Identification of aldosterone-induced proteins in the toad's urinary bladder

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.

Methylation of cytosolic proteins may be a possible biochemical pathway of early aldosterone action in cultured renal collecting duct cells

The common model of aldosterone-dependent sodium transport is that the hormone increases sodium transport during the "early" and "late" response phases by inducing specific proteins (AIPs). However, in actual biochemical studies, AIPs were mostly detected 6-24 h after aldosterone application. Regarding the physiological early response phase, this implies temporal dissociation of the physiological and biochemical events. The discrepancy raises the question as to whether other biochemical events, such as protein modifications, may be involved in addition to the novo protein synthesis. Labelling of cultured renal collecting duct epithelia for 1-5 h with a radioactive methylgroup donor, S-adenosyl methionine (SAM), following tissue fractionation, resulted in progressive methylations of specific cytosolic proteins. Aldosterone-dependent methylations increased consistently with time, and accounted for a 60% increase in total cytosolic protein content as compared to controls after 5 h labelling. The different methylated proteins showed a molecular weight of 220, 97 and 75 kd and comprised groups of proteins with an isoelectric point of 5.1-5.7 and 6.0-7.5. Methylation of identical proteins was obtained by incubation of the epithelia with unlabelled SAM instead of aldosterone. SAM-induced as well as aldosterone-induced methylation of proteins with an isoelectric point of 6.0-7.5 could be inhibited by the methylation inhibitor S-adenosylhomocysteine. The results indicate that aldosterone may influence the SAM cycle in cultured collecting-duct epithelia during increase of the Na+-transport.

Increased chloride permeability of amphibian epithelia treated with aldosterone

The transepithelial flux of chloride was increased by aldosterone treatment of amphibian skin and bladder and this was reflected by increased "shunt" conductance. The hormonal effect depended on the presence of chloride on the epithelial side of the preparation. These changes in tissue conductance and chloride permeability appear to be a direct effect of aldosterone as they did not occur when sodium transport was stimulated with vasopressin or hypotonicity. Chloride efflux was reduced in magnitude by indacrinone and DIDS, as well as after removal of chloride from the solution on the epithelial side of the preparations. These results suggest that, rather than merely diffusing along (a) paracellular pathway(s), chloride flows through (a) cellular structure(s), notably mitochondria-rich cells. These cells can therefore be considered as targets for aldosterone.

Thyroid hormone antagonizes an aldosterone-induced protein: a candidate mediator for the late mineralocorticoid response

In the urinary bladder of the toad Bufo marinus, the basal rate of synthesis of a number of proteins was modulated in a bidirectional way (i.e., induced or repressed) by aldosterone and by triiodothyronine (T3). Each hormone was therefore characterized by a distinct domain of response. When both hormones were added simultaneously, the two domains consistently overlapped at least for one protein, termed AIP-1, or aldosterone-induced protein 1 (Mr approximately 65 kilodaltons, pi = 6.7, as analyzed by two-dimension gel electrophoresis). The physiological role of AIP-1 is unknown, but could be related to the late mineralocorticoid response. In five experiments, T3 (60 nM, 18-hr incubation) consistently repressed AIP-1, while aldosterone-dependent sodium transport (late response) was significantly inhibited, as previously described. The repression of AIP-1 was also observed as early as 6 hr after aldosterone addition. In addition, sodium butyrate (3 mM), which was previously shown to also selectively inhibit the late mineralocorticoid response, was also able to repress AIP-1. Our results suggest that AIP-1 is one of the proteins involved in the mediation of the late mineralocorticoid response.

Steroid-induced protein synthesis in giant-toad (Bufo marinus) urinary bladders. Correlation with natriferic activity

We have identified a group of proteins (Mr approximately 70 000-80 000; pI approximately 5.5-6.0) in giant-toad (Bufo marinus) urinary bladders whose synthesis appears to be related to aldosterone-stimulated Na+ transport. Spironolactone, a specific mineralocorticoid antagonist in renal epithelia, inhibits the synthesis of these proteins as well as the natriferic effect of the hormone. Since a variety of other steroids (some of which are traditionally considered to be glucocorticoids) also stimulate Na+ transport in toad urinary bladders, we examined whether their natriferic activity was expressed in a fashion similar to that of aldosterone. Short-circuit current was used to measure Na+ transport, and epithelial-cell protein synthesis was detected with high-resolution two-dimensional polyacrylamide-gel electrophoresis and autoradiography. At a concentration of approximately 100 nM, dexamethasone, corticosterone and aldosterone were equinatriferic. Dexamethasone and aldosterone had identical dose-response curves, maximal and half-maximal activity being evident at concentrations of approximately 100 nM and 10 nM respectively. In contrast, at a concentration of approximately 10 nM, corticosterone had no effect on Na+ transport. The natriferic activities of these three steroids correlate with their known affinities for the putative mineralocorticoid receptor in toad urinary bladders. Natriferic concentrations of dexamethasone and corticosterone (140 nM) induced the synthesis of proteins with characteristics identical with those induced by aldosterone. Spironolactone, at an antagonist/agonist ratio of 2000:1, inhibited steroid-induced Na+ transport and the synthesis of these proteins. Thus it appears that all natriferic steroids share a common mechanism of action in toad urinary bladders. Natriferic activity can be correlated not only with relative steroid-receptor affinity but also with the induction of a specific group of epithelial-cell proteins.

Energetics of sodium transport in the urinary bladder of the toad. Effect of aldosterone and sodium cyanide

Experiments were designed to determine whether the stimulatory effect of aldosterone on sodium transport involves an increase in tissue ATP. Urinary bladders that were removed from toads presoaked in 0.6% saline for 48-72 h, mounted as sacs, and maintained in open circuit except for brief observation of short circuit current every 30 min responded to 100 nM aldosterone added to the serosal bath with an increase in short circuit current to 170% of control hemibladders, which plateaus at 2-3 h. Tissue (ATP)/(ADP) X (Pi) measured in perchloric acid extracts increased to a maximum of 208% of controls (P less than 0.001) and ATP increased to 116% of controls (P less than 0.01) at 180 min. The short circuit current response to aldosterone paralleled the increase in ATP and (ATP)/(ADP) X (Pi) measured at 75, 120, 180, and 240 min. In bladders clamped at -150 mV, the short circuit current response to aldosterone was greater: 280% of controls (P less than 0.001) and tissue (ATP)/(ADP) X (Pi) increased to 191% of controls (P less than 0.001). In continuously short circuited bladders and bladders clamped at +75 mV, the short circuit current response to aldosterone and the change in ATP, ADP, or Pi were markedly diminished. 100 microM amiloride added to mucosal bath decreased the short circuit current to zero and inhibited the short circuit current response to aldosterone, whereas tissue ATP increased to 141% (P less than 0.05). 100, 250, and 500 microM NaCN dropped the short circuit current to 59, 35, and 24% of control values, respectively. Concurrently, tissue ATP measured at 60 min after the addition of NaCN dropped to 79, 66, and 56% of control values, respectively, and tissue ATP/ADP dropped to 68, 50, and 40%, respectively. The data revealed significant correlation between the change in the rate of sodium transport produced by aldosterone or NaCN as measured by the short circuit current and the concentration of ATP (r = 0.96, P less than 0.001), as well as ATP/ADP (r = 0.95, P less than 0.001). In conclusion, these results support the view that the stimulatory effects of aldosterone on sodium transport involve an increase in ATP or (ATP)/(ADP) X (Pi).

Insulin-induced proteins in the toad urinary bladder

Insulin increases active sodium transport by the toad urinary bladder within 15 min, an effect which persists for an least 20 h. In tissues pre-treated with inhibitors of transcription or translation, sodium transport briefly increases after insulin addition but returns to basal levels within 60-90 min. We have studied the effects of insulin on the incorporation of radioactive amino acids into mucosal cell proteins. Insulin had no detectable effect on the uptake of amino acids nor on their incorporation into total protein; however, using a dual label technique, we found that insulin increases the incorporation of amino acids into specific soluble and plasma membrane proteins of the granular mucosal cell. The time course and dose-response relationship of the induction of protein synthesis by insulin suggest that these proteins may play a role in the sustained elevation of sodium transport by insulin.

Role of induced proteins in insulin-stimulated sodium transport

Insulin increases active sodium transport by the toad urinary bladder, an effect that begins with 15 minutes and persists for at least 20 hours. Although pretreatment of bladders with inhibitors of RNA or protein synthesis has no effect on the response to insulin within the first hour, these agents block the long-term component of insulin-stimulated sodium transport. To examine the relationship of protein synthesis to the sustained increase in sodium transport elicited by insulin, we have studied the effects of the hormone upon the incorporation of radioactive amino acids into mucosal cell proteins. There is no detectable effect of insulin on the uptake of aminoisobutyric acid into mucosal cells or on the incorporation of labeled precursors into total protein; however, using a dual-label technique, we find that insulin increases the incorporation of amino acids into specific soluble and plasma-membrane proteins of granular mucosal cells. Insulin has no discernible effect upon the incorporation of amino acids into proteins of mitochondria-rich mucosal cells. Thus the effects of insulin upon sodium transport appear to be the result of two separate mechanisms, (1) a short-term response that is independent of protein synthesis and (2) a long-term response that is expressed after the first hour of hormone treatment and that requires the synthesis of one or more specific proteins in the granular cell.

Decreased sensitivity to amiloride of amphibian epithelia treated with aldosterone. Further evidence for an apical hormonal effect

The reversible inhibition of transepithelial sodium transport achieved with amiloride (and triamterene) was evaluated in amphibian preparations stimulated with aldosterone so as to provide further information regarding a possible influence of this hormone on the apical border of target cells. When aldosterone secretion was enhanced by withdrawal of sodium from toad (Bufo marinus) habitat, sensitivity of abdominal skin to amiloride decreased; the same occurred in skin and bladder preparations incubated with aldosterone for several hours. Amiloride proved a less efficient blocker of sodium transport by toad skin exposed to vasopressin and to ouabain; both substances are capable or raising cell sodium content. It is therefore proposed that the decrease in sensitivity to amiloride of amphibian epithelial treated with aldosterone results from an increase in target cell sodium, itself due to a hormone-induced increas in sodium conductance at the apical cell border. Glucose, which enhanced markedly the rate of sodium transport in preparations treated with aldosterone for several hours, failed to decrease any further the response to amiloride; this is taken as an argument for an additional (? secondary) influence of aldosterone on the cell's metabolic machinery connected with the operation of the sodium 'pump'.

Microfilament-rich cells in the toad bladder epithelium

Basal cells of the bladder epithelium of Bufo marinus have been found heterogenous and consist of microfilament-rich cells (MFR-cell) and undifferentiated cells (Un-cell). The MFR-cell, which represents approximately 20% of the epithelial cell population, lies between the epithelial layer lining the urinary space and the basement membrane; it extends under several epithelial cells by processes of varying widths and lengths which contact, via desmosomes, other MFR-cells, as well as cells in the superficial layer, i.e., granular and mitochondria-rich cells. The cytoplasm of MFR-cell is filled with intermediate filaments arranged in bundles which run parallel to the plane of the epithelium and no dense granules, typical of granular cells, have been detected. Strong immunofluorescence for actin is associated with cells which occupy the same basal position as MFR-cells. Undifferentiated cells have no contact via desmosomes with adjacent cells and their cytoplasm is filled with free ribosomes; they lack bundles of intermediate filaments and possess no specialized organelles. After a 4-hr pulse of 3H-thymidine, 1.5% of epithelial cells incorporate thymidine into nuclear DNA, out of which 3/4 are basally and 1/4 are apically located. Identification of cell types by electron microscopy reveals that approximately 10% of undifferentiated basal cells are labeled, whereas less than 0.1% of granular cells and no MFR-cells incorporate 3H-thymidine into DNA. When dissociated from the epithelium and separated by isopycnic centrifugation, MFR-cells possess a mean buoyant density of approximately 1.025, cosediment with mitochondria-rich cells and exhibit a strong immunofluorescence for actin. The function of MFR-cells remains unknown; however, they may play a role in cell coupling and responses to hormonal and physical factors.

Effect of aldosterone on incorporation of amino acids into renal medullary proteins

Studies on the effects of pretreatment with aldosterone on the incorporation of 3H leucine or 3H methionine into proteins in renal slices were carried out in Joklik-modified minimal essential medium. Administration of aldosterone (2microgram/100 g body wt) to adrenalectomized rats increased 3H leucine incorporation into trichloroacetic acid insoluble fractions of crude homogenates of cortical slices by 15.5 +/- 0.4% and of medullary slices by 53.5 +/- 1.3%. No increase in isotope incorporation was observed in slices of renal papilla or spleen prepared from the same rats. Aldosterone had no effect on the 3H-leucine content of the trichloroacetic acid-soluble fractions of all three renal zones and the spleen. The dose of aldosterone that elicited a half-maximal increase in 3H-methionine incorporation into proteins of renal medullary slices (0.45 microgram of aldosterone/100 g body wt) was indistinguishable from that needed to elicit a halt-maximal increase in the urinary K+/Na+ ratio (0.35 microgram of aldosterone/100 g body wt). Dexamethasone, a potent glucocorticoid, at a dose of 0.8 microgram/100 g body wt did not augment 3H-leucine incorporation into renal medullary proteins but was effective at 8 microgram/100 g body wt. Spirolactone (SC-26304), a potent anti-mineralocorticoid, abolished the effect of aldosterone on amino acid incorporation into medullary proteins when administered at a 100-fold higher dosage [i.e., 80 microgram (per 100 g body wt)]. These results imply that the action of aldosterone on amino acid incorporation is mediated by the mineralocorticoid rather than the glucocorticoid pathway, presumably the mineralocorticoid receptors. Moreover, pretreatment of the rats with actinomycin D (70--80 microgram/100 g body wt) erased the effect of aldosterone (0.8 microgram/100 g body wt) on amino acid incorporation into medullary proteins. In paired experiments with 3H and 35S methionine, aldosterone (0.8 microgram/100 g body wt) increased methionine incorporation into trichloroacetic acid precipitable proteins of subcellular fractions of the renal medulla. The effect of aldosterone on incorporation of methionine into medullary cytosol proteins was analyzed further by polyacrylamide gel electrophoresis at pH 8.3 in tris-glycine buffer. The gel profiles indicate that aldosterone significantly increased methionine incorporation into at least one protein (independent of the isotope) with a molecular weight of approximately 31,000. This increase was inhibited by either pretreatment of the rat with actinomycin D (70--80 microgram/100 g body wt or SC-26304 (80 microgram/100 g body wt). Dexamethasone (0.8 microgram/100 g body wt) did not increase incorporation of methinine into the medullary cytosol proteins resolved by polyacrylamide gel electrophoresis.

Rapid isolation of chloride cells from pinfish gill

Enriched fractions of chloride cells with good ultrastructural integrity have been obtained from gill filaments of the euyhaline teleost, Lagodon rhomboides. The branchial epithelium from seawater-adapted fish was dissociated by gentle mechanical means in a Ca++, Mg++-free balanced salt solution. Density gradient centrifugation of the mixed cell suspensions through a Ficoll gradient yielded a fraction containing between 50 and 70% chloride cells. This fraction showed a 3- to 4-fold enrichment over comparable gill homogenate values for sodium plus potassium-activated adenosinetriphosphatase, (Na+, K+ ATPase), an enzyme concentrated in chloride cells. Isolation of chloride cells from fish adapted to one-third seawater was less successful, due to the smaller size and reduced number of these cells, although fractions with at least a 2-fold enrichment of the enzyme were obtained. These results continue to support the belief that chloride cells are responsible for osmoregulatory activity associated with the branchial epithelium of teleosts and that this vital function is mediated through the activity of the transport associated enzyme, Na+, K+-ATPase, the specific activity of which increases with osmotic stress.

Role of RNA in the action of aldosterone on Na+ transport

Recent data describing the effects of aldosterone on the induction of messenger RNA (= mRNA) and ribosomal RNA (= rRNA) are reviewed. In the urinary bladder of the toad, aldosterone induces a few specific polyadenylated mRNAs (= poly(A)(+)mRNA) during the latent period, i.e., 30 to 60 min after hormone addition. Later i.e., 90 to 240 min after aldosterone addition, 18S and 28S cytoplasmic rRNA subunits are also induced. The effect of poly(A)(+)mRNA is mineralocorticoid-specific and correlates well with the aldosterone-dependent Na+ transport. Actinomycin D which inhibits both poly(A)(+)mRNA and nonpolyadenylated mRNA (= POLY(A)(-)mRNA) totally abolishes the response to aldosterone on Na+ transport. 3'deoxyadenosine (cordycepin), which inhibits poly(A)(+)mRNA but not poly(A)(-)mRNA, only inhibits 50 to 60% of the physiological response. These differential effects suggest that an intact poly(A)(-)mRNA pathway is also an important factor in mediating the action of aldosterone. In contrast, 3'deoxycytidine, which inhibits rRNA but not mRNA, does not impair the mineralocorticoid response, at least during the first 3 hr of aldosterone action.

Comparison of toad bladder aldosterone-induced proteins and proteins synthesized in vitro using aldosterone-induced messenger RNA as template

Using double-labeled isotope techniques, it can be shown that aldosterone induces the synthesis of several proteins in the mitochondria-rich (MR) cells of the toad's urinary bladder. Induced proteins have been identified both in the plasma membrane (mol wt = 170,000, 85,000 and 12,000) and the cytosol (mol wt = 36,000, 12,000 and 6,000) fractions of these mucosal cells. We have also shown that aldosterone (Aldo) induces the synthesis of a class of RNA having the properties of messenger RNA (mRNA). mRNA isolated from Aldo-treated mucosal cells was used as template in a cell-free protein-synthesis system prepared from rabbit reticulocytes. Preparations charged with mRNA from Aldo-treated cells synthesized two proteins that were not labeled when mRNA from control tissues was used as template. The electrophoretic mobility of one of these proteins was similar to an Aldo-induced membrane protein (mol wt = 70,000) found in the intact tissue.

Effect of aldosterone on ion transport by rabbit colon in vitro

Segments of descending colon obtained from rabbits, that had been maintained on drinking water containing 25 mM NaCl and an artificial diet which contains 1% Na and is nominally K-free, respond to aldosterone in vitro (after a 30 to 60-min lag period) with a marked increase in the short-circuit current (Isc), an equivalent increase in the rate of active Na absorption (JNa net) and a decline in tissue resistance (Rt). Aldosterone also brings about a marked increase in the unidirection influx of Na into the cells across the mucosal membrane ("zero-time" rate of uptake) which does not differ significantly from the inrease m Isc. Treatment of control tissues with amphotericin B brings about sustained increases in Isc and JNa net to levels observed in aldosterone-treated tissues. However, addition of amphotericin B to the mucosal solution of aldosterone-treated tissues does not result in a sustained increase in Isc or JNa net and these values do not differ markedly from those observed in control tissues treated with amphotericin B. These findings, together with other evidence that Na entry in the presence of amphotericin B is sufficiently rapid to saturate the active Na extrusion mechanism at the baso-lateral membrane, are consistent with the notion that the aldosterone-induced protein increases the permeability of the mucosal membrane to Na but does not increase the "saturation level" of the active Na "pump" within the time-frame of these studies (3 hr). Finally, aldosterone has no effect on the bidirectional or net transepithelial movements of K under short-circuit conditions, suggesting that the enhanced secretion of K observed in vivo is the result of increased diffusion of K from plasma to lumen via paracellular pathways in response to an increased transepithelial electrical potential difference (lumen negative).

Spironolactone antagonism of aldosterone action on Na+ transport and RNA metabolism in toad bladder epithelium

In earlier studies, aldosterone increased the incorporation of precursors into a class of cytoplasmic RNA with the characteristics of messenger RNA (mRNA), in toad bladder epithelium. In the present studies, this effect was analyzed further with a competitive antagonist, spironolactone (SC-9420). Paired hemibladders were labeled with 3H-uridine (30 min pulse - 140 min chase), with or without aldosterone (3.5 x 10(-8) M, 7 X 10(-8) M) in the presence or absence of SC-9420 (7 X 10(-6) M, 2.5 X 10(-5) M) at molar ratios of 200:1 to 280:1. Cytoplasmic RNA, either the total phenol-SDS extract or polyadenylated-RNA (poly(A)(+)-RNA) obtained by oligo-deoxythymidylate-cellulose (oligo(dT)-cellulose) chromatography was analyzed in linear 5 -- 20% sucrose gradients. Eight sets of experiments were completed in which the short-circuit current (scc) was monitored for 180 min and the incorporation of 3H-uridine (30 min pulse -- 150 min chase) was simultaneously determined on pools of epithelia from 5 to 10 hemibladders. The fractional change in scc correlated linearly with the fractional change in 3H-uridine of 12S cytoplasmic RNA (r=0.95, p less than 0.001). The poly(A)(+)-RNA fraction had no detectable rRNA or tRNA and gave a heterogeneous pattern, typical of mRNA, in the sucrose gradients. In the presence of exogenous aldosterone, SC-9420 inhibited the incorporation of 3H-uridine into poly(A)(+)-RNA (particularly 12S). These results support the inference that induction of mRNA mediates the action of aldosterone on Na+ transport.

Estimation of the lifespan of amiloride binding sites in the membranes of toad bladder epithelial cells

1. Sodium entry sites in the membranes of isolated epithelial cells prepared from bladders of toads (Bufo marinus) have been labelled with amiloride. The number of binding sites remained constant in suspensions for up to 100 hr. 2. In the presence of a protein synthesis inhibitor (cycloheximide, 0-5 mug/ml.) there was a decline in the density of binding sites was approximately exponential. Regression analysis gave a half-life of approximately 60 hr. 3. Aldosterone (5 X 10(-8) M) caused a significant (P less than 0-001) increase (50%) in the density of amiloride binding sites. Cells which had been treated with aldosterone had populations of binding sites which declined, in the presence of cycloheximide, at rates indistinguishable from those of untreated cells.