Macromolecular synthesis in human fibroblasts at 37 degrees and 42 degrees C during aging in vitro

A normal strain of human skin fibroblasts was pulse labelled at 42 degrees and 37 degrees C during intervals of the in vitro lifespan with 3H-thymidine, 3H-uridine and 14C-phenylalanine, precursors of DNA, RNA and protein synthesis, respectively. No increased heat lability could be detected in the relative profiles of precursor incorporation at the two temperatures until terminal stages of passage when a minor increase in heat lability appeared in RNA and protein synthesis. Early passage cells, grown in the presence of the substrate analogues, p-fluorophenylalanine or 5-fluorouracil, showed a moderate increase in relative heat lability but comparable patterns were not evident in drug-free cultures, even at terminal stages of passage. Although the results indicate that defects exist in enzyme pathways for macromolecular synthesis at terminal stages of growth and in the presence of analogues, they do not support the hypothesis that such defects are causal to the onset of cellular aging in vitro.