ROLE OF MICROBIOTA IN DIABETES-ASSOCIATED COLORECTAL CANCER

Abstract

Diabetes mellitus is a complex disease that affects the whole-body metabolism and energy utilization. The disturbances of these vital functions affect body organs in different drastic ways. Not long ago, several studies have reported an association between diabetes mellitus and cancer. However, the mechanistic link between these two diseases is still speculative. Diabetes mellitus is accompanied by a chronic state of inflammation that can eventually lead to many types of cancers including colorectal cancer (CRC). Diabetes changes the gastrointestinal environment and hence the microbiota of the host, a condition known as dysbiosis. Dysbiosis have emerged to be an important contributing factor for the pathogenesis of diabetes and CRC. Throughout this PhD work, we focused on: identifying the microbial signature associated with diabetes and CRC; and identifying the signaling mechanism altered by dysbiosis and leading to the progression of CRC in diabetes. MKR mice that can spontaneously develop type 2 diabetes were used in our work. For CRC induction, another subset of mice was treated with azoxymethane and dextran sulfate sodium, to identify the link between both diabetes and CRC. Fecal samples were collected at different time points to detect the differences in microbiota that occur along the course of the diseases. In parallel experiments, a subset of control mice was depleted of their endogenous microbiota after weaning, then mice were inoculated with fecal microbial transplant from diabetic mice and mice with CRC. After 5 weeks of transplantation, all groups of mice were treated with azoxymethane and dextran sulfate sodium. Further, and to verify if dysbiosis is playing a role in the observed pathogenesis, a subset of the diabetic mice and diabetic mice with CRC were treated with probiotics or butyrate to determine the beneficial effect of probiotics on the progression of these chronic conditions. At the end of the treatment, 16S rRNA sequencing was performed to identify different microbial communities in the fecal samples. Besides, at sacrifice, blood was collected, and colons were harvested for molecular, anatomical, histological, and biochemical analysis. Our analysis focused on inflammatory, and reactive oxygen species (ROS) production pathways. Our results show that diabetes is associated with a defined microbial signature that is characterized by reduction of butyrate-forming bacteria. This dysbiosis is associated with gastrointestinal complications reflected by a reduction in colon lengths. These changes are reversed upon treatment with probiotics or butyrate, which rectified the observed dysbiosis. Inoculation of control mice with diabetic microbiota and cancer microbiota resulted in the development of more aggressive CRC and higher number of polyps after chemical induction of CRC. Diabetes-associated dysbiosis resulted in less production of beneficial butyrate in both cecal and fecal contents, which consequently led to over-activation of histone deacetylase (HDAC)3 enzymes. Our data also show that the downstream effectors of HDAC3, namely inflammatory cytokines (mainly interleukin (IL)-1) and NADPH oxidase (NOX)4 are over-expressed in diabetic mice as a result of activation of HDAC3 in colon tissue. Collectively our data suggest that diabetes is associated with dysbiosis characterized by lower abundance of butyrate-forming bacteria leading to less butyrate production and activation of HDAC3 resulting in over-expression of inflammatory cytokines and NOX4 leading to gastrointestinal complications and CRC.