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J. Biol. Chem., Vol. 279, Issue 51, 52881-52892, December 17, 2004

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Cell Cycle Dependence of Group VIA Calcium-independent Phospholipase A₂ Activity^*

Alex D. Manguikian and Suzanne E. Barbour

From the Department of Biochemistry, Virginia Commonwealth University, Richmond, Virginia 23298-0614

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.

Received for publication, September 16, 2004

^* This work was supported by Grant No. 0212213 from the National Science Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: Box 980614, Dept. of Biochemistry, Virginia Commonwealth University, Richmond, VA 23298-0614. Tel.: 804-828-2308; Fax: 804-828-1473; E-mail: sbarbour{at}hsc.vcu.edu.

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