Structural determinants and significance of regulation of electrogenic Na(+)-HCO(3)(-) cotransporter stoichiometry

Potential Theranostic Roles of SLC4 Molecules in Human Diseases

The solute carrier family 4 (SLC4) is an important protein responsible for the transport of various ions across the cell membrane and mediating diverse physiological functions, such as the ion transporting function, protein-to-protein interactions, and molecular transduction. The deficiencies in SLC4 molecules may cause multisystem disease involving, particularly, the respiratory system, digestive, urinary, endocrine, hematopoietic, and central nervous systems. Currently, there are no effective strategies to treat these diseases. SLC4 proteins are also found to contribute to tumorigenesis and development, and some of them are regarded as therapeutic targets in quite a few clinical trials. This indicates that SLC4 proteins have potential clinical prospects. In view of their functional characteristics, there is a critical need to review the specific functions of bicarbonate transporters, their related diseases, and the involved pathological mechanisms. We summarize the diseases caused by the mutations in SLC4 family genes and briefly introduce the clinical manifestations of these diseases as well as the current treatment strategies. Additionally, we illustrate their roles in terms of the physiology and pathogenesis that has been currently researched, which might be the future therapeutic and diagnostic targets of diseases and a new direction for drug research and development.

Polyamines Counteract Carbonate-Driven Proteasome Stalling in Alkaline Conditions

Cancer cells tend to increase intracellular pH and, at the same time, are known to intensively produce and uptake polyamines such as spermine. Here, we show that various amines, including biogenic polyamines, boost the activity of proteasomes in a dose-dependent manner. Proteasome activity in the classical amine-containing buffers, such as 2-(N-morpholino)ethanesulfonic acid (MES), Tris, (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), glycylglycine, bis-Tris propane, and bicine, has a skewed distribution with a maximum at pH of 7.0–8.0. The activity of proteasomes in buffers containing imidazole and bis-Tris is maintained almost on the same level, in the pH range of 6.5–8.5. The third type of activation is observed in buffers based on the amino acids arginine and ornithine, as well as the natural polyamines spermine and spermidine. Proteasome activity in these buffers is dramatically increased at pH values greater than 7.5. Anionic buffers such as phosphate or carbonate, in contrast, inhibit proteasome activity during alkalization. Importantly, supplementation of a carbonate–phosphate buffer with spermine counteracts carbonate-driven proteasome stalling in alkaline conditions, predicting an additional physiological role of polyamines in maintaining the metabolism and survival of cancer cells.

Expression, Localization, and Regulation of the Sodium Bicarbonate Cotransporter NBCe1 in the Thyroid

BACKGROUND

The intrafollicular space of thyroid follicles is the storage compartment for thyroid hormones. Its pH has been established at around 7.6 at least after thyrotropin (TSH) stimulation. This alkaline intrafollicular pH is thought to be critical for iodide coupling to thyroglobulin and internalization of iodinated thyroglobulin. At least in mice, this alkalinization requires the expression of pendrin (Slc26a4) within the apical membrane, and a lack of pendrin results in acidic follicular lumen pH. Yet, the mechanism importing HCO₃^- into the cytoplasm is unknown. This study investigated whether the rather ubiquitous sodium bicarbonate cotransporter NBCe1 (SLC4A4) might play this role. It also examined which variant was expressed and where it was localized in both rat and human thyroid tissue. Lastly, the dependence of its expression on TSH was studied.

METHODS

Reverse transcription polymerase chain reaction, immunofluorescence, and Western blotting were used to test whether TSH stimulated NBCe1 protein expression in vivo. Subcellular localization of NBCe1 was performed using immunofluorescence in both rat and human thyroid. Cultured thyroid cells were also used to attempt to define how TSH affects NBCe1 expression.

RESULTS

Only transcripts of the NBCe1-B variant were detected in both rat and human thyroid. Of interest, NBCe1-C was not detected in human tissues, not even in the brain. On immunofluorescence microscopy, the immunostaining of NBCe1 mainly appeared in the basolateral membrane upon stimulation with TSH. This TSH induction of basolateral membrane expression of NBCe1 protein was confirmed in vivo in rat thyroid and in vitro on human thyroid slices.

CONCLUSIONS

This study demonstrates the expression of the sodium bicarbonate cotransporter NBCe1-B in rat and human thyroid. Additionally, the data suggest that TSH blocks the degradation of NBCe1 protein by trafficking it to the basolateral membrane. Hence, TSH increases NBCe1 half-life without increasing its synthesis.

Mouse models of SLC4-linked disorders of HCO3--transporter dysfunction

The SLC4 family Cl^-/[Formula: see text] cotransporters (NBCe1, NBCe2, NBCn1, and NBCn2) contribute to a variety of vital physiological processes including pH regulation and epithelial fluid secretion. Accordingly, their dysfunction can have devastating effects. Disorders such as epilepsy, hemolytic anemia, glaucoma, hearing loss, osteopetrosis, and renal tubular acidosis are all genetically linked to SLC4-family gene loci. This review summarizes how studies of Slc4-modified mice have enhanced our understanding of the etiology of SLC4-linked pathologies and the interpretation of genetic linkage studies. The review also surveys the novel disease signs exhibited by Slc4-modified mice which could either be considered to presage their description in humans, or to highlight interspecific differences. Finally, novel Slc4-modified mouse models are proposed, the study of which may further our understanding of the basis and treatment of SLC4-linked disorders of [Formula: see text]-transporter dysfunction.

A novel delta current method for transport stoichiometry estimation

Background

The ion transport stoichiometry (q) of electrogenic transporters is an important determinant of their function. q can be determined by the reversal potential (E_rev) if the transporter under study is the only electrogenic transport mechanism or a specific inhibitor is available. An alternative approach is to calculate delta reversal potential (ΔE_rev) by altering the concentrations of the transported substrates. This approach is based on the hypothesis that the contributions of other channels and transporters on the membrane to E_rev are additive. However, E_rev is a complicated function of the sum of different conductances rather than being additive.

Results

We propose a new delta current (ΔI) method based on a simplified model for electrogenic secondary active transport by Heinz (Electrical Potentials in Biological Membrane Transport, 1981). ΔI is the difference between two currents obtained from altering the external concentration of a transported substrate thereby eliminating other currents without the need for a specific inhibitor. q is determined by the ratio of ΔI at two different membrane voltages (V₁ and V₂) where q = 2RT/(F(V₂ –V₁))ln(ΔI₂/ΔI₁) + 1. We tested this ΔI methodology in HEK-293 cells expressing the elctrogenic SLC4 sodium bicarbonate cotransporters NBCe2-C and NBCe1-A, the results were consistent with those obtained with the E_rev inhibitor method. Furthermore, using computational simulations, we compared the estimates of q with the ΔE_rev and ΔI methods. The results showed that the ΔE_rev method introduces significant error when other channels or electrogenic transporters are present on the membrane and that the ΔI equation accurately calculates the stoichiometric ratio.

Conclusions

We developed a ΔI method for estimating transport stoichiometry of electrogenic transporters based on the Heinz model. This model reduces to the conventional reversal potential method when the transporter under study is the only electrogenic transport process in the membrane. When there are other electrogenic transport pathways, ΔI method eliminates their contribution in estimating q. Computational simulations demonstrated that the ΔE_rev method introduces significant error when other channels or electrogenic transporters are present and that the ΔI equation accurately calculates the stoichiometric ratio. This new ΔI method can be readily extended to the analysis of other electrogenic transporters in other tissues.

Short-term regulation of murine colonic NBCe1-B (electrogenic Na+/HCO3(-) cotransporter) membrane expression and activity by protein kinase C

The colonic mucosa actively secretes HCO3(-), and several lines of evidence point to an important role of Na+/HCO3(-) cotransport (NBC) as a basolateral HCO3(-) import pathway. We could recently demonstrate that the predominant NBC isoform in murine colonic crypts is electrogenic NBCe1-B, and that secretagogues cause NBCe1 exocytosis, which likely represents a component of NBC activation. Since protein kinase C (PKC) plays a key role in the regulation of ion transport by trafficking events, we asked whether it is also involved in the observed NBC activity increase. Crypts were isolated from murine proximal colon to assess PKC activation as well as NBC function and membrane abundance using fluorometric pHi measurements and cell surface biotinylation, respectively. PKC isoform translocation and phosphorylation occurred in response to PMA-, as well as secretagogue stimulation. The conventional and novel PKC inhibitors Gö6976 or Gö6850 did not alter NBC function or surface expression by themselves, but stimulation with forskolin (10(-5) M) or carbachol (10(-4) M) in their presence led to a significant decrease in NBC-mediated proton flux, and biotinylated NBCe1. Our data thus indicate that secretagogues lead to PKC translocation and phosphorylation in murine colonic crypts, and that PKC is necessary for the increase in NBC transport rate and membrane abundance caused by cholinergic and cAMP-dependent stimuli.

Role of AE2 for pHi regulation in biliary epithelial cells

The Cl⁻/HCO⁻₃anion exchanger 2 (AE2) is known to be involved in intracellular pH (pH_i) regulation and transepithelial acid-base transport. Early studies showed that AE2 gene expression is reduced in liver biopsies and blood mononuclear cells from patients with primary biliary cirrhosis (PBC), a disease characterized by chronic non-suppurative cholangitis associated with antimitochondrial antibodies (AMA) and other autoimmune phenomena. Microfluorimetric analysis of the Cl⁻/HCO⁻₃ anion exchange (AE) in isolated cholangiocytes showed that the cAMP-stimulated AE activity is diminished in PBC compared to both healthy and diseased controls. More recently, it was found that miR-506 is upregulated in cholangiocytes of PBC patients and that AE2 may be a target of miR-506. Additional evidence for a pathogenic role of AE2 dysregulation in PBC was obtained with Ae2^−/−_a,b mice, which develop biochemical, histological, and immunologic alterations that resemble PBC (including development of serum AMA). Analysis of HCO⁻₃ transport systems and pH_i regulation in cholangiocytes from normal and Ae2^−/−_a,b mice confirmed that AE2 is the transporter responsible for the Cl⁻/HCO⁻₃exchange in these cells. On the other hand, both Ae2^+/+_a,b and Ae2^−/−_a,b mouse cholangiocytes exhibited a Cl⁻-independent bicarbonate transport system, essentially a Na⁺-bicarbonate cotransport (NBC) system, which could contribute to pH_i regulation in the absence of AE2.

Loss of the AE3 Cl(-)/HCO(-) 3 exchanger in mice affects rate-dependent inotropy and stress-related AKT signaling in heart

Cl(-)/HCO(-) 3 exchangers are expressed abundantly in cardiac muscle, suggesting that HCO(-) 3 extrusion serves an important function in heart. Mice lacking Anion Exchanger Isoform 3 (AE3), a major cardiac Cl(-)/HCO(-) 3 exchanger, appear healthy, but loss of AE3 causes decompensation in a hypertrophic cardiomyopathy (HCM) model. Using intra-ventricular pressure analysis, in vivo pacing, and molecular studies we identified physiological and biochemical changes caused by loss of AE3 that may contribute to decompensation in HCM. AE3-null mice had normal cardiac contractility under basal conditions and after β-adrenergic stimulation, but pacing of hearts revealed that frequency-dependent inotropy was blunted, suggesting that AE3-mediated HCO(-) 3 extrusion is required for a robust force-frequency response (FFR) during acute biomechanical stress in vivo. Modest changes in expression of proteins that affect Ca(2+)-handling were observed, but Ca(2+)-transient analysis of AE3-null myocytes showed normal twitch-amplitude and Ca(2+)-clearance. Phosphorylation and expression of several proteins implicated in HCM and FFR, including phospholamban (PLN), myosin binding protein C, and troponin I were not altered in hearts of paced AE3-null mice; however, phosphorylation of Akt, which plays a central role in mechanosensory signaling, was significantly higher in paced AE3-null hearts than in wild-type controls and phosphorylation of AMPK, which is affected by Akt and is involved in energy metabolism and some cases of HCM, was reduced. These data show loss of AE3 leads to impaired rate-dependent inotropy, appears to affect mechanical stress-responsive signaling, and reduces activation of AMPK, which may contribute to decompensation in heart failure.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Ion transporting proteins of human bronchial epithelium

The electrolyte transport system across human airway epithelium followed by water movement is essential for the normal mucociliary clearance that allows the maintenance of the aseptic condition of the respiratory tract. The function of epithelial cells is to control and regulate ionic composition and volume of fluids in the airways. Various types of proteins taking part in assuring effective ions and water transport in apical and basolateral membranes of the airway epithelium have been found (e.g., CFTR, ENaC, CaCC, ORCC, potassium channels, NaKATPase, aquaporins). The paper reviews the current state of the art in the field of ion channels, transporters, and other signaling proteins identified in the human bronchial epithelium.

HCO(3)(-)-independent conductance with a mutant Na(+)/HCO(3)(-) cotransporter (SLC4A4) in a case of proximal renal tubular acidosis with hypokalaemic paralysis

The renal electrogenic Na(+)/HCO(3)(−) cotransporter (NBCe1-A) contributes to the basolateral step of transepithelial HCO(3)(−) reabsorption in proximal tubule epithelia, contributing to the buffering of blood pH. Elsewhere in the body (e.g. muscle cells) NBCe1 variants contribute to, amongst other processes, maintenance of intracellular pH. Others have described a homozygous mutation in NBCe1 (NBCe1-A p.Ala799Val) in an individual with severe proximal renal tubular acidosis (pRTA; usually associated with defective HCO(3)(−) reabsorption in proximal tubule cells) and hypokalaemic periodic paralysis (hypoPP; usually associated with leaky cation channels in muscle cells). Using biotinylation and two-electrode voltage-clamp on Xenopus oocytes expressing NBCe1, we demonstrate that the mutant NBCe1-A (A(A799V)) exhibits a per-molecule transport defect that probably contributes towards the observed pRTA. Furthermore, we find that A(A799V) expression is associated with an unusual HCO(3)(−)-independent conductance that, if associated with mutant NBCe1 in muscle cells, could contribute towards the appearance of hypokalaemic paralysis in the affected individual. We also study three novel lab mutants of NBCe1-A: p.Ala799Ile, p.Ala799Gly and p.Ala799Ser. All three exhibit a per-molecule transport defect, but only A(A799I) exhibits an A(A799V)-like ion conductance. A(A799G) and A(A799S) exhibit unusual outward rectification in their HCO(3)(−)-dependent conductance and A(A799G) exhibits reduced sensitivity to both DIDS and tenidap. A799G is the first mutation shown to affect the apparent tenidap affinity of NBCe1. Finally we show that A(A799V) and A(A799I), which accumulate poorly in the plasma membrane of oocytes, exhibit signs of abnormal intracellular accumulation in a non-polarized renal cell-line.

Phosphorylation of carbonic anhydrase IX controls its ability to mediate extracellular acidification in hypoxic tumors

In the hypoxic regions of a tumor, carbonic anhydrase IX (CA IX) is an important transmembrane component of the pH regulatory machinery that participates in bicarbonate transport. Because tumor pH has implications for growth, invasion, and therapy, determining the basis for the contributions of CA IX to the hypoxic tumor microenvironment could lead to new fundamental and practical insights. Here, we report that Thr443 phosphorylation at the intracellular domain of CA IX by protein kinase A (PKA) is critical for its activation in hypoxic cells, with the fullest activity of CA IX also requiring dephosphorylation of Ser448. PKA is activated by cAMP, which is elevated by hypoxia, and we found that attenuating PKA in cells disrupted CA IX-mediated extracellular acidification. Moreover, following hypoxia induction, CA IX colocalized with the sodium-bicarbonate cotransporter and other PKA substrates in the leading edge membranes of migrating tumor cells, in support of the concept that bicarbonate metabolism is spatially regulated at cell surface sites with high local ion transport and pH control. Using chimeric CA IX proteins containing heterologous catalytic domains derived from related CA enzymes, we showed that CA IX activity was modulated chiefly by the intracellular domain where Thr443 is located. Our findings indicate that CA IX is a pivotal mediator of the hypoxia-cAMP-PKA axis, which regulates pH in the hypoxic tumor microenvironment.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

AE2 Cl-/HCO3- exchanger is required for normal cAMP-stimulated anion secretion in murine proximal colon

Anion secretion by colonic epithelium is dependent on apical CFTR-mediated anion conductance and basolateral ion transport. In many tissues, the NKCC1 Na(+)-K(+)-2Cl(-) cotransporter mediates basolateral Cl(-) uptake. However, additional evidence suggests that the AE2 Cl(-)/HCO(3)(-) exchanger, when coupled with the NHE1 Na(+)/H(+) exchanger or a Na(+)-HCO(3)(-) cotransporter (NBC), contributes to HCO(3)(-) and/or Cl(-) uptake. To analyze the secretory functions of AE2 in proximal colon, short-circuit current (I(sc)) responses to cAMP and inhibitors of basolateral anion transporters were measured in muscle-stripped wild-type (WT) and AE2-null (AE2(-/-)) proximal colon. In physiological Ringer, the magnitude of cAMP-stimulated I(sc) was the same in WT and AE2(-/-) colon. However, the I(sc) response in AE2(-/-) colon exhibited increased sensitivity to the NKCC1 inhibitor bumetanide and decreased sensitivity to the distilbene derivative SITS (which inhibits AE2 and some NBCs), indicating that loss of AE2 results in a switch to increased NKCC1-supported anion secretion. Removal of HCO(3)(-) resulted in robust cAMP-stimulated I(sc) in both AE2(-/-) and WT colon that was largely mediated by NKCC1, whereas removal of Cl(-) resulted in sharply decreased cAMP-stimulated I(sc) in AE2(-/-) colon relative to WT controls. Inhibition of NHE1 had no effect on cAMP-stimulated I(sc) in AE2(-/-) colon but caused a switch to NKCC1-supported secretion in WT colon. Thus, in AE2(-/-) colon, Cl(-) secretion supported by basolateral NKCC1 is enhanced, whereas HCO(3)(-) secretion is diminished. These results show that AE2 is a component of the basolateral ion transport mechanisms that support anion secretion in the proximal colon.

AE2 Cl-/HCO3- exchanger is required for normal cAMP-stimulated anion secretion in murine proximal colon

Anion secretion by colonic epithelium is dependent on apical CFTR-mediated anion conductance and basolateral ion transport. In many tissues, the NKCC1 Na(+)-K(+)-2Cl(-) cotransporter mediates basolateral Cl(-) uptake. However, additional evidence suggests that the AE2 Cl(-)/HCO(3)(-) exchanger, when coupled with the NHE1 Na(+)/H(+) exchanger or a Na(+)-HCO(3)(-) cotransporter (NBC), contributes to HCO(3)(-) and/or Cl(-) uptake. To analyze the secretory functions of AE2 in proximal colon, short-circuit current (I(sc)) responses to cAMP and inhibitors of basolateral anion transporters were measured in muscle-stripped wild-type (WT) and AE2-null (AE2(-/-)) proximal colon. In physiological Ringer, the magnitude of cAMP-stimulated I(sc) was the same in WT and AE2(-/-) colon. However, the I(sc) response in AE2(-/-) colon exhibited increased sensitivity to the NKCC1 inhibitor bumetanide and decreased sensitivity to the distilbene derivative SITS (which inhibits AE2 and some NBCs), indicating that loss of AE2 results in a switch to increased NKCC1-supported anion secretion. Removal of HCO(3)(-) resulted in robust cAMP-stimulated I(sc) in both AE2(-/-) and WT colon that was largely mediated by NKCC1, whereas removal of Cl(-) resulted in sharply decreased cAMP-stimulated I(sc) in AE2(-/-) colon relative to WT controls. Inhibition of NHE1 had no effect on cAMP-stimulated I(sc) in AE2(-/-) colon but caused a switch to NKCC1-supported secretion in WT colon. Thus, in AE2(-/-) colon, Cl(-) secretion supported by basolateral NKCC1 is enhanced, whereas HCO(3)(-) secretion is diminished. These results show that AE2 is a component of the basolateral ion transport mechanisms that support anion secretion in the proximal colon.

Modular structure of sodium-coupled bicarbonate transporters

Mammalian genomes contain 10 SLC4 genes that, between them, encode three Cl-HCO(3) exchangers, five Na(+)-coupled HCO(3) transporters (NCBTs), one reported borate transporter, and what is reported to be a fourth Cl-HCO(3) exchanger. The NCBTs are expressed throughout the body and play important roles in maintaining intracellular and whole-body pH, as well as contributing to transepithelial transport processes. The importance of NCBTs is underscored by the genetic association of dysfunctional NCBT genes with blindness, deafness, epilepsy, hypertension and metal retardation. Key to understanding the action and regulation of NCBTs is an appreciation of the diversity of NCBT gene products. The transmembrane domains of human NCBT paralogs are 50-84% identical to each other at the amino acid level, and are capable of a diverse range of actions, including electrogenic Na/HCO(3) cotransport (i.e. NBCe1 and NBCe2) and electroneutral Na/HCO(3) cotransport (i.e. NBCn1 and NBCn2), as well as Na(+)-dependent Cl-HCO(3) exchange (i.e. NDCBE). Furthermore, by the use of alternative promoters and alternative-splicing events, individual SLC4 genes have the potential to generate multiple splice variants (as many as 16 in the case of NBCn1), each of which could have unique temporal and spatial patterns of distribution, unitary transporter activity (i.e. flux mediated by one molecule), array of protein-binding partners, and complement of regulatory stimuli. In the first section of this review, we summarize our present knowledge of the function and distribution of mammalian NCBTs and their multiple variants. In the second section of this review we consider the molecular consequences of NCBT variation.

Augmentation and ionic mechanism of effect of beta-N-methylamino-L-alanine in presence of bicarbonate on membrane potential of Retzius nerve cells of the leech Haemopis sanguisuga

The role of neurotoxic non-protein amino acid beta-N-methylamino-L-alanine (L-BMAA) as a putative causative agent of Western pacific amyotrophic lateral sclerosis/Parkinsonism dementia complex (ALS/PDC) has recently been reinvigorated. In view of this data we have investigated the strength and mechanism of effect of L-BMAA in presence of 20 mmol/L bicarbonate (a cofactor for BMAA) on membrane potential of the Leech Haemopis sanguisuga. Our results show that L-BMAA has excitatory effect in bicarbonate containing solution, which is more potent than in nominally bicarbonate free solution. This potentiation by bicarbonate is L-BMAA specific, as it was not exhibited by beta-N-oxalylamino-L-alanine. The effect of L-BMAA was partially blocked by non-NMDA receptor antagonist CNQX. Application of L-BMAA caused a decrease in input membrane resistance, an increase of intracellular sodium activity, and a decrease of intracellular potassium activity. Present findings indicate that BMAA could initiate excitotoxicity through activation of non-NMDA ionotropic glutamate receptors.

Formation of carbonato and hydroxo complexes in the reaction of platinum anticancer drugs with carbonate

The second-generation Pt(II) anticancer drug carboplatin is here shown to react with carbonate, which is present in blood, interstitial fluid, cytosol, and culture medium, to produce platinum-carbonato and -hydroxo complexes. Using [(1)H-(15)N] HSQC NMR and (15)N-labeled carboplatin, we observe that cis-[Pt(CBDCA-O)(OH)(NH(3))(2)](-), cis-[Pt(OH)(2)(NH(3))(2)], cis-[Pt(CO(3))(OH)(NH(3))(2)](-), and what may be cis-[Pt(CO(3))(NH(3))(2)] are produced when 1 is allowed to react in 23.8 mM carbonate buffer. When (15)N-labeled carboplatin is allowed to react in 0.5 M carbonate buffer, these platinum species, as well as other hydroxo and carbonato species, some of which may be dinuclear complexes, are produced. Furthermore, we show that the carbonato species cis-[Pt(CO(3))(OH)(NH(3))(2)](-) is also produced when cisplatin is allowed to react in carbonate buffer. The study outlines the conditions under which carboplatin and cisplatin form carbonato and aqua/hydroxo species in carbonate media.

Bicarbonate transport in cell physiology and disease

The family of mammalian bicarbonate transport proteins are involved in a wide-range of physiological processes. The importance of bicarbonate transport follows from the biochemistry of HCO(3)(-) itself. Bicarbonate is the waste product of mitochondrial respiration. HCO(3)(-) undergoes pH-dependent conversion into CO(2) and in doing so converts from a membrane impermeant anion into a gas that can diffuse across membranes. The CO(2)-HCO(3)(-) equilibrium forms the most important pH buffering system of our bodies. Bicarbonate transport proteins facilitate the movement of membrane-impermeant HCO(3)(-) across membranes to accelerate disposal of waste CO(2), control cellular and whole-body pH, and to regulate fluid movement and acid/base secretion. Defects of bicarbonate transport proteins manifest in diseases of most organ systems. Fourteen gene products facilitate mammalian bicarbonate transport, whose physiology and pathophysiology is discussed in the present review.

cAMP-dependent and cholinergic regulation of the electrogenic intestinal/pancreatic Na+/HCO3- cotransporter pNBC1 in human embryonic kidney (HEK293) cells

Background

The renal (kNBC1) and intestinal (pNBC1) electrogenic Na⁺/HCO₃^- cotransporter variants differ in their primary structure, transport direction, and response to secretagogues. Previous studies have suggested that regulatory differences between the two subtypes can be partially explained by unique consensus phosphorylation sites included in the pNBC1, but not the kNBC1 sequence. After having shown activation of NBC by carbachol and forskolin in murine colon, we now investigated these pathways in HEK293 cells transiently expressing a GFP-tagged pNBC1 construct.

Results

Na⁺- and HCO₃^--dependent pH_i recovery from an acid load (measured with BCECF) was enhanced by 5-fold in GFP-positive cells compared to the control cells in the presence of CO₂/HCO₃^-. Forskolin (10^-5 M) had no effect in untransfected cells, but inhibited the pH_i recovery in cells expressing pNBC1 by 62%. After preincubation with carbachol (10^-4 M), the pH_i recovery was enhanced to the same degree both in transfected and untransfected cells, indicating activation of endogenous alkalizing ion transporters. Acid-activated Na⁺/HCO₃^- cotransport via pNBC1 expressed in renal cells is thus inhibited by cAMP and not affected by cholinergic stimulation, as opposed to the findings in native intestinal tissue.

Conclusion

Regulation of pNBC1 by secretagogues appears to be not solely dependent on its primary structure, but also on properties of the cell type in which it is expressed.

Molecular expression of SLC4-derived Na+-dependent anion transporters in selected human tissues

NaHCO(3) transporters are involved in maintenance of intracellular pH and transepithelial HCO(3)(-) movement in many rodent tissues. To establish the human relevance of the many investigations on rodents, this study aimed to map these transporters and a related polypeptide, NaBC1 [solute carrier 4 (SLC4)A11], to several human tissues by using PCR on reverse transcribed human mRNA and immunoperoxidase histochemistry. The mRNA encoding the electroneutral Na(+):HCO(3)(-) cotransporter (NBCe1; SLC4A4), was expressed in renal cortex, renal medulla, stomach, duodenum, jejunum, ileum, colon, pancreas, choroid plexus, cerebellum, cerebrum, and hippocampus. NBCe2 (SLC4A5) and NBCn1 (SLC4A7) mRNAs were mainly found in kidney and brain tissues, as was mRNA encoding the Na(+)-dependent anion exchangers NCBE (SLC4A10) and NDCBE1 (SLC4A8). In addition to previous findings, NBCn1 protein was localized to human renal medullary thick ascending limbs and duodenal epithelial villus cells and NBCe2 protein to renal collecting ducts. Finally, the message encoding NaBC1 was found in kidney, stomach, duodenum, pancreas, and brain, and the corresponding protein in the anterior and posterior corneal epithelia, renal corpuscules, proximal tubules, collecting ducts, pancreatic ducts, and the choroid plexus epithelium. In conclusion, the selected human tissues display distinct expression patterns of HCO(3)(-) transporters, which closely resemble that of rodent tissues.

The activation of platinum(II) antiproliferative drugs in carbonate medium evaluated by means of a DNA-biosensor

We report on the binding of cisplatin, carboplatin and oxaliplatin to double-stranded DNA in two different (phosphate and carbonate) buffers, using an electrochemical DNA-biosensor. The propensity of the electrophilic agent produced by hydrolysis to interact with DNA was measured as a function of the decrease of guanine oxidation signal of the metal-DNA adduct immobilized on a screen-printed electrode, by using square wave voltammetry. The results obtained confirm that carbonate reacts with platinum drugs to form activated carbonato complexes, which are able to react readily with DNA.

Establishing a definitive stoichiometry for the Na+/monocarboxylate cotransporter SMCT1

Several different stoichiometries have been proposed for the Na(+)/monocarboxylate cotransporter SMCT1, including variable Na(+)/substrate stoichiometry. In this work, we have definitively established an invariant 2:1 cotransport stoichiometry for SMCT1. By using two independent means of assay, we first showed that SMCT1 exhibits a 2:1 stoichiometry for Na(+)/lactate cotransport. Radiolabel uptake experiments proved that, unlike lactate, propionic acid diffuses passively through oocyte membranes and, consequently, propionate is a poor candidate for stoichiometric determination by these methods. Although we previously determined SMCT1 stoichiometry by measuring reversal potentials, this technique produced erroneous values, because SMCT1 simultaneously mediates both an inwardly rectifying cotransport current and an outwardly rectifying anionic leak current; the leak current predominates in the range where reversal potentials are observed. We therefore employed a method that compared the effect of halving the external Na(+) concentration to the effect of halving the external substrate concentration on zero-current potentials. Both lactate and propionate were cotransported through SMCT1 using 2:1 stoichiometries. The leak current passing through the protein has a 1 osmolyte/charge stoichiometry. Identification of cotransporter stoichiometry is not always a trivial task and it can lead to a much better understanding of the transport activity mediated by the protein in question.

Interactions between Na+channels and Na+-HCO3−cotransporters in the freshwater fish gill MR cell: a model for transepithelial Na+uptake

Isolated mitochondria-rich (MR) cells from the rainbow trout gill epithelium were subjected to intracellular pH (pHi) imaging with the pH-sensitive dye BCECF-AM. MR cells were categorized into two distinct functional subtypes based on their ability to recover pHifrom an NH4Cl-induced acidification in the absence of Na+. An apparent link between resting pHiand Na+-independent pHirecovery was made. We observed a unique pHiacidification event that was induced by extracellular Na+addition. This further classified the mixed MR cell population into two functional subtypes: the majority of cells (77%) demonstrated the Na+-induced pHiacidification, whereas the minority (23%) demonstrated an alkalinization of pHiunder the same circumstances. The focus of this study was placed on the Na+-induced acidification and pharmacological analysis via the use of amiloride and phenamil, which revealed that Na+uptake was responsible for the intracellular acidification. Further experiments revealed that pHiacidification could be abolished when Na+was allowed entry into the cell, but the activity of an electrogenic Na+-HCO3−cotransporter (NBC) was inhibited by DIDS. The electrogenic NBC activity was supported by a DIDS-sensitive, Na+-induced membrane potential depolarization as observed via imaging of the voltage-sensitive dye bis-oxonol. We also demonstrated NBC immunoreactivity via Western blotting and immunohistochemistry in gill tissue. We propose a model for transepithelial Na+uptake occurring via an apical Na+channel linked to a basolateral, electrogenic NBC in one subpopulation of MR cells.

Carbachol increases Na+-HCO3- cotransport activity in murine colonic crypts in a M3-, Ca2+/calmodulin-, and PKC-dependent manner

The Na(+)-HCO(3)(-) cotransporter (NBC) mediates HCO(3)(-) import into the colonocyte via its pNBC1 isoform. Whereas renal kNBC1 is inhibited by increased cAMP levels, pNBC1 is stimulated. Cholinergic stimulation activates renal NBC, but the effect on intestinal NBC is unknown. Therefore, crypts were isolated from the murine proximal colon by Ca(2+) chelation and loaded with the pH-sensitive dye 2',7'-bis-carboxyethyl-5,6-carboxyfluorescein. Na(+)-HCO(3)(-) cotransport activity was calculated from the dimethylamiloride-insensitive (500 microM) intracellular pH recovery from an acid load in the presence of CO(2)-HCO(3)(-) and the intracellular buffering capacity. Carbachol strongly increased Na(+)-HCO(3)(-) cotransport activity compared with control rates. Ca(2+) chelation with BAPTA-AM, blockade of the M(3) subtype of muscarinergic receptors with 4-diphenylacetoxy-N-methylpiperidine methiodide, and inhibition of Ca(2+)/calmodulin kinase II with KN-62 all caused significant inhibition of the carbachol-induced NBC activity increase. Furthermore, PKC inhibition with Gö-6976 and Gö-6850 significantly reduced the carbachol effect, which may be related to the unique NH(2)-terminal consensus site for PKC-dependent phosphorylation of pNBC1. We conclude that NBC in the murine colon is thus activated by carbachol, consistent with its presumed function as an anion uptake pathway during intestinal anion secretion, but that the signal transductions pathways are distinct from those involved in the cholinergic activation of renal NBC1.

SLC4 base (HCO3 -, CO3 2-) transporters: classification, function, structure, genetic diseases, and knockout models

In prokaryotic and eukaryotic organisms, biochemical and physiological processes are sensitive to changes in H(+) activity. For these processes to function optimally, a variety of proteins have evolved that transport H(+)/base equivalents across cell and organelle membranes, thereby maintaining the pH of various intracellular and extracellular compartments within specific limits. The SLC4 family of base (HCO(3)(-), CO(3)(2(-))) transport proteins plays an essential role in mediating Na(+)- and/or Cl(-)-dependent base transport in various tissues and cell types in mammals. In addition to pH regulation, specific members of this family also contribute to vectorial transepithelial base transport in several organ systems including the kidney, pancreas, and eye. The importance of these transporters in mammalian cell biology is highlighted by the phenotypic abnormalities resulting from spontaneous SLC4 mutations in humans and targeted deletions in murine knockout models. This review focuses on recent advances in our understanding of the molecular organization and functional properties of SLC4 transporters and their role in disease.

Activation of carboplatin by carbonate

Carboplatin, [Pt(NH3)2(CBDCA-O,O')], 1, where CBDCA is cyclobutane-1,1-dicarboxylate, is in wide clinical use for the treatment of ovarian, lung, and other types of cancer. Because carboplatin is relatively unreactive toward nucleophiles, an important question concerning the drug is the mechanism by which it is activated in vivo. Using [1H,15N] heteronuclear single quantum coherance spectroscopy (HSQC) NMR and 15N-labeled carboplatin, we show that carboplatin reacts with carbonate ion in carbonate buffer to produce ring-opened products, the nature of which depends on the pH of the medium. The assignment of HSQC NMR resonances was facilitated by studying the reaction of carboplatin in strong acid, which also produces a ring-opened product. The HSQC NMR spectra and UV-visible difference spectra show that reaction of carboplatin with carbonate at pH > 8.6 produces mainly cis-[Pt(NH3)2(CO3(-2))(CBDCA-O)]-2, 5, which contains the mono-dentate CBDCA ligand and mono-dentate carbonate. At pH 6.7, the primary product is the corresponding bicarbonato complex, which may be in equilibrium with its decarboxylated hydroxo analogue. The UV-visible absorption data indicate that the pKb for the protonation of 5 is approximately 8.6. Thus, the reaction of carboplatin with carbonate produces a mixture of ring-opened species that are anions at physiological pH. HSQC NMR studies on 15N-labeled carboplatin in RPMI culture media containing 10% fetal bovine serum with and without added carbonate suggest that carbonate is the attacking nucleophile in culture media. However, because the rate of reaction of carbonate with carboplatin at physiological pH is small, NMR peaks for ring-opened carboplatin were not detected with HSQC NMR. The rate of disappearance of carboplatin in culture medium containing 9 x 10(8) Jurkat cells is essentially the same as that in carbonate buffer, indicating that the ring-opening reaction is not affected by the presence of cells. This work shows that carbonate at concentrations found in culture media, blood, and the cytosol readily displaces one arm of the CBDCA ligand of carboplatin to give a ring-opened product, which at physiological pH is a mixture of anions. These ring-opened species may be important in the uptake, antitumor properties, and toxicity of carboplatin.

Electrophysiological characterization of native Na+-HCO3- cotransporter current in bovine parotid acinar cells

Using patch-clamp and molecular biological techniques, we identified and characterized membrane currents most likely generated by an electrogenic Na+-HCO3- cotransporter (NBCe) in acutely dissociated bovine parotid acinar (BPA) cells. When BPA cells were dialysed with a N-methyl-D-glucamine (NMDG)-glutamate-rich pipette solution, switching a Na-glutamate-rich, nominally HCO3--free bath solution to the one containing 25 mM HCO3-, but not Cl-, elicited a whole-cell current with a linear current-voltage relation. The HCO3- evoked current was abolished by total replacement of extracellular Na+ (Na+o) with NMDG or by 0.5 mM 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS), and was only partially supported by Li+o, but not by K+o, Cs+o, and cholineo. The reversal potential shift of DIDS (0.5 mM)-sensitive current induced by a change of [Na+]o corresponded to an apparent coupling ratio of HCO3- to Na+ of 2. RT-PCR analysis showed the presence of transcripts of NBCe1-B, but not NBCe1-A in BPA cells. Electrophysiological and pharmacological properties of whole-cell currents recorded from HEK293 cells transfected with the NBCe1-B, which was cloned from BPA cells resembled those of the native currents. Non-invasive measurements of membrane potential changes in the cell-attached patch configuration indicated that an NBCe activity is present in intact unstimulated BPA cells bathed in a 25 mM HCO3--containing solution. Collectively, these results not only suggest that an NBCe is present, functional and may be mediated, at least in part, by NBCe1-B in BPA cells, but also provide the first electrophysiological characterization of transport properties of NBCe expressed in native exocrine glands.

Critical amino acid residues involved in the electrogenic sodium-bicarbonate cotransporter kNBC1-mediated transport

We have previously reported a topological model of the electrogenic Na(+)-HCO(3)(-) cotransporter (NBC1) in which the cotransporter spans the plasma membrane 10 times with N- and C-termini localized intracellularly. An analysis of conserved amino acid residues among members of the SLC4 superfamily in both the transmembrane segments (TMs) and intracellular/extracellular loops (ILs/ELs) provided the basis for the mutagenesis approach taken in the present study to determine amino acids involved in NBC1-mediated ion transport. Using large-scale mutagenesis, acidic and basic amino acids putatively involved in ion transport mediated by the predominant variant of NBC1 expressed in the kidney (kNBC1) were mutated to neutral and/or oppositely charged amino acids. All mutant kNBC1 cotransporters were expressed in HEK-293T cells and the Na(+)-dependent base flux of the mutants was determined using intracellular pH measurements with 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Critical glutamate, aspartate, lysine, arginine and histidine residues in ILs/ELs and TMs were detected that were essential for kNBC1-mediated Na(+)-dependent base transport. In addition, critical phenylalanine, serine, tyrosine, threonine and alanine residues in TMs and ILs/ELs were detected. Furthermore, several amino acid residues in ILs/ELs and TMs were shown to be essential for membrane targeting. The data demonstrate asymmetry of distribution of kNBC1 charged amino acids involved in ion recognition in putative outward-facing and inward-facing conformations. A model summarizing key amino acid residues involved in kNBC1-mediated ion transport is presented.

Molecular biology of major components of chloride cells

Current understanding of chloride cells (CCs) is briefly reviewed with emphasis on molecular aspects of their channels, transporters and regulators. Seawater-type and freshwater-type CCs have been identified based on their shape, location and response to different ionic conditions. Among the freshwater-type CCs, subpopulations are emerging that are implicated in the uptake of Na(+), Cl(-) and Ca(2+), respectively, and can be distinguished by their shape of apical crypt and affinity for lectins. The major function of the seawater CC is transcellular secretion of Cl(-), which is accomplished by four major channels and transporters: (1). CFTR Cl(-) channel, (2). Na(+),K(+)-ATPase, (3). Na(+)/K(+)/2Cl(-) cotransporter and (4). a K(+) channel. The first three components have been cloned and characterized, but concerning the K(+) channel that is essential for the continued generation of the driving force by Na(+),K(+)-ATPase, only one candidate is identified. Although controversial, freshwater CCs seem to perform the uptake of Na(+), Cl(-) and Ca(2+) in a manner analogous to but slightly different from that seen in the absorptive epithelia of mammalian kidney and intestine since freshwater CCs face larger concentration gradients than ordinary epithelial cells. The components involved in these processes are beginning to be cloned, but their CC localization remains to be established definitively. The most important yet controversial issue is the mechanism of Na(+) uptake. Two models have been postulated: (i). the original one involves amiloride-sensitive electroneutral Na(+)/H(+) exchanger (NHE) with the driving force generated by Na(+),K(+)-ATPase and carbonic anhydrase (CA) and (ii). the current model suggests that Na(+) uptake occurs through an amiloride-sensitive epithelial sodium channel (ENaC) electrogenically coupled to H(+)-ATPase. While fish ENaC remains to be identified by molecular cloning and database mining, fish NHE has been cloned and shown to be highly expressed on the apical membrane of CCs, reviving the original model. The CC is also involved in acid-base regulation. Analysis using Osorezan dace (Tribolodon hakonensis) living in a pH 3.5 lake demonstrated marked inductions of Na(+),K(+)-ATPase, CA-II, NHE3, Na(+)/HCO(3)(-) cotransporter-1 and aquaporin-3 in the CCs on acidification, leading to a working hypothesis for the mechanism of Na(+) retention and acid-base regulation.

Molecular mechanism of kNBC1-carbonic anhydrase II interaction in proximal tubule cells

We have recently shown that carbonic anhydrase II (CAII) binds in vitro to the C-terminus of the electrogenic sodium bicarbonate cotransporter kNBC1 (kNBC1-ct). In the present study we determined the molecular mechanisms for the interaction between the two proteins and whether kNBC1 and CAII form a transport metabolon in vivo wherein bicarbonate is transferred from CAII directly to the cotransporter. Various residues in the C-terminus of kNBC1 were mutated and the effect of these mutations on both the magnitude of CAII binding and the function of kNBC1 expressed in mPCT cells was determined. Two clusters of acidic amino acids, L(958)DDV and D(986)NDD in the wild-type kNBC1-ct involved in CAII binding were identified. In both acidic clusters, the first aspartate residue played a more important role in CAII binding than others. A significant correlation between the magnitude of CAII binding and kNBC1-mediated flux was shown. The results indicated that CAII activity enhances flux through the cotransporter when the enzyme is bound to kNBC1. These data are the first direct evidence that a complex of an electrogenic sodium bicarbonate cotransporter with CAII functions as a transport metabolon.

Differential expression of electrogenic NBC1 (SLC4A4) variants in rat kidney and pancreas

The purpose of this study was to determine expression and localization of NH(2)-terminal variants of the electrogenic Na(+)-HCO(3)(-) co-transporter NBC1 (SLC4A4) in the rat kidney and pancreas. We generated two anti-peptide antibodies: alpha333 against the "mste" start (kidney; kNBC1) and alpha332 against the "mede" start (pancreas; pNBC1). Transcripts for both NBC1 variants were detected in kidney and pancreas by RT-PCR, though kNBC1 was more prominent in the kidney and pNBC1 was more prominent in the pancreas. Similar protein expression levels were detected by immunoblotting of plasma membranes (PM) from kidney cortex and pancreas. Immunohistochemistry with alpha333 recognized the "mste"-epitope in the basolateral plasma membrane (BLM) of renal proximal tubule. The "mede"-protein (alpha332) was similarly localized although staining was much less and more diffuse. In the pancreas, alpha332 stained BLM of acinar and duct cells. Some isolated duct cells were also stained at the apical PM. The "mste"-protein (alpha333) was absent in acinar cells but was located at the apical PM of duct cells. The data indicate that the two NH(2)-terminal NBC1 variants are co-expressed in kidney and pancreas, where they may contribute to HCO(3)(-) transport and pH regulation.

Identification and cloning of the Na/HCO(3-) cotransporter (NBC) in human corneal endothelium

Fluid secretion by the corneal endothelium is associated with the net flux of HCO(3)(-) from basolateral (stromal) to apical (anterior chamber) sides of the tissue. In this study we asked if Na(+)/HCO(3)(-) cotransporter (NBC-1) protein expression and functional activity are present in freshly isolated human corneal endothelium. Immunoblot analysis using a polyclonal antibody to NBC-1 showed a single band at approximately 130 kDa. Indirect immunofluorescence indicated that NBC-1 is expressed on the basolateral, but not apical side of human corneal endothelium. RT-PCR was used to determine whether the kidney or pancreatic isoform of NBC-1 is expressed. Using the specific primers for pNBC and kNBC isoforms, RT-PCR showed that only pNBC could be detected in human corneal endothelium. The product was cloned and confirmed by sequencing. Full-length NBC-1 was also cloned from human corneal endothelium. This clone (hcNBC) is 100% identical to the longer, more common form of NBC [pNBC; 1079 amino acids (aa); 122 kDa in human heart, pancreas and prostate]. To test for functional activity of NBC-1, freshly isolated endothelium was loaded with the pH sensitive fluorescent dye BCECF and HCO(3)(-) fluxes were measured. HCO(3)(-) fluxes were Na(+)-dependent, electrogenic and H(2)-DIDS sensitive. We conclude that the long isoform of the sodium bicarbonate cotransporter (pNBC-1) is expressed on the basolateral side of fresh human corneal endothelium (hcNBC). The shorter form, kNBC, could not be detected. As in bovine corneal endothelium, hcNBC is instrumental in loading HCO(3)(-) into endothelial cells from the basolateral membrane.

Blindness and auditory impairment caused by loss of the sodium bicarbonate cotransporter NBC3

Normal sensory transduction requires the efficient disposal of acid (H+) generated by neuronal and sensory receptor activity. Multiple highly sensitive transport mechanisms have evolved in prokaryotic and eukaryotic organisms to maintain acidity within strict limits. It is currently assumed that the multiplicity of these processes provides a biological robustness. Here we report that the visual and auditory systems have a specific requirement for H+ disposal mediated by the sodium bicarbonate cotransporter NBC3 (refs. 7,8). Mice lacking NBC3 develop blindness and auditory impairment because of degeneration of sensory receptors in the eye and inner ear as in Usher syndrome. Our results indicate that in certain sensory organs, in which the requirement to transduce specific environmental signals with speed, sensitivity and reliability is paramount, the choice of the H+ disposal mechanism used is limited.

Phosphorylation-induced modulation of pNBC1 function: distinct roles for the amino- and carboxy-termini

The human NBC1 (SLC4A4) gene encodes the electrogenic sodium bicarbonate cotransporters kNBC1 and pNBC1, which are highly expressed in the kidney and pancreas, respectively. The HCO3-:Na+ stoichiometry of these cotransporters is an important determinant of the direction of ion flux. Recently we showed in a mouse proximal tubule (mPCT) cell line expressing kNBC1, that 8-Br-cAMP shifts the stoichiometry of the cotransporter from 3:1 to 2:1 via protein kinase A (PKA)-dependent phosphorylation of Ser982. pNBC1 has the identical carboxy-terminal consensus phosphorylation PKA site (KKGS1026), and an additional site in its amino-terminus (KRKT49). In this study we determined the potential role of these sites in regulating the function of pNBC1. The results demonstrated that in mPCT cells expressing pNBC1, PKA-dependent phosphorylation of Ser1026 following 8-Br-cAMP treatment shifted the stoichiometry from 3:1 to 2:1. The effect was electrostatic in nature as replacing Ser1026 with Asp resulted in a similar stoichiometry shift. In addition to shifting the stoichiometry, 8-Br-cAMP caused a significant increase in the 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS)-sensitive basolateral membrane conductance (GDS) of cells expressing pNBC1, but not kNBC1. Although, the effect did not involve phosphorylation of Thr49, which was endogenously phosphorylated, replacing this residue with Asp or Ala abolished the 8-Br-cAMP-induced increase in GDS. In the mPEC pancreatic duct cell line, where endogenous pNBC1 functions with a HCO3-:Na+ stoichiometry of 2:1, 8-Br-cAMP increased GDS by ~90 % without altering the stoichiometry or inducing phosphorylation of the cotransporter. The results demonstrate that phosphorylation of Ser1026 mediates the cAMP-dependent shift in the stoichiometry of pNBC1, whereas Thr49 plays an essential role in the cAMP-induced increase in GDS.

Reduction of intracellular pH inhibits constitutive expression of Cyclooxygenase-2 in human colon cancer cells

Cyclooxygenase-2 (COX-2) over-expression is critically involved in tumor formation. Intracellular pH (pHi) has been shown to be alkaline in cancer cells, and to be an important trigger for cell proliferation. This study therefore analyzed the relationship between pHi and COX-2 expression. HRT-18 and Caco-2 cells cultured in medium with bicarbonate maintained a pHi of approximately 7.6, which is higher than that of non-neoplastic cells. Cells grown in bicarbonate-free medium with a pH at 6.8 showed a reduction in pHi to approximately 7.0. Importantly, reduction of pHi resulted in a complete inhibition of COX-2 mRNA and protein expression. When cells were grown in bicarbonate-supplemented medium at pH 6.8, pHi maintained at approximately 7.6 and COX-2 expression was not inhibited. Additionally, analysis utilizing protein synthesis inhibitor cycloheximide demonstrated that pHi mediated inhibition of COX-2 mRNA expression requires de novo protein synthesis of regulatory protein(s). These data strongly suggest that an alkaline pHi is an important trigger for constitutive COX-2 expression. Defining pHi-mediated mechanisms that govern the constitutive COX-2 expression may help in developing new strategies to block COX-2 over-expression in cancer cells.