Abstract:
Persistent organic pollutants (POPs), such as polybrominated diphenyl ethers (PBDEs), are ubiquitously detected in the environment despite being banned from commercial and industrial use. Due to their lipophilic nature, PBDEs bioaccumulate in the human environment, including food (e.g. milk, meat, seafood), soil, and water. In humans, such exposures have been linked to inflammation and metabolic disorders. The liver is a vital organ responsible for xenobiotic metabolism and nutrient homeostasis. It functions as a central hub for digestive and systemic regulation, playing crucial roles in the metabolism of carbohydrates, amino acids, nucleotides, fatty acids, cholesterol, hormones, and especially, xenobiotics including drugs and toxicants present in our environment such as PBDEs. In close interactions with the liver, the gut microbiome can influence the host’s xenobiotic biotransformation capacity. Disruption of the microbial ecosystem–gut dysbiosis–is now recognized as a critical factor influencing disease susceptibility by modulating host metabolism, immunity, and response to environmental exposures. However, very little is known regarding how maternal exposure to PBDEs modulate the metabolic signatures involved in obesity/diabetes and inflammation later in life, and how microbial metabolites, such as indole-3-propionic acid (IPA) could modify the effect of PBDEs during the interplay between endogenous versus toxicological pregnane X receptor (PXR) activation, an important nuclear receptor for xenobiotic biotransformation. Therefore the goal of my dissertation is to investigate how developmental PBDE exposure disrupts the gut microbiome and microbial metabolism of tryptophan (Aim 1), as well as to investigate whether microbial indole metabolites and indole derivatives reduce inflammation and improve diabetic- or inflammation-related phenotype following developmental PBDE exposure (Aim 2), and lastly to determine whether targeting gut microbiome mechanistically contributes to developmental PBDE exposure-mediated disruption of PXR signaling and delayed onset of obesity and inflammation (Aim 3). I hypothesized that maternal PBDE exposure induces acute and persistent gut dysbiosis, disrupting the interplay between endogenous PXR activation (by indoles) and xenobiotic PXR activation (by PBDEs), ultimately contributing to the delayed onset of obesity and inflammation. To achieve Aim 1, breeders of conventional (CV) humanized PXR-transgenic (hPXR-TG) mice were exposed to vehicle, 0.1 mg/kg/day DE-71 via diet, DE-71+IPA (20 mg/kg/day via drinking water), or IPA from 4 weeks preconception until end of lactation, whereas weaned pups were fed standard chow with no DE-71 with or without IPA supplementation via drinking water. Organs were collected at postnatal day (PND) 21, 3 months, and 6 months of age. Targeted and untargeted metabolomic responses were analyzed with metagenomic shotgun sequencing to identify differentially abundant bacterial taxa and predict microbial functional changes associated with PBDE exposure. For Aim 2, serum immunoassays, cytokine and chemokine levels in the intestine and liver, and liver histopathological analysis were assessed in PND21, 3 months, and 6 months of age. Overall, the maternal DE-71 exposure on the gut microbiome of pups were amplified over time. The regulation of hepatic cytokines and prototypical xenobiotic-sensing transcription factor target genes by DE-71 and IPA was age- and sex-dependent, where DE-71-induced mRNA increased selected cytokines (Il10, Il12p40, Il1β [both sexes]). The hepatic mRNA of the aryl hydrocarbon receptor (AhR) target gene Cyp1a2 was increased by maternal DE-71 and DE-71+IPA exposure at PND 21 but intestinal Cyp1a1 was not altered by any of the exposures and ages. In addition, maternal DE-71 exposure increased serum indole, a known AhR ligand, in age- and sex-dependent manner. To achieve Aim 3, 3 months- and 6 months-old germ-free (GF) hPXR-TG mice were used for fecal microbiota transplantation (FMT) using fecal donors from CV control and DE-71-exposed mice were utilized. To investigate whether gut microbiome perturbations contribute to PBDE-mediated disruption of PXR signaling and delayed-onset inflammation, analysis of inflammatory signaling in metabolic organs, xenobiotic-sensing transcription factor-target gene expression, RNA sequencing, metagenomic shotgun sequencing, and targeted and untargeted metabolomics were assessed across multiple tissues. Overall, the microbiome from DE-71 exposed donors produced more pronounced changes in the hepatic transcriptomes of the female recipients than males, with enriched pathways linked to sterol and cholesterol metabolism and stress-activated protein kinase signaling cascades. Interestingly, exposed GF male recipients exhibited enrichment of pathways related to xenobiotic metabolism and fatty acid metabolic processes. In the large intestinal tissues, transcripts of tight junction genes Tjp1 and Tjp2 were consistently downregulated, indicative of compromised intestinal barrier integrity, or “leaky gut,” likely driven by DE-71-induced gut microbiome dysbiosis. Furthermore, hepatic inflammatory markers were more pronounced in female recipients compared to males, suggesting sex-specific susceptibility to inflammation following microbial transfer. In conclusion, my findings demonstrate that maternal PBDE exposure induces a proinflammatory signature along the gut-liver axis, characterized by gut dysbiosis, disrupted tryptophan metabolism, and attenuated PXR signaling in postweaned hPXR-TG offspring over time. Notably, these effects were partially mitigated by IPA supplementation, highlighting the potential of microbiome-derived metabolites in counteracting environmentally-induced metabolic dysfunction.
Persistent organic pollutants (POPs), such as polybrominated diphenyl ethers (PBDEs), are ubiquitously detected in the environment despite being banned from commercial and industrial use. Due to their lipophilic nature, PBDEs bioaccumulate in the human environment, including food (e.g. milk, meat, seafood), soil, and water. In humans, such exposures have been linked to inflammation and metabolic disorders. The liver is a vital organ responsible for xenobiotic metabolism and nutrient homeostasis. It functions as a central hub for digestive and systemic regulation, playing crucial roles in the metabolism of carbohydrates, amino acids, nucleotides, fatty acids, cholesterol, hormones, and especially, xenobiotics including drugs and toxicants present in our environment such as PBDEs. In close interactions with the liver, the gut microbiome can influence the host’s xenobiotic biotransformation capacity. Disruption of the microbial ecosystem–gut dysbiosis–is now recognized as a critical factor influencing disease susceptibility by modulating host metabolism, immunity, and response to environmental exposures. However, very little is known regarding how maternal exposure to PBDEs modulate the metabolic signatures involved in obesity/diabetes and inflammation later in life, and how microbial metabolites, such as indole-3-propionic acid (IPA) could modify the effect of PBDEs during the interplay between endogenous versus toxicological pregnane X receptor (PXR) activation, an important nuclear receptor for xenobiotic biotransformation. Therefore the goal of my dissertation is to investigate how developmental PBDE exposure disrupts the gut microbiome and microbial metabolism of tryptophan (Aim 1), as well as to investigate whether microbial indole metabolites and indole derivatives reduce inflammation and improve diabetic- or inflammation-related phenotype following developmental PBDE exposure (Aim 2), and lastly to determine whether targeting gut microbiome mechanistically contributes to developmental PBDE exposure-mediated disruption of PXR signaling and delayed onset of obesity and inflammation (Aim 3). I hypothesized that maternal PBDE exposure induces acute and persistent gut dysbiosis, disrupting the interplay between endogenous PXR activation (by indoles) and xenobiotic PXR activation (by PBDEs), ultimately contributing to the delayed onset of obesity and inflammation. To achieve Aim 1, breeders of conventional (CV) humanized PXR-transgenic (hPXR-TG) mice were exposed to vehicle, 0.1 mg/kg/day DE-71 via diet, DE-71+IPA (20 mg/kg/day via drinking water), or IPA from 4 weeks preconception until end of lactation, whereas weaned pups were fed standard chow with no DE-71 with or without IPA supplementation via drinking water. Organs were collected at postnatal day (PND) 21, 3 months, and 6 months of age. Targeted and untargeted metabolomic responses were analyzed with metagenomic shotgun sequencing to identify differentially abundant bacterial taxa and predict microbial functional changes associated with PBDE exposure. For Aim 2, serum immunoassays, cytokine and chemokine levels in the intestine and liver, and liver histopathological analysis were assessed in PND21, 3 months, and 6 months of age. Overall, the maternal DE-71 exposure on the gut microbiome of pups were amplified over time. The regulation of hepatic cytokines and prototypical xenobiotic-sensing transcription factor target genes by DE-71 and IPA was age- and sex-dependent, where DE-71-induced mRNA increased selected cytokines (Il10, Il12p40, Il1β [both sexes]). The hepatic mRNA of the aryl hydrocarbon receptor (AhR) target gene Cyp1a2 was increased by maternal DE-71 and DE-71+IPA exposure at PND 21 but intestinal Cyp1a1 was not altered by any of the exposures and ages. In addition, maternal DE-71 exposure increased serum indole, a known AhR ligand, in age- and sex-dependent manner. To achieve Aim 3, 3 months- and 6 months-old germ-free (GF) hPXR-TG mice were used for fecal microbiota transplantation (FMT) using fecal donors from CV control and DE-71-exposed mice were utilized. To investigate whether gut microbiome perturbations contribute to PBDE-mediated disruption of PXR signaling and delayed-onset inflammation, analysis of inflammatory signaling in metabolic organs, xenobiotic-sensing transcription factor-target gene expression, RNA sequencing, metagenomic shotgun sequencing, and targeted and untargeted metabolomics were assessed across multiple tissues. Overall, the microbiome from DE-71 exposed donors produced more pronounced changes in the hepatic transcriptomes of the female recipients than males, with enriched pathways linked to sterol and cholesterol metabolism and stress-activated protein kinase signaling cascades. Interestingly, exposed GF male recipients exhibited enrichment of pathways related to xenobiotic metabolism and fatty acid metabolic processes. In the large intestinal tissues, transcripts of tight junction genes Tjp1 and Tjp2 were consistently downregulated, indicative of compromised intestinal barrier integrity, or “leaky gut,” likely driven by DE-71-induced gut microbiome dysbiosis. Furthermore, hepatic inflammatory markers were more pronounced in female recipients compared to males, suggesting sex-specific susceptibility to inflammation following microbial transfer. In conclusion, my findings demonstrate that maternal PBDE exposure induces a proinflammatory signature along the gut-liver axis, characterized by gut dysbiosis, disrupted tryptophan metabolism, and attenuated PXR signaling in postweaned hPXR-TG offspring over time. Notably, these effects were partially mitigated by IPA supplementation, highlighting the potential of microbiome-derived metabolites in counteracting environmentally-induced metabolic dysfunction.