PKC-beta and PKC-zeta mediate opposing effects on proximal tubule Na+,K+-ATPase activity

Sensational site: the sodium pump ouabain-binding site and its ligands

Cardiotonic steroids (CTS), used by certain insects, toads, and rats for protection from predators, became, thanks to Withering's trailblazing 1785 monograph, the mainstay of heart failure (HF) therapy. In the 1950s and 1960s, we learned that the CTS receptor was part of the sodium pump (NKA) and that the Na+/Ca2+ exchanger was critical for the acute cardiotonic effect of digoxin- and ouabain-related CTS. This "settled" view was upended by seven revolutionary observations. First, subnanomolar ouabain sometimes stimulates NKA while higher concentrations are invariably inhibitory. Second, endogenous ouabain (EO) was discovered in the human circulation. Third, in the DIG clinical trial, digoxin only marginally improved outcomes in patients with HF. Fourth, cloning of NKA in 1985 revealed multiple NKA α and β subunit isoforms that, in the rodent, differ in their sensitivities to CTS. Fifth, the NKA is a cation pump and a hormone receptor/signal transducer. EO binding to NKA activates, in a ligand- and cell-specific manner, several protein kinase and Ca2+-dependent signaling cascades that have widespread physiological effects and can contribute to hypertension and HF pathogenesis. Sixth, all CTS are not equivalent, e.g., ouabain induces hypertension in rodents while digoxin is antihypertensinogenic ("biased signaling"). Seventh, most common rodent hypertension models require a highly ouabain-sensitive α2 NKA and the elevated blood pressure is alleviated by EO immunoneutralization. These numerous phenomena are enabled by NKA's intricate structure. We have just begun to understand the endocrine role of the endogenous ligands and the broad impact of the ouabain-binding site on physiology and pathophysiology.

Long-QT mutations in KCNE1 modulate the 17β-estradiol response of Kv7.1/KCNE1

Estradiol (17[Formula: see text]-E2) is implicated in higher arrhythmia risk of women with congenital or acquired long-QT syndrome (LQTS) compared to men. However, the underlying mechanisms remain poorly understood, and little is known about the impact of LQTS-associated mutations. We show that 17[Formula: see text]-E2 inhibits the human cardiac Kv7.1/KCNE1 channel expressed in Xenopus oocytes. We find that the 17[Formula: see text]-E2 effect depends on the Kv7.1 to KCNE1 stoichiometry, and we reveal a critical function of the KCNE1 carboxyl terminus for the effect. LQTS-associated mutations in the KCNE1 carboxyl terminus show a range of responses to 17[Formula: see text]-E2, from a wild-type like response to impaired or abolished response. Together, this study increases our understanding of the mechanistic basis for 17[Formula: see text]-E2 inhibition of Kv7.1/KCNE1 and demonstrates mutation-dependent responses to 17[Formula: see text]-E2. These findings suggest that the 17[Formula: see text]-E2 effect on Kv7.1/KCNE1 might contribute to the higher arrhythmia risk of women, particularly in carriers with specific LQTS-associated mutations.

The tubular hypothesis of nephron filtration and diabetic kidney disease

Kidney size and glomerular filtration rate (GFR) often increase with the onset of diabetes, and elevated GFR is a risk factor for the development of diabetic kidney disease. Hyperfiltration mainly occurs in response to signals passed from the tubule to the glomerulus: high levels of glucose in the glomerular filtrate drive increased reabsorption of glucose and sodium by the sodium-glucose cotransporters SGLT2 and SGLT1 in the proximal tubule. Passive reabsorption of chloride and water also increases. The overall capacity for proximal reabsorption is augmented by growth of the proximal tubule, which (alongside sodium-glucose cotransport) further limits urinary glucose loss. Hyperreabsorption of sodium and chloride induces tubuloglomerular feedback from the macula densa to increase GFR. In addition, sodium-glucose cotransport by SGLT1 on macula densa cells triggers the production of nitric oxide, which also contributes to glomerular hyperfiltration. Although hyperfiltration restores sodium and chloride excretion it imposes added physical stress on the filtration barrier and increases the oxygen demand to drive reabsorption. Tubular growth is associated with the development of a senescence-like molecular signature that sets the stage for inflammation and fibrosis. SGLT2 inhibitors attenuate the proximal reabsorption of sodium and glucose, normalize tubuloglomerular feedback signals and mitigate hyperfiltration. This tubule-centred model of diabetic kidney physiology predicts the salutary effect of SGLT2 inhibitors on hard renal outcomes, as shown in large-scale clinical trials.

Regulation of Neuronal Na+/K+-ATPase by Specific Protein Kinases and Protein Phosphatases

The Na⁺/K⁺-ATPase (NKA) is a ubiquitous membrane-bound enzyme responsible for generating and maintaining the Na⁺ and K⁺ electrochemical gradients across the plasmalemma of living cells. Numerous studies in non-neuronal tissues have shown that this transport mechanism is reversibly regulated by phosphorylation/dephosphorylation of the catalytic α subunit and/or associated proteins. In neurons, Na⁺/K⁺ transport by NKA is essential for almost all neuronal operations, consuming up to two-thirds of the neuron's energy expenditure. However, little is known about its cellular regulatory mechanisms. Here we have used an electrophysiological approach to monitor NKA transport activity in male rat hippocampal neurons in situ We report that this activity is regulated by a balance between serine/threonine phosphorylation and dephosphorylation. Phosphorylation by the protein kinases PKG and PKC inhibits NKA activity, whereas dephosphorylation by the protein phosphatases PP-1 and PP-2B (calcineurin) reverses this effect. Given that these kinases and phosphatases serve as downstream effectors in key neuronal signaling pathways, they may mediate the coupling of primary messengers, such as neurotransmitters, hormones, and growth factors, to the NKAs, through which multiple brain functions can be regulated or dysregulated.SIGNIFICANCE STATEMENT The Na⁺/K⁺-ATPase (NKA), known as the "Na⁺ pump," is a ubiquitous membrane-bound enzyme responsible for generating and maintaining the Na⁺ and K⁺ electrochemical gradients across the plasma membrane of living cells. In neurons, as in most types of cells, the NKA generates the negative resting membrane potential, which is the basis for almost all aspects of cellular function. Here we used an electrophysiological approach to monitor physiological NKA transport activity in single hippocampal pyramidal cells in situ We have found that neuronal NKA activity is oppositely regulated by phosphorylation and dephosphorylation, and we have identified the main protein kinases and phosphatases mediating this regulation. This fundamental form of NKA regulation likely plays a role in multiple brain functions.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

Crosstalk between dopamine receptors and the Na⁺/K⁺-ATPase (review)

Dopamine (DA) receptors, which belong to the G protein-coupled receptor family, are the target of ~50% of all modern medicinal drugs and constitute a large and diverse class of proteins whose primary function is to transduce extracellular stimuli into intracellular signals. Na+/K+-ATPase (NKA) is ubiquitous and crucial for the maintenance of intracellular ion homeostasis and excitability. Furthermore, it plays a critical role in diverse effects, including clinical cardiotonic and cardioprotective effects, ischemic preconditioning in the brain, natriuresis, lung edema clearance and other processes. NKA regulation is of physiological and pharmacological importance and has species- and tissue-specific variations. The activation of DA receptors regulates NKA expression/activity and trafficking in various tissues and cells, for example in the kidney, lung, intestine, brain, non-pigmented ciliary epithelium and the vascular bed. DA receptor-mediated regulation of NKA mediates a diverse range of cellular responses and includes endocytosis/exocytosis, phosphorylation/dephosphorylation of the α subunit of NKA and multiple signaling pathways, including phosphatidylinositol (PI)-phospholipase C/protein kinase (PK) C, cAMP/PKA, PI3K, adaptor protein 2, tyrosine phosphatase and mitogen-activated protein kinase/extracellular signal-regulated protein kinase. Furthermore, in brain and HEK293T cells, D1 and D2 receptors exist in a complex with NKA. Among D1 and D2 receptors and NKA, regulations are reciprocal, which leads to crosstalk between DA receptors and NKA. In the present study, the current understanding of signaling mechanisms responsible for the crosstalk between DA receptors and NKA, as well as with specific consequent functions, is reviewed.

Renal function in diabetic disease models: the tubular system in the pathophysiology of the diabetic kidney

Diabetes mellitus affects the kidney in stages. At the onset of diabetes mellitus, in a subset of diabetic patients the kidneys grow large, and glomerular filtration rate (GFR) becomes supranormal, which are risk factors for developing diabetic nephropathy later in life. This review outlines a pathophysiological concept that focuses on the tubular system to explain these changes. The concept includes the tubular hypothesis of glomerular filtration, which states that early tubular growth and sodium-glucose cotransport enhance proximal tubule reabsorption and make the GFR supranormal through the physiology of tubuloglomerular feedback. The diabetic milieu triggers early tubular cell proliferation, but the induction of TGF-β and cyclin-dependent kinase inhibitors causes a cell cycle arrest and a switch to tubular hypertrophy and a senescence-like phenotype. Although this growth phenotype explains unusual responses like the salt paradox of the early diabetic kidney, the activated molecular pathways may set the stage for tubulointerstitial injury and diabetic nephropathy.

Dopamine D1 receptor-mediated inhibition of NADPH oxidase activity in human kidney cells occurs via protein kinase A-protein kinase C cross talk

Dopamine cellular signaling via the D(1) receptor (D(1)R) involves both protein kinase A (PKA) and protein kinase C (PKC), but the PKC isoform involved has not been determined. Therefore, we tested the hypothesis that the D(1)R-mediated inhibition of NADPH oxidase activity involves cross talk between PKA and a specific PKC isoform(s). In HEK-293 cells heterologously expressing human D(1)R (HEK-hD(1)), fenoldopam, a D(1)R agonist, and phorbol 12-myristate 13-acetate (PMA), a PKC activator, inhibited oxidase activity in a time- and concentration-dependent manner. The D(1)R-mediated inhibition of oxidase activity (68.1±3.6%) was attenuated by two PKA inhibitors, H89 (10μmol/L; 88±8.1%) and Rp-cAMP (10μmol/L; 97.7±6.7%), and two PKC inhibitors, bisindolylmaleimide I (1μmol/L; 94±6%) and staurosporine (10nmol/L; 93±8%), which by themselves had no effect (n=4-8/group). The inhibitory effect of PMA (1μmol/L) on oxidase activity (73±3.2%) was blocked by H89 (100±7.8%; n=5 or 6/group). The PMA-mediated inhibition of NADPH oxidase activity was accompanied by an increase in PKCθ(S676), an effect that was also blocked by H89. Fenoldopam (1μmol/L) also increased PKCθ(S676) in HEK-hD(1) and human renal proximal tubule (RPT) cells. Knockdown of PKCθ with siRNA in RPT cells prevented the inhibitory effect of fenoldopam on NADPH oxidase activity. Our studies demonstrate for the first time that cross talk between PKA and PKCθ plays an important role in the D(1)R-mediated negative regulation of NADPH oxidase activity in human kidney cells.

The paradox of dopamine and angiotensin II-mediated Na(+), K(+)-ATPase regulation in renal proximal tubules

Accumulated studies reported that the natruretic dopamine (DA) and the anti-natruretic angiotensin II (Ang II) represent an important mechanism to regulate renal Na(+) and water excretion through intracellular secondary messengers to inhibit or activate renal proximal tubule (PT) Na(+), K(+)-ATPase (NKA). The antagonistic actions were mediated by the phosphorylation of different position of NKA α₁-subunit and different Pals-associated tight junction protein (PATJ) PDZ domains, the different protein kinase C (PKC) isoforms (PKC-β, PKC-ζ), the common adenylyl cyclase (AC) pathway, and the crosstalk and balance between DA and Ang II to NKA regulation. Besides, Ang II-mediated NKA modulation has bi-phasic effects.

Dopamine regulation of Na+-K+-ATPase requires the PDZ-2 domain of sodium hydrogen regulatory factor-1 (NHERF-1) in opossum kidney cells

Na(+)-K(+)-ATPase activity in renal proximal tubule is regulated by several hormones including parathyroid hormone (PTH) and dopamine. The current experiments explore the role of Na(+)/H(+) exchanger regulatory factor 1 (NHERF-1) in dopamine-mediated regulation of Na(+)-K(+)-ATPase. We measured dopamine regulation of ouabain-sensitive (86)Rb uptake and Na(+)-K(+)-ATPase α1 subunit phosphorylation in wild-type opossum kidney (OK) (OK-WT) cells, OKH cells (NHERF-1-deficient), and OKH cells stably transfected with full-length human NHERF-1 (NF) or NHERF-1 constructs with mutated PDZ-1 (Z1) or PDZ-2 (Z2) domains. Treatment with 1 μM dopamine decreased ouabain-sensitive (86)Rb uptake, increased phosphorylation of Na(+)-K(+)-ATPase α1-subunit, and enhanced association of NHERF-1 with D1 receptor in OK-WT cells but not in OKH cells. Transfection with wild-type, full-length, or PDZ-1 domain-mutated NHERF-1 into OKH cells restored dopamine-mediated regulation of Na(+)-K(+)-ATPase and D1-like receptor association with NHERF-1. Dopamine did not regulate Na(+)-K(+)-ATPase or increase D1-like receptor association with NHERF-1 in OKH cells transfected with mutated PDZ-2 domain. Dopamine stimulated association of PKC-ζ with NHERF-1 in OK-WT and OKH cells transfected with full-length or PDZ-1 domain-mutated NHERF-1 but not in PDZ-2 domain-mutated NHERF-1-transfected OKH cells. These results suggest that NHERF-1 mediates Na(+)-K(+)-ATPase regulation by dopamine through its PDZ-2 domain.

Pals-associated tight junction protein functionally links dopamine and angiotensin II to the regulation of sodium transport in renal epithelial cells

BACKGROUND AND PURPOSE

Dopamine inhibits renal cell Na(+),K(+)-ATPase activity and cell sodium transport by promoting the internalization of active molecules from the plasma membrane, whereas angiotensin II (ATII) stimulates its activity by recruiting new molecules to the plasma membrane. They achieve such effects by activating multiple and distinct signalling molecules in a hierarchical manner. The purpose of this study was to investigate whether dopamine and ATII utilize scaffold organizer proteins as components of their signalling networks, in order to avoid deleterious cross talk.

EXPERIMENTAL APPROACH

Attention was focused on a multiple PDZ domain protein, Pals-associated tight junction protein (PATJ). Ectopic expression of PATJ in renal epithelial cells in culture was used to study its interaction with components of the dopamine signalling cascade. Similarly, expression of PATJ deletion mutants was employed to analyse its functional relevance during dopamine-, ATII- and insulin-dependent regulation of Na(+),K(+)-ATPase.

KEY RESULTS

Dopamine receptors and components of its signalling cascade mediating inhibition of Na(+),K(+)-ATPase interact with PATJ. Inhibition of Na(+),K(+)-ATPase by dopamine was prevented by expression of mutants of PATJ lacking PDZ domains 2, 4 or 5; whereas the stimulatory effect of ATII and insulin on Na(+),K(+)-ATPase was blocked by expression of PATJ lacking PDZ domains 1, 4 or 5.

CONCLUSIONS AND IMPLICATIONS

A multiple PDZ domain protein may add functionality to G protein-coupled and tyrosine kinase receptors signalling during regulation of Na(+),K(+)-ATPase. Signalling molecules and effectors can be integrated into a functional network by the scaffold organizer protein PATJ via its multiple PDZ domains.

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.

Inhibitory effect of D1-like and D3 dopamine receptors on norepinephrine-induced proliferation in vascular smooth muscle cells

The sympathetic nervous system plays an important role in the regulation of blood pressure. There is increasing evidence for positive and negative interactions between dopamine and adrenergic receptors; the activation of the alpha-adrenergic receptor induces vasoconstriction, whereas the activation of dopamine receptor induces vasorelaxation. We hypothesize that the D1-like receptor and/or D3 receptor also inhibit alpha1-adrenergic receptor-mediated proliferation in vascular smooth muscle cells (VSMCs). In this study, VSMC proliferation was determined by measuring [3H]thymidine incorporation, cell number, and uptake of 3-(4,5-dimethylthiazol-2-yl)-diphenyltetrazolium bromide (MTT). Norepinephrine increased VSMC number and MTT uptake, as well as [3H]thymidine incorporation via the alpha1-adrenergic receptor in aortic VSMCs from Sprague-Dawley rats. The proliferative effects of norepinephrine were attenuated by the activation of D1-like receptors or D3 receptors, although a D1-like receptor agonist, fenoldopam, and a D3 receptor agonist, PD-128907, by themselves, at low concentrations, had no effect on VSMC proliferation. Simultaneous stimulation of both D1-like and D3 receptors had an additive inhibitory effect. The inhibitory effect of D3 receptor was via protein kinase A, whereas the D1-like receptor effect was via protein kinase C-zeta. The interaction between alpha1-adrenergic and dopamine receptors, especially D1-like and D3 receptors in VSMCs, could be involved in the pathogenesis of hypertension.

Dysregulation of dopamine-dependent mechanisms as a determinant of hypertension: studies in dopamine receptor knockout mice

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and by interacting with vasoactive hormones/humoral factors, such as aldosterone, angiotensin, catecholamines, endothelin, oxytocin, prolactin pro-opiomelancortin, reactive oxygen species, renin, and vasopressin. Dopamine receptors are classified into D(1)-like (D(1) and D(5)) and D(2)-like (D(2), D(3), and D(4)) subtypes based on their structure and pharmacology. In recent years, mice deficient in one or more of the five dopamine receptor subtypes have been generated, leading to a better understanding of the physiological role of each of the dopamine receptor subtypes. This review summarizes the results from studies of various dopamine receptor mutant mice on the role of individual dopamine receptor subtypes and their interactions with other G protein-coupled receptors in the regulation of blood pressure.

High CO2 levels impair alveolar epithelial function independently of pH

Background

In patients with acute respiratory failure, gas exchange is impaired due to the accumulation of fluid in the lung airspaces. This life-threatening syndrome is treated with mechanical ventilation, which is adjusted to maintain gas exchange, but can be associated with the accumulation of carbon dioxide in the lung. Carbon dioxide (CO₂) is a by-product of cellular energy utilization and its elimination is affected via alveolar epithelial cells. Signaling pathways sensitive to changes in CO₂ levels were described in plants and neuronal mammalian cells. However, it has not been fully elucidated whether non-neuronal cells sense and respond to CO₂. The Na,K-ATPase consumes ∼40% of the cellular metabolism to maintain cell homeostasis. Our study examines the effects of increased pCO₂ on the epithelial Na,K-ATPase a major contributor to alveolar fluid reabsorption which is a marker of alveolar epithelial function.

Principal Findings

We found that short-term increases in pCO₂ impaired alveolar fluid reabsorption in rats. Also, we provide evidence that non-excitable, alveolar epithelial cells sense and respond to high levels of CO₂, independently of extracellular and intracellular pH, by inhibiting Na,K-ATPase function, via activation of PKCζ which phosphorylates the Na,K-ATPase, causing it to endocytose from the plasma membrane into intracellular pools.

Conclusions

Our data suggest that alveolar epithelial cells, through which CO₂ is eliminated in mammals, are highly sensitive to hypercapnia. Elevated CO₂ levels impair alveolar epithelial function, independently of pH, which is relevant in patients with lung diseases and altered alveolar gas exchange.

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.

Simple transporter trafficking model for amphetamine-induced dopamine efflux

Amphetamine and its derivatives are important drugs of abuse causing both short-term excitatory and long-term addictive effects. The short-term excitatory effects are linked to amphetamine's ability to maintain high levels of dopamine (DA) outside the cell both by inhibiting DA reuptake after synaptic transmission and by enhancing the efflux of DA from the dopaminergic cells. The molecular mechanisms by which amphetamine elicits the efflux of DA and similar monoamines are still unclear. Recent literature data suggest that trafficking of the monoamine transporters is a phenomenon that underlies observed changes in amphetamine-induced monoamine reuptake and efflux. We develop an ordinary differential equation model incorporating the diverse mechanistic details behind amphetamine-induced DA efflux and demonstrate its utility in describing our experimental data. We also demonstrate an experimental method to track the time-varying concentration of membrane-bound transporter molecules from the DA efflux data. The good fit between our model and the experimental data supports the hypothesis that amphetamine-induced transporter trafficking is necessary to produce extended efflux of DA. This model can explain the relative significance of different processes associated with DA efflux at different times and at different concentration ranges of amphetamine and DA.

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.

Leishmania amazonensis: PKC-like protein kinase modulates the (Na++K+)ATPase activity

The present study aimed to identify the presence of protein kinase C-like (PKC-like) in Leishmania amazonensis and to elucidate its possible role in the modulation of the (Na(+)+K(+))ATPase activity. Immunoblotting experiments using antibody against a consensus sequence (Ac 543-549) of rabbit protein kinase C (PKC) revealed the presence of a protein kinase of 80 kDa in L. amazonensis. Measurements of protein kinase activity showed the presence of both (Ca(2+)-dependent) and (Ca(2+)-independent) protein kinase activity in plasma membrane and cytosol. Phorbol ester (PMA) activation of the Ca(2+)-dependent protein kinase stimulated the (Na(+)+K(+))ATPase activity, while activation of the Ca(2+)-independent protein kinase was inhibitory. Both effects of protein kinase on the (Na(+)+K(+))ATPase of the plasma membrane were lower than that observed in intact cells. PMA induced the translocation of protein kinase from cytosol to plasma membrane, indicating that the maximal effect of protein kinase on the (Na(+)+K(+))ATPase activity depends on the synergistic action of protein kinases from both plasma membrane and cytosol. This is the first demonstration of a protein kinase activated by PMA in L. amazonensis and the first evidence for a possible role in the regulation of the (Na(+)+K(+))ATPase activity in this trypanosomatid. Modulation of the (Na(+)+K(+))ATPase by protein kinase in a trypanosomatid opens up new possibilities to understand the regulation of ion homeostasis in this parasite.

FRET analysis reveals a critical conformational change within the Na,K-ATPase α1 subunit N-terminus during GPCR-dependent endocytosis

Dopamine is a major regulator of sodium reabsorption in proximal tubule epithelia. It induces the endocytosis of plasma membrane Na,K-ATPase molecules, and this results in a reduced capacity of the cells to transport sodium. Dopamine induces the phosphorylation of Ser-18 in the alpha1-subunit of Na,K-ATPase. Fluorescence resonance energy transfer analysis of cells expressing YFP-alpha1 and beta1-CFP reveals that treatment of the cells with dopamine increases energy transfer between CFP and YFP. This is consistent with a protein conformational change that results in the N-terminal end of alpha1 moving closer to the internal face of the plasma membrane.

Phosphorylation of adaptor protein-2 mu2 is essential for Na+,K+-ATPase endocytosis in response to either G protein-coupled receptor or reactive oxygen species

Activation of G protein-coupled receptor by dopamine and hypoxia-generated reactive oxygen species promote Na+,K+-ATPase endocytosis. This effect is clathrin dependent and involves the activation of protein kinase C (PKC)-zeta and phosphorylation of the Na+,K+-ATPase alpha-subunit. Because the incorporation of cargo into clathrin vesicles requires association with adaptor proteins, we studied whether phosphorylation of adaptor protein (AP)-2 plays a role in its binding to the Na+,K+-ATPase alpha-subunit and thereby in its endocytosis. Dopamine induces a time-dependent phosphorylation of the AP-2 mu2 subunit. Using specific inhibitors and dominant-negative mutants, we establish that this effect was mediated by activation of the adaptor associated kinase 1/PKC-zeta isoform. Expression of the AP-2 mu2 bearing a mutation in its phosphorylation site (T156A) prevented Na+,K+-ATPase endocytosis and changes in activity induced by dopamine. Similarly, in lung alveolar epithelial cells, hypoxia-induced endocytosis of Na+,K+-ATPase requires the binding of AP-2 to the tyrosine-based motif (Tyr-537) located in the Na+,K+-ATPase alpha-subunit, and this effect requires phosphorylation of the AP-2 mu2 subunit. We conclude that phosphorylation of AP-2 mu2 subunit is essential for Na+,K+-ATPase endocytosis in response to a variety of signals, such as dopamine or reactive oxygen species.

The dopamine paradox in lung and kidney epithelia: sharing the same target but operating different signaling networks

Stimulation of dopamine receptors in the lung or kidney epithelia has distinct and opposite effects on the function of Na,K-ATPase, which results in increased Na(+) absorption across the alveolar epithelium and increased sodium excretion via the kidney epithelium. In the lung, dopamine increases Na,K-ATPase by increasing cell basolateral surface expression of Na(+),K(+)-ATPase molecules, whereas in the kidney epithelia it decreases Na(+),K(+)-ATPase activity by removing active units from the plasma membrane by endocytosis. The opposite effects of dopamine over the same target (the Na(+),K(+)-ATPase) involve the activation of a distinct signaling network that it is target specific, and has a different spatial resolution. Understanding the specific signaling pathways involved in these actions of dopamine and their hierarchical organization may facilitate the drug discovery process that could lead to the design of new therapeutic approaches to clear lung edema in patients with acute lung injury and to decrease fluid overload during congestive heart failure and hypertension.

Atypical PKC-zeta and PKC-iota mediate opposing effects on MCF-7 Na+/K+ATPase activity

We demonstrated previously that in serum-starved MCF-7 breast cancer cell line, Ang II increased Na+/K+ATPase activity and activated the protein kinase C zeta (PKC-zeta) (Muscella et al., 2002 J Endocrinol 173:315-323; 2003 J Cell Physiol 197:61-68.). The aim of the present study was to investigate the modulation of the activity of the Na+/K+ATPase by PKC-zeta in MCF-7 cells. Here, using serum-starved MCF-7 cells, we have demonstrated that the effect of Ang II on the Na+/K+ATPase activity was inhibited by a synthetic myristoylated peptide with sequences based on the endogenous PKC-zeta pseudosubstrate region (zeta-PS) and by high doses of GF109203X, inhibitor of PKCs. When MCF-7 cells, grown in 10% fetal bovine serum (FBS), were stimulated with Ang II a dose- and time-dependent inhibition of the Na+/K+ATPase activity was obtained. Under this growth condition we found that mRNAs for AT1, AT2, and for Na+/K+ATPase alpha1 and alpha3 subunits were unchanged; besides both the activity of the Na+/K+ATPase and the level of PKC-zeta also were unaffected by the serum. The atypical PKC-iota level (present in very low abundance in serum-starved MCF-7) was increased and Ang II provoked its translocation from the cytosol to plasma membrane. PKC-zeta was localized to the membrane, and upon Ang II treatment its cellular localization did not change. The Ang II-mediated decrease of the Na+/K+ATPase activity was inhibited by high doses of GF109203X but not by zeta-PS, thus indicating that such effect was not due to PKC-zeta activity. The treatment of cells with PKC-iota antisense oligodeoxynucleotides inhibited the effects of Ang II on the Na+/K+ATPase activity. Additionally, the effect of Ang II on Na+/K+ATPase activity was also blocked by the phosphatidylinositol 3-kinase (PI3K) inhibitors, wortmannin and LY294002, and by the actin depolymerizing agents, cytochalasin D. In conclusion, in MCF-7 cells Ang II modulates the Na+/K+ATPase activity by both atypical PKC-zeta/-iota. The effects of Ang II are opposite depending upon the presence of the serum-sensitive PKC-iota, with the inhibitory effect possibly due to the redistribution of sodium pump from plasma membrane to the inactive intracellular pool.

Contrary to Rat-Type, Human-Type Na,K-ATPase Is Phosphorylated at the Same Amino Acid by Hormones that Produce Opposite Effects on Enzyme Activity

Renal sodium homeostasis is a major determinant of BP and is regulated by several natriuretic and antinatriuretic hormones. These hormones, acting through intracellular secondary messengers, either activate or inhibit proximal tubule Na,K-ATPase. It was shown previously that phorbol esters and angiotensin II and serotonin induce the phosphorylation of both Ser-11 and Ser-18 of the Na,K-ATPase alpha-subunit. This results in the recruitment of Na,K-ATPase molecules to the plasma membrane and an increased capacity to transport sodium ions. Treatment of the same cells with dopamine leads to phosphorylation of the Na,K-ATPase alpha-subunit Ser-18. The subsequent internalization of Na,K-ATPase molecules results in a reduced capacity to transport sodium ions. These effects are observed in cells that express the rat-type Na,K-ATPase. However, the Na,K-ATPase alpha1-subunit of several species, such as human, pig, and mouse, does not have a Ser-18 in their N-terminal region. Therefore, the possibility exists that, in those species, the Na,K-ATPase is not regulated by the hormones that regulate natriuresis. This study presents evidence that in cells that express the human-type Na,K-ATPase, dopamine inhibits and phorbol esters activate the Na,K-ATPase-mediated transport. These opposite effects are mediated by the phosphorylation of the same amino acid residue, Ser-11 of Na,K-ATPase alpha1, and the presence of alpha1 Ser-18 is not essential for the hormonal regulation of Na,K-ATPase activity in LLCPK1 cells. It was observed that, whereas the regulatory stimulation of Na,K-ATPase is mediated by protein kinase Cbeta, the regulatory inhibition is mediated by protein kinase Czeta. This is similar to what was demonstrated previously in cells that express the rat-type Na,K-ATPase.

Ouabain induces an increase of retinal ganglion cell survival in vitro: the involvement of protein kinase C

Ouabain is a steroid derivative that can regulate many cellular events such as growth and proliferation. It modulates Na+,K+-ATPase activity leading to the activation of different intracellular pathways through protein-protein interactions that have been characterized during the last few years. The aim of this work was to study the role of ouabain in rat retinal ganglion cell survival after 48 h in culture. Our results demonstrated that ouabain significantly induced an increase in retinal ganglion cell survival. The effect was dose-dependent and was maximal with 3.0 nM. The blockade of protein kinase C activity by 1.25 microM chelerythrine chloride abolished the ouabain effect, indicating an involvement of this intracellular pathway. None of the protein kinase inhibitors usually employed in the study of ouabain-driven intracellular pathways (PD98059, Ly294002, herbimycin, and genistein) was able to influence neuronal survival induced by ouabain. The data presented suggest that ouabain may be the trigger of an intracellular pathway responsible for neuronal survival.

Modulation of the (Na(+)+K+)ATPase activity by Angiotensin-(1-7) in MDCK cells

In the present paper the effect of Ang-(1-7) on the distal tubule (Na(+)+K+)ATPase activity was evaluated by using MDCK cells as a model. Confluent cell monolayers were incubated with increasing concentrations of Ang-(1-7) for 30 min. Thereafter, the (Na(+)+K+)ATPase activity was evaluated and a dose-dependent (from 10(-12) to 10(-7) M) inhibition was observed. The maximal inhibitory effect (54%) was reached at the concentration of 10(-8) M. The inhibitory effect of Ang-(1-7) was not affected by the AT2 receptor selective antagonist PD123319 (from 10(-10) to 10(-7) M) but was blocked in a dose-dependent manner by the AT1 receptor selective antagonists losartan (10(-10) M), candesartan (10(-17) M), irbesartan (2 x 10(-12) M) and telmisartan (2 x 10(-16) M). The signaling pathway triggered by stimulation of the AT(1) receptor was also investigated. The PI-phospholipase C (PI-PLC) inhibitor U73122 (5 x 10(-8) M) blocked the inhibitory effect elicited by Ang-(1-7). Involvement of the protein kinase C (PKC) was evidenced by the sensitivity of the inhibitory effect of Ang-(1-7) to calphostin C (6.32 x 10(-7) M) and the lack of additive effects when the cells were co-incubated with Ang-(1-7) and 3.2 x 10(-8) M PMA. Altogether, these results demonstrate that Ang-(1-7) inhibits the (Na(+)+K+)ATPase activity of the prototypic distal tubule cell MDCK through the AT1 receptor-mediated stimulation of PI-PLC/PKC signaling pathway.

Functional genomics of the dopaminergic system in hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. Under normal conditions, D(1)-like receptors (D(1) and D(5)) inhibit sodium transport in the kidney and intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats (SHRs) and in humans with essential hypertension, the D(1)-like receptor-mediated inhibition of epithelial sodium transport is impaired because of an uncoupling of the D(1)-like receptor from its G protein/effector complex. The uncoupling is receptor specific, organ selective, nephron-segment specific, precedes the onset of hypertension, and cosegregates with the hypertensive phenotype. The defective transduction of the renal dopaminergic signal is caused by activating variants of G protein-coupled receptor kinase type 4 (GRK4: R65L, A142V, A486V). The GRK4 locus is linked to and GRK4 gene variants are associated with human essential hypertension, especially in salt-sensitive hypertensive subjects. Indeed, the presence of three or more GRK4 variants impairs the natriuretic response to dopaminergic stimulation in humans. In genetically hypertensive rats, renal inhibition of GRK4 expression ameliorates the hypertension. In mice, overexpression of GRK4 variants causes hypertension either with or without salt sensitivity according to the variant. GRK4 gene variants, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic factors (e.g., angiotensin II type 1 receptor) to predominate, may be responsible for salt sensitivity. Subclasses of hypertension may occur because of additional perturbations caused by variants of other genes, the quantitative interaction of which may vary depending upon the genetic background.

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.

Antecedent ethanol ingestion prevents postischemic leukocyte adhesion and P-selectin expression by a protein kinase C-dependent mechanism

The aim of this study was to determine whether protein kinase C (PKC) contributed to the effects of ethanol ingestion to prevent P-selectin expression, leukocyte rolling (LR), and stationary leukocyte adhesion (LA) induced by subjecting the small bowel to ischemia and reperfusion (I/R) 24 hr later. I/R increased P-selectin expression, LR, and LA, effects that were largely abolished by antecedent ethanol consumption. Exposing the bowel to a specific but nonisoform-selective PKC inhibitor (chelerythrine or bisindolylmaleimide I) during the period of ethanol exposure did not alter the anti-inflammatory effects induced by ethanol ingestion 24 hr prior to I/R. Go-6976, a PKC inhibitor that exhibits a high degree of selectivity for the calcium-dependent PKC isoforms, markedly reduced the effectiveness of antecedent ethanol exposure to abrogate these postischemic inflammatory responses. Our data indicate that antecedent ethanol exposure prevents postischemic P-selectin expression, LR, and LA by a mechanism that involves activation of calcium-dependent PKC isotypes.

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.

Regulation of amphetamine-stimulated dopamine efflux by protein kinase C beta

Evidence suggests that protein kinase C (PKC) and intracellular calcium are important for amphetamine-stimulated outward transport of dopamine in rat striatum. In this study, we examined the effect of select PKC isoforms on amphetamine-stimulated dopamine efflux, focusing on Ca(2+)-dependent forms of PKC. Efflux of endogenous dopamine was measured in superfused rat striatal slices; dopamine was measured by high performance liquid chromatography. The non-selective classical PKC inhibitor Gö6976 inhibited amphetamine-stimulated dopamine efflux, whereas rottlerin, a specific inhibitor of PKC delta, had no effect. A highly specific PKC beta inhibitor, LY379196, blocked dopamine efflux that was stimulated by either amphetamine or the PKC activator, 12-O-tetradecanoylphorbol-13-acetate. None of the PKC inhibitors significantly altered [3H]dopamine uptake. PKC beta(I) and PKC beta(II), but not PKC alpha or PKC gamma, were co-immunoprecipitated from rat striatal membranes with the dopamine transporter (DAT). Conversely, antisera to PKC beta(I) and PKC beta(II) but not PKC alpha or PKCg amma were able to co-immunoprecipitate DAT. Amphetamine-stimulated dopamine efflux was significantly enhanced in hDAT-HEK 293 cells transfected with PKC beta(II) as compared with hDAT-HEK 293 cells alone, or hDAT-HEK 293 cells transfected with PKCa lpha or PKC beta(I). These results suggest that classical PKC beta(II) is physically associated with DAT and is important in maintaining the amphetamine-stimulated outward transport of dopamine in rat striatum.

Parathyroid hormone-mediated regulation of Na+-K+-ATPase requires ERK-dependent translocation of protein kinase Calpha

Parathyroid hormone (PTH) inhibits Na+-K+-ATPase activity by serine phosphorylation of the alpha1 subunit through protein kinase C (PKC)- and extracellular signal-regulated kinase (ERK)-dependent pathways. Based on previous studies we postulated that PTH regulates sodium pump activity through isoform-specific PKC-dependent activation of ERK. In the present work utilizing opossum kidney cells, a model of renal proximal tubule, PTH stimulated membrane translocation of PKCalpha by 102 +/- 16% and PKCbetaI by 41 +/- 7% but had no effect on PKCbetaII and PKCzeta. Both PKCalpha and PKCbetaI phosphorylated the Na+-K+-ATPase alpha1 subunit in vitro. PTH increased the activity of PKCalpha but not PKCbetaI. Coimmunoprecipitation assays demonstrated that treatment with PTH enhanced the association between Na+-K+-ATPase alpha1 subunit and PKCalpha, whereas the association between Na+-K+-ATPase alpha1 subunit and PKCbetaI remained unchanged. A PKCalpha inhibitory peptide blocked PTH-stimulated serine phosphorylation of the Na+-K+-ATPase alpha1 subunit and inhibition of Na+-K+-ATPase activity. Pharmacologic inhibition of MEK-1 blocked PTH-stimulated translocation of PKCalpha, whereas transfection of constitutively active MEK-1 cDNA induced translocation of PKCalpha and increased phosphorylation of the Na+-K+-ATPase alpha1 subunit. In contrast, PTH-stimulated ERK activation was not inhibited by pretreatment with the PKCalpha inhibitory peptide. Inhibition of PKCalpha expression by siRNA did not inhibit PTH-mediated ERK activation but significantly reduced PTH-mediated phosphorylation of the Na+-K+-ATPase alpha1 subunit. Pharmacologic inhibition of phosphoinositide 3-kinase blocked PTH-stimulated ERK activation, translocation of PKCalpha, and phosphorylation of the Na+-K+-ATPase alpha1 subunit. We conclude that PTH stimulates Na+-K+-ATPase phosphorylation and decreases the activity of Na+-K+-ATPase by ERK-dependent activation of PKCalpha.

Specific PKC isoforms regulate blastocoel formation during mouse preimplantation development

During early mammalian development, blastocyst morphogenesis is achieved by epithelial differentiation of trophectoderm (TE) and its segregation from the inner cell mass (ICM). Two major interrelated features of TE differentiation required for blastocoel formation include intercellular junction biogenesis and a directed ion transport system, mediated by Na+/K+ ATPase. We have examined the relative contribution of intercellular signalling mediated by protein kinase C (PKC) and gap junctional communication in TE differentiation and blastocyst cavitation. The distribution pattern of four (delta, theta, iota/lambda, zeta) PKC isoforms and PKCmicro/PKD1 showed partial colocalisation with the tight junction marker ZO-1alpha+ in TE and all four PKCs (delta, theta, iota/lambda, zeta) showed distinct TE/ICM staining patterns (predominantly at the cell membrane within the TE and cytoplasmic within the ICM), indicating their potential contribution to TE differentiation and blastocyst morphogenesis. Specific inhibition of PKCdelta and zeta activity significantly delayed blastocyst formation. Although modulation of these PKC isoforms failed to influence the already established programme of epithelial junctional differentiation within the TE, Na+/K+ ATPase alpha1 subunit was internalised from membrane to cytoplasm. Inhibition of gap junctional communication, in contrast, had no influence on any of these processes. Our results demonstrate for the first time that distinct PKC isotypes contribute to the regulation of cavitation in preimplantation embryos via target proteins including Na+/K+ ATPase.

Regulation of spontaneous meiosis resumption in mouse oocytes by various conventional PKC isozymes depends on cellular compartmentalization

In this study, we investigated the spatio-temporal distribution of conventional protein kinases C (cPKC) isoforms PKC-alpha, PKC-betaI, PKC-betaII and PKC-gamma in mouse oocytes. The cPKCs were present in the cytoplasm at the start of the process and migrated to the nucleus (or germinal vesicle) before germinal vesicle breakdown, except for PKC-gamma which remained cytoplasmic. In both compartments, the fully phosphorylated form corresponding to the 'mature' enzyme was revealed for PKC-alpha, PKC-betaI and PKC-betaII. Microinjection of specific antibodies against each isozyme in one or the other cell compartment at different times of the meiotic process, permitted us to observe the following: (1) When located in the cytoplasm at the beginning of the process, PKC-alpha is not implicated in germinal vesicle breakdown, PKC-betaI and PKC-gamma are involved in maintaining the meiotic arrest, and PKC-betaII plays a role in meiosis reinitiation. Furthermore, just before germinal vesicle breakdown, these cytoplasmic cPKCs were no longer implicated. (2) When located in the germinal vesicle, PKC-alpha, PKC-betaI and PKC-betaII are involved in meiosis reinitiation. Our data highlight not only the importance of the nuclear pathways in the cell cycle progression, but also their independence of the cytoplasmic ones. Further investigations are however necessary to discover the molecular targets of these cPKCs to better understand the links with the cell cycle progression.

Transport protein trafficking in polarized cells

In order to carry out their physiological functions, ion transport proteins must be targeted to the appropriate domains of cell membranes. Regulation of ion transport activity frequently involves the tightly controlled delivery of intracellular populations of transport proteins to the plasma membrane or the endocytic retrieval of transport proteins from the cell surface. Transport proteins carry signals embedded within their structures that specify their subcellular distributions and endow them with the capacity to participate in regulated membrane trafficking processes. Recently, a great deal has been learned about the biochemical nature of these signals, as well as about the cellular machinery that interprets them and acts upon their messages.

Overexpression of PKC-betaI and -delta contributes to higher PKC activity in the proximal tubules of old Fischer 344 rats

Previously, we reported that natriuretic and diuretic response to dopamine is diminished in old Fischer 344 rats, which is due to higher basal protein kinase C (PKC) activity and hyperphosphorylation of Na-K-ATPase in the proximal tubules (PTs) of old rats. The present study was conducted to determine whether higher PKC activity could be due to altered expression of some of the PKC isoforms in the superficial cortex (rich in PTs) of old rats. Fluorimetric measurement showed almost twofold increase in the PKC activities in homogenates and membranes of old (24 mo) compared with adult (6 mo) rats. Interestingly, in the basal state PKC-betaI was overexpressed in the membranes, whereas PKC-delta expression was increased in the cytosol of old compared with adult rats. Treatment of the cortical slices with either SKF-38393, a D1-like agonist, or PDBu, a direct activator of PKC, caused translocation of PKC-betaI from cytosol to membranes in adult but not in old rats. Both of these drugs caused translocation of PKC-delta from membranes to cytosol in adult but not in old rats. These drugs had no effect on translocation of PKC-zeta in both adult and old rats. Both PKC-betaI and -delta co-immunoprecipitated with alpha1-subunit of Na-K-ATPase in adult and old rats. These observations suggest that both SKF-38393 and PDBu differentially regulate PKC-betaI and -delta in adult but not in old rats. Also, PKC-betaI and -delta seem to interact with Na-K-ATPase in these animals. The overexpression of both PKC-betaI and -delta in old rats could be responsible for a higher basal PKC activity, which causes the hyperphosphorylation of Na-K-ATPase and contributes to the diminished inhibition of Na-K-ATPase activity by dopamine in old rats.

Bradykinin B1 receptor stimulates the proximal tubule Na+-ATPase activity through protein kinase C pathway

Recently, our group described a B1-mediated stimulatory effect of des-Arg(9)-bradykinin (DABK) on the Na(+)-ATPase activity of proximal tubule basolateral membranes (BLM) [Biochim. Biophys. Acta 1431 (1999) 483.]. Data in the present report suggest the participation of a phosphatidylinositol-specific PLC (PI-PLC)/protein kinase C (PKC) pathway as the molecular mechanism of DABK-mediated stimulation of the Na(+)-ATPase activity since (i) 10(-8) M DABK activates PI-PLC activity; (ii) 10(-9) M U73122, a PI-PLC inhibitor, abolishes the effect of 10(-8) M DABK on the Na(+)-ATPase activity; (iii) 10(-8) M DABK increases phosphoprotein formation by 34%. This effect is completely reversed by 10(-7) M calphostin C, an inhibitor of PKC; (iv) 20 ng/ml TPA, an activator of PKC, and 10(-8) M DABK stimulate the Na(+)-ATPase activity in a similar and nonadditive manner. Furthermore, the effect of 10(-8) M DABK is completely reversed by calphostin C; (v) 10(-8) M DABK increases phosphoserine residue levels by 54%. This effect is completely reversed by 10(-7) M calphostin C.

PKC-zeta is required for angiotensin II-induced activation of ERK and synthesis of C-FOS in MCF-7 cells

We examined the signalling pathways responsible for the Ang II induction of growth in MCF-7 human breast cancer cells. Ang II in MCF-7 cells induced: (a) the translocation from the cytosol to membrane and nucleus of atypical protein kinase C-zeta (PKC-zeta) but not of PKC-alpha, -delta, - epsilon and -eta; (b) the expression of c-fos mRNA and protein; (c) the phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). All these effects were due to the activation of the Ang II type I receptor (AT1) since they were blocked by the AT1 antagonist losartan. The Ang II-stimulated ERK1/2 phosphorylation was blocked by (a) high doses of staurosporine, inhibitor of PKC-zeta, and by a synthetic myristoylated peptide with sequences based on the endogenous PKC-zeta pseudosubstrate region (zeta-PS); (b) PD098059, a mitogen-activated protein kinase kinase inhibitor (MAPKK/MEK); and, moreover, (c) the inhibitors of phosphoinositide 3-kinases (PI3K), LY294002 and wortmannin, thus indicating that PI3K may act upstream of ERK1/2. The Ang II-evoked c-fos induction was blocked only by high doses of staurosporine and by zeta-PS whilst PD098059, LY294002 and wortmannin were ineffective, thus indicating that c-fos induction is not due to ERK1/2 activity. When the epidermal growth factor-receptor (EGFR) tyrosine kinase activity was inhibited by the use of its inhibitor AG1478, Ang II was still able to induce ERK1/2 phosphorylation and c-fos expression, therefore proving that the transactivation of EGFR was not required for these Ang II effects in MCF-7 cells. The previously reported proliferation of MCF-7 cells induced by Ang II was blocked by PD098059 and by wortmannin in a dose-dependent manner, thereby indicating that in MCF-7 cells the PI3K and ERK pathways mediate the mitogenic signalling of AT1. Our results suggest that in MCF-7 cells Ang II activates multiple signalling pathways involving PKC-zeta, PI3K and MAPK; of these pathways only PKC-zeta appears responsible for the induction of c-fos.

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.

Omega-3 fatty acids modulate ATPases involved in duodenal Ca absorption

Dietary supplementation with fish oil that contains omega-3 polyunsaturated fatty acids has been shown to enhance bone density as well as duodenal calcium uptake in rats. The latter process is supported by membrane ATPases. The present in vitro study was undertaken to test the effect of omega-3 fatty acids on ATPase activity in isolated basolateral membranes from rat duodenal enterocytes. Ca-ATPase in calmodulin-stripped membranes was activated in a biphasic manner by docosahexanoic acid (DHA) (10-30 microg/ml) but not by eicosapentanoic acid (EPA). This effect was blocked partially by 0.5 microM calphostin (a protein kinase C blocker). DHA inhibited Na,K-ATPase (-49% of basal activity, [DHA]=30 microg/ml, P <0.01). This effect could be reversed partially by 50 microM genistein, a tyrosine kinase blocker. EPA also inhibited Na,K-ATPase: (-47% of basal activity, [EPA]=30 microg/ml, P <0.01), this effect was partially reversed by 100 microM indomethacin, a cyclo-oxygenase blocker. Omega-3 fatty acids are thus involved in multiple signalling effects that effect ATPases in BLM.

Isoform-specific regulation of Na+,K+-ATPase endocytosis and recruitment to the plasma membrane

The Na(+),K(+)-ATPase traffics between the plasma membrane and intracellular compartments in response to acute changes in membrane receptor activation. These effects are accomplished by a time-dependent interaction of the Na(+),K(+)-ATPase alpha-subunit with specific intracellular signaling molecules either at the plasma membrane (endocytosis) or at the endosome's membranes (recruitment). Most of these studies have been performed in rat renal epithelial cells in which only the alpha(1) isoenzyme is present. Studies in neurons from the neostriatum in which all three alpha-subunit isoforms are present indicate that neurotransmitter-dependent regulation of Na(+),K(+)-ATPase activity displays isoform specificity and also suggest a more complex organization of the intracellular signaling networks controlling Na(+),K(+)-ATPase traffic in mammalian cells.

Hypoxia-induced endocytosis of Na,K-ATPase in alveolar epithelial cells is mediated by mitochondrial reactive oxygen species and PKC-zeta

During ascent to high altitude and pulmonary edema, the alveolar epithelial cells (AEC) are exposed to hypoxic conditions. Hypoxia inhibits alveolar fluid reabsorption and decreases Na,K-ATPase activity in AEC. We report here that exposure of AEC to hypoxia induced a time-dependent decrease of Na,K-ATPase activity and a parallel decrease in the number of Na,K-ATPase alpha(1) subunits at the basolateral membrane (BLM), without changing its total cell protein abundance. These effects were reversible upon reoxygenation and specific, because the plasma membrane protein GLUT1 did not decrease in response to hypoxia. Hypoxia caused an increase in mitochondrial reactive oxygen species (ROS) levels that was inhibited by antioxidants. Antioxidants prevented the hypoxia-mediated decrease in Na,K-ATPase activity and protein abundance at the BLM. Hypoxia-treated AEC deficient in mitochondrial DNA (rho(0) cells) did not have increased levels of ROS, nor was the Na,K-ATPase activity inhibited. Na,K-ATPase alpha(1) subunit was phosphorylated by PKC in hypoxia-treated AEC. In AEC treated with a PKC-zeta antagonist peptide or with the Na,K-ATPase alpha(1) subunit lacking the PKC phosphorylation site (Ser-18), hypoxia failed to decrease Na,K-ATPase abundance and function. Accordingly, we provide evidence that hypoxia decreases Na,K-ATPase activity in AEC by triggering its endocytosis through mitochondrial ROS and PKC-zeta-mediated phosphorylation of the Na,K-ATPase alpha(1) subunit.

Analysis of Na+,K+-ATPase motion and incorporation into the plasma membrane in response to G protein-coupled receptor signals in living cells

Dopamine (DA) increases Na(+),K(+)-ATPase activity in lung alveolar epithelial cells. This effect is associated with an increase in Na(+),K(+)-ATPase molecules within the plasma membrane (). Analysis of Na(+),K(+)-ATPase motion was performed in real-time in alveolar cells stably expressing Na(+),K(+)-ATPase molecules carrying a fluorescent tag (green fluorescent protein) in the alpha-subunit. The data demonstrate a distinct (random walk) pattern of basal movement of Na(+),K(+)-ATPase-containing vesicles in nontreated cells. DA increased the directional movement (by 3.5 fold) of the vesicles and an increase in their velocity (by 25%) that consequently promoted the incorporation of vesicles into the plasma membrane. The movement of Na(+),K(+)-ATPase-containing vesicles and incorporation into the plasma membrane were microtubule dependent, and disruption of this network perturbed vesicle motion toward the plasma membrane and prevented the increase in the Na(+),K(+)-ATPase activity induced by DA. Thus, recruitment of new Na(+),K(+)-ATPase molecules into the plasma membrane appears to be a major mechanism by which dopamine increases total cell Na(+),K(+)-ATPase activity.

Renal dopamine receptors and hypertension

Dopamine has been recognized as an important modulator of central as well as peripheral physiologic functions in both humans and animals. Dopamine receptors have been identified in a number of organs and tissues, which include several regions within the central nervous system, sympathetic ganglia and postganglionic nerve terminals, various vascular beds, the heart, the gastrointestinal tract, and the kidney. The peripheral dopamine receptors influence cardiovascular and renal function by decreasing afterload and vascular resistance and promoting sodium excretion. Within the kidney, dopamine receptors are present along the nephron, with highest density on proximal tubule epithelial cells. It has been reported that there is a defective dopamine receptor, especially D(1) receptor function, in the proximal tubule of various animal models of hypertension as well as in humans with essential hypertension. Recent reports have revealed the site of and the molecular mechanisms responsible for the defect in D(1) receptors in hypertension. Moreover, recent studies have also demonstrated that the disruption of various dopamine receptor subtypes and their function produces hypertension in rodents. In this review, we present evidence that dopamine and dopamine receptors play an important role in regulating renal sodium excretion and that defective renal dopamine production and/or dopamine receptor function may contribute to the development of various forms of hypertension.

C-peptide stimulates Na+,K+-ATPase activity via PKC alpha in rat medullary thick ascending limb

AIMS/HYPOTHESIS

C-peptide, the cleavage product of proinsulin processing exerts physiological effects including stimulation of Na(+),K(+)-ATPase in erythrocytes and renal proximal tubules. This study was undertaken to assess the physiological effects of connecting peptide on Na(+),K(+)-ATPase activity in the medullary thick ascending limb of Henle's loop.

METHODS

Na(+),K(+)-ATPase activity was measured as the ouabain-sensitive generation of (32)Pi from gamma[(32)P]-ATP and (86)Rb uptake on isolated rat medullary thick ascending limbs. The cell-surface expression of Na(+),K(+)-ATPase was evaluated by Western blotting of biotinylated proteins, and its phosphorylation amount was measured by autoradiography. The membrane-associated fraction of protein kinase C isoforms was evaluated by Western blotting.

RESULTS

Rat connecting peptide concentration-dependently stimulated Na(+),K(+)-ATPase activity with a threshold at 10(-9) mol/l and a maximal effect at 10(-7) mol/l. C-peptide (10(-7) mol/l) already stimulates Na(+),K(+)-ATPase activity after 5 min with a plateau from 15 to 60 min. C-peptide (10(-7) mol/l) stimulated Na(+),K(+)-ATPase activity and (86)Rb uptake to the same extent, but did not alter Na(+),K(+)-ATPase cell surface expression. The stimulation of Na(+),K(+)-ATPase activity was associated with an increase in Na(+),K(+)-ATPase alpha-subunit phosphorylation and both effects were abolished by a specific protein kinase C inhibitor. Furthermore, connecting peptide induced selective membrane translocation of PKC-alpha.

CONCLUSION/INTERPRETATION

This study provides evidence that in rat medullary thick ascending limb, C-peptide stimulates Na(+),K(+)-ATPase activity within a physiological concentration range. This effect is due to an increase in Na(+),K(+)-ATPase turnover rate that is most likely mediated by protein kinase C-alpha phosphorylation of the Na(+),K(+)-ATPase alpha-subunit, suggesting that C-peptide could control Na(+) reabsorption during non-fasting periods.

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.

Relevance of dopamine signals anchoring dynamin-2 to the plasma membrane during Na+,K+-ATPase endocytosis

Clathrin-dependent endocytosis of Na(+),K(+)-ATPase in response to dopamine regulates its catalytic activity in intact cells. Because fission of clathrin-coated pits requires dynamin, we examined the mechanisms by which dopamine receptor signals promote dynamin-2 recruitment and assembly at the site of Na(+),K(+)-ATPase endocytosis. Western blotting revealed that dopamine increased the association of dynamin-2 with the plasma membrane and with phosphatidylinositol 3-kinase. Dopamine inhibited Na(+),K(+)-ATPase activity in OK cells and in those overexpressing wild type dynamin-2 but not in cells expressing a dominant-negative mutant. Dephosphorylation of dynamin is important for its assembly. Dopamine increased protein phosphatase 2A activity and dephosphorylated dynamin-2. In cells expressing a dominant-negative mutant of protein phosphatase 2A, dopamine failed to dephosphorylate dynamin-2 and to reduce Na(+),K(+)-ATPase activity. Dynamin-2 is phosphorylated at Ser(848), and expression of the S848A mutant significantly blocked the inhibitory effect of dopamine. These results demonstrate a distinct signaling network originating from the dopamine receptor that regulates the state of dynamin-2 phosphorylation and that promotes its location (by interaction with phosphatidylinositol 3-kinase) at the site of Na(+),K(+)-ATPase endocytosis.