The AQP-3 water channel and the ClC-3 chloride channel coordinate the hypotonicity-induced swelling volume in nasopharyngeal carcinoma cells

QCM sensor provides insight into the role of pivotal ions in cellular regulatory volume decrease

All vertebrate cells generally self-regulate for sustaining homeostasis and cell functions. As a major regulatory mechanism, regulatory volume decrease (RVD) occurs in hypotonicity-induced cell swelling, and then shrinking by the efflux of intracellular osmolytes and water, in which the ions K⁺, Cl^-, and Ca²⁺ play a key role in the RVD process. We observed that these pivotal ions could result in novel RVD behaviors under repeatedly hypotonic stimulation. However, there is a lack of valid means for assessing the effect of pivotal ions on RVD. In this work, we proposed an effective measurement process based on a quartz crystal microbalance (QCM) combined with cell function of RVD for revealing acute variations in cell volume regulation induced by the pivotal ions. A QCM sensor was implemented by adhering MCF-7 cells to a poly-l-lysine-modified gold chip and cyclic stimulation with hypotonic NaCl medium, in which a frequency shift (Δf) showed the superior feasibility of the technique in exhibiting RVD behaviors. With the increase in the number of cycles, the RVD values decreased progressively under three stimulation cycles with hypotonic NaCl alone. Compared with the first cycle, the RVD level in the second and third cycles declined by 60.7±1.7% and 82.1±1.6% (n=3), respectively; conversely, it recovered in NaCl-KCl solution, but was significantly enhanced by 52.2±0.8% in NaCl-CaCl₂ solution. Moreover, the inhibition of chloride channels to block Cl^- efflux also decreased the RVD level by 56.2±3.0%. The results indicate that these ions (K⁺, Cl^-, Ca²⁺) are all able to affect the function of RVD, among which intracellular Cl^- depletion reduced RVD during measurement, but which recovered with K⁺ supplement, and Ca²⁺ enhanced RVD due to activation of ion channels. Therefore, this work provides a comprehensive assessment of cellular behavior and offers an innovative method for gaining insight into cellular functions and mechanisms. A novel strategy was conducted by integrating a quartz crystal microbalance (QCM) with the function of cell volume regulation for analyzing the role of the pivotal ions ( K⁺, Cl^-, Ca²⁺) in NaCl media on the behaviors of regulatory cell volume decrease (RVD).

Insight into the Mammalian Aquaporin Interactome

Aquaporins (AQPs) are a family of transmembrane water channels expressed in all living organisms. AQPs facilitate osmotically driven water flux across biological membranes and, in some cases, the movement of small molecules (such as glycerol, urea, CO₂, NH₃, H₂O₂). Protein-protein interactions play essential roles in protein regulation and function. This review provides a comprehensive overview of the current knowledge of the AQP interactomes and addresses the molecular basis and functional significance of these protein-protein interactions in health and diseases. Targeting AQP interactomes may offer new therapeutic avenues as targeting individual AQPs remains challenging despite intense efforts.

Autophagy induction regulates aquaporin 3-mediated skin fibroblasts aging

BACKGROUND

Long- and short-term ultraviolet (UV) exposure have distinct biological effects on human fibroblasts.

OBJECTIVES

This study aimed to elucidate the underlying mechanisms of the biological effects of UV exposure on human skin fibroblasts.

METHOD

We subjected human skin fibroblast cells with or without AQP3, DEDD, or Beclin1 manipulation to UVA treatment and evaluated autophagy and senescence/aging in them.

RESULTS

Short-term UVA irradiation induced autophagy and upregulated AQP3 but not senescence, whereas long-term UVA irradiation inhibited autophagy, AQP3, and senescence/aging in vitro and in vivo. Silencing AQP3 abolished short-term UVA irradiation-induced autophagy and led to cellular senescence, whereas AQP3 overexpression partially rescued the senescence and autophagy inhibition induced by long-term UVA exposure in vitro. Mechanistically, the transcription factor JUN was found to bind to the AQP3 promoter to activate its transcription following short-term UVA exposure. Subsequently, AQP3 interacted with DEDD to induce its ubiquitination-mediated degradation and promote autophagy, and bound to Beclin1 to directly activate autophagy. Finally, autophagy induced by AQP3 overexpression robustly prevented UVA-induced senescence/aging in vitro and in vivo.

CONCLUSIONS

Thus, our study indicates that AQP3 controls skin fibroblasts photoaging by regulating autophagy and represents a potential target for future interventions against skin aging.

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.

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.

High expression of leucine‑rich repeat‑containing 8A is indicative of a worse outcome of colon cancer patients by enhancing cancer cell growth and metastasis

To survive, cells need to avoid excessive volume change that jeopardizes structural integrity and stability of the intracellular milieu. Searching for the molecular identity of volume‑regulated anion channel (VRAC) has yielded multiple potential candidates, but none has been confirmed. Recently, it is reported that leucine‑rich repeat‑containing 8A (LRRC8A) is a main molecular determinant of VRAC current. The biological functions of LRRC8 family proteins are poorly understood, particularly in cancer. In the present study, we investigated LRRC8A in the most common cancers of the digestive system. LRRC8A proteins were found to be abundantly expressed in the esophagus, stomach, duodenum, colon, rectum, liver and pancreas. LRRC8A was elevated in 60% of colorectal cancer patient tissues, which was higher than that in patients with cancer of the esophagus, stomach, duodenum, liver and pancreas. Colon cancer patients with high‑ expressed LRRC8A had a survival time of 54.9±5.5 months, shorter than that of patients with low‑expressed LRRC8A (77.1±3.7). Moreover, survival time (52.6±7.3 months) of patients with metastases in the lymph nodes was shorter than that of patients without positive lymph nodes (72.2±3.6); patients with positive lymph nodes and an elevated LRRC8A expression had the highest mortality rate (~80%). These rates were not observed in rectal cancer. After LRRC8A protein was knocked down in colon cancer HCT116 cells, VRAC currents, migration and tumorigenesis in nude mice were significantly inhibited. In conclusion, we propose that LRRC8A could be a novel prognostic biomarker for colon cancer patient survival, and that the elevated expression of LRRC8A may enhance cancer cell growth and metastasis, and worsen the outcome of patients.

Roles for Xenopus aquaporin-3b (aqp3.L) during gastrulation: Fibrillar fibronectin and tissue boundary establishment in the dorsal margin

Aquaporins and aquaglyceroporins are a large family of membrane channel proteins that allow rapid movement of water and small, uncharged solutes into and out of cells along concentration gradients. Recently, aquaporins have been gaining recognition for more complex biological roles than the regulation of cellular osmotic homeostasis. We have identified a specific expression pattern for Xenopus aqp3b (also called aqp3.L) during gastrulation, where it is localized to the sensorial (deep) layer of the blastocoel roof and dorsal margin. Interference with aqp3b expression resulted in loss of fibrillar fibronectin matrix in Brachet's cleft at the dorsal marginal zone, but not on the free surface of the blastocoel. Detailed observation showed that the absence of fibronectin matrix correlated with compromised border integrities between involuted mesendoderm and noninvoluted ectoderm in the marginal zone. Knockdown of aqp3b also led to delayed closure of the blastopore, suggesting defects in gastrulation movements. Radial intercalation was not affected in aqp3b morphants, while the data presented are consistent with impeded convergent extension movements of the dorsal mesoderm in response to loss of aqp3b. Our emerging model suggests that aqp3b is part of a mechanism that promotes proper interaction between cells and the extracellular matrix, thereby playing a critical role in gastrulation.

Regulating the Membrane Transport Activity and Death of Cells via Electroosmotic Manipulation

Although the volume of living cells has been known to heavily influence their behavior and fate, a method allowing us to control the cell size in a programmable manner is still lacking. Here, we develop a technique in which precise changes in the cellular volume can be conveniently introduced by varying the voltage applied across a Nafion membrane that separates the culture medium from a reservoir. It is found that, unlike sudden osmotic shocks, active ion transport across the membrane of leukemia K562 cells will not be triggered by a gradual change in the extracellular osmolarity. Furthermore, when subjected to the same applied voltage, different lung and nasopharyngeal epithelial cancer cells will undergo larger volumetric changes and have a 5-10% higher death rate compared to their normal counterparts. We show that such distinct response is largely caused by the overexpression of aquaporin-4 in tumor cells, with knockout of this water channel protein resulting in a markedly reduced change in the cellular volume. Finally, by taking into account the exchange of water/ion molecules across the Nafion film and the cell membrane, a theoretical model is also proposed to describe the voltage-induced size changes of cells, which explain our experimental observations very well.

The AQP-3 water channel is a pivotal modulator of glycerol-induced chloride channel activation in nasopharyngeal carcinoma cells

Aquaporin (AQP) and chloride channels are ubiquitous in virtually all living cells, playing pivotal roles in cell proliferation, migration and apoptosis. We previously reported that AQP-3 aquaglyceroporin and ClC-3 chloride channels could form complexes to regulate cell volume in nasopharyngeal carcinoma cells. In this study, the roles of AQP-3 in their hetero-complexes were further investigated. Glycerol entered the cells via AQP-3 and induced two different Cl(-) currents through cell swelling-dependent or -independent pathways. The swelling-dependent Cl(-) current was significantly inhibited by pretreatment with CuCl2 and AQP-3-siRNA. After siRNA-induced AQP-3 knock-down, the 140 mM glycerol isoosmotic solution swelled cells by 22% (45% in AQP-3-intact cells) and induced a smaller Cl(-) current; this current was smaller than that activated by 8% cell volume swelling, which induced by the 140 mM glycerol hyperosmotic solution in AQP-3-intact cells. This suggests that the interaction between AQP-3 and ClC-3 plays an important role in cell volume regulation and that AQP-3 may be a modulator that opens volume-regulated chloride channels. The swelling-independent Cl(-) current, which was activated by extracellular glycerol, was reduced by CuCl2 and AQP-3-siRNA pretreatment. Dialyzing glycerol into cells via the pipette directly induced the swelling-independent Cl(-) current; however this current was blocked by AQP-3 down-regulation, suggesting AQP-3 is essential for the opening of chloride channels. In conclusion, AQP-3 is the pathway for water, glycerol and other small solutes to enter cells, and it may be an essential modulator for the gating of chloride channels.

Beyond water homeostasis: Diverse functional roles of mammalian aquaporins

BACKGROUND

Aquaporin (AQP) water channels are best known as passive transporters of water that are vital for water homeostasis.

SCOPE OF REVIEW

AQP knockout studies in whole animals and cultured cells, along with naturally occurring human mutations suggest that the transport of neutral solutes through AQPs has important physiological roles. Emerging biophysical evidence suggests that AQPs may also facilitate gas (CO2) and cation transport. AQPs may be involved in cell signalling for volume regulation and controlling the subcellular localization of other proteins by forming macromolecular complexes. This review examines the evidence for these diverse functions of AQPs as well their physiological relevance.

MAJOR CONCLUSIONS

As well as being crucial for water homeostasis, AQPs are involved in physiologically important transport of molecules other than water, regulation of surface expression of other membrane proteins, cell adhesion, and signalling in cell volume regulation.

GENERAL SIGNIFICANCE

Elucidating the full range of functional roles of AQPs beyond the passive conduction of water will improve our understanding of mammalian physiology in health and disease. The functional variety of AQPs makes them an exciting drug target and could provide routes to a range of novel therapies.

Neuronal ClC-3 Splice Variants Differ in Subcellular Localizations, but Mediate Identical Transport Functions

ClC-3 is a member of the CLC family of anion channels and transporters, for which multiple functional properties and subcellular localizations have been reported. Since alternative splicing often results in proteins with diverse properties, we investigated to what extent alternative splicing might influence subcellular targeting and function of ClC-3. We identified three alternatively spliced ClC-3 isoforms, ClC-3a, ClC-3b, and ClC-3c, in mouse brain, with ClC-3c being the predominant splice variant. Whereas ClC-3a and ClC-3b are present in late endosomes/lysosomes, ClC-3c is targeted to recycling endosomes via a novel N-terminal isoleucine-proline (IP) motif. Surface membrane insertion of a fraction of ClC-3c transporters permitted electrophysiological characterization of this splice variant through whole-cell patch clamping on transfected mammalian cells. In contrast, neutralization of the N-terminal dileucine-like motifs was required for functional analysis of ClC-3a and ClC-3b. Heterologous expression of ClC-3a or ClC-3b carrying mutations in N-terminal dileucine motifs as well as WTClC-3c in HEK293T cells resulted in outwardly rectifying Cl(-) currents with significant capacitive current components. We conclude that alternative splicing of Clcn3 results in proteins with different subcellular localizations, but leaves the transport function of the proteins unaffected.