Effects of Air Pollution Exposure on Inflammation, Oxidative Stress, and DNA Methylation Pathways in the Placenta

Abstract

Prenatal exposure to ambient air pollution has been consistently associated with adverse pregnancy outcomes, including low birth weight, preterm birth, and preeclampsia; however, the biological mechanisms underlying these associations remain incompletely understood. The placenta is a transient yet essential organ responsible for nutrient and oxygen transfer and is critical for normal fetal growth and healthy gestation for both the mother and the fetus. Recent evidence linking air pollution to placental biology, including the detection of black carbon on the fetal side of the placenta, has heightened interest in the placenta as a key target of environmental exposures. Under normal conditions, placental inflammation versus anti-inflammation, oxidative stress versus antioxidation, and DNA methylation at imprinted genes are tightly regulated processes. Disruption of these processes may have detrimental consequences for both maternal and fetal health. This dissertation research aims to examine associations between gestational exposure to fine particulate matter (PM₂.₅), nitrogen dioxide (NO₂), and polycyclic aromatic hydrocarbons (PAHs) and placental inflammation, oxidative stress, and DNA methylation at imprinted genes in a relatively healthy pregnancy cohort. This includes identifying sensitive windows of exposure across multiple temporal scales, including gestational days, weeks, months, and trimesters. The dissertation also investigated the utility of a urinary PAH biomarker in reflecting source-specific air pollution exposures in the study cohort. Data were drawn from the Understanding Pregnancy Signals and Infant Development (UPSIDE) birth cohort in Rochester, New York, part of the NIH Environmental Influences on Child Health Outcomes (ECHO) program. Ambient air pollution exposures were estimated using daily concentrations of ambient PM₂.₅ and NO₂ at participants’ residences. Internal exposure to PAHs was assessed using biomarkers including urinary 1-hydroxypyrene (1-OHP) reflecting short-term exposure and PAH–hemoglobin adducts reflecting longer-term exposure. Urinary 1-OHP was quantified using high-performance liquid chromatography with ultraviolet detection (HPLC-UV), and PAH–hemoglobin adducts were measured using liquid chromatography–tandem mass spectrometry. Placental inflammatory markers interleukin-6 (IL-6) and tumor necrosis factor–alpha (TNF-α) were measured using enzyme-linked immunosorbent assays. Oxidative stress was assessed via malondialdehyde (MDA) concentrations using HPLC-UV. DNA methylation at imprinted gene differentially methylated regions (iDMRs) was quantified using pyrosequencing. Statistical analyses included linear regression, linear mixed-effects models, and distributed lag nonlinear models with penalized spline functions. Results demonstrated that transportation-related air pollution sources in Rochester, New York, including proximity to the airport, residential traffic density within 300 meters, and a rail switching yard, were significant predictors of urinary 1-OHP concentrations. These findings support the use of 1-OHP as a biomarker of air pollution exposure in a low-pollution setting, where mean PM₂.₅ concentrations averaged 6.6 µg/m³. Gestational PM₂.₅ exposure was associated with decreased placental IL-6 concentrations during the first trimester, while urinary 1-OHP was associated with increased TNF-α during the same period. For DNA methylation, PM₂.₅ exposure across the full gestational period was associated with similar exposure–response patterns for IGF2 and SNRPN across cohorts, with less than 0.01% difference in methylation. When exposure was restricted to the first 28 days of gestation, IGF2 methylation patterns remained comparable, while NO₂ exposure during this window was significantly associated with similar exposure–response relationships for MEG3, with less than 0.25% difference in methylation. Associations between PAH–hemoglobin adducts and iDMR methylation corroborated findings observed for PM₂.₅ and were more evident and stronger in placentas from male fetuses, indicating sex-specific susceptibility. Although week-specific distributed lag estimates were not statistically significant, PM₂.₅ exposure during gestational weeks 16–20 was positively associated with placental MDA concentrations. A one-interquartile-range increase in second-trimester PM₂.₅ exposure was associated with a 24.8% increase in MDA (95% CI: 3.3–50.7), and exposure during the fifth month of pregnancy was associated with a 13.7% increase in placental MDA (95% CI: 0.1–29.2). PAH–hemoglobin adducts of benzo[a]pyrene showed positive but non-significant associations with placental MDA, while no significant associations were observed for NO₂. In summary, this dissertation demonstrated that even low levels of ambient air pollution were associated with changes in placental inflammation, epigenetic regulation, and oxidative stress. The research identified early pregnancy, particularly the first trimester, as a critical window of vulnerability to air pollution effects on placental pathophysiology. The findings add to evidence that alterations in the placenta’s inflammatory and oxidative status, as well as alterations in the DNA methylation of imprinted genes may be effective by prenatal air pollution exposure.