(Hypertension. 1995;26:665-669.)
© 1995 American Heart Association, Inc.
Articles |
From the Second Department of Internal Medicine, Kansai Medical University, Osaka, Japan.
Correspondence to Hiroaki Matsubara, MD, PhD, Second Department of Internal Medicine, Kansai Medical University, Fumizonocho 1, Moriguchi, Osaka 570, Japan.
| Abstract |
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Key Words: RNA, messenger receptors, vasopressin gene expression glucocorticoids
| Introduction |
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1ß-adrenergic5 and
angiotensin II6 receptors is increased by
glucocorticoids in vascular smooth muscle cells, which may cause the
vascular hyperreactivity to catecholamines and
angiotensin II in patients with Cushing's
syndrome7 and in experimental animals.8
Arginine vasopressin (AVP) receptors are expressed in vascular smooth
muscle cells and play an important physiological
role in vasoconstriction.9 10 Although previous studies
have demonstrated the contribution of AVP to the hypertension process
in several forms of hypertension, particularly in deoxycorticosterone
acetatesalt hypertension11 and in spontaneously
hypertensive rats and stroke-prone spontaneously hypertensive
rats,12 13 the involvement of AVP in
glucocorticoid-induced hypertension remains unknown. AVP has diverse actions, including the contraction of smooth muscle, stimulation of liver glycogenolysis, modulation of corticotropin release from the pituitary, and inhibition of diuresis.14 These physiological effects are mediated through the binding of AVP to specific membrane receptors of the target cells. AVP receptors are G protein-coupled and have been divided into at least three types (V1a, V1b, and V2). The V1a (vascular/hepatic) and V1b (anterior pituitary) receptors act through phosphatidylinositol hydrolysis to mobilize intracellular Ca2+.15 The V1a receptor mediates physiological effects such as cell contraction and proliferation, platelet aggregation, coagulation factor release, and glycogenolysis. The V1b receptor exists in the anterior pituitary to stimulate corticotropin release. The V2 receptors are found mainly in the kidney, are linked to adenylate cyclase and the production of cAMP, and are associated with antidiuresis.16 All of these receptors have been cloned recently17 18 19 and belong to the family of "seven membrane spanning" receptors, which signal through G proteins.
A previous study demonstrated that adrenalectomy reduced the AVP-sensitive adenylate cyclase activity in rat kidneys and that injection of aldosterone into adrenalectomized rats partially restored this response.20 Similarly, dexamethasone has also been shown to increase AVP-sensitive adenylate cyclase in epithelial cells derived from the kidney of Xenopus laevis.21 Colson et al22 have shown that glucocorticoids upregulate the rat V1a receptor number in a rat mammary tumor cell line. Although these findings suggest that glucocorticoids regulate AVP receptor expression, the molecular mechanisms responsible for the regulation have not been clarified. In the present study we report that dexamethasone upregulates V1a receptor expression by increasing mRNA stability and that de novo protein synthesis is involved in this regulation.
| Methods |
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[3H]AVP Binding to Cells in Monolayer
A10 cells were washed with ice-cold binding buffer
(Dulbecco's phosphate-buffered saline with 10 mmol/L
MgCl2, 0.7 mmol/L CaCl2, 0.1%
glucose, and 0.2% bovine serum albumin), and binding was
initiated with 1 mL binding buffer containing 0.5 to 55 nmol/L
[3H]AVP (curies per millimole, New England Nuclear) with
or without 10 µmol/L AVP for 90 minutes at 4°C, as described by
Stassen et al.9 Binding was terminated by aspirating the
solution, and the unbound radioligand was removed by
washing twice with ice-cold binding buffer. Bound radioactivity was
removed by adding 1 mL of 0.2% sodium dodecyl sulfate
(SDS)/0.2 mol/L NaOH for 5 minutes and washing with an additional 0.5
mL SDS/NaOH. Radioactivity was measured with a gamma counter (LKB
Produkter). Counts were corrected for background radioactivity, and
specific binding was determined by subtracting the radioactivity bound
in the presence of 10 µmol/L unlabeled AVP.
Bmax and Kd values were
calculated from Scatchard plots.24 25
Northern Blotting
Total cellular RNA, isolated by the CsCl
centrifugation method, was denatured with 6%
formaldehyde, fractionated by 1% agarose gel electrophoresis,
transferred to a nylon filter, and hybridized at 42°C for 12 to 16
hours to the V1a receptor cDNA probe labeled with
[32P]dCTP.23 26 The used probe was the cDNA
fragment corresponding to nucleotides -91 to +1343 of
a V1a receptor cDNA clone17 relative to the
ATG initiation codon. The cDNA fragment was obtained by reverse
transcriptase and polymerase chain reaction as follows. Adult rat liver
total RNA (1 µg) was transcribed with random hexamers (2.5 µmol/L),
dNTPs (1 mmol/L), and RNase inhibitors (1 U/µL) with the
use of Moloney Murine Leukemia Virus reverse transcriptase (2.5 U/µL)
for 45 minutes at 42°C as previously reported.24 27 The
resultant single-strand cDNA was amplified with the forward primer
in the 5'-noncoding region (5'-GCGCAGAGCTTAGAACTCGGATCCTCCGGT-3') and
reverse primer in the 3'-noncoding region
(5'-CTTTGGACGCAGTCTTGCAGGAGATGGCC-3') with the use of Taq
polymerase (Takara Shuzo). The resultant polymerase chain reaction
product was radiolabeled by random oligonucleotide
primer extension with the use of [
-32P]dCTP and was
used as a probe.23 After hybridization the filter was
washed in 0.1x SSC plus 0.1% SDS at 65°C and exposed to Kodak XAR-5
film with an intensifying screen. The used filters were boiled in
0.01x SSC plus 0.01% SDS and rehybridized to the ß-actin cDNA
probe. The washing and exposure to the film were performed in the same
way as that for the V1a receptor cDNA probe.
Transcript Stability Analysis
The stability of V1a receptor mRNA in
dexamethasone-treated and control cells was measured by
incubation with actinomycin D (5 µg/mL) to block
transcription.24 28 After various incubation times, total
RNA was isolated from individual dishes, and the disappearance of mRNA
abundance was determined by Northern blots as mentioned above.
Nuclear Run-off Assay
The preparation of nuclei and run-off assays were
performed as described previously.24 28 Nuclei were
incubated for 20 minutes at 30°C in the presence of 50 mmol/L Tris-Cl
(pH 7.9); 100 mmol/L KCl; 12.5% glycerol; 6 mmol/L MgCl2;
0.2 mmol/L EDTA; 0.5 mmol/L dithiothreitol; 4 mmol/L each ATP, GTP, and
CTP; 1 U/µL RNAsin; and 200 µCi [
-32P]UTP. After
RNase-free DNase I and proteinase K digestion, the reaction
products were extracted with guanidinium isothiocyanate (4 mol/L)
and phenol/chloroform, and unincorporated [
-32P]UTP
was removed by trichloroacetic acid precipitation and filtration. The
radiolabeled RNA (3x106 cpm) was hybridized at
42°C for 48 hours with linearized pBluescript II KS(-) plasmids
containing rat V1a receptor cDNA (15 µg), rat
angiotensin II type 1 receptor cDNA (15 µg), or
ß-actin cDNA fragments (5 µg). The membrane was washed in 2x
SSC plus 0.1% SDS at room temperature for 5 minutes three times and
0.2x SSC plus 0.1% SDS at 65°C for 15 minutes twice, and then the
bound radioactivity was determined by scintillation counting.
Reagents and Statistical Methods
All reagents were purchased from Sigma Chemical Co unless
otherwise indicated. Results are expressed as mean±SEM. ANOVA and
Fisher's protected least significant difference test were used for
multigroup comparisons. Values of P<.05 were considered
statistically significant.
| Results |
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The steady-state V1a receptor mRNA level was measured by Northern blot analyses. Three sizes of mRNA signals were observed in A10 cells (2.8, 2.1, and 1.9 kb), and they showed the same relative proportions throughout the experiments involving dexamethasone treatment (Fig 1B and 1C). These signals were positive even after the filters were washed at high stringency in 0.1x SSC at 65°C. Morel et al17 showed that a single signal of 2.1-kb mRNA was exclusively expressed in rat liver, kidneys, spleen, and testis; they did not study the expression profile in vascular smooth muscle cells such as A10 cells. As shown in Fig 1B, the addition of dexamethasone induced a significant increase in each of the three different sizes of the V1a receptor mRNA. The maximal accumulation was found at 12 hours; this gradually declined over the next few hours. Throughout the experimental period the abundance of ß-actin mRNA did not change (Fig 1B); therefore, we used ß-actin mRNA as an internal control for minor fluctuations in mRNA applied to the gel. Fig 1C illustrates the relative increase in the ratio of V1a receptor mRNA and ß-actin mRNA. The maximal increases in the 2.8-, 2.1-, and 1.9-kb V1a receptor mRNAs were 2.7-fold, 3.1-fold, and 2.8-fold, respectively, at 12 hours (data for 2.8-kb mRNA are shown in Fig 1C). RU38486, a specific glucocorticoid antagonist, completely inhibited the increase in mRNA levels observed after exposure to dexamethasone (Fig 1B and 1C).
To examine whether the glucocorticoid-mediated effects required new protein synthesis, we pretreated A10 cells with cycloheximide (5 µg/mL) for 2 hours and then added dexamethasone to the medium for 12 hours. The yield of mRNA from the cycloheximide-treated cells was significantly increased compared with that in the cells not exposed to the protein synthesis inhibitor. Nevertheless, the proportion of the alternatively spliced signals was unchanged in the cycloheximide-treated cells. Dexamethasone failed to induce an increase in V1a receptor mRNA in cells treated with cycloheximide (Fig 1B and 1C).
Effects of Glucocorticoids on V1a Receptor mRNA
Stability and Gene Transcription
We examined V1a receptor mRNA stability by inhibiting
new mRNA transcription with actinomycin D. After the cells had been
incubated 12 hours with dexamethasone, actinomycin D (5
µg/mL) was added and the disappearance of V1a receptor
mRNA with time was measured by Northern blots. Fig 2
shows the effect of dexamethasone on V1a
receptor mRNA turnover. Half-life periods for the 2.8-, 2.1-, and
1.9-kb V1a receptor mRNAs were 6.2, 6.8, and 6.6 hours,
respectively, in the absence of dexamethasone; these
half-life periods increased to 14.0, 14.8, and 14.4 hours (about
2.2-fold) in the presence of dexamethasone (data for the
2.8-kb transcript are shown in Fig 2). In addition,
the effect of cycloheximide on V1a receptor mRNA turnover
was also examined, since cycloheximide affected the steady-state
level of the V1a receptor mRNA, as indicated in Fig 1. Fig 2 shows that the half-life
periods for the 2.8-, 2.1-, and 1.9-kb transcripts increased to 9.4,
9.4, and 9.6 hours, respectively, on exposure to cycloheximide.
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We next examined the effect of dexamethasone on the transcriptional level using a nuclear run-off assay as reported previously.24 28 The results of the run-off assay were normalized to the transcription rate of the ß-actin gene, which was unchanged by exposure to dexamethasone. As shown in Fig 3, the transcription rate of the V1a receptor gene was unchanged in the cells that had been exposed to dexamethasone for 12 hours compared with that of control cells. Similar results were obtained in the cells incubated with dexamethasone for 6 or 24 hours (data not shown). On the other hand, the relative transcription rate of the rat angiotensin II type 1 receptor gene was significantly increased (2.3±0.1-fold, n=3) in these dexamethasone-treated cells, as previously observed in cardiac fibroblasts24 and vascular smooth muscle cells,6 confirming the validity of our nuclear run-off assay.
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| Discussion |
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For most genes regulated by steroids, gene expression is mainly influenced by a change in transcription rate,29 but significant differences in mRNA stability have been reported. Glucocorticoids stabilize the growth hormone,30 fibronectin,31 and phosphoenolpyruvate carboxykinase mRNAs32 and destabilize interleukin-1ß33 and type 1 procollagen34 mRNAs. Structures at the 3' end enhance or diminish the stability of the specific mRNAs: Glucocorticoids stabilize growth hormone mRNA by increasing the length of the poly(A) tail,30 and the 3' noncoding region of phosphoenolpyruvate carboxykinase mRNA contains a glucocorticoid-responsive mRNA-stabilizing element.32 Of particular interest are the A plus Urich regions, many of which contain the sequence AUUUA (in a single or, more frequently, a multiple repeat) upstream of and relatively near the poly(A) tail.35 The rat V1a receptor mRNA includes this sequence in its 3'-untranslated tail (S.M. et al, unpublished observation, 1995). Thus, different mechanisms appear to be involved in the stabilization process for various mRNAs, and the present study could not define which mechanism is responsible for the glucocorticoid-induced stabilization in rat V1a receptor mRNA. The finding that cycloheximide affects the steady-state level of V1a receptor mRNA by increasing its mRNA stability suggests that de novo synthesis of a destabilizing factor is involved in the mRNA degradation. The failure of dexamethasone to induce an increase in mRNA abundance on exposure to cycloheximide raises the possibility that dexamethasone upregulates V1a receptor mRNA level by increasing synthesis of a putative stabilizing factor.
Gene transcriptional regulation by steroids requires the
interaction of the hormone-intracellular receptor complex with
cis-regulatory sequences.36 The transcriptional
induction by glucocorticoids through the glucocorticoid response
element is generally rapid and does not require new protein
synthesis.37 Expression of the structurally related
ß2-adrenergic receptor is regulated by glucocorticoids at
the gene transcriptional level.38 For the
ß2-adrenergic receptor, mRNA abundance rises rapidly
(occurring within 15 minutes), and the maximal accumulation occurs
between 1 and 2 hours. In the present study the maximal increase in
V1a receptor mRNA accumulation was observed after a 12-hour
exposure to glucocorticoids, and new protein synthesis was involved in
the steady-state level as well as the glucocorticoid-induced
increase in V1a receptor mRNA. In addition, the nuclear
run-off assay clearly established that the gene transcription of
the V1a receptor is not stimulated by glucocorticoids.
Using the nuclear run-off assay, we have shown that the gene
transcription of the angiotensin II receptor is upregulated
by glucocorticoids in cardiac fibroblasts27 and A10 cells
(Fig 3). Expression of
1ß-adrenergic5 and
ß2-adrenergic38 receptors has also been
shown to be upregulated by glucocorticoids on the gene transcriptional
level. Thus, the V1a receptor gene may belong to a distinct
group in seven transmembranetype receptors with respect to the
response to glucocorticoids.
In conclusion, we have demonstrated that glucocorticoids induce the expression of V1a receptor mRNA and protein by increasing mRNA stability and that de novo protein synthesis is involved in this regulation. Although the established A10 smooth muscle cell line is derived from rat thoracic aorta and may not reflect with fidelity events in relevant vascular smooth muscles, the number of receptors and mRNA abundance remained increased even after a 48-hour exposure to glucocorticoids, suggesting that AVP could at least in part contribute to the enhancement of vascular responsiveness observed in patients with Cushing's syndrome.
| Acknowledgments |
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Received January 25, 1995; first decision March 23, 1995; accepted June 20, 1995.
| References |
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1ß adrenergic receptor gene in
DTT1 MF-2 smooth muscle cells. J Clin
Invest.. 1991;88:385-389.
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