Colon transcriptomic profile during colitis

irrespective of the type of diet or age. These findings were supported by other studies

using Il10-/- _{mice [6, 116, 305, 342, 343, 353]. The induced expression of genes such as}

REG3B, S100A9 and multiple members of the C-X-C chemokine subfamily in the colon

of Il10-/- _{mice has also been shown in UC patients compared to healthy subjects [359].}

Calprotectin is a complex formed by the proteins S100A8 and S100A9, highly abundant in the cytosolic fraction of neutrophils [360], and are elevated at local sites of inflammation. Faecal calprotectin was suggested as a biomarker of IBD [143], however, the EPA diet did not have an effect on the expression of genes S100A8 or S100A9 in the

colon of Il10-/- _mice.

The induction of genes in inflammatory pathways in the colon of Il10-/- _{mice was}

also associated with antigen presentation in those mice compared to C57BL/6J mice, regardless of diet or age. The antigen presentation pathway is vital for the development of innate and adaptive immunity, and a dysregulation in these responses has been suggested to be part of the pathogenesis in IBD [361]. The loss of intestinal barrier integrity in IBD may result in increased infiltration of bacteria into the mucosa, which

could increase antigen presentation (MHC class II) by antigen presenting cells to CD4+ _T

cells [362]. Following activation, CD4+ _{T cells differentiate into one of several Th cells}

that further drive the immune response by releasing cytokines, recruiting other immune cells to the site and assisting in maturation of other white blood cells such as B cells.

Another study in Il10-/- _{mice reported an abnormally high number of colonic mucosal}

CD4+ _{cells, with predominant Th1 activity that produced large amounts of cytokines}

[109], thus supporting the current findings.

Myeloid differentiation primary response gene 88 (MYD88), an important

regulator of the commensal microbiota along the GIT [271], was increased in expression

Il10-/- mice at 9 weeks of age, as observed here, may be an attempt to control the separation of microbiota and the epithelial surface during early stages of colitis.

Genes involved in digestive/absorptive processes and related to metabolism of, for example, amino acids, carbohydrates, lipids, and cofactors and vitamins were

decreased in expression in the colon of Il10-/- _{mice. This may suggest perturbations of}

numerous biochemical processes underlying colitis [266] and has been reported in other studies using mice with experimental colitis [6, 265, 268, 343]. Similar observations were made in the colon of IBD patients, where decreased expression of several genes involved in metabolism of fatty acids was reported (e.g. FATP 3 and 4 and FABP 2 and 6 [267]). Martínez-Augustin et al. [265] suggested that the reduced functionality of various metabolic processes in colitis may result in an energy deficiency for mucosal cells. This was supported by decreased expression of genes in energy metabolism in the colon of

Il10-/- mice, regardless of age, and also reported in other studies [265, 268]. It is further

proposed that this may cause a shortage of energy for processes which are energy- demanding, for example, selective permeability or xenobiotic metabolism.

The KEGG pathway Metabolism of xenobiotics by cytochrome P450 was

decreased in expression in the colon of Il10-/- _{mice compared to C57BL/6J mice when}

both were fed the OA diet. The GIT surface is the largest interface between the body and the outside environment and is constantly exposed to foreign substances. The CYP- mediated metabolism of xenobiotics is crucial to detoxify these foreign substances which

could otherwise lead to severe destruction of the tissue [262, 264]. In Il10-/- _{mice, loss of}

detoxification could further result in increased epithelial permeability, triggering the inflammatory response by infiltrating toxins and bacteria.

The metabolism of tryptophan was affected in the colon of Il10-/- _{mice compared}

to C57BL/6J mice at 9 and 12 weeks of age, with increased IDO1-mediated degradation of tryptophan to xanthurenic acid via kynurenine, but reduced genes that mediate the synthesis of tryptamine and serotonin. IDO1 was among the genes with the largest

increase in the colon of Il10-/- _{mice and has reportedly been elevated in expression in}

lesional biopsies from CD patients [365, 366]. It has been suggested that an increased IDO1 gene expression by antigen-presenting cells of the lamina propria may suppress T cell responses to induce immune tolerance, while in epithelial cells, IDO1 expression may specifically be induced to limit microbial invasion as a result of ongoing inflammation

[366]. Concomitantly with the induction of IDO1 gene expression, the expression of genes that lead to the synthesis of tryptamine and tryptamine-derived metabolites from tryptophan (e.g. DDC, INMT, AOC1, MAOA and MAOB), and serotonin from tryptophan

(e.g. TPH1 and DDC [367-369]), were decreased in expression in the colon of Il10-/- _mice

(vs. C57BL/6J mice), regardless of age. Similar observations were made in another study among UC patients, where levels of serotonin in colon biopsies were reduced compared to healthy control subjects [370]. The authors attributed these reductions to the gene TPH1, the rate-limiting enzyme in the biosynthesis of serotonin. Together this supports the findings of increased IDO1-mediated catabolism of kynurenine from tryptophan, concomitantly with lowered expression of genes along the serotonin and tryptamine pathways.

3.5.3.2

Colon transcriptomic profile in response to the EPA diet (vs. OA diet)