Heme oxygenase-2 is a hemoprotein and binds heme through heme regulatory motifs that are not involved in heme catalysis

Heme metabolism in nonerythroid cells

Heme is an iron-containing prosthetic group necessary for the function of several proteins termed "hemoproteins." Erythrocytes contain most of the body's heme in the form of hemoglobin and contain high concentrations of free heme. In nonerythroid cells, where cytosolic heme concentrations are 2 to 3 orders of magnitude lower, heme plays an essential and often overlooked role in a variety of cellular processes. Indeed, hemoproteins are found in almost every subcellular compartment and are integral in cellular operations such as oxidative phosphorylation, amino acid metabolism, xenobiotic metabolism, and transcriptional regulation. Growing evidence reveals the participation of heme in dynamic processes such as circadian rhythms, NO signaling, and the modulation of enzyme activity. This dynamic view of heme biology uncovers exciting possibilities as to how hemoproteins may participate in a range of physiologic systems. Here, we discuss how heme is regulated at the level of its synthesis, availability, redox state, transport, and degradation and highlight the implications for cellular function and whole organism physiology.

Altered Expression of Heme Oxygenase 2 in Heme Oxygenase 1-deficient Mouse Embryos

Heme oxygenases (Hmoxs) are enzymes that catalyze the first and rate-limiting step in the degradation of heme to carbon monoxide, iron, and biliverdin. The two main isozymes, namely Hmox1 and Hmox2, are encoded by two different genes. Mutation of the Hmox1 gene in mice is known to cause extensive prenatal lethality, and limited information is available about the expression of Hmox proteins in developing mouse embryos. In this study, immunohistochemistry was used to perform a detailed investigation comparing Hmox proteins in Hmox1 wild-type and knockout (KO) mouse embryos collected from wild-type and heterozygous timed-matings. Western analysis for Hmoxs was also done in the organs of late-gestation embryos. The results demonstrated cytoplasmic and nuclear localization of Hmoxs in all the organs examined in wild-type embryos. Interestingly, Hmox2 immunoreactive protein signals were significantly low in most of the organs of mid- and late-gestation Hmox1-KO embryos. Furthermore, relative levels of Hmox2 were revealed to be significantly lower in the lung and kidney of late-gestation Hmox1-KO embryos by western analysis, which complemented the immunohistochemistry findings in these two organs. The current study provides detailed immunoexpression patterns of Hmox proteins in wild-type and Hmox1-KO mouse embryos in mid- and late-gestation.

Shapes and Patterns of Heme-Binding Motifs in Mammalian Heme-Binding Proteins

Heme is a double-edged sword. On the one hand, it has a pivotal role as a prosthetic group of hemoproteins in many biological processes ranging from oxygen transport and storage to miRNA processing. On the other hand, heme can transiently associate with proteins, thereby regulating biochemical pathways. During hemolysis, excess heme, which is released into the plasma, can bind to proteins and regulate their activity and function. The role of heme in these processes is under-investigated, with one problem being the lack of knowledge concerning recognition mechanisms for the initial association of heme with the target protein and the formation of the resulting complex. A specific heme-binding sequence motif is a prerequisite for such complex formation. Although numerous short signature sequences indicating a particular protein function are known, a comprehensive analysis of the heme-binding motifs (HBMs) which have been identified in proteins, concerning specific patterns and structural peculiarities, is missing. In this report, we focus on the evaluation of known mammalian heme-regulated proteins concerning specific recognition and structural patterns in their HBMs. The Cys-Pro dipeptide motifs are particularly emphasized because of their more frequent occurrence. This analysis presents a comparative insight into the sequence and structural anomalies observed during transient heme binding, and consequently, in the regulation of the relevant protein.

Carotid body responses to O2 and CO2 in hypoxia-tolerant naked mole rats

AIM

Naked mole rats (NMRs) exhibit blunted hypoxic (HVR) and hypercapnic ventilatory responses (HCVR). The mechanism(s) underlying these responses are largely unknown. We hypothesized that attenuated carotid body (CB) sensitivity to hypoxia and hypercapnia contributes to the near absence of ventilatory responses to hypoxia and CO₂ in NMRs.

METHODS

We measured ex vivo CB sensory nerve activity, phrenic nerve activity (an estimation of ventilation), and blood gases in urethane-anesthetized NMRs and C57BL/6 mice breathing normoxic, hypoxic, or hypercapnic gases. CB morphology, carbon monoxide, and H₂ S levels were also determined.

RESULTS

Relative to mice, NMRs had blunted CB and HVR. Morphologically, NMRs have larger CBs, which contained more glomus cells than in mice. Furthermore, NMR glomus cells form a dispersed pattern compared to a clustered pattern in mice. Hemeoxygenase (HO)-1 mRNA was elevated in NMR CBs, and an HO inhibitor increased CB sensitivity to hypoxia in NMRs. This increase was blocked by an H₂ S synthesis inhibitor, suggesting that interrupted gas messenger signaling contributes to the blunted CB responses and HVR in NMRs. Regarding hypercapnia, CB and ventilatory responses to CO₂ in NMRs were larger than in mice. Carbonic anhydrase (CA)-2 mRNA is elevated in NMR CBs, and a CA inhibitor blocked the augmented CB response to CO₂ in NMRs, indicating CA activity regulates augmented CB response to CO₂ .

CONCLUSIONS

Consistent with our hypothesis, impaired CB responses to hypoxia contribute in part to the blunted HVR in NMRs. Conversely, the HCVR and CB are more sensitive to CO₂ in NMRs.

Heme Oxygenase-1: An Anti-Inflammatory Effector in Cardiovascular, Lung, and Related Metabolic Disorders

The heme oxygenase (HO) enzyme system catabolizes heme to carbon monoxide (CO), ferrous iron, and biliverdin-IXα (BV), which is reduced to bilirubin-IXα (BR) by biliverdin reductase (BVR). HO activity is represented by two distinct isozymes, the inducible form, HO-1, and a constitutive form, HO-2, encoded by distinct genes (HMOX1, HMOX2, respectively). HO-1 responds to transcriptional activation in response to a wide variety of chemical and physical stimuli, including its natural substrate heme, oxidants, and phytochemical antioxidants. The expression of HO-1 is regulated by NF-E2-related factor-2 and counter-regulated by Bach-1, in a heme-sensitive manner. Additionally, HMOX1 promoter polymorphisms have been associated with human disease. The induction of HO-1 can confer protection in inflammatory conditions through removal of heme, a pro-oxidant and potential catalyst of lipid peroxidation, whereas iron released from HO activity may trigger ferritin synthesis or ferroptosis. The production of heme-derived reaction products (i.e., BV, BR) may contribute to HO-dependent cytoprotection via antioxidant and immunomodulatory effects. Additionally, BVR and BR have newly recognized roles in lipid regulation. CO may alter mitochondrial function leading to modulation of downstream signaling pathways that culminate in anti-apoptotic, anti-inflammatory, anti-proliferative and immunomodulatory effects. This review will present evidence for beneficial effects of HO-1 and its reaction products in human diseases, including cardiovascular disease (CVD), metabolic conditions, including diabetes and obesity, as well as acute and chronic diseases of the liver, kidney, or lung. Strategies targeting the HO-1 pathway, including genetic or chemical modulation of HO-1 expression, or application of BR, CO gas, or CO donor compounds show therapeutic potential in inflammatory conditions, including organ ischemia/reperfusion injury. Evidence from human studies indicate that HO-1 expression may represent a biomarker of oxidative stress in various clinical conditions, while increases in serum BR levels have been correlated inversely to risk of CVD and metabolic disease. Ongoing human clinical trials investigate the potential of CO as a therapeutic in human disease.

Regulation of protein function and degradation by heme, heme responsive motifs, and CO

Heme is an essential biomolecule and cofactor involved in a myriad of biological processes. In this review, we focus on how heme binding to heme regulatory motifs (HRMs), catalytic sites, and gas signaling molecules as well as how changes in the heme redox state regulate protein structure, function, and degradation. We also relate these heme-dependent changes to the affected metabolic processes. We center our discussion on two HRM-containing proteins: human heme oxygenase-2, a protein that binds and degrades heme (releasing Fe²⁺ and CO) in its catalytic core and binds Fe³⁺-heme at HRMs located within an unstructured region of the enzyme, and the transcriptional regulator Rev-erbβ, a protein that binds Fe³⁺-heme at an HRM and is involved in CO sensing. We will discuss these and other proteins as they relate to cellular heme composition, homeostasis, and trafficking. In addition, we will discuss the HRM-containing family of proteins and how the stability and activity of these proteins are regulated in a dependent manner through the HRMs. Then, after reviewing CO-mediated protein regulation of heme proteins, we turn our attention to the involvement of heme, HRMs, and CO in circadian rhythms. In sum, we stress the importance of understanding the various roles of heme and the distribution of the different heme pools as they relate to the heme redox state, CO, and heme binding affinities.

Growing the molecular architecture of imidazole-like ligands in HO-1 complexes

Up-regulation of HO-1 had been frequently reported in different cases and types of human malignancies. Since poor clinical outcomes are reported in these cases, this enzyme's inhibition is considered a valuable and proven anticancer approach. To identify novel HO-1 inhibitors suitable for drug development, we report a structure-guided fragment-based approach to identify new lead compounds. Different parts of the selected molecules were analyzed, and the different series of novel compounds were virtually evaluated. The growing experiments of the classical HO-1 inhibitors structure led us to different hit-compounds. A synthetic pathway for six selected molecules was designed, and the compounds were synthesized. The biological activity revealed that molecules 10 and 12 inhibit the HO-1 activity with an IC₅₀ of 1.01 and 0.90 μM, respectively. This study suggested that our growing approach was successful, and these results are ongoing for further development.

Significance of Heme and Heme Degradation in the Pathogenesis of Acute Lung and Inflammatory Disorders

The heme molecule serves as an essential prosthetic group for oxygen transport and storage proteins, as well for cellular metabolic enzyme activities, including those involved in mitochondrial respiration, xenobiotic metabolism, and antioxidant responses. Dysfunction in both heme synthesis and degradation pathways can promote human disease. Heme is a pro-oxidant via iron catalysis that can induce cytotoxicity and injury to the vascular endothelium. Additionally, heme can modulate inflammatory and immune system functions. Thus, the synthesis, utilization and turnover of heme are by necessity tightly regulated. The microsomal heme oxygenase (HO) system degrades heme to carbon monoxide (CO), iron, and biliverdin-IXα, that latter which is converted to bilirubin-IXα by biliverdin reductase. Heme degradation by heme oxygenase-1 (HO-1) is linked to cytoprotection via heme removal, as well as by activity-dependent end-product generation (i.e., bile pigments and CO), and other potential mechanisms. Therapeutic strategies targeting the heme/HO-1 pathway, including therapeutic modulation of heme levels, elevation (or inhibition) of HO-1 protein and activity, and application of CO donor compounds or gas show potential in inflammatory conditions including sepsis and pulmonary diseases.

The heme-regulatory motifs of heme oxygenase-2 contribute to the transfer of heme to the catalytic site for degradation

Heme-regulatory motifs (HRMs) are present in many proteins that are involved in diverse biological functions. The C-terminal tail region of human heme oxygenase-2 (HO2) contains two HRMs whose cysteine residues form a disulfide bond; when reduced, these cysteines are available to bind Fe3+-heme. Heme binding to the HRMs occurs independently of the HO2 catalytic active site in the core of the protein, where heme binds with high affinity and is degraded to biliverdin. Here, we describe the reversible, protein-mediated transfer of heme between the HRMs and the HO2 core. Using hydrogen-deuterium exchange (HDX)-MS to monitor the dynamics of HO2 with and without Fe3+-heme bound to the HRMs and to the core, we detected conformational changes in the catalytic core only in one state of the catalytic cycle-when Fe3+-heme is bound to the HRMs and the core is in the apo state. These conformational changes were consistent with transfer of heme between binding sites. Indeed, we observed that HRM-bound Fe3+-heme is transferred to the apo-core either upon independent expression of the core and of a construct spanning the HRM-containing tail or after a single turnover of heme at the core. Moreover, we observed transfer of heme from the core to the HRMs and equilibration of heme between the core and HRMs. We therefore propose an Fe3+-heme transfer model in which HRM-bound heme is readily transferred to the catalytic site for degradation to facilitate turnover but can also equilibrate between the sites to maintain heme homeostasis.

Heme oxygenase-1/carbon monoxide as modulators of autophagy and inflammation

Heme oxygenase-1 (HO-1) catalyzes heme degradation to generate biliverdin-IXα, carbon monoxide (CO), and iron. The HO-1/CO system confers cytoprotection in animal models of organ injury and disease, via modulation of inflammation and apoptosis. Recent studies have uncovered novel anti-inflammatory targets of HO-1/CO including regulation of the autophagy and inflammasome pathways. Autophagy is a lysosome-dependent program for the turnover of cellular organelles such as mitochondria, proteins, and pathogens; which may downregulate inflammatory processes. Therapeutic modulation of autophagy by CO has been demonstrated in models of sepsis. The nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3 (NLRP3) inflammasome regulates the maturation of pro-inflammatory cytokines. CO can regulate NLRP3 inflammasome activation and associated pro-inflammatory cytokines production and promote the resolution of inflammation by upregulating the synthesis of specialized pro-resolving mediators (SPMs). Mitochondria may represent a proximal target of HO-1/CO action. HO-1 may localize to mitochondria in response to stress, while CO can moderate mitochondrial dysfunction and regulate mitochondrial autophagy (mitophagy) and biogenesis. The interplay between mitochondrial autophagy, mitochondrial dysfunction, and the regulation and resolution of inflammation may make important contributions to the protection afforded by HO-1/CO in cellular and organ injury models. Recent studies have continued to explore the potential of CO for clinical applications.

Role of Heme Oxygenase as a Modulator of Heme-Mediated Pathways

The heme oxygenase (HO) system is essential for heme and iron homeostasis and necessary for adaptation to cell stress. HO degrades heme to biliverdin (BV), carbon monoxide (CO) and ferrous iron. Although mostly beneficial, the HO reaction can also produce deleterious effects, predominantly attributed to excessive product formation. Underrated so far is, however, that HO may exert effects additionally via modulation of the cellular heme levels. Heme, besides being an often-quoted generator of oxidative stress, plays also an important role as a signaling molecule. Heme controls the anti-oxidative defense, circadian rhythms, activity of ion channels, glucose utilization, erythropoiesis, and macrophage function. This broad spectrum of effects depends on its interaction with proteins ranging from transcription factors to enzymes. In degrading heme, HO has the potential to exert effects also via modulation of heme-mediated pathways. In this review, we will discuss the multitude of pathways regulated by heme to enlarge the view on HO and its role in cell physiology. We will further highlight the contribution of HO to pathophysiology, which results from a dysregulated balance between heme and the degradation products formed by HO.

Heme Oxygenase-2 (HO-2) as a therapeutic target: Activators and inhibitors

Heme oxygenase (HO) enzymes are involved in heme catabolism and several physiological functions. Among the different HO isoforms, HO-2 stands out for its neuroprotective properties and modulatory activity in male reproduction. However, unlike the HO-1 ligands, the potential therapeutic applications of HO-2 inhibitors/activators have not been extensively explored yet. Moreover, the physiological role of HO-2 is still unclear, mostly due to the lack of highly selective HO-2 chemical probes. To boost the interest on this intriguing target, the present review updates the knowledge on the structure-activity relationships of HO-2 inhibitors and activators, as well as their potential therapeutic applications. To the best of our knowledge, among HO-2 inhibitors, clemizole derivatives are the most selective HO-2 inhibitors reported so far (IC₅₀ HO-1 >100 μM, IC₅₀ HO-2 = 3.4 μM), while the HO-2 nonselective inhibitors described herein possess IC₅₀ HO-2 values ≤ 10 μM. Furthermore, the development of HO-2 activators, such as menadione analogues, helped to understand the critical moieties required for HO-2 activation. Recent advances in the potential therapeutic applications of HO-2 inhibitors/activators cover the fields of neurodegenerative, cardiovascular, inflammatory, and reproductive diseases further stimulating the interest towards this target.

Redox Regulation of Heme Oxygenase-2 and the Transcription Factor, Rev-Erb, Through Heme Regulatory Motifs

SIGNIFICANCE

Heme binds to and serves as a cofactor for a myriad of proteins that are involved in diverse biological processes. Hemoproteins also exhibit varying modes of heme binding, suggesting that the protein environment contributes to the functional versatility of this prosthetic group. The subject of this review is a subset of hemoproteins that contain at least one heme regulatory motif (HRM), which is a short sequence containing a Cys-Pro core that, in many cases, binds heme with the Cys acting as an axial ligand. Recent Advances: As more details about HRM-containing proteins are uncovered, some underlying commonalities are emerging, including a role in regulating protein stability. Further, the cysteines of some HRMs have been shown to form disulfide bonds. Because the cysteines must be in the reduced, dithiol form to act as a heme axial ligand, heme binds at these sites in a redox-regulated manner, as demonstrated for heme oxygenase-2 (HO2) and Rev-erbβ.

CRITICAL ISSUES

HRM-containing proteins have wide variations in heme affinity, utilize different axial ligand schemes, and exhibit differences in the ability to act as a redox sensor-all while having a wide variety of biological functions. Here, we highlight HO2 and Rev-erbβ to illustrate the similarities and differences between two hemoproteins that contain HRMs acting as redox sensors.

FUTURE DIRECTIONS

HRMs acting as redox sensors may be applicable to other HRM-containing proteins as many contain multiple HRMs and/or other cysteine residues, which may become more evident as the functional significance of HRMs is probed in additional proteins.

Carbon monoxide and its donors - their implications for medicine

Inhalation of high concentrations of carbon monoxide (CO) is known to lead to serious systemic complications and neuronal disturbances. However, it has been found that not only is CO produced endogenously, but also that low concentrations can bestow beneficial effects which may be of interest in biology and medicine. As translocation of CO through the human organism is difficult, small molecules known as CO-releasing molecules (CORMs) deliver controlled amounts of CO to biological systems, and these are of great interest from a medical point of view. These actions may prevent vascular dysfunction, regulate blood pressure, inhibit blood platelet aggregation or have anti-inflammatory effects. This review summarizes the functions of various CO-releasing molecules in biology and medicine.

Brain-derived erythropoietin protects from intermittent hypoxia-induced cardiorespiratory dysfunction and oxidative stress in mice

Study Objectives

Based on the fact that erythropoietin (Epo) administration in rodents protects against spatial learning and cognitive deficits induced by chronic intermittent hypoxia (CIH)-mediated oxidative damage, here we tested the hypothesis that Epo in the brain protects against cardiorespiratory disorders and oxidative stress induced by CIH in adult mice.

Methods

Adult control and transgenic mice overexpressing Epo in the brain only (Tg21) were exposed to CIH (21%-10% O2-10 cycles/hour-8 hours/day-7 days) or room air. After CIH exposure, we used the tail cuff method to measure arterial pressure, and whole-body plethysmography to assess the frequency of apneic episodes at rest, minute ventilation, and ventilatory responses to hypoxia and hypercapnia. Finally, the activity of pro-oxidant (XO-xanthine oxidase, and NADPH) and antioxidant (super oxide dismutase) enzymes was evaluated in the cerebral cortex and brainstem.

Results

Exposure of control mice to CIH significantly increased the heart rate and arterial pressure, the number of apneic events, and the ventilatory response to hypoxia and hypercapnia. Furthermore, CIH increased the ratio of pro-oxidant to antioxidant enzymes in cortex and brainstem tissues. Both physiological and molecular changes induced by CIH were prevented in transgenic Tg21 mice.

Conclusions

We conclude that the neuroprotective effect of Epo prevents oxidative damage in the brain and cardiorespiratory disorders induced by CIH. Considering that Epo is used in clinics to treat chronic kidney disease and stroke, our data show convincing evidence suggesting that Epo may be a promising alternative drug to treat sleep-disorder breathing.

Repurposing and repositioning neurosteroids in the treatment of traumatic brain injury: A report from the trenches

The field of neuroprotection after brain injuries has been littered with failed clinical trials. Finding a safe and effective treatment for acute traumatic brain injury remains a serious unmet medical need. Repurposing drugs that have been in use for other disorders is receiving increasing attention as a strategy to move candidate drugs more quickly to trial while reducing the very high cost of new drug development. This paper describes our own serendipitous discovery of progesterone's neuroprotective potential, and the strategies we are using in repurposing and developing this hormone for use in brain injuries-applications very different from its classical uses in treating disorders of the reproductive system. We have been screening and testing a novel analog that maintains progesterone's therapeutic properties while overcoming its physiochemical challenges, and testing progesterone in combination treatment with another pleiotropic hormone, vitamin D. Finally, our paper, in the context of the problems and pitfalls we have encountered, surveys some of the factors we found to be critical in the clinical translation of repurposed drugs. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

Reactive oxygen radicals and gaseous transmitters in carotid body activation by intermittent hypoxia

Sleep apnea is a prevalent respiratory disease characterized by periodic cessation of breathing during sleep causing intermittent hypoxia (IH). Sleep apnea patients and rodents exposed to IH exhibit elevated sympathetic nerve activity and hypertension. A heightened carotid body (CB) chemoreflex has been implicated in causing autonomic abnormalities in IH-treated rodents and in sleep apnea patients. The purpose of this article is to review the emerging evidence showing that interactions between reactive oxygen species (ROS) and gaseous transmitters as a mechanism cause hyperactive CB by IH. Rodents treated with IH exhibit markedly elevated ROS in the CB, which is due to transcriptional upregulation of pro-oxidant enzymes by hypoxia-inducible factor (HIF)-1 and insufficient transcriptional regulation of anti-oxidant enzymes by HIF-2. ROS, in turn, increases cystathionine γ-lyase (CSE)-dependent H₂S production in the CB. Blockade of H₂S synthesis prevents IH-evoked CB activation. However, the effects of ROS on H₂S production are not due to direct effects on CSE enzyme activity but rather due to inactivation of heme oxygenase-2 (HO-2), a carbon monoxide (CO) producing enzyme. CO inhibits H₂S production through inactivation of CSE by PKG-dependent phosphorylation. During IH, reduced CO production resulting from inactivation of HO-2 by ROS releases the inhibition of CO on CSE thereby increasing H₂S. Inhibiting H₂S synthesis prevented IH-evoked sympathetic activation and hypertension.

Cardiovascular effects of gasotransmitter donors

Gasotransmitters represent a subfamily of the endogenous gaseous signaling molecules that include nitric oxide (NO), carbon monoxide (CO), and hydrogen sulphide (H(2)S). These particular gases share many common features in their production and function, but they fulfill their physiological tasks in unique ways that differ from those of classical signaling molecules found in tissues and organs. These gasotransmitters may antagonize or potentiate each other's cellular effects at the level of their production, their downstream molecular targets and their direct interactions. All three gasotransmitters induce vasodilatation, inhibit apoptosis directly or by increasing the expression of anti-apoptotic genes, and activate antioxidants while inhibiting inflammatory actions. NO and CO may concomitantly participate in vasorelaxation, anti-inflammation and angiogenesis. NO and H(2)S collaborate in the regulation of vascular tone. Finally, H(2)S may upregulate the heme oxygenase/carbon monoxide (HO/CO) pathway during hypoxic conditions. All three gasotransmitters are produced by specific enzymes in different cell types that include cardiomyocytes, endothelial cells and smooth muscle cells. As translational research on gasotransmitters has exploded over the past years, drugs that alter the production/levels of the gasotransmitters themselves or modulate their signaling pathways are now being developed. This review is focused on the cardiovascular effects of NO, CO, and H(2)S. Moreover, their donors as drug targeting the cardiovascular system are briefly described.

NPY binds with heme to form a NPY–heme complex: enhancing peroxidase activity in free heme and promoting NPY nitration and inactivation

NPY binding with heme enhances the peroxidase activity of free heme, resulting in the important tyrosine nitration, which will attenuate its bioactivity.

Oxygen Sensing and Homeostasis

The discovery of carotid bodies as sensory receptors for detecting arterial blood oxygen levels, and the identification and elucidation of the roles of hypoxia-inducible factors (HIFs) in oxygen homeostasis have propelled the field of oxygen biology. This review highlights the gas-messenger signaling mechanisms associated with oxygen sensing, as well as transcriptional and non-transcriptional mechanisms underlying the maintenance of oxygen homeostasis by HIFs and their relevance to physiology and pathology.

Heme Oxygenases in Cardiovascular Health and Disease

Heme oxygenases are composed of two isozymes, Hmox1 and Hmox2, that catalyze the degradation of heme to carbon monoxide (CO), ferrous iron, and biliverdin, the latter of which is subsequently converted to bilirubin. While initially considered to be waste products, CO and biliverdin/bilirubin have been shown over the last 20 years to modulate key cellular processes, such as inflammation, cell proliferation, and apoptosis, as well as antioxidant defense. This shift in paradigm has led to the importance of heme oxygenases and their products in cell physiology now being well accepted. The identification of the two human cases thus far of heme oxygenase deficiency and the generation of mice deficient in Hmox1 or Hmox2 have reiterated a role for these enzymes in both normal cell function and disease pathogenesis, especially in the context of cardiovascular disease. This review covers the current knowledge on the function of both Hmox1 and Hmox2 at both a cellular and tissue level in the cardiovascular system. Initially, the roles of heme oxygenases in vascular health and the regulation of processes central to vascular diseases are outlined, followed by an evaluation of the role(s) of Hmox1 and Hmox2 in various diseases such as atherosclerosis, intimal hyperplasia, myocardial infarction, and angiogenesis. Finally, the therapeutic potential of heme oxygenases and their products are examined in a cardiovascular disease context, with a focus on how the knowledge we have gained on these enzymes may be capitalized in future clinical studies.

H2S production by reactive oxygen species in the carotid body triggers hypertension in a rodent model of sleep apnea

Sleep apnea is a prevalent respiratory disease in which episodic cessation of breathing causes intermittent hypoxia. Patients with sleep apnea and rodents exposed to intermittent hypoxia exhibit hypertension. The carotid body senses changes in blood O2 concentrations, and an enhanced carotid body chemosensory reflex contributes to hypertension in sleep apnea patients. A rodent model of intermittent hypoxia that mimics blood O2 saturation profiles of patients with sleep apnea has shown that increased generation of reactive oxygen species (ROS) in the carotid body enhances the chemosensory reflex and triggers hypertension. CO generated by heme oxygenase-2 (HO-2) induces a signaling pathway that inhibits hydrogen sulfide (H2S) production by cystathionine γ-lyase (CSE), leading to suppression of carotid body activity. We found that ROS inhibited CO generation by HO-2 in the carotid body and liver through a mechanism that required Cys(265) in the heme regulatory motif of heterologously expressed HO-2. We showed that ROS induced by intermittent hypoxia inhibited CO production and increased H2S concentrations in the carotid body, which stimulated its neural activity. In rodents, blockade of H2S synthesis by CSE, by either pharmacologic or genetic approaches, inhibited carotid body activation and hypertension induced by intermittent hypoxia. Thus, our results indicate that oxidant-induced inactivation of HO-2, which leads to increased CSE-dependent H2S production in the carotid body, is a critical trigger of hypertension in rodents exposed to intermittent hypoxia.

Propitious Therapeutic Modulators to Prevent Blood-Spinal Cord Barrier Disruption in Spinal Cord Injury

The blood-spinal cord barrier (BSCB) is a specialized protective barrier that regulates the movement of molecules between blood vessels and the spinal cord parenchyma. Analogous to the blood-brain barrier (BBB), the BSCB plays a crucial role in maintaining the homeostasis and internal environmental stability of the central nervous system (CNS). After spinal cord injury (SCI), BSCB disruption leads to inflammatory cell invasion such as neutrophils and macrophages, contributing to permanent neurological disability. In this review, we focus on the major proteins mediating the BSCB disruption or BSCB repair after SCI. This review is composed of three parts. Section 1. SCI and the BSCB of the review describes critical events involved in the pathophysiology of SCI and their correlation with BSCB integrity/disruption. Section 2. Major proteins involved in BSCB disruption in SCI focuses on the actions of matrix metalloproteinases (MMPs), tumor necrosis factor alpha (TNF-α), heme oxygenase-1 (HO-1), angiopoietins (Angs), bradykinin, nitric oxide (NO), and endothelins (ETs) in BSCB disruption and repair. Section 3. Therapeutic approaches discusses the major therapeutic compounds utilized to date for the prevention of BSCB disruption in animal model of SCI through modulation of several proteins.

BACH1, the master regulator gene: A novel candidate target for cancer therapy

BACH1 (BTB and CNC homology 1, basic leucine zipper transcription factor 1) is a transcriptional factor and a member of cap 'n' collar (CNC) and basic region leucine zipper factor family. In contrast to other bZIP family members, BACH1 appeared as a comparatively specific transcription factor. It acts as transcription regulator and is recognized as a recently hypoxia regulator and functions as an inducible repressor for the HO-1 gene in many human cell types in response to stress oxidative. In regard to studies lately, although, BACH1 has been related to the regulation of oxidative stress and heme oxidation, it has never been linked to invasion and metastasis. Recent studies have showed that BACH1 is involved in bone metastasis of breast cancer by up-regulating vital metastatic genes like CXCR4 and MMP1. This newly discovered aspect of BACH1 gene provides new insight into cancer progression study and stands on its master regulator role in metastasis process, raising the possibility of considering it as a potential target for cancer therapy.

Cysteine-independent activation/inhibition of heme oxygenase-2

Reactive thiols of cysteine (cys) residues in proteins play a key role in transforming chemical reactivity into a biological response. The heme oxygenase-2 (HO-2) isozyme contains two cys residues that have been implicated in binding of heme and also the regulation of its activity. In this paper, we address the question of a role for cys residues for the HO-2 inhibitors or activators designed in our laboratory. We tested the activity of full length recombinant human heme oxygenase-2 (FL-hHO-2) and its analog in which cys265 and cys282 were both replaced by alanine to determine the effect on activation by menadione (MD) and inhibition by QC-2350. Similar inhibition by QC-2350 and almost identical activation by MD was observed for both recombinant FL-hHO-2s. Our findings are interpreted to mean that thiols of FL-hHO-2s are not involved in HO-2 activation or inhibition by the compounds that have been designed and identified by us. Activation or inhibition of HO-2 by our compounds should be attributed to a mechanism other than altering binding affinity of HO-2 for heme through cys265 and cys282.

Oxidative and nitrosative stress pathways in the brain of socially isolated adult male rats demonstrating depressive- and anxiety-like symptoms

Various stressors may disrupt the redox homeostasis of an organism by causing oxidative and nitrosative stress that may activate stressor-specific pathways and provoke specific responses. Chronic social isolation (CSIS) represents a mild chronic stress that evokes a variety of neurobehavioral changes in rats similar to those observed in people with psychiatric disorders, including depression. Most rodent studies have focused on the effect of social isolation during weaning or adolescence, while its effect in adult rats has not been extensively examined. In this review, we discuss the current knowledge regarding the involvement of oxidative/nitrosative stress pathways in the prefrontal cortex and hippocampus of adult male rats exposed to CSIS, focusing on hypothalamic-pituitary-adrenocortical (HPA) axis activity, behavior parameters, antioxidative defense systems, stress signaling mediated by nuclear factor-kappa B (NF-κB), and mitochondria-related proapoptotic signaling. Although increased concentrations of corticosterone (CORT) have been shown to induce oxidative and nitrosative stress, we suggest a mechanism underlying the glucocorticoid paradox whereby a state of oxidative/nitrosative stress may exist under basal CORT levels. This review also highlights the differential susceptibility of prefrontal cortex and hippocampus to oxidative stress following CSIS and suggests a possible cellular pathway of stress tolerance that preserves the hippocampus from molecular damage and apoptosis. The differential regulation of the transcriptional factor NF-κB, and the enzymes inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) following CSIS may be one functional difference between the response of the prefrontal cortex and hippocampus, thus identifying potentially relevant targets for antidepressant treatment.

Deletion of the hemopexin or heme oxygenase-2 gene aggravates brain injury following stroma-free hemoglobin-induced intracerebral hemorrhage

Background

Following intracerebral hemorrhage (ICH), red blood cells release massive amounts of toxic heme that causes local brain injury. Hemopexin (Hpx) has the highest binding affinity to heme and participates in its transport, while heme oxygenase 2 (HO2) is the rate-limiting enzyme for the degradation of heme. Microglia are the resident macrophages in the brain; however, the significance and role of HO2 and Hpx on microglial clearance of the toxic heme (iron-protoporphyrin IX) after ICH still remain understudied. Accordingly, we postulated that global deletion of constitutive HO2 or Hpx would lead to worsening of ICH outcomes.

Methods

Intracerebral injection of stroma-free hemoglobin (SFHb) was used in our study to induce ICH. Hpx knockout (Hpx^−/−) or HO2 knockout (HO2^−/−) mice were injected with 10 μL of SFHb in the striatum. After injection, behavioral/functional tests were performed, along with anatomical analyses. Iron deposition and neuronal degeneration were depicted by Perls’ and Fluoro-Jade B staining, respectively. Immunohistochemistry with anti-ionized calcium-binding adapter protein 1 (Iba1) was used to estimate activated microglial cells around the injured site.

Results

This study shows that deleting Hpx or HO2 aggravated SFHb-induced brain injury. Compared to wild-type littermates, larger lesion volumes were observed in Hpx^−/− and HO2^−/− mice, which also bear more degenerating neurons in the peri-lesion area 24 h postinjection. Fewer Iba1-positive microglial cells were detected at the peri-lesion area in Hpx^−/− and HO2^−/− mice, interestingly, which is associated with markedly increased iron-positive microglial cells. Moreover, the Iba1-positive microglial cells increased from 24 to 72 h postinjection and were accompanied with improved neurologic deficits in Hpx^−/− and HO2^−/− mice. These results suggest that Iba1-positive microglial cells could engulf the extracellular SFHb and provide protective effects after ICH. We then treated cultured primary microglial cells with SFHb at low and high concentrations. The results show that microglial cells actively take up the extracellular SFHb. Of interest, we also found that iron overload in microglia significantly reduces the Iba1 expression level and resultantly inhibits microglial phagocytosis.

Conclusions

This study suggests that microglial cells contribute to hemoglobin-heme clearance after ICH; however, the resultant iron overloads in microglia appear to decrease Iba1 expression and to further inhibit microglial phagocytosis.

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.

Comparison of the Mechanisms of Heme Hydroxylation by Heme Oxygenases-1 and -2: Kinetic and Cryoreduction Studies

The two isoforms of human heme oxygenase (HO1 and HO2) catalyze oxidative degradation of heme to biliverdin, Fe, and CO. Unlike HO1, HO2 contains two C-terminal heme regulatory motifs (HRMs) centered at Cys265 and Cys282 that act as redox switches and, in their reduced dithiolate state, bind heme (Fleischhacker et al., Biochemistry , 2015 , 54 , 2693 - 2708 ). Here, we describe cryoreduction/annealing and electron paramagnetic resonance spectroscopic experiments to study the structural features of the oxyheme moiety in HO2 and to elucidate the initial steps in heme degradation. We conclude that the same mechanism of heme hydroxylation to α-meso-hydroxyheme is employed by both isoforms and that the HRMs do not affect the physicochemical properties of the oxy-Fe(II) and HOO-Fe(III) states of HO2. However, the absorption spectrum of oxy-Fe(II)-HO2 is slightly blue-shifted relative to that of HO1. Furthermore, heme hydroxylation proceeds three times more slowly, and the oxy-Fe(II) state is 100-fold less stable in HO2 than in HO1. These distinctions are attributed to slight structural variances in the two proteins, including differences in equilibrium between open versus closed conformations. Kinetic studies revealed that heme oxygenation by HO2 occurs solely at the catalytic core in that a variant of HO2 lacking the C-terminal HRM domain exhibits the same specific activity as one containing both the catalytic core and HRM domain; furthermore, a truncated variant containing only the HRM region binds but cannot oxidize heme. In summary, HO1 and HO2 share similar catalytic mechanisms, and the HRMs do not play a direct role in the HO2 catalytic cycle.

Spectroscopic studies reveal that the heme regulatory motifs of heme oxygenase-2 are dynamically disordered and exhibit redox-dependent interaction with heme

Heme oxygenase (HO) catalyzes a key step in heme homeostasis: the O2- and NADPH-cytochrome P450 reductase-dependent conversion of heme to biliverdin, Fe, and CO through a process in which the heme participates both as a prosthetic group and as a substrate. Mammals contain two isoforms of this enzyme, HO2 and HO1, which share the same α-helical fold forming the catalytic core and heme binding site, as well as a membrane spanning helix at their C-termini. However, unlike HO1, HO2 has an additional 30-residue N-terminus as well as two cysteine-proline sequences near the C-terminus that reside in heme regulatory motifs (HRMs). While the role of the additional N-terminal residues of HO2 is not yet understood, the HRMs have been proposed to reversibly form a thiol/disulfide redox switch that modulates the affinity of HO2 for ferric heme as a function of cellular redox poise. To further define the roles of the N- and C-terminal regions unique to HO2, we used multiple spectroscopic techniques to characterize these regions of the human HO2. Nuclear magnetic resonance spectroscopic experiments with HO2 demonstrate that, when the HRMs are in the oxidized state (HO2(O)), both the extra N-terminal and the C-terminal HRM-containing regions are disordered. However, protein NMR experiments illustrate that, under reducing conditions, the C-terminal region gains some structure as the Cys residues in the HRMs undergo reduction (HO2(R)) and, in experiments employing a diamagnetic protoporphyrin, suggest a redox-dependent interaction between the core and the HRM domains. Further, electron nuclear double resonance and X-ray absorption spectroscopic studies demonstrate that, upon reduction of the HRMs to the sulfhydryl form, a cysteine residue from the HRM region ligates to a ferric heme. Taken together with EPR measurements, which show the appearance of a new low-spin heme signal in reduced HO2, it appears that a cysteine residue(s) in the HRMs directly interacts with a second bound heme.

The C-terminal heme regulatory motifs of heme oxygenase-2 are redox-regulated heme binding sites

Heme oxygenase-2 (HO2), an enzyme that catalyzes the conversion of heme to biliverdin, contains three heme regulatory motifs (HRMs) centered at Cys127, Cys265, and Cys282. Previous studies using the soluble form of human HO2 spanning residues 1-288 (HO2sol) have shown that a disulfide bond forms between Cys265 and Cys282 and that, in this oxidized state, heme binds to the catalytic site of HO2sol via His45. However, various mutational and spectroscopic studies have confirmed the involvement of cysteine in Fe(3+)-heme binding upon reduction of the disulfide bond. In an effort to understand how the HRMs are involved in binding of heme to disulfide-reduced HO2sol, in the work described here, we further investigated the properties of Fe(3+)-heme bound to HO2. Specifically, we investigated binding of Fe(3+)-heme to a truncated form of soluble HO2 (residues 213-288; HO2tail) that spans the C-terminal HRMs of HO2 but lacks the catalytic core. We found that HO2tail in the disulfide-reduced state binds Fe(3+)-heme and accounts for the spectral features observed upon binding of heme to the disulfide-reduced form of HO2sol that cannot be attributed to heme binding at the catalytic site. Further analysis revealed that while HO2sol binds one Fe(3+)-heme per monomer of protein under oxidizing conditions, disulfide-reduced HO2sol binds slightly more than two. Both Cys265 and Cys282 were identified as Fe(3+)-heme ligands, and His256 also acts as a ligand to the Cys265-ligated heme. Additionally, Fe(3+)-heme binds with a much weaker affinity to Cys282 than to Cys265, which has an affinity much weaker than that of the His45 binding site in the catalytic core. In summary, disulfide-reduced HO2 has multiple binding sites with varying affinities for Fe(3+)-heme.

Protein kinase G-regulated production of H2S governs oxygen sensing

Reflexes initiated by the carotid body, the principal O2-sensing organ, are critical for maintaining cardiorespiratory homeostasis during hypoxia. O2 sensing by the carotid body requires carbon monoxide (CO) generation by heme oxygenase-2 (HO-2) and hydrogen sulfide (H2S) synthesis by cystathionine-γ-lyase (CSE). We report that O2 stimulated the generation of CO, but not that of H2S, and required two cysteine residues in the heme regulatory motif (Cys(265) and Cys(282)) of HO-2. CO stimulated protein kinase G (PKG)-dependent phosphorylation of Ser(377) of CSE, inhibiting the production of H2S. Hypoxia decreased the inhibition of CSE by reducing CO generation resulting in increased H2S, which stimulated carotid body neural activity. In carotid bodies from mice lacking HO-2, compensatory increased abundance of nNOS (neuronal nitric oxide synthase) mediated O2 sensing through PKG-dependent regulation of H2S by nitric oxide. These results provide a mechanism for how three gases work in concert in the carotid body to regulate breathing.

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.

Dysregulated heme oxygenase-ferritin system in pterygium pathogenesis

PURPOSE

Cyclooxygenase (COX)-, lipoxygenase (LOX)-, and cytochrome P450 monooxygenase (CYP)-derived eicosanoids have been implicated in ocular surface inflammation and neovascularization. These eicosanoids are subjected to regulation by enzymes, such as heme oxygenases (HOs) and ferritin.

METHODS

Quantitative polymerase chain reaction and lipidomics based on liquid chromatography-tandem mass spectrometry were performed on pterygia from patients undergoing surgical pterygium excision. Control tissues consisted of donor corneas. In addition, lipidomics based on liquid chromatography-tandem mass spectrometry was performed on tears collected from patients before the surgery.

RESULTS

Messenger RNA (mRNA) expression of HO-2, the constitutive HO isoform, was upregulated by 40% in pterygia compared with control tissue, whereas the mRNA level of the inducible form, HO-1, was downregulated by more than 50%. Levels of CYP4B1 mRNA showed an approximate 2-fold increase in pterygia compared with control. Lipidomic analysis of tissues indicated a moderate elevation in Prostaglandin E2 and thromboxane B2 levels in pterygia compared with control. Among the LOX-derived metabolites, the antiinflammatory-hydroxyeicosatetraenoic acid (15-HETE) levels were significantly reduced in pterygia (79.3 ± 48.11 pg/mg protein) compared with control (586.2 ± 213.5 pg/mg protein), whereas the proinflammatory LOX- and CYP4B1-derived 12-HETE levels were 10-fold higher in pterygia (2768 ± 832.3 pg/mg protein) compared with control (231.4 ± 87.35 pg/mg protein). Prostaglandin E2 and HETEs were also present in tears from patients with pterygium but were not detected in tears from healthy volunteers. The mRNA expression levels of both light and heavy chain ferritin were 60% and 30% lower, respectively, in pterygia compared with control.

CONCLUSIONS

We believe that a dysfunctional HO-ferritin system leads to increased levels of proinflammatory mediators, thus contributing to the inflammation characteristic of pterygia.

Carbon Monoxide and the brain: time to rethink the dogma

Carbon Monoxide (CO), long thought to be a simple environmental pollutant is now known to have a critical role in cellular functions ranging from vasodilation to circadian rhythms. In this review, we will begin with a discussion of the enzyme responsible for CO production: heme oxygenase. Because this review will focus on the effects of CO in the brain, we will transition to CO toxicology and determine if this simple diatomic gas has really earned its nefarious reputation. An in depth analysis of the roles for CO in circadian rhythms and as a gasotransmitter will be provided in the neurological functional role section, followed by its vascular effects derived mainly from interactions with soluble guanylyl cyclase. We will then describe the evidence for CO's protective roles through the MAPK pathway, and finally touch upon the potential therapeutic roles for CO in neurological diseases including ischemic stroke, multiple sclerosis, and neuropathic pain.

Mitochondria-targeted heme oxygenase-1 induces oxidative stress and mitochondrial dysfunction in macrophages, kidney fibroblasts and in chronic alcohol hepatotoxicity

The inducible form of Heme Oxygenase-1 (HO-1), a major endoplasmic reticulum (ER) associated heme protein, is known to play important roles in protection against oxidative and chemical stress by degrading free heme released from degradation of heme proteins. In this study we show that induced expression of HO-1 by subjecting macrophage RAW-264.7 cells to chemical or physiological hypoxia resulted in significant translocation of HO-1 protein to mitochondria. Transient transfection of COS-7 cells with cloned cDNA also resulted in mitochondrial translocation of HO-1. Deletion of N-terminal ER targeting domain increased mitochondrial translocation under the transient transfection conditions. Mitochondrial localization of both intact HO-1 and N-terminal truncated HO-1 caused loss of heme aa-3 and cytochrome c oxidase (CcO) activity in COS-7 cells. The truncated protein, which localizes to mitochondria at higher levels, induced substantially steeper loss of CcO activity and reduced heme aa3 content. Furthermore, cells expressing mitochondria targeted HO-1 also induced higher ROS production. Consistent with dysfunctional state of mitochondria induced by HO-1, the mitochondrial recruitment of autophagy markers LC-3 and Drp-1 was also increased in these cells. Chronic ethanol feeding in rats also caused an increase in mitochondrial HO-1 and decrease in CcO activity. These results show that as opposed to the protective effect of the ER associated HO-1, mitochondria targeted HO-1 under normoxic conditions induces mitochondrial dysfunction.

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Under hypoxia, the inducible Heme Oxygenase-1 (HO-1) is localized in mitochondria.

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N-terminal truncated HO-1 is more efficiently translocated to mitochondria.

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Mitochondria targeted HO-1 induces oxidative stress and CcO dysfunction.

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Mitochondrial HO-1 content is increased in alcohol treated rat livers.

Heme sensor proteins

Heme is a prosthetic group best known for roles in oxygen transport, oxidative catalysis, and respiratory electron transport. Recent years have seen the roles of heme extended to sensors of gases such as O2 and NO and cell redox state, and as mediators of cellular responses to changes in intracellular levels of these gases. The importance of heme is further evident from identification of proteins that bind heme reversibly, using it as a signal, e.g. to regulate gene expression in circadian rhythm pathways and control heme synthesis itself. In this minireview, we explore the current knowledge of the diverse roles of heme sensor proteins.

Coordinated expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 and heme oxygenase 2: evidence for a regulatory link between glycolysis and heme catabolism

Heme is an essential requirement for cell survival. Heme oxygenase (HO) is the rate-limiting enzyme in heme catabolism and consists of two isozymes, HO-1 and HO-2. To identify the protein that regulates the expression or function of HO-1 or HO-2, we searched for proteins that interact with both isozymes, using protein microarrays. We thus identified 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4) that synthesizes or degrades fructose-2,6-bisphosphate, a key activator of glycolysis, depending on cellular microenvironments. Importantly, HO-2 and PFKFB4 are predominantly expressed in haploid spermatids. Here, we show a drastic reduction in expression levels of PFKFB4 mRNA and protein and HO-2 mRNA in HepG2 human hepatoma cells in responses to glucose deprivation (≤ 2.5 mM), which occurred concurrently with remarkable induction of HO-1 mRNA and protein. Knockdown of HO-2 expression in HepG2 cells, using small interfering RNA, caused PFKFB4 mRNA levels to decrease with a concurrent increase in HO-1 expression. Thus, in HepG2 cells, HO-1 expression was increased, when expression levels of HO-2 and PFKFB4 mRNAs were decreased. Conversely, overexpression of HO-2 in HepG2 cells caused the level of co-expressed PFKFB4 protein to increase. These results suggest a potential regulatory role for HO-2 in ensuring PFKFB4 expression. Moreover, in D407 human retinal pigment epithelial cells, glucose deprivation decreased the expression levels of PFKFB4, HO-1, and HO-2 mRNAs. Thus, glucose deprivation consistently down-regulated the expression of PFKFB4 and HO-2 mRNAs in both HepG2 cells and RPE cells. We therefore postulate that PFKFB4 and HO-2 are expressed in a coordinated manner to maintain glucose homeostasis.

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.

An HPLC method to detect heme oxygenase activity

This unit presents a method to calculate heme oxygenase enzymatic activity from the formation of bilirubin equivalents [biliverdin-Ix alpha (BV) and bilirubin-IX alpha (BR)]. The BV and BR generated in the reaction are separated by reversed-phase HPLC and detected using visible absorbance spectroscopy. Since both metabolites of heme degradation are directly quantifiable, the assay eliminates the requirement for biliverdin reductase supplementation.

Emerging roles of cadmium and heme oxygenase in type-2 diabetes and cancer susceptibility

Many decades after an outbreak of severe cadmium poisoning, known as Itai-itai disease, cadmium continues to pose a significant threat to human health worldwide. This review provides an update on the effects of this environmental toxicant cadmium, observed in numerous populations despite modest exposure levels. In addition, it describes the current knowledge on the link between heme catabolism and glycolysis. It examines novel functions of heme oxygenase-2 (HO-2) that protect against type 2-diabetes and obesity, which have emerged from diabetic/obese phenotypes of the HO-2 knockout mouse model. Increased cancer susceptibility in type-2 diabetes has been noted in several large cohorts. This is a cause for concern, given the high prevalence of type-2 diabetes worldwide. A lifetime exposure to cadmium is associated with pre-diabetes, diabetes, and overall cancer mortality with sex-related differences in specific types of cancer. Liver and kidney are target organs for the toxic effects of cadmium. These two organs are central to the maintenance of blood glucose levels. Further, inhibition of gluconeogenesis is a known effect of heme, while cadmium has the propensity to alter heme catabolism. This raises the possibility that cadmium may mimic certain HO-2 deficiency conditions, resulting in diabetic symptoms. Intriguingly, evidence has emerged from a recent study to suggest the potential interaction and co-regulation of HO-2 with the key regulator of glycolysis: 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4). HO-2 could thus be critical to a metabolic switch to cancer-prone cells because the enzyme PFKFB and glycolysis are metabolic requirements for cell proliferation and resistance to apoptosis.

Heme and HO-1 Inhibition of HCV, HBV, and HIV

Hepatitis C virus, human immunodeficiency virus, and hepatitis B virus are chronic viral infections that cause considerable morbidity and mortality throughout the world. In the decades following the identification and sequencing of these viruses, in vitro experiments demonstrated that heme oxygenase-1, its oxidative products, and related compounds of the heme oxygenase system inhibit replication of all 3 viruses. The purpose of this review is to critically evaluate and summarize the seminal studies that described and characterized this remarkable behavior. It will also discuss more recent work that discovered the antiviral mechanisms and target sites of these unique antiviral agents. In spite of the fact that these viruses are diverse pathogens with quite profound differences in structure and life cycle, it is significant that heme and related compounds show striking similarity for viral target sites across all three species. Collectively, these findings strongly indicate that we should move forward and develop heme and related tetrapyrroles into versatile antiviral agents that could be used therapeutically in patients with single or multiple viral infections.

Redox, haem and CO in enzymatic catalysis and regulation

The present paper describes general principles of redox catalysis and redox regulation in two diverse systems. The first is microbial metabolism of CO by the Wood-Ljungdahl pathway, which involves the conversion of CO or H2/CO2 into acetyl-CoA, which then serves as a source of ATP and cell carbon. The focus is on two enzymes that make and utilize CO, CODH (carbon monoxide dehydrogenase) and ACS (acetyl-CoA synthase). In this pathway, CODH converts CO2 into CO and ACS generates acetyl-CoA in a reaction involving Ni·CO, methyl-Ni and acetyl-Ni as catalytic intermediates. A 70 Å (1 Å=0.1 nm) channel guides CO, generated at the active site of CODH, to a CO 'cage' near the ACS active site to sequester this reactive species and assure its rapid availability to participate in a kinetically coupled reaction with an unstable Ni(I) state that was recently trapped by photolytic, rapid kinetic and spectroscopic studies. The present paper also describes studies of two haem-regulated systems that involve a principle of metabolic regulation interlinking redox, haem and CO. Recent studies with HO2 (haem oxygenase-2), a K+ ion channel (the BK channel) and a nuclear receptor (Rev-Erb) demonstrate that this mode of regulation involves a thiol-disulfide redox switch that regulates haem binding and that gas signalling molecules (CO and NO) modulate the effect of haem.

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

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

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.