Trimethylamine N-Oxide: A Link among Diet, Gut Microbiota, Gene Regulation of Liver and Intestine Cholesterol Homeostasis and HDL Function

Abstract

Recent evidence, including massive gene-expression analysis and a wide-variety of other multi-omics approaches, demonstrates an interplay between gut microbiota and the regulation of plasma lipids. Gut microbial metabolism of choline and l-carnitine results in the formation of trimethylamine (TMA) and concomitant conversion into trimethylamine-N-oxide (TMAO) by liver flavin monooxygenase 3 (FMO3). The plasma level of TMAO is determined by the genetic variation, diet and composition of gut microbiota. Multiple studies have demonstrated an association between TMAO plasma levels and the risk of atherothrombotic cardiovascular disease (CVD). We aimed to review the molecular pathways by which TMAO production and FMO3 exert their proatherogenic effects. TMAO may promote foam cell formation by upregulating macrophage scavenger receptors, deregulating enterohepatic cholesterol and bile acid metabolism and impairing macrophage reverse cholesterol transport (RCT). Furthermore, FMO3 may promote dyslipidemia by regulating multiple genes involved in hepatic lipogenesis and gluconeogenesis. FMO3 also impairs multiple aspects of cholesterol homeostasis, including transintestinal cholesterol export and macrophage-specific RCT. At least part of these FMO3-mediated effects on lipid metabolism and atherogenesis seem to be independent of the TMA/TMAO formation. Overall, these findings have the potential to open a new era for the therapeutic manipulation of the gut microbiota to improve CVD risk.

Keywords: FMO3; atherosclerosis and cardiovascular disease; cholesterol homeostasis; intestinal microbiota; reverse cholesterol transport; trimethylamine; trimethylamine-N-oxide.

Figure 1

A schematic diagram of macrophage-to-feces reverses cholesterol transport pathways. ApoA-I is synthesized by the liver and small intestine, and it acquires phospholipids to become partially lipidated preβ-high-density lipoprotein (HDL) particles at its nascent stage. Preβ-HDL particles acquire free cholesterol from macrophages via the adenosine triphosphate-binding cassette (ABC) A1 transporter. The free cholesterol is converted into cholesteryl ester within the HDL particle by the action of Lecithin: cholesterol acyltransferase (LCAT), thereby resulting in mature HDL. ApoA-I is the major HDL protein and activates LCAT, whereas apoA-II, the second HDL protein, displaces apoA-I from the HDL particles. Both the macrophage scavenger receptor class B type I (SR-BI) and ABCG1 facilitate the cholesterol efflux process from macrophages to mature HDL. The phospholipid transfer protein (PLTP) promotes the transfer of phospholipids and free cholesterol from triglyceride-rich lipoproteins into HDL, producing a remodeling process where preβ-HDL particles can be generated. HDL cholesterol esters can be transferred to very low density lipoprotein (VLDL) or low-density lipoprotein (LDL) by the cholesteryl ester transfer protein (CETP) and be returned to the liver through the low-density lipoprotein receptor (LDLR) or other LDL and VLDL receptors. Another function of the liver is to take up HDL-associated cholesterol selectively via SR-BI. Cholesterol can be secreted into bile as unesterified cholesterol via the ABCG5/G8 heterodimer or used to synthesize bile acids (BA). The bile salt export pump (BSEP) is involved in the bile acid transport to bile. Niemann-Pick C1-like 1 (NPC1L1) is of crucial importance for absorbing macrophage-derived cholesterol in the small intestine. Cholesterol may also be excreted back to the lumen by the intestinal ABCG5/G8 heterodimer. The transintestinal cholesterol export (TICE) route promotes the flow of cholesterol from plasma to enterocytes and the intestinal lumen.

Figure 2

A schematic representation of pathways linking the gut microbiota, the formation of trimethylamine-N-oxide (TMAO) by flavin monooxygenase (FMO) 3 and the regulation of enterohepatic bile acid (BA) and cholesterol metabolism and macrophage reverse cholesterol transport (RCT) pathways. Black arrows indicate movement of TMA/TMAO and cholesterol through the body. Blunt-end arrows indicate activation (+) or inhibition (-) of specified receptors and transporters or pathways. Gut microbiota metabolism of choline and l-carnitine results in the formation of trimethylamine (TMA). In the liver, FMO3 converts TMA into TMAO. The potential effects of TMAO on the proatherogenic pathways include the promotion of foam cell formation by increasing macrophage scavenger receptors (↑) and the downregulation of the main liver bile acid (BA) synthetic enzymes, cyp7a1 and cyp27a1. Downregulation of these rate-limiting enzymes reduces intracellular levels of BA (↓). The BA pool size could impact the farnesoid X receptor (FXR)-mediated regulation of FMO3. In turn, this enzyme may regulate the liver X receptor (LXR) and peroxisome proliferator-activated receptor (PPAR) α signaling pathways, reducing liver inflammation (↓) and promoting hepatic lipogenesis and gluconeogenesis (↓). FMO3 impairs the cholesterol flux into the nonbiliary transintestinal cholesterol export (TICE) pathway (↓). TMAO also reduces Niemann-Pick C1-Like 1 (NPC1L1) and adenosine triphosphate-binding cassette (ABC) G5/G8 expression (↓), and inhibits intestinal cholesterol absorption. Collectively, the effects of TMAO/FMO3 on BA homeostasis and TICE impair the macrophage-to-feces RCT (↓).