Subunit composition of VRAC channels determines substrate specificity and cellular resistance to Pt‐based anti‐cancer drugs

Although platinum‐based drugs are widely used chemotherapeutics for cancer treatment, the determinants of tumor cell responsiveness remain poorly understood. We show that the loss of subunits LRRC8A and LRRC8D of the heteromeric LRRC8 volume‐regulated anion channels (VRACs) increased resistance to clinically relevant cisplatin/carboplatin concentrations. Under isotonic conditions, about 50% of cisplatin uptake depended on LRRC8A and LRRC8D, but neither on LRRC8C nor on LRRC8E. Cell swelling strongly enhanced LRRC8‐dependent cisplatin uptake, bolstering the notion that cisplatin enters cells through VRAC. LRRC8A disruption also suppressed drug‐induced apoptosis independently from drug uptake, possibly by impairing VRAC‐dependent apoptotic cell volume decrease. Hence, by mediating cisplatin uptake and facilitating apoptosis, VRAC plays a dual role in the cellular drug response. Incorporation of the LRRC8D subunit into VRAC substantially increased its permeability for cisplatin and the cellular osmolyte taurine, indicating that LRRC8 proteins form the channel pore. Our work suggests that LRRC8D‐containing VRACs are crucial for cell volume regulation by an important organic osmolyte and may influence cisplatin/carboplatin responsiveness of tumors.     

Biophysics and Structure-Function Relationships of LRRC8-Formed Volume-Regulated Anion Channels

Volume-regulated anion channels (VRACs) are key players in regulatory volume decrease of vertebrate cells by mediating the extrusion of chloride and organic osmolytes. They play additional roles in various physiological processes beyond their role in osmotic volume regulation. VRACs are formed by heteromers of LRRC8 proteins; LRRC8A (also called SWELL1) is an essential subunit that combines with any of its paralogs, LRRC8B–E, to form hexameric VRAC complexes. The subunit composition of VRACs determines electrophysiological characteristics of their anion transport such as single-channel conductance, outward rectification, and depolarization-dependent inactivation kinetics. In addition, differently composed VRACs conduct diverse substrates, such as LRRC8D enhancing VRAC permeability to organic substances like taurine or cisplatin. Here, after a recapitulation of the biophysical properties of VRAC-mediated ion and osmolyte transport, we summarize the insights gathered since the molecular identification of VRACs. We describe the recently solved structures of LRRC8 complexes and discuss them in terms of their structure-function relationships. These studies open up many potential avenues for future research.     

VRACs swallow platinum drugs

Platinum‐based drugs such as cisplatin and carboplatin are on the WHO model list of essential medicines, as highly effective chemotherapeutic drugs for the treatment of various solid tumors. These drugs react with purine residues in DNA, thereby causing DNA damage, inhibition of cell division, and eventually cell death. However, the mechanisms whereby platinum‐based drugs enter cancer cells remained poorly understood. In this issue, Planells‐Cases et al ( 2015 ) provide evidence that cells take up cisplatin and carboplatin via volume‐regulated anion channels (VRACs), more specifically VRACs composed of LRRC8A and LRRC8D subunits.     

LRRC8A is essential for volume‐regulated anion channel in smooth muscle cells contributing to cerebrovascular remodeling during hypertension

ObjectivesRecent studies revealed LRRC8A to be an essential component of volume‐regulated anion channel (VRAC), which regulates cellular volume homeostasis. However, evidence for the contribution of LRRC8A‐dependent VRAC activity in vascular smooth muscle cells (VSMCs) is still lacking, and the relevant functional role of LRRC8A in VSMCs remains unknown. The primary goal of this study was to elucidate the role of LRRC8A in VRAC activity in VSMCs and the functional role of LRRC8A in cerebrovascular remodeling during hypertension.Materials and MethodssiRNA‐mediated knockdown and adenovirus‐mediated overexpression of LRRC8A were used to elucidate the electrophysiological properties of LRRC8A in basilar smooth muscle cells (BASMCs). A smooth muscle–specific overexpressing transgenic mouse model was used to investigate the functional role of LRRC8A in cerebrovascular remodeling.ResultsLRRC8A is essential for volume‐regulated chloride current (I _Cl, Vol) in BASMCs. Overexpression of LRRC8A induced a voltage‐dependent Cl⁻ current independently of hypotonic stimulation. LRRC8A regulated BASMCs proliferation through activation of WNK1/PI3K‐p85/AKT axis. Smooth muscle‐specific upregulation of LRRC8A aggravated Angiotensin II‐induced cerebrovascular remodeling in mice.ConclusionsLRRC8A is an essential component of VRAC and is required for cell volume homeostasis during osmotic challenge in BASMCs. Smooth muscle specific overexpression of LRRC8A increases BASMCs proliferation and substantially aggravates basilar artery remodeling, revealing a potential therapeutic target for vascular remodeling in hypertension.

The schematic diagram for LRRC8A role in cerebrovascular remodeling. LRRC8A is an essential component of VRAC in BASMCs. During the challenge of hypertension, the activated LRRC8A channel‐mediated‐Cl⁻ efflux increases WNK1 phosphorylation, which in turn triggers AKT phosphorylation and promotes BASMCs proliferation, eventually exacerbates hypertension‐induced cerebrovascular vascular remodeling.     

Role of volume-regulated and calcium-activated anion channels in cell volume homeostasis,cancer and drug resistance

Volume-regulated channels for anions (VRAC) / organic osmolytes (VSOAC) play essential roles in cell volume regulation and other cellular functions, e.g. proliferation, cell migration and apoptosis. LRRC8A, which belongs to the leucine rich-repeat containing protein family, was recently shown to be an essential component of both VRAC and VSOAC. Reduced VRAC and VSOAC activities are seen in drug resistant cancer cells. ANO1 is a calcium-activated chloride channel expressed on the plasma membrane of e.g., secretory epithelia. ANO1 is amplified and highly expressed in a large number of carcinomas. The gene, encoding for ANO1, maps to a region on chromosome 11 (11q13) that is frequently amplified in cancer cells. Knockdown of ANO1 impairs cell proliferation and cell migration in several cancer cells. Below we summarize the basic biophysical properties of VRAC, VSOAC and ANO1 and their most important cellular functions as well as their role in cancer and drug resistance.     

Specific and essential but not sufficient roles of LRRC8A in the activity of volume-sensitive outwardly rectifying anion channel (VSOR)

The broadly expressed volume-sensitive outwardly rectifying anion channel (VSOR, also called VRAC) plays essential roles in cell survival and death. Recent findings have suggested that LRRC8A is a core component of VSOR in human cells. In the present study, VSOR currents were found to be largely reduced by siRNA against LRRC8A in mouse C127 cells as well. In contrast, LRRC8A knockdown never affected activities of 4 other types of anion channel activated by acid, Ca²⁺, patch excision or cAMP. While cisplatin-resistant KCP-4 cells poorly expressed endogenous VSOR activity, molecular expression levels of LRRC8A, LRRC8D and LRRC8E were indistinguishable between VSOR-deficient KCP-4 cells and the parental VSOR-rich KB cells. Furthermore, overexpression of LRRC8A alone or together with LRRC8D or LRRC8E in KCP-4 cells failed to restore VSOR activity. These results show that deficiency of VSOR currents in KCP-4 cells is not due to insufficient expression of the LRRC8A/D/E gene, suggesting an essential involvement of some other factor(s), and indicate that further study is required to better understand the complexities of the molecular determinants of VSOR, including the precise role of LRRC8 proteins.     

Deficient of LRRC8A attenuates hypoxia-induced necrosis in 3T3-L1 cells

ABSTRACT

Under acute hypoxia, multiple ion channels on the cell membrane are activated, causing cell swelling and eventually necrosis. LRRC8A is an indispensable protein of the volume-regulated anion channel (VRAC), which participates in swelling and the acceleration of cell necrosis. In this study, we revealed a dynamic change in the expression level of the LRRC8 family during hypoxia in 3T3-L1 cells. The disruption of LRRC8A in 3T3-L1 cells was also associated with a significant anti-necrotic phenotype upon hypoxia accompanied by the reduced expression of necrosis-related genes. In vivo, differential expression of LRRC8 family members was also identified between high-altitude pigs and their low-altitude relatives. Taken these findings together, this study demonstrates the involvement of LRRC8A in hypoxia-induced cell necrosis.     

Crosstalk between Na+,K+-ATPase and a volume-regulated anion channel in membrane microdomains of human cancer cells

Low concentrations of cardiac glycosides including ouabain, digoxin, and digitoxin block cancer cell growth without affecting Na⁺,K⁺-ATPase activity, but the mechanism underlying this anti-cancer effect is not fully understood. Volume-regulated anion channel (VRAC) plays an important role in cell death signaling pathway in addition to its fundamental role in the cell volume maintenance. Here, we report cardiac glycosides-induced signaling pathway mediated by the crosstalk between Na⁺,K⁺-ATPase and VRAC in human cancer cells. Submicromolar concentrations of ouabain enhanced VRAC currents concomitantly with a deceleration of cancer cell proliferation. The effects of ouabain were abrogated by a specific inhibitor of VRAC (DCPIB) and knockdown of an essential component of VRAC (LRRC8A), and they were also attenuated by the disruption of membrane microdomains or the inhibition of NADPH oxidase. Digoxin and digitoxin also showed anti-proliferative effects in cancer cells at their therapeutic concentration ranges, and these effects were blocked by DCPIB. In membrane microdomains of cancer cells, LRRC8A was found to be co-immunoprecipitated with Na⁺,K⁺-ATPase α1-isoform. These ouabain-induced effects were not observed in non-cancer cells. Therefore, cardiac glycosides were considered to interact with Na⁺,K⁺-ATPase to stimulate the production of reactive oxygen species, and they also apparently activated VRAC within membrane microdomains, thus producing anti-proliferative effects.     

The Protein Synthesis Inhibitor Blasticidin S Enters Mammalian Cells via Leucine-rich Repeat-containing Protein 8D

Leucine-rich repeat-containing 8 (LRRC8) proteins have been identified as putative receptors involved in lymphocyte development and adipocyte differentiation. They remain poorly characterized, and no specific function has been assigned to them. There is no consensus on how this family of proteins might function because homology searches suggest that members of the LRRC8 family act not as plasma membrane receptors, but rather as channels that mediate cell-cell signaling. Here we provide experimental evidence that supports a role for LRRC8s in the transport of small molecules. We show that LRRC8D is a mammalian protein required for the import of the antibiotic blasticidin S. We characterize localization and topology of LRRC8A and LRRC8D and demonstrate that LRRC8D interacts with LRRC8A, LRRC8B, and LRRC8C. Given the suggested involvement in solute transport, our results support a model in which LRRC8s form one or more complexes that may mediate cell-cell communication by transporting small solutes.     

Distinct contributions of LRRC8A and its paralogs to the VSOR anion channel from those of the ASOR anion channel

Volume- and acid-sensitive outwardly rectifying anion channels (VSOR and ASOR) activated by swelling and acidification exhibit voltage-dependent inactivation and activation time courses, respectively. Recently, LRRC8A and some paralogs were shown to be essentially involved in the activity and inactivation kinetics of VSOR currents in human colonic HCT116 cells. In human cervix HeLa cells, here, inactivation of VSOR currents was found to become accelerated by RNA silencing only of LRRC8A but never decelerated by that of any LRRC8 isoform. These data suggest that LRRC8A is associated with the deceleration mechanism of VSOR inactivation, while none of LRRC8 members is related to the acceleration mechanism. Activation kinetics of ASOR currents was unaffected by knockdown of any LRRC8 family member. Double, triple and quadruple gene-silencing studies indicated that combinatory expression of LRRC8A with LRRC8D and LRRC8C is essential for VSOR activity, whereas none of LRRC8 family members is involved in ASOR activity.     

Regulators of cell volume: The structural and functional properties of anion channels of the LRRC8 family

Members of the LRRC8 family participate in the response of vertebrate cells to osmotic changes in their environment. These proteins form heteromeric assemblies composed of the obligatory subunit LRRC8A and at least one of the other four homologs, which together function as anion-selective channels with distinct properties that are activated upon cell-swelling. The hexameric complexes share a conserved architecture consisting of a membrane-inserted pore domain with an ion permeation path located at the axis of symmetry and cytoplasmic leucine-rich repeat domains that regulate the open probability of the channel. In this review, we summarize the current understanding of structure–function relationships of these unusual ion channels whose mechanisms are, despite their large physiological importance, still poorly understood.     

Carbenoxolone inhibits volume-regulated anion conductance in cultured rat cortical astroglia

Accumulating evidence indicate that the gap-junction inhibitor carbenoxolone (CBX) regulates neuronal synchronization, depresses epileptiform activity and has a neuroprotective action. These CBX effects do not depend solely on its ability to inhibit gap junction channels formed by connexins (Cx), but the underlying mechanisms remain to be elucidated. Here we addressed the questions whether CBX modulates volume-regulated anion channels (VRAC) involved in the regulatory volume decrease and regulates the associated release of excitatory amino acids in cultured rat cortical astrocytes. We found that CBX inhibits VRAC conductance with potency comparable to that able to depress the activity of the most abundant astroglial gap junction protein connexin43 (Cx43). However, the knock down of Cx43 with small interfering RNA (siRNA) oligonucleotides and the use of various pharmacological tools revealed that VRAC inhibition was not mediated by interaction of CBX with astroglial Cx proteins. Comparative experiments in HEK293 cells stably expressing another putative target of CBX, the purinergic ionotropic receptor P2X7, indicate that the presence of this receptor was not necessary for CBX-mediated depression of VRAC. Finally, we show that in COS-7 cells, which are not endowed with pannexin-1 protein, another astroglial plasma membrane interactor of CBX, VRAC current retained its sensitivity to CBX. Complementary analyses indicate that the VRAC-mediated release of excitatory amino acid aspartate was decreased by CBX. Collectively, these findings support the notion that CBX could affect astroglial ability to modulate neuronal activity by suppressing excitatory amino acid release through VRAC. They also provide a possible mechanistic clue for the neuroprotective effect of CBX in vivo.     

LRRC8 proteins share a common ancestor with pannexins, and may form hexameric channels involved in cell-cell communication

Leucine-rich repeat-containing 8 (LRRC8) proteins are composed of four transmembrane helices and 17 leucine-rich repeats (LRR). Although LRRC8 proteins have been associated with important processes, like maturation of B cells or adipocyte differentiation, their biology and molecular function are largely unknown. We found that LRRC8 proteins originated from the combination of a pannexin and an LRR domain (most likely related to the SHOC2, LAP, RSU1 and LRRIQ4 protein families) before the diversification of chordates. We propose that, like pannexins, LRRC8 proteins form hexameric channels, which participate in cell-cell communication processes. According to the inferred topological model, and contrary to what was previously assumed, the six LRR domains are located in the cytoplasm, and could participate in the organisation of signalling cascades. By compiling available proteomics and gene expression data, and on the basis of the LRRC8 proposed hexameric channel structure, we present clues to the function of this family.     

Airway Surface Dehydration by Transforming Growth Factor β (TGF-β) in Cystic Fibrosis Is Due to Decreased Function of a Voltage-dependent Potassium Channel and Can Be Rescued by the Drug Pirfenidone

Transforming growth factor β1 (TGF-β1) is not only elevated in airways of cystic fibrosis (CF) patients, whose airways are characterized by abnormal ion transport and mucociliary clearance, but TGF-β1 is also associated with worse clinical outcomes. Effective mucociliary clearance depends on adequate airway hydration, governed by ion transport. Apically expressed, large-conductance, Ca²⁺- and voltage-dependent K⁺ (BK) channels play an important role in this process. In this study, TGF-β1 decreased airway surface liquid volume, ciliary beat frequency, and BK activity in fully differentiated CF bronchial epithelial cells by reducing mRNA expression of the BK γ subunit leucine-rich repeat-containing protein 26 (LRRC26) and its function. Although LRRC26 knockdown itself reduced BK activity, LRRC26 overexpression partially reversed TGF-β1-induced BK dysfunction. TGF-β1-induced airway surface liquid volume hyper-absorption was reversed by the BK opener mallotoxin and the clinically useful TGF-β signaling inhibitor pirfenidone. The latter increased BK activity via rescue of LRRC26. Therefore, we propose that TGF-β1-induced mucociliary dysfunction in CF airways is associated with BK inactivation related to a LRRC26 decrease and is amenable to treatment with clinically useful TGF-β1 inhibitors.     

Studies of the Mechanism of Activation of the Volume-Regulated Anion Channel in Rat Pancreatic β-Cells

There is evidence that depolarization of the pancreatic β cell by glucose involves cell swelling and activation of the volume-regulated anion channel (VRAC). However, it is unclear whether cell swelling per se or accompanying changes in intracellular osmolality and/or ionic strength are responsible for VRAC activation. VRAC activity was measured in rat β cells by conventional or perforated patch whole-cell recording. Cell volume was measured by video imaging. In conventional whole-cell recordings, VRAC activation was achieved by exposure of the cells to a hyposmotic bath solution, by application of positive pressure to the pipette, or by use of a hyperosmotic pipette solution. Increased concentrations of intracellular CsCl also caused channel activation, but with delayed kinetics. In perforated patch recordings, VRAC activation was induced by isosmotic addition of the permeable osmolytes urea, 3-Ο-methyl glucose, arginine, and NH₄Cl. These effects were all accompanied by β-cell swelling. It is concluded that increased cell volume, whether accompanied by raised intracellular osmolality or ionic strength, is a major determinant of VRAC activation in the β cell. However, increased intracellular ionic strength markedly reduced the rate of VRAC activation. These findings are consistent with the hypothesis that the accumulation of glucose metabolites in the β cell, and the resultant increase in cell volume, provides a signal coupling glucose metabolism with VRAC activation.     

Dual effect of fluid shear stress on volume-regulated anion current in bovine aortic endothelial cells

The key mechanism responsible formaintaining cell volume homeostasis is activation ofvolume-regulated anion current (VRAC). The role of hemodynamicshear stress in the regulation of VRAC in bovine aortic endothelialcells was investigated. We showed that acute changes in shear stresshave a biphasic effect on the development of VRAC. A shear stress stepfrom a background flow (0.1 dyn/cm²) to 1 dyn/cm² enhanced VRAC activation induced by an osmoticchallenge. Flow alone, in the absence of osmotic stress, did not induceVRAC activation. Increasing the shear stress to 3 dyn/cm²,however, resulted in only a transient increase of VRAC activity followed by an inhibitory phase during which VRAC was gradually suppressed. When shear stress was increased further (5-10dyn/cm²), the current was immediately strongly suppressed.Suppression of VRAC was observed both in cells challenged osmoticallyand in cells that developed spontaneous VRAC under isotonic conditions. Our findings suggest that shear stress is an important factor inregulating the ability of vascular endothelial cells to maintain volume homeostasis.

     

Functional down-regulation of volume-regulated anion channels in AQP4 knockdown cultured rat cortical astrocytes

In the brain, the astroglial syncytium is crucially involved in the regulation of water homeostasis. Accumulating evidence indicates that a dysregulation of the astrocytic processes controlling water homeostasis has a pathogenetic role in several brain injuries. Here, we have analysed by RNA interference technology the functional interactions occurring between the most abundant water channel in the brain, aquaporin-4 (AQP4), and the swelling-activated Cl(-) current expressed by cultured rat cortical astrocytes. We show that in primary cultured rat cortical astrocytes transfected with control small interfering RNA (siRNA), hypotonic shock promotes an increase in cellular volume accompanied by augmented membrane conductance mediated by volume-regulated anion channels (VRAC). Conversely, astroglia in which AQP4 was knocked down (AQP4 KD) by transfection with AQP4 siRNA changed their morphology from polygonal to process-bearing, and displayed normal cell swelling but reduced VRAC activity. Pharmacological manipulations of actin cytoskeleton in rat astrocytes, and functional analysis in mouse astroglial cells, which retain their morphology upon knockdown of AQP4, suggest that stellation of AQP4 KD rat cortical astrocytes was not causally linked to reduction of VRAC current. Molecular analysis of possible candidates of swelling-activated Cl(-) current provided evidence that in AQP4 KD astrocytes, there was a down-regulation of chloride channel-2 (CIC-2), which, however, was not involved in VRAC conductance. Inclusion of ATP in the intracellular saline restored VRAC activity upon hypotonicity. Collectively, these results support the view that in cultured astroglial cells, plasma membrane proteins involved in cell volume homeostasis are assembled in a functional platform.     

The CLIC1 Chloride Channel Is Regulated by the Cystic Fibrosis Transmembrane Conductance Regulator when Expressed in Xenopus Oocytes

CLIC proteins comprise a family of chloride channels whose physiological roles are uncertain. To gain further insight into possible means of CLIC1 channel activity regulation, this protein was expressed in Xenopus oocytes alone or in combination with the cystic fibrosis transmembrane conductance regulator (CFTR). Whole-cell currents were determined using two-electrode voltage-clamp methods. Expression of CLIC1 alone did not increase whole-cell conductance either at rest or in response to increased intracellular cyclic adenosine monophosphate (cAMP). However, expression of CLIC1 with CFTR led to increased cAMP-activated whole-cell currents compared to expression from the same amount of CFTR mRNA alone. IAA-94 is a drug known to inhibit CLIC family channels but not CFTR. In oocytes expressing both CLIC1 and CFTR, a fraction of the cAMP-activated whole-cell current was sensitive to IAA-94, whereas in oocytes expressing CFTR alone, the cAMP-stimulated current was resistant to the drug. Cell fractionation studies revealed that the presence of CFTR conferred cAMP-stimulated redistribution of a fraction of CLIC1 from a soluble to a membrane-associated form. We conclude that when expressed in Xenopus oocytes CFTR confers cAMP regulation to CLIC1 activity in the plasma membrane and that at least part of this regulation is due to recruitment of CLIC1 from the cytoplasm to the membrane.     

Cell cycle-dependent activity of the volume- and Ca2+-activated anion currents in Ehrlich lettre ascites cells

Recent evidence implicates the volume-regulated anion current (VRAC) and other anion currents in control or modulation of cell cycle progression; however, the precise involvement of anion channels in this process is unclear. Here, Cl- currents in Ehrlich Lettre Ascites (ELA) cells were monitored during cell cycle progression, under three conditions: (i) after osmotic swelling (i.e., VRAC), (ii) after an increase in the free intracellular Ca2+ concentration (i.e., the Ca2+-activated Cl- current, CaCC), and (iii) under steady-state isotonic conditions. The maximal swelling-activated VRAC current decreased in G1 and increased in early S phase, compared to that in G0. The isotonic steady-state current, which seems to be predominantly VRAC, also decreased in G1, and increased again in early S phase, to a level similar to that in G0. In contrast, the maximal CaCC current (500 nM free Ca2+ in the pipette), was unaltered from G0 to G1, but decreased in early S phase. A novel high-affinity anion channel inhibitor, the acidic di-aryl-urea NS3728, which inhibited both VRAC and CaCC, attenuated ELA cell growth, suggesting a possible mechanistic link between cell cycle progression and cell cycle-dependent changes in the capacity for conductive Cl- transport. It is suggested that in ELA cells, entrance into the S phase requires an increase in VRAC activity and/or an increased potential for regulatory volume decrease (RVD), and at the same time a decrease in CaCC magnitude.