The effects of metformin on colorectal cancer growth and the involvement of the gut microbiome

Abstract

Metformin is the most common type 2 diabetes therapy, and may also reduce colorectal cancer growth. Currently, two mechanisms driving reduced cancer growth are considered: 1) Regulation of glucose and insulin levels, which may support cancer growth, and 2) Direct entry into cancer cells to activate the AMP-activated kinase (AMPK) protein, and inhibit cell growth pathways. The gut microbiome is the community of commensal microorganisms in the gastrointestinal tract. It is also affected by metformin, and may elevate production of short-chain fatty acids (SCFAs). Therefore, this thesis aimed to clarify how metformin may inhibit colorectal cancer growth and if the microbiome is involved. The hyperglycemic-responsive, murine-derived MC38 colon cancer cell line was used to test these effects. This model was confirmed to experience growth stimulation caused by high-fat diet (HFD) feeding in mice. Daily i.p. injections of metformin (100mg/kg) had no measurable effect on glucose and insulin sensitivity, or MC38 tumor growth. Oral metformin (250mg/kg) improved glucose tolerance and inhibited MC38 tumor growth in HFD-fed mice. To see if the gut microbiome is required for this effect, the antibiotic ampicillin was used to limit the gut microbiome. The addition of ampicillin blunted metformin’s glucose sensitization and tumor inhibition effects. A fecal microbiome transfer model was then used to isolate the role of the microbiome. Conventional mice fed HFD and gavaged with feces from metformin-treated donors experienced no glucose or insulin tolerance improvements. However, tumor growth was decreased by 30%, and serum SCFAs concentrations were elevated. The SCFA butyrate inhibited in vitro MC38 colony growth, but did not activate AMPK. These data suggest that metformin alters the gut microbiome, and fecal transfer from metformin-treated animals can uncouple MC38 tumor growth inhibition from the glucose homeostasis effects of metformin. These novel findings support a new mechanism for metformin to prevent cancer growth and development.