Na(+)-dependent and Na(+)-independent systems of choline transport by plasma membrane vesicles of A549 cell line

Involvement of H+-gradient dependent transporter in PGE2 release from A549 cells

The purpose of this study was to identify the transporter involved in the release of prostaglandin E₂ (PGE₂). In the present study, transport assays were conducted using membrane vesicles prepared from human lung adenocarcinoma A549 cells, thus enabling identification of the novel exporter present in A549 cells. PGE₂ transport into A549 vesicles was higher in the presence of a proton (H⁺)-gradient, thus suggesting the involvement of PGE₂H⁺ symporter in PGE₂ transport. Results from our experiments showed enhanced PGE₂ release in A549 cells in the presence of H⁺-gradient ([H⁺]_{extracellular} < [H⁺]_{intracellular}). Moreover, in vesicular transport assays, H⁺-gradient-dependent transport of PGE₂ did not show saturation up to 500 μM PGE_2, and 10 mM aromatic monocarboxylic acids (acetylsalicylic acid, salicylic acid, and p-nitrobenzoic acid) significantly inhibited PGE₂ transport by 62-70%. These results suggest, the involvement of monocarboxylate transporters in the H⁺-gradient-dependent PGE₂ export.

Choline transporter-like protein 4 (CTL4) links to non-neuronal acetylcholine synthesis

Synthesis of acetylcholine (ACh) by non-neuronal cells is now well established and plays diverse physiologic roles. In neurons, the Na(+) -dependent, high affinity choline transporter (CHT1) is absolutely required for ACh synthesis. In contrast, some non-neuronal cells synthesize ACh in the absence of CHT1 indicating a fundamental difference in ACh synthesis compared to neurons. The aim of this study was to identify choline transporters, other than CHT1, that play a role in non-neuronal ACh synthesis. ACh synthesis was studied in lung and colon cancer cell lines focusing on the choline transporter-like proteins, a five gene family choline-transporter like protein (CTL)1-5. Supporting a role for CTLs in choline transport in lung cancer cells, choline transport was Na(+) -independent and CTL1-5 were expressed in all cells examined. CTL1, 2, and 5 were expressed at highest levels and knockdown of CTL1, 2, and 5 decreased choline transport in H82 lung cancer cells. Knockdowns of CTL1, 2, 3, and 5 had no effect on ACh synthesis in H82 cells. In contrast, knockdown of CTL4 significantly decreased ACh secretion by both lung and colon cancer cells. Conversely, increasing expression of CTL4 increased ACh secretion. These results indicate that CTL4 mediates ACh synthesis in non-neuronal cell lines and presents a mechanism to target non-neuronal ACh synthesis without affecting neuronal ACh synthesis.

Organic cation transporters in the blood-air barrier: expression and implications for pulmonary drug delivery

Studies concerning the impact that hepatic, renal and intestinal transporters have on drug disposition have been frequently reported in the literature. Surprisingly, however, little is known regarding the distribution and function of drug-transporter proteins of the lung epithelium. Many drugs (delivered to the lung) have a net positive charge and, thus, are potential substrates of organic cation transporters; currently marketed compounds (e.g., bronchodilators), as well as novel drug candidates in development, are such substrates. It is the aim of this review to summarize the current state of organic cation-transporter expression analysis in the lung and in in vitro models of bronchial and alveolar barriers. Moreover, activity of selected transporters in lung epithelium in situ and in vitro will be highlighted, and their potential role in pulmonary drug disposition will be addressed. One example included here is the transporter-dependent absorption of beta2-agonists in respiratory epithelial cells.

miR-9 and NFATc3 regulate myocardin in cardiac hypertrophy

Myocardial hypertrophy is frequently associated with poor clinical outcomes including the development of cardiac systolic and diastolic dysfunction and ultimately heart failure. To prevent cardiac hypertrophy and heart failure, it is necessary to identify and characterize molecules that may regulate the hypertrophic program. Our present study reveals that nuclear factor of activated T cells c3 (NFATc3) and myocardin constitute a hypertrophic pathway that can be targeted by miR-9. Our results show that myocardin expression is elevated in response to hypertrophic stimulation with isoproterenol and aldosterone. In exploring the molecular mechanism by which myocardin expression is elevated, we identified that NFATc3 can bind to the promoter region of myocardin and transcriptionally activate its expression. Knockdown of myocardin can attenuate hypertrophic responses triggered by NFATc3, suggesting that myocardin can be a downstream mediator of NFATc3 in the hypertrophic cascades. MicroRNAs are a class of small noncoding RNAs that mediate post-transcriptional gene silencing. Our data reveal that miR-9 can suppress myocardin expression. However, the hypertrophic stimulation with isoproterenol and aldosterone leads to a decrease in the expression levels of miR-9. Administration of miR-9 could attenuate cardiac hypertrophy and ameliorate cardiac function. Taken together, our data demonstrate that NFATc3 can promote myocardin expression, whereas miR-9 is able to suppress myocardin expression, thereby regulating cardiac hypertrophy.

Characterization of an extensive transverse tubular network in sheep atrial myocytes and its depletion in heart failure

BACKGROUND

In ventricular myocytes, the majority of structures that couple excitation to the systolic rise of Ca(2+) are located at the transverse tubular (t-tubule) membrane. In the failing ventricle, disorganization of t-tubules disrupts excitation contraction coupling. The t-tubule membrane is virtually absent in the atria of small mammals resulting in spatiotemporally distinct profiles of intracellular Ca(2+) release on stimulation in atrial and ventricular cells. The aims of this study were to determine (i) whether atrial myocytes from a large mammal (sheep) possess t-tubules, (ii) whether these are functionally important, and (iii) whether they are disrupted in heart failure.

METHODS AND RESULTS

Sheep left atrial myocytes were stained with di-4-ANEPPS. Nearly all control cells had an extensive t-tubule network resulting in each voxel in the cell being nearer to a membrane (sarcolemma or t-tubule) than would otherwise be the case. T-tubules decrease the distance of 50% of voxels from a membrane from 3.35 + or - 0.15 to 0.88 + or- 0.04 microm. During depolarization, intracellular Ca(2+) rises simultaneously at the cell periphery and center. In heart failure induced by rapid ventricular pacing, there was an almost complete loss of atrial t-tubules. The distance of 50% of voxels from a membrane increased to 2.04 + or - 0.08 microm, and there was a loss of early Ca(2+) release from the cell center.

CONCLUSIONS

Sheep atrial myocytes possess a substantial t-tubule network that synchronizes the systolic Ca(2+) transient. In heart failure, this network is markedly disrupted. This may play an important role in changes of atrial function in heart failure.

Expression and functional characterization of choline transporter in human keratinocytes

Choline is essential for synthesis of the major membrane phospholipid phosphatidylcholine. Moreover, it serves as a precursor for synthesis of the neurotransmitter acetylcholine (ACh). Keratinocytes of the epidermis synthesize and release ACh. The uptake of choline is the rate-limiting step in both ACh synthesis and choline phospholipid metabolism, and it is a prerequisite for keratinocyte proliferation. However, the nature of the choline transport system in keratinocytes is poorly understood. In this study, we examined the molecular and functional characterization of choline uptake into cultured human keratinocytes. Choline uptake into keratinocytes was independent of extracellular Na(+), saturable, and mediated by a single transport system with an apparent Michaelis-Menten constant of 12.3 muM. Choline uptake was reduced when the keratinocyte membrane potential was depolarized by high K(+). These results provide evidence that the choline transport activity is potential-sensitive. Various organic cations inhibit the choline transport system. RT-PCR demonstrated that keratinocytes expressed mRNA for choline transporter-like protein 1 (CTL1), mainly the CTL1a subtype. The present biochemical and pharmacological data suggest that CTL1a is functionally expressed in human keratinocytes and is responsible for the uptake of choline and organic cations in these cells.

The epithelial cholinergic system of the airways

Acetylcholine (ACh), a classical transmitter of parasympathetic nerve fibres in the airways, is also synthesized by a large number of non-neuronal cells, including airway surface epithelial cells. Strongest expression of cholinergic traits is observed in neuroendocrine and brush cells but other epithelial cell types--ciliated, basal and secretory--are cholinergic as well. There is cell type-specific expression of the molecular pathways of ACh release, including both the vesicular storage and exocytotic release known from neurons, and transmembrane release from the cytosol via organic cation transporters. The subcellular distribution of the ACh release machineries suggests luminal release from ciliated and secretory cells, and basolateral release from neuroendocrine cells. The scenario as known so far strongly suggests a local auto-/paracrine role of epithelial ACh in regulating various aspects on the innate mucosal defence mechanisms, including mucociliary clearance, regulation of macrophage function and modulation of sensory nerve fibre activity. The proliferative effects of ACh gain importance in recently identified ACh receptor disorders conferring susceptibility to lung cancer. The cell type-specific molecular diversity of the epithelial ACh synthesis and release machinery implies that it is differently regulated than neuronal ACh release and can be specifically targeted by appropriate drugs.

Natural agents targeting the alpha7-nicotinic-receptor in NSCLC: a promising prospective in anti-cancer drug development

Nicotinic acetylcholine receptors (nAChR) are expressed on normal bronchial epithelial and nonsmall cell lung cancer (NSCLC) cells and are involved in cell growth regulation. Nicotine induced cell proliferation. The purpose of this study was to determine if interruption of autocrine nicotinic cholinergic signaling might inhibit A549 NSCLC cell growth. For this purpose alpha-Cobratoxin (alpha-CbT), a high affinity alpha7-nAChR antagonist was studied. Cell growth decrease was evaluated by Clonogenic and MTT assays. Evidence of apoptosis was identified staining cell with Annexin-V/PI. Characterization of the basal NF-kappaB activity was done using the Trans-AM NF-kappaB assay colorimetric kit. "In vivo" antitumour activity was evaluated in orthotopically transplanted nude mice monitored by In vivo Imaging System technology. alpha-CbT caused concentration-dependent cell growth decrease, mitochondrial apoptosis caspases-9 and 3-dependent, but caspase-2 and p53-independent and down-regulation of basal high levels of activated NF-kappaB. alpha-CbT treatment determines a significant reduction of tumor growth in nude mice orthotopically engrafted with A549-luciferase cells (4.6% of living cells vs. 31% in untreated mice). No sign of toxicity was reported related to treatment. These findings suggest that alpha7-nAChR antagonists namely alpha-CbT may be useful adjuvant for treatment of NSCLC and potentially other cancers.

Choline transporters in human lung adenocarcinoma: expression and functional implications

Choline is an essential nutrient for cell survival and proliferation, however, the expression and function of choline transporters have not been well identified in cancer. In this study, we detected the mRNA and protein expression of organic cation transporter OCT3, carnitine/cation transporters OCTN1 and OCTN2, and choline transporter-like protein CTL1 in human lung adenocarcinoma cell lines A549, H1299 and SPC-A-1. Their expression pattern was further confirmed in 25 human primary adenocarcinoma tissues. The choline uptake in these cell lines was significantly blocked by CTL1 inhibitor, but only partially inhibited by OCT or OCTN inhibitors. The efficacy of these inhibitors on cell proliferation is closely correlated with their abilities to block choline transport. Under the native expression of these transporters, the total choline uptake was notably blocked by specific PI3K/AKT inhibitors. These results describe the expression of choline transporters and their relevant function in cell proliferation of human lung adenocarcinoma, thus providing a potential choline-starvation strategy of cancer interference through targeting choline transporters, especially CTL1.

Adenovirus stimulates choline efflux by increasing expression of organic cation transporter-2

We examined the effect of wild-type human adenovirus (Ad5) on choline transport in murine lung epithelia (MLE) and in rodent primary alveolar type II cells. Cells were active in pH-sensitive, reversible transport of choline, a process blocked pharmacologically with phenoxybenzamine, an inhibitor of organic cation transporters (OCT). PCR products for the choline transporters, OCT-1 and OCT-2, were detected, but only OCT-2 protein was robustly expressed within MLE and primary alveolar epithelial cells. Ad5 produced a two- to threefold increase in choline efflux from cells, resulting in a significant reduction in intracellular choline content and its major product, phosphatidylcholine. Effects of Ad5 on choline efflux were inhibited with phenoxybenzamine, and choline efflux was attenuated by OCT-2 small interfering RNA. Adenovirus also produced a dose-dependent increase in immunoreactive OCT-2 levels concomitant with increased cellular OCT-2 steady-state mRNA. These results indicate that adenoviruses can significantly disrupt choline trafficking in lung epithelia by upregulating expression of an alveolar protein involved in organic cation transport.

Innervation of the levator ani and coccygeus muscles of the female rat

In humans, the pelvic floor skeletal muscles support the viscera. Damage to innervation of these muscles during parturition may contribute to pelvic organ prolapse and urinary incontinence. Unfortunately, animal models that are suitable for studying parturition-induced pelvic floor neuropathy and its treatment are rare. The present study describes the intrapelvic skeletal muscles (i.e., the iliocaudalis, pubocaudalis, and coccygeus) and their innervation in the rat to assess its usefulness as a model for studies of pelvic floor nerve damage and repair. Dissection of rat intrapelvic skeletal muscles demonstrated a general similarity with human pelvic floor muscles. Innervation of the iliocaudalis and pubocaudalis muscles (which together constitute the levator ani muscles) was provided by a nerve (the "levator ani nerve") that entered the pelvic cavity alongside the pelvic nerve, and then branched and penetrated the ventromedial (i.e., intrapelvic) surface of these muscles. Innervation of the rat coccygeus muscle (the "coccygeal nerve") was derived from two adjacent branches of the L6-S1 trunk that penetrated the muscle on its rostral edge. Acetylcholinesterase staining revealed a single motor endplate zone in each muscle, closely adjacent to the point of nerve penetration. Transection of the levator ani or coccygeal nerves (with a 2-week survival time) reduced muscle mass and myocyte diameter in the iliocaudalis and pubocaudalis or coccygeus muscles, respectively. The pudendal nerve did not innervate the intrapelvic skeletal muscles. We conclude that the intrapelvic skeletal muscles in the rat are similar to those described in our previous studies of humans and that they have a distinct innervation with no contribution from the pudendal nerve.

Cadmium transport through type II alveolar cell monolayers: contribution of transcellular and paracellular pathways in the rat ATII and the human A549 cells

Cadmium (CD) transport in alveolar type II (ATII) cells has been studied using two in vitro models widely used to investigate lung function: primary cultures of rat ATII cells and the human cell line A549. Nonlinear regression analyses of the uptake time-course of (109)Cd revealed: a zero-time accumulation, a fast process of accumulation which proceeds within minutes, and a much slower process which takes hours. This three-step mechanism was characterized by different parameter values under dishes-or filter-growth conditions. A higher initial uptake rate (v(i)) and equilibrium accumulation (A(max)) of (109)Cd were found in the rat ATII cells; these differences were not related to a higher level of adsorption onto the external surface of the cell membrane. Specific transport systems of similar capacity but different affinity (threefold higher in rat cells) were characterized. A significant transepithelial transport of (109)Cd, with similar P(coeff) in both cell models, could not be exclusively related to cellular metal release. Results on 3H-mannitol permeability together with (109)Cd efflux data strongly suggest a greater contribution of the paracellular pathways in Cd transport through A549 cell monolayers. These differences in transport properties between the two lung cell models may modify the dose-response curve for Cd toxicity.

The transport of choline

Choline has many physiological functions throughout the body that are dependent on its available local supply. However, since choline is a charged hydrophilic cation, transport mechanisms are required for it to cross biological membranes. Choline transport is required for cellular membrane construction and is the rate-limiting step for acetylcholine production. Transport mechanisms include: (1) sodium-dependent high-affinity uptake mechanism in synaptosomes, (2) sodium-independent low-affinity mechanism on cellular membranes, and (3) unique choline uptake mechanisms (e.g., blood-brain barrier choline transport). A comprehensive overview of choline transport studies is provided. This review article examines landmark and current choline transport studies, molecular mapping, and molecular identification of these carriers. Information regarding the choline-binding site is presented by reviewing choline structural analog (hemicholinium-3 and 15, and other nitrogen/methyl-hydroxyl compounds) inhibition studies. Choline transport in Alzheimer's disease, brain ischemic events, and aging is also discussed. Emphasis throughout the article is placed on targeting the choline transporter in disease and use of this carrier as a drug delivery vector.

Sarcoplasmic reticulum Ca2+ regulatory protein gene expression in human right atrium under hemodynamic overload

Sarcoplasmic reticulum (SR) Ca2+-adenosine triphosphatase (ATPase) mRNA expression is reduced in the failing human myocardium. However, it is not known whether SR Ca2+-regulatory protein gene expression is altered in human myocardial tissue subjected to pressure overload or volume overload. We sought to determine whether SR Ca2+-regulatory protein gene expression is altered in human atrial tissue subjected to mechanical overload. We obtained right atrial myocardial tissue (about 250mg) at open-heart surgery from three groups of patients: no hemodynamic overload to the right atrium (control group; 12 patients), atrial septal defect (ASD group; 8 patients), and tricuspid regurgitation (TR group; 7 patients). We measured the myocyte size, the area of interstitial fibrosis, SR Ca2+,-ATPase, and ryanodine receptor mRNA abundance. The isolated cardiocyte area and the percent area of interstitial fibrosis were in the order TR > ASD > control (P < 0.05). The SR Ca2+-ATPase mRNA level in TR was significantly decreased (P = 0.004) compared with the control, whereas in the ASD group it did not differ significantly from control. There were no significant differences in ryanodine receptor mRNA levels among the three groups. SR Ca2+-ATPase gene expression was downregulated in human atrial tissue with TR but not in ASD, which might have resulted from the differences in the degree and/or the type of hemodynamic overload to the myocardium.

Calcineurin expression, activation, and function in cardiac pressure-overload hypertrophy

BACKGROUND

Vascular hypertension resulting in increased cardiac load is associated with left ventricular hypertrophy and is a leading predicator for progressive heart disease. The molecular signaling pathways that respond to increases in cardiac load are poorly understood. One potential regulator of the hypertrophic response is the calcium-sensitive phosphatase calcineurin.

METHODS AND RESULTS

We showed that calcineurin enzymatic activity is increased 3. 2-fold in the heart in response to pressure-overload hypertrophy induced by abdominal aortic banding in the rat. Western blot analysis further demonstrates that calcineurin A (catalytic subunit) protein content and association with calmodulin are increased in response to pressure-overload hypertrophy. This increase in calcineurin protein content was prevented by administration of the calcineurin inhibitor cyclosporine A (CsA). CsA administration attenuated load-induced cardiac hypertrophy in a dose-dependent manner over a 14-day treatment protocol. CsA administration also partially reversed pressure-overload hypertrophy in aortic-banded rats after 14 days. CsA also attenuated the histological and molecular indexes of pressure-overload hypertrophy.

CONCLUSIONS

These data suggest that calcineurin is an important upstream regulator of load-induced hypertrophy.

Biophysical properties and molecular characterization of amiloride-sensitive sodium channels in A549 cells

Amiloride-sensitive Na(+) channels, present in fetal and adult alveolar epithelial type II (ATII) cells, play a critical role in the reabsorption of fetal fluid shortly after birth and in limiting the extent of alveolar edema across the adult lung. Because of the difficulty in isolating and culturing ATII cells, there is considerable interest in characterizing the properties of ion channels and their response to injury of ATII cell-like cell lines such as A549 that derive from a human alveolar cell carcinoma. A549 cells were shown to contain alpha-, beta-, and gamma-epithelial Na(+) channel mRNAs. In the whole cell mode of the patch-clamp technique (bath, 145 mM Na(+); pipette, 145 mM K(+)), A549 cells exhibited inward Na(+) currents reversibly inhibited by amiloride, with an inhibition constant of 0.83 microM. Ion substitution studies showed that these channels were moderately selective for Na(+) (Na(+)-to-K(+) permeability ratio = 6:1). Inward Na(+) currents were activated by forskolin (10 microM) and inhibited by nitric oxide (300 nM) and cGMP. Recordings in cell-attached mode revealed the presence of an amiloride-sensitive Na(+) channel with a unitary conductance of 8.6 +/- 0.04 (SE) pS. Channel activity was increased by forskolin and decreased by nitric oxide and the cGMP analog 8-bromo-cGMP. These data demonstrate that A549 cells contain amiloride-sensitive Na(+) channels with biophysical properties similar to those of ATII cells.

Conductive choline transport by alveolar epithelial plasma membrane vesicles

Choline is an important substrate in alveolar epithelia for both surfactant production and cellular maintenance. The underlying mechanisms of uptake and sites of membrane transport remain uncertain. To test the hypothesis that choline transport occurs at the basolateral side of alveolar epithelia by both Na+-independent and -dependent mechanisms, plasma membrane vesicles were prepared from the apical and basolateral membranes of mature porcine type II pneumocytes. Choline+ transport was assayed by uptake of [3H]choline+ by enriched apical or basolateral vesicles. In the presence of imposed, inside-negative charge gradients, basolateral vesicles exhibited early overshoot of [3H]choline+ uptake unaffected by the presence or absence of external Na+ (541 +/- 53 vs 564 +/- 79 pmol/mg protein (NS)). High sensitivity to hemicholinium-3 was observed in the presence or absence of Na+. In the absence of inside-negative charge gradients, uptake was reduced 12-fold in the presence or absence of Na+, and external choline+ induced internal alkalization of acidified basolateral vesicles. Accumulative [3H]choline+ uptakes by apical vesicles in the presence or absence of inside-negative charge gradients and Na+ were insignificant. We conclude that predominant choline+ uptake by type II pneumocytes occurs at the basolateral membrane by Na+-independent, electrogenic choline+ conductance. The presence of electroneutral choline+/H+ exchange is suggested.

Betaine as an osmolyte in rat liver: metabolism and cell-to-cell interactions

Betaine was recently identified as an osmolyte in rat liver macrophages (Kupffer cells [KCs]) and sinusoidal endothelial cells (SECs). Betaine interferes with KC functions, such as phagocytosis, cytokine, and prostaglandin syntheses. As betaine is derived from choline, the present study was undertaken to evaluate osmosensitivity and cell heterogeneity of choline metabolism in rat liver. In the perfused rat liver after in vivo prelabeling with [14C]-choline, hypoosmotic stress induced a radioactivity release into the perfusate which was identified as [14C]-betaine by high-performance liquid chromatography (HPLC) analysis and which was inhibited by the anion exchanger inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Choline metabolism was studied in cultured liver parenchymal cells, (PCs), KCs, and SECs. Choline was taken up by all but betaine formation from choline was only detectable in PCs and not in KCs and SECs. Betaine formation in PCs was not stimulated by hyperosmolarity; rather, betaine has a role as an osmolyte in KCs and SECs but is of minor importance in PCs, as evidenced by only minor hyperosmolarity-induced betaine uptake. Thus, liver PCs can produce and release betaine derived from choline, and, thereby, possibly supply the osmolyte important for KC and SEC cell function. This may be another example for cell-to-cell interaction in the liver.