OVEREXPRESSION OF PHOSPHATIDYLETHANOLAMINE N-METHYLTRANSFERASE-2 IN CHO-K1 CELLS DOES NOT ATTENUATE THE ACTIVITY OF THE CDP-CHOLINE PATHWAYFOR PHOSPHATIDYLCHOLINE BIOSYNTHESIS

Chinese hamster ovary (CHO) cells express only a trace amount of phosp hatidylethanolamine N-methyltransferase (PEMT) activity. CHO cells mak e their phosphatidylcholine (PC) via the CDP-choline pathway. We inves tigated whether or not overexpression of PEMT2, an isoform of PEMT, in these cells would down-regulate the activity of the CDP-choline pathw ay. Transfection of CHO cells with PEMT2 cDNA behind the cytomegalovir us promoter resulted in a series of cen lines that overexpressed PEMT2 . Phospholipid-metabolism was characterized in cell lines that express ed a medium (281 pmol/min per mg of protein) and a high (1300 pmol/min per mg of protein) level of PEMT activity. The expression of the regu lated enzyme (CTP:phosphocholine cytidylyltransferase) in the CDP-chol ine pathway was increased, not decreased, in these cell lines as judge d by immunoblot analysis and enzymic activity. Conversion of phosphati dylethanolamine to PC was enhanced in CHO cells that expressed PEMT2 a ctivity. PC mass was not increased in the transfected compared with th e control cells. The rate of PC catabolism made by either the CDP-chol ine; methylation pathways was unaffected by PEMT2 expression. We concl ude that expression of PEMT2 in CHO cells does not down-regulate, but rather enhances, the expression of CTP:phosphocholine cytidylyl-transf erase.