Volume expansion-sensing outward-rectifier Cl- channel: fresh start to the molecular identity and volume sensor

Induction by transforming growth factor-beta1 of epithelial to mesenchymal transition is a rare event in vitro

INTRODUCTION

Transforming growth factor (TGF)-beta1 is proposed to inhibit the growth of epithelial cells in early tumorigenesis, and to promote tumor cell motility and invasion in the later stages of carcinogenesis through the induction of an epithelial to mesenchymal transition (EMT). EMT is a multistep process that is characterized by changes in cell morphology and dissociation of cell-cell contacts. Although there is growing interest in TGF-beta1-mediated EMT, the phenotype is limited to only a few murine cell lines and mouse models.

METHODS

To identify alternative cell systems in which to study TGF-beta1-induced EMT, 18 human and mouse established cell lines and cultures of two human primary epithelial cell types were screened for TGF-beta1-induced EMT by analysis of cell morphology, and localization of zonula occludens-1, E-cadherin, and F-actin. Sensitivity to TGF-beta1 was also determined by [3H]thymidine incorporation, flow cytometry, phosphorylation of Smad2, and total levels of Smad2 and Smad3 in these cell lines and in six additional cancer cell lines.

RESULTS

TGF-beta1 inhibited the growth of most nontransformed cells screened, but many of the cancer cell lines were insensitive to the growth inhibitory effects of TGF-beta1. In contrast, TGF-beta1 induced Smad2 phosphorylation in the majority of cell lines, including cell lines resistant to TGF-beta1-mediated cell cycle arrest. Of the cell lines screened only two underwent TGF-beta1-induced EMT.

CONCLUSION

The results presented herein show that, although many cancer cell lines have lost sensitivity to the growth inhibitory effect of TGF-beta1, most show evidence of TGF-beta1 signal transduction, but only a few cell lines undergo TGF-beta1-mediated EMT.

Differential effects of tyrosine kinase inhibitors on volume-sensitive chloride current in human atrial myocytes: evidence for dual regulation by Src and EGFR kinases

To determine whether protein tyrosine kinase (PTK) modulates volume-sensitive chloride current (I_Cl.vol) in human atrial myocytes and to identify the PTKs involved, we studied the effects of broad-spectrum and selective PTK inhibitors and the protein tyrosine phosphatase (PTP) inhibitor orthovanadate (VO₄⁻³). I_Cl.vol evoked by hyposmotic bath solution (0.6-times isosmotic, 0.6T) was enhanced by genistein, a broad-spectrum PTK inhibitor, in a concentration-dependent manner (EC₅₀ = 22.4 μM); 100 μM genistein stimulated I_Cl.vol by 122.4 ± 10.6%. The genistein-stimulated current was inhibited by DIDS (4,4′-diisothiocyanostilbene-2,2′-disulfonic acid, 150 μM) and tamoxifen (20 μM), blockers of I_Cl.vol. Moreover, the current augmented by genistein was volume dependent; it was abolished by hyperosmotic shrinkage in 1.4T, and genistein did not activate Cl⁻ current in 1T. In contrast to the stimulatory effects of genistein, 100 μM tyrphostin A23 (AG 18) and A25 (AG 82) inhibited I_Cl.vol by 38.2 ± 4.9% and 40.9 ± 3.4%, respectively. The inactive analogs, daidzein and tyrphostin A63 (AG 43), did not alter I_Cl.vol. In addition, the PTP inhibitor VO₄⁻³ (1 mM) reduced I_Cl.vol by 53.5 ± 4.5% (IC₅₀ = 249.6 μM). Pretreatment with VO₄⁻³ antagonized genistein-induced augmentation and A23- or A25-induced suppression of I_Cl.vol. Furthermore, the selective Src-family PTK inhibitor PP2 (5 μM) stimulated I_Cl.vol, mimicking genistein, whereas the selective EGFR (ErbB-1) kinase inhibitor tyrphostin B56 (AG 556, 25 μM) reduced I_Cl.vol, mimicking A23 and A25. The effects of both PP2 and B56 also were substantially antagonized by pretreatment with VO₄⁻³. The results suggest that I_Cl.vol is regulated in part by the balance between PTK and PTP activity. Regulation is complex, however. Src and EGFR kinases, distinct soluble and receptor-mediated PTK families, have opposing effects on I_Cl.vol, and multiple target proteins are likely to be involved.

Altered properties of volume-sensitive osmolyte and anion channels (VSOACs) and membrane protein expression in cardiac and smooth muscle myocytes from Clcn3-/- mice

ClC-3, a member of the large superfamily of ClC voltage-dependent Cl(-) channels, has been proposed as a molecular candidate responsible for volume-sensitive osmolyte and anion channels (VSOACs) in some cells, including heart and vascular smooth muscle. However, the reported presence of native VSOACs in at least two cell types from transgenic ClC-3 disrupted (Clcn3(-/-)) mice casts considerable doubt on this proposed role for ClC-3. We compared several properties of native VSOACs and examined mRNA transcripts and membrane protein expression profiles in cardiac and pulmonary arterial smooth muscle cells from Clcn3(+/+) and Clcn3(-/-) mice to: (1) test the hypothesis that native VSOACs are unaltered in cells from Clcn3(-/-) mice, and (2) test the possibility that targeted inactivation of the Clcn3 gene using a conventional murine global knock-out approach may result in compensatory changes in expression of other membrane proteins. Our experiments demonstrate that VSOAC currents in myocytes from Clcn3(+/+) and Clcn3(-/-) mice are remarkably similar in terms of activation and inactivation kinetics, steady-state current densities, rectification, anion selectivity (I(-) > Cl(-)>> Asp(-)) and sensitivity to block by glibenclamide, niflumic acid, DIDS and extracellular ATP. However, additional experiments revealed several significant differences in other fundamental properties of native VSOACs recorded from atrial and smooth muscle cells from Clcn3(-/-) mice, including: differences in regulation by endogenous protein kinase C, differential sensitivity to block by anti-ClC-3 antibodies, and differential sensitivities to [ATP](i) and free [Mg(2+)](i). These results suggest that in response to Clcn3 gene deletion, there may be compensatory changes in expression of other proteins that alter VSOAC channel subunit composition or associated regulatory subunits that give rise to VSOACs with different properties. Consistent with this hypothesis, in atria from Clcn3(-/-) mice compared to Clcn3(+/+) mice, quantitative analysis of ClC mRNA expression levels revealed significant increases in transcripts for ClC-1, ClC-2, and ClC-3, and protein expression profiles obtained using two-dimensional polyacrylamide gel electrophoresis revealed complex changes in at least 35 different unidentified membrane proteins in cells from Clcn3(-/-) mice. These findings emphasize that caution needs to be exercised in simple attempts to interpret the phenotypic consequences of conventional global Clcn3 gene inactivation.

Expression of the Skate (Raja erinacea) AE1 Osmolyte Channel in Xenopus laevis Oocytes: Monovalent Cation Permeability

The aim of this study was to express the cloned skate anion exchanger 1 (skAE1) in Xenopus oocytes and determine whether the differences in monovalent cation permeabilities in hypotonically stimulated skate and trout erythrocytes could be due to differences in the presence or absence of intracellular channel regulators between the two species or in the intrinsic permeability properties of the channels themselves. The expressed protein (skAE1) was inserted into the oocyte cell membrane and facilitated both Cl- exchange and taurine transport. Expression of skAE1 in oocytes showed similar monovalent cation permeabilities as previously reported for skate erythrocytes and different from both trout erythrocytes and trAE1 expressed in Xenopus oocytes. These results show that the skAE1 expressed in oocytes functions in a manner similar to that of the osmolyte channel in hypotonically activated skate erythrocytes and supports the hypothesis that differences in the monovalent cation permeabilities of the osmolyte channels in skate and trout RBCs resides in the differences in permeability properties of the channels between the two species.

Cell volume regulation in response to hypotonicity is impaired in HeLa cells expressing a protein kinase Calpha mutant lacking kinase activity

The chloride conductance (G(Cl,swell)) that participates in the regulatory volume decrease process triggered by osmotic swelling in HeLa cells was impaired by removal of extracellular Ca(2+), depletion of intracellular Ca(2+) stores with thapsigargin, or by preloading the cells with BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid). Furthermore, overnight exposure to the phorbol ester tetradecanoyl phorbol acetate and acute incubation with inhibitors of the conventional protein kinase C (PKC) isoforms bisindolylmaleimide I and Gö6976 inhibited G(Cl,swell). Treatment of HeLa cells with U73122, a phospholipase C inhibitor, also prevented G(Cl,swell). Hypotonicity induced selective PKC alpha accumulation in the membrane/cytoskeleton fraction in fractionation experiments and translocation of a green fluorescent protein-PKC alpha fusion protein to the plasma membrane of transiently transfected HeLa cells. To further explore the role of PKCs in hypotonicity-induced G(Cl,swell), HeLa clones stably expressing either a kinase-dead dominant negative variant of the Ca(2+)-dependent PKC isoform alpha (PKC alpha K386R) or of the atypical PKC isoform zeta (PKCzeta K275W) were generated. G(Cl,swell) was significantly reduced in HeLa cells expressing the dominant negative PKC alpha mutant but remained unaltered in cells expressing dominant negative PKCzeta. These findings strongly implicate PKC alpha as a critical regulatory element that is required for efficient regulatory volume decrease in HeLa cells.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Relation between food intake and visual analogue scale ratings of appetite and other sensations in healthy older and young subjects

OBJECTIVE

Visual analogue scales are widely used in appetite research, yet the validity of these scales to evaluate appetite and mood has not been assessed in older subjects. The aim of this study was to determine the relations between food intake and visual analogue scale (VAS) ratings of appetite and nonappetite sensations in healthy older and young subjects.

DESIGN

Retrospective combined analysis of four single-blind, randomised, controlled appetite studies.

SETTING

All studies were conducted in the University of Adelaide, Department of Medicine, Adelaide, Australia.

SUBJECTS

A total of 45 healthy young men (n=24) and women (n=21) aged 18-35 y and 45 healthy older men (n=24) and women (n=21) aged 65-85 y were recruited by advertisement.

INTERVENTIONS

Oral, intraduodenal or intravenous administration of treatments which suppressed food intake were compared to control. Up to 90 min after treatment, a test meal was offered and subjects ate freely for between 30 and 60 min. Perceptions were assessed by 100-mm visual analogue scales administered at regular intervals.

RESULTS

Food intake at the test meal was positively related to perceptions of hunger, drowsiness, and calmness at both baseline and premeal (r>0.16, P<0.05), and inversely related to premeal ratings of fullness (r> 0.2, P<0.05) in both older and young subjects. Food intake was related to VAS ratings at least as strongly, if not more so, in older as in young subjects.

CONCLUSIONS

These observations (i) confirm that food intake is related to perceptions of hunger and fullness as assessed by VAS in healthy older and young subjects, and (ii) suggest that sensations, not obviously associated with appetite, including 'drowsiness' and 'calmness', are also associated with food intake.

The fall and rise of active chloride transport: implications for regulation of intraocular pressure

Early study of transepithelial salt transfer focused on Cl(-) and not Na(+), partly because Cl(-) was readily measureable. The advent of flame photometry and tracer techniques brought Na(+) to the fore, especially since short-circuited frog skin (Rana temporaria) produces baseline net movement of Na(+) and not of Cl(-). Zadunaisky was among the first to describe what is currently termed secondary active Cl(-) transport, helping stimulate interest in Cl(-) handling by other tissues, notably the thick ascending limb of the loop of Henle important in renal counter-current multiplication. More recently, molecules responsible for electroneutral and electrogenic Cl(-) transfer have been cloned, and specific diseases resulting from their faulty expression have been identified. The clinical importance of transepithelial Cl(-) transfer is illustrated by studies of aqueous humor formation by the eye's bilayered ciliary epithelium. NaCl is taken up from the stroma by the pigmented ciliary epithelial (PE) layer, diffuses through gap junctions into the nonpigmented ciliary epithelial (NPE) layer, and is released into the aqueous humor largely through Na(+) pumps and Cl(-) channels. ATP released by NPE cells can be ecto-enzymatically metabolized to adenosine. Adenosine can mediate paracrine/autocrine stimulation of Cl(-) channels and aqueous humor secretion by occupying A(3) adenosine receptors (ARs). A(3)AR agonists indeed elevate, and A(3)AR antagonists lower, intraocular pressure (IOP) in wild-type mice. A(3)AR knockout mice have low IOP and their responses to A(3)AR agonists and antagonists are blunted; this suggests that reducing Cl(-)-channel activity with A(3)AR antagonists may provide a novel approach for treating glaucoma.

Epithelial-mesenchymal transition and its implications for fibrosis

Epithelial to mesenchymal transition (EMT) is a central mechanism for diversifying the cells found in complex tissues. This dynamic process helps organize the formation of the body plan, and while EMT is well studied in the context of embryonic development, it also plays a role in the genesis of fibroblasts during organ fibrosis in adult tissues. Emerging evidence from studies of renal fibrosis suggests that more than a third of all disease-related fibroblasts originate from tubular epithelia at the site of injury. This review highlights recent advances in the process of EMT signaling in health and disease and how it may be attenuated or reversed by selective cytokines and growth factors.

Hypotonic cell swelling stimulates permeability to cAMP in a rat colonic cell line

This study characterized the membrane permeability to cAMP in a cell line derived from the rat colon (CC531(mdr+)) by comparison of fluxes of 3H-cAMP, 3H-8-bromo-cAMP, 3H-taurine, 3H-adenosine and 3H-5'AMP under various experimental conditions including cell membrane depolarization and hypotonic cell swelling. Cell volume was modified by changing the osmolality and composition of the extracellular medium. Incubation in iso- and hypotonic KCl media induced graded increases in cell volume and stable activation of volume-sensitive channels that was reflected in an increased efflux of 3H-taurine. Incubation in hypotonic KCl solution also enhanced the efflux of 3H-8-Br-cAMP (a non-hydrolysable analogue of cAMP). Both the efflux of 3H-taurine and of 3H-8-Br-cAMP were inhibited by 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB, 100 microM) suggesting the involvement of volume-sensitive anion channels. To gain further insight into the route mediating cAMP permeability, the uptakes of 3H-cAMP, 3H-8-Br-cAMP and 3H-taurine were determined over short (5-min) periods. Uptakes of these substrates demonstrated close similarities: comparable increases were observed that correlated with the increases in cell volume in iso- and hypoosmotic KCl media; they were inhibited strongly by NPPB (100 microM) and metabolic inhibitors (deoxyglucose, 20 mM together with the mitochondrial uncoupler carbonylcyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 10 microM) while barely reduced by dipyridamole (100 microM) and they were not affected by adenosine (1 mM). In contrast, the uptakes of 3H-adenosine and 3H-5'AMP had strikingly different properties; they were insensitive to cell swelling; barely inhibited by NPPB (100 microM) and metabolic inhibitors (deoxyglucose and FCCP) while strongly reduced by dipyridamole (100 micro M). Unlike the uptakes of 3H-cAMP, 3H-8-Br-cAMP and 3H-taurine, the uptakes of 3H-adenosine and 3H-5'AMP were reduced in Na(+)-free media, suggesting the presence in this cell line of two different adenosine carriers, one sodium-dependent and one sodium-independent. Taken together the present data show that in this rat colonic cell line, cAMP permeability is increased by cell swelling in hypotonic KCl medium and inhibited by NPPB and metabolic inhibitors. The similarity of these characteristics to those of taurine permeability suggests the involvement of a volume-sensitive anion pathway.

Cell-Volume Regulation by Swelling-Activated Chloride Current in Guinea-Pig Ventricular Myocytes

The cell-volume regulation by swelling-activated Cl- current (I(Cl,swell)) was studied in guinea pig ventricular myocytes, using a microscopic video-image analysis. We have previously shown that in ventricular cells depolarized in high-K+ ([K+]o>45 mM) solution, an activation of the cyclic AMP-dependent Cl- current (I(Cl,cAMP)) leads to cell swelling. We first investigated the mechanism underlying the I(Cl,cAMP)-independent recovery (shrinkage) of the swollen cells. They shrank when the membrane potential (Vm) was made negative to the equilibrium potential of Cl- (ECl) by lowering [K+]o or [Cl-]o in the high-K+ solution. This shrinkage was attenuated by the inhibitors (DIDS, glibenclamide, furosemide) of swelling-activated Cl- current (I(Cl,swell)). These findings suggested an involvement of I(Cl,swell) in the observed isosmotic cell shrinkage. On the other hand, an application of hyposmotic (70% of control) solution to the cells at normal [K+]o (ECl>Vm) induced a cell swelling, and the swollen cells underwent a slight but definite spontaneous cell shrinkage during hyposmotic challenge, indicating the operation of the mechanism of regulatory volume decrease (RVD). This RVD was pronounced at low [Cl-]o, at which ECl was much more positive than Vm. On the contrary, when the hyposmotic solution was applied to the cells at high [K+]o, at which ECl was negative to Vm, the cells swelled vigorously and monotonically without showing RVD, the swelling being much greater than that seen at normal [K+]o. Both the RVD at normal [K+]o and the extra cell swelling at high [K+]o were suppressed by DIDS. These results suggest that I(Cl,swell) activated by cell swelling can shrink or inflate the cardiac cells under hyposmotic as well as isosmotic conditions, depending on Vm and ECl.

Streptomycin and its analogues are potent inhibitors of the hypotonicity-induced Ca2+ entry and Cl- channel activity

Streptomycin is a common antibiotic used in culture media. It is also a known blocker of stretch-activated and mechanosensitive ion channels in neurons and cardiac myocytes. But very little information is available on its effect in the regulation of epithelial ion channels. Osmotic swelling is a kind of mechanical stretch. The opening of stretch-activated Ca(2+) channels contributes to hypotonicity-induced Ca(2+) influx which is necessary for the activation of volume-regulated Cl(-) channels in human cervical cancer cells. This study aimed to investigate the role of streptomycin in cell volume regulation. Treatment of cervical cancer SiHa cells with streptomycin and its analogues (gentamicin and netilmicin) did not affect the basal cytosolic Ca(2+) ([Ca(2+)](i)) level. But it attenuated the hypotonicity-stimulated increase of [Ca(2+)](i) in a dose-dependent manner with half-maximal inhibitory concentrations (IC(50)) of 25, 90 and 200 microM for streptomycin, gentamicin and netilmicin, respectively, when measured at room temperature. In contrast, under free extracellular Ca(2+) condition, hypotonic stress only induced a small, progressive increase of [Ca(2+)](i), while 500 microM streptomycin did not affect this Ca(2+) signaling. Streptomycin and its analogues (gentamicin and netilmicin) also inhibited the activation of volume-regulated Cl(-) channels in a dose-dependent manner with IC(50) of 30, 95 and 250 microM at room temperature, respectively. Chronic culture with 50 microM streptomycin downregulates the activity of volume-regulated Cl(-) channels and retards the process of regulatory volume decrease in SiHa cells and MDCK cells. We suggest that using cells chronically cultured with streptomycin to study epithelial ion channels risks studying cellular and molecular pathology rather than physiology.

2-APB inhibits volume-regulated anion channels independently from intracellular calcium signaling modulation

It has previously been suggested that volume-regulated anion channels (VRACs) and store-operated channels (SOCs) interact with each other according to their expected colocalization in the plasma membrane of LNCaP cells. In order to study interactions between these two channels, we used 2-aminoethoxydiphenyl borate (2-APB) as a regular SOC inhibitor. Surprisingly 2-APB reduced VRAC activity in a dose-dependent manner (IC(50)=122.8 microM), but not 2,2-diphenyltetrahydrofuran (a structural analog of 2-APB). This effect was also present in keratinocytes. We conclude that 2-APB is an inhibitor of the VRAC family, and is also a potent tool to study the SOC-VRAC interaction in LNCaP cells.

The role of renal proximal tubular cells in diabetic nephropathy

We now know that the rate of progression of diabetic nephropathy, like all progressive renal disease, correlates with the degree of corticointerstitial fibrosis. Therefore, much interest has focused on the contribution of the resident cells in the renal cortex to this process. This article reviews the evidence that the epithelial cells of the proximal tubule are major players in orchestrating events in the corticointerstitium in diabetic nephropathy. More specifically, it addresses their role in extracellular matrix turnover, generation of cytokines, and recruitment of inflammatory cells, as well as examining the concept that they are the source of the interstitial myofibroblasts, which are the principal mediators of the fibrotic process.

ATP-dependent regulation of SK4/IK1-like currents in rat submandibular acinar cells: possible role of cAMP-dependent protein kinase

SK4/IK1 encodes an intermediate conductance, Ca2+ -activated K+ channel and fulfills a variety of physiological functions in excitable and nonexcitable cells. Although recent studies have provided evidence for the presence of SK4/IK1 channels in salivary acinar cells, the regulatory mechanisms and the physiological function of the channel remain unknown in these cells. Using molecular and electrophysiological techniques, we examined whether cytosolic ATP-dependent regulation of native SK4/IK1-like channel activity would involve endogenous cAMP-dependent protein kinase (PKA) in rat submandibular acinar (RSA) cells. Electrophysiological properties of tetraethylammonium (TEA) (10 mM)-insensitive, Ca2+ -dependent K+ currents in macropatches excised from RSA cells matched those of whole cell currents recorded from human embryonic kidney-293 cells heterologously expressing rat SK4/IK1 (rSK4/IK1) cloned from RSA cells. In outside-out macropatches, activity of native SK4/IK1-like channels, defined as a charybdotoxin (100 nM)-blockable current in the presence of TEA (10 mM) in the bathing solution, ran down unless both ATP and Mg2+ were present in the pipette solution. The nonhydrolyzable ATP analog AMP-PNP failed to support the channel activity as ATP did. The addition of Rp-cAMPS (10 microM), a PKA inhibitor, to the pipette solution containing ATP/Mg2+ induced a rundown of the Ca2+ -dependent K+ currents. Inclusion of cAMP (1 mM) into the pipette solution (1 microM free Ca2+) containing ATP/Mg2+ caused a gradual increase in the currents, the effect being pronounced for the currents induced by 0.1 microM free Ca2+. Forskolin (1 microM), an adenylyl cyclase activator, also increased the currents induced by 0.1 microM free Ca2+. In inside-out macropatches, cytosolic ATP/Mg2+ increased both the maximum current (proportional to the maximum channel activity) and Ca2+ sensitivity of current activation. Collectively, these results suggest that ATP-dependent regulation of native SK4/IK1-like channels, at least in part, is mediated by endogenous PKA in RSA cells.

Role of cell volume in K+-induced Ca2+ signaling by rat adrenal glomerulosa cells

The involvement of cell volume in the K+-evoked Ca2+ signaling was studied in cultured rat glomerulosa cells. Previously we reported that hyposmosis (250 mOsm) increased the amplitude of T-type Ca2+ current and, accordingly, enhanced the Ca2+ response of cultured rat glomerulosa cells to K+. In the present study we found that this enhancement is not influenced by the cytoskeleton-disrupting drugs cytochalasin-D (20 microM) and colchicine (100 microM). Elevation of extracellular potassium concentration ([K+]e) from 3.6 to 4.6-8.6 mM induced cell swelling, which had slower kinetics than the Ca2+ signal. Cytoplasmic Ca2+ signal measured in single glomerulosa cells in response to stimulation with 5 mm K+ for 2 min showed two phases: after a rapid rise reaching a plateau within 20-30 sec, [Ca2+]c increased further slowly by approximately one third. When 5 mM K+ was coapplied with elevation of extracellular osmolarity from 290 to 320 mOsm, the second phase was prevented. These results indicate that cell swelling evoked by physiological elevation of [K+]e may contribute to the generation of sustained Ca2+ signals by enhancing voltage-activated Ca2+ influx.

Functional effects of novel anti-ClC-3 antibodies on native volume-sensitive osmolyte and anion channels in cardiac and smooth muscle cells

Whether ClC-3 encodes volume-sensitive organic osmolyte and anion channels (VSOACs) remains controversial. We have shown previously that native VSOACs in some cardiac and vascular myocytes were blocked by a commercial anti-ClC-3 carboxy terminal antibody (Alm C592-661 antibody), although recent studies have raised questions related to the specificity of Alm C592-661 antibody. Therefore, we have developed three new anti-ClC-3 antibodies and investigated their functional effects on native VSOACs in freshly isolated canine pulmonary artery smooth muscle cells (PASMCs) and guinea pig cardiac myocytes. These new antibodies produced a common prominent immunoreactive band with an apparent molecular mass of 90-92 kDa in the guinea pig heart and PASMCs, and a similar molecular mass immunoreactive band was observed in the brain from homozygous Clcn3+/+ mice but not from homozygous Clcn3-/- mice. VSOACs elicited by hypotonic cell swelling in PASMCs and guinea pig atrial myocytes were nearly completely abolished by intracellular dialysis with two new anti-ClC-3 antibodies specifically targeting the ClC-3 carboxy (C670-687 antibody) and amino terminus (A1-14 antibody). This inhibition of native VSOACs can be attributed to a specific interaction with endogenous ClC-3, because 1) preabsorption of the antibodies with corresponding antigens prevented the inhibitory effects, 2) extracellular application of a new antibody raised against an extracellular epitope (Ex133-148) of ClC-3 failed to inhibit native VSOACs in PASMCs, 3) intracellular dialysis with an antibody targeting Kv1.1 potassium channels failed to inhibit native VSOACs in guinea pig atrial myocytes, and 4) anti-ClC-3 C670-687 antibody had no effects on swelling-induced augmentation of the slow component of the delayed rectifying potassium current in guinea pig ventricular myocytes, although VSOACs in the same cells were inhibited by the antibody. These results confirm that endogenous ClC-3 is an essential molecular entity responsible for native VSOACs in PASMCs and guinea pig cardiac myocytes.

Role for integrin-linked kinase in mediating tubular epithelial to mesenchymal transition and renal interstitial fibrogenesis

Under pathologic conditions, renal tubular epithelial cells can undergo epithelial to mesenchymal transition (EMT), a phenotypic conversion that is believed to play a critical role in renal interstitial fibrogenesis. However, the underlying mechanism that governs this process remains largely unknown. Here we demonstrate that integrin-linked kinase (ILK) plays an important role in mediating tubular EMT induced by TGF-beta1. TGF-beta1 induced ILK expression in renal tubular epithelial cells in a time- and dose-dependent manner, which was dependent on intracellular Smad signaling. Forced expression of ILK in human kidney proximal tubular epithelial cells suppressed E-cadherin expression and induced fibronectin expression and its extracellular assembly. ILK also induced MMP-2 expression and promoted cell migration and invasion in Matrigel. Conversely, ectopic expression of a dominant-negative, kinase-dead form of ILK largely abrogated TGF-beta1-initiated tubular cell phenotypic conversion. In vivo, ILK was markedly induced in renal tubular epithelia in mouse models of chronic renal diseases, and such induction was spatially and temporally correlated with tubular EMT. Moreover, inhibition of ILK expression by HGF was associated with blockade of tubular EMT and attenuation of renal fibrosis. These findings suggest that ILK is a critical mediator for tubular EMT and likely plays a crucial role in the pathogenesis of chronic renal fibrosis.

Calcium-dependent, swelling-activated K+ conductance in human neuroblastoma cells

In most mammalian cells, regulatory volume decrease (RVD) is mediated by swelling-activated Cl(-) and K(+) channels. Previous studies in the human neuroblastoma cell line CHP-100 have demonstrated that exposure to hypoosmotic solutions activates Cl(-) channels which are sensitive to Ca(2+). Whether a Ca(2+)-dependent K(+) conductance is activated after cell swelling was investigated in the present studies. Reducing the extracellular osmolarity from 290 to 190 mOsm/kg H(2)O rapidly activated 86Rb effluxes. Hypoosmotic stress also increased cytosolic Ca(2+) in fura-2 loaded cells. Pretreatment with 2.5 mM EGTA and nominally Ca(2+) free extracellular solution significantly decreased the hypoosmotically induced rise in cytosolic Ca(2+) and the swelling-activated 86Rb efflux. In cell-attached patch-clamp studies, decreasing the extracellular osmolarity activated a K(+) conductance that was blocked by Ba(2+). In addition, the swelling-activated K(+) channels were significantly inhibited in the presence of nominally free extracellular Ca(2+) and 2.5mM EGTA. These results suggest that in response to hypoosmotic stress, a Ca(2+)-dependent K(+) conductance is activated in the human neuroblastoma cell line CHP-100.

Propofol inhibits volume-sensitive chloride channels in human coronary artery smooth muscle cells

Volume-sensitive chloride channels (VSCC) play an important role in regulation of cell volume and electrical activity. Activation of vascular smooth muscle VSCC causes smooth muscle depolarization and contraction. We investigated the effects of propofol on VSCC in cultured human coronary artery smooth muscle cells by using the chloride-sensitive dye 6-methoxy-N-ethylquinolinium (MEQ). To activate VSCC, cells were superfused for 2 min with hypotonic gluconate solutions and then potassium thiocyanate solution. The percentage reduction in MEQ fluorescence during 60 s in the presence of potassium thiocyanate was measured and used as an index of VSCC activity. 5-Nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), a well characterized chloride channel blocker, and propofol were dissolved in hypotonic gluconate solution to test their effect on VSCC activity. The reduction in fluorescence was inversely related to osmolality, indicating that activation of VSCC is osmolality dependent. Hypotonic gluconate solution (210 mOsm/kg H(2)O) reduced fluorescence by 38.9% +/- 2.6% of the baseline value. The reduction in fluorescence was dose-dependently inhibited by NPPB. Propofol at 0.3, 1, 3, 10, 30, and 100 micro g/mL significantly inhibited the reduction in fluorescence to 23.6% +/- 4.8%, 19.7% +/- 7.4%, 18.2% +/- 3.5%, 17.6% +/- 5.0%, 15.8% +/- 3.1%, and 10.3% +/- 3.9% of the baseline value, respectively. Our results indicate that propofol inhibits VSCC in a dose-dependent manner in human coronary artery smooth muscle cells.

[Ionic mechanisms underlying the regulation of cell proliferation, differentiation and death]

Ion channels and transporters act as major components that regulate membrane excitability in neurons, muscles, and some secretory glands, but may also contribute to the regulation of proliferation, differentiation, and death in a greater variety of cells including non-excitable ones. The molecular basis of ionic mechanisms underlying the later regulation has been partly identified in the last several years and is a hot issue now. In this short review, some of the molecular mechanisms underlying these regulations and novel compounds acting on the mechanisms were introduced as exciting topics in this area. Several types of transient receptor potential (TRP), identified as Ca(2+)-permeable, non-selective cation channels, may play obligatory roles in functional complexes, which regulate multiple signal transduction pathways triggering proliferation, differentiation, or death of many cell types. In addition, the relation between Cl(-) pump activity and the induction of beta-amyloid protein toxicity for neuronal cell death in Alzheimer disease was described. Unique functions of H(+) channel and pump in osteoclasts in bone mineral homeostasis and remodeling were also discussed. Finally, topics about activation of specific types of Cl(-) channels and K(+) channels, which are responsible for the induction of apoptosis or proliferation in several types of cells, were introduced.

Potentiation of the osmosensitive taurine release and cell volume regulation by cytosolic Ca2+rise in cultured cerebellar astrocytes

Hyposmolarity (-30%) in cultured cerebellar astrocytes raised cytosolic Ca2+ concentration ([Ca2+]i) from 160 to 400 nM and activated the osmosensitive taurine release (OTR) pathway. Although OTR is essentially [Ca2+]i-independent, further increase in [Ca2+]i by ionomycin strongly enhanced OTR, with a more robust effect at low and mild osmolarity reductions. Ionomycin did not affect isosmotic taurine efflux. OTR was decreased by tyrphostin A25 and increased by ortho-vanadate, suggesting a modulation by tyrosine kinase or phosphorylation state. Inhibition of phosphatidylinositol-3-kinase activity by wortmannin markedly decreased OTR and the ionomycin increase. Conversely, OTR and the ionomycin effect were independent of ERK1/ERK2 activation. OTR and its potentiation by ionomycin differed in their sensitivity to CaM and CaMK blockers and in the requirement of an intact cytoskeleton for the ionomycin effect, but not for normal OTR. Changes in the actin cytoskeleton organization elicited by hyposmolarity were not observed in ionomycin-treated cells, which may permit the operation of CaM/CaMK pathways involved in the OTR potentiation by [Ca2+]i rise. OTR potentiation by [Ca2+]i requires the previous or simultaneous activation/operation of the taurine release mechanism and is not modifying its set point, but rather increasing the effectiveness of the pathway, resulting in a more efficient volume regulation. This may have a beneficial effect in pathological situations with concurrent swelling and [Ca2+]i elevation in astrocytes.

BMP-7 counteracts TGF-beta1-induced epithelial-to-mesenchymal transition and reverses chronic renal injury

Bone morphogenic protein (BMP)-7 is a 35-kDa homodimeric protein and a member of the transforming growth factor (TGF)-beta superfamily. BMP-7 expression is highest in the kidney, and its genetic deletion in mice leads to severe impairment of eye, skeletal and kidney development. Here we report that BMP-7 reverses TGF-beta1-induced epithelial-to-mesenchymal transition (EMT) by reinduction of E-cadherin, a key epithelial cell adhesion molecule. Additionally, we provide molecular evidence for Smad-dependent reversal of TGF-beta1-induced EMT by BMP-7 in renal tubular epithelial cells and mammary ductal epithelial cells. In the kidney, EMT-induced accumulation of myofibroblasts and subsequent tubular atrophy are considered key determinants of renal fibrosis during chronic renal injury. We therefore tested the potential of BMP-7 to reverse TGF-beta1-induced de novo EMT in a mouse model of chronic renal injury. Our results show that systemic administration of recombinant human BMP-7 leads to repair of severely damaged renal tubular epithelial cells, in association with reversal of chronic renal injury. Collectively, these results provide evidence of cross talk between BMP-7 and TGF-beta1 in the regulation of EMT in health and disease.

Upregulation of swelling-activated Cl- channel sensitivity to cell volume by activation of EGF receptors in murine mammary cells

Whole-cell recordings showed that, in mouse mammary C127 cells transfected with the full genome of the bovine papilloma virus (BPV), a hypotonic challenge induced the activation of outwardly rectifying Cl- currents with a peak amplitude 2.7 times greater than that in control C127 cells. Cell-attached single-channel recordings showed that BPV-induced augmentation of the peak amplitude of the whole-cell current could not chiefly be explained by a small increase (1.2 times) in unitary conductance. There was no difference between control and BPV-transfected cells in the osmotic cell swelling rate, and hence, osmotic water permeability. However, a plot of the whole-cell current density as a function of cell volume, which was measured simultaneously, showed that the BPV-transfected cells had a strikingly greater volume sensitivity than control cells. Since the E5 protein of BPV has been reported to induce constitutive activation of the epidermal growth factor (EGF) receptor and platelet-derived growth factor (PDGF) receptor in a variety of cell lines including C127 cells, effects of the growth factors on volume-sensitive outwardly rectifying (VSOR) Cl- currents were examined in C127 cells. Application of PDGF peptides failed to affect the Cl- currents in control and BPV-transfected cells, although C127 cells are known to endogenously express PDGF receptors. In contrast, EGF peptides significantly increased the VSOR Cl- current in control cells. However, they failed to induce further augmentation of the current in BPV-transfected cells. VSOR Cl- currents were inhibited by tyrphostin B46, an inhibitor of the EGF receptor tyrosine kinase, in both control and BPV-transfected cells. The IC50 value in BPV-transfected cells (12 micro M) was lower than that in control cells (31 micro M). However, the VSOR Cl- currents in both cell types were insensitive to tyrphostin AG1296, an inhibitor of the PDGF receptor tyrosine kinase. The rate of regulatory volume decrease (RVD) was markedly diminished by tyrphostin B46 but not significantly affected by tyrphostin AG1296. We thus conclude that the EGF receptor tyrosine kinase upregulates the activity of the VSOR Cl- channel, mainly by enhancing the volume sensitivity.

Cell volume changes during rapid temperature shifts

The effect of a rapid temperature increase on the volume of different types of cells was investigated. Experiments were carried out using continuous microscopic image analysis. Volume variation of yeast cells, yeast spheroplasts and human leukaemia cells was measured during the transient phase after a thermal shift. The thermal shift was found to induce rapid increase in cell volume for cells lacking a cell wall (yeast spheroplasts and human leukaemia cells). This increase in cell volume is assumed to be a main cause of the heat shock-induced cell death. A theoretical mechanistic model that explains the behaviour of these cells is finally proposed.

Secretory activation of basolateral membrane Cl- channels in guinea pig distal colonic crypts

Cell-attached recordings revealed Cl(-) channel activity in basolateral membrane of guinea pig distal colonic crypts isolated from basement membrane. Outwardly rectified currents ((gp)Cl(or)) were apparent with a single-channel conductance (gamma) of 29 pS at resting membrane electrical potential; another outward rectifier with gamma of 24 pS was also observed ( approximately 25% of (gp)Cl(or)). At a holding potential of -80 mV gamma was 18 pS for both (gp)Cl(or) currents, and at +80 mV gamma was 67 and 40 pS, respectively. Identity as Cl(-) channels was confirmed in excised patches by changing bath ion composition. From reversal potentials, relative permeability of K(+) over Cl(-) (P(K)/P(Cl)) was 0.07 +/- 0.03, with relative permeability of Na(+) over Cl(-) (P(Na)/P(Cl)) = 0.08 +/- 0.04. A second type of Cl(-) channel was seen with linear current-voltage (I-V) relations ((gp)Cl(L)), having subtypes with gamma of 21, 13, and 8 pS. Epinephrine or forskolin increased the number of open (gp)Cl(or) and (gp)Cl(L). Open probabilities (P(o)) of (gp)Cl(or), (gp)Cl(L21), and (gp)Cl(L13) were voltage dependent in cell-attached patches, higher at more positive potentials. Kinetics of (gp)Cl(or) were more rapid with epinephrine activation than with forskolin activation. Epinephrine increased P(o) at the resting membrane potential for (gp)Cl(L13). Secretagogue activation of these Cl(-) channels may contribute to stimulation of electrogenic K(+) secretion across colonic epithelium by increasing basolateral membrane Cl(-) conductance that permits Cl(-) exit after uptake via Na(+)-K(+)-2Cl(-) cotransport.

Localisation and phenotypical characterisation of collagen-producing cells in TGF-beta 1-induced renal interstitial fibrosis

Transforming growth factor beta 1 (TGF-beta 1) contributes to the accumulation of extracellular matrix (ECM) in the tubulointerstitial space in chronic renal diseases. Identification of target cells and the contribution of epithelial-mesenchymal transformation (EMT) in TGF-beta 1-induced fibrosis in vivo are currently under investigation. We have developed a transgenic model of slowly developing TGF-beta 1-driven tubulointerstitial fibrosis (TIF). By using this model our aim was to localise the ECM-producing cells, to investigate the temporal and spatial distribution of the cellular markers alpha-smooth muscle cell actin (alpha SM-actin), Fsp1 and Hsp47 and to explore the possible involvement of EMT in TGF-beta1-induced TIF in vivo. We utilised a combination of in situ hybridisation, immunohistochemistry and western blotting techniques and found that alpha SM-actin-positive interstitial cells are the main source of collagen types I and III and fibronectin, whereas collagen type IV(alpha 1/alpha 2) originates mainly from the tubular epithelial cells. Furthermore, macrophages are not important combatants during the early course of TGF-beta 1-induced TIF. Finally, EMT is not necessary for the initiation of TGF-beta 1-induced TIF. We conclude, that intervention directed against the recruitment of activated interstitial cells may avoid the development of end-stage renal disease.

Amino acid-permeable anion channels in early mouse embryos and their possible effects on cleavage

Effects of several Cl(-) channel blockers on ionic currents in mouse embryos were studied using whole-cell patch-clamp and microelectrode methods. Microelectrode measurements showed that the resting membrane potential of early embryonic cells (1-cell stage) was -23 mV and that reduction of extracellular Cl(-) concentration depolarized the membrane, suggesting that Cl(-) conductance is a major contributor for establishing the resting membrane potential. Membrane currents recorded by whole-cell voltage clamp showed outward rectification and confirmed that a major component of these embryonic currents are carried by Cl(-) ions. A Cl(-) channel blocker, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), suppressed the outward rectifier current in a voltage- and concentration-dependent manner. Other Cl(-) channel blockers (5-nitro-2-[3-phenylpropyl-amino] benzoic acid and 2-[3-(trifluoromethyl)-anilino] nicotinic acid [niflumic acid]) similarly inhibited this current. Simultaneous application of niflumic acid with DIDS further suppressed the outward rectifier current. Under high osmotic condition, niflumic acid, but not DIDS, inhibited the Cl(-)current, suggesting the presence of two types of Cl(-) channels: a DIDS-sensitive (swelling-activated) channel, and a DIDS-insensitive (niflumic acid-sensitive) Cl(-) channel. Anion permeability of the DIDS-insensitive Cl(-) current differed from that of the compound Cl(-) current: Rank order of anion permeability of the DIDS-sensitive Cl(-) channels was I(-) = Br(-) > Cl(-) > gluconate(-), whereas that of the DIDS-insensitive Cl(-) channel was I(-) = Br(-) > Cl(-) >> gluconate(-). These results indicate that early mouse embryos have a Cl(-) channel that is highly permeable to amino acids, which may regulate intracellular amino acid concentration.

Recovery from lactacidosis-induced glial cell swelling with the aid of exogenous anion channels

Hypotonic challenge induces transient swelling in glial cells, which is typically followed by a regulatory volume decrease (RVD). In contrast, lactic acidosis (lactacidosis) induces persistent cell swelling in astrocytes without an accompanying RVD. In the present study, we studied the mechanisms by which lactacidosis interferes with normal volume regulation in rat astrocytic glioma C6 cells. Following exposure of C6 cells to a hypotonic challenge, a current was detected that exhibited properties consistent with those of volume-sensitive outwardly rectifying (VSOR) anion channels. When exposed to in vitro conditions designed to simulate lactacidosis, C6 cells failed to respond to hypotonic stress with an RVD, and VSOR anion currents were not activated. When added to C6 cells, an anion channel-forming protein purified from Helicobacter pylori, VacA, was found to form anion-selective channels in the plasma membrane, and the activity of the VacA channel was not affected by lactacidosis (pH 6.2). Cells preincubated with VacA and then exposed to lactacidotic conditions underwent transient swelling followed by RVD. In contrast, application of a cation ionophore, gramicidin, failed to inhibit lactacidosis-induced persistent cell swelling. From these results, we conclude that inhibition of a volume-sensitive anion channel contributes to persistent swelling induced by lactacidosis in glial cells. Introduction of anion channel activity into glial cells might provide a novel approach for treating cerebral edema, which is associated with lactacidosis in cerebral ischemia or head injury.

Amazing chloride channels: an overview

AIM

This review describes molecular and functional properties of the following Cl- channels: the ClC family of voltage-dependent Cl- channels, the cAMP-activated transmembrane conductance regulator (CFTR), Ca2+ activated Cl- channels (CaCC) and volume-regulated anion channels (VRAC). If structural data are available, their relationship with the function of Cl- channels will be discussed. We also describe shortly some recently discovered channels, including high conductance Cl- channels and the family of bestrophins. We illustrate the growing physiological importance of these channels in the plasma membrane and in intracellular membranes, including their involvement in transepithelial transport, pH regulation of intracellular organelles, regulation of excitability and volume regulation. Finally, we discuss the role of Cl- channels in various diseases and describe the pathological phenotypes observed in knockout mice models.

Effects of osmotic changes on the chemoreceptor cell of rat carotid body

The carotid body plays a crucial role in cardiorespiratory regulation. In the present study we investigated the effect of osmotic changes on cytoplasmic calcium concentration ([Ca(2+)](c)) and pH (pH(i)) of isolated chemoreceptor cells of the rat carotid body. In CO(2)/HCO(3)(-)-buffered medium, reduction of osmolality from the control level of 300 mosmol kg(-1) to 250-285 mosmol kg(-1) resulted in a rise in [Ca(2+)](c), as measured with Indo-1, whereas elevation of osmolality to 350 mosmol kg(-1) had no effect. The Ca(2+) response required extracellular Ca(2+) and was reduced by application of the L-type Ca(2+) channel antagonist nifedipine (10 microM). The hyposmosis-induced Ca(2+) response could be prevented by application of niflumic acid (300 microM), an inhibitor of the swelling-activated Cl(-) channel. In whole-cell patch-clamp experiments niflumic acid abolished the swelling-activated Cl(-) current but only slightly depressed the Ca(2+) current. The inhibition of Ca(2+) current by niflumic acid does not account for its action in preventing of hyposmosis-induced Ca(2+) response, which seems to be initiated by Cl(-)-mediated depolarisation. Withdrawal of CO(2)/HCO(3)(-) also prevented the Ca(2+) response. Reduction of the osmotic concentration by 50 mosmol kg(-1) induced a small but sustained decrease in pH(i), while elevation by 50 mosmol kg(-1) had an inverse effect, as measured fluorimetrically with carboxy SNARF-1. Our conclusion is that in the rat chemoreceptor cell the activation of Cl(-) channels, e.g. by hyposmotic challenge, induces depolarisation, which, in turn, activates voltage-gated Ca(2+) channels.

IK channels are involved in the regulatory volume decrease in human epithelial cells

Parallel activation of Ca(2+)-dependent K(+) channels and volume-sensitive Cl(-) channels is known to be responsible for KCl efflux during regulatory volume decrease (RVD) in human epithelial Intestine 407 cells. The present study was performed to identify the K(+) channel type. RT-PCR demonstrated mRNA expression of Ca(2+)-activated, intermediate conductance K(+) (IK), but not small conductance K(+) (SK1) or large conductance K(+) (BK) channels in this cell line. Whole cell recordings showed that ionomycin or hypotonic stress activated inwardly rectifying K(+) currents that were reversibly blocked by IK channel blockers [clotrimazole (CLT) and charybdotoxin] but not by SK and BK channel blockers (apamin and iberiotoxin). Inside-out recordings revealed the existence of CLT-sensitive single K(+)-channel activity, which exhibited an intermediate unitary conductance (30 pS at -100 mV). The channel was activated by cytosolic Ca(2+) in inside-out patches and by a hypotonic challenge in cell-attached patches. The RVD was suppressed by CLT, but not by apamin or iberiotoxin. Thus we conclude that the IK channel is involved in the RVD process in these human epithelial cells.

Impaired activity of volume-sensitive anion channel during lactacidosis-induced swelling in neuronally differentiated NG108-15 cells

Acidosis coupled to lactate accumulation, called lactacidosis, occurs in cerebral ischemia or trauma and is known to cause persistent swelling in neuronal and glial cells. It is therefore possible that mechanisms of cell volume regulation are impaired during lactacidosis. Here we tested this possibility using neuronally differentiated NG108-15 cells. These cells responded to a hypotonic challenge with osmotic swelling followed by a regulatory volume decrease (RVD) under physiological pH conditions in the absence of lactate. Under normotonic conditions, sustained cell swelling without subsequent RVD was induced by exposure to lactate-containing solution with acidic pH (6.4 or 6.2), but not with physiological pH (7.4). Under whole-cell patch-clamp, osmotic swelling was found to activate outwardly rectifying Cl(-) currents in cells exposed to control hypotonic solution. A Cl(-) channel blocker, NPPB, inhibited both RVD and the swelling-activated Cl(-) current. RVD and the volume-sensitive Cl(-) current were also markedly inhibited by lactacidosis (pH 6.4 or 6.2), but neither by application of lactate with physiological pH (7.4) nor by acidification without lactate (pH 6.2). RT-PCR analysis showed mRNA expression of two isoforms of proton-coupled monocarboxylate transporters, MCT1 and MCT8, in differentiated NG108-15 cells. Thus, we conclude that persistence of neuronal cell swelling under lactacidosis is coupled to an impairment of the activity of the volume-sensitive Cl(-) channel and to dysfunction of RVD. It is also suggested that the volume-sensitive Cl(-) channel is inhibited by intracellular acidification induced by MCT-mediated proton influx under lactacidosis.

Maxi K+ channel mediates regulatory volume decrease response in a human bronchial epithelial cell line

The cell regulatory volume decrease (RVD) response triggered by hypotonic solutions is mainly achieved by the coordinated activity of Cl- and K+ channels. We now describe the molecular nature of the K(+) channels involved in the RVD response of the human bronchial epithelial (HBE) cell line 16HBE14o-. These cells, under isotonic conditions, present a K+ current consistent with the activity of maxi K+ channels, confirmed by RT-PCR and Western blot. Single-channel and whole cell maxi K+ currents were readily and reversibly activated following the exposure of HBE cells to a 28% hypotonic solution. Both maxi K+ current activation and RVD response showed calcium dependency, inhibition by TEA, Ba2+, iberiotoxin, and the cationic channel blocker Gd3+ but were insensitive to clofilium, clotrimazole, and apamin. The presence of the recently cloned swelling-activated, Gd3+-sensitive cation channels (TRPV4, also known as OTRPC4, TRP12, or VR-OAC) was detected by RT-PCR in HBE cells. This channel, TRPV4, which senses changes in volume, might provide the pathway for Ca2+ influx under hypotonic solutions and, consequently, for the activation of maxi K+ channels.

Regulation of volume-sensitive outwardly rectifying anion channels in pulmonary arterial smooth muscle cells by PKC

We tested the possible role of endogenous protein kinase C (PKC) in the regulation of native volume-sensitive organic osmolyte and anion channels (VSOACs) in acutely dispersed canine pulmonary artery smooth muscle cells (PASMC). Hypotonic cell swelling activated native volume-regulated Cl(-) currents (I(Cl.vol)) which could be reversed by exposure to phorbol 12,13-dibutyrate (0.1 microM) or by hypertonic cell shrinkage. Under isotonic conditions, calphostin C (0.1 microM) or Ro-31-8425 (0.1 microM), inhibitors of both conventional and novel PKC isozymes, significantly activated I(Cl.vol) and prevented further modulation by subsequent hypotonic cell swelling. Bisindolylmaleimide (0.1 microM), a selective conventional PKC inhibitor, was without effect. Dialyzing acutely dispersed and cultured PASMC with epsilon V1-2 (10 microM), a translocation inhibitory peptide derived from the V1 region of epsilon PKC, activated I(Cl.vol) under isotonic conditions and prevented further modulation by cell volume changes. Dialyzing PASMC with beta C2-2 (10 microM), a translocation inhibitory peptide derived from the C2 region of beta PKC, had no detectable effect. Immunohistochemistry in cultured canine PASMC verified that hypotonic cell swelling is accompanied by translocation of epsilon PKC from the vicinity of the membrane to cytoplasmic and perinuclear locations. These data suggest that membrane-bound epsilon PKC controls the activation state of native VSOACs in canine PASMC under isotonic and anisotonic conditions.

Osmotic shrinkage of human cervical cancer cells induces an extracellular Cl- -dependent nonselective cation channel, which requires p38 MAPK

This study is to integrate a functional role of nonselective cation (NSC) channels into a model of volume regulation on osmotic shrinkage for human cervical cancer cells. Application of a hypertonic solution (400 mosm kg(-1)) induced cell shrinkage, which was accompanied by a 7-fold increase of inward currents at -80 mV from -4.1 +/- 0.4 pA pF(-1) to -29 +/- 1.1 pA pF(-1) (n = 36, p < 0.001). There is a good correlation of channel activity and cell volume changes. Replacement of bath Na(+) by K(+), Cs(+), Li(+), or Rb(+) did not affect the stimulated inward current significantly, but replacement by Ca(2+), Ba(2+), or the impermeable cation N-methyl-d-glucamine abolished the inward current; this demonstrates that the shrinkage-induced currents discriminate poorly between monovalent cations but are not carried by divalent cations. Replacement of extracellular Cl(-) by gluconate abolished the shrinkage-induced currents in a concentration-dependent manner without changing the reversal potential. Gadolinium (Gd(3+)) inhibited the stimulated current, whereas bumetanide and amiloride had no inhibitory effect. Cell shrinkage triggered mitogen-activated protein (MAP) kinase cascades leading to the activation of MAP/extracellular signal-regulated kinase 1/2 (ERK1/2) kinase (MEK1/2), and p38 kinase. Interference with p38 MAPK by either the specific inhibitor (SB202190), or a dominant-negative mutant profoundly suppressed the activation of the shrinkage-induced NSC channels. In contrast, the regulatory mechanism of shrinkage-induced NSC channels was independent of the volume-responsive MEK1/2 signaling pathway. More importantly, the cell volume response to hypertonicity was inhibited significantly in p38 dominant-negative mutant or by SB202190. Therefore, p38 MAPK is critically involved in the activation of a shrinkage-induced NSC channel, which plays an important role in the volume regulation of human cervical cancer cells.

Control of Cl- transport in the operculum epithelium of Fundulus heteroclitus: long- and short-term salinity adaptation

The eurohaline fish, Fundulus heteroclitus, adapts rapidly to enhanced salinity by increasing the ion secretion by gill chloride cells. An increase of approximately 70 mOsm in plasma osmolarity was previously found during the transition. To mimic this in vitro, isolated opercular epithelia of seawater-adapted Fundulus mounted in a modified Ussing chamber were exposed to an increase in NaCl and/or osmolarity on the basolateral side, which immediately increased I(SC). Various Cl(-) channel blockers as well as the K(+) channel blocker Ba(2+) added to the basolateral side all inhibited the steady-state as well as the hypertonic stimulation of I(SC). The exists -agonist isoproterenol stimulates I(SC) in standard Ringer solutions. In contrast, when cell volume was kept at the larger value by simultaneous addition of water, the stimulation with isoproterenol was abolished, suggesting that the key process for activation of the Na(+), K(+), 2Cl(-) cotransporter is cell shrinkage. The protein kinase C (PKC) inhibitor chelerythrine and the myosin light chain kinase (MLCK) inhibitor ML-7 had strong inhibitory effects on the mannitol activation of I(SC), thus both MLCK and PKC are involved. The two specific protein kinase A (PKA) inhibitors H-89 and KT 5720 had no effect after mannitol addition whereas isoproterenol stimulation was completely blocked by H-89. This indicates that PKA is involved in the activation of the apical Cl(-) channel via c-AMP whereas the shrinkage activation of the Na(+), K(+), 2Cl(-) cotransporter is independent of PKA activation. The steady-state Cl(-) secretion was stimulated by an inhibitor of serine/threonine phosphatases of the PP-1 and PP-2A type and inhibited by a PKC inhibitor but not by a PKA inhibitor. Thus, it seems to be determined by continuous phosphorylation and dephosphorylation involving PKC but not PKA. The steady-state Cl(-) secretion and the maximal obtainable Cl(-) secretion were measured in freshwater-adapted fish and in fish retransferred to saltwater. No I(SC) could be measured in freshwater-adapted fish or in the fish within the first 18 h after transfer to saltwater. As evidenced from Western blot analysis using antiserine-antibodies, a heavily serine phosphorylated protein of about 190 kDa was consistently observed in the saltwater-acclimated fish, but was only weakly present in freshwater-acclimated fish. This observation indicates that acclimatization to saltwater stimulates the expression of this 190-kDa protein and/or a serine/threonine kinase, which subsequently phosphorylates the protein.

ClC-3 is a fundamental molecular component of volume-sensitive outwardly rectifying Cl- channels and volume regulation in HeLa cells and Xenopus laevis oocytes

Volume-sensitive osmolyte and anion channels (VSOACs) are activated upon cell swelling in most vertebrate cells. Native VSOACs are believed to be a major pathway for regulatory volume decrease (RVD) through efflux of chloride and organic osmolytes. ClC-3 has been proposed to encode native VSOACs in Xenopus laevis oocytes and in some mammalian cells, including cardiac and vascular smooth muscle cells. The relationship between the ClC-3 chloride channel, the native volume-sensitive osmolyte and anion channel (VSOAC) currents, and cell volume regulation in HeLa cells and X. laevis oocytes was investigated using ClC-3 antisense. In situ hybridization in HeLa cells, semiquantitative and real-time PCR, and immunoblot studies in HeLa cells and X. laevis oocytes demonstrated the presence of ClC-3 mRNA and protein, respectively. Exposing both cell types to hypotonic solutions induced cell swelling and activated native VSOACs. Transient transfection of HeLa cells with ClC-3 antisense oligonucleotide or X. laevis oocytes injected with antisense cRNA abolished the native ClC-3 mRNA transcript and protein and significantly reduced the density of native VSOACs activated by hypotonically induced cell swelling. In addition, antisense against native ClC-3 significantly impaired the ability of HeLa cells and X. laevis oocytes to regulate their volume. These results suggest that ClC-3 is an important molecular component underlying VSOACs and the RVD process in HeLa cells and X. laevis oocytes.

Cell cycle-dependent expression of volume-activated chloride currents in nasopharyngeal carcinoma cells

Patch-clamping and cell image analysis techniques were used to study the expression of the volume-activated Cl(-) current, I(Cl(vol)), and regulatory volume decrease (RVD) capacity in the cell cycle in nasopharyngeal carcinoma cells (CNE-2Z). Hypotonic challenge caused CNE-2Z cells to swell and activated a Cl(-) current with a linear conductance, negligible time-dependent inactivation, and a reversal potential close to the Cl(-) equilibrium potential. The sequence of anion permeability was I(-) > Br(-) > Cl(-) > gluconate. The Cl(-) channel blockers tamoxifen, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), and ATP inhibited I(Cl(vol)). Synchronous cultures of cells were obtained by the mitotic shake-off technique and by a double chemical-block (thymidine and hydroxyurea) technique. The expression of I(Cl(vol)) was cell cycle dependent, being high in G(1) phase, downregulated in S phase, but increasing again in M phase. Hypotonic solution activated RVD, which was cell cycle dependent and inhibited by the Cl(-) channel blockers NPPB, tamoxifen, and ATP. The expression of I(Cl(vol)) was closely correlated with the RVD capacity in the cell cycle, suggesting a functional relationship. Inhibition of I(Cl(vol)) by NPPB (100 microM) arrested cells in G(0)/G(1). The data also suggest that expression of I(Cl(vol)) and RVD capacity are actively modulated during the cell cycle. The volume-activated Cl(-) current associated with RVD may therefore play an important role during the cell cycle progress.

Regulatory volume decrease is actively modulated during the cell cycle

Nasopharyngeal carcinoma cells, CNE-2Z, when swollen by 47% hypotonic solution, exhibited a regulatory volume decrease (RVD). The RVD was inhibited by extracellular applications of the chloride channel blockers tamoxifen (30 microM; 61% inhibition), 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 100 microM; 60% inhibition), and ATP (10 mM; 91% inhibition). The level and time constant of RVD varied greatly between cells. Most cells conducted an incomplete RVD, but a few had the ability to recover their volume completely. There was no obvious correlation between cell volume and RVD capacity. Flow cytometric analysis showed that highly synchronous cells were obtained by the mitotic shake-off technique and that the cells progressed through the cell cycle synchronously when incubated in culture medium. Combined application of DNA synthesis inhibitors, thymidine and hydroxyurea arrested cells at the G1/S boundary and 87% of the cells reached S phase 4 h after being released. RVD capacity changed significantly during the cell cycle progression in cells synchronized by shake-off technique. RVD capacity being at its highest in G1 phase and lowest in S phase. The RVD capacity in G1 (shake-off cells sampled after 4 h of incubation), S (obtained by chemical arrest), and M cells (selected under microscope) was 73, 33, and 58%, respectively, and the time constants were 435, 769, and 2,000 sec, respectively. We conclude that RVD capacity is actively modulated in the cell cycle and RVD may play an important role in cell cycle progress.

Evidence that fibroblasts derive from epithelium during tissue fibrosis

Interstitial fibroblasts are principal effector cells of organ fibrosis in kidneys, lungs, and liver. While some view fibroblasts in adult tissues as nothing more than primitive mesenchymal cells surviving embryologic development, they differ from mesenchymal cells in their unique expression of fibroblast-specific protein-1 (FSP1). This difference raises questions about their origin. Using bone marrow chimeras and transgenic reporter mice, we show here that interstitial kidney fibroblasts derive from two sources. A small number of FSP1(+), CD34(-) fibroblasts migrate to normal interstitial spaces from bone marrow. More surprisingly, however, FSP1(+) fibroblasts also arise in large numbers by local epithelial-mesenchymal transition (EMT) during renal fibrogenesis. Both populations of fibroblasts express collagen type I and expand by cell division during tissue fibrosis. Our findings suggest that a substantial number of organ fibroblasts appear through a novel reversal in the direction of epithelial cell fate. As a general mechanism, this change in fate highlights the potential plasticity of differentiated cells in adult tissues under pathologic conditions.

Human salivary histatin 5 causes disordered volume regulation and cell cycle arrest in Candida albicans

Human salivary histatin 5 (Hst 5) is a nonimmune salivary protein with antifungal activity against an important human pathogen, Candida albicans. The candidacidal activity of histatins appears to be a distinctive multistep mechanism involving depletion of the C. albicans intracellular ATP content as a result of nonlytic ATP efflux. Hst 5 caused a loss of cell viability concomitant with a decrease in cellular volume as determined both by a classical candidacidal assay with exogenous Hst 5 and by using a genetically engineered C. albicans strain expressing Hst 5. Preincubation of C. albicans cells with pharmacological inhibitors of anion transport provided complete or substantial protection from Hst 5-induced killing and volume reduction of cells. Moreover, intracellular expression of Hst 5 resulted in a reduction in the population mean cell volume that was accompanied by an increase in the percentage of unbudded cells and C. albicans cells in the G(1) phase. Following expression of Hst 5, the smallest cells sorted by fluorescence-activated cell sorting from the total population did not replicate and were exclusively in the G(1) phase. Cells with intracellularly expressed Hst 5 had greatly reduced G(1) cyclin transcript levels, indicating that they arrested in the G(1) phase before the onset of Start. Our data demonstrate that a key determinant in the mechanism of Hst 5 toxicity in C. albicans cells is the disruption of regulatory circuits for cell volume homeostasis that is closely coupled with loss of intracellular ATP. This novel process of fungicidal activity by a human salivary protein has highlighted potential interactions of Hst 5 with volume regulatory mechanisms and the process of yeast cell cycle control.

Cell cycle- and swelling-induced activation of a Caenorhabditis elegans ClC channel is mediated by CeGLC-7alpha/beta phosphatases

ClC voltage-gated anion channels have been identified in bacteria, yeast, plants, and animals. The biophysical and structural properties of ClCs have been studied extensively, but relatively little is known about their precise physiological functions. Furthermore, virtually nothing is known about the signaling pathways and molecular mechanisms that regulate channel activity. The nematode Caenorhabditis elegans provides significant experimental advantages for characterizing ion channel function and regulation. We have shown previously that the ClC Cl- channel homologue CLH-3 is expressed in C. elegans oocytes, and that it is activated during meiotic maturation and by cell swelling. We demonstrate here that depletion of intracellular ATP or removal of Mg2+, experimental maneuvers that inhibit kinase function, constitutively activate CLH-3. Maturation- and swelling-induced channel activation are inhibited by type 1 serine/threonine phosphatase inhibitors. RNA interference studies demonstrated that the type 1 protein phosphatases CeGLC-7alpha and beta, both of which play essential regulatory roles in mitotic and meiotic cell cycle events, mediate CLH-3 activation. We have suggested previously that CLH-3 and mammalian ClC-2 are orthologues that play important roles in heterologous cell-cell interactions, intercellular communication, and regulation of cell cycle-dependent physiological processes. Consistent with this hypothesis, we show that heterologously expressed rat ClC-2 is also activated by serine/threonine dephosphorylation, suggesting that the two channels have common regulatory mechanisms.

Peroxynitrite enhances astrocytic volume-sensitive excitatory amino acid release via a src tyrosine kinase-dependent mechanism

Volume-regulated anion channels (VRACs) are critically important for cell volume homeostasis, and under pathological conditions contribute to neuronal damage via excitatory amino (EAA) release. The precise mechanisms by which brain VRACs are activated and/or modulated remain elusive. In the present work we explored the possible involvement of nitric oxide (NO) and NO-related reactive species in the regulation of VRAC activity and EAA release, using primary astrocyte cultures. The NO donors sodium nitroprusside and spermine NONOate did not affect volume-activated d-[3H]aspartate release. In contrast, the peroxynitrite (ONOO-) donor 3-morpholinosydnomine hydrochloride (SIN-1) increased volume-dependent EAA release by approx. 80-110% under identical conditions. Inhibition of ONOO- formation with superoxide dismutase completely abolished the effects of SIN-1. Both the volume- and SIN-1-induced EAA release were sensitive to the VRAC blockers NPPB and ATP. Further pharmacological analysis ruled out the involvement of cGMP-dependent reactions and modification of sulfhydryl groups in the SIN-1-inducedmodulation of EAA release. The src family tyrosine kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo [3,4-d]pyrimidine (PP2), but not its inactive analog PP3, abolished the effects of SIN-1. A broader spectrum tyrosine kinase inhibitor tyrphostin A51, also completely eliminated the SIN-1-induced EAA release. Our data suggest that ONOO- up-regulates VRAC activity via a src tyrosine kinase-dependent mechanism. This modulation may contribute to EAA-mediated neuronal damage in ischemia and other pathological conditions favoring cell swelling and ONOO- production.

Proteolytic modification of swelling-activated Cl- current in LNCaP prostate cancer epithelial cells

The effects of intracellular application of trypsin on the Cl- current induced by hypotonic cell swelling (I(Cl,swell)) in human prostate cancer epithelial cells (LNCaP) was studied using the patch-clamp technique. In cells predialyzed with 1 mg/mL trypsin, I(Cl,swell)) developed and diminished in response to the application and withdrawal of hypotonic solution about three times faster than that in control cells. In trypsin-infused cells, I(Cl,swell)) also had about twofold higher current density and displayed considerably slowed voltage-dependent inactivation, which was quite pronounced in control cells at potentials above +60 mV. Trypsin-induced modification of I(Cl,swell)) could be prevented by coinfusion of 10 mg/mL soybean trypsin inhibitor, suggesting that proteolytic cleavage of essential intracellular structural domains of the I(Cl,swell))-carrying volume-regulated anion channel (VRAC) was responsible for this functional modification. The effect of trypsin was not dependent on the presence of intracellular ATP. We conclude that VRACs, similarly to voltage-gated Na+, K+, and Cl- channels, possess intracellular inactivation domain(s) subjected to proteolytic cleavage that may function in conformity with the classical "ball-and-chain" inactivation model.