Upregulation of aquaporin-3 is involved in keratinocyte proliferation and epidermal hyperplasia

Downregulation of aquaporin 3 inhibits cellular proliferation, migration and invasion in the MDA-MB-231 breast cancer cell line

Aquaporins are membrane proteins that regulate cellular water flow. Recently, aquaporins have been proposed as mediators of cancer cell biology. A subset of aquaporins, referred to as aquaglyceroporins are known to facilitate the transport of glycerol. The present study describes the effect of gene knockdown of the aquaglyceroporin AQP3 on MDA-MB-231 breast cancer cell proliferation, migration, invasion, adherence and response to the chemotherapeutic agent 5-fluorouracil. shRNA mediated AQP3 gene knockdown induced a 28% reduction in cellular proliferation (P<0.01), a 39% decrease in migration (P<0.0001), a 24% reduction in invasion (P<0.05) and a 25% increase in cell death at 100 µM 5-FU (P<0.01). Analysis of cell permeability to water and glycerol revealed that MDA-MB-231 cells with knocked down AQP3 demonstrated a modest decrease in water permeability (17%; P<0.05) but a more marked decrease in glycerol permeability (77%; P<0.001). These results suggest that AQP3 has a role in multiple aspects of breast cancer cell pathophysiology and therefore represents a novel target for therapeutic intervention.

Aquaporins as Targets of Dietary Bioactive Phytocompounds

Plant-derived bioactive compounds have protective role for plants but may also modulate several physiological processes of plant consumers. In the last years, a wide spectrum of phytochemicals have been found to be beneficial to health interacting with molecular signaling pathways underlying critical functions such as cell growth and differentiation, apoptosis, autophagy, inflammation, redox balance, cell volume regulation, metabolic homeostasis, and energy balance. Hence, a large number of biologically active phytocompounds of foods have been isolated, characterized, and eventually modified representing a natural source of novel molecules to prevent, delay or cure several human diseases. Aquaporins (AQPs), a family of membrane channel proteins involved in many body functions, are emerging among the targets of bioactive phytochemicals in imparting their beneficial actions. Here, we provide a comprehensive review of this fast growing topic focusing especially on what it is known on the modulatory effects played by several edible plant and herbal compounds on AQPs, both in health and disease. Phytochemical modulation of AQP expression may provide new medical treatment options to improve the prognosis of several diseases.

Forensic application of epidermal AQP3 expression to determination of wound vitality in human compressed neck skin

In forensic practices, it is often difficult to determine wound vitality in compression marks of the neck with naked eyes. AQP1 and AQP3 are the major water channels associated with skin. Thus, we immunohistochemically examined the expression of AQP1 and AQP3 in neck skin samples to discuss their forensic applicability to determination of the wound vitality. Skin samples were obtained from 56 neck compression cases (hanging, 35 cases; strangulation, 21 cases). The intact skin from the same individual was taken as a control. Although AQP1 was immnunostained in dermal capillaries in both the neck compression marks and intact skin samples, there was no significant difference in the magnitude of AQP1 expression between both groups. On the contrary, AQP3-positive signals could be faintly detected in uninjured skin samples, and the positive signals seemed more intense in the keratinocytes in compression regions. Morphometrical analyses revealed that the ratio of AQP3-expressed keratinocytes was significantly enhanced in neck compression regions, compared with control groups. From the viewpoints of forensic pathology, immunohistochemical detection of AQP3 in the neck skin can be considered a valuable marker to diagnose the trace of antemortem compression.

18ß-glycyrrhetinic acid derivative promotes proliferation, migration and aquaporin-3 expression in human dermal fibroblasts

Licorice (Glycyrrhiza) species have been widely used as a traditional medicine and a natural sweetener in foods. The 18β-glycyrrhetinic acid (18β-GA) is a bioactive compound in licorice that exhibits potential anti-cancer, anti-inflammatory, and anti-microbial activities. Many synthesized derivatives of 18β-GA have been reported to be cytotoxic and suggested for the treatment of malignant diseases. In this study, we explored the possible pharmacological roles of an 18β-GA derivative in skin biology using primary human dermal fibroblasts and HaCaT keratinocytes as cell models. We found that this 18β-GA derivative did not cause cell death, but significantly enhanced the proliferation of dermal fibroblasts and HaCaT keratinocytes. A scratch wound healing assay revealed that the 18β-GA derivative promoted the migration of fibroblasts. Due to the important role of aquaporin-3 in cell migration and proliferation, we also investigated the expression of aquaporin-3 and found this compound up-regulated the expression of aquaporin-3 in dermal fibroblasts and HaCaT keratinocytes. In dermal fibroblasts, the 18β-GA derivative induced the phosphorylation of Akt, ERK, and p38. The inhibitor of Akt predominantly suppressed the 18β-GA derivative-induced expression of aquaporin-3. Collectively, this compound had a positive effect on the proliferation, migration, and aquaporin-3 expression of skin cells, implying its potential role in the treatment of skin diseases characterized by impaired wound healing or dermal defects.

The etiopathogenesis of atopic dermatitis: barrier disruption, immunological derangement, and pruritus

Atopic dermatitis (AD) is a common chronic skin inflammatory disorder characterized by recurrent eczema accompanied by an intractable itch that leads to an impaired quality of life. Extensive recent studies have shed light on the multifaceted pathogenesis of the disease. The complex interplay among skin barrier deficiency, immunological derangement, and pruritus contributes to the development, progression, and chronicity of the disease. Abnormalities in filaggrin, other stratum corneum constituents, and tight junctions induce and/or promote skin inflammation. This inflammation, in turn, can further deteriorate the barrier function by downregulating a myriad of essential barrier-maintaining molecules. Pruritus in AD, which may be due to hyperinnervation of the epidermis, increases pruritogens, and central sensitization compromises the skin integrity and promotes inflammation. There are unmet needs in the treatment of AD. Based on the detailed evidence available to date, certain disease mechanisms can be chosen as treatment targets. Numerous clinical trials of biological agents are currently being conducted and are expected to provide treatments for patients suffering from AD in the future. This review summarizes the etiopathogenesis of the disease and provides a rationale for choosing the novel targeted therapy that will be available in the future.

Silencing of AQP3 induces apoptosis of gastric cancer cells via downregulation of glycerol intake and downstream inhibition of lipogenesis and autophagy

Gastric cancer (GC) has a poor prognosis and is a leading cause of cancer-related death. Optimal therapeutic targets have not been identified. AQP3 is capable of transporting glycerol across the cytomembrane. Previous studies have shown that AQP3 is involved in proliferation, invasion and migration by regulating glycerol and lipid metabolism in diverse cancer cell types. However, the potential roles of glycerol and lipid metabolism in AQP3-related cell apoptosis in GC remain unclear. In this study, we observed that AQP3 expression was upregulated in tumor tissues, and positively correlated with tumor size, lymph node metastasis and glycerol concentration in human GC samples. Silencing of AQP3 resulted in decreased glycerol intake and impaired lipid synthesis, which contributed to increased cell apoptosis. Furthermore, inhibition of autophagy induced by AQP3 knockdown promoted cell apoptosis. Administration of either glycerol or rapamycin restored cell viability, and overexpression of AQP3 increased cell viability by upregulating cellular glycerol metabolism and autophagy. Our study demonstrates that the increase in cell apoptosis of AQP3-deficient GC cells is a consequence of reduced glycerol uptake and lipogenesis and is associated with autophagy inhibition induced by AQP3 deficiency.

Regulation of the Glycerol Transporter, Aquaporin-3, by Histone Deacetylase-3 and p53 in Keratinocytes

Aquaporin- (AQP) 3, a water and glycerol channel, plays an important role in epidermal function, with studies showing its involvement in keratinocyte proliferation, differentiation, and migration and in epidermal wound healing and barrier repair. Increasing speculation about the use of histone deacetylase (HDAC) inhibitors to treat skin diseases led us to investigate HDAC's role in the regulation of AQP3. The broad-spectrum HDAC inhibitor suberoylanilide hydroxamic acid induced AQP3 mRNA and protein expression in a dose- and time-dependent manner in normal keratinocytes. The SAHA-induced increase in AQP3 levels resulted in enhanced [³H]glycerol uptake in normal but not in AQP3-knockout keratinocytes, confirming that the expressed AQP3 was functional. Use of HDAC inhibitors with different specificities limited our exploration of the responsible HDAC member to HDAC1, HDAC2, or HDAC3. Cre-recombinase-mediated knockdown and overexpression of HDAC3 suggested a role for HDAC3 in suppressing AQP3 expression basally. Further investigation implicated p53 as a transcription factor involved in regulating HDAC inhibitor-induced AQP3 expression. Thus, our study supports the regulation of AQP3 expression by HDAC3 and p53. Because suberoylanilide hydroxamic acid is already approved to treat cutaneous T-cell lymphoma, it could potentially be used as a therapy for skin diseases like psoriasis, where AQP3 is abnormally expressed.

Overexpression of AQP3 and AQP10 in the skin exacerbates psoriasiform acanthosis

We previously observed that aquaporin-3 and aquaporin-10 are upregulated in the epidermis of hand dermatitis patients (Med. Hypotheses, 84, 2015, 498). To address the functional relevance of this upregulation, we overexpressed AQP3/AQP10 in mice using the human K1 promoter. Combining imiquimod with detergent-containing water challenge, a common trigger in hand and other dermatitis, resulted in an increase in acanthosis in mice overexpressing AQP3 or AQP3 and AQP10. Aquaporin overexpression also drove a trend towards greater weight loss in these animals. These data support a role for cutaneous aquaporins in the pathogenesis of dermatitis and as a potential target in their treatment.

TRPV1 and TRPA1 in cutaneous neurogenic and chronic inflammation: pro-inflammatory response induced by their activation and their sensitization

Cutaneous neurogenic inflammation (CNI) is inflammation that is induced (or enhanced) in the skin by the release of neuropeptides from sensory nerve endings. Clinical manifestations are mainly sensory and vascular disorders such as pruritus and erythema. Transient receptor potential vanilloid 1 and ankyrin 1 (TRPV1 and TRPA1, respectively) are non-selective cation channels known to specifically participate in pain and CNI. Both TRPV1 and TRPA1 are co-expressed in a large subset of sensory nerves, where they integrate numerous noxious stimuli. It is now clear that the expression of both channels also extends far beyond the sensory nerves in the skin, occuring also in keratinocytes, mast cells, dendritic cells, and endothelial cells. In these non-neuronal cells, TRPV1 and TRPA1 also act as nociceptive sensors and potentiate the inflammatory process. This review discusses the role of TRPV1 and TRPA1 in the modulation of inflammatory genes that leads to or maintains CNI in sensory neurons and non-neuronal skin cells. In addition, this review provides a summary of current research on the intracellular sensitization pathways of both TRP channels by other endogenous inflammatory mediators that promote the self-maintenance of CNI.

Yeast aquaporin regulation by 4-hydroxynonenal is implicated in oxidative stress response

Reactive oxygen species, especially hydrogen peroxide (H₂ O₂ ), contribute to functional molecular impairment and cellular damage, but also are necessary in normal cellular metabolism, and in low doses play stimulatory role in cell proliferation and stress resistance. In parallel, reactive aldehydes such as 4-hydroxynonenal (HNE), are lipid peroxidation breakdown products which also contribute to regulation of numerous cellular processes. Recently, channeling of H₂ O₂ by some mammalian aquaporin isoforms has been reported and suggested to contribute to aquaporin involvement in cancer malignancies, although the mechanism by which these membrane water channels are implicated in oxidative stress is not clear. In this study, two yeast models with increased levels of membrane polyunsaturated fatty acids (PUFAs) and aquaporin AQY1 overexpression, respectively, were used to evaluate their interplay in cell's oxidative status. In particular, the aim of the study was to investigate if HNE accumulation could affect aquaporin function with an outcome in oxidative stress response. The data showed that induction of aquaporin expression by PUFAs results in increased water permeability in yeast membranes and that AQY1 activity is impaired by HNE. Moreover, AQY1 expression increases cellular sensitivity to oxidative stress by facilitating H₂ O₂ influx. On the other hand, AQY1 expression has no influence on the cellular antioxidant GSH levels and catalase activity. These results strongly suggest that aquaporins are important players in oxidative stress response and could contribute to regulation of cellular processes by regulation of H₂ O₂ influx. © 2017 IUBMB Life, 69(5):355-362, 2017.

RNA interference influenced the proliferation and invasion of XWLC-05 lung cancer cells through inhibiting aquaporin 3

OBJECTIVE

The objective of this study was to construct a recombinant vector expressing siRNA targetedly inhibiting aquaporin 3 (AQP3), and to evaluate the effects of AQP3 inhibition on the proliferation and invasion of XWLC-05 human lung cancer cells.

METHODS

We obtained human AQP3 sequence from the Genbank and established the recombinant vector expressing siRNA targeting AQP3. After the transfection of the recombinant vectors, the expression of AQP3 was determined by RT-PCR and western blot. The MTS assay, flow cytometry and Transwell assay were conducted to detect the proliferation, cell cycle process, apoptosis and invasion of XWLC-05 cells. Then the activity of metal matrix proteinase (MMP) 2 was determined by gelatin zymography. Tumor formation in vivo experiments were also conducted in nude mice.

RESULTS

RNA interference (RNAi) of AQP3 substantially suppressed the XWLC-05 cell proliferation and invasion, blocked the cell cycle progressing and promoted cell apoptosis. In addition, the activity of MMP2 was remarkably attenuated in RNAi group. AQP3 RNAi did not affect the tumor formation rate in nude mice but reduced the tumor growth.

CONCLUSION

The inhibition of AQP3 retarded the growth and invasiveness of XWLC-05 lung cancer cells and decreased the activity of MMP2.

Preventive effect of dietary astaxanthin on UVA-induced skin photoaging in hairless mice

Astaxanthin, a carotenoid found mainly in seafood, has potential clinical applications due to its antioxidant activity. In this study, we evaluated the effect of dietary astaxanthin derived from Haematococcus pluvialis on skin photoaging in UVA-irradiated hairless mice by assessing various parameters of photoaging. After chronic ultraviolet A (UVA) exposure, a significant increase in transepidermal water loss (TEWL) and wrinkle formation in the dorsal skin caused by UVA was observed, and dietary astaxanthin significantly suppressed these photoaging features. We found that the mRNA expression of lympho-epithelial Kazal-type-related inhibitor, steroid sulfatase, and aquaporin 3 in the epidermis was significantly increased by UVA irradiation for 70 days, and dietary astaxanthin significantly suppressed these increases in mRNA expression to be comparable to control levels. In the dermis, the mRNA expression of matrix metalloprotease 13 was increased by UVA irradiation and significantly suppressed by dietary astaxanthin. In addition, HPLC-PDA analysis confirmed that dietary astaxanthin reached not only the dermis but also the epidermis. Our results indicate that dietary astaxanthin accumulates in the skin and appears to prevent the effects of UVA irradiation on filaggrin metabolism and desquamation in the epidermis and the extracellular matrix in the dermis.

Molecular Biology of Aquaporins

Aquaporins (AQPs ) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, thirteen AQPs , which are distributed widely in specific cell types in various organs and tissues, have been characterized in humans. Four AQP monomers, each of which consists of six membrane-spanning alpha-helices that have a central water-transporting pore, assemble to form tetramers, forming the functional units in the membrane. AQP facilitates osmotic water transport across plasma membranes and thus transcellular fluid movement. The cellular functions of aquaporins are regulated by posttranslational modifications , e.g. phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation, and protein interactions. Insight into the molecular mechanisms responsible for regulated aquaporin trafficking and synthesis is proving to be fundamental for development of novel therapeutic targets or reliable diagnostic and prognostic biomarkers.

Epigenetic Signatures at AQP3 and SOCS3 Engage in Low-Grade Inflammation across Different Tissues

BACKGROUND

Elevated levels of C-reactive protein (CRP, determined by a high-sensitivity assay) indicate low-grade inflammation which is implicated in many age-related disorders. Epigenetic studies on CRP might discover molecular mechanisms underlying CRP regulation. We aimed to identify DNA methylation sites related to CRP concentrations in cells and tissues regulating low-grade inflammation.

RESULTS

Genome-wide DNA methylation was measured in peripheral blood in 1,741 participants of the KORA F4 study using Illumina HumanMethylation450 BeadChip arrays. Four CpG sites (located at BCL3, AQP3, SOCS3, and cg19821297 intergenic at chromosome 19p13.2, P ≤ 1.01E-07) were significantly hypomethylated at high CRP concentrations independent of various confounders including age, sex, BMI, smoking, and white blood cell composition. Findings were not sex-specific. CRP-related top genes were enriched in JAK/STAT pathways (Benjamini-Hochberg corrected P < 0.05). Results were followed-up in three studies using DNA from peripheral blood (EPICOR, n = 503) and adipose tissue (TwinsUK, n = 368) measured as described above and from liver tissue (LMU liver cohort, n = 286) measured by MALDI-TOF mass spectrometry using EpiTYPER. CpG sites at the AQP3 locus (significant p-values in peripheral blood = 1.72E-03 and liver tissue = 1.51E-03) and the SOCS3 locus (p-values in liver < 2.82E-05) were associated with CRP in the validation panels.

CONCLUSIONS

Epigenetic modifications seem to engage in low-grade inflammation, possibly via JAK/STAT mediated pathways. Results suggest a shared relevance across different tissues at the AQP3 locus and highlight a role of DNA methylation for CRP regulation at the SOCS3 locus.

Chemokine levels in serum of children with atopic dermatitis with regard to severity and sensitization status

BACKGROUND

Many infants with atopic dermatitis (AD) are sensitized against food or airborne allergens. The severity of AD, using the SCORAD, seems to correlate with elevated serum levels of TARC/CCL17. Other chemokines, such as CCL20 or CCL25, have been described in the context of allergic inflammation. The aim of this study was to analyze whether chemokine serum levels differ within a cohort of infants suffering from varying severities of AD with or without allergic sensitization.

METHODS

Chemokine serum levels (CCL8, CCL17, CCL20, CCL25) as well as food and airborne allergen-specific IgE were analyzed in infants with AD.

RESULTS

About 60.9% (78/128) infants with AD (median age 8.8 months, 49 (38%) girls and 79 (62%) boys) showed a positive screening test to common food allergens and 26.6% to common airborne allergens. There was a strong correlation between serum levels of CCL17 and SCORAD in food-sensitized infants (r(s) = 0.646, p = <1e-04) and airborne-sensitized infants (r(s) = 0.587, p = 0.00065) in contrast to non-sensitized ones. Moreover, food-sensitized infants showed significantly higher levels of CCL25 compared to non-food-sensitized ones (p = 0.007).

CONCLUSION

The strong correlation between TARC/CCL17 and SCORAD in infants with specific sensitizations may be accounted for by the impaired skin barrier. As TARC/CCL17 has been found mainly in the (inflamed) skin but not in the gut, the detection of significantly higher levels of CCL25, ligand of CCR9, localized primarily in the gastrointestinal tract, suggests its impact on food allergen-induced inflammation processes in food-sensitized infants.

Polyphenols as Modulators of Aquaporin Family in Health and Disease

Polyphenols are bioactive molecules widely distributed in fruits, vegetables, cereals, and beverages. Polyphenols in food sources are extensively studied for their role in the maintenance of human health and in the protection against development of chronic/degenerative diseases. Polyphenols act mainly as antioxidant molecules, protecting cell constituents against oxidative damage. The enormous number of polyphenolic compounds leads to huge different mechanisms of action not fully understood. Recently, some evidence is emerging about the role of polyphenols, such as curcumin, pinocembrin, resveratrol, and quercetin, in modulating the activity of some aquaporin (AQP) isoforms. AQPs are integral, small hydrophobic water channel proteins, extensively expressed in many organs and tissues, whose major function is to facilitate the transport of water or glycerol over cell plasma membranes. Here we summarize AQP physiological functions and report emerging evidence on the implication of these proteins in a number of pathophysiological processes. In particular, this review offers an overview about the role of AQPs in brain, eye, skin diseases, and metabolic syndrome, focusing on the ability of polyphenols to modulate AQP expression. This original analysis can contribute to elucidating some peculiar effects exerted by polyphenols and can lead to the development of an innovative potential preventive/therapeutic strategy.

Aquaporin-3-mediated hydrogen peroxide transport is required for NF-κB signalling in keratinocytes and development of psoriasis

Aquaporin 3 (AQP3), a water/glycerol channel protein, has been found to transport hydrogen peroxide (H2O2). Here, we show that H2O2, imported via AQP3, is involved in nuclear factor-κB (NF-κB) signalling in keratinocytes and in the pathogenesis of psoriasis. IL-23-mediated induction of psoriasis is reduced in AQP3 knockout mice (AQP3(-/-)), and is accompanied by impaired NF-κB activation and intracellular H2O2 accumulation. In primary keratinocyte cultures, cellular import of H2O2 produced by membrane NADPH oxidase 2 (Nox2) in response to TNF-α is facilitated by AQP3 and required for NF-κB activation by regulation of protein phosphatase 2A. As AQP3 associates with Nox2, we propose that this interplay constitutes H2O2-mediated signalling in response to TNF-α stimulation. Collectively, these data indicate that AQP3-facilitated H2O2 transport is required for NF-κB activation in keratinocytes in the development of psoriasis.

The key role of aquaporin 3 and aquaporin 10 in the pathogenesis of pompholyx

Pompholyx remains a chronic skin affliction without a compelling pathophysiological explanation. The disease is characterized by the sudden onset of vesicles exclusively in the palms and soles which generally resolves. However, the disease may progress and the vesicles may expand and fuse; with chronicity there is deep fissuring. Multiple therapeutic approaches are available, but the disease is often resistant to conventional treatments. Currently, oral alitretinoin is used for patients with resistant chronic disease; however, this therapy is only approved for use in the UK, Europe and Canada. In this paper we wish to put forward a hypothesis: exposure to water and the subsequent steep osmotic gradient imbalance are key factors driving skin dehydration seen in pompholyx patients once the disease becomes chronic. The mechanistic explanation for the epidermal fissuring might lie in the over-expression across the mid and upper epidermis, including the stratum corneum, of two water/glycerol channel proteins aquaporin 3 and aquaporin 10, expressed in the keratinocytes of afflicted pompholyx patients. The over-expression of these two aquaporins may bridge the abundantly hydrated dermis and basal epidermis to the outer environment allowing cutaneous water and glycerol to flow outward. The beneficial effects reported in alitretinoin-treated patients with chronic hand eczemas may be due potential regulation of aquaporin 3 and aquaporin 10 by alitretinoin.

Epidermal aquaporin-3 is increased in the cutaneous burn wound

INTRODUCTION

Aquaporins (AQP) are a family of transmembrane proteins that transport water and small solutes such as glycerol across cell membranes. It is a mediator of transcellular water flow and plays an important role in maintaining intra/extracellular fluid homeostasis by facilitating water transport in response to changing osmotic gradients. In the skin, AQPs permit rapid, regulated, and selective water permeability and have been demonstrated to play a role in skin hydration, cell proliferation, migration, immunity, and wound healing. However, the expression of AQP-3 in the cutaneous burn wound has never been elucidated. We sought to assess the expression of AQP-3 in patients with burn wounds.

METHODS

A fresh full thickness biopsy sample was taken from the center of the burn wound, the burn wound edge, and the graft donor site in 7 patients (n=21), approximately 3-7 days post injury. Fixed, paraffin embedded sections were stained using AQP-3 specific antibody and examined by immunofluorescence. Fresh samples were processed to quantify AQP-3 protein expression with Western blot analysis.

RESULTS

The central portion of the burn wound revealed destruction of the epidermis and dermis with no AQP-3 present. Along the burn wound edge where the epidermal architecture was disrupted, there was robust AQP-3 staining. Western blot analysis demonstrated deeper staining along the burn wound edge compared to unburned skin (control). Quantification of the protein shows a significant amount of AQP-3 expression along the burn wound edge (3.6±0.34) compared to unburned skin (2.1±0.28, N=7, *p<0.05). There is no AQP-3 expression in the burn wound center.

CONCLUSION

AQP-3 expression is increased in the burn wound following injury. While its role in wound healing has been defined, we report for the first time the effect of cutaneous burns on AQP-3 expression. Our data provides the first step in determining its functional role in burn wounds. We hypothesize that development of AQP3 targeted therapies may improve burn wound healing.

Aquaporin-3 re-expression induces differentiation in a phospholipase D2-dependent manner in aquaporin-3-knockout mouse keratinocytes

Aquaporin-3 (AQP3) is a water and glycerol channel expressed in epidermal keratinocytes. Despite many studies, controversy remains about the role of AQP3 in keratinocyte differentiation. Previously, our laboratory has shown co-localization of AQP3 and phospholipase D2 (PLD2) in caveolin-rich membrane microdomains. We hypothesized that AQP3 transports glycerol and "funnels" this primary alcohol to PLD2 to form a pro-differentiative signal, such that the action of AQP3 to induce differentiation should require PLD2. To test this idea, we re-expressed AQP3 in mouse keratinocytes derived from AQP3-knockout mice. The re-expression of AQP3, which increased [3H]glycerol uptake, also induced mRNA and protein expression of epidermal differentiation markers such as keratin 1, keratin 10, and loricrin, with or without the induction of differentiation by an elevated extracellular calcium concentration. Re-expression of AQP3 had no effect on the expression of the proliferation markers keratin 5 and cyclin D1. Furthermore, a selective inhibitor of PLD2, CAY10594, and a lipase-dead (LD) PLD2 mutant, but not a LD PLD1 mutant, significantly inhibited AQP3 re-expression-induced differentiation marker expression with calcium elevation, suggesting a role for PLD2 in this process. Thus, our results indicate that AQP3 has a pro-differentiative role in epidermal keratinocytes and that PLD2 activity is necessary for this effect.

Defective channels lead to an impaired skin barrier

Channels are integral membrane proteins that form a pore, allowing the passive movement of ions or molecules across a membrane (along a gradient), either between compartments within a cell, between intracellular and extracellular environments or between adjacent cells. The ability of cells to communicate with one another and with their environment is a crucial part of the normal physiology of a tissue that allows it to carry out its function. Cell communication is particularly important during keratinocyte differentiation and formation of the skin barrier. Keratinocytes in the skin epidermis undergo a programme of apoptosis-driven terminal differentiation, whereby proliferating keratinocytes in the basal (deepest) layer of the epidermis stop proliferating, exit the basal layer and move up through the spinous and granular layers of the epidermis to form the stratum corneum, the external barrier. Genes encoding different families of channel proteins have been found to harbour mutations linked to a variety of rare inherited monogenic skin diseases. In this Commentary, we discuss how human genetic findings in aquaporin (AQP) and transient receptor potential (TRP) channels reveal different mechanisms by which these channel proteins function to ensure the proper formation and maintenance of the skin barrier.

Forensic diagnosis of ante- and postmortem burn based on aquaporin-3 gene expression in the skin

In order to diagnose death associated with fire, it is essential to show that the person was exposed to heat while still alive. We investigated both AQP1 and AQP3 expression in the skin of an experimental burn model, as well as in forensic autopsy cases, and discuss its role in the differential diagnosis of ante- and postmortem burns. In animal experiments, there was no difference in AQP1 gene expression among four groups (n=4): antemortem burn, postmortem burn, mechanical wound, and control. However, AQP3 expression in the antemortem burn was increased significantly compared with that of the other groups even at 5min after burn. Water content of the skin was decreased significantly by the burn procedure. Consistent with animal experiments, AQP3 gene expression in the skin of antemortem burn cases was increased significantly compared with postmortem burns, mechanical wounds, and controls (n=12 in each group). These observations suggest that dermal AQP3 gene expression was increased to maintain water homeostasis in response to dehydration from burn. Finally, our results suggest that AQP3 gene expression may be useful for forensic molecular diagnosis of antemortem burn.

Urea transport mediated by aquaporin water channel proteins

Aquaporins (AQPs) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, thirteen aquaporins have been characterized. They are distributed wildly in specific cell types in multiple organs and tissues. Each AQP channel consists of six membrane-spanning alpha-helices that have a central water-transporting pore. Four AQP monomers assemble to form tetramers, which are the functional units in the membrane. Some of AQPs also transport urea, glycerol, ammonia, hydrogen peroxide, and gas molecules. AQP-mediated osmotic water transport across epithelial plasma membranes facilitates transcellular fluid transport and thus water reabsorption. AQP-mediated urea and glycerol transport is involved in energy metabolism and epidermal hydration. AQP-mediated CO2 and NH3 transport across membrane maintains intracellular acid-base homeostasis. AQPs are also involved in the pathophysiology of a wide range of human diseases (including water disbalance in kidney and brain, neuroinflammatory disease, obesity, and cancer). Further work is required to determine whether aquaporins are viable therapeutic targets or reliable diagnostic and prognostic biomarkers.

An aquaporin 3-notch1 axis in keratinocyte differentiation and inflammation

Aquaporin 3 (AQP3) is an aquaglyceroporin which transports water, glycerol and small solutes across the plasma membrane. Its functions are not limited to fluid transport but also involve the regulation of cell proliferation, migration, skin hydration, wound healing and tumorigenesis. While AQP3 has been reported to play an important role in keratinocyte proliferation, its role in differentiation remains controversial. Our study demonstrated that the expression of AQP3 was regulated during differentiation and that it participated in keratinocyte differentiation control. We further revealed that AQP3 was a transcriptional target of Notch signaling, a critical pathway regulating keratinocyte differentiation and tumor suppression, and it regulated differentiation through a reciprocal negative feedback loop with Notch1. When the expression level of AQP3 was elevated, impaired barrier integrity and increased pro-inflammatory cytokine production ensued, mimicking the pathological conditions in Notch deficient mice and in atopic dermatitis. Dysregulation of AQP3 and Notch receptors has been reported in several skin diseases, including skin cancer. Our discovery of the novel AQP3-Notch1 axis may provide insight into epidermal homeostasis control and possible translational applications, including its potential use as a biomarker for molecular diagnosis in environmental studies.

Phospho-ΔNp63α regulates AQP3, ALOX12B, CASP14 and CLDN1 expression through transcription and microRNA modulation

Cisplatin-induced and ATM-phosphorylated (p)-ΔNp63α regulates the expression of epidermal differentiation and skin barrier regulators (AQP3, CASP14, ALOX12B, and CLDN1) in squamous cell carcinoma (SCC) cells by dual transcriptional and post-transcriptional mechanisms. We found that p-ΔNp63α bound to target gene promoters, and regulated the activity of the tested promoters in vitro. P-ΔNp63α was shown to upregulate miR-185-5p and downregulate let7-5p, which subsequently modulated AQP3, CASP14, ALOX12B and CLDN1 through their respective 3'-untranslated regions. The introduction of miR-185-5p into resistant SCC-11M cells, which are unable to phosphorylate ΔNp63α, render these cells more sensitive to cisplatin treatment. Further studies of the AQP3, CASP14, ALOX12B, and CLDN1 contributions to chemoresistance may assist in developing novel microRNA-based therapies for human SCC.

The ion channel TRPA1 is required for chronic itch

Chronic itch is a debilitating condition that affects one in 10 people. Little is known about the molecules that mediate chronic itch in primary sensory neurons and skin. We demonstrate that the ion channel TRPA1 is required for chronic itch. Using a mouse model of chronic itch, we show that scratching evoked by impaired skin barrier is abolished in TRPA1-deficient animals. This model recapitulates many of the pathophysiological hallmarks of chronic itch that are observed in prevalent human diseases such as atopic dermatitis and psoriasis, including robust scratching, extensive epidermal hyperplasia, and dramatic changes in gene expression in sensory neurons and skin. Remarkably, TRPA1 is required for both transduction of chronic itch signals to the CNS and for the dramatic skin changes triggered by dry-skin-evoked itch and scratching. These data suggest that TRPA1 regulates both itch transduction and pathophysiological changes in the skin that promote chronic itch.

In vivo studies of aquaporins 3 and 10 in human stratum corneum

Aquaporins (AQPs) constitute one family of transmembrane proteins facilitating transport of water across cell membranes. Due to their specificity, AQPs have a broad spectrum of physiological functions, and for keratinocytes there are indications that these channel proteins are involved in cell migration and proliferation with consequences for the antimicrobial defense of the skin. AQP3 and AQP10 are aqua-glyceroporins, known to transport glycerol as well as water. AQP3 is the predominant AQP in human skin and has previously been demonstrated in the basal layer of epidermis in normal human skin, but not in stratum corneum (SC). AQP10 has not previously been identified in human skin. Previous studies have demonstrated the presence of AQP3 and AQP10 mRNA in keratinocytes. In this study, our aim was to investigate if these aquaporin proteins were actually present in human SC cells. This can be seen as a first step toward elucidating the possible functional role of AQP3 and AQP10 in SC hydration. Specifically we investigate the presence of AQP3 and AQP10 in vivo in human SC using "minimal-invasive" technique for obtaining SC samples. SC samples were obtained from six healthy volunteers. Western blotting and immunohistochemistry were used to demonstrate the presence of AQP3 as well as AQP10. The presence of AQP3 and AQP10 was verified by Western blotting, allowing for detection of proteins by specific antibodies. Applying immunohistochemistry, cell-like structures in the shape of corneocytes were identified in all samples by AQP3 and AQP10 antibodies. In conclusion, identification of AQP3 and AQP10 protein in SC in an in vivo model is new. Together with the new "minimal-invasive" method for SC collection presented, this opens for new possibilities to study the role of AQPs in relation to function of the skin barrier.

Reduction of Skin pH during Treatment for Palmoplantar Hyperhidrosis: A Conjecture on the Role of pH-Regulated Water Channel, i.e. Aquaporin

Primary palmoplantar hyperhidrosis (PPH) is a disorder that involves excessive sweating on the palms and soles. Although the pathophysiology of PPH remains unknown, some treatments, including topical aluminum chloride (AC) and tap water iontophoresis (TWI), are effective at suppressing the perspiration. Herein, we report the kinetics of the skin pH of two cases of PPH treated with AC and TWI. We found that the skin pH decreased in accordance with the reduction in sweating. This finding indicates that the reduction in sweating may be attributed to the reduction of skin pH in AC and TWI. Whether or not the pH-regulated function of aquaporin can explain this finding remains unknown.

Critical role of aquaporin-3 in epidermal growth factor-induced migration of colorectal carcinoma cells and its clinical significance

Aquaporins (AQPs) are a family of small, integral membrane proteins that have been shown to play an important role in tumor development and metastasis. Several studies have demonstrated that expression of AQP3 contributes to the enhanced migration of epithelial cells and is related to differentiation, metastasis and vascular invasion in lung and gastric cancer. Therefore, we investigated whether AQP3 could enhance human colorectal carcinoma cell migration and we examined the role of AQP3 in the prognosis of colorectal carcinoma. Our results showed that human epidermal growth factor (hEGF) increased the expression of AQP3 and, subsequently, the migration ability of human colorectal carcinoma cells HCT116 in a dose- and time-dependent manner. The enhanced migration ability of HCT116 cells was blocked by the AQP3 inhibitor, CuSO(4). Overexpression of AQP3 induced by hEGF was inhibited by a PI3K/AKT inhibitor, LY294002, but the ERK inhibitor U0126 had a minor effect on the hEGF-induced AQP3 upregulation. Immunohistochemical staining of the cancer tissues and corresponding normal tissues showed that AQP3 expression in cancer tissue was higher compared to that in normal tissue. The expression intensity of AQP3 was associated with the differentiation, lymph node and distant metastasis of colorectal carcinoma patients. Our results suggest that AQP3 overexpression could facilitate colorectal carcinoma cell migration and AQP3 may be considered a potential indicator and therapeutic target for colon tumor metastasis and prognosis.

Aquaporins in clinical medicine

The aquaporins are a family of membrane water channels, some of which also transport glycerol. They are involved in a wide range of physiological functions (including water/salt homeostasis, exocrine fluid secretion, and epidermal hydration) and human diseases (including glaucoma, cancer, epilepsy, and obesity). At the cellular level, aquaporin-mediated osmotic water transport across cell plasma membranes facilitates transepithelial fluid transport, cell migration, and neuroexcitation; aquaporin-mediated glycerol transport regulates cell proliferation, adipocyte metabolism, and epidermal water retention. Genetic diseases caused by loss-of-function mutations in aquaporins include nephrogenic diabetes insipidus and congenital cataracts. The neuroinflammatory demyelinating disease neuromyelitis optica is marked by pathogenic autoantibodies against astrocyte water channel aquaporin-4. There remain broad opportunities for the development of aquaporin-based diagnostics and therapeutics. Disease-relevant aquaporin polymorphisms are beginning to be explored. There is great promise in the development of small-molecule aquaporin modulators for therapy of some types of refractory edema, brain swelling, neuroinflammation, glaucoma, epilepsy, cancer, pain, and obesity.

Changes in transepidermal water loss and skin hydration according to expression of aquaporin-3 in psoriasis

Background

Aquaporins (AQPs) are a family of water transporting proteins present in many mammalian epithelial and endothelial cell types. Among the AQPs, AQP3 is known to be a water/glycerol transporter expressed in human skin.

Objective

The relationship between the expression level of AQP3 and transpidermal water loss (TEWL) in the lesional and peri-lesional skin of psoriasis-affected patients, and skin hydration in the lesional and peri-lesional skin of psoriasis patients, was investigated.

Methods

The expression of AQP3 in psoriasis-affected and healthy control skin was determined using immunohistochemical and immunofluroscence staining. TEWL and skin hydration were measured using a Tewameter® TM210 (Courage & Khazaka, Cologne, Germany) and a Corneometer® CM 820 (Courage & Khazaka), respectively.

Results

AQP3 was mainly expressed in the plasma membrane of stratum corneum and the stratum spinosum in normal epidermis. Unlike the normal epidermis, AQP3 showed decreased expression in the lesional and peri-lesional epidermis of psoriasis. TEWL was increased, and skin hydration was decreased, in the lesional and peri-lesional skin of psoriasis patients, compared with the healthy control sample.

Conclusion

Although various factors contribute to reduced skin hydration in the lesional and peri-lesional skin of psoriasis, AQP3 appears to be a key factor in the skin dehydration of psoriasis-affected skin.

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

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

The channel physiology of the skin

During embryonic development, the skin, the largest organ of the human body, and nervous system are both derived from the neuroectoderm. Consequently, several key factors and mechanisms that influence and control central or peripheral nervous system activities are also present and hence involved in various regulatory mechanisms of the skin. Apparently, this is the case for the ion and non-ion selective channels as well. Therefore, in this review, we shall focus on delineating the regulatory roles of the channels in skin physiology and pathophysiology. First, we introduce key cutaneous functions and major characteristics of the channels in question. Then, we systematically detail the involvement of a multitude of channels in such skin processes (e.g. skin barrier formation, maintenance, and repair, immune mechanisms, exocrine secretion) which are mostly defined by cutaneous non-neuronal cell populations. Finally, we close by summarizing data suggesting that selected channels are also involved in skin diseases such as e.g. atopic dermatitis, psoriasis, non-melanoma cancers and malignant melanoma, genetic and autoimmune diseases, etc., as well as in skin ageing.

Abnormal aquaporin-3 protein expression in hyperproliferative skin disorders

Non-melanoma skin cancers (NMSCs) and psoriasis represent common hyperproliferative skin disorders, with approximately one million new NMSC diagnoses each year in the United States alone and a psoriasis prevalence of about 2% worldwide. We recently demonstrated that the glycerol channel, aquaporin-3 (AQP3) and the enzyme phospholipase D2 (PLD2) interact functionally in epidermal keratinocytes of the skin to inhibit their proliferation. However, others have suggested that AQP3 is pro-proliferative in keratinocytes and is upregulated in the NMSC, squamous cell carcinoma (SCC). To evaluate the AQP3/PLD2 signaling module in skin diseases, we determined their levels in SCC, basal cell carcinoma (BCC) and psoriasis as compared to normal epidermis. Skin biopsies with the appropriate diagnoses (10 normal, 5 SCC, 13 BCC and 10 plaque psoriasis samples) were obtained from the pathology archives and examined by immunohistochemistry using antibodies recognizing AQP3 and PLD2. In normal epidermis AQP3, an integral membrane protein, was localized mainly to the plasma membrane and PLD2 to the cell periphery, particularly in suprabasal layers. In BCC, AQP3 and PLD2 levels were reduced as compared to the normal-appearing overlying epidermis. In SCC, AQP3 staining was "patchy," with areas of reduced AQP3 immunoreactivity exhibiting positivity for Ki67, a marker of proliferation. PLD2 staining was unchanged in SCC. In psoriasis, AQP3 staining was usually observed in the cytoplasm rather than in the membrane. Also, in the majority of psoriatic samples, PLD2 showed weak immunoreactivity or aberrant localization. These results suggest that abnormalities in the AQP3/PLD2 signaling module correlate with hyperproliferation in psoriasis and the NMSCs.

Aquaporin-3 in keratinocytes and skin: its role and interaction with phospholipase D2

Aquaporin 3 (AQP3) is an aquaglyceroporin that transports water and glycerol and is expressed in the epidermis, among other epithelial tissues. We have recently shown that there is an association between this glycerol channel and phospholipase D2 (PLD2) in caveolin-rich membrane microdomains. While PLD2 is able to hydrolyze membrane phospholipids to generate phosphatidic acid, this enzyme also catalyzes, in the presence of primary alcohols, a transphosphatidylation reaction to produce a phosphatidylalcohol. We have proposed that AQP3 associated with PLD2 provides the physiological primary alcohol glycerol to PLD2 for use in the transphosphatidylation reaction to generate phosphatidylglycerol (PG). Further, we have proposed that PG functions as a signaling molecule to mediate early epidermal keratinocyte differentiation, and manipulation of this signaling module inhibits keratinocyte proliferation and enhances differentiation. In contrast, other investigators have suggested a proliferative role for AQP3 in keratinocytes. In addition, AQP3 knockout mice exhibit an epidermal phenotype, characterized by dry skin, decreased elasticity and delayed barrier repair and wound healing, which can be corrected by glycerol but not other humectants. AQP3 levels have also been found to be altered in human skin diseases. In this article the evidence supporting a role for AQP3 in the epidermis will be discussed.