Mass spectrometry- and nuclear magnetic resonance-based metabolomic studies comparing diseased versus healthy individuals have shown that microbial metabolites are often the compounds most markedly altered in the disease state. Recent studies suggest that several of these metabolites that derive from microbial transformation of dietary components have significant effects on physiological processes such as gut and immune homeostasis, energy metabolism, vascular function, and neurological behavior. Here, we review several of the most intriguing diet-dependent metabolites that may impact host physiology and may therefore be appropriate targets for therapeutic interventions, such as short-chain fatty acids, trimethylamine N-oxide, tryptophan and tyrosine derivatives, and oxidized fatty acids. Such interventions will require modulating either bacterial species or the bacterial biosynthetic enzymes required to produce these metabolites, so we briefly describe the current understanding of the bacterial and enzymatic pathways involved in their biosynthesis and summarize their molecular mechanisms of action. We then discuss in more detail the impact of these metabolites on health and disease, and review current strategies to modulate levels of these metabolites to promote human health. We also suggest future studies that are needed to realize the full therapeutic potential of targeting the gut microbiota.

Purpose of the review To summarize recent evidence supporting the use of reactive carbonyl species scavengers in the prevention and treatment of disease. Recent findings The newly developed 2-aminomethylphenol class of scavengers shows great promise in preclinical trials for a number of diverse conditions including neurodegenerative diseases and cardiovascular disease. In addition, new studies with the thiol-based and imidazole-based scavengers have found new applications outside of adjunctive therapy for chemotherapeutics. Summary Reactive oxygen species (ROS) generated by cells and tissues act as signaling molecules and as cytotoxic agents to defend against pathogens, but ROS also cause collateral damage to vital cellular components. The polyunsaturated fatty acyl chains of phospholipids in the cell membranes are particularly vulnerable to damaging peroxidation by ROS. Evidence suggests that the breakdown of these peroxidized lipids to reactive carbonyls species plays a critical role in many chronic diseases. Antioxidants that abrogate ROS-induced formation of reactive carbonyl species also abrogate normal ROS signaling and thus exert both beneficial and adverse functional effects. The use of scavengers of reactive dicarbonyl species represent an alternative therapeutic strategy to potentially mitigate the adverse effects of ROS without abrogating normal signaling by ROS. In this review, we focus on three classes of reactive carbonyl species scavengers: thiol-based scavengers (2-mercaptoethanesulfonate and amifostine), imidazole-based scavengers (carnosine and its analogs), and 2-aminomethylphenols-based scavengers (pyridoxamine, 2-hydroxybenzylamine, and 5’-O-pentyl-pyridoxamine) that are either undergoing pre-clinical studies, advancing to clinical trials, or are already in clinical use.

Recent evidence suggest that cardiovascular disease (CVD) risk depends on levels of functional HDL particles, not HDL-cholesterol. In CVD, increased oxidative stress generates reactive lipid species that alter HDL function. Isolevuglandins (isoLGs), generated in parallel to isoprostanes, are extremely reactive to lysine residues of proteins and headgroups of phosphatidylethanolamine (PE). Importantly, IsoLG protein and PE adducts are elevated in atherosclerosis. Recently, our group observed a 42% reduction of atherosclerotic lesion size when salicylamine (SAM), a small molecule scavenger of reactive dicarbonyls including IsoLG, was administered to LDLr -/- mice. Little is known about the consequences of IsoLG to HDL function. The aim of this study is to compare effects of IsoLG on apolipoprotein crosslinking, morphology and size of HDL to its functions: cholesterol efflux, apoA-I exchange and anti-inflammation. Human HDL was incubated overnight at 37°C with IsoLG. Thioglycolate-induced intraperitoneal macrophages were harvested from apoE -/- mice. IsoLG crosslinked structural apolipoproteins, apoA-I and apoA-II, starting at 0.3 mol IsoLG per mol apoA-I (0.3 eq). HDL modified with 3 eq IsoLG formed subpopulations of two distinct sizes, 6-13 nm and 16-23 nm. A 40.6±0.04% decrease in 3 H-cholesterol efflux from macrophages was observed at 1 eq IsoLG compared to unmodified control HDL. At this IsoLG concentration, HDL-ApoA-I exchange was reduced (P<0.01, n=4), from 47.4±2.8% with control HDL to only 24.8±5.8%, suggesting that IsoLG inhibited apoA-I from disassociating from HDL to interact with ABCA1. Intriguingly, IsoLG inhibited HDL’s protection against LPS-stimulated inflammatory response in macrophages at 0.03 eq as shown by IL-1β and TNFα mRNA expression comparable to LPS alone. At 0.1 eq IsoLG, HDL becomes pro-inflammatory, as indicated by a 927±309% increase in IL-1β mRNA expression (P<0.001). Unlike cholesterol efflux, these effects occurred independent of HDL apolipoprotein crosslinking. We report a novel pathway by which HDL becomes dysfunctional, by mechanisms involving IsoLG-mediated alterations of HDL proteins and structure. Future studies will pinpoint how IsoLG modifies HDL proteins (or lipids) to alter its function.

Background: The rise in obesity in the United States has led to a concomitant rise in prevalence of non-alcoholic fatty liver disease (NAFLD). The four stages of NAFLD include accumulation of triglyceride (hepatosteatosis), development of chronic inflammation (non-alcoholic steatohepatitis, NASH), fibrosis, and finally cirrhosis. Unlike wildtype C57BL6 mice, low density lipoprotein receptor (LDLR) -/- mouse fed a diet enriched in fat and cholesterol (Western Diet) progress to NASH and fibrotic stages of NAFLD. We showed that incorporating engineered bacteria expressing N-acyl phosphatidylethanolamine (NAPE) into the gut microbiota can inhibit development of obesity. NAPE is a precursor of N-acylethanolamines, which are bioactive lipids with anti-inflammatory functions. Here, we test the hypothesis that administering these NAPE-expressing bacteria inhibits development of NASH and fibrosis. Methods: NAPE-expressing E. coli Nissle 1917 (pNAPE-EcN, n=10), control Nissle 1917 (pEcN, n=10), or vehicle (veh, n=10) were given via drinking water to LDLR -/- mice fed a Western diet for 12 weeks. LDLR -/- mice fed a low fat diet (LFD) (n=10) were included for comparison. Results: pNAPE-EcN reduced adiposity by 26% compared with pEcN and veh (P<0.05). pNAPE-EcN also dramatically reduced hepatic triglyceride levels by 45% (p<0.05) and lipid droplet size, as well as the hepatic expressions of tissue necrosis factor α (TNFα, p<0.05), chemokine receptor 2 (CCR2, p<0.01), and tissue inhibitor of matrix metalloproteinase (TIMP1, p<0.05), consistent with reduced NASH and fibrosis. Sirius Red staining of liver sections further demonstrated reduced fibrosis. Because fatty liver is associated with atherosclerosis, we checked to see if pNAPE-EcN was able to reduce the development of atherosclerotic lesions. While serum cholesterol was reduced by 23% with pNAPE-EcN treatment (p<0.05), atherosclerotic lesion size in proximal or en face aortas only tended to be reduced (20%, 18.6%) but was not statistically significant. Conclusions: Our results demonstrate that incorporating therapeutically modified bacteria into the gut microbiota has potential to inhibit the development of NAFLD.