Comparative heterokaryon study of cellular senescence and the serum-deprived state

Autoradiographic patterns of DNA replication in serum-deprived human diploid fibroblast-like cells (HDFC) and “senescent” HDFC have been compared in two types of heterokaryons. Each was fused to low passage, proliferating HDFC and, in separate experiments, to HeLa cells. Sequential 1 h pulses with ³H]thymidine were initiated at short intervals following fusion. In all hybridizations serum-deprived and senescent cells behaved identically. Upon fusion to HeLa cells, DNA synthesis in the quiescent nuclei occurred in a wave between 3 and 30 h after fusion. When either serum-deprived or senescent HDFC were fused to young proliferating HDFC, the nuclei of the latter were blocked from entering the S phase if fusion occurred at least 3 h before the G/S boundary. These findings are consistent with the interpretation that one or more crucial steps in G0 occurs 3 h before the G1/S interface. That young serum-deprived (G0) HDFC behave identically to senescent cells in these hybridization studies suggests that the mechanism of arrest in each state might share a final common pathway, and a model based on these observations is proposed.