Analysis of the G1 arrest position of senescent WI38 cells by quinacrine dihydrochloride nuclear fluorescence. Evidence for a late G1 arrest.

Senescence of the human diploid fibroblast-like cell line, WI38, is characterized by a loss of proliferative activity and an arrest of cells with a 2C DNA content (G1 or G0). To examine the specific region within G1 in which senescent cells arrest, senescent cells were stained with quinacrine dihydrochloride (QDH) and their nuclear fluorescence was compared with that of young cultures arrested in early and late G1 by serum deprivation and hydroxyurea exposure, respectively. Release of these G1-arrested young cultures from their blocking conditions and timing the kinetics of their entry into the S phase by autoradiographic detection of [3H]thymidine incorporation revealed that serum-deprived cells entered the S phase within 15-18 h, whereas hydroxyurea-exposed cells entered the S phase within 1.5 h, thus confirming their relative G1-arrest positions. QDH-stained, serum-deprived and hydroxyurea-exposed young cells exhibited relative nuclear fluorescence intensities of 51.7 and 23.9, respectively. Senescent cells exhibited a relative nuclear fluorescence intensity of 17.4, closely resembling the fluorescence of young cultures arrested in late G1 by hydroxyurea exposure. These data support the concept that senescent cells are arrested from further progression in the cell cycle in late G1.