From Mammalian Cell Culture to Aquatic Species: Deciphering the role of the Kynurenine-Tryptophan Ratio under Environmental Stress

Abstract

Monitoring the impact of anthropogenic activities, particularly in industrial regions, requires ecological screening tools and frameworks that provide a comprehensive understanding of ecosystem responses to environmental changes. Biological indicators, organisms like algae, insects, fish, and sentinel mammals, are critical for assessing ecosystem health, particularly in areas of high industrial activity. The aim of this thesis was to identify a cross-species biomarker that can assess organismal health and environmental stress across various species, organs, and biological matrices. A range of biological systems and signaling pathways related to xenobiotic metabolism, energy homeostasis, immune responses, and stress adaptation were explored, leading to the identification of the Tryptophan-Kynurenine Pathway, which consumes 60-90% of tryptophan in vertebrates. Tryptophan and its metabolites play key roles in diverse physiological processes, including cell growth and maintenance, immunity, disease states, and the coordination of adaptive responses to environmental and dietary cues. This adaptive response suggests that kynurenine-tryptophan ratio (KTR) may serve as a marker for exposure to a variety of environmental stress conditions, including toxicants, nutrient scarcity, predatory stress, and habitat loss—stressors that are prevalent in areas of high industrial activity. In recent years, the KTR is increasingly recognized as a sensitive biomarker in human diseases induced or exacerbated by stress; however, its role in environmental exposure and wildlife health remains unexplored. This thesis explores the question of whether KTR can be utilized as a cross-species biomarker for environmental stress or environmental exposure to toxicants, particularly focusing on the Athabasca Oil Sands Region (AOSR). In vitro studies with mammalian hepatocytes exposed to polycyclic aromatic compounds (PACs): benzo[a]pyrene (BaP), and a Bitumen Water Accommodated Fraction (BitWAF) demonstrated that KTR increases were driven by elevated kynurenine levels, indicating disruption of tryptophan metabolism via the aryl hydrocarbon receptor (AhR). Further studies using acid extractable organics from Oil Sands Process-Affected Water (OSPW), Naphthenic Acid Fraction Components (NAFCs) showed metabolic reprogramming, including altered glucose and fatty acid uptake and mitochondrial dysfunction, mediated through PPARα activation and upregulation of Tdo2, the enzyme responsible for kynurenine production. In vivo studies of longnose and white suckers from the AOSR were conducted to assess the relationship between KTR and CYP1 enzyme activity (EROD). These studies revealed species-specific responses, with an inverse correlation between KTR and EROD in longnose suckers and a direct correlation in white suckers. These findings validate KTR as a biomarker for environmental exposure in wildlife, with significant implications for monitoring ecosystem health. Collectively, this work demonstrates the potential of KTR as a novel biomarker for environmental toxicology, offering a valuable tool for assessing organismal stress across species in response to environmental contaminants.