Ceramide: a new lipid "second messenger"?

Alterations in β-Cell Sphingolipid Profile Associated with ER Stress and iPLA2β: Another Contributor to β-Cell Apoptosis in Type 1 Diabetes

Type 1 diabetes (T1D) development, in part, is due to ER stress-induced β-cell apoptosis. Activation of the Ca²⁺-independent phospholipase A₂ beta (iPLA₂β) leads to the generation of pro-inflammatory eicosanoids, which contribute to β-cell death and T1D. ER stress induces iPLA₂β-mediated generation of pro-apoptotic ceramides via neutral sphingomyelinase (NSMase). To gain a better understanding of the impact of iPLA₂β on sphingolipids (SLs), we characterized their profile in β-cells undergoing ER stress. ESI/MS/MS analyses followed by ANOVA/Student’s t-test were used to assess differences in sphingolipids molecular species in Vector (V) control and iPLA₂β-overexpressing (OE) INS-1 and Akita (AK, spontaneous model of ER stress) and WT-littermate (AK-WT) β-cells. As expected, iPLA₂β induction was greater in the OE and AK cells in comparison with V and WT cells. We report here that ER stress led to elevations in pro-apoptotic and decreases in pro-survival sphingolipids and that the inactivation of iPLA₂β restores the sphingolipid species toward those that promote cell survival. In view of our recent finding that the SL profile in macrophages—the initiators of autoimmune responses leading to T1D—is not significantly altered during T1D development, we posit that the iPLA₂β-mediated shift in the β-cell sphingolipid profile is an important contributor to β-cell death associated with T1D.

Podocyte Lysosome Dysfunction in Chronic Glomerular Diseases

Podocytes are visceral epithelial cells covering the outer surface of glomerular capillaries in the kidney. Blood is filtered through the slit diaphragm of podocytes to form urine. The functional and structural integrity of podocytes is essential for the normal function of the kidney. As a membrane-bound organelle, lysosomes are responsible for the degradation of molecules via hydrolytic enzymes. In addition to its degradative properties, recent studies have revealed that lysosomes may serve as a platform mediating cellular signaling in different types of cells. In the last decade, increasing evidence has revealed that the normal function of the lysosome is important for the maintenance of podocyte homeostasis. Podocytes have no ability to proliferate under most pathological conditions; therefore, lysosome-dependent autophagic flux is critical for podocyte survival. In addition, new insights into the pathogenic role of lysosome and associated signaling in podocyte injury and chronic kidney disease have recently emerged. Targeting lysosomal functions or signaling pathways are considered potential therapeutic strategies for some chronic glomerular diseases. This review briefly summarizes current evidence demonstrating the regulation of lysosomal function and signaling mechanisms as well as the canonical and noncanonical roles of podocyte lysosome dysfunction in the development of chronic glomerular diseases and associated therapeutic strategies.

Podocyte pathology and nephropathy - sphingolipids in glomerular diseases

Sphingolipids are components of the lipid rafts in plasma membranes, which are important for proper function of podocytes, a key element of the glomerular filtration barrier. Research revealed an essential role of sphingolipids and sphingolipid metabolites in glomerular disorders of genetic and non-genetic origin. The discovery that glucocerebrosides accumulate in Gaucher disease in glomerular cells and are associated with clinical proteinuria initiated intensive research into the function of other sphingolipids in glomerular disorders. The accumulation of sphingolipids in other genetic diseases including Tay-Sachs, Sandhoff, Fabry, hereditary inclusion body myopathy 2, Niemann-Pick, and nephrotic syndrome of the Finnish type and its implications with respect to glomerular pathology will be discussed. Similarly, sphingolipid accumulation occurs in glomerular diseases of non-genetic origin including diabetic kidney disease (DKD), HIV-associated nephropathy, focal segmental glomerulosclerosis (FSGS), and lupus nephritis. Sphingomyelin metabolites, such as ceramide, sphingosine, and sphingosine-1-phosphate have also gained tremendous interest. We recently described that sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) is expressed in podocytes where it modulates acid sphingomyelinase activity and acts as a master modulator of danger signaling. Decreased SMPDL3b expression in post-reperfusion kidney biopsies from transplant recipients with idiopathic FSGS correlates with the recurrence of proteinuria in patients and in experimental models of xenotransplantation. Increased SMPDL3b expression is associated with DKD. The consequences of differential SMPDL3b expression in podocytes in these diseases with respect to their pathogenesis will be discussed. Finally, the role of sphingolipids in the formation of lipid rafts in podocytes and their contribution to the maintenance of a functional slit diaphragm in the glomerulus will be discussed.

An inhibitor of glucosylceramide synthase inhibits the human enzyme, but not enzymes from other organisms

Specific inhibitors of glucosylceramide biosynthesis are used as drugs for the treatment of some human diseases correlated to glycosphingolipid metabolism. The target of the presently available inhibitors is the human glucosylceramide synthase (GCS), but effects on enzymes from other organisms have not been studied. We expressed cDNAs encoding GCS enzymes from lower animals, plants, fungi, and bacteria in the yeast P. pastoris. In vitro GCS assays with the GCS inhibitor D-threo-1-(3',4'-ethylenedioxy)phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol showed that this inhibitor did not affect non-human GCS enzymes.

Glycero- versus sphingo-phospholipids: correlations with human and non-human mammalian lens growth

The human lens differs from other mammalian lenses in its very slow growth and unusual phospholipid composition of its cell membranes. Dihydrosphingomyelins (DHSMs) make up about half of all phospholipids in adult human fiber membranes. In all other membranes, sphingomyelins(SMs) with a trans double bond in their backbone, are prevalent. In our quest to understand the biological implications of such elevated DHSM levels, we analyzed membranes from various regions of human, elephant, giraffe, polar bear, pig and cow lenses. The levels of DHSMs were minor in non-human lens membranes. A strong correlation was observed between growth rate and relative contents of phosphatidylcholines(PCs) in epithelia and outer cortical fibers. Sphingomyelins became increasingly predominant in differentiated fibers and this increase was age dependent. Indeed, nuclear fiber membranes of aged non-human mammals were composed, almost exclusively, of (SMs). Although human lens membranes followed comparable compositional trends, the magnitude of the changes was much smaller. We postulate that the high relative contents of DHSMs provide a biochemically inert matrix in which only small amounts of PCs and SMs and their metabolites, known to promote and arrest growth, respectively, are present. This compositional difference is proposed to contribute to the slow multiplication and elongation of human lens cells.

Modulation of osteopontin post-translational state by 1, 25-(OH)2-vitamin D3. Dependence on Ca2+ influx

In osteoblastic ROS 17/2.8 cells, 1,25-(OH)2-vitamin D3 stimulates transcription of the extracellular matrix phosphoprotein osteopontin (OPN). We now show post-translational regulation of OPN production by 1,25-(OH)2D3. Prior to transcriptional up-regulation of OPN, 1, 25-(OH)2D3 induces a shift in OPN isoelectric point (pI) from 4.6 to 5.1. Loading equal amounts of OPN recovered from ROS 17/2.8 cells exposed to 1,25-(OH)2D3 or carrier for 3 h reveals that the pI shift represents reduced phosphorylation. Trypsin cleavage patterns of OPN produced after 1,25-(OH)2D3 treatment indicates phosphorylation changes in the resulting peptides. Using structural analogs to 1, 25-(OH)2D3, we found that analog AT (25-(OH)-16-ene-23-yne-D3), which triggers Ca2+ influx but does not bind to the vitamin D receptor, mimicked the OPN pI shift, whereas analog BT (1, 25-(OH)2-22-ene-24-cyclopropyl-D3), which binds to the vitamin D receptor without triggering Ca2+ influx, did not. Likewise, inclusion of the Ca2+ channel blocker nifedipine blocks the charge conversion of OPN. Isolation of OPN from rat femurs and tibiae demonstrates the existence of two OPN charge forms in vivo. We conclude that 1,25-(OH)2D3 regulates OPN not only at the transcriptional level, but also modulates OPN phosphorylation state. The latter involves a short term (<3 h) treatment and is associated with membrane-initiated Ca2+ influx.

Sphingomyelin, glycosphingolipids and ceramide signalling in cells exposed to P-fimbriated Escherichia coli

Uropathogenic Escherichia coli attach to epithelial cells through P fimbriae that bind Galalpha1-4Galbeta-oligosaccharide sequences in cell surface glycosphingolipids. The binding of P-fimbriated E. coli to uroepithelial cells causes the release of ceramide, activation of the ceramide signalling pathway and a cytokine response in the epithelial cells. The present study examined the molecular source of ceramide in human kidney A498 cells exposed to P-fimbriated E. coli. Agonists such as TNF-alpha and IL-1beta released ceramide from sphingomyelin by the activation of endogenous sphingomyelinases and hydrolysis of sphingomyelin, and triggered an IL-6 response. P-fimbriated E. coli caused a slight increase in endogenous sphingomyelinase activity, but there was no associated sphingomyelin hydrolysis. Instead, the concentration of galactose-containing glycolipids decreased. We propose that P-fimbriated E. coli differ from other activators of the ceramide pathway, in that release of ceramide is from receptor glycolipids and not from sphingomyelin. Receptor breakdown may be an efficient host defence strategy, as it reduces the concentration of cell surface receptors, releases soluble receptor analogues and activates an inflammatory response.

The influence of dietary lipids on the composition and membrane fluidity of rat hepatocyte plasma membrane

Weanling male Wistar rats were fed for five weeks on standard rat chow (23 g fat/kg diet) or one of four synthetic diets with butterfat, coconut oil, corn oil, or fish oil as the main lipid source (100 g fat/kg diet). In all diets, 10% of the fat was provided as corn oil to prevent essential fatty acid deficiency. Significant differences were observed in the saturated, monounsaturated, and polyunsaturated fatty acid composition, and in the ratio of cholesterol to phospholipid, in the hepatocyte membranes. The fluidity of hepatocyte plasma membranes was assessed using the fluorescence recovery after photobleaching technique and steady-state fluorescence anisotropy of diphenylhexatriene. No significant differences were found in the fluidity of plasma membranes between animals on the different fat diets, despite diet-induced changes in their fatty acid composition. However, the proportion of lipid free to diffuse in the plasma membrane varied with diet, being significantly greater (P < 0.05) in animals fed chow (63.7%), coconut oil (61.5%), and butterfat (57.6%) diets than in those fed the corn oil (47.3%) diet. Animals fed fish oil showed an intermediate (50.0%) proportion of lipid free to diffuse. The data support the hypothesis that dietary lipids can change both the chemical composition and lateral organization (lipid domain structure) of rat hepatocyte plasma membranes.

Phosphatidate phosphohydrolase catalyzes the hydrolysis of ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate

A Mg2+-independent phosphatidate phosphohydrolase was purified from rat liver plasma membranes in two distinct forms, an anionic protein and a cationic protein. Both forms of the enzyme dephosphorylated phosphatidate, ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate. When assayed at a constant molar ratio of lipid to Triton X-100 of 1:500, the apparent Km values of the anionic phosphohydrolase for the lipid substrates was 3.5, 1.9, 0.4, and 4.0 microM, respectively. The relative catalytic efficiency of the enzyme for phosphatidate, ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate was 0.16, 0.14, 0.48, and 0.04 liter (min x mg)-1, respectively. The hydrolysis of phosphatidate was inhibited competitively by ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate. The Ki(app) values were 5.5, 5.9, and 4.0 microM, respectively. The hydrolysis of phosphatidate by the phosphohydrolase conformed to a surface dilution kinetic model. It is concluded that the enzyme is a lipid phosphomonoesterase that could modify the balance of phosphatidate, ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate relative to diacylglycerol, ceramide, monoacylglycerol, and sphingosine, respectively. The enzyme could thus play an important role in regulating cell activation and signal transduction.

Ceramide signalling and the immune response

Ceramide, produced through either the induction of SM hydrolysis or synthesized de novo transduces signals mediating differentiation, growth, growth arrest, apoptosis, cytokine biosynthesis and secretion, and a variety of other cellular functions. A generalized ceramide signal transduction scheme is shown in Fig. 2 in which ceramide is generated through the activation of distinct SMases residing in separate subcellular compartments in response to specific stimuli. Clearly, specificity of cellular responses to ceramide depends upon many factors which include the nature of the stimulus, co-stimulatory signals and the cell type involved. Ceramide derived from neutral SMase activation is thought to be involved in modulating CAPK and MAP kinases, PLA2 (arachidonic acid mobilization), and CAPP while ceramide generated through acid SMase activation appears to be primarily involved in NF-kappa B activation. While there is no apparent cross-talk between these two ceramide-mediated signalling pathways, there is likely to be significant cross-talk between ceramide signalling and other signal transduction pathways (e.g., the PKC and MAP kinase pathways). Other downstream targets for ceramide action include Cox, IL-6 and IL-2 gene expression, PKC zeta, Vav, Rb, c-Myc, c-Fos, c-Jun and other transcriptional regulators. Many, if not all, of these ceramide-mediated signalling events have been identified in the various cells comprising the immune system and are integral to the optimal functioning of the immune system. Although the role of the SM pathway and the generation of ceramide in T and B lymphocytes have only recently been recognized, it is clear from these studies that signal transduction through SM and ceramide can strongly affect the immune response, either directly through cell signalling events, or indirectly through cytokines produced by other cells as the result of signalling through the SM pathway. An overview of the signalling mechanisms coupling ceramide to the modulation of the immune response is depicted in Fig. 3 and shows how ceramide may play pivotal roles in regulating a number of complex processes. The SM pathway represents a potentially valuable focal point for therapeutic control of immune responses, perhaps for either enhancement of the activity of T cells in the elimination of tumors, or the down-regulation of lymphocyte function in instances of autoimmune disease. The recent explosion of knowledge regarding ceramide signalling notwithstanding, a number of critical questions need to be answered before a comprehensive, mechanistic understanding can be formulated relative to the incredibly varied effects of ceramide on cell function. For example, (i) how is a structurally simple molecule like ceramide able to mediate so many different, and sometimes paradoxical, physiological responses ranging from cell proliferation and differentiation to inhibition of cell growth and apoptosis, (ii) what are the molecular identities and modes of activation of the various SMase isoforms, (iii) what determines the distribution of the unique isoforms of SMase in cells of different lineages or at different stages of differentiation, (iv) what is the relative contribution of ceramide generated through SM hydrolysis versus de novo synthesis, and (v) by what means does ceramide interact with specific intracellular targets? Although a number of ceramide-activatable kinases, phosphatases, and their protein substrates have been identified, a more extensive search for additional cellular targets will be indispensable in determining the phosphorylation cascades linking the activation of the SM pathway to the regulation of nuclear events. Clearly, cross-talk between ceramide-induced signal transduction cascades and other signalling pathways adds to the inherent difficulty in distinguishing the specific effects of complex, intertwining signalling pathways.

(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)-D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol (D-erythro-MAPP). Addition of 5 microM D-e-MAPP to HL-60 human promyelocytic leukemia cells resulted in a concentration- and time-dependent growth suppression accompanied by an arrest in the G0/G1 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 ceremidase to an extent similar to that seen with C16-ceramide. D-e-MAPP was not metabolized. Instead, D-e-MAPP inhibited alkaline ceramidase activity in vitro with an IC50 of 1-5 microM. 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 (IC50>500 microM). 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.

C2-ceramide primes specifically for the superoxide anion production induced by N-formylmethionylleucyl phenylalanine (fMLP) in human neutrophils

Activated sphingomyelinases release ceramide molecules believed to be involved in intracellular signalling. The present study investigated whether soluble C2-ceramide modulates some of the effects of N-formylmethionylleucyl phenylalanine (fMLP) and other agonists on human neutrophils (or polymorphonuclear leukocytes-PMN); principally superoxide anion (O2-) production. The preincubation of PMN for 15 min with C2-ceramide increased by up to almost 3-fold the amounts of O2- generated in response to 0.1 and 1 microM fMLP. Priming was detected at C2-ceramide concentrations of 2 microM to 4 microM per million PMN. Though less potent than C2-ceramide, C6-ceramide (N-hexanoylsphingosine) could prime for O2- generated in response to 0.1 microM fMLP, with maximal effects obtained at 10-20 microM. In contrast, micromolar concentrations of sphingosine, dihydroceramide, and ceramide-phosphate, failed to exert any potentiating effect on fMLP-induced O2- generation. As expected, TNF-alpha (1000 U/ml), also primed for fMLP-induced O2- production; however, the combination of TNF-alpha and C2-ceramide showed no additive effect. Moreover, S. aureus sphingomyelinase (0.1 U/ml), was unable to reproduce the priming effects of C2-ceramide and TNF-alpha. C2-ceramide at 2 microM did not enhance the production of O2- induced by 100 nM recombinant human interleukin-8 (IL-8), leukotriene B4 (LTB4), platelet-activating factor (PAF) or 20 mM sodium fluoride (NaF). Furthermore, C2-ceramide (2 microM) did not enhance the mobilization of calcium, the release of arachidonic acid or the accumulation of phosphatidylethanol, induced by 100 nM fMLP. This suggests that probably neither phospholipases C, A2 or D (PLC, PLA2, PLD) were involved in the priming effect by C2-ceramide. However, C2-ceramide inhibited in a dose-related manner the production of O2- induced by phorbol 12-myristate 13-acetate (PMA) and mezerein. Furthermore, PMA-stimulated PLD activity was also significantly reduced by a preincubation of PMN with C2-ceramide. The priming of O2- production by C2-ceramide could involve yet unidentified mechanisms specific for fMLP, or it might imply that cytokines such as TNF-alpha have different mechanisms than C2-ceramide.

Interaction of ceramides, sphingosine, and sphingosine 1-phosphate in regulating DNA synthesis and phospholipase D activity

C2- and C6-ceramides (N-acetylsphingosine and N-hexanoylsphingosine, respectively) abolished the stimulation of DNA synthesis by sphingosine 1-phosphate in rat fibroblasts. This inhibition by ceramide was partially prevented by insulin. C2-ceramide did not alter the stimulation of DNA synthesis by insulin and decreased the sphingosine-induced stimulation by only 16%. The ceramides did not significantly modify the actions of sphingosine or sphingosine 1-phosphate in decreasing cAMP concentrations. C2- and C6-ceramides blocked the activation of phospholipase D by sphingosine 1-phosphate, and this inhibition was not affected by insulin. Okadaic acid decreased the activation of phospholipase D by sphingosine 1-phosphate and did not reverse the inhibitory effect of C2-ceramide on this activation. Therefore, this effect of C2-ceramide is unlikely to involve the stimulation of phosphoprotein phosphatase activity. Sphingosine did not activate phospholipase D activity significantly after 10 min. C2-ceramide stimulated the conversion of exogenous [3H]sphingosine 1-phosphate to sphingosine and ceramide in fibroblasts. Ceramides can inhibit some effects of sphingosine 1-phosphate by stimulating its degradation via a phosphohydrolase that also hydrolyzes phosphatidate. Furthermore, C2- and C6-ceramides stimulated ceramide production from endogenous lipids, and this could propagate the intracellular signal. This work demonstrates that controlling the production of ceramide versus sphingosine and sphingosine 1-phosphate after sphingomyelinase activation could have profound effects on signal transduction.

Purification and biochemical characterization of membrane-bound epidermal ceramidases from guinea pig skin

Ceramidase (CDase) catalyzes the hydrolysis of ceramides to yield sphingosine and fatty acid. In this paper, two forms of membrane-bound alkaline ceramidase, have been, for the first time, purified from guinea pig epidermis by chromatography on DEAE-cellulose, phenyl-Superose, HCA-hyroxyapatite, isoelectric focusing, Mono Q, and TSK-3000SW column. One species (CDase-I) migrated upon SDS-polyacrylamide gel electrophoresis as a single band with an apparent molecular mass of 60 kDa; the other (CDase-II) was only partially purified with apparent M(r) of about 148,000 estimated by gel filtration. The specific activities of the two species increased by 1.130- (for CDase-I) and 400-fold (for CDase-II) over the original tissue extract. The activity of both enzymes for ceramide species decreased in the order of linoleoyl > oleoyl > palmitoylsphingosine. The optimal pH for enzyme activity was approximately 7.0-9.0 for CDase-I and 7.5-8.5 for CDase-II. Interestingly, both enzymes were inhibited by the reaction product sphingosine with a concentration for half-maximal inhibition (ID50) of 100-130 microM, compared to the apparent kinetic parameters with CDase-I (Km = 90 microM, Vmax = 0.62 unit) and CDase-II (Km = 140 microM, Vmax = 0.50 units). Some lipids, such as phosphatidylcholine and sphingomyelin, are also inhibitory with IC50 values of 50-250 microM, suggesting well controlled CDase activity by sphingolipid metabolites. These studies begin to elucidate a regulatory mechanism for the balance of the ratio of ceramide/sphingosine which can serve as an intracellular effector molecule in epidermis.

Evidence that ceramide selectively inhibits protein kinase C-alpha translocation and modulates bradykinin activation of phospholipase D

Sphingomyelinase (SMase) treatment (0.1 unit/ml for up to 30 min) of mouse epidermal (HEL-37) or human skin fibroblast (SF 3155) cells preincubated with [3H]serine to label the sphingomyelin pool caused the accumulation of labeled ceramide but not sphingosine or ceramide 1-phosphate. Incubation of HEL-37 cells with dioctanoylglycerol (diC8) or SF 3155 cells with bradykinin caused translocation of calcium/phosphatidylserine-dependent protein kinase C (PKC) activity to particulate material. In both cell lines the translocation was blocked by SMase treatment of the cells or by incubation with the cell-permeable ceramide analogue N-acetylsphingosine (C2-Cer). Western blot analysis indicated that treatment of HEL-37 cells with diC8 or SF 3155 cells with bradykinin resulted in the translocation of both PKC-alpha and PKC-espilon to particulate material. Treatment with SMase or C2-Cer specifically blocked the translocation of PKC-alpha but not that of PKC-epsilon. Pretreatment of cells with SMase or C2-Cer also inhibited the activation of phospholipase D activity induced by either diC8 (HEL-37 cells) or bradykinin (SF 3155 cells). The data provide strong evidence that ceramide can negatively regulate the translocation of PKC-alpha but not PKC-epsilon and further suggest that PKC-alpha may be involved in regulating phospholipase D activity.

Modulation of cytosolic protein phosphorylation by sphingosylphosphorylcholine

Sphingosylphosphorylcholine (SPC), a putative product of sphingomyelin N-deacylation, has been shown to exert potent mitogenic activity (Desai, N.N. and Spiegel, S. (1991) Biochem. Biophys. Res. Commun. 181, 361-366). In order to explore potential mechanisms of action of SPC, the effects of SPC on endogenous protein phosphorylation was examined in vitro. In cytosol derived from Jurkat T cells, SPC was found to exert dual effects on protein phosphorylation. SPC (10-100 microM) induced the phosphorylation of a number of protein substrates with molecular weights of 32, 35, and 87 kDa. Higher concentrations of SPC (50-200 microM) inhibited the phosphorylation of proteins with estimated molecular weights of 22, 56, and 60 kDa by inhibiting the activity of endogenous protein kinases. Stimulation of protein kinases by SPC required distinct structural features (amino base and the hydrophobic character) from those required for inhibition of protein kinases (the choline phosphate headgroup as well as the hydrophobic character). The SPC-dependent protein kinases were distinct from protein kinase C, cyclic-nucleotide-dependent protein kinases, and calcium-dependent protein kinases, but may be related to casein kinase II. These studies suggest that SPC may act, at least in part, by modulating the activity of endogenous protein kinases.

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

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

Regulation of sphingosine-activated protein kinases: selectivity of activation by sphingoid bases and inhibition by non-esterified fatty acids

Sphingosine has been shown to activate protein kinases in Jurkat T cell cytosol [Pushkareva, Khan, Alessenko, Sahyoun and Hannun (1992) J. Biol. Chem. 267, 15246-15251]. In this study, two sphingosine-activated protein kinases were distinguished by their substrate specificity, their dose-response to sphingosine and the specificity of their activation by sphingosine and dihydrosphingosine stereoisomers. A p32-sphingosine-activated protein kinase responded to low concentrations of D-erythrosphingosine with an initial activation observed at 2.5 microM and a peak activity at 10-20 microM. This kinase showed a modest specificity for D-erythro-sphingosine over other sphingosine stereoisomers, and a preference for sphingosines over dihydrosphingosines. Phosphorylation of a p18 substrate required higher concentrations of sphingosine (20-100 microM) and showed a significant preference for the erythro isomers of sphingosine and dihydrosphingosine over the threo isomers. The ability of other lipids to modulate sphingosine activation of these kinases was also examined. Oleic acid, but not oleic alcohol or the methyl ester, induced the phosphorylation of a distinct set of substrates (probably through the activation of protein kinase C), and inhibited sphingosine-induced phosphorylation with an IC50 of approximately 20 microM. Oleic anhydride failed to induce changes in basal protein phosphorylation but inhibited sphingosine-activated protein kinases, thus distinguishing the effects of fatty acids on protein kinase C from the inhibition of sphingosine-induced phosphorylation. These studies define two distinct sphingosine-activated protein kinases and reveal an important interaction between two classes of putative lipid second messengers.

Distribution and excretion of [14C]fumonisin B1 in male Sprague-Dawley rats

[14C]Fumonisin B1 was biosynthetically produced by the addition of [14C]methyl methionine to a liquid culture of Fusarium moniliforme. The labeled toxin was then administered to rats intragastrically in one study and intravenously in another. The rats were killed at intervals up to 96 hr after dosing. In a third study, rats were dosed intragastrically 3 times at 24 hr intervals, and killed at intervals up to 144 hr after the first dose. After intragastric administration, up to 80 percent of the radiolabel was recovered in feces and up to 3% in urine. The remainder of the radioactivity was distributed in tissues, with the liver, kidney, and blood having the highest percentages. The radioactivity appeared to persist in these tissues for the duration of the experiment. This observation was duplicated in rats dosed intravenously, as well as the fact that urinary excretion of systemic [14C]fumonisin B1 takes place. Also observed during the intravenous study was the elimination of up to 35% of the radiolabel in feces, indicating that fumonisin B1 and/or its metabolites undergoes biliary excretion. The results obtained suggest a portion of the fumonisin B1 that is absorbed is persistent in the target organs, liver and kidney, for up to 96 hr.

Crosstalk among multiple signal-activated phospholipases

Transduction of extracellular signals across the plasma membrane often involves activation of several phospholipases that generate multiple, sometimes interconvertible, lipid-derived messengers. Coordination and integration of these signal-activated phospholipases may require crosstalk between both the messengers and target protein constituents of these pathways.