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Cardiovascular Research 2004 61(1):152-158; doi:10.1016/j.cardiores.2003.09.013
© 2004 by European Society of Cardiology
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Copyright © 2004, European Society of Cardiology

YC-1 prevents sodium nitroprusside-mediated apoptosis in vascular smooth muscle cells

Shiow L Pana, Jih H Guhb, Ya L Changa, Sheng C Kuoc, Fang Y Leed and Che M Teng*,a

aPharmacological Institute, College of Medicine, National Taiwan University, No. 1, Jen-Ai Road, Section 1, 100 Taipei, Taiwan
bSchool of Pharmacy, College of Medicine, National Taiwan University, Taipei, Taiwan
cGraduate Institute of Pharmaceutical Chemistry, China Medical College, Taichung, Taiwan
dYung-Shin Pharmaceutical Industry Co., Ltd., Taichung, Taiwan

* Corresponding author. Tel./fax: +886-2-2322-1742. cmteng{at}ha.mc.ntu.edu.tw

Received 25 April 2003; revised 27 August 2003; accepted 9 September 2003


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: Nitric oxide signaling pathways are of central importance in both the maintenance of vascular homeostasis and the progression of vascular disease. Since smooth muscle cell apoptosis is associated with numerous vascular disorders, the authors investigated whether YC-1, a soluble guanylyl cyclase (sGC) activator, regulates apoptosis in vascular smooth muscle cells (VSMC). Methods and results: Sodium nitroprusside (SNP) (1 mM) induced cGMP (guanosine 3':5'-cyclic monophosphate)-independent apoptosis in rat vascular smooth muscle cells using MTT assay and TUNEL-reaction techniques. Furthermore, sodium nitroprusside induced apoptosis via Bcl-2 down-regulation, cytochrome c release reaction, and caspase-3 activation by Western blotting analysis and enzymatic assay methods. YC-1 abolished these apoptotic signaling cascades and prevented apoptosis through a cGMP-involved pathway, and phosphatidylinositol (PI) 3-kinase behaved a downstream event in this pathway. Conclusions: These results suggest that YC-1 inhibits sodium nitroprusside-induced vascular smooth muscle cells apoptosis via a cGMP- and phosphatidylinositol 3-kinase-involved inhibition on Bcl-2 down-regulation/cytochrome c release/caspase-3 activation cascades. The ability of YC-1 to prevent smooth muscle cell apoptosis may play an important role in blocking lesion formation at sites of vascular injury.

KEYWORDS YC-1; Vascular smooth muscle cells; Apoptosis; Nitric oxide


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
Apoptosis, programmed cell death, is an important mechanism in the formation of vascular lesion [1]. In recent years, apoptosis of vascular smooth muscle cells (VSMC) has been implicated in both development and outcome of atherosclerosis. The altered balance between apoptosis and proliferation appears to promote disease development [2]. Identification of both the negative and positive modulators of apoptosis may lead to novel therapeutic approaches in treating vascular disease. NO inhibits key events that promote atherogenesis, including alterations in endothelial redox state, platelet and monocyte adhesion to the vessel wall and migration and proliferation of VSMC [3,4]. The small amount of NO has shown antiapoptotic effects via cGMP (guanosine 3':5'-cyclic monophosphate)-mediated interruption of apoptotic signaling pathways and direct inhibition of caspase activity [5,6]. Higher NO concentrations promote apoptosis, which has been studied in a variety of cell types including macrophages, chondrocytes, fibroblasts, and smooth muscle cells [7–10]. Under these circumstances, the proapoptotic effects of NO seem to be independent of cGMP accumulation [10].

There are numerous lines of evidence suggesting that the lipid kinase phosphatidylinositol (PI) 3-kinase plays an important role on the regulation of cell death in a lot of types of cells and it becomes clear that PI 3-kinase is also a determinant of VSMC fate [11]. Recent studies provide evidence that Akt/PKB (protein kinase B) is a critical downstream effector of PI 3-kinase [12]. Furthermore, it has been defined that VSMC fate is dependent on a cell-survival signaling cascade regulated by PI 3-kinase and the activation of Akt/PKB [11].

YC-1 (3-(5'-hydroxymethyl-2'-furyl)-1-benzyl-indazol) was discovered in our laboratory as a novel NO-independent type of soluble guanylyl cyclase (sGC) activator [13]. It is now recognized that YC-1 may have beneficial effects on cardiovascular function [14]. YC-1 inhibits cell proliferation of VSMC [15], inhibits glucose transport of cardiomyocytes [16], and attenuates the development of intimal hyperplasia in animal models of balloon-catheter vascular injury [15]. In the current study, we investigated the role of YC-1 in VSMC apoptosis induced by high concentrations of sodium nitroprusside (SNP) in vitro.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
2.1 Cell culture
The investigation conforms with Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1996). Wistar rats were euthanatized with intraperitoneal administration of pentobarbital following the guidelines for animal studies at our institution. VSMC were prepared and cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% FBS, 100 units/ml penicillin and 100 µg/ml streptomycin (Gibco, Grand Island, NY) as previously described [17]. Cells from passages 3 through 7 were used for all studies. The cells were characterized as smooth muscle cells by morphology and immunostaining with monoclonal antibody specific for smooth muscle {alpha}-actin.

2.2 Cytotoxicity assay
The cytotoxicity assay was carried out using the mitochondrial reduction activity assay. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, Sigma, St. Louis, MO) was dissolved in phosphate-buffered saline (PBS) at a concentration of 5 mg/ml and filtered (Millipore, Bedford, MA). From this stock solution, 10 µl/100 µl of medium was added to each well, and plates was gently shaken and incubated at 37 °C for 2 h. After the incubation period, the cells were lysed with dimethylsulphoxide (DMSO) and quantified by the measurement of OD550 with an enzyme-linked immunosorbent assay (ELISA) reader.

2.3 In situ labeling of apoptotic cells
In situ detection of apoptotic cells was carried out by using a terminal deoxynucleotidyl transferase (TdT) dUTP nick-end labeling (TUNEL) method with an apoptotic detection kit (Promega, Madison, WI, USA) as described previously [18] and photomicrographs were obtained with a fluorescence microscope (Nikon).

2.4 Assay of cGMP contents
At confluence, monolayer cells were incubated with indicated agents for 10 min. Then, cells were washed twice with ice-cold PBS and lysed with 0.5 ml NaOH (0.1 M). A 0.5 ml HCl (0.1 M) was then added to neutralize the assay solution. After the centrifugation (3000 x g for 3 min), the supernatant was used for the detection of cGMP content by using a cGMP ELISA kit.

2.5 Determination of caspase-3 activity
The caspase-3 activity was assayed with the caspase-3 colorimetric assay kit (R&D Systems, Minneapolis, MN). After the treatment of cells with indicated agents for 10 h, cells were harvested and the cell pellet was re-suspended in pre-cooled lysis buffer. After a 10-min incubation on ice, cell homogenates were centrifuged at 10,000 x g for 1 min and supernatants were removed for the determination of caspase-3 activity. Proteolytic reactions were performed in a total volume of 100 µl reaction buffer containing 50 µl of cytosolic extracts and 5 µl DEVD-pNA. The reaction mixture was incubated at 37 °C for 1 to 2 h and the formation of p-nitroaniline was measured at 405 nm by an ELISA reader.

2.6 Preparation of cellular cytosol fraction
Cells were harvested and the cell pellet was re-suspended in 50 µl of extraction buffer (20 mM HEPES, pH 7.5, 10 mM KCl, 1.5 mM MgCl2, 1 mM EDTA, 1 mM EGTA, 1 mM dithithretol, and 1 mM PMSF) and incubated for 3 min on ice. Cells were managed with 30 strokes and centrifuged at 15,000 x g for 15 min at 4 °C. The supernatant was obtained for the detection of cytochrome c.

2.7 Western blot analysis
After the exposure of cells to the indicated agents and time courses, cells were washed twice with ice-cold PBS and reaction was terminated by the addition of 100 µl ice-cold lysis buffer (10 mM Tris–HCl, pH 7.4, 150 mM NaCl, 1 mM EGTA, 0.5 mM phenylmethylsulfonyl fluoride (PMSF), 10 µg/ml aprotinin, 10 µg/ml leupeptin, and 1% Triton X-100). For the detection of Akt and phosphorylated Akt, 1 mM Na3VO4, 1 mM NaF, 50 mM tetrasodium pyrophosphate, 10 nM okadaic acid, 0.25% sodium deoxycholate were included in the lysis buffer. The cell lysates (25 µg/lane) were electrophoresized on 10–15% SDS-polyacrylamide gels and the Western blot analysis was carried out as we previously described [18]. Detection of signal was performed with an enhanced chemiluminescence detection kit (ECL; Amersham International, Little Chalfont, UK).

2.8 Measurement of reactive oxygen species (ROS)
Cells were incubated in the absence or presence of SNP and/or YC-1 for the indicated time courses. Thirty minutes before the termination of incubation period, DCFH-DA (10 µM) was added to the cells and incubated for the last 30 min at 37 °C. Then, cells were harvested for the detection of ROS accumulation using FACS analysis.

2.9 Statistical analysis
Data are presented as the mean±S.E.M. for the indicated number of separate experiment. Statistical analysis of data was performed with one-way analysis of variance (ANOVA) followed by a t-test and P-values less than 0.05 were considered significant.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
3.1 YC-1 prevents SNP-induced apoptosis in VSMCs
We examined the effect of YC-1 on SNP-induced apoptosis in cultured VSMC. MTT assay method and TUNEL-reaction technique showed that a high concentration of SNP (1 mM) induced profound cell apoptosis, but YC-1 (30 µM) completely abolished these SNP-induced effects (Fig. 1A). ODQ, an inhibitor of sGC, had no influence on SNP-induced apoptosis but significantly reversed YC-1-mediated action (Table 1), suggesting that the activation of sGC is involved in YC-1-mediated antiapoptotic activity other than SNP-induced apoptotic reaction. Additionally, YC-1 is able to reverse the apoptosis induced by other NO generating systems, such as 3-morpholino-sydnonimine (SIN-1) and S-nitroso-N-acetylpenicillamine (SNAP) (Fig. 1B).


Figure 1
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  		Fig. 1 Effects of SNP and YC-1 on VSMC cell survival. Cells were pretreated with or without YC-1 (A, 30 µM; B, 10 and 30 µM) for 30 min. Then vehicle, SNP (1 mM, A), SIN-1 (100 µM, B) or SNAP (100 µM, B) was added for another 24 h. After the incubation period, the cell viability was assayed using MTT assay method and TUNEL-techniques (as showed in the photographs) as described in the Methods section. Data are expressed as mean±S.E.M. of five determinations (each performed in triplicate).

 

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  		Table 1 Effects of sodium nitroprusside, YC-1, and ODQ on the regulation of cell survival in rat aortic smooth muscle cells

 
Intracellular cGMP levels were also assayed in this study. As shown in Fig. 2, YC-1 alone induced a marked increase in cGMP synthesis (4.2±0.8 vs. baseline value of 2.3±0.3 fmol/well). Additionally, the combination of SNP and YC-1 synergistically evoked more than 60-fold increase of this cyclic nucleotide formation. However, ODQ significantly inhibited the effects of YC-1 alone and the combination action of SNP and YC-1 (Fig. 2). Furthermore, the cell-permeable cGMP analogue dibutyl-cGMP (300 µM) effectively reversed SNP-induced apoptosis (52.6% and 21.4% reduction of 100 µM and 1 mM SNP-induced effect, respectively). Taken together, these data suggest that SNP induces a cGMP-independent apoptosis, while YC-1 prevents the SNP action through a cGMP-involved signaling pathway.


Figure 2
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  		Fig. 2 Effects of SNP, YC-1, and ODQ on cGMP synthesis. Cells were treated with the indicated agents for 10 min, and intracellular cGMP was detected as described in the Methods section. Data are expressed as mean±S.E.M. of six determinations. *P<0.05 and **P<0.001 vs. control; #P<0.05 vs. YC-1; +P<0.01 vs. SNP plus YC-1.

 
3.2 YC-1 action involves the activation of PI 3-kinase
As demonstrated in Fig. 3, wortmannin (a PI 3-kinase inhibitor) but not PD98059 (a MEK specific inhibitor) significantly reversed the YC-1 action indicating that the activation of PI 3-kinase might be involved in YC-1-mediated effects. In a further identification, the results showed that YC-1 alone and in combination with SNP induced a profound increase in phosphorylated Akt expression. These effects were significantly inhibited by ODQ and wortmannin (Fig. 4), suggesting that the YC-1-induced activation of PI 3-kinase is a downstream event of cGMP synthesis, and that the activation of PI 3-kinase might play a central role in this YC-1-induced antiapoptotic effect.


Figure 3
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  		Fig. 3 Effects of several agents on VSMC cell survival. Cells were pretreated with or without YC-1 (30 µM) in the absence or presence of the indicated agent for 30 min, and vehicle or SNP (1 mM) was added for 24 h. After the incubation period, the cell viability was assayed using the MTT assay method as described in the Methods section. Data are expressed as mean±S.E.M. of five determinations (each performed in triplicate).

 

Figure 4
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  		Fig. 4 Effects of several agents on the expression of phosphorylated Akt. After the treatment of the indicated agents for 15 min, cells were harvested and cell lysates (25 µg/lane) were prepared for the detection of Akt (internal control) and phosphorylated Akt expression. The proteins were separated and detected using Western blotting method.

 
3.3 YC-1 prevents SNP-induced Bcl-2 down-regulation and cytochrome c release reaction
Exposure of cells to SNP (1 mM) caused a profound down-regulation of Bcl-2 expression and increased cytochrome c release into the cytosol. The SNP-induced effects were completely prevented by YC-1. However, ODQ and wortmannin significantly reversed YC-1-mediated effects (Fig. 5). These data suggest that the prevention of SNP-induced Bcl-2 down-regulation and cytochrome c release reaction contributed to the cGMP- and PI 3-kinase-involved signaling pathways to YC-1 action. Furthermore, we also examined the involvement of Bad, a pro-apoptotic Bcl-2 family member, on SNP-mediated apoptotic pathway. However, SNP as well as YC-1 had little effect on Bad protein expression suggesting that Bad did not play a role on the apoptotic signaling in this study (data not shown).


Figure 5
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  		Fig. 5 Effect of several agents on the expressions of Bcl-2 and cytosolic cytochrome c. After the treatment of the indicated agents, cells were harvested and prepared for the detection of Bcl-2 and cytosolic cytochrome c expressions. The proteins were separated and detected using Western blotting method. {alpha}-Tubulin was used as the internal standard.

 
3.4 YC-1 inhibits SNP-induced caspase-3 activation
Apoptotic processes are stimulated by a variety of stimuli converge on the activation of the caspase family. Among these caspases, the activation of caspase-3 is the crucial event that leads to the apoptosis in a variety of cells. We measured the caspase-3 activity after the exposure of cells to SNP (1 mM), and found that although SNP significantly increased the caspase-3 activity in VSMC, YC-1 completely inhibited this response (Fig. 6). Moreover, this YC-1-mediated inhibitory effect was significantly reversed by ODQ and wortmannin (Fig. 6).


Figure 6
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  		Fig. 6 Effects of several agents on the caspase-3 activities and their correlation with cytotoxic effects. After the treatment of cells with indicated agents, cells were washed, trypsinized, and lysed for the determination of caspase-3 activity as described in the Methods section. Data are expressed as mean±S.E.M. of five determinations.

 
3.5 YC-1 inhibits SNP-induced ROS production
To determine whether SNP-induced cytotoxicity is mediated by ROS, two experiments have been carried out during the revised presses. At first, we examined the generation of ROS by means of the fluorescent probe DCFH-DA and flow cytometric analysis. This cell-permeable dye DCFH-DA, once inside the cells, is cleaved by endogenous esterase into DCFH. The intracellular nonfluorescent form of DCFH is oxidized, commonly by hydrogen peroxide, into the fluorescent form, DCF. The fluorescence intensity was measured after the exposure of cells to the indicated agents for 4 h. The data showed that SNP (1 mM) induced a significant increase of fluorescence intensity (3.16±0.05, n = 3, P<0.001 compared with vehicle control of 1±0). YC-1 alone (30 µM) had little effect on ROS production (0.94±0.24, n = 3) but completely abolished SNP-mediated effect. These data demonstrate that SNP-induced cytotoxic effect might involve the production of ROS and the ROS-reducing effect of YC-1 might explain its inhibition on SNP-induced cytotoxicity.


   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
The results of the current study strongly suggest that VSMC apoptosis, an event that has been well studied in atherosclerosis and neointimal formation postinjury [19], is environmentally dependent. The actions of NO on apoptosis are dependent on cell type, concentration, radical circumstances, and the redox state of cells [9,20]. Our data indicate that the NO donor SNP induces VSMC apoptosis through a cGMP-independent pathway, since ODQ did not reverse and dibutyl-cGMP did not mimic the SNP-evoked effects. These findings supported similar studies of VSMC [9]. In the current study, YC-1 prevented an SNP-induced apoptotic effect in a cGMP-dependent manner, in that YC-1 in combination with SNP synergistically increased cGMP synthesis, dibutyl-cGMP efficiently mimicked YC-1-mediated effect, and ODQ significantly reversed the YC-1 action. However, approximately 20% of ODQ-irresponsible action remained in the YC-1-mediated effect. It has been suggested that high concentrations of YC-1 could also inhibit phosphodiesterase activity [21], but further investigations are needed to determine if the ODQ-resistant response to YC-1 action results from its regulation of phosphodiesterase activity.

The mechanisms suggested for NO-induced cytotoxicity include inactivation of the mitochondrial respiratory chain, DNA damage, and Bcl-2 down-regulation/Bax up-regulation [22,23]. The balance between prosurvival and proapoptotic members of Bcl-2 family can regulate significant events in apoptosis, such as cytochrome c release into the cytosol and the activation of caspase-9 and downstream caspases (especially caspase-3). These events account for most of the apoptotic mechanisms in various cell types [24]. In this study, SNP induced a significant down-regulation of Bcl-2 proteins other than the influence on Bax expression (data not shown), and stimulated the release reaction of cytochrome c into the cytosol and the activation of caspase-3 activity. These data demonstrate the regulation of Bcl-2/cytochrome c/caspase-3 signaling pathways in the SNP-mediated apoptotic mechanism in VSMC. However, YC-1 almost completely blocked all of these apoptotic responses to SNP activity. Furthermore, ODQ profoundly reversed these YC-1-mediated effects, revealing that YC-1 has a cGMP-dependent antiapoptotic influence.

In recent years, considerable lines of evidence suggest that PI 3-kinase and p42/44 MAPK are involved in the survival regulation of several cell types [25]. It has been suggested that the increase in cyclic nucleotide synthesis and the following activation of PI 3-kinase play a central role on the prevention of apoptotic reaction [26]. Furthermore, it has been reported that in some cell types, such as cytokine-activated mesangial cells, cGMP may regulate the activation of p42/44 MAPK by NO [27]. In the current study, we found that wortmannin but not PD98059 reversed the YC-1-mediated effects. Further, the Akt phosphorylation was markedly induced in the presence of YC-1 and this action was diminished by ODQ. These results suggest that the PI 3-kinase is a downstream effector of sGC activation after YC-1 application and involves an antiapoptotic mechanism. In contrast, the p42/44 MAPK pathway is not relevant in YC-1-mediated survival in VSMC. However, there was some ODQ-resistant Akt phosphorylation being observed in this study, but whether the ODQ-resistant response is caused by the regulation of phosphodiesterase activity to YC-1 action remains unknown. YC-1 alone induced a modest (albeit statistically nonsignificant) increase in cell number (11%) in VSMC (Table 1 and Fig. 3). Both ODQ and PD98059 completely inhibited the YC-1-induced cell proliferation, implying the involvement of sGC and p42/44 MAPK activities. Furthermore, it is worth noting that SNP (1 mM) was capable of inducing a significant increase of ROS production in Rat VSMCs while YC-1 completely abolished SNP-mediated effect, indicating that SNP-induced cytotoxic effect might involve the production of ROS and the ROS-reducing effect of YC-1 might, at least partly, explain its inhibition on SNP-induced cytotoxicity.

We conclude that SNP induces VSMC apoptosis via a cGMP-independent signaling cascade, such as Bcl-2 down-regulation, cytochrome c release reaction, and caspase-3 activation. YC-1 produces its antiapoptotic effect through cGMP- and PI 3-kinase-involved inhibition of these events. In this study, we used sodium nitroprusside as a research tool to elucidate the pathological role of high concentrations of nitric oxide or related mediators and also investigate the protective potential of YC-1. In several severe inflammatory sites or circulation, large amounts of nitric oxide could be detected, such as sepsis [28], acute respiratory distress syndrome [29], and atherosclerosis [30]. The ability of YC-1 to block VSMC apoptosis may play a fundamental role in reducing the damage made by large amounts of nitric oxide.


   Acknowledgements
 
This work was supported by a research grant of the National Science Council of the Republic of China (NSC 91-2320-B-002-157).


   Notes
 
Time for primary review 20 days


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

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