F-actin modulates swelling-activated chloride current in cultured chick cardiac myocytes

The Important Role of Ion Transport System in Cervical Cancer

Cervical cancer is a significant gynecological cancer and causes cancer-related deaths worldwide. Human papillomavirus (HPV) is implicated in the etiology of cervical malignancy. However, much evidence indicates that HPV infection is a necessary but not sufficient cause in cervical carcinogenesis. Therefore, the cellular pathophysiology of cervical cancer is worthy of study. This review summarizes the recent findings concerning the ion transport processes involved in cell volume regulation and intracellular Ca²⁺ homeostasis of epithelial cells and how these transport systems are themselves regulated by the tumor microenvironment. For cell volume regulation, we focused on the volume-sensitive Cl⁻ channels and K⁺-Cl⁻ cotransporter (KCC) family, important regulators for ionic and osmotic homeostasis of epithelial cells. Regarding intracellular Ca²⁺ homeostasis, the Ca²⁺ store sensor STIM molecules and plasma membrane Ca²⁺ channel Orai proteins, the predominant Ca²⁺ entry mechanism in epithelial cells, are discussed. Furthermore, we evaluate the potential of these membrane ion transport systems as diagnostic biomarkers and pharmacological interventions and highlight the challenges.

The Relationship Between Actin Cytoskeleton and Membrane Transporters in Cisplatin Resistance of Cancer Cells

Cisplatin [cis-diamminedichloroplatinum (II)] is a platinum-based anticancer drug widely used for the treatment of various cancers. It forms interstrand and intrastrand cross-linking with DNA and block DNA replication, resulting in apoptosis. On the other hand, intrinsic and acquired cisplatin resistance restricts its therapeutic effects. Although some studies suggest that dramatic epigenetic alternations are involved in the resistance triggered by cisplatin, the mechanism is complicated and remains poorly understood. Recent studies reported that cytoskeletal structures regulate cisplatin sensitivity and that activities of membrane transporters contribute to the development of resistance to cisplatin. Therefore, we focus on the roles of actin filaments and membrane transporters in cisplatin-induced apoptosis. In this review, we summarize the relationship between actin cytoskeleton and membrane transporters in the cisplatin resistance of cancer cells.

Impaired actin filaments decrease cisplatin sensitivity via dysfunction of volume-sensitive Cl- channels in human epidermoid carcinoma cells

Cisplatin is a widely used platinum-based anticancer drug in the chemotherapy of numerous human cancers. However, cancer cells acquire resistance to cisplatin. So far, functional loss of volume-sensitive outwardly rectifying (VSOR) Cl^- channels has been reported to contribute to cisplatin resistance of cancer cells. Here, we analyzed protein expression patterns of human epidermoid carcinoma KB cells and its cisplatin-resistant KCP-4 cells. Intriguingly, KB cells exhibited higher β-actin expression and clearer actin filaments than KCP-4 cells. The β-actin knockdown in KB cells decreased VSOR Cl^- currents and inhibited the regulatory volume decrease (RVD) process after cell swelling. Consistently, KB cells treated with cytochalasin D, which depolymerizes actin filaments, showed smaller VSOR Cl^- currents and slower RVD. Cytochalasin D also inhibited cisplatin-triggered apoptosis in KB cells. These results suggest that the disruption of actin filaments cause the dysfunction of VSOR Cl^- channels, which elicits resistance to cisplatin in human epidermoid carcinoma cells.

Reconstituted HDL for the acute treatment of acute coronary syndrome

PURPOSE OF REVIEW

New therapeutic strategies are needed for the rapid stabilization of acute coronary syndrome (ACS) patients by treating nonculprit lesions. Reconstituted HDL (rHDL), which is apoA-I combined with phospholipids, is currently being tested in clinical trials for this purpose and is the subject of this review.

RECENT FINDINGS

At least four different formulations (SRC-rHDL, CSL-111, CSL-112 and ETC-216) have been tested in clinical trials. The various rHDL preparations have been shown to be effective in the rapid mobilization of excess cholesterol from cells and in regressing atherosclerotic plaques in animal models. Two of the rHDL agents, namely ETC-216 and CSL-111, have been shown to be effective after only a few treatments in reducing plaque volume in ACS patients, as assessed by intravascular ultrasound, but no clinical trials assessing clinical endpoints have yet been completed.

SUMMARY

rHDL is a promising new potential therapy for ACS patients, but much work remains to be done, and there are many unresolved questions. Progress in developing rHDL into a therapy will depend on improving our understanding of their mechanism of action, determining the optimum formulation and delivery and how to monitor rHDL therapy.

Hypotonic activation of volume-sensitive outwardly rectifying anion channels (VSOACs) requires coordinated remodeling of subcortical and perinuclear actin filaments

Cell volume regulation requires activation of volume-sensitive outwardly rectifying anion channels (VSOACs). The actin cytoskeleton may participate in the activation of VSOACs but the roles of the two major actin pools remain undefined. We hypothesized that structural reorganization of both subcortical and perinuclear actin filaments (F-actin) contributes to the hypotonic activation of VSOACs. Hypotonic stress of pulmonary artery smooth muscle cells (PASMCs) was associated with reorganization of both peripheral and perinuclear F-actin, and with activation of VSOACs. Preincubation with cytochalasin D caused prominent dissociation of perinuclear, but not of subcortical F-actin. Cytochalasin D failed to induce isotonic activation and delayed the hypotonic activation of VSOACs. F-actin stabilization by phalloidin delayed both the hypotonic stress-induced dissociation of membrane-associated actin filaments and the activation kinetics of VSOACs. PKCepsilon, which was proposed to phosphorylate and inhibit VSOACs, colocalized primarily with F-actin and the net kinase activity remained unchanged during hypotonic cell swelling. In conclusion, normal hypotonic activation of VSOACs requires disruption of peripheral F-actin but intact perinuclear F-actin; interference with this pattern of actin reorganization delays the activation kinetics of VSOACs. The cell swelling-induced peripheral actin dissociation may underlie the observed translocation of PKCepsilon, which leads to a net decrease of PKCepsilon inhibitory activity in submembranous sites. Thus, reorganization of actin and PKCepsilon may establish conditions for mechano- and/or signal transduction-mediated activation of VSOACs.

High-density lipoproteins: an emerging target in the prevention of cardiovascular disease

High-density lipoproteins (HDLs) have been well established to protect against the development of atherosclerotic cardiovascular disease. It has become apparent that in addition to the promotion of reverse cholesterol transport, HDLs possess a number of additional functional properties that may contribute to their beneficial influence on the arterial wall. A number of exciting therapeutic strategies have been developed that target HDL and its ability to protect against the development of atherosclerotic plaque. This paper will review how the promotion of the functional properties of HDL inhibits the formation of atherosclerotic plaque and stabilises lesions in patients with established disease.

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.

Oxidized low-density lipoprotein inhibits endothelium-dependent vasodilation by an antioxidant-sensitive, lysophosphatidylcholine-independent mechanism

Previous studies have shown that oxidized low-density lipoprotein (LDL) can impair endothelial function and that this can be overcome in vivo by administration of vitamin E. However, it is unclear whether this effect of oxidized LDL is due to lysophosphatidylcholine or other components of oxidized LDL, and it is also uncertain if the protective effect of vitamin E is related to its antioxidant action. The objectives of the current study were to define how much of the effect of extensively oxidized LDL on endothelium-dependent relaxation (EDR) was in fact due to lysophosphatidylcholine, to determine if the effect of oxidized LDL involved oxidant stress to the endothelium, and, if so, to ascertain if this could be blocked by oxyradical scavengers or antioxidants. Endothelial function was assessed by measuring vasodilation in preconstricted rat mesenteric artery rings in response to acetylcholine. In the presence of 100 microg/mL oxidized LDL, 25-fold higher concentrations of acetylcholine were required for the same degree of vasorelaxation. Similar concentrations of native LDL or acetyl LDL had no effect, but 100 microg/mL phospholipase A(2)-treated LDL or 20 microM lysophosphatidylcholine produced a similar inhibition of EDR. Removal of more than 90% of lysophosphatidylcholine from oxidized LDL did not affect its ability to inhibit EDR, nor did treatment of oxidized LDL with borohydride. This effect of oxidized LDL was blocked by preincubation of arterial rings with vitamin E, probucol, or BO-653 (a potent lipophilic antioxidant), but not by superoxide dismutase. In contrast, the inhibition of EDR by lysophosphatidylcholine was unaffected by antioxidants. Calphostin C prevented the inhibition of EDR by oxidized LDL and lysophosphatidylcholine. These studies demonstrate that at least part of the effect of oxidized LDL on EDR is independent of lysophosphatidylcholine, lipid peroxides, and superoxide release but appears to involve intracellular oxidative stress and protein kinase C activation.

Regulation of swelling-activated chloride channels in embryonic chick heart cells

Swelling-activated Cl- currents, I(Cl,swell) were measured during hyposmotic shock in white Leghorn embryonic chick heart cells using the whole-cell recording of patch-clamp technique. Genistein, an inhibitor of protein tyrosine kinase (PTK), suppressed I(Cl,swell). Under isosmotic condition phorbol 12-myristate 13-acetate (PMA), an activator of PKC, elicited the Cl- current similar to that in hyposmotic solution, whereas hyposmotic shock did not elicit I(Cl,swell) in chelerythrine chloride(an inhibitor of PKC)-treated cells. Confocal microscopy experiments using FITC-phalloidin as a fluorescent label of F-actin showed that the actin network was moved from cortical region of the cell to the center after hyposmotic shock as compared with the image under isosmotic condition. When the cells were treated with cytochalasin B (CB) or cytochalasin D (CD) under isosmotic condition the disruption of the F-actin integrity was observed, and I(Cl,swell) was not elicited. With combination treatment of CB with PMA, hyposmotic solution could not elicited I(Cl,swell). The results suggested that the role of PTK, probably receptor tyrosine kinase, for regulation of I(Cl,swell) appeared to be at upstream site related to the role of F-actin. Then PKC signal pathway was activated somehow and finally change in the polymerization state of cytoskeleton led to activate the swelling-activated Cl- channels. These results demonstrate clearly that PTK, PKC and F-actin are important factors for regulation of I(Cl,swell), in embryonic chick heart cells as compared with often controversial results reported in different cell types.

Keratin mutations of epidermolysis bullosa simplex alter the kinetics of stress response to osmotic shock

The intermediate filament cytoskeleton is thought to confer physical resilience on tissue cells, on the basis of extrapolations from the phenotype of cell fragility that results from mutations in skin keratins. There is a need for functional cell assays in which the impact of stress on intermediate filaments can be induced and analyzed. Using osmotic shock, we have induced cytoskeleton changes that suggest protective functions for actin and intermediate filament systems. Induction of the resulting stress response has been monitored in keratinocyte cells lines carrying K5 or K14 mutations, which are associated with varying severity of epidermolysis bullosa simplex. Cells with severe mutations were more sensitive to osmotic stress and took longer to recover from it. Their stress-activated response pathways were induced faster, as seen by early activation of JNK, ATF-2 and c-Jun. We demonstrate that the speed of a cell's response to hypotonic stress, by activation of the SAPK/JNK pathway, is correlated with the clinical severity of the mutation carried. The response to hypo-osmotic shock constitutes a discriminating stress assay to distinguish between the effects of different keratin mutations and is a potentially valuable tool in developing therapeutic strategies for keratin-based skin fragility disorders.

On the role of the macromolecular phase transitions in biology in response to change in solution volume or macromolecular composition: action as an entropy buffer

We have used numerical simulation to demonstrate the potential for macromolecular precipitate solution phase transitions existing within the cell, to play a role in the minimization of changes in location or quaternary state of other macromolecular components, predicted to accompany changes in cell volume. For our modeling we have employed thermodynamic relations that take into account the large effects upon the thermodynamic activity coefficient produced by a solution environment that is highly volume occupied due to the presence of high concentrations of soluble macromolecule. The theoretical approach adopted, along with the simulated results, provide a framework for the interpretation of certain proteins' behavior (e.g. cytoskeletal elements such as tubulin and actin and possibly some prion structures) in response to cell volume change.

Functional reconstitution of an eicosanoid-modulated Cl- channel from bovine tracheal smooth muscle

We describe the biochemical properties of an eicosanoid-modulated Cl- channel and assess the mechanisms by which the epoxyeicosatrienoic acids (EETs) alter both its unitary conductance and its open probability (P(o)). After a purification protocol involving wheat-germ agglutinin affinity and anion-exchange chromatography, the proteins were sequentially inserted into liposomes, which were then fused into PLBs. Functional and biochemical characterization tests confirm that the Cl- channel is a 55-kDa glycosylated monomer with voltage- and Ca(2+) concentration-independent activity. 5,6- and 8,9-EET decreased the conductance of the native channel (control conductance: 70 +/- 5 pS in asymmetrical 50 mM trans/250 mM cis CsCl) in a concentration-dependent manner, with respective 50% inhibitory concentration values of 0.31 and 0.42 microM. These regioisomers similarly decreased the conductance of the purified channel (control conductance value: 75 +/- 5 pS in asymmetrical 50 mM trans/250 mM cis CsCl), which had been stripped of its native proteic and lipidic environment. On the other hand, 5,6- and 8,9-EETs decreased the P(o) of the native channel with respective 50% inhibitory concentration values of 0.27 and 0.30 microM but failed to alter the P(o) of the purified protein. Thus we suggest that the effects of these EETs on channel conductance likely result from direct interactions of EET- anions with the channel pore, whereas the alteration of P(o) requires a lipid environment of specific composition that is lost on solubilization and purification of the protein.

The cytoskeleton and cell volume regulation

Although the precise mechanisms have yet to be elucidated, early events in osmotic signal transduction may involve the clustering of cell surface receptors, initiating downstream signaling events such as assembly of focal adhesion complexes, and activation of, e.g. Rho family GTPases, phospholipases, lipid kinases, and tyrosine- and serine/threonine protein kinases. In the present paper, we briefly review recent evidence regarding the possible relation between such signaling events, the F-actin cytoskeleton, and volume-regulatory membrane transporters, focusing primarily on our own work in Ehrlich ascites tumer cells (EATC). In EATC, cell shrinkage is associated with an increase, and cell swelling with a decrease in F-actin content, respectively. The role of the F-actin cytoskeleton in cell volume regulation in various cell types has largely been investigated using cytochalasins to disrupt F-actin and highly varying effects have been reported. Findings in EATC show that the effect of cytochalasin treatment cannot always be assumed to be F-actin depolymerization, and that, moreover, there is no well-defined correlation between effects of cytochalasins on F-actin content and their effects on F-actin organization and cell morphology. At a concentration verified to depolymerize F-actin, cytochalasin B (CB), but not cytochalasin D (CD), inhibited the regulatory volume decrease (RVD) and regulatory volume increase (RVI) processes in EATC. This suggests that the effect of CB is related to an effect other than F-actin depolymerization, possibly its F-actin severing activity.

Regions in the carboxy terminus of alpha-bENaC involved in gating and functional effects of actin

Gating differences occur between the alpha-subunits of the bovine and rat clones of an amiloride-sensitive epithelial Na+ channel (ENaC). Deletion of the carboxy terminus of bovine alpha-ENaC (alpha-bENaC) at R567 converted the gating properties to that of rat alpha-ENaC (alpha-rENaC). The equivalent truncation in alpha-rENaC was without effect on the gating of the rat homologue. The addition of actin to ENaC channels composed of either alpha-rENaC or alpha-bENaC alone produced a twofold reduction in conductance and an increase in open probability. Neither alpha-rENaC (R613X) nor alpha-bENaC (R567X) was responsive to actin. Using a chimera consisting of alpha-rENaC1-615 and alpha-bENaC570-650, we examined several different carboxy-terminal truncation mutants plus and minus actin. When incorporated into planar bilayers, the gating pattern of this construct was identical to wild-type (wt) alpha-bENaC. Premature stop mutations proximal to E685X produced channels with gating patterns like alpha-rENaC. Actin had no effect on the E631X truncation, whereas more distal truncations all interacted with actin, as did wt alpha-bENaC. Key findings were confirmed using channels expressed in Xenopus oocytes and studied by cell-attached patch-clamp recording. Our results suggest that the site of actin regulation at the carboxy terminus of the chimera is located between residues 631 and 644.

A moderate decrease in temperature inhibits the calcium signaling mechanism(s) of the regulatory volume decrease in chick embryo cardiomyocytes

Chick cardiomyocytes, when submitted to hyposmotic swelling, exhibit a partial regulatory volume decrease (RVD). A Ca2+ influx by stretch-activated channels signals a taurine efflux and the RVD at 37 degrees C. We evaluated the cell's performance at room temperature. Cardiomyocytes isolated and cultured from 11-day-old chick embryos were submitted to a hyposmotic solution (180 mOsm/kg H2O) at 37 degrees C and at room temperature (26 degrees C). Under these conditions we measured the changes in cell volume as well as the intracellular free Ca2+ (using fura-2). During hyposmotic swelling, cells at 37 degrees C displayed a peak relative volume of 1.61 +/- 0.03 and recovery to 1.22 +/- 0.04 (N = 14), while cells at 26 degrees C presented a peak swell relative volume of 1.74 +/- 0.06 and did not recover (1.59 +/- 0.09, N = 9). Transient increases in intracellular Ca2+, which are characteristic of the normal RVD, were observed at both temperatures (29.1 +/- 4.5% (N = 8) and 115.2 +/- 42.8% (N = 5) increase at 37 degrees and 26 degrees C (P<0.05), respectively). A delay in the Ca2+ transient increase was also observed when the cells were at 26 degrees C (109 +/- 34 s compared to 38 +/- 9 s at 37 degrees C, P<0.05). At room temperature the RVD does not occur because the calcium transient increase, which is an early event in the signaling of the RVD, is delayed. Also, free calcium is not cleared as in the 37 degrees C RVD. In the normal RVD the free calcium returns to baseline levels. The very high and persistent free calcium levels seen at room temperature can lead to unregulated enzyme activities and may promote irreversible injury and cell death.

The role of ClC-3 in volume-activated chloride currents and volume regulation in bovine epithelial cells demonstrated by antisense inhibition

1. A chloride current with mild outward rectification was induced in the native bovine non-pigmented ciliary epithelial (NPCE) cells by a 23 % hypotonic solution. The current showed no or little inactivation at depolarized steps. 2. ATP blocked 88 and 61 % of the outward and inward components of the volume-activated chloride current (ICl,vol) with an IC50 of 5.3 and 9.6 mM, respectively. 3. The volume-activated chloride current was decreased and the activation of the current was delayed by inhibiting endogenous ClC-3 expression using a ClC-3 antisense oligonucleotide. The inhibition of the current as a function of antisense concentration was asymptotic with a maximum about 60 %. The remaining current was probably not derived from ClC-3 and was inhibited by ATP. 4. ClC-3 expression in the bovine NPCE cells was verified by immunofluorescence studies. ClC-3 immunofluorescence was distributed throughout the cells but with the predominant location within the nucleus. The expression of ClC-3 protein was diminished by the ClC-3 antisense oligonucleotide with the greatest diminution occurring in the nuclear region. 5. The size of the volume-activated chloride current was positively correlated with the ClC-3 immunofluorescence level. 6. Regulatory volume decrease of the NPCE cells was reduced by ClC-3 antisense oligonucleotide. 7. We conclude that endogenous ClC-3 is associated with the volume-activated chloride current and is involved in cell volume regulation, but that it can only contribute towards a proportion of the current in NPCE cells. 8. The nuclear predominance of ClC-3 immunofluorescence in NPCE cells, the absence of basal activity of chloride current and the marked pharmacological differences between IClC-3 and ICl,vol argue against ClC-3 being the only, or even the main, volume-activated chloride channel in NPCE cells.

Comparisons of different stages of chick embryonic development by the physiological regulatory response to hyposmotic challenge

Cardiac myocytes isolated and cultured from 11 day chick embryos present a Ca(2+)-dependent regulatory volume decrease (RVD) when exposed to hyposmotic stimulus. The RVD of myocytes from different embryonic stages were analyzed to evaluate their physiological performance through development. Among the several embryonic stages analyzed (6, 11, 16 and 19 days) only 19 day cardiac myocytes present a greater RVD when compared with 11 day (considered as control), the other ages showed no difference in the regulatory response. As it is known that RVD is Ca(2+) dependent, we decided to investigate the transient free Ca(2+) response during the hyposmotic swelling of the 11 and 19 day stages. The 11 day cardiac myocyte showed a transient 40% increase in intracellular free Ca(2+) when submitted to hyposmotic solutions, and the free Ca(2+) returned to baseline levels while the cells remained in hyposmotic buffer. However, the intracellular free Ca(2+) transient in the 19 day cells during hyposmotic challenge increases 100% and instead of returning to baseline levels, declines to 55% above control, well after the 11 day transient has returned to baseline. Also, quantitative fluorescence microscopy revealed that 19 day cardiac myocytes have more sarcoplasmic reticulum (SR) Ca(2+) ATPase sites per cell as compared to the 11 day cells. Our findings suggest that 19 day cells have more developed intracellular Ca(2+) stores (SR). By evoking the mechanism of Ca(2+) induced Ca(2+) release, the cells have more free Ca(2+) available for signaling the RVD during hyposmotic swelling.

A peptide derived from pICln induced a strong hypotonic resistance in Escherichia coli cells

We demonstrated previously that expression of rat pICln in Escherichia coli conferred a strong resistance to hypotonic stress. To define the intramolecular functional domain responsible for the resistance, molecular dissection of pICln was performed and the obtained peptides were expressed in E. coli. The cells expressing the peptides were exposed to a hypotonic solution, and their 'survival rates' were observed. The cells expressing only the peptides including the second acidic domain of pICln exhibited significantly higher 'survival rates' after hypotonic stress. The functional domain against hypotonicity was finally narrowed down to a peptide consisting of a 46-amino acid residue, P107-152. We conclude that the expression of P107-152 in E. coli cells could enhance their resistance to a hypotonic environment.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Modulation of volume-sensitive Cl - channels and cell volume by actin filaments and microtubules in human cervical cancer HT-3 cells

Hypotonicity activates volume-sensitive Cl- currents, which are implicated in the regulatory volume decrease (RVD) responses and transport of taurine in human cervical cancer HT-3 cells. In this study, the role of cytoskeleton in the regulation of volume-sensitive Cl- channels and RVD responses in HT-3 cells was studied. Cells were incubated with various compounds, which depolymerized or polymerized cytoskeletal elements, i.e. actin filaments and microtubules. The hypotonicity-induced changes in Cl- conductance and in cell volume were measured by whole-cell voltage clamping and cell size monitoring, respectively. Our results show that in HT-3 cells hypotonicity activated an outward rectified Cl- current that was abrogated by Cl- channel blockers. Cytochalasin B, an actin-depolymerizing compound, induced a substantial increase in Cl- conductance under isotonic condition and potentiated the expression of Cl- currents in hypotonic stress. Phorbol 12-myristate 13-acetate (PMA) significantly inhibited the cytochalasin B-induced activation of Cl- conductance under isotonic condition. On the other hand, treatment with cytochalasin B significantly prolonged the RVD responses. Phalloidin, a stabilizer of actin polymerization, did not change the basal currents under isotonic condition, but completely abolished the increase in whole-cell Cl- conductance elicited by hypotonicity and retarded the cell volume recovery. Colchicine, a microtubule-assembly inhibitor, had no effect on either basal Cl- conductance or volume-sensitive Cl- current and was unable to inhibit the RVD responses. Taxol, a microtubule-stabilizing compound, did not alter the basal Cl- conductance, but inhibited the activation of volume-sensitive Cl- channels as well as the process of RVD in a dose-dependent manner. These data support the notion that functional integrity of actin filaments and microtubules plays critical roles in maintaining the RVD responses and activation of Cl- channels in human cervical cancer HT-3 cells.

Relationships between the actin cytoskeleton and cell volume regulation

The actin cytoskeleton mediates a variety of essential biological functions in cells, including division, shape changes, and movement. A number of studies have suggested that the abundant submembranous actin cytoskeleton present in the cortex of many cell types is involved in the regulation of cell volume. This relationship is supported by numerous works which document the changes in the structural organization of the actin cytoskeleton which accompany cell volume changes and the F-actin-dependence of the regulatory volume responses. In addition, other studies demonstrate structural and functional relationships between the actin cytoskeleton and the membrane transporters known to be involved in cell volume homeostasis. This review provides a summary of the current level of knowledge in this area and discusses the mechanisms which may underlie the linkage between the actin cytoskeleton and cell volume regulation.

Impairment of endothelium-dependent arterial relaxation by high-fat feeding in ApoE-deficient mice: toward normalization by human ApoA-I expression

BACKGROUND

Atherogenic lipoproteins can impair the endothelium-dependent arterial relaxation, and circumstantial evidence suggests a beneficial role of plasma high density lipoproteins and apolipoprotein (apo) A-I in counteracting the endothelium dysfunction. In the present study, vascular reactivity was determined in control, apoE-deficient mice (apoE-KO mice), and apoE-deficient mice expressing human apoA-I (apoE-KO/HuAITg mice).

METHODS AND RESULTS

In the first part of the study, control and apoE-KO mice were fed a low-fat or a high-fat diet for 23 weeks, and the vasoactive responses of isolated thoracic aortic segments to norepinephrine, sodium nitroprusside, and acetylcholine (ACh) were determined. Whereas norepinephrine, sodium nitroprusside, and ACh evoked similar vascular responses in control and apoE-KO mice fed the low-fat diet, high-fat feeding in apoE-KO mice produced a significant 3-fold increase in the mean concentration required to produce a half-maximal relaxing effect (EC(50)) of ACh as compared with control mice. This reflects a weaker sensitivity to ACh of the aortic segments from the apoE-deficient animals. In the second part of the study, the mean EC(50) for ACh after high-fat feeding was found to be 4.4-fold lower in apoE-KO/HuAITg mice than in apoE-KO mice, indicating that the reduced sensitivity to ACh of the thoracic aorta from the apoE-KO mice fed the high-fat diet is improved by the expression of human apoA-I.

CONCLUSIONS

The present study demonstrates that the endothelium-dependent arterial relaxation is impaired in apoE-KO mice fed the high-fat diet. The endothelium dysfunction tends to be normalized by human apoA-I expression.

F-actin as a functional target for retro-retinoids: a potential role in anhydroretinol-triggered cell death

The retro-retinoids, metabolites of vitamin A (retinol), belong to a family of lipophilic signalling molecules implicated in regulation of cell growth and survival. Growth-promoting properties have been ascribed to 14-hydroxy-retro-retinol (14HRR), while anhydroretinol (AR) was discovered to act as a natural antagonist triggering growth arrest and death by apoptosis. Based on morphological studies and inhibition of apoptosis by the kinase blocker, herbimycin A, it has been suggested that retro-retinoids exhibit their function in the cytosolic compartment. F-actin emerged as a functional target for retro-retinoid action. By FACS analysis and fluorescence microscopy of phalloidin-FITC labeled cells we demonstrated that F-actin reorganization was an early event in AR-triggered apoptosis. Fluorescence images of AR-treated fibroblasts displayed short, thick, stick-like and punctate structures, and membrane ruffles at the cell periphery along with an increased diffuse staining pattern. Reversal of the AR effect by 14HRR or retinol indicates that F-actin is a common site for regulation by retro-retinoids. Inhibition of both cell death and actin depolymerisation by bcl-2 implies that cytoskeleton reorganization is downstream of bcl-2-related processes. Furthermore, stabilization of microfilaments by jasplakinolide increased the survival potential of AR treated cells, while weakening the cytoskeleton by cytochalasin B abetted apoptosis. Thus the cytoskeleton is an important way station in a communication network that decides whether a cell should live or die.

Functional and molecular identification of a novel chloride conductance in canine colonic smooth muscle

Swelling-activated or volume-sensitive Cl- currents are found in numerous cell types and play a variety of roles in their function; however, molecular characterization of the channels is generally lacking. Recently, the molecular entity responsible for swelling-activated Cl- current in cardiac myocytes has been identified as ClC-3. The goal of our study was to determine whether such a channel exists in smooth muscle cells of the canine colon using both molecular biological and electrophysiological techniques and, if present, to characterize its functional and molecular properties. We hypothesized that ClC-3 is present in colonic smooth muscle and is regulated in a manner similar to the molecular entity cloned from heart. Indeed, the ClC-3 gene was expressed in colonic myocytes, as demonstrated by reverse transcriptase polymerase chain reaction performed on isolated cells. The current activated by decreasing extracellular osmolarity from 300 to 250 mosM was outwardly rectifying and dependent on the Cl- gradient. Current magnitude increased and reversed at more negative potentials when Cl- was replaced by I- or Br-. Tamoxifen ([Z]-1-[p-dimethylaminoethoxy-phenyl]-1,2-diphenyl-1-butene; 10 microM) and DIDS (100 microM) inhibited the current, whereas 25 microM niflumic acid, 10 microM nicardipine, and Ca2+ removal had no effect. Current was inhibited by 1 mM extracellular ATP in a voltage-dependent manner. Cl- current was also regulated by protein kinase C, as phorbol 12,13-dibutyrate (300 nM) decreased Cl- current magnitude, while chelerythrine chloride (30 microM) activated it under isotonic conditions. Our findings indicate that a current activated by hypotonic solution is present in colonic myocytes and is likely mediated by ClC-3. Furthermore, we suggest that the ClC-3 may be an important mechanism controlling depolarization and contraction of colonic smooth muscle under conditions that impose physical stress on the cells.

Characterization of pI(Cln) binding proteins: identification of p17 and assessment of the role of acidic domains in mediating protein-protein interactions

pICln is a ubiquitous and abundant 27 kDa soluble protein that is localized primarily to the cytoplasm. The protein has been proposed to be a swelling-activated anion channel or a channel regulator. Recent studies, however, have cast significant doubt on these hypotheses, and the function of pI(Cln) therefore remains unknown. To further characterize the physiological role of pI(Cln), we have begun to identify the proteins that bind to it and the amino acid domains that mediate pICln protein-protein interactions. Using affinity assays and immunoprecipitation we have identified three proteins in C6 glioma cells with molecular masses of 17 kDa, 29 kDa and 72 kDa that bind selectively to pI(Cln). Microsequencing revealed that p17 is the non-muscle isoform of the alkali myosin light chain. pI(Cln) contains three acidic amino acid domains termed AD1, AD2 and AD3. Mutation of AD1 and/or AD2 had no effect on p17, p29 and p72 binding. However, binding of p72 was lost when four acidic amino acid residues were mutated in AD3, which is located at the carboxy terminus. A truncation peptide containing the last 29 amino acids of pI(Cln) was able to bind p72 normally. These results indicate that the carboxy terminus is necessary for p72-pI(Cln) interaction. Based on these and other findings, we propose that pI(Cln) is a protein responsible for regulating the structure and function of the cytoskeleton, and/or a protein involved in mediating interactions between components of intracellular signal transduction pathways.

Intracellular ionic strength regulates the volume sensitivity of a swelling-activated anion channel

Cell swelling activates an outwardly rectifying anion channel termed VSOAC (volume-sensitive organic osmolyte/anion channel). Regulation of VSOAC by intracellular electrolytes was characterized in Chinese hamster ovary cells by whole cell patch clamp. Elevation of intracellular CsCl concentration from 40 to 180 mM resulted in a concentration-dependent decrease in channel activation. Activation of VSOAC was insensitive to the salt gradient across the plasma membrane, the intracellular concentration of specific anions or cations, and the total intracellular concentration of cations, anions, or electrolytes. Comparison of cells dialyzed with either CsCl or Na2SO4 solutions demonstrated directly that VSOAC activation is modulated by intracellular ionic strength (microi). The relative cell volume at which VSOAC current activation was triggered, termed the channel volume set point, decreased with decreasing ionic strength. At microi = 0.04, VSOAC activation occurred spontaneously in shrunken cells. The rate of VSOAC activation was nearly 50-fold higher in cells with microi = 0.04 vs. those with microi = 0.18. We propose that microi modulates the volume sensor responsible for channel activation.