Involvement of phosphodiesterase-cGMP-PKG pathway in intracellular Ca2+ oscillations in pituitary GH3 cells

TRH Regulates the Synthesis and Secretion of Prolactin in Rats with Adenohypophysis through the Differential Expression of miR-126a-5p

Prolactin (PRL) is an important hormone that is secreted by the pituitary gland and plays an important role in the growth, development and reproduction of organisms. Thyrotropin-releasing hormone (TRH) is a common prolactin-releasing factor that regulates the synthesis and secretion of prolactin. In recent studies, microRNAs (miRNAs) have been found to play a key role in the regulation of pituitary hormones. However, there is a lack of systematic studies on the regulatory role that TRH plays on the pituitary transcriptome, and the role of miRNAs in the regulation of PRL synthesis and secretion by TRH lacks experimental evidence. In this study, we first investigated the changes in PRL synthesis and secretion in the rat pituitary gland after TRH administration. The results of transcriptomic analysis after TRH treatment showed that 102 genes, including those that encode Nppc, Fgf1, PRL, Cd63, Npw, and Il23a, were upregulated, and 488 genes, including those that encode Lats1, Cacna2d1, Top2a, and Tfap2a, were downregulated. These genes are all involved in the regulation of prolactin expression. The gene expression of miR-126a-5p, which regulates the level of PRL in the pituitary gland, was screened by analysis prediction software and by a dual luciferase reporter system. The data presented in this study demonstrate that TRH can regulate prolactin synthesis and secretion through miR-126a-5p, thereby improving our understanding of the molecular mechanism of TRH-mediated PRL secretion and providing a theoretical basis for the role of miRNAs in regulating the secretion of pituitary hormones.

The changing landscape of voltage-gated calcium channels in neurovascular disorders and in neurodegenerative diseases

It is a common belief that voltage-gated calcium channels (VGCC) cannot carry toxic amounts of Ca²⁺ in neurons. Also, some of them as L-type channels are essential for Ca²⁺-dependent regulation of prosurvival gene-programs. However, a wealth of data show a beneficial effect of drugs acting on VGCCs in several neurodegenerative and neurovascular diseases. In the present review, we explore several mechanisms by which the “harmless” VGCCs may become “toxic” for neurons. These mechanisms could explain how, though usually required for neuronal survival, VGCCs may take part in neurodegeneration. We will present evidence showing that VGCCs can carry toxic Ca²⁺ when: a) their density or activity increases because of aging, chronic hypoxia or exposure to β-amyloid peptides or b) Ca²⁺-dependent action potentials carry high Ca²⁺ loads in pacemaker neurons. Besides, we will examine conditions in which VGCCs promote neuronal cell death without carrying excess Ca²⁺. This can happen, for instance, when they carry metal ions into the neuronal cytoplasm or when a pathological decrease in their activity weakens Ca²⁺-dependent prosurvival gene programs. Finally, we will explore the role of VGCCs in the control of nonneuronal cells that take part to neurodegeneration like those of the neurovascular unit or of microglia.

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

Involvement of the nitric oxide/protein kinase G pathway in polychlorinated biphenyl-induced cell death in SH-SY 5Y neuroblastoma cells

Polychlorinated biphenyls (PCB) are persistent environmental contaminants whose chronic exposure can affect nervous system development and function. The cellular and molecular mechanisms underlying neuronal damage are not yet clear. In the present study, we investigated whether nitric oxide (NO) could be involved in aroclor 1254 (A1254; a PCB mixture)-induced cytotoxicity in SH-SY5Y human neuroblastoma cells. Prolonged exposure (24 hr) to A1254 (10-100 microg/ml) caused a dose-dependent reduction of cell viability that was attenuated in the presence of a calcium entry blocker, gadolinum (Gd(3+)) at 10 microM, a concentration able to block voltage-sensitive calcium channels. In addition, A1254 caused an increase of cytosolic calcium that was dependent on extracellular calcium, as measured by fura-2 videomicroscopy. A1254-induced calcium rise may stimulate NO production through an activation of neuronal NOS (nNOS). Indeed, the concomitant addition of the selective nNOS inhibitor N(omega)-propyl-L-arginine (NPLA) and A1254 prevented cell injury, suggesting that NO production plays a major role in A1254-evoked cell injury. Furthermore, the exposure (14 hr) to A1254 (30 microg/ml) produced an up-regulation of the expression of beta isoform of nNOS. This up-regulation was calcium dependent and was accompanied by an enhancement of NO production as demonstrated by an increase of nitrite formation. Moreover, A1254-induced cell injury was prevented when KT 5823, a selective cGMP/PKG inhibitor, was added concomitantly to 30 microg/ml A1254. These results suggest that PCB-induced cell death in neuroblastoma cells is mediated by an activation of the cGMP/PKG pathway triggered by NO production.

BAY 41‐2272, a potent activator of soluble guanylyl cyclase, stimulates calcium elevation and calcium‐activated potassium current in pituitary GH3cells

The effects of BAY 41-2272, a nitric oxide-independent activator of soluble guanylyl cyclase, on Ca2+ signalling and ion currents were investigated in pituitary GH3 cells. Intracellular Ca2+ concentrations ([Ca2+]i) in these cells were increased by BAY 41-2272. Removing extracellular Ca2+ abolished the BAY 41-2272-induced increase in [Ca2+]i. After [Ca2+]i was elevated by BAY 41-2272 (300 nmol/L), subsequent application of 1-benzyl-3-(5'-hydroxymethyl-2'-furyl) indazole (YC-1; 1 micromol/L) did not increase [Ca2+]i further. In whole-cell recordings, BAY 41-2272 reversibly stimulated Ca2+-activated K+ current (I(K(Ca))) with an EC50 of 225 +/- 8 nmol/L. At 3 micromol/L, BAY 41-2272 slightly and significantly decreased L-type Ca2+ current. In the cell-attached configuration, BAY 41-2272 (300 nmol/L) enhanced the activity of large-conductance Ca2+-activated K+ (BK(Ca)) channels. After BK(Ca) channel activity was stimulated by spermine NONOate (30 micromol/L) or YC-1 (10 micromol/L) in cell-attached patches, subsequent application of BAY 41-2272 (300 nmol/L) further increased the channel open probability. In the inside-out configuration, BAY 41-2272 applied to the intracellular surface of excised patches enhanced BK(Ca) channel activity. Unlike 1 micromol/L paxilline, 1H-[1,2,4]oxadiazolol-[4,3a] quinoxalin-1-one (ODQ; 10 micromol/L) or heme (10 micromol/L) had no effect on BAY 41-2272-stimulated channel activity. BAY 41-2272 caused no shift in the activation curve of BK(Ca) channels; however, it did increase the Ca2+ sensitivity of these channels. At 300 nmol/L, BAY 41-2272 reduced the firing rate of spontaneous action potentials stimulated by thyrotropin-releasing hormone (10 micromol/L). The BK(Ca) channel activity was also enhanced by 300 nmol/L BAY 41-2272 in neuroblastoma IMR-32 cells. Therefore, the BAY 41-2272-induced increase in [Ca2+]i is primarily explained by an increase in Ca2+ influx. The BAY 41-2272-mediated simulation of IK(Ca) may result from direct activation of BKCa channels and indirectly as a result of elevated [Ca2+]i.

Reciprocal regulation and integration of signaling by intracellular calcium and cyclic GMP

Calcium and guanosine-3',5'-cyclic monophosphate (cGMP) are second messenger molecules that regulate opposing physiological functions, reflected in the reciprocal regulation of their intracellular concentrations, in many systems. Indeed, cGMP and Ca2+ constitute discrete points of integration between multiple cell signaling cascades in both convergent and parallel pathways. This chapter describes the molecular mechanisms regulating intracellular Ca2+ and cGMP, and their integration in specific cellular responses.

Activation of endogenous nitric oxide synthase coupled with methacholine-induced exocytosis in rat parotid acinar cells

Methacholine (MCh) interacted with M(3) muscarinic receptors in rat parotid tissue slices and induced amylase secretion. MCh- and calcimycin-induced exocytosis was completely inhibited by N-[2-(N-(4-chlorocinnamyl)-N-methylaminomethyl)phenyl]-N-[2-hydroxyethyl]-4-methoxybenzenesulfonamide, N(G)-nitro-L-arginine methylester (L-NAME), 1H-(1,2,4)-oxadiazolo[4,3-a]quinoxaline-1-one, and 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, suggesting that activations of calmodulin (CaM) kinase II, nitric oxide synthase (NOS), and cGMP-dependent protein kinase (PKG) were coupled with the exocytosis. These suggestions were supported by the results that exposure of the slices to MCh induced a rapid increase in these enzyme activities. Western blot analysis showed that neuronal NOS (nNOS) was expressed in isolated parotid acinar cells of rats. To measure nitric oxide (NO) production in response to the stimulation with MCh in real time, the isolated parotid acinar cells had been preloaded with 4,5-diaminofluorescein diacetate and incubated with the agonist. MCh (1 microM) induced a fast increase in 4,5-diaminofluorescein fluorescence, corresponding to an increase in the NO synthesis in the presence of extracellular Ca(2+) but not in the absence of it. When the isolated parotid acinar cells preloaded with L-NAME or 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethylester) were treated simultaneously with MCh, the increase in the fluorescence also was not observed. The MCh-induced increase in the fluorescence was not observed in the cells incubated in the absence of extracellular calcium, showing the importance of Ca(2+) entry from extracellular sites for MCh-induced NOS activation. These results indicate that nNOS is endogenously present in rat parotid acinar cells and that the rapid activation of this enzyme together with those of CaM kinase II and PKG contributes to MCh-induced amylase secretion.

Vinpocetine-induced stimulation of calcium-activated potassium currents in rat pituitary GH3 cells

The effects of vinpocetine, an inhibitor of cyclic GMP phosphodiesterase, on ionic currents were examined in rat pituitary GH3 lactotrophs with the aid of the patch-clamp technique. In GH3 cells bathed in normal Tyrode's solution, vinpocetine (10 microM) reversibly increased the amplitude of Ca2+-activated K+ current (I(K)Ca) with an EC50 value of 4 microM. When the recording pipettes were filled with 10 mM EGTA, vinpocetine also stimulated I(K)Ca. In the cell-attached configuration, application of vinpocetine to the bath increased the activity of large-conductance Ca2+-activated K+ (BK(Ca)) channels. In excised membrane patches, application of vinpocetine (10 microM) to the bath did not change the single-channel conductance of BK(Ca) channels; however, it did increase channel activity. In the inside-out configuration, neither 8-bromo cyclic GMP nor YC-1 applied intracellularly affected BK(Ca) channel activity. The vinpocetine-induced change in the kinetic behavior of BK(Ca) channels was due to an increase in mean open time and a decrease in mean closed time. Vinpocetine (10 microM) caused a leftward shift in the midpoint for the voltage-dependent opening. Under the current-clamp mode, vinpocetine (10 microM) decreased the firing rate of spontaneous action potentials induced by thyrotropin-releasing hormone (10 microM) in GH3 cells. In pheochromocytoma PC12 cells, vinpocetine (10 microM) applied intracellularly also enhanced the activity of BK(Ca) channels without altering single-channel conductance. Thus, the present study suggests that vinpocetine-mediated stimulation of I(K)Ca may result from the direct activation of BK(Ca) channels and indirectly from elevated cytosolic Ca2+.

Histamine induces exocytosis and IL-6 production from human lung macrophages through interaction with H1 receptors

Increasing evidence suggests that a continuous release of histamine from mast cells occurs in the airways of asthmatic patients and that histamine may modulate functions of other inflammatory cells such as macrophages. In the present study histamine (10(-9)-10(-6) M) increased in a concentration-dependent fashion the basal release of beta-glucuronidase (EC(50) = 8.2 +/- 3.5 x 10(-9) M) and IL-6 (EC(50) = 9.3 +/- 2.9 x 10(-8) M) from human lung macrophages. Enhancement of beta-glucuronidase release induced by histamine was evident after 30 min and peaked at 90 min, whereas that of IL-6 required 2-6 h of incubation. These effects were reproduced by the H(1) agonist (6-[2-(4-imidazolyl)ethylamino]-N-(4-trifluoromethylphenyl)heptane carboxamide but not by the H(2) agonist dimaprit. Furthermore, histamine induced a concentration-dependent increase of intracellular Ca(2+) concentrations ([Ca(2+)](i)) that followed three types of response, one characterized by a rapid increase, a second in which [Ca(2+)](i) displays a slow but progressive increase, and a third characterized by an oscillatory pattern. Histamine-induced beta-glucuronidase and IL-6 release and [Ca(2+)](i) elevation were inhibited by the selective H(1) antagonist fexofenadine (10(-7)-10(-4) M), but not by the H(2) antagonist ranitidine. Inhibition of histamine-induced beta-glucuronidase and IL-6 release by fexofenadine was concentration dependent and displayed the characteristics of a competitive antagonism (K(d) = 89 nM). These data demonstrate that histamine induces exocytosis and IL-6 production from human macrophages by activating H(1) receptor and by increasing [Ca(2+)](i) and they suggest that histamine may play a relevant role in the long-term sustainment of allergic inflammation in the airways.

Dependence of soluble guanylyl cyclase activity on calcium signaling in pituitary cells

The role of nitric oxide (NO) in the stimulation of soluble guanylyl cyclase (sGC) is well established, but the mechanism by which the enzyme is inactivated during the prolonged NO stimulation has not been characterized. In this paper we studied the interactions between NO and intracellular Ca(2+) in the control of sGC in rat anterior pituitary cells. Experiments were done in cultured cells, which expressed neuronal and endothelial NO synthases, and in cells with elevated NO levels induced by the expression of inducible NO synthase and by the addition of several NO donors. Basal sGC-dependent cGMP production was stimulated by the increase in NO levels in a time-dependent manner. In contrast, depolarization of cells by high K(+) and Bay K 8644, an L-type Ca(2+) channel agonist, inhibited sGC activity. Depolarization-induced down-regulation of sGC activity was also observed in cells with inhibited cGMP-dependent phosphodiesterases but not in cells bathed in Ca(2+)-deficient medium. This inhibition was independent from the pattern of Ca(2+) signaling (oscillatory versus nonoscillatory) and NO levels, and was determined by averaged concentration of intracellular Ca(2+). These results indicate that inactivation of sGC by intracellular Ca(2+) serves as a negative feedback to break the stimulatory action of NO on enzyme activity in intact pituitary cells.

Pharmacological blockade of ERG K(+) channels and Ca(2+) influx through store-operated channels exerts opposite effects on intracellular Ca(2+) oscillations in pituitary GH(3) cells

In the present study, the effects on intracellular calcium concentration ([Ca(2+)](i)) oscillations of the blockade of ether-a-go-go-related gene (ERG) K(+) channels and of Ca(2+) influx through store-operated channels (SOC) activated by [Ca(2+)](i) store depletion have been studied in GH(3) cells by means of a combination of single-cell fura-2 microfluorimetry and whole-cell mode of the patch-clamp technique. Nanomolar concentrations (1-30 nM) of the piperidinic second-generation antihistamines terfenadine and astemizole and of the class III antiarrhythmic methanesulfonanilide dofetilide, by blocking ERG K(+) channels, increased the frequency and the amplitude of [Ca(2+)](i) oscillations in resting oscillating GH(3) cells. These compounds also induced the appearance of an oscillatory pattern of [Ca(2+)](i) in a subpopulation of nonoscillating GH(3) cells. The effects of ERG K(+) channel blockade on [Ca(2+)](i) oscillations appeared to be due to the activation of L-type Ca(2+) channels, because they were prevented by 300 nM nimodipine. By contrast, the piperazinic second-generation antihistamine cetirizine (0.01-30 microM), which served as a negative control, failed to affect ERG K(+) channels and did not interfere with [Ca(2+)](i) oscillations in GH(3) cells. Interestingly, micromolar concentrations of terfenadine and astemizole (0.3-30 microM), but not of dofetilide (10-100 microM), produced an inhibition of the spontaneous oscillatory pattern of [Ca(2+)](i) changes. This effect was possibly related to an inhibition of SOC, because these compounds inhibited the increase of [Ca(2+)](i) achieved by extracellular calcium reintroduction after intracellular calcium store depletion with the sarcoplasmic or endoplasmic reticulum calcium ATPase pump inhibitor thapsigargin (10 microM) in an extracellular calcium-free medium. The same inhibitory effect on [Ca(2+)](i) oscillations and SOC was observed with the first-generation antihistamine hydroxyzine (1-30 microM), the more hydrophobic metabolic precursor of cetirizine. Collectively, the results of the present study obtained with compounds that interfere in a different concentration range with ERG K(+) channels or SOC suggest that 1) ERG K(+) channels play a relevant role in controlling the oscillatory pattern of [Ca(2+)](i) in resting GH(3) cells and 2) the inhibition of SOC might induce an opposite effect, i.e., an inhibition of [Ca(2+)](i) oscillations.

Nitric oxide modulates stretch activation of mitogen-activated protein kinases in mesangial cells

BACKGROUND

In vivo, intraglomerular hypertension results in resident cell hypertrophy, proliferation and matrix protein production, leading to glomerulosclerosis. Mesangial cells (MCs) exposed to in vitro stretch also proliferate and produce matrix. We have shown activation of Jun N-terminal kinase/stress-activated protein kinase (SAPK) and p42/44 mitogen-activated protein kinase (MAPK) in stretched MCs and have also demonstrated that L-arginine decreases resident cell proliferation and protects against glomerulosclerosis in remnant kidney glomeruli, presumably by increasing nitric oxide (NO) production. Consequently, we studied whether NO could affect SAPK and p42/44 MAPK activation in stretched MCs.

METHODS

MCs (passages 5 to 10) cultured on type 1 collagen-coated, flexible-bottom plates were exposed to 0 to 30 minutes of cyclic strain (60 cycles per minute) by computer-driven generation of vacuum of -27 kPa, inducing 28% elongation in the diameter of the surface. Control MCs were grown on coated, flexible-bottom plates. Protein levels (by Western blot) and activity assays for SAPK/JNK and p42/44 MAPK were performed under these conditions. As maximal activation was at 10 minutes, with decay by 30 minutes, the effect of NO on kinase activation was studied at 0, 2, 5, and 10 minutes by preincubation with 70 micromol/L s-nitroso-n-acetylpenicillamine (SNAP; an NO donor) or 1 mmol/L 8-bromo cyclic guanosine monophosphate (8-bromo-cGMP). Downstream events in response to stretch and NO were studied at the time of maximal response (10 minutes) by examining nuclear translocation of SAPK with immunofluorescence microscopy and transcription factor activator protein-1 nuclear protein binding by gel mobility shift assay. The effect of kinase inhibition by NO donors on MC proliferation was studied by Western blotting for proliferating cell nuclear antigen (PCNA).

RESULTS

Cyclic MC stretch led to prompt SAPK and p42/44 MAPK activation, which was maximal at 10 minutes. Preincubation with either SNAP or 8-bromo-cGMP decreased this by 50 and 70%, respectively (N = 4), suggesting that the effect of NO was through cGMP generation. Nuclear translocation of both phosphorylated kinases was seen after 10 minutes of stretch and was largely prevented by 8-bromo-cGMP. Increased DNA binding of activator protein-1 proteins was observed in the nuclei of stretched MCs at 10 minutes by mobility shift assay (N = 4), which was also largely prevented by 8-bromo-cGMP. Stretch increased PCNA expression by MCs, and this was inhibited by 8-bromo-cGMP.

CONCLUSIONS

Stretch-induced activation of SAPK and p42/44 MAPK in MCs can be inhibited by NO. The effect of NO is mediated by the generation of cGMP. These mechanisms may be responsible, at least in part, for the protective effect of NO in animal models of glomerular injury characterized by glomerular capillary hypertension.

Participation of a K(+) channel modulated directly by cGMP in the speract-induced signaling cascade of strongylocentrotus purpuratus sea urchin sperm

Speract, a decapeptide from Strongylocentrotus purpuratus sea urchin eggs, transiently stimulates a membrane guanylyl cyclase and activates a K(+)-selective channel that hyperpolarizes sperm. However, previous studies of sperm and of sperm membrane vesicles reached conflicting conclusions about the mechanisms that open these channels. We find that speract hyperpolarizes and increases the cGMP content of flagellar vesicles. We confirm previous findings that intravesicular GTPgammaS and GTP enhance this hyperpolarization, but not GDPbetaS. The G protein activators AlF(-)(4) and mastoparan also are ineffective. Thus, it is unlikely that a G protein participates in the speract response. In contrast, hyperpolarization responses to speract are increased by 3-isobutyl-1-methylxanthine, which preferentially inhibits cGMP-selective phosphodiesterases of sperm, and the 8Br-cGMP derivative hyperpolarizes vesicles in the absence of speract. The responses to speract and to 8Br-cGMP have similar ionic selectivities (K(+) > Rb(+) > > Li(+) > Na(+)) and sensitivities to the channel blockers 4-aminopiridine and 3, 4-dichlorobenzamil, indicating that they likely result from opening of the same K(+) channel. Inhibitors that preferentially inhibit cAMP-selective phosphodiesterases do not alter responses to speract, and permeant cAMP analogs do not hyperpolarize vesicles. In addition, inhibitors of protein kinases and phosphatases fail to alter vesicle hyperpolarization by speract. The increase in vesicular cGMP content produced by speract therefore may directly mediate opening of the channel that hyperpolarizes sperm membrane vesicles. Similar mechanisms presumably operate in intact sperm.

Characterization of a plasma membrane calcium oscillator in rat pituitary somatotrophs

In excitable cells, oscillations in intracellular free calcium concentrations ([Ca(2+)](i)) can arise from action-potential-driven Ca(2+) influx, and such signals can have either a localized or global form, depending on the coupling of voltage-gated Ca(2+) influx to intracellular Ca(2+) release pathway. Here we show that rat pituitary somatotrophs generate spontaneous [Ca(2+)](i) oscillations, which rise from fluctuations in the influx of external Ca(2+) and propagate within the cytoplasm and nucleus. The addition of caffeine and ryanodine, modulators of ryanodine-receptor channels, and the depletion of intracellular Ca(2+) stores by thapsigargin and ionomycin did not affect the global nature of spontaneous [Ca(2+)](i) signals. Bay K 8644, an L-type Ca(2+) channel agonist, initiated [Ca(2+)](i) signaling in quiescent cells, increased the amplitude of [Ca(2+)](i) spikes in spontaneously active cells, and stimulated growth hormone secretion in perifused pituitary cells. Nifedipine, a blocker of L-type Ca(2+) channels, decreased the amplitude of spikes and basal growth hormone secretion, whereas Ni(2+), a blocker of T-type Ca(2+) channels, abolished spontaneous [Ca(2+)](i) oscillations. Spiking was also abolished by the removal of extracellular Na(+) and by the addition of 10 mM Ca(2+), Mg(2+), or Sr(2+), the blockers of cyclic nucleotide-gated channels. Reverse transcriptase-polymerase chain reaction and Southern blot analyses indicated the expression of mRNAs for these channels in mixed pituitary cells and purified somatotrophs. Growth hormone-releasing hormone, an agonist that stimulated cAMP and cGMP productions in a dose-dependent manner, initiated spiking in quiescent cells and increased the frequency of spiking in spontaneously active cells. These results indicate that in somatotrophs a cyclic nucleotide-controlled plasma membrane Ca(2+) oscillator is capable of generating global Ca(2+) signals spontaneously and in response to agonist stimulation. The Ca(2+)-signaling activity of this oscillator is dependent on voltage-gated Ca(2+) influx but not on Ca(2+) release from intracellular stores.