Cross talk of cAMP and flavone in regulation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel and Na+/K+/2Cl- cotransporter in renal epithelial A6 cells

Regulation of Cl- Electrolyte Permeability in Epithelia by Active Traditional Chinese Medicine Monomers for Diarrhea

The epithelial layer, lining the inner surface of the mammalian alveolar, kidney, brain and colon, is a typical electrolyte transporting tissue. Large quantities of salt and fluid are actively moved from the mucosal side toward the blood vessel. Transepithelial salt re-absorption in epithelial tissues plays an important role in maintaining fluid homeostasis. In absorptive epithelium, fluid and salt flux is controlled by the machinery mainly composed of epithelial sodium channel, cystic fibrosis transmembrane conductance regulator, Na+-K+-2Cl- cotransporter, Na+/H+ exchanger, and Na+/K+-ATPase. Dysregulation of salt permeability across epithelium contributes to the pathogenesis of organ edema. In numerous ion transporters, epithelial Cl- transportation plays an important role in water secretion across epithelial tissues and regulation of body fluid content. Many traditional Chinese medicines treat diarrhea by regulating the Cl- electrolyte transport. We systematically summarized the recent progress regarding the traditional Chinese medicine on Cl- electrolyte transport in the intestinal epithelial tissues. The pharmaceutical relevance of developing advanced strategies to mitigate edematous disorders is also implicated. In conclusion, the crosstalk between Cl- electrolyte transport and active traditional Chinese medicine monomers may lead to the development of new strategies for diarrhea by manipulating the function and expression of ion channels.

Role of Quercetin in Modulating Chloride Transport in the Intestine

Epithelial chloride channels provide the pathways for fluid secretion in the intestine. Cystic fibrosis transmembrane conductance regulator (CFTR) and calcium-activated chloride channels (CaCCs) are the main chloride channels in the luminal membrane of enterocytes. These transmembrane proteins play important roles in many physiological processes. In this study, we have identified a flavonoid quercetin as a modulator of CaCC chloride channel activity. Fluorescence quenching assay showed that quercetin activated Cl⁻ transport in a dose-dependent manner, with EC₅₀ ~37 μM. Short-circuit current analysis confirmed that quercetin activated CaCC-mediated Cl⁻ currents in HT-29 cells that can be abolished by CaCC_inh-A01. Ex vivo studies indicated that application of quercetin to mouse ileum and colon on serosal side resulted in activation of CFTR and CaCC-mediated Cl⁻ currents. Notably, we found that quercetin exhibited inhibitory effect against ANO1 chloride channel activity in ANO1-expressing FRT cells and decreased mouse intestinal motility. Quercetin-stimulated short-circuit currents in mouse ileum was multi-component, which included elevation of Ca²⁺ concentration through L-type calcium channel and activation of basolateral NKCC, Na⁺/K⁺-ATPase, and K⁺ channels. In vivo studies further revealed that quercetin promoted fluid secretion in mouse ileum. The modulatory effect of quercetin on CaCC chloirde channels may therefore represent a potential therapeutic strategy for treating CaCC-related diseases like constipation, secretory diarrhea and hypertension. The inverse effects of quercetin on CaCCs provided evidence that ANO1 and intestinal epithelial CaCCs are different calcium-activated chloride channels.

Distinct action of flavonoids, myricetin and quercetin, on epithelial Cl⁻ secretion: useful tools as regulators of Cl⁻ secretion

Epithelial Cl⁻ secretion plays important roles in water secretion preventing bacterial/viral infection and regulation of body fluid. We previously suggested that quercetin would be a useful compound for maintaining epithelial Cl⁻ secretion at a moderate level irrespective of cAMP-induced stimulation. However, we need a compound that stimulates epithelial Cl⁻ secretion even under cAMP-stimulated conditions, since in some cases epithelial Cl⁻ secretion is not large enough even under cAMP-stimulated conditions. We demonstrated that quercetin and myricetin, flavonoids, stimulated epithelial Cl⁻ secretion under basal conditions in epithelial A6 cells. We used forskolin, which activates adenylyl cyclase increasing cytosolic cAMP concentrations, to study the effects of quercetin and myricetin on cAMP-stimulated epithelial Cl⁻ secretion. In the presence of forskolin, quercetin diminished epithelial Cl⁻ secretion to a level similar to that with quercetin alone without forskolin. Conversely, myricetin further stimulated epithelial Cl⁻ secretion even under forskolin-stimulated conditions. This suggests that the action of myricetin is via a cAMP-independent pathway. Therefore, myricetin may be a potentially useful compound to increase epithelial Cl⁻ secretion under cAMP-stimulated conditions. In conclusion, myricetin would be a useful compound for prevention from bacterial/viral infection even under conditions that the amount of water secretion driven by cAMP-stimulated epithelial Cl⁻ secretion is insufficient.

Inhibitory effect of luteolin on the odorant-induced cAMP level in HEK293 cells expressing the olfactory receptor

Luteolin is a flavonoid in many fruits and vegetables. Although luteolin has important biological functions, including antioxidant, anti-inflammatory, antimicrobial, and neuroprotective activities, little is known about the functions of luteolin in the olfactory system. Various odorants can be detected and distinguished by using several molecular processes, including the binding of odorants to odorant receptors, activation of adenylyl cyclase (AC), changes of cyclic adenosine monophosphate (cAMP) and Ca(2+) levels in olfactory sensory neurons, as well as changes in membrane potentials and the transmission of electric signals to the brain. Because AC-cAMP signal transduction plays a pivotal role in the olfactory system, we evaluated the effects of luteolin on the AC-cAMP pathway that had been stimulated by the odorant eugenol. We demonstrated that eugenol caused an upregulation of the cAMP level and the phosphorylation of phosphokinase A (PKA, a downstream target of cAMP) in human embryonic kidney 293 (HEK293) cells expressing the murine eugenol receptor. This upregulation significantly decreased in the presence of luteolin, suggesting that luteolin inhibited the odorant-induced production of cAMP and affected the downstream phosphorylation of PKA.

Genistein enhances the NGF-induced neurite outgrowth

In the present report, we studied if an isoflavone, genistein, enhances the nerve growth factor (NGF)-induced neurite outgrowth of PC12 cells. Application of genistein enhanced the NGF-induced neurite outgrowth. Knockdown of Na+/K+/2Cl- cotransporter isoform 1 (NKCC1) abolished the stimulatory effect of genistein on the neurite outgrowth. These observations indicate that NKCC1 is essential for genistein to stimulate the NGF-induced neurite outgrowth, although genistein had no effect on the protein expression of NKCC1. On the other hand, genistein activates NKCC1 as shown in our previous study. Taken together, these observations indicate that genistein enhanced the NGF-induced neurite outgrowth in PC12 cells via activation of NKCC1.

Quercetin stimulates NGF-induced neurite outgrowth in PC12 cells via activation of Na(+)/K(+)/2Cl(-) cotransporter

We have recently reported that Na(+)/K(+)/2Cl(-) cotransporter isoform 1 (NKCC1) plays an essential role in nerve growth factor (NGF)-induced neurite outgrowth in PC12D cells. On the other hand, it has been reported that dietary flavonoids, such as quercetin, apigenin, and luteolin, stimulate various ion transporters. In the present report, we investigated the effect of quercetin, a flavonoid, on NGF-induced neurite outgrowth in PC12 cells (the parental strain of PC12D cells). Quercetin stimulated the NGF-induced neurite outgrowth in a dose-dependent manner. Knockdown of NKCC1 by RNAi methods abolished the stimulatory effect of flavonoid. Quercetin stimulated NKCC1 activity (measured as bumetanide-sensitive (86)Rb influx) without any increase in the expression level of NKCC1 protein. The stimulatory effect of quercetin on neurite outgrowth was dependent upon extracellular Cl(-). These observations indicate that quercetin stimulates the NGF-induced neurite outgrowth via an increase in Cl(-) incorporation into the intracellular space by activating NKCC1 in PC12 cell.

The flavonone naringenin inhibits chloride secretion in isolated colonic epithelia

Studies investigating the activating and inhibitory actions of bioflavonoids on colonic function have yielded conflicting results. At low concentrations, flavonoids may stimulate chloride secretion while at higher concentrations they may have antisecretory actions in the colon. Naringenin (4',5,7-trihydroxyflavanone), found predominantly in citrus fruits, confers a protective effect against colorectal cancer and is purported to modulate secretory function in colonic cell lines. The aim of this study was to investigate the effects of naringenin on ion transport in rat and human colonic mucosae. Naringenin inhibited basal and stimulated chloride secretion in rat and human colonic mucosae mounted in Ussing chambers (IC(50) 330 μMol/L and 360 μMol/L respectively) and did not alter intracellular cAMP generation. Naringenin inhibited chloride secretion in MQAE (N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide) loaded crypts stimulated with forskolin. In BCECF (2',7'-bis-(2-carboxyethyl)-5-(and 6)-carboxyfluorescein acetoxymethyl ester) loaded crypts, naringenin caused an intracellular acidification (ΔpH/min=0.05 ± 0.004) which was sensitive to the Na-K-Cl co-transporter (NKCC) inhibitor bumetanide. In addition, the antisecretory effect of naringenin was not inhibited by blockade of barium sensitive basolateral K(+) transporters or by inhibition of Na+/H(+) exchange by amiloride. We propose that the antisecretory action of naringenin is due to inhibition of basolateral NKCC1 in rat and human colon.

Regulation of paracellular Na+ and Cl(-) conductances by hydrostatic pressure

The effect of hydrostatic pressure on the paracellular ion conductance (Gp) composed of the Na(+) conductance (G(Na)) and the Cl(-) conductance (G(Cl)) has been Investigated. Gp, G(Na) and G(Cl) were time-dependently increased after applying an osmotic gradient generated by NaCl with basolateral hypotonicity. Hydrostatic pressure (1-4cm H2O) applied from the basolateral side enhanced the osmotic gradient-induced increase in Gp, G(Na) and G(Cl) in a magnitude-dependent manner, while the hydrostatic pressure applied from the apical side diminished the osmotic gradient-induced increase in Gp, G(Na) and G(Cl). How the hydrostatic pressure influences Gp, G(Na) and G(Cl) under an isosmotic condition was also investigated. Gp, G(Na) and G(Cl) were stably constant under a condition with basolateral application of sucrose canceling the NaCl-generated osmotic gradient (an isotonic condition). Even under this stable condition, the basolaterally applied hydrostatic pressure drastically elevated Gp, G(Na) and G(Cl), while apically applied hydrostatic pressure had little effect on Gp, G(Na) or G(Cl). Taken together, these observations suggest that certain factors controlled by the basolateral osmolality and the basolaterally applied hydrostatic pressure mainly regulate the Gp, G(Na) and G(Cl).

Quercetin stimulates Na+/K+/2Cl- cotransport via PTK-dependent mechanisms in human airway epithelium

We investigated regulatory mechanisms of Cl(-) secretion playing an essential role in the maintenance of surface fluid in human airway epithelial Calu-3 cells. The present study reports that quercetin (a flavonoid) stimulated bumetanide-sensitive Cl(-) secretion with reduction of apical Cl(-) conductance, suggesting that quercetin stimulates Cl(-) secretion by activating an entry step of Cl(-) across the basolateral membrane through Na(+)/K(+)/2Cl(-) cotransporter (NKCC1). To clarify the mechanism stimulating NKCC1 by quercetin, we verified involvement of protein kinase (PK)A, PKC, protein tyrosine kinase (PTK), and cytosolic Ca(2+)-dependent pathways. A PKA inhibitor (PKI-14-22 amide), a PKC inhibitor (Gö 6983) or a Ca(2+) chelating agent did not affect the quercetin-stimulated Cl(-) secretion. On the other hand, a PTK inhibitor (AG18) significantly diminished the stimulatory action of quercetin on Cl(-) secretion without inhibitory effects on apical Cl(-) conductance, suggesting that a PTK-mediated pathway is involved in the stimulatory action of quercetin. The quercetin action on Cl(-) secretion was suppressed with brefeldin A (BFA, an inhibitor of vesicular transport from ER to Golgi), and the BFA-sensitive Cl(-) secretion was not observed in the presence of an epidermal growth factor receptor (EGFR) kinase inhibitor (AG1478), suggesting that quercetin stimulates Cl(-) secretion by causing the EGFR kinase-mediated translocation of NKCC1 or an NKC1-activating factor to the basolateral membrane in human airway epithelial Calu-3 cells. However, the surface density of NKCC1 was not increased by quercetin, but quercetin elevated the activity of NKCC1. These observations indicate that quercetin stimulates Cl(-) secretion by activating NKCC1 via translocation of an NKCC1-activating factor through an EGFR kinase-dependent pathway.

Regulation of paracellular ion conductances by NaCl gradients in renal epithelial cells

In the present study, we clarified how the NaCl gradient across the epithelial cells regulates the paracellular ion conductance. Under isotonic conditions, the absorption-directed NaCl gradient elevated the paracellular conductances of Na(+) (G(Na)) and Cl(-) (G(Cl)), while the secretion-directed NaCl gradient diminished the G(Na) and G(Cl). We further investigated the paracellular ionic conductances of NMDG (G(NMDG)) and gluconate (G(gluconate)) by replacing Na(+) with NMDG or Cl(-) with gluconate. The G(NMDG) was lower than the G(Na) and the replacement of Na(+) with NMDG decreased the G(Cl). The G(gluconate) was lower than the G(Cl) and the replacement of Cl(-) with gluconate also decreased the G(Na). These observations suggest the interaction of cations and anions on paracellular ionic conductances; i.e., cations affect paracellular anion conductances and anions affect paracellular cation conductances.

Aldosterone-induced modification of osmoregulated ENaC trafficking

Aldosterone and osmotic stress are well known to regulate the epithelial Na(+) channel (ENaC)-mediated Na(+) transport in renal epithelial cells. However, we have no information on how aldosterone and osmotic stress interact on stimulation of ENaC-mediated Na(+) transport in renal epithelium. In the present report, we studied how application of aldosterone (1 microM for 1 day) modifies the action of hypotonic stress on the ENaC-mediated Na(+) transport in renal A6 epithelial cells by measuring the benzamil (a specific inhibitor for ENaC)-sensitive short-circuit current. The present study suggests that: (1) most ENaCs in cells without aldosterone treatment are translocated to Golgi apparatus, (2) major parts of aldosterone-generated ENaCs are located at the endoplasmic reticulum, (3) aldosterone diminishes the endocytosis rate of ENaCs from the apical membrane without any significant changes in the insertion rate of ENaCs into the apical membrane, and (4) application of sucrose after hypotonic stress stimulates the endocytosis of ENaCs, and elongates the functional life time of ENaCs by enhancing recycle of ENaCs into the endoplasmic reticulum in a retrograde manner.

Action of neltenexine on anion secretion in human airway epithelia

Neltenexine has been applied to human lung diseases such as chronic obstructive pulmonary disease (COPD) as a mucolytic agent. However, we have no information on the neltenexine action in bronchial epithelial cells. We studied the neltenexine action on the ion transport in human submucosal serous Calu-3 cells. Under a hyper-secreting condition caused by terbutaline (a beta2-adrenergic agonist), neltenexine diminished anion secretion by inhibiting the Cl- and HCO3- uptake via Na+/K+/2Cl- cotransporter and Na+/HCO3- cotransporter without blockade of the cystic fibrosis transmembrane conductance regulator (CFTR) channel, and also diminished anion secretion via stimulation of Cl-/HCO3- exchanger, which facilitates the extrusion of more CFTR-permeant anion, Cl-, with the uptake of less CFTR-permeant anion, HCO3-. Thus, neltenexine reduced the hyper-secretion to keep an appropriate fluid level in the airway, providing a possibility that neltenexine can be an effective drug in airway obstructive diseases by decreasing the airway resistance under a hyper-secreting condition.

Cyclic nucleotides induce long-term augmentation of glutamate-activated chloride current in molluscan neurons

1. Literature data indicate that serotonin induces the long-term potentiation of glutamate (Glu) response in molluscan neurons. The aim of present work was to elucidate whether cyclic nucleotides can cause the same effect. 2. Experiments were carried out on isolated neurons of the edible snail (Helix pomatia) using a two-microelectrode voltage-clamp method. 3. In the majority of the cells examined, the application of Glu elicited a Cl- -current. The reversal potential (Er) of this current lied between -35 and -55 mV in different cells. 4. Picrotoxin, a blocker of Cl- -channels, suppressed this current equally on both sides of Er. Furosemide, an antagonist of both Cl- -channels and the Na+/K+/Cl- -cotransporter, had a dual effect on Glu-response: decrease in conductance, and shift of Er to negative potentials. 5. A short-term (2 min) cell treatment with 8-Br-cAMP or 8-Br-cGMP caused long-term (up to 30 min) change in Glu-response. At a holding potential of -60 mV, which was close to the resting level, an increase in Glu-activated inward current was observed. This potentiation seems to be related to the right shift of Er of Glu-activated Cl- -current rather than to the increase in conductance of Cl- -channels. The blocking effect of picrotoxin rested after 8-Br-cAMP treatment. 6. The change in the Cl- -homeostasis as a possible mechanism for the observed effect of cyclic nucleotides is discussed.

Ambroxol-induced modification of ion transport in human airway Calu-3 epithelia

Ambroxol is often used as a mucolytic agent in various lung diseases. However, it is unclear how ambroxol acts on bronchial epithelial cells. To clarify the action of ambroxol, we studied the effects of ambroxol on the ion transport in human Calu-3 cells, a human submucosal serous cell line, measuring the transepithelial short-circuit current and conductance across monolayers of Calu-3 cells. Ambroxol of 100 microM diminished the terbutaline (a beta2-adrenergic agonist)-stimulated Cl-/HCO3(-)-dependent secretion without any decreases in the conductance of cystic fibrosis transmembrane conductance regulator (CFTR) channel locating on the apical membrane. On the other hand, under the basal (unstimulated) condition ambroxol increased the Cl(-)-dependent secretion with no significant change in the apical CFTR channel conductance and decreased the HCO3- secretion associated with a decrease in the apical CFTR channel conductance. Ambroxol had no major action on the epithelial Na+ channel (ENaC) or the ENaC-mediated Na+ absorption. These results indicate that in Calu-3 cells: (1) under the basal (unstimulated) condition ambroxol increases Cl- secretion by stimulating the entry step of Cl- and decreases HCO3- secretion by diminishing the activity of the CFTR channel and/or the Na+/HCO3(-)-dependent cotransporter, (2) under the adrenergic agonist-stimulated condition, ambroxol decreases Cl- secretion by acting on the Cl-/HCO3- exchanger, and (3) ambroxol has a more powerful action than the adrenergic agonist on the Cl-/HCO3- exchanger, leading fluid secretion to a moderately stimulated level from a hyper-stimulated level.

Quercetin and NPPB-induced diminution of aldosterone action on Na+ absorption and ENaC expression in renal epithelium

In renal epithelial A6 cells, aldosterone applied for 24 h increased the transepithelial Cl- secretion over 30-fold due to activation of the Na+/K+/2Cl- cotransporter and stimulated the transepithelial Na+ absorption, activity of epithelial Na+ channel (ENaC), and alpha-ENaC mRNA expression. The stimulatory action of aldosterone on the transepithelial Na+ absorption, ENaC activity, and alpha-ENaC mRNA expression was diminished by 24h-pretreatment with quercetin (an activator of Na+/K+/2Cl- cotransporter participating in Cl- entry into the cytosolic space) or 5-nitro 2-(3-phenylpropylamino)benzoate (NPPB) (a blocker of Cl- channel participating in Cl- release from the cytosolic space), while 24h-pretreatment with bumetanide (a blocker of Na+/K+/2Cl- cotransporter) enhanced the stimulatory action of aldosterone on transepithelial Na+ absorption. On the other hand, under the basal (aldosterone-unstimulated) condition, quercetin, NPPB or bumetanide had no effect on transepithelial Na+ absorption, activity of ENaC or alpha-ENaC mRNA expression. These observations suggest that although aldosterone shows overall its stimulatory action on ENaC (transepithelial Na+ transport), aldosterone has an inhibitory action on ENaC (transepithelial Na+ transport) via activation of the Na+/K+/2Cl- cotransporter, and that modification of activity of Cl- transporter/channel participating in the transepithelial Cl- secretion influences the aldosterone-stimulated ENaC (transepithelial Na+ transport).

Synergic action of insulin and genistein on Na+/K+/2Cl − cotransporter in renal epithelium

Transepithelial Cl(-) secretion in polarized renal A6 cells is composed of two steps: (1) Cl(-) entry step across the basolateral membrane mediated by Na(+)/K(+)/2Cl(-) cotransporter (NKCC) and (2) Cl(-) releasing step across the apical membrane via cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. We estimated CFTR Cl(-) channel activity and transcellular Cl(-) secretion by measuring 5-nitro 2-(3-phenylpropylamino)benzoate (NPPB, a blocker of CFTR Cl(-) channel)-sensitive transepithelial conductance (Gt) and short-circuit current (Isc), respectively. Pretreatment with 1 microM insulin for 24 h had no effects on NPPB-sensitive Gt or Isc. On the other hand, in A6 cells treated with carbobenzoxy-L-leucyl-leucyl-L-leucinal (MG132; 100 microM for 2 h) that inhibits endocytosis of proteins at the plasma membrane into the cytosolic space, insulin pretreatment increased the NPPB-sensitive Isc with no effects on NPPB-sensitive Gt. Genistein (100 microM) induced sustained increases in NPPB-sensitive Gt and Isc, which were diminished by brefeldin A (a blocker of protein translocation to Golgi apparatus from endoplasmic reticulum). Co-application of insulin and genistein synergically stimulated the NPPB-sensitive Isc without any effects on NPPB-sensitive Gt. These observations suggest that: (1) insertion and endocytosis of NKCC are stimulated by insulin, (2) the insulin-induced stimulation of NKCC insertion into the basolateral membrane is offset by the stimulatory action on NKCC endocytosis from the basolateral membrane, (3) genistein stimulates insertion of both CFTR Cl(-) channel into the apical membrane and NKCC into the basolateral membrane, and (4) insulin and genistein synergically stimulated NKCC insertion into the basolateral membrane.

Effect of the flavonol quercetin on membrane conductances in rat colonic crypt cells

The plant polyphenol quercetin was shown to induce Cl- secretion in rat colon. This study was performed to investigate the alterations of membrane conductances in isolated epithelial cells induced by quercetin. Whole-cell patch-clamp recordings were performed in isolated crypts from rat distal colon. In cells of the crypt basis, quercetin significantly hyperpolarized the membrane potential at concentrations > or =3 microM and increased the K+ conductance without visibly altering the Cl- conductance. Thus, quercetin induces Cl- secretion merely by activation of K+ channels in the colon epithelium.

Involvement of cytosolic Cl- in osmoregulation of alpha-ENaC gene expression

Hypotonicity stimulates transepithelial Na(+) reabsorption in renal A6 cells, but the mechanism for this stimulation is not fully understood. In the present study, we found that hypotonicity stimulated Na(+) reabsorption through increases in mRNA expression of the alpha-subunit of the epithelial Na(+) channel (alpha-ENaC). Hypotonicity decreases cytosolic Cl(-) concentration; therefore, we hypothesized that hypotonicity-induced decreases in cytosolic Cl(-) concentration could act as a signal to regulate Na(+) reabsorption through changes in alpha-ENaC mRNA expression. Treatment with the flavone apigenin, which activates the Na(+)-K(+)-2Cl(-) cotransporter and increases cytosolic Cl(-) concentration, markedly suppressed the hypotonicity-induced increase in alpha-ENaC mRNA expression. On the other hand, blockade of the Na(+)-K(+)-2Cl(-) cotransporter decreases cytosolic Cl(-) concentration and increased alpha-ENaC mRNA expression and Na(+) reabsorption. Blocking Cl(-) channels with 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) inhibited the hypotonicity-induced decrease in cytosolic Cl(-) concentration and suppressed the hypotonicity-induced increase in alpha-ENaC mRNA expression. Coapplication of NPPB and apigenin synergistically suppressed alpha-ENaC mRNA expression. Thus, in every case, changes in cytosolic Cl(-) concentration were associated with changes in alpha-ENaC mRNA expression and changes in Na(+) reabsorption: decreases in cytosolic Cl(-) concentration increased alpha-ENaC mRNA and increased Na(+) reabsorption, whereas increases in cytosolic Cl(-) concentration decreased alpha-ENaC mRNA and decreased Na(+) reabsorption. These findings support the hypothesis that changes in cytosolic Cl(-) concentration are an important and novel signal in hypotonicity-induced regulation of alpha-ENaC expression and Na(+) reabsorption.