p53, the Cellular Gatekeeper for Growth and Division

The p53 tumour-suppressor protein exerts antiproliferative effects, including growth arrest and apoptosis, in response to various types of stress. The activity of p53 is abrogated by mutations that occur frequently in tumours, as well as by several viral and cellular proteins. The Mdm2 oncoprotein is a potent inhibitor of p53. Mdm2 binds the transcriptional activation domain of p53 and blocks its ability to regulate target genes and to exert antiproliferative effects. On the other hand, p53 activates the expression of the mdm2 gene in an autoregulatory feedback loop. The interval between p53 activation and consequent Mdm2 accumulation defines a time window during which p53 exerts its effects. We now report that Mdm2 also promotes the rapid degradation of p53 under conditions in which p53 is otherwise stabilized. This effect of Mdm2 requires binding of p53; moreover, a small domain of p53, encompassing the Mdm2-binding site, confers Mdm2-dependent detstabilization upon heterologous proteins. Raised amounts of Mdm2 strongly repress mutant p53 accumulation in tumour-derived cells. During recovery from DNA damage, maximal Mdm2 induction coincides with rapid p53 loss. We propose that the Mdm2-promoted degradation of p53 provides a new mechanism to ensure effective termination of the p53 signal.

Mdm2 promotes the rapid degradation of p53

The tumour-suppressor p53 is a short-lived protein that is maintained at low, often undetectable, levels in normal cells. Stabilization of the protein in response to an activating signal, such as DNA damage, results in a rapid rise in p53 levels and subsequent inhibition of cell growth. Tight regulation of p53 function is critical for normal cell growth and development, and one mechanism by which p53 function is controlled is through interaction with the Mdm2 protein. Mdm2 inhibits p53 cell-cycle arrest and apoptic functions and we show here that interaction with Mdm2 can also result in a large reduction in p53 protein levels through enhanced proteasome-dependent degradation. Endogenous levels of Mdm2 are sufficient to regulate p53 stability, and overexpression of Mdm2 can reduce the amount of endogenous p53. Because mdm2 is transcriptionally activated by p53, this degradative pathway may contribute to the maintenance of low p53 concentrations in normal cells. Furthermore, mechanisms regulating the Mdm2-induced degradation of p53 may play a role in controlling the extent and duration of the p53 response.

Regulation of p53 stability by Mdm2

Compared with many normal tissues, cancer cells are highly sensitized to apoptotic signals, and survive only because they have acquired lesions — such as loss of p53 — that prevent or impede cell death. We are now beginning to understand the complex mechanisms that regulate whether or not a cell dies in response to p53 — insights that will ultimately contribute to the development of therapeutic strategies to repair the apoptotic p53 response in cancers.

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Live or let die: the cell's response to p53

As the tale of p53 unfolds, it becomes ever more intriguing. Although our understanding of the critical and complex roles played by p53 is progressing rapidly, new findings continue to pose new paradoxes. Here we present some of these recent advances in p53 research and discuss how they either shed light on or add to the complexities of p53. Therefore, we only briefly summarize some of the key developments leading to our current state of knowledge. For further information, the reader is referred to several excellent reviews that have focused on p53 research (see Donehower and Bradley 1993; Levine 1993; Greenblatt et al. 1994; Oren 1994; Prives 1994; Kinzler and Vogelstein 1996).

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p53: puzzle and paradigm.

The gene p53 encodes a transcriptional activator of genes involved in growth arrest, DNA repair and apoptosis. Loss of p53 function contributes to tumour development in vivo. The transcriptional activation function of p53 is inactivated by interaction with the mdm2 gene product. Amplification of mdm2 has been observed in 36% of human sarcomas, indicating that it may represent an alternative mechanism of preventing p53 function in tumour development. To study mdm2 function in vivo, we generated an mdm2 null allele by homologous recombination. Mdm2 null mice are not viable, and further analysis revealed embryonic lethality around implantation. To examine the importance of the interaction of MDM2 with p53 in vivo, we crossed mice heterozygous for mdm2 and p53 and obtained progeny homozygous for both p53 and mdm2 null alleles. Rescue of the mdm2-/- lethality in a p53 null background suggests that a critical in vivo function of MDM2 is the negative regulation of p53 activity.

Roberta Montes de Oca Luna

Papers

Rescue of early embryonic lethality in mdm2-deficient mice by deletion of p53