E3 Ligase MDM2 Research Articles

Abstract 4613: MDM4 SNP 34091 (rs4245739) effect on risk of breast, colon, lung, prostate, endometrial and ovarian cancer

Abstract Background MDM4 enhances MDM2's E3 ligase activity causing ubiquitin-proteasome-dependent degradation of p53. Thus, elevated levels of both MDM4 and MDM2 may result in p53 inactivation and elevated cancer risk. A single nucleotide polymorphism in the MDM4 3′ UTR (SNP34091A>C; rs4245739) has been found to exert biological function as the SNP34091C allele creates a putative target site for hsa-miR-191 leading to specific hsa-miR-191 down-regulation of MDM4. Further, the SNP34091AA genotype is associated with increased risk for both recurrence and tumor related death in estrogen negative ovarian cancer patients. In the present study we assessed the potential effect of MDM4 SNP34091A>C on cancer risk in six major cancer forms. Materials and methods We analyzed 1717 breast-, 1331 lung-, 1531 colon-, 2501 prostate cancer cases and 3747 healthy controls form the same population based study (CONOR) as well as 1404 endometrial- and 1699 ovarian cancer cases from hospital based studies. All samples were genotyped for MDM4 SNP34091A>C using custom Light-SNiP assay on a LightCycler 480 II instrument. Potential associations between MDM4 SNP34091 and cancer risk were estimated by calculating Odds Ratio (OR) with 95% confidence intervals (CI). All statistical analyses were performed using the IBM SPSS 19 software. Results We observed no significant association between MDM4 SNP34091A>C status and cancer risk in any of the analyzed cancer forms. Interestingly, stratifying the ovarian cancer samples according to grade and histology, we observed a reduced risk of high grade serous ovarian cancer in patients harboring the MDM4 SNP34091AA genotype (OR = 0.80; 95% CI = 0.65 - 0.98). Stratifying according to MDM2 SNP309 status we found the MDM4 SNP34091A allele to be associated with increased risk for breast cancer (OR = 2.10; 95% CI = 1.08 - 4.10) in patients carrying the SNP309 GG genotype. Conclusions The data presented here indicate that the effect of MDM4 SNP34091 on cancer risk may be tissue specific and that there may be cooperative effects with MDM2 SNP309. Citation Format: Liv B. Gansmo, Merete Bjørnslett, Anne Dørum, Helga Salvesen, Pål Romundstad, Kristian Hveem, Lars Vatten, Per Eystein Lønning, Stian Knappskog. MDM4 SNP 34091 (rs4245739) effect on risk of breast, colon, lung, prostate, endometrial and ovarian cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4613. doi:10.1158/1538-7445.AM2015-4613

Abstract 418: Mdm2 E3 Ligase-mediated Ubiquitination of Histone Deacetylase 1 in Vascular Calcification

Vascular calcification (VC) often associates with many cardiovascular and metabolic diseases. Although VC is the cause of high morbidity and mortality, molecular mechanisms have yet to be elucidated. Here we report that MDM2-induced ubiquitination of histone deacetylase 1 (HDAC1) mediates VC. Loss of HDAC1 activity enhanced VC in vivo and in vitro . HDAC1 protein was reduced in cell and animal calcification models and in human calcified coronary artery and this reduction preceded VC. Calcification stresses induced MDM2 E3 ligase, which resulted in HDAC1 K74 ubiquitination. Forced expression of MDM2 enhanced VC, whereas loss of MDM2 blunted it. A decoy peptide spanning HDAC1 K74 prevented VC. These results demonstrate a previously unknown ubiquitination pathway as well as the involvement of HDAC1 in VC. Our results suggest MDM2-mediated HDAC1 ubiquitination as a new therapeutic target in VC.

Regulation of MDM2 Stability After DNA Damage.

Cells in our body are constantly exposed to various stresses and threats to their genomic integrity. The tumor suppressor protein p53 plays a critical role in successful defense against these threats by inducing apoptotic cell death or cell cycle arrest. In unstressed conditions, p53 levels and activity must be kept low to prevent lethal activation of apoptotic and senescence pathways. However, upon DNA damage or other stressors, p53 is released from its inhibitory state to induce an array of apoptosis and cell cycle genes. Conversely, inactivation of p53 could promote unrestrained tumor proliferation and failure to appropriately undergo apoptotic cell death, which could, in turn, lead to carcinogenesis. The ubiquitin E3 ligase MDM2 is the most critical inhibitor of p53 that determines the cellular response to various p53-activating agents, including DNA damage. MDM2 activity is controlled by post-translational modifications, especially phosphorylation. However, accumulating evidence suggests that MDM2 is also regulated at the level of protein stability, which is controlled by the ubiquitin-proteasome pathway. Here, we discuss how MDM2 can be regulated in response to DNA damage with particular focus on the regulation of MDM2 protein stability.

Ubiquitinated Sirtuin 1 (SIRT1) Function Is Modulated during DNA Damage-induced Cell Death and Survival

Downstream signaling of physiological and pathological cell responses depends on post-translational modification such as ubiquitination. The mechanisms regulating downstream DNA damage response (DDR) signaling are not completely elucidated. Sirtuin 1 (SIRT1), the founding member of Class III histone deacetylases, regulates multiple steps in DDR and is closely associated with many physiological and pathological processes. However, the role of post-translational modification or ubiquitination of SIRT1 during DDR is unclear. We show that SIRT1 is dynamically and distinctly ubiquitinated in response to DNA damage. SIRT1 was ubiquitinated by the MDM2 E3 ligase in vitro and in vivo. SIRT1 ubiquitination under normal conditions had no effect on its enzymatic activity or rate of degradation; hypo-ubiquitination, however, reduced SIRT1 nuclear localization. Ubiquitination of SIRT1 affected its function in cell death and survival in response to DNA damage. Our results suggest that ubiquitination is required for SIRT1 function during DDR.

Heat shock protein gp96 decreases p53 stability by regulating Mdm2 E3 ligase activity in liver cancer

The resistance to apoptosis displayed by liver cancer plays a key role in hepatocarcinogenesis, tumor progression, and resistance to chemo- or radio-therapy. In this study, we uncovered the potential role and mechanism of heat shock protein gp96 in regulating liver tumor cell growth and apoptosis. P53 protein was identified as a gp96 client protein by profiling apoptosis-related proteins in gp96-knockdown liver cancer cells. Overexpression and knockdown studies both demonstrated that gp96 decreases p53 protein levels, and gp96 regulated cell apoptosis in a p53-dependent manner. We further provide evidence that gp96 interacts with both p53 and Mdm2 to enhance Mdm2-mediated p53 ubiquitination and degradation. Moreover, targeting gp96 with siRNA induced cell apoptosis and led to the suppression of liver tumor growth in vivo. In conclusion, we elucidated an underlying mechanism by which gp96 promotes p53 degradation via increasing Mdm2 E3 ligase activity and provided a new therapeutic strategy to target the gp96-mediated anti-apoptotic characteristic of hepatocellular carcinoma.

Rational design of promiscuous binding modulators of p53 inducing E3(Ub)-ligases (Mdm2 and Pirh2) as anticancer agents: an in silico approach

Twelve promiscuous binding p53 inducing E3(Ub) ligases (Mdm2 and Pirh2) hit candidates have been identified by structure based virtual screening.

Abstract 2735: Multi-point targeting of the synthetic lethal interactions between Myc, ribosome biogenesis and ribosome function cooperates to treat B-cell lymphoma

Abstract We recently demonstrated that transcription of the ribosomal genes (rRNA) by RNA Polymerase I (Pol I) can be therapeutically targeted with a novel small molecule, CX-5461, to selectively kill B-lymphoma cells in vivo while maintaining a viable wild-type B-cell population (Bywater et al Cancer Cell 2012; Bywater et al Nature Reviews Cancer 2013). The therapeutic effect was a consequence of nucleolar disruption, activation of ribosomal protein (Rp)-MDM2-P53 nucleolar stress response and apoptosis. We have recently launched a first-in-human clinical trial of CX-5461 in patients with hematological malignancies and although our pre-clinical data indicate immense potential of Pol I targeting for cancer therapy, some cancers still develop resistance. We hypothesized that simultaneously targeting the ribosome at multiple steps will extend survival. Thus we tested pharmacological inhibitors of PI3K/AKT/mTOR signalling in combination with CX-5461 as the former pathway is known to potently regulate both translational activity (Jefferies et al EMBO J, 1997; Pourdehnad M et al PNAS 2013) and ribosome biogenesis (Chan et al Science Signaling 2011; Devlin et al FEBS J 2013; Wall et al Cancer Discovery 2013). Using the Eμ-Myc model of B-cell lymphoma we demonstrate that multiple pharmacological inhibitors of the PI3K/AKT/mTOR pathway suppress transcription of the rRNA genes and induce cell death similar to CX-5461. Unexpectedly however, PI3K/AKT/mTOR pathway blockade is not associated with nucleolar disruption, or activation of the Rp/MDM2/p53 nucleolar stress pathway. This is because inhibition of PI3K/AKT/mTOR signalling suppresses both rRNA synthesis and ribosomal protein synthesis equally and therefore does not increase the pool of free Rps necessary to suppress MDM2 E3 ligase that regulates p53 stability. Furthermore, we demonstrate that combined treatment of Eμ-Myc tumor-bearing mice with CX-5461 and Everolimus delayed relapse compare to single agent and significantly extended survival of tumor bearing mice. These data demonstrate that dual targeting of the ribosome by selectively inhibiting Pol I transcription and inhibition of key signaling molecules regulating ribosome synthesis and function combine to potently treat MYC driven tumors. It therefore provides a rationale to combine such drugs in the clinic for the treatment of MYC driven cancer. Citation Format: Richard B. Pearson, Jennifer R. Devlin, Katherine M. Hannan, Nadine Hein, Megan J. Bywater, Gretchen Poortinga, Donald Cameron, Denis Drygin, Sean O'Brien, Carleen Cullinane, Grant A. McArthur, Ross D. Hannan. Multi-point targeting of the synthetic lethal interactions between Myc, ribosome biogenesis and ribosome function cooperates to treat B-cell lymphoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2735. doi:10.1158/1538-7445.AM2014-2735

Abstract LB-309: Interplay between E3 ligases modulate Per2 stability

Abstract Circadian oscillation relies on autoregulatory loops generated by sequential phases of transcription/translation, protein modification, and degradation. Interestingly, disruption of the clock gene period2 (per2) in mice is associated with a hyperplasic phenotype and loss of apoptotic response. When irradiated, per2 knockout mice show a significantly higher frequency of tumor development most likely due to loss of normal circadian control of genes concerned with the regulation of the DNA-damage response as well as partial resistance to radiation-induced apoptosis. Although evidence suggests that the per2 gene functions in tumor suppression, no mechanism of Per2 action is known. Our work show direct evidences of crosstalk mechanisms that involve Per2 and the oncogenic E3-ligase Mdm2. Whereas we know much about the role of b-TrCP in modulating the oscillatory levels of Per2 in the cell, the functional relevance of Mdm2 targeting of Per2 has just been uncovered. Bacterial two-hybrid screenings led to the identification of Mdm2 as a new PER2 binding partner, an interaction that was later validated by immunoprecipitation studies from both ectopicaly expressed and endogenous proteins. In vitro experiment show that recombinantly expressed Per2 serves as a substrate for Mdm2 and polyubiquitination takes place shortly after interaction. These results were later confirmed when Mdm2 was overexpressed and ubiquitination of Per2 was detected in immunoprecipitated samples. Interestingly, knock down of either beta-TrCP or Mdm2 dampened PER2 oscillation, although the mechanism by either E3 ligase seems to act relies in the subcellular localization of the substrate., Lastly, there is an interplay between these two ligases as we showed that downregulation of Mdm2 influences the total levels of beta-TrCP and direct interaction between these protein was confirmed by immunoprecipitation. Moreover, experiments carried out with constitutively negative active forms of each ligase show they are each other targeted for ubiquitination and that Per2 exerts a very relevant control on stability of b-TrCP by binding to Mdm2 and inhibiting its E3-ligase activity. Citation Format: jingjing liu, Carla V. Finkielstein. Interplay between E3 ligases modulate Per2 stability. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-309. doi:10.1158/1538-7445.AM2014-LB-309

Inhibition of the MDM2 E3 Ligase induces apoptosis and autophagy in wild-type and mutant p53 models of multiple myeloma, and acts synergistically with ABT-737.

Intracellular proteolytic pathways have been validated as rational targets in multiple myeloma with the approval of two proteasome inhibitors in this disease, and with the finding that immunomodulatory agents work through an E3 ubiquitin ligase containing Cereblon. Another E3 ligase that could be a rational target is the murine double minute (MDM) 2 protein, which plays a role in p53 turnover. A novel inhibitor of this complex, MI-63, was found to induce apoptosis in p53 wild-type myeloma models in association with activation of a p53-mediated cell death program. MI-63 overcame adhesion-mediated drug resistance, showed anti-tumor activity in vivo, enhanced the activity of bortezomib and lenalidomide, and also overcame lenalidomide resistance. In mutant p53 models, inhibition of MDM2 with MI-63 also activated apoptosis, albeit at higher concentrations, and this was associated with activation of autophagy. When MI-63 was combined with the BH3 mimetic ABT-737, enhanced activity was seen in both wild-type and mutant p53 models. Finally, this regimen showed efficacy against primary plasma cells from patients with newly diagnosed and relapsed/refractory myeloma. These findings support the translation of novel MDM2 inhibitors both alone, and in combination with other novel agents, to the clinic for patients with multiple myeloma.

The Transcription Factor MEF/Elf4 Is Dually Modulated by p53-MDM2 Axis and MEF-MDM2 Autoregulatory Mechanism

Myeloid Elf-1-like factor (MEF) or Elf4 is an ETS transcription factor that activates innate immunity-associated genes such as lysozyme (LYZ), human β-defensin 2 (HβD2), and interleukin-8 (IL-8) in epithelial cells and is also known to influence cell cycle progression. MEF is transcriptionally activated by E2F1, but the E2F1-mediated transcriptional activation is inhibited by p53 through E2F1-p53 protein interaction. Although the transcriptional activation of MEF has been investigated in depth, its post-translational regulation is not well explored. By overexpressing MEF cDNA in human cell lines, here we show that MEF protein expression is suppressed by p53. By screening a number of E3 ligases regulated by p53, we found that MDM2 is involved in the effect of p53 on MEF. MDM2 is transcriptionally activated by p53 and interacts with MEF protein to enhance MEF degradation. MDM2 reduces MEF protein expression, as well as stability and function of MEF as transcriptional activator. Furthermore, MDM2 was able to down-regulate MEF in the absence of p53, indicating a p53-independent effect on MEF. Notably, MEF transcriptionally activates MDM2, which was previously demonstrated to be the mechanism by which MEF suppresses the p53 protein. These results reveal that in addition to the potential of MEF to down-regulate p53 by transcriptionally activating E3 ligase MDM2, MEF participates with MDM2 in a novel autoregulatory feedback loop to regulate itself. Taken together with the findings on the effect of p53 on MEF, these data provide evidence that the p53-MDM2-MEF axis is a feedback mechanism that exquisitely controls the balance of these transcriptional regulators.

DNMT3B Overexpression by Deregulation of FOXO3a-Mediated Transcription Repression and MDM2 Overexpression in Lung Cancer

DNA methyltransferase 3B (DNMT3B) contributes to de novo DNA methylation and its overexpression promotes tumorigenesis. However, whether DNMT3B is upregulated by transcriptional deregulation remains unclear. We studied the transcriptional repression of DNMT3B by forkhead O transcription factor 3a (FOXO3a) in lung cancer cell, animal, and clinical models. The results of luciferase reporter assay showed that FOXO3a negatively regulated DNMT3B promoter activity by preferentially interacting with the binding element FOXO3a-E (+166 to +173) of DNMT3B promoter. Ectopically overexpressed FOXO3a or combined treatment with doxorubicin to induce FOXO3a nuclear accumulation further bound at the distal site, FOXO3a-P (-249 to -242) by chromatin-immunoprecipitation assay. Knockdown of FOXO3a resulted in an open chromatin structure and high DNMT3B mRNA and protein expression. Abundant FOXO3a repressed DNMT3B promoter by establishing a repressed chromatin structure. Note that FOXO3a is a degradation substrate of MDM2 E3-ligase. Cotreatment with doxorubicin and MDM2 inhibitor, Nutlin-3, further enforced abundant nuclear accumulation of FOXO3a resulting in decrease expression of DNMT3B leading to synergistic inhibition of tumor growth and decrease of methylation status on tumor suppressor genes in xenograft specimens. Clinically, lung cancer patients with DNMT3B high, FOXO3a low, and MDM2 high expression profile correlated with poor prognosis examined by immunohistochemistry and Kaplan-Meier survival analysis. We reveal a new mechanism that FOXO3a transcriptionally represses DNMT3B expression and this regulation can be attenuated by MDM2 overexpression in human lung cancer model. Cotreatment with doxorubicin and Nutlin-3 is a novel therapeutic strategy through epigenetic modulation.

Autoactivation of the MDM2 E3 ligase by intramolecular interaction.

The RING domain ubiquitin E3 ligase MDM2 is a key regulator of p53 degradation and a mediator of signals that stabilize p53. The current understanding of the mechanisms by which MDM2 posttranslational modifications and protein binding cause p53 stabilization remains incomplete. Here we present evidence that the MDM2 central acidic region is critical for activating RING domain E3 ligase activity. A 30-amino-acid minimal region of the acidic domain binds to the RING domain through intramolecular interactions and stimulates the catalytic function of the RING domain in promoting ubiquitin release from charged E2. The minimal activation sequence is also the binding site for the ARF tumor suppressor, which inhibits ubiquitination of p53. The acidic domain-RING domain intramolecular interaction is modulated by ATM-mediated phosphorylation near the RING domain or by binding of ARF. These results suggest that MDM2 phosphorylation and association with protein regulators share a mechanism in inhibiting the E3 ligase function and stabilizing p53 and suggest that targeting the MDM2 autoactivation mechanism may be useful for therapeutic modulation of p53 levels.

Regulation of p53 by Mdm2 E3 Ligase Function Is Dispensable in Embryogenesis and Development, but Essential in Response to DNA Damage

Mdm2 E3 ubiquitin ligase-mediated p53 degradation is generally accepted as the major mechanism for p53 regulation; nevertheless, the invivo significance of this function has not been unequivocally established. Here, we have generated an Mdm2(Y487A) knockin mouse; Mdm2(Y487A) mutation inactivates Mdm2 E3 ligase function without affecting its ability to bind its homolog MdmX. Unexpectedly, Mdm2(Y487A/Y487A) mice were viable and developed normally into adulthood. While disruption of Mdm2 E3 ligase function resulted in p53 accumulation, p53 transcriptional activity remained low; however, exposure to sublethal stress resulted in hyperactive p53 and p53-dependent mortality in Mdm2(Y487A/Y487A) mice. These findings reveal a potentially dispensable nature for Mdm2 E3 ligase function in p53 regulation, providing insight that may affect how this pathway is targeted therapeutically.

AKT regulates NPM dependent ARF localization and p53mut stability in tumors.

Nucleophosmin (NPM) is known to regulate ARF subcellular localization and MDM2 activity in response to oncogenic stress, though the precise mechanism has remained elusive. Here we describe how NPM and ARF associate in the nucleoplasm to form a MDM2 inhibitory complex. We find that oligomerization of NPM drives nucleolar accumulation of ARF. Moreover, the formation of NPM and ARF oligomers antagonizes MDM2 association with the inhibitory complex, leading to activation of MDM2 E3-ligase activity and targeting of p53. We find that AKT phosphorylation of NPM-Ser48 prevents oligomerization that results in nucleoplasmic localization of ARF, constitutive MDM2 inhibition and stabilization of p53. We also show that ARF promotes p53 mutant stability in tumors and suppresses p73 mediated p21 expression and senescence. We demonstrate that AKT and PI3K inhibitors may be effective in treatment of therapeutically resistant tumors with elevated AKT and carrying gain of function mutations in p53. Our results show that the clinical candidate AKT inhibitor MK-2206 promotes ARF nucleolar localization, reduced p53(mut) stability and increased sensitivity to ionizing radiation in a xenograft model of pancreatic cancer. Analysis of human tumors indicates that phospho-S48-NPM may be a useful biomarker for monitoring AKT activity and in vivo efficacy of AKT inhibitor treatment. Critically, we propose that combination therapy involving PI3K-AKT inhibitors would benefit from a patient stratification rationale based on ARF and p53(mut) status.

Nucleolar Stress Induces Ubiquitination-independent Proteasomal Degradation of PICT1 Protein

The nucleolar protein PICT1 regulates tumor suppressor p53 by tethering ribosomal protein L11 within the nucleolus to repress the binding of L11 to the E3 ligase MDM2. PICT1 depletion results in the release of L11 to the nucleoplasm to inhibit MDM2, leading to p53 activation. Here, we demonstrate that nucleolar stress induces proteasome-mediated degradation of PICT1 in a ubiquitin-independent manner. Treatment of H1299 cells with nucleolar stress inducers, such as actinomycin D, 5-fluorouridine, or doxorubicin, induced the degradation of PICT1 protein. The proteasome inhibitors MG132, lactacystin, and epoxomicin blocked PICT1 degradation, whereas the inhibition of E1 ubiquitin-activating enzyme by a specific inhibitor and genetic inactivation fail to repress PICT1 degradation. In addition, the 20 S proteasome was able to degrade purified PICT1 protein in vitro. We also found a PICT1 mutant showing nucleoplasmic localization did not undergo nucleolar stress-induced degradation, although the same mutant underwent in vitro degradation by the 20 S proteasome, suggesting that nucleolar localization is indispensable for the stress-induced PICT1 degradation. These results suggest that PICT1 employs atypical proteasome-mediated degradation machinery to sense nucleolar stress within the nucleolus.

The anaphase-promoting complex/cyclosome is an E3 ubiquitin ligase for Mdm2

The Mdm2 proto-oncoprotein is the primary negative regulator for p53. While it is believed that Mdm2 degradation is regulated via its own E3 ubiquitin ligase activity, recent development of knock-in mouse models demonstrates that Mdm2 E3 ligase function is dispensable for self-degradation in vivo. Here, we show that the anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase for Mdm2 degradation. We demonstrate that APC2, a scaffold subunit of APC/C, binds to Mdm2 and is required for Mdm2 polyubiquitination and proteasomal degradation. Downregulation of APC2 by RNAi results in transcription-independent accumulation of Mdm2 and attenuation of stress-induced p53 stabilization, leading to decreased senescence and increased cell survival. Furthermore, APC2 expression is frequently downregulated in human cancers; in tumor cell lines, APC2 downregulation correlates with Mdm2 overexpression. Our study shows the regulation of Mdm2 by the E3 ubiquitin ligase APC/C and has important therapeutic implications for tumors with Mdm2 overexpression.

A nanotechnological, molecular-modeling, and immunological approach to study the interaction of the anti-tumorigenic peptide p28 with the p53 family of proteins.

p28 is an anionic, amphipathic, cell-penetrating peptide derived from the cupredoxin azurin that binds to the DNA-binding domain (DBD) of the tumor suppressor protein, p53, and induces a post-translational increase in the level of wild type and mutated p53 in a wide variety of human cancer cells. As p63 and p73, additional members of the p53 superfamily of proteins, also appear to be involved in the cellular response to cancer therapy and are reportedly required for p53-induced apoptosis, we asked whether p28 also binds to p63 and p73. Atomic force spectroscopy demonstrates that p28 forms a stable, high-affinity complex with full-length p63, the DBD of p63, and full-length p73. Exposure to p28 decreased the level of TAp63α and ΔNp63α, the truncated form of p63, in p53 wild type and mutated human breast cancer cells, respectively. p28 increased the level of TAp73α, but not ΔNp73α, in the same breast cancer cell lines. In contrast, p28 increased the level of the TA and ΔN isoforms of p63 in p53 wild type, but not in p53 mutated melanoma cells, while decreasing TA p73α in p53 wild type and mutated human melanoma cells. All changes were mirrored by an associated change in the expression of the HECT E3 ligases Itch/AIP4, AIP5, and the RING E3 ligase Pirh2, but not in the receptor for activated C kinase or the RING E3 ligases Mdm2 and Cop1. Collectively, the data suggest that molecules such as p28 bind with high affinity to the DBD of p63 and p73 and alter their expression independent of the Mdm2 and Cop1 pathways.

Regulation of p53 by Mdm2 E3 Ligase Function Is Dispensable in Embryogenesis and Development, but Essential in Response to DNA Damage

Regulation of p53 by Mdm2 E3 Ligase Function Is Dispensable in Embryogenesis and Development, but Essential in Response to DNA Damage

XPO1 Inhibition Disrupts Ribosomal Subunits Assembly and Induces Multiple Myeloma (MM) Cell Death

BackgroundChromosomal region maintenance (CRM1), also known as exportin 1 (XPO1) plays an important role in the nuclear-cytoplasmic shuttling. The nuclear export receptor, XPO1, is considered as a regulator of subcellular distribution of several proteins involved in the regulation of centrosome duplication such as nucleophosmin (NPM), breast and ovarian cancer susceptibility protein 1 (BRCA1) and many tumor suppressor proteins (p53, p21, FOXO and pRB). Furthermore, XPO1 is required for the export of assembled ribosomal subunits (60S & 40S) from the nucleolus back into the cytoplasm. Inhibition of XPO1 triggers a ribosomal stress response that may result in the death of transformed cells with stressed ribogenesis. Silencing of XPO1 is reported to be synthetically lethal in MM cells, however the mechanisms that mediate this effect are not fully elucidated. Methods and ResultsTo determine the effect of XPO1 inhibition in MM, cells were exposed to different doses of KPT330 (Karyopharm), a selective inhibitor of nuclear-cytoplasmic transport by irreversibly binding to the XPO1 cargo recognition site. Nanomolar concentrations of KPT330 (50-150 nM) induced apoptosis (Puma up-regulation and caspase 3 cleavage) and suppressed the proliferation of myeloma cell lines MM1S, OPM2 while KMS11 cells were more resistant. Mechanistically treatment with KPT330 up-regulated the expression of p53, as well as p21, p27 and MDM2 at the protein and RNA levels and significantly decreased the expression of c-Myc and IRF4. Cognizant of the role XPO1 in cytoplasmic-nuclear shuttling of ribosomal subunits, we reasoned that c-Myc downregulation and p53 induction in MM cells exposed to KPT330 results from ribosomal biogenesis stress. Therefore we analyzed the cellular co-localization of ribosomal proteins (RPL5, RPL11), c-Myc and MDM2 in presence of KPT330. A shifting of RPL11 and RPL5 from the nucleolus to the nucleoplasm and cytosol was observed in presence of KPT330 where they accumulated in ribosome-free cellular fractions. Co-immunoprecipitation studies showed that RPL11 and RPL5 released from the nucleolus bind MDM2 and c-Myc. This binding of RPL11 and RPL5 to MDM2 and c-Myc is known to suppress their function and expression. Therefore our data explain the suppression of MDM2 E3 ligase activity with p53 stabilization and reduction of c-Myc at the post-transcriptional levels. Study of ribosome fractions with sucrose gradients showed that in the presence of KPT330, 40S and the polysomes were completely suppressed while 60S and 80S subunits were significantly downregulated in OPM2 and MM1S cell lines. Consistent with the disruption of ribosomal function and the translational machinery, c-Myc mRNA levels were significantly decreased in 40S, 60S and 80S fractions after treatment with KPT330. Confirming the role of the ribosomal stress response in KPT330-mediated MM cells' death, silencing of ribosomal proteins RPL11 or RPL5 fully protected them from KPT330 cytotoxicity. Furthermore silencing RPL11 or RPL5, suppressed the effects of KPT330 on MDM2, p53, p21 and c-MYC. ConclusionInhibition of XPO1 induces a perturbation in the ribosome subunits transfer, disruption of ribosomal assembly and the induction of a ribosomal stress response in MM cells. Perturbation on the nucleolar-cytoplasmic shuttling by KPT330 and the targeting of the translational factory represents a novel therapeutic approach in multiple myeloma. Disclosures:No relevant conflicts of interest to declare.

Lenalidomide Induces A Ribosomal Stress Response In Multiple Myeloma (MM) Cells

BackgroundDeregulation of ribosome biogenesis is associated with carcinogenesis and in MM two of the most common recurrent mutations (DIS3 and FAM46C) are implicated in ribosomal decay and translational control. Beside its role in the regulation of protein synthesis, the importance of ribosome biogenesis is underscored by the observation that the impairment of this process leads to a ribosomal stress response with induction of p53, inhibition of c-Myc and cell cycle arrest. These effects are mediated by the direct binding of ribosomal proteins (RPs), particularly RPL11 and RPL5 to Mdm2 and c-Myc. Immunomodulatory drugs (IMiDs) anti-MM effects require their binding to Cereblon (CRBN), an adaptor protein of the Cul4A-DDB1-ROC1 ubiquitin E3 ligase complex with an ensuing down-regulation of c-Myc and up-regulation of p21. In the present study, we delineated the mechanisms through which IMiDs mediate these effects in MM cells. Methods and ResultsIn order to identify ubiquitylated substrates that are modified by lenalidomide (Len) treatment, we performed a ubiquitin-proteome pull-down using Tandem Ubiquitin Binding Entity (Lifesensors) coupled with quantitative mass-spectroscopy proteomics (iTRAQ) in OPM2 cells exposed to Len (10 μM). Several ribosomal proteins (RPs) - S25, S26, S20 & S28 - were increased in Len treated samples suggesting that the Cul4a-CRBN complex may regulate RPs stability and hence IMiDs binding to cereblon may trigger a ribosomal stress response. Immunoblot analysis of MM cells exposed to Len lead to a rapid decrease in c-Myc expression (within 30 min) that significantly preceded the downregulation of IRF4 consistent with an IRF4-independent mechanism for c-Myc down-regulation. Furthermore, Len treatment transiently stabilised and subsequently down-regulated MDM2 expression with up-regulation of p53 and its down-stream targets (p21, PUMA). Under ribosomal stress conditions, polysome-free RPs are released into the nucleoplasm where they bind Mdm2 and suppress its E3 ligase activity. Consistent with this effect, in Len treated cells MDM2 co-immunoprecipitated with RPL11 and RPL5 with p53 protein stabilization (no change in p53 mRNA). In addition, an increase in the interaction between RPL11 and c-Myc was also observed consistent with the reported role for RPL11 in the post-transcriptional regulation and suppression of c-Myc. Furthermore, treatment with Lenalidomide supressed ribosomal RNA (rRNA) transcription with inhibition of pre-rRNA (47S) processing. The examination of polysome fractions (sucrose gradients) in Len treated MM cells (OPM2 and MM1S) revealed a striking reduction of 60S and 80S as well as polysomes fractions, an effect similar to that Actinomycin D (5 nM), a known RNA PolI inhibitor and potent ribosome stress inducer. Furthermore Len treatment significant reduced RPL11 within the 60S and 80S ribosome fractions. Of note, CRBN knockdown abrogated all these effects suggesting that Len binding to CRBN is upstream of this ribosome stress response. In order to investigate whether the down-regulation of c-Myc is the primary event leading to the disruption of ribogenesis, we transiently and stably silenced RPL11 in MM cells. RPL11 silencing nearly fully protected MM cells from the effects of Len and importantly it completely reversed Len-induced Mdm2 E3 ligase inhibition with up-regulation of p53, p21 and suppression of c-Myc and IRF4. ConclusionTaken together our data indicate that treatment with lenalidomide suppresses ribogenesis and induces a ribosomal stress response downstream of Cul4a-CRBN but upstream of c-Myc suppression and p53 induction. These effects likely result from the IMiDs-induced modification of the Cul4a-CRBN ubiquitome and the regulation of RPs integration into ribosomal subunits. These findings also explain the observed clinical activity of IMiDs in other ribosomopathies like 5q- MDS and Diamond-Blackfan anemia Disclosures:No relevant conflicts of interest to declare.