Short-term regulation of the proximal tubule Na+,K+-ATPase: increased/decreased Na+,K+-ATPase activity mediated by protein kinase C isoforms

Unique Regulation of Na-K-ATPase during Growth and Maturation of Intestinal Epithelial Cells

Na-K-ATPase on the basolateral membrane provides the favorable transcellular Na gradient for the proper functioning of Na-dependent nutrient co-transporters on the brush border membrane (BBM) of enterocytes. As cells mature from crypts to villus, Na-K-ATPase activity doubles, to accommodate for the increased BBM Na-dependent nutrient absorption. However, the mechanism of increased Na-K-ATPase activity during the maturation of enterocytes is not known. Therefore, this study aimed to determine the mechanisms involved in the functional transition of Na-K-ATPase during the maturation of crypts to villus cells. Na-K-ATPase activity gradually increased as IEC-18 cells matured in vitro from day 0 (crypts) through day 4 (villus) of post-confluence. mRNA abundance and Western blot studies showed no change in the levels of Na-K-ATPase subunits α1 and β1 from 0 to 4 days post-confluent cells. However, Na-K-ATPase α1 phosphorylation levels on serine and tyrosine, but not threonine, residues gradually increased. These data indicate that as enterocytes mature from crypt-like to villus-like in culture, the functional activity of Na-K-ATPase increases secondary to altered affinity of the α1 subunit to extracellular K⁺, in order to accommodate the functional preference of the intestinal cell type. This altered affinity is likely due to increased phosphorylation of the α1 subunit, specifically at serine and tyrosine residues.

Protein phosphatase 2A interacts with the Na,K-ATPase and modulates its trafficking by inhibition of its association with arrestin

BACKGROUND

The P-type ATPase family constitutes a collection of ion pumps that form phosphorylated intermediates during ion transport. One of the best known members of this family is the Na⁺,K⁺-ATPase. The catalytic subunit of the Na⁺,K⁺-ATPase includes several functional domains that determine its enzymatic and trafficking properties.

METHODOLOGY/PRINCIPAL FINDINGS

Using the yeast two-hybrid system we found that protein phosphatase 2A (PP2A) catalytic C-subunit is a specific Na⁺,K⁺-ATPase interacting protein. PP-2A C-subunit interacted with the Na⁺,K⁺-ATPase, but not with the homologous sequences of the H⁺,K⁺-ATPase. We confirmed that the Na⁺,K⁺-ATPase interacts with a complex of A- and C-subunits in native rat kidney. Arrestins and G-protein coupled receptor kinases (GRKs) are important regulators of G-protein coupled receptor (GPCR) signaling, and they also regulate Na⁺,K⁺-ATPase trafficking through direct association. PP2A inhibits association between the Na⁺,K⁺-ATPase and arrestin, and diminishes the effect of arrestin on Na⁺,K⁺-ATPase trafficking. GRK phosphorylates the Na⁺,K⁺-ATPase and PP2A can at least partially reverse this phosphorylation.

CONCLUSIONS/SIGNIFICANCE

Taken together, these data demonstrate that the sodium pump belongs to a growing list of ion transport proteins that are regulated through direct interactions with the catalytic subunit of a protein phosphatase.

Blocking the salt-inducible kinase 1 network prevents the increases in cell sodium transport caused by a hypertension-linked mutation in human alpha-adducin

OBJECTIVES

Because a newly described salt-inducible kinase 1 (SIK1) network is responsible for increases in active cell sodium transport in response to elevated intracellular sodium, we hypothesized that this network could mediate the effects of the mutant (hypertensive) form of alpha-adducin on Na,K-ATPase activity.

METHODS

Studies were performed in normotensive and hypertensive Milan rats and in a cell line of proximal tubule origin expressing transiently variants of alpha-adducin (human G460W/S586C; rat F316Y) that are associated with elevated blood pressure and result in increased Na,K-ATPase activity. Na,K-ATPase activity was determined as ouabain-sensitive rubidium transport.

RESULTS

SIK1 activity (T182 phosphorylation) was significantly elevated in renal proximal tubule cells from Milan hypertensive rats (carrying a alpha-adducin mutation) when compared with normotensive controls. Similarly, SIK1 activity (T182 phosphorylation) was elevated in a normal renal proximal tubule cell line when transfected with the alpha-adducin variant carrying the human hypertensive mutation. Blocking the SIK1 network using negative mutants as well as different stages of its activation pathway prevented the effects induced by the hypertensive alpha-adducin.

CONCLUSION

The SIK1 network may constitute an alternative target by which agents can modulate active sodium transport in renal epithelia and avoid the increases in systemic blood pressure that are associated with genetic mutations in the human alpha-adducin molecule.

Trafficking of Na-K-ATPase and dopamine receptor molecules induced by changes in intracellular sodium concentration of renal epithelial cells

Most of the transepithelial transport of sodium in proximal tubules occurs through the coordinated action of the apical sodium/proton exchanger and the basolateral Na-K-ATPase. Hormones that regulate proximal tubule sodium excretion regulate the activities of these proteins. We have previously demonstrated that the level of intracellular sodium concentration modulates the regulation of Na-K-ATPase activity by angiotensin II and dopamine. An increase of a few millimolars in intracellular sodium concentration leads to increased Na-K-ATPase activity without a statistically significant increase in the number of plasma membrane Na-K-ATPase molecules, as determined by cell surface protein biotinylation. Using total internal reflection fluorescence, we detected an increased number of Na-K-ATPase molecules in cytosolic compartments adjacent to the plasma membrane, suggesting that the increased intracellular sodium concentration induces a movement of Na-K-ATPase molecules toward the plasma membrane. While intracellular compartments containing Na-K-ATPase molecules are very close to the plasma membrane, compartments containing type 1 dopamine receptors (D1Rs) are distributed in different parts of the cell cytosol. Fluorescence determinations indicate that an increased intracellular sodium concentration induces the increased colocalization of dopamine receptors with Na-K-ATPase molecules in the region of the plasma membrane. We propose that under in vivo conditions, in response to a sodium load in the lumen of proximal tubules, an increased level of intracellular sodium in epithelial cells is an early event that triggers the cellular response that leads to dopamine inhibition of proximal tubule sodium reabsorption.

Regulation of sodium pump endocytosis by cardiotonic steroids: Molecular mechanisms and physiological implications

We have previously shown that ouabain and other cardiotonic steroids interact with the plasmalemmal Na/K-ATPase and cause a time and dose dependent endocytosis of the Na/K-ATPase. This endocytosis is demonstrable using fluorescence imaging as well as conventional biochemical and biophysical cell separation methods. In proximal tubule cells, this process appears to regulate the density of basolateral Na/K-ATPase expression directly as well as indirectly modulate transepithelial sodium transport. Work with genetic manipulations, as well as pharmacological agents with cell culture models, have demonstrated that the cardiotonic steroid stimulated endocytosis of the plasmalemmal Na/K-ATPase requires caveolin and clathrin as well as the activation of c-Src, transactivation of the EGFR and activation of PI3K. Interestingly c-Src, EGFR and ERK1/2 all appear to be endocytosed along with the plasmalemmal Na/K-ATPase. These observations suggest a close analogy between a subset of plasmalemmal Na/K-ATPase and signaling companions with conventional receptor tyrosine kinases. While further studies are necessary to delineate the role of this endocytosis in the generation as well as the limit of signal transduction through the Na/K-ATPase signal cascade, we propose that it has an important role in the regulation of renal sodium handling as well as other important processes.

Impairment of Na(+),K(+)-ATPase in CD95(APO-1)-induced human T-cell leukemia cell apoptosis mediated by glutathione depletion and generation of hydrogen peroxide

Human T-cell leukemia is a malignant disease that needs various regimens of cytotoxic chemotherapy to overcome drug resistance. Recently, Na(+),K(+)-ATPase has emerged as a potential target for cancer therapy. However, its exact signaling pathway in human T-cell leukemia cell death has not been well defined. In the current study, we found CD95(APO-1) was able to trigger the internalization of plasma membrane Na(+),K(+)-ATPase in Jurkat cells or primary T cells as a mechanism to suppress its activity. This internalization was closely relevant to intracellular glutathione (GSH) depletion in Jurkat cells downstream of Fas-associated death domain protein (FADD) and caspase 8. GSH depletion in Fas L-treated Jurkat cells induced the generation of hydrogen peroxide (H(2)O(2)), which subsequently increased the serine phosphorylation of Na(+),K(+)-ATPase alpha1 subunit. Exogenous H(2)O(2) even mimicked the effect of Fas L to upregulate the serine phosphorylation of Na(+),K(+)-ATPase alpha1 subunit and suppress Na(+),K(+)-ATPase activity. Overall, our results indicate that CD95(APO-1) induces the FADD- and caspase 8-dependent internalization of Na(+),K(+)-ATPase through intracellular GSH loss, and the subsequent generation of H(2)O(2)-mediated serine phosphorylation of Na(+),K(+)-ATPase alpha1 subunit. Taken together, this study presents a novel regulatory mechanism of Na(+),K(+)-ATPase in CD95(APO-1)-mediated human T-leukemia cell apoptosis.

Localization of intracellular compartments that exchange Na,K-ATPase molecules with the plasma membrane in a hormone-dependent manner

BACKGROUND AND PURPOSE

Dopamine is a major regulator of sodium reabsorption in proximal tubule epithelia. By binding to D1-receptors, dopamine induces endocytosis of plasma membrane Na,K-ATPase, resulting in a reduced capacity of the cells to transport sodium, thus contributing to natriuresis. We have previously demonstrated several aspects of the molecular mechanism by which dopamine induces Na,K-ATPase endocytosis; however, the location of intracellular compartments containing Na,K-ATPase molecules has not been identified.

EXPERIMENTAL APPROACH

In this study, we used different approaches to determine the localization of Na,K-ATPase-containing intracellular compartments. By expression of fluorescent-tagged Na,K-ATPase molecules in opossum kidney cells, a cell culture model of proximal tubule epithelia, we used fluorescence microscopy to determine cellular distribution of the fluorescent molecules and the effects of dopamine on this distribution. By labelling cell surface Na,K-ATPase molecules from the cell exterior with either biotin or an epitope-tagged antibody, we determined the localization of the tagged Na,K-ATPase molecules after endocytosis induced by dopamine.

KEY RESULTS

In cells expressing fluorescent-tagged Na,K-ATPase molecules, there were intracellular compartments containing Na,K-ATPase molecules. These compartments were in very close proximity to the plasma membrane. Upon treatment of the cells with dopamine, the fluorescence labelling of these compartments was increased. The labelling of these compartments was also observed when the endocytosis of biotin- or antibody-tagged plasma membrane Na,K-ATPase molecules was induced by dopamine.

CONCLUSIONS AND IMPLICATIONS

The intracellular compartments containing Na,K-ATPase molecules are located just underneath the plasma membrane.

Role of 20-HETE in D1/D2 dopamine receptor synergism resulting in the inhibition of Na+-K+-ATPase activity in the proximal tubule

Previous studies propose 20-hydroxyeicosatetraenoic acid (20-HETE), a major arachidonic acid metabolite of cytochrome P-450 (CYP), as a possible mediator of Na(+)-K(+)-ATPase inhibition by dopamine (DA). The aim of this study was to investigate the intracellular mechanisms involved in this effect and to elucidate the DA receptor associated with the 20-HETE pathway in the rat kidney. DA (10(-5) M) inhibited Na(+)-K(+)-ATPase activity in microdissected tubular segments to 59.4 +/- 3.8% of control activity. This response was suppressed by the CYP4A inhibitor 17-octadecynoic acid (10(-6) M), which had no effect per se, thus confirming the participation of CYP arachidonic acid metabolites in DA-induced Na(+)-K(+)-ATPase inhibition. We next examined whether 20-HETE is involved in the signaling pathways triggered by either D(1) or D(2) receptors. Neither fenoldopam nor quinpirole (D(1) and D(2) agonists, respectively, both 10(-5) M) modified Na(+)-K(+)-ATPase activity when tried alone. However, coincubation of a threshold concentration of 20-HETE (10(-9) M) with fenoldopam resulted in a synergistic inhibition of Na(+)-K(+)-ATPase activity (66 +/- 2% of control activity), while 20-HETE plus quinpirole had no effect. Furthermore, 20-HETE (10(-9) M) synergized with forskolin (10(-5) M) and with the diacylglycerol analog 1-oleoyl-2-acetoyl-sn-glycerol (OAG; 10(-11) M; 62.0 +/- 5.3 and 69.9 +/- 2.0% of control activity, respectively), indicating a cooperative role of 20-HETE with the D(1)-triggered pathways. In line with these results, no additive effect was observed when OAG and 20-HETE were combined at concentrations which per se produced maximal inhibition (10(-6) M). These results demonstrate that the inhibition of Na(+)-K(+)-ATPase activity by DA in the proximal tubule may be the result of the synergism between 20-HETE and the D(1) signaling pathway.

Na,K‐ATPase α1‐subunit dephosphorylation by protein phosphatase 2A is necessary for its recruitment to the plasma membrane

In alveolar epithelial cells, G-protein coupled-receptors agonists (GPCR) induce the recruitment of the Na,K-ATPase to the plasma membrane. Here we report that for the recruitment of the Na,K-ATPase to occur, dephosphorylation of its alpha1-subunit at serine 18 is necessary, as demonstrated by in vitro phosphorylation, mutation of the serine 18 to alanine, and use of a specific phospho-antibody. Several approaches strongly suggest dephosphorylation to be mediated by protein phosphatase 2A (PP2A): 1) Na,K-ATPase dephosphorylation and recruitment were prevented by okadaic acid (OA); 2) the Na,K-ATPase alpha1-subunit is an in vitro substrate for PP2A; and 3) glutathione S-transferase (GST)-fusion proteins binding assays demonstrate a direct interaction between the catalytic subunit of PP2A and the first 90 amino acids of the Na,K-ATPase alpha1-subunit. Finally, GPCR agonists induced a rapid translocation of PP2A from the cytosol to the membrane fraction, which corresponded with increased coimmunoprecipitation and colocalization of PP2A and the Na,K-ATPase. Accordingly, we provide evidence that GPCR agonists promote PP2A translocation to the membrane fraction, leading to the dephosphorylation of the Na,K-ATPase alpha1-subunit at the serine 18 residue and its recruitment to the cell plasma membrane, which is of biological and physiological importance.

Inhibition of Na,K-ATPase by Dopamine in Proximal Tubule Epithelial Cells

In the current report we review the results that lay grounds for the model of intracellular sodium-mediated dopamine-induced endocytosis of Na,K-ATPase. Under conditions of a high salt diet, dopamine activates PKCzeta, which phosphorylates NKA alpha1 Ser-18. The phosphorylation produces a conformational change of alpha1 NH2-terminus, which through interaction with other domains of alpha1 exposes PI3K- and AP-2-binding domains. PI3K bound to the NKA alpha1 induces the recruitment and activation of other proteins involved in endocytosis, and PI3K-generated 3-phosphoinositides affect the local cytoskeleton and modify the biophysical conditions of the membrane for development of clathrin-coated pits. Plasma membrane phosphorylated NKA is internalized to specialized intracellular compartments where the NKA will be dephosphorylated. The NKA internalization results in a reduced Na+ transport by proximal tubule epithelial cells.

The 14-3-3 protein translates the NA+,K+-ATPase {alpha}1-subunit phosphorylation signal into binding and activation of phosphoinositide 3-kinase during endocytosis

Clathrin-dependent endocytosis of Na(+),K(+)-ATPase molecules in response to G protein-coupled receptor signals is triggered by phosphorylation of the alpha-subunit and the binding of phosphoinositide 3-kinase. In this study, we describe a molecular mechanism linking phosphorylation of Na(+),K(+)-ATPase alpha-subunit to binding and activation of phosphoinositide 3-kinase. Co-immunoprecipitation studies, as well as experiments using confocal microscopy, revealed that dopamine favored the association of 14-3-3 protein with the basolateral plasma membrane and its co-localization with the Na(+),K(+)-ATPase alpha-subunit. The functional relevance of this interaction was established in opossum kidney cells expressing a 14-3-3 dominant negative mutant, where dopamine failed to decrease Na(+),K(+)-ATPase activity and to promote its endocytosis. The phosphorylated Ser-18 residue within the alpha-subunit N terminus is critical for 14-3-3 binding. Activation of phosphoinositide 3-kinase by dopamine during Na(+),K(+)-ATPase endocytosis requires the binding of the kinase to a proline-rich domain within the alpha-subunit, and this effect was blocked by the presence of a 14-3-3 dominant negative mutant. Thus, the 14-3-3 protein represents a critical linking mechanism for recruiting phosphoinositide 3-kinase to the site of Na(+),K(+)-ATPase endocytosis.

Evidence for a role of protein kinase C-alpha in urine concentration

In mouse kidney, the conventional protein kinase C (PKC) isoenzyme alpha is expressed in glomeruli, the cortical collecting duct (intercalated cells only), and medullary collecting duct. To get insights on its function, PKC-alpha knockout (-/-) and wild-type (+/+) mice were studied. When provided free access to water, PKC-alpha -/- mice showed approximately 50% greater urine flow rate and lower urinary osmolality in 24-h metabolic cage experiments despite a greater urinary vasopressin-to-creatinine ratio vs. PKC-alpha +/+ mice. Renal albumin excretion was not different. Clearance experiments under inactin/ketamine anesthesia revealed a modestly reduced glomerular filtration rate and showed a reduced absolute and fractional renal fluid reabsorption in PKC-alpha -/- mice. The sodium-restricting response to a low-sodium diet was unaffected in PKC-alpha -/- mice. Urinary osmolality was reduced to similar hypotonic levels in PKC-alpha -/- and +/+ mice during acute oral water loading or application of the vasopressin V(2)-receptor antagonist SR-121463. In comparison, the lower urinary osmolality observed in PKC-alpha -/- mice vs. wild-type mice under basal conditions persisted during water restriction for 36 h. In conclusion, PKC-alpha appears not to play a major role in renal sodium reabsorption but, consistent with its expression in the medullary collecting duct, contributes to urinary concentration in mice. Considering that PKC-beta I and -beta II are coexpressed with PKC-alpha in mouse medullary collecting duct, the present results indicate that conventional PKC isoenzymes cannot fully compensate for each other.

Relationship between PPARalpha activation and NO on proximal tubular Na+ transport in the rat

BACKGROUND

Nitric oxide (NO) regulates renal proximal tubular (PT) Na+ handling through modulation of Na+-K+ ATPase. Peroxisome Proliferator Activated Receptor alpha (PPARalpha), a nuclear transcription factor, is expressed in PTs and has been reported to influence NO generation/activity in renal tissues. This study tested the hypothesis that PPARalpha interacts with NO and thereby affects renal tubular Na+ transport. Urinary excretion of nitrite (UNOXV) and Na+ (UNaV) and PT Na+ transport (Na+-K+ ATPase activity) were determined in rats treated with clofibrate (250 mg/kg i.p) or WY14643 (45 mg/kg; i.p.), a PPARalpha ligand, 2% NaCl (orally), clofibrate/NaCl, L-NAME, an inhibitor of NO production (100 mg/kg; orally), L-NAME/Clofibrate.

RESULTS

Clofibrate or WY14643 increased PPARalpha expression by 106 +/- 7% (p < 0.05) and 113 +/- 8% (p < 0.05), respectively. Similarly, clofibrate and WY14643 increased expression of MCAD, a downstream target protein of PPARalpha by 123 +/- 8% (p < 0.05) and 143 +/- 8% (p < 0.05), respectively. L-NAME attenuated clofibrate-induced increase in PPARalpha expression by 27 +/- 2% (p < 0.05) but did not affect MCAD expression. UNOXV excretion increased 3-4 fold in rats treated with clofibrate, WY14643 or NaCl from 44 +/- 7 to 170 +/- 15, 144 +/- 18 or 132 +/- 11 nmol/24 hr, respectively (p < 0.05). Similarly, clofibrate, WY14643 or NaCl elicited a 2-5 fold increase in UNaV. L-NAME significantly reduced basal UNOXV and UNaV and abolished the clofibrate-induced increase. Clofibrate, WY14643, NaCl or clofibrate + NaCl treatment reduced Na+-K+-ATPase activity in the PT by 89 +/- 23, 62 +/- 10, 43 +/- 9 and 82 +/- 15% (p < 0.05), respectively. On the contrary, L-NAME or ODQ, inhibitor of sGC, abolished the inhibition of Na+-K+-ATPase activity by clofibrate (p < 0.05). Clofibrate either alone or with NaCl elicited approximately 2-fold increase in the expression of the alpha1 subunit of Na+-K+ ATPase in the PT while L-NAME abolished clofibrate-induced increase in Na+-K+ ATPase expression.

CONCLUSION

These data suggest that PPARalpha activation, through increased NO generation promotes renal excretion of Na+ through reduced Na+-K+ ATPase activity in the PT probably via post translational modification of Na+-K+-ATPase.

Basic fibroblast growth factor (bFGF) regulation of the plasma membrane calcium ATPase (PMCA) as part of an anti-apoptotic mechanism of action

Basic fibroblast growth factor (bFGF) preserves the viability of at least 13 different cells, including epithelial, endothelial, smooth muscle and neuronal cells. In spite of this profound and rather universal effect on cell viability, detailed studies regarding the mechanism of bFGF's action have not been conducted. Rather, most studies have simply shown that bFGF inhibits cells from undergoing programmed cell death (i.e. apoptosis). The most mechanistic studies to date have been conducted on either neurons or ovarian (granulosa) cells. These studies have shown that bFGF prevents apoptosis through both genomic and acute actions. Basic FGF's acute actions involved the maintenance of normal levels of intracellular free calcium levels ([Ca(2+)](i)). In granulosa cells, bFGF maintained [Ca(2+)](i) through a protein kinase C(delta) (PKCdelta)-dependent mechanism. Further, bFGF-activated PKCdelta maintained [Ca(2+)](i) by stimulating calcium efflux. The ability of bFGF to stimulate calcium efflux involved the plasma membrane calcium ATPase (PMCA). Interestingly, bFGF-activated PKCdelta appeared to regulate PMCA activity in part by promoting its membrane localization.

Regulation of Na,K-ATPase Function in the Lens

The two cell types in the lens, epithelium and fiber, have a very different specific activity of Na,K-ATPase; activity is much higher in the epithelium. However, judged by Western blot, fibers and epithelium express a similar amount of both Na,K-ATPase alpha and beta subunit proteins. Na,K-ATPase protein abundance does not tally with Na,K-ATPase activity. Studies were conducted to examine whether protein synthesis plays a role in maintenance of the high Na,K-ATPase activity in lens epithelium. An increase of cytoplasmic sodium was found to increase Na,K-ATPase protein expression in the epithelium, but not in the fibers. The findings illustrate the ability of lens epithelium to synthesize new Na,K-ATPase protein as a way to boost Na,K-ATPase in response to cell damage or pathological events. Methionine incorporation studies suggested Na,K-ATPase synthesis may also play a role in day to day preservation of high Na,K-ATPase activity. Na,K-ATPase protein in lens epithelial cells appeared to be continually synthesized and degraded. Experiments with cycloheximide suggest that specific activity of Na,K-ATPase in the lens epithelium may depend on the ability of the cells to continuously synthesize fresh Na,K-ATPase proteins. However, other factors such as phosphorylation of Na,K-ATPase alpha subunit may also influence Na,K-ATPase activity. When intact lenses were exposed to the agonist thrombin, Na,K-ATPase activity was diminished, but the response was suppressed by inhibitors of the Src family of non-receptor tyrosine kinases. Thrombin elicited tyrosine phosphorylation of lens epithelium membrane proteins, including a 100 kDa protein band thought to be the Na,K-ATPase alpha 1 subunit. It remains to be determined whether a tyrosine phosphorylation mechanism contributes to the low activity of Na,K-ATPase in lens fibers.

Intracellular Na+ regulates dopamine and angiotensin II receptors availability at the plasma membrane and their cellular responses in renal epithelia

The balance and cross-talk between natruretic and antinatruretic hormone receptors plays a critical role in the regulation of renal Na+ homeostasis, which is a major determinant of blood pressure. Dopamine and angiotensin II have antagonistic effects on renal Na+ and water excretion, which involves regulation of the Na+,K+-ATPase activity. Herein we demonstrate that angiotensin II (Ang II) stimulation of AT1 receptors in proximal tubule cells induces the recruitment of Na+,K+-ATPase molecules to the plasmalemma, in a process mediated by protein kinase Cbeta and interaction of the Na+,K+-ATPase with adaptor protein 1. Ang II stimulation led to phosphorylation of the alpha subunit Ser-11 and Ser-18 residues, and substitution of these amino acids with alanine residues completely abolished the Ang II-induced stimulation of Na+,K+-ATPase-mediated Rb+ transport. Thus, for Ang II-dependent stimulation of Na+,K+-ATPase activity, phosphorylation of these serine residues is essential and may constitute a triggering signal for recruitment of Na+,K+-ATPase molecules to the plasma membrane. When cells were treated simultaneously with saturating concentrations of dopamine and Ang II, either activation or inhibition of the Na+,K+-ATPase activity was produced dependent on the intracellular Na+ concentration, which was varied in a very narrow physiological range (9-19 mm). A small increase in intracellular Na+ concentrations induces the recruitment of D1 receptors to the plasma membrane and a reduction in plasma membrane AT1 receptors. Thus, one or more proteins may act as an intracellular Na+ concentration sensor and play a major regulatory role on the effect of hormones that regulate proximal tubule Na+ reabsorption.

Regulation and critical role of potassium homeostasis in apoptosis

Programmed cell death or apoptosis is broadly responsible for the normal homeostatic removal of cells and has been increasingly implicated in mediating pathological cell loss in many disease states. As the molecular mechanisms of apoptosis have been extensively investigated a critical role for ionic homeostasis in apoptosis has been recently endorsed. In contrast to the ionic mechanism of necrosis that involves Ca(2+) influx and intracellular Ca(2+) accumulation, compelling evidence now indicates that excessive K(+) efflux and intracellular K(+) depletion are key early steps in apoptosis. Physiological concentration of intracellular K(+) acts as a repressor of apoptotic effectors. A huge loss of cellular K(+), likely a common event in apoptosis of many cell types, may serve as a disaster signal allowing the execution of the suicide program by activating key events in the apoptotic cascade including caspase cleavage, cytochrome c release, and endonuclease activation. The pro-apoptotic disruption of K(+) homeostasis can be mediated by over-activated K(+) channels or ionotropic glutamate receptor channels, and most likely, accompanied by reduced K(+) uptake due to dysfunction of Na(+), K(+)-ATPase. Recent studies indicate that, in addition to the K(+) channels in the plasma membrane, mitochondrial K(+) channels and K(+) homeostasis also play important roles in apoptosis. Investigations on the K(+) regulation of apoptosis have provided a more comprehensive understanding of the apoptotic mechanism and may afford novel therapeutic strategies for apoptosis-related diseases.

Direct activation of gastric H,K-ATPase by N-terminal protein kinase C phosphorylation. Comparison of the acute regulation mechanisms of H,K-ATPase and Na,K-ATPase

In this study we compared the protein kinase dependent regulation of gastric H,K-ATPase and Na,K-ATPase. The protein kinase A/protein kinase C (PKA/PKC) phosphorylation profile of H,K-ATPase was very similar to the one found in the Na,K-ATPase. PKC phosphorylation was taking place in the N-terminal part of the alpha-subunit with a stoichiometry of approximately 0.6 mol Pi/mole alpha-subunit. PKA phosphorylation was in the C-terminal part and required detergent, as is also found for the Na,K-ATPase. The stoichiometry of PKA-induced phosphorylation was approximately 0.7 mol Pi/mole alpha-subunit. Controlled proteolysis of the N-terminus abolished PKC phosphorylation of native H,K-ATPase. However, after detergent treatment additional C-terminal PKC sites became exposed located at the beginning of the M5M6 hairpin and at the cytoplasmic L89 loop close to the inner face of the plasma membrane. N-terminal PKC phosphorylation of native H,K-ATPase alpha-subunit was found to stimulate the maximal enzyme activity by 40-80% at saturating ATP, depending on pH. Thus, a direct modulation of enzyme activity by PKC phosphorylation could be demonstrated that may be additional to the well-known regulation of acid secretion by recruitment of H,K-ATPase to the apical membranes of the parietal cells. Moreover, a distinct difference in the regulation of H,K-ATPase and Na,K-ATPase is the apparent absence of any small regulatory proteins associated with the H,K-ATPase.

Hormonal-dependent recruitment of Na+,K+-ATPase to the plasmalemma is mediated by PKC beta and modulated by [Na+]i

1. The present study demonstrates that stimulation of hormonal receptors of proximal tubule cells with the serotonin-agonist 8-hydroxy-2-(di-n-propylamino) tetraline (8-OH-DPAT) induces an augmentation of Na(+),K(+)-ATPase activity that results from the recruitment of enzyme molecules to the plasmalemma. 2. Cells expressing the rodent wild-type Na(+),K(+)-ATPase alpha-subunit had the same basal Na(+),K(+)-ATPase activity as cells expressing the alpha-subunit S11A or S18A mutants, but stimulation of Na(+),K(+)-ATPase activity was completely abolished in either mutant. 3. 8-OH-DPAT treatment of OK cells led to PKC(beta)-dependent phosphorylation of the alpha-subunit Ser-11 and Ser-18 residues, and determination of enzyme activity with the S11A and S18A mutants indicated that both residues are essential for the agonist-dependent stimulation of Na(+),K(+)-ATPase activity. 4. When cells were treated with both dopamine and 8-OH-DPAT, an activation of Na(+),K(+)-ATPase was observed at basal intracellular sodium concentration (approximately 9 mM), and this activation was gradually reduced and became a significant inhibition as the concentration of intracellular sodium gradually increased from 9 to 19 mM. Thus, besides the antagonistic effects of dopamine and 8-OH-DPAT, intracellular sodium modulates whether an activation or an inhibition of Na(+),K(+)-ATPase is produced.

Biochemistry of Na,K-ATPase

The Na,K-ATPase or sodium pump carries out the coupled extrusion and uptake of Na and K ions across the plasma membranes of cells of most higher eukaryotes. It is a member of the P-type ATPase superfamily. This heterodimeric integral membrane protein is composed of a 100-kDa alpha-subunit with ten transmembrane segments and a heavily glycosylated beta subunit of about 55 kDa, which is a type II membrane protein. Current ideas on how the protein achieves active transport are based on a fusion of results of transport physiology, protein chemistry, and heterologous expression of mutant proteins. Recently acquired high resolution structural information provides an important new avenue for a more complete understanding of this protein. In this review, the current status of knowledge of Na,K-ATPase is discussed, and areas where there is still considerable uncertainty are highlighted.