Keywords
placenta - obesity - inflammation - mitochondria - oxidative stress - epigenetics
Obesity is a global health concern among adults and children of both sexes and has
major societal and economic costs. The number of women of reproductive age who are
overweight (body mass index [BMI] = 25–30 kg/m2) or obese (BMI > 30 kg/m2) continues to increase, with the incidence of obesity among pregnant women now estimated
at between 18.5 and 38.3%.[1] The economic cost of obesity in pregnancy is greater than $100 million annually.[2] Maternal obesity affects the continuum of pregnancy. Fertility and fecundity rates
are lower among overweight and obese women, in spontaneous conception as well as in
artificial reproductive techniques.[1] During pregnancy, these women are more susceptible to hypertensive disorders, gestational
diabetes, respiratory complications, and thromboembolic events.[1]
[3]
[4]
[5]
[6] While unknown cause remains the most common contributor to stillbirth with 40% of
these cases occurring in late gestation,[7] up to 25% of stillbirths between 37 and 42 weeks of gestation are perhaps due to
obesity.[8] Indeed obesity may explain the increased morbidity including the increased risk
(2.8 fold) of late stillbirth, particularly in males, in such pregnancies.[9] Overweight women also have a slower labor progression rate; higher rates of cesarean
deliveries[10]; and more surgery-related complications such as difficult spinal, epidural, or general
anesthesia, wound infection, and endometritis.[1]
[3] From the fetal and newborn perspective, complications include congenital malformations,[11] large-for-gestational-age infants,[12] intrauterine growth restriction, stillbirth, and shoulder dystocia. Finally, the
effects are not confined to pregnancy alone. Obese and also gestational diabetic women
have greater rates of type 2 diabetes[13] and cardiovascular disease later in life.
Obesity and Developmental Programing
Obesity and Developmental Programing
By virtue of its location and roles at the interface between the mother and fetus,
the placenta is the key regulator of fetal growth and development.[14] The placenta not only conveys the maternal metabolic environment to the fetus but
can also become both a target and a source of pathogenic factors affecting the fetus.[15] Developmental programing occurs when the normal developmental pattern is disrupted
by inappropriate or ill-timed signals reaching the fetus or neonate which is then
set on an altered developmental trajectory that can lead to disease in adult life.[16] A large body of evidence[17] shows that an adverse or altered intrauterine or early postnatal environment, including
obesity, can program for disease in adult life including cardiovascular disease, obesity
and metabolic syndrome, diabetes, osteoporosis, cancer, and disorders of the hypothalamic/pituitary/adrenal
axis.[1]
[3]
[18]
[19]
[20] Hence, while obese women may have babies within the normal birth weight range, with
normal sized placentas and apparently a normal outcome there may be a programing effect
on the fetus that is only revealed subsequently. While it is clear that maternal obesity
programs the fetus, the mechanism and physiological consequences of the adverse metabolic
and inflammatory environment of obesity for placental function and fetal development
are just now being elucidated.
Sexual Dimorphism and Developmental Programing
Sexual Dimorphism and Developmental Programing
It is clear that male and female fetuses respond differently to the adverse intrauterine
environment. This may then relate to their risk of developing disease in adult life
where differences in incidence of various diseases are clearly documented. Even in
“normal” pregnancy and development, there is a sexually dimorphic effect. Male fetuses
grow faster and are usually larger than females.[21] However, male fetuses are at much higher risk during pregnancy and show greater
incidences of preterm birth, preterm premature rupture of membranes, placenta previa,
lagging lung development, greater incidence of macrosomia with maternal glucose intolerance,
and more late stillbirths associated with pregestational diabetes.[22] The female neonate can more readily adapt to ex utero life even when delivered in
a highly immature state at midgestation, an effect possibly mediated by in utero adaptations
to an adverse environment prior to delivery.[23] The male fetus is claimed “to live dangerously in the womb” to maximize its growth
potential but with consequent high risk when faced with additional adverse events.[22] It is likely that there is a complex interaction between the adverse environment
of obesity and fetal sex.
Sexual Dimorphism and the Placenta
Sexual Dimorphism and the Placenta
The placenta is a fetal tissue that shows sexual dimorphism. Microarray analysis revealed
distinct sexually dimorphic profiles of gene expression in the human placenta; in
particular immune genes were expressed at higher level in female placenta compared
with male.[24] Gene expression in the placenta also responds to maternal inflammatory status in
sex-dependent manner.[25] Expression of 59 genes was changed in the placenta of women with asthma versus no
asthma with a female fetus compared with only 6 genes changed in those with asthma
with a male fetus.[26] Some of these genes were associated with growth, inflammatory, and immune pathways.
Changes in diet provide distinctive signature of sexually dimorphic genes in placenta
with expression generally higher in genes in female than in male placenta.[27] The male placenta has higher toll-like receptor 4 (TLR4) expression and a greater
production of tumor necrosis factor (TNF)-α in response to lipopolysaccharide (LPS)
than the female placenta, which can underlie the propensity to preterm birth in males.[28] The mechanisms of sexual dimorphism in placenta with obesity remain unstudied; however,
evidence from other complicated pregnancies links sex differences to gonadal steroids.
Women with preeclampsia have increased plasma testosterone levels compared with those
of healthy pregnant women, with significantly higher levels in male- than in female-bearing
preeclamptic pregnancies.[29] At the same time, the placental levels of aromatase, a rate-limiting enzyme converting
androgens to estrogens, varied depending on fetal sex: it was much higher in the preeclamptic
placentas with female than male fetuses.[30] Interestingly, aromatase can be downregulated by TNFα, hypoxia, insulin, and leptin,
which mirror the actual conditions of the placenta in the context of maternal obesity.[31]
[32]
[33]
[34]
Inflammation in Pregnancy with Obesity
Inflammation in Pregnancy with Obesity
Pregnancy per se is an inflammatory state.[35] This is enhanced in pregnancies complicated by obesity,[36] where increased concentrations of inflammatory cytokines can be seen in maternal
plasma[37] and the placenta.[38] The increased placental inflammation in obese pregnancy may be stimulated by endotoxin,[36] lipids,[39] reactive oxygen species (ROS),[40] or oxidized lipids.[41] Chronic low-grade inflammation in obese women prior to pregnancy initiates a cascade
of events which translate into an inflammatory in utero environment. Significant accumulation
of subsets of macrophages has been shown in placentas from obese patients resulting
in production of proinflammatory cytokines and adipokines including interleukin-6
(IL-6), leptin, TNF-α, monocyte chemotactic protein 1, and TLR4.[42]
[43]
[44] Uncontrolled placental inflammation leads to the impairment of overall placental
function such as increased free fatty acid (FFA) delivery to the fetal circulation,
which is expected to alter fetal growth and development.[45] We found that TNF-α, used to simulate the inflammatory milieu of obesity, decreases
trophoblast mitochondrial respiration but in a sexually dimorphic manner. The effect
is seen only in trophoblasts of a female placenta and is mediated by the transcription
factor NFκB1.[46]
Effect of Obesity on Maternal and Placental Metabolism in Pregnancy
Effect of Obesity on Maternal and Placental Metabolism in Pregnancy
Pregnancy is a state of profound metabolic changes characterized by increased fat
mass, insulin resistance, low-grade inflammation,[35] and mild hyperlipidemia,[47] where phospholipids, total LDL and HDL cholesterol, and triglycerides all increase.
The metabolic changes become exacerbated with pregravid obesity.[48] Obese pregnant women are characterized by high levels of FFA, higher circulating
levels of leptin, TNF-α, IL-1, IL-6, IL-8, oxidative stress, and reduced levels of
adiponectin.[49]
The placenta, particularly syncytiotrophoblast, has tremendous oxygen consumption[50] and metabolic activity, the energy for which is derived from ATP mainly generated
by oxidative phosphorylation in mitochondria. Glucose was traditionally thought of
as the major (if not sole) substrate for energy generation in fetus and placenta.[51] However, the placenta does not appear to utilize anaerobic glycolysis to generate
energy during periods of anoxia.[52] Work in the past 10 years has shown that the placenta can generate energy from fatty
acids[53] via fatty acid oxidation (FAO).[54] Long chain fatty acids necessary for placental FAO are abundant in maternal plasma
in late gestation but are markedly increased with obesity and thought to play a role
in insulin resistance.[55] Deficiencies in placenta of enzymes involved in FAO lead to accumulation of toxic
long chain metabolites and are associated with maternal HELLP syndrome and with preeclampsia.[56] Saturated fatty acids, palmitate and stearate, activate inflammatory signaling pathways
via interaction with TLRs and via secretion of cytokines including TNF-α, Il-1β, and
IL-6.[57] We find significantly increased level of TNF-α in the placenta of female fetuses
of obese women.[46] Fatty acids also reduce mitochondrial function through induction of proinflammatory
cytokines, and chronically elevated fatty acids are associated with increased production
of reactive oxygen and nitrogen species.[58] There has not been an investigation of FAO in the placenta of obese pregnancies,
nor has it been studied in relation to circulating maternal saturated or unsaturated
fatty acids, inflammatory cytokines, or oxidative stress or to fetal outcomes.
Free Fatty Acids and Lipid Transport in Placenta during Maternal Obesity
Free Fatty Acids and Lipid Transport in Placenta during Maternal Obesity
Although maternal hyperglycemia has long been associated with increased fetal growth,[59] maternal triglycerides also contribute with aberrant fetal growth seen with gestational
diabetes mellitus (GDM) despite good glucose control. Indeed in multivariate analysis,
increased birth weight positively correlates only with hypertriglyceridemia in women
with GDM.[60] However, such studies have not been performed in pregnancies complicated by obesity
alone, nor in relation to fetal adiposity.
Placental uptake of FFAs from the maternal circulation provides fatty acids both for
placental metabolism and delivery to the fetus.[61] Cells involved in active lipid trafficking express discrete fatty acid binding proteins
(FABP), implicated in cellular uptake and transport of fatty acids and coordination
of metabolic and inflammatory pathways.[62]
[63] FABP1, FABP3, FABP4, FABP5, and FABP plasma membranes are expressed in human trophoblasts.[64] Maternal obesity can alter lipid content and increase the expression of FABP4 in
trophoblasts.[65] An ovine model of maternal obesity showed significantly higher concentration of
FFA in the fetal circulation of obese ewes at midgestation than in control ewes.[45] In addition, the level of peroxisome proliferator-activated receptor gamma which
is known to be essential for placental development and placental uptake of fatty acids
was found to be activated in the placenta of obese ewes.[61] As fatty acids are ligands for TLR4, which drives the inflammatory response,[66] it was postulated that excessive fatty acids in the fetal circulation in the setting
of maternal obesity would activate TLR4 signaling, resulting in inflammation of fetal
tissues.
Placental Oxidative/Nitrative Stress and Obesity
Placental Oxidative/Nitrative Stress and Obesity
Pregnancy is a state of oxidative stress. Mitochondria are the major source of ROS
under physiologic conditions. Increased metabolic activity in placental mitochondria
and the reduced scavenging power of antioxidants may be responsible for rapid ROS
generation by different placental cell types.[67]
[68] At the same time, mitochondrial function itself can be compromised by severe and/or
prolonged oxidative stress. Oxidative inactivation of mitochondrial DNA polymerase
gamma could slow down mitochondrial DNA (mtDNA) replication and eventually lead to
inhibition of oxidative phosphorylation.[69] The placenta can also produce nitric oxide (NO.) and this molecule in combination
with excess superoxide can result in the production of peroxynitrite (ONOO−), leading to nitrative stress. Peroxynitrite is a powerful prooxidant that can modify
tyrosine residues within a protein sequence to give nitrotyrosine, or protein nitration.
Covalent modification of proteins by nitration may be a physiologic regulatory mechanism
in redox regulation for signaling pathways.[70] Nitrotyrosine residues have been demonstrated in the placenta of pregnancies complicated
by preeclampsia,[71] pregestational diabetes,[72] and chronic hypoxia at high altitude.[73] We have previously shown nitration of several proteins in the human placenta, and
demonstrated that the extent of nitration is increased in obese compared with lean
and overweight placentas.[40] Koeck et al[74] provided evidence for rapid and selective oxygen-regulated protein tyrosine denitration/nitration
in the mitochondria. Nitrated proteins can be eliminated from mitochondria during
hypoxia/anoxia and regenerated during reoxygenation. This nitration/denitration in
mitochondria may affect cellular energy and redox homeostasis and therefore cell and
tissue viability.
Placental Mitochondrial Energetics and Obesity
Placental Mitochondrial Energetics and Obesity
As stated previously, the placenta can generate energy from fatty acids[53] following FAO and generation of acetyl CoA. We have shown that with increasing maternal
adiposity, there is a significant fall in mitochondrial respiration by oxidative phosphorylation
and in ATP generation in the placenta[75] that is not compensated for by glycolysis. In galactose-containing medium, the trophoblast
from obese pregnancies cannot increase oxidative phosphorylation, that is, they show
metabolic inflexibility. This would suggest that with obesity, the generation of acetyl
CoA by FAO is compromised. Fatty acids also reduce mitochondrial function perhaps
via proinflammatory cytokines and/or increased production of reactive oxygen and nitrogen
species.[58] In turn, mitochondrial dysfunction can lead to a reduction in mitochondrial FAO.[76] Saturated fatty acids (palmitate, stearate) may be more damaging while unsaturated
fatty acids (oleic, DHA) may be beneficial.
Mitochondria generate most of the cell's supply of ATP, used as a source of chemical
energy which are also involved in a range of other processes, such as signaling, cellular
differentiation, apoptosis and programed cell death, control of the cell cycle and
cell growth, regulation of the membrane potential, regulation of cellular metabolism,
and steroid synthesis. Damage, reduced content, and functional capacity of mitochondria
are involved in neurodegenerative and cardiovascular diseases,[77] obesity and diabetes.[78]
[79] Diminished FAO and greater dependence on glucose for ATP synthesis,[80] ectopic lipid accumulation in skeletal muscle, the liver, and other cells[81] and low basal ATP concentrations[43] are seen with obesity. Mitochondrial oxidative capacity is decreased in skeletal
muscle of obese individuals,[76] in the kidney of high-fat diet (HFD)-fed mice,[82] as well as in the liver and the heart of ob/ob mice.[83]
[84] An isoenergetic HFD in healthy young men for only 3 days was sufficient to reduce
the expression of genes involved in mitochondrial complexes I and II, and mitochondrial
carriers.[85] While oxidative stress and mitochondrial dysfunction are often proposed as mechanisms
mediating dysfunction in various organs in obesity models, little data are available
for the placenta.
Sexual Dimorphism in the Effect of Inflammation on Placental Mitochondria
Sexual Dimorphism in the Effect of Inflammation on Placental Mitochondria
MicroRNAs (miRNAs) are conserved, regulatory molecules that have an important role
in the posttranscriptional regulation of target gene expression by promoting mRNA
instability or translational inhibition.[86] MicroRNA-210, which has been traditionally linked to hypoxia,[87] targets and decreases activity of mitochondrial subunits in placenta,[88] hence reducing cellular respiration. We have shown that expression of miR-210 was
significantly increased in placentas of obese and overweight women conceived with
female, but not male, fetuses compared with female placentas of lean women.[46] We also demonstrated increased TNF-α in female but not male placentas of overweight
and obese women, and that via NFκB1 (p50) signaling this resulted in activation of
miR-210 expression. Chromatin immunoprecipitation assay showed that NFkB1 binds to
placental miR-210 promoter in a fetal sex-dependent manner such that female but not
male trophoblast treated with TNF-α showed overexpression of miR-210, reduction of
mitochondrial target genes, and decreased mitochondrial respiration. Overall, our
data suggest that the inflammatory intrauterine environment associated with maternal
obesity induces an NFκB1-mediated increase in miR-210 in a fetal sex-dependent manner,
leading to inhibition of mitochondrial respiration and placental dysfunction in the
placentas of female fetuses. We propose that impaired mitochondrial function in placenta
and hence altered placental metabolism can evoke changes in the fetus and may potentially
link maternal obesity to metabolic and cardiovascular disease in the offspring.
We have recently shown that increasing maternal adiposity is associated with increased
generation of ROS and decreased mitochondrial respiration in the placenta.[75] Total antioxidant capacity and activity of superoxide dismutase are significantly
greater in the lean male placentas than in lean female placentas or placentas of either
sex from an obese mother (unpublished data, L. Myatt PhD, 2015), that is, there is
sexual dimorphism and an effect of obesity. The connection of oxidative stress to
mitochondrial dysfunction has refocused use of antioxidants in pregnancy toward alleviation
of mitochondrial dysfunction. Selenium is a trace element necessary for normal cellular
function and which protects trophoblast mitochondria against oxidative stress[89] by upregulating activity of antioxidant enzymes glutathione peroxidases, thioredoxin
reductases, and iodothyronine deiodinases.[90]
Obesity and Epigenetics in the Placenta
Obesity and Epigenetics in the Placenta
Epigenetics describes heritable changes in gene expression that are not mediated by
DNA sequence alterations[91] but are susceptible to environmental influences.[92] Several diverse factors epigenetically regulate genes, including age, lifestyle,
inflammation, gender, genotype, stress, nutrition, metabolism, drugs, and infection.[93] Epigenetic information is conveyed in mammals via a synergistic interaction between
mitotically heritable patterns of DNA methylation[94] and chromatin structure.[95] Local chromatin conformation regulates specific methylation patterns to control
gene transcription.[96] Epigenetic mechanisms have been postulated to have a role in developmental programing
of obesity and type 2 diabetes in offspring by the intrauterine environment[97] and may therefore also regulate placental function. There are several mechanisms
that regulate epigenetic changes.
Gene expression can be altered via posttranslational covalent modifications of chromatin
by histone methylation or acetylation which determines accessibility to transcription
factors[98] leading to transcriptionally repressive or permissive chromatin structures.[99]
[100] Repressive histone modifications seem to confer short-term, flexible silencing important
for developmental plasticity, whereas DNA methylation is believed to be a more stable,
long-term silencing mechanism.[101] Differential histone modification occurs in a gender-specific manner,[102] and in primates[103] and rats,[104] consumption of a maternal HFD gave altered histone modifications of fetal hepatic
genes accompanied by alterations in hepatic gene expression. There is, however, relatively
little data[105] on histone modification in the human placenta with pregnancy complications.
Hypermethylation of DNA in promoter regions typically is associated with transcriptional
repression of genes, whereas hypomethylation leads to gene activity.[106] Global DNA methylation in the placenta increases with advancing gestational age,[107] but with greater interindividual variation in the third trimester suggesting environmental
factors may influence methylation, gene expression, and function of the placenta.
Variations in DNA methylation profiles in the term placenta are seen in relation to
pregnancy outcome (reviewed in Koukoura et al[108]). Recently, a novel modification DNA hydroxymethylation has been described.[109] Ten-eleven translocase (TET) enzymes convert 5mC to 5hmC. Although 5mC is repressive,
5hmC is permissive for gene expression. Therefore, the balance of 5mC to 5hmc at particular
CpGs may control gene expression. Alpha ketoglutarate (αKG) and ascorbate are cofactors
for TET enzymes, suggesting a link between cellular metabolism and epigenetic regulation
of cellular activity as αKG is produced in the citric acid cycle. Maternal nutritional
status may alter the epigenetic state of the fetal genome and imprinted gene expression.[110] Hyperglycemia induces demethylation of specific cytosines throughout the genome[111] with altered gene expression.
In mammalian genomes, DNA methyltransferase (DNMT) enzymes mediate the transfer of
methyl groups from S-adenosylmethionine to cytosine,[112] establish and maintain DNA methylation patterns at specific regions of the genome,
and contribute to gene regulation. DNMT1 is primarily a maintenance methyltransferase
preserving methylation patterns during cell division, while DNMT3 enzymes are responsible
for de novo methylation. The metabolic/inflammatory milieu of obesity increases DNMT3a
expression of DNMT3a in adipose tissue of obese mice[113] and correlates with gene suppression. There is little data available on DNMTs in
human placenta.
The Influence of Nutrition and the Metabolic Environment on Epigenetic Modifications
The Influence of Nutrition and the Metabolic Environment on Epigenetic Modifications
While there is increasing evidence from other tissues that metabolic regulation of
epigenetic mechanisms occurs, it is relatively unstudied in placenta. Tight regulation
of epigenetic changes is essential especially in the early phase of gestation where
global DNA demethylation in the zygote is seen but may subsequently be influenced
by the maternal metabolic environment. Chromatin-modifying enzymes including DNMTs
can sense and respond to alterations to the nutritional environment through their
effects on intermediary metabolites.[114] Differences in DNA methylation have been reported in individuals exposed to famine
during the Dutch Hunger Winter.[115]
[116] In later life, the epigenome appears to be capable of responding to changes in nutrients
including deficiencies in methyl donors,[117] folic acid supplementation,[118] as well as fat[119] and caloric restriction.[120] The dramatic changes in methylation seen in early gestation and the relative hypomethylation
of the placenta suggest it to be susceptible to dietary influences. Recently, intrauterine
caloric restriction in mice, which programs male offspring for glucose intolerance,
increased fat mass, and hypercholesterolemia, gave a significant decrease in overall
methylation throughout the placental genome.[121] The level of demethylation was greater in placentas of male mice than in placentas
of female mice and imprinted genes appeared to be more susceptible to methylation
changes.
Conclusion
The intrauterine environment found in the obese women is associated with poor pregnancy
outcomes and importantly with programing the fetus for disease in later life. This
effect is mediated via the placenta ([Fig. 1]), which displays altered function and compromised energetics related to the obese
environment of hyperlipidemia, heightened inflammation, and oxidative stress. Evidence
that the metabolic environment of obesity causes epigenetic changes is accumulating
and needs to be studied in the placenta to link cellular metabolism to changes in
gene expression and cellular function. There is also an overarching effect of fetal
and placental sex, which now needs to be considered when studying placental function.
Fig. 1 Mechanisms linking maternal obesity to placental dysfunction and developmental programing.
