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

Review of Eukaryote Cellular Membrane Lipid Composition, with Special Attention to the Fatty Acids

Biological membranes, primarily composed of lipids, envelop each living cell. The intricate composition and organization of membrane lipids, including the variety of fatty acids they encompass, serve a dynamic role in sustaining cellular structural integrity and functionality. Typically, modifications in lipid composition coincide with consequential alterations in universally significant signaling pathways. Exploring the various fatty acids, which serve as the foundational building blocks of membrane lipids, provides crucial insights into the underlying mechanisms governing a myriad of cellular processes, such as membrane fluidity, protein trafficking, signal transduction, intercellular communication, and the etiology of certain metabolic disorders. Furthermore, comprehending how alterations in the lipid composition, especially concerning the fatty acid profile, either contribute to or prevent the onset of pathological conditions stands as a compelling area of research. Hence, this review aims to meticulously introduce the intricacies of membrane lipids and their constituent fatty acids in a healthy organism, thereby illuminating their remarkable diversity and profound influence on cellular function. Furthermore, this review aspires to highlight some potential therapeutic targets for various pathological conditions that may be ameliorated through dietary fatty acid supplements. The initial section of this review expounds on the eukaryotic biomembranes and their complex lipids. Subsequent sections provide insights into the synthesis, membrane incorporation, and distribution of fatty acids across various fractions of membrane lipids. The last section highlights the functional significance of membrane-associated fatty acids and their innate capacity to shape the various cellular physiological responses.

Lipid Metabolic Events Underlying the Formation of the Corneocyte Lipid Envelope

Cornified cells of the stratum corneum have a monolayer of an unusual lipid covalently attached to the outer surface. This is referred to as the corneocyte lipid envelope (CLE). It consists of a monolayer of ω-hydroxyceramides covalently attached to the outer surface of the cornified envelope. The CLE is essential for proper barrier function of the skin and is derived from linoleate-rich acylglucosylceramides synthesized in the viable epidermis. Biosynthesis of acylglucosylceramide and its conversion to the cornified envelope is complex. Acylglucosylceramide in the bounding membrane of the lamellar granule is the precursor of the CLE. The acylglucosylceramide in the limiting membrane of the lamellar granule may be oriented with the glucosyl moiety on the inside. Conversion of the acylglucosylceramide to the CLE requires removal of the glucose by action of a glucocerebrosidase. The ester-linked fatty acid may be removed by an as yet unidentified esterase, and the resulting ω-hydroxyceramide may become ester linked to the outer surface of the cornified envelope through action of transglutaminase 1. Prior to removal of ester-linked fatty acids, linoleate is oxidized to an epoxy alcohol through action of 2 lipoxygenases. This can be further oxidized to an epoxy-enone, which can spontaneously attach to the cornified envelope through Schiff's base formation. Mutations of genes coding for enzymes involved in biosynthesis of the CLE result in ichthyosis, often accompanied by neurologic dysfunction. The CLE is recognized as essential for barrier function of skin, but many questions about details of this essentiality remain. What are the relative roles of the 2 mechanisms of lipid attachment? What is the orientation of acylglucosylceramide in the bounding membrane of lamellar granules? Some evidence supports a role for CLE as a scaffold upon which intercellular lamellae unfold, but other evidence does not support this role. There is also controversial evidence for a role in stratum corneum cohesion. Evidence is presented to suggest that covalently bound ω-hydroxyceramides serve as a reservoir for free sphingosine that can serve in communicating with the viable epidermis and act as a potent broad-acting antimicrobial at the skin surface. Many questions remain.

Cutting Edge of the Pathogenesis of Atopic Dermatitis: Sphingomyelin Deacylase, the Enzyme Involved in Its Ceramide Deficiency, Plays a Pivotal Role

Atopic dermatitis (AD) is characterized clinically by severe dry skin and functionally by both a cutaneous barrier disruption and an impaired water-holding capacity in the stratum corneum (SC) even in the nonlesional skin. The combination of the disrupted barrier and water-holding functions in nonlesional skin is closely linked to the disease severity of AD, which suggests that the barrier abnormality as well as the water deficiency are elicited as a result of the induced dermatitis and subsequently trigger the recurrence of dermatitis. These functional abnormalities of the SC are mainly attributable to significantly decreased levels of total ceramides and the altered ceramide profile in the SC. Clinical studies using a synthetic pseudo-ceramide (pCer) that can function as a natural ceramide have indicated the superior clinical efficacy of pCer and, more importantly, have shown that the ceramide deficiency rather than changes in the ceramide profile in the SC of AD patients plays a central role in the pathogenesis of AD. Clinical studies of infants with AD have shown that the barrier disruption due to the ceramide deficiency is not inherent and is essentially dependent on postinflammatory events in those infants. Consistently, the recovery of trans-epidermal water loss after tape-stripping occurs at a significantly slower rate only at 1 day post-tape-stripping in AD skin compared with healthy control (HC) skin. This resembles the recovery pattern observed in Niemann-Pick disease, which is caused by an acid sphingomyelinase (aSMase) deficiency. Further, comparison of ceramide levels in the SC between before and after tape-stripping revealed that whereas ceramide levels in HC skin are significantly upregulated at 4 days post-tape-stripping, their ceramide levels remain substantially unchanged at 4 days post-tape-stripping. Taken together, the sum of these findings strongly suggests that an impaired homeostasis of a ceramide-generating process may be associated with these abnormalities. We have discovered a novel enzyme, sphingomyelin (SM) deacylase, which cleaves the N-acyl linkage of SM and glucosylceramide (GCer). The activity of SM deacylase is significantly increased in AD lesional epidermis as well as in the involved and uninvolved SC of AD skin, but not in the skin of patients with contact dermatitis or chronic eczema, compared with HC skin. SM deacylase competes with aSMase and β-glucocerebrosidase (BGCase) to hydrolyze their common substrates, SM and GCer, to yield their lysoforms sphingosylphosphorylcholine (SPC) and glucosylsphingosine (GSP), respectively, instead of ceramide. Consistently, those reaction products (SPC and GSP) accumulate to a greater extent in the involved and uninvolved SC of AD skin compared with chronic eczema or contact dermatitis skin as well as HC skin. Successive chromatographies were used to purify SM deacylase to homogeneity with a single band of ≈43 kDa and with an enrichment of >14,000-fold. Analysis of a protein spot with SM deacylase activity separated by 2D-SDS-PAGE using MALDI-TOF MS/MS allowed its amino acid sequence to be determined and to identify it as the β-subunit of acid ceramidase (aCDase), an enzyme consisting of α- and β-subunits linked by amino-bonds and a single S-S bond. Western blotting of samples treated with 2-mercaptoethanol revealed that whereas recombinant human aCDase was recognized by antibodies to the α-subunit at ≈56 and ≈13 kDa and the β-subunit at ≈43 kDa, the purified SM deacylase was detectable only by the antibody to the β-subunit at ≈43 kDa. Breaking the S-S bond of recombinant human aCDase with dithiothreitol elicited the activity of SM deacylase with an apparent size of ≈40 kDa upon gel chromatography in contrast to aCDase activity with an apparent size of ≈50 kDa in untreated recombinant human aCDase. These results provide new insights into the essential role of SM deacylase as the β-subunit aCDase that causes the ceramide deficiency in AD skin.

Sphingosine Kinases as Druggable Targets

There is substantial evidence that the enzymes, sphingosine kinase 1 and 2, which catalyse the formation of the bioactive lipid sphingosine 1-phosphate, are involved in pathophysiological processes. In this chapter, we appraise the evidence that both enzymes are druggable and describe how isoform-specific inhibitors can be developed based on the plasticity of the sphingosine-binding site. This is contextualised with the effect of sphingosine kinase inhibitors in cancer, pulmonary hypertension, neurodegeneration, inflammation and sickling.

Cutaneous permeability barrier function in signal transducer and activator of transcription 6-deficient mice is superior to that in wild-type mice

BACKGROUND

Th2 cytokines exhibit a variety of inhibitory effects on permeability barrier function via signal transducer and activator of transcription 6 (STAT6). However, the role of STAT6 signaling on the construction and/or homeostasis of permeability barrier function in the physiological state has not been fully assessed.

OBJECTIVE

We compared permeability barrier function between Stat6-deficient and wild-type C57BL/6 mice at steady state.

METHODS AND RESULTS

Measurement of transepidermal water loss and quantitative penetration assay revealed that permeability barrier function was superior in Stat6-deficient mice. Accordingly, expressions of loricrin, acidic sphingomyelinase (aSMase) and β-glucocerebrosidase (β-GlcCer'ase) in epidermis and ceramide levels in stratum corneum were elevated in STAT6-deficient mice. On the other hands, up-regulations of loricrin, aSMase and β-GlcCer'ase were not observed in 3-dimensionally cultured human keratinocytes transfected with siRNA for STAT6. Meanwhile, number of mast cells in the dermis was decreased in Stat6-deficient mice.

CONCLUSIONS

These results suggest that STAT6 signaling negatively affects permeability barrier function in vivo, even in the physiological state. However, the superior permeability barrier function in Stat6-deficient mice may be a secondary effect exerted via cells other than keratinocytes, such as mast cells, since mast cells are known to influence permeability barrier function in vivo. Blockade of STAT6 signaling might be a strategy to augment the permeability barrier function.

Degradation of glycosphingolipids in oyster: ceramide glycanase and ceramidase in the hepatopancreas of oyster, Crassostrea virginica

The hepatopancreas of oyster, Crassostrea virginica, was found to contain two unique glycosphingolipid (GSL) cleaving enzymes, ceramide glycanase (CGase) and ceramidase. These two enzymes were found to be tightly associated together through the consecutive purification steps including gel filtration, hydrophobic interaction and cation-exchange chromatographies. They were separated only by preparatory SDS-PAGE. The purified CGase was found to have a molecular mass of 52 kDa and pH optimum of 3.2-3.3. This enzyme prefers to hydrolyze the acidic GSLs, II³SO₃LacCer and gangliosides over the neutral GSLs. Oyster ceramidase was found to have a molecular mass of 88 kDa and pH optimum of 4-4.5. Since oyster ceramidase greatly prefers ceramides with C₆ to C₈ fatty acids, C₆-ceramide (N-hexanoyl-D-sphingosine) was used as the substrate for its purification and characterization. The oyster acid ceramidase also catalyzed the synthesis of ceramide from a sphingosine and a fatty acid. For the synthesis, C₁₆ and C₁₈ fatty acids were the best precursors. The amino acid sequences of the two cyanogenbromide peptides derived from the purified ceramidase were found to have similarities to those of several neutral and alkaline ceramidases reported. The tight association of CGase and ceramidase may indicate that CGase in oyster hepatopancreas acts as a vehicle to release ceramide from GSLs for subsequent generation of sphingosines and fatty acids by ceramidase to serve as signaling factors and energy source.

The emerging role of peptides and lipids as antimicrobial epidermal barriers and modulators of local inflammation

Skin is complex and comprised of distinct layers, each layer with unique architecture and immunologic functions. Cells within these layers produce differing amounts of antimicrobial peptides and lipids (sphingoid bases and sebaceous fatty acids) that limit colonization of commensal and opportunistic microorganisms. Furthermore, antimicrobial peptides and lipids have distinct, concentration-dependent ancillary innate and adaptive immune functions. At 0.1-2.0 μM, antimicrobial peptides induce cell migration and adaptive immune responses to coadministered antigens. At 2.0-6.0 μM, they induce cell proliferation and enhance wound healing. At 6.0-12.0 μM, they can regulate chemokine and cytokine production and at their highest concentrations of 15.0-30.0 μM, antimicrobial peptides can be cytotoxic. At 1-100 nM, lipids enhance cell migration induced by chemokines, suppress apoptosis, and optimize T cell cytotoxicity, and at 0.3-1.0 μM they inhibit cell migration and attenuate chemokine and pro-inflammatory cytokine responses. Recently, many antimicrobial peptides and lipids at 0.1-2.0 μM have been found to attenuate the production of chemokines and pro-inflammatory cytokines to microbial antigens. Together, both the antimicrobial and the anti-inflammatory activities of these peptides and lipids may serve to create a strong, overlapping immunologic barrier that not only controls the concentrations of cutaneous commensal flora but also the extent to which they induce a localized inflammatory response.

Inhibitory Effect of Polyphenol-Rich Fraction from the Bark of Acacia mearnsii on Itching Associated with Allergic Dermatitis

We examined the inhibitory effect of polyphenol-rich aqueous extract from the bark of Acacia mearnsii (PrA) on itching associated with atopic dermatitis (AD). HR-1 mice were fed a normal diet, special diet (AD group), or special diet containing 3% PrA (PrA group) for 6 weeks. In the AD group, itching frequency and transepidermal water loss increased compared to the control group. In the PrA group, an improvement in atopic dermatitis symptoms was observed. Ceramide expression in the skin decreased in the AD group compared to the control group, but no decrease was observed in the PrA group. mRNA expression of ceramidase decreased in the PrA group compared to the AD group. The results of this study have revealed that PrA inhibits itching in atopic dermatitis by preventing the skin from drying. It is considered that the mechanism by which PrA prevents the skin from drying involves the inhibition of increased ceramidase expression associated with atopic dermatitis.

Drug targeting of sphingolipid metabolism: sphingomyelinases and ceramidases

Sphingolipids represent a class of diverse bioactive lipid molecules that are increasingly appreciated as key modulators of diverse physiologic and pathophysiologic processes that include cell growth, cell death, autophagy, angiogenesis, and stress and inflammatory responses. Sphingomyelinases and ceramidases are key enzymes of sphingolipid metabolism that regulate the formation and degradation of ceramide, one of the most intensely studied classes of sphingolipids. Improved understanding of these enzymes that control not only the levels of ceramide but also the complex interconversion of sphingolipid metabolites has provided the foundation for the functional analysis of the roles of sphingolipids. Our current understanding of the roles of various sphingolipids in the regulation of different cellular processes has come from loss-of-function/gain-of-function studies utilizing genetic deletion/downregulation/overexpression of enzymes of sphingolipid metabolism (e.g. knockout animals, RNA interference) and from the use of pharmacologic inhibitors of these same enzymes. While genetic approaches to evaluate the functional roles of sphingolipid enzymes have been instrumental in advancing the field, the use of pharmacologic inhibitors has been equally important in identifying new roles for sphingolipids in important cellular processes.The latter also promises the development of novel therapeutic targets with implications for cancer therapy, inflammation, diabetes, and neurodegeneration. In this review, we focus on the status and use of pharmacologic compounds that inhibit sphingomyelinases and ceramidases, and we will review the history, current uses and future directions for various small molecule inhibitors, and will highlight studies in which inhibitors of sphingolipid metabolizing enzymes have been used to effectively treat models of human disease.

Purification and characterization of a second type of neutral ceramidase from rat brain: a second more hydrophobic form of rat brain ceramidase

Ceramidases (CDase) are enzymes that catalyze the hydrolysis of N-acyl linkage of ceramide (Cer) to generate sphingosine and free fatty acids. In this study we report the purification and characterization of a novel second type of neutral ceramidase from rat brain (RBCDase II). Triton X-100 protein extract from rat brain membrane was purified sequentially using Q-Sepharose, HiLoad16/60 Superdex 200pg, heparin-Sepharose, phenyl-Sepharose HP, and Mono Q columns. After Mono Q, the specific activity of the enzyme increased by ~15,000-fold over that of the rat brain homogenate. This enzyme has pH optima of 7.5, and it has a larger apparent molecular weight (110kDa) than the previously purified (90kDa) and characterized neutral rat brain CDase (RBCDase I). De-glycosylation experiments show that the differences in molecular mass of RBCDase I and II on SDS-PAGE are not due to the heterogeneity with N-glycan. RBCDase II is partially stimulated by Ca(2+) and is inhibited by pyrimidine mono nucleotides such as TMP and UMP. This finding is significant as it demonstrates for the first time an effect by nucleotides on a CDase activity. The enzyme was also inhibited by both oxidized and reduced GSH. The effects of metal ions were examined, and we found that the enzyme is very sensitive to Hg(2+) and Fe(3+), while it is not affected by Mn(2+). EDTA was somewhat inhibitory at a 20mM concentration.

Remodeling of sphingolipids by plasma membrane associated enzymes

The sphingolipid plasma membrane content and pattern is the result of several processes, among which the main, in term of quantity, are: neo-biosynthesis in endoplasmic reticulum and Golgi apparatus, membrane turnover with final catabolism in lysosomes and membrane shedding. In addition to this, past and recent data suggest that the head group of sphingolipids can be opportunely modified at the plasma membrane level, probably inside specific membrane lipid domains, by the action of enzymes involved in the sphingolipids metabolism, working directly at the cell surface. The number of membrane enzymes, hydrolases and transferases, acting on membrane sphingolipids is growing very rapidly. In this report we describe some properties of these enzymes.

Effect of topically applied sphingomyelin-based liposomes on the ceramide level in a three-dimensional cultured human skin model

Sphingomyelin-based liposomes were prepared and applied to the stratum corneum side or basal layer side of a three-dimensional (3D) cultured human skin model, and the increase in the type II ceramide (ceramide II) content of the cultured skin model was evaluated. The sphingomyelin-based liposomes were prepared by a high-pressure emulsification method, and the obtained liposomes were characterized; the particle diameter and zeta potential of the liposomes were 155.3 nm and -11.4 mV, respectively. Their spherical shape and lamella structure were observed by transmission electron microscopy. The sphingomyelin-based liposomes or saline were applied to the cultured skin model, and ceramide II was extracted from the skin model. The extracted ceramide II was separated by high-performance thin-layer chromatography and quantified by a densitometer. The amount of ceramide II in the cultured skin model was significantly increased by the application of the sphingomyelin-based liposomes, compared with the nonapplication group. Thus, sphingomyelin-based liposomes are useful for enriching the ceramide level in 3D cultured skin models.

Increase of skin-ceramide levels in aged subjects following a short-term topical application of bacterial sphingomyelinase from Streptococcus thermophilus

Several studies have demonstrated that ceramides play an essential role in both the barrier and water-holding functions of healthy stratum corneum, suggesting that the dysfunction of the stratum corneum associated with ageing as well that observed in patients with several skin diseases could result from a ceramide deficiency. In a previous study our group reported a significant increase in skin ceramide levels in healthy subjects after treatment in vivo with a cream containing a preparation of Streptococcus thermophilus. The presence of high levels of neutral sphingomyelinase activity in this organism was responsible for the observed increase of stratum corneum ceramide levels, thus leading to an improvement in barrier function and maintenance of stratum corneum flexibility. The aim of the present work is to investigate the effects of the topical treatment of a Streptococcus thermophilus-containing cream on ceramide levels of stratum corneum of healthy elderly women. The ceramide levels, transepidermal water loss and capacitance were evaluated on stratum corneum sheets from the forearms of 20 healthy female subjects treated with a base cream or the same cream containing a sonicated preparation of the lactic acid bacterium Streptococcus thermophilus. A 2-week topical application of a sonicated Streptococcus thermophilus preparation led to significant and relevant increase of stratum corneum ceramide levels. Moreover, the hydration values of the treated forearm of each subject was significantly higher than control sites. These results suggest that the experimental cream was able to improve the lipid barrier and to increase a resistance against ageing-associated xerosis.

Thematic review series: skin lipids. Antimicrobial lipids at the skin surface

The skin surface represents our interface with the external environment, and as such, is our first line of defense against microbial colonization and infection. Lipids at the skin surface are thought to underlie at least part of an antimicrobial barrier. Some of these lipids are synthesized in the epidermis and are carried to the surface as cells differentiate, whereas others are secreted onto the surface from the sebaceous glands. One such group, free sphingoid bases, are known to have broad antimicrobial activity, and our previous studies demonstrate their presence at the skin surface. Free sphingoid bases may be generated by enzymatic hydrolysis of epidermal ceramides. In addition, our preliminary results demonstrate potent antibacterial activity associated with two specific fatty acids derived from sebaceous triglycerides. Most remarkably, one of these fatty acids (sapienic acid, C16:1Delta6), in combination with a low concentration of ethanol, is very effective against methicillin-resistant Staphylococcus aureus (MRSA). In fact, this combination was far more effective than mupirocin with or without ethanol. Mupirocin is a "gold standard" for activity against MRSA.

Synthesis of a Novel Ceramide Analogue and its Use in a High-Throughput Fluorogenic Assay for Ceramidases

Several investigations have shown that acid ceramidase inhibitors are potential antiproliferative and cytostatic drugs for cancer chemotherapy. The combinatorial chemistry approach for the discovery of acid ceramidase inhibitors requires the availability of a high-throughput enzyme assay. The synthesis of a novel fluorogenic ceramidase substrate, and its processing both in vitro and in cultured cells in a microtiter plate layout, are reported in this article. This coumarinic substrate was hydrolyzed in vitro (rat liver lysosomes) with Km and Vmax values of 113 microM and 3.6 pmol min-1 mg-1, respectively. Similarly, hydrolysis occurred in intact cultured cells that overexpressed acidic ceramidase. The assay was validated for the identification and characterization of acidic ceramidase inhibitors by using several alpha-ketoamide ceramide analogues, whose inhibitory activity had been previously described.

Kinetic Characteristics of Acidic and Alkaline Ceramidase in Human Epidermis

It has recently become evident that at least five ceramidase (CDase) isoforms are present in human epidermis, and that specifically acidic CDase (aCDase) and alkaline CDase (alkCDase) activities increase during keratinocyte differentiation, and thus might play a pivotal role(s) in permeability barrier function. Prior to investigating their possible roles in the epidermal barrier function, it is necessary to characterize basic kinetic parameters for these enzymes, as well as to determine the effects of the established CDase inhibitors and their activities. In this study, assays for both aCDase and alkCDase activities in fully differentiated human epidermis were optimized using a radiolabeled substrate. These studies revealed that aCDase activity is substantially higher than alkCDase activity, and that both isoenzymes are inhibited by a CDase inhibitor N-oleylethanolamine. These findings were also confirmed using an in situ enzyme assay.

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.

Differentiation-associated expression of ceramidase isoforms in cultured keratinocytes and epidermis

Ceramides (Cers) accumulate within the interstices of the outermost epidermal layers, or stratum corneum (SC), where they represent critical components of the epidermal permeability barrier. Although the SC contains substantial sphingol, indicative of ceramidase (CDase) activity, which CDase isoforms are expressed in epidermis remains unresolved. We hypothesized here that CDase isoforms are expressed within specific epidermal compartments in relation to functions that localize to these layers. Keratinocytes/epidermis express all five known CDase isoforms, of which acidic and alkaline CDase activities increase significantly with differentiation, persisting into the SC. Conversely, neutral and phytoalkaline CDase activities predominate in proliferating keratinocytes. These differentiation-associated changes in isoform activity/protein are attributed to corresponding, differentiation-associated changes in mRNA levels (by quantitative RT-PCR). Although four of the five known CDase isoforms are widely expressed in cutaneous and extracutaneous tissues, alkaline CDase-1 occurs almost exclusively in epidermis. These results demonstrate large, differentiation-associated, and tissue-specific variations in the expression and activities of all five CDase isoforms. Because alkaline CDase-1 and acidic CDase are selectively upregulated in the differentiated epidermal compartment, they could regulate functions that localize to the distal epidermis, such as permeability barrier homeostasis and antimicrobial defense.

Acid and neutral sphingomyelinase, ceramide synthase, and acid ceramidase activities in cutaneous aging

In aged skin, decreased levels of stratum corneum ceramides have been described. Epidermal ceramides are generated by sphingomyelin hydrolysis or synthesis from sphingosin and fatty acids and are degraded by ceramidase. We recently showed that epidermal acid sphingomyelinase (A-SMase) generates ceramides with structural function in the stratum corneum lipid bilayers, which provide for the permeability barrier function of the skin. Here, we examined the activities of epidermal A-SMase, ceramide synthase, and ceramidase in chronologically aged versus young hairless mouse skin. We found reduced A-SMase and ceramide synthase activities in the epidermis of aged mice. However, studies on enzyme localization revealed unchanged, ongoing high A-SMase activity in the outer epidermis, which correlated with reported normal barrier function found in aged skin under basal conditions. Reduced A-SMase and ceramide synthase activity was noted in the inner epidermis, correlating with reduced capacity for permeability barrier repair in aging. Ceramidase activity was not age dependent. In summary, we found reduced activities of ceramide-generating SMase and ceramide synthase in the inner epidermis of aged skin, explaining its reduced capacity in barrier repair. In contrast, A-SMase activity in the outer epidermis was unchanged, indicating that this enzyme is crucially involved in basal permeability barrier homeostasis.

Interleukin-4 suppresses the enhancement of ceramide synthesis and cutaneous permeability barrier functions induced by tumor necrosis factor-alpha and interferon-gamma in human epidermis

Ceramide is an integral part of the extracellular lipid bilayer of the stratum corneum (SC) that forms the permeability barrier of the skin. The production of SC ceramides is catalyzed by sphingomyelinase (SMase) and glucocerebrosidase (GCase). Acid-ceramidase (acid-CDase) catalyzes the hydrolysis of ceramide in the SC. We examined the effects of T helper (Th)1 and Th2 cytokines on levels of transcripts of genes for acid-CDase, acid-SMase, and GCase, on levels of ceramide, and on the extent of transepidermal water loss (TEWL) in the human epidermis in an effort to determine whether these cytokines affect the permeability barrier functions. Levels of transcripts for acid-SMase and GCase and the amount of ceramide in human epidermal sheets were enhanced by tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma and these effects were inhibited in the presence of interleukin (IL)-4. In epidermal keratinocytes cultured under submerged conditions, however, no similar inhibitory effects of IL-4 were observed. Consistent with these results, TEWL was suppressed by TNF-alpha and IFN-gamma, and these effects were also inhibited by IL-4. The balance between Th1 and Th2 might affect the construction and/or the repair of the epidermal permeability barrier via regulation of the production of ceramide.

Analogs of the dihydroceramide desaturase inhibitor GT11 modified at the amide function: synthesis and biological activities

Dihydroceramide desaturase is the last enzyme in the biosynthesis of ceramide de novo. The cyclopropene-containing sphingolipid GT11 is a competitive inhibitor of dihydroceramide desaturase. The biological effects of chemical modification of the GT11 amide linkage are reported in this article. Either N-methyl substitution or replacement of the amide alpha-carbonyl methylene by oxygen result in inactive compounds. In contrast, both urea (3) and thiourea (4) analogs of GT11, as well as three alpha-ketoamides (5-7), did inhibit the desaturation of N-octanoylsphinganine to N-octanoylsphingosine, although with significantly lower potency than GT11. Furthermore, the alpha-ketoamides 5-7 inhibit the acidic ceramidase with similar potencies (IC50 52-83 microM). Inhibition of the neutral/alkaline ceramidase by these compounds requires around 20-fold higher concentrations. Structure-activity relationships and the biological interest of these compounds are discussed.

Effect of the lactic acid bacterium Streptococcus thermophilus on stratum corneum ceramide levels and signs and symptoms of atopic dermatitis patients

A reduced amount of total ceramides could be responsible for functional abnormalities of the skin of atopic dermatitis (AD) patients. The ability of an experimental cream containing sonicated Streptococcus thermophilus to increase skin ceramide levels in healthy subjects has been previously reported. The aim of the present work was to investigate the effects of the topical administration of a S. thermophilus-containing cream on ceramide levels of stratum corneum from AD patients. A 2-week application of the cream, containing a sonicated preparation of the lactic acid bacterium S. thermophilus, in the forearm skin of 11 patients led to a significant and relevant increase of skin ceramide amounts, which could have resulted from the sphingomyelin hydrolysis through the bacterial sphingomyelinase. Moreover, in all patients the topical application of our experimental cream also resulted in the improvement of the signs and symptoms characteristic of AD skin (i.e. erythema, scaling, pruritus).

Neutral ceramidase gene: role in regulating ceramide-induced apoptosis

The sphingolipid, ceramide, is a natural dietary constituent and a potent mediator of apoptosis. If left undegraded, it may induce apoptosis and cause disruption of cellular integrity. A potential mechanism to prevent ceramide-induced apoptosis in various organs may involve ceramidases that facilitate the degradation of ceramide. In this study, we first isolated and characterized the murine neutral ceramidase (N-CDase) gene, mapped its chromosomal location and determined its developmental and organ-specific expression. Then we used cultured mesangial cells as our in vitro model and mouse gastrointestinal (GI) tract as the in vivo model to determine the effects of an inhibitor of N-CDase, D-erythro-MAPP, to delineate whether N-CDase plays a role in preventing ceramide-induced apoptosis. Our results show that: (i) the structure of the murine neutral ceramidase gene is virtually identical to that of the human gene; (ii) it is localized on chromosome 19 at bands C2-C3 that is syntenic to human chromosome 10q24-26; (iii) N-CDase expression is developmentally regulated and it is expressed at high levels in cultured mesangial cells and in specific regions of the mouse small intestine; (iv) inhibition of N-CDase by D-erythro-MAPP leads to increased ceramide levels and consequent apoptosis in cultured mesangial cells; (v) mice treated with D-erythro-MAPP alone also caused apoptosis in the small intestine; and (vi) mice treated with D-erythro-MAPP prior to feeding C2 ceramide manifest markedly elevated levels of apoptosis in the GI tract raising the possibility that neutral ceramidase plays a detoxifying role against inadvertent stimulation of ceramide-induced apoptosis in organs that come in contact with this sphingolipid. We propose that N-CDase is an essential component of an innate detoxifying mechanism to prevent ceramide-induced apoptosis.

Molecular cloning and characterization of the alkaline ceramidase from Pseudomonas aeruginosa PA01

Ceramidase (CDase) hydrolyzes the amide bond in ceramides to yield free fatty acid and sphingosine. From a 3-L Pseudomonas aeruginosa PA01 culture, 70 microg of extracellular alkaline, Ca(2+)-dependent CDase, was purified to homogeneity, the N-terminal sequence was determined, and the CDase gene was cloned. The CDase gene encodes a 670 amino acid protein with a 26 amino acid signal peptide. CDase was expressed in five prokaryotic and eukaryotic expression systems. Small amounts of recombinant active extracellular CDase were expressed by Pseudomonas putida KT2440. In Pichia pastoris GS115 low amounts of recombinant extracellular glycosylated CDase were expressed. High levels of intracellular CDase were expressed by Escherichia coli DH5alpha and E. coli BL21 cells under control of the lac-promoter and T7-promoter, respectively. From a 3-L E. coli DH5alpha culture, 280 microg of pure CDase was obtained after a three-step purification protocol. Under control of the T7-promotor CDase, without its signal peptide, was produced in inclusion bodies in E. coli BL21 cells. After refolding, 1.8 mg of pure active CDase was obtained from a 2.4-L culture after ammonium sulfate precipitation and gel filtration. Both the recombinant and wild-type CDases have a pH optimum of 8.5. The recombinant enzyme was partially characterized. This is the first report of a high yield CDase production system allowing detailed characterization of the enzyme at the molecular level.

Abnormal expression of the novel epidermal enzyme, glucosylceramide deacylase, and the accumulation of its enzymatic reaction product, glucosylsphingosine, in the skin of patients with atopic dermatitis

To clarify mechanisms underlying acylceramide deficiency as an causative factor of the permeability barrier disruption seen in the skin of patients with atopic dermatitis (AD), we hypothesized and then demonstrated the presence of a novel epidermal enzyme, termed glucosylceramide (GC) deacylase. This enzyme hydrolyzes (acyl)GC at the N-acyl site to yield its lysoform, glucosylsphingosine (GS), instead of the formation of (acyl)ceramides by beta-glucocerebrosidase. Assays of enzymatic activity using [palmitic acid-(14)C] GC as a substrate revealed that extracts from the stratum corneum and from the epidermis (but not from the dermis) of patients with AD have the significantly higher potential to hydrolyze GC at the N-acyl site to release (14)C-labeled free fatty acid than of healthy controls. To determine the in vivo physiologic function of this novel enzyme, we measured the metabolic product GS in the upper stratum corneum. In both the involved and the uninvolved stratum corneum from patients with AD, there were significant increases in the amounts of GS compared with healthy controls and there was a significant inverse correlation with the decreased content of ceramides or ceramide-1 (acylceramide). Thus, collectively these results strongly suggest the physiologic relevance of GC deacylase to the acylceramide deficiency seen in the stratum corneum of patients with AD.

Decreased levels of sphingosine, a natural antimicrobial agent, may be associated with vulnerability of the stratum corneum from patients with atopic dermatitis to colonization by Staphylococcus aureus

The stratum corneum of the skin of patients with atopic dermatitis is highly susceptible to colonization by various bacteria, including Staphylococcus aureus. The defense system of the skin against bacterial invasion appears to be significantly disrupted in atopic dermatitis skin, but little is known about the defense mechanism(s) involved. As one sphingolipid metabolite, sphingosine is known to exert a potent antimicrobial effect on S. aureus at physiologic levels, and it may play a significant role in bacterial defense mechanisms of healthy normal skin. Because of the altered ceramide metabolism in atopic dermatitis, the possible alteration of sphingosine metabolism might be associated with the acquired vulnerability to colonization by S. aureus in patients with atopic dermatitis. In this study, we measured the levels of sphingosine in the upper stratum corneum from patients with atopic dermatitis, and then compared that with the colonization levels of bacteria in the same subjects. Levels of sphingosine were significantly downregulated in uninvolved and in involved stratum corneum of patients with atopic dermatitis compared with healthy controls. This decreased level of sphingosine was relevant to the increased numbers of bacteria including S. aureus present in the upper stratum corneum from the same subjects. This suggests the possibility that the increased colonization of bacteria found in patients with atopic dermatitis may result from a deficiency of sphingosine as a natural antimicrobial agent. As for the mechanism involved in the decreased production of sphingosine in atopic dermatitis, analysis of the activities of ceramidases, major sphingosine-producing enzymes, revealed that, whereas the activity of alkaline ceramidase did not differ between patients with atopic dermatitis and healthy controls, the activity of acid ceramidase was significantly reduced in patients with atopic dermatitis and this had obvious relevance to the increased colonization of bacteria in those subjects. Further, there was a close correlation between the level of sphingosines and acid ceramidase (r = 0.65, p < 0.01) or ceramides (r = 0.70, p < 0.01) in the upper stratum corneum from the same patients with atopic dermatitis. Collectively, our results suggest the possibility that vulnerability to bacterial colonization in the skin of patients with atopic dermatitis is associated with reduced levels of a natural antimicrobial agent, sphingosine, which results from decreased levels of ceramides as a substrate and from diminished activities of its metabolic enzyme, acid ceramidase.

Synthesis of a novel fluorescent ceramide analogue and its use in the characterization of recombinant ceramidase from Pseudomonas aeruginosa PA01

Ceramidase (CDase) hydrolyses the N-acyl linkage of the sphingolipid ceramide. We synthesized the non-fluorescent ceramide analogue (4E,2S,3R)-2-N-(10-pyrenedecanoyl)-1,3,17-trihydroxy-17-(3,5-dinitrobenzoyl)-4-heptadecene (10) that becomes fluorescent upon hydrolysis of its N-acyl bond. This novel substrate was used to study several kinetic aspects of the recombinant CDase from the pathogenic bacterium Pseudomonas aeruginosa PA01. Maximum CDase activity was observed above 1.5 microM substrate, with an apparent K(m) of 0.5+/-0.1 microM and a turnover of 5.5 min(-1). CDase activity depends on divalent cations without a strong specificity. CDase is inhibited by sphingosine and by several sphingosine analogues. The lack of inhibition by several mammalian CDase inhibitors such as D-erythro-MAPP, L-erythro-MAPP or N-oleoylethanolamine points to a novel active site and/or substrate binding region. The CDase assay described here offers the opportunity to develop and screen for specific bacterial CDase inhibitors of pharmaceutical interest.

Human acid ceramidase: processing, glycosylation, and lysosomal targeting

The biosynthesis of human acid ceramidase (hAC) starts with the expression of a single precursor polypeptide of approximately 53-55 kDa, which is subsequently processed to the mature, heterodimeric enzyme (40 + 13 kDa) in the endosomes/lysosomes. Secretion of hAC by either fibroblasts or acid ceramidase cDNA-transfected COS cells is extraordinarily low. Both lysosomal targeting and endocytosis critically depend on a functional mannose 6-phosphate receptor as judged by the following criteria: (i) hAC-precursor secretion by NH(4)Cl-treated fibroblasts and I-cell disease fibroblasts, (ii) inhibition of the formation of mature heterodimeric hAC in NH(4)Cl-treated fibroblasts or in I-cell disease fibroblasts, and (iii) blocked endocytosis of hAC precursor by mannose 6-phosphate receptor-deficient fibroblasts or the addition of mannose 6-phosphate. The influence of the six individual potential N-glycosylation sites of human acid ceramidase on targeting, processing, and catalytic activity was determined by site-directed mutagenesis. Five glycosylation sites (sites 1-5 from the N terminus) are used. The elimination of sites 2, 4, and 6 has no influence on lysosomal processing or enzymatic activity of recombinant ceramidase. The removal of sites 1, 3, and 5 inhibits the formation of the heterodimeric enzyme form. None of the mutant ceramidases gave rise to an increased rate of secretion, suggesting that lysosomal targeting does not depend on one single carbohydrate chain.

Structural requirements of ceramide and sphingosine based inhibitors of mitochondrial ceramidase

The effects of structural analogues of ceramide on rat brain mitochondrial ceramidase (mt-CDase) were investigated. Design of target compounds was mainly based on modifications of the key elements in ceramide and sphingosine, including stereochemistry, the primary and secondary hydroxyl groups, the trans double bond in the sphingosine backbone, and the amide bond. Mt-CDase was inhibited by (1) all stereoisomers of D-erythro-ceramide (D-e-Cer) with an IC50 of 0.11, 0.21, and 0.26 mol % for the L-threo, D-threo, and L-erythro isomers, respectively; (2) all stereoisomers of sphingosine with IC50 ranging from 0.04 to 0.14 mol %, N-methyl-D-erythro-sphingosine (N-Me-Sph, IC50 0.13 mol %); and (3) D-erythro-urea-C16-ceramide (C16-urea-Cer IC50 0.33 mol %). The enzyme was not inhibited by N-methyl ceramide (N-Me-C16-Cer), 1-O-methyl ceramide (1-O-Me-C16-Cer), 3-O-methyl ceramide (3-O-Me-C16-Cer), cis-D-erythro ceramide (cis-D-e-C16-Cer) and 3-O-methyl-D-erythro-sphingosine (3-O-Me-Sph). It was less potently inhibited by D-erythro-sphinganine (D-e-dh-Sph, IC50 0.20 mol %), D-erythro-dehydro sphingosine (D-e-deh-Sph, IC50 0.25 mol %), (2S)-3-keto-sphinganine (3-keto-dh-Sph, IC50 0.34 mol %), (2S) 3-keto-ceramide (3-keto-C16-Cer, IC50 0.60 mol %), and ceramine (C18-ceramine, IC50 0.62 mol %), 1-O-methyl-D-erythro-sphingosine (1-O-Me-Sph), cis-D-erythro-sphingosine (cis-D-e-Sph), (2S)-3-ketosphingosine (3-keto-Sph), (2S)-3-keto-dehyrosphingosine (3-keto-deh-Sph), and N,N-dimethyl-D-erythrosphingosine (N,N-diMe-Sph) were weak inhibitors whereas ceramide-1-phosphate (Cer-1-P) and sphingosine-1-phosphate (Sph-1-P) stimulated the enzyme. Thus, for inhibition, the enzyme requires the primary and secondary hydroxyl groups, the C4-C5 double bond, the trans configuration of this double bond, and the NH-protons from either the amide of ceramide or the amine of sphingosine. Therefore, these results provide important information on the requirements for ceramide-enzyme interaction, and they suggest that ligand interaction with the enzyme occurs in a high affinity low specificity manner, in contrast to catalysis which is highly specific for D-erythro-ceramide (D-e-Cer) but occurs with a lower affinity. In addition, this study identifies two competitive inhibitors of mt-CDase; urea-ceramide (C16-urea-Cer) and ceramine (C18-ceramine) that may be further developed and used to understand the mechanism of mt-CDase in vitro and in biologic responses.

Evidence for specific ceramidase present in the intestinal contents of rats and humans

A neutral ceramidase activity stimulated by bile salt was previously identified in the intestinal content. Recently, bile salt stimulated lipase (BSSL) was found to have ceramidase activity. It is unknown whether the ceramidase activity previously found is attributable to BSSL. To address this question, we compared the behaviors of high quaternary aminoethyl (HQ) anion exchange chromatography, the distributions, the stability, and the responses to lipase inhibitor between ceramidase and pancreatic BSSL. The proteins from whole small intestinal contents of humans and rats were precipitated by acetone and dissolved in 20 mM Tris buffer pH 8.2. These proteins had neutral ceramidase activity but not BSSL activity against p-nitrophenyl acetate. When the proteins were subject to HQ chromatography, two peaks of ceramidase activity were identified, which had acid and neutral pH optima, respectively. Neither of them had BSSL activity against p-nitrophenyl acetate. Western blot using BSSL antiserum failed to identify BSSL protein in the fractions with high neutral ceramidase activity. In rat intestinal tract, pancreatic BSSL activity was high in the duodenum and declined rapidly in the small intestine, whereas neutral ceramidase activity was low in the duodenum and maintained a high level until the distal part of the small intestine. In addition, orlistat, the inhibitor of lipase, abolished human BSSL activity against p-nitrophenyl acetate and slightly reduced its activity against ceramide but had no inhibitory effect on ceramidase activity isolated by HQ chromatography. In conclusion, we provide the evidence for a specific ceramidase other than pancreatic BSSL present in the intestinal content. The enzyme may play important roles in digestion of dietary sphingolipids.

Purification, characterization, molecular cloning, and subcellular distribution of neutral ceramidase of rat kidney

Previously, we reported two types of neutral ceramidase in mice, one solubilized by freeze-thawing and one not. The former was purified as a 94-kDa protein from mouse liver, and cloned (Tani, M., Okino, N., Mori, K., Tanigawa, T., Izu, H., and Ito, M. (2000) J. Biol. Chem. 275, 11229--11234). In this paper, we describe the purification, molecular cloning, and subcellular distribution of a 112-kDa membrane-bound neutral ceramidase of rat kidney, which was completely insoluble by freeze-thawing. The open reading frame of the enzyme encoded a polypeptide of 761 amino acids having nine putative N-glycosylation sites and one possible transmembrane domain. In the ceramidase overexpressing HEK293 cells, 133-kDa (Golgi-form) and 113-kDa (endoplasmic reticulum-form) Myc-tagged ceramidases were detected, whereas these two proteins were converted to a 87-kDa protein concomitantly with loss of activity when expressed in the presence of tunicamycin, indicating that the N-glycosylation process is indispensable for the expression of the enzyme activity. Immunohistochemical analysis clearly showed that the ceramidase was mainly localized at the apical membrane of proximal tubules, distal tubules, and collecting ducts in rat kidney, while in liver the enzyme was distributed with endosome-like organelles in hepatocytes. Interestingly, the kidney ceramidase was found to be enriched in the raft microdomains with cholesterol and GM1 ganglioside.

High-expression of sphingomyelin deacylase is an important determinant of ceramide deficiency leading to barrier disruption in atopic dermatitis

We have previously demonstrated that there is abnormal expression of sphingomyelin (SM) deacylase-like enzyme in the epidermis of patients with atopic dermatitis (AD), which results in decreased levels of ceramides in their involved and uninvolved stratum corneum. For quantitation of the expression of SM deacylase in AD, we synthesized 16-(9-anthroyloxy) hexadecanoylsphingosylphosphorylcholine or [palmitic acid-14C] SM and used them as substrates to directly measure the activity of SM deacylase by detecting the release of labeled free fatty acid. Direct enzymatic measurements demonstrated that stratum corneum from lesional forearm skin (volar side) of AD patients has an extremely high SM deacylase activity that is at least five times higher than in the stratum corneum from healthy controls. In stratum corneum from nonlesional skin of AD patients, SM deacylase activity is still at least three times higher than in healthy controls. In contrast, stratum corneum from contact dermatitis patients shows levels of SM deacylase similar to healthy controls. In extracts of whole epidermis biopsies from AD patients, SM deacylase activities are significantly (3-fold) increased over healthy controls in the particulate fraction, whereas there is no significant difference in the activity of sphingomyelinase between AD and healthy control. In peripheral blood lymphocytes of AD patients, there is no increase in activity compared with healthy controls, indicating a possibility that the high expression of SM deacylase is highly associated with the skin of AD patients. These findings suggest that, in contrast to changes in sphingolipid metabolism due to aging, the hitherto undiscovered enzyme SM deacylase, is highly expressed in the epidermis of AD patients, and competes with sphingomyelinase or beta-glucocerebrosidase for the common substrate SM or glucosylceramide, which leads to the ceramide deficiency of the stratum corneum in AD.

Cloning of an alkaline ceramidase from Saccharomyces cerevisiae. An enzyme with reverse (CoA-independent) ceramide synthase activity

Ceramide is not only a core intermediate of sphingolipids but also an important modulator of many cellular events including apoptosis, cell cycle arrest, senescence, differentiation, and stress responses. Its turnover may be tightly regulated. However, little is known about the regulation of its metabolism because most enzymes responsible for its synthesis and breakdown have yet to be cloned. Here we report the cloning and characterization of the yeast gene YPC1 (YBR183w) by screening Saccharomyces cerevisiae genes whose overexpression bestows resistance to fumonisin B1. We demonstrate that the yeast gene YPC1 encodes an alkaline ceramidase activity responsible for the breakdown of dihydroceramide and phytoceramide but not unsaturated ceramide. YPC1 ceramidase activity was confirmed by in vitro studies using an Escherichia coli expression system. Importantly, YPC1p also has reverse activity, catalyzing synthesis of phytoceramide from palmitic acid and phytosphingosine. This ceramide synthase activity is CoA-independent and is resistant to fumonisin B1, thus explaining why YPC1 was cloned as a fumonisin B1-resistant gene.

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.

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.

Purification and characterization of a membrane-bound nonlysosomal ceramidase from rat brain

We have purified a membrane bound ceramidase 22,300-fold to apparent homogeneity. The purification scheme included Triton X-100 extraction of membranes followed by Q-Sepharose, blue Sepharose, phenyl-Sepharose, and MonoS column chromatography. The purified enzyme showed an apparent molecular mass of 90 kDa as estimated by SDS-polyacrylamide gel electrophoresis under reducing conditions and 95 kDa by chromatography on Superose 12. Using C(16)-ceramide as substrate, the enzyme showed a broad pH optimum in the neutral to alkaline range. A mixed micelle assay was developed, and using Triton X-100/ceramide mixed micelles, the enzyme exhibited classical Michaelis-Menten kinetics, with a K(m) of 1.29 mol % and a V(max) of 4.4 micromol/min/mg. When dihydroceramide was used as substrate, these values were 3.84 mol % and 1.2 micromol/min/mg, respectively, indicating that the enzyme hydrolyzes ceramides preferentially. The activity of the purified ceramidase did not require cations, and it was inhibited by reducing agents. Phosphatidylcholine and sphingomyelin were without effect on the enzyme activity, whereas phosphatidic acid and phosphatidylserine stimulated the activity 3-fold. Sphingosine acted as a competitive inhibitor with an IC(50) of 5-10 microM. These results indicate that the purified enzyme is a novel ceramidase.

Effect of the lactic acid bacterium Streptococcus thermophilus on ceramide levels in human keratinocytes in vitro and stratum corneum in vivo

The effects of Streptococcus thermophilus on ceramide levels either in vitro on cultured human keratinocytes or in vivo on stratum corneum, have been investigated. In vitro, Streptococcus thermophilus enhanced the levels of ceramides in keratinocytes in a time-dependent way. The presence of high levels of neutral, glutathione-sensitive, sphingomyelinase in Streptococcus thermophilus could be responsible for the observed ceramide increase. The application of a base cream containing sonicated Streptococcus thermophilus in the forearm skin of 17 healthy volunteers for 7 d also led to a significant and relevant increase of skin ceramide amounts, which could be due to the sphingomyelin hydrolysis through bacterial neutral sphingomyelinase. Indeed, similar results were obtained with a base cream containing purified bacterial neutral sphingomyelinase. In addition, the inhibition of bacterial neutral sphingomyelinase activity through glutathione blocked the skin ceramide increase observed after the treatment. The topical application of a sonicated Streptococcus thermophilus preparation, leading to increased stratum corneum ceramide levels, could thus result in the improvement of lipid barrier and a more effective resistance against xerosis.

Differential effects of glycolipid biosynthesis inhibitors on ceramide-induced cell death in neuroblastoma cells

An in vitro model of Gaucher's disease in murine neuroblastoma x rat glioma NG108-15 cells was used to investigate the physiological effects of two specific inhibitors of glucosylceramide synthase, d,l-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (d,l-PDMP) and N-butyldeoxynojirimycin (NB-DNJ), which have been suggested as agents for treatment of glycolipid storage disorders. Incubation of NG108-15 cells with conduritol-B-epoxide, a covalent inhibitor of glucosylceramidase, raised the intracellular concentration of glucosylceramide (GC) by more than fourfold, indicating a glycolipid composition equivalent to that of Gaucher's cells. The level of GC was decreased, and the cells were depleted of gangliosides by postincubation with d,l-PDMP or NB-DNJ. Treatment with d,l-PDMP, but not with NB-DNJ, resulted in a dose-dependent reduction of the growth rate and eventually caused cell death in NG108-15 cells on reaching confluency. An in situ detection assay using terminal nucleotidyltransferase indicated that cell degeneration was accompanied by apoptosis. Lipid analysis by high-performance TLC revealed that on incubation with d,l-PDMP, but not with NB-DNJ, the concentration of endogenous ceramide was elevated by threefold. Ceramide elevation and apoptosis were also observed when NG108-15 cells were incubated with daunorubicin, which was previously reported to induce programmed cell death by stimulation of ceramide synthesis. Structural characterization by HPLC and subsequent laser desorption mass spectrometry revealed that the endogenous ceramide contained fatty acids with chain lengths ranging from C14:0 to C24:0. The results indicate that elevation of levels of these ceramide species by incubation with d,l-PDMP or daunorubicin induces programmed cell death in NG108-15 cells. Because ceramide accumulation and cell death were not observed on incubation with NB-DNJ, its use is suggested to be less toxic than that of d,l-PDMP for treatment of Gaucher's disease and other sphingolipid storage disorders.

Sphingosylphosphorylcholine is a potent inducer of intercellular adhesion molecule-1 expression in human keratinocytes

We recently reported that the epidermis of patients with atopic dermatitis contains an abnormally expressed sphingomyelin deacylase that yields a large amount of sphingosylphosphorylcholine (SPC) rather than ceramide. In this study, we characterize inflammatory roles of newly discovered chemicals in the epidermis by elucidating biologic effects of SPC on intercellular adhesion molecules-1 (ICAM-I) expression by human keratinocytes in culture in comparison with other sphingolipids. Using fluorescence-activated cell sorter analysis, we found that SPC treatment at concentrations of 10-20 microM significantly enhanced the expression of ICAM-I by cultured human keratinocytes in a dose-dependent manner after incubation for 15-24 h, and, using northern blot analysis, that this was accompanied by increased expression of ICAM-1 mRNA within 4 h of incubation. Transforming necrosis factor-alpha (TNF-alpha) levels in the medium of keratinocytes treated at a 10 microM concentration of SPC were significantly increased by 200%. Furthermore, the SPC-induced ICAM-1 expression was partially abolished by the concomitant addition of anti-TNF-alpha, suggesting a partial autocrine involvement of TNF-alpha in ICAM-1 expression. Assay of mitogen-activated protein kinase revealed that 10 microM SPC induced a rapid activation of mitogen-activated protein kinase in human keratinocytes, including an increase in its phosphorylation within 5 min, which then declined to the baseline control level after 30 min. In contrast, sphingomyelin or sphingosine had no significant potential to activate mitogen-activated protein kinase at the same concentration. These findings suggest that SPC plays an important role in the inflammatory process of epidermis in skin diseases, such as atopic dermatitis, with high expression of sphingomyelin deacylase.

Diversity and complexity of ceramide signalling in apoptosis

Sphingolipid ceramide has emerged as a lipid messenger of cell functions including differentiation and apoptosis. Diverse kinds of stresses (ultraviolet, irradiation, heat shock and hypoxia) and biological factors (TNF-alpha, IFN-gamma and Fas antibody) require ceramide generation to execute apoptosis. The review summarises the diversity and complexity of up- and downstream of ceramide signalling in apoptosis and clinical implications of ceramide-induced apoptosis.

Purification and characterization of a novel ceramidase from Pseudomonas aeruginosa

We report here a novel type of ceramidase of Pseudomonas aeruginosa AN17 isolated from the skin of a patient with atopic dermatitis. The enzyme was purified 83,400-fold with an overall yield of 21.1% from a culture supernatant of strain AN17. After being stained with a silver staining solution, the purified enzyme showed a single protein band, and its molecular mass was estimated to be 70 kDa on SDS-polyacrylamide gel electrophoresis. The enzyme showed quite wide specificity for various ceramides, i.e. it hydrolyzed ceramides containing C12:0-C18:0 fatty acids and 7-nitrobenz-2-oxa-1, 3-diazole-labeled dodecanoic acid, and not only ceramide containing sphingosine (d18:1) or sphinganine (d18:0) but also phytosphingosine (t18:0) as the long-chain base. However, the enzyme did not hydrolyze galactosylceramide, sulfatide, GM1, or sphingomyelin, and thus was clearly distinguished from a Pseudomonas sphingolipid ceramide N-deacylase (Ito, M., Kurita, T., and Kita, K. (1995) J. Biol. Chem. 270, 24370-24374). This bacterial ceramidase had a pH optimum of 8.0-9.0, an apparent Km of 139 microM, and a Vmax of 5.3 micromol/min/mg using N-palmitoylsphingosine as the substrate. The enzyme appears to require Ca2+ for expression of the activity. Interestingly, the 70-kDa protein catalyzed a reversible reaction in which the N-acyl linkage of ceramide was either cleaved or synthesized. Our study demonstrated that ceramidase is widely distributed from bacteria to mammals.