GLUCOSE AND GLUTAMINE-METABOLISM IN C6 GLIOMA-CELLS STUDIED BY CARBON-13 NMR

The question as to whether glutamine and glucose are both required for optimal growth of glioma cells is studied through the role of these s ubstrates on the metabolism of the cells. C6 rat glioma cells grow onl y very slowly when glutamine is omitted from the culture medium. The r ates of glucose consumption and lactate production on confluent cells in glutamine free medium were 0.88 +/- 0.99 and 1.06 +/- 0.25 mu mol/h /mg protein, respectively. In the presence of 4 mM glutamine,,glucose utilization increased to 60% leading to a 45% increase of lactate prod uction. We have studied the kinetics of enrichment of intracellular gl utamate at C-2, C-3 and C-4 positions on cells incubated witt; 5 mM 99 % enriched [1-C-13]glucose in the presence or the absence of glutamine in the incubation medium. The specific enrichments at metabolic stead y state of all carbon positions were the same under both conditions, b ut we observed a significantly reduced rate of C-13 incorporation in t he presence of glutamine, showing an isotopic dilution of tricarboxyli c acid cycle intermediates and indicating the use of this amino acid a s an anaplerotic substrate. The fact that no dilution occurred at the level of pyruvate suggests strongly the lack of glutaminolysis in thes e cells. The main conclusion from this work is that glutamine metaboli sm in C6 cells appears complementary to that of glucose as far as ener gy production and carbon sources for the growing of the cells are conc erned: glutamine is mainly utilized for anaplerosis as carbon donor to replenish the tricarboxylic acid cycle; it is not a substrate for ene rgy metabolism. In contrast, glucose is poorly anaplerotic and is esse ntially used as energetic fuel by the C6 cells.