Identifying blood biomarkers of oxidative stress and a leaky gut in patients with Inflammatory Bowel Disease and in healthy individuals

Inflammatory bowel diseases (IBD), encompassing Crohn’s disease (CD) and ulcerative colitis (UC), are chronic, progressive inflammatory diseases of the gastrointestinal (GI) tract, and are characterized by an inappropriate and uncontrolled immune response triggered by the gut microbiome in genetically susceptible individuals. (Abraham et al., 2009; Ananthakrishnan et al., 2015) The etiology and pathogenesis of IBD is thought to be of multifactorial origin, consisting of a complex interplay between host genetics, the gut microbiota, the host immune system and environmental triggers, e.g. lifestyle and dietary factors. (Cosnes et al., 2011; Fiocchi et al., 1998)
In patients with IBD, the intestinal mucosa is often characterized by inflamed regions with infiltration of numerous inflammatory cells. (Naito et al., 2007) Active intestinal inflammation is accompanied by the generation and release of free radicals or reactive species by immune cells, which have important physiological roles as they contribute to their immunological functions. (Pereira et al., 2015) Reactive species are highly reactive molecules that participate in a variety of cellular signaling processes and are responsible for multiple protein modifications. On the other hand, overproduction of reactive species can be harmful and their continuous release in the local microenvironment of actively inflamed intestinal regions can lead to extensive cellular and molecular damage, and eventually, tissue destruction. (Campbell et al., 2019; Biasi et al., 2013) At the level of the intestinal mucosa, both inflammation and oxidative stress can lead to diminished functioning of the intestinal barrier. This barrier is crucial to protect the host against adhesion and infiltration of intraluminal antigens into the underlying mucosa and is governed by the mucosal immune system. (Antoni et al., 2014) Although it is not yet fully understood whether a defective intestinal barrier promotes intestinal inflammation and oxidative stress or that a disrupted barrier integrity is the result of inflammation and oxidative stress, it is clear that these processes are closely linked in terms of disease pathophysiology. (Ahmad et al., 2017)
Oxidative stress, which can be defined as a pathological imbalance between the production and clearance of reactive oxygen species (ROS), is a key pathophysiological mechanism in IBD. (Bourgonje et al., 2020) Oxidative stress not only occurs at the level of the intestinal mucosa, as locally-produced reactive species in the intestine also enter the bloodstream, resulting in gut-derived oxidative stress to be ‘mirrored’ within the systemic circulation. (Guan et al., 2018; Moura et al., 2015; Bourgonje et al., 2019) Oxidative stress can be quantified by measuring specific components of the systemic redox status. (Banne et al., 2003) In addition to ROS, also other reactive species, such as reactive nitrogen species (RNS) and reactive sulfur species (RSS), participate in the chemical reactions that determine an individual’s redox status. (Cortese-Krott et al, 2017)
Over the past decades, many attempts have been made to quantify oxidative stress in patients with IBD. (Pereira et al., 2015; Zhu et al., 2012; Kruidenier et al., 2002) Many previous studies on oxidative stress quantification in IBD focused on the measurement of oxidized molecules in either intestinal tissue or blood. For instance, these oxidized molecules included oxidized proteins, lipid peroxidation products, oxidative byproducts of DNA metabolism, or oxidized forms of antioxidant substances (e.g. cysteine and glutathione). However, it is not always clear what particular processes all these biomarkers actually mark within the redox signaling network, which could mean that these individual quantitative assessments may not be fully representative of the overall redox state, but instead reflect spill-over products that originate from local inflammatory processes. (Cortese-Krott et al., 2017) In other words, a comprehensive understanding of their (patho)physiological roles and mutual interactions is lacking. (Santolini et al., 2019) In this respect, it may be more informative to focus on a wider panel of redox biomarkers instead of only one or two biomarkers to accurately determine the dynamics of oxidative stress in IBD.
Recently, advances in “multi-omics” technologies led to an increased advocacy of a systems-biology approach in order to gain more comprehensive insight into host genetics, the gut microbiome, host metabolism, and biochemistry, from which several factors may underlie the pathogenesis of IBD. (Metwaly et al., 2019) For instance, recent breakthroughs have been made by combining metagenomics, metatranscriptomics, metaproteomics, and metabolomics data in patients with IBD. (Lloyd-Price et al., 2019) Given the variety of biomolecules affected by oxidative stress (DNA, proteins and lipids) and the variability in the oxidative stress-inflicted changes, no high throughput “omics” platform is yet available. (Cumpstey et al., 2019) Still, detailed analysis of biomarkers of oxidative stress and gut barrier integrity is very promising to improve our understanding of the pathophysiology of IBD. For instance, readouts of redox signaling and gut barrier integrity could be combined with other multi-omics datasets and detailed phenotypic data to unravel the interplay between the host redox system and to discover its potential determinants from different biological angles (e.g. genomics, transcriptomics, proteomics and the gut microbiome). (Cortese-Krott et al., 2017; Bourgonje et al., 2020). For this proposal, we made a comprehensive selection of documented biomarkers of oxidative stress and gut barrier integrity, which will be analyzed in a most efficient way to determine their association with IBD disease factors.