Alicja Bielawska

Long Chain Ceramides Activate Protein Phosphatase-1 and Protein Phosphatase-2A ACTIVATION IS STEREOSPECIFIC AND REGULATED BY PHOSPHATIDIC ACID

The search for potential targets for ceramide action led to the identification of ceramide-activated protein phosphatases, which include protein phosphatase-2A (PP2A) and protein phosphatase-1 (PP1) with roles in regulating apoptosis and cell growth. Thus far, in vitro studies on ceramide-activated protein phosphatases have been restricted to the use of short chain ceramides, limiting the extent of mechanistic insight. In this study, we show that the long chaind-erythro-C18-ceramide activated PP2A (AB′C trimer), PP2Ac (catalytic subunit of PP2A), and PP1γc and -αc (catalytic subunits of PP1γ and -1α isoforms, respectively) 2–6-fold in the presence of dodecane, a lipid-solubilizing agent, with 50% maximal activation achieved at approximately 10 μm d-erythro-C18-ceramide. The diastereoisomers ofd-erythroC18-ceramide,d-threo-, andl-threo-C18-ceramide, as well as the enantiomericl-erythro-C18-ceramide, did not activate PP1 or PP2A, but they inhibited PP1 and PP2A activity. The addition of phosphatidic acid decreased the basal activity of PP1c but also increased the stimulation byd-erythro-C18-ceramide from 1.8- to 2.8-fold and decreased the EC50 ofd-erythro-C18-ceramide to 4.45 μm. The addition of 150 mm KCl decreased the basal activity of PP1 and the dose ofd-erythro-C18-ceramide necessary to activate PP1c (EC50 = 6.25 μm) and increased the ceramide responsiveness up to 10–17-fold. These studies disclose stereospecific activation of PP1 and PP2A by long chain natural ceramides under near physiologic ionic strengths in vitro. The implications of these studies for mechanisms of ceramide action are discussed.

(1S,2R)-D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol as an inhibitor of ceramidase

In this study, we have examined the cellular and biochemical activities of the ceramide analog (1S,2R)-Derythro-2-(N-myristoylamino)-1-phenyl-1-propanol (Derythro-MAPP). Addition of 5 μMD-e-MAPP to HL-60 human promyelocytic leukemia cells resulted in a concentration- and time-dependent growth suppression accompanied by an arrest in the G/G phase of the cell cycle; thus mimicking the action of exogenous ceramides. Its enantiomer L-e-MAPP was without effect. Two lines of evidence suggested that D-e-MAPP may not function as a direct analog of ceramide. First, D-e-MAPP possesses a stereochemical configuration opposite to that of D-erythro-ceramide. Second, D-e-MAPP failed to activate ceramide-activated protein phosphatase in vitro. Therefore, we examined if D-e-MAPP functioned indirectly by modulating endogenous ceramide levels. The addition of D-e-MAPP to cells, but not L-e-MAPP, caused a time- and concentration-dependent elevation in endogenous ceramide levels reaching greater than 3-fold over baseline following 24 h of treatment. Both D-e-MAPP and L-e-MAPP underwent similar uptake by HL-60 cells. D-e-MAPP was poorly metabolized, and remained intact in cells, whereas L-e-MAPP underwent a time- and concentration-dependent metabolism; primarily through N-deacylation. In vitro, L-e-MAPP was metabolized by alkaline ceramidase to an extent similar to that seen with C-ceramide. D-e-MAPP was not metabolized. Instead, D-e-MAPP inhibited alkaline ceramidase activity in vitro with an IC of 1-5 μM. D-e-MAPP did not modulate the activity of other ceramide metabolizing enzymes in vitro or in cells, and it was a poor inhibitor of acid ceramidase (IC > 500 μM). Finally, D-e-MAPP inhibited the metabolism of L-e-MAPP in cells. These studies demonstrate that D-e-MAPP functions as an inhibitor of alkaline ceramidase in vitro and in cells resulting in elevation in endogenous levels of ceramide with the consequent biologic effects of growth suppression and cell cycle arrest. These studies point to an important role for ceramidases in the regulation of endogenous levels of ceramide.

Cloning of an alkaline ceramidase from Saccharomyces cerevisiae. An enzyme with reverse (CoA-independent) ceramide synthase activity.

Ceramide is not only a core intermediate of sphingolipids but also an important modulator of many cellular events including apoptosis, cell cycle arrest, senescence, differentiation, and stress responses. Its turnover may be tightly regulated. However, little is known about the regulation of its metabolism because most enzymes responsible for its synthesis and breakdown have yet to be cloned. Here we report the cloning and characterization of the yeast gene YPC1 (YBR183w) by screeningSaccharomyces cerevisiae genes whose overexpression bestows resistance to fumonisin B1. We demonstrate that the yeast geneYPC1 encodes an alkaline ceramidase activity responsible for the breakdown of dihydroceramide and phytoceramide but not unsaturated ceramide. YPC1 ceramidase activity was confirmed byin vitro studies using an Escherichia coliexpression system. Importantly, YPC1p also has reverse activity, catalyzing synthesis of phytoceramide from palmitic acid and phytosphingosine. This ceramide synthase activity is CoA-independent and is resistant to fumonisin B1, thus explaining why YPC1was cloned as a fumonisin B1-resistant gene.

Ceramide Kinase Mediates Cytokine- and Calcium Ionophore-induced Arachidonic Acid Release

Despite the importance of prostaglandins, little is known about the regulation of prostanoid synthesis proximal to the activation of cytosolic phospholipase A2, the initial rate-limiting step. In this study, ceramide-1-phosphate (C-1-P) was shown to be a specific and potent inducer of arachidonic acid (AA) and prostanoid synthesis in cells. This study also demonstrates that two well established activators of AA release and prostanoid synthesis, the cytokine, interleukin-1β (IL-1β), and the calcium ionophore, A23187, induce an increase in C-1-P levels within the relevant time-frame of AA release. Furthermore, the enzyme responsible for the production of C-1-P in mammalian cells, ceramide kinase, was activated in response to IL-1β and A23187. RNA interference targeted to ceramide kinase specifically down-regulated ceramide kinase mRNA and activity with a concomitant decrease of AA release in response to IL-1β and A23187. Down-regulation of ceramide kinase had no effect on AA release induced by exogenous C-1-P. Collectively, these results indicate that ceramide kinase, via the formation of C-1-P, is an upstream modulator of phospholipase A2 activation. This study identifies previously unknown roles for ceramide kinase and its product, C-1-P, in AA release and production of eicosanoids and provides clues for potential new targets to block inflammatory responses.

Cloning and Characterization of a Saccharomyces cerevisiae Alkaline Ceramidase with Specificity for Dihydroceramide

In a previous study, we reported that theSaccharomyces cerevisiae gene YPC1 encodes an alkaline ceramidase with a dual activity, catalyzing both hydrolysis and synthesis of yeast ceramide (Mao, C., Xu, R., Bielawska, A., and Obeid, L. M. (2000) J. Biol. Chem. 275, 6876–6884). In this study, we have identified a YPC1homologue in S. cerevisiae that also encodes an alkaline ceramidase. We show that these two ceramidases have different substrate specificity, such that YPC1p preferentially hydrolyzes phytoceramide, whereas the new ceramidase YDC1p hydrolyzes dihydroceramide preferentially and phytoceramide only slightly. Neither enzyme hydrolyzes unsaturated mammalian-type ceramide. In contrast to YPC1p, YDC1p had only minor in vitro reverse activity of catalyzing dihydroceramide formation from a free fatty acid and dihydrosphingosine and no activity with phytosphingosine. Overexpression of YDC1p had no reverse activity in non-stressed yeast cells, but like YPC1p suppressed the inhibition of growth by fumonisin B1 albeit more modestly. Deletion of YDC1 andYPC1 or both did not apparently affect growth, suggesting neither gene is essential. However, the Δydc1 deletion mutant but not the Δypc1 deletion mutant was sensitive to heat stress, indicating a role for dihydroceramide but not phytoceramide in heat stress responses, and suggesting that the two enzymes have distinct physiological functions.

Blood sphingolipidomics in healthy humans: impact of sample collection methodology

We used a HPLC-MS/MS methodology for determination of a basic metabolomic profile (18:1,18:0 sphingoid backbone, C14-C26 N-acyl part) of “normal” sphingolipid levels in human serum and plasma. Blood was collected from healthy males and nonpregnant females under fasting and nonfasting conditions with and without anticoagulants. Sphingolipids analyzed included sphingoid bases, sphingosine and dihydrosphingosine, their 1-phosphates (S1P and dhS1P), molecular species (Cn-) of ceramide (Cer), sphingomyelin (SM), hexosylceramide (HexCer), lactosylceramide (LacCer), and Cer 1-phosphate (Cer1P). SM, LacCer, HexCer, Cer, and Cer1P constituted 87.7, 5.8, 3.4, 2.8, and 0.15% of total sphingolipids, respectively. The abundant circulating SM was C16-SM (64.0 µM), and it increased with fasting (100 µM). The abundant LacCer was C16-LacCer (10.0 µM) and the abundant HexCer was C24-HexCer (2.5 µM). The abundant Cer, C24-Cer (4.0 µM), was not influenced by fasting; however, levels of C16-C20 Cers were decreased in response to fasting. S1P levels were higher in serum than plasma (0.68 µM vs. 0.32 µM). We also determined levels of sphingoid bases and SM species in isolated lipoprotein classes. HDL3 was the major carrier of S1P, dhS1P, and Sph, and LDL was the major carrier of Cer and dhSph. Per particle, VLDL contained the highest levels of SM, Cer, and S1P. HPLC-MS/MS should provide a tool for clinical testing of circulating bioactive sphingolipids in human blood.

Identification of ISC1 (YER019w) as Inositol Phosphosphingolipid Phospholipase C inSaccharomyces cerevisiae

Sphingolipids have emerged as novel bioactive mediators in eukaryotic cells including yeast. It has been proposed that sphingomyelin (SM) hydrolysis and the concomitant generation of ceramide are involved in various stress responses in mammalian cells. The yeast Saccharomyces cerevisiae has inositol phosphosphingolipids (IPS) instead of SM and glycolipids, and synthesis of IPS is indispensable to its growth. Although the genes responsible for the synthesis of IPS have been identified, the gene(s) for the degradation of IPS has not been reported. Here we show thatISC1 (YER019w), which has homology to bacterial neutral sphingomyelinase (SMase), encodes IPS phospholipase C (IPS-PLC). First, we observed that overexpression ofISC1 greatly increased neutral SMase activity, and this activity was dependent on the presence of phosphatidylserine. Cells deleted in ISC1 demonstrated negligible neutral SMase activity. Because yeast cells have IPS instead of SM, we investigated whether IPS are the physiologic substrates of this enzyme. Lysates ofISC1-overexpressing cells demonstrated very high PLC activities on IPS. Deletion of ISC1 eliminated endogenous IPS-PLC activities. Labeling yeast cells with [3H]dihydrosphingosine showed that IPS were increased in the deletion mutant cells. This study identifies the first enzyme involved in catabolism of complex sphingolipids in S. cerevisiae.

Ceramide inhibits phospholipase D in a cell-free system.

Recent evidence in whole cells has implicated ceramide in the regulation of phospholipase D (PLD). In intact HL-60 cells, phorbol myristate acetate (PMA) activated PLD as measured by [3H]palmitate-labeled phosphatidylcholine conversion to phosphatidylethanol in the presence of 2% ethanol. C6-Ceramide completely inhibited PLD activation after 4 h of treatment and was maximally active at 10 μM. The activity was structurally specific in that the structural analogs 4,5-dihydro-C6-ceramide and dioctanoylglycerol were inactive. Although ceramide inhibited PMA-induced activation of PLD, it did not inhibit translocation of protein kinase C (PKC) to the membrane in response to PMA. In a cell-free system, we confirmed that PLD is activated by guanosine 5′-O-(3-thiotriphosphate (GTPγS); however, ceramide had no effect on this activity under a variety of conditions. Activation of PLD by GTPγS was synergistically enhanced by the addition of PKC activators. This upstream effect was inhibited rapidly and specifically by ceramide (30 μM). Recombinant ARF plus PKCα substituted for crude cytosol in the activation of PLD, and this activity was inhibited by C6-ceramide. Taken together, these data show that ceramide interferes with PKC-mediated activation of PLD.

Involvement of Dihydroceramide Desaturase in Cell Cycle Progression in Human Neuroblastoma Cells

The role of dihydroceramide desaturase as a key enzyme in the de novo pathway of ceramide generation was investigated in human neuroblastoma cells (SMS-KCNR). A novel assay using water-soluble analogs of dihydroceramide, dihydroceramidoids (d-erythro-dhCCPS analogs), was used to measure desaturase activity in situ. Conversion of d-erythro-2-N-[12′-(1″-pyridinium)-dodecanoyl]-4,5-dihydrosphingosine bromide (C12-dhCCPS) to its 4,5-desaturated counterpart, d-erythro-2-N-[12′-(1″-pyridinium)dodecanoyl]sphingosine bromide (C12-CCPS), was determined by liquid chromatography/mass spectrometry analysis. The validity of the assay was confirmed using C8-cyclopropenylceramide, a competitive inhibitor of dihydroceramide desaturase. A human homolog (DEGS-1) of the Drosophila melanogaster des-1 gene was recently identified and reported to have desaturase activity. Transfection of SMS-KCNR cells with small interfering RNA to DEGS-1 significantly blocked the conversion of C12-dhCCPS to C12-CCPS. The associated accumulation of endogenous dihydroceramides confirmed DEGS-1 as the main active dihydroceramide desaturase in these cells. The partial loss of DEGS-1 inhibited cell growth, with cell cycle arrest at G0/G1. This was accompanied by a significant decrease in the amount of phosphorylated retinoblastoma protein. This hypophosphorylation was inhibited by tautomycin and not by okadaic acid, suggesting the involvement of protein phosphatase 1. Additionally, we found that treatment of SMS-KCNR cells with fenretinide inhibited desaturase activity in a dose-dependent manner. An increase in dihydroceramides (but not ceramides) paralleled this process as measured by liquid chromatography/mass spectrometry. There were no effects on the mRNA or protein levels of DEGS-1, suggesting that fenretinide acts at the post-translational level as an inhibitor of this enzyme. Tautomycin was also able to block the hypophosphorylation of the retinoblastoma protein observed upon fenretinide treatment. These findings suggest a novel biological function for dihydroceramides.

Phosphatidic Acid Is a Potent And Selective Inhibitor of Protein Phosphatase 1 and an Inhibitor of Ceramide-mediated Responses

In the present study, we report that phosphatidic acid (PA) functions as a novel, potent, and selective inhibitor of protein phosphatase 1 (PP1). The catalytic subunit of PP1α was inhibited by PA dose-dependently in a noncompetitive manner with a K i value of 80 nm. The inhibition by PA was specific to PP1 as PA failed to inhibit protein phosphatase 2A (PP2A) or PP2B. Furthermore, PA was the most effective and potent inhibitor of PP1 compared with other phospholipids. Because we recently showed that ceramides activated PP1, we next examined the effects of PA on ceramide stimulation of PP1. PA inhibited both basal and ceramide-stimulated PP1 activities, and ceramide showed potent and stereoselective activation of PP1 in the presence of PA. Next, the effects of PA on ceramide-induced responses were examined. Molt-4 cells took up PA dose- and time-dependently such that by 1 and 3 h, uptake of PA was 0.37 and 0.65% of total PA added, respectively. PA at 30 μm and calyculin A at 10 nm (an inhibitor of PP1 and PP2A at low concentrations), but not okadaic acid at 10 nm (a PP2A inhibitor at low concentrations) prevented poly(ADP-ribose) polymerase proteolysis induced by C6-ceramide. Moreover, the combination of PA with okadaic acid prevented retinoblastoma gene product dephosphorylation induced by C6-ceramide. These data suggest that PA functions as a specific regulator of PP1 and may reverse or counteract those effects of ceramide that are mediated by PP1, such as apoptosis and retinoblastoma gene product dephosphorylation.