The TMAO Generating Enzyme Flavin Monooxygenase 3 is a Central Regulator of Cholesterol Balance

Circulating levels of the gut microbe-derived metabolite trimethylamine-N-oxide(TMAO) have recently been linked to cardiovascular disease (CVD) risk. Here we performed transcriptional profiling in mouse models of altered reverse cholesterol transport (RCT), and serendipitously identified the TMAO-generating enzyme flavin monooxygenase 3 (FMO3) as a powerful modifier of cholesterol metabolism and RCT. Knockdown of FMO3 in cholesterol-fed mice alters biliary lipid secretion, blunts intestinal cholesterol absorption, and limits the production of hepatic oxysterols and cholesteryl esters. Furthermore, FMO3 knockdown stimulates basal and liver X receptor (LXR)-stimulated macrophage RCT, thereby improving cholesterol balance. Conversely, FMO3 knockdown exacerbates hepatic ER stress and inflammation in part by decreasing hepatic oxysterol levels and subsequent LXR activation. FMO3 is thus identified as a central integrator of hepatic cholesterol and triacylglycerol metabolism, inflammation, and ER stress. These studies suggest that the gut microbiota-driven TMA/FMO3/TMAO pathway is a key regulator of lipid metabolism and inflammation. To identify potential regulators of macrophage reverse cholesterol transport (RCT), liver was isolated from two independent mouse models where the non-biliary RCT pathway known as transintestinal cholesterol excretion (TICE) was either chronically (NPC1L1-liver-transgenic mice) or acutely (ACAT2 antisense oligonucleotide treatment) stimulated. Total RNA was isolated and gene expression levels were profiled on the Affymetrix GeneAtlas MG-430 PM Array Strip (Affymetrix; Santa Clara, CA, USA)