Monensin inhibits synthesis of plasma membrane sphingomyelin by blocking transport of ceramide through the Golgi: evidence for two sites of sphingomyelin synthesis in BHK cells

Metal-Organic Framework-Based Composites for the Detection and Monitoring of Pharmaceutical Compounds in Biological and Environmental Matrices

The production of synthetic drugs is considered a huge milestone in the healthcare sector, transforming the overall health, aging, and lifestyle of the general population. Due to the surge in production and consumption, pharmaceutical drugs have emerged as potential environmental pollutants that are toxic with low biodegradability. Traditional chromatographic techniques in practice are time-consuming and expensive, despite good precision. Alternatively, electroanalytical techniques are recently identified to be selective, rapid, sensitive, and easier for drug detection. Metal-organic frameworks (MOFs) are known for their intrinsic porous nature, high surface area, and diversity in structural design that provides credible drug-sensing capacities. Long-term reusability and maintaining chemo-structural integrity are major challenges that are countered by ligand-metal combinations, optimization of synthetic conditions, functionalization, and direct MOFs growth over the electrode surface. Moreover, chemical instability and lower conductivities limited the mass commercialization of MOF-based materials in the fields of biosensing, imaging, drug release, therapeutics, and clinical diagnostics. This review is dedicated to analyzing the various combinations of MOFs used for electrochemical detection of pharmaceutical drugs, comprising antibiotics, analgesics, anticancer, antituberculosis, and veterinary drugs. Furthermore, the relationship between the composition, morphology and structural properties of MOFs with their detection capabilities for each drug species is elucidated.

Cell lipid metabolism modulators 2-bromopalmitate, D609, monensin, U18666A and probucol shift discoidal HDL formation to the smaller-sized particles: implications for the mechanism of HDL assembly

ATP-binding cassette transporter A1 (ABCA1) mediates formation of disc-shaped high-density lipoprotein (HDL) from cell lipid and lipid-free apolipoprotein A-I (apo A-I). Discoidal HDL particles are heterogeneous in physicochemical characteristics for reasons that are understood incompletely. Discoidal lipoprotein particles similar in characteristics and heterogeneity to cell-formed discoidal HDL can be reconstituted from purified lipids and apo A-I by cell-free, physicochemical methods. The heterogeneity of reconstituted HDL (rHDL) is sensitive to the lipid composition of the starting lipid/apo A-I mixture. To determine whether the heterogeneity of cell-formed HDL is similarly sensitive to changes in cell lipids, we investigated four compounds that have well-established effects on cell lipid metabolism and ABCA1-mediated cell cholesterol efflux. 2-Bromopalmitate, D609, monensin and U18666A decreased formation of the larger-sized, but dramatically increased formation of the smaller-sized HDL. 2-Bromopalmitate did not appear to affect ABCA1 activity, subcellular localization or oligomerization, but induced dissolution of the cholesterol-phospholipid complexes in the plasma membrane. Arachidonic and linoleic acids shifted HDL formation to the smaller-sized species. Tangier disease mutations and inhibitors of ABCA1 activity wheat germ agglutinin and AG 490 reduced formation of both larger-sized and smaller-sized HDL. The effect of probucol was similar to the effect of 2-bromopalmitate. Taking rHDL formation as a paradigm, we propose that ABCA1 mutations and activity inhibitors reduce the amount of cell lipid available for HDL formation, and the compounds in the 2-bromopalmitate group and the polyunsaturated fatty acids change cell lipid composition from one that favors formation of the larger-sized HDL particles to one that favors formation of the smaller-sized species.

Differential partitioning and trafficking of GM gangliosides and cholesterol-rich lipid rafts in thymic and splenic CD4 T cells

The GM gangliosides and cholesterol components of plasma membrane lipid rafts play an important role in the recruitment and signaling of protein receptors in eukaryotic cells. Herein, we have analyzed at the single-cell level the partitioning and intracellular trafficking of GM gangliosides and cholesterol in quiescent (CD4+CD69-) and CD3-activated (CD4+CD69+) thymic and splenic T cells. First, regardless the gender and the quiescent or activated status of T cells, the GM and cholesterol content in cytosol and plasma membrane as well as the expression levels of GM synthase, Sphingomyelin phosphodiestarase 2 and HMG Co-A reductase genes involved in GM and cholesterol synthesis were constantly lower in CD4 thymocytes than in CD4 splenocytes. Second, we detected variations in the balance between GM and cholesterol in plasma membrane depending on aging, and found that deprivation of cellular cholesterol does not necessarily affect the GM content in both quiescent CD4 thymocytes and splenocytes. Third, CD3 stimulation up-regulated the GM and little if any the cholesterol content in both thymic and splenic CD4 T cells, suggesting a cross talk between the CD3 signaling and GM but not cholesterol biosynthesis pathway. Fourth, partitioning and trafficking of GM to the plasma membrane depended on the transport of ceramide precursors from endoplasmic reticulum to Golgi network, as well as on the synthesis, glycosylation and vesicular assembly in trans-Golgi, and less on the cytoskeleton architecture in both quiescent and activated CD4 thymic and splenic T cells. Together, these findings suggest that the differential partitioning and intracellular trafficking of GM and cholesterol in thymic and splenic CD4 T cells may account for the stage of functional maturation.

Cellular cholesterol depletion triggers shedding of the human interleukin-6 receptor by ADAM10 and ADAM17 (TACE)

Interleukin-6 (IL-6) activates cells by binding to the membrane-bound IL-6 receptor (IL-6R) and subsequent formation of a glycoprotein 130 homodimer. Cells that express glycoprotein 130, but not the IL-6R, can be activated by IL-6 and the soluble IL-6R which is generated by shedding from the cell surface or by alternative splicing. Here we show that cholesterol depletion of cells with methyl-beta-cyclodextrin increases IL-6R shedding independent of protein kinase C activation and thus differs from phorbol ester-induced shedding. Contrary to cholesterol depletion, cholesterol enrichment did not increase IL-6R shedding. Shedding of the IL-6R because of cholesterol depletion is highly dependent on the metalloproteinase ADAM17 (tumor necrosis factor-alpha-converting enzyme), and the related ADAM10, which is identified here for the first time as an enzyme involved in constitutive and induced shedding of the human IL-6R. When combined with protein kinase C inhibition by staurosporine or rottlerin, breakdown of plasma membrane sphingomyelin or enrichment of the plasma membrane with ceramide also increased IL-6R shedding. The effect of cholesterol depletion was confirmed in human THP-1 and Hep3B cells and in primary human peripheral blood monocytes, which naturally express the IL-6R. For decades, high cholesterol levels have been considered harmful. This study indicates that low cholesterol levels may play a role in shedding of the membrane-bound IL-6R and thereby in the immunopathogenesis of human diseases.

Synthesis and transport of different sphingomyelin species in rat Sertoli cells

Cellular phospholipids of Sertoli cells from immature rats were labeled with [14C]-choline. Two sphingomyelin bands (SM1 and SM2) were identified by TLC. The incorporation of [14C]-choline over a 45 h period of incubation demonstrated that there are differences in labeling kinetics between SM1 and SM2. The subcellular location of SM1 and SM2 was investigated by accessibility to bacterial sphingomyelinase. The results showed the existence of two SM pools in Sertoli cells, but an equal cellular distribution of SM1 and SM2. SM2 is characterized by a relatively high content of unsaturated fatty acids. The inhibition of vesicular flow by monensin determines a decrease of about 60-70% in incorporation into SM1 and SM2, suggesting the existence of at least two sites of sphingomyelin synthesis. Pulse-chase and time-course experiments indicated a phosphatidylcholine --> SM precursor product relationship and differences in kinetic properties between SM1 and SM2. Resynthesis experiments showed that monensin had only a partial inhibitory effect on SM1 resynthesis, and a second sphingomyelinase treatment demonstrated that the resynthesized fraction reached the outer leaflet of the plasma membrane. The 60-70% inhibition of SM synthesis by monensin showed that the trans-Golgi cisternae and the trans-Golgi network are the most likely sites of bulk SM synthesis, and that about 15% of SM was synthesized in the cis/medial Golgi apparatus. Additionally the results indicated that plasma membrane SM synthase activity could be the site of about 15% of SM synthesis in Sertoli cells.

De novo synthesis and recycling pathways of sphingomyelin in rat Sertoli cells

Sertoli cells from 19-day-old rats have two molecular species of sphingomyelin (SM1 and SM2) with different kinetic characteristics and fatty acid composition. Here, we have studied the incorporation of [14C]-choline and [14C]-palmitic acid into SM in presence or absence of fumonisin B1, an inhibitor of ceramide synthesis, and beta-chloroalanine, an inhibitor of sphinganine synthesis. The contributions of de novo synthesis and recycling pathways were estimated by analysis of the inhibition caused by these drugs. SM1 was synthesized more by sphingosine recycling, and SM2 was synthesized principally by ceramide recycling than SM1. De novo synthesis seems to be important for the two SM types, but our results showed that this pathway is more extensively utilized by SM2. In conclusion, using Sertoli cell cultures, we have shown for the first time that in the same cell different molecular species of SM are synthesized by different pathways.

Sphingolipid transport in eukaryotic cells

Sphingolipids constitute a sizeable fraction of the membrane lipids in all eukaryotes and are indispensable for eukaryotic life. First of all, the involvement of sphingolipids in organizing the lateral domain structure of membranes appears essential for processes like protein sorting and membrane signaling. In addition, recognition events between complex glycosphingolipids and glycoproteins are thought to be required for tissue differentiation in higher eukaryotes and for other specific cell interactions. Finally, upon certain stimuli like stress or receptor activation, sphingolipids give rise to a variety of second messengers with effects on cellular homeostasis. All sphingolipid actions are governed by their local concentration. The intricate control of their intracellular topology by the proteins responsible for their synthesis, hydrolysis and intracellular transport is the topic of this review.

Rapid replenishment of sphingomyelin in the plasma membrane upon degradation by sphingomyelinase in NIH3T3 cells overexpressing the phosphatidylinositol transfer protein beta

In order to study the in vivo function of the phosphatidylinositol transfer protein beta (PI-TPbeta), mouse NIH3T3 fibroblasts were transfected with cDNA encoding mouse PI-TPbeta. Two stable cell lines were isolated (SPIbeta2 and SPIbeta8) in which the levels of PI-TPbeta were increased 16- and 11-fold respectively. The doubling time of the SPIbeta cells was about 1.7 times that of the wild-type (wt) cells. Because PI-TPbeta expresses transfer activity towards sphingomyelin (SM) in vitro, the SM metabolism of the overexpressors was investigated. By measuring the incorporation of [methyl-(3)H]choline chloride in SM and phosphatidylcholine (PtdCho), it was shown that the rate of de novo SM and PtdCho synthesis was similar in transfected and wt cells. We also determined the ability of the cells to resynthesize SM from ceramide produced in the plasma membrane by the action of bacterial sphingomyelinase (bSMase). In these experiments the cells were labelled to equilibrium (60 h) with [(3)H]choline. At relatively low bSMase concentrations (50 munits/ml), 50% of [(3)H]SM in wt NIH3T3 cells was degraded, whereas the levels of [(3)H]SM in SPIbeta cells appeared to be unaffected. Since the release of [(3)H]choline phosphate into the medium was comparable for both wt NIH3T3 and SPIbeta cells, these results strongly suggest that breakdown of SM in SPIbeta cells was masked by rapid resynthesis of SM from the ceramide formed. By increasing the bSMase concentrations to 200 munits/ml, a 50% decrease in the level of [(3)H]SM in SPIbeta cells was attained. During a recovery period of 6 h (in the absence of bSMase) the resynthesis of SM was found to be much more pronounced in these SPIbeta cells than in 50% [(3)H]SM-depleted wt NIH3T3 cells. After 6 h of recovery about 50% of the resynthesized SM in the SPIbeta cells was available for a second hydrolysis by bSMase. When monensin was present during the recovery period, the resynthesis of SM in bSMase-treated SPIbeta cells was not affected. However, under these conditions 100% of the resynthesized SM was available for hydrolysis. On the basis of these results we propose that, under conditions where ceramide is formed in the plasma membrane, PI-TPbeta plays an important role in restoring the steady-state levels of SM.

Advances in the signal transduction of ceramide and related sphingolipids

Recently, the sphingolipid metabolites ceramide, sphingosine, ceramide 1-P, and sphingosine 1-P have been implicated as second messengers involved in many different cellular functions. Publications on this topic are appearing at a rapidly increasing rate and new developments in this field are also appearing rapidly. It is thus important to summarize the results obtained from many different laboratories and from different fields of research to obtain a clearer picture of the importance of sphingolipid metabolites. This article reviews the studies from the last few years and includes the effects of a variety of extracellular agents on sphingolipid signal transduction pathways in different tissues and cells and on the mechanisms of regulation. Sphingomyelin exists in a number of functionally distinct pools and is composed of distinct molecular species. Sphingomyelin metabolites may be formed by many different pathways. For example, the generation of ceramide from sphingomyelin can be catalyzed by at least five different sphingomyelinases. A large variety of stimuli can induce the generation of ceramide, leading to activation or inhibition of various cellular events such as proliferation, differentiation, apoptosis, and inflammation. The effect of ceramide on these physiological processes is due to its many different downstream targets. It can activate ceramide-activated protein kinases and ceramide-activated protein phosphatases. It also activates or inhibits PKCs, PLD, PLA2, PC-PLC, nitric oxide synthase, and the ERK and SAPK/JNK signaling cascades. Ceramide activates or inhibits transcription factors, modulates calcium homeostasis and interacts with the retinoblastoma protein to regulate cell cycle progression. Most of the work in this field has involved the study of ceramide effects, but the roles of the other three sphingomyelin metabolites is now attracting much attention. The complex interactions between signaling components and ceramide and the controls regulating these interactions are now being identified and are presented in this review.

The rate of sphingomyelin synthesis de novo is influenced by the level of cholesterol in cultured human skin fibroblasts

Plasma membrane sphingomyelin (SM) is known to affect the cellular distribution of cholesterol. The aim of this work was to examine how SM homoeostasis in human skin fibroblasts is affected by alterations in the level of cholesterol in the cell. The cellular cholesterol level was decreased by exposing cells to 2-hydroxypropyl-beta-cyclodextrin, and increased by exposing cells to cholesterol-methyl-beta-cyclodextrin inclusion complexes. A lowering of the cellular unesterified cholesterol content by 20% was shown to increase the incorporation of [14C]palmitic acid into SM by 70%. Subsequently, the cellular SM mass was shown to be increased (24% increase after a 24 h period). Since l-cycloserine completely abolished the increased incorporation of [14C]palmitic acid into SM in cholesterol-depleted cells, we concluded that the de novo synthesis of the sphingosine backbone of SM was activated in cholesterol-depleted cells. This conclusion was further verified by performing a cell-free assay of serine C-palmitoyltransferase (SPT) in cholesterol-depleted cells, which showed that the activity of the enzyme was increased by 30% after cholesterol depletion. Most of the newly synthesized SM in cholesterol-depleted cells was susceptible to degradation by sphingomyelinase, indicating that it was transported efficiently to the cell surface. Loading of fibroblasts with cholesterol had essentially the opposite effects on SM homoeostasis to those of cholesterol depletion, i.e. 20-30% decreased incorporation of [14C]palmitic acid into SM and decreased activity of SPT. The results of this study show that cellular cholesterol levels have marked effects on the homoeostasis of SM.

Sphingomyelin synthase, a potential regulator of intracellular levels of ceramide and diacylglycerol during SV40 transformation. Does sphingomyelin synthase account for the putative phosphatidylcholine-specific phospholipase C?

Sphingomyelin synthase (SMS), an enzyme involved in sphingomyelin (SM) and ceramide metabolism, can potentially regulate, in opposite directions, the levels of ceramide and diacylglycerol. In this study SMS activity was investigated in normal and SV40-transformed human lung fibroblasts (WI38). The addition of [3H]C2-ceramide to cells resulted in a time-dependent formation of [3H]C2-SM. At 24 h after treatment, normal WI38 cells cleared 17% of [3H]C2-ceramide producing [3H]C2-SM, which accounted for 13% of total radioactivity. On the other hand, SV40-transformed cells cleared 45% of [3H]C2-ceramide and produced C2-SM, which accounted for 24% of total radioactivity. This enhanced production of C2-SM was also supported by an increase in the total SMS activity of cells (measured in vitro), such that SV40-transformed cells had SMS activity of 222 pmol/mg of protein/h, whereas wild type cells had 78 pmol/mg of protein/h of activity. Additional studies aimed at examining the SMS activity directed at ceramide produced in the plasma membrane. Treatment of cells with exogenous bacterial sphingomyelinase (SMase) for 25 min resulted in cleavage of 90-95% of total SM and the concomitant generation of ceramide. After bacterial SMase treatment, wild type WI38 cells cleared ceramide very slowly (19.2 pmol of ceramide/nmol of phosholipid Pi after 6 h of incubation) and hardly regenerated any SM. On the other hand, SV40-transformed cells cleared ceramide much faster (41.1 pmol/nmol of Pi after 6 h of incubation) and regenerated approximately 80% of the original SM. These results show that the enhanced SMS activity of transformed cells is particularly pronounced when ceramide is produced in the plasma membrane. Finally, several observations led us to consider the relationship of SMS to the "putative" phosphatidylcholine-specific phospholipase C (PC-PLC). We, therefore, tested the effects of D609, a purported PC-PLC-specific inhibitor on the activity of SMS. D609 inhibited SMS activity in vitro. In addition, cellular studies showed that SMS activity was dramatically inhibited by concentrations of D609 used previously to study PC-PLC (10-50 microg/ml). These results suggest SMS as an important biochemical target for D609, and they raise the distinct possibility that many of the roles of PC-PLC, especially in cell transformation, may be attributable to SMS.

The subcellular sites of sphingomyelin synthesis in BHK cells

The subcellular distributions of the enzymes which synthesise sphingomyelin (SM) and glucosylceramide (GluCer) from ceramide have been assessed in BHK cells. On a sucrose density gradient GluCer synthase (a marker of the cis/medial Golgi apparatus) and the trans-Golgi marker galactosyltransferase showed an similar monotonic distribution. In contrast, SM synthase showed two peaks of activity, a minor one which migrated with the Golgi markers and a major one which had a density close to that of plasma membrane markers (sphingomyelin, cholesterol, PtdSer, ganglioside GM3 and alkaline phosphodiesterase). When cell homogenates were treated with digitonin, the sedimentation characteristics of the Golgi markers was largely unaffected whereas the plasma membrane markers and the main peak of SM synthase activity were shifted to higher density. In contrast, when cells were treated with brefeldin A (BFA) the Golgi markers were shifted to higher density but not the plasma membrane markers or the main peak of SM synthase. These results suggest that the bulk of SM synthase activity in BHK cells is not associated with the Golgi cisternae but with a cell compartment which is relatively rich in cholesterol (e.g., plasma membrane, endosomes or trans-Golgi network.) Further experiments in which cells were treated with sphingomyelinase provided evidence that SM synthase activity was in an internal compartment and not at the plasma membrane.

Sphingomyelin synthase is absent from endosomes

Both the Golgi and the endosomes have recently been proposed as the main site of SM-synthase, the enzyme responsible for sphingomyelin (SM) biosynthesis. To settle this confusion, we studied the subcellular distribution of SM-synthase in human liver-derived HepG2 and baby hamster kidney BHK-21 cells. To discriminate between Golgi and endosomes we made use of 3,3-diaminobenzidine (DAB) cytochemistry. Cells were incubated with a conjugate of transferrin (Tf) and horseradish peroxidase (HRP), or with unconjugated HRP, to label the recycling pathway and the complete endocytic pathway (including lysosomes) with peroxidase activity, respectively. After cell homogenization, the peroxidase activity was used to induce a local deposition of DAB-polymer. The total SM-synthase activity was not affected by this procedure, and, in contrast to endosomes labeled with (125)I-Tf, organelles containing SM-synthase did not increase in buoyant density as determined by Percoll density gradient fractionation. Thus, little, if any, SM-synthase localizes to the endocytic pathway of HepG2 and BHK-21 cells. In experiments performed at low temperature to inhibit vesicular transport, we found less than 10% of newly synthesized short-chain SM at the cell surface. We conclude that most SM-synthase activity is present in the Golgi, and to a small extent at the cell surface.

Comparative study of the metabolic pools of sphingomyelin and phosphatidylcholine sensitive to tumor necrosis factor

The metabolism and localization of the pools of sphingomyelin and phosphatidylcholine (PtdCho) which are hydrolyzed upon activation of the sphingomyelin signal transduction pathway were studied in human skin fibroblasts treated with tumor necrosis factor alpha (TNF-alpha). In a first series of experiments, cellular phospholipids were labeled with [3H]choline under conditions that inhibit the vesicular traffic to the plasma membrane. Thus, in human fibroblasts metabolically labeled in the presence of brefeldin A, monensin or at 20 degree C, the arrival of newly synthesized sphingomyelin to the cell surface was prevented, supporting previous conclusions for a vesicular mechanism of sphingomyelin transport to the plasma membrane. Under these conditions, TNF-alpha induced the hydrolysis of PtdCho but did not promote the hydrolysis of 3H-labeled sphingomyelin, suggesting that the sphingomyelin signaling pool resides in a compartment distal to the Golgi apparatus, and possibly in the plasma membrane. TNF was also unable to trigger the breakdown of a radioactive sphingomyelin, [ceramide-3H]sphingomyelin, exogenously added to the cells to label the exoplasmic side of the cell surface. However, TNF caused PtdCho and sphingomyelin degradation in fibroblasts that had been treated with bacterial sphingomyelinase to degrade the sphingomyelin pool of the external leaflet of the plasma membrane. A similar result was obtained at 4 degree C, i.e. under conditions which inhibit endocytosis, thereby excluding the endosomes as a potential site for TNF-induced sphingomyelin hydrolysis. Altogether, these results strongly argue for a localization of the sphingomyelin signaling pool at the inner leaflet of the plasma membrane, but neither in the endolyso-somal nor the Golgi compartments. In addition, when [3H]choline-labeled fibroblasts were treated under non-lytic conditions with bacterial phospholipase C to degrade the external pool of PtdCho, TNF was still able to stimulate the hydrolysis of PtdCho. This demonstrates that the pool of PtdCho involved in TNF-alpha signaling (and which is hydrolyzed concurrently with sphingomyelin to generate diacylglycerol), is not located in the outer leaflet of the plasma membrane.

Sphingomyelin is synthesized at the plasma membrane of oligodendrocytes and by purified myelin membranes: a study with fluorescent- and radio-labelled ceramide analogues

In most cell types sphingomyelin is synthesized predominantly in the cis-medial compartments of the Golgi stacks whereas the contribution of the plasma membrane is much lower. The aim of this study was to assess the contribution of both compartments to the synthesis of sphingomyelin in myelinating cells. Therefore, oligodendrocytes from rat spinal cord were incubated in culture with fluorescently- or radiolabelled ceramides, and the effects of a block in the vesicular flow (monensin, brefeldin A, low temperature) on surface synthesis of sphingomyelin were evaluated. The results indicate that approximately 50% of the sphingomyelin synthase is present at the plasma and myelin membranes of oligodendrocytes.

Inhibition of acyl-CoA:cholesterol acyltransferase in Chinese hamster ovary (CHO) cells by short-chain ceramide and dihydroceramide

The biological activity of ceramide, an intermediate in the synthesis and catabolism of sphingolipids, has been shown to be mimicked by short-chain N-acyl analogues. A potential role for ceramide in modulating cholesterol esterification was investigated using a series of short-chain ceramides and dihydroceramides. Acyl-CoA:cholesterol acyltransferase (ACAT) in CHO cells was inhibited rapidly (< 30 min) and in a dose-dependent fashion by two N-acyl analogues of naturally occurring D-erythro-ceramide, N-acetyl-sphingosine (D-erythro-C2-ceramide) and N-hexanoyl-sphingosine (D-erythro-C6-ceramide). At 10 microM D-erythro-C2-ceramide, esterification of cholesterol was inhibited by 95% in CHO cells grown in delipidated serum, and 80-85% in cells grown in 25-hydroxycholesterol or human low-density lipoprotein (LDL). D-erythro-C2-Ceramide did not inhibit [14C]oleate-labelling of triacylglycerol and phospholipid. Inhibition of cholesterol esterification in cells and isolated membranes required the D-erythro (2S,3R) configuration (the L-threo isomer of C2-ceramide was not inhibitory) and an N-acyl group (sphingosine and sphinganine did not inhibit). DL-erythro-C2-Dihydroceramide was also a potent ACAT inhibitor in isolated membranes (IC50 0.2 microM) and cells indicating lack of requirement for a 4-trans double bond. Consistent with results for C2-ceramides, DL-threo-C2-dihydroceramide was not inhibitory in cells or in vitro. Long-chain ceramide and N-palmitoyl-dihydroceramide did not inhibit ACAT in isolated membranes. Compared to D-erythro-C2-ceramide, D-erythro-C6- and C4-ceramide were slightly weaker inhibitors of ACAT in isolated membranes. Thus, N-acyl chain length could influence inhibition, either by altering the effective concentration of ceramide in membranes or affinity for the ACAT enzyme. Short-chain ceramides and dihydroceramides are the first ACAT inhibitors described with structural similarity to a naturally occurring compound.

Monensin and brefeldin A inhibit high density lipoprotein-mediated cholesterol efflux from cholesterol-enriched cells. Implications for intracellular cholesterol transport

Mechanisms and pathways of excess cholesterol removal from intracellular sites of accumulation to extracellular cholesterol acceptors remain poorly defined. To gain further insights, compounds known to affect cellular protein transport pathways were tested for their effects on high density lipoprotein (HDL)-mediated cholesterol efflux from cultured cells enriched with cholesterol. Monensin, nigericin, and brefeldin A inhibited the ability of HDL to decrease cellular cholesterol esterification, stimulate sterol biosynthesis, and promote the efflux of labeled cholesterol and cholesterol mass from fibroblasts and smooth muscle cells. HDL-mediated decrease in cell cholesterol esterification was inhibited up to 80% by these compounds compared with control incubations over an HDL concentration of 5-100 micrograms/ml and up to 18 h of incubation. Up-regulation of sterol biosynthesis after depletion of cell cholesterol by HDL increased over 10-fold; however, inclusion of monensin or brefeldin A during the incubation completely prevented the increase of sterol biosynthesis by HDL. Efflux of [3H]cholesterol to HDL from prelabeled cells was inhibited up to 40% by these compounds, and this effect persisted when cholesterol esterification was blocked. Similarly, monensin and brefeldin A inhibited up to 50% of HDL-mediated cholesterol mass efflux relative to controls. Treatment of cells with cholesterol oxidase demonstrated an increase of intracellular cholesterol after exposure to monensin or nigericin and to a lesser extent with brefeldin A. These data show that monensin, nigericin, and brefeldin A sequester cholesterol from sites normally available for efflux by HDL. Since these compounds act by disruption of Golgi complex structure and function, a role for this intracellular organelle in transport of cholesterol between intracellular sites and the plasma membrane for eventual removal by extracellular acceptors such as HDL is suggested.

Peripheral nerve sphingomyelin and cerebroside are both formed via two metabolically and kinetically distinct pathways in vivo

We have studied the labeling kinetics of peripheral nerve sphingolipids in vivo. The kinetic analysis of the labeling profiles observed for the various sphingolipids demonstrated that 90% of cerebrosides, but only 30% of sphingomyelin, were synthesized via a de novo synthesized ceramide intermediate following the injection of 1-4 pmol [3H]palmitate into mouse sciatic nerves. The remaining sphingolipid labeling (30% of the total) was due to direct acylation events, using free fatty acids originating from a pool different from those implicated in the de novo ceramide pathway. Direct acylation events ceased within 1 h following substrate administration, while labeling via the ceramide pathway continued through 5 h. The results provide the first in vivo demonstration that the formation of cerebrosides and sphingomyelin in peripheral nerves in situ can be simultaneously assured via two metabolically and kinetically distinct pathways that employ different fatty acid pools.

Is plasma membrane lipid composition defined in the exocytic or the endocytic pathway?

Compared with intracellular membranes, the plasma membrane is rich in cholesterol and sphingomyelin. How does this distinct composition arise? Here David Allan and Karl-Josef Kallen take a critical view of the belief that these lipids arrive at the plasma membrane via vesicular traffic from the Golgi complex and propose instead that they may be accreted in the endocytic recycling pathway.

Synthesis of surface sphingomyelin in the plasma membrane recycling pathway of BHK cells

Sphingomyelin, which has been degraded at the BHK cell surface by exogenous sphingomyelinase, is converted back into sphingomyelin with kinetics similar to those of plasma membrane recycling. Resynthesis of sphingomyelin under these conditions proceeds at a rate about 4-fold higher than normal biosynthesis of sphingomyelin. Neither resynthesis of sphingomyelin nor its return to the surface is inhibited by brefeldin A (BFA), which is a potent blocker of vesicular transport through the Golgi but has no effect on plasma membrane recycling. However, resynthesis of plasma membrane sphingomyelin is greatly decreased in cells undergoing mitosis or energy depletion, where endocytosis is inhibited. We conclude that the main site of surface sphingomyelin synthesis in BHK cells could be in recycling endosomes and not in the Golgi apparatus as proposed previously. We also suggest a model pathway by which cholesterol may reach the plasma membrane via recycling endosomes.

Plasmodium falciparum exports the Golgi marker sphingomyelin synthase into a tubovesicular network in the cytoplasm of mature erythrocytes

This work describes two unusual features of membrane development in a eukaryotic cell. (a) The induction of an extensive network of tubovesicular membranes by the malaria parasite Plasmodium falciparum in the cytoplasm of the mature erythrocyte, and its visualization with two ceramide analogues C5-DMB-ceramide and C6-NBD-ceramide. "Sectioning" of the infected erythrocytes using laser confocal microscopy has allowed the reconstruction of detailed three-dimensional images of this novel membrane network. (b) The stage-specific export of sphingomyelin synthase, a biosynthetic activity concentrated in the Golgi of mammalian cells, to this tubovesicular network. Evidence is presented that in the extracellular merozoite stage the parasite retains sphingomyelin synthase within its plasma membrane. However, intracellular ring- and trophozoite-stage parasites export a substantial fraction (approximately 26%) of sphingomyelin synthase activity to membranes beyond their plasma membrane. Importantly we do not observe synthesis of new enzyme during these intracellular stages. Taken together these results strongly suggest that the export of this classic Golgi enzyme is developmentally regulated in Plasmodium. We discuss the significance of this export and the tubovesicular network with respect to membrane development and function in the erythrocyte cytosol.

Transport and sorting of membrane lipids

The lipid composition of cellular membranes may seem unnecessarily complex. However, the lipid composition of each membrane is carefully regulated by local metabolism and specificity in transport, marking the functional significance for the cell. Recent research has revealed unexpected discoveries concerning the topology of lipid synthesis, specificity in lipid transport, and the function of lipid and protein microdomains in sorting.

The intriguing link between modulation of both multidrug resistance and ligand-toxin conjugate cytotoxicity

Pharmacological agents which possess a chemosensitizing activity (i.e. the ability to modulate the multidrug resistance phenotype) can equally enhance ligand-toxin conjugate cytotoxicity. By confronting results obtained in both fields of research it appears that quite a number of agents, which are structurally unrelated, possess this bilateral effect. We have therefore attempted to provide a brief review of the literature and to discuss a hypothesis by which a common mechanism such as modifications in intracellular vesicle sorting and/or lipid metabolism may be implicated. We believe that these observations may provide clues for future research.