Several mouse models with deletion of choline-related genes have given insight into the mechanisms
of NAFLD. In severalmousemodels, deletion of genes needed to use choline as a methyl donor (Bhmt [33&]
and Chdh [34&]), deletion of genes needed to form the choline moiety endogenously (Pemt [35]), or deletion
of genes needed tomake S-adenosylmethionine (Mat1 [36]) result in fatty liver. In humans, polymorphisms
in PEMT [37,38] are associated with NAFLD. These observations suggest that the methyl-donation function
of choline is important in the mechanism of NAFLD. Earlier, we discussed the hypothesis that
phosphatidylcholine was required for normal VLDL secretion from liver. The genetic data suggest that
it is phosphatidylcholine that is derived from the PEMT methylation pathway which is important
(rather than phosphatidylcholine derived from preformed choline); mouse studies support this
conclusion [39].
Endoplasmic reticulum (ER) stress is a condition whereby excess unfolded proteins lead to a cascade
of stress responses. If stress is chronic, cell death can occur. ER stress is believed to play a role in the
pathogenesis of NAFLD [51]. In mice fed methionine–choline-deficient diets for up to 21 days, hepatic
steatosis was associated with inducing specific ER stress cascades upstream of the unfolded protein
response. The integrated ER stress response was unable to cause liver injury in the absence of steatosis,
suggesting a coordinated mechanism is necessary for liver disease progression [52]. Another link
between choline, NAFLD, and ER stress was found when metabolomic and proteomic studies in obese,
leptin-deficient mice revealed that the obese phenotype is characterized by ER stress, increased
expression of proteins involved in lipogenesis and phospholipid metabolism (including PEMT), and a
distinct lipid profile characterized by increased monounsaturated fatty acids and an increased phosphatidylcholine
to phosphatidylethanolamine concentration ratio. This altered ratio impairs calcium
signaling and ER homeostasis [53&].
The study of the influence of the gut microbiome on human health has advanced tremendously.
The gut microbiome integrates many important pathways, including those related to
enterohepatic circulation of bile, cholesterol, and phospholipids [56]. The gut flora modulates host
immunity [57], glucose, lipid, and energy metabolism [58], and choline availability [59], all of which
play a role in NAFLD [60]. Gut microbiome composition is influenced by multiple factors such as
maternal diet, lifelong diet, environmental exposures, and genetics [61]. Gammaproteobacteria and
Erysipelotrichi within the gut microbiome were directly associated with changes in liver fat in
humans during choline depletion. Levels of these bacteria, change in amount of liver fat, and a singlenucleotide
polymorphism (PEMT rs12325817) that affects choline biosynthesis were combined into a model that accurately predicted the degree to which
patients developed fatty liver on a choline-deficient diet [4&]. This suggests that understanding the
effects of the microbiome can enhance current paradigms defining NAFLD risk and progression
