Transcriptional control of sodium transport in tight epithelial by adrenal steroids

Activation of mitogen-activated protein kinase (mitogen-activated protein kinase/extracellular signal-regulated kinase) cascade by aldosterone

Aldosterone in some tissues increases expression of the mRNA encoding the small monomeric G protein Ki-RasA. Renal A6 epithelial cells were used to determine whether induction of Ki-ras leads to concomitant increases in the total as well as active levels of Ki-RasA and whether this then leads to subsequent activation of its effector mitogen-activated protein kinase (MAPK/extracellular signal-regulated kinase) cascade. The molecular basis and cellular consequences of this action were specifically investigated. We identified the intron 1-exon 1 region (rasI/E1) of the mouse Ki-ras gene as sufficient to reconstitute aldosterone responsiveness to a heterologous promotor. Aldosterone increased reporter gene activity containing rasI/E1 threefold. Aldosterone increased the absolute and GTP-bound levels of Ki-RasA by a similar extent, suggesting that activation resulted from mass action and not effects on GTP binding/hydrolysis rates. Aldosterone significantly increased Ki-RasA and MAPK activity as early as 15 min with activation peaking by 2 h and waning after 4 h. Inhibitors of transcription, translation, and a glucocorticoid receptor antagonist attenuated MAPK signaling. Similarly, rasI/E1-driven luciferase expression was sensitive to glucocorticoid receptor blockade. Overexpression of dominant-negative RasN17, addition of antisense Ki-rasA and inhibition of mitogen-activated protein kinase kinase also attenuated steroid-dependent increases in MAPK signaling. Thus, activation of MAPK by aldosterone is dependent, in part, on a genomic mechanism involving induction of Ki-ras transcription and subsequent activation of its downstream effectors. This genomic mechanism has a distinct time course from activation by traditional mitogens, such as serum, which affect the GTP-binding state and not absolute levels of Ras. The result of such a genomic mechanism is that peak activation of the MAPK cascade by adrenal corticosteroids is delayed but prolonged.

New ideas about aldosterone signaling in epithelia

The systemic actions of aldosterone are well documented; however, in comparison, our understanding of the cellular and molecular mechanisms by which aldosterone orchestrates these actions is rudimentary. Aldosterone exerts most of its physiological actions by modifying gene expression. It is now apparent that aldosterone represses almost as many genes as it induces. Several aldosterone-sensitive genes, including serum and glucocorticoid-inducible kinase (sgk) and small, monomeric Kirsten Ras GTP-binding protein (Ki-ras) have recently been identified. The molecular mechanisms and elements bestowing corticosteroid sensitivity on these and many other genes are becoming clear. Induction of Ki-Ras and Sgk is necessary and sufficient for some portion of aldosterone action in epithelia. These two signaling factors are components of a converging pathway with phosphatidylinositol 3-kinase positioned between them that enables both stabilizing the epithelial Na(+) channel (ENaC) in the open state as well as increasing the number of ENaC in the apical membrane. This aldosterone-induced signaling pathway contains many potential sites for feedback regulation and cross talk from other cascades and potentially impinges directly on the activity of transport proteins and/or cellular differentiation to modify electrolyte transport.

Short term effect of aldosterone on Na,K-ATPase cell surface expression in kidney collecting duct cells

Aldosterone controls extracellular volume and blood pressure by regulating Na+ reabsorption, in particular by epithelia of the distal nephron. A main regulatory site of this transcellular transport is the epithelial sodium channel (ENaC) that mediates luminal Na+ influx. The Na,K-ATPase (Na+ pump) that coordinately extrudes Na+ across the basolateral membrane is known to be regulated by short term aldosterone as well. We now show that in the cortical collecting duct (CCD) from adrenalectomized rats, the increase in Na,K-ATPase activity (approximately 3-fold in 3 h), induced by a single aldosterone injection, can be fully accounted by the increase in Na,K-ATPase cell surface expression (+ 497 +/- 35%). The short term aldosterone action was further investigated in cultured mouse collecting duct principal cells mpkCCD(cl4). Within 2 h, maximal Na,K-ATPase function assessed by Na+ pump current (I(p)) measurements and Na,K-ATPase cell surface expression were increased by 20-50%. Aldosterone did not modify the Na+ dependence of the Na+ pumps and induced transcription- and translation-dependent actions on pump surface expression and current independently of ENaC-mediated Na+ influx. In summary, short term aldosterone directly increases the cell surface expression of pre-existing Na+ pumps in kidney CCD target cells. Thus, aldosterone controls Na+ reabsorption in the short term not only by regulating the apical cell surface expression of ENaC (Loffing, J., Zecevic, M., Feraille, E., Kaissling, B., Asher, C., Rossier, B. C., Firestone, G. L., Pearce, D., and Verrey, F. (2001) Am. J. Physiol. 280, F675-F682) but also by coordinately acting on the basolateral cell surface expression of the Na,K-ATPase.

The role of SGK1 in hormone-regulated sodium transport

Ion transport in epithelia is regulated by a variety of hormonal and nonhormonal factors, including mineralocorticoids, insulin, shear stress and osmotic pressure. In mammals, the mineralocorticoid aldosterone is the principal regulator of sodium homeostasis and hence is central to the control of extracellular fluid volume and blood pressure. Aldosterone acts through a member of the nuclear receptor superfamily, the mineralocorticoid receptor (MR), to control the transcriptional activity of specific target genes. Recently, a serine/threonine kinase, SGK1 (serum and glucocorticoid-regulated kinase isoform 1) was identified as a candidate mediator of aldosterone action in the colon and distal nephron. The aldosterone-activated MR increases SGK1 gene transcription and SGK1, in turn, strongly stimulates the activity of the epithelial sodium channel (ENaC). Interestingly, other factors appear to regulate SGK1 gene expression and kinase activity. Insulin, for example, stimulates SGK1 activity (but not gene transcription) through its effects on phosphatidylinositol-3-kinase and osmotic shock appears to stimulate both SGK1 activity and gene transcription. Hence, SGK1 might integrate the effects of multiple hormonal and nonhormonal regulators of Na(+) transport in tight epithelia and thereby play a key role in volume homeostasis. It is interesting to speculate that SGK1 might be implicated in medical conditions, such as the insulin resistance syndrome, hypertension and congestive heart failure.

Modulation of aldosterone-induced stimulation of ENaC synthesis by changing the rate of apical Na+ entry

Primary cultures of immunodissected rabbit connecting tubule and cortical collecting duct cells were used to investigate the effect of apical Na+ entry rate on aldosterone-induced transepithelial Na+ transport, which was measured as benzamil-sensitive short-circuit current (I(sc)). Stimulation of the apical Na+ entry, by long-term short-circuiting of the monolayers, suppressed the aldosterone-stimulated benzamil-sensitive I(sc) from 320 +/- 49 to 117 +/- 14%, whereas in the presence of benzamil this inhibitory effect was not observed (335 +/- 74%). Immunoprecipitation of [(35)S]methionine-labeled beta-rabbit epithelial Na+ channel (rbENaC) revealed that the effects of modulation of apical Na+ entry on transepithelial Na+ transport are exactly mirrored by beta-rbENaC protein levels, because short-circuiting the monolayers decreased aldosterone-induced beta-rbENaC protein synthesis from 310 +/- 51 to 56 +/- 17%. Exposure to benzamil doubled the beta-rbENaC protein level to 281 +/- 68% in control cells but had no significant effect on aldosterone-stimulated beta-rbENaC levels (282 +/- 68%). In conclusion, stimulation of apical Na+ entry suppresses the aldosterone-induced increase in transepithelial Na+ transport. This negative-feedback inhibition is reflected in a decrease in beta-rbENaC synthesis or in an increase in beta-rbENaC degradation.

Aquaporin-2, a regulated water channel, is expressed in apical membranes of rat distal colon epithelium

Aquaporin-2 (AQP-2) is the vasopressin-regulated water channel expressed in the apical membrane of principal cells in the collecting duct and is involved in the urinary concentrating mechanism. In the rat distal colon, vasopressin stimulates water absorption through an unknown mechanism. With the hypothesis that AQP-2 could contribute to this vasopressin effect, we studied its presence in rat colonic epithelium. We used RT-PCR, in situ hybridization, immunoblotting, and immunocytochemistry to probe for AQP-2 expression. An AQP-2 amplicon was obtained through RT-PCR of colon epithelium RNA, and in situ hybridization revealed AQP-2 mRNA in colonic crypts and, to a lesser extent, in surface absorptive epithelial cells. AQP-2 protein was localized to the apical membrane of surface absorptive epithelial cells, where it colocalized with H(+)-K(+)-ATPase but not with Na(+)-K(+)-ATPase. AQP-2 was absent from the small intestine, stomach, and liver. Water deprivation increased the hybridization signal and the protein level (assessed by Western blot analysis) for AQP-2 in distal colon. This was accompanied by increased p-chloromercuriphenylsulfonic acid-sensitive water absorption. These results indicate that AQP-2 is present in the rat distal colon, where it might be involved in a water-sparing mechanism. In addition, these results support the idea that AQP-2, and probably other aquaporins, are involved in water absorption in the colon.

A new human MR splice variant is a ligand-independent transactivator modulating corticosteroid action

Aldosterone effects are mediated by the MR, which possesses the same affinity for mineralocorticoids and glucocorticoids. In addition to the existence of mechanisms regulating intracellular hormone availability, we searched for human MR splice variants involved in tissue-specific corticosteroid function. We have identified a new human MR isoform, hMRDelta5,6, resulting from an alternative splicing event skipping exons 5 and 6 of the human MR gene. hMRDelta5,6 mRNAs are expressed in several human tissues at different levels compared with wild-type human MR, as shown by real time PCR. Introduction of a premature stop codon results in a 75-kDa protein lacking the entire hinge region and ligand binding domain. Interestingly, hMRDelta5,6 is still capable of binding to DNA and acts as a ligand-independent transactivator, with maximal transcriptional induction corresponding to approximately 30-40% of aldosterone-activated wild-type human MR. Coexpression of hMRDelta5,6 with human MR or human GR increases their transactivation potential at high doses of hormone. Finally, hMRDelta5,6 is able to recruit the coactivators, steroid receptor coactivator 1, receptor interacting protein 140, and transcription intermediary factor 1alpha, which enhance its transcriptional activity. Ligand-independent transactivation and enhancement of both wild-type MR and GR activities by hMRDelta5,6 suggests that this new variant might play a role in modulating corticosteroid effects in target tissues.

S-adenosyl-L-homocysteine hydrolase is necessary for aldosterone-induced activity of epithelial Na(+) channels

The A6 cell line was used to study the role of S-adenosyl-L-homocysteine hydrolase (SAHHase) in the aldosterone-induced activation of the epithelial Na(+) channel (ENaC). Because aldosterone increases methylation of several different molecules, and because this methylation is associated with increased Na(+) reabsorption, we tested the hypothesis that aldosterone increases the expression and activity of SAHHase protein. The rationale for this work is that general methylation may be promoted by activation of SAHHase, the only enzyme known to metabolize SAH, a potent end-product inhibitor of methylation. Although aldosterone increased SAHHase activity, steroid did not affect SAHHase expression. Antisense SAHHase oligonucleotide decreased SAHHase expression and activity. Moreover, this oligonucleotide, as well as a pharmacological inhibitor of SAHHase, decreased aldosterone-induced activity of ENaC via a decrease in ENaC open probability. The kinetics of ENaC in cells treated with antisense plus aldosterone were similar to those reported previously for the channel in the absence of steroid. This is the first report showing that active SAHHase, in part, increases ENaC open probability by reducing the transition rate from open states in response to aldosterone. Thus aldosterone-induced SAHHase activity plays a critical role in shifting ENaC from a gating mode with short open and closed times to one with longer open and closed times.

Aldosterone mediates the changes in hexose transport induced by low sodium intake in chicken distal intestine

1. In chickens, low Na+ diets markedly decrease the hexose transport in the rectal segment of the large intestine; transport in the ileum shows a lower, but significant reduction and transport in the jejunum is unaffected. These effects involve both apical (SGLT1) and basolateral (GLUT2) hexose transporters. 2. The role of the renin-angiotensin-aldosterone axis (RAAS) in the epithelial response to Na+ intake was studied in chickens fed high-NaCl (HS) and low-NaCl (LS) diets. The V(max) of alpha-methyl-D-glucoside and D-glucose were determined in vesicles from the brush-border (BBMVs) and basolateral (BLMVs) membranes, respectively. The binding of phlorizin to BBMV and cytochalasin B to BLMV were used as indicators of the abundance of SGLT1 and GLUT2, respectively. 3. In HS-adapted chickens, the serum concentration of aldosterone (means +/- S.E.M.) was 35 +/- 5 pg ml(-1) (n = 6) and that of renin was 20 +/- 2 ng ml(-1) (n = 3). In LS-fed birds, these values were 166 +/- 12 pg ml(-1) (n = 6) and 122 +/- 5 ng ml(-1) (n = 3), respectively. Administration of captopril, the inhibitor of the angiotensin-converting enzyme (ACE), to LS-chickens lowered the aldosterone serum concentration without affecting the renin concentration. Captopril also prevented the reduction of apical and basolateral hexose transport in ileum and rectum characteristic of the intestinal response to LS adaptation. 4. Administration of the aldosterone antagonist spironolactone to LS-adapted chickens did not affect the serum concentrations of aldosterone, but prevented the effects of LS intake on hexose transport in both apical and basolateral membranes. This suggests that the effects of aldosterone are mediated by cytosolic mineralcorticoid receptors. 5. Administration of exogenous aldosterone to HS-fed birds induced hexose transport and binding properties typical of the LS-adapted animals. These findings support the view that aldosterone, besides its primary role in controlling intestinal Na+ absorption, can also modulate the expression of apical and basolateral glucose transporters in the chicken distal intestine.

Contrasting effects of cPLA2 on epithelial Na+ transport

Activity of the epithelial Na+ channel (ENaC) is the limiting step for discretionary Na+ reabsorption in the cortical collecting duct. Xenopus laevis kidney A6 cells were used to investigate the effects of cytosolic phospholipase A2 (cPLA2) activity on Na+ transport. Application of aristolochic acid, a cPLA2 inhibitor, to the apical membrane of monolayers produced a decrease in apical [3H]arachidonic acid (AA) release and led to an approximate twofold increase in transepithelial Na+ current. Increased current was abolished by the nonmetabolized AA analog 5,8,11,14-eicosatetraynoic acid (ETYA), suggesting that AA, rather than one of its metabolic products, affected current. In single channel studies, ETYA produced a decrease in ENaC open probability. This suggests that cPLA2 is tonically active in A6 cells and that the end effect of liberated AA at the apical membrane is to reduce Na+ transport via actions on ENaC. In contrast, aristolochic acid applied basolaterally inhibited current, and the effect was not reversed by ETYA. Basolateral application of the cyclooxygenase inhibitor ibuprofen also inhibited current. Both effects were reversed by prostaglandin E2 (PGE2). This suggests that cPLA2 activity and free AA, which is metabolized to PGE2, are necessary to support transport. This study supports the fine-tuning of Na+ transport and reabsorption through the regulation of free AA and AA metabolism.

Aldosterone induces rapid apical translocation of ENaC in early portion of renal collecting system: possible role of SGK

Aldosterone controls sodium reabsorption and potassium secretion in the aldosterone-sensitive distal nephron (ASDN). Although clearance measurements have shown that aldosterone induces these transports within 30--60 min, no early effects have been demonstrated in vivo at the level of the apical epithelial sodium channel (ENaC), the main effector of this regulation. Here we show by real-time RT-PCR and immunofluorescence that an aldosterone injection in adrenalectomized rats induces alpha-ENaC subunit expression along the entire ASDN within 2 h, whereas beta- and gamma-ENaC are constitutively expressed. In the proximal ASDN portions only, ENaC is shifted toward the apical cellular pole and the apical plasma membrane within 2 and 4 h, respectively. To address the question of whether the early aldosterone-induced serum and glucocorticoid-regulated kinase (SGK) might mediate this apical shift of ENaC, we analyzed SGK induction in vivo. Two hours after aldosterone, SGK was highly induced in all segment-specific cells of the ASDN, and its level decreased thereafter. In Xenopus laevis oocytes, SGK induced ENaC activation and surface expression by a kinase activity-dependent mechanism. In conclusion, the rapid in vivo accumulation of SGK and alpha-ENaC after aldosterone injection takes place along the entire ASDN, whereas the translocation of alpha,beta,gamma-ENaC to the apical plasma membrane is restricted to its proximal portions. Results from oocyte experiments suggest the hypothesis that a localized activation of SGK may play a role in the mediation of ENaC translocation.

Mineralocorticoid and glucocorticoid receptors inhibit UCP expression and function in brown adipocytes

Uncoupling proteins (UCP), specific mitochondrial proton transporters that function by uncoupling oxidative metabolism from ATP synthesis, are involved in thermoregulation and control of energy expenditure. The hibernoma-derived T37i cells, which possess functional endogenous mineralocorticoid receptors (MR), can undergo differentiation into brown adipocytes. In differentiated T37i cells, UCP1 mRNA levels increased 10- to 20-fold after retinoic acid or beta-adrenergic treatment. Interestingly, UCP2 and UCP3 mRNA was also detected. Aldosterone treatment induced a drastic decrease in isoproterenol- and retinoic acid-stimulated UCP1 mRNA levels in a time- and dose-dependent manner (IC(50) approximately 1 nM aldosterone). This inhibition was unaffected by cycloheximide and did not modify UCP1 mRNA stability (half-life time = 5 h), indicating that it occurs at the transcriptional level. It involves both the MR and/or the glucocorticoid receptor (GR), depending on the retinoic or catecholamine induction pathway. Basal UCP3 expression was also significantly reduced by aldosterone, whereas UCP2 mRNA levels were not modified. Finally, as demonstrated by JC1 aggregate formation in living cells, aldosterone restored mitochondrial membrane potential abolished by isoproterenol or retinoic acid. Our results demonstrate that MR and GR inhibit expression of UCP1 and UCP3, thus participating in the control of energy expenditure.

Impaired sodium excretion during mental stress in mild essential hypertension

In hypertensive rats, environmental stress causes sodium retention by an exaggerated increase in renal sympathetic nerve activity, which is modulated by angiotensin II. We tested whether similar effects can be observed in humans. In 66 normotensive subjects (half of them with a family history of hypertension) and 36 subjects with mild essential hypertension, urinary sodium excretion and renal hemodynamics were examined at rest and during mental stress treated either with placebo or ACE inhibition in a double-blind, randomized, cross-over design. Despite a marked increase in glomerular filtration rate in response to mental stress (Deltaglomerular filtration rate, 4.3+/-7.7 mL/min in normotensives without versus 5.6+/-8.4 mL/min in normotensives with a family history versus 10.1+/-5.7 mL/min in patients with mild essential hypertension; P:<0.002), the increase in urinary sodium excretion was blunted in patients with mild essential hypertension (Deltaurinary sodium excretion, 0.12+/-0.17 mmol/min versus 0.10+/-0.14 mmol/min versus 0.05+/-0.14 mmol/min; P:<0.05). ACE inhibition corrected the natriuretic response to mental stress in subjects with mild essential hypertension (Deltaurinary sodium excretion, 0.05+/-0.14 mmol/min with placebo versus 0.13+/-0.19 mmol/min with ACE inhibition; P:<0.01); thus, after ACE inhibition, urinary sodium excretion increased similarly in all 3 groups. In conclusion, impaired sodium excretion occurs during mental stress in human essential hypertension but not in subjects with positive family history of hypertension. This abnormality in sodium handling during activation of the sympathetic nervous system appears to be mediated by angiotensin II.

Transcriptome of a mouse kidney cortical collecting duct cell line: effects of aldosterone and vasopressin

Aldosterone and vasopressin are responsible for the final adjustment of sodium and water reabsorption in the kidney. In principal cells of the kidney cortical collecting duct (CCD), the integral response to aldosterone and the long-term functional effects of vasopressin depend on transcription. In this study, we analyzed the transcriptome of a highly differentiated mouse clonal CCD principal cell line (mpkCCD(cl4)) and the changes in the transcriptome induced by aldosterone and vasopressin. Serial analysis of gene expression (SAGE) was performed on untreated cells and on cells treated with either aldosterone or vasopressin for 4 h. The transcriptomes in these three experimental conditions were determined by sequencing 169,721 transcript tags from the corresponding SAGE libraries. Limiting the analysis to tags that occurred twice or more in the data set, 14,654 different transcripts were identified, 3,642 of which do not match known mouse sequences. Statistical comparison (at P < 0.05 level) of the three SAGE libraries revealed 34 AITs (aldosterone-induced transcripts), 29 ARTs (aldosterone-repressed transcripts), 48 VITs (vasopressin-induced transcripts) and 11 VRTs (vasopressin-repressed transcripts). A selection of the differentially-expressed, hormone-specific transcripts (5 VITs, 2 AITs and 1 ART) has been validated in the mpkCCD(cl4) cell line either by Northern blot hybridization or reverse transcription-PCR. The hepatocyte nuclear transcription factor HNF-3-alpha (VIT39), the receptor activity modifying protein RAMP3 (VIT48), and the glucocorticoid-induced leucine zipper protein (GILZ) (AIT28) are candidate proteins playing a role in physiological responses of this cell line to vasopressin and aldosterone.

Structure and regulation of amiloride-sensitive sodium channels

Amiloride-sensitive Na+ channels constitute a new class of proteins known as the ENaC-Deg family of ion channels. All members in this family share a common protein structure but differ in their ion selectivity, their affinity for the blocker amiloride, and in their gating mechanisms. These channels are expressed in many tissues of invertebrate and vertebrate organisms where they serve diverse functions varying from Na+ absorption across epithelia to being the receptors for neurotransmitters in the nervous system. Here, we review progress made during the last years in the characterization, regulation, and cloning of new amiloride-sensitive Na+ channels.

Differential effects of protein kinase C on the levels of epithelial Na+ channel subunit proteins

Regulation of epithelial Na(+) channel (ENaC) subunit levels by protein kinase C (PKC) was investigated in A6 cells. PKC activation altered ENaC subunit levels, differentially decreasing the levels of both beta and gamma, but not alphaENaC. Temporal regulation of beta and gammaENaC by PKC differed; gammaENaC decreased with a time constant of 3.7 +/- 1.0 h, whereas betaENaC decreased in 13.9 +/- 3. 0 h. Activation of PKC also resulted in a decrease in trans-epithelial Na(+) reabsorption for up to 48 h. PMA activation of PKC resulted in negative feedback inhibition of PKC protein levels beginning within 4 h. Both beta and gammaENaC levels, as well as transport tended toward pretreatment values after 48 h of PMA treatment. PKC inhibitors attenuated the effects of PMA on ENaC subunit levels and Na(+) transport. These results directly show for the first time that PKC differentially regulates ENaC subunit levels by decreasing the levels of beta and gamma but not alphaENaC protein. These results imply a PKC-dependent, long term decrease in Na(+) reabsorption.

Is the mineralocorticoid receptor in Brown Norway rats constitutively active?

In a previous study using corticosterone treatment of adrenalectomized rats, we hypothesized that mineralocorticoid receptor (MR)-related mechanisms are constitutively active and that glucocorticoid receptor (GR)-mediated mechanisms are more efficient in Brown Norway rats compared to Fischer 344 (F344) rats. In order to discriminate the mineralocorticoid from the glucocorticoid actions exerted by corticosterone, F344 and Brown Norway adrenalectomized rats were treated with increasing doses (1, 5 and 25 microg/ml of drinking water) of deoxycorticosterone (DOC, MR-specific ligand) or RU 28362 (GR-specific ligand). These rats were compared with long-term adrenalectomized (ADX) untreated rats and sham-ADX rats. This study confirms our previous results, notably the lack of effect of ADX on body weight and fluid intake in Brown Norway rats. Moreover, DOC treatment had no effect in Brown Norway rats whereas the higher dose restored fluid intake of the F344 ADX group to sham values. These results support the hypothesis of a constitutive activation of the MR and therefore the insensitivity of this receptor to its ligand in Brown Norway rats. Alternatively, RU 28362 treatment induced greater weight loss, decrease in food intake, anxiolysis, thymus involution, and decrease in plasma transcortin concentration and pituitary corticosteroid receptor densities in Brown Norway rats than in F344 rats, which is consistent with greater efficiency of GR mechanisms in Brown Norway rats than in F344 rats. Therefore, these strains are of great utility to disentangle MR and GR effects on complex phenotypes.

Regulation of the epithelial Na+ channel by aldosterone: open questions and emerging answers

Aldosterone is the principal adrenal steroid controlling Na+ retention in amphibians and mammalians. It acts primarily by increasing the apical Na+ permeability through activation of the epithelial Na+ channel (ENaC). The cellular events mediating the hormonal action are mostly unknown. Early studies have provided evidence that the hormone functions to activate or translocate pre-existing channels by a yet undefined mechanism. In addition, enhanced de novo channel synthesis appears to take place as well. The molecular cloning of the three ENaC subunits has provided new powerful tools for testing and confirming this hypothesis, as well as for characterizing mechanisms by which ENaC is regulated. Another important development is the recent identification of several cDNAs corresponding to aldosterone-induced and suppressed mRNAs. The study of these genes and their putative interactions with ENaC is likely to provide important clues to the mechanisms by which aldosterone controls the apical Na+ permeability of tight epithelia. This article reviews recent developments in the field that may lead to the elucidation of the mechanisms by which the hormone controls Na+ transport.

Mineralocorticoid receptor knockout mice: lessons on Na+ metabolism

The mineralocorticoid receptor (MR) binds aldosterone and glucocorticoids with equal affinity. In aldosterone target tissues, like the epithelial cells of the distal colon and the principal cells of the collecting ducts in the kidney, the MR is protected from glucocorticoids by the action of the enzyme 11beta-hydroxysteroid-dehydrogenase type 2 (11betaOHSD2), allowing aldosterone to specifically activate the receptor. However, in MR-expressing cells, which lack 11betaOHSD2, like the neurons of the limbic system in the brain, MR is mainly activated by glucocorticoids. MR knockout mice die in the second week after birth, showing at day 8 symptoms of pseudohypoaldosteronism with hyponatremia, hyperkalemia, high renal salt wasting, and a strongly activated renin-angiotensin-aldosterone system (RAAS). The activity of the amiloride-sensitive epithelial Na+ channel (ENaC) is strongly reduced in colon and kidney, but there is no down-regulation of the mRNA abundance of the three ENaC subunits. Daily subcutaneous injections of isotonic NaCl solution until weaning and continued oral NaCl supply lead to survival of the MR knockout mice. The NaCl-rescued MR knockout mice display a strongly enhanced fractional renal excretion of Na+, hyperkalemia, and a persistently strongly activated RAAS. There is almost no renal ENaC activity. The renal mRNA abundance of alphaENaC is reduced by 30%, whereas betaENaC and gammaENaC are not altered.

Pleiotropic action of aldosterone in epithelia mediated by transcription and post-transcription mechanisms

The aldosterone-induced increase in sodium reabsorption across tight epithelia can be divided schematically into two functional phases: an early regulatory phase starting after a lag period of 20 to 60 minutes, during which the pre-existing transport machinery is activated, and a late phase (>2.5 h), which can be viewed as an anabolic action leading to a further amplification/differentiation of the Na+ transport machinery. At the transcriptional level, both early and late responses are initiated during the lag period, but the functional impact of newly synthesized regulatory proteins is faster than that of the structural ones. K-Ras2 and SGK were identified as the first early aldosterone-induced regulatory proteins in A6 epithelia. Their mRNAs also were shown to be regulated in vivo by aldosterone, and their expression (constitutively active K-Ras2 and wild-type SGK) was shown to increase the function of ENaC coexpressed in Xenopus oocytes. Recently, aldosterone was also shown to act on transcription factors in A6 epithelia: It down-regulates the mRNAs of the proliferation-promoting c-Myc, c-Jun, and c-Fos by a post-transcriptional mechanism, whereas it up-regulates that of Fra-2 (c-Fos antagonist) at the transcriptional level. Together, these new data illustrate the complexity of the regulatory network controlled by aldosterone and support the view that its early action is mediated by the induction of key regulatory proteins such as K-Ras2 and SGK. These early induced proteins are sites of convergence for different regulatory inputs, and thus, their aldosterone-regulated expression level tunes the impact of other regulatory cascades on sodium transport. This suggests mechanisms for the escape from aldosterone action.

Regulation of sgk by aldosterone and its effects on the epithelial Na(+) channel

Aldosterone is the major corticosteroid regulating Na(+) absorption in tight epithelia and acts primarily by activating the epithelial Na(+) channel (ENaC) through unknown induced proteins. Recently, it has been reported that aldosterone induces the serum- and glucocorticoid-dependent kinase sgk and that coexpressing ENaC with this kinase in Xenopus laevis oocytes increases the amiloride-sensitive Na(+) current (Chen SY, Bhargava A, Mastroberardino L, Meijer OC, Wang J, Buse P, Firestone GL, Verrey F, and Pearce D. Proc Natl Acad Sci USA 96: 2514-2519, 1999). The present study was done to further characterize regulation of sgk by aldosterone in native mammalian epithelia and to examine its effect on ENaC. With both in vivo and in vitro protocols, an almost fivefold increase in the abundance of sgk mRNA has been demonstrated in rat kidney and colon but not in lung. Induction of sgk by aldosterone was detected in kidney cortex and medulla, whereas the papilla expressed a constitutively high level of the kinase. The increase in sgk mRNA was detected as early as 30 min after the hormonal application and was independent of de novo protein synthesis. The observed aldosterone dose-response relationships suggest that the response is mediated, at least in part, by occupancy of the mineralocorticoid receptor. Coexpressing sgk and ENaC in Xenopus oocytes evoked a fourfold increase in the amiloride-blockable Na(+) channel activity. A point mutation in the beta-subunit known to impair regulation of the channel by Nedd4 (Y618A) had no significant effect on the response to sgk.

Mineralocorticoid selectivity: molecular and cellular aspects

Aldosterone acts in mineralocorticoid-sensitive cells by binding to the mineralocorticoid receptor (MR). Because the MR displays similar affinity for aldosterone and glucocorticoid hormones and because these latter hormones are 100- to 1000-fold more abundant than aldosterone in the plasma, mechanisms are required to avoid permanent illicit occupancy of MR by glucocorticoid hormones. The main mechanism of mineralocorticoid selectivity is enzymatic: the 11beta hydroxysteroid dehydrogenase (HSD2) metabolizes glucocorticoid hormones into derivatives with a low affinity for MR. The cell biology and regulation of HSD2 are reviewed in this article, as well as its implications in human hypertension. Other factors play a role in mineralocorticoid selectivity: the MR itself, the possibility to form homodimers (MR-MR), or heterodimers (with the glucocorticoid receptor). All of these cellular events participate to successive dynamic equilibriums, which allow fine tuning of transcriptional regulation of target genes, depending on the target tissue and the hormonal status.

Mechanisms of inactivation of the action of aldosterone on collecting duct by TGF-beta

The purpose of these experiments was to investigate the mechanisms whereby transforming growth factor-beta (TGF-beta) antagonizes the action of adrenocorticoid hormones on Na(+) transport by the rat inner medullary collecting duct in primary culture. Steroid hormones 1) increased Na(+) transport by three- to fourfold, 2) increased the maximum capacity of the Na(+)-K(+) pump by 30-50%, 3) increased the steady-state levels of the alpha(1)-subunit of the Na(+)-K(+)-ATPase by approximately 30%, and 4) increased the steady-state levels of the alpha-subunit of the rat epithelial Na(+) channel (alpha-rENaC) by nearly fourfold. TGF-beta blocked the effects of steroids on the increase in Na(+) transport and the stimulation of the Na(+)-K(+)-ATPase and pump capacity. However, there was no effect of TGF-beta on the steroid-induced increase in mRNA levels of alpha-rENaC. The effects of TGF-beta were not secondary to the decrease in Na(+) transport per se, inasmuch as benzamil inhibited the increase in Na(+) transport but did not block the increase in pump capacity or Na(+)-K(+)-ATPase mRNA. The results indicate that TGF-beta does not inactivate the steroid receptor or its translocation to the nucleus. Rather, they indicate complex pathways involving interruption of the enhancement of pump activity and activation/inactivation of pathways distal to the steroid-induced increase in the transcription of alpha-rENaC.

The serum and glucocorticoid kinase sgk increases the abundance of epithelial sodium channels in the plasma membrane of Xenopus oocytes

The serum- and glucocorticoid-induced kinase (sgk) is a serine and threonine kinase that stimulates amiloride-sensitive sodium transport in Xenopus oocytes. Because aldosterone induces phosphorylation on serine/threonine (Ser/Thr) residues in the carboxyl termini of beta and gamma subunits of epithelial sodium channels (ENaCs) and causes an increase in the sgk transcript in mammalian and amphibian renal epithelial cells, it seems likely that sgk mediates the action of aldosterone to stimulate sodium transport. Experiments were performed in Xenopus oocytes to determine the mechanism by which sgk increases sodium conductance by examining its effect on phosphorylation, kinetics, and membrane abundance of ENaC. Our results demonstrate that deletions of the carboxyl termini of the three subunits do not inhibit sgk-induced sodium current, indicating that the effect of sgk is not mediated via phosphorylation within the carboxyl termini of ENaC. They also show no evidence that sgk reduces the removal of ENaC from the plasma membrane because mutations of tyrosine residues in the sequences necessary for endocytosis and degradation did not affect the response to sgk. Further studies performed with the patch-clamp technique indicated that sgk did not increase the open probability or changed the kinetics of ENaC. These studies, however, showed a 3-fold increase in the abundance of ENaC in the plasma membrane in the presence of sgk compared with control. Together, the experiments indicate that sgk stimulates electrogenic sodium transport by increasing the number of ENaCs at the cell surface and suggest that sgk may mediate the early increase in aldosterone-induced sodium current.

Regulation of Na(+) reabsorption by the aldosterone-induced small G protein K-Ras2A

Xenopus laevis A6 cells were used as model epithelia to test the hypothesis that K-Ras2A is an aldosterone-induced protein necessary for steroid-regulated Na(+) transport. The possibility that increased K-Ras2A alone is sufficient to mimic aldosterone action on Na(+) transport also was tested. Aldosterone treatment increased K-Ras2A protein expression 2.8-fold within 4 h. Active Ras is membrane associated. After aldosterone treatment, 75% of K-Ras was localized to the plasma membrane compared with 25% in the absence of steroid. Aldosterone also increased the amount of active (phosphorylated) mitogen-activated protein kinase kinase likely through K-Ras2A signaling. Steroid-induced K-Ras2A protein levels and Na(+) transport were decreased with antisense K-ras2A oligonucleotides, showing that K-Ras2A is necessary for the natriferic actions of aldosterone. Aldosterone-induced Na(+) channel activity, was decreased from 0.40 to 0.09 by pretreatment with antisense ras oligonucleotide, implicating the luminal Na(+) channel as one final effector of Ras signaling. Overexpression of K-Ras2A increased Na(+) transport approximately 2.2-fold in the absence of aldosterone. These results suggest that aldosterone signals to the luminal Na(+) channel via multiple pathways and that K-Ras2A levels are limiting for a portion of the aldosterone-sensitive Na(+) transport.

Early aldosterone action: toward filling the gap between transcription and transport

The mineralocorticoid hormone aldosterone stimulates transcellular Na+ reabsorption across target epithelia after a lag period of 20 to 60 min by first activating preexisting channels (epithelial sodium channels, ENaC) and pumps (Na-K-ATPase) and, subsequently, increasing the overall transport capacity of the cells. Both these early regulatory and late anabolic-type actions depend on the transcriptional regulation exerted by hormone-activated mineralocorticoid and/or glucocorticoid receptors (MR and/or GR). Starting at the transcriptional side of the aldosterone action, recent studies have identified the small G protein K-Ras2 and the kinase sgk as the first early aldosterone-induced gene products potentially regulating Na+ transport. At the level of the Na+ transport effectors, much knowledge about ENaC and Na-K-ATPase structure-function relationship and regulation has accumulated. However, the regulatory pathway(s) that link the transcriptional action of aldosterone to these Na+ transport proteins is still to a large extent unknown. The available data suggest that the early regulatory action of aldosterone is pleiotropic, similarly to the late anabolic-type action. The early Na+ transport stimulation would be mediated by the rapid induction of gene products belonging to the regulatory network that integrates the inputs of diverse pathways and finally controls the function of the Na+ transport machinery.

Immortalized kidney epithelial cells as tools for hormonally regulated ion transport studies

The development of transgenic mice carrying the simian virus-40 large T antigen gene or the temperature-sensitive simian virus-40 large T antigen gene, either alone or placed under the control of the 5'-regulatory regions of tissue-specific or ubiquitous genes, has permitted the production of differentiated, polarized kidney epithelial cells. This review covers the immortalized cell lines issued from the various parts of the renal tubule and, in particular, the recently established collecting duct cell lines that have been used as ex-vivo cell models to analyze the regulation of ion transport processes by hormones.

Ca-sensitive Na transport in sheep omasum

Na transport across a preparation of sheep omasum was studied. All tissues exhibited a serosa-positive short-circuit current (Isc), with a range of 1-4 microeq. h-1. cm-2. A Michaelis-Menten-type kinetic was found between the Na concentration and the Isc (Michaelis-Menten constant for transport of Na = 6.7 mM; maximal transport capacity of Na = 4.16 microeq. h-1. cm-2). Mucosal amiloride (1 mM), phenamil (1 or 10 microM), or serosal aldosterone (1 microM for 6 h) did not change Isc. Removal of divalent cations (Ca and Mg) enhanced Isc considerably from 2.61 +/- 0.24 to a peak value of 11.18 +/- 1.1 microeq. h-1. cm-2. The peak Isc (overshoot) immediately declined to a plateau Isc of approximately 6-7 microeq. h-1. cm-2. Na flux measurements showed a close correlation between changes in Isc and Na transport. Transepithelial studies demonstrated that K, Cs, Rb, and Li are transported, indicating putative nonselective cation channels, which are inhibited by divalent cations (including Ca, Mg, Sr, Ba) and by (trivalent) La. Intracellular microelectrode recordings from the luminal side clearly showed changes of voltage divider ratio when mucosal divalent cations were removed. The obtained data support the assumption of a distinct electrogenic Na transport mechanism in sheep omasum.

Regulation of Na+ pump function by aldosterone is alpha-subunit isoform specific

1. During its early 'genomic' phase of action (< 3 h), aldosterone activates pre-existing Na+ pumps (Na+,K+-ATPase) in epithelia formed by Xenopus laevis A6 kidney cells. 2. To test whether this action also applies to pumps containing mammalian alpha-subunits of different isoforms, we generated A6 cell lines expressing the naturally ouabain-resistant rat alpha1 subunit or the rat alpha2* and alpha3* subunits made ouabain resistant by site-directed mutagenesis. 3. Cell lines were obtained which expressed the exogenous alpha-subunits in active, basolateral Na+ pumps, such that ouabain-resistant pump current (Ip) could be measured following apical permeabilization with amphotericin B. 4. The inhibition constants (Ki) for ouabain of the current carried by the pumps containing exogenous rat alpha-subunits were similar to those reported previously for ATPase activity inhibition. The apparent Michaelis constant (Km) for Na+ (K+ replacement) was slightly higher for pumps containing the rat alpha1 than for those containing the alpha2* subunit (34.9 +/- 1.9 versus 26.3 +/- 2.6 mM). 5. At a Na+ concentration of 10 mM, aldosterone (2.5 h) increased the pump current carried by endogenous pumps as well as that carried by pumps containing the exogenous rat alpha1 subunit (by 1.8- to 2.2-fold). In contrast, the current carried by pumps containing the exogenous rat alpha2* subunit remained unchanged. 6. The fact that this early transcriptionally mediated activation of Na+ pumps by aldosterone is specific for pumps containing an alpha1 subunit should permit the identification in this subunit of structures involved in its regulation.

Aldosterone action: induction of p21(ras) and fra-2 and transcription-independent decrease in myc, jun, and fos

Adrenal steroids induce an increase in transcellular Na+ reabsorption across Xenopus laevis A6 cell epithelia that requires the action of transcriptionally regulated gene products. In a previous study we identified K-ras2 as an aldosterone-upregulated mRNA in A6 epithelia. Here, we show that in vivo injection of aldosterone in Xenopus (2.5 h) increases K-ras2 mRNA specifically in the kidney (2.5-fold) and that in A6 epithelia aldosterone (2.5 h) increases Ras protein synthesis ( approximately 6-fold). Xl-ras, another ras mRNA expressed at a low level in A6 cells, was also induced (2-fold). Aldosterone was shown to regulate the mRNA levels of several transcription factors as well. After 2 h of aldosterone treatment, fra-2 mRNA was upregulated by 130%, whereas c-myc, c-jun, c-fos, and glucocorticoid receptor mRNAs were downregulated by 23-43%. After 16 h, c-fos and GR mRNAs were further decreased, whereas levels of fra-2, c-jun, and c-myc began to return to control levels. Interestingly, the downregulation of the protooncogene mRNAs was independent of transcription. These results support the view that aldosterone exerts complex pleiotropic transcriptional and nontranscriptional actions that involve the regulation of signaling cascade elements (i.e., K-Ras2) as well as that of transcription factors.

The structure of the rat amiloride-sensitive epithelial sodium channel gamma subunit gene and functional analysis of its promoter

Prolonged dietary Na+ depletion and chronic administration of adrenal steroids increase steady-state mRNA levels of the gamma subunit of the epithelial sodium channel (gammaENaC) in rat colon. This increase correlates with a marked increase in transepithelial Na+ transport and is thought to occur via transcriptional regulation. To begin to evaluate these mechanisms in detail, we determined the organization of the rat gammaENaC gene. A rat genomic library was screened and overlapping lambda clones that together span the gene (approximately 36 kb) and contain at least 1 kb of 5' flanking genomic DNA were isolated. As in the human gene, the rat gammaENaC gene contains 13 exons and a CpG island at the 5' end of the gene. A single transcription start site was identified in rat kidney by nuclease protection assay defining a 5' untranslated region of 126 nt. The translation initiation codon was identified within the second exon and the entire 3' UTR (approximately 1 kb) was within the last exon. 800 bp of 5' flanking sequence, as well as the 3.4 kb first intron, were sequenced and analyzed for transcriptional regulatory motifs. Analogous to the human gammaENaC gene [Thomas, C.P., Doggett, N.A., Fisher, R., Stokes J.B., 1996. Genomic organization and the 5' flanking region of the gamma subunit of the human amiloride-sensitive epithelial sodium channel. J. Biol. Chem. 271, 26 062--26 066], two GC boxes were seen at -30 and -61 to the transcription start site. In addition, putative AP-1, AP-2, CRE, Sp1 and GATA-1 and GRE motifs were identified elsewhere in the 5' flanking region or the first intron. Two mammalian-wide interspersed repeats and a rodent-specific B1 repeat were also identified within the first intron. Fragments containing the putative GRE motifs coupled to luciferase did not confer a glucocorticoid-stimulated response in two cell lines that contained a functional glucocorticoid receptor. However, a 76 nt rat gammaENaC 5' flanking fragment (-76 to +68) directed expression of luciferase in the epithelial cell lines H441 and FRTL5, suggesting that this minimal region that contained both GC boxes was sufficient for promoter activity.

Epithelial sodium channel regulated by aldosterone-induced protein sgk

Sodium homeostasis in terrestrial and freshwater vertebrates is controlled by the corticosteroid hormones, principally aldosterone, which stimulate electrogenic Na+ absorption in tight epithelia. Although aldosterone is known to increase apical membrane Na+ permeability in target cells through changes in gene transcription, the mechanistic basis of this effect remains poorly understood. The predominant early effect of aldosterone is to increase the activity of the epithelial sodium channel (ENaC), although ENaC mRNA and protein levels do not change initially. Rather, the open probability and/or number of channels in the apical membrane are greatly increased by unknown modulators. To identify hormone-stimulated gene products that modulate ENaC activity, a subtracted cDNA library was generated from A6 cells, a stable cell line of renal distal nephron origin, and the effect of candidates on ENaC activity was tested in a coexpression assay. We report here the identification of sgk (serum and glucocorticoid-regulated kinase), a member of the serine-threonine kinase family, as an aldosterone-induced regulator of ENaC activity. sgk mRNA and protein were strongly and rapidly hormone stimulated both in A6 cells and in rat kidney. Furthermore, sgk stimulated ENaC activity approximately 7-fold when they were coexpressed in Xenopus laevis oocytes. These data suggest that sgk plays a central role in aldosterone regulation of Na+ absorption and thus in the control of extracellular fluid volume, blood pressure, and sodium homeostasis.

S-adenosyl-L-homocysteine hydrolase regulates aldosterone-induced Na+ transport

Aldosterone-induced Na+ reabsorption, in part, is regulated by a critical methyl esterification; however, the signal transduction pathway regulating this methylation remains unclear. The A6 cell line was used as a model epithelia to investigate regulation of aldosterone-induced Na+ transport by S-adenosyl-L-homocysteine hydrolase (SAHHase), the only enzyme in vertebrates known to catabolize S-adenosyl-L-homocysteine (SAH), an end product inhibitor of methyl esterification. Sodium reabsorption was decreased within 2 h by 3-deazaadenosine, a competitive inhibitor of SAHHase, with a half inhibitory concentration between 40 and 50 microM. Aldosterone increased SAH catabolism by activating SAHHase. Increased SAH catabolism was associated with a concomitant increase in S-adenosylmethionine catabolism. Moreover, SAH decreased substrate methylation. Antisense oligonucleotide complementary to SAHHase mRNA decreased SAHHase activity and Na+ current by approximately 50%. Overexpression of SAHHase increased SAHHase activity and dependent substrate methyl esterification. Whereas basal Na+ current was not affected by overexpression of SAHHase, aldosterone-induced current in SAHHase-overexpressing cells was significantly potentiated. These results demonstrate that aldosterone induction of SAHHase activity is necessary for a concomitant relief of the methylation reaction from end product inhibition by SAH and the subsequent increase in Na+ reabsorption. Thus, regulation of SAHHase activity is a control point for aldosterone signal transduction, but SAHHase is not an aldosterone-induced protein.

Molecular and cellular determinants of mineralocorticoid selectivity

Aldosterone plays a major role in the regulation of renal sodium reabsorption, of extracellular fluid volume and blood pressure. Such specific mineralocorticoid physiological adaptations occur despite the large prevalence of glucocorticoid hormones over aldosterone in the plasma. Indeed both classes of hormones bind with the same affinity to the mineralocorticoid receptor, but several mechanisms allow selective and tissue-specific aldosterone effects. They represent a series of mutually interacting selectivity filters, which have not yet been fully documented. The main determinants of aldosterone selective effects include an enzymatic protection of the mineralocorticoid receptor, the intrinsic properties of the mineralocorticoid receptor towards different ligands, and numerous possibilities of interaction between corticosteroid receptors (forming different homo or heterodimers) and other transcription factors.

Ras pathway activates epithelial Na+ channel and decreases its surface expression in Xenopus oocytes

The small G protein K-Ras2A is rapidly induced by aldosterone in A6 epithelia. In these Xenopus sodium reabsorbing cells, aldosterone rapidly activates preexisting epithelial Na+ channels (XENaC) via a transcriptionally mediated mechanism. In the Xenopus oocytes expression system, we tested whether the K-Ras2A pathway impacts on XENaC activity by expressing XENaC alone or together with XK-Ras2A rendered constitutively active (XK-Ras2AG12V). As a second control, XENaC-expressing oocytes were treated with progesterone, a sex steroid that induces maturation of the oocytes similarly to activated Ras. Progesterone or XK-Ras2AG12V led to oocyte maturation characterized by a decrease in surface area and endogenous Na+ pump function. In both conditions, the surface expression of exogenous XENaC's was also decreased; however, in comparison with progesterone-treated oocytes, XK-ras2AG12V-coinjected oocytes expressed a fivefold higher XENaC-mediated macroscopic Na+ current that was as high as that of control oocytes. Thus, the Na+ current per surface-expressed XENaC was increased by XK-Ras2AG12V. The chemical driving force for Na+ influx was not changed, suggesting that XK-Ras2AG12V increased the mean activity of XENaCs at the oocyte surface. These observations raise the possibility that XK-Ras2A, which is the first regulatory protein known to be transcriptionally induced by aldosterone, could play a role in the control of XENaC function in aldosterone target cells.

Adrenal-dependent modulation of the catalytic subunit isoforms of the Na+-K+-ATPase in aorta

Na+-K+-ATPase gene expression and activity were studied in aortas from adrenalectomized (ADX) rats and ADX rats with deoxycorticosterone supplement (ADX-DOCA). Northern analysis of RNA from ADX rats revealed a significant decrease in alpha2-mRNA levels (38.5 +/- 8.3% of control, P < 0.01) that was prevented by DOCA (P < 0.05). A decrease to 55.8 +/- 7.7% in alpha2-isoform protein was observed 8 days after adrenal removal (P < 0.05); DOCA reversed this effect (90.8 +/- 10.5%). Adrenalectomy induced a decrease of 68.5 +/- 4.5% in beta1-mRNA (P < 0.01) and 52.7 +/- 8.3% in ADX-DOCA rats (P < 0.01). Also, a reduction in beta1-isoform protein that was not prevented by DOCA was detected after adrenalectomy (47.1 +/- 11%, P < 0.01). In contrast, no differences in alpha1-mRNA or -protein levels were observed. Vascular sodium pump activity was reduced to 59.8 +/- 4.6% of control values after adrenalectomy (P < 0.01); this reduction was reversed by DOCA. Our data indicate that corticosteroids regulate Na+-K+-ATPase isoform expression and activity in vascular tissue in vivo, suggesting a mineralocorticoid-dependent modulation of alpha2-Na+-K+-ATPase gene expression in aorta, with beta1-isoform expression dependent on the presence of glucocorticoids.

Epithelial sodium channel regulatory proteins identified by functional expression cloning

We describe here our current strategy for identifying and cloning proteins involved in the regulation of the epithelial sodium channel (ENaC). We have set up a complementation functional assay in the Xenopus laevis oocyte expression system. Using this assay, we have been able to identify a channel-activating protease (CAP-1) that can increase ENaC activity threefold. We propose a novel extracellular signal transduction pathway controlling ionic channels of the ENaC gene family that include genes involved in mechanotransduction (degenerins), in peptide-gated channels involved in neurotransmission (FaNaCh), in proton-gated channels involved in pH sensing (ASIC) or pain sensation (DRASIC).

Mineralocorticoid receptor knockout mice: pathophysiology of Na+ metabolism

Mineralocorticoid receptor (MR)-deficient mice were generated by gene targeting. These animals had a normal prenatal development. During the first week of life, MR-deficient (-/-) mice developed symptoms of pseudohypoaldosteronism. They finally lost weight and eventually died at around day 10 after birth from dehydration by renal sodium and water loss. At day 8, -/- mice showed hyperkalemia, hyponatremia, and a strong increase in renin, angiotensin II, and aldosterone plasma concentrations. Methods were established to measure renal clearance and colonic transepithelial Na+ reabsorption in 8-day-old mice in vivo. The fractional renal Na+ excretion was elevated >8-fold. The glomerular filtration rate in -/- mice was not different from controls. The effect of amiloride on renal Na+ excretion and colonic transepithelial voltage reflects the function of amiloide-sensitive epithelial Na+ channels (ENaC). In -/- mice, it was reduced to 24% in the kidney and to 16% in the colon. There was, however, still significant residual ENaC-mediated Na+ reabsorption in both epithelia. RNase protection analysis of the subunits of ENaC and (Na++ K+)-ATPase did not reveal a decrease in -/- mice. The present data indicate that MR-deficient neonates die because they are not able to compensate renal Na+ loss. Regulation of Na+ reabsorption via MR is not achieved by transcriptional control of ENaC and (Na+ + K+)-ATPase in RNA abundance but by transcriptional control of other as yet unidentified genes. MR knockout mice will be a suitable tool for the search of these genes.

Regulation of rENaC mRNA by dietary NaCl and steroids: organ, tissue, and steroid heterogeneity

Rats on a low-NaCl diet have a high Na+ channel activity in colon and kidney. To address the mechanism of this increased activity, we measured mRNA levels of three Na+ channel subunits in epithelial tissue (rENaC) from rats having been fed either a low (0.13%)- or high (8%)-NaCl diet for 2-3 wk. The size of the mRNA for each of the rENaC subunits as determined by Northern blot was unaffected by diet. RNase protection assay showed heterogeneity of response by organs and subunit. In lung, there was no effect of diet on any of the three subunits. In descending colon, the low-NaCl diet increased beta- and gamma-rENaC mRNA, with no effect on alpha-rENaC mRNA. In the kidney, the response to dietary NaCl was dependent on the region. In cortex and outer medulla, diet had no effect on any of the subunits. Rats fed the low-NaCl diet had greater alpha-rENaC in inner medulla but not beta- or gamma-rENaC mRNA. We next asked whether acute administration of pure glucocorticoid (GC) or mineralocorticoid (MC) hormones to adrenalectomized rats reproduced the effects of a low-NaCl diet. Six hours after administration of GC or MC, a somewhat different heterogeneity occurred. In lung, alpha-rENaC mRNA was increased but only in response to GC. In colon, either GC or MC increased beta- or gamma-rENaC, and there was no effect on alpha-rENaC. In kidney, either GC or MC increased alpha-rENaC, without an effect on beta- or gamma-rENaC. In contrast to the response to a low-NaCl diet, all three regions were similarly affected by acute steroids. These results demonstrate a striking heterogeneity in response to physiological stimuli that regulate ENaC function. The mRNA levels of each of the rENaC subunits can be determined by the type of steroid and by factors unique to the organ and even to the specific region of the kidney.

Subunit stoichiometry of the epithelial sodium channel

The epithelial Na+ Channel (ENaC) mediates Na+ reabsorption in a variety of epithelial tissues. ENaC is composed of three homologous subunits, termed alpha, beta, and gamma. All three subunits participate in channel formation as the absence of any one subunit results in a significant reduction or complete abrogation of Na+ current expression in Xenopus oocytes. To determine the subunit stoichiometry, a biophysical assay was employed utilizing mutant subunits that display significant differences in sensitivity to channel blockers from the wild type channel. Our results indicate that ENaC is a tetrameric channel with an alpha2 beta gamma stoichiometry, similar to that reported for other cation selective channels, such as Kv, Kir, as well as voltage-gated Na+ and Ca2+ channels that have 4-fold internal symmetry.

Renal potassium transport: mechanisms and regulation

The regulation of potassium metabolism involves mechanisms for the appropriate distribution between the intra- and extracellular fluid compartments and for the excretion by the kidney. Clearance and single nephron studies show that renal excretion is determined by regulated potassium secretion and potassium reabsorption, respectively, in principal and intercalated cells of the distal nephron. Measurement of the electrochemical driving forces acting on potassium transport across individual cell membranes and characterization of several ATPases and potassium channels provide insights into the transport and regulation of renal potassium excretion.

In vivo phosphorylation of the epithelial sodium channel

The activity of the epithelial sodium channel (ENaC) in the distal nephron is regulated by an antidiuretic hormone, aldosterone, and insulin, but the molecular mechanisms that mediate these hormonal effects are mostly unknown. We have investigated whether aldosterone, insulin, or activation of protein kinases has an effect on the phosphorylation of the channel. Experiments were performed in an epithelial cell line generated by stable cotransfection of the three subunits (alpha, beta, and gamma) of ENaC. We found that beta and gamma, but not the alpha subunit, are phosphorylated in the basal state. Aldosterone, insulin, and protein kinases A and C increased phosphorylation of the beta and gamma subunits in their carboxyl termini, but none of these agents induced de novo phosphorylation of alpha subunits. Serines and threonines but not tyrosines were found to be phosphorylated. The results suggest that aldosterone, insulin, and protein kinases A and C modulate the activity of ENaC by phosphorylation of the carboxyl termini of the beta and gamma subunits.

In vivo treatment of bullfrog tadpoles with aldosterone potentiates ACh-receptor channels, but not amiloride-blockable Na+ channels in the skin

Amiloride-blockable Na(+) channels participate in active Na(+) transport across adult, but not larval, bullfrog skin. Their development is induced in vitro by culturing the tadpole skin with aldosterone. When tadpoles were raised in aldosterone (5 x 10(-7) M) for 2 weeks, however, neither development of such channels nor localization of antigen A, a marker of adult-type epidermis, was seen, the skin still being of the larval type. In contrast, aldosterone treatment did potentiate (by a factor of two) the activity of the acetylcholine receptor (ACh-receptor) channel, a functional marker of larval-type skin. The short-circuit current (SCC) across the skin, far from being inhibited by amiloride, was stimulated by both amiloride and ACh. The nystatin-stimulated SCC was about twice its control amplitude, suggesting that the aldosterone treatment also potentiated the activity of the Na(+) pump.

Vasopressin potentiates mineralocorticoid selectivity by stimulating 11 beta hydroxysteroid deshydrogenase in rat collecting duct

Arginine vasopressin (AVP) and corticosteroid hormones are involved in sodium reabsorption regulation in the renal collecting duct. Synergy between AVP and aldosterone has been well documented, although its mechanism remains unclear. Both aldosterone and glucocorticoid hormones bind to the mineralocorticoid receptor (MR), and mineralocorticoid selectivity depends on the MR-protecting enzyme 11 beta hydroxysteroid deshydrogenase (11-HSD), which metabolizes glucocorticoids into derivatives with low affinity for MR. We have investigated whether the activity of 11-HSD could be influenced by AVP and corticosteroid hormones. This study shows that in isolated rat renal collecting ducts, AVP increases 11-HSD catalytic activity. This effect is maximal at 10(-8) M AVP (a concentration clearly above the normal physiological range of AVP concentrations) and involves the V2 receptor pathway, while activation of protein kinase C or changes in intracellular calcium are ineffective. The stimulatory effect of AVP on 11-HSD is largely reduced after adrenalectomy, and is selectively restored by infusion of aldosterone, not glucocorticoids. We conclude that this synergy between AVP and aldosterone in controlling the activity of 11-HSD is likely to play a pivotal role in resetting mineralocorticoid selectivity, and hence sodium reabsorption capacities of the renal collecting duct.

An aldosterone regulated chicken intestine protein with high affinity to amiloride

The pattern of chicken intestine amiloride-binding proteins was determined using the photoreactive amiloride analogue 2'-methoxy-5'-nitrobenzamil (NMBA) and a polyclonal anti-amiloride antibody. At 10(-7)M, NMBA inhibits approximately 62% of the Na+ channel activity. At this concentration the amiloride analogue labels a number of membrane proteins, and in particular a 40-45 kDa polypeptide denoted ABP40. Incorporation of NMBA into ABP40 could be prevented by a 100-fold excess of benzamil, but not by a 1000-fold excess of 5-(N-ethyl-N-isopropyl)-amiloride. Labeling of ABP40 was intense in membranes derived from salt-deprived chickens and approximately 5-fold weaker in membranes from salt-repleted animals. Because of its small size, ABP40 is not likely to be an avian Na+ channel subunit, yet this amiloride-binding protein could be involved in the response to aldosterone.

Regulation of nephron acidification by corticosteroids

The present paper reviews work from our laboratories evaluating the importance of adrenal cortical hormones in acidification by proximal and cortical distal tubules. Proximal acidification was determined by stationary microperfusion, and measurement of bicarbonate reabsorption using luminal pH determination was performed with H(+)-ionsensitive microelectrodes. Rats were adrenalectomized (ADX) 48 h before the experiments, and corticosteroids (aldosterone (A), corticosterone (B), and 18-OH corticosterone (18-OH-B)) were injected intramuscularly 100 and 40 min before the experiments. In ADX rats stationary pH increased significantly to 7.03 as compared to sham-operated rats (6.78). Bicarbonate reabsorption decreased from 2.65 +/- 0.18 in sham-operated rats to 0.50 +/- 0.07 nmol cm-2 s-1 after ADX. The administration of the three hormones stimulated proximal tubule acidification, reaching, however, only 47.2% of the sham values in aldosterone-treated rats. Distal nephron acidification was studied by measuring urine minus blood pCO2 differences (U-B pCO2) in bicarbonate-loaded rats treated as above. This pCO2 difference is used as a measure of the distal nephron ability to secrete H+ ions into an alkaline urine. U-B pCO2 decreased significantly from 39.9 +/- 1.26 to 11.9 +/- 1.99 mmHg in ADX rats. When corticosteroids were given to ADX rats before the experiment, U-B pCO2 increased significantly, but reached control levels only when aldosterone (two 3-microgram doses per rat) plus corticosterone (220 micrograms) were given together. In order to control for the effect of aldosterone on distal transepithelial potential difference one group of rats was treated with amiloride, which blocks distal sodium channels. Amiloride-treated rats still showed a significant reduction in U-B pCO2 after ADX. Only corticosterone and 18-OH-B but not aldosterone increased U-B pCO2 back to the levels of sham-operated rats. These results show that corticosteroids stimulate renal tubule acidification both in proximal and distal nephrons and provide some clues about the mechanism of action of these steroids.