Molecular and functional characterization of sodium--hydrogen exchanger in skin as well as cultured keratinocytes and melanocytes

Membrane Transporters and Channels in Melanoma

Recent research has revealed that ion channels and transporters can be important players in tumor development, progression, and therapy resistance in melanoma. For example, members of the ABC family were shown to support cancer stemness-like features in melanoma cells, while several members of the TRP channel family were reported to act as tumor suppressors.Also, many transporter proteins support tumor cell viability and thus suppress apoptosis induction by anticancer therapy. Due to the high number of ion channels and transporters and the resulting high complexity of the field, progress in understanding is often focused on single molecules and is in total rather slow. In this review, we aim at giving an overview about a broad subset of ion transporters, also illustrating some aspects of the field, which have not been addressed in detail in melanoma. In context with the other chapters in this special issue on "Transportome Malfunctions in the Cancer Spectrum," a comparison between melanoma and these tumors will be possible.

Acidic Stress Triggers Sodium-Coupled Bicarbonate Transport and Promotes Survival in A375 Human Melanoma Cells

Melanoma cells preserve intracellular pH (pH_i) within a viable range despite an acidic ambient pH that typically falls below pH 7.0. The molecular mechanisms underlying this form of acidic preservation in melanoma remain poorly understood. Previous studies had demonstrated that proton transporters including the monocarboxylate transporter (MCT), the sodium hydrogen exchanger (NHE), and V-Type ATPase mediate acid extrusion to counter intracellular acidification in melanoma cells. In this report, the expression and function of the Sodium-Coupled Bicarbonate Transporter (NCBT) family of base loaders were further characterized in melanoma cell lines. NCBT family members were found to be expressed in three different melanoma cell lines – A375, MeWo, and HS695T – and included the electrogenic sodium-bicarbonate cotransporter isoforms 1 and 2 (NBCe1 and NBCe2), the electroneutral sodium-bicarbonate cotransporter (NBCn1), and the sodium-dependent chloride-bicarbonate exchanger (NDCBE). These transporters facilitated 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS)-dependent pH_i recovery in melanoma cells, in response to intracellular acidification induced by ammonium chloride prepulse. Furthermore, the expression of NCBTs were upregulated via chronic exposure to extracellular acidification. Given the current research interest in the NCBTs as a molecular driver of tumourigenesis, characterising NCBT in melanoma provides impetus for developing novel therapeutic targets for melanoma treatment.

Cutaneous pH landscape as a facilitator of melanoma initiation and progression

Melanoma incidence is on the rise and currently causes the majority of skin cancer-related deaths. Yet, therapies for metastatic melanoma are still insufficient so that new concepts are essential. Malignant transformation of melanocytes and melanoma progression are intimately linked to the cutaneous pH landscape and its dysregulation in tumour lesions. The pH landscape of normal skin is characterized by a large pH gradient of up to 3 pH units between surface and dermis. The Na⁺ /H⁺ exchanger NHE1 is one of the major contributors of acidity in superficial skin layers. It is also activated by the most frequent mutation in melanoma, BRAF^{V 600E} , thereby causing pH dysregulation during melanoma initiation. Melanoma progression is supported by an extracellular acidification and/or NHE1 activity which promote the escape of single melanoma cells from the primary tumour, migration and metastatic spreading. We propose that viewing melanoma against the background of the acid-base physiology of the skin provides a better understanding of the pathophysiology of this disease and allows the development of novel therapeutic concepts.

Acidic tumor microenvironment in human melanoma

One characteristic of solid tumors such as malignant melanoma is the acidification of the tumor microenvironment. The deregulation of cancer cell metabolism is considered a main cause of extracellular acidosis. Here, cancer cells utilize aerobic glycolysis instead of oxidative phosphorylation even under normoxic conditions, as originally described by Otto Warburg. These metabolic alterations cause enhanced acid production, especially of lactate and carbon dioxide (CO2 ). The extensive production of acidic metabolites and the enhanced acid export to the extracellular space cause a consistent acidification of the tumor microenvironment, thus promoting the formation of an acid-resistant tumor cell population with increased invasive and metastatic potential. As melanoma is one of the deadliest and most metastatic forms of cancer, understanding the effects of this extracellular acidosis on human melanoma cells with distinct metastatic properties is important. The aim of this review was to summarize recent studies of the acidification of the tumor microenvironment, focusing on the specific effects of the acidic milieu on melanoma cells and to give a short overview of therapeutic approaches.

Selective potentiation of 2-APB-induced activation of TRPV1-3 channels by acid

Temperature-sensitive TRP channels are important for responses to pain and inflammation, to both of which tissue acidosis is a major contributing factor. However, except for TRPV1, acid-sensing by other ThermoTRP channels remains mysterious. We show here that unique among TRPV1–3 channels, TRPV3 is directly activated by protons from cytoplasmic side. This effect is very weak and involves key cytoplasmic residues L508, D512, S518, or A520. However, mutations of these residues did not affect a strong proton induced potentiation of TRPV3 currents elicited by the TRPV1–3 common agonist, 2-aminoethoxydiphenyl borate (2-APB), no matter if the ligand was applied from extracellular or cytoplasmic side. The acid potentiation was common among TRPV1–3 and only seen with 2-APB-related ligands. Using ¹H-nuclear magnetic resonance to examine the solution structures of 2-APB and its analogs, we observed striking structural differences of the boron-containing compounds at neutral/basic as compared to acidic pH, suggesting that a pH-dependent configuration switch of 2-APB-based drugs may underlie their functionality. Supporting this notion, protons also enhanced the inhibitory action of 2-APB on TRPM8. Collectively, our findings reveal novel insights into 2-APB action on TRP channels, which should facilitate the design of new drugs for these channels.

Hydration status regulates sodium flux and inflammatory pathways through epithelial sodium channel (ENaC) in the skin

Although it is known that the inflammatory response that results from disruption of epithelial barrier function after injury results in excessive scarring, the upstream signals remain unknown. It has also been observed that epithelial disruption results in reduced hydration status and that the use of occlusive dressings that prevent water loss from wounds decreases scar formation. We hypothesized that hydration status changes sodium homeostasis and induces sodium flux in keratinocytes, which result in activation of pathways responsible for keratinocyte-fibroblast signaling and ultimately lead to activation of fibroblasts. Here, we demonstrate that perturbations in epithelial barrier function lead to increased sodium flux in keratinocytes. We identified that sodium flux in keratinocytes is mediated by epithelial sodium channels (ENaCs) and causes increased secretion of proinflammatory cytokines, which activate fibroblast via the cyclooxygenase 2 (COX-2)/prostaglandin E2 (PGE2) pathway. Similar changes in signal transduction and sodium flux occur by increased sodium concentration, which simulates reduced hydration, in the media in epithelial cultures or human ex vivo skin cultures. Blockade of ENaC, prostaglandin synthesis, or PGE2 receptors all reduce markers of fibroblast activation and collagen synthesis. In addition, employing a validated in vivo excessive scar model in the rabbit ear, we demonstrate that utilization of either an ENaC blocker or a COX-2 inhibitor results in a marked reduction in scarring. Other experiments demonstrate that the activation of COX-2 in response to increased sodium flux is mediated through the PIK3/Akt pathway. Our results indicate that ENaC responds to small changes in sodium concentration with inflammatory mediators and suggest that the ENaC pathway is a potential target for a strategy to prevent fibrosis.

The marine toxin palytoxin induces necrotic death in HaCaT cells through a rapid mitochondrial damage

Palytoxin (PLTX) is one of the most toxic algal biotoxin known so far. It transforms the Na(+)/K(+)-ATPase into a cationic channel inducing a massive intracellular Na(+) influx. However, from a mechanistic point of view, the features and the intracellular pathways leading to PLTX-induced cell death are still not completely characterized. This study on skin HaCaT keratinocytes demonstrates that PLTX induces necrosis since propidium iodide uptake was observed already after 1 h toxin exposure, an effect that was not lowered by toxin removal. Furthermore, necrotic-like morphological alterations were evidenced by confocal microscopy. Apoptosis occurrence was excluded since no caspases 3/7, caspase 8, and caspase 9 activation as well as no apoptotic bodies formation were recorded. Necrosis was preceded by a very early mitochondrial damage as indicated by JC-1 fluorescence shift, recorded already after 5 min toxin exposure. This shift was totally abolished when Na(+) and Ca(2+) ions were withdrawn from culture medium, whereas cyclosporine-A was ineffective, excluding the occurrence of a controlled biochemical response. These results clearly establish necrosis as the primary mechanism for PLTX-induced cell death in HaCaT cells. The rapidity of mitochondrial damage and the consequent irreversible necrosis rise serious concerns about the very fast onset of PLTX toxic effects.

Luminescent dual sensors reveal extracellular pH-gradients and hypoxia on chronic wounds that disrupt epidermal repair

Wound repair is a quiescent mechanism to restore barriers in multicellular organisms upon injury. In chronic wounds, however, this program prematurely stalls. It is known that patterns of extracellular signals within the wound fluid are crucial to healing. Extracellular pH (pH_e) is precisely regulated and potentially important in signaling within wounds due to its diverse cellular effects. Additionally, sufficient oxygenation is a prerequisite for cell proliferation and protein synthesis during tissue repair. It was, however, impossible to study these parameters in vivo due to the lack of imaging tools. Here, we present luminescent biocompatible sensor foils for dual imaging of pH_e and oxygenation in vivo. To visualize pH_e and oxygen, we used time-domain dual lifetime referencing (tdDLR) and luminescence lifetime imaging (LLI), respectively. With these dual sensors, we discovered centripetally increasing pH_e-gradients on human chronic wound surfaces. In a therapeutic approach, we identify pH_e-gradients as pivotal governors of cell proliferation and migration, and show that these pH_e-gradients disrupt epidermal barrier repair, thus wound closure. Parallel oxygen imaging also revealed marked hypoxia, albeit with no correlating oxygen partial pressure (pO₂)-gradient. This highlights the distinct role of pH_e-gradients in perturbed healing. We also found that pH_e-gradients on chronic wounds of humans are predominantly generated via centrifugally increasing pH_e-regulatory Na⁺/H⁺-exchanger-1 (NHE1)-expression. We show that the modification of pH_e on chronic wound surfaces poses a promising strategy to improve healing. The study has broad implications for cell science where spatial pH_e-variations play key roles, e.g. in tumor growth. Furthermore, the novel dual sensors presented herein can be used to visualize pH_e and oxygenation in various biomedical fields.

Regulation of intracellular pH in cnidarians: response to acidosis in Anemonia viridis

The regulation of intracellular pH (pHi) is a fundamental aspect of cell physiology that has received little attention in studies of the phylum Cnidaria, which includes ecologically important sea anemones and reef-building corals. Like all organisms, cnidarians must maintain pH homeostasis to counterbalance reductions in pHi, which can arise because of changes in either intrinsic or extrinsic parameters. Corals and sea anemones face natural daily changes in internal fluids, where the extracellular pH can range from 8.9 during the day to 7.4 at night. Furthermore, cnidarians are likely to experience future CO₂-driven declines in seawater pH, a process known as ocean acidification. Here, we carried out the first mechanistic investigation to determine how cnidarian pHi regulation responds to decreases in extracellular and intracellular pH. Using the anemone Anemonia viridis, we employed confocal live cell imaging and a pH-sensitive dye to track the dynamics of pHi after intracellular acidosis induced by acute exposure to decreases in seawater pH and NH₄Cl prepulses. The investigation was conducted on cells that contained intracellular symbiotic algae (Symbiodinium sp.) and on symbiont-free endoderm cells. Experiments using inhibitors and Na⁺-free seawater indicate a potential role of Na⁺/H⁺ plasma membrane exchangers (NHEs) in mediating pHi recovery following intracellular acidosis in both cell types. We also measured the buffering capacity of cells, and obtained values between 20.8 and 43.8 mM per pH unit, which are comparable to those in other invertebrates. Our findings provide the first steps towards a better understanding of acid-base regulation in these basal metazoans, for which information on cell physiology is extremely limited.

sPLA2 and the epidermal barrier

The mammalian epidermis provides both an interface and a protective barrier between the organism and its environment. Lipid, processed into water-impermeable bilayers between the outermost layers of the epidermal cells, forms the major barrier that prevents water from exiting the organism, and also prevents toxins and infectious agents from entering. The secretory phospholipase 2 (sPLA2) enzymes control important processes in skin and other organs, including inflammation and differentiation. sPLA2 activity contributes to epidermal barrier formation and homeostasis by generating free fatty acids, which are required both for formation of lamellar membranes and also for acidification of the stratum corneum (SC). sPLA2 is especially important in controlling SC acidification and establishment of an optimum epidermal barrier during the first postnatal week. Several sPLA2 isoforms are present in the epidermis. We find that two of these isoforms, sPLA2 IIA and sPLA2 IIF, localize to the upper stratum granulosum and increase in response to experimental barrier perturbation. sPLA2F(-/-) mice also demonstrate a more neutral SC pH than do their normal littermates, and their initial recovery from barrier perturbation is delayed. These findings confirm that sPLA2 enzymes perform important roles in epidermal development, and suggest that the sPLA2IIF isoform may be central to SC acidification and barrier function. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Evidence for NHE3-mediated Na transport in sheep and bovine forestomach

Na absorption across the cornified, multilayered, and squamous rumen epithelium is mediated by electrogenic amiloride-insensitive transport and by electroneutral Na transport. High concentrations of amiloride (>100 μM) inhibit Na transport, indicating Na+/H+ exchange (NHE) activity. The underlying NHE isoform for transepithelial Na absorption was characterized by mucosal application of the specific inhibitor HOE642 for NHE1 and S3226 for NHE3 in Ussing chamber studies with isolated epithelia from bovine and sheep forestomach. S3226 (1 μM; NHE3 inhibitor) abolished electroneutral Na transport under control conditions and also the short-chain fatty acid-induced increase of Na transport via NHE. However, HOE642 (30 μM; NHE1 inhibitor) did not change Na transport rates. NHE3 was immunohistochemically localized in membranes of the upper layers toward the lumen. Expression of NHE1 and NHE3 has been previously demonstrated by RT-PCR, and earlier experiments with isolated rumen epithelial cells have shown the activity of both NHE1 and NHE3. Obviously, both isoforms are involved in the regulation of intracellular pH, pHi. However, transepithelial Na transport is only mediated by apical uptake via NHE3 in connection with extrusion of Na by the basolaterally located Na-K-ATPase. The missing involvement of NHE1 in transepithelial Na transport suggests that the proposed “job sharing” in epithelia between these two isoforms probably also applies to forestomach epithelia: NHE3 for transepithelial transport and NHE1 for, among others, pHi and volume regulation.

Acidification in the epidermis and the role of secretory phospholipases

The function of the epidermis is to form an effective barrier between the dry, external environment and the interior of the body. The barrier specifically resides in the extracellular lipid membranes of the stratum corneum (SC) and an acidic pH is necessary to maintain its competency against various insults. The purpose of this review is to explore the mechanisms which are postulated to contribute to the acidification of the stratum corneum, including both exogenous and endogenous sources. However, recent research as pointed to several endogenous mechanisms as the major source of acidification, including a sodium/proton pump (NHE1) and free fatty acid conversion from phospholipids by secretory phospholipase A₂ (sPLA₂). sPLA₂ has been shown to play a central role in the formation of the SC “acid mantle” in the early maturation of the epidermis postnatally. Many aspects of this enzyme family are complex and still being elucidated in research and the most recent findings on the localization and functions of sPL A₂-IB, -IIA, -IIC, -IID, -IIE, -IIF, -III, -V, -X and -XII in the epidermis are presented here. Given their role in inflammatory dermatoses, such as psoriasis and atopic dermatitis, understanding this complex enzyme family can lead to novel, life-changing therapies.

Zinc released from injured cells is acting via the Zn2+-sensing receptor, ZnR, to trigger signaling leading to epithelial repair

A role for Zn(2+) in accelerating wound healing is established, yet, the signaling pathways linking Zn(2+) to tissue repair are not well known. We show that in the human HaCaT keratinocytes extracellular Zn(2+) induces a metabotropic Ca(2+) response that is abolished by silencing the expression of the G-protein-coupled receptor GPR39, suggesting that this Zn(2+)-sensing receptor, ZnR, is mediating the response. Keratinocytic-ZnR signaling is highly selective for Zn(2+) and can be triggered by nanomolar concentrations of this ion. Interestingly, Zn(2+) was also released following cellular injury, as monitored by a specific non-permeable fluorescent Zn(2+) probe, ZnAF-2. Chelation of Zn(2+) and scavenging of ATP from conditioned medium, collected from injured epithelial cultures, was sufficient to eliminate the metabotropic Ca(2+) signaling. The signaling triggered by Zn(2+), via ZnR, or by ATP further activated MAP kinase and induced up-regulation of the sodium/proton exchanger NHE1 activity. Finally, activation of ZnR/GPR39 signaling or application of ATP enhanced keratinocytes scratch closure in an in vitro model. Thus our results indicate that extracellular Zn(2+), which is either applied or released following injury, activates ZnR/GPR39 to promote signaling leading to epithelial repair.

The polymerization of melanin: a poorly understood phenomenon with egregious biological implications

Several hypotheses have explicitly implicated the role of an altered redox status of melanin in the aetiology of melanoma and macular degeneration. The balance between the intrinsic anti-oxidant and pro-oxidant properties of melanin is lost, resulting in an altered redox phenotype. We propose that such an alteration of the redox status of melanin may arise, in part, due to suboptimal conditions for the effective polymerization of melanin precursors. We suggest that a decrease in the degree of polymerization or molecular weight of the melanin polymer may cause an alteration of the redox status of the polymer towards a more pro-oxidant state. A higher propensity of smaller oligomers to complex metals, coupled with an upregulation of metallothionein expression, results in increased production of free radicals including the superoxide anion. This, in association with an increase in the rate of tyrosinase degradation, a decrease in the rate of tyrosinase activation, alterations to template protein structure or alterations in the kinetics of the oxidation of tyrosine via the Raper-Mason pathway, may result in an overcoming of the cellular anti-oxidant pool, an increased susceptibility to oxidative stress and alterations to the reaction kinetics of melanogenesis, thus setting up a cycle of increasing oxidative stress and proliferation leading to the leakage of melanin monomers outside the organelle, thereby causing cytotoxicity and necrosis.

Extracellular pH Controls NHE1 expression in epidermis and keratinocytes: implications for barrier repair

We have previously shown that the Na+/H+ antiporter (NHE1) is an essential endogenous pathway responsible for stratum corneum (SC) acidification. Since the epidermis must re-establish its epidermal barrier after acute barrier perturbations, we asked whether the NHE1 was, in turn, regulated by changes in barrier status. We found that in vivo epidermal NHE1 expression was upregulated within hours of barrier disruption. We next asked whether NHE1 was regulated by barrier status per se, or by the SC alkalinization that accompanies barrier perturbation. NHE1 was upregulated by alkalinizing SC pH, whereas this antiporter was downregulated by acidifying SC pH, independent of changes in barrier status. Moreover, acidifying SC pH overrode the effects of barrier break in regulating NHE1 expression, suggesting that SC alkalinization is the major stimulus for increased NHE1 expression. Finally, we confirmed that the keratinocyte NHE1 antiporter is regulated by extracellular pH independent of barrier status, by demonstrating that NHE1 was upregulated in cultured keratinocytes exposed to pH 8.3 medium and downregulated in cultured keratinocytes exposed to pH 6.3 medium. These data suggest that the keratinocyte NHE1 is regulated by extracellular pH. SC barrier break also upregulates NHE1 expression, but this response seems to be mediated by concomitant changes in SC pH.

Stratum corneum defensive functions: an integrated view

Most epidermal functions can be considered as protective, or more specifically, as defensive in nature. Yet, the term "barrier function" is often used synonymously with only one such defensive function, though arguably its most important, i.e., permeability barrier homeostasis. Regardless of their relative importance, these protective cutaneous functions largely reside in the stratum corneum (SC). In this review, I first explore the ways in which the multiple defensive functions of the SC are linked and interrelated, either by their shared localization or by common biochemical processes; how they are co-regulated in response to specific stressors; and how alterations in one defensive function impact other protective functions. Then, the structural and biochemical basis for these defensive functions is reviewed, including metabolic responses and signaling mechanisms of barrier homeostasis. Finally, the clinical consequences and therapeutic implications of this integrated perspective are provided.

The relationship between Na+/H+ exchanger expression and tyrosinase activity in human melanocytes

The activity of melanosome-associated tyrosinase in human melanocytes differs based on racial skin type. In melanocytes from Black skin, tyrosinase activity is high while in White melanocytes the activity of the enzyme is low. Recent studies suggest that low tyrosinase activity in White melanocytes may be due to an acidic pH environment within the melanosome. Because sodium/hydrogen (Na(+)/H(+)) exchangers (NHEs) are known to regulate intracellular pH, melanocytes were treated with NHE inhibitors to determine what effect this inhibition might have on tyrosinase activity. Treatment of Black melanocytes with ethyl-isopropyl amiloride (EIPA) caused a rapid dose-dependent inhibition of tyrosinase activity. This inhibition was not due to either direct enzyme inhibition or to a decrease in tyrosinase abundance. In contrast, treatment of White melanocytes with EIPA, cimetidine, or clonidine resulted in little inhibition of tyrosinase activity. Reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis showed that both Black and White melanocytes expressed mRNA and protein for NHE-1, NHE-3, NHE-5, NHE-6, and NHE-7. Immunohistochemical analysis showed that NHE-7 and NHE-3 co-localized with the melanosomal protein, Tyrosinase Related Protein-1 (TRP-1). In addition, the vesicular proton pump, vesicular ATPase (V-ATPase), was found to be present in both White and Black melanosomes, indicating that organelles from both racial skin types are capable of being acidified. The results suggest that one or more NHEs may help regulate melanosome pH and tyrosinase activity in human melanocytes.

NHE1 regulates the stratum corneum permeability barrier homeostasis. Microenvironment acidification assessed with fluorescence lifetime imaging

The outermost epidermal layer, the stratum corneum (SC), exhibits an acidic surface pH, whereas the pH at its base approaches neutrality. NHE1 is the only Na(+)/H(+) antiporter isoform in keratinocytes and epidermis, and has been shown to regulate intracellular pH. We now demonstrate a novel function for NHE1, as we find that it also controls acidification of extracellular "microdomains" in the SC that are essential for activation of pH-sensitive enzymes and the formation of the epidermal permeability barrier. NHE1 expression in epidermis is most pronounced in granular cell layers, and although the surface pH of NHE1 knockout mice is only slightly more alkaline than normal using conventional pH measurements, a more sensitive method, fluorescence lifetime imaging, demonstrates that the acidic intercellular domains at the surface and of the lower SC disappear in NHE1 -/- animals. Fluorescence lifetime imaging studies also reveal that SC acidification does not occur through a uniform gradient, but through the progressive accumulation of acidic microdomains. These findings not only visualize the spatial distribution of the SC pH gradient, but also demonstrate a role for NHE1 in the generation of acidic extracellular domains of the lower SC, thus providing the acidification of deep SC interstices necessary for lipid processing and barrier homeostasis.

A rapid method for measuring intracellular pH using BCECF-AM

A rapid intracellular pH (pH(i)) measurement method based on initial rate of increase of fluorescence ratio of 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein upon dye addition to a cell suspension in growth medium is reported. A dye transport model that describes dye concentration and fluorescence values in intracellular and extracellular spaces provides the mathematical basis for the approach. Experimental results of ammonium chloride challenge response of the two suspension cells, Spodoptera frugiperda and Chinese hamster ovary (CHO) cells, successfully compared with results obtained using traditional perfusion method. Since the cell suspension does not require any preparation, measurement of pH(i) can be completed in about 1 min minimizing any potential errors due to dye leakage.