Selective activation of particulate guanylate cyclase by a specific class of porphyrins

What is next in nitric oxide research? From cardiovascular system to cancer biology

The broad role of nitric oxide (NO) and cyclic GMP in biochemistry and biology as important messenger molecules is evident from the numerous publications in this research field. NO and cGMP have been known as components of the key signaling pathway in regulating numerous processes such as vascular dilation, blood pressure, neurotransmission, cardiovascular function, and renal function. In spite of almost 150,000 publications with nitric oxide and cyclic GMP, there are few publications regarding the effects of these messenger molecules on gene regulation, cell differentiation and cell proliferation. Our research data with embryonic stem cells and several cancer cell lines suggest that nitric oxide, its receptor soluble guanylyl cyclase (sGC) and sGC's product cyclic GMP can regulate the processes of proliferation and differentiation. Furthermore, we have found that undifferentiated stem cells and some malignant tumors such as human glioma have decreased levels of sGC and translocation of the sGCβ1 subunit to the nucleus. We propose that sGC and cyclic GMP function as tumor suppressors. An understanding of the mechanisms of the translocation of the sGCβ1 subunit into the nucleus and the possible regulation of gene expression of NO and/or cyclic CMP could lead to novel and innovative approaches to cancer therapy and stem cell proliferation and differentiation.

Toxins which activate guanylate cyclase: heat-stable enterotoxins

Certain enteropathogenic bacteria, including strains of Escherichia coli, Klebsiella pneumoniae and Yersinia enterocolitica, elicit their diarrhoeagenic effects by elaborating small molecular weight, heat-stable enterotoxins (STs). Structural and functional characteristics indicate that ST peptides are heterogeneous and two major subtypes, STa and STb, have been identified. Molecules of STa, unlike those of STb, are methanol-soluble and elicit their pathogenic effects by activating host cell guanylate cyclase activity and thereby increasing tissue cyclic GMP content: this increase in cyclic GMP causes fluid secretion. STa binds to specific proteinaceous receptors on intestinal cells but the nature of STa-receptor coupling to guanylate cyclase is poorly understood. The actions of STa, including binding to its receptor, activation of guanylate cyclase and stimulation of electrolyte transport, are rapid, reversible and tissue-specific. STa activates only particulate and not soluble guanylate cyclase. It alters the Vmax but not the apparent Km of this enzyme for Mg-GTP or Mn-GTP. At concentrations above 0.5-1 mM, calcium inhibits the STa activation of guanylate cyclase. The effects of calmodulin antagonists such as chlorpromazine on the activation of guanylate cyclase by STa are less clear. Inhibitors of phospholipid and arachidonic acid cascade pathways interfere with both basal and STa-stimulated guanylate cyclase. Membrane integrity is essential for STa activation of guanylate cyclase and the STa-receptor complex may activate the enzyme by intramembrane protein-protein interactions and/or perturbations. Interference with membrane phospholipid could alter such coupling.

Nitric oxide mediates natural polyphenol-induced Bcl-2 down-regulation and activation of cell death in metastatic B16 melanoma

Intravenous administration to mice of trans-pterostilbene (t-PTER; 3,5-dimethoxy-4'-hydroxystilbene) and quercetin (QUER; 3,3',4',5,6-pentahydroxyflavone), two structurally related and naturally occurring small polyphenols, inhibits metastatic growth of highly malignant B16 melanoma F10 (B16M-F10) cells. t-PTER and QUER inhibit bcl-2 expression in metastatic cells, which sensitizes them to vascular endothelium-induced cytotoxicity. However, the molecular mechanism(s) linking polyphenol signaling and bcl-2 expression are unknown. NO is a potential bioregulator of apoptosis with controversial effects on Bcl-2 regulation. Polyphenols may affect NO generation. Short-term exposure (60 min/day) to t-PTER (40 microM) and QUER (20 microM) (approximate mean values of the plasma concentrations measured within the first hour after intravenous administration of 20 mg of each polyphenol/kg) down-regulated inducible NO synthetase in B16M-F10 cells and up-regulated endothelial NO synthetase in the vascular endothelium and thereby facilitated endothelium-induced tumor cytotoxicity. Very low and high NO levels down-regulated bcl-2 expression in B16M-F10 cells. t-PTER and QUER induced a NO shortage-dependent decrease in cAMP-response element-binding protein phosphorylation, a positive regulator of bcl-2 expression, in B16M-F10 cells. On the other hand, during cancer and endothelial cell interaction, t-PTER- and QUER-induced NO release from the vascular endothelium up-regulated neutral sphingomyelinase activity and ceramide generation in B16M-F10 cells. Direct NO-induced cytotoxicity and ceramide-induced mitochondrial permeability transition and apoptosis activation can explain the increased endothelium-induced death of Bcl-2-depleted B16M-F10 cells.

Small cytoplasmic domain peptides of natriuretic peptide receptor-C attenuate cell proliferation through Gialpha protein/MAP kinase/PI3-kinase/AKT pathways

The present studies were undertaken to investigate the effect of C-atrial natriuretic peptide (ANP)(4-23) and several peptide fragments containing 12 amino acids from different regions of the cytoplasmic domain of natriuretic peptide receptor (NPR)-C on cell proliferation in the absence or presence of angiotensin (ANG) II, endothelin (ET)-1, and arginine vasopressin (AVP) in A-10 vascular smooth muscle cells (VSMC). The peptide fragments used have either complete G(i) activator sequences K(461)-H(472) (peptide 1) and H(481)-H(492) (peptide 3) or partial G(i) activator sequences R(469)-K(480) (peptide 2) and I(465)-H(472) (peptide Y) with truncated COOH or NH(2) terminus, respectively. The other peptide used had no structural specificity (Q(473)-K(480), peptide X) or was the scrambled peptide control for peptide 1 (peptide Z). ANG II, ET-1 and AVP significantly stimulated DNA synthesis in these cells as determined by [(3)H]thymidine incorporation that was inhibited by peptides 1, 2, and 3 and not by peptides X, Y, and Z in a concentration-dependent manner, with an apparent K(i) between 1 and 10 nM. In addition, C-ANP(4-23), which interacts with NPR-C, also inhibited DNA synthesis stimulated by vasoactive peptides; however, the inhibition elicited by C-ANP(4-23) was not additive with the inhibition elicited by peptide 1. On the other hand, basal DNA synthesis in these cells was not inhibited by C-ANP(4-23) or the peptide fragments. Furthermore, vasoactive peptide-induced stimulation of DNA synthesis was inhibited by PD-98059 and wortmannin, and this inhibition was potentiated by peptide 1. In addition, peptide 1 also inhibited vasoactive peptide-induced phosphorylation of ERK1/2 and AKT and enhanced expression of G(i)alpha proteins. These data suggest that C-ANP(4-23) and small peptide fragments containing 12 amino acids irrespective of the region of the cytoplasmic domain of NPR-C inhibit proliferative responses of vasoactive peptides through G(i)alpha protein and MAP kinase/phosphatidylinositol 3-kinase/AKT pathways.

Hemin induces active chloride secretion in Caco-2 cells

Enterocytes maintain fluid-electrolyte homeostasis by keeping a tight barrier and regulating ion channels. Carbon monoxide (CO), a product of heme degradation, modulates electrolyte transport in kidney and lung epithelium, but its role in regulating intestinal fluid-electrolyte homeostasis has not been studied. The major source of endogenous CO formation comes from the degradation of heme via heme oxygenase. We hypothesized that heme activates electrolyte transport in intestinal epithelial cells. Basolateral hemin treatment increased baseline Caco-2 cell short-circuit currents (I(sc)) twofold (control = 1.96 +/- 0.14 microA/cm(2) vs. hemin = 4.07 +/- 0.16 microA/cm(2), P < 0.01); apical hemin had no effect. Hemin-induced I(sc) was caused by Cl- secretion because it was inhibited in Cl- -free medium, with ouabain, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), or DIDS. Apical electrogenic Na+ channel inhibitor benzamil had no effect on hemin-induced I(sc). Hemin did not alter the ability of Caco-2 cells to respond maximally to forskolin, but a soluble guanylate cyclase inhibitor, [1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) inhibited the effects of hemin. A CO-releasing molecule, tricarbonyldichlororuthenium II, induced active Cl- secretion that was also inhibited with ODQ. We conclude that hemin induces active Cl- secretion in Caco-2 cells via a cGMP-dependent pathway. These effects are probably the consequence of CO formation. Heme and CO may be important regulators of intestinal fluid-electrolyte homeostasis.

Methemoglobin is a supplement for in vitro culture of human nasopharyngeal epithelial cells transformed by human papillomavirus type 16 DNA

NPC-N cells were normal human nasopharvngeal epithelial cells transformed by transfection with human papillomavirus type 16 deoxyribonucleic acid. Bovine pituitary extract (BPE) was one of the indispensable ingredients for in vitro culture of NPC-N cells in a serum-free medium. Chromatographic fractionation of BPE and subsequent immunoblotting analyses identified the hemoglobin growth-stimulating factor. Methemoglobin (metHb) was then synthesized, and also found to be growth stimulating. The growth-stimulating effect of metHb was abolished when NPC-N cells were cultured in a medium that also contained haptoglobin, a molecule that binds to hemoglobin. A defined medium consisting of insulin and metHb was then developed for optimal growth of NPC-N cells. MetHb kept under the conditions identical to those of cell culture released hemin which also enhanced the cell growth. Though all the degradation products of hemin are currently known to be physiologically significant. only ferric iron derived from metHb or hemin could stimulate the growth of NPC-N cells. Abnormal vasculature showing leaky walls and hemorrhage is a common feature of malignant tumors. Hemoglobin originating from extravasated red blood cells and subsequently oxidized to metHb because of the presence of activated inflammatory cells might contribute to the increased proliferation of cancerous cells.

Methaemoglobin enhances the proliferation of transformed human epithelial cells: a possible outcome of neovascularisation and haemorrhage in tumours?

The effect of human methaemoglobin (metHb), possibly derived from extravasated red blood cells in tumours showing neovascularisation and haemorrhage, on the growth of transformed human epithelial cells was investigated. MetHb stimulated the growth of immortalised epithelial cells or transformed cells at precrisis stage (cells have bypassed M1, but not M2, the two mortality checkpoints). The stimulatory effect was due to the release of haemin from metHb that was isolated by a Sephadex column and identified by its characteristic light absorption spectrum. Although all the degradation products of haemin are currently known to be physiologically significant, only ferric iron derived from metHb or haemin could stimulate cell growth. High concentrations of metHb or haemin inhibited cell growth possibly due to the generation of high concentrations of bilirubin. However, bilirubin formed in the cells of human body is known to be transported to the liver for further processing and excretion. Haemoglobin oxidised to where tumours show neovascularisation and haemorrhage likely contributes significantly to the increased proliferation of cancerous cells.

Platelet cyclic GMP. A potentially useful indicator to evaluate the effects of nitroglycerin and nitrate tolerance

BACKGROUND

The present study was designed to investigate the intracellular production of cyclic GMP (cGMP) in platelets in response to nitroglycerin and to determine the potential clinical value of platelet cGMP as an indicator of the effects of nitroglycerin and nitrate tolerance.

METHODS AND RESULTS

Platelet cGMP levels and the diameters of the coronary arteries before and 2 minutes after intracoronary injection of 200 micrograms nitroglycerin were measured in 15 patients who had previously received nitrates (nitrates group) and in 16 who had not received any nitrates (no-nitrates group). Platelet cGMP levels increased significantly after nitroglycerin injection in the two groups, but plasma cGMP levels and plasma atrial natriuretic peptide levels did not change. The percent increase in platelet cGMP levels and the percent dilatation of the left anterior descending (LAD) and left circumflex (LCx) coronary arteries after nitroglycerin injection were higher in the no-nitrates group than in the nitrates group (platelet cGMP levels: artery, 74.2 +/- 18.3% versus 11.5 +/- 4.2%, P < .01; vein, 73.6 +/- 22.9% versus 9.0 +/- 3.1%, P < .01; coronary dilatation: LAD, 46.7 +/- 6.0% versus 9.9 +/- 2.5%, P < .01, LCx, 51.2 +/- 8.7% versus 6.1 +/- 3.0%, P < .01). The percent increase in platelet cGMP levels was significantly correlated with the percent dilatation of the coronary arteries (LAD: r = .90, P < .01; LCx: r = .92, P < .01) in the no-nitrates group and not in the nitrates group.

CONCLUSIONS

These results indicate that platelet cGMP can be used as an indicator for in situ evaluation of nitroglycerin effects and that patients who have received nitrates develop nitrate tolerance, which affects intracellular production of cGMP and vasodilation in the response to nitroglycerin.

Adenine nucleotide regulation of particulate guanylate cyclase from rat lung

Adenine nucleotides activate basal particulate guanylate cyclase in rat lung membranes. Activation is specific for adenine and not guanine, cytidine or uridine nucleotides. The concentration of adenine nucleotides yielding half-maximum activation of particulate guanylate cyclase is 0.1 mM and this nucleotide activates the enzyme by increasing maximum velocity 11-fold without altering affinity for substrate. Activation is specific for particulate guanylate cyclase, since soluble enzyme is inhibited by adenine nucleotides. Similarly, activation is specific for magnesium as the enzyme substrate cation cofactor, since adenine nucleotides inhibit particulate guanylate cyclase when manganese is used. Adenine nucleotide regulation of particulate guanylate cyclase may occur by a different molecular mechanism compared to other activators, since the effects of these nucleotides are synergistic with those of detergent, hemin and atrial natriuretic peptides. Cystamine inhibits adenine nucleotide activation of particulate guanylate cyclase at concentrations having minimal effects on basal enzyme activity suggesting a role for critical sulfhydryls in mechanisms underlying nucleotide regulation of particulate guanylate cyclase. Purification and quantitative recovery of particulate guanylate cyclase by substrate affinity chromatography results in the loss of adenine nucleotide regulation. These data suggest that adenine nucleotides may be important in the regulation of basal and activated particulate guanylate cyclase and may be mediated by an adenine nucleotide-binding protein which is separate from that enzyme.

Pharmacological activities of delta-aminolaevulinic acid, protoporphyrin IX and haemin in isolated preparations of rabbit gastric fundus and jejunum

1. Pharmacological effects of delta-aminolaevulinic acid (ALA), protoporphyrin IX and haemin were examined in isolated preparations of rabbit jejunum and gastric fundus suspended in oxygenated Ringer-Locke solution at pH 7.0. 2. In jejunal preparations, delta-aminolaevulinic acid (3.0-4.5 mM), protoporphyrin IX (1.1-2.2 mM) and haemin (3.0-4.5 mM) dose-dependently reduced the amplitude of contractions and increased resting length. Pretreatment with prazosin (10(-7) M) inhibited effects produced by delta-aminolaevulinic acid (3 mM) and protoporphyrin IX (1.1 mM) but not those of haemin (3 mM). 3. In fundic preparations, dose-dependent contracture occurred in response to delta-aminolaevulinic acid (0.1-3.0 mM) protoporphyrin IX (0.1-2.2 mM) and haemin (0.6-6.3 mM). Effects qualitatively resembled those of noradrenaline (0.1-0.4 microM). Prazosin (10(-7) M) attenuated these effects, depressing the maximum response and causing a rightward shift of the concentration-response curves. 4. It is concluded that actions of delta-aminolaevulinic acid at alpha 1-adrenoceptor sites are unlikely to be related to the autonomic neuropathy of acute porphyria. Its in vitro effects occurred only at comparatively high concentrations and were mimicked by protoporphyrin IX and haemin. It is suggested that ALA is more likely to modify autonomic functions by an indirect action, since it is known at low dose levels to influence GABA-ergic functioning.

The effects of nitroprusside and putative agonists on guanylate cyclase activity in squid giant axons

cGMP content of axoplasm from the giant axon of Loligo forbesi was investigated after subjecting the axon to various treatments. Repetitive electrical stimulation or depolarisation by high K+ caused no change in cGMP content. Glutamate and serotonin were also without effect. The nicotinic agonist carbachol (100 microM) increased cGMP levels by 90% (n = 5). A large transient elevation of cGMP content was evoked by external nitroprusside (10 nM-20 microM in intact axons. Nitroprusside injected into both extruded axoplasm and intact axons also increased cGMP content, the stimulation being considerably higher in intact axons where the axolemma was also present. Nitroprusside was also active in axons where the soluble cytoplasmic components were washed out by internal perfusion.

Regulation of intestinal mucosa guanylate cyclase by hemin, heme and protoporphyrin IX

Mg2+-dependent activity of intestinal brush border guanylate cyclase was stimulated 4-5-fold by 50-100 microM hemin. Higher concentrations were inhibitory. In the presence of 25% dimethyl sulfoxide, which stimulated activity 9-times, 50 microM hemin further increased activity 1.7-fold. However, when activity was stimulated 32-fold by the Escherichia coli heat-stable enterotoxin, or 26-fold by Lubrol PX, hemin produced only concentration-dependent inhibition. The first type of activation was more sensitive to hemin than the second. Reduction of hemin by dithiothreitol eliminated stimulation of basal activity, while inhibition of Lubrol PX-stimulated activity remained. Protoporphyrin IX also had no effect on basal activity, however, it inhibited enterotoxin- and Lubrol PX-stimulated activities similarly, but only to half the extent of hemin. Substitution of Mn2+ for Mg2+ elevated basal activity 15-fold, and this Mn2+-dependent activity was inhibited by hemin. Mn2+-dependent activity was stimulated (43%) by enterotoxin, however, the stimulated activity was more sensitive to hemin inhibition than the basal Mn2+-dependent activity and both inhibition curves were congruent above 50 microM hemin. Hemin inhibition of Lubrol PX-stimulated activity was much less with Mn2+ than with Mg2+. These results were interpreted as suggesting two sites of hemin inhibition; on an inhibitory regulator and on the enzyme. We also found that the secretory effect of enterotoxin in the suckling mouse bioassay was reduced 56% by the oral administration of hemin.

Coexistence of guanylate cyclase and atrial natriuretic factor receptor in a 180-kD protein

Atrial natriuretic factor (ANF) is a peptide hormone that is released from atria and regulates a number of physiological processes, including steroidogenesis in adrenal cortex and testes. The parallel stimulation of membrane guanylate cyclase and corticosterone production in isolated fasciculata cells of rat adrenal cortex has supported the hypothesis of a mediatory role for cyclic guanosine monophosphate (cyclic GMP) in signal transduction. A novel particulate guanylate cyclase tightly coupled with ANF receptor was purified approximately 273,000-fold by two-step affinity chromatography. The enzyme had a molecular size of 180 kilodaltons and was acidic in nature with a pI of 4.7. Its specific activity was 1800 nanomoles of cyclic GMP formed per minute per milligram of protein. The purified enzyme bound ANF with a specific binding activity of 4.01 nanomoles per milligram of protein, a value that is close to the theoretical binding activity of 5.55 nanomoles per milligram of protein for 1 mole of the ligand binding 1 mole of the receptor protein. These results indicate that the guanylate cyclase-coupled ANF receptor exists in a 180-kilodalton protein of rat adrenocortical carcinoma and represent a step toward the elucidation of the basic mechanism of cyclic GMP-mediated transmembrane signal transduction in response to a hormone.

Pharmacological effects of haem biosynthetic intermediates in isolated intestinal preparations

Spontaneous contractile activity of isolated rabbit jejunal preparations bathed in oxygenated Ringer-Locke buffered to pH 7.0 was inhibited by haemin and by all haem biosynthetic intermediates tested, there being decreases in tone or amplitude of contractions. Effects were concentration-dependent. The minimal effective concentrations were 0.4 mM for porphobilinogen, 0.6 mM for protoporphyrin IX, 0.8 mM for haemin, 3.0 mM for delta-aminolaevulinic acid (ALA), 4.5 mM for coproporphyrin I, 3.0 mM for glycine and 6.0 mM for succinate. Inhibition was short-lasting, other than for protoporphyrin (2.3 mM) and haemin (3.0 mM). Leucine at concentrations up to 12 mM had no significant effect. Inhibitory effects of ALA were followed by contractions of increased amplitude; pretreatment of preparations with indomethacin (56 microM) prevented this enhancement of contractile activity. Tone in isolated preparations of human taenia coli bathed in oxygenated Ringer-Locke was decreased by ALA (1.5-6.0 mM). A significant increase of tone followed the initial inhibitory effect. The relevance of these findings to acute porphyria is discussed.

Properties of digitonin-solubilized calmodulin-dependent guanylate cyclase from the plasma membranes of Tetrahymena pyriformis NT-1 cells

Calmodulin-dependent guanylate cyclase from Tetrahymena plasma membranes was solubilized in about a 22% yield by using digitonin in the presence of 0.2 mM CaCl2 and 20% glycerol. The detergent, when present in the assay at concentrations above 0.05%, diminished the basal and calmodulin-stimulated activity of the enzyme. Guanylate cyclase solubilized with digitonin was eluted from DEAE-cellulose with 200 mM KCl in a yield of 50%. Properties of the solubilized enzyme were similar to those of the native membrane-bound enzyme. The Kms for Mg-GTP and Mn-GTP were 140 and 30 microM, respectively. The enzyme required Mn2+ for maximum activity, the relative activity in the presence of Mg2+ being 30% of the activity with Mn2+. The solubilized enzyme retained the ability to be activated by calmodulin, with its extent being reduced as compared to the membrane-bound enzyme. The presence of a Ca2+-dependent calmodulin-binding site on the solubilized enzyme was shown by the Ca2+-dependent retention of the enzyme on a calmodulin-Sepharose-4B column.

Desensitization to nitroglycerin in vascular smooth muscle from rat and human

Guanylate cyclase in high speed supernatant fractions obtained from rat thoracic aorta or human coronary arteries pretreated with nitroglycerin exhibited a marked desensitization to activation by nitroglycerin, nitroprusside, and nitric oxide. However, activation of soluble guanylate cyclase by arachidonic acid was unaffected by pretreatment of vessels with nitroglycerin. Furthermore, activation of soluble guanylate cyclase by protoporphyrin IX was increased 4-fold when vessels were pretreated with nitroglycerin. Soluble guanylate cyclase partially purified from nitroglycerin-pretreated rat thoracic aorta by immunoprecipitation with a specific monoclonal antibody exhibited persistent desensitization to nitrate-induced activation. These data suggest that nitroglycerin-induced desensitization of guanylate cyclase to activation by nitrovasodilators represents a stable alteration of the enzyme. In contrast, activation by protoporphyrin IX of guanylate cyclase immunoprecipitated from nitroglycerin-pretreated or control vessels was not significantly different. This suggests that the mechanism of protoporphyrin activation of guanylate cyclase is different than the mechanism with nitrovasodilators. Activation of particulate guanylate cyclase by Lubrol-PX, hemin, or atrial natriuretic factor was not significantly different with enzyme prepared from nitroglycerin-pretreated or control vessels from rat and human. Thus, nitroglycerin-induced desensitization of rat thoracic aorta or human coronary artery results in a relatively stable molecular alteration of soluble guanylate cyclase such that the enzyme is specifically less sensitive to activation by nitrovasodilators whereas the effects of other activators of the enzyme are either unchanged or increased.

Effects of atriopeptin on particulate guanylate cyclase from rat adrenal

Atriopeptin II activated particulate guanylate cyclase 5-10-fold in a concentration- and time-dependent fashion in crude membranes obtained from homogenates of rat adrenal cortex or medulla. Similar effects were observed with other atriopeptin analogs. Soluble guanylate cyclase and adenylate cyclase in these preparations were not activated. Accumulation of cyclic GMP in minces of adrenal cortex or medulla was increased 6-8-fold due to atriopeptin II activation of particulate guanylate cyclase. Several thiol-reactive agents blocked the activation of particulate guanylate cyclase, suggesting that free thiol groups on membrane proteins may be important in atriopeptin receptor-guanylate cyclase coupling.

Exposure to toxic agents: the heme biosynthetic pathway and hemoproteins as indicator

The heme biosynthetic pathway is closely controlled by levels of the end product of the pathway, namely, heme, and porphyrins are normally formed in only trace amounts. When control mechanisms are disturbed by xenobiotics, porphyrins accumulate and serve as a signal of the interaction between a xenobiotic and the heme biosynthetic pathway. For example, an increase in erythrocyte protoporphyrin is a useful measurement for early detection of exposure to lead and porphyrinuria was an early manifestation of a hexachlorobenzene-induced porphyria in Turkey. In recent years a variety of additional xenobiotics has been shown to interact with the heme biosynthetic pathway, namely, halogenated aromatic hydrocarbons, pesticides, sulfides, and a variety of metals. Moreover, different xenobiotics (e.g., dihydropyridines and compounds containing unsaturated carbon-carbon bonds) interact with the prosthetic heme of cytochrome P-450 forming novel N-alkylporphyrins.