Current Drug Targets (v.11, #12)

The heme oxygenase (HO) enzyme reaction has initially been described more that 40 years ago [1]. HO catalyzes the degradation of the redox-reactive molecule heme into equimolar amounts of bilirubin, carbon monoxide (CO) and iron. Two genetically distinct isozymes of HO, HO-1 and HO-2, have been identified and exhibit distinct patterns of cell- and tissue- specific gene expression. In contrast to the constitutive isoform HO-2, which is mainly found in brain and testis [2], the inducible isoform HO-1 is expressed in almost all cells and tissues and is up-regulated by its substrate heme and a wide variety of oxidative stress stimuli. Up-regulation of HO-1 has therefore been considered for many years as a general antioxidant protective response against unfavourable cellular conditions including, but not limited to oxidative stress. Major advances in understanding the wide-ranging physiological functions of HO-1 have been made in studies on HO-1 knockout mouse models [3]. Such studies not only confirmed the antioxidant cytoprotective role of HO-1, but also provided evidence that HO-1 has major anti-inflammatory and immunomodulatory effects. Importantly, phenotypical alterations in the so far only known human case of genetic HO-1 deficiency were highly similar to those observed in HO-1 knockout mice [4]. Further insights into the cell type- and cell context-specific functions of HO-1 have recently been given in a mouse model, in which the HO-1 gene has been specifically deleted in myeloid cells. Animals with myeloid HO-1 deficiency exhibited a defect of the early innate immune response and an exacerbated phenotype of an experimental autoimmune disease [5]. Future experimental approaches along this line might help to further elucidate the anti-inflammatory functions of HO-1, as HO-1 appears to be crucially involved in the regulatory mechanisms that link various forms of cellular stress (endoplasmic reticulum stress and oxidative stress) with the pathogenesis of inflammatory diseases [6, 7]. In addition to the advances in understanding principal biological functions of HO-1, this enzyme has attracted major attention in recent years as a therapeutic target [8]. A rapidly growing body of evidence has demonstrated that specific induction of HO-1 may provide novel therapeutic options for the treatment of multiple experimental disease models ranging from inflammatory disorders to transplantation and cancer. This Special Issue Edition of Current Drug Targets on Therapeutic Applications of the Heme Oxygenase System deals with pertinent aspects on the basic physiological functions of HO-1, in particular how the HO system and its products are specifically involved in controlling the physiological homeostasis of different organs and organ systems (e.g. lung, liver, cardiovascular system and central nervous system). Moreover, current concepts will be discussed that show how targeted modulation of HO-1 may be applicable for specific therapeutic interventions in various disease entities (e.g. neurodegenerative disease, atherosclerosis, sepsis, cancer and wound healing). Finally, emerging roles of the bilirubin producing enzyme biliverdin reductase and that of synthetic CO-releasing molecules (CORMs), which may release the gaseous HO product CO for therapeutic purposes, are addressed in more detail. In conclusion, although HO-1 and its products have raised many expectations for the development of novel pharmacological compounds, critical questions remain to be answered before targeted interventions might be available for clinical applications.

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) remain major causes of morbidity and mortality in critical care medicine despite advances in therapeutic modalities. ALI can be associated with sepsis, trauma, pharmaceutical or xenobiotic exposures, high oxygen therapy (hyperoxia) and mechanical ventilation. The stress protein heme oxygenase-1 (HO-1) provides an inducible defense mechanism that can protect lung cells and tissues against injury. HO-1 degrades heme to biliverdin-IXand#945;, carbon monoxide (CO), and iron. Each of these reaction products has been implicated in the cytoprotection associated with HO-1 expression. At low concentrations, CO can confer cyto-protective and tissue-protective effects involving the inhibition of inflammatory, proliferative, and apoptotic signaling. Lung protection by HO-1 has been demonstrated in vitro and in vivo in several models of experimental ALI and sepsis. Recent studies have also explored the protective effects of pharmacological or inhalation CO therapy in animal models of ALI/sepsis. CO has shown therapeutic potential in models of oxidative and acid-induced lung injury, ventilator-induced lung injury, endotoxin challenge, and cecal-ligation and puncture induced-sepsis. Despite therapeutic benefit in animal model studies, the efficacy of CO in humans with these conditions remains unclear, and awaits further controlled clinical studies. This review will summarize recent findings on the therapeutic applications of HO-1 and its end-product CO in the lung, with an emphasis on lung injury models relevant to critical care medicine.

Regulatory Role of Anesthetics on Heme Oxygenase-1 by Alexander Hoetzel, Rene Schmidt (1495-1503).
As an enzyme, heme oxygenase (HO) can provide substantial cellular protection. By eliminating free heme and generating iron, biliverdin, as well as carbon monoxide, HO exerts anti-inflammatory, anti-proliferative, antioxidative, and vasodilatory effects. The inducible form of HO, heme oxygenase-1 (HO-1) can be upregulated by harmful stimuli in most human cell types. In such a way, cells utilize HO-1 as a mechanism of self-protection. Many studies have shown that upregulation of HO-1 prior to injurious stimuli conferred protection to cells and organs against subsequent injury. Therefore, manipulation of HO-1 gene expression might represent a valuable strategy for the prevention of organ dysfunction. In recent studies, intravenous and inhaled anesthetics (e.g., ketamine, propofol, opioids, isoflurane, sevoflurane, desflurane, etc.) not only upregulate HO-1 to varying extents, but account for organ protection via the HO pathway. The major advantage of anesthetics over other HO-inducing agents is related to their clinically proven safety. Another important issue is that patients receiving anesthetics in anesthesia or intensive care medicine are often suffering from pathological conditions involving pro-oxidative or pro-inflammatory states. Therefore, it would be interesting to know whether the impact of anesthetics on HO-1 regulation might influence outcome of these patients. This overview summarizes the effects of different anesthetics on HO-1 regulation and function in disease models.

Targeting Heme Oxygenase-1 in Vascular Disease by William Durante (1504-1516).
Heme oxygenase-1 (HO-1) metabolizes heme to generate carbon monoxide (CO), biliverdin, and iron. Biliverdin is subsequently metabolized to bilirubin by biliverdin reductase. HO-1 has recently emerged as a promising therapeutic target in the treatment of vascular disease. Pharmacological induction or gene transfer of HO-1 ameliorates vascular dysfunction in animal models of atherosclerosis, post-angioplasty restenosis, vein graft stenosis, thrombosis, myocardial infarction, and hypertension, while inhibition of HO-1 activity or gene deletion exacerbates these disorders. The vasoprotection afforded by HO-1 is largely attributable to its end products: CO and the bile pigments, biliverdin and bilirubin. These end products exert potent anti-inflammatory, antioxidant, anti-apoptotic, and anti-thrombotic actions. In addition, CO and bile pigments act to preserve vascular homeostasis at sites of arterial injury by influencing the proliferation, migration, and adhesion of vascular smooth muscle cells, endothelial cells, endothelial progenitor cells, or leukocytes. Several strategies are currently being developed to target HO-1 in vascular disease. Pharmacological induction of HO-1 by heme derivatives, dietary antioxidants, or currently available drugs, is a promising near-term approach, while HO-1 gene delivery is a long-term therapeutic goal. Direct administration of CO via inhalation or through the use of COreleasing molecules and/or CO-sensitizing agents provides an attractive alternative approach in targeting HO-1. Furthermore, delivery of bile pigments, either alone or in combination with CO, presents another avenue for protecting against vascular disease. Since HO-1 and its products are potentially toxic, a major challenge will be to devise clinically effective therapeutic modalities that target HO-1 without causing any adverse effects.

Heme oxygenase-1 (HO-1), an enzyme degrading heme to carbon monoxide, free iron, and biliverdin, participates in the cell defence against oxidative stress and it has been speculated that it might be a new therapeutic target for neuroprotection. In this review, we discuss recent findings on the regulation of the HO-1 gene, Hmox1, in the brain with particular focus on the transcription factors Nrf2 and HIF-1. Functional polymorphisms in Hmox1 have been associated with high risk for Alzheimer's and Parkinson's disease. Hence, we review the current knowledge on the role of HO-1 and its enzymatic products on these two pathologies as well as ischemic brain injury. HO-1 modulates the inflammatory response in several scenarios, and therefore we discuss its role in modulation of the innate immune cell of the brain, microglia. From the therapeutic side, the blood brain barrier represents an obstacle to directly modulate heme oxygenase activity, but drugs activating the transcription actor Nrf2, which have a very diverse molecular structure, may be good candidates to induce HO-1 in concert with other antioxidant and detoxification enzymes. A more complete understanding on the mechanisms regulating HO-1 expression in brain cells and how these mechanisms are involved in neuropathological changes will be essential to develop these new therapeutic approaches.

Heme Oxygenase-1 in Lung Disease by Chintan M. Raval, Patty J. Lee (1532-1540).
The lungs are a major target for various inflammatory, oxidative, carcinogenic or infectious stressors, which result in a range of lung diseases. Induction of heme oxygenase-1 (HO-1) during acute and chronic lung processes is a crucial defense mechanism. HO-1 catalyzes the degradation of free cellular heme to iron, carbon monoxide (CO) and biliverdin which is eventually converted to bilirubin by biliverdin reductase. In addition to the degradation of free heme, a pro-oxidant, HO-1 exerts anti-oxidant, anti-inflammatory and anti-apoptotic properties via its reaction products. This review summarizes the regulation and protective roles of HO-1 and its reaction products in several in vitro and in vivo lung disease models, including acute lung injury, ischemia-reperfusion (IR)-induced lung injury, cigarette smoke and chronic obstructive pulmonary disease (COPD), pulmonary arterial hypertension (PAH), lung cancer and asthma. The therapeutic applications of HO-1 in the lung as well as potential complications of excessive HO-1 induction are also covered. In summary, the HO-1 system is a powerful endogenous defense strategy with immense therapeutic potential against a range of lung diseases if optimal levels and tissue targeting can be achieved.

Heme Oxygenase-1 and Iron in Liver Inflammation: A Complex Alliance by Stephan Immenschuh, Eveline Baumgart-Vogt, Sebastian Mueller (1541-1550).
Heme oxygenase (HO)-1 is the inducible isoform of the first and rate-limiting enzyme of heme degradation. HO-1 has potent antioxidant and also anti-inflammatory functions, the underlying mechanisms of which are not well understood. Together with antioxidant carbon monoxide and biliverdin, HO produces reactive iron, which unambiguously connects this enzyme with the iron metabolism and its potential toxicity. A link between HO-1 and iron homeostasis has been demonstrated in HO-1 knockout mice, which develop major hemosiderosis in solid organs such as liver and kidney. Moreover, genetic HO-1 deficiency causes a chronic inflammatory condition in these animals. As the liver plays a crucial role for the body's iron homeostasis (e.g. via secretion of the iron regulatory hormone hepcidin) and also for systemic inflammation, hepatic HO-1 may be important for the regulation of both systems. In particular, cell-specific functions of HO-1 in liver tissue macrophages (Kupffer cells) might be of major significance, because these cells play a key role in iron recycling during erythrophagocytosis and also in the control of hepatic and systemic inflammatory responses. This review discusses the current knowledge on interactions of HO-1 with iron metabolism in the context of systemic as well as hepatic inflammatory disorders. Recent advances in the understanding of the functional role of HO-1 in inflammatory liver diseases, namely viral hepatitis, alcoholic liver disease and non-alcoholic steatohepatitis are summarized. Finally, it is highlighted, how targeted modulation of HO-1 may provide specific protection in these inflammatory disorders.

Heme Oxygenase-1 in Tumor Biology and Therapy by Halina Was, Jozef Dulak, Alicja Jozkowicz (1551-1570).
Heme oxygenase-1 (HO-1) degrades heme to carbon monoxide (CO), biliverdin, and ferrous iron. As HO-1 expression is highly increased by stressful conditions, the major role of the enzyme is the protection against oxidative injury. Additionally, it regulates cell proliferation, modulates inflammatory response and facilitates angiogenesis. Beneficial activities of HO-1 have been recognized in many pathological states e.g. atherosclerosis, diabetes, ischemia/reperfusion injury or organ transplantation. Interestingly HO-1 expression is very often boosted in tumor tissues and could be further elevated in response to radio-, chemo-, or photodynamic therapy. A growing body of evidence suggests that HO-1 may play a role in tumor induction and can potently improve the growth and spread of tumors. This review discusses the implications of HO-1 properties for tumor proliferation and cell death, differentiation, angiogenesis and metastasis, and tumor-related inflammation. Finally, it suggests that pharmacological agents that regulate HO activity or HO-1 gene silencing may become powerful tools for preventing the onset or progression of various cancers and sensitize them to anticancer therapies.

The Heme-Heme Oxygenase System in Wound Healing; Implications for Scar Formation by Frank A.D.T.G. Wagener, Alwin Scharstuhl, Rex M. Tyrrell, Johannes W. Von den Hoff, Alicja Jozkowicz, Jozef Dulak, Frans G.M. Russel, Anne Marie Kuijpers-Jagtman (1571-1585).
Wound healing is an intricate process requiring the concerted action of keratinocytes, fibroblasts, endothelial cells, and macrophages. Here, we review the literature on normal wound healing and the pathological forms of wound healing, such as hypertrophic or excessive scar formation, with special emphasis on the heme-heme oxygenase (HO) system and the versatile effector molecules that are formed after HO-mediated heme degradation. Excessive scar formation following wounding is thought to relate to prolonged oxidative and inflammatory stress in the skin. Evidence is accumulating that the heme-HO system forms a novel and important target in the control of wound healing. Heme-protein derived heme can act as a potent oxidative and inflammatory stress inducer, and excess levels of heme may thus contribute to delayed resolution of oxidative and inflammatory insults in the skin. This emphasizes the need for a timely reduction of the levels of heme. Heme-binding proteins, heme transporters, and the heme degrading protein, HO, form therefore a necessary defense. Deficiencies in these defense proteins or a disturbed redox status, as in diabetic patients, may render individuals more prone to heme-induced deleterious effects. A better understanding of the heme-heme oxygenase system as target during wound healing may result in novel strategies to reduce scar formation.

The range and diversity of functions of biliverdin reductase (BVR) are unmatched by any enzyme characterized to date. BVR is the sole catalyst for the conversion of biliverdin-IXand#945;, the activity product of the stress-inducible HO-1 and the constitutive HO-2, to bilirubin-IXand#945;. Bilirubin is both cytoprotective and cytotoxic, quenches reactive oxygen species (ROS) and inhibits inflammatory and mitogen-induced ROS-mediated responses, and its elevated levels in the newborn adversely effect neuronal cells. Thus, BVR occupies a center stage in cellular defense mechanisms. As a dual specificity (serine/threonine/tyrosine) kinase the human (h) BVR influences transduction of extracellular stimuli to kinases downstream of the insulin/IGF-1(insulin-like growth factor-1)/MAPK/PI3-K signaling pathways. As a bZip-type transcription factor it binds to AP-1 (activator protein-1) and CRE (cAMP response element) sites and stimulates stressinducible gene expression; as a scaffold protein, it is a platform for interaction of kinases; while acting as an intracellular shuttle, it transports regulatory factors to their target sites. hBVR promoter has consensus sequences with several regulatory elements. The gene is subject to negative and positive regulation, respectively, by TNF-and#945; (tumor necrosis factor-and#945;) and oxidative stress/hypoxia. Small human BVR – based peptides effectively duplicate polypeptide's activating influence on kinases, or mimic inhibitors of cell signaling. This, points to a realistic prospect of their use in clinical settings. The present review will briefly update cytoprotective activity and cytotoxicity of bile pigments and will focus on findings that link hBVR to cell signaling.

The perception that carbon monoxide (CO) is poisonous and life-threatening for mammalian organisms stems from its intrinsic propensity to bind iron in hemoglobin, a reaction that ultimately leads to impaired oxygen delivery to tissues. From evolutionary and chemical perspectives, however, CO is one of the most essential molecules in the formation of biological components and its interaction with transition metals is at the origin of primordial cell signaling. Not surprisingly, mammals have gradually evolved systems to finely control the synthesis and the sensing of this gaseous molecule. Cells are indeed continuously exposed to small quantities of CO produced endogenously during the degradation of heme by constitutive and inducible heme oxygenase enzymes. We have gradually learnt that heme oxygenase-derived carbon monoxide (CO) serves as a ubiquitous signaling mediator which could be exploited for therapeutic purposes. The development of transition metal carbonyls as prototypic carbon monoxide-releasing molecules (CO-RMs) represents a novel stratagem for a safer delivery of CO-based pharmaceuticals in the treatment of various pathological disorders. This review looks back at evolution to analyze and argue that a dynamic interaction of CO with specific intracellular metal centers is the common denominator for the diversified beneficial effects mediated by this gaseous molecule.

Prostanoid Receptors as Possible Targets for Anti-Allergic Drugs: Recent Advances in Prostanoids on Allergy and Immunology by Tetsuya Honda, Yoshiki Tokura, Yoshiki Miyachi, Kenji Kabashima (1605-1613).
Prostanoids, consisting of prostaglandins and thromboxane, are cyclooxygenase metabolites of arachidonic acid released in various pathophysiological conditions which exert a range of actions mediated through their respective receptors expressed on target cells. Although it has been difficult to analyze the physiological role of prostanoids, recent developments in both the disruption of the respective gene and receptor selective compounds have enabled us to investigate the physiological roles for each receptor. It has been demonstrated that each prostanoid receptor has multiple functions, and that their expression is regulated in a context-dependent manner that sometimes results in opposite, excitatory and inhibitory, outcomes. The balance of prostanoid production and receptor expression has been revealed to be important for homeostasis of the human body. Here, we review new findings on the roles of prostanoids in allergic and immune diseases, focusing on contact dermatitis, atopic dermatitis, asthma, rheumatoid arthritis, and encephalomyelitis, and also discuss the clinical potentials of receptor-selective drugs.

Targeting Trypanothione Metabolism in Trypanosomatid Human Parasites by Viridiana Olin-Sandoval, Rafael Moreno-Sanchez, Emma Saavedra (1614-1630).
The diseases caused by the trypanosomatid parasites Trypanosoma brucei, Trypanosoma cruzi and Leishmania are widely distributed throughout the world. Because of the toxic side-effects and the economically unviable cost of the currently used pharmaceutical treatments, the search for new drug targets continues. Since the antioxidant metabolism in these parasites relies on trypanothione [T(SH)2], a functional analog of glutathione, most of the pathway enzymes involved in its synthesis, utilization and reduction have been proposed as drug targets for therapeutic intervention. In the present review, the antioxidant metabolism and the phenotypic effects of inhibiting by genetic (RNA interference, knockout) or chemical approaches, the T(SH)2 and polyamine pathway enzymes in the parasites are analyzed. Although the genetic strategies are helpful in identifying essential genes for parasite survival/infectivity, they are less useful for drugtarget validation. The effectiveness of targeting each pathway enzyme was evaluated by considering (i) the enzyme kinetic properties and antioxidant metabolite concentrations and (ii) the current knowledge and experimental approaches to the study of the control of fluxes and intermediary concentrations in metabolic pathways. The metabolic control analysis indicates that highly potent and specific inhibitors have to be designed for trypanothione reductase and the peroxide detoxification system, and hence other enzymes emerge (and#947;-glutamylcysteine synthetase, trypanothione synthetase, ornithine decarboxylase, S-adenosylmethionine decarboxylase and polyamine transporters) as alternative more suitable and effective drug targets in the antioxidant metabolism of trypanosomatids.