Importance of ribonucleotide availability to proliferating T-lymphocytes from healthy humans. Disproportionate expansion of pyrimidine pools and contrasting effects of de novo synthesis inhibitors

Abstract

Sensitive high performance liquid chromatography techniques, which differentiate between purine and pyrimidine ribonucleoside and deoxyribonucleoside triphosphates, were used to quantify pools in phytohemagglutinin-stimulated T-lymphocytes (98% CD4+ and CD8+) from healthy volunteers. The importance of de novo synthesis and salvage was evaluated by incubating the cells with 14C-radiolabeled precursors (40 microM), azaserine (20 microM; a glutamine antagonist), and ribavirin (50 microM; an IMP dehydrogenase inhibitor). We confirmed that resting T-lymphocytes meet their metabolic requirements by salvage. Noteworthy observations were as follows. First, nucleotide pool expansion over 72 h is disproportionate, with that for purines (ATP and GTP) being 2-fold compared with up to 8-fold for pyridine (NAD) or pyrimidine (UTP, UDP-Glc, and CTP) pools. This supports an additional role for the latter in membrane lipid biosynthesis, protein glycosylation, and strand break repair. Second, intact de novo pathways are essential for such expansion. Azaserine not only inhibited purine synthesis (confirmed by N-formylglycinamide polyphosphate accumulation), but also reduced expansion of pyrimidine and NAD pools by 70%. Ribavirin depleted GTP pools by 40% and reduced pyrimidine pool expansion by 40% at 72 h. These findings underline the importance of pyrimidine ribonucleotide availability as well as GTP synthesis de novo to proliferating T-lymphocytes. They also demonstrate an absence of coordinate regulation between de novo purine and pyrimidine biosynthesis.