HUMAN-DIPLOID FIBROBLASTS WITH ALTERATIONS IN RIBONUCLEOTIDE REDUCTASE-ACTIVITY, DEOXYRIBONUCLEOTIDE POOLS AND INVITRO LIFESPAN

Three drug resistant human diploid fibroblast clones were isolated which contained elevated levels of ribonucleotide reductase activity when compared to wild type fibroblasts. The drug resistant cells do not appear to possess an enzyme with altered affinity for hydroxyurea. The increase in ezyme activity can entirely account for cellular drug resistance. In keeping with the observed changes in reductase activity in drug resistant fibroblasts, deoxyribonucleotide pools were also found to be altered. Most significantly, there was a 1.8-fold expansion of the dCTP pool size, suggesting that elevation in intracellular dCTP concentrations plays an important role in cellular resistance. The drug resistant fibroblasts exhibited substantial reductions in their replicative abilities, suggesting that the regulation of ribonucleotide reductase and the accompanying deoxyribonucleotide pools in human diploid cells is involved in aspects of cellular senescence.