Mobile lipid production after confluence and pH stress in perfused C6 cells

NMR-visible mobile lipid (ML) has been observed in aggressive tumors and also in in vitro tumor cell models subjected to growth-inhibiting conditions, such as confluence or low-pH stress. The aim of the present study was to determine if ML production after confluence or low pH stress in a cultured cell model of brain tumor is due to growth arrest alone. ML was observed in situ by one- and two-dimensional (1)H NMR in viable but growth-arrested C6 glioma cells superfused for a period of 48 h after harvesting. The rate of ML production in cells harvested at subconfluence was compared to the rate in cells confluent for one cell cycle and to the rate in subconfluent-harvested cells superfused at low pH (pH 6.1). Confluent-harvested cells produced ML at a markedly greater rate than that of cells harvested at subconfluence, suggesting the involvement of prior cell-cell contact rather than simple growth arrest. A high rate was also observed in subconfluent-harvested cells subjected to low pH, indicating that ML in pH-stressed cells also does not arise from growth arrest alone. Furthermore, two-dimensional data on the degree of unsaturation of the ML fatty acyl chains and one-dimensional (31)P and two-dimensional (1)H NMR data on the GPC content of the cells suggest distinct metabolic pathways for the production of ML following confluence and low-pH stress.