Different regulation of c-Myc- and E2F-1-induced apoptosis during the ongoing cell cycle

The transcription factors c-Myc and E2F-1 have been shown to harbour both mitogenic and apoptotic properties. Both factors have been implicated in the regulation of the transition from the G1 phase to the S phase in the mammalian cell cycle. However, whether cell death triggered by these molecules is dependent on the cell's position in the ongoing cell cycle remained elusive. Using centrifugal elutriation we here show for the first time that c-Myc induces apoptosis in G1 and in G2 phase, whereas E2F-1-induced apoptosis specifically occurs in G1. S phase cells are resistant to cell death triggered by these factors. We demonstrate that this is not a general phenomenon, since S phase cells are susceptible to apoptosis induced by treatment with actinomycin D and to the anti-apoptotic activity of Bcl-2. Our data indicate that S phase cells harbour specific protective activities against c-Myc- and E2F-1-induced apoptosis. Our results demonstrate that these transcription factors, although probably sharing specific apoptotic pathways, also take distinct routes to induce cell death and that apoptosis can occur at different phases of the cell cycle depending on the apoptotic stimulus. In this report we present the usefulness of a new approach to determine the regulation of apoptosis in the ongoing unperturbated cell cycle. This approach has clear implications for the identification of target genes involved in the regulation of cell death.