FIG. 5.

Activation of E2F-1, E2F-2, or E2F-3 results in S-phase entry and apoptosis. (A) E2F-1, E2F-2, or E2F-3 activation is sufficient for S-phase induction. Rat1 cells expressing ERE2F-1, ERE2F-2, or ERE2F-3 were grown in medium containing 0.1% serum for 48 h and subsequently induced with OHT. Cells were harvested for FACS analysis at the indicated times after addition of OHT, and the cell cycle profile was determined. (B) E2F-1, E2F-2, and E2F-3 induce cell death. Cells were serum starved for 48 h in medium with 0.1% serum, and OHT was subsequently added to the starvation medium. At the indicated times, cells were harvested and living cells were counted after being stained with Trypan blue. The percentage of surviving cells was calculated by using the number serum-starved cells in the absence of OHT as the reference point. (C) Activation of E2F-1, E2F-2, or E2F-3 induces apoptosis. FACS analysis of Rat1 cells expressing ERE2F-1, ERE2F-2, or ERE2F-3, showing the percentage of apoptotic cells (sub-G₁ fraction) at 0 and 24 h after OHT addition. The cells were grown in medium with 0.1% serum for 48 h prior to addition of OHT.