Early events in murine erythroleukemia cells induced to differentiate: variation of the cell cycle parameters in relation to p53 accumulation.

Among the early events of induced differentiation of murine erythroleukemia cells that we studied was the variations of cell distribution in the cell cycle as a function of the time of induction. Flow-cytofluorimetry measurements of DNA content and BrdU incorporation allowed for a precise determination of the variations of the cell cycle parameters. Cells underwent a transient arrest in both G1 and G2 + M between 6 to 16 h of induction. The progression of the cells through S phase seems not to be affected during this period. After this time cells escaped from G1 and reentered the S phase. We described previously [S. Khochbin et al. (1988) J. Mol. Biol. 200, 55-64], that p53 decreased continuously during the induction of MELC and remained at a steady-state level after 18 to 20 h of induction. In order to look for a possible redistribution of the protein along the cell cycle during the induction process, we measured the accumulation of the protein along the cell cycle. In noninduced cells there were four steps in the accumulation of the protein throughout the cell cycle: the amount of p53 was constant during G1 and it increased as cells progressed through S phase, which is characterized by an increased accumulation at the G1/S transition and a more moderate accumulation during progression through the rest of the S phase. A constant level in G2/M, approximately twice that obtained in G1, was achieved. There was no change in this distribution that correlated with the various modifications of the cell cycle in induced cells. It seems then, that p53 is associated neither with the progression of the cells in the S phase nor with the resumption of the DNA synthesis after the G1 block.