Bioavailable Microbial Metabolites of Flavanols Demonstrate Highly Individualized Bioactivity on In Vitro β-Cell Functions Critical for Metabolic Health

Dietary flavanols are known for disease preventative properties but are often poorly absorbed. Gut microbiome flavanol metabolites are more bioavailable and may exert protective activities. Using metabolite mixtures extracted from the urine of rats supplemented with flavanols and treated with or without antibiotics, we investigated their effects on INS-1 832/13 β-cell glucose stimulated insulin secretion (GSIS) capacity. We measured insulin secretion under non-stimulatory (low) and stimulatory (high) glucose levels, insulin secretion fold induction, and total insulin content. We conducted treatment-level comparisons, individual-level dose responses, and a responder vs. non-responder predictive analysis of metabolite composition. While the first two analyses did not elucidate treatment effects, metabolites from 9 of the 28 animals demonstrated significant dose responses, regardless of treatment. Differentiation of responders vs. non-responder revealed that levels of native flavanols and valerolactones approached significance for predicting enhanced GSIS, regardless of treatment. Although treatment-level patterns were not discernable, we conclude that the high inter-individual variability shows that metabolite bioactivity on GSIS capacity is less related to flavanol supplementation or antibiotic treatment and may be more associated with the unique microbiome or metabolome of each animal. These findings suggest flavanol metabolite activities are individualized and point to the need for personalized nutrition practices.

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

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.