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

Author

M K Storey, D M Byers, H W Cook, and N D Ridgway

Subjects
Biochemistry, QD415-436
Abstract

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.

Published
1997
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2. Golgi localization and phosphorylation of oxysterol binding protein in Niemann-Pick C and U18666A-treated cells

Author

A, Mohammadi, R J, Perry, M K, Storey, H W, Cook, D M, Byers, and N D, Ridgway

Subjects
Niemann-Pick Diseases, Receptors, Steroid, Golgi Apparatus, CHO Cells, Cholesterol, LDL, Fibroblasts, Cholinergic Antagonists, Protein Transport, Cholesterol, Cricetinae, Mutagenesis, Site-Directed, Animals, Humans, Androstenes, Phosphorylation
Abstract

Oxysterol binding protein (OSBP) translocation between Golgi and vesicular/cytoplasmic compartments is affected by conditions that alter cholesterol and sphingomyelin homeostasis, indicating a role in lipid and sterol regulation in this organelle. In this study, we show that OSBP dissociation from the Golgi apparatus was inhibited when LDL cholesterol efflux from lysosomes was blocked in Niemann-Pick C (NPC) or U18666A [3-beta-[2-(diethylamino)ethoxy]androst-5-en-17-one]-treated fibroblasts. Dissociation of OSBP from the Golgi apparatus in response to LDL was independent of de novo cholesterol biosynthesis. OSBP did not localize with filipin-stained lysosomal cholesterol, and the NPC defect did not alter OSBP expression or phosphorylation. However, OSBP in the Golgi apparatus was progressively dephosphorylated (as assessed by a molecular mass shift on SDS-PAGE) in U18666A-treated fibroblasts or Chinese hamster ovary cells as a result of combined inhibition of LDL cholesterol transport and de novo cholesterol synthesis. In vivo phosphopeptide mapping and mutagenesis of OSBP was used to identify the cholesterol-sensitive phosphorylation sites at serines 381, 384, and 387 that were responsible for the altered mobility on SDS-PAGE. NPC-1 protein-mediated release of LDL-derived cholesterol and de novo biosynthesis regulates OSBP localization and phosphorylation. This indicates that OSBP responds to or senses altered cellular sterol content and transport.

Published
2001

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