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

Late adolescence mortality in mice with brain-specific deletion of the volume-regulated anion channel subunit LRRC8A

The leucine-rich repeat-containing family 8 member A (LRRC8A) is an essential subunit of the volume-regulated anion channel (VRAC). VRAC is critical for cell volume control, but its broader physiological functions remain under investigation. Recent studies in the field indicate that Lrrc8a disruption in the brain astrocytes reduces neuronal excitability, impairs synaptic plasticity and memory, and protects against cerebral ischemia. In the present work, we generated brain-wide conditional LRRC8A knockout mice (LRRC8A bKO) using Nestin^Cre -driven Lrrc8a^flox/flox excision in neurons, astrocytes, and oligodendroglia. LRRC8A bKO animals were born close to the expected Mendelian ratio and developed without overt histological abnormalities, but, surprisingly, all died between 5 and 9 weeks of age with a seizure phenotype, which was confirmed by video and EEG recordings. Brain slice electrophysiology detected changes in the excitability of pyramidal cells and modified GABAergic inputs in the hippocampal CA1 region of LRRC8A bKO. LRRC8A-null hippocampi showed increased immunoreactivity of the astrocytic marker GFAP, indicating reactive astrogliosis. We also found decreased whole-brain protein levels of the GABA transporter GAT-1, the glutamate transporter GLT-1, and the astrocytic enzyme glutamine synthetase. Complementary HPLC assays identified reduction in the tissue levels of the glutamate and GABA precursor glutamine. Together, these findings suggest that VRAC provides vital control of brain excitability in mouse adolescence. VRAC deletion leads to a lethal phenotype involving progressive astrogliosis and dysregulation of astrocytic uptake and supply of amino acid neurotransmitters and their precursors.

LRRC8A influences the growth of gastric cancer cells via the p53 signaling pathway

BACKGROUND

Leucin-rich repeat containing protein A (LRRC8A), a component of the volume-regulated anion channel (VRAC), is activated by cell swelling and mediates regulatory volume decrease. We previously reported the expression of and important roles for several ion transporters in various gastrointestinal cancers, which have potential as novel targets for cancer treatment; however, the significance of LRRC8A in gastric cancer (GC) remains unclear.

MATERIALS AND METHODS

Knockdown experiments were performed by transfecting human GC cell lines with LRRC8A siRNA. Gene expression was then assessed using microarray analysis. Samples from 132 patients with GC were subjected to immunohistochemistry (IHC) for LRRC8A, and its relationships with clinicopathological factors and prognosis were examined.

RESULTS

The knockdown of LRRC8A suppressed the proliferation and movement of cells and enhanced apoptosis. The results of the microarray analysis showed the up- or down-regulated expression of genes related to the p53 signaling pathway (JNK, p53, p21, Bcl-2, and FAS) in LRRC8A-knockdown cells. IHC revealed a correlation between the expression of LRRC8A and the pT status (p = 0.015), and multivariate analysis identified the strong expression of LRRC8A as an independent prognostic factor for 5-year survival in GC patients (p = 0.0231).

CONCLUSIONS

The present results indicate that LRRC8A functions as a mediator of and/or biomarker for GC.

Beyond Single-Cell Analysis of Metallodrugs by ICP-MS: Targeting Cellular Substructures

Platinum compounds such as cisplatin (cisPt) embody the backbone of combination chemotherapy protocols against advanced lung cancer. However, their efficacy is primarily limited by inherent or acquired platinum resistance, the origin of which has not been fully elucidated yet, although of paramount interest. Using single cell inductively coupled plasma mass spectrometry (SC-ICP-MS), this study quantifies cisPt in single cancer cells and for the first time in isolated nuclei. A comparison of cisPt uptake was performed between a wild type (wt) cancer cell line and related resistant sublines. In both, resistant cells, wt cells, and their nuclei, cisPt uptake was measured at different incubation times. A lower amount of cisPt was found in resistant cell lines and their nuclei compared to wt cells. Moreover, the abundance of internalized cisPt decreased with increasing resistance. Interestingly, concentrations of cisPt found within the nuclei were higher than compared to cellular concentrations. Here, we show, that SC-ICP-MS allows precise and accurate quantification of metallodrugs in both single cells and cell organelles such as nuclei. These findings pave the way for future applications investigating the potency and efficacy of novel metallodrugs developed for cancer treatment.

On the molecular nature of large-pore channels

Membrane transport is a fundamental means to control basic cellular processes such as apoptosis, inflammation, and neurodegeneration and is mediated by a number of transporters, pumps, and channels. Accumulating evidence over the last half century has shown that a type of so-called "large-pore channel" exists in various tissues and organs in gap-junctional and non-gap-junctional forms in order to flow not only ions but also metabolites such as ATP. They are formed by a number of protein families with little or no evolutionary linkages including connexin, innexin, pannexin, leucine-rich repeat-containing 8 (LRRC8), and calcium homeostasis modulator (CALHM). This review summarizes the history and concept of large-pore channels starting from connexin gap junction channels to the more recent developments in innexin, pannexin, LRRC8, and CALHM. We describe structural and functional features of large-pore channels that are crucial for their diverse functions on the basis of available structures.

Sphingosine-1-Phosphate Induces ATP Release via Volume-Regulated Anion Channels in Breast Cell Lines

High interstitial level of ATP and its lysate adenosine in the cancer microenvironment are considered a halo mark of cancer. Adenosine acts as a strong immune suppressor. However, the source of ATP release is unclear. We clarified the release of ATP via volume-regulated anion channels (VRACs) in breast cell lines using an ATP luminescence imaging system. We detected a slowly rising diffuse pattern of ATP release that was only observed in undifferentiated cells, not in differentiated primary cultured cells. This was confirmed by suppression with DCPIB, a blocker of VRACs, and shRNA for LRRC8A, an indispensable subunit of VRACs. We herein demonstrated that the inflammatory mediator sphingosine-1-phosphate (S1P), which exists abundantly in the cancer microenvironment, induced a diffuse pattern of ATP release isovolumetrically. The response was dose-dependent and suppressed by the knock-down of LRRC8A. It was also suppressed by blockers of S1P receptor 1 and 2 (W146 and JTE013, respectively). RTqPCR demonstrated the prominent presence of S1PR1 and S1PR2 mRNAs. We discussed the roles of S1P-induced ATP release in the cancer microenvironment.

Targeting DNA Damage Response and Repair to Enhance Therapeutic Index in Cisplatin-Based Cancer Treatment

Platinum-based chemotherapies, such as cisplatin, play a large role in cancer treatment. The development of resistance and treatment toxicity creates substantial barriers to disease control, yet. To enhance the therapeutic index of cisplatin-based chemotherapy, it is imperative to circumvent resistance and toxicity while optimizing tumor sensitization. One of the primary mechanisms by which cancer cells develop resistance to cisplatin is through upregulation of DNA repair pathways. In this review, we discuss the DNA damage response in the context of cisplatin-induced DNA damage. We describe the proteins involved in the pathways and their roles in resistance development. Common biomarkers for cisplatin resistance and their utilization to improve patient risk stratification and treatment personalization are addressed. Finally, we discuss some of the current treatments and future strategies to circumvent the development of cisplatin resistance.

Potassium and Chloride Ion Channels in Cancer: A Novel Paradigm for Cancer Therapeutics

Cancer is a collection of diseases caused by specific changes at the genomic level that support cell proliferation indefinitely. Traditionally, ion channels are known to control a variety of cellular processes including electrical signal generation and transmission, secretion, and contraction by controlling ionic gradients. However, recent studies had brought to light important facts on ion channels in cancer biology.In this review we discuss the mechanism linking potassium or chloride ion channel activity to biochemical pathways controlling proliferation in cancer cells and the potential advantages of targeting ion channels as an anticancer therapeutic option.

Ternary Copper (II) Complex Induced Apoptosis and Cell Cycle Arrest in Colorectal Cancer Cells

BACKGROUND

The lack of specificity, severe side effects, and development of drug resistance have largely limited the use of platinum-based compounds in cancer treatment. Therefore, copper complexes have emerged as potential alternatives to platinum-based compounds.

OBJECTIVE

Ternary copper (II) complex incorporated with 1-10-phenanthroline and L-tyrosine was investigated for its anti-cancer effects in HT-29 colorectal cancer cells.

METHODS

Cytotoxic effects of ternary copper (II) complex in HT-29 cells were evaluated using MTT assay, Real-Time Cell Analysis (RTCA), and lactate dehydrogenase (LDH) assay. Cell cycle analysis was performed using flow cytometry. Apoptosis induction was studied by Annexin V-FITC/propidium iodide (PI) staining and mitochondrial membrane potential analysis (JC-10 staining) using flow cytometry. Intracellular reactive oxygen species (ROS) were detected by DCFH-DA assay. The expression of proteins involved in the apoptotic signalling pathway (p53, caspases, and PARP-1) was evaluated by western blot analysis.

RESULTS

Ternary copper (II) complex reduced the cell viability of HT-29 cells in a time- and dose-dependent manner, with IC50 of 2.4 ± 0.4 and 0.8 ± 0.04 µM at 24 and 48 hours, respectively. Cell cycle analysis demonstrated induction of S-phase cell cycle arrest. Morphological evaluation and Annexin V-FITC/PI flow cytometry analysis confirmed induction of apoptosis that was further supported by cleavage and activation of caspase-8, caspase-9, caspase-3, and PARP-1. Mutant p53 was also downregulated in a dose-dependent manner. No LDH release, mitochondrial membrane potential disruption, and ROS production were observed.

CONCLUSION

Ternary copper (II) complex holds great potential to be developed for colorectal cancer treatment.

Flow fluorometry quantification of anion channel VRAC subunit LRRC8A at the membrane of living U937 cells

Assessing the expression of channels on the cell membrane is a necessary step in studying the functioning of ion channels in living cells. We explore, first, if endogenous VRAC can be assayed using flow cytometry and a commercially available antibody against an extracellular loop of the LRRC8A, also known as SWELL1, subunit of the VRAC channel. The second goal is to determine if an increase in the number of VRAC channels at the cell membrane is responsible for an increase in chloride permeability of the membrane in two well-known cases: during staurosporine (STS)-induced apoptosis and after water balance disturbance caused by hypotonic medium. Human suspension lymphoid cells U937 were used as they are suitable for flow fluorometry and because we have recently studied their membrane chloride permeability during apoptosis. We found that surface expression of endogenous LRRC8A subunits can be quantified in living U937 cells using flow fluorometry with the Alomone Lab antibody. Further, we revealed that treatment of cells for 1 hour using STS or a hypotonic solution did not change the number of LRRC8A subunits to the extent that would correspond to changes in the membrane chloride permeability determined by ion content analysis. This indicates that prolonged increase in chloride permeability of the cell membrane during apoptotic cell shrinkage or cell volume regulation under hypotonicity in U937 cells occurs without altering cell surface expression of VRAC.

Targeting the Copper Transport System to Improve Treatment Efficacies of Platinum-Containing Drugs in Cancer Chemotherapy

The platinum (Pt)-containing antitumor drugs including cisplatin (cis-diamminedichloroplatinum II, cDDP), carboplatin, and oxaliplatin, have been the mainstay of cancer chemotherapy. These drugs are effective in treating many human malignancies. The major cell-killing target of Pt drugs is DNA. Recent findings underscored the important roles of Pt drug transport system in cancer therapy. While many mechanisms have been proposed for Pt-drug transport, the high-affinity copper transporter (hCtr1), Cu chaperone (Atox1), and Cu exporters (ATP7A and ATP7B) are also involved in cDDP transport, highlighting Cu homeostasis regulation in Pt-based cancer therapy. It was demonstrated that by reducing cellular Cu bioavailable levels by Cu chelators, hCtr1 is transcriptionally upregulated by transcription factor Sp1, which binds the promoters of Sp1 and hCtr1. In contrast, elevated Cu poisons Sp1, resulting in suppression of hCtr1 and Sp1, constituting the Cu-Sp1-hCtr1 mutually regulatory loop. Clinical investigations using copper chelator (trientine) in carboplatin treatment have been conducted for overcoming Pt drug resistance due in part to defective transport. While results are encouraging, future development may include targeting multiple steps in Cu transport system for improving the efficacies of Pt-based cancer chemotherapy. The focus of this review is to delineate the mechanistic interrelationships between Cu homeostasis regulation and antitumor efficacy of Pt drugs.

Zinc pyrithione activates the volume-regulated anion channel through an antioxidant-sensitive mechanism

Leucine-rich repeat-containing 8 (LRRC8) volume-regulated anion channels (VRACs) play important physiological roles in diverse cell types and may represent therapeutic targets for various diseases. To date, however, the pharmacological tools for evaluating the druggability of VRACs have been limited to inhibitors, as no activators of the channel have been reported. We therefore performed a fluorescence-based high-throughput screening (HTS) of 1,184 Food and Drug Administration-approved drugs for compounds that increase VRAC activity. The most potent VRAC potentiator identified was zinc pyrithione (ZPT), which is used commercially as an antifouling agent and for treating dandruff and other skin disorders. In intracellular Yellow Fluorescent Protein YFP(F46L/H148Q/I152L)-quenching assays, ZPT potentiates the rate and extent of swelling-induced iodide influx dose dependently with a half-maximal effective concentration (EC₅₀) of 5.7 µM. Whole cell voltage-clamp experiments revealed that coapplication of hypotonic solution and 30 µM ZPT to human embryonic kidney 293 or human colorectal carcinoma 116 cells increases the rate of swelling-induced VRAC activation by approximately 10-fold. ZPT potentiates swelling-induced VRAC currents after currents have reached a steady state and activates currents in the absence of cell swelling. Neither ZnCl₂ nor free pyrithione activated VRAC; however, treating cells with a mixture of ZnCl₂ and pyrithione led to robust channel activation. Finally, the effects of ZPT on VRAC were inhibited by reactive oxygen species (ROS) scavenger N-acetylcysteine (NAC) and NAD(P)H oxidase inhibitor diphenyleneiodonium chloride, suggesting the mechanism of action involves ROS generation. The discovery of ZPT as a potentiator/activator of VRAC demonstrates the utility of HTS for identifying small-molecule modulators of VRAC and adds to a growing repertoire of pharmacological tool compounds for probing the molecular physiology and regulation of this important channel.

Oxidant-resistant LRRC8A/C anion channels support superoxide production by NADPH oxidase 1

KEY POINTS

LRRC8A-containing anion channels associate with NADPH oxidase 1 (Nox1) and regulate superoxide production and tumour necrosis factor-α (TNFα) signalling. Here we show that LRRC8C and 8D also co-immunoprecipitate with Nox1 in vascular smooth muscle cells. LRRC8C knockdown inhibited TNFα-induced O₂ ^•- production, receptor endocytosis, nuclear factor-κB (NF-κB) activation and proliferation while LRRC8D knockdown enhanced NF-κB activation. Significant changes in LRRC8 isoform expression in human atherosclerosis and psoriasis suggest compensation for increased inflammation. The oxidant chloramine-T (ChlorT, 1 mM) weakly (∼25%) inhibited LRRC8C currents but potently (∼80%) inhibited LRRC8D currents. Substitution of the extracellular loop (EL1, EL2) domains of 8D into 8C conferred significantly stronger (69%) ChlorT-dependent inhibition. ChlorT exposure impaired subsequent current block by DCPIB, which occurs through interaction with EL1, further implicating external oxidation sites. LRRC8A/C channels most effectively sustain Nox1 activity at the plasma membrane. This may result from their ability to remain active in an oxidized microenvironment.

ABSTRACT

Tumour necrosis factor-α (TNFα) activates NADPH oxidase 1 (Nox1) in vascular smooth muscle cells (VSMCs), producing superoxide (O₂ ^•- ) required for subsequent signalling. LRRC8 family proteins A-E comprise volume-regulated anion channels (VRACs). The required subunit LRRC8A physically associates with Nox1, and VRAC activity is required for Nox activity and the inflammatory response to TNFα. VRAC currents are modulated by oxidants, suggesting that channel oxidant sensitivity and proximity to Nox1 may play a physiologically relevant role. In VSMCs, LRRC8C knockdown (siRNA) recapitulated the effects of siLRRC8A, inhibiting TNFα-induced extracellular and endosomal O₂ ^•- production, receptor endocytosis, nuclear factor-κB (NF-κB) activation and proliferation. In contrast, siLRRC8D potentiated NF-κB activation. Nox1 co-immunoprecipitated with 8C and 8D, and colocalized with 8D at the plasma membrane and in vesicles. We compared VRAC currents mediated by homomeric and heteromeric LRRC8C and LRRC8D channels expressed in HEK293 cells. The oxidant chloramine T (ChlorT, 1 mM) weakly inhibited 8C, but potently inhibited 8D currents. ChlorT exposure also impaired subsequent current block by the VRAC blocker DCPIB, implicating external sites of oxidation. Substitution of the 8D extracellular loop domains (EL1, EL2) into 8C conferred significantly stronger ChlorT-mediated inhibition of 8C currents. Our results suggest that LRRC8A/C channel activity can be effectively maintained in the oxidized microenvironment expected to result from Nox1 activation at the plasma membrane. Increased ratios of 8D:8C expression may potentially depress inflammatory responses to TNFα. LRRC8A/C channel downregulation represents a novel strategy to reduce TNFα-induced inflammation.

Regulation of Anion Channel LRRC8 Volume-Regulated Anion Channels in Transport of 2'3'-Cyclic GMP-AMP and Cisplatin under Steady State and Inflammation

The recently identified anion channel LRRC8 volume-regulated anion channels (VRACs) are heteromeric hexamers constituted with the obligate LRRC8A subunit paired with at least one of the accessory LRRC8B to LRRC8E subunits. In addition to transport chloride, taurine, and glutamate, LRRC8 VRACs also transport the anticancer agent cisplatin and STING agonists 2'3'-cyclic GMP-AMP (cGAMP) and cyclic dinucleotides; hence, they are implicated in a variety of physiological and pathological processes, such as cell swelling, stroke, cancer, and viral infection. Although the subunit composition largely determines VRAC substrate specificity, the opening of various VRAC pores under physiological and pathological settings remains enigmatic. In this study, we demonstrated that VRACs comprising LRRC8A and LRRC8E (LRRC8A/E-containing VRACs), specialized in cGAMP transport, can be opened by a protein component present in serum under resting condition. Serum depletion ablated the tonic activity of LRRC8A/E-containing VRACs, decreasing cGAMP transport in various human and murine cells. Also, heating or proteinase K treatment abolished the ability of serum to activate VRAC. Genetic analyses revealed a crucial role for cGAMP synthase (cGAS) in serum/TNF-promoted VRAC activation. Notably, the presence of cGAS on the plasma membrane, rather than its DNA-binding or enzymatic activity, enabled VRAC activation. Moreover, phospholipid PIP2 seemed to be instrumental in the membrane localization of cGAS and its association with VRACs. Corroborating a role for LRRC8A/D-containing VRACs in cisplatin transport, serum and TNF markedly potentiated cisplatin uptake and killing of cancer cells derived from human or mouse. Together, these observations provide new insights into the complex regulation of VRAC activation and suggest a novel approach to enhance the efficacy of cGAMP and cisplatin in treating infection and cancer.

The Role of Nucleotide Excision Repair in Cisplatin-Induced Peripheral Neuropathy: Mechanism, Prevention, and Treatment

Platinum-based chemotherapy-induced peripheral neuropathy (CIPN) is one of the most common dose-limiting effects of cancer treatment and results in dose reduction and discontinuation of life-saving chemotherapy. Its debilitating effects are often permanent and lead to lifelong impairment of quality of life in cancer patients. While the mechanisms underlying the toxicity are not yet fully defined, dorsal root ganglia sensory neurons play an integral role in symptom development. DNA-platinum adducts accumulate in these cells and inhibit normal cellular function. Nucleotide excision repair (NER) is integral to the repair of platinum adducts, and proteins involved in its mechanism serve as potential targets for future therapeutics. This review aims to highlight NER’s role in cisplatin-induced peripheral neuropathy, summarize current clinical approaches to the toxicity, and discuss future perspectives for the prevention and treatment of CIPN.

Ectonucleotidases in Acute and Chronic Inflammation

Ectonucleotidases are extracellular enzymes with a pivotal role in inflammation that hydrolyse extracellular purine and pyrimidine nucleotides, e.g., ATP, UTP, ADP, UDP, AMP and NAD⁺. Ectonucleotidases, expressed by virtually all cell types, immune cells included, either as plasma membrane-associated or secreted enzymes, are classified into four main families: 1) nucleoside triphosphate diphosphohydrolases (NTPDases), 2) nicotinamide adenine dinucleotide glycohydrolase (NAD glycohydrolase/ADP-ribosyl cyclase/cyclic ADP-ribose hydrolase 1), 3) ecto-5′-nucleotidase (NT5E), and 4) ecto-nucleotide pyrophosphatase/phosphodiesterases (NPPs). Concentration of ATP, UTP and NAD⁺ can be increased in the extracellular space thanks to un-regulated, e.g., cell damage or cell death, or regulated processes. Regulated processes include secretory exocytosis, connexin or pannexin hemichannels, ATP binding cassette (ABC) transporters, calcium homeostasis modulator (CALMH) channels, the ATP-gated P2X7 receptor, maxi-anion channels (MACs) and volume regulated ion channels (VRACs). Hydrolysis of extracellular purine nucleotides generates adenosine, an important immunosuppressant. Extracellular nucleotides and nucleosides initiate or dampen inflammation via P2 and P1 receptors, respectively. All these agents, depending on their level of expression or activation and on the agonist concentration, are potent modulators of inflammation and key promoters of host defences, immune cells activation, pathogen clearance, tissue repair and regeneration. Thus, their knowledge is of great importance for a full understanding of the pathophysiology of acute and chronic inflammatory diseases. A selection of these pathologies will be briefly discussed here.

Cell Death Induction and Protection by Activation of Ubiquitously Expressed Anion/Cation Channels. Part 1: Roles of VSOR/VRAC in Cell Volume Regulation, Release of Double-Edged Signals and Apoptotic/Necrotic Cell Death

Cell volume regulation (CVR) is essential for survival and functions of animal cells. Actually, normotonic cell shrinkage and swelling are coupled to apoptotic and necrotic cell death and thus called the apoptotic volume decrease (AVD) and the necrotic volume increase (NVI), respectively. A number of ubiquitously expressed anion and cation channels are involved not only in CVD but also in cell death induction. This series of review articles address the question how cell death is induced or protected with using ubiquitously expressed ion channels such as swelling-activated anion channels, acid-activated anion channels and several types of TRP cation channels including TRPM2 and TRPM7. The Part 1 focuses on the roles of the volume-sensitive outwardly rectifying anion channels (VSOR), also called the volume-regulated anion channel (VRAC), which is activated by cell swelling or reactive oxygen species (ROS) in a manner dependent on intracellular ATP. First we describe phenotypical properties, the molecular identity, and physical pore dimensions of VSOR/VRAC. Second, we highlight the roles of VSOR/VRAC in the release of organic signaling molecules, such as glutamate, glutathione, ATP and cGAMP, that play roles as double-edged swords in cell survival. Third, we discuss how VSOR/VRAC is involved in CVR and cell volume dysregulation as well as in the induction of or protection from apoptosis, necrosis and regulated necrosis under pathophysiological conditions.

KCa3.1-dependent uptake of the cytotoxic DNA-binding dye Hoechst 33258 into cancerous but not healthy cervical cells

The poor and nonselective penetration of current chemotherapeutics across the plasma membranes of cancer cells, which is necessary for the targeted disruption of the intracellular machinery, remains a major pharmaceutical challenge. In several cell types, including mast cells and macrophages, exposure to extracellular ATP is known to stimulate passive entry of large and otherwise membrane impermeable cationic dyes, which is usually attributed to conduction through ionotropic P2X receptors. Here, we report that elevations in cytosolic Ca2+ stimulate the rapid uptake and nuclear accumulation of a DNA-binding fluorescent cation, Hoechst 33258 (H33258), in cervical cancer cells. The H33258 uptake was dependent on activation of intermediate conductance Ca2+-activated K+ channels (KCa3.1), and direct stimulation of the channel with the activators SKA 31 and DCEBIO was sufficient to induce cellular uptake of H33258 directly. In contrast to the results from cancerous cervical cells, KCa3.1-dependent H33258 uptake was rarely observed in epithelial cells derived from the ectocervix and transformation zone of healthy cervical tissue. Furthermore, whole-cell patch clamp experiments and assessment of membrane potential using the slow voltage-sensitive dye bis-(1,3-diethylthiobarbituric acid)trimethine oxonol revealed a significant difference in functional KCa3.1 activity between cancerous and healthy cervical epithelial cells, which correlated strongly with the incidence of KCa3.1-dependent H33258 uptake. Finally, we show that activation of KCa3.1 channels caused a modest but significant sensitization of cancer cells to the growth suppressant effects of H33258, lending plausibility to the idea of using KCa3.1 channel activators to enhance cell penetration of small cationic toxins into cancer cells expressing these channels.

The rediscovery of platinum-based cancer therapy

Platinum (Pt) compounds entered the clinic as anticancer agents when cisplatin was approved in 1978. More than 40 years later, even in the era of precision medicine and immunotherapy, Pt drugs remain among the most widely used anticancer drugs. As Pt drugs mainly target DNA, it is not surprising that recent insights into alterations of DNA repair mechanisms provide a useful explanation for their success. Many cancers have defective DNA repair, a feature that also sheds new light on the mechanisms of secondary drug resistance, such as the restoration of DNA repair pathways. In addition, genome-wide functional screening approaches have revealed interesting insights into Pt drug uptake. About half of cisplatin and carboplatin but not oxaliplatin may enter cells through the widely expressed volume-regulated anion channel (VRAC). The analysis of this heteromeric channel in tumour biopsies may therefore be a useful biomarker to stratify patients for initial Pt treatments. Moreover, Pt-based approaches may be improved in the future by the optimization of combinations with immunotherapy, management of side effects and use of nanodelivery devices. Hence, Pt drugs may still be part of the standard of care for several cancers in the coming years.

Cellular basis of ClC-2 Cl- channel-related brain and testis pathologies

The ClC-2 chloride channel is expressed in the plasma membrane of almost all mammalian cells. Mutations that cause the loss of ClC-2 function lead to retinal and testicular degeneration and leukodystrophy, whereas gain-of-function mutations cause hyperaldosteronism. Leukodystrophy is also observed with a loss of GlialCAM, a cell adhesion molecule that binds to ClC-2 in glia. GlialCAM changes the localization of ClC-2 and opens the channel by altering its gating. We now used cell type-specific deletion of ClC-2 in mice to show that retinal and testicular degeneration depend on a loss of ClC-2 in retinal pigment epithelial cells and Sertoli cells, respectively, whereas leukodystrophy was fully developed only when ClC-2 was disrupted in both astrocytes and oligodendrocytes. The leukodystrophy of Glialcam-/- mice could not be rescued by crosses with Clcn2op/op mice in which a mutation mimics the "opening" of ClC-2 by GlialCAM. These data indicate that GlialCAM-induced changes in biophysical properties of ClC-2 are irrelevant for GLIALCAM-related leukodystrophy. Taken together, our findings suggest that the pathology caused by Clcn2 disruption results from disturbed extracellular ion homeostasis and identifies the cells involved in this process.

LRRC8A homohexameric channels poorly recapitulate VRAC regulation and pharmacology

Swelling-activated volume-regulated anion channels (VRACs) are heteromeric channels comprising LRRC8A and at least one other LRRC8 paralog. Cryoelectron microscopy (cryo-EM) structures of nonnative LRRC8A and LRRC8D homohexamers have been described. We demonstrate here that LRRC8A homohexamers poorly recapitulate VRAC functional properties. Unlike VRACs, LRRC8A channels heterologously expressed in Lrr8c^-/- HCT116 cells are poorly activated by low intracellular ionic strength (µ) and insensitive to cell swelling with normal µ. Combining low µ with swelling modestly activates LRRC8A, allowing characterization of pore properties. VRACs are strongly inhibited by 10 µM 4-[(2-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]butanoic acid (DCPIB) in a voltage-independent manner. In contrast, DCPIB block of LRRC8A is weak and voltage sensitive. Cryo-EM structures indicate that DCPIB block is dependent on arginine 103. Consistent with this, LRRC8A R103F mutants are insensitive to DCPIB. However, an LRRC8 chimeric channel in which R103 is replaced by a leucine at the homologous position is inhibited ∼90% by 10 µM DCPIB in a voltage-independent manner. Coexpression of LRRC8A and LRRC8C gives rise to channels with DCPIB sensitivity that is strongly µ dependent. At normal intracellular µ, LRRC8A + LRRC8C heteromers exhibit strong, voltage-independent DCPIB block that is insensitive to R103F. DCPIB inhibition is greatly reduced and exhibits voltage dependence with low intracellular µ. The R103F mutation has no effect on maximal DCPIB inhibition but eliminates voltage dependence under low µ conditions. Our findings demonstrate that the LRRC8A cryo-EM structure and the use of heterologously expressed LRRC8 heteromeric channels pose significant limitations for VRAC mutagenesis-based structure-function analysis. Native VRAC function is most closely mimicked by chimeric LRRC8 homomeric channels.

Ca2+ Dependence of Volume-Regulated VRAC/LRRC8 and TMEM16A Cl- Channels

All vertebrate cells activate Cl- currents (ICl ,swell) when swollen by hypotonic bath solution. The volume-regulated anion channel VRAC has now been identified as LRRC8/SWELL1. However, apart from VRAC, the Ca2+-activated Cl- channel (CaCC) TMEM16A and the phospholipid scramblase and ion channel TMEM16F were suggested to contribute to cell swelling-activated whole-cell currents. Cell swelling was shown to induce Ca2+ release from the endoplasmic reticulum and to cause subsequent Ca2+ influx. It is suggested that TMEM16A/F support intracellular Ca2+ signaling and thus Ca2+-dependent activation of VRAC. In the present study, we tried to clarify the contribution of TMEM16A to ICl ,swell. In HEK293 cells coexpressing LRRC8A and LRRC8C, we found that activation of ICl ,swell by hypotonic bath solution (Hypo; 200 mosm/l) was Ca2+ dependent. TMEM16A augmented the activation of LRRC8A/C by enhancing swelling-induced local intracellular Ca2+ concentrations. In HT29 cells, knockdown of endogenous TMEM16A attenuated ICl ,swell and changed time-independent swelling-activated currents to VRAC-typical time-dependent currents. Activation of ICl ,swell by Hypo was attenuated by blocking receptors for inositol trisphosphate and ryanodine (IP3R; RyR), as well as by inhibiting Ca2+ influx. The data suggest that TMEM16A contributes directly to ICl ,swell as it is activated through swelling-induced Ca2+ increase. As activation of VRAC is shown to be Ca2+-dependent, TMEM16A augments VRAC currents by facilitating Hypo-induced Ca2+ increase in submembraneous signaling compartments by means of ER tethering.

Ions, the Movement of Water and the Apoptotic Volume Decrease

The movement of water across the cell membrane is a natural biological process that occurs during growth, cell division, and cell death. Many cells are known to regulate changes in their cell volume through inherent compensatory regulatory mechanisms. Cells can sense an increase or decrease in their cell volume, and compensate through mechanisms known as a regulatory volume increase (RVI) or decrease (RVD) response, respectively. The transport of sodium, potassium along with other ions and osmolytes allows the movement of water in and out of the cell. These compensatory volume regulatory mechanisms maintain a cell at near constant volume. A hallmark of the physiological cell death process known as apoptosis is the loss of cell volume or cell shrinkage. This loss of cell volume is in stark contrast to what occurs during the accidental cell death process known as necrosis. During necrosis, cells swell or gain water, eventually resulting in cell lysis. Thus, whether a cell gains or loses water after injury is a defining feature of the specific mode of cell death. Cell shrinkage or the loss of cell volume during apoptosis has been termed apoptotic volume decrease or AVD. Over the years, this distinguishing feature of apoptosis has been largely ignored and thought to be a passive occurrence or simply a consequence of the cell death process. However, studies on AVD have defined an underlying movement of ions that result in not only the loss of cell volume, but also the activation and execution of the apoptotic process. This review explores the role ions play in controlling not only the movement of water, but the regulation of apoptosis. We will focus on what is known about specific ion channels and transporters identified to be involved in AVD, and how the movement of ions and water change the intracellular environment leading to stages of cell shrinkage and associated apoptotic characteristics. Finally, we will discuss these concepts as they apply to different cell types such as neurons, cardiomyocytes, and corneal epithelial cells.

Looking back at multidrug resistance (MDR) research and ten mistakes to be avoided when writing about ABC transporters in MDR

This paper presents a personal, selective, and sometimes critical retrospective of the history of ABC transporters in multidrug resistance (MDR) of cancer cells, overrepresenting discoveries of some early pioneers, long forgotten, and highlights of research in Amsterdam, mainly focussing on discoveries made with disruptions of ABC genes in mice (KO mice) and on the role of ABC transporters in causing drug resistance in a mouse model of mammary cancer. The history is complemented by a list of erroneous concepts often found in papers and grant applications submitted anno 2020.

LRRC8A is essential for hypotonicity-, but not for DAMP-induced NLRP3 inflammasome activation

The NLRP3 inflammasome is a multi-molecular protein complex that converts inactive cytokine precursors into active forms of IL-1β and IL-18. The NLRP3 inflammasome is frequently associated with the damaging inflammation of non-communicable disease states and is considered an attractive therapeutic target. However, there is much regarding the mechanism of NLRP3 activation that remains unknown. Chloride efflux is suggested as an important step in NLRP3 activation, but which chloride channels are involved is still unknown. We used chemical, biochemical, and genetic approaches to establish the importance of chloride channels in the regulation of NLRP3 in murine macrophages. Specifically, we identify LRRC8A, an essential component of volume-regulated anion channels (VRAC), as a vital regulator of hypotonicity-induced, but not DAMP-induced, NLRP3 inflammasome activation. Although LRRC8A was dispensable for canonical DAMP-dependent NLRP3 activation, this was still sensitive to chloride channel inhibitors, suggesting there are additional and specific chloride sensing and regulating mechanisms controlling NLRP3.

LRRC8 channel activation and reduction in cytosolic chloride concentration during early differentiation of C2C12 myoblasts

Leucine-rich repeat containing family 8 (LRRC8) proteins form the volume-regulated anion channel (VRAC). Recently, they were shown to be required for normal differentiation and fusion of C2C12 myoblasts, by promoting membrane hyperpolarization and intracellular Ca²⁺ signals. However, the mechanism by which they are involved remained obscure. Here, using a FRET-based sensor for VRAC activity, we show temporary activation of VRAC within the first 2 h of myogenic differentiation. During this period, we also observed a significant decrease in the intracellular Cl^- concentration that was abolished by the VRAC inhibitor carbenoxolone. However, lowering the intracellular Cl^- concentration by extracellular Cl^- depletion did not promote differentiation as judged by the percentage of myogenin-positive nuclei or total myogenin levels in C2C12 cells. Instead, it inhibited myosin expression and myotube formation. Together, these data suggest that VRAC is activated and mediates Cl^- efflux early on during myogenic differentiation, and a moderate intracellular Cl^- concentration is necessary for myoblast fusion.

LRRC8A:C/E Heteromeric Channels Are Ubiquitous Transporters of cGAMP

Extracellular 2'3'-cyclic-GMP-AMP (cGAMP) is an immunotransmitter exported by diseased cells and imported into host cells to activate the innate immune STING pathway. We previously identified SLC19A1 as a cGAMP importer, but its use across human cell lines is limited. Here, we identify LRRC8A heteromeric channels, better known as volume-regulated anion channels (VRAC), as widely expressed cGAMP transporters. LRRC8A forms complexes with LRRC8C and/or LRRC8E, depending on their expression levels, to transport cGAMP and other 2'3'-cyclic dinucleotides. In contrast, LRRC8D inhibits cGAMP transport. We demonstrate that cGAMP is effluxed or influxed via LRRC8 channels, as dictated by the cGAMP electrochemical gradient. Activation of LRRC8A channels, which can occur under diverse stresses, strongly potentiates cGAMP transport. We identify activator sphingosine 1-phosphate and inhibitor DCPIB as chemical tools to manipulate channel-mediated cGAMP transport. Finally, LRRC8A channels are key cGAMP transporters in resting primary human vasculature cells and universal human cGAMP transporters when activated.

LRRC8 family proteins within lysosomes regulate cellular osmoregulation and enhance cell survival to multiple physiological stresses

LRRC8 family proteins on the plasma membrane play a critical role in cellular osmoregulation by forming volume-regulated anion channels (VRACs) necessary to prevent necrotic cell death. We demonstrate that intracellular LRRC8 proteins acting within lysosomes also play an essential role in cellular osmoregulation. LRRC8 proteins on lysosome membranes generate large lysosomal volume-regulated anion channel (Lyso-VRAC) currents in response to low cytoplasmic ionic strength conditions. When a double-leucine L⁷⁰⁶L⁷⁰⁷ motif at the C terminus of LRRC8A was mutated to alanines, normal plasma membrane VRAC currents were still observed, but Lyso-VRAC currents were absent. We used this targeting mutant, as well as pharmacological tools, to demonstrate that Lyso-VRAC currents are necessary for the formation of large lysosome-derived vacuoles, which store and then expel excess water to maintain cytosolic water homeostasis. Thus, Lyso-VRACs allow lysosomes of mammalian cells to act as the cell`s "bladder." When Lyso-VRAC current was selectively eliminated, the extent of necrotic cell death to sustained stress was greatly increased, not only in response to hypoosmotic stress, but also to hypoxic and hypothermic stresses. Thus Lyso-VRACs play an essential role in enabling cells to mount successful homeostatic responses to multiple stressors.

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.

Platinum-Based Combination Therapy: Molecular Rationale, Current Clinical Uses, and Future Perspectives

Platinum drugs are common in chemotherapy, but their clinical applications have been limited due to drug resistance and severe toxic effects. The combination of platinum drugs with other drugs with different mechanisms of anticancer action, especially checkpoint inhibitors, is increasingly popular. This combination is the leading strategy to improve the therapeutic efficiency and minimize the side effects of platinum drugs. In this review, we focus on the mechanistic basis of the combinations of platinum-based drugs with other drugs to inspire the development of more promising platinum-based combination regimens in clinical trials as well as novel multitargeting platinum drugs overcoming drug resistance and toxicities resulting from current platinum drugs.

Functional Radiogenetic Profiling Implicates ERCC6L2 in Non-homologous End Joining

Using genome-wide radiogenetic profiling, we functionally dissect vulnerabilities of cancer cells to ionizing radiation (IR). We identify ERCC6L2 as a major determinant of IR response, together with classical DNA damage response genes and members of the recently identified shieldin and CTC1-STN1-TEN1 (CST) complexes. We show that ERCC6L2 contributes to non-homologous end joining (NHEJ), and it may exert this function through interactions with SFPQ. In addition to causing radiosensitivity, ERCC6L2 loss restores DNA end resection and partially rescues homologous recombination (HR) in BRCA1-deficient cells. As a consequence, ERCC6L2 deficiency confers resistance to poly (ADP-ribose) polymerase (PARP) inhibition in tumors deficient for both BRCA1 and p53. Moreover, we show that ERCC6L2 mutations are found in human tumors and correlate with a better overall survival in patients treated with radiotherapy (RT); this finding suggests that ERCC6L2 is a predictive biomarker of RT response.

Generation of Stable cisPt Resistant Lung Adenocarcinoma Cells

Platinum compounds represent the backbone of combined chemotherapy protocols for advanced lung cancer. The mechanisms responsible for its frequent primary or acquired resistance to cisplatin (cisPt)-based chemotherapy remains enigmatic. The availability of two cell lines of the same origin, one resistant and the other sensitive, will facilitate research to reveal the mechanism of resistance formation. Lung adenocarcinoma cells, A240286S (A24), were cultivated in increasing cisPt concentrations over a prolonged time. After a significant increase in IC₅₀ was measured, cultivation of the cells was continued in absence of cisPt and IC₅₀s determined over a long period (>7 months). As a result, a cell line with lasting, high-level cisPt resistance, designated (D-)A24cisPt8.0, was obtained. The cells were cross-resistant to oxaliplatin and to pemetrexed at a low level. Previous publications have claimed that Leucine-rich repeat-containing protein 8 (LRRC8A and LRRC8D) of the volume-regulated anion channels (VRACs) affect cellular resistance to cisPt. Even though cisPt decreased LRRC8D expression levels, we showed by knockdown and overexpression experiments with LRRC8A and D that these proteins do not govern the observed cisPt resistance. The tumor cell sublines described here provide a powerful model to study the mechanisms of resistance to cisPt in lung cancer cells and beyond.

Cryo-EM structure of the volume-regulated anion channel LRRC8D isoform identifies features important for substrate permeation

Members of the leucine-rich repeat-containing 8 (LRRC8) protein family, composed of the five LRRC8A-E isoforms, are pore-forming components of the volume-regulated anion channel (VRAC). LRRC8A and at least one of the other LRRC8 isoforms assemble into heteromers to generate VRAC transport activities. Despite the availability of the LRRC8A structures, the structural basis of how LRRC8 isoforms other than LRRC8A contribute to the functional diversity of VRAC has remained elusive. Here, we present the structure of the human LRRC8D isoform, which enables the permeation of organic substrates through VRAC. The LRRC8D homo-hexamer structure displays a two-fold symmetric arrangement, and together with a structure-based electrophysiological analysis, revealed two key features. The pore constriction on the extracellular side is wider than that in the LRRC8A structures, which may explain the increased permeability of organic substrates. Furthermore, an N-terminal helix protrudes into the pore from the intracellular side and may be critical for gating.

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.

A genome-wide enrichment screen identifies NUMA1-loss as a resistance mechanism against mitotic cell-death induced by BMI1 inhibition

BMI1 is a core protein of the polycomb repressive complex 1 (PRC1) that is overexpressed in several cancer types, making it a promising target for cancer therapies. However, the underlying mechanisms and interactions associated with BMI1-induced tumorigenesis are often context-dependent and complex. Here, we performed a drug resistance screen on mutagenized human haploid HAP1 cells treated with BMI1 inhibitor PTC-318 to find new genetic and mechanistic features associated with BMI1-dependent cancer cell proliferation. Our screen identified NUMA1-mutations as the most significant inducer of PTC-318 cell death resistance. Independent validations on NUMA1-proficient HAP1 and non-small cell lung cancer cell lines exposed to BMI1 inhibition by PTC-318 or BMI1 knockdown resulted in cell death following mitotic arrest. Interestingly, cells with CRISPR-Cas9 derived NUMA1 knockout also showed a mitotic arrest phenotype following BMI1 inhibition but, contrary to cells with wildtype NUMA1, these cells were resistant to BMI1-dependent cell death. The current study brings new insights to BMI1 inhibition-induced mitotic lethality in cancer cells and presents a previously unknown role of NUMA1 in this process.

Transfer of cGAMP into Bystander Cells via LRRC8 Volume-Regulated Anion Channels Augments STING-Mediated Interferon Responses and Anti-viral Immunity

The enzyme cyclic GMP-AMP synthase (cGAS) senses cytosolic DNA in infected and malignant cells and catalyzes the formation of 2'3'cGMP-AMP (cGAMP), which in turn triggers interferon (IFN) production via the STING pathway. Here, we examined the contribution of anion channels to cGAMP transfer and anti-viral defense. A candidate screen revealed that inhibition of volume-regulated anion channels (VRACs) increased propagation of the DNA virus HSV-1 but not the RNA virus VSV. Chemical blockade or genetic ablation of LRRC8A/SWELL1, a VRAC subunit, resulted in defective IFN responses to HSV-1. Biochemical and electrophysiological analyses revealed that LRRC8A/LRRC8E-containing VRACs transport cGAMP and cyclic dinucleotides across the plasma membrane. Enhancing VRAC activity by hypotonic cell swelling, cisplatin, GTPγS, or the cytokines TNF or interleukin-1 increased STING-dependent IFN response to extracellular but not intracellular cGAMP. Lrrc8e^-/- mice exhibited impaired IFN responses and compromised immunity to HSV-1. Our findings suggest that cell-to-cell transmission of cGAMP via LRRC8/VRAC channels is central to effective anti-viral immunity.

LRRCA8A and ANO1 contribute to serum-induced VRAC in a Ca2+-dependent manners

The volume-regulated anion channel (VRAC) plays a central role in maintaining cell volume in response to osmotic stress. Leucine-rich repeat-containing 8A (LRRC8A) was recently identified as an essential component of VRAC although other Cl^- channels were also suggested to contribute to VRAC. VRAC is activated when a cell is challenged with a hypotonic environment or even in isotonic conditions challenged with different stimuli. It is not clear how VRAC is activated and whether activation of VRAC in hypotonic and isotonic conditions share the same mechanism. In this present study, we investigated relative contribution of LRRC8A and anoctamin 1(ANO1) to VRAC currents activated by fetal bovine serum (FBS) in isotonic condition, and studied the role of intracellular Ca²⁺ in this activation. We used CRISPR/Cas9 gene editing approach, electrophysiology, and pharmacology approaches to show that VRAC currents induced by FBS is mostly mediated by LRRC8A in HEK293 cells, but also with significant contribution from ANO1. FBS induces Ca²⁺ transients and these Ca²⁺ signals are required for the activation of VRAC by serum. These findings will help to further understand the mechanism in activation of VRAC.

Functions and mechanisms of circular RNAs in cancer radiotherapy and chemotherapy resistance

Circular RNAs (circRNAs), one type of non-coding RNA, were initially misinterpreted as nonfunctional products of pre-mRNA mis-splicing. Currently, circRNAs have been proven to manipulate the functions of diverse molecules, including non-coding RNAs, mRNAs, DNAs and proteins, to regulate cell activities in physiology and pathology. Accumulating evidence indicates that circRNAs play critical roles in tumor genesis, development, and sensitivity to radiation and chemotherapy. Radiotherapy and chemotherapy are two primary types of intervention for most cancers, but their therapeutic efficacies are usually retarded by intrinsic and acquired resistance. Thus, it is urgent to develop new strategies to improve therapeutic responses. To achieve this, clarification of the underlying mechanisms affecting therapeutic responses in cancer is needed. This review summarizes recent progress and mechanisms of circRNAs in cancer resistance to radiation and chemotherapy, and it discusses the limitations of available knowledge and potential future directions.

Chloride transporters and channels in β-cell physiology: revisiting a 40-year-old model

It is accepted that insulin-secreting β-cells release insulin in response to glucose even in the absence of functional ATP-sensitive K+ (KATP)-channels, which play a central role in a 'consensus model' of secretion broadly accepted and widely reproduced in textbooks. A major shortcoming of this consensus model is that it ignores any and all anionic mechanisms, known for more than 40 years, to modulate β-cell electrical activity and therefore insulin secretion. It is now clear that, in addition to metabolically regulated KATP-channels, β-cells are equipped with volume-regulated anion (Cl-) channels (VRAC) responsive to glucose concentrations in the range known to promote electrical activity and insulin secretion. In this context, the electrogenic efflux of Cl- through VRAC and other Cl- channels known to be expressed in β-cells results in depolarization because of an outwardly directed Cl- gradient established, maintained and regulated by the balance between Cl- transporters and channels. This review will provide a succinct historical perspective on the development of a complex hypothesis: Cl- transporters and channels modulate insulin secretion in response to nutrients.

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

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.

Leucine-rich repeat containing 8A contributes to the expansion of brain ventricles in zebrafish embryos

The sodium osmotic gradient is necessary for the initiation of brain ventricle inflation, but a previous study predicted that organic and inorganic osmolytes play equivalently important roles in osmotic homeostasis in astrocytes. To test whether organic osmoregulation also plays a role in brain ventricle inflation, the core component for volume-regulated anion and organic osmolyte channel, lrrc8a, was investigated in the zebrafish model. RT-PCR and whole-mount in situ hybridization indicated that both genes were ubiquitously expressed through to 12 hpf, and around the ventricular layer of neural tubes and the cardiogenic region at 24 hpf. Knocking down either one lrrc8a paralog with morpholino oligos resulted in abnormalities in circulation at 32 hpf. Morpholino oligos or CRISPR interference against either paralog led to smaller brain ventricles at 24 hpf. Either lrrc8aa or lrrc8ab mRNA rescued the phenotypic penetrance in both lrrc8aa and lrrc8ab morphants. Supplementation of taurine in the E3 medium and overexpression csad mRNA also rescued lrrc8aa and lrrc8ab morphants. Our results indicate that the two zebrafish lrrc8a paralogs are maternal message genes and are ubiquitously expressed in early embryos. The two genes play redundant roles in the expansion of brain ventricles and the circulatory system and taurine contributes to brain ventricle expansion via the volume-regulated anion and organic osmolyte channels.

What biologists want from their chloride reporters – a conversation between chemists and biologists

Impaired chloride transport affects diverse processes ranging from neuron excitability to water secretion, which underlie epilepsy and cystic fibrosis, respectively. The ability to image chloride fluxes with fluorescent probes has been essential for the investigation of the roles of chloride channels and transporters in health and disease. Therefore, developing effective fluorescent chloride reporters is critical to characterizing chloride transporters and discovering new ones. However, each chloride channel or transporter has a unique functional context that demands a suite of chloride probes with appropriate sensing characteristics. This Review seeks to juxtapose the biology of chloride transport with the chemistries underlying chloride sensors by exploring the various biological roles of chloride and highlighting the insights delivered by studies using chloride reporters. We then delineate the evolution of small-molecule sensors and genetically encoded chloride reporters. Finally, we analyze discussions with chloride biologists to identify the advantages and limitations of sensors in each biological context, as well as to recognize the key design challenges that must be overcome for developing the next generation of chloride sensors.

Impact of the histone deacetylase inhibitor trichostatin A on active uptake, volume-sensitive release of taurine, and cell fate in human ovarian cancer cells

The histone deacetylase inhibitor trichostatin A (TSA) reduces cell viability in cisplatin-sensitive (A2780WT) and cisplatin-resistant (A2780RES) human ovarian cancer cells due to progression of apoptosis (increased caspase-9 activity), autophagy (increased LC3-II expression), and cell cycle arrest (increased p21 expression). The TSA-mediated effect on p21 and caspase-9 is mainly p53 independent. Cisplatin increases DNA-damage (histone H2AX phosphorylation) in A2780WT cells, whereas cisplatin, due to reduced uptake [inductively coupled-plasma-mass spectrometry (Pt) analysis], has no DNA-damaging effect in A2780RES cells. TSA has no effect on cisplatin accumulation or cisplatin-induced DNA-damage in A2780WT/A2780RES cells. Tracer technique indicates that TSA inhibits the volume-sensitive organic anion channel (VSOAC) in A2780WT/A2780RES cells and that the activity is restored by exogenous H₂O₂. As TSA reduces NOX4 mRNA accumulation and concomitantly increases catalase mRNA/protein accumulation, we suggest that TSA increases the antioxidative defense in A2780 cells. Inhibition of the kinase mTOR (rapamycin, palomid, siRNA), which is normally associated with cell growth, reduces VSOAC activity synergistically to TSA. However, as TSA increases mTOR activity (phosphorylation of 4EBP1, S6 kinase, S6, ULK1, SGK1), the effect of TSA on VSOAC activity does not reflect the shift in mTOR signaling. Upregulation of the protein expression and activity of the taurine transporter (TauT) is a phenotypic characteristic of A2780RES cells. However, TSA reduces TauT protein expression in A2780RES cells and activity to values seen in A2780WT cells. It is suggested that therapeutic benefits of TSA in A2780 do not imply facilitation of cisplatin uptake but more likely a synergistic activation of apoptosis/autophagy and reduced TauT activity.

Pannexin 1 activation and inhibition is permeant-selective

KEY POINTS

The large-pore channel pannexin 1 (Panx1) is expressed in many cell types and can open upon different, yet not fully established, stimuli. Panx1 permeability is often inferred from channel permeability to fluorescent dyes, but it is currently unknown whether dye permeability translates to permeability to other molecules. Cell shrinkage and C-terminal cleavage led to a Panx1 open-state with increased permeability to atomic ions (current), but did not alter ethidium uptake. Panx1 inhibitors affected Panx1-mediated ion conduction differently from ethidium permeability, and inhibitor efficiency towards a given molecule therefore cannot be extrapolated to its effects on the permeability of another. We conclude that ethidium permeability does not reflect equal permeation of other molecules and thus is no measure of general Panx1 activity.

ABSTRACT

Pannexin 1 (Panx1) is a large-pore membrane channel connecting the extracellular milieu with the cell interior. While several activation regimes activate Panx1 in a variety of cell types, the selective permeability of an open Panx1 channel remains unresolved: does a given activation paradigm increase Panx1's permeability towards all permeants equally and does fluorescent dye flux serve as a proxy for biological permeation through an open channel? To explore permeant-selectivity of Panx1 activation and inhibition, we employed Panx1-expressing Xenopus laevis oocytes and HEK293T cells. We report that different mechanisms of activation of Panx1 differentially affected ethidium and atomic ion permeation. Most notably, C-terminal truncation or cell shrinkage elevated Panx1-mediated ion conductance, but had no effect on ethidium permeability. In contrast, extracellular pH changes predominantly affected ethidium permeability but not ionic conductance. High [K⁺ ]_o did not increase the flux of either of the two permeants. Once open, Panx1 demonstrated preference for anionic permeants, such as Cl^- , lactate and glutamate, while not supporting osmotic water flow. Panx1 inhibitors displayed enhanced potency towards Panx1-mediated currents compared to that of ethidium uptake. We conclude that activation or inhibition of Panx1 display permeant-selectivity and that permeation of ethidium does not necessarily reflect an equal permeation of smaller biological molecules and atomic ions.

Ion Channel Dysregulation in Head and Neck Cancers: Perspectives for Clinical Application

Head and neck cancers are a highly complex and heterogeneous group of malignancies that involve very diverse anatomical structures and distinct aetiological factors, treatments and clinical outcomes. Among them, head and neck squamous cell carcinomas (HNSCC) are predominant and the sixth most common cancer worldwide with still low survival rates. Omic technologies have unravelled the intricacies of tumour biology, harbouring a large diversity of genetic and molecular changes to drive the carcinogenesis process. Nonetheless, this remarkable heterogeneity of molecular alterations opens up an immense opportunity to discover novel biomarkers and develop molecular-targeted therapies. Increasing evidence demonstrates that dysregulation of ion channel expression and/or function is frequently and commonly observed in a variety of cancers from different origin. As a consequence, the concept of ion channels as potential membrane therapeutic targets and/or biomarkers for cancer diagnosis and prognosis has attracted growing attention. This chapter intends to comprehensively and critically review the current state-of-art ion channel dysregulation specifically focusing on head and neck cancers and to formulate the major challenges and research needs to translate this knowledge into clinical application. Based on current reported data, various voltage-gated potassium (K_v) channels (i.e. K_v3.4, K_v10.1 and K_v11.1) have been found frequently aberrantly expressed in HNSCC as well as precancerous lesions and are highlighted as clinically and biologically relevant features in both early stages of tumourigenesis and late stages of disease progression. More importantly, they also emerge as promising candidates as cancer risk markers, tumour markers and potential anti-proliferative and anti-metastatic targets for therapeutic interventions; however, the oncogenic properties seem to be independent of their ion-conducting function.

The Role of the Reactive Oxygen Species Scavenger Agent, Astaxanthin, in the Protection of Cisplatin-Treated Patients Against Hearing Loss

Emerging evidence of significant hearing loss occurring shortly after cisplatin administration in cancer patients has stimulated research into the causes and treatment of this side effect. Although the aetiology of cisplatin-induced hearing loss (CIHL) remains unknown, an increasing body of research suggests that it is associated with excessive generation of intracellular reactive oxygen species (ROS) in the cochlea. Astaxanthin, a xanthophyll carotenoid, has powerful anti-oxidant, anti-inflammatory, and anti-apoptotic properties based on its unique cell membrane function, diverse biological activities, and ability to permeate the blood-brain barrier. In this review, we summarize the role of ROS in CIHL and the effect of astaxanthin on inhibiting ROS production. We focus on investigating the mechanism of action of astaxanthin in suppressing excessive production of ROS.

LRRC8/VRAC channels exhibit a noncanonical permeability to glutathione, which modulates epithelial-to-mesenchymal transition (EMT)

Volume-regulated anion channels (VRAC) are chloride channels activated in response to osmotic stress to regulate cellular volume and also participate in other cellular processes, including cell division and cell death. Recently, members of the LRRC8 family have been identified as the main contributors of VRAC conductance. LRRC8/VRAC is permeable to chloride ions but also exhibits significant permeability to various substrates that vary strongly in charge and size. In this study, we explored the intriguing ability of LRRC8/VRAC to transport glutathione (GSH), the major cellular reactive oxygen species (ROS) scavenger, and its involvement in epithelial-to-mesenchymal transition (EMT), a cellular process in which cellular oxidative status is a crucial step. First, in HEK293-WT cells, we showed that a hypotonic condition induced LRRC8/VRAC-dependent GSH conductance (P_GSH/P_Cl of ~0.1) and a marked decrease in intracellular GSH content. GSH currents and GSH intracellular decrease were both inhibited by DCPIB, an inhibitor of LRRC8/VRAC, and were not observed in HEK293-LRRC8A KO cells. Then, we induced EMT by exposing renal proximal tubule epithelial cells to the pleiotropic growth factor TGFβ1, and we measured the contribution of LRRC8/VRAC in this process by measuring (i) EMT marker expression (assessed both at the gene and protein levels), (ii) cell morphology and (iii) the increase in migration ability. Interestingly, pharmacologic targeting of LRRC8/VRAC (DCPIB) or RNA interference-mediated inhibition (LRRC8A siRNA) attenuated the TGFβ1-induced EMT response by controlling GSH and ROS levels. Interestingly, TGFβ1 exposure triggered DCPIB-sensitive chloride conductance. These results suggest that LRRC8/VRAC, due to its native permeability to GSH and thus its ability to modulate ROS levels, plays a critical role in EMT and might contribute to other physiological and pathophysiological processes associated with oxidative stress.

Absolute Protein Amounts and Relative Abundance of Volume-regulated Anion Channel (VRAC) LRRC8 Subunits in Cells and Tissues Revealed by Quantitative Immunoblotting

The volume-regulated anion channel (VRAC) plays an important role in osmotic cell volume regulation. In addition, it is involved in various physiological processes such as insulin secretion, glia-neuron communication and purinergic signaling. VRAC is formed by hetero-hexamers of members of the LRRC8 protein family, which consists of five members, LRRC8A-E. LRRC8A is an essential subunit for physiological functionality of VRAC. Its obligate heteromerization with at least one of its paralogues, LRRC8B-E, determines the biophysical properties of VRAC. Moreover, the subunit composition is of physiological relevance as it largely influences the activation mechanism and especially the substrate selectivity. However, the endogenous tissue-specific subunit composition of VRAC is unknown. We have now developed and applied a quantitative immunoblot study of the five VRAC LRRC8 subunits in various mouse cell lines and tissues, using recombinant protein for signal calibration. We found tissue-specific expression patterns of the subunits, and generally relative low expression of the essential LRRC8A subunit. Immunoprecipitation of LRRC8A also co-precipitates an excess of the other subunits, suggesting that non-LRRC8A subunits present the majority in hetero-hexamers. With this, we can estimate that in the tested cell lines, the number of VRAC channels per cell is in the order of 10,000, which is in agreement with earlier calculations from the comparison of single-channel and whole-cell currents.