Properties of acid ceramidase from human spleen

Molecular Mechanism of Inhibition of Acid Ceramidase by Carmofur

Human acid ceramidase (AC) is a lysosomal cysteine amidase, which has received a great deal of interest in recent years as a potential target for the development of new therapeutics against melanoma and glioblastoma tumors. Despite the strong interest in obtaining structural information, only the structures of the apo-AC enzyme in its zymogen and activated conformations are available. In this work, the crystal structure of AC in complex with the covalent carmofur inhibitor is presented. Carmofur is an antineoplastic drug containing an electrophilic carbonyl reactive group that targets the catalytic cysteine. This novel structural data explains the basis of the AC inhibition, provides insights into the enzymatic properties of the protein, and is a great aid toward the structure-based drug design of potent inhibitors for AC, providing the detailed mechanism, which has eluded the scientific community for more than 30 years, of carmofur's mysterious 5-fluorouracil-independent antitumor activity.

Structural basis for the activation of acid ceramidase

Acid ceramidase (aCDase, ASAH1) hydrolyzes lysosomal membrane ceramide into sphingosine, the backbone of all sphingolipids, to regulate many cellular processes. Abnormal function of aCDase leads to Farber disease, spinal muscular atrophy with progressive myoclonic epilepsy, and is associated with Alzheimer’s, diabetes, and cancer. Here, we present crystal structures of mammalian aCDases in both proenzyme and autocleaved forms. In the proenzyme, the catalytic center is buried and protected from solvent. Autocleavage triggers a conformational change exposing a hydrophobic channel leading to the active site. Substrate modeling suggests distinct catalytic mechanisms for substrate hydrolysis versus autocleavage. A hydrophobic surface surrounding the substrate binding channel appears to be a site of membrane attachment where the enzyme accepts substrates facilitated by the accessory protein, saposin-D. Structural mapping of disease mutations reveals that most would destabilize the protein fold. These results will inform the rational design of aCDase inhibitors and recombinant aCDase for disease therapeutics.

Acid ceramidase (aCDase) hydrolyzes lysosomal membrane ceramide into sphingosine and its dysfunction leads to a variety of disease phenotypes. Here, the authors present structures of aCDase in its proenzyme and autocleaved forms, which provides insight into its mechanism of action.

Molecular targeting of acid ceramidase: implications to cancer therapy

Increasingly recognized as bioactive molecules, sphingolipids have been studied in a variety of disease models. The impact of sphingolipids on cancer research facilitated the entry of sphingolipid analogues and enzyme modulators into clinical trials. Owing to its ability to regulate two bioactive sphingolipids, ceramide and sphingosine-1-phosphate, acid ceramidase (AC) emerges as an attractive target for drug development within the sphingolipid metabolic pathway. Indeed, there is extensive evidence supporting a pivotal role for AC in lipid metabolism and cancer biology. In this article, we review the current knowledge of the biochemical properties of AC, its relevance to tumor promotion, and its molecular targeting approaches.

Inhibitors of sphingolipid metabolism enzymes

Sphingolipids are a family of lipids that play essential roles both as structural cell membrane components and in cell signalling. The cellular contents of the various sphingolipid species are controlled by enzymes involved in their metabolic pathways. In this context, the discovery of small chemical entities able to modify these enzyme activities in a potent and selective way should offer new pharmacological tools and therapeutic agents.

Design, synthesis and activity as acid ceramidase inhibitors of 2-oxooctanoyl and N-oleoylethanolamine analogues

The synthesis of novel N-acylethanolamines and their use as inhibitors of the aCDase is reported here. The compounds are either 2-oxooctanamides or oleamides of sphingosine analogs featuring a 3-hydroxy-4,5-hexadecenyl tail replaced by ether or thioether moieties. It appears that, within the 2-oxooctanamide family, the C3-OH group of the sphingosine molecule is required for inhibition both in vitro and in cultured cells. Furthermore, although the (E)-4 double bond is not essential for inhibitory activity, the (E) configuration is required, since the analogue with a (Z)-4 unsaturation was not inhibitory. None of the oleamides inhibited the aCDase in vitro. Conversely, with the exception of N-oleoylethanolamine and its analogs with S-decyl and S-hexadecyl substituents, all the synthesized oleamides inhibited the aCDase in cultured cells, although with a relatively low potency. We conclude that novel aCDase inhibitors can evolve from N-acylation of sphingoid bases with electron deficient-acyl groups. In contrast, chemical modification of the N-oleoylsphingosine backbone does not seem to offer an appropriate strategy to obtain aCDase inhibitors.

Interfacial regulation of acid ceramidase activity. Stimulation of ceramide degradation by lysosomal lipids and sphingolipid activator proteins

The lysosomal degradation of ceramide is catalyzed by acid ceramidase and requires sphingolipid activator proteins (SAP) as cofactors in vivo. The aim of this study was to investigate how ceramide is hydrolyzed by acid ceramidase at the water-membrane interface in the presence of sphingolipid activator proteins in a liposomal assay system. The degradation of membrane-bound ceramide was significantly increased both in the absence and presence of SAP-D when anionic lysosomal phospholipids such as bis(monoacylglycero)phosphate, phosphatidylinositol, and dolichol phosphate were incorporated into substrate-bearing liposomes. Higher ceramide degradation rates were observed in vesicles with increased membrane curvature. Dilution assays indicated that acid ceramidase remained bound to the liposomal surface during catalysis. Not only SAP-D, but also SAP-C and SAP-A, were found to be stimulators of ceramide hydrolysis in the presence of anionic phospholipids. This finding was confirmed by cell culture studies, in which SAP-A, -C, and -D reduced the amount of ceramide storage observed in fibroblasts of a patient suffering from prosaposin deficiency. Strong protein-lipid interactions were observed for both SAP-D and acid ceramidase in surface plasmon resonance experiments. Maximum binding of SAP-D and acid ceramidase to lipid bilayers occurred at pH 4.0. Our results demonstrate that anionic, lysosomal lipids are required for efficient hydrolysis of ceramide by acid ceramidase.

Impairment of skin barrier function in NC/Nga Tnd mice as a possible model for atopic dermatitis

BACKGROUND

The pathogenesis and aetiology of atopic dermatitis (AD) remain unclear. Establishment of suitable animal models should aid elucidation of the pathogenesis and development of therapy.

OBJECTIVES

We focused on biophysical and biochemical parameters in the skin of NC/Nga Tnd mice to evaluate similarities to and differences from AD.

METHODS

Biophysical (transepidermal water loss and skin surface conductance) and biochemical parameters (ceramide contents and activity of ceramide-metabolizing enzymes) were measured in NC/Nga Tnd mice in which spontaneous dermatitis appeared under ambient laboratory conditions (ALC).

RESULTS

Biophysical parameters suggested impairment of water retention properties and barrier function. The amount of ceramide in NC/Nga Tnd mice under ALC decreased significantly. These dermatological features resembled those of AD, as did the clinical signs and histological changes.

CONCLUSIONS

The results described here and previous immunological studies on AD suggest that the NC/Nga Tnd mouse may be a suitable model for certain aspects of AD.

Sphingomyelin metabolites in vascular cell signaling and atherogenesis

The atherosclerotic lesion most probably develops through a number of cellular events which implicate all vascular cell types and include synthesis of extracellular proteins, cell proliferation, differentiation and death. Sphingolipids and sphingolipid metabolizing enzymes may play important roles in atherogenesis, not only because of lipoprotein alterations but also by mediating a number of cellular events which are believed to be crucial in the development of the vascular lesions such as proliferation or cell death. Exogenous sphingolipids may mediate various biological effects such as apoptosis, mitogenesis or differentiation depending on the cell type. Moreover, several molecules present in the atherogenic lesion, such as oxidized LDL, growth factors or cytokines, which activate intracellular signaling pathways leading to vascular cell modifications, can stimulate sphingomyelin hydrolysis and generation of ceramide (and other metabolites as sphingosine-1-phosphate). Here we review the potential implication of the sphingomyelin/ceramide cycle in vascular cell signaling related to atherosclerosis, and more generally the role of sphingolipids in the events observed during the atherosclerotic process as cell differentiation, migration, adhesion, retraction, proliferation and death.

Purification and characterization of a neutral ceramidase from mouse liver. A single protein catalyzes the reversible reaction in which ceramide is both hydrolyzed and synthesized

We report here a novel ceramidase that was purified more than 150, 000-fold from the membrane fraction of mouse liver. The enzyme was a monomeric polypeptide having a molecular mass of 94 kDa and was highly glycosylated with N-glycans. The amino acid sequence of a fragment obtained from the purified enzyme was homologous to those deduced from the genes encoding an alkaline ceramidase of Pseudomonas aeruginosa and a hypotheical protein of the slime mold Dictyostelium discoideum. However, no significant sequence similarities were found in other known functional proteins including acid ceramidases of humans and mice. The enzyme hydrolyzed various N-acylsphingosines but not galactosylceramide, sulfatide, GM1a, or sphingomyelin. The enzyme exhibited the highest activity around pH 7.5 and was thus identified as a type of neutral ceramidase. The apparent K(m) and V(max) values for C12-4-nitrobenzo-2-oxa-1, 3-diazole-ceramide and C16-(14)C-ceramide were 22.3 microM and 29.1 micromol/min/mg and 72.4 microM and 3.6 micromol/min/mg, respectively. This study also clearly demonstrated that the purified 94-kDa ceramidase catalyzed the condensation of fatty acid to sphingosine to generate ceramide, but did not catalyze acyl-CoA-dependent acyl-transfer reaction.

Retrovirus-mediated correction of the metabolic defect in cultured Farber disease cells

Farber disease is a rare severe lysosomal storage disorder due to a deficient activity of the enzyme acid ceramidase (AC). Patients have granulomas along with lipid-laden macrophages that accumulate in a number of tissues, leading to multiple diverse clinical symptoms. There is no therapy for the disorder and most patients succumb to the disease in early childhood. The severity of the disease progression seems to correlate with the amount of the accumulated ceramide substrate. Since the cDNA for human AC has been elucidated we sought to establish if genetic transfer of this sequence would lead to enzymatic and, especially, functional correction of the catabolic defect in Farber patient cells. To do this, a novel amphotropic recombinant retrovirus was constructed that engineers transfer of the human AC cDNA. On infection of patient fibroblasts, AC enzyme activity in cell extracts was completely restored. Further, substrate-loading assays of intact living cells showed a fully normalized catabolism of lysosomal ceramide. Lastly, as reported for some other corrected enzymatic defects of lysosomes, metabolic cooperativity was seen, in that functionally corrected patient fibroblasts secreted AC that was taken up through the mannose 6-phosphate receptor and used by uncorrected fibroblasts as well as recipient Farber lymphoblastoid cells. This overall transduction and uptake scenario for Farber disease allows future treatment of this severe disorder to be envisioned using gene transfer approaches.

[14C]ceramide synthesis by sphingolipid ceramide N-deacylase: new assay for ceramidase activity detection

Sphingolipid ceramide N-deacylase (SCDase) is an enzyme which hydrolyzes the N-acyl linkage between fatty acid and sphingosine in ceramide of various glycosphingolipids and sphingomyelin. Recently the enzyme was found to catalyze the hydrolysis and its reverse reaction under different conditions. We report here an innovative method for synthesis of radioisotope-labeled ceramide with high specific activity using the reverse hydrolysis reaction of SCDase. More than 80% of free fatty acid was transferred to sphingosine when 1 nmol [14C]stearic acid and 2 nmol sphingosine were incubated with 5 microU SCDase at 37 degrees C for 20 h in 10 microliters of 25 mM phosphate buffer, pH 7.0, containing 0.3% Triton X-100. Free [14C]fatty acid and sphingosine were easily separated from synthesized [14C]ceramide by using a Sep-Pak Plus Silica and a Sep-Pak CM cartridge, respectively. We also developed a sensitive assay method for ceramidase using the [14C]ceramide prepared. The method consists of separation of the [14C]fatty acid released from [14C]ceramide by the action of enzyme on thin-layer chromatography followed by analysis and quantification with an imaging analyzer (BAS1000). This method was sensitive and qualitative and enabled the detection of ceramidase activity in invertebrates for the first time as well as in several human cancer cell lines.

Use of fumonisin B1 to test the intercellular transfer of sphingosine

Sphingosine, a core unit of sphingolipids, has been shown to mediate intracellular signaling events. It is conceivable that sphingosine could act as an intracellular messenger as well as an intracellular modulator, if sphingosine were to be transferred intercellularly. Murine interleukin-2 (IL-2) dependent T-lymphocyte CTLL cells, murine fibroblasts Swiss 3R3 cells, and murine fibroblast BALB/C A31 cells metabolize exogenously added sphingosine to ceramide. Fumonisin B2, a mycotoxin produced by Fusarium moniliforme, blocks the conversion of sphingosine to ceramide. In the study described here, CTLL cells exhibiting the conversion of sphingosine to ceramide were used as recipient cells, whereas fumonisin B1-treated cells (A31 cells, Swiss 3T3 cells and CTLL cells), in which the conversion was blocked were used as donor cells. To demonstrate intercellular transfer of sphingosine through the conversion pathway of sphingosine to ceramide, fumonisin B1-treated donor cells incorporating radioactive sphingosine were co-incubated with CTLL cells and the ceramide response was examined. These experiments demonstrated that it is possible to prove the intercellular transfer of sphingosine by using different activities in the cellular conversion pathway of sphingosine to ceramide.

A simple method for screening for Farber disease on cultured skin fibroblasts

Farber disease is an inborn lysosomal storage disorder characterized by accumulation of ceramide in the patient's tissues due to the deficient activity of acid ceramidase. Currently, confirmation of the diagnosis is performed in an extremely limited number of laboratories. We therefore developed a procedure which does not require any particular sphingolipid substrate and is based on the quantitation of ceramide levels in cultured skin fibroblasts. In the method we devised, the ceramide present in cellular lipid extracts subjected to mild alkaline hydrolysis was quantified using the commercially available diacylglycerol kinase kit. We show that both primary cultures of skin fibroblasts and SV40-transformed fibroblasts derived from a series of patients with Farber disease exhibit ceramide excess as compared to their normal counterparts (2345-17 153 pmol/mg cell protein in Farber cells vs. 432-1298 pmol/mg cell protein in controls). Use of this simple method should greatly facilitate the biochemical diagnosis of Farber disease.

Neurodegenerative course in ceramidase deficiency (Farber disease) correlates with the residual lysosomal ceramide turnover in cultured living patient cells

Farber's lipogranulomatosis is an inborn lipid storage disease characterized by tissue accumulation of ceramide due to deficient activity of lysosomal ceramidase. Symptoms include painful swelling of joints, subcutaneous nodules, a hoarse cry, hepatosplenomegaly and nervous system dysfunction of markedly variable degree. In most cases the neural dysfunction rather than the general dystrophy, seems to limit the duration of Farber disease. We examined whether the severity can be shown as a function of ceramide turnover by lysosomal ceramidase. The lysosomal degradation of sphingomyelin-derived ceramide was studied in situ in patient skin fibroblasts and lymphoid cells loaded with LDL-associated radioactive sphingomyelin. We could show for the first time a significant correlation between the ceramide accumulated in situ and the severity of Farber disease. Our method provides an alternative means for determining ceramide degradation by lysosomal ceramidase, but in intact cells. The relatively simple method is at least of the same diagnostic use for Farber disease as the in vitro assay of acid ceramidase using cell homogenates and may also have some prognostic use.

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.

The in situ degradation of ceramide, a potential lipid mediator, is not completely impaired in Farber disease

The time course of degradation of a radiolabelled natural ceramide has been studied in intact, living lymphoid cells and skin fibroblasts from normal individuals and from patients affected with Farber disease, an inborn disorder of ceramide metabolism due to deficient activity of lysosomal ceramidase. The hydrolysis of ceramide in lysosomes was selectively followed by examining the turnover of an LDL-associated radioactive sphingomyelin. This permitted to estimate accurately the effective lysosomal ceramidase activity and to demonstrate: (i) a very active catabolism of ceramide in normal cells; and (ii) the absence of a complete block of ceramide degradation in Farber cells. The possible implication of ceramide as a lipid mediator of the pathogenesis of Farber disease is discussed.

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.