E2F1-specific induction of apoptosis and p53 accumulation, which is blocked by Mdm2.

Previous work has demonstrated a role for E2F transcription factor activity in the regulation of cell growth during the G0/G1-S phase transition. Indeed, overexpression of E2F proteins, including the E2F1 and E2F2 products, induces DNA synthesis in quiescent fibroblasts. Other experiments have shown that E2F1 expression also induces apoptosis, dependent on p53. Although this could represent a response to aberrant cell cycle progression, we show that only E2F1 induces apoptosis and that this coincides with an ability of E2F1 to induce accumulation of p53 protein. We also find that coexpression of Mdm2, which is known to regulate p53 activity, blocks the E2F1-mediated induction of apoptosis and also blocks the E2F1-mediated accumulation of p53. We propose that E2F1 acts as a specific signal for the induction of apoptosis by affecting the accumulation of p53, which under normal proliferative conditions may be controlled by Mdm2.