascular and nervous systems. In a continuing and highly successful collaboration with Dr. Sarah Spiegel at Virginia Commonwealth University School of Medicine, we are elucidating the mechanisms by which S1P is produced by two sphingosine kinases (SphK1 and SphK2), how its levels are regulated, and how it mediates such diverse actions. S1P is a ligand for five specific G protein-coupled receptors (named S1Psub(1-5)) that regulate many vital cellular processes and account for the pleiotropic effects of S1P. In fact, no cell in the body has been found that does not express as least one S1P receptor. Although they were long considered to be merely structural components of membranes, in the last decades it has become apparent that sphingolipids have other important functions. More recently, S1P and its precursors, sphingosine and ceramide, have been implicated in the regulation of many aspects of neuronal proliferation, differentiation, survival and apoptosis.[unreadable] Sphingolipids, such as S1P, are highly enriched in the CNS and recent data has substantiated their importance in development and functions of the brain. We recently reviewed current knowledge of regulation of SphK1 and SphK2 on both transcriptional and post-translational levels and the functions of these isozymes and their product S1P and its receptors in the central nervous system. In this regard, we recently examined the role of S1P in glioblastoma cells. Patients with gliomas expressing high levels of epidermal growth factor receptor (EGFR) and plasminogen activator inhibitor-1 (PAI-1) have a shorter overall survival prognosis. Although multiple signaling cascades are activated by EGF in glioma cells, we showed for the first time that EGF enhances expression of PAI-1 via sequential activation of c-Src, PKCdelta, and SphK1, the enzyme that produces S1P, which have all been implicated in regulating motility and invasion of glioma cells.[unreadable] We previously found that SphK1 was important for cell growth and survival and SphK2 seemed to inhibit proliferation and promote cell death. SphK1 and SphK2 have different cellular localizations and have opposing roles in the regulation of sphingolipid metabolism suggesting that the location of S1P production in the cell dictates its functions. We have now identified filamin A (FLNa), an actin crosslinking protein involved in cell movement, as a SphK1-interacting protein and showed that FLNa links SphK1 and the S1P1 receptor to locally influence the dynamics of actin cytoskeletal structures by orchestrating the concerted actions of the triumvirate of SphK1, FLNa, and PAK1, at lamellipodia to promote cell movement.[unreadable] Lysophosphatidic acid (LPA), like S1P, mediates similar diverse cellular processes important for cancer progression. We recently found that LPA markedly enhanced SphK1 expression and S1P formation in gastric cancer cells and in other human cancer cells that express the LPA1 receptor. Our results suggest that SphK1 is a convergence point of multiple cell surface receptors for three different ligands, LPA, EGF, and S1P, which have all been implicated in regulation of motility and invasiveness of cancer cells.[unreadable] We have been developing specific SphK1 inhibitors as potential therapeutic agenst. A sphingosine analogue we called SK1-I decreased growth and survival of human leukemia cell lines and primary leukemia cells from patients, while sparing normal peripheral blood mononuclear cells. Importantly, SK1-I markedly reduced growth of xenograft tumors. Our results suggest that specific inhibitors of SphK1 warrant attention as potential additions to the therapeutic armamentarium in leukemia.[unreadable] We also made the unexpected finding that treatment of leukemia cells with a non-specific SphK inhibitor caused a large increase in expression of SphK1 concomitant with induction of apoptosis. The chemotherapeutic drug doxorubicin, a potent inducer of apoptosis in these cells, also stimulated SphK1 expression and activity and promoted S1P secretion. We found that secreted S1P potently stimulated chemotaxis of leukemia cells and primary monocytes and macrophages and suggested that apoptotic cells may up-regulate SphK1 to produce and secrete S1P (inside-out signaling) that serves as a "come-and-get-me" signal for scavenger cells to engulf them in order to prevent necrosis.[unreadable] In two invited reviews, we discussed how inside-out signaling of S1P could be a therapeutic target and wju targeting SphK1 could become a new strategy against cancer.[unreadable] Mast cells are tissue-dwelling pivotal early effectors of allergic responses, which produce and secrete S1P, that acts in an autocrine or paracrine manner to influence many facets of the immune system. The relevance of mast cells in the etiology of allergic disorders, asthma and anaphylaxis is well established. In contrast to its weak effect on degranulation of murine mast cells, we found that S1P potently induced degranulation of a human mast-cell line and of cord blood-derived human mast cells (hMCs). S1P also stimulated production and secretion of cytokines, TNF-alpha and IL-6, and markedly enhanced secretion of a chemokine, CCL2/MCP-1, important modulators of inflammation. Production of S1P by SphK1 but not SphK2 was critical for IgE/Ag-induced degranulation, migration toward antigen, and CCL2 secretion from hMCs, although both isoenzymes were required for TNF-alpha secretion. In sum, our data suggest that differential formation of S1P by SphK1 and SphK2 has distinct and important actions in hMCs.