Abstract
Phosphatidylcholine (PtdCho) is the major membrane phospholipid in mammalian cells, and its synthesis is controlled by the activity of CDP:phosphocholine cytidylyltransferase (CCT). Enforced CCT expression accelerated the rate of PtdCho synthesis. However, the amount of cellular PtdCho did not increase as a result of the turnover of both the choline and glycerol components of PtdCho. Metabolic labeling experiments demonstrated that cells compensated for elevated CCT activity by the degradation of PtdCho to glycerophosphocholine (GPC). Phospholipase D-mediated PtdCho hydrolysis and phosphocholine formation were unaffected. Most of the GPC produced in response to excess phospholipid production was secreted into the medium. Cells also degraded the excess membrane PtdCho to GPC when phospholipid formation was increased by exposure to exogenous lysophosphatidylcholine or lysophosphatidylethanolamine. The replacement of the acyl moiety at the 1-position of PtdCho with a non-hydrolyzable alkyl moiety prevented degradation to GPC. Accumulation of alkylacyl-PtdCho was associated with the inhibition of cell proliferation, demonstrating that alternative pathways of degradation will not substitute. GPC formation was blocked by bromoenol lactone, implicating the calcium-independent phospholipase A2 as a key participant in the response to excess phospholipid. Owing to the fact that PtdCho is biosynthetically converted to PtdEtn, excess PtdCho resulted in overproduction and exit of GPE as well as GPC. Thus, general membrane phospholipid homeostasis is achieved by a balance between the opposing activities of CCT and phospholipase A2.
Highlights
Phosphoprotein [1] that is phosphorylated on multiple serine residues within its carboxyl-terminal domain [7]
In transiently transfected COS cells, there was a concomitant increase in GPC and PCho, suggesting that elevated PtdCho degradation compensated for increased synthesis [9]
PtdCho synthesis increased in G1; the total phospholipid mass was maintained as a result of a concomitant increase in PtdCho degradation
Summary
Phosphoprotein [1] that is phosphorylated on multiple serine residues within its carboxyl-terminal domain [7]. Elevated pools of two PtdCho breakdown products, PCho and GPC, were observed in these experiments [11,12,13, 15,16,17,18], the activation of Cho kinase makes a significant contribution to the PCho pool [12]. Another example of balanced PtdCho synthesis and degradation was found during the G1 stage of the cell cycle [19]. The goals of this study were to determine if the coupling between CCT activity and PtdCho turnover is a manifestation of a general mechanism for phospholipid homeostasis and to investigate the phospholipase(s) responsible for regulated PtdCho degradation
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