Ubiquitous Induction of p53 in Tumor Cells by Antisense Inhibition of MDM2 Expression

Virtual Screening Based on Ensemble Docking Targeting Wild-Type p53 for Anticancer Drug Discovery

The tumor-suppressor function of p53 makes it an attractive drug target. Efforts were mostly put on stabilization of the functional p53 or reactivation of mutated p53. Previous studies have shown that small molecules targeting Loop1/Sheet3 (L1/S3) can reactivate the R175H-p53 and stabilize p53 in vitro. Since the L1/S3 pocket is shared by the mutate and the wild type (WT) p53, virtual screening is introduced to identify natural products targeting the L1/S3 of WT p53. Considering the high flexibility of Loop1, ensemble docking method is utilized for different clusters of the L1/S3. Seven conformations were chosen for docking. As one of the 181 selected candidates, torilin not only improved p53 activity, but also increased p21 protein expression level, which lies downstream of p53, therefore suppressing HCT116 cancer cell growth. Torilin may covalently bind to Cys124 of p53 by 2-methyl-2-butenal (2M2B) group, as torilin derivatives, which do not contain the 2M2B group, were not able to increase the p53 transcription activity. In conclusion, this study demonstrated that L1/S3 of WT-p53 is a druggable pocket, and torilin has a potential cytotoxicity through activating the p53 pathway.

Experimental Therapy of Advanced Breast Cancer: Targeting NFAT1-MDM2-p53 Pathway

Advanced breast cancer, especially advanced triple-negative breast cancer, is typically more aggressive and more difficult to treat than other breast cancer phenotypes. There is currently no curable option for breast cancer patients with advanced diseases, highlighting the urgent need for novel treatment strategies. We have recently discovered that the nuclear factor of activated T cells 1 (NFAT1) activates the murine double minute 2 (MDM2) oncogene. Both MDM2 and NFAT1 are overexpressed and constitutively activated in breast cancer, particularly in advanced breast cancer, and contribute to its initiation, progression, and metastasis. MDM2 regulates cancer cell proliferation, cell cycle progression, apoptosis, migration, and invasion through both p53-dependent and -independent mechanisms. We have proposed to target the NFAT1-MDM2-p53 pathway for the treatment of human cancers, especially breast cancer. We have recently identified NFAT1 and MDM2 dual inhibitors that have shown excellent in vitro and in vivo activities against breast cancer, including triple-negative breast cancer. Herein, we summarize recent advances made in the understanding of the oncogenic functions of MDM2 and NFAT1 in breast cancer, as well as current targeting strategies and representative inhibitors. We also propose several strategies for inhibiting the NFAT1-MDM2-p53 pathway, which could be useful for developing more specific and effective inhibitors for breast cancer therapy.

Assay for Ubiquitin Ligase Activity: High-Throughput Screen for Inhibitors of HDM2

An assay for the autoubiquitination activity of the E3 ligaseHDM2 (Mdm2) was developed and adapted to a high-throughput format to identify inhibitors of this activity. The assay can also be used tomeasure the activity of other E3s andmay be useful in finding both inhibitors and activators of a wide range of different ubiquitin ligases.

Mutant p53 aggregates into prion-like amyloid oligomers and fibrils: implications for cancer

Over 50% of all human cancers lose p53 function. To evaluate the role of aggregation in cancer, we asked whether wild-type (WT) p53 and the hot-spot mutant R248Q could aggregate as amyloids under physiological conditions and whether the mutant could seed aggregation of the wild-type form. The central domains (p53C) of both constructs aggregated into a mixture of oligomers and fibrils. R248Q had a greater tendency to aggregate than WT p53. Full-length p53 aggregated into amyloid-like species that bound thioflavin T. The amyloid nature of the aggregates was demonstrated using x-ray diffraction, electron microscopy, FTIR, dynamic light scattering, cell viabilility assay, and anti-amyloid immunoassay. The x-ray diffraction pattern of the fibrillar aggregates was consistent with the typical conformation of cross β-sheet amyloid fibers with reflexions of 4.7 Å and 10 Å. A seed of R248Q p53C amyloid oligomers and fibrils accelerated the aggregation of WT p53C, a behavior typical of a prion. The R248Q mutant co-localized with amyloid-like species in a breast cancer sample, which further supported its prion-like effect. A tumor cell line containing mutant p53 also revealed massive aggregation of p53 in the nucleus. We conclude that aggregation of p53 into a mixture of oligomers and fibrils sequestrates the native protein into an inactive conformation that is typical of a prionoid. This prion-like behavior of oncogenic p53 mutants provides an explanation for the negative dominance effect and may serve as a potential target for cancer therapy.

Identification of the spiro(oxindole-3,3'-thiazolidine)-based derivatives as potential p53 activity modulators

Here, we report the design of new analogues of spirooxoindolepyrrolidine nucleus as modulators of p53 activity. Compounds (3R,7aR)-6-(4-chlorobenzyl)-1H-spiro[imidazo[1,5-c]thiazole-3,3'-indoline]-2',5,7(6H,7aH)-trione (9c) and (3R,7aR)-5'-methyl-6-(3,4,5-trimethoxybenzyl)-1H-spiro[imidazo[1,5-c]thiazole-3,3'-indoline]-2',5,7(6H,7aH)-trione (10d) are the most potent compounds of this series, inhibiting cell growth of different human tumor cells at submicromolar and micromolar concentrations, respectively. Compound 9c induces apoptotic cell death in human melanoma cell line M14 at 24 h, while in the same condition, treatment with 10d showes a clear arrest at G2/M phase inducing delay of cell cycle progression. Possibly, these activities may be due to inhibition of p53-MDM2 interaction and subsequent p53 release and activation.

Modulation of mdm2 pre-mRNA splicing by 9-aminoacridine-PNA (peptide nucleic acid) conjugates targeting intron-exon junctions

Background

Modulation of pre-mRNA splicing by antisense molecules is a promising mechanism of action for gene therapeutic drugs. In this study, we have examined the potential of peptide nucleic acid (PNA) 9-aminoacridine conjugates to modulate the pre-mRNA splicing of the mdm2 human cancer gene in JAR cells.

Methods

We screened 10 different 15 mer PNAs targeting intron2 at both the 5' - and the 3'-splice site for their effects on the splicing of mdm2 using RT-PCR analysis. We also tested a PNA (2512) targeting the 3'-splice site of intron3 with a complementarity of 4 bases to intron3 and 11 bases to exon4 for its splicing modulation effect. This PNA2512 was further tested for the effects on the mdm2 protein level as well as for inhibition of cell growth in combination with the DNA damaging agent camptothecin (CPT).

Results

We show that several of these PNAs effectively inhibit the splicing thereby producing a larger mRNA still containing intron2, while skipping of exon3 was not observed by any of these PNAs. The most effective PNA (PNA2406) targeting the 3'-splice site of intron2 had a complementarity of 4 bases to intron2 and 11 bases to exon3. PNA (2512) targeting the 3'-splice site of intron3 induced both splicing inhibition (intron3 skipping) and skipping of exon4. Furthermore, treatment of JAR cells with this PNA resulted in a reduction in the level of MDM2 protein and a concomitant increase in the level of tumor suppressor p53. In addition, a combination of this PNA with CPT inhibited cell growth more than CPT alone.

Conclusion

We have identified several PNAs targeting the 5'- or 3'-splice sites in intron2 or the 3'-splice site of intron3 of mdm2 pre-mRNA which can inhibit splicing. Antisense targeting of splice junctions of mdm2 pre-mRNA may be a powerful method to evaluate the cellular function of MDM2 splice variants as well as a promising approach for discovery of mdm2 targeted anticancer drugs.

Enhancement of transcriptional activity of mutant p53 tumor suppressor protein through stabilization of tetramer formation by calix[6]arene derivatives

Li-Fraumeni syndrome, a hereditary disorder characterized by familial clusters of early-onset multiple tumors, is caused by mutation of the TP53 gene, which encodes the p53 tumor suppressor protein. Mutation of Arg337 to histidine in the tetramerization domain of p53 is most frequently observed in Li-Fraumeni syndrome. This mutation is reported to destabilize the tetrameric structure of p53. We designed and synthesized calix[6]arene derivatives, which have six imidazole or pyrazole groups at the upper rim. In this study, we report, for the first time, the enhancement of the in vivo transcriptional activity of the most common Li-Fraumeni p53 mutant by imidazole-calix[6]arene through stabilization of the oligomer formation.

Concurrent overexpression of serum p53 mutation related with Helicobacter pylori infection

Background & Aims

In the province of Cadiz (Spain), the adjusted mortality rate for gastric cancer in the coastal town of Barbate is 10/100.000 inhabitants, whereas in the inland town of Ubrique, the rate is twice as high. The rate of Helicobacter pylori (H. pylori) infection (H. pylori antibodies) in the normal population was 54% in Ubrique, but only 32% in Barbate. In the two decades since its original discovery, p53 has found a singularly prominent place in our understanding of human gastric cancer and H. pylori cause accumulation of reactive oxygen species in the mucosa compartment. This study was designed to compare serum levels of p53 in a population characterized by high mortality due to stomach cancer and a high prevalence of H. pylori infection and another population in which mortality from this cause and the prevalence of H. pylori infection are low.

Materials and methods

319 subjects from the low mortality population and 308 from the high mortality population were studied, as were 71 patients with stomach cancer. We measured serum immunoglobulin G antibody to H. pylori and serum mutant p53 protein and ceruloplasmin.

Results

The difference between the two populations in the prevalence of H. pylori infection was significant (p < 0.001). Of the seropositive, 81% had elevated values of mutant p53, in comparison with 11% of the seronegative (p < 0.0001). Serum concentration of ceruloplasmin was significantly higher in seropositive with elevated mutant p53 protein than in seronegative with normal levels of p53 (p < 0.05).

Conclusions

There is a significant association between infection with H. pylori, elevated titers of H. pylori antibodies, and positivity for serum mutant p53 protein. Such information can significantly increase our basic knowledge in molecular pathology of gastric cancer and protection against H. pylori infection.

MDM2 controls the timely expression of cyclin A to regulate the cell cycle

Overexpression of MDM2 has been related to oncogenesis. In this communication, we present evidence to show that MDM2 controls the cell cycle-dependent expression of cyclin A by using a pathway that ensures its timely expression. MDM2 does not inhibit cyclin D or E expression. Silencing of endogenous MDM2 expression elevates cyclin A expression. The p53-binding domain of MDM2 harbors a SWIB region homologous to a conserved domain of a chromosome remodeling factor BRG1-associated protein. The SWIB domain of MDM2 inhibits cyclin A expression in a p53- and BRG1-dependent fashion, suggesting that MDM2 interferes with p53 binding of the BRG1 complex freeing it to repress cyclin A expression. Silencing of cyclin-dependent kinase (cdk) inhibitor p16 prevents MDM2-mediated inhibition of cyclin A expression, implicating its role in the process. MDM2-mediated repression of cyclin A expression induces G(1)-S arrest, which can be rescued by ectopic expression of cyclin A. Cancer cells lacking p53, p16, or BRG1 escape MDM2-mediated repression of cyclin A expression and growth arrest. Our data propose a novel mechanism by which MDM2 controls the cell cycle in normal cells and how cancer cells may escape this important safety barrier.

Small-molecule inhibitors of the p53-HDM2 interaction for the treatment of cancer

BACKGROUND

The hdm2 oncogene product, HDM2 (also known as MDM2), is an ubiquitin protein ligase that suppresses the transcriptional activity of the tumor suppressor p53 and promotes its degradation. Approximately 50% of all human tumors harbor mutations or deletions in the TP53 gene. In the remaining half of all human cancers that express the wild-type protein, aberrations of p53 regulators such as HDM2 account for p53 inhibition. Therefore, small-molecule inhibitors of the HDM2-p53 protein-protein interaction appear to offer an attractive strategy for cancer therapy.

OBJECTIVE

This review focuses on recent progress in the field of small-molecule inhibitors of the p53-HDM2 protein-protein interaction for the treatment of cancer.

RESULTS/CONCLUSION

The development of pharmacological inhibitors has been challenging. Although many small-molecule HDM2 inhibitors have shown potent in vitro activity, only a limited number of compounds have displayed acceptable pharmacokinetic properties for in vivo evaluation. To date, the most studied chemotypes have been cis-imidazolines (e.g., Nutlins), benzodiazepines (BDPs) and spiro-oxindoles. The cis-imidazolines were the first reported potent, selective small-molecule inhibitors of the p53-MDM2 interaction, and continue to show therapeutic potential. Additionally, p53-based strategies involving inhibition of MDM2-mediated p53 ubiquitylation and restoration of DNA-binding activity of mutant p53 protein, as well as combination therapies, will be briefly described. Finally, a structurally distinct chemotype currently in Phase I clinical trials will be presented.

Discovery of new pyridoacridine alkaloids from Lissoclinum cf. badium that inhibit the ubiquitin ligase activity of Hdm2 and stabilize p53

Compounds that stabilize p53 could suppress tumors providing a additional tool to fight cancer. Mdm2, and the human ortholog, Hdm2 serve as ubiquitin E3 ligases and target p53 for ubiquitylation and degradation. Inhibition of Hdm2 stabilizes p53, inhibits cell proliferation and induces apoptosis. Using HTS to discover inhibitors, we identified three new alkaloids, isolissoclinotoxin B, diplamine B, and lissoclinidine B from Lissoclinum cf. badium. Lissoclinidine B inhibited ubiquitylation and degradation of p53, and selectively killed transformed cells harboring wild-type p53, suggesting this compound could be used to develop new treatments.

E2F-1 transcriptional activity is a critical determinant of Mdm2 antagonist-induced apoptosis in human tumor cell lines

Nutlin-3 is a selective inhibitor of the p53-Mdm2 interaction, and inhibits growth in most tumor cells with wild-type p53. However, it only induces apoptosis in subsets of tumor cells. We report that the apoptotic response induced by Nutlin-3 correlates with its antitumor effects in xenograft models in athymic mice. We have investigated signals that sensitize cells to undergo apoptosis induced by Nutlin-3. We demonstrate that adenovirus E1A increases Nutlin-3-induced apoptosis through pRb inhibition in mouse embryonic fibroblast cells in a p53-dependent manner. Consistent with this, pRb depletion by siRNA transfection with Nutlin-3 synergistically increases apoptosis in HCT116 human colon cancer cells, which are insensitive to induction of apoptosis by Nutlin-3 alone. As pRb is a key negative regulator of E2F, we asked whether E2F transcriptional activity determines the apoptotic response of cancer cells to Nutlin-3. We demonstrate that transcriptional activity of E2F correlates with the apoptotic response to Nutlin-3 in various tumor cells and depletion of E2F-1 suppresses Nutlin-3-induced apoptosis in cells possessing high transcriptional activity of E2F, including retinoblastoma cells harboring mutated Rb with wild-type p53. Furthermore, we report that expression of the p53 homologue p73, a target of E2F-1, is markedly increased by Nutlin-3 in Rb-mutated tumor cells harboring wild-type p53. Depletion of p73 by siRNA transfection suppresses Nutlin-3-induced apoptosis in these cells. Taken together, our results demonstrate that E2F-1 transcriptional activity is a critical determinant of Mdm2 antagonist-induced apoptosis and p73 is important for E2F-1-mediated apoptosis induced by Nutlin-3, especially in tumor cells with mutated Rb. Furthermore, our results suggest that tumor cells, including Rb mutated cells, which harbor wild-type p53 and high E2F transcriptional activity, could be a good target for Mdm2 antagonist therapy.

Nuclear to cytoplasmic shift of p33(ING1b) protein from normal oral mucosa to oral squamous cell carcinoma in relation to clinicopathological variables

PURPOSE

p33(ING1b), as a candidate tumour suppressor gene, has been found to be expressed a proportion of oral squamous cell carcinomas (OSCCs), however, its clinicopathological significance is not studied yet. Our aim was to investigate association of p33(ING1b) expression with clinicopathological variables and particularly interesting new cysteine-histidine rich protein (PINCH) in OSCCs.

METHODS

p33(ING1b) expression was immunohistochemically examined in 20 normal oral mucosa specimens and 49 OSCCs.

RESULTS

Normal squamous cells showed only p33(ING1b )nuclear expression (no cytoplasmic expression), with a rate of 90% positive cases. While 24% of OSCCs appeared cytoplasmic expression (11 of them with weak nuclear staining) and the rest tumours (76%) were negative for p33(ING1b). Furthermore, the cases having lymph node metastasis showed a higher frequency of positive cytoplasmic expression than those without metastasis (P = 0.03). The p33(ING1b) cytoplasmic expression was positively related to PINCH expression (P = 0.04), the cases positive for both proteins had a high rate of the metastasis (P = 0.03).

CONCLUSIONS

The transfer of p33(ING1b) protein from the nucleus to the cytoplasm may result in loss of normal cellular function of the protein, which might play a role in the tumourigenesis and metastasis of OSCCs.

Restoration of wild-type p53 function in human cancer: relevance for tumor therapy

BACKGROUND

In the majority of human cancers, the tumor suppressor activity of p53 is impaired because of mutational events or interactions with other proteins (ie, MDM2). The loss of p53 function is responsible for increased aggressiveness of cancers, while tumor chemoresistance and radioresistance are dependent upon the expression of mutant p53 proteins.

METHODS

Review of the literature indicates that p53 acts primarily as a transcription factor whose function is subject to a complex and diverse array of covalent post-translational modifications that markedly influence the expression of p53 target genes responsible for cellular responses such as growth arrest, senescence, or apoptosis. The ability of p53 to induce apoptosis in cancer cells is believed essential for cancer therapy.

RESULTS

Numerous data indicate that p53 dependent apoptosis is a relevant factor in determining the efficacy of anticancer treatments. Thus, the development of new strategies for restoration of p53 function in human tumors is considered an important issue. Two main approaches for restoration of p53 function have been pursued that impact anticancer treatments: (a) de novo expression of wild-type p53 (wt-p53) through gene therapy and (b) identification of small molecules reactivating wt-p53 function.

CONCLUSIONS

The extensive body of knowledge acquired has identified manipulations of p53 signaling as a relevant issue for successful therapies. In this context, the recognition of p53 status in cancer cells is significant and would help considerably in the selection of an appropriate therapeutic approach. p53 manipulations for cancer therapy have revealed the need for specificity of p53 activation and ability to spare body tissues. Furthermore, the promising results obtained by using molecules competent to reactivate wt-p53 functions in cancer cells provide the basis for the design of new molecules with lower side effects and higher anti-tumor efficiency. The reexpression and reactivation of p53 protein in human cancer cells would increase tumor susceptibility to radiation or chemotherapy enhancing the efficacy of standard therapeutic protocols.

Strategies for targeting protein-protein interactions with synthetic agents

The development of small-molecule modulators of protein-protein interactions is a formidable goal, albeit one that possesses significant potential for the discovery of novel therapeutics. Despite the daunting challenges, a variety of examples exists for the inhibition of two large protein partners with low-molecular-weight ligands. This review discusses the strategies for targeting protein-protein interactions and the state of the art in the rational design of molecules that mimic the structures and functions of their natural targets.

p53—A Natural Cancer Killer: Structural Insights and Therapeutic Concepts

Every single day, the DNA of each cell in the human body is mutated thousands of times, even in absence of oncogenes or extreme radiation. Many of these mutations could lead to cancer and, finally, death. To fight this, multicellular organisms have evolved an efficient control system with the tumor-suppressor protein p53 as the central element. An intact p53 network ensures that DNA damage is detected early on. The importance of p53 for preventing cancer is highlighted by the fact that p53 is inactivated in more than 50 % of all human tumors. Thus, for good reason, p53 is one of the most intensively studied proteins. Despite the great effort that has been made to characterize this protein, the complex function and the structural properties of p53 are still only partially known. This review highlights basic concepts and recent progress in understanding the structure and regulation of p53, focusing on emerging new mechanistic and therapeutic concepts.

Expression proteomics to p53 mutation reactivation with PRIMA-1 in breast cancer cells

PRIMA-1 has emerged as a small molecule that restores the wild type function to mutant p53. To identify molecular targets that are involved in PRIMA-1-induced apoptosis, we used a proteomics approach with two-dimensional gel electrophoresis coupled with liquid chromatography-tandem mass spectrometry for protein identification. By comparing the proteome of the PRIMA-1-treated MDA-231 breast carcinoma cells with that of MCF-7 cells, we have identified seven proteins that upregulated only in MDA-231 cells as a result of PRIMA-1-induced apoptosis. The identified proteins are involved in anaerobic glycolysis and in mitochondrial intrinsic apoptosis. Treatment of MDA-231 cells with PRIMA-1 resulted in the release of mitochondrial cytochrome c as well as the activation of caspase-3, which are essential for the execution of apoptosis. We present evidence to suggest that PRIMA-1-induced apoptosis in breast cancer cells with mutated p53 function involved the expression of proteins required for the activation of mitochondrial intrinsic pathway that is glycolysis-relevant.

Benzodiazepinedione inhibitors of the Hdm2:p53 complex suppress human tumor cell proliferation in vitro and sensitize tumors to doxorubicin in vivo

The activity and stability of the p53 tumor suppressor are regulated by the human homologue of the mouse double minute 2 (Hdm2) oncoprotein. It has been hypothesized that small molecules disrupting the Hdm2:p53 complex would allow for the activation of p53 and result in growth suppression. We have identified small-molecule inhibitors of the Hdm2:p53 interaction using our proprietary ThermoFluor microcalorimetry technology. Medicinal chemistry and structure-based drug design led to the development of an optimized series of benzodiazepinediones, including TDP521252 and TDP665759. Activities were dependent on the expression of wild-type (wt) p53 and Hdm2 as determined by lack of potency in mutant or null p53-expressing cell lines or cells engineered to no longer express Hdm2 and wt p53. TDP521252 and TDP665759 inhibited the proliferation of wt p53-expressing cell lines with average IC(50)s of 14 and 0.7 micromol/L, respectively. These results correlated with the direct cellular dissociation of Hdm2 from wt p53 observed within 15 minutes in JAR choriocarcinoma cells. Additional activities of these inhibitors in vitro include stabilization of p53 protein levels, up-regulation of p53 target genes in a DNA damage-independent manner, and induction of apoptosis in HepG2 cells. Administration of TDP665759 to mice led to an increase in p21(waf1/cip1) levels in liver samples. Finally, TDP665759 synergizes with doxorubicin both in culture and in an A375 xenograft model to decrease tumor growth. Taken together, these data support the potential utility of small-molecule inhibitors of the Hdm2:p53 interaction for the treatment of wt p53-expressing tumors.

ATM and Chk2-dependent phosphorylation of MDMX contribute to p53 activation after DNA damage

The p53 tumor suppressor is activated after DNA damage to maintain genomic stability and prevent transformation. Rapid activation of p53 by ionizing radiation is dependent on signaling by the ATM kinase. MDM2 and MDMX are important p53 regulators and logical targets for stress signals. We found that DNA damage induces ATM-dependent phosphorylation and degradation of MDMX. Phosphorylated MDMX is selectively bound and degraded by MDM2 preceding p53 accumulation and activation. Reduction of MDMX level by RNAi enhances p53 response to DNA damage. Loss of ATM prevents MDMX degradation and p53 stabilization after DNA damage. Phosphorylation of MDMX on S342, S367, and S403 were detected by mass spectrometric analysis, with the first two sites confirmed by phosphopeptide-specific antibodies. Mutation of MDMX on S342, S367, and S403 each confers partial resistance to MDM2-mediated ubiquitination and degradation. Phosphorylation of S342 and S367 in vivo require the Chk2 kinase. Chk2 also stimulates MDMX ubiquitination and degradation by MDM2. Therefore, the E3 ligase activity of MDM2 is redirected to MDMX after DNA damage and contributes to p53 activation.

Proteomic identification of heat shock protein 90 as a candidate target for p53 mutation reactivation by PRIMA-1 in breast cancer cells

Introduction

A loss of p53 function resulting from mutation is prevalent in human cancers. Thus, restoration of p53 function to mutant p53 using small compounds has been extensively studied for cancer therapy. We previously reported that PRIMA-1 (for 'p53 reactivation and induction of massive apoptosis') restored the transcriptional activity of p53 target genes in breast cancer cells with a p53 mutation. By using functional proteomics approach, we sought to identify molecular targets that are involved in the restoration of normal function to mutant p53.

Methods

PRIMA-1 treated cell lysates were subjected to immunoprecipitation with DO-1 primary antibody against p53 protein, and proteins bound to p53 were separated on a denaturing gel. Bands expressed differentially between control and PRIMA-1-treated cells were then identified by matrix-assisted laser desorption ionization-time-of-flight spectrometry. Protein expression in whole cell lysates and nuclear extracts were confirmed by Western blotting. The effect of combined treatment of PRIMA-1 and adriamycin in breast cancer cells was determined with a cytotoxicity assay in vitro.

Results

PRIMA-1 treated cells distinctly expressed a protein band of 90 kDa that was identified as heat shock protein 90 (Hsp90) by the analysis of the 90 kDa band tryptic digest. Immunoblotting with isoform-specific antibodies against Hsp90 identified this band as the α isoform of Hsp90 (Hsp90α). Co-immunoprecipitation with anti-Hsp90α antibody followed by immunoblotting with DO-1 confirmed that p53 and Hsp90α were interacting proteins. PRIMA-1 treatment also resulted in the translocation of Hsp90α to the nucleus by 8 hours. Treatment of cells with PRIMA-1 alone or in combination with adriamycin, a DNA-targeted agent, resulted in increased sensitivity of tumor cells.

Conclusion

The studies demonstrate that PRIMA-1 restores the p53-Hsp90α interaction, enhances the translocation of the p53-Hsp90α complex and reactivates p53 transcriptional activity. Our preliminary evidence also suggests that PRIMA-1 could be considered in combination therapy with DNA-targeted agents for the treatment of breast cancer, especially for tumors with aberrant p53 function.

p53 α-Helix mimetics antagonize p53/MDM2 interaction and activate p53

Overexpression or hyperactivation of MDM2 contributes to functional inactivation of wild-type p53 in nearly 50% of tumors. Inhibition of p53 by MDM2 depends on binding between an NH(2)-terminal (residues 16-28) p53 alpha-helical peptide and a hydrophobic pocket on MDM2, presenting an attractive target for development of inhibitors against tumors expressing wild-type p53. Here we report that novel p53 alpha-helical peptide mimics based on a terphenyl scaffold can inhibit MDM2-p53 binding in vitro and activate p53 in vivo. Several active compounds have been identified that inhibit MDM2-p53 binding in an ELISA assay with IC(50) of 10 to 20 micromol/L and induce p53 accumulation and activation in cell culture at 15 to 40 micromol/L. These results suggest that p53 alpha-helical mimetics based on the terphenyl scaffold may be developed into potent p53 activators.

Targeting the p53-MDM2 interaction to treat cancer

The tumour suppressor p53 is a transcription factor with powerful antitumour activity that is controlled by its negative regulator MDM2 (mouse double minute 2, also termed HDM2 in humans) through a feedback mechanism. MDM2, which is overproduced in many tumours, binds p53 and inhibits its function by modulating its transcriptional activity and stability. Activation of p53 in tumour cells by inhibiting its physical interaction with MDM2 has been in the focus of cancer drug discovery. However, development of nonpeptidic MDM2 antagonists turned out to be challenging. Recently, the first potent and selective small-molecule antagonists of MDM2, the Nutlins, have been identified. Studies with Nutlins provided in vitro and in vivo proof-of-principle for targeting p53–MDM2 interaction for cancer therapy.

Radiosensitization by antisense anti-MDM2 mixed-backbone oligonucleotide in in vitro and in vivo human cancer models

PURPOSE

The MDM2 oncogene, amplified or overexpressed in many human cancers, has been suggested to be a novel target for cancer therapy. We have demonstrated a second-generation antisense antihuman-MDM2 oligonucleotide to have antitumor activity when administered alone or in combination with cancer chemotherapeutic agents. In the present study, we investigated the effect of the antisense oligonucleotide on radiation therapy.

EXPERIMENTAL DESIGN

The in vitro radiosensitization activity was determined in cell lines of human cancers of prostate (LNCaP and PC3), breast (MCF-7 and MDA-MB-468), pancreas (PANC-1), and glioma (U87-MG and A172) and its in vivo radiosensitization activity in xenograft models of LNCaP, PC3, MCF-7, MDA-MB-468, and PANC-1.

RESULTS

In cells containing at least one functional p53 allele (LNCaP, U87-MG, and A172), after specific inhibition of MDM2 expression, p53 and p21 levels were elevated. In LNCaP cells, the Bax level was increased, and Bcl-2 and E2F1 levels were decreased. In PC3 cells that are p53 null, after inhibition of MDM2 expression, Bax and p21 levels were elevated, and E2F1 levels were decreased. On the basis of in vitro clonogenic assay, the antisense oligonucleotide, in a sequence-specific manner, significantly increased radiation-induced antiproliferation effects. It also increased radiation-induced inhibitory effects on tumor growth in SCID or nude mice bearing LNCaP, PC3, MCF-7, MDA-MB-468, and PANC-1 xenografts.

CONCLUSIONS

These results suggest that MDM2 has a role in radiation therapy of human cancers, regardless of p53 status, providing a basis for future development of MDM2 inhibitors, such as antisense oligonucleotides, as radiosensitizers.

Nuclear accumulation of p53 following inhibition of transcription is not due to diminished levels of MDM2

A common mechanism by which the tumor suppressor p53 accumulates in the nucleus following cellular stress is through the attenuation of its interaction with MDM2, a protein involved in the nuclear export and degradation of p53. This is accomplished by induced modifications of p53, MDM2 or both. We have previously found that the kinase and mRNA synthesis inhibitor DRB (5,6-dichloro-1-b-D-ribofuranosylbenzimidazole) induces the nuclear accumulation of p53 without concomitant phosphorylation of the ser15 site of p53, which is thought to be a modification important for the attenuation of p53-MDM2 interaction. It has been proposed that the mechanism by which p53 accumulates following blockage of transcription involves the downregulation of MDM2 expression. In this study, we tested this hypothesis and found that after DRB treatment, p53 accumulated despite the fact that MDM2 levels remained high in human cells. Furthermore, over expression of MDM2 did not prevent the accumulation of p53 following DRB treatment. In, addition, p53 accumulating in the nucleus after DRB treatment was able to interact with MDM2 and was ubiquitylated. These findings suggest that blockage of transcription induce the nuclear accumulation of p53 without breaking the p53-MDM2 regulation loop.

Chemosensitization and radiosensitization of human cancer by antisense anti-MDM2 oligonucleotides: in vitro and in vivo activities and mechanisms

MDM2 oncogene is overexpressed in many human cancers including breast, colon, and prostate cancer, and MDM2 levels are associated with poor prognosis in patients with cancer. Here, we summarize the investigation of the functions of MDM2 oncogene in human cancer growth and the value of MDM2 as a drug target for prostate cancer therapy by using antisense to inhibit MDM2 expression. Antisense anti-human-MDM2 oligonucleotides and mismatch controls were tested in in vitro and in vivo human cancer models for antitumor activity. Targeted gene products and related proteins were analyzed and the antitumor activity was determined when the oligonucleotides were used alone or in combination with cancer chemotherapeutics and radiation therapy. The antisense oligonucleotide specifically inhibited MDM2 expression in a dose- and time-dependent manner, resulting in significant antitumor activity in vitro and in vivo. The antisense oligonucleotides also potentiated the effects of p53 activation and p21 induction by chemotherapeutic agents 10-hydroxycamptothecin, adriamycin, 5-fluorouracil, and paclitaxel. In a dose-dependent manner, the antisense oligonucleotide showed antitumor activity in nude mice bearing human cancer xenografts and increased therapeutic effectiveness of the chemotherapeutic agents irinotecan, paclitaxel, and Rituxan and radiation therapy. These results indicate that MDM2 has a role in various tumor growth through both p53-dependent and p53-independent mechanisms, indicating that MDM2 inhibitors have a broad spectrum of antitumor activities in human cancers regardless of p53 status. These results provide a basis for clinical evaluation of antisense anti-MDM2 oligonucleotides as chemosensitizer and radiosensitizer.

Expression of MDM2 protein and mRNA in condyloma acuminata

In order to investigate the role of murine double minute 2 (MDM2) in cell proliferation and carcinogenesis in condyloma acuminatum (CA), immunohistochemistry and in situ hybridization were used to detect the expression of MDM2 protein and mRNA in normal skin and skin lesions of CA of vulva. PCR was also used to detect HPV types. The results showed that in 32 observed CA specimens, the expression of MDM2 protein and mRNA was detected in 18 (56.25%) and 22 (68.75%) respectively, while the co-expression of MDM2 protein and mRNA was found in 14. PCR results revealed that HPV6/11 and HPV16/18 subtypes were shown in 28(87.5%) and 4 (12.5%) respectively out of 32 CA specimens. Out of the 18 positive specimens expressing MDM2 protein, HPV6/11 subtypes were shown in 15 and HPV16/18 subtypes in 3. In 22 positive specimens expressing MDM2 mRNA, HPV6/11 subtypes were shown in 18 and HPV16/18 subtypes in 4. No expression of MDM2 protein and MDM2 mRNA was observed in normal skin. Our study indicated that the overexpression of MDM2 might be involved in malignant proliferation and carcinogenesis of CA.

MDM2 and Its Splice Variant Messenger RNAs: Expression in Tumors and Down-Regulation Using Antisense Oligonucleotides

Alternative splicing has an important role in expanding protein diversity. An example of a gene with more than one transcript is the MDM2 oncogene. To date, more than 40 different splice variants have been isolated from both tumor and normal tissues. Here, we review what is known about the alteration of MDM2 mRNA expression, focusing on alternative splicing and potential functions of different MDM2 isoforms. We also discuss the progress that has been made in the development of antisense oligonucleotides targeted to MDM2 for use as a potential cancer therapy.

Antisense therapeutics: from theory to clinical practice

The use of antisense (AS) oligonucleotides as therapeutic agents was proposed as far back as the 1960s/1970s when the AS strategy was initially developed. However, it has taken almost a quarter of a century for this potential to be realized. The last few years has seen a rapid increase in the number of AS molecules progressing past Phase I in clinical trials, due in part to our increased knowledge of their structure and chemistry. Here, we describe the most prominent of these modifications with respect to clinical applicability. However, the main focus of this review is clinical application, with a focus on cancer. We will discuss in detail both the status of the current AS clinical trials and the molecules that are likely to be the targets of the next group of AS molecules entering the clinic.

Experimental therapy of human prostate cancer by inhibiting MDM2 expression with novel mixed-backbone antisense oligonucleotides: in vitro and in vivo activities and mechanisms

BACKGROUND

MDM2 oncogene is overexpressed in many human cancers including prostate cancer and MDM2 levels are associated with poor prognosis. This study was undertaken to investigate the functions of MDM2 oncogene in prostate cancer growth and the value of MDM2 as a drug target for prostate cancer therapy by inhibiting MDM2 expression.

METHODS

Antisense anti-human-MDM2 mixed-backbone oligonucleotide and its mismatch control were tested in in vitro and in vivo human prostate cancer models (LNCaP, DU 145, and PC-3) for anti-tumor activity. Targeted gene products and related proteins were analyzed and the anti-tumor activity was determined when the oligonucleotides were used alone or in combination with cancer therapeutics.

RESULTS

The antisense oligonucleotide specifically inhibited MDM2 expression in a dose- and time-dependent manner, resulting in significant anti-tumor activity in vitro and in vivo. In LNCaP cells, p53 and p21 levels were elevated. The antisense oligonucleotide also potentiated the effects of p53 activation and p21 induction by chemotherapeutic agents 10-hydroxycamptothecin, adriamycin, 5-fluorouracil, and paclitaxel. In DU145 cells, following inhibition of MDM2 expression, p21 levels were elevated although p53 levels remained unchanged. In both cell lines, the antisense oligonucleotide inhibited tumor cell growth and induced apoptosis in vitro. In a dose-dependent manner, the antisense oligonucleotide showed anti-tumor activity in nude mice bearing DU145 or PC-3 xenografts. It significantly increased therapeutic effectiveness of the chemotherapeutic agent irinotecan and slightly improved the effects of paclitaxel and Rituxan.

CONCLUSIONS

These results indicate that MDM2 has a role in prostate tumor growth through both p53-dependent and p53-independent mechanisms, indicating that MDM2 inhibitors have a broad spectrum of anti-tumor activities in human prostate cancers regardless of p53 status.

RING finger ubiquitin protein ligases: implications for tumorigenesis, metastasis and for molecular targets in cancer

Covalent modification of proteins with ubiquitin regulates almost all aspects of eukaryotic cellular function. Ubiquitin protein ligases (E3s) play central regulatory roles in that they provide substrate specificity to this process and therefore, represent attractive molecular targets for disease therapy. We summarize recent advances in our understanding of RING finger and RING finger-related E3s with emphasis on BRCA1 and the tumor autocrine motility factor receptor (gp78), as well as discuss the potential for components of the ubiquitin pathway for proteasomal degradation as molecular targets.

Zebrafish as a model organism for the identification and characterization of drugs and genes affecting p53 signaling

p53 and its main negative regulator, Mdm2, are key players in mammalian cancer development. Activation of the transcription factor p53 through DNA damage or other stresses can result in cell cycle arrest, apoptosis, or both. Because of the absence of characterized p53 signaling in zebrafish (Danio rerio), we have studied the roles of Mdm2 and p53 in zebrafish by generating early embryonic knockdowns and examined the involvement of p53 in DNA damage-induced apoptosis. p53-deficient embryos, induced by injection of antisense morpholinos, were morphologically indistinguishable from control embryos, when unperturbed, whereas Mdm2 knockdown embryos were severely apoptotic and arrested very early in development. Double knockdowns showed that p53 deficiency rescued Mdm2-deficient embryos completely, similar to observations in mice. p53 deficiency also markedly decreased DNA damage-induced apoptosis, elicited by ultraviolet irradiation or by the anti-cancer compound camptothecin. p21/Waf/Cip-1 appeared to be a downstream target of zebrafish p53, as revealed relative p21 mRNA levels determined via TaqMan analysis. In contrast to mammals, zebrafish may regulate p53 activity by using an internal polyA signal site. We conclude that zebrafish represents a promising model organism for future compound-based and genetic screens and believe that it will help to identify and characterize new anticancer drugs and new targets for cancer treatment.

Study of the cytotoxic effect of a peptidic inhibitor of the p53-hdm2 interaction in tumor cells

Inhibitors of the p53-hdm2 interaction are attractive molecules for stimulating the p53 pathway in tumors. In this report, an inhibitor of the p53-hdm2 interaction, the AP peptide, is used to activate p53 in tumor cells expressing various levels of hdm2 protein. It induces apoptosis only in cells expressing high endogenous levels of hdm2 protein. The absence of apoptosis in tumor cells with low hdm2 levels is due not to alterations in the p53-dependent apoptotic pathway but to a different regulation of this pathway. The peptide is also less toxic for non-tumor cells than for tumor cells overexpressing the hdm2 protein.

p53 Mutations are present in colorectal cancer with cytoplasmic p53 accumulation

Previous studies have shown that nuclear p53 over-expression is an indicator of p53 mutations whereas cytoplasmic p53 accumulation is related to wild-type p53 in several kinds of tumors. Cytoplasmic p53 accumulation has been demonstrated to be an independent prognostic factor in colorectal adenocarcinomas. The purpose was to examine whether mutations occur in cases with p53 accumulated in the cytoplasm and whether there are any differences in the frequency and characteristics of p53 mutations in different staining patterns. In the present study, we identified p53 mutations using PCR single-strand conformation polymorphism (SSCP) and DNA sequencing in 75 primary colorectal adenocarcinomas with different staining patterns (negative, nucleus, cytoplasm, nucleus and cytoplasm). The results show that the frequency and nature of mutations in tumors with cytoplasmic p53 accumulation were similar to those with nuclear p53 expression. However, the tumors with accumulation in both the nucleus and cytoplasm demonstrated a higher mutation rate. We suppose that the role of cytoplasmic p53 accumulation in predicting prognosis in patients with colorectal cancer may be dependent on both mutational and non-mutational mechanisms.

Apoptosis of tumoral and nontumoral lymphoid cells is induced by both mdm2 and p53 antisense oligodeoxynucleotides

Following stress signals, the p53 tumor suppressor protein plays a critical role in regulation of cell proliferation, mainly through induction of growth arrest or apoptosis. Therefore, this protein needs to be strictly regulated and numerous studies have shown that the MDM2 protein is an essential element for p53 regulation in normal cells and, most importantly, that overexpression of MDM2 is responsible for p53 inactivation in various types of tumors. A previous study showed that this is the case in some Burkitt lymphoma (BL) cell lines, where enhanced translation of mdm2 messenger RNA results in overexpression of the protein that complexes and inactivates wild-type p53. To further investigate the role of the p53/MDM2 complex in these BL cells, as well as in other lymphoid cells that do not overexpress MDM2, this study used antisense oligodeoxynucleotides directed either against mdm2 or against p53. Results show that the mdm2 antisense oligodeoxynucleotide induces apoptosis of cells that express a high or low level of MDM2 protein, only if they contain wild-type p53. Moreover, apoptosis is independent of the accumulation of p53 following mdm2 antisense treatment. Finally, the p53 antisense oligodeoxynucleotide, which inhibits the expression of wild-type p53, also induces a decrease of the MDM2 level in cells, whether or not they overexpress this protein, and causes apoptosis of these cells. These results indicate that decreasing the MDM2 protein level by directly or indirectly targeting its biosynthesis is a potent tool for the induction of apoptosis.

ATM complexes with HDM2 and promotes its rapid phosphorylation in a p53-independent manner in normal and tumor human cells exposed to ionizing radiation

To further understand the mechanism(s) by which DNA damage activates p53, we analysed the expression levels of p53 and HDM2 (the human homolog of murine MDM2) in various human diploid fibroblast and tumor cell strains during the period that precedes activation of known downstream effectors of p53. In X-irradiated human cells, HDM2 protein was rapidly phosphorylated in serine/threonine residues in a p53, p14ARF and p73-independent manner. In p53 wild-type cells, HDM2 phosphorylation precedes a detectable increase in the levels of p53 and is not observed in ataxia telangiectasia (AT) fibroblasts. The transfection of AT cells with a vector expressing ATM restored the ability to rapidly phosphorylate HDM2 following X-irradiation, confirming a role for ATM in its phosphorylation. We also show that ATM complexes with HDM2. The DNA lesions signaling the early rapid phosphorylation of HDM2 are a result of X-ray and not UV-type damage. The ATM-promoted early covalent modification of HDM2 in X-irradiated human cells may provide a mechanism to activate p53.

A novel MDM2 anti-sense oligonucleotide has anti-tumor activity and potentiates cytotoxic drugs acting by different mechanisms in human colon cancer

MDM2 is over-expressed in several human tumors. Its product is a negative-feedback regulator of p53, which interferes with the control of cell proliferation and apoptosis, interacting not only with p53 but also with retinoblastoma (Rb) and E2F. Moreover, mutations in the ARF-Ink4a locus may also allow MDM2 to override p53 functions. In this study, we have used a novel oligonucleotide anti-sense MDM2, with mixed-backbone structure and demonstrate that it causes inhibition of MDM2 expression, induction of both p53 and p21/WAF1 expression and a dose-dependent, growth-inhibitory effect in human GEO colon-cancer cells. We also show that anti-sense MDM2 has a co-operative growth-inhibitory effect with different classes of cytotoxic drugs acting by different mechanisms. Moreover, anti-sense MDM2 induces apoptosis and markedly enhances the apoptotic activity of different cytotoxic drugs. Finally, we show that anti-sense MDM2 has anti-tumor activity in vivo in nude mice bearing GEO xenografts and potentiates the anti-tumor effect of cytotoxic drugs. Indeed, despite the short treatment period, the combination of anti-sense MDM2 and cytotoxic drugs causes a marked delay in tumor growth and prolongation of mice survival, lasting several months after treatment cessation. The anti-tumor effect is associated with inhibition of MDM2 expression in tumor specimens of animals treated with anti-sense MDM2, alone or in combination with a cytotoxic drug. Our results provide the rationale for development of a novel mixed-backbone anti-sense MDM2 into a clinical setting in therapeutic combination strategies with conventional cytotoxic drugs.

The contribution of the RING finger domain of MDM2 to cell cycle progression

The MDM2 oncoprotein binds to p53 and abrogates p53-mediated G1 arrest and apoptosis. We show that MDM2 over-expression accelerates cell cycle progression of RPM12650 cells by overcoming the negative effect of endogenous wild type p53 at the G1/S checkpoint. The interaction with p53 and transcription inhibition are necessary but not sufficient. The RING finger domain of MDM2 is also required for the positive effect of MDM2 on the cell cycle. Surprisingly, several point mutants in the zinc binding sites of the RING finger are fully competent for cell cycle stimulation even though they abolish MDM2-directed degradation of p53 and MDM2 E3-ligase activity. In contrast, alterations in and around the cryptic nucleolar localization sequence (KR motif) inhibit MDM2-mediated cell cycle progression as well as p53 degradation and MDM2 E3 ligase activity. We found that all the RING mutants decrease inhibition of both p53 dependent reporters and endogenous p21CIP1/WAF1/SDI1. These results indicate that the RING finger of MDM2 has a role in the regulation of the cell cycle that is independent of p53 degradation and endogenous p21CIP1/WAF1/SDI1 regulation.

Disrupting the p53-mdm2 interaction as a potential therapeutic modality

P53 and mdm2 are linked to each other through a negative feedback loop. P53 transactivates mdm2, but mdm2, in turn, is a major opponent of p53. Mdm2 promotes p53 degradation through a ubiquitin-dependent pathway on 26S proteasomes and is thought to be largely responsible for the very low levels of p53 protein in unstressed cells. The rationale for targeting the p53-mdm2 interaction therapeutically lies in the ability to activate p53 in all those tumors that retain wild type p53. Copyright 2000 Harcourt Publishers Ltd.

Current opinion in germ cell cancer 2000

Despite its relative rarity compared with the common adult cancers, scientific and clinical interest in germ cell cancer is increasing. From the point of view of epidemiology, the controversy about the relative importance of intrauterine versus postpubertal risk factors has continued. Evidence to support the importance of intrauterine factors comes from reports from Norway, Canada, and the US, confirming the Danish observation that the rising incidence of germ cell cancer is linked to a birth cohort effect; evidence in support of the importance of postpubertal risk comes from three case/control studies demonstrating increased risk linked to postpubertal exposures such as pesticides, plastics, electromagnetic radiation, trauma, and infections. There has been increasing interest in human endogenous retrovirus K10 as a possible factor explaining genetic susceptibility and providing a linkage between the two groups of risk factors. In cytogenetics, progress was reported in identifying the deletion point of the suspected tumor suppressor gene responsible for the i12p marker chromosome abnormality and development of FISH probes for diagnostic purposes. In molecular biology, the importance of DNA repair deficiency in normal germ cells as a factor in the exquisite chemosensitivity of germ cell cancer has been high-lighted by a report demonstrating a low level of the xeroderma pigmentosa group A (XPA) protein and induction of resistance in vitro by adding XPA. In the clinic, progress in positron emission tomography scanning and laparoscopic lymph node staging are leading to changes in outlook on management of stage 1 cases and patients with small residual masses postchemotherapy. Salvage chemotherapy regimens integrating dose dense and vertical dose intensification strategies reported 60% progression-free survival. New drugs such as gemcitabine demonstrated continued therapeutic potential for chemotherapy in these tumors. A report demonstrating the inadequacies of hormone replacement after bilateral orchidectomy and a report of the first child born after testis-conserving therapy highlight the need for more attention to testis conservation as a quality of life issue. With the cure rates so high, the need for central referral is once again debated both for stage 1 and metastatic disease. With new ways of defining poor risk stage 1 patients and reports on impact of experience highlighting the worse outcome of patients treated in centers treating small numbers, views on this issue remain clearcut.

The p53 tumor suppressor protein does not regulate expression of its own inhibitor, MDM2, except under conditions of stress

MDM2 is an important regulator of the p53 tumor suppressor protein. MDM2 inhibits p53 by binding to it, physically blocking its ability to transactivate gene expression, and stimulating its degradation. In cultured cells, mdm2 expression can be regulated by p53. Hence, mdm2 and p53 can interact to form an autoregulatory loop in which p53 activates expression of its own inhibitor. The p53/MDM2 autoregulatory loop has been elucidated within cultured cells; however, regulation of mdm2 expression by p53 has not been demonstrated within intact tissues. Here, we examine the role of p53 in regulating mdm2 expression in vivo in order to test the hypothesis that the p53/MDM2 autoregulatory loop is the mechanism by which low levels of p53 are maintained. We demonstrate that basal expression of mdm2 in murine tissues is p53 independent, even in tissues that express functional p53. Transcription of mdm2 is induced in a p53-dependent manner following gamma irradiation, indicating that p53 regulates mdm2 expression in vivo following a stimulus. The requirement for a stimulus to activate p53-dependent regulation of mdm2 expression in vivo appeared to differ from the situation in early-passage mouse embryo fibroblasts, where mdm2 expression is enhanced by the presence of p53. Analysis of mdm2 expression in intact and dispersed embryos revealed that establishment of mouse embryo fibroblasts in culture induces p53-dependent mdm2 expression, suggesting that an unknown stimulus activates p53 function in cultured cells. Together, these results indicate that p53 does not regulate expression of its own inhibitor, except in response to stimuli.

Colony formation of soft tissue sarcoma cells is inhibited by lipid-mediated antisense oligodeoxynucleotides targeting the human mdm2 oncogene

More than one third of human soft tissue sarcoma (STS) have elevated levels of the MDM2 oncoprotein, resulting either from gene amplification or alternate mechanisms. MDM2 functions as a negative feedback regulator of the tumor suppressor p53. The aim of the present study was to investigate whether mdm2-antisense oligodeoxyribonucleotides (AS-ODNs) can influence the growth characteristics of two MDM2-overexpressing STS cell lines (US8-93, LMS6-93) where both have heterozygous p53 non-missense mutations. Cells were treated with lipofectamine-complexed mdm2 AS-ODNs complementary to a sequence of the mdm2 cDNA initiation site in comparison to sense control ODNs. After seeding and cultivation of a defined cell number the clonogenic survival was performed. The treatment of US8-93 cells with AS-ODNs, but not with sense ODNs, decreased the number of colonies up to > 80%. Western blot analysis demonstrated a significant decreasing of MDM2 protein level in AS-ODN transfected cells indicating an AS-specific inhibition of mdm2 transcription in US8-93 cells. Additionally, an increase of the G2/M population was found. In contrast, in the LMS6-93 cells treated with AS-ODNs only a decrease in clonogenic survival up to 26%, no change in MDM2 protein level and no cell cycle alterations were seen. All these factors taken together into consideration can be suggest that lipid-mediated mdm2 AS-ODNs could be as an effective therapeutic strategy for STS with an abnormal mdm2 overexpression.

The roles of E6-AP and MDM2 in p53 regulation in human papillomavirus-positive cervical cancer cells

The p53 tumor suppressor is regulated by the MDM2 oncoprotein through a negative feedback mechanism. MDM2 promotes the ubiquitination and proteasome-dependent degradation of p53, possibly by acting as a ubiquitin ligase. In cervical cancer cells containing high-risk human papillomaviruses (HPV), p53 is also targeted for degradation by the HPV E6 oncoprotein in combination with the cellular E6-AP ubiquitin ligase. In this report, we describe the identification of efficient antisense oligonucleotides against human E6-AP. The roles of MDM2 and E6-AP in p53 regulation were investigated using a novel E6-AP antisense oligonucleotide and a previously characterized MDM2 antisense oligonucleotide. In HPV16-positive and HPV-18 positive cervical cancer cells, inhibition of E6-AP, but not MDM2, expression results in significant induction of p53. In HPV-negative tumor cells, p53 is activated by inhibition of MDM2 but not E6-AP. Furthermore, treatment with both E6-AP and MDM2 antisense oligonucleotides in HPV-positive cells does not lead to further induction of p53 over inhibition of E6-AP alone. Therefore, E6-AP-mediated degradation is dominant over MDM2 in cervical cancer cells but does not have a significant role in HPV-negative cells.

Antisense therapeutics: is it as simple as complementary base recognition?

Antisense oligonucleotides provide a simple and efficient approach for developing target-selective drugs because they can modulate gene expression sequence-specifically. Antisense oligonucleotides have also become efficient molecular biological tools to investigate the function of any protein in the cell. As the application of antisense oligonucleotides has expanded, multiple mechanisms of oligonucleotides have been characterized that impede their routine use. Here, we discuss different mechanisms of action of oligonucleotides and the possible ways of minimizing non-antisense-related [corrected] effects to improve their specificity.