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

Cellular, Structural Basis, and Recent Progress for Targeting Murine Double Minute X (MDMX) in Tumors

Murine double minute X (MDMX) is an oncoprotein that mainly has a negative regulatory effect on the tumor suppressor p53 to induce tumorigenesis. As MDMX is highly expressed in various types of tumor cells, targeting and inhibiting MDMX are becoming a promising strategy for treating cancers. However, the high degree of structural homology between MDMX and its homologous protein murine double minute 2 (MDM2) is a great challenge for the development of MDMX-targeted therapies. This review introduces the structure, distribution, and regulation of the MDMX, summarizes the structural features and structure-activity relationships (SARs) of MDMX ligands, and focuses on the differences between MDMX and MDM2 in these aspects. Our purpose of this work is to propose potential strategies to achieve the specific targeting of MDMX.

Acute exposure to dihydroxyacetone promotes genotoxicity and chromosomal instability in lung, cardiac, and liver cell models

Inhalation exposures to dihydroxyacetone (DHA) occur through spray tanning and e-cigarette aerosols. Several studies in skin models have demonstrated that millimolar doses of DHA are cytotoxic, yet the genotoxicity was unclear. We examined the genotoxicity of DHA in cell models relevant to inhalation exposures. Human bronchial epithelial cells BEAS-2B, lung carcinoma cells A549, cardiomyocyte Ac16, and hepatocellular carcinoma HepG3 were exposed to DHA, and low millimolar doses of DHA were cytotoxic. IC90 DHA doses induced cell cycle arrest in all cells except the Ac16. We examined DHA's genotoxicity using strand break markers, DNA adduct detection by Repair Assisted Damage Detection (RADD), metaphase spreads, and a forward mutation assay for mutagenesis. Similar to results for skin, DHA did not induce significant levels of strand breaks. However, RADD revealed DNA adducts were induced 24 h after DHA exposure, with BEAS-2B and Ac16 showing oxidative lesions and A549 and HepG3 showing crosslink-type lesions. Yet, only low levels of reactive oxygen species or advanced glycation end products were detected after DHA exposure. Metaphase spreads revealed significant increases in chromosomal aberrations in the BEAS-2B and HepG3 with corresponding changes in ploidy. Finally, we confirmed the mutagenesis observed using the supF reporter plasmid. DHA increased the mutation frequency, consistent with methylmethane sulfonate, a mutagen and clastogen. These data demonstrate DHA is a clastogen, inducing cell-specific genotoxicity and chromosomal instability. The specific genotoxicity measured in the BEAS-2B in this study suggests that inhalation exposures pose health risks to vapers, requiring further investigation.

MDMX in Cancer: A Partner of p53 and a p53-Independent Effector

The p53 tumor suppressor protein plays an important role in physiological and pathological processes. MDM2 and its homolog MDMX are the most important negative regulators of p53. Many studies have shown that MDMX promotes the growth of cancer cells by influencing the regulation of the downstream target gene of tumor suppressor p53. Studies have found that inhibiting the MDMX-p53 interaction can effectively restore the tumor suppressor activity of p53. MDMX has growth-promoting activities without p53 or in the presence of mutant p53. Therefore, it is extremely important to study the function of MDMX in tumorigenesis, progression and prognosis. This article mainly reviews the current research progress and mechanism on MDMX function, summarizes known MDMX inhibitors and provides new ideas for the development of more specific and effective MDMX inhibitors for cancer treatment.

MDM4 polymorphisms associated with the risk but not the prognosis of esophageal cancer in Cixian high-incidence region from northern China

AIM

The mouse double minute 4 (MDM4) may contribute to tumorgenesis by inhibiting p53 tumor suppressor activity. This study was designed to investigate whether MDM4 polymorphisms could affect susceptibility to esophageal squamous cell carcinoma (ESCC) and the survival of ESCC patients in a population from Cixian high-incidence region of northern China, which has not been explored.

METHODS

MDM4 rs1380576 and rs4245739 were genotyped by polymerase chain reaction-ligase detection reaction (PCR-LDR) in 568 ESCC patients and 578 controls.

RESULTS

Compared to rs1380576 C/C genotype, C/G genotype was associated with decreased risk of ESCC (odds ratio [OR] = 0.761, 95% confidence interval [CI] = 0.595-0.973). Compared to rs4245739 A/A genotype, A/C or C/C genotype was related to increased susceptibility to ESCC (OR = 1.551, 95% CI = 1.001-2.402). Individuals with GC haplotype had significantly higher risk of ESCC than those with CA or GA haplotype (OR = 1.598, 95% CI = 1.048-2.438). Neither rs1380576 nor rs4245739 influenced the survival of ESCC patients.

CONCLUSION

rs1380576 and rs4245739 may be used to predict susceptibility to ESCC for population in Cixian high-incidence region.

SNPs in miRNAs and Target Sequences: Role in Cancer and Diabetes

miRNAs are fascinating molecular players for gene regulation as individual miRNA can control multiple targets and a single target can be regulated by multiple miRNAs. Loss of miRNA regulated gene expression is often reported to be implicated in various human diseases like diabetes and cancer. Recently, geneticists across the world started reporting single nucleotide polymorphism (SNPs) in seed sequences of miRNAs. Similarly, SNPs are also reported in various target sequences of these miRNAs. Both the scenarios lead to dysregulated gene expression which may result in the progression of diseases. In the present paper, we explore SNPs in various miRNAs and their target sequences reported in various human cancers as well as diabetes. Similarly, we also present evidence of these mutations in various other human diseases.

The roles and regulation of MDM2 and MDMX: it is not just about p53

In this review, Klein et al. discuss the p53-independent roles of MDM2 and MDMX. First, they review the structural and functional features of MDM2 and MDMX proteins separately and together that could be relevant to their p53-independent activities. Following this, they summarize how these two proteins are regulated and how they can function in cells that lack p53.

Most well studied as proteins that restrain the p53 tumor suppressor protein, MDM2 and MDMX have rich lives outside of their relationship to p53. There is much to learn about how these two proteins are regulated and how they can function in cells that lack p53. Regulation of MDM2 and MDMX, which takes place at the level of transcription, post-transcription, and protein modification, can be very intricate and is context-dependent. Equally complex are the myriad roles that these two proteins play in cells that lack wild-type p53; while many of these independent outcomes are consistent with oncogenic transformation, in some settings their functions could also be tumor suppressive. Since numerous small molecules that affect MDM2 and MDMX have been developed for therapeutic outcomes, most if not all designed to prevent their restraint of p53, it will be essential to understand how these diverse molecules might affect the p53-independent activities of MDM2 and MDMX.

MDMX inhibits casein kinase 1α activity and stimulates Wnt signaling

Casein kinase 1 alpha (CK1α) is a serine/threonine kinase with numerous functions, including regulating the Wnt/β-catenin and p53 pathways. CK1α has a well-established role in inhibiting the p53 tumor suppressor by binding to MDMX and stimulating MDMX-p53 interaction. MDMX purified from cells contains near-stoichiometric amounts of CK1α, suggesting that MDMX may in turn regulate CK1α function. We present evidence that MDMX is a potent competitive inhibitor of CK1α kinase activity (K_i  = 8 nM). Depletion of MDMX increases CK1α activity and β-catenin S45 phosphorylation, whereas ectopic MDMX expression inhibits CK1α activity and β-catenin phosphorylation. The MDMX acidic domain and zinc finger are necessary and sufficient for binding and inhibition of CK1α. P53 binding to MDMX disrupts an intramolecular auto-regulatory interaction and enhances its ability to inhibit CK1α. P53-null mice expressing the MDMX^{W 200S/W201G} mutant, defective in CK1α binding, exhibit reduced Wnt/β-catenin target gene expression and delayed tumor development. Therefore, MDMX is a physiological inhibitor of CK1α and has a role in modulating cellular response to Wnt signaling. The MDMX-CK1α interaction may account for certain p53-independent functions of MDMX.

Mdm4 supports DNA replication in a p53-independent fashion

The Mdm4 (alias MdmX) oncoprotein, like its paralogue and interaction partner Mdm2, antagonizes the tumor suppressor p53. p53-independent roles of the Mdm proteins are emerging, and we have reported the ability of Mdm2 to modify chromatin and to support DNA replication by suppressing the formation of R-loops (DNA/RNA-hybrids). We show here that the depletion of Mdm4 in p53-deficient cells compromises DNA replication fork progression as well. Among various deletion mutants, only full-length Mdm4 was able to support DNA replication fork progression. Co-depletion of Mdm4 and Mdm2 further impaired DNA replication, and the overexpression of each partially compensated for the other's loss. Despite impairing replication, Mdm4 depletion only marginally hindered cell proliferation, likely due to compensation through increased firing of replication origins. However, depleting Mdm4 sensitized p53-/- cells to the nucleoside analog gemcitabine, raising the future perspective of using Mdm4 inhibitors as chemosensitizers. Mechanistically, Mdm4 interacts with members of the Polycomb Repressor Complexes and supports the ubiquitination of H2A, thereby preventing the accumulation of DNA/RNA-hybrids. Thus, in analogy to previously reported activities of Mdm2, Mdm4 enables unperturbed DNA replication through the avoidance of R-loops.

MDM2 and MDMX promote ferroptosis by PPARα-mediated lipid remodeling

Here, Venkatesh et al. investigated the p53-independent roles of MDMX and the MDM2–MDMX complex. They found that MDM2 and MDMX facilitate ferroptosis in cells with or without p53, and that PPARα activity is essential for MDM2 and MDMX to promote ferroptosis, suggesting that the MDM2–MDMX complex regulates lipids through altering PPARα activity.

MDM2 and MDMX, negative regulators of the tumor suppressor p53, can work separately and as a heteromeric complex to restrain p53's functions. MDM2 also has pro-oncogenic roles in cells, tissues, and animals that are independent of p53. There is less information available about p53-independent roles of MDMX or the MDM2–MDMX complex. We found that MDM2 and MDMX facilitate ferroptosis in cells with or without p53. Using small molecules, RNA interference reagents, and mutant forms of MDMX, we found that MDM2 and MDMX, likely working in part as a complex, normally facilitate ferroptotic death. We observed that MDM2 and MDMX alter the lipid profile of cells to favor ferroptosis. Inhibition of MDM2 or MDMX leads to increased levels of FSP1 protein and a consequent increase in the levels of coenzyme Q₁₀, an endogenous lipophilic antioxidant. This suggests that MDM2 and MDMX normally prevent cells from mounting an adequate defense against lipid peroxidation and thereby promote ferroptosis. Moreover, we found that PPARα activity is essential for MDM2 and MDMX to promote ferroptosis, suggesting that the MDM2–MDMX complex regulates lipids through altering PPARα activity. These findings reveal the complexity of cellular responses to MDM2 and MDMX and suggest that MDM2–MDMX inhibition might be useful for preventing degenerative diseases involving ferroptosis. Furthermore, they suggest that MDM2/MDMX amplification may predict sensitivity of some cancers to ferroptosis inducers.

Mutual regulation of MDM4 and TOP2A in cancer cell proliferation

MDM4 and topoisomerase IIα (TOP2A) are overexpressed in various human cancers. MDM4 acts as an oncoprotein which promotes cancer progression by inhibiting tumor suppressor p53. As a DNA replication- and cell division-regulating enzyme, TOP2A is the main target of many anticancer therapy regimens; however, the exact role of TOP2A in cancer remains elusive. Herein, we report that MDM4 and TOP2A bind to each other and are mutually upregulated at the post-translational level, leading to TOP2A protein stabilization, inhibition of p53, and increased tumor-cell proliferation. We demonstrate that the C-terminal region (CTR) of TOP2A binds to a unique sequence (residues: 188-238) of MDM4, which contains an auto-inhibitory segment regulating the MDM4-p53 interaction. TOP2A binding in turn activates MDM4 for p53 binding, resulting in enhanced inhibition of p53 and cancer cell proliferation. Conversely, binding of the MDM4 sequence to the CTR of TOP2A stabilizes TOP2A protein, leading to increased TOP2A protein expression. These results reveal novel functions of MDM4 and TOP2A as well as their interactions in oncogenesis, suggesting that inhibition of the MDM4-TOP2A interaction may represent a novel strategy in specifically and simultaneously targeting TOP2A and MDM4 for cancer treatment.

MDMX acidic domain inhibits p53 DNA binding in vivo and regulates tumorigenesis

The MDM2 homolog MDMX oncoprotein is indispensable for inhibition of p53 during normal embryonic development and malignant transformation, yet how MDMX harnesses p53 functions is unclear. In addition to a canonical N-terminal p53-binding domain, recent work suggests the central acidic domain of MDMX regulates p53 interaction through intramolecular mimicry and engages in second-site interaction with the p53 core domain in vitro. To test the physiological relevance of these interactions, we generated an MDMX knockin mouse having substitutions in a conserved WW motif necessary for these functions (W201S/W202G). Notably, MDMX^SG cells have normal p53 level but increased p53 DNA binding and target gene expression, and rapidly senesce. In vivo, MDMX^SG inhibits early-phase disease in Eµ-Myc transgenic mice but accelerates the onset of lethal lymphoma and shortens overall survival. Therefore, MDMX is an important regulator of p53 DNA binding, which complements the role of MDM2 in regulating p53 level. Furthermore, the results suggest that the WW motif has dual functions that regulate p53 and inhibit Myc-driven lymphomas independent of p53.

Otub1 stabilizes MDMX and promotes its proapoptotic function at the mitochondria

Otub1 regulates p53 stability and activity via non-canonical inhibition of UbcH5, the MDM2 cognate ubiquitin-conjugating enzyme (E2). However, whether Otub1 regulates MDMX stability and activity is not clear. Here we report that Otub1 also suppresses MDM2-mediated MDMX ubiquitination in cells and in vitro, independently of its deubiquitinating enzyme activity. Consequently, overexpression of Otub1 markedly stabilized MDMX and increased its levels, whereas knockdown of Otub1 reduced the levels of MDMX. Interestingly, MDMX induced by Otub1 can localize to mitochondria in addition to the cytosol, enhance p53 phosphorylation at S46 (p53S46P) and promote mitochondria-mediated apoptotic pathway. Knockdown of MDMX reduced Otub1-induced p53S46P, which was shown to be critical for p53's mitochondrial function and apoptotic activity. Furthermore, Otub1 promotes UV-irradiation-induced p53S46P and apoptosis, which can be significantly inhibited by MDMX depletion. Together, these results suggest that Otub1 stabilizes MDMX and promotes p53S46P and mitochondria-mediated apoptosis, providing an alternative mechanism of Otub1's role in apoptosis.

MDM4 actively restrains cytoplasmic mTORC1 by sensing nutrient availability

BACKGROUND

Many tumor-related factors have shown the ability to affect metabolic pathways by paving the way for cancer-specific metabolic features. Here, we investigate the regulation of mTORC1 by MDM4, a p53-inhibitor with oncogenic or anti-survival activities depending on cell growth conditions.

METHOD

MDM4-mTOR relationship was analysed through experiments of overexpression or silencing of endogenous proteins in cell culture and using purified proteins in vitro. Data were further confirmed in vivo using a transgenic mouse model overexpressing MDM4. Additionally, the Cancer Genome Atlas (TCGA) database (N = 356) was adopted to analyze the correlation between MDM4 and mTOR levels and 3D cultures were used to analyse the p53-independent activity of MDM4.

RESULTS

Following nutrient deprivation, MDM4 impairs mTORC1 activity by binding and inhibiting the kinase mTOR, and contributing to maintain the cytosolic inactive pool of mTORC1. This function is independent of p53. Inhibition of mTORC1 by MDM4 results in reduced phosphorylation of the mTOR downstream target p70S6K1 both in vitro and in vivo in a MDM4-transgenic mouse. Consistently, MDM4 reduces cell size and proliferation, two features controlled by p70S6K1, and, importantly, inhibits mTORC1-mediated mammosphere formation. Noteworthy, MDM4 transcript levels are significantly reduced in breast tumors characterized by high mTOR levels.

CONCLUSION

Overall, these data identify MDM4 as a nutrient-sensor able to inhibit mTORC1 and highlight its metabolism-related tumor-suppressing function.

Targeting MDM4 Splicing in Cancers

MDM4, an essential negative regulator of the P53 tumor suppressor, is frequently overexpressed in cancer cells that harbor a wild-type P53. By a mechanism based on alternative splicing, the MDM4 gene generates two mutually exclusive isoforms: MDM4-FL, which encodes the full-length MDM4 protein, and a shorter splice variant called MDM4-S. Previous results suggested that the MDM4-S isoform could be an important driver of tumor development. In this short review, we discuss a recent set of data indicating that MDM4-S is more likely a passenger isoform during tumorigenesis and that targeting MDM4 splicing to prevent MDM4-FL protein expression appears as a promising strategy to reactivate p53 in cancer cells. The benefits and risks associated with this strategy are also discussed.

Role of Mdm2 and Mdmx in DNA repair

Mdm2 and Mdmx are critical regulators of the p53 tumour suppressor and are overexpressed in many human malignancies. However, in recent years, their impact on genome instability was shown to be at least, in part, independent of p53. Both Mdm2 and Mdmx inhibit DNA break repair through their association with the Mre11/Rad50/Nbs1 DNA repair complex. Recent evidence indicates that harnessing Mdm2 and/or Mdmx-mediated inhibition of DNA break repair in cancer cells could provide a therapeutic opportunity, particularly for those malignancies that have lost functional p53.

The Zn-finger domain of MdmX suppresses cancer progression by promoting genome stability in p53-mutant cells

The MDMX (MDM4) oncogene is amplified or overexpressed in a significant percentage of human tumors. MDMX is thought to function as an oncoprotein by binding p53 tumor suppressor protein to inhibit p53-mediated transcription, and by complexing with MDM2 oncoprotein to promote MDM2-mediated degradation of p53. However, down-regulation or loss of functional MDMX has also been observed in a variety of human tumors that are mutated for p53, often correlating with more aggressive cancers and a worse patient prognosis. We have previously reported that endogenous levels of MdmX can suppress proliferation and promote pseudo-bipolar mitosis in primary and tumor cells derived from p53-deficient mice, and that MdmX-p53 double deficient mice succumb to spontaneously formed tumors more rapidly than p53-deficient mice. These results suggest that the MdmX oncoprotein may act as a tumor-suppressor in cancers with compromised p53 function. By using orthotopic transplantation and lung colonization assays in mice we now establish a p53-independent anti-oncogenic role for MdmX in tumor progression. We also demonstrate that the roles of MdmX in genome stability and in proliferation are two distinct functions encoded by the separate MdmX protein domains. The central Zn-finger domain suppresses multipolar mitosis and chromosome loss, whereas the carboxy-terminal RING domain suppresses proliferation of p53-deficient cells. Furthermore, we determine that it is the maintenance of genome stability that underlies MdmX role in suppression of tumorigenesis in hyperploid p53 mutant tumors. Our results offer a rationale for the increased metastatic potential of p53 mutant human cancers with aberrant MdmX function and provide a caveat for the application of anti-MdmX treatment of tumors with compromised p53 activity.

Genome Stability Requires p53

It is now clear that functional p53 is critical to protect the genome from alterations that lead to tumorigenesis. However, with the myriad of cellular stresses and pathways linked to p53 activation, much remains unknown about how p53 maintains genome stability and the proteins involved. The current understanding of the multiple ways p53 contributes to genome stability and how two of its negative regulators, Mdm2 and Mdmx, induce genome instability will be described.

The Roles of MDM2 and MDMX in Cancer

For more than 25 years, MDM2 and its homolog MDMX (also known as MDM4) have been shown to exert oncogenic activity. These two proteins are best understood as negative regulators of the p53 tumor suppressor, although they may have additional p53-independent roles. Understanding the dysregulation of MDM2 and MDMX in human cancers and how they function either together or separately in tumorigenesis may improve methods of diagnosis and for assessing prognosis. Targeting the proteins themselves, or their regulators, may be a promising therapeutic approach to treating some forms of cancer.

Antisense oligonucleotide-mediated MDM4 exon 6 skipping impairs tumor growth

MDM4 is a promising target for cancer therapy, as it is undetectable in most normal adult tissues but often upregulated in cancer cells to dampen p53 tumor-suppressor function. The mechanisms that underlie MDM4 upregulation in cancer cells are largely unknown. Here, we have shown that this key oncogenic event mainly depends on a specific alternative splicing switch. We determined that while a nonsense-mediated, decay-targeted isoform of MDM4 (MDM4-S) is produced in normal adult tissues as a result of exon 6 skipping, enhanced exon 6 inclusion leads to expression of full-length MDM4 in a large number of human cancers. Although this alternative splicing event is likely regulated by multiple splicing factors, we identified the SRSF3 oncoprotein as a key enhancer of exon 6 inclusion. In multiple human melanoma cell lines and in melanoma patient-derived xenograft (PDX) mouse models, antisense oligonucleotide-mediated (ASO-mediated) skipping of exon 6 decreased MDM4 abundance, inhibited melanoma growth, and enhanced sensitivity to MAPK-targeting therapeutics. Additionally, ASO-based MDM4 targeting reduced diffuse large B cell lymphoma PDX growth. As full-length MDM4 is enhanced in multiple human tumors, our data indicate that this strategy is applicable to a wide range of tumor types. We conclude that enhanced MDM4 exon 6 inclusion is a common oncogenic event and has potential as a clinically compatible therapeutic target.

Antisense oligonucleotide-mediated MDM4 exon 6 skipping impairs tumor growth

MDM4 is a promising target for cancer therapy, as it is undetectable in most normal adult tissues but often upregulated in cancer cells to dampen p53 tumor-suppressor function. The mechanisms that underlie MDM4 upregulation in cancer cells are largely unknown. Here, we have shown that this key oncogenic event mainly depends on a specific alternative splicing switch. We determined that while a nonsense-mediated, decay-targeted isoform of MDM4 (MDM4-S) is produced in normal adult tissues as a result of exon 6 skipping, enhanced exon 6 inclusion leads to expression of full-length MDM4 in a large number of human cancers. Although this alternative splicing event is likely regulated by multiple splicing factors, we identified the SRSF3 oncoprotein as a key enhancer of exon 6 inclusion. In multiple human melanoma cell lines and in melanoma patient-derived xenograft (PDX) mouse models, antisense oligonucleotide-mediated (ASO-mediated) skipping of exon 6 decreased MDM4 abundance, inhibited melanoma growth, and enhanced sensitivity to MAPK-targeting therapeutics. Additionally, ASO-based MDM4 targeting reduced diffuse large B cell lymphoma PDX growth. As full-length MDM4 is enhanced in multiple human tumors, our data indicate that this strategy is applicable to a wide range of tumor types. We conclude that enhanced MDM4 exon 6 inclusion is a common oncogenic event and has potential as a clinically compatible therapeutic target.

Inhibition of AKT/FoxO3a signaling induced PUMA expression in response to p53-independent cytotoxic effects of H1: A derivative of tetrandrine

PUMA (p53 unregulated modulator of apoptosis), a BH3-only Bcl-2 family member, can be induced by p53-dependent and p53-independent manners. It plays an important role as regulator of cellular apoptosis. Herein, we evaluate the effects of H1 (a derivative of tetrandrine) on induction of PUMA and underlie its potential mechanism in p53-independent cytotoxic response. Anti-proliferative activity and evidently cytotoxic activity of H1 were observed in wild-type and p53 null cells. Further studies demonstrated that H1 resulted in an increase of cleaved PARP, decease of survivin and elevation of p-H2AX. What is more, H1 significantly induced PUMA expression in a concentration- and time-dependent manner and caused an increase of Bax/Bcl-2 ratio in p53 null cells. Of note, knockdown of PUMA attenuated cytotoxic activity of H1. Further studies demonstrated that inhibition of AKT/FoxO3a signaling contributed to H1-mediated PUMA induction. Targeted suppression of AKT/FoxO3a signaling by siRNA could overcome H1-mediated PUMA induction. In addition, H1 significantly suppressed NF-κB activity and caused an increase of early apoptotic and late apoptotic cells, and elevated caspase-3 activity. Taken together, we found that inhibition of AKT/FoxO3a signaling may contribute to H1-mediated PUMA induction, suggesting that inhibition of AKT/FoxO3a signaling result in PUMA expression in response to p53-independent cytotoxic effects of H1.

Mdmx promotes genomic instability independent of p53 and Mdm2

The oncogene Mdmx is overexpressed in many human malignancies, and together with Mdm2, negatively regulates the p53 tumor suppressor. However, a p53-independent function of Mdmx that impacts genome stability has been described, but this function is not well understood. In the present study, we determined that of the 13 different cancer types evaluated, 6-90% of those that had elevated levels of Mdmx had concurrent inactivation (mutated or deleted) of p53. We show elevated levels of Mdmx-inhibited double-strand DNA break repair and induced chromosome and chromatid breaks independent of p53, leading to genome instability. Mdmx impaired early DNA damage-response signaling, such as phosphorylation of the serine/threonine-glutamine motif, mediated by the ATM kinase. Moreover, we identified Mdmx associated with Nbs1 of the Mre11-Rad50-Nbs1 (MRN) DNA repair complex, and this association increased upon DNA damage and was detected at chromatin. Elevated Mdmx levels also increased cellular transformation in a p53-independent manner. Unexpectedly, all Mdmx-mediated phenotypes also occurred in cells lacking Mdm2 and were independent of the Mdm2-binding domain (RING) of Mdmx. Therefore, Mdmx-mediated inhibition of the DNA damage response resulted in delayed DNA repair and increased genome instability and transformation independent of p53 and Mdm2. Our results reveal a novel p53- and Mdm2-independent oncogenic function of Mdmx that provides new insight into the many cancers that overexpress Mdmx.

Mice engineered for an obligatory Mdm4 exon skipping express higher levels of the Mdm4-S isoform but exhibit increased p53 activity

Mdm4, a protein related to the ubiquitin-ligase Mdm2, is an essential inhibitor of tumor suppressor protein p53. In both human and mouse cells, the Mdm4 gene encodes two major transcripts: one encodes the full-length oncoprotein (designated below as Mdm4-FL), whereas the other, resulting from a variant splicing that skips exon 6, encodes the shorter isoform Mdm4-S. Importantly, increased Mdm4-S mRNA levels were observed in several human cancers, and correlated with poor survival. However, the role of Mdm4-S in cancer progression remains controversial, because the Mdm4-S protein appeared to be a potent p53 inhibitor when overexpressed, but the splice variant also leads to a decrease in Mdm4-FL expression. To unambiguously determine the physiological impact of the Mdm4-S splice variant, we generated a mouse model with a targeted deletion of the Mdm4 exon 6, thereby creating an obligatory exon skipping. The mutant allele (Mdm4(ΔE6)) prevented the expression of Mdm4-FL, but also led to increased Mdm4-S mRNA levels. Mice homozygous for this allele died during embryonic development, but were rescued by a concomitant p53 deficiency. Furthermore in a hypomorphic p53(ΔP/ΔP) context, the Mdm4(ΔE6) allele led to p53 activation and delayed the growth of oncogene-induced tumors. We next determined the effect of Mdm4(+/ΔE6) heterozygosity in a hypermorphic p53(+/Δ31) genetic background, recently shown to be extremely sensitive to Mdm4 activity. Mdm4(+/ΔE6) p53(+/Δ31) pups were born, but suffered from aplastic anemia and died before weaning, again indicating an increased p53 activity. Our results demonstrate that the main effect of a skipping of Mdm4 exon 6 is not the synthesis of the Mdm4-S protein, but rather a decrease in Mdm4-FL expression. These and other data suggest that increased Mdm4-S mRNA levels might correlate with more aggressive cancers without encoding significant amounts of a potential oncoprotein. Hypotheses that may account for this apparent paradox are discussed.

EEF1A2 inactivates p53 by way of PI3K/AKT/mTOR-dependent stabilization of MDM4 in hepatocellular carcinoma

Mouse Double Minute homolog 4 (MDM4) gene up-regulation often occurs in human hepatocellular carcinoma (HCC), but the molecular mechanisms responsible for its induction remain poorly understood. Here we investigated the role of the phosphoinositide-3-kinase/v-akt murine thymoma viral oncogene homolog/mammalian target of rapamycin (PI3K/AKT/mTOR) axis in the regulation of MDM4 levels in HCC. The activity of MDM4 and the PI3K/AKT/mTOR pathway was modulated in human HCC cell lines by way of silencing and overexpression experiments. Expression of main pathway components was analyzed in an AKT mouse model and human HCCs. MDM4 inhibition resulted in growth restraint of HCC cell lines both in vitro and in vivo. Inhibition of the PI3K-AKT and/or mTOR pathways lowered MDM4 protein levels in HCC cells and reactivated p53-dependent transcription. Deubiquitination by ubiquitin-specific protease 2a and AKT-mediated phosphorylation protected MDM4 from proteasomal degradation and increased its protein stability. The eukaryotic elongation factor 1A2 (EEF1A2) was identified as an upstream inducer of PI3K supporting MDM4 stabilization. Also, we detected MDM4 protein up-regulation in an AKT mouse model and a strong correlation between the expression of EEF1A2, activated/phosphorylated AKT, and MDM4 in human HCC (each rho > 0.8, P < 0.001). Noticeably, a strong activation of this cascade was associated with shorter patient survival.

CONCLUSION

The EEF1A2/PI3K/AKT/mTOR axis promotes the protumorigenic stabilization of the MDM4 protooncogene in human HCC by way of a posttranscriptional mechanism. The activation level of the EEF1A2/PI3K/AKT/mTOR/MDM4 axis significantly influences the survival probability of HCC patients in vivo and may thus represent a promising molecular target.

The Mdm network and its regulation of p53 activities: a rheostat of cancer risk

The potent transcriptional activity of p53 (Trp53, TP53) must be kept in check for normal cell growth and survival. Tumors, which drastically deviate from these parameters, have evolved multiple mechanisms to inactivate TP53, the most prevalent of which is the emergence of TP53 missense mutations, some of which have gain-of-function activities. Another important mechanism by which tumors bypass TP53 functions is via increased levels of two TP53 inhibitors, MDM2, and MDM4. Studies in humans and in mice reveal the complexity of TP53 regulation and the exquisite sensitivity of this pathway to small changes in regulation. Here, we summarize the factors that impinge on TP53 activity and thus cell death/arrest or tumor development.

Mdm2 and MdmX involvement in human cancer

Discovered in 1987 and 1997 respectively, Mdm2 and MdmX represent two critical cellular regulators of the p53 tumor suppressor. This chapter reviews each from initial discovery to our current understanding of their deregulation in human cancer with a focus on how each regulator impacts p53 function. While p53 independent activities of Mdm2 and MdmX are noted the reader is directed to other reviews on this topic. The chapter concludes with an examination of the various mechanisms of Mdm-deregulation and an assessment of the current therapeutic approaches to target Mdm2 and MdmX overexpression.

RABL6A, a novel RAB-like protein, controls centrosome amplification and chromosome instability in primary fibroblasts

RABL6A (RAB-like 6 isoform A) is a novel protein that was originally identified based on its association with the Alternative Reading Frame (ARF) tumor suppressor. ARF acts through multiple p53-dependent and p53-independent pathways to prevent cancer. How RABL6A functions, to what extent it depends on ARF and p53 activity, and its importance in normal cell biology are entirely unknown. We examined the biological consequences of RABL6A silencing in primary mouse embryo fibroblasts (MEFs) that express or lack ARF, p53 or both proteins. We found that RABL6A depletion caused centrosome amplification, aneuploidy and multinucleation in MEFs regardless of ARF and p53 status. The centrosome amplification in RABL6A depleted p53−/− MEFs resulted from centrosome reduplication via Cdk2-mediated hyperphosphorylation of nucleophosmin (NPM) at threonine-199. Thus, RABL6A prevents centrosome amplification through an ARF/p53-independent mechanism that restricts NPM-T199 phosphorylation. These findings demonstrate an essential role for RABL6A in centrosome regulation and maintenance of chromosome stability in non-transformed cells, key processes that ensure genomic integrity and prevent tumorigenesis.

Molecular pathways: targeting Mdm2 and Mdm4 in cancer therapy

The p53 tumor suppressor is activated in response to cellular stresses to induce cell-cycle arrest, cellular senescence, and apoptosis. The p53 gene is inactivated by mutations in more than 50% of human tumors. In addition, tumor cells dampen p53 activities via overexpression of p53-negative regulators, in particular 2 structurally related proteins, Mdm2 and Mdm4. And yet, Mdm2 and Mdm4 possess p53-independent activities, which also contribute to tumor formation and progression. Given that Mdm2 and Mdm4 inhibit p53 activities to promote tumor development, small molecules and peptides were developed to abrogate the inhibition of p53 by Mdm proteins. Antitumor activities of these molecules have already been confirmed in preclinical studies and early-phase clinical trials. These research endeavors and clinical advances constitute the main focus of this review.

Protecting the genome from mdm2 and mdmx

The contribution of Mdm2, and its recently identified family member Mdmx (Mdm4), to tumorigenesis has primarily focused on their negative regulation of the p53 tumor suppressor. Although Mdm2 and Mdmx clearly inhibit p53, which can lead to tumor development, both have also been shown to affect tumorigenesis independent of p53. Given that Mdm2 and/or Mdmx overexpression is common and likely underestimated in human cancers, understanding the functions of these proteins beyond p53 control is critical. In recent years, new functions of Mdm2 and Mdmx that lead to genome instability, a hallmark of malignancy, have emerged. Specifically, roles in the DNA damage response that are distinct from their regulation of p53 have been identified. Inhibition of p53 as well as other components of the DNA damage response by Mdm2 and Mdmx can result in delayed DNA repair and increased genome instability, making Mdm2 and Mdmx a danger to the genome when aberrantly expressed. However, the genome instability caused by altered levels of Mdm2 and Mdmx could be used therapeutically for the treatment of cancer. Specifically, drugs/small molecules that target the interaction between Mdm2 and p53 can stabilize Mdm2, resulting in negative consequences on the genome that could be exploited for cancer treatment, particularly malignancies lacking functional p53.

Mdm2 and MdmX as Regulators of Gene Expression

p53 is an important tumor suppressor, functioning as a transcriptional activator and repressor. Upon receiving signals from multiple stress related pathways, p53 regulates numerous activities such as cell cycle arrest, senescence, and cell death. When p53 activities are not required, the protein is held in check by interacting with 2 key homologous regulators, Mdm2 and MdmX, and a search for inhibitors of these interactions is well underway. However, it is now recognized that Mdm2 and MdmX function beyond simple inhibition of p53, and a complete understanding of Mdm2 and MdmX functions is ever more important. Indeed, increasing evidence suggests that Mdm2 and MdmX affect p53 target gene specificity and influence the activity of other transcription factors, and Mdm2 itself may even function as a transcription co-factor through post-translational modification of chromatin. Additionally, Mdm2 affects post-transcriptional activities such as mRNA stability and translation of a variety of transcripts. Thus, Mdm2 and MdmX influence the expression of many genes through a wide variety of mechanisms, which are discussed in this review.

Using Mouse Models to Explore MDM-p53 Signaling in Development, Cell Growth, and Tumorigenesis

The p53 transcription factor regulates the expression of numerous genes whose products affect cell proliferation, senescence, cellular metabolism, apoptosis, and DNA repair. These p53-mediated effects can inhibit the growth of stressed or mutated cells and suppress tumorigenesis in the organism. However, the various growth-inhibitory properties of p53 must be kept in check in nondamaged cells in order to facilitate proper embryogenesis or the homeostatic maintenance of adult tissues. This requisite inhibition of p53 is performed primarily by the MDM oncoproteins, Mdm2 and MdmX. These p53-binding proteins limit p53 activity both in normal cells and in stressed cells seeking to promote resolution of their p53-stress response. Many mouse models bearing genetic alterations in Mdm2 or MdmX have been generated to explore the function and regulation of MDM-p53 signaling in development, in tissue homeostasis, in aging, and in cancer. These models not only have demonstrated a critical need for Mdm2 and MdmX in normal cell growth and in development but more recently have identified the MDM-p53 signaling axis as a key regulator of the cellular response to a wide variety of genetic or metabolic stresses. In this review, we discuss what has been learned from various studies of these Mdm2 and MdmX mouse models and highlight a few of the many important remaining questions.

Alternate splicing of the p53 inhibitor HDMX offers a superior prognostic biomarker than p53 mutation in human cancer

Conventional high-grade osteosarcoma is the most common primary bone malignancy. Although altered expression of the p53 inhibitor HDMX (Mdmx/Mdm4) is associated with cancer risk, progression, and outcome in other tumor types, little is known about its role in osteosarcoma. High expression of the Hdmx splice variant HDMX-S relative to the full-length transcript (the HDMX-S/HDMX-FL ratio) correlates with reduced HDMX protein expression, faster progression, and poorer survival in several cancers. Here, we show that the HDMX-S/HDMX-FL ratio positively correlates with less HDMX protein expression, faster metastatic progression, and a trend to worse overall survival in osteosarcomas. We found that the HDMX-S/HDMX-FL ratio associated with common somatic genetic lesions connected with p53 inhibition, such as p53 mutation and HDM2 overexpression in osteosarcoma cell lines. Interestingly, this finding was not limited to osteosarcomas as we observed similar associations in breast cancer and a variety of other cancer cell lines, as well as in tumors from patients with soft tissue sarcoma. The HDMX-S/HDMX-FL ratio better defined patients with sarcoma with worse survival rates than p53 mutational status. We propose a novel role for alternative splicing of HDMX, whereby it serves as a mechanism by which HDMX protein levels are reduced in cancer cells that have already inhibited p53 activity. Alternative splicing of HDMX could, therefore, serve as a more effective biomarker for p53 pathway attenuation in cancers than p53 gene mutation.

Rpl27a mutation in the sooty foot ataxia mouse phenocopies high p53 mouse models

Ribosomal stress is an important, yet poorly understood, mechanism that results in activation of the p53 tumour suppressor. We present a mutation in the ribosomal protein Rpl27a gene (sooty foot ataxia mice), isolated through a sensitized N-ethyl-N-nitrosourea (ENU) mutagenesis screen for p53 pathway defects, that shares striking phenotypic similarities with high p53 mouse models, including cerebellar ataxia, pancytopenia and epidermal hyperpigmentation. This phenocopy is rescued in a haploinsufficient p53 background. A detailed examination of the bone marrow in these mice identified reduced numbers of haematopoietic stem cells and a p53-dependent c-Kit down-regulation. These studies suggest that reduced Rpl27a increases p53 activity in vivo, further evident with a delay in tumorigenesis in mutant mice. Taken together, these data demonstrate that Rpl27a plays a crucial role in multiple tissues and that disruption of this ribosomal protein affects both development and transformation.

Functions of MDMX in the modulation of the p53-response

The MDM family proteins MDM2 and MDMX are two critical regulators of the p53 tumor suppressor protein. Expression of both proteins is necessary for allowing the embryonal development by keeping the activity of p53 in check. Upon stresses that need to activate p53 to perform its function as guardian of the genome, p53 has to be liberated from these two inhibitors. In this review, we will discuss the various mechanisms by which MDMX protein levels are downregulated upon various types of stress, including posttranslational modifications of the MDMX protein and the regulation of mdmx mRNA expression, including alternative splicing. In addition, the putative function(s) of the described MDMX splice variants, particularly in tumor development, will be discussed. Lastly, in contrast to common belief, we have recently shown the existence of a p53-MDMX feedback loop, which is important for dampening the p53-response at later phases after genotoxic stress.

An illegitimate microRNA target site within the 3' UTR of MDM4 affects ovarian cancer progression and chemosensitivity

Overexpression of MDM4 (also known as MDMX or HDMX) is thought to promote tumorigenesis by decreasing p53 tumor suppressor function. Even modest decrease in Mdm4 levels affects tumorigenesis in mice, suggesting that genetic variants of MDM4 might have similar effects in humans. We sequenced the MDM4 gene in a series of ovarian cancer cell lines and carcinomas to identify mutations and/or single nucleotide polymorphisms (SNPs). We identified an SNP (SNP34091) in the 3'-UTR of MDM4 that creates a putative target site for hsa-miR-191, a microRNA that is highly expressed in normal and tumor tissues. Biochemical evidence supports specific miR-191-dependent regulation of the MDM4-C, but not MDM4-A, variant. Consistently, the A-allele was associated with statistically significant increased expression of MDM4 mRNA and protein levels in ovarian carcinomas. Importantly, the wild-type genotype (A/A) is more frequent (57.8% vs. 42.2% for A/C and C/C, respectively) in patients with high-grade carcinomas than in patients with low-grade carcinomas (47.2% vs. 52.5% for A/A and A/C + C/C, respectively). Moreover, A/A patients who do not express the estrogen receptor had a 4.2-fold [95% confidence interval (CI) = 1.2-13.5; P = 0.02] increased risk of recurrence and 5.5-fold (95% CI = 1.5-20.5; P = 0.01) increased risk of tumor-related death. Unexpectedly, the frequency of p53 mutations was not significantly lower in A/A patients. We conclude that acquisition of an illegitimate miR-191 target site causes downregulation of MDM4 expression, thereby significantly delaying ovarian carcinoma progression and tumor-related death. Importantly, these effects appear to be, at least partly, independent of p53.

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

Three decades of p53 research have led to many advances in understanding the basic biology of normal and cancer cells. Nonetheless, the detailed functions of p53 in normal cells, and even more so in cancer cells, remain obscure. A major breakthrough is the realization that mutant p53 has a life of its own: it contributes to cancer not only through loss of activity, but also through gain of specific 'mutant functions'. This new focus on mutant p53 is the rationale behind the meeting series dedicated to advances on mutant p53 biology. This review provides an overview of results presented at the Fourth International Workshop on Mutant p53, held in Akko, Israel in March 2009. New roles and functions of p53 relevant for tumor suppressions were presented, including the regulation of microRNAs networks, the modulation of cell-stroma interactions and the induction of senescence. A main focus of the meeting was the rapidly growing body of knowledge on autonomous properties of mutant p53 and on their oncogenic 'gain of function' impact. Importantly, the meeting highlighted that, 30 years after p53 discovery, research on mutant p53 is entering the clinical and translational era. Two major steps forward in this respect are a better understanding of the active mechanism of small drugs targeting mutant p53 in tumor cells and an improved definition of the prognostic and predictive value of mutant p53 in human cancer.

Phosphorylation and degradation of MdmX is inhibited by Wip1 phosphatase in the DNA damage response

MdmX and Mdm2 regulate p53 tumor suppressor functions by controlling p53 transcriptional activity and/or stability in cells exposed to DNA damage. Accumulating evidence indicates that ATM-mediated phosphorylation and degradation of Mdm2 and MdmX may be the initial driving force that induces p53 activity during the early phase of the DNA damage response. We have recently determined that a novel protein phosphatase, Wip1 (or PPM1D), contributes to p53 regulation by dephosphorylating Mdm2 to close the p53 activation loop initiated by the ATM/ATR kinases. In the present study, we determine that Wip1 directly dephosphorylates MdmX at the ATM-targeted Ser403 and indirectly suppresses phosphorylation of MdmX at Ser342 and Ser367. Wip1 inhibits the DNA damage-induced ubiquitination and degradation of MdmX, leading to the stabilization of MdmX and reduction of p53 activities. Our data suggest that Wip1 is an important component in the ATM-p53-MdmX regulatory loop.

Murine double minute 2: p53-independent roads lead to genome instability or death

The oncoprotein murine double minute 2 (Mdm2) is frequently overexpressed in many types of human malignancies. Although Mdm2 has an essential role in negatively regulating the p53 tumor suppressor, it also has less well characterized p53-independent functions that influence pathways that are crucial for controlling tumorigenesis. In addition to the impact Mdm2 has on p53-independent apoptosis, mounting evidence is linking increased Mdm2 levels to altered cell-cycle regulation, DNA replication and DNA repair leading to loss of genome stability. Mdm2 involvement in pathways that influence chromosome stability and cell death, distinct from its role in the p53 pathway, strengthens the position of Mdm2 as a desirable therapeutic target for the treatment of human cancers.

Residues in the alternative reading frame tumor suppressor that influence its stability and p53-independent activities

The Alternative Reading Frame (ARF) protein suppresses tumorigenesis through p53-dependent and p53-independent pathways. Most of ARF's anti-proliferative activity is conferred by sequences in its first exon. Previous work showed specific amino acid changes occurred in that region during primate evolution, so we programmed those changes into human p14ARF to assay their functional impact. Two human p14ARF residues (Ala(14) and Thr(31)) were found to destabilize the protein while two others (Val(24) and Ala(41)) promoted more efficient p53 stabilization and activation. Despite those effects, all modified p14ARF forms displayed robust p53-dependent anti-proliferative activity demonstrating there are no significant biological differences in p53-mediated growth suppression associated with simian versus human p14ARF residues. In contrast, p53-independent p14ARF function was considerably altered by several residue changes. Val(24) was required for p53-independent growth suppression whereas multiple residues (Val(24), Thr(31), Ala(41) and His(60)) enabled p14ARF to block or reverse the inherent chromosomal instability of p53-null MEFs. Together, these data pinpoint specific residues outside of established p14ARF functional domains that influence its expression and signaling activities. Most intriguingly, this work reveals a novel and direct role for p14ARF in the p53-independent maintenance of genomic stability.

The aneuploidy paradox in cell growth and tumorigenesis

Aneuploidy, an abnormal chromosome number, is a frequent characteristic of malignant cells, leading to the suggestion that aneuploidy drives tumorigenesis. In a recent issue of Science, Williams et al. identified a paradoxical relationship between aneuploidy and its linkage to tumorigenesis: chromosome gains in nontransformed cells are antiproliferative, despite frequently occurring in human tumors.

MdmX regulates transformation and chromosomal stability in p53-deficient cells

The cellular homologues Mdm2 and MdmX play critical roles in regulating the activity of the p53 tumor suppressor in damaged and non-damaged cells and during development in mice. Recently, we have utilized genetically defined primary cells and mice to reveal that endogenous levels of MdmX can also suppress multipolar mitosis and transformation in hyperploid p53-deficient cells and tumorigenesis in p53-deficient mice. These MdmX functions are not shared by Mdm2, and are distinct from the well-established ability of MdmX to complex with and inhibit p53 activity. Here we discuss some of the ramifications of MdmX loss in p53-deficient cells and mice, and we explore further the fate of MdmX/p53-double null embryonic fibroblasts undergoing multi-polar cell division using time-lapse video microscopy. We also discuss the relationship between chromosomal loss, cell proliferation, and the tumorigenic potential of p53-deficient cells lacking MdmX.

Analysis of human MDM4 variants in papillary thyroid carcinomas reveals new potential markers of cancer properties

A wild-type (wt) p53 gene characterizes thyroid tumors, except for the rare anaplastic histotype. Because p53 inactivation is a prerequisite for tumor development, alterations of p53 regulators represent an alternative way to impair p53 function. Indeed, murine double minute 2 (MDM2), the main p53 negative regulator, is overexpressed in many tumor histotypes including those of the thyroid. A new p53 regulator, MDM4 (a.k.a. MDMX or HDMX) an analog of MDM2, represents a new oncogene although its impact on tumor properties remains largely unexplored. We estimated levels of MDM2, MDM4, and its variants, MDM4-S (originally HDMX-S) and MDM4-211 (originally HDMX211), in a group of 57 papillary thyroid carcinomas (PTC), characterized by wt tumor protein 53, in comparison to matched contra-lateral lobe normal tissue. Further, we evaluated the association between expression levels of these genes and the histopathological features of tumors. Quantitative real-time polymerase chain reaction revealed a highly significant downregulation of MDM4 mRNA in tumor tissue compared to control tissue (P < 0.0001), a finding confirmed by western blot on a subset of 20 tissue pairs. Moreover, the tumor-to-normal ratio of MDM4 levels for each individual was significantly lower in late tumor stages, suggesting a specific downregulation of MDM4 expression with tumor progression. In comparison, MDM2 messenger RNA (mRNA) and protein levels were frequently upregulated with no correlation with MDM4 levels. Lastly, we frequently detected overexpression of MDM4-S mRNA and presence of the aberrant form, MDM4-211 in this tumor group. These findings indicate that MDM4 alterations are a frequent event in PTC. It is worthy to note that the significant downregulation of full-length MDM4 in PTC reveals a novel status of this factor in human cancer that counsels careful evaluation of its role in human tumorigenesis and of its potential as therapeutic target.