[Role of NO-cGMP-PKG axis in pulmonary arterial hypertension]

The nitric oxide (NO) - cyclic guanosine monophosphate (cGMP) - protein kinase G (PKG) axis is a critical signaling cascade in the development of pulmonary arterial hypertension (PAH). Dysregulation of the NO-cGMP-PKG axis results in pulmonary vascular inflammation, thrombosis and constriction, and ultimately leads to PAH. The PDE5 inhibitors such as sildenafil and tadalafil block the breakdown of cGMP. The resultant increase in cGMP concentration leads to relaxation of the smooth muscle and vasodilation. These effects are dependent on NO availability and sGC activity. The sGC stimulator riociguat has a dual mode of action, sensitizing sGC to endogenous NO by stabilizing NO-sGC binding and directly stimulating sGC via a different binding site. While there are clear pharmacokinetic and pharmacodynamic differences between these agents, it is still difficult to determine which agent is most appropriate for a specific PAH patient. Some patients respond better to sGC stimulator than a PDE5 inhibitor and vice versa. This chapter describes the role of the NO-cGMP-PKG pathway in PAH, potential and established treatment modalities to target this pathway, and their clinical applications.

Puerarin promotes the osteogenic differentiation of rat dental follicle cells by promoting the activation of the nitric oxide pathway

Puerarin regulates the osteoblast differentiation of umbilical cord mesenchymal stem cells. This study, hereby, explored the effects of puerarin on the osteogenic differentiation of dental follicle cells (DFCs) for the first time. Rat DFCs (rDFCs) were isolated and identified. After the rDFCs were treated by Puerarin and cultured in osteogenic induction medium, the viability, osteogenic differentiation, and the activities of alkaline phosphatase (ALP) and nitric oxide (NO) were detected. Besides, the secretion of cyclic guanosine monophosphate (cGMP) and expressions of collagen I, osteocalcin (OC), osteopontin (OPN), runt-related transcription factor 2 (RUNX2), soluble guanylate cyclase (SGC), and protein kinase G 1 (PKG-1) were further determined or quantified. Puerarin enhanced the viability and osteogenic differentiation, and increased the activities of ALP, NO, and cGMP and the expressions of Collagen I, OC, OPN, RUNX2, SGC, and PKG-1 in rDFCs. After the co-treatment with puerarin and L-NMMA (NO synthase inhibitor), the promotive effects of Puerarin on cell viability, osteogenic differentiation, and the expressions of collagen I, OC, OPN, RUNX2, SGC, and PKG-1 in rDFCs were reversed by L-NMMA. Puerarin boosted the osteogenic differentiation of rDFCs by activating the NO pathway.

An herbal preparation ameliorates heart failure with preserved ejection fraction by alleviating microvascular endothelial inflammation and activating NO-cGMP-PKG pathway

BACKGROUND

Heart failure with preserved ejection fraction (HFpEF) is a heterogeneous disease presenting a substantial challenge to clinicians. Currently, there is no safe and efficacious HFpEF treatment. In this study, we reported a standardized herbal medicinal product, QiShenYiQi (QSYQ), that can be used in the treatment of HFpEF.

METHODS

HFpEF mice were established by infusing a combination of N^ω-nitro-L-arginine methyl ester (L-NAME) and feeding them a high-fat diet for 14 weeks. In the 10th week, the HFpEF mice were given dapagliflozin or QSYQ via oral gavage for four weeks. The blood pressure, echocardiography, hemodynamics, leukocyte infiltration, and oxidative stress in HFpEF mice were evaluated. Besides, inflammatory factors, endothelial adhesion factors, and endothelial-mesenchymal transformation (EndMT) markers were investigated.

RESULTS

QSYQ significantly attenuated concentric cardiac remodeling while improving diastolic function and left ventricular compliance in HFpEF mice. QSYQ also inhibited inflammation and immunocyte recruitment during HFpEF. The infiltration of CD8⁺, CD4⁺ T cells, and CD11b/c⁺ monocytes was substantially mitigated in the myocardium of QSYQ-treated mice. TNF-α, MCP-1, NF-κB, and NLRP3 levels also reduced after QSYQ treatment. Furthermore, QSYQ significantly reversed the elevated expression of endothelial adhesion factors and EndMT occurrence. These effects of QSYQ were demonstrated by the activation of NO-cGMP-PKG pathway and reduction of eNOS uncoupling in the HFpEF heart.

CONCLUSION

These results provide novel evidence that QSYQ treatment improves HFpEF by inhibiting microvascular endothelial inflammation and activating NO-cGMP-PKG pathway.

Impact of Interatrial Shunts on Invasive Hemodynamics and Exercise Tolerance in Patients With Heart Failure

Approximately 50% of patients with heart failure have preserved ejection fraction. Although a wide variety of conditions cause or contribute to heart failure with preserved ejection fraction, elevated left ventricular filling pressures, particularly during exercise, are common to all causes. Acute elevation in left-sided filling pressures promotes lung congestion and symptoms of dyspnea, while chronic elevations often lead to pulmonary vascular remodeling, right heart failure, and increased risk of mortality. Pharmacologic therapies, including neurohormonal modulation and drugs that modify the nitric oxide/cyclic GMP-protein kinase G pathway have thus far been limited in reducing symptoms or improving outcomes in patients with heart failure with preserved ejection fraction. Hence, alternative means of reducing the detrimental rise in left-sided heart pressures are being explored. One proposed method of achieving this is to create an interatrial shunt, thus unloading the left heart at rest and during exercise. Currently available studies have shown 3- to 5-mm Hg decreases of pulmonary capillary wedge pressure during exercise despite increased workload. The mechanisms underlying the hemodynamic changes are just starting to be understood. In this review we summarize results of recent studies aimed at elucidating the potential mechanisms of improved hemodynamics during exercise tolerance following interatrial shunt implantation and the current interatrial shunt devices under investigation.

Scutellarin attenuates endothelium-dependent aasodilation impairment induced by hypoxia reoxygenation, through regulating the PKG signaling pathway in rat coronary artery

Scutellarin (SCU), a flavonoid from a traditional Chinese medicinal plant. Our previous study has demonstrated that SCU relaxes mouse aortic arteries mainly in an endothelium-depend-ent manner. In the present study, we investigated the vasoprotective effects of SCU against HR-induced endothelial dysfunction (ED) in isolated rat CA and the possible mechanisms involving cyclic guanosine monophosphate (cGMP) dependent protein kinase (PKG). The isolated endothelium-intact and endothelium-denuded rat CA rings were treated with HR injury. Evaluation of endothelium-dependent and -independent vasodilation relaxation of the CA rings were performed using wire myography and the protein expressions were assayed by Western blotting. SCU (10-1 000 μmol·L(-1)) could relax the endothelium-intact CA rings but not endothelium-denuded ones. In the intact CA rings, the PKG inhibitor, Rp-8-Br-cGMPS (PKGI-rp, 4 μmol·L(-1)), significantly blocked SCU (10-1 000 μmol·L(-1))-induced relaxation. The NO synthase (NOS) inhibitor, NO-nitro-L-arginine methylester (L-NAME, 100 μmol·L(-1)), did not significantly change the effects of SCU (10-1 000 μmol·L(-1)). HR treatment significantly impaired ACh-induced relaxation, which was reversed by pre-incubation with SCU (500 μmol·L(-1)), while HR treatment did not altered NTG-induced vasodilation. PKGI-rp (4 μmol·L(-1)) blocked the protective effects of SCU in HR-treated CA rings. Additionally, HR treatment reduced phosphorylated vasodilator-stimulated phosphoprotein (p-VASP, phosphorylated product of PKG), which was reversed by SCU pre-incubation, suggesting that SCU activated PKG phosphorylation against HR injury. SCU induces CA vasodilation in an endothelium-dependent manner to and repairs HR-induced impairment via activation of PKG signaling pathway.

Inorganic nitrate promotes the browning of white adipose tissue through the nitrate-nitrite-nitric oxide pathway

Inorganic nitrate was once considered an oxidation end product of nitric oxide metabolism with little biological activity. However, recent studies have demonstrated that dietary nitrate can modulate mitochondrial function in man and is effective in reversing features of the metabolic syndrome in mice. Using a combined histological, metabolomics, and transcriptional and protein analysis approach, we mechanistically defined that nitrate not only increases the expression of thermogenic genes in brown adipose tissue but also induces the expression of brown adipocyte-specific genes and proteins in white adipose tissue, substantially increasing oxygen consumption and fatty acid β-oxidation in adipocytes. Nitrate induces these phenotypic changes through a mechanism distinct from known physiological small molecule activators of browning, the recently identified nitrate-nitrite-nitric oxide pathway. The nitrate-induced browning effect was enhanced in hypoxia, a serious comorbidity affecting white adipose tissue in obese individuals, and corrected impaired brown adipocyte-specific gene expression in white adipose tissue in a murine model of obesity. Because resulting beige/brite cells exhibit antiobesity and antidiabetic effects, nitrate may be an effective means of inducing the browning response in adipose tissue to treat the metabolic syndrome.

Cyclic-AMP-dependent proliferation of a human osteoblast cell line (HOS cells) induced by hydroxyapatite: effect of exogenous nitric oxide

BACKGROUND AND AIMS OF THE WORK: Nitric oxide (NO) has been reported to enhance the production of cAMP by hydroxyapatite (HA)-induced a human osteoblast cell line (HOS cells). The aim of the present study was to test the hypothesis that exogenous NO may up-regulate the proliferation of hydroxyapatite (HA)-induced HOS cells via the cyclic-AMP-protein kinase A (PKA) pathway.

METHODS

HOS cells were pre-incubated with ODQ (guanylyl cyclase inhibitor), SQ22536 (adenylyl cyclase inhibitor), forskolin (adenylyl cyclase activator), IBMX [phosphodiesterase (PDE) inhibitor], siguazodan (PDE3 inhibitor), rolipram (PDE4 inhibitor), or KT5720 (PKA inhibitor), and then, cultured on the surface of HA with or without the presence of SNAP (NO donor). The HOS cell cultures on the HA surface were added with br-cGMP (cGMP analogue), db-cAMP (cAMP analogue) with or without SNAP. The cell proliferation was assessed by a colorimetric assay.

RESULTS

The up-regulatory effect of SNAP on HA-induced HOS cell proliferation was suppressed by SQ22536 and KT5720, but enhanced by db-cAMP, IBMX, and rolipram. The HA-induced HOS cell proliferation with or without the presence of SNAP was unaltered by ODQ br-cGMP and siguazodan.

CONCLUSION

These results suggest, therefore, that HA-induced HOS cell proliferation may be mediated by the cAMP-PKA pathway regulated by PDE4 and that exogenous NO may amplify this cyclic nucleotide pathway, thereby augmenting HA-induced HOS cell proliferation.

Serotonin transporter phosphorylation by cGMP-dependent protein kinase is altered by a mutation associated with obsessive compulsive disorder

Human serotonin transporter (hSERT) activity expressed in HeLa cells was stimulated by agents that release nitric oxide, stimulate soluble guanylyl cyclase, or activate cGMP-dependent protein kinase (PKG). This stimulation was blocked by a PKG inhibitor. A naturally occurring mutation, I425V, associated with obsessive-compulsive disorder and other neuropsychiatric disorders, activated hSERT and eliminated stimulation via the PKG pathway. Inhibitors of soluble guanylyl cyclase or PKG decreased activity of the I425V mutant, but not wild type, indicating that both wild-type and mutant transporters could exist in both high and low activity forms. Mutation of Thr-276 in the fifth transmembrane domain (TM5) to alanine or aspartate prevented activation of wild-type hSERT through the PKG pathway and also blocked the inhibition of I425V activity by inhibitors of the pathway. The accessibility of positions in TM5 near Thr-276 was modified in T276D, but not in I425V. These results are consistent with the hypothesis that PKG phosphorylates hSERT at Thr-276 and increases its activity by modifying the substrate permeation pathway formed, in part, by TM5. The effect of the I425V mutation may shift the balance of hSERT toward the phosphorylated form, possibly by interfering with the action of a phosphatase. However, association of hSERT with protein phosphatase 2A was not decreased in the I425V mutant.

Interaction of fibroblast growth factor and C-natriuretic peptide signaling in regulation of chondrocyte proliferation and extracellular matrix homeostasis

Overexpression of C-natriuretic peptide (CNP) in cartilage partially rescues achondroplasia in the mouse. Here, we studied the interaction of fibroblast growth factor (FGF) and CNP signaling in chondrocytes. CNP antagonized FGF2-induced growth arrest of rat chondrosarcoma (RCS) chondrocytes by inhibition of the Erk mitogen activated protein kinase pathway. This effect of CNP was protein kinase G-dependent and was mimicked by the cGMP analog pCPT-cGMP. FGF2-mediated activation of both MEK and Raf-1 but not Ras or FRS2 was abolished by CNP demonstrating that CNP blocks the Erk pathway at the level of Raf-1. CNP also counteracted the FGF2-mediated degradation of RCS extracellular matrix. CNP partially antagonized FGF2-induced expression, release and activation of several matrix-remodeling molecules including matrix metalloproteinase 2 (MMP2), MMP3, MMP9, MMP10 and MMP13. In addition, CNP compensated for FGF2-mediated matrix loss by upregulation of matrix production independent of its interference with FGF signaling. We conclude that CNP utilizes both direct and indirect ways to counteract the effects of FGF signaling in a chondrocyte environment.

Tenellones A and B from a Diaporthe sp.: two highly substituted benzophenone inhibitors of parasite cGMP-dependent protein kinase activity

Parasite cGMP-dependent protein kinase (PKG) has been recently validated as a biochemical target for the treatment of coccidiosis. To discover new anticoccidial leads, we have screened our library of natural product extracts for inhibitors of parasite PKG. Bioassay-guided fractionation of the microbial extracts has led to the discovery of tenellones A (2) and B (3), two new highly substituted benzophenones. The isolation, structure, and activity of these compounds are described.

Chronic inhibition of cyclic GMP phosphodiesterase 5A prevents and reverses cardiac hypertrophy

Sustained cardiac pressure overload induces hypertrophy and pathological remodeling, frequently leading to heart failure. Genetically engineered hyperstimulation of guanosine 3',5'-cyclic monophosphate (cGMP) synthesis counters this response. Here, we show that blocking the intrinsic catabolism of cGMP with an oral phosphodiesterase-5A (PDE5A) inhibitor (sildenafil) suppresses chamber and myocyte hypertrophy, and improves in vivo heart function in mice exposed to chronic pressure overload induced by transverse aortic constriction. Sildenafil also reverses pre-established hypertrophy induced by pressure load while restoring chamber function to normal. cGMP catabolism by PDE5A increases in pressure-loaded hearts, leading to activation of cGMP-dependent protein kinase with inhibition of PDE5A. PDE5A inhibition deactivates multiple hypertrophy signaling pathways triggered by pressure load (the calcineurin/NFAT, phosphoinositide-3 kinase (PI3K)/Akt, and ERK1/2 signaling pathways). But it does not suppress hypertrophy induced by overexpression of calcineurin in vitro or Akt in vivo, suggesting upstream targeting of these pathways. PDE5A inhibition may provide a new treatment strategy for cardiac hypertrophy and remodeling.

Nitric oxide provokes tumor necrosis factor-alpha expression in adult feline myocardium through a cGMP-dependent pathway

BACKGROUND

The mechanism(s) responsible for the persistent coexpression of tumor necrosis factor-alpha (TNF-alpha) and nitric oxide (NO) in the failing heart is unknown.

METHODS AND RESULTS

To determine whether NO was sufficient to provoke TNF-alpha biosynthesis, we examined the effects of an NO donor, S-nitroso-N-acetyl penicillamine (SNAP), in buffer-perfused Langendorff hearts. SNAP (1 micromol/L) treatment resulted in a time- and dose-dependent increase in myocardial TNF-alpha mRNA and protein biosynthesis in adult cat hearts. The effects of SNAP were completely abrogated by a NO quenching agent, 2-(4-carboxyphenyl)-4, 4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (C-PTIO), and mimicked by sodium nitroprusside. Electrophoretic mobility shift assays demonstrated that SNAP treatment led to the rapid induction of nuclear factor kappa-beta (NF-kappaB) but not AP-1. The importance of the cGMP pathway in terms of mediating NO-induced TNF-alpha biosynthesis was shown by studies that demonstrated that 8-bromo-cGMP mimicked the effects of SNAP and that the effects of SNAP could be completely abrogated using a cGMP antagonist, 1H-(1,2, 4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), or protein kinase G antagonist (Rp-8-Br-cGMPS). SNAP and 8-Br-cGMP were both sufficient to lead to the site-specific phosphorylation (serine 32) and degradation of IkappaBalpha in isolated cardiac myocytes. Finally, protein kinase G was sufficient to directly phosphorylate IkappaBalpha on serine 32, a critical step in the activation of NF-kappaB.

CONCLUSIONS

These studies show that NO provokes TNF-alpha biosynthesis through a cGMP-dependent pathway, which suggests that the coincident expression of TNF-alpha and NO may foster self-sustaining positive autocrine/paracrine feedback inflammatory circuits within the failing heart.

Role of protein kinases in the prolactin-induced intracellular calcium rise in Chinese hamster ovary cells expressing the prolactin receptor

There is still only limited understanding of the early steps of prolactin signal transduction in target cells. It has been shown that prolactin actions are associated with cell protein phosphorylation, Ca2+ increases, and so on. However, the link between the activation of kinases and calcium influx or intracellular Ca2+ mobilization has not yet been clearly established. Chinese hamster ovary (CHO) cells, stably transfected with the long form of rabbit mammary gland prolactin receptor (PRL-R) cDNA were used for PRL-R signal transduction studies. Spectrofluorimetric techniques were used to measure intracellular calcium ([Ca2+]i) in cell populations with Indo1 as a calcium fluorescent probe. We demonstrate that, although protein kinase C activation (PMA or DiC8) caused a calcium influx in CHO cells, prolactin-induced PKC activation was not responsible for the early effect of prolactin on [Ca2+]i. Activation of protein kinase A (PKA) or protein kinase G did not modify [Ca2+]i and inhibition of PKA pathway did not affect the prolactin response. In the same way, phosphatidylinositol-3 kinaseinhibition had no effect on the prolactin-induced Ca2+ increase. On the other hand, tyrosine kinase inhibitors (herbimycin A, lavendustin A, and genistein) completely blocked the effect of prolactin on [Ca2+]i (influx and release). W7, a calmodulin-antagonist, and a specific inhibitor of calmodulin kinases (KN-62), only blocked prolactin-induced Ca2+ influx but had no significant effect on Ca2+ release. Using pharmacological agents, we present new data concerning the involvement of protein phosphorylations in the early effects of prolactin on ionic channels in CHO cells expressing the long form of PRL-R. Our results suggest that, at least in the very early steps of prolactin signal transduction, serine-threonine phosphorylation does not participate in the prolactin-induced calcium increase. On the other hand, tyrosine phosphorylation is a crucial, very early step, since it controls K+ channel activation, calcium influx, and intracellular calcium mobilization. Calmodulin acts later, since its inhibition only blocks the prolactin-induced Ca2+ influx.

Phosphorylation of telokin by cyclic nucleotide kinases and the identification of in vivo phosphorylation sites in smooth muscle

The Ca(2+)-independent acceleration of dephosphorylation of the regulatory light chain of smooth muscle myosin and relaxation of smooth muscle by telokin are enhanced by cyclic nucleotide-activated protein kinase(s) [Wu et al. (1998) J. Biol. Chem. 273, 11362-113691. The purpose of this study was to determine the in vivo site(s) and in vitro rates of telokin phosphorylation and to evaluate the possible effects of sequential phosphorylation by different kinases. The in vivo site(s) of phosphorylation of telokin were determined in rabbit smooth muscles of longitudinal ileum and portal vein. Following stimulation of ileum with forskolin (20 microM) the serine at position 13 was the only amino acid to exhibit increased phosphorylation. Rabbit portal vein telokin was phosphorylated on both Ser-13 and -19 as a result of forskolin and GTPgammaS stimulation in vivo. Point mutation of Ser-13 (to Ala or Asp) abolished in vitro phosphorylation by cyclic nucleotide-dependent protein kinases.

Extracellular ATP inhibits the small-conductance K channel on the apical membrane of the cortical collecting duct from mouse kidney

We have used the patch-clamp technique to study the effects of changing extracellular ATP concentration on the activity of the small-conductance potassium channel (SK) on the apical membrane of the mouse cortical collecting duct. In cell-attached patches, the channel conductance and kinetics were similar to its rat homologue. Addition of ATP to the bathing solution of split-open single cortical collecting ducts inhibited SK activity. The inhibition of the channel by ATP was reversible, concentration dependent (K(i) = 64 microM), and could be completely prevented by pretreatment with suramin, a specific purinergic receptor (P(2)) blocker. Ranking of the inhibitory potency of several nucleotides showed strong inhibition by ATP, UTP, and ATP-gamma-S, whereas alpha, beta-Me ATP, and 2-Mes ATP failed to affect channel activity. This nucleotide sensitivity is consistent with P(2)Y(2) purinergic receptors mediating the inhibition of SK by ATP. Single channel analysis further demonstrated that the inhibitory effects of ATP could be elicited through activation of apical receptors. Moreover, the observation that fluoride mimicked the inhibitory action of ATP suggests the activation of G proteins during purinergic receptor stimulation. Channel inhibition by ATP was not affected by blocking phospholipase C and protein kinase C. However, whereas cAMP prevented channel blocking by ATP, blocking protein kinase A failed to abolish the inhibitory effects of ATP. The reduction of K channel activity by ATP could be prevented by okadaic acid, an inhibitor of protein phosphatases, and KT5823, an agent that blocks protein kinase G. Moreover, the effect of ATP was mimicked by cGMP and blocked by L-NAME (N(G)-nitro-l-arginine methyl ester). We conclude that the inhibitory effect of ATP on the apical K channel is mediated by stimulation of P(2)Y(2) receptors and results from increasing dephosphorylation by enhancing PKG-sensitive phosphatase activity.

Role of phosphodiesterase and protein kinase G on nitric oxide-induced inhibition of prolactin release from the rat anterior pituitary

OBJECTIVE

In order to determine the mechanism by which nitric oxide (NO) inhibits prolactin release, we investigated the participation of cGMP-dependent cAMP-phosphodiesterases (PDEs) and protein kinase G (PKG) in this effect of NO.

METHODS

Anterior pituitary glands of male rats were incubated with inhibitors of PDE and PKG with or without sodium nitroprusside (NP). Prolactin release, and cAMP and cGMP concentrations were determined by RIA.

RESULTS AND CONCLUSIONS

The inhibitory effect of NP (0.5 mmol/l) on prolactin release and cAMP concentration was blocked by EHNA (10(-4)mol/l) and HL-725 (10(-4)mol/l), inhibitors of cGMP-stimulated cAMP-PDE (PDE2). 8-Br-cGMP (10(-4) and 10(-3)mol/l), which mimics cGMP as a mediator of NP effects on prolactin release, also decreased cAMP concentration. Zaprinast (10(-4)mol/l), a selective inhibitor of specific cGMP-PDE (PDE5), potentiated the NP effect on cAMP concentration. Rp-8-[(4-chlorophenyl)thio]-cGMP triethylamine (Rp-8-cGMP, 10(-7)-10(-6)mol/l), an inhibitor of PKG, reversed the effect of NP on prolactin release. The present study suggests that several mechanisms are involved in the inhibitory effect of NO on prolactin release. The activation of PDE2 by cGMP may mediate the inhibitory effect of NO on cAMP concentration and therefore on prolactin release. NO-activated PKG may also be participating in the inhibitory effect of NO on prolactin release.

Nitric oxide/cyclic GMP pathway attenuates ATP-evoked intracellular calcium increase in supporting cells of the guinea pig cochlea

We demonstrate here that nitric oxide (NO) attenuates ATP-evoked calcium transients in Deiters' and Hensen's cells, "supporting" (nonsensory) cells of the guinea pig cochlea, by means of activation of soluble guanylyl cyclase and protein kinase G. The enzymatic activities associated with the nitric oxide/cGMP/protein kinase G pathway had previously been demonstrated to be present in Deiters' and Hensen's cells. We now isolate these cells and measure changes in intracellular free calcium by using the calcium indicator fluo-3. In Deiters' cells, calcium increased rapidly in response to the application of ATP. The increase was attenuated when the pathway was stimulated by NO donors (diethylamine NONOate or sodium nitroprusside) or the cyclic GMP analog, 8-bromo-cyclic GMP. When the activation of the pathway was blocked by the additional presence of inhibitors of soluble guanylyl cyclase (LY83583) or protein kinase G (Rp-8-bromo-cyclic GMP or KT5823), the response to ATP was restored. The reactions also occurred in calcium-free media. Hensen's cells responded similarly. These results provide evidence that intracellular calcium is regulated by the NO/cGMP/protein kinase G pathway in the inner ear.

Mechanisms of reduced nitric oxide/cGMP-mediated vasorelaxation in transgenic mice overexpressing endothelial nitric oxide synthase

NO, constitutively produced by endothelial NO synthase (eNOS), plays a key regulatory role in vascular wall homeostasis. We generated transgenic (Tg) mice overexpressing eNOS in the endothelium and reported the presence of reduced NO-elicited relaxation. The purpose of this study was to clarify mechanisms of the reduced response to NO-mediated vasodilators in eNOS-Tg mice. Thoracic aortas of Tg and control mice were surgically isolated for vasomotor studies. Relaxations to acetylcholine and sodium nitroprusside were significantly reduced in Tg vessels compared with control vessels. Relaxations to atrial natriuretic peptide and 8-bromo-cGMP were also significantly reduced in Tg vessels. Reduced relaxations to these agents were restored by chronic N(G)-nitro-L-arginine methyl ester treatment. Basal cGMP levels of aortas were higher in Tg mice than in control mice, whereas soluble guanylate cyclase (sGC) activity in Tg vessels was approximately 50% of the activity in control vessels. Moreover, cGMP-dependent protein kinase (PKG) protein levels and PKG enzyme activity were decreased in Tg vessels. These observations indicate that chronic overexpression of eNOS in the endothelium resulted in resistance to the NO/cGMP-mediated vasodilators and that at least 2 distinct mechanisms might be involved: one is reduced sGC activity, and the other is a decrease in PKG protein levels. We reported for the first time that increased NO release from the endothelium reduces sGC and PKG activity in mice. These data may provide a new insight into the mechanisms of nitrate tolerance and cross tolerance to nitrovasodilators.

Cyclic GMP-dependent feedback inhibition of AMPA receptors is independent of PKG

In central neurons, the second messenger cGMP is believed to induce long-term changes in efficacy at glutamatergic synapses through activation of protein kinase G (PKG). Stimulating nitric oxide synthase, activating soluble guanylyl cyclase or elevating concentrations of intracellular cGMP depressed excitatory synaptic transmission in CA1 hippocampal neurons. Unexpectedly, intracellular cGMP depressed responses of AMPA receptors and inhibited excitatory postsynaptic currents in hippocampal neurons independently of phosphorylation. Our findings demonstrate that cGMP's modulation of excitatory transmission may involve a coupling of AMPA channel activity to levels of cGMP.

Inhibitory effect of nitric oxide on voltage-dependent calcium currents in rat dorsal root ganglion cells

The effect of nitric oxide (NO) on calcium current (I(Ca)) and intracellular calcium concentration ([Ca(2+)](i)) in primarily cultured dorsal root ganglion (DRG) neurons was investigated from neonatal rats. I(Ca) and [Ca(2+)](i) were simultaneously recorded using perforated-patch technique in combination with fluorescence measurement from single DRG neurons. NO donors, sodium nitroprusside (SNP) and S-nitro-N-acetylpenicillamine (SNAP), inhibited I(Ca) in small-diameter neurons without significant change in voltage-dependence of activation and activation time constants. SNP and SNAP also reduced the transient [Ca(2+)](i) peak accompanied by I(Ca). Inhibition by NO was reproducible, but gradually desensitized. In some DRG neurons, SNP and SNAP increased basal [Ca(2+)](i) in concentration of 10 microM with little effect on NO-induced inhibition of I(Ca). 8-Br-cGMP, a permeable cGMP analog, mimicked the effects of SNP and SNAP. These results suggest that, in DRG neurons, NO has inhibitory effect on I(Ca), which is independent of NO-induced increase of basal [Ca(2+)](i), through cGMP-dependent pathway.

Phosphorylation of phosphodiesterase-5 by cyclic nucleotide-dependent protein kinase alters its catalytic and allosteric cGMP-binding activities

In addition to its cGMP-selective catalytic site, cGMP-binding cGMP-specific phosphodiesterase (PDE5) contains two allosteric cGMP-binding sites and at least one phosphorylation site (Ser92) on each subunit [Thomas, M.K., Francis, S.H. & Corbin, J.D. (1990) J. Biol. Chem. 265, 14971-14978]. In the present study, prior incubation of recombinant bovine PDE5 with a phosphorylation reaction mixture [cGMP-dependent protein kinase (PKG) or catalytic subunit of cAMP-dependent protein kinase (PKA), MgATP, cGMP, 3-isobutyl-1-methylxanthine], shown earlier to produce Ser92 phosphorylation, caused a 50-70% increase in enzyme activity and also increased the affinity of cGMP binding to the allosteric cGMP-binding sites. Both effects were associated with increases in its phosphate content up to 0.6 mol per PDE5 subunit. Omission of any one of the preincubation components caused loss of stimulation of catalytic activity. Addition of the phosphorylation reaction mixture to a crude bovine lung extract, which contains PDE5, also produced a significant increase in cGMP PDE catalytic activity. The increase in recombinant PDE5 catalytic activity brought about by phosphorylation was time-dependent and was obtained with 0.2-0.5 microM PKG subunit, which is approximately the cellular level of this enzyme in vascular smooth muscle. Significantly greater stimulation was observed using cGMP substrate concentrations below the Km value for PDE5, although stimulation was also seen at high cGMP concentrations. Considerably higher concentration of the catalytic subunit of PKA than of PKG was required for activation. There was no detectable difference between phosphorylated and unphosphorylated PDE5 in median inhibitory concentration for the PDE5 inhibitors, sildenafil, or zaprinast 3-isobutyl-1-methylxanthine. Phosphorylation reduced the cGMP concentration required for half-maximum binding to the allosteric cGMP-binding sites from 0.13 to 0.03 microM. The mechanism by which phosphorylation of PDE5 by PKG could be involved in physiological negative-feedback regulation of cGMP levels is discussed.

A protein kinase G-sensitive channel mediates flow-induced Ca(2+) entry into vascular endothelial cells

The hemodynamic force generated by blood flow is considered to be the physiologically most important stimulus for the release of nitric oxide (NO) and prostacyclin (PGI(2)) from vascular endothelial cells (1). NO and PGI(2) then act on the underlying smooth muscle cells, causing vasodilation and thus lowering blood pressure (2, 3). One critical early event occurring in this flow-induced regulation of vascular tone is that blood flow induces Ca(2+) entry into vascular endothelial cells, which in turn leads to the formation of NO (4, 5). Here we report a mechanosensitive Ca(2+)-permeable channel in vascular endothelial cells. The activity of the channel was inhibited by 8-Br-cGMP, a membrane-permeant activator of protein kinase G (PKG), in cell-attached membrane patches. The inhibition could be reversed by PKG inhibitor KT5823 or H-8. A direct application of active PKG in inside-out patches blocked the channel activity. Gd(3+), Ni(2+), or SK&F-96365 also inhibited the channel activity. A study of fluorescent Ca(2+) entry revealed a striking pharmacological similarity between the Ca(2+) entry elicited by flow and the mechanosensitive Ca(2+)-permeable channel we identified, suggesting that this channel is the primary pathway mediating flow-induced Ca(2+) entry into vascular endothelial cells.

A protein phosphatase is involved in the cholinergic suppression of the Ca(2+)-activated K(+) current sI(AHP) in hippocampal pyramidal neurons

The slow calcium-activated potassium current sI(AHP) underlies spike-frequency adaptation and has a substantial impact on the excitability of hippocampal CA1 pyramidal neurons. Among other neuromodulatory substances, sI(AHP) is modulated by acetylcholine acting via muscarinic receptors. The second-messenger systems mediating the suppression of sI(AHP) by muscarinic agonists are largely unknown. Both protein kinase C and A do not seem to be involved, whereas calcium calmodulin kinase II has been shown to take part in the muscarinic action on sI(AHP). We re-examined the mechanism of action of muscarinic agonists on sI(AHP) combining whole-cell recordings with the use of specific inhibitors or activators of putative constituents of the muscarinic pathway. Our results suggest that activation of muscarinic receptors reduces sI(AHP) in a G-protein-mediated and phospholipase C-independent manner. Furthermore, we obtained evidence for the involvement of the cGMP-cGK pathway and of a protein phosphatase in the cholinergic suppression of sI(AHP), whereas release of Ca(2+) from IP(3)-sensitive stores seems to be relevant neither for maintenance nor for modulation of sI(AHP).

Effects of natriuretic peptides (ANP, BNP, CNP) on catecholamine synthesis and TH mRNA levels in PC12 cells

Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) are present in adrenal chromaffin cells, and are co-secreted with catecholamines suggesting that these natriuretic peptides (NPs) may modulate functions of chromaffin cells in an autocrine and/or paracrine manner. Therefore, we investigated the effects of NPs on tyrosine hydroxylase (TH: a rate-limiting enzyme in biosynthesis of catecholamine) mRNA in rat pheochromocytoma PC12 cells. It was also determined whether the cyclic GMP/cGMP-dependent protein kinase (cGMP/PKG) pathway was involved in theses effects. Finally, we examined the effects of NPs on intracellular catecholamine content to confirm increase of catecholamine synthesis following TH mRNA induction. NPs (0.1 microM) induced significant increases of the TH mRNA (ANP= BNP> CNP). Also, the effects of NPs on TH mRNA were mimicked by 8-bromo cyclic GMP (1mM), and were blocked by KT5823 (1 microM) (inhibitor PKG) or LY83583 (1 microM) (guanylate cyclase inhibitor). Moreover, NPs were shown to induce significant increases of intracellular catecholamine contents (ANP= BNP> CNP). These findings suggest that NPs induced increases of TH mRNA through cGMP/PKG dependent mechanisms, which, in turn, resulted in stimulation of catecholamine synthesis in PC12 cells.

Nitric oxide increases p21(Waf1/Cip1) expression by a cGMP-dependent pathway that includes activation of extracellular signal-regulated kinase and p70(S6k)

Nitric oxide (NO) regulates the expression of p21(Waf1/Cip1) in several cell types. The present study examined the role of both the extracellular signal-regulated kinase (ERK) and p70 S6 kinase (p70(S6k)) in the NO-induced increase in p21 expression that occurred in adventitial fibroblasts during the cell cycle. Both ERK and p70(S6k) were phosphorylated in response to the NO donor S-nitroso-N-acetylpenicillamine (SNAP) and the activation was rapid, transient, and preceded increased p21 expresion under defined conditions where serum was present. Addition of a selective inhibitor of ERK phosphorylation (PD98059) prevented the subsequent phosphorylation of p70(S6k) and the increase in p21 protein. Both cGMP and cAMP activated both ERK and p70(S6k), whereas only selective inhibitors of protein kinase G prevented the activation of the kinases by SNAP. A complex between ERK and p70(S6k) was documented by immunoprecipitation procedures. Rapamycin blocked p70(S6k) phosphorylation induced by NO and also inhibited p53 phosphorylation and p21 expression whereas PD98059 only prevented the NO-induced increase in p21 protein without influencing either p53 activation or p21 mRNA expression. The studies show a unique relationship between NO, ERK, and p70(S6k) and also provide evidence for a novel role of p70(S6k) in the activation of p53.

Semaphorin 3A is a chemoattractant for cortical apical dendrites

The apical dendrites of pyramidal neurons integrate inputs from various cortical layers and are central to information processing. Here we show that the growth of apical dendrites towards the pial surface is regulated by a diffusible chemoattractant present at high levels near the marginal zone. A major component of this signal is semaphorin 3A (Sema3A), which was previously characterized as a chemorepellant for cortical axons. Soluble guanylate cyclase is asymmetrically localized to the developing apical dendrite, and is required for the chemoattractive effect of Sema3A. Thus the asymmetric localization of soluble guanylate cyclase confers distinct Sema3A responses to axons and dendrites. These observations reveal a mechanism by which a single chemotropic signal can pattern both axons and dendrites during development.

Nitric oxide reduces vascular smooth muscle cell elastase activity through cGMP-mediated suppression of ERK phosphorylation and AML1B nuclear partitioning

Nitric oxide (NO) reduces the severity of pulmonary vascular disease in rats as do elastase inhibitors. We therefore hypothesized that NO inhibits elastase by suppressing mitogen-activated protein kinases that trans-activate AML1B, a transcription factor for elastase. We used cultured pulmonary artery smooth muscle cells in which serum-treated elastin (STE) induces a > threefold increase in elastase activity as evaluated by solubilization of [(3)H]-elastin. NO donors (SNAP and DETA NONOate) inhibited elastase in a dose-dependent manner as did a cGMP mimetic (8-pCPT-cGMP). SNAP inhibition of elastase was reversed by coadministration of a cGMP-PKG inhibitor (Rp-8-pCPT-cGMP). The STE-induced increase in phospho-ERK was suppressed by NO donors and the cGMP mimetic, and reversed by cGMP-PKG inhibitor, as was expression of AML1B and DNA binding in nuclear extracts. A concomitant increase in p38 phosphorylation was also inhibited by SNAP, but whereas MEK inhibitor (PD98059) suppressed elastase and AML1B-DNA binding, a p38 inhibitor (SB202190) did not. Our study uniquely links NO with inhibition of elastase-dependent matrix remodeling in vascular disease by suggesting a cGMP-PKG-related mechanism suppressing ERK-mediated partitioning of AML1B in nuclear extracts.

Cellular mechanisms involved in carotid body inhibition produced by atrial natriuretic peptide

Atrial natriuretic peptide (ANP) and its analog, atriopeptin III (APIII), inhibit carotid body chemoreceptor nerve activity evoked by hypoxia. In the present study, we have examined the hypothesis that the inhibitory effects of ANP and APIII are mediated by cyclic GMP and protein kinase G (PKG) via the phosphorylation and/or dephosphorylation of K(+) and Ca(2+) channel proteins that are involved in regulating the response of carotid body chemosensory type I cells to low-O(2) stimuli. In freshly dissociated rabbit type I cells, we examined the effects of a PKG inhibitor, KT-5823, and an inhibitor of protein phosphatase 2A (PP2A), okadaic acid (OA), on K(+) and Ca(2+) currents. We also investigated the effects of these specific inhibitors on intracellular Ca(2+) concentration and carotid sinus nerve (CSN) activity under normoxic and hypoxic conditions. Voltage-dependent K(+) currents were depressed by hypoxia, and this effect was significantly reduced by 100 nM APIII. The effect of APIII on this current was reversed in the presence of either 1 microM KT-5823 or 100 nM OA. Likewise, these drugs retarded the depression of voltage-gated Ca(2+) currents induced by APIII. Furthermore, APIII depressed hypoxia-evoked elevations of intracellular Ca(2+), an effect that was also reversed by OA and KT-5823. Finally, CSN activity evoked by hypoxia was decreased in the presence of 100 nM APIII, and was partially restored when APIII was presented along with 100 nM OA. These results suggest that ANP initiates a cascade of events involving PKG and PP2A, which culminates in the dephosphorylation of K(+) and Ca(2+) channel proteins in the chemosensory type I cells.

Loss of expression of the beta subunit of soluble guanylyl cyclase prevents nitric oxide-mediated inhibition of DNA synthesis in smooth muscle cells of old rats

We compared the effects of NO donors and cGMP analogues on the growth of aortic smooth muscle cells (SMCs) derived from newborn, adult (aged 3 months), and old (aged 2 years) rats. We found that the NO donor S-nitroso-N-acetylpenicillamine failed to block DNA synthesis in SMCs from old rats but was effective in SMCs from newborn and adult rats. However, cGMP analogues were inhibitory in all 3 SMC types. We demonstrated that in SMCs from old rats, NO was unable to increase the concentration of intracellular cGMP, suggesting that either cGMP synthesis was defective or cGMP degradation was enhanced. Western blot analysis revealed that SMCs from old rats do not express the beta subunit of soluble guanylyl cyclase. To confirm the importance of this observation in vivo, we balloon-injured the carotid arteries of adult and old rats. Whereas soluble guanylyl cyclase was expressed at the same level in the media of injured vessels and uninjured vessels of both groups, its expression in the intimas of old rats was reduced by 70% compared with intimas from adult animals. Furthermore, N(omega)-nitro-L-arginine, an inhibitor of NO synthesis, enhanced the intimal thickening in injured vessels in adult rats but not in old rats. We conclude that the loss of NO responsiveness in aged rats is due to the lack of the beta subunit of soluble guanylyl cyclase, and we speculate that this defect contributes to the enhanced intimal thickening in response to injury in old animals.

Store-operated calcium entry in vascular endothelial cells is inhibited by cGMP via a protein kinase G-dependent mechanism

Store-operated Ca(2+) entry in vascular endothelial cells not only serves to refill the intracellular Ca(2+) stores, but also acts to stimulate the synthesis of nitric oxide, a key vasodilatory factor. In this study, we examined the role of cGMP in regulating the store-operated Ca(2+) entry in aortic endothelial cells. Cyclopiazonic acid (CPA) and thapsigargin, two selective inhibitors of endoplasmic reticulum Ca(2+)-ATPase, were used to induce store-operated Ca(2+) entry. 8-Bromo-cGMP, an activator of protein kinase G, inhibited the CPA- or thapsigargin-induced Ca(2+) entry in a concentration-dependent manner. An inhibitor of protein kinase G, KT5823 (1 microM) or H-8 (10 microM), abolished the inhibitory action of 8-bromo-cGMP and resumed Ca(2+) entry. Addition of S-nitroso-N-acetylpenicillamine (a nitric oxide donor) or dipyridamole (a cGMP phosphodiesterase inhibitor) during CPA treatment elevated cellular cGMP levels, stimulated protein kinase G activity, and at the same time reduced Ca(2+) influx due to CPA. Patch clamp study confirmed the existence of a CPA-activated Ca(2+)-permeable channel sensitive to cGMP inhibition. These results suggest that cGMP via a protein kinase G-dependent mechanism may play a key role in the regulation of the store-operated Ca(2+) entry in vascular endothelial cells.

Structure-activity relationship of staurosporine analogs in regulating expression of endothelial nitric-oxide synthase gene

In human umbilical vein endothelial cells and in human umbilical vein endothelial cell-derived EA.hy 926 cells, staurosporine (Stsp) and its glycosidic indolocarbazole analogs 7-hydroxystaurosporine (UCN-01) and 4'-N-benzoyl staurosporine (CGP 41251) enhanced nitric-oxide synthase (NOS) III mRNA expression (analyzed by RNase protection assay), protein expression (determined by Western blot), and activity [measured by rat fetal lung fibroblast (RFL-6) reporter cell assay] in a concentration- and time-dependent manner. In contrast, the bisindolylmaleimide analogs GF 109203X, Ro 31-8220 and Gö 6983 had no effect on NOS III expression, and Gö 6976, a methyl- and cyanoalkyl-substituted nonglycosidic indolocarbazole derivative of Stsp, even reduced NOS III expression in a concentration-dependent fashion. The up-regulation of NOS III expression by Stsp and analogs appears to be a transcriptional event because Stsp, 7-hydroxystaurosporine, and CGP 41251 enhanced the activity of a 1.6-kb human NOS III promoter fragment transiently transfected into EA.hy 926 endothelial cells. Stsp and analogs did not affect the stability of the NOS III mRNA. Stsp is known as a potent protein kinase (PK) inhibitor. Data obtained with other kinase inhibitors (and stimulators) indicated, however, that the effect of Stsp and analogs on NOS III expression was unrelated to the activities of PKC, PKA, PKG, or tyrosine kinase(s). Stsp analogs such as CGP 41251 also counteracted the NOS III mRNA-decreasing effect of tumor necrosis factor-alpha. These findings demonstrate that Stsp analogs represent a new class of compounds positively interacting with the transcription of the endothelial NOS III gene. Such compounds may prove useful in the prophylaxis and therapy of vascular disease.

Effects of selective protein kinase C inhibitors on the proteolytic down-regulation of L-selectin from chemoattractant-activated neutrophils

The signaling factors that direct the rapid shedding of L-selectin from neutrophils upon chemoattractant stimulation are poorly understood. Protein kinase C (PKC) has been implicated, yet previous studies have relied on the use of phorbol esters and nonselective kinase inhibitors. We treated neutrophils with various selective kinase inhibitors to evaluate their effects on N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced L-selectin shedding. We found that three selective inhibitors of PKC, structurally related to staurosporine, largely blocked both fMLP- and phorbol 12-myristate 13-acetate (PMA)-induced L-selectin shedding; however, these inhibitors did not affect fMLP-induced up-regulation of Mac-1 (CD11b/CD18) expression, which has been shown not to involve PKC. Other selective serine, threonine, and tyrosine kinase inhibitors were found not to block fMLP-induced L-selectin shedding. These findings provide more definitive evidence for the role of PKC in chemoattractant-induced L-selectin proteolysis. It is interesting that certain highly selective PKC inhibitors, not structurally related to staurosporine, were found to directly induce L-selectin shedding from neutrophils.

Membrane hyperpolarization removes inactivation of Ca2+ channels, leading to Ca2+ influx and subsequent initiation of sperm motility in the common carp

Change of osmolality surrounding spawned sperm from isotonic to hypotonic causes the initiation of sperm motility in the common carp. Here we show that membrane-permeable cAMP does not initiate motility of carp sperm that is quiescent in isotonic solution, and that motility of the demembranated sperm can be reactivated without cAMP. Furthermore, the cAMP level does not change during the initiation of sperm motility, and inhibitors of protein kinase do not affect sperm motility, suggesting that no cAMP-dependent system is necessary for the regulation of sperm motility. Sperm motility could not be initiated in Ca(2+)-free hypoosmotic solutions, and significant increase in the intracellular Ca(2+) level was observed by a Ca-sensitive fluorescence dye during hypoosmolality-induced active motion period. The demembranated sperm cells were fully reactivated in the solutions containing 10(-7) to 10(-5) M Ca(2+). Ca(2+) channel blockers such as verapamil and omega-conotoxin reversibly inhibited the initiation of sperm motility, suggesting that Ca(2+) influx is the prerequisite for the initiation of carp sperm motility. Motility of intact sperm was completely blocked; however, that of the demembranated sperm was not inhibited by the calmodulin inhibitor W7, suggesting that the calmodulin bound close to the plasma membrane participated in the initiation of sperm motility. Flow cytometric membrane potential measurements and spectrophotometric measurements by using fluorescence dyes showed transient membrane hyperpolarization on hypoosmolality-induced motility. This article discusses the role of membrane hyperpolarization on removal of inactivation of Ca(2+) channels, leading to Ca(2+) influx at the initiation of carp sperm motility.

Regulation of phenobarbital induction of the cytochrome P450 2b9/10 genes in primary mouse hepatocyte culture. Involvement of calcium- and cAMP-dependent pathways

Phenobarbital (PB) has long been known as an inducer of drug-metabolizing enzymes in liver, but the molecular mechanism underlying this induction is still poorly understood. Using primary mouse hepatocyte culture, we have investigated the possible involvement of different regulatory pathways in PB action, by exposing PB-treated cells to various protein kinase/phosphatase modulators. Our results showed a negative role of the cAMP-dependent pathway, as treatment with cAMP-dependent protein kinase (PKA) activators (10 microM dibutyryl-cAMP and 50 microM forskolin) dramatically inhibited PB-induced Cyp2b9/10 mRNA accumulation, whereas PKA inhibitor potentiated the PB responsiveness of this gene. The cGMP-dependent protein kinase (PKG) seems to play a positive role as PKG inhibitor reduced the PB-induced level of Cyp2b9/10 mRNA. We also obtained two lines of evidence for the involvement of Ca2+ in modulating PB action. Firstly, measurements of intracellular Fura-2 fluorescence ratio in murine hepatocytes showed that long-term PB incubation (24 and 48 h) led to a significant increase of [Ca2+]i. Secondly, treatment with an intracellular Ca2+ chelator (BAPTA-AM) nearly completely abolished PB-induced Cyp2b9/10 expression. Ca2+ thus appeared to mediate PB action likely via Ca2+/calmodulin-dependent protein kinase II, as KN62, a specific inhibitor of this enzyme, also dramatically inhibited PB induction of the Cyp2b9/10 genes.

Protein kinase and phosphatase modulation of quail brain GABA(A) and non-NMDA receptors co-expressed in Xenopus oocytes

The GABA(A) receptor and the non-NMDA subtype of the ionotropic glutamate receptor were co-expressed in Xenopus oocytes by injection of quail brain mRNA. The oocytes were treated with various protein kinase (PK) and protein phosphatase (PP) activators and inhibitors and the effects on receptor functioning were monitored. Two phorbol esters, 4-beta-phorbol 12-myristate-13-acetate (PMA) and 4-beta-phorbol 12,13-dibutyrate (PDBu); the cGMP-dependent PK activators sodium nitroprusside (SNP) and S-nitrosoglutathione (SNOG); and the PP inhibitor okadaic acid (OA) reduced the amplitude of the GABA-induced currents, whilst the PK inhibitor staurosporine potentiated it. In addition, PMA, PDBu, SNP, and OA reduced the desensitization of the GABA-induced response. Identical treatments generally had similar but less pronounced effects on responses generated by kainate (KA) but the desensitization characteristic of the non-NMDA receptor was not affected. None of the treatments had any effect on the reversal potentials of the induced currents. Immunoblots revealed that the oocytes express endogenous PKG and guanylate cyclase. The results are discussed in terms of the molecular structures of GABA(A) and non-NMDA receptors and the potential functional consequences of phosphorylation/dephosphorylation.

Tumor necrosis factor-alpha-induced activating protein-1 activity is modulated by nitric oxide-mediated protein kinase G activation

We tested the hypothesis that protein kinase (PK)G activation in response to nitric oxide ((*)NO) mediates tumor necrosis factor (TNF)-alpha-induced activation of the transcription factor activating protein-1 (AP-1) in pulmonary microvessel endothelial monolayers (PEM). The DNA-binding activity of AP-1 was assessed using the electrophoretic mobility shift assay. TNF treatment (1,000 U/ml) for 4 h induced a significant increase in DNA binding of AP-1. The effects of TNF were prevented by the superoxide radical scavenger superoxide dismutase (SOD) (100 U/ml), the (*)NO synthase inhibitor aminoguanidine (100 microM), the guanylate cyclase inhibitor ODQ (100 microM), and the PKG inhibitors KT5823 (1 microM) and 8-bromo-cyclic guanosine monophosphate (cGMP)-thioate (100 microM). Spermine-NO (1 microM) and L-arginine (400 microM) prevented the aminoguanidine-induced ablation of AP-1 activation in response to TNF. Phosphorylation of H-Arg-Lys-Ile-Ser-Ala-Ser-Glu-Phe-Asp-Arg-Pro-Leu-Arg-OH (BPDEtide), a specific substrate for PKG, measured the activity of cGMP-dependent protein kinase (PKG). TNF for 0.5 h induced an increase in PKG activity that was prevented by aminoguanidine, ODQ, KT5823, and 8-bromo-cGMP-thioate; however, SOD had no effect. The PKG agonist 8-bromo-cGMP (100 microM), when given alone, increased PKG activity but induced significant DNA-binding activity of AP-1 only when given in the ODQ + TNF Group. SIN-1 (1 mM, a peroxynitrite agonist) increased DNA-binding activity of AP-1. SOD prevented SIN-1-induced AP-1 activation, a response similar to that of the SOD + TNF Group. PEM were transfected with the chloramphenicol acetyltransferase (CAT) reporter plasmid pBLCAT2, which contains a regulation sequence responsive to AP-1. The pharmacologic profile of TNF-induced CAT activity was identical to TNF-induced DNA binding by AP-1. Thus, TNF-induced AP-1-dependent gene transcription is modulated by (*)NO-dependent mediated activation of PKG.

Shear-induced increase in hydraulic conductivity in endothelial cells is mediated by a nitric oxide-dependent mechanism

This study addresses the role of nitric oxide (NO) and its downstream mechanism in mediating the shear-induced increase in hydraulic conductivity (L(p)) of bovine aortic endothelial cell monolayers grown on porous polycarbonate filters. Direct exposure of endothelial monolayers to 20-dyne/cm(2) shear stress induced a 4. 70+/-0.20-fold increase in L(p) at the end of 3 hours. Shear stress (20 dyne/cm(2)) also elicited a multiphasic NO production pattern in which a rapid initial production was followed by a less rapid, sustained production. In the absence of shear stress, an exogenous NO donor, S-nitroso-N-acetylpenicillamine, increased endothelial L(p) 2.23+/-0.14-fold (100 micromol/L) and 4.8+/-0.66-fold (500 micromol/L) at the end of 3 hours. In separate experiments, bovine aortic endothelial cells exposed to NO synthase inhibitors, N(G)-monomethyl-L-arginine and N(G)-nitro-L-arginine methyl ester, exhibited significant attenuation of shear-induced increase in L(p) in a dose-dependent manner. Inhibition of guanylate cyclase (GC) with LY-83,583 (1 micromol/L) or protein kinase G (PKG) with KT5823 (1 micromol/L) failed to attenuate the shear-induced increase in L(p). Furthermore, direct addition of a stable cGMP analogue, 8-bromo-cGMP, had no effect in altering baseline L(p), indicating that the GC/cGMP/PKG pathway is not involved in shear stress-NO-L(p) response. Incubation with iodoacetate (IAA), a putative inhibitor of glycolysis, dose-dependently increased L(p). Addition of IAA at levels that did not affect baseline L(p) greatly potentiated the response of L(p) to 20-dyne/cm(2) shear stress. Finally, both shear stress-induced and IAA-induced increases in L(p) could be reversed with the addition of dibutyryl cAMP. However, additional metabolic inhibitors, 2 deoxyglucose (10 mmol/L) and oligomycin (1 micromol/L), or reactive oxygen species scavengers, deferoxamine (1 mmol/L) and ascorbate (10 mmol/L), failed to alter shear-induced increases in L(p). Our results show that neither the NO/cGMP/PKG pathway nor a metabolic pathway mediates the shear stress-L(p) response. An alternate mechanism downstream from NO that is sensitive to IAA must mediate this response.

Odorants induce the phosphorylation of the cAMP response element binding protein in olfactory receptor neurons

Although odorants are known to activate olfactory receptor neurons through cAMP, the long-term effects of odorant detection are not known. Our recent findings indicate that there is also a delayed and sustained cAMP response, with kinetics sufficient to mediate long-term cellular responses. This cAMP response is mediated by cGMP through activation of adenylyl cyclase by protein kinase G (PKG). Therefore, we investigated the ability of odorants to regulate gene expression in rat olfactory epithelium. The cAMP-responsive binding protein (CREB) is a well-characterized transcription factor regulated by cAMP. We examined CREB activity in rat olfactory epithelium and olfactory receptor neurons (ORNs) after stimulation with odorants. Odorants increased levels of phosphorylated CREB in olfactory epithelium in vivo, and this increase was localized to ORNs in vitro. Incubation with 8-bromo-cGMP or sodium nitroprusside, a guanylyl cyclase activator, also increased phosphorylated CREB. In vitro, cAMP-dependent protein kinase phosphorylated CREB. In contrast, PKG failed to phosphorylate CREB directly in vitro. Our results demonstrate that the delayed odorant-induced cAMP signal activates CREB, which in turn may modulate gene expression in ORNs. In addition, cGMP indirectly affects CREB activation. This effect of cGMP on CREB activity through cAMP provides another mechanism for the modulation of CREB.

Inhibition of spinal protein kinase C reduces nerve injury-induced tactile allodynia in neuropathic rats

We investigated the effects of inhibiting spinal protein kinases including PKC, PKA and PKG on tactile allodynia in rats with a unilateral tight ligation on L5/L6 spinal nerves (Chung model). The intrathecal (IT) delivery of GF109203X, a PKC inhibitor, produced a potent and long lasting anti-allodynic effect. The effect was dose-dependent and stereospecific. Bisindolymaleimide V, an inactive homologue of GF, had no effect. Additionally, two other PKC inhibitors, PKC19-31 and chelerythrine, displayed significant anti-allodynic action. Spinal PKA, but not PKG, is likely involved in Chung tactile allodynia, since H89 (a PKA inhibitor) showed anti-allodynic activity, while KT5823 (a PKG inhibitor) had only a minor effect. These data emphasize that spinal PKC plays an important role in nerve injury-induced tactile allodynia. Other protein kinases such as PKA may also contribute to this phenomenon.

Nitric oxide signaling contributes to late-phase LTP and CREB phosphorylation in the hippocampus

Long-term potentiation (LTP) in the hippocampus has an early phase (E-LTP) that can be induced by one- or two-train tetanization, lasts approximately 1 hr, and is cAMP-dependent protein kinase (PKA) and protein synthesis independent and a late phase (L-LTP) that can be induced by three- or four-train tetanization, lasts >3 hr, and is reduced by inhibitors of PKA and of protein or RNA synthesis. Nitric oxide (NO) is thought to be involved in E-LTP, but until now there has been no information about the role of the NO-signaling pathway in L-LTP. We examined this question at the Schaffer collateral-CA1 synapses in slices of mouse hippocampus. An inhibitor of NO synthase blocked L-LTP induced by three-train tetanization and reduced L-LTP induced by four-train tetanization, whereas an inhibitor of PKA was more effective in blocking four-train L-LTP than three-train L-LTP. Three-train L-LTP was also blocked by inhibitors of guanylyl cyclase or cGMP-dependent protein kinase (PKG). Conversely, either NO or cGMP analogs paired with one-train tetanization produced late-phase potentiation, and the cGMP-induced potentiation was blocked by inhibitors of protein or RNA synthesis and an inhibitor of PKG, but not by an inhibitor of PKA. To test a possible downstream target of PKG, we examined changes in phospho-CRE-binding protein (phospho-CREB) immunofluorescence in the CA1 cell body area and obtained results similar to those of the electrophysiology experiments. These results suggest that NO contributes to L-LTP by stimulating guanylyl cyclase and cGMP-dependent protein kinase, which acts in parallel with PKA to increase phosphorylation of the transcription factor CREB.

Guanosine 5',3'-cyclic monophosphate enhances lipopolysaccharide-induced nitric oxide synthase expression in mixed glial cell cultures of rat

Primary mixed glial cell cultures treated with lipopolysaccharide (LPS; 1.0 microg/ml) showed biphasic increases of inducible nitric oxide synthase (iNOS) mRNA expression 6 h and 24-36 h after LPS treatment. Dibutyryl-guanosine 5',3'-cyclic monophosphate (db-cGMP; 1.0 mM) enhanced the second phase of the LPS-induced iNOS expression 24 and 30 h after LPS stimulation. KT5823 (1.0 microM), a protein kinase G (PKG) inhibitor, inhibited the LPS-induced iNOS expressions at 24 and 30 h and their enhancements caused by db-cGMP. In astrocyte-enriched cultures with reduced microglial contamination, the LPS-induced iNOS expression was decreased, though slightly enhanced by db-cGMP. These results suggest that cGMP/PKG signaling may be involved in the second phase of the LPS-induced glial iNOS expression and its upregulation, which are apparent in the presence of microglial cells.

Immunohistochemical evidence for the NO cGMP signaling pathway in respiratory ciliated epithelia of rat

Airway epithelia play a crucial role in protecting the lung from the external environment. Ciliated airway epithelial cells contribute to mucociliary transport systems via ciliary beating and electrolyte transport mechanisms to defend against respiratory tract infection. Both of these activities are regulated by nitric oxide (NO)-dependent mechanisms. To better understand the role of the NO-cGMP signal transduction cascade in these responses, we investigated the localization of endothelial nitric oxide synthase (eNOS), soluble guanylyl cyclase (sGC), cGMP-dependent protein kinase (PKG) I-alpha, and PKG I-beta in the tracheas and lungs of normal rats by immunohistochemistry. Mouse anti-eNOS, rabbit anti-sGC, PKG I-alpha, and PKG I-beta antibodies were used. Strong immunostaining for eNOS was detected in ciliated tracheal, bronchial, and bronchiolar epithelia, in Clara cells, and in Type II alveolar cells. The pattern of sGC and PKG I-beta immunostaining showed striking parallels with that of eNOS staining. No staining was detectable in ciliated epithelium with the anti-PKG I-alpha antibody. Taken together, these observations suggest that PKG I-beta might transduce NO-sGC signaling into biological responses in ciliated respiratory epithelia.(J Histochem Cytochem 47:1369-1374, 1999)

Nitric oxide signaling pathway mediates the L-arginine-induced cardiovascular effects in the nucleus tractus solitarii of rats

We have previously demonstrated that L-arginine produces profound cardiovascular effects when microinjected into the nucleus tractus solitarii (NTS) of the rat. The present study extended our earlier work and examined further the underlying mechanisms of action of L-arginine in the NTS. Our results showed that intra-NTS microinjection of L-arginine (0.1-10 nmol) elicited dose-dependent depressor and bradycardic effects that were not significantly evoked by equivalent doses of D-arginine. The effects of L-arginine were blocked by pre-injection of 7-nitroindazole (0.02-1 nmol), a neuronal nitric oxide synthase inhibitor. Additionally, application of the calmodulin inhibitor W-7 (0.01-0.33 nmol) reduced cardiovascular responses to L-arginine (10 nmol) in a dose-dependent manner. Pre-injections of soluble guanylyl cyclase inhibitors, LY83583 (0.01-0.33 nmol) and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 0.03-1 pmol) both suppressed the L-arginine-induced depressor and bradycardic effects. Finally, the cardiovascular effects of L-arginine in the NTS were attenuated by HA1004 (0.1-1 nmol), a cGMP-dependent protein kinase inhibitor, but not by the protein kinase C inhibitor H-7 (1 nmol). Taken together, the results indicate that the cardiovascular effects produced by L-arginine in the NTS are inhibited by pharmacological interventions that block nitric oxide production and cGMP-PKG signaling pathway within the nucleus.

NO induces a cGMP-independent release of cytochrome c from mitochondria which precedes caspase 3 activation in insulin producing RINm5F cells

Exposure of RINm5F cells to interleukin-1beta and to several chemical NO donors such as sodium nitroprusside (SNP), SIN-1 and SNAP induce apoptotic events such as the release of cytochrome c from mitochondria, caspase 3 activation, Bcl-2 downregulation and DNA fragmentation. SNP exposure led to transient activation of soluble guanylate cyclase (sGC) and prolonged protein kinase G (PKG) activation but apoptotic events were not attenuated by inhibition of the sGC/PKG pathway. Prolonged activation of the cGMP pathway by exposing cells to the dibutyryl analogue of cGMP for 12 h induced both apoptosis and necrosis, a response that was abolished by the PKG inhibitor KT5823. These results suggest that NO-induced apoptosis in the pancreatic beta-cell line is independent of acute activation of the cGMP pathway.

Chemically induced, activity-independent LTD elicited by simultaneous activation of PKG and inhibition of PKA

Although it is widely agreed that cyclic AMP is necessary for the full expression of long-term potentiation of synaptic strength, it is unclear whether cyclic AMP or cyclic AMP-dependent protein kinase (PKA) play roles in the induction of long-term depression (LTD). We show here that two PKA inhibitors, H-89 (10 microM) and KT5720 (1 microM), are unable to block induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices in vitro. Rather, H-89 enhanced the magnitude of LTD induced by submaximal low-frequency stimulation. Raising [cGMP] with zaprinast (20 microM), a selective type V phosphodiesterase inhibitor, reversibly depressed synaptic potentials. However, coapplication of H-89 plus zaprinast converted this to a robust LTD that depended critically on activation of cyclic GMP-dependent protein kinase (PKG). Chemically induced LTD is activity-independent because it could be induced without stimulation and in tetrodotoxin (0.5 microM). Additionally, chemical LTD did not require activation of N-methyl-D-aspartate or GABA receptors and could be reversed by LTP. Stimulus-induced LTD occluded chemical LTD, suggesting a common expression mechanism. In contrast to bath application, postsynaptic infusion of H-89 into CA1 pyramidal neurons did not enhance LTD, suggesting a presynaptic site of action. Further evidence for a presynaptic locus was supplied by experiments where H-89 applied postsynaptically along with bath application of zaprinast was unable to produce chemical LTD. Thus simultaneous presynaptic generation of cyclic GMP and inhibition of PKA is sufficient to induce LTD of synaptic transmission at Schaffer collateral-CA1 synapses.

Role of protein kinase G in nitric oxide- and cGMP-induced relaxation of newborn ovine pulmonary veins

In a variety of systemic blood vessels, protein kinase G (PKG) plays a critical role in mediating relaxation induced by agents that elevate cGMP, such as nitric oxide. The role of PKG in nitric oxide- and cGMP-induced relaxation is less certain in the pulmonary circulation. In the present study, we examined the effects of inhibitors of PKG on the responses of isolated fourth-generation pulmonary veins of newborn lambs (10 +/- 1 days of age) to nitric oxide and cGMP. In vessels preconstricted with endothelin-1, nitric oxide and 8-bromo-cGMP (a cell-membrane-permeable cGMP analog) induced concentration-dependent relaxation. The relaxation was significantly attenuated by beta-phenyl-1, N(2)-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothionate (Rp-8-Br-PET-cGMPS; a PKG inhibitor) and N-[2-(methylamino)ethyl]5-isoquinolinesulfonamide [H-8; an inhibitor of PKG and protein kinase A (PKA)] but was not affected by KT-5720 (a PKA inhibitor). Biochemical study showed that PKG activity in newborn ovine pulmonary veins was inhibited by 8-Br-PET-cGMPS and H-8 but not by KT-5720. PKA activity was not affected by 8-Br-PET-cGMPS but was inhibited by H-8 and KT-5720. These results suggest that PKG is involved in relaxation of pulmonary veins of newborn lambs induced by nitric oxide and cGMP.