Gut Metabolite Trimethylamine N-oxide Protects β Cell Insulin Secretion by Reducing Oxidative Stress and Maintaining Insulin Granule Formation

Elevated circulating levels of the dietary metabolite trimethylamine N-oxide (TMAO) is associated with chronic diseases including cardiovascular disease (CVD) and obesity. While TMAO production via the gut microbiome-liver axis and distribution through the circulation is clear, its molecular effects on metabolic tissues are still unclear. Some clinical studies suggest that elevated TMAO levels increase the risk of type 2 diabetes (T2D) where pancreatic β cell insulin secretion is insufficient for blood glucose management. T2D promoting mechanisms limit functional β cell mass by reducing β cell viability and survival, inhibiting proliferation or decreasing insulin secretory function. We hypothesized that TMAO decreases functional β cell mass by one of these mechanisms to aggravate the T2D phenotype. Using the INS-1 832/13 β cell line and primary murine islets, we screened the effect of various TMAO concentrations on cell viability, proliferation, and function. These parameters were measured under standard and glucolipotoxic (GLT) culture conditions to mimic T2D. We investigated TMAO effects, GLT effects and combined effects. TMAO minimally affected viability, proliferation or function under standard culture conditions across 96-hours of treatment. Culturing with GLT impaired viability, proliferation and function after 24 hours of treatment, mimicking T2D onset. Interestingly, adding 40–80 μM TMAO protected against GLT mediated functional impairments in cells and islets. Further, GLT increased oxidative stress by 2.5-fold and adding TMAO was significantly protective. Electron microscopy reveals that GLT alters insulin granule density whereas TMAO maintains proper granule structure. These results reject our hypothesis. While TMAO has minor effects on β cells in standard culture conditions, TMAO is sufficient to improve GLT mediated β cell damage by decreasing oxidative stress and maintaining insulin granule formation. These results suggest an early compensatory role for TMAO in countering oxidative damage caused by glucolipotoxicity in β cell function during T2D onset. Funding for this study was provided by the Beatson Foundation and the US Department of Agriculture.