Fig. 6. Oncogenic Ras partially reverses p16-independent senescence. (A) S-WI cells do not synthesize DNA in response to oncogenic Ras. S-WI cells were infected with the indicated lentiviruses, and 72 h later assessed for ability to synthesize DNA (% LN) and proliferate (% growth), as described in Materials and methods. (B) S-BJ cells synthesize DNA and undergo limited proliferation in response to oncogenic Ras. S-BJ cells were infected with the indicated lentiviruses, and assessed for % LN and % growth. The asterisk indicates that the cells underwent limited proliferation, amounting to 3 PDs or less. (C) Mitogenic effects of Ras are concentration- and p16-dependent. S-BJ cells were infected with 1× (low Ras) or 3× (high Ras) lenti-Ras virus concentrations (determined by p24 levels, as described in Materials and methods) and % LN was measured. Where indicated, S-BJ cells were infected with lenti-p16 5 days prior to subsequent infection with high lenti-Ras. (D) Ras immunostaining. S-BJ cells were mock infected or infected with lenti-Ras at 1× or 3× virus concentrations, and immunostained for Ras. Nuclei were identified by DAPI staining.