Alternatively spliced forms of the cGMP-gated channel in human keratinocytes

Effects of trans-2-nonenal and olfactory masking odorants on proliferation of human keratinocytes

Malodorous compounds induce stress responses, mood changes, an increase of skin conductance, activation of the sympathetic nervous system and other physiological changes, and it has been suggested that sensing malodors could provide warning of danger to health. Furthermore, the human body secretes various malodorous compounds as waste products of metabolism, including trans-2-nonenal ((E)-2-nonenal), the amount of which increases with aging. In the present study, we examined the effects of some endogenous malodorous compounds ((E)-2-nonenal, nonanal, pentanal, hexanal, hexanoic acid, hexylamine and isovaleric acid) on cultured human keratinocytes. (E)-2-Nonenal decreased the viability and promoted apoptosis of cultured keratinocytes. It also reduced the thickness and the number of proliferative cells in a three-dimensional epidermal equivalent model. Co-application of masking odorants (dihydromycenol, benzaldehyde, linalool, phenethyl alcohol, benzyl acetate and anisaldehyde), but not non-masking odorants (1,8-cineol, β-damascone, and o-t-butylcyclohexyl acetate), reduced the effect of (E)-2-nonenal on keratinocyte proliferation, and restored the thickness and number of proliferative cells in a three-dimensional epidermal equivalent model.

Two olfactory receptors-OR2A4/7 and OR51B5-differentially affect epidermal proliferation and differentiation

Olfactory receptors (ORs), which belong to the G-protein coupled receptor family, are expressed in various human tissues, including skin. Cells in non-olfactory tissues tend to express more than one individual OR gene, but function and interaction of two or more ORs in the same cell type has only been marginally analysed. Here, we revealed OR2A4/7 and OR51B5 as two new ORs in human skin cells and identified cyclohexyl salicylate and isononyl alcohol as agonists of these receptors. In cultured human keratinocytes, both odorants induce strong Ca²⁺ signals that are mediated by OR2A4/7 and OR51B5, as demonstrated by the receptor knockdown experiments. Activation of corresponding receptors induces a cAMP-dependent pathway. Localization studies and functional characterization of both receptors revealed several differences. OR2A4/7 is expressed in suprabasal keratinocytes and basal melanocytes of the epidermis and influences cytokinesis, cell proliferation, phosphorylation of AKT and Chk-2 and secretion of IL-1. In contrast, OR51B5 is exclusively expressed in suprabasal keratinocytes, supports cell migration and regeneration of keratinocyte monolayers, influences Hsp27, AMPK1 and p38MAPK phosphorylation and interestingly, IL-6 secretion. These findings underline that different ORs perform diverse functions in cutaneous cells, and thus offering an approach for the modulated treatment of skin diseases and wound repair.

A synthetic sandalwood odorant induces wound-healing processes in human keratinocytes via the olfactory receptor OR2AT4

As the outermost barrier of the body, the skin is exposed to multiple environmental factors, including temperature, humidity, mechanical stress, and chemical stimuli such as odorants that are often used in cosmetic articles. Keratinocytes, the major cell type of the epidermal layer, express a variety of different sensory receptors that enable them to react to various environmental stimuli and process information in the skin. Here we report the identification of a novel type of chemoreceptors in human keratinocytes, the olfactory receptors (ORs). We cloned and functionally expressed the cutaneous OR, OR2AT4, and identified Sandalore, a synthetic sandalwood odorant, as an agonist of this receptor. Sandalore induces strong Ca(2+) signals in cultured human keratinocytes, which are mediated by OR2AT4, as demonstrated by receptor knockdown experiments using RNA interference. The activation of OR2AT4 induces a cAMP-dependent pathway and phosphorylation of extracellular signal-regulated kinases (Erk1/2) and p38 mitogen-activated protein kinases (p38 MAPK). Moreover, the long-term stimulation of keratinocytes with Sandalore positively affected cell proliferation and migration, and regeneration of keratinocyte monolayers in an in vitro wound scratch assay. These findings combined with our studies on human skin organ cultures strongly indicate that the OR 2AT4 is involved in human keratinocyte re-epithelialization during wound-healing processes.

Calcium regulation of keratinocyte differentiation

Calcium is the major regulator of keratinocyte differentiation in vivo and in vitro. A calcium gradient within the epidermis promotes the sequential differentiation of keratinocytes as they traverse the different layers of the epidermis to form the permeability barrier of the stratum corneum. Calcium promotes differentiation by both outside-in and inside-out signaling. A number of signaling pathways involved with differentiation are regulated by calcium, including the formation of desmosomes, adherens junctions and tight junctions, which maintain cell-cell adhesion and play an important intracellular signaling role through their activation of various kinases and phospholipases that produce second messengers that regulate intracellular free calcium and PKC activity, critical for the differentiation process. The calcium receptor plays a central role by initiating the intracellular signaling events that drive differentiation in response to extracellular calcium. This review will discuss these mechanisms.

TRPC7 is a receptor-operated DAG-activated channel in human keratinocytes

Muscarinic and purinergic receptors expressed in keratinocytes are an important part of a functional system for cell growth. While several aspects of this process are clearly dependent on Ca(2+) homeostasis, less is known about the mechanisms controlling Ca(2+) entry during epidermal receptor stimulation. We used patch-clamp technique to study responses to carbachol (CCh) and adenosine triphosphate (ATP) in HaCaT human keratinocytes. Both agonists induced large currents mediated by cation-selective channels about three times more permeable to Ca(2+) than Na(+), suggesting that they play an important role in receptor-operated Ca(2+) entry. CCh- and ATP-induced currents were inhibited by 1-[6-([(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino)hexyl]-1H-pyrrole-2,5-dione, a phospholipase C (PLC) blocker. Investigation of the pathways downstream of PLC activation revealed that InsP(3) did not affect the agonist responses. In contrast, 1-oleoyl-2-acetyl-sn-glycerol (OAG), a membrane-permeable analog of 1,2-diacylglycerol (DAG), evoked a similar cation current. This action appears to be direct, since the effects of activators or inhibitors of protein kinase C were comparatively small. Finally, transient receptor potential canonical 7 (TRPC7) specific knockdown by antisense oligonucleotides led to a decrease in ATP- and CCh-induced calcium entry, as well as OAG-evoked current. We concluded that activation of both muscarinic and purinergic receptors via a common DAG-dependent link opens Ca(2+)-permeable TRPC7 channels.

Vitamin D and skin cancer

Skin cancer is the most common cancer afflicting humans. These cancers include melanomas and 2 types of malignant keratinocytes: basal-cell carcinomas (BCC) and squamous-cell carcinomas (SCC). UV light exposure is linked to the incidence of these cancers. On the other hand, the skin is the major source of vitamin D-3 (cholecalciferol) and UV light is critical for its formation. Keratinocytes can convert vitamin D-3 to its hormonal form, 1,25 dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] (calcitriol). 1,25(OH)(2)D(3) in turn stimulates the differentiation of keratinocytes, raising the hope that 1,25(OH)(2)D(3) may prevent the development of malignancies in these cells. We identified a number of mechanisms by which 1,25(OH)(2)D(3) regulates the differentiation of keratinocytes and explored where this regulation breaks down in SCCs. 1,25(OH)(2)D(3) regulates gene expression by activating the vitamin D receptor (VDR). When activated, the VDR binds to one of two coactivator complexes: DRIP or p160/SRC. Binding to DRIP occurs in the undifferentiated keratinocyte, but, as the cell differentiates, DRIP(205) levels fall and p160/SRC binding takes over as SRC3 expression increases. SCCs fail to respond to the prodifferentiating actions of 1,25(OH)(2)D(3). These cells have normal levels of VDR and normal binding of VDR to vitamin D response elements. However, they overexpress DRIP(205) such that the p160/SRC complex is blocked from binding to VDR. We hypothesize that failure of 1,25(OH)(2)D(3) to induce differentiation in SCCs lies at least in part with its failure to induce the replacement of the DRIP complex with the SRC complex in the promoters of genes required for differentiation.

Vitamin D regulated keratinocyte differentiation

The epidermis is the largest organ in the body. It is comprised primarily of keratinocytes which are arranged in layers that recapitulates their programmed life cycle. Proliferating keratinocytes are on the bottom-the stratum basale. As keratinocytes leave the stratum basale they begin to differentiate, culminating in the enucleated stratum corneum which has the major role of permeability barrier. Calcium and the active metabolite of vitamin D, 1,25(OH)(2)D(3), play important roles in this differentiation process. The epidermis has a gradient of calcium with lowest concentrations in the stratum basale, and highest concentrations in the stratum granulosum where proteins critical for barrier function are produced. Vitamin D is made in different layers of the epidermis, but 1,25(OH)(2)D(3) is made primarily in the stratum basale. Together calcium and 1,25(OH)(2)D(3) regulate the ordered differentiation process by the sequential turning on and off the genes producing the elements required for differentiation as well as activating those enzymes involved in differentiation. Animal models in which the sensing mechanism for calcium, the receptor for 1,25(OH)(2)D(3), or the enzyme producing 1,25(OH)(2)D(3) have been rendered inoperative demonstrate the importance of these mechanisms for the differentiation process, although each animal model has its own phenotype. This review will examine the mechanisms by which calcium and 1,25(OH)(2)D(3) interact to control epidermal differentiation.

Aberrant signalling and transcription factor activation as an explanation for the defective growth control and differentiation of keratinocytes in psoriasis: a hypothesis

Psoriasis is a chronic inflammatory skin disease characterized by the accumulation of red, scaly plaques on the skin. The plaques result from hyperproliferation and incomplete differentiation of keratinocytes (KC) in a process that seems to be driven, in part by skin-infiltrating leucocytes. We believe that the KC have inherent defects in intracellular signalling which could be usefully targeted to allow the development of more effective therapies. We suggest that there are defects in the regulation of the transcription factors: signal transducer and activator of transcription (STAT-1alpha), interferon regulated factor-1 (IRF-1) and NFkappaB which lead to loss of growth and differentiation control when the cells are subjected to physico-chemical and immunological stress. We also highlight recent studies that suggest that peroxisome proliferator-activated receptors, the notch receptor and defects in calcium and other ion transporting proteins may contribute to impairment in the ability of psoriatic KC to differentiate. The role of these systems in the development of the psoriatic phenotype and tests of these hypotheses are proposed.

Sequential activation of store-operated currents in human gingival keratinocytes

Calcium ion store-activated currents in undifferentiated human gingival keratinocytes were measured with the whole cell patch clamp and fura techniques. Thapsigargin or intracellular inositol 1,4,5-trisphosphate and BAPTA rapidly induced an early transient current with I(CRAC) (calcium release activated calcium ion current) characteristics, and several later, larger sustained currents that depended on the mode of store depletion. Thapsigargin activated two currents within minutes of I(CRAC) activation. The first was a nonspecific cation current, I(NSC). A second conducted Na+ and Cs+, and was partially inhibited by thapsigargin (INa1). Dialysis with inositol 1,4,5-trisphosphate and BAPTA induced a later current that also conducted Na+ and Cs+, but was inhibited by extracellular calcium ion (INa2), with properties consistent with an epithelial Na+ channel current in some cells, and a calcium ion-insensitive Na+ current (INa3). Comparison of thapsigargin-evoked current changes with fura-2/AM results from separate cells indicated that both the I(CRAC) and the later, larger calcium ion conducting currents contributed to changes in intracellular calcium ion concentration, and likely play important parts in calcium ion signaling in undifferentiated keratinocytes.

Phospholipase Cdelta1 is required for skin stem cell lineage commitment

Phosphoinositide-specific phospholipase C (PLC) is a key enzyme in phosphoinositide turnover and is involved in a variety of physiological functions. Here we report that PLCdelta(1)-deficient mice undergo progressive hair loss in the first postnatal hair cycle. Epidermal hyperplasia was observed, and many hairs in the skin of PLCdelta(1)-deficient mice failed to penetrate the epidermis and became zigzagged owing to occlusion of the hair canal. Two major downstream signals of PLC, calcium elevation and protein kinase C activation, were impaired in the keratinocytes and skin of PLCdelta(1)-deficient mice. In addition, many cysts that had remarkable similarities to interfollicular epidermis, as well as hyperplasia of sebaceous glands, were observed. Furthermore, PLCdelta(1)-deficient mice developed spontaneous skin tumors that had characteristics of both interfollicular epidermis and sebaceous glands. From these results, we conclude that PLCdelta(1) is required for skin stem cell lineage commitment.

A large conductance [Ca(2+)](i)-independent K(+) channel expressed in HaCaT keratinocytes

Patch-clamp recordings were carried out in the inside-out configuration in human keratinocytes of the cell line HaCaT. Patch pipettes were filled with 150 mM KCl, 1 mM CaCl(2) and 10 mM HEPES. In symmetrical KCl solutions, single channel currents from a large conductance channel (about 170 pS) were measured. Replacement of 120 mM KCl by K-aspartate had only a minor influence on the single channel conductance and on the reversal potential. In intracellular solution in which K(+) has been replaced by Na(+) or NMDG(+), the reversal potential shifted to > + 40 mV indicating K(+) as the main charge carrier. The channels were neither dependent on intracellular Ca(2+) (between 0.8 nM and 10 micro M), ATP (at 0 and 1 mM) nor Mg(2+) (at 0 and 0.5 mM). The mean current showed an outward rectification that can be mainly attributed to the voltage dependence of the open probability. The channels displayed bursting kinetics with a mean open time of about 2 ms and closed times of about 0.2, 2 and 20 ms. The mean open probability was usually low (0.05) but increased occasionally (0.6) mainly due to a lower probability of long closings. We conclude that these K(+) channels contribute to the resting potential of human keratinocytes which may control the Ca(2+) influx and thereby their proliferation and differentiation.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Epithelial sodium channels are upregulated during epidermal differentiation

Terminal differentiation of epidermal keratinocytes is linked to transmembrane ion flux. Previously, we have shown that amiloride, an inhibitor of epithelial sodium channels, blocks synthesis of differentiation-specific proteins in normal human keratinocytes. Here, we have identified the specific subunits of amiloride-sensitive human epithelial sodium channels in relation to differentiation of cultured human keratinocytes, as well as to epidermal development. As assessed by northern hybridization, RNase protection assay, and reverse transcription-polymerase chain reaction, transcripts encoding functional alpha and regulatory beta subunits of human epithelial sodium channels were expressed both in cultured keratinocytes and in epidermis at levels comparable with the kidney. The mRNA expression of both human epithelial sodium channel-alpha and -beta increased during calcium-induced keratinocyte differentiation. Whereas the beta subunit of human epithelial sodium channel was induced by elevated concentrations of calcium, the alpha subunit increased with duration of culture. The regulatory gamma subunit was less abundant but also expressed in epidermis. Both human epithelial sodium channel-alpha and -beta were localized throughout the nucleated layers of human adult epidermis, but these channels were not detected in early stages of fetal epidermal development. This co-ordinated expression of subunits suggests that epithelial sodium channels may play an important part in both epidermal differentiation and skin development, presumably by modulating ion transport required for epidermal terminal differentiation.

Cyclic nucleotide-gated cation channel expression in embryonic chick brain

Cyclic nucleotide-gated cation channels mediate sensory transduction in vertebrate photoreceptors and olfactory epithelium. These channels are also present in some non-sensory cells, but little is known of their physiological roles outside sensory systems. Using in situ hybridization we found that cyclic nucleotide channel mRNA is expressed specifically in the embryonic chicken forebrain, thalamus, optic tectum, basal midbrain and hindbrain, as well as in the branchial arches, limb buds and skin. Cyclic nucleotide gated channels may thus contribute to development or to cellular differentiation in the brain and in other tissues.

The calcium sensing receptor and its alternatively spliced form in keratinocyte differentiation

We have recently reported the presence of the calcium sensing receptor (CaR) in keratinocytes and suggested that it signaled calcium-induced differentiation of these cells. cDNA clones encoding the full-length CaR were isolated from human keratinocytes. In addition, an alternatively spliced form that lacks exon 5, encoding a portion of the extracellular domain, also was found. The in frame deletion of 231 nucleotides of exon 5 resulted in the loss of function of the CaR as measured by calcium-stimulated production of inositol phosphates when transfected into HEK293 cells or keratinocytes. This variant produced a smaller CaR protein with an altered glycosylation pattern compared with the full-length CaR. Coexpression of the spliced variant with the full-length CaR reduced the function of the full-length CaR. The full-length CaR was expressed in undifferentiated keratinocytes consistent with their greater response to elevated extracellular calcium in terms of increased intracellular free calcium and production of inositol phosphates. The full-length CaR decreased as the keratinocytes differentiated with an increase in the ratio of the spliced variant to the full-length form. The relative proportions of these two forms of CaR may regulate the calcium responsiveness of keratinocytes during their differentiation.

Substituted cGMP analogs can act as selective agonists of the rod photoreceptor cGMP-gated cation channel

Cyclic nucleotide-gated (CNG) channels are expressed in many cell types in both the nervous system and nonexcitable tissues. In order to understand the roles of cGMP-gated channels, and to distinguish actions of cGMP mediated through CNG channels from those through cGMP-dependent protein kinase (G-kinase), several new cGMP analogs were tested for potency as CNG channel agonists. Using Xenopus oocytes expressing the rat rod cGMP-gated ion channel alpha-subunit, we showed that an analog containing a pCPT group at the 8-position, 8-pCPT-cGMP, was 80 times more potent than cGMP and 14 times more potent than 8-Br-cGMP. 8-pCPT-cGMP is the most potent CNG channel agonist so far described and also has the advantages of much better membrane permeability as well as much higher resistance to PDE-hydrolysis, as compared with 8-Br-cGMP. Modification of both 8-Br-cGMP and 8-pCPT-cGMP by introduction of a sulphur atom into the cyclic phosphate group gave smaller changes in agonist efficiency. Both Sp-8-Br-cGMPS and Sp-8-pCPT-cGMPS acted as agonists of CNG channels and are also G-kinase activators. In contrast, Rp-8-Br-cGMPS was a channel agonist, with an EC50 of 173.5 microM, but a G-kinase antagonist with a Ki of 4 microM. Finally, Rp-8-pCPT-cGMPS was a channel agonist and showed additional noncompetitive antagonist activity at higher concentrations. The results suggest that 8-pCPT-cGMPS is a highly potent photoreceptor CNG channel agonist with high membrane permeability and PDE-resistance and furthermore Rp-8-Br-cGMPS can be used to test whether the actions of cGMP are selectively mediated by CNG channels.