Inducible heme oxygenase in the kidney: a model for the homeostatic control of hemoglobin catabolism

Metalloporphyrins as Tools for Deciphering the Role of Heme Oxygenase in Renal Immune Injury

Renal immune injury is a frequent cause of end-stage renal disease, and, despite the progress made in understanding underlying pathogenetic mechanisms, current treatments to preserve renal function continue to be based mainly on systemic immunosuppression. Small molecules, naturally occurring biologic agents, show considerable promise in acting as disease modifiers and may provide novel therapeutic leads. Certain naturally occurring or synthetic Metalloporphyrins (Mps) can act as disease modifiers by increasing heme oxygenase (HO) enzymatic activity and/or synthesis of the inducible HO isoform (HO-1). Depending on the metal moiety of the Mp employed, these effects may occur in tandem or can be discordant (increased HO-1 synthesis but inhibition of enzyme activity). This review discusses effects of Mps, with varying redox-active transitional metals and cyclic porphyrin cores, on mechanisms underlying pathogenesis and outcomes of renal immune injury.

Hematoma Resolution In Vivo Is Directed by Activating Transcription Factor 1

RATIONALE

The efficient resolution of tissue hemorrhage is an important homeostatic function. In human macrophages in vitro, heme activates an AMPK (AMP-activated protein kinase)/ATF1 (activating transcription factor-1) pathway that directs Mhem macrophages through coregulation of HO-1 (heme oxygenase-1; HMOX1) and lipid homeostasis genes.

OBJECTIVE

We asked whether this pathway had an in vivo role in mice.

METHODS AND RESULTS

Perifemoral hematomas were used as a model of hematoma resolution. In mouse bone marrow-derived macrophages, heme induced HO-1, lipid regulatory genes including LXR (lipid X receptor), the growth factor IGF1 (insulin-like growth factor-1), and the splenic red pulp macrophage gene Spic. This response was lost in bone marrow-derived macrophages from mice deficient in AMPK (Prkab1^-/-) or ATF1 (Atf1^-/-). In vivo, femoral hematomas resolved completely between days 8 and 9 in littermate control mice (n=12), but were still present at day 9 in mice deficient in either AMPK (Prkab1^-/-) or ATF1 (Atf1^-/-; n=6 each). Residual hematomas were accompanied by increased macrophage infiltration, inflammatory activation and oxidative stress. We also found that fluorescent lipids and a fluorescent iron-analog were trafficked to lipid-laden and iron-laden macrophages respectively. Moreover erythrocyte iron and lipid abnormally colocalized in the same macrophages in Atf1^-/- mice. Therefore, iron-lipid separation was Atf1-dependent.

CONCLUSIONS

Taken together, these data demonstrate that both AMPK and ATF1 are required for normal hematoma resolution. Graphic Abstract: An online graphic abstract is available for this article.

Detection of Occult Acute Kidney Injury in Glucose-6-Phosphate Dehydrogenase Deficiency Anemia

BACKGROUND

Glucose-6-phosphate dehydrogenase (G6PD) deficiency anemia is associated with intravascular hemolysis. The freely filtered hemoglobin can damage the kidney. We aimed to assess any subclinical renal injury in G6PD children.

METHODS

Sixty children were included. Thirty G6PD deficiency anemia children were enrolled during the acute hemolytic crisis and after the hemolytic episode had elapsed. Another thirty healthy children were included as controls. Serum cystatin C, creatinine levels, and urinary albumin/creatinine (A/C) ratio were measured, and the glomerular filtration rate (GFR) was calculated.

RESULTS

Significantly higher urinary A/C ratio (p=0.001,0.002 respectively) and lower GFR (p=0.001 for both) were found during hemolysis and after the hemolytic episode compared to the controls. Also, significant higher serum cystatin C (p=0.001), creatinine (p=0.05) and A/C (p= 0.001) ratio and insignificant lower GFR (p=0.3) during acute hemolytic crisis compared to the same children after the hemolytic episode subsided.

CONCLUSIONS

G6PD deficiency anemia is associated with a variable degree of acute renal injury during acute hemolytic episodes which may persist after elapsing of the hemolytic crises.

Sodium nitrite potentiates renal oxidative stress and injury in hemoglobin exposed guinea pigs

Methemoglobin-forming drugs, such as sodium nitrite (NaNO2), may exacerbate oxidative toxicity under certain chronic or acute hemolytic settings. In this study, we evaluated markers of renal oxidative stress and injury in guinea pigs exposed to extracellular hemoglobin (Hb) followed by NaNO2 at doses sufficient to simulate clinically relevant acute methemoglobinemia. NaNO2 induced rapid and extensive oxidation of plasma Hb in this model. This was accompanied by increased renal expression of the oxidative response effectors nuclear factor erythroid 2-derived-factor 2 (Nrf-2) and heme oxygenase-1 (HO-1), elevated non-heme iron deposition, lipid peroxidation, interstitial inflammatory cell activation, increased expression of tubular injury markers kidney injury-1 marker (KIM-1) and liver-fatty acid binding protein (L-FABP), podocyte injury, and cell death. Importantly, these indicators of renal oxidative stress and injury were minimal or absent following infusion of Hb or NaNO2 alone. Together, these results suggest that the exposure to NaNO2 in settings associated with increased extracellular Hb may potentiate acute renal toxicity via processes that are independent of NaNO2 induced erythrocyte methemoglobinemia.

Drugs and acute porphyrias: reasons for a hazardous relationship

The porphyrias are a group of metabolic diseases caused by inherited or acquired enzymatic deficiency in the metabolic pathway of heme biosynthesis. Simplistically, they can be considered as storage diseases, because the partial enzymatic defect gives rise to a metabolic "bottleneck" in the biosynthetic pathway and hence to an accumulation of different metabolic intermediates, potentially toxic and responsible for the various (cutaneous or neurovisceral) clinical manifestations observed in these diseases. In the acute porphyrias (acute intermittent porphyria, hereditary coproporphyria, variegate porphyria, and the very rare delta-aminolevulinic acid dehydratase ALAD-d porphyria), the characteristic severe neurovisceral involvement is mainly ascribed to a tissue accumulation of delta-aminolevulinic acid, a neurotoxic nonporphyrin precursor. Many different factors, both endogenous and exogenous, may favor the accumulation of this precursor in patients who are carriers of an enzymatic defect consistent with an acute porphyria, thus contributing to trigger the serious (and potentially fatal) clinical manifestations of the disease (acute porphyric attacks). To date, many different drugs are known to be able to precipitate an acute porphyric attack, so that the acute porphyrias are also considered as pharmacogenetic or toxygenetic diseases. This article reviews the different biochemical mechanisms underlying the capacity of many drugs to precipitate a porphyric acute attack (drug porphyrogenicity) in carriers of genetic mutations responsible for acute porphyrias, and addresses the issue of prescribing drugs for patients affected by these rare, but extremely complex, diseases.

A computational model of liver iron metabolism

Iron is essential for all known life due to its redox properties; however, these same properties can also lead to its toxicity in overload through the production of reactive oxygen species. Robust systemic and cellular control are required to maintain safe levels of iron, and the liver seems to be where this regulation is mainly located. Iron misregulation is implicated in many diseases, and as our understanding of iron metabolism improves, the list of iron-related disorders grows. Recent developments have resulted in greater knowledge of the fate of iron in the body and have led to a detailed map of its metabolism; however, a quantitative understanding at the systems level of how its components interact to produce tight regulation remains elusive. A mechanistic computational model of human liver iron metabolism, which includes the core regulatory components, is presented here. It was constructed based on known mechanisms of regulation and on their kinetic properties, obtained from several publications. The model was then quantitatively validated by comparing its results with previously published physiological data, and it is able to reproduce multiple experimental findings. A time course simulation following an oral dose of iron was compared to a clinical time course study and the simulation was found to recreate the dynamics and time scale of the systems response to iron challenge. A disease state simulation of haemochromatosis was created by altering a single reaction parameter that mimics a human haemochromatosis gene (HFE) mutation. The simulation provides a quantitative understanding of the liver iron overload that arises in this disease. This model supports and supplements understanding of the role of the liver as an iron sensor and provides a framework for further modelling, including simulations to identify valuable drug targets and design of experiments to improve further our knowledge of this system.

Iron is an essential nutrient required for healthy life but, in excess, is the cause of debilitating and even fatal conditions. The most common genetic disorder in humans caused by a mutation, haemochromatosis, results in an iron overload in the liver. Indeed, the liver plays a central role in the regulation of iron. Recently, an increasing amount of detail has been discovered about molecules related to iron metabolism, but an understanding of how they work together and regulate iron levels (in healthy people) or fail to do it (in disease) is still missing. We present a mathematical model of the regulation of liver iron metabolism that provides explanations of its dynamics and allows further hypotheses to be formulated and later tested in experiments. Importantly, the model reproduces accurately the healthy liver iron homeostasis and simulates haemochromatosis, showing how the causative mutation leads to iron overload. We investigate how best to control iron regulation and identified reactions that can be targets of new medicines to treat iron overload. The model provides a virtual laboratory for investigating iron metabolism and improves understanding of the method by which the liver senses and controls iron levels.

Cytoprotection behind heme oxygenase-1 in renal diseases

Renal insults are considered a public health problem and are linked to increased rates of morbidity and mortality worldwide. The heme oxygenase (HO) system consists of evolutionary specialized machinery that degrades free heme and produces carbon monoxide, biliverdin and free iron. In this sense, the inducible isoform HO-1 seems to develop an important role and is widely studied. The reaction involved with the HO-1 molecule provides protection to injured tissue, directly by reducing the toxic heme molecule and indirectly by the release of its byproducts. The up regulation of HO-1 enzyme has largely been described as providing antioxidant, antiapoptotic, anti-inflammatory and immunomodulatory properties. Several works have explored the importance of HO-1 in renal diseases and they have provided consistent evidence that its overexpression has beneficial effects in such injuries. So, in this review we will focus on the role of HO-1 in kidney insults, exploring the protective effects of its up regulation and the enhanced deleterious effects of its inhibition or gene deletion.

Differential induction of renal heme oxygenase and ferritin in ascorbate and nonascorbate producing species transfused with modified cell-free hemoglobin

Abstract Heme catabolism and iron sequestration systems play an important role in regulating the response to extracellular hemoglobin (Hb). We previously reported that extracellular Hb oxidizes more readily in the circulation of guinea pigs, a nonascorbate (AA)-producing species with similar plasma and tissue antioxidant status to humans, compared to rats, an AA-producing species. To determine whether these two species exhibit differences in heme catabolism and iron sequestration at the level of the kidney, we examined heme oxygenase (HO), H- and L-ferritin expression, nonheme iron deposition, and renal AA content following transfusion with polymerized bovine hemoglobin (HbG). Both species showed similar rates of hemoglobinuria but urinary HbG was significantly more oxidized in guinea pigs. HbG enhanced HO activity in both species but appeared greater and more sustained in guinea pigs. Conversely, rats showed a greater and more rapid induction of H- and L-ferritin as well as greater iron accumulation and AA content. Furthermore, ferrous and ferric iron deposits were detected in rats while only ferric iron was observed in guinea pigs. These findings suggest significant differences in the renal handling of HbG which may be important for understanding how endogenous antioxidant defenses may modulate the renal response to extracellular Hb.

Effect of Gossypol in Association with Chromium Protoporphyrin on Heme Metabolic Enzymes

Gossypol prevents the liberation of oxygen from oxyhemoglobin and exerts a hemolytic effect on erythrocytes. In excessive dosages of gossypol, an extreme burden is placed upon the respiratory and circulatory organs owing to the reduced oxygen carrying capacity of blood. Chromium protoporphyrin (CrPP) has been shown to either competitively suppress or to significantly ameliorate a variety of naturally occurring or experimentally induced forms of jaundice in animals and man. In this communication, a novel tissue dependent response to gossypol (50 micromol/kg bw) and gossypol in association with CrPP (50 micromol/kg bw) is described. Our results revealed that gossypol stimulated the hepatic, splenic, and renal delta-aminolevulinic acid synthase (ALA-S) activity, the heme biosynthetic enzyme, and simultaneous administration of CrPP and gossypol synergized the gossypol-mediated increase of ALA-S activity. Gossypol was found to be a potent stimulator of heme oxygenase (HMOX) activity in rat liver and kidney to varying degrees. This tissue response contrasted with that of the spleen, where gossypol decreased the activity of the enzyme. In consonance with the increased hepatic and renal HMOX activity, a marked increase was observed in total serum bilirubin concentration in gossypol treated rats. When rats were given CrPP simultaneously with gossypol, the gossypol mediated increase in hepatic and renal HMOX activity was effectively blocked. Furthermore, the increase in enzymatic activity was accomplished by a decline in the total microsomal protein content on gossypol administration. These findings emphasize the toxic effect of gossypol in eliciting increased heme degradation by stimulating HMOX activity in the liver and the kidney and the potential usefulness of CrPP in experimental and perhaps clinical conditions in which hyperbilirubinemia occurs.

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

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

Comparison of apo- and heme-bound crystal structures of a truncated human heme oxygenase-2

Heme oxygenase (HO) catalyzes the first step in the heme degradation pathway. The crystal structures of apo- and heme-bound truncated human HO-2 reveal a primarily alpha-helical architecture similar to that of human HO-1 and other known HOs. Proper orientation of heme in HO-2 is required for the regioselective oxidation of the alpha-mesocarbon. This is accomplished by interactions within the heme binding pocket, which is made up of two helices. The iron coordinating residue, His(45), resides on the proximal helix. The distal helix contains highly conserved glycine residues that allow the helix to flex and interact with the bound heme. Tyr(154), Lys(199), and Arg(203) orient the heme through direct interactions with the heme propionates. The rearrangements of side chains in heme-bound HO-2 compared with apoHO-2 further elucidate HO-2 heme interactions.

Hemoglobin-based oxygen carrier induces hepatic heme oxygenase 1 expression in Kupffer cells

BACKGROUND

Kupffer cells (liver macrophages) are a key initiator of inflammation following hepatic insults such as infection, ischemia/reperfusion, and rejection. Heme oxygenase 1 (HO-1) is protective against inflammatory injury. A hemoglobin-based oxygen carrier (HBOC) has been shown to prevent organ inflammation from hemorrhagic shock as well as induce HO-1 at the cellular level. Therefore, we hypothesize that HBOC can induce Kupffer cell HO-1 production.

METHODS

Mice administered 20% blood volume HBOC or saline intravenously were sacrificed at 0, 12, 24, 48 hours (n = 4-6/group). Hepatic protein underwent Western blotting for HO-1 and heat shock protein 72. Hepatic frozen sections underwent immunofluorescent staining for HO-1/CD68.

RESULTS

Following HBOC injection, hepatic HO-1 fold change peaked at 12 hours (7.3 +/- 0.8) (p < .01), remained increased at 24 hours (4.7 +/- 0.4) (p < .01), and returned to baseline by 48 hours. HSP72 expression was unaffected in all groups. Twleve-hour liver section immunostaining confirmed significant induction of HO-1 by HBOC. Double staining for HO-1 and CD68 identified Kupffer cells as the majority of cells expressing HO-1.

CONCLUSION

HBOC induces hepatic HO-1 expression in Kupffer cells without heat shock protein response. These data provide the basis for further investigation into a clinical therapy to induce Kupffer cell HO-1 expression with the goal of attenuating the hepatic immunoresponse to various insults.

Down-regulation of heme oxygenase-2 is associated with the increased expression of heme oxygenase-1 in human cell lines

Intracellular heme concentrations are maintained in part by heme degradation, which is catalyzed by heme oxygenase. Heme oxygenase consists of two structurally related isozymes, HO-1 and HO-2. Recent studies have identified HO-2 as a potential oxygen sensor. To gain further insights into the regulatory role of HO-2 in heme homeostasis, we analyzed the expression profiles of HO-2 and the biochemical consequences of HO-2 knockdown with specific short interfering RNA (siRNA) in human cells. Both HO-2 mRNA and protein are expressed in the eight human cancer cell lines examined, and HO-1 expression is detectable in five of the cell lines, including HeLa cervical cancer and HepG2 hepatoma. Down-regulation of HO-2 expression with siRNA against HO-2 (siHO-2) caused induction of HO-1 expression at both mRNA and protein levels in HeLa and HepG2 cells. In contrast, knockdown of HO-1 expression did not noticeably influence HO-2 expression. HO-2 knockdown prolonged the half-life of HO-1 mRNA twofold in HeLa cells. Transient transfection assays in HeLa cells revealed that the 4.5-kb human HO-1 gene promoter was activated with selective knockdown of HO-2 in a sequence-dependent manner. Moreover, HO-2 knockdown caused heme accumulation in HeLa and HepG2 cells only when exposed to exogenous hemin. HO-2 knockdown may mimic a certain physiological change that is important in the maintenance of cellular heme homeostasis. These results suggest that HO-2 may down-regulate the expression of HO-1, thereby directing the co-ordinated expression of HO-1 and HO-2.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

Protein kinase Akt/PKB phosphorylates heme oxygenase-1 in vitro and in vivo

Heme oxygenase-1 (HO-1) is a stress response protein that protects cells against diverse noxious stimuli. Although regulation of HO-1 occurs mainly at the transcriptional level, its posttranslational modifications remain unexplored. We have identified a putative consensus sequence for phosphorylation by Akt/PKB of HO-1 at Ser188. Recombinant human and rat HO-1, but not mutant HO-1(S188A), are phosphorylated in vitro by Akt/PKB. Isotopic 32P-labeling of HEK293T cells confirmed that HO-1 is a phosphoprotein and that the basal HO-1 phosphorylation is increased by Akt1 activation. HO-1(S188D), a single point mutant equivalent to the phosphorylated protein, exhibited over 1.6-fold higher activity than wild type HO-1. Fluorescence resonance energy transfer (FRET) studies indicated that HO-1(S188D) bound to cytochrome P450 reductase (CPR) and biliverdin reductase (BVR) with a slightly lower Kd than wild-type HO-1. Although the changes in activity are small, this study provides the first evidence for a role of the survival kinase Akt in the regulation of HO-1.

The story so far: Molecular regulation of the heme oxygenase-1 gene in renal injury

Heme oxygenases (HOs) catalyze the rate-limiting step in heme degradation, resulting in the formation of iron, carbon monoxide, and biliverdin, the latter of which is subsequently converted to bilirubin by biliverdin reductase. Recent attention has focused on the biological effects of product(s) of this enzymatic reaction, which have important antioxidant, anti-inflammatory, and cytoprotective functions. Two major isoforms of the HO enzyme have been described: an inducible isoform, HO-1, and a constitutively expressed isoform, HO-2. A third isoform, HO-3, closely related to HO-2, has also been described. Several stimuli implicated in the pathogenesis of renal injury, such as heme, nitric oxide, growth factors, angiotensin II, cytokines, and nephrotoxins, induce HO-1. Induction of HO-1 occurs as an adaptive and beneficial response to these stimuli, as demonstrated by studies in renal and non-renal disease states. This review will focus on the molecular regulation of the HO-1 gene in renal injury and will highlight the interspecies differences, predominantly between the rodent and human HO-1 genes.

Heme oxygenase and the kidney

Heme plays a significant pathogenic role in several diseases involving the kidney. The cellular content of heme, derived either from the delivery of filtered heme proteins such as hemoglobin and myoglobin, or from the breakdown of ubiquitous intracellular heme proteins, is regulated via the heme oxygenase enzyme system. Heme oxygenases catalyze the rate-limiting step in heme degradation, resulting in the formation of iron, carbon monoxide, and biliverdin, which is subsequently converted to bilirubin by biliverdin reductase. Recent attention has focused on the biological effects of product(s) of this enzymatic reaction, which have important antioxidant, anti-inflammatory, and cytoprotective functions. Three isoforms of heme oxygenase (HO) enzyme have been described: an inducible isoform, HO-1, and two constitutively expressed isoforms, HO-2 and HO-3. Induction of HO-1 occurs as an adaptive and beneficial response to several injurious stimuli, and has been implicated in many clinically relevant disease states including atherosclerosis, transplant rejection, endotoxic shock, hypertension, acute lung injury, acute renal injury, as well as others. This review will focus predominantly on the role of HO-1 in the kidney.

Changes in (Na + K)-adenosine triphosphatase activity and ultrastructure of lung and kidney associated with oxidative stress induced by acute ethanol intoxication

STUDY AND OBJECTIVES: (Na + K)-adenosine triphosphatase (ATPase) activity, oxidative stress parameters, and morphologic characteristics of the lung and kidney of rats under acute ethanol intoxication were assessed to investigate the pathogenic mechanism of tissue damage.

DESIGN AND INTERVENTIONS

Adult rats were given ethanol (5.5 g/kg) 3 h before performing the biochemical and morphologic studies. Oxidative stress was assessed by measuring the levels of reduced glutathione (GSH) and glutathione disulfide (GSSG), the activities of key antioxidant enzymes (ie, catalase [CAT], superoxide dismutase [SOD], and glutathione peroxidase [GSH-Px]) and malondialdehyde production. (Na + K)-ATPase, a membrane-bound enzyme, also was assayed.

RESULTS

In the lung, ethanol increased MDA production by 60%, decreased GSH levels by 33%, decreased SOD and GSH-Px activity by 10%, and decreased (Na + K)-ATPase activity by 55%, whereas CAT activity was unaltered. Impaired surfactant secretion and cell adhesion of lung epithelial cells were found. In the kidney, ethanol did not influence the activity of (Na + K)-ATPase or lipid peroxidation, despite the reduction of both GSH and the GSH/GSSG ratio. Focally thickened glomerular basement membrane, apoptosis of foot processes, and tubulointerstitial fibrosis were found.

CONCLUSIONS

These data suggest that oxidative stress plays a role in mediating the ethanol-induced down-regulation of lung (Na + K)-ATPase. GSH depletion seems to be a major determinant of this effect. Independent mechanisms seem to account for the morphologic alterations of these organs.

Heme biosynthesis and oogenesis in the blood-sucking bug, Rhodnius prolixus

We have previously shown that the pathway of porphyrin synthesis operates in the blood feeding triatomine bug Rhodnius prolixus but not in the cattle tick Boophilus microplus. In the present paper we studied the correlation between heme synthesis and egg development in Rhodnius. There is a sharp increase heme biosynthetic capability in the fat body (160%) and in the ovaries (360%) in response to a blood meal, as evaluated from the activity of the enzyme delta-aminolevulinate dehydratase (EC 4.2.1.24). The in vivo inhibition of ALA-D by succinyl acetone results in a dose dependent decrease of oviposition. Oviposition is recovered when porphobilinogen, the product of the impaired reaction, is added to the succinyl acetone enriched blood. Taken together, these results show that heme biosynthesis is a fundamental event to vitellogenic females. The demand for heme in this metabolic juncture cannot be supplied by the heme eventually absorbed during blood digestion and associated with Rhodnius heme binding protein (RHBP), which is then incorporated into growing oocytes. Inhibition of heme biosynthesis results in lower levels of RHBP in the hemolymph, suggesting that the synthesis of this protein is controlled by heme availability.

Oxidative stress and induction of heme oxygenase-1 in the kidney in sickle cell disease

Chronic nephropathy is a recognized complication of sickle cell disease. Using a transgenic sickle mouse, we examined whether oxidative stress occurs in the sickle kidney, the origins and functional significance of such oxidant stress, and the expression of the oxidant-inducible, potentially protective gene, heme oxygenase-1 (HO-1); we also examined the expression of HO-1 in the kidney and in circulating endothelial cells in sickle patients. We demonstrate that this transgenic sickle mouse exhibits renal enlargement, medullary congestion, and a reduced plasma creatinine concentration. Oxidative stress is present in the kidney as indicated by increased amounts of lipid peroxidation; heme content is markedly increased in the kidney. Exacerbation of oxidative stress by inhibiting glutathione synthesis with buthionine-sulfoximine dramatically increased red blood cell sickling in the sickle kidney: in buthionine-sulfoximine-treated sickle mice, red blood cell sickling extended from the medulla into the cortical capillaries and glomeruli. HO activity is increased in the sickle mouse kidney, and is due to induction of HO-1. In the human sickle kidney, HO-1 is induced in renal tubules, interstitial cells, and in the vasculature. Expression of HO-1 is increased in circulating endothelial cells in patients with sickle cell disease. These results provide the novel demonstration that oxidative stress occurs in the sickle kidney, and that acute exacerbation of oxidative stress in the sickle mouse precipitates acute vaso-occlusive disease. Additionally, the oxidant-inducible, heme-degrading enzyme, HO-1, is induced regionally in the murine and human sickle kidney, and systemically, in circulating endothelial cells in sickle patients.

New features of renal lesion induced by stroma free hemoglobin

This study focused on the subacute renal lesions resulting from the infusion of stroma free hemoglobin (SFH), which remains under evaluation as a potential blood substitute despite limited renal toxicity observed in acute infusion. Four groups of rats received different doses of SFH (0.03, 0.48, 0.96, and 1.46 g, respectively) and were monitored, on alternate days, for their glomerular filtration rate over the course of 10 days. Another group of 6 rats receiving 0.96 g SFH was sacrificed at day 10 for examination of renal morphology. The low dose (0.03 g) of SFH infusion did not alter the creatinine clearance (Clcr) over 10 days. The Clcr decreased in rats receiving 0.48 g SFH but fully recovered at day 10. A persistent decrease in Clcr was observed in the groups of rats receiving 0.96 and 1.68 g of SFH. Tubular necrosis was the most prominent renal lesion distributed in the proximal tubules, especially in the convoluted segment of the juxtamedullary nephrons. Pearls' stained cytoplasmic granules and electron-dense lysosomal granules were found in surviving proximal tubules. Necrosis was the predominant mechanism of cell death. This study revealed for the first time proliferation of smooth endoplasmic reticulum in the proximal tubules after SFH treatment, where it appeared as nodular aggregates of tubulovesicular structures. The effect of SFH on the proximal tubule appeared to be a direct toxicity, and this toxicity was shown to be dose dependent. The presence of reversible toxicity indicated that a safety limit dosage for SFH infusion exists and that tolerance dose of SFH can be determined for clinical applications.

Involvement of haemoxygenase-1 in ozone-induced airway inflammation and hyperresponsiveness

Haemoxygenase catalyses the degradation of haem to bilirubin, and the inducible form of haemoxygenase, haemoxygenase-1, is highly induced in response to oxidative stress in vitro. The effect of haemoxygenase-1 in oxidant stress in vivo is not known. We determined the effect of exposure to ozone on haemoxygenase-1 expression, and the modulation of haemoxygenase-1 expression on ozone-induced lung neutrophilia and bronchial hyperresponsiveness in rats. Ozone caused a significant induction of lung haemoxygenase-1. Pretreatment of rats with haemoglobin, a potent inducer of haemoxygenase-1, resulted in a large induction of haemoxygenase-1 expression, and inhibited ozone-induced neutrophilia and bronchial hyperresponsiveness. Tin protoporphyrin, a competitive inhibitor of haemoxygenase, reduced the expression of haemoxygenase-1 induced by haemoglobin. It enhanced ozone-induced neutrophilia, but not the bronchial hyperresponsiveness, and reduced the protective effect of haemoglobin. Overall, there was an association between bronchial hyperresponsiveness and the neutrophilic response. These data indicate that haemoxygenase-1 plays an important role in modulating the effects of an oxidant, such as ozone in the lungs.

Prior induction of heat shock proteins by a nitric oxide donor attenuates cardiac ischemia/reperfusion injury in the rat

BACKGROUND

Recent studies have demonstrated that nitric oxide (NO) releasers considerably increase heat shock proteins (HSPs) in the in vitro cell system, providing resistance to oxidant damage. This study was designed to examine the cellular responses of HSPs induced by prior administration of an NO releaser, FK409 (FK), in an in vivo transplantation model.

METHODS

Lewis rats received either saline or FK solution intravenously administered at different time points before graft harvesting (10 micromol/kg) or for 15 min during reperfusion (0.66 micromol/kg/min). Tissue specimens were taken to determine HSP70 and heme oxygenase-1/HSP32 (HO-1) expression, and glutathione content. After 24-hr preservation with University of Wisconsin solution, heterotopic cardiac transplantations were performed, and graft survival was determined at 14 days. Tissue samples for end labeling of nuclear DNA fragments (TdT-mediated d-uridine triphosphate biotin nick end labeling; TUNEL) and propidium iodide staining were taken 15 min after reperfusion.

RESULTS

The gene and protein expression of HSP70 after FK administration peaked at 12 min and 60-90 min, whereas those of HO-1 peaked at 6 min and 90 min, respectively. Then, representative cardiac grafts taken 60 min after FK treatment were examined for further assay. Localization of induced HSP70 and HO-1 molecules were observed in the myocardium and vascular endothelium, respectively. Prior treatment of FK was effective in preventing the reduction of tissue glutathione contents compared with control (P<0.05). Fewer TUNEL and propidium iodide-positive cells were also observed in the FK group (P<0.0005, vs. control). The graft survival rate was higher in the FK group (9/10 vs. 1/10 of control; P<0.001), whereas the groups either harvested 10 min after FK pretreatment or continuously infused for 15 min during reperfusion were inferior, similar to that of control.

CONCLUSION

Prior induction of HSP70 and HO-1 with a relatively low dose of FK administration attenuates ischemia and reperfusion injury, which was due to antioxidant and antiapoptotic activities augmented by such stress proteins. Thus, NO releasers as a pharmacological maneuver may provide an innovative approach for the prevention of ischemia and reperfusion injury.

Acute renal failure after cardiopulmonary bypass in related to decreased serum ferritin levels

Acute renal failure (ARF) requiring dialysis occurs in up to 4% of patients after cardiopulmonary bypass (CPB). CPB leads to the generation of intravascular free hemoglobin, resulting in increased endothelial and renal tubular cell free iron, which is associated with renal injury. Conversely, renoprotection is conferred by processes that upregulate heme and iron sequestration pathways, such as ferritin. This study evaluates the influence of free hemoglobin generation during CPB and the capacity to sequester free iron on the occurrence of post-CPB renal insufficiency. Thirty consecutive patients undergoing CPB were enrolled in the study. Serum creatinine, free hemoglobin, and ferritin were measured preoperatively, at the end of bypass, and 24 and 48 h after surgery. Renal injury, as determined by an increase in the serum creatinine of > or =25% (ARF) by 48 h after surgery, occurred in 40% (12 of 30) of patients, and dialysis was necessary in 6.6% (2 of 30). Free hemoglobin levels increased in all patients but did not correlate with postoperative ARF. However, patients with preoperative serum ferritin levels < or =130 microg/L, the median value for the group, had a sixfold greater likelihood of developing ARF compared to patients with levels above this value (P = 0.03). Lower serum ferritin levels appear to be associated with the development of ARF. Serum ferritin levels may signify intravascular as well as endothelial and renal epithelial cell ability to bind free iron generated during CPB-induced hemolysis, and thus may help provide information regarding the risk for ARF.

Identification of enzymes responsible for the metabolism of heme in human platelets

The major enzymes involved in the degradation of heme were identified in human platelets. It was determined that heme oxygenase activity levels in umbilical cord blood platelets were higher, whereas biliverdin reductase activity levels were comparable with that found in platelets from adults. In membranes prepared from adenosine diphosphate-activated platelets, UDP-glucuronic acid-dependent bilirubin conjugation was detected, whereas activity was negligible in unactivated platelets and undetected in serum and heat-inactivated platelets, and in platelets prepared from umbilical cord blood. Platelet fractions were analyzed by Western blot and shown to express heme oxygenase, biliverdin reductase, and UDP-glucuronosyltransferases, and there was concordance with known developmental profiles found in other tissues. Heme oxygenase expression was higher, whereas UGT expression was lower, in neonatal compared with adult platelets. These data suggest that platelets are involved in multiple steps of heme and bilirubin metabolism and that developmental regulation of these enzymes may be similar to that in other human tissues.

Lung-specific induction of heme oxygenase-1 and hyperoxic lung injury

Heme oxygenase (HO)-1, which catalyzes heme breakdown, is induced by oxidative stress and may protect against oxidative injury. We hypothesized that induction of HO-1 by hemoglobin (Hb) in the lung would protect the rat from pulmonary O2 toxicity. Rats given intratracheal (i.t.) Hb showed lung-specific induction of HO-1 by 8 h by Western analysis. Rats were then pretreated for 8 h before 60 h of exposure to 100% O2 with either IT normal saline, HB, or Hb plus the HO-1 inhibitor tinprotoporphyrin (SnPP). Both the Hb + O2 and Hb + O2 + SnPP animals had less lung injury than normal saline controls as indicated by lower pleural fluid volumes and wet-to-dry weight ratios (P < 0.01). The improvement in injury in the two Hb-treated groups was the same despite a 61% decrease in HO enzyme activity in the Hb + SnPP group after 60 h of O2. In addition, inhibition of HO activity with SnPP alone before O2 exposure did not augment the extent of hyperoxic lung injury. These results demonstrate that IT Hb induces lung HO-1 in the rat and protects against hyperoxia; however, the protection is not mediated by increased HO enzyme activity.

Glomerular inflammation induces resistance to tubular injury in the rat. A novel form of acquired, heme oxygenase-dependent resistance to renal injury

Considerable attention is directed to a surprising biologic phenomenon wherein tissues exposed to one insult acquire resistance to another. We identify a novel example of acquired resistance to acute renal failure and a mechanism that contributes to such resistance. Nephrotoxic serum, administered to rats 24 h before the induction of glycerol-induced acute renal failure, reduces functional and structural injury that occurs in this model. Since heme oxygenase, the rate-limiting enzyme in heme degradation, protects against heme protein-induced renal injury, we questioned whether induction of heme oxygenase underlies the protection afforded by nephrotoxic serum. Kidney heme oxygenase (HO-1) mRNA was induced 6 h after nephrotoxic serum and renal tubules were identified as the site of expression of heme oxygenase protein. Induction of heme oxygenase was accompanied by increased renal content of ferritin but not by induction of other antioxidant enzymes. Inhibition of heme oxygenase prevented the protection afforded by nephrotoxic serum. Nephrotoxic serum did not protect against ischemic acute renal failure, a model in which heme oxygenase is not induced. Thus, nephrotoxic serum protects against glycerol-induced acute renal failure by inducing heme oxygenase in tubules. This study provides the first demonstration of resistance to tubular injury acquired from glomerular inflammation, uncovers a mechanism for such resistance, and exposes the dialogue that occurs between glomeruli and tubules.

Renal oxidant injury and oxidant response induced by mercury

The role of oxidative stress in mercuric chloride (HgCl2)-induced nephrotoxicity is uncertain and controversial. We demonstrate that I.L.C-PK1 cells, exposed to HgCl2, generate massive amounts of hydrogen peroxide, the latter completely quenched by the hydrogen peroxide scavenger, pyruvate. HgCl2 exerts a dose-dependent cytotoxicity which is attenuated by pyruvate and catalase. Cellular generation of hydrogen peroxide arises, at least in part, from mitochondria since mitochondrial rates of generation of hydrogen peroxide increase in response to HgCl2; HgCl2 also provokes a shift in absorbance spectra in rhodamine 123 loaded-mitochondria and stimulates mitochondrial state 4 respiration. HgCl2, applied for one hour, impairs cellular vitality as demonstrated by the MTT assay, an assay dependent in part on mitochondrial function. HgCl2 impairs function in other organelles such as lysosomes that maintain a transmembrane proton gradient; these latter effects are partially attenuated by pyruvate. We complement these in vitro findings with in vivo evidence demonstrating that HgCl2 stimulates renal generation of hydrogen peroxide. The functional significance of such generation of hydrogen peroxide was evaluated in rats deficient in selenium and vitamin E, a nutrient deficiency that impairs the scavenging of hydrogen peroxide and promotes the toxicity of this oxidant. In these rats serum creatinine values were significantly higher on sequential days following the administration of HgCl2. To probe the renal response to oxidative stress induced by HgCl2, we examined hydrogen peroxide-scavenging enzymes and redox-sensitive genes. Catalase activity was unaltered whereas glutathione peroxidase activity was decreased, effects that may contribute to the net renal generation of hydrogen peroxide. The redox sensitive enzyme, heme oxygenase, was markedly up-regulated in the kidney in response to HgCl2. HgCl2 also induced members of the bcl family, bcl2 and bclx, genes that protect against apoptosis and oxidant injury. In another model of oxidant-induced renal injury, the glycerol model, bcl2 mRNA was not induced at 6 and 24 hours after the administration of glycerol. In summary, we demonstrate that HgCl2 potently stimulates renal generation of hydrogen peroxide in vitro and in vivo and such generation of peroxide contributes to renal dysfunction in vitro and in vivo. We also demonstrate that in response to HgCl2, redox sensitive genes are expressed including heme oxygenase and members of the bcl family.

Identification of a cis-regulatory element and putative trans-acting factors responsible for 12-O-tetradecanoylphorbol-13-acetate (TPA)-mediated induction of heme oxygenase expression in myelomonocytic cell lines

The human monocytic leukemia cell line THP-1 differentiates into macrophage-like cells when treated with a variety of agents, including 12-O-tetradecanoylphorbol-13-acetate (TPA). We show here that during this process, the expression of heme oxygenase, a rate-limiting enzyme in heme catabolism, is induced. Treatment with TPA increases heme oxygenase mRNA in other myelomonocytic cell lines also, but not in cell lines of other lineages, such as HeLa cells. Increased heme oxygenase activity may represent one of the functions of activated macrophages, which sequestrate senescent erythrocytes and degrade heme derived from hemoglobin. This cell-type-specific induction by TPA treatment further investigated with respect to transcriptional regulation. We defined a cis-regulatory element, 5'-GTCATATGAC-3', located in the 5'-flanking region (positions -156 to -147) of the human heme oxygenase gene, which confers inducibility by TPA in THP-1 cells but not in HeLa cells. Nuclear proteins that bind to this element, which may be responsible for the cell specificity, were identified in THP-1 nuclear extracts. This element contains the consensus motif CANNTG, to which a large family of basic helix-loop-helix proteins binds. Our results suggest a novel mechanism of TPA-mediated transcriptional regulation in myelomonocytic cell lines.

Identification of a cis-regulatory element and putative trans-acting factors responsible for 12-O-tetradecanoylphorbol-13-acetate (TPA)-mediated induction of heme oxygenase expression in myelomonocytic cell lines

The human monocytic leukemia cell line THP-1 differentiates into macrophage-like cells when treated with a variety of agents, including 12-O-tetradecanoylphorbol-13-acetate (TPA). We show here that during this process, the expression of heme oxygenase, a rate-limiting enzyme in heme catabolism, is induced. Treatment with TPA increases heme oxygenase mRNA in other myelomonocytic cell lines also, but not in cell lines of other lineages, such as HeLa cells. Increased heme oxygenase activity may represent one of the functions of activated macrophages, which sequestrate senescent erythrocytes and degrade heme derived from hemoglobin. This cell-type-specific induction by TPA treatment further investigated with respect to transcriptional regulation. We defined a cis-regulatory element, 5'-GTCATATGAC-3', located in the 5'-flanking region (positions -156 to -147) of the human heme oxygenase gene, which confers inducibility by TPA in THP-1 cells but not in HeLa cells. Nuclear proteins that bind to this element, which may be responsible for the cell specificity, were identified in THP-1 nuclear extracts. This element contains the consensus motif CANNTG, to which a large family of basic helix-loop-helix proteins binds. Our results suggest a novel mechanism of TPA-mediated transcriptional regulation in myelomonocytic cell lines.

Endothelial activation and the kidney: vasomediator modulation and antioxidant strategies

We have come to appreciate that the endothelium plays a major role in regulation of renal hemodynamics and excretory function. In the normal state, the endothelium maintains an intricate balance of interacting relaxing and contracting factors that can influence vasomotor tone and renal sodium handling, but also plays a role in the control of the coagulation system and cellular proliferation. Studies of reactive oxygen species as mediators of endothelial injury have shown that the perturbed endothelium can respond to such a threat, calling on intrinsic protective mechanisms such as induction of heme oxygenase and ferritin synthesis. In vivo studies have demonstrated that these mechanisms may confer protection in experimental models of acute renal injury. However, when endothelial injury or dysfunction does occur, adverse renal hemodynamic consequences, systemic hypertension, enhanced platelet aggregation, and mesangial cell proliferation could all contribute to progressive renal dysfunction. The role of the endothelium in modulation of normal renal function and in the pathogenesis of renal diseases will be the focus of future research efforts.

Identification of a nuclear protein that constitutively recognizes the sequence containing a heat-shock element. Its binding properties and possible function modulating heat-shock induction of the rat heme oxygenase gene

Heme oxygenase is an essential enzyme in heme catabolism, and also known as a 32-kDa heat-shock protein in rat. The rat heme-oxygenase gene promoter contains a functional heat-shock element (HSE) designated as HSE1 (-290 to -276 from the transcriptional initiation site), which consists of three copies of a 5-bp unit (5'-NGAAN-3';-->) in alternating orientation. Here we identified a putative HSE (-221 to -212), designated as HSE2, consisting of an inverted repeat of this 5-bp unit (<==>). Using transient expression assays, we show that HSE1 is sufficient to confer the heat-inducibility (a three fold to fourfold increase) on the reporter gene located downstream from the rat heme-oxygenase gene promoter, but HSE2 alone is not, suggesting that HSE2, a HSE of a tail-to-tail configuration, is not functional in vivo. However, the presence of both HSE1 and HSE2 in the promoter region increased the heat-mediated induction of the reporter-gene expression by more than 15-fold. Gel mobility-shift assays indicate that both HSE1 and HSE2 are recognized by activated heat-shock factor present only in heat-shocked rat glioma cells. Interestingly, the sequence containing HSE2 is also bound by a protein that is present in nuclear extracts prepared from either heat-shocked or non-shocked glioma cells, but this nuclear protein is unable to bind to HSE1. We suggest that a protein binding to the sequence containing HSE2 may be involved in transcriptional regulation of the rat heme oxygenase gene under thermal stress.

Induction of heme oxygenase is a rapid, protective response in rhabdomyolysis in the rat

Heme proteins such as myoglobin or hemoglobin, when released into the extracellular space, can instigate tissue toxicity. Myoglobin is directly implicated in the pathogenesis of renal failure in rhabdomyolysis. In the glycerol model of this syndrome, we demonstrate that the kidney responds to such inordinate amounts of heme proteins by inducing the heme-degradative enzyme, heme oxygenase, as well as increasing the synthesis of ferritin, the major cellular repository for iron. Prior recruitment of this response with a single preinfusion of hemoglobin prevents kidney failure and drastically reduces mortality (from 100% to 14%). Conversely, ablating this response with a competitive inhibitor of heme oxygenase exacerbates kidney dysfunction. We provide the first in vivo evidence that induction of heme oxygenase coupled to ferritin synthesis is a rapid, protective antioxidant response. Our findings suggest a therapeutic strategy for populations at a high risk for rhabdomyolysis.

Renal heme metabolism in hereditary tyrosinemia: use of succinylacetone in rat renal tubules

Succinylacetone (SA), a metabolic end-product found in urine from individuals with hereditary tyrosinemia and associated renal Fanconi syndrome and a known inhibitor of hepatic 5-aminolevulinic acid dehydratase (ALAD), has been used to study heme metabolism in isolated rat renal tubules. Heme biosynthetic porphyrin precursors are increased selectively in the presence of 4 mmol/1 SA. Total porphyrin content of the tubules are increased approximately 2-fold, while both ferrochelatase and heme oxygenase activities remain unaffected by SA. Nonetheless, total heme content is reduced, as was incorporation of radioactive label from amino[14C]levulinic acid. Cytochrome P-450 content remained unaffected. Impairment of iron uptake and/or transport within the cell or enhancement of heme catabolism via a non-heme oxygenase-dependent pathway could explain the observations.

Status of hepatic heme and heme oxygenase during Plasmodium yoelii nigeriensis infection in mice

Plasmodium yoelii nigeriensis infection in albino mice caused a significant increase in hepatic heme level, the increase being concomitant with a rise in parasitemia. This elevated heme was found to be associated with all the subcellular fractions except the cytosol, where its content remained unaltered. Activity of heme oxygenase, the key enzyme responsible for catabolism of heme, also increased progressively with rise in parasitemia. Treatment of normal mice with cobalt chloride [60 mg (kg body wt)-1; subcutaneously] brought about a 150% increase in the level of heme oxygenase; similar treatment of infected mice at low parasitemia could induce the enzyme activity while at high parasitemia the enzymic activity remained unaltered as compared to untreated infected mice. In spite of an increased level of heme oxygenase in the cobalt-treated mice, the level of heme did not show any noticeable change. Oral administration of chloroquine [64 mg (kg body wt)-1 x 4 days] brought about a 56% reduction in the level of heme oxygenase of normal animals but there was no change in infected animals when compared with the corresponding untreated infected mice. However, the amount of chloroquine present in livers of normal and infected animals was not significantly different.

Heme oxygenase-2 mRNA: developmental expression in the rat liver and response to cobalt chloride

We have previously identified two isozymes of heme oxygenase, HO-1 and HO-2, in the rat liver exhibiting vastly different molecular biochemical properties, including their responses to various chemical inducers. In the present study of the livers of fetal and newborn rats (-1 day to 21 days) we observed marked differences in the developmental pattern of expression of the transcripts for the two forms of heme oxygenase. In addition, the transcripts for HO-1 and HO-2 vary in their number and response to treatment with cobalt chloride. Specifically, using a full length cDNA probe for HO-2, we observed the 1.3-kb mRNA previously shown to encode HO-2 in the testis, as well as a second less abundant transcript of 1.9 kb. An HO-1 cDNA probe detected only the anticipated single transcript of 1.8 kb. The abundance of each HO-2 homologous transcript was unaffected by cobalt chloride treatment (4 or 24 h) in newborn (7 days old) and adult rats. The 1.8-kb HO-1 mRNA, however, was markedly increased in abundance in both age groups by this treatment. The 1.3-kb HO-2 mRNA level nearly doubled after parturition, but remained essentially unchanged during the first 2 weeks of life, and only modestly increased by age 21 days. At all stages of development, the relative abundance of the 1.3- and 1.9-kb transcripts remained unchanged. In the case of 1.8-kb HO-1 mRNA, the level of mRNA was relatively constant during the first week of life, but was substantially reduced by age 14 days, and was further decreased by age 21 days and in the adult animals. In adult rats, the abundance of the 1.3-kb HO-2 mRNA did not markedly increase over that detected in the course of the early postparturition developmental period. The data suggest that the abundance of the two HO-2 homologous transcripts is not readily subject to regulation by chemicals at any stage of development. The expression of HO-1, on the other hand, is subject to such regulation throughout life, and its high transcript abundance in newborn rats may well reflect availability of an endogenous inducer.

Regulation of heme oxygenase gene expression by cobalt in rat liver and kidney

The effect of heavy metals such as cobalt chloride and the corresponding metalloporphyrin on the transcription of the heme oxygenase gene in tissues was examined using cDNA for rat heme oxygenase as the probe. An increase in heme oxygenase mRNA level was observed in response to cobalt chloride and cobalt protoporphyrin treatment in both liver and kidney. Quantitative evaluation of the heme oxygenase transcript was obtained by determining the intensity of mRNA bands by scanning densitometry, and indicated that cobalt chloride increased heme oxygenase mRNA by 40-60-fold after 2 h of metal exposure. Accumulation of heme oxygenase mRNA after cobalt chloride administration was prevented by co-administration of actinomycin D or cycloheximide. These results indicate that the increased expression of heme oxygenase by cobalt chloride required de novo protein synthesis and was regulated at the transcriptional level. The time course of heme oxygenase transcript accumulation following administration of cobalt protoporphyrin was different from that of cobalt chloride. There was a sharp increase in heme oxygenase mRNA after cobalt chloride administration at 2 h and cobalt protoporphyrin at 10 h. Heme, to which cobalt protoporphyrin is structurally analogous, acted as a potent inducer of heme oxygenase transcripts in both liver and kidney. Variation in heme oxygenase mRNA levels resulting from enhanced transcription of the heme oxygenase gene was evaluated by nuclear runoff assay using isolated rat liver nuclei after cobalt chloride administration. Quantification of specific nuclear RNAs labeled during the in vitro transcription revealed active heme oxygenase gene transcription in liver nuclei from cobalt-chloride-treated rats. Transcription of heme oxygenase is greatly increased within 1 h of administration of cobalt chloride in rat liver, as evidenced by the level of [alpha-32P]UTP incorporation into nuclear RNA. The transcription was increased by 40-fold after 3 h of cobalt chloride administration. The activation of the heme oxygenase gene by metal ions is the most rapid transcriptional response to heavy metals yet described and highlights the regulatory role of heme oxygenase in heme degradation during deviating environmental conditions. On the other hand, cobalt protoporphyrin and heme arginate increase transcription of the heme oxygenase gene in a similar pattern but at a slower rate than that of the heavy metal, suggesting that the heme oxygenase promotor region may contain additional elements conferring the inducing effect of these two agents.

Interaction of upstream stimulatory factor with the human heme oxygenase gene promoter

Upstream stimulatory factor (USF), originally identified in HeLa cells, interacts with the upstream promoter sequence of adenovirus 2 major late promoter (Ad2MLP) and activates its transcription. USF is present in uninfected HeLa cells and appears to be involved in the transcription of cellular genes related to stress. Recently, we have proposed that the rat heme oxygenase gene, newly identified heat-shock protein gene, is regulated at least in partly by a rat homolog of USF [Sato, M., Fukushi, Y., Ishizawa, S., Okinaga, S., Muller, R.M. & Shibahara, S. (1989) J. Biol. Chem. 264, 10251-10260]. We therefore confirm that the heme oxygenase gene is expressed in HeLa cells and its expression is increased by cadmium, suggesting that human heme oxygenase is a stress protein similar to the metallothioneins. Using partially purified USF from HeLa cells, we show that USF binds to the human heme oxygenase gene promoter and stimulates its cell-free transcription. The cis-acting element, identified as CACGTGACCCG, is located 34 bp upstream from the transcription initiation site, and contains the core sequence of the upstream promoter sequence of Ad2MLP. We propose that USF contributes to the transcription of the human heme oxygenase gene.

Induction of haem oxygenase as a defence against oxidative stress

Cells respond to metabolic perturbations by producing specific stress proteins. Exposure of mammalian cells to various forms of oxidative stress induces haem oxygenase, the rate-limiting enzyme in haem degradation. This response is proposed to represent an antioxidant defence operating at two different stages simultaneously. It (i) decreases the levels of the potential pro-oxidants haem and haem proteins such as cytochrome P-450 and protoporphyrinogen oxidase, and (ii) increases the tissue concentrations of antioxidatively active bile pigments.

Structural organization of the human heme oxygenase gene and the function of its promoter

Human heme oxygenase is induced by its substrate heme, but not induced by heat shock [Yoshida et al. (1988) Eur. J. Biochem. 171, 457-461]. In order to study the molecular mechanisms of heme-mediated induction of human heme oxygenase, we have isolated and characterized the genomic clones for heme oxygenase. The human heme oxygenase gene (HO gene) is about 14 kb long and organized into five exons. The transcription initiation site was identified by S1 nuclease mapping and primer-extension analyses. Using HeLa whole cell extracts, we confirmed that the transcription of the cloned HO gene is initiated accurately at the assigned initiation site. In its 5'-flanking region, a potential heat-shock element (HSE) is present 367 bp upstream from the initiation site, although, in contrast to rat heme oxygenase, human enzyme is not induced by heat shock. We therefore analyzed the effects of heat shock on the transient expression of chimeric fusion genes harboring the promoter of the human HO gene ligated to the Escherichia coli gene gpt in mouse amelanotic melanoma cells. The 5'-flanking region of the human HO gene bearing a potential HSE failed to confer the heat-inducibility of gpt RNA production, suggesting that the HSE of the human HO gene is not functional.

Properties of human kidney heme oxygenase: inhibition by synthetic heme analogues and metalloporphyrins

Heme oxygenase activities in human kidney microsomes were found to be from 0.238 to 0.620 nmol of bilirubin/mg/hr (mean 0.375, SD 0.134), which represent approximately 30% of activities determined for human adult liver. There was interindividual variation in heme oxygenase activity of a 2-5-fold difference. Rabbits were immunized with purified human liver heme oxygenase and the resulting antibody preparation was used to examine the species specificity of the enzyme. Microsomal protein with a molecular weight of 32,000 from human kidney was identified on Western blots by its reaction with the anti-heme oxygenase liver antibody similar to the purified enzyme protein. Thus, a homology exists between human hepatic and kidney heme oxygenase. The enzyme activity was sensitive to inhibition by metalloporphyrins, such as tin-protoporphyrin IX and, to a lesser degree, by zinc and cobalt protoporphyrin IX. In a study of different synthetic heme analogues for in vitro inhibition of heme oxygenase, we found that replacement of iron by zinc in deuteroporphyrin IX 2,4 bis glycol dramatically potentiated the inhibition of heme oxygenase activity. This finding demonstrated that zinc deuteroporphyrin IX 2,4 bis glycol is a most potent inhibitor of heme oxygenase activity.