Novel in vitro bioactivity of (poly)phenolic human metabolites

Abstract

Several epidemiological studies support a role of a diet rich in fruits and vegetables in the prevention of different chronic diseases. This potential has been partially attributed to the high content of these dietary items in (poly)phenolic compounds. (Poly)phenols are one of the most copious and ubiquitous groups of plant secondary metabolites, counting more than 8000 structures. Therefore, they are highly present in the diet, occurring in a wide number of foods and beverages, especially fruits, vegetables, coffee, tea and red wine. A large body of evidence has been produced in the last two decades, describing the anti-obesity, anti-diabetic, antihypertensive and antihyperlipidemic effects of (poly)phenolic compounds, therefore supporting the role of these phytochemicals in the prevention of metabolic syndrome and its related pathologies. Indeed, metabolic syndrome is defined as a combination of interrelated risk factors for the development of cardiovascular diseases and type II diabetes, namely central obesity, insulin resistance, dyslipidaemia and hypertension. The exact cellular mechanisms responsible for the health benefits of (poly)phenols are still elusive. Many in vitro studies have tried to address their bioactivity, but in most cases a misleading approach has been used. Actually, most studies evaluated either (poly)phenol‐rich food extract or native compound properties, without taking into consideration the extensive metabolism that these compounds undergo in humans and that likely modifies their biological effects. Based on this evidence, this PhD thesis aims to evaluate the in vitro bioactivity of some selected (poly)phenolic metabolites, in the framework of the metabolic syndrome and its related pathologies. The first part of the thesis focuses on the role of (poly)phenols in the control of obesity. Being obesity the result of the imbalance between energy intake and energy expenditure, the enhancement of energy expenditure is a promising solution to promote weight loss. Activated brown adipose tissue (BAT) can dissipate energy in the form of heat, so increasing BAT mass represents a potential therapeutic approach to develop new anti-obesity treatments, as supported by recent studies reporting highly metabolically active BAT in humans. Relevant in vivo evidence suggests that the anti-obesity effects of a specific class of flavonoids, flavan-3-ols, may be related to their capacity to enhance energy expenditure and activate BAT. Therefore, their most relevant colonic metabolites, phenyl-γ-valerolactones, have been tested in a cell line of brown adipocytes. Despite the in vivo evidence, these compounds did not affect brown adipocyte differentiation nor activation. However, in a condition of oxidative stress, they protected brown adipocytes from increased reactive oxygen species production. These results do not exclude the physiological relevance of phenyl-γ-valerolactones, but rather suggest addressing their bioactivity in other cell types, important in the framework of obesity, such as white adipocytes, macrophages, hepatocytes or muscle cells. In the second part of the thesis, the protective effects of (poly)phenols in the development of atherosclerosis, one of the main pathologies related to metabolic syndrome, have been investigated. Atherosclerosis is characterized by impaired endothelial function and lipid metabolism, leading to accumulation of cholesterol-loaded macrophages (foam cells) in the intima of blood vessels. Ellagitannins are a class of (poly)phenolic compounds that have been suggested to play a role in cardiovascular health. The in vitro bioactivity of both ellagic acid and its colonic metabolites, urolithins, have been evaluated on different key components of the atherosclerotic lesion development. Our results support the hypothesis of a protective role of this class of (poly)phenols in the framework of atherosclerosis development and progression. Actually, some urolithins and ellagic acid were able to reduce macrophage cholesterol accumulation and to exert anti-inflammatory effects on endothelial cells, limiting their adhesiveness to monocytes.