The phosphatidylethanolamine N-methyltransferase pathway is quantitatively not essential for biliary phosphatidylcholine secretion

Abstract

The phosphatidylethanolamine N-methyltransferase (PEMT) pathway of phosphatidylcholine (PC) biosynthesis is not essential for the highly specific acyl chain composition of biliary PC. We evaluated whether the PEMT pathway is quantitatively important for biliary PC secretion in mice under various experimental conditions. Biliary bile salt and PC secretion were determined in mice in which the gene encoding PEMT was inactivated (Pemt−/−) and in wild-type mice under basal conditions, during acute metabolic stress (intravenous infusion of the bile salt tauroursodeoxycholate), and during chronic metabolic stress (feeding a taurocholate-containing diet for 1 week). The activity of CTP:phosphocholine cytidylyltransferase, the rate-limiting enzyme of PC biosynthesis via the CDP-choline pathway, and the abundance of multi-drug-resistant protein 2 (Mdr2; encoded by the Abcb4 gene), the canalicular membrane flippase essential for biliary PC secretion, were determined. Under basal conditions, Pemt−/− and wild-type mice exhibited similar biliary secretion rates of bile salt and PC (∼145 and ∼28 nmol/min/100 g body weight, respectively). During acute or chronic bile salt administration, the biliary PC secretion rates increased similarly in Pemt−/− and control mice. Mdr2 mRNA and protein abundance did not differ between Pemt−/− and wild-type mice. The cytidylyltransferase activity in hepatic lysates was increased by 20% in Pemt−/− mice fed the basal (bile salt-free) diet (P < 0.05). We conclude that the biosynthesis of PC via the PEMT pathway is not quantitatively essential for biliary PC secretion under acute or chronic bile salt administration.

Keywords

• biliary lipids
• bile salts
• phosphatidylcholine biosynthesis
• liver
• cholesterol