Induction of heme oxygenase-1 in monocytes suppresses angiotensin II-elicited chemotactic activity through inhibition of CCR2: role of bilirubin and carbon monoxide generated by the enzyme

Electronic and Tobacco Cigarettes Alter Polyunsaturated Fatty Acids and Oxidative Biomarkers

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Vascular and Macrophage Heme Oxygenase-1 in Hypertension: A Mini-Review

Hypertension is one predictive factor for stroke and heart ischemic disease. Nowadays, it is considered an inflammatory disease with elevated cytokine levels, oxidative stress, and infiltration of immune cells in several organs including heart, kidney, and vessels, which contribute to the hypertension-associated cardiovascular damage. Macrophages, the most abundant immune cells in tissues, have a high degree of plasticity that is manifested by polarization in different phenotypes, with the most well-known being M1 (proinflammatory) and M2 (anti-inflammatory). In hypertension, M1 phenotype predominates, producing inflammatory cytokines and oxidative stress, and mediating many mechanisms involved in the pathogenesis of this disease. The increase in the renin-angiotensin system and sympathetic activity contributes to the macrophage mobilization and to its polarization to the pro-inflammatory phenotype. Heme oxygenase-1 (HO-1), a phase II detoxification enzyme responsible for heme catabolism, is induced by oxidative stress, among others. HO-1 has been shown to protect against oxidative and inflammatory insults in hypertension, reducing end organ damage and blood pressure, not only by its expression at the vascular level, but also by shifting macrophages toward the anti-inflammatory phenotype. The regulatory role of heme availability for the synthesis of enzymes involved in hypertension development, such as cyclooxygenase or nitric oxide synthase, seems to be responsible for many of the beneficial HO-1 effects; additionally, the antioxidant, anti-inflammatory, antiapoptotic, and antiproliferative effects of the end products of its reaction, carbon monoxide, biliverdin/bilirubin, and Fe²⁺, would also contribute. In this review, we analyze the role of HO-1 in hypertensive pathology, focusing on its expression in macrophages.

Angiotensin II and leukocyte trafficking: New insights for an old vascular mediator. Role of redox-signaling pathways

Inflammation and activation of the immune system are key molecular and cellular events in the pathogenesis of cardiovascular diseases, including atherosclerosis, hypertension-induced target-organ damage, and abdominal aortic aneurysm. Angiotensin II (Ang-II) is the main effector peptide hormone of the renin-angiotensin system. Beyond its role as a potent vasoconstrictor and regulator of blood pressure and fluid homeostasis, Ang-II is intimately involved in the development of vascular lesions in cardiovascular diseases through the activation of different immune cells. The migration of leukocytes from circulation to the arterial subendothelial space is a crucial immune response in lesion development that is mediated through a sequential and coordinated cascade of leukocyte-endothelial cell adhesive interactions involving an array of cell adhesion molecules present on target leukocytes and endothelial cells and the generation and release of chemoattractants that activate and guide leukocytes to sites of emigration. In this review, we outline the key events of Ang-II participation in the leukocyte recruitment cascade, the underlying mechanisms implicated, and the corresponding redox-signaling pathways. We also address the use of inhibitor drugs targeting the effects of Ang-II in the context of leukocyte infiltration in these cardiovascular pathologies, and examine the clinical data supporting the relevance of blocking Ang-II-induced vascular inflammation.

Association Between Serum Bilirubin and the Progression of Carotid Atherosclerosis in Type 2 Diabetes

Objective

This study investigated whether serum bilirubin levels can predict the progression of carotid atherosclerosis in individuals with type 2 diabetes mellitus (T2DM).

Methods

This observational study included 1,381 subjects with T2DM in whom serial measurements of carotid intima-media thickness (CIMT) were made at 1- to 2-year intervals for 6–8 years. The progression of carotid atherosclerosis was defined as newly detected plaque lesions on repeat ultrasonography. After dividing total serum bilirubin levels into tertiles, the association between total serum bilirubin at baseline and plaque progression status was analyzed.

Results

Among 1,381 T2DM patients, 599 (43.4%) were categorized as having plaque progression in their carotid arteries. Those with plaque progression were significantly older; showed a higher prevalence of hypertension, abdominal obesity, and chronic kidney disease; and had a longer duration of T2DM, higher levels of total cholesterol (TC), triglycerides, and insulin resistance, and lower total bilirubin concentrations than those with no plaque progression. When total serum bilirubin levels were divided into tertiles, the highest tertile group was younger than the lowest tertile group, with higher levels of TC and high-density lipoprotein cholesterol. Multiple logistic regression analysis demonstrated that higher serum bilirubin levels were associated with a significantly lower risk of CIMT progression (odds ratio, 0.584; 95% confidence interval, 0.392–0.870; p=0.008). Age (p<0.001), body mass index (p=0.023), and TC (p=0.019) were also associated with the progression of carotid atherosclerosis in T2DM patients.

Conclusion

Total serum bilirubin is independently associated with progression of atherosclerosis in the carotid arteries in T2DM patients.

Tempol reduces inflammation and oxidative damage in cigarette smoke-exposed mice by decreasing neutrophil infiltration and activating the Nrf2 pathway

Cigarette smoke is a complex mixture capable of triggering inflammation and oxidative damage in animals at pulmonary and systemic levels. Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) reduces tissue injury associated with inflammation in vivo by mechanisms that are not completely understood. Here we evaluated the effect of tempol on inflammation and oxidative damage induced by acute exposure to cigarette smoke in vivo. Male C57BL/6 mice (n = 32) were divided into 4 groups (n = 8 each): 1) control group exposed to ambient air (GC), 2) animals exposed to cigarette smoke for 5 days (CSG), mice treated 3) prior or 4) concomitantly with tempol (50 mg/kg/day) and exposed to cigarette smoke for 5 days. The results showed that the total number of leukocytes and neutrophils increased in the respiratory tract and lung parenchyma of mice exposed to cigarette smoke. Likewise, MPO levels and activity as well as lipid peroxidation and lung protein nitration and carbonylation also increased. Administration of tempol before or during exposure to cigarette smoke inhibited all the above parameters. Tempol also reduced the pulmonary expression of the inflammatory cytokines Il-6, Il-1β and Il-17 to basal levels and of Tnf-α by approximately 50%. In contrast, tempol restored Il-10 and Tgf-β levels and enhanced the expression of Nrf2-associated genes, such as Ho-1 and Gpx2. Accordingly, total GPx activity increased in lung homogenates of tempol-treated animals. Taken together, our results show that tempol protects mouse lungs from inflammation and oxidative damage resulting from exposure to cigarette smoke, likely through reduction of leukocyte infiltration and increased transcription of some of the Nrf2-controlled genes.

Heme-Oxygenase-1 Is Decreased in Circulating Monocytes and Is Associated With Impaired Phagocytosis and ROS Production in Lupus Nephritis

Lupus nephritis (LN) is one of the most serious manifestations of systemic lupus erythematosus (SLE). Based on studies showing the potential role of heme oxygenase-1 (HO-1), an enzyme that catalyzes the degradation of heme and has anti-inflammatory properties in SLE development, we decided to explore HO-1 in LN. Accordingly, we evaluated HO-1 levels and function in circulating and infiltrating monocytes and neutrophils of LN patients. HO-1 levels were assessed in peripheral monocytes of LN patients and controls by flow cytometry and immunofluorescence microscopy. Phagocytosis and the production of reactive oxygen species (ROS) were evaluated to determine the effect of HO-1 in monocyte function. In addition, renal biopsies with proliferative LN were used to identify HO-1 in infiltrating cells and renal tissue by immunofluorescence and immunohistochemistry. Biopsies of healthy controls (HC) and patients who underwent nephrectomy were included as controls. Circulating pro-inflammatory monocytes and activated neutrophils were increased in LN patients. HO-1 levels were decreased in all subsets of monocytes and in activated neutrophils. LN monocytes showed increased phagocytosis and higher production of ROS than those of HC. When HO-1 was induced, phagocytosis and ROS levels became similar to those of HC. HO-1 was mostly expressed in renal tubular epithelial cells (RTEC). Renal tissue of LN patients showed lower levels of HO-1 than HC, whereas infiltrating immune cells of LN showed lower levels of HO-1 than biopsies of patients who had renal surgery. HO-1 is decreased in circulating monocytes and activated neutrophils of LN patients. HO-1 levels modulate the phagocytosis of LN monocytes and ROS production. HO-1 expression in RTEC might be an attempt of self-protection from inflammation.

Monocytes as immune targets in arterial hypertension

The role of myelomonocytic cells appears to be critical for the initiation, progression and manifestation of arterial hypertension. Monocytes can induce vascular inflammation as well as tissue remodelling and (mal)adaptation by secreting chemokines and cytokines, producing ROS, expressing coagulation factors and transforming into macrophages. A multitude of adhesion molecules promote the infiltration and accumulation of monocytes into the kidney, heart, brain and vasculature in hypertension. All these facets offer the possibility to pharmacologically target monocytes and may represent novel therapeutic ways to treat hypertension, attenuate hypertension-associated end organ damage or prevent the development or worsening of high blood pressure. LINKED ARTICLES: This article is part of a themed section on Immune Targets in Hypertension. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.12/issuetoc.

Relation of Direct, Indirect, and Total bilirubin to Adverse Long-term Outcomes Among Patients With Acute Coronary Syndrome

Bilirubin is known as an antioxidant. However, there have been controversies over whether bilirubin is protective against cardiovascular disease or not. In addition, no study has examined the association between subtypes of total bilirubin (direct bilirubin [DB] and indirect bilirubin [IDB]) and long-term outcomes of acute coronary syndrome (ACS) patients. We included 533 consecutive patients with ACS. All the patients were followed up for the composite end point of cardiac death, revascularization, and acute heart failure. At a median follow-up of 2.4 years, Kaplan-Meier curve demonstrated that higher serum DB levels were significantly associated with major adverse cardiac events (MACE) (p <0.05). However, total bilirubin (TB) and IDB were not associated with MACE by Kaplan-Meier analysis. Cox analysis showed that high TB and DB were associated with increased risk of MACE in ACS even after adjustment of cardiovascular risk factors. The receiver operating characteristic curve illustrated that DB had a predictive value of MACE in ACS. In conclusion, we firstly reported that high TB and DB but not IDB were associated with increased risk of MACE in Chinese ACS, and the prognostic value of DB was superior to that of TB or IDB.

The role of chemokines in hypertension and consequent target organ damage

Immune cells infiltrate the kidney, vasculature, and central nervous system during hypertension, consequently amplifying tissue damage and/or blood pressure elevation. Mononuclear cell motility depends partly on chemokines, which are small cytokines that guide cells through an increasing concentration gradient via ligation of their receptors. Tissue expression of several chemokines is elevated in clinical and experimental hypertension. Likewise, immune cells have enhanced chemokine receptor expression during hypertension, driving immune cell infiltration and inappropriate inflammation in cardiovascular control centers. T lymphocytes and monocytes/macrophages are pivotal mediators of hypertensive inflammation, and these cells migrate in response to several chemokines. As powerful drivers of diapedesis, the chemokines CCL2 and CCL5 have long been implicated in hypertension, but experimental data highlight divergent, context-specific effects of these chemokines on blood pressure and tissue injury. Several other chemokines, particularly those of the CXC family, contribute to blood pressure elevation and target organ damage. Given the significant interplay and chemotactic redundancy among chemokines during disease, future work must not only describe the actions of individual chemokines in hypertension, but also characterize how manipulating a single chemokine modulates the expression and/or function of other chemokines and their cognate receptors. This information will facilitate the design of precise chemotactic immunotherapies to limit cardiovascular and renal morbidity in hypertensive patients.

Innovative biomarkers for predicting type 2 diabetes mellitus: relevance to dietary management of frailty in older adults

Type 2 diabetes mellitus (T2DM) increases in prevalence in the elderly. There is evidence for significant muscle loss and accelerated cognitive impairment in older adults with T2DM; these comorbidities are critical features of frailty. In the early stages of T2DM, insulin sensitivity can be improved by a "healthy" diet. Management of insulin resistance by diet in people over 65 years of age should be carefully re-evaluated because of the risk for falling due to hypoglycaemia. To date, an optimal dietary programme for older adults with insulin resistance and T2DM has not been described. The use of biomarkers to identify those at risk for T2DM will enable clinicians to offer early dietary advice that will delay onset of disease and of frailty. Here we have used an in silico literature search for putative novel biomarkers of T2DM risk and frailty. We suggest that plasma bilirubin, plasma, urinary DPP4-positive microparticles and plasma pigment epithelium-derived factor merit further investigation as predictive biomarkers for T2DM and frailty risk in older adults. Bilirubin is screened routinely in clinical practice. Measurement of specific microparticle frequency in urine is less invasive than a blood sample so is a good choice for biomonitoring. Future studies should investigate whether early dietary changes, such as increased intake of whey protein and micronutrients that improve muscle function and insulin sensitivity, affect biomarkers and can reduce the longer term complication of frailty in people at risk for T2DM.

Heme oxygenase-1 suppresses a pro-inflammatory phenotype in monocytes and determines endothelial function and arterial hypertension in mice and humans

AIMS

Heme oxygenase-1 (HO-1) confers protection to the vasculature and suppresses inflammatory properties of monocytes and macrophages. It is unclear how HO-1 determines the extent of vascular dysfunction in mice and humans.

METHODS AND RESULTS

Decreased HO-1 activity and expression was paralleled by increased aortic expression and activity of the nicotinamide dinucleotide phosphate oxidase Nox2 in HO-1 deficient Hmox1⁻/⁻ and Hmox1(⁺/⁻) compared with Hmox1⁺/⁺ mice. When subjected to angiotensin II-infusion, streptozotocin-induced diabetes mellitus and aging, HO-1 deficient mice showed increased vascular dysfunction inversely correlated with HO activity. In a primary prevention population-based cohort, we assessed length polymorphisms of the HMOX1 promoter region and established a bipolar frequency pattern of allele length (long vs. short repeats) in 4937 individuals. Monocytic HMOX1 mRNA expression was positively correlated with flow-mediated dilation and inversely with CD14 mRNA expression indicating pro-inflammatory monocytes in 733 hypertensive individuals of this cohort. Hmox1⁻/⁻ mice showed drastically increased expression of the chemokine receptor CCR2 in monocytes and the aorta. Angiotensin II-infused Hmox1⁻/⁻ mice had amplified endothelial inflammation in vivo, significantly increased aortic infiltration of pro-inflammatory CD11b⁺ Ly6C(hi) monocytes and Ly6G⁺ neutrophils and were marked by Ly6C(hi) monocytosis in the circulation and an increased blood pressure response. Finally, individuals with unfavourable HMOX1 gene promoter length had increased prevalence of arterial hypertension and reduced cumulative survival after a median follow-up of 7.23 years.

CONCLUSIONS

Heme oxygenase-1 is a regulator of vascular function in hypertension via determining the phenotype of inflammatory circulating and infiltrating monocytes with possible implications for all-cause mortality.

Effects of serum bilirubin on atherosclerotic processes

This review highlights the protective roles of bilirubin against the atherosclerotic process. Bilirubin belongs to the superfamily of tetrapyrrolic compounds formed during heme catabolism. Although for decades bilirubin was considered to be a harmful waste product, recent epidemiologic studies have shown that serum bilirubin levels have consistently been inversely associated with cardiovascular disease (CVD), as well as cardiovascular risk factors such as metabolic syndrome and diabetes. These clinical studies are supported by in vitro and in vivo experimental data and have demonstrated that bilirubin not only has an ability to scavenge overproduced reactive oxygen species and inhibit vascular smooth muscle cell proliferation but, additionally, has anti-inflammatory effects. In this review, we will discuss the inverse association of serum bilirubin and CVD and cardiovascular risk factors established in various clinical studies. We also review detailed experimental studies about the effect of bilirubin on atherosclerotic processes. In vitro, animal and human studies have proved that bilirubin inhibits oxidation of cholesterol which is an important step of atherosclerosis. Bilirubin attenuates chemotactic activity of monocytes and strongly inhibits adhesion of leukocytes to venule and production of pro-inflammatory cytokines. Bilirubin has inhibited serum-driven smooth muscle cell cycle progression at the G1 phase. Lastly, we will discuss briefly the influence of bilirubin on lipoprotein composition and endothelial dysfunction.

The mononuclear phagocyte system in homeostasis and disease: a role for heme oxygenase-1

SIGNIFICANCE

Heme oxygenase-1 (HO-1) is a potential therapeutic target in many diseases, especially those mediated by oxidative stress and inflammation. HO-1 expression appears to regulate the homeostatic activity and distribution of mononuclear phagocytes (MP) in lymphoid tissue under physiological conditions. It also regulates the ability of MP to modulate the inflammatory response to tissue injury.

RECENT ADVANCES

The induction of HO-1 within MP-particularly macrophages and dendritic cells-modulates the effector functions that they acquire after activation. These effector functions include cytokine production, surface receptor expression, maturation state, and polarization toward a pro- or anti-inflammatory phenotype. The importance of HO-1 in MP is emphasized by their expression of specific receptors that primarily function to ingest heme-containing substrate and deliver it to HO-1.

CRITICAL ISSUES

MP are the first immunological responders to tissue damage. They critically affect the outcome of injury to many organ systems, yet few therapies are currently available to specifically target MP during disease pathogenesis. Elucidation of the role of HO-1 expression in MP may help to direct broadly applicable therapies to clinical use that are based on the immunomodulatory capabilities of HO-1.

FUTURE DIRECTIONS

Unraveling the complexities of HO-1 expression specifically within MP will more completely define how HO-1 provides cytoprotection in vivo. The use of models in which HO-1 expression is specifically modulated in bone marrow-derived cells will allow for a more complete characterization of its immunoregulatory properties.

Heme oxygenase-1 promotes granuloma development and protects against dissemination of mycobacteria

Non-tuberculous mycobacterial (NTM) infections occur in both immunocompromised and immunocompetent hosts and are an increasingly recognized cause of morbidity and mortality. The hallmark of pulmonary mycobacterial infections is the formation of granuloma in the lung. Our study focuses on the role of heme oxygenase-1 (HO-1), a cytoprotective enzyme, in the regulation of granuloma development and maturation following infection with Mycobacterium avium. We examined the role of HO-1 in regulating monocyte chemoattractant protein-1 (MCP-1) and chemokine receptor 2 (CCR2), two molecules involved in monocyte-macrophage cell trafficking after infection. We showed that RAW 264.7 mouse monocytes exposed to M. avium expressed HO-1 and MCP-1. Inhibition of HO by zinc protoporphyrin-IX led to inhibition of MCP-1 and increased expression of CCR2, its cognate receptor. HO-1⁻/⁻ mice did not develop organized granuloma in their lungs, had higher lung colony forming unit of M. avium when infected with intratracheal M. avium, and had loose collections of inflammatory cells in the lung parenchyma. Mycobacteria were found only inside defined granulomas but not outside granuloma in the lungs of HO-1⁺/⁺ mice. In HO-1⁻/⁻ mice, mycobacteria were also found in the liver and spleen and showed increased mortality. Peripheral blood monocytes isolated from GFP⁺ mice and given intravenously to HO-1⁺/⁺ mice localized into tight granulomas, while in HO-1⁻/⁻ mice they remained diffusely scattered in areas of parenchymal inflammation. Higher MCP-1 levels were found in bronchoalveolar lavage fluid of M. avium infected HO-1(-/-) mice and CCR2 expression was higher in HO-1⁻/⁻ alveolar macrophages when compared with HO-1⁺/⁺ mice. CCR2 expression localized to granuloma in HO-1⁺/⁺ mice but not in the HO-1⁻/⁻ mice. These findings strongly suggest that HO-1 plays a protective role in the control of M. avium infection.

Protective role of heme oxygenase-1 against inflammation in atherosclerosis

Heme oxygenase-1 (HO-1) catalyzes the first and rate-limiting step in the metabolism of free heme into equimolar amounts of ferrous iron, carbon monoxide (CO), and biliverdin. Biliverdin is subsequently converted to bilirubin by biliverdin reductase. HO-1 has recently been identified as a promising therapeutic target in the treatment of vascular inflammatory disease, including atherosclerosis. HO-1 represses inflammation by removing the pro-inflammatory molecule heme and by generating CO and the bile pigments, biliverdin and bilirubin. These HO-1 reaction products are capable of blocking innate and adaptive immune responses by modifying the activation, differentiation, maturation, and/or polarization of numerous immune cells, including endothelial cells, monocytes/macrophages, dendritic cells, T lymphocytes, mast cells, and platelets. These cellular actions by CO and bile pigments result in diminished leukocyte recruitment and infiltration, and pro-inflammatory mediator production within atherosclerotic lesions. This review highlights the mechanisms by which HO-1 suppresses vascular inflammation in atherosclerosis, and explores possible therapeutic modalities by which HO-1 and its reaction products can be employed to ameliorate vascular inflammatory disease.

Complement activation and disease: protective effects of hyperbilirubinaemia

Complement, an important effector mechanism of the immune system, is an enzymatic cascade of approx. 30 serum proteins leading to the amplification of a specific humoral response. It can be activated through the classical or alternative pathways, or through the mannose-binding lectin pathway. The activation of the classical pathway is initiated by the binding of the C1 component to antigen-bound antibodies, known as immunocomplexes. C1 is a complex of one molecule of C1q, two molecules of C1r and two molecules of C1s. C1q contains three copies of a Y-shaped fundamental unit with globular heads included in its structure, which play a major role in the interaction with the Fc portion of immunoglobulins. Deficient or exacerbated activation of the complement system leads to diseases of variable severity, and pharmacological inhibition of the complement system is considered as a therapeutic strategy to ameliorate the inflammatory effects of exacerbated complement activation. Bilirubin is a product of haem degradation by the concerted action of haem oxygenase, which converts haem into biliverdin, and biliverdin reductase, which reduces biliverdin to UCB (unconjugated bilirubin). UCB exerts both cytoprotective and cytotoxic effects in a variety of tissues and cells, acting either as an antioxidant at low concentrations or as an oxidant at high concentrations. In the present review, we describe in detail the anti-complement properties of bilirubin, occurring at levels above the UCB concentrations found in normal human serum, as a beneficial effect of potential clinical relevance. We provide evidence that UCB interferes with the interaction between C1q and immunoglobulins, thus inhibiting the initial step in the activation of complement through the classical pathway. A molecular model is proposed for the interaction between UCB and C1q.

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.

Heme oxygenase-1 and the vascular bed: from molecular mechanisms to therapeutic opportunities

Heme oxygenase-1, an enzyme degrading heme to carbon monoxide, iron, and biliverdin, has been recognized as playing a crucial role in cellular defense against stressful conditions, not only related to heme release. HO-1 protects endothelial cells from apoptosis, is involved in blood-vessel relaxation regulating vascular tone, attenuates inflammatory response in the vessel wall, and participates in blood-vessel formation by means of angiogenesis and vasculogenesis. The latter functions link HO-1 not only to cardiovascular ischemia but also to many other conditions that, like development, wound healing, or cancer, are dependent on neovascularization. The aim of this comprehensive review is to address the mechanisms of HO-1 regulation and function in cardiovascular physiology and pathology and to demonstrate some possible applications of the vast knowledge generated so far. Recent data provide powerful evidence for the involvement of HO-1 in the therapeutic effect of drugs used in cardiovascular diseases. Novel studies open the possibilities of application of HO-1 for gene and cell therapy. Therefore, research in forthcoming years should help to elucidate both the real role of HO-1 in the effect of drugs and the clinical feasibility of HO-1-based cell and gene therapy, creating the effective therapeutic avenues for this refined antioxidant system.

Angiotensin AT1 receptor antagonists exert anti-inflammatory effects in spontaneously hypertensive rats

BACKGROUND AND PURPOSE

Although the main therapeutic effect of angiotensin AT1 receptor antagonists is to decrease blood pressure, they also exert anti-inflammatory effects in the cardiovascular system. However, the underlying mechanisms remain unclear. We investigated the inhibitory effect of AT1 antagonists on the chemokine monocyte chemoattractant protein 1 (MCP-1) and its receptor C-C chemokine receptor 2 (CCR2) in rat monocytes and aortas.

EXPERIMENTAL APPROACH

Spontaneous hypertensive rats (SHRs) were treated with the AT1 antagonists losartan or telmisartan for 4 weeks, and Wistar-Kyoto rats (WKYs) were used as normotensive controls. Systolic arterial pressure was measured, and the number of macrophages in the aortic vessel wall was assessed by anti-ED-1 antibody immunolabelling.

KEY RESULTS

Compared with WKYs, SHRs showed significantly increased ED-1 positive macrophages in the aortic wall, which were decreased after high doses of losartan or telmisartan. Low doses of losartan did not improve blood pressure significantly as did the high doses, but markedly decreased macrophage infiltration in the vessel wall. AT1 antagonists, particularly at high doses, improved aortic remodeling in SHR. At the molecular level, AT1 antagonists attenuated the expression of MCP-1 and CCR2 in the aorta and peripheral blood monocytes and lowered the serum level of MCP-1. In addition, Western blotting showed that AT1 antagonists inhibited the phosphorylation of Akt in mouse monocytes.

CONCLUSIONS AND IMPLICATIONS

AT1 antagonism inhibited vessel wall inflammation and inhibition of PI3K/Akt may be involved in the modulation of the MCP-1/CCR2 system by AT1 antagonists in SHRs.

''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.

Reduced expression of heme oxygenase-1 in patients with coronary atherosclerosis

Heme oxigenase-1 (HO-1) is known to be an inducible cytoprotective enzyme that copes with oxidative stress. However, changes in HO-1 expression and their association with human diseases have not been studied. To test the hypothesis that the capacity to upregulate HO-1 in response to oxidative stress is an intrinsic marker for susceptibility to coronary atherosclerosis, we assessed stimulation-induced change in HO-1 expression in blood cells in 110 patients who underwent coronary angiography, comparing the results with the extent of coronary atherosclerosis and (GT)(n) repeat polymorphism in the HO-1 gene promoter region, which is believed to affect the gene expression level. The extent of coronary atherosclerosis was assessed by coronary score. Mononuclear cells were incubated with 10 micromol/l hemin or vehicle for 4 h to maximally stimulate HO-1 expression, then the HO-1 expression level was determined by real-time polymerase chain reaction (PCR). The difference between the HO-1 mRNA levels of hemin- and vehicle-treated cells (DeltaHO-1 mRNA) was taken as an index of the capacity to upregulate HO-1 mRNA. The coefficient of variance of DeltaHO-1 mRNA was 7.2%. Consistent with previous studies, DeltaHO-1 mRNA was significantly lower in patients carrying a long (GT)(n) repeat. DeltaHO-1 mRNA negatively and significantly correlated with the coronary score (r(2)=0.50, p<0.01). In conclusion, the capacity to upregulate HO-1 expression may be determined, at least in part, by genetics, and reduced ability to induce HO-1 may be involved in the mechanism of coronary atherosclerosis.

Role of Oxidative Stress in the Pathogenesis of Abdominal Aortic Aneurysms

The role of inflammation in the pathogenesis of abdominal aortic aneurysms (AAA) is well established. The inflammatory process leads to protease-mediated degradation of the extracellular matrix and apoptosis of smooth muscle cells (SMC), which are the predominant matrix synthesizing cells of the vascular wall. These processes act in concert to progressively weaken the aortic wall, resulting in dilatation and aneurysm formation. Oxidative stress is invariably increased in, and contributes importantly to, the pathophysiology of inflammation. Moreover, reactive oxygen species (ROS) play a key role in regulation of matrix metalloproteinases and induction of SMC apoptosis. ROS may also contribute to the pathogenesis of hypertension, a risk factor for AAA. Emerging evidence suggests that ROS and reactive nitrogen species (RNS) are associated with AAA formation in animal models and in humans. Although experimental data are limited, several studies suggest that modulation of ROS production or activity may suppress AAA formation and improve experimental outcome in rodent models. Although a number of enzymes can produce injurious ROS in the vasculature, increasing evidence points toward a role for NADPH oxidase as a source of oxidative stress in the pathogenesis of AAA.

Heme oxygenase-2 is a critical determinant for execution of an acute inflammatory and reparative response

Heme oxygenase (HO) represents an intrinsic anti-inflammatory system based on its ability to regulate leukocyte function and inhibit expression of proinflammatory cytokines. This anti-inflammatory function is linked to the inducible isoform HO-1; the role of the constitutive isoform HO-2 is unknown. The current study was undertaken to investigate the role of HO-2 in the regulation of the acute inflammatory and reparative response by using HO-2-null mice and well-established animal models of epithelial injury and antigen-induced peritonitis. Here we show that in vivo deletion of HO-2 disables execution of the acute inflammatory and reparative response after epithelial injury and leads to an exaggerated inflammatory response in antigen-induced peritonitis. HO-2 deletion was associated with impaired HO-1 induction, indicating that HO-2 is critical for HO-1 expression and that the subsequent failure to up-regulate the HO system may contribute to unresolved inflammation and the development of chronic inflammatory conditions. Indeed, supplementation with the HO bioactive product, biliverdin, rescued the acute inflammatory and reparative response in HO-2-null mice. Thus, HO-2 sets in place a basal tone of anti-inflammatory signals that may be a prerequisite for the ordered execution of an inflammatory and reparative response.

Influence of bradykinin B1 and B2 receptors in the immune response triggered by renal ischemia-reperfusion injury

Bradykinin B1 receptors are exclusively expressed in inflamed tissues. For this reason, they have been related with the outcomes of several pathologies. Ischemia-reperfusion injury is caused by the activation of inflammatory and cytoprotective genes, such as macrophage chemoattractant protein-1 and heme oxygenase-1, respectively. This study was aimed to analyze the involvement of bradykinin B1 and B2 receptors (B1R and B2R) in tissue response after renal ischemia-reperfusion injury. For that, B1R (B1-/-), B2R (B2-/-) knockout animals and its control (wild-type mice, B1B2+/+) were subjected to renal bilateral ischemia, followed by 24, 48 and 120 h of reperfusion. At these time points, blood serum samples were collected for creatinine and urea dosages. Kidneys were harvested for histology and molecular analyses by real-time PCR. At 24 and 48 h of reperfusion, B1-/- group resulted in the lowest serum creatinine and urea levels, indicating less renal damage, which was proved by renal histology. Renal protection associated with B1-/- mice was also related with higher expression of HO-1 and lower expression of MCP-1. In conclusion, the absence of B1R had a protective role against inflammatory responses developed after renal ischemia-reperfusion injury.

Influence of TH1/TH2 Switched Immune Response on Renal Ischemia-Reperfusion Injury

BACKGROUND/AIMS

Recent evidence shows a critical role of the CD4+ T cell with the Th1/Th2 paradigm as a possible effector mechanism in ischemia and reperfusion injury. We hypothesize that a polarized Th1 activation response may negatively influence the renal IRI through its relationship with chemokine production (MCP-1) and with a protective tissue response (HO-1).

METHODS

We subjected mice to renal ischemia for 45 min using IL-4 and IL-12 knockout C57BL/6. We then measured serum urea levels, performed histomorphometric analysis for tubular necrosis and regeneration, and evaluated the mRNA expression of HO-1, t-bet, Gata-3 and MCP-1 by real-time PCR at 24, 48 and 120 h after surgery.

RESULTS/CONCLUSIONS

The IL-4 knockout mice had a statistically significant rise in serum urea levels post IRI compared with control animals. The IL-12-deficient mice were not affected. The IL-4-deficient mice had a statistically significant increase in tubular injury and impairment in cell regeneration. The IRI in IL-4-deficient mice was accompanied by higher levels of HO-1, t-bet and later up-regulation of MCP-1. These findings suggest that the deleterious effects of the Th1 cell involve increased production of chemokines such as MCP-1.

Selective expansion of CD16highCCR2- subpopulation of circulating monocytes with preferential production of haem oxygenase (HO)-1 in response to acute inflammation

Monocytes are composed of two distinct subpopulations in the peripheral blood as determined by their surface antigen expressions, profiles of cytokine production and functional roles played in vivo. We attempted to delineate the unique functional roles played by a minor CD16(high)CCR2(-) subpopulation of circulating monocytes. They produced significant levels of interleukin (IL)-6 and tumour necrosis factor (TNF)-alpha, but very low levels of IL-10 upon in vitro stimulation. Characteristic profiles of cytokine production were confirmed by stimulating purified subpopulations of monocytes after cell sorting. It was noteworthy that freshly isolated CD16(high)CCR2(-) monocyte subpopulations produced significant levels of haem oxygenase (HO)-1, whereas the major CD16(low)CCR2(+) subpopulation produced little. These results were contrary to the generally accepted notion that the CD16(high)CCR2(-) monocyte subpopulation plays a predominantly proinflammatory role in vivo. The CD16(high)CCR2(-) subpopulation increased in Kawasaki disease and influenza virus infection. In accord with this, HO-1 mRNA expression by mononuclear cells was significantly increased in these illnesses. These results indicate that CD16(high)CCR2(-) subpopulations are of a distinct lineage from CD16(low)CCR2(+) monocytes. More importantly, they may represent a monocyte subpopulation with a unique functional role to regulate inflammation by producing HO-1 in steady state in vivo.

Heme Oxygenase and Atherosclerosis

Overproduction of reactive oxygen species under pathophysiological conditions, including dyslipidemia, hypertension, diabetes, and smoking, is integral in the development of cardiovascular diseases (CVD). The reactive oxygen species released from all types of vascular cells regulate various signaling pathways that mediate not only vascular inflammation in atherogenesis but also antioxidative and antiinflammatory responses. One such protective and stress-induced protein is heme oxygenase (HO). HO is the first rate-limiting enzyme in heme breakdown to generate equimolar quantities of carbon monoxide, biliverdin, and free ferrous iron. Accumulating evidence has shown that inducible HO (HO-1) and its products function as adaptive molecules against oxidative insults. The proposed mechanisms by which HO-1 exerts its cytoprotective effects include its abilities to degrade the pro-oxidative heme, to release biliverdin and subsequently convert it bilirubin, both of which have antioxidant properties, and to generate carbon monoxide, which has antiproliferative and antiinflammatory as well as vasodilatory properties. Herein, I highlight the relationship of HO and cardiovascular disease, especially atherosclerosis, gene-targeting approaches in animal models, and the potential for and concern about HO-1 as a novel therapeutic target for cardiovascular diseases.