Regulation of Trimethylamine N-oxide in Treatment of Cardiometabolic Diseases. Doctoral Thesis

Abstract

Trimethylamine N-oxide (TMAO) is a gut microbiota-derived metabolite synthesised in host organisms from specific food constituents, such as choline, carnitine and betaine, that are highly abundant in products of animal origin. During the last decade, elevated TMAO level has been proposed as biomarker to estimate the risk of cardiometabolic diseases. In addition, elevated concentrations of TMAO have been associated with type 2 diabetes, heart failure, incidence of major adverse cardiovascular events and all-cause mortality. However, there is still no consensus on the exact role of TMAO in the pathogenesis of cardiovascular diseases, since regular consumption of TMAO-rich seafood, is considered beneficial for the primary prevention of cardiovascular events. Therefore, the objective of the present Thesis was to study TMAO-mediated signalling pathways in the pathophysiology of cardiometabolic diseases and to identify possible intervention options. To assess this aim, 3 preclinical studies and 1 clinical study were conducted. First, the impact of short-term treatment with high TMAO concentrations (up to 1 mM) on cardiac and vascular functionality was evaluated in ex vivo and in vivo rodent models. Second, the effects of long-term TMAO supplementation (120 mg/kg, 14 weeks) on cardiac functionality and energy metabolism were assessed in a monocrotaline-induced right ventricular heart failure model in rats. Next, the effect of metformin (250 mg/kg daily), the most widely prescribed oral antidiabetic drug worldwide, on TMAO reduction was tested in experimental model of type 2 diabetes in mice. Additional experiments with bacterial monocultures were performed to test the possible TMAO regulatory mechanisms of action of metformin. Lastly, a fasting mimicking diet was tested for 5 days in healthy volunteers to observe the potential changes in plasma TMAO levels and general markers of metabolic health. An acute 2.5-fold increase in TMAO did not affect vascular functionality, but a higher input of fatty acid oxidation was observed in vascular energy metabolism. In cardiac tissue, acute elevation of the TMAO level did not affect cardiac function. However, in detrimental conditions of right ventricular heart failure, long-term administration of TMAO surprisingly prevented impairment of cardiac mitochondrial energy metabolism and preserved right ventricular function. Regarding potential approaches to reduce TMAO levels, metformin was able to decrease plasma concentrations of TMAO in the type 2 diabetes model in mice nearly twofold; these effects can be attributed to the impact of metformin on the composition and activity of intestinal microbiota. In the dietary intervention study, a 5-day cycle of fasting mimicking diet with limited animal-derived protein intake and caloric restriction was effective in reducing TMAO levels and improving overall metabolic health of the volunteers. To summarise, the obtained results indicate that a short-term increase in TMAO concentrations does not activate detrimental signalling pathways in cardiac and vascular tissues. Meanwhile, long-term elevation of TMAO levels can even serve as a preconditioning factor and protect cardiac function in the right ventricular heart failure model. Furthermore, the results of the present thesis provide evidence on the possibility of targeting TMAO levels using pharmacological approaches, namely metformin, and lifestyle approaches.