GLUCOSE CONTRIBUTION TO NUCLEIC-ACID BASE SYNTHESIS IN PROLIFERATING HEPATOMA-CELLS - A GLYCINE-BIOSYNTHESIS-MEDIATED PATHWAY

The coupling of glycolysis to serine and glycine metabolism was studie d in fast-growing Zajdela hepatoma cultured cells. During the exponent ial phase of growth, occurring between 12 and 72 h, cells exhibited a decreased glycogen content together with a high glycolytic activity. G lycogen labelling, evaluated by 1 h-pulse experiments with [U-C-14]glu cose (5.5 mM), was minimal during the first 48 h and increased 2.5-fol d at 72 h and 8-fold at 96 h, at which times it was also stimulated 2- fold by 10 nM insulin. [U-C-14]Glucose carbons were incorporated into nucleic acid bases, with maximal incorporation at 72 h, the rate of nu cleotide base labelling exceeding that of glycogen during the first 2 days of culture. Incubation of the cells with [U-C-14]glucose resulted in the release into the medium of C-14-labelled glycine, the first in termediate formed on the route from serine to DNA. The rate of release per cell decreased as a function of cell growth, concomitantly with a n increased rate of glucose carbon incorporation into nucleotide bases . The latter implied the intermediary formation of amino acids since t he transaminase inhibitor cycloserine (10 mM), which totally inhibited [C-14]glycine release, decreased by 65% nucleotide labelling from [U- C-14]glucose. A dose-dependent inhibition by serine of the rate of [U- C-14]glucose carbon incorporation into nucleotide bases was observed, which was maximal at 5 mM serine. These metabolic flux measurements in dicate that glucose can be used as a precursor of nucleic acid synthes is. These results strongly suggest that this process is to a large ext ent mediated by a serine/glycine-biosynthesis-mediated pathway, and re inforce the hypothesis that glycolysis contributes to enhancing the pr ovision of precursors required for cell proliferation.