Regulation of phosphatidylcholine homeostasis by calcium-independent phospholipase A(2)

Phosphatidylcholine (PtdCho) is the most abundant phospholipid in mammalian cell membranes and is essential for cell viability. The levels of this lip id must be tightly controlled to maintain homeostasis. Therefore, changes i n the rate of PtdCho synthesis are generally balanced by changes in PtdCho catabolism and vice versa. It is commonly accepted that the rate of PtdCho synthesis is regulated by CTP:phosphocholine cytidylyltransferase (CT). How ever, it is not certain if PtdCho mass is regulated by specific catabolic e nzyme(s). Our goal is to determine if PtdCho homeostasis is regulated by a phospholipase A(2) (PLA(2)), To this end, we have prepared Chinese hamster ovary (CHO) cell lines that overexpress CT. CT activity is 7-10-fold higher in the transfected cells than in parental CHO cells. This increase in CT a ctivity is associated with increases in both PtdCho synthesis and PtdCho ca tabolism. Glycerophosphocholine is the PtdCho catabolite that accumulates i n the transfected cells, which suggests that PtdCho turnover is mediated by a phospholipase A(2) (PLA(2)). Indeed, higher levels of calcium-independen t PLA(2) activity are measured in the cytosols of the CHO cells that overex press CT, compared to parental CHO cells. The elevated calcium-independent PLA(2) activity is associated with increases in the expression of the 80-kD a calcium-independent PLA(2) (iPLA(2)). Together, these data suggest that t he 80-kDa iPLA(2) may be modulated in response to changes in PtdCho levels and therefore is involved in the regulation of PtdCho homeostasis in CHO ce lls. (C) 1999 Elsevier Science B.V. All rights reserved.