Bilirubin: a natural inhibitor of vascular smooth muscle cell proliferation

Heme oxygenase-1, oxidation, inflammation, and atherosclerosis

Atherosclerosis is an inflammatory process of the vascular wall characterized by the infiltration of lipids and inflammatory cells. Oxidative modifications of infiltrating low-density lipoproteins and induction of oxidative stress play a major role in lipid retention in the vascular wall, uptake by macrophages and generation of foam cells, a hallmark of this disorder. The vasculature has a plethora of protective resources against oxidation and inflammation, many of them regulated by the Nrf2 transcription factor. Heme oxygenase-1 (HO-1) is a Nrf2-regulated gene that plays a critical role in the prevention of vascular inflammation. It is the inducible isoform of HO, responsible for the oxidative cleavage of heme groups leading to the generation of biliverdin, carbon monoxide, and release of ferrous iron. HO-1 has important antioxidant, antiinflammatory, antiapoptotic, antiproliferative, and immunomodulatory effects in vascular cells, most of which play a significant role in the protection against atherogenesis. HO-1 may also be an important feature in macrophage differentiation and polarization to certain subtypes. The biological effects of HO-1 are largely attributable to its enzymatic activity, which can be conceived as a system with three arms of action, corresponding to its three enzymatic byproducts. HO-1 mediated vascular protection may be due to a combination of systemic and vascular local effects. It is usually expressed at low levels but can be highly upregulated in the presence of several proatherogenic stimuli. The HO-1 system is amenable for use in the development of new therapies, some of them currently under experimental and clinical trials. Interestingly, in contrast to the HO-1 antiatherogenic actions, the expression of its transcriptional regulator Nrf2 leads to proatherogenic effects instead. This suggests that a potential intervention on HO-1 or its byproducts may need to take into account any potential alteration in the status of Nrf2 activation. This article reviews the available evidence that supports the antiatherogenic role of HO-1 as well as the potential pathways and mechanisms mediating vascular protection.

Heme oxygenase-1, oxidation, inflammation, and atherosclerosis

Atherosclerosis is an inflammatory process of the vascular wall characterized by the infiltration of lipids and inflammatory cells. Oxidative modifications of infiltrating low-density lipoproteins and induction of oxidative stress play a major role in lipid retention in the vascular wall, uptake by macrophages and generation of foam cells, a hallmark of this disorder. The vasculature has a plethora of protective resources against oxidation and inflammation, many of them regulated by the Nrf2 transcription factor. Heme oxygenase-1 (HO-1) is a Nrf2-regulated gene that plays a critical role in the prevention of vascular inflammation. It is the inducible isoform of HO, responsible for the oxidative cleavage of heme groups leading to the generation of biliverdin, carbon monoxide, and release of ferrous iron. HO-1 has important antioxidant, antiinflammatory, antiapoptotic, antiproliferative, and immunomodulatory effects in vascular cells, most of which play a significant role in the protection against atherogenesis. HO-1 may also be an important feature in macrophage differentiation and polarization to certain subtypes. The biological effects of HO-1 are largely attributable to its enzymatic activity, which can be conceived as a system with three arms of action, corresponding to its three enzymatic byproducts. HO-1 mediated vascular protection may be due to a combination of systemic and vascular local effects. It is usually expressed at low levels but can be highly upregulated in the presence of several proatherogenic stimuli. The HO-1 system is amenable for use in the development of new therapies, some of them currently under experimental and clinical trials. Interestingly, in contrast to the HO-1 antiatherogenic actions, the expression of its transcriptional regulator Nrf2 leads to proatherogenic effects instead. This suggests that a potential intervention on HO-1 or its byproducts may need to take into account any potential alteration in the status of Nrf2 activation. This article reviews the available evidence that supports the antiatherogenic role of HO-1 as well as the potential pathways and mechanisms mediating vascular protection.

Preservation solution supplemented with biliverdin prevents lung cold ischaemia/reperfusion injury

OBJECTIVES

Biliverdin (BV), one of the byproducts of heme catalysis through the heme oxygenase system, is a known scavenger of the reactive oxygen species. We hypothesized that adding BV to the perfusate and cold storage solution could protect rat lung grafts from oxidative injuries via its antioxidant efficacies.

METHODS

Orthotopic left lung transplantation was performed in a syngenic Lewis-to-Lewis rat combination under 100% oxygen. Grafts were preserved in low-potassium dextran (LPD; Perfadex) at 4°C for 6 h with or without supplementation of 1 or 10 μM of BV into LPD.

RESULTS

Prolonged cold storage and reperfusion resulted in a considerable deterioration of graft functions associated with massive apoptosis in the grafts after reperfusion. The untreated grafts exhibited the early up-regulations of mRNA for inflammatory mediators and an increase in a marker of lipid peroxidation, showing oxidative injuries. Although BV supplementation of LPD at a lower concentration (1 μM) did not improve the graft gas exchange, the grafts treated with BV (10 μM) showed a significant improvement of oxygenation and less inflammatory responses as well as reduced lipid peroxidation and apoptosis. Although the rapid activations of mitogen-activated protein kinases (MAPKs) were seen 30 min after reperfusion in the grafts stored in control LPD, BV treatment significantly reduced phosphorylated-MAPK protein expression.

CONCLUSIONS

This study demonstrates that the exposure of the lung grafts to BV during cold storage can impart potent cytoprotective effects to lung cold ischaemia/reperfusion injury and significantly improve the lung graft function following extended cold preservation and transplantation by the mechanism of a reduction in oxidative injury and following inflammatory events.

Transgenic expression of human heme oxygenase-1 in pigs confers resistance against xenograft rejection during ex vivo perfusion of porcine kidneys

BACKGROUND

The major immunological hurdle to successful porcine-to-human xenotransplantation is the acute vascular rejection (AVR), characterized by endothelial cell (EC) activation and perturbation of coagulation. Heme oxygenase-1 (HO-1) and its derivatives have anti-apoptotic, anti-inflammatory effects and protect against reactive oxygen species, rendering HO-1 a promising molecule to control AVR. Here, we report the production and characterization of pigs transgenic for human heme oxygenase-1 (hHO-1) and demonstrate significant protection in porcine kidneys against xenograft rejection in ex vivo perfusion with human blood and transgenic porcine aortic endothelial cells (PAEC) in a TNF-α-mediated apoptosis assay.

METHODS

Transgenic and non-transgenic PAEC were tested in a TNF-α-mediated apoptosis assay. Expression of adhesion molecules (ICAM-1, VCAM-1, and E-selectin) was measured by real-time PCR. hHO-1 transgenic porcine kidneys were perfused with pooled and diluted human AB blood in an ex vivo perfusion circuit. MHC class-II up-regulation after induction with IFN-γ was compared between wild-type and hHO-1 transgenic PAEC.

RESULTS

Cloned hHO-1 transgenic pigs expressed hHO-1 in heart, kidney, liver, and in cultured ECs and fibroblasts. hHO-1 transgenic PAEC were protected against TNF-α-mediated apoptosis. Real-time PCR revealed reduced expression of adhesion molecules like ICAM-1, VCAM-1, and E-selectin. These effects could be abrogated by the incubation of transgenic PAECs with the specific HO-1 inhibitor zinc protoporphorine IX (Zn(II)PPIX, 20 μm). IFN-γ induced up-regulation of MHC class-II molecules was significantly reduced in PAECs from hHO-1 transgenic pigs. hHO-1 transgenic porcine kidneys could successfully be perfused with diluted human AB-pooled blood for a maximum of 240 min (with and without C1 inh), while in wild-type kidneys, blood flow ceased after ∼60 min. Elevated levels of d-Dimer and TAT were detected, but no significant consumption of fibrinogen and antithrombin was determined. Microthrombi could not be detected histologically.

CONCLUSIONS

These results are encouraging and warrant further studies on the biological function of heme oxygenase-I expression in hHO-1 transgenic pigs in the context of xenotransplantation.

Heme cytotoxicity and the pathogenesis of immune-mediated inflammatory diseases

Heme, iron (Fe) protoporphyrin IX, functions as a prosthetic group in a range of hemoproteins essential to support life under aerobic conditions. The Fe contained within the prosthetic heme groups of these hemoproteins can catalyze the production of reactive oxygen species. Presumably for this reason, heme must be sequestered within those hemoproteins, thereby shielding the reactivity of its Fe-heme. However, under pathologic conditions associated with oxidative stress, some hemoproteins can release their prosthetic heme groups. While this heme is not necessarily damaging per se, it becomes highly cytotoxic in the presence of a range of inflammatory mediators such as tumor necrosis factor. This can lead to tissue damage and, as such, exacerbate the pathologic outcome of several immune-mediated inflammatory conditions. Presumably, targeting “free heme” may be used as a therapeutic intervention against these diseases.

Bilirubin inhibits neointima formation and vascular smooth muscle cell proliferation and migration

Bilirubin is a heme metabolite generated by the concerted action of the enzymes heme oxygenase and biliverdin reductase. Although long considered a toxic byproduct of heme catabolism, recent preclinical, and clinical studies indicate the bilirubin exerts beneficial effects in the circulation. In the present study, we determined whether local administration of bilirubin attenuates neointima formation following injury of rat carotid arteries. In addition, the ability of bilirubin to regulate the proliferation and migration of human arterial smooth muscle cells (SMCs) was investigated. Local perivascular administration of bilirubin immediately following balloon injury of rat carotid arteries significantly attenuated neointima formation. Bilirubin-mediated inhibition of neointimal thickening was associated with a significant decrease in ERK activity and cyclin D1 and A protein expression, and an increase in p21 and p53 protein expression in injured blood vessels. Treatment of human aortic SMCs with bilirubin inhibited proliferation and migration in a concentration-dependent manner without affecting cell viability. In addition, bilirubin resulted in a concentration-dependent increase in the percentage of cells in the G₀/G₁ phase of the cell cycle and this was paralleled by a decrease in the fraction of cells in the S and G₂M phases of the cell cycle. Finally, bilirubin had no effect on mitochondrial function and ATP content of vascular SMCs. In conclusion, these studies demonstrate that bilirubin inhibits neointima formation after arterial injury and this is associated with alterations in the expression of cell cycle regulatory proteins. Furthermore, bilirubin blocks proliferation and migration of human arterial SMCs and arrests SMCs in the G₀/G₁ phase of the cell cycle. Bilirubin represents an attractive therapeutic agent in treating occlusive vascular disease.

Go green: the anti-inflammatory effects of biliverdin reductase

Biliverdin (BV) has emerged as a cytoprotective and important anti-inflammatory molecule. Conversion of BV to bilirubin (BR) is catalyzed by biliverdin reductase (BVR) and is required for the downstream signaling and nuclear localization of BVR. Recent data by others and us make clear that BVR is a critical regulator of innate immune responses resulting from acute insult and injury and moreover, that a lack of BVR results in an enhanced proinflammatory phenotype. In macrophages, BVR is regulated by its substrate BV which leads to activation of the PI3K–Akt-IL-10 axis and inhibition of TLR4 expression via direct binding of BVR to the TLR4 promoter. In this review, we will summarize recent findings on the role of BVR and the bile pigments in inflammation in context with its activity as an enzyme, receptor, and transcriptional regulator.

Bile pigments in pulmonary and vascular disease

The bile pigments, biliverdin, and bilirubin, are endogenously derived substances generated during enzymatic heme degradation. These compounds have been shown to act as chemical antioxidants in vitro. Bilirubin formed in tissues circulates in the serum, prior to undergoing hepatic conjugation and biliary excretion. The excess production of bilirubin has been associated with neurotoxicity, in particular to the newborn. Nevertheless, clinical evidence suggests that mild states of hyperbilirubinemia may be beneficial in protecting against cardiovascular disease in adults. Pharmacological application of either bilirubin and/or its biological precursor biliverdin, can provide therapeutic benefit in several animal models of cardiovascular and pulmonary disease. Furthermore, biliverdin and bilirubin can confer protection against ischemia/reperfusion injury and graft rejection secondary to organ transplantation in animal models. Several possible mechanisms for these effects have been proposed, including direct antioxidant and scavenging effects, and modulation of signaling pathways regulating inflammation, apoptosis, cell proliferation, and immune responses. The practicality and therapeutic-effectiveness of bile pigment application to humans remains unclear.

Therapeutic potential of heme oxygenase-1/carbon monoxide in lung disease

Heme oxygenase (HO), a catabolic enzyme, provides the rate-limiting step in the oxidative breakdown of heme, to generate carbon monoxide (CO), iron, and biliverdin-IXα. Induction of the inducible form, HO-1, in tissues is generally regarded as a protective mechanism. Over the last decade, considerable progress has been made in defining the therapeutic potential of HO-1 in a number of preclinical models of lung tissue injury and disease. Likewise, tissue-protective effects of CO, when applied at low concentration, have been observed in many of these models. Recent studies have expanded this concept to include chemical CO-releasing molecules (CORMs). Collectively, salutary effects of the HO-1/CO system have been demonstrated in lung inflammation/acute lung injury, lung and vascular transplantation, sepsis, and pulmonary hypertension models. The beneficial effects of HO-1/CO are conveyed in part through the inhibition or modulation of inflammatory, apoptotic, and proliferative processes. Recent advances, however, suggest that the regulation of autophagy and the preservation of mitochondrial homeostasis may serve as additional candidate mechanisms. Further preclinical and clinical trials are needed to ascertain the therapeutic potential of HO-1/CO in human clinical disease.

Essential roles of Raf/extracellular signal-regulated kinase/mitogen-activated protein kinase pathway, YY1, and Ca2+ influx in growth arrest of human vascular smooth muscle cells by bilirubin

The biological effects of bilirubin, still poorly understood, are concentration-dependent ranging from cell protection to toxicity. Here we present data that at high nontoxic physiological concentrations, bilirubin inhibits growth of proliferating human coronary artery smooth muscle cells by three events. It impairs the activation of Raf/ERK/MAPK pathway and the cellular Raf and cyclin D1 content that results in retinoblastoma protein hypophosphorylation on amino acids S608 and S780. These events impede the release of YY1 to the nuclei and its availability to regulate the expression of genes and to support cellular proliferation. Moreover, altered calcium influx and calpain II protease activation leads to proteolytical degradation of transcription factor YY1. We conclude that in the serum-stimulated human vascular smooth muscle primary cell cultures, bilirubin favors growth arrest, and we propose that this activity is regulated by its interaction with the Raf/ERK/MAPK pathway, effect on cyclin D1 and Raf content, altered retinoblastoma protein profile of hypophosphorylation, calcium influx, and YY1 proteolysis. We propose that these activities together culminate in diminished 5 S and 45 S ribosomal RNA synthesis and cell growth arrest. The observations provide important mechanistic insight into the molecular mechanisms underlying the transition of human vascular smooth muscle cells from proliferative to contractile phenotype and the role of bilirubin in this transition.

Functional induction of the cystine-glutamate exchanger system Xc(-) activity in SH-SY5Y cells by unconjugated bilirubin

We have previously reported that exposure of SH-SY5Y neuroblastoma cells to unconjugated bilirubin (UCB) resulted in a marked up-regulation of the mRNA encoding for the Na⁺ -independent cystine∶glutamate exchanger System X_c⁻ (SLC7A11 and SLC3A2 genes). In this study we demonstrate that SH-SY5Y cells treated with UCB showed a higher cystine uptake due to a significant and specific increase in the activity of System X_c⁻, without the contribution of the others two cystine transporters (X_AG⁻ and GGT) reported in neurons. The total intracellular glutathione content was 2 folds higher in the cells exposed to bilirubin as compared to controls, suggesting that the internalized cystine is used for gluthathione synthesis. Interestingly, these cells were significantly less sensitive to an oxidative insult induced by hydrogen peroxide. If System X_c⁻ is silenced the protection is lost. In conclusion, these results suggest that bilirubin can modulate the gluthathione levels in neuroblastoma cells through the induction of the System X_c⁻, and this renders the cell less prone to oxidative damage.

The Relationship among Homocysteine, Bilirubin, and Diabetic Retinopathy

BACKGROUND

Diabetic retinopathy is a common microvascular complication of diabetes mellitus (DM) and the leading cause of blindness in adults. Homocysteine, a risk factor with toxic effects on vascular endothelial cells, and bilirubin, a protectant with antioxidant and anti-inflammatory properties on the vasculature, have been reported to be linked to vaso-occlusive disorders. Therefore, the author of the present study investigated the association between the levels of plasma homocysteine and serum total bilirubin and the incidence of diabetic retinopathy as a chronic microvascular complication in patients with type 2 diabetes mellitus (T2DM).

METHODS

A total of 102 patients with T2DM who visited our hospital from January 2009 to January 2010 were assessed.

RESULTS

Of the 102 patients, the prevalence of diabetic retinopathy was 67 cases (65.7%) according to clinical ophthalmic examination. The duration of DM (P<0.001), age (P=0.003), fasting blood glucose (P=0.045) and urine albumin-creatinine ratio (P=0.015) in univariate analysis and plasma homocysteine level (P=0.038), duration of DM (P=0.001), and total bilirubin level (P=0.012) in multiple logistic regression analysis were statistically significantly associated with the incidence of diabetic retinopathy.

CONCLUSION

The present study indicates that homocysteine and bilirubin may be useful biomarkers for increased risk of diabetic retinopathy since retinopathy in patients with T2DM was linked to higher plasma homocysteine level and decreased serum total bilirubin level.

Serum Bilirubin Level is Negatively Correlated With Disease Progression of Peripheral Arterial Disease

We determined whether low bilirubin level is a risk factor for peripheral arterial disease (PAD). We recruited 318 patients with PAD and 100 healthy volunteers. Patients were divided into 4 groups by the Fontaine classification for PAD, namely, group 1 (grade 1, n = 4); group 2 (grade 2, n = 114), group 3 (grade 3, n = 164), and group 4 (grade 4, n = 36). Total bilirubin (T-BIL), direct bilirubin (D-BIL), and indirect bilirubin (I-BIL) levels were compared using stepwise multiple regressions adjusted for selected factors. After adjusting for gender, age, smoking, and diastolic blood pressure, serum levels of T-BIL, D-BIL, and I-BIL were significantly lower in the PAD group ( P < .05). Patients with grade 4 PAD showed significantly ( P < .05) lower levels of T-BIL when compared with grade 2 patients. We concluded that serum bilirubin levels are negatively correlated with the severity and progression of PAD.

A central role of heme oxygenase-1 in cardiovascular protection

The intrinsic defense mechanisms of the body are critical in protecting tissues from injury in response to pathological stress. Heme oxygenase-1 (HO-1), a stress response protein, is induced in response to various pathological stimuli to serve a cytoprotective function. By degrading the oxidant heme and generating the antioxidant bilirubin and anti-inflammatory molecule carbon monoxide, HO-1 may protect cell from injury due to oxidative and pathological stress. Oxidative stress in the heart caused by ischemia and reperfusion leads to cardiomyocyte death and subsequent myocardial infarction. Vascular diseases including atherosclerosis, graft failure, and restenosis are all associated with reactive oxygen species-induced injury and inflammation. Given that cardiovascular disease is the leading cause of death worldwide, there is considerable interest in developing new strategies for preventing and treating cardiovascular disease. Since HO-1 is induced in the heart and blood vessels in response to various stresses, a role of HO-1 has been implicated in cardiovascular homeostasis. Numerous studies using pharmacological method or genetic approach have since demonstrated the cardiovascular protective function of HO-1. Importantly, a number of studies have associated human HO-1 gene promoter polymorphisms with risk for vascular diseases. Taken together, HO-1 has a great therapeutic potential for cardiovascular disease.

Association of Serum Total Bilirubin with Serum High Sensitivity C-reactive Protein in Middle-aged Men

Background

It has been suggested that bilirubin has an inverse association with cardiovascular disease (CVD) due to its antioxidant properties. However, there are few data regarding the relationship between serum total bilirubin (sTB) and risk factors for CVD in Koreans. This study aimed to evaluate the relationship between sTB and high sensitivity C-reactive protein (hsCRP), which is an independent risk factor for CVD.

Methods

We performed a cross sectional study in 6,800 men who were examined at a health promotion center at a university hospital in Korea between May 2005 and June 2006. We grouped the subjects according to values of serum hsCRP (above or below 1.0 mg/L) and compared the characteristics of the two groups. To evaluate the relationship between sTB and hsCRP, we classified the subjects according to quartile values of sTB. Multivariate logistic regression analyses were used to analyze the relationship of levels of sTB and hsCRP after adjusting for known risk factors for CVD.

Results

Serum hsCRP was significantly associated with body mass index (BMI), smoking, diabetes, hypertension, fasting plasma glucose, systolic blood pressure, alanine aminotransferase, and total cholesterol/high density lipoprotein (TC/HDL-C) ratio, but not with age or alcohol use. As levels of sTB increased, there was a decrease in age, numbers of smokers, BMI, and TC/HDL ratio. Compared to the lowest quartile of sTB, levels of hsCRP decreased with odds ratios of 0.82 (95% CI, 0.71 to 0.96), 0.75 (95% CI, 0.65 to 0.88), and 0.63 (95% CI, 0.54 to 0.74) in the 2nd, 3rd, and 4th quartiles of bilirubin, respectively.

Conclusion

Bilirubin may be inversely associated with hsCRP

Greater effectiveness of ε-viniferin in red wine than its monomer resveratrol for inhibiting vascular smooth muscle cell proliferation and migration

Resveratrol is a strong candidate for explaining an irreversible correlation between red wine consumption and coronary heart disease. The present study examined the effect of ε-viniferin, a dehydrodimer of resveratrol, on vascular smooth muscle cells (VSMCs), because ε-viniferin functions are poorly understood in spite of its comparable content to resveratrol in red wines and grapes. Both ε-viniferin and resveratrol inhibited platelet-derived growth factor-induced cell proliferation, migration, and reactive oxygen species (ROS) production, in addition to inducing nitric oxide generation. ε-Viniferin was more effective than resveratrol in these effects, except for inhibiting ROS production. The compounds also increased the expression of the antioxidant enzyme, hemeoxygenase-1, via transcription factor Nrf2. The phosphatidylinositol 3-kinase-Akt pathway was implicated in resveratrol-dependent nuclear Nrf2 accumulation, whereas extracellular signal-regulated kinase and p38 were involved in ε-viniferin-induced Nrf2 accumulation. These data suggest that ε-viniferin may function more effectively than resveratrol in different mechanisms and cooperatively with resveratrol in preventing atherosclerosis.

Relationship between bilirubin and C-reactive protein

BACKGROUND

Epidemiologic studies have shown that serum total bilirubin is inversely related to an increased risk of coronary artery disease. However, the number of studies which have focused on the relationship between bilirubin and C-reactive protein (CRP), which is a marker reflecting chronic vascular inflammation, are limited.

METHOD

This cross-sectional study consisted of 2307 Korean adults who visited the Center for Health Promotion for a routine health check-up from January 2009 to December 2009.

RESULT

CRP level showed a declining tendency as total bilirubin increased according to total bilirubin quartiles. Negative relations of CRP with both total and direct bilirubin were found after adjustment of age, body mass index, hypertension, diabetes, hypercholesterolemia, cardiovascular disease, taking aspirin, smoking, alcohol drinking and regular exercise and total bilirubin or direct bilirubin.

CONCLUSIONS

This study found that elevation of the two types of bilirubin (total and direct) have a relationship with a low serum CRP level among apparently healthy Korean adults. It is hypothesized that a low serum CRP level may be due to the antioxidant and anti-inflammatory effects of bilirubin metabolism.

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.

Regulation of vulnerable plaque development by the heme oxygenase/carbon monoxide system

Plaque rupture and luminal thrombosis is the most common cause of coronary occlusion that leads to acute coronary syndromes. High-risk plaques, or vulnerable plaques, are defined as lesions that are prone to rupture, also known as thin cap fibroatheroma (TCFA), or lesions prone to erosion or with calcified cores. This review will focus mainly on the vulnerable plaque, which is thought to be the precursor of the thrombogenic or ruptured plaque. Heme oxygenase 1 (HO-1) protein expression is specifically increased in lesions with a vulnerable plaque phenotype resembling TCFAs and correlates with a rise in expression levels of intimal proinflammatory markers. Data from several human and animal studies imply an important function for HO-1 in the genetic regulation of early, as well as late atherogenesis, and plaque destabilization toward a vulnerable phenotype. Although a direct association between HO-1, vulnerable plaque development, and clinical outcome is for now missing, the correlations that have been reported for HO-1 and coronary artery disease point to a possible link.

Haem oxygenase-1 and cardiovascular disease: mechanisms and therapeutic potential

Cardiovascular disease remains the leading cause of death worldwide. Despite progress in management, there remain a significant number of patients who are not eligible for current treatment options. Traditionally, HO-1 (haem oxygenase-1), one of two isoenzymes that initiate haem catabolism, was thought to only play a metabolic role. However, HO-1 is now recognized to have additional protective activities in states of heightened noxious stimuli or stress such as acute coronary syndromes. The present review article provides an overview of the mode of action of HO-1 in vascular protection, with particular emphasis on its atheroprotective, anti-inflammatory and antioxidative properties, as well as its role in vascular repair. Furthermore, we present evidence for the protective effects of HO-1 in CVD (cardiovascular disease) in both animal and human studies. Given its potential in vascular protection and repair, strategies aimed at inducing HO-1 emerge as a novel and alternative therapeutic target in the management of CVD.

Targeting heme oxygenase-1 in vascular disease

Heme oxygenase-1 (HO-1) metabolizes heme to generate carbon monoxide (CO), biliverdin, and iron. Biliverdin is subsequently metabolized to bilirubin by biliverdin reductase. HO-1 has recently emerged as a promising therapeutic target in the treatment of vascular disease. Pharmacological induction or gene transfer of HO-1 ameliorates vascular dysfunction in animal models of atherosclerosis, post-angioplasty restenosis, vein graft stenosis, thrombosis, myocardial infarction, and hypertension, while inhibition of HO-1 activity or gene deletion exacerbates these disorders. The vasoprotection afforded by HO-1 is largely attributable to its end products: CO and the bile pigments, biliverdin and bilirubin. These end products exert potent anti-inflammatory, antioxidant, anti-apoptotic, and anti-thrombotic actions. In addition, CO and bile pigments act to preserve vascular homeostasis at sites of arterial injury by influencing the proliferation, migration, and adhesion of vascular smooth muscle cells, endothelial cells, endothelial progenitor cells, or leukocytes. Several strategies are currently being developed to target HO-1 in vascular disease. Pharmacological induction of HO-1 by heme derivatives, dietary antioxidants, or currently available drugs, is a promising near-term approach, while HO-1 gene delivery is a long-term therapeutic goal. Direct administration of CO via inhalation or through the use of CO-releasing molecules and/or CO-sensitizing agents provides an attractive alternative approach in targeting HO-1. Furthermore, delivery of bile pigments, either alone or in combination with CO, presents another avenue for protecting against vascular disease. Since HO-1 and its products are potentially toxic, a major challenge will be to devise clinically effective therapeutic modalities that target HO-1 without causing any adverse effects.

Heme oxygenase-1 does not mediate the effects of extracellular acidosis on vascular smooth muscle cell proliferation, migration, and susceptibility to apoptosis

BACKGROUND

Unbalanced vascular smooth muscle cell (VSMC) proliferation, migration, and apoptosis contribute to vascular disorders such as atherosclerosis, restenosis, and pulmonary hypertension. The effect of extracellular acidosis (EA) on VSMC homeostasis is incompletely understood but we previously reported that EA increases heme oxygenase-1 (HO-1) expression in VSMCs. Since HO-1 regulates VSMC proliferation and apoptosis we sought to define the role of HO-1 in VSMC responses to EA.

METHODS

Mouse aortic smooth muscle cells (MASMCs) were isolated from wild-type and HO-1-null mice. Cell proliferation and migration assays were done in a physiologic pH (7.4) or EA (pH 6.8). VSMC apoptosis in response to hydrogen peroxide was assessed by JC-1 staining, caspase-3 cleavage, annexin V, and Hoechst staining.

RESULTS

Wild-type MASMCs showed decreased proliferation and migration at pH 6.8 compared to pH 7.4. This observation was also true in HO-1-null MASMCs. Although wild-type and HO-1-null cells showed differences in the mode and kinetics of cell death, both genotypes exhibited increased susceptibility to hydrogen peroxide-induced apoptosis at pH 6.8 compared to 7.4.

CONCLUSIONS

EA inhibits VSMC proliferation and migration and increases susceptibility to oxidant-induced apoptosis. These effects of acidosis on VSMC homeostasis are independent of HO-1.

Interplay between heme oxygenase-1 and the multifunctional transcription factor yin yang 1 in the inhibition of intimal hyperplasia

RATIONALE

induction of heme oxygenase (HO)-1 protects against experimental atherosclerotic diseases, and certain pharmacological HO-1 inducers, like probucol, inhibit the proliferation of vascular smooth muscle cells and, at the same time, promote the growth of endothelial cells in vivo and in vitro.

OBJECTIVE

because such cell-specific effects are reminiscent of the action of the transcription factor Yin Yang (YY)1, we tested the hypothesis that there is a functional relationship between HO-1 and YY1.

METHODS AND RESULTS

we report that probucol increases the number of YY1(+) cells in rat carotid artery following balloon injury at a time coinciding with increased HO-1 expression. The drug also induces the expression of YY1 mRNA and protein in rat aortic smooth muscle cells (RASMCs) in vitro, as do other known HO-1 inducers (tert-butylhydroquinone and hemin) and overexpression of HO-1 using a human HMOX1 cDNA plasmid. Conversely, overexpression of YY1 induces expression of HO-1 in RASMCs. Induction of YY1 expression is dependent on HO-1 enzyme activity and its reaction product CO, because pharmacological inhibition of heme oxygenase activity or CO scavenging block, whereas exposure of RASMCs to a CO-releasing molecule increases, YY1 expression. Furthermore, RNA interference knockdown of YY1 prevents probucol or adeno-HO-1 from inhibiting RASMC proliferation in vitro and neointimal formation in vivo.

CONCLUSIONS

our findings show, for the first time, that HO-1 functionally interplays with the multifunctional transcription factor YY1 and that this interplay explains some of the protective activities of HO-1.

Traditional laboratory measures of cardiovascular risk in hereditary spherocytosis

BACKGROUND

Individuals who have undergone splenectomy may be at an increased risk of arterial and venous thrombosis. We sought to determine if splenectomy affects surrogate laboratory measures of cardiovascular risk in persons with hereditary spherocytosis (HS).

PROCEDURE

We conducted a prospective cross-sectional study of 21 children and 36 adults with HS. Fasting blood samples were collected for complete blood count and plasma lipid panel, homocysteine, lipoprotein (a), C-reactive protein, and fibrinogen. The variables were compared between the groups with and without prior splenectomy by Mann-Whitney tests.

RESULTS

Subjects with prior splenectomy had higher hemoglobin, white blood cell and platelet counts and lower reticulocyte counts and total serum bilirubin concentrations (P < 0.001). Subjects not having had splenectomy had lower than normal levels of total and LDL-cholesterol (LDL-C). Total and LDL-C values were significantly higher, as were fibrinogen and homocysteine concentrations, in the post-splenectomy subjects than in individuals with intact spleen.

CONCLUSION

Various lipid levels and other measures of cardiovascular risk are affected by splenectomy in persons with HS. Further investigations are indicated to more clearly define the balance of the potential benefits of hemolysis and anemia versus the deleterious effects of splenectomy in HS.

Role of heme oxygenase-1 in transplantation

Heme oxygenase-1 (HO-1) is the rate-limiting enzyme in heme catabolism that converts heme to Fe++, carbon monoxide and biliverdin. HO-1 acts anti-inflammatory and modulates apoptosis in many pathological conditions. In transplantation, HO-1 is overexpressed in organs during brain death, when undergoing ischemic damage and rejection. However, intentionally induced, it ameliorates pathological processes like ischemia reperfusion injury, allograft, xenograft or islet rejection, facilitates donor specific tolerance and alleviates chronic allograft changes. We herein consistently summarize the huge amount of data on HO-1 and transplantation that have been generated in multiple laboratories during the last 15years and suggest possible clinical implications and applications for the near future.

High serum bilirubin is associated with the reduced risk of diabetes mellitus and diabetic nephropathy

Several studies have suggested a potential effect of serum bilirubin as an antioxidant and cytoprotectant factor. For the results presented here, we evaluated the correlation between serum bilirubin and diabetes mellitus (DM) or chronic kidney disease originated from DM (DMCKD) in a Korean population. We used a cross-sectional, population-based design to examine 93,909 subjects (aged 18-96 years, 53.0% male). The trend of P values in the odds ratios for being DM and DMCKD was calculated using patients separated into five groups based on individual serum bilirubin concentrations. The prevalence of DM and DMCKD was 6.7% and 0.8%, respectively. Higher serum bilirubin levels were significantly associated with decreased prevalence of DM in both men (P trend < 0.001) and women (P trend = 0.014). The risk of DMCKD also decreased as bilirubin levels increased in women (P trend = 0.011), but not in men (P trend = 0.467). Serum bilirubin level was inversely related to insulin resistance using the homeostasis model assessment (HOMA-IR), serum insulin, and C-reactive protein (CRP) levels in multiple linear regression analyses. The regression coefficients (B) of log-HOMA-IR, log-insulin, and log-CRP were as follows: -0.09, -0.13, and -0.60 in men; -0.07, -0.09, and -0.50 in women, respectively. All the regressions were statistically significant (P < 0.001). These results indicate that serum bilirubin might have some protective function against DM and DMCKD, although the association between high serum bilirubin and decreased prevalence of DMCKD is observed only in women.

Establishment of a novel hemodynamic cerebral ischemia model of atherosclerotic rabbit

BACKGROUND

Atherosclerosis is the most common cause of ischemic stroke. Until now, there has been no ideal animal model for studying the hemodynamic ischemia caused by atherosclerosis in posterior circulation.

OBJECTIVE

This study was conducted to establish a cerebral ischemic model in an atherosclerotic rabbit by subclavian artery (SA) ligation or embolization, emphasizing the role of hemodynamics.

MATERIALS AND METHODS

Thirty-two male New Zealand rabbits (weight, 3.5 to 4.0 kg; provided by Animal Laboratory of Tongji Hospital of Tongji University) were randomly divided into four groups. All the groups, except the controlled group, were fed with high lipid diet for three months and two of the groups were submitted for right SA ligation or left SA embolization and the remaining one group to non-operated group.

RESULTS

A SA steal syndrome was established successfully with delayed visualization and prolonged circulation time in the posterior circulation. The circulation time was different when SA was ligated or embolized (P < 0.001), and also there was statistically significant difference between controlled and non-operated groups when angiography of either, left and right, VA was performed (P = 0.009, P = 0.003, respectively). However, there was no statistical significant difference between the ligation and embolization group either before or after the procedure (ligation or embolization) (P = 0.402, P = 0.068, respectively). In the control and non-operated group, angiography of either left or right VA showed no statistically significant difference (P = 0.285, P = 0.197, respectively). In the groups fed with high lipid diet for three months, atherosclerotic plaques were found in the common carotid arteries (CCAs), aortic arch (AAs) and variably in the internal carotid arteries (ICAs).

CONCLUSIONS

This model could, to some extent, simulate the clinical hemodynamic state of atherosclerotic stroke. and it is an ideal model for studying the hemodynamic cerebral ischemia in posterior circulation.

Mechanisms of cell protection by heme oxygenase-1

Heme oxygenases (HO) catabolize free heme, that is, iron (Fe) protoporphyrin (IX), into equimolar amounts of Fe(2+), carbon monoxide (CO), and biliverdin. The stress-responsive HO-1 isoenzyme affords protection against programmed cell death. The mechanism underlying this cytoprotective effect relies on the ability of HO-1 to catabolize free heme and prevent it from sensitizing cells to undergo programmed cell death. This cytoprotective effect inhibits the pathogenesis of a variety of immune-mediated inflammatory diseases.

Role of heme oxygenase in modulating endothelial function in mesenteric small resistance arteries of spontaneously hypertensive rats

It has been proposed that endothelial dysfunction is due to the excessive degradation of nitric oxide (NO) by oxidative stress. The enzyme heme-oxygenase (HO) seems to exert a protective effect on oxidative stress in the vasculature, both in animal models and in humans. The objective of this study is to evaluate the effects of inhibition or activation of HO on endothelial function in mesenteric small resistance arteries of spontaneously hypertensive rats (SHR). Six SHR were treated with cobalt protoporphyrin IX 50 mg/Kg (CoPP), an activator of HO; six SHR with stannous mesoporphyrin 30 mg/Kg (SnMP), an inhibitor of HO, and six SHR with saline. As controls, six Wistar-Kyoto rats (WKY) were treated with CoPP, six WKY with SnMP, and six WKY with saline. Drugs were injected in the peritoneum once a week for 2 weeks. Systolic blood pressure (SBP) was measured (tail cuff method) before and after treatment. Mesenteric small resistance arteries were mounted on a micromyograph. Endothelial function was evaluated as a cumulative concentration-response curve to acetylcholine (ACH), before and after preincubation with N(G)-methyl-L-arginine (L-NMMA, inhibitor of NO synthase), and to bradykinin (BK). In SHR treatment with CoPP, improved ACH-and BK-induced vasodilatation (ANOVA p < 0.001) and this improvement was abolished by L-NMMA (ANOVA p < 0.001). SnMP was devoid of effects on endothelial function. In WKY, both activation and inhibition of HO did not substantially affect endothelium-mediated vasodilatation. The stimulation of HO seems to induce an improvement of endothelial dysfunction in SHR by possibly reducing oxidative stress and increasing NO availability.

A transcriptome analysis identifies molecular effectors of unconjugated bilirubin in human neuroblastoma SH-SY5Y cells

BACKGROUND

The deposition of unconjugated bilirubin (UCB) in selected regions of the brain results in irreversible neuronal damage, or Bilirubin Encephalopathy (BE). Although UCB impairs a large number of cellular functions in other tissues, the basic mechanisms of neurotoxicity have not yet been fully clarified. While cells can accumulate UCB by passive diffusion, cell protection may involve multiple mechanisms including the extrusion of the pigment as well as pro-survival homeostatic responses that are still unknown.

RESULTS

Transcriptome changes induced by UCB exposure in SH-SY5Y neuroblastoma cell line were examined by high density oligonucleotide microarrays. Two-hundred and thirty genes were induced after 24 hours. A Gene Ontology (GO) analysis showed that at least 50 genes were directly involved in the endoplasmic reticulum (ER) stress response. Validation of selected ER stress genes is shown by quantitative RT-PCR. Analysis of XBP1 splicing and DDIT3/CHOP subcellular localization is presented.

CONCLUSION

These results show for the first time that UCB exposure induces ER stress response as major intracellular homeostasis in surviving neuroblastoma cells in vitro.

HO-1 and CO decrease platelet-derived growth factor-induced vascular smooth muscle cell migration via inhibition of Nox1

OBJECTIVE

Heme oxygenase-1 (HO-1), via its enzymatic degradation products, exhibits cell and tissue protective effects in models of vascular injury and disease. The migration of vascular smooth muscle cells (VSMC) from the medial to the intimal layer of blood vessels plays an integral role in the development of a neointima in these models. Despite this, there are no studies addressing the effect of increased HO-1 expression on VSMC migration. Results and Methods- The effects of increased HO-1 expression, as well as biliverdin, bilirubin, and carbon monoxide (CO), were studied in in vitro models of VSMC migration. Induction of HO-1 or CO, but not biliverdin or bilirubin, inhibited VSMC migration. This effect was mediated by the inhibition of Nox1 as determined by a range of approaches, including detection of intracellular superoxide, nicotinamide adenine dinucleotide phosphate oxidase activity measurements, and siRNA experiments. Furthermore, CO decreased platelet-derived growth factor-stimulated, redox-sensitive signaling pathways.

CONCLUSIONS

Herein, we demonstrate that increased HO-1 expression and CO decreases platelet-derived growth factor-stimulated VSMC migration via inhibition of Nox1 enzymatic activity. These studies reveal a novel mechanism by which HO-1 and CO may mediate their beneficial effects in arterial inflammation and injury.

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.

After vascular injury, heme oxygenase-1/carbon monoxide enhances re-endothelialization via promoting mobilization of circulating endothelial progenitor cells

BACKGROUND

Heme oxygenase-1 (HO-1), a heme degradation enzyme with multiple vasoprotective functions, is systemically induced in pathophysiological states associated with oxidative stress.

OBJECTIVES

To evaluate the impact of systemic HO-1 expression on circulating endothelial progenitor cells (EPCs) and re-endothelialization after vascular injury in an animal model.

METHODS

Mice received an intravenous (i.v.) injection of the adenovirus-bearing HO-1 gene (Adv-HO-1). The serum levels of vascular endothelial growth factor (VEGF) and stromal cell-derived factor-1 (SDF-1) were determined by ELISA and gene expression examined by quantitative real-time PCR. Circulating EPCs were characterized by flow cytometry and in vitro culture. EPC recruitment and re-endothelialization in injured arteries were assessed in mice receiving GFP+-bone marrow transplantation and guide wire-induced carotid injury. The effect of carbon monoxide (CO), a byproduct from heme degradation by HO-1, was assessed by exposing mice to 250 p.p.m. CO for 2 h day(-1).

RESULTS

Systemic HO-1 induction led to elevated serum levels of VEGF and SDF-1 and an increase in circulating EPCs. The re-endothelialization of denuded vessels was accelerated in mice with systemic HO-1 overexpression. A further experiment demonstrated that both EPC mobilization and re-endothelialization were significantly attenuated in mice with HO-1 deficiency. The increase in EPC mobilization and enhanced re-endothelialization was also observed in mice exposed to CO prior to carotid injury. The CO-mediated effect was associated with an increase in circulating SDF-1 but not VEGF.

CONCLUSION

These findings support a vital role of HO-1 and its reaction byproduct, CO, in vascular repair through enhancing EPC mobilization.

Heme oxygenase-1/carbon monoxide: from metabolism to molecular therapy

Heme oxygenase-1 (HO-1), a ubiquitous inducible stress-response protein, serves a major metabolic function in heme turnover. HO activity cleaves heme to form biliverdin-IXalpha, carbon monoxide (CO), and iron. Genetic experiments have revealed a central role for HO-1 in tissue homeostasis, protection against oxidative stress, and in the pathogenesis of disease. Four decades of research have witnessed not only progress in elucidating the molecular mechanisms underlying the regulation and function of this illustrious enzyme, but also have opened remarkable translational applications for HO-1 and its reaction products. CO, once regarded as a metabolic waste, can act as an endogenous mediator of cellular signaling and vascular function. Exogenous application of CO by inhalation or pharmacologic delivery can confer cytoprotection in preclinical models of lung/vascular injury and disease, based on anti-apoptotic, anti-inflammatory, and anti-proliferative properties. The bile pigments, biliverdin and bilirubin, end products of heme degradation, have also shown potential as therapeutics in vascular disease based on anti-inflammatory and anti-proliferative activities. Further translational and clinical trials research will unveil whether the HO-1 system or any of its reaction products can be successfully applied as molecular medicine in human disease.

Heme oxygenase-1, a critical arbitrator of cell death pathways in lung injury and disease

Increases in cell death by programmed (i.e., apoptosis, autophagy) or nonprogrammed mechanisms (i.e., necrosis) occur during tissue injury and may contribute to the etiology of several pulmonary or vascular disease states. The low-molecular-weight stress protein heme oxygenase-1 (HO-1) confers cytoprotection against cell death in various models of lung and vascular injury by inhibiting apoptosis, inflammation, and cell proliferation. HO-1 serves a vital metabolic function as the rate-limiting step in the heme degradation pathway and in the maintenance of iron homeostasis. The transcriptional induction of HO-1 occurs in response to multiple forms of chemical and physical cellular stress. The cytoprotective functions of HO-1 may be attributed to heme turnover, as well as to beneficial properties of its enzymatic reaction products: biliverdin-IXalpha, iron, and carbon monoxide (CO). Recent studies have demonstrated that HO-1 or CO inhibits stress-induced extrinsic and intrinsic apoptotic pathways in vitro. A variety of signaling molecules have been implicated in the cytoprotection conferred by HO-1/CO, including autophagic proteins, p38 mitogen-activated protein kinase, signal transducer and activator of transcription proteins, nuclear factor-kappaB, phosphatidylinositol 3-kinase/Akt, and others. Enhanced HO-1 expression or the pharmacological application of HO end-products affords protection in preclinical models of tissue injury, including experimental and transplant-associated ischemia/reperfusion injury, promising potential future therapeutic applications.

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.

Divergent cardiopulmonary actions of heme oxygenase enzymatic products in chronic hypoxia

Background

Hypoxia and pressure-overload induce heme oxygenase-1 (HO-1) in cardiomyocytes and vascular smooth muscle cells (VSMCs). HO-1^−/− mice exposed to chronic hypoxia develop pulmonary arterial hypertension (PAH) with exaggerated right ventricular (RV) injury consisting of dilation, fibrosis, and mural thrombi. Our objective was to indentify the HO-1 product(s) mediating RV protection from hypoxic injury in HO-1^−/− mice.

Methodology/Principal Findings

HO-1^−/− mice were exposed to seven weeks of hypoxia and treated with inhaled CO or biliverdin injections. CO reduced right ventricular systolic pressure (RVSP) and prevented hypoxic pulmonary arteriolar remodeling in both HO-1^−/− and control mice. Biliverdin had no significant effect on arteriolar remodeling or RVSP in either genotype. Despite this, biliverdin prevented RV failure in the hypoxic HO-1^−/− mice (0/14 manifested RV wall fibrosis or thrombus), while CO-treated HO-1^−/− mice developed RV insults similar to untreated controls. In vitro, CO inhibited hypoxic VSMC proliferation and migration but did not prevent cardiomyocyte death from anoxia-reoxygenation (A-R). In contrast, bilirubin limited A-R-induced cardiomyocyte death but did not inhibit VSMC proliferation and migration.

Conclusions/Significance

CO and bilirubin have distinct protective actions in the heart and pulmonary vasculature during chronic hypoxia. Moreover, reducing pulmonary vascular resistance may not prevent RV injury in hypoxia-induced PAH; supporting RV adaptation to hypoxia and preventing RV failure must be a therapeutic goal.

Bilirubin inhibits the TNFalpha-related induction of three endothelial adhesion molecules

Since an increased serum unconjugated bilirubin (UCB) level has been proposed as an independent protective factor against atherosclerotic disease, we investigated the molecular events at the basis of this effect. HUVEC and H5V cells were treated with TNFalpha and UCB and the effects assessed on E-selectin, VCAM-1 and ICAM-1. In HUVEC cells, UCB blunted the TNFalpha-induced gene upregulation of E-selectin VCAM-1 and ICAM-1. The same pattern was observed in H5V cells except for ICAM-1. UCB also inhibited the PMN endothelial adhesion in HUVEC H5V cells. Western blot and FACS analysis confirmed that UCB prevented TNFalpha-induced over-expression of adhesion molecules proteins in H5V cells. These data contribute to further explain the protective effect of bilirubin against development of atherosclerosis.