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

Abstract

Objectives 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. Methods 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. Results 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. Conclusions 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 Sources Funding for this study was provided by the Beatson Foundation and the US Department of Agriculture.

LC-QToF-Based Metabolomics Identifies Aberrant Tissue Metabolites Associated with a Higher-Fat Diet and Their ‘Reversion to Healthy’ with Dietary Probiotic Supplementation

“…Similarly, TMAO is an oxidation product of trimethylamine, a gut microbiome metabolite derived from either betaine, choline, or carnitine [ 41 , 42 , 43 ]. While an elevation in circulating TMAO has been linked to an increase in cardiovascular disease, this does not necessarily reflect its metabolic effect within the tissues [ 44 ]. For example, within the pancreas, TMAO supplementation has been shown to improve islet cell impairment in Type 2 diabetes [ 44 ].…”

“…While an elevation in circulating TMAO has been linked to an increase in cardiovascular disease, this does not necessarily reflect its metabolic effect within the tissues [ 44 ]. For example, within the pancreas, TMAO supplementation has been shown to improve islet cell impairment in Type 2 diabetes [ 44 ]. Hence, the reduction of TMAO within the pancreas of the higher-fat cohort of pigs may reflect altered microbiome activity and a reduction in islet cell functionality.…”