p53 facilitates pRb cleavage in IL-3-deprived cells: novel pro-apoptotic activity of p53.

In the interleukin-3 (IL-3)-dependent lymphoid cell line DA-1, functional p53 is required for efficient apoptosis in response to IL-3 withdrawal. Activation of p53 in these cells, by either DNA damage or p53 overexpression, results in a vital growth arrest in the presence of IL-3 and in accelerated apoptosis in its absence. Thus, IL-3 can control the choice between p53-dependent cell-cycle arrest and apoptosis. Here we report that the cross-talk between p53 and IL-3 involves joint control of pRb cleavage and degradation. Depletion of IL-3 results in caspase-mediated pRb cleavage, occurring preferentially within cells which express functional p53. Moreover, pRb can be cleaved efficiently by extracts prepared from DA-1 cells but not from their derivatives which lack p53 function. Inactivation of pRb through expression of the human papillomavirus (HPV) E7 oncogene overrides the effect of IL-3 in a p53-dependent manner. Our data suggest a novel role for p53 in the regulation of cell death and a novel mechanism for the cooperation between p53 and survival factor deprivation. Thus, p53 makes cells permissive to pRb cleavage, probably by controlling the potential activity of a pRb-cleaving caspase, whereas IL-3 withdrawal provides signals that turn on this potential activity and lead to the actual cleavage and subsequent degradation of pRb. Elimination of a presumptive anti-apoptotic effect of pRb may then facilitate conversion of p53-mediated growth arrest into apoptosis.