MdmX Promotes Bipolar Mitosis To Suppress Transformation and Tumorigenesis in p53-Deficient Cells and Mice

Abstract

Mdm2 and MdmX are structurally related p53-binding proteins that function as critical negative regulators of p53 activity in embryonic and adult tissue. The overexpression of Mdm2 or MdmX inhibits p53 tumor suppressor functions in vitro, and the amplification of Mdm2 or MdmX is observed in human cancers retaining wild-type p53. We now demonstrate a surprising role for MdmX in suppressing tumorigenesis that is distinct from its oncogenic ability to inhibit p53. The deletion of MdmX induces multipolar mitotic spindle formation and the loss of chromosomes from hyperploid p53-null cells. This reduction in chromosome number, not observed in p53-null cells with Mdm2 deleted, correlates with increased cell proliferation and the spontaneous transformation of MdmX/p53-null mouse embryonic fibroblasts in vitro and with an increased rate of spontaneous tumorigenesis in MdmX/p53-null mice in vivo. These results indicate that MdmX has a p53-independent role in suppressing oncogenic cell transformation, proliferation, and tumorigenesis by promoting centrosome clustering and bipolar mitosis.Although genetic and biochemical studies clearly indicate that Mdm2 and MdmX are key regulators of p53 activity, there are distinct differences in their mechanisms of p53 inhibition. Mdm2 forms a complex with p53 and functions as an E3 ligase to target p53 for ubiquitination and proteosomal degradation (16,18,25), thereby inhibiting p53's transactivation of genes whose products are involved in the regulation of cell growth and apoptosis (47). MdmX complexes with p53 and inhibits p53 transactivation without altering p53 stability (11,46), and in contrast to that of Mdm2, MdmX expression is not regulated by p53. Regardless of these differences, Mdm2 and MdmX act as critical negative regulators of p53 function in development. The developmental block imposed by the loss of Mdm2 or MdmX can be relieved by the deletion of p53 (9,21,28,30,32) or by Mdm2 amplification (20,22) or, in the case of MdmX, partially rescued by the deletion of the p53 downstream effector p21 (43). These data indicate that the primary role of Mdm2 or MdmX in development is to regulate p53.The amplification and overexpression of either Mdm2 (31) or MdmX (7, 36) have been observed in a variety of human cancers, including sarcoma, glioma, and, in the case of MdmX, retinoblastoma (24), suggesting that either Mdm2 or MdmX can function as an oncogene to inhibit p53 activity and promote tumorigenesis. Since many of these Mdm-overexpressing tumors retain wild-type p53 alleles, the reactivation of p53 by small-molecule inhibition of the Mdm2-p53 or MdmX-p53 interaction is an attractive strategy for treating these cancers (26,27).The results of experiments in vitro or in vivo involving the forced overexpression of Mdm proteins suggest that Mdm2 and MdmX may also have p53-independent roles in promoting cell growth (13,23,29,38,41). However, molecular targets for Mdm2 or MdmX activity other than p53 have yet to be confirmed. Furthermore, it remains unclear if physiologic levels of eit...

Alternate Splicing of the p53 Inhibitor HDMX Offers a Superior Prognostic Biomarker than p53 Mutation in Human Cancer

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Understanding wild-type and mutant p53 activities in human cancer: new landmarks on the way to targeted therapies

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Antisense oligonucleotide–mediated MDM4 exon 6 skipping impairs tumor growth

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