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Journal of Lipid Research, Vol 38, 711-722, Copyright © 1997 by Lipid Research, Inc.

ARTICLES

Decreased phosphatidylcholine biosynthesis and abnormal distribution of CTP:phosphocholine cytidylyltransferase in cholesterol auxotrophic Chinese hamster ovary cells

MK Storey, DM Byers, HW Cook and ND Ridgway
Atlantic Research Centre, Dalhousie University, Halifax, Nova Scotia, Canada.

Phosphatidylcholine (PtdCho) biosynthesis was examined in the sterol regulatory defective (SRD) Chinese hamster ovary (CHO) cell line SRD 6. SRD 6 cells do not display transcriptional activation of sterol- regulated genes and are cholesterol auxotrophs. Compared to CHO 7 cells (parental line from which the SRD cells were derived), incorporation of [3H]choline during a 2-h pulse into PtdCho and sphingomyelin was reduced 3- and 4.5-fold, respectively, in SRD 6 cells grown with or without cholesterol. SRD 6 cells grown in cholesterol-free medium for 24 h had 8% less phosphatidylcholine (PtdCho) mass compared to CHO 7 cells. Consistent with impaired CTP:phosphocholine cytidylyltransferase activity, [3H]choline-labeled SRD 6 cells had elevated [3H]phosphocholine and delayed conversion to [3H]PtdCho during a 2-h chase period. Compared to CHO 7 cells, cytosolic cytidylyltransferase activity was elevated 2- to 3-fold in SRD 6 cells grown in the absence of cholesterol, but activity in the total membrane fraction was normal. Immunoblot analysis confirmed that cytidylyltransferase mass was increased 2-fold in SRD 6 total cell extracts and cytosol, but not membranes. The amount of [32P]phosphate-labeled cytidylyltransferase in cytosol and membranes of SRD 6 cells were similar to controls. Likewise, cytidylyltransferase mRNA levels were not significantly different between SRD 6 and CHO 7. The defect in PtdCho synthesis in SRD 6 cells could be overcome by treatment with 150 microns oleate, such that after 5 h [3H] choline incorporation into PtdCho and phosphocholine in SRD 6 and CHO 7 cells was similar. Cholesterol auxotrophic SRD 6 cells display reduced PtdCho mass and synthesis and elevated levels of cytosolic cytidylyltransferase, defects that were only partially corrected by growth in exogenous cholesterol. These results indicate a requirement for normal cholesterol regulation and synthesis in the maintenance PtdCho levels and activity of cytidylyltransferase.
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