Catabolism of exogenous and endogenous sphingomyelin and phosphatidylcholine by homogenates and subcellular fractions of cultured neuroblastoma cells. Effects of anesthetics

Isoflurane posttreatment reduces neonatal hypoxic-ischemic brain injury in rats by the sphingosine-1-phosphate/phosphatidylinositol-3-kinase/Akt pathway

BACKGROUND AND PURPOSE

Isoflurane, administered before or during cerebral ischemia, has been shown to exhibit neuroprotection in animal models of ischemic stroke. However, the underlying mechanism remains to be elucidated. In the present study, we determined whether isoflurane posttreatment provides neuroprotection after neonatal hypoxia-ischemia (HI) in rats and evaluated the role of the sphingosine-1-phosphate/phosphatidylinositol-3-kinase/Akt pathway in this volatile anesthetic-mediated neuroprotection.

METHODS

HI was induced in postnatal day 10 (P10) rat pups by unilateral carotid ligation and 2 hours of hypoxia. For treatment, 2% isoflurane was administered immediately after HI for 1 hour. As pharmacological interventions, the sphingosine-1-phosphate antagonist VPC23019, phosphatidylinositol-3-kinase inhibitor wortmannin, or opioid antagonist naloxone was administered before HI. Isoflurane posttreatment was evaluated for effects on infarct volume at 48 hours after HI and brain atrophy and neurological outcomes at 4 weeks after HI. The expression of phosphorylated Akt and cleaved caspase-3 was determined by Western blotting and immunofluorescence analysis.

RESULTS

Isoflurane posttreatment significantly reduced infarct volume at 48 hours after HI. VPC23019 or wortmannin abrogated the neuroprotective effect of isoflurane, whereas naloxone did not inhibit the isoflurane-induced neuroprotection. Isoflurane posttreatment significantly preserved phosphorylated Akt expression and decreased cleaved caspase-3 levels. These effects were reversed by VPC23019 and wortmannin, respectively. Isoflurane also confers long-term neuroprotective effects against brain atrophy and neurological deficits at 4 weeks after HI.

CONCLUSIONS

Isoflurane posttreatment provides lasting neuroprotection against hypoxic-ischemic brain injury in neonatal rats. Activation of the sphingosine-1-phosphate/phosphatidylinositol-3-kinase/Akt pathway may play a key role in isoflurane posttreatment-induced neuroprotection.

Isoflurane mediates protection from renal ischemia-reperfusion injury via sphingosine kinase and sphingosine-1-phosphate-dependent pathways

The inhalational anesthetic isoflurane has been shown to protect against renal ischemia-reperfusion (IR) injury. Previous studies demonstrated that isoflurane modulates sphingolipid metabolism in renal proximal tubule cells. We sought to determine whether isoflurane stimulates sphingosine kinase (SK) activity and synthesis of sphingosine-1-phosphate (S1P) in renal proximal tubule cells to mediate renal protection via the S1P signaling pathway. Isoflurane anesthesia reduced the degree of renal failure and necrosis in a murine model of renal IR injury. This protection with isoflurane was reversed by SK inhibitors (DMS and SKI-II) as well as an S1P(1) receptor antagonist (VPC23019). In addition, mice deficient in SK1 enzyme were not protected from IR injury with isoflurane. SK activity as well as SK1 mRNA expression increased in both cultured human proximal tubule cells (HK-2) and mouse kidneys after exposure to isoflurane. Finally, isoflurane increased the generation of S1P in HK-2 cells. Taken together, our findings indicate that isoflurane activates SK in renal tubule cells and initiates S1P-->S1P(1) receptor signaling to mediate the renal protective effects. Our findings may help to unravel the cellular signaling pathways of volatile anesthetic-mediated renal protection and lead to new therapeutic applications of inhalational anesthetics during the perioperative period.

Isoflurane alters proximal tubular cell susceptibility to toxic and hypoxic forms of attack

BACKGROUND

Fluorinated anesthetics can profoundly alter plasma membrane structure and function, potentially impacting cell injury responses. Because major surgery often precipitates acute renal failure, this study assessed whether the most commonly used fluorinated anesthetic, isoflurane, alters tubular cell responses to toxic and hypoxic attack.

METHODS

Mouse proximal tubule segments were incubated under control conditions or with a clinically relevant isoflurane dose. Cell viability (lactate dehydrogenase release), deacylation (fatty acid, such as C20:4 levels), and adenosine triphosphate (ATP) concentrations were assessed under one or more of the following conditions: (a) exogenous phospholipase A2 (PLA2) or C20:4 addition, (b) Ca2+ overload (A23187 ionophore), (c) increased metabolic work (Na ionophore), and (d) hypoxia- or antimycin A-induced attack. Isoflurane's effect on NBD phosphatidylserine uptake (an index of plasma membrane aminophospholipid translocase activity) was also assessed.

RESULTS

Isoflurane alone caused trivial deacylation and no lactate dehydrogenase release. However, it strikingly sensitized to both PLA2- and A23187-induced deacylation and cell death. Isoflurane also exacerbated C20:4's direct membrane lytic effect. Under conditions of mild ATP depletion (Na ionophore-induced increased ATP consumption; PLA2-induced mitochondrial suppression), isoflurane provoked moderate/severe ATP reductions and cell death. Conversely, under conditions of maximal ATP depletion (hypoxia, antimycin), isoflurane conferred a modest cytoprotective effect. Isoflurane blocked aminophospholipid translocase activity, which normally maintains plasma membrane lipid asymmetry (that is, preventing its "flip flop").

CONCLUSIONS

Isoflurane profoundly and differentially affects tubular cell responses to toxic and hypoxic attack. Direct drug-induced alterations in lipid trafficking/plasma membrane orientation and in cell energy production are likely involved. Although the in vivo relevance of these findings remains unknown, they have potential implications for intraoperative renal tubular cell structure/function and how cells may respond to superimposed attack.

Fluorinated anesthetic exposure "activates" the renal cortical sphingomyelinase cascade

BACKGROUND

Previous studies indicate that fluorinated anesthetics can enhance sphingomyelin (SM) hydrolysis in in vitro neuronal extracts. Renal cortex has substantial SM content. Hence, this study assessed whether in vivo fluorinated anesthetic use stimulates renal SM hydrolysis, causing accumulation of ceramide, an important signaling molecule.

METHODS

Mice were anesthetized with isoflurane or desflurane (fluorinated anesthetics). Pentobarbital anesthetized mice served as controls. After six hours, kidney cortex was assayed for ceramide. In selected experiments, renal cortical sphingosine and sphingomyelinase (SMase) levels were also determined. Isoflurane's effects on ceramide levels in cultured human proximal tubule (HK-2) cells/isolated mouse proximal tubule segments (PTS), and on in vitro 14C-SM hydrolysis were also assessed.

RESULTS

Isoflurane and desflurane, but not pentobarbital, increased renal cortical ceramide levels (such as, 65% with isoflurane, P < 0.003). Isoflurane also raised PTS/HK-2 ceramide levels (by 25 to 35%). Ceramidase inhibition (fumonisin B1) did not block this ceramide accumulation in HK-2 cells. Isoflurane did not increase renal cortical/PTS SMase levels. However, it directly enhanced the ability of (acidic) SMase to effect in vitro 14C-SM hydrolysis. Isoflurane raised renal cortical sphingosine (and not just ceramide) levels, implying ongoing ceramidase activity.

CONCLUSIONS

Fluorinated anesthetics can stimulate renal cortical/tubule ceramide expression, presumably by stimulating SMase-mediated SM hydrolysis. Since ceramide is a potential mediator of tubule apoptosis/necrosis, these findings have potential relevance for the development of intra/post-operative acute renal failure.

The effect of different molecular species of sphingomyelin on phospholipase C delta 1 activity

Bovine brain sphingomyelin was separated into different molecular species using a reverse phase column. PLC delta 1 was inhibited by all molecular species of sphingomyelin. The extent of this inhibition was dependent on the hydrophobicity. Based on fatty acid analysis, we conclude that the inhibition of PLC delta 1 depends on the chain length and degree of unsaturation of the fatty acid moiety of SM. N-palmitoyl-D-sphingomyelin and N-stearoyl-D-sphingomyelin inhibited PLC delta 1 less then N-oleoyl-D-sphingomyelin. In the absence of Ca2+ (1 mM EGTA) all tested molecular species of SM inhibited weakly the enzyme. The sensitivity of PLC delta 1 to inhibition by SM increased with increasing Ca2+ concentration. The shape of calcium curve differed for molecular species with saturated and unsaturated fatty acids. Inhibition of PLC delta 1 by N-palmitoyl-D-sphingomyelin and N-stearoyl-D-sphingomyelin reached a maximum at 0.2 microM Ca2+, while inhibition by N-oleoyl-D-sphingomyelin reached maximum at 2 microM Ca2+. PLC delta 1 is more sensitive to inhibition by SM when it is maximally activated by spermine and calcium and the extent of this inhibition depends on the length and degree of fatty acid unsaturation of the molecular species.

Sphingoid bioregulators in the differentiation of cells of neural origin

The involvement of ceramide in the differentiation of two neuroblastoma cell lines, Neuro2a and SH-SY5Y, and cerebellar granule cells in primary culture was investigated. The following results were obtained: (a) the cellular content of ceramide markedly increased with induced differentiation of Neuro2a cells (inducers: RA, FCS deprivation), SH-SY5Y cells (inducers: RA, PMA), and spontaneous differentiation of cerebellar granule cells; (b) all the investigated cells in the differentiated form displayed a higher ability to produce ceramide from exogenously administered [3H]Sph-SM and expressed a higher content of neutral sphingomyelinase and, in the case of cerebellar granule cells, also of acidic sphingomyelinase; (c) inhibition of ceramide biosynthesis by Fumonisin B1 blocked the process of differentiation in Neuro2a and cerebellar granule cells; and (d) treatments capable of enhancing ceramide level (administration of sphingosine or C2-Ceramide) induced differentiation in both Neuro2a and SH-SY5Y cells. The data obtained support the notion that ceramide plays a general biomodulatory role in neural cell differentiation.

A mediator role of ceramide in the regulation of neuroblastoma Neuro2a cell differentiation

Current studies indicate that ceramide is involved in the regulation of important cell functions, namely cell growth, differentiation, and apoptosis. In the present study, the possible role of ceramide in the differentiation of neuroblastoma Neuro2a cells was investigated. The following results were obtained. (a) Ceramide content of Neuro2a cells, induced to differentiate by retinoic acid (RA) treatment rapidly increased after addition of RA, was maintained at high levels in RA-differentiated cells and returned to the starting levels with removal of RA and reversal of differentiation; under the same conditions, the sphingosine content remained unchanged. (b) After a short pulse with [3H]sphingomyelin or [3H]sphingosine or L-[3H]serine, the metabolic formation of ceramide was markedly higher and more rapid in RA-differentiated than undifferentiated cells. (c) Inhibitors of ceramide biosynthesis (Fumonisin B1, beta-chloroalanine and L-cycloserine) diminished the extent of the differentiating effect of RA and concomitantly Cer content decreased. (d) The activity of neutral sphingomyelinase increased after addition of RA, maintained high levels in RA-differentiated cells, and returned to the initial levels with removal of RA. (e) Experimental conditions that cause an elevation of ceramide content (treatment with sphingosine or ceramide or C2-ceramide or bacterial sphingomyelinase) inhibited cell proliferation and stimulated neurite outgrowth; dihydro-analogues of sphingosine, ceramide, and C2-ceramide had no effect on differentiation. (f) treatment with Fumonisin B1 completely inhibited sphingosine-induced differentiation. These data suggest a specific bioregulatory function of ceramide in the control of Neuro2a cell growth and differentiation and pose the general hypothesis of a mediator role of ceramide in the differentiation of cells of neural origin.

Very long chain fatty acids in higher animals--a review

Fatty acids with greater than 22 carbon atoms (very long chain fatty acids, VLCFA) are present in small amounts in most animal tissues. Saturated and monoenoic VLCFA are major components of brain, while the polyenoic VLCFA occur in significant amounts in certain specialized animal tissues such as retina and spermatozoa. Biosynthesis of VLCFA occurs by carbon chain elongation of shorter chain fatty acid precursors while beta-oxidation takes place almost exclusively in peroxisomes. Mitochondria are unable to oxidize VLCFA because they lack a specific VLCFA coenzyme A synthetase, the first enzyme in the beta-oxidation pathway. VLCFA accumulate in the tissues of patients with inherited abnormalities in peroxisomal assembly, and also in individuals with defects in enzymes catalyzing individual reactions along the beta-oxidation pathway. It is believed that the accumulation of VLCFA in patient tissues contributes to the severe pathological changes which are a feature of these conditions. However, little is known of the role of VLCFA in normal cellular processes, and of the molecular basis for their contribution to the disease process. The present review provides an outline of the current knowledge of VLCFA including their biosynthesis, degradation, possible function and involvement in human disease.

Ceramide: a new lipid "second messenger"?

1-Alpha, 25-dihydroxyvitamin D3, tumor necrosis factor alpha, and gamma-interferon induce sphingomyelin turnover to ceramide in HL-60 cells. The time frame of the turnover is consistent with ceramide formation being a mediator of growth and differentiation; furthermore, differentiation can be induced by addition of exogenous ceramides alone. Hence, the action of the biologically active form of vitamin D and other nutritionally relevant factors may be mediated by lipid second messenger pathways involving sphingolipids.

Sphingomyelin and derivatives as cellular signals

This comprehensive review was necessitated by recent observations suggesting that sphingomyelin and derivatives may serve second messenger functions. It has attempted to remain true to the theme of cellular signalling. Hence, it has focussed on the lipids involved primarily with respect to their metabolism and properties in mammalian systems. The enzymology involved has been emphasized. An attempt was made to define directions in which signals may be flowing. However, the evidence presented to date is insufficient to conclusively designate the mechanisms of stimulated lipid metabolism. Hence, the proposed pathways must be viewed as preliminary. Further, the biologic functions of these lipids is for the most part uncertain. Thus, it is difficult to presently integrate this sphingomyelin pathway into the greater realm of cell biology. Nevertheless, the present evidence appears to suggest that a sphingomyelin pathway is likely to possess important bioregulatory functions. Hopefully, interest in this novel pathway will grow and allow a more complete understanding of the roles of these sphingolipids in physiology and pathology.

An update of the enzymology and regulation of sphingomyelin metabolism

Sphingomyelin is found in plasma membranes and related organelles (such as endocytic vesicles and lysosomes) of all tissues, as well as in lipoproteins. Abnormalities in sphingomyelin metabolism have been associated with atherosclerosis, cancer and genetically transmitted diseases; however, except for Niemann-Pick disease, little is known about the mechanism for these disorders. Sphingomyelin biosynthesis de novo involves ceramide formation from serine and two mol of fatty acyl-CoA followed by addition of the phosphocholine headgroup. The headgroup appears to come from phosphatidylcholine, but other sources have not been ruled out. Factors that influence the rate of sphingomyelin synthesis include the availability of serine and palmitic acid, plus the relative activities of key enzymes of this pathway. Sphingomyelin turnover involves removal of the headgroup and amide-linked fatty acid by sphingomyelinases and ceramidases, respectively, which have been found in both lysosomes (with acidic pH optima) and plasma membranes (with neutral to alkaline pH optima). The enzymes of sphingomyelin turnover release ceramide and free sphingosine from endogenous substrates, which may have implications for the participation of a sphingomyelin/sphingosine cycle as another 'lipid second messenger' system.

Occurrence of unusual molecular species of sphingomyelin containing 28-34-carbon polyenoic fatty acids in ram spermatozoa

The high levels of very long chain fatty acids found in ram spermatozoa are located almost exclusively in one of two separable species of sphingomyelin. Mass spectral analysis, including fast atom bombardment of the purified sphingomyelin, has shown the fatty acids to have a carbon chain length of between 28 and 34, with between four and six double bonds, and to belong predominantly to the n-3 series.

Dantrolene and mepacrine antagonize the hemolysis of human red blood cells by halothane and bee venom phospholipase A2

Dantrolene is an effective antagonist of anesthesia-induced malignant hyperthermia due to a poorly understood action on skeletal muscle. The present study examines whether the red blood cell can be used as a model to investigate the mechanism of dantrolene action. Halothane (4.7 mM) caused 9% hemolysis of red blood cells. Phospholipase A2 (1 microM) alone caused less than 2% hemolysis, despite high levels (54%) of phosphatidylcholine hydrolysis. Incubation of red blood cells with halothane and phospholipase A2 caused 72% hemolysis. Halothane addition caused 100% hydrolysis of all diacylphosphoglycerides by phospholipase A2, suggesting a mutual potentiation. The major products of phospholipase A2 activity, arachidonic acid and lysophosphatidylcholine, when exogenously added, also greatly increased hemolysis induced by halothane, with arachidonic acid most closely resembling the synergism observed with phospholipase A2. Dantrolene (10 microM) and mepacrine (10 microM) significantly antagonized hemolysis induced by halothane and phospholipase A2 or halothane and exogenously added arachidonic acid and lysophosphatidylcholine. Dantrolene and mepacrine did not antagonize phospholipid hydrolysis or free fatty acid levels. Dantrolene and mepacrine antagonized the synergism between halothane and phospholipase A2 most likely by reducing the lytic action of halothane in the presence of arachidonic acid. The red blood cell is a useful model for studying the antagonism of halothane and phospholipase A2 toxicity by dantrolene and mepacrine.

Turnover of phospholipid fatty acyl chains in cultured neuroblastoma cells: involvement of deacylation-reacylation and de novo synthesis in plasma membranes

Cultured neuroblastoma cells (NIE-115) rapidly incorporated the essential fatty acid, linoleic acid (18:2 (n = 6), into membrane phospholipids. Fatty acid label appeared rapidly (2-10 min) in plasma membrane phospholipids without evidence of an initial lag. Specific activity (nmol fatty acid/mumol phospholipid) was 1.5-2-fold higher in microsomes than in plasma membrane. In these membrane fractions phosphatidylcholine had at least 2-fold higher specific activity than other phospholipids. With 32P as radioactive precursor, the specific activity of phosphatidylinositol was 2-fold higher compared to other phospholipids in both plasma membrane and microsomes. Thus a differential turnover of fatty acyl and head group moieties of both phospholipids was suggested. This was confirmed in dual-label (3H fatty acid and 32P), pulse-chase studies that showed a relatively rapid loss of fatty acyl chains compared to the head group of phosphatidylcholine; the opposite occurred with phosphatidylinositol. A high loss of fatty acyl chain relative to phosphorus indicated involvement of deacylation-reacylation in fatty acyl chain turnover. The patterns of label loss in pulse-chase experiments at 37 and 10 degrees C indicated some independent synthesis and modification of plasma membrane phospholipids at the plasma membrane. Lysophosphatidylcholine acyltransferase and choline phosphotransferase activities were demonstrated in isolated plasma membrane in vitro. Thus, studies with intact cells and with isolated membrane fractions suggested that neuroblastoma plasma membranes possess enzyme activities capable of altering phospholipid fatty acyl chain composition by deacylation-reacylation and de novo synthesis at the plasma membrane itself.

Phospholipid dependence of the neutral sphingomyelinase in rat liver plasma membranes

Investigations have been carried out on the influence of the phospholipid composition of rat liver plasma membranes and of their physico-chemical properties on the activity of membrane-bound neutral sphingomyelinase. The membrane phospholipid composition was modified by the incorporation of different phospholipids into the membrane bilayer by means of lipid transfer proteins, n-butanol delipidation or exogenous sphingomyelinase (Staphylococcus aureus) treatment. The results indicate that the activity of neutral sphingomyelinase in liver plasma membranes depends upon phosphatidyl choline presence in the membrane bilayer and not upon membrane fluidity.

Interindividual heterogeneity of molecular weight of human brain neutral sphingomyelinase determined by radiation inactivation method

The molecular weight (Mr) of the membrane-bound neutral sphingomyelinase from human brain was determined using the radiation inactivation procedure. Previous studies on three human brains suggested a Mr of 165 +/- 25 kDa (J. Neurochem. 1985, 45:630-632). We now report that in another human brain the neutral sphingomyelinase had a Mr of 740 +/- 100 kDa; this higher Mr was not accompanied by differences in enzymatic properties nor heat-stability.