Altered oxidative stress and antioxidant biomarkers concentrations in pregnant individuals exposed to oil and gas sites in northeastern British Columbia by Matthew W Day, Coreen Daley, Yifan Wu, Maduomethaa Pathmaraj, Marc-André Verner, and Élyse Caron-Beaudoin, June 19, 2024, Toxicological Sciences, kfae080
https://doi.org/10.1093/toxsci/kfae080
Correspondence to:
Élyse Caron-Beaudoin
Department of Health and Society
University of Toronto Scarborough
1265 Military Trail, Scarborough, Ontario, Canada, M1C 1A4
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Abstract
Northeastern British Columbia is a region of prolific unconventional oil and gas activity (UOG). UOG activity can release volatile organic compounds (VOCs) which can elevate oxidative stress and disrupt antioxidant activity in exposed pregnant individuals, potentially increasing the risk of adverse pregnancy outcomes.
This study measured biomarkers of oxidative stress and antioxidant activity in pooled urine samples of 85 pregnant individuals living in Northeastern British Columbia, to analyze associations between indoor air VOCs, oil and gas well density and proximity metrics, and biomarker concentrations. Concentrations of catalase (CAT), superoxide dismutase (SOD), glutathione S-transferase (GST), total antioxidant capacity (TAC), 6-hydroxymelatonin sulfate (aMT6s), malondialdehyde (MDA), 8-hydroxy-2′-deoxyguanosine (8-OHdG), and 8-isoprostane (8-IP) were measured using assay kits. Associations between exposure metrics and biomarker concentrations were determined using multiple linear regression models adjusted for biomarker-specific covariables.
UOG proximity was associated with decreased SOD and 8-OHdG. Decreased 8-OHdG was associated with increased proximity to all wells. Decreased aMT6s was observed with increased indoor air hexanal concentrations. MDA was negatively associated with indoor air 1,4-dioxane concentrations. No statistically significant associations were found between other biomarkers and exposure metrics. While some associations linked oil and gas activity to altered oxidative stress and antioxidant activity, the possibility of chance findings due to the large number of tests can not be discounted.
This study shows that living near UOG wells may alter oxidative stress and antioxidant activity in pregnant individuals. More research is needed to elucidate underlying mechanisms and to what degree UOG activity affects oxidative stress and antioxidant activity.
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The median density of conventional wells within 10 km and 5 km of the participants residence was 46 wells (range: 1-326) and 10 wells (range: 0-46), respectively.
The median density of unconventional wells within 10 km and 5 km of the participants residence was 160 wells (range: 0-578) and 12 wells (range: 0-210) respectively.
The phase-specific ID2W values for UOG wells in pad preparation, drilling, stimulation and production phases are detailed elsewhere (Caron-Beaudoin et al. 2023). No statistically significant differences were observed between Indigenous and non-Indigenous participants in terms of oil and gas wells density and proximity metrics.
Ranges of indoor air concentrations for selected VOCs can be seen in Table 1. It is noteworthy that chloroform concentrations were significantly higher in Indigenous compared to non-Indigenous participants as defined by the Mann-Whitney U test (Caron-Beaudoin et al. 2022).
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4. Discussion
Previous studies by our group reported associations between oil and gas wells metrics and
increased concentrations of VOCs in indoor air and tap water in EXPERIVA (Caron-Beaudoin et al. 2022). Using birth records data, we also reported non-monotonic associations between oil and gas well proximity/density metrics and lower birthweight as well as increased odds of preterm birth (Caron-Beaudoin et al. 2021). The results of the current study shed some light on the potential implication of oxidative stress as a mediating mechanism for these associations.
4.1 Antioxidant Activity
Some studies suggest that the degree of oxidative stress could be a determining factor in how
concentrations of antioxidant biomarkers change in response to exposure to oxidative stress-inducing pollutants (Gechev et al. 2002; Wei and Lee 2002; Rodriguez et al. 2004). In a proteomics study investigating macrophage cell lines, a hierarchical oxidative stress response was found (Xiao et al. 2003). The Nrf2 pathway, responsible for the transcription of antioxidant enzymes, responds to moderate levels of oxidative stress but beyond a threshold, high ROS concentrations overwhelm the antioxidant capacity resulting in the consumption of antioxidants and the downregulation of Nrf2 (Faraonio et al., 2006; Liu et al., 2008; Myatt & Cui, 2004; Xiao et al., 2003). Chronically high levels of oxidative stress have been shown to directly inactivate antioxidant enzymes, reducing their overall concentrations, and impairing the ability to mount an antioxidant defense (Pigeolet et al. 1990; Sultana et al. 2017). Our study population differs from most oxidative stress studies as all participants were pregnant. Blood samples from previous studies have demonstrated that pregnant individuals have higher basal levels of oxidative stress and altered antioxidant activity (Wisdom et al. 1991; Morris et al. 1998; Llurba et al. 2004; Myatt and Cui 2004). These unique characteristics may affect how pregnant individuals respond to similar levels of exposure-induced oxidative stress compared to non-pregnant individuals.
In this study, SOD concentrations were found to decrease with increased proximity to UOG wells. This study is not the first to observe decreases in SOD in a population exposed to oxidative stress-inducing pollutants. Karagözler et al. (2002) observed significantly lower blood concentrations of SOD and a 2-fold increase in blood CAT concentrations in response to chronic VOC exposure in house painters. Hormozi et al. (2018) performed a similar study examining the effects of lead and cadmium exposure on antioxidant enzymes in tile glazers. Statistically significant decreases in SOD and TAC were found alongside increases in CAT in blood samples (Hormozi et al. 2018). Both studies noted that explanatory mechanisms were difficult to pinpoint, but they postulate that decreased SOD may be a result of VOC-induced enzyme inhibition or depletion of substrate molecules (Karagözler et al. 2002). This is supported by Pigeolet et al. (1990) who found that high ROS concentrations can result in the direct inactivation of antioxidant enzymes. Studies examining epithelial and cardiac cells have previously found a separate mitochondrial CAT production pathway, potentially explaining CAT increases in previous studies and the increasing CAT trends with proximity to oil and gas wells in the present study when other antioxidant enzyme concentrations are decreased (Bai & Cederbaum, 2001; Rindler et al., 2013).
The observed decrease in SOD may also be driven by a stress-induced decrease in aMT6s.
Melatonin is an activator of the Nrf2 antioxidant pathway which transcribes antioxidant enzymes (Ahmadi and Ashrafizadeh 2020; Shaw and Chattopadhyay 2020; Chen et al. 2022). Decreased melatonin would therefore result in reduced transcription of antioxidant enzymes. Urinary aMT6s correlates strongly with blood melatonin concentrations (Bojkowski et al. 1987; Graham et al. 1998; Braam and Spruyt 2022), so it is reasonable to assume that the observed decreasing trend in aMT6s in response to elevated proximity to wells corresponds with a decrease in blood melatonin concentrations.
This decrease in blood melatonin may be stress-induced as populations living near oil and gas wells have previously reported increased stress due to noise, light pollution, environmental pollution, traffic, and stressed community infrastructures (Elliott et al. 2018; Malin 2020; Aker et al. 2022).
Increased perceived stress has previously been linked to increased blood cortisol levels (van Eck and Nicolson 1994; Van Uum et al. 2008; Bozovic et al. 2013; Etwel et al. 2014). Melatonin and cortisol are inversely related in the human body (Fei et al. 2004; Claustrat et al. 2005) and murine models have shown cortisol-mediated stress-induced inhibition of melatonin synthesis in the pineal gland (López-Patiño et al. 2014). This pathway may explain the observed decrease in melatonin in participants living near oil and gas wells but more research investigating melatonin-cortisol interactions in humans will be needed before drawing conclusions.
The results from both mechanistic and real-world studies may indicate that, up to a threshold,
oxidative stress activates the antioxidant response, but chronically high levels of oxidative stress may overwhelm the antioxidant response resulting in a decrease in antioxidant activity through the activation of other protective pathways, the consumption of antioxidants, and the direct inactivation of antioxidant enzymes. Decreases in SOD may also be driven by stress-induced decreases in blood melatonin due to proximity to stress-inducing oil and gas sites.
4.2 Oxidative Stress
While no associations were found between oil and gas proximity metrics and biomarkers of lipid peroxidation, MDA and 8-IP, there were associations found with some indoor air VOC concentrations. Hexanal was significantly associated to both an increase in 8-IP and a decrease in aMT6s. Both hexanal and 8-IP are byproducts of lipid peroxidation (Janicka et al. 2010; Elisia and Kitts 2011) so it is not surprising to find some association between the two. However, while hexanal has been measured in exhaled breath, indoor air hexanal concentrations are often significantly higher than what can be accounted for by human respiration alone, indicating a fraction coming from exogenous sources (Mochalski et al.
2013). An analysis of hexanal source attribution could help determine the impacts of endogenous vs. exogenous indoor air hexanal concentrations on urinary 8-IP concentrations. Increased indoor air concentrations of 1,4-dioxane were associated with a decrease in MDA. 1,4-dioxane, which has previously been identified in flowback water from UOG activity (Lester et al. 2015), is classified as a probable carcinogen (Godri Pollitt et al. 2019) and has been shown to increase oxidative stress (Kurt et al. 2021). MDA would therefore be expected to increase in association with 1,4-dioxane exposure, but the opposite was observed.
Our analysis found that 8-OHdG significantly decreased with elevated density and proximity of all wells and UOG wells. This contradicts other studies which find increases in 8-OHdG with elevated exposure to VOCs (Lu et al. 2007; Chuang et al. 2007; Qian et al. 2021). One possible explanation may lie in the mechanisms of DNA repair. When DNA is oxidized, 8-OHdG is formed within the DNA strand and upon repair, 8-OHdG is released (Halliwell 2000). Without DNA repair, 8-OHdG is not released despite potential DNA damage. Melatonin plays a key role in repairing DNA segments and may affect the urinary excretion of 8-OHdG (Morioka et al. 1999). A previous study demonstrated positive correlations between 8-OHdG and aMT6s concentrations (He et al. 2020), matching our results. Bhatti et al. (2017) investigated melatonin-deficient night-shift workers and found that decreases in 8-OHdG mirrored decreases in melatonin.
If our participants do indeed have lower aMT6s concentrations due to the increased stress associated with living near oil and gas wells, the association between urinary aMT6s and 8-OHdG concentrations shown in the current literature may explain why this study found a decrease in urinary 8-OHdG in response to increased exposure where other studies found the opposite.
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4.4 Conclusion
Some associations were found between oil and gas wells metrics, indoor air VOC concentrations, and altered biomarker concentrations of oxidative stress and antioxidant activity. Among these associations, proximity to oil and gas wells were associated with decreasing urinary concentrations of SOD, aMT6s, and 8-OHdG. Indoor hexanal concentrations were associated with 8-IP. Indoor 1,4-dioxane concentrations were negatively associated with MDA, contradicting current literature. While associations were found linking proximity to oil and gas wells to altered oxidative stress and antioxidant activity, the degree to which UOG activity impacts oxidative stress and antioxidant activity remains unclear. These
findings along with growing epidemiological evidence linking UOG exposure and negative birth outcomes reinforce the need to assess the health risks associated with this industry, especially in vulnerable populations like pregnant individuals.
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