Effect of nitric oxide on DNA replication induced by angiotensin II in rat cardiac fibroblasts

Drugs of the future for Peyronie's disease

With the increasing awareness of Peyronie's disease (PD), the interest in new concept medications to treat the disorder is escalating. Profibrogenic factors such as transforming growth factor (TGF)-beta1, endothelin (ET-1), connective tissue growth factor (CTGF), angiotensin (Ang) II and platelet derived growth factor (PDGF), all appear to be involved in the pathogenesis of PD. β-Thymosins, pirfenidone, nitric oxide (NO) donors, phosphodiesterase (PDE)-5 inhibitors, matrix metalloproteinases (MMPs)/anti-tissue inhibitor of metalloproteinases (TIMP)-1 reduce collagen synthesis, while decorin, follistatin, and Smad 7 exert antifibrotic effects; all have been proposed for the treatment of PD. Alternative and/or novel approaches for the treatment of PD are needed in part because of the recognized multifactorial etiology of this complex disorder. A comprehensive approach for translating available experimental information into clinically effective drug trials for the treatment of PD is needed. We propose a multi-faceted approach for drug development to generate novel drug products for the treatment of PD.

Cyclic nucleotide phosphodiesterase 1A: a key regulator of cardiac fibroblast activation and extracellular matrix remodeling in the heart

Cardiac fibroblasts become activated and differentiate to smooth muscle-like myofibroblasts in response to hypertension and myocardial infarction (MI), resulting in extracellular matrix (ECM) remodeling, scar formation and impaired cardiac function. cAMP and cGMP-dependent signaling have been implicated in cardiac fibroblast activation and ECM synthesis. Dysregulation of cyclic nucleotide phosphodiesterase (PDE) activity/expression is also associated with various diseases and several PDE inhibitors are currently available or in development for treating these pathological conditions. The objective of this study is to define and characterize the specific PDE isoform that is altered during cardiac fibroblast activation and functionally important for regulating myofibroblast activation and ECM synthesis. We have found that Ca(2+)/calmodulin-stimulated PDE1A isoform is specifically induced in activated cardiac myofibroblasts stimulated by Ang II and TGF-β in vitro as well as in vivo within fibrotic regions of mouse, rat, and human diseased hearts. Inhibition of PDE1A function via PDE1-selective inhibitor or PDE1A shRNA significantly reduced Ang II or TGF-β-induced myofibroblast activation, ECM synthesis, and pro-fibrotic gene expression in rat cardiac fibroblasts. Moreover, the PDE1 inhibitor attenuated isoproterenol-induced interstitial fibrosis in mice. Mechanistic studies revealed that PDE1A modulates unique pools of cAMP and cGMP, predominantly in perinuclear and nuclear regions of cardiac fibroblasts. Further, both cAMP-Epac-Rap1 and cGMP-PKG signaling was involved in PDE1A-mediated regulation of collagen synthesis. These results suggest that induction of PDE1A plays a critical role in cardiac fibroblast activation and cardiac fibrosis, and targeting PDE1A may lead to regression of the adverse cardiac remodeling associated with various cardiac diseases.

Nitric oxide attenuates endothelin-1-induced activation of ERK1/2, PKB, and Pyk2 in vascular smooth muscle cells by a cGMP-dependent pathway

Nitric oxide (NO), in addition to its vasodilator action, has also been shown to antagonize the mitogenic and hypertrophic responses of growth factors and vasoactive peptides such as endothelin-1 (ET-1) in vascular smooth muscle cells (VSMCs). However, the mechanism by which NO exerts its antimitogenic and antihypertrophic effect remains unknown. Therefore, the aim of this study was to determine whether NO generation would modify ET-1-induced signaling pathways involved in cellular growth, proliferation, and hypertrophy in A-10 VSMCs. Treatment of A-10 VSMCs with S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP), two NO donors, attenuated the ET-1-enhanced phosphorylation of several key components of growth-promoting and hypertrophic signaling pathways such as ERK1/2, PKB, and Pyk2. On the other hand, inhibition of the endogenous NO generation with N(G)-nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor, increased the ET-1-induced phosphorylation of these signaling components. Since NO mediates its effect principally through a cGMP-soluble guanylyl cyclase (sGC) pathway, we investigated the role of these molecules in NO action. 8-Bromoguanosine 3',5'-cyclic monophosphate, a nonmetabolizable and cell-permeant analog of cGMP, exhibited a effect similar to that of SNAP and SNP. Furthermore, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of sGC, reversed the inhibitory effect of NO on ET-1-induced responses. SNAP treatment also decreased the protein synthesis induced by ET-1. Together, these data demonstrate that NO, in a cGMP-dependent manner, attenuated ET-1-induced phosphorylation of ERK1/2, PKB, and Pyk2 and also antagonized the hypertrophic effects of ET-1. It may be suggested that NO-induced generation of cGMP contributes to the inhibition of ET-1-induced mitogenic and hypertrophic responses in VSMCs.

Arginine vasopressin increases iNOS–NO system activity in cardiac fibroblasts through NF-κB activation and its relation with myocardial fibrosis

Previous studies have shown that arginine vasopressin (AVP) promotes myocardial fibrosis (MF), whereas nitric oxide (NO) inhibits MF. Cardiac fibroblasts (CFs) are the main target cells of MF. However, the modulatory effect of AVP on NO production in CFs and the role of this effect in MF are still unknown. In the present study, CFs obtained from Sprague-Dawley rats were stimulated with or without AVP and pyrrolidine dithiocarbamate (PDTC), a specific inhibitor of nuclear factor kappa-B (NF-kappaB). NO production and NOS activity were detected with absorption spectrometry, inducible nitric oxide synthase (iNOS) protein with Western blot analysis, iNOS mRNA with real-time PCR, CF collagen synthesis with [(3)H]proline incorporation, and NF-kappaB activation with immunofluorescence staining and Western blot analysis. The results showed that AVP increased NO production in a dose- and time-dependent manner, with maximal effects at 10(-7) mol/l after 24-h stimulation. AVP also increased NOS activity, protein and mRNA levels of iNOS in a coincident manner. Furthermore, AVP also increased CF collagen synthesis in a dose- and time-dependent manner. In addition, it was found that NF-kappaB was activated by AVP, and that PDTC could inhibit NO production, NOS activity, protein and mRNA levels of iNOS stimulated by AVP in a dose-dependent manner. The inhibitory effects of PDTC on NF-kappaB translocation were coincident with the effects of PDTC on iNOS-NO system activity. It is suggested that AVP increases NO production via the regulation of iNOS gene expression, and the upregulation of iNOS gene expression stimulated by AVP is mediated through NF-kappaB activation. NO production induced by AVP may counteract the profibrotic effects of AVP, thus the development of MF perhaps depends on the balance between profibrotic AVP and antifibrotic NO effects on MF.

Resveratrol inhibits proliferation of cultured rat cardiac fibroblasts: correlated with NO-cGMP signaling pathway

Rhizoma polygoni cuspidate, used as a traditional Chinese herb, offered the therapeutic potential for cardiovascular diseases. Resveratrol, extracted from root of the rhizoma polygoni cuspidate has sparked increasing interest in therapeutic application. Resveratrol was shown to exert a variety of pharmacological effects including cardioprotective and cancer chemopreventive properties. However, its mechanisms of the action are not completely understood. The aim of this study was to investigate the molecular mechanism of resveratrol on preventing cardiac fibroblasts from proliferative and hypertrophic response induced by angiotensin II. Cell proliferation and cytotoxicity were detected by methyl thiazolyl tetrazolium (MTT) and lactate dehydrogenase (LDH) release assay, respectively. Hypertrophic response of cardiac fibroblasts was measured by mRNA expression of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). Resveratrol (25, 50, 75, and 100 microM) inhibited cardiac fibroblasts proliferation in a dose- and time-dependent manner compared with angiotensin II group (P<0.01), and the inhibitory effects were blocked by pretreatment with N(G)-nitro-l-arginine methyl ester (L-NAME) and 1H-[1,2,4]-oxadiazole-[4,3-a]-quinoxalin-1-one (ODQ). Resveratrol increased nitric oxide (NO) and nitric oxide synthase (NOS) levels in culture medium, increased intracellular cyclic GMP (cGMP) level in cardiac fibroblasts, and decreased ANP and BNP levels in culture medium. The mRNA expression of ANP and BNP was suppressed by resveratrol. These results suggested that resveratrol inhibited cardiac fibroblasts proliferation induced by angiotensin II, and the inhibitory effect might be associated with the activation of NO-cGMP signaling pathway.

Inhibitory effect of 14-3-3 proteins on serum-induced proliferation of cardiac fibroblasts

Proliferation in cardiac fibroblasts (CFs) can be induced by a wide variety of growth factors that recruit multiple signal transduction pathways, including mitogen-activated protein kinase, phosphatidylinositol 3-kinase and protein kinase C. As a family of dimeric phophoserine-binding proteins, 14-3-3s are associated with a multitude of proteins that regulate signal transduction, apoptosis and checkpoint control pathways. However, it remains unknown whether the 14-3-3 proteins play an active role in cardiac proliferation and alter their expression patterns in response to growth factors in CFs. R18 peptide, an isoform-independent 14-3-3 inhibitor, was used to disrupt 14-3-3 function by adenovirus-mediated transfer of R18-EYFP (AdR18). Our results demonstrate that the 14-3-3 isoforms gamma, zeta and epsilon were highly expressed in CFs and the expression of 14-3-3 epsilon was elevated following serum stimulation. Inhibition of 14-3-3 proteins by AdR18 potentiated mitogen-induced DNA synthesis in CFs. This potentiation was presumably due to the increased inactivated glycogen synthase kinase-3 beta by Ser9 phosphorylation and nuclear factor of activated T-cell nuclear accumulation. However, AdR18 had no effect on extracellular signal-regulated kinase phosphorylation and reduced p70 S6 kinase (p70S6K) phosphorylation upon mitogenic stimulation. Furthermore, though R18 can block 14-3-3 binding abilities, it did not affect the serum-induced upregulation of 14-3-3 epsilon protein. Collectively, these findings reveal that the expression of 14-3-3 epsilon can be upregulated by serum in CFs and 14-3-3s may exert an inhibitory effect on serum-induced proliferation.

Hydrogen sulfide potentiates interleukin-1beta-induced nitric oxide production via enhancement of extracellular signal-regulated kinase activation in rat vascular smooth muscle cells

Hydrogen sulfide (H(2)S) and nitric oxide (NO) are endogenously synthesized from l-cysteine and l-arginine, respectively. They might constitute a cooperative network to regulate their effects. In this study, we investigated whether H(2)S could affect NO production in rat vascular smooth muscle cells (VSMCs) stimulated with interleukin-1beta (IL-1beta). Although H(2)S by itself showed no effect on NO production, it augmented IL-beta-induced NO production and this effect was associated with increased expression of inducible NO synthase (iNOS) and activation of nuclear factor (NF)-kappaB. IL-1Beta activated the extracellular signal-regulated kinase 1/2 (ERK1/2), and this activation was also enhanced by H(2)S. Inhibition of ERK1/2 activation by the selective inhibitor U0126 inhibited IL-1beta-induced NF-kappaB activation, iNOS expression, and NO production either in the absence or presence of H(2)S. Our findings suggest that H(2)S enhances NO production and iNOS expression by potentiating IL-1beta-induced NF-kappaB activation through a mechanism involving ERK1/2 signaling cascade in rat VSMCs.

Activation of the PI3-K/Akt pathway mediates cGMP enhanced-neurogenesis in the adult progenitor cells derived from the subventricular zone

The intracellular mechanisms that regulate neurogenesis remain unclear. Using neurospheres isolated from the subventricular zone (SVZ) of the adult rat, we investigated the effect of cyclic guanosine monophosphate (cGMP) and its signaling pathway on the induction of neurogenesis. Neurospheres expressed phosphodiesterase 5 (PDE5) and treatment of neurospheres with Sildenafil, a specific inhibitor of PDE5, significantly increased cGMP levels and neurogenesis. In addition, incubation of neurospheres with Sildenafil significantly phosphorylated Akt, which was associated with an increase of phosphorylation of glycogen synthase kinase 3 (GSK-3), a downstream target of Akt. Coincubation of neurospheres with Sildenafil and LY 294002, a pharmacological inhibitor of PI3-K/Akt, abolished Sildenafil-induced phosphorylated Akt and GSK-3. Furthermore, LY 294002 blocked Sildenafil-increased SVZ cell proliferation. These data suggest that Sildenafil-enhanced neurogenesis likely occurs through activation of the PI3-K/Akt/GSK-3 pathway.

Quantitative analysis of the cardiac fibroblast transcriptome-implications for NO/cGMP signaling

Cardiac fibroblasts regulate tissue repair and remodeling in the heart. To quantify transcript levels in these cells we performed a comprehensive gene expression study using serial analysis of gene expression (SAGE). Among 110,169 sequenced tags we could identify 30,507 unique transcripts. A comparison of SAGE data from cardiac fibroblasts with data derived from total mouse heart revealed a number of fibroblast-specific genes. Cardiac fibroblasts expressed a specific collection of collagens, matrix proteins and metalloproteinases, growth factors, and components of signaling pathways. The NO/cGMP signaling pathway was represented by the mRNAs for alpha(1) and beta(1) subunits of guanylyl cyclase, cGMP-dependent protein kinase type I (cGK I), and, interestingly, the G-kinase-anchoring protein GKAP42. The expression of cGK I was verified by RT-PCR and Western blot. To establish a functional role for cGK I in cardiac fibroblasts we studied its effect on cell proliferation. Selective activation of cGK I with a cGMP analog inhibited the proliferation of serum-stimulated cardiac fibroblasts, which express cGK I, but not higher passage fibroblasts, which contain no detectable cGK I. Currently, our data suggest that cGK I mediates the inhibitory effects of the NO/cGMP pathway on cardiac fibroblast growth. Furthermore the SAGE library of transcripts expressed in cardiac fibroblasts provides a basis for future investigations into the pathological regulatory mechanisms underlying cardiac fibrosis.

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Effect of nitric oxide on the differentiation of fibroblasts into myofibroblasts in the Peyronie's fibrotic plaque and in its rat model

The myofibroblast shares phenotypic features of both fibroblasts and smooth muscle cells. It plays a critical role in collagen deposition and wound healing and disappears by apoptosis when the wound is closed. Its abnormal persistence leads to hypertrophic scar formation and other fibrotic conditions. Myofibroblasts are present in the fibrotic plaque of the tunica albuginea (TA) of the penis in men with Peyronie's disease (PD), a localized fibrosis that is accompanied by a spontaneous induction of the inducible nitric oxide synthase (iNOS), also observed in the TGFbeta1-elicited, PD-like lesion in the rat model. iNOS expression counteracts fibrosis, by producing nitric oxide (NO) that reduces collagen deposition in part by neutralization of profibrotic reactive oxygen species. In this study we investigated whether fibroblast differentiation into myofibroblasts is enhanced in the human and rat PD-like plaque and in cultures of human tissue fibroblasts. We also examined whether NO reduces this cell differentiation and collagen synthesis. The myofibroblast content in the fibroblast population was measured by quantitative immunohistochemistry as the ratio between alpha-smooth muscle actin (ASMA; myofibroblast marker) and vimentin (general fibroblast marker) levels. We found that myofibroblast content was considerably increased in the human and TGFbeta1-induced rat plaques as compared to control TA. Inhibition of iNOS activity by chronic administration of L-iminoethyl-L-lysine to rats with TGFbeta1-induced TA lesion increased myofibroblast abundance and collagen I synthesis measured in plaque and TA homogenates from animals injected with a collagen I promoter construct driving the expression of beta-galactosidase. Fibroblast differentiation into myofibroblasts occurred with passage in the cell cultures from the human PD plaque, but was minimal in cultures from the TA. Induction of iNOS in PD and TA cultures with a cytokine cocktail and a NO donor, S-nitroso-N-acetyl penicillamine (SNAP), was detected by immunohistochemistry. Both treatments reduced the total number of cells and the number of ASMA positive cells, whereas only SNAP decreased collagen I immunostaining. These results support the hypotheses that myofibroblasts play a role in the development of the PD plaque and that the antifibrotic effects of NO may be mediated at least in part by the reduction of myofibroblast abundance and lead to a reduction in collagen I synthesis.

Antifibrotic role of inducible nitric oxide synthase

Long-term treatment in rats with l-NAME, an isoform-non-specific inhibitor of nitric oxide synthase (NOS), leads to fibrosis of the heart and kidney, suggesting that nitric oxide (NO) may play a role in preventing tissue fibrosis. In this process, a likely target of NO is the quenching of reactive oxygen species (ROS) through peroxynitrite formation, and one possible source for this NO is inducible NOS (iNOS). Using Peyronie's disease (PD) tissue from both human specimens and from a rat model of PD as the source of fibrotic tissue, we investigated if NO derived from iNOS could act as such an antifibrogenic defense mechanism by determining whether: (a) tunical ROS and iNOS are increased in PD; and (b) the long-term inhibition of iNOS activity decreases the NO/ROS balance in the tunica albuginea thereby promoting collagen deposition. It was determined that in the human PD plaque, iNOS mRNA and protein, ROS, collagen, and the peroxynitrite marker, nitrotyrosine, were all increased in comparison to the normal tunica. In the rat model of PD, the fibrotic plaque also showed significant increases in iNOS mRNA and protein, nitrotyrosine, ROS as measured by heme oxygenase-1, and collagen when compared with the normal control tunica. When a selective inhibitor of iNOS, L-NIL, was given to rats with the PD-like plaque, this resulted in a decrease in nitrotyrosine levels but intensified ROS levels and collagen deposition. These data demonstrate that: (a) iNOS induction occurs in both the human and rat PD fibrotic plaque; and (b) that the NO derived from iNOS appears to counteract ROS formation and collagen deposition. Because the inhibition of iNOS activity leads to a decrease in the NO/ROS ratio, thereby favoring the development of fibrosis, it is proposed that iNOS induction in this tissue may be a protective mechanism against fibrosis and abnormal wound healing.

Changes in NOS activity and protein expression during acute and prolonged ANG II administration

The aim of this study was to assess the effects of acute or prolonged increases of ANG II on nitric oxide synthase (NOS) activities and protein expression in mesenteric resistance vessels, left ventricle, renal cortex, and renal medulla. The response of NOS activities to ANG II is compared with that induced by phenylephrine. ANG II or phenylephrine were infused over either 3 h or 3 days to conscious rats. NOS activity was examined by measuring the rate of conversion of L-[14C]arginine to L-[14C]citrulline. Protein levels of endothelial (e) and neuronal (n) NOS were determined by Western blot analysis. Arterial pressure (AP) increased (P < 0.05) during acute and prolonged ANG II infusion. Ca2+-dependent NOS activity values (pmol of citrulline x min(-1) x g wet wt(-1)) for control rats were 21 +/- 9 in mesenteric arteries, 13 +/- 7 in left ventricle, 14 +/- 8 in renal cortex, and 411 +/- 70 in renal medulla. Acute ANG II infusion increased (P < 0.05) Ca2+-dependent NOS activity in renal cortex and renal medulla (81 +/- 18 and 611 +/- 48, respectively), but no differences were found in mesenteric arteries and left ventricle with respect to control rats. In contrast to the renal changes in NOS activity, acute ANG II infusion did not modify eNOS or nNOS expression in any of the tissues examined. Prolonged ANG II infusion increased (P < 0.05) Ca2+-dependent NOS activity in mesenteric arteries (70 +/- 17), renal cortex (104 +/- 31), and left ventricle (49 +/- 8) and did not elicit changes in renal medulla. After a prolonged ANG II infusion, eNOS and nNOS levels increased in all tissues examined with the exception of eNOS in the mesenteric arteries and nNOS in the left ventricle, which were not altered. Acute and prolonged phenylephrine infusion elevated AP to a similar extent as ANG II but only elicited significant increments of Ca2+-dependent NOS activity in renal cortex. These data indicate that acute and prolonged elevations in ANG II upregulate Ca2+-dependent NOS activity and protein expression in different tissues related to the control of blood pressure. However, these ANG II effects are heterogeneous with respect to the tissue implicated, the time course of the stimulation, and the NOS isoform involved. Phenylephrine only induces a significant elevation of Ca2+-dependent NOS activity in renal cortex.

Shear level influences resistance artery remodeling: wall dimensions, cell density, and eNOS expression

The magnitude of shear stimulus has been shown to determine the level of growth factor expression in cell culture. However, little is known regarding what effect shear level has on specific arterial wall remodeling events in vivo. We have hypothesized that the rate of luminal diameter change and specific remodeling events within the arterial wall layers are dependent on shear level. Selective ligations were made to alter the number of microvascular perfusion units of mesenteric arteries within the same animal to approximately 50%, 200%, and 400% of control. Arterial blood flow and wall shear rate were correlated with the degree of alteration in perfusion units. Luminal diameters were decreased in 50% arteries by day 2 and increased approximately 17% and 33% respectively, in 200% and 400% arteries at day 7. The rate of diameter change was greatest in 50% and 400% arteries. Wall areas (medial +37%; intimal +18% at day 2) and cell densities (intimal +26%; adventitial +44% at day 2) were altered only in the 400% arteries. A positive correlation existed by day 2 between endothelial staining for endothelial nitric oxide synthase and shear level. The results demonstrate that shear level influences the rate of luminal expansion, specific remodeling events within each wall layer, and the degree of endothelial gene expression. A greater understanding of how shear level influences specific remodeling events within each wall layer should aid in the development of targeted therapies to manipulate the remodeling process in health and disease.

Angiotensin II induces contraction and proliferation of human hepatic stellate cells

BACKGROUND & AIMS

Circulating levels of angiotensin II (ANGII), a powerful vasoconstrictor factor, are frequently increased in chronic liver diseases. In these conditions, hepatic stellate cells (HSCs) proliferate and acquire contractile properties. This study investigated the presence of receptors for ANGII and the effects of ANGII in human HSCs activated in culture.

METHODS

The presence of ANGII receptors was assessed by binding studies. The effects of ANGII on intracellular calcium concentration ([Ca(2+)](i)), cell contraction, and cell proliferation were also assessed.

RESULTS

Binding studies showed the presence of ANGII receptors of the AT1 subtype. ANGII elicited a marked dose-dependent increase in [Ca(2+)](i) and cell contraction. Moreover, ANGII stimulated DNA synthesis and increased cell number. All these effects were totally blocked by losartan and reduced by nitric oxide donors or prostaglandin E(2). The effects of ANGII were barely detectable in quiescent cells (2 days in culture), suggesting that phenotypic transformation of HSCs is associated with a marked increase in the effects of ANGII.

CONCLUSIONS

ANGII induces contraction and is mitogenic for human-activated HSCs by acting through AT1 receptors. These results suggest that activated HSCs are targets of the vasoconstrictor action of ANGII in the intrahepatic circulation.

Pressor response to pulsatile compression of the rostral ventrolateral medulla mediated by nitric oxide and c-fos expression

It has been reported that neurovascular compression of the rostral ventrolateral medulla might be causally related to essential hypertension. Recently, we found that pulsatile compression of the rostral ventrolateral medulla increases sympathetic nerve activity and elevates arterial pressure via activation of glutamate receptors in rats. We also found that increases in sympathetic and cardiovascular activities by microinjection of L-glutamate into the rostral ventrolateral medulla are mediated by c-fos expression-related substance(s) following activation of the nitric oxide-cyclic GMP pathway. Herein, we investigated whether responses to pulsatile compression are mediated by local activation of the nitric oxide-cyclic GMP pathway and/or c-fos expression-related substance(s) in rats. Increases in arterial pressure (15+/-1 mmHg), heart rate (9+/-1 b.p.m.), and sympathetic nerve activity (% change: 8.5+/-1.1%) induced by pulsatile compression were partially but significantly inhibited after local microinjection of a nitric oxide synthase inhibitor, L-N(G)-nitroarginine methyl ester (8+/-2 mmHg, 1+/-1 b.p.m., 4.0+/-1.3%; P<0.05 vs compression without pretreatment) or 7-nitroindazole (7+/-2 mmHg, 2+/-1 b.p.m., 4.0+/-1. 5%; P<0.05), or a soluble guanylate cyclase inhibitor, methylene blue (9+/-1 mmHg, 4+/-1 b.p.m., 4.1+/-1.4%; P<0.05). In addition, increases in arterial pressure, heart rate, and sympathetic nerve activity by pulsatile compression were significantly reduced 6 h after microinjection of antisense oligodeoxynucleotide to c-fos mRNA (2+/-2 mmHg, 2+/-1 b.p.m., 1.0+/-1.0%; P<0.05 vs sense oligodeoxynucleotide). These results suggest that increases in sympathetic and cardiovascular activities induced by pulsatile compression of the rostral ventrolateral medulla are mediated, at least in part, by local activation of the nitric oxide-cyclic GMP pathway and c-fos expression-related substance(s) in rats.

The myocardial matrix and the development and progression of ventricular remodeling

Our conceptual framework of chronic heart failure is based upon the neurohormonal model. In this construct, neurohormonal systems that provide short-term homeostasis remain activated after a myocardial injury, producing progressive ventricular dysfunction and worsening heart failure. However, this model fails to explain several important clinical phenomena, that can be explained by an expanded model of heart failure that focuses on myocardial matrix events as the triggers for disease progression. This model embraces the neurohormonal model and integrates the roles of the immune system and the myocardial fibroblast within the matrix to more fully describe the initiation and progression of the disease.

Angiotensin II (ATII)-inducible platelet-derived growth factor A-chain gene expression is p42/44 extracellular signal-regulated kinase-1/2 and Egr-1-dependent and mediated via the ATII type 1 but not type 2 receptor. Induction by ATII antagonized by nitric oxide

Angiotensin II (ATII) and platelet-derived growth factor (PDGF) are two vasoconstrictors implicated in the maintenance of normal vascular homeostasis. PDGF A-chain levels increase in cultured vascular smooth muscle cells (SMCs) exposed to ATII. The molecular mechanisms underlying this induction are not known. We used transient transfection analysis to show that ATII can increase reporter gene activity driven by fragments of the PDGF-A promoter bearing recognition elements for the transcription factor, Egr-1. Nuclear run-off experiments indicate that ATII induces Egr-1 expression at the level of transcription. Gel shift and supershift studies show that Egr-1 protein accumulates in the nuclei of SMCs exposed to ATII and binds to the proximal region of the PDGF-A promoter in a specific, time-dependent manner. ATII induced extracellular-signal regulated kinase (p42/44 ERK) activity as did phorbol 12-myristate 13-acetate. The specific MEK1/2 inhibitor, PD98059, suppressed both PDGF-A and Egr-1 endogenous and promoter-dependent expression inducible by ATII. The ATII type 1 receptor (AT1) antagonist, Losartan, inhibited ATII-induction of p42/44 ERK, as well as Egr-1 and PDGF-A, whereas neither PD123319, an AT2 receptor antagonist, nor wortmannin, an inhibitor of phosphatidylinositol 3-kinase and c-Jun N-terminal kinase, had any effect. ATII-induction of Egr-1 and PDGF-A was blocked by SIN-1, a NO donor. In addition, this pathway was blocked by overexpression of NO synthase. Collectively, these findings demonstrate that ATII activation of the PDGF-A promoter is mediated via the MEK/ERK/Egr-1 pathway and AT1 receptor and that this process is antagonized by NO.

Reciprocal antagonism coordinates C-type natriuretic peptide and mitogen-signaling pathways in fibroblasts

The fibroblast, a cell central to effective wound remodeling, not only contains various growth factor receptors but also high activities of a guanylyl cyclase receptor (GC-B). Here we demonstrate that marked elevations of cyclic GMP induced by C-type natriuretic peptide (CNP), the ligand of GC-B, blocks activation of the mitogen-activated protein kinase cascade in fibroblasts. We also show that platelet-derived growth factor, fibroblast growth factor, serum, or Na3VO4 rapidly (within 5 min) and extensively (up to 85% inhibition) disrupt CNP-dependent elevations of cyclic GMP. In addition, the mitogens also lower cyclic GMP concentrations (50% decrease) in cells not treated with CNP. Cytoplasmic forms of guanylyl cyclase, in contrast to the CNP-stimulated pathway, are not antagonized by the various mitogens. The effects of the mitogens on cellular cyclic GMP are fully explained by a direct and stable inactivation of GC-B. Homogenates obtained from fibroblasts treated with or without the various mitogens contain equivalent amounts of GC-B protein, but both ligand-dependent and ligand-independent activity are markedly (up to 90% inhibition of CNP-dependent activity) decreased after mitogen addition. The stable inactivation is correlated with the dephosphorylation of phosphoserine and phosphothreonine residues of the cyclase receptor. These results not only establish a specific and reciprocal antagonistic relationship between mitogen-activated and GC-B-regulated signaling pathways in the fibroblast but also suggest that one of the earliest events following mitogen activation of a fibroblast is an interruption of cyclic GMP production from this receptor.

Nitric oxide and N-acetylcysteine inhibit the activation of mitogen-activated protein kinases by angiotensin II in rat cardiac fibroblasts

Angiotensin II acts on the cardiac fibroblast to produce a mitogenic response. Nitric oxide and N-acetylcysteine have been used to determine if oxidative stress influenced the effects of angiotensin II on the cardiac fibroblast. Angiotensin II activated the mitogen-activated protein kinases designated extracellular signal-regulated kinases within 5 min by interacting with the AT1 receptor. This activation was completely independent of protein kinase C and was inhibited when farnesylation was blocked, implicating Ras involvement. Pretreatment of cardiac fibroblasts with either N-acetylcysteine for 8 h or nitric oxide for 10 min suppressed this activation by angiotensin II in a dose-dependent manner. However, when both agents were added, inhibition was essentially complete. This combined effect of N-acetylcysteine and nitric oxide to block ERKs activation also was found if the activity was stimulated by either another growth factor (platelet-derived growth factor) or by the addition of phorbol ester, suggesting the effect was not limited to the receptor site alone. The results are consistent with the hypothesis that hormonal activation of mitogenic steps such as ERKs is influenced by increased oxidative stress, which is reduced by the combined effects of N-acetylcysteine and nitric oxide.