Regulatory volume decrease in human esophageal epithelial cells

OEsophageal Ion Transport Mechanisms and Significance Under Pathological Conditions

Ion transporters play an important role in several physiological functions, such as cell volume regulation, pH homeostasis and secretion. In the oesophagus, ion transport proteins are part of the epithelial resistance, a mechanism which protects the oesophagus against reflux-induced damage. A change in the function or expression of ion transporters has significance in the development or neoplastic progression of Barrett’s oesophagus (BO). In this review, we discuss the physiological and pathophysiological roles of ion transporters in the oesophagus, highlighting transport proteins which serve as therapeutic targets or prognostic markers in eosinophilic oesophagitis, BO and esophageal cancer. We believe that this review highlights important relationships which might contribute to a better understanding of the pathomechanisms of esophageal diseases.

Review: Experimental models for Barrett's esophagus and esophageal adenocarcinoma

Several different cell culture systems and laboratory animal models have been used over the years to study Barrett's esophagus (BE) and esophageal adenocarcinoma (EAC). Most of the existing models have key differences with the human esophagus and complex pathogenesis of disease. None of the models offers an ideal system for the complex study of environmental exposure, genetic risk, and prevention strategies. In fact, different model systems may be required to answer different specific research questions about the pathogenesis of BE and EAC. Given the high mortality associated with EAC and the fact that current screening strategies miss most cases of EAC, advances in basic and translational science related to esophageal injury, repair, and carcinogenesis are clearly needed. This review describes several of the existing and potential model systems for BE and EAC with their benefits and disadvantages.

Acid inhibits TRPV4-mediated Ca²⁺ influx in mouse esophageal epithelial cells

BACKGROUND

The transient receptor potential vanilloid 4 (TRPV4), a thermo-sensitive stretch-activated cation channel, is expressed in the skin stratified squamous epithelium, contributing to the acquisition of barrier function. Similarly, functional TRPV4 may be located in the stratified squamous epithelial lining of the esophagus, being involved in the pathogenesis of gastroesophageal reflux disease (GERD). Here we investigated the expression of TRPV4 in the mouse esophageal epithelium.

METHODS

TRPV4 expression at the mRNA and protein levels was examined by reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization, and immunohistochemistry. A calcium imaging technique and ATP assay were used to evaluate the functionality of TRPV4 in freshly isolated esophageal epithelial cells.

KEY RESULTS

Transcripts and proteins encoding TRPV4 were colocalized in the basal and intermediate layers of the esophageal epithelium. Both 4α-phorbol 12,13- didecanoate (4α-PDD), a selective agonist for TRPV4, and hypo-osmolar solution (160 mOsm) elevated the intracellular calcium concentration ([Ca(2+) ](i) ) in a subset of the isolated cells (70%). These [Ca(2+) ](i) increases were potently inhibited by ruthenium red (RuR), a TRPV4 channel antagonist, and were suppressed by extracellular protons (pH 5.0). Finally, application of 4α-PDD evoked ATP release in primary esophageal epithelial cells.

CONCLUSIONS & INFERENCES

Acid-sensitive TRPV4 channels were mainly expressed in the esophageal epithelial cells of the basal and intermediate layers. Direct exposure of TRPV4-expressing cells to gastric acid, as would occur in cases of GERD, could influence their cellular functions, possibly aggravating the disease state.

A microfluidic chip for real-time studies of the volume of single cells

We report a microfluidic chip that is capable of measuring volume changes in single cells in real-time. Single eukaryotic cells were immobilized in the sensing area and changes in volume in response to hypotonic challenges and drugs were measured using the electrical impedance method. Experiments on MDCK cells showed that the maximum swelling and the time course of swelling vary between individual cells following hypotonic stimulation. The microfluidic chip allows, rapid and convenient change of solutions, enabling detailed studies of various drugs and chemicals that may play important role in cell physiology at the single cell level.

The extracellular calcium-sensing receptor (CaSR) on human esophagus and evidence of expression of the CaSR on the esophageal epithelial cell line (HET-1A)

Gastrointestinal reflux disease and eosinophilic esophagitis are characterized by basal cell hyperplasia. The extracellular calcium-sensing receptor (CaSR), a G protein-coupled receptor, which may be activated by divalent agonists, is expressed throughout the gastrointestinal system. The CaSR may regulate proliferation or differentiation, depending on cell type and tissue. The current experiments demonstrate the expression of the CaSR on a human esophageal epithelial cell line (HET-1A) and the location and expression of the CaSR in the human esophagus. CaSR immunoreactivity was seen in the basal layer of normal human esophagus. CaSR expression was confirmed in HET-1A cells by RT-PCR, immunocytochemistry, and Western blot analysis. CaSR stimulation by extracellular calcium or agonists, such as spermine or Mg(2+), caused ERK1 and 2 activation, intracellular calcium concentration ([Ca(2+)](i)) mobilization (as assessed by microspecfluorometry using Fluo-4), and secretion of the multifunctional cytokine IL-8 (CX-CL8). HET-1A cells transiently transfected with small interfering (si)RNA duplex against the CaSR manifested attenuated responses to Ca(2+) stimulation of phospho- (p)ERK1 and 2, [Ca(2+)](i) mobilization, and IL-8 secretion, whereas responses to acetylcholine (ACh) remained sustained. An inhibitor of phosphatidylinositol-specific phospholipase C (PI-PLC) (U73122) blocked CaSR-stimulated [Ca(2+)](i) release. We conclude that the CaSR is present on basal cells of the human esophagus and is present in a functional manner on the esophageal epithelial cell line, HET-1A.

Electrical parameters and ion species for active transport in human esophageal stratified squamous epithelium and Barrett's specialized columnar epithelium

The human esophagus is lined by stratified squamous epithelium (ESSE), and in some subjects with reflux disease the distal esophagus becomes lined by Barrett's specialized columnar epithelium (BSCE). ESSE and BSCE differ both histologically and functionally, the latter evident by differences in their in vivo transmural electrical potential difference (PD), ESSE averaging -15 mV and BSCE being greater than -25 mV. In this report we examine the basis for this difference in PD. This is done by mounting endoscopic biopsies of ESSE from 25 subjects without esophageal disease and BSCE from 19 with Barrett's esophagus in mini-Ussing chambers for electrical recordings basally and after bathing solution ion replacement. The results show that the PD of human ESSE reflects a low level of active ion transport (5.1 +/- 0.8 muA/cm(2)) combined with a high level of tissue (electrical) resistance (344 +/- 34 Omega.cm(2)) and that of BSCE reflects a high level of active transport (43.6 +/- 11.6 muA/cm(2)) combined with a low level of resistance (69 +/- 8 Omega.cm(2)). Furthermore, active transport in ESSE was principally due to sodium absorption whereas in BSCE it was equally divided between sodium absorption and anion (chloride/bicarbonate) secretion, the latter through an apical membrane, 4-acetamido4'-isothiocyano-2,2'-stilbenedisulfonic acid-sensitive anion channel. As an anion-secreting tissue with bicarbonate secretory capacity more than fivefold greater than ESSE, BSCE is better suited than ESSE for defense of the esophagus against reflux disease.

Cancer chemopreventive properties of orally bioavailable flavonoids--methylated versus unmethylated flavones

Poor oral bioavailability has been a major limitation for the successful use of dietary flavonoids as cancer chemopreventive agents. In this study, we examined fully methylated flavones as promising improved agents. In the human oral SCC-9 cancer cells, 5,7-dimethoxyflavone and 5,7,4'-trimethoxyflavone were both 10 times more potent inhibitors of cell proliferation (IC(50) values 5-8 microM) than the corresponding unmethylated analogs chrysin and apigenin. Flow cytometry indicated that both methylated flavones arrested the SCC-9 cells in the G1 phase with a concomitant decrease in the S phase, dramatically different from the unmethylated analogs, which promoted G2/M phase arrest. Both methylated compounds inhibited the proliferation of two other cancer cell lines with very little effect on two immortalized normal cell lines. Examination of additional flavone structures indicated that methylated flavones in general have antiproliferative properties. Finally, we demonstrated that 5,7-dimethoxyflavone, in contrast to its unmethylated analog chrysin, was well absorbed and had high oral bioavailability as well as tissue accumulation in vivo in the rat. Thus, fully methylated flavones appear to have great potential as cancer chemopreventive/chemotherapeutic agents, in particular in oral cancer.

Volume regulation following hyposmotic shock in isolated turbot (Scophthalmus maximus) hepatocytes

Regulatory volume decrease (RVD) following hyposmotic stimulation was studied in isolated turbot, Scophthalmus maximus, hepatocytes. Exposed to a reduced osmolality (from 320 to 240 mosm kg(-1)), cells first swelled and then exhibited a RVD. Volume regulation was significantly inhibited in presence of NPPB, 9-AC, acetazolamide, DIDS and barium. Taken together, these results could suggest that RVD operated via separate K+ and Cl- channels and probably Cl-/HCO3(-) exchanger in turbot hepatocytes. The K+/Cl- cotransporter could also be involved as furosemide and DIOA strongly inhibited the process whereas NEM, a K+/Cl- cotransporter activator, added under isosmotic conditions, led to cell shrinkage. RVD in turbot hepatocytes appeared also to depend on proteins p38 MAP kinase and tyrosine kinase but not on proteins ERK 1/2. Arachidonic acid and leukotrienes could also be involved since inhibition of synthesis of both these compounds by quinacrine and NDGA, respectively, inhibited the volume regulation. Likewise, Ca2+ has been proved to be an essential messenger as RVD was prevented in absence of Ca2+. Finally, this work provides bases for novel studies on cell volume regulation in marine teleosteans.

Nonselective cation transport in native esophageal epithelia

Rabbit esophageal epithelia actively transport Na(+) in a manner similar to that observed in classic electrically tight Na(+)-absorbing epithelia, such as frog skin. However, the nature of the apical entry step is poorly understood. To address this issue, we examined the electrophysiological and biochemical nature of this channel. Western blotting experiments with epithelial Na(+) channel (ENaC) subunit-specific antibodies revealed the presence of all three ENaC subunits in both native and immortalized esophageal epithelial cells. The amino acid sequence of the rabbit alpha-ENaC cloned from native rabbit esophageal epithelia was not significantly different from that of other published alpha-ENaC homologs. To characterize the electrophysiological properties of this native apical channel, we utilized nystatin permeabilization to eliminate the electrical contribution of the basolateral membrane in isolated native epithelia mounted in Ussing-type chambers. We find that the previously described apical Na(+) channel is nonselective for monovalent cations (Li(+), Na(+), and K(+)). Moreover, this channel was not blocked by millimolar concentrations of amiloride. These findings document the presence of a nonselective cation channel in a native Na(+) transporting epithelia, a finding that hereto has been thought to be limited to artificial culture conditions. Moreover, our data are consistent with a potential role of ENaC subunits in the formation of a native nonselective cation channel.