The pathway of cell senescence: WI-38 cells arrest in late G₁ and are unable to traverse the cell cycle from a true G₀ state

Senescent human diploid fibroblasts have an undefined arrest state partially characterized by the differential expression of cell cycle- regulated genes and a failure to complete the mitogen-stimulated cascade of signalling events that lead to DNA synthesis. We present evidence that this arrest state precludes the entry of senescent fibroblasts into a normally reversible G₀ or quiescent state. Both nuclear association kinetics and quinacrine dihydrochloride nuclear fluorescence show chromatin condensation patterns consistent with arrest in late G₁ and exclusion of senescent cells from the G₀ phase of the cell cycle. Steady-state thymidine kinase mRNA levels indicate that some of the signalling cascades initiated from a functional G₀ state may be intact in senescent cells, at least qualitatively, and that this expression may represent an abortive attempt to complete pathways required for DNA replication. Taken together, the evidence suggests that growth arrest in senescent cells likely occurs in a physiologic state fundamentally distinct from that of the G₀, quiescent state that is achieved by nonproliferating young cells. A full response to serum or growth factor addition, leading from quiescence to DNA synthesis, may require cells to initiate this traverse from a true G₀ state. If so, senescent cells would be excluded from this pathway.