The antioxidant role of bile pigments evaluated by chemical tests

Toward nanotechnology-enabled application of bilirubin in the treatment and diagnosis of various civilization diseases

Bilirubin, an open chain tetrapyrrole, has powerful antioxidant, anti-inflammatory, immuno-suppressive, metabolic-modulating and anti-proliferative activities. Bilirubin is a natural molecule that is produced and metabolized within the human body, making it highly biocompatible and well suited for clinical use. However, the use of bilirubin has been hampered by its poor water solubility and instability. With advanced construction strategies, bilirubin-derived nanoparticles (BRNPs) have not only overcome the disadvantages of bilirubin but also enhanced its therapeutic effects by targeting damaged tissues, passing through physiological barriers, and ensuring controlled sustained release. We review the mechanisms underlying the biological activities of bilirubin, BRNP preparation strategies and BRNP applications in various disease models. Based on their superior performance, BRNPs require further exploration of their efficacy, biodistribution and long-term biosafety in nonhuman primate models that recapitulate human disease to promote their clinical translation.

The Extent of Intracellular Accumulation of Bilirubin Determines Its Anti- or Pro-Oxidant Effect

Background: Severe hyperbilirubinemia can cause permanent neurological damage in particular in neonates, whereas mildly elevated serum bilirubin protects from various oxidative stress-mediated diseases. The present work aimed to establish the intracellular unconjugated bilirubin concentrations (iUCB) thresholds differentiating between anti- and pro-oxidant effects. Methods: Hepatic (HepG2), heart endothelial (H5V), kidney tubular (HK2) and neuronal (SH-SY5Y) cell lines were exposed to increasing concentration of bilirubin. iUCB, cytotoxicity, intracellular reactive oxygen species (ROS) concentrations, and antioxidant capacity (50% efficacy concentration (EC₅₀)) were determined. Results: Exposure of SH-SY5Y to UCB concentration > 3.6 µM (iUCB of 25 ng/mg) and >15 µM in H5V and HK2 cells (iUCB of 40 ng/mg) increased intracellular ROS production (p < 0.05). EC₅₀ of the antioxidant activity was 21 µM (iUCB between 5.4 and 21 ng/mg) in HepG2 cells, 0.68 µM (iUCB between 3.3 and 7.5 ng/mg) in SH-SY5Y cells, 2.4 µM (iUCB between 3 and 6.7 ng/mg) in HK2 cells, and 4 µM (iUCB between 4.7 and 7.5 ng/mg) in H5V cells. Conclusions: In all the cell lines studied, iUCB of around 7 ng/mg protein had antioxidant activities, while iUCB > 25 ng/mg protein resulted in a prooxidant and cytotoxic effects. UCB metabolism was found to be cell-specific resulting in different iUCB.

Multiple origins of green coloration in frogs mediated by a novel biliverdin-binding serpin

Many vertebrates have distinctive blue-green bones and other tissues due to unusually high biliverdin concentrations-a phenomenon called chlorosis. Despite its prevalence, the biochemical basis, biology, and evolution of chlorosis are poorly understood. In this study, we show that the occurrence of high biliverdin in anurans (frogs and toads) has evolved multiple times during their evolutionary history, and relies on the same mechanism-the presence of a class of serpin family proteins that bind biliverdin. Using a diverse combination of techniques, we purified these serpins from several species of nonmodel treefrogs and developed a pipeline that allowed us to assemble their complete amino acid and nucleotide sequences. The described proteins, hereafter named biliverdin-binding serpins (BBS), have absorption spectra that mimic those of phytochromes and bacteriophytochromes. Our models showed that physiological concentration of BBSs fine-tune the color of the animals, providing the physiological basis for crypsis in green foliage even under near-infrared light. Additionally, we found that these BBSs are most similar to human glycoprotein alpha-1-antitrypsin, but with a remarkable functional diversification. Our results present molecular and functional evidence of recurrent evolution of chlorosis, describe a biliverdin-binding protein in vertebrates, and introduce a function for a member of the serpin superfamily, the largest and most ubiquitous group of protease inhibitors.

Plasma metals and serum bilirubin levels in workers from manganese-exposed workers healthy cohort (MEWHC)

Few studies specifically address the possible associations between multiple-metal exposures and liver damage among the occupational population. This study aimed to explore the cross-sectional relationships of plasma metals with liver function parameters. For 571 on-the-spot workers in the manganese-exposed workers healthy cohort (MEWHC), we determined liver function parameters: total bilirubin (TBILI), direct bilirubin (DBILI), indirect bilirubin (IBILI), alanine transaminase (ALT) and aspartate transaminase (AST). Total concentrations of 22 plasma metals were measured by ICP-MS. The LASSO (least absolute shrinkage and selection operator) penalized regression model was applied for selecting plasma metals independently associated with liver function parameters. Multiple linear regression analyses and restricted cubic spline (RCS) were utilized for identifying the exposure-response relationship of plasma metals with liver function parameters. After adjusting for covariates and selected metals, a 1-SD increase in log-10 transformed levels of iron was associated with increases in the levels of TBILI, DBILI and IBILI by 20.3%, 12.1% and 23.7%, respectively; similar increases in molybdenum for decreases in levels of TBILI, DBILI and IBILI by 6.1%, 2.6% and 8.3%, respectively. The effect of a 1-SD increase in plasma copper corresponded decreases of 3.2%, 3.4% and 5.0% in TBILI, AST and ALT levels, respectively. The spline analyses further clarified the non-linear relationships between plasma iron and bilirubin whilst negative linear relationships for plasma molybdenum and bilirubin. Plasma iron was positively whilst plasma molybdenum was negatively associated with increased serum bilirubin levels. Further studies are needed to validate these associations and uncover the underlying mechanisms.

Role of inflammation and inflammasome activation in human bile cast nephropathy

Bile cast nephropathy (BCN) is an underdiagnosed cause of acute kidney injury (AKI). The precise pathogenesis of bilirubin tubular toxicity remains unknown. The aim of this study is to explore the cellular and molecular pathophysiology of human BCN. Paraffin-embedded sections of renal biopsy tissue from a BCN patient were stained by immunohistochemistry (IHC) for oxidative stress (4-hydroxynonenal), immune cell subpopulations, including dendritic cells (CD1c), macrophages (CD68) and T cells (CD3), and inflammasome activation by staining for active-caspase-1 and the inflammasome adaptor protein, ASC (apoptosis-associated speck-like protein containing a caspase activation and recruitment domain). Quantitative analyses of IHC staining were compared to healthy renal cortical tissue. We identified yellow to brown granular casts within the BCN case, consistent with the presence of bile pigment. The presence of bile pigment was associated with strong tubular 4-hydroxynonenal staining intensity, a marker of oxidative stress. Diffuse tubulointerstitial inflammatory cell infiltrate was detected, with elevated CD1c, CD68 and CD3 staining. Foci of inflammasome activity were co-localized with this intense immune cell infiltration, with increased active-caspase-1 and ASC staining. Our findings are the first to suggest that bile casts may lead to oxidative stress and trigger the inflammasome signalling cascade, leading to interstitial inflammation and driving AKI pathobiology. SUMMARY AT A GLANCE The report suggests that bile casts may lead to oxidative stress and trigger the inflammasome signalling cascade, leading to interstitial inflammation and driving bile cast nephropathy pathobiology.

Multiple origins of green blood in New Guinea lizards

Several species of lizards from the megadiverse island of New Guinea have evolved green blood. An unusually high concentration of the green bile pigment biliverdin in the circulatory system of these lizards makes the blood, muscles, bones, tongue, and mucosal tissues bright green in color, eclipsing the crimson color from their red blood cells. This is a remarkable physiological feature because bile pigments are toxic physiological waste products of red blood cell catabolism and, when chronically elevated, cause jaundice in humans and all other vertebrates. Although these lizards offer a promising system to examine the evolution of extraordinary physiological characteristics, little is known about the phylogenetic relationships of green-blooded lizards or the evolutionary origins of green blood. We present the first extensive phylogeny for green-blooded lizards and closely related Australasian lizards using thousands of genomic regions to examine the evolutionary history of this unusual trait. Maximum likelihood ancestral character state reconstruction supports four independent origins of green blood. Our results lay the phylogenetic foundation necessary to determine the role, if any, of natural selection in shaping this enigmatic physiological trait as well as understanding the genetic, proteomic, and biochemical basis for the lack of jaundice in those species that have independently evolved green blood.

Association of serum bilirubin with renal outcomes in Han Chinese patients with chronic kidney disease

BACKGROUND

Oxidative stress and inflammation play pivotal roles in chronic kidney disease (CKD). Bilirubin is an endogenous anti-inflammatory antioxidant. However, the relationship between serum bilirubin and renal outcomes in CKD is controversial. We explored the association of serum bilirubin levels with renal outcomes in Han Chinese patients with CKD.

METHODS

Clinical and laboratory data were collected from 316 patients with CKD. The primary clinical endpoint was renal replacement therapy or death. The association between serum bilirubin and clinical parameters was assessed by correlation analysis. Multiple Cox regression analysis was used to explore the relationship between serum bilirubin and renal outcomes in patients with CKD.

RESULTS

Serum total and indirect bilirubin were positively correlated with estimated glomerular filtration rate, but negatively correlated with 24-h urine protein in patients with CKD. Serum total and indirect bilirubin were inversely associated with CKD stages in patients with CKD stages 1-5. Multiple Cox regression analysis demonstrated that the higher concentration of serum total bilirubin was independently associated with better renal outcomes in CKD.

CONCLUSIONS

Our results suggest that serum total bilirubin may have protective effects on kidneys.

Protein structure. Structure and activity of tryptophan-rich TSPO proteins

Translocator proteins (TSPOs) bind steroids and porphyrins, and they are implicated in many human diseases, for which they serve as biomarkers and therapeutic targets. TSPOs have tryptophan-rich sequences that are highly conserved from bacteria to mammals. Here we report crystal structures for Bacillus cereus TSPO (BcTSPO) down to 1.7 Å resolution, including a complex with the benzodiazepine-like inhibitor PK11195. We also describe BcTSPO-mediated protoporphyrin IX (PpIX) reactions, including catalytic degradation to a previously undescribed heme derivative. We used structure-inspired mutations to investigate reaction mechanisms, and we showed that TSPOs from Xenopus and man have similar PpIX-directed activities. Although TSPOs have been regarded as transporters, the catalytic activity in PpIX degradation suggests physiological importance for TSPOs in protection against oxidative stress.

The pharmacological features of bilirubin: the question of the century

This review looks at the toxicity and metabolism of bilirubin in terms of its pharmacological potential. Its role has gained importance as more research has revealed the functional significance and interrelationship between the gasotransmitters nitric oxide and carbon monoxide. The biological actions of bilirubin have mostly been characterized in the high micromolar range where toxic effects occur. However, it could also prove to be an important cytoprotector for brain tissue, which is inherently less equipped for antioxidant defense. Plasma bilirubin levels negatively correlate to a number of disease states. Higher levels of bilirubin that are still within the normal range provide a protective effect to the body. The effects on various disorders could be tested using controlled pharmacological upregulation of the molecule with animal models. At nanomolar concentrations, considerable benefits have been obtained when the molecule was delivered pharmacologically under in vitro or in vivo test conditions, particularly in neurodegenerative disorders and after tissue or organ transplantation. The induction of heme oxygenase-1 (HMOX-1) via the activation of nuclear factor erythroid 2-related factor or the use of bile pigments in the harvesting of diseased tissue are novel applications, and like every new therapy, should be used with caution. HMOX-1 is tissue specific, and in exceptional states, such as schizophrenia and specific types of renal disorder, the same therapy may have disastrous effects.

Higher serum direct bilirubin levels were associated with a lower risk of incident chronic kidney disease in middle aged Korean men

Background

The association between serum bilirubin levels and incident chronic kidney disease (CKD) in the general population is unknown. We aimed to examine the association between serum bilirubin concentration (total, direct, and indirect) and the risk of incident CKD.

Methods and Findings

Longitudinal cohort study of 12,823 Korean male workers 30 to 59 years old without CKD or proteinuria at baseline participating in medical health checkup program in a large worksite. Study participants were followed for incident CKD from 2002 through 2011. Estimated glomerular filtration rate (eGFR) was estimated by using the CKD-EPI equation. CKD was defined as eGFR <60 mL/min per 1.73 m². Parametric Cox models and pooled logistic regression models were used to estimate adjusted hazard ratios for incident CKD. We observed 238 incident cases of CKD during 70,515.8 person-years of follow-up. In age-adjusted models, the hazard ratios for CKD comparing quartiles 2–4 vs. quartile 1 of serum direct bilirubin were 0.93 (95% CI 0.67–1.28), 0.88 (0.60–1.27) and 0.60 (0.42–0.88), respectively. In multivariable models, the adjusted hazard ratio for CKD comparing the highest to the lowest quartile of serum direct bilirubin levels was 0.60 (95% CI 0.41–0.87; P trend = 0.01). Neither serum total nor indirect bilirubin levels were significantly associated with the incidence of CKD.

Conclusions

Higher serum direct bilirubin levels were significantly associated with a lower risk of developing CKD, even adjusting for a variety of cardiometabolic parameters. Further research is needed to elucidate the mechanisms underlying this association and to establish the role of serum direct bilirubin as a marker for CKD risk.

Heme oxygenase 1-generated carbon monoxide and biliverdin attenuate the course of experimental necrotizing pancreatitis

OBJECTIVE

The cytoprotective enzyme heme oxygenase 1 (HO-1) is highly up-regulated in acute pancreatitis (AP). In this study, we tested its metabolites as potential therapeutic agents for AP in rats.

METHODS

Acute necrotizing pancreatitis was induced by retrograde intraductal injection of sodium taurocholate in rats. Biliverdin hydrochloride (BV HCl) (50 μmol/kg subcutaneously), the carbon monoxide, donor methylene chloride (MC) (500 mg/kg orally), or iron-chelating desferrioxamine (DFO) (125 mg/kg subcutaneously) were administered in a therapeutic manner starting with the first dose 4 hours after taurocholate injection to mimic the effects of HO-1 metabolites.

RESULTS

Administration of BV HCl, MC, or DFO showed significant reduction of inflammatory activity in comparison to controls leading to lower myeloperoxidase activity in the pancreas, less edema, lower ascites volumes, and preservation of tissue integrity (P < 0.05). Administration of either BV HCl or MC markedly increased 5-day survival rate (70% and 75% vs 40%; P < 0.05), whereas DFO had no significant effect on survival (60%). When given in therapeutic manner, all 3 substances led to diminished nuclear factor κB activity in the pancreas (P < 0.05).

CONCLUSIONS

Therapeutic use of BV HCl and MC led to marked reduction of mortality in experimental pancreatitis. Thus, HO-1 metabolites may present a novel therapeutic approach in AP treatment.

Structural insights into human heme oxygenase-1 inhibition by potent and selective azole-based compounds

The development of heme oxygenase (HO) inhibitors, especially those that are isozyme-selective, promises powerful pharmacological tools to elucidate the regulatory characteristics of the HO system. It is already known that HO has cytoprotective properties and may play a role in several disease states, making it an enticing therapeutic target. Traditionally, the metalloporphyrins have been used as competitive HO inhibitors owing to their structural similarity with the substrate, heme. However, given heme's important role in several other proteins (e.g. cytochromes P450, nitric oxide synthase), non-selectivity is an unfortunate side-effect. Reports that azalanstat and other non-porphyrin molecules inhibited HO led to a multi-faceted effort to develop novel compounds as potent, selective inhibitors of HO. This resulted in the creation of non-competitive inhibitors with selectivity for HO, including a subset with isozyme selectivity for HO-1. Using X-ray crystallography, the structures of several complexes of HO-1 with novel inhibitors have been elucidated, which provided insightful information regarding the salient features required for inhibitor binding. This included the structural basis for non-competitive inhibition, flexibility and adaptability of the inhibitor binding pocket, and multiple, potential interaction subsites, all of which can be exploited in future drug-design strategies.

A novel, "double-clamp" binding mode for human heme oxygenase-1 inhibition

The development of heme oxygenase (HO) inhibitors is critical in dissecting and understanding the HO system and for potential therapeutic applications. We have established a program to design and optimize HO inhibitors using structure-activity relationships in conjunction with X-ray crystallographic analyses. One of our previous complex crystal structures revealed a putative secondary hydrophobic binding pocket which could be exploited for a new design strategy by introducing a functional group that would fit into this potential site. To test this hypothesis and gain further insights into the structural basis of inhibitor binding, we have synthesized and characterized 1-(1H-imidazol-1-yl)-4,4-diphenyl-2-butanone (QC-308). Using a carbon monoxide (CO) formation assay on rat spleen microsomes, the compound was found to be ∼15 times more potent (IC₅₀ = 0.27±0.07 µM) than its monophenyl analogue, which is already a potent compound in its own right (QC-65; IC₅₀ = 4.0±1.8 µM). The crystal structure of hHO-1 with QC-308 revealed that the second phenyl group in the western region of the compound is indeed accommodated by a definitive secondary proximal hydrophobic pocket. Thus, the two phenyl moieties are each stabilized by distinct hydrophobic pockets. This “double-clamp” binding offers additional inhibitor stabilization and provides a new route for improvement of human heme oxygenase inhibitors.

Unconjugated bilirubin contributes to early inflammation and edema after intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a stroke subtype with significant mortality and morbidity. The role of unconjugated bilirubin (UBR) in ICH brain injury is not well understood. Therefore, we studied the effects of UBR on brain injury markers and inflammation, as well as mechanisms involved therein. We induced ICH in mice by infusion of autologous whole blood with vehicle (dimethyl sulfoxide) or UBR. We found that UBR led to an increase in edema (P≤0.05), but a decrease in nitrate/nitrite formation (7.0±0.40  nmol/mg versus 5.2±0.70  nmol/mg protein, P≤0.05) and no change in protein carbonyls. Unconjugated bilirubin was also associated with an increase in neutrophil infiltration compared with ICH alone, as determined by both immunofluorescence and flow cytometry (36%±3.2% versus 53%±1.3% of CD45(+) cells, P≤0.05). In contrast, we observed reduced perihematomal microglia immunoreactivity in animals receiving UBR (P≤0.05). Using in vitro techniques, we show neutrophil activation by UBR and also show that protein kinase C participates in this signaling pathway. Finally, we found that UBR was associated with an increased expression of the leukocyte adhesion molecule intercellular adhesion molecule-1. Our results suggest that UBR possesses complex immune-modulatory and antioxidant effects.

Glutathione biosynthesis via activation of the nuclear factor E2-related factor 2 (Nrf2)--antioxidant-response element (ARE) pathway is essential for neuroprotective effects of sulforaphane and 6-(methylsulfinyl) hexyl isothiocyanate

Oxidative stress plays pivotal roles in aging, neurodegenerative disease, and pathological conditions such as ischemia. We investigated the effect of sulforaphane and 6-(methysulfinyl) hexyl isothiocyanate (6-HITC), a naturally occurring isothiocyanate, on oxidative stress-induced cytotoxicity using primary neuronal cultures of rat striatum. Pretreatment with sulforaphane and 6-HITC significantly protected against H(2)O(2)- and paraquat-induced cytotoxicity in a concentration-dependent manner. Sulforaphane and 6-HITC induced the translocation of nuclear factor E2-related factor 2 (Nrf2) into the nucleus and increased the expression of γ-glutamylcysteine synthetase (γ-GCS), a rate-limiting enzyme in glutathione synthesis, and the intracellular glutathione content. Treatment with reduced glutathione (GSH) and N-acetyl-L-cysteine, a substance for glutathione synthesis, significantly prevented the cytotoxicity induced by H(2)O(2) and paraquat. Moreover, exposure to L-buthionine-sulfoximine, an irreversible inhibitor of γ-GCS, suppressed the protective effects of sulforaphane and 6-HITC. In contrast, sulforaphane and 6-HITC increased heme oxygenase-1 (HO-1) expression in neurons. However, zinc-protophorphyrin IX, a competitive inhibitor of HO-1, did not influence the protective effects of sulforaphane and 6-HITC. These results suggest that sulforaphane and 6-HITC prevent oxidative stress-induced cytotoxicity in rat striatal cultures by raising the intracellular glutathione content via an increase in γ-GCS expression induced by the activation of the Nrf2-antioxidant response element pathway.

The effects of azole-based heme oxygenase inhibitors on rat cytochromes P450 2E1 and 3A1/2 and human cytochromes P450 3A4 and 2D6

Heme oxygenases (HOs) catalyze the degradation of heme to biliverdin, carbon monoxide (CO), and free iron. The two major isoforms, HO-1 (inducible) and HO-2 (constitutive), are involved in a variety of physiological functions, including inflammation, apoptosis, neuromodulation, and vascular regulation. Major tools used in exploring these actions have been metalloporphyrin analogs of heme that inhibit the HOs. However, these tools are limited by their lack of selectivity; they affect other heme-dependent enzymes, such as cytochromes P450 (P450s), soluble guanylyl cyclase (sGC), and nitric-oxide synthase (NOS). Our laboratory has successfully synthesized a number of nonporphyrin azole-based HO inhibitors (QC-xx) that had little or no effect on sGC and NOS activity. However, their effects on various P450 isoforms have yet to be fully elucidated. To determine the effects of the QC-xx inhibitors on P450 enzyme activity, microsomal preparations of two rat P450 isoforms (2E1 and 3A1/3A2) and two human P450 supersome isoforms (3A4 and 2D6) were incubated with varying concentrations of HO inhibitor, and the activity was determined by spectrophotometric or fluorometric analysis. Results indicated that some QC compounds demonstrated little to no inhibition of the P450s, whereas others did inhibit these P450 isoforms. Four structural regions of QC-xx were analyzed, leading to the identification of structures that confer a decreased effect on both rat and human P450 isoforms studied while maintaining an inhibitory effect on the HOs.

Heme oxygenase-1 and its metabolites affect pancreatic tumor growth in vivo

Background

Pancreatic cancer (PaCa) is a fatal human cancer due to its exceptional resistance to all current anticancer therapies. The cytoprotective enzyme heme oxygenase-1 (HO-1) is significantly overexpressed in PaCa and seems to play an important role in cancer resistance to anticancer treatment. The inhibition of HO-1 sensitized PaCa cells to chemo- and radiotherapy in vitro.

Therefore, we investigated the effects of HO-1 and its metabolites biliverdin, carbon monoxide and iron on PaCa cells.

PaCa cell lines with divergent HO-1 expression patterns were used in a murine orthotopic cancer model. HO-1 expression and activity was regulated by zinc (inhibition) and cobalt (induction) protoporphyrin. Furthermore, the influence of cellular HO-1 levels and its metabolites on effects of standard chemotherapy with gemcitabine was tested in vivo and in vitro.

Results

High HO-1 expression in PaCa cell lines was associated with increased chemoresistance in vitro. Chemoresistance to gemcitabine was increased during HO-1 induction in PaCa cells expressing low levels of HO-1. The inhibition of HO-1 activity in pancreatic tumors with high HO-1 boosted chemotherapeutic effects in vivo significantly. Furthermore, biliverdin and iron promoted PaCa resistance to chemotherapy. Consequently, specific iron chelation by desferrioxamine revealed profound anticancerous effects.

Conclusion

In summary, the inhibition of HO-1 and the chelation of iron in PaCa cells were associated with increased sensitivity and susceptibility of pancreatic tumors to chemotherapy in vivo. The metabolites biliverdin and iron seem to be involved in HO-1-mediated resistance to anticancer treatment. Therefore, HO-1 inhibition or direct interference with its metabolites may evolve new PaCa treatment strategies.

UGT1A1 gene polymorphism as a potential factor inducing iron overload in the pathogenesis of type 1 hereditary hemochromatosis

Aim Hereditary hemochromatosis is a common genetic disorder characterized by iron overload and subsequent organ damage. It is caused in most cases by HFE gene mutations which penetrance can be affected by many factors. The aim of this study was to establish the role of UGT1A1 gene polymorphism and serum bilirubin concentration in the pathogenesis of hereditary hemochromatosis. Methods Biochemical, histopathological and genetic data indicating iron excess and serum total bilirubin concentration were determined in 32 patients with the type 1 hereditary hemochromatosis. Fluorescent molecular probes assays were used for genotyping of UGT1A1*28 and UGT1A1*60 mutations in these individuals. Results High incidence and a significant correlation of UGT1A1 gene mutations with increased serum bilirubin level and lower grades of liver tissue inflammatory activity were observed in study participants. UGT1A1*28 and UGT1A1*60 mutations were strongly linked together. Two of the subjects presented very rare genotypes of UGT1A1 gene: (TA)(5/7) and c.-64G>C heterozygotes. Conclusions UGT1A1 gene polymorphism and as its consequence of high serum bilirubin level may promote iron accumulation in hemochromatosis patients by reducing the activity of inflammation. We proposed a possible mechanism of this interaction.

Inhibitors of the heme oxygenase - carbon monoxide system: on the doorstep of the clinic?

The past decade has seen substantial developments in our understanding of the physiology, pathology, and pharmacology of heme oxygenases (HO), to the point that investigators in the field are beginning to contemplate therapies based on administration of HO agonists or HO inhibitors. A significant amount of our current knowledge is based on the judicious application of metalloporphyrin inhibitors of HO, despite their limitations of selectivity. Recently, imidazole-based compounds have been identified as potent and more selective HO inhibitors. This 'next generation' of HO inhibitors offers a number of desirable characteristics, including isozyme selectivity, negligible effects on HO protein expression, and physicochemical properties favourable for in vivo distribution. Some of the applications of HO inhibitors that have been suggested are treatment of hyperbilirubinemia, neurodegenerative disorders, certain types of cancer, and bacterial and fungal infections. In this review, we address various approaches to altering HO activity with a focus on the potential applications of second-generation inhibitors of HO.

Biliverdin reductase mediates hypoxia-induced EMT via PI3-kinase and Akt

Chronic hypoxia in the renal parenchyma is thought to induce epithelial-to-mesenchymal transition (EMT), leading to fibrogenesis and ultimately end-stage renal failure. Biliverdin reductase, recently identified as a serine/threonine/tyrosine kinase that may activate phosphatidylinositol 3-kinase (PI3K) and Akt, is upregulated in response to reactive oxygen species that may accompany hypoxia. We investigated this potential role of biliverdin reductase in hypoxia-induced renal tubular EMT. Expression of biliverdin reductase was upregulated in a human proximal tubule cell line (HK-2) cultured in hypoxic conditions (1% O2), and this was accompanied by reduced expression of E-cadherin and increased expression of the mesenchymal marker vimentin. Inhibiting PI3K reversed these changes, consistent with EMT. In normoxic conditions, overexpression of biliverdin reductase promoted similar characteristics of EMT, which were also reversed by inhibiting PI3K. Furthermore, using small interfering RNA (siRNA) to knockdown biliverdin reductase, we demonstrated that the enzyme associates with phosphorylated Akt and mediates the hypoxia-induced EMT phenotype. In vivo, expression of biliverdin reductase increased in the tubular epithelia of 5/6-nephrectomized rats, and immunohistochemistry of serial sections demonstrated similar localization of phosphorylated Akt and biliverdin reductase. In conclusion, biliverdin reductase mediates hypoxia-induced EMT through a PI3K/Akt-dependent pathway.

''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.

Bilirubin as an antioxidant in micelles and lipid bilayers: its contribution to the total antioxidant capacity of human blood plasma

The antioxidant capacities, antioxidant activities, k(inh), and stoichiometric factors, n, of water-soluble derivatives of bilirubin (BR), BR-human serum albumin (BR-HSA), and BR-ditaurate disodium conjugate (BRC) were determined in aqueous/lipid dispersions of sodium dodecyl sulfate (SDS) micelles/methyl linoleate and in bilayers of dilinoleoylphosphatidylcholine (DLPC) during initiation by water-soluble azo-bis-amidinopropane dihydrochloride (ABAP). The inhibition rate constants for BRC and BR-HSA were similar in micelles (k(inh) approximately 1.3 x 10(4) M(-1) s(-1)), where n approximately 2, whereas the k(inh) for BR-HSA dropped by (1/2) in bilayers. The dimethyl ester of bilirubin (BRDE) gave a k(inh) only one-tenth that of the vitamin E analog, pentamethylhydroxychroman (PMHC) in SDS micelles/methyl linoleate when initiated by lipid-soluble azo-bis-2,4-dimethylvaleronitrile (DMVN). Biliverdin hydrochloride (BVHCl) was NOT an effective peroxyl radical-trapping agent in the micellar phase during initiation by ABAP or DMVN containing methyl linoleate but it inhibited oxygen uptake in the aqueous phase. Both BRC and BR-HSA extended the total radical antioxidant parameter (TRAP) of human blood plasma and their contribution to TRAP was in the range of 5-10% of the natural TRAP of blood plasma, depending on the BR content determined in the blood plasma.

Induction of heme oxygenase-1 in vivo suppresses NADPH oxidase derived oxidative stress

Our previous studies suggest that heme oxygenase (HO)-1 induction and/or subsequent bilirubin generation in endothelial cells may suppress superoxide generation of from reduced nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase. In this study, we examined the consequence of HO-1 induction in vivo on NADPH oxidase activity. Three doses of hemin (25 mg x kg(-1), IP, every 48 hours), with or without cotreatment with the HO inhibitor tin protoporphyrin-IX (15 mg x kg(-1), IP), were given to apolipoprotein E-deficient mice, which display vascular oxidative stress. Hemin treatment increased HO-1 expression and activity in aorta (undetectable at baseline) and kidney (by 3-fold) and significantly reduced both NADPH oxidase activity (by approximately 25% to 50%) and superoxide generation in situ. The increase in HO-1 activity and inhibition of NADPH oxidase activity by hemin were reversed by tin protoporphyrin-IX and were not associated with changes in Nox2 or Nox4 protein levels. Hemin also reduced plasma F(2)-isoprostane levels by 23%. The inhibition of NADPH oxidase activity by hemin in the aorta was mimicked by bilirubin in vitro (0.01 to 1 micromol/L). Bilirubin also concentration-dependently reduced NADPH oxidase-dependent superoxide production stimulated by angiotensin II in rat vascular smooth muscle cells and by phorbol 12-myristate 13-acetate in human neutrophil-like HL-60 cells. HO-1 overexpression by plasmid-mediated gene transfer in rat vascular smooth muscle cells decreased NADPH-stimulated superoxide production. Thus, systemic expression of HO-1 suppresses NADPH oxidase activity by mechanisms at least partly mediated by the bile pigment bilirubin, thereby reducing oxidative stress.

Effect of cigarette smoke on placental antioxidant enzyme expression

Cigarette smoking is associated with systemic oxidative stress leading to an upregulation of antioxidant systems [superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and heme oxygenase (HO)] in some tissues, but the response in the human placenta is unknown. The aim of this study was to determine the effect of cigarette smoke exposure on placental antioxidant expression in vivo, as well as the effect on antioxidant expression in the human trophoblast choriocarcinoma (HTR)-8SVNeo cell line. In the in vivo experiment, normal-term placentas were obtained following elective caesarean section. The chorionic villi (CV), anchoring villi (AV), and basal plate (BP) were dissected, and Western blot analysis was carried out for HO-1, HO-2, SOD, CAT, and GPx. In vitro experiment, a cigarette smoke extract (CSE) was prepared by bubbling the smoke form three cigarettes through 15 ml of RPMI. This 100% CSE was syringe filtered and diluted to 0.1, 0.5, 1, 2, 5, and 10% concentrations. HTR-8SVNeo cells were cultured with the CSE for 48 h. The cells were harvested, protein was extracted, and run on SDS-PAGE gels, and Western blot analysis was carried out for HO-1, HO-2, SOD, and CAT. Immunofluorescence for HTR-8SVNeo cells HO-1 was carried out following increasing concentrations of CSE. In the in vivo experiment, HO-1 and HO-2 expression was increased in the BP of placentas from smokers compared with nonsmokers. CAT, GPx, and SOD levels in all placental regions, as well as HO-1 and HO-2 expression in the AV and CV were unchanged. In the in vitro experiment, The 5%, 10%, and 20% dilutions were toxic to the cells. The 0.1% CSE solution did not significantly alter HO-1 expression. Treatment with the 0.5%, 1% and 2% CSE solutions resulted in a dose-dependent increase in HO-1 expression. None of the CSE treatments resulted in a significant alteration in HO-2, SOD, GPx, or CAT expression. HO-1 immunoflourescence confirmed the HO-1 expression studies. Cigarette smoke exposure increases HO-1 and HO-2 expression in the placental basal plate and increases HO-1 expression in the HTR-8SVNeo cell line. Increased HO-1 and HO-2 protein expression may increase the production of the antioxidants biliverdin and bilirubin, which are products of heme metabolism. This could function to reduce the oxidative load that is released into the maternal plasma from the preeclamptic placenta and may contribute to the observed decreased incidence of preeclampsia in smokers.

The Heme Catabolic Pathway and its Protective Effects on Oxidative Stress‐Mediated Diseases

Bilirubin, the principal bile pigment, is the end product of heme catabolism. For many years, bilirubin was thought to have no physiological function other than that of a waste product of heme catabolism--useless at best and toxic at worst. Although hyperbilirubinemia in neonates has been shown to be neurotoxic, studies performed during the past decade have found that bilirubin has a number of new and interesting biochemical and biological properties. In addition, there is now a strong body of evidence suggesting that bilirubin may have a beneficial role in preventing oxidative changes in a number of diseases including atherosclerosis and cancer, as well as a number of inflammatory, autoimmune, and degenerative diseases. The results also suggest that activation of the heme oxygenase and heme catabolic pathway may have beneficiary effects on disease prevention either through the action of bilirubin or in conjunction with bilirubin. If so, it may be possible to therapeutically induce heme oxygenase, increase bilirubin concentrations, and lower the risk of oxidative stress-related diseases.

Protective effect of carbon monoxide in transplantation

During the last decades due to the development of new immunosuppressive agents and improvements in organ preservation methods, surgical techniques, and postoperative care, organ transplantation has become an ultimate therapeutic option for irreversible organ failure. Early graft survival has significantly improved; however, the long-term outcome remains unsatisfactory. Multiple factors, both immunogenic and non-immunogenic etiologies, are involved in the deterioration of the allografts, and the recent use of expanded criteria donors to overcome the organ shortage may also contribute to the graft losses. Carbon monoxide (CO) is commonly viewed as a poison in high concentrations due to its ability to interfere with oxygen delivery. However, CO is endogenously produced in the body as a byproduct of heme degradation by the heme oxygenase (HO) and has recently received notable attention as a gaseous regulatory molecule. In fact, an augmentation of endogenous CO by induction of HO-1 or exogenously added CO is known to have potent cytoprotective effects in various disease models. Several recent reports have demonstrated that CO provides potent cytoprotective effects in the field of organ and cell transplantation. CO is able to prevent ischemia/reperfusion injury, allograft rejection, and xenograft rejection via its anti-inflammatory, anti-apoptotic and anti-proliferation effects, suggesting that CO might be a valuable therapeutic option in the field of transplantation. Based on the recent advancement of our understanding of CO as a new therapeutic molecule, this review attempts to summarize the functional roles as well as biological and molecular mechanisms of CO in transplantation and discusses potential CO application to the clinical transplant setting.

A protective role for heme oxygenase-1 in INS-1 cells and rat islets that are exposed to high glucose conditions

Heme oxygenase-1 (HO-1) has been described as an inducible protein that is capable of cytoprotection via radical scavenging and the prevention of apoptosis. Chronic exposure to hyperglycemia can lead to cellular dysfunction that may become irreversible over time, and this process has been termed glucose toxicity. Yet little is known about the relation between glucose toxicity and HO-1 in the islets. The purposes of the present study were to determine whether prolonged exposure of pancreatic islets to a supraphysiologic glucose concentration disrupts the intracellular balance between reactive oxygen species (ROS) and HO-1, and so this causes defective insulin secretion; we also wanted to evaluate a protective role for HO-1 in pancreatic islets against high glucose levels. The intracellular peroxide levels of the pancreatic islets (INS-1 cell, rat islet) were increased in the high glucose media (30 mM glucose or 50 mM ribose). The HO-1 expression was induced in the INS-1 cells by the high glucose levels. Both the HO-1 expression and glucose stimulated insulin secretion (GSIS) was decreased simultaneously in the islets by treatment of the HO-1 antisense. The HO-1 was upregulated in the INS-1 cells by hemin, an inducer of HO-1. And, HO-1 upregulation induced by hemin reversed the GSIS in the islets at a high glucose condition. These results suggest HO-1 seems to mediate the protective response of pancreatic islets against the oxidative stress that is due to high glucose conditions.

Biliverdin administration protects against endotoxin-induced acute lung injury in rats

Given the high morbidity and mortality rates associated with pulmonary inflammation in sepsis, there is a pressing need for new therapeutic modalities to prevent acute respiratory distress. The enzyme heme oxygenase-1 (HO-1) provides potent cytoprotection against lung injury; however, the mechanism by which it does so is unclear. HO-1 catabolizes heme into biliverdin (BV), which is rapidly converted to bilirubin by BV reductase. We tested the hypothesis that BV administration could substitute for the effects observed with HO-1. Using the well-described rat model of LPS-induced shock, we demonstrate that exposure to BV imparts a potent defense against lethal endotoxemia systemically, as well as in the lungs, and effectively abrogates the inflammatory response. BV administration before a lethal dose of LPS leads to a significant improvement in long-term survival: 87% vs. 20% in sham-treated controls. BV treatment suppressed LPS-induced increases in lung permeability and lung alveolitis and significantly reduced serum levels of the LPS-induced proinflammatory cytokine IL-6. Moreover, bilirubin administered just after LPS also abrogated lung inflammation. BV treatment also augmented expression of the anti-inflammatory cytokine IL-10. Similar effects on production were observed with BV treatment in vitro in mouse lung endothelial cells and RAW 264.7 macrophages treated with LPS. In conclusion, these data demonstrate that BV can modulate the inflammatory response and suppress pathophysiological changes in the lung and may therefore have therapeutic application in inflammatory disease states of the lung.

Unique Properties of Polyphenol Stilbenes in the Brain: More than Direct Antioxidant Actions; Gene/Protein Regulatory Activity

The 'French Paradox' has been typically associated with moderate consumption of wine, especially red wine. A polyphenol 3,4',5-trihydroxy-trans-stilbene (a member of the non-flavonoids family), better known as resveratrol, has been purported to have many health benefits. A number of these valuable properties have been attributed to its intrinsic antioxidant capabilities, although the potential level of resveratrol in the circulation is likely not enough to neutralize free radical scavenging. The brain and the heart are uniquely vulnerable to hypoxic conditions and oxidative stress injuries. Recently, evidence suggests that resveratrol could act as a signaling molecule within tissues and cells to modulate the expression of genes and proteins. Stimulation of such proteins and enzymes could explain some the intracellular antioxidative properties. The modulation of genes could suffice as an explanation of some of resveratrol's cytoprotective actions, as well as its influence on blood flow, cell death, and inflammatory cascades. Resveratrol stimulation of the expression of heme oxygenase is one example. Increased heme oxygenase activity has led to significant protection against models of in vitro and in vivo oxidative stress injury. Resveratrol could provide cellular resistance against insults; although more work is necessary before it is prescribed as a potential prophylactic in models of either acute or chronic conditions, such as stroke, amyotrophic lateral sclerosis, Parkinson, Alzheimer, and a variety of age-related vascular disorders.

A facile and versatile preparation of bilindiones and biladienones from tetraarylporphyrins

Bilindiones and biladienones carrying aryl groups at the meso positions were prepared using coupled oxidation reactions of iron tetraarylporphyrins in 20-63% yield.

Differential induction of heme oxygenase and other stress proteins in cultured hippocampal astrocytes and neurons by inorganic lead

We examined the effects of exposure to inorganic lead (Pb2+) on the induction of stress proteins in cultured hippocampal neurons and astrocytes, with particular emphasis on the induction of heme oxygenase-1 (HO-1). In radiolabeled neuronal cultures, Pb2+ exposure had no significant effect on the synthesis of any protein at any concentration (up to 250 microM) or duration of exposure (up to 4 days). In radiolabeled astrocyte cultures, however, Pb2+ exposure (100 nM to 100 microM; 1-4 days) increased synthesis of proteins with approximate molecular weights of 23, 32, 45, 57, 72, and 90 kDa. Immunoblot experiments showed that Pb2+ exposure (100 nM to 10 microM, 1-14 days) induces HO-1 synthesis in astrocytes, but not in neurons; this is probably the 32-kDa protein. The other heme oxygenase isoform, HO-2, is present in both neurons and astrocytes, but is not inducible by Pb2+ at concentrations up to 100 microM. HO-1 can be induced by a variety of stimuli. We found that HO-1 induction in astrocytes is increased by combined exposure to Pb2+ and many other stresses, including heat, nitric oxide, H2O2, and superoxide. One of the stimuli that may induce HO-1 is oxidative stress. Lead exposure causes oxidative stress in many cell types, including astrocytes. Induction of HO-1 by Pb2+ is reduced by the hydroxyl radical scavengers dimethylthiourea (DMTU) and mannitol, but not by inhibitors of calmodulin, calmodulin-dependent protein kinases, protein kinase C, or extracellular signal-regulated kinases (ERK). Therefore, we conclude that oxidative stress is an important mechanism by which Pb2+ induces HO-1 synthesis in astrocytes.

Kaposi sarcoma-associated herpesvirus (KSHV) induces heme oxygenase-1 expression and activity in KSHV-infected endothelial cells

Kaposi sarcoma (KS) is the most common AIDS-associated malignancy and is characterized by angiogenesis and the presence of spindle cells. Kaposi sarcoma-associated herpesvirus (KSHV) is consistently associated with all clinical forms of KS, and in vitro infection of dermal microvascular endothelial cells (DMVECs) with KSHV recapitulates many of the features of KS, including transformation, spindle cell proliferation, and angiogenesis. To study the molecular mechanisms of KSHV pathogenesis, we compared the protein expression profiles of KSHV-infected and uninfected DMVECs. This comparison revealed that heme oxygenase-1 (HO-1), the inducible enzyme responsible for the rate-limiting step in heme catabolism, was up-regulated in infected endothelial cells. Recent evidence suggests that the products of heme catabolism have important roles in endothelial cell biology, including apoptosis and angiogenesis. Here we show that HO-1 mRNA and protein are up-regulated in KSHV-infected cultures. Comparison of oral and cutaneous AIDS-KS tissues with normal tissues revealed that HO-1 mRNA and protein were also up-regulated in vivo. Increased HO-1 enzymatic activity in vitro enhanced proliferation of KSHV-infected DMVECs in the presence of free heme. Treatment with the HO-1 inhibitor chromium mesoporphyrin IX abolished heme-induced proliferation. These data suggest that HO-1 is a potential therapeutic target for KS that warrants further study.

Prooxidant and antioxidant activities of bilirubin and its metabolic precursor biliverdin: a structure-activity study

Bilirubin, which is derived from its metabolic precursor biliverdin, is the end product of heme catabolism. It has been proposed as a physiological antioxidant present in human extracellular fluids. We have earlier shown that bilirubin in the presence of the transition metal ion Cu(II) causes strand cleavage in DNA through generation of reactive oxygen species, particularly the hydroxyl radical. Thus bilirubin possesses both antioxidant and prooxidant properties. In order to understand the chemical basis of various biological properties of bilirubin, we have studied the structure-activity relationship between bilirubin and its precursor biliverdin. The latter has also been reported to possess both antioxidant and toxic properties. In the present studies bilirubin was found to be more effective in the DNA cleavage reaction and a more efficient reducer of Cu(II). The rate of formation of hydrogen peroxide and hydroxyl radicals by the compounds also showed a similar pattern. The relative antioxidant activity was also examined by studying the effect of these compounds on DNA cleavage by a hydroxyl radical generating system and their quenching effect on hydroxyl radicals. The results indicate that bilirubin is more active both as an antioxidant as well as an oxidative DNA cleaving agent. A model for binding of copper to bilirubin has been proposed where two copper ions are bound to two molecules of bilirubin through their terminal pyrrole nitrogens. In order to account for the enhanced copper reducing capacity of bilirubin we have further proposed that an additional copper binding site is provided for in the case of bilirubin due to the absence of a double bond between pyrrole rings II and III. Further it would appear that the structural features of the bilirubin molecule which are important for its prooxidant action are also the ones that render it a more effective antioxidant.

Scavenging of peroxynitrite by phycocyanin and phycocyanobilin from Spirulina platensis: protection against oxidative damage to DNA

Peroxynitrite (ONOO(-)) is known to inactivate important cellular targets and also mediate oxidative damage in DNA. The present study has demonstrated that phycocyanin, a biliprotein from spirulina platensis and its chromophore, phycocyanobilin (PCB), efficiently scavenge ONOO(-), a potent physiological inorganic toxin. Scavenging of ONOO(-) by phycocyanin and PCB was established by studying their interaction with ONOO(-) and quantified by using competition kinetics of pyrogallol red bleaching assay. The relative antioxidant ratio and IC(50) value clearly indicate that phycocyanin is a more efficient ONOO(-) scavenger than PCB. The present study has also shown that PCB significantly inhibits the ONOO(-)-mediated single-strand breaks in supercoiled plasmid DNA in a dose-dependent manner with an IC(50) value of 2.9 +/- 0.6 microM. These results suggest that phycocyanin, has the ability to inhibit the ONOO(-)-mediated deleterious biological effects and hence has the potential to be used as a therapeutic agent.

Bilirubin, formed by activation of heme oxygenase-2, protects neurons against oxidative stress injury

Heme oxygenase (HO) catalyzes the conversion of heme to carbon monoxide, iron, and biliverdin, which is immediately reduced to bilirubin (BR). Two HO active isozymes exist: HO1, an inducible heat shock protein, and HO2, which is constitutive and highly concentrated in neurons. We demonstrate a neuroprotective role for BR formed from HO2. Neurotoxicity elicited by hydrogen peroxide in hippocampal and cortical neuronal cultures is prevented by the phorbol ester, phorbol 12-myristate 13-acetate (PMA) via stimulation of protein kinase C. We observe phosphorylation of HO2 through the protein kinase C pathway with enhancement of HO2 catalytic activity and accumulation of BR in neuronal cultures. The neuroprotective effects of PMA are prevented by the HO inhibitor tin protoporphyrin IX and in cultures from mice with deletion of HO2 gene. Moreover, BR, an antioxidant, is neuroprotective at nanomolar concentrations.

A protective role for heme oxygenase expression in pancreatic islets exposed to interleukin-1beta

Heme oxygenase (HO)-1 expression was investigated in rat isolated pancreatic islets. Freshly isolated islets showed no evidence of HO-1 expression. After a 20-h culture, there was a small increase in HO-1 in control islets, and interleukin-1beta (IL-1beta) induced HO-1 expression above control levels. N(G)-monomethyl-L-arginine inhibited the IL-1beta-induced increase in HO-1. Sodium nitroprusside-generated nitric oxide also increased HO-1 expression. CoCl2 induced a concentration- and time-dependent increase in HO-1, but not heat shock protein 70, expression. Cobalt chloride (CoCl2) protected islets from the inhibitory effects of IL-1beta on glucose-stimulated insulin release and glucose oxidation. Nickel chloride did not mimic the effects of CoCl2. An inhibitor of HO-1 activity, zinc-protoporphyrin IX (ZnPP), prevented the protective effect of CoCl2 on insulin release with IL-1beta but did not affect HO-1 expression or the inhibitory response to IL-1beta alone. ZnPP also inhibited the protective effect of hemin in IL-1beta-treated islets. CoCl2 inhibited the marked increase in islet nitrite production in response to IL-1beta. Cobalt-protoporphyrin IX (CoPP), which increased HO expression and activity, also protected islets from the inhibitory effects of IL-1beta, even though IL-1beta largely blocked the CoPP-induced increase in HO-1 expression. In betaHC9 cells, CoCl2 increased HO-1 expression and HO activity, whereas CoPP directly activated HO. ZnPP inhibited basal and CoCl2-stimulated HO activity. Thus, increased HO-1 expression and/or HO activity in response to CoCl2, CoPP, and hemin, seems to mediate protective responses of pancreatic islets against IL-1beta. HO-1 may be protective of beta-cells because of the scavenging of free heme, the antioxidant effects of the end-product bilirubin, or the generation of carbon monoxide, which might have insulin secretion-promoting effects and inhibitory effects on nitric oxide synthase.

Potential protective effect of calcium carbonate as liming agent against copper toxicity in the African tilapia Oreochromis mossambicus

The lipid peroxidative effects of copper sulfate singly (4 mg/l CuSO4.5H2O) and in combination with calcium carbonate (4 mg/l CuSO4.5H2O + 50 mg/l CaCO3) were determined in the liver of the African tilapia Oreochromis mossambicus following exposures of the fish to the chemicals for 24, 48, 72, 96 and 168 h. Lipid peroxidative effects of the treatment with calcium carbonate (50 mg/l CaCO3) and with a known hepatotoxicant, carbon tetrachloride (0.25 ml/l CCl4) were also determined. Fish not exposed to any chemical served as negative controls. The extent of lipid peroxidation was based on hepatic malondialdehyde (MDA) levels as assayed using the thiobarbituric acid reaction test. Results suggested the lipid peroxidative property of the copper salt which was associated with the toxic nature of the heavy metal, although, this effect was not as potent as that of CCl4. Findings also indicated a measure of protection against copper hepatotoxicity provided by the addition of calcium carbonate as a liming agent in the water.

Cooperative induction of c-fos and heme oxygenase gene products under oxidative stress in human fibroblastic cells

Heme oxygenase-1 is a stress responsive enzyme and implicated in a protective function of cellular damage. We investigated cellular events leading to the heme oxygenase-1 gene expression induced by sublethal concentrations of glutathione depletors, phorone and diethyl maleate, in human fibroblastic cells. Accumulation of heme oxygenase-1 mRNA by glutathione depletors was canceled by simultaneous treatment with cycloheximide, an inhibitor of protein synthesis; however, the inhibitory effect decreased when the inhibitor was added 30 min later. Among the inducible early response genes, the c-fos expression was significantly elevated with a peak at 30 min after the agents. Accumulation of heme oxygenase-1 and c-fos transcripts was abrogated in cells pretreated with 1,4-diazabicyclo[2.2.2]octane, an oxygen-free radical quencher. Decrease in glutathione levels preferentially activated extracellular-signal regulated kinases rather than other stress-activated protein kinases such as c-Jun N-terminal kinases and p38 MAP kinase. Pretreatment of cells with PD 98059, an inhibitor of the extracellular-signal regulated kinase cascade, or the c-fos antisense oligodeoxynucleotide inhibited the heme oxygenase-1 induction elicited by glutathione depletion. These observations indicated that c-Fos protein plays a role in heme oxygenase-1 gene expression induced by glutathione depletion-mediated oxidative stress in human fibroblasts.

New physiological importance of two classic residual products: carbon monoxide and bilirubin

Heme oxygenase the rate-limiting step in the degradation of heme to bilirubin, generates carbon monoxide. This gaseous molecule plays important roles in neuronal signaling and modulation of vascular tone. Additionally, carbon monoxide is involved in some pathological conditions (e.g., ischemia, endotoxic shock, excitotoxicity) as a protective or toxic factor. Bilirubin, another heme metabolite, exhibits intriguing biological activities as an antioxidant, an antimutagen, and an anti-complement agent. Vital functions and the dual nature displayed by these two heme metabolites are discussed.

Bilirubin, REM sleep, and phototransduction of environmental time cues. A hypothesis

The prevailing hypothesis for phototransduction is that visual (rod or cone) pigments mediate light's primary effects on biological clock systems. Common light-responsive chronobiological behavioral properties of plants and animals and some common molecular structures of plants and animals suggest the possibility that heme moieties and bile pigments in animals mediate some nonvisual influences of light on neuroactive gases and biological rhythms. As plant phytochrome resets the plant biological clock, the similar chromophore in bile pigments is proposed to transduce environmental light zeitgeber signals to endogenous biological clocks. The temporal association of plasma bilirubin and rapid-eye-movement (REM) sleep in populations, the correlation of secretion of biliary bilirubin with REM sleep among 10 different species (Spearman r = 0.89, p < 0.002), and the known responses of bilirubin to light lead to the hypothesis that bilirubin, in particular, plays an evolutionary role in the regulation of REM sleep and in mediating some of light's antidepressant effects.

Higher serum bilirubin is associated with decreased risk for early familial coronary artery disease

Mildly increased serum bilirubin has recently been suggested as a protective factor, possibly reducing the risk of coronary artery disease (CAD) by acting as an antioxidant. We tested this hypothesis by examining serum bilirubin concentrations and other coronary risk factors in 120 men and 41 women with early familial CAD and 155 control subjects. At screening, both cases and control subjects were 38 to 68 years old. Early familial CAD patients had experienced myocardial infarction, coronary artery bypass grafting, or coronary angioplasty by age 55 years for men and 65 for women and had another sibling similarly affected. The average total serum bilirubin concentration was 8.9 +/- 6.1 mumol/L in cases and 12.4 +/- 8.1 mumol/L in control subjects (P = .0001 for difference). In univariate analysis stratified by sex, serum bilirubin was strongly and inversely related to CAD risk, with relative odds of 0.4 to 0.1 (relative to the lowest quintile, P = .04 to .00001) in both men and women as bilirubin increased into the upper two quintiles. Multiple logistic regression analysis was performed including age, sex, smoking, body mass index, diabetes, hypertension, plasma measured LDL cholesterol, HDL cholesterol, triglycerides, and serum bilirubin as potential risk factors. Bilirubin entered as an independent protective factor with an odds ratio of 0.25 (P = .0015) for an increase of 17 mumol/L (1 mg/dL). The standardized logistic regression coefficient for bilirubin was -.33 compared with -.34 for HDL, suggesting that the protective effect of bilirubin on CAD risk in the population is comparable to that of HDL cholesterol. A history of cigarette smoking was associated with significantly lower serum bilirubin concentration and appeared to attenuate the protective effect of bilirubin.