Serine Utilization As a Precursor of Phosphatidylserine and Alkenyl-(plasmenyl)-, Alkyl-, and Acylethanolamine Phosphoglycerides in Cultured Glioma Cells

In several tissues and cell lines, serine utilized for phosphatidylserine (PS) synthesis is an eventual precursor of the base moiety of ethanolamine phosphoglycerides (PE). We investigated the biosynthesis and decarboxylation of PS in cultured C6 glioma cells, with particular attention to 1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine (plasmenylethanolamine) biosynthesis. Incorporation of [3H]serine into PS reached a maximum within 4-8 h, and label in nonplasmenylethanolamine phosphoglyceride (NP-PE) and plasmenylethanolamine was maximal by 12-24 h and 48 h, respectively. After 8 h, label in PS decreased even though 40-60% of initial label remained in the culture medium. Serial additions of fresh [3H]serine restored PS synthesis to higher levels of labeled PS accumulation followed by a subsequent decrease in 4-8 h. High performance liquid chromatographic analyses confirmed that medium serine was depleted by 8 h, and thereafter metabolites, including acetate and formate, accounted for radioactivity in the medium. The rapid but transient appearance of labeled glycine and ATP inside the cells indicated conversion of serine by hydroxymethyltransferase. 78-85% of label from serine was in headgroup of PS or of PE formed by decarboxylation. A precursor-product relationship was suggested for label from [3H]serine appearing in the headgroup of diacyl, alkylacyl, and alkenylacyl subclasses of PE. By 48 h, a constant specific activity, ratio of approximately 1:1 was reached between plasmenylethanolamine and NP-PE, similar to the molar distribution of these lipids. In contrast, equilibrium was not achieved in cells incubated with [1,2-14C]ethanolamine; plasmenylethanolamine had 2-fold greater specific activity than labeled NP-PE by 72-96 h. These observations indicate that in cultured glioma cells 1) serine serves as a precursor of the head group of PS and of both plasmenyl and non-plasmenyl species of PE; 2) exchange of headgroup between NP-PE and plasmenylethanolamine may involve different donor pools of PE depending on whether the headgroup originates with exogenous serine or ethanolamine; 3) serine is rapidly converted to other metabolites, which limits exogenous serine as a direct phospholipid precursor.