Can dietary polyphenols prevent adverse pregnancy outcomes?

Abstract

GDM and spontaneous preterm birth are significant pregnancy complications that impact up to 20% of pregnancies worldwide. Despite current interventions, these rates are increasing. Both GDM and preterm birth leave a legacy of negative health consequences on maternal and fetal health post-pregnancy. Further, both conditions create significant economic burden on the healthcare system that continues beyond pregnancy. GDM and spontaneous preterm birth are seemingly disparate conditions; however, both exhibit a maternal inflammatory response that drives their pathophysiology. For example, inflammation is positively correlated with insulin resistance associated with GDM. Additionally, GDM pregnancies are characterised by increased oxidative stress. Inflammation is also increased in pregnancies that culminate in spontaneous preterm birth, with several studies identifying a contributing role for pro-inflammatory cytokines to myometrial activation and fetal membrane weakening. Current interventions, however, target only the downstream symptoms of GDM and preterm birth, and fail to adequately target the underlying inflammation.

Given the role of inflammation in their pathophysiology, there is an urgent need for anti-inflammatory therapeutic that can prevent the development of GDM and preterm birth. Recent epidemiological evidence suggests that plant-based diets, rich in polyphenols, are associated with reduced incidence of GDM and preterm birth. Polyphenols are bioactive plant compounds found in fruits, vegetables, nuts and legumes and are classified according to four categories (flavonoids, phenolic acids, stilbenes and lignans). Importantly, polyphenols possess anti-inflammatory properties. However, the specific effect of polyphenols in the context of GDM or spontaneous preterm birth is unknown. Thus, this thesis employs a series of in vitro and in vivo models to determine the effect of dietary polyphenols on the expression of mediators involved in the development of GDM and preterm birth.

To determine the effect of polyphenols as therapeutic interventions for GDM, three studies in this thesis investigated the effect of flavonoids (naringenin and nobiletin) and phenolic acids (punicalagin and curcumin) in in vitro and in vivo models of GDM. In vitro, human placenta, adipose tissue (visceral and subcutaneous) and skeletal muscle were treated with or without the pro-inflammatory cytokine TNF. In vivo, pregnant heterozygous leptin receptor deficient db/+ mice were used to model GDM. These chapters found that naringenin, nobiletin and the phenolic acids exerted anti-inflammatory effects in placenta and adipose tissue in vitro and in vivo. Naringenin and nobiletin were also found to improve human skeletal muscle glucose uptake, as well as lower fasting blood glucose of pregnant GDM mice. Naringenin and the phenolic acids also reduced oxidative stress associated with GDM by regulating antioxidant mRNA expression and reducing hydrogen peroxide levels in placenta and adipose tissue in vitro and in vivo. Interestingly, using a proteomic approach, nobiletin was found to differentially regulate metabolic signalling pathways in the placenta, pancreas, adipose tissue and skeletal muscle of pregnant GDM mice. The findings from these chapters indicate that polyphenols can target inflammation, insulin resistance and oxidative stress associated with GDM.

To determine the effect of polyphenols as therapeutic interventions for spontaneous preterm birth, two studies in this thesis investigated the effect of the phenolic acids gallic acid and punicalagin in in vitro and in vivo models of intrauterine inflammation associated with spontaneous preterm birth. In vitro, primary cells were isolated from myometrium and fetal membranes (decidua, amnion mesenchyme and amnion epithelia) and stimulated with or without either pro-inflammatory cytokine IL1B or TNF. In vivo, pregnant mice were treated with LPS via intraperitoneal injection on gestational day 16.5 to induce intrauterine inflammation associated with preterm birth. These chapters found that gallic acid and punicalagin exerted anti- inflammatory effects in primary human myometrial, decidual and amnion cells. Gallic acid and punicalagin also downregulated the expression of prostaglandin pathway proteins PTGS2 and PGF2a in both myometrium and decidua in vitro and in vivo. Notably, using an in situ 3D collagen gel assay, gallic acid and punicalagin inhibited cytokine-stimulated myometrial cell contractility. Both phenolic acids also differentially regulated MMP expression associated with fetal membrane weakening in vitro. Using a proteomic approach, gallic acid was also found to differentially regulate expression of collagen proteins and cytoskeletal proteins associated with cell contractility. Altogether, these findings indicate that polyphenols can target pro- inflammatory and pro-labor mediators involved in myometrial contractility and fetal membrane weakening associated with spontaneous preterm birth.

Taken together, these studies demonstrate that polyphenols exert anti-inflammatory properties in in vitro and in vivo models of GDM and preterm birth. Additionally, polyphenols may target other pathological features, including insulin resistance and oxidative stress associated with GDM; and myometrial activation and fetal membrane weakening associated with spontaneous preterm birth. Polyphenols may represent alternative therapeutic options to prevent GDM and spontaneous preterm birth. Further research is warranted to investigate the effect of polyphenols on maternal and fetal health post-pregnancy.