The phospholipases A of epidermis

Natural skin surface pH is on average below 5, which is beneficial for its resident flora

Variable skin pH values are being reported in literature, all in the acidic range but with a broad range from pH 4.0 to 7.0. In a multicentre study (N = 330), we have assessed the skin surface pH of the volar forearm before and after refraining from showering and cosmetic product application for 24 h. The average pH dropped from 5.12 +/- 0.56 to 4.93 +/- 0.45. On the basis of this pH drop, it is estimated that the 'natural' skin surface pH is on average 4.7, i.e. below 5. This is in line with existing literature, where a relatively large number of reports (c. 50%) actually describes pH values below 5.0; this is in contrast to the general assumption, that skin surface pH is on average between 5.0 and 6.0. Not only prior use of cosmetic products, especially soaps, have profound influence on skin surface pH, but the use of plain tap water, in Europe with a pH value generally around 8.0, will increase skin pH up to 6 h after application before returning to its 'natural' value of on average below 5.0. It is demonstrated that skin with pH values below 5.0 is in a better condition than skin with pH values above 5.0, as shown by measuring the biophysical parameters of barrier function, moisturization and scaling. The effect of pH on adhesion of resident skin microflora was also assessed; an acid skin pH (4-4.5) keeps the resident bacterial flora attached to the skin, whereas an alkaline pH (8-9) promotes the dispersal from the skin.

Lipides et inflammation cutanée : place des phospholipases A2

Phospholipases A2 (PLA2) are enzymes that catalyse the hydrolysis of glycerophospholipids at the sn-2 position, generating free fatty acids and lysophospholipids. At present, PLA2 family consists of 12 groups. PLA2 are involved in many pathophysiological processes such as barrier function, eicosanoid production, and inflammation. They are implicated in inflammatory diseases of the skin: psoriasis, eczema, atopy. The presence of PLA2 activity has been demonstrated several years ago, however the precise localization of all these PLA2 in the epidermis and its appendages has to be determined. Further studies have shown that these enzymes are expressed in various layers of epidermis. This differential localization suggests different roles for each PLA2 in skin physiology and during inflammation.

Flavonoids differentially inhibit guinea pig epidermal cytosolic phospholipase A2

Cytosolic phospholipase A2 (cPLA2) is believed to involve the regulation of essential cellular processes. Like other cell types, epidermal cPLA2 may participate in various metabolic processes including eicosanoid generation. In this investigation, we demonstrated the presence of cPLA2 in guinea pig epidermis. The epidermal cPLA2 is Ca2+-dependent, active at micromolar concentration of Ca2+ and resistant to disulfide-reducing agents. Furthermore, it is inhibited by methyl arachidonyl fluorophosphonate (MAFP), a selective inhibitor of cPLA2, while 12-epi-scalardial (a sPLA2 inhibitor) did not cause inhibition. A test of several flavonoids revealed that quercetin (flavonol) weakly inhibited cPLA2, while flavanone had negligible inhibitory activity. In contrast, amentoflavone and ginkgetin (biflavones) markedly inhibited cPLA2 activity in the epidermis. These results underscore that different flavonoids do vary in their capability to exert differential effects on arachidonate metabolism in the skin via modulation of epidermal cPLA2 activity.

Generation of free fatty acids from phospholipids regulates stratum corneum acidification and integrity

There is evidence that the "acid mantle" of the stratum corneum is important for both permeability barrier formation and cutaneous antimicrobial defense. The origin of the acidic pH of the stratum corneum remains conjectural, however. Both passive (e.g., eccrine/sebaceous secretions, proteolytic) and active (e.g., proton pumps) mechanisms have been proposed. We assessed here whether the free fatty acid pool, which is derived from phospholipase-mediated hydrolysis of phospholipids during cornification, contributes to stratum corneum acidification and function. Topical applications of two chemically unrelated secretory phospholipase sPLA2 inhibitors, bromphenacylbromide and 1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol, for 3 d produced an increase in the pH of murine skin surface that was paralleled not only by a permeability barrier abnormality but also altered stratum corneum integrity (number of strippings required to break the barrier) and decreased stratum corneum cohesion (protein weight removed per stripping). Not only stratum corneum pH but also all of the functional abnormalities normalized when either palmitic, stearic, or linoleic acids were coapplied with the inhibitors. Moreover, exposure of intact murine stratum corneum to a neutral pH for as little as 3 h produced comparable abnormalities in stratum corneum integrity and cohesion, and further amplified the inhibitor-induced functional alterations. Furthermore, short-term applications of an acidic pH buffer to inhibitor-treated skin also reversed the abnormalities in stratum corneum integrity and cohesion, despite the ongoing decrease in free fatty acid levels. Finally, the secretory-phospholipase-inhibitor-induced alterations in integrity/cohesion were in accordance with premature dissolution of desmosomes, demonstrated both by electron microscopy and by reduced desmoglein 1 levels in the stratum corneum (shown by immunofluorescence staining and visualized by confocal microscopy). Together, these results demonstrate: (i) the importance of phospholipid-to-free-fatty-acid processing for normal stratum corneum acidification; and (ii) the potentially important role of this pathway not only for barrier homeostasis but also for the dual functions of stratum corneum integrity and cohesion.

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.

Identification of pancreatic type I secreted phospholipase A2 in human epidermis and its determination by tape stripping

Phospholipases A2 (PLA2) catalyse the release of fatty acids from the sn-2 position of phospholipids and have been suggested to play a key part in permeability barrier homeostasis. Using a sensitive and versatile fluorometric method, significant PLA2 activity has been detected in both human skin homogenates and tape strippings of stratum corneum. Based on various properties (resistance to heat and sulphuric acid treatment, neutral optimal pH, absolute requirement for millimolar calcium concentrations, inhibition by dithiothreitol and p-bromophenacyl bromide, and resistance to a trifluoromethyl ketone derivative of arachidonic acid, AACOCF3, a specific inhibitor of cytosolic PLA2), this enzyme was characterized as a secretory PLA2 (sPLA2). Immunohistochemistry revealed strong labelling of type I pancreatic sPLA2 at the stratum corneum-stratum granulosum junction, type II sPLA2 being undetectable. An increase in PLA2 activity in tape-stripped material from the deepest level of the stratum corneum was correlated with partial morphological disappearance of type I sPLA2 immunolabelling. Our data thus provide the first convincing evidence that pancreatic sPLA2 is significantly expressed in human epidermis, where it might participate in the accumulation of free fatty acids contributing to the permeability barrier. In addition, our method for determining PLA2 activity in easily available tape strippings should allow further clinical studies aimed to explore possible PLA2 abnormalities in various dermatoses.

Melanocyte-stimulating properties of secretory phospholipase A2

Phospholipase A2 (PLA2) catalyzes the release of free fatty acids from membrane phospholipids, and its products derived from these fatty acids, such as prostaglandins and leukotrienes, significantly up-regulate the key melanogenic enzyme, tyrosinase, in melanocytes. This has led to suggestions that PLA2 itself triggers melanin synthesis in melanogenesis following UV irradiation or inflammation. We have examined the effect of secretory PLA2 (sPLA2) on melanogenesis in cultured human melanocytes. Secretory PLA2 stimulated DNA synthesis and melanin synthesis, and these phenomena were completely inhibited by treatment with a phospholipase inhibitor, p-bromophenacyl bromide, demonstrating that the catalytic activity of sPLA2 is required for melanogenesis. Secretory PLA2 also stimulated tyrosinase activity, increased the amount of tyrosinase-related protein-1 and up-regulated the expression of both mRNA. These findings suggest that sPLA2 is an important mediator of UV-induced or postinflammatory pigmentation.

Human keratinocytes possess an sn-2 acylhydrolase that is biochemically similar to the U937-derived 85-kDa phospholipase A2

The phospholipase A2 (PLA2) activities that are localized in the keratinocyte cytosolic and microsomal fractions were biochemically and pharmacologically characterized. The cytosol and to a lesser extent the microsome were sensitive to heat treatment and stable in the presence of sulfhydryl reducing agents. Both fractions were almost totally inactivated by reduction of pH to 2. The cytosolic activity demonstrated a sevenfold preference for arachidonic acid over oleic acid in the sn-2 position of substrate phospholipid and the microsome exhibited a fourfold preference. Neither the cytosol nor the microsome was inactivated by a neutralizing mouse monoclonal antibody 3F10 generated against recombinant human (rh) type II 14-kDa PLA2. Western immunoblot analysis of both fractions identified a high-molecular-mass protein in keratinocyte cytosol but not the microsome that migrated with rh 85-kDa PLA2. Neither the cytosol nor the microsome possessed immunoreactive bands that migrated with rh type II 14-kDa PLA2 when probed with monoclonal antibody 3F10. Further analysis of the cytosolic activity showed that it was activated by submicromolar concentrations of Ca2+, reduced by arachidonyl trifloromethylketone, a selective 85-kDa PLA2 inhibitor, but was unaffected by C-7 phosphonate phospholipid, a selective 14-kDa PLA2 transition state inhibitor. Taken together, the data supports the existence of a PLA2 activity in the cytosol that displays characteristics that are indistinguishable from those exhibited by the 85-kDa PLA2. Alternatively, both the cytosol and microsome were devoid of type II 14-kDa-like PLA2 activity. The failure of 12-epi scalaradial, a 14-kDa PLA2 inhibitor, to modify A23187-stimulated keratinocyte prostaglandin E2 release, was consistent with the biochemistry and suggests that the 85-kDa PLA2 may play an important role in keratinocyte prostaglandin E2 formation.

Intracellular target for alpha-terthienyl photosensitization: involvement of lysosomal membrane damage

Intracellular targets for the photosensitizer alpha-terthienyl (alpha T) were examined by fluorescence microscopy and microfluorospectrometry using human nonkeratinized buccal cells. Intracellular distribution of alpha T was observed as fluorescent patches widely dispersed in the cytoplasm. The distribution of the fluorescent patches was compared with that of acid phosphatase activity visualized as an azo dye produced by the fast garnet 2-methyl-4-[(2-methyl-phenyl)azo]benzenediasonium sulfate reaction. Because both the distribution sites coincided, lysosomes were the likely sites of intracellular affinity of alpha T. However, because acid phosphatase is not a specific lysosomal marker, we tried to detect another lysosomal enzyme, beta-galactosidase, to confirm if the fluorescent patches were lysosomes, using fluorescein-di-(beta-D-galactopyranoside) (FDG) as a fluorogenic substrate. Without UV-A (320-400 nm) irradiation of the cells after uptake of alpha T and FDG, no significant fluorescence was observed. In contrast, with prior UV-A irradiation in the presence of alpha T and FDG, the bright yellow fluorescence of fluorescein, which is the digested product of FDG, was clearly detected in the cells by fluorescence microscopy. This observation implied that inflow of external FDG into the lysosomes is caused by lysosomal membrane damage on alpha T photosensitization. The present results indicated that lysosomes are the primary photosensitization site of alpha T.

Glucocorticoid-induced modulation of the beta-adrenergic adenylate cyclase response of epidermis: its relation to epidermal phospholipase A2 activity

It has been suggested that glucocorticoids produce their biologic effects through the synthesis of phospholipase A2 inhibitor protein (lipocortin) in various cell systems. Recent studies from our laboratory revealed that glucocorticoids augment the beta-adrenergic adenylate cyclase response of epidermis and that this effect depends on a protein synthesis mechanism. In order to elucidate the possible mechanism of this glucocorticoid effect in terms of phospholipase A2 activity, an in vitro pig skin incubation system was employed. Mepacrine, a phospholipase A2 inhibitor, augmented the beta-adrenergic adenylate cyclase response of epidermis as glucocorticoids. The effect of mepacrine was stronger and was observed earlier than that of glucocorticoid (hydrocortisone). The addition of both mepacrine and hydrocortisone at their optimal concentrations in the incubation medium, resulted in neither an additive nor a synergistic effect on the beta-adrenergic augmentation. On the other hand, melittin, a phospholipase A2 stimulator, depressed the beta-adrenergic adenylate cyclase response. The addition of both melittin and hydrocortisone in the incubation medium resulted in the inhibition of the hydrocortisone-induced beta-adrenergic augmentation effect. Following long-term incubation with hydrocortisone, the epidermal phospholipase A2 activity was significantly decreased. These results indicate that glucocorticoids might affect the beta-adrenergic adenylate cyclase response of epidermis through the synthesis of phospholipase A2 inhibitor protein (lipocortin) as in other cell systems.

Epidermal permeability barrier: transformation of lamellar granule-disks into intercellular sheets by a membrane-fusion process, a freeze-fracture study

Freeze-fracture replication of lamellar granules and intercellular sheets of the horny layer in mouse, chicken, and snake epidermis reveals a pattern of serial fracture faces which is highly suggestive of polar lipids in a bilayer configuration. The occurrence of alternating wide and narrow fracture faces separated by intervening steps supports the view that epidermal barrier bilayers display lipid asymmetry similar to membranes. Within the lamellar granules, bilayers arrange to form disks which in fact are equivalent to flattened unilamellar liposomes. Stacking of the disks in turn gives rise to the lamellar pattern. After exocytosis into the intercellular space, the disks are arranged parallel to the cell membranes. In tangentially fractured specimens, the cleavage plane jumps back and forth from the plasma membrane to a disk-bilayer, thereby giving rise to the known phenomenon of EF-ridges (on the extracellular fracture face) and PF-grooves (in the plasmatic fracture face) which both represent the level of the plasma membrane sur- or subjacent to the aisles between disks. Concomitantly with the upward movement of the keratinocytes, the ridges and grooves become narrower until they fade away by the second or third cell layer of the stratum corneum. This phenomenon is explained by the fusion of adjacent disks at their highly curved brims due to a mechanism similar to the process of membrane fusion which causes the formation of wide, uninterrupted sheets.

A new approach to the measurement of phospholipase A2 in tissue homogenates and its application to human skin

We describe a new approach to the quantification of tissue phospholipase A2 based on the observation that the Vmax is increased by several orders of magnitude in the presence of high concentrations of dimethyl sulphoxide. The enzyme from human skin showed optimal activity at 30% dimethyl sulphoxide and pH 6-7.5, and had an absolute dependence on Ca2+ (apparent Km = 1.4 mmol/l). The relationship between reaction velocity and substrate concentration did not follow Michaelis-Menten kinetics. The enzyme was inhibited by p-bromophenacyl bromide, half-maximal inhibition occurring at 2.4 mumol/l. Surprisingly, assay of specimens of various human tissues indicated that, with the exception of pancreas, skin contained higher levels of phospholipase A2 activity than any other tissue investigated. Equally surprising was that primate skin (in particular human) seems considerably more active than that of other mammals. Speculatively, these findings may be related to the high levels of free fatty acids occurring on the skin surface.

Lipid composition and acid hydrolase content of lamellar granules of fetal rat epidermis

Lipids and acid hydrolases have been characterized in a subcellular fraction, enriched with lamellar granules (LG), derived from fetal rat epidermis. This fraction contains 23% glycosyl ceramides and ceramides, 15% free sterols, and 34% phospholipids. The lipid/protein ratio is 2.0. The sterols and sphingolipids were present in proportions similar to those previously reported in stratum corneum. These findings provide direct biochemical evidence for the widely accepted hypothesis that stratum corneum lipids are derived from exocytosis of lamellar granules into the intercellular space. The LG fraction was enriched in certain acid hydrolases including glucosidase, acid phosphatase, phospholipases A, and sphingomyelinase; other acid hydrolases, i.e., amino-glycosidases, glactosidase and aryl sulfatase (pH 5.5), and steroid sulfatase were not preferentially localized in this fraction. By modulation of phospholipids, glycolipids, and proteins in the membrane regions of stratum corneum, the acid hydrolases of LG may play a role relevant to the function and desquamation of stratum corneum.

Lamellar body-enriched fractions from neonatal mice: preparative techniques and partial characterization

Several problems have frustrated the isolation of lamellar bodies (LB) from mammalian epidermis. We obtained pellets enriched in intact LB by utilizing the staphylococcal epidermolytic toxin to provide intact, outer epidermal sheets, by controlled homogenization in a cell disrupter, and by passage of homogenates through a graded series of nuclepore filters (Science 221:962, 1983). Such preparations contained more intact LB than did fractions prepared by a variety of differential or sucrose/metrizamide discontinuous centrifugation methods. Initial characterization of the enzymatic content of this fraction revealed it to be enriched in certain hydrolytic enzymes (acid phosphatase, carboxypeptidase, cathepsin B, acid lipase, sphingomyelinase, and phospholipase A), but strikingly depleted in all sulfatases, beta-glucuronidase, and the non-lysosomal protease, plasminogen activator. Thus, LB show some properties of lysosomes, although certain characteristic lysosomal enzymes are strikingly absent. Lamellar body fractions contained 2-3 times more lipid per unit weight than did homogenates, and were enriched in phospholipids, free sterols, and glycosphingolipids, but not in other neutral lipids or ceramides. In summary, whereas some of the enzymes in LB could participate in the metabolism of LB lipid precursors to hydrophobic barrier constituents, others may attack intercellular constituents, ultimately resulting in desquamation. The lipid profile of these organelles suggests that they deliver precursors of permeability barrier lipids to intercellular domains.

Tumor promoter-stimulated protein phosphorylation in mouse epidermis is inhibited by bromophenacyl bromide

Phosphorylation of mouse epidermal proteins by endogenous protein kinases and production of arachidonic acid are processes stimulated by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate. In mouse epidermal homogenates, each process, as well as production of diacylglycerol (the physiologic activator of protein kinase C), was inhibited by bromophenacyl bromide at concentrations equivalent to those that inhibit tumor promotion. These results lend support to the hypothesis that tumor promotion in mouse skin may be mediated by activation of protein kinase C.

Characterization and activity of phospholipase A2 in normal human epidermis and in lesion-free epidermis of patients with psoriasis or eczema

The kinetic properties of phospholipase A2 isolated from single large specimens of normal human epidermis and 'uninvolved' (lesion-free) psoriatic epidermis were determined. The enzymes from the two sources behaved identically with respect to changes in protein concentration, Ca2+ concentration and pH, but the enzymes responded differently to changes in substrate concentration. Furthermore, the specific activity of the enzyme derived from lesion-free psoriatic epidermis was higher than that from normal epidermis under all conditions used. Increased specific activity of the enzyme in the lesion-free epidermis was also found when biopsy specimens taken from thirty-five patients with psoriasis vulgaris at varying severity were compared with biopsies of normal epidermis from thirty-one control volunteers (P less than 0.001). Mixing experiments, in which homogenates of lesion-free psoriatic epidermis and control epidermis were combined, suggested that the relatively low activity of the enzyme in normal epidermis was due to the presence of an inhibitor. As the activity of the enzyme was not elevated in the lesion-free epidermis from twelve cases of eczema, which is also an inflammatory condition of the epidermis and superficial dermis, it is suggested that the raised phospholipase A2 activity demonstrated in the lesion-free epidermis of psoriasis may play an important role in the pathogenesis of the disease.

Utilization of epidermal phospholipase A2 inhibition to monitor topical steroid action

The effect of several steroid creams on epidermal phospholipase A2 (PLA2) activity in symptomless psoriatic and normal epidermis was studied. The magnitude of PLA2 inhibition produced by the steroids was directly proportional to the initial level of the enzyme activity. This differential inhibition resulted in PLA2 activity approaching or attaining the normal range regardless of its initial level. Clobetasol propionate 0.05% (Dermovate) produced more enzyme inhibition than betamethasone valerate 0.1% (Betnovate) but there was no difference in inhibition between this latter steroid and clobetasone butyrate 0.05% (Eumovate). All were more inhibitory than hydrocortisone I% (Efcortelan).

Percutaneous absorption: a new physicochemical predictive model for maximum human in vivo penetration rates

A diffusion model for stratum corneum-limited percutaneous absorption based on the interaction of the diffusate with the stratum corneum was derived. Two types of interactions were proposed, ion-dipole and lipid-lipid, based on current knowledge of the stratum corneum and on irreversible thermodynamic arguments. The resulting flux equations predict a linear dependence of flux on the dipole moment and ln X of the diffusates , where X is the mole fraction solubility. These flux equations were tested on 21 different diffusates whose human percutaneous absorption rates in vivo had been previously determined. A solubility method was used to classify the interaction pathway for each diffusate . Correlation of the maximum absorption rate for the lipid and polar pathways give correlation coefficients of 0.946 and 0.998, respectively. It is believed that these studies provide a starting point for the ultimate goal of percutaneous absorption research: to be able to bypass in vivo and in vitro studies and to predict absorption solely on the basis of the physicochemical properties of the diffusates .

Acid hydrolases of the epidermis: subcellular localization and relationship to cornification

Three lysosomal-type acid hydrolases were examined in subcellular fractions of the developing epidermis of fetal rats to assess the relationship of degradative enzymes to cornification. As the granular layer developed and cornified between 18 and 20 days (D) of gestation, epidermal acid phosphatase increased, acid phospholipase A remained constant, and beta-glucuronidase activity declined. The enzymes were present in 3,000, 17,000, and 100,000 g particulate fractions and soluble cytoplasm. However distribution differed: acid phosphatase and phospholipase A were more preferentially localized than was glucuronidase in the 17,000 g fraction which excluded mitochondria and ribosomes and was enriched in lamellar granules. The findings suggested that acid phosphatase and phospholipase were present in membrane-bound organelles (e.g., lamellar granules) in the granular layer. Particulate acid phosphatase increased with granular layers on days 19 and 20 while a 7-fold increase in soluble enzyme coincided with cornification on day 20. As shown by isoelectric focusing, the enzyme became more heterogeneous at day 20 than at day 18, suggesting increased glycosylation. The particulate fraction displayed lysosomal characteristics with respect to release of acid phosphatase, which was inhibited by hydrocortisone and enhanced by retinol. When fetal epidermis was allowed to cornify in organ cultures, similar increases in acid phosphatase occurred. The presence of hydrocortisone did not affect increase in total enzyme but a greater proportion remained in the particulate fraction. The findings suggest that particulate acid phosphatase and phospholipase are compartmentalized in organelles with lysosomal characteristics during development of granular cells and that release of phosphatase is coincident with cornification. This may reflect not only exocytosis of lamellar granules but also intracellular release of the hydrolytic enzyme.