Mitochondria and microRNAs in obesity and after weight loss

Abstract

Colorectal cancer (CRC) is the 3rd most common cancer worldwide. CRC is initiated in colonocytes and, with advancing age, colonocytes accumulate mitochondrial mutations which contribute to age-related dysfunction and to increased cancer risk. Obesity, and its lifestyle determinants, physical inactivity and poor diet, also increase CRC risk. However, the effects of weight loss by bariatric surgery on mitochondrial defects in human colonoctyes and on CRC risk are unclear. Epigenetic mechanisms involving microRNAs that lead to dysregulated gene expression may mediate the effects of obesity and weight loss on CRC risk. I hypothesised that mitochondrial defects and microRNAs are i) elevated and aberrantly expressed in obese individuals compared with healthy non-obese individuals and ii) reduced and modulated by significant weight loss following bariatric surgery, respectively. Colorectal mucosal biopsies of obese patients listed for bariatric surgery were collected at baseline and six months post-surgery and at baseline only from non-obese Controls. Mitochondrial oxidative phosphorylation proteins complex I and IV and mitochondrial mass were quantified by immunofluorescence. Using Next Generation Sequencing and bioinformatics i) mitochondrial DNA was sequenced and ii) a panel of 8 microRNAs was selected and validated by quantitative PCR in colorectal mucosal biopsies. Greater adiposity and advancing age resulted in significantly more complex I and IV deficient crypts in the human colorectal mucosa but, at least after 6 months, weight loss following bariatric surgery had no significant effect on these mitochondrial defects. Neither excess adiposity nor significant weight loss resulted in differences in mtDNA mutations between the study groups. Expression of miR-31, miR-215, miR-3196 and miR-4516 was significantly higher in obese than in non-obese individuals. Weight loss reduced expression of miR-31, miR-215 and miR-3196 significantly to expression levels that were comparable with those in Controls. These differentially expressed microRNAs are implicated in pathways linked with inflammation, obesity and cancer. This research enables the broadening of our knowledge on the mechanistic pathways of obesity related CRC risk and provides novel evidence on the effects of intentional weight loss by bariatric surgery on these biomarkers in the colon.