Regulation of intracellular free calcium in normal murine keratinocytes

Decreased Calcium-Sensing Receptor Expression Controls Calcium Signaling and Cell-To-Cell Adhesion Defects in Aged Skin

The calcium-sensing receptor (CaSR) drives essential calcium ion (Ca2+) and E-cadherin‒mediated processes in the epidermis, including differentiation, cell-to-cell adhesion, and epidermal barrier homeostasis in cells and in young adult mice. We now report that decreased CaSR expression leads to impaired Ca2+ signal propagation in aged mouse (aged >22 months) epidermis and human (aged >79 years, donor age) keratinocytes. Baseline cytosolic Ca2+ concentrations were higher, and capacitive Ca2+ entry was lower in aged than in young keratinocytes. As in Casr-knockout mice (EpidCaSR-/-), decreased CaSR expression led to decreased E-cadherin and phospholipase C-γ expression and to a compensatory upregulation of STIM1. Pretreatment with the CaSR agonist N-(3-[2-chlorophenyl]propyl)-(R)-alpha-methyl-3-methoxybenzylamine normalized Ca2+ propagation and E-cadherin organization after experimental wounding. These results suggest that age-related defects in CaSR expression dysregulate normal keratinocyte and epidermal Ca2+ signaling, leading to impaired E-cadherin expression, organization, and function. These findings show an innovative mechanism whereby Ca2+- and E-cadherin‒dependent functions are impaired in aging epidermis and suggest a new therapeutic approach by restoring CaSR function.

Phosphoprotein Phosphatase 1 Is Required for Extracellular Calcium-Induced Keratinocyte Differentiation

Extracellular calcium is a major regulator of keratinocyte differentiation in vitro and appears to play that role in vivo, but the mechanism is unclear. We have previously demonstrated that, following calcium stimulation, PIP5K1α is recruited by the E-cadherin-β-catenin complex to the plasma membrane where it provides the substrate PIP2 for both PI3K and PLC-γ1. This signaling pathway is critical for calcium-induced generation of second messengers including IP3 and intracellular calcium and keratinocyte differentiation. In this study, we explored the upstream regulatory mechanism by which calcium activates PIP5K1α and the role of this activation in calcium-induced keratinocyte differentiation. We found that treatment of human keratinocytes in culture with calcium resulted in an increase in serine dephosphorylation and PIP5K1α activation. PP1 knockdown blocked extracellular calcium-induced increase in serine dephosphorylation and activity of PIP5K1α and induction of keratinocyte differentiation markers. Knockdown of PLC-γ1, the downstream effector of PIP5K1α, blocked upstream dephosphorylation and PIP5K1α activation induced by calcium. Coimmunoprecipitation revealed calcium induced recruitment of PP1 to the E-cadherin-catenin-PIP5K1α complex in the plasma membrane. These results indicate that PP1 is recruited to the extracellular calcium-dependent E-cadherin-catenin-PIP5K1α complex in the plasma membrane to activate PIP5K1α, which is required for PLC-γ1 activation leading to keratinocyte differentiation.

Epigallocatechin-3-O-(3-O-methyl)-gallate-induced differentiation of human keratinocytes involves klotho-mediated regulation of protein kinase-cAMP responsive element-binding protein signaling

(−)-Epigallocatechin-3-O-gallate (EGCG) has long been known as a potent inducer of keratinocyte differentiation. Although its molecular mechanisms have been extensively studied, its actions on human skin remain to be elucidated. In this study, we demonstrated that methylated EGCG and EGCG increase the expression of klotho, and that klotho functions as a downstream target of EGCG and methylated EGCG in keratinocyte differentiation. We demonstrated that methylated EGCG3 and EGCG induce morphological changes in normal human epidermal keratinocytes (NHEKs) that are related to up-regulation of klotho expression. We also demonstrated that a klotho-induced keratinocyte differentiation marker in NHEKs is inhibited by H-89, a protein kinase (PKA) inhibitor. These results suggest that methylated EGCG and EGCG may function as inducers of keratinocyte differentiation via transcriptional regulation of the klotho protein.

Sodium-sulfate/carboxylate cotransporters (SLC13)

The SLC13 gene family is comprised of five sequence related proteins that are found in animals, plants, yeast and bacteria. Proteins encoded by the SLC13 genes are divided into the following two groups of transporters with distinct anion specificities: the Na(+)-sulfate (NaS) cotransporters and the Na(+)-carboxylate (NaC) cotransporters. Members of this gene family (in ascending order) are: SLC13A1 (NaS1), SLC13A2 (NaC1), SLC13A3 (NaC3), SLC13A4 (NaS2) and SLC13A5 (NaC2). SLC13 proteins encode plasma membrane polypeptides with 8-13 putative transmembrane domains, and are expressed in a variety of tissues. They are all Na(+)-coupled symporters with strong cation preference for Na(+), and insensitive to the stilbene 4, 4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS). Their Na(+):anion coupling ratio is 3:1, indicative of electrogenic properties. They have a substrate preference for divalent anions, which include tetra-oxyanions for the NaS cotransporters or Krebs cycle intermediates (including mono-, di- and tricarboxylates) for the NaC cotransporters. This review will describe the molecular and cellular mechanisms underlying the biochemical, physiological and structural properties of the SLC13 gene family.

TIG3: a regulator of type I transglutaminase activity in epidermis

Keratinocytes undergo a process of terminal cell differentiation that results in the construction of a multilayered epithelium designed to produce a structure that functions to protect the body from dehydration, abrasion and infection. These protective properties are due to the production of a crosslinked layer of protein called the cornified envelope. Type I transglutaminase (TG1), an enzyme that catalyzes the formation of epsilon-(gamma-glutamyl)lysine bonds, is the key protein responsible for generation of the crosslinks. The mechanisms that lead to activation of transglutaminase during terminal differentiation are not well understood. We have identified a protein that interacts with TG1 and regulates its activity. This protein, tazarotene-induced gene 3 (TIG3), is expressed in the differentiated layers of the epidermis and its expression is associated with transglutaminase activation and cornified envelope formation. We describe a novel mechanism whereby TIG3 regulates TG1 activity.

Phosphatidylinositol-4-phosphate 5-kinase 1alpha mediates extracellular calcium-induced keratinocyte differentiation

Extracellular calcium (Cao) is a major regulator of keratinocyte differentiation, but the mechanism is unclear. Phosphatidylinositol-4-phosphate 5-kinase 1alpha (PIP5K1alpha) is critical in synthesizing phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. In this study, we sought to determine whether PIP5K1alpha plays a role in mediating the ability of Cao to induce keratinocyte differentiation. We found that treatment of human keratinocytes in culture with Cao resulted in increased PIP5K1alpha level and activity, as well as PI(4,5)P2 level, binding of phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P3] to and activation of phospholipase C-gamma1 (PLC-gamma1), with the resultant increase in inositol 1,4,5-trisphosphate (IP3) and intracellular calcium (Cai). Knockdown of PIP5K1alpha in human keratinocytes blocked Cao-induced increases in the binding of PI(3,4,5)P3 to PLC-gamma1; PLC-gamma1 activity; levels of PI(4,5)P2, IP3, and Cai; and induction of keratinocyte differentiation markers. Coimmunoprecipitation and confocal studies revealed that Cao stimulated PIP5K1alpha recruitment to the E-cadherin-catenin complex in the plasma membrane. Knockdown of E-cadherin or beta-catenin blocked Cao-induced activation of PIP5K1alpha. These results indicate that after Cao stimulation PIP5K1alpha is recruited by the E-cadherin-catenin complex to the plasma membrane where it provides the substrate PI(4,5)P2 for both PI3K and PLC-gamma1. This signaling pathway is critical for Cao-induced generation of the second messengers IP3 and Cai and keratinocyte differentiation.

Oat5 and NaDC1 protein abundance in kidney and urine after renal ischemic reperfusion injury

The aim of this study was to evaluate the abundance of the organic anion transporter 5 (Oat5) and the sodium-dicarboxylate cotransporter 1 (NaDC1) in kidney and urine after renal ischemic reperfusion injury. Renal injury was induced in male Wistar rats by occlusion of both renal pedicles for 0 (Group Sham), 5 (Group I5R60), or 60 (Group I60R60) min. The studies were performed after 60 min of reperfusion. The expression of Oat5 and NaDC1 was evaluated by IHC and Western blotting. Oat5 and NaDC1 abundance and alkaline phosphatase activity (AP) were assayed in urine. A decreased expression in renal homogenates and apical membranes and an increase in urinary excretion of Oat5 and NaDC1 were observed in I60R60 rats, as well as alterations of other widely used parameters for renal dysfunction and injury (plasma creatinine, urinary AP activity, kidney weight, histological lesions). In contrast, in the I5R60 group, only an increase in urinary excretion of Oat5 and mild histopathological damage was detected. This is the first study on Oat5 and NaDC1 detection in urine. These results suggest that urinary excretion of Oat5 might be an early indicator of renal dysfunction, which is useful for detection of even minor alterations in renal structural and functional integrity.

Localization of the TIG3 transglutaminase interaction domain and demonstration that the amino-terminal region is required for TIG3 function as a keratinocyte differentiation regulator

Tazarotene-induced gene 3 (TIG3) regulates keratinocyte terminal differentiation by activating type I transglutaminase (TG1). TIG3 consists of an amino-terminal (N-terminal) segment, that encodes several conserved motifs, and a carboxy-terminal (C-terminal) membrane-anchoring domain. By producing a series of truncation mutants that remove segments of the N-terminal region, and monitoring the ability of each mutant to co-precipitate TG1, function as a TG1 substrate, or functionally localize with TG1 in cells, we show that the TIG3 domain that interacts with TG1 is located within a TIG3 segment spanning amino acids 112-164. Although they bind TG1, TIG3 mutants lacking the conserved N-terminal region drive apoptosis-like cell death characterized by cell rounding, membrane blebbing, cytochrome c release, procaspase-3 and poly(ADP-ribose)polymerase (PARP) cleavage, and reduced p53 and p21 levels. Compared with TIG3, these truncated mutants have an increased tendency to associate with membranes. A mutant lacking the C-terminal membrane-anchoring domain is inactive. These findings suggest that TIG3 interaction with TG1 does not require the N-terminal conserved domains, that the TIG3 N-terminal region is required for TIG3-dependent keratinocyte differentiation, that its removal converts TIG3 into a proapoptotic protein, and that this change in action of TIG3 is associated with an intracellular redistribution.

Altered calcium handling following the recombinant overexpression of protein kinase C isoforms in HaCaT cells

Both changes in intracellular calcium concentration ([Ca(2+)](i)) and activation of certain protein kinase C (PKC) isoforms play a crucial role in keratinocyte functions. To better understand the interaction between these two signalling pathways we investigated the resting [Ca(2+)](i) and the extracellular ATP-induced changes in [Ca(2+)](i) on HaCaT cell clones overexpressing either the classical alpha or the beta PKC isoform. These PKC isoenzymes were previously shown to decrease (alpha) or increase (beta) cell proliferation and augment (alpha) or suppress (beta) cell differentiation. Keratinocyte clones with decreased proliferation rate were found to have unaltered resting [Ca(2+)](i), but responded with greater calcium transients to the application of 180 mum of ATP. In contrast, clones with increased proliferation rate had elevated resting [Ca(2+)](i) and suppressed calcium responses to ATP. Calcium transients on PKCbeta clones displayed a faster falling phase. Each clone had a distinct purinergic receptor expression pattern, some of which paralleled the altered proliferation rate and calcium handling. Keratinocytes overexpressing PKCbeta revealed decreased P2X1 and increased P2Y1 receptor expression as compared with the control or PKCalpha clones. The expression level of P2X7 was significantly increased in keratinocytes overexpressing PKCalpha. On the other hand neither the P2X2 nor the P2Y2 expression was altered significantly in the cell types investigated. These data indicate that a modified proliferation and differentiation pattern is associated with altered calcium handling in keratinocytes. The observations also suggest that different PKC isoenzymes have different effects on the phosphatidyl-inositol signalling pathway.

Acute and Chronic Regulation of the Renal Na+/H+ Exchanger NHE3 in Rats with STZ-Induced Diabetes mellitus

BACKGROUND

Early stages of diabetic nephropathy are characterized by alterations of glomerular filtration, increased tubular sodium and water reabsorption, and systemic volume expansion, which may be a major cause for the development of hypertension. As a significant fraction of renal salt and water transport is mediated by the proximal tubular Na+/H+ exchanger NHE3, we investigated its regulation in rats with STZ-induced diabetes mellitus.

METHODS

Male Sprague-Dawley rats were injected +/- streptozotocin (STZ, 60 mg/kg), and sacrificed after 2, 7 or 14 days. Renal cortical BBM vesicles were prepared to measure Na+/H+ exchange (NHE) activity and NHE3 protein abundance. Cortical NHE3 mRNA was extracted to perform Northern blot analysis. Pharmacological inhibitors were used in vivo and in vitro in order to identify isoform specificity conferring changes in NHE activity mediated by the diabetic milieu.

RESULTS

Compared to control rats, STZ rats were clearly hyperglycemic at all time points studied. NHE activity was significantly increased by 40 and 37% in diabetic rats after 7 and 14 days, respectively, but not after 2 days. The increase in Na+/H+ exchange activity was not inhibited by HOE-642 (3 microM). Administration of exogenous insulin to diabetic rats resulted in lower blood sugars, but not NHE activity. Moreover, serum glucose concentration did not correlate with NHE activity in any subgroup nor in all animals analyzed together. However, in STZ rats supplemented with exogenous insulin NHE activity was positively correlated with serum insulin concentrations (r = 0.86, p < 0.01). In vivo, the increase in NHE activity induced by STZ could be completely inhibited when rats were fed 6 ppm of HOE-642 with the diet over 14 days. The changes in Na+/H+ exchange activity were not paralleled by changes in NHE3 protein or mRNA abundance in diabetic rats at any of the time points investigated.

CONCLUSIONS

These results suggest that proximal tubular Na/H exchange activity is modified in the early stage of diabetes mellitus.

Direct effect of CoC12 and NiCl2 on citrate uptake by the rat renal brush border membrane

Co and Ni are essential but relatively rare elements as to organisms. In the mammalian membrane, these metals are transported by the same carrier proteins. The aim of this study was to investigate the direct effects of CoCl2 and NiCl2 on citrate uptake by rat renal brush border membrane vesicles (BBMV). BBMV were prepared by the divalent cation precipitation methods, and citrate uptake was measured by the Millipore rapid membrane filtration technique. The time course of citrate uptake during 120-min of incubation with 1 mM CoCl2 and NiCl2 showed a rapid significant inhibition at the early phase and a slight recover at the late phase. Incubation for 1 min of BBMV with 1, 5 and 25 mM CoCl2 and NiCl2, respectively, significantly inhibited citrate uptake in a concentration-dependent manner compared with that of 0 mM. We discuss these findings from the point of view that Co and Ni are located in Group VIII of the periodic table.

Human epidermal keratinocytes undergo (−)-epigallocatechin-3-gallate-dependent differentiation but not apoptosis

Epigallocatechin-3-gallate (EGCG) is an important chemopreventive agent derived from green tea. We recently reported that EGCG treatment enhances keratinocyte differentiation as evidenced by increased human involucrin promoter activity [Balasubramanian,S., Efimova,T. and Eckert,R.L. (2002) J. Biol. Chem., 277, 1828-1836]. In the present paper, we extend these findings and show that EGCG also increases the expression of other differentiation markers-procaspase 14 and type I transglutaminase (TG1). Both TG1 mRNA and protein level, and activity are increased by treatment with EGCG. Increased TG1 activity is evidenced by a direct transglutaminase assay, and by the ability of EGCG to stimulate the covalent incorporation of fluorescein cadaverine substrate into crosslinked intracellular structures. In contrast, type II transglutaminase levels are not altered by EGCG treatment. We also assessed whether EGCG promotes keratinocyte apoptosis. We show that EGCG treatment does not promote the cleavage of procaspase-3, -8, -9 or poly(ADP-ribose) polymerase. Moreover, treatment with the pan-caspase inhibitor, Z-VAD-FMK, does not reverse the EGCG-associated reduction in cell viability. In addition, there is no increase in cells having sub-G(1)/S DNA content, and no evidence for the release of cytochrome c from the mitochondria. These findings confirm, using several endpoints, that EGCG treatment enhances normal keratinocyte differentiation but does not promote apoptosis.

Phenotypes of SERCA and PMCA knockout mice

P-type Ca2+-ATPases of the sarco(endo)plasmic reticulum (SERCAs) and plasma membrane (PMCAs) are responsible for maintaining the Ca2+ gradients across cellular membranes that are required for regulation of Ca2+-mediated signaling and other biological processes. Gene-targeting studies of SERCA isoforms 1, 2, and 3 and PMCA isoforms 1, 2, and 4 have confirmed some of the general functions proposed for these pumps, such as a major role in excitation-contraction coupling for SERCA1 and SERCA2 and housekeeping functions for PMCA1 and SERCA2, but have also revealed some unexpected phenotypes. These include squamous cell cancer and plasticity in the regulation of Ca2+-mediated exocytosis in SERCA2 heterozygous mutant mice, modulation of Ca2+ signaling in SERCA3-deficient mice, deafness and balance disorders in PMCA2 null mice, and male infertility in PMCA4 null mice. These unique phenotypes provide new information about the cellular functions of these pumps, the requirement of their activities for higher order physiological processes, and the pathophysiological consequences of pump dysfunction.

An Overview of Divalent Cation and Citrate Handling by the Kidney

Urinary calcium, magnesium and citrate levels are important in promoting or inhibiting renal stone formation. Here we review current information on the tubular handling of these ions. Most filtered calcium is reabsorbed in the proximal tubule and the thick ascending limb (TAL) of the loop of Henle, largely paracellularly; most of the remainder is reabsorbed in the distal tubule, transcellularly. Calcium reabsorption in the TAL and distal tubule is stimulated by parathyroid hormone and vitamin D; other factors influencing its renal handling include extracellular volume status and acid-base balance. Little filtered magnesium is reabsorbed in the proximal tubule; the bulk is reabsorbed paracellularly in the TAL, while most of the remainder is reabsorbed transcellularly in the distal tubule. Dietary intake, peptide hormones and chronic potassium depletion can all influence magnesium reabsorption in the TAL and distal tubule. Most filtered citrate is taken up across the apical membrane of the proximal tubule via a sodium-dicarboxylate co-transporter (NaDC-1). It also enters proximal tubular cells across the basolateral membrane; citrate contributes to the cells' oxidative metabolism. Citrate excretion is affected by acid-base balance, acetazolamide treatment, chronic potassium depletion and urinary excretion of calcium and magnesium. Where possible, we have indicated the mechanisms of these complex interactions.

Role of Nutrition in the Formation of Calcium-Containing Kidney Stones

Diet plays an important role in the pathogenesis of calcium-containing kidney stones. Although much work has demonstrated that specific dietary components alter urinary composition and supersaturation, relatively few studies link the ingestion of these components with actual nephrolithiasis. This article reviews the dietary factors thought to promote or inhibit the formation of calcium stones and discusses the current controversies in the field of nutrition and nephrolithiasis. Special attention is paid to the roles of dietary calcium, supplemental calcium, oxalate, phytate, and n-3 fatty acids. We offer dietary recommendations to individuals who have suffered from a calcium-containing kidney stone, and emphasize that a patient's 24-hour urine chemistries should be used to help guide dietary intervention.

Human Keratinocyte ATP2C1 Localizes to the Golgi and Controls Golgi Ca2+ Stores

Hailey-Hailey disease (MIM16960) is a blistering skin disease caused by mutations in the Ca2+ ATPase ATP2C1. We found that the abnormal Ca2+ signaling seen in Hailey-Hailey disease keratinocytes correlates with decreased protein levels of ATP2C1. Human ATP2C1 protein approximated 115 kDa in size. The ATP2C1 is localized to the Golgi apparatus in human keratinocytes, similar to its localization in yeast and Caenorhabditis elegans. To test whether the ATP2C1 controls Golgi Ca2+ stores, we measured intraorganelle Ca2+ concentrations using specifically targeted aequorins. Whereas normal keratinocytes display Golgi Ca2+ levels comparable to other epithelial cells, Hailey-Hailey disease keratinocyte Golgi Ca2+ refill is slower, and the maximum Ca2+ concentration reached is significantly lower. These findings were replicated in vivo, because clinically normal Hailey-Hailey disease epidermis contained lower Ca2+ stores and displayed an abnormal Ca2+ gradient. In this report we localize the ATP2C1, demonstrate its physiologic relevance in mammalian cells, and measure intraorganelle Golgi Ca2+ in keratinocytes.

Calcium: a potential central regulator in wound healing in the skin

Calcium has an established role in the normal homeostasis of mammalian skin and serves as a modulator in keratinocyte proliferation and differentiation. Gradients of calcium concentration increasing from 0.5 mM in the basal layer to > 1.4 mM in the stratum granulosum are consistent with migration patterns in response to minor abrasion (normal wear). Dermal fibroblasts require calcium but are approximately 100 times less sensitive than keratinocytes. Normal calcium metabolism in the skin is dependent on cell membrane and cytosolic calcium binding proteins (calmodulin, cadherins, etc.), but their modulation through parathyroid hormone, vitamin D or growth factors in normal or damaged tissue is not well documented. In wound repair, calcium is predominantly involved as Factor IV in the hemostatic phase, but it is expected to be required in epidermal cell migration and regeneration patterns in later stages of healing. Calcium alginate dressings are designed to liberate calcium early in the acute phase to promote hemostasis, but it is presently unclear whether the supplementary calcium influences the intracellular environment at later stages of wound repair, notably during the remodeling phase. Although experimental studies suggest that control of calcium is obligatory in wound management, we know very little as to how calcium in the wound bed is modulated through hormones, vitamin D, or various growth factors. Also, there is limited information as to how calcium released either from dressings, platelets, or from the circulation through the action of parathyroid hormone, growth factors or other modulators influences cell migration and remodeling in skin wounds, although experimental models suggest that management of calcium is essential in wound management.

Regulation of the proximal tubular sodium/proton exchanger NHE3 in rats with puromycin aminonucleoside (PAN)-induced nephrotic syndrome

Excessive proteinuria due to loss of glomerular permselectivity in nephrotic syndrome can cause disturbances in renal salt and water handling with edema formation. Apart from oncotic and hydrostatic mechanisms associated with hypoalbuminemia, primary derangements in renal tubular sodium transport may contribute to the pathogenesis of nephrotic edema. Whereas there is evidence for an increase of cortical collecting duct sodium reabsorption in nephrotic rats, it remains controversial whether proximal tubule sodium transport may also be activated in this condition. The regulation of the cortical Na/H exchanger NHE3, the main pathway for Na reabsorption in the proximal tubule (PT), was investigated in rats with puromycin aminonucleoside (PAN)-induced nephrotic syndrome. PAN rats developed reduced GFR, severe proteinuria, and sodium retention within 3 d. After 10 d, immunoblots of brush border vesicles revealed a decreased abundance of NHE3 in nephrotic animals. However, the Na/H antiporter activity in the same vesicle preparations was not significantly altered. Antiporter activity normalized for NHE3 protein was increased by 88% in nephrotic animals (P = 0.025). Immunohistochemistry with the same polyclonal antibody as for immunoblots revealed a decrease of NHE3 abundance in PT. In contrast, immunoreactivity for the monoclonal antibody 2B9, which specifically recognizes the non-megalin-associated, transport-competent pool of NHE3, was higher in PAN-treated rats than in controls. In conclusion, increased sodium reabsorption might be associated with a shift of NHE3 from an inactive pool to an active pool, thus contributing to sodium retention in a state of proteinuria.

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.

Squamous cell tumors in mice heterozygous for a null allele of Atp2a2, encoding the sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2 Ca2+ pump

Mutations in the human ATP2A2 gene, encoding sarco(endo)plasmic reticulum Ca(2+)-ATPase isoform 2 (SERCA2), cause Darier disease, an autosomal dominant skin disease characterized by multiple keratotic papules in the seborrheic regions of the body. Mice with a single functional Atp2a2 allele (the mouse homolog of ATP2A2) were shown previously to have reduced levels of SERCA2 in heart and mildly impaired cardiac contractility and relaxation. Here we show that aged heterozygous mutant (Atp2a2(+/-)) mice develop squamous cell tumors of the forestomach, esophagus, oral mucosa, tongue, and skin. Squamous cell tumors occurred in 13/14 Atp2a2(+/-) mice but were not observed in age- and sex-matched wild-type controls. Hyperkeratinized squamous cell papillomas and carcinomas of the upper digestive tract were the most frequent finding among Atp2a2(+/-) mice, and many animals had multiple tumors. Western blot analyses showed that SERCA2 protein levels were reduced in skin and other affected tissues of heterozygous mice. The development of squamous cell tumors in aged Atp2a2(+/-) mice indicates that SERCA2 haploinsufficiency predisposes murine keratinocytes to neoplasia. These findings provide the first direct demonstration that a perturbation of Ca(2+) homeostasis or signaling can serve as a primary initiating event in cancer.

Endothelin-1/endothelin-B receptor-mediated increases in NHE3 activity in chronic metabolic acidosis

Decreases in blood pH activate NHE3, the proximal tubular apical membrane Na/H antiporter. In cultured renal epithelial cells, activation of the endothelin-B (ET(B)) receptor increases NHE3 activity. To examine the role of the ET(B) receptor in the response to acidosis in vivo, the present studies examined ET(B) receptor-deficient mice, rescued from neonatal lethality by expression of a dopamine beta-hydroxylase promoter/ET(B) receptor transgene (Tg/Tg:ET(B)(-/-) mice). In proximal tubule suspensions from Tg/Tg:ET(B)(+/-) mice, 10(-8) M endothelin-1 (ET-1) increased NHE3 activity, but this treatment had no effect on tubules from Tg/Tg:ET(B)(-/-) mice. Acid ingestion for 7 days caused a greater decrease in blood HCO(3)(-) concentration in Tg/Tg:ET(B)(-/-) mice compared with Tg/Tg:ET(B)(+/+) and Tg/Tg:ET(B)(+/-) mice. Whereas acid ingestion increased apical membrane NHE3 by 42-46% in Tg/Tg:ET(B)(+/+) and Tg/Tg:ET(B)(+/-) mice, it had no effect on NHE3 in Tg/Tg:ET(B)(-/-) mice. In C57BL/6 mice, excess acid ingestion increased renal cortical preproET-1 mRNA expression 2.4-fold and decreased preproET-3 mRNA expression by 37%. On a control diet, Tg/Tg:ET(B)(-/-) mice had low rates of ammonium excretion, which could not be attributed to an inability to acidify the urine, as well as hypercitraturia, with increased titratable acid excretion. Acid ingestion increased ammonium excretion, citrate absorption, and titratable acid excretion to the same levels in Tg/Tg:ET(B)(-/-) and Tg/Tg:ET(B)(+/+) mice. In conclusion, metabolic acidosis increases ET-1 expression, which increases NHE3 activity via the ET(B) receptor.

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.

Effects of sulfur mustard on the basal cell adhesion complex

Among the most intriguing questions about sulfur mustard (di(2-chloroethyl) sulfide) is why basal cells are the primary targets of its vesicating lesions. To investigate this problem, replicate cultures of human epidermal keratinocytes (HEK) were grown from normal skin and exposed to 400 microM sulfur mustard (HD) for 5 min. Using fluorescein isothiocyanate (FITC)-conjugated antibodies, confocal laser microscopy and image analyses, we found that in early passages, sham-treated HEK maintained in a 0.15 mM Ca2+ medium continued to express keratins K5 and K14 as well as alpha6beta4-integrin. Both K5 and K14 are intermediate filaments characteristic of basal cells and linked with attachment mechanisms effecting epidermolysis bullosa simplex, a family of blistering skin diseases. Acute exposure to HD caused a statistically significant (P < 0.01) 30.74% decrease in K14 fluorescence within 1 h of exposure. Within 2 h of exposure, K14 fluorescence decreased to near-zero values. The loss in expression of K14 was progressive and occurred well before the expected appearance of in vivo blisters, which have a dose-dependent, clinical latent phase of 8-24 h. Acute exposure to HD also caused a statistically significant (P < 0.002) decrease in expression of beta4, an integrin which has been associated with junctional epidermolysis bullosa (JEB). Disruption of K14 and alpha6beta4-integrin may be early events in the HD injury pathway; however, they had no immediate or obvious effect on cell to substrate attachment.

Effect of intracellular calcium modulation on sulfur mustard cytotoxicity in cultured human neonatal keratinocytes

Previous studies in human skin keratinocyte cultures have shown that sulfur mustard (HD) induces an immediate and irreversible increase in internal free calcium levels that was independent of external calcium concentrations. These findings suggested a role for calcium in the aetiology of HD-induced cell death and that modulation of intracellular calcium concentrations may assist in providing protection against this agent. In the current work, actively proliferating and confluent cultures of first passage neonatal human skin keratinocytes were used to assess the effect of altered intra- and extracellular calcium levels on HD toxicity. Treatment of cultures with the endoplasmic reticulum calcium ATPase inhibitor thapsigargin, or the calcium chelator BAPTA-AM, which reduce HD-induced elevation of intracellular free calcium, did not modulate the toxicity of HD. Furthermore, alteration of external calcium concentrations during these same experiments failed to elicit any change in the viability of HD-exposed cells. Treatment of confluent cultures with ionomycin at either low (100 microM) or high (1.2 mM) external calcium concentrations also failed to modulate the toxicity of HD in any way. It appears that in neonatal human skin keratinocytes in culture, HD-induced intracellular calcium perturbation does not play a major role in HD-induced cytotoxicity.

Inhibition of cadherin function differentially affects markers of terminal differentiation in cultured human keratinocytes

Cadherin function is required for normal keratinocyte intercellular adhesion and stratification. In the present study, we have investigated whether cadherin-cadherin interactions may also modulate keratinocyte differentiation, as evidenced by alterations in the levels of several differentiation markers. Confluent keratinocyte cultures, propagated in low Ca(2+) medium in which cadherins are not active, were pre-incubated with antibodies that block the function of E-cadherin and/or P-cadherin; Ca(2+)was then elevated to 1 mM to activate the cadherins and induce differentiation. In control cultures (incubated with no antibody or with antibodies to other cell surface molecules), Ca(2+) elevation induced an increase in type 1 transglutaminase, profilaggrin, and loricrin, as measured by western blotting and in agreement with previous results. However, the concurrent addition of antibodies against both E- and P-cadherin prevented this increase in transglutaminase 1 protein. Incubation with either antibody alone had no consistent effect. Profilaggrin and loricrin, which are later markers of keratinocyte differentiation, responded differently from transglutaminase 1 to addition of antibodies. In the presence of anti-E-cadherin antibody, both loricrin and profilaggrin levels were dramatically enhanced compared to the high Ca(2+) control cells, while addition of antibody to P-cadherin slightly attenuated the Ca(2+)-induced increase. In the presence of both antibodies, loricrin and profilaggrin protein levels were intermediate between those observed in the presence of either antibody alone. The expression of involucrin, however, was unaffected by addition of antibodies. In addition, effects of the anti-cadherin antibodies were not secondary to alterations in proliferation or programmed cell death, as determined by several independent assays of these processes. Thus, the consequences of cadherin inhibition depend upon both the particular cadherin and the differentiation marker under study. Taken together, these data suggest that E-cadherin and P-cadherin contribute to the orderly progression of terminal differentiation in the epidermis in multiple ways.

Phospholipase C-gamma1 is required for calcium-induced keratinocyte differentiation

Phospholipase C-gamma1 is the most abundant member of the phospholipase C family in keratinocytes and is induced by calcium. Phospholipase C-gamma1, therefore, may be involved in the signal transduction system leading to calcium regulation of keratinocyte differentiation. To test this hypothesis, expression of phospholipase C-gamma1 in human keratinocytes was blocked by transfecting cells with the antisense human phospholipase C-gamma1 cDNA construct. These cells demonstrated a dramatic reduction in phospholipase C-gamma1 protein level compared with the empty vector-transfected cells and a marked reduction in the mRNA and protein levels of the differentiation markers involucrin and transglutaminase following administration of calcium. Similarly, cotransfection of antisense phospholipase C-gamma1 constructs with a luciferase reporter vector containing involucrin or transglutaminase promoters led to a substantial reduction in calcium-stimulated involucrin and transglutaminase promoter activities. Similar results were seen following treatment with a specific phospholipase C inhibitor U73122. To determine whether phospholipase C-gamma1 regulated differentiation by controlling intracellular calcium, we examined the ability of antisense phospholipase C-gamma1 to block the calcium-induced rise in intracellular calcium and found that it could. These findings indicate that phospholipase C-gamma1 is a critical component of the signaling pathway mediating calcium regulation of keratinocyte differentiation via its mobilization of intracellular calcium.

Molecular and functional analysis of SDCT2, a novel rat sodium-dependent dicarboxylate transporter

Kidney proximal tubule cells take up Krebs cycle intermediates for metabolic purposes and for secretion of organic anions through dicarboxylate/organic anion exchange. Alteration in reabsorption of citrate is closely related to renal stone formation. The presence of distinct types of sodium-coupled dicarboxylate transporters has been postulated on either side of the polarized epithelial membrane in the kidney proximal tubule. Using a PCR-based approach, we isolated a novel member of the sodium-dependent dicarboxylate/sulfate transporter called SDCT2. SDCT2 is a 600-amino acid residue protein that has 47-48% amino acid identity to SDCT1 and NaDC-1, previously identified in kidney and intestine. Northern analysis gave a single band of 3.3 kb for SDCT2 in kidney, liver, and brain. In situ hybridization revealed that SDCT2 is prominently expressed in kidney proximal tubule S3 segments and in perivenous hepatocytes, consistent with the sites of high-affinity dicarboxylate transport identified based on vesicle studies. A signal was also detected in the meningeal layers of the brain. SDCT2 expressed in Xenopus oocytes mediated sodium-dependent transport of di- and tricarboxylates with substrate preference for succinate rather than citrate, but excluding monocarboxylates. SDCT2, unlike SDCT1, displayed a unique pH dependence for succinate transport (optimal pH 7.5-8.5) and showed a high affinity for dimethylsuccinate, two features characteristic of basolateral transport. These data help to interpret the mechanisms of renal citrate transport, their alteration in pathophysiological conditions, and their role in the elimination of organic anions and therapeutic drugs.

Sodium-coupled transporters for Krebs cycle intermediates

Krebs cycle intermediates such as succinate, citrate, and alpha-ketoglutarate are transferred across plasma membranes of cells by secondary active transporters that couple the downhill movement of sodium to the concentrative uptake of substrate. Several transporters have been identified in isolated membrane vesicles and cells based on their functional properties, suggesting the existence of at least three or more Na+/dicarboxylate cotransporter proteins in a given species. Recently, several cDNAs, called NaDC-1, coding for the low-affinity Na+/dicarboxylate cotransporters have been isolated from rabbit, human, and rat kidney. The Na+/dicarboxylate cotransporters are part of a distinct gene family that includes the renal and intestinal Na+/sulfate cotransporters. Other members of this family include a Na(+)- and Li(+)-dependent dicarboxylate transporter from Xenopus intestine and a putative Na+/dicarboxylate cotransporter from rat intestine. The current model of secondary structure in NaDC-1 contains 11 transmembrane domains and an extracellular N-glycosylated carboxy terminus.

Differentiation of cultured human epidermal keratinocytes at high cell densities is mediated by endogenous activation of the protein kinase C signaling pathway

Normal human epidermal keratinocytes (NHEK) grown in serum-free medium on a plastic substrate spontaneously differentiate at high cell densities in vitro. Because protein kinase C (PKC) regulates murine keratinocyte differentiation triggered by a variety of stimuli, we examined the role of this signaling pathway in density-dependent activation of NHEK differentiation. Relative to subconfluent cultures, confluent NHEK expressed markedly higher levels of multiple differentiation markers assayed by immunoblotting, including keratin 1, loricrin, filaggrin, involucrin, TGK, and SPR-1. Expression of several of these markers continued to increase for several days after cells reached confluency. The total level of several PKC isoforms was not substantially altered in NHEK harvested at different cell densities, based on immunoblotting; however, subcellular fractionation revealed that PKCalpha underwent a redistribution to the particulate fraction in confluent and postconfluent NHEK cultures, suggesting that this isozyme was activated under these conditions and may be involved in triggering the terminal differentiation program. Supporting this concept, inhibition of PKC function using bryostatin 1 or GF 109203X blocked the induction of keratinocyte differentiation markers at high cell densities. These data suggest that endogenous activation of PKC is responsible for cell density-mediated stimulation of NHEK differentiation, establishing a critical role for this pathway in regulating human as well as murine keratinocyte differentiation.

Characterization of a rat Na+-dicarboxylate cotransporter

The metabolism of Krebs cycle intermediates is of fundamental importance for eukaryotic cells. In the kidney, these intermediates are transported actively into epithelial cells. Because citrate is a potent inhibitor for calcium stone formation, excessive uptake results in nephrolithiasis due to hypocitraturia. We report the cloning and characterization of a rat kidney dicarboxylate transporter (SDCT1). In situ hybridization revealed that SDCT1 mRNA is localized in S3 segments of kidney proximal tubules and in enterocytes lining the intestinal villi. Signals were also detected in lung bronchioli, the epididymis, and liver. When expressed in Xenopus oocytes, SDCT1 mediated electrogenic, sodium-dependent transport of most Krebs cycle intermediates (Km = 20-60 microM), including citrate, succinate, alpha-ketoglutarate, and oxaloacetate. Of note, the acidic amino acids L- and D-glutamate and aspartate were also transported, although with lower affinity (Km = 2-18 mM). Transport of citrate was pH-sensitive. At pH 7.5, the Km for citrate was high (0.64 mM), whereas at pH 5.5, the Km was low (57 microM). This is consistent with the concept that the -2 form of citrate is the transported species. In addition, maximal currents at pH 5.5 were 70% higher than those at pH 7.5, and our data show that the -3 form acts as a competitive inhibitor. Simultaneous measurements of substrate-evoked currents and tracer uptakes under voltage-clamp condition, as well as a thermodynamic approach, gave a Na+ to citrate or a Na+ to succinate stoichiometry of 3 to 1. SDCT1-mediated currents were inhibited by phloretin. This plant glycoside also inhibited the SDCT1-specific sodium leak in the absence of substrate, indicating that at least one Na+ binds to the transporter before the substrate. The data presented provide new insights into the biophysical characteristics and physiological implications of a cloned dicarboxylate transporter.

Sulfur mustard induces markers of terminal differentiation and apoptosis in keratinocytes via a Ca2+-calmodulin and caspase-dependent pathway

Sulfur mustard (SM) induces vesication via poorly understood pathways. The blisters that are formed result primarily from the detachment of the epidermis from the dermis at the level of the basement membrane. In addition, there is toxicity to the basal cells, although no careful study has been performed to determine the precise mode of cell death biochemically. We describe here two potential mechanisms by which SM causes basal cell death and detachment: namely, induction of terminal differentiation and apoptosis. In the presence of 100 microM SM, terminal differentiation was rapidly induced in primary human keratinocytes that included the expression of the differentiation-specific markers K1 and K10 and the cross-linking of the cornified envelope precursor protein involucrin. The expression of the attachment protein, fibronectin, was also reduced in a time- and dose-dependent fashion. Features common to both differentiation and apoptosis were also induced in 100 microM SM, including the rapid induction of p53 and the reduction of Bcl-2. At higher concentrations of SM (i.e., 300 microM), formation of the characteristic nucleosome-sized DNA ladders, TUNEL-positive staining of cells, activation of the cysteine protease caspase-3/apopain, and cleavage of the death substrate poly(ADP-ribose) polymerase, were observed both in vivo and in vitro. Both the differentiation and the apoptotic processes appeared to be calmodulin dependent, because the calmodulin inhibitor W-7 blocked the expression of the differentiation-specific markers, as well as the apoptotic response, in a concentration-dependent fashion. In addition, the intracellular Ca2+ chelator, BAPTA-AM, blocked the differentiation response and attenuated the apoptotic response. These results suggest a strategy for designing inhibitors of SM vesication via the Ca2+-calmodulin or caspase-3/PARP pathway.

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

Alternatively spliced forms of the alpha subunit of the cGMP-gated channel have been cloned from human keratinocytes. One form encodes a complete channel which is almost identical to the rod photoreceptor. A second spliced variant would encode a protein missing a portion of the intracellular hydrophilic domain and the putative first transmembrane domain. Both complete and spliced variants of the channel also were found in epidermis. The expression of the complete form of the channel was induced by levels of extracellular calcium which promote keratinocyte differentiation. The cGMP-gated channel may play an important role in calcium induced keratinocyte differentiation by mediating Ca2+ entry.

Decrease of Ca(2+)-ATPase activity in human keratinocytes during calcium-induced differentiation

Ca2+ regulates keratinocyte differentiation by increasing intracellular Ca2+ levels. Ca(2+)-ATPase in the Ca(2+)-induced differentiation of human keratinocytes was investigated by measuring Ca(2+)-ATPase mRNA, protein, and activity levels. Human keratinocytes were grown in Keratinocyte Growth Medium containing 0.03, 0.1, or 1.2 mM Ca2+ and assayed on days 2, 5, 7, 14, and 21. Ca(2+)-ATPase mRNA levels were found to be modestly increased in 5-, 7-, and 14-day cultured cells as compared with 2-day cultured cells, but levels fell below that of the 2-day cultured cells in the 21-day cultured cells. The Ca(2+)-ATPase mRNA levels were not affected by Ca2+ levels. A 135-kDa protein in human keratinocytes cross reacted with the monoclonal antibody against human erythrocyte Ca(2+)-ATPase. The level of this protein was decreased by Ca2+ and lost during differentiation, in parallel with the loss of enzymatic activity. Ca2+ influx of postconfluent 1.2 mM Ca(2+)-grown cells was higher than that of cells grown in lower Ca2+ concentrations. Ca2+ efflux from postconfluent cells grown in 0.03 mM Ca2+ was less than that from cells grown in stronger Ca2+ concentrations. These results suggest that the loss of the plasma membrane Ca(2+)-ATPase with time in culture contributes to the rise in intracellular Ca2+, thus promoting keratinocyte differentiation.

Keratinocyte K+ channels mediate Ca2+-induced differentiation

K+ channel activation has been associated with growth or differentiation in many cells. We have previously identified a 70-pS K+ channel that was found only in differentiated involucrin-positive cells. In this study we examined the role of K+ channels in Ca2+-induced keratinocyte differentiation. Consistent with our previous report, we found that a K+ conductance developed only in cells cultured in high extracellular Ca2+. Addition of charybdotoxin or verapamil blocked these K+ channels and inhibited Ca2+-induced differentiation, as assessed by cornified envelope formation or transglutaminase activity. These results suggest that K+ channel activation is necessary for Ca2+-induced differentiation. Finally, we used (125)I-labeled charybdotoxin to demonstrate the presence of K+ channels in intact human and mouse epidermis, hair follicles, and eccrine glands, indicating that these channels are found in keratinocytes both in vitro and in vivo. Thus K+ channels may moderate Ca2+ influx in more differentiated keratinocytes and may play a central role in keratinocyte differentiation.

Thapsigargin induces rapid, transient growth inhibition and c-fos expression followed by sustained growth stimulation in mouse keratinocyte cultures

Although the sesquiterpene lactone thapsigargin has been shown to possess hyperplastic and tumor-promoting activities when applied topically to mouse skin in vivo, the cellular mechanism(s) which underlie these effects are unclear. We show here that thapsigargin treatment of Primary mouse epidermal keratinocytes increased intracellular free Ca2+ concentration (Cai) in a concentration-dependent manner. Thapsigargin induced a rapid, transient elevation in keratinocyte Cai, in part due to the release of Ca2+ from intracellular stores. This response was followed by a sustained elevation in Ca2+, resulting entirely from calcium influx. Thapsigargin elicited a biphasic effect on keratinocyte DNA synthesis: a rapid inhibitory effect (50-60% inhibition at 4-8 h), followed by a very marked and sustained elevation. Prolonged treatment of keratinocytes with thapsigargin at relatively high concentrations resulted in cytotoxicity (inhibition of neutral red uptake). The rapid antiproliferative effect of thapsigargin was not associated with cytotoxicity, as determined by either neutral red uptake or by trypan blue exclusion, and was not blocked by pretreatment with Ro 31-7349, a selective inhibitor of protein kinase C. The rapid antiproliferative effect of thapsigargin was associated with rapid, transient activation of keratinocyte c-fos expression and rapid inhibition of total protein synthesis. Taken together, these findings raise the possibility that the hyperplastic and tumor-promoting activities of thapsigargin on epidermis in vivo result from direct keratinocyte growth stimulation as a consequence of a prolonged elevation in levels of Cai.

The effect of hydrocortisone and thyroxine on development of calcium homeostasis in embryonic intestinal epithelium

Cytoplasmic Ca2+ concentration of epithelial cells from 14-day embryonic chick duodena decreased during 72 h of organ culture to a value 54% of that found at 17 days in vivo. The ability of cells to maintain a constant Ca2+ concentration when challenged with high extracellular calcium was also significantly reduced. Addition of 1 microM hydrocortisone during culture restored both parameters of Ca2+ homeostasis to that of 16-day uncultured duodena, and rise in cytoplasmic Ca2+ was significant within 4 h of hormone treatment. Thyroxine influenced epithelial Ca2+ similarly, but to a lesser degree and only after 48-72 h of culture. These data indicate that glucocorticoids, and possibly thyroid hormones, influence the development of calcium homeostasis in intestinal epithelium.

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.

Suspension-induced murine keratinocyte differentiation is mediated by calcium

Modulating extracellular Ca2+ (Cao) and suspension culture are two frequently used methods to induce maturation of cultured human and mouse keratinocytes. To determine if the two methods share a common mechanism, changes in Ca2+ metabolism were studied in suspension cultures of mouse keratinocytes. Spontaneously detached and suspension- cultured keratinocytes in 0.05 microM Ca2+ medium express markers of suprabasal differentiation, while 0.05 microM Ca2+ is not permissive for marker expression by attached keratinocytes. Intracellular free Ca2+ (Cai) increased rapidly after placing keratinocytes in suspension in 0.05 microM Ca2+, reaching levels up to 3- to 4-fold higher than Cai in attached cells after 4-5 h. In suspended cells, the increase in Cai was associated with a 2- to 6- fold increase in Ca2+ transport across plasma membrane as well as depletion of intracellular Ca2+ -stores. Differentiation marker expression and terminal differentiation were inhibited in suspension-cultured keratinocytes by preventing the rise of Cai using either 1,2-bis (o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid to chelate intracellular Ca2+ or ethyleneglycol-bis (beta-aminoethyl ether)- N,N,N',N' -tetraacetic acid to reduce Cao. Together these results indicate that a rise in CAi is a common mechanism controlling differentiation in cultured mouse keratinocytes, and suspension of keratinocytes enhances Ca2+ transport and alters intracellular Ca2+ sequestration producing a rise in Cai.

Expression of the renal Na+/dicarboxylate cotransporter, NaDC-1, in COS-7 cells

The cloned rabbit renal Na+/dicarboxylate cotransporter, NaDC-1, was transiently expressed in the mammalian monkey kidney cell line, COS-7. Cells transfected with the plasmid pSV-201, containing the cDNA for the Na+/dicarboxylate cotransporter, expressed sodium-dependent succinate and citrate transport after 48 h. Cells transfected with control plasmid, pSV-SPORT, did not express sodium-dependent transport of succinate or citrate. The transport of succinate in cells expressing NaDC-1 was inhibited by di-and tricarboxylic acids, but not by monocarboxylic acids. Sodium-dependent transport of succinate was insensitive to changes in pH, while sodium-dependent citrate transport was stimulated by acidic pH. Succinate transport by NaDC-1 was saturable with an apparent Michaelis constant, Km, around 0.5 mM. The kinetics of sodium activation of succinate transport by NaDC-1 were sigmoidal, with an apparent Hill coefficient of 2.9, indicating that three sodium ions are involved in the transport of each succinate. Succinate transport by NaDC-1 was inhibited by lithium. The functional characteristics of NaDC-1 expressed in COS-7 cells correspond to those of the Na+/dicarboxylate cotransporter of the apical membrane of the renal proximal tubule.

Transcription factor regulation of epidermal keratinocyte gene expression

The epidermis is a tissue that undergoes a very complex and tightly controlled differentiation program. The elaboration of this program is generally flawless, resulting in the production of an effective protective barrier for the organism. Many of the genes expressed during keratinocyte differentiation are expressed in a coordinate manner; this suggests that common regulatory models may emerge. The simplest model envisions a 'common regulatory element' that is possessed by all genes that are regulated together (e.g., involucrin and transglutaminase type 1). Studies to date, however, have not identified any such elements and, if anything, the available studies suggest that appropriate expression of each gene is achieved using sometime subtly and sometime grossly different mechanisms. Recent studies indicate that a variety of transcription factors (AP1, AP2, POU domain. Sp1, STAT factors) are expressed in the epidermis and, in many cases, multiple members of several families are present (e.g., AP1 and POU domain factors). The simultaneous expression of multiple members of a single transcription factor family provides numerous opportunities for complex regulation. Some studies suggest that specific members of these families interact with specific keratinocyte genes. The best studied of these families in epidermis is the AP1 family of factors. All of the known AP1 factors are expressed in epidermis [52] and each is expressed in a specific spatial pattern that suggests the potential to regulate multiple genes. It will be important to determine the role of each of these members in regulating keratinocyte gene expression. Finally, information is beginning to emerge regarding signal transduction in keratinocytes. Some of the early events in signal transduction have been identified (e.g., PLC and PKC activation, etc.) and some of the molecular targets of these pathways (e.g., AP1 transcription factors) are beginning to be identified. Eventually we can expect to elucidation of all of the steps between the interaction of the stimulating agent with its receptor and the activation of target gene expression.

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.

Chelation of intracellular Ca2+ inhibits murine keratinocyte differentiation in vitro

The role of intracellular Ca2+ in the regulation of Ca(2+)-induced terminal differentiation of mouse keratinocytes was investigated using the intracellular Ca2+ chelator 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA). A cell permeable acetoxymethyl (AM) ester derivative BAPTA (BAPTA/AM) was loaded into primary mouse keratinocytes in 0.05 mM Ca2+ medium, and then the cells were induced to differentiate by medium containing 0.12 or 0.5 mM Ca(2+) Intracellular BAPTA loaded by BAPTA/AM (15-30 microM) inhibited the expression of epidermal differentiation-specific proteins keratin 1 (K1), keratin 10 (K10), filaggrin and loricrin as detected by immunoblotting. The differentiation-associated redistribution of E-cadherin on the cell membrane was delayed but not inhibited as determined by immunofluorescence. BAPTA also inhibited the expression of K1, K10 and loricrin mRNA. Furthermore, BAPTA prevented the decrease in DNA synthesis induced by 0.12 and 0.5 mM Ca2+, indicating the drug was inhibiting differentiation but was not toxic to keratinocytes. To evaluate the influence of BAPTA on intracellular Ca2+, the concentration of intracellular free Ca2+ (Cai) in BAPTA-loaded keratinocytes was examined by digital image analysis using the Ca(2+)-sensitive fluorescent probe fura-2, and Ca2+ influx was measured by 45Ca2+ uptake studies. Increase in extracellular Ca2+ (Cao) in the culture medium of keratinocytes caused a sustained increase in both Cai and Ca2+ localized to ionomycin-sensitive intracellular stores in keratinocytes. BAPTA lowered basal Cai concentration and prevented the Cai increase. After 12 hours of BAPTA treatment, the basal level of Cai returned to the control value, but the Ca2+ localized in intracellular stores was substantially decreased. 45Ca2+ uptake was initially (within 30 min) increased in BAPTA-loaded cells. However, the total 45Ca2+ accumulation over 24 hours in BAPTA-loaded cells remained unchanged from control values. These results indicate that keratinocytes can maintain Cai and total cellular Ca2+ content in the presence of increased amount of intracellular Ca2+ buffer (e.g., BAPTA) by depleting intracellular Ca2+ stores over a long period. The inhibition by BAPTA of keratinocyte differentiation marker expression may result from depletion of the Ca(2+)-stores since this is the major change in intracellular Ca2+ detected at the time keratinocytes express the differentiation markers. In contrast, the redistribution of E-cadherin on the cell membrane may be more directly associated with Cai change.

Cell density and culture factors regulate keratinocyte commitment to differentiation and expression of suprabasal K1/K10 keratins

Irreversible growth arrest and commitment to differentiation are among the earliest events in the program of cellular terminal differentiation. The transition from highly proliferative human keratinocytes in subconfluent culture to stationary cells in confluent culture was studied in a serum-free culture system to identify conditions that regulate the initiation of terminal differentiation in keratinocytes. We observed that culture confluence strongly induced commitment to terminal differentiation, as demonstrated by a dramatic loss of keratinocyte clonogenicity. Commitment was accompanied by the rapid induction of early differentiation markers, represented by expression of suprabasal keratin 1 (K1) and 10 (K10) genes. Induction of differentiation was independent of low (0.1 mM) or high (1.5 mM) calcium concentration in the medium. Epidermal growth factor suppressed expression of K1 and K10 mRNA in cultures induced to differentiate. Suspension of keratinocytes in methylcellulose medium failed to induce in subconfluent cultures, or enhance in confluent cultures, the expression of K1 and K10 genes. Subconfluent cells cultured in medium containing high calcium and no exogenous growth factor induced expression of K1 and K10 transcripts, but commitment and loss of proliferative potential were not observed. Confluent cell density primarily controlled keratinocyte commitment to terminal differentiation and differentiated keratin gene expression. However, suprabasal K1 and K10 gene expression also was regulated by medium calcium and exogenous growth-factor concentrations in subconfluent cultures that promoted cell-cell association. Epidermal growth factor inhibited the expression of suprabasal keratins but not the commitment to terminal differentiation mediated by cell confluence. Control of keratinocyte commitment and expression of selected differentiation genes are mediated by cell confluence and, at subconfluence, by specific culture factors.

Regulation of fos-lacZ fusion gene expression in primary mouse epidermal keratinocytes isolated from transgenic mice

The expression of a fos-lacZ fusion gene was studied in primary mouse epidermal keratinocytes obtained from transgenic mice. This gene construct contains the entire upstream regulatory sequence of c-fos, and expression of the endogenous and fusion gene was shown by Northern analysis to correlate upon induction with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA). Using a chromogenic substrate of beta-galactosidase, we also demonstrated that expression of the fusion gene product, like that of Fos, was localized to the cell nucleus. In addition, we showed that epidermal keratinocytes responded to dialysed fetal bovine serum (FBS), TPA and high-calcium medium with enhanced Fos-lacZ expression and an inhibition of proliferation. The time course of induction of Fos-lacZ expression was similar for dialysed FBS and TPA, with a peak approximately 2 h after exposure. Exposure for approximately 24 h to an elevated extracellular calcium concentration was required to elicit an increase in Fos-lacZ expression. The lack of an immediate effect of raising medium calcium levels on Fos-lacZ expression contrasted with the rapidity of its effect on DNA synthesis, which was significantly inhibited within 6-8 h. In addition, we found that the protein kinase C inhibitor Ro 31-7549 blocked Fos-lacZ expression induced by TPA but had little or no effect on that elicited by high calcium levels. Thus, although our results indicate that the fos gene product may be involved in mediating epidermal keratinocyte growth arrest in response to differentiative agents such as FBS, TPA and high medium calcium levels, the exact role of this gene product remains unclear.

Chronic hypocalcemia of vitamin D deficiency leads to lower intracellular calcium concentrations in rat hepatocytes

Several lines of evidence indicate that calcium deficiency is associated with cellular defects in many tissues and organs. Owing to the large in vivo gradient between ionized extra- and intracellular Ca2+ concentrations ([Ca2+]i), it is generally recognized that the prevailing circulating Ca2+ does not significantly affect resting cytosolic Ca2+. To probe the consequences of hypocalcemia on [Ca2+]i, a model of chronic hypocalcemia secondary to vitamin D (D) deficiency was used. Hepatocytes were isolated from livers of hypocalcemic D-deficient, of normocalcemic D3-repleted, or of normal control rats presenting serum Ca2+ of 0.78 +/- 0.02, 1.24 +/- 0.03, or 1.25 +/- 0.01 mM, respectively (P < 0.0001). [Ca2+]i was measured in cell couplets using the fluorescent probe Fura-2. Hepatocytes of normocalcemic D3-repleted and of normal controls exhibited similar [Ca2+]i of 227 +/- 10 and 242 +/- 9 nM, respectively (NS), whereas those of hypocalcemic rats had significantly lower resting [Ca2+]i (172 +/- 10 nM; P < 0.0003). Stimulation of hepatocytes with the alpha 1-adrenoreceptor agonist phenylephrine illicited increases in cytosolic Ca2+ leading to similar [Ca2+]i and phosphorylase a (a Ca(2+)-dependent enzyme) activity in all groups but in contrast to normocalcemia, low extracellular Ca2+ was often accompanied by a rapid decay in the sustained phase of the [Ca2+]i response. When stimulated with the powerful hepatic mitogen epidermal growth factor (EGF), hepatocytes isolated from hypocalcemic rat livers responded with a blunted maximal [Ca2+]i of 237.6 +/- 18.7 compared with 605.2 +/- 89.9 nM (P < 0.0001) for their normal counterparts, while the EGF-mediated DNA synthesis response was reduced by 50% by the hypocalcemic condition (P < 0.03). Further studies on the possible mechanisms involved in the perturbed [Ca2+]i homeostasis associated with chronic hypocalcemia revealed the presence of an unchanged plasma membrane Ca2+ ATPase but of a significant decrease in agonist-stimulated Ca2+ entry as indicated using Mn2+ as surrogate ion (P < 0.03). Our data, thus indicate that, in rat hepatocytes, the in vivo calcium status significantly affects resting [Ca2+]i, and from this we raise the hypothesis that this lower than normal [Ca2+]i may be linked, in calcium disorders, to inappropriate cell responses mediated through the calcium signaling pathway as illustrated by the response to phenylephrine and EGF.