Dual cell cycle-specific mechanisms mediate the antimitogenic effects of nitric oxide in vascular smooth muscle cells

Roles of vascular endothelial and smooth muscle cells in the vasculoprotective effect of insulin in a mouse model of restenosis

BACKGROUND

Insulin exerts vasculoprotective effects on endothelial cells (ECs) and growth-promoting effects on vascular smooth muscle cells (SMCs) in vitro, and suppresses neointimal growth in vivo. Here we determined the role of ECs and SMCs in the effect of insulin on neointimal growth.

METHODS

Mice with transgene CreERT2 under the control of EC-specific Tie2 (Tie2-Cre) or SMC-specific smooth muscle myosin heavy chain promoter/enhancer (SMMHC-Cre) or littermate controls were crossbred with mice carrying a loxP-flanked insulin receptor (IR) gene. After CreERT2-loxP-mediated recombination was induced by tamoxifen injection, mice received insulin pellet or sham (control) implantation, and underwent femoral artery wire injury. Femoral arteries were collected for morphological analysis 28 days after wire injury.

RESULTS

Tamoxifen-treated Tie2-Cre+ mice showed lower IR expression in ECs, but not in SMCs, than Tie2-Cre- mice. Insulin treatment reduced neointimal area after arterial injury in Tie2-Cre- mice, but had no effect in Tie2-Cre+ mice. Tamoxifen-treated SMMHC-Cre+ mice showed lower IR expression in SMCs, but not in ECs, than SMMHC-Cre- mice. Insulin treatment reduced neointimal area in SMMHC-Cre- mice, whereas unexpectedly, it failed to inhibit neointima formation in SMMHC-Cre+ mice.

CONCLUSION

Insulin action in both ECs and SMCs is required for the "anti-restenotic" effect of insulin in vivo.

Nitric oxide signaling inhibits microglia proliferation by activation of protein kinase-G

Microglia population is primarily determined by a finely-regulated proliferation process during early development of the central nervous system (CNS). Nitric oxide (NO) is known to inhibit proliferation in numerous cell types. However, how NO signaling regulates microglia proliferation remains elusive. Using wildtype (WT) and inducible nitric oxide synthase knockout (iNOS^-/-) mice, this study investigated the role and underlying mechanisms of iNOS/NO signaling in microglia proliferation. Here we reported that iNOS^-/- mice displayed significantly more BrdU-labeled proliferating microglia in the cortex than that in WT mice at postnatal day 10. Compared to microglia isolated from WT mouse cortex, significantly more iNOS^-/- microglia displayed the specific cell-cycle markers Ki67 and phospho-histone H3 (pH3) in their nuclei. In addition, treating WT microglia with the NOS inhibitor LNAME drastically increased the percentage of cells expressing Ki67 and pH3, whereas treating iNOS^-/- microglia with NOC18, a slow-release NO-donor, significantly decreased the percentage of microglia expressing the two cell-cycle markers. Moreover, inhibition of protein kinase-G (PKG) in WT microglia increased the proportion of microglia expressing Ki67 and pH3, whereas activation of PKG signaling using 8Br-cGMP in iNOS^-/- microglia significantly decreased the fraction of microglia displaying Ki67 and pH3. Interestingly, in the presence of a PKG inhibitor, NOC18 increased the quantity of iNOS^-/- microglia expressing Ki67 and pH3. Together, these results indicate that basal activity of iNOS/NO signaling impedes microglial cell-cycle progression and attenuates proliferation through activation of the cGMP-PKG pathway. However, NO increases microglia cell-cycle progression in the absence of cGMP-PKG signaling.

Inhibition of intimal thickening after vascular injury with a cocktail of vascular endothelial growth factor and cyclic Arg-Gly-Asp peptide

BACKGROUND

Percutaneous coronary intervention is widely used for the treatment of coronary artery disease; however, significant challenges such as restenosis remain. Key to solving these problems is to inhibit smooth muscle cell activation while enhancing re-endothelialization. Early growth response-1 (Egr-1) is a transcription factor that regulates vascular smooth muscle cell (SMC) proliferation and migration through its control of an array of downstream genes.

METHODS

A "cocktail" of vascular endothelial growth factor (VEGF)-A, VEGF-D and cyclic RGD was tested for its ability to inhibit neointima formation and accelerate re-endothelialization following balloon injury to carotid arteries of rats.

RESULTS

In vitro, the cocktail stimulated endothelial cell growth yet inhibited smooth muscle cell growth. In vivo, cocktail-treated injured arteries exhibited reduced intimal thickening by >50% (P<0.05). It increased both re-endothelialization and endothelial nitric oxide synthase (NOS) expression. Cocktail reduced Egr-1 expression, an effect blocked by the NOS inhibitor L-N(G)-nitroarginine methyl ester (L-NAME) that also prevented cocktail inhibition of neointima inhibition.

CONCLUSIONS

This combination may potentially be useful for the treatment of restenosis with concomitant stimulation of revascularization.

Recent developments in the effects of nitric oxide-donating statins on cardiovascular disease through regulation of tetrahydrobiopterin and nitric oxide

Since the discovery of the importance of nitric oxide (NO) to the human body three decades ago, numerous laboratory and clinical studies have been done to explore its potential therapeutic actions on many organs. In the cardiovascular system, NO works as a volatile signaling molecule regulating the vascular permeability and vascular tone, preventing thrombosis and inflammation, as well as inhibiting the smooth muscle hyperplasia. Thus, NO is important in the prevention and treatment of cardiovascular disease. NO is synthesized by NO synthase (NOS) with tetrahydrobiopterin (BH4) as the crucial cofactor. Many studies have been done to form nitric oxide donors so as to deliver NO directly to the vessel walls. In addition, NO moieties have been incorporated into existing therapeutic agents to enhance the NO bioavailability, including statins. Statins are inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme (HMG-CoA), the rate-limiting enzyme of the mevalonate pathway. By inhibiting this pathway, statins lower blood cholesterol and exert their pleiotropic effects through activity in reaction cascades, such as Rho/ROCK and Rac 1/NADPH oxidase pathways. Statins have also been observed to implement their non-lipid effects by promoting BH4 synthesis with increase of NO bioavailability. Furthermore, NO-donating statins in laboratory studies have demonstrated to produce better therapeutic effects than their parent's drugs. They offer better anti-inflammatory, anti-proliferative and antithrombotic actions on cardiovascular system. They also cause better revascularization in peripheral ischemia and produce greater enhancement in limb reperfusion and salvage. In addition, it has been shown that NO-donating statin caused less myotoxicity, the most common side effect related to treatment with statins. The initial studies have demonstrated the superior therapeutic effects of NO-donating statins while producing fewer side effects.

Effects of nitric oxide on cell proliferation: novel insights

Nitric oxide (NO) has been suggested to be a pathophysiological modulator of cell proliferation, cell cycle arrest, and apoptosis. In this context, NO can exert opposite effects under diverse conditions. Indeed, several studies have indicated that low relative concentrations of NO seem to favor cell proliferation and antiapoptotic responses and higher levels of NO favor pathways inducing cell cycle arrest, mitochondria respiration, senescence, or apoptosis. Here we report the effects of NO on both promotion and inhibition of cell proliferation, in particular in regard to cardiovascular disease, diabetes, and stem cells. Moreover, we focus on molecular mechanisms of action involved in the control of cell cycle progression, which include both cyclic guanosine monophosphate-dependent and -independent pathways. This growing field may lead to broad and novel targeted therapies against cardiovascular diseases, especially concomitant type 2 diabetes, as well as novel bioimaging NO-based diagnostic tools.

Cyclic strain upregulates VEGF and attenuates proliferation of vascular smooth muscle cells

Objective

Vascular smooth muscle cell (VSMC) hypertrophy and proliferation occur in response to strain-induced local and systemic inflammatory cytokines and growth factors which may contribute to hypertension, atherosclerosis, and restenosis. We hypothesize VSMC strain, modeling normotensive arterial pressure waveforms in vitro, results in attenuated proliferative and increased hypertrophic responses 48 hrs post-strain.

Methods

Using Flexcell Bioflex Systems we determined the morphological, hyperplastic and hypertrophic responses of non-strained and biomechanically strained cultured rat A7R5 VSMC. We measured secretion of nitric oxide, key cytokine/growth factors and intracellular mediators involved in VSMC proliferation via fluorescence spectroscopy and protein microarrays. We also investigated the potential roles of VEGF on VSMC strain-induced proliferation.

Results

Protein microarrays revealed significant increases in VEGF secretion in response to 18 hours mechanical strain, a result that ELISA data corroborated. Apoptosis-inducing nitric oxide (NO) levels also increased 43% 48 hrs post-strain. Non-strained cells incubated with exogenous VEGF did not reproduce the antimitogenic effect. However, anti-VEGF reversed the antimitogenic effect of mechanical strain. Antibody microarrays of strained VSMC lysates revealed MEK1, MEK2, phospo-MEK1^{T385, T291, T298}, phospho-Erk1/2^{T202+Y204/T185+T187}, and PKC isoforms expression were universally increased, suggesting a proliferative/inflammatory signaling state. Conversely, VSMC strain decreased expression levels of Cdk1, Cdk2, Cdk4, and Cdk6 by 25-50% suggesting a partially inhibited proliferative signaling cascade.

Conclusions

Subjecting VSMC to cyclic biomechanical strain in vitro promotes cell hypertrophy while attenuating cellular proliferation. We also report an upregulation of MEK and ERK activation suggestive of a proliferative phenotype. Hhowever, the proliferative response appears to be aborogated by enhanced antimitogenic cytokine VEGF, NO secretion and downregulation of Cdk expression. Although exogenous VEGF alone is not sufficient to promote the quiescent VSMC phenotype, we provide evidence suggesting that strain is a necessary component to induce VSMC response to the antimitogenic effects of VEGF. Taken together these data indicate that VEGF plays a critical role in mechanical strain-induced VSMC proliferation and vessel wall remodeling. Whether VEGF and/or NO inhibit signaling distal to Erk 1/2 is currently under investigation.

Intimal hyperplasia in balloon dilated coronary arteries is reduced by local delivery of the NO donor, SIN-1 via a cGMP-dependent pathway

Background

To elucidate the mechanism by which local delivery of 3-morpholino-sydnonimine (SIN-1) affects intimal hyperplasia after percutaneous transluminal coronary angioplasty (PTCA).

Methods

Porcine coronary arteries were treated with PTCA and immediately afterwards locally treated for 5 minutes, with a selective cytosolic guanylate cyclase inhibitor, 1 H-(1,2,4)oxadiazole(4,3-alpha)quinoxaline-1-one (ODQ) + SIN-1 or only SIN-1 using a drug delivery-balloon. Arteries were angiographically depicted, morphologically evaluated and analyzed after one and eight weeks for actin, myosin and intermediate filaments (IF) and nitric oxide synthase (NOS) contents.

Results

Luminal diameter after PCI in arteries treated with SIN-1 alone and corrected for age-growth was significantly larger as compared to ODQ + SIN-1 or to controls (p < 0.01). IF/actin ratio after one week in SIN-1 treated segments was not different compared to untreated segments, but was significantly reduced compared to ODQ + SIN-1 treated vessels (p < 0.05). Expression of endothelial NADPH diaphorase activity was significantly lower in untreated segments and in SIN-1 treated segments compared to controls and SIN-1 + ODQ treated arteries (p < 0.01). Restenosis index (p < 0.01) and intimal hyperplasia (p < 0.01) were significantly reduced while the residual lumen was increased (p < 0.01) in SIN-1 segments compared to controls and ODQ + SIN-1 treated vessels.

Conclusions

After PTCA local delivery of high concentrations of the NO donor SIN-1 for 5 minutes inhibited injury induced neointimal hyperplasia. This favorable effect was abolished by inhibition of guanylyl cyclase indicating mediation of a cyclic guanosine 3',5'-monophosphate (cGMP)-dependent pathway. The momentary events at the time of injury play crucial role in the ensuring development of intimal hyperplasia.

In vitro evaluation of the safe margin, antithrombotic and antiproliferative actions for the treatment of restenosis: Nitric oxide donor and polymers

Drug-eluting stents (DES) were developed to combat the problem of in-stent restenosis, and evaluating the biological activity from DES systems is critical for its safety and efficacy. To test the cytotoxicity of nitric oxide (NO) donor-containing polymers for their potential use in DES applications, S-nitrosoglutathione (GSNO) or in combination with poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) in an aqueous polymeric solution (PVA/PVP/GSNO) was investigated using Balb/c 3T3 and Rabbit arterial smooth muscle (RASM) cells. The sensitivity of 3T3 cells to the cytotoxicity effects induced by GSNO was higher than that of RASM cells, while RASM cells were more susceptible to alterations in membrane permeability. Cell growth assays showed that GSNO and PVA/PVP/GSNO induced antiproliferative effects in RASM cells. Moreover, the presence of polymers can reduce the cytotoxicity and enhance the antiproliferative effects of GSNO. Dose-dependent inhibition of platelet aggregation was similar for both PVA/PVP/GSNO (EC50 of 3.4 ± 2.3 µM) and GSNO (EC50 of 2.8 ± 1.1 µM) solutions. Platelet adhesion assays showed that the inhibition caused by GSNO (EC50 of 5.0 mM) was dependent on the presence of plasma. These results demonstrate that the methodology adopted here is suitable to establish safety margins and evaluate the antithrombotic potential and antiproliferative effects of NO-eluting biomaterials and polymeric solutions for the new cardiovascular devices, and also to emphasize the importance of using more specific cell lines in these evaluations.

Total Ginsenosides suppress the neointimal hyperplasia of rat carotid artery induced by balloon injury

Ginsenosides, the active components found in Panax ginseng, have been reported to inhibit the cardiac hypertrophy in rats. This study aims to observe the potential effect of total ginsenosides (TG) on the hypertrophic vascular diseases. The model of vascular neointimal hyperplasia was established by rubbing the endothelia of the common carotid artery with a balloon in male Sprague Dawley rats. TG (15 mg/kg/day, 45 mg/kg/day), L-arginine (L-arg) 200 mg/kg/day, and NG-nitro-L-arginine-methyl ester (L-NAME) 100 mg/kg/day used with the same dose of L-arg or TG 45 mg/kg/day were given for 7 and 14 consecutive days after surgery. TG and L-arg administrations significantly ameliorated the histopathology of injured carotid artery, which was abolished or blunted by L-NAME, an NOS inhibitor; TG and L-arg could also remarkably reduce the expression of proliferating cell nuclear antigen (PCNA), a proliferation marker of vascular smooth muscle cells(VSMCs), in neointima of the injured artery wall. Further study indicated that balloon injury caused a decreased superoxide dismutase (SOD) activity and an elevated malondialdehyde (MDA) content in plasma, and reduced the cGMP level in the artery wall, which were reversed by TG. It was concluded that TG suppress the rat carotid artery neointimal hyperplasia induced by balloon injury, which may be involved in its anti-oxidative action and enhancing the inhibition effects of NO/cGMP on VSMC proliferation.

Effects of endogenous nitric oxide and of DETA NONOate in arteriogenesis

Previous studies showed that targeted endothelial nitric oxide synthase (eNOS) disruption in mice with femoral artery occlusion does not impede and transgenic eNOS overexpression does not stimulate collateral artery growth after femoral artery occlusion, suggesting that nitric oxide from eNOS does not play a role in arteriogenesis. However, pharmacologic nitric oxide synthase inhibition with L-NAME markedly blocks arteriogenesis, suggestive of an important role of nitric oxide. To solve the paradox, we studied targeted deletion of eNOS and of inducible nitric oxide synthase (iNOS) in mice and found that only iNOS knockout could partially inhibit arteriogenesis. However, the combination of eNOS knockout and treatment with the iNOS inhibitor L-NIL completely abolished arteriogenesis. mRNA transcription studies (reverse transcriptase-polymerase chain reaction) performed on collateral arteries of rats showed that eNOS and especially iNOS (but not neural nitric oxide synthase) become upregulated in shear stress-stimulated collateral vessels, which supports the hypothesis that nitric oxide is necessary for arteriogenesis but that iNOS plays an important part. This was strengthened by the observation that the nitric oxide donor DETA NONOate strongly stimulated collateral artery growth, activated perivascular monocytes, and increased proliferation markers. Shear stress-induced nitric oxide may activate the innate immune system and activate iNOS. In conclusion, arteriogenesis is completely dependent on the presence of nitric oxide, a large part of it coming from mononuclear cells.

The extracellular signal-regulated kinase is involved in the effects of sildenafil on pulmonary vascular remodeling

Pulmonary hypertension is a group of diseases comprising vascular constriction and obstructive changes of the pulmonary vasculature. Phosphodiesterase type 5 inhibitors, for example, sildenafil, can alleviate vascular remodeling in the monocrotaline pulmonary hypertension model in rats. We investigate the mechanisms of sildenafil on the pulmonary vascular remodeling of pulmonary hypertension induced by monocrotaline (MCT) in rats. Thirty Sprague-Dawley rats (weighing 200-220 g) were administered with MCT abdominal cavity injection or equivalent volume of normal saline (NS) (which were treated as C group n = 10) to induce pulmonary hypertension model. Fourteen days later, 20 MCT treated rats were randomly fed with sildenafil (25mg/kg/day) or placebo as S, P group (10 rats for each group), respectively. Another 6 weeks later, mean pulmonary artery pressure (mPAP), index of right ventricular hypertrophy (RV/LV+S) of all animals were measured under general anesthesia. Pulmonary tissue was collected to investigate pathological features of pulmonary arteries and to measure protein expression of ERK(1)/ERK(2) and MKP1. After 6 weeks, there were significant elevated mPAP and RV/LV+S in both P and S groups. The ratio of wall thickness to vessel diameter in pulmonary arteries with diameters <200 microm were increased in both P and S groups. But the ratio of wall thickness to vessel diameter was smaller in S group than that in P group. The phosphorylation level of ERK(1)/ERK(2) were elevated in both P and S groups, but the level of phosphorlation ERK(1)/ERK(2) were lower in S group than that in P group. Intriguingly, the expression level of MKP1 was significantly increased in both S and P groups, while it was higher in S group than that in P group. The sildenafil can decrease mPAP and inhibit the progress of pulmonary vascular remodeling in pulmonary hypertension rats. The ERK-MAP kinase signaling pathway might play a role during this process.

Effects of isorhynchophylline on angiotensin II-induced proliferation in rat vascular smooth muscle cells

Proliferation of vascular smooth muscle cells (VSMCs) is a crucial event in cardiovascular diseases. Isorhynchophylline, an alkaloid from a traditional Chinese medicine Gambirplant, has been used to treat cardiovascular diseases. The aim of this study was to investigate the effects of isorhynchophylline on angiotensin II (Ang II)-induced proliferation of rat VSMCs. VSMCs were isolated from rat artery and cultured for 14 days before experimentation. The effect of isorhynchophylline on Ang II-induced proliferation was evaluated by cell number, MTT assay and flow cytometry, and nitric oxide (NO) content and activity of NO synthase (NOS) were measured. The expression of proto-oncogene c-fos, osteopontin (OPN) and proliferating cell nuclear antigen (PCNA) mRNAs was measured by real-time RT-PCR. VSMC cultures were verified by morphology and immunostaining with α-smooth muscle actin. Isorhynchophylline (0.1–10.0 μM) was not toxic to VSMCs, but markedly decreased Ang II (1.0 μm)-enhanced cell number and MTT intensity, and blocked cell transition from G0/G1 to S phase. Furthermore, isorhynchophylline increased the NO content and NOS activity, and suppressed Ang II-induced over-expression of c-fos, OPN and PCNA. Thus, isorhynchophylline was effective against Ang-II induced cell proliferation, an effect that appears to be due, at least in part, to increased NO production, regulation of the cell cycle, and depressed expression of c-fos, OPN and PCNA related to VMSC proliferation.

The roles of nitric oxide synthase and eIF2alpha kinases in regulation of cell cycle upon UVB-irradiation

In response to ultraviolet light (UV)-induced damage, cells initiate cellular recovery mechanisms including activation of repair genes and redistribution of cell cycle phases. While most studies have focused on DNA damage-inducible transcriptional regulation of cell cycle checkpoints, translational regulation also plays an important role in control of cell cycle progression upon UV-irradiation. UV-irradiation activates two kinases, PERK and GCN2, which phosphorylate the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) and subsequently inhibit protein synthesis. We recently identified an upstream regulator, nitric oxide synthase (NOS), which controls the activation of both PERK and GCN2 upon UVB-irradiation. Our data suggested that UVB induces NOS activation and NO(.) production, which reacts with superoxide (O(2)(*-)) to form peroxynitrite (ONOO(-)) and activate PERK. The NO(*) production also leads to L-Arg depletion and GCN2 activation. The elevation of nitric oxide and activation of PERK/GCN2 have been shown to play roles in regulation of cell cycle upon UVB irradiation. In the present study, we show that the cell cycle phases were redistributed by inhibition of NOS activation or reduction of oxidative stress upon UVB irradiation, indicating the roles of NO(*) and its oxidative products in regulation of cell cycle. We also demonstrate that both PERK and GCN2 were involved in regulation of cell cycle upon UVB-irradiation, but the regulation is independent of eIF2alpha phosphorylation. While the mechanism for UVB-induced cell cycle control is yet to be unraveled, we here discuss the differential roles of NOS, PERK and GCN2 in regulation of cell cycle upon UVB-irradiation.

Role of redox signaling and poly (adenosine diphosphate-ribose) polymerase activation in vascular smooth muscle cell growth inhibition by nitric oxide and peroxynitrite

PURPOSE

The vascular mediator, nitric oxide regulates vascular smooth muscle cell proliferation and can react with superoxide to form peroxynitrite, a highly reactive free radical. The intracellular mechanisms by which nitric oxide and peroxynitrite inhibit smooth muscle cell growth remain undefined, as is the potential role of peroxynitrite formation in the antiproliferative effects of nitric oxide. We sought to define the intracellular effects and signaling mechanisms of nitric oxide and peroxynitrite in smooth muscle cells.

METHODS

Cultured rat aortic smooth muscle cells were treated with exogenous nitric oxide or peroxynitrite and inhibitors of nitric oxide and redox signaling pathways. Cell growth, DNA synthesis, apoptosis, cyclic guanosine 3'-5' monophosphate (cGMP) levels, poly(adenosine diphosphate [ADP]-ribose) polymerase (PARP) activity, and cytotoxicity were assayed. Peroxynitrite formation was determined by nitrotyrosine immunoblotting. Vasoreactivity was assessed in isolated rat aortic rings after treatment with nitric oxide/peroxynitrite and redox agents.

RESULTS

Both exogenous nitric oxide and peroxynitrite decreased cell growth and DNA synthesis of cultured rat aortic smooth muscle cells, but peroxynitrite-induced growth arrest was irreversible and associated with apoptosis and cytotoxicity. Inhibition of guanylate cyclase, PARP activity, mitogen-activated protein kinase, or bypass of ornithine decarboxylase did not reverse growth arrest by nitric oxide. The antioxidants N-acetylcysteine, ascorbate, and glutathione selectively reversed growth inhibition by nitric oxide but not by peroxynitrite. Antioxidants did not impair nitric oxide-induced cGMP generation in smooth muscle cells or nitric oxide-induced vasodilatation of isolated aortic rings. Nitric oxide treatment did not result in peroxynitrite formation and augmentation of superoxide levels did not induce peroxynitrite-like effects. Peroxynitrite-induced cytotoxicity and apoptosis were not reversed by antioxidants or PARP inhibition, because peroxynitrite activated PARP in J774 macrophages but failed to activate PARP in smooth muscle cells.

CONCLUSIONS

Exogenous nitric oxide induces reversible cytostasis in smooth muscle cells by a redox-sensitive mechanism independent of peroxynitrite formation and distinct from the nitric oxide vasodilating mechanism. Peroxynitrite does not activate PARP selectively in smooth muscle cells and induces redox-independent smooth muscle cell cytotoxicity and apoptosis. Thus, the antiproliferative effects of nitric oxide and peroxynitrite on smooth muscle cells use divergent intracellular pathways with distinct redox sensitivities. These findings are relevant to the pathogenesis of vascular disease and the potential application of nitric oxide-based therapy for vascular disease.

CLINICAL RELEVANCE

Vascular smooth muscle cell proliferation is an important component of atherosclerosis, vein graft failure, and arterial restenosis, and is known to be regulated by the vascular signaling molecule nitric oxide. Nitric oxide can combine with the free radical superoxide to form the unstable metabolite peroxynitrite, which has been detected in human vascular lesions. This study examines the role of peroxynitrite in mediating the antiproliferative effects of nitric oxide. We identify important differences in the effects and intracellular mechanisms of nitric oxide and peroxynitrite in regulating vascular smooth muscle cell proliferation and programmed cell death. Defining the differential effects of these free radicals in vascular cells is important to our understanding of the pathogenesis of vascular disease and the development of novel therapy aimed at treating proliferative vascular lesions.

Arterial adaptations to chronic changes in haemodynamic function: coupling vasomotor tone to structural remodelling

Healthy mature arteries are usually extremely quiescent tissues with cell proliferation rates much below 1%/day and with extracellular matrix constituents exhibiting half-lives of years to decades. However, chronic physiological or pathological changes in haemodynamic function elicit arterial remodelling processes that may involve substantial tissue synthesis, degradation or turnover. Although these remodelling processes accommodate changing demands placed upon the cardiovascular system by physiological adaptations, they can compromise further perfusion in the context of arterial occlusive disease and they entrench hypertension and may exacerbate its progression. Recent findings indicate that some of the most important such remodelling responses involve the integrated effects of persistently altered vascular tone that feed into restructuring responses, with common signalling pathways frequently interacting in the control of both phases of the response. Current efforts to define these signals and their targets may provide new directions for therapeutic interventions to treat important vascular disorders.

Nitric oxide and cell proliferation

Nitric oxide (NO*) has been proposed to be a physiological modulator of cell proliferation, able to promote in most cases cell cycle arrest. In this review I explore the molecular basis of this mechanism of action. The modulatory action of NO* on the intracellular concentration of cGMP and the machinery directly involved in the control of cell cycle progression, including the expression and activity of diverse cyclins and cyclin-dependent kinases, their physiological inhibitors, and the master transcriptional regulator retinoblastoma protein, will be discussed. The role of NO* in proliferation mediated by tyrosine kinase receptors such as the epidermal growth factor receptor and downstream signalling pathways will also be considered. Finally, the involvement of NO* in proliferative processes relevant for normal development will be outlined.

Regulation of Neuronal Nitric-oxide Synthase Activity by Somatostatin Analogs following SST5 Somatostatin Receptor Activation

Somatostatin receptor SST5 is an inhibitory G protein-coupled receptor that exerts a strong cytostatic effect on various cell types. We reported previously that the SST5 anti-proliferative effect results in the inhibition of mitogen-induced increases in intracellular cGMP levels and MAPK activity. This study was conducted to define the early molecular events accountable for the SST5-mediated anti-proliferative effect. Here, we demonstrate that, in Chinese hamster ovary cells expressing SST5 (CHO/SST5 cells), somatostatin inhibited cell proliferation induced by nitric oxide donors and overexpression of the neuronal nitric-oxide synthase (nNOS) protein isoform. Accordingly, nNOS activity and dimerization were strongly inhibited following SST5 activation by the somatostatin analog RC-160. In CHO/SST5 cells, nNOS was dynamically recruited by the SST5 receptor and phosphorylated at tyrosyl residues following RC-160 treatment. RC-160 induced SST5-p60(src) kinase complex formation and subsequent p60(src) kinase activation. Coexpression of an inactive p60(src) kinase mutant with SST5 blocked RC-160-induced nNOS phosphorylation and inactivation and prevented the SST5-mediated anti-proliferative effect. In CHO/SST5 cells, p60(src) kinase associated with nNOS to induce its inactivation by phosphorylation at tyrosyl residues following RC-160 treatment. Using recombinant proteins, we demonstrated that such phosphorylation prevented nNOS homodimerization. Next, surface plasmon resonance and mutation analysis revealed that p60(src) directly associated with nNOS phosphorylated Tyr604. SST5-mediated inhibition of nNOS activity was demonstrated to be essential to the RC-160 anti-proliferative effect on pancreatic endocrine tumor-derived cells. We therefore identified nNOS as a new p60(src) kinase substrate essential for SST5-mediated anti-proliferative action.

Multiple potassium channels mediate nitric oxide-induced inhibition of rat vascular smooth muscle cell proliferation

Several nitric oxide (NO) effects in the cardiovascular system are mediated by soluble guanylate cyclase (sGC) activation but potassium channels (KC) are also emerging as important effectors of NO actions. We investigated the relationship among vascular smooth muscle cell proliferation, NO, cyclic GMP, and KC using the A7r5 smooth muscle cell line derived from rat aorta. NO donors (two nitrosothiols, S-nitroso-acetyl-d,l-penicillamine, SNAP, and S-nitroso-glutathione, GSNO, and an organic nitrate, glyceryl trinitrate, GTN; 1-1000 microM) dose-dependently inhibited cell proliferation. ODQ (a selective inhibitor of sGC; 0.1 and 1 microM) and KT5823 (a selective inhibitor of cGMP-dependent protein kinase, 1 microM) prevented NO effects, confirming that sGC is a key target. In this report, we show that tetraethylammonium (TEA, a non-selective blocker of KC, 300 microM), and 4-aminopyridine (a selective blocker of voltage-dependent KC, 100 microM) prevented SNAP inhibitory effects on cell proliferation, whereas glibenclamide (a selective blocker of ATP-dependent KC, 1 microM) was ineffective. Iberiotoxin (a selective blocker of high conductance calcium-activated KC, 100 nM), as well charybdotoxin (a blocker of high and intermediate conductance calcium-activated KC, 100 nM) and apamine (a selective blocker of small conductance calcium-activated KC, 100 nM), blocked the antiproliferative effect induced by SNAP. NS1619 (an opener of high conductance calcium-activated KC, 1-100 microM), inhibited cell proliferation. In addition, sub-effective concentrations of ODQ (100 nM) and TEA (10 microM) synergized in blocking SNAP antiproliferative effects. Thus, voltage-dependent and calcium-activated but not ATP-dependent KC appear to have a prominent role, besides sGC activation, in NO-induced inhibition of vascular smooth muscle cell proliferation.

Anti-Atherogenic Effect of Insulin in vivo

Metabolic syndrome is a risk factor for atherosclerosis and restenosis. In metabolic syndrome, insulin resistance coexists with hyperinsulinemia and hyperlipidemia. Hyperlipidemia has growth-promoting effects, whereas insulin has both growth-promoting and growth-inhibitory effects on vascular smooth muscle cells in vitro. The objective of this study was to investigate the effects of hyperlipidemia and hyperinsulinemia on vascular cell growth in vivo after arterial injury. Rats fed a low-fat diet were treated with either subcutaneous blank (LFC) or insulin (LFI) implants. Rats fed a high-fat diet also received blank (HFC) or insulin (HFI) implants. After 3 days, rats received balloon carotid injury, and 14 days later they were sacrificed to measure neointimal area and proliferation. Hyperinsulinemia was present in LFI and HFI and hyperlipidemia was present in HFC and HFI. Neointimal area was higher in HFC (0.153 +/- 0.009 mm(2), p < 0.05) but lower in LFI (0.098 +/- 0.005, p < 0.01) than LFC (0.127 +/- 0.005). In HFI (0.142 +/- 0.008, p < 0.05) neointimal area was not different from HFC or LFC. In conclusion, insulin reduced neointimal growth, but the effect of insulin was diminished by the high-fat diet. Thus, our results demonstrate an anti-atherogenic effect of insulin in vivo and suggest that in metabolic syndrome insulin resistance rather than hyperinsulinemia is the atherogenic risk factor.

Gene transfer using the mature form of VEGF-D reduces neointimal thickening through nitric oxide-dependent mechanism

Gene transfer to the vessel wall using vascular endothelial growth factors (VEGFs) has shown therapeutic potential for the treatment of restenosis. In this study, we evaluated the effect of catheter-mediated adenoviral (Ad) gene transfer of the mature form of VEGF-D (VEGF-D(DeltaNDeltaC)) in balloon-denuded cholesterol-fed rabbit aorta. AdLacZ was used as a control. Transduced VEGF-D(DeltaNDeltaC) mRNA was detectable in the arterial wall with RT-PCR at 6, 14 and 28 days. Gene transfer efficiency as detected with X-gal staining 6 days after the AdLacZ transduction was 1.91 +/- 1.32% in intima. AdVEGF-D(DeltaNDeltaC) gene transfer led to 52% reduction in intima/media ratio (I/M) as compared to the AdLacZ controls at 14 days time point. At 6 days there were no differences in I/M, but the number of macrophages in the vessel wall was 85% lower in the AdVEGF-D(DeltaNDeltaC) group as compared to the controls. The therapeutic effect was no longer detectable 28 days after the gene transfer. The therapeutic effect of VEGF-D(DeltaNDeltaC) was nitric oxide (NO)-dependent as the feeding of NO synthase inhibitor, L-NAME, blocked the reduction in intimal thickening. It is concluded that AdVEGF-D(DeltaNDeltaC) gene transfer reduces intimal thickening and macrophage influx into the vessel wall in balloon-denuded rabbit aortas.

Exogenous nitric oxide upregulates p21(waf1/cip1) in pulmonary microvascular smooth muscle cells

The histopathology of chronic pulmonary hypertension includes microvascular proliferation and neointimal formation. Nitric oxide (NO) has been implicated in the regulation of these mechanisms, but how NO controls microvascular proliferation and its effect on pulmonary microvascular cells is still unclear. In this study, we characterized the in vitro effects of NO on rat pulmonary microvascular smooth muscle cell (PMVSMC) proliferation and investigated the contribution of the p42/44 mitogen-activated protein kinase (MAPK) pathway and p21(waf1/cip1) induction to this response. NO donors inhibited PMVSMC proliferation in a dose-dependent manner. In the presence of hypoxia, the degree of inhibition was significantly enhanced. This inhibition was reversible and independent of apoptosis. The soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) had no impact on proliferation rates, suggesting a cyclic guanosine monophosphate-independent process. Administration of MEK1/2 inhibitors failed to abrogate the antimitotic effect of NO. There was a two- fold induction of the cyclin-dependent kinase inhibitor p21 in PMVSMC treated with NO donors. Under hypoxic conditions, NO caused a three-fold increase in p21 levels. These results demonstrate that NO inhibits PMVSMC proliferation and that this inhibition is not the result of p42/44 MAPK activation. The ability of NO to induce p21 upregulation may be a mechanism by which it exerts antiproliferative effects in PMVSMC.

Nitric oxide, an iceberg in cardiovascular physiology:

The endothelium is now recognized not only as a physical barrier between blood and vascular wall, but also as an important and strategically located organ with multiple endocrine and paracrine functions. By releasing vasoactive substances, the endothelium acts as an inhibitory regulator of vascular contraction, leukocyte adhesion, vascular smooth muscle cell growth, and platelet aggregation. This review intends to demonstrate how much the picture of the biological functions of nitric oxide has changed in cardiovascular physiology, extending beyond its vessel-relaxing activity, as well as to highlight new insights into the factors affecting its bioavailability and regulation in relation with many cardiovascular diseases.

Dual functionality of cyclooxygenase-2 as a regulator of tumor necrosis factor-mediated G1 shortening and nitric oxide-mediated inhibition of vascular smooth muscle cell proliferation

BACKGROUND

Cyclooxygenase (COX)-2 contributes to vascular smooth muscle cell (VSMC) proliferation induced by tumor necrosis factor (TNF) and angiotensin II. The present study demonstrates, however, that depending on prevailing conditions, COX-2-derived prostanoids may also inhibit VSMC proliferation.

METHODS AND RESULTS

TNF-alpha stimulated proliferation of VSMCs by shortening the G1 phase of the cell cycle. This effect was abolished by NS-398, a selective COX-2 inhibitor. Addition of TNF did not affect the protein-to-DNA ratio, measured by flow cytometry, suggesting that TNF does not induce VSMC hypertrophy. Inhibition of nitric oxide synthase (NOS) activity attenuated TNF-mediated increases in prostaglandin (PG) I2 synthesis, whereas thromboxane (TX) A2 production and COX-2 protein expression were unaffected. Moreover, inhibition of NOS activity increased TNF-mediated proliferation by approximately 23%. Thus, NO preferentially stimulates PGI2 production, suggesting that production of NO by VSMCs challenged with TNF limits the ability of the cytokine to increase proliferation. NO donors increased COX-2 protein expression and PGI2 synthesis, had no effect on TXA2 production, and decreased cell numbers by 50%, indicating that expression of COX-2 per se might not be sufficient to support proliferation. The effects of NO donors were prevented when COX-2 activity was inhibited with NS-398.

CONCLUSIONS

The COX-2-dependent proliferative response of VSMCs to TNF was modulated in an NO-dependent manner, and PGI2 derived from COX-2 might contribute to the antiproliferative effect of NO donors.

S-nitrosothiols inhibit uterine smooth muscle cell proliferation independent of metabolism to NO and cGMP formation

S-nitrosothiols (RSNOs) are important mediators of nitric oxide (NO) biology. The two mechanisms that appear to dominate in their biological effects are metabolism leading to the formation of NO and S-nitrosation of protein thiols. In this study we demonstrate that RSNOs inhibit uterine smooth muscle cell proliferation independent of NO. The antiproliferative effects of NO on vascular smooth muscle are well defined, with the classic NO-dependent production of cGMP being demonstrated as the active pathway. However, less is known on the role of NO in mediating uterine smooth muscle cell function, a process that is important during menstruation and pregnancy. The RSNOs S-nitrosoglutathione and S-nitroso-N-acetyl pencillamine inhibited growth factor-dependent proliferation of human and rat uterine smooth muscle cells (ELT-3). Interestingly, these cells reduced RSNOs to generate NO. However, use of NO donors and other activators of the cGMP pathway failed to inhibit proliferation. These findings demonstrate the tissue-specific nature of responses to NO and demonstrate the presence of a RSNO-dependent but NO-independent pathway of inhibiting DNA synthesis in uterine smooth muscle cells.

Cyclic stretch induces vascular smooth muscle cell alignment via NO signaling

We investigated the effects of cyclic stretch on vascular smooth muscle cell (VSMC) alignment and potential overlap of signaling modalities with stretch-induced proliferation. VSMC were subjected to graded stretch (1 Hz at 100-124% of resting length) for 48 h. Graded stretch resulted in graded VSMC alignment from a minimum of completely random orientation to a maximum of ~80-85 degrees to the stretch vector. Alignment was reversible within 48 h of stretch cessation and independent of signaling modalities mediating stretch-induced proliferation: modulation of IGF-1, MAPK, phosphatidylinositol 3-kinase, tyrosine kinase, and stretch-activated calcium channels did not affect alignment. Nitric oxide (NO) synthase (NOS) blockade uncoupled alignment. Neither the NO donor, cytokine-induced NOS activity, nor L-citrulline affected alignment, but inhibited VSMC proliferation. Therefore, stretch-induced proliferation and alignment are differentially regulated, with NO a common signaling molecule for both. Targeting NOS in states such as restenosis and hypertension may prove to be beneficial.

Nitric oxide induces apoptosis in NALM-6, a leukaemia cell line with low cyclin E protein levels

Intracellular nitric oxide levels may differ in resting and stimulated cells and contribute to the regulation of cell survival and proliferation through a variety of mechanisms and effects. We exposed two B-cell lines to a range of S-nitroso-N-acetyl-D,L-penicillamine (SNAP) concentrations in order to examine their susceptibility to exogenous nitric oxide and the participation of nitric oxide as modulator of cell proliferation. Although both FLEB and NALM-6 decreased their levels of thymidine incorporation, only NALM-6 cells were induced to undergo G1 arrest, phosphatidyl serine exposure and DNA fragmentation when cultured in the presence of 250 microm SNAP. This higher sensitivity of NALM-6 coincided with initially low cyclin E protein levels which were increased 7.8-fold after culture for 24 h with 250 microm SNAP. In contrast, there was no difference in cyclins A and D3, Bcl-2 and actin levels, neither at the beginning nor at the end of the 24 h culture. Our study reveals that FLEB and NALM-6 exhibit different response to the same concentration of nitric oxide, that nitric oxide can simultaneously induce cell cycle alterations and apoptosis, and further suggests an association between these two processes, with the involvement of cell cycle regulatory molecules.

Activation of mitogenic and antimitogenic pathways in cyclically stretched arterial smooth muscle

Biophysical forces regulate vascular smooth muscle cell (VSMC) physiology and evoke vascular remodeling. Two VSMC autocrine molecules, insulin-like growth factor I (IGF-I) and nitric oxide (NO), are implicated in remodeling attributable to VSMC hyperplasia. We investigated the role of in vitro cyclic stretch on rat VSMC IGF-I, NO, and cellular growth. Cyclic stretch (1 Hz at 120% resting length for 48 h) stimulated VSMC proliferation 2.5-fold vs. unstretched cells and was accompanied by a 1.8-fold increase in VSMC IGF-I secretion. Despite activation of this proliferative pathway, cyclic stretch induced inducible (i) nitric oxide synthase (NOS) expression and a twofold increase in NO secretion, a molecule with documented antiproliferative effects. Cytokine treatment enhanced iNOS expression and NO secretion while inhibiting vascular growth by approximately 50% in static cells. Cytokine treatment of stretched VSMC enhanced NO secretion 2.5-fold while inhibiting growth by approximately 80%. Exogenous IGF-I increased NOS activity 1.5-fold and NO secretion 8.5-fold in static cells. In turn, iNOS inhibition increased IGF-I secretion 1.6-fold and enhanced VSMC growth 1.6-fold in stretched cells. An NO donor (sodium nitroprusside) similarly inhibited VSMC proliferation in static (24%) and stretched (50%) VSMC while also inhibiting IGF-I secretion from stretched cells by approximately 35%. Thus cyclic stretch stimulates mitogenic (IGF-I) and antimitogenic (NO) pathways in VSMC. These two molecules regulate each other's secretory rates, providing tight regulation of VSMC proliferation. These data may have profound implications in understanding vascular growth alterations in vascular injury and hypertension.

Nitric oxide improves cisplatin cytotoxicity in head and neck squamous cell carcinoma

OBJECTIVE

To test whether nitric oxide (NO) enhances the cytotoxicity of cisplatin in a head and neck squamous cell carcinoma (HNSCC) cell line.

BACKGROUND

Cisplatin is one of the most frequently used chemotherapeutic agents in the treatment of HNSCC. NO has been shown to play an important role in regulating tumor growth. Previous studies demonstrate that NO can enhance the cytotoxicity of cisplatin in Chinese hamster lung fibroblasts. In this report, we examined the in vitro interaction of NO and cisplatin in a HNSCC cell line.

MATERIALS AND METHODS

CCL23 cells were pretreated with three different NO donors: PAPA/NO (t 1/2 = 15 min), DPTA/NO (t 1/2 = 3 h), and DETA/NO (t 1/2 = 20 h). The cells were rinsed and exposed for 6 hours to a culture medium containing cisplatin. Cell survival and LD50 of cisplatin were calculated with and without NO pretreatment.

RESULTS

PAPA/NO and DPTA/NO did not show any cytotoxic activity and did not change the LD50 of cisplatin. DETA/NO when used alone resulted in 25.6% cell death at its peak dose (100 microM). Pretreatment with DETA/NO resulted in almost a threefold reduction of the LD50 of cisplatin (6.8 vs. 2.4 microg/mL). Pretreatment with DETA/NO sensitized the HNSCC cells to subsequent cisplatin activity (two-sided P =.00016).

CONCLUSION

Pretreatment of HNSCC cells with long-acting NO donors enhances cisplatin activity. Short- and medium-acting NO donors do not exert a toxic effect and do not augment the activity of cisplatin. NO agonists should be considered in the future as a possible adjunct to cisplatin in the treatment of HNSCC. Further studies with animal models are necessary to further clarify this relationship.

Mechanisms mediating the antiproliferative effects of nitric oxide in cultured human airway smooth muscle cells

We have characterised the mechanisms involved in the antiproliferative effect of NO in human airway smooth muscle cells (HASMC). S-Nitroso-N-acetyl penicillamine, a nitric oxide donor, inhibited proliferation in both G(1) and S phases of the cell cycle. Additionally, experiments with 8-bromo-cGMP, haemoglobin, a NO scavenger and zaprinast, a cGMP-specific phosphodiesterase inhibitor, showed that both effects were NO-mediated. The G(1) phase inhibition was cGMP-dependent whereas the S phase inhibition was due to a cGMP-independent inhibition of ribonucleotide reductase. These results demonstrate that NO inhibits HASMC proliferation by cGMP-dependent and -independent mechanisms acting at distinct points in the cell cycle.

Neuronal nitric oxide synthase: a substrate for SHP-1 involved in sst2 somatostatin receptor growth inhibitory signaling

Somatostatin receptor sst2 is an inhibitory G protein-coupled receptor, which inhibits normal and tumor cell growth by a mechanism involving the tyrosine phosphatase SHP-1. We reported previously that SHP-1 associates transiently with and is activated by sst2 and is a critical component for sst2 growth inhibitory signaling. Here, we demonstrate that in Chinese hamster ovary cells expressing sst2, SHP-1 is associated at the basal level with the neuronal nitric oxide synthase (nNOS). Following sst2 activation by the somatostatin analog RC-160, SHP-1 rapidly recruits nNOS tyrosine dephosphorylates and activates it. The resulting NO activates guanylate cyclase and inhibits cell proliferation. Coexpression of a catalytically inactive SHP-1 mutant with sst2 blocks RC-160-induced nNOS dephosphorylation and activation, as well as guanylate cyclase activation. In mouse pancreatic acini, RC-160 treatment reduces nNOS tyrosine phosphorylation accompanied by an increase of its activity. By opposition, in acini from viable motheaten (mev/mev) mice, which express a markedly inactive SHP-1, RC-160 has no effect on nNOS activity. Finally, expression of a dominant-negative form of nNOS prevents both RC-160-induced p27 up-regulation and cell proliferation inhibition. We therefore identified nNOS as a novel SHP-1 substrate critical for sst2-induced cell-growth arrest.

Sildenafil inhibits hypoxia-induced pulmonary hypertension

BACKGROUND

This study investigated the effect of the phosphodiesterase 5 inhibitor sildenafil on the pulmonary vascular response to hypoxia in humans and mice.

METHODS AND RESULTS

In a randomized, double-blind study, sildenafil 100 mg or placebo was given orally to 10 healthy volunteers 1 hour before breathing 11% O(2) for 30 minutes. Pulmonary artery pressure (PAP) was measured with an indwelling right heart catheter. The acute 56% increase in mean PAP produced by hypoxia during placebo treatment (mean PAP [mean+/-SD mm Hg]: normoxia 16.0+/-2.1 versus hypoxia 25.0+/-4.8) was almost abolished by sildenafil (normoxia 16.0+/-2.1 versus hypoxia 18.0+/-3.6), with no significant effect on systemic blood pressure. In the isolated perfused lung of wild-type and endothelial nitric oxide synthase (eNOS)-deficient mice, sildenafil markedly blunted acute hypoxic pulmonary vasoconstriction. Wild-type mice dosed orally with the drug (25 mg. kg(-1). d(-1)) throughout 3 weeks of exposure to hypoxia (10% O(2)) exhibited a significant reduction in right ventricular systolic pressure (placebo versus sildenafil: 43.3+/-9.9 versus 29.9+/-9.7 mm Hg, P<0.05) coupled with a small reduction in right ventricular hypertrophy and inhibition of pulmonary vascular remodeling. In eNOS mutant mice, sildenafil attenuated the increase in right ventricular systolic pressure but without a significant effect on right ventricular hypertrophy or vascular remodeling.

CONCLUSIONS

Sildenafil attenuates hypoxia-induced pulmonary hypertension in humans and mice and offers a novel approach to the treatment of this condition. The eNOS-NO-cGMP pathway contributes to the response to sildenafil, but other biochemical sources of cGMP also play a role. Sildenafil has beneficial pulmonary hemodynamic effects even when eNOS activity is impaired.

Mechanisms of nitric oxide-induced cytotoxicity in normal human hepatocytes

Chronic exposure of hepatocytes to reactive nitrogen species (RNS) following liver injury and inflammation leads not only to functional and morphological alterations in the liver but also to degenerative liver diseases and hepatocellular carcinoma. Previously, we showed that S-nitroso-N-acetylpenicillamine-amine (SNAP), which generates nitric oxide, and 3-morpholinosydnonimine (Sin-1), which generates equal molar concentrations of superoxide and nitric oxide resulting in peroxynitrite production, exhibited different levels of cytotoxicity to normal human hepatocytes in culture. The aim of the present study was to elucidate some of the molecular and cellular pathways leading to hepatocyte cell death induced by RNS. Following treatment of the hepatocytes with SNAP or Sin-1, gene-specific DNA damage was measured in mtDNA and a hprt gene fragment using a quantitative Southern blot analysis. Both agents induced dose-dependent increases in DNA damage that was alkaline labile, but not sensitive to both formamidopyrimidine-DNA glycosylase (fpg) and endonuclease III, which recognize 8-oxoguanine, thymine glycol, and other oxidized pyrimidines. DNA damage was two- to fivefold greater in mtDNA than in the hprt gene fragment. There was a persistent and marked increase in DNA damage posttreatment that appeared to arise from the disruption of electron transport in the mitochondria, generating reactive species that saturated the repair system. DNA damage induced by Sin-1 and SNAP led to cell-cycle arrest in the S-phase, growth inhibition, and apoptosis. The data support the hypothesis that the functional and morphological changes observed in liver following chronic exposure to RNS are, in part, the result of persistent mitochondrial and nuclear DNA damage.

eNOS-deficient mice show reduced pulmonary vascular proliferation and remodeling to chronic hypoxia

Pulmonary hypertension is characterized by structural and morphological changes to the lung vasculature. To determine the potential role of nitric oxide in the vascular remodeling induced by hypoxia, we exposed wild-type [WT(+/+)] and endothelial nitric oxide synthase (eNOS)-deficient [(-/-)] mice to normoxia or hypoxia (10% O(2)) for 2, 4, and 6 days or for 3 wk. Smooth muscle alpha-actin and von Willebrand factor immunohistochemistry revealed significantly less muscularization of small vessels in hypoxic eNOS(-/-) mouse lungs than in WT(+/+) mouse lungs at early time points, a finding that correlated with decreases in proliferating vascular cells (5-bromo-2'-deoxyuridine positive) at 4 and 6 days of hypoxia in the eNOS(-/-) mice. After 3 wk of hypoxia, both mouse types exhibited similar percentages of muscularized small vessels; however, only the WT(+/+) mice exhibited an increase in the percentage of fully muscularized vessels and increased vessel wall thickness. eNOS protein expression was increased in hypoxic WT(+/+) mouse lung homogenates at all time points examined, with significantly increased percentages of small vessels expressing eNOS protein after 3 wk. These results indicate that eNOS deficiency causes decreased muscularization of small pulmonary vessels in hypoxia, likely attributable to the decrease in vascular cell proliferation observed in these mice.

Suppression of proliferation of human coronary artery smooth muscle cells by the nitric oxide donor, S-nitrosoglutathione, is cGMP-independent

Nitric oxide (NO), delivered by a single addition of S-nitrosoglutathione (GSNO, IC(50) = 60-75 microM), causes the prolonged, multi-day suppression of proliferation of asynchronous, logarithmically growing human (hCASMC, two cell strains), and porcine (porCASMC) coronary artery smooth muscle cells. The inhibition is not cytotoxic, but cytostatic and reversible. Transient exposure (>4-12 h) to GSNO is sufficient to elicit prolonged suppression, but a less than 4 h exposure produces little or no inhibition. Unlike porCASMC and rat and rabbit aortic SMC, hCASMC synthesize little cGMP in response to GSNO stimulation, suggesting loss of NO responsive guanylate cyclase in vitro. The guanylate cyclase inhibitor, ODQ, blocks the slight cGMP synthesis induced by GSNO in hCASMC, but does not prevent GSNO suppression of proliferation. These data support a cGMP independent mechanism for NO induced suppression of hCASMC proliferation which may be significant in the treatment of proliferative coronary artery diseases.

Mechanisms of vascular atrophy and fibrous cap disruption

The process of plaque destabilization and rupture remains an area of intense investigation. While reductions in lumen cross-sectional area induced by early, non-occlusive lesions are compensated by remodeling and expansion of the artery, further plaque enlargement leads to an uncompensated reduction in lumen area and an increase in surface shearing forces. We hypothesize that these local increases in wall shear stress lead to a reduction in smooth muscle cell proliferation and increase in cell death. Using a primate prosthetic graft model, we have observed that alterations in nitric oxide and platelet-derived growth factor metabolism are important regulators of intimal growth and regression. We suggest that these factors may also be influential in the process of fibrous cap atrophy and plaque rupture.

A redox-based mechanism for nitric oxide-induced inhibition of DNA synthesis in human vascular smooth muscle cells

1. The current study explored potential redox mechanisms of nitric oxide (NO)-induced inhibition of DNA synthesis in cultured human and rat aortic smooth muscle cells. 2. Exposure to S-nitrosothiols, DETA-NONOate and NO itself inhibited ongoing DNA synthesis and S phase progression in a concentration-dependent manner, as measured by thymidine incorporation and flow cytometry. Inhibition by NO donors occurred by release of NO, as detected by chemiluminescence and judged by the effects of NO scavengers, haemoglobin and cPTIO. 3. Co-incubation with redox compounds, N-acetyl-L-cysteine, glutathione and L-ascorbic acid prevented NO inhibition of DNA synthesis. These observations suggest that redox agents may alternatively attenuate NO bioactivity extracellularly, interfere with intracellular actions of NO on the DNA synthesis machinery or restore DNA synthesis after established inhibition by NO. 4. Recovery of DNA synthesis after inhibition by NO was similar with and without redox agents suggesting that augmented restoration of DNA synthesis is an unlikely mechanism to explain redox regulation. 5. Study of extracellula interactions revealed that all redox agents potentiated S-nitrosothiol decomposition and NO release. 6. Examination of intracellular NO bioactivity showed that as opposed to attenuation of NO inhibition of DNA synthesis by redox agents, there was no inhibition (potentiation in the presence of ascorbic acid) of soluble guanylate cyclase (sGC) activation judged by cyclic GMP accumulation in rat cells. 7. These data provide evidence that NO-induced inhibition of ongoing DNA synthesis is sensitive to redox environment. Redox processes might protect the DNA synthesis machinery from inhibition by NO, in the setting of augmented liberation of biologically active NO from NO donors.

Transplant atherosclerosis: role of phenotypic modulation of vascular smooth muscle by nitric oxide

Occlusive accelerated atherosclerosis of coronary grafts is the predominant factor that limits longevity of heart transplant recipients. This form of vascular disease affects both the large epicardial and the smaller intramyocardial vessels, leading to characteristic clinical presentation that necessitates the use of sophisticated techniques for their accurate detection. Accelerated atherosclerosis after transplantation is a multifactorial disease with many events contributing to its progression. The initial vascular injury associated with ischemia-reperfusion appears to aggravate preexisting conditions in the donor vasculature in addition to activation of new immunological and nonimmunological mechanisms. Throughout these events, the endothelium remains a primary target of cell- and humoral-mediated injury. Changes in the vascular intima leads to alterations in vascular smooth muscle cell (VSMC) physiology, resulting in VSMC phenotypic modulation with the orchestration of a broad spectrum of growth and inflammatory reactions, which might be a healing response to vascular injury. Endogenous nitric oxide (NO) pathways regulate a multiplicity of cellular mechanisms that play a major role in determining the structure and function of the vessel wall during normal conditions and during remodeling associated with accelerated atherosclerosis. Recently identified signaling pathways, including mitogen-activated protein kinase, cGMP-dependent protein kinase, phosphatidylinositol 3-kinase, and transcriptional events in which nuclear factor kappa B and activator protein 1 take part, can be associated with NO modulation of cell cycle perturbations and phenotypic alteration of VSMC during accelerated atherosclerosis. This article reviews recent progress covering the aforementioned matters. We start by summarizing the clincal aspects and pathogenesis of accelerated atherosclerosis associated with transplantation, including clinical presentation and detection. This summary is followed by a discussion of the multiple factors of the disease process, including immunological and nonimmunolgical contributions. The next section focuses on cellular responses of the VSMCs relevant to lesion formation, with special emphasis on classical and recent paradigms of phenotypic modulation of these cells. To examine the influence of NO on VSMC phenotypic modulation and consequent lesion development, we briefly overview characteristics of NO production in the normal coronary vascular bed and the changes in endogenous NO release and activity during atherosclerosis. This overview is followed by a section covering molecular mechanisms whereby NO regulates a range of signaling pathways, transcriptional events underlying cell cycle perturbation, and phenotypic alteration of VSMC in accelerated atherosclerosis.

Nitric oxide-induced increase in p21(Sdi1/Cip1/Waf1) expression during the cell cycle in aortic adventitial fibroblasts

This study was performed to investigate whether the expression of p21(Sdi1/Cip1/Waf1), one of the cyclin-dependent kinase inhibitor proteins, could be regulated by nitric oxide (NO) and might account for the antiproliferative effect of NO. Quiescent adventitial fibroblasts were stimulated to proliferate by serum addition and by NO donors added during different phases of the cell cycle. [(3)H]Thymidine incorporation was markedly reduced by S-nitroso-N-acetyl-penicillamine (SNAP) added either with serum at quiescence or at later time point in the cell cycle. Northern and Western blot analyses showed that addition of SNAP either at quiescence or 15 hours after serum addition induced a rapid induction of p21 mRNA and protein. Immunoprecipitation studies and electrophoretic mobility shift analysis indicate that the treatment of cells with SNAP induced the phosphorylation of p53 (a tumor suppressor protein) and enhanced the ability of p53 to bind DNA when SNAP was added during the cell cycle. The increased expression of p21 mRNA or p53 activation during late G(1) or S phase was also caused by addition of 8-bromo-cGMP and effectively blocked by a specific inhibitor of the soluble guanylate cyclase. Furthermore, this response to SNAP was blocked by an inhibitor of protein kinase G. These studies implicate NO as a potential regulator of the cell cycle in aortic adventitial fibroblasts through a cGMP-mediated transcriptional mechanism involving the induction of p21.

Differential regulation of DNA synthesis by nitric oxide and hydroxyurea in vascular smooth muscle cells

We investigated the influence of nitrovasodilators on DNA synthesis in cultured human aortic smooth muscle cells and explored the hypothesis that nitric oxide (NO) is directly involved in mediating the inhibitory effects of hydroxyurea on DNA synthesis. Both NO and hydroxyurea inhibited ongoing DNA synthesis and S phase progression in our cells. Exogenous deoxynucleosides partially reversed this inhibition, suggesting that ribonucleotide reductase is a primary target for both NO and hydroxyurea. Nitrovasodilators inhibited DNA synthesis by releasing NO, as detected by chemiluminescence and as shown by the reversal of DNA synthesis inhibition by NO scavengers. This inhibition appears to occur via a cGMP-independent mechanism. In contrast, hydroxyurea did not produce a detectable NO signal, and NO scavengers had no influence on its inhibition of DNA synthesis, suggesting that NO does not mediate the inhibitory action of hydroxyurea in our system. Furthermore, the action of nitrovasodilators and hydroxyurea on DNA synthesis differed according to redox sensitivity. The redox agents N-acetyl-L-cysteine and ascorbate reversed NO inhibition of DNA synthesis and had no effect on DNA synthesis inhibition caused by hydroxyurea.

E-4010, a selective phosphodiesterase 5 inhibitor, attenuates hypoxic pulmonary hypertension in rats

The purpose of this study was to determine whether E-4010, a newly synthesized potent and selective orally active phosphodiesterase (PDE) 5 inhibitor, would prevent the development of chronic hypoxia-induced pulmonary hypertension in rats. In conscious, pulmonary hypertensive rats, a single oral administration of E-4010 (1.0 mg/kg) caused an acute, long-lasting reduction in mean pulmonary arterial pressure (PAP), with no significant effects on systemic arterial pressure, cardiac output, and heart rate. In rats that received food containing 0.01 or 0.1% E-4010 during the 3-wk exposure to hypoxia, mean PAP was significantly decreased (mean PAP 24.0 +/- 0.9, 16.2 +/- 0.8, and 12.8 +/- 0.5 mmHg in rats treated with 0, 0.01, and 0.1% E-4010-containing food, respectively), whereas mean systemic arterial pressure was unchanged and cardiac output was slightly increased compared with chronically hypoxic control rats. Right ventricular hypertrophy, medial wall thickness in pulmonary arteries corresponding to the respiratory and terminal bronchioles, and the degree of muscularization of more distal arteries were less severe in E-4010-treated rats. Long-term treatment with E-4010 caused an increase in cGMP levels in lung tissue and plasma but not in aortic tissue and no significant change in cAMP levels in either lung, aorta, or plasma. These results suggest that long-term oral treatment with E-4010 reduced the increase in PAP, right ventricular hypertrophy, and pulmonary arterial remodeling induced by exposure to chronic hypoxia, probably through increasing cGMP levels in the pulmonary vascular smooth muscle.

Effects of treatment with FK409, a nitric oxide donor, on collar-induced intimal thickening and vascular reactivity

Intimal thickening in arteries is considered a site of predilection for atherosclerosis. In a rabbit model of early atherosclerosis, a silastic collar was placed around the carotid artery, which resulted in the formation of intimal thickening. We investigated whether the oral application of FK409 ((+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide , 10 mg kg(-1) day(-1), p.o.), a nitric oxide donor, inhibited the collar-induced intimal thickening as well as accompanying reactivity changes in rabbit carotid artery. The intimal thickening was significantly inhibited by FK409. The collar treatment increased the pD2 value of 5-hydroxytryptamine (5-HT) whereas it decreased those of phenylephrine and acetylcholine and did not significantly alter that of nitroglycerine. Maximal contractile force development in response to potassium chloride (KCl), 5-HT and phenylephrine was decreased in collared arteries. The collar did not alter the maximal relaxant effects of acetylcholine and nitroglycerine. Despite the significant reduction of intimal thickening, FK409 treatment did not affect these collar-induced modifications in vascular reactivity.

Effects of somatostatin, somatostatin analogs, and endothelial cell somatostatin gene transfer on smooth muscle cell proliferation in vitro

OBJECTIVE

Somatostatin analogs inhibit neointimal hyperplasia and smooth muscle cell (SMC) proliferation in vivo. The gene transfer of somatostatin to endothelial cells (ECs) represents a potential means of local delivery of somatostatin to areas of arterial injury. This study tested the hypothesis that the retroviral gene transfer of somatostatin to ECs would inhibit SMC proliferation in vitro and evaluated the effects of somatostatin analogs on DNA synthesis and the growth of SMCs.

METHODS

Media transfer and coculture were used to determine the effects of somatostatin-producing ECs on SMC proliferation in vitro. The effects of a variety of somatostatin isoforms and analogs on the proliferation of SMCs, mitogenesis of serum-restimulated quiescent SMCs, and arterial explants were measured.

RESULTS

Despite the production of biologically relevant concentrations of somatostatin by ECs, no inhibition of SMC proliferation was noted. Somatostatin analogs inhibited DNA synthesis in arterial explants but did not inhibit either DNA synthesis or growth of cultured SMCs, which showed a likely effect of somatostatin on the initial transition in SMC phenotype.

CONCLUSION

Somatostatin exerts inhibitory effects on SMC proliferation only during the early transition to a proliferative phenotype. There are significant differences between this in vivo transition and the standard serum-restimulated model of cultured SMCs. These differences may account for the failure of somatostatin to inhibit SMC proliferation in the standard in vitro models.

Growth-inhibitory effect of cyclic GMP- and cyclic AMP-dependent vasodilators on rat vascular smooth muscle cells: effect on cell cycle and cyclin expression

1. The possibility that the antiproliferative effect of cyclic GMP- and cyclic AMP-dependent vasodilators involves an impaired progression of vascular smooth muscle cells (VSMC) through the cell cycle and expression of cyclins, which in association with the cyclin-dependent kinases control the transition between the distinct phases of the cell cycle, was examined. 2. FCS (10%) stimulated the transition of quiescent VSMC from the G0/G1 to the S phase (maximum within 18-24 h and then to the G2/M phase (maximum within 22-28 h). Sodium nitroprusside and 8-Br-cyclic GMP, as well as forskolin and 8-Br-cyclic AMP markedly reduced the percentage of cells in the S phase after FCS stimulation. 3. FCS stimulated the low basal protein expression of cyclin D1 (maximum within 8-24 h) and E (maximum within 8-38 h) and of cyclin A (maximum within 14-30 h). The stimulatory effect of FCS on cyclin D1 and A expression was inhibited, but that of cyclin E was only minimally affected by the vasodilators. 4. FCS increased the low basal level of cyclin D1 mRNA after a lag phase of 2 h and that of cyclin A after 12 h. The vasodilators significantly reduced the FCS-stimulated expression of cyclin D1 and A mRNA. 5. These findings indicate that cyclic GMP- and cyclic AMP-dependent vasodilators inhibit the proliferation of VSMC by preventing the progression of the cell cycle from the G0/G1 into the S phase, an effect which can be attributed to the impaired expression of cyclin D1 and A.

Does nitric oxide regulate smooth muscle cell proliferation? A critical appraisal

Smooth muscle proliferation is involved in the pathogenesis of atherosclerosis, restenosis after angioplasty and vein graft failure due to neointimal hyperplasia. Nitric oxide (NO) inhibits smooth muscle cell growth in vitro and experimental neointimal hyperplasia in vivo, suggesting a role for NO as a regulator of smooth muscle cell proliferation. NO is also involved in the control of numerous other vascular functions including platelet and inflammatory cell adhesion, vascular reactivity and endothelial permeability. This review critically examines the experimental and clinical evidence that supports a role for NO as a modulator of smooth muscle cell proliferation, with an emphasis on the multiple mechanisms by which NO acts on vascular lesions.