Extracellular calcium-dependent regulation of transmembrane calcium fluxes in murine keratinocytes

Connexin hemichannels influence genetically determined inflammatory and hyperproliferative skin diseases

Connexin mutations underlie numerous human genetic diseases. Several connexin genes have been linked to skin diseases, and mechanistic studies have indicated that a gain of abnormal channel function may be responsible for pathology. The topical accessibility of the epidermal connexins, the existence of several mouse models of human skin disease, and the ongoing identification of pharmacological inhibitors targeting connexins provide an opportunity to test new therapeutic approaches.

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.

Active regulation of the epidermal calcium profile

A distinct calcium profile is strongly implicated in regulating the multi-layered structure of the epidermis. However, the mechanisms that govern the regulation of this calcium profile are currently unclear. It clearly depends on the relatively impermeable barrier of the stratum corneum (passive regulation) but may also depend on calcium exchanges between keratinocytes and extracellular fluid (active regulation). Using a mathematical model that treats the viable sublayers of unwounded human and murine epidermis as porous media and assumes that their calcium profiles are passively regulated, we demonstrate that these profiles are also actively regulated. To obtain this result, we found that diffusion governs extracellular calcium motion in the viable epidermis and hence intracellular calcium is the main source of the epidermal calcium profile. Then, by comparison with experimental calcium profiles and combination with a hypothesised cell velocity distribution in the viable epidermis, we found that the net influx of calcium ions into keratinocytes from extracellular fluid may be constant and positive throughout the stratum basale and stratum spinosum, and that there is a net outflux of these ions in the stratum granulosum. Hence, the calcium exchange between keratinocytes and extracellular fluid differs distinctly between the stratum granulosum and the underlying sublayers, and these differences actively regulate the epidermal calcium profile. Our results also indicate that plasma membrane dysfunction may be an early event during keratinocyte disintegration in the stratum granulosum.

The recruitment of phosphatidylinositol 3-kinase to the E-cadherin-catenin complex at the plasma membrane is required for calcium-induced phospholipase C-gamma1 activation and human keratinocyte differentiation

Calcium induces epidermal keratinocyte differentiation, but the mechanism is not completely understood. We have previously demonstrated that calcium-induced human keratinocyte differentiation requires an intracellular calcium rise caused by phosphatidylinositol 3-kinase (PI3K)-dependent activation of phospholipase C-gamma1. In this study we sought to identify the upstream signaling pathway necessary for calcium activation of PI3K and its subsequent activation of phospholipase C-gamma1. We found that calcium induces the recruitment of PI3K to the E-cadherin-catenin complex at the plasma membrane of human keratinocytes. Knocking-down E-cadherin, beta-catenin, or p120-catenin expression blocked calcium activation of PI3K and phospholipase C-gamma1 and calcium-induced keratinocyte differentiation. However, knocking-down gamma-catenin expression had no effect. Calcium-induced PI3K recruitment to E-cadherin stabilized by p120-catenin at the plasma membrane requires beta-catenin but not gamma-catenin. These data indicate that the recruitment of PI3K to the E-cadherin/beta-catenin/p120-catenin complex via beta-catenin at the plasma membrane is required for calcium-induced phospholipase C-gamma1 activation and, ultimately, keratinocyte differentiation.

Characterization of large-conductance Ca2+-dependent and -independent K+ channels in HaCaT keratinocytes

To characterize ion channels expressed in cell membrane of human keratinocytes, patch-clamp recordings were carried out in HaCaT cells. Two types of large-conductance K(+) channels (about 250 pS) were measured. One type was activated by micromolar concentrations of intracellular Ca(2+) ions ([Ca(2+)](i)) and membrane depolarization, the other was [Ca(2+)](i) independent. The channels were neither dependent on intracellular ATP nor Mg(2+) nor on membrane stretch. We conclude that HaCaT keratinocytes express Ca(2+)-dependent maxi K(+) channels and still unknown large Ca(2+)-independent K(+) channels. These K(+) channels may affect the proliferation and differentiation of human keratinocytes by the influence on the resting potential, which may control the Ca(2+) influx across the cell membrane.

Calcium-induced human keratinocyte differentiation requires src- and fyn-mediated phosphatidylinositol 3-kinase-dependent activation of phospholipase C-gamma1

We have previously demonstrated that phospholipase C (PLC)-gamma1 is required for calcium-induced human keratinocyte differentiation. In the present study, we investigated whether the activation of PLC-gamma1 by nonreceptor kinases such as src and fyn plays a role in mediating this process. Our results showed that the combination of dominant negative src and fyn blocked calcium-stimulated PLC-gamma1 activity and human keratinocyte differentiation, whereas each separately has little effect. However, unlike the activation of PLC-gamma1 by epidermal growth factor, calcium-induced activation of PLC-gamma1 was not a result of direct tyrosine phosphorylation. Therefore, we examined an alternative mechanism, in particular phosphatidylinositol 3,4,5-triphosphate (PIP3) formed as a product of phosphatidylinositol 3-kinase (PI3K) activity. PIP3 binds to and activates PLC-gamma1. The combination of dominant negative src and fyn blocked calcium-induced tyrosine phosphorylation of the regulatory subunit of PI3K, p85alpha, and the activity of the catalytic subunit of PI3K. PI3K inhibitors blocked calcium activation of PLC-gamma1 as well as the induction of keratinocyte differentiation markers involucrin and transglutaminase. These data indicate that calcium activates PLC-gamma1 via increased PIP3 formation mediated by c-src- and fyn-activated PI3K. This activation is required for calcium-induced human keratinocyte differentiation.

The role of the calcium-sensing receptor in epidermal differentiation

Calcium regulates the proliferation and differentiation of keratinocytes both in vivo and in vitro. Elevated extracellular Ca2+ concentration ([Ca2+]o) raises the intracellular free calcium ([Ca2+]i) and activates differentiation-related genes. Cells lacking the calcium-sensing receptor (CaR) fail to respond to [Ca2+]o and to differentiate, indicating a role for CaR in keratinocyte differentiation. These concepts derived from in vitro experiments have been tested and confirmed in two mouse models.

ATP priming of macrophage-derived chemokine responses in CHO cells expressing the CCR4 receptor

The mechanism by which ATP primes for subsequent macrophage-derived chemokine (MDC) mediated intracellular calcium (Ca2+(i)) responses at the human CCR4 receptor stably expressed in Chinese hamster ovary (CHO) cells was investigated. MDC alone was unable to elicit a Ca2+(i) response, but pre-stimulation of cells with ATP enabled a subsequent MDC-mediated Ca2+(i) response with a pEC50 of 8.66+/-0.16. The maximal response elicited by MDC was dependent upon the concentration of ATP used to prime, but the pEC50 was stable at all ATP concentrations tested. Pertussis toxin pre-treatment did not effect the ATP response, but abolished that to MDC, demonstrating that priming with ATP did not alter G protein-coupling specificity of the CCR4 receptor. Ionomycin and thapsigargin both increased Ca2+(i) concentrations (pEC50s of 7.59+/-0.57 and 6.81+/-0.31 respectively), but were unable to prime for MDC responses, suggesting the priming mechanism was not dependent upon increases in Ca2+(i) concentrations. Priming of the MDC response was still observed when experiments were performed with low Ca2+(e) (70 microM), indicating that Ca2+ influx was not required for ATP to prime the CCR4 receptor. Neither Ro31-8220 nor wortmannin affected priming, suggesting that protein kinase C and phosphoinositol 3-kinase were not involved. In conclusion, pre-stimulation of endogenous P2Y receptors with ATP facilitates Ca2+ signalling at the recombinant CCR4 receptor in CHO cells, although the mechanism by which this occurs remains to be defined.

Hg2+ and small-sized polyethylene glycols have inverse effects on membrane permeability, while both impair neutrophil cell motility

Toxic effects after exposure to mercury are well documented in human. Little is, however, known about how Hg(2+) affect host defense in general and neutrophil functions in particular. We show here that exposure of human neutrophils to HgCl(2) dose-dependently impairs chemoattractant-stimulated motility. Long-term exposure (5-10 min) to Hg(2+) yields a rapid influx of extracellular Ca(2+) followed by leakage of cytosolic fluorophores, as assessed using fura-2 and ratio imaging microscopy. The inhibition on motility was partly reversible, since pre-treated neutrophils placed in an Hg(2+)-free environment displayed higher migration rates. The Hg(2+)-induced fluxes were prevented by addition of small-sized polyethylene glycols (PEG 200-400), which also dose-dependently inhibited neutrophil transmigration. Localized, minute micropipette additions of Hg(2+) or PEG caused retraction of the leading edge and redirection of cell migration. Since Hg(2+) increases and PEGs decrease membrane permeability in a partially competitive manner, we suggest that the known aquaporin-inhibitor Hg(2+) alters membrane permeability by affecting the bidirectional flux through the leukocyte aquaporin-9 (AQP9) while small-sized PEGs yield decreased membrane permeability by becoming trapped in the promiscuous channel. The local additions of Hg(2+) or PEG probably force other cell regions to take over from those with blocked AQPs. Hence, the cells turn direction of motility away from the micromanipulator needle.

Calcium channel blockers inhibit galvanotaxis in human keratinocytes

Directed migration of keratinocytes is essential for wound healing. The migration of human keratinocytes in vitro is strongly influenced by the presence of a physiological electric field and these cells migrate towards the negative pole of such a field (galvanotaxis). We have previously shown that the depletion of extracellular calcium blocks the directional migration of cultured human keratinocytes in an electric field (Fang et al., 1998; J Invest Dermatol 111:751-756). Here we further investigate the role of calcium influx on the directionality and migration speed of keratinocytes during electric field exposure with the use of Ca(2+) channel blockers. A constant, physiological electric field strength of 100 mV/mm was imposed on the cultured cells for 1 h. To determine the role of calcium influx during galvanotaxis we tested the effects of the voltage-dependent cation channel blockers, verapamil and amiloride, as well as the inorganic Ca(2+) channel blockers, Ni(2+) and Gd(3+) and the Ca(2+) substitute, Sr(2+), on the speed and directionality of keratinocyte migration during galvanotaxis. Neither amiloride (10 microM) nor verapamil (10 microM) had any effect on the galvanotaxis response. Therefore, calcium influx through amiloride-sensitive channels is not required for galvanotaxis, and membrane depolarization via K(+) channel activity is also not required. In contrast, Sr(2+) (5 mM), Ni(2+) (1-5 mM), and Gd(3+) (100 microM) all significantly inhibit the directional migratory response to some degree. While Sr(2+) strongly inhibits directed migration, the cells exhibit nearly normal migration speeds. These findings suggest that calcium influx through Ca(2+) channels is required for directed migration of keratinocytes during galvanotaxis and that directional migration and migration speed are probably controlled by separate mechanisms.

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.

Cross talk among calcineurin, Sp1/Sp3, and NFAT in control of p21(WAF1/CIP1) expression in keratinocyte differentiation

Calcium functions as a trigger for the switch between epithelial cell growth and differentiation. We report here that the calcium/calmodulin-dependent phosphatase calcineurin is involved in this process. Treatment of primary mouse keratinocytes with cyclosporin A, an inhibitor of calcineurin activity, suppresses the expression of terminal differentiation markers and of p21(WAF1/Cip1) and p27(KIP1), two cyclin-dependent kinase inhibitors that are usually induced with differentiation. In parallel with down-modulation of the endogenous genes, suppression of calcineurin function blocks induction of the promoters for the p21(WAF1/Cip1) and loricrin differentiation marker genes, whereas activity of these promoters is enhanced by calcineurin overexpression. The calcineurin- responsive region of the p21 promoter maps to a 78-bp Sp1/Sp3-binding sequence next to the TATA box, and calcineurin induces activity of the p21 promoter through Sp1/Sp3-dependent transcription. We find that the endogenous NFAT-1 and -2 transcription factors, major downstream targets of calcineurin, associate with Sp1 in keratinocytes in a calcineurin-dependent manner, and calcineurin up-regulates Sp1/Sp3-dependent transcription and p21 promoter activity in synergism with NFAT1/2. Thus, our study reveals an important role for calcineurin in control of keratinocyte differentiation and p21 expression, and points to a so-far-unsuspected interconnection among this phosphatase, NFATs, and Sp1/Sp3-dependent transcription.

Calcium- and vitamin D-regulated keratinocyte differentiation

Calcium and 1,25 dihydroxyvitamin D (1,25(OH)(2)D) regulate the differentiation of keratinocytes. We have examined the mechanisms by which such regulation takes place, focusing primarily on the events leading to cornified envelope (CE) formation, in particular the mechanisms by which calcium and 1,25(OH)(2)D regulate the induction of involucrin, a component of the CE, and transglutaminase, the enzyme cross-linking involucrin and other substrates to form the CE. Both extracellular calcium (Ca(o)) and 1,25(OH)(2)D raise intracellular free calcium (Ca(i)) as a necessary step toward stimulating differentiation. Cells lacking the calcium sensing receptor (CaR) or phospholipase C-gamma 1 (PLC-gamma 1) fail to respond to Ca(o) or 1,25(OH)(2)D with respect to differentiation. Residing in the promoter of involucrin is a region responsive to calcium and 1,25(OH)(2)D, the calcium response element (CaRE). The CaRE contains an AP-1 site, mutations of which result in loss of responsiveness to Ca(o) and 1,25(OH)(2)D, indicating a role for protein kinases C (PKC). PKC alpha is the major PKC isozyme involved at least for calcium-induced differentiation. Thus, the regulation of keratinocyte differentiation by calcium and 1,25(OH)(2)D involves a number of signaling pathways including PLC and PKC activation, leading to the induction of proteins required for the differentiation process.

Ras-independent activation of the Raf/MEK/ERK pathway upon calcium-induced differentiation of keratinocytes

MAPKs are crucially involved in the regulation of growth and differentiation of a variety of cells. To elucidate the role of MAPKs in keratinocyte differentiation, activation of ERK, JNK, and p38 in response to stimulation with extracellular calcium was analyzed. We provide evidence that calcium-induced differentiation of keratinocytes is associated with rapid and transient activation of the Raf/MEK/ERK pathway. Stimulation of keratinocytes with extracellular calcium resulted in activation of Raf isozymes and their downstream effector ERK within 10-15 min, but did not increase JNK or p38 activity. Calcium-induced ERK activation differed in kinetics from mitogenic ERK activation by epidermal growth factor and could be modulated by alterations of intracellular calcium levels. Interestingly, calcium stimulation led to down-regulation of Ras activity at the same time that ERK activation was initiated. Expression of a dominant-negative mutant of Ras also did not significantly impair calcium-induced ERK activation, indicating that calcium-mediated ERK activation does not require active Ras. Despite the transient nature of ERK activation, calcium-induced expression of the cyclin-dependent kinase inhibitor p21/Cip1 and the differentiation marker involucrin was sensitive to MEK inhibition, which suggests a role for the Raf/MEK/ERK pathway in early stages of keratinocyte differentiation.

Signal transduction pathways controlling the switch between keratinocyte growth and differentiation

Self-renewing epithelia are characterized by a high turnover rate and a fine balance between growth and differentiation. Such a balance is influenced by many exogenous factors, including gradients of diffusible molecules, cell/substrate adhesion contacts, and direct cell-cell communication. The inter-connection between these various extracellular signals and underlying intracellular pathways is clearly of great interest. Primary keratinocytes of either human or murine origin provide an ideal experimental system to elucidate early signaling events involved in the control of epithelial differentiation. Relative to established cell lines, use of a primary system eliminates the possibility of alterations in critical regulatory events which may occur during prolonged propagation in culture. Primary keratinocytes are easily grown in large numbers, and their differentiation can be induced under well-defined culture conditions. The ensuing rapid and homogeneous response is amenable to careful biochemical analysis. Gene transfer technology (transient transfections, adenoviral and retroviral vectors), together with the use of keratinocytes derived from gene knockout and transgenic mice, makes it possible to assess the specific contribution of individual genes to the control of the differentiation process. This review focuses on the significant progress that has been made over the last few years in our understanding of the specific signals that trigger keratinocyte differentiation, the underlying signaling pathways, and how they impinge on specific transcription and cell-cycle control mechanisms associated with the onset of keratinocyte differentiation. Recent developments and future directions in this important area of research will be highlighted.

Activation of keratinocyte nicotinic cholinergic receptors stimulates calcium influx and enhances cell differentiation

Human epidermal keratinocytes synthesize, secrete, and degrade acetylcholine and use their cell-surface nicotinic and muscarinic cholinergic receptors to mediate the autocrine and paracrine effects of acetyl-choline. Because acetylcholine modulates transmembrane Ca2+ transport and intracellular metabolism in several types of cells, we hypothesized that cholinergic agents might have similar effects on keratinocytes. Nicotine increased in a concentration-dependent manner the amount of 45Ca2+ taken up by keratinocytes isolated from human neonatal fore-skins. This effect was abolished in the presence of the specific nicotinic antagonist mecamylamine, indicating that it was mediated by keratinocyte nicotinic acetylcholine receptor(s). The sequences encoding the alpha 5 and alpha 7 nicotinic receptor subunits were amplified from cDNA isolated from cultured keratinocytes. These subunits, as well as the alpha 3, beta 2, and beta 4 subunits previously found in keratinocytes, can be components of Ca(2+)-permeable nicotinic receptor channels. To learn how activation of keratinocyte nicotinic receptors affected the rate of cell differentiation, we measured the nicotinic cholinergic effects on the expression of differentiation markers by cultured keratinocytes. Long-term incubations with micromolar concentrations of nicotine markedly increased the number of cells forming cornified envelopes and the number of cells staining with antibodies to suprabasal keratin 10, transglutaminase type I, involucrin, and filaggrin. The increased production of these differentiation-associated proteins was verified by Western blotting. Because nicotinic cholinergic stimulation causes transmembrane Ca2+ transport into keratinocytes, and because changes in concentrations of intracellular Ca2+ are known to alter various keratinocyte functions, including differentiation, the subcellular mechanisms mediating the autocrine and paracrine actions of epidermal acetylcholine on keratinocytes may involve Ca2+ as a second messenger.

Changes in calcium responsiveness and handling during keratinocyte differentiation. Potential role of the calcium receptor

Extracellular calcium concentrations (Cao) > 0.1 mM are required for the differentiation of normal human keratinocytes in culture. Increments in Cao result in acute and sustained increases in the intracellular calcium level (Cai), postulated to involve both a release of calcium from intracellular stores and a subsequent increase in calcium influx through nonspecific cation channels. The sustained rise in Cai appears to be necessary for keratinocyte differentiation. To understand the mechanism by which keratinocytes respond to Cao, we measured the acute effects of Cao on Cai and calcium influx in keratinocytes at various stages of differentiation. We then demonstrated the existence of the calcium receptor (CaR) in keratinocytes and determined the effect of calcium-induced differentiation on its mRNA levels. Finally, we examined the role of Cai in regulating both the initial rise in Cai after the switch to higher Cao and the activity of the nonspecific cation channel through which calcium influx occurs. Our data indicate that the acute Cai response to Cao is lost as the cells differentiate and increase their basal Cai. These data correlated with the decrease in CaR mRNA levels in cells grown in low calcium. However, calcium influx as measured by 45Ca uptake increased with differentiation in 1.2mM calcium, consistent with the increase in CaR mRNA in these cells as well as the calcium-induced opening of the nonspecific cation channels. We conclude that the keratinocyte contains a CaR that regulates both the initial release of Cai from intracellular stores and the subsequent increase in calcium flux through nonspecific calcium channels. A rising level of Cai may turn off the release of calcium from intracellular stores while potentiating the influx through the nonspecific cation channels. Differentiation of keratinocytes appears to increase the CaR, which may facilitate the maintenance of the high Cai required for differentiation.

Intracellular ionic changes induced by bullous pemphigoid IgG subclasses

To ascertain whether membrane signal transduction is induced by bullous pemphigoid (BP) antibody and whether cell lysis is induced by its complement activation, we assessed the intracellular Ca2+ concentration ([Ca2+]i), intracellular pH, membrane potential and morphology of living cells by following the time course of fluorescence intensity of Fluo-3/AM, Snaff-1/AM, Dioc-5 and Luciffer yellow, respectively. A transient increase of Fluo-3 fluorescence intensity in DJM-1 cells (a squamous cell carcinoma line) was revealed when the cells were incubated with 2 of five IgG1 BP antibodies. However, no transient increase of Fluo-3 fluorescence intensity was revealed when the cells were incubated with IgG2 and IgG4 BP antibodies. A transient increase of Fluo-3 fluorescence intensity was revealed in DJM-1 cells incubated with 3 of seven IgG1 and 1 of four IgG2 BP antibodies in an EGTA-containing low-Ca2+ medium. On the other hand, the Dioc-5 fluorescence intensity did not change significantly, though the increase of Fluo-3 fluorescence intensity was observed. The increase of Snarf-1 fluorescence intensity was revealed in DJM-1 cells incubated with 2 of five IgG1 BP antibodies, but was not revealed in the cells incubated with IgG2 or IgG4 of BP antibodies. Study of complement activation by BP IgG1 showed a transient increase of Fluo-3 fluorescence intensity of with 3 of five IgG1 BP antibodies when DJM-1 cells were incubated with complement-supplemented normal-Ca2+ medium. At the same time, however, endocytosis and cell lysis were not observed with 2 IgG1 BP antibodies which did induce an increase of Fluo-3 fluorescence intensity when Lucifer-yellow-loaded DJM-1 cells were incubated with complement-supplemented normal-Ca2+ medium. We examined next whether anti-180 kD BP antigen monoclonal antibodies (mAbs R-223 and 233) induce an increase of Fluo-3 fluorescence intensity. MAb R-223 did not induce any increase of Fluo-3 fluorescence intensity in DJM-1 cells, when incubated with normal- and low-Ca2+ media However, mAb R-223 induced a transient increase of Fluo-3 fluorescence intensity in DJM-1 cells when incubated with complement-supplemented normal-Ca2+ medium. MAb 233 did not induced an increase of Fluo-3 fluorescence intensity in DJM-1 cells when incubated with normal- and low-Ca2+ media. These results suggest that the BP IgG1 induces Ca2+ release from intracellular storage sites, however, the complement activated by BP IgG1 does not induce cell lysis. It could not be confirmed that anti-180 kD BP antigen antibody induced Ca2+ release from intracellular storage sites.

fyn tyrosine kinase is involved in keratinocyte differentiation control

Induction of tyrosine phosphorylation is an early and specific event which is required for mouse keratinocyte differentiation to occur, in response to both calcium and TPA (12-0-tetradecanoylphorbol-13-acetate). We report here that there is an increase of tyrosine kinase activity immunoprecipitable with anti-phosphotyrosine antibodies specifically in response to calcium--and a number of other divalent cations--within 2 min of exposure. Such an activity does not correspond to any of the known tyrosine kinases that were tested. A second tyrosine kinase activity is induced in response to both calcium and TPA, and has been identified as fyn, a nonreceptor tyrosine kinase of the src family. fyn activation is induced in keratinocytes within 6 hr of calcium exposure, but already within 2 min of TPA treatment. Cortactin, a p80-85 substrate of src- and fyn-related kinases that localizes with actin at cell adhesion sites, is increasingly tyrosine phosphorylated in calcium- and TPA-induced differentiation, with a time course which parallels that of fyn activation. Keratinocytes with a specific disruption of the fyn, but not yes kinase gene show no induction of phosphorylation of p80-85 proteins, and are significantly altered in their differentiation response both in vitro and in vivo. Thus, at least two tyrosine kinase activities are induced in keratinocyte differentiation, one of which has been identified as fyn and shown to be specifically involved in this process.

Amiloride blocks a keratinocyte nonspecific cation channel and inhibits Ca(++)-induced keratinocyte differentiation

Proliferation and differentiation in many cells are linked to specific changes in transmembrane ion fluxes. Previously, we have identified a nonspecific cation channel in keratinocytes, which is permeable to and activated by Ca++. To test whether this cation channel might serve as a pathway for Ca++ entry, we examined the effect of blocking this channel on membrane currents, markers of differentiation, and intracellular Ca++. In patch clamp studies, 10(-8) to 10(-6) M amiloride decreased the single-channel open probability. The same concentrations of amiloride inhibited the calcium-induced formation of cornified envelopes and activity of transglutaminase in a dose-dependent fashion. Amiloride inhibited the long-term rise of intracellular Ca++ induced by raised extracellular Ca++, without blocking the initial increase of intracellular Ca++. Amiloride at concentrations of 10(-7) to 10(-3) M did not change the resting intracellular pH of keratinocytes, although concentrations of 10(-6) M or greater inhibited the recovery from NH4(+)-induced acidification. To test whether the effect of amiloride was toxic, we measured DNA synthesis in the presence or absence of amiloride. DNA synthesis was unchanged, suggesting that amiloride's actions were not due to toxic effects. Although the exact mechanisms of amiloride's action remains to be determined, these experiments suggest that this compound may inhibit keratinocyte differentiation by blocking the nonspecific cation channel.

Intracellular calcium modulates the responses of human melanocytes to melanogenic stimuli

Ultraviolet radiation (UVR), the synthetic diacyglycerol (DAG), 1-oleoyl-2-acetylglycerol (OAG), and cyclic AMP (cAMP) stimulants, including cholera toxin (CT) have all been shown to increase melanogenesis in cultured human melanocytes. Indirect evidence suggests that an increase in intracellular free Ca2+ ([Ca2+]i) may be important in stimulated melanogenesis. Therefore, to determine whether melanogenic responses are modulated by [Ca2+]i, the Ca2+ in the culture medium of melanocytes ([Ca2+]o) was raised from 70 microM to 1 mM. This switch in [Ca2+]o was associated with a biphasic increase in [Ca2+]i, with an early transient rise, over minutes, and a delayed sustained rise in [Ca2+]i, over hours. The early increase was blocked by nickel chloride (NiCl2), but not affected by depletion of [Ca2+]i stores by thapsigargin, suggesting that this [Ca2+]i rise was due to Ca2+ entry across the plasma membrane. Melanocytes cultured in the absence of CT had a reduced basal melanin content following the switch to 1 mM [Ca2+]o, but in the presence of CT, which acts by stimulating cAMP synthesis, the basal level was increased. Raising [Ca2+]o resulted in enhanced melanogenic responses to UVR and OAG, in the presence or absence of CT, suggesting that Ca(2+)-dependent mechanisms are important. UVR also stimulated a delayed rise in [Ca2+]i, over 24 h, but OAG did not. These results indicate that while [Ca2+]i is not essential for melanogenesis, it plays an important role in modulating the responses of melanocytes to melanogenic stimuli.

Calcium-inducible transmodulation of receptor tyrosine kinase activity

Calcium is a potent mitogen and transmodulator of growth factor receptor activity, but does not activate tyrosine kinases in ligand-deprived cells (Epstein et al. 1992) Cell Growth Different. 3, 157-164). In this study the mitogenic and transcriptional effects of increased extracellular calcium and ionophore are shown to be identical in 3T3 cells, consistent with mediation of these effects via increased intracellular calcium availability. Near-maximal mitogenic and transcriptional effects are seen after brief exposure to increased extracellular calcium or ionophore, while additive effects occur with co-administration of calcium and platelet-derived growth factor (PDGF). Exposure of PDGF-primed cells to calcium or ionophore is associated with a substantial enhancement of receptor tyrosine autophosphorylation which is abrogated by calcium channel blockade or intracellular calcium chelation. In contrast, pretreatment of quiescent cells with calcium or ionophore significantly diminishes subsequent PDGF-inducible receptor autophosphorylation. Intracellular calcium thus appears to potentiate the kinase activity of ligand-stimulated PDGF receptors while inhibiting ligand-inducible activation of unstimulated receptors. These findings suggest a model of receptor tyrosine kinase regulation involving calcium-dependent positive and negative feedback loops which vary with the activation state of the receptor.

Extracellular Ca2+ sensing by the osteoblast-like cell line, MC3T3-E1

The present study was undertaken to clarify the role of extracellular calcium on osteoblast activation. It was found that bradykinin and thrombin induced synthesis of prostaglandin E2 was strongly dependent on the concentration of extracellular calcium in the osteoblast-like cell line, MC3T3-E1. Moreover, this effect was not related to Ca2+ influx, since it was even potentiated by Ni2+ and Co2+, which was not due to intracellular activity of Ni2+, as judged by studies with 63Ni2+. Ba2+, Mg2+ and Sr2+ had no effect. Cd2+ caused dose-dependent synthesis of prostaglandin E2, which was shown to correlate with its cytotoxic properties. The results thus strongly suggest the presence of a divalent cation sensor in osteoblast-like MC3T3-E1 cells.

Role of Ca2+ in stimulation of DNA synthesis by epidermal growth factor and tumor promoters in cultured rat hepatocytes

This study examines the effects of extracellular Ca2+ concentrations, [Ca2+]o, and of treatments known to modulate intracellular Ca2+ levels on the extent and timing of DNA synthesis in primary cultures of adult rat hepatocytes. In cultures exposed to insulin and EGF, the extent of DNA synthesis between 40 h and 70 h in culture was independent of [Ca2+]o in the range 25-1,800 microM, although the peak of DNA synthesis occurred 5-10 h earlier with 1.2 mM Ca2+ than with 25 microM Ca2+. Complete removal of extracellular Ca2+ using EGTA blocked DNA synthesis if Ca2+ was removed on the second day after EGF addition but not if Ca2+ was absent only on day 1. Treatment of cultures in 1.2 mM Ca(2+)-containing media with Ca(2+)-ionophore A23187 or with thapsigargin, agents expected to raise cytosolic [Ca2+], failed to augment the stimulation of DNA synthesis by EGF. These observations suggest that hepatocytes may have a permissive requirement for [Ca2+]o > 0 at least late in the sequence of events leading from growth factor stimulation to DNA synthesis. However, sustained elevation of cytosolic [Ca2+] does not appear to be important as an early signalling event either in mediating or augmenting EGF action in hepatocytes. The ability of liver tumor promoters alpha-hexachlorocyclohexane or DDT to stimulate DNA synthesis in combination with EGF was independent of [Ca2+]o. By contrast, the skin tumor-promoting phorbol ester, TPA, or liver tumor promoter, phenobarbital, were without effect or inhibitory at low [Ca2+]o but in combination with EGF, stimulated DNA synthesis at [Ca2+]o > 0.4 mM, suggesting that Ca2+ may have some role in mediating or modulating the stimulatory effects of these agents.