Isolation and reconstitution of furosemide-binding proteins from Ehrlich ascites tumor cells

Regulation of Na-K-2Cl cotransport by phosphorylation and protein-protein interactions

The Na-K-2Cl cotransporter plays important roles in cell ion homeostasis and volume control and is particularly important in mediating the movement of ions and thus water across epithelia. In addition to being affected by the concentration of the transported ions, cotransport is affected by cell volume, hormones, growth factors, oxygen tension, and intracellular ionized Mg(2+) concentration. These probably influence transport through three main routes acting in parallel: cotransporter phosphorylation, protein-protein interactions and cell Cl(-) concentration. Many effects are mediated, at least in part, by changes in protein phosphorylation, and are disrupted by kinase and phosphatase inhibitors, and manoeuvres that reduce cell ATP content. In some cases, phosphorylation of the cotransporter itself on serine and threonine (but not tyrosine) is associated with changes in transport rate, in others, phosphorylation of associated proteins has more influence. Analysis of the stimulation of cotransport by calyculin A, arsenite and deoxygenation suggests that the cotransporter is phosphorylated by several kinases and dephosphorylated by several phosphatases. These kinases and phosphatases may themselves be regulated by phosphorylation of residues including tyrosine, with Src kinases possibly playing an important role. Protein-protein interactions also influence cotransport activity. Cotransporter molecules bind to each other to form high molecular weight complexes, they also bind to other members of the cation-chloride cotransport family, to a variety of cytoskeletal proteins, and to enzymes that are part of regulatory cascades. Many of these interactions affect transport and may override the effects of cotransporter phosphorylation. Cell Cl(-) may also directly affect the way the cotransporter functions independently of its role as substrate.

Occlusion of K+ in the Na+/K+/2Cl- cotransporter of Ehrlich ascites tumor cells

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb+.K+ displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K+ occlusion. The following observations indicate that the Na+/K+/2Cl- cotransporter is responsible for this occlusion: (1) Na+ does not compete for the K+ binding site, but rather stimulates 86Rb+ occlusion. (2) K+ occlusion saturates with increasing [Na+] and [K+], the respective K0.5 values being 50 +/- 7 microM for Na+ and 371 +/- 63 microM for K+. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0.5 value for K+ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na+. (4) The K+ occlusion is sensitive to [Cl-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2Cl- cotransporter.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. Na-K-Cl cotransporters are present in a wide variety of cells and tissues, including reabsorptive and secretory epithelia, nerve and muscle cells, endothelial cells, fibroblasts, and blood cells. Na-K-Cl cotransport plays a vital role in renal salt reabsorption and in salt secretion by intestinal, airway, salivary gland, and other secretory epithelia. Cotransport function also appears to be important in the maintenance and regulation of cell volume and of ion gradients by both epithelial and nonepithelial cells. Na-K-Cl cotransport activity is inhibited by "loop" diuretics, including the clinically efficacious agents bumetanide and furosemide. The regulation of Na-K-Cl cotransport is mediated, at least in some cases, through direct phosphorylation of the cotransport protein. Cotransporter regulation is highly tissue specific, perhaps in part related to the presence of different Na-K-Cl cotransporter isoforms. In epithelia, both absorptive (kidney-specific) and secretory isoforms have been identified by cDNA cloning and sequencing and Northern blot analysis; alternatively spliced variants of the kidney-specific isoform have also been identified. The absorptive and secretory isoforms exhibit approximately 60% identity at the amino acid sequence level; these sequences in turn show approximately 45% overall homology with those of thiazide-sensitive, bumetanide-insensitive, Na-Cl cotransport proteins of winter flounder urinary bladder and mammalian kidney. This review focuses on recent developments in the identification of Na-K-Cl cotransport proteins in epithelial and on the regulation of epithelial Na-K-Cl cotransporter function at cellular and molecular levels.

Activation of protein kinase C during cell volume regulation in Ehrlich mouse ascites tumor cells

We have previously demonstrated that in Ehrlich cells a bumetanide-sensitive Na+,K+,2Cl- cotransporter is activated during regulatory volume decrease after cell shrinkage (hypertonic conditions) as well as during the late phase of regulatory volume decrease (hypotonic conditions). It is, however, quiescent under isotonic conditions. Using a protein kinase C assay system (Amersham, UK) it is here demonstrated that hypertonic cell shrinkage results in an increase in protein kinase C activity to 174% within the first minute, concomitant with the activation of the Na+,K+,2Cl- cotransporter. Hypotonic cell swelling results in a late activation of protein kinase C concomitant with a late activation of the Na+,K+,2Cl- cotransporter. The activation of protein kinase C during hypertonic as well as hypotonic conditions is inhibited by H-7. The more specific protein kinase C inhibitor chelerythrine inhibited protein kinase C as well as the Na+,K+,2Cl- cotransporter to the same extent as did H-7. These results indicate the involvement of protein kinase C in the regulation of the Na+,K+,2Cl- cotransporter in Ehrlich ascites tumor cells during cell volume regulation.

Na+, K+, Cl- cotransport and its regulation in Ehrlich ascites tumor cells. Ca2+/calmodulin and protein kinase C dependent pathways

Net Cl- uptake as well as unidirectional 36Cl influx during regulatory volume increase (RVI) require external K+. Half-maximal rate of bumetanide-sensitive 36Cl uptake is attained at about 3.3 mM external K+. The bumetanide-sensitive K+ influx found during RVI is strongly dependent on both Na+ and Cl-. The bumetanide-sensitive unidirectional Na+ influx during RVI is dependent on K+ as well as on Cl-. The cotransporter activated during RVI in Ehrlich cells, therefore, seems to transport Na+, K+ and Cl-. In the presence of ouabain and Ba+ the stoichiometry of the bumetanide-sensitive net fluxes can be measured at 1.0 Na+, 0.8 K+, 2.0 Cl- or approximately 1:Na, 1:K, 2:Cl. Under these circumstances the K+ and Cl- flux ratios (influx/efflux) for the bumetanide-sensitive component were estimated at 1.34 +/- 0.08 and 1.82 +/- 0.15 which should be compared to the gradient for the Na+, K+, 2Cl- cotransport system at 1.75 +/- 0.24. Addition of sucrose to hypertonicity causes the Ehrlich cells to shrink with no signs of RVI, whereas shrinkage with hypertonic standard medium (all extracellular ion concentrations increased) results in a RVI response towards the original cell volume. Under both conditions a bumetanide-sensitive unidirectional K+ influx is activated. During hypotonic conditions a small bumetanide-sensitive K+ influx is observed, indicating that the cotransport system is already activated. The cotransport is activated 10-15 fold by bradykinin, an agonist which stimulates phospholipase C resulting in release of internal Ca2+ and activation of protein kinase C. The anti-calmodulin drug pimozide inhibits most of the bumetanide-sensitive K+ influx during RVI. The cotransporter can be activated by the phorbol ester TPA. These results indicate that the stimulation of the Na+, K+, Cl- cotransport involves both Ca2+/calmodulin and protein kinase C.

Activation of the Na+/K+/Cl- cotransport system by reorganization of the actin filaments in Ehrlich ascites tumor cells

Reorganization (disassembly) of the actin filaments in Ehrlich ascites tumor cells, either by hypotonic treatment in the presence of Ca2+ or by addition of cytochalasin B, results in activation of the Na+/K+/Cl- cotransport system. However, other regulatory processes, some of which may be dependent on an intact filament system, are responsible for the activation of the Na+/K+/Cl- cotransport system after cell shrinkage.

Secondary active transport of water across ventricular cell membrane of choroid plexus epithelium of Necturus maculosus

1. The interaction between Cl-, K+ and H2O fluxes were studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion-selective microelectrodes. The flux of H2O was measured by means of K+ electrodes as the dilution or concentration of intracellular choline ions, Ch+i. 2. In one series of experiments Cl- was readministered to the ventricular solution of tissues incubated in media with low Cl- concentrations. The resulting influx of Cl- was associated with an instantaneous influx of K+ and H2O. 3. Both the Cl- and the K+ influxes were reduced by the diuretic furosemide but were unaffected by inhibitors of Na+, K(+)-ATPase or changes in membrane potentials induced by Ba2+. Since the influx of K+ proceeds against its electrochemical gradient and is unaffected by changes in membrane potentials, the membrane exhibits secondary active, electroneutral transport of K+. 4. The influx of water, initiated simultaneously with the influx of K+ and Cl-, commenced before these ions had changed the osmolarity of the intracellular solution significantly. The influx of H2O could proceed against an osmotic gradient. The influx stopped when 100 mmol l-1 of mannitol was added to the ventricular solution at the same time as the Cl- ions. The influx of H2O was inhibited by K+ removal, furosemide or high external Ba2+ (10 mmol l-1), but not by strophanthidin, ouabain or low concentrations of Ba2+ (0.5 mmol l-1). The influx could not continue with other permeable anions, NO3-, acetate- or SCN-, replacing Cl-. 5. In another series of experiments Cl- was removed from the ventricular solution of tissues bathed in saline solutions with normal concentrations of Cl-. The resulting efflux of Cl- was associated with an instantaneous efflux of K+ and H2O. This efflux of H2O could proceed against an osmotic gradient of up to 70 mosmol l-1. This effect was inhibited by furosemide, in which case the water fluxes were entirely dependent on the osmotic gradients and the osmotic water permeability Lp of the ventricular membrane. 6. The data suggest that there is a coupling between the flux of KCl and of water in the ventricular membrane, which implies that the reflection coefficient sigma for KCl under the given circumstances is less than one. I suggest that the ability of leaky epithelia to transport against osmotic gradients depends on such a coupling, which derives from the properties of the proteins through which K+, Cl- and H2O leave the cell.

Solubilization and partial purification of the thiazide diuretic receptor from rabbit renal cortex

This study was designed to solubilize, characterize and begin to purify the thiazide-sensitive Na/Cl transporter from mammalian kidney. Metolazone, a thiazide-like diuretic drug, binds to receptors in rat renal cortex closely related to the thiazide-sensitive Na/Cl transport pathway of the renal distal tubule. In the current study, [3H]metolazone bound to receptors in rabbit renal cortical microsomes. The portion of [3H]metolazone binding that was inhibited by hydrochlorothiazide reflected binding to a high-affinity class of receptor. The affinity (Kd 2.0 +/- 0.1 nM) and number (Bmax = 0.9 +/- 0.4 pmol/mg protein) of high-affinity receptors in rabbit renal cortical membranes were similar to values reported previously for rat. When proximal and distal tubule fragments were separated by Percoll gradient centrifugation, receptors were restricted to the fraction that contained distal tubules. When compared with cortical homogenates, receptor density was enriched 12-fold by magnesium precipitation and differential centrifugation. The zwitterionic detergent CHAPS solubilized 25-35% of the receptors (at 6 mM). Chloride inhibited and Na stimulated binding of [3H]metolazone to solubilized high-affinity receptors. The receptors could be purified significantly by hydroxyapatite chromatography and size exclusion high performance liquid chromatography (HPLC). The combination of magnesium precipitation and differential centrifugation, hydroxyapatite chromatography, and size exclusion HPLC resulted in a 213-fold enrichment of receptors, compared to renal cortical homogenate. The current results indicate that thiazide receptors from rabbit kidney share characteristics with receptors from rat, and that rabbit receptors can be solubilized in CHAPS and purified significantly by hydroxyapatite chromatography and size exclusion HPLC.

Inhibition of Na-K-C1 cotransport in Ehrlich ascites cells by antiserum against purified proteins of the cotransporter

Two proteins were purified earlier from solubilized membranes of Ehrlich ascites cells by using a bumetanide-Sepharose affinity column. These proteins were proposed to be constituents of the Na-K-C1 cotransporter. However, the specificity of binding of bumetanide to the cotransporter was insufficient evidence for this proposal. We now have direct evidence that the purified protein contains components of the cotransporter. Antiserum raised against the bumetanide-binding proteins strongly inhibits Na-K-C1 cotransport measured by two independent methods. Cotransport was induced by hypertonic challenge and was measured as the bumetanide-sensitive portion of unidirectional C1 influx and as regulatory cell volume increase. In both assays, cotransport was strongly inhibited by the antiserum. Fab fragments of the antibodies inhibited cotransport to a similar extent.

Mercuric chloride activates latent, anion-dependent cation transport systems in the plasma membrane of Ehrlich ascites tumour cells

Electronic cell sizing of Ehrlich ascites tumour cells is presented as a biological test system for assessment of membrane associated effects of toxic compounds. Ehrlich ascites tumour cells readjust their cell volume after osmotic swelling in hypotonic media. This regulatory process (Regulatory Volume Decrease, RVD) involves a net loss of KCl from the cells. Addition of HgCl2 (1 microM) results in a Cl- -dependent acceleration of RVD in hypotonic medium. Cells in isotonic Cl- -containing medium shrink upon addition of HgCl2 due to a Cl- -dependent net loss of K+. In addition, a Cl- -dependent net uptake of Na+ was also seen in the presence of HgCl2. It is concluded that HgCl2 activates a latent K+, Cl- cotransport as well as Na+, Cl- cotransport in Ehrlich cells.