Where have all the Na+ channels gone? In search of functional ENaC in exocrine pancreas

Pantoprazole and riluzole target H+/K+-ATPases and pH-sensitive K+ channels in pancreatic cancer cells

Pancreatic ductal adenocarcinoma (PDAC) remains the most lethal cancer type. PDAC is characterized by fibrotic, hypoxic, and presumably acidic tumor microenvironment (TME). Acidic TME is an important player in tumor development, progression, aggressiveness, and chemoresistance. The dysregulation of ductal ion transporters/channels might contribute to extracellular pH (pHe) acidification and PDAC progression. Our aim was to test whether H+/K+-ATPases and pH-sensitive K+ channels contribute to these processes and could be targeted by clinically approved drugs. We used human pancreatic cancer cells adapted to various pHe conditions and grown in monolayers and spheroids. First, we created cells expressing pHoran4 at the outer plasma membrane and showed that pantoprazole, the H+/K+-ATPase inhibitor, alkalinized pHe. Second, we used FluoVolt to monitor the membrane voltage (Vm) and showed that riluzole hyperpolarized Vm, most likely by opening of pH-sensitive K+ channels such as TREK-1. Third, we show that pantoprazole and riluzole inhibited cell proliferation and viability of monolayers and spheroids of cancer cells adapted to various pHe conditions. Most importantly, combination of the two drugs had significantly larger inhibitory effects on PDAC cell survival. We propose that co-targeting H+/K+-ATPases and pH-sensitive K+ channels by re-purposing of pantoprazole and riluzole could provide novel acidosis-targeted therapies of PDAC.

Upregulation of α-ENaC induces pancreatic β-cell dysfunction, ER stress, and SIRT2 degradation

Islet beta cells (β-cells) produce insulin in response to high blood glucose levels, which is essential for preserving glucose homeostasis. Voltage-gated ion channels in β-cells, including Na +, K +, and Ca 2+ channels, aid in the release of insulin. The epithelial sodium channel alpha subunit (α-ENaC), a voltage-independent sodium ion channel, is also expressed in human pancreatic endocrine cells. However, there is no reported study on the function of ENaC in the β-cells. In the current study, we found that α-ENaC was expressed in human pancreatic glandule and pancreatic islet β-cells. In the pancreas of db/db mice and high-fat diet-induced mice, and in mouse islet β-cells (MIN6 cells) treated with palmitate, α-ENaC expression was increased. When α-ENaC was overexpressed in MIN6 cells, insulin content and glucose-induced insulin secretion were significantly reduced. On the other hand, palmitate injured islet β-cells and suppressed insulin synthesis and secretion, but increased α-ENaC expression in MIN6 cells. However, α-ENaC knockout ( Scnn1a -/-) in MIN6 cells attenuated β-cell disorder induced by palmitate. Furthermore, α-ENaC regulated the ubiquitylation and degradation of sirtuin 2 in β-cells. α-ENaC also modulated β-cell function in correlation with the inositol-requiring enzyme 1 alpha/X-box binding protein 1 (IRE1α/XBP1) and protein kinase RNA-like endoplasmic reticulum kinase/C/EBP homologous protein (PERK/CHOP) endoplasmic reticulum stress pathways. These results suggest that α-ENaC may play a novel role in insulin synthesis and secretion in the β-cells, and the upregulation of α-ENaC promotes islet β-cell dysfunction. In conclusion, α-ENaC may be a key regulator involved in islet β-cell damage and a potential therapeutic target for type 2 diabetes mellitus.

Human pancreatic ductal organoids with controlled polarity provide a novel ex vivo tool to study epithelial cell physiology

Epithelial ion and fluid secretion determine the physiological functions of a broad range of organs, such as the lung, liver, or pancreas. The molecular mechanism of pancreatic ion secretion is challenging to investigate due to the limited access to functional human ductal epithelia. Patient-derived organoids may overcome these limitations, however direct accessibility of the apical membrane is not solved. In addition, due to the vectorial transport of ions and fluid the intraluminal pressure in the organoids is elevated, which may hinder the study of physiological processes. To overcome these, we developed an advanced culturing method for human pancreatic organoids based on the removal of the extracellular matrix that induced an apical-to-basal polarity switch also leading to reversed localization of proteins with polarized expression. The cells in the apical-out organoids had a cuboidal shape, whereas their resting intracellular Ca²⁺ concentration was more consistent compared to the cells in the apical-in organoids. Using this advanced model, we demonstrated the expression and function of two novel ion channels, the Ca²⁺ activated Cl^- channel Anoctamin 1 (ANO1) and the epithelial Na⁺ channel (ENaC), which were not considered in ductal cells yet. Finally, we showed that the available functional assays, such as forskolin-induced swelling, or intracellular Cl^- measurement have improved dynamic range when performed with apical-out organoids. Taken together our data suggest that polarity-switched human pancreatic ductal organoids are suitable models to expand our toolset in basic and translational research.

Synergistic effects of agonists and two-pore-domain potassium channels on secretory responses of human pancreatic duct cells Capan-1

Mechanisms of synergistic agonist stimulation and modulation of the electrochemical driving force for anion secretion are still not fully explored in human pancreatic duct epithelial cells. The first objective of this study was therefore to test whether combined agonist stimulation augments anion transport responses in the Capan-1 monolayer model of human pancreatic duct epithelium. The second objective was to test the influence of H⁺,K⁺-ATPase inhibition on anion transport in Capan-1 monolayers. The third objective was to analyze the expression and function of K⁺ channels in Capan-1, which could support anion secretion and cooperate with H⁺,K⁺-ATPases in pH and potassium homeostasis. The human pancreatic adenocarcinoma cell line Capan-1 was cultured conventionally or as polarized monolayers that were analyzed by Ussing chamber electrophysiological recordings. Single-cell intracellular calcium was assayed with Fura-2. mRNA isolated from Capan-1 was analyzed by use of the nCounter assay or RT-PCR. Protein expression was assessed by immunofluorescence and western blot analyses. Combined stimulation with different physiological agonists enhanced anion transport responses compared to single agonist stimulation. The responsiveness of Capan-1 cells to histamine was also revealed in these experiments. The H⁺,K⁺-ATPase inhibitor omeprazole reduced carbachol- and riluzole-induced anion transport responses. Transcript analyses revealed abundant TASK-2, TWIK-1, TWIK-2, TASK-5, K_Ca3.1, and KCNQ1 mRNA expression. KCNE1 mRNA and TREK-1, TREK-2, TASK-2, and KCNQ1 protein expression were also shown. This study shows that the Capan-1 model recapitulates key physiological aspects of a bicarbonate-secreting epithelium and constitutes a valuable model for functional studies on human pancreatic duct epithelium.

Ion Channel Signature in Healthy Pancreas and Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is the fourth most common cause of cancer-related deaths in United States and Europe. It is predicted that PDAC will become the second leading cause of cancer-related deaths during the next decades. The development of PDAC is not well understood, however, studies have shown that dysregulated exocrine pancreatic fluid secretion can contribute to pathologies of exocrine pancreas, including PDAC. The major roles of healthy exocrine pancreatic tissue are secretion of enzymes and bicarbonate rich fluid, where ion channels participate to fine-tune these biological processes. It is well known that ion channels located in the plasma membrane regulate multiple cellular functions and are involved in the communication between extracellular events and intracellular signaling pathways and can function as signal transducers themselves. Hereby, they contribute to maintain resting membrane potential, electrical signaling in excitable cells, and ion homeostasis. Despite their contribution to basic cellular processes, ion channels are also involved in the malignant transformation from a normal to a malignant phenotype. Aberrant expression and activity of ion channels have an impact on essentially all hallmarks of cancer defined as; uncontrolled proliferation, evasion of apoptosis, sustained angiogenesis and promotion of invasion and migration. Research indicates that certain ion channels are involved in the aberrant tumor growth and metastatic processes of PDAC. The purpose of this review is to summarize the important expression, localization, and function of ion channels in normal exocrine pancreatic tissue and how they are involved in PDAC progression and development. As ion channels are suggested to be potential targets of treatment they are furthermore suggested to be biomarkers of different cancers. Therefore, we describe the importance of ion channels in PDAC as markers of diagnosis and clinical factors.

Mouse pancreatic ductal organoid culture as a relevant model to study exocrine pancreatic ion secretion

Pancreatic exocrine secretory processes are challenging to investigate on primary epithelial cells. Pancreatic organoid cultures may help to overcome shortcomings of the current models, however the ion secretory processes in pancreatic organoids-and therefore their physiological relevance or their utility in disease modeling-are not known. To answer these questions, we provide side-by-side comparison of gene expression, morphology, and function of epithelial cells in primary isolated pancreatic ducts and organoids. We used mouse pancreatic ductal fragments for experiments or were grown in Matrigel to obtain organoid cultures. Using PCR analysis we showed that gene expression of ion channels and transporters remarkably overlap in primary ductal cells and organoids. Morphological analysis with scanning electron microscopy revealed that pancreatic organoids form polarized monolayers with brush border on the apical membrane. Whereas the expression and localization of key proteins involved in ductal secretion (cystic fibrosis transmembrane conductance regulator, Na⁺/H⁺ exchanger 1 and electrogenic Na⁺/HCO₃^- cotransporter 1) are equivalent to the primary ductal fragments. Measurements of intracellular pH and Cl^- levels revealed no significant difference in the activities of the apical Cl^-/HCO₃^- exchange, or in the basolateral Na⁺ dependent HCO₃^- uptake. In summary we found that ion transport activities in the mouse pancreatic organoids are remarkably similar to those observed in freshly isolated primary ductal fragments. These results suggest that organoids can be suitable and robust model to study pancreatic ductal epithelial ion transport in health and diseases and facilitate drug development for secretory pancreatic disorders like cystic fibrosis, or chronic pancreatitis.

Both Ways at Once: Keeping Small Airways Clean

The small airways of the lungs are under constant assault from the pathogens and debris in the air that they must conduct to alveoli. Although hygiene is of paramount importance for respiratory health, the underlying principles of airway clearance have not been well integrated or established. Newly emerging concepts of simultaneous absorption and secretion of airway surface liquid (ASL) and the role of [Formula: see text] in the maturation of mucins have advanced from experimental evidence as well as observations from the congenital disease cystic fibrosis (CF) to present a novel model that integrates microanatomy with organ physiology to meet the constant challenge of cleaning small airways.

The cystic fibrosis of exocrine pancreas

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is highly expressed in the pancreatic duct epithelia and permits anions and water to enter the ductal lumen. This results in an increased volume of alkaline fluid allowing the highly concentrated proteins secreted by the acinar cells to remain in a soluble state. This work will expound on the pathophysiology and pathology caused by the malfunctioning CFTR protein with special reference to ion transport and acid-base abnormalities both in humans and animal models. We will also discuss the relationship between cystic fibrosis (CF) and pancreatitis, and outline present and potential therapeutic approaches in CF treatment relevant to the pancreas.

Ion transport in human pancreatic duct epithelium, Capan-1 cells, is regulated by secretin, VIP, acetylcholine, and purinergic receptors

OBJECTIVES

The objective of the study was to establish a solid model of polarized epithelium for human pancreatic ducts, where electrical parameters could be measured as indicators of ion transport. Further, we aimed to determine functional expression of several receptors, in particular, purinergic receptors, and determine their effects on ion transport.

METHODS

Human adenocarcinoma cell line Capan-1 cells were grown on permeable supports and set in Ussing chambers for electrophysiological recordings. Transepithelial voltage (Vte), resistance, and short-circuit currents (Isc) were measured in response to agonists.

RESULTS

Secretin, vasoactive intestinal peptide (VIP), acetylcholine, forskolin, ionomycin, adenosine 5'-triphosphate (ATP), uridine 5'-triphosphate (UTP), 3'-O-(4-benzoyl)benzoyl ATP, and adenosine induced lumen negative Vte and Isc. These changes were consistent with anion secretion, as verified in forskolin-stimulated preparations. Extracellular nucleotides, ATP, and UTP, applied from luminal and basolateral sides, caused largest responses: Vte increased up to -5 mV, Isc increased to 20 to 30 μA/cm, and resistance decreased by up to 200 Ω·cm.

CONCLUSIONS

Transepithelial transport in human pancreatic duct epithelium, Capan-1 cells, is regulated by secretin, VIP, acetylcholine, adenosine, and purinergic P2 receptors; and this human model has a good potential for studies of physiology and pathophysiology of pancreatic duct ion transport.

The epithelial sodium channel δ-subunit: new notes for an old song

Amiloride-sensitive epithelial Na(+) channels (ENaCs) can be formed by different combinations of four homologous subunits, named α, β, γ, and δ. In addition to providing an apical entry pathway for transepithelial Na(+) reabsorption in tight epithelia such as the kidney distal tubule and collecting duct, ENaCs are also expressed in nonepithelial cells, where they may play different functional roles. The δ-subunit of ENaC was originally identified in humans and is able to form amiloride-sensitive Na(+) channels alone or in combination with β and γ, generally resembling the canonical kidney ENaC formed by α, β, and γ. However, δ differs from α in its tissue distribution and channel properties. Despite the low sequence conservation between α and δ (37% identity), their similar functional characteristics provide an excellent model for exploring structural correlates of specific ENaC biophysical and pharmacological properties. Moreover, the study of cellular mechanisms modulating the activity of different ENaC subunit combinations provides an opportunity to gain insight into the regulation of the channel. In this review, we examine the evolution of ENaC genes, channel subunit composition, the distinct functional and pharmacological features that δ confers to ENaC, and how this can be exploited to better understand this ion channel. Finally, we briefly consider possible functional roles of the ENaC δ-subunit.

A new role for bicarbonate secretion in cervico-uterine mucus release

Cervical mucus thinning and release during the female reproductive cycle is thought to rely mainly on fluid secretion. However, we now find that mucus released from the murine reproductive tract critically depends upon concurrent bicarbonate (HCO(3)(-)) secretion. Prostaglandin E(2) (PGE(2))- and carbachol-stimulated mucus release was severely inhibited in the absence of serosal HCO(3)(-), HCO(3)(-) transport, or functional cystic fibrosis transmembrane conductance regulator (CFTR). In contrast to mucus release, PGE(2)- and carbachol-stimulated fluid secretion was not dependent on bicarbonate or on CFTR, but was completely blocked by niflumic acid. We found stimulated mucus release was severely impaired in the cystic fibrosis F508 reproductive tract, even though stimulated fluid secretion was preserved. Thus, CFTR mutations and/or poor bicarbonate secretion may be associated with reduced female fertility associated with abnormal mucus and specifically, may account for the increased viscosity and lack of cyclical changes in cervical mucus long noted in women with cystic fibrosis.

Delta-subunit confers novel biophysical features to alpha beta gamma-human epithelial sodium channel (ENaC) via a physical interaction

Native amiloride-sensitive Na+ channels exhibit a variety of biophysical properties, including variable sensitivities to amiloride, different ion selectivities, and diverse unitary conductances. The molecular basis of these differences has not been elucidated. We tested the hypothesis that co-expression of delta-epithelial sodium channel (ENaC) underlies, at least in part, the multiplicity of amiloride-sensitive Na+ conductances in epithelial cells. For example, the delta-subunit may form multimeric channels with alpha beta gamma-ENaC. Reverse transcription-PCR revealed that delta-ENaC is co-expressed with alpha beta gamma-subunits in cultured human lung (H441 and A549), pancreatic (CFPAC), and colonic epithelial cells (Caco-2). Indirect immunofluorescence microscopy revealed that delta-ENaC is co-expressed with alpha-, beta-, and gamma-ENaC in H441 cells at the protein level. Measurement of current-voltage that cation selectivity ratios for the revealed relationships Na+/Li+/K+/Cs+/Ca2+/Mg2+, the apparent dissociation constant (Ki) for amiloride, and unitary conductances for delta alpha beta gamma-ENaC differed from those of both alpha beta gamma- and delta beta gamma-ENaC (n = 6). The contribution of the delta subunit to P(Li)/P(Na) ratio and unitary Na+ conductance under bi-ionic conditions depended on the injected cRNA concentration. In addition, the EC50 for proton activation, mean open and closed times, and the self-inhibition time of delta alpha beta gamma-ENaC differed from those of alpha beta gamma- and delta beta gamma-ENaC. Co-immunoprecipitation of delta-ENaC with alpha- and gamma-subunits in H441 and transfected COS-7 cells suggests an interaction among these proteins. We, therefore, concluded that the interactions of delta-ENaC with other subunits could account for heterogeneity of native epithelial channels.

Heightened epithelial Na+ channel-mediated Na+ absorption in a murine polycystic kidney disease model epithelium lacking apical monocilia

The Tg737 degrees (rpk) autosomal recessive polycystic kidney disease (ARPKD) mouse carries a hypomorphic mutation in the Tg737 gene. Because of the absence of its protein product Polaris, the nonmotile primary monocilium central to the luminal membrane of ductal epithelia, such as the cortical collecting duct (CCD) principal cell (PC), is malformed. Although the functions of the renal monocilium remain elusive, primary monocilia or flagella on neurons act as sensory organelles. Thus we hypothesized that the PC monocilium functions as a cellular sensor. To test this hypothesis, we assessed the contribution of Polaris and cilium structure and function to renal epithelial ion transport electrophysiology. Properties of Tg737 degrees (rpk) mutant CCD PC clones were compared with clones genetically rescued with wild-type Tg737 cDNA. All cells were grown as polarized cell monolayers with similarly high transepithelial resistance on permeable filter supports. Three- to fourfold elevated transepithelial voltage (V(te)) and short-circuit current (I(sc)) were measured in mutant orpk monolayers vs. rescued controls. Pharmacological and cell biological examination of this enhanced electrical end point in mutant monolayers revealed that epithelial Na(+) channels (ENaCs) were upregulated. Amiloride, ENaC-selective amiloride analogs (benzamil and phenamil), and protease inhibitors (aprotinin and leupeptin) attenuated heightened V(te) and I(sc). Higher concentrations of additional amiloride analogs (ethylisopropylamiloride and dimethylamiloride) also revealed inhibition of V(te). Cell culture requirements and manipulations were also consistent with heightened ENaC expression and function. Together, these data suggest that ENaC expression and/or function are upregulated in the luminal membrane of mutant, cilium-deficient orpk CCD PC monolayers vs. cilium-competent controls. When the genetic lesion causes loss or malformation of the monocilium, ENaC-driven Na(+) hyperabsorption may explain the rapid emergence of severe hypertension in a majority of patients with ARPKD.

Characterization of vectorial chloride transport pathways in the human pancreatic duct adenocarcinoma cell line HPAF

Pancreatic duct cells express a Ca2+-activated Cl- conductance (CaCC), upregulation of which may be beneficial to patients with cystic fibrosis. Here, we report that HPAF, a human pancreatic ductal adenocarcinoma cell line that expresses CaCC, develops into a high-resistance, anion-secreting epithelium. Mucosal ATP (50 microM) caused a fourfold increase in short-circuit current (Isc), a hyperpolarization of transepithelial potential difference (from -4.9 +/- 0.73 to -8.5 +/- 0.84 mV), and a fall in resistance to less than one-half of resting values. The effects of ATP were inhibited by mucosal niflumic acid (100 microM), implicating an apical CaCC in the response. RT-PCR indicated expression of hClC-2, hClC-3, and hClC-5, but surprisingly not hCLCA-1 or hCLCA-2. K+ channel activity was necessary to maintain the ATP-stimulated Isc. Using a pharmacological approach, we found evidence for two types of K+ channels in the mucosal and serosal membranes of HPAF cells, one activated by chlorzoxazone (500 microM) and sensitive to clotrimazole (30 microM), as well as one blocked by clofilium (100 microM) but not chromanol 293B (5 microM). RT-PCR indicated expression of the Ca2+-activated K+ channel KCNN4, as well as the acid-sensitive, four transmembrane domain, two pore K+ channel, KCNK5 (hTASK-2). Western blot analysis verified the expression of CLC channels, as well as KCNK5. We conclude that HPAF will be a useful model system for studying channels pertinent to anion secretion in human pancreatic duct cells.