α2-Adrenergic Receptors and the Na+/H+ Exchanger in the Intestinal Epithelial Cell Line, HT-29

Adrenaline induces chemoresistance in HT-29 colon adenocarcinoma cells

Psychological distress and its ensuing chronic elevation of plasma catecholamines (adrenaline and noradrenaline) lead to poor response of tumors to chemotherapy, and constitute a poor prognostic factor for survival. Colorectal cancer patients suffer from various forms of psychological stress reflected in elevated plasma catecholamines, and their cancer cells express adrenergic receptors. Our objective was to investigate whether adrenergic activation contributes to the chemoresistance of colon cancers, and to explore the signal transduction pathway involved in the activation. The mRNA expression of the ABCB1 gene (previously MDR1) in human colon carcinoma HT-29 cell line was measured after treatment with an adrenergic receptor agonist (adrenaline) and various antagonists (propranolol, prazosin, and yohimbine). The function of P-glycoprotein, the protein product of the ABCB1 gene, was assessed by rhodamine 123 (Rh123)-retention assay, and chemosensitivity was determined by evaluating the cytotoxicity of 5-fluorouracil (5-FU) on the tumor cells. Increased ABCB1 mRNA expression and P-glycoprotein function levels in HT-29 cells by adrenaline was dose-dependent. This was accompanied by promotion of Rh123 efflux, and resistance to the growth-inhibiting effect of 5-FU in the tumor cells. The alpha2-adrenergic receptor antagonist yohimbine completely abolished the induction of ABCB1 mRNA, the stimulatory effect of adrenaline on Rh123 efflux, and the growth-inhibiting effect of 5-FU. The alpha1-adrenergic receptor and beta-adrenergic receptor antagonists did not inhibit the induction of ABCB1. The stimulating effects were coupled with extracellular receptor kinase 1/2 (Erk1/2) phosphorylation, but were not associated with protein kinase A activity. We conclude that adrenaline induces multidrug resistance in colon cancer cells by upregulating ABCB1 gene expression via alpha2-adrenergic receptors, and such effects were associated with the mitogen activated protein kinase (MAPK) pathway.

Therapeutic potential of H(3)-receptor agonists in myocardial infarction

Sympathetic over-activity accompanied by excessive noradrenaline (NA) release within the heart is a recognised cause of dysfunction in myocardial ischaemia. Myocardial infarction is often accompanied by arrhythmias with high morbidity and mortality. Indeed, NA enhances intracellular Ca(2+) by increasing its influx through voltage-dependent channels, mobilising it from intracellular stores and favouring its inward transport by Na(+)/Ca(2+) exchange. Ca(2+) overload eventually results in dysrhythmia and uncoordinated myocyte contraction. Moreover, NA increases metabolic demand. In concert with other contributing factors, this will aggravate the primary ischaemia and initiate a vicious cycle that can culminate in myocardial damage and heart failure. Therefore, reduction of NA release from cardiac sympathetic nerves is an important protective measure. Adrenergic nerves possess inhibitory receptors, such as alpha(2)-adrenoceptors, adenosine A(1)-receptors and histamine H(3)-receptors (H(3)R). In myocardial infarction, NA is released by both exocytotic (Ca(2+)-dependent) and carrier-mediated (Na(+)/H(+) exchange-dependent) mechanisms, associated with short-term and protracted ischaemia, respectively. Unlike alpha(2)-adrenoceptor agonists that reduce NA exocytosis, but enhance carrier-mediated NA release, H(3)R agonists inhibit both exocytotic and carrier-mediated NA release. Moreover, unlike adenosine A(1)-receptor agonists, H(3)R agonists do not depress sinoatrial and atrioventricular nodes, nor cause bronchoconstriction. Therefore, stimulation of H(3)R on cardiac sympathetic nerve endings is an important new way to protect the heart from the consequences of ischaemia and infarction. Although H(3)R agonists alleviate reperfusion arrhythmias in isolated hearts by reducing NA release, this protective action needs to be demonstrated in classical in vivo models of occlusion/reperfusion. Regardless, H(3)R agonists offer the promise of a novel strategy in the treatment of myocardial ischaemia and infarction.

Relationship between imidazoline/guanidinium receptive sites and monoamine oxidase A and B

Imidazoline binding sites or imidazoline/guanidinium receptive sites (IGRS) recognize bioactive endogenous substances and a variety of pharmacologically active compounds containing imidazoline or guanidinium moieties. The family of imidazoline binding proteins consists of multiple membrane-associated proteins that differ in their tissue/subcellular localization, M(r) and ligand recognition properties. Two of the imidazoline binding proteins are identical to the mitochondrial enzyme monoamine oxidase (MAO) A and B isoforms, which contain imidazoline binding domains distinct from the enzyme active site. The relationship between the imidazoline binding proteins and monoamine oxidases was further characterized in the present report using a covalent probe (2-[3-azido-4[125I]iodophenoxy] methyl imidazoline, [125I]-AZIPI) to label the imidazoline binding proteins in different species and following transient expression of MAO- A and -B in COS 7 cells. Species homologues of MAO-A and -B in rat and human differ in their apparent molecular weight by approximately 2000 Da. In rat and human liver [125I]-AZIPI identified peptides with apparent molecular weights similar to those of the species homologues of MAO. Peptides of M(r) approximately 63,000 (MAO-A) and approximately 59,000 (MAO-B) were also photolabeled in membranes prepared from COS-7 cells transfected with human cDNA clones encoding MAO-A or -B. Additional experiments indicate that the imidazoline binding domains on MAO-A and -B exhibit different ligand recognition properties. The covalent labeling of human liver MAO-B was more sensitive than that of placenta MAO-A to inhibition by the imidazoline 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224). These data indicate that the A and B isoforms of MAO possess imidazoline binding domains that differ in their ligand recognition properties. Allosteric regulation of the activity of MAO via the imidazoline binding domains may be of significance in various disease states associated with elevated enzyme expression or in which the enzyme is a therapeutic target.

Activation of histamine H3-receptors inhibits carrier-mediated norepinephrine release during protracted myocardial ischemia. Comparison with adenosine A1-receptors and alpha2-adrenoceptors

We previously showed that prejunctional histamine H3-receptors downregulate norepinephrine exocytosis, which is markedly enhanced in early myocardial ischemia. In the present study, we investigated whether H3-receptors modulate nonexocytotic norepinephrine release during protracted myocardial ischemia. In this setting, decreased pH(i) in sympathetic nerve endings sequentially leads to a compensatory activation of the Na+-H+ antiporter (NHE), accumulation of intracellular Na+, reversal of the neuronal uptake of norepinephrine, and thus carrier-mediated release of norepinephrine. Accordingly, norepinephrine overflow from isolated guinea pig hearts undergoing 20-minute global ischemia and 45-minute reperfusion was attenuated approximately 80% by desipramine (10 nmol/L) and 70% by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA, 10 micromol/L), inhibitors of norepinephrine uptake and NHE, respectively. The H3-receptor agonist imetit (0.1 micromol/L) decreased carrier-mediated norepinephrine release by approximately 50%. This effect was blocked by the H3-receptor antagonist thioperamide (0.3 micromol/L), indicating that H-receptor activation inhibits carrier-mediated norepinephrine release. At lower concentrations, imetit (10 nmol/L) or EIPA (3 micromol/L) did not inhibit carrier-mediated norepinephrine release. However, a 25% inhibition occurred with imetit (10 nmol/L) and EIPA (3 micromol/L) combined. This synergism suggests an association between H-receptors and NHE. Conceivably, activation of H-receptors may lead to inhibition of NHE. In fact, alpha2-adrenoceptor activation, which is known to stimulate NHE, enhanced norepinephrine release, whereas alpha2-adrenoceptor blockade attenuated it. Furthermore, activation of adenosine A1-receptors markedly attenuated norepinephrine release, whereas their inhibition potentiated it. Because norepinephrine directly correlated with the severity of reperfusion arrhythmia and imetit reduced the incidence of ventricular fibrillation by 50%, our findings with H-receptor agonists may further the development of novel pharmacological means to reduce reperfusion arrhythmias in the clinical setting.

Factors determining specificity of signal transduction by G-protein-coupled receptors. Regulation of signal transfer from receptor to G-protein

Among subfamilies of G-protein-coupled receptors, agonists initiate several cell signaling events depending on the receptor subtype (R) and the type of G-protein (G) or effector molecule (E) expressed in a particular cell. Determinants of signaling specificity/efficiency may operate at the R-G interface, where events are influenced by cell architecture or accessory proteins found in the receptor's microenvironment. This issue was addressed by characterizing signal transfer from R to G following stable expression of the alpha 2A/D adrenergic receptor in two different membrane environments (NIH-3T3 fibroblasts and the pheochromocytoma cell line, PC-12). Receptor coupling to endogenous G-proteins in both cell types was eliminated by pertussis toxin pretreatment and R-G signal transfer restored by reconstitution of cell membranes with purified brain G-protein. Thus, the receptor has access to the same population of G-proteins in the two different environments. In this signal restoration assay, agonist-induced activation of G was 3-9-fold greater in PC-12 as compared with NIH-3T3 alpha 2-adrenergic receptor transfectants. The cell-specific differences in signal transfer were observed over a range of receptor densities or G-protein concentration. The augmented signal transfer in PC-12 versus NIH-3T3 transfectants occurred despite a 2-3-fold lower level of receptors existing in the R-G-coupled state (high affinity, guanyl-5'-yl imidodiphosphate-sensitive agonist binding), suggesting the existence of other membrane factors that influence the nucleotide binding behavior of G-protein in the two cell types. Detergent extraction of PC-12 but not NIH-3T3 membranes yielded a heat-sensitive, macromolecular entity that increased 35S-labeled guanosine 5'-O-(thiotriphosphate) binding to brain G-protein in a concentration-dependent manner. These data indicate that the transfer of signal from R to G is regulated by a cell type-specific, membrane-associated protein that enhances the agonist-induced activation of G.

pH regulation in HT29 colon carcinoma cells

The pH regulation in HT29 colon carcinoma cells has been investigated using the pH-sensitive fluorescent indicator 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). Under control conditions, intracellular pH (pHi) was 7.21 +/- 0.07 (n = 22) in HCO3(-)-containing and 7.21 +/- 0.09 (n = 12) in HCO3(-)-free solution. HOE-694 (10 mumol/l), a potent inhibitor of the Na+/H+ exchanger, did not affect control pHi. As a means to acidify cells we used the NH4+/NH3 (20 mmol/l) prepulse technique. The mean peak acidification was 0.37 +/- 0.07 pH units (n = 6). In HCO3(-)-free solutions recovery from acid load was completely blocked by HOE-694 (1 mumol/l), whereas in HCO3(-)-containing solutions a combination of HOE-694 and 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS, 0.5 mmol/l) was necessary to show the same effect. Recovery from acid load was Na(+)-dependent in HCO3(-)-containing and HCO3(-)-free solutions. Removal of external Cl- caused a rapid, DIDS-blockable alkalinization of 0.33 +/- 0.03 pH units (n = 15) and of 0.20 +/- 0.006 pH units (n = 5), when external Na+ was removed together with Cl-. This alkalinization was faster in HCO3(-)-containing than in HCO3(-)-free solutions. The present observations demonstrate three distinct mechanisms of pHi regulation in HT29 cells: (a) a Na+/H+ exchanger, (b) a HCO3-/Cl- exchanger and (c) a Na(+)-dependent HCO3- transporter, probably the Na(+)-HCO3-/Cl- antiporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmitter-induced changes of the membrane voltage of HT29 cells

The colonic carcinoma cell line HT29 was used to examine the influence of agonists increasing cytosolic cAMP and Ca2+ activity on the conductances and the cell membrane voltage (Vm). HT29 cells were grown on glass cover-slips. Cells were impaled by microelectrodes 4-10 days after seeding, when they had formed large plaques. In 181 impalements Vm was -51 +/- 1 mV. An increase in bath K+ concentration from 3.6 mmol/l to 18.6 mmol/l or 0.5 mmol/l Ba2+ depolarized the cells by 10 +/- 1 mV (n = 49) or by 9 +/- 2 mV (n = 3), respectively. A decrease of bath Cl- concentration from 145 to 30 mmol/l depolarized the cells by 11 +/- 1 mV (n = 24). Agents increasing intracellular cAMP such as isobutylmethylxanthine (0.1 mmol/l), forskolin (10 mumol/l) or isoprenaline (10 mumol/l) depolarized the cells by 6 +/- 1 (n = 13), 15 +/- 3 (n = 5) and 6 +/- 2 (n = 3) mV, respectively. In hypoosmolar solutions (225 mosmol/l) cells depolarized by 9 +/- 1 mV (n = 6). Purine and pyrimidine nucleotides depolarized the cells dose-dependently with the following potency sequence: UTP greater than ATP greater than ITP greater than GTP greater than TTP greater than CTP = 0. The depolarization by ATP was stronger than that by ADP and adenosine. The muscarinic agonist carbachol led to a sustained depolarization by 27 +/- 6 mV (n = 5) at 0.1 mmol/l, and to a transient depolarization by 12 +/- 4 mV (n = 5) at 10 mumol/l. Neurotensin depolarized with a half-maximal effect at around 5 nmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Na(+)-H+ exchanger subtypes: a predictive review

In recent years, many reports have appeared describing distinct heterogeneity of proteins that heretofore were considered to be a single species or type. The division of proteins into different classes or subtypes is aided by pharmacological tools such as selective ligands, functional measurements such as those examining kinetic or regulatory differences, and molecular biological approaches that have identified distinct genes coding for similar yet distinguishable gene products. Currently, much effort is directed toward understanding the significance of these sometimes subtle differences in terms of functional consequences for the cells in which they exist. Although most reports to date involve hormone and neurotransmitter receptor subtypes, it is also possible that other cell surface molecules such as ion transporters exist as multiple subtypes. In this paper we review the current evidence that Na(+)-H+ exchange activity is mediated by different Na(+)-H+ exchanger subtypes. Although subtypes have not been identified with certainty, we can predict certain distinguishing characteristics that these putative subtypes may have that may be of value in correlating predicted gene products obtained from cDNA cloning with previously characterized Na(+)-H+ exchangers.

Effects of clonidine and other imidazole-receptor binding agents on second messenger systems and calcium influx in bovine adrenal chromaffin cells

Clonidine and related imidazoline compounds bind to alpha 2-adrenergic as well as to newly described non-adrenergic imidazole/imidazoline receptors in brain and peripheral tissues. The present study was undertaken to identify the signal transduction mechanism coupled to this new class of receptors (imidazole receptors) using bovine adrenal chromaffin cells. Clonidine did not modify the basal or forskolin-stimulated production of cyclic AMP (cAMP), suggesting the absence of functionally active alpha 2-adrenergic receptors in adrenal chromaffin cells. Clonidine also failed to modify the basal and GTP gamma S- or carbachol-stimulated increase in phosphoinositide hydrolysis. However, clonidine increased significantly the production of cyclic GMP (cGMP) as well as the uptake of 45Ca2+. The cGMP response to clonidine was slower (peak at 15 min) and smaller (only about 50% over control) than the response to acetylcholine and was not shared by other agents that bind to imidazole receptors. In contrast, all agents that bind to imidazole receptors increased the influx of 45Ca2+ into chromaffin cells. It is concluded that (a) alpha 2-adrenergic and imidazole receptors are functionally distinct and linked to different signal transduction mechanisms; (b) the classical G-protein coupled soluble second messenger systems are not coupled to imidazole receptors; (c) clonidine may increase cGMP by a non-receptor-mediated intracellular action; and (d) imidazole receptors may regulate intracellular calcium levels through an ion regulating system that may be different from calcium channels.

Inhibition of the rat renal Na+/H+ exchanger by beta-adrenergic antagonists

The beta-adrenergic antagonists, alprenolol and propranolol, inhibit the Na+/H+ exchanger in rat renal brush-border membrane vesicles. Half-maximal inhibition occurs at 86 microM alprenolol and 36 microM propranolol. Similar to amiloride and Na+, propranolol protects the Na+/H+ exchanger from irreversible inhibition by the carboxyl group reagent, N,N'-dicyclohexyl-carbodiimide (DCCD). Protection is incomplete, depends on propranolol concentration, and reaches a maximum at 0.4 mM propranolol. With a comparable dose dependence, propranolol protects a 65 kDa band from labeling with [14C]DCCD. The data indicate that beta-adrenergic antagonists specifically interact with the proximal tubular Na+/H+ exchanger.

Alpha 1-adrenoceptor stimulation increases intracellular pH and Ca2+ in cardiomyocytes through Na+/H+ and Na+/Ca2+ exchange

The effects of alpha 1-adrenergic stimulation on intracellular pH (pHi) and Ca2+ concentration ([Ca2+]i) were investigated in isolated rat cardiomyocytes with fluorescence dyes, BCECF and fura-2, respectively. In the presence of 5 or 25 mM HCO3- norepinephrine (NE) increased pHi in a dose-dependent manner. Intracellular alkalinization was inhibited by prazosin and phentolamine but not by yohimbine. NE-induced alkalinization was inhibited in the presence of a Na+/H+ exchange inhibitor (5-(N,N-hexamethylene) amiloride (HMA)), a C kinase inhibitor (H-7) or a calmodulin inhibitor (W-7), or in the absence of extracellular Na+. NE also increased [Ca2+]i following the pHi increase, which was abolished in the absence of extracellular Na+ or Ca2+. This Ca2+ influx was inhibited by HMA but not by diltiazem (10(-5) M). Thus, we conclude that alpha 1-adrenergic stimulation enhances Na+/H+ exchange by activation of C kinase, thereby allowing intracellular alkalinization, and that subsequent activation of Na+/Ca2+ exchange increases Ca2+ influx.