ALTERATIONS IN CONTACT AND DENSITY-DEPENDENT ARREST STATE IN SENESCENT WI-38 CELLS

Normal human Wi-38 fibroblast-like cells in culture undergo a process of senescence, one feature of which is a gradual decline in proliferat ive capacity. As these cells reach the end of their replicative life s pan they exhibit decreases in the fraction of cells able to synthesize DNA, in the number of doublings per passage (constant seeding density ), and in the cell harvest and saturation densities, They also display increased average cell cycle times, largely at the expense of longer G, intervals. These alterations are accompanied by morphologic changes , including cell enlargement. Before the end of the replicative life s pan or phase-out, there is a highly reproducible (55/58 sublines) cell loss of approximately 50%; however, a stable population survives that can exist in a viable yet nonproliferative state for many months. Thi s stable population maintains an extremely low saturation density, rep resenting <5% of that achieved by early passage cultures. Further, we show that maximum harvest densities achieved by senescent cells are lo wer, irrespective of seeding densities, i.e. when placed at cell densi ties higher than those normally achieved by senescent cultures they di splay a net decline in cell number. This decline continues until the c ell density approximates the density that would have been achieved had the cultures been seeded at standard density (I X 10(4) cells/cm(2)). By measuring the accumulation of an mRNA species, EPC-I, that is expr essed when early passage cultures reach a growth-arrested state via de nsity-dependent contact inhibition, we also show that senescent cells are unable to produce this transcript at either their normal confluent density or at high cell density obtained by overseeding. The above re sults suggest that there are significant alterations in cell-to-cell c ontact sensitivity and arrest state of senescent cultures. These chang es result in both a cytotoxic response to crowding and failure to expr ess at least one molecular marker which is induced as young cells appr oach growth arrest by contact inhibition.