Expression of phosphatidylethanolamine N-methyltransferase-2 in McArdle-RH7777 hepatoma cells inhibits the CDP-choline pathway for phosphatidylcholine biosynthesis via decreased gene expression of CTP:phosphocholine cytidylyltransferase

Abstract

Phosphatidylethanolamine N-methyltransferase-2 (PEMT2) of rat liver was expressed in McArdle-RH7777 rat hepatoma cells, which lack endogenous PEMT activity. Expression of the enzyme was confirmed by assay of PEMT activity and immunoblotting. There was no change in the amount of phosphatidylcholine in the transfected cells [Cu, Houweling and Vance (1994) J. Biol. Chem. 269, 24531-24533], even though the expression of PEMT2 caused an increased incorporation of [methyl-3H]methionine and [3H]ethanolamine into phosphatidylcholine. In contrast, [3H]serine incorporation into phosphatidylcholine was only marginally enhanced by PEMT2 expression. Incorporation of [methyl-3H]choline into phosphatidylcholine was decreased by greater than 60%, suggesting that the CDP-choline pathway was inhibited as a result of PEMT2 expression. CTP:phosphocholine cytidylyltransferase (CT) activities in transfected cell lines were decreased in proportion to the level of expression of PEMT2. Immunoblot analyses showed a decrease in CT mass as a function of PEMT2 expression. In contrast, there was no change in the mass of protein disulphide-isomerase or the relative amounts of most proteins expressed in the PEMT2-transfected, compared with control, cells. Similarly, the expression of CT mRNA was decreased in PEMT2-expressing cells, whereas the mRNAs for protein disulphide-isomerase and actin were unchanged. When cell growth was slowed by incubating McArdle-RH7777 cells at 25 degrees C, compared with 37 degrees C, there was no difference in the specific activity of the CT. These results argue that PEMT2 expression down-regulates the CDP-choline pathway by decreasing the expression of the gene for the CT. The decreased activity of the CDP-choline pathway might contribute to the slower rate of cell division in PEMT2-transfected hepatoma cells.