Mineralocorticoid receptor antagonists do not block rapid ERK activation by aldosterone

New Perspectives on Sex Steroid and Mineralocorticoid Receptor Signaling in Cardiac Ischemic Injury

The global burden of ischemic heart disease is burgeoning for both men and women. Although advances have been made, the need for new sex-specific therapies targeting key differences in cardiovascular disease outcomes in men and women remains. Mineralocorticoid receptor directed treatments have been successfully used for blood pressure control and heart failure management and represent a potentially valuable therapeutic option for ischemic cardiac events. Clinical and experimental data indicate that mineralocorticoid excess or inappropriate mineralocorticoid receptor (MR) activation exacerbates ischemic damage, and many of the intracellular response pathways activated in ischemia and subsequent reperfusion are regulated by MR. In experimental contexts, where MR are abrogated genetically or mineralocorticoid signaling is suppressed pharmacologically, ischemic injury is alleviated, and reperfusion recovery is enhanced. In the chronic setting, mineralocorticoid signaling induces fibrosis, oxidative stress, and inflammation, which can predispose to ischemic events and exacerbate post-myocardial infarct pathologies. Whilst a range of cardiac cell types are involved in mineralocorticoid-mediated regulation of cardiac function, cardiomyocyte-specific MR signaling pathways are key. Selective inhibition of cardiomyocyte MR signaling improves electromechanical resilience during ischemia and enhances contractile recovery in reperfusion. Emerging evidence suggests that the MR also contribute to sex-specific aspects of ischemic vulnerability. Indeed, MR interactions with sex steroid receptors may differentially regulate myocardial nitric oxide bioavailability in males and females, potentially determining sex-specific post-ischemic outcomes. There is hence considerable impetus for exploration of MR directed, cell specific therapies for both women and men in order to improve ischemic heart disease outcomes.

Non-genomic steroid signaling through the mineralocorticoid receptor: Involvement of a membrane-associated receptor?

Corticosteroid receptors in the mammalian brain mediate genomic as well as non-genomic actions. Although receptors mediating genomic actions were already cloned 35 years ago, it remains unclear whether the same molecules are responsible for the non-genomic actions or that the latter involve a separate class of receptors. Here we focus on one type of corticosteroid receptors, i.e. the mineralocorticoid receptor (MR). We summarize some of the known properties and the current insight in the localization of the MR in peripheral cells and neurons, especially in relation to non-genomic signaling. Previous studies from our own and other labs provided evidence that MRs mediating non-genomic actions are identical to the ones involved in genomic signaling, but may be translocated to the plasma cell membrane instead of the nucleus. With fixed cell imaging and live cell imaging techniques we tried to visualize these presumed membrane-associated MRs, using antibodies or overexpression of MR-GFP in COS7 and hippocampal cultured neurons. Despite the physiological evidence for MR location in or close to the cell membrane, we could not convincingly visualize membrane localization of endogenous MRs or GFP-MR molecules. However, we did find punctae of labeled antibodies intracellularly, which might indicate transactivating spots of MR near the membrane. We also found some evidence for trafficking of MR via beta-arrestins. In beta-arrestin knockout mice, we didn't observe metaplasticity in the basolateral amygdala anymore, indicating that internalization of MRs could play a role during corticosterone activation. Furthermore, we speculate that membrane-associated MRs could act indirectly via activating other membrane located structures like e.g. GPER and/or receptor tyrosine kinases.

ERK1,2 Signalling Pathway along the Nephron and Its Role in Acid-base and Electrolytes Balance

Mitogen-activated protein kinases (MAPKs) are intracellular molecules regulating a wide range of cellular functions, including proliferation, differentiation, apoptosis, cytoskeleton remodeling and cytokine production. MAPK activity has been shown in normal kidney, and its over-activation has been demonstrated in several renal diseases. The extracellular signal-regulated protein kinases (ERK 1,2) signalling pathway is the first described MAPK signaling. Intensive investigations have demonstrated that it participates in the regulation of ureteric bud branching, a fundamental process in establishing final nephron number; in addition, it is also involved in the differentiation of the nephrogenic mesenchyme, indicating a key role in mammalian kidney embryonic development. In the present manuscript, we show that ERK1,2 signalling mediates several cellular functions also in mature kidney, describing its role along the nephron and demonstrating whether it contributes to the regulation of ion channels and transporters implicated in acid-base and electrolytes homeostasis.

Signaling pathways involved in the rapid biphasic effect of aldosterone on Na+/H+ exchanger in rat proximal tubule cells

The receptors and signaling pathways for nongenomic effects of aldosterone (Aldo) on the proximal Na⁺/H⁺ exchanger are still unknown; therefore, the aim of this study was to investigate the mineralocorticoid receptor (MR) and/or glucocorticoid receptor (GR) participation in rapid Aldo effects on NHE1 (basolateral Na⁺/H⁺ exchanger isoform) and cytosolic calcium concentration ([Ca²⁺]_i). In addition, phospholipase C (PLC), protein kinase C (PKC), and mitogen-activated protein kinase kinase (MEK) involvement in signaling pathways of such effects was evaluated, using immortalized proximal tubule cells of rat (IRPTC) as an experimental model. MR and GR expression was investigated using reverse transcription polymerase chain reaction and immunoblotting. The intracellular pH recovery rate (after acid loading) and [Ca²⁺]_i were determined by the probes BCECF-AM and FURA 2-AM, respectively. Aldo (10^-12 M) promoted a moderate increase in [Ca²⁺]_i and stimulation of NHE1, whereas Aldo (10^-6 M) greatly increased the [Ca²⁺]_i, but inhibited the NHE1. BAPTA-AM (a calcium chelator), GR antagonism and inhibition of PLC, PKC and MEK pathway abolished the biphasic and dose-dependent effect of Aldo on NHE1 and decreased the [Ca²⁺]_i; whereas MR do not appear to participate in this rapid signaling in IRPTC cells. The reduction of GR content, by gene silencing, abolished the Aldo effect on NHE1, in low concentration, confirming the importance of this receptor in the rapid modulation of proximal sodium and hydrogen transports.

Aldosterone-induced protein kinase signalling and the control of electrolyte balance

Aldosterone acts through the mineralocorticoid receptor (MR) to modulate gene expression in target tissues. In the kidney, the principal action of aldosterone is to promote sodium conservation in the distal nephron and so indirectly enhance water conservation under conditions of hypotension. Over the last twenty years the rapid activation of protein kinase signalling cascades by aldosterone has been described in various tissues. This review describes the integration of rapid protein kinase D signalling responses with the non-genomic actions of aldosterone and transcriptional effects of MR activation.

The function and regulation of acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC): IUPHAR Review 19

Acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC) are both members of the ENaC/degenerin family of amiloride-sensitive Na(+) channels. ASICs act as proton sensors in the nervous system where they contribute, besides other roles, to fear behaviour, learning and pain sensation. ENaC mediates Na(+) reabsorption across epithelia of the distal kidney and colon and of the airways. ENaC is a clinically used drug target in the context of hypertension and cystic fibrosis, while ASIC is an interesting potential target. Following a brief introduction, here we will review selected aspects of ASIC and ENaC function. We discuss the origin and nature of pH changes in the brain and the involvement of ASICs in synaptic signalling. We expose how in the peripheral nervous system, ASICs cover together with other ion channels a wide pH range as proton sensors. We introduce the mechanisms of aldosterone-dependent ENaC regulation and the evidence for an aldosterone-independent control of ENaC activity, such as regulation by dietary K(+) . We then provide an overview of the regulation of ENaC by proteases, a topic of increasing interest over the past few years. In spite of the profound differences in the physiological and pathological roles of ASICs and ENaC, these channels share many basic functional and structural properties. It is likely that further research will identify physiological contexts in which ASICs and ENaC have similar or overlapping roles.

Role of Nongenomic Signaling Pathways Activated by Aldosterone During Cardiac Reperfusion Injury

Aldosterone (Aldo) activates both genomic and nongenomic signaling pathways in the cardiovascular system. Activation of genomic signaling pathways contributes to the adverse cardiac actions of Aldo during reperfusion injury; however, the extent nongenomic signaling pathways contribute has been difficult to identify due to lack of a specific ligand that activates only nongenomic signaling pathways. Using a pegylated aldosterone analog, aldosterone-3-carboxymethoxylamine-TFP ester conjugated to methoxypegylated amine (Aldo-PEG), we are able for the first time to distinguish between nongenomic and genomic cardiac actions of Aldo. We confirm Aldo-PEG activates phosphorylation of ERK1/2 in rat cardiomyocyte H9c2 cells similar to Aldo and G protein-coupled receptor 30 (GPR30 or GPER) agonist G1. GPER antagonist, G36, but not mineralocorticoid receptor (MR) antagonist spironolactone, prevented ERK1/2 phosphorylation by Aldo, Aldo-PEG, and G1. The selective nongenomic actions of Aldo-PEG are confirmed, with Aldo-PEG increasing superoxide production in H9c2 cells to similar levels as Aldo but having no effect on subcellular localization of MR. Striatin serves as a scaffold for GPER and MR, with GPER antagonist G36, but not spironolactone, restoring MR-striatin complexes. Aldo-PEG had no effect on MR-dependent transcriptional activation, whereas Aldo increased transcript levels of serum-regulated kinase 1 and plasminogen activator inhibitor-1. Using our ex vivo experimental rat model of myocardial infarction, we found aggravated infarct size and apoptosis by Aldo but not Aldo-PEG. Our studies confirm that in the heart, activation of nongenomic signaling pathways alone are not sufficient to trigger the deleterious effects of aldosterone during myocardial reperfusion injury.

Cardiac effects of aldosterone: does gender matter?

Ischemic heart disease (IHD) continues to be the most common cause of death globally, although mortality rates are decreasing with significant advances in treatment. Higher prevalence of co-morbidities in women only partly explains the lack of decrease in mortality rates in younger women due to. Until recently there has been gender bias in pre-clinical studies and many clinical trials, resulting in a significant gap in knowledge whether there are differential responses to therapy for women, particularly younger women. There is increasing evidence that there are significant gender-specific differences in the outcome of post-infarction remodelling, prevalence of hypertension and sudden cardiac death. These differences indicate that cardiac tissue in females displays significant physiological and biochemical differences compared to males. However, the mechanisms mediating these differences, and how they change with age, are poorly understood. Circulating levels and physiological effects of aldosterone vary across the menstrual cycle suggesting female steroid sex hormones may not only regulate production of, but also responses to, aldosterone in pre-menopausal women. This modified tissue response may foster a homeostatic environment where higher levels of aldosterone are tolerated without adverse cardiac effect. Moreover, there is limited data on the direct regulation of this signalling axis by androgens in female animals/subjects. This review explores the relationship between gender and the effects of aldosterone in cardiovascular disease (CVD), an issue of significant need that may lead to changes in best practice to optimise clinical care and improve outcomes for females with CVD.

Mineralocorticoid receptor signaling: crosstalk with membrane receptors and other modulators

The mineralocorticoid receptor (MR) belongs to the steroid receptor superfamily. Classically, it acts as a ligand-bound transcription factor in epithelial tissues, where it regulates water and electrolyte homeostasis and controls blood pressure. Additionally, the MR has been shown to elicit pathophysiological effects including inflammation, fibrosis and remodeling processes in the cardiovascular system and the kidneys and MR antagonists have proven beneficial for patients with certain cardiovascular and renal disease. The underlying molecular mechanisms that mediate MR effects have not been fully elucidated but very likely rely on interactions with other signaling pathways in addition to genomic actions at hormone response elements. In this review we will focus on interactions of MR signaling with different membrane receptors, namely receptor tyrosine kinases and the angiotensin II receptor because of their potential relevance for disease. In addition, GPR30 is discussed as a new aldosterone receptor. To gain insights into the problem why the MR only seems to mediate pathophysiological effects in the presence of additional permissive factors we will also briefly discuss factors that lead to modulation of MR activity as well. Overall, MR signaling is part of an intricate network that still needs to be investigated further.

Mineralocorticoids in the heart and vasculature: new insights for old hormones

The mineralocorticoid aldosterone is a key regulator of water and electrolyte homeostasis. Numerous recent developments have advanced the field of mineralocorticoid pharmacology—namely, clinical trials have shown the beneficial effects of aldosterone antagonists in chronic heart failure and post-myocardial infarction treatment. Experimental studies using cell type-specific gene targeting of the mineralocorticoid receptor (MR) gene in mice have revealed the importance of extrarenal aldosterone signaling in cardiac myocytes, endothelial cells, vascular smooth cells, and macrophages. In addition, several molecular pathways involving signal transduction via the classical MR as well as the G protein-coupled receptor GPER mediate the diverse spectrum of effects of aldosterone on cells. This knowledge has initiated the development of new pharmacological ligands to specifically interfere with targets on different levels of aldosterone signaling. For example, aldosterone synthase inhibitors such as LCI699 and the novel nonsteroidal MR antagonist BAY 94-8862 have been tested in clinical trials. Interference with the interaction between MR and its coregulators seems to be a promising strategy toward the development of selective MR modulators.

Aldosterone stimulates vacuolar H(+)-ATPase activity in renal acid-secretory intercalated cells mainly via a protein kinase C-dependent pathway

Urinary acidification in the collecting duct is mediated by the activity of H(+)-ATPases and is stimulated by various factors including angiotensin II and aldosterone. Classically, aldosterone effects are mediated via the mineralocorticoid receptor. Recently, we demonstrated a nongenomic stimulatory effect of aldosterone on H(+)-ATPase activity in acid-secretory intercalated cells of isolated mouse outer medullary collecting ducts (OMCD). Here we investigated the intracellular signaling cascade mediating this stimulatory effect. Aldosterone stimulated H(+)-ATPase activity in isolated mouse and human OMCDs. This effect was blocked by suramin, a general G protein inhibitor, and GP-2A, a specific G(αq) inhibitor, whereas pertussis toxin was without effect. Inhibition of phospholipase C with U-73122, chelation of intracellular Ca(2+) with BAPTA, and blockade of protein kinase C prevented the stimulation of H(+)-ATPases. Stimulation of PKC by DOG mimicked the effect of aldosterone on H(+)-ATPase activity. Similarly, aldosterone and DOG induced a rapid translocation of H(+)-ATPases to the luminal side of OMCD cells in vivo. In addition, PD098059, an inhibitor of ERK1/2 activation, blocked the aldosterone and DOG effects. Inhibition of PKA with H89 or KT2750 prevented and incubation with 8-bromoadenosine-cAMP mildly increased H(+)-ATPase activity. Thus, the nongenomic modulation of H(+)-ATPase activity in OMCD-intercalated cells by aldosterone involves several intracellular pathways and may be mediated by a G(αq) protein-coupled receptor and PKC. PKA and cAMP appear to have a modulatory effect. The rapid nongenomic action of aldosterone may participate in the regulation of H(+)-ATPase activity and contribute to final urinary acidification.

Aldosterone up-regulates 12- and 15-lipoxygenase expression and LDL oxidation in human vascular smooth muscle cells

Several lines of evidence suggest that aldosterone excess may have detrimental effects in the cardiovascular system, independent of its interaction with the renal epithelial cells. Here we examined the possibility that aldosterone modulates 12- and/or 15-lipoxygenase (LO) expression/activity in human vascular smooth muscle cells (VSMC), in vitro, thereby potentially contributing to both vascular reactivity and atherogenesis. Following 24 h treatment of VSMC with aldosterone (1 nmol/L), there was a approximately 2-fold increase in the generation rate of 12 hydroxyeicosatetraenoic acid (12-HETE), 70% increase in platelet type 12-LO mRNA expression (P < 0.001) along with a approximately 3-fold increase in 12-LO protein expression, which were blocked by the mineralocorticoid receptor (MR) antagonists spironolactone (100 nmol/L) and eplerelone (100 nmol/ml). Additionally, aldosterone (1 nmol/L; 24 h) increased the production of 15-HETE (50%; P < 0.001) and the expression of 15-LO type 2 mRNA (50%; P < 0.05) (in VSMC). Aldosterone also increased the 12- and 15-LO type 2 mRNA expression in a line of human aortic smooth muscle cells (T/G HA-VSMC) (60% and 50%, respectively). Aldosterone-induced 12- and 15-LO type 2 mRNA expressions were blocked by the EGF-receptor antagonist AG 1478 and by the MAPK-kinase inhibitor UO126. Aldosterone-treated VSMC also showed increased LDL oxidation, (approximately 2-fold; P < 0.001), which was blocked by spironolactone. In conclusion, aldosterone increased 12- and 15-LO expression in human VSMC, in association with increased 12- and 15-HETE generation and enhanced LDL oxidation and may directly augment VSMC contractility, hypertrophy, and migration through 12-HETE and promote LDL oxidation via the pro-oxidative properties of these enzymes.

Aldosterone as a renal growth factor

Aldosterone regulates blood pressure through its effects on the cardiovascular system and kidney. Aldosterone can also contribute to the development of hypertension that leads to chronic pathologies such as nephropathy and renal fibrosis. Aldosterone directly modulates renal cell proliferation and differentiation as part of normal kidney development. The stimulation of rapidly activated protein kinase cascades is one facet of how aldosterone regulates renal cell growth. These cascades may also contribute to myofibroblastic transformation and cell proliferation observed in pathological conditions of the kidney. Polycystic kidney disease is a genetic disorder that is accelerated by hypertension. EGFR-dependent proliferation of the renal epithelium is a factor in cyst development and trans-activation of EGFR is a key feature in initiating aldosterone-induced signalling cascades. Delineating the components of aldosterone-induced signalling cascades may identify novel therapeutic targets for proliferative diseases of the kidney.

Interaction between mineralocorticoid receptor and cAMP/CREB signaling

Besides regulating water and electrolyte homeostasis, the mineralocorticoid receptor (MR) elicits pathophysiological effects in the renocardiovascular system. Although the MR's closest relative, the glucocorticoid receptor (GR), acts as a transcription factor at the same hormone-response-element (HRE), activated glucocorticoid receptor mediates very different effects. One explanation for this discrepancy is that the MR interacts with additional signaling pathways in the cytosol. In the literature, there are several indications for an interaction between aldosterone/MR and the cAMP/CREB signaling. Here we summarize the current knowledge of the cross-talk between the two signaling pathways, including some unpublished observations of our own that indicate that MR/CREB signaling is mediated by calcineurin and has potentially pathophysiological consequences.

Nongenomic actions of adrenal steroids in the central nervous system

Mineralocorticoids and glucocorticoids are steroid hormones that are released by the adrenal cortex in response to stress and hydromineral imbalance. Historically, adrenocorticosteroid actions are attributed to effects on gene transcription. More recently, however, it has become clear that genome-independent pathways represent an important facet of adrenal steroid actions. These hormones exert nongenomic effects throughout the body, although a significant portion of their actions are specific to the central nervous system. These actions are mediated by a variety of signalling pathways, and lead to physiologically meaningful events in vitro and in vivo. We review the nongenomic effects of adrenal steroids in the central nervous system at the levels of behaviour, neural system activity, individual neurone activity and subcellular signalling activity. A clearer understanding of adrenal steroid activity in the central nervous system will lead to a better ability to treat human disease as well as reduce the side-effects of the steroid treatments already in use.

Protein kinase D stabilizes aldosterone-induced ERK1/2 MAP kinase activation in M1 renal cortical collecting duct cells to promote cell proliferation

Aldosterone elicits transcriptional responses in target tissues and also rapidly stimulates the activation of protein kinase signalling cascades independently of de novo protein synthesis. Here we investigated aldosterone-induced cell proliferation and extra-cellular regulated kinase 1 and 2 (ERK1/2) mitogen activated protein (MAP) kinase signalling in the M1 cortical collecting duct cell line (M1-CCD). Aldosterone promoted the proliferative growth of M1-CCD cells, an effect that was protein kinase D1 (PKD1), PKCdelta and ERK1/2-dependent. Aldosterone induced the rapid activation of ERK1/2 with peaks of activation at 2 and 10 to 30 min after hormone treatment followed by sustained activation lasting beyond 120 min. M1-CCD cells suppressed in PKD1 expression exhibited only the early, transient peaks in ERK1/2 activation without the sustained phase. Aldosterone stimulated the physical association of PKD1 with ERK1/2 within 2 min of treatment. The mineralocorticoid receptor (MR) antagonist RU28318 inhibited the early and late phases of aldosterone-induced ERK1/2 activation, and also aldosterone-induced proliferative cell growth. Aldosterone induced the sub-cellular redistribution of ERK1/2 to the nuclei at 2 min and to cytoplasmic sites, proximal to the nuclei after 30 min. This sub-cellular distribution of ERK1/2 was inhibited in cells suppressed in the expression of PKD1.

New aspects of rapid aldosterone signaling

Aldosterone, the endogenous ligand of the mineralocorticoid receptor (MR) in humans, is a steroid hormone that regulates salt and water homeostasis. Recently, additional pathophysiological effects in the renocardiovascular system have been identified. Besides genomic effects mediated by activated MR, rapid aldosterone actions that are independent of translation and transcription have been documented. While these nongenomic actions influence electrolyte homeostasis, pH and cell volume in classical MR target organs, they also participate in pathophysiological effects in the renocardiovascular system causing endothelial dysfunction, inflammation and remodeling. The mechanisms conveying these rapid effects consist of a multitude of signaling molecules and include a cross-talk with genomic aldosterone effects as well as with angiotensin II and epidermal growth factor receptor signaling. Rapid corticosteroid signaling via the MR has also been demonstrated in the brain. Altogether, the function of nongenomic aldosterone effects seems to be to modulate other signaling cascades, depending on the surrounding milieu.

Enhancement of aldosterone-induced catecholamine production by bone morphogenetic protein-4 through activating Rho and SAPK/JNK pathway in adrenomedullar cells

Here we investigated the effects of mineralocorticoid in the regulation of catecholamine biosynthesis using rat pheochromocytoma PC12 cells. Expression of mineralocorticoid receptor (MR) was confirmed in undifferentiated PC12 cells. Aldosterone stimulated dopamine production by PC12 cells without any increase in cAMP activity. Aldosterone-induced dopamine accumulation was enhanced in accordance with the increase in the rate-limiting enzyme tyrosine hydroxylase (TH). Blocking MR with eplerenone suppressed aldosterone-induced increases of TH mRNA and dopamine production. A glucocorticoid receptor (GR) antagonist, RU-486, attenuated dexamethasone- but not aldosterone-induced TH expression. Cycloheximide reduced both aldosterone- and dexamethasone-induced TH mRNA. A SAPK/JNK inhibitor, SP600125, suppressed aldosterone-induced TH mRNA expression; however, the aldosterone-induced TH expression was not affected by inhibition of ERK1/2, p38-MAPK, Rho-kinase, PI 3-kinase, and PKC. It was of note that cotreatment with eplerenone and SP600125 restored aldosterone-induced TH mRNA expression to basal levels. To investigate the involvement of bone morphogenetic protein (BMP) actions in aldosterone-induced catecholamine production, we examined the effects of BMP-4 and BMP-7, which are expressed in the adrenal medulla, on catecholamine biosynthesis. BMP-4 preferentially enhanced aldosterone-induced TH mRNA and dopamine production, although BMP-4 alone did not affect TH expression. The BMP-4 enhancement of aldosterone-induced TH expression was not observed in cells treated with eplerenone. BMP-4 did not affect MR expression of PC12 cells; however, it did enhance aldosterone-induced SAPK/JNK phosphorylation. Inhibition of SAPK/JNK or Rho suppressed BMP-4 enhancement of aldosterone-induced TH expression. Collectively, our findings demonstrate that aldosterone stimulates catecholamine biosynthesis in adrenomedullar cells via MR through genomic action and partly through nongenomic action by Rho-SAPK/JNK signaling, the latter of which is facilitated by BMP-4. A functional link between MR actions and endogenous BMP may be involved in the catecholamine production.

Aldosterone induces collagen synthesis via activation of extracellular signal-regulated kinase 1 and 2 in renal proximal tubules

AIM

Aldosterone plays a crucial role in renal fibrosis by inducing mesangial cell proliferation and promoting collagen synthesis in renal fibroblasts. However, renal proximal tubule involvement in aldosterone-induced collagen synthesis has not yet been identified. The aim of this study was to examine the potential role of aldosterone in collagen expression and its possible mineralocorticoid receptor (MR)-dependent pathway, mediated by activation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) in cultured human renal proximal tubular epithelial (HKC) cells.

METHODS

After HKC cells were stimulated by aldosterone with different concentrations for various time and periods, the gene expression and protein synthesis of collagen I, II, III and IV were measured by real-time polymerase chain reaction and western blot, respectively. ERK1/2 activation, alpha-smooth muscle actin (alpha-SMA), and E-cadherin were also detected by western blot.

RESULTS

Aldosterone can increase ERK1/2 phosphorylation of human renal proximal tubular epithelial cells in a time- and dose-dependent manner. Although aldosterone had no effect on collagen I and II expression, it increased expression of alpha-SMA and collagen III and IV and decreased that of E-cadherin in HKC cells after 48 h. These effects could be prevented by a ERK pathway inhibitor, U0126, or by a selective MR antagonist, spironolactone.

CONCLUSION

The results suggest that aldosterone plays a pivotal role in tubulointerstitial fibrosis by promoting tubular epithelial-mesenchymal transition and collagen synthesis in proximal tubular cells. The process is MR-dependent, and mediated by ERK1/2 mitogen-activated protein kinase pathway.

Immunohistochemical demonstration of the mineralocorticoid receptor, 11beta-hydroxysteroid dehydrogenase-1 and -2, and hexose-6-phosphate dehydrogenase in rat ovary

An IHC survey using several monoclonal antibodies against different portions of the rat mineralocorticoid receptor (MR) molecule demonstrated significant specific MR immunoreactivity in the ovary, prompting further study of the localization of MR and of determinants of extrinsic MR ligand specificity, 11beta-hydroxysteroid dehydrogenase (11beta-HSD) types 1 and 2, and hexose-6-phosphate dehydrogenase (H6PDH). MR expression (real-time RT-PCR and Western blot) did not differ significantly in whole rat ovaries at early diestrus, late diestrus, estrus, and a few hours after ovulation. MR immunostaining was most intense in corporal lutea cells, light to moderate in oocytes and granulosa cells, and least intense in theca cells. Light immunoreactivity for 11beta-HSD2 occurred in most cells, with some mural granulosa cells of mature follicles staining more strongly. The distribution of immunoreactivity for 11beta-HSD1 and H6PDH required to generate NADPH, the cofactor required for reductase activity of 11beta-HSD1, was similar, with the most-intense staining in the cytoplasm of corporal lutea and theca cells and light or no staining in the granulosa and oocytes. MR function in the ovary is as yet unclear, but distinct patterns of distribution of 11beta-HSD1 and -2 and H6PDH suggest that the ligand for MR activation in different cells of the ovary may be differentially regulated.

Classic versus non-classic receptors for nongenomic mineralocorticoid responses: emerging evidence

Mineralocorticoids, which are synthesized locally in the central nervous system in addition to their adrenal production, trigger both genomic and nongenomic responses. Several functions of mineralocorticoids in the CNS are known to date, which are reviewed along with nongenomic responses in other tissues. A controversy regarding the identity of receptors that mediate nongenomic, transcription-independent cellular responses to steroids is presently attracting considerable scientific interest. While there is strong evidence for classic receptors belonging to the nuclear receptor superfamily to mediate nongenomic steroid effects in some cases, it does not exist for others. Recent findings on new and unexpected properties of classic receptors have partially withdrawn the interest from novel, non-classic membrane receptors, which are being progressively identified at present. This has been facilitated by the robust and elaborate toolkit for classic receptor studies in contrast to the comparably immature research tools for alternative receptors. To know the nature of receptors involved may be the key to beneficial medical translation of specific and targeted steroid responses.

Rapid responses to aldosterone in the kidney and colon

Aldosterone is a crucial modulator of ion transport across high resistance epithelia and regulates whole body electrolyte balance through its effects on the kidney and colon. The net consequence of aldosterone release is to promote salt conservation. The genomic mechanism of aldosterone action is relatively well characterized and the role of the classical mineralocorticoid receptor as a ligand-dependent transcription factor is well established. The rapid effects of aldosterone on target tissues are less well understood and there is still controversy over the identity of the aldosterone non-genomic receptor. Greater understanding of the physiological consequences of aldosterone's rapid responses in the kidney and colon has been achieved through the identification of definite and putative membrane targets and their signaling regulators.

Aldosterone rapidly activates protein kinase D via a mineralocorticoid receptor/EGFR trans-activation pathway in the M1 kidney CCD cell line

Aldosterone elicits physiological responses through the modulation of gene expression and by stimulating signaling processes. Here we investigated the activation pathway of protein kinase D1 (PKD1) by aldosterone in the murine M1 renal cortical collecting duct cell line. Aldosterone stimulated a rapid increase in PKD1 activity peaking at 2-5 min and at 30 min after treatment that was insensitive to inhibitors of transcription or translation. PKD1 was not activated by aldosterone in MR null NIH-3T3 fibroblasts or M1-CCD cells propagated without dexamethasone, which did not express MR. PKD1 activation was sensitive to the MR antagonists spironolactone and RU28318 but not to the glucocorticoid receptor antagonist RU486. Aldosterone activation of PKD1 was inhibited by the epidermal growth factor (EGFR) antagonist tyrphostin AG1478 and by the c-Src inhibitor PP2. Western blotting revealed EGFR phosphorylation following aldosterone treatment at the c-Src tyrosine kinase-specific residue Tyr845. The activation of c-Src was dependent on its interaction with HSP84, since HSP84 antagonist 17-AAG inhibited both the phosphorylation of EGFR in response to aldosterone by c-Src and also the subsequent activation of PKD1.

Rapid Responses to Steroid Hormones in the Kidney

Rapid signalling responses stimulated by steroid hormones have been detected in various tissues including the nephron. The significance of these responses in modulating the physiological effects elicited by mineralocorticoids, glucocorticoids and the reproductive hormones in the kidney is now becoming more evident. This review outlines how rapid signalling responses stimulated by these hormones are coupled to the regulation of membrane transport targets that impact upon the reabsorptive and excretory functions of the kidney.

Antagonistic effects of bone morphogenetic protein-4 and -7 on renal mesangial cell proliferation induced by aldosterone through MAPK activation

Aldosterone and angiotensin II (ANG II) contribute to the development and progression of renal damage. Here we investigated the effects of bone morphogenetic proteins (BMPs) on renal cell proliferation evoked by aldosterone and ANG II with mouse mesangial cells, which express mineralocorticoid receptors (MR), ANG II type 1 receptors, and BMP signaling molecules. Aldosterone and ANG II stimulated mesangial cell mitosis and activated ERK1/2 and SAPK/JNK signaling. These aldosterone effects were neutralized by the MR antagonist eplerenone and inhibition of transcription or translation, suggesting the involvement of genomic activation via MR. BMP-4 and BMP-7 stimulated Smad1, -5, -8 signaling more potently than BMP-2 and BMP-6, leading to suppression of mesangial cell mitosis and MR expression. MAPK inhibitors including U-0126 and SP-600125, but not SB-203580, suppressed aldosterone-induced cellular DNA synthesis, implying that ERK1/2 and SAPK/JNK pathways play crucial roles in mesangial cell proliferation. BMP-4 and BMP-7 inhibited phosphorylation of ERK1/2 and SAPK/JNK induced by aldosterone while activating p38 pathway, resulting in inhibition of aldosterone-induced cell mitosis. In contrast, aldosterone modulated the mesangial BMP system by decreasing expression of ALK-3, BMP-4, and BMP-7 while increasing inhibitory Smad6 expression. Thus novel functional cross talk between the mesangial BMP system and aldosterone signaling was uncovered, in which inhibition of MAPK signaling and MR expression by BMP-4 and BMP-7 may be involved in ameliorating renal damage due to mesangial proliferation caused by aldosterone.

Effect of acute aldosterone administration on gene expression profile in the heart

Aldosterone is known to have a number of direct adverse effects on the heart, including fibrosis and myocardial inflammation. However, genetic mechanisms of aldosterone action on the heart remain unclear. This paper describes an investigation of temporal changes in gene expression profile of the whole heart induced by acute administration of a physiologic dose of aldosterone in the mouse. mRNA levels of 34,000 known mouse genes were measured at eight time points after aldosterone administration using oligonucleotide microarrays and compared with those of the control animals who underwent a sham injection. A novel software tool (CAGED) designed for analysis of temporal microarray experiments using a Bayesian approach was used to identify genes differentially expressed between the aldosterone-injected and control group. CAGED analysis identified 12 genes as having significant differences in their temporal profiles between aldosterone-injected and control groups. All of these genes exhibited a decrease in expression level 1-3 h after aldosterone injection followed by a brief rebound and a return to baseline. These findings were validated by quantitative RT-PCR. The differentially expressed genes included phosphatases, regulators of steroid biosynthesis, inactivators of reactive oxygen species, and structural proteins. Several of these genes are known to functionally mediate biochemical phenomena previously observed to be triggered by aldosterone administration, such as phosphorylation of ERK1/2. These results provide the first description of cardiac genetic response to aldosterone and identify several potential mediators of known biochemical sequelae of aldosterone administration in the heart.

Aldosterone inhibits apical NHE3 and HCO3- absorption via a nongenomic ERK-dependent pathway in medullary thick ascending limb

Although aldosterone influences a variety of cellular processes through nongenomic mechanisms, the significance of nongenomic pathways for aldosterone-induced regulation of epithelial function is not understood. Recently, we demonstrated that aldosterone inhibits transepithelial HCO(3)(-) absorption in the medullary thick ascending limb (MTAL) through a nongenomic pathway. This inhibition is mediated through a direct cellular action of aldosterone to inhibit the apical membrane NHE3 Na(+)/H(+) exchanger. The present study was designed to identify the intracellular signaling pathway(s) responsible for this aldosterone-induced transport regulation. In rat MTALs perfused in vitro, addition of 1 nM aldosterone to the bath decreased HCO(3)(-) absorption by 30%. This inhibition was not mediated by cAMP/PKA and was not prevented by inhibitors of PKC or PI3-K, pertussis toxin, or rapamycin. The inhibition of HCO(3)(-) absorption by aldosterone was largely eliminated by the MEK/ERK inhibitors U-0126 and PD-98059. Aldosterone increased ERK activity 1.8-fold in microdissected MTALs. This ERK activation is rapid (</=5 min) and is blocked by U-0126 or PD-98059 but is unaffected by spironolactone or actinomycin D. Pretreatment with U-0126 to block ERK activation prevented the effect of aldosterone to inhibit apical NHE3. These data demonstrate that aldosterone inhibits NHE3 and HCO(3)(-) absorption in the MTAL through rapid activation of the ERK signaling pathway. The results identify NHE3 as a target for nongenomic regulation by aldosterone and establish a role for ERK in the acute regulation of NHE3 and its epithelial absorptive functions.

Molecular Mechanisms and Therapeutic Strategies of Chronic Renal Injury: Renoprotective Effects of Aldosterone Blockade

Recent clinical and pre-clinical studies have indicated the utility of mineralocorticoid receptor (MR) antagonists in renal injury. We have demonstrated in rats that chronic treatment with aldosterone results in severe proteinuria and renal injury, characterized by glomerular changes, tubulointerstitial fibrosis, and collagen accumulation. We also observed increased reactive oxygen species (ROS) generation and mitogen-activated protein kinases (MAPKs) activity in renal cortical tissues. Treatment with a selective MR antagonist, eplerenone, prevented elevation of ROS levels and MAPK activity, as well as ameliorating renal injury. In vitro studies revealed that MRs are highly expressed in rat glomerular mesangial cells (RMC) and rat renal fibroblasts. In RMC, aldosterone induces cellular injuries through NADPH oxidase-dependent ROS production and/or MAPK activation. Aldosterone-induced renal cellular injuries were markedly attenuated by treatment with eplerenone. These data suggest that aldosterone induces renal injury through activation of MRs and support the notion that MR blockade has beneficial effects on aldosterone-dependent renal injury through mechanisms that cannot be simply explained by hemodynamic changes. In this review, we summarized our recent findings pertaining to the roles of aldosterone and MRs in the pathogenesis of renal injury. Potential molecular mechanisms responsible for aldosterone/MR-induced renal injury were also discussed.

Aldosterone Stimulates Collagen Gene Expression and Synthesis Via Activation of ERK1/2 in Rat Renal Fibroblasts

Recently, we demonstrated that in rats treated chronically with aldosterone and salt, severe tubulointerstitial fibrosis is associated with the activation of mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinases (ERK1/2). Here, we investigated whether aldosterone stimulates collagen synthesis via ERK1/2-dependent pathways in cultured rat renal fibroblasts. Gene expression of mineralocorticoid receptor (MR) and types I, II, III, and IV collagen was measured by real-time polymerase chain reaction (PCR). MR protein expression and ERK1/2 activity were evaluated by Western blotting analysis with anti-MR and anti–phospho-ERK1/2 antibodies, respectively. Collagen synthesis was determined by [ 3 H]-proline incorporation. Significant levels of MR mRNA and protein expression were observed in rat renal fibroblasts. Treatment with aldosterone (0.1 to 10 nmol/L) increased ERK1/2 phosphorylation in a concentration-dependent manner with a peak at 5 minutes. Aldosterone (10 nmol/L) also increased the mRNA levels of types I, III, and IV collagen at 36 hours but had no effect on the type II collagen mRNA level. [ 3 H]-proline incorporation was significantly increased by aldosterone in both the medium and cell layer at 48 hours. Aldosterone-induced ERK1/2 phosphorylation was markedly attenuated by pretreatment with eplerenone (10 μmol/L), a selective MR antagonist, or PD98059 (10 μmol/L), a specific inhibitor of MAPK kinase/ERK kinase, which is the upstream activator of ERK1/2. In addition, both eplerenone and PD98059 prevented the aldosterone-induced increases in types I, III, and IV collagen mRNA and [ 3 H]-proline incorporation. These results suggest that aldosterone stimulates collagen gene expression and synthesis via MR-mediated ERK1/2 activation in renal fibroblasts, which may contribute to the progression of aldosterone-induced tubulointerstitial fibrosis.

Aldosterone and angiotensin II synergistically induce mitogenic response in vascular smooth muscle cells

Interaction between aldosterone (Aldo) and angiotensin II (Ang II) in the cardiovascular system has been highlighted; however, its detailed signaling mechanism is poorly understood. Here, we examined the cross-talk of growth-promoting signaling between Aldo and Ang II in vascular smooth muscle cells (VSMC). Treatment with a lower dose of Aldo (10(-12) mol/L) and with a lower dose of Ang II (10(-10) mol/L) significantly enhanced DNA synthesis, whereas Aldo or Ang II alone at these doses did not affect VSMC proliferation. This effect of a combination of Aldo and Ang II was markedly inhibited by a selective AT1 receptor blocker, olmesartan, a mineralocorticoid receptor antagonist, spironolactone, an MEK inhibitor, PD98059, or an EGF receptor tyrosine kinase inhibitor, AG1478. Treatment with Aldo together with Ang II, even at noneffective doses, respectively, synergistically increased extracellular signal-regulated kinase (ERK) activation, reaching 2 peaks at 10 to 15 minutes and 2 to 4 hours. The early ERK peak was effectively blocked by olmesartan or an EGF receptor kinase inhibitor, AG1478, but not by spironolactone, whereas the late ERK peak was completely inhibited by not only olmesartan, but also spironolactone. Combined treatment with Aldo and Ang II attenuated mitogen-activated protein kinase phosphatase-1 (MKP-1) expression and increased Ki-ras2A expression. The late ERK peak was not observed in VSMC treated with Ki-ras2A-siRNA. Interestingly, the decrease in MKP-1 expression and the increase in Ki-ras2A expression were restored by PD98059 or AG1478. These results suggest that Aldo exerts a synergistic mitogenic effect with Ang II and support the notion that blockade of both Aldo and Ang II could be more effective to prevent vascular remodeling.

The nongenomic actions of aldosterone

Aldosterone has physiological effects to regulate fluid and electrolyte homeostasis across epithelia and proinflammatory effects on a variety of nonepithelial cells in the context of inappropriate salt status. These effects are mediated by mineralocorticoid receptors, members of a large family of nuclear transcription factors, by DNA-directed, RNA-mediated protein synthesis. Rapid effects of aldosterone, insensitive to actinomycin D or cycloheximide and thus clearly nongenomic, have been convincingly documented in a variety of epithelial and nonepithelial tissues. Despite strenuous attempts, isolation of a nonclassical membrane receptor for aldosterone has proven unsuccessful, and rapid nongenomic effects mediated by classical mineralocorticoid receptors are increasingly recognized in the kidney, heart, and vascular wall. The mechanism of rapid nongenomic actions of aldosterone may vary between tissues in terms of pathways; in addition, what remains to be established is the physiological role of aldosterone action via such rapid nongenomic mechanisms and how they might synergize with the longer time course genomic actions of mineralocorticoids.

Involvement of Aldosterone and Mineralocorticoid Receptors in Rat Mesangial Cell Proliferation and Deformability

We demonstrated recently that chronic administration of aldosterone to rats induces glomerular mesangial injury and activates mitogen-activated protein kinases including extracellular signal-regulated kinases 1/2 (ERK1/2). We also observed that the aldosterone-induced mesangial injury and ERK1/2 activation were prevented by treatment with a selective mineralocorticoid receptor (MR) antagonist, eplerenone, suggesting that the glomerular mesangium is a potential target for injuries induced by aldosterone via activation of MR. In the present study, we investigated whether MR is expressed in cultured rat mesangial cells (RMCs) and involved in aldosterone-induced RMC injury. MR expression and localization were evaluated by Western blotting analysis and fluorolabeling methods. Cell proliferation and micromechanical properties were determined by [ 3 H]-thymidine uptake measurements and a nanoindentation technique using an atomic force microscope cantilever, respectively. ERK1/2 activity was measured by Western blotting analysis with an anti-phospho–ERK1/2 antibody. Protein expression and immunostaining revealed that MR was abundant in the cytoplasm of RMCs. Aldosterone (1 to 100 nmol/L) dose-dependently activated ERK1/2 in RMCs with a peak at 10 minutes. Pretreatment with eplerenone (10 μmol/L) significantly attenuated aldosterone-induced ERK1/2 phosphorylation. Aldosterone (100 nmol/L) treatment for 30 hours increased [ 3 H]-thymidine incorporation and decreased the elastic modulus, indicating cellular proliferative and deforming effects of aldosterone, respectively. These aldosterone-induced changes in cellular characteristics were prevented by pretreatment with eplerenone or an ERK (MEK) inhibitor, PD988059 (100 μmol/L). The results indicate that aldosterone directly induces RMC proliferation and deformability through MR and ERK1/2 activation, which may contribute to the pathogenesis of glomerular mesangial injury.

Aldosterone Activates Vascular p38MAP Kinase and NADPH Oxidase Via c-Src

Increasing evidence indicates that aldosterone elicits vascular effects through nongenomic signaling pathways. We tested the hypothesis that aldosterone induces activation of vascular mitogen-activated protein (MAP) kinases and NADPH oxidase via c-Src–dependent mechanisms in vascular smooth muscle cells (VSMCs). Aldosterone effects on activation of c-Src, p38MAP kinase, and NADPH oxidase, and incorporation of [ 3 H]proline, an index of collagen synthesis, were assessed in cultured rat VSMCs. Studies were performed in the absence and presence of eplerenone, a selective mineralocorticoid receptor blocker, PP2, a selective Src inhibitor, and SB212190, a selective p38MAPK inhibitor. Phosphorylation of c-Src was dose-dependently increased by aldosterone, with maximal responses obtained at 10 −7 mol/L. Aldosterone increased p38MAP kinase phosphorylation, NAD(P)H oxidase activation, and [ 3 H]proline incorporation. These responses were abrogated by eplerenone and almost abolished by PP2. Aldosterone-stimulated incorporation of [ 3 H]proline was significantly reduced by SB212190, indicating that p38MAP kinase plays a role in profibrotic actions of aldosterone. To unambiguously demonstrate the importance of aldosterone in c-Src signaling, VSMCs from c-Src +/+ and c-Src +/− mice were also studied. Aldosterone increased phosphorylation of c-Src, p38MAP kinase, and cortactin, a Src-specific substrate, in c-Src +/+ VSMCs, but not in c-Src-deficient cells. Taken together, our findings demonstrate that nongenomic signaling by aldosterone occurs through c-Src–dependent pathways. These processes may play an important role in profibrotic actions of aldosterone.

Effects of Aldosterone and Mineralocorticoid Receptor Blockade on Intracellular Electrolytes

Genomic mechanisms of mineralocorticoid action have been increasingly elucidated over the past four decades. In renal epithelia, the main effect is an increase in sodium transport through activation and de novo synthesis of epithelial sodium channels. This leads to increased concentrations of intracellular sodium activating sodium-potassium-ATPase molecules mainly at the basolateral membrane which extrude sodium back into the blood stream. In contrast, rapid steroid actions have been widely recognized only recently. The present article summarizes both traditional and rapid effects of mineralocorticoid hormones on intracellular electrolytes, e.g. free intracellular calcium in vascular smooth muscle cells as determined by fura 2 spectrofluorometry in single cultured cells from rat aorta. Latter effects are almost immediate, reach a plateau after only 3 to 5 minutes and are characterized by high specificity for mineralocorticoids versus glucocorticoids. The effect of aldosterone is blocked by neomycin and short-term treatment with phorbol esters but augmented by staurosporine, indicating an involvement of phospholipase C and protein kinase C. The Ca(2+) effect appears to involve the release of intracellular Ca(2+), as shown by the inhibitory effect of thapsigargin. This mechanism operates at physiological subnanomolar aldosterone concentrations and appears to result in rapid fine tuning of cardiovascular responsivity. As a landmark feature of these rapid effects, insensitivity to classic antimineralocorticosteroids, e.g. spironolactone or canrenone has been found in the majority of observations. In an integrated view, mineralocorticoids seem to mainly effect intracellular electrolytes genomically to induce transepithelial transport, and induce nongenomically mediated alterations of cell function (e.g. vasoconstriction) by rapid effects on intracellular electrolytes such as free intracellular calcium.