Protein prenylation is required for aldosterone-stimulated Na+ transport

30 YEARS OF THE MINERALOCORTICOID RECEPTOR: Nongenomic effects via the mineralocorticoid receptor

The mineralocorticoid receptor (MR) belongs to the steroid hormone receptor family and classically functions as a ligand-dependent transcription factor. It is involved in water-electrolyte homeostasis and blood pressure regulation but independent from these effects also furthers inflammation, fibrosis, hypertrophy and remodeling in cardiovascular tissues. Next to genomic effects, aldosterone elicits very rapid actions within minutes that do not require transcription or translation and that occur not only in classical MR epithelial target organs like kidney and colon but also in nonepithelial tissues like heart, vasculature and adipose tissue. Most of these effects can be mediated by classical MR and its crosstalk with different signaling cascades. Near the plasma membrane, the MR seems to be associated with caveolin and striatin as well as with receptor tyrosine kinases like EGFR, PDGFR and IGF1R and G protein-coupled receptors like AT1 and GPER1, which then mediate nongenomic aldosterone effects. GPER1 has also been named a putative novel MR. There is a close interaction and functional synergism between the genomic and the nongenomic signaling so that nongenomic signaling can lead to long-term effects and support genomic actions. Therefore, understanding nongenomic aldosterone/MR effects is of potential relevance for modulating genomic aldosterone effects and may provide additional targets for intervention.

Physiological and anatomic evidence for functional subclasses of serotonergic raphe magnus cells

Serotonergic cells in the medullary nucleus raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC) project to the spinal cord where they are likely to modulate nociceptive transmission. Previous studies have suggested that these cells are physiologically and anatomically heterogeneous. In the present investigation, we examined whether subclasses of serotonergic RM and NRMC cells can be delineated based on their response to a visceral stimulus, and whether any such subclasses are morphologically distinct. Most RM and NRMC serotonergic cells tested (81 of 116) responded to retraction of the descending aorta into a polyethylene tube (the snare stimulus) with 57% of all cells tested excited and 13% inhibited. Responses of serotonergic cells to the snare outlasted the stimulus, were not reflective of evoked cardiovascular changes, and were observed in sino-aortic deafferented rats, evidence that the snare stimulus does not influence serotonergic cell discharge through activation of baroreceptors. Because serotonergic cells responsive to the snare were also responsive to mechanical brushing within the retroperitoneum, the snare is likely to change serotonergic cell discharge by means of the activation of mechanosensitive visceral afferents. Intracellular labeling of physiologically characterized serotonergic RM and NRMC cells showed that cells that were responsive to the snare stimulus had simpler axonal collateralization patterns than cells that were unresponsive to the snare stimulus. This association between morphological and physiological properties provides additional evidence that subpopulations of serotonergic cells exist and serve varied physiological functions.

Carboxyl-methylation of Rab3D in the rat pancreatic acinar tumor cell line AR42J

Rab3D is a small GTPase implicated in regulated exocytosis, and is a marker of secretory granules in exocrine cells. We have previously shown that rab3D undergoes reversible carboxyl-methylation in adult rat pancreatic acinar cells, and that carboxyl-methylation of rab3D is developmentally regulated concomitantly with the maturation of the regulated secretory apparatus in rat pancreas. We also observed that dexamethasone treatment of the rat pancreatic acinar tumor cell line, AR42J, led to a significant increase in the size of the unmethylated pool of a rab3-like protein. The current study was designed to further characterize this rab3-like protein. Here we show that AR42J cells express rab3D, and that the protein focuses on 2D gels as two spots with pI values of 4.9 and 5.0. Treatment of AR42J cells with N-acetyl-S-geranylgeranyl-l-cysteine, an inhibitor of carboxyl-methylation, led to a decrease in the basic form of rab3D and a proportional increase in the acidic form. In contrast, N-acetyl-S-farnesyl-l-cysteine, which inhibits carboxyl-methylation of farnesylated proteins, had no effect. Lovastatin, an inhibitor of geranylgeranylation, also induced an accumulation of the acidic form of rab3D. Taken together, these data indicate that rab3D can undergo reversible carboxyl-methylation in AR42J cells by a geranylgeranyl-specific methyltransferase. The 2D gel and immunoblotting analyses indicated that dexamethasone treatment of AR42J cells led to an increase in the proportion of the unmethylated form of rab3D concurrent to inducing a regulated secretory pathway, similar to the rab3D profile change in developing rat pancreas. Our data, along with previous studies done on developing rat pancreas, indicate that the tumor cell line AR42J represents a good model system for studying the regulated secretory pathway, and that carboxyl-methylation of rab3D may play a role in the acquisition of stimulus-secretion coupling.

Aldosterone induces ras methylation in A6 epithelia

Aldosterone increases Na(+) reabsorption by renal epithelial cells: the acute actions (<4 h) appear to be promoted by protein methylation. This paper describes the relationship between protein methylation and aldosterone's action and describes aldosterone-mediated targets for methylation in cultured renal cells (A6). Aldosterone increases protein methylation from 7.90 +/- 0.60 to 20.1 +/- 0.80 methyl ester cpm/microg protein. Aldosterone stimulates protein methylation by increasing methyltransferase activity from 14.0 +/- 0.64 in aldosterone-depleted cells to 31.8 +/- 2.60 methyl ester cpm/microg protein per hour in aldosterone-treated cells. Three known methyltransferase inhibitors reduce the aldosterone-induced increase in methyltransferase activity. One of these inhibitors, the isoprenyl-cysteine methyltransferase-specific inhibitor, S-trans, trans-farnesylthiosalicylic acid, completely blocks aldosterone-induced protein methylation and also aldosterone-induced short-circuit current. Aldosterone induces protein methylation in two molecular weight ranges: near 90 kDa and around 20 kDa. The lower molecular weight range is the weight of small G proteins, and aldosterone does increase both Ras protein 1.6-fold and Ras methylation almost 12-fold. Also, Ras antisense oligonucleotides reduce the activity of Na(+) channels by about fivefold. We conclude that 1) protein methylation is essential for aldosterone-induced increases in Na(+) transport; 2) one target for methylation is p21(ras); and 3) inhibition of Ras expression or Ras methylation inhibits Na(+) channel activity.

Toward Understanding the Role of Methylation in Aldosterone-Sensitive Na(+) Transport

Proper endocrine regulation of Na(+) reabsorption by renal principal cells is the primary means in mammals for maintaining blood pressure. Aldosterone increases Na(+) reabsorption by activating luminal Na(+) channels; however, the signal transduction pathway of aldosterone is not fully understood. Cellular methylation is necessary for aldosterone signaling to the luminal Na(+) channel. We describe the enzymes, regulators, and effectors of aldosterone-mediated methylation relevant to Na(+) reabsorption.

Methylation increases the open probability of the epithelial sodium channel in A6 epithelia

We used single channel methods on A6 renal cells to study the regulation by methylation reactions of epithelial sodium channels. 3-Deazaadenosine (3-DZA), a methyltransferase blocker, produced a 5-fold decrease in sodium transport and a 6-fold decrease in apical sodium channel activity by decreasing channel open probability (P(o)). 3-Deazaadenosine also blocked the increase in channel open probability associated with addition of aldosterone. Sodium channel activity in excised "inside-out" patches usually decreased within 1-2 min; in the presence of S-adenosyl-l-methionine (AdoMet), activity persisted for 5-8 min. Sodium channel mean time open (t(open)) before and after patch excision was higher in the presence of AdoMet than in untreated excised patches but less than t(open) in cell-attached patches. Sodium channel activity in excised patches exposed to both AdoMet and GTP usually remained stable for more than 10 min, and P(o) and the number of active channels per patch were close to values in cell-attached patches from untreated cells. These findings suggest that a methylation reaction contributes to the activity of epithelial sodium channels in A6 cells and is directed to some regulatory element closely connected with the channel, whose activity also depends on the presence of intracellular GTP.

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.

Isoprenylcysteine-O-carboxyl methyltransferase regulates aldosterone-sensitive Na(+) reabsorption

The Xenopus laevis distal tubule epithelial cell line A6 was used as a model epithelia to study the role of isoprenylcysteine-O-carboxyl methyltransferase (pcMTase) in aldosterone-mediated stimulation of Na(+) transport. Polyclonal antibodies raised against X. laevis pcMTase were immunoreactive with a 33-kDa protein in whole cell lysate. These antibodies were also reactive with a 33-kDa product from in vitro translation of the pcMTase cDNA. Aldosterone application increased pcMTase activity resulting in elevation of total protein methyl esterification in vivo, but pcMTase protein levels were not affected by steroid, suggesting that aldosterone increased activity independent of enzyme number. Inhibition of pcMTase resulted in a reduction of aldosterone-induced Na(+) transport demonstrating the necessity of pcMTase-mediated transmethylation for steroid induced Na(+) reabsorption. Transfection with an eukaryotic expression construct containing pcMTase cDNA increased pcMTase protein level and activity. This resulted in potentiation of the natriferic actions of aldosterone. However, overexpression did not change Na(+) reabsorption in the absence of steroid, suggesting that pcMTase activity is not limiting Na(+) transport in the absence of steroid, but that subsequent to aldosterone addition, pcMTase activity becomes limiting. These results suggest that a critical transmethylation is necessary for aldosterone-induction of Na(+) transport. It is likely that the protein catalyzing this methylation is isoprenylcysteine-O-carboxyl methyltransferase and that aldosterone activates pcMTase without affecting transferase expression.

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.

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.