Selection of metalloporphyrin heme oxygenase inhibitors based on potency and photoreactivity

"Light" on Phototherapy-Complications and Strategies for Shortening Its Duration, A Review of the Literature

Neonatal hyperbilirubinemia is an extremely common metabolic complication of the neonatal period which may be associated with bilirubin encephalopathy and even death. Adverse neurological consequences are preventable if a timely diagnosis and treatment are provided. Phototherapy is usually the preferred option to decrease hyperbilirubinemia. Although considered to be safe, evidence in recent years has shown that this treatment may not be free of side effects and short- and long-term unfavorable outcomes. These are usually mild or rare, but should be decreased or avoided if possible. Many useful complementary measures and treatments have been described that could shorten the duration of exposure to phototherapy. However, there is no current unequivocal recommendation to use any of the methods presented in this review. Our review aims to depict the wide range of possible complementary treatments to phototherapy, and to provide the scientific and clinical evidence available regarding their usefulness. It is essential that, while utilizing the full potential of phototherapy to treat hyperbilirubinemia, caregivers are aware of its side effects and possible inherent dangers, and seek ways to minimize the exposure to phototherapy to what is really needed for the newborn. Further studies are needed to clarify the preferred complementary treatments that could reduce the duration of exposure to phototherapy without impairing its effectiveness.

Targeting ferroptosis opens new avenues for the development of novel therapeutics

Ferroptosis is an iron-dependent form of regulated cell death with distinct characteristics, including altered iron homeostasis, reduced defense against oxidative stress, and abnormal lipid peroxidation. Recent studies have provided compelling evidence supporting the notion that ferroptosis plays a key pathogenic role in many diseases such as various cancer types, neurodegenerative disease, diseases involving tissue and/or organ injury, and inflammatory and infectious diseases. Although the precise regulatory networks that underlie ferroptosis are largely unknown, particularly with respect to the initiation and progression of various diseases, ferroptosis is recognized as a bona fide target for the further development of treatment and prevention strategies. Over the past decade, considerable progress has been made in developing pharmacological agonists and antagonists for the treatment of these ferroptosis-related conditions. Here, we provide a detailed overview of our current knowledge regarding ferroptosis, its pathological roles, and its regulation during disease progression. Focusing on the use of chemical tools that target ferroptosis in preclinical studies, we also summarize recent advances in targeting ferroptosis across the growing spectrum of ferroptosis-associated pathogenic conditions. Finally, we discuss new challenges and opportunities for targeting ferroptosis as a potential strategy for treating ferroptosis-related diseases.

Heme Oxygenase-1 as Therapeutic Target for Diabetic Foot Ulcers

A diabetic foot ulcer (DFU) is one of the major complications of diabetes. Wound healing under diabetic conditions is often impaired. This is in part due to the excessive oxidative stress, prolonged inflammation, immune cell dysfunction, delayed re-epithelialization, and decreased angiogenesis present at the wound site. As a result of these multifactorial impaired healing pathways, it has been difficult to develop effective therapeutic strategies for DFU. Heme oxygenase-1 (HO-1) is the rate-limiting enzyme in heme degradation generating carbon monoxide (CO), biliverdin (BV) which is converted into bilirubin (BR), and iron. HO-1 is a potent antioxidant. It can act as an anti-inflammatory, proliferative, angiogenic and cytoprotective enzyme. Due to its biological functions, HO-1 plays a very important role in wound healing, in part mediated through the biologically active end products generated by its enzymatic activity, particularly CO, BV, and BR. Therapeutic strategies involving the activation of HO-1, or the topical application of its biologically active end products are important in diabetic wound healing. Therefore, HO-1 is an attractive therapeutic target for DFU treatment. This review will provide an overview and discussion of the importance of HO-1 as a therapeutic target for diabetic wound healing.

Efficacy and Safety Concerns with Sn-Mesoporphyrin as an Adjunct Therapy in Neonatal Hyperbilirubinemia: A Literature Review

Neonatal hyperbilirubinemia is a frequently observed clinical situation that, sometimes, may result in complications ranging from mild neurodevelopment impairment to serious outcome of kernicterus. The rationale logic of heme oxygenase enzyme inhibition to lower bilirubin levels is intriguing. In compliance with that rationale, metalloporphyrin was discovered. After successful results in in-vitro and animal studies, tin mesoporphyrin is now under phase II clinical trial to test for preventive and therapeutic efficacy in unconjugated hyperbilirubinemia. This review evaluates in-vitro studies, animal studies, and clinical trials for the efficacy and safety of tin analogues of metalloporphyrin. Few alternatives to metalloporphyrins are also available, synchronizing with the same rationale logic of inhibition of bilirubin production, which need further research.

The brain heme oxygenase/biliverdin reductase system as a target in drug research and development

INTRODUCTION

The heme oxygenase/biliverdin reductase (HO/BVR) system is involved in heme metabolism. The inducible isoform of HO (HO-1) and BVR both exert cytoprotective effects by enhancing cell stress response. In this context, some xenobiotics, which target HO-1, including herbal products, behave as neuroprotectants in several experimental models of neurodegeneration. Despite this, no drug having either HO-1 or BVR as a main target is currently available.

AREAS COVERED

After a description of the brain HO/BVR system, the paper analyzes the main classes of drugs acting on the nervous system, with HO as second-level target, and their neuroprotective potential. Finally, the difficulties that exist for the development of drugs acting on HO/BVR and the possible ways to overcome these hurdles are examined.

EXPERT OPINION

Although the limited clinical evidence has restricted the translational research on the HO/BVR system, mainly because of the dual nature of its by-products, there has been growing interest in the therapeutic potential of these enzymes. Scientists should boost the translational research on the HO/BVR system which could be supported by the significant evidence provided by preclinical studies.

Computational Designing and Prediction of ADMET Properties of Four Novel Imidazole-Based Drug Candidates Inhibiting Heme Oxygenase-1 Causing Cancers

The overexpression of heme oxygenase-1 (HO-1) contributes to the development of several types of cancers. The inhibition of HO-1 through imidazole-based drugs, which is non-competitive with heme, is a focus of anticancer drug research. We designed the four following novel HO-1 inhibiting compounds: 2-(1-cyclopentyl-4-(1H-imidazol-4-yl)butan-2-yl)pyrazine (M11), 2-[(2-chloro-3-methylcyclohexyl)methyl]-1H-imidazole (M26), 2-(2-phenethyl-1H-imidazol-4-yl)ethanesulfonamide (M28), and 5-chloro-2-[2-(2,5-dihydro-1H-imidazol-2-yl)propan-2-yl]-1H-imidazole (M31). All compounds showed a strong binding affinity with HO-1 in molecular docking studies. The in silico absorption, distribution, metabolism, excretion and toxicity (ADMET) data showed that the compounds would be available orally in an acceptable manner. The bioactivity scores revealed that they were moderately active substances. They were found as non-mutagen, non-tumorigenic, non-irritant, and non-detrimental to the reproductive system. Finally, the drug-likeness values of the compounds were obtained as -0.71, -1.64, -2.04, and 0.4 respectively, with the final drug-score of 0.60, 0.54, 0.51, and 0.77 respectively.

Clinical Significance of Heme Oxygenase 1 in Tumor Progression

Heme oxygenase 1 (HO-1) plays a key role in cell adaptation to stressors through the antioxidant, antiapoptotic, and anti-inflammatory properties of its metabolic products. For these reasons, in cancer cells, HO-1 can favor aggressiveness and resistance to therapies, leading to poor prognosis/outcome. Genetic polymorphisms of HO-1 promoter have been associated with an increased risk of cancer progression and a high degree of therapy failure. Moreover, evidence from cancer biopsies highlights the possible correlation between HO-1 expression, pathological features, and clinical outcome. Indeed, high levels of HO-1 in tumor specimens often correlate with reduced survival rates. Furthermore, HO-1 modulation has been proposed in order to improve the efficacy of antitumor therapies. However, contrasting evidence on the role of HO-1 in tumor biology has been reported. This review focuses on the role of HO-1 as a promising biomarker of cancer progression; understanding the correlation between HO-1 and clinical data might guide the therapeutic choice and improve the outcome of patients in terms of prognosis and life quality.

Heme oxygenase‑1 improves the survival of ischemic skin flaps (Review)

Heat shock protein 32 (Hsp32), also known as heme oxygenase‑1 (HO‑1), is an enzyme that exists in microsomes. HO‑1 can be induced by a variety of stimuli, including heavy metals, heat shock, inflammatory stimuli, heme and its derivatives, stress, hypoxia, and biological hormones. HO‑1 is the rate‑limiting enzyme of heme catabolism, which splits heme into biliverdin, carbon monoxide (CO) and iron. The metabolites of HO‑1 have anti‑inflammatory and anti‑oxidant effects, and provide protection to the cardiovascular system and transplanted organs. This review summarizes the biological characteristics of HO‑1 and the functional significance of its products, and specifically elaborates on its protective effect on skin flaps. HO‑1 improves the survival rate of ischemic skin flaps through anti‑inflammatory, anti‑oxidant and vasodilatory effects of enzymatic reaction products. In particular, this review focuses on the role of carbon monoxide (CO), one of the primary metabolites of HO‑1, in flap survival and discusses the feasibility and existing challenges of HO‑1 in flap surgery.

Immune Modulation by Inhibitors of the HO System

The heme oxygenase (HO) system involves three isoforms of this enzyme, HO-1, HO-2, and HO-3. The three of them display the same catalytic activity, oxidating the heme group to produce biliverdin, ferrous iron, and carbon monoxide (CO). HO-1 is the isoform most widely studied in proinflammatory diseases because treatments that overexpress this enzyme promote the generation of anti-inflammatory products. However, neonatal jaundice (hyperbilirubinemia) derived from HO overexpression led to the development of inhibitors, such as those based on metaloproto- and meso-porphyrins inhibitors with competitive activity. Further, non-competitive inhibitors have also been identified, such as synthetic and natural imidazole-dioxolane-based, small synthetic molecules, inhibitors of the enzyme regulation pathway, and genetic engineering using iRNA or CRISPR cas9. Despite most of the applications of the HO inhibitors being related to metabolic diseases, the beneficial effects of these molecules in immune-mediated diseases have also emerged. Different medical implications, including cancer, Alzheimer´s disease, and infections, are discussed in this article and as to how the selective inhibition of HO isoforms may contribute to the treatment of these ailments.

Modulation of Inflammation and Immune Responses by Heme Oxygenase-1: Implications for Infection with Intracellular Pathogens

Heme oxygenase-1 (HO-1) catalyzes the degradation of heme molecules releasing equimolar amounts of biliverdin, iron and carbon monoxide. Its expression is induced in response to stress signals such as reactive oxygen species and inflammatory mediators with antioxidant, anti-inflammatory and immunosuppressive consequences for the host. Interestingly, several intracellular pathogens responsible for major human diseases have been shown to be powerful inducers of HO-1 expression in both host cells and in vivo. Studies have shown that this HO-1 response can be either host detrimental by impairing pathogen control or host beneficial by limiting infection induced inflammation and tissue pathology. These properties make HO-1 an attractive target for host-directed therapy (HDT) of the diseases in question, many of which have been difficult to control using conventional antibiotic approaches. Here we review the mechanisms by which HO-1 expression is induced and how the enzyme regulates inflammatory and immune responses during infection with a number of different intracellular bacterial and protozoan pathogens highlighting mechanistic commonalities and differences with the goal of identifying targets for disease intervention.

Carbon Monoxide, a Retrograde Messenger Generated in Postsynaptic Mushroom Body Neurons, Evokes Noncanonical Dopamine Release

Dopaminergic neurons innervate extensive areas of the brain and release dopamine (DA) onto a wide range of target neurons. However, DA release is also precisely regulated. In Drosophila melanogaster brain explant preparations, DA is released specifically onto α3/α'3 compartments of mushroom body (MB) neurons that have been coincidentally activated by cholinergic and glutamatergic inputs. The mechanism for this precise release has been unclear. Here we found that coincidentally activated MB neurons generate carbon monoxide (CO), which functions as a retrograde signal evoking local DA release from presynaptic terminals. CO production depends on activity of heme oxygenase in postsynaptic MB neurons, and CO-evoked DA release requires Ca²⁺ efflux through ryanodine receptors in DA terminals. CO is only produced in MB areas receiving coincident activation, and removal of CO using scavengers blocks DA release. We propose that DA neurons use two distinct modes of transmission to produce global and local DA signaling.SIGNIFICANCE STATEMENT Dopamine (DA) is needed for various higher brain functions, including memory formation. However, DA neurons form extensive synaptic connections, while memory formation requires highly specific and localized DA release. Here we identify a mechanism through which DA release from presynaptic terminals is controlled by postsynaptic activity. Postsynaptic neurons activated by cholinergic and glutamatergic inputs generate carbon monoxide, which acts as a retrograde messenger inducing presynaptic DA release. Released DA is required for memory-associated plasticity. Our work identifies a novel mechanism that restricts DA release to the specific postsynaptic sites that require DA during memory formation.

Heme Oxygenase 1 in Schwann Cells Regulates Peripheral Nerve Degeneration Against Oxidative Stress

During Wallerian degeneration, Schwann cells lose their characteristic of myelinating axons and shift into the state of developmental promyelinating cells. This recharacterized Schwann cell guides newly regrowing axons to their destination and remyelinates reinnervated axons. This Schwann cell dynamics during Wallerian degeneration is associated with oxidative events. Heme oxygenases (HOs) are involved in the oxidative degradation of heme into biliverdin/bilirubin, ferrous iron, and carbon monoxide. Overproduction of ferrous iron by HOs increases reactive oxygen species, which have deleterious effects on living cells. Thus, the key molecule for understanding the exact mechanism of Wallerian degeneration in the peripheral nervous system is likely related to oxidative stress-mediated HOs in Schwann cells. In this study, we demonstrate that demyelinating Schwann cells during Wallerian degeneration highly express HO1, not HO2, and remyelinating Schwann cells during nerve regeneration decrease HO1 activation to levels similar to those in normal myelinating Schwann cells. In addition, HO1 activation during Wallerian degeneration regulates several critical phenotypes of recharacterized repair Schwann cells, such as demyelination, transdedifferentiation, and proliferation. Thus, these results suggest that oxidative stress in Schwann cells after peripheral nerve injury may be regulated by HO1 activation during Wallerian degeneration and oxidative-stress-related HO1 activation in Schwann cells may be helpful to study deeply molecular mechanism of Wallerian degeneration.

Progress in the development of selective heme oxygenase-1 inhibitors and their potential therapeutic application

Heme oxygenases (HOs) are a family of enzymes involved in the selective catabolism of free circulating heme. While HO-2 is constitutively expressed, HO-1 is strongly overexpressed under stressful stimuli (e.g., oxidative stress). Under these conditions, HO-1 exerts its strong cytoprotective activities and plays a crucial role in stimulating cell survival by removing the pro-oxidant heme and by producing carbon monoxide and biliverdin (promptly reduced to bilirubin). Unfortunately, the broad spectrum of HO-1 cytoprotective effects has been well experimentally documented both in normal and tumor cells, where the enzyme can be overexpressed, making it an exciting target in the management of some type of tumors. Development of non-competitive HO-1 inhibitors dates back in 2002 with the discovery of Azalanstat. Since then, many efforts have been devoted to the identification of selective HO-1 and HO-2 inhibitors and to unravel the molecular determinants responsible for selectivity. Molecular modeling studies supported the identification of chemical features involved in the recognition and inhibition of these enzymes. Herein, medicinal chemistry aspects and in silico studies related to the development of HO inhibitors will be discussed. The purpose of this review is to highlight recent advances in the development of new selective HO-1 and HO-2 inhibitors and covers the last six years (2013-2018).

Heme Oxygenase-1 Inhibitors Induce Cell Cycle Arrest and Suppress Tumor Growth in Thyroid Cancer Cells

Heme oxygenase-1 (HO-1) is induced by a variety of stimuli and plays a multifaceted role in cellular protection. We have shown that HO-1 is overexpressed in thyroid cancer and is associated with tumor aggressiveness. Therefore, we set out to assess the effects of HO-1 inhibitors on the biology of thyroid cancer cells. Two different classes of HO-1 inhibitors were used, including a metalloporphyrin, zinc protoporphyrin-IX (ZnPP), and an azole antifungal agent, ketoconazole. The viability and colony formation of thyroid cancer cells decreased in a concentration- and time-dependent fashion following treatment with HO-1 inhibitors. Cancer cells exhibited a higher sensitivity to HO-1 inhibitors than non-malignant cells. HO-1 inhibitors induced a G0/G1 arrest accompanied by decreased cyclin D1 and CDK4 expressions and an increase in levels of p21 and p27. HO-1 inhibitors significantly increased intracellular ROS levels and suppressed cell migration and invasion. Oxygen consumption rate and mitochondrial mass were increased with ZnPP treatment. Mice treated with ZnPP had a reduced xenograft growth and diminished cyclin D1 and Ki-67 staining in tumor sections. Taken together, HO-1 inhibitors might have therapeutic potential for inducing cell cycle arrest and promoting growth suppression of thyroid cancer cells in vitro and in vivo.

Heme oxygenase inhibition in cancers: possible tools and targets

Heme oxygenase-1 (HO-1, encoded by HMOX1) through degradation of pro-oxidant heme into carbon monoxide (CO), ferrous ions (Fe²⁺) and biliverdin, exhibits cytoprotective, anti-apoptotic and anti-inflammatory properties. All of these potentially beneficial functions of HO-1 may play an important role in tumors’ development and progression. Moreover, HO-1 is very often upregulated in tumors in comparison to healthy tissues, and its expression is further induced upon chemo-, radio- and photodynamic therapy, what results in decreased effectiveness of the treatment. Consequently, HO-1 can be proposed as a therapeutic target for anticancer treatment in many types of tumors. Nonetheless, possibilities of specific inhibition of HO-1 are strongly limited. Metalloporphyrins are widely used in in vitro studies, however, they are unselective and may exert serious side effects including an increase in HMOX1 mRNA level. On the other hand, detailed information about pharmacokinetics and biodistribution of imidazole-dioxolane derivatives, other potential inhibitors, is lacking. The genetic inhibition of HO-1 by RNA interference (RNAi) or CRISPR/Cas9 approaches provides the possibility to specifically target HO-1; however, the potential therapeutic application of those methods are distant at best. In summary, HO-1 inhibition might be the valuable anticancer approach, however, the ideal strategy for HO-1 targeting requires further studies.

Ethnopharmacological Approaches for Therapy of Jaundice: Part I

Jaundice is a very common symptom especially in the developing countries. It is associated with several hepatic diseases which are still major causes of death. There are many different approaches to jaundice treatment and the growing number of ethnomedicinal studies shows the plant pharmacology as very promising direction. Many medicinal plants are used for the treatment of jaundice, however a comprehensive review on this subject has not been published. The use of medicinal plants in drug discovery is highly emphasized (based on their traditional and safe uses in different folk medicine systems from ancient times). Many sophisticated analytical techniques are emerging in the pharmaceutical field to validate and discover new biologically active chemical entities derived from plants. Here, we aim to classify and categorize medicinal plants relevant for the treatment of jaundice according to their origin, geographical location, and usage. Our search included various databases like Pubmed, ScienceDirect, Google Scholar. Keywords and phrases used for these searches included: "jaundice," "hyperbilirubinemia," "serum glutamate," "bilirubin," "Ayurveda." The first part of the review focuses on the variety of medicinal plant used for the treatment of jaundice (a total of 207 medicinal plants). In the second part, possible mechanisms of action of biologically active secondary metabolites of plants from five families for jaundice treatment are discussed.

Ethnopharmacological Approaches for Therapy of Jaundice: Part I

Jaundice is a very common symptom especially in the developing countries. It is associated with several hepatic diseases which are still major causes of death. There are many different approaches to jaundice treatment and the growing number of ethnomedicinal studies shows the plant pharmacology as very promising direction. Many medicinal plants are used for the treatment of jaundice, however a comprehensive review on this subject has not been published. The use of medicinal plants in drug discovery is highly emphasized (based on their traditional and safe uses in different folk medicine systems from ancient times). Many sophisticated analytical techniques are emerging in the pharmaceutical field to validate and discover new biologically active chemical entities derived from plants. Here, we aim to classify and categorize medicinal plants relevant for the treatment of jaundice according to their origin, geographical location, and usage. Our search included various databases like Pubmed, ScienceDirect, Google Scholar. Keywords and phrases used for these searches included: “jaundice,” “hyperbilirubinemia,” “serum glutamate,” “bilirubin,” “Ayurveda.” The first part of the review focuses on the variety of medicinal plant used for the treatment of jaundice (a total of 207 medicinal plants). In the second part, possible mechanisms of action of biologically active secondary metabolites of plants from five families for jaundice treatment are discussed.

Heme Oxygenase 1 Up-Regulates Glomerular Decay Accelerating Factor Expression and Minimizes Complement Deposition and Injury

Complement-activation controllers, including decay accelerating factor (DAF), are gaining emphasis as they minimize injury in various dysregulated complement-activation disorders, including glomerulopathies. Heme oxygenase (HO)-1 overexpression or induction has been shown to attenuate injury in complement-dependent models of glomerulonephritis. This study investigated whether up-regulation of DAF by heme oxygenase 1 (HO-1) is an underlying mechanism by using Hmox-1-deficient rats (Hmox1^+/-; Hmox1^-/-) or rats with HO-1 overexpression targeted to glomerular epithelial cells (GEC^HO-1), which are particularly vulnerable to complement-mediated injury owing to their terminally differentiated nature. Constitutively expressed DAF was decreased in glomeruli of Hmox1^-/- rats and augmented in glomeruli of GEC^HO-1 rats. In GEC^HO-1 rats with anti-glomerular basement membrane antibody mediated, complement-dependent injury, complement component C3 fragment b (C3b) deposition was reduced, whereas proteinuria was diminished. In glomeruli of wild-type rats, the natural Hmox substrate, hemin, induced glomerular DAF. This effect was attenuated in glomeruli of Hmox1^-/- rats and augmented in glomeruli of GEC^HO-1 rats. Hemin analogues differing in either metal or porphyrin ring functionalities, acting as competitive Hmox-substrate inhibitors, also increased glomerular DAF and reduced C3b deposition after spontaneous complement activation. In the presence of a DAF-blocking antibody, the reduction in C3b deposition was reversed. These observations establish HO-1 as a physiologic regulator of glomerular DAF and identify hemin analogues as inducers of functional glomerular DAF able to minimize C3b deposition.

18O-Tracer Metabolomics Reveals Protein Turnover and CDP-Choline Cycle Activity in Differentiating 3T3-L1 Pre-Adipocytes

The differentiation of precursor cells into mature adipocytes (adipogenesis) has been an area of increased focus, spurred by a rise in obesity rates. Though our understanding of adipogenesis and its regulation at the cellular level is growing, many questions remain, especially regarding the regulation of the metabolome. The 3T3-L1 cell line is the most well characterized cellular model of adipogenesis. Using a time course metabolomics approach, we show that the 3T3-L1 preadipocyte metabolome is greatly altered during the first 48 hours of differentiation, where cells go through about two rounds of cell division, a process known as mitotic clonal expansion. Short-chain peptides were among several small molecules that were increased during mitotic clonal expansion. Additional indicators of protein turnover were also increased, including bilirubin, a degradation product of heme-containing proteins, and 3-methylhistidine, a post-translationally modified amino acid that is not reutilized for protein synthesis. To study the origin of the peptides, we treated differentiating preadipocytes with ¹⁸O labeled water and found that ¹⁸O was incorporated into the short chain peptides, confirming them, at least in part, as products of hydrolysis. Inhibitors of the proteasome or matrix metalloproteinases affected the peptide levels during differentiation, but inhibitors of autophagy or peptidases did not. ¹⁸O was also incorporated into several choline metabolites including cytidine 5'-diphosphocholine (CDP-choline), glycerophosphocholine, and several phosphatidylcholine species, indicative of phosphatidylcholine synthesis/degradation and of flux through the CDP-choline cycle, a hallmark of proliferating cells. ¹⁸O-Tracer metabolomics further showed metabolic labeling of glutamate, suggestive of glutaminolysis, also characteristic of proliferating cells. Together, these results highlight the utility of ¹⁸O isotope labeling in combination with metabolomics to uncover changes in cellular metabolism that are not detectable by time-resolved metabolomics.

Targeting heme oxygenase-1 and carbon monoxide for therapeutic modulation of inflammation

The heme oxygenase-1 (HO-1) enzyme system remains an attractive therapeutic target for the treatment of inflammatory conditions. HO-1, a cellular stress protein, serves a vital metabolic function as the rate-limiting step in the degradation of heme to generate carbon monoxide (CO), iron, and biliverdin-IXα (BV), the latter which is converted to bilirubin-IXα (BR). HO-1 may function as a pleiotropic regulator of inflammatory signaling programs through the generation of its biologically active end products, namely CO, BV and BR. CO, when applied exogenously, can affect apoptotic, proliferative, and inflammatory cellular programs. Specifically, CO can modulate the production of proinflammatory or anti-inflammatory cytokines and mediators. HO-1 and CO may also have immunomodulatory effects with respect to regulating the functions of antigen-presenting cells, dendritic cells, and regulatory T cells. Therapeutic strategies to modulate HO-1 in disease include the application of natural-inducing compounds and gene therapy approaches for the targeted genetic overexpression or knockdown of HO-1. Several compounds have been used therapeutically to inhibit HO activity, including competitive inhibitors of the metalloporphyrin series or noncompetitive isoform-selective derivatives of imidazole-dioxolanes. The end products of HO activity, CO, BV and BR may be used therapeutically as pharmacologic treatments. CO may be applied by inhalation or through the use of CO-releasing molecules. This review will discuss HO-1 as a therapeutic target in diseases involving inflammation, including lung and vascular injury, sepsis, ischemia-reperfusion injury, and transplant rejection.

Heme Oxygenase Improves Renal Function by Potentiating Podocyte-Associated Proteins in Nω-Nitro-l-Arginine-Methyl Ester (l-NAME)-Induced Hypertension

BACKGROUND

Although heme-oxygenase (HO) is cytoprotective, its effects on podocyte regulators like podocalyxin, podocin, CD2-associated protein (CD2AP) in renal dysfunction in N (ω)-nitro-l-arginine-methyl ester (l-NAME) hypertension are largely unclear.

METHODS

Hypertension was induced in normotensive Sprague Dawley rats by administering l-NAME for 4 weeks. Enzyme immunoassay, enzyme-linked immunosorbent, histology/morphology, spectrophotometry, and western immunoblotting were used. HO was enhanced with heme-arginate (HA) or inhibited with chromium mesoporphyrin (CrMP).

RESULTS

Treatment with heme-arginate reduced several renal histo-pathological lesions including renal arteriolar thickening, glomerular abnormalities, tubular cast, tubular atrophy/fibrosis, and mononuclear cell infiltration in l-NAME-hypertensive rats. Similarly, HA abated the elevated levels of renal extracellular matrix/profibrotic proteins like collagen and fibronectin that deplete nephrin, a fundamental transmembrane protein that forms the scaffoldings of the podocyte slit diaphragm permitting small ions to filter, but not massive excretion of proteins, hence proteinuria. Correspondingly, HA enhanced the aberrant expression of nephrin alongside other important regulators of podocyte like podocalyxin, podocin, and CD2AP, and improved renal function by reducing albuminuria/proteinuria, while increasing creatinine clearance. The renoprotection by HA were accompanied by significant reduction of inflammatory/oxidative mediators including nuclear factor-kappaB, macrophage inflammatory protein-1-alpha, macrophage chemoattractant protein-1, tumor necrosis factor-alpha, interleukin (IL)-6, IL1β, 8-isoprostane, endothelin-1, and aldosterone. These were associated with increased levels of adiponectin, HO-1, HO activity, cyclic guanosine monophosphate, and atrial natriuretic peptide (ANP), whereas the HO inhibitor, CrMP annulled the renoprotection and exacerbated renal dysfunction.

CONCLUSIONS

HA improves renal function by attenuating histopathological lesions, suppressing inflammatory/oxidative mediators, abating profibrotic/extracellular matrix proteins, and reducing albuminuria/proteinuria, while concomitantly potentiating the HO-adiponectin-ANP axis, enhancing nephrin, podocin, podocalyxin, CD2AP and increasing creatinine clearance. Our study underscores the benefit of potentiating the HO-adiponectin-ANP against nephropathy.

A review on hemeoxygenase-2: focus on cellular protection and oxygen response

Hemeoxygenase (HO) system is responsible for cellular heme degradation to biliverdin, iron, and carbon monoxide. Two isoforms have been reported to date. Homologous HO-1 and HO-2 are microsomal proteins with more than 45% residue identity, share a similar fold and catalyze the same reaction. However, important differences between isoforms also exist. HO-1 isoform has been extensively studied mainly by its ability to respond to cellular stresses such as hemin, nitric oxide donors, oxidative damage, hypoxia, hyperthermia, and heavy metals, between others. On the contrary, due to its apparently constitutive nature, HO-2 has been less studied. Nevertheless, its abundance in tissues such as testis, endothelial cells, and particularly in brain, has pointed the relevance of HO-2 function. HO-2 presents particular characteristics that made it a unique protein in the HO system. Since attractive results on HO-2 have been arisen in later years, we focused this review in the second isoform. We summarize information on gene description, protein structure, and catalytic activity of HO-2 and particular facts such as its cellular impact and activity regulation. Finally, we call attention on the role of HO-2 in oxygen sensing, discussing proposed hypothesis on heme binding motifs and redox/thiol switches that participate in oxygen sensing as well as evidences of HO-2 response to hypoxia.

Effects of light on metalloporphyrin-treated newborn mice

Zinc protoporphyrin (ZnPP) is a promising metalloporphyrin with sufficient potency, but has poor solubility and is not absorbed well orally. Intragastric administration of ZnPP microparticles (30 μmol/kg) to 3-day-old mice resulted in a twofold increase in potency and no signs of phototoxicity.

CONCLUSION

The use of polymeric particulate delivery systems can improve the stability and enhance intestinal absorption of ZnPP, while retaining HO inhibitory potency without photosensitising effects, and thus is potentially useful in treating neonatal hyperbilirubinemia.

Heme oxygenase suppresses markers of heart failure and ameliorates cardiomyopathy in L-NAME-induced hypertension

Heart failure and related cardiac complications remains a great health challenge. We investigated the effects of upregulating heme-oxygenase (HO) on myocardial histo-pathological lesions, proinflammatory cytokines/chemokines, oxidative mediators and important markers of heart failure such as osteopontin and osteoprotergerin in N(ω)-nitro-l-arginine methyl ester (L-NAME)-induced hypertension. Treatment with the HO-inducer, heme-arginate improved myocardial morphology in L-NAME hypertensive rats by attenuating subendocardial injury, interstitial fibrosis, mononuclear-cell infiltration and cardiomyocyte hypertrophy. These were associated with the reduction of several inflammatory/oxidative mediators including chemokines/cytokines such as macrophage inflammatory protein-1 alpha (MIP-1α), macrophage chemoattractant protein-1 (MCP-1), tumor necrosis factor alpha (TNF-α), interleukin (IL)-6, IL-1β, endothelin-1, 8-isoprostane, nitrotyrosine, and aldosterone. Similarly, heme-arginate abated the elevated levels of extracellular matrix/remodeling proteins including transforming-growth factor beta (TGF-β1) and collagen-IV in the myocardium. These were accompanied by significant reduction of proteins of heart failure such as osteopontin and osteoprotegerin. Interestingly, the cardio-protective effects of heme-arginate were associated with the potentiation of adiponectin, atrial-natriuretic peptide (ANP), HO-1, HO-activity, cyclic gnanosine monophosphate (cGMP) and the total-anti-oxidant capacity, whereas the HO-inhibitor, chromium-mesoporphyrin nullified the effects of heme-arginate, exacerbating inflammatory injury and oxidative insults. We conclude that heme-arginate therapy protects myocardial damage by potentiating the HO-adiponectin-ANP axis, which in turn suppressed the elevated levels of aldosterone, pro-inflammatory chemokines/cytokines, mononuclear-cell infiltration and oxidative stress, with concomitant reduction of extracellular matrix/remodeling proteins and heart failure proteins. These data suggest a cardio-protective role of the HO system against L-NAME-induced hypertension that could be explored in the design of novel strategies against cardiomyopathy.

Hemin therapy improves kidney function in male streptozotocin-induced diabetic rats: role of the heme oxygenase/atrial natriuretic peptide/adiponectin axis

Diabetic nephropathy is characterized by elevated macrophage infiltration and inflammation. Although heme-oxygenase (HO) is cytoprotective, its role in macrophage infiltration and nephropathy in type 1 diabetes is not completely elucidated. Administering the HO inducer, hemin, to streptozotocin-diabetic rats suppressed renal proinflammatory macrophage-M1 phenotype alongside several proinflammatory agents, chemokines, and cytokines including macrophage inflammatory protein 1α (MIP-1α), macrophage-chemoattractant protein-1 (MCP-1), TNF-α, IL-1β, IL-6, nuclear factor-κB (NF-κB), and aldosterone, a stimulator of the inflammatory/oxidative transcription factor, NF-κB. Similarly, hemin therapy attenuated extracellular matrix/profibrotic proteins implicated in renal injury including fibronectin, collagen-IV, and TGF-β1 and reduced several renal histopathological lesions such as glomerulosclerosis, tubular necrosis, tubular vacuolization, and interstitial macrophage infiltration. Furthermore, hemin reduced markers of kidney dysfunction like proteinuria and albuminuria but increased creatinine clearance, suggesting improved kidney function. Correspondingly, hemin significantly enhanced the antiinflammatory macrophage-M2 phenotype, IL-10, adiponectin, HO-1, HO activity, and atrial natriuretic-peptide (ANP), a substance that abates TNF-α, IL-6, and IL-1β, with parallel increase of urinary cGMP, a surrogate marker of ANP. Contrarily, coadministering the HO inhibitor, chromium-mesoporphyrin with the HO-inducer, hemin nullified the antidiabetic and renoprotective effects, whereas administering chromium-mesoporphyrin alone abrogated basal HO activity, reduced basal adiponectin and ANP levels, aggravated hyperglycemia, and further increased MCP-1, MIP-1α, aldosterone, NF-κB, TNF-α, IL-6, IL-1β, proteinuria/albuminuria, and aggravated creatinine clearance, thus exacerbating renal dysfunction, suggesting the importance of the basal HO-adiponectin-ANP axis in renoprotection and kidney function. Collectively, these data suggest that hemin ameliorates diabetic nephropathy by selectively enhancing the antiinflammatory macrophage-M2 phenotype and IL-10 while concomitantly abating the proinflammatory macrophage-M1 phenotype and suppressing extracellular matrix/profibrotic factors with reduction of renal lesions including interstitial macrophage infiltration. Because aldosterone stimulate NF-κB, which activates cytokines like TNF-α, IL-6, IL-1β that in turn stimulate chemokines such as MCP-1 and MIP-1α to promote macrophage-M1 infiltration, the hemin-dependent potentiation of the HO-adiponectin-ANP axis may account for reduced macrophage infiltration and inflammatory insults in streptozotocin-diabetic rats.

Upregulation of heme oxygenase-1 expression may facilitate memory and learning in mice

Heme oxygenase (HO)-1 is highly expressed in the hippocampus. Its expression is induced by many factors including hemes, whose metabolites play an important role in neuron protection and learning development. In the present study, the correlation between HO-1 and learning ability was investigated in mice. Behavioral tests were used to evaluate the effects of altering HO-1 on learning ability in mature mice. In order to determine the function of HO-1 in the immature mice, a dark-reared model was constructed. Either the HO-1 inducer hemin or the HO-1 inhibitor Zn protoporphyrin IX (ZnPPIX) was injected into the left lateral ventricle prior to a behavior test. Results showed that neither hemin nor ZnPPIX affected the learning ability of adult mice reared in normal conditions. The hippocampal HO-1 of dark-reared mice was decreased while it was increased in the behavioral training group. In general, HO-1 had no effect on established learning ability but it may be upregulated by behavioral training and is beneficial for the development of memory and learning ability in neonatal mice.

Overview of heme degradation pathway

Heme oxygenase 1 and 2 activities are responsible for initiating most of the degradation of heme, although other enzyme pathways play a role as well. The degradation pathway also includes biliverdin reductase, the activity of which is coupled to oxidation of NADH and NADPH. This overview discusses the pathways and enzymes involved in heme degradation.

Treatment with heme arginate alleviates adipose tissue inflammation and improves insulin sensitivity and glucose metabolism in a rat model of Human primary aldosteronism

Visceral adiposity and insulin resistance are common pathophysiological denominators in patients with primary aldosteronism. Although we recently reported the antidiabetic effects of heme oxygenase (HO), no study has examined the effects of upregulating HO on visceral adiposity in uninephrectomized (UnX) deoxycorticosterone acetate (DOCA-salt) hypertensive rats, a model of human primary aldosteronism characterized by elevated endothelin (ET-1) and oxidative/inflammatory events. Here, we report the effects of the HO inducer heme arginate and the HO blocker chromium mesoporphyrin (CrMP) on visceral adipose tissue obtained from retroperitoneal fat pads of UnX DOCA-salt rats. UnX DOCA-salt rats were hypertensive but normoglycemic. Heme arginate reduced visceral adiposity and enhanced HO activity and cGMP in the adipose tissue, but suppressed ET-1, nuclear-factor κB (NF-κB), activating-protein (AP-1), c-Jun-NH2-terminal kinase (JNK), macrophage chemoattractant protein-1 (MCP-1), intercellular adhesion molecule-1 (ICAM-1), and 8-isoprostane. These were associated with reduced glycemia, increased insulin, and the insulin-sensitizing protein adiponectin, with corresponding reduction in insulin resistance. In contrast, the HO inhibitor, CrMP, abolished the effects of heme arginate, aggravating insulin resistance, suggesting a role for the HO system in insulin signaling. Importantly, the effects of the HO system on ET-1, NF-κB, AP-1, JNK, MCP-1, and ICAM-1 in visceral or retroperitoneal adiposity in UnX-DOCA-salt rats have not been reported. Because 8-isoprostane stimulates ET-1 to enhance oxidative insults, and increased oxidative events deplete adiponectin and insulin levels, the suppression of oxidative/inflammatory mediators such as 8-isoprostane, NF-κB, AP-1, MCP-1, ICAM-1, and JNK, an inhibitor of insulin biosynthesis, may account for the potentiation of insulin signaling/glucose metabolism by heme arginate. These data indicate that although UnX DOCA-salt rats were normoglycemic, insulin signaling was impaired, suggesting that dysfunctional insulin signaling may be a forerunner to overt diabetes in primary aldosteronism.

Hemin induction of HO-1 protects against LPS-induced septic ileus

BACKGROUND

Heme oxygenase (HO-1) protects against inflammation. In this study, we investigated the protective function of hemin-induced HO-1 against lipopolysaccharide (LPS)-induced ileus.

METHODS

Rats received LPS intraperitoneally 24 h after intraperitoneal hemin pretreatment or placebo. We also injected zinc protoporphyrin (ZnPP, 3rd group), an inhibitor of HO-1, intraperitoneally 2 h before LPS administration. To assess intestinal muscle function, we examined muscularis strip contractility in an organ bath and measured gastrointestinal transit in vivo. We investigated inflammation within the muscularis using polymerase chain reaction (interleukin [IL]-6, inducible nitric oxide synthase (iNOS), HO-1 and IL-10) 6 and 24 h after LPS.

RESULTS

Hemin significantly improved in vitro intestinal muscularis contractility (P < 0.001). In addition, hemin prevented LPS-induced dysmotility in vivo (gastrointestinal transit, geometric center: 8.39 ± 0.33 versus 5.68 ± 0.44; P < 0.001). In Zinc protoporphyrin (ZnPP)-treated animals, both parameters were significantly decreased compared with the hemin group. Messenger RNA expression demonstrated a significant reduction in IL-6 (6 h, hemin: 127.6 ± 36.7 versus LPS: 14,431 ± 5407; 24 h: 1.58 ± 0.39 versus 11.15 ± 2.59; P < 0.01) and iNOS (6 h: 2516 ± 985 versus 50,771 ± 13,321; 24 h: 55.11 ± 10.55 versus 257.1 ± 43.18; P < 0.001) in hemin-treated animals. Anti-inflammatory HO-1 messenger RNA levels (6 h, hemin: 116.3 ± 18.55 versus LPS: 26.02 ± 3.64; 24 h: 18.46 ± 2.69 versus 2.80 ± 0.32; P < 0.001) were increased. There was no significant difference in IL-10 levels at 6 and 24 h. ZnPP reversed the anti-inflammatory hemin effects.

CONCLUSIONS

Hemin induction of HO-1 diminishes LPS-induced sepsis. Heme oxygenase-1 has a central role in preventing sepsis-induced ileus. This benefit is reversed by HO-1 inhibition with ZnPP.

Effect of light exposure on metalloporphyrin-treated newborn mice

BACKGROUND

Neonatal hyperbilirubinemia arises from increased bilirubin production and decreased bilirubin elimination. Although phototherapy safely and effectively reduces bilirubin levels, recent evidence shows that it has adverse effects. Therefore, alternative treatments are warranted. Metalloporphyrins, competitive inhibitors of heme oxygenase (HO), the rate-limiting enzyme in bilirubin production, effectively reduce bilirubin formation; however, many are photoreactive. Here, we investigated possible photosensitizing effects of chromium mesoporphyrin (CrMP) and zinc deuteroporphyrin bis-glycol (ZnBG).

METHODS AND RESULTS

Administration of CrMP or ZnBG to 3-d-old mouse pups (3.75-30.0 μmol/kg intraperitoneally) and exposure to cool white (F20T12CW) and blue (TL20W/52) fluorescent lights (+L) for 3 h, resulted in a dose-dependent mortality (50% lethal dose (LD50) = 21.5 and 19.5 μmol/kg, respectively). In contrast to ZnBG, there was no significant difference in survival between the CrMP+L and CrMP groups. Following 30 μmol/kg ZnBG+L, we found significant weight loss, decreased liver antioxidant capacities, and increased aspartate aminotransaminase levels. At 6-d post-light exposure, ZnBG+L-treated pups showed gross and histologic skin changes at doses >7.5 μmol/kg. No lethality was observed following treatment with 30 μmol ZnBG/kg plus exposure to blue light-emitting diodes. Phototoxicity of ZnBG was dependent on light source, emission spectrum, and irradiance.

CONCLUSION

Low doses of ZnBG (<3.75 μmol/kg) retained maximal HO inhibitory potency without photosensitizing effects, and therefore are potentially useful in treating neonatal hyperbilirubinemia.

Metalloporphyrins - an update

Metalloporphyrins are structural analogs of heme and their potential use in the management of neonatal hyperbilirubinemia has been the subject of considerable research for more than three decades. The pharmacological basis for using this class of compounds to control bilirubin levels is the targeted blockade of bilirubin production through the competitive inhibition of heme oxygenase (HO), the rate-limiting enzyme in the bilirubin production pathway. Ongoing research continues in the pursuit of identifying ideal metalloporphyrins, which are safe and effective, by defining therapeutic windows and targeted interventions for the treatment of excessive neonatal hyperbilirubinemia.

Bis(aryloxo) derivatives of tin(IV) porphyrins: synthesis, spectroscopy and redox activity

Spectroscopic, electrochemical and fluorescence properties of five octahedral tin(IV) porphyrins ([( TpTP ) Sn ( O -m,p- C 6 H 3( CH 3)2)2] (1), [( TpTP ) Sn ( O -p- C 6 H 4( CH 3))2] (2), [( TpTP ) Sn ( O - C 6 H 5)2] (3), [( TpTP ) Sn ( O -p- C 6 H 4 O -o,p- C 6 H 3( NO 2)2)2] (4) and [( TpTP ) Sn ( O -p- C 6 H 4( NO 2))2] (5)) bearing electron donating/electron withdrawing aryloxo subunits as the axial ligands are investigated in detail. These complexes have been synthesized by the reaction of 5,10,15,20-tetra(4-methylphenyl)porphyrinato tin(IV) dihydroxide ([( TpTP ) Sn ( OH )2]) and either 3, 4-dimethylphenol, p-cresol, phenol, 4-(2,4-dinitrophenoxy)phenol or p-nitrophenol and were isolated in good-to-moderate yields. Analysis of the spectral data (FAB mass, IR, UV-vis and 1H NMR) of 1 - 5 suggests that the two trans axial aryloxo ligands are strongly bound in a symmetric manner at the tin center in these complexes. Specifically, the general symmetry observed in the 1H NMR spectra and the ring current effect experienced by the protons present on the axial ligands are revealing in this regard. Each porphyrin ring undergoes two successive, reversible/quasi-reversible, one-electron reductions in CH 2 Cl 2, 0.1 M TBAP. The bound nitroaromatic axial ligands of complexes 4 and 5 could also be reduced under the same set of experimental conditions. The wavelengths of maximum emission, the singlet energies and the Stokes’ shifts observed in the fluorescence spectra of 1 - 5 are close to the corresponding parameters of the reference porphyrin, [( TpTP ) Sn ( OH )2]. On the other hand, fluorescence intensities of complexes 1 - 4 are quenched in comparison with that of [( TpTP ) Sn ( OH )2] in three different solvents. A detailed analysis of the emission and redox potential data indicates that a photoinduced electron transfer from the axial aromatic subunit to singlet state of the basal tin(IV) porphyrin can, in principle, explain the fluorescence quenching observed in these donor-acceptor systems. The spectroscopic and redox features of these tin(IV) porphyrins are compared with those of the analogous phosphorus(V) porphyrins reported by us earlier.

Effects of zinc deuteroporphyrin bis glycol on newborn mice after heme loading

Infants with hemolytic diseases frequently develop hyperbilirubinemia and are treated with phototherapy, which only eliminates bilirubin after its production. A better strategy might be to directly inhibit heme oxygenase (HO), the rate-limiting enzyme in bilirubin production. Metalloporphyrins (Mps) are heme analogs that competitively inhibit HO activity in vitro and in vivo and suppress plasma bilirubin levels in vivo. A promising Mp, zinc deuteroporphyrin bis glycol (ZnBG), is orally absorbed and effectively inhibits HO activity at relatively low doses. We determined the I(50) (the dose needed to inhibit HO activity by 50%) of orally administered ZnBG in vivo and then evaluated ZnBG's effects on in vivo bilirubin production, HO activity, HO protein levels, and HO-1 gene expression in newborn mice after heme loading, a model analogous to a hemolytic infant. The I(50) of ZnBG was found to be 4.0 μmol/kg body weight (BW). At a dose of 15 μmol/kg BW, ZnBG reduced in vivo bilirubin production, inhibited heme-induced liver HO activity and spleen HO activity to and below baseline, respectively, transiently induced liver and spleen HO-1 gene transcription, and induced liver and spleen HO-1 protein levels. We conclude that ZnBG may be an attractive compound for treating severe neonatal hyperbilirubinemia caused by hemolytic disease.

Bilirubin production and the risk of bilirubin neurotoxicity

Neonatal jaundice usually occurs in the transitional period after birth, presenting as an elevation of circulating bilirubin. Bilirubin neurotoxicity can occur if the levels of bilirubin become excessive (hyperbilirubinemia). This pathologic phenotype of newborn jaundice can develop because of excessive bilirubin production or impaired conjugation, with the risk for developing bilirubin-induced neurologic dysfunction, depending on the degree of the resultant bilirubin load. The plasma bilirubin level thus can be used to assess an infant's risk for developing bilirubin neurotoxicity relative to an infant's age in hours. Because all infants have an impaired conjugation ability, infants at greatest risk are those who have increased bilirubin production rates, because of hemolysis, for example. Therefore, developing potential preventive strategies as well as noninvasive technologies to treat and to identify infants with increased bilirubin production rates, respectively, are tantamount to reducing the incidence of bilirubin-induced neurologic dysfunction.

In vitro inhibition of heme oxygenase isoenzymes by metalloporphyrins

OBJECTIVE

Neonatal jaundice results from an increased bilirubin production and decreased hepatic bilirubin conjugation and excretion. Severe hyperbilirubinemia is currently treated with phototherapy or exchange transfusion; however, its prevention by inhibiting bilirubin formation is a more logical strategy. Heme oxygenase (HO), with inducible (HO-1) and constitutive (HO-2) isoenzymes, is the rate-limiting enzyme in heme catabolism, producing equimolar amounts of bilirubin and carbon monoxide (CO). Metalloporphyrins (Mps) are heme derivatives that competitively inhibit HO and thereby suppress hyperbilirubinemia. No systematic studies have been reported evaluating whether the HO isoenzymes are inhibited differentially by various Mps. Identification of Mps that selectively inhibit the inducible HO-1 without affecting the 'housekeeping' HO-2 isoenzyme might be desirable in the clinical setting of hemolytic disease, in which the Hmox1 gene is greatly induced. Although bilirubin production is due to the activity of both HO-1 and HO-2, the inhibition of HO-1 with a relative sparing of HO-2 activity might provide the most selective approach for the treatment of hemolytic disease.

STUDY DESIGN

We determined for the deutero-, proto-, meso- and bis-glycol porphyrins with zinc, tin and chromium as central atoms, respectively, the concentration needed for 50% inhibition (I(50)) of HO-1 and HO-2 activities in rat spleen and brain tissue.

RESULT

For a given Mp, HO-1 activity was less inhibited than that of HO-2. The order of inhibitor potency of each Mp was nearly identical for both isoenzymes. Tin mesoporphyrin was the most potent inhibitor for both isoenzymes. HO-2 selectivity was greatest for tin protoporphyrin. Conversely, the Zn compounds were least inhibitory toward HO-2. No Mp preferentially inhibited HO-1.

CONCLUSION

Mps that produce a less inhibitory effect on HO-2, while limiting the response of the inducible HO-1, such as ZnPP, may be a useful clinical tool.

Inhibition of heme oxygenase augments tubular sodium reabsorption

Heme oxygenase (HO) catalyzes the degradation of heme to form iron, biliverdin, and carbon monoxide (CO). The vascular actions of CO include direct vasodilation of vascular smooth muscle and indirect vasoconstriction through inhibition of nitric oxide synthase (NOS). This study was performed to examine the effects in the kidney of inhibition of heme oxygenase alone or combined with NOS inhibition. Chromium mesoporphyrin (CrMP; 45 μmol/kg ip), a photostable HO inhibitor, was given to control rats and N(G)-nitro-l-arginine methyl ester (l-NAME)-treated hypertensive rats (50 mg·kg⁻¹·day⁻¹), 12 h, 4 days). In control animals, CrMP decreased CO levels, renal HO-1 levels, urine volume, and sodium excretion, but had no effect on arterial pressure, renal blood flow (RBF), plasma renin activity (PRA), or glomerular filtration rate (GFR). In l-NAME-treated hypertensive rats, CrMP decreased endogenous CO and renal HO-1 levels and had no effect on arterial pressure, RBF, or GFR but decreased sodium and water excretion in a similar manner to control animals. An increase in PRA was observed in untreated rats but not in l-NAME-infused rats, indicating that this effect is associated with an absent NO system. The results suggest that inhibition of HO promotes water and sodium excretion by a direct tubular action that is independent of renal hemodynamics or the NO system.

Recombinant truncated and microsomal heme oxygenase-1 and -2: differential sensitivity to inhibitors

Recombinant truncated forms of heme oxygenase-1 and -2 (HO-1 and HO-2) were compared with their crude microsomal counterparts from brain and spleen tissue of adult male rats with respect to their inhibition by azole-based, nonporphyrin HO inhibitors. The drugs tested were an imidazole-alcohol, an imidazole-dioxolane, and a triazole-ketone. Both the recombinant and crude forms of HO-2 were similarly inhibited by the 3 drugs. The crude microsomal spleen form of HO-1 was more susceptible to inhibition than was the truncated recombinant form. This difference is attributed to the extra amino acids in the full-length enzyme. These observations may be relevant in the design of drugs as inhibitors of HO and other membrane proteins.

H2S mediates O2 sensing in the carotid body

Gaseousmessengers, nitric oxide and carbon monoxide, have been implicated in O2 sensing by the carotid body, a sensory organ that monitors arterial blood O2 levels and stimulates breathing in response to hypoxia. We now show that hydrogen sulfide (H2S) is a physiologic gasotransmitter of the carotid body, enhancing its sensory response to hypoxia. Glomus cells, the site of O2 sensing in the carotid body, express cystathionine gamma-lyase (CSE), an H2S-generating enzyme, with hypoxia increasing H2S generation in a stimulus-dependent manner. Mice with genetic deletion of CSE display severely impaired carotid body response and ventilatory stimulation to hypoxia, as well as a loss of hypoxia-evoked H2S generation. Pharmacologic inhibition of CSE elicits a similar phenotype in mice and rats. Hypoxia-evoked H2S generation in the carotid body seems to require interaction of CSE with hemeoxygenase-2, which generates carbon monoxide. CSE is also expressed in neonatal adrenal medullary chromaffin cells of rats and mice whose hypoxia-evoked catecholamine secretion is greatly attenuated by CSE inhibitors and in CSE knockout mice.

Antineoplastic effects of gamma linolenic Acid on hepatocellular carcinoma cell lines

The aim of this study was to investigate the effect and the mechanism of gamma linolenic acid (GLA) treatment on human hepatocellular (HCC) cell lines. The human HCC cell line HuH7 was exposed to GLA. Cell proliferation and reactive oxygen species (ROS) generation including lipid peroxidation and apoptosis were compared. We then used a cDNA microarray analysis to investigate the molecular changes induced by GLA. GLA treatment significantly reduced cell proliferation, generated ROS, and induced apoptosis. After 24 h exposure of Huh7 cells to GLA, we identified several genes encoding the antioxidant proteins to be upregulated: heme oxygenase-1 (HO-1), aldo-keto reductase 1 family C1 (AKR1C1), C4 (AKR1C4), and thioredoxin (Trx). The HO-1 protein levels were overexpressed in Huh7 cells after GLA exposure using a Western blot analysis. Furthermore, chromium mesoporphyrin (CrMP), an inhibitor of HO activity, significantly potentiated GLA cytotoxicity. GLA treatment has induced cell growth inhibition, ROS generation including lipid peroxidation, and HO-1 production for antioxidant protection against oxidative stress caused by GLA in Huh7 cells. GLA treatment should be considered as a therapeutic modality in patients with advanced HCC.

In vivo inhibition of renal heme oxygenase with an imidazole-dioxolane inhibitor

Recent studies have identified imidazole-dioxolane based compounds as novel heme oxygenase (HO) inhibitors. While these compounds have been demonstrated to be specific HO inhibitors in vitro, they have yet to be used to inhibit renal HO activity in vivo. The goal of this study was to determine the effectiveness of the imidazole-dioxolane HO-1 inhibitor, QC-13, in the inhibition of renal HO activity in vivo. HO-1 was induced in mice by treatment with cobalt protoporphyrin (CoPP). After 5 days, QC-13 was delivered either by continuous intrarenal medullary interstitial infusion (IRMI) into one kidney at several concentrations for 72 h or by two intraperitoneal injections over a 48-h period. IRMI infusion of QC-13 at a concentration of 25 microM resulted in a significant decrease in medullary but not cortical HO activity as compared to CoPP treated kidneys. IRMI infusion of QC-13 at a lower concentration (2.5 microM) had no effect on either medullary or cortical HO activity in CoPP treated mice. In contrast, administration of QC-13 at a higher concentration (250 microM) resulted in a significant decrease in both medullary and cortical HO activity in CoPP treated mice. Systemic administration of QC-13 resulted in significant decrease both renal cortical and medullary HO activity in CoPP treated mice. In contrast to classical porphyrin based HO inhibitors, IRMI infusion of QC-13 did not induce HO-1 protein levels as determined by Western blot analysis of medullary protein samples. Our results demonstrated that imidazole-dioxolane inhibitors are renal HO inhibitors in vivo and can inhibit HO activity independent of HO-1 induction. These inhibitors may be useful tools to elucidate the role of renal HO-1 in numerous physiologic and pathophysiologic conditions.

Up-regulating the heme oxygenase system with hemin improves insulin sensitivity and glucose metabolism in adult spontaneously hypertensive rats

Accumulating clinical evidence indicates that impaired glucose tolerance is a common phenomenon in essential hypertension. Although recent evidence underscores the role of heme-oxygenase (HO) in diabetes, its effects on insulin sensitivity and glucose metabolism in spontaneously hypertensive rat (SHR), a model of essential hypertension with characteristics of metabolic syndrome including insulin resistance/impaired glucose metabolism remains largely unclear. Here we report the effects of the HO inducer, hemin, and the HO blocker, chromium-mesoporphyrin on insulin sensitivity and glucose metabolism in SHRs. Adult SHRs were severely hypertensive but normoglycemic. Hemin therapy lowered blood pressure, increased plasma insulin, decreased glycemia, and enhanced insulin sensitivity by improving glucose tolerance (ip glucose tolerance test) and insulin tolerance (ip insulin tolerance test) but reduced insulin resistance (homeostasis model assessment index). These effects were accompanied by increased gastrocnemius muscle HO-1, HO activity, cGMP, cAMP alongside antioxidants including bilirubin, ferritin, superoxide dismutase, catalase, and the total antioxidant capacity, whereas oxidative/inflammatory mediators like 8-isoprostane, nuclear-factor-kappaB, activating-protein-1, activating-protein-2, c-Jun-NH2-terminal-kinase, and heme were abated. Furthermore, hemin reduced proteinuria/albuminuria and enhanced the depressed levels of adiponectin, AMP-activated protein-kinase, and glucose transporter-4 in SHRs, suggesting that although SHRs are normoglycemic, insulin signaling and renal function may be impaired. Contrarily, the HO inhibitor chromium-mesoporphyrin exacerbated oxidative stress, aggravated insulin resistance, glucose tolerance, insulin tolerance and nephropathy. Hemin also enhanced HO signaling in Wistar Kyoto and Sprague Dawley rats and increased insulin sensitivity albeit less intensely than in SHRs, suggesting greater selectivity of HO in SHRs with dysfunctional insulin signaling. These results suggest that perturbations of insulin signaling may be a forerunner to hyperglycemia in essential hypertension. By concomitantly potentiating insulin-sensitizing agents, suppressing insulin/glucose intolerance, and abating oxidative stress, HO inducers may prevent metabolic and cardiovascular complications in essential hypertension.

The heme oxygenase system attenuates pancreatic lesions and improves insulin sensitivity and glucose metabolism in deoxycorticosterone acetate hypertension

Recent clinical reports indicate that impaired glucose tolerance is a common phenomenon in primary aldosteronism. Aldosterone stimulates NF-κB and activating protein-1 (AP-1) to cause oxidative injury. Elevated oxidative stress impairs insulin signaling. We recently showed that the heme oxygenase (HO) system lowers blood pressure (BP) in deoxycorticosterone-acetate (DOCA)+salt hypertension, a model of primary aldosteronism. However, the effect of the HO system on insulin sensitivity in this model remains largely unclear. Here we report the effects of the HO-inducer hemin and the HO-blocker [chromium mesoporphyrin (CrMP)] on insulin sensitivity/glucose metabolism. Our experimental design included the following 10 groups: (A) controls [(i) surgery-free or normal Sprague-Dawley (SD), (ii) uninephrectomized (UnX)-sham, (iii) UnX+salt (0.9%NaCl+0.2%KCl) and (iv) UnX+DOCA]; (B) DOCA+salt; (C) hemin+DOCA+salt; (D) hemin+CrMP+DOCA+salt; (E) CrMP+DOCA+salt; (F) vehicle-treated rats and (G) normal SD+hemin. Hemin therapy lowered BP and increased plasma insulin and the insulin-sensitizing protein adiponectin with slight but significant reduction of glycemia, while CrMP abolished the hemin effects. Furthermore, hemin improved intraperitoneal glucose and insulin tolerance, suggesting that although DOCA+salt-hypertensive rats were normoglycemic, insulin signaling may be impaired. In contrast, the HO-inhibitor CrMP aggravated insulin resistance and exacerbated glucose and insulin tolerance. Interestingly, the enhanced insulin sensitization in hemin-treated animals was accompanied by reduced urinary/gastrocnemius muscle 8-iso-prostaglandin F2α (8-isoprostane), inflammatory/oxidative transcription factors like NF-κB, AP-1, JNK, and heme content, whereas HO-1, HO-activity, cGMP, and plasma/gastrocnemius muscle antioxidants including bilirubin, ferritin, SOD, catalase, and the total antioxidant capacity were increased. Similarly, hemin enhanced pancreatic HO, cGMP, and cAMP but suppressed 8-isoprostane and attenuated pancreatic histopathological lesions including fibrosis, interstitial edema, acinar cell necrosis, vacuolization, and mononuclear cell infiltration, with corresponding improvement of insulin production. Our results suggest that impaired insulin signaling may be a forerunner to hyperglycemia in aldosteronism. By preserving pancreatic morphology, potentiating insulin signaling, and lowering BP, the HO system may prevent metabolic and cardiovascular complications in aldosteronism.

Growth-dependent changes in the contribution of carbon monoxide to arteriolar function

BACKGROUND/AIMS

Endothelium-dependent dilation of skeletal muscle arterioles is mediated by unknown factors in very young rats. We assessed the possible contribution of carbon monoxide (CO) to this dilation and to dilation in older animals.

METHODS

The effects of de-endothelialization or various pharmacological inhibitors on responses to CO or endothelium-dependent dilators were studied in gracilis muscle arterioles from rats at 3-4 weeks ('weanlings') and 6-7 weeks ('juveniles').

RESULTS

Exogenous CO constricted, rather than dilated, arterioles from both age groups. This constriction was reduced by endothelial removal or NOS inhibition in juvenile, but not weanling, arterioles. In contrast, this constriction was abolished by K(+) channel inhibition in weanling, but not juvenile, arterioles. The heme precursor delta-aminolevulinic acid constricted juvenile arterioles but did not affect weanling arterioles. The heme oxygenase inhibitor chromium (III) mesoporphyrin IX abolished the endothelium-dependent dilation of juvenile arterioles to simvastatin, and reduced ACh- and simvastatin-induced dilations of weanling arterioles.

CONCLUSION

These findings suggest that relatively high concentrations of exogenous CO can cause constriction by inhibiting endothelium-derived NO in juvenile arterioles and inhibiting K(+) channels in weanling arterioles. Endogenous CO produced at lower concentrations can contribute to endothelium-dependent dilation in both age groups.

Heme Arginate Suppresses Cardiac Lesions and Hypertrophy in Deoxycorticosterone Acetate-Salt Hypertension

In hypertension, elevated levels of oxidative/inflammatory mediators including nuclear factor kappaB (NF-κB), activating protein (AP-1), c-Jun-NH2-terminal kinase (JNK), and cell-regulatory proteins such as transforming growth factor beta (TGF-β), trigger the mobilization of extracellular matrix (ECM) leading to fibrosis, hypertrophy and impairment of cardiac function. Although the heme oxygenase (HO) system is cytoprotective, its effects on cardiac fibrosis and hypertrophy in deoxycorticosterone acetate (DOCA-salt) hypertension are not completely elucidated. Here, we report cardioprotection by the HO inducer, heme arginate against histopathological lesions in DOCA-hypertension. Treatment with heme arginate restored physiological blood pressure, and abated cardiac hypertrophy (3.75 ± 0.12 vs. 3.19 ± 0.09 g/kg body wt; n =16, P < 0.01), left-to-right ventricular ratio (6.67 ± 0.62 vs. 4.39 ± 0.63; n = 16, P < 0.01), left ventricular mass (2.48 ± 0.14 vs. 2.01 ± 0.09 g/kg body wt; n = 16, P < 0.01) and left-ventricular wall thickness (2.82 ± 0.16 vs. 1.98 ± 0.14 mm; n = 16, P < 0.01), whereas the HO inhibitor, chromium mesoporphyrin, exacerbated hypertrophy and cardiac lesions. The suppression of cardiac hypertrophy was accompanied by a robust increase in HO-1, HO activity, cyclic guanosine monophosphate (cGMP), ferritin and the total antioxidant capacity, whereas 8-isoprostane, NF-κB, JNK, AP-1, TGF-β, fibronectin and collagen-I were significantly abated. Correspondingly, histopathological parameters that depict progressive cardiac damage, including fibrosis, interstitial/perivascular collagen deposition, scarring, muscle-fiber thickness, muscular hypertrophy and coronary-arteriolar thickening were abated. Our study suggests that upregulating the HO system lowers blood pressure, potentiates the antioxidant status in tissues, suppresses oxidative stress/mediators such as NF-κB, AP-1 and cJNK, and suppresses the mobilization of ECM proteins like TGF-β, collagen and fibronectin, with corresponding reduction of cardiac histopathological lesion and hypertrophy.

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

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

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

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

Upregulating the heme oxygenase system suppresses left ventricular hypertrophy in adult spontaneously hypertensive rats for 3 months

BACKGROUND

Aldosterone and phospholipase C (PLC) stimulate nuclear factor-kappaB (NF-kappaB) and activating-protein (AP-1), causing fibrosis and hypertrophy. Besides harboring binding sites for NF-kappaB and AP-1, heme oxygenase (HO-1) generates cytoprotective products, including bilirubin and ferritin. The multifaceted interaction between HO-1 and aldosterone-PLC profibrotic axis in cardiac hypertrophy of spontaneously hypertensive rats (SHR) was studied.

METHODS AND RESULTS

HO-1 was induced with hemin or blocked with chromium mesoporphyrin (CrMP). The study groups included: (A) controls (SHR, WKY, and SD), (B) SHR+hemin, (C) SHR+hemin+CrMP, (D) SHR+CrMP, and (E) SHR+vehicle. Histological and morphological/morphometrical, quantitative reverse transcription-polymerase chain reaction, Western blot, enzyme immunoassay, and spectrophotometric assays were used to assess the effect of the HO system on cardiac hypertrophy. Hemin therapy evoked a 3-month enduring cardioprotection in adult SHR by lowering blood pressure, and reducing left-to-right ventricular ratio, left ventricular wall-thickness, and left ventricle-to-body-weight ratio, whereas CrMP exacerbated cardiac fibrosis/hypertrophy. The cardioprotection was accompanied by reduced aldosterone, PLC, inositol-triphosphate, NF-kappaB, AP-1, heme, and 8-isoprostane, a marker of oxidative stress, whereas HO-1, HO activity, cGMP, bilirubin, ferritin, superoxide dismutase, and the total antioxidant capacity were increased. Correspondingly, extracellular matrix/remodeling proteins such as fibronectin, collagen-1, collagen-IV, alongside cardiac histopathological lesions including fibrosis, scarring, muscular-hypertrophy, coronary-arteriolar thickening, and interstitial/perivascular collagen deposition were attenuated.

CONCLUSIONS

Our study unveils sustained cardioprotection by hemin that may have clinical relevance.

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

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

Inhibition of heme oxygenase activity in newborn mice by azalanstat

Inhibition of heme oxygenase (HO), the rate-limiting enzyme in heme catabolism, may be an ideal strategy for preventing neonatal jaundice. Although natural and synthetic heme analogs, called metalloporphyrins (Mps), have been extensively investigated for this purpose, some Mps are phototoxic, affect the activity of other enzymes, or induce HO-1 transcription-properties that may limit their clinical use. Another class of compounds, imidazole-dioxolanes, has been shown to selectively inhibit the inducible isozyme HO-1. Therefore, we investigated the efficacy of azalanstat (AZA), an imidazole-dioxolane, towards inhibiting HO activity in 7-day-old mice. We found that a single dose of AZA at 500 micromol.kg(-1) body mass (BM) administered i.p. significantly inhibited HO activity and reduced in vivo bilirubin production. In the spleen, HO inhibition (>50%) was observed within 0.25-3 h after administration. After 24 h, however, spleen HO activity, HO-1 protein, and HO-1 mRNA levels significantly increased 1.2-, 2.4-, and 4.0-fold, respectively. We conclude that AZA effectively inhibits in vivo HO activity only at a high dose and that it also induces spleen HO-1 gene transcription. Therefore, other imidazole-dioxolanes should be evaluated to determine whether they are more potent than AZA for use in treating neonatal jaundice.