Seminars in Cancer Biology 13 (2003) 49–58 The p53–Mdm2 module and the ubiquitin system Dan Michael, Moshe Oren ∗ Department of Molecular Cell Biology, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel Abstract The p53 tumor suppressor protein is a short-lived protein, which is stabilized in response to cellular stress. The ubiquitination and degradation of p53 are largely controlled by Mdm2, an oncogenic E3 ligase. Stress signals lead to p53 stabilization either by induction of covalent modifications in Mdm2 and p53, or through altered protein–protein interactions. Mdm2 also harbors a post-ubiquitination function, probably enabling efficient targeting of ubiquitinated p53 to the proteasome. p53 ubiquitination is associated with its export from the nucleus into the cytoplasm. However, the exact site of degradation of p53 is presently under debate. p53 may be targeted by other E3 ligases besides Mdm2, as well as by non-proteasomal mechanisms. Despite extensive information about p53 degradation, many important aspects remain unresolved. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Hdm2; Mdm2; p53; Proteasome; Ubiquitin 1. Introduction The p53 tumor suppressor protein has been characterized primarily as a sequence-specific transcription factor [1–6]. Its activity is either disabled or attenuated in the vast major- ity of human cancers. Its inactivation often occurs through mutations affecting the p53 gene locus directly. Its role in transcriptional activation is directed towards a constantly growing number of target genes and results in numerous biological outcomes. Among the most characterized cel- lular outcomes are transient and reversible growth arrest, apoptosis, as well as differentiation and senescence [1–3,7]. However, in some cases p53 has been shown to actually increase the survival of cultured cells exposed to particular types of stress [8–10]. This is also highlighted by recent work showing that p53 can extend the lifespan of C. ele- gans under stress conditions [11]. Therefore, p53 has been suggested to function as an “optimizer of cell fate” [1]. Many stress-inducing factors such as DNA damage, chro- mosomal aberrations, telomere erosion, oncogene activation, and hypoxia, lead to an increase in the transcription of var- ious p53 downstream genes [3]. A key component of this process is an increase in the levels of the p53 protein it- self. These changes in p53 levels are often associated with Abbreviations: ARF, alternative reading frame; Hdm2, human double minute 2; Mdm2, mouse double minute 2; NES, nuclear export sequence; NLS, nuclear localization sequence; RB, retinoblastoma protein ∗ Corresponding author. Tel.: +972-8-9342358; fax: +972-8-9346004. E-mail address: moshe.oren@weizmann.ac.il (M. Oren). post-translational modifications, primarily phosphorylation and acetylation [2–4]. One of the best-characterized target genes of p53 is the mdm2 gene. p53 binds to two adjacent p53-responsive elements located within this gene, and promotes the pro- duction of mdm2 transcripts [12–15]. However, unlike other p53 target gene products, the Mdm2 protein does not act downstream of p53 to mediate the biological effects of this tumor suppressor. Rather, it actually interacts with p53 it- self, and inhibits its function in a number of ways [14–16]. In fact, Mdm2 is now believed to be the major physiologi- cal antagonist of p53. The autoregulatory negative feedback loop, where p53 induces Mdm2 expression whereas Mdm2 represses p53 activity, most probably serves as a pivotal mechanism for restraining p53 function in normal cells, in the absence of stress. In line with this notion, inappropriate excess of Mdm2 may result in exaggerated silencing of p53, abrogating its protective tumor suppressor effects. Indeed, at least 5–10% of all human tumors possess inappropri- ate Mdm2 overexpression, due to either gene amplifica- tion or transcriptional and post-transcriptional mechanisms [14,15,17]. As predicted, in many of those cases the p53 gene remains in its wild-type configuration, presumably be- cause Mdm2 overexpression alleviates the selective pressure for direct mutational inactivation of the p53 gene. The ability of Mdm2 to inactivate p53 relies on a di- rect physical interaction between the two proteins. The N-terminal portion of Mdm2 contains the p53-binding do- main (Fig. 1). Mdm2 also contains a central acidic region, capable of interacting with the ribosomal protein L5 [18,19], 1044-579X/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S1044-579X(02)00099-8