TNF-induced modulations of phospholipid metabolism in human breast cancer cells

Tumor necrosis factor α (TNF) is a cytokine that is cytocidal for certain tumor cells and induces necrotic and apoptotic forms of cell death. Flow cytometry and transmission electron microscopy analysis demonstrated that in human breast cancer cells (MCF7) TNF induces cell cycle arrest in G₀+G₁/S, accompanied by apoptosis. ³¹P and ¹³C NMR spectroscopy was applied to study cellular metabolism of MCF7 cells during TNF-induced signal to apoptosis. Deuterated choline and ²H NMR spectroscopy were utilized to monitor the kinetics of the rate limiting reactions in phosphocholine metabolism. The NMR measurements revealed that immediately after administration of TNF, choline transport was inhibited by 52±6%. Later (∼15 h), the activity of phosphocholine:cytidine triphosphate cytidylyltransferase, a key enzyme in the biosynthesis of phosphatidylcholine, was enhanced two-fold. These two opposing changes led to a decrease in the level of phosphocholine. Throughout these changes the energetic state of the cells, determined by the level of nucleoside triphosphates and the rate of glucose metabolism via glycolysis, remained constant. The results indicate that TNF specifically modulates the kinetics of membrane-bound enzymes of the rate determining steps in phosphatidylcholine biosynthesis, possibly as part of early events involved in apoptosis.