Decreased level of prosaposin in atopic skin

Nanodelivery Strategies for Skin Diseases with Barrier Impairment: Focusing on Ceramides and Glucocorticoids

The human epidermis has a characteristic lipidic composition in the stratum corneum, where ceramides play a crucial role in the skin barrier homeostasis and in water-holding capacity. Several skin diseases, such as atopic dermatitis and psoriasis, exhibit a dysfunction in the lipid barrier with altered ceramide levels and increased loss of transepidermal water. Glucocorticoids are normally employed in the therapeutical management of these pathologies. However, they have shown a poor safety profile and reduced treatment efficiency. The main objective of this review is to, within the framework of the limitations of the currently available therapeutical approaches, establish the relevance of nanocarriers as a safe and efficient delivery strategy for glucocorticoids and ceramides in the topical treatment of skin disorders with barrier impairment.

The Pathogenic and Therapeutic Implications of Ceramide Abnormalities in Atopic Dermatitis

Ceramides play an essential role in forming a permeability barrier in the skin. Atopic dermatitis (AD) is a common chronic skin disease associated with skin barrier dysfunction and immunological abnormalities. In patients with AD, the amount and composition of ceramides in the stratum corneum are altered. This suggests that ceramide abnormalities are involved in the pathogenesis of AD. The mechanism underlying lipid abnormalities in AD has not yet been fully elucidated, but the involvement of Th2 and Th1 cytokines is implicated. Ceramide-dominant emollients have beneficial effects on skin barrier function; thus, they have been approved as an adjunctive barrier repair agent for AD. This review summarizes the current understanding of the mechanisms of ceramide abnormalities in AD. Furthermore, the potential therapeutic approaches for correcting ceramide abnormalities in AD are discussed.

Skin of atopic dermatitis patients shows disturbed β-glucocerebrosidase and acid sphingomyelinase activity that relates to changes in stratum corneum lipid composition

Patients with Atopic Dermatitis (AD) suffer from inflamed skin and skin barrier defects. Proper formation of the outermost part of the skin, the stratum corneum (SC), is crucial for the skin barrier function. In this study we analyzed the localization and activity of lipid enzymes β-glucocerebrosidase (GBA) and acid sphingomyelinase (ASM) in the skin of AD patients and controls. Localization of both the expression and activity of GBA and ASM in the epidermis of AD patients was altered, particularly at lesional skin sites. These changes aligned with the altered SC lipid composition. More specifically, abnormal localization of GBA and ASM related to an increase in specific ceramide subclasses [AS] and [NS]. Moreover we related the localization of the enzymes to the amounts of SC ceramide subclasses and free fatty acids (FFAs). We report a correlation between altered localization of active GBA and ASM and a disturbed SC lipid composition. Localization of antimicrobial peptide beta-defensin-3 (HBD-3) and AD biomarker Thymus and Activation Regulated Chemokine (TARC) also appeared to be diverging in AD skin compared to control. This research highlights the relation between correct localization of expressed and active lipid enzymes and a normal SC lipid composition for a proper skin barrier.

Permeability Barrier and Microstructure of Skin Lipid Membrane Models of Impaired Glucosylceramide Processing

Ceramide (Cer) release from glucosylceramides (GlcCer) is critical for the formation of the skin permeability barrier. Changes in β-glucocerebrosidase (GlcCer'ase) activity lead to diminished Cer, GlcCer accumulation and structural defects in SC lipid lamellae; however, the molecular basis for this impairment is not clear. We investigated impaired GlcCer-to-Cer processing in human Cer membranes to determine the physicochemical properties responsible for the barrier defects. Minor impairment (5-25%) of the Cer generation from GlcCer decreased the permeability of the model membrane to four markers and altered the membrane microstructure (studied by X-ray powder diffraction and infrared spectroscopy), in agreement with the effects of topical GlcCer in human skin. At these concentrations, the accumulation of GlcCer was a stronger contributor to this disturbance than the lack of human Cer. However, replacement of 50-100% human Cer by GlcCer led to the formation of a new lamellar phase and the maintenance of a rather good barrier to the four studied permeability markers. These findings suggest that the major cause of the impaired water permeability barrier in complete GlcCer'ase deficiency is not the accumulation of free GlcCer but other factors, possibly the retention of GlcCer bound in the corneocyte lipid envelope.

The role of epidermal sphingolipids in dermatologic diseases

Sphingolipids, a group of lipids containing the sphingoid base, have both structural and biological functions in human epidermis. Ceramides, as a part of extracellular lipids in the stratum corneum, are important elements of the skin barrier and are involved in the prevention of transepidermal water loss. In addition, ceramides regulate such processes as proliferation, differentiation and apoptosis of keratinocytes. Another important sphingolipid, sphingosine-1-phosphate (S1P), inhibits proliferation and induces differentiation of keratinocytes. A recent clinical study of the efficacy and safety of ponesimod (a selective modulator of the S1P receptor 1) suggested that sphingolipid metabolism may become a new target for the pharmacological treatment of psoriasis. The role of sphingolipids in some dermatologic diseases, including psoriasis, atopic dermatitis and ichthyoses was summarized in this article.

The structure, function, and importance of ceramides in skin and their use as therapeutic agents in skin-care products

Ceramides (CERs) are epidermal lipids that are important for skin barrier function. Much research has been devoted to identifying the numerous CERs found in human skin and their function. Alterations in CER content are associated with a number of skin diseases such as atopic dermatitis. Newer formulations of skin-care products have incorporated CERs into their formulations with the goal of exogenously applying CERs to help skin barrier function. CERs are a complex class of molecules and because of their growing ubiquity in skin-care products, a clear understanding of their role in skin and use in skin-care products is essential for clinicians treating patients with skin diseases. This review provides an overview of the structure, function, and importance of skin CERs in diseased skin and how CERs are being used in skin-care products to improve or restore skin barrier function.

Atopic dermatitis in dogs is associated with a high heterogeneity in the distribution of protein-bound lipids within the stratum corneum

The stratum corneum (SC) was taken from five atopic dogs by tape stripping (12 strips) of non-lesional areas of the abdomen. The free and protein-bound lipids were extracted and analyzed by thin-layer chromatography after fractionation on aminopropyl-bonded silica gel columns. A very frequent feature was the heterogeneity in the lipid content of consecutive layers. This was even more accentuated for the covalently bound lipids, with variations from one layer to another in the concentrations of cholesterol, omega hydroxylated ceramides and omega hydroxylated long-chain fatty acids. Among the free lipids, large amounts of glucosylceramides were present in canine atopic SC although they are nearly absent from the SC of normal dogs. A heterogeneous distribution of lipids was seen in canine atopic SC. These results suggest that strikingly deep variations occur in the lipid metabolism of keratinocytes in the skin of atopic dogs. In order to gain insight into this phenomenon, further studies should be focused on the activity of enzymes involved in both biosynthetic and catabolic processes.

Murine atopic dermatitis responds to peroxisome proliferator-activated receptors alpha and beta/delta (but not gamma) and liver X receptor activators

BACKGROUND

Atopic dermatitis (AD) is a chronic inflammatory dermatosis now increasingly linked to mutations that alter the structure and function of the stratum corneum. Activators of peroxisome proliferator-activated receptors (PPARs) alpha, beta/delta, and gamma and liver X receptor (LXR) regulate epidermal protein and lipid production, leading to superior barrier function. Additionally, some of these activators exhibit potent antihyperplastic and anti-inflammatory activity in irritant contact dermatitis and acute allergic contact dermatitis murine models.

OBJECTIVE

We evaluated the efficacy of PPAR/LXR activation in a hapten (oxazolone [Ox])-induced AD-like model (Ox-AD) in hairless mice.

METHODS

Ox-AD was established with 10 Ox challenges (every other day) on the flank. After the establishment of Ox-AD, twice-daily topical application with individual PPAR/LXR activators was then performed for 4 days, with continued Ox challenges every other day. The efficacy of topical PPAR/LXR activators to reduce parameters of Ox-AD was assessed physiologically, morphologically, and immunologically.

RESULTS

Certain topical activators of PPARalpha, PPARbeta/delta, and LXR, but not activators of PPARgamma, reversed the clinical dermatosis, significantly improved barrier function, and increased stratum corneum hydration in Ox-AD mice. In addition, the same activators, but again not PPARgamma, largely reversed the immunologic abnormalities in Ox-AD mice, including the increased T(H)2 markers, such as tissue eosinophil/mast cell density, serum thymus and activation-related chemokine levels, the density of chemoattractant receptor-homologous molecule expressed on T(H)2-positive lymphocytes (but not serum IgE levels), and reduced IL-1alpha and TNF-alpha activation, despite ongoing hapten challenges.

CONCLUSION

These results suggest that topical applications of certain activators/ligands of PPARalpha, PPARbeta/delta, and LXR could be useful for the treatment of AD in human subjects.

Epidermal sphingolipids: Metabolism, function, and roles in skin disorders

Mammalian epidermis produces and delivers large quantities of glucosylceramide and sphingomyelin precursors to stratum corneum extracellular domains, where they are hydrolyzed to corresponding ceramide species. This cycle of lipid precursor formation and subsequent hydrolysis represents a mechanism that protects the epidermis against potentially harmful effects of ceramide accumulation within nucleated cell layers. Prominent skin disorders, such as psoriasis and atopic dermatitis, have diminished epidermal ceramide levels, reflecting altered sphingolipid metabolism, that may contribute to disease severity/progression. Enzymatic processes in the hydrolysis of glucosylceramide and sphingomyelin, and the roles of sphingolipids in skin diseases, are the focus of this review.

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.

Role of ceramides in barrier function of healthy and diseased skin

Stratum corneum intercellular lipids play an important role in the regulation of skin water barrier homeostasis and water-holding capacity. Modification of intercellular lipid organization and composition may impair these properties. Patients with skin diseases such as atopic dermatitis, psoriasis, contact dermatitis, and some genetic disorders have diminished skin barrier function. Lipid composition in diseased skin is characterized by decreased levels of ceramide and altered ceramide profiles. To clarify mechanisms underlying ceramides as a causative factor of skin disease, investigators have examined the activity of enzymes in the stratum corneum on ceramide production and degradation. The activities of ceramidase, sphingomyelin deacylase, and glucosylceramide deacylase are increased in epidermal atopic dermatitis. Investigators have also compared the expression levels of sphingolipid activator protein in the epidermis of normal and diseased skin. A decreased level of prosaposin has been identified in both atopic dermatitis and psoriasis. These results indicate that decreased ceramide level is a major etiologic factor in skin diseases. Hence, topical skin lipid supplementation may provide opportunities for controlling ceramide deficiency and improving skin condition.

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.

The stratum corneum: structure and function in health and disease

Our understanding of the formation, structure, composition, and maturation of the stratum corneum (SC) has progressed enormously over the past 30 years. Today, there is a growing realization that this structure, while faithfully providing a truly magnificent barrier to water loss, is a unique, intricate biosensor that responds to environmental challenges and surface trauma by initiating a series of biologic processes which rapidly seek to repair the damage and restore barrier homeostasis. The detailed ultrastructural, biochemical, and molecular dissection of the classic "bricks and mortar" model of the SC has provided insights into the basis of dry, scaly skin disorders that range from the cosmetic problems of winter xerosis to severe conditions such as psoriasis. With this knowledge comes the promise of increasingly functional topical therapies.

Permeability barrier disruption increases the level of serine palmitoyltransferase in human epidermis

Sphingolipids play an important role in the homeostasis and barrier function of human stratum corneum. A disturbance of sphingolipid formation is supposed to be a crucial factor for the increased transepidermal water loss in common skin diseases like atopic eczema or psoriasis. The key enzyme for de novo sphingolipid synthesis is serine palmitoyltransferase, which consists of two different subunits, named LCB1 and LCB2 proteins. In order to investigate the induction of LCB2 synthesis in human epidermis, skin barrier disruption was performed by tape stripping on the forearm of healthy volunteers enough to obtain a 3-4-fold increase in transepidermal water loss. Skin punch biopsies were taken before and 0.5, 2, 4, and 8 h after tape stripping by each volunteer to measure LCB2 at the mRNA level. Additional biopsies taken before and 12 h after tape stripping were used to evaluate LCB2 at the protein level. Our results show that 0.5 and 2 h after tape stripping the LCB2 mRNA expression was decreased compared to control in all cases. A significant increase in LCB2 mRNA expression was detectable 4 h after barrier disruption, with individual variations; no further increase was detectable 8 h after tape stripping. Immunohistochemical analysis 12 h after barrier disruption showed increased LCB2 immunolocalization in the inner epidermis, whereas in the outer epidermis it was similar to control. LCB2 mRNA expression preceded the expression of the corresponding protein by 4-8 h. Our findings support the concept that an increase in transepidermal water loss is an obligatory trigger for the upregulation of serine palmitoyltransferase mRNA expression in humans.

The level of prosaposin is decreased in the skin of patients with psoriasis vulgaris

Ceramides are the most abundant lipids constituting the intercellular matrix of the skin stratum corneum and their critical role in skin homeostasis has been extensively documented. Their concentration in the skin highly depends on the rate of availability of the enzymes involved in ceramide generation. The aim of this study was to investigate whether the concentration of prosaposin was altered in the skin of patients with psoriasis vulgaris. Prosaposin, the precursor of saposins (sphingolipid activator proteins), was measured in lesional and nonlesional skin of psoriatic patients and in normal skin from surgical patients, both at the mRNA and at the protein level. Densitometric analysis of reverse transcriptase-polymerase chain reaction bands separated by gel-electrophoresis showed a progressive decrease of prosaposin mRNA expression in nonlesional and lesional psoriatic skin, being substantially decreased in lesional psoriatic skin compared with normal control skin. Immunohistochemical analysis showed a significant decrease of prosaposin level in the stratum corneum of psoriatic lesional skin (both in active-type and in chronic-type plaque) compared with nonlesional and with normal skin (p < 0.01), and in psoriatic nonlesional skin compared with normal control (p < 0.05). Immunolocalization of sphingomyelinase in lesional and nonlesional psoriatic skin showed a decrease in the level of this enzyme in the stratum corneum of psoriatic lesional, compared with nonlesional skin. These results support the concept that disturbance of epidermal barrier function caused by derangement in ceramide generation can be crucial for the development of psoriatic skin diseases.

Sphingolipid activator proteins are required for epidermal permeability barrier formation

The epidermal permeability barrier is maintained by extracellular lipid membranes within the interstices of the stratum corneum. Ceramides, the major components of these multilayered membranes, derive in large part from hydrolysis of glucosylceramides mediated by stratum corneum beta-glucocerebrosidase (beta-GlcCerase). Prosaposin (pSAP) is a large precursor protein that is proteolytically cleaved to form four distinct sphingolipid activator proteins, which stimulate enzymatic hydrolysis of sphingolipids, including glucosylceramide. Recently, pSAP has been eliminated in a mouse model using targeted deletion and homologous recombination. In addition to the extracutaneous findings noted previously, our present data indicate that pSAP deficiency in the epidermis has significant consequences including: 1) an accumulation of epidermal glucosylceramides together with below normal levels of ceramides; 2) alterations in lipids that are bound by ester linkages to proteins of the cornified cell envelope; 3) a thickened stratum lucidum with evidence of scaling; and 4) a striking abnormality in lamellar membrane maturation within the interstices of the stratum corneum. Together, these results demonstrate that the production of pSAP, and presumably mature sphingolipid activator protein generation, is required for normal epidermal barrier formation and function. Moreover, detection of significant amounts of covalently bound omega-OH-GlcCer in pSAP-deficient epidermis suggests that deglucosylation to omega-OH-Cer is not a requisite step prior to covalent attachment of lipid to cornified envelope proteins.

Localization of sphingomyelinase in lesional skin of atopic dermatitis patients

Because it has been suggested that the majority of the activity hydrolysing [N-methyl-14C] sphingomyelin is due to sphingomyelin acylase in the lesional skins of atopic dermatitis (AD), in this study we used immunologic techniques to localize and quantitate sphingomyelinase in AD lesional and normal skin. A polyclonal antibody raised against a synthetic polypeptide corresponding to a portion of the amino acid sequence deduced from the cDNA of human acid sphingomyelinase, cross-reacted with a 58 kDa, pI 5.8 human epidermal protein in an immunoblot analysis. The 58 kDa protein-rich fraction, partially purified by immunoprecipitation, converted [N-methyl-14C]-sphingomyelin to 14C-phosphorylcholine and ceramides. The reaction products were immunohistochemically observed in the intercellular domain from the upper spinous cell layer to the upper stratum corneum cell layers in the lesional skin of AD patients. Immunoelectron-microscopically, gold particles appeared to be concentrated in the intercellular domains of the granular-upper stratum corneum cells in the lesional skin of AD patients. The total amount of the 58 kDa protein in a 7 mm2 area of the skin was measured by quantitative immunoblot analysis; and was slightly increased in the lesional skin samples [3.5 +/- 0.3 microg per 7 mm2 (n = 7)], as compared with the nonlesional skin samples of AD patients [2.8 +/- 0.19 microg per 7 mm2 (n = 10)] and with the normal skin samples [2.7 +/- 0.22 microg per 7 mm2 (n = 10)]. This difference (between the lesional skin of AD and the nonlesional skin of AD or the normal control) was significant (nonpaired student's t test, p < 0.05).