Mechanisms by which grape polyphenols promote metabolic resilience
Creators: name: Mezhibovsky, Esther
Publisher: name: No Publisher Supplied
Year: 2023

description: Epidemiological, clinical, and preclinical studies indicate that dietary polyphenols are protective against chronic metabolic disease, such as metabolic syndrome (MetS) and type-2 diabetes (T2D). Exact mechanisms have been elusive as polyphenols are poorly absorbed into circulation but reach high levels in the gastrointestinal tract, where they interact with luminal contents and modify gut microbial communities, with some bacteria increasing in relative abundance while others are decreased due to the polyphenol-mediated antimicrobial activities. Gut bacteria in turn metabolize polyphenols to metabolites that are more bioavailable to the host. Dietary polyphenol supplementation in humans and rodent models have been shown to attenuate symptoms and markers of metabolic disease, likely through the combined mechanisms of reducing inflammatory tone and gut dysbiosis. Concord grape skins and seeds are particularly abundant in proanthocyanidin (PAC) compounds. PAC-rich grape polyphenol (GP) extracts have been associated with reduced symptoms of MetS/T2D in association with marked changes in the gut microbial community. Notably, GP supplemented mice showed a bloom in Akkermansia muciniphila, a gut microbe generally associated with healthy metabolic phenotypes. Gut microbiota biotransform PACs into PAC-derived microbial metabolites (MM) that may be absorbed into circulation, become substrates for other bacteria via cross-feeding, or act as signaling molecules that communicate with the intestinal epithelium and peripheral organs. By modulating the gut microbial community, GP/PAC-supplementation may also alter the production of secondary bile acids (SBAs) and short chain fatty acids (SCFAs), which signal to host cell receptors regulating nutrient and energy metabolism Thus, polyphenol-induced alteration of gut microbial communities can affect host health indirectly via bioactive MM. The overarching goal of the following work was to investigate mechanisms by which GP and PAC-metabolites may alter the host intestine, resident gut bacteria, and resulting metabolite products to promote metabolic resilience. The studies detailed in chapters 2, 3, 4, and 5 provide preclinical evidence for therapeutic potential of GPs. Previous studies suggested GP-supplementation reduced metabolic endotoxemia, as evidenced by increased expression of tight junction proteins, reduced levels of serum lipopolysaccharide (LPS) , and reduced markers of intestinal inflammation. Based on these data, it was hypothesized that GPs would increase the mucus layer that provides protective separation between the intestinal epithelium and gut bacteria. Surprisingly, the Chapter 2 study showed that mice supplemented with GPs had a depletion of the intestinal mucus layer in association with increased relative abundance of A. muciniphila and reduced intestinal gene expression of LPS binding protein (LBP), suggesting reduced circulating LPS and reduced intestinal inflammation. This data suggested that GP-induced changes to the intestinal milieu may mitigate threats within the gut lumen and reduce the need for a thick mucus layer and elevated LBP expression. To investigate the hypothesis that GPs improved metabolic health in a manner associated with microbial metabolites in Chapter 3 mice were fed a Western-style diet (WD) high in butter fat and sucrose and levels of short chain fatty acids were measured in cecal content. We showed that compared to WD-fed mice, mice fed WD supplemented with GPs showed significantly less body weight and hepatic steatosis and elevated energy expenditure, though had similar glucose tolerance. GP-supplementation were found to reduce butyrate levels in cecal content in association with increased gene markers for carbohydrate oxidation by the intestinal epithelium. These results led to the hypothesis that GPs could protect from fatty liver by increasing intestinal carbohydrate disposal and reducing delivery of lipogenic butyrate and fructose to the liver. In chapter 4 we investigated the effect GPs on metabolic phenotypes, i.e., body composition, glucose tolerance and energy expenditure using a 0.5% w/w GP diet rather than the 1% GP dose previously used. This study showed mice on a high lard fat diet supplemented with 0.5% w/w GPs had reduced liver fat and body weight with elevated energy expenditure but had no significant changes to glucose metabolism or SCFA levels. Mice supplemented with 0.5% GPs showed significant alterations in bile acid (BA) profile with reduced concentrations of several BAs with hydrophobic and cytotoxic properties. Notably, species able to inhibit the hypothalamic pituitary adrenal axis (HPAA) were reduced and several markers of improved HPAA homeostasis were increased by GP supplementation. These data suggested that GPs could preserve energy balance by preventing high fat diet induced dampening of the HPAA.In Chapter 5, an in-silico approach was used to investigate PAC and PAC-derived MM as competitive inhibitors for SARS-CoV-2 viral proteins and host immune proteins. Epidemiological studies indicated that, although positive for SARS-CoV-2, people who consumed diets rich in fruits and vegetables had less severe or no symptoms of COVID-19. AutoDock Vina docking and Yasara molecular dynamics showed that several polyphenols interacted with target viral or host complement proteins, as effectively as known drug inhibitors. These findings provide a mechanism by which dietary polyphenols may reduce viral infection/replication and the cytokine storm. The final project included a longitudinal human intervention study, designed to obtain a biological signature of GP supplementation in healthy subjects. A comprehensive metabolic panel (CMP) was obtained before and after GP intervention to assess metabolic health. Levels of PAC-derived MM were quantified across feces, urine, and blood. Whole-genome sequencing and metaproteomics was performed on fecal gDNA to measure bacterial community and functions in response to GPs. GP supplementation resulted in a reduction in fasting blood glucose that was consistent with increases to hyocholic acid (HCA) and ursodeoxycholic acid (UDCA), BAs associated with improved glucose metabolism and liver health. Sexual dimorphic changes to BA were observed. While metabolite changes were sustained, GPs induced mainly transient changes to bacterial guilds and proteins. This study provided insight into candidate bioactive PP, MM, and bile acids, and important modulations to gut bacteria that could confer metabolic health in humans.