Topical Application of Neuronal Nitric Oxide Synthase Inhibitor Accelerates Cutaneous Barrier Recovery and Prevents Epidermal Hyperplasia Induced by Barrier Disruption

Dietary nitrate supplementation increases nitrate and nitrite concentrations in human skin interstitial fluid

Acute dietary nitrate (NO₃^-) supplementation can increase [NO₃^-], but not nitrite ([NO₂^-]), in human skeletal muscle, though its effect on [NO₃^-] and [NO₂^-] in skin remains unknown. In an independent group design, 11 young adults ingested 140 mL of NO₃^--rich beetroot juice (BR; 9.6 mmol NO₃^-), and 6 young adults ingested 140 mL of a NO₃^--depleted placebo (PL). Skin dialysate, acquired through intradermal microdialysis, and venous blood samples were collected at baseline and every hour post-ingestion up to 4 h to assess dialysate and plasma [NO₃^-] and [NO₂^-]. The relative recovery rate of NO₃^- and NO₂^- through the microdialysis probe (73.1% and 62.8%), determined in a separate experiment, was used to estimate skin interstitial [NO₃^-] and [NO₂^-]. Baseline [NO₃^-] was lower, whereas baseline [NO₂^-] was higher in the skin interstitial fluid relative to plasma (both P < 0.001). Acute BR ingestion increased [NO₃^-] and [NO₂^-] in the skin interstitial fluid and plasma (all P < 0.001), with the magnitude being smaller in the skin interstitial fluid (e.g., 183 ± 54 vs. 491 ± 62 μM for Δ[NO₃^-] from baseline and 155 ± 190 vs. 217 ± 204 nM for Δ[NO₂^-] from baseline at 3 h post BR ingestion, both P ≤ 0.037). However, due to the aforementioned baseline differences, skin interstitial fluid [NO₂^-] post BR ingestion was higher, whereas [NO₃^-] was lower relative to plasma (all P < 0.001). These findings extend our understanding of NO₃^- and NO₂^- distribution at rest and indicate that acute BR supplementation increases [NO₃^-] and [NO₂^-] in human skin interstitial fluid.

Role of nitric oxide in regulating epidermal permeability barrier function

Nitric oxide (NO), a free radical molecule synthesized by nitric oxide synthases (NOS), regulates multiple cellular functions in a variety of cell types. These NOS, including endothelial NOS (eNOS), inducible NOS (iNOS) and neural NOS (nNOS), are expressed in keratinocytes. Expression levels of both iNOS and nNOS decrease with ageing, and insufficient NO has been linked to the development of a number of disorders such as diabetes and hypertension, and to the severity of atherosclerosis. Conversely, excessive NO levels can induce cellular oxidative stress, but physiological levels of NO are required to maintain the normal functioning of cells, including keratinocytes. NO also regulates cutaneous functions, including epidermal permeability barrier homeostasis and wound healing, through its stimulation of keratinocyte proliferation, differentiation and lipid metabolism. Topical applications of a diverse group of agents which generate nitric oxide (called NO donors) such as S-nitroso-N-acetyl-D,L-penicillamine (SNAP) can delay permeability barrier recovery in barrier-disrupted skin, but iNOS is still required for epidermal permeability barrier homeostasis. This review summarizes the regulatory role that NO plays in epidermal permeability barrier functions and the underlying mechanisms involved.

Topical and transdermal delivery with diseased human skin: passive and iontophoretic delivery of hydrocortisone into psoriatic and eczematous skin

Psoriasis and atopic dermatitis (eczema) are both common immune-mediated inflammatory skin diseases associated with changes in skin's stratum corneum lipid structure and barrier functionality. The present study aimed to investigate healthy, eczematous, and psoriatic excised human tissue for the effect of non-infectious skin diseases on skin characteristics (surface color, pH, transepidermal water loss, electrical resistance, and histology), as well as on permeation and retention profile of hydrocortisone. Further, differences in percutaneous absorption on application of iontophoresis on healthy and diseased skin were also investigated. Measurements of transepidermal water loss and electrical resistance showed a significant difference in psoriasis skin samples indicating a damaged barrier function. In vitro permeation studies on full-thickness human skin using vertical diffusion cells further confirmed these results as the drug amount retained in the psoriatic tissue was significantly higher when compared with the other groups. Despite no significant difference, the presence of the drug in the receptor chamber in both diseased groups can be concerning as it suggests the increased possibility of systemic absorption and adverse reactions associated with it in the use of topical corticosteroids. Application of anodal iontophoresis resulted in greater distribution of hydrocortisone into deeper layers of skin and the receptor chamber, in comparison to passive permeation. However, no significant differences were observed due to the healthy or diseased condition of skin.

Inducible nitric oxide synthase is required for epidermal permeability barrier homeostasis in mice

Nitric oxide (NO) regulates a variety of epidermal functions, including epidermal proliferation, differentiation and cutaneous wound healing. However, whether nitric oxide (NO) and its synthetic enzymes regulate epidermal permeability barrier homeostasis is not clear. In the present study, we employed inducible nitric oxide synthase (iNOS) KO mice to explore the role of iNOS in epidermal permeability barrier homeostasis. Our results showed that iNOS mice displayed a comparable levels of basal transepidermal water loss rates, stratum corneum hydration and skin surface pH to their wild-type mice, but epidermal permeability barrier recovery was significantly delayed both 2 and 4 hours after acute barrier disruption by tape stripping. In parallel, expression levels of mRNA for epidermal differentiation-related proteins and lipid synthetic enzymes were lower in iNOS KO mice versus wild-type controls. Topical applications of two structurally unrelated NO donors to iNOS KO mice improved permeability barrier recovery kinetics and upregulated expression levels of mRNA for epidermal differentiation-related proteins and lipid synthetic enzymes. Together, these results indicate that iNOS and its product regulate epidermal permeability barrier homeostasis in mice.

Sub-erythemal ultraviolet radiation reduces metabolic dysfunction in already overweight mice

Exposure to sunlight may limit cardiometabolic risk. In our previous studies, regular exposure to sub-erythemal (non-burning) ultraviolet radiation (UVR) reduced signs of adiposity and cardiometabolic dysfunction in mice fed a high-fat diet. Some of the observed effects were dependent on skin release of nitric oxide after UVR exposure. Here, we examine the effects of sub-erythemal UVR on signs of adiposity and metabolic dysfunction in already overweight mice, comparing the effects of two sunlamps with distinct emitted light spectra. Mice were fed a high-fat diet from 8 weeks of age, with UVR administered twice a week from 14 weeks of age until they were killed at 20 weeks of age. Mice were irradiated with the same dose of UVB radiation (1 kJ/m²) from either FS40 (65% UVB, 35% UVA) or CLEO (4% UVB, 96% UVA) sunlamps, but substantially more UVA from the latter. FS40 UVR (but not CLEO UVR) significantly reduced mouse weights and weight gain, compared to mice fed a high-fat diet (only). These effects were dependent on nitric oxide. Conversely, CLEO UVR (but not FS40 UVR) significantly reduced circulating LDL cholesterol. Both light sources reduced fasting insulin levels, and the extent of hepatic steatosis; the latter was reversed by topical application of cPTIO, suggesting an important role for skin release of nitric oxide in preventing hepatic lipid accumulation. These results suggest that there may be a number of benefits achieved by regular exposure to safe (non-burning) levels of sunlight or UV-containing phototherapy, with effects potentially dependent on the predominance of the wavelengths of UVR administered.

Ultraviolet radiation suppresses obesity and symptoms of metabolic syndrome independently of vitamin D in mice fed a high-fat diet

The role of vitamin D in curtailing the development of obesity and comorbidities such as the metabolic syndrome (MetS) and type 2 diabetes has received much attention recently. However, clinical trials have failed to conclusively demonstrate the benefits of vitamin D supplementation. In most studies, serum 25-hydroxyvitamin D [25(OH)D] decreases with increasing BMI above normal weight. These low 25(OH)D levels may also be a proxy for reduced exposure to sunlight-derived ultraviolet radiation (UVR). Here we investigate whether UVR and/or vitamin D supplementation modifies the development of obesity and type 2 diabetes in a murine model of obesity. Long-term suberythemal and erythemal UVR significantly suppressed weight gain, glucose intolerance, insulin resistance, nonalcoholic fatty liver disease measures; and serum levels of fasting insulin, glucose, and cholesterol in C57BL/6 male mice fed a high-fat diet. However, many of the benefits of UVR were not reproduced by vitamin D supplementation. In further mechanistic studies, skin induction of the UVR-induced mediator nitric oxide (NO) reproduced many of the effects of UVR. These studies suggest that UVR (sunlight exposure) may be an effective means of suppressing the development of obesity and MetS, through mechanisms that are independent of vitamin D but dependent on other UVR-induced mediators such as NO.

Non-invasive assessment of tryptophan fluorescence and confocal microscopy provide information on skin barrier repair dynamics beyond TEWL

The stratum corneum (SC) serves a primary function of skin barrier and understanding the kinetics of SC formation may provide great insight for skin diagnosis and evaluation of therapies. Besides trans-epidermal water loss (TEWL), few methods have been characterized to assess skin barrier non-invasively in vivo, particularly for dynamic measurements on the same specimen over time. The objective of this study was to characterize alternative non-invasive methods to evaluate the dynamic processes involved in the recovery of normal human SC after total removal. TEWL, tryptophan fluorescence and reflectance confocal microscopy (RCM) were used to determine skin barrier function, cell turnover and epidermal morphology over a period of 10 days after total removal of the SC by tape stripping. The results show a biphasic recovery of TEWL over time, which contrasted with a linear increase of 2.3 μm/day in SC thickness. Tryptophan assessment of cell turnover also demonstrated a biphasic pattern attaining a maximum three to four times the levels of the control site 3 days after injury that slowly returned to baseline and displayed great correlation (R(2)  > 0.95) to viable epidermis thickness that also achieved a maximum about 3 days after injury with an approximate increase of 55%. When plotting the change of TEWL versus SC thickness, a single exponential function is observed [Δ-TEWL = 55 exp (-0.157×)] which contrasts with other proposed models. These methods were able to present rates for SC recovery processes beyond skin barrier (TEWL) that may provide new insights on kinetics of barrier formation for evaluation of skin conditions and treatments.

Enhanced profiling of flavonol glycosides in the fruits of sea buckthorn (Hippophae rhamnoides)

Use of enhanced LC-MS/MS methods to identify common glycosyl groups of flavonoid glycosides enabled better characterization of the flavonoids in fruits of sea buckthorn (Hippophae rhamnoides). The saccharide moieties of 48 flavonol O-glycosides detected in a methanol extract were identified by these methods. Several of the flavonol glycosides were acylated, two of which were isolated and found to be new compounds. Their structures were determined using spectroscopic and chemical methods as isorhamnetin 3-O-(6-O-E-sinapoyl-β-D-glucopyranosyl)-(1→2)-β-D-glucopyranoside-7-O-α-L-rhamnopyranoside (24) and isorhamnetin 3-O-(6-O-E-feruloyl-β-D-glucopyranosyl)-(1→2)-β-D-glucopyranoside-7-O-α-L-rhamnopyranoside (30). Analysis of the acylated glycosyl groups of 24 and 30 by serial mass spectrometry provided evidence to suggest the acylation position of 11 other minor flavonol glycosides acylated with hydroxycinnamic or hydroxybenzoic acids. The nitric oxide scavenging activities of 24 and 30 were compared with those of other flavonoids and with ascorbic acid and the potassium salt of 2-(4-carboxyphenyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3-oxide (carboxy-PTIO).

Regulation of permeability barrier homeostasis

A major function of the skin is to provide a barrier to the movement of water and electrolytes, which is required for life in a terrestrial environment. This permeability barrier is localized to the stratum corneum and is mediated by extracellular lipid-enriched lamellar membranes, which are delivered to the extracellular spaces by the secretion of lamellar bodies by stratum granulosum cells. A large number of factors have been shown to regulate the formation of this permeability barrier. Specifically, lamellar body secretion and permeability barrier formation are accelerated by decreases in the calcium content in the stratum granulosum layer of the epidermis. In addition, increased expression of cytokines and growth factors and the activation of nuclear hormone receptors (peroxisome proliferator-activated receptors, liver X receptors, vitamin D receptor) accelerate permeability barrier formation. In contrast, nitric oxide, protease-activated receptor 2 activation, glucocorticoids, and testosterone inhibit permeability barrier formation. The ability of a variety of factors to regulate permeability barrier formation allows for a more precise and nuanced regulation.

Morphological and functional differences in coculture system of keratinocytes and dorsal-root-ganglion-derived cells depending on time of seeding

Previous study indicated that in a coculture system of keratinocytes and dorsal-root-ganglion-derived (DRG) cells, mechanical stimulation of keratinocytes induced ATP-mediated calcium propagation and excitation of DRG cells. Here, we examined two different coculture systems of keratinocytes and DRG cells. In one, we seeded keratinocytes first and then seeded DRG cells on the keratinocytes. In this system, nerve fibres from DRG cells passed between keratinocytes. Mechanical stimulation of keratinocytes did not induce excitation of DRG cells. In the other, we seeded both cell types together. At first, each cell type grew separately, forming cell aggregates. Then, nerve fibres grew out from the DRG cell aggregates to keratinocyte aggregates and penetrated into them. In this system, mechanical stimulation of keratinocytes induced excitation of the nerve fibres, but the excitation was not completely blocked by apyrase, an ATP-degrading enzyme. These results suggest that coculture of keratinocytes and DRG can generate a variety of structures, depending on the seeding conditions.

Roles of transient receptor potential proteins (TRPs) in epidermal keratinocytes

Epidermal keratinocytes are the epithelial cells of mammalian skin. At the basal layer of the epidermis, these cells proliferate strongly, and as they move towards the skin surface, differentiation proceeds. At the uppermost layer of the epidermis, keratinocytes undergo apoptosis and die, forming a thin, water-impermeable layer called the stratum corneum. Peripheral blood vessels do not reach the epidermis, but peripheral nerve fibers do penetrate into it. Until recently, it was considered that the main role of epidermal keratinocytes was to construct and maintain the water-impermeable barrier function. However, since the functional existence of TRPV1, which is activated by heat and low pH, in epidermal keratinocytes was identified, our understanding of the role of keratinocytes has changed enormously. It has been found that many TRP channels are expressed in epidermal keratinocytes, and play important roles in differentiation, proliferation and barrier homeostasis. Moreover, because TRP channels expressed in keratinocytes have the ability to sense a variety of environmental factors, such as temperature, mechanical stress, osmotic stress and chemical stimuli, epidermal keratinocytes might form a key part of the sensory system of the skin. The present review deals with the potential roles of TRP channels expressed in epidermal keratinocytes and focuses on the concept of the epidermis as an active interface between the body and the environment.

Phosphodiesterase inhibitors block the acceleration of skin permeability barrier repair by red light

We previously demonstrated that exposure to red light (550-670 nm) accelerates epidermal permeability barrier recovery after barrier disruption. Furthermore, we showed that photosensitive proteins, originally found in retina, are also expressed in epidermis. In retina, transducin and phosphodiesterase 6 play key roles in signal transmission. In this study, we evaluate the role of phosphodiesterese 6 in the acceleration by red light of epidermal permeability barrier recovery. Immunohistochemical study and reverse transcription-PCR assays confirmed the expression of both transducin and phosphodiesterase 6 in epidermal keratinocytes. Topical application of 3-isobutyl-1-methylxanthine, a non-specific phosphodiesterase inhibitor, blocked the acceleration of the barrier recovery by red light. Topical application of zaprinast, a specific inhibitor of phosphodiesterases 5 and 6, also blocked the acceleration, whereas T0156, a specific inhibitor of phosphodiesterase 5, had no effect. Red light exposure reduced the epidermal hyperplasia induced by barrier disruption under low humidity, and the effect was blocked by pretreatment with zaprinast. Our results indicate phosphodiesterase 6 is involved in the recovery-accelerating effect of red light on the disrupted epidermal permeability barrier.

Calcium ion propagation in cultured keratinocytes and other cells in skin in response to hydraulic pressure stimulation

We have previously suggested that a variety of environmental factors might be first sensed by epidermal keratinocytes, which represent the frontier of the body. To further examine this idea, in the present study, we examined the intracellular calcium responses of cultured keratinocytes to external hydraulic pressure. First, we compared the responses of undifferentiated and differentiated keratinocytes with those of fibroblasts, vascular endothelial cells (VEC), and lymphatic endothelial cells. Elevation of intracellular calcium was observed after application of pressure to keratinocytes, fibroblasts, and VEC. The calcium propagation extended over a larger area and continued for a longer period of time in differentiated keratinocytes, as compared with the other cells. The response of the keratinocytes was dramatically reduced when the cells were incubated in medium without calcium. Application of a non-selective transient receptor potential (TRP) channel blocker also attenuated the calcium response. These results suggest that differentiated keratinocytes are sensitive to external pressure and that TRP might be involved in the mechanism of their response.

Mechanical-stimulation-evoked calcium waves in proliferating and differentiated human keratinocytes

Calcium dynamics in the epidermis play a crucial role in barrier homeostasis and keratinocyte differentiation. We have recently suggested that the electro-physiological responses of the keratinocyte represent the frontier of the skin sensory system for environmental stimuli. In the present study, we have evaluated the responses of proliferating and differentiated human keratinocytes to mechanical stress by measuring the intracellular calcium level. Before differentiation, mechanical stress induces a calcium wave over a limited area; this is completely blocked by apyrase, which degrades ATP. In the case of differentiated keratinocytes, the calcium wave propagates over a larger area. Application of apyrase does not completely inhibit this wave. Thus, in differentiated cells, the induction of calcium waves might involve not only ATP, but also another factor. Immunohistochemical studies indicate that connexins 26 and 43, both components of gap junctions, are expressed in the cell membrane of differentiated keratinocytes. Application of octanol or carbenxolone, which block gap junctions, significantly reduces calcium wave propagation in differentiated keratinocytes. Thus, signaling via gap junctions might be involved in the induction of calcium waves in response to mechanical stress at the upper layer of the epidermis.

Skin irritation and sensitization: mechanisms and new approaches for risk assessment. 1. Skin irritation

Cutaneous irritation presents a major health problem with serious social and occupational impact. The interaction between an irritant and the human skin depends on multiple factors: the intrinsic properties and the nature of the irritant itself, and specific individual- and environment-related variables. The main pathological mechanisms of irritancy include skin barrier disruption, induction of a cytokine cascade and involvement of the oxidative stress network; all of them resulting in a visible or subclinical inflammatory reaction. In vivo, different non-invasive parameters for the evaluation of skin irritation and irritant potential of compounds and their specific formulations have been introduced, such as epidermal barrier function, skin hydration, surface pH, lipid composition, skin colour and skin blood flow. The diverse physiological changes caused by irritating agents require implementation of a multiparametric approach in the evaluation of cutaneous irritancy.

The regulation of permeability barrier homeostasis

Disruption of the permeability barrier stimulates a repair response that leads to the restoration of barrier function. Previous studies demonstrated that changes in ions, particularly calcium, and cytokines are positive signals, whereas serine protease activation of proteinase-activated receptor 2 is a negative signal regulating barrier recovery. Ikeyama and colleagues provide data that the nitric oxide signaling pathway also regulates barrier homeostasis.