Hemeoxygenase-1 inhibits human myometrial contractility via carbon monoxide and is upregulated by progesterone during pregnancy

Mid-pregnancy maternal blood nitric oxide-related gene and miRNA expression are associated with preterm birth

Aim: The nitric oxide (NO) pathway modulates inflammation and may influence birth timing. Patients & methods: Case-control analysis of 136 pregnant women with RNA obtained <28 weeks; n = 212 mRNAs and n = 108 miRNAs in the NO pathway were evaluated. NO-pathway mRNA and miRNA transcript counts in women delivering preterm versus at term were compared, miRNA-mRNA expression levels correlated and prediction models generated. Results: Fourteen genes were differentially expressed in women delivering <37 weeks; 13/14 were also differentially expressed in those delivering <34 weeks (q <0.10) versus term births. Multiple miRNA-mRNA pairs were correlated. Models with gene expression better predicted prematurity than models with only clinical or nongenomic predictors. Conclusion: Maternal blood NO pathway-related mRNA and miRNA expression is associated with prematurity.

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.

MZe786, a hydrogen sulfide-releasing aspirin prevents preeclampsia in heme oxygenase-1 haplodeficient pregnancy under high soluble flt-1 environment

Preeclampsia affects one in twelve of the 130 million pregnancies a year. The lack of an effective therapeutic to prevent or treat it is responsible for an annual global cost burden of 100 billion US dollars. Preeclampsia also affects these women later in life as it is a recognised risk factor for cardiovascular disease, stroke and vascular dementia. Our laboratory demonstrated that preeclampsia is associated with high soluble fms-like tyrosine kinase 1 (sFlt-1) and low heme oxygenase-1 (HO1/Hmox1) expression. Here we sought to determine the therapeutic value of a novel H₂S-releasing aspirin (MZe786) in HO-1 haploid deficient (Hmox1^+/−) pregnant mice in a high sFlt-1 environment. Pregnant Hmox1^+/− mice were injected with adenovirus encoding sFlt-1 or control virus at gestation day E11.5. Subsequently, Hmox1^+/− dams were treated daily with a number of treatment regimens until E17.5, when maternal and fetal outcomes were assessed. Here we show that HO-1 compromised mice in a high sFlt-1 environment during pregnancy exhibit severe preeclampsia signs and a reduction in antioxidant genes. MZe786 ameliorates preeclampsia by reducing hypertension and renal damage possibly by stimulating antioxidant genes. MZe786 also improved fetal outcome in comparison with aspirin alone and appears to be a better therapeutic agent at preventing preeclampsia than aspirin alone.

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Partial loss of heme oxygenase-1 under high soluble Flt-1 causes severe preeclampsia compared to high sFlt-1 alone.

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MZe786, hydrogen sulfide releasing aspirin prevents preeclampsia by suppressing maternal hypertension and kidney injury.

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MZe786 is able to rescue pregnancy and improves fetal outcome despite the persistent high levels of sFlt-1.

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MZe786 is a superior therapeutic candidate than aspirin in preventing preeclampsia.

Gasotransmitters in pregnancy: from conception to uterine involution

Gasotransmitters are endogenous small gaseous messengers exemplified by nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S or sulfide). Gasotransmitters are implicated in myriad physiologic functions including many aspects of reproduction. Our objective was to comprehensively review basic mechanisms and functions of gasotransmitters during pregnancy from conception to uterine involution and highlight future research opportunities. We searched PubMed and Web of Science databases using combinations of keywords nitric oxide, carbon monoxide, sulfide, placenta, uterus, labor, and pregnancy. We included English language publications on human and animal studies from any date through August 2018 and retained basic and translational articles with relevant original findings. All gasotransmitters activate cGMP signaling. NO and sulfide also covalently modify target protein cysteines. Protein kinases and ion channels transduce gasotransmitter signals, and co-expressed gasotransmitters can be synergistic or antagonistic depending on cell type. Gasotransmitters influence tubal transit, placentation, cervical remodeling, and myometrial contractility. NO, CO, and sulfide dilate resistance vessels, suppress inflammation, and relax myometrium to promote uterine quiescence and normal placentation. Cervical remodeling and rupture of fetal membranes coincide with enhanced oxidation and altered gasotransmitter metabolism. Mechanisms mediating cellular and organismal changes in pregnancy due to gasotransmitters are largely unknown. Altered gasotransmitter signaling has been reported for preeclampsia, intrauterine growth restriction, premature rupture of membranes, and preterm labor. However, in most cases specific molecular changes are not yet characterized. Nonclassical signaling pathways and the crosstalk among gasotransmitters are emerging investigation topics.

Estrogen Receptors and Estrogen-Induced Uterine Vasodilation in Pregnancy

Normal pregnancy is associated with dramatic increases in uterine blood flow to facilitate the bidirectional maternal–fetal exchanges of respiratory gases and to provide sole nutrient support for fetal growth and survival. The mechanism(s) underlying pregnancy-associated uterine vasodilation remain incompletely understood, but this is associated with elevated estrogens, which stimulate specific estrogen receptor (ER)-dependent vasodilator production in the uterine artery (UA). The classical ERs (ERα and ERβ) and the plasma-bound G protein-coupled ER (GPR30/GPER) are expressed in UA endothelial cells and smooth muscle cells, mediating the vasodilatory effects of estrogens through genomic and/or nongenomic pathways that are likely epigenetically modified. The activation of these three ERs by estrogens enhances the endothelial production of nitric oxide (NO), which has been shown to play a key role in uterine vasodilation during pregnancy. However, the local blockade of NO biosynthesis only partially attenuates estrogen-induced and pregnancy-associated uterine vasodilation, suggesting that mechanisms other than NO exist to mediate uterine vasodilation. In this review, we summarize the literature on the role of NO in ER-mediated mechanisms controlling estrogen-induced and pregnancy-associated uterine vasodilation and our recent work on a “new” UA vasodilator hydrogen sulfide (H₂S) that has dramatically changed our view of how estrogens regulate uterine vasodilation in pregnancy.

Maternal disease and gasotransmitters

The three known gasotransmitters, nitric oxide, carbon monoxide, and hydrogen sulfide are involved in key processes throughout pregnancy. Gasotransmitters are known to impact on smooth muscle tone, regulation of immune responses, and oxidative state of cells and their component molecules. Failure of the systems that tightly regulate gasotransmitter production and downstream effects are thought to contribute to common maternal diseases such as preeclampsia and preterm birth. Normal pregnancy-related changes in uterine blood flow depend heavily on gasotransmitter signaling. In preeclampsia, endothelial dysfunction is a major contributor to aberrant gasotransmitter signaling, resulting in hypertension after 20 weeks gestation. Maintenance of pregnancy to term also requires gasotransmitter-mediated uterine quiescence. As the appropriate signals for parturition occur, regulation of gasotransmitter signaling must work in concert with those endocrine signals in order for appropriate labor and delivery timing. Like preeclampsia, preterm birth may have origins in abnormal gasotransmitter signaling. We review the evidence for the involvement of gasotransmitters in preeclampsia and preterm birth, as well as mechanistic and molecular signaling targets.

Rosiglitazone Regulates TLR4 and Rescues HO-1 and NRF2 Expression in Myometrial and Decidual Macrophages in Inflammation-Induced Preterm Birth

INTRODUCTION

Elevated inflammation accounts for approximately 30% of preterm birth (PTB) cases. We previously reported that targeting the peroxisome proliferator-activated receptor gamma (PPARγ) pathway reduced the incidence of PTB in the mouse model of endotoxin-induced PTB. The PPARγ has proven anti-inflammatory functions and its activation via rosiglitazone significantly downregulated the systemic inflammatory response and reduced PTB and stillbirth rate by 30% and 41%, respectively, in our model. Oxidative stress is inseparable from inflammation, and rosiglitazone has a reported antioxidative activity. In the current study, we therefore aimed to evaluate whether rosiglitazone treatment had effects outside of inflammatory pathway, specifically on the antioxidation pathway in our model.

METHODS

Pregnant C57BL/6J mice (E16.5) were treated with phosphate-buffered saline (PBS), rosiglitazone (Rosi), lipopolysaccharide (LPS; 10µg in 200µL 1XPBS), or LPS + Rosi (6 hours after the LPS injection). The myometrial and decidual tissues were collected and processed for macrophage isolation using magnetic cell sorting and F4/80+ antibody. Expression levels of antioxidative factors- Nrf2 and Ho-1-along with the LPS receptor Tlr4 were quantified by quantitative polymerase chain reaction. The protein levels were assessed by immunofluorescence staining.

RESULTS

Both the decidual and myometrial macrophages from the LPS-treated animals showed significantly lowered expression of Ho-1 and Nrf2 and higher expression of Tlr4 when compared to the PBS control group. The macrophages from the animals in the LPS + Rosi group had significantly elevated expression of Ho-1 and Nrf2 and downregulated expression of Tlr4 when compared to the LPS group.

CONCLUSION

Rosiglitazone administration prevents PTB by downregulating inflammation and upregulating antioxidative response.

Heme oxygenase/carbon monoxide in the female reproductive system: an overlooked signalling pathway

For a long time, carbon monoxide (CO) was known for its toxic effect on organisms. But there are still many things left to discover on that molecule. CO is formed directly in the body by the enzymatic activity of heme oxygenase (HO). CO plays an important role in many physiological processes, such as cell protections (against various stress factors), and the regulation of metabolic processes. Recent research proves that CO also operates in the female reproductive system. At the centre of interest is the importance of CO for gestation. During the gestation period, CO is an important element affecting the proper function of the feto-placental unit and generally affects fetal survivability rates. Gestation is one of the most important processes of successful reproduction, although there are more relevant processes that need to be researched. While already proven that CO influences steroidogenesis and the corpus luteum survivability rate, our knowledge concerning the function and importance of CO in the reproductive system is still relatively limited. As an example, our knowledge of CO function in an oocyte, the most important cell for reproduction, is almost non-existent. The aim of this review is to summarize our current knowledge concerning the function of CO in the female reproductive system.

Increased Heme Oxygenase-1 and Nuclear Factor Erythroid 2-Related Factor-2 in the Placenta Have a Cooperative Action on Preeclampsia

BACKGROUND

Previous studies have shown that oxidative stress is an important factor in preeclampsia (PE). Heme oxygenase-1 (HO-1) and nuclear factor erythroid 2-related factor-2 (Nrf2) are protective proteins that are involved in combating oxidative stress in the body. Nrf2 is also an essential upstream transcription factor regulating HO-1. This study was aimed at exploring the physiological roles of HO-1 and Nrf2 in PE.

METHODS

Serum and placenta were collected from 30 patients who presented with severe PE and 30 healthy pregnant females. HO-1 and Nrf2 levels in placenta were measured. Following stimulation of the HTR-8/SVneo cell line with tert-butylhydroquinone (tBHQ), an Nrf2 activator, nuclear Nrf2 protein and HO-1 mRNA levels were determined.

RESULTS

Compared with the healthy pregnancy group, HO-1 protein and mRNA levels were increased in placental samples obtained from the severe PE group (p < 0.01, p < 0.05). Similar increases were also observed for Nrf2 protein levels (p < 0.01). Nuclear Nrf2 protein and HO-1 mRNA levels were both increased in the HTR-8/SVneo cell line following stimulation with tBHQ (p < 0.05).

CONCLUSION

Patients with severe PE may be protected against oxidative injury following an elevation in HO-1 and Nrf2 levels. Nrf2 is likely to have a synergistic effect on HO-1 in PE.

Unravelling the theories of pre-eclampsia: are the protective pathways the new paradigm?

Pre-eclampsia is a vascular disorder of pregnancy where anti-angiogenic factors, systemic inflammation and oxidative stress predominate, but none can claim to cause pre-eclampsia. This review provides an alternative to the ‘two-stage model’ of pre-eclampsia in which abnormal spiral arteries modification leads to placental hypoxia, oxidative stress and aberrant maternal systemic inflammation. Very high maternal soluble fms-like tyrosine kinase-1 (sFlt-1 also known as sVEGFR) and very low placenta growth factor (PlGF) are unique to pre-eclampsia; however, abnormal spiral arteries and excessive inflammation are also prevalent in other placental disorders. Metaphorically speaking, pregnancy can be viewed as a car with an accelerator and brakes, where inflammation, oxidative stress and an imbalance in the angiogenic milieu act as the ‘accelerator’. The ‘braking system’ includes the protective pathways of haem oxygenase 1 (also referred as Hmox1 or HO-1) and cystathionine-γ-lyase (also known as CSE or Cth), which generate carbon monoxide (CO) and hydrogen sulphide (H₂S) respectively. The failure in these pathways (brakes) results in the pregnancy going out of control and the system crashing. Put simply, pre-eclampsia is an accelerator–brake defect disorder. CO and H₂S hold great promise because of their unique ability to suppress the anti-angiogenic factors sFlt-1 and soluble endoglin as well as to promote PlGF and endothelial NOS activity. The key to finding a cure lies in the identification of cheap, safe and effective drugs that induce the braking system to keep the pregnancy vehicle on track past the finishing line.

Evidence-Based Revised View of the Pathophysiology of Preeclampsia

Preeclampsia is a life-threatening vascular disorder of pregnancy due to a failing stressed placenta. Millions of women risk death to give birth each year and globally each year, almost 300,000 lose their life in this process and over 500,000 babies die as a consequence of preeclampsia. Despite decades of research, we lack pharmacological agents to treat it. Maternal endothelial oxidative stress is a central phenomenon responsible for the preeclampsia phenotype of high maternal blood pressure and proteinuria. In 1997, it was proposed that preeclampsia arises due to the loss of VEGF activity, possibly due to elevation in anti-angiogenic factor, soluble Flt-1 (sFlt-1). Researchers showed that high sFlt-1 and soluble endoglin (sEng) elicit the severe preeclampsia phenotype in pregnant rodents. We demonstrated that heme oxygenase-1 (HO-1)/carbon monoxide (CO) pathway prevents placental stress and suppresses sFlt-1 and sEng release. Likewise, hydrogen sulphide (H₂S)/cystathionine-γ-lyase (Cth) systems limit sFlt-1 and sEng and protect against the preeclampsia phenotype in mice. Importantly, H₂S restores placental vasculature, and in doing so improves lagging fetal growth. These molecules act as the inhibitor systems in pregnancy and when they fail, preeclampsia is triggered. In this review, we discuss what are the hypotheses and models for the pathophysiology of preeclampsia on the basis of Bradford Hill causation criteria for disease causation and how further in vivo experimentation is needed to establish 'proof of principle'. Hypotheses that fail to meet the Bradford Hill causation criteria include abnormal spiral artery remodelling and inflammation and should be considered associated or consequential to the disorder. In contrast, the protection against cellular stress hypothesis that states that the protective pathways mitigate cellular stress by limiting elevation of anti-angiogenic factors or oxidative stress and the subsequent clinical signs of preeclampsia appear to fulfil most of Bradford Hill causation criteria. Identifying the candidates on the roadmap to this pathway is essential in developing diagnostics and therapeutics to target the pathogenesis of preeclampsia.

Chloride channels mediate sodium sulphide-induced relaxation in rat uteri

BACKGROUND AND PURPOSE

Hydrogen sulphide reduces uterine contractility and is of potential interest as a treatment for uterine disorders. The aim of this study was to explore the mechanism of sodium sulphide (Na2 S)-induced relaxation of rat uterus, investigate the importance of redox effects and ion channel-mediated mechanisms, and any interactions between these two mechanisms.

EXPERIMENTAL APPROACH

Organ bath studies were employed to assess the pharmacological effects of Na2 S in uterine strips by exposing them to Na2 S with or without Cl(-) channel blockers (DIDS, NFA, IAA-94, T16Ainh-A01, TA), raised KCl (15 and 75 mM), K(+) channel inhibitors (glibenclamide, TEA, 4-AP), L-type Ca(2+) channel activator (S-Bay K 8644), propranolol and methylene blue. The activities of antioxidant enzymes were measured in homogenates of treated uteri. The expression of bestrophin channel 1 (BEST-1) was determined by Western blotting and RT-PCR.

KEY RESULTS

Na2 S caused concentration-dependent reversible relaxation of spontaneously active and calcium-treated uteri, affecting both amplitude and frequency of contractions. Uteri exposed to 75 mM KCl were less sensitive to Na2 S compared with uteri in 15 mM KCl. Na2 S-induced relaxations were abolished by DIDS, but unaffected by other modulators or by the absence of extracellular HCO3 (-) , suggesting the involvement of chloride ion channels. Na2 S in combination with different modulators provoked specific changes in the anti-oxidant profiles of uteri. The expression of BEST-1, both mRNA and protein, was demonstrated in rat uteri.

CONCLUSIONS AND IMPLICATIONS

The relaxant effects of Na2 S in rat uteri are mediated mainly via a DIDS-sensitive Cl(-) -pathway. Components of the relaxation are redox- and Ca(2+) -dependent.

Heme oxygenase and the immune system in normal and pathological pregnancies

Normal pregnancy is an immunotolerant state. Many factors, including environmental, socioeconomic, genetic, and immunologic changes by infection and/or other causes of inflammation, may contribute to inter-individual differences resulting in a normal or pathologic pregnancy. In particular, imbalances in the immune system can cause many pregnancy-related diseases, such as infertility, abortions, pre-eclampsia, and preterm labor, which result in maternal/fetal death, prematurity, or small-for-gestational age newborns. New findings imply that myeloid regulatory cells and regulatory T cells (Tregs) may mediate immunotolerance during normal pregnancy. Effector T cells (Teffs) have, in contrast, been implicated to cause adverse pregnancy outcomes. Furthermore, feto-maternal tolerance affects the developing fetus. It has been shown that the Treg/Teff balance affects litter size and adoptive transfer of pregnancy-induced Tregs can prevent fetal rejection in the mouse. Heme oxygenase-1 (HO-1) has a protective role in many conditions through its anti-inflammatory, anti-apoptotic, antioxidative, and anti-proliferative actions. HO-1 is highly expressed in the placenta and plays a role in angiogenesis and placental vascular development and in regulating vascular tone in pregnancy. In addition, HO-1 is a major regulator of immune homeostasis by mediating crosstalk between innate and adaptive immune systems. Moreover, HO-1 can inhibit inflammation-induced phenotypic maturation of immune effector cells and pro-inflammatory cytokine secretion and promote anti-inflammatory cytokine production. HO-1 may also be associated with T-cell activation and can limit immune-based tissue injury by promoting Treg suppression of effector responses. Thus, HO-1 and its byproducts may protect against pregnancy complications by its immunomodulatory effects, and the regulation of HO-1 or its downstream effects has the potential to prevent or treat pregnancy complications and prematurity.

Progesterone and HMOX-1 promote fetal growth by CD8+ T cell modulation

Intrauterine growth restriction (IUGR) affects up to 10% of pregnancies in Western societies. IUGR is a strong predictor of reduced short-term neonatal survival and impairs long-term health in children. Placental insufficiency is often associated with IUGR; however, the molecular mechanisms involved in the pathogenesis of placental insufficiency and IUGR are largely unknown. Here, we developed a mouse model of fetal-growth restriction and placental insufficiency that is induced by a midgestational stress challenge. Compared with control animals, pregnant dams subjected to gestational stress exhibited reduced progesterone levels and placental heme oxygenase 1 (Hmox1) expression and increased methylation at distinct regions of the placental Hmox1 promoter. These stress-triggered changes were accompanied by an altered CD8+ T cell response, as evidenced by a reduction of tolerogenic CD8+CD122+ T cells and an increase of cytotoxic CD8+ T cells. Using progesterone receptor- or Hmox1-deficient mice, we identified progesterone as an upstream modulator of placental Hmox1 expression. Supplementation of progesterone or depletion of CD8+ T cells revealed that progesterone suppresses CD8+ T cell cytotoxicity, whereas the generation of CD8+CD122+ T cells is supported by Hmox1 and ameliorates fetal-growth restriction in Hmox1 deficiency. These observations in mice could promote the identification of pregnancies at risk for IUGR and the generation of clinical interventional strategies.

The real-time in vivo electrochemical measurement of nitric oxide and carbon monoxide release upon direct epidural electrical stimulation of the rat neocortex

This study reports real-time, in vivo functional measurement of nitric oxide (NO) and carbon monoxide (CO), two gaseous mediators in controlling cerebral blood flow. A dual electrochemical NO/CO microsensor enables us to probe the complex relationship between NO and CO in regulating cerebrovascular tone. Utilizing this dual sensor, we monitor in vivo change of NO and CO simultaneously during direct epidural electrical stimulation of a living rat brain cortex. Both NO and CO respond quickly to meet physiological needs. The neural system instantaneously increases the released amounts of NO and CO to compensate the abrupt, yet transient hypoxia that results from epidural electrical stimulation. Intrinsic-signal optical imaging confirms that direct electrical stimulation elicits robust, dynamic changes in cerebral blood flow, which must accompany NO and CO signaling. The addition of l-arginine (a substrate for NO synthase, NOS) results in increased NO generation and decreased CO production compared to control stimulation. On the other hand, application of the NOS inhibitor, l-N(G)-nitroarginine methyl ester (l-NAME), results in decreased NO release but increased CO production of greater magnitude. This observation suggests that the interaction between NO and CO release is likely not linear and yet, they are tightly linked vasodilators.

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.

Involvement of the Heme Oxygenase System in the Development of Preeclampsia and as a Possible Therapeutic Target

The enzyme heme oxygenase (HO) is an important regulatory molecule present in most nucleated mammalian cells which functions to break down the pro-oxidant molecule heme into three products, carbon monoxide (CO), biliverdin and free iron. The HO system has been associated with many physiologic functions, including vascular tone, regulation of inflammation and apoptosis, angiogenesis and antioxidant capabilities. Deficiencies in HO are associated with several pregnancy disorders, including preeclampsia. With no present cure, this disorder continues to affect 5–7% of all pregnancies worldwide, leading to maternal and fetal morbidity and mortality. Researchers continue to strive for therapeutic potentials and this review will outline the possible use of the HO/CO system as a target treatment/prevention of preeclampsia in the future.

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.

Statins prevent cervical remodeling, myometrial contractions and preterm labor through a mechanism that involves hemoxygenase-1 and complement inhibition

Preterm birth (PTB) is a major public health problem, with a global prevalence of 9.6% and over a million annual neonatal deaths. In a mouse model of preterm labor (PTL) induced by intravaginal administration of a subclinical dose of lipopolysaccharide (LPS), we previously demonstrated that LPS ascends to the cervix, inducing complement activation, cervical remodeling and PTL. Here we show that complement activation also plays a role in myometrial contractions during PTL in this model. Increased levels of C5a were detected in the myometrium of LPS-treated mice but not in age-matched control or term myometrium. Human and mouse myometrium incubated with C5a showed increased frequency of contractions and expression of connexin 43, suggesting that C5a is an uterotonic molecule. Statins, which showed beneficial effects in preventing complement-mediated pregnancy complications, prevented cervical remodeling, myometrial contractions and PTL in the LPS model. The protective effects of statins in PTL were associated with increased synthesis, expression and activity of heme oxygenase (HO-1) in myometrium and cervix. Coadministration of HO-1 inhibitor tin-protoporphyrin-IX with pravastatin abrogated the protective effects of pravastatin on cervical remodeling and myometrial contractions leading to PTB. In addition, pravastatin inhibited complement activation in the cervix by increasing the synthesis and expression of complement inhibitor decay-accelerating factor. This study in mice suggests that statins might be useful to prevent PTL in humans. Clinical trials in humans are needed and if these results are confirmed, they may form the basis for a new clinical approach to prevent PTB.

Edaravone inhibits hypoxia-induced trophoblast-soluble Fms-like tyrosine kinase 1 expression: a possible therapeutic approach to preeclampsia

OBJECTIVE

To investigate the effects of edaravone, a potent free radical scavenger used clinically, on hypoxia-induced trophoblast-soluble Fms-like tyrosine kinase 1 (sFlt-1) expression.

METHODS

A trophoblast cell line (HRT-8/SVneo) impaired by cobalt chloride (CoCl2) was used as the cell model under hypoxic conditions. 3-[4, 5-dimethylthiazol-2-yl]-2, 5-diphenyl tetrazolium bromide (MTT) was used to measure the viability of cells exposed to CoCl2 and edaravone. The levels of intracellular reactive oxygen species (ROS) were analyzed by flow cytometry. mRNA expression of sFlt-1, vascular endothelial growth factor (VEGF), and placental growth factor (PlGF) in trophoblasts was measured by real-time polymerase chain reaction, and the secretion of sFlt-1, VEGF, and PlGF proteins was analyzed by enzyme-linked immunosorbent assays (ELISAs). A human umbilical vein endothelial cell (HUVEC) tube-formation assay was performed to identify the effects of CoCl2 and edaravone on vascular development.

RESULTS

CoCl2 treatment caused the loss of trophoblast viability, the formation of ROS, and sFlt-1 mRNA and protein expression in a dose-dependent manner. Pretreatment with edaravone significantly inhibited hypoxia-induced oxidative stress formation and sFlt-1 expression in trophoblasts. Neither PlGF nor VEGF mRNA or protein expression was increased by CoCl2. In the in vitro tube formation assay, edaravone showed a protective role in vascular development under hypoxic conditions.

CONCLUSION

This study demonstrated that hypoxia leading to increased sFlt-1 release in trophoblasts may contribute to the placental vascular formation abnormalities observed in preeclampsia and suggested that the free radical scavenger edaravone could be a candidate for the effective treatment of preeclampsia.

Therapeutic potential of statins and the induction of heme oxygenase-1 in preeclampsia

Heme oxygenase (Hmox) is an endogenous system that offers protection against placental cytotoxic damage associated with preeclampsia. The Hmox1/carbon monoxide (CO) pathway inhibits soluble Flt-1 (sFlt-1) and soluble Endoglin (sEng). More importantly, statins induce Hmox1 and suppress the release of sFlt-1 and sEng; thus, statins and Hmox1 activators are potential novel therapeutic agents for treating preeclampsia. The contribution of the Hmox system to the pathogenesis of preeclampsia has been further indicated by the incidence of preeclampsia being reduced by a third in smokers, who had reduced levels of circulating sFlt-1. Interestingly, preeclamptic women exhale less CO compared with women with healthy pregnancies. Hmox1 is reduced prior to the increase in sFlt-1 as Hmox1 mRNA expression in the trophoblast is decreased in the first trimester in women who go on to develop preeclampsia. Induction of Hmox1 or exposure to CO or bilirubin has been shown to inhibit the release of sFlt-1 and sEng in animal models of preeclampsia. The functional benefit of statins and Hmox1 induction in women with preeclampsia is valid not only because they inhibit sFlt-1 release, but also because statins and Hmox1 are associated with anti-apoptotic, anti-inflammatory, and anti-oxidant properties. The StAmP trial is the first randomized control trial (RCT) evaluating the use of pravastatin to ameliorate severe preeclampsia. This proof-of-concept study will pave the way for future global RCT, the success of which will greatly contribute to achieving the United Nations Millennium Development Goals (MDG4 and MDG5) and offering an affordable and easily accessible therapy for preeclampsia.

Hormonal Fluctuations during the Estrous Cycle Modulate Heme Oxygenase-1 Expression in the Uterus

Deletion of the heme oxygenase-1 (HO-1) (Hmox1) locus in mice results in intrauterine lethality. The expression of the heme catabolizing enzyme encoded by this gene, namely HO-1, is required to successfully support reproductive events. We have previously observed that HO-1 acts at several key events in reproduction ensuring pregnancy. HO-1 defines ovulation, positively influences implantation and placentation, and ensures fetal growth and survival. Here, we embarked on a study aimed to determine whether hormonal changes during the estrous cycle in the mouse define HO-1 expression that may influence receptivity. We analyzed the serum levels of progesterone and estrogen by ELISA and HO-1 mRNA expression in uterus by real time RT-PCR at the metestrus, proestrus, estrus, and diestrus phases of the estrous cycle. Further, we studied the HO-1 protein expression by western blot upon hormone addition to cultured uterine AN3 cells. We observed that HO-1 variations in uterine tissue correlated to changes in hormonal levels at different phases of the estrus cycle. In vitro, HO-1 protein levels in AN3 cells augmented after the addition of physiological concentrations of progesterone and estradiol, which confirmed our in vivo observations. Our data suggest an important role for hormones in HO-1 regulation in uterus during receptivity, a process known to have a significant impact in receptivity and later on blastocyst implantation.

Heme oxygenase in pregnancy and preeclampsia

PURPOSE OF REVIEW

Emerging evidence supports an important role for the heme oxygenase system in the maintenance of a healthy pregnancy. This review attempts to collect these wide-ranging data and summarize the recent progress in the field.

RECENT FINDINGS

New studies looking at heme oxygenase depletion in a variety of animal models have demonstrated that the heme oxygenase system is an important regulator of placental development, particularly in vascular structure. Furthermore, emerging studies demonstrate a role for heme oxygenase in the maintenance of pregnancy, especially during pathological challenge. Intriguingly, it now appears that the heme oxygenase system can be dramatically altered by pathological disorders of pregnancy, in particular preeclampsia, perhaps functionally in the symptomatic phase of the disorder. Promisingly, however, recent data suggest that induction of the heme oxygenase system, or administration of its bioactive metabolites, could provide a promising novel therapeutic approach to the management of this currently untreatable disease.

SUMMARY

Long considered a molecular housekeeping system, the heme oxygenase system is now known to be an important stress response pathway. New evidence suggests that it is also an important player in pregnancy and preeclampsia. However, the evidence now also suggests that it may provide a therapeutic approach for this common disease with few management options.

Inflammatory markers: exhaled nitric oxide and carbon monoxide during the ovarian cycle

Nitric oxide (NO) production and carbon monoxide (CO) production are increased in inflammatory lung diseases. Although there are some pieces of evidence for hormonal modulation by estrogen, little is known about exhaled NO and CO during the ovarian cycle. In 23 subjects, we measured exhaled NO and CO by an online analyzer. Significantly higher levels of exhaled NO were found at the midcycle compared with those in the premenstrual period or during menstruation. Higher levels of CO were after ovulation and reached a peak in the premenstrual phase. The lowest levels of CO were observed in the first days of the estrogen phase. In males, there was no significant variation in exhaled NO and CO. Exhaled NO and CO levels vary during the ovarian cycle in women, and this fact should be taken into account during serial measurements of these markers in the female population.

A deficiency in haem oxygenase-1 induces foetal growth restriction by placental vasculature defects

Haem oxygenase-1 (HO-1), the rate-limiting enzyme in haem degradation, plays a role in angiogenesis and vasculogenesis and is highly expressed in the placenta. Deficiencies in HO-1 are associated with several pregnancy disorders, such as recurrent miscarriages and pre-eclampsia. The unique combination of tissue protective, smooth muscle relaxing and angiogenesis regulatory properties makes HO-1 a key player in the maintenance of a healthy pregnancy through a direct effect on placental structural and vascular development, thus affecting foetal development.

CONCLUSION

  Therefore, we conclude that HO-1 plays an important role in placental vasculature development and a deficiency in HO-1 may contribute to pregnancy complications, such as pre-eclampsia, spontaneous abortions and premature births.

Improved electrochemical microsensor for the real-time simultaneous analysis of endogenous nitric oxide and carbon monoxide generation

An amperometric dual NO/CO microsensor was developed on the basis of a working electrode incorporating dual Pt microdisks (each diameter, 76 μm) and a Ag/AgCl reference electrode covered with a gas permeable membrane. One of the Pt disks was sequentially electrodeposited with Pt and Sn; the other Pt disk was deposited with Pt-Fe(III) oxide nanocomposites. The first showed activity for the oxidation of both NO and CO; the second showed activity only for NO oxidation. In the copresence of NO and CO, the currents measured at each electrode, respectively, represented the concentrations of CO and NO. The sensor showed high stability during the monitoring of organ tissue for at least 2.5 h and high selectivity to NO over CO at the Pt-Fe(III) oxide working electrode. Real-time coupled dynamic changes of NO and CO generated by a living C57 mouse kidney were monitored simultaneously and quantitatively in response to a NO synthase inhibitor (N(G)-nitro-l-arginine methyl ester), for the first time. CO was found to increase and NO decreased upon addition of the inhibitor, suggesting a possible reciprocal interaction between these endogenous gases.

Interactions of multiple gas-transducing systems: hallmarks and uncertainties of CO, NO, and H2S gas biology

The diverse physiological actions of the "biologic gases," O2, CO, NO, and H2S, have attracted much interest. Initially viewed as toxic substances, CO, NO, and H2S play important roles as signaling molecules. The multiplicity of gas actions and gas targets and the difficulty in measuring local gas concentrations obscures detailed mechanisms whereby gases exert their actions, and many questions remain unanswered. It is now readily apparent, however, that heme-based proteins play central roles in gas-generation/reception mechanisms and provide a point where multiple gases can interact. In this review, we consider a number of key issues related to "gas biology," including the effective tissue concentrations of these gases and the importance and significance of the physical proximity of gas-producing and gas-receptor/sensors. We also take an integrated approach to the interaction of gases by considering the physiological significance of CO, NO, and H2S on mitochondrial cytochrome c oxidase, a key target and central mediator of mitochondrial respiration. Additionally, we consider the effects of biologic gases on mitochondrial biogenesis and "suspended animation." By evaluating gas-mediated control functions from both in vitro and in vivo perspectives, we hope to elaborate on the complex multiple interactions of O2, NO, CO, and H2S.

Metalloporphyrins in the management of neonatal hyperbilirubinemia

Neonatal jaundice in the first week of life is a common problem in newborns. It is due to an imbalance of bilirubin production and its elimination, which can lead to significantly elevated levels of circulating bilirubin or hyperbilirubinemia. Use of phototherapy and/or exchange transfusion are the current modes for treating neonatal hyperbilirubinemia and preventing any neurologic damage. These strategies, however, only remove bilirubin that has already been formed. Preventing the production of excess bilirubin may be a more logical approach. Synthetic heme analogs, metalloporphyrins, are competitive inhibitors of heme oxygenase, the rate-limiting enzyme in bilirubin production, and their use has been proposed as an attractive alternative strategy for preventing or treating severe hyperbilirubinemia.

Understanding neonatal jaundice: a perspective on causation

Neonatal jaundice can be best understood as a balance between the production and elimination of bilirubin, with a multitude of factors and conditions affecting each of these processes. When an imbalance results because of an increase in circulating bilirubin (or the bilirubin load) to significantly high levels (severe hyperbilirubinemia), it may cause permanent neurologic sequelae (kernicterus). In most infants, an increase in bilirubin production (e.g., due to hemolysis) is the primary cause of severe hyperbilirubinemia, and thus reducing bilirubin production is a rational approach for its management. The situation can become critical in infants with an associated impaired bilirubin elimination mechanism as a result of a genetic deficiency and/or polymorphism. Combining information about bilirubin production and genetic information about bilirubin elimination with the tracking of bilirubin levels means that a relative assessment of jaundice risk might be feasible. Information on the level of bilirubin production and its rate of elimination may help to guide the clinical management of neonatal jaundice.

Effect of heme oxygenase-1 deficiency on placental development

Heme oxygenase (HO) is the rate-limiting enzyme in the heme catabolic pathway and highly expressed in the placenta. Deficiencies in HO-1, the inducible isoform, have been associated with pregnancy disorders, such as recurrent miscarriages, intrauterine growth retardation, and pre-eclampsia. The aim of this study was to identify if a deficiency in HO-1 affects placental development using a mouse model. When HO-1 heterozygote (Het, HO-1(+/-)) mice were cross-bred, an extremely low birth rate in homozygote (Mut, HO-1(-/-)) offspring (2.4%) and small litter sizes were observed. Placentas and fetuses from Het cross-breedings were relatively smaller and weighed less than those from wild-type (WT) cross-breedings at E12.5 and E15.5. Furthermore, Het placentas had significantly less HO-1 mRNA and protein levels than WT placentas, but no significant differences in placental HO activity. Interestingly, HO-2, the constituitive HO isoform, as well as iNOS and eNOS expression were significantly upregulated in Het placentas. Histological examination showed that the junctional zone (JZ) of Het placentas were markedly thinner than those of WT placentas and appeared to be due to an increase in apoptosis. Immunohistochemistry revealed that HO-1-expressing cells were located primarily in the JZ of Het placentas, specifically in the spongiotrophoblast layer. In addition, diastolic blood pressures and plasma soluble VEGFR-1 (sFlt-1) levels were significantly elevated in pregnant Het mice. We conclude that a partial deficiency in HO-1 is associated with morphological changes in the placenta and elevations in maternal diastolic blood pressure and plasma sFlt-1 levels, despite a compensatory increase in HO-2 expression.

Acute effect of cigarette smoking on placental circulation - a study by carbon-monoxide measurement and Doppler assessment

OBJECTIVE

Carbon-monoxide (CO) decreases placental vascular impedance. We assessed the consequences of smoking-induced temporary maternal CO-increase on fetal and placental circulation.

STUDY DESIGN

In a prospective study twenty-nine smoking pregnant women and their fetuses were evaluated. We determined the changes in maternal blood CO levels after smoking, and the concomitant changes in maternal and fetal circulation. Changes in fetal heart rate, uterine artery (UTA), middle cerebral artery (MCA), and descending aorta (DA) flow were measured by Doppler velocimetry. Changes in maternal CO level and umbilical flow value were assessed by paired t-test. The correlation between CO level and placental flow was assessed by partial correlation test.

RESULTS

CO level increased (mean +/-SD 1.7 +/- 0.065% vs. 2.36 +/- 0.89, p<0.0001). Nicotine-related maternal circulatory parameters changed significantly, but uterine flow values remained unchanged. Fetal heart rate increased, while flow in MCA and DA showed no change. CO-dependent umbilical artery impedance remained unchanged (Pulsatility Index: 0.956 +/- 0.18 vs. 0.948 +/- 0.21). Partial correlation between CO level and umbilical arterial impedance showed no significance (r:-0.324).

CONCLUSION

Despite significant CO elevation, the mainly CO-regulated placental flow remained unchanged.

The endogenous production of hydrogen sulphide in intrauterine tissues

BACKGROUND

Hydrogen sulphide is a gas signalling molecule which is produced endogenously from L-cysteine via the enzymes cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CSE). The possible role of hydrogen sulphide in reproduction has not yet been fully investigated. It has been previously demonstrated that hydrogen sulphide relaxes uterine smooth muscle in vitro. The aim of the present study was to investigate the endogenous production of hydrogen sulphide in rat and human intrauterine tissues in vitro.

METHODS

The production of hydrogen sulphide in rat and human intrauterine tissues was measured in vitro using a standard technique. The expression of CBS and CSE was also investigated in rat and human intrauterine tissues via Western blotting. Furthermore, the effects of nitric oxide (NO) and low oxygen conditions on the production rates of hydrogen sulphide were investigated.

RESULTS

The order of hydrogen sulphide production rates (mean +/- SD, n = 4) for rat tissues were: liver (777 +/- 163 nM/min/g) > uterus (168 +/- 100 nM/min/g) > fetal membranes (22.3 +/- 15.0 nM/min/g) > placenta (11.1 +/- 4.7 nM/min/g), compared to human placenta (200 +/- 102 nM/min/g). NO significantly increased hydrogen sulphide production in rat fetal membranes (P < 0.05). Under low oxygen conditions the production of hydrogen sulphide was significantly elevated in human placenta, rat liver, uterus and fetal membranes (P < 0.05). Western blotting (n = 4) detected the expression of CBS and CSE in all rat intrauterine tissues, and in human placenta, myometrium, amnion and chorion.

CONCLUSION

Rat and human intrauterine tissues produce hydrogen sulphide in vitro possibly via CBS and CSE enzymes. NO increased the production of hydrogen sulphide in rat fetal membranes. The augmentation of hydrogen sulphide production in human intrauterine tissues in a low oxygen environment could have a role in pathophysiology of pregnancy.

Negative regulation of soluble Flt-1 and soluble endoglin release by heme oxygenase-1

BACKGROUND

Preeclampsia is characterized clinically by hypertension and proteinuria. Soluble Flt-1 (sFlt-1; also known as soluble vascular endothelial growth factor receptor-1 [VEGFR-1]) and soluble endoglin (sEng) are elevated in preeclampsia, and their administration to pregnant rats elicits preeclampsia-like symptoms. Heme oxygenase-1 (HO-1) and its metabolite carbon monoxide (CO) exert protective effects against oxidative stimuli. Thus, we hypothesized that HO-1 upregulation may offer protection against preeclampsia by inhibiting sFlt-1 and sEng release.

METHODS AND RESULTS

Preeclamptic villous explants secreted high levels of sFlt-1 and sEng. Adenoviral overexpression of HO-1 in endothelial cells inhibited VEGF-mediated sFlt-1 release and interferon-gamma- and tumor necrosis factor-alpha-induced sEng release, whereas HO-1 inhibition potentiated sFlt-1 and sEng production from endothelial cells and placental villous explants. Consistent with these findings, mice lacking HO-1 produced higher levels of sFlt-1 and sEng compared with wild-type mice. Using selective ligands (VEGF-E and placental growth factor) and a receptor-specific inhibitor (SU-1498), we demonstrated that VEGF-induced sFlt-1 release was VEGFR-2 dependent. Furthermore, CO-releasing molecule-2 (CORM-2) or CO decreased sFlt-1 release and inhibited VEGFR-2 phosphorylation. Treatment of endothelial cells with statins upregulated HO-1 and inhibited the release of sFlt-1, whereas vitamins C and E had no effect.

CONCLUSIONS

The present study demonstrates that the HO-1/CO pathway inhibits sFlt-1 and sEng release, providing compelling evidence for a protective role of HO-1 in pregnancy, and identifies HO-1 as a novel target for the treatment of preeclampsia.

CO as a cellular signaling molecule

Many biological functions of heme oxygenase (HO), such as cytoprotection against oxidative stress, vasodilation, neurotransmission in the central or peripheral nervous systems, and anti-inflammatory, anti-apoptotic, or anti-proliferative potential, have been attributed to its enzymatic byproduct carbon monoxide (CO), although roles for biliverdin/bilirubin and iron have also been proposed. In addition to these well-characterized effects, recent findings reveal that HO-derived CO may act as an oxygen sensor and circadian modulator of heme biosynthesis. In lymphocytes, CO may participate in regulatory T cell function. A number of the known signaling effects of CO depend on stimulation of soluble guanylate cyclase and/or activation of mitogen-activated protein kinases (MAPK). Furthermore, modulation of caveolin-1 status may serve as an essential component of certain aspects of CO action, such as growth control. In this review, we summarize recent findings of the beneficial or detrimental effects of endogenous CO with an emphasis on the signaling pathways and downstream targets that trigger the action of this gas.

Heme oxygenase-carbon monoxide (HO-CO) system in rat uterus: effect of sexual steroids and prostaglandins

Pregnancy maintenance is a very complex phenomenon, and the mechanisms that allow the survival of the fetus within the maternal uterus are still poorly understood. Our objectives were to analyze heme oxygenase (HO) activity and expression in the pregnant rat and to study its association with steroid hormones and prostaglandins. Uterine tissues were obtained from non-pregnant and from time-mated rats at days 5, 13, 18-22 of pregnancy and postpartum. HO activity was significantly higher at days 5 and 20 while HO-1 protein levels measured by Western blot, were significantly elevated from days 19 to 22. In ovariectomized rats, estrogen and progesterone in estrogenized animals increased HO activity and expression. Cyclooxygenase inhibitors augmented HO activity and HO-1 expression. Pre-incubation with prostaglandin F2alpha (PGF2alpha) diminished the enzymatic activity in ovariectomized rat uterus. Tin protoporphyrin IX, an HO inhibitor, significantly decreased uterine cGMP accumulation. Bilirubin decreased uterine thiobarbituric acid substances levels (an index of lipid peroxidation). These results demonstrate a uterine gestational pattern of HO activity and expression in the rat. In addition, these results suggest that uterine HO activity could regulate uterine quiescence in pregnancy via cGMP and it may contribute to the defense against oxidative stress.

Carbon monoxide and hydrogen sulfide: gaseous messengers in cerebrovascular circulation

This review focuses on two gaseous cellular messenger molecules, CO and H2S, that are involved in cerebrovascular flow regulation. CO is a dilatory mediator in active hyperemia, autoregulation, hypoxic dilation, and counteracting vasoconstriction. It is produced from heme by a constitutively expressed enzyme [heme oxygenase (HO)-2] expressed highly in the brain and by an inducible enzyme (HO-1). CO production is regulated by controlling substrate availability, HO-2 catalytic activity, and HO-1 expression. CO dilates arterioles by binding to heme that is bound to large-conductance Ca2+-activated K+ channels. This binding elevates channel Ca2+ sensitivity, that increases coupling of Ca2+ sparks to large-conductance Ca2+-activated K+ channel openings and, thereby, hyperpolarizes the vascular smooth muscle. In addition to dilating blood vessels, CO can either inhibit or accentuate vascular cell proliferation and apoptosis, depending on conditions. H2S may also function as a cerebrovascular dilator. It is produced in vascular smooth muscle cells by hydrolysis of l-cysteine catalyzed by cystathione gamma-lyase (CSE). H2S dilates arterioles at physiologically relevant concentrations via activation of ATP-sensitive K+ channels. In addition to dilating blood vessels, H2S promotes apoptosis of vascular smooth muscle cells and inhibits proliferation-associated vascular remodeling. Thus both CO and H2S modulate the function and the structure of circulatory system. Both the HO-CO and CSE-H2S systems have potential to interact with NO and prostanoids in the cerebral circulation. Much of the physiology and biochemistry of HO-CO and CSE-H2S in the cerebral circulation remains open for exploration.

Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications

Over the last decade, studies have unraveled many aspects of endogenous production and physiological functions of carbon monoxide (CO). The majority of endogenous CO is produced in a reaction catalyzed by the enzyme heme oxygenase (HO). Inducible HO (HO-1) and constitutive HO (HO-2) are mostly recognized for their roles in the oxidation of heme and production of CO and biliverdin, whereas the biological function of the third HO isoform, HO-3, is still unclear. The tissue type-specific distribution of these HO isoforms is largely linked to the specific biological actions of CO on different systems. CO functions as a signaling molecule in the neuronal system, involving the regulation of neurotransmitters and neuropeptide release, learning and memory, and odor response adaptation and many other neuronal activities. The vasorelaxant property and cardiac protection effect of CO have been documented. A plethora of studies have also shown the importance of the roles of CO in the immune, respiratory, reproductive, gastrointestinal, kidney, and liver systems. Our understanding of the cellular and molecular mechanisms that regulate the production and mediate the physiological actions of CO has greatly advanced. Many diseases, including neurodegenerations, hypertension, heart failure, and inflammation, have been linked to the abnormality in CO metabolism and function. Enhancement of endogenous CO production and direct delivery of exogenous CO have found their applications in many health research fields and clinical settings. Future studies will further clarify the gasotransmitter role of CO, provide insight into the pathogenic mechanisms of many CO abnormality-related diseases, and pave the way for innovative preventive and therapeutic strategies based on the physiologic effects of CO.

Increased expression and activity of heme oxygenase-2 in pregnant rat aorta is not involved in attenuated vasopressin-induced contraction

Pregnancy is associated with attenuated vascular reactivity to a variety of contractile agonists. Heme oxygenases are expressed in the placenta, and it has been suggested that the heme oxygenase/carbon monoxide (HO/CO) pathway plays a significant role in regulating blood flow through the feto-placental unit. In this study we investigated the possible involvement of heme oxygenases in the reduced vascular reactivity associated with pregnancy. Arginine vasopressin (AVP) (10(-10)-3x10(-7) M) induced concentration-dependent contraction of aortic ring segments from non-pregnant and pregnant (16-19 days) rats. Pregnancy did not alter the sensitivity to AVP (pD2=8.5+/-0.1 and pD2=8.4+/-0.2 in non-pregnant and pregnant rats, respectively) but significantly reduced the maximum response (107.9+/-12.7% and 38.6+/-7.4%, respectively, relative to noradrenaline-induced contraction). Western blot analysis revealed the expression of HO-2 but not HO-1 isoform in both groups. There was a significant increase in the expression and activity of HO-2 protein in aortic tissues from pregnant rats compared with those from age-matched non-pregnant rats. In the presence of L-NAME to inhibit nitric oxide (NO) synthesis, tin protoporphyrin IX (SnPP-IX, 10(-5) M), an inhibitor of heme oxygenase, did not significantly affect AVP-induced contraction in aorta segments from pregnant and non-pregnant rats. It was concluded that, though pregnancy increased the expression and activity of HO-2 in the aorta, HO-2 was not involved in the attenuated response to AVP.

Heme oxygenase as a therapeutic target in immunological pregnancy complications

The allogeneic fetus has been considered to be an allograft and the tolerance mechanisms involved in pregnancy maintenance resemble those leading to graft acceptance. Up-regulation of Heme Oxygenase-1 (HO-1) promotes graft acceptance. Additionally, HO-1 has been proposed to have tissue-protective properties. Previous studies reported the presence of HO-1 and HO-2 in mammalian placenta and postulated a protective role for HO during pregnancy. Here, we analyze HO-1 and HO-2 expression at the feto-maternal interface from normal pregnant and abortion mice and correlate these findings with inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS) expression as well as with Th1/Th2 cytokine production by immune cells. DBA/2-mated CBA/J females undergoing abortion and BALB/c-mated CBA/J females having normal pregnancies were included in our study. The mice received no treatment. On day 14 of pregnancy, the mice were sacrificed, the abortion rate was calculated and the ex vivo Th1/Th2 production by decidual immune cells was analyzed by flow cytometry. The expression of HO-1 and HO-2, iNOS and eNOS was analyzed by immunohistochemistry (IHC) and Western blot in placenta samples. The Th1/Th2 cytokines ratio was augmented in decidua from abortion mice. We further observed a significant down-regulation of HO-1, HO-2, iNOS and eNOS molecules in placental tissue from mice undergoing abortion when compared to normal pregnant mice. Since we found diminished HOs and nitric oxide synthase (NOS) levels at the feto-maternal interface from abortion mice when compared to normal pregnant mice, which were further associated with increased Th1/Th2 cytokine production, we propose HO as a putative therapeutic target in immunological abortions. Up-regulation of HO-1 or HO-2 would favour the Th2-cytokine production, which could avoid abortion onset.

In vitro responsiveness of human post-menopausal myometrium to endothelin-1 and ovarian steroids

It has been shown in vitro that endothelin 1 (ET1) differentially affects the human myometrial contractility according to the hormonal profile of women. Our purpose was to test the hypothesis that ovarian steroids influence the ET1 induced myometrial contractility. We performed three types of experiments. Myometrial tissues were exposed to 17beta-oestradiol (E), progesterone (P), E plus P (E+P) in concentrations 10(-10)M, 10(-8)M, 10(-7)M, 10(-6)M and 10(-4)M (Type I), ET1 in concentrations 10(-10)M, 10(-9)M, 10(-8)M, 10(-7)M and 10(-6)M (Type II) and E+ET1, P+ET1 and E+P+ET1 in concentrations ranging from 10(-10)M to 10(-6)M (Type III). Tissue exposure to E, P and E+P did not significantly alter the pattern of spontaneous myometrial motility. ET1 (10(-6)M) induced a sustained long-lasting contraction, the initial part of which lasted 34 +/- 4 min, elevating the initial baseline by 190 +/- 20%. This was followed by ripples of gradually increasing amplitude with no regular contractions up to the end of the period of observation (120 min). Addition of P or E+P to ET1 markedly restricted (p<0.05) the elevation of initial baseline (P+ET1: 68 +/- 8%, P+E+ET1: 67 +/- 8%), and significantly shortened (p<0.01) the duration of the alterations (P+ET1: 21 +/- 3 min, P+E+ET1: 26 +/- 3 min). These results demonstrate the lack of any significant effect of E and P or their combinations on the pattern of spontaneous myometrial motility in post-menopausal women. However, P alone or in combination with E exerted an inhibitory action on ET1 -induced contractility on human post-menopausal myometrium. The physiological significance of these findings remains to be clarified.

HO in pregnancy

The enzyme heme oxygenase (HO) has been implicated in several physiological functions throughout the body including control of vascular tone and regulation of the inflammatory and apoptotic cascades as well as contributing to the antioxidant capabilities in several organ systems. These various properties attributed to HO are carried out through the catalytic products of heme degradation, namely carbon monoxide (CO), biliverdin, and free iron (Fe2+). As the newly emerging roles of HO in normal organ function have come to light, researchers in several disciplines have assessed the role of this enzyme in various physiological and pathological changes taking place in the human body over a lifetime. Included in this new wave of interest is the involvement of HO, and its by-products, in the normal function of the vital organ of pregnancy, the placenta. In this review the role of HO, and its catalytic products, will be examined in the context of pregnancy. The different isoforms of the HO enzyme (HO-1, HO-2, HO-3) have been localized throughout placental tissue, and have been shown to be physiologically active. The HO protein and more specifically its catalytic by-products (CO, biliverdin, and Fe2+) have been postulated to be involved in the maintenance of uterine quiescence throughout gestation, regulation of hemodynamic control within the uterus and placenta, regulation of the apoptotic and inflammatory cascades in trophoblast cells, and the maintenance of a balance of the oxidant-antioxidant status within the placental tissues. The association between this enzyme system, and its above-noted roles throughout pregnancy, with the hypertensive disorder of pregnancy preeclampsia (PET), will also be examined. It is hypothesized that a decrease in HO expression and/or activity throughout gestation would be capable of initiating several pathological processes involved in the etiology of PET. This hypothesis has led to further discussion emphasizing the possibility of novel therapeutic designs targeting this enzyme system for the treatment of PET.