Regulation of MDMX Expression by Mitogenic Signaling

Abstract

MDMX is an important regulator of p53 transcriptional activity and stress response. MDMX overexpression and gene amplification are implicated in p53 inactivation and tumor development. Unlike MDM2, MDMX is not inducible by p53, and little is known about its regulation at the transcriptional level. We found that MDMX levels in tumor cell lines closely correlate with promoter activity and mRNA level. Activated K-Ras and insulin-like growth factor 1 induce MDMX expression at the transcriptional level through mechanisms that involve the mitogen-activated protein kinase and c-Ets-1 transcription factors. Pharmacological inhibition of MEK results in down-regulation of MDMX in tumor cell lines. MDMX overexpression was detected in ϳ50% of human colon tumors and showed strong correlation with increased extracellular signal-regulated kinase phosphorylation. Therefore, MDMX expression is regulated by mitogenic signaling pathways. This mechanism may protect normal proliferating cells from p53 but also hamper p53 response during tumor development.MDMX is a p53 binding protein and a regulator of p53 response to DNA damage and ribosomal stress (7,14,19). The physiological importance of MDMX was revealed by the embryonic lethality of MDMX-null mice, which can be rescued by the simultaneous knockout of p53 (13,30,33). MDMX overexpression has been reported in 40% of tumor cell lines (36) and 18.5% of breast, colon, and lung tumor samples (10). It is amplified in 4% of glioblastomas (37) and 5% of breast tumors (10). More recently, ϳ60% of retinoblastomas have been found to have MDMX overexpression or gene amplification (22). MDMX overexpression prevents oncogenic Ras-induced premature senescence in mouse fibroblasts and cooperates with activated Ras to confer tumorigenic potential in nude mice (10). Taken together the evidence suggests that MDMX overexpression can efficiently suppress p53 and may alleviate or delay the selection for p53 mutation during tumor initiation and progression.MDMX is structurally similar to its homolog and binding partner MDM2 (45). Unlike MDM2, which promotes p53 degradation, MDMX does not have significant intrinsic E3 ligase activity (46). MDMX forms heterodimers with MDM2 through C-terminal RING domain interactions (40, 47), which stimulates the ability of MDM2 to ubiquitinate and degrade p53 (16,26). MDM2 can also ubiquitinate itself as well as MDMX. Under nonstress conditions, MDM2 and p53 have short halflives whereas MDMX is relatively stable. Following DNA damage, both p53 and MDM2 are phosphorylated by several kinases (44). Most notably, ATM phosphorylates p53 on serine 15 and enhances its transcriptional activity (2). ATM also activates Chk2, which in turn phosphorylates p53 on serine 20 and inhibits binding to MDM2 (2,6,42,43). ATM phosphorylation of MDM2 also inhibits its ability to degrade p53. The combination of these modifications results in the stabilization and activation of p53.Although MDMX can regulate p53 stability by heterodimerization with MDM2, its impact on p53 level is moderate c...

MDM4 (MDMX) is overexpressed in chronic lymphocytic leukaemia (CLL) and marks a subset of p53wild‐type CLL with a poor cytotoxic response to Nutlin‐3

“…This observation is in keeping with studies describing the over-expression of MDM4 in primary samples from several solid tumours, including glioblastoma, retinoblastoma, breast, colon, and lung cancers (Gilkes et al, 2008), and may contribute to identify MDM4 as a potentially useful new therapeutic target also for CLL. Preclinical studies indicating the anti-neoplastic effects of MDM4 down-regulation in murine lymphoma models (Terzian et al, 2007) are of further support for this hypothesis.…”

Rare MDM4 gene amplification in colorectal cancer: The principle of a mutually exclusive relationship between MDM alteration and TP53 inactivation is not applicable

“…these tumors were located in the rectum or canalis analis, which is consistent with previous studies demonstrating that the incidence of MSI in distal colonic and rectal tumors is lower than that in proximal tumors (33). It has also been reported that MDM4 expression is induced in CRC by activated kRAS and insulin-like growth factor 1 (IGF1) and that the expression is activated through extracellular signal-related kinase (eRk) phosphorylation (32). Although wild-type KRAS was detected in two of the three MDM4-amplified tumors (cases 2 and 3), the level of MDM4 protein was only able to be assessed in one of these.…”

“…We may have to consider the content of MDM4-positive nonepithelial normal cells such as infiltrating cells in the stroma. on the contrary, an immunohistochemical analysis of MDM4 in CRC has been reported previously, which demonstrated that MDM4 expression was increased in aggressive tumors and that it was independent of the TP53 mutation status (32). The roles of MDM4 in crc tumorigenesis and progression remain to be elucidated.…”

MdmX Protein Is Essential for Mdm2 Protein-mediated p53 Polyubiquitination

“…Because MdmX is an essential component for Mdm2-driven p53 polyubiquitnation and degradation, DNA damage triggered removal of MdmX would be consistent with the rapid uncoupling of the p53/Mdm2 regulatory loop, which heralds p53 accumulation and nuclear accumulation in response to DNA damage (23)(24)(25). In a more cancer-relevant context, MdmX was shown to be transcriptionally up-regulated by mitogenic signaling (45), suggesting its involvement in procancer inactivation of p53.…”

“…In light of the critical role of MdmX in p53 down-regulation and the fact that MdmX expression is regulated by mitogenic signaling (45), targeting MdmX will undoubtedly restore p53 activity in a more robust way for therapeutic treatment of cancer. Indeed, it has been reported that the levels of MdmX dictate the sensitivity of normal and transformed cells to the Mdm2 inhibitor Nutlin-3 (46), and efficient p53 activation and apoptosis can be achieved by simultaneous disruption of binding to Mdm2 and MdmX (47).…”