The growth inhibitory functions of p53 are controlled in unstressed cells by rapid degradation of the p53 protein. One of the principal regulators of p53 stability is MDM2, a RING finger protein that functions as an E3 ligase to ubiquitinate p53. MDM2 promotes p53 nuclear export, and in this study, we show that ubiquitination of the C terminus of p53 by MDM2 contributes to the efficient export of p53 from the nucleus to the cytoplasm. In contrast, MDM2 did not promote nuclear export of the p53-related protein, p73. p53 nuclear export was enhanced by overexpression of the export receptor CRM1, although no significant relocalization of MDM2 was seen in response to CRM1. However, nuclear export driven by CRM1 overexpression did not result in the degradation of p53, and nuclear export was not essential for p53 degradation. These results indicate that MDM2 mediated ubiquitination of p53 contributes to both nuclear export and degradation of p53 but that these activities are not absolutely dependent on each other.The p53 tumor suppressor protein plays an important role in preventing malignant development, and p53 function is lost or compromised in most human cancers (37). One of the principal functions of p53 is to inhibit cell growth, and p53 shows strong cell cycle arrest and apoptotic activities (43). While these functions play an important role in preventing the growth of abnormal or damaged cells, p53 activity must be tightly regulated in normal tissue to allow growth and development. One of the principal regulators of p53 is the MDM2 protein (26), and loss of MDM2 results in early embryonic lethality associated with deregulated p53-mediated apoptosis (10). MDM2 expression is transcriptionally activated by p53, establishing a feedback loop in which p53 controls expression of its own negative regulator (4, 45).MDM2 shows several functions that contribute to the inhibition of p53 activity. p53 is a transcription factor, and the activation of cell cycle arrest and apoptotic responses to p53 are dependent, at least in part, on the expression of p53 target genes (43). The p53 protein contains domains for sequencespecific DNA binding and an N-terminal transactivation domain that forms direct contacts with a number of proteins that are involved in transcriptional control (22). Since the MDM2 binding site is also within the N-terminal region of p53 (7), one of the consequences of MDM2 binding is to inhibit p53-mediated transcriptional activity by blocking the p53-transcriptional coactivator interactions (31,33,44). This effect may be further enhanced by MDM2-mediated inhibition of the acetylation of p53 by factors such as p300 (21, 23) and an ability of MDM2 to function directly as a transcriptional repressor (42).Another function of MDM2 that efficiently abolishes all p53 activity is the ability of MDM2 to target p53 for degradation through the ubiquitin-dependent proteasome pathway (17,24). Interaction between MDM2 and p53 leads to the ubiquitination and degradation of p53, and this is likely to play a key role in maint...

The p53 tumour suppressor protein inhibits malignant progression by mediating cell cycle arrest, apoptosis or repair following cellular stress. One of the major regulators of p53 function is the MDM2 protein, and multiple forms of cellular stress activate p53 by inhibiting the MDM2-mediated degradation of p53. Mutations in p53, or disruption of the pathways that allow activation of p53, seem to be a general feature of all cancers. Here we review recent advances in our understanding of the pathways that regulate p53 and the pathways that are induced by p53, as well as their implications for cancer therapy. © 2001 Cancer Research Campaign http://www.bjcancer.com

The function of the p53 tumor suppressor protein is regulated by interaction with Mdm2, which targets p53 for ubiquitin dependent degradation. We show here that like p53, p73a forms an interaction with Mdm2, both in vitro and in cells, but this does not result in the degradation of the p73a protein. The human papillomavirus E6 protein also fails to degrade p73a, suggesting that the mechanisms governing p73a stability are distinct from those known to regulate p53 stability. However, the interaction of Mdm2 with 73a is sucient to impede p73a transcriptional function, despite the lack of degradation.

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