The HDMX protein is closely related to HDM2 with which it shares different structural domains, particularly the p53 binding domain and the ring finger domain, where the two HDM proteins interact. Several oncogenic forms derived from splicing of HDM2 have been described in cancer. This work aimed at investigating whether analogous forms of HDMX exist in human tumors. Here, we report the characterization of an aberrantly spliced form of HDMX, HDMX211, isolated from the thyroid tumor cell line, ARO. HDMX211 binds and stabilizes the HDM2 protein. Although it lacks the p53 binding domain, HDMX211 also stabilizes p53 by counteracting its degradation by HDM2. However, the resulting p53 is transcriptionally inactive and increasingly associated to its inhibitor HDM2. Expression of HDMX211 strongly enhances the colony-forming ability of human cells in the presence or absence of wild-type p53. Conversely, depletion of HDMX211 by small interfering RNA significantly reduces the growth of ARO cells and increases their sensitivity to chemotherapy. Screening of lung cancer biopsies shows the presence of HDMX211 in samples that overexpress HDM2 protein, supporting a pathologic role for this new protein. This is the first evidence of a variant form of HDMX that has oncogenic potential independently of p53. HDMX211 reveals a new mechanism for overexpression of the oncoprotein HDM2. Most interestingly, it outlines a possible molecular explanation for a yet unclarified tumor phenotype, characterized by simultaneous overexpression of HDM2 and wild-type p53. (Cancer Res 2005; 65(21): 9687-94)

MDMX is a p53 binding protein, which shares a high degree of homology with MDM2, a negative regulator of the tumor suppressor p53. MDMX has been shown to counteract MDM2-dependent p53 degradation and to stabilize p53 in its inactive form. In this study: we identify two MDMX proteolytic pathways that control its intracellular levels, and show that MDMX posttranslational processing may be regulated by p53. Mouse MDMX is cleaved in vitro and in vivo by caspase activity, between aminoacids 358 and 361, producing a p54 minor form. In addition, MDMX is subjected to proteasome-mediated degradation, which concurs to MDMX proteolysis mainly through degradation of p54. A D361A-MDMX mutant, resistant to caspase cleavage, exhibits prolonged intracellular lifetime in comparison to wild-type protein, indicating that caspase cleavage aects stability of MDMX protein probably by modulating its further degradation. Overexpression of exogenous p53 increases the intracellular levels of p54 product. Similarly, activation of endogenous p53 by adriamycin enhances MDMX cleavage and produces a marked decrease of its intracellular levels, while not aecting the D361A-MDMX mutant. In addition, the D361A-MDMX mutant lacks the ability to inhibit p53 transactivation in respect to wild-type MDMX, suggesting that MDMX caspase cleavage play an important functional role. In conclusion, our results demonstrate that, in analogy to MDM2, MDMX may be subjected to proteolytic modi®cations that regulate its intracellular levels. Moreover, decrease of MDMX protein levels following p53 activation suggests a p53-dependent regulatory feedback of MDMX function.

Rescue of embryonic lethality in MDM4؊/؊ mice through concomitant loss of p53 has revealed a functional partnership between the two proteins. Biochemical studies have suggested that MDM4 may act as a negative regulator of p53 levels and activity. On the other hand, MDM4 overexpression has been reported to stabilize p53 levels and to counteract MDM2-degradative activity. We have investigated the functional role of MDM4 overexpression on cell behavior. In both established and primary cells cultured under stress conditions, overexpression of MDM4 significantly increased p53-dependent cell death, in correlation with enhanced induction of the endogenous p53 protein levels. This phenomenon was associated with induced p53 transcriptional activity and increased levels of the proapoptotic protein, Bax. Further, p53 stabilization was accompanied by decreased association of the protein to its negative regulator, MDM2. These findings reveal a novel role for MDM4 by demonstrating that in non-tumor cells under stress conditions it may act as a positive regulator of p53 activity, mainly by controlling p53 levels. They also indicate a major distinction between the biological consequences of MDM4 and MDM2 overexpression.

<div>Abstract<p>The HDMX protein is closely related to HDM2 with which it shares different structural domains, particularly the p53 binding domain and the ring finger domain, where the two HDM proteins interact. Several oncogenic forms derived from splicing of HDM2 have been described in cancer. This work aimed at investigating whether analogous forms of HDMX exist in human tumors. Here, we report the characterization of an aberrantly spliced form of HDMX, HDMX211, isolated from the thyroid tumor cell line, ARO. HDMX211 binds and stabilizes the HDM2 protein. Although it lacks the p53 binding domain, HDMX211 also stabilizes p53 by counteracting its degradation by HDM2. However, the resulting p53 is transcriptionally inactive and increasingly associated to its inhibitor HDM2. Expression of HDMX211 strongly enhances the colony-forming ability of human cells in the presence or absence of wild-type p53. Conversely, depletion of HDMX211 by small interfering RNA significantly reduces the growth of ARO cells and increases their sensitivity to chemotherapy. Screening of lung cancer biopsies shows the presence of HDMX211 in samples that overexpress HDM2 protein, supporting a pathologic role for this new protein. This is the first evidence of a variant form of HDMX that has oncogenic potential independently of p53. HDMX211 reveals a new mechanism for overexpression of the oncoprotein HDM2. Most interestingly, it outlines a possible molecular explanation for a yet unclarified tumor phenotype, characterized by simultaneous overexpression of HDM2 and wild-type p53.</p></div>

Supplementary Figure 2 from Identification of an Aberrantly Spliced Form of HDMX in Human Tumors: A New Mechanism for HDM2 Stabilization