The effect of lethal acid stress on Na+/H+ exchanger isoforms in cultured inner medullary collecting duct cells: deletion of NHE-2 and over expression of NHE-1

Antitumor effects of lactate transport inhibition on esophageal adenocarcinoma cells

As a consequence of altered glucose metabolism, cancer cell intake is increased, producing large amounts of lactate which is pumped out the cytosol by monocarboxylate transporters (MCTs). MCT 1 and MCT4 are frequently overexpressed in tumors, and recently, MCT inhibition has been reported to exert antineoplastic effects. In the present study, MCT1 and MCT4 levels were assessed in esophageal adenocarcinoma (EAC) cells and the effects of the MCT-1 selective inhibitor AZD3965, hypoxia, and a glucose overload were evaluated in vitro. Two EAC cell lines (OE33 and OACM5.1C) were treated with AZD3965 (10-100 nM) under different conditions (normoxia/hypoxia) and also different glucose concentrations, and parameters of cytotoxicity, oxidative stress, intracellular pH (pHi), and lactate levels were evaluated. MCT1 was present in both cell lines whereas MCT4 was expressed in OE33 cells and only in a small proportion of OACM5.1C cells. Glucose addition did not have any effect on apoptosis nor cell proliferation. AZD3965 increased apoptosis and reduced proliferation of OACM5.1C cells, effects which were abrogated when cells were growing in hypoxia. MCT1 inhibition increased intracellular lactate levels in all the cells evaluated, but this increase was higher in cells expressing only MCT1 and did not affect oxidative stress. AZD3965 induced a decrease in pHi of cells displaying low levels of MCT4 and also increased the sodium/hydrogen exchanger 1 (NHE-1) expression on these cells. These data provide in vitro evidence supporting the potential of MCT inhibitors as novel antineoplastic drugs for EAC and highlight the importance of achieving a complete MCT inhibition.

Influence of ANG II on cytoplasmic sodium in cultured rabbit nonpigmented ciliary epithelium

Angiotensin (ANG) II receptors have been reported in the nonpigmented ciliary epithelium (NPE) of the eye. In cultured NPE, we found ANG II caused a dose-dependent rise of cytoplasmic sodium. The sodium increase was inhibited by the AT(1)-AT(2) receptor antagonist saralasin (IC(50) = 3.7 nM) and the AT(1) antagonist losartan (IC(50) = 0.6 nM) but not by the AT(2) antagonist PD-123319. ANG II also caused a dose-dependent increase in the rate of ouabain-sensitive (86)Rb uptake. The ANG II-induced cell sodium increase and (86)Rb uptake increase were reduced by dimethylamiloride (DMA; 10 microM). On the basis of this finding, we propose that Na(+)/H(+) exchange is stimulated by ANG II. Simultaneously, ANG II appears to inhibit H(+)-ATPase-mediated proton export. Thus Ang II (10 nM) did not alter the baseline cytoplasmic pH (pH(i)) but reduced pH(i) in cells that were also exposed to 10 microM DMA. Consistent with the notion of H(+)-ATPase inhibition in ANG II-treated NPE, bafilomycin A(1) (100 nM) (BAF) and ANG II were both observed to suppress the pH(i) increase that occurs upon exposure to a mixture of epinephrine (1 microM) and acetylcholine (10 microM) and the pH(i) increase elicited by depolarization. In ATP hydrolysis measurements, H(+)-ATPase activity (bafilomycin A(1)-sensitive ATP hydrolysis) was reduced significantly in cells that had been pretreated 10 min with 10 nM ANG II. In summary, these studies suggest that ANG II causes H(+)-ATPase inhibition and an increase of cell sodium due to activation of Na(+)/H(+) exchange.

Molecular and functional characterization of sodium--hydrogen exchanger in skin as well as cultured keratinocytes and melanocytes

The sodium--hydrogen (Na(+)/H(+)) exchanger is one of the few transporter proteins involved in the regulation and maintenance of intracellular pH and cell volume in most eukaryotic cell types. The current study investigates the expression of isoforms of the Na(+)/H(+) exchanger (NHE) in human skin and in cultured keratinocytes, melanocytes, and melanoma cells by reverse transcription-polymerase chain reaction (RT--PCR), immunohistochemical analysis and functional studies. Neonatal foreskins were used to isolate RNA from epidermis and dermis, and to initiate cultures of keratinocytes and melanocytes. RT--PCR on RNA isolated from epidermis, dermis, keratinocytes, melanocytes and melanoma cells using PCR primers specific for NHE-1 yielded a 463 bp PCR product. RT--PCR performed using primers specific for NHE isoforms 2, 3, 4 and 5 did not yield any products. Western blotting analysis (of keratinocyte and melanocyte cell cultures) and indirect immunohistochemistry on neonatal foreskin, keratinocytes, melanocytes and melanoma cells using a NHE-1-specific polyclonal antibody demonstrated NHE-1 expression at the protein level. Physiological regulation of intracellular pH using a pH-sensitive dye, BCECF, detected an amiloride-sensitive NHE activity in human keratinocyte, melanocyte and melanoma cell cultures. These results indicate that cultures of human keratinocytes and melanocytes established from human skin and melanoma cells express the NHE-1 isoform of the sodium--hydrogen exchanger.

Two uncompetitive, activated, and transport sites of the Na+/H+ exchanger for pH regulation in perfused rat kidney

The purpose of this study is to assess the effect of an apparent alteration in intracellular pH and the effect of amiloride on the activity of the Na+/H+ antiporter in perfused rat kidney. Rat kidney-Na+ retention was determined using tracer 22Na in perfusate composed of HCl-glycine buffer (pH 3.80 to pH 5.92) or NH4OH-glycine buffer (pH 6.22-7.95) containing Na+ to match physiologic concentrations. Plotting renal Na+ retention for 10 min versus pH in absence of amiloride showed two classical uncompetitive activator curves for H+, one curve from pH 4.19 to 5.10 and another from pH 6.22 to 7.95. H+ acts as an uncompetitive reversible binding substrate with the receptor triggering activation of the exchanger already sequestered with Na+, thus yielding two Ka values for the exchanger suggesting non-first order kinetics. Using an equation derived for uncompetitive-activation binding of Nao+ and Hi+, plotting [mM Na+ mg protein-1 10 min-1]-1 versus [H+], two linear plots are observed on Cartesian coordinates with abscissa intersecting at 47 +/- 1 microM, pKa = 4.32 +/- 0.02 (pH 4.19-5.10) and 4.21 +/- 0.02 microM, pKa = 5.38 +/- 0.01 (pH 6.22-7.95), respectively. Perfusing buffer containing 2 mM amiloride, completely inactivated the antiporter showing stronger inhibition between pH 3.80 and 5.92. Results suggest the presence of two uncompetitive binding sites for H+ with the Na+/H+ exchanger. One is a high affinity binding site at physiological intracellular apparent pH, and another is a low affinity binding site at ischaemic apparent pH, implying the existence of two titration sites for intracellular pH regulation.

Characterization of Na+/HCO-3 cotransporter isoform NBC-3

Na+-HCO-3 cotransporters mediate the transport of HCO-3 into or out of the cell. Two Na+-HCO-3 cotransporters (NBC) have been identified previously, which are referred to as NBC-1 and NBC-2. A cDNA library from uninduced human NT-2 cells was screened with an NBC-2 cDNA probe. Several clones were identified and isolated. Sequence analysis of these clones identified a partial coding region (2 kb) of a novel NBC (called here NBC-3), which showed 53% and 72% identity with NBC-1 and NBC-2, respectively. Northern blot analysis revealed that NBC-3 encodes a 4.4-kb mRNA with a tissue distribution pattern distinct from NBC-1 and NBC-2. NBC-3 is highly expressed in brain and spinal column, with moderate levels in trachea, thyroid, and kidney. In contrast with NBC-1, NBC-3 shows low levels of expression in pancreas and kidney cortex. In the kidney, NBC-3 expression is predominantly limited to the medulla. Cultured mouse inner medullary collecting duct (mIMCD-3) cells showed high levels of NBC-1 and low levels of NBC-3 mRNA expression. Subjecting the mutagenized mIMCD-3 cells to sublethal acid stress decreased the mRNA expression of NBC-1 by approximately 90% but increased the Na+-dependent HCO-3 cotransport activity by approximately 7-fold (as assayed by DIDS-sensitive, Na+-dependent, HCO-3-mediated intracellular pH recovery). This increase was associated with approximately 5.5-fold enhancement of NBC-3 mRNA levels. NBC showed significant affinity for Li+ in the mutant but not the parent mIMCD-3 cells. On the basis of the widespread distribution of NBC-3, we propose that this isoform is likely involved in cell pH regulation by transporting HCO-3 from blood to the cell. We further propose that enhanced expression of NBC-3 in severe acid stress could play an important role in cell survival by mediating the influx of HCO-3 into the cells.

The Na+/H+ antiporter potentiates growth and retinoic acid-induced differentiation of P19 embryonal carcinoma cells

The Na+/H+ exchanger is a ubiquitous plasma membrane protein that is responsible for pH regulation and is activated by growth factors. We examined the role of the Na+/H+ exchanger in cell growth and differentiation. Treatment of P19 cells with the Na+/H+ exchanger inhibitor Hoe 694 eliminated retinoic acid-induced differentiation in this cell line. We developed a P19 embryonal carcinoma cell line that was deficient in the Na+/H+ antiporter. Na+/H+ exchanger-deficient cells were reduced in the rate of cell growth and this effect was enhanced by the removal of added HCO3- and by reducing extracellular pH. The antiporter-deficient cells were also markedly deficient in their ability to differentiate to neuronal-like cells and recovered this ability when the Na+/H+ antiporter was reintroduced. The results show that the absence of Na+/H+ antiport as a pH regulatory mechanism can result in deficiencies in both cell growth and differentiation in embryonal carcinoma cells.

Functional upregulation of H+-ATPase by lethal acid stress in cultured inner medullary collecting duct cells

The response of H+-ATPase to lethal acid stress is unknown. A mutant strain (called NHE2d) was derived from cultured inner medullary collecting duct cells (mIMCD-3 cells) following three cycles of lethal acid stress. Cells were grown to confluence on coverslips, loaded with 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein, and monitored for intracellular pH (pHi) recovery from an acid load. The rate of Na+-independent pHi recovery from an acid load in mutant cells was approximately fourfold higher than in parent cells (P < 0.001). The Na+-independent H+ extrusion was ATP dependent and K+ independent and was completely inhibited in the presence of diethylstilbestrol, N,N'-dicyclohexylcarbodiimide, or N-ethylmaleimide. These results indicate that the Na+-independent H+ extrusion in cultured medullary cells is mediated via H+-ATPase and is upregulated in lethal acidosis. Northern hybridization experiments demonstrated that mRNA levels for the 16- and 31-kDa subunits of H+-ATPase remained unchanged in mutant cells compared with parent cells. We propose that lethal acid stress results in increased H+-ATPase activity in inner medullary collecting duct cells. Upregulation of H+-ATPase could play a protective role against cell death in severe intracellular acidosis.