Introduction

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Although life on this planet almost certainly began under anaerobic conditions, the environment changed ~ 2.5 billion years ago with the addition of oxygen to the atmosphere by early oxygenic photoautotrophic cells. Unwittingly, these cyanobacteria created an environment hostile to their very survival. The subsequent evolution of aerobic respiration made the best of a bad situation and allowed for the evolution of higher life forms, but our cells continue to contend with the toxic effects of oxygen. Over the millennia, cellular defense mechanisms have evolved as protection against oxidative stress. Prototypical antioxidants include superoxide dismutases, catalases, peroxidase, and nonenzymatic antioxidants such as vitamin C and vitamin E. One of the less well known but critical defenders of cellular homeostasis is the microsomal enzyme heme oxygenase.

Heme oxygenase (HO) made its debut onto the scientific stage in 1964, when Wise and coworkers first demonstrated the in vitro degradation of heme to biliverdin (1). This finding was confirmed by Tenhunen and colleagues (2, 3), who identified HO as the enzyme responsible for the catabolism of heme and went on to characterize the enzyme and its cellular localization. The enzyme settled into relative obscurity until the late 1980s, when an inducible form of HO was discovered (4). This inducible form, called heme oxygenase-1 (HO-1), proved to be identical to heat shock protein 32 (5); this observation sparked new interest in HO as a stress-responsive protein.

Cells respond to metabolic perturbations by producing specific stress proteins. Noting that HO-1 is induced by exposure to various forms of oxidative stress, Stocker proposed in 1990 that this enzyme might provide cellular protection (6). This view is supported by the observation that both HO and its substrate, heme, are highly conserved molecules across almost all forms of life, from algae to mammals. Molecules so evolutionarily conserved and ubiquitous generally serve a necessary and fundamental purpose. The first report of human HO-1 deficiency served to underscore this point: The boy in question suffered from growth failure, anemia, tissue iron deposition, lymphadenopathy, leukocytosis, and increased sensitivity to oxidant injury. He ultimately succumbed to a premature death (7). In an animal model of HO-1 deficiency, mice lacking the gene frequently die in utero, and those surviving to term display a phenotype similar to the HO-1-deficient boy (8). Clearly, HO-1 is necessary to the survival of organisms, consistent with Stocker's hypothesis that the enzyme protects against oxidative stress.

Beneficial effects of HO-1 have now been described in diseases as diverse as atherosclerosis and pre-eclampsia (Table 1). The mechanism by which this enzyme confers protection is only beginning to be unraveled, and this puzzle currently engages the attention of researchers worldwide. If the interest of the scientific community can be measured by the volume of publications dealing with a particular subject, then interest in heme oxygenase is increasing exponentially (Figure 1). The appeal is readily apparent: if we can understand how cells are able to protect themselves from oxidative stress, then our understanding and ability to intervene in disease processes will be immeasurably advanced.

View larger version (18K): [in this window] [in a new window]   Figure 1.   Graph illustrating the number of publications concerning HO-1 cataloged ny Medline from 1990 to the present.

At least part of the antioxidant function of HO-1 may depend upon the prevention of free heme from participating in pro-oxidant reactions (5). A major focus of interest, however, is in the products of enzymatic heme breakdown as mediators of cytoprotection. HO accomplishes the first rate-limiting step of heme degradation as it cleaves the -meso carbon bridge of heme molecules to yield equimolar quantities of biliverdin IXa, free iron, and carbon monoxide (CO) (Figure 2). Biliverdin is subsequently converted to bilirubin through the action of biliverdin reductase, and free iron is sequestered by ferritin. The three products of this reactionbilirubin, CO, and ferritin induced by free iron releaseall have antioxidant function (9). These molecules are candidate effectors of the anti-inflammatory, antiapoptotic, and antiproliferative functions of HO-1, and they represent targets of active research.

View larger version (6K): [in this window] [in a new window]   Figure 2.   Catalytic reaction of HO.

As previously noted, there exist several isoforms of HO, all products of separate genes. Two of these isoforms have been extensively characterized: HO-1, an inducible isoform, and HO-2, which is constitutively synthesized. A third isoform, HO-3, was more recently identified; this enzyme is structurally similar to HO-2, but acts as a less efficient heme catalyst. HO-1 is the principle focus of this review. Since this subject was last reviewed in a 1996 issue of AJRCMB (12), a great deal of progress has been made in the study of HO-1, particularly with regard to function. This article is not exhaustive but seeks to summarize what is known about the induction, regulation, and function of HO-1, ending with a discussion of HO-1 in specific human diseases.

	Induction and Regulation of HO-1

Top Abstract Introduction Induction and Regulation of... HO-1 and Signal Transduction HO-1 and Apoptosis HO-1 and Nitric Oxide HO-1 and Diseases of... Diagnostic and Therapeutic Uses... Conclusion References

HO-1 is inducible by more diverse stimuli than any other enzyme described to date (13). The common characteristic of these many inducers is their ability to cause oxidative stress. These include, but are not limited to: heme, hyperoxia, hypoxia, heat shock, endotoxin, hydrogen peroxide, cytokines, UV light, heavy metals, and nitric oxide (14). HO-1 expression is primarily regulated at the transcriptional level (12, 23), although there are interspecies variations in the regulation of HO-1. For example, the rat HO-1 promoter has a heat shock-responsive element; during heat shock increased binding of heat shock nuclear factor to the promoter region of the gene causes HO-1 message stabilization (27). By contrast, the heat shock element is not functional in the human HO-1 gene (28).

Various regulatory elements have been identified in the promoter region of HO-1, including binding sites for oxidative stress-responsive transcription factors such as nuclear factor (NF)-B, which is present in the human HO-1 gene, and activator protein-1 (AP-1) (12, 29, 30). The role of the AP-1 family of transcription factors in the regulation of HO-1 is currently being investigated, but it would appear that NF-E2-related factor 2 (Nrf2) is of particular importance (31). Work is ongoing to demonstrate which HO-1 gene elements mediate activation in response to particular stimuli. The mouse gene, which is the best characterized with regard to inducer-dependent transcriptional regulation, contains one proximal and two distal enhancers. The first distal enhancer is located 4 kb upstream from the transcription initiation site (32), and the other is located 10 kb upstream (33). Some stimuli, such as lipopolysaccharide (LPS), require the distal enhancers for gene activation (34), whereas others such as hypoxia do not (16). The mouse promoter does not function alone in response to a number of stimuli tested, including cadmium, heme, and 12-O-tetradecanoylphorbol-13-acetate (32, 35). The human HO-1 gene is not regulated in an identical fashion; the promoter has been shown to be functionally active in response to various inducers such as UV irradiation, cadmium, and heme (36). Iron- and hyperoxia-responsive regions appear to reside outside the promoter region, however, implying that the human HO-1 gene requires distal enhancer elements for induction by some agents (25).

Within the enhancer regions of the mouse HO-1 gene, there is a 10-bp sequence that is necessary for induction of the gene by all agents except hypoxia. This sequence is referred to as the stress response element (StRE). The mouse StRE contains the binding site for the activator protein-1 (AP-1) family of transcription factors. Mutation of the AP-1 binding site within the StREs abolishes gene activation by heavy metals, hydrogen peroxide, arsenate, and LPS in vitro (14, 33), providing evidence of the role of AP-1 proteins in HO-1 gene regulation. Lee and coworkers have presented both in vitro and in vivo data demonstrating that that AP-1 activation represents one mechanism mediating hyperoxia-induced HO-1 gene transcription in the lung (37), and that AP-1 acts in concert with signal transducer and activator of transcription (STAT) (38). More recently, Rensing and colleagues have demonstrated that the induction of HO-1 after hemorrhagic shock and resuscitation is mainly regulated by AP-1 in a reactive oxygen species (ROS)-dependent manner (39).

Other cis-acting DNA elements that have been studied include hypoxia-inducible factor-1 (HIF-1) and interleukin (IL)-6 response element. HIF-1 is a transcriptional activator of many oxygen-sensitive genes, and has been shown to bind to hypoxia response elements in the mouse HO-1 gene (16). Mutation of the hypoxia response elements abolishes HIF-1 binding and hypoxia-dependent gene activation (16), confirming that this activator mediates HO-1 expression in response to hypoxia. Yang and coworkers have corroborated this finding with their studies using rat renal medullary interstitial cells (40). IL-6 is one of several cytokines known to activate HO-1 gene transcription (23). A sequence motif in the HO-1 5' flanking region that conforms to the consensus IL-6 response element has been identified (41).

AP-1 is a well-known redox-sensitive transcription factor activating a number of genes involved in adaptive responses to oxidative stress, including HO-1. AP-1 belongs to the family of basic region/leucine zipper (bZIP) transcription factors which includes Jun-Jun and Jun-Fos dimers, and the more recently described NF-E2, Nrf1, and Nrf2. There is good evidence that Nrf2 modulates HO-1 gene expression by binding to a recognition site for Cap'n'Collar/basic leucine zipper (CNC-bZIP) in one of the inducible enhancers (E1) (42, 43). Results obtained using a yeast 2 hybrid system further suggest that Nrf2 may heterodimerize with activating transcription factor 4 to exert its effect (44).

	HO-1 and Signal Transduction

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The MAP kinases (mitogen-activated protein kinases) are components of signaling cascades that respond to extracellular stimuli by targeting transcription factors, resulting in the modulation of gene expression. Three major MAP kinase subfamilies have been described: extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 (Figure 3). ERK was the first cascade to be elucidated; it is activated by mitogens and is primarily involved in the regulation of growth and proliferation. The remaining two cascades respond to cellular stress signals and are hence sometimes referred to as stress-activated protein kinases. There is a good deal of overlap among the cascades both in their targets and upstream activators. The specifics of these interactions have been extensively reviewed elsewhere (45).

View larger version (22K): [in this window] [in a new window]   Figure 3.   Simplified diagram illustrating signal transduction by MAP kinases.

Given that both HO-1 and the MAP kinases are activated by stressful stimuli, and that tyrosine phosphorylation is involved in the induction of HO-1 (48), it is reasonable to postulate a relationship between the MAP kinases and HO-1. Lu and coworkers have reported that constitutive activation of ERK and p38 pathway components results in increased HO-1 reporter gene activity, and that dominant negative components abrogate arsenite-induced reporter gene activity (49, 50). These findings were confirmed using chemical inhibitors of the ERK and p38 pathways (51). The JNK pathway does not appear to be involved in the response of HO-1 to arsenite (50).

There is evidence of MAP kinase pathway involvement downstream of HO-1 activation as well. CO, one of the products of heme catabolism by HO-1, has cytoprotective and antiapoptotic effects that appear to be mediated through MAP kinase pathways, specifically the p38 pathway (11, 52, 86). Sethi and colleagues confirmed that CO can upregulate the p38 MAP kinase pathway while simultaneously downregulating ERK (53).

	HO-1 and Apoptosis

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One common cellular response to oxidative injury is apoptosis, and the involvement of reactive oxygen species in programmed cell death makes HO-1 a good candidate as an antiapoptotic molecule. Fibroblasts (54) and neurons (55) overexpressing HO-1 are resistant to stress-mediated cell death, and fibroblasts deficient in the HO-1 gene are particularly susceptible to stressful or toxic insults (55). Similarly, human renal epithelial cells resist cisplatin-induced apoptosis and necrosis when HO-1 activity is enhanced by chemical induction or gene overexpression, and mice lacking the gene for HO-1 exhibit increased susceptibility to apoptosis and necrosis after treatment with cisplatin (56). The authors of these studies have postulated that HO-1 inhibits apoptosis by regulating cellular pro-oxidant iron. Ferris and coworkers correlated protection of cells by HO-1 with a decrease in intracellular iron amounts and reproduced the protective effect by iron chelation (54).

Using mouse fibroblasts, Petrache and colleagues have demonstrated that conditional overexpression of HO-1 prevents tumor necrosis factor (TNF)- induced apoptosis. The antiapoptotic effect is lost in the presence of tin protoporphyrin, a specific inhibitor of HO activity, and in cells overexpressing antisense HO-1. These authors reported that the exogenous administration of CO also prevents TNF-- induced apoptosis, suggesting that the antiapoptotic effect of HO-1 may be mediated via CO (57). Corroborating a role for CO in the antiapoptotic function of HO-1, Brouard and coworkers have shown that exposure of endothelial cells to exogenous CO prevents TNF--induced apoptosis. They have further demonstrated that this effect is mediated via the p38 MAP kinase pathway (52).

Whether HO-1 exerts its antiapoptotic effect principally through the action of CO, the regulation of cellular iron, or even the generation of bilirubin (58) is a matter for further study. It is possible that two or more of the byproducts of HO-1 are required for full protection, or that the requirements differ depending on cell type and apoptotic stimulus. Given the relevance of apoptosis to so many disease processes, this will doubtless be an area of active investigation in the future.

	HO-1 and Nitric Oxide

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When nitric oxide (NO) was unveiled as the long-sought endothelium-derived relaxing factor in the 1980s (59), the concept of a gaseous molecule acting as a second messenger was entirely novel. It is now clear that NO has numerous vital biologic roles, including neurotransmission, host defense, inhibition of platelet aggregation, and regulation of blood flow (60). The generation of NO by nitric oxide synthase (NOS) bears an uncanny resemblance to the production of CO by HO. Both diatomic gasses are produced endogenously by enzymes that have constitutive and inducible forms. Both NO and CO share affinity for the heme molecule, and both are capable of activating guanylyl cyclase (63). One important difference between NO and CO is that NO is a highly reactive gas by virtue of its unpaired electron, whereas CO is inert. These similarities (and this difference) between the two molecules make interactions between NO and HO a natural subject for investigation.

There is some evidence that NO and CO have parallel signaling actions via soluble guanylyl cyclase (sGC) in select tissues. sGC acts as a receptor for NO, mediating many of its biologic effects. NO stimulates sGC by binding to the prosthetic heme of the enzyme leading to as much as a 400-fold activation of the purified enzyme (63, 66). CO, like NO, binds to the heme group of sGC with high affinity but leads to a far lower level of activation of the purified enzyme (67). CO has been shown to stimulate cyclic 3'5'-guanosine monophosphate (cGMP) production and to promote cGMP-dependent activities such as inhibition of platelet aggregation and smooth muscle relaxation (64, 65, 68). In the nervous system, HO-2 colocalizes with cGMP and/or sGC in cells with little or no NOS expression, supporting a link between HO and sGC. Moreover, in isolated neural cell preparations, a direct relationship between HO activity and cGMP production has been observed (69).

The inducible form of NOS (iNOS) and HO-1 are upregulated by common stimuli such as ROS and cytokines (70). NO itself has also been shown to induce HO-1 expression in various cell types by a cGMP-independent pathway (26, 71). This induction may depend upon changes in mRNA stability (71, 72), or may be transcriptionally regulated (26); the mechanism appears to vary with the cell type and the NO exposure conditions. Given the overlap in the biologic functions of CO and NO, it is possible that some actions of NO are exerted by the induction of HO-1 and elaboration of CO.

It has also been proposed that HO activity may serve under some circumstances to modulate NO production. Chemical inhibition of HO activity by zinc protoporphyrin-IX (ZnPP) in isolated smooth muscle strips (74) and macrophages (75) results in increased NO production, suggesting that HO may exert an inhibitory effect on NOS. This suggestion is further supported by the observation that induction of HO-1 in macrophages suppresses NO generation (75). As NOS is a hemoprotein, it is reasonable to postulate that CO generated by HO activity could bind to NOS, causing inactivation. It has been demonstrated that CO is in fact capable of binding to NOS (76), and that exogenously administered CO inhibits NOS activity (77). Inhibition of NOS by HO could be important in view of the fact that NO is a free radical, and as such is able to react nonspecifically with many cellular components, causing unwanted effects (78, 79). Maines has proposed that in the case of NO interaction with sGC heme, HO may promote a reversal of the reaction (13). This could prevent tonic activation of sGC by NO, which dissociates from heme at an exceedingly slow rate (80).

	HO-1 and Diseases of the Lung and Vascular System

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The salutary effects of HO-1 activation have been studied in nearly every organ system and a growing number of diseases (Table 1). What follows is a brief summary of some of the work that has been done to date relating to diseases of the lung and cardiovascular system.

HO-1 and the Lung

As an organ that provides an interface between the environment and the circulation, the lung affords an ideal environment for studying the effects of oxidative stress. HO-1 is readily induced in the lung by numerous stresses and has been implicated in the maintenance of homeostasis in the face of oxidative injury.

One common model of pulmonary oxidative stress is exposure to hyperoxia. Increased HO-1 activity has been observed in the lungs of rodents exposed to hyperoxic conditions (81), and overexpression of HO-1 in isolated pulmonary epithelial cells (81) and rat fetal lung cells (82) has been shown to confer protection against hyperoxic injury. This same protective phenomenon has been demonstrated in vivo with adenoviral transfer of HO-1 into rat lungs (83): rats overexpressing HO-1 demonstrate marked resistance to oxygen-induced lung injury and survive longer in a hyperoxic environment. However, there may be a threshold effect for cytoprotection by HO-1. Suttner and coworkers observed that although moderate overexpression of HO-1 in fibroblasts conferred protection against oxidative injury, higher levels of HO-1 were detrimental (82).

There is evidence that the CO elaborated during heme catabolism by HO-1 accounts for at least some of the protective effects of HO-1 in the lung. Otterbein and colleagues reported that rats exposed to hyperoxia in the presence of a low concentration of CO (250 ppm) exhibit less lung injury than control rats exposed to oxygen alone (84). A similar study by Clayton and associates demonstrated a statistically significant reduction in pulmonary edema with exposure to CO and hyperoxia, but no difference in other markers of lung injury (85). Interestingly, the latter study did find that inhaled CO abrogated a hyperoxia-induced increase in HO-1 protein expression. The mechanism by which CO might provide pulmonary cytoprotection has not been entirely elucidated, but downregulation of proinflammatory cytokines such as TNF- and IL-1 along with augmentation of the anti-inflammatory cytokine IL-10 appear to play a role (86). The potential role for IL-10 has been further supported by a recent murine study demonstrating resistance to LPS-induced lung injury and enhanced IL-10 production by alveolar macrophages after transfer of HO-1 cDNA (87).

As anti-inflammatory actions of HO-1 continue to be described, there is growing interest in the role of HO-1 in asthma, which is now understood to be an inflammatory disease involving activated T lymphocytes and their cytokine products (88, 89). Exhaled CO, a marker of HO activity, is elevated in humans with asthma (90). The inflammatory cells of patients with asthma have increased HO-1 protein content (91), and enhanced airway HO-1 immunostaining has also been described in a murine model of asthma (92). Given the evidence for HO-1 involvement in asthma and the proposed anti-inflammatory properties of CO (86), Chapman and coworkers sought an immunoregulatory role for CO in aeroallergen-induced inflammation in mice (93). The authors demonstrated a reduction in bronchoalveolar lavage levels of IL-5, prostaglandin E2, leukotriene B4, and eosinophils in mice treated with inhaled CO. Although these findings do not definitively indicate that symptoms of asthma will respond to exogenous or endogenous CO, they do suggest that HO and CO play a role in the modulation of allergic inflammation.

In the past year, HO-1 has been implicated in a number of other pulmonary diseases. Smokers are now known to have increased airway expression of HO-1 (94), and microsatellite polymorphisms in the HO-1 gene promoter have been linked with increased susceptibility to emphysema (95). Hypoxia-induced pulmonary hypertension, vascular remodeling, and inflammation have been attenuated through enhanced HO-1 expression in rodent models (96, 97). Influenza virus-induced lung injury in mice has also been prevented through transfer of HO-1 cDNA (98). Oxidative injury contributes to all of these disease processes, and it stands to reason that HO-1 would be of central importance in the maintenance of homeostasis under these circumstances. HO-1 will likely prove to play a role in the full range of pulmonary disease processes and, perhaps one day, in their prevention or cure.

HO-1 and the Cardiovascular System

A potential role for HO in the regulation of vascular tone was first surmised in 1984, when McGrath and Smith (99) and McFaul and McGrath (100) exposed isolated perfused rat hearts to exogenous CO and noted an increase in coronary blood flow. They concluded that CO caused dilation of the coronary arteries and that the effect was reversible because coronary flow returned to control level when the CO was removed. Several years later, other investigators reported that heme, a potent inducer of HO-1, lowers blood pressure in spontaneously hypertensive rats (101) and that inhibition of HO activity increases mean arterial blood pressure and total peripheral resistance in rats (102). It has since been demonstrated that CO promotes arterial vasodilation through activation of guanylyl cyclase in a manner similar to but independent of NO (103). The physiologic significance of a stress-induced mediator of vasodilation separate from the NOS/NO system remains to be fully elucidated, but one potential teleologic explanation is that HO-1 serves to maintain perfusion after vascular injury.

This thesis is supported by several findings related to HO-1 in the cardiovascular system. First, HO-1 expression has been shown to inhibit the growth of vascular smooth muscle cells in vitro and in vivo (107). Vascular smooth muscle cell-derived CO has been shown to inhibit endothelial cell platelet derived growth factor and endothelin-1 expression resulting in inhibited smooth muscle cell growth (108). CO can also inhibit hypoxia-induced vascular endothelial growth factor induction in smooth muscle cells (109). These observations have obvious implications with regard to vascular remodeling after injury. Next, ischemia-reperfusion injury, shear stress, and hypoxia are potent inducers of HO-1 (103, 110). Third, HO-1 induction has been shown to inhibit leukocyte adhesion through the action of bilirubin (113) and platelet aggregation via the production of CO (112). Fourth, and most convincingly, HO-1 provides protection against ischemia-reperfusion tissue injury and vascular balloon injury in a number of models (114).

The studies demonstrating a protective effect of HO-1 in vascular injury have generated a great deal of excitement because of their relevance to so many human disease processes. Yet and coworkers recently showed that HO-1 overexpression confers protection against ischemia-reperfusion-induced myocardial tissue injury (114). A role for bilirubin has been implicated in the amelioration of postischemic myocardial dysfunction conferred by HO-1 (115). Interestingly, several studies in humans have correlated low serum bilirubin levels with ischemic heart disease (116- 118). In a model of ischemic lung injury, it is CO that appears to protect against lethality through modulation of the fibrinolytic axis (119). CO has also been implicated in the prolonged survival of mouse-to-rat cardiac transplants expressing HO-1. Exposure of graft donor and recipient animals to exogenous CO is associated with suppression of graft rejection; this effect is associated with inhibition of platelet aggregation, thrombosis, myocardial infarction, and apoptosis in the transplanted organ (120). Balloon injury has been used as a model of vascular injury to demonstrate that HO-1 is induced locally in the vessel (121), that preinduction of HO-1 can prevent the injury (121, 122), that inhibition of HO-1 can worsen the injury (121), and that overexpression of HO-1 through adenovirus-mediated delivery can inhibit injury-induced vascular neointima formation (123). These findings have been further supported by a human study demonstrating that HO-1 gene promoter microsatellite polymorphisms are associated with restenosis after percutaneous transluminal angioplasty (124).

	Diagnostic and Therapeutic Uses of HO-1 and Its Byproducts

Top Abstract Introduction Induction and Regulation of... HO-1 and Signal Transduction HO-1 and Apoptosis HO-1 and Nitric Oxide HO-1 and Diseases of... Diagnostic and Therapeutic Uses... Conclusion References

What practical applications can we foresee in the near future for HO-1 and its byproducts? Based on our current knowledge of the function, regulation, and properties of HO-1, several approaches have been proposed for diagnostic and therapeutic purposes.

From a diagnostic point of view, there is growing interest in monitoring HO-1 activity in disease states. It is clear from the work cited above that HO-1 is activated in many pathologic situations, and it may be possible to use this association to predict disease flares or monitor therapy. Because CO generated by HO-1 is excreted by the lungs, quantification of exhaled CO can be used to track enzymatic activity. Elevated levels of CO in the breath have been described in asthma (125, 126), cystic fibrosis (127), diabetes (128), and critical illness (129). Increases in the amount of exhaled CO have also been correlated with disease exacerbations (130, 131). Standardization of equipment and reference values for the measurement of exhaled CO is being undertaken by researchers in this field; their efforts will lead to a better appreciation for how CO monitoring may fit into clinical practice.

The obvious target and ultimate goal of most research in HO-1 is to find a therapeutic use, and there have been several approaches to this issue. Gene transfer has been attempted in animals with promising results (83, 98, 123), but current limitations to this approach in humans are widely appreciated. The antioxidant and cytoprotective effects of CO, bilirubin, and ferritin have been demonstrated experimentally, but given the potential toxicities of these products of HO-1, they have yet to be used therapeutically in human studies. As noted above, high serum bilirubin levels correlate with less cardiovascular disease in humans; whether this is cause and effect, and whether bilirubin levels could safely be manipulated in humans, is unknown. Administration of exogenous CO has been shown to protect against lung injury and transplant rejection (84, 120), among other ills, but whether this poisonous gas could be given harmlessly to humans is an unanswered question. Low concentrations of inhaled CO are currently used diagnostically to estimate lung diffusing capacity in patients, so it is not unthinkable that CO could be used therapeutically. Motterlini and coworkers have described the use of chemical CO donors in the form of transition metal carbonyls (132), which represents a novel approach to administering this gas. Treatment of organ grafts with CO before transplant is another use of the gas which does not involve direct inhalation. The future will show which (if any) of these approaches will ultimately be of benefit to humans.

	Conclusion

Top Abstract Introduction Induction and Regulation of... HO-1 and Signal Transduction HO-1 and Apoptosis HO-1 and Nitric Oxide HO-1 and Diseases of... Diagnostic and Therapeutic Uses... Conclusion References

Oxidative injury is now understood to be the common denominator in manyindeed, mostpathologic processes. An antidote to oxidative damage has become a holy grail of medical research. The discovery of a molecule which is ubiquitous in nature, cytoprotective, antiapoptotic, and anti-inflammatory is cause for great excitement. It is to be hoped that investigators will one day understand the mechanisms by which HO-1 exerts its remarkable effects, and that this understanding will lead to cures for the many infirmities caused by oxidative stress. Until that time, the quest to delineate the complex interplay of HO-1 and its enzymatic products with biologic systems will continue to be pursued by a growing number of researchers in every medical discipline.