Glucosylceramides stimulate murine epidermal hyperproliferation

Mammalian Epidermis: A Compendium of Lipid Functionality

Mammalian epidermis is a striking example of the role of lipids in tissue biology. In this stratified epithelium, highly specialized structures are formed that leverage the hydrophobic properties of lipids to form an impermeable barrier and protect the humid internal environment of the body from the dry outside. This is achieved through tightly regulated lipid synthesis that generates the molecular species unique to the tissue. Beyond their fundamental structural role, lipids are involved in the active protection of the body from external insults. Lipid species present on the surface of the body possess antimicrobial activity and directly contribute to shaping the commensal microbiota. Lipids belonging to a variety of classes are also involved in the signaling events that modulate the immune responses to environmental stress as well as differentiation of the epidermal keratinocytes themselves. Recently, high-resolution methods are beginning to provide evidence for the involvement of newly identified specific lipid molecules in the regulation of epidermal homeostasis. In this review we give an overview of the wide range of biological functions of mammalian epidermal lipids.

Dysregulated sphingolipid metabolism and autophagy in granulosa cells of women with endometriosis

We evaluated metabolic profiles between cumulus cells (CCs) and mural granulosa cells (MGCs) derived from women with endometriosis to identify their correlations with oocyte quality. CCs and MGCs were collected from women with and without endometriosis undergoing in vitro fertilization/intracytoplasmic sperm injection treatment. The metabolomics of CCs and MGCs were measured by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) followed by a quantitative polymerase chain reaction to further confirm the genes involved in the metabolic results. LC-MS/MS analysis revealed differences in 24 metabolites of CCs and 71 metabolites of MGCs between groups. Among them, five metabolites were upregulated and 19 metabolites were downregulated in CCs with endometriosis, whereas three metabolites were upregulated and 68 metabolites were downregulated in MGCs with endometriosis. Metabolites related to sphingolipid metabolism, which included palmitic acid (PA) and docosahexaenoic acid, increased significantly only in CCs with endometriosis, whereas sphingosine and PA were significantly downregulated in MGCs with endometriosis compared with CCs and MGCs without endometriosis. Gene expression involved in ceramide synthesis (CERS1, SPTL1, and SMPD1) and autophagy (BECN1, LAMP, and PC3) were significantly higher in CCs with endometriosis according to FASN, BECN1, and LAMP protein expressions. However, gene expression involved in ceramide synthesis (SPHK1, ASAH1, and SGPP1) and autophagy (BECN1, LAMP, and PC3) were significantly lower in MGCs with endometriosis, whereas CERS1 and UGCG expression increased. There are differences in sphingolipid metabolites in CCs and MGCs with endometriosis compared with women without endometriosis. These differences seem to be involved in the regulation of autophagic cell death in preovulatory follicles.

Glucosylceramide and galactosylceramide, small glycosphingolipids with significant impact on health and disease

Numerous clinical observations and exploitation of cellular and animal models indicate that glucosylceramide (GlcCer) and galactosylceramide (GalCer) are involved in many physiological and pathological phenomena. In many cases, the biological importance of these monohexosylcermides has been shown indirectly as the result of studies on enzymes involved in their synthesis and degradation. Under physiological conditions, GalCer plays a key role in the maintenance of proper structure and stability of myelin and differentiation of oligodendrocytes. On the other hand, GlcCer is necessary for the proper functions of epidermis. Such an important lysosomal storage disease as Gaucher disease (GD) and a neurodegenerative disorder as Parkinson’s disease are characterized by mutations in the GBA1 gene, decreased activity of lysosomal GBA1 glucosylceramidase and accumulation of GlcCer. In contrast, another lysosomal disease, Krabbe disease, is associated with mutations in the GALC gene, resulting in deficiency or decreased activity of lysosomal galactosylceramidase and accumulation of GalCer and galactosylsphingosine. Little is known about the role of both monohexosylceramides in tumor progression; however, numerous studies indicate that GlcCer and GalCer play important roles in the development of multidrug-resistance by cancer cells. It was shown that GlcCer is able to provoke immune reaction and acts as a self-antigen in GD. On the other hand, GalCer was recognized as an important cellular receptor for HIV-1. Altogether, these two molecules are excellent examples of how slight differences in chemical composition and molecular conformation contribute to profound differences in their physicochemical properties and biological functions.

Trends in Glucocerebrosides Research: A Systematic Review

Glucocerebrosides are sphingolipid components of cell membranes that intervene in numerous cell biological processes and signaling pathways and that deregulation is implicated in human diseases such as Gaucher disease and Parkinson's disease. In the present study, we conducted a systematic review using document co-citation analysis, clustering and visualization tools to explore the trends and knowledge structure of glucocerebrosides research as indexed in the Science Citation Index Expanded database (1956-present). A co-citation network of 5,324 publications related to glucocerebrosides was constructed. The analysis of emerging categories and keywords suggested a growth of research related to neurosciences over the last decade. We identified ten major areas of research (e.g., clusters) that developed over time, from the oldest (i.e., on glucocerebrosidase protein or molecular analysis of the GBA gene) to the most recent ones (i.e., on drug resistance in cancer, pharmacological chaperones, or Parkinson's disease). We provided for each cluster the most cited publications and a description of their intellectual content. We moreover identified emerging trends in glucocerebrosides research by detecting the surges in the rate of publication citations in the most recent years. In conclusion, this study helps to apprehend the most significant lines of research on glucocerebrosides. This should strengthen the connections between scientific communities studying glycosphingolipids to facilitate advances, especially for the most recent researches on cancer drug resistance and Parkinson's disease.

The Disturbance of Hepatic and Serous Lipids in Aristolochic Acid Ι Induced Rats for Hepatotoxicity Using Lipidomics Approach

Aristolochic acid I (AAI) was regarded as the major toxic component of aristolochic acid (AA). In addition to aristolochic acid nephropathy (AAN), liver cancers induced by AAI has aroused increasing attention recently. In this paper, the discovery of diagnostic biomarkers for AAI-induced liver injury has been studied, especially for the lipid markers. From the histopathological characteristics, the injury was observed clearly in the liver apart from the kidney after 30 mg/kg of AAΙ treatment for one week, while the lesion alleviated after AAΙ discontinuance. The serum biochemical indexes were manifested to the normal tendency after AAΙ discontinuance for two weeks. According to the evaluation of pathology slices and serum biochemical indexes, they indicated that the hepatotoxicity induced by AAΙ was reversible to some extent. A total of 44 lipid markers were identified in the liver, as well as 59 in the serum. Twenty-six common lipid markers were observed in both serum and liver. Furthermore, nine out of 26 lipids exhibited the excellent diagnostic ability to differentiate the control group from the AAΙ group and AAΙ discontinuance group with high sensitivity and specificity. The changed lipid markers might serve as characteristics to explain the mechanisms of pathogenesis and progression in hepatotoxicity induced by AAΙ.

Sphingolipid synthesis and role in uterine epithelia proliferation

Sphingolipids are involved in the regulation of cell proliferation. It has been reported that diacylglycerol and sphingosine-1-phosphate generation, during the synthesis of phospho-sphingolipids, is necessary for both, G1-S transition of cell cycle during the sustained activation of protein kinase C in various cell models (MDCK,SaccharomycesandEntamoeba) and AKT pathway activation. During the estrous cycle of the rat, AKT signaling is the main pathway involved in the regulation of uterine cell proliferation. The aim of the present study was to investigate the role of sphingolipid synthesis during proliferation of uterine cells in the estrous cycle of the rat. On metestrus day, when both luminal and glandular uterine epithelia present the maximal BrdU-labeled cells (S phase cells), there was an increase in the relative abundance of total sphingomyelins, as compared to estrus day. Myriocin, a sphingolipid synthesis inhibitor administered on estrus day, before the new cell cycle of epithelial cells is initiated, decreased the abundance of sphingomyelin, accompanied by proliferation arrest in uterine epithelial cells on metestrus day. In order to study the sphingolipid signaling pathway affected by myriocin, we evaluated the activation of the PKC-AKT-GSK3b-Cyclin D3 pathway. We observed that total and phosphorylated protein kinase C diminished in uterine epithelial cells of myriocin treated animals. Interestingly, cyclin D3 nuclear localization was blocked by myriocin, concomitantly with a decrease in nuclear pRb expression. In conclusion, we demonstrate that sphingolipid synthesis and signaling are involved in uterine epithelial cell proliferation during the estrous cycle of the rat.

Impact of sphingolipids on osteoblast and osteoclast activity in Gaucher disease

Gaucher disease (GD) is an inherited disorder in which mutations in the GBA1 gene lead to deficient β-glucocerebrosidase activity and accumulation of its substrate glucosylceramide. Bone disease is present in around 84% of GD patients, ranging from bone loss including osteopenia and osteonecrosis to abnormal bone remodelling in the form of Erlenmeyer flask formation. The range of severity and variety of types of bone disease found in GD patients indicate the involvement of several mechanisms. Here we investigate the effects of exogenous sphingolipids on osteoclasts, osteoblasts, plasma cells and mesenchymal stem cells (MSC) and the interactions between these cell types. Osteoclasts were differentiated from the peripheral blood of Gaucher patients and control subjects. Osteoblasts were differentiated from mesenchymal stem cells isolated from bone marrow aspirates of Gaucher patients and control subjects. The human osteoblast cell line SaOS-2 was also investigated. Osteoclasts, osteoblasts and a human myeloma plasma cell line NCI-H929 were cultured with relevant exogenous sphingolipids to assess effects on cellular viability and function. Calcium deposition by osteoblasts differentiated from Gaucher patient MSC's was on average only 11.4% of that deposited by control subject osteoblasts. Culture with glucosylsphingosine reduced control subject MSC viability by 10.4%, SaOS-2 viability by 17.4% and plasma cell number by 40%. Culture with glucosylceramide decreased calcium deposition by control MSC-derived osteoblasts while increasing control subject osteoclast generation by 55.6%, Gaucher patient osteoclast generation by 37.6% and plasma cell numbers by up to 29.7%. Excessive osteoclast number and activity and reduced osteoblast activity may have the overall effect of an uncoupling between osteoclasts and osteoblasts in the GD bone microenvironment.

Dysregulated sphingolipid metabolism in endometriosis

BACKGROUND

In endometriosis, the establishment and subsistence of ectopic lesions outside the endometrium suggest an altered cellular state for pathological hyperplasia. Sphingolipids are bioactive compounds, and their biosynthesis and metabolism modulate a range of cellular processes including proliferation, migration and apoptosis. We demonstrate that aberrations in sphingolipid metabolism occur in women with endometriosis.

METHODS

Targeted mass spectrometry on >120 sphingolipids were measured in the sera (n = 62), peritoneal fluid (n = 63), and endometrial tissue (n = 14) of women with and without endometriosis. Quantitative RT-PCR and immunohistochemistry were performed on endometrial tissues determine the expression levels of sphingolipid enzymes.

RESULTS

Sphingolipidomics identified the in vivo accumulation of numerous sphingolipids, including the functionally antagonistic glucosylceramides and ceramides in the serum and PF of women with endometriosis. We found upregulation of specific sphingolipid enzymes, namely sphingomyelin synthase 1 (SMS1), sphingomyelinase 3 (SMPD3), and glucosylceramide synthase (GCS) in the endometrium of endometriotic women with corresponding increased GlcCer, decreased sphingomyelin levels, and decreased apoptosis in the endometrium.

CONCLUSIONS

Our sphingolipidomics approach provided evidence of altered sphingolipid metabolism flux in serum, peritoneal fluid, and endometrial tissue in women with endometriosis. The results provide new information on how sphingolipids and eutopic endometrium may contribute to the pathophysiology of endometriosis. The results also have implications for the use of sphingolipids as potential biomarkers.

Reduced glucosylceramide in the mouse model of Fabry disease: correction by successful enzyme replacement therapy

Fabry disease is an X-linked lysosomal storage disease (LSD) caused by deficient activity of α-Galactosidase A (α-Gal A). As a result, glycosphingolipids, mainly globotriaosylceramide (Gb3), progressively accumulate in body fluids and tissues. Studies aiming at the identification of secondary lipid alterations in Fabry disease may be potentially useful for the monitorization of the response to enzyme replacement therapy (ERT) and development of future therapies. The focus of this study was to evaluate if α-Gal A deficiency has an effect on two key groups of molecules of sphingolipids metabolism: glucosylceramides (GlucCers) and ceramides (Cers). Studies performed in a mouse model of Fabry disease showed reduced level of GlucCer and normal level of Cer in plasma, liver, spleen, kidney and heart. Moreover, analysis of GlucCer isoforms in Fabry knockout mice showed that GlucCer isoforms are unequally reduced in different tissues of these animals. ERT had a specific effect on the liver's GlucCer levels of Fabry knockout mice, increasing hepatic GlucCer to the levels observed in wild type mice. In contrast to Fabry knockout mice, plasma of Fabry patients had normal GlucCer and Cer but an increased GlucCer/Cer ratio. This alteration showed a positive correlation with plasma globotriaosylsphingosine (lyso-Gb3) concentration. In conclusion, this work reveals novel secondary lipid imbalances caused by α-Gal A deficiency.

The impact of sphingosine kinase-1 in head and neck cancer

Head and neck squamous cell carcinoma (HNSCC) has a high reoccurrence rate and an extremely low survival rate. There is limited availability of effective therapies to reduce the rate of recurrence, resulting in high morbidity and mortality of advanced cases. Late presentation, delay in detection of lesions, and a high rate of metastasis make HNSCC a devastating disease. This review offers insight into the role of sphingosine kinase-1 (SphK1), a key enzyme in sphingolipid metabolism, in HNSCC. Sphingolipids not only play a structural role in cellular membranes, but also modulate cell signal transduction pathways to influence biological outcomes such as senescence, differentiation, apoptosis, migration, proliferation, and angiogenesis. SphK1 is a critical regulator of the delicate balance between proliferation and apoptosis. The highest expression of SphK1 is found in the advanced stage of disease, and there is a positive correlation between SphK1 expression and recurrent tumors. On the other hand, silencing SphK1 reduces HNSCC tumor growth and sensitizes tumors to radiation-induced death.  Thus, SphK1 plays an important and influential role in determining HNSCC proliferation and metastasis. We discuss roles of SphK1 and other sphingolipids in HNSCC development and therapeutic strategies against HNSCC.

Epidermal barrier: Adverse and beneficial changes induced by ultraviolet B irradiation depending on the exposure dose and time (Review)

Exposure of the skin to ultraviolet (UV) radiation induces various harmful effects in the tissues, particularly disruption of the epidermal barrier. However, ultraviolet B (UVB) irradiation has been applied in the treatment of atopic dermatitis, a skin disease in which the epidermal barrier is defective. We reviewed the homeostasis of the epidermal barrier and several studies investigating the adverse and beneficial effects caused by different doses of UVB irradiation in the epidermal barrier. It may be concluded that, despite the harmful effects of UVB irradiation on the skin, UVB irradiation is able to exert beneficial effects in the epidermal barrier when administered in suberythemal doses and over a relatively short period of time, with no clinically evident inflammation or barrier disruption. This may be a useful therapeutic strategy for the use of UVB irradiation in the treatment of skin diseases with a disrupted epidermal barrier, such as atopic dermatitis, while reducing or avoiding the side-effects.

Age-dependent changes in the sphingolipid composition of mouse CD4+ T cell membranes and immune synapses implicate glucosylceramides in age-related T cell dysfunction

To determine whether changes in sphingolipid composition are associated with age-related immune dysfunction, we analyzed the core sphingolipidome (i.e., all of the metabolites through the first headgroup additions) of young and aged CD4⁺ T cells. Since sphingolipids influence the biophysical properties of membranes, we evaluated the compositions of immune synapse (IS) and non-IS fractions prepared by magnetic immuno-isolation. Broadly, increased amounts of sphingomyelins, dihydrosphingomyelins and ceramides were found in aged CD4⁺ T cells. After normalizing for total sphingolipid content, a statistically significant decrease in the molar fraction of glucosylceramides was evident in both the non-IS and IS fractions of aged T cells. This change was balanced by less dramatic increases in the molar fractions of sphingomyelins and dihydrosphingomyelins in aged CD4⁺ T cells. In vitro, the direct or enzymatic enhancement of ceramide levels decreased CD4⁺ T cell proliferation without regard for the age of the responding T cells. In contrast, the in vitro inhibition of glucosylceramidase preferentially increased the proliferation of aged CD4⁺ T cells. These results suggest that reductions in glucosylceramide abundance contribute to age-related impairments in CD4⁺ T cell function.

Neurofibromatosis-like phenotype in Drosophila caused by lack of glucosylceramide extension

Glycosphingolipids (GSLs) are of fundamental importance in the nervous system. However, the molecular details associated with GSL function are largely unknown, in part because of the complexity of GSL biosynthesis in vertebrates. In Drosophila, only one major GSL biosynthetic pathway exists, controlled by the glycosyltransferase Egghead (Egh). Here we discovered that loss of Egh causes overgrowth of peripheral nerves and attraction of immune cells to the nerves. This phenotype is reminiscent of the human disorder neurofibromatosis type 1, which is characterized by disfiguring nerve sheath tumors with mast cell infiltration, increased cancer risk, and learning deficits. Neurofibromatosis type 1 is due to a reduction of the tumor suppressor neurofibromin, a negative regulator of the small GTPase Ras. Enhanced Ras signaling promotes glial growth through activation of phosphatidylinositol 3-kinase (PI3K) and its downstream kinase Akt. We find that overgrowth of peripheral nerves in egh mutants is suppressed by down-regulation of the PI3K signaling pathway by expression of either dominant-negative PI3K, the tumor suppressor PTEN, or the transcription factor FOXO in the subperineurial glia. These results show that loss of the glycosyltransferase Egh affects membrane signaling and activation of PI3K signaling in glia of the peripheral nervous system, and suggest that glycosyltransferases may suppress proliferation.

Skin ultrastructural findings in type 2 Gaucher disease: diagnostic implications

BACKGROUND

Type 2 Gaucher disease is a rare and progressive subtype of this lysosomal storage disorder, marked by rapid, early-onset neurodegeneration. Distinguishing type 2 from types 1 and 3 Gaucher disease has remained challenging, due to the lack of a clear correlation between phenotype and enzymatic activity or genotype. β-glucocerebrosidase, the enzyme deficient in Gaucher disease, also has an essential role in maintaining epidermal permeability function, by regulating the ratio of ceramides to glucosylceramides in the stratum corneum of the skin.

OBJECTIVES

To further assess the diagnostic utility of epidermal evaluations in distinguishing patients with type 2 Gaucher disease in an expanded cohort.

STUDY DESIGN

Epidermal samples were evaluated from twenty children with type 2, three patients with type 3 Gaucher disease and two adults with type 1 Gaucher disease with different clinical manifestations and genotypes. Electron microscopy on ruthenium tetroxide post-fixed tissue was performed.

RESULTS

Compared to controls and subjects with type 1 and type 3 Gaucher disease, only patients with type 2 Gaucher disease displayed characteristic electron dense, non-lamellar clefts and immature-lamellar membranes.

CONCLUSION

The appearance of characteristic alterations in epidermal ultrastructure provides an early and specific diagnostic tool to help in distinguishing type 2 from the other types of Gaucher disease.

Pathogenesis of permeability barrier abnormalities in the ichthyoses: inherited disorders of lipid metabolism

Many of the ichthyoses are associated with inherited disorders of lipid metabolism. These disorders have provided unique models to dissect physiologic processes in normal epidermis and the pathophysiology of more common scaling conditions. In most of these disorders, a permeability barrier abnormality "drives" pathophysiology through stimulation of epidermal hyperplasia. Among primary abnormalities of nonpolar lipid metabolism, triglyceride accumulation in neutral lipid storage disease as a result of a lipase mutation provokes a barrier abnormality via lamellar/nonlamellar phase separation within the extracellular matrix of the stratum corneum (SC). Similar mechanisms account for the barrier abnormalities (and subsequent ichthyosis) in inherited disorders of polar lipid metabolism. For example, in recessive X-linked ichthyosis (RXLI), cholesterol sulfate (CSO(4)) accumulation also produces a permeability barrier defect through lamellar/nonlamellar phase separation. However, in RXLI, the desquamation abnormality is in part attributable to the plurifunctional roles of CSO(4) as a regulator of both epidermal differentiation and corneodesmosome degradation. Phase separation also occurs in type II Gaucher disease (GD; from accumulation of glucosylceramides as a result of to beta-glucocerebrosidase deficiency). Finally, failure to assemble both lipids and desquamatory enzymes into nascent epidermal lamellar bodies (LBs) accounts for both the permeability barrier and desquamation abnormalities in Harlequin ichthyosis (HI). The barrier abnormality provokes the clinical phenotype in these disorders not only by stimulating epidermal proliferation, but also by inducing inflammation.

Glucosylceramide transfer from lysosomes--the missing link in molecular pathology of glucosylceramidase deficiency: a hypothesis based on existing data

Gaucher disease (GD), deficiency of acid glucosylceramidase (GlcCer-ase) is characterized by deficient degradation of beta-glucosylceramide (GlcCer). It is well known that, in GD, the lysosomal accumulation of uncleaved GlcCer is limited to macrophages, which are gradually converted to storage cells with well known cytology--Gaucher cells (GCs). On the basis of previous studies of the disorder and of a comparison with other lysosomal enzymopathies affecting degradation of the GlcCer-based glycosphingolipid series, it is hypothesized that in other cell types (i.e. non-macrophage cells) the uncleaved GlcCer, in GlcCer-ase deficiency, is transferred to other cell compartments, where it may be processed and even accumulated to various degrees. The consequence of the abnormal extralysosomal load may differ according to the cell type and compartment targeted and may be influenced by genetically determined factors, by a number of acquired conditions, including the current metabolic situation. The sequelae of the uncleaved GlcCer extralysosomal transfer may range from probably innocent or positive stimulatory, to the much more serious, in which it interferes with a variety of cell functions, and in extreme cases, can lead to cell death. This alternative processing of uncleaved GlcCer may help to explain tissue alterations seen in GD that have, so far, resisted explanation based simply on the presence of GCs. Paralysosomal alternative processing may thus go a long way towards filling a long-standing gap in the understanding of the molecular pathology of the disorder. The impact of this alternative process will most likely be inversely proportional to the level of residual GlcCer-ase activity. Lysosomal sequestration of GlcCer in these cells is either absent or in those exceptional cases where it does occur, it is exceptional and rudimentary. It is suggested that paralysosomal alternative processing of uncleaved GlcCer is the main target for enzyme replacement therapy. The mechanism responsible for GlcCer transfer remains to be elucidated. It may also help in explaining the so far unclear origin of glucosylsphingosine (GlcSph) and define the mutual relation between these two processes.

An accumulation of glucosylceramide in the stratum corneum due to attenuated activity of beta-glucocerebrosidase is associated with the early phase of UVB-induced alteration in cutaneous barrier function

Epidermal glucosylceramide (GlcCer) metabolism is essential to the maintenance of skin homeostasis. Although exposure of the skin to ultraviolet (UV) radiation elicits dramatic physiological and biological changes in the epidermis attributable to barrier perturbation, wrinkle formation and inflammation, little is known about UV-induced changes in GlcCer metabolism. In this study, we have assessed beta-glucocerebrosidase (GlcCer'ase) activity in murine epidermis before and after a single UVB irradiation and have compared it with GlcCer and ceramide (Cer) levels. GlcCer'ase enzymatic activity was significantly suppressed in a dose-dependent manner one day after UVB (70 mJ/cm(2)) irradiation despite a significant increase in GlcCer'ase mRNA. The marked decrease in enzyme activity was followed by an accumulation of GlcCer in the stratum corneum, which peaked at day 2. This decreased level of GlcCer'ase activity returned to 80% of the control level by day 3 followed by a return of GlcCer level to the control level by day 4. In the whole epidermis, significant increases in Cer and GlcCer levels occurred on day 3 and on day 2, respectively. These results suggest that UVB irradiation dramatically affects the metabolism of GlcCer to Cer in the epidermis (including the stratum corneum) and that this may be closely associated with the early and minor phase of UVB-induced alteration in cutaneous barrier function.

Glucosylceramide accumulates preferentially in lamellar bodies in differentiated keratinocytes

BACKGROUND

Sphingolipids, e.g. ceramide (Cer), glucosylceramide (GlcCer) and sphingomyelin (SM), are important bulk constituents of plasma membranes in mammalian cells. In addition, these lipids are also enriched in certain intracellular organelles, as well as in the epidermal lamellar bodies (LBs) of differentiating keratinocytes (KCs). Epidermal Cer, which comprises a heterogeneous family of at least 10 members, is a key component of the stratum corneum (SC) lipids, and regulates permeability barrier function. Levels of GlcCer, but not SM, significantly increase during epidermal differentiation, and then both GlcCer and SM are enzymatically hydrolysed to Cer at and just above the transition from the stratum granulosum to the SC.

OBJECTIVES

To determine: (i) whether the GlcCer contained in different pools, i.e. the membrane fraction or the LB fraction, has different metabolic fates; and (ii) whether specific molecular species of GlcCer localize to distinct subcellular pools.

METHODS

To study the metabolic fate of specific molecular fractions of GlcCer and Cer, we first ascertained the full spectrum of molecular species present in cultured normal human KCs (CHK) in a differentiated condition as shown in vivo in epidermis. Cer species were analysed in CHK using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Next, the metabolic fate of the GlcCer was studied by pulse-labelling of CHK with L-[14C]-serine.

RESULTS

The GlcCer of undifferentiated KCs comprised GlcCer B and Cer NS (or Cer 2), which contain nonhydroxy fatty acid (FA) as the amide-linked FA, while differentiated KCs displayed further heterogeneity of both GlcCer and Cer, including the presence of acylGlcCer and acylCer. The metabolic fates of these sphingolipids were determined. The GlcCer B level decreased over 2 days and then plateaued between days 3 and 5 following pulse-labelling of sphingolipids for 24 h. As GlcCer B declined, Cer NS (Cer 2) increased in a similar time-dependent manner. In contrast, both acylGlcCer and acylCer increased continuously in parallel over this experimental period.

CONCLUSIONS

Distinct GlcCer pools segregated to those that were either hydrolysable or nonhydrolysable in differentiated KCs. We assume that the latter pool appears to be LB enriched, and also sequestrates acylGlcCer from other cellular membrane fractions.

Alterations of glucosylceramide-beta-glucosidase levels in the skin of patients with psoriasis vulgaris

Hydrolysis of glucosylceramides by the enzyme glucosylceramide-beta-glucosidase (GlcCer'ase) results in ceramide, a critical component of the intercellular lamellae that mediates the epidermal permeability barrier. A disturbance of ceramide formation is supposed to influence the transepidermal water loss in common skin diseases like atopic eczema or psoriasis. The aim of this study was to investigate whether GlcCer'ase levels were altered in the skin of subjects with psoriasis vulgaris. Skin punch biopsies were taken from lesional and non-lesional psoriatic skin and GlcCer'ase was evaluated both at the RNA and at the protein level. Normal skin from surgical patients provided the baseline GlcCer'ase expression in healthy subjects. Our results show that GlcCer'ase mRNA expression was decreased in psoriatic non-lesional skin compared to normal controls in all cases. Interestingly, in lesional psoriatic skin the level of GlcCer'ase was increased compared to non-lesional skin in all cases. For the immunohistochemical analysis, we used a newly synthesized monoclonal antibody anti-human GBC (GlcCer'ase-GST fusion protein). The results confirmed that GlcCer'ase, mainly present in the upper epidermis, was decreased in psoriatic skin compared to normal control and was increased in lesional compared to non-lesional psoriatic skin. Our findings support the concept that alteration in water permeability barrier in lesional psoriatic skin can serve as a trigger for the upregulation of the expression of enzymes like GlcCer'ase with consequent stimulation of ceramide generation.

Ceramides and skin function

Ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the stratum corneum. These lamellar sheets are thought to provide the barrier property of the epidermis. It is generally accepted that the intercellular lipid domain is composed of approximately equimolar concentrations of free fatty acids, cholesterol, and ceramides. Ceramides are a structurally heterogeneous and complex group of sphingolipids containing derivatives of sphingosine bases in amide linkage with a variety of fatty acids. Differences in chain length, type and extent of hydroxylation, saturation etc. are responsible for the heterogeneity of the epidermal sphingolipids. It is well known that ceramides play an essential role in structuring and maintaining the water permeability barrier function of the skin. In conjunction with the other stratum corneum lipids, they form ordered structures. An essential factor is the physical state of the lipid chains in the nonpolar regions of the bilayers. The stratum corneum intercellular lipid lamellae, the aliphatic chains in the ceramides and the fatty acids are mostly straight long-chain saturated compounds with a high melting point and a small polar head group. This means that at physiological temperatures, the lipid chains are mostly in a solid crystalline or gel state, which exhibits low lateral diffusional properties and is less permeable than the state of liquid crystalline membranes, which are present at higher temperatures. The link between skin disorders and changes in barrier lipid composition, especially in ceramides, is difficult to prove because of the many variables involved. However, most skin disorders that have a diminished barrier function present a decrease in total ceramide content with some differences in the ceramide pattern. Formulations containing lipids identical to those in skin and, in particular, some ceramide supplementation could improve disturbed skin conditions. Incomplete lipid mixtures yield abnormal lamellar body contents, and disorder intercellular lamellae, whereas complete lipid mixtures result in normal lamellar bodies and intercellular bilayers. The utilization of physiological lipids according to these parameters have potential as new forms of topical therapy for dermatoses. An alternative strategy to improving barrier function by topical application of the various mature lipid species is to enhance the natural lipid-synthetic capability of the epidermis through the topical delivery of lipid precursors.

Human epidermal glucosylceramides are major precursors of stratum corneum ceramides

Ceramides are the major component of the stratum corneum, accounting for 30%-40% of stratum corneum lipids by weight, and are composed of at least seven molecular groups (designated ceramides 1-7). Stratum corneum ceramides, together with cholesterol and fatty acids, form extracellular lamellae that are responsible for the epidermal permeability barrier. Previous studies indicated that beta-glucocerebrosidase- and sphingomyelinase-dependent ceramide production from glucosylceramides and sphingomyelins, respectively, is important for epidermal permeability barrier homeostasis. A recent study indicated that sphingomyelins are precursors of two stratum corneum ceramide molecular groups (ceramides 2 and 5). In this study, we have examined the role of glucosylceramides in the generation of each of the seven stratum corneum ceramide molecular groups. First, the structures of various glucosylceramide species in human epidermis were determined by gas chromatography-mass spectrometry, fast atom bombardment-mass spectrometry, and nuclear magnetic resonance. The results indicate that total epidermal glucosylceramides are composed of six distinct molecular groups, glucosylceramides 1-6. Glucosylceramide 1 contains sphingenine and nonhydroxy fatty acids, glucosylceramide 2, phytosphingosine and nonhydroxy fatty acids, glucosylceramide 3, phytosphingosine with one double bond and nonhydroxy fatty acids, glucosylceramide 4, sphingenine and alpha-hydroxy fatty acids, glucosylceramide 5, phytosphingosine and alpha-hydroxy fatty acids, and glucosylceramide 6, phytosphingosine with one double bond and alpha-hydroxy fatty acids. The nonhydroxy fatty acids typically have 16-24-carbon-length chains, whereas alpha-hydroxy fatty acids are limited to 24-, 25-, and 26-carbon chains. The sphingosine bases are C18 or C20 chains. Next, acylglucosylceramides and glucosylceramides were treated with beta-glucocerebrosidase and the ceramides released were compared with stratum corneum ceramides. Ceramide moieties of acylglucosylceramides and glucosylceramides 1, 2, 4-6 correspond to stratum corneum ceramides 1-7. These results, together with those of our previous reports characterizing epidermal sphingomyelins, indicate that all ceramide species, including omega-hydroxy fatty-acid-containing ceramides, are derived from glucosylceramides, and fractions of ceramides 2 and 5 are from sphingomyelins. Furthermore, structural analysis of glucosylceramides revealed that human epidermal glycosphingolipids display a unique lipid profile that is rich in very long chain hydroxylated (alpha- and omega-hydroxy) fatty acids and phytosphingosine.

Altered sphingolipid metabolism in multidrug-resistant ovarian cancer cells is due to uncoupling of glycolipid biosynthesis in the Golgi apparatus

Multidrug-resistant tumor cells display enhanced levels of glucosylceramide. In this study, we investigated how this relates to the overall sphingolipid composition of multidrug-resistant ovarian carcinoma cells and which mechanisms are responsible for adapted sphingolipid metabolism. We found in multidrug-resistant cells substantially lower levels of lactosylceramide and gangliosides in sharp contrast to glucosylceramide, galactosylceramide, and sphingomyelin levels. This indicates a block in the glycolipid biosynthetic pathway at the level of lactosylceramide formation, with concomitant accumulation of glucosylceramide. A series of observations exclude regulation at the enzyme level as the underlying mechanism. First, reduced lactosylceramide formation occurred only in intact resistant cells whereas cell-free activity of lactosylceramide synthase was higher compared with the parental cells. Second, the level of lactosylceramide synthase gene expression was equal in both phenotypes. Third, glucosylceramide synthase (mRNA and protein) expression and activity were equal or lower in resistant cells. Based on the kinetics of sphingolipid metabolism, the observation that brefeldin A does not restore lactosylceramide synthesis, and altered localization of lactosylceramide synthase fused to green fluorescent protein, we conclude that lactosylceramide biosynthesis is highly uncoupled from glucosylceramide biosynthesis in the Golgi apparatus of resistant cells.

The aged epidermal permeability barrier: basis for functional abnormalities

Aged epidermis develops an abnormality in permeability barrier homeostasis, which is accentuated further in photoaged skin. The biochemical basis is a global reduction in stratum corneum lipids and profound abnormality in cholesterol synthesis. Various cytokine/growth factor signaling pathways are abnormal in aged skin, particularly in the interleukin-1 family. Barrier repair therapy can be effective in restoring normal function if a cholesterol-dominant mixture of the three key physiologic lipids, including ceramides and free fatty acids, is emphasized.

Ceramide glycosylation potentiates cellular multidrug resistance

Ceramide glycosylation, through glucosylceramide synthase (GCS), allows cellular escape from ceramide-induced programmed cell death. This glycosylation event confers cancer cell resistance to cytotoxic anticancer agents [Liu, Y. Y., Han, T. Y., Giuliano, A. E., and M. C. Cabot. (1999) J. Biol. Chem. 274, 1140-1146]. We previously found that glucosylceramide, the glycosylated form of ceramide, accumulates in adriamycin-resistant breast carcinoma cells, in vinblastine-resistant epithelioid carcinoma cells, and in tumor specimens from patients showing poor response to chemotherapy. Here we show that multidrug resistance can be increased over baseline and then totally reversed in human breast cancer cells by GCS gene targeting. In adriamycin-resistant MCF-7-AdrR cells, transfection of GCS upgraded multidrug resistance, whereas transfection of GCS antisense markedly restored cellular sensitivity to anthracyclines, Vinca alkaloids, taxanes, and other anticancer drugs. Sensitivity to the various drugs by GCS antisense transfection increased 7- to 240-fold and was consistent with the resumption of ceramide-caspase-apoptotic signaling. GCS targeting had little influence on cellular sensitivity to either 5-FU or cisplatin, nor did it modify P-glycoprotein expression or rhodamine-123 efflux. GCS antisense transfection did enhance rhodamine-123 uptake compared with parent MCF-7-AdrR cells. This study reveals that GCS is a novel mechanism of multidrug resistance and positions GCS antisense as an innovative force to overcome multidrug resistance in cancer chemotherapy.

Overexpression of CD1d by keratinocytes in psoriasis and CD1d-dependent IFN-gamma production by NK-T cells

The MHC class I-like protein CD1d is a nonpolymorphic molecule that plays a central role in development and activation of a subset of T cells that coexpress receptors used by NK cells (NK-T cells). Recently, T cells bearing NK receptors were identified in acute and chronic lesions of psoriasis. To determine whether NK-T cells could interact with epidermal cells, we examined the pattern of expression of CD1d in normal skin, psoriasis, and related skin disorders, using a panel of CD1d-specific mAbs. CD1d was expressed by keratinocytes in normal skin, although expression was at a relatively low level and was generally confined to upper level keratinocytes immediately beneath the lipid-rich stratum corneum. In contrast, there was overexpression of CD1d in chronic, active psoriatic plaques. CD1d could be rapidly induced on keratinocytes in normal skin by physical trauma that disrupted barrier function or by application of a potent contact-sensitizing agent. Keratinocytes displayed enhanced CD1d following exposure to IFN-gamma. Combining CD1d-positive keratinocytes with human NK-T cell clones resulted in clustering of NK-T cells, and while no significant proliferation ensued, NK-T cells became activated to produce large amounts of IFN-gamma. We conclude that CD1d can be expressed in a functionally active form by keratinocytes and is up-regulated in psoriasis and other inflammatory dermatoses. The ability of IFN-gamma to enhance keratinocyte CD1d expression and the subsequent ability of CD1d-positive keratinocytes to activate NK-T cells to produce IFN-gamma, could provide a mechanism that contributes to the pathogenesis of psoriasis and other skin disorders.

Oxysterols induce differentiation in human keratinocytes and increase Ap-1-dependent involucrin transcription

Ligands and activators of the nuclear hormone receptor superfamily are important in the regulation of epidermal development and differentiation. Previously, we showed that naturally occurring fatty acids, as well as synthetic ligands for the peroxisome proliferator-activated receptor, induce keratinocyte differentiation in vitro. Here we asked whether oxysterols, another class of lipids formed de novo in the epidermis and that activate liver X-activated receptor, regulate keratinocyte differentiation. mRNA and protein levels of involucrin and transglutaminase 1, markers of differentiation, increased 2- to 3-fold in normal human keratinocytes incubated in the presence of 25- or 22R-hydroxycholesterol in low calcium. In high calcium, which alone induces differentiation, mRNA levels were further increased by oxysterols. Rates of cornified envelope formation, an indicator of terminal differentiation, also increased 2-fold with oxysterol treatment. In contrast, the rate of DNA synthesis was inhibited approximately 50% by oxysterols. Transcriptional regulation was assessed in keratinocytes transfected with either transglutaminase 1 or involucrin promoter-luciferase constructs. 22R-hydroxycholesterol increased transglutaminase 1 and involucrin promoter activity 2- to 3-fold. Either deletion of the -2452 bp to -1880 bp region of the involucrin promoter, or mutation of the AP-1 site within this region, abolished oxysterol responsiveness. Moreover, increased AP-1 DNA binding was observed in oxysterol-treated keratinocytes by gel shift analyses. Finally, we demonstrated the presence of liver X-activated receptor alpha and beta mRNAs, and showed that oxysterols stimulate a liver X-activated receptor response element transfected into keratinocytes. These data suggest that oxysterols induce keratinocyte differentiation, in part through increased AP-1-dependent transcription of the involucrin gene, an effect that may be mediated by liver X-activated receptor.

Cloning and expression of glycolipid transfer protein from bovine and porcine brain

Glycolipid transfer protein (GLTP) is a small (23-24 kDa), basic protein (pI congruent with 9.0) that accelerates the intermembrane transfer of various glycolipids. Here, we report the first cloning of cDNAs that encode the bovine and porcine GLTPs. The cDNA open reading frame for bovine GLTP was constructed by bridge-overlapping extension polymerase chain reaction (PCR) after obtaining partial coding cDNA clones by hot start, seminested, and rapid amplification of cDNA ends-PCR. The cDNA open reading frame for porcine GLTP was constructed by reverse transcriptase-PCR. The encoded amino acid sequences in the full-length bovine and porcine cDNAs were identical, consisting of 209 amino acid residues, and were nearly the same as the published sequence determined by Edman degradation. The cDNA encoded one additional amino acid at the N terminus (methionine), arginine at positions 10 and 200 instead of lysine, and threonine at position 65 instead of alanine. Expression of GLTP-cDNA in Escherichia coli using pGEX-6P-1 vector resulted in glutathione S-transferase (GST)-GLTP fusion protein. Regulation of growth and induction conditions led to approximately 50% of expressed fusion protein being soluble and active. Proteolytic cleavage of GST-GLTP fusion protein (bound to GST-Sepharose) and affinity purification resulted in fully active GLTP. Northern blot analyses of bovine tissues showed a single transcript of approximately 2.2 kilobases and the following hierarchy of mRNA levels: cerebrum > kidney > spleen congruent with lung congruent with cerebellum > liver > heart muscle. Reverse transcriptase-PCR analyses of mRNA levels supported the Northern blot results.

Inhibition of P-glycoprotein activity and chemosensitization of multidrug-resistant ovarian carcinoma 2780AD cells by hexanoylglucosylceramide

In the present study we show that neutral hexanoyl-(glyco)sphingolipids inhibit P-glycoprotein (Pgp) activity in human ovarian 2780AD cells. By contrast, hexanoylceramide and the gangliosides GM(3) and GM(2) had no effect on Pgp activity, whereas sphingosine had a stimulating effect. In the case of hexanoylglucosylceramide, inhibition of Pgp activity by was reflected by a regained doxorubicin sensitivity of cells, which were grown in medium supplemented with the lipid. Our results lead to the conclusion that a direct transmodulation of Pgp activity by glycolipids occurs, depending on lipid headgroup structure, which can result in reduced resistance to the chemotherapeutic agent doxorubicin.

Preferential killing of multidrug-resistant KB cells by inhibitors of glucosylceramide synthase

This study has compared the preferential killing of three multidrug-resistant (MDR) KB cell lines, KB-C1, KB-A1 and KB-V1 by two inhibitors of glucosylceramide synthase, 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) and 1-phenyl-2-hexadecanoylamino-3-pyrrolidino-1-propanol (PPPP), to the killing produced by these compounds in the drug-sensitive cell line, KB-3-1. Both of the inhibitors caused much greater induction of apoptosis in each of the three MDR cell lines than in the drug-sensitive cell line, as judged by morphological assay and confirmed by poly-(ADP-ribose)-polymerase cleavage. The highest level of apoptosis was produced following 24-h exposure to 5 microM PPPP. This treatment produced 75.8 (+/- 7.1)%, 73.6 (+/- 9.8)% and 75.3 (+/- 6.4)% apoptotic cells in the three MDR cell lines respectively, compared to 19.0 (+/- 9.8)% in the drug-sensitive cell line. A reduction in glucosylceramide level following inhibitor treatment occurred in KB-3-1 cells as well as in the MDR cell lines, suggesting that the increased apoptotic response in the MDR cells reflected a different downstream response to changes in the levels of this lipid in these cells compared to that in the drug-sensitive cells. These results suggest that the manipulation of glucosylceramide levels may be a fruitful way of causing the preferential killing of MDR cells in vitro and possibly in vivo.

Multidrug resistance modulators and doxorubicin synergize to elevate ceramide levels and elicit apoptosis in drug-resistant cancer cells

BACKGROUND

To provide insight for the development of more effective clinical agents, the authors attempted to elucidate the mechanisms of action of multidrug resistance (MDR) modulators. Previously, the authors found that MDR modulators blocked the conversion of ceramide to glucosylceramide in MDR cells, thereby enhancing cytotoxicity. Because ceramide is a critical component of the apoptosis signaling cascade, the current study examined the impact of therapy using agents that elicit ceramide formation combined with agents that block ceramide glycosylation.

METHODS

Doxorubicin-resistant human breast carcinoma cells (MCF-7-AdrR) were treated with either doxorubicin, tamoxifen, cyclosporine A, or the cyclosporine A analog SDZ PSC 833 (PSC 833) or with combinations thereof, and ceramide and glucosylceramide metabolisms were measured by cell radiolabeling. Cell viability was quantitated spectrophotometrically and apoptosis was evaluated analyzing DNA integrity by gel electrophoresis.

RESULTS

Whereas cyclosporine A blocked the generation of glucosylceramide in MCF-7-AdrR cells, a chemical cousin, PSC 833, elicited a 3-fold increase in glucosylceramide and a 5-fold increase in ceramide levels at 24 hours. The PSC 833 response was time-dependent(as early as 30 minutes) and dose-dependent (as low as 0.1 microM). The appearance of ceramide foreran the generation of glucosylceramide. Sphingomyelin levels were not decreased in response to PSC 833; however, Fumonisin B1, a ceramide synthase inhibitor, blocked PSC 833-induced ceramide generation. Adding tamoxifen, which blocks ceramide glycosylation, to the PSC 833 regimen boosted ceramide levels 11-fold over controls and caused DNA fragmentation. A 3-component regimen comprised of tamoxifen, doxorubicin, and PSC 833 increased ceramide levels 26-fold and brought cell viability to zero.

CONCLUSIONS

These results demonstrate that MDR modulators can be used separately, in combination, or in conjunction with chemotherapy at clinically relevant concentrations to manipulate cellular ceramide levels and restore sensitivity in the drug resistant setting. As such, this represents a new direction in the treatment of cancer.

Regulation of glycosphingolipid metabolism in liver during the acute phase response

The host response to infection is associated with multiple alterations in lipid and lipoprotein metabolism. We have shown recently that endotoxin (lipopolysaccharide (LPS)) and cytokines enhance hepatic sphingolipid synthesis, increase the activity and mRNA levels of serine palmitoyltransferase, the first committed step in sphingolipid synthesis, and increase the content of sphingomyelin, ceramide, and glucosylceramide (GlcCer) in circulating lipoproteins in Syrian hamsters. Since the LPS-induced increase in GlcCer content of lipoproteins was far greater than that of ceramide or sphingomyelin, we have now examined the effect of LPS and cytokines on glycosphingolipid metabolism. LPS markedly increased the mRNA level of hepatic GlcCer synthase, the enzyme that catalyzes the first glycosylation step of glycosphingolipid synthesis. The LPS-induced increase in GlcCer synthase mRNA levels was seen within 2 h, sustained for 8 h, and declined to base line by 24 h. LPS-induced increase in GlcCer synthase mRNA was partly accounted for by an increase in its transcription rate. LPS produced a 3-4-fold increase in hepatic GlcCer synthase activity and significantly increased the content of GlcCer (the immediate product of GlcCer synthase reaction) as well as ceramide trihexoside and ganglioside GM3 (products distal to the GlcCer synthase step) in the liver. Moreover, both tumor necrosis factor-alpha and interleukin-1beta, cytokines that mediate many of the metabolic effects of LPS, increased hepatic GlcCer synthase mRNA levels in vivo as well as in HepG2 cells in vitro, suggesting that these cytokines can directly stimulate glycosphingolipid metabolism. These results indicate that LPS and cytokines up-regulate glycosphingolipid metabolism in vivo and in vitro. An increase in GlcCer synthase mRNA levels and activity leads to the increase in hepatic GlcCer content and may account for the increased GlcCer content in circulating lipoproteins during the acute phase response.

Expression of glucosylceramide synthase, converting ceramide to glucosylceramide, confers adriamycin resistance in human breast cancer cells

Multidrug-resistant cancer cells display elevated levels of glucosylceramide (Lavie, Y., Cao, H. T., Volner, A., Lucci, A., Han, T. Y., Geffen, V., Giuliano, A. E., and Cabot, M. C. (1997) J. Biol. Chem. 272, 1682-1687). In this study, we have introduced glucosylceramide synthase (GCS) into wild type MCF-7 breast cancer cells using a retroviral tetracycline-on expression system, and we developed a cell line, MCF-7/GCS. MCF-7/GCS cells expressed an 11-fold higher level of GCS activity compared with the parental cell line. Interestingly, the transfected cells demonstrated strong resistance to adriamycin and to ceramide, whereas both agents were highly cytotoxic to MCF-7 cells. The EC50 values of adriamycin and ceramide were 11-fold (p < 0.0005) and 5-fold (p < 0.005) higher, respectively, in MCF-7/GCS cells compared with MCF-7 cells. Ceramide resistance displayed by MCF-7/GCS cells closely paralleled the activity of expressed GCS with a correlation coefficient of 0.99. In turn, cellular resistance and GCS activity were dependent upon the concentration of the expression mediator doxycycline. Adriamycin resistance in MCF-7/GCS cells was related to the hyperglycosylation of ceramide and was not related to shifts in the levels of either P-glycoprotein or Bcl-2. This work demonstrates that overexpression of GCS, which catalyzes ceramide glycosylation, induces resistance to adriamycin and ceramide in MCF-7 breast cancer cells.

Inherited disorders of epidermal keratinization

There have been a number of major discoveries recently in the field of dermatological science which have enabled us to determine the causes of inherited skin diseases of previously unknown etiology. In this paper we will review some important aspects of the biology of epidermal differentiation and the recent advances in understanding of the molecular mechanism underlying genetic diseases of keratinization.

Sphingolipids in atherosclerosis and vascular biology

Sphingolipids and their metabolic products are now known to have second-messenger functions in a variety of cellular signaling pathways. Lactosylceramide (LacCer), a glycosphingolipid (GSL) present in vascular cells such as endothelial cells, smooth muscle cells, macrophages, neutrophils, platelets, and monocytes, contributes to atherosclerosis. Large amounts of LacCer accumulate in fatty streaks, intimal plaque, and calcified intimal plaque, along with oxidized low density lipoproteins (Ox-LDLs), growth factors, and proinflammatory cytokines. A possible role for LacCer in vascular cell biology was suggested when this GSL was found to stimulate the proliferation in vitro of aortic smooth muscle cells (ASMCs). A further link of LacCer in atherosclerosis was uncovered by the finding that Ox-LDLs stimulated specifically the biosynthesis of LacCer. Ox-LDL-stimulated endogenous synthesis of LacCer by activation of UDP-Gal:GlcCer,beta1-4galtransferase (GalT-2) is an early step in this signaling pathway. In turn, LacCer serves as a lipid second messenger that orchestrates a signal transduction pathway, ultimately leading to cell proliferation. This signaling pathway includes LacCer-mediated activation of NADPH oxidase that produces superoxide. Such superoxide molecules stimulate the GTP loading of p21(ras). Subsequently, the kinase cascade (Raf-1, Mek2, and p44MAPK [mitogen-activated protein kinase]) is activated. The phosphorylated form of p44MAPK translocates from the cytoplasm to the nucleus and engages in c-fos expression, proliferating cell nuclear antigen (PCNA) such as cyclin activation, and cell proliferation takes place. Interestingly, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of GalT-2, can abrogate the Ox-LDL-mediated activation of GalT-2, the signal kinase cascade noted above, as well as cell proliferation. Additional studies have revealed that LacCer mediates the tumor necrosis factor-alpha (TNF-alpha)-induced nuclear factor-kappaB expression and intercellular adhesion molecule (ICAM-1) expression in vascular endothelial cells via the redox-dependent transcriptional pathway. LacCer also stimulates the expression of CD11/CD8, or Mac-1, on the surface of human neutrophils. Collectively, this phenomenon may contribute to the adhesion of neutrophils or monocytes to the endothelial cell surface and thus initiate the process of atherosclerosis. In addition, the LacCer-mediated proliferation of ASMCs may contribute to the progression of atherosclerosis. On the other hand, programmed cell death (apoptosis) by proinflammatory cytokines such as TNF-alpha, interleukin-1, and high concentrations of Ox-LDL occur via activation of a cell membrane-associated neutral sphingomyelinase (N-SMase). N-SMase hydrolyzes sphingomyelin into ceramide and phosphocholine. In turn, ceramide or a homologue serves as an important stress-signaling molecule. Interestingly, an antibody against N-SMase can abrogate Ox-LDL- and TNF-alpha-induced apoptosis and therefore may be useful for in vivo studies of apoptosis in experimental animals. Because plaque stability is an integral aspect of atherosclerosis management, activation of N-SMase and subsequent apoptosis may be vital events in the onset of plaque rupture, stroke, or heart failure. Interestingly, in human liver cells, N-SMase action mediates the TNF-alpha-induced maturation of the sterol regulatory-element binding protein. Moreover, a cell-permeable ceramide can reconstitute the phenomenon above in a sterol-independent fashion. Such findings may provide new avenues for therapy for patients with atherosclerosis. The findings described here indicate an important role for sphingolipids in vascular biology and provide an exciting opportunity for further research in vascular disease and atherosclerosis.

The relationship between hyperproliferation and epidermal thickening in a mouse model for BCIE

Epidermal thickening is a phenomenon common to many genodermatoses but little is known about the underlying causes. We have recently created a mouse model for the human skin disease bullous congenital ichthyosiform erythroderma by gene targeting. Mice heterozygous for a truncated keratin 10 gene exhibit acanthosis and hyperkeratosis as seen in the human disease. The degree of epidermal thickening is highly variable, offering a novel opportunity to investigate how epidermal homeostasis is modulated in keratin disorders by comparing epidermis from different body regions. We have performed bromodeoxyuridine labeling experiments and detected proliferation antigens by immunohistochemical means to compare proliferation in the epidermis of wild-type and heterozygous mice. These results have been compared with the expression of epidermal differentiation markers and of the "hyperproliferation associated" keratins K6 and K16. These experiments indicated that hyperproliferation is only partly responsible for the morphologic changes and that other mechanisms such as decreased desquamation are likely to be involved.

Glucosylceramides stimulate mitogenesis in aged murine epidermis

Glucosylceramides (GlcCer) and ceramides (Cer) appear to have opposite effects on epidermal growth and differentiation. Whereas Cer inhibit mitosis and induce terminal differentiation and apoptosis in cultured keratinocytes, GlcCer is mitogenic in young murine epidermis. Using a recently described murine model of chronologic senescence we explored whether GlcCer is mitogenic in aged epidermis. Epidermal GlcCer content increases following topical applications of either conduritol-B epoxide (CBE), an inhibitor of GlcCer hydrolysis, or exogenous GlcCer in a penetration-enhancing vehicle. During chronologic aging in the hairless mouse, baseline epidermal DNA synthesis rates remain normal until 18 mo, but decline significantly at 24 mo. Topical CBE stimulates a 1.5- to 1.9-fold increase in epidermal DNA synthesis in all age groups (i.e., 1-2, 18, and 24 mo). Although the CBE induced increase in [3H]thymidine incorporation in 24 mo old animals is significant (p < 0.01), it is not sufficient to reach the absolute levels reached in similarly treated, younger mouse epidermis. Moreover, topical GlcCer induced mitogenesis is both dose dependent and hexose specific in young (1-2 mo old) animals, and remains effective in aged (< or = 24 mo old) animals. Furthermore, the CBE induced increase in DNA synthesis in aged epidermis is sufficient to produce epidermal hyperplasia. Finally, although an increased GlcCer:Cer ratio can alter stratum corneum barrier function and membrane structure, neither stratum corneum function nor extracellular membrane structure change under these experimental conditions, and therefore the mitogenic effects of increased epidermal GlcCer cannot be attributed to effects on the stratum corneum. These results show that: (i) elevations in endogenous GlcCer are mitogenic for aged as well as young murine epidermis; (ii) topical GlcCer is also mitogenic when delivered in an enhancing vehicle; and (iii) despite the putative importance of epidermal DNA synthesis for barrier homeostasis, these mitogenic alterations do not alter stratum corneum function.

Effect of an inhibitor of glucosylceramide synthesis on cultured human keratinocytes

Glucosylceramide (GlcCer) is a major glycosphingolipid component of epidermis, which is thought to be related to the barrier function of skin permeability. However, the role of glycosphingolipids in keratinocyte growth and differentiation has not been fully clarified. It has been reported that D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), an inhibitor of GlcCer synthase (EC 2.4.1.80), depletes cells of glycosphingolipids. This inhibitor has been used as a tool for elucidating their functions. In the present study, the effect of PDMP on cultured normal human keratinocytes was investigated. The cells were treated with various concentrations of PDMP. Forty-eight hours later cell growth, thymidine incorporation, and lipid content were studied. The cell growth and the incorporation of thymidine into cells were inhibited by PDMP in a dose dependent manner. The synthesis of GlcCer was strongly inhibited by PDMP treatment, whereas no significant changes in ceramide level were observed. We concluded that GlcCer in epidermis may play an important role in regulating epidermal growth and suggested that PDMP may be beneficial for treating proliferative skin disorders in the future.

Galactocerebroside and not glucocerebroside or ceramide stimulate epidermal beta-glucocerebrosidase activity

Glycosphingolipids including glucocerebroside (GluCer) and galactocerebroside (GalCer) have been recognized as bioreguratory lipids by our group and others. In addition, our recent study demonstrated that GalCer corrects dry skin conditions in humans. The processing of stratum corneum lipids, which occurs when beta-glucocerebrosidase (beta-GluCer'ase) changes GluCer to ceramide (Cer), is required to form the epidermal permeability barrier. We herein investigated the effects of GluCer, GalCer and Cer on the processing of GluCer to Cer by assaying epidermal beta-GluCer'ase in mice (155%, P < 0.01) when compared to vehicle treated controls, while neither GluCer nor Cer had this effect. Studies using inhibitors of beta-GluCer'ase or beta-galactosidase and measuring the optimum pH of the enzyme verified that GalCer specifically activated beta-GluCer'ase. We confirmed that GalCer significantly increased beta-GluCer'ase activity in the outer epidermal fraction (172%, P < 0.01) and that the activation of beta-GluCer'ase is not due to a direct activating effect of GalCer on the enzyme. Furthermore, the induction of beta-GluCer'ase activity by GalCer was also observed in cultured normal human deratinocytes (123%, P < 0.01). Finally, acylceramide content in stratum corneum was increased in mice treated with GalCer (194%, P < 0.0005). These results indicate that GalCer appears to affect the Cer construct in the stratum corneum by the activation of beta-GluCer'ase, which ultimately contribute to an enhancement of barrier formation.

The formation of competent barrier lipids in reconstructed human epidermis requires the presence of vitamin C

Our analysis of epidermal lipids revealed that (glucosyl)ceramide profiles in various human skin equivalents are different from those of native tissue. The main difference is the reduced content in skin equivalents of ceramides 4-7 and especially the very low content of the most polar ceramides 6 and 7, which contain hydroxylated sphingoid base and/or fatty acid. To facilitate hydroxylation, the culture medium was supplemented with vitamins C and E. Although in vitamin E-supplemented medium lipogenesis was not affected, in vitamin C-supplemented medium the content of glucosylceramides and of ceramides 6 and 7 was markedly increased, both in the presence and absence of serum and irrespective the substrate used (inert or natural, populated or not with fibroblasts). The improvement of the lipid profile was accompanied by a marked improvement of the barrier formation as judged from extensive production of lamellar bodies, their complete extrusion at the stratum granulosum/stratum corneum interface, and the formation of multiple broad lipid lamellar structures in the intercorneocyte space. The presence of well-ordered lipid lamellar phases was confirmed by small-angle x-ray diffraction. Some differences between native and reconstructed epidermis, however, were noticed. Although the long-range lipid lamellar phase was present in both the native and the reconstructed epidermis, the short lamellar phase was present only in native tissue. It remains to be established whether these differences can be ascribed to small differences in relative amounts of individual ceramides, to differences in fatty acid profiles, or to differences in cholesterol sulfate, pH, or calcium gradients. The results indicate the key role vitamin C plays in the formation of stratum corneum barrier lipids.