MdmX promotes bipolar mitosis to suppress transformation and tumorigenesis in p53-deficient cells and mice

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.     

MdmX binding to ARF affects Mdm2 protein stability and p53 transactivation

Regulation of p53 involves a complex network of protein interactions. The primary regulator of p53 protein stability is the Mdm2 protein. ARF and MdmX are two proteins that have recently been shown to inhibit Mdm2-mediated degradation of p53 via distinct associations with Mdm2. We demonstrate here that ARF is capable of interacting with MdmX and in a manner similar to its association with Mdm2, sequestering MdmX within the nucleolus. The sequestration of MdmX by ARF results in an increase in p53 transactivation. In addition, the redistribution of MdmX by ARF requires that a nucleolar localization signal be present on MdmX. Although expression of either MdmX or ARF leads to Mdm2 stabilization, coexpression of both MdmX and ARF results in a decrease in Mdm2 protein levels. Similarly, increasing ARF protein levels in the presence of constant MdmX and Mdm2 leads to a dose-dependent decrease in Mdm2 levels. Under these conditions, ARF can synergistically reverse the ability of Mdm2 and MdmX to inhibit p53-dependent transactivation. Finally, the association and redistribution of MdmX by ARF has no effect on the protein stability of either ARF or MdmX. Taken together, these results demonstrate that the interaction between MdmX and ARF represents a novel pathway for regulating Mdm2 protein levels. Additionally, both MdmX and Mdm2, either individually or together, are capable of antagonizing the effects of the ARF tumor suppressor on p53 activity.     

Modification of p53 with O-linked N-acetylglucosamine regulates p53 activity and stability

Post-translational addition of O-linked N-acetylglucosamine (O-GlcNAc) to p53 is known to occur, but the site of O-GlcNAcylation and its effects on p53 are not understood. Here, we show that Ser 149 of p53 is O-GlcNAcylated and that this modification is associated with decreased phosphorylation of p53 at Thr 155, which is a site that is targeted by the COP9 signalosome, resulting in decreased p53 ubiquitination. Accordingly, O-GlcNAcylation at Ser 149 stabilizes p53 by blocking ubiquitin-dependent proteolysis. Our results indicate that the dynamic interplay between O-GlcNAc and O-phosphate modifications coordinately regulate p53 stability and activity.     

p53 regulates the proliferation, differentiation and spontaneous transformation of mesenchymal stem cells

Mesenchymal stem cells (MSC) have been extensively studied and gained wide popularity due to their therapeutic potential. Spontaneous transformation of MSC, from both human and murine origin, has been reported in many studies. MSC transformation depends on the culture conditions, the origin of the cells and the time on culture; however, the precise biological characteristics involved in this process have not been fully defined yet. In this study, we investigated the role of p53 in the biology and transformation of murine bone marrow (BM)-derived MSC. We demonstrate that the MSC derived from p53KO mice showed an augmented proliferation rate, a shorter doubling time and also morphologic and phenotypic changes, as compared to MSC derived from wild-type animals. Furthermore, the MSC devoid of p53 had an increased number of cells able to generate colonies. In addition, not only proliferation but also MSC differentiation is controlled by p53 since its absence modifies the speed of the process. Moreover, genomic instability, changes in the expression of c-myc and anchorage independent growth were also observed in p53KO MSC. In addition, the absence of p53 implicates the spontaneous transformation of MSC in long-term cultures. Our results reveal that p53 plays a central role in the biology of MSC.     

MdmX protects p53 from Mdm2-mediated degradation

The p53 tumor suppressor protein is stabilized in response to cellular stress, resulting in activation of genes responsible for either cell cycle arrest or apoptosis. The cellular pathway for releasing normal cells from p53-dependent cell cycle arrest involves the Mdm2 protein. Recently, a p53-binding protein with homology to Mdm2 was identified and called MdmX. Like Mdm2, MdmX is able to bind p53 and inhibit p53 transactivation; however, the ability of MdmX to degrade p53 has yet to be examined. We report here that MdmX is capable of associating with p53 yet is unable to facilitate nuclear export or induce p53 degradation. In addition, expression of MdmX can reverse Mdm2-targeted degradation of p53 while maintaining suppression of p53 transactivation. Using a series of MdmX deletions, we have determined that there are two distinct domains of the MdmX protein that can stabilize p53 in the presence of Mdm2. One domain requires MdmX interaction with p53 and results in the retention of both proteins within the nucleus and repression of p53 transactivation. The second domain involves the MdmX ring finger and results in stabilization of p53 and an increase in p53 transactivation. The potential basis for stabilization and increased p53 transactivation by the MdmX ring finger domain is discussed. Based on these observations, we propose that the MdmX protein may function to maintain a nuclear pool of p53 protein in undamaged cells.     

Preferential killing of p53-deficient cancer cells by reversine

Reversine is a small synthetic molecule that inhibits multiple mitotic kinases, including MPS1 as well as Aurora kinase A and B (AURKA and AURKB). Here, we investigated the effects of reversine on p53-deficient vs p53-proficient cancer cells. We found that low doses (~0.5 µM) of reversine, which selectively inhibit MPS1 and hence impair the spindle assembly checkpoint, kill human TP53^?/? colon carcinoma cells less efficiently than their wild-type counterparts. In sharp contrast, high doses (~5 µM) of reversine induced hyperploidization and apoptosis to a much larger extent in TP53^?/? than in TP53^+/+ cells. Such a selective cytotoxicity could not be reproduced by the knockdown of MPS1, AURKA and AURKB, neither alone nor in combination, suggesting that it involves multiple (rather than a few) molecular targets of reversine. Videomicroscopy-based cell fate profiling revealed that, in response to high-dose reversine, TP53^?/? (but not TP53^+/+) cells undergo several consecutive rounds of abortive mitosis, resulting in the generation of hyperpolyploid cells that are prone to succumb to apoptosis upon the activation of mitotic catastrophe. In line with this notion, the depletion of anti-apoptotic proteins of the BCL-2 family sensitized TP53^?/? cells to the toxic effects of high-dose reversine. Moreover, the knockdown of BAX or APAF-1, as well as the chemical inhibition of caspases, limited the death of TP53^?/? cells in response to high-dose reversine. Altogether, these results suggest that p53-deficient cells are particularly sensitive to the simultaneous inhibition of multiple kinases, including MPS1, as it occurs in response to high-dose reversine.     

Dancing with p53: The role of p38MAPK in mitosis of p53-deficient tetraploid cells

Comment on: Vitale I, et al. Cell Cycle 2010; 9:2823-9.     

MdmX protein is essential for Mdm2 protein-mediated p53 polyubiquitination

Genetic evidence has implicated both Mdm2 and MdmX as essential in negative regulation of p53. However, the exact role of MdmX in this Mdm2-dependent protein degradation is not well understood. Most, if not all, previous Mdm2 studies used GST-Mdm2 fusion proteins in the in vitro assays. Here, we show that the p53 polyubiquitination activity of GST-Mdm2 is conferred by the GST tag and non-GST-tagged Mdm2 only catalyzes monoubiquitination of p53 even at extremely high concentrations. We further demonstrate that MdmX is a potent activator of Mdm2, facilitating dose-dependent p53 polyubiquitination. This activation process requires the RING domains of both MdmX and Mdm2 proteins. The polyubiquitination activity of Mdm2/MdmX is Mdm2-dependent. Unlike Mdm2 or MdmX overexpression alone, co-overexpression of MdmX and Mdm2 consistently triggered p53 degradation in cells. Moreover, cellular polyubiquitination of p53 was only observable in the cytoplasm where both Mdm2 and MdmX are readily detectable. Importantly, RNAi knockdown of MdmX increased levels of endogenous p53 accompanied by reduced p53 polyubiquitination. In conclusion, our work has resolved a major confusion in the field derived from using GST-Mdm2 and demonstrated that MdmX is the cellular activator that converts Mdm2 from a monoubiquitination E3 ligase to a polyubiquitination E3 ligase toward p53. Together, our findings provide a biochemical basis for the requirement of both Mdm2 and MdmX in the dynamic regulation of p53 stability.     

Malignant transformation of p53-deficient astrocytes is modulated by environmental cues in vitro.

The early incidence of p53 mutation in astrocytomas suggests that it plays an important role in astrocyte transformation. Astrocytes isolated from homozygous p53 knockout mice grow rapidly, lack contact inhibition, and are immortal. Here we tested whether the loss of p53 is sufficient for progression to tumorigenicity of astrocytes. We grew primary astrocytes under three conditions for over 120 population doublings and assessed their antigenic phenotype, chromosome number, and expression of glioma-associated genes as well as their ability to form colonies in soft agarose and tumors s.c. and intracranially in nude mice. Under two conditions (10% FCS and 0.5% FCS plus 20 ng/ml EGF), cells acquired the ability to form colonies in soft agarose and tumors in nude mice, and this was accompanied by the expression of genes, including epidermal growth factor receptor, platelet-derived growth factor receptor alpha and beta, protein kinase Cdelta, and vascular endothelial growth factor, which are known to be aberrantly regulated in human astrocytomas. Under the third condition (0.5% FCS plus 10 ng/ml basic fibroblast growth factor), astrocytes gained the ability to form colonies in soft agarose and had abnormal chromosome numbers similar to cells in the first two conditions but did not form tumors in nude mice or overexpress glioma-associated genes. These data provide experimental evidence for the idea that the malignant progression initiated by the loss of p53 may be subject to modulation by extracellular environmental influences.     

Chk1 suppresses bypass of mitosis and tetraploidization in p53-deficient cancer cells

Many cancer cells are unable to maintain a numerically stable chromosome complement. It is well established that aberrant cell division can generate progeny with increased ploidy, but the genetic factors required for maintenance of diploidy are not well understood. Using an isogenic model system derived by gene targeting, we examined the role of Chk1 in p53-proficient and -deficient cancer cells. Targeted inactivation of a single CHK1 allele in stably diploid cells caused an elevated frequency of mitotic bypass if p53 was naturally mutated or experimentally disrupted by homologous recombination. CHK1-haploinsufficient, p53-deficient cells frequently underwent sequential rounds of DNA synthesis without an intervening mitosis. These aberrant cell cycles resulted in whole-genome endoreduplication and tetraploidization. The unscheduled bypass of mitosis could be suppressed by targeted reversion of a p53 mutation or by exogenous expression of Cdk1. In contrast, the number of tetraploid cells was not increased in isogenic cell populations that harbor hypomorphic ATR mutations, suggesting that suppression of unscheduled mitotic bypass is a distinct function of Chk1. These results are consistent with a recently described role for Chk1 in promoting the expression of genes that promote cell cycle transitions and demonstrate how Chk1 might prevent tetraploidization during the cancer cell cycle.     

p53RDL1 regulates p53-dependent apoptosis

Although a number of targets for p53 have been reported, the mechanism of p53-dependent apoptosis still remains to be elucidated. Here we report a new p53 target-gene, designated p53RDL1 (p53-regulated receptor for death and life; also termed UNC5B). The p53RDL1 gene product contains a cytoplasmic carboxy-terminal death domain that is highly homologous to rat Unc5H2, a dependence receptor involved in the regulation of apoptosis, as well as in axon guidance and migration of neural cells. We found that p53RDL1 mediated p53-dependent apoptosis. Conversely, when p53RDL1 interacted with its ligand, Netrin-1, p53-dependent apoptosis was blocked. Therefore, p53RDL1 seems to be a previously un-recognized target of p53 that may define a new pathway for p53-dependent apoptosis. We suggest that p53 might regulate the survival of damaged cells by balancing the regulation of Netrin-p53RDL1 signalling, and cell death through cleavage of p53RDL1 for apoptosis.     

Chk1 suppresses bypass of mitosis and tetraploidization in p53-deficient cancer cells

Many cancer cells are unable to maintain a numerically stable chromosome complement. It is well established that aberrant cell division can generate progeny with increased ploidy, but the genetic factors required for maintenance of diploidy are not well understood. Using an isogenic model system derived by gene targeting, we examined the role of Chk1 in p53-proficient and -deficient cancer cells. Targeted inactivation of a single CHK1 allele in stably diploid cells caused an elevated frequency of mitotic bypass if p53 was naturally mutated or experimentally disrupted by homologous recombination. CHK1-haploinsufficient, p53-deficient cells frequently underwent sequential rounds of DNA synthesis without an intervening mitosis. These aberrant cell cycles resulted in whole-genome endoreduplication and tetraploidization. The unscheduled bypass of mitosis could be suppressed by targeted reversion of a p53 mutation or by exogenous expression of Cdk1. In contrast, the number of tetraploid cells was not increased in isogenic cell populations that harbor hypomorphic ATR mutations, suggesting that suppression of unscheduled mitotic bypass is a distinct function of Chk1. These results are consistent with a recently described role for Chk1 in promoting the expression of genes that promote cell cycle transitions and demonstrate how Chk1 might prevent tetraploidization during the cancer cell cycle.