has a role in the progression of steatosis to steatohepatitis that was
observed in Pemt−/− mice fed the CD diet for 3 days [59].
An abnormal ratio of PC/PE also can impair other aspects of liver
function. In Pemt−/− mice, a decreased PC/PE ratio was associated
with lower survival rates after partial hepatectomy [66]. Moreover,
the ratio of PC to PE was inversely correlated with the development of
non-alcoholic fatty liver disease (NAFLD) in mice fed a high fat diet.
The reduction in survival after partial hepatectomy and the development
of NAFLD were reversed when the PC/PE ratio was normalized
by supplementation of the diet with additional choline. In contrast,
the PC to PE ratiowas increased in livers fromobese mice that exhibited
ER stress and impaired calcium homeostasis [67]. Moreover, when
the activity of hepatic PEMT was attenuated by 50–70% by shRNA, the
PC/PE ratio was normalized and calcium transport in the ER was improved.
Thus, an optimal level of PC and PE appears to be required for
normal hepatic function.
As mentioned in Section 5, the supply of AdoMet can modulate PE
methylation. In recent experiments a very large (40-fold) increase in
AdoMet stimulated the conversion of PE to PC [68].
This increase in AdoMet occurred in mice that lacked glycine N-methyltransferase, an
enzyme that normally consumes large amounts of AdoMet. This large
increase in AdoMet stimulated the conversion of PE to PC via PEMT.
Moreover, the increased activity of PEMT enhanced the hepatic secretion
of PC into plasma lipoproteins and stimulated the conversion of
PEMT-derived PC to triacylglycerol (TG) leading to steatosis. Supplementation
of the mice lacking glycine N-methyltransferase with a
methionine-deficient diet normalized hepatic AdoMet levels and restored
hepatic lipids to control levels [68].
PC can also be a source of hepatic TGas previously reported in rat hepatocytes
[69]. More recent studies have demonstrated that PC from
both low density and high density lipoproteins can be delivered to hepatocytes
[70,71]. Much of this PC is converted into TG. A quantitative
estimation of the amount of PC delivered from lipoproteins to the liver
inmice suggests that ~50% of hepatic PC is derived fromplasma lipoproteins,
and remarkably that 30% of this lipoprotein-derived PC is converted
into TG [72]. Thus, lipoprotein derived PC is an important source of
both PC and TG in the liver.