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J. Biol. Chem., Vol. 279, Issue 23, 23916-23924, June 4, 2004

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A Choline-deficient Diet in Mice Inhibits neither the CDP-choline Pathway for Phosphatidylcholine Synthesis in Hepatocytes nor Apolipoprotein B Secretion^*

Agnes Kulinski, Dennis E. Vance¶||, and Jean E. Vance**

From the Canadian Institutes of Health Research Group on the Molecular and Cell Biology of Lipids and the Departments of Medicine and ^¶Biochemistry, University of Alberta, Edmonton, Alberta T6G 2S2, Canada

Phosphatidylcholine is a major component of very low density lipoproteins (VLDLs) secreted by the liver. Hepatic phosphatidylcholine is synthesized from choline via the CDP-choline pathway and from the phosphatidylethanolamine N-methyltransferase pathway. Elimination of the methyltransferase in male mice reduces hepatic VLDL secretion. Our objective was to determine whether inhibition of the CDP-choline pathway for phosphatidylcholine synthesis (by restricting the supply of choline) also impaired VLDL secretion. In mice fed a choline-deficient (CD), compared with a choline-supplemented, diet for 21 days, the amounts of plasma apolipoproteins (apo) B100 and B48 were reduced and the liver triacylglycerol content was increased. Hepatocytes were isolated from male mice that had been fed the CD diet for 3 or 21 days, and the cells were incubated with or without choline. The secretion of apoB100 and B48 from CD hepatocytes was not reduced, and triacylglycerol secretion was only modestly decreased, compared with that from cells supplemented with choline. Remarkably, in light of widely held assumptions, the rate of phosphatidylcholine synthesis from the CDP-choline pathway was not decreased in CD hepatocytes. Rather, there was a trend toward increased phosphatidylcholine synthesis that might be explained by enhanced CTP:phosphocholine cytidylyltransferase activity. Although the concentration of phosphocholine in CD hepatocytes was reduced, the size of the phosphocholine pool remained well above the K for the cytidylyltransferase. Moreover, the amount and m activity of the cytidylyltransferase and methyltransferase were increased. The reduction in plasma apoB in mice deprived of dietary choline cannot, therefore, be attributed to decreased apoB secretion.

Received for publication, November 19, 2003 , and in revised form, February 20, 2004.

^* This work was supported in part by a grant from the Heart and Stroke Foundation of Alberta, Northwest Territories and Nunavut. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^|| Holder of the Canada Research Chair in Molecular and Cell Biology of Lipids and Medical Scientist of the Alberta Heritage Foundation for Medical Research.

^** To whom correspondence should be addressed: 328 HMRC, University of Alberta, Edmonton, Alberta T6G 2S2, Canada. Tel.: 780-492-7250; Fax: 780-492-3383; E-mail: jean.vance{at}ualberta.ca.

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