Colorectal cancer in inflammatory bowel disease: a review of the role of gut microbiota and bacterial biofilms in disease pathogenesis

The risk of colorectal cancer (CRC) is increased in patients with inflammatory bowel disease (IBD), particularly in extensive ulcerative colitis (UC) and Crohn's colitis. Gut microbiota have been implicated in the pathogenesis of CRC via multiple mechanisms, including the release of reactive oxygen species and genotoxins, and induction of inflammation as well as activation of the immune response. Gut microbiota can enhance their carcinogenic and pro-inflammatory properties by organizing into biofilms, potentially making them more resistant to the host's immune system and to antibiotics. Colonic biofilms have the capacity to invade colonic tissue and accelerate tumorigenesis in tumor-prone models of mice. In the context of IBD, the prevalence of biofilms has been estimated to be up to 95%. Although the relationship between chronic inflammation and molecular mediators that contribute to IBD-associated CRC is well established, the role of gut microbiota and biofilms in this sequence is not fully understood. Because CRC can still arise in the absence of histologic inflammation, there is a growing interest in identifying chemopreventive agents against IBD-associated CRC. 5-aminosalicylates, commonly used in the treatment of UC, have antimicrobial and anti-carcinogenic properties that might have a role in the chemoprevention of CRC via the inhibition or modulation of carcinogenic gut microbiota and potentially biofilm formation. Whether biologics and other IBD-targeted therapies can decrease the progression towards dysplasia and CRC via mechanisms independent of inflammation is still unknown. Further research is warranted to identify potential new microbial targets of therapy for chemoprevention of dysplasia and CRC in IBD.

Crohn's Disease-Associated Pathogenic Mutation in the Manganese Transporter ZIP8 Shifts the Ileal and Rectal Mucosal Microbiota Implicating Aberrant Bile Acid Metabolism

BACKGROUND

A pathogenic mutation in the manganese transporter ZIP8 (A391T; rs13107325) increases the risk of Crohn's disease. ZIP8 regulates manganese homeostasis and given the shared need for metals between the host and resident microbes, there has been significant interest in alterations of the microbiome in carriers of ZIP8 A391T. Prior studies have not examined the ileal microbiome despite associations between ileal disease and ZIP8 A391T.

METHODS

Here, we used the Pediatric Risk Stratification Study (RISK)  cohort to perform a secondary analysis of 16S ribosomal RNA gene sequencing data obtained from ileal and rectal mucosa to study associations between ZIP8 A391T carrier status and microbiota composition.

RESULTS

We found sequence variants mapping to Veillonella were decreased in the ileal mucosa of ZIP8 A391T carriers. Prior human studies have demonstrated the sensitivity of Veillonella to bile acid abundance. We therefore hypothesized that bile acid homeostasis is differentially regulated in carriers of ZIP8 A391T. Using a mouse model of ZIP8 A391T, we demonstrate an increase in total bile acids in the liver and stool and decreased fibroblast growth factor 15 (Fgf15) signaling, consistent with our hypothesis. We confirmed dysregulation of FGF19 in the 1000IBD cohort, finding that plasma FGF19 levels are lower in ZIP8 A391T carriers with ileocolonic Crohn's disease.

CONCLUSIONS

In the search for genotype-specific therapeutic paradigms for patients with Crohn's disease, these data suggest targeting the FGF19 pathway in ZIP8 A391T carriers. Aberrant bile acid metabolism may precede development of Crohn's disease and prioritize study of the interactions between manganese homeostasis, bile acid metabolism and signaling, and complicated ileal Crohn's disease.

Induction of the metal transporter ZIP8 by interferon gamma in intestinal epithelial cells: Potential role of metal dyshomeostasis in Crohn's disease

Transition metals are required for intestinal homeostasis and provide essential nutrients for the resident microbiota. Abnormalities in metal homeostasis are common in Crohn's disease (CD), but remain poorly defined and causes appear multifactorial. There has been renewed interest in understanding these mechanisms with the discovery of an association between a coding variant in SLC39A8 (rs13107325; ZIP8 A391T) and increased CD risk. SLC39A8 encodes the protein ZIP8, a metal transporter that is induced under inflammatory stimuli; however, studies of its gut-specific functions are lacking. Here, we show that SLC39A8 mRNA is differentially expressed in active CD with a high positive correlation with markers of disease severity, including CXCL8, TNFα, IFNγ, and calprotectin. SLC39A8 expression exhibits a negative correlation with SLC39A4 and SLC39A5, two key zinc importers in absorptive enterocytes, and a lack of correlation with two manganese transporters, SLC39A14 and SLC11A2. Immunohistochemistry demonstrates ZIP8 expression in intestinal epithelial cells and immune cells of the lamina propria. Patients with CD exhibit variable patterns of ZIP8 subcellular localization within IECs. In ileal enteroids, SLC39A8 was induced by IFNγ and IFNγ + TNFα, but not by TNFα alone, independent of NF-κB activation. IFNγ also down-regulated SLC39A5. To explore the functional implications of disease-associated genetic variation, in over-expression experiments in HEK293A cells, ZIP8 A391T was associated with increased TNFα-induced NF-κB activation, consistent with a loss of negative regulation. Taken together, these results suggest a potential role for ZIP8 in intestinal inflammation, induced by IFNγ in the intestinal epithelial compartment, and that perturbations in negative regulation of NF-κB by ZIP8 A391T may contribute to CD pathogenesis.