Effects of various lysosphingolipids on cell growth, morphology and lipid composition in three neuroblastoma cell lines

Lysosomal Re-acidification Prevents Lysosphingolipid-Induced Lysosomal Impairment and Cellular Toxicity

Neurodegenerative lysosomal storage disorders (LSDs) are severe and untreatable, and mechanisms underlying cellular dysfunction are poorly understood. We found that toxic lipids relevant to three different LSDs disrupt multiple lysosomal and other cellular functions. Unbiased drug discovery revealed several structurally distinct protective compounds, approved for other uses, that prevent lysosomal and cellular toxicities of these lipids. Toxic lipids and protective agents show unexpected convergence on control of lysosomal pH and re-acidification as a critical component of toxicity and protection. In twitcher mice (a model of Krabbe disease [KD]), a central nervous system (CNS)-penetrant protective agent rescued myelin and oligodendrocyte (OL) progenitors, improved motor behavior, and extended lifespan. Our studies reveal shared principles relevant to several LSDs, in which diverse cellular and biochemical disruptions appear to be secondary to disruption of lysosomal pH regulation by specific lipids. These studies also provide novel protective strategies that confer therapeutic benefits in a mouse model of a severe LSD.

Inhibition of autophagy promoted sphingosylphosphorylcholine induced cell death in non-small cell lung cancer cells

Sphingosylphosphorylcholine (SPC) is a bioactive lipid mediated popular cell apoptosis in cancer cells. As a cell-specific sphingolipid, its function in lung cancer cells is unknown. Here we showed that SPC treatment triggered necrosis and autophagy but inhibited apoptosis in two non-small cell lung cancer cell lines: A549 cell line and H157 cell line. Then 3-methyladenine (3-MA), an autophagy inhibitor, was introduced to clarify the relationships between autophagy and necrosis or apoptosis. 3MA suppressed the survival furtherly by promoting apoptosis while had no influence on necrosis. Subsequent studies revealed that activity of AKT and mammalian target of rapamycin (mTOR) complex 1 (mTORC1) were downregulated during autophagy. Furthermore, SPC failed to promote autophagy in p53 deleted cells. Thus SPC induced autophagy in non-small cell lung cancer cells was through AKT/mTORC1 and P53 signal pathway. Besides, SPC reduced both the mitochondria membrane potential and ROS level in A549 cells. These findings provided a molecular basis of SPC-stimulated A549 cell death and support the notion that inhibition of autophagy is likely a novel anticancer mechanism.

Pathology and current treatment of neurodegenerative sphingolipidoses

Sphingolipidoses constitute a large subgroup of lysosomal storage disorders (LSDs). Many of them are associated with a progressive neurodegeneration. As is the case for LSDs in general, most sphingolipidoses are caused by deficiencies in lysosomal hydrolases. However, accumulation of sphingolipids can also result from deficiencies in proteins involved in the transport or posttranslational modification of lysosomal enzymes, transport of lipids, or lysosomal membrane proteins required for transport of lysosomal degradation end products. The accumulation of sphingolipids in the lysosome together with secondary changes in the concentration and localization of other lipids may cause trafficking defects of membrane lipids and proteins, affect calcium homeostasis, induce the unfolded protein response, activate apoptotic cascades, and affect various signal transduction pathways. To what extent, however, these changes contribute to the pathogenesis of the diseases is not fully understood. Currently, there is no cure for sphingolipidoses. Therapies like enzyme replacement, pharmacological chaperone, and substrate reduction therapy, which have been shown to be efficient in non-neuronopathic LSDs, are currently evaluated in clinical trials of neuronopathic sphingolipidoses. In the future, neural stem cell therapy and gene therapy may become an option for these disorders.

Lysophospholipids and ATP mutually suppress maturation and release of IL-1 beta in mouse microglial cells using a Rho-dependent pathway

The P2X7 receptor (P2X7R), an ATP-gated ion channel, plays essential roles in the release and maturation of IL-1beta in microglial cells in the brain. Previously, we found that lysophosphatidylcholine (LPC) potentiated P2X7R-mediated intracellular signals in microglial cells. In this study, we determined whether the lysophospholipids, i.e., LPC and sphingosylphosphorylcholine (SPC), modulate the ATP-induced release and processing of IL-1beta mediated by P2X7R in mouse MG6 microglial cells. LPC or SPC alone induced the release of precursor (pro-IL-1beta) and mature IL-1beta (mIL-1beta) from LPS-primed MG6 cells, possibly due to lytic functions. However, these lysophospholipids inhibited ATP-induced caspase-1 activation that is usually followed by the release of mIL-1beta. Conversely, ATP inhibited the release of pro-IL-1beta and mIL-1beta induced by LPC/SPC. This suggests that lysophospholipids and ATP mutually suppressed each function to release IL-1beta. P2X7R activation resulted in microtubule reorganization in the MG6 cells that was blocked in the presence of LPC and SPC. LPC/SPC reduced the amount of activated RhoA after stimulation with ATP, implying that these lysophospholipids block ATP-induced microtubule reorganization by interfering with RhoA activation. In addition, the microtubule inhibitor colchicine inhibited ATP-induced release of mIL-1beta similar to that of LPC and SPC. This suggests that the impairment of the microtubule reassembly may be associated with the inhibitory effects of LPC/SPC on ATP-induced mIL-1beta release. Mutual suppression by ATP and LPC/SPC on the maturation of IL-1beta was observed in LPS-primed primary microglia. Collectively, these data suggest opposing functions by lysophospholipids, either proinflammatory or anti-inflammatory, in regard to the maturation and release of IL-1beta from microglial cells.

An immunosuppressive effect by synthetic sulfonolipids deduced from sulfonoquinovosyl diacylglycerols of sea urchin

BACKGROUND

It is important to develop new immunosuppressive agents without clinical drawbacks. In this article, we reveal the possibility of a chemically synthetic sulfonolipid that acts as a novel immunosuppressive drug.

METHODS

We evaluated the immunosuppressive effect of 3-O-(6-deoxy-6-sulfono-beta-D-glucopyranosyl)-1,2-di-O-acylglycerol (beta-SQDG) that contains a saturated C18 fatty acid, which is designated as beta-SQDG(18:0) by mixed lymphocyte reaction (MLR) and rat allogeneic skin graft. Then, we investigated the mechanism of immunosuppressive effect of beta-SQDG(18:0).

RESULTS

beta-SQDG(18:0) inhibited human MLR in a dose-dependent manner without overt cytotoxic effect and prolonged rat skin allograft rejection in vivo. beta-SQDG(18:0) did not inhibit the direct activation of responder T. This reagent could not affect the expression of either major histocompatibility antigen complex (MHC) class I or class II molecules on the cell surface of the stimulator cells, antigen-presenting cells. In contrast, beta-SQDG(18:0) was demonstrated to inhibit the binding among allogeneic lymphocytes. However, the expression of known cell surface accessory and adhesion molecules, such as CD4, CD28, leukocyte function-associated antigen 1, intercellular adhesion molecule 1, and CTLA-4, was not affected by beta-SQDG(18:0) treatment.

CONCLUSIONS

beta-SQDG(18:0) might be a new class of the immunosuppressive reagent, and the inhibition of responder T-lymphocyte activation in MLR by beta-SQDG(18:0) may be responsible for certain three-dimensional structures of this reagent or its quinovose binding to sulfonic acid.

Anti-tumor effect of chemically synthesized sulfolipids based on sea urchin's natural sulfonoquinovosylmonoacylglycerols

We recently reported that 3'-sulfonoquinovosyl-1'-monoacylglycerol (designated A-5) extracted from sea urchin intestine was effective in suppressing the growth of solid tumors. Although the major fatty acid component of A-5 was a saturated C(16) acid, there were five other fatty acids, 14:0, 18:0, 14:1, 16:1, and 18:1, which constitute minor components of A-5. Therefore, it remains unclear as to which of these six fatty acid components of A-5 has the anti-tumor effect. In this study, we synthesized sulfolipids each containing only one of these six fatty acids and tested their cytotoxicity against tumor cells and in vivo anti-tumor effects on nude-mice bearing solid tumors of human lung adenocarcinoma cell line A-549. The IC(50) values of all products against tumor cells were more than 10(-5) M, suggesting weak cytotoxic activity compared with other chemotherapeutic compounds for cancer. On the other hand, in vivo anti-tumor assay showed that sulfoquinovosylmonoacylglycerols (SQMG) composed of 14:1 and 18:1 (designated SQMG(14:1) and SQMG(18:1), respectively) were significantly effective in suppressing the growth of solid tumors. Our data suggested that these two SQMGs had a substantial anti-tumor effect in vivo, and they are of interest as candidate drugs for anti-cancer treatment.

Sphingosylphosphorylcholine activates an amiloride-insensitive Na+-H+-exchange mechanism in GH4C1 cells

The effect of sphingosylphosphorylcholine (SphPCho) on the intracellular pH (pHi) in GH4C1 cells was investigated. SphPCho evoked a very slow increase in basal pHi. In cells acidified with nigericin, SphPCho induced a rapid alkalinization of the cells. The effect was inhibited in a Na+-free buffer solution, but was insensitive to ethylisopropyl amiloride, a potent inhibitor of Na+-H+ exchangers (NHE). Reverse transcription and PCR showed that the predominant isoform of the antiport expressed in GH4C1 cells is NHE-1. The rate of alkalinization after stimulation with propionate, and after addition of Na+ to cells acidified with NH4Cl, was enhanced in cells treated with SphPCho. The initial rate of alkalinization after addition of Na+ to acidified cells treated with SphPCho gave an apparent Km value of 15 +/- 2 mM for Na+. The Vmax value was 9 +/- 2 mM H+/min. The effect was insensitive to ouabain, staurosporine and bafilomycin A. However, the SphPCho-evoked alkalinization was abolished in cells treated with 2-deoxy-D-glucose. The effect was not due to the charge of the molecule, as stearylamine increased pHi in Na+-containing and Na+-free buffer. The results show that SphPCho may activate Na+-H+ exchange, and that this effect is mediated via an amiloride-insensitive exchange mechanism.

Peroxisome proliferator-induced acyl-CoA thioesterase from rat liver cytosol: molecular cloning and functional expression in Chinese hamster ovary cells

We have isolated and cloned a cDNA that codes for one of the peroxisome proliferator-induced acyl-CoA thioesterases of rat liver. The deduced amino acid sequence corresponds to the major induced isoform in cytosol. Analysis and comparison of the deduced amino acid sequence with the established consensus sequences suggested that this enzyme represents a novel kind of esterase with an incomplete lipase serine active site motif. Analyses of mRNA and its expression indicated that the enzyme is significantly expressed in liver only after peroxisome proliferator treatment, but isoenzymes are constitutively expressed at high levels in testis and brain. The reported cDNA sequence is highly homologous to the recently cloned brain acyl-CoA thioesterase [Broustas, Larkins, Uhler and Hajra (1996) J. Biol. Chem. 271, 10470-10476], but subtle differences throughout the sequence, and distinct differences close to the resulting C-termini, suggest that they are different enzymes, regulated in different manners. A full-length cDNA clone was expressed in Chinese hamster ovary cells and the expressed enzyme was characterized. The palmitoyl-CoA hydrolysing activity (Vmax) was induced approx. 9-fold to 1 micromol/min per mg of cell protein, which was estimated to correspond to a specific activity of 250 micromol/min per mg of cDNA-expressed enzyme. Both the specific activity and the acyl-CoA chain length specificity were very similar to those of the purified rat liver enzyme.

In vivo anti-tumour effect of 3'-sulphonoquinovosyl 1'-monoacylglyceride isolated from sea urchin (Strongylocentrotus intermedius) intestine

Extracts from sea urchin intestine were screened for new anti-tumour drugs. Four glycolipids, 3'-sulphonoquinovosyl-1', 2'-diacylglyceride (A-4), 3'-sulphonoquinovosyl-1'-monoacylglyceride (2'-lyso A-4, A-5), NeuGc(alpha)2-6Glc(beta)1-1ceramide (A-6) and HSO3-8NeuGc(alpha)2-6Glc(beta)1-1ceramide (A-7), were isolated from the intestine of sea urchin, Strongylocentrotus intermedius, and characterized by means of proton nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. When tested for cytotoxic activity against tumour cells in vitro, A-5 showed significant activity, but A-4, -6 and -7 did not. In addition, the hydrophilic derivatives of A-4 or -5 had no cytotoxicity. Furthermore, the anti-tumour effects on nude mice bearing solid tumours of a human lung adenocarcinoma cell line A-549 were evaluated in vivo using A-4 and -5. As a result, A-5 was found to be significantly effective in suppressing the growth of solid tumours, whereas A-4 had no effect. Pathologically, the solid tumours showed haemorrhagic necrosis areas after treatment with A-5. In this study, we have demonstrated the anti-tumour effect of sulphonoquinovosyl-lysoglyceride (A-5), which provides important information that this sulpholipid could be a useful drug for cancer chemotherapy.

Sphingosylphosphocholine, a signaling molecule which accumulates in Niemann-Pick disease type A, stimulates DNA-binding activity of the transcription activator protein AP-1

Sphingosylphosphocholine (SPC) is the deacylated derivative of sphingomyelin known to accumulate in neuropathic Niemann-Pick disease type A. SPC is a potent mitogen that increases intracellular free Ca2+ and free arachidonate through pathways that are only partly protein kinase C-dependent. Here we show that SPC increased specific DNA-binding activity of transcription activator AP-1 in electrophoretic mobility-shift assays. Increased DNA-binding activity of AP-1 was detected after only 1-3 min, was maximal after 6 hr, and remained elevated at 12-24 hr. c-Fos was found to be a component of the AP-1 complex. Northern hybridization revealed an increase in c-fos transcripts after 30 min. Since the increase in AP-1 binding activity preceded the increase in c-fos mRNA, posttranslational modifications may be important in mediating the early SPC-induced increases in AP-1 DNA-binding activity. Western analysis detected increases in nuclear c-Jun and c-Fos proteins following SPC treatment. SPC also transactivated a reporter gene construct through the AP-1 recognition site, indicating that SPC can regulate the expression of target genes. Thus, SPC-induced cell proliferation may result from activation of AP-1, linking signal transduction by SPC to gene expression. Since the expression of many proteins with diverse functions is known to be regulated by AP-1, SPC-induced activation of AP-1 may contribute to the pathophysiology of Niemann-Pick disease.

Sphingolipid breakdown products: anti-proliferative and tumor-suppressor lipids

The sphingolipids are a family of lipids found ubiquitously in eukaryotic cell membranes. Within the last decade sphingolipids have emerged as active participants in the regulation of cell growth, differentiation, transformation, and cell-cell contact. A prototypic sphingolipid signalling pathway is the 'sphingomyelin cycle,' in which membrane sphingomyelin is hydrolyzed in response to extracellular stimuli, generating the putative second messenger ceramide. Ceramide, in turn, is thought to propagate the signal into the cell interior by the activation of a phosphatase. It is likely that other sphingolipids are components of similar signalling cycles, generating a variety of lipid messengers which participate in as yet undefined pathways. Sphingosine, for example, is a potential breakdown product of all sphingolipids, and is well-known for its pharmacologic inhibition of protein kinase C. However, it is becoming apparent that sphingosine is active in multiple signalling cascades that are independent of protein kinase C, including effects on fibroblast cell growth and the regulation of the retinoblastoma tumor suppressor protein. Similarly, lyso-sphingolipids, while comprising only a minor fraction of the cell's total sphingolipids, are turning out to have biological effects which warrant their investigation as potential signalling molecules. A distinguishing characteristic of sphingolipid breakdown products is their apparent participation in anti-proliferative pathways of cell regulation. Thus, sphingolipid breakdown products can be found to play roles in growth inhibition, induction of differentiation, and programmed cell death. In coordination with other cellular signal transduction pathways, the sphingolipid breakdown products may be the harnesses on cell growth and may also contribute to the suppression of oncogenesis.

Signaling pathways for sphingosylphosphorylcholine-mediated mitogenesis in Swiss 3T3 fibroblasts

Sphingosylphosphorylcholine (SPC), or lysophingomyelin, a wide-spectrum growth promoting agent for a variety of cell types (Desai, N. N., and S. Spiegel. 1991. Biochem. Biophys. Res. Comm. 181: 361-366), stimulates cellular proliferation of quiescent Swiss 3T3 fibroblasts to a greater extent than other known growth factors or than the structurally related molecules, sphingosine and sphingosine-1-phosphate. SPC potentiated the mitogenic effect of an activator of protein kinase C, 12-O-tetradecanoylphorbol 13-acetate, and did not compete with phorbol esters for binding to protein kinase C in intact Swiss 3T3 fibroblasts. However, downregulation of protein kinase C, by prolonged treatment with phorbol ester, reduced, but did not eliminate, the ability of SPC to stimulate DNA synthesis, indicating that SPC may act via both protein kinase C-dependent and -independent signaling pathways. SPC induced a rapid rise in intracellular free calcium ([Ca2+]i) in viable 3T3 fibroblasts determined with a digital imaging system. Although the increases in [Ca2+]i were observed even in the absence of calcium in the external medium, no increase in the levels of inositol phosphates could be detected in response to mitogenic concentrations of SPC. Furthermore, in contrast to sphingosine or sphingosine-1-phosphate, the mitogenic effect of SPC was not accompanied by increases in phosphatidic acid levels or changes in cAMP levels. SPC, but not sphingosine or sphingosine-1-phosphate, stimulates the release of arachidonic acid. Therefore, the ability of SPC to act an extremely potent mitogen may be due to activation of signaling pathway(s) distinct from those used by sphingosine or sphingosine-1-phosphate.

Inhibition of phospholipase C delta by hexadecylphosphorylcholine and lysophospholipids with antitumor activity

The antineoplastic compound hexadecylphosphorylcholine (HPC) was shown to be a highly effective inhibitor of phospholipase C delta (PLC delta 1), with an I50 of about 30 nmol/mL (30 microM) in the presence and absence of 200 microM spermine. A number of lysophospholipids, of which HPC can be considered to be a structural analog, also inhibited PLC. Lysosphingomyelin, lysophosphatidylserine, and lysophosphatidylcholine exhibited I50 values of 15, 10, and 7 nmol/mL, respectively, in the presence of 200 microM spermine. The I50 values were increased to 21-53 nmol/mL in the absence of spermine. N,N-Dimethylsphingosine and N,N,N-trimethylsphingosine, which inhibit the metastatic potential of human and murine tumor cells, were weak activators of PLC delta 1. It is postulated that HPC is more effective as an antineoplastic agent than lysophospholipids because HPC is metabolized slowly, while the lysophospholipids are metabolized rapidly in vivo.

Sphingosylphosphorylcholine is a remarkably potent mitogen for a variety of cell lines

The effect of spingosylphosphorylcholine on cellular proliferation was investigated in a variety of cell types. Spingosylphosphorylcholine at low concentrations greatly stimulated DNA synthesis and cell division in quiescent Swiss 3T3 fibroblasts. The increased DNA synthesis was also accompanied by pronounced morphological alterations. Spingosylphosphorylcholine was remarkably more potent than other known growth factors and also acted synergistically with insulin, epidermal growth factor, fibroblast growth factor, and the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate, to induce cellular proliferation. Sphingosylphosphorylcholine was less effective in stimulating DNA synthesis in rapidly growing normal and transformed cells. Spingosylphosphorylcholine appears to be a new type of potent, wide-spectrum growth promoting agent.