(Hypertension. 2000;35:219.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
From the Department of Physiology, School of Medicine, University of Florida, Gainesville.
Correspondence to Dr M. Ian Phillips, Department of Physiology, School of Medicine, University of Florida, Box 100274, 1600 SW Archer Rd, Gainesville, FL 32610. E-mail mip{at}phys.med.ufl.edu
| Abstract |
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Key Words: antisense ß1-adrenergic receptors blood pressure renin-angiotensin system kidney
| Introduction |
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We have previously shown that antisense oligodeoxynucleotide (ß1-AS-ODN) significantly inhibits ß1-AR expression in the cardiac ventricles, which results in suppressed inotropic response to adrenergic activation and thereby contributes to hypotension.3 In addition to inducing positive inotropy and chronotropy in the heart, ß1-ARs are also responsible for mediating the sympathetic stimulation of renin expression and secretion from juxtaglomerular cells of the renal cortex. Infusion of isoproterenol has been shown to increase renin expression and secretion and plasma renin activity (PRA) in rats.4 ß-Blockers reduce PRA in patients.5 However, despite the evidence that ß-blockers are more effective in patients with high renin profiles,6 the importance of ß-blockerinduced decreases in renin release has been debated.7 The present study had 2 goals. One was to investigate whether ß1-AS-ODN reduces renin expression and secretion and whether the RAS is involved in the antihypertensive impact of ß1-AS-ODN. The second was to test whether the antihypertensive effect of ß1-AS-ODN could be improved by delivery with cationic liposomes and to determine the optimal charge ratio of liposome:ODN.
| Methods |
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-dioleoyl phosphatidylethanolamine (DOPE, Avanti Polar Lipids) at 1:1 molar ratio was used to deliver ODNs in a single intravenous injection into the tongue vein. ODN-liposome complex was prepared on the day of use by mixing desired amounts of ODNs with DOTAP/DOPE to a final DNA concentration of 300 µg/mL in 5% (wt/vol) dextrose in water and incubating at room temperature for 60 minutes.
Animals
Adult male SHR (4 to 6 months old, Harlan, Indianapolis, Ind) were kept in cages in a room with a 12-hour light-dark cycle. Animals were fed standard laboratory rat chow and tap water ad libitum. Tail blood was collected for determination of PRA and angiotensin II (Ang II) levels. All procedures were approved by the University of Florida Animal Care Committee.
Blood Pressure Measurement
Blood pressure was measured by the tail-cuff method as previously described.3 Systolic blood pressure (SBP) was determined as the first pulsatile oscillation on the descending side of the pressure curve. Data values of each rat were taken as an average of
4 stable readings. Baseline was determined by averaging 3 days of measurements before antisense administration.
Membrane Preparation and ß-Adrenergic Receptor Binding Assay
Four days after intravenous injection of saline (n=6) or 0.5 mg/kg inverted ODN (n=6) or 4, 10, 18, and 40 days after injection of 0.5 mg/kg ß1-AS-ODN (n=24), animals were euthanized, and membranes were prepared from the renal cortex of left kidneys as previously described.9 For saturation experiments, 100 µg membrane protein was incubated in triplicate with 6 concentrations of [125I](-)-iodocyanopindolol (ICYP, NEN Life Science, 6.25 to 100 pmol/L) in a total volume of 250 µL containing 50 mmol/L Tris-HCl (pH 7.4) and 5 mmol/L MgCl2 at 36°C for 60 minutes. The nonspecific and ß2-adrenergic receptor binding levels were determined in the presence of 1 µmol/L (±)-alprenolol and 150 nmol/L CGP20712A (RBI), respectively. Then the reaction mixture was passed through a Whatman GF/B glass fiber filter with a Brandel harvester, and the bound radioactivity was counted for 1 minute.
Tissue Preparation and Quantitative Autoradiography
Four days after injection of 0.5 mg/kg ß1-AS-ODN (n=6) or saline (n=6), rats were euthanized, and the right kidneys were removed and frozen in dry ice. Sagittal sections of kidney (20 µm) were cut on a cryostat (Microm) at -20°C and mounted on microscope slides. Every seventh slide was stained with hematoxylin and eosin for histology. Receptor autoradiography was performed as described10 with 100 pmol/L ICYP at 25°C for 150 minutes in the presence of 1 µmol/L (-)-propranolol, 100 nmol/L ICI118,551 (ß2-selective antagonist), or 100 nmol/L CGP20712A (ß1-selective antagonist) to distinguish nonspecific, ß1-, and ß2-bindings. The images were quantified with a computerized image analysis system (MCID, Imaging Research) and normalized with 125I standards. Nonspecific binding was <10% of total binding.
Reverse TranscriptionPolymerase Chain Reaction and Southern Blotting
At different time points after the single injection of ß1-AS-ODN or inverted ODN, rats were euthanized, and the renal cortex was dissected from the left kidneys, immediately dipped into RNAlater tissue storage buffer (Ambion), and stored at -20°C. Total RNA was extracted with RNAwiz reagent (Ambion) and quantified by spectrophotometer. RNA samples from 4 to 5 rats from each time point were pooled. RNA (5 µg) was digested by DNase I and reverse-transcribed by Superscript reverse transcriptase (GIBCO BRL) at 42°C for 50 minutes, and 1/20 of the reverse transcription (RT) product was used to run a polymerase chain reaction (PCR) for 20 cycles. PCR primers for ß1-AR were 5'-CTCCGAAGCTCGGCATGG-3' (forward) and 5'-GCACGTCTACCGAAGTCCAGA-3' (reverse) and yielded products of 432 bp, which spanned the AUG start codon. Primers for preprorenin were 5'-AGGCAGTGACCCTCAACATTACCAG-3' (forward) and 5'-CCAGTATGCACAGGTCATCGTTCCT-3' (reverse) and yielded products of 362 bp. Primers for GAPDH were 5'-ATCAAATGGGGTGATGCTGGTGCTG-3' (forward) and 5'-CAGGTTTCTCCAGGCGGCATGTCAG-3' (reverse) and yielded products of 505 bp.11 RT-PCR products were subjected to Southern blotting, hybridized with psoralen-biotinlabeled cDNA probes, and detected with nonisotopic kits (Ambion). After the membranes had been exposed to x-ray films, the intensity of ß1-AR and preprorenin mRNAs was quantified by densitometry and normalized with GAPDH mRNA levels. The experiments were repeated at least twice.
PRA and Plasma Ang II Levels
PRA was determined with an angiotensin I (125I) radioimmunoassay kit (DuPont). Plasma Ang II levels were measured by radioimmunoassay as previously described.12
Statistical Analysis
Values were expressed as mean±SEM. Differences were considered statistically significant at a value of P<0.05. One-way repeated ANOVA and Tukeys test were used to compare blood pressure before and after AS-ODN treatment. Unpaired t test was used to compare Bmax, PRA, and plasma Ang II levels in 2 groups.
| Results |
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2.0 achieved the best gene delivery in vitro and in vivo.13 14 Therefore, we chose to test 5 charge ratios of DOTAP:ODN ranging from 0 to 3.5 to deliver 0.5 mg/kg ß1-AS-ODN intravenously. It was noticed that different batches of liposome mixture varied slightly in structure and particle size, which may influence the delivery efficiency. Figure 1 shows the effect of different liposome:ODN charge ratios on blood pressure of SHR (n=6 for each ratio) in a representative experiment. ß1-AS-ODN alone, ie, at ratio 0, did not change SBP, whereas ratio 0.5 significantly reduced SBP by up to 38 mm Hg for 7 to 8 days. When the ratio was increased, the duration of the hypotensive impact was drastically prolonged to 20 days at ratio 1.5 and 33 days at ratio 2.5 and 3.5, varying with liposome preparations. But the maximum drop in SBP was greater at ratio 1.5 and 2.5 (
35 mm Hg) than at ratio 3.5 (
25 mm Hg) (Table). Accordingly, the optimal charge ratio of DOTAP:ODN was determined to be 2.5. In the subsequent experiments, SHR (n=24) injected with 0.5 mg/kg ß1-AS-ODN with liposomes at a charge ratio of 2.0 were analyzed for the time course of changes in SBP, receptor levels, and peripheral RAS.
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Effects of ß1-AS-ODN on ß-Adrenergic Receptors in Renal Cortex
Scatchard analysis of ß-AR binding in renal cortex (Figure 2) indicated that ß1-AR was the major subtype in the control rats, composing 70% of total ß-ARs. After ß1-AS-ODN injection, the Bmax of ß1-ARs was diminished significantly, by 35% on day 4 (P<0.05), 29% on day 10 (P<0.05), and 23% on day 18, and completely restored on day 40. ß1-AR reduction in kidney coincided with that in heart,3 and both were accompanied by a significant drop in SBP (P<0.01) (Figure 2A). In contrast, the ß2-AR level was not affected (Figure 2B), nor was the affinity of either subtype (data not shown). Inverted ODN had no effect on either subtype (Figure 2C).
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Kidney slices from the same rats were subject to quantitative autoradiography to display the structural distribution of ß-ARs (Figure 3). ß1-Subtype composed
60% of the ß-AR levels, which was localized predominantly in the renal cortex and the outer band of the medulla. ß2-Subtype was more diffusely distributed in the kidney at a lower level. This result was consistent with previous reports.15 Four days after ß1-AS-ODN treatment, the overall density of ß1-subtype in kidney was significantly reduced from 23.5±2.1 to 15.4±3.3 fmol/mg (P<0.05). The diminution in renal cortex was particularly conspicuous because of the higher basal level. As expected, the distribution and concentration of ß2-subtype remained unchanged in accord with binding results. This further confirms the specificity of the inhibitory effect of ß1-AS-ODN on ß1-subtype.
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In an effort to demonstrate whether ß1-AS-ODN decreases the mRNA level of ß1-AR by inducing RNase H digestion, a pair of primers flanking the AUG start codon where ß1-AS-ODN was targeted was used to run a semiquantitative RT-PCR for 20 cycles, followed by Southern blotting. Figure 4 shows that ß1-AS-ODN did not reduce the level of steady-state ß1-AR mRNA in renal cortex, indicating that the inhibition of ß1-AR expression was not at the transcriptional level.
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Effects of ß1-AS-ODN on Peripheral RAS
RT-PCR (Figure 4) revealed that the preprorenin mRNA level in renal cortex was transiently decreased to 62% of control 4 days after ß1-AS-ODN injection. It was completely reversed by day 18 (Figure 5A). Conversely, PRA and plasma Ang II levels showed different patterns of reduction, which were significantly decreased on day 10 and day 18 (P<0.01) but not on day 4. Thus, PRA and Ang II seemed to have a delayed action relative to the reduction in renin mRNA (Figure 5).
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| Discussion |
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ß-Blockers have been used to treat hypertension for 3 decades. The reasons for their antihypertensive effects remain largely unclear, but the inhibition of renin release is regarded as a primary mechanism. Many ß-blockers can reduce PRA in patients and experimental animals.5 16 They are found to be more effective in patients with higher renin profiles.6 In our study, we find that ß1-AS-ODN effectively decreased PRA and Ang II in the long term. But the decrease in PRA and Ang II did not occur until
10 days after ß1-AS-ODN injection, in contrast to the rapid drop in cardiac output 2 days after injection.3 Thus, it appears that the effects of ß1-AS-ODN on the kidney renin and the circulating RAS are more delayed than cardiac action. We speculate that the suppression of cardiac output may account for the early phase of the antihypertensive effect of ß1-AS-ODN, whereas the inhibition of renin-angiotensin activity acts as the secondary mechanism underlying the sustained reduction of blood pressure in SHR.
Receptor binding assay showed that ß1-AS-ODN reduced the ß1-AR levels in renal cortex by
30% for 18 days. This is consistent with the decrease of ß1-AR in heart ventricles in magnitude and time course.3 This suggests that ß1-AS-ODN delivered by cationic liposomes is rapidly transported to peripheral organs after intravenous injection and effectively taken up into heart and kidney cells to a comparable extent. Several mechanisms have been proposed for the AS-ODN inhibition of the expression of target proteins. One involves the decrease in mRNA levels resulting from RNase H digestion of the RNA strand of the RNA-DNA duplex.17 18 To test this hypothesis, we designed a pair of primers that flanked the AUG start codon of ß1-AR mRNA where AS-ODN bound to perform semiquantitative RT-PCR. As shown in the Results, there was no reduction in the RT-PCR products, indicating the absence of RNase H action. Therefore, the inhibition of ß1-AR expression probably occurs in posttranscriptional steps.
By decreasing ß1-AR levels in kidney, ß1-AS-ODN significantly reduced PRA and the subsequent plasma Ang II levels. This is unlikely to be through the inhibition of renin expression, however, because there is no long-term diminution of renin mRNA levels. Instead, ß1-AS-ODN may exert its inhibitory impact on renin secretion or the conversion of inactive renin to active renin. This hypothesis is consistent with the observation that ß-adrenergic stimulation of renin expression had a time course different from that of renin secretion.4 19 Furthermore, ß-blockers have been shown to reduce prorenin processing to active renin without changing total renin (prorenin+PRA) levels in plasma.5
Efficient gene delivery is vital to the therapeutic application of AS-ODN in vivo. Among nonviral vectors, cationic liposomes are the most widely used. They are safe, nonimmunogenic, and easy to produce on a large scale. However, relatively low transfection efficiency has been obtained after intravenous administration, mainly because of the inactivation of cationic liposome by serum. It was recently shown that increasing the charge ratio (±) of liposome to DNA and inducing the maturation of liposome-DNA complex by prolonging incubation time can overcome this problem.13 20 The optimal charge ratio of DOTAP:DNA was demonstrated to be
2.13 14 Thus, we chose to test 5 charge ratios ranging from 0 to 3.5 to optimize the AS-ODN delivery. As shown in the results, increasing the charge ratio not only improved the delivery efficiency but also prolonged the duration of ß1-AS-ODN action. The best antihypertensive result (-35 mm Hg for 33 days) was consistently achieved at ratio 2.5.
In summary, ß1-AS-ODN delivered with cationic liposomes at a single intravenous injection achieves a marked and sustained hypotensive effect (30 to 35 mm Hg for 33 days) in SHR. The ß1-AS-ODN is clearly longer-lasting than any current drug, does not inhibit ß2-adrenergic receptors or cross the blood-brain barrier, and has negligible effect on heart rate.3 Therefore, the antisense is likely to have fewer side effects than currently used ß-blockers. Inhibition of cardiac contractility initially followed by reduced renin release are important mechanisms contributing to its antihypertensive effects.
| Acknowledgments |
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Received September 14, 1999; first decision October 29, 1999; accepted November 16, 1999.
| References |
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5. Blumenfeld JD, Sealey JE, Mann SJ, Bragat A, Marion R, Pecker MS, Sotelo J, August P, Pickering TG, Laragh JH. Beta-adrenergic receptor blockade as a therapeutic approach for suppressing the renin-angiotensin-aldosterone system in normotensive and hypertensive subjects. Am J Hypertens. 1999;12:451459.[Medline] [Order article via Infotrieve]
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