Aart G. Jochemsen

Inactivation of the p53 pathway in retinoblastoma.

Most human tumours have genetic mutations in their Rb and p53 pathways, but retinoblastoma is thought to be an exception. Studies suggest that retinoblastomas, which initiate with mutations in the gene retinoblastoma 1 (RB1), bypass the p53 pathway because they arise from intrinsically death-resistant cells during retinal development. In contrast to this prevailing theory, here we show that the tumour surveillance pathway mediated by Arf, MDM2, MDMX and p53 is activated after loss of RB1 during retinogenesis. RB1-deficient retinoblasts undergo p53-mediated apoptosis and exit the cell cycle. Subsequently, amplification of the MDMX gene and increased expression of MDMX protein are strongly selected for during tumour progression as a mechanism to suppress the p53 response in RB1-deficient retinal cells. Our data provide evidence that the p53 pathway is inactivated in retinoblastoma and that this cancer does not originate from intrinsically death-resistant cells as previously thought. In addition, they support the idea that MDMX is a specific chemotherapeutic target for treating retinoblastoma.

MDMX : A NOVEL P53-BINDING PROTEIN WITH SOME FUNCTIONAL PROPERTIES OF MDM2

Here we report the isolation of a cDNA encoding a new p53‐associating protein. This new protein has been called MDMX on the basis of its structural similarity to MDM2, which is especially notable in the p53‐binding domain. In addition, the putative metal binding domains in the C‐terminal part of MDM2 are completely conserved in MDMX. The middle part of the MDMX and MDM2 proteins shows a low degree of conservation. We can show by co‐immunoprecipitation that the MDMX protein interacts specifically with p53 in vivo. This interaction probably occurs with the N‐terminal part of p53, because the activity of the transcription activation domain of p53 was inhibited by co‐transfection of MDMX. Northern blotting showed that MDMX, like MDM2, is expressed in all tissues tested, and that several mRNAs for MDMX can be detected. Interestingly, the level of MDMX mRNA is unchanged after UV irradiation, in contrast to MDM2 transcription. This observation suggests that MDMX may be a differently regulated modifier of p53 activity in comparison with MDM2. Our study indicates that at least one additional member of the MDM protein family exists which can modulate p53 function.

Rescue of embryonic lethality in Mdm4-null mice by loss of Trp53 suggests a nonoverlapping pathway with MDM2 to regulate p53

The p53 protein can inhibit cell cycling or induce apoptosis, and is thus a critical regulator of tumorigenesis. This protein is negatively regulated by a physical interaction with MDM2, an E3 ubiquitin ligase. This interaction is critical for cell viability; loss of Mdm2 causes cell death in vitro and in vivo in a p53-dependent manner. The recently discovered MDM2-related protein MDM4 (also known as MDMX) has some of the same properties as MDM2. MDM4 binds and inhibits p53 transcriptional activity in vitro. Unlike MDM2, however, MDM4 does not cause nuclear export or degradation of p53 (refs. 9,10). To study MDM4 function in vivo, we deleted Mdm4 in mice. Mdm4-null mice died at 7.5–8.5 dpc, owing to loss of cell proliferation and not induction of apoptosis. To assess the importance of p53 in the death of Mdm4−/− embryos, we crossed in the Trp53-null allele. The loss of Trp53 completely rescued the Mdm4−/− embryonic lethality. Thus, MDM2 and MDM4 are nonoverlapping critical regulators of p53 in vivo. These data define a new pathway of p53 regulation and raise the possibility that increased MDM4 levels and the resulting inactivation of p53 contribute to the development of human tumors.

Amplification of Mdmx (or Mdm4) directly contributes to tumor formation by inhibiting p53 tumor suppressor activity

ABSTRACT Human tumors are believed to harbor a disabled p53 tumor suppressor pathway, either through direct mutation of the p53 gene or through aberrant expression of proteins acting in the p53 pathway, such as p14ARF or Mdm2. A role for Mdmx (or Mdm4) as a key negative regulator of p53 function in vivo has been established. However, a direct contribution of Mdmx to tumor formation remains to be demonstrated. Here we show that retrovirus-mediated Mdmx overexpression allows primary mouse embryonic fibroblast immortalization and leads to neoplastic transformation in combination with HRasV12. Furthermore, the human Mdmx ortholog, Hdmx, was found to be overexpressed in a significant percentage of various human tumors and amplified in 5% of primary breast tumors, all of which retained wild-type p53. Hdmx was also amplified and highly expressed in MCF-7, a breast cancer cell line harboring wild-type p53, and interfering RNA-mediated reduction of Hdmx markedly inhibited the growth potential of these cells in a p53-dependent manner. Together, these results make Hdmx a new putative drug target for cancer therapy.

Mdmx stabilizes p53 and Mdm2 via two distinct mechanisms

The p53 protein maintains genomic integrity through its ability to induce cell cycle arrest or apoptosis in response to various forms of stress. Substantial regulation of p53 activity occurs at the level of protein stability, largely determined by the activity of the Mdm2 protein. Mdm2 targets both p53 and itself for ubiquitylation and subsequent proteasomal degradation by acting as an ubiquitin ligase, a function that needs an intact Mdm2 RING finger. For efficient degradation of p53 nuclear export appears to be required. The Mdmx protein, structurally homologous to Mdm2, does not target p53 for degradation, but even stabilizes both p53 and Mdm2, an activity most likely mediated by heterodimerization of the RING fingers of Mdm2 and Mdmx. Here we show that Mdmx expression leads to accumulation of ubiquitylated, nuclear p53 but does not significantly affect the Mdm2‐mediated ubiquitylation of p53. In contrast, Mdmx stabilizes Mdm2 by inhibiting its self‐ubiquitylation.

Distinct gene mutation profiles among luminal-type and basal-type breast cancer cell lines.

Breast cancer has for long been recognized as a highly diverse tumor group, but the underlying genetic basis has been elusive. Here, we report an extensive molecular characterization of a collection of 41 human breast cancer cell lines. Protein and gene expression analyses indicated that the collection of breast cancer cell lines has retained most, if not all, molecular characteristics that are typical for clinical breast cancers. Gene mutation analyses identified 146 oncogenic mutations among 27 well-known cancer genes, amounting to an average of 3.6 mutations per cell line. Mutations in genes from the p53, RB and PI3K tumor suppressor pathways were widespread among all breast cancer cell lines. Most important, we have identified two gene mutation profiles that are specifically associated with luminal-type and basal-type breast cancer cell lines. The luminal mutation profile involved E-cadherin and MAP2K4 gene mutations and amplifications of Cyclin D1, ERBB2 and HDM2, whereas the basal mutation profile involved BRCA1, RB1, RAS and BRAF gene mutations and deletions of p16 and p14ARF. These subtype-specific gene mutation profiles constitute a genetic basis for the heterogeneity observed among human breast cancers, providing clues for their underlying biology and providing guidance for targeted pharmacogenetic intervention in breast cancer patients.

The large E1B protein together with the E4orf6 protein target p53 for active degradation in adenovirus infected cells.

It has recently been shown that an adenovirus mutant lacking expression of the large E1B protein (ΔE1B) selectively replicates in p53 deficient cells. However, apart from the large E1B protein the adenovirus early region encodes the E1A and E4orf6 proteins which also have been reported to affect p53 expression as well as its functioning. After infection with wild-type adenovirus we observed a dramatic decrease in wild-type p53 expression while no down-regulation of p53 could be detected after infection with the ΔE1B virus. The different effects of the wild-type and ΔE1B adenovirus on p53 expression were not only found in cells expressing wild-type p53 but were also observed when tumor cells expressing highly stabilized mutant p53 were infected with these two viruses. Infection with different adenovirus mutants indicated the importance of a direct interaction between p53 and the large E1B protein for reduced p53 expression after infection. Moreover, coexpression of the E4orf6 protein was found to be required for this phenomenon, while expression of E1A is dispensable. In addition, we provide evidence that p53 is actively degraded in wild-type adenovirus-infected cells but not in ΔE1B-infected cells.

MDM4 is a key therapeutic target in cutaneous melanoma

The inactivation of the p53 tumor suppressor pathway, which often occurs through mutations in TP53 (encoding tumor protein 53) is a common step in human cancer. However, in melanoma—a highly chemotherapy-resistant disease—TP53 mutations are rare, raising the possibility that this cancer uses alternative ways to overcome p53-mediated tumor suppression. Here we show that Mdm4 p53 binding protein homolog (MDM4), a negative regulator of p53, is upregulated in a substantial proportion (∼65%) of stage I–IV human melanomas and that melanocyte-specific Mdm4 overexpression enhanced tumorigenesis in a mouse model of melanoma induced by the oncogene Nras. MDM4 promotes the survival of human metastatic melanoma by antagonizing p53 proapoptotic function. Notably, inhibition of the MDM4-p53 interaction restored p53 function in melanoma cells, resulting in increased sensitivity to cytotoxic chemotherapy and to inhibitors of the BRAF (V600E) oncogene. Our results identify MDM4 as a key determinant of impaired p53 function in human melanoma and designate MDM4 as a promising target for antimelanoma combination therapy.

Comparative study of the p53-mdm2 and p53-MDMX interfaces.

Mdm2 and MDMX are two structurally related p53-binding proteins which show the highest level of sequence similarity in the N-terminal p53-binding domains. Apart from its ability to inhibit p53 mediated transcription, a feature it shares with mdm2, very little is known about the physiological functions of MDMX. It is clearly distinct from mdm2 since its expression appears not to be regulated by p53 and it cannot compensate for lack of mdm2 in early development. We present data on the structural similarity between the p53 binding pockets of mdm2 and MDMX using p53- and phage-selected peptides. From the results we conclude that our recently devised innovative approach to reverse the mdm2-mediated inhibition of p53s transactivation function in vivo would probably target MDMX as well. Strategies for selectively targeting mdm2 and MDMX are suggested and a possible mechanism for regulating the p53-mdm2/MDMX interactions by protein phosphorylation is discussed.

Identification and characterization of the first small molecule inhibitor of MDMX.

The p53 pathway is disrupted in virtually every human tumor. In ∼50% of human cancers, the p53 gene is mutated, and in the remaining cancers, the pathway is dysregulated by genetic lesions in other genes that modulate the p53 pathway. One common mechanism for inactivation of the p53 pathway in tumors that express wild-type p53 is increased expression of MDM2 or MDMX. MDM2 and MDMX bind p53 and inhibit its function by distinct nonredundant mechanisms. Small molecule inhibitors and small peptides have been developed that bind MDM2 in the p53-binding pocket and displace the p53 protein, leading to p53-mediated cell cycle exit and apoptosis. To date, peptide inhibitors of MDMX have been developed, but no small molecule inhibitors have been reported. We have developed biochemical and cell-based assays for high throughput screening of chemical libraries to identify MDMX inhibitors and identified the first MDMX inhibitor SJ-172550. This compound binds reversibly to MDMX and effectively kills retinoblastoma cells in which the expression of MDMX is amplified. The effect of SJ-172550 is additive when combined with an MDM2 inhibitor. Results from a series of biochemical and structural modeling studies suggest that SJ-172550 binds the p53-binding pocket of MDMX, thereby displacing p53. This lead compound is a useful chemical scaffold for further optimization of MDMX inhibitors that may eventually be used to treat pediatric cancers and various adult tumors that overexpress MDMX or have similar genetic lesions. When combined with selective MDM2 inhibitors, SJ-172550 may also be useful for treating tumors that express wild-type p53.