Endogenous carbon monoxide formation by chorionic villi of term human placenta

Carbon Monoxide Poisoning: From Occupational Health to Emergency Medicine

Carbon monoxide poisoning remains a leading cause of accidental poisoning worldwide (both at home and at work), and it is also a cause of suicidal poisoning. Such poisoning can arise following prolonged exposure to low levels of CO or following brief exposure to high concentrations of the gas. In fact, despite exposure limits, high safety standards, and the availability of CO alarms, nearly 50,000 people in the United States visit the emergency department each year due to poisoning. Additionally, CO poisoning in the United States causes up to 500 deaths each year. Despite the widespread nature of this form of poisoning, known about for centuries and whose damage mechanisms have been recognized (or rather hypothesized about) since the 1800s, early recognition, especially of late complications, and treatment remain a medical challenge. A well-designed therapeutic diagnostic process is necessary so that indication for hyperbaric or normobaric therapy is correctly made and so that patients are followed up even after acute exposure to diagnose late complications early. Furthermore, it is necessary to consider that in the setting of emergency medicine, CO poisoning can be part of a differential diagnosis along with other more frequent conditions, making its recognition difficult. The last thirty years have been marked by a significant increase in knowledge regarding the toxicity of CO, as well as its functioning and its importance at physiological concentrations in mammalian systems. This review, taking into account the significant progress made in recent years, aims to reconsider the pathogenicity of CO, which is not trivially just poisonous to tissues. A revision of the paradigm, especially as regards treatment and sequelae, appears necessary, and new studies should focus on this new point of view.

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.

Endogenous carbon monoxide production by menadione

INTRODUCTION

Smokers have a significantly decreased risk of pre-eclampsia (PE), possibly attributed to an increase in blood carbon monoxide (CO) concentrations. At physiological concentrations, CO has been demonstrated to have placental vasodilatory and anti-inflammatory properties. Increasing endogenous CO production may have therapeutic potential to either prevent or treat PE. Menadione (MD), synthetic vitamin K3, increases CO in rat microsomes. Our objective was to investigate MD's ability to increase endogenous CO concentrations in pregnancy.

METHODS

Three experiments were completed. First, in vitro CO production was measured using isolated GD15 placentas. Second, non-pregnant normotensive mice received no, 1.5, 4.0 or 6.5 g/L MD for 7 days. Lastly, pregnant normotensive mice received either no or 6.5 g/L MD in water from GD10.5 to GD17.5. Consumption was measured as average daily water intake per gram of body weight. Maternal and fetal CO levels in the blood and tissue were quantified using headspace gas chromatography.

RESULTS

MD significantly increased CO production in isolated GD15 placentas. In both pregnant and non-pregnant experiments, splenic CO, hepatic CO, and splenic mass were higher in treated mice compared to controls (all p < 0.05). Maternal %COHb and Hb in treated dams were not significantly different compared to controls. The fetal:placental mass ratio was significantly lower in the treatment group (p = 0.002).

DISCUSSION

Placental CO production was observed in GD15 placentas after co-incubation with MD. MD administration increased CO in the liver and spleens of pregnant mice. Further investigation into different doses of MD is required to identify one without demonstrable fetal/placental effects.

Heme oxygenase expression in selected regions of term human placenta

Carbon monoxide (CO), formed during heme oxygenase (HO)-catalyzed oxidation of heme, has been proposed to play a complementary role with nitric oxide in the regulation of placental hemodynamics. The objective of this study was to elucidate HO enzymatic activity and HO-1 (inducible) and HO-2 (constitutive) protein content in the microsomal subcellular fraction of homogenate of selected regions of placenta from normotensive and mild pre-eclamptic pregnancies. HO enzymatic activity was measured under optimized conditions by gas chromatography using CO formation as an index of activity, and HO-1 and HO-2 protein content were determined by Western immunoblot analysis. Microsomal HO activity in each of the four placental regions was not different between normotensive and mild pre-eclamptic pregnancies. Microsomal HO-2 protein content was not different between normotensive and mild pre-eclamptic pregnancies, whereas there was increased expression of microsomal HO-1 protein in chorionic villi and fetal membranes from pre-eclamptic pregnancy compared with normotensive pregnancy. Microsomal HO enzymatic activity correlated with HO-2, but not HO-1, protein content.

Carbon monoxide decreases perfusion pressure in isolated human placenta

Carbon monoxide (CO) is one of the metabolites formed via heme oxidation catalysed by the enzyme heme oxygenase (HO). Endogenous formation of CO, mediated by HO, has been noted in both placental and umbilical vessels. In blood vessels from different mammalian sources, it has been proposed that the vasodilator effect of CO is mediated via stimulation of soluble guanylyl cyclase (sGC) and consequent increased cGMP formation. The purpose of the present study was to determine the effect of exogenous CO on placental cotyledon perfusion pressure and to determine the role of sGC in the CO-induced decrease of perfusion pressure using the in vitro human placental perfusion preparation. A thromboxane A2 mimetic (U46619) was added to the foetal perfusion medium to constrict the placental blood vessels. Carbon monoxide was added to the foetal perfusion medium in increasing concentrations to determine its effect on placental perfusion pressure. Carbon monoxide produced a concentration-dependent decrease in placental perfusion pressure. The addition of ODQ, a sGC inhibitor, attenuated the CO-induced decrease in placental perfusion pressure, while addition of YC-1, an activator of sGC, augmented the CO-induced decrease in placental perfusion pressure. The data indicate that CO causes vasorelaxation of placental resistance blood vessels, in large part, via activation of sGC.

Heme oxygenase activity in placenta: direct dependence on oxygen availability

Carbon monoxide (CO), which is formed endogenously from heme catalyzed by heme oxygenase (HO), is proposed to play a role in vascular control. The mRNA and protein expression of the inducible isoform of HO (HO-1) increases in response to hypoxia, and it has been assumed that HO activity also increases. This assumption requires evaluation because the catalytic activity of HO requires three molecules of O(2) for each molecule of CO formed from heme, and HO activity may be limited by O(2) availability. To test the hypothesis that low physiological O(2) concentrations limit HO activity, heme-derived CO formation by microsomal fractions of homogenates of chorionic villi of human placentas was determined after exposure to 0, 1, 5, or 21% O(2). Results revealed that HO activity was directly dependent on O(2) concentration. Thus, although hypoxia may increase HO protein and mRNA expression, there is a progressive decrease in HO activity with decreasing O(2) concentration and the dependence of HO activity on O(2) concentration is similar in chorionic villi from noninfarcted areas of preeclamptic and normotensive placenta.