Ceramide inhibition of phospholipase D and its relationship to RhoA and ARF1 translocation in GTP gamma S-stimulated polymorphonuclear leukocytes

Ceramide induces a multicomponent intracellular calcium increase triggering the acrosome secretion in human sperm

Exocytosis of spermatozoon's secretory vesicle, named acrosome reaction (AR), is a regulated event that plays a central role in fertilization. It is coupled to a complex calcium signaling. Ceramide is a multitasking lipid involved in exocytosis. Nevertheless, its effect on secretion is controversial and the underlying cellular and molecular mechanisms remain unknown. Human spermatozoa are useful to dissect the role of ceramide in secretion given that the gamete is not capable to undergo any trafficking mechanisms other than exocytosis. We report for the first time, the presence of sphingolipid metabolism enzymes such as neutral-sphingomyelinase and ceramide synthase in sperm. Ceramidases are also present and active. Both the addition of cell-permeable ceramide and the rise of the endogenous one, increase intracellular calcium acting as potent inducers of exocytosis. Ceramide triggers AR in capacitated spermatozoa and enhances the gamete response to progesterone. The lipid induces physiological ultrastructural changes in the acrosome and triggers an exocytosis-signaling cascade involving protein tyrosine phosphatase 1B and VAMP2. Real-time imaging showed an increment of calcium in the cytosol upon ceramide treatment either in the absence or in the presence of extracellular calcium. Pharmacological experiments demonstrate that at early stages the process involves ryanodine receptors, CatSper (calcium channel of sperm), and store-operated calcium channels. We set out the signaling sequence of events that connect ceramide to internal calcium mobilization and external calcium signals during secretion. These results allow the coordination of lipids and proteins in a pathway that accomplishes secretion. Our findings contribute to the understanding of ceramide's role in regulated exocytosis and fertilization.

Membrane lipids and their degradation compounds control GM2 catabolism at intralysosomal luminal vesicles

The catabolism of ganglioside GM2 is dependent on three gene products. Mutations in any of these genes result in a different type of GM2 gangliosidosis (Tay-Sachs disease, Sandhoff disease, and the B1 and AB variants of GM2 gangliosidosis), with GM2 as the major lysosomal storage compound. GM2 is also a secondary storage compound in lysosomal storage diseases such as Niemann-Pick disease types A-C, with primary storage of SM in type A and cholesterol in types B and C, respectively. The reconstitution of GM2 catabolism at liposomal surfaces carrying GM2 revealed that incorporating lipids into the GM2-carrying membrane such as cholesterol, SM, sphingosine, and sphinganine inhibits GM2 hydrolysis by β-hexosaminidase A assisted by GM2 activator protein, while anionic lipids, ceramide, fatty acids, lysophosphatidylcholine, and diacylglycerol stimulate GM2 catabolism. In contrast, the hydrolysis of the synthetic, water-soluble substrate 4-methylumbelliferyl-6-sulfo-2-acetamido-2-deoxy-β-d-glucopyranoside was neither significantly affected by membrane lipids such as ceramide or SM nor stimulated by anionic lipids such as bis(monoacylglycero)phosphate added as liposomes, detergent micelles, or lipid aggregates. Moreover, hydrolysis-inhibiting lipids also had an inhibiting effect on the solubilization and mobilization of membrane-bound lipids by the GM2 activator protein, while the stimulating lipids enhanced lipid mobilization.

Membrane lipids regulate ganglioside GM2 catabolism and GM2 activator protein activity

Ganglioside GM2 is the major lysosomal storage compound of Tay-Sachs disease. It also accumulates in Niemann-Pick disease types A and B with primary storage of SM and with cholesterol in type C. Reconstitution of GM2 catabolism with β-hexosaminidase A and GM2 activator protein (GM2AP) at uncharged liposomal surfaces carrying GM2 as substrate generated only a physiologically irrelevant catabolic rate, even at pH 4.2. However, incorporation of anionic phospholipids into the GM2 carrying liposomes stimulated GM2 hydrolysis more than 10-fold, while the incorporation of plasma membrane stabilizing lipids (SM and cholesterol) generated a strong inhibition of GM2 hydrolysis, even in the presence of anionic phospholipids. Mobilization of membrane lipids by GM2AP was also inhibited in the presence of cholesterol or SM, as revealed by surface plasmon resonance studies. These lipids also reduced the interliposomal transfer rate of 2-NBD-GM1 by GM2AP, as observed in assays using Förster resonance energy transfer. Our data raise major concerns about the usage of recombinant His-tagged GM2AP compared with untagged protein. The former binds more strongly to anionic GM2-carrying liposomal surfaces, increases GM2 hydrolysis, and accelerates intermembrane transfer of 2-NBD-GM1, but does not mobilize membrane lipids.

Kinase-dependent activation of voltage-gated Ca2+ channels by ET-1 in pulmonary arterial myocytes during chronic hypoxia

Exposure to chronic hypoxia (CH) causes pulmonary hypertension. The vasoconstrictor endothelin-1 (ET-1) is thought to play a role in the development of hypoxic pulmonary hypertension. In pulmonary arterial smooth muscle cells (PASMCs) from chronically hypoxic rats, ET-1 signaling is altered, with the ET-1-induced change in intracellular calcium concentration (Δ[Ca(2+)](i)) occurring through activation of voltage-dependent Ca(2+) channels (VDCC) even though ET-1-induced depolarization via inhibition of K(+) channels is lost. The mechanism underlying this response is unclear. We hypothesized that activation of VDCCs by ET-1 following CH might be mediated by protein kinase C (PKC) and/or Rho kinase, both of which have been shown to phosphorylate and activate VDCCs. To test this hypothesis, we examined the effects of PKC and Rho kinase inhibitors on the ET-1-induced Δ[Ca(2+)](i) in PASMCs from rats exposed to CH (10% O(2), 3 wk) using the Ca(2+)-sensitive dye fura 2-AM and fluorescent microscopy techniques. We found that staurosporine and GF109203X, inhibitors of PKC, and Y-27632 and HA 1077, Rho kinase inhibitors, reduced the ET-1-induced Δ[Ca(2+)](i) by >70%. Inhibition of tyrosine kinases (TKs) with genistein or tyrphostin A23, or combined inhibition of PKC, TKs, and Rho kinase, reduced the Δ[Ca(2+)](i) to a similar extent as inhibition of either PKC or Rho kinase alone. The ability of PKC or Rho kinase to activate VDCCs in our cells was verified using phorbol 12-myristate 13-acetate and GTP-γ-S. These results suggest that following CH, the ET-1-induced Δ[Ca(2+)](i) in PASMCs occurs via Ca(2+) influx through VDCCs mediated primarily by PKC, TKs, and Rho kinase.

An obligate role for membrane-associated neutral sphingomyelinase activity in orienting chemotactic migration of human neutrophils

For polymorphonuclear neutrophils (PMNs) to orient migration to chemotactic gradients, weak external asymmetries must be amplified into larger internal signaling gradients. Lipid mediators, associated with the plasma membrane and within the cell, participate in generating these gradients. This study examined the role in PMN chemotaxis of neutral sphingomyelinase (N-SMase), a plasma membrane-associated enzyme that converts sphingomyelin to ceramide. A noncompetitive N-SMase inhibitor, GW4869 (5 mM, 5 minutes), did not inhibit PMN motility (as percentage of motile cells, or mean cell velocity), but it abrogated any orientation of movement toward the source of the chemotaxin, formylmethionylleucylphenylanaline (FMLP) (net displacement along the gradient axis in micrometers, or as percentage of total migration distance). This defect could be completely reversed by treatment with lignoceric ceramide (5 μg/ml, 15 minutes). Immunolocalization studies demonstrated that N-SMase (1) distributes preferentially toward the leading edge of some elongated cells, (2) is associated with the plasma membrane, (3) is more than 99.5% localized to the cytofacial aspect of the plasma membrane, (4) is excluded from pseudopodial extensions, and (5) increases rapidly in response to FMLP. Morphologically, the inhibition of N-SMase limited cellular spreading and the extension of sheet-like pseudopods. Elongated PMNs demonstrated a polarized distribution of GTPases, with Rac 1/2 accumulated at, and RhoA excluded from, the front of the cell. This polarity was negated by N-SMase inhibition and restored by lignoceric ceramide. We conclude that N-SMase at the cytofacial plasma membrane is an essential element for the proper orientation of PMNs in FMLP gradients, at least in part by polarizing the distribution of Rac 1/2 and RhoA GTPases.

Understanding phospholipase D (PLD) using leukocytes: PLD involvement in cell adhesion and chemotaxis

Phospholipase D (PLD) is an enzyme that catalyzes the conversion of membrane phosphatidylcholine to choline and phosphatidic acid (PA; a second messenger). PLD is expressed in nearly all types of leukocytes and has been associated with phagocytosis, degranulation, microbial killing, and leukocyte maturation. With the application of recently developed molecular tools (i.e., expression vectors, silencing RNA, and specific antibodies), the demonstration of a key role for PLD in those and related cellular actions has contributed to a better awareness of its importance. A case in point is the recent findings that RNA interference-mediated depletion of PLD results in impaired leukocyte adhesion and chemotaxis toward a gradient of chemokines, implying that PLD is necessary for leukocyte movement. We forecast that based on results such as those, leukocytes may prove to be useful tools to unravel still-unresolved mechanistic issues in the complex biology of PLD. Three such issues are considered here: first, whether the cellular actions of PLD are mediated entirely by PA (the product of its enzymatic reaction) or whether PLD by itself interacts with other protein signaling molecules; second, the current difficulty of defining a "PA consensus site" in the various intracellular protein targets of PA; and third, the resolution of specific PLD location (upstream or downstream) in a particular effector signaling cascade. There are reasons to expect that leukocytes and their leukemic cell line counterparts will continue yielding invaluable information to cell biologists to resolve standing molecular and functional issues concerning PLD.

Inhibition of de novo ceramide synthesis upregulates phospholipase D and enhances myogenic differentiation

In L6 skeletal myoblasts induced to differentiate by Arg8-vasopressin treatment, a short-lived lowering of ceramide levels was observed, followed by a long-lasting elevation that was prevented by inhibitors of the de novo synthesis pathway, fumonisin B1 and myriocin. Both inhibitors increased the expression of myogenic differentiation markers and cell fusion rate, whereas short-chain ceramides inhibited these responses. Similar drug effects were observed on primary mouse satellite cell differentiation. Furthermore, bacterial sphingomyelinase overexpression suppressed myogenin nuclear accumulation in L6 cells. These data suggested that endogenous ceramide mediates a negative feedback mechanism limiting myogenic differentiation, and that inhibitors of ceramide synthesis promoted myogenesis by removing this control. Phospholipase D (PLD), a recognized target of ceramide, is required for myogenesis, as shown by the negative effects of PLD1 isoform depletion obtained by siRNA treatment. Fumonisin induced an increase in PLD activity of L6 cells, whereas C6-ceramide decreased it. The expression of PLD1 mRNA transcripts was selectively decreased by C6-ceramide, and increased by ceramide synthesis inhibitors. An early step of myogenic response is the PLD1-dependent formation of actin stress fiber-like structures. C6-ceramide addition or overexpression of sphingomyelinase impaired actin fiber formation. Ceramide might thus regulate myogenesis through downregulation of PLD1 expression and activity.

Phagocyte cell migration is mediated by phospholipases PLD1 and PLD2

We have investigated whether the signaling protein phospholipase D is implicated in leukocyte cell motility. Treating differentiated HL-60 cells with small interfering RNAs (siRNAs), to deplete endogenous expression of the PLD1 isoform, led to an abolishment of basal chemokinesis that could not be rescued with chemoattractants ENA-78, FMLP, and IL-8. Transient overexpression of PLD1 increased both chemokinesis and chemotaxis toward IL-8 and FMLP but not toward ENA-78. Chemokinesis was not mediated by the enzymatic activity of PLD1, but the chemotactic response was, because a lipase-inactive mutant (PLD1-K830R) negated all chemokine-induced potentiating actions and because IL-8 and FMLP increased activity in vitro. Gene expression silencing of the other mammalian isoform, PLD2, also led to cell migration arrest, whereas ENA-78 selectively increased endogenous PLD2 activity and chemotaxis of HL-60 cells overexpressing a myc-pcDNA-PLD2 construct. Thus, PLD1 is differentially activated by CXCR-1, whereas CXCR-2 (and possibly CXCR-1) mediates PLD2 activation. Finally, immunofluorescence microscopy showed that both isoforms were associated with cell polarity and directionality concomitantly with adhesion and F-actin polymerization in response to IL-8. These data represent the first demonstration of the involvement of PLD and its enzymatic activity toward chemokines in the key physiologic process of leukocyte migration.

1Alpha,25-dihydroxyvitamin D3-mediated stimulation of steroid sulphatase activity in myeloid leukaemic cell lines requires VDRnuc-mediated activation of the RAS/RAF/ERK-MAP kinase signalling pathway

1Alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) stimulates the activity of steroid sulphatase (STS) in myeloid cells [Hughes et al., 2001, 2005]. This was attenuated by inhibitors of phospholipase D (PLD) (n-butanol, 2,3-diphosphoglyceric acid, C(2)-ceramide) and phosphatidate phosphohydrolase (PAP) (propranolol and chlorpromazine), but was unaffected by inhibitors of phospholipase C. The 1alpha,25(OH)(2)D(3)-induced STS activity was also attenuated by inhibitors of protein kinase Calpha and protein kinase Cdelta (Go 6976, HBDDE and rottlerin), but not by an inhibitor of protein kinase Cbeta (LY379196). Additionally, 1alpha,25(OH)(2)D(3)-induced STS activity was attenuated by inhibitors of RAS (manumycin A), RAF (GW5074), MEK (PD098059 and U1026) and JNK (SP600125), but not p38 (PD169316). 1alpha,25(OH)(2)D(3) produced a rapid and long lasting stimulation of the ERK-MAP kinase signalling cascade in HL60 myeloid leukaemic cells. This 'non-genomic' effect of 1alpha,25(OH)(2)D(3) blocked by pharmacological antagonists of nuclear vitamin D receptors (VDR(nuc)) and does not appear to require hetero-dimerisation with the retinoid-X receptor (RXR). Inhibitors of the Src tyrosine kinase (PP1), RAS (manumycin A), RAS-RAF interactions (sulindac sulphide and RAS inhibitory peptide), RAF (GW5074 or chloroquine), and protein kinase Calpha (HBDDE) abrogated the 1alpha,25(OH)(2)D(3)-stimulated increase in ERK-MAP kinase activity. Taken together, these results show that 1alpha,25(OH)(2)D(3)/VDR(nuc) activation of the RAS/RAF/ERK-MAP kinase signalling pathway plays an important role in augmenting STS activity in human myeloid leukaemic cell lines.

Retinoid-mediated stimulation of steroid sulfatase activity in myeloid leukemic cell lines requires RARalpha and RXR and involves the phosphoinositide 3-kinase and ERK-MAP kinase pathways

All-trans retinoic acid and 9-cis-retinoic acid stimulate the activity of steroid sulfatase in HL60 acute myeloid leukemia cells in a concentration- and time-dependent manner. Neither of these 'natural retinoids' augmented steroid sulfatase activity in a HL60 sub-line that expresses a dominant-negative retinoic acid receptor alpha (RARalpha). Experiments with synthetic RAR and RXR agonists and antagonists suggest that RARalpha/RXR heterodimers play a role in the retinoid-stimulated increase in steroid sulfatase activity. The retinoid-driven increase in steroid sulfatase activity was attenuated by inhibition of phospholipase D (PLD), but not by inhibitors of phospholipase C. Experiments with inhibitors of protein kinase C (PKC) show that PKCalpha and PKCdelta play an important role in modulating the retinoid-stimulation of steroid sulfatase activity in HL60 cells. Furthermore, we show that pharmacological inhibition of the RAF-1 and ERK MAP kinases blocked the retinoid-stimulated increase in steroid sulfatase activity in HL60 cells and, by contrast, inhibition of the p38-MAP kinase or JNK-MAP kinase had no effect. Pharmacological inhibitors of the phosphatidylinositol 3-kinase, Akt, and PDK-1 also abrogated the retinoid-stimulated increase in steroid sulfatase activity in HL60 cells. These results show that crosstalk between the retinoid-stimulated genomic and non-genomic pathways is necessary to increase steroid sulfatase activity in HL60 cells.

Phagocytic signaling molecules in lipid rafts of COS-1 cells transfected with FcgammaRIIA

COS-1 cells bearing FcgammaRIIA were used as a model to demonstrate co-localization of several enzymes previously shown to regulate neutrophil phagocytosis. In COS-1 cells, phospholipase D (PLD) in the membrane fraction was activated during phagocytosis. PLD was found almost exclusively in lipid rafts, along with RhoA and ARF1. Protein kinase C-delta (PKCdelta) and Raf-1 translocated to lipid rafts. In neutrophils, ceramide levels increase during phagocytosis, indicating that FcgammaRIIA engagement initiates ceramide generation. Applying this model, we transfected COS-1 cells with FcgammaRIIA that had been mutated in the ITAM region, rendering them unable to ingest particles. When the mutant receptors were engaged, ceramide was generated and MAPK was activated normally, thus these processes did not require actual ingestion of particles. These results indicate that signaling proteins for phagocytosis are either constitutively present in, or are recruited to, lipid rafts where they are readily available to activate one another.

Sphingolipids and the balancing of immune cell function: lessons from the mast cell

Recent studies reveal that metabolites of sphingomyelin are critically important for initiation and maintenance of diverse aspects of immune cell activation and function. The conversion of sphingomyelin to ceramide, sphingosine, or sphingosine-1-phosphate (S1P) provides interconvertible metabolites with distinct biological activities. Whereas ceramide and sphingosine function to induce apoptosis and to dampen mast cell responsiveness, S1P functions as a chemoattractant and can up-regulate some effector responses. Many of the S1P effects are mediated through S1P receptor family members (S1P(1-5)). S1P(1), which is required for thymocyte emigration and lymphocyte recirculation, is also essential for Ag-induced mast cell chemotaxis, whereas S1P(2) is important for mast cell degranulation. S1P is released to the extracellular milieu by Ag-stimulated mast cells, enhancing inflammatory cell functions. Modulation of S1P receptor expression profiles, and of enzymes involved in sphingolipid metabolism, particularly sphingosine kinases, are key in balancing mast cell and immune cell responses. Current efforts are unraveling the complex underlying mechanisms regulating the sphingolipid pathway. Pharmacological intervention of these key processes may hold promise for controlling unwanted immune responses.