Charles W. Leffler

Nox4 NADPH oxidase mediates oxidative stress and apoptosis caused by TNF-α in cerebral vascular endothelial cells

Inflammatory brain disease may damage cerebral vascular endothelium leading to cerebral blood flow dysregulation. The proinflammatory cytokine TNF-alpha causes oxidative stress and apoptosis in cerebral microvascular endothelial cells (CMVEC) from newborn pigs. We investigated contribution of major cellular sources of reactive oxygen species to endothelial inflammatory response. Nitric oxide synthase and xanthine oxidase inhibitors (N(omega)-nitro-l-arginine and allopurinol) had no effect, while mitochondrial electron transport inhibitors (CCCP, 2-thenoyltrifluoroacetone, and rotenone) attenuated TNF-alpha-induced superoxide (O(2)(*-)) and apoptosis. NADPH oxidase inhibitors (diphenylene iodonium and apocynin) greatly reduced TNF-alpha-evoked O(2)(*-) generation and apoptosis. TNF-alpha rapidly increased NADPH oxidase activity in CMVEC. Nox4, the cell-specific catalytic subunit of NADPH oxidase, is highly expressed in CMVEC, contributes to basal O(2)(*-) production, and accounts for a burst of oxidative stress in response to TNF-alpha. Nox4 small interfering RNA, but not Nox2, knockdown prevented oxidative stress and apoptosis caused by TNF-alpha in CMVEC. Nox4 is colocalized with HO-2, the constitutive isoform of heme oxygenase (HO), which is critical for endothelial protection against TNF-alpha toxicity. The products of HO activity, bilirubin and carbon monoxide (CO, as a CO-releasing molecule, CORM-A1), inhibited Nox4-generated O(2)(*-) and apoptosis caused by TNF-alpha stimulation. We conclude that Nox4 is the primary source of inflammation- and TNF-alpha-induced oxidative stress leading to apoptosis in brain endothelial cells. The ability of CO and bilirubin to combat TNF-alpha-induced oxidative stress by inhibiting Nox4 activity and/or by O(2)(*-) scavenging, taken together with close intracellular compartmentalization of HO-2 and Nox4 in cerebral vascular endothelium, may contribute to HO-2 cytoprotection against inflammatory cerebrovascular disease.

Heme Is a Carbon Monoxide Receptor for Large-Conductance Ca2+-Activated K+ Channels

Carbon monoxide (CO) is an endogenous paracrine and autocrine gaseous messenger that regulates physiological functions in a wide variety of tissues. CO induces vasodilation by activating arterial smooth muscle large-conductance Ca2+-activated potassium (BKCa) channels. However, the mechanism by which CO activates BKCa channels remains unclear. Here, we tested the hypothesis that CO activates BKCa channels by binding to channel-bound heme, a BKCa channel inhibitor, and altering the interaction between heme and the conserved heme-binding domain (HBD) of the channel α subunit C terminus. Data obtained using thin-layer chromatography, spectrophotometry, mass spectrometry (MS), and MS-MS indicate that CO modifies the binding of reduced heme to the α subunit HBD. In contrast, CO does not alter the interaction between the HBD and oxidized heme (hemin), to which CO cannot bind. Consistent with these findings, electrophysiological measurements of native and cloned (cbv) cerebral artery smooth muscle BKCa channels show that CO reverses BKCa channel inhibition by heme but not by hemin. Site-directed mutagenesis of the cbv HBD from CKACH to CKASR abolished both heme-induced channel inhibition and CO-induced activation. Furthermore, on binding CO, heme switches from being a channel inhibitor to an activator. These findings indicate that reduced heme is a functional CO receptor for BKCa channels, introduce a unique mechanism by which CO regulates the activity of a target protein, and reveal a novel process by which a gaseous messenger regulates ion channel activity.

Carbon Monoxide Dilates Cerebral Arterioles by Enhancing the Coupling of Ca2+ Sparks to Ca2+-Activated K+ Channels

Abstract— Carbon monoxide (CO) is generated endogenously by the enzyme heme oxygenase. Although CO is a known vasodilator, cellular signaling mechanisms are poorly understood and are a source of controversy. The goal of the present study was to investigate mechanisms of CO dilation in porcine cerebral arterioles. Data indicate that exogenous or endogenously produced CO is a potent activator of large-conductance Ca2+-activated K+ (KCa) channels and Ca2+ spark–induced transient KCa currents in arteriole smooth muscle cells. In contrast, CO is a relatively poor activator of Ca2+ sparks. To understand the apparent discrepancy between potent effects on transient KCa currents and weak effects on Ca2+ sparks, regulation of the coupling relationship between these events by CO was investigated. CO increased the percentage of Ca2+ sparks that activated a transient KCa current (ie, the coupling ratio) from ≈62% in the control condition to 100% and elevated the slope of the amplitude correlation between these events ≈2.6-fold, indicating that Ca2+ sparks induced larger amplitude transient KCa currents in the presence of CO. This signaling pathway for CO is physiologically relevant because ryanodine, a ryanodine-sensitive Ca2+ release channel blocker that inhibits Ca2+ sparks, abolished CO dilation of pial arterioles in vivo. Thus, CO dilates cerebral arterioles by priming KCa channels for activation by Ca2+ sparks. This study presents a novel dilatory signaling pathway for CO in the cerebral circulation and appears to be the first presents of a vasodilator that acts by increasing the effective coupling of Ca2+ sparks to KCa channels.

Carbon monoxide and cerebral microvascular tone in newborn pigs.

The present study addresses the hypothesis that CO produced from endogenous heme oxygenase (HO) can dilate newborn cerebral arterioles. HO-2 protein was highly expressed in large and small blood vessels, as well as parenchyma, of newborn pig cerebrum. Topically applied CO dose-dependently dilated piglet pial arterioles in vivo over the range 10(-11)-10(-9) M (maximal response). CO-induced cerebrovascular dilation was abolished by treatment with the Ca2+-activated K+ channel inhibitors tetraethylammonium chloride and iberiotoxin. The HO substrate heme-L-lysinate also produced tetraethylammonium-inhibitable, dose-dependent dilation from 5 x 10(-8) to 5 x 10(-7) M (maximal). The HO inhibitor chromium mesoporphyrin blocked dilation of pial arterioles in response to heme-L-lysinate. In addition to inhibiting dilation to heme-L-lysinate, chromium mesoporphyrin also blocked pial arteriolar dilations in response to hypoxia but did not alter responses to hypercapnia or isoproterenol. We conclude that CO dilates pial arterioles via activation of Ca2+-activated K+ channels and that endogenous HO-2 potentially can produce sufficient CO to produce the dilation.The present study addresses the hypothesis that CO produced from endogenous heme oxygenase (HO) can dilate newborn cerebral arterioles. HO-2 protein was highly expressed in large and small blood vessels, as well as parenchyma, of newborn pig cerebrum. Topically applied CO dose-dependently dilated piglet pial arterioles in vivo over the range 10-11-10-9M (maximal response). CO-induced cerebrovascular dilation was abolished by treatment with the Ca2+-activated K+ channel inhibitors tetraethylammonium chloride and iberiotoxin. The HO substrate heme-l-lysinate also produced tetraethylammonium-inhibitable, dose-dependent dilation from 5 × 10-8 to 5 × 10-7 M (maximal). The HO inhibitor chromium mesoporphyrin blocked dilation of pial arterioles in response to heme-l-lysinate. In addition to inhibiting dilation to heme-l-lysinate, chromium mesoporphyrin also blocked pial arteriolar dilations in response to hypoxia but did not alter responses to hypercapnia or isoproterenol. We conclude that CO dilates pial arterioles via activation of Ca2+-activated K+ channels and that endogenous HO-2 potentially can produce sufficient CO to produce the dilation.

The Onset of Breathing at Birth Stimulates Pulmonary Vascular Prostacyclin Synthesis

Summary: The purpose of the present study was to determine if pulmonary prostacyclin synthesis was stimulated by spontaneous onset of breathing by unanesthetized fetuses at birth. Cannulae were implanted and flow cuffs placed in fetal lambs and goats (0.93 term). Fetuses were delivered by cesarean section at 0.95 term and began breathing spontaneously. Prostacyclin in blood was determined by radioimmunoassay of its hydrolysis product, 6-ketoprostaglandin F1α using methods that produced the same values in duplicate samples as did gas chromatography with electron capture detection. Fetal pulmonary prostacyclin production (left lung) [(left pulmonary venous concentration — pulmonary arterial concentration) × left pulmonary blood flow] was undetectable [−1.7 ± 1.0 (SEM) ng PGI2·kg−1·min−1] and fetal pulmonary vascular resistance (left lung) high (5.1 ± 0.9 mm Hg · kg·min · ml−1). Pulmonary prostacyclin production increased to 30.1 ± 12.3 ngPGI2 · kg−1 · min−1 and pulmonary vascular resistance declined to 0.5 ± 0.1 mm Hg · kg · min · ml−1 15 min after-birth. Pulmonary vascular resistance remained low even though pulmonary prostacyclin production fell 2-5 h after birth. These results, coupled with earlier studies using indomethacin to inhibit prostaglandin synthesis, support the hypothesis that pulmonary prostacyclin synthesis participates in the decline of pulmonary vascular resistance that accompanies the onset of ventilation at birth, but may be less important in maintenance of low pulmonary vascular resistance once reduced pulmonary vascular tone has been established.

Carbon monoxide as an endogenous vascular modulator

Carbon monoxide (CO) is produced by heme oxygenase (HO)-catalyzed heme degradation to CO, iron, and biliverdin. HO has two active isoforms, HO-1 (inducible) and HO-2 (constitutive). HO-2, but not HO-1, is highly expressed in endothelial and smooth muscle cells and in adjacent astrocytes in the brain. HO-1 is expressed basally only in the spleen and liver but can be induced to a varying extent in most tissues. Elevating heme, protein phosphorylation, Ca(2+) influx, and Ca(2+)/calmodulin-dependent processes increase HO-2 activity. CO dilates cerebral arterioles and may constrict or dilate skeletal muscle and renal arterioles. Selected vasodilatory stimuli, including seizures, glutamatergic stimulation, hypoxia, hypotension, and ADP, increase CO, and the inhibition of HO attenuates the dilation to these stimuli. Astrocytic HO-2-derived CO causes glutamatergic dilation of pial arterioles. CO dilates by activating smooth muscle cell large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels. CO binds to BK(Ca) channel-bound heme, leading to an increase in Ca(2+) sparks-to-BK(Ca) channel coupling. Also, CO may bind directly to the BK(Ca) channel at several locations. Endothelial nitric oxide and prostacyclin interact with HO/CO in circulatory regulation. In cerebral arterioles in vivo, in contrast to dilation to acute CO, a prolonged exposure of cerebral arterioles to elevated CO produces progressive constriction by inhibiting nitric oxide synthase. The HO/CO system is highly protective to the vasculature. CO suppresses apoptosis and inhibits components of endogenous oxidant-generating pathways. Bilirubin is a potent reactive oxygen species scavenger. Still many questions remain about the physiology and biochemistry of HO/CO in the circulatory system and about the function and dysfunction of this gaseous mediator system.

Effects of indomethacin upon cerebral hemodynamics of newborn pigs.

ABSTRACT: Treatment of unanesthetized newborn pigs with indomethacin trihydrate (5 ± 1 mg/kg, intravenous) decreased cerebral blood flow uniformly throughout the brain by 18-28% without changing cardiac output, arterial pressure, or arterial blood gases and pH. Breathing 10% O2, 9% CO2 with the balance N2 (hypoxia/hypercapnia) caused cerebral blood flow to increase from 102 ± 12 to 218 ± 19 ml/100 g-min. Intravenous administration of indomethacin during hypoxia/hypercapnia caused a uniform decrease in cerebral flow throughout the brain to levels (94 ± 5 ml/100 g-min) indistinguishable from those when the piglet was breathing ambient air. Further, 2.5 h later, the cerebral hyperemia caused by hypoxia/hypercapnia was attenuated markedly (129 ± 19 ml/100 g-min). Vehicle treatment did not alter resting cerebral blood flow or cerebral hyperemia in response to hypoxia/hypercapnia. Measurements of 6-keto-prostaglandin F1α, thromboxane B2, and prostaglandin E2 demonstrated that intravenously administered indomethacin crossed the blood-brain barrier of newborn pigs in sufficient quantity to inhibit prostanoid release into the cerebrospinal fluid passing over the surface of the brain. The mechanism by which indomethacin reduces cerebral blood flow and attenuates cerebral hyperemia cannot be determined from the present experiments. We conclude that intravenous administration of indomethacin decreases cerebral blood flow and attenuates cerebral hyperemia induced by severe, combined hypoxia/hypercapnia in newborn pigs.

Brain superoxide anion generation in asphyxiated piglets and the effect of indomethacin at therapeutic dose.

ABSTRACT: We have previously shown that generation of superoxide anion occurs in cerebral cortex during asphyxia/reventilation in newborn pigs and that a high dose of indornethacin (5 mg/kg i.v.) abolishes superoxide anion production. The purposes of this study were 1) to determine whether the generation of superoxide anion occurs primarily during asphyxia or whether reventilation must take place, 2) to investigate the effects of indornethacin pretreatment at a therapeutic dose of 0.2 mg/kg i.v. on superoxide anion generation, and 3) to investigate the effects of oxypurinol, an oxygen free radical scavenger, on superoxide anion production during asphyxia/reventilation. Superoxide anion production on cerebral cortex was determined by superoxide dismutase-inhibitable nitroblue tetrazolium (NBT) reduction using closed cranial windows. Superoxideanion generation during asphyxia without reventilation was 4 ± 2 pmol NBT/mm2 per 20 min, which was significantly lower than during asphyxia/reventilation (16 ± 4 pmol NBT/mm2 per 20 min) but comparable to the control group (3 ± 1 pmol NBT/mm2 per 20 min). Indomethacin given at therapeutic dosage before asphyxia/reventilation decreased superoxide anion production to 3 ± 1 pmol NBT/mm2 per 20 min, values not significantly different from the control group and from piglets pretreated with oxypurinol at a dose of 50 mg/kg i.v. (4 ± 2 pmol NBT/mm2 per 20 min). We conclude that in newborn pigs 1) superoxide anions are generated largely during reventilation rather than during asphyxia; 2) the therapeutic dose of indomethacin (0.2 mg/kg) is effective in inhibiting the superoxide anion generation during asphyxia/reventilation; and 3) oxypurinol reduces the superoxide anion accumulation on cerebral cortex during asphyxia/reventilation.

Prostanoids in cortical subarachnoid cerebrospinal fluid and pial arterial diameter in newborn pigs.

These studies were designed to investigate the relationship between cerebral prostanoid synthesis and pial arterial caliber in chloralose-anesthetized newborn pigs with normal blood gases and pH and during combined arterial hypoxia and hypercapnia. Piglets less than 5 days old were equipped with closed cranial windows to allow direct observation of pial vessels, application of prostaglandin E2, and sampling of cortical subarachnoid cerebrospinal fluid. We found that prostanoids accumulate in cerebrospinal fluid on the cortical surface. The only prostanoid detected in arterial blood was 6-keto-prostaglandin F1 alpha [442 +/- 74 pg/ml (radioimmunoassay)]. Only small quantities of 6-keto-prostaglandin F1 alpha (214 +/- 53 pg/ml) and thromboxane B2 (122 +/- 18 pg/ml) were found in cerebrospinal fluid from the cisterna magna. Higher concentrations of 6-keto-prostaglandin F1 alpha (1056 +/- 159 pg/ml), thromboxane B2 (229 +/- 64 pg/ml), and prostaglandin E2 (4235 +/- 269 pg/ml) were found in cortical subarachnoid fluid. In contrast to arterial and cisternal concentrations, the concentrations of 6-keto-prostaglandin F1 alpha, thromboxane B2, and prostaglandin E2 in cortical subarachnoid fluid were increased reversibly by ventilation with 9% carbon dioxide, 10% oxygen, (6-keto-prostaglandin F1 alpha, 5436 +/- 1576 pg/ml; thromboxane B2, 694 +/- 122 pg/ml; and, prostaglandin E2, 12,455 +/- 3688 pg/ml). Further, pial arteries dilated in response to topical application of prostaglandin E2 at the concentration that was found in cortical subarachnoid fluid during combined hypoxia and hypercapnia. Systemic administration of indomethacin trihydrate (5 mg/kg) markedly reduced cortical subarachnoid fluid prostanoid concentrations and attenuated the pial artery vasodilation induced by combined hypoxia and hypercapnia.(ABSTRACT TRUNCATED AT 250 WORDS)

Maintenance of cerebral circulation during hemorrhagic hypotension in newborn pigs: role of prostanoids.

The possibility that the prostanoid system contributes to the capability of the newborn piglet to maintain cerebral blood flow and cerebral metabolic rate during hypotension was investigated. The effect of hemorrhage on net (arterial-to-venous) cerebral prostacyclin production and the effects of indomethacin on cerebral hemodynamic response to hemorrhage and on the cerebral oxygen utilization following hemorrhage were determined in chronically Instrumented, unanesthetized newborn pigs. Hemorrhage decreased arterial pressure about 35% but did not affect cerebral blood flow or cerebral O2 consumption. Hemorrhage was accompanied by an increase in net cerebral 6-ket0-PGFlα production from 4.0 ±1.1 to 15.3 ± 4.9 ng/100gmin (mean ± SEM). Indomethacin treatment of piglets following hemorrhage inhibited the net cerebral production of 6-keto-PGFla and caused a decrease in blood flow (= 40%) to all brain regions within 20 minutes. The decrease in cerebral blood flow was the result of an increase in cerebral vascular resistance of 57 and 180%, 20 and 40 minutes post treatment, respectively. Cerebral O2 consumption was reduced from 2.5 ± 0.3 ml/100 g. min to 1.5 ± 0.3 ml/100 gmin 20 minutes following treatment of hemorrhaged piglets with indomethacin and to 1.1 ± 0.3 ml/100 g. min 40 minutes after treatment. Six of 8 piglets for whom the data were recorded that were administered indomethacin following hemorrhage became comatose with cerebral O2 consumption of 0.4 ± 0.1 ml O2/100 g-min by 40 minutes after treatment. These data are consistent with the hypothesis that the prostanoid system contributes to the maintenance of cerebral blood flow and cerebral metabolic rate during hypotension in the newborn.