Pregnancy is a state of increased requirement of macro and micronutrients and malnourishment before and during pregnancy can lead to adverse perinatal outcomes [1]. Maternal diet specifically, the intakes of antioxidants and polyunsaturated fats are implicated in the development of oxidative stress with subsequent effects on offspring health [2, 3]. Although oxidative stress is required for the normal progression of embryonic, placental, fetal growth and development [4-7], it is also implicated in the pathophysiology of many adverse pregnancy outcomes such as miscarriage, diabetes-related congenital malformations, spontaneous abortions, preterm birth, preeclampsia, fetal growth restriction and low birth weight [8-11]. Oxidative stress-induced placental dysfunction [5], suppression of placental angiogenesis, endothelial damage, altered vascular function [12, 13], immune malfunction [14], myometrium damage [15] are suggested to underlie pregnancy complications.
A series of our studies have shown altered folate, vitamin B12 and reduced DHA, levels leading to increased homocysteine and oxidative stress in pregnancy complications such as preeclampsia and preterm birth [16-20]. We have further shown that DHA is negatively associated with homocysteine concentrations in preeclampsia [19]. The increased oxidative stress was associated with altered angiogenesis leading to poor birth outcomes in preeclampsia [21, 22]. In preterm deliveries, we have shown that increased oxidative stress and reduced DHA levels lead to changes in the circulating levels of maternal and cord brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase B (TrkB) levels [23] as well as alterations in placental matrix metalloproteinases [24, 25]. We have hypothesized that altered maternal micronutrients (folic acid, vitamin B12) and the consequent increased homocysteine and oxidative stress levels may cause epigenetic modifications that contribute to preterm birth and poor fetal outcome [26]. Reduced maternal LCPUFA levels were also observed in mothers delivering LBW babies [27]. We have recently shown in a rat model that an imbalance in maternal micronutrients like folic acid and vitamin B12 leads to increased oxidative stress [28].
Maternal micronutrients like folic acid and vitamin B12 are important determinants of one-carbon metabolism and can influence intrauterine epigenetic programming [29, 30]. Deficiency of these micronutrients leads to increased homocysteine which is also a marker for the presence of pathological oxidative stress [31]. These micronutrients mediate the remethylation of homocysteine, which affects the production of the universal methyl donor, S-adenosylmethionine (SAM) [32]. SAM maintains methyl group supply for various macromolecules like DNA, neurotransmitters, proteins and membrane phospholipids [33]. Inadequate enzyme activities and imbalances of substrates and cofactors in one-carbon metabolism may cause homocysteine and S-adenosylhomocysteine (SAH) accumulation [34]. Although a lot of importance has been given to the methyl donors, the methyl acceptors also play an important role in one-carbon metabolism. Membrane phospholipids are major methyl group acceptors and in earlier reports we have proposed that reduced docosahexaenoic acid (DHA) levels may result in diversion of methyl groups towards DNA ultimately resulting in DNA methylation [35].
The role of oxidative stress in fetal programming via epigenetic mechanisms has been reviewed previously [36, 37]. The cellular redox status influences epigenetic mechanisms, gene expression and cell differentiation [38]. Reactive oxygen species (ROS) serve as signaling factors in transcription thereby affecting gene expression [39]. They can cause oxidative damage leading to DNA breaks providing access to sites for DNA methyltransferases, which promote DNA methylation [40]. Further, ROS can act as signaling molecules in modifying histone function and can also directly interact with histones, thereby altering chromatin structure and the binding of transcription factors and RNA polymerase to DNA resulting in disruption of normal gene expression [41].
In this review, we discuss the possibility of ameliorating oxidative stress during pregnancy by modulation of the maternal one carbon cycle.
Oxidative Stress and Preeclampsia
Pregnancy characterized by high blood pressure and the excretion of protein in the urine of a previously healthy woman is referred to as preeclampsia [42] and is a major contributor to maternal and fetal morbidity and mortality [43]. Early placental development involves invasion of the uterine spiral arteries [44, 45] allowing adequate maternal blood flow to the placenta [46]. In preeclampsia there is inadequate conversion of maternal uterine spiral arteries leading to poor blood supply to the growing feto-placental unit leading to placental hypoxia/ ischemia, endothelial dysfunction, altered expression of angiogenic factors [47-49] and oxidative stress [50-52]. Literature suggests that placental oxidative stress (imbalance between pro- and anti-oxidants) is one of the key factors involved in the etiopathology of preeclampsia [53, 54]. The reactive oxygen species can lead to a poorly developed feto-placental vasculature affecting blood flow and oxygen supply to the fetus [13, 55]. Oxidative stress has been shown to affect the levels of the antiangiogenic factor sEndoglin from villous and amniotic tissues in vitro leading to endothelial dysfunction [56].Our study in preeclampsia has shown that dysregulation of angiogenic factors may be associated with increased maternal oxidative stress [21]. We further propose that altered placental LCPUFA may result in altered membrane lipid fatty acid composition leading to increased release of sFlt-1 in circulation [22].
Preeclampsia is also suggested to result from a widespread inflammatory response followed by oxidative stress [57] which induces the release of pro-inflammatory factors [58-60]. Several studies show that proinflammatory cytokines lead to endothelial cell dysfunction thereby contributing to preeclampsia [47].
Increased lipid peroxidation and decreased levels of antioxidants are suggested to play an important role in the pathogenesis of preeclampsia [61-65]. Increased malondialdehyde (MDA) levels may be helpful in the early prediction of preeclampsia [66]. Antioxidant enzymes are important in scavenging ROS. Oxidative stress associated with preeclampsia may be a consequence of reduced antioxidant defense pathways.It has been suggested that reduced glutathione, glutathione peroxidase and antioxidant vitamin status contribute to the endothelial dysfunction and hypertension of preeclampsia [67, 68]. Reduced levels of enzymatic antioxidants like superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx) and glutathione reductase (GSR) have been observed in preeclampsia [69, 70]. We have earlier demonstrated reduced levels of antioxidants and increased oxidative stress in preeclampsia [16].
Oxidative stress and Preterm birth
Oxidant stress may predispose to membrane rupture, a major cause of preterm birth [71]. Inflammation is considered an important underlying pathological mechanism of premature delivery in mother and fetus [72]. Biondi et al. [11] have suggested that oxidative stress during pregnancy can lead to the improper activation of inflammatory changes, thus triggering premature membrane rupture. Oxidative damage to fetal membranes susceptible to may lead to membrane rupture [71]. Further, oxidative stress has been suggested to be a common link underlying the associations between adverse fetal growth/preterm birth and elevated risks of adult diseases [37]. Women with preterm labor have decreased total antioxidant status compared with uncomplicated pregnancies in similar gestational weeks [73]. We have hypothesized that altered maternal micronutrients (folic acid, vitamin B12), omega 3 fatty acids and consequent oxidative stress lead to altered expression of matrix metalloproteinases and neurotrophic factors in preterm pregnancy [24, 26]. Recently, we have shown that increased oxidative stress and reduced DHA levels lead to changes in the circulating levels of maternal and cord brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase B (TrkB) levels [23] as well as alterations in placental matrix metalloproteinases [25].
Oxidative stress and Spontaneous abortion
Oxidative stress is also associated with early pregnancy loss [74]. Increased susceptibility to lipid peroxidation has been reported in women with early pregnancy failure [75]. Oxidative stress-induced damage has been hypothesized to play a role in spontaneous and recurrent abortions, idiopathic recurrent pregnancy loss, and defective embryogenesis [76, 77]. Syncytiotrophoblastic oxidative damage is suggested to contribute to miscarriage [78].
Gestational diabetes and Oxidative stress:
Diabetes during pregnancy affects >5% of all pregnancies and oxidative stress mechanisms have been implicated in morbidity and mortality of the fetus [79]. Gestational diabetes mellitus is reported to be associated with a higher risk of oxidative stress and pregnancy complications [80, 81]. Others suggest that enhanced lipid peroxidation and and reduced antioxidant capacity are not the cause but the consequence of GDM [82]. Elevated urinary 8-hydroxydeoxyguanosine concentrations in early pregnancy are suggested to be associated with increased risk of GDM [83].
One-Carbon Metabolism and Oxidative Stress
It is well established that the B-vitamins like folate, vitamin B12 and vitamin B6 are components of the one carbon cycle. Aberrations in one-carbon metabolism have been suggested to induce oxidative stress either by affecting the folate pool or by impairing remethylation of homocysteine [84]. Hyperhomocysteinemia has been associated with oxidative stress [85, 86] and is a factor in the pathogenesis of preeclampsia, intrauterine growth retardation, preterm birth, spontaneous abortions and intrauterine fetal death [87-91]. It has been suggested that thiol group of homocysteine can rapidly auto-oxidize in circulation in the presence of ceruloplasmin, the major copper binding protein in plasma, to form homocysteine and hydrogen peroxide (H2O2), thereby generating oxidative stress [92]. Further, the synthesis of the cellular antioxidant, glutathione (GSH) is linked to one-carbon metabolic pathway. Homocysteine can indirectly result in oxidative stress by decreasing the transcription, translation and catalytic activity of glutathione peroxidase [93]. Any aberrant disruptions in one-carbon metabolism can result in potentially deleterious effects including cell death as a result of an imbalance in the cellular redox state. Chronic methyl group deficiency leads to an imbalance in cellular antioxidant defense systems, increased oxidative stress, and apoptosis [94].
Discussion
Increased oxidative stress in pathologic pregnancies has important implications on the placental function and fetal well-being [6]. Adverse influences during fetal life alter the structure and function of distinct cells, organ systems or homoeostatic pathways, thereby contributing to the pathogenesis of disorders in the perinatal period and adult life [95-98]
Antioxidant therapy has been suggested to counteract the deleterious effect of ROS in women with preeclampsia and those delivering preterm [10], although it has been suggested that there is the need to investigate the optimum dosing and timing of antioxidants administration [99, 100]. Vitamins A, C, E are potent antioxidants [9, 101, 102] however, their effect on oxidative stress markers has been inconsistent [103-106]. Supplementation of these vitamins during pregnancy to reduce the risk of preeclampsia and preterm birth has not been consistent [104, 107-115]. Recent reports indicate that vitamin B12 and folic acid supplementation lowers homocysteine and oxidative stress during pregnancy and also in other disease conditions [116-118]. Studies also suggest that Vitamin B12 and folic acid supplementation can prevent an increase in TBARS in rats [119, 120].
Our earlier studies in animals for the first time have demonstrated that altered maternal micronutrients (folic acid and vitamin B12) can lead to increased oxidative stress both in the mother and the offspring [28]. Others have shown that supplementation of vitamins B12, folic acid and B6 may lower oxidative stress through their homocysteine lowering effect in addition to their antioxidant effects [121]. We have earlier discussed the possible beneficial effects of supplementing women with antioxidants and omega 3 fatty acids during pregnancy to counteract oxidative stress and thereby prevent or delay the onset of preeclampsia ultimately improving the health of the mother and baby [16]. Our earlier studies have shown that increased oxidative stress may reduce cord DHA levels and increase sFlt-1 levels, leading to poor birth outcomes in preeclampsia [21, 22]. Previous reports indicate that modest levels of DHA can alleviate oxidative DNA damage whereas high levels of DHA accelerate lipid peroxidation [122]. A recent study suggests that DHA may counteract oxidative stress and improve antioxidant status [123].
Changes in maternal micronutrients such as folate, vitamin B12 and omega-3 fatty acids could alter the availability of these key metabolites thereby affecting methylation reactions and homocysteine levels. DHA is a major methyl group acceptor in the one-carbon cycle and has recently been shown to have antioxidant activity [123]. Oxidative stress can also affect DNA methylation patterns [124]. We have shown changes in DNA methylation in response to altered one carbon metabolism in both animals and humans [19, 125]. Recent study indicates that oxidative stress may be one of the key mechanisms of both fetal metabolic alterations and subsequent development of non-transmissible chronic diseases [126]. Our lab has therefore undertaken studies to examine the epigenetic changes occurring at the gene specific level for antioxidant enzymes.
To summarize, altered one carbon cycle leads to increased homocysteine, thereby increasing the oxidative stress and altered DNA methylation patterns. This may further lead to increased lipid peroxidation, altered expression of antioxidant enzymes, angiogenic factors, altered inflammatory response ultimately leading to pregnancy complications (Figure 1). Thus a series of our studies both in animals and in women with preeclampsia suggest that the manipulation of one-carbon metabolism through maternal diet could be a potential mechanism to counteract oxidative stress. However there is a need for detailed mechanistic studies to elucidate their role in alleviating oxidative stress and reduce the risk of adverse pregnancy outcomes.
