Epidermal growth factor regulation in adult rat alveolar type II cells of amiloride-sensitive cation channels

Epidermal growth factor strongly affects epithelial Na+ transport and barrier function in fetal alveolar cells, with minor sex-specific effects

Male sex remains an independent risk factor for respiratory distress syndrome (RDS) in preterm infants. Insufficient Na+ transport-mediated alveolar fluid clearance contributes to RDS development and we previously demonstrated sex-specific differences in Na+ transport. The epidermal growth factor (EGF) is important during fetal lung development with possible influence on Na+ transport. Sex-specific effects of EGF during surfactant synthesis were shown. We thus determined whether EGF exerts sex-specific effects on Na+ transport in fetal alveolar cells. We analyzed sex-specific fetal distal lung epithelial (FDLE) cells exposed to EGF and related ligands with Ussing chambers, RT-qPCR and Western blots. EGF strongly reduced the epithelial Na+ channel (ENaC) mRNA levels in both male and female FDLE cells. This was corroborated by a markedly reduced ENaC activity, while amiloride-insensitive pathways as well as barrier function were raised by EGF. In contrast to chronic effects, acute effects of EGF were sex-specific, because Na+ transport was reduced only in males. AKT phosphorylation was elevated only in female cells, while pERK1/2 was increased in both male and female cells. EGF showed certain sex- and time-dependent effects in FDLE cells. Nevertheless, the results suggest that EGF is an unlikely cause for the sex-specific differences in Na+ transport.

Alveolar epithelium: beyond the barrier

I am deeply honored to have been awarded an American Thoracic Society Recognition Award for Scientific Accomplishment for 2014. Over the last 20 years, it has become clear that the alveolar epithelium, my area of research focus, is not simply a gas exchange surface and barrier to leakage of fluid and protein into the alveoli, but is an active participant in the pathogenesis of a number of lung diseases, including pulmonary fibrosis. Recognition by this Award stimulates a review of the awardee's contributions to the field, as summarized in this perspective.

Regulation of Epithelial Na+ Channel (ENaC) in the Salivary Cell Line SMG-C6

Glucocorticoids and mineralocorticoids modulate Na+ transport via epithelial Na+ channels (ENaC). The rat submandibular epithelial cell line, SMG-C6, expresses α-ENaC mRNA and protein and exhibits amiloride-sensitive Na+ transport when grown in low-serum (2.5%) defined medium, therefore, we examined the effects of altering the composition of the SMG-C6 cell growth medium on ENaC expression and function. No differences in basal or amiloride-sensitive short-circuit current (Isc) were measured across SMG-C6 monolayers grown in the absence of thyroid hormone, insulin, transferrin, or EGF. In the absence of hydrocortisone, basal and amiloride-sensitive Isc significantly decreased. Similarly, monolayers grown in 10% serum-supplemented medium had lower basal Isc and no response to amiloride. Adding hydrocortisone (1.1 μM) to either the low or 10% serum medium increased basal and amiloride-sensitive Isc, which was blocked by RU486, the glucocorticoid and progesterone receptor antagonist. Aldosterone also induced an increase in α-ENaC expression and Na+ transport, which was also blocked by RU486 but not by the mineralocorticoid receptor antagonist spironolactone. Thus, in the SMG-C6 cell line, hydrocortisone and aldosterone increased ENaC expression and basal epithelial Na+ transport. The absence of endogenous ENaC expression in culture conditions devoid of steroids makes the properties of this cell line an excellent model for investigating pathways regulating ENaC expression and Na+ transport.

Biphasic regulation of ENaC by TGF-{alpha} and EGF in renal epithelial cells

The epithelial sodium channel (ENaC) is regulated by epidermal growth factor (EGF). We investigate whether ENaC is regulated by another EGF receptor (EGFR) ligand, transforming growth factor-alpha (TGF-alpha). We show that chronic (24 h) treatment with TGF-alpha inhibits ENaC in Xenopus laevis kidney cells 20 times more strongly than EGF. By using single-channel measurements, we show that TGF-alpha significantly reduces the number of ENaC per patch. The open probability (P(o)) is unchanged by 24-h treatment with TGF-alpha. alpha-, beta-, and gamma-ENaC mRNA levels are significantly reduced by TGF-alpha or EGF. TGF-alpha or EGF reduces alpha- and gamma-ENaC proteins in the membrane; however, beta-ENaC is unchanged. TGF-alpha or EGF inhibits ENaC by activating EGFR since the EGFR inhibitor AG1478 blocks the effects of both. The MAPK 1/2 inhibitor U0126 also blocks the effect of TGF-alpha or EGF on ENaC, indicating that the MAPK1/2 pathway is involved in the TGF-alpha- or EGF-induced inhibition of ENaC. Interestingly, acute treatment (<1 h) with TGF-alpha or EGF does not inhibit ENaC current; it enhances ENaC activity by increasing P(o). Pretreatment of the cells with U0126 potentiates the acute TGF-alpha- or EGF-induced stimulation of ENaC. This TGF-alpha- or EGF-induced increase in sodium current is abolished by a phosphatidylinositol 3-kinase (PI-3 kinase) inhibitor, LY294002, suggesting that PI-3 kinase is involved in the activation of sodium transport. In conclusion, chronic treatment with TGF-alpha or EGF inhibits ENaC by decreasing the number of channels in the membrane transcriptionally through MAPK1/2 pathways, but acute treatment with TGF-alpha or EGF activates ENaC by increasing P(o) via PI-3 kinase.

Electrophysiological characterization of rat type II pneumocytes in situ

Optimal aeration of the lungs is dependent on an alveolar fluid clearance, a process that is governed by Na+ and Cl- transport. However, the specific contribution of various ion channels in different alveolar cell types under basal or stimulated conditions is not exactly known. We established a novel functional model of rat lung slices suitable for nystatin-perforated whole-cell patch-clamp experiments. Lung slices retained a majority of live cells for up to 72 hours. Type II pneumocytes in situ had a mean capacitance of 8.8 +/- 2.5 pF and a resting membrane potential of -4.4 +/- 1.9 mV. Bath replacement of Na+ with NMDG+ decreased inward whole-cell currents by 70%, 21% and 52% of which were sensitive to 10 microM and 1 mM of amiloride, respectively. Exposure of slices to 0.5 microM dexamethasone for 1 hour did not affect ion currents, while chronic exposure (0.5 microM, 24-72 h) induced an increase in both total Na+-entry currents and amiloride-sensitive currents. Under acute exposure to 100 microM cpt-cAMP, Type II cells in situ rapidly hyperpolarized by 25-30 mV, due to activation of whole-cell Cl- currents sensitive to 0.1 mM of 5-Nitro-2-(3-phenylpropylamino)benzoic acid. In addition, in the presence of cpt-cAMP, total sodium currents and currents sensitive to 10 microM amiloride increased by 32% and 70%, respectively. Thus, in Type II pneumocytes in situ: (1) amiloride-sensitive sodium channels contribute to only half of total Na+-entry and are stimulated by chronic exposure to glucocorticoids; (2) acute increase in cellular cAMP content simultaneously stimulates the entry of Cl- and Na+ ions.

Alveolar epithelial transport in the adult lung

The alveolar surface comprises >99% of the internal surface area of the lungs. At birth, the fetal lung rapidly converts from a state of net fluid secretion, which is necessary for normal fetal lung development, to a state in which there is a minimal amount of alveolar liquid. The alveolar surface epithelium facing the air compartment is composed of TI and TII cells. The morphometric characteristics of both cell types are fairly constant over a range of mammalian species varying in body weight by a factor of approximately 50,000. From the conservation of size and shape across species, one may infer that both TI and TII cells also have important conserved functions. The regulation of alveolar ion and liquid transport has been extensively investigated using a variety of experimental models, including whole animal, isolated lung, isolated cell, and cultured cell model systems, each with their inherent strengths and weaknesses. The results obtained with different model systems and a variety of different species point to both interesting parallels and some surprising differences. Sometimes it has been difficult to reconcile results obtained with different model systems. In this section, the primary focus will be on aspects of alveolar ion and liquid transport under normal physiologic conditions, emphasizing newer data and describing evolving paradigms of lung ion and fluid transport. We will highlight some of the unanswered questions, outline the similarities and differences in results obtained with different model systems, and describe some of the complex and interweaving regulatory networks.

Alveolar epithelium and Na,K-ATPase in acute lung injury

Active transport of sodium across the alveolar epithelium, undertaken in part by the Na,K-adenosine triphosphatase (Na,K-ATPase), is critical for clearance of pulmonary edema fluid and thus the outcome of patients with acute lung injury. Acute lung injury results in disruption of the alveolar epithelial barrier and leads to impaired clearance of edema fluid and altered Na,K-ATPase function. There has been significant progress in the understanding of mechanisms regulating alveolar edema clearance and signaling pathways modulating Na,K-ATPase function during lung injury. The accompanying review by Morty et al. focuses on intact organ and animal models as well as clinical studies assessing alveolar fluid reabsorption in alveolar epithelial injury. Elucidation of the mechanisms underlying regulation of active Na⁺ transport, as well as the pathways by which the Na,K-ATPase regulates epithelial barrier function and edema clearance, are of significance to identify interventional targets to improve outcomes of patients with acute lung injury.

Mechanisms of pulmonary edema clearance

The mechanisms of pulmonary edema resolution are different from those regulating edema formation. Absorption of excess alveolar fluid is an active process that involves vectorial transport of Na+out of alveolar air spaces with water following the Na+osmotic gradient. Active Na+transport across the alveolar epithelium is regulated via apical Na+and chloride channels and basolateral Na-K-ATPase in normal and injured lungs. During lung injury, mechanisms regulating alveolar fluid reabsorption are inhibited by yet unclear pathways and can be upregulated by pharmacological means. Better understanding of the mechanisms that regulate edema clearance may lead to therapeutic interventions to improve the ability of lungs to clear fluid, which is of clinical significance.

Catecholamine clearance from alveolar spaces of rat and human lungs

BACKGROUND

Although aerosolized beta-adrenergic agonists have been used as a therapy for the resolution of pulmonary edema, the mechanisms of catecholamine clearance from the alveolar spaces of the lung are not well known.

OBJECTIVE

To determine whether catecholamine clearance from the alveolar spaces is correlated with the fluid transport capacity of the lung.

METHODS

Albumin solution containing epinephrine (10(-7)M) or norepinephrine (10(-7)M) was instilled into the alveolar spaces of isolated rat and human lungs. Alveolar fluid clearance rate was estimated by the progressive increase in the albumin concentration over 1 h. Catecholamine clearance rate was estimated by the changes in catecholamine concentration and alveolar fluid volume over 1 h.

RESULTS

The norepinephrine clearance rate was faster than the epinephrine clearance rate in the rat and human lungs. In the rat lungs, amiloride (a sodium channel blocker) caused a greater decrease in alveolar fluid clearance and epinephrine clearance rate than propranolol (a nonselective beta-adrenergic antagonist). Although propranolol and phentolamine (an alpha-adrenergic antagonist), and 5-(N-ethyl-N-isoprophyl)amiloride (a Na+/H+ antiport blocker) changed neither the alveolar fluid clearance nor the norepinephrine clearance rate, amiloride and benzamil (a sodium channel blocker) decreased both clearance rates. As in the rat lungs, amiloride decreased alveolar fluid and norepinephrine clearance rates in the human lungs.

CONCLUSION

These results indicate that the catecholamine clearance rate from the alveolar spaces is correlated with alveolar fluid clearance in rat and human lungs.

Chronic exposure to EGF affects trafficking and function of ENaC channel in cystic fibrosis cells

Using the whole-cell patch-clamp technique, we identified an amiloride (AMI)-sensitive Na(+) current in cystic fibrosis cells, JME/CF15, growing in standard medium. The reversal potential of this current depended on Na(+) concentrations and the cation selectivity was much higher for Na(+) than for K(+), indicating that the current is through ENaC channels. In contrast, cells from EGF-containing medium lacked AMI-sensitive Na(+) currents. In permeabilized cells growing in EGF-containing medium, alphaENaC was mainly detected in a perinuclear region, while in cells from standard medium it was distributed over the cell body. Western-blot analysis showed that in standard medium cells expressed fast-migrating EndoH-insensitive and slow-migrating EndoH-sensitive alphaENaC fractions, while in cells growing in the presence of EGF, alphaENaC was only detected as the fast-migrating EndoH-insensitive fraction. Long-term incubation of cells with EGF resulted in an increased basal Ca(2+) level, [Ca(2+)](i). A similar increase of [Ca(2+)](i) was also observed in the presence of 2muM thapsigargin, resulting in inhibition of ENaC function. Thus, in JME/CF15 cells inhibition of the ENaC function by chronic incubation with EGF is a Ca(2+)-mediated process that affects trafficking and surface expression of ENaC channels.

Inhibition by cytoplasmic nucleotides of a new cation channel in freshly isolated human and rat type II pneumocytes

Here we report a 26- to 29-pS cation channel abundantly expressed in freshly isolated and primary cultured type II cells from rat or healthy human lungs. The channel was never spontaneously active in cell-attached patches but could be activated by cell permeabilization with beta-escin. Excised patch-clamp experiments revealed activation by Ca(2+) concentrations at the cytoplasmic side in the micromolar range. High concentrations of amiloride (>10 microM) at the extracellular side did not inhibit. The channel was equally permeable for K(+) and Na(+) but was essentially impermeable for Cl(-), Ca(2+), and Mg(2+). It was blocked by adenosine nucleotides (cytoplasmic side) with the following order of potency: AMP approximately ADP (EC(50) </= 10 microM) > ATP >> adenosine >> cyclic AMP. The blocking effect of ATP was reproduced by its nonhydrolyzable analogs AMPPNP or ATP-gamma-S. GTP did not inhibit. Cd(2+) blocked the channel with an EC(50) approximately 55.5 nM. We conclude that type II cells express a Ca(2+)-dependent, nucleotide-inhibited, nonselective, and Ca(2+)-impermeable cation channel (NSC(Ca/AMP)) with tonically suppressed activity. RT-PCR confirmed expression of TRPM4b, a channel with functional characteristics almost identical with NSC(Ca/AMP). Potential physiological roles are discussed.

Epidermal growth factor decreases PEPT2 transport capacity and expression in the rat kidney proximal tubule cell line SKPT0193 cl.2

The renal peptide transporter PEPT2 plays an important role in absorption of di- and tripetides in the proximal tubule; however, knowledge of regulation of PEPT2 by growth factors and hormones is limited. In the present study, we examined the effects of epidermal growth factor (EGF) on PEPT2 transport capacity and expression in the rat proximal tubule cell line SKPT0193 cl.2 (SKPT), which expresses rat PEPT2 (rPEPT2) in the apical membrane. Treatment of SKPT cells with EGF during cell culture growth caused a dose-dependent decrease in rPEPT2 transport capacity and expression, as determined by studies of apical uptake of [14C]glycylsarcosine, rPepT2 mRNA levels, and immunostaining of SKPT cells with a rPEPT2-specific antibody. On the contrary, apical uptake of glucose and lysine was increased in EGF-treated cells, indicating that EGF was not acting generally to decrease apical nutrient uptake mechanisms in the proximal tubule cells. Our findings indicate that EGF decreases rPEPT2 expression by lowering transcription of the rat PepT2 gene or by decreasing rat PepT2 mRNA stability. Previous investigators routinely used SKPT cell culture media with a high (10 ng/ml) EGF concentration. Our study suggests that this might be disadvantageous when studying PEPT2-mediated transport phenomena. These findings demonstrate for the first time EGF-mediated regulation of PEPT2 expression in a kidney cell line. The relevance for kidney regulation of peptide transport activity in physiological and/or pathophysiological situations, where EGF and EGF receptor levels change drastically, remains to be established.

Modulation of ion conductance and active transport by TGF-beta 1 in alveolar epithelial cell monolayers

Transforming growth factor-beta1 (TGF-beta 1) may be a critical mediator of lung injury and subsequent remodeling during recovery. We evaluated the effects of TGF-beta 1 on the permeability and active ion transport properties of alveolar epithelial cell monolayers. Rat alveolar type II cells plated on polycarbonate filters in defined serum-free medium form confluent monolayers and acquire the phenotypic characteristics of alveolar type I cells. Exposure to TGF-beta 1 (0.1-100 pM) from day 0 resulted in a concentration- and time-dependent decrease in transepithelial resistance (Rt) and increase in short-circuit current (Isc). Apical amiloride or basolateral ouabain on day 6 inhibited Isc by 80 and 100%, respectively. Concurrent increases in expression of Na+-K+-ATPase alpha 1- and beta 1-subunits were observed in TGF-beta 1-treated monolayers. No change in the alpha-subunit of the rat epithelial sodium channel (alpha-rENaC) was seen. Exposure of confluent monolayers to TGF-beta 1 from day 4 resulted in an initial decrease in Rt within 6 h, followed by an increase in Isc over 72-96 h. These results demonstrate that TGF-beta 1 modulates ion conductance and active transport characteristics of the alveolar epithelium, associated with increased Na+-K+-ATPase, but without a change in alpha-rENaC.

Downregulation of ENaC activity and expression by TNF-alpha in alveolar epithelial cells

Sodium absorption by an amiloride-sensitive channel is the main driving force of lung liquid clearance at birth and lung edema clearance in adulthood. In this study, we tested whether tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine involved in several lung pathologies, could modulate sodium absorption in cultured alveolar epithelial cells. We found that TNF-alpha decreased the expression of the alpha-, beta-, and gamma-subunits of epithelial sodium channel (ENaC) mRNA to 36, 43, and 16% of the controls after 24-h treatment and reduced to 50% the amount of alpha-ENaC protein in these cells. There was no impact, however, on alpha(1) and beta(1) Na(+)-K(+)-ATPase mRNA expression. Amiloride-sensitive current and ouabain-sensitive Rb(+) uptake were reduced, respectively, to 28 and 39% of the controls. A strong correlation was found at different TNF-alpha concentrations between the decrease of amiloride-sensitive current and alpha-ENaC mRNA expression. All these data show that TNF-alpha, a proinflammatory cytokine present during lung infection, has a profound influence on the capacity of alveolar epithelial cells to transport sodium.

Epidermal growth factor inhibits amiloride-sensitive sodium absorption in renal collecting duct cells

The effects of the ERK pathway on electrogenic transepithelial Na(+) absorption by renal collecting duct cells were determined. Approximately 90% of the unstimulated short-circuit current (15 +/- 1 microA/cm(2), n = 10) across conditionally immortalized murine collecting duct epithelial cells (mCT1) is amiloride sensitive and is likely mediated by apical epithelial Na(+) channels. Chronic exposure (24 h) of the epithelial monolayers to either EGF (50 ng/ml) or transforming growth factor-alpha (TGF-alpha; 20 ng/ml) reduced amiloride-sensitive short-circuit current by >60%. The inhibitory effect of EGF on Na(+) absorption was not due to inhibition of basolateral Na(+)-K(+)-ATPase, because the pump current elicited by permeabilization of apical membrane with nystatin was not reduced by EGF. Chronic exposure of the mCT1 cells to EGF (20 ng/ml, 24 h) elicited a 70-85% decrease in epithelial Na(+) channel subunit mRNA levels. Exposure of mCT1 cells to either EGF (20 ng/ml) or PMA (150 nM) induced rapid phosphorylation of p42/p44 (ERK1/2) and pretreatment of the monolayers with PD-98059 (an ERK kinase inhibitor; 30 microM) prevented phosphorylation of p42/p44. Similarly, pretreatment of mCT1 monolayers with PD-98059 prevented the EGF- and PMA-induced inhibition of amiloride-sensitive Na(+) absorption. The results of these studies demonstrate that amiloride-sensitive Na(+) absorption by renal collecting duct cells is regulated by the ERK pathway. This pathway may play a role in alterations in ion transport that occur in polycystic kidney disease.

Invited review: Active fluid clearance from the distal air spaces of the lung

Active ion transport drives iso-osmolar alveolar fluid clearance, a hypothesis originally suggested by in vivo studies in sheep 20 yr ago. Over the last two decades, remarkable progress has been made in establishing a critical role for active sodium transport as a primary mechanism that drives fluid clearance from the distal air spaces of the lung. The rate of fluid transport can be increased in most species, including the human lung, by cAMP stimulation. Catecholamine-independent mechanisms, including hormones, growth factors, and cytokines, can also upregulate epithelial fluid clearance in the lung. The new insights into the role of the distal lung epithelium in actively regulating lung fluid balance has important implications for the resolution of clinical pulmonary edema.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Re-evaluating the Na(+) conductance of adult rat alveolar type II pneumocytes: evidence for the involvement of cGMP-activated cation channels

1. Alveolar epithelial type II pneumocytes were isolated and purified from adult rat lung by elastase digestion and differential adhesion, and cultured in serum-free medium for approximately 2 days on glass coverslips for subsequent patch-clamp studies employing symmetrical sodium isethionate solutions. 2. Whole-cell Na(+) currents exhibited essentially linear current-voltage relationships which were mildly inhibited (by approximately 25 %) by 10 microM amiloride. In contrast, 1 mM Zn(2+) inhibited the currents by approximately 55 % with an IC(50) of approximately 134 microM and maximal blockade achieved between 5 and 10 mM. The effects of Zn(2+) and amiloride were additive, and independent of the order of blocker addition. 3. Gd(2+), Zn(2+) and La(3+) at 10 mM were all effective at rapidly, reversibly and significantly blocking the amiloride-insensitive currents by approximately 60%. in contrast, Ni(2+) was a very weak inhibitor (30 % inhibition at 10 mM). 4. Pimozide (10 microM) caused inhibition of whole-cell cation conductance by approximately 55 %. The inhibitory effect of pimozide was concentration dependent with an IC(50) of approximately 1 microM and was maximally effective between 10 and 30 microM. Sequential addition of Zn(2+) and pimozide, in either order, revealed no overlapping inhibitory effect on the amiloride-insensitive conductance, and supported the notion that the Zn(2+)- and pimozide-sensitive currents are identical. 5. The amiloride-insensitive, Zn(2+)-blockable conductance was characterised by a Na(+)/K(+) permeability ratio (P(Na)/P(K)) of 0.73 +/- 0.02. 6. 8Br-cGMP (100 microM), a membrane-permeable analogue of cGMP, evoked a robust activation of whole-cell cation conductance to 220 % of control. This activation was apparent in either the absence or the presence of 10 microM amiloride, but was completely abolished in the presence of Zn(2+). 7. These data support the in vivo and in situ observations of a substantial amiloride-resistant Na(+) conductance, demonstrate directly that cyclic nucleotide-gated non-selective cation channels are functionally expressed in alveolar epithelial type II cells, and suggest that these channels may contribute to the fluid-reabsorptive driving force in adult lung.

Selectivity properties of a Na-dependent amino acid cotransport system in adult alveolar epithelial cells

We investigated the amino acid specificity of a Na-dependent amino acid cotransport system that contributes to transepithelial Na absorption in the apical membrane of cultured adult rat alveolar epithelial cell monolayers. Short-circuit current was increased by basic, uncharged polar, and nonpolar amino acids but not by L-aspartic acid or L-proline. EC(50) values for L-lysine and L-histidine were 0.16 and 0.058 mM, respectively. The L-lysine-stimulated short-circuit current was Na dependent, with a concentration causing a half-maximal stimulation by Na of 44.24 mM. L-Serine, L-glutamine, and L-cysteine had EC(50) values of 0.095, 0.25, and 0.12 mM, respectively. L-Alanine had the highest affinity, with an EC(50) of 0.027 mM. We conclude that monolayer cultures of adult rat alveolar epithelial cells possess a broad-specificity Na-dependent amino acid cotransport system with properties consistent with system B(0,+). We suggest that this cotransport system plays a critical role in recycling of constituent amino acids that make up glutathione, thus ensuring efficient replenishment of this important antioxidant within the alveolar fluid.