Cell cycle dependence of group VIA calcium-independent phospholipase A 2 activity

Abstract

Homeostasis of phosphatidylcholine (PC) is regulated by the opposing actions between CTP:phosphocholine cytidylyltransferase (CT) and the group VIA Ca2+-independent phospholipase A2 (iPLA2). We investigated this process during the cell cycle. PC mass doubles during late G1 and early S phase when its rate of catabolism is lowest. We show that iPLA2 activity is cell cycle-dependent with peak activity during G2/M and late S phase. iPLA2 activity declines during G1 and is lowest at the G1/S transition and early S phase. The accumulation of PC correlates with decreased iPLA2 activity, suggesting that regulation of this enzyme contributes to phospholipid accumulation. The levels of 80 kDa iPLA2 protein do not change and thus cannot account for changes in enzyme activity. Reverse transcriptase and real-time PCR experiments show that splice variant iPLA2 mRNAs are preferentially expressed during G2/M. Immunoblot analyses with an antibody directed against the N terminus of iPLA2 revealed a ∼50 kDa protein that is of appropriate size to be the truncated protein encoded by the ankyrin-iPLA2-1 splice variant mRNA. The levels of truncated iPLA2 protein were high in cells in late G1 and S phase cells that had low iPLA2 activity and low in G2/M cells that had high iPLA2 activity. The truncated protein co- immunoprecipitated with full-length iPLA2, indicating a physical interaction between the two proteins. Together, these data suggest that truncated iPLA2 proteins associate with active iPLA2 and down-regulate its activity during G1. This down-regulation may contribute to phospholipid accumulation during the cell cycle.