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Department of Internal Medicine, Justus Liebig University Giessen, Giessen
Schering Deutschland, Berlin, Germany
ABSTRACT
Rationale: Inhaled iloprost is an effective therapy for pulmonary arterial hypertension (PAH). However, no study to date has addressed the effects of inhaled iloprost on changes to pulmonary vascular structure that occur in PAH. Objectives: The present study was designed to investigate chronic antiremodeling effects of inhaled iloprost in monocrotaline (MCT)-induced PAH in rats. Methods: Four weeks after a single injection of MCT, after full establishment of PAH, rats were nebulized with iloprost at a dose of 6 e · kgeC1 · dayeC1, or underwent sham nebulization with saline. Results: After 2 weeks of inhalation therapy, right ventricular pressure and pulmonary vascular resistance were reversed in rats treated with iloprost, but not in sham-treated control animals. Systemic arterial pressure was unaffected. In addition, right heart hypertrophy, the degree of pulmonary artery muscularization, and the medial wall thickness of intraacinar pulmonary arteries regressed in response to iloprost. Furthermore, the MCT-induced increase in matrix metalloproteinase-2 and -9 activities and tenascin-C expression was suppressed. Conclusions: We conclude that the inhalation of iloprost reverses PAH and vascular structural remodeling in MCT-treated rats. This regimen suggests the possibility of an antiremodeling therapy in PAH.
Key Words: iloprost; monocrotaline; pulmonary hypertension; remodeling
Pulmonary arterial hypertension (PAH) is a severe disease, characterized by elevated pulmonary artery pressure and pulmonary vascular resistance that ultimately leads to high mortality due to right heart failure (1). Infused epoprostenol has proven beneficial in the long-term treatment of patients with pulmonary hypertension (2). However, because of the nonselective route of application, systemic side effects accompany the pulmonary vasodilatory effect. Inhalation of aerosolized iloprost has been demonstrated to promote selective pulmonary vasodilation in severe pulmonary hypertension (3). Iloprost is a stable prostacyclin analog with strong vasodilatory (4) and antithrombotic (4) properties, and the long-term beneficial effects of daily repetitive iloprost inhalation have been documented (5eC9). However, it remains controversial whether such beneficial long-term effects are due to the sustained pulmonary vasodilatory effects of the prostanoid, or whether they indicate reversal of structural changes that underlie the pathophysiology of PAH.
In the present study, we employed monocrotaline (MCT), a toxin derived from plants of the genus Crotalaria (10), to induce pulmonary artery smooth muscle hypertrophy and PAH in a rat model, as described previously (11). We then examined the acute pulmonary vasodilatory efficacy of inhaled iloprost in this model, and the antiremodeling effects of long-term inhalation of this agent.
METHODS
Study Protocols
Experiments were performed on Wistar rats (body weight, 300eC350 g; Charles River, Sulzfeld, Germany). PAH was induced by a subcutaneous injection of MCT (60 mg · kgeC1; Sigma, Deisenhofen, Germany), dissolved in 0.1 M NaOH and adjusted to pH 7.40 with 0.1 M HCl, as described previously (12eC16). The stable prostacyclin analog iloprost, or vehicle, was administered in nebulized form every day for 14 days, beginning 28 days after MCT injection. In a separate set of experiments, the acute hemodynamic effects of iloprost were investigated in animals 28 days after MCT injection. The experiments were performed according to institutional guidelines for the use and care of animals.
Study Groups
The animals were classified into the following four groups: (1) rats injected with saline (control, n = 10); (2) MCT-injected rats, 28 days postinjection (MCT28, n = 14); (3) MCT-injected rats, 42 days postinjection and undergoing sham nebulization with saline from Days 28 to 42 (MCT42, n = 14); and (4) MCT-injected rats, 42 days postinjection and undergoing inhalation of iloprost from Days 28 to 42 (MCT42/Ilo, n = 10). For acute hemodynamic studies, 24 MCT28 rats (rats injected with MCT, 28 days postinjection) were used (inhalation of saline, n = 6; inhalation of iloprost at three different doses, n = 6 each).
Inhalation of Iloprost
Four weeks after a single MCT injection, rats were subjected to inhalation of iloprost in an unrestrained, whole body aerosol exposure system. The agent was aerosolized, via a jet nebulizer (Pari LC Star; Pari, Starnberg, Germany) connected to the cage, at a constant flow rate (6 L · minuteeC1) for 15 minutes. This nebulizer was characterized by a mass median aerodynamic diameter of 2.8 e and a geometric standard deviation of 2.5 (determined by laser difractometric measurements, as described previously [17]). Total lung deposition of nebulized material was 0.5%. This deposition fraction was determined in preceding experiments by nebulization of 99mTc-DTPA in the same nebulization system and subsequent quantification of 99mTc-DTPA in the lavage fluid of exposed rats. This value is in accordance with 0.3% lung deposition, which has been demonstrated previously in this model (16).
To assess the acute vasodilatory effects of inhaled iloprost, hemodynamic testing in response to the drug was performed 4 weeks after MCT injection. Hemodynamics were measured before and after a single inhalation of the agent (iloprost: 0.13, 0.65, and 1.3 e · kgeC1 · minuteeC1 for 15 minutes), using an ultrasonic nebulizer with a mass median aerodynamic diameter of 4.0 e and a geometric standard deviation of 2.1 as described previously (18).
For assessment of chronic effects of inhaled saline or iloprost (total deposited dose, 6.0 e · kgeC1 · dayeC1), 15-minute nebulizations were repeated 12 times per day for 2 weeks in rats that had already developed PAH due to MCT injection 4 weeks previously. Hemodynamic studies and assessment of structural changes were performed after completion of the 2-week period of inhalative therapy.
Surgical Preparation and Tissue Preparation
For monitoring hemodynamics, animals were anesthetized with ketamineeCxylazine and tracheostomized, as described previously (13, 15). A polyethylene catheter was inserted into the left carotid artery to measure arterial pressure. A right heart catheter (PE-50 tubing) was inserted into the right ventricle through the right jugular vein for measurement of right ventricular systolic pressure with fluid-filled force transducers. Cardiac output was measured by a thermodilution technique. Briefly, a thermistor (1.5 F) was placed into the ascending thoracic aorta via the right carotid artery for measurement of transpulmonary thermodilution cardiac output (Cardiotherm 500-X; Hugo Sachs Electronik-Harvard Apparatus, March-Hugstetten, Germany). The cardiac output was averaged from three consecutive determinations and indexed to the weight of the animal to obtain cardiac index as described previously (13). After exsanguination, the left lung was fixed for histology in 10% neutral buffered formalin and the right lung was frozen in nitrogen.
Right Ventricular Hypertrophy
The right ventricle (RV) was dissected from the left ventricle (LV) and the septum (S) and weighed to determine the extent of RV hypertrophy from the ratio RV/(LV + S); this was done by an observer blinded to the treatment.
Histologic Examination of Lungs
The degree of muscularization of small peripheral pulmonary arteries was assessed by double-staining 3-e sections with an antieC-smooth muscle actin antibody (diluted 1:900; clone 1A4; Sigma, St. Louis, MO) and antihuman von Willebrand factor antibody (diluted 1:900; DakoCytomation, Hamburg, Germany). Sections were counterstained with hematoxylin and examined by light microscopy, using a computerized morphometric system (QWin; Leica Microsystems, Bensheim, Germany). In detail, at x400 magnification 80 small pulmonary vessels of each animal ranging from 10 to 50 e in external diameter were counted and noted as muscular, partially muscular, or nonmuscular by an observer blinded to treatment. To assess the degree of muscularization, the amount of -smooth muscle actineCpositive vessel wall area was determined in each vessel by the QWin image-processing system (Leica Imaging Systems, Bensheim, Germany). Nonmuscular arterioles were detected by endothelial antieCvon Willebrand factor staining. Arteries that contained more than 70% -actineCpositive vessel wall area were defined as muscular; arteries with less than 4% -actineCpositive vessel area were defined as nonmuscular. Arteries that contained between 4 and 70% -actineCpositive vessel area were defined as partially muscularized. For all muscular arteries with an external diameter of 50 to 100 e, the wall thickness of the media (i.e., the distance between external and internal elastic laminae) was measured along the shortest curvature and expressed as a percentage of medial wall thickness calculated as (2 x media thickness/external diameter) x 100. Media thickness was defined as the distance between the lamina elastica interna and lamina elastica externa.
Gelatin Zymography
Lung tissues were homogenized (20 mg of tissue weight per milliliter of buffer) at 4°C in 20 mM Tris-HCl, 150 mM NaCl, 1% (vol/vol) Triton X-100, 2.5 mM sodium pyrophosphate, 1 mM -glycerophosphate, 10 e E-64, pH 7.5. The matrix metalloproteinase (MMP) activity was analyzed by sodium dodecyl sulfateeCpolyacrylamide gel electrophoresis zymography, in which the enzymes hydrolyze the gelatin substrate present in the gel to form a clear band. After staining for protein in the gel, samples were separated on a 10% polyacrylamide gel copolymerized with gelatin (1 mg · mleC1) at 4°C. The gel was subsequently incubated for 1 hour at 25°C in 2.5% (vol/vol) Triton X-100. The gel was washed with water (twice, 20 minutes each time) before incubating it overnight at 37°C in 50 mM Tris-HCl, pH 7.5. The gel was kept in fixative containing 40% (vol/vol) methanol and 7% (vol/vol) acetic acid for 1 hour, after which the gel was stained with 0.25% (vol/vol) Coomassie Brilliant Blue R250 dye for at least 1 hour and then destained in 10% (vol/vol) methanol and 7% (vol/vol) acetic acid. The gel was visualized with a BioDocAnalyze system (Whatman Biometra, Goettingen, Germany), in which activity of MMP-2 and MMP-9 was shown as a clear white band against a blue background.
Reverse TranscriptioneCPolymerase Chain Reaction
To quantify tenascin-C expression changes in the various treatment groups, semiquantitative reverse transcriptioneCpolymerase chain reaction was employed. For this, total RNA was isolated from rat lungs with Trizol reagent (Life Technologies, Rockville, MD), and first-strand cDNA was synthesized with the ImProm-II reverse transcription system (Promega, Madison, WI), using oligo(dT)12eC18 primers according to the manufacturer's instructions. Afterward, 1 e of cDNA product was used as a template in polymerase chain reaction amplifications together with appropriate primers as described. The final products were electrophoresed in a 2% agarose gel and detected by ethidium bromide staining. The expression levels of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were monitored as a loading control. Primers designed specifically for rat tenascin-C and GAPDH were as follows: for tenascin-C: forward primer, ATGTTGAATGGCGACAC; and reverse primer, CGGTCTCCAAACCCAG; and for GAPDH: forward primer, ACAGCCTGTTTCTGGT; and reverse primer, GATGCTGGATGCCTTTTAT.
Data Analysis
All data are given as means ± SEM. Differences between the groups were assessed by one-way analysis of variance and a StudenteCNewmaneCKeuls test for multiple comparisons, with a p value < 0.05 regarded as significant.
RESULTS
Acute Effects of Aerosolized Iloprost Administered to Rats 28 Days after MCT Treatment
Aerosolized iloprost dose dependently reduced right ventricular systolic pressure (Figure 1). As depicted, this pulmonary vasodilatation was accompanied by a slight decrease in systemic arterial pressure at the higher end of the dose range.
Chronic Effects of Aerosolized Iloprost: Hemodynamics
After injection of MCT, PAH developed in all rats (right ventricular systolic pressure on Day 28 = 68.9 ± 3.2 mm Hg [n = 11] and on Day 42 = 74.9 ± 5.1 mm Hg [n = 9], as compared with 27.3 ± 3.2 mm Hg in controls; Figure 2). No significant changes in systemic arterial pressure were observed. When compared with control animals (cardiac index, 36.5 ± 3.45 ml · minuteeC1 · 100 g bodyweight), the cardiac index was slightly decreased on Day 28 (27.4 ± 2.7 ml · minuteeC1 · 100 g bodyweight) and on Day 42 (31.8 ± 1.3 ml · minuteeC1 · 100 g bodyweight). The pulmonary vascular resistance index thus substantially increased, from 0.91 ± 0.05 to 2.41 ± 0.29 mm Hg · minute · mleC1 · 100 g bodyweight on Day 28 and to 2.36 ± 0.15 mm Hg · minute · mleC1 · 100 g bodyweight on Day 42. Inhaled iloprost (n = 8) decreased right ventricular systolic pressure to 52.9 ± 4.8 mm Hg (p < 0.05 vs. MCT42 and MCT28), increased cardiac output, and decreased the pulmonary vascular resistance index to 1.48 ± 0.18 mm Hg · minute · mleC1 · 100 g bodyweight (p < 0.05 vs. MCT42 and MCT28).
Chronic Effects of Aerosolized Iloprost: Right Ventricular Hypertrophy
Four weeks after injection of MCT, animals demonstrated significant right heart hypertrophy, indicated by an increase in the ratio of right ventricular weight to left ventricular plus septum weight (RV/[LV + S]) from 0.29 ± 0.02 (control animals) to 0.62 ± 0.02 (Figure 3). Rats that received inhaled vehicle for 2 weeks, from Days 28 to 42, showed further progression of right ventricular hypertrophy (RV/[LV + S] = 0.78 ± 0.07). Inhaled iloprost not only prevented this further progression, but also caused a significant regression of right ventricular hypertrophy as compared with Day 28 (MCT42/Ilo = 0.51 ± 0.02).
Chronic Effects of Aerosolized Iloprost: Histopathology
Evaluation of the extent of muscularization of pulmonary arteries with external diameters of 15 to 50 e demonstrated a significant reduction in normally nonmuscularized pulmonary arteries and a subsequent increase in fully muscularized vessels in response to MCT (Figures 4 and 5). Medial wall thickness was significantly increased in the MCT28 and MCT42 groups compared with the control group. We analyzed the medial wall thickness of arteries with external diameters of 50 to 100 e (Figure 6). In comparison with control animals, the medial wall thickness (as a percentage of total wall thickness) increased significantly from 7.2 ± 1.8 to 28.0 ± 4.8% (MCT28) and to 32.0 ± 5.7% (MCT42). Both the percentage of fully muscularized pulmonary arteries and medial wall thickness (21.8 ± 2.1%) were significantly reduced by aerosolized iloprost (p < 0.05 vs. MCT42 and MCT28).
Chronic Effects of Aerosolized Iloprost: MMPs and Tenascin-C
The activity of MMP-2 and MMP-9 was monitored by zymography. Significantly elevated gelatinolytic capacities were observed in response to MCT treatment, with MMP-2 having the major effect (Figure 7). The MMP-2 activity in lungs from MCT-treated rats was observed as a triplet, with molecular mass values of 66, 62, and 59 kD, with 62 kD being the predominant form, as previously described (19, 20). Activity of MMP-2 and MMP-9 decreased significantly in the iloprost treatment group, compared with both MCT42 and MCT28. Similar results were obtained for tenascin-C, which was fivefold upregulated in MCT-treated rats (p < 0.05) as compared with control animals (Figure 8) and downregulated by 25% in the iloprost-treated animals.
DISCUSSION
In the present study, we demonstrate that daily repetitive iloprost inhalation significantly improved pulmonary hemodynamics and reversed changes in vascular structure in MCT-induced PAH in rats. Similar to the abnormalities in human PAH, MCT treatment in rats is known to provoke endothelial injury, proliferation and hypercontraction of vascular smooth muscle cells, and inflammatory sequelae (21, 22). The animals die as a result of a progressive increase in precapillary lung vascular resistance with right heart failure. The inhalative therapy was commenced after establishment of severe PAH, 4 weeks after application of the toxin, which is in contrast to most previous investigations, in which the MCT and the agent of interest were coapplied (e.g., endothelin antagonist [23], prostaglandin E1 [24], beraprost [14], or a phosphodiesterase type 5 inhibitor [14]). The limitation of such a preventive approach is that several pharmacologic interventions will result in a mitigation of PAH, because most of these compounds have some antiinflammatory potency (e.g., prostanoids, endothelin antagonists). However, as proven in many studies from our group and others, the inflammatory response levels off within the first week after MCT injection and is then replaced by a chronic vascular remodeling process, which in the later course of the disease appears to be no longer driven by inflammation. Therefore, more recent studies have applied therapeutic interventions once the disease was fully established (i.e., Rho kinase inhibitors [25], elastase inhibitors [12], statins [26], or phosphodiesterase inhibitors [13, 15]).
Preceding in vitro studies have already suggested substantial antiproliferative potency of stable prostacyclin analogs in human pulmonary artery smooth muscle cells (27). The effects of the prostacyclin analogs are mediated by cell surface prostanoid receptors, in particular the prostaglandin I2 (prostacyclin) receptor (28). This receptor is coupled to adenylyl cyclase via G proteins and increases cAMP in many cell types, including pulmonary artery smooth muscle cells (27, 29). Interestingly, distal human pulmonary artery smooth muscle cells, isolated from pulmonary arteries (less than 1 mm in external diameter), seem to be more susceptible to prostacyclin analog-induced inhibition of proliferation than are pulmonary artery smooth muscle cells from proximal pulmonary arteries (greater than 8 mm in external diameter) (29). These observations are well in line with our observation that MCT-induced structural changes in the intraacinar small pulmonary arteries (15eC50 e), representing the decisive segment for precapillary lung vascular resistance, were significantly improved by inhalation of iloprost.
The key finding of the study is the fact that regression of PAH was achieved by the inhalative prostanoid regimen. This is true for both hemodynamics and structural changes. As to hemodynamics, right ventricular systolic pressure, cardiac index, pulmonary vascular resistance, arterial oxygenation, and central venous oxygenation in the MCT42/Ilo animals were significantly improved in comparison with both the sham-treated MCT42 animals and the MCT28 animals representing those before onset of therapy. Concerning structural changes, corresponding treatment effects were observed for right heart hypertrophy, the degree of muscularization (nonmuscular versus partially muscular versus fully muscular) of the small intraacinar pulmonary arteries, and the thickness of the medial wall of these vessels. These findings are supported by the observed reductions in MMP-2 and MMP-9 expression that we observed after iloprost application, because these two metalloproteinases are about four- to fivefold increased in MCT-treated rats. Both, MMP-2 and MMP-9 are regulated by intracellular cAMP levels (30, 31) and it has been shown that prostaglandin E2 and dibutyryl-cAMP suppress membrane type 1 MMP, which activates MMP-2 (32). Furthermore, cAMP upregulates expression of tissue inhibitors of metalloproteinases (33, 34), which may further contribute to this reduced MMP-2 and MMP-9 activity observed in response to iloprost. It has been shown that pulmonary vascular remodeling is associated with induction of tenascin-C, which is a mitogenic cofactor produced through the action of MMPs (35). The reduction in tenascin expression that we found in response to iloprost may thus be directly linked to the downregulation of MMP-2 and MMP-9. In addition, it has been shown that inhibition of MMPs and vascular elastases suppresses tenascin-C expression (36, 37).
In conclusion, repeated inhalation of aerosolized iloprost reversed MCT-induced PAH, with respect to both hemodynamics and structural changes of the small intraacinar pulmonary arteries. The in vivo antiproliferative potency of inhaled iloprost, which even caused regression of structural remodeling of the lung vessels, has not been demonstrated previously, but is well in line with the beneficial effects reported for long-term treatment with inhaled iloprost in clinical studies (6eC9). In addition to its potent lung vasodilatory effects, these findings suggest that inhaled iloprost also represents an effective antiremodeling therapy in the rat model of MCT-induced PAH.
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