Effects of hypoxia on heme oxygenase expression in human chorionic villi explants and immortalized trophoblast cells

Haem oxygenases play a pivotal role in placental physiology and pathology

BACKGROUND

Haem oxygenases (HO) catabolise haem, which is the prosthetic group of numerous haemoproteins. Thus, multiple primary cellular pathways and functions rely on haem availability. HO exists in two isoforms, both expressed in the placenta, namely HO-1 and HO-2, the first being inducible. Haem oxygenases, particularly HO-1, have garnered specific interest in the field of physiological and pathological placental function. These enzymes mediate haem degradation by cleaving the alpha methene bridge to produce biliverdin, which is subsequently converted to bilirubin, carbon monoxide and iron. HO-1 has anti-inflammatory and antioxidant activities.

SEARCH METHODS

An initial literature analysis was performed using PubMed on 3 October 2018 using key terms such as 'haem oxygenase and pregnancy', 'haem oxygenase and placenta', 'HO-1 and pregnancy', 'HO-1 and placenta', 'HO and placenta', 'HO and pregnancy', 'genetic variant and HO', 'CO and pregnancy', 'CO and placenta', 'Bilirubin and pregnancy', 'Iron and pregnancy' and 'PPAR and Haem', selecting consensus conferences, recommendations, meta-analyses, practical recommendations and reviews. A second literature analysis was performed, including notable miscarriages, foetal loss and diabetes mellitus, on 20 December 2019. The three authors studied the publications independently to decipher whether they should be included in the manuscript.

OBJECTIVE AND RATIONALE

This review aimed to summarise current pieces of knowledge of haem oxygenase location, function and regulation in the placenta, either in healthy pregnancies or those associated with miscarriages and foetal loss, pre-eclampsia, foetal growth restriction and diabetes mellitus.

OUTCOMES

HO-1 exerts some protective effects on the placentation, probably by a combination of factors, including its interrelation with the PGC-1α/PPAR pathway and the sFlt1/PlGF balance, and through its primary metabolites, notably carbon monoxide and bilirubin. Its protective role has been highlighted in numerous pregnancy conditions, including pre-eclampsia, foetal growth restriction, gestational diabetes mellitus and miscarriages.

WIDER IMPLICATIONS

HO-1 is a crucial enzyme in physiological and pathological placentation. This protective enzyme is currently considered a potential therapeutic target in various pregnancy diseases.

Effects of selenium on the survival and invasion of trophoblasts

Objective

Placental oxidative stress is known to be a factor that contributes to pregnancy failure. The aim of this study was to determine whether selenium could induce antioxidant gene expression and regulate invasive activity and mitochondrial activity in trophoblasts, which are a major cell type of the placenta.

Methods

To understand the effects of selenium on trophoblast cells exposed to hypoxia, the viability and invasive activity of trophoblasts were analyzed. The expression of antioxidant enzymes was assessed by reverse-transcription polymerase chain reaction. In addition, the effects of selenium treatment on mitochondrial activity were evaluated in terms of adenosine triphosphate production, mitochondrial membrane potential, and reactive oxygen species levels.

Results

Selenium showed positive effects on the viability and migration activity of trophoblast cells when exposed to hypoxia. Interestingly, the increased heme oxygenase 1 expression under hypoxic conditions was decreased by selenium treatment, whereas superoxide dismutase expression was increased in trophoblast cells by selenium treatment for 72 hours, regardless of hypoxia. Selenium-treated trophoblast cells showed increased mitochondrial membrane potential and decreased reactive oxygen species levels under hypoxic conditions for 72 hours.

Conclusion

These results will be used as basic data for understanding the mechanism of how trophoblast cells respond to oxidative stress and how selenium promotes the upregulation of related genes and improves the survival rate and invasive ability of trophoblasts through regulating mitochondrial activity. These results suggest that selenium may be used in reproductive medicine for purposes including infertility treatment.

Heme oxygenase-1 in pregnancy and cancer: similarities in cellular invasion, cytoprotection, angiogenesis, and immunomodulation

Pregnancy can be defined as a “permissible” process, where a semi-allogeneic fetus and placenta are allowed to grow and survive within the mother. Similarly, in tumor growth, antigen-specific malignant cells proliferate and evade into normal tissues of the host. The microenvironments of the placenta and tumors are amazingly comparable, sharing similar mechanisms exploited by fetal or cancer cells with regard to surviving in a hypoxic microenvironment, invading tissues via degradation and vasculogenesis, and escaping host attack through immune privilege. Heme oxygease-1 (HO-1) is a stress-response protein that has antioxidative, anti-apoptotic, pro-angiogenic, and anti-inflammatory properties. Although a large volume of research has been published in recent years investigating the possible role(s) of HO-1 in pregnancy and in cancer development, the molecular mechanisms that regulate these “yin-yang” processes have still not been fully elucidated. Here, we summarize and compare pregnancy and cancer development, focusing primarily on the function of HO-1 in cellular invasion, cytoprotection, angiogenesis, and immunomodulation. Due to the similarities of both processes, a thorough understanding of the molecular mechanisms of each process may reveal and guide the development of new approaches to prevent not only pregnancy disorders; but also, to study cancer.

Heme oxygenase-1 is critically involved in placentation, spiral artery remodeling, and blood pressure regulation during murine pregnancy

The onset of pregnancy implies the appearance of a new organ, the placenta. One main function of the placenta is to supply oxygen to the fetus via hemoproteins. In this review, we highlight the importance of the enzyme heme oxygenase-1 (HO-1) for pregnancy to be established and maintained. HO-1 expression is pivotal to promote placental function and fetal development, thus determining the success of pregnancy. The deletion of the gene Hmox1 in mice leads to inadequate remodeling of spiral arteries and suboptimal placentation followed by intrauterine growth restriction (IUGR) and fetal lethality. A partial Hmox1 deletion leads to IUGR as well, with heterozygote and wild-type fetuses being born, but Hmox1 (-/-) significantly below the expected Mendelian rate. This strong phenotype is associated with diminished number of pregnancy-protective uterine natural killer (uNK) cells. Pregnant heterozygote females develop gestational hypertension. The protective HO-1 effects on placentation and fetal growth can be mimicked by the exogenous administration of carbon monoxide (CO), a product of heme catalyzed by HO-1. CO application promotes the in situ proliferation of uNK cells, restores placentation and fetal growth, while normalizing blood pressure. Similarly, HO-1 inhibition provokes hypertension in pregnant rats. The HO-1/CO axis plays a pivotal role in sustaining pregnancy and aids in the understanding of the biology of pregnancy and reveals a promising therapeutic application in the treatment of pregnancy complications.

The heme oxygenases: important regulators of pregnancy and preeclampsia

The heme oxygenase system has long been believed to act largely as a housekeeping unit, converting prooxidant free heme from heme protein degradation into the benign bilirubin for conjugation and safe excretion. In recent decades, however, heme oxygenases have emerged as important regulators of cardiovascular function, largely through the production of their biologically active metabolites: carbon monoxide, bilirubin, and elemental iron. Even more recently, a number of separate lines of evidence have demonstrated an important role for the heme oxygenases in the establishment and maintenance of pregnancy. Early preclinical and clinical studies have associated defects in the heme oxygenase with the obstetrical complication preeclampsia, as well as failure to establish adequate placental blood flow, an underlying mechanism of the disorder. Several recent preclinical studies have suggested, however, that the heme oxygenase system could serve as a valuable therapeutic tool for the management of preeclampsia, which currently has few pharmacological options. This review will summarize the role of heme oxygenases in pregnancy and highlight their potential in advancing the management of patients with preeclampsia.

Methaemalbumin formation in sickle cell disease: effect on oxidative protein modification and HO-1 induction

Normally, cell free haemoglobin is bound by haptoglobin and efficiently cleared. However, the chronic haemolysis in sickle cell disease (SCD) overwhelms haptoglobin binding capacity and protein turnover, resulting in elevated cell free haemoglobin. Cell free haemoglobin acts as both a scavenger of vasoactive nitric oxide and a pro-oxidant. In addition, methaemoglobin (metHb) releases the haem moiety, which can bind to albumin to form methaemalbumin (metHSA). This study used electron paramagnetic resonance to detect metHSA in SCD plasma and demonstrated that haptoglobin prevents haem transfer from metHb to HSA. MetHSA may either provide a second line of defence against haemoglobin/haem-mediated oxidation or contribute to the pro-oxidant environment of SCD plasma. We demonstrated that HSA inhibited oxidative protein modification induced by metHb. Additionally, we showed that while metHb induced haem oxygenase 1 (HO-1), an indicator of oxidative stress, HSA attenuated metHb induction of this enzyme, thereby limiting the potential benefits of HO-1. Furthermore, HO-1 induction by metHSA was less than HO-1 induction by equimolar metHb not bound to albumin. Our findings confirm the presence of metHSA in SCD and suggest that haem transfer from metHb to HSA reduces the oxidative effects of free haemoglobin/haem on endothelium with both beneficial (reduced protein oxidation) and potentially harmful (reduced HO-1 induction) outcomes.

Increased circulating cell-free hemoglobin levels reduce nitric oxide bioavailability in preeclampsia

Contrasting with increased nitric oxide (NO) formation during healthy pregnancy, reduced NO bioavailability plays a role in preeclampsia. However, no study has examined whether increased NO consumption by enhanced circulating levels of cell-free hemoglobin plays a role in preeclampsia. We studied 82 pregnant women (38 healthy pregnant and 44 with preeclampsia). To assess NO bioavailability, we measured plasma and whole blood nitrite concentrations using an ozone-based chemiluminescence assay. Plasma ceruloplasmin concentrations and plasma NO consumption (pNOc) were assessed and plasma hemoglobin (pHb) concentrations were measured with a commercial immunoassay. We found lower whole blood and plasma nitrite concentrations in preeclamptic patients (-48 and -39%, respectively; both P<0.05) compared with healthy pregnant women. Plasma samples from preeclamptic women consumed 63% more NO (P=0.003) and had 53% higher pHb and 10% higher ceruloplasmin levels than those found in healthy pregnant women (P<0.01). We found significant positive correlations between pHb and pNOc (r=0.61; P<0.0001), negative correlations between pNOc and whole blood or plasma nitrite concentrations (P=0.02; r=-0.32 and P=0.01; r=-0.34, respectively), and negative correlations between pHb and whole blood or plasma nitrite concentrations (P=0.03; r=-0.36 and P=0.01; r=-0.38, respectively). These findings suggest that increased pHb levels lead to increased NO consumption and lower NO bioavailability in preeclamptic compared with healthy pregnant women.

Heme oxygenase expression in human placental villous tissue in response to exposure to in vitro ischemia-reperfusion injury

Heme oxygenases (HO-1 and HO-2) are responsible for the production of carbon monoxide, a vasodilator. The products of heme oxygenase are also anti-oxidants. HO is expressed within the placenta and is important in controlling placental blood flow. HO can be sensitive to oxygen, with responses differing depending on the cell type. Recent studies have suggested that in preeclampsia, the placenta would be subjected to fluctuations in oxygen tension analogous to an ischemia-reperfusion injury. Thus the present study tested the hypothesis that HO-1 and or HO-2 expression in placental villous explants would be altered by an ischemic-reperfusion insult. Human term placental explants were exposed to hypoxia then re-oxygenation in 5% or 20% O(2) or repeated cycles of hypoxia-re-oxygenation. HO protein concentrations were assessed by Western blotting. No changes in HO-1 or HO-2 were found with any treatment protocol. Chemical induction of HO-1 was possible in explants showing that HO-1 induction in explants is possible. The results suggest that cells in term placental villous tissue do not respond to hypoxia-re-oxygenation by altering the amount of HO-1 or HO-2 protein.

Heme oxygenase-1 and the vascular bed: from molecular mechanisms to therapeutic opportunities

Heme oxygenase-1, an enzyme degrading heme to carbon monoxide, iron, and biliverdin, has been recognized as playing a crucial role in cellular defense against stressful conditions, not only related to heme release. HO-1 protects endothelial cells from apoptosis, is involved in blood-vessel relaxation regulating vascular tone, attenuates inflammatory response in the vessel wall, and participates in blood-vessel formation by means of angiogenesis and vasculogenesis. The latter functions link HO-1 not only to cardiovascular ischemia but also to many other conditions that, like development, wound healing, or cancer, are dependent on neovascularization. The aim of this comprehensive review is to address the mechanisms of HO-1 regulation and function in cardiovascular physiology and pathology and to demonstrate some possible applications of the vast knowledge generated so far. Recent data provide powerful evidence for the involvement of HO-1 in the therapeutic effect of drugs used in cardiovascular diseases. Novel studies open the possibilities of application of HO-1 for gene and cell therapy. Therefore, research in forthcoming years should help to elucidate both the real role of HO-1 in the effect of drugs and the clinical feasibility of HO-1-based cell and gene therapy, creating the effective therapeutic avenues for this refined antioxidant system.

Placental expression profiling in preeclampsia: local overproduction of hemoglobin may drive pathological changes

OBJECTIVE

To create a library enriched in cDNAs from preeclamptic placentas to print onto microarrays for placental profiling of preeclampsia (PE) and high risk pregnancies.

DESIGN

Prospective study.

SETTING

University women's clinic and academic research laboratory.

PATIENT(S)

Ten patients with PE, 5 with PE and bilateral notching, 5 with bilateral notching without PE, and 15 normotensive patients were recruited.

INTERVENTION(S)

Placenta and placenta bed biopsies were collected after delivery.

MAIN OUTCOME MEASURE(S)

Subtracted libraries of PE transcripts were produced, and cDNAs from these libraries were used to make PE-specific cDNA arrays. Results were verified quantitatively using real-time polymerase chain reaction (PCR) and histologically using in situ hybridization and immunohistochemistry.

RESULT(S)

Thirty genes were significantly altered in at least one group comparison. Differences in two candidate genes were confirmed using quantitative real-time PCR. Hemoglobin alpha2 and gamma transcripts were significantly overexpressed in the PE placenta. Scattered cells in the placenta and placental blood vessels were shown to express genes encoding these hemoglobin chains.

CONCLUSION(S)

We demonstrate increased hemoglobin production in the PE placenta. The hemoglobin may be released into the placenta blood vessel lumen. Free heme and hemoglobin are potent toxins that cause endothelial damage and inflammation.

Hypoxia and heme oxygenases: oxygen sensing and regulation of expression

Heme is an essential molecule for life, as it is involved in sensing and using oxygen. Heme must be synthesized and degraded within an individual nucleated cell. Physiologic heme degradation is catalyzed by two functional isozymes of heme oxygenase, heme oxygenase-1 (HO-1) and HO-2, yielding carbon monoxide, iron, and biliverdin, an immediate precursor to bilirubin. HO-1 is an inducible enzyme, but the expression level of HO-2 is maintained in a narrow range. Characteristically, human HO-1 contains no Cys residue, whereas human HO-2 contains three Cys residues, each of which might be involved in heme binding. These features suggest separate physiologic roles of HO-1 and HO-2. Recent studies have shown that the expression levels of HO-1 and HO-2 are reduced under hypoxia, depending on the cell types. Moreover, we have proposed HO-2 as a potential O(2) sensor, because HO-2-deficient mice show hypoxemia and a blunted hypoxic ventilatory response with normal hypercapnic ventilatory response. HO-2-deficient mice also show hypertrophy of the pulmonary venous myocardium and enlargement of the carotid body. These morphometric changes are attributable to chronic hypoxemia. Here, we update the understanding of the regulation of HO-1 and HO-2 expression and summarize the regulatory role of HO-2 in the intercellular communication.

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.

Hypoxia reduces the expression of heme oxygenase-2 in various types of human cell lines. A possible strategy for the maintenance of intracellular heme level

Heme oxygenase consists of two structurally related isozymes, heme oxygenase-1 and and heme oxygenase-2, each of which cleaves heme to form biliverdin, iron and carbon monoxide. Expression of heme oxygenase-1 is increased or decreased depending on cellular microenvironments, whereas little is known about the regulation of heme oxygenase-2 expression. Here we show that hypoxia (1% oxygen) reduces the expression levels of heme oxygenase-2 mRNA and protein after 48 h of incubation in human cell lines, including Jurkat T-lymphocytes, YN-1 and K562 erythroleukemia, HeLa cervical cancer, and HepG2 hepatoma, as judged by northern blot and western blot analyses. In contrast, the expression level of heme oxygenase-1 mRNA varies under hypoxia, depending on the cell line; it was increased in YN-1 cells, decreased in HeLa and HepG2 cells, and remained undetectable in Jurkat and K562 cells. Moreover, heme oxygenase-1 protein was decreased in YN-1 cells under the conditions used, despite the induction of heme oxygenase-1 mRNA under hypoxia. The heme oxygenase activity was significantly decreased in YN-1, K562 and HepG2 cells after 48 h of hypoxia. To explore the mechanism for the hypoxia-mediated reduction of heme oxygenase-2 expression, we showed that hypoxia shortened the half-life of heme oxygenase-2 mRNA (from 12 h to 6 h) in YN-1 cells, without affecting the half-life of heme oxygenase-1 mRNA (9.5 h). Importantly, the heme contents were increased in YN-1, HepG2 and HeLa cells after 48 h of incubation under hypoxia. Thus, the reduced expression of heme oxygenase-2 may represent an important adaptation to hypoxia in certain cell types, which may contribute to the maintenance of the intracellular heme level.

Dynamic changes in expression of heme oxygenases in mouse heart and liver during hypoxia

Heme oxygenase cleaves heme to form biliverdin, carbon monoxide (CO), and iron, and consists of two structurally related isozymes, HO-1 and HO-2. HO-2 is also known as a potential oxygen sensor. Here we show that the relative CO content in arterial blood, which reflects the total amount of endogenous heme degradation, dynamically changes in mice during acclimatization to normobaric hypoxia (10% O2), with the two peaks at 1 day and 21 days of hypoxia. The expression levels of HO-1 and HO-2 proteins were decreased by 20% and 40%, respectively, in the mouse liver at 7 days of hypoxia, which returned to the basal levels at 14 days. On the other hand, HO-1 and HO-2 proteins were increased 2-fold and 1.3-fold, respectively, in the heart at 28 days of hypoxia. Thus, hypoxia induces or represses the expression of HO-1 and HO-2 in vivo, depending on cellular microenvironments.

PL74, a novel member of the transforming growth factor-beta superfamily, is overexpressed in preeclampsia and causes apoptosis in trophoblast cells

PL74, a novel member of the TGFbeta superfamily that has highest expression in placenta, is a multifunctional peptide that can induce differentiation, inhibit inflammatory stimulation of TNFalpha, and execute apoptosis after p53 overexpression and cytotoxic injury. To study its expression and function in placenta and preeclampsia, we first determined mRNA expression in nine normal and 10 preeclamptic placentas. PL74 mRNA was overexpressed by 57.3% in preeclampsia. Transfection of PL74 into term cytotrophoblasts resulted in increased apoptosis by terminal uridine deoxynucleotidyl nick end labeling labeling (control, 2.8 +/- 0.5%; PL74, 19.1 +/- 0.2%; P < 0.005). Addition of PL74 protein to HTR8/SVneo extravillous cytotrophoblast cells showed a dose-response (0-100 ng/ml) inhibition of [3H]thymidine uptake and increase in apoptosis shown by terminal uridine deoxynucleotidyl nick end labeling and histone-associated DNA fragment ELISA (control, 0.11 +/- 0.01 absorbance units; PL74, 0.21 +/- 0.01; P < 0.01). PL74 did not alter cytotrophoblast invasion using a Matrigel in vitro invasion assay. Cytokine regulation of PL74 mRNA expression in term cytotrophoblasts showed that epidermal growth factor and IFNgamma increased PL74 expression, but TGFbeta and TNFalpha had no effect. Transfection of antisense PL74 into term cytotrophoblast cells resulted in an inhibition of spontaneous differentiation at 2 and 24 h of culture (control vector, 30.8 +/- 3.1% and 26.4 +/- 1.2%; antisense PL74, 17.6 +/- 1.8%and 12.6 +/- 1.4% syncytial units, at 2 and 24 h respectively; P < 0.01). We conclude that PL74 is overexpressed in preeclampsia and may thus promote apoptosis of cytotrophoblasts at the expense of differentiation. PL74 secretion is induced by IFNgamma and may play a role in abnormal placental responses in preeclampsia.

Heme oxygenase expression in cultured human trophoblast cells during in vitro differentiation: effects of hypoxia

Heme oxygenases (HO-1 and HO-2) are responsible for the production of carbon monoxide, a vasodilator. HO is important in controlling placental blood flow and expression can be sensitive to oxygen. We previously reported a reduction in HO-2 expression in placentae obtained from patients with pre-eclampsia or living at high altitude, both associated with placental hypoxia. Thus we hypothesized that HO expression in cultured trophoblasts would be altered by exposure to hypoxia. HO-1 and HO-2 expression was assessed in trophoblast cell cultures following exposure to different oxygen environments. Western blot analyses showed that HO-1 expression in syncytiotrophoblast was significantly lower than in cytotrophoblasts in standard conditions (p < 0.05). There was no difference in HO-1 expression in cytotrophoblasts transferred to 2% O2 for various times. However, exposure of syncytiotrophoblast cultures to hypoxia for 12 h resulted in a significant reduction in HO-1 expression (p < 0.05). HO-2 expression was not affected by exposure to hypoxia in either cytotrophoblast or syncytiotrophoblast cultures. Possible interpretations of these findings are that chronic hypoxia alone is not responsible for reduced HO-2 expression or a much longer exposure to chronic hypoxia (perhaps months) is required. This study also reinforces the complexities of HO regulation by oxygen.

Regulation of heme oxygenase-1 gene transcription: recent advances and highlights from the International Conference (Uppsala, 2003) on Heme Oxygenase

Recent investigations into the regulation of heme oxygenase-1 gene (hmox-1) transcription have exposed mechanisms of increasing diversity and complexity worthy of a gene whose expression is modulated by a seemingly endless array of physiological, pathophysiological, and nonphysiological agents and conditions. For instance, contrary to initial and prevalent assumptions that inducer-dependent gene stimulation is mediated principally by the positive action of transcription activators, it now appears that such induction may occur secondarily to deactivation of the repressor protein, Bach1. As a further complication, heme and cadmium, two potent inducers of the hmox-1 gene, inhibit Bach1 function by different mechanisms-by inhibition of DNA binding or promotion of nuclear export, respectively. Bach1 also plays a role in signal-dependent hmox-1 gene repression, an increasingly appreciated phenomenon that is manifested in a species- and cell-specific manner. Although extreme concentrations of the heme oxygenase-1 protein resulting from the opposing phenomena of gene activation and repression have physiological consequences, even minor modulation in the level of this enzyme, as elicited by variations in the length of a dinucleotide repeat region within the human hmox-1 promoter, may be of clinical relevance. Finally, mechanistic diversity is also apparent in the type and combination of protein kinase-dependent, signal transduction pathways used during hmox-1 gene activation.

The heme oxygenase dilemma in cellular homeostasis: new insights for the feedback regulation of heme catabolism

Heme must be synthesized and degraded within an individual nucleated cell. Heme degradation is catalyzed by the two isozymes of heme oxygenase, heme oxygenase-1 (HO-1) and HO-2, eventually yielding biliverdin/bilirubin, CO, and iron. These products possess important physiological roles but are potentially toxic to cells. Characteristically, human HO-1 contains no Cys residues, whereas HO-2 contains the potential heme-binding motifs of the Cys-Pro dipeptide. Expression of HO-1 is inducible or repressible, depending on cell types or cellular microenvironments, but expression levels of HO-2 are fairly constant. Thus, the main regulation of heme catabolism is a problem of the balance between induction and repression of HO-1. Notably, HO-1 expression is induced by heme in all mammalian cells examined, but is repressed by hypoxia in certain types of cultured human cells. The recent discovery of Bach1 as a heme-regulated and hypoxia-inducible repressor for transcription of the HO-1 gene has provided a missing link in the feedback control of heme catabolism. On the other hand, the human HO-1 gene promoter contains the (GT)n repeat polymorphism and a single nucleotide polymorphism (-427A --> T), both of which may contribute to fine-tuning of the transcription. Importantly, long (GT)n alleles are associated with susceptibility to smoking-induced emphysema or coronary artery disease, but may provide with resistance to cerebral malaria. The latter finding suggests a novel therapeutic strategy with inhibitors of HO-1 for the treatment of cerebral malaria. We discuss the potential regulatory role of Bach1 and HO-2 in heme catabolism and update the understanding of the regulation of HO-1 expression.

Effect of glucose and oxygen deprivation on heme oxygenase expression in human chorionic villi explants and immortalized trophoblast cells

Although hypoxia induces heme oxygenase (HO)-1 mRNA and protein expression in many cell types, recent studies in our laboratory using human placental tissue have shown that a preexposure to hypoxia does not affect subsequent HO enzymatic activity for optimized assay conditions (20% O2; 0.5 mM NADPH; 25 microM methemalbumin) or HO-1 protein content. One of the consequences of impaired blood flow is glucose deprivation, which has been shown to be an inducer of HO-1 expression in HepG2 hepatoma cells. The objective of the present study was to test the effects of a 24-h preexposure to glucose-deprived medium, in 0.5 or 20% O2, on HO protein content and enzymatic activity in isolated chorionic villi and immortalized HTR-8/SVneo first-trimester trophoblast cells. HO protein content was determined by Western blot analysis, and microsomal HO enzymatic activity was measured by assessment of the rate of CO formation. HO enzymatic activity was increased (P < 0.05) in both placental models after 24-h preexposure to glucose-deficient medium in 0.5 or 20% O2. Preexposure (24 h) in a combination of low O2 and low glucose concentrations decreased the protein content of the HO-1 isoform by 59.6% (P < 0.05), whereas preexposure (24 h) to low glucose concentration alone increased HO-2 content by 28.2% in chorionic villi explants (P < 0.05). In this preparation, HO enzymatic activity correlated with HO-2 protein content (r = 0.825). However, there was no correlation between HO-2 protein content and HO enzymatic activity in HTR-8/SVneo trophoblast cells preexposed to 0.5% O2 and low glucose concentration for 24 h. These findings indicate that the regulation of HO expression in the human placenta is a complex process that depends, at least in part, on local glucose and oxygen concentrations.