Phosphatidylcholine homeostasis and liver failure

In mammals, the only endogenous pathway for choline biosynthesis is the methylation of phosphatidylethanolamine to phosphatidylcholine (PC) by phosphatidylethanolamine N-methyltransferase (PEMT) coupled to PC degradation. Complete choline deprivation in mice by feeding Pemt^-/- mice a choline-deficient (CD) diet decreases hepatic PC by 50% and is lethal within 5 days. PC secretion into bile is mediated by a PC-specific flippase, multiple drug-resistant protein 2 (MDR2). Here, we report that mice that lack both PEMT and MDR2 and are fed a CD diet survive for >90 days. Unexpectedly, the amount of PC also decreases by 50% in the livers of Mdr2^-/-/Pemt ^-/- mice. The Mdr2^-/-/Pemt^-/- mice adapt to the severe choline deprivation via choline recycling by induction of phospholipase A₂, choline kinase, and CTP:phosphocholine cytidylyltransferase activities and by a strikingly decreased expression of choline oxidase. The ability of Mdr2^-/-/Pemt^-/- mice to survive complete choline deprivation suggests that acute lethality in CD-Pemt^-/- mice results from rapid depletion of hepatic PC via biliary secretion.