Published online before print March 17, 2008, doi: 10.1161/HYPERTENSIONAHA.107.105718
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(Hypertension. 2008;51:1372.)
© 2008 American Heart Association, Inc.
Original Articles |
Glucocorticoid-Related Signaling Effects in Vascular Smooth Muscle Cells
From the Medical Faculty of the Charité (G.A.M., G.D., S.W., F.C.L., D.N.M., A.F.), Experimental and Clinical Research Center and Max Delbrück Center, Franz Volhard Clinic, HELIOS Klinikum-Berlin, Berlin, Germany; Department of Nephrology and Clinical Immunology (P.R.M.), University Hospital Rheinisch-Westfälische Technische Hochschule-Aachen, Aachen, Germany; Department of Internal Medicine-Nephrology (C.L., T.K.), Hannover University Medical School, Hannover, Germany; 2nd Department of Medicine and Nephrological Center (G.A.M.), University of Pecs, Pecs, Hungary; Section Nephrology/Intensive Care (M.G.), Campus Virchow, Charité, Berlin, Germany; and Section Clinical Endocrinology (M.Q.), Campus Mitte, Charité, Berlin, Germany.
Correspondence to Anette Fiebeler, ECRC and MDC, Robert-Rössle Str 10, 13125 Berlin, Germany. E-mail fiebeler{at}charite.de
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Abstract |
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Mineralocorticoid receptor blockade protects from angiotensin II–induced target-organ damage. 11β-Hydroxysteroid dehydrogenase type 2 protects the mineralocorticoid receptor from activation by glucocorticoids; however, high glucocorticoid concentrations and absent 11β-hydroxysteroid dehydrogenase type 2 in some tissues make glucocorticoids highly relevant mineralocorticoid receptor ligands. We investigated the effects of corticosterone (10–6 to 10–12 mol/L) on early vascular mineralocorticoid receptor signaling by Western blotting, confocal microscopy, and myography. Corticosterone initiated extracellular signal–regulated kinase 1/2 phosphorylation in rat vascular smooth muscle cells at
10–11 mol/L doses. Protein synthesis inhibitors had no effect, indicating a nongenomic action. Corticosterone also stimulated c-Jun N-terminal kinase, p38, Src, and Akt phosphorylation at 15 minutes and enhanced angiotensin II–induced signaling at 5 minutes. A specific epidermal growth factor receptor blocker, AG1478, as well as the Src inhibitor PP2, markedly reduced corticosterone-induced extracellular signal–regulated kinase 1/2 phosphorylation, as did preincubation of cells with the mineralocorticoid receptor antagonist spironolactone. Silencing mineralocorticoid receptor with small interfering RNA abolished corticosterone-induced effects. Corticosterone (10–9 mol/L) enhanced phenylephrine-induced contraction of intact aortic rings. These effects were dependent on the intact endothelium, mineralocorticoid receptor, and mitogen-activated protein kinase kinase 1/extracellular signal-regulated kinase signaling. We conclude that corticosterone induces rapid mineralocorticoid receptor signaling in vascular smooth muscle cells that involves mitogen-activated protein kinase kinase/extracellular signal–regulated kinase–dependent pathways. These new mineralocorticoid receptor–dependent signaling pathways suggest that glucocorticoids may contribute to vascular disease via mineralocorticoid receptor signaling, independent of circulating aldosterone.
Key Words: corticosterone • angiotensin • phenylephrine • mineralocorticoid receptor • epidermal growth factor receptor
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Introduction |
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Clinical trials have shown that mineralocorticoid receptor (MR) blockers reduce morbidity and mortality1,2 and protect against progressive renal disease.3 Although aldosterone has been seen as the culprit, glucocorticoids bind to the MR with the same affinity at physiological concentrations and exert similar effects on various targets, including vessels.4–6 These observations are consistent with the fact that Cushing’s syndrome patients having a high cardiovascular mortality.7,8 The enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) converts MR-active cortisol to cortisone, which does not bind to the MR. 11β-HSD2 inactivity occurs in the syndrome of apparent mineralocorticoid excess or in licorice abuse and allows cortisol to activate the MR.9 Affected persons show enhanced vasoconstrictor dermal vessel responses to cortisol and symptoms mimicking locally high aldosterone levels.5 Because cortisol levels are >1000-fold higher than aldosterone levels, cortisol is an MR ligand even under physiological conditions, particularly in tissues not expressing 11β-HSD2. We tested the hypothesis that corticosterone, the physiological glucocorticoid in rats and mice, may exert aldosterone-like effects on early signaling in the vasculature and that these effects are mediated via MR and mitogen-activated protein kinase kinase (MEK)/extracellular signal–regulated kinase (ERK)–dependent pathways. Because rapid and protein synthesis-independent aldosterone signaling has been described previously,10,11 we reasoned that corticosterone might also signal along these lines. We also examined corticosterone responses and signaling in aortic rings.
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Materials and Methods |
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Cell Culture
Aortic vascular smooth muscle cells (vascular smooth muscle cells [VSMCs]) were isolated from Sprague-Dawley rats as described previously.12 We followed all of the requirements of the American Physiological Society, and local authorities approved the studies. We used passages 3 to 8 after phenotyping by staining VSMCs for muscle-specific
-actin (Dako) and desmin (Boehringer-Mannheim). Cells were treated with corticosterone, angiotensin (Ang II; both Sigma), or aldosterone (Clinalfa). The following blockers were used as indicated: AG 1478, LY294002, PD98059, PP2, actinomycin, cycloheximide (all Calbiochem), spironolactone, RU 486, and carbenoxolone (CBX; all Sigma). All of the experiments were performed under 24-hour serum-free conditions. For small interfering RNA (siRNA) experiments, cells were transfected with specific siRNA (Dharmacon) directed against MR or nontargeting siRNA following the manufacturer’s protocol (Amaxa). The following sequences were used as designed and provided by Dharmacon: MR-directed siRNAs: CCAAGUCAUUGCAGUGUAA, CGGCAAAUCUCAACAACUC, GUGAAGAGCCCUAUCAUCU, and CUUAGGAGCUCGAAAGUCA; siSTABLE Non Targeting siRNA control and siSTABLE Cyclophilin B siRNA served as controls. Three days after transfection, cells were stimulated as described.
Immunohistochemistry
Confocal microscopy was performed as described previously.12 At least 50 to 80 cells from 3 experiments were examined at each treatment point by 2 blinded investigators. Quantification was by histogram functions using the MRC laser sharp software. The subcellular regions were outlined manually, and mean fluorescence intensities were obtained for the regions delineated with data presented as mean fluorescence intensity in each cell area. Rabbit and mouse antiphospho-ERK1/2 antibodies (Cell Signaling; 1:200) or phospho-Src (Cell Signaling; 1:100) was used in the studies.
Western Blot
The following primary antibodies were used: polyclonal ERK1/2 (Cell Signaling; 1:1000), phospho-ERK1/2 (Cell Signaling; 1:1000), phospho–c-Jun N-terminal kinase (Cell Signaling; 1:1000), phospho-epidermal growth factor receptor (EGFR; Cell Signaling; 1:1000), phospho-Akt (Cell Signaling; 1:500), phospho-Src (Cell Signaling; 1:500), and phospho-p38 mitogen-activated protein kinase (Cell Signaling; 1:500). Peroxidase-conjugated secondary antibodies were purchased from Dianova (1:10000). Blots were developed with a chemiluminescent substrate and visualized on Kodak film. Three to 6 different cell stimulation experiments were performed and quantified for each protocol. For quantification, the intensity of the control was defined as 100%, and other bands were calculated as a percentage thereof.
11β-HSD2 Activity
VSMCs were transferred to serum-free DMEM without phenol. 11β-HSD2 enzyme reactions were started by addition of 10–11 to 10–7 mol/L corticosterone or 10–7 mol/L corticosterone and 10–8 M CBX (both from Sigma), spiked with 200 000 cpm of [1,2,6,7-3H] corticosterone (GE Healthcare Ltd). After incubation at 37°C for 15 minutes or 40 minutes, steroids were extracted with dichloromethane, separated by thin layer chromatography on plates coated with silica gel 60 (F254; Merck Ltd) with a mobile phase of ethanol and chloroform (8:92), and quantified on a Typhoon 8600 scanner. All of the experiments were performed in duplicate.
Aortic Ring Contraction
All of the animal procedures were in accordance with institutional guidelines corresponding with those of the American Physiological Society. Male Sprague-Dawley rats (250 to 300 g, Charles River, Berlin, Germany) were killed, and the thoracic aorta removed, quickly transferred to cold (4°C) oxygenated (95% O2/5% CO2) physiological salt solution, and dissected into 5-mm rings as described previously, with perivascular fat and connective tissue removed.13 After 1 hour of equilibration, aortic ring contractile force was measured isometrically by standard bath procedures as described previously.13 The composition of physiological salt solution (in mmol/L) was 119 NaCl, 4.7 KCl, 1.2 KH2PO4, 25 NaHCO3, 1.2 MgSO4, 11.1 glucose, and 1.6 CaCl2, and the volume of the bath solution was 20 mL. Cumulative dose-response curves were obtained for phenylephrine in the absence or presence of corticosterone or spironolactone, with tension expressed as a percentage of the steady-state tension (100%) found in isotonic external KCl at 60 mmol/L. In some rings, the endothelial layer was disrupted by gently rubbing the luminal surface with a forceps’ tip. Endothelium was accepted as "nonfunctional" if no relaxation was seen with 10 µmol/L of acetylcholine.
Statistics
Data were analyzed by SPSS 13.0 and those with a normal distribution (Kolmogorov-Smirnov test) are shown as means±SEMs. Statistical significance was tested by unpaired t test, or, in case of multiple groups, ANOVA with posthoc correction according to Bonferroni. A value of P<0.05 was considered statistically significant.
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Results |
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Corticosterone Stimulates ERK1/2 Phosphorylation in VSMCs
The high supraphysiological dose of corticosterone (10–6 mol/L) used in the initial studies on ERK1/2 phosphorylation showed that ERK1/2 phosphorylation was maximal at 15 minutes (Figure 1A). In the steroid classification system, corticosterone has been called "Kendall’s" compound B. We used "B" to designate corticosterone in the figure. At 10–9 mol/L, corticosterone induced ERK1/2 phosphorylation that peaked within 10 minutes; similar results were found by immunohistochemistry and confocal microscopy (n=4: control: 1.0; B: 10 minutes; 3.25±1.03). Lower and physiologically relevant free corticosterone doses (10–9 to 10–11 mol/L) induced ERK1/2 signaling at 15 minutes (supplement Figure S1, available online at http://hyper.ahajournals.org). When the corticosterone concentration was reduced to 10–12 mol/L in a second series of dose-response studies, we observed diminished ERK1/2 phosphorylation (Figure 1B). We also investigated corticosterone-induced ERK1/2 phosphorylation in the presence of the transcription and protein synthesis inhibitors actinomycin D (preincubation: 30 minutes; 1 µg/mL; n=3 to 5: control: 0.75±0.25, B: 3.03±0.49, actinomycin D: 2.56±0.32, actinomycin D+corticosterone: 3.98±0.22) and cycloheximide (preincubation: 30 minutes; 5 µg/mL; n=3 to 5: control: 1.2±0.38, corticosterone: 3.22±0.55, cycloheximide: 3.41±0.12, cycloheximide+corticosterone, 4.06±0.17). Neither inhibitor affected short-term ERK1/2 phosphorylation, supporting a nongenomic effect of corticosterone.
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Corticosterone Stimulates c-Jun N-Terminal Kinase, p-38 Mitogen-Activated Protein Kinase, Akt, and Src Phosphorylation in VSMCs
We next investigated downstream corticosterone-stimulated rapid pathways. Stimulation of VSMCs with corticosterone (10–7 mol/L) for 15 minutes induced c-Jun N-terminal kinase phosphorylation (Figure S2A), p38 mitogen-activated protein kinase phosphorylation (Figure S2B), and Src phosphorylation (Figure S2C), with eIF-4e used as a loading control. Corticosterone-induced Src phosphorylation was completely inhibited when cells were preincubated with the tyrosine kinase inhibitor PP2 (10–5 mol/L) for 30 minutes (Figure S2D). In addition, corticosterone induced Akt phosphorylation at 15 minutes (Figure 2A), which was completely abolished by preincubating the cells with the phosphatidylinositol 3-kinase inhibitor LY294002 (10–5 mol/L) for 30 minutes. LY294002 did not affect corticosterone-induced ERK1/2 phosphorylation (Figure 2B).
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11β-HSD2 Activity Does not Affect Corticosterone Levels
We determined the 11β-HSD2 activity under our experimental conditions. The percentage of substrate metabolized from corticosterone to 11-dehydrocorticosterone was <1% in each experiment. Furthermore, we used the 11β-HSD2 inhibitor CBX (preincubation for 30 minutes). CBX at a dose of 10–9 mol/L did not affect corticosterone-induced ERK1/2 phosphorylation (n=12: control: 4.71±0.18, corticosterone: 9.10±0.69, CBX: 4.48±0.16, CBX+B: 8.77±0.29). Thus, 11β-HSD2 was not active under the conditions of our experiments.
Corticosterone-Induced Signaling Is Mediated Through MR
We preincubated VSMCs for 30 minutes with spironolactone (10–7 mol/L). We chose this low dose according to experiences of other groups, that observed partial agonist effects of spironolactone at higher doses on smooth muscle contraction.10,14,15 We suggest that equimolar spironolactone doses blocked ERK phosphorylation and that these doses may be related to our experimental setting with a 30-minute preincubation with spironolactone given before stimulation. We then exposed the cells to corticosterone (10–7 mol/L) for 15 minutes. Spironolactone prevented corticosterone-induced ERK1/2 phosphorylation at 15 minutes, as shown by immunoblotting (Figure S3A) and by confocal microscopy (Figure S3B). Similar results were observed with the more specific MR blocker eplerenone at 10–5 mol/L (Figure S3C). To confirm the findings, we used siRNA targeting the MR. Successful silencing of the MR was shown by quantitative RT-PCR measuring mRNA expression of the MR, which was reduced by 80% in the MR siRNA-transfected cells compared with cells transfected with nontargeting control RNA (Figure SS4). siRNA against the MR prevented corticosterone, aldosterone, and Ang II–induced ERK1/2 phosphorylation but did not affect the response to phorbol ester. In VSMCs transfected with nontargeting control siRNA, aldosterone, Ang II, and corticosterone stimulated ERK1/2 phosphorylation at the expected time points (corticosterone: 10–7 mol/L, 15 minutes; aldosterone: 10–9 mol/L, 15 minutes; Ang II: 10–7 mol/L, 5 minutes; phorbolester: 10–7 mol/L, 5 minutes; Figure 3). The glucocorticoid receptor (GR) blocker mifepristone (RU486; 10–6 mol/L; preincubation for 30 minutes) had no effect on B-induced ERK1/2 phosphorylation at 15 minutes (Figure SS5).
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MEK1, Src, and EGFR Mediate Corticosterone-Induced Signaling; Ang II–Induced ERK1/2 Phosphorylation Is Enhanced
We preincubated VSMCs with the specific MEK1 inhibitor PD98059 (10–5 mol/L, 30 minutes), PP2 (10–5 mol/L, 30 minutes), and with specific EGFR blocker AG1478 (10–7 mol/L, 30 minutes) before adding corticosterone (10–7 mol/L). Under these conditions, all of the inhibitors abrogated ERK1/2 phosphorylation completely (Figure 4A through 4C). To test the hypothesis that corticosterone potentiates Ang II–induced effects, we stimulated VSMCs with Ang II (10–7 mol/L) and corticosterone (10–7 mol/L) for 5 minutes. Although corticosterone alone, as expected from our time course studies (compare Figure 1A), did not stimulate ERK1/2 phosphorylation at this early time point, it significantly enhanced the Ang II–induced effect (Figure 4D).
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Corticosterone Affects Aortic Ring Contraction In Vitro
The contractions with corticosterone were compared with those evoked by 60 mmol/L of KCl and are expressed as a percentage. At 10–9 mol/L, corticosterone alone did not affect vascular tone (data not shown). However, when added to phenylephrine, the phenylephrine-induced vasoconstriction was enhanced in a dose-dependent manner from 50 to 200 nmol/L (Figure 5A). In these experiments, corticosterone was added simultaneously with phenylephrine, beginning at 50 nmol/L; the dose of phenylephrine was cumulatively increased to 200 nmol/L within 10 minutes. The effects of corticosterone on phenylephrine-dependent contraction were completely abolished by pretreatment with NG-nitro-L-arginine methyl ester (300 µmol/L, 20 minutes) plus indomethacin (5 µmol/L, 20 minutes), suggesting an endothelium-dependent effect of corticosterone (200 nmol/L phenylephrine 104±2 versus 200 nmol/L phenylephrine plus corticosterone 106±1). We next mechanically denuded the aorta of endothelium, repeated the experiment, and found no effect on phenylephrine-induced aortic ring contraction in denuded vessels (Figure 5B). Preincubation of the rings with spironolactone (10–6 mol/L) for 30 minutes partially reduced the corticosterone-dependent effects (Figure 5C) but did not affect phenylephrine-dependent contractions in the absence of corticosterone (data not shown). Similarly, preincubation with the GR blocker RU486 (10–6 mol/L) reduced the corticosterone-dependent effects (200 nM phenylephrine plus corticosterone without RU486 92±3 versus with RU486 75±6), although, RU486 did not enhance suppressive effects of spironolactone.
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To test the role of MEK1/ERK1/2 on corticosterone-induced enhancement of phenylephrine-dependent contraction, we preincubated rings with the specific MEK1 inhibitor PD98059 (10–5 mol/L, 30 minutes). PD98059 reduced phenylephrine-related effects on phenylephrine-dependent vasoconstriction but did not inhibit the phenylephrine-dependent contraction in the absence of corticosterone (Figure 5D).
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Discussion |
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Our data suggest that corticosterone exerts rapid effects on signaling in VSMCs in a fashion similar to aldosterone. Corticosterone-induced effects involve signaling pathways that include phosphorylation of mitogen-activated protein kinases, Src, and Akt. ERK1/2-induced signaling depended on an intact MR and a functional EGFR. Corticosterone-induced effects on aortic ring contraction were opposite to aldosterone-induced effects, at least ex vivo. Although aldosterone diminishes phenylephrine-induced contraction,10 corticosterone enhanced phenylephrine-induced aortic ring contraction, an effect dependent on an intact endothelium and partially reduced by MR blockade or an MEK1-ERK1/2 blocker.
The MR and GR both belong to the same nuclear hormone receptor superfamily, and they share high sequence identity.28 Both receptors bind glucocorticoids, cortisol in humans, and corticosterone in rats and mice with high affinity. However, aldosterone binds to the MR with high affinity, whereas its affinity for the GR is much lower.17 Because aldosterone plasma concentrations are 3 orders of magnitude lower than cortisol or corticosterone concentrations, cortisol should occupy most of the MR. This excess is diminished 10-fold by the stronger plasma protein binding of cortisol by a corticosteroid-binding globulin (3% unbound cortisol) compared with the weak protein binding of aldosterone by albumin (30% unbound aldosterone).
In transfection studies, cortisol showed a 10-fold weaker transactivation activity of the MR compared with aldosterone despite an equal binding affinity.18 In addition, the cortisol-MR receptor complex seems to be less stable than the aldosterone-MR receptor complex because of a different receptor conformation. This state of affairs leads to the dissociation of cortisol from the MR that is 2 to 4 times faster than that of aldosterone.19 Furthermore, the exact mechanism regarding how cortisol and aldosterone enter the cell is not clear, and there could be further differences between these steroids because of the 11-18 hemiketal group of aldosterone. However, to allow exclusive aldosterone activation of MR in epithelial cells, the enzyme 11β-HSD2 is expressed and metabolizes cortisol and corticosterone to their inactive metabolites cortisone and 11-dehydrocorticosterone.17,20 In nonepithelial cells, which express MR but not 11β-HSD2, the MR is probably occupied by glucocorticoids. To complicate matters further, several studies have shown that cortisol can block the actions of aldosterone, suggesting that, in many circumstances, cortisol occupies the MR and acts as an MR antagonist.21,22 To what extent further regulatory mechanism, such as coactivator complexes and conformational interactions of MR domains, influence the ligand specificity, transactivation, or repression at the MR, as well as the exact ligand concentration at the intracellular steroid receptor, is unknown.23,24
Our data indicate that corticosterone activates MR in VSMCs to trigger the phosphorylation of proteins that are central to cell proliferation and differentiation. We believe that these effects are mediated through the classical MR, because MR silencing, in addition to the MR antagonists spironolactone and eplerenone, prevented corticosterone signaling. We interpret these data as indicating that corticosterone-induced effects could mimic aldosterone-induced events. These findings may extend our understanding of the MR and how the receptor mediates unexplained pathways. They raise the possibility that glucocorticoids activate MR signaling in nonepithelial tissue and could thereby increase cardiovascular risk. Moreover, the data could have implications in terms of clinical MR blockade with currently available drugs.
The rapid effects of corticosterone on protein phosphorylation are in line with results of Gros et al,14 who showed that cortisol stimulated myosin light-chain phosphorylation in VSMCs within 30 minutes and that this effect was blocked by spironolactone. Corticosterone can be an agonist for both MRs and for GRs, and both receptor classes are expressed in VSMCs.25,26 They also have a common evolutionary precursor,27 display high sequence homology, and their DNA binding domains bind to the same regulatory elements.28 GRs mediate anti-inflammatory and immunosuppressive signals, and in nonepithelial tissue MR activation produces the opposite effects.16,29 To test whether the GR plays a role in the rapid effects, we showed that RU486 did not inhibit corticosterone-induced signaling in VSMCs. However, RU486 partially inhibited corticosterone-induced effects on aortic ring contraction, suggesting that in vivo neighboring cells and coexisting factors may modulate the response to corticosterone. In vivo, 11β-HSD2 is expressed in VSMCs and converts to cortisol and to receptor-inactive 11-keto products.25 Alzamora et al36 demonstrated that the 11β-HSD2 inhibitor, CBX, significantly enhanced cortisol-induced pH changes at doses as low as 0.5 nmol/L in endothelium-denuded aortic rings. Their findings demonstrated the principle of cortisol-relevant effects on the vasculature. Nonetheless, whether and how 11β-HSD2 influences composition or stability of steroid-MR complexes with associated coactivators or repressors to modulate MR activity chronically in vivo remain to be investigated.
Sato et al26 and Ullian et al30 observed that corticosterone upregulates Ang II type 1 receptor mRNA and protein levels in cultured VSMCs. However, these effects cannot explain the enhancing responses that we observed for corticosterone on Ang II–induced effects after 5 minutes. We suggest that ligand binding to the MR initiates shifting and redistribution of different receptors forming a protein complex, thereby facilitating transmission of an activating signal. We have shown previously that the Ang II type 1 receptor and the MR form a functional protein complex and that MR blockade reduces Ang II–induced ERK1/2 phosphorylation.11 When we stimulated VSMCs after MR silencing with Ang II, ERK1/2 phosphorylation was markedly reduced in siRNA-transfected cells but was unaffected in control-transfected cells. We conclude that corticosterone can activate MR producing rapid signaling and, similar to aldosterone, enhances Ang II–induced signaling via MR. A third likely candidate member of the receptor complex is EGFR in that AG1478, which specifically inhibits EGFR kinase activity, prevented corticosterone-induced signaling, as described earlier for aldosterone.11 All 3 of the receptors, Ang II type 1 receptor, EGFR, and MR, mediate activation of mitogen-activated kinases, Src, and the phosphatidylinositol 3-kinase cascade. In addition, EGFR forms complexes with other extranuclear steroid receptors, as well as with Src.31 Such a receptor complex and its interaction with other ligands and their receptors may explain how corticosterone, even at low concentrations (10–11 mol/L), can induce signaling. We suggest that additional stimuli at other receptors involved in the complex may also influence MR activity.
Perspectives
Our data have clinical implications. Hoen et al32 reported recently that cortisol administered to patients with traumatic shock increases the sensitivity to 1-adrenoceptor stimulation independent of adrenal reserve, thus supporting the view that glucocorticoids modulate vascular function and tone. Tauchmanova et al33 showed that patients with symptoms of even a minor degree of Cushing’s syndrome have increased cardiovascular risk. Güder et al34 observed that high cortisol levels in heart failure patients were independent predictors of increased mortality risk. Total cortisol and its metabolite concentrations are higher in hypertensive persons with glucose intolerance than in normotensive control subjects.35 Our data could contribute to the understanding of the role of MR in the progression of cardiovascular disease.
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Acknowledgments |
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Mineralocorticoid receptor–detecting antibodies were a kind gift from Celso Gomez-Sanchez. We thank Petra Quass, Jana Czyci, Michaela Beese and Kristin Wyss for their excellent technical assistance.
Sources of Funding
The Deutscher Akademischer Austauschdienst supported G.A.M. The Deutsche Forschungsgemeinschaft supported A.F., P.R.M., M.G., F.C.L., and D.N.M. EuReGene also supported F.C.L. and D.N.M.
Disclosures
None.
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Footnotes |
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The first 2 authors contributed equally to this work.
Received November 21, 2007; first decision December 19, 2007; accepted February 12, 2008.
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