Carbon monoxide and hypertension

Heme oxygenase: the key to renal function regulation

Heme oxygenase (HO) plays a critical role in attenuating the production of reactive oxygen species through its ability to degrade heme in an enzymatic process that leads to the production of equimolar amounts of carbon monoxide and biliverdin/bilirubin and the release of free iron. The present review examines the beneficial role of HO-1 (inducible form of HO) that is achieved by increased expression of this enzyme in renal tissue. The influence of the HO system on renal physiology, obesity, vascular dysfunction, and blood pressure regulation is reviewed, and the clinical potential of increased levels of HO-1 protein, HO activity, and HO-derived end products of heme degradation is discussed relative to renal disease. The use of pharmacological and genetic approaches to investigate the role of the HO system in the kidney is key to the development of therapeutic approaches to prevent the adverse effects that accrue due to an impairment in renal function.

Haem oxygenase-1 induction protects against tumour necrosis factor alpha impairment of endothelial-dependent relaxation in rat isolated pulmonary artery

BACKGROUND AND PURPOSE

Disturbances in pulmonary vascular reactivity are important components of inflammatory lung disease. Haem oxygenase-1 (HO-1) is an important homeostatic enzyme upregulated in inflammation. Here we have investigated the potentially protective effect of HO-1 against cytokine-induced impairment in pulmonary artery relaxation.

EXPERIMENTAL APPROACH

Haem oxygenase-1 protein levels were assessed by immunofluorescence. HO activity was assessed by conversion of haemin to bilirubin. Rings of rat isolated pulmonary artery in organ baths were used to measure relaxant responses to the endothelium-dependent agent ACh and the endothelium-independent agent sodium nitroprusside (SNP). Production of nitric oxide (NO) and reactive oxygen species (ROS) was assessed by confocal fluorescence microscopy and fluorescent probes.

KEY RESULTS

Haem oxygenase-1 protein expression was strongly induced in pulmonary artery after 24-h incubation with either haemin (5 microM) or curcumin (2 microM), accompanied by a significant increase in HO activity. Incubation with tumour necrosis factor alpha (TNFalpha, 1 ng.mL(-1), 2 h) significantly decreased relaxation of arterial rings to ACh, without affecting responses to SNP. Induction of HO-1 by curcumin or haemin protected against TNFalpha-induced hyporesponsiveness to ACh. The competitive HO inhibitor, tin protoporphyrin (20 microM), abolished the protective effect of haemin. HO-1 induction prevented a TNFalpha-induced increase in NO generation without affecting the TNFalpha-induced increase in ROS generation. HO-1 induction prevented the TNFalpha-induced decrease in ACh-stimulated NO generation.

CONCLUSIONS AND IMPLICATIONS

Induction of HO-1 protected against TNFalpha impairment of endothelium-dependent relaxation in pulmonary artery, by a mechanism involving a reduction in inducible NO synthase-derived NO production.

Targeting heme oxygenase: therapeutic implications for diseases of the cardiovascular system

Heme oxygenase (HO) is important in attenuating the overall production of reactive oxygen species through its ability to degrade heme and to produce carbon monoxide, biliverdin/bilirubin, and release of free iron. Excess free heme catalyzes the formation of reactive oxygen species, which leads to endothelial cell (EC) dysfunction as seen in numerous pathologic vascular conditions including systemic hypertension and diabetes, as well as in ischemia/reperfusion injury.The up-regulation of HO-1 can be achieved through the use of pharmaceutical agents such as metalloporphyrins and statins. In addition, atrial natriuretic peptide and nitric oxide donors are important modulators of the heme-HO system, either through induction of HO-1 or the increased biologic activity of its products. Gene therapy and gene transfer, including site- and organ-specific targeted gene transfer have become powerful tools for studying the potential role of the 2 isoforms of HO, HO-1/HO-2, in the treatment of cardiovascular disease, as well as diabetes. HO-1 induction by pharmacological agents or the in vitro gene transfer of human HO-1 into ECs increases cell cycle progression and attenuates angiotensin II, tumor necrosis factor-alpha, and heme-mediated DNA damage; administration in vivo corrects blood pressure elevation after angiotensin II exposure. Delivery of human HO-1 to hyperglycemic rats significantly lowers superoxide levels and prevents EC damage and sloughing of vascular EC into the circulation. In addition, administration of human HO-1 to rats in advance of ischemia/reperfusion injury considerably reduces tissue damage.The ability to up-regulate HO-1 either through pharmacological means or through the use of gene therapy may offer therapeutic strategies for the prevention of cardiovascular disease in the future. This review discusses the implications of HO-1 delivery during the early stages of cardiovascular system injury or in early vascular pathology, and suggests that pharmacological agents that regulate HO activity or HO-1 gene delivery itself may become powerful tools for preventing the onset or progression of various cardiovascular diseases.

Up-regulating the hemeoxygenase system enhances insulin sensitivity and improves glucose metabolism in insulin-resistant diabetes in Goto-Kakizaki rats

Insulin-mediated signal transduction is positively correlated to adiponectin, adenosine monophosphate-activated protein kinase (AMPK), and glucose-transporter-4 (GLUT4) but negatively to oxidative/inflammatory mediators such as nuclear factor-kappaB, activating-protein (AP)-1, AP-2, and c-Jun-N-terminal-kinase. Although hemeoxygenase (HO) suppresses oxidative insults, its effects on insulin-sensitizing agents like AMPK and GLUT4 remains unclear and were investigated using Goto-Kakizaki rats (GK), a nonobese insulin-resistant type-2 diabetic model. HO was induced with hemin or inhibited with chromium mesoporphyrin (CrMP). The application of hemin to GK rats evoked a 3-month antidiabetic effect, whereas the HO-inhibitor, CrMP, exacerbated hyperglycemia and nullified insulin-signaling/glucose metabolism. Interestingly, the antidiabetic was accompanied by a paradoxical increase of insulin alongside the potentiation of insulin-sensitizing agents such as adiponectin, AMPK, and GLUT4 in the gastrocnemius muscle. Furthermore, hemin enhanced mediators/regulators of insulin signaling like cGMP and cAMP and suppressed oxidative insults by up-regulating HO-1, HO activity, superoxide dismutase, catalase, and the total antioxidant capacity in the gastrocnemius muscle. Accordingly, oxidative markers/mediators including nuclear factor-kappaB, AP-1, AP-2, c-Jun-N-terminal-kinase, and 8-isoprostane were abated, whereas CrMP annulled the cytoprotective and antidiabetic effects of hemin. Correspondingly, ip glucose tolerance, insulin tolerance, and homeostasis model assessment insulin resistance analyses revealed improved glucose tolerance, reduced insulin intolerance, enhanced insulin sensitivity, and reduced insulin resistance in hemin-treated GK rats. In contrast, CrMP, abolished the insulin-sensitizing effects and restored and/or exacerbated insulin resistance. Our study unveils a 3-month enduring antidiabetic effect of hemin and unmasks the synergistic interaction among the HO system, adiponectin, AMPK, and GLUT4 that could be explored to enhance insulin signaling and improve glucose metabolism in insulin-resistant diabetes.

Upregulation of the heme oxygenase system ameliorates postprandial and fasting hyperglycemia in type 2 diabetes

In type 2 diabetes (T2D), postprandial and fasting hyperglycemia are important predictors of cardiovascular diseases; however, few drugs are currently available to simultaneously suppress these conditions. Here, we report an enduring antidiabetic effect of the heme oxygenase (HO) inducer hemin on Goto-Kakizaki rats (GK), a nonobese insulin-resistant T2D model. HO breaks down the heme-moiety-generating antioxidants (biliverdin/bilirubin and ferritin) and carbon monoxide, which stimulate insulin secretion. Hemin induces HO-1 to potentiate HO activity and the HO-derived products. Chronically applied hemin (30 mg/kg ip) for a month reduced and maintained fasting glucose at physiological levels for 3 mo. Before therapy, glucose levels were 9.3 +/- 0.3 mmol/l (n = 14). At 1, 2, and 3 mo posttherapy, we recorded 6.7 +/- 0.13, 5.9 +/- 0.2, and 7.2 +/- 0.2 mmol/l, respectively. Hemin was also effective against postprandial hyperglycemia (14.6 +/- 1.1 vs. 7.5 +/- 0.4 mmol/l; n = 14; P < 0.01), and the effect remained sustained for 3 mo after therapy. The reduction of hyperglycemia was accompanied by enhanced HO-1, HO activity, and cGMP of the soleus muscle, alongside increased plasma bilirubin, ferritin, SOD, total antioxidant capacity, and insulin levels, whereas markers/mediators of oxidative stress like urinary-8-isoprostane and soleus muscle nitrotyrosine, NF-kappaB, and activator protein-1 and -2 were abated. Furthermore, inhibitors of insulin signaling including soleus muscle glycogen synthase kinase-3 and JNK were reduced, while the insulin-sensitizing adipokine, adiponectin, alongside AMPK were increased. Correspondingly, hemin improved glucose tolerance, suppressed insulin intolerance, reduced insulin resistance, and overturned the inability of insulin to enhance glucose transporter 4, a protein required for glucose uptake. Hemin also upregulated HO-1/HO activity and cGMP and lowered glucose in euglycemic Sprague-Dawley control rats albeit less intensely, suggesting greater selectivity of the HO system in diabetic conditions. In conclusion, reduced oxidative stress alongside the concomitant and paradoxical enhancement of insulin secretion and insulin-sensitizing pathways may account for the 3-mo-enduring antidiabetic effect. The synergistic interaction among HO, adiponectin, and GLUT4 may be explored against insulin-resistant diabetes.

The heme oxygenase system abates hyperglycemia in Zucker diabetic fatty rats by potentiating insulin-sensitizing pathways

Emerging evidence indicates that aldosterone causes oxidative stress by stimulating proinflammatory/oxidative mediators, including nuclear factor-kappaB, activating protein (AP-1), and c-Jun N-terminal kinase. Thus, in insulin-resistant type 2 diabetes (T2D), oxidative stress generated by hyperglycemia and aldosterone would potentiate the oxidative destruction of tissue and important regulators of glucose metabolism like adiponectin and insulin. Although heme oxygenase (HO)-1 is cytoprotective, its effects on T2D have not been fully characterized. Here we report an enduring antidiabetic effect of the HO inducer, hemin, on Zucker diabetic-fatty rat (ZDF), a model of insulin-resistant T2D. Chronically applied hemin to ZDF reduced and maintained significantly low fasting and postprandial hyperglycemia for 4 months after therapy. The antidiabetic effect was accompanied by enhanced HO activity, catalase, cyclic GMP, bilirubin, ferritin, total antioxidant capacity, and insulin. In contrast, reduced aldosterone alongside markers/mediators of oxidative stress, including 8-isoprostane, c-Jun N-terminal kinase, nuclear factor-kappaB, AP-1, and AP-2 were observed. Interestingly, in hemin-treated ZDF, inhibitory proteins of insulin-signaling, such as glycogen synthase kinase-3 and protein-tyrosine phosphatase-1B were reduced, whereas agents that promote insulin signaling including adiponectin, cAMP, AMP-activated protein kinase, aldolase-B, and glucose transporter-4 (GLUT4), were robustly increased. Correspondingly, hemin improved ip glucose tolerance, reduced insulin intolerance, and lowered insulin resistance (homeostasis model assessment of insulin resistance), and the inability of insulin to enhance GLUT4 was overturned. These results suggest that the suppression of hyperglycemia and aldosterone-induced oxidative stress alongside the potentiation of insulin-sensitizing pathways may account for the 4-month enduring antidiabetic effect. The synergistic interaction between the HO system, aldolase-B, adiponectin, AMP-activated protein kinase, and GLUT4 may be explored for novel strategies against postprandial/fasting hyperglycemia and insulin-resistant T2D.

Heme oxygenase system enhances insulin sensitivity and glucose metabolism in streptozotocin-induced diabetes

Hyperglycemia-induced oxidative stress is a common phenomenon in diabetes. Since oxidative stress depletes adiponectin and insulin levels, we investigated whether an upregulated heme oxygenase (HO) system would attenuate the oxidative destruction of adiponectin/insulin and improve insulin sensitivity and glucose metabolism in streptozotocin (STZ)-induced type 1 diabetes. HO was upregulated with hemin (15 mg/kg ip) or inhibited with chromium mesoporphyrin (CrMP, 4 micromol/kg ip). Administering hemin to STZ-diabetic rats reduced hyperglycemia and improved glucose metabolism, whereas the HO inhibitor CrMP annulled the antidiabetic effects and/or exacerbated fasting/postprandial hyperglycemia. Interestingly, the antidiabetic effects of hemin lasted for 2 mo after termination of therapy and were accompanied by enhanced HO-1 and HO activity of the soleus muscle, along with potentiation of plasma antioxidants like bilirubin, ferritin, and superoxide dismutase, with corresponding elevation of the total antioxidant capacity. Importantly, hemin abated c-Jun NH2-terminal kinase (JNK), a substance known to inhibit insulin biosynthesis, and suppressed markers/mediators of oxidative stress including 8-isoprostane, nuclear-factor (NF)-kappaB, activating protein (AP)-1, and AP-2 of the soleus muscle. Furthermore, hemin therapy significantly attenuated pancreatic histopathological lesions including acinar cell necrosis, interstitial edema, vacuolization, fibrosis, and mononuclear cell infiltration. Correspondingly, hemin increased plasma insulin and potentiated agents implicated in insulin sensitization and insulin signaling such as adiponectin, adenosine monophosphate-activated protein kinase (AMPK), cAMP, cGMP, and glucose transporter (GLUT)4, a protein required for glucose uptake. These were accompanied by improved glucose tolerance [intraperitoneal glucose tolerance text (IPGTT)], decreased insulin intolerance [intraperitoneal insulin tolerance test (IPITT)], and reduced insulin resistance [homeostasis model assessment of insulin resistance (HOMA-IR) index], whereas CrMP nullified the hemin-dependent antidiabetic and insulin-sensitizing effects. In conclusion, by concomitantly enhancing insulin and paradoxically potentiating insulin sensitivity, this study unveils a novel, unique, and long-lasting antidiabetic characteristic of upregulating HO with hemin that could be exploited against insulin-resistant and insulin-dependent diabetes.

H2S as a physiologic vasorelaxant: hypertension in mice with deletion of cystathionine gamma-lyase

Studies of nitric oxide over the past two decades have highlighted the fundamental importance of gaseous signaling molecules in biology and medicine. The physiological role of other gases such as carbon monoxide and hydrogen sulfide (H2S) is now receiving increasing attention. Here we show that H2S is physiologically generated by cystathionine gamma-lyase (CSE) and that genetic deletion of this enzyme in mice markedly reduces H2S levels in the serum, heart, aorta, and other tissues. Mutant mice lacking CSE display pronounced hypertension and diminished endothelium-dependent vasorelaxation. CSE is physiologically activated by calcium-calmodulin, which is a mechanism for H2S formation in response to vascular activation. These findings provide direct evidence that H2S is a physiologic vasodilator and regulator of blood pressure.

Use of carbon monoxide as a therapeutic agent: promises and challenges

As a by-product of heme catabolism by the heme oxygenase system, carbon monoxide (CO) has been neglected for many years, and only recently has its role as an essential signaling molecule been appreciated. In the past decade, the use of CO gas in pre-clinical experimental models of disease has produced some remarkable data indicating that its therapeutic delivery to mammals could alleviate inflammatory processes and cardiovascular disorders. However, the inherent toxic nature of CO cannot be ignored, knowing that inhalation of uncontrolled amounts of this gas can ultimately lead to serious systemic complications and neuronal derangements. From a clinical perspective, a key question is whether a safe and therapeutically effective threshold of CO can be reached locally in organs and tissues without delivering potentially toxic amounts through the lung. The advent of CO-releasing molecules (CO-RMs), a group of compounds capable of carrying and liberating controlled quantities of CO in cellular systems, appears a plausible alternative in the attempt to overcome the limitations of CO gas. Although in its infancy and far from being used for clinical applications, the CO-RMs technology is supported by very encouraging biological results and reflected by the chemical versatility of these compounds and their endless potential to be transformed into CO-based pharmaceuticals.

Pharmacological and clinical aspects of heme oxygenase

This review is intended to stimulate interest in the effect of increased expression of heme oxygenase-1 (HO-1) protein and increased levels of HO activity on normal and pathological states. The HO system includes the heme catabolic pathway, comprising HO and biliverdin reductase, and the products of heme degradation, carbon monoxide (CO), iron, and biliverdin/bilirubin. The role of the HO system in diabetes, inflammation, heart disease, hypertension, neurological disorders, transplantation, endotoxemia and other pathologies is a burgeoning area of research. This review focuses on the clinical potential of increased levels of HO-1 protein and HO activity to ameliorate tissue injury. The use of pharmacological and genetic probes to manipulate HO, leading to new insights into the complex relationship of the HO system with biological and pathological phenomena under investigation, is reviewed. This information is critical in both drug development and the implementation of clinical approaches to moderate and to alleviate the numerous chronic disorders in humans affected by perturbations in the HO system.

Regulation of maternal and fetal hemodynamics by heme oxygenase in mice

Heme oxygenase (HMOX) regulates vascular tone and blood pressure through the production of carbon monoxide (CO), a vasodilator derived from the heme degradation pathway. During pregnancy, the maternal circulation undergoes significant adaptations to accommodate the hemodynamic demands of the developing fetus. Our objective was to investigate the role of HMOX on maternal and fetal hemodynamics during pregnancy in a mouse model. We measured and compared maternal tissue and placental HMOX activity and endogenous CO production, represented by excreted CO and carboxyhemoglobin levels, during pregnancy (Embryonic Days 12.5-15.5) to nonpregnant controls. Micro-ultrasound was used to monitor maternal abdominal aorta diameters as well as blood flow velocities and diameters of fetal umbilical arteries. Tin mesoporphyrin, a potent HMOX inhibitor, was used to inhibit HMOX activity. Changes in maternal vascular tone were monitored by tail cuff blood pressure measurements. Effects of HMOX inhibition on placental structures were assessed by histology. We showed that maternal tissue and placental HMOX activity and CO production were significantly elevated during pregnancy. When HMOX in the placenta was inhibited, maternal and fetal hemodynamics underwent significant changes, with maternal blood pressures increasing. We concluded that increases in maternal tissue and placental HMOX activity contribute to the regulation of peripheral vascular resistance and therefore are important for the maintenance of normal maternal vascular tone and fetal hemodynamic functions during pregnancy.

''Iatrogenic Gilbert syndrome''--a strategy for reducing vascular and cancer risk by increasing plasma unconjugated bilirubin

The catabolism of heme, generating biliverdin, carbon monoxide, and free iron, is mediated by heme oxygenase (HO). One form of this of this enzyme, heme oxygenase-1, is inducible by numerous agents which promote oxidative stress, and is now known to provide important antioxidant protection, as demonstrated in many rodent models of free radical-mediated pathogenesis, and suggested by epidemiology observing favorable health outcomes in individuals carrying high-expression alleles of the HO-1 gene. The antioxidant impact of HO-1 appears to be mediated by bilirubin, generated rapidly from biliverdin by ubiquitously expressed biliverdin reductase. Bilirubin efficiently scavenges a wide range of physiological oxidants by electron donation. In the process, it is often reconverted to biliverdin, but biliverdin reductase quickly regenerates bilirubin, thereby greatly boosting its antioxidant potential. There is also suggestive evidence that bilirubin inhibits the activity or activation of NADPH oxidase. Increased serum bilirubin is associated with reduced risk for atherogenic disease in epidemiological studies, and more limited data show an inverse correlation between serum bilirubin and cancer risk. Gilbert syndrome, a genetic variant characterized by moderate hyperbilirubinemia attributable to reduced hepatic expression of the UDP-glucuronosyltransferase which conjugates bilirubin, has been associated with a greatly reduced risk for ischemic heart disease and hypertension in a recent study. Feasible strategies for boosting serum bilirubin levels may include administration of HO-1 inducers, supplementation with bilirubin or biliverdin, and administration of drugs which decrease the efficiency of hepatic bilirubin conjugation. The well-tolerated uricosuric drug probenecid achieves non-competitive inhibition of hepatic glucuronidation reactions by inhibiting the transport of UDP-glucuronic acid into endoplasmic reticulum; probenecid therapy is included in the differential diagnosis of hyperbilirubinemia, and presumably could be used to induce an ''iatrogenic Gilbert syndrome''. Other drugs, such as rifampin, can raise serum bilirubin through competitive inhibition of hepatocyte bilirubin uptake--although unfortunately rifampin is not as safe as probenecid. Measures which can safely achieve moderate serum elevations of bilirubin may prove to have value in the prevention and/or treatment of a wide range of disorders in which oxidants play a prominent pathogenic role, including many vascular diseases, cancer, and inflammatory syndromes. Phycobilins, algal biliverdin metabolites that are good substrates for biliverdin reductase, may prove to have clinical antioxidant potential comparable to that of bilirubin.

Propofol attenuation of renal ischemia/reperfusion injury involves heme oxygenase-1

AIM

To examine the protective effect of propofol in renal ischemia/reperfusion (I/R) injury and the role of heme oxygenase-1 (HO-1) in this process.

METHODS

Sprague-Dawley rats were randomly divided into 3 groups: (i) sham-operated group; (ii) I/R group; and (iii) propofol group. Bilateral renal warm ischemia for 45 min was performed. After 2, 6, and 24 h reperfusion, blood samples and kidneys were collected for assessment of renal injury, and HO-1 expressions were analyzed by immunohistochemical analysis, RT-PCR and Western blotting.

RESULTS

Blood urea nitrogen and serum creatinine levels in the propofol group were significantly lower than that in the I/R group at 24 h after reperfusion. The mean histological score by Palleros standard showed that propofol significantly attenuated renal I/R injury after 6 h reperfusion. Propofol increased HO-1 mRNA and protein levels 2 h after reperfusion, whereas HO-1 expressions were present at exceedingly low levels in the I/R group and the sham-operated group at same time point. Propofol also markedly increased HO-1 mRNA and protein levels than I/R at 6 and 24 h after reperfusion.

CONCLUSION

These results suggest that propofol mitigates renal I/R injury in rats. This protection may be partly through the induction of the HO-1 expression.

Influence of the heme-oxygenase pathway on cerebrocortical blood flow

Heme-oxygenase (HO)-derived carbon monoxide (CO) is generated in the cardiovascular and in the central nervous systems. Endogenous CO exerts direct vascular effects and has also been shown to inhibit nitric oxide synthase (NOS). In the current study, the heme-oxygenase blockade [zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG), 45 micromol/kg intraperitoneally] decreased cerebral CO production and increased cerebrocortical blood flow (CBF) in anesthetized rats. This latter effect was abrogated by the NOS blockade (50 mg/kg L-NAME intravenously). Furthermore, inhibition of CO production had no effect on stepwise hypoxia/hypercapnia-stimulated increases in CBF. Our results indicate that endogenous CO reduces the resting CBF via inhibition of NOS but fails to influence the CBF response to hypoxia and hypercapnia in adult rats.

Continuous inhalation of carbon monoxide induces right ventricle ischemia and dysfunction in rats with hypoxic pulmonary hypertension

We aimed to investigate the toxicity of carbon monoxide (CO) in rats with right ventricle (RV) remodeling induced by hypoxic pulmonary hypertension (PHT). A group of Wistar rats was exposed to 3-wk hypobaric hypoxia (H). A second group was exposed to 50 ppm CO for 1 wk (CO). A third group was exposed to chronic hypoxia including 50 ppm CO during the third week (H+CO). These groups were compared with controls. RV and left ventricle (LV) functions were assessed by echocardiography and transparietal catheterization. Ventricular perfusion was estimated with the fluorescent microsphere method. Results were confirmed by histology. PHT induced RV hypertrophy and function enhancement. In the H group, RV shortening fraction (RVSF; 71 +/- 12% vs. 41 +/- 2%) and RV end-systolic pressure (RVESP; 54 +/- 6 vs. 19 +/- 2 mmHg) were increased compared with controls. Moreover, myocardial perfusion was increased in the RV (36 +/- 2% vs. 22 +/- 2%) and decreased in the LV (64 +/- 3% vs. 78 +/- 2%). In the H+CO group, RVSF (45 +/- 3% vs. 71 +/- 12%) and RVESP (38 +/- 3 vs. 54 +/- 6 mmHg) were decreased compared with the H group. RV perfusion was decreased in the H+CO group compared with the H group (21 +/- 5% vs. 36 +/- 2%), and LV perfusion was increased (79 +/- 5% vs. 64 +/- 3%). PHT and RV hypertrophy were still present in the H+CO group, and fibroses localized in the RV were detected. Similar lesions were observed in an additional group exposed simultaneously to hypoxia and 50 ppm CO over 3 wk. We demonstrated that rats with established PHT were more sensitive to CO, which dramatically alters the RV adaptive response to PHT, leading to ischemic lesions.

An analysis of the DOCA-salt model of hypertension in HO-1-/- mice and the Gunn rat

Heme oxygenase-1 (HO-1) is induced in the vasculature in the DOCA-salt model of hypertension in rats. Whereas the HO system and its products may exert vasodilator effects, recent studies have suggested that the HO system may predispose to hypertension. The present study examined the effects of selected components of the HO system, specifically, the HO-1 isozyme and the product bilirubin in the DOCA-salt model of systemic hypertension; the experimental approach employed mutant rodent models, namely, the HO-1(-/-) mouse and the hyperbilirubinemic Gunn rat. DOCA-salt induced HO-1 protein in the aorta in HO-1(+/+) mice and provoked a significant rise in systolic arterial pressure in HO-1(-/-) mice but not in HO-1(+/+) mice; this effect could not be ascribed to impaired urinary sodium excretion or impaired glomerular filtration rate in the DOCA-salt-treated HO-1(-/-) mice. The administration of DOCA salt to uninephrectomized rats significantly increased systolic arterial pressure in wild-type rats, an effect that was attenuated in the mutant Gunn rat; this reduction in systemic hypertension in the DOCA-salt-treated Gunn rat was not due to a greater induction of HO-1 in the vasculature or to a more avid urinary sodium excretion. DOCA-salt impaired endothelium-dependent and endothelium-independent vasorelaxation in wild-type rats but not in Gunn rats; prior exposure to bilirubin repaired the defect in endothelium-dependent vasorelaxation in aortic rings in DOCA-salt-treated rats. DOCA salt stimulated vascular production of superoxide anion in wild-type but not in Gunn rats. We suggest that HO-1 and the product bilirubin may exert a countervailing effect in the DOCA-salt model of systemic hypertension.

Heme oxygenase inhibition by 2-oxy-substituted 1-(1H-imidazol-1-yl)-4-phenylbutanes: effect of halogen substitution in the phenyl ring

A series of 2-oxy-substituted 1-(1H-imidazol-1-yl)-4-phenylbutanes comprising imidazole-ketones, imidazole-dioxolanes, and imidazole-alcohols substituted with halogens in the phenyl ring were synthesized and evaluated as novel inhibitors of heme oxygenase which are structurally distinct from metalloporphyrins. The entire library of compounds was found to be highly active, with the bromine- and iodine-substituted derivatives being the most potent. The imidazole-dioxolanes were all selective for the HO-1 isozyme (inducible) and exhibited substantially lower activity toward the HO-2 isozyme (constitutive). The corresponding imidazole-ketones and imidazole-alcohols showed selectivity toward HO-1 to a lesser degree than the similarly substituted imidazole-dioxolanes.

Role of carbon monoxide in cardiovascular function

Carbon monoxide (CO) is an endogenously derived gas formed from the breakdown of heme by the enzyme heme oxygenase. Although long considered an insignificant and potentially toxic waste product of heme catabolism, CO is now recognized as a key signaling molecule that regulates numerous cardiovascular functions. Interestingly, alterations in CO synthesis are associated with many cardiovascular disorders, including atherosclerosis, septic shock, hypertension, metabolic syndrome, and ischemia-reperfusion injury. Significantly, restoration of physiologic CO levels exerts a beneficial effect in many of these settings, suggesting a crucial role for CO in maintaining cardiovascular homeostasis. In this review, we outline the actions of CO in the cardiovascular system and highlight this gas as a potential therapeutic target in treating a multitude of cardiovascular disorders.

Role of Renal Medullary Heme Oxygenase in the Regulation of Pressure Natriuresis and Arterial Blood Pressure

Recent studies have demonstrated that inhibition of renal medullary heme oxygenase (HO) activity and carbon monoxide (CO) significantly decreases renal medullary blood flow and sodium excretion. Given the crucial role of renal medullary blood flow in the control of pressure natriuresis, the present study was designed to determine whether renal medullary HO activity and resulting CO production participate in the regulation of pressure natriuresis and thereby the long-term control of arterial blood pressure. In anesthetized Sprague-Dawley rats, increases in renal perfusion pressure induced significant elevations of CO concentrations in the renal medulla. Renal medullary infusion of chromium mesoporphyrin (CrMP), an inhibitor of HO activity, remarkably inhibited HO activity and the renal perfusion pressure-dependent increases in CO levels in the renal medulla and significantly blunted pressure natriuresis. In conscious Sprague-Dawley rats, continuous infusion of CrMP into the renal medulla significantly increased mean arterial pressure (129±2.5 mm Hg in CrMP group versus 118±1.6 mm Hg in vehicle group) when animals were fed a normal salt diet (1% NaCl). After rats were switched to a high-salt diet (8% NaCl) for 10 days, CrMP-treated animals exhibited further increases in mean arterial pressure compared with CrMP-treated animals that were kept on normal salt diet (152±4.1 versus 130±4.2 mm Hg). These results suggest that renal medullary HO activity plays a crucial role in the control of pressure natriuresis and arterial blood pressure and that impairment of this HO/CO-mediated antihypertensive mechanism in the renal medulla may result in the development of hypertension.

Heme oxygenase activity in fetal and adult sheep is not altered by acclimatization to high altitude hypoxia

Hypoxic stress has been reported to induce the expression of stress proteins such as heme oxygenase (HO), which catalyze the breakdown of heme to generate biliverdin, ferrous iron, and carbon monoxide. These degradation products play a role in the regulation of a variety of processes such as vascular tone, inflammation, and central nervous system function. In mammals, there are 2 catalytically functional HO isozymes, HO-1 (inducible) and HO-2 (constitutive). HO-1 expression is regulated by an array of nonphysiological and physiological stimuli including acute hypoxemia. As relatively little is known of the HO response to prolonged hypoxia in whole animals other than small laboratory rodents, the aim of this work was to examine the effect of long-term hypoxia on total HO activity in fetal and adult ovine tissue. Sheep were maintained at high altitude (3820 m), after which the following tissues were harvested from near-term fetal and non-pregnant ewes for in vitro measurement of HO activity: left ventricle, renal papilla, lung apex, pulmonary artery, carotid artery, mesenteric artery, placental cotyledon, spleen, and brain frontal cortex. There were no significant differences between HO activities in tissues from hypoxic fetal and adult sheep compared with their normoxic controls. Fetal heart HO activities were higher than those of adult tissue (p < 0.05), whereas adult spleen HO activity was significantly higher than that of fetal tissue (p < 0.05). In conclusion, these data indicate that long-term exposure to high altitude hypoxia does not have a persistent effect on HO activity in ovine tissues. Also, except for the spleen where there is a high expression of HO-1 under normal conditions, tissue HO activity is correlated with the expression of HO-2, the constitutive isozyme.

Heme oxygenase-1: a provenance for cytoprotective pathways in the kidney and other tissues

Heme oxygenase (HO) is the rate-limiting enzyme in the degradation of heme, converting heme to biliverdin, during which iron is released and carbon monoxide (CO) is emitted; biliverdin is subsequently converted to bilirubin by biliverdin reductase. At least two isozymes possess HO activity: HO-1 represents the isozyme induced by diverse stressors, including ischemia, nephrotoxins, cytokines, endotoxin, oxidants, and vasoactive substances; HO-2 is the constitutive, glucocorticoid-inducible isozyme. HO-1 is upregulated in the kidney in assorted conditions and diseases. Interest in HO is driven by the capacity of this system to protect the kidney against injury, a capacity likely reflecting, at least in part, the cytoprotective properties of its products: in relatively low concentrations, CO exerts vasorelaxant, antiapoptotic, and anti-inflammatory effects while bile pigments are antioxidant and anti-inflammatory metabolites. This article reviews the HO system and the extent to which it influences the function of the healthy kidney; it summarizes conditions and stimuli that elicit HO-1 in the kidney; and it explores the significance of renal expression of HO-1 as induced by ischemia, nephrotoxins, nephritides, transplantation, angiotensin II, and experimental diabetes. This review also points out the tissue specificity of the HO system, and the capacity of HO-1 to induce renal injury in certain settings. Studies of HO in other tissues are discussed insofar as they aid in elucidating the physiologic and pathophysiologic significance of the HO system in the kidney.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

CO as a cellular signaling molecule

Many biological functions of heme oxygenase (HO), such as cytoprotection against oxidative stress, vasodilation, neurotransmission in the central or peripheral nervous systems, and anti-inflammatory, anti-apoptotic, or anti-proliferative potential, have been attributed to its enzymatic byproduct carbon monoxide (CO), although roles for biliverdin/bilirubin and iron have also been proposed. In addition to these well-characterized effects, recent findings reveal that HO-derived CO may act as an oxygen sensor and circadian modulator of heme biosynthesis. In lymphocytes, CO may participate in regulatory T cell function. A number of the known signaling effects of CO depend on stimulation of soluble guanylate cyclase and/or activation of mitogen-activated protein kinases (MAPK). Furthermore, modulation of caveolin-1 status may serve as an essential component of certain aspects of CO action, such as growth control. In this review, we summarize recent findings of the beneficial or detrimental effects of endogenous CO with an emphasis on the signaling pathways and downstream targets that trigger the action of this gas.

Exercise and Vasorelaxing Effects of CO-Releasing Molecules in Hypertensive Rats

PURPOSE

Because carbon monoxide (CO) has been reported able to induce relaxation, we aimed to investigate the effects of exercise training on the rat thoracic aorta responsiveness to a CO-releasing molecule (CORM), tricarbonyldichlororuthenium ([Ru(CO)3Cl2]2).

METHODS

Male Wistar rats (N = 32) were divided in hypertensive and normotensive groups using the two-kidney, one-clip model of Goldblatt or SHAM surgery. Hypertensive and normotensive groups were assigned to an exercise training protocol on a level treadmill over a 10-wk period or were assigned to remain sedentary. After the exercise training protocol, blood pressure and cardiac tissue weight were assessed. The responsiveness of endothelium-denuded thoracic aortic rings to [Ru(CO)3Cl2]2 was evaluated by isometric contractions recordings.

RESULTS

Systolic, diastolic, and mean blood pressures were significantly increased in hypertensive rats compared with control rats. Exercise training did not significantly alter blood pressure but decreased pulse pressure in hypertensive animals compared with sedentary hypertensive rats. In all groups, application of [Ru(CO)3Cl2]2 induced relaxation in precontracted aortic rings. Compared with normotensive rats, CO-induced relaxation was significantly decreased in hypertensive rats. Nevertheless, training exercise increased relaxation markedly in response to [Ru(CO)3Cl2]2 application in hypertensive rats, whereas it remained without effect in control rings. Pretreatment with TEA, a nonselective K+ channel inhibitor, decreased [Ru(CO)3Cl2]2-induced relaxation in all groups that became similar. In trained hypertensive rats, iberiotoxin had effects similar to those of TEA.

CONCLUSIONS

This finding supports the concept that the CORM [Ru(CO)3Cl2]2 can induce relaxation in both normotensive and hypertensive rats with an impairment of the CO-induced relaxation during hypertension. However, exercise training improves the aorta's ability to relax in response to [Ru(CO)3Cl2]2 during hypertension, probably by increasing K+ channel activity.

Effects of carbon monoxide and heme oxygenase inhibitors in cerebral vessels of rats and mice

Carbon monoxide (CO) has been postulated to be a signaling molecule in many tissues, including the vasculature. We examined vasomotor responses of adult rat and mouse cerebral arteries to both exogenously applied and endogenously produced CO. The diameter of isolated, pressurized, and perfused rat middle cerebral arteries (MCAs) was not altered by authentic CO (10(-6) to 10(-4) M). Mouse MCAs, however, dilated by 21 +/- 10% at 10(-4) M CO. Authentic nitric oxide (NO., 10(-10) to 10(-7) M) dilated both rat and mouse MCAs. At 10(-8) M NO., rat vessels dilated by 84 +/- 4%, and at 10(-7) M NO., mouse vessels dilated by 59 +/- 9%. Stimulation of endogenous CO production through heme oxygenase (HO) with the heme precursor delta-aminolevulinic acid (10(-10) to 10(-4) M) did not dilate the MCAs of either species. The metalloporphyrin HO inhibitor chromium mesoporphyrin IX (CrMP) caused profound constriction of the rat MCA (44 +/- 2% at 3 x 10(-5) M). Importantly, this constriction was unaltered by exogenous CO (10(-4) M) or CO plus 10(-5) M biliverdine (both HO products). In contrast, exogenous CO (10(-4) M) reversed CrMP-induced constriction in rat gracilis arterioles. Control mouse MCAs constricted by only 3 +/- 1% in response to 10(-5) M CrMP. Magnesium protoporphyrin IX (10(-5) M), a weak HO inhibitor used to control for nonspecific effects of metalloporphyrins, also constricted the rat MCA to a similar extent as CrMP. We conclude that, at physiological concentrations, CO is not a dilator of adult rodent cerebral arteries and that metalloporphyrin HO inhibitors have nonspecific constrictor effects in rat cerebral arteries.

Carbon monoxide: endogenous production, physiological functions, and pharmacological applications

Over the last decade, studies have unraveled many aspects of endogenous production and physiological functions of carbon monoxide (CO). The majority of endogenous CO is produced in a reaction catalyzed by the enzyme heme oxygenase (HO). Inducible HO (HO-1) and constitutive HO (HO-2) are mostly recognized for their roles in the oxidation of heme and production of CO and biliverdin, whereas the biological function of the third HO isoform, HO-3, is still unclear. The tissue type-specific distribution of these HO isoforms is largely linked to the specific biological actions of CO on different systems. CO functions as a signaling molecule in the neuronal system, involving the regulation of neurotransmitters and neuropeptide release, learning and memory, and odor response adaptation and many other neuronal activities. The vasorelaxant property and cardiac protection effect of CO have been documented. A plethora of studies have also shown the importance of the roles of CO in the immune, respiratory, reproductive, gastrointestinal, kidney, and liver systems. Our understanding of the cellular and molecular mechanisms that regulate the production and mediate the physiological actions of CO has greatly advanced. Many diseases, including neurodegenerations, hypertension, heart failure, and inflammation, have been linked to the abnormality in CO metabolism and function. Enhancement of endogenous CO production and direct delivery of exogenous CO have found their applications in many health research fields and clinical settings. Future studies will further clarify the gasotransmitter role of CO, provide insight into the pathogenic mechanisms of many CO abnormality-related diseases, and pave the way for innovative preventive and therapeutic strategies based on the physiologic effects of CO.